ACTIVATION OF CYTOSOLIC PHOSPHOLIPASE-A2 BY TRANSFORMING GROWTH-FACTOR-ALPHA IN HEL-30 KERATINOCYTES

In the mouse keratinocyte line HEL-30 the epidermal mitogen transformi ng growth factor-alpha (TGF-alpha) stimulated the rapid release of ara chidonic acid in a dose- and time-dependent manner. The liberation of arachidonic acid was due to the activation of a Ca2+-dependent cytosol ic phospholipase A2 (cPLA2). The activation mechanism critically depen ded on a functionally active epidermal growth factor receptor tyrosine kinase and occurred independently of phospholipase C-mediated increas es in cellular diacylglycerol and inositol 1,4,5-trisphosphate concent rations and protein kinase C activation. The activation included an in crease in cytosolic PLA2 (cPLA2) activity and an association of the en zyme with the membrane fraction. Both activation steps apparently occu rred in the presence of basal cytoplasmic Ca2+ concentrations. Moreove r, cPLA2 or a closely associated protein was found to be phosphorylate d on tyrosine upon TGF-alpha challenge of the cells. The data suggest that tyrosine phosphorylation is involved in the TGF-alpha-induced act ivation of cPLA2.