Cisplatin resistance in a murine leukemia cell line is associated with a defective apoptotic process

Apoptosis is characterized by typical morphological changes and most frequently fragmentation of DNA into oligonucleosome-size fragments. In order to investigate whether an alteration in the mechanisms involved in the process of apoptosis could contribute to cellular resistance, induction of apoptosis was studied in a cisplatin-resistant cell line (L1210/DDP) derived from a L1210 murine leukemia cell line (L1210/0). Treatments of the parental L1210/0 cell line with two DNA damaging agents (cisplatin and 5-azacytidine) or a protein kinase C inhibitor (staurosporine) led to biochemical events characteristic of apoptosis (as determined by the cell morphology and the oligonucleosomal DNA fragmentation). In contrast, the cisplatin-resistant L1210/DDP subline, which was cross-resistant to 5-azacytidine, did not exhibit any DNA fragmentation or morphological changes typical of apoptosis when exposed to toxic concentrations of either cisplatin or 5-azacytidine. The failure of these cells to undergo apoptosis upon cisplatin or 5-azacytidine exposure has been correlated with the lack of a nuclear endonuclease activity present in wild-type cell nuclei. However, staurosporine, which exerted the same toxicity on both cell lines, induced the internucleosomal DNA fragmentation and morphological features of apoptosis in both of them. This indicates that a functional pathway for apoptosis is preserved in the resistant cells. The induction of this pathway can be correlated with the presence of a cytoplasmic endonuclease activity whose specificity seems different from that operating in L1210/0 cells in terms of cation and pH dependence. Therefore, in these cell lines, different endonucleases are possibly involved in apoptosis. In response to treatment with drugs having different targets, the apoptotic cell death may operate through different signaling pathways, one of them being possibly defective in the L1210/DDP-resistant cells.