Susceptibility to TNF in the presence of inhibitors of transcription or translation is dependent on the activity of cytosolic phospholipase A2 in human melanoma tumor cells.

In this study, we have examined the relationship between the expression of the high molecular weight, cytosolic form of PLA2 (cPLA2) and ability of inhibitors of transcription or translation (ITT) to induce susceptibility to TNF. S Susceptibility to lysis was assayed by 51CR release, and the expression of cPLA2 was assayed by activity assay and by Western blot. The panel of cells that we examined included two murine cell lines, six human melanoma-derived cell lines, two samples of freshly explanted melanoma tumor tissue, and a culture of normal epidermal melanocytes. Our experiments revealed a near perfect correlation between the activity of cPLA2 per cell and susceptibility to TNF in the presence of either cycloheximide (CHI) or actinomycin D (r = 0.97). These results suggest that the activity of cPLA2 is both necessary and rate-limiting in this form of programmed cell death, conclusions that were confirmed in transfection experiments and in experiments with antisense oligonucleotides. Over-expression of cPLA2 in two melanoma-derived cell lines, WM793 and SK-MEL-131, led to enhanced susceptibility to TNF and CHI. Conversely, suppression of cPLA2 with antisense oligonucleotides dramatically decreased susceptibility to TNF and CHI in C3HA fibroblasts. These experiments also revealed a coupled, transformation-released change in the expression of cPLA2 and susceptibility to lysis. Normal melanocytes contained the lowest levels of cPLA2 and were completely resistant to sensitization with ITT. In contrast, all of the melanoma-derived cell lines and samples of melanoma tumor tissue we examined has higher levels of cPLA2 and could be killed, to some extent, by treatment with TNF and ITT.