Role of the TSC1-TSC2 complex in the integration of insulin and glucose signaling involved in pancreatic beta-cell proliferation.
Tuberous sclerosis complex proteins 1-2 (TSC1-TSC2) complex integrates both nutrient and hormonal signaling and is a critical negative regulator of mammalian target of rapamycin (mTOR) complex 1. The use of different beta-cell lines expressing or not the insulin receptor (IR(+/+) and IR(-/-)) or with a reconstituted expression of IR isoform A or B (Rec A and Rec B) revealed that both phosphatidylinositol 3-kinase/Akt/TSC/mTOR complex 1 and MAPK kinase/ERK pathways mediate insulin signaling in IR(+/+)-, IRA-, or IRB-expressing cells. However, glucose signaling was mediated by MAPK kinase/ERK and AMP-activated protein kinase pathways as assessed in IR(-/-) cells. The effect of insulin on Akt phosphorylation was completely inhibited by the use of the phosphatidylinositol 3-kinase inhibitor wortmannin in IR(+/+) and Rec B cells, a partial inhibitory effect being observed in Rec A cell line. The knockdown of TSC2 expression up-regulated the downstream basal phosphorylation of 70-kDa ribosomal protein S6 kinase (p70S6K) and mTOR. More importantly, upregulation of p70S6K signaling impaired insulin-stimulated phosphorylation of Akt Ser(473) and p70S6K in IR(+/+) and Rec B but not in Rec A cell lines. In fact, insulin receptor substrate-1 Ser(307) phosphorylation signal in Rec B was stronger than in Rec A cell line during insulin action. Rec A cells induced a higher proliferation rate compared with Rec B or IR(+/+) during serum stimulation. Thus, we propose that the regulation of TSC2 phosphorylation by insulin or glucose independently integrates beta-cell proliferation signaling, the relative expression of IRA or IRB isoforms in pancreatic beta cells playing a major role.
Bartolomé
Alberto
A
Departamento de Bioquímica y Biología Molecular, Facultad de Farmacia, Universidad Complutense, 28040 Madrid, Spain.
Guillén
Carlos
C
Benito
Manuel
M
D000730:Androstadienes / Q000494:pharmacology; D000818:Animals; D015153:Blotting, Western; D002453:Cell Cycle / Q000187:drug effects; D002460:Cell Line; D005947:Glucose / Q000494:pharmacology*; D007150:Immunohistochemistry; D007328:Insulin / Q000494:pharmacology*; D050417:Insulin-Secreting Cells / Q000187:drug effects* / Q000378:metabolism*; D051379:Mice; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010766:Phosphorylation / Q000187:drug effects; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D034741:RNA, Small Interfering; D011972:Receptor, Insulin / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D000077004:Tuberous Sclerosis Complex 1 Protein; D000077005:Tuberous Sclerosis Complex 2 Protein; D025521:Tumor Suppressor Proteins / Q000235:genetics / Q000378:metabolism*; D000077191:Wortmannin
D000730:Androstadienes; D007328:Insulin; D000081082:Phosphoinositide-3 Kinase Inhibitors; D034741:RNA, Small Interfering; C000624651:Tsc1 protein, mouse; C000624654:Tsc2 protein, mouse; D000077004:Tuberous Sclerosis Complex 1 Protein; D000077005:Tuberous Sclerosis Complex 2 Protein; D025521:Tumor Suppressor Proteins; D011972:Receptor, Insulin; D051057:Proto-Oncogene Proteins c-akt; D005947:Glucose; D000077191:Wortmannin