The diabetes-susceptible gene SLC30A8/ZnT8 regulates hepatic insulin clearance
Motoyuki TamakiYoshio FujitaniAkemi HaraToyoyoshi UchidaYoshifumi TamuraKageumi TakenoMinako KawaguchiTakahiro WatanabeTakeshi OgiharaAyako FukunakaTomoaki ShimizuTomoya MitaAkio KanazawaM. ImaizumiTakaya AbeHiroshi KiyonariShintaro HojyoToshiyuki FukadaTakeshi KawauchiShinya NagamatsuToshio HiranoRyuzo KawamoriHirotaka Watada
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Abstract:
Recent genome-wide association studies demonstrated that common variants of solute carrier family 30 member 8 gene (SLC30A8) increase susceptibility to type 2 diabetes. SLC30A8 encodes zinc transporter-8 (ZnT8), which delivers zinc ion from the cytoplasm into insulin granules. Although it is well known that insulin granules contain high amounts of zinc, the physiological role of secreted zinc remains elusive. In this study, we generated mice with β cell-specific Slc30a8 deficiency (ZnT8KO mice) and demonstrated an unexpected functional linkage between Slc30a8 deletion and hepatic insulin clearance. The ZnT8KO mice had low peripheral blood insulin levels, despite insulin hypersecretion from pancreatic β cells. We also demonstrated that a substantial amount of the hypersecreted insulin was degraded during its first passage through the liver. Consistent with these findings, ZnT8KO mice and human individuals carrying rs13266634, a major risk allele of SLC30A8, exhibited increased insulin clearance, as assessed by c-peptide/insulin ratio. Furthermore, we demonstrated that zinc secreted in concert with insulin suppressed hepatic insulin clearance by inhibiting clathrin-dependent insulin endocytosis. Our results indicate that SLC30A8 regulates hepatic insulin clearance and that genetic dysregulation of this system may play a role in the pathogenesis of type 2 diabetes.This chapter contains sections titled: Insulin resistance – definitions and role in the natural history of type 2 diabetes Metabolic syndrome and non - alcoholic fatty liver disease Insulin resistance in the liver Insulin resistance in adipose tissue Insulin resistance in skeletal muscle Resistance to other actions of insulin Conclusions References
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Insulin receptor substrate
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In muscle, physiologic hyperinsulinemia, presumably acting on endothelial cells (ECs), dilates arterioles and regulates both total blood flow and capillary recruitment, which in turn influences glucose disposal. In cultured ECs, however, supraphysiological (e.g. >or=10 nM) insulin concentrations are typically used to study insulin receptor (IR) signaling pathways and nitric oxide generation. IGF-I receptors (IGF-IRs) are more abundant than IR in ECs, and they also respond to high concentrations of insulin. To address whether IR mediates responses to physiologic insulin stimuli, we examined the insulin concentration dependence of IR and IGF-IR-mediated insulin signaling in bovine aortic ECs (bAECs). We also assessed whether insulin/IGF-I hybrid receptors were present in bAECs. Insulin, at 100-500 pM, significantly stimulated the phosphorylation of IRbeta, Akt1, endothelial isoform of nitric oxide synthase, and ERK 1/2 but not the IGF-IRbeta subunit. At concentrations 1-5 nm or greater, insulin dose-dependently enhanced the tyrosine phosphorylation of IGF-IRbeta, and this was inhibited by IGF-IR neutralizing antibody. In addition, immunoprecipitation of IRbeta pulled down the IGF-IRbeta, and the IRbeta immunocytochemically colocalized with IGF-IRbeta, suggesting that ECs have insulin/IGF-I hybrid receptors. We conclude that: 1) insulin at physiological concentrations selectively activates IR signaling in bAECs; 2) bAECs express IGF-IR and insulin/IGF-I hybrid receptors in addition to IR; 3) high concentrations of insulin (>or=1-5 nM) activate IGF-IR and hybrid receptors as well as IR; and 4) this crossover activation can confound interpretation of studies of insulin action in ECs when high insulin concentrations are used.
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Nonalcoholic fatty liver disease (NAFLD), hepatic insulin resistance, and type 2 diabetes are all strongly associated and are all reaching epidemic proportions. Whether there is a causal link between NAFLD and hepatic insulin resistance is controversial. This review will discuss recent studies in both humans and animal models of NAFLD that have implicated increases in hepatic diacylglycerol (DAG) content leading to activation of novel protein kinase Cϵ (PKCϵ) resulting in decreased insulin signaling in the pathogenesis of NAFLD-associated hepatic insulin resistance and type 2 diabetes. The DAG-PKCϵ hypothesis can explain the occurrence of hepatic insulin resistance observed in most cases of NAFLD associated with obesity, lipodystrophy, and type 2 diabetes.
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IRS1
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Specific binding sites for insulin have been identified and characterized for the human erythroleukemia cell line K-562. The binding of [125I]-insulin to the cells increased as a function of time, reaching a maximum at 20 min when incubation was performed at 37°C. The binding of [125I]-insulin was dose-dependently inhibited by insulin or proinsulin. Scatchard plot of the binding data was curvilinear, and the number of insulin receptors was approximately 39,000. Insulin at concentrations of 0.05–10.0 ng/ml stimulated CO2 production and DNA and protein synthesis in K-562 cells in a dose-dependent manner, indicating that the insulin binding sites are functionally important in mediating these biochemical events induced by insulin. Maximal insulin responses were elicited at concentrations of <5 ng/ml, when (at most) 10% of the insulin receptors were occupied. After binding to the cells, [125I]-insulin was degraded in a time- and temperature-dependent manner. As reported for other types of cells, unlabeled insulin also downregulated insulin receptors in K-562 cells. When the cells were incubated with 1 × 10−7 M unlabeled insulin for 24 h, the number of insulin receptors decreased by 50% without a change of affinity. K-562 cells may be useful in studying the role of insulin receptors in cell functions induced by insulin.
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We thank Prof. Dogru and colleagues for their interest in our recent article on the importance of adipocyte fatty acid binding protein (AFABP) in nonalcoholic fatty liver disease (NAFLD).1 It is now well accepted that NAFLD is the hepatic manifestation of the metabolic syndrome and as such is intimately associated with insulin resistance, visceral obesity, and dyslipidemia.2 Insulin resistance, which is a key characteristic of both conditions, has also been associated with NAFLD progression from simple steatosis to nonalcoholic steatohepatitis.3 We used the homeostasis model assessment of insulin resistance (HOMA-IR) to reflect the spectrum of insulin sensitivity. This index has been shown to correlate with the results of euglycemic-hyperinsulinemic clamp in patients without diabetes and with type 2 diabetes, including those treated with metformin and other oral hypoglycemic agents.4, 5 We were careful to exclude those taking thiazolidenediones, which have been shown to significantly affect circulating adipokine levels,6, 7 in contrast to sulfonylureas and metformin, which have not.7, 8 Type 2 diabetes eventually ensues in many subjects with increasing insulin resistance and is associated with more progressive fatty liver disease9; therefore, we decided not to exclude subjects with type 2 diabetes in our study. Although we agree it would be interesting to further subclassify subjects by glucose dysregulation status, the use of post hoc subset analysis is far less robust statistically and prone to type 2 error. We agree that metabolic variables and, in particular, measures of insulin resistance are important to consider when interpreting data on adipocytokines, which are intimately related to these factors. Indeed, our data showed the close association between both AFABP and lipocalin-2 to insulin resistance, body mass index, and waist circumference. To ensure our findings were independent of key confounders, we performed multivariate analysis for each histological endpoint in NAFLD using those factors significant on univariate analysis.1 We demonstrated that the association between AFABP and necroinflammatory and fibrotic activity is independent of abdominal obesity (the waist-hip ratio), cholesterol, high-density lipoprotein, and insulin resistance. For further clarity, as suggested by Prof. Dogru, we provide in Table 1 the relationship between AFABP and disease severity in NAFLD, directly controlled for insulin resistance and the key metabolic variables of body mass index, low-density lipoprotein, high-density lipoprotein, triglycerides, and glucose. This conclusively demonstrates that AFABP plays an important role in the pathogenesis of NAFLD independent of metabolic confounders. David van der Poorten*, Kerry-Lee Milner , Donald J. Chisholm , Jacob George*, * Storr Liver Unit, Westmead Millennium Institute, University of Sydney, Sydney, Australia, Garvan Institute of Medical Research, University of New South Wales, Sydney, Australia.
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