Transcriptional and epigenetic regulation in human islets

FOXM1 cooperates with ERα to regulate functional β-cell mass

The transcription factor forkhead box (FOX)M1 regulates β-cell proliferation and insulin secretion. Our previous work demonstrates that expressing a constitutively active form of FOXM1 (FOXM1*) in β-cells increases β-cell function, proliferation, and mass in male mice. However, in contrast to what is observed in males, we demonstrate here that in female mice expression of FOXM1* in β-cells does not affect β-cell proliferation or glucose tolerance. Similarly, FOXM1* transduction of male but not female human islets enhances insulin secretion in response to elevated glucose. We therefore examined the mechanism behind this sexual dimorphism. Estrogen contributes to diabetes susceptibility differences between males and females, and estrogen receptor (ER)α is the primary mediator of β-cell estrogen signaling. Moreover, in breast cancer cells, ERα and FOXM1 work together to drive gene expression. We therefore examined whether FOXM1 and ERα functionally interact in β-cells. FOXM1* rescued elevated fasting glucose, glucose intolerance, and homeostatic model assessment of β-cell function (HOMA-B) in female mice with a β-cell-specific ERα deletion. Furthermore, in the presence of estrogen, the FOXM1 and ERα cistromes exhibit significant overlap in βTC6 β-cells. In addition, FOXM1 and ERα binding sites frequently occur in complex enhancers co-occupied by other islet transcription factors. These data indicate that FOXM1 and nuclear ERα cooperate to regulate β-cell function and suggest a general mechanism contributing to the lower incidence of diabetes observed in women.NEW & NOTEWORTHY Here we investigate why the effects of increasing FOXM1 activity in β-cells observed in male mice are not seen in female mice. ERα likely collaborates with FOXM1 and other transcription factors to enhance gene expression related to β-cell function. Higher estrogen levels in females may contribute to their increased insulin secretion and the more severe consequences of losing transcription factors like FOXM1 in males. Overall, these findings shed light on sex differences in diabetes susceptibility.

FOXM1 acts sexually dimorphically to regulate functional β-cell mass

The transcription factor FOXM1 regulates β-cell proliferation and insulin secretion. Our previous work demonstrates that expressing an activated form of FOXM1 (FOXM1*) in β cells increases β-cell proliferation and mass in aged male mice. Additionally, FOXM1* enhances β-cell function even in young mice, in which no β-cell mass elevation occurs. Here, we demonstrate that FOXM1 acts in a sexually dimorphic manner in the β cell. Expression of FOXM1* in female mouse β cells does not affect β-cell proliferation or glucose tolerance. Transduction of male but not female human islets with FOXM1* enhances insulin secretion in response to elevated glucose. Estrogen contributes to diabetes susceptibility differences between males and females, and the estrogen receptor (ER)α is the primary mediator of β-cell estrogen signaling. We show that FOXM1* can rescue impaired glucose tolerance in female mice with a pancreas-wide ERα deletion. Further, FOXM1 and ERα binding sites overlap with each other and with other β-cell-enriched transcription factors, including ISL1, PAX6, MAF, and GATA. These data indicate that FOMX1 and ERα cooperate to regulate β-cell function and suggest a general mechanism contributing to the lower incidence of diabetes observed in women.

The postnatal pancreatic microenvironment guides β cell maturation through BMP4 production

Glucose homeostasis depends on regulated insulin secretion from pancreatic β cells, which acquire their mature phenotype postnatally. The functional maturation of β cells is regulated by a combination of cell-autonomous and exogenous factors; the identity of the latter is mostly unknown. Here, we identify BMP4 as a critical component through which the pancreatic microenvironment regulates β cell function. By combining transgenic mouse models and human iPSCs, we show that BMP4 promotes the expression of core β cell genes and is required for proper insulin production and secretion. We identified pericytes as the primary pancreatic source of BMP4, which start producing this ligand midway through the postnatal period, at the age β cells mature. Overall, our findings show that the islet niche directly promotes β cell functional maturation through the timely production of BMP4. Our study highlights the need to recapitulate the physiological postnatal islet niche for generating fully functional stem-cell-derived β cells for cell replacement therapy for diabetes.

Altered islet prohormone processing: A cause or consequence of diabetes?

Peptide hormones are first produced as larger precursor prohormones that require endoproteolytic cleavage to liberate the mature hormones. A structurally conserved but functionally distinct family of nine prohormone convertase enzymes (PCs) are responsible for cleavage of protein precursors of which PC1/3 and PC2 are known to be exclusive to neuroendocrine cells and responsible for prohormone cleavage. Differential expression of PCs within tissues define prohormone processing; whereas glucagon is the major product liberated from proglucagon via PC2 in pancreatic α-cells, proglucagon is preferentially processed by PC1/3 in intestinal L cells to produce glucagon-like peptides 1 and 2 (GLP-1, GLP-2). Beyond our understanding of processing of islet prohormones in healthy islets, there is convincing evidence that proinsulin, proIAPP, and proglucagon processing is altered during prediabetes and diabetes. There is predictive value of elevated circulating proinsulin or proinsulin : C-peptide ratio for progression to type 2 diabetes and elevated proinsulin or proinsulin : C-peptide is predictive for development of type 1 diabetes in at risk groups. After onset of diabetes, patients have elevated circulating proinsulin and proIAPP and proinsulin may be an autoantigen in type 1 diabetes. Further, preclinical studies reveal that α-cells have altered proglucagon processing during diabetes leading to increased GLP-1 production. We conclude that despite strong associative data, current evidence is inconclusive on the potential causal role of impaired prohormone processing in diabetes, and suggest that future work should focus on resolving the question of whether altered prohormone processing is a causal driver or merely a consequence of diabetes pathology.

Employing core regulatory circuits to define cell identity

The interplay between extrinsic signaling and downstream gene networks controls the establishment of cell identity during development and its maintenance in adult life. Advances in next-generation sequencing and single-cell technologies have revealed additional layers of complexity in cell identity. Here, we review our current understanding of transcription factor (TF) networks as key determinants of cell identity. We discuss the concept of the core regulatory circuit as a set of TFs and interacting factors that together define the gene expression profile of the cell. We propose the core regulatory circuit as a comprehensive conceptual framework for defining cellular identity and discuss its connections to cell function in different contexts.

β-Cells are not uniform after all-Novel insights into molecular heterogeneity of insulin-secreting cells

While the β-cells of the endocrine pancreas are defined as cells with high levels of insulin production and tight stimulus-secretion coupling, the existence of functional heterogeneity among them has been known for decades. Recent advances in molecular technologies, in particular single-cell profiling on both the protein and messenger RNA level, have uncovered that β-cells exist in several antigenically and molecularly definable states. Using antibodies to cell surface markers or multidimensional clustering of β-cells using more than 20 protein markers by mass cytometry, 4 distinct groups of β-cells could be differentiated. However, whether these states represent permanent cell lineages or are readily interconvertible from one group to another remains to be determined. Nevertheless, future analysis of the pathogenesis of type 1 and type 2 diabetes will certainly benefit from a growing appreciation of β-cell heterogeneity. Here, we aim to summarize concisely the recent advances in the field and their possible impact on our understanding of β-cell physiology and pathophysiology.

Deletion of histone deacetylase 3 in adult beta cells improves glucose tolerance via increased insulin secretion

Objective

Histone deacetylases are epigenetic regulators known to control gene transcription in various tissues. A member of this family, histone deacetylase 3 (HDAC3), has been shown to regulate metabolic genes. Cell culture studies with HDAC-specific inhibitors and siRNA suggest that HDAC3 plays a role in pancreatic β-cell function, but a recent genetic study in mice has been contradictory. Here we address the functional role of HDAC3 in β-cells of adult mice.

Methods

An HDAC3 β-cell specific knockout was generated in adult MIP-CreERT transgenic mice using the Cre-loxP system. Induction of HDAC3 deletion was initiated at 8 weeks of age with administration of tamoxifen in corn oil (2 mg/day for 5 days). Mice were assayed for glucose tolerance, glucose-stimulated insulin secretion, and islet function 2 weeks after induction of the knockout. Transcriptional functions of HDAC3 were assessed by ChIP-seq as well as RNA-seq comparing control and β-cell knockout islets.

Results

HDAC3 β-cell specific knockout (HDAC3βKO) did not increase total pancreatic insulin content or β-cell mass. However, HDAC3βKO mice demonstrated markedly improved glucose tolerance. This improved glucose metabolism coincided with increased basal and glucose-stimulated insulin secretion in vivo as well as in isolated islets. Cistromic and transcriptomic analyses of pancreatic islets revealed that HDAC3 regulates multiple genes that contribute to glucose-stimulated insulin secretion.

Conclusions

HDAC3 plays an important role in regulating insulin secretion in vivo, and therapeutic intervention may improve glucose homeostasis.

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HDAC3 ablation in adult mouse β-cells improves glucose tolerance.

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Improved glucose tolerance is due to increased insulin secretion.

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HDAC3 targets multiple genes involved in potentiating insulin secretion.

Role of long non-coding RNAs in the determination of β-cell identity

Pancreatic β-cells are highly specialized cells committed to secrete insulin in response to changes in the level of nutrients, hormones and neurotransmitters. Chronic exposure to elevated concentrations of glucose, fatty acids or inflammatory mediators can result in modifications in β-cell gene expression that alter their functional properties. This can lead to the release of insufficient amount of insulin to cover the organism's needs, and thus to the development of diabetes mellitus. Although most of the studies carried out in the last decades to elucidate the causes of β-cell dysfunction under disease conditions have focused on protein-coding genes, we now know that insulin-secreting cells also contain thousands of molecules of RNA that do not encode polypeptides but play key roles in the acquisition and maintenance of a highly differentiated state. In this review, we will highlight the involvement of long non-coding RNAs (lncRNAs), a particular class of non-coding transcripts, in the differentiation of β-cells and in the regulation of their specialized tasks. We will also discuss the crosstalk between the activities of lncRNAs and microRNAs and present the emerging evidence of a potential contribution of particular lncRNAs to the development of both type 1 and type 2 diabetes.

The role of butyrate, a histone deacetylase inhibitor in diabetes mellitus: experimental evidence for therapeutic intervention

The contribution of epigenetic mechanisms in diabetes mellitus (DM), β-cell reprogramming and its complications is an emerging concept. Recent evidence suggests that there is a link between DM and histone deacetylases (HDACs), because HDAC inhibitors promote β-cell differentiation, proliferation, function and improve insulin resistance. Moreover, gut microbes and diet-derived products can alter the host epigenome. Furthermore, butyrate and butyrate-producing microbes are decreased in DM. Butyrate is a short-chain fatty acid produced from the fermentation of dietary fibers by microbiota and has been proven as an HDAC inhibitor. The present review provides a pragmatic interpretation of chromatin-dependent and independent complex signaling/mechanisms of butyrate for the treatment of Type 1 and Type 2 DM, with an emphasis on the promising strategies for its drugability and therapeutic implication.

A cistrome roadmap for understanding pancreatic islet biology

Although dozens of common variants have been associated with increased risk of type 2 diabetes (T2D), the mechanisms by which these variants increase disease susceptibility are largely unknown. A new study mapping the human pancreatic islet cistrome provides a roadmap for exploring the effects of these variants and suggests that altered enhancer function might be a common contributor to the genetic risk of T2D.