Understanding of basic mechanisms of β-cell function and survival

Type 1 and type 2 diabetes are both diseases of insulin insufficiency, although they develop by distinct pathways. The recent surge in the incidence of type 2 diabetes and the chronic ailments confronted by patients with either form of the disease highlight the need for better understanding of β-cell biology. In this review, we present recent work focused on this goal. Our hope is that basic research being conducted in this and other laboratories will ultimately contribute to the development of methods for enhancing β-cell function and survival in the context of both major forms of diabetes. Our strategy for understanding the β-cell involves a multidisciplinary approach in which tools from the traditional fields of biochemistry, enzymology, and physiology are teamed with newer technologies from the fields of molecular biology, gene discovery, cell and developmental biology, and biophysical chemistry. We have focused on two important aspects of β-cell biology in our studies: β-cell function, specifically the metabolic regulatory mechanisms involved in glucose-stimulated insulin secretion, and β-cell resistance to immune attack, with emphasis on resistance to inflammatory cytokines and reactive oxygen species.     

Eicosanoids, β-cell function, and diabetes

Arachidonic acid (AA) is metabolized by cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P450 (CYP) enzymes into eicosanoids, which are involved in diverse diseases, including type 1 and type 2 diabetes. During the last 30 years, evidence has been accumulated that suggests important functions for eicosanoids in the control of pancreatic β-cell function and destruction. AA metabolites of the COX pathway, especially prostaglandin E(2) (PGE(2)), appear to be significant factors to β-cell dysfunction and destruction, participating in the pathogenesis of diabetes and its complications. Several elegant studies have contributed to the sorting out of the importance of 12-LOX eicosanoids in cytokine-mediated inflammation in pancreatic β cells. The role of CYP eicosanoids in diabetes is yet to be explored. A recent publication has demonstrated that stabilizing the levels of epoxyeicosatrienoic acids (EETs), CYP eicosanoids, by inhibiting or deleting soluble epoxide hydrolase (sEH) improves β-cell function and reduces β-cell apoptosis in diabetes. In this review we summarize recent findings implicating these eicosanoid pathways in diabetes and its complications. We also discuss the development of animal models with targeted gene deletion and specific enzymatic inhibitors in each pathway to identify potential targets for the treatment of diabetes and its complications.     

Cholesterol metabolism,pancreatic β-cell function and diabetes

Cholesterol plays an essential role in determining cell membrane physico-chemical characteristics and functions. A proper membrane structure is critical in pancreatic β-cells for glucose-mediated insulin secretion, and alterations in cellular cholesterol content may negatively affect this process, leading to β-cell dysfunction. The low density lipoprotein receptor (LDL-R) appears to play a relevant role in ß-cell dysfunction due to cholesterol accumulation. This observation raised the question of whether hypocholesterolemic drugs which increase LDL-R expression might bear diabetogenic properties, thus increasing the risk of new-onset diabetes or worsen glycaemic parameters in diabetic patients.Being at higher cardiovascular risk, diabetic patients are usually treated with hypolipidemic drugs to correct the atherogenic dyslipidemia characteristic of this pathological condition. Statin therapy has been associated with an increased incidence of new-onset diabetes (NOD), being the diabetogenic effect depending on the type and dose of statin. However, it is worth noting that the benefits on cardiovascular mortality largely exceed the increased risk associated with the development of diabetes. Although genetic variants associated with lower levels of LDL-C are also associated with an increased NOD risk, clinical trials with lipid-lowering drugs other than statins, namely ezetimibe or monoclonal antibodies against PCSK9, did not observe an increase of developing diabetes.In summary, molecular evidence clearly points to a key role for cholesterol homeostasis in pancreatic β-cell function which, in humans, is negatively affected by statins. Available data exclude that this could be the case for other hypocholesterolemic approaches, but long-term studies are warranted to explore this critical aspect.     

Altered insulin receptor signalling and β-cell cycle dynamics in type 2 diabetes mellitus

Insulin resistance, reduced β-cell mass, and hyperglucagonemia are consistent features in type 2 diabetes mellitus (T2DM). We used pancreas and islets from humans with T2DM to examine the regulation of insulin signaling and cell-cycle control of islet cells. We observed reduced β-cell mass and increased α-cell mass in the Type 2 diabetic pancreas. Confocal microscopy, real-time PCR and western blotting analyses revealed increased expression of PCNA and down-regulation of p27-Kip1 and altered expression of insulin receptors, insulin receptor substrate-2 and phosphorylated BAD. To investigate the mechanisms underlying these findings, we examined a mouse model of insulin resistance in β-cells--which also exhibits reduced β-cell mass, the β-cell-specific insulin receptor knockout (βIRKO). Freshly isolated islets and β-cell lines derived from βIRKO mice exhibited poor cell-cycle progression, nuclear restriction of FoxO1 and reduced expression of cell-cycle proteins favoring growth arrest. Re-expression of insulin receptors in βIRKO β-cells reversed the defects and promoted cell cycle progression and proliferation implying a role for insulin-signaling in β-cell growth. These data provide evidence that human β- and α-cells can enter the cell-cycle, but proliferation of β-cells in T2DM fails due to G1-to-S phase arrest secondary to defective insulin signaling. Activation of insulin signaling, FoxO1 and proteins in β-cell-cycle progression are attractive therapeutic targets to enhance β-cell regeneration in the treatment of T2DM.     

The role of autophagy in pancreatic β-cell and diabetes

Pancreatic β-cells play a key role in glucose homeostasis in mammals. Although large-scale protein synthesis and degradation occur in pancreatic β-cells, the mechanism underlying dynamic protein turnover in β-cells remains largely unknown. We found low-level constitutive autophagy in β-cells of C57BL/6 mice fed a standard diet; however, autophagy was markedly upregulated in mice fed a high-fat diet. β-cells of diabetic db/db mice contained large numbers of autophagosomes, compared with non-diabetic db/misty controls. The functional importance of autophagy was analyzed using β-cell-specific Atg7 knockout mice. Autophagy-deficient mice showed degeneration of β-cells and impaired glucose tolerance with reduced insulin secretion. While a high-fat diet stimulated β-cell autophagy in control mice, it induced a profound deterioration of glucose intolerance in β-cell autophagy-deficient mutants, partly because of the lack of a compensatory increase in β-cell mass. These results suggest that the degradation of unnecessary cellular components by autophagy is essential for maintenance of the architecture and function of β-cells. Autophagy also serves as a crucial element of stress responses to protect β-cells under insulin resistant states. Impairment of autophagic machinery could thus predispose individuals to type 2 diabetes.     

Adipose tissue derived-factors impaired pancreatic β-cell function in diabetes

Inflammatory factors produced and secreted by adipose tissue, in particular peri-pancreatic adipose tissue (P-WAT), may influence pancreatic β-cell dysfunction. Using the ZDF Rat model of diabetes, we show the presence of infiltrating macrophage (ED1 staining) on pancreatic tissue and P-WAT in the pre-diabetes stage of the disease. Then, when the T2D is installed, infiltrating cells decreased. Meanwhile, the P-WAT conditioned-medium composition, in terms of inflammatory factors, varies during the onset of the T2D. Using chemiarray technology, we observed an over expression of CXCL-1, -2, -3, CCL-3/MIP-1α and CXCL-5/LIX and TIMP-1 in the 9?weeks old obese ZDF pre-diabetic rat model. Surprisingly, the expression profile of these factors decreased when animals become diabetic (12?weeks obese ZDF rats). The expression of these inflammatory proteins is highly associated with inflammatory infiltrate. P-WAT conditioned-medium from pre-diabetes rats stimulates insulin secretion, cellular proliferation and apoptosis of INS-1 cells. However, inhibition of conditioned-medium chemokines acting via CXCR2 receptor do not change cellular proliferation apoptosis and insulin secretion of INS-1 cells induced by P-WAT conditioned-medium. Taken together, these results show that among the secreted chemokines, increased expression of CXCL-1, -2, -3 and CXCL-5/LIX in P-WAT conditioned-medium is concomitant with the onset of the T2D but do not exerted a direct effect on pancreatic β-cell dysfunction.     

Chronic alcohol consumption potentiates the development of diabetes through pancreatic β-cell dysfunction

Chronic ethanol consumption is well established as a major risk factor for type-2 diabetes(T2D), which is evidenced by impaired glucose metabolism and insulin resistance. However, the relationships between alcoholconsumption and the development of T2 D remain controversial. In particular, the direct effects of ethanol consumption on proliferation of pancreatic β-cell and the exact mechanisms associated with ethanolmediated β-cell dysfunction and apoptosis remain elusive. Although alcoholism and alcohol consumption are prevalent and represent crucial public health problems worldwide, many people believe that low-tomoderate ethanol consumption may protect against T2 D and cardiovascular diseases. However, the J- or U-shaped curves obtained from cross-sectional and large prospective studies have not fully explained the relationship between alcohol consumption and T2 D. This review provides evidence for the harmful effects of chronic ethanol consumption on the progressive development of T2 D, particularly with respect to pancreatic β-cell mass and function in association with insulin synthesis and secretion. This review also discusses a conceptual framework for how ethanolproduced peroxynitrite contributes to pancreatic β-cell dysfunction and metabolic syndrome.     

Progressive histopathological changes and β-cell loss in the pancreas of a new spontaneous rat model of type 2 diabetes

The eSMT rat is a new spontaneous model of type 2 diabetes that develops a progressive diabetic syndrome with a stronger incidence in males than in females. We decide to investigate the progression of the pancreatic histopathological changes during the lifespan of the eSMT rat, especially those associated with islet cell populations. Besides that, some plasmatic parameters were evaluated in order to correlate them with the morphological findings. Male eSMT and Sprague-Dawley control rats were used.The results showed a dramatic decrease of the volume density (VD) of endocrine tissue in the eSMT rats without evidence of insulitis. Islets became fragmented structures with strong presence of interstitial fibrosis. Consequently, plasma insulin levels showed a significant decrease, while plasma glucose, cholesterol and triglyceride levels were increased. Normal rats showed no significant changes in the VD of endocrine tissue, except for the older animals, where the VD of β-cell population was increased.Early derangements observed in islets, together with the progressive decrease of endocrine tissue and the metabolic disorders described, would be responsible for an irreversible pathologic condition which avoids the animal survival beyond about 18 months of age.However, there is still a need to investigate the causes of endocrine tissue decrease and its possible association with an inflammatory process that it could be associated with the development and progression of fibrosis.     

Bcl-2 proteins in diabetes: mitochondrial pathways of β-cell death and dysfunction

Diabetes is a metabolic disease affecting nearly 300 million individuals worldwide. Both types of diabetes (1 and 2) are characterized by loss of functional pancreatic β-cell mass causing different degrees of insulin deficiency. The Bcl-2 family has a double-edged effect in diabetes. These proteins are crucial controllers of the mitochondrial pathway of β-cell apoptosis induced by pro-inflammatory cytokines or lipotoxicity. In parallel, some Bcl-2 members also regulate glucose metabolism and β-cell function. In this review, we describe the role of Bcl-2 proteins in β-cell homeostasis and death. We focus on how these proteins interact, their contribution to the crosstalk between endoplasmic reticulum stress and mitochondrial permeabilization, their context-dependent usage following different pro-apoptotic stimuli, and their role in β-cell physiology.     

log(TG)/HDL-C is related to both residual cardiometabolic risk and β-cell function loss in type 2 diabetes males

Background

To study the relationship between the intima-media thickness (IMT) of the carotid artery and the stage of chronic kidney disease (CKD) based on the estimated glomerular filtration rate (eGFR) and diabetic nephropathy graded by the urinary albumin excretion (UAE) in the patients with type 2 diabetes mellitus.

Methods

A cross-sectional study was performed in 338 patients with type 2 diabetes mellitus. The carotid IMT was measured using an ultrasonographic examination.

Results

The mean carotid IMT was 1.06 ± 0.27 mm, and 42% of the subjects showed IMT thickening (≥ 1.1 mm). Cerebrovascular disease and coronary heart disease were frequent in the patients with IMT thickening. The carotid IMT elevated significantly with the stage progression of CKD (0.87 ± 0.19 mm in stage 1, 1.02 ± 0.26 mm in stage 2, 1.11 ± 0.26 mm in stage 3, and 1.11 ± 0.27 mm in stage 4+5). However, the IMT was not significantly different among the various stages of diabetic nephropathy. The IMT was significantly greater in the diabetic patients with hypertension compared to those without hypertension. The IMT positively correlated with the age, the duration of diabetes mellitus, and the brachial-ankle pulse wave velocities (baPWV), and negatively correlated with the eGFR. In a stepwise multivariate regression analysis, the eGFR and the baPWV were independently associated with the carotid IMT.

Conclusions

Our study is the first report showing a relationship between the carotid IMT and the renal parameters including eGFR and the stages of diabetic nephropathy with a confirmed association between the IMT and diabetic macroangiopathy. Our study further confirms the importance of intensive examinations for the early detection of atherosclerosis and positive treatments for hypertension, dyslipidaemia, obesity, as well as hyperglycaemia are necessary when a reduced eGFR is found in diabetic patients.     

β-Casomorphin-7 attenuates the development of nephropathy in type I diabetes via inhibition of epithelial-mesenchymal transition of renal tubular epithelial cells

This study was designed to investigate the putative protective effect of β-casomorphin-7 on diabetic nephropathy in a rat model, and to explore the possible mechanism of this effect. SD rats were randomly divided into the following three groups: control group, diabetes group and β-casomorphin-7-treatment group. All rats were euthanized after 30 days with or without β-casomorphin-7 treatment. Biochemical parameters including blood glucose and renal function were quantified. The concentration of plasma TGF-β1 was measured by ELISA. Histopathological changes to the kidney were studied by Masson and Sirius red staining. Expressions of α-smooth muscle actin (α-SMA), E-cadherin, vimentin, cytokeratin19 and TGF-β1 mRNA in rat renal cortices were analyzed by real-time PCR. Changes in α-SMA and E-cadherin protein expression in rat renal cortices were quantified by Western blot. β-Casomorphin-7 treatment of diabetic rats reduced urinary glucose, urinary protein, serum creatinine, blood urinary nitrogen, plasma TGF-β1 and the ratio of kidney: body weight. Masson and Sirius red staining showed that β-casomorphin-7 treatment attenuated renal interstitial fibrosis in diabetic rats. Compared to the control rats, diabetic rats had elevated expressions of α-SMA, vimentin and TGF-β1 mRNA and α -SMA protein and decreased expression of E-cadherin and cytokeratin19 mRNA, and E-cadherin protein. β-Casomorphin-7 treatment of diabetic rats partially normalized these changes. Our results suggest that administration of β-casomorphin-7 attenuates renal interstitial fibrosis caused by diabetes. This protective effect may be associated, in part, with down regulation of epithelial-mesenchymal transition of renal tubular epithelial cells.     

The role of TRPM2 in pancreatic β-cells and the development of diabetes

TRPM2 is a Ca²⁺-permeable non-selective cation channel that can be activated by adenosine dinucleotides, hydrogen peroxide, or intracellular Ca²⁺. The protein is expressed in a wide variety of cells, including neurons in the brain, immune cells, endocrine cells, and endothelial cells. This channel is also well expressed in β-cells in the pancreas. Insulin secretion from pancreatic β-cells is the primary mechanism by which the concentration of blood glucose is reduced. Thus, impairment of insulin secretion leads to hyperglycemia and eventually causes diabetes. Glucose is the principal stimulator of insulin secretion. The primary pathway involved in glucose-stimulated insulin secretion is the ATP-sensitive K⁺ (K_ATP) channel to voltage-gated Ca²⁺ channel (VGCC)-mediated pathway. Increases in the intracellular Ca²⁺ concentration are necessary for insulin secretion, but VGCC is not sufficient to explain [Ca²⁺]_i increases in pancreatic β-cells and the resultant secretion of insulin. In this review, we focus on TRPM2 as a candidate for a [Ca²⁺]_i modulator in pancreatic β-cells and its involvement in insulin secretion and development of diabetes. Although further analyses are needed to clarify the mechanism underlying TRPM2-mediated insulin secretion, TRPM2 could be a key player in the regulation of insulin secretion and could represent a new target for diabetes therapy.     

Antihyperglycemic effect of ginsenoside Rh2 by inducing islet β-cell regeneration in mice

The present study was designed to determine the antihyperglycemic function of ginsenoside Rh2 (GS-Rh2) by the regeneration of β-cells in mice that underwent 70% partial pancreatectomy (PPx), and to explore the mechanisms of GS-Rh2-induced β-cell proliferation. Adult C57BL/6J mice were subjected to PPx or a sham operation. Within 14 days post-PPx, mice that underwent PPx received GS-Rh2 (1?mg/kg body weight) or saline injection. GS-Rh2-treated mice exhibited an improved glycemia and glucose tolerance, an increased serum insulin levels, and β-cell hyperplasia. Meanwhile, increased β-cell proliferation percentages and decreased β-cell apoptosis percentages were also observed in GS-Rh2-treated mice. Further studies on the Akt/Foxo1/PDX-1 signaling pathway revealed that GS-Rh2 probably induced β-cell proliferation via activation of Akt and PDX-1 and inactivation of Foxo1. Studies on the abundance and activity of cell cycle proteins suggested that GS-Rh2-induced β-cell proliferation may ultimately be achieved through the regulation of cell cycle proteins. These findings demonstrate that GS-Rh2 administration could inhibit the tendency of apoptosis, and reverse the impaired β-cell growth potential by modulating Akt/Foxo1/PDX-1 signaling pathway and regulating cell cycle proteins. Induction of islet β-cell proliferation by GS-Rh2 suggests its therapeutic potential in the treatment of diabetes.     

Epigenetics: The missing link to understanding β-cell dysfunction in the pathogenesis of type 2 diabetes

Type 2 diabetes (T2D) is a growing health problem worldwide. While peripheral insulin resistance is common during obesity and aging in both animals and people, progression to T2D is largely due to insulin secretory dysfunction and significant apoptosis of functional β-cells, leading to an inability to compensate for insulin resistance. It is recognized that environmental factors and nutrition play an important role in the pathogenesis of diabetes. However, our knowledge surrounding molecular mechanisms by which these factors trigger β-cell dysfunction and diabetes is still limited. Recent discoveries raise the possibility that epigenetic changes in response to environmental stimuli may play an important role in the development of diabetes. In this paper, we review emerging knowledge regarding epigenetic mechanisms that may be involved in β-cell dysfunction and pathogenesis of diabetes, including the role of nutrition, oxidative stress and inflammation. We will mainly focus on the role of DNA methylation and histone modifications but will also briefly review data on miRNA effects on the pancreatic islets. Further studies aimed at better understanding how epigenetic regulation of gene expression controls β-cell function may reveal potential therapeutic targets for prevention and treatment of diabetes.     

Regulation of pancreatic β-cell function and mass dynamics by prostaglandin signaling

Prostaglandins (PGs) are signaling lipids derived from arachidonic acid (AA), which is metabolized by cyclooxygenase (COX)-1 or 2 and class-specific synthases to generate PGD₂, PGE₂, PGF_2α, PGI₂ (prostacyclin), and thromboxane A₂. PGs signal through G-protein coupled receptors (GPCRs) and are important modulators of an array of physiological functions, including systemic inflammation and insulin secretion from pancreatic islets. The role of PGs in β-cell function has been an active area of interest, beginning in the 1970s. Early studies demonstrated that PGE₂ inhibits glucose-stimulated insulin secretion (GSIS), although more recent studies have questioned this inhibitory action of PGE₂. The PGE₂ receptor EP3 and one of the G-proteins that couples to EP3, Gα_Z, have been identified as negative regulators of β-cell proliferation and survival. Conversely, PGI₂ and its receptor, IP, play a positive role in the β-cell by enhancing GSIS and preserving β-cell mass in response to the β-cell toxin streptozotocin (STZ). In comparison to PGE₂ and PGI₂, little is known about the function of the remaining PGs within islets. In this review, we discuss the roles of PGs, particularly PGE₂ and PGI₂, PG receptors, and downstream signaling events that alter β-cell function and regulation of β-cell mass.     

Epigenetics: The missing link to understanding β-cell dysfunction in the pathogenesis of type 2 diabetes

Type 2 diabetes (T2D) is a growing health problem worldwide. While peripheral insulin resistance is common during obesity and aging in both animals and people, progression to T2D is largely due to insulin secretory dysfunction and significant apoptosis of functional β-cells, leading to an inability to compensate for insulin resistance. It is recognized that environmental factors and nutrition play an important role in the pathogenesis of diabetes. However, our knowledge surrounding molecular mechanisms by which these factors trigger β-cell dysfunction and diabetes is still limited. Recent discoveries raise the possibility that epigenetic changes in response to environmental stimuli may play an important role in the development of diabetes. In this paper, we review emerging knowledge regarding epigenetic mechanisms that may be involved in β-cell dysfunction and pathogenesis of diabetes, including the role of nutrition, oxidative stress and inflammation. We will mainly focus on the role of DNA methylation and histone modifications but will also briefly review data on miRNA effects on the pancreatic islets. Further studies aimed at better understanding how epigenetic regulation of gene expression controls β-cell function may reveal potential therapeutic targets for prevention and treatment of diabetes.     

Experimental diabetes treated with trigonelline: effect on key enzymes related to diabetes and hypertension, β-cell and liver function

Type 2 diabetes is quite diverse, including the improvement of insulin sensitivity by dipeptidylpeptidase-4 (DPP-4) inhibitor, α-glucosidase inhibitors, and the protection of β-cells islet. The aim of this study was to search the effect of trigonelline (Trig) on DPP-4, α-glucosidase and angiotensin converting enzyme (ACE) activities as well as β-cells architecture, and starch and glucose tolerance test. In surviving diabetic rats, the supplement of Trig potentially inhibited DPP-4 and α-glucosidase activities in both plasma and small intestine. The pancreas islet and less β-cells damage were observed after the administration of trig to diabetic rats. The increase of GLP-1 in surviving diabetic rats suppressed the increase of blood glucose level and improved results in the oral glucose and starch tolerance test. Trig also normalized key enzyme related to hypertension as ACE and improved the hemoglobin A1c and lipid profiles (plasma triglyceride, HDL-cholesterol, LDL-cholesterol, and total cholesterol), and liver indices toxicity. Therefore, these results revealed that Trig was successful in improving glycemic control, metabolic parameters, and liver function in diabetic rats. It is therefore suggested that Trig may be a potential agent for the treatment of type 2 diabetes.