Characterization of group X phospholipase A(2) as the major enzyme secreted by human keratinocytes and its regulation by the phorbol ester TPA

HaCaT as well as human primary keratinocytes constitutively expressed mRNA of the human secreted phospholipase A(2) subtype groups X, V, IIA, and IID. A similar expression pattern was also found in human skin biopsies. Protei n analysis showed that under serum-free conditions only group X secreted ph ospholipase Az is secreted into cell culture supernatants of HaCaT as well as human primary keratinocytes, whereas the other secreted phospholipases A (2) were not detectable at protein level. HaCaT keratinocytes constitutivel y released secreted phospholipase A(2) activity into the cell culture super natant, being reflected by a constant release of fatty acids. The phorbol e ster 12-O-tetradecanoylphorbol-13-acetate, which is a potent inducer of inf lammation in skin, drastically reduced the mRNA level of group X secreted p hospholipase A(2) and other secreted phospholipase A(2) subtypes as well as secreted phospholipase A(2) activity in cell culture supernatants. This su ggests that inhibition of secreted phospholipase A(2) expression and activi ty as well as of fatty acid release by 12-O-tetradecanoylphorbol-13-acetate treatment might be a critical step impairing the integrity of the epidermi s during phorbol-ester-induced pathologic processes in skin. The results sh ow that group X secreted phospholipase A(2) represents the major secreted p hospholipase A(2) subtype in human keratinocytes and thus may indicate a ph ysiologic role for this enzyme in epidermis in vivo.