Activation of high levels of endogenous phospholipase A2 in cultured cells

Abstract

Activatable cellular phospholipase A2 (PLase; phosphatide 2-acyl-hydrolase, EC 3.1.1.4) has been proposed to constitute the first and rate-limiting step in prostaglandin synthesis and to regulate membrane function by altering the levels in the membrane of the detergent lipids lysolecithin and free fatty acids. We have observed that a wide variety of cells in culture contain high levels of endogenous PLase that can be activated by polypeptide toxins, such as melittin purified from bee venom and direct lytic factor purified from the venom of African Ringhals cobra (Hemachatus hemachatus). Activation of PLase by sublytic concentrations of these agents results in the synthesis and release of prostaglandins. Melittin concentrations greater than or equal to 10 microgram/ml activate sufficient PLase in 3T3-4a mouse fibroblasts to hydrolyze 10% of the cellular lecithin in less than 5 min and virtually all of it within 30 min, demonstrating the existence of sufficient activatable PLase to provide the basis for the proposed mechanism of regulation of membrane function by alteration of membrane lipid composition. Lipases, phospholipases B and C, and sphingomyelinases are not activated by melittin. The PLase activated in 3T3-4a cells exhibits little, if any, specificity for individual phosphoglycerides. The PLase activated by direct lytic factor exhibits a Ca2+ dependence characteristic of lysosomal PLase, wherease the Ca2+ dependence of PLase activated by melittin is consistent with the activation of a cell-surface enzyme. The extent of cell death correlates with percent of maximal PLase activation.