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Quercetin depletes glutathione in U87 glioblastoma and A549 lung
carcinoma cell lines: Implications for use in combination with tumor cytotoxic
therapies
Christopher T. Chen, Angela D. Colby, Phyllis R. Wachsberger, Stephanie N.
Lavorgna, Adam P. Dicker, Randy Burd
Thomas
Jefferson University, Philadelphia, PA
Quercetin (Qct) is a
phenolic phytochemical belonging to a class of bioflavonoids that are found in
fruits, vegetables and teas.
Phenolic phytochemicals such as Qct can have both
antioxidant and pro-oxidant effects depending on their cellular concentrations.
The oxidation of Qct by enzymes such as peroxidases or tyrosinases form
pro-oxidant quinoid products that can react with glutathione (GSH) to form
quinone methide-GSH adducts.
The present study was undertaken to test the
hypothesis that Qct, through its pro-oxidative metabolism, can deplete GSH,
rendering cells sensitive to oxidative stress.
We investigated the effects of
Qct on the U87 human glioblastoma and A549 lung carcinoma cell lines based on a
method developed by Biaglow et al., 2000 (Analyt. Biochem. 281, 77-86).
Because
hydroxyethyl disulfide (HEDS) is primarily bioreduced by GSH, the bioreduction
of HEDS serves as an indirect measure of GSH levels.
Therefore, a decrease in
the rate of HEDS bioreduction would indicate cellular GSH depletion.
HEDS
bioreduction was measured in cells grown on 6- well plates and incubated in the
presence or absence of 75 uM Qct for 1 hr.
The data presented are the means of
at least 3 independent experiments +/- standard deviation.
GSH depletion by Qct
was greater in U87 cells than in A549 cells.
In the presence of Qct, the rate of
HEDS bioreduction decreased to 31 +/- 7% and 53 +/- 2% of control in U87 and
A549 cells, respectively (p<0.05 when either rate was compared to the
untreated control).
Since the formation of Qct-GSH adducts is increased at low
pH, we lowered the pH of the cellular incubation medium.
In U87 cells, a
decrease in extracellular pH to 6.7 or 6.5 decreased the rate of HEDS
bioreduction to 20 +/- 4 % and 16 +/- 3% of control, respectively.
In A549
cells, decreasing the extracellular pH to 6.7 or 6.5 decreased the rate of HEDS
bioreduction to 30 +/- 1% and 25 +/- 5% of control, respectively.
Cell viability
as determined by trypan blue exclusion was compromised starting 2 hrs after
incubation with Qct.
The U87 cells were more sensitive to cell death by Qct than
A549, which may be a reflection of the greater GSH depletion by Qct in U87
cells.
These results indicate that although Qct may act as a free radical
scavenger, it can also have significant cytotoxic effects.
Depletion of GSH by
administration of Qct has the potential to be utilized clinically to improve
therapies such as chemotherapy and/or radiation therapy.
Studies are ongoing to
determine the effect of Qct in combination with these other cytotoxic agents.
Copyright © 2003 American
Association for Cancer Research. All rights reserved.
Source: http://aacr03.agora.com/planner/displayabstract.asp?presentationid=7067
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