Possible involvement of a tyrosine kinase-dependent pathway in the regulation of phosphoinositide metabolism by vanadate in normal mouse islets

Abstract

The potential roles of protein tyrosine kinases (TKs) and of phosphotyrosine phosphatases (PTPs) in pancreatic islet function are not known. In this study, we investigated whether vanadate, a potent PTP inhibitor, affects phosphoinositide (PI) metabolism by a TK-dependent pathway in isolated mouse islets. To avoid the confounding effects of changes in Ca²⁺ influx, all experiments were performed in the absence of Ca²⁺. In the presence of 15 mM glucose, vanadate, acetylcholine (ACh) or [Arg]vasopressin (AVP) strongly stimulated InsP production. Vanadate also increased PtdInsP levels in membranes. The TK inhibitor genistein (not its inactive analogues genistin and daidzein) significantly reduced vanadate effects, but was without effect in the absence of stimulation or in the presence of ACh or AVP. Islet proteins resolved by SDS/PAGE were analysed by immunoblotting with anti-phosphotyrosine antibody. Under control conditions, several phosphotyrosyl-proteins (PYPs) were present. Vanadate increased phosphotyrosine residues on several PYPs, notably two proteins of 145 and 85 kDa. This effect was prevented by genistein. p145 and p85 could correspond to phospholipase Cγ (PLCγ) and the regulatory subunit of PtdIns-3-kinase (PtdIns-3K) respectively. Both proteins are expressed in islets, as revealed by immunoblots with specific antibodies. Tungstate, another PTP inhibitor, reproduced vanadate effects, but inhibition of PtdIns-3K by wortmannin failed to affect vanadate-increased PtdInsP levels. Incubation of the islets in the presence of 10% (v/v) fetal calf serum instead of BSA increased InsP production and this effect was prevented by genistein. These results suggest that inhibition of PTP increases InsP production in mouse islets by a TK-dependent pathway. They also provide evidence for a potential role of TK and PTP in pancreatic B-cell function.