Cellular regulation of cytosolic group IV phospholipase A2 by phosphatidylinositol bisphosphate levels

Cytosolic group IV phospholipase A(2) (cPLA(2)) is a ubiquitously expressed enzyme with key roles in intracellular signaling. The current paradigm for activation of cPLA(2) by stimuli proposes that both an increase in intracellular calcium and mitogen activated protein kinase-mediated phosphorylation occur together to fully activate the enzyme. Calcium is currently thought to be needed for translocation of the cPLA(2) to the membrane via a C2 domain, whereas the role of cPLA(2) phosphorylation is less clearly defined. Herein, we report that brief exposure of P388D(1) macrophages to UV radiation results in a rapid, cPLA(2)-mediated arachidonic acid mobilization, without increases in intracellular calcium. Thus, increased Ca2+ availability is a dispensable signal for cPLA(2) activation, which suggests the existence of alternative mechanisms for the enzyme to efficiently interact with membranes. Our previous in vitro data suggested the importance of phosphatidylinositol 4,5-bisphosphate (PtdInsP(2)) in the association of cPLA(2) to model membranes and hence in the regulation of cPLA(2) activity. Experiments described herein show that PtdInsP(2) also serves a similar role in vivo. Moreover, inhibition of PtdInsP(2) formation during activation conditions leads to inhibition of the cPLA(2)-mediated arachidonic acid mobilization. These results suggest that cellular PtdInsP(2) levels are involved in the regulation of group IV cPLA(2) activation.