Expression of Ca2+-independent and Ca2+-dependent phospholipases A2 and cyclooxygenases in human melanocytes and malignant melanoma cell lines

We provide novel evidence that human melanoma cell lines (M10, M14, SK-MEL28, SK-MEL93, 243MEL, 1074MEL, OCM-1, and COLO38) expressed, at mRNA and protein levels, either Ca2+-independent phospholipase A2 (iPLA2) or cytosolic phospholipase A2 (cPLA2) and its phosphorylated form. Normal human melanocytes contained the lowest levels of both PLA2s. Cyclooxygenase-1 and -2 (COX-1 and COX-2) were also expressed in cultured tumor cells as measured by Western blots. The most pronounced overexpression of iPLA2 and COX-1 was found in two melanoma-derived cells, M14 and COLO38. Normal human melanocytes and the M10 melanoma cell line displayed no COX-2 expression. Using subcellular fractionation, Western blot and confocal microcopy analyses, in paradigmatic SK-MEL28 and SK-MEL93 cells we showed that iPLA2, COX-1 and even cPLA2 were equally located in the cytosol, membrane structures and perinuclear region while COX-2 was preferentially associated with the cytosol. Specific inhibitors of these three enzymes significantly reduced the basal proliferation rate either in melanocytes or in melanoma cell lines. These results, coupled with the inhibition of the cell proliferation by electroporation of melanoma cells with cPLA2 or COX-2 antibodies, demonstrate that a possible correlation between PLA2-COX expression and tumor cell proliferation in the melanocytic system does exist. In addition, the high expression level of both PLA2s and COXs suggests that eicosanoids modulate cell proliferation and tumor invasiveness.