The ubiquitin-like modifier FAT10 is induced in MASLD and impairs the lipid-regulatory activity of PPARα.

BACKGROUND AND AIMS: Peroxisome Proliferator-Activated Receptor α (PPARα) is a key regulator of hepatic lipid metabolism and therefore a promising therapeutic target against Metabolic-dysfunction Associated Steatotic Liver Diseases (MASLD). However, its expression and activity decrease during disease progression and several of its agonists did not achieve sufficient efficiency in clinical trials with, surprisingly, a lack of steatosis improvement. Here, we identified the Human leukocyte antigen-F Adjacent Transcript 10 (FAT10) as an inhibitor of PPARα lipid metabolic activity during MASLD progression. APPROACH AND RESULTS: In vivo, the expression of FAT10 is upregulated in human and murine MASLD livers upon disease progression and correlates negatively with PPARα expression. The increase of FAT10 occurs in hepatocytes in which both proteins interact. FAT10 silencing in vitro in hepatocytes increases PPARα target gene expression, promotes fatty acid oxidation and decreases intra-cellular lipid droplet content. In line, FAT10 overexpression in hepatocytes in vivo inhibits the lipid regulatory activity of PPARα in response to fasting and agonist treatment in conditions of physiological and pathological hepatic lipid overload. CONCLUSIONS: FAT10 is induced during MASLD development and interacts with PPARα resulting in a decreased lipid metabolic response of PPARα to fasting or agonist treatment. Inhibition of the FAT10-PPARα interaction may provide a means to design potential therapeutic strategies against MASLD.

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.

Impaired Glucose Homeostasis in a Tau Knock-In Mouse Model.

Alzheimer's disease (AD) is the leading cause of dementia. While impaired glucose homeostasis has been shown to increase AD risk and pathological loss of tau function, the latter has been suggested to contribute to the emergence of the glucose homeostasis alterations observed in AD patients. However, the links between tau impairments and glucose homeostasis, remain unclear. In this context, the present study aimed at investigating the metabolic phenotype of a new tau knock-in (KI) mouse model, expressing, at a physiological level, a human tau protein bearing the P301L mutation under the control of the endogenous mouse Mapt promoter. Metabolic investigations revealed that, while under chow diet tau KI mice do not exhibit significant metabolic impairments, male but not female tau KI animals under High-Fat Diet (HFD) exhibited higher insulinemia as well as glucose intolerance as compared to control littermates. Using immunofluorescence, tau protein was found colocalized with insulin in the ß cells of pancreatic islets in both mouse (WT, KI) and human pancreas. Isolated islets from tau KI and tau knock-out mice exhibited impaired glucose-stimulated insulin secretion (GSIS), an effect recapitulated in the mouse pancreatic ß-cell line (MIN6) following tau knock-down. Altogether, our data indicate that loss of tau function in tau KI mice and, particularly, dysfunction of pancreatic ß cells might promote glucose homeostasis impairments and contribute to metabolic changes observed in AD.

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.

Leptin brain entry via a tanycytic LepR-EGFR shuttle controls lipid metabolism and pancreas function.

Metabolic health depends on the brain's ability to control food intake and nutrient use versus storage, processes that require peripheral signals such as the adipocyte-derived hormone, leptin, to cross brain barriers and mobilize regulatory circuits. We have previously shown that hypothalamic tanycytes shuttle leptin into the brain to reach target neurons. Here, using multiple complementary models, we show that tanycytes express functional leptin receptor (LepR), respond to leptin by triggering Ca2+ waves and target protein phosphorylation, and that their transcytotic transport of leptin requires the activation of a LepR-EGFR complex by leptin and EGF sequentially. Selective deletion of LepR in tanycytes blocks leptin entry into the brain, inducing not only increased food intake and lipogenesis but also glucose intolerance through attenuated insulin secretion by pancreatic ß-cells, possibly via altered sympathetic nervous tone. Tanycytic LepRb-EGFR-mediated transport of leptin could thus be crucial to the pathophysiology of diabetes in addition to obesity, with therapeutic implications.

Improved synthesis, resolution, absolute configuration determination and biological evaluation of HLM006474 enantiomers.

An improved green synthesis of the E2F inhibitor HLM0066474 is described, using solvent-free and microwave irradiation conditions. The two enantiomers are separated using semi-preparative separation on Chiralpak ID and their absolute configuration is determined by vibrational circular dichroism (VCD) analysis. Biological evaluation of both enantiomers on E2F1 transcriptional activity reveals that the (+)-R, but not the (-)-S enantiomer is biologically active in repressing E2F1 transcriptional activity.