The many lives of Myc in the pancreatic ß-cell.

Diabetes results from insufficient numbers of functional pancreatic ß-cells. Thus, increasing the number of available functional ß-cells ex vivo for transplantation, or regenerating them in situ in diabetic patients, is a major focus of diabetes research. The transcription factor, Myc, discovered decades ago lies at the nexus of most, if not all, known proliferative pathways. Based on this, many studies in the 1990s and early 2000s explored the potential of harnessing Myc expression to expand ß-cells for diabetes treatment. Nearly all these studies in ß-cells used pathophysiological or supraphysiological levels of Myc and reported enhanced ß-cell death, dedifferentiation, or the formation of insulinomas if cooverexpressed with Bcl-xL, an inhibitor of apoptosis. This obviously reduced the enthusiasm for Myc as a therapeutic target for ß-cell regeneration. However, recent studies indicate that "gentle" induction of Myc expression enhances ß-cell replication without induction of cell death or loss of insulin secretion, suggesting that appropriate levels of Myc could have therapeutic potential for ß-cell regeneration. Furthermore, although it has been known for decades that Myc is induced by glucose in ß-cells, very little is known about how this essential anabolic transcription factor perceives and responds to nutrients and increased insulin demand in vivo. Here we summarize the previous and recent knowledge of Myc in the ß-cell, its potential for ß-cell regeneration, and its physiological importance for neonatal and adaptive ß-cell expansion.

Transcriptional activation of the Myc gene by glucose in ß-cells requires a ChREBP-dependent 3-D chromatin interaction between the Myc and Pvt1 genes.

OBJECTIVE: All forms of diabetes result from insufficient functional ß-cell mass. Thus, achieving the therapeutic goal of expanding ß-cell mass requires a better mechanistic understanding of how ß-cells proliferate. Glucose is a natural ß-cell mitogen that mediates its effects in part through the glucose-responsive transcription factor, carbohydrate response element binding protein (ChREBP) and the anabolic transcription factor, MYC. However, mechanistic details by which glucose activates Myc at the transcriptional level are poorly understood. METHODS: Here, siRNA was used to test the role of ChREBP in the glucose response of MYC, ChIP and ChIPseq to identify potential regulatory binding sites, chromatin conformation capture to identify DNA/DNA interactions, and an adenovirus was constructed to expresses x-dCas9 and an sgRNA that specifically disrupts the recruitment of ChREBP to a specific targeted ChoRE. RESULTS: We found that ChREBP is essential for glucose-mediated transcriptional induction of Myc, and for increases in Myc mRNA and protein abundance. Further, ChIPseq revealed that the carbohydrate response element (ChoRE) nearest to the Myc transcriptional start site (TSS) is immediately upstream of the gene encoding the lncRNA, Pvt1, 60,000 bp downstream of the Myc gene. Chromatin Conformation Capture (3C) confirmed a glucose-dependent interaction between these two sites. Transduction with an adenovirus expressing x-dCas9 and an sgRNA specifically targeting the highly conserved Pvt1 ChoRE, attenuates ChREBP recruitment, decreases Myc-Pvt1 DNA/DNA interaction, and decreases expression of the Pvt1 and Myc genes in response to glucose. Importantly, isolated and dispersed rat islet cells transduced with the ChoRE-disrupting adenovirus also display specific decreases in ChREBP-dependent, glucose-mediated expression of Pvt1 and Myc, as well as decreased glucose-stimulated ß-cell proliferation. CONCLUSIONS: The mitogenic glucose response of Myc is mediated via glucose-dependent recruitment of ChREBP to the promoter of the Pvt1 gene and subsequent DNA looping with the Myc promoter.

Nrf2 Regulates ß-Cell Mass by Suppressing ß-Cell Death and Promoting ß-Cell Proliferation.

Finding therapies that can protect and expand functional ß-cell mass is a major goal of diabetes research. Here, we generated ß-cell-specific conditional knockout and gain-of-function mouse models and used human islet transplant experiments to examine how manipulating Nrf2 levels affects ß-cell survival, proliferation, and mass. Depletion of Nrf2 in ß-cells results in decreased glucose-stimulated ß-cell proliferation ex vivo and decreased adaptive ß-cell proliferation and ß-cell mass expansion after a high-fat diet in vivo. Nrf2 protects ß-cells from apoptosis after a high-fat diet. Nrf2 loss of function decreases Pdx1 abundance and insulin content. Activating Nrf2 in a ß-cell-specific manner increases ß-cell proliferation and mass and improves glucose tolerance. Human islets transplanted under the kidney capsule of immunocompromised mice and treated systemically with bardoxolone methyl, an Nrf2 activator, display increased ß-cell proliferation. Thus, by managing reactive oxygen species levels, Nrf2 regulates ß-cell mass and is an exciting therapeutic target for expanding and protecting ß-cell mass in diabetes.

Maladaptive positive feedback production of ChREBPß underlies glucotoxic ß-cell failure.

Preservation and expansion of ß-cell mass is a therapeutic goal for diabetes. Here we show that the hyperactive isoform of carbohydrate response-element binding protein (ChREBPß) is a nuclear effector of hyperglycemic stress occurring in ß-cells in response to prolonged glucose exposure, high-fat diet, and diabetes. We show that transient positive feedback induction of ChREBPß is necessary for adaptive ß-cell expansion in response to metabolic challenges. Conversely, chronic excessive ß-cell-specific overexpression of ChREBPß results in loss of ß-cell identity, apoptosis, loss of ß-cell mass, and diabetes. Furthermore, ß-cell "glucolipotoxicity" can be prevented by deletion of ChREBPß. Moreover, ChREBPß-mediated cell death is mitigated by overexpression of the alternate CHREBP gene product, ChREBPα, or by activation of the antioxidant Nrf2 pathway in rodent and human ß-cells. We conclude that ChREBPß, whether adaptive or maladaptive, is an important determinant of ß-cell fate and a potential target for the preservation of ß-cell mass in diabetes.

Nrf2: The Master and Captain of Beta Cell Fate.

Prolonged hyperglycemia is toxic to pancreatic ß cells, generating excessive reactive oxygen species, defective glucose-stimulated insulin secretion, decreased insulin production, and eventually ß cell death and diabetes. Nrf2 is a master regulator of cellular responses to counteract dangerous levels of oxidative stress. Maintenance of ß cell mass depends on Nrf2 to promote the survival, function, and proliferation of ß cells. Indeed, Nrf2 activation decreases inflammation, increases insulin sensitivity, reduces body weight, and preserves ß cell mass. Therefore, numerous pharmacological activators of Nrf2 are being tested in clinical trials for the treatment of diabetes and diabetic complications. Modulating Nrf2 activity in ß cells is a promising therapeutic approach for the treatment of diabetes.

HB-EGF Signaling Is Required for Glucose-Induced Pancreatic ß-Cell Proliferation in Rats.

The molecular mechanisms of ß-cell compensation to metabolic stress are poorly understood. We previously observed that nutrient-induced ß-cell proliferation in rats is dependent on epidermal growth factor receptor (EGFR) signaling. The aim of this study was to determine the role of the EGFR ligand heparin-binding EGF-like growth factor (HB-EGF) in the ß-cell proliferative response to glucose, a ß-cell mitogen and key regulator of ß-cell mass in response to increased insulin demand. We show that exposure of isolated rat and human islets to HB-EGF stimulates ß-cell proliferation. In rat islets, inhibition of EGFR or HB-EGF blocks the proliferative response not only to HB-EGF but also to glucose. Furthermore, knockdown of HB-EGF in rat islets blocks ß-cell proliferation in response to glucose ex vivo and in vivo in transplanted glucose-infused rats. Mechanistically, we demonstrate that HB-EGF mRNA levels are increased in ß-cells in response to glucose in a carbohydrate-response element-binding protein (ChREBP)-dependent manner. In addition, chromatin immunoprecipitation studies identified ChREBP binding sites in proximity to the HB-EGF gene. Finally, inhibition of Src family kinases, known to be involved in HB-EGF processing, abrogated glucose-induced ß-cell proliferation. Our findings identify a novel glucose/HB-EGF/EGFR axis implicated in ß-cell compensation to increased metabolic demand.