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Accepted article online:
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Issue=20 online:
30 Oct 2007

Received: April 28, 2007; Accepted: June 29,=20 2007

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Fig.=20 1 Scheme showing the critical functions assumed by cancer = progenitor cells and the bone-marrow ...

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Fig.=20 2 Scheme showing the possible oncogenic cascades involved = in the=20 stimulation of sustained growt...

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Table=20 1 Characterization of specific biomarkers and functional=20 properties of human cancer progenitor...

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Table=20 2 Characterization of specific biomarkers and functional=20 properties of Hoechst dye-low side po...

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Table=20 3 Specific inhibitors of growth factor cascades and = signaling=20 elements involved in sustained g...

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Journal of Cellular and Molecular = Medicine

Volume 11 Issue 5 Page 981-1011, = September/October 2007=20

To cite this article: M. Mimeault, R. Hauke, P. P. = Mehta, S.=20 K. Batra (2007) Recent advances in cancer stem/progenitor cell research: = therapeutic implications for overcoming resistance to the most = aggressive=20 cancers
Journal of Cellular and Molecular Medicine 11 (5) , = 981=E2=80=931011=20 doi:10.1111/j.1582-4934.2007.00088.x

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Stem Cells Review Series

Recent advances in cancer stem/progenitor = cell=20 research: therapeutic implications for overcoming resistance to the most = aggressive cancers

^aDepartment of = Biochemistry and=20 Molecular Biology, Eppley Institute of Cancer and Allied Diseases, = University=20 of Nebraska Medical Center, Omaha, NE, USA ^bDepartment of Pathology and Microbiology, Eppley = Institute=20 of Cancer and Allied Diseases, University of Nebraska Medical Center, = Omaha,=20 NE, USA ^cDivision of Hematology and = Oncology,=20 Department of Internal Medicine, Eppley Institute of Cancer and Allied = Diseases, University of Nebraska Medical Center, Omaha, NE, USA = ^dEppley Institute of Cancer and Allied = Diseases,=20 University of Nebraska Medical Center, Omaha, NE, = USA

*Correspondence to:=20 Murielle MIMEAULT and Surinder K. BATRA
Department of Biochemistry = and=20 Molecular Biology, Eppley Institute of Cancer and Allied Diseases, = University of=20 Nebraska Medical Center, Omaha, Nebraska 68198-5870, USA. Tel: (40 2)55 = 9-5455;=20 Fax: (40 2)55 9-6650 E-mail: mmimeault@unmc.edu or sbatra@unmc.edu

=E2=80=A2	Introduction
=E2=80=A2	Functions of cancer progenitor cells in = the cancer=20 initiation and progression
-	New model of carcinogenesis based on the = cancer=20 progenitor cells
-	Isolation and ex vitro and in = vivo=20 characterization of functional properties of cancer progenitor=20 cells
-	Functions of cancer progenitor cells in = the cancer=20 development
-	Influence of local tumour microenvironment = on the=20 behavior of cancer progenitor cells
=E2=80=A2	Cancer types originating from cancer = progenitor=20 cells
-	New concepts of the heterogeneity of = cancers derived=20 from distinct cancer progenitor cells
-	Implication of cancer progenitor cells in = bone=20 marrow-derived cancers
-	Leukaemias
-	Sarcomas
-	Implication of cancer progenitor cells in = pediatric=20 and adult brain tumors
-	Implication of cancer progenitor cells in = other=20 cancer types
=E2=80=A2	Novel cancer therapies by molecular = targeting of=20 cancer progenitor cells and their microenvironment
-	New concepts on the functions of cancer = progenitor=20 cells in the resistance to current cancer therapies
-	New combination therapies against the = aggressive and=20 recurrent cancers
-	Targeting cancer progenitor = cells
-	Targeting the local microenvironment of = cancer=20 progenitor cells
=E2=80=A2	Conclusions
=E2=80=A2	Future directions and=20 perspectives

Abstract

Overcoming intrinsic and acquired resistance of cancer=20 stem/progenitor cells to current clinical treatments represents a major=20 challenge in treating and curing the most aggressive and metastatic = cancers.=20 This review summarizes recent advances in our understanding of the = cellular=20 origin and molecular mechanisms at the basis of cancer initiation and=20 progression as well as the heterogeneity of cancers arising from the = malignant=20 transformation of adult stem/progenitor cells. We describe the critical=20 functions provided by several growth factor cascades, including = epidermal growth=20 factor receptor (EGFR), platelet-derived growth factor receptor (PDGFR), = stem=20 cell factor (SCF) receptor (KIT), hedgehog and Wnt/=CE=B2 -catenin = signalling=20 pathways that are frequently activated in cancer progenitor cells and = are=20 involved in their sustained growth, survival, invasion and drug = resistance. Of=20 therapeutic interest, we also discuss recent progress in the development = of new=20 drug combinations to treat the highly aggressive and metastatic cancers=20 including refractory/relapsed leukaemias, melanoma and head and neck, = brain,=20 lung, breast, ovary, prostate, pancreas and gastrointestinal cancers = which=20 remain incurable in the clinics. The emphasis is on new therapeutic = strategies=20 consisting of molecular targeting of distinct oncogenic signalling = elements=20 activated in the cancer progenitor cells and their local = microenvironment during=20 cancer progression. These new targeted therapies should improve the = efficacy of=20 current therapeutic treatments against aggressive cancers, and thereby=20 preventing disease relapse and enhancing patient survival.

Introduction

Major improvements in the treatment of cancer patients have been = accomplished=20 in the few last years. The development of more effective diagnostic and=20 prognostic methods is leading to an earlier therapeutic intervention in = the=20 clinics [1=E2=80=9317].=20 Among the clinical treatments, tumour surgical ablation, hormonal = therapy,=20 radiotherapy and chemotherapy, alone or in combination, are the most = currently=20 used therapies for treating the patients diagnosed with diverse cancers=20 including leukaemias and malignant solid tumours, such as skin, head and = neck,=20 brain, lung, kidney, bladder, prostate, breast, ovary, pancreas and=20 gastrointestinal cancers [= 1=E2=80=933,=20 5=E2=80=939, 12=E2=80=9324]. In general, the patients diagnosed with = localized cancers that=20 are treated with these conventional clinical therapies have a high = response rate=20 and a good outcome. Although these conventional therapies are effective = in=20 initial phase of treatment, the cancer progression to locally invasive = and=20 metastatic states is often associated with resistance to treatments and = disease=20 relapse, which leads to the death of the patients [1=E2=80=933,=20 5=E2=80=937, 9, 12, 13, 15=E2=80=9317, 21=E2=80=9323, = 25=E2=80=9331]. The cancer recurrence phenomenon has=20 been associated with the accumulating genetic and/or epigenic = alterations in=20 cancer cells that may contribute to their uncontrolled growth, survival = and=20 invasion as well as their intrinsic or acquired resistance to clinical=20 treatments [1=E2=80= =933,=20 13, 16, 26=E2=80=9328, 31=E2=80=9341]. The altered activation and/or = overexpression of=20 numerous growth factors, adenosine 5' -triphosphate (ATP)-binding = cassette (ABC)=20 multidrug efflux transporters, anti-apoptotic factors (MYC, Bcl-2, = NF-=CE=BA B and=20 survivin) as well as a decreased expression or activity of tumour = suppressor=20 genes (p53 and phosphatase and tensin homolog, PTEN) in cancer cells may = contribute to the drug resistance and disease relapse [1,=20 16, 26=E2=80=9328, 32=E2=80=9346].

Recently, new concepts have been proposed on the = critical=20 implication of highly leukaemic or tumourigenic cancer progenitor cells = also=20 designated as cancer stem cells or cancer-initiating cells, in cancer = initiation=20 and progression to metastatic disease states and resistance to = conventional=20 therapies [16,=20 35, 39, 40, 47=E2=80=9353]. These cancer progenitor cell-based = concepts may=20 partially explain the recurrence of the most aggressive cancers to = current=20 clinical treatments. More specifically, the reactivation of diverse=20 developmental signalling cascades (epidermal growth factor (EGF)/EGFR, = stem cell=20 factor (SCF)/KIT, platelet-derived growth factor (PDGF)/PDGFR, sonic = hedgehog=20 (SHH/PTCH/GLI transcription factor) and/or Wnt/=CE=B2 -catenin) combined = with the=20 increased DNA repair mechanisms and ABC transporter-mediated multi-drug = efflux=20 in cancer progenitor cells may be responsible, at least in part, for = their=20 resistance to current clinical therapies [

Functions of cancer progenitor cells in the cancer = initiation and=20 progression

New model of carcinogenesis based on the cancer progenitor = cells

Numerous investigations have provided evidence that the = genetic=20 and/or epigenic alterations occurring in the multi-potent = tissue-specific adult=20 stem cells and/or their early progenies may lead to their malignant=20 transformation into cancer progenitor cells also designated as cancer = stem cells=20 or cancer-initiating cells (Fig.=20 1 and 2) [3,=20 13, 37, 39=E2=80=9341, 47=E2=80=9353, 55, 57, 60, 62=E2=80=9365]. A = small population of=20 undifferentiated- or poorly differentiated cancer progenitor cells, = which=20 possesses the stem cell-like properties including their self-renewal = ability and=20 capacity to give rise to the bulk mass of further differentiated = malignant=20 cells, appears to represent the principal cancer cells that are = responsible for=20 leukaemia or tumour formation [13,=20 39, 40, 47, 59, 63, 66=E2=80=9376]. Accumulating genetic and/or = epigenic alterations=20 in leukaemic or tumourigenic cancer progenitor cells occurring during = cancer=20 progression, and more particularly the acquisition of a migratory = phenotype by=20 tumour cells during epithelial-mesenchymal transition (EMT) program, may = also=20 confer to them the invasive properties that are essential for their = migration to=20 distant metastatic sites [39,=20 40, 49, 64, 72, 73, 77=E2=80=9381]. This cancer progenitor cell = model of=20 carcinogenesis is notably supported by the fact that the poorly = differentiated=20 and highly-leukaemic or tumourigenic cancer progenitor cells isolated = from=20 patients'malignant tissue specimens may give rise to the bulk mass of = further=20 differentiated cancer cells ex vitro and in vivo, and thereby be = responsible for=20 leukaemia or tumour development (Fig. 1;=20 Table=20 1, 2) [47,=20 63, 66=E2=80=9370, 72, 73, 75, 76]. Recent investigations have also = revealed that a=20 very small sub-population of cancer progenitor cells with stem cell-like = properties may be isolated from well-established cancer cell lines and=20 maintained under an undifferentiated or poorly differentiated state = during long=20 years in culture, and this even after multiple passages in medium = containing=20 serum [47,=20 59, 65=E2=80=9369, 72, 73, 75, 76, 82=E2=80=9384]. In regard with = this, we describe here the=20 recent investigations that have led to the isolation of cancer = progenitor cells=20 from numerous cancer types and the establishment of their functional = properties=20 ex vitro and in animal models in vivo.

Isolation and ex vitro and in vivo = characterization of=20 functional properties of cancer progenitor cells

Significant advancements have been made in the identification of the = specific=20 biomarkers of multi-potent tissue-specific adult stem cells. Researchers = have=20 been able to isolate these adult stem cells as well as their malignant=20 counterparts, cancer progen-itor cells from total cell mass in cancer = patients=E2=80=99=20 malignant tissue specimens and well-established cancer cell lines for = their ex=20 vitro and in vivo functional characterization (Table=20 1, 2) [47,=20 59, 65=E2=80=9369, 71=E2=80=9373, 75, 76, 82=E2=80=9389]. Hence, the = isolated small sub-population=20 of cancer progenitor cells may be subsequently expanded ex vivo in = serum-free=20 medium and further characterized by the non-adherent spheroid generation = and=20 clonogenicity assays for establishing their self-renewal and = multi-lineage=20 capacities in vitro. The implantation and serial transplantations assays = may=20 also be carried out with the isolated cancer progenitor cells in animal = models=20 for estimating their leukaemic or tumouri-genic potential and = self-renewal=20 ability in vivo [47,=20 59, 65=E2=80=9369, 71=E2=80=9373, 75, 76, 82=E2=80=9387]. More = particularly, a very small=20 sub-population of human cancer progenitor cells expressing the specific = stem=20 cell-like surface markers has been successfully isolated from malignant = tissues=20 and/or well-established cancer cell lines. Among the cancer types = harbouring a=20 sub-population of cancer progenitor cells, there are the acute myeloid=20 leukaemia, multiple myeloma, melanoma, head and neck, brain, breast, = ovary,=20 prostate, pancreas and colorectal cancers (Table 1) [47, = 66=E2=80=9369, 71=E2=80=9373, 75, 76, 82=E2=80=9387, 90=E2=80=9392]. = It has been shown that all these cancer=20 progenitor cells, which possess a self-renewal capacity, are able to = give rise=20 in vitro and/or in vivo to the bulk mass of further differentiated = cancer cells=20 that recapitulates the cellular heterogeneity and morphological = characteristics=20 of cancer tissues from which they originate [66=E2=80=9369,=20 71=E2=80=9373, 75, 76, 82=E2=80=9384, 86]. The fact that the = engrafted leukaemic or=20 tumourigenic cells could be serially transplanted into other mice has = also=20 provided further experimental evidence of the self-renewal capacity of = these=20 cancer-initiating cells [67=E2=80=9369,7= 3,76].=20 Particularly, a small number of these poorly differentiated cancer = progenitor=20 cells showed a higher leukaemic or tumourigenic potential in animal = models in=20 vivo as compared to their further differentiated progenies [67=E2=80=9369,=20 72, 75, 76, 83, 84, 86, 87]. For instance, a sub-population of = non-adherent=20 melanoma spheroid cells expressing CD20⁺ antigen has been = isolated=20 from human metastatic melanoma tissues and established primary WM115 and = metastatic WM239A melanoma cell lines derived from a same patient [70]. The multi-potent individual = cells=20 within these non-adherent melanoma spheres established from metastatic = melanoma=20 tissues were able to give rise to multiple mesenchymal cell lineages = including=20 melanocytes, adipocytes, osteocytes and chondrocytes ex vitro and in = vivo (Table 1) [70]. These cells forming = non-adherent=20 melanoma spheres were also more tumourigenic than their adherent = melanoma cell=20 counterpart in severe combined immunodeficient (SCID) mice in vivo [70]. Similarly, a single clone = (A2)=20 expressing different markers, such as CD44, Oct-3/4, Nanog, EGFR, = vimentin and=20 E-cadherin and able to form the multi-layered spheroids ex vivo, has = been=20 isolated from the total cancer cell population of the ascites of a = patient with=20 advanced ovarian cancer [63].=20 Additionally, another clone A4-T derived from multi-layered spheroids = that=20 underwent a spontaneous transformation in culture has also been = isolat-ed. A4-T=20 was characterized by an expression marker profile comparable to that of = A2 clone=20 with the exception that it expressed a detectable level of the = transcriptional=20 repressor of E-cadherin, Snail but not E-cadherin suggesting that this = clone may=20 have to undergo a more complete EMT program [63]. Importantly, the A4-T clone = was more=20 potential than the A2 clone to form the tumours with an architecture = resembling=20 to the patients=E2=80=99 original tumours, and undergo metastases in = nude mice in vivo=20 (Table 1) [63]. On the other hand, by using a = specific=20 antibody directed against the embryonic stem cell-like marker Oct-3/4, = which is=20 a POU family transcription factor, combined with the analyses of its = expression=20 level by RT-PCR, it has been observed that the human breast, liver, = pancreas,=20 kidney, mesenchymal and gastric adult stem cells, and a few = keratinocytes in the=20 basal layer of human skin epidermis express significant levels of = Oct-3/4 [93]. Moreover, the human, dog and = rat=20 tumours and HeLa and MCF-7 cancer cell lines also expressed a detectable = level=20 of its stem cell-related marker Oct-3/4, while the differentiated cells = did not,=20 supporting thereby the cancer progenitor cell concept of tumourigenesis = [93].

In addition, the enrichment of undifferentiated or poorly = differentiated=20 cancer progenitor cells expressing the stemness genes from patient's = cancer=20 samples or well-established cancer cell lines has also been performed by = the=20 Hoechst dye efflux technique, which is particularly useful when the=20 tissue-specific stem cell markers are not well established. Hence, a = very small=20 fraction of cancer cells designated as a =E2=80=98side = population=E2=80=99 (SP), and which=20 possesses a high ability to actively efflux the fluorescent DNA-binding = dye,=20 Hoechst 33342 due to elevated expression levels of ABC multi-drug efflux = pumps,=20 has been identified in several mammalian malignant tumour tissues and=20 well-established cancer cell lines (Table=20 2) [54,=20 74=E2=80=9376, 80, 86=E2=80=9389, 94, 96=E2=80=9398, 101, 103]. For = instance, the FACS sorted SP=20 fraction from the C6 glioma cell line may differentiate in vitro and in = vivo=20 into neurons and glia expressing the neuronal (low molecular weight=20 neurofilament [NF-L] or =CE=B2III-tubulin) and glial (glial fibrillary = acidic=20 protein, [GFAP]) markers, respectively, and form the metastatic tumours = when=20 injected intraperitonally in nude mice in vivo [96,97]. Importantly, it has = been noted=20 that the number of cancer cells detected in the SP population versus the = non-SP=20 fraction may be significantly influenced by the experimental conditions=20 including the source of the patients'malignant tissues (untreated, = treated=20 and/or relapsed patients) and the culture conditions used (cell density = and=20 growth factors) [54, 97,=20 102, 105].

In light of these observations, it appears that several = types of=20 malignant tissues and well-established cancer cell lines contain a very = small=20 population of undifferentiated or poorly differentiated cancer = progenitor cells=20 that may be responsible for the generation of leukaemia or tumour = formation in=20 animal models in vivo. Hence, the most aggressive human cancers may = originate=20 from this small highly leukaemic or tumourigenic cancer progenitor cell=20 sub-population possessing stem cell-like properties and an aberrant=20 differentiating ability. Based on these observations, it appears = important to=20 consider the presence of these putative cancer progenitor cells in = certain=20 cancer cell lines which have been initially established from patients = with=20 leukaemia and malignant primary or secondary neoplasms. More = particularly, this=20 small sub-population of cancer progenitor cells isolated from = patients'malignant=20 tissue specimens or well-established cancer cell lines may represent a = more=20 appropriate experimental model for the basic cancer research and drug = testing in=20 vitro and in vivo.

Functions of cancer progenitor cells in the cancer = development

Numerous recent investigations indicated that the most = cancers may=20 arise from the malignant transformation of multi-potent tissue-specific = adult=20 stem cells and/or their early progenitors into cancer progenitor cells.=20 Furthermore, the accumulation of different genetic and/or epigenic = alterations=20 in cancer progenitor cells during cancer progression also seems to be = associated=20 with the occurrence of highly aggressive cancer subtypes. However, one = of the=20 major challenges now is to determine more precisely whether the cancers = may=20 originate in the majority of cases from accumulating oncogenic events = occurring=20 in the cancer progenitor cells that may lead to the generation of all = the=20 further differentiated cancer cells or whether the more committed and=20 differentiated cell lineage precursors with the stem-cell-like = properties could=20 be responsible of the initiation of certain cancer subtypes. The = available=20 experimental lines of evidence appear to corroborate the possibility = that these=20 two processes are not exclusive and may be dependent on the cancer = subtypes [69,=20 90, 106=E2=80=93113]. In this matter, the familial genetic = alterations, such as=20 germinal mutations may notably promote the incidence of certain cancers = derived=20 from the embryonic or adult stem cells [114=E2=80=93123].=20 For instance, it has been proposed that the deregulation of the hedgehog = signalling pathway associated with the germinal mutations in PTCH = receptor gene may lead to the developmental defects and a = pre-disposition to=20 develop certain tumours, such as basal cell carcinoma, medulloblastoma,=20 meningioma, foetal rhabdomyoma as well as embryonal rhabdomyosarcoma, = which=20 appears to originate in myogenic satellite cells [118, 119]. Intriguingly, = it has also=20 been suggested that the transformed cells possessing the biomarkers and=20 differentiating properties like embryonic hemangioblasts, which = represent the=20 putative common embryonic precursor to both haematopoietic and = endothelial cell=20 lineages (Fig. 1), may = contribute to the=20 development of certain malignancies in adult life [123, 124]. Among them, the = ocular=20 and central nervous system (CNS) hemangioblastomas, which are highly = vascular=20 tumours that may occur in patients with inherited von Hippel-Lindau = (VHL)=20 disease, have been proposed to likely originate from a = haematopoi-etic/vascular=20 cell lineage [123, = 125]. More=20 specifically, it has been proposed that the malignant cells that are at = the=20 origin of the hemangioblastoma formation within CNS in patients with VHL = syndrome could represent the mesoderm-derived embryologically arrested=20 hemangioblasts that have persisted in adults, and which may be further=20 transformed under specific conditions (Fig.=20 1) [123]. In addition, the = primitive=20 neuroectodermal tumours (PNETs) including neuroblastoma, = pheochromocytoma,=20 ependymoblastoma and pineoblastoma may also originate during the = embryonic=20 development from neuroecto-dermal stem cells such as neural crest stem = cells (Fig. 1) [10, = 11, 121,=20 122, 126, 127]. For instance, it has been reported that the immature = and=20 tumouri-genic human neuroblastoma stem cells termed as intermediate type = (I-type) cells isolated from neuroblastoma tissues or cell lines were = able to=20 give rise to the neuronal cells, melanocytes and Schwann cell precursors = that=20 constitute the tumours [128=E2=80=93130]. = Although further=20 genetic studies are necessary to confirm the possible implication of = embryonic=20 cells in the development of certain cancers, it is noteworthy that = several=20 cancer progenitor cells derived from adult stem cells also may = re-express the=20 markers associated with the more primitive pluripotent embryonic stem = cells. In=20 addition to these oncogenic changes occurring in cancer progenitor = cells, the=20 local tumour microenvironment may also influence their behaviour during = the=20 multiple steps of carcinogenesis.

Influence of local tumour microenvironment on the behaviour = of cancer=20 progenitor cells

The functional properties of cancer progenitor cells may = be=20 influenced through external signals mediated by other further = differentiated=20 cancer cells and host stromal cells including activated fibroblasts and=20 infiltrating immune cells, such as macrophages and endothelial cells [131=E2=80=93134]. Among the=20 diverse growth factors, chemokines and angiogenic substances released by = stromal=20 cells, there are matrix-degrading enzymes, matrix metalloproteinases = (MMPs) and=20 urokinase-type plasminogen activator (uPA), stromal cell-derived = factor-1=20 (SDF-1) acting as CXC chemokine receptor 4 (CXCR4) ligand and vascular=20 endothelial growth factor (VEGF) [132, 133, 135, = 136].=20 All these soluble factors can influence, of autocrine or paracrine = manner, the=20 tumour cell behaviour and neovascularization process during cancer = progression.=20 In fact, the tumour and stromal cells may actively collaborate during = each step=20 of the tumour formation at primary and secondary sites in mediating the=20 pro-angiogenic signals (VEGF and SDF-1) that contribute to the = mobilization and=20 recruitment of the bone-marrow (BM)-derived cells into peripheral = circulation,=20 and their subsequent migration to distant tumoural sites (Fig. 1) [134, 136]. Among the cells = that may=20 be attracted from BM and peripheral circulation into primary or = secondary=20 tumours, there are the circulating endothelial progenitor cells (EPCs)=20 characterized by different biomarkers, including CD34⁺ or=20 CD34^=E2=88=92, CD133, VEGF receptor-2 (VEGFR-2), also = designated as Flk-1=20 (foetal liver kinase-1), SCF receptor (KIT) and CXCR4. Moreover, the=20 haematopoietic progenitor cells expressing VEGFR-1 (Flt-1, = fms-like=20 tyrosine kinase) also can participate to the new vessel formation within = tumour=20 [131,=20 133=E2=80=93135, 137=E2=80=93139]. More specifically,the = multi-potent BM-derived EPCs=20 possessing the stem cell-like properties may migrate into peripheral = circulation=20 and reach the tumoural sites where they can contribute to the tumour=20 neovascularization process (Fig. = 1). The=20 circulating EPCs can promote the sprouting of pre-existent endothelial = cells=20 within tumours and/or participate in the formation of new = microvasculature in=20 differentiating into mature endothelial cells with the=20 CD34⁺/CD133^=E2=88=92/VEGFR-2⁺/KIT^{+<=
/SUP>/CXCR4⁺=20
phenotype that physically incorporate to new tumour microvasculature (Fig. 1) [131,=20
134, 137, 138, 140=E2=80=93142]. In regard with this, it has been =
observed that the=20
BM-derived EPCs were recruited into the tumour site and incorporated in=20
lymphoma, melanoma, brain, lung, liver, pancreas, breast and prostate =
tumour=20
vessels in the mouse models in vivo (Fig.=20
1) [135,=20
142=E2=80=93144]. It has been observed that a small number of EPCs =
may be detected=20
in circulation and within tumours in the patients diagnosed with =
Hodgkin's=20
lymphoma and non-small cell lung cancer (NSCLC) as well as head and =
neck,=20
thyroid, mucoepidermoid, osteogenic and breast carcinomas [138, 145]. Although it =
appears that=20
the number of circulating EPCs that migrate to tumour sites and =
contribute to=20
tumour vascularization is low, an enhanced number of EPCs in peripheral=20
circulation in NSCLC patients has been associated with a weak response =
rate to=20
antitumoural therapy and a poor clinical behaviour and overall survival =
[141]. Moreover, the possibility =
that a part=20
of the EPCs that migrates to tumour sites can give rise to further=20
differentiated endothelial cells within neo-plasms, and thereby may be=20
undetectable, in peripheral circulation should also be considered. =
Further=20
investigations by the engineering of specific modification in EPCs are =
therefore=20
necessary to estimate more precisely the number of EPCs that can migrate =
to the=20
tumour site as well as the amount of EPC-derived-endothelial cells that =
are=20
incorporated in new tumour microvessels in diverse human cancer types. =
In=20
addition, recent studies has also revealed that the EPCs including=20
CD133⁺ EPCs with mesenchymal stem cell (MSC)-like =
characteristics and=20
MSCs could also reside within the specific niches in certain normal =
human=20
tissues [146=E2=80=93148].=20
Interestingly, the resident CD133⁺ renal progenitor cells, =
which are=20
committed to differentiate into endothelial cells, were able to form the =
new=20
vessels in the presence of tumour-derived growth factors, and thereby =
can=20
promote the tumour growth established by co-transplanted renal carcinoma =
cells=20
in SCID mice in vivo [148]. =
Moreover,=20
the injection of BM-derived human =
Flk⁺/CD34^=E2=88=92 MSCs as well=20
as the differentiated human endothelial cells into severe SCID mice =
engrafted=20
with human malignant melanoma, resulted in their incorporation into new =
tumour=20
vessels as confirmed by an immunohistochemical analysis of the walls of =
murine=20
tumour vessels [149]. On the =
other hand,=20
the circulating VEGFR-1 positive haematopoietic progenitor cells that =
migrate to=20
tumour sites may release the growth factors that promote the =
neoangiogenic=20
process, and thereby contribute to the stabilization of the =
microenvironment=20
that favours the incorporation of EPC-derived endothelial cells in =
tumour=20
microvasculature [134, 138,=20
139]. The changes within the local tumour microenvironment combined =
with=20
different oncogenic events occurring in cancer progenitor cells during =
cancer=20
progression may also lead to the occurrence of distinct cancer=20
subtypes.}

Cancer types originating from cancer progenitor = cells

New concepts of the heterogeneity of cancers derived from = distinct=20 cancer progenitor cells

According to the hierarchical organization model of=20 carcinogenesis, the leukaemic or tumourigenic cancer progenitor cells = with=20 unlimited potential for self-renewal and altered differentiating = capacity are=20 considered to be able to generate the total cancer cell mass of further=20 differentiated progenies that constitutes the leukaemia or tumour [16,=20 39, 40, 48, 57, 65, 69, 112, 150=E2=80=93154]. Based on this cancer = progenitor cell=20 concept, it is likely that the differences between the phenotypic and = functional=20 properties of cancer progenitor cells from which a cancer originates may = be at=20 the basis of the heterogeneity of cancers [13,=20 16, 39, 40, 42, 49, 64, 73, 77, 155]. More specifically, the = differentiating=20 potential of cancer progenitor cells from which a cancer originates may=20 corroborate the differentiation status of the resulting cancer sub-type. = For=20 instance, certain undifferentiated- to poorly-differentiated cancer = subtypes=20 could arise from the malignant transformation of primitive multi-potent=20 stem/progenitor cells that acquire an aberrant differentiating potential = resulting in a maturation arrest at an early stage of cell lineage=20 differentiation [39, = 152,=20 156]. In contrast, other moderately- to well-differentiated cancer = subtypes=20 could derive either from multi-potent cancer progenitor cells with a = high=20 differentiating capacity that are able to give rise to further = differentiated=20 cancer cells or more committed differentiated cancer progenitor cells = [39, = 106, 112,=20 113, 152, 156]. Furthermore, the differences between the specific = oncogenic=20 events activated in cancer progenitor cells, such as a gain of new = oncogenic=20 product expression and/or repression of tumour suppressor genes combined = with=20 the changes in their microenvironment during the progression from = localized=20 cancers into invasive forms may lead to the development of distinct = cancer=20 subtypes or be responsible for the intratumoural heterogeneity noticed = for a=20 certain cancer type [39,=20 40, 42, 64, 69, 72, 73, 77=E2=80=9381, 111, 157, 158]. More = particularly, the=20 acquisition of a more malignant behaviour by poorly- or moderately=20 differentiated cancer progenitor cells during the EMT program including = a=20 migratory potential may result in highly aggressive cancer forms [39,=20 40, 42, 64, 72, 73, 77=E2=80=9381]. In support with these new = concepts on the=20 heterogeneity of cancers, several recent lines of evidence revealed that = distinct poorly- to moderately differentiated cancer progenitor cells = with=20 different stem cell-like properties may be at the origin of cancer = subtypes. In=20 this regard, we describe here the recent observations supporting the=20 heterogeneity of cancers derived from distinct cancer progenitor cells = with a=20 particular emphasis on the cellular origin of leukaemia, brain and = breast cancer=20 subtypes.

Implication of cancer progenitor cells in bone = marrow-derived=20 cancers

Leukaemias

Leukaemias are a type of cancer that arise in the blood-forming = tissue, the=20 BM and result in the production of a large number of immature blood = cells termed=20 as leukaemic blasts that accumulate in the BM and within the = bloodstream, and=20 which did not provide the functions of normal blood cells (Fig. 1). Several experimental lines = of=20 evidence suggested that the leukaemias, including acute = myeloid/myelogenous=20 leukaemia (AML), acute promyelocytic leukaemia (APL), chronic=20 myeloid/myelogenous leukaemia (CML), acute lymphoblastic leukaemia (ALL) = and=20 chronic lymphocytic leukaemia (ALL) may originate from the malignant=20 transformation of multi-potent haematopoietic stem cells (HSCs) or their = early=20 progenies in BM into leukaemic stem cells (LSCs) also designated as=20 leukaemia-initiating cells (Fig. = 1) [21,=20 22, 69, 90, 94, 95, 108, 109, 112, 154, 159, 160]. Among the = frequent=20 malignant transforming events occurring in HSCs or their more committed=20 progenitors, there are the gene re-arrangements leading to the = generation of=20 chimeric fusion oncoproteins and aberrant activation of diverse = haematopoietic=20 growth factor and cytokine signalling cascades involved in the stringent = regulation of self-renewal and/or differentiation of HSCs, such as = hedgehog,=20 Wnt/=CE=B2 -catenin, Notch, KIT and/or FMS-like tyrosine kinase 3 (FLT3) = pathways [48,=20 58, 108, 109, 150, 161=E2=80=93165]. Hence, the generation of LSCs, = which possess a=20 self-renewal capacity and aberrant differentiating potential, can give = rise to a=20 heterogeneous population of leukaemic blasts containing different = haematopoietic=20 cell lineage precursors and further differentiated cells that vary with = the=20 leukaemia subtype (Fig. 1) [65, 90, 108, = 112].=20 Especially, the accumulating genetic alterations including gene = translocations=20 and/or mutations in CD34^=E2=88=92 or=20 CD34⁺/CD38^=E2=88=92/LIN^=E2=88=92 = primitive HSCs or their early=20 progenitors may result in the cell sub-populations that display strong=20 repopulating activity and leukaemic-initiating-capacity in non-obese = diabetic=20 (NOD)/SCID mice in vivo [90, 94,=20 166]. In this matter, since it has been observed that the primitive = CD34=20 HSCs can give rise to CD34⁺ cells in animal models in vivo, = it is=20 therefore likely that these two HSC sub-populations may initiate certain = types=20 of leukaemia under specific conditions [94, 95, 167].

The specific chromosomal translocations generating different fusion=20 oncoproteins, such as TEL-AML1, MLL-ENL, MN1-TEL and AML1-ETO and = mutations in=20 the genes encoding transcriptional factors such as Spi-1/PU.1 that are = involved=20 in regulation of the differentiation of haematopoietic progenitor cells = may=20 notably inhibit their differentiation into mature myeloid or lymphoid = cells, and=20 hence contribute to the induction and progression of certain AML or ALL = subtypes=20 [1,=20 153, 154, 168=E2=80=93172]. Among the initiating events associated = with APL which is=20 an AML subtype, there is a gene re-arrangement involving the retinoic = acid=20 receptor-=CE=B1 gene (RAR-=CE=B1) with the promyelocytic = leukaemia gene=20 (PML), t(15;17) chromosomal translocation that generates the=20 PML-RAR-fusion protein. PML-RAR-=CE=B1 oncoprotein may induce an arrest = of=20 granulopoiesis concomitant with the accumulation of immature = granulocytes=20 designated as promyelocytes [1]. = Additional molecular oncogenic events that frequently occur during=20 leukaemogenesis include the activating mutations in K- and N-Ras and the = members=20 of class III receptor tyrosine kinase family, such as FLT3, KIT and = PDGFR as=20 well as tumour suppressor gene p53 mutations and enhanced = expression of=20 Bcl-2 and Wilms tumour gene product (WT1) [1, 161, 170, = 172]. These=20 events may enhance the proliferation and/or inhibit the apoptosis in AML = or ALL=20 cells, and thereby promote the risk of relapse [1, 161, 170, = 172].=20 Importantly, it has also been noted that the occurrence of one = transforming=20 event that leads to an arrest of the differentiation combined with = another event=20 inducing a proliferative effect in haematopoietic progenitor cells may=20 co-operate to induce the more malignant leukaemiasubtypes [1, 170, 173].

Accumulating evidence has indicated that certain leukaemia subtypes = could=20 originate from additional genetic and/or epigenic alterations occurring = in the=20 intermediate haematopoietic progenitor cells, the pre-cancerous-LSCs = (pre-LSCs)=20 also designated as pre-cancerous stem cells (p-CSCs) (Fig. 1) [106, 112, 113]. The = pre-LSCs,=20 which may derive from more primitive HSCs, could possess a more limited=20 self-renewal and multi-potent differentiating ability. In support with = this, it=20 has been noted that the = CD34⁺/CD38^=E2=88=92/LIN^=E2=88=92 LSCs=20 in the most AMLs did not express the Thy-1 surface marker like normal=20 Thy-1⁺ HSCs, suggesting that these leukaemia types = could be=20 accompanied either by the mutations leading to the down-regulation of = its=20 expression during the malignant transformation of primitive = Thy-1⁺=20 HSCs, or derived from more committed leukaemic progenitor cells (Fig. 1) [90, 106, 112]. For = instance,=20 the analyses of BM cells from AML patients showing long-term remission = after=20 chemotherapeutic treatment, alone or combined with autologous mobilized = blood=20 transplantation (MBT), have revealed the persistence of AML-ETO = chromosomal=20 translocation in Thy-1⁺ HSCs [106]. This suggests that this = genetic=20 alteration may be insufficient to cause the cancer initiation. In = contrast, the=20 same patient harboured a population of Thy-1^=E2=88=92 LSCs at = diagnosis=20 corroborating the fact that this cancer subtype may have arisen from the = additional oncogenic events in more differentiated leukaemic progenitor = cells=20 [106]. The single clones of = p-CSCs=20 (pre-LSCs) with=20 CD45^=E2=88=92/KIT^=E2=88=92/Sca-1^{=E2=88=92/LIN^=E2=88=92/CD44^high=20
phenotype, which have been established from the spleen of a mouse with =
dendritic=20
cell-like leukaemia, also retained an incomplete multi-potency and can=20
differentiate into non-malignant or malignant cells depending on their=20
microenvironment [113]. The =
malignant=20
transformation of pre-LSCs was associated with the up-regulation of KIT =
and=20
Sca-1, suggesting that pre-LSCs may represent a cell population with an=20
intermediate phenotype between the normal HSCs and LSCs (Fig. 1) [113]. Additionally, further =
oncogenic=20
events in LSCs during cancer progression, as observed for the cancer =
progenitor=20
cells during EMT in solid tumours, may also be accompanied by the =
acquisition of=20
more malignant phenotypes [174]. For=20
instance, during the initial chronic phase of CML, a t(9;22) =
BCR-ABL=20
chromosomal translocation resulting in a shortened chromosome 22, =
designated the=20
Philadelphia (Ph⁺) chromosome occurs in the HSCs and =
generates the=20
chimeric BCR-ABL fusion oncoprotein with an enhanced constitutive =
tyrosine=20
kinase activity in CML-LSCs [108,=20
175]. Moreover, the initial phase of CML may be accompanied by the=20
overexpression of Bcl-2 and/or decreased Jun B expression [108, 175]. These molecular =
events=20
may enhance the rate of growth factor-independent proliferation and =
reduce=20
apoptotic signals in haematopoietic progenitor cells [174]. This initial chronic phase =
is=20
followed by an accelerated phase driven by the occurrence of further =
oncogenic=20
transformation of LSCs leading ultimately to the Ph⁺ B =
cell-ALL and=20
the terminal stage of the disease designated as blast-crisis (Bc)-CML =
[108, 164, 175]. =
More=20
specifically, the late-stage Bc-CML is accompanied by an expansion of=20
granulo-cyte-macrophage progenitors (GMPs) expressing an enhanced level =
of=20
=CE=B2-catenin whose oncogenic event appears to be associated with an =
enhanced=20
self-renewal capacity and leukaemic potential of human GMPs [108, 109]. On the other =
hand, it has=20
also been observed that the BCR-ABL transduction in mouse committed =
myeloid=20
progenitors does not induce their malignant transformation in vitro =
whereas=20
MOZ-TIF2 or MLL-ENL fusion proteins were transforming for both mouse =
HSCs and=20
their more committed myeloid progenitors [107, 108, 110].=20
BCR-ABL-transduced mouse committed myeloid progenitors also did not =
generate an=20
AML in animal model in vivo while MOZ-TIF2 or MLL-ENL-transduced-mouse =
myeloid=20
progenitors were able to give rise to an AML in irradiated mouse in vivo =
after=20
serial transplantations [107,=20
108, 110]. This suggests then that only certain types of genetic =
alterations=20
may contribute to the malignant transformation of HSCs and/or the more =
committed=20
haematopoietic cells during leukaemia initiation and progression.}

Despite the initial implication of the primitive HSCs = and/or their=20 early progeny (pre-LSCs) in the occurrence of the most the leukaemias, = these=20 observations suggest that the loss of the multi-potent or bipotent=20 differentiating potential of LSCs may occur at an early or later stage = of the=20 disease depending on the oncogenic events occurring in the malignant = cells=20 during cancer development. Particularly, the fact that certain more = committed=20 haematopoietic cells may acquire a more malignant phenotype, including = an=20 enhanced self-renewal capacity during the cancer progression, underlines = the=20 importance of also considering their implication for a more appropriate=20 therapeutic intervention. Further investigations on the genetic/epigenic = alterations occurring in primitive human HSCs and their early progenies = versus=20 the more committed progenitor cells should establish more precisely the=20 molecular oncogenic events associated with each particular case of=20 leukaemias.

Sarcomas

Adult BM also constitutes a homing site for several = other types of=20 non-haematopoietic multi-potent stem cells including EPCs and = mesenchymal=20 stem/progenitor cells (MSCs also designated MPCs) which provide the = specific=20 functions (Fig. 1) [176]. For instance, MSCs can give = rise to=20 diverse mature cell lineages, such as adipocytes, chondrocytes and = osteocytes.=20 Several lines of evidence have revealed that the bone sarcomas, which = are among=20 the most aggressive mesenchymal malignancies in childhood and young = adults, may=20 originate from the malignant transformation of primitive stem cell-like = MSCs [114, 115]. More = specifically, the=20 osteoblastic sarcoma, which is the most common type of primary bone = cancer, may=20 result in distant metastases to lungs [114, 115]. The germinal = mutations in=20 retinoblastoma (Rb) and p53 tumour suppressor genes may = notably=20 promote the incidence of osteoblastic sarcoma [114=E2=80=93117]. Since MSCs=20 are the bone-forming osteogenic progenitor cells, they may then = represent the=20 putative BM adult stem cells that may be at the origin of osteoblastic = sarcoma=20 initiation. In regard with this, it has been reported that a small = population of=20 bone sarcoma cells that are able to form the sarcospheres in culture = expressed=20 several markers of MSCs (Stro-1, CD105 and CD4) as well as the = pluripotent=20 embryonic cells (Oct-3/4 and Nanog) [177]. This supports the potential = implication of MSCs in bone sarcoma development. More recently, it has = been=20 observed that the cultured murine MSC co-infused with BM into irradiated = allogeneic host could drive the development of foci of sarcoma in the = lungs and=20 extremities in certain mice in vivo [178]. The infusion of MSC-derived = sarcoma=20 cells having a cytogenetic profile comparable to human sarcoma also = resulted in=20 the malignant lesions in secondary recipients [178]. Additionally, Ewing's = sarcoma, which=20 is a member of Ewing's family tumours (EFTs), and the second most common = solid=20 bone and soft tissue malignancy of childhood and young adults, also = appears to=20 originate from MSCs [179]. The = EFTs are=20 generally associated with the occurrence of a chromosomal translocation = that=20 involves the fusion of a 5' segment of the EWS gene with a 3' segment of = the=20 ETS transcription factor family gene FLI-1, given the = EWS-FLI-1=20 fusion protein. EWS-FLI-1 oncoprotein acts as an aberrant = transcriptional=20 activator and contributes to tumour development. In support with this, = it has=20 been shown that EWS-FLI-1 can transform the primary BM MSCs and generate = tumours=20 in animal models in vivo showing the phenotypic characteristics that = resemble=20 Ewing's sarcoma [179].

Implication of cancer progenitor cells in paediatric and = adult brain=20 tumours

Primary malignant CNS tumours are the most frequent forms of solid = malignant=20 tumours diagnosed in children and the glioblastoma multiformes (GBMs) = represent=20 one of the more frequent aggressive tumour types in adults [7, 11, 15, 29]. = The=20 cellular origin of paediatric and adult brain tumour types has not yet = been=20 precisely established. The paediatric brain tumours constitute a diverse = group=20 in terms of their localization, prognosis and response to various = therapeutic=20 treatments relative to adult brain tumours (Fig. 1) [10, 11, 29, = 126,=20 127]. Gliomas, which are the most common malignant primary brain = tumours,=20 include astrocytomas (grade I slow-growing pilocytic astrocytomas, grade = II=20 astrocytomas, grade III anaplastic astrocytomas and grade IV GBMs),=20 oligo-dendrogliomas, ependymomas and mixed gliomas, which may contain = the=20 astrocytes, oligodendrocytes and/or ependymal cell-like cells in = different=20 proportions (Fig. 1) [7,15]. Several recent lines = of evidence=20 support the concept that certain brain cancers including = medulloblastomas,=20 astrocytomas, GBMs, oligodendrogliomas and ependymomas could derive from = the=20 malignant transformation of neural stem cells (NSCs) or more = restricted-neuronal=20 and glial cell lineage precursors into brain cancer progenitor cells = also=20 designated as brain tumour stem cells (BTSCs) [15,=20 41, 52, 66, 67, 72, 73, 82, 180=E2=80=93182]. BTSCs could possess a = self-renewal=20 capacity but showed an abnormal ability to differentiate (Fig. 1). More specifically, the=20 infratentorial medulloblastomas, which are the most common brain tumours = diagnosed in childhood, may originate either from NSCs within the = ventricular=20 zone germinal layer or neuronal progenitors designated as cerebellar = granule=20 cell precursors (GCPs) located in the external granular layer within the = cerebellum (Fig. 1) [37, 127, 157, = 158].=20 Aberrant activation of SHH pathway (SHH/PTCH/GLI), which is recognized = to=20 triggering normal growth of the cerebellum in the foetus, induced = through the=20 mutations in patched receptor (PTCH), smoothened (SMO) or Sfu, may = notably=20 contribute to uncontrolled growth of GCPs in childhood and young = children, and=20 thereby induce the medulloblastoma formation [37,=20 41, 62, 111, 127, 157,158, 183]. The p53 inactivation or = sustained=20 activation of Wnt/=CE=B2 -catenin pathway may also result in an = increased expression=20 of downstream oncogenic gene products that contribute to development of = certain=20 human medulloblastoma subtypes [41, 111, 157, = 158]. In=20 addition, the data from a recent study have revealed that the pilocytic=20 astrocytomas and ependymomas arising in the supratentorial (cortex and=20 sub-cortex) versus infratentorial (cerebellum and brainstem) = regions of=20 the brain show specific genetic signatures, which were also detected in = NSCs=20 localized in these regions [184]. This=20 suggests then that these brain tumours may derive from the malignant=20 transformation of a common precursor that possesses different = gene-expression=20 patterns depending on its brain localization. Interestingly the = tumourigenic=20 CD133⁺/nestin⁺ BTSCs isolated from ependymomas = display a=20 phenotype resembling to radial glia-like cells, which are the neuronal = precursor=20 cells that may give rise to mature ependymal cells lining membrane of = brain=20 ventricles (Fig. 1) [92]. This observation supports the = fact that=20 certain ependymoma subtypes may derive from the malignant transformation = of=20 radial glia-like cells. Similarly, the secondary or progressive GBMs may = also=20 arise from the malignant transformation of NSCs into BTSCs that give = rise to a=20 heterogeneous population of further differentiated cancer cells (Fig. 1) [41, 64]. The secondary GBMs, = which=20 develop slowly from low-grade tumours, are often accompanied by the = p53=20 tumour suppressor gene mutations. In this matter, it has been observed = that the=20 inactivation both of p53 and neurofibromatosis type 1 = (NF1), which=20 is a tumour suppressor gene that is involved in negative regulation of = Ras=20 pathway, may co-operate to induce the development of malignant = astrocytomas from=20 early pre-symptomatic lesions within sub-ventricular zone containing = NSCs in=20 animal model in vivo [185]. In = contrast,=20 other subgroup of aggressive primary GBM tumours, which progress rapidly = without=20 evidence of a transitory step of lower grade tumour, are frequently = accompanied=20 by the overexpression of EGFR and acquisition of mesenchymal properties = by=20 cancer progenitor cells [41, 64, 72, 73]. = The=20 results from a recent study have also revealed that an inhibition of the = nuclear=20 co-repressor (N-CoR) of astroglial differentiation may contribute to de = novo GBM=20 development in mice suggesting that this molecular event may occur in = certain=20 GBM tumours in human [182].

In addition, the isolation of CD133/nestin positive-tumour cells from = patients=E2=80=99 malignant tissues has revealed that these primitive = cancer progenitor=20 cells can give rise in vitro and in vivo to different neural cell = lineages,=20 including neuron and glial cell-like cells (astrocytes, oligodendrocytes = and=20 ependymal cells), but in different proportion in respect to the normal = NSCs [15,=20 41, 52, 66, 67, 72, 73, 82]. For instance, it has been observed that = the=20 neurospheres established from the paediatric medulloblastomas, = astrocytomas and=20 GBMs were able to differentiate majoritarly ex vitro and in = vivo=20 into cells expressing the neural markers of original tumour cell = phenotype,=20 including the neuronal (=CE=B2-III tubulin) and astrocytic GFAP markers = [66, 67, 82]. = Similarly, the=20 multi-potent neurosphere-like clusters isolated from adult primary GBMs = can also=20 give rise to further differentiated progenitors expressing the specific = markers=20 to three neural cell lineages including neurons (=CE=B2-III tubulin), = astrocytes=20 (GFAP) and/or oligodendrocytes (myelin or galactocerebroside) ex = vitro=20 and in vivo resembling to the cellular heterogeneity = characterizing the=20 initial GBMs [72, 73]. An = immunohistochemical analysis of brain tissue specimens from astrocytomas = of=20 different grades has also revealed that the number of tumour cells = expressing a=20 NSC-like phenotype (doublecortin, =CE=B2-III-tubulin, collapsing = response-mediated=20 protein-4 [CRMP-4/TUC-4] and nestin) and proliferation marker (Ki67 = antigen),=20 was increased in high-grade astrocytomas including GBMs relative to = low-grade=20 neoplasms [186]. This suggests = then that=20 a high proliferative capacity of BTSCs could be related with the = occurrence of=20 highly aggressive tumours. In regard with this, it has been observed = that the=20 co-injection of primary human endothelial cells with CD133⁺ = BTSCs=20 from primary medulloblastomas in the cerebral cortex of = immuno-compromised mice,=20 markedly increased the tumour growth derived from CD133⁺ = cells in=20 vivo (Fig. 1) [187]. Based on this observation, = it has=20 been proposed that the perivascular niche of self-renewing BTSCs could = assume an=20 important role for their expansion during cancer development [187]. In addition, the results = from a=20 recent study revealed that certain primary glioblastoma subtypes may = also=20 derived from a small sub-population of CD133^=E2=88=92 = tumour cells=20 with stem cell-like properties and showing a distinct gene expression = pattern=20 relative to CD133⁺ cancer stem/progenitor cells [188]. Hence, since these = observations=20 suggest that different cancer stem/progenitor cells may be at origin = from=20 certain brain cancer subtypes, further investigations are then necessary = to=20 establish more precisely their specific properties and sensitivity to=20 therapeutic treatments.

Implication of cancer progenitor cells in other cancer = types

The isolation of cancer progenitor cells with the stem = cell-like=20 properties from diverse solid tumour specimens and cancer cell lines = also=20 supports the fact that melanoma, skin, head and neck, thyroid, cervix, = lung,=20 liver, breast, ovary, prostate, pancreas, gastrointestinal and retinal = cancers=20 may arise from the malignant transformation of tissue-specific adult = stem cells=20 and/or their early progenies (Table=20 1, 2) [39, = 40,47, 49, 50, 54, 59, 61, 65, 68, 70, 71, 75, 76, 80, 83=E2=80=9387, = 96, 98=E2=80=93105,=20 189]. Particularly, the re-activation of distinct growth factor = signalling=20 including EGFR, hedgehog, Wnt/=CE=B2 -catenin, Notch and/or integrin = pathways, which=20 frequently occurs in cancer progenitor cells during the cancer = initiation and=20 EMT program, may lead to different cancer subtypes (Fig. 1, 2) [13,=20 31, 35=E2=80=9337, 39, 40, 42, 49, 54=E2=80=9357, 59=E2=80=9362, 64, = 102, 190, 191]. For instance,=20 the occurrence of different malignant transforming events in breast stem = cells=20 during cancer initiation and/or accumulating of distinct = genetic/epigenic=20 alterations in breast cancer progenitor cells and/or their early = progenies=20 during cancer progression may result in the formation of different = breast cancer=20 subtypes [39,=20 42, 47, 56, 60, 68, 75, 77, 190, 192=E2=80=93196]. It has been = observed that the=20 targeted expression of stabilized -catenin in the basal epithelial cells = of=20 mouse mammary epithelium resulted in an enhanced proliferation of = basal-type=20 cell-like progenitor cells possessing an abnormal differentiation = potential,=20 whose oncogenic event led to development of invasive basal-type = carcinomas [192]. Based on the gene = expression=20 signatures detected in distinct breast cancers including the expression = levels=20 of cytokeratin 5/6 (CK5/6), erbB2/HER-2/Neu, estrogen receptor = (ER=CE=B1) and/or=20 progesterone receptor (PR), the invasive breast cancers have been = classified at=20 least five subtypes [8,=20 14, 156, 197=E2=80=93201]. Among them, there are the basal-like = (CK5/6⁺,=20 ER=CE=B1^=E2=88=92, PR^=E2=88=92, = erbB2^{=E2=88=92/low}, EGFR⁺,=20 vimentin⁺ and KIT⁺); erbB2⁺ = overexpressing=20 (ER=CE=B1^=E2=88=92 and PR^=E2=88=92; luminal A = (ER=CE=B1⁺ and/or=20 PR⁺ and erbB2^=E2=88=92); luminal B (ER = ⁺ and/or=20 PR⁺ and erbB2⁺) and normal breast-cancer subtype = (high=20 expression of normal epithelium genes and low expression of luminal = epithelial=20 gene products) [8,=20 14, 152, 197=E2=80=93201]. It has been observed that the ER=CE=B1 = -negative breast cancer=20 subtypes including the basal-like breast cancers and = erbB2-overexpressing breast=20 cancers, which are among the most aggressive breast cancer forms = diagnosed, are=20 generally associated with a poor prognosis and survival of patients to = current=20 clinical therapies [197= =E2=80=93200,=20 202, 203]. Despite the activation of estrogen-ER=CE=B1 signalling = cascade may=20 contribute to ER=CE=B1 ⁺-epithelial cell proliferation, it = has been=20 observed that the ovariectomy had no effect on the size of the mouse = mammary=20 EGFR⁺ stem cell-enriched population, which did not express = ER=CE=B1, PR or=20 erbB2 [204, 205]. = Moreover, the=20 mouse ER=CE=B1 ^=E2=88=92breast cancer cells may express a = lower level of=20 E-cadherin than the ER=CE=B1 ⁺ breast cancer cells, = and therefore=20 they can display a higher migratory capacity [206]. Although further studies = are=20 essential to confirm the implication of estrogens/ER=CE=B1 ⁺ = cascade for=20 the proliferation of human ER ^=E2=88=92 breast cancer = progenitor cells, it=20 appears likely that the different breast cancer subtypes may respond = differently=20 to the therapies consisting to targeting ER=CE=B1, erbB2 and/or EGFR [202, 203, 207]. In = this=20 matter, we are reporting the specific functional properties of cancer = progenitor=20 cells that may contribute to their resistance to current=20 therapies.

Novel cancer therapies by molecular targeting of cancer = progenitor=20 cells and their microenvironment

New concepts on the functions of cancer progenitor cells in = the=20 resistance to current cancer therapies

Accumulating evidence revealed that the intrinsic or acquired = resistance of=20 poorly differentiated and leukaemic or tumourigenic cancer progenitor = cells to=20 current clinical therapies may lead to their persistence in primary and=20 secondary neoplasms after treatments, and thereby contribute to cancer=20 recurrence [13,=20 16, 35, 39, 40, 55, 56, 59, 62, 64, 71, 78, 81, 88, 89, 155]. In = this=20 regard, certain ABC multi-drug efflux transporters including = P-glycoprotein=20 (P-gp) encoded by the MDR1 (ABCB1) gene, the multi-drug = resistance-associated=20 protein 1 (MRP1) and breast cancer resistance protein-1 (mouse = BCRP-1)/human=20 ABCG2/MXR gene products are frequently overex-pressed in cancer = progenitor cells=20 [16= , 35,=20 46, 88, 89, 95, 208]. Since the SP cells, which possess the stem = cell-like=20 properties, generally express higher levels of ABC multi-drug efflux = pump(s)=20 than the non-SP cells, it is likely that this phenotype may be also = related to=20 the intrinsic resistance of certain cancer progenitor cells to = chemotherapeutic=20 treatments. In support with this, several data have revealed that the SP = cells=20 from malignant tissues or cancer cell lines were more resistant than the = non-SP=20 cells to the chemotherapeutic drugs, and the enhanced drug efflux in the = SP=20 cells was associated with a high rate of survival [54, 80, 88, = 89,=20 209]. For instance, the SP cells isolated from HuH7 hepatocellular = cancer=20 cells, which expressed the high levels of ABCG2 and ABCB1 multi-drug = efflux=20 pumps as well as carcinoembryonic antigen-related cell adhesion molecule = (CEACAM6), which may contribute to gemcitabine chemoresis-tance, = displayed a=20 greater resistance to doxorubicin, 5-fluorouracil (5-Fu) and gemcitabine = treatment than the non-SP fraction [80, 210]. Similarly, the SP = cells=20 from the mouse ovarian MOVCAR 7 cancer cell line, which express the = BCRP-1 gene=20 product, were also more resistant to the anti-proliferative effect of=20 doxorubicin which acts as a substrate of BCRP-1 efflux pump than the = non-SP=20 fraction [98]. The ABCG2 = gene=20 product overexpression in mitoxantrone-resistant MCF-7/MitoR breast = cancer cells=20 also resulted in a substantial decrease of intracellular accumulation of = Hoechst=20 33342 dye and enhanced the number of SP cells with an acquired = multi-drug=20 resistance phenotype as compared to parental MCF-7 cell line [208]. Thus, the selection of=20 multi-drug-resistant cancer progenitor cells by continuous drug = treatment may=20 result in an increase of their proportion in total cancer cell mass, and = thereby=20 contribute to the cancer progression to highly aggressive cancers and = disease=20 relapse [54]. In addition, the = enhanced=20 expression/activation of many growth factors and anti-apoptotic = signalling=20 elements may also contribute to the drug resistance and survival of = cancer=20 progenitor cells [13,=20 16, 30, 31, 35=E2=80=9337, 39=E2=80=9342, 46, 54=E2=80=9361, 78, = 89]. For instance, many changes in=20 the apoptotic signalling elements have been observed in the chemotherapy = resistant ex vivo selected =E2=80=98survivor cell=E2=80=99 population = which was enriched for=20 CD34⁺ LSCs [211]. = Similarly,=20 CD133⁺ BTSCs isolated from primary cell lines established = from=20 glioblastoma patients, which expressed higher levels of CD90, CD44, = CXCR4,=20 Nestin, Msi1 and MELK relative to CD133^=E2=88=92 cancer = cells, were also=20 resistant to diverse chemotherapeutic drugs such as temo-zolomide, = carboplatin,=20 etoposide and paclitaxel [78].

On the other hand, the cancer stem/progenitor cell model = of=20 carcinogenesis may also explain the differences of response of distinct = cancer=20 subtypes to current therapies as well as the dormancy phenomenon and = disease=20 relapse, which may be associated with a higher resistance of cancer = progenitor=20 cells to conventional therapies under specific conditions prevalent in = primary=20 and/or secondary neoplasms relative to their further differentiated = progeny [14,=20 25, 39, 40, 46, 79, 207, 212]. Importantly, it has been observed = that the=20 CD44/CD24^{=E2=88=92/low} mammosphere cell cultures established = from human MCF-7=20 breast cancer cells were more resistant to radiation than the adherent = breast=20 cancer cell fraction and that their number was enhanced after = radiotherapy [213]. Similarly, = Sca-1⁺=20 multi-potent progenitor cells from an immortalized COMMA-D=CE=B2 -geo = murine mammary=20 gland cell line expressing a high Wnt/=CE=B2 -catenin level were also = resistant to=20 radiation at a clinically relevant dose [212]. Moreover, the = CD133⁺=20 breast cancer stem cell population from glioblastomas was also enriched = after=20 ionizing radiation treatment whose radioresistance phenomenon may be = associated=20 with a preferential activation of the DNA damage checkpoint response = concomitant=20 with an increase in DNA repair capacity relative to the CD133 fraction = [79]. Additionally, since most of = the solid=20 tumours are molecularly heterogeneous and distinct cancer cells may = possess=20 specific hyper-activated intracellular signalling cascades, it appears = that the=20 targeting of distinct oncogenic pathways may represent a more effective = strategy=20 to eradicate the total cancer cell population in certain aggressive and=20 recurrent cancers. Consistent with this, it has been observed that the=20 undifferentiated-type areas of heterogeneous gastric tumours display a = higher=20 GLI1 nuclear immunostaining than the differentiated-type areas = suggesting that=20 the undifferentiated gastric cancer subtype could be more sensitive to = the=20 agents targeting hedgehog cascade [214].=20 In analogy with the BM-resident HSCs which possess long-term in = vivo=20 re-populating abilities and are mainly under a quiescent state, the = primitive=20 quiescent LSCs may also be more resistant than diving LSCs to the = agents, such=20 as 5-FU or imatinib mesylate that principally act on the proliferative = cells (Fig. 1) [78, 81]. Hence, all these = stem=20 cell-like properties attributed to cancer progenitor cells may provide = them with=20 a higher resistance to current cancer therapies, and thereby constitute = a=20 substantial obstacle to the successful treatment of cancer patients. = Together,=20 these observations underline then the critical importance of targeting = the=20 cancer progenitor cells and their early progenies as well as their local = microenvironment in the earlier stages of cancer treatment to counteract = the=20 rapid progression of certain cancer types and prevent the metastatic = spread at=20 distant sites. In regard with this, we describe here the new cancer = therapeutic=20 strategies based on targeting of different oncogenic cascades activated = in=20 highly leukaemic or tumourigenic cancer progenitor cells, and which must = now be=20 considered for improving the current therapeutic = treatments.

New combination therapies against the aggressive and = recurrent=20 cancers

Targeting cancer progenitor cells

The simultaneous blockade of several oncogenic cascades activated in = cancer=20 progenitor cells during cancer progression may be essential for = improving the=20 current clinical treatments against high-risk, metastatic or relapsed=20 leukaemias, multiple myelomas and numerous solid cancers, including=20 neuroblastomas, medulloblastomas, GBMs and skin, lung, head and neck, = breast,=20 ovary, prostate, liver, pancreas and gastrointestinal cancers [1,=20 12, 13, 15=E2=80=9317, 30, 31, 35=E2=80=9342, 55=E2=80=9357, = 59=E2=80=9362, 163, 164, 190, 191, 214, 215].=20 Among them, the molecular targeting of diverse developmental cascades = including=20 hedgehog, Wnt/=CE=B2 -catenin, Notch, EGFR, PDGFR and KIT pathways = and/or oncogenic=20 signalling elements (telomerase, Src, ABL, PI3K/Akt, MYC, NF-=CE=BA B = and survivin)=20 which assume a critical function in regulating the self-renewal, = survival and=20 invasion of cancer progenitor cells as well as in drug resistance and = disease=20 relapse, is of therapeutic interest [1,=20 12, 13, 16, 17, 26, 30, 31, 35=E2=80=9345, 49, 55=E2=80=9359, 61, 62, = 64, 78, 102, 164, 190,=20 191, 214, 216, 217]. In this matter, one of the major significant = advances=20 in cancer therapeutics over the few last years has been the development = of a=20 number of novel anti-carcinogenic agents, including the antibodies and = small=20 chemical molecules such as the protein tyrosine kinase inhibitors (TKIs) = (Table 3) [12,=20 13, 16, 17, 24, 26, 30, 36, 37, 39, 40, 57, 216, 218=E2=80=93220]. = These=20 pharmacological agents, which target one or several oncogenic products = involved=20 in the malignant transformation of human cells, may be used alone or in=20 combination with conventional therapeutic treatments against certain = aggressive=20 cancers. For instance, certain anti-carcinogenic agents may induce the = apoptotic=20 death and/or a differentiating effect on primitive cancer-initiating = cells, and=20 thereby may constitute the useful tools for the development of more = effective=20 cancer therapies [13,=20 16, 26, 30, 35, 37, 39=E2=80=9341, 48, 55, 57, 59, 150, 215, 221]. = The combination=20 of high-dose chemotherapy or radiation with autologous or allogenic stem = cell=20 transplantation also represent another alternative for the patients = diagnosed=20 with advanced and refractory/relapsed leukaemias, multiple myeloma, = lymphomas=20 and sarcoma as well as brain, lung, breast, kidney, ovary and colorectal = cancers=20 [13, 16, = 222=E2=80=93224].

Among the current clinical targeted therapies, the use of imatinib = mesylate,=20 which is a TKI targeting BCR-ABL⁺ oncoprotein, has provoked = great=20 interest based on its high efficacy and safe profile and successful use = for the=20 first-line clinical treatment of patients with BCR-ABL⁺=20 (Ph⁺) CMLs in initial chronic phase [16, 24, 27, = 28, 33].=20 Moreover, imatinib mesylate may also act as a potential therapeutic = agent for=20 the treatment of Ph⁺ AML, FIP1L1-PDGFRA positive chronic = eosinophilic=20 leukaemia and gastrointestinal stromal tumours which frequently harbour = the=20 activating mutations in KIT and PDGFRA tyrosine kinase receptors [24, 27, 28, = 33,=20 225]. Particularly, the inhibition of the tyrosine kinase activity = of=20 chimeric BCR-ABL fusion oncoprotein by imatinib mesylate may lead to a = complete=20 cytogenetic remission after initiation treatment in the most patients = diagnosed=20 with BCR-ABL⁺ (Ph⁺) CMLs [27, 28, 33]. However, = the=20 persistence of quiescent CML-LSCs resistant to ima-tinib treatment as = well as=20 the progression to an advanced and late-stage CML disease, accelerated = phase or=20 terminal blast crisis phase, is often accompanied by acquisition of a = more=20 malignant transformed phenotype by LSCs and disease relapse [27,=20 28, 32=E2=80=9334, 81, 226=E2=80=93228]. The mutations in BCL-ABL = and other independent=20 BCR-ABL mechanisms, such as the up-regulation of Scr-related LYN kinase, = SHH,=20 Wnt/=CE=B2 -catenin, anti-apoptotic factor Bcl-2, MDR1/ABCB1 and = ABCG2/BCRP muli-drug=20 efflux transporters and/or cellular organic cation transporter Oct-1 = involved in=20 the uptake of imatinib may contribute to the CML recurrence [16,=20 27, 28, 32=E2=80=9334, 81, 164, 226=E2=80=93228]. Therefore, the = combined use of distinct=20 anti-leukaemic agents that are able to inhibit diverse oncogenic = cascades that=20 are activated in LSCs is of particular interest for overcoming imatinib=20 resistance in the refractory/relapsed CML patients. Among them, there = are the=20 inhibitors of BCR-ABL (dasatinib and nilotinib), Scr-related LYN kinase=20 (PD180970), anti-apoptotic factor Bcl-2 (ABT-737) or Oct-1 transporter=20 (prazosin) which may be used alone or incombination with imatinib = mesylate=20 and/or other cytotoxic drugs, such as interferon-=CE=B1 [27,=20 28, 34, 175, 227, 229=E2=80=93232]. Recently, it has also been = observed that the=20 exposure of quiescent BCR-ABL⁺ CML-LSCs to the growth factors = such as=20 granulo-cyte-colony stimulating factor (G-CSF) and granulocyte = macrophage=20 colony-stimulating factor (GM-CSF) may decrease the number of = non-dividing=20 BCR-ABL⁺ CML-LSCs, and thereby could also constitute an = alternative=20 strategy for improving the ima-tinib mesylate-based therapies [233, 234]. Similarly, the = current=20 treatment for APL, which consists of the combined use of all-trans = retinoic acid=20 (ATRA) and anthracycline-based chemotherapy, generally results in a high = rate of=20 complete cytogenetic remission (about 90%) and an overall survival rate = of 80%=20 in the initial phase. Certain patients with high-risk APL may, however, = relapse=20 [21, 23]. In these cases, = the=20 efficacy of clinical treatment of high-risk and relapsed = PML-RAR⁺ APL=20 may also be improved by the combined used of a differentiating agent, = ATRA plus=20 cytotoxic drugs such as arsenic trioxide that may co-operate to = eradicate the=20 total population of PML-RAR⁺ APL-LSCs [21=E2=80=9323]. = Additionally, it has=20 also been observed that the use of an inhibitor of =CE=B3-secretase = (DAPT), a=20 protease that is involved in the activation of Notch receptor-mediated=20 intracellular signalling by its lig-and Jagged-2, which is overexpressed = in the=20 LSCs from AML samples, may inhibit the growth of CD34⁺=20 CD38^=E2=88=92 AML-LSC fraction in colony formation assays in = vitro [235]. The molecular mechanisms = underlying=20 the cytotoxic effect of DAPT on LSCs appears to be due in an alteration = of the=20 AML-LSC fate and the interference of this agent with the transportation = of LSCs=20 into the stem cell-supportive microenvironmen-tal niche [45]. Interestingly, a recent study = has also=20 indicated that the targeting of LSCs at the origin of certain AML = subtypes by=20 using an activating mono-clonal antibody directed against the cell=20 surface-adhesion molecule CD44 results in a reduction of leukaemic = population in=20 human AML-LSC cells transplanted NOD-SCID mice in vivo as well as = in the=20 absence of leukaemia development after subsequent serial = transplantations in=20 other mice [236]. The data = from another=20 study carried out on a murine leukaemia model have also revealed that=20 CALM/AF10-positive AML can be propagated by a transformed leukaemic = progenitor=20 cells, with lym-phoid characteristics including CD45RA (B220) expression = and Ig=20 DH-JH re-arrangement, which can be targeted by antibodies that do not=20 cross-react with normal HSCs [237].

Interestingly, recent studies have also revealed the = possibility=20 of using the agents targeting the EGFR, hedgehog, Wnt/=CE=B2 -catenin = and/or Notch=20 cascades to inhibit the ABC muli-drug efflux transporters and/or = eliminate the=20 cancer progenitor cells (Fig. = 2;Table 3) [57,=20 89, 102, 191, 215, 238=E2=80=93240]. For instance, it has been = observed that the EGF=20 treatment of human metastatic UMSCC10B and HN12 head and neck squamous = carcinoma=20 cell lines was accompanied by an increase of number of the SP cells and = the add=20 of gefitinib reduced the SP population [102]. The cyclopamine treatment = of poorly=20 differentiated and highly-tumourigenic CD44⁺ PC3 metastatic = prostate=20 cancer cells, which possess an intermediate phenotype (androgen receptor = negative, CK5/18), was also accompanied by a decrease of the expression = levels=20 of MDR1 and BCRP/ABCG2 suggesting that the activation of hedgehog = signalling may=20 contribute to MDR in these cancer cells [239]. Importantly, the continuous = cyclopamine treatment of PC3 cell-xenografts established in nude mice = in=20 vivo has also been observed to result in apoptotic cell death and = tumour=20 regression [191]. Also, no = sign of the=20 tumour recurrence and secondary effects on normal cells was detected 72 = hrs=20 after the cessation of treatment, suggesting that the cancer-initiating = cells=20 may have been eliminated [191].=20 Similarly, it has also been reported that the hedgehog ligand induces a = clonal=20 expansion of multiple myeloma (MM) stem cells while cyclopamine = treatment=20 counteracts the expansion of purified MM stem cells but has little = effect on the=20 growth of their further differentiated malignant plasma cells [238]. The number of SP cells = detected in=20 the NCL-H929 MM cell line was also decreased after the cyclopamine = treatment [238]. Of particular therapeutic = interest,=20 cyclopamine alone also inhibited the self-renewal ability of = CD133⁺=20 glioma cancer stem cell cultures established from human GBM tumour = samples in=20 vitro, and induced an additive or synergistic anti-proliferative and = apoptotic=20 effect at a lower concentration in combination with the current = therapeutic=20 drug, temozolomide on the gliomasphere cells [240]. Long-term treatment of = gliomasphere=20 cells expressing a high expression of the stemness genes (GLI1, PTCH1, = Nanog,=20 Oct-4, SOX2, BMI1 and PCNA) with the cyclopamine alone also killed all = these=20 cancer stem cells in culture, and induced the regression of glioma = tumours=20 established from the gliomasphere cells in nude mice in vivo, = without=20 detectable secondary effects [240].=20 Moreover, the radioresistant property of CD133⁺ glioblastoma = stem=20 cells could also be reversed by using a specific inhibitor of the CDK1 = and CDK2=20 checkpoint kinases [79]. = Additionally,=20 the suppressing of =CE=B2-catenin expression in Sca-1⁺ = multi-potent=20 progenitor cells from an immortalized COMMA-D =CE=B2-geo murine mammary = gland cell=20 line has been observed to decrease their self-renewal capacity [212]. The results from a recent=20 investigation have also indicated that the Notch signalling blockade by = using an=20 inhibitor of =CE=B3-secretase (GSI-18) significantly reduced the = CD133⁺=20 cell fraction and eliminated the SP cells detected in medulloblastoma = cell mass,=20 which express a high Notch signalling levels [215]. Since the medulloblas-toma = cells=20 treated with Notch inhibitor did not effectively form the tumour = xenografts=20 in vivo, it has been suggested that the cancer-initiating cells = have been=20 eradicated [215].

Targeting the local microenvironment of cancer progenitor = cells

The development of more effective antitumoural = strategies also=20 implicates the inhibition of the neoangiogenic process which is = necessary for=20 the tumour vascularization and growth. Many anti-angiogenic agents that = are able=20 to interfere with the VEGF-VEGFR transduction system, including the = anti-VEGF or=20 VEGFR antibody, VEGFR antagonists and the soluble truncated form of = VEGFR have=20 been designed and observed to effectively counteract the tumour growth = in animal=20 models in vivo (Table 3) = [30,=20 131, 132, 134=E2=80=93136, 140, 187, 241]. Particularly, it has been = observed that=20 the targeting of VEGFRs may be more effective than the inhibition of the = single=20 receptor to prevent and/or counteract the tumour formation in animal = models=20 in vivo [135, = 137].=20 Interestingly, the treatment of the mice bearing ortho-topic U87 glioma = cell=20 xenografts with anti-VEGF monoclonal antibody, bevacizumab markedly = reduced the=20 microvasculature density and tumour growth and this anti-angiogenic = effect was=20 also accompanied by a decrease of the number of vessel-associated = self-renewing=20 CD133⁺/nestin⁺ tumour cells [187]. Based on these = observations, it has=20 been suggested that the bevacizumab may act, at least in part, by = disrupting the=20 vascular microenvironment =E2=80=98niche=E2=80=99 of BTSCs, and thereby = impairs their=20 self-renewal capacity (Fig. 1) = [187, 242]. In addition, = several=20 lines of evidence have revealed the potential benefit of targeting the=20 tumour-specific vascular precursor cells including the circulating=20 VEGFR-2⁺-EPCs and VEGFR-1⁺-PMC which may be = recruited=20 within the activated stromal compartment in the primary and secondary = neoplasms=20 for preventing and/or counteracting the neovascularization process = associated=20 with tumour development (Fig. 1) = [134, 136, 137, = 140].=20 More particularly, it has been shown that the use of BM-derived EPCs = engineered=20 to express an anti-angiogenic gene product, a soluble truncated form of = VEGFR-2,=20 may impair tumour growth in vivo [140]. The identification of = further=20 specific biomarkers to the tumour-specific EPCs should therefore help to = develop=20 the more effective anti-angio-genic treatments based on their molecular=20 targeting.

Conclusions

Altogether, these recent investigations have revealed = that the=20 most aggressive cancers may originate from the malignant transformation = of=20 embryonic or adult stem/progenitor cells into cancer progenitor cells. = The=20 cancer progenitor cells can provide critical functions in cancer = initiation and=20 progression into metastatic and recurrent disease states. Based on these = observations, the molecular targeting of highly leukaemic or = tumourigenic cancer=20 progenitor cells must be considered for improving the efficacy of the = current=20 cancer therapies. New drug combinations targeting cancer progenitor = cells have=20 been developed and shown to be effective against certain aggressive = cancers in=20 the clinical settings. Additional investigations to identify the = specific=20 biomarkers and resistance mechanisms of cancer progenitor cells versus = their=20 further differentiated malignant progenies and their normal = counterparts, adult=20 stem cells should lead to more effective and safe therapeutic treatments = for the=20 most the aggressive and incurable cancers.

Future directions and perspectives

The technical advancements in the adult stem cell = biology have=20 provided the methods to isolate and characterize the ex vitro and = in=20 vivo properties of cancer stem/progenitor cells. However, the = evaluation of=20 their functional properties ex vivo must be considered with caution = since=20 multiple extrinsic factors may influence their behaviour, and more = particularly,=20 the experimental culture conditions did not necessarily reflect their = local=20 microenvironment within the tumours. Further investigations of cancer = progenitor=20 cells in more vigorous experimental conditions are required, and thereby = should=20 help to elucidate the molecular mechanisms that provide a critical role = for=20 their uncontrolled self-renewal and/or aberrant differentiation = capacity. The=20 differences between the specific biomarkers and functional properties of = cancer=20 progenitor cells versus their normal counterpart, adult stem cells = should=20 notably be exploited for the specific targeting of cancer progenitor = cells, and=20 thereby preserve the integrity of normal tissue-specific adult stem = cells. The=20 cancer progenitor cells also exhibit a differential gene expression = pattern, as=20 compared to the bulk mass of further cancer cells, and therefore a = re-analyse of=20 the oncogenic gene products activated specifically in cancer progenitor = cells=20 appears to be necessary. Further investigations by using the cancer = progenitor=20 cells isolated from primary cancer patient's malignant tissues at = different=20 stages during the cancer progression and metastatic disease should help = to=20 identify new biomarkers for the development of more effective diagnostic = and=20 prognostic methods and targeted therapies against the aggressive and = metastatic=20 cancers. The development of new effective and safe targeted therapies by = eradicating the total population of cancer progenitor cells and their = further=20 differentiated progenies at the primary and secondary neoplasms should = allow us=20 to improve the current cancer treatments, prevent the disease relapse = and=20 thereby induce a complete cytogenetic remission and cure of cancer = patients in=20 the clinics.

Acknowledgements

We apologize to researchers that have contributed to the = advancements in cancer stem/progenitor cell research and whose works = have not=20 been cited due to space limitations. The authors are supported by the = grants=20 from the U. S. Department of Defense (PC04502, OC04110) and the National = Institutes of Health (CA78590, CA72712, CA111294). We thank Ms. Kristi = L. Berger=20 for editing the manuscript.

References

. Lowenberg B, Griffin JD, Tallman MS. Acute = myeloid=20 leukemia and acute promyelocytic leukemia. Hematology=20 Am Soc Hematol Educ. Program 2003; 82=E2=80=93101.

Medline

. Mimeault M, Brand RE, Sasson AA, Batra SK. = Recent=20 advances on the molecular mechanisms involved in pancreatic cancer = progression=20 and therapies. Pancreas. 2005;31:301=E2=80=9316.

CrossRef,=20 Medline,=20 ISI,=20 Chemport

. Mimeault M, Batra SK. Recent advances on = multiple=20 tumorigenic cascades involved in prostatic cancer progression and = targeting=20 therapies. Carcinogenesis. 2006;27:1=E2=80=9322.

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Journal of Cellular and Molecular = Medicine

Stem Cells Review Series

Recent advances in cancer stem/progenitor = cell=20 research: therapeutic implications for overcoming resistance to the most = aggressive cancers

Abstract

Introduction

Functions of cancer progenitor cells in the cancer = initiation and=20 progression

New model of carcinogenesis based on the cancer progenitor = cells

Isolation and ex vitro and in vivo = characterization of=20 functional properties of cancer progenitor cells

Functions of cancer progenitor cells in the cancer = development

Influence of local tumour microenvironment on the behaviour = of cancer=20 progenitor cells

Cancer types originating from cancer progenitor = cells

New concepts of the heterogeneity of cancers derived from = distinct=20 cancer progenitor cells

Implication of cancer progenitor cells in bone = marrow-derived=20 cancers

Leukaemias

Sarcomas

Implication of cancer progenitor cells in paediatric and = adult brain=20 tumours

Implication of cancer progenitor cells in other cancer = types

Novel cancer therapies by molecular targeting of cancer = progenitor=20 cells and their microenvironment

New concepts on the functions of cancer progenitor cells in = the=20 resistance to current cancer therapies

New combination therapies against the aggressive and = recurrent=20 cancers

Targeting cancer progenitor cells

Targeting the local microenvironment of cancer progenitor = cells

Conclusions

Future directions and perspectives

Acknowledgements

References

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