Effects of permeability transition inhibition and decrease in cytochrome c content on doxorubicin toxicity in K562 cells

As mitochondria play a key role in the commitment to cell death, we have investigated the mitochondrial consequences of resistance to doxorubicin ( DOX) in K562 cells. We found that the permeability transition pore ( PTP) inhibitor cyclosporine A ( CsA) failed to inhibit PTP opening in the resistant clone. Moreover, the Ca2+ loading capacity in the resistant clone was identical to that observed in the parent cells in the presence of CsA, suggesting that the PTP was already inhibited in a CsA-like manner in the resistant cells. In agreement with this proposal, the mitochondrial target of CsA cyclophilin D ( CyD) decreased by half in the resistant cells. The levels of adenine nucleotide translocator, voltage anion-dependent channel, Bax, Bcl-2, Bcl-x(L), AIF and Smac/Diablo, were similar in both cell lines, whereas cytochrome c content was divided by three in the resistant cells. Since P-glycoprotein inhibition did not restore DOX toxicity in the resistant cells, while DOX-induced cell death in the parent cells was prevented by either PTP inhibition or siRNA-induced decrease in cytochrome c content, we conclude that the inhibition of PTP opening and the decrease in cytochrome c content participate in the mechanism that makes K562 cells resistant to DOX.