Protein kinase Cdelta participates in insulin-induced activation of PKB via PDK1.

PKCdelta has been shown to be activated by insulin and to interact with insulin receptor and IRS. PKB(Akt) plays an important role in glucose transport and glycogen synthesis. In this study, we investigated the possibility that PKCdelta may be involved in insulin-induced activation of PKB. Studies were conducted on primary cultures of rat skeletal muscle. PKB was activated by insulin stimulation within 5min and reached a peak by 15-30min. Insulin also increased the physical association between PKCdelta with PKB and with PDK1. The insulin-induced PKCdelta-PKB association was PI3K dependent. PKB-PKCdelta association was accounted for by the involvement of PDK1. Overexpression of dominant negative PKCdelta abrogated insulin-induced association of PKCdelta with both PKB and PDK1. Blockade of PKCdelta also decreased insulin-induced Thr308 PKB phosphorylation and PKB translocation. Moreover, PKCdelta inhibition reduced insulin-induced GSK3 phosphorylation. The results indicate that insulin-activated PKCdelta interacts with PDK1 to regulate PKB.

Regulation of insulin secretion and proinsulin biosynthesis by succinate.

Succinate stimulates insulin secretion and proinsulin biosynthesis. We studied the effects of reduced nicotinamide adenine dinucleotide phosphate (NADPH)-modulating pathways on glucose- and succinate-stimulated insulin secretion and proinsulin biosynthesis in the rat and the insulin-resistant Psammomys obesus. Disruption of the anaplerotic pyruvate/malate shuttle by phenylacetic acid inhibited glucose- and succinate-stimulated insulin secretion and succinate-stimulated proinsulin biosynthesis in both species. In contrast, phenylacetic acid failed to inhibit glucose-stimulated proinsulin biosynthesis in P. obesus islets. Inhibition of the NADPH-consuming enzyme neuronal nitric oxide synthase (nNOS) with l-N(G)-nitro-l-arginine methyl ester or with N(G)-monomethyl-l-arginine(G) doubled succinate-stimulated insulin secretion in rat islets, suggesting that succinate- and nNOS-derived signals interact to regulate insulin secretion. In contrast, nNOS inhibition had no effect on succinate-stimulated proinsulin biosynthesis in both species. In P. obesus islets, insulin secretion was not stimulated by succinate in the absence of glucose, whereas proinsulin biosynthesis was increased 5-fold. Conversely, under stimulating glucose levels, succinate doubled insulin secretion, indicating glucose-dependence. Pyruvate ester and inhibition of nNOS partially mimicked the permissive effect of glucose on succinate-stimulated insulin secretion, suggesting that anaplerosis-derived signals render the beta-cells responsive to succinate. We conclude that beta-cell anaplerosis via pyruvate carboxylase is important for glucose- and succinate-stimulated insulin secretion and for succinate-stimulated proinsulin biosynthesis. In P. obesus, pyruvate/malate shuttle dependent and independent pathways that regulate proinsulin biosynthesis coexist; the latter can maintain fuel stimulated biosynthetic activity when the succinate-dependent pathway is inhibited. nNOS signaling is a negative regulator of insulin secretion, but not of proinsulin biosynthesis.