Statins are beneficial in the treatment of cardiovascular disease (CVD), but these lipid-lowering drugs are associated with increased incidence of new on-set diabetes. The cellular mechanisms behind the development of diabetes by statins are elusive. Here we have treated mice on normal diet (ND) and high fat diet (HFD) with rosuvastatin. Under ND rosuvastatin lowered blood glucose through improved insulin sensitivity and increased glucose uptake in adipose tissue. In vitro rosuvastatin reduced insulin secretion and insulin content in islets. In the beta cell Ca2 + signaling was impaired and the density of granules at the plasma membrane was increased by rosuvastatin treatment. HFD mice developed insulin resistance and increased insulin secretion prior to administration of rosuvastatin. Treatment with rosuvastatin decreased the compensatory insulin secretion and increased glucose uptake. In conclusion, our data shows dual effects on glucose homeostasis by rosuvastatin where insulin sensitivity is improved, but beta cell function is impaired.
Altered microRNA profiles have been demonstrated in experimental models of type 2 diabetes, including in islets of the diabetic Goto-Kakizaki (GK) rat. Our bioinformatic analysis of conserved sequences in promoters of microRNAs, previously observed to be up-regulated in GK rat islets, revealed putative CGCG-core motifs on the promoter of the miR-212/miR-132 cluster, overexpression of which has been shown to increase insulin secretion. These motifs are possible targets of calmodulin binding transcription activators Camta1 and Camta2 that have been recognized as integrators of stress responses. We also identified putative NKE elements, possible targets of NK2 homeobox proteins like the essential islet transcription factor Nkx2-2. As Camtas can function as co-activators with NK2 proteins in other tissues, we explored the role of Camta1, Camta2, and Nkx2-2 in the regulation of the miR-212/miR-132 cluster and insulin secretion. We demonstrate that exposure of control Wistar or GK rat islets to 16.7 mM glucose increases miR-212/ miR-132 expression but significantly less so in the GK rat. In addition, Camta1, Camta2, and Nkx2-2 were down-regulated in GK rat islets, and knockdown of Camta1 reduced miR-212/ miR-132 promoter activity and miR-212/miR-132 expression, even under cAMP elevation. Knockdown of Camta1 decreased insulin secretion in INS-1 832/13 cells and Wistar rat islets but increased insulin content. Furthermore, knockdown of Camta1 reduced K ؉ -induced insulin secretion and voltage-dependent Ca 2؉ currents. We also demonstrate Camta1 and Nkx2-2 protein interaction. These results indicate that Camta1 is required not only for expression of the miR-212/miR-132 cluster but at multiple levels for regulating beta cell insulin content and secretion.
MicroRNAs are small non-coding RNAs, which negatively regulate the expression of target genes. They have emerged as important modulators in beta cell compensation upon increased metabolic demand, failure of which leads to reduced insulin secretion and type 2 diabetes. To elucidate the function of miRNAs in beta cells, insulin-secreting cell lines, such as the rat insulinoma INS-1 832/13 and the human EndoC-βH1, are widely used. Previous studies in the cancer field have suggested that miRNA expression is influenced by confluency of adherent cells. We therefore aimed to investigate whether one of the most enriched miRNAs in the pancreatic endocrine cells, miR-375, and two of its validated targets in mouse, Cav1 and Aifm1, were differentially-expressed in cell cultures with different confluences. Additionally, we measured the expression of other miRNAs, such as miR-152, miR-130a, miR-132, miR-212 and miR-200a, with known roles in beta cell function. We did not see any significant expression changes of miR-375 nor any of the two targets, in both the rat and human beta cell lines at different confluences. Interestingly, among the other miRNAs measured, the expression of miR-132 and miR-212 positively correlated with confluence, but only in the INS-1 832/13 cells. Our results show that the expression of miR-375 and other miRNAs with known roles in beta cell function is independent of, or at least minimally influenced by the density of proliferating adherent cells, especially within the confluence range optimal for functional assays to elucidate miRNA-dependent regulatory mechanisms in the beta cell.
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