Opposing Effects of Protein Kinase C on IgE-Dependent Exocytosis and InsP₃ Formation

Abstract

Aggregation of the receptors for Immunoglobulin E (IgE) by a multivalent antigen (Ag) leads to activation of phospholipase C (PLC) and secretion of serotonin from Rat Basophilic Leukemia (RBL) cells. The phorbol ester 12-0-tetradecanoyl-phorbol-13-acetate (TPA), exerts dual actions on the IgE-dependent responses in RBL cells: TPA potentiates IgE- induced secretion but it inhibits IgE-induced InsP₃ formation and the subsequent rise in internal Ca²⁺ concentrations. In addition, TPA synergizes with Ca²⁺ ionophores to trigger exocytosis. We have undertaken this study to examine whether the dual actions of TPA are both mediated by protein kinase C (PKC). It is relevant to evaluate the participation of PKC in TPA actions since some reports suggest that not all TPA actions are consequences of TPA binding and activation of PKC (Zick et al. (1985); Maraganore, J.M. (1987)). In addition, PKC consists of a family of enzymes that while closely related in structure (reviewed in Nishizuka, Y. (1988)), they differ in their cellular and intracellular disributions as well as in their responses to cofactors such as Ca²⁺, diacylglycerol (DAG), phosphatidylserine (PS) and fatty acids (Nishizuka, Y. (1988)). We have therefore analyzed the possibility that different isozymes of PKC may be involved in mediating the opposing effects of TPA on the IgE-mediated responses in RBL cells.