Development of diabetes in obese, insulin-resistant mice: essential role of dietary carbohydrate in beta cell destruction

AIMS/HYPOTHESIS

The role of dietary carbohydrate in the pathogenesis of type 2 diabetes is still a subject of controversial debate. Here we analysed the effects of diets with and without carbohydrate on obesity, insulin resistance and development of beta cell failure in the obese, diabetes-prone New Zealand Obese (NZO) mouse.

MATERIALS AND METHODS

NZO mice were kept on a standard diet (4% [w/w] fat, 51% carbohydrate, 19% protein), a high-fat diet (15, 47 and 17%, respectively) and a carbohydrate-free diet in which carbohydrate was exchanged for fat (68 and 20%, respectively). Body composition and blood glucose were measured over a period of 22 weeks. Glucose tolerance tests and euglycaemic-hyperinsulinaemic clamps were performed to analyse insulin sensitivity. Islet morphology was assessed by immunohistochemistry.

RESULTS

Mice on carbohydrate-containing standard or high-fat diets developed severe diabetes (blood glucose >16.6 mmol/l, glucosuria) due to selective destruction of pancreatic beta cells associated with severe loss of immunoreactivity of insulin, glucose transporter 2 (GLUT2) and musculoaponeurotic fibrosarcoma oncogene homologue A (MafA). In contrast, mice on the carbohydrate-free diet remained normoglycaemic and exhibited hyperplastic islets in spite of a morbid obesity associated with severe insulin resistance and a massive accumulation of macrophages in adipose tissue.

CONCLUSIONS/INTERPRETATION

These data indicate that the combination of obesity, insulin resistance and the inflammatory response of adipose tissue are insufficient to cause beta cell destruction in the absence of dietary carbohydrate.

Fatty acid synthase gene expression in human adipose tissue: association with obesity and type 2 diabetes

AIMS/HYPOTHESIS

Increased expression and activity of the lipogenic pathways in adipose tissue may contribute to the development of obesity. As a central enzyme in lipogenesis, the gene encoding fatty acid synthase (FASN) was identified as a candidate gene for determining body fat. In the present study we tested the hypothesis that increased FASN expression links metabolic alterations of excess energy intake, including hyperinsulinaemia, dyslipidaemia and altered adipokine profile to increased body fat mass.

SUBJECTS AND METHODS

In paired samples of visceral and subcutaneous adipose tissue from 196 participants (lean or obese), we investigated whether FASN mRNA expression (assessed by PCR) in adipose tissue is increased in obesity and related to visceral fat accumulation, measures of insulin sensitivity (euglycaemic-hyperinsulinaemic clamp) and glucose metabolism.

RESULTS

FASN mRNA expression was increased by 1.7-fold in visceral vs subcutaneous fat. Visceral adipose tissue FASN expression was correlated with FASN protein levels, subcutaneous FASN expression, visceral fat area, fasting plasma insulin, serum concentrations of IL-6, leptin and retinol-binding protein 4 (RBP4), and inversely with measures of insulin sensitivity, independently of age, sex and BMI. Moreover, we found significant correlations between FASN expression and markers of renal function, including serum creatinine and urinary albumin excretion.

CONCLUSIONS/INTERPRETATION

Increased FASN gene expression in adipose tissue is linked to visceral fat accumulation, impaired insulin sensitivity, increased circulating fasting insulin, IL-6, leptin and RBP4, suggesting an important role of lipogenic pathways in the causal relationship between consequences of excess energy intake and the development of obesity and type 2 diabetes.

A SNP haplotype of the forkhead transcription factor FOXO1A gene may have a protective effect against type 2 diabetes in German Caucasians

AIM

The human protein encoded by the FOXO1A gene functions as a transcription factor of insulin signaling key genes. In this study we investigated the role of genetic variation in the FOXO1A gene in susceptibility to type 2 diabetes (T2D) and relevant metabolic traits.

METHODS

We genotyped six haplotype tagging single nucleotide polymorphisms (SNPs) for association analyses in German Caucasians (593 patients with T2D and 760 non-diabetics, who included 594 normoglycemics and 166 individuals with impaired glucose tolerance).

RESULTS

In a case control study involving all type 2 diabetics and healthy controls with normal glucose tolerance, none of the FOXO1A SNPs showed any association with T2D. However, the frequency of the [C-C-G-A-A-A] haplotype comprising six FOXO1A SNPs was 36.5% in normoglycemic non-diabetic controls compared to 31.0% in type 2 diabetic patients (P=0.004). Consistent with this, the same haplotype was significantly associated with lower fasting plasma insulin, BMI, HbA(1C), free fatty acids and % body fat in all non-diabetic subjects (all adjusted P<0.05).

CONCLUSION

In conclusion, our study suggests a protective effect of FOXO1A haplotype [C-C-G-A-A-A] on T2D development and relevant intermediate phenotypes which predispose for T2D.

Shortcomings in methodology complicate measurements of serum retinol binding protein (RBP4) in insulin-resistant human subjects

AIMS/HYPOTHESIS

Levels of retinol binding protein (RBP4) are increased in the serum of insulin-resistant human subjects even before overt diabetes develops. RBP4 levels correlate with insulin resistance, BMI, WHR, dyslipidaemia and hypertension. Improvement of insulin sensitivity with exercise training is associated with reduction in serum RBP4 levels. Therefore serum RBP4 may be useful for early diagnosis of insulin resistance and for monitoring improvements in insulin sensitivity. We sought to determine the performance of assays for this application.

SUBJECTS AND METHODS

We compared quantitative western blotting and three commercially available multiwell immunoassays in parallel measurements of RBP4 concentrations in serum from insulin-sensitive subjects and from insulin-resistant subjects with impaired glucose tolerance or type 2 diabetes.

RESULTS

The assays yielded different absolute values and magnitudes of elevation of serum RBP4. Western blotting and a sandwich ELISA reported RBP4 concentrations that highly inversely correlated with insulin sensitivity measured by euglycaemic-hyperinsulinaemic clamp. However, western blotting yielded concentrations with a greater dynamic range and less overlap between control and insulin-resistant subjects. Two competitive enzyme-linked immunoassays undervalued serum RBP4 concentrations in insulin-resistant subjects, possibly due to assay saturation. Poor linearity of dilution also limited assay utility. All assays tested exhibited greater immunoreactivity with urinary (C-terminal proteolysed) RBP4 than with full-length RBP4, the predominant form in serum.

CONCLUSIONS/INTERPRETATIONS

These findings support the use of quantitative western blotting standardised to full-length RBP4 protein as a 'gold standard' method for measuring serum RBP4 in insulin-resistant states. Other assays should use full-length RBP4 and be extensively cross-validated using other methods.

Differential regulation of visfatin and adiponectin in pregnancies with normal and abnormal placental function

OBJECTIVE

There is compelling evidence that insulin resistance may play a pivotal role in the development of pregnancy complications including pre-eclampsia and intrauterine growth restriction (IUGR). As dysregulation of visfatin and adiponectin is found in insulin resistance, both adipokines might contribute to pregnancy disorders.

PATIENTS AND METHODS

Plasma levels of visfatin and adiponectin were quantified in second-trimester pregnancies with abnormal uterine perfusion and in third-trimester pregnancies with IUGR as compared to healthy controls.

RESULTS

Mean plasma adiponectin levels were significantly increased in women with pathological perfusion (7078 +/- 789 ng/ml) as compared to normal uterine perfusion (4481 +/- 1183 ng/ml) (P < 0.05) whereas visfatin plasma concentrations were not different between these groups. Visfatin levels were significantly elevated in women with IUGR (140.1 +/- 10 ng/ml) as compared to patients with normal outcome (65.5 +/- 11 ng/ml) (P < 0.05). By contrast, adiponectin plasma concentrations were not significantly altered in women with IUGR.

CONCLUSIONS

Our data support the view that the adipokines visfatin and adiponectin are differentially regulated in pregnancy complications.

Biologie des viszeralen Fetts

Visceral obesity is an independent risk factor for the development of cardiovascular diseases and type 2 diabetes. This is likely to be due to biological characteristics of visceral tissue, which are different from those of subcutaneous adipose tissue in terms of decreased insulin sensitivity and increased lipolytic activity. In addition, the anatomical site of visceral fat could be one potential reason for the increased cardio-metabolic risk associated with visceral obesity. Visceral adipose tissue drains into the portal vein and therefore the liver is exposed to the undiluted metabolites and adipokines released from visceral fat. There are profound differences between visceral and subcutaneous adipocytes in the metabolism, expression of specific receptors and secretion of a specific adipokine pattern, which could contribute to the adverse consequences of visceral obesity.

Rolle von Muskulatur und Fettgewebe in der Pathogenese des Typ-2-Diabetes

In the last years type 2 diabetes has reached almost epidemic proportions. More than 170 million individuals are affected worldwide, about 6 million in Germany. In the pathogenesis of type 2 diabetes, insulin resistance in liver, fat and muscle as well as the inability of the pancreatic beta-cell to fully compensate for this insulin resistance are the central pathophysiological events. Both genetic and environmental factors, such as lack of physical exercise and hypercaloric nutrition play a major role in this process, although the precise mechanisms for type 2 diabetes development remain largely unknown. In the characterization of the role of liver, adipose tissue and skeletal muscle in the pathogenesis of type 2 diabetes, tissue specific knockout mouse models have challenged our concepts of glucose homeostasis.

Impact of the Thr789Ala variant of the von Willebrand factor levels, on ristocetin co-factor and collagen binding capacity and its association with coronary heart disease in patients with diabetes mellitus type 2

A Thr789Ala variant in the von Willebrand Factor (vWF) gene is associated with increased vWF plasma concentrations and might therefore affect the risk of coronary heart disease (CHD) in the general population. Patients with type 2 diabetes have an increased risk for premature atherosclerosis and are characterized by alterations of the coagulation system. However, it is not known whether the Thr789Ala variant in the vWF gene contributes to the increased CHD risk in patients with type 2 diabetes. We therefore investigated the potential relationship between the Thr789Ala variant in the vWF gene and the occurrence of CHD in 356 patients with type 2 diabetes, either with (DM+/CHD+, n = 204) or without evidence for CHD (DM+/CHD-, n = 152). In addition, two control groups without type 2 diabetes, with (DM-/CHD+, n = 22) or without CHD (DM-/CHD-, n = 100), were investigated. Individuals with the vWF Thr789Ala variant have significantly higher von Willebrand factor plasma concentrations (p < 0.001). In addition, ristocetin co-factor was significantly increased in vWF Thr789Ala variant carriers (p < 0.05). Ristocetin co-factor levels and collagen binding capacity were also increased in individuals affected with either type 2 diabetes, CHD or both (DM+/CHD+, DM+/CHD-, DM-/CHD+) as compared to healthy controls (DM-/CHD-) (p < 0.001). However, we did not find an association between the vWF Thr789Ala variant and the occurrence of CHD in patient with type 2 diabetes (p = 0.34). In conclusion, although the Thr789Ala vWF gene variant is associated with increased plasma concentrations of vWF, ristocetin co factor levels and collagen binding capacity in patients with type 2 diabetes and CHD, a direct effect of this variant on the occurrence of CHD in patients with type 2 diabetes, could not be detected.

Glucose regulates expression of the nerve growth factor (NGF) receptors TrkA and p75NTR in rat islets and INS-1E beta-cells

BACKGROUND

The function and survival of pancreatic beta-cells strongly depend on glucose concentration and on autocrine secretion of peptide growth factors. NGF and its specific receptors TrkA and p75NTR play a pivotal role in islet survival and glucose-dependent insulin secretion. We therefore investigated whether or not glucose concentration influences expression of TrkA and p75NTR in rat islets and in INS-1E beta-cells at the mRNA and protein level (INS-1E).

METHODS

Gene expression of the NGF receptors TrkA and p75NTR but also of the metabolic gene liver-type pyruvate kinase (L-PK) and the neurotrophin receptors TrkB and TrkC was studied by semi-quantitative PCR and by real-time PCR in islets and INS-1E beta-cells.

RESULTS

In rat islets, high glucose exposure (25 mmol/l) increased gene expression of TrkA, p75NTR and L-PK. Expression of TrkA, p75NTR and L-PK reflected insulin secretion at the respective glucose concentration. In rat INS-1E insulinoma cells, expression of L-PK and p75NTR was suppressed by low glucose as in the islets, while expression of TrkA was strongly increased by low glucose levels and thus was regulated differently than in islets. Expression of TrkB and TrkC was not regulated by glucose concentration at all. TrkA protein was regulated in the same fashion as its mRNA expression, while p75NTR protein was not significantly regulated within 24 h.

CONCLUSION

Glucose interacts with gene expression of TrkA and p75NTR that are strongly involved in beta-cell growth and glucose-dependent insulin secretion. The fact that TrkA expression is regulated the opposite way in islets and in INS-1E beta-cells might reflect their specific grade of differentiation and tendency to proliferate.

Association of interleukin-6, C-reactive protein, interleukin-10 and adiponectin plasma concentrations with measures of obesity, insulin sensitivity and glucose metabolism

Abnormal glucose tolerance is associated with subclinical chronic inflammation in patients with type 2 diabetes. The aim of this study was to investigate whether plasma concentrations of inflammatory markers are associated with measures of obesity, insulin sensitivity, and hyperglycemia. IL-6, adiponectin, CRP, and IL-10 plasma concentrations were evaluated in 142 patients with a wide range of obesity, insulin sensitivity and glucose tolerance. In parallel with the impairment of glucose tolerance, there was a significant increase in IL-6, and CRP, and a significant decrease in adiponectin and IL-10 plasma concentrations. There were significant correlations between the plasma concentrations of all inflammatory markers and % body fat, insulin sensitivity, and fasting plasma glucose. However, multivariate linear regression analysis identified insulin sensitivity as determined by glucose infusion rate during the steady state of an euglycemic-hyperinsulinemic clamp as the strongest predictor of adiponectin, CRP, IL-6, and IL-10 plasma concentrations. In addition, fasting plasma glucose was a significant determinant of adiponectin, CRP, and IL-6 plasma concentrations, whereas body fat content was only a significant predictor of CRP plasma concentration. In conclusion, our data suggest that abnormal inflammatory markers in patients with type 2 diabetes are primarily related to decreased insulin sensitivity.