Induction of apoptosis in AK-5 tumor cells by a serum factor from tumor rejecting animals: cytochrome c release independent of Bcl-2 and caspases

The ability to selectively induce apoptosis in tumor cells is the prime goal in cancer immunotherapy and aims at identifying potential molecular targets, regulating this process. Here we show that the sera from the animals which had spontaneously rejected the AK-5 tumor (a rat histiocytoma) had an effective and potent ability to counteract and kill tumor cells by inducing apoptosis, with a high degree of specificity. Apoptosis induced by the serum factor involved the activation of caspases and cytochrome c release to the cytosol. A reduction in mitochondrial transmembrane potential (Delta psi(m)) occurred considerably later than cytochrome c translocation. The anti-apoptotic protein Bcl-2 and the pancaspase inhibitor zVAD-fmk did not prevent cytochrome c release, but completely blocked the reduction in Delta psi(m), DNA fragmentation and apoptosis. Cyclosporin A (CsA), an inhibitor of the mitochondrial permeability transition (MPT) pore had no effect on cytochrome c release and apoptosis mediated by serum factor in AK-5 cells, suggesting that apoptosis was independent of MPT. Taken together these results suggest that the serum factor in conjunction with the immune cells may be participating in the efficient rejection of the tumor in syngeneic hosts and Delta psi(m) disruption but not cytochrome c release, is a critical and decisive event to trigger apoptotic cell death induced by the serum factor in AK-5 tumor cells.