Insulin-Mediated Phosphorylation of the Proline-Rich Akt Substrate PRAS40 Is Impaired in Insulin Target Tissues of High-Fat Diet-Fed Rats

VU University Amsterdam, Amsterdamo, North Holland, Netherlands
Diabetes (Impact Factor: 8.1). 01/2007; 55(12):3221-8. DOI: 10.2337/db05-1390
Source: PubMed


Clinical insulin resistance is associated with decreased activation of phosphatidylinositol 3'-kinase (PI3K) and its downstream substrate protein kinase B (PKB)/Akt. However, its physiological protein substrates remain poorly characterized. In the present study, the effect of in vivo insulin action on phosphorylation of the PKB/Akt substrate 40 (PRAS40) was examined. In rat and mice, insulin stimulated PRAS40-Thr246 phosphorylation in skeletal and cardiac muscle, the liver, and adipose tissue in vivo. Physiological hyperinsulinemia increased PRAS40-Thr246 phosphorylation in human skeletal muscle biopsies. In cultured cell lines, insulin-mediated PRAS40 phosphorylation was prevented by the PI3K inhibitors wortmannin and LY294002. Immunohistochemical and immunofluorescence studies showed that phosphorylated PRAS40 is predominantly localized to the nucleus. Finally, in rats fed a high-fat diet (HFD), phosphorylation of PRAS40 was markedly reduced compared with low-fat diet-fed animals in all tissues examined. In conclusion, the current study identifies PRAS40 as a physiological target of in vivo insulin action. Phosphorylation of PRAS40 is increased by insulin in human, rat, and mouse insulin target tissues. In rats, this response is reduced under conditions of HFD-induced insulin resistance.

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    • "The ribosomal protein PRAS40, abundant in steatosis patients has been identified as a crucial regulator of insulin signaling and also as mediator of AKT signals to mTOR (Vander Haar et al., 2007). Furthermore, it has been shown that PRAS40 phosphorylation is decreased under insulin-resistant conditions (Nascimento et al., 2006). Therefore, reduced AKT/mTOR signaling of skin fibroblasts derived from steatosis patients has revealed that the insulin-resistant phenotype is present not only in insulin-metabolizing central organs, e.g., the liver of steatosis patients, but is also manifested in skin fibroblasts derived from these patients. "
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    ABSTRACT: Non-alcoholic fatty liver disease comprises a broad spectrum of disease states ranging from simple steatosis to non-alcoholic steatohepatitis. As a result of increases in the prevalences of obesity, insulin resistance, and hyperlipidemia, the number of people with hepatic steatosis continues to increase. Differences in susceptibility to steatohepatitis and its progression to cirrhosis have been attributed to a complex interplay of genetic and external factors all addressing the intracellular network. Increase in sugar or refined carbohydrate consumption results in an increase of insulin and insulin resistance that can lead to the accumulation of fat in the liver. Here we demonstrate how a multidisciplinary approach encompassing cellular reprogramming, transcriptomics, proteomics, metabolomics, modeling, network reconstruction, and data management can be employed to unveil the mechanisms underlying the progression of steatosis. Proteomics revealed reduced AKT/mTOR signaling in fibroblasts derived from steatosis patients and further establishes that the insulin-resistant phenotype is present not only in insulin-metabolizing central organs, e.g., the liver, but is also manifested in skin fibroblasts. Transcriptome data enabled the generation of a regulatory network based on the transcription factor SREBF1, linked to a metabolic network of glycerolipid, and fatty acid biosynthesis including the downstream transcriptional targets of SREBF1 which include LIPIN1 (LPIN) and low density lipoprotein receptor. Glutathione metabolism was among the pathways enriched in steatosis patients in comparison to healthy controls. By using a model of the glutathione pathway we predict a significant increase in the flux through glutathione synthesis as both gamma-glutamylcysteine synthetase and glutathione synthetase have an increased flux. We anticipate that a larger cohort of patients and matched controls will confirm our preliminary findings presented here.
    Full-text · Article · Sep 2012 · Frontiers in Physiology
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    • "Raubenheimer et al96 fed rats with a 45% fat diet for 8 weeks and observed an increase in body weight, triacylglycerol levels of 117%, hyperinsulinemia, and glucose intolerance. Nascimento et al97 demonstrated that animals fed with hyperlipidic diets featured lower phosphorylation of one of the proteins of the insulin signaling mechanisms (protein kinase B/Akt), a denotation of reduced insulin action, and resistance induced by diet. "
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    • "The importance of the AKT for amphetamine responsivity is demonstrated by the ability of the AKT1 inhibitor LY294002 to block amphetamineinduced dopamine release and reuptake (Williams et al. 2007). There is also evidence that phosphorylation of the AKT substrate PRAS40 is markedly reduced in rats fed a high-fat diet (Nascimento et al. 2007), suggesting a mechanism through which diet may influence DAT expression and dopamine release. Given this combination of clinical and preclinical data, AKT1 may be an important candidate for understanding genetic relations between dopamine functioning and overeating. "
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