SUSCEPTIBILITY TO TNF IN THE PRESENCE OF INHIBITORS OF TRANSCRIPTION OR TRANSLATION IS DEPENDENT ON THE ACTIVITY OF CYTOSOLIC PHOSPHOLIPASEA(2) IN HUMAN-MELANOMA TUMOR-CELLS

In this study, we have examined the relationship between the expressio n of the high molecular weight, cytosolic form of PLA(2) (cPLA(2)) and ability of inhibitors of transcription or translation (ITT) to induce susceptibility to TNF. Susceptibility to lysis was assayed by Cr-51 r elease, and the expression of cPLA(2) was assayed by activity assay an d by Western blot, The panel of cells that we examined included two mu rine cell lines, six human melanoma-derived cell lines, two samples of freshly explanted melanoma tumor tissue, and a culture of normal epid ermal melanocytes. Our experiments revealed a near perfect correlation between the activity of cPLA(2) per cell and susceptiblity to TNF in the presence of either cycloheximide (CHI) or actinomycin D (r = 0.97) . These results suggest that the activity of cPLA, is both necessary a nd rate-limiting in this form of programmed cell death, conclusions th at were confirmed in transfection experiments and in experiments with antisense oligonucleotides. Overexpression of cPLA, in two melanoma-de rived cell lines, WM793 and SK-MEL-131, led to enhanced susceptibility to TNF and CHI, Conversely, suppression of cPLA(2) with antisense oli gonucleotides dramatically decreased susceptibility to TNF and CHI in C3HA fibroblasts. These experiments also revealed a coupled, transform ation-related change in the expression of cPLA(2) and susceptibility t o lysis. Normal melanocytes contained the lowest levels of cPLA(2) and were completely resistant to sensitization with ITT. In contrast, all of the melanoma-derived cell lines and samples of melanoma tumor tiss ue we examined had higher levels of cPLA(2) and could be killed, to so me extent, by treatment with TNF and ITT.