Mutation at position -132 in the islet amyloid polypeptide ( IAPP) gene promoter enhances basal transcriptional activity through a new CRE-like binding site

CREB mediates the insulinotropic and anti-apoptotic effects of GLP-1 signaling in adult mouse β-cells

Objective

Glucagon-like peptide-1 (GLP-1) plays a major role in pancreatic β-cell function and survival by increasing cytoplasmic cAMP levels, which are thought to affect transcription through activation of the basic leucine zipper (bZIP) transcription factor CREB. Here, we test CREB function in the adult β-cell through inducible gene deletion.

Methods

We employed cell type-specific and inducible gene ablation to determine CREB function in pancreatic β-cells in mice.

Results

By ablating CREB acutely in mature β-cells in tamoxifen-treated Creb^loxP/loxP;Pdx1-CreERT2 mice, we show that CREB has little impact on β-cell turnover, in contrast to what had been postulated previously. Rather, CREB is required for GLP-1 to elicit its full effects on stimulating glucose-induced insulin secretion and protection from cytokine-induced apoptosis. Mechanistically, we find that CREB regulates expression of the pro-apoptotic gene p21 (Cdkn1a) in β-cells, thus demonstrating that CREB is essential to mediating this critical aspect of GLP-1 receptor signaling.

Conclusions

In sum, our studies using conditional gene deletion put into question current notions about the importance of CREB in regulating β-cell function and mass. However, we reveal an important role for CREB in the β-cell response to GLP-1 receptor signaling, further validating CREB as a therapeutic target for diabetes.

Involvement of ATP-sensitive potassium (K(ATP)) channels in the loss of beta-cell function induced by human islet amyloid polypeptide

Islet amyloid polypeptide (IAPP) is a major component of amyloid deposition in pancreatic islets of patients with type 2 diabetes. It is known that IAPP can inhibit glucose-stimulated insulin secretion; however, the mechanisms of action have not yet been established. In the present work, using a rat pancreatic beta-cell line, INS1E, we have created an in vitro model that stably expressed human IAPP gene (hIAPP cells). These cells showed intracellular oligomers and a strong alteration of glucose-stimulated insulin and IAPP secretion. Taking advantage of this model, we investigated the mechanism by which IAPP altered beta-cell secretory response and contributed to the development of type 2 diabetes. We have measured the intracellular Ca(2+) mobilization in response to different secretagogues as well as mitochondrial metabolism. The study of calcium signals in hIAPP cells demonstrated an absence of response to glucose and also to tolbutamide, indicating a defect in ATP-sensitive potassium (K(ATP)) channels. Interestingly, hIAPP showed a greater maximal respiratory capacity than control cells. These data were confirmed by an increased mitochondrial membrane potential in hIAPP cells under glucose stimulation, leading to an elevated reactive oxygen species level as compared with control cells. We concluded that the hIAPP overexpression inhibits insulin and IAPP secretion in response to glucose affecting the activity of K(ATP) channels and that the increased mitochondrial metabolism is a compensatory response to counteract the secretory defect of beta-cells.

Amylin and hypertension: association of an amylin -G132A gene mutation and hypertension in humans and amylin-induced endothelium dysfunction in rats

CONTEXT

Amylin has been linked to the development of hypertension in several pathological states related to hypertension and insulin resistance, although there is scant data regarding its potential mechanisms of action. The -132 G/A mutation located within an activator domain of the amylin gene's promoter was first identified in a small cohort of Spanish patients with type 2 diabetes.

OBJECTIVE

The objective of the study was to test the interference of amylin peptide with endothelium-dependent responses as an added potential mechanism for amylin-induced hypertension.

DESIGN

A total of 384 patients with type 2 diabetes and 207 healthy controls were subjected to clinical analysis and genetic screening for the -132 G/A mutation of the amylin gene. The effect of amylin on endothelium-dependent responses was analyzed in aortic rings and mesenteric microvessels from nondiabetic rats.

RESULTS

The prevalence of the mutation was 10.1 vs. 0.9% in the control population (P<0.001). Hypertension was higher in a diabetic population carrying the mutation than in diabetic noncarriers (74 vs. 57%; P<0.05). Diabetic carriers showed higher fasting amylin levels than diabetic noncarriers (11.4+/-7 vs. 8.2+/-3 pmol/liter; P<0.05). Preincubation with 20 pmol/liter amylin impaired the relaxant responses induced by acetylcholine in rat aorta and mesenteric microvessels. This effect was abolished in both vascular beds in the presence of 100 micromol/liter NG-nitro-L-arginine methyl ester.

CONCLUSIONS

We propose that amylin levels and hypertension may be linked by a novel mechanism involving the capacity of amylin to induce endothelial dysfunction by interfering with nitric oxide-mediated responses.

Islet amyloid polypeptide gene promoter polymorphisms are not associated with Type 2 diabetes or with the severity of islet amyloidosis

The over-expression of the islet amyloid polypeptide (IAPP) gene could be a causal factor for islet amyloidosis and beta-cell destruction in Type 2 diabetes (T2DM). An IAPP gene promoter polymorphism, IAPP-132G to A, has been associated with T2DM in Spain. To investigate this polymorphism in other cohorts and in relation to therapy, DNA from 425 T2DM and 279 unrelated, non-diabetic UK subjects (ND) and 102 T2DM and 80 ND Finnish subjects was examined. The relationship of amyloid severity (percent amyloid/islet) to prevalence (number of islets affected) and the association of IAPP-132G/A with amyloid was determined in post-mortem pancreas from 38 T2DM subjects. The -132G/A was not associated with T2DM in the UK cohorts (4.5% T2DM; 3.2% ND) or associated with requirement for insulin therapy by 6 years. The mutation was and undetected in the Finnish samples but a new variant, -166T/C, was identified in 2 Finnish T2DM subjects. -132G/A was found in 2/38 diabetic, amyloid-containing and 3/19 ND, amyloid-free subjects. The islet amyloid severity was linearly correlated with the prevalence in T2DM. The IAPP-132G/A promoter polymorphism is not associated with T2DM, a requirement for insulin therapy or with the degree of islet amyloidosis in cohorts from the UK or Finland.