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High glucose and insulin in combination cause insulin receptor substrate-1 and -2 depletion and protein kinase B desensitisation in primary cultured rat adipocytes: possible implications for insulin resistance in type 2 diabetes
Umeå University, Faculty of Medicine, Department of Public Health and Clinical Medicine, Medicine.
Umeå University, Faculty of Medicine, Department of Public Health and Clinical Medicine, Medicine.
Umeå University, Faculty of Medicine, Department of Public Health and Clinical Medicine, Medicine.
Umeå University, Faculty of Medicine, Department of Public Health and Clinical Medicine, Medicine.
2003 (English)In: European Journal of Endocrinology, ISSN 0804-4643, E-ISSN 1479-683X, Vol. 148, 157-167 p.Article in journal (Refereed) Published
Abstract [en]

OBJECTIVE: The purpose of this study was to investigate the cellular effects of long-term exposure to high insulin and glucose levels on glucose transport and insulin signalling proteins. DESIGN AND METHODS: Rat adipocytes were cultured for 24 h in different glucose concentrations with 10(4) microU/ml of insulin or without insulin. After washing, (125)I-insulin binding, basal and acutely insulin-stimulated d-[(14)C]glucose uptake, and insulin signalling proteins and glucose transporter 4 (GLUT4) were assessed. RESULTS: High glucose (15 and 25 mmol/l) for 24 h induced a decrease in basal and insulin-stimulated glucose uptake compared with control cells incubated in low glucose (5 or 10 mmol/l). Twenty-four hours of insulin treatment decreased insulin binding capacity by approximately 40%, and shifted the dose-response curve for insulin's acute effect on glucose uptake 2- to 3-fold to the right. Twenty-four hours of insulin treatment reduced basal and insulin-stimulated glucose uptake only in the presence of high glucose (by approximately 30-50%). At high glucose, insulin receptor substrate-1 (IRS-1) expression was downregulated by approximately 20-50%, whereas IRS-2 was strongly upregulated by glucose levels of 10 mmol/l or more (by 100-400%). Insulin treatment amplified the suppression of IRS-1 when combined with high glucose and also IRS-2 expression was almost abolished. Twenty-four hours of treatment with high glucose or insulin, alone or in combination, shifted the dose-response curve for insulin's effect to acutely phosphorylate protein kinase B (PKB) to the right. Fifteen mmol/l glucose increased GLUT4 in cellular membranes (by approximately 140%) compared with 5 mmol/l but this was prevented by a high insulin concentration. CONCLUSIONS: Long-term exposure to high glucose per se decreases IRS-1 but increases IRS-2 content in rat adipocytes and it impairs glucose transport capacity. Treatment with high insulin downregulates insulin binding capacity and, when combined with high glucose, it produces a marked depletion of IRS-1 and -2 content together with an impaired sensitivity to insulin stimulation of PKB activity. These mechanisms may potentially contribute to insulin resistance in type 2 diabetes.

Place, publisher, year, edition, pages
European Society of Endocrinology , 2003. Vol. 148, 157-167 p.
Identifiers
URN: urn:nbn:se:umu:diva-3932DOI: 10.1530/eje.0.1480157OAI: oai:DiVA.org:umu-3932DiVA: diva2:142845
Available from: 2002-12-06 Created: 2002-12-06 Last updated: 2010-06-17Bibliographically approved
In thesis
1. Glucose and lipid metabolism in insulin resistance: an experimental study in fat cells
Open this publication in new window or tab >>Glucose and lipid metabolism in insulin resistance: an experimental study in fat cells
2003 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Type 2 diabetes is usually caused by a combination of pancreatic β-cell failure and insulin resistance in target tissues like liver, muscle and fat. Insulin resistance is characterised by an impaired effect of insulin to reduce hepatic glucose production and to promote glucose uptake in peripheral tissues. The focus of this study was to further elucidate cellular mechanisms for insulin resistance that may be of relevance for type 2 diabetes in humans. We used rat and human adipocytes as an established model of insulin’s target cells.

Glucocorticoids, e.g. cortisol, can induce insulin resistance in vivo. In the present study, pretreatment of rat adipocytes in vitro for 24 h with the cortisol analogue dexamethasone produced a downregulation of glucose uptake capacity as well as a marked depletion of cellular insulin receptor substrate 1 (IRS-1) and protein kinase B (PKB), two proteins suggested to play a critical role in the intracellular signal transduction pathway of insulin. The amount of phosphorylated PKB in response to acute insulin treatment was decreased in parallel to total PKB content. The basal rate of lipolysis was enhanced, but insulin’s antilipolytic effect was not consistently altered following dexamethasone pretreatment.

Alterations in blood glucose as well as insulin levels may be of great importance for cellular as well as whole-body insulin resistance. High glucose (≥15 mM) for 24 h induced a decrease in glucose uptake capacity in rat adipocytes and IRS-1 content was reduced whereas IRS-2 was increased. Long-term pretreatment with a high insulin concentration downregulated insulin binding capacity and when combined with high glucose, it produced a pronounced

reduction of cellular IRS-1 and 2 content together with insensitivity to insulin’s effect to activate PKB and a decrease in glucose uptake capacity. A common denominator for a decrease in glucose uptake capacity in our rat adipocyte studies seems to be a decrease in IRS-1 content.

Adipocytes from type 2 diabetes patients are insulin-resistant, but in our work the insulin resistance could be reversed by incubation of the cells at a physiological glucose level for 24 h. Insulin resistance in fresh adipocytes from type 2 diabetes patients was associated with in vivo insulin resistance and glycemic level and with adipocyte cell size and waist-hip ratio

(WHR).

As a potential mechanism for postprandial dyslipidemia in type 2 diabetes, we examined the nutritional regulation of subcutaneous adipose tissue lipoprotein lipase (LPL) activity. It was upregulated by ~40-50 % after a standardised lipid-enriched meal and this was very similar in type 2 diabetes patients and control subjects, suggesting that the postprandial

hypertriglyceridemia found in type 2 diabetes is not explained by an altered nutritional regulation of LPL in subcutaneous fat.

In conclusion, the present work provides evidence for novel interactions between glucocorticoids and insulin in the regulation of glucose metabolism that may potentially contribute to the development of insulin resistance. High levels of glucose and insulin produce perturbations in the insulin signalling pathway that may be of relevance for human type 2 diabetes. Cellular insulin resistance may be secondary to the diabetic state in vivo, e.g. via glucotoxicity. This is supported by our finding that insulin resistance in adipocytes from type 2 diabetes patients can be reversed after incubation at a physiological glucose level.

Key words: adipocyte, insulin resistance, type 2 diabetes, insulin signalling, glucose uptake,

insulin, glucose, dexamethasone, insulin receptor substrate, protein kinase B, GLUT4,

lipoprotein lipase.

Place, publisher, year, edition, pages
Umeå: Folkhälsa och klinisk medicin, 2003. 63 p.
Series
Umeå University medical dissertations, ISSN 0346-6612 ; 817
Keyword
Public health, adipocyte, insulin resistance, type 2 diabetes, insulin signalling, glucose uptake, insulin, glucose, dexamethasone, insulin receptor substrate, protein kinase B, GLUT4, lipoprotein lipase, Folkhälsomedicin
National Category
Public Health, Global Health, Social Medicine and Epidemiology
Research subject
Medicine
Identifiers
urn:nbn:se:umu:diva-26 (URN)91-7305-359-7 (ISBN)
Public defence
2002-12-06, Umeå, 13:00
Available from: 2002-12-06 Created: 2002-12-06 Last updated: 2010-06-17Bibliographically approved

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