Effects of melittin on adipocyte metabolism unrelated to lysophospholipid accumulation

Release of carrot plasma membrane-associated phosphatidylinositol kinase by phospholipase A2 and activation by a 70 kDa protein

Plasma membranes were isolated from carrot (Daucus carota L.) cells grown in suspension culture and treated with phospholipase A2 from snake or bee venom for 10 min. As a result of this treatment, phosphatidylinositol kinase activity was recovered in the soluble fraction. There was no detectable diacylglycerol kinase or phosphatidylinositol monophosphate kinase activity released from the membranes after the phospholipase A2 treatment. Treating the plasma membranes with phospholipase C or D did not release PI kinase activity. The phospholipase A2-released PI kinase was activated over 2-fold by a heat stable, soluble 70 kDa protein. The partially purified 70 kDa activator increases the Vmax but does not affect the Km of the phospholipase A2-released PI kinase.

Melittin stimulates phosphoinositide hydrolysis and placental lactogen release: arachidonic acid as a link between phospholipase A2 and phospholipase C signal-transduction pathways

Previous investigations from this laboratory have implicated both phospholipase A2 and phospholipase C in the regulation of human placental lactogen release from human trophoblast. To study further the role of endogenous phospholipase A2 and the relationship between phospholipase A2 activation and phosphoinositide metabolism, we examined hPL and [3H]-inositol release from trophoblast cells in response to agents that stimulate or inhibit the endogenous enzyme. Melittin (0.5-2.0 micrograms/ml) stimulated rapid, dose-dependent, and reversible increases in the release of hPL, prostaglandin E, and [3H]-inositol. Mepacrine (0.1-0.25 mM) inhibited this stimulation. However, mepacrine had no effect on the stimulation of hPL and [3H]-inositol release by exogenous arachidonic acid (AA). These results indicate that the stimulation by melittin of phosphoinositide metabolism and hPL release is mediated by initial activation of phospholipase A2. Furthermore, the results support the possibility that AA, released as a consequence of phospholipase A2 activation, can act as a second messenger linking the two phospholipase pathways.