Angiotensin II inhibits insulin-stimulated phosphorylation of eukaryotic initiation factor 4E-binding protein-1 in proximal tubular epithelial cells.

Interaction between angiotensin II, which binds a G-protein-coupled receptor, and insulin, a ligand for receptor tyrosine kinase, was examined in renal proximal tubular epithelial cells. Augmented protein translation by insulin involves activation of eukaryotic initiation factor 4E (eIF4E) which follows the release of the factor from a heterodimeric complex by phosphorylation of its binding protein, 4E-BP1. Angiotensin II (1 nM) or insulin (1 nM) individually stimulated 4E-BP1 phosphorylation. However, pre-incubation with angiotensin II abrogated insulin-induced phosphorylation of 4E-BP1, resulting in persistent binding to eIF4E. Although angiotensin II and insulin individually activated phosphoinositide 3-kinase and extracellular signal-regulated kinase (ERK)-1/-2-type mitogen-activated protein (MAP) kinase, pre-incubation with angiotensin II abolished insulin-induced stimulation of these kinases, suggesting more proximal events in insulin signalling may be intercepted. Pretreatment with angiotensin II markedly inhibited insulin-stimulated tyrosine phosphorylation of insulin-receptor beta-chain and insulin-receptor substrate 1. Losartan prevented angiotensin II inhibition of insulin-induced ERK-1/-2-type MAP kinase activation and 4E-BP1 phosphorylation, suggesting mediation of the effect of angiotensin II by its type 1 receptor. Insulin-stimulated de novo protein synthesis was also abolished by pre-incubation with angiotensin II. These data show that angiotensin II inhibits 4E-BP1 phosphorylation and stimulation of protein synthesis induced by insulin by interfering with proximal events in insulin signalling. Our data provide a mechanistic basis for insulin insensitivity induced by angiotensin II.

Activation of renal signaling pathways in db/db mice with type 2 diabetes.

BACKGROUND: Altered regulation of signaling pathways may contribute to the pathogenesis of renal disease. We examined renal cortical signaling pathways in type 2 diabetes. METHODS: The status of renal cortical signaling pathways was examined in control and db/db mice with type 2 diabetes in the early phase of diabetic nephropathy associated with renal matrix expansion and albuminuria. RESULTS: Tyrosine phosphorylation of renal cortical proteins was increased in diabetic mice. Renal cortical activities of phosphatidylinositol 3-kinase (PI 3-kinase) in antiphosphotyrosine immunoprecipitates, Akt (PKB), and ERK1/2-type mitogen-activated protein (MAP) kinase activities were significantly augmented sixfold (P < 0.01), twofold (P < 0.0003), and sevenfold (P < 0.001), respectively, in diabetic mice compared with controls. A part of the increased renal cortical PI 3-kinase activity was due to insulin receptor activation, as PI 3-kinase activity associated with beta chain of the insulin receptor was increased nearly fourfold (P < 0.0235). Additionally, the kinase activity of the immunoprecipitated insulin receptor beta chain was augmented in the diabetic renal cortex, and tyrosine phosphorylation of the insulin receptor was increased. In the liver, activities of PI 3-kinase in the antiphosphotyrosine immunoprecipitates and Akt also were increased threefold (P < 0.05) and twofold (P < 0.0002), respectively. However, there was no change in the hepatic insulin receptor-associated PI 3-kinase activity. Additionally, the hepatic ERK1/2-type MAP kinase activity was inhibited by nearly 50% (P < 0.01). CONCLUSIONS: These studies demonstrate that a variety of receptor signaling pathways are activated in the renal cortex of mice with type 2 diabetes, and suggest a role for augmented insulin receptor activity in nephropathy of type 2 diabetes.