PHORBOL ESTER TPA-INDUCED AND BRADYKININ-INDUCED ARACHIDONIC-ACID RELEASE FROM KERATINOCYTES IS CATALYZED BY A CYTOSOLIC PHOSPHOLIPASE-A2 (CPLA2)

In a previous paper, we have shown that bradykinin (Bk) and the phorbo l ester 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulate arachidon ic acid release from HEL-30 keratinocytes along a Bk-B2 receptor G-pro tein-coupled pathway or a protein kinase C-dependent mechanism, respec tively. Here we show a cytosolic PLA2 (cPLA2) to be responsible for th is effect. This enzyme exhibited a marked acyl-group specificity towar ds arachidonic acid. It was activated by Ca++ in micromolar concentrat ions and partially translocated from the cytoplasmic to the membrane f raction upon Ca++ treatment. Translocation was also observed upon trea tment of cells with either Bk or TPA. However, only with Bk was a corr esponding increase of the cytoplasmic Ca++ level observed, whereas TPA -induced translocation occurred at basal Ca++ concentrations. Indirect evidence for a G protein to be involved in Bk- but not TPA-dependent cPLA2 activation was provided using non-hydrolyzable GTP derivatives. It is concluded that keratinocyte cPLA2 plays a critical role in the i nitiation by exogenous and endogenous factors of the eicosanoid cascad e in skin.