Beta-cell GLUT-2 loss and non-insulin-dependent diabetes mellitus: current status of the hypothesis

Sox17 regulates insulin secretion in the normal and pathologic mouse β cell

SOX17 is a key transcriptional regulator that can act by regulating other transcription factors including HNF1β and FOXA2, which are known to regulate postnatal β cell function. Given this, we investigated the role of SOX17 in the developing and postnatal pancreas and found a novel role for SOX17 in regulating insulin secretion. Deletion of the Sox17 gene in the pancreas (Sox17-paLOF) had no observable impact on pancreas development. However, Sox17-paLOF mice had higher islet proinsulin protein content, abnormal trafficking of proinsulin, and dilated secretory organelles suggesting that Sox17-paLOF adult mice are prediabetic. Consistant with this, Sox17-paLOF mice were more susceptible to aged-related and high fat diet-induced hyperglycemia and diabetes. Overexpression of Sox17 in mature β cells using Ins2-rtTA driver mice resulted in precocious secretion of proinsulin. Transcriptionally, SOX17 appears to broadly regulate secretory networks since a 24-hour pulse of SOX17 expression resulted in global transcriptional changes in factors that regulate hormone transport and secretion. Lastly, transient SOX17 overexpression was able to reverse the insulin secretory defects observed in MODY4 animals and restored euglycemia. Together, these data demonstrate a critical new role for SOX17 in regulating insulin trafficking and secretion and that modulation of Sox17-regulated pathways might be used therapeutically to improve cell function in the context of diabetes.

Blood glucose control for individuals with type-2 diabetes: acute effects of resistance exercise of lower cardiovascular-metabolic stress

This study compared the effects of resistance exercise (RE) intensities on blood glucose (GLUC) of individuals without (ND) and with type-2 diabetes (T2D). Nine individuals with T2D and 10 ND performed: (a) RE circuit at 23% of 1 maximal repetition (1RM) (RE_L); (b) RE circuit at 43% 1RM (RE_M); and (c) control (CON) session. Blood lactate (LAC) and GLUC were measured before, during, and postinterventions. Double product (DP) and rate of perceived exertion (RPE) were recorded. The area under the curve (AUC) revealed the effects of RE circuits in reducing GLUC in individuals with T2D (RE_L: 12,556 ± 3,269 vs. RE_M: 13,433 ± 3,054 vs. CON: 14,576 ± 3,922 mg.dl(-1).145 minutes; p < 0.05) with a lower AUC of GLUC in RE_L in comparison to RE_M. Similarly, for ND the RE_L reduced the AUC of GLUC when compared with RE_M and CON (RE_L: 10,943 ± 956 vs. RE_M: 12,156 ± 1,062 vs. CON: 11,498 ± 882 mg.dl(-1).145 minutes; p < 0.05). The AUC of GLUC was higher for T2D compared with ND on CON condition (p = 0.02). However, after RE circuits the difference between groups for AUC of GLUC was abolished. The RE_M for T2D was more stressful when compared with RE_L for LAC (CON: 1.3 ± 0.5 vs. RE_L: 5.5 ± 1.5 vs. RE_M: 6.8 ± 1.3 mmol·L(-1); p < 0.05), DP (CON: 8,415 ± 1,223 vs. RE_L: 15,980 ± 2,007 vs. RE_M: 18,047 ± 3,693 mmHg.bpm(-1); p < 0.05), and RPE (RE_L: 11 ± 2 vs. RE_M: 13 ± 2 Borg Scale; p < 0.05). We concluded that RE_L and RE_M were effective in reducing GLUC for individuals with T2D, with lower cardiovascular-metabolic and perceptual stress being observed for RE_L. These data suggest that acute RE sessions at light or moderate intensities are effective for controlling GLUC in individuals with T2D.

Enhanced mitochondrial metabolism may account for the adaptation to insulin resistance in islets from C57BL/6J mice fed a high-fat diet

AIM/HYPOTHESIS

Hyperinsulinaemia maintains euglycaemia in insulin-resistant states. The precise cellular mechanisms by which the beta cells adapt are still unresolved. A peripherally derived cue, such as increased circulating fatty acids, may instruct the beta cell to initiate an adaptive programme to maintain glucose homeostasis. When this fails, type 2 diabetes ensues. Because mitochondria play a key role in beta cell pathophysiology, we tested the hypothesis that mitochondrial metabolism is critical for beta cell adaptation to insulin resistance.

METHODS

C57BL/6J mice were given high-fat (HF) diet for 12 weeks. We then analysed islet hormone secretion, metabolism in vivo and in vitro, and beta cell morphology.

RESULTS

HF diet resulted in insulin resistance and glucose intolerance but not frank diabetes. Basal insulin secretion was elevated in isolated islets from HF mice with almost no additional response provoked by high glucose. In contrast, a strong secretory response was seen when islets from HF mice were stimulated with fuels that require mitochondrial metabolism, such as glutamate, glutamine, alpha-ketoisocaproic acid and succinate. Moreover, while glucose oxidation was impaired in islets from HF mice, oxidation of glutamine and palmitate was enhanced. Ultrastructural analysis of islets in HF mice revealed an accumulation of lipid droplets in beta cells and a twofold increase in mitochondrial area.

CONCLUSIONS/INTERPRETATION

We propose that beta cells exposed to increased lipid flux in insulin resistance respond by increasing mitochondrial volume. This expansion is associated with enhanced mitochondrial metabolism as a means of beta cell compensation.

Identification of two novel amino acid polymorphisms in beta-cell/liver (GLUT2) glucose transporter in Japanese subjects

The beta-cell/liver glucose transporter (GLUT2) gene was screened for mutations using single-strand conformation polymorphism analysis (SSCP) in 30 Japanese subjects with non-insulin dependent diabetes mellitus (NIDDM). Analysis of all exons and adjacent intron regions identified six SSCP polymorphisms, three of which resulted in amino acid substitutions: V101I, T110I and G519E. The V101I and G519E, substitutions represent new polymorphisms in this gene. The six polymorphisms were observed in both NIDDM and control groups and there were no significant differences in allele frequencies between groups. A portion of the insulin receptor substrate 1 gene in 30 NIDDM subjects and in normal control subjects was also screened for mutations. Two SSCP variants that change the sequence of the protein, delta S686/687 (deletion of the codons for serine-686 and 687) and G972R, were identified in two different NIDDM subjects, both whom were also heterozygous for the V101I polymorphisms in GLUT2. The GLUT2 and IRS1 amino acid polymorphisms did not show a simple pattern of co-inheritance with NIDDM in the families of these subjects suggesting that neither polymorphism is sufficient to cause NIDDM but may increase diabetes-susceptibility through their interaction with other loci and environmental factors.

The pancreatic beta-cell glucose sensor

Pancreatic beta cells secrete insulin in response to an increase in the level of blood glucose above 5 mM, which is characteristic of the fasting state. Glucose metabolism is essential for glucose sensing, and both the high-Km glucose transporter GLUT2 and the high-Km glucose phosphorylating enzyme glucokinase have been implicated in coupling insulin secretion to extracellular glucose levels. Experiments in isolated islets, immortalized beta-cell lines and transgenic animals, together with findings in humans with maturity-onset diabetes of the young, indicate that the primary beta-cell glucose sensor is glucokinase. Although the level of GLUT2 is frequently reduced in animal models of type II diabetes, GLUT2 does not limit glucose metabolism in beta cells and does not appear to regulate glucose induction of insulin secretion.