Pharmacological HDAC inhibition impairs pancreatic ß-cell function through an epigenome-wide reprogramming.

Histone deacetylases enzymes (HDACs) are chromatin modifiers that regulate gene expression through deacetylation of lysine residues within specific histone and non-histone proteins. A cell-specific gene expression pattern defines the identity of insulin-producing pancreatic ß cells, yet molecular networks driving this transcriptional specificity are not fully understood. Here, we investigated the HDAC-dependent molecular mechanisms controlling pancreatic ß-cell identity and function using the pan-HDAC inhibitor trichostatin A through chromatin immunoprecipitation assays and RNA sequencing experiments. We observed that TSA alters insulin secretion associated with ß-cell specific transcriptome programming in both mouse and human ß-cell lines, as well as on human pancreatic islets. We also demonstrated that this alternative ß-cell transcriptional program in response to HDAC inhibition is related to an epigenome-wide remodeling at both promoters and enhancers. Our data indicate that HDAC activity could be required to protect against loss of ß-cell identity with unsuitable expression of genes associated with alternative cell fates.

ß-Cell-Specific E2f1 Deficiency Impairs Glucose Homeostasis, ß-Cell Identity, and Insulin Secretion.

The loss of pancreatic ß-cell identity has emerged as an important feature of type 2 diabetes development, but the molecular mechanisms are still elusive. Here, we explore the cell-autonomous role of the cell-cycle regulator and transcription factor E2F1 in the maintenance of ß-cell identity, insulin secretion, and glucose homeostasis. We show that the ß-cell-specific loss of E2f1 function in mice triggers glucose intolerance associated with defective insulin secretion, altered endocrine cell mass, downregulation of many ß-cell genes, and concomitant increase of non-ß-cell markers. Mechanistically, epigenomic profiling of the promoters of these non-ß-cell upregulated genes identified an enrichment of bivalent H3K4me3/H3K27me3 or H3K27me3 marks. Conversely, promoters of downregulated genes were enriched in active chromatin H3K4me3 and H3K27ac histone marks. We find that specific E2f1 transcriptional, cistromic, and epigenomic signatures are associated with these ß-cell dysfunctions, with E2F1 directly regulating several ß-cell genes at the chromatin level. Finally, the pharmacological inhibition of E2F transcriptional activity in human islets also impairs insulin secretion and the expression of ß-cell identity genes. Our data suggest that E2F1 is critical for maintaining ß-cell identity and function through sustained control of ß-cell and non-ß-cell transcriptional programs. ARTICLE HIGHLIGHTS: ß-Cell-specific E2f1 deficiency in mice impairs glucose tolerance. Loss of E2f1 function alters the ratio of α- to ß-cells but does not trigger ß-cell conversion into α-cells. Pharmacological inhibition of E2F activity inhibits glucose-stimulated insulin secretion and alters ß- and α-cell gene expression in human islets. E2F1 maintains ß-cell function and identity through control of transcriptomic and epigenetic programs.

The transcription factor E2F1 controls the GLP-1 receptor pathway in pancreatic ß cells.

The glucagon-like peptide 1 (Glp-1) has emerged as a hormone with broad pharmacological potential in type 2 diabetes (T2D) treatment, notably by improving ß cell functions. The cell-cycle regulator and transcription factor E2f1 is involved in glucose homeostasis by modulating ß cell mass and function. Here, we report that ß cell-specific genetic ablation of E2f1 (E2f1ß-/-) impairs glucose homeostasis associated with decreased expression of the Glp-1 receptor (Glp1r) in E2f1ß-/- pancreatic islets. Pharmacological inhibition of E2F1 transcriptional activity in nondiabetic human islets decreases GLP1R levels and blunts the incretin effect of GLP1R agonist exendin-4 (ex-4) on insulin secretion. Overexpressing E2f1 in pancreatic ß cells increases Glp1r expression associated with enhanced insulin secretion mediated by ex-4. Interestingly, ex-4 induces retinoblastoma protein (pRb) phosphorylation and E2f1 transcriptional activity. Our findings reveal critical roles for E2f1 in ß cell function and suggest molecular crosstalk between the E2F1/pRb and GLP1R signaling pathways.

Glucose Regulates m6A Methylation of RNA in Pancreatic Islets.

Type 2 diabetes is characterized by chronic hyperglycemia associated with impaired insulin action and secretion. Although the heritability of type 2 diabetes is high, the environment, including blood components, could play a major role in the development of the disease. Amongst environmental effects, epitranscriptomic modifications have been recently shown to affect gene expression and glucose homeostasis. The epitranscriptome is characterized by reversible chemical changes in RNA, with one of the most prevalent being the m6A methylation of RNA. Since pancreatic ß cells fine tune glucose levels and play a major role in type 2 diabetes physiopathology, we hypothesized that the environment, through variations in blood glucose or blood free fatty acid concentrations, could induce changes in m6A methylation of RNAs in pancreatic ß cells. Here we observe a significant decrease in m6A methylation upon high glucose concentration, both in mice and human islets, associated with altered expression levels of m6A demethylases. In addition, the use of siRNA and/or specific inhibitors against selected m6A enzymes demonstrate that these enzymes modulate the expression of genes involved in pancreatic ß-cell identity and glucose-stimulated insulin secretion. Our data suggest that environmental variations, such as glucose, control m6A methylation in pancreatic ß cells, playing a key role in the control of gene expression and pancreatic ß-cell functions. Our results highlight novel causes and new mechanisms potentially involved in type 2 diabetes physiopathology and may contribute to a better understanding of the etiology of this disease.

Cdkn2a deficiency promotes adipose tissue browning.

OBJECTIVES: Genome-wide association studies have reported that DNA polymorphisms at the CDKN2A locus modulate fasting glucose in human and contribute to type 2 diabetes (T2D) risk. Yet the causal relationship between this gene and defective energy homeostasis remains elusive. Here we sought to understand the contribution of Cdkn2a to metabolic homeostasis. METHODS: We first analyzed glucose and energy homeostasis from Cdkn2a-deficient mice subjected to normal or high fat diets. Subsequently Cdkn2a-deficient primary adipose cells and human-induced pluripotent stem differentiated into adipocytes were further characterized for their capacity to promote browning of adipose tissue. Finally CDKN2A levels were studied in adipocytes from lean and obese patients. RESULTS: We report that Cdkn2a deficiency protects mice against high fat diet-induced obesity, increases energy expenditure and modulates adaptive thermogenesis, in addition to improving insulin sensitivity. Disruption of Cdkn2a associates with increased expression of brown-like/beige fat markers in inguinal adipose tissue and enhances respiration in primary adipose cells. Kinase activity profiling and RNA-sequencing analysis of primary adipose cells further demonstrate that Cdkn2a modulates gene networks involved in energy production and lipid metabolism, through the activation of the Protein Kinase A (PKA), PKG, PPARGC1A and PRDM16 signaling pathways, key regulators of adipocyte beiging. Importantly, CDKN2A expression is increased in adipocytes from obese compared to lean subjects. Moreover silencing CDKN2A expression during human-induced pluripotent stem cells adipogenic differentiation promoted UCP1 expression. CONCLUSION: Our results offer novel insight into brown/beige adipocyte functions, which has recently emerged as an attractive therapeutic strategy for obesity and T2D. Modulating Cdkn2a-regulated signaling cascades may be of interest for the treatment of metabolic disorders.