miR-146a regulates insulin sensitivity via NPR3.

The pathogenesis of obesity-related metabolic diseases has been linked to the inflammation of white adipose tissue (WAT), but the molecular interconnections are still not fully understood. MiR-146a controls inflammatory processes by suppressing pro-inflammatory signaling pathways. The aim of this study was to characterize the role of miR-146a in obesity and insulin resistance. MiR-146a-/- mice were subjected to a high-fat diet followed by metabolic tests and WAT transcriptomics. Gain- and loss-of-function studies were performed using human Simpson-Golabi-Behmel syndrome (SGBS) adipocytes. Compared to controls, miR-146a-/- mice gained significantly more body weight on a high-fat diet with increased fat mass and adipocyte hypertrophy. This was accompanied by exacerbated liver steatosis, insulin resistance, and glucose intolerance. Likewise, adipocytes transfected with an inhibitor of miR-146a displayed a decrease in insulin-stimulated glucose uptake, while transfecting miR-146a mimics caused the opposite effect. Natriuretic peptide receptor 3 (NPR3) was identified as a direct target gene of miR-146a in adipocytes and CRISPR/Cas9-mediated knockout of NPR3 increased insulin-stimulated glucose uptake and enhanced de novo lipogenesis. In summary, miR-146a regulates systemic and adipocyte insulin sensitivity via downregulation of NPR3.

Elevated ß-cell stress levels promote severe diabetes development in mice with MODY4.

Maturity-onset diabetes of the young (MODY) is a group of monogenetic forms of diabetes mellitus caused by mutations in genes regulating ß-cell development and function. MODY represents a heterogeneous group of non-insulin-dependent diabetes arising in childhood or adult life. Interestingly, clinical heterogeneity in MODY patients like variable disease onset and severity is observed even among individual family members sharing the same mutation, an issue that is not well understood. As high blood glucose levels are a well-known factor promoting ß-cell stress and ultimately leading to cell death, we asked whether additional ß-cell stress might account for the occurrence of disease heterogeneity in mice carrying a MODY4 mutation. In order to challenge ß-cells, we established a MODY4 animal model based on Pdx1 (pancreatic and duodenal homeobox 1) haploinsufficiency, which allows conditional modulation of cell stress by genetic inhibition of the stress-responsive IKK/NF-κB signalling pathway. While Pdx1+/- mice were found glucose intolerant without progressing to diabetes, additional challenge of ß-cell function by IKK/NF-κB inhibition promoted rapid diabetes development showing hyperglycaemia, hypoinsulinemia and loss of ß-cell mass. Disease pathogenesis was characterized by deregulation of genes controlling ß-cell homeostasis and function. Importantly, restoration of normal IKK/NF-κB signalling reverted the diabetic phenotype including normalization of glycaemia and ß-cell mass. Our findings implicate that the avoidance of additional ß-cell stress can delay a detrimental disease progression in MODY4 diabetes. Remarkably, an already present diabetic phenotype can be reversed when ß-cell stress is normalized.

Long-term IKK2/NF-κB signaling in pancreatic ß-cells induces immune-mediated diabetes.

Type 1 diabetes is a multifactorial inflammatory disease in genetically susceptible individuals characterized by progressive autoimmune destruction of pancreatic ß-cells initiated by yet unknown factors. Although animal models of type 1 diabetes have substantially increased our understanding of disease pathogenesis, heterogeneity seen in human patients cannot be reflected by a single model and calls for additional models covering different aspects of human pathophysiology. Inhibitor of κB kinase (IKK)/nuclear factor-κB (NF-κB) signaling is a master regulator of inflammation; however, its role in diabetes pathogenesis is controversially discussed by studies using different inhibition approaches. To investigate the potential diabetogenic effects of NF-κB in ß-cells, we generated a gain-of-function model allowing conditional IKK2/NF-κB activation in ß-cells. A transgenic mouse model that expresses a constitutively active mutant of human IKK2 dependent on Pdx-1 promoter activity (IKK2-CA(Pdx-1)) spontaneously develops full-blown immune-mediated diabetes with insulitis, hyperglycemia, and hypoinsulinemia. Disease development involves a gene expression program mimicking virus-induced diabetes and allergic inflammatory responses as well as increased major histocompatibility complex class I/II expression by ß-cells that could collectively promote diabetes development. Potential novel diabetes candidate genes were also identified. Interestingly, animals successfully recovered from diabetes upon transgene inactivation. Our data give the first direct evidence that ß-cell-specific IKK2/NF-κB activation is a potential trigger of immune-mediated diabetes. Moreover, IKK2-CA(Pdx-1) mice provide a novel tool for studying critical checkpoints in diabetes pathogenesis and mechanisms governing ß-cell degeneration/regeneration.