Altered Expression Levels and Impaired Steps in the Pathway to Phosphatidylinositol 3-Kinase Activation via Insulin Receptor Substrates 1 and 2 in Zucker Fatty Rats

Anai, Motonobu; Funaki, Makoto; Ogihara, Takehide; Terasaki, Jungo; Inukai, Kouichi; Katagiri, Hideki; Fukushima, Yasushi; Yazaki, Yoshio; Kikuchi, Masatoshi; Oka, Yoshitomo; Asano, Tomohiko

doi:10.2337/diab.47.1.13

Cited by 146 publications

(132 citation statements)

References 44 publications

(63 reference statements)

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“…As observed in the liver [44], a moderate decrease in IRS-1 and IRS-2 protein expression occurs in the hypothalamus of the obese rats. In parallel, there is also a decrease in IRS-1 and IRS-2 tyrosine phosphorylation, reflecting the decrease in protein levels as well as a decrease in the stoichiometry of phosphorylation.…”

Section: Discussionsupporting

confidence: 59%

Selective impairment of insulin signalling in the hypothalamus of obese Zucker rats

et al. 2003

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Aim/hypothesis. By acting in the brain, insulin suppresses food intake. However, little is known with regard to insulin signalling in the hypothalamus in insulin-resistant states. Methods. Western blotting, immunohistochemistry and polymerase chain reaction assays were combined to compare in vivo hypothalamic insulin signalling through the PI3-kinase and MAP kinase pathways between lean and obese Zucker rats. Results. Intracerebroventricular insulin infusion reduced food intake in lean rats to a greater extent than that observed in obese rats, and pre-treatment with PI3-kinase inhibitors prevented insulin-induced anorexia. The relative abundance of IRS-2 was considerably higher than that of IRS-1 in hypothalamus of both lean and obese rats. Insulin-stimulated phosphorylation of IR, IRS-1/2, the associations of PI 3-kinase to IRS-1/2 and phosphorylation of Akt in hypothalamus were decreased in obese rats compared to lean rats. These effects seem to be mediated by increased phosphoserine content of IR, IRS-1/2 and decreased protein levels of IRS-1/2 in obese rats. In contrast, insulin stimulated the phosphorylation of MAP kinase equally in lean and obese rats. Conclusion/interpretation. This study provides direct measurements of insulin signalling in hypothalamus, and documents selective resistance to insulin signalling in hypothalamus of Zucker rats. These findings provide support for the hypothesis that insulin could have anti-obesity actions mediated by the PI3-kinase pathway, and that impaired insulin signalling in hypothalamus could play a role in the development of obesity in this animal model of insulin-resistance. [Diabetologia (2003[Diabetologia ( ) 46:1629[Diabetologia ( -1640 Keywords Obesity, insulin resistance, hypothalamus, anorexia, signal transduction, phosphatidylinositol 3-kinase/antagonists & inhibitors/*metabolism, mitogen-activated protein kinase. Abbreviations: ERK, extracellular signal-regulated kinase; Grb2, growth factor receptor binding protein 2; IR, insulin receptor; IRS, insulin receptor substrate; MAPK, mitogen-activated protein kinase; PI 3-kinase, phosphatidylinositol 3-kinase; PKC, Protein kinase C; Shc, Src-homology and collagen homology; SHP2, Src-homology phosphatase 2; TNF-α, Tumor-necrosis factor-α.

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Section: Discussionsupporting

confidence: 59%

Selective impairment of insulin signalling in the hypothalamus of obese Zucker rats

et al. 2003

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“…In this context we speculated that, as peripheral tissues become resistant to circulating insulin as a consequence of reduced signaling via the PI3K pathway (Kim et al 1999, Cusi et al 2000, the same biochemical perturbation at the level of the -cell evokes a linked and compensatory insulin secretory response from these cells. We also predicted that wortmannin should be less effective in stimulating release from islets of Zucker fatty rats, a model of hyperglycemia, hyperinsulinemia and insulin resistance in which in vivo reductions in PI3K activity have been demonstrated (Anai et al 1998). This prediction was confirmed (Zawalich & Zawalich 2000).…”

Section: Discussionsupporting

confidence: 63%

Inhibitors of phosphatidylinositol 3-kinase amplify insulin release from islets of lean but not obese mice

Zawalich

Tesz²,

Zawalich³

2002

Journal of Endocrinology

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We examined the effects of phosphatidylinositol 3-kinase (PI3K) inhibition by wortmannin or LY294002 on glucose-induced secretion from mouse islets. Islets were collagenase isolated and perifused or subjected to Western blot analyses and probed for insulin receptor-signaling components. In agreement with previous studies, mouse islets, when compared with rat islets, were minimally responsive to 10 mM glucose stimulation. The inclusion of 50 nM wortmannin or 10 µM LY294002 significantly amplified 10 mM glucose-induced release from mouse islets. The effect of wortmannin was abolished by the calcium channel antagonist nitrendipine or by lowering the glucose level to 3 mM. Wortmannin had no effect on 10 mM -ketoisocaproate-induced secretion. In contrast to its potentiating effect on islets from CD-1 mice, wortmannin had no effect on 10 mM glucose-induced release from ob/ob mouse islets. Western blot analyses revealed the presence of the insulin receptor, insulin receptor substrate proteins 1 and 2 and PI3K in CD-1 islets. These results support the concept that a PI3K-dependent signaling pathway exists in -cells and that it may function to restrain glucose-induced insulin secretion from -cells. They also suggest that, as insulin resistance develops in peripheral tissues, a potential result of impaired PI3K activation, the same biochemical anomaly in -cells promotes a linked increase in insulin secretion to maintain glucose homeostasis.

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“…Previous studies on Zucker fatty rats and ob/ob mice, an hyperinsulinemic and insulin-resistant rodent models, respectively, show decreased IRS-1 expression levels in skeletal muscle (41,42). Likewise, IRS-1 was found to be down-regulated in adipose tissue from diabetic patients (28) and in 3T3-L1 adipocytes treated chronically with insulin (43).…”

Section: Desensitization Of Insulin Signaling Is Associated With a Pimentioning

confidence: 85%

Phosphoinositide 3-Kinase-mediated Reduction of Insulin Receptor Substrate-1/2 Protein Expression via Different Mechanisms Contributes to the Insulin-induced Desensitization of Its Signaling Pathways in L6 Muscle Cells

Pirola¹,

Bonnafous²,

Johnston³

et al. 2003

Journal of Biological Chemistry

104

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Impaired glucose tolerance precedes type 2 diabetes and is characterized by hyperinsulinemia, which develops to balance peripheral insulin resistance. To gain insight into the deleterious effects of hyperinsulinemia on skeletal muscle, we studied the consequences of prolonged insulin treatment of L6 myoblasts on insulin-dependent signaling pathways. A 24-h long insulin treatment desensitized the phosphoinositide 3-kinase (PI3K)/ protein kinase B (PKB) and p42/p44 MAPK pathways toward a second stimulation with insulin or insulin-like growth factor-1 and led to decreased insulin-induced glucose uptake. Desensitization was correlated to a reduction in insulin receptor substrate (IRS)-1 and IRS-2 protein levels, which was reversed by the PI3K inhibitor LY294002. Co-treatment of cells with insulin and LY294002, while reducing total IRS-1 phosphorylation, increased its phosphotyrosine content, enhancing IRS-1/PI3K association. PDK1, mTOR, and MAPK inhibitors did not block insulin-induced reduction of IRS-1, suggesting that the PI3K serine-kinase activity causes IRS-1 serine phosphorylation and its commitment to proteasomal degradation. Contrarily, insulin-induced IRS-2 down-regulation occurred via a PI3K/mTOR pathway. Suppression of IRS-1/2 down-regulation by LY294002 rescued the responsiveness of PKB and MAPK toward acute insulin stimulation. Conversely, adenoviraldriven expression of constitutively active PI3K induced an insulin-independent reduction in IRS-1/2 protein levels. IRS-2 appears to be the chief molecule responsible for MAPK and PKB activation by insulin, as knockdown of IRS-2 (but not IRS-1) by RNA interference severely impaired activation of both kinases. In summary, (i) PI3K mediates insulin-induced reduction of IRS-1 by phosphorylating it while a PI3K/mTOR pathway controls insulin-induced reduction of IRS-2, (ii) in L6 cells, IRS-2 is the major adapter molecule linking the insulin receptor to activation of PKB and MAPK, (iii) the mechanism of IRS-1/2 down-regulation is different in L6 cells compared with 3T3-L1 adipocytes. In conclusion, the reduction in IRS proteins via different PI3K-mediated mechanisms contributes to the development of an insulin-resistant state in L6 myoblasts.

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Altered Expression Levels and Impaired Steps in the Pathway to Phosphatidylinositol 3-Kinase Activation via Insulin Receptor Substrates 1 and 2 in Zucker Fatty Rats

Cited by 146 publications

References 44 publications

Selective impairment of insulin signalling in the hypothalamus of obese Zucker rats

Selective impairment of insulin signalling in the hypothalamus of obese Zucker rats

Inhibitors of phosphatidylinositol 3-kinase amplify insulin release from islets of lean but not obese mice

Phosphoinositide 3-Kinase-mediated Reduction of Insulin Receptor Substrate-1/2 Protein Expression via Different Mechanisms Contributes to the Insulin-induced Desensitization of Its Signaling Pathways in L6 Muscle Cells

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