Mechanisms by which elevated intracellular calcium induces S49 cell membranes to become susceptible to the action of secretory phospholipase A(2)

Exposure of S49 lymphoma cells to exogenous group IIA or V secretory phosph olipase A(2) (sPLA(2)) caused an initial release of fatty acid followed by resistance to further hydrolysis by the enzyme. This refractoriness was ove rcome by exposing cells to palmitoyl lysolecithin. This effect was specific in terms of lysophospholipid structure, Induction of membrane susceptibili ty by lysolecithin involved an increase in cytosolic calcium and was duplic ated by incubating the cells with calcium ionophores such as ionomycin. Lys olecithin also activated cytosolic phospholipase A(2) (cPLA(2)). Inhibition of this enzyme attenuated the ability of lysolecithin (but not ionomycin) to induce susceptibility to sPLC(2). Lysolecithin or ionomycin caused concu rrent hydrolysis of both phosphatidylethanolamine and phosphatidylcholine i mplying that transbilayer movement of phosphatidylethanolamine occurred upo n exposure to these agents but that susceptibility is not simply due to exp osure of a preferred substrate (i.e. phosphatidylethanolamine) to the enzym e. Microvesicles were apparently released from the cells upon addition of l ysolecithin or ionomycin. Both these vesicles and the remnant cell membrane s were susceptible to sPLA(2). Together these data suggest that lysolecithi n induces susceptibility through both cPLA(2)-dependent and -independent pa thways. Whereas elevated cytosolic calcium was required for both pathways, it was sufficient only for the cPLA(2)-independent pathway. This cPLA(2)-in dependent pathway involved changes in cell membrane structure associated wi th transbilayer phospholipid migration and microvesicle release.