Troglitazone reduces glyoxalase I protein expression in glioma and potentiates the effects of chemotherapeutic agents

Abstract

Despite resistance of most gliomas to chemotherapy, approximately 2/3 of oligodendrogliomas show sensitivity to such agents. This sensitivity has been associated with deletions on chromosome 1p alone or in combination with 19q. Higher expression of the enzyme glyoxalase I has been found in oligodendrogliomas with chromosome 1p intact compared to those with a deletion. Higher expression of this enzyme is also associated with tumor chemoresistance in other cancers. The present study tested whether the drug troglitazone would make a glioma cell line more sensitive to chemotherapeutic agents. This drug was chosen because it has been shown to decrease glyoxalase I enzyme activity in cells. Treatment with troglitazone decreased expression of glyoxalase I, and potentiated cell death when used in combination with chemotherapeutic agents. This decrease in glyoxalase I protein may be one mechanism by which this potentiation occurs, and troglitazone may be a candidate for use in glioma therapy.

Figure 1

Western blot demonstrating GLO-1 protein expression in four commercial glioma cell lines.

Figure 2

(a) Representative Western blots showing GLO-1 protein expression after treatment of U-373 cells with 10, 25, 50, and 100 μM troglitazone (TRG) for various time periods. Note loss of β-actin control at 72 hrs for 100 μM TRG concentration, reflecting loss of cell viability at this time point. (b) A representative gel showing semiquantitative reverse transcription PCR of GLO-1 following treatment of U-373 cells with 50 μM TRG. (c) Densitometry analysis of Western blots showing GLO-1 expression after U-373 cells were treated with concentrations of TRG. Asterisks demark time points with a significance of P < .01 compared to control based on Student's two-tailed t-test. Each data point is the mean and SEM of three separate experiments performed at the specified condition.

Figure 3

A representative experiment demonstrating U-373 cell viability after 72 (a) and 96 hours (b) of treatment with TRG, 100 nM DOX, and TRG/100 nM DOX together at the specified concentration of TRG. (c) Time course of treatment of U-373 cells with 25 μM troglitazone (TRG), 100 nM doxorubicin (DOX), and TRG/100 nM DOX together. Combination treatments found to potentiate cytotoxicity are marked with an asterisk; other concentrations were found to be additive. Each data point is the mean and SEM of seven wells treated at the specified condition.

Figure 4

A representative experiment showing cell viability time course with treatment of U-373 cells with TRG, 300 μM BCNU, and TRG/300 μM BCNU together at 25 (a), 50 (b), and 100 (c) μM concentrations of TRG. Combination treatments found to potentiate cytotoxicity are marked with an asterisk; other concentrations were found to be additive. Each data point is the mean and SEM of seven wells treated at the specified condition.