BRANCHED-CHAIN AMINO-ACIDS ARE ESSENTIAL IN THE REGULATION OF PHAS-I AND P70 S6 KINASE BY PANCREATIC BETA-CELLS - A POSSIBLE ROLE IN PROTEIN TRANSLATION AND MITOGENIC SIGNALING

Amino acids have been identified as important signaling molecules invo lved in pancreatic beta-cell proliferation, although the cellular mech anism responsible for this effect is not well defined. We previously r eported that amino acids are required for glucose or exogenous insulin to stimulate phosphorylation of PHAS-I (phosphorylated heat- and acid -stable protein regulated by insulin), a recently discovered regulator of translation initiation during cell mitogenesis. Here we demonstrat e that essential amino acids, in particular branched-chain amino acids (leucine, valine, and isoleucine), are largely responsible for mediat ing this effect. The transamination product of leucine, alpha-ketoisoc aproic acid, also stimulates PHAS-I phosphorylation although the trans amination products of isoleucine and valine are ineffective, Since ami no acids are secretagogues for insulin secretion by beta-cells, we inv estigated whether endogenous insulin secreted by beta-cells is involve d. Interestingly, branched-chain amino acids stimulate phosphorylation of PHAS-I independent of endogenous insulin secretion since genistein (10 mu M) and herbimycin A (1 mu M), two tyrosine kinase inhibitors i n the insulin signaling pathway, exert no effect on amino acid-induced phosphorylation of PHAS-I, Furthermore, branched-chain amino acids re tain their ability to induce phosphorylation of PHAS-I under condition s that block insulin secretion from beta-cells. In exploring the signa ling pathway responsible for these effects, we find that rapamycin (25 nM) inhibits the ability of branched-chain amino acids to stimulate t he phosphorylation of PHAS-I and p70(s6) kinase, suggesting that the m ammalian target of rapamycin signaling pathway is involved. The branch ed-chain amino acid, leucine, also exerts similar effects on PHAS-I ph osphorylation in isolated pancreatic islets. In addition, we find that amino acids are necessary for insulin-like growth factor (IGF-I) to s timulate the phosphorylation of PHAS-I indicating that a requirement f or amino acids may be essential for other beta-cell growth factors in addition to insulin and IGF-I to activate this signaling pathway. We p ropose that amino acids, in particular branched-chain amino acids, may promote beta-cell proliferation either by stimulating phosphorylation of PHAS-I and p70(S6k) via the mammalian target of rapamycin pathway and/or by facilitating the proliferative effect mediated by growth fac tors such as insulin and IGF-I.