P-glycoprotein expression does not change the apoptotic pathway induced by curcumin in HL-60 cells

Purpose. One of the mechanisms responsible for the multidrug resistance (MDR) phenotype of cancer cells is overexpression of so-called ATP-dependent drug efflux proteins: the 170-kDa P-glycoprotein (P-gp) encoded by the MDR1 gene and the 190-kDa multidrug resistance-associated protein 1 encoded by the MRP1 gene. The purpose of the present study was to verify the hypothesis postulating that P-gp expression, apart from enabling drug efflux, confers on the cells resistance to apoptosis by inhibiting caspase-8 and caspase-3. Materials and methods. Human HL-60 cells, either drug-sensitive or with the MDR phenotype caused by overexpression of P-gp (HL-60/Vinc) or MRP1 (HL-60/Adr), were treated with the natural dye curcumin at 50 muM or with UVC to induce apoptosis. Symptoms of cell death were assessed by morphological observation after Hoechst staining, DNA fragmentation was measured by flow cytometry and the TUNEL method, and caspase-8 and caspase-3 activation and cytochrome c release from mitochondria were measured by Western blotting. Results. Curcumin induced cell death in HL-60 cells, both sensitive and with the MDR phenotype, which could be classified as caspase-3-dependent apoptosis, together with cytochrome c release, activation of caspase-3 and oligonucleosomal DNA fragmentation. No active caspase-8 was detected. Also UVC caused caspase-3 activation in both the sensitive and the MDR HL-60 cells. Conclusions. Our findings show that there was no correlation between P-gp expression and resistance to caspase-3-dependent apoptosis induced by curcumin and UVC, at least in HL-60 cells. However, we cannot exclude the possibility of parallel P-gp expression and caspase-3 inhibition in some other cell lines, as cancer cells can acquire many different apoptosis-resistance mechanisms.