Reversal of P-glycoprotein-mediated multidrug resistance in SGC7901/VCR cells by PPARγ activation by troglitazone

Over-expression of P-glycoprotein (P-gp), an ATP-dependent drug efflux pump, represents one of the major mechanisms that contribute to multidrug resistance (MDR) in cancer cells. This study examined the effects of troglitazone, a ligand of peroxisome proliferator-activated receptor gamma (PPARγ), on P-gp-mediated MDR in SGC7901/VCR cells (a vincristine-resistant human gastric cancer cell line). The expression of P-gp was detected by RT-PCR and Western blotting, respectively. The SGC7901/VCR cells were treated with 0.1 mg/L vincristine (VCR) alone or in combination with 1, 5, 10 μmol/L troglitazone for 24 h. PPARγ was measured by electrophoretic mobility shift assay (EMSA). The intracellular concentration of Rhodamine123 (Rh123, a fluorescent P-gp substrate) was assayed to evaluate the activity of P-gp. The cell cycle and apoptosis were measured by flow cytometry. The results showed that the P-gp was increasingly expressed in SGC7901, BGC823 and SGC7901/VCR cells in turn, suggesting that MDR in the SGC7901/VCR cells was mediated by the increased expression of P-gp. In the SGC7901/VCR cells, the expression level of total PPARγ was increased, however, the protein level and activity of PPARγ in the nuclei of cells decreased significantly. Troglitazone elevated the PPARγ activity in SGC7901/VCR cells in a dose-dependent manner. Troglitazone decreased the P-gp expression and markedly enhanced the accumulation of Rh123 in SGC7901/VCR cells in a dose-dependent manner. We also found that troglitazone significantly increased the percentage of SGC7901/VCR cells in the G₂/M phase and decreased the cell percentage in G₁ and S phase in a dose-dependent manner. Troglitazone significantly increased the apoptotic rate of SGC7901/VCR cells treated by VCR or ADR in a dose-dependent manner. It was concluded that P-gp-overexpressed SGC7901/VCR cells have minor endogenous PPARγ activity. Elevation of the PPARγ activity by troglitazone can reverse P-gp-mediated MDR via down-regulating the expression and activity of P-gp in SGC7901/VCR cells. It was suggested that troglitazone can dramatically enhance the sensitivity of P-gp-mediated MDR cancer cells to chemotherapeutic agents.