Cellular events involved in the sensitization of etoposide resistant cells by inhibitors of calcium-calmodulin-dependent processes - Role for effects on apoptosis, DNA cleavable complex, and phosphorylation

Inhibitors of calcium calmodulin dependent processes, 1-[N,O-bis(1,5-isoquinolinesulfonyl)-N- methyl-L-tyrosyl]-4-piperazine KN-62 and trifluoperazine (TFP), at non-cytotoxic concentrations (2 and 5 mu M, respectively) enhanced etoposide (VP-16) cytotoxicity in Adriamycin(R)-resistant (HL-60/ADR0.05) cells (3- to >50-fold). In contrast to TFP, the inhibitor KN-62 was able to reverse resistance in HL-60/ADR0.05 cells at VP-16 concentrations that produced equivalent cytotoxicity in sensitive (HL-60/S) cells. Unlike TFP, the cellular accumulation of VP-16 in the presence of KN-62 was enhanced 1.5- to 2-fold in HL-60/S (MDR1 -ve) and HL-60/ADR0.05 (MDR1 +ve) cells. To achieve equivalent cytotoxicity, levels of VP-16 in the resistant cells were >4-fold lower in the presence of KN-62 compared with treatment with VP-16 alone. The sensitizing effects of both KN-62 and TFP were due to enhancement (2- to 4-fold) of VP-16 induced topoisomerase II (TOPO II)-mediated DNA cleavable complex formation, and depletion of the 170 kDa (alpha) TOPO II isoform. The DNA damage induced by VP-16 in the presence of KN-62 or TFP resulted in the rapid induction of apoptosis and depletion of cells in ''S'' phase of the cell cycle. Both 5 mu M TFP and 2 mu M KN-62 enhanced the phosphorylation of 170 kDa TOPO II 1.6-fold and 1.5-fold, respectively. Results suggest that the inhibitory effect of KN-62 or TFP on calcium-calmodulin-dependene processes may be mechanistically involved in sensitizing resistant cells to VP-16 by enhancing TOPO II-mediated DNA damage. Copyright (C) 1996 Elsevier Science Inc.