Evidence for Presence and Functional Effects of Kv1.1 Channels in β-Cells: General Survey and Results from mceph/mceph Mice

BACKGROUND:Voltage-dependent K(+) channels (Kv) mediate repolarisation of β-cell action potentials, and thereby abrogate insulin secretion. The role of the Kv1.1 K(+) channel in this process is however unclear. We tested for presence of Kv1.1 in different species and tested for a functional role of Kv1.1 by assessing pancreatic islet function in BALB/cByJ (wild-type) and megencephaly (mceph/mceph) mice, the latter having a deletion in the Kv1.1 gene. METHODOLOGY/PRINCIPAL FINDINGS:Kv1.1 expression was detected in islets from wild-type mice, SD rats and humans, and expression of truncated Kv1.1 was detected in mceph/mceph islets. Full-length Kv1.1 protein was present in islets from wild-type mice, but, as expected, not in those from mceph/mceph mice. Kv1.1 expression was localized to the β-cell population and also to α- and δ-cells, with evidence of over-expression of truncated Kv1.1 in mceph/mceph islets. Blood glucose, insulin content, and islet morphology were normal in mceph/mceph mice, but glucose-induced insulin release from batch-incubated islets was (moderately) higher than that from wild-type islets. Reciprocal blocking of Kv1.1 by dendrotoxin-K increased insulin secretion from wild-type but not mceph/mceph islets. Glucose-induced action potential duration, as well as firing frequency, was increased in mceph/mceph mouse β-cells. This duration effect on action potential in β-cells from mceph/mceph mice was mimicked by dendrotoxin-K in β-cells from wild-type mice. Observations concerning the effects of both the mceph mutation, and of dendrotoxin-K, on glucose-induced insulin release were confirmed in pancreatic islets from Kv1.1 null mice. CONCLUSION/SIGNIFICANCE:Kv1.1 channels are expressed in the β-cells of several species, and these channels can influence glucose-stimulated insulin release

Activation of hepatic AMPK by 17β-estradiol suppresses both nuclear receptor Nr2c2/TR4 and its downstream lipogenic targets, reduces gluconeogenic genes and improves insulin signaling

Estrogen replacement reduces the frequency of type 2 diabetes (T2D), an effect which involves the suppression of hepatic glucose production. The objective of this study was to identify the hepatic mechanisms involved in the beneficial effects of 17β-estradiol (E2) on insulin sensitivity in mice exposed to long term (10 months) high fat diet (HFD) feeding. E2 treatment in HFD mice led to significant improvements in glycemic control. In the livers from these animals, Western blotting studies showed that E2 treatment led to significant increases in the activation state of the AMP-activated protein kinase (AMPK), in association with reduction of the nuclear receptor Nr2c2/TR4 both mRNA and protein levels, and suppression of downstream lipogenic gene expression. These effects were in parallel with up-regulation of hormone sensitive lipase (lipe) expression by E2. Increased fasting glucose levels following HFD feeding were associated with increases in the expression levels of the gluconeogenic genes, g6pt1 and pyruvate carboxylase, whilst E2 treatment significantly reduced their expression levels. The insulin signaling pathway was studied in the liver after acute insulin intervention. The phosphorylation states of AKT2 and FOXO1 were both decreased in HFD mice, and E2 treatment reversed these changes. In conclusion, E2 treatment reduced body weight and improved glycemic control in association with activation of hepatic AMPK, reduced expression of its downstream target Nr2c2/TR4, and consequent decreases in lipogenic gene expression. Together with increased triglyceride mobilization, these changes paralleled improved hepatic insulin signaling and reduced gluconeogenic gene expression. [Dis Mol Med 2016; 4(4.000): 55-67

Mechanisms of antidiabetogenic and body weight-lowering effects of estrogen in high-fat diet-fed mice

The high-fat diet (HFD)-fed mouse is a model of obesity, impaired glucose tolerance, and insulin resistance. The main objective of this study was to elucidate the molecular mechanisms underlying the antidiabetogenic and weight-lowering effects of 17β-estradiol (E2) in this mouse model. C57BL/6 female mice (8 wk old) were fed on a HFD for 10 mo. E2, given daily (50 μg/kg sc) during the last month of feeding, decreased body weight and markedly improved glucose tolerance and insulin sensitivity. Plasma levels of insulin, leptin, resistin, and adiponectin were decreased. We demonstrated that E2 treatment decreased the expression of genes encoding resistin and leptin in white adipose tissue (WAT), whereas adiponectin expression was unchanged. Furthermore, in WAT we demonstrated decreased expression levels of sterol regulatory element-binding protein 1c (SREBP1c) and its lipogenic target genes, such as fatty acid synthase and stearoyl-CoA desaturase 1 (SCD1). In the liver, the expression levels of transcription factors such as liver X receptor α and SREBP1c were not changed by E2 treatment, but the expression of the key lipogenic gene SCD1 was reduced. This was accompanied by decreased hepatic triglyceride content. Importantly, E2 decreased the hepatic expression of glucose-6-phosphatase (G-6-Pase). We conclude that E2 treatment exerts antidiabetic and antiobesity effects in HFD mice and suggest that this is related to decreased expression of lipogenic genes in WAT and liver and suppression of hepatic expression of G-6-Pase. Decreased plasma levels of resistin probably also play an important role in this context

Monomeric G-protein, Rhes, is not an imidazoline-regulated protein in pancreatic β-cells

The monomeric G-protein, Rhes, is a candidate imidazoline-regulated molecule involved in mediating the insulin secretory response to efaroxan [S.L. Chan, L.K. Monks, H. Gao, P. Deaville, N.G. Morgan, Identification of the monomeric G-protein, Rhes, as an efaroxan-regulated protein in the pancreatic beta-cell, Br. J. Pharmacol. 136 (1) (2002) 31–36]. This suggestion was based on observations regarding changes in Rhes mRNA expression in rat islets and pancreatic β-cells after prolonged culture with efaroxan, leading to desensitization of the insulin response to the compound. To verify this report, we have evaluated the effects of the imidazoline compounds efaroxan and BL11282 on Rhes mRNA expression in isolated rat pancreatic islets maintained in conditions identical to those used by Chan et al. The results demonstrate that desensitization of the insulin response to efaroxan, or to another imidazoline, BL11282, does not change Rhes mRNA expression levels. Transfection of MIN6 cells with plasmids containing Rhes or Rhes-antisense also does not alter efaroxan- or BL11282-induced insulin secretion. Together, these data do not support the hypothesis that Rhes is an imidazoline-regulated protein