Roles of ß-Cell Hypoxia in the Progression of Type 2 Diabetes.

Type 2 diabetes is a chronic disease marked by hyperglycemia; impaired insulin secretion by pancreatic ß-cells is a hallmark of this disease. Recent studies have shown that hypoxia occurs in the ß-cells of patients with type 2 diabetes and hypoxia, in turn, contributes to the insulin secretion defect and ß-cell loss through various mechanisms, including the activation of hypoxia-inducible factors, induction of transcriptional repressors, and activation of AMP-activated protein kinase. This review focuses on advances in our understanding of the contribution of ß-cell hypoxia to the development of ß-cell dysfunction in type 2 diabetes. A better understanding of ß-cell hypoxia might be useful in the development of new strategies for treating type 2 diabetes.

Hypoxia causes pancreatic ß-cell dysfunction and impairs insulin secretion by activating the transcriptional repressor BHLHE40.

Hypoxia can occur in pancreatic ß-cells in type 2 diabetes. Although hypoxia exerts deleterious effects on ß-cell function, the associated mechanisms are largely unknown. Here, we show that the transcriptional repressor basic helix-loop-helix family member e40 (BHLHE40) is highly induced in hypoxic mouse and human ß-cells and suppresses insulin secretion. Conversely, BHLHE40 deficiency in hypoxic MIN6 cells or ß-cells of ob/ob mice reverses defects in insulin secretion. Mechanistically, BHLHE40 represses the expression of Mafa, encoding the transcription factor musculoaponeurotic fibrosarcoma oncogene family A (MAFA), by attenuating the binding of pancreas/duodenum homeobox protein 1 (PDX1) to its enhancer region. Impaired insulin secretion in hypoxic ß-cells was recovered by MAFA re-expression. Collectively, our work identifies BHLHE40 as a key hypoxia-induced transcriptional repressor in ß-cells that inhibit insulin secretion by suppressing MAFA expression.

SIRT7 suppresses energy expenditure and thermogenesis by regulating brown adipose tissue functions in mice.

Brown adipose tissue plays a central role in the regulation of the energy balance by expending energy to produce heat. NAD+-dependent deacylase sirtuins have widely been recognized as positive regulators of brown adipose tissue thermogenesis. However, here we reveal that SIRT7, one of seven mammalian sirtuins, suppresses energy expenditure and thermogenesis by regulating brown adipose tissue functions. Whole-body and brown adipose tissue-specific Sirt7 knockout mice have higher body temperature and energy expenditure. SIRT7 deficiency increases the protein level of UCP1, a key regulator of brown adipose tissue thermogenesis. Mechanistically, we found that SIRT7 deacetylates insulin-like growth factor 2 mRNA-binding protein 2, an RNA-binding protein that inhibits the translation of Ucp1 mRNA, thereby enhancing its inhibitory action on Ucp1. Furthermore, SIRT7 attenuates the expression of batokine genes, such as fibroblast growth factor 21. In conclusion, we propose that SIRT7 serves as an energy-saving factor by suppressing brown adipose tissue functions.

SIRT7 Deficiency Protects against Aging-Associated Glucose Intolerance and Extends Lifespan in Male Mice.

Sirtuins (SIRT1-7 in mammals) are evolutionarily conserved nicotinamide adenine dinucleotide-dependent lysine deacetylases/deacylases that regulate fundamental biological processes including aging. In this study, we reveal that male Sirt7 knockout (KO) mice exhibited an extension of mean and maximum lifespan and a delay in the age-associated mortality rate. In addition, aged male Sirt7 KO mice displayed better glucose tolerance with improved insulin sensitivity compared with wild-type (WT) mice. Fibroblast growth factor 21 (FGF21) enhances insulin sensitivity and extends lifespan when it is overexpressed. Serum levels of FGF21 were markedly decreased with aging in WT mice. In contrast, this decrease was suppressed in Sirt7 KO mice, and the serum FGF21 levels of aged male Sirt7 KO mice were higher than those of WT mice. Activating transcription factor 4 (ATF4) stimulates Fgf21 transcription, and the hepatic levels of Atf4 mRNA were increased in aged male Sirt7 KO mice compared with WT mice. Our findings indicate that the loss of SIRT7 extends lifespan and improves glucose metabolism in male mice. High serum FGF21 levels might be involved in the beneficial effect of SIRT7 deficiency.

Methionine metabolism regulates pluripotent stem cell pluripotency and differentiation through zinc mobilization.

Pluripotent stem cells (PSCs) exhibit a unique feature that requires S-adenosylmethionine (SAM) for the maintenance of their pluripotency. Methionine deprivation in the medium causes a reduction in intracellular SAM, thus rendering PSCs in a state potentiated for differentiation. In this study, we find that methionine deprivation triggers a reduction in intracellular protein-bound Zn content and upregulation of Zn exporter SLC30A1 in PSCs. Culturing PSCs in Zn-deprived medium results in decreased intracellular protein-bound Zn content, reduced cell growth, and potentiated differentiation, which partially mimics methionine deprivation. PSCs cultured under Zn deprivation exhibit an altered methionine metabolism-related metabolite profile. We conclude that methionine deprivation potentiates differentiation partly by lowering cellular Zn content. We establish a protocol to generate functional pancreatic ß cells by applying methionine and Zn deprivation. Our results reveal a link between Zn signaling and methionine metabolism in the regulation of cell fate in PSCs.

SIRT7 regulates lipogenesis in adipocytes through deacetylation of PPARγ2.

AIMS/INTRODUCTION: Peroxisome proliferator-activated receptor (PPAR)-γ2 is a transcription factor crucial for regulating adipogenesis and glucose/lipid metabolism, and synthetic PPARγ ligands, such as thiazolidinediones, are effective oral medication for type 2 diabetes. Sirtuin 7 (SIRT7), a nicotinamide adenine dinucleotide-dependent deacetylase, also controls metabolism. However, it is not known whether SIRT7 regulates the function of PPARγ2 by its deacetylation. MATERIALS AND METHODS: Physical interaction between SIRT7 and PPARγ2, the effect of SIRT7 on PPARγ2 acetylation, and the deacetylation residue targeted by SIRT7 were investigated. The effects of PPARγ2 K382 acetylation on lipid accumulation, gene expression in C3H10T1/2 cell-derived adipocytes, and ligand-dependent transactivation activity were also evaluated. RESULTS: We demonstrated that SIRT7 binds to PPARγ2 and deacetylates PPARγ2 at K382. C3H10T1/2-derived adipocytes expressing PPARγ2K382Q (a mimic of acetylated K) accumulated much less fat than adipocytes expressing wild-type PPARγ2 or PPARγ2K382R (a mimic of nonacetylated K). Global gene expression analysis of adipocytes expressing PPARγ2K382Q revealed that K382Q caused the dysregulation of a set of genes involved in lipogenesis, including Srebp1c, Acaca, Fasn, and Scd1. The rosiglitazone-dependent transcriptional activity of PPARγ2K382Q was reduced compared with that of PPARγ2K382R . CONCLUSION: Our findings indicate that SIRT7-dependent PPARγ2 deacetylation at K382 controls lipogenesis in adipocytes.

HNF1α controls glucagon secretion in pancreatic α-cells through modulation of SGLT1.

Hepatocyte nuclear factor 1α (HNF1α) is a transcription factor required for normal insulin secretion and maintenance of ß-cell number in the pancreas. HNF1α is also expressed in pancreatic α-cells, but its role in these cells is unknown. The aim of this study was to clarify the role of HNF1α in α-cells. Male Hnf1a+/- mice with a mixed background were backcrossed to outbred ICR mice. Glucose tolerance, glucagon and insulin secretion, islet histology, and gene expression were investigated in ICR Hnf1a-/- and Hnf1a+/+ mice. Regulation of Slc5a1 (encoding sodium glucose cotransporter 1 [SGLT1]) expression by HNF1α and the effect of SGLT1 inhibition on glucagon secretion were also explored. ICR Hnf1a-/- mice were glucose intolerant and exhibited impaired glucose-stimulated insulin secretion. The ß-cell area of ICR mice was decreased in Hnf1a-/- mice, but the α-cell area in the pancreas was similar between Hnf1a-/- and Hnf1a+/+ mice. Hnf1a-/- mice showed higher fasting glucagon levels and exhibited inadequate suppression of glucagon after glucose load. In addition, glucagon release in response to hypoglycemia was impaired in Hnf1a-/- mice, and glucagon secretion after 1.1 mM glucose administration, was also decreased in Hnf1a-/- islets. Slc5a1 expression was decreased in Hnf1a-/- islets, while HNF1α activated the Slc5a1 promoter in αTC1-6 cells. Inhibition of SGLT1 suppressed 1.1 mM glucose-stimulated glucagon secretion in islets and αTC1-6 cells, but SGLT1 inhibition had no additional inhibitory effect in HNF1α-deficient cells. Our findings indicate that HNF1α modulates glucagon secretion in α-cells through the regulation of Slc5a1.

Brown adipocyte-derived exosomal miR-132-3p suppress hepatic Srebf1 expression and thereby attenuate expression of lipogenic genes.

Recent evidence has revealed a novel signaling mechanism through which brown adipose tissue (BAT)-derived exosomal microRNAs (miRNAs) influence hepatic gene expression. Here, we uncover neuronal control of these miRNAs and identify exosomal miR-132-3p as a regulator of hepatic lipogenesis under cold stress conditions. Norepinephrine, a sympathetic nervous system neurotransmitter mediating cold-induced BAT activation, altered the composition of brown adipocyte (BAC)-derived exosomal miRNAs; among them, miR-132-3p was significantly induced. The isolated BAC-derived exosomes suppressed expression of hepatic Srebf1, a predicted target of miR-132-3p. In an indirect co-culture system, BACs suppressed expression of hepatic Srebf1 and its target lipogenic genes; this effect was not seen with miR-132-3p-inhibited BACs. Srebf1 was experimentally validated as an miR-132-3p target. Cold stimuli consistently induced miR-132-3p expression in BAT and attenuated Srebf1 expression in the liver. Our results suggest that BAT-derived exosomal miR-132-3p acts as an endocrine factor that regulates hepatic lipogenesis for cold adaptation.

Hypoxia reduces HNF4α/MODY1 protein expression in pancreatic ß-cells by activating AMP-activated protein kinase.

Hypoxia plays a role in the deterioration of ß-cell function. Hepatocyte nuclear factor 4α (HNF4α) has an important role in pancreatic ß-cells, and mutations of the human HNF4A gene cause a type of maturity-onset diabetes of the young (MODY1). However, it remains unclear whether hypoxia affects the expression of HNF4α in ß-cells. Here, we report that hypoxia reduces HNF4α protein expression in ß-cells. Hypoxia-inducible factor was not involved in the down-regulation of HNF4α under hypoxic conditions. The down-regulation of HNF4α was dependent on the activation of AMP-activated protein kinase (AMPK), and the reduction of HNF4α protein expression by metformin, an AMPK activator, and hypoxia was inhibited by the overexpression of a kinase-dead (KD) form of AMPKα2. In addition, hypoxia decreased the stability of the HNF4α protein, and the down-regulation of HNF4α was sensitive to proteasome inhibitors. Adenovirus-mediated overexpression of KD-AMPKα2 improved insulin secretion in metformin-treated islets, hypoxic islets, and ob/ob mouse islets. These results suggest that down-regulation of HNF4α could be of importance in ß-cell dysfunction by hypoxia.

Definitive Endoderm Differentiation of Human Embryonic Stem Cells Combined with Selective Elimination of Undifferentiated Cells by Methionine Deprivation.

Human embryonic stem cells (ESCs) show a characteristic feature in that they are highly dependent on methionine metabolism. Undifferentiated human ESCs cannot survive under condition that methionine is deprived from culture medium. We describe here a procedure for definitive endoderm differentiation from human ESCs, in which human ESCs are subject to 10 days' (d) differentiation combined with methionine deprivation between differentiation days (d) 8 to (d) 10. Methionine deprivation results in elimination of undifferentiated cells from the culture with no significant loss of definitive endoderm cells, as compared to those cultured under complete condition throughout the whole culture period.

Definitive Endoderm Differentiation of Human Embryonic Stem Cells Combined with Selective Elimination of Undifferentiated Cells by Methionine Deprivation.

Human embryonic stem cells (ESCs) show a characteristic feature in that they are highly dependent on methionine metabolism. Undifferentiated human ESCs cannot survive under the condition that methionine is deprived from culture medium. We describe here a procedure for definitive endoderm differentiation from human ESCs, in which human ESCs are subject to 10 days (d) differentiation combined with methionine deprivation between differentiation day (d) 8 to d10. Methionine deprivation results in elimination of undifferentiated cells from the culture with no significant loss of definitive endoderm cells, as compared to those cultured under complete condition throughout the whole culture period.

Beneficial effect of insulin treatment on islet transplantation outcomes in Akita mice.

Islet transplantation is a promising potential therapy for patients with type 1 diabetes. The outcome of islet transplantation depends on the transplantation of a sufficient amount of ß-cell mass. However, the initial loss of islets after transplantation is problematic. We hypothesized the hyperglycemic status of the recipient may negatively affect graft survival. Therefore, in the present study, we evaluated the effect of insulin treatment on islet transplantation involving a suboptimal amount of islets in Akita mice, which is a diabetes model mouse with an Insulin 2 gene missense mutation. Fifty islets were transplanted under the left kidney capsule of the recipient mouse with or without insulin treatment. For insulin treatment, sustained-release insulin implants were implanted subcutaneously into recipient mice 2 weeks before transplantation and maintained for 4 weeks. Islet transplantation without insulin treatment did not reverse hyperglycemia. In contrast, the group that received transplants in combination with insulin treatment exhibited improved fasting blood glucose levels until 18 weeks after transplantation, even after insulin treatment was discontinued. The group that underwent islet transplantation in combination with insulin treatment had better glucose tolerance than the group that did not undergo insulin treatment. Insulin treatment improved graft survival from the acute phase (i.e., 1 day after transplantation) to the chronic phase (i.e., 18 weeks after transplantation). Islet apoptosis increased with increasing glucose concentration in the medium or blood in both the in vitro culture and in vivo transplantation experiments. Expression profile analysis of grafts indicated that genes related to immune response, chemotaxis, and inflammatory response were specifically upregulated when islets were transplanted into mice with hyperglycemia compared to those with normoglycemia. Thus, the results demonstrate that insulin treatment protects islets from the initial rapid loss that is usually observed after transplantation and positively affects the outcome of islet transplantation in Akita mice.

Methionine metabolism regulates maintenance and differentiation of human pluripotent stem cells.

Mouse embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) are in a high-flux metabolic state, with a high dependence on threonine catabolism. However, little is known regarding amino acid metabolism in human ESCs/iPSCs. We show that human ESCs/iPSCs require high amounts of methionine (Met) and express high levels of enzymes involved in Met metabolism. Met deprivation results in a rapid decrease in intracellular S-adenosylmethionine (SAM), triggering the activation of p53-p38 signaling, reducing NANOG expression, and poising human iPSC/ESCs for differentiation, follow by potentiated differentiation into all three germ layers. However, when exposed to prolonged Met deprivation, the cells undergo apoptosis. We also show that human ESCs/iPSCs have regulatory systems to maintain constant intracellular Met and SAM levels. Our findings show that SAM is a key regulator for maintaining undifferentiated pluripotent stem cells and regulating their differentiation.