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 involved in pancreatic beta-cell proliferation, although the cellular mechanism responsible for this effect is not well defined. We previously reported 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 demonstrate that essential amino acids, in particular branched-chain amino acids (leucine, valine, and isoleucine), are largely responsible for mediating this effect. The transamination product of leucine, alpha-ketoisocaproic acid, also stimulates PHAS-I phosphorylation although the transamination products of isoleucine and valine are ineffective. Since amino acids are secretagogues for insulin secretion by beta-cells, we investigated whether endogenous insulin secreted by beta-cells is involved. Interestingly, branched-chain amino acids stimulate phosphorylation of PHAS-I independent of endogenous insulin secretion since genistein (10 microM) and herbimycin A (1 microM), two tyrosine kinase inhibitors in the insulin signaling pathway, exert no effect on amino acid-induced phosphorylation of PHAS-I. Furthermore, branched-chain amino acids retain their ability to induce phosphorylation of PHAS-I under conditions that block insulin secretion from beta-cells. In exploring the signaling pathway responsible for these effects, we find that rapamycin (25 nM) inhibits the ability of branched-chain amino acids to stimulate the phosphorylation of PHAS-I and p70(s6) kinase, suggesting that the mammalian target of rapamycin signaling pathway is involved. The branched-chain amino acid, leucine, also exerts similar effects on PHAS-I phosphorylation in isolated pancreatic islets. In addition, we find that amino acids are necessary for insulin-like growth factor (IGF-I) to stimulate the phosphorylation of PHAS-I indicating that a requirement for amino acids may be essential for other beta-cell growth factors in addition to insulin and IGF-I to activate this signaling pathway. We propose 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 factors such as insulin and IGF-I.
Xu
G
G
Department of Pathology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
Kwon
G
G
Marshall
C A
CA
Lin
T A
TA
Lawrence
J C
JC
McDaniel
M L
ML
D000597:Amino Acids, Branched-Chain / Q000494:pharmacology*; D000818:Animals; D002352:Carrier Proteins; D002455:Cell Division; D004357:Drug Synergism; D005786:Gene Expression Regulation; D006133:Growth Substances / Q000494:pharmacology; D007328:Insulin / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology; D047908:Intracellular Signaling Peptides and Proteins; D007515:Islets of Langerhans / Q000187:drug effects*; D008297:Male; D010750:Phosphoproteins / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D014176:Protein Biosynthesis; D051381:Rats; D017207:Rats, Sprague-Dawley; D019893:Ribosomal Protein S6 Kinases / Q000378:metabolism*; D015398:Signal Transduction; D020123:Sirolimus / Q000494:pharmacology
D000597:Amino Acids, Branched-Chain; D002352:Carrier Proteins; C499526:Eif4ebp1 protein, rat; D006133:Growth Substances; D007328:Insulin; D047908:Intracellular Signaling Peptides and Proteins; D010750:Phosphoproteins; D007334:Insulin-Like Growth Factor I; D019893:Ribosomal Protein S6 Kinases; D020123:Sirolimus