Pancreatic β-cell regeneration: From molecular mechanisms to therapy

Pancreatic β cells are a type of cells that are present in the islets of Langerhans. These cells are highly specialized for the secretion of insulin in response to low increasing of blood glucose levels. Hence, pancreatic β cells could contribute to maintaining systemic glucose homeostasis. Increasing evidence has revealed that a variety of internal (ie, genetic and epigenetic factors) and external factors (ie, radical-oxidative stress) are involved in the protection and/or regeneration of pancreatic β cells. The pathways regulating β-cell replication have been intensely investigated. Glucose has an important role in cell cycle entry of quiescent β cells, which exerts its effect via glucose metabolism and unfolded proteins. A variety of growth factors, hormones, and signaling pathways (ie, calcium-calcineurin nuclear factor of activated T cells) are others factors that could affect β-cell replication under different conditions. Therefore, a greater understanding of the underlying pathways involved in the regeneration and protection of pancreatic β cells could lead to finding and developing new therapeutic approaches. Utilization of stem cells and various phytochemical agents have provided new aspects for preventing β-cell degeneration and stimulating the endogenous regeneration of islets. Thus, these therapeutic platforms could be used as potential therapies in the treatment of insulin-dependent diabetes mellitus. Here, we summarized the various mechanisms involved in pancreatic β-cell regeneration. Moreover, we highlighted different therapeutic approaches which could be used for the regeneration of pancreatic β cells.     

Pancreatic β-cell death in response to pro-inflammatory cytokines is distinct from genuine apoptosis

A reduction in functional β-cell mass leads to both major forms of diabetes; pro-inflammatory cytokines, such as interleukin-1beta (IL-1β) and gamma-interferon (γ-IFN), activate signaling pathways that direct pancreatic β-cell death and dysfunction. However, the molecular mechanism of β-cell death in this context is not well understood. In this report, we tested the hypothesis that individual cellular death pathways display characteristic phenotypes that allow them to be distinguished by the precise biochemical and metabolic responses that occur during stimulus-specific initiation. Using 832/13 and INS-1E rat insulinoma cells and isolated rat islets, we provide evidence that apoptosis is unlikely to be the primary pathway underlying β-cell death in response to IL-1β+γ-IFN. This conclusion was reached via the experimental results of several different interdisciplinary strategies, which included: 1) tandem mass spectrometry to delineate the metabolic differences between IL-1β+γ-IFN exposure versus apoptotic induction by camptothecin and 2) pharmacological and molecular interference with either NF-κB activity or apoptosome formation. These approaches provided clear distinctions in cell death pathways initiated by pro-inflammatory cytokines and bona fide inducers of apoptosis. Collectively, the results reported herein demonstrate that pancreatic β-cells undergo apoptosis in response to camptothecin or staurosporine, but not pro-inflammatory cytokines.     

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.     

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.     

Recombinant disintegrin targets α(v) β(3) integrin and leads to mediator production

Integrin αvβ3 is most likely the foremost modulator of angiogenesis among all known integrins. Recombinant disintegrin DisBa-01, originally obtained from snake venom glands, binds to αvβ3, thereby significantly inhibiting adhesion and generating in vivo anti-metastatic ability. However, its function in mediator production is not clear. Here, we observed that the mediators VEGF-A, IL-8, and TGF-β are not produced by human umbilical vein endothelial cells (HUVEC cell line) or monocyte/macrophage cells (SC cell line) when cells adhered to vitronectin. However, when exposed to DisBa-01, HUVECs produced higher levels of TGF-β, and SC cells produced higher levels of VEGF-A. Nonetheless, HUVECs also showed an enhancement of apoptosis after losing adherence when exposed to disintegrin, which is a characteristic of anoikis. We propose that disintegrin DisBa-01 could be used to modulate integrin αvβ3 functions.     

Recombinant disintegrin targets α(v) β(3) integrin and leads to mediator production

Integrin αvβ3 is most likely the foremost modulator of angiogenesis among all known integrins. Recombinant disintegrin DisBa-01, originally obtained from snake venom glands, binds to αvβ3, thereby significantly inhibiting adhesion and generating in vivo anti-metastatic ability. However, its function in mediator production is not clear. Here, we observed that the mediators VEGF-A, IL-8, and TGF-β are not produced by human umbilical vein endothelial cells (HUVEC cell line) or monocyte/macrophage cells (SC cell line) when cells adhered to vitronectin. However, when exposed to DisBa-01, HUVECs produced higher levels of TGF-β, and SC cells produced higher levels of VEGF-A. Nonetheless, HUVECs also showed an enhancement of apoptosis after losing adherence when exposed to disintegrin, which is a characteristic of anoikis. We propose that disintegrin DisBa-01 could be used to modulate integrin αvβ3 functions.     

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.     

Targeting hIAPP fibrillation: A new paradigm to prevent β-cell death?

Loss of pancreatic β-cell mass is deleterious for type 2 diabetes patients since it reduces insulin production, critical for glucose homeostasis. The main research axis developed over the last few years was to generate new pancreatic β-cells or to transplant pancreatic islets as occurring for some specific type 1 diabetes patients. We evaluate here a new paradigm consisting in preservation of β-cells by prevention of human islet amyloid polypeptide (hIAPP) oligomers and fibrils formation leading to pancreatic β-cell death. We review the hIAPP physiology and the pathology that contributes to β-cell destruction, deciphering the various cellular steps that could be involved. Recent progress in understanding other amyloidosis such as Aβ, Tau, α-synuclein or prion, involved in neurodegenerative processes linked with inflammation, has opened new research lines of investigations to preserve neuronal cells. We evaluate and estimate their transposition to the pancreatic β-cells preservation. Among them is the control of reactive oxygen species (ROS) production occurring with inflammation and the possible implication of the mitochondrial translocator protein as a diagnostic and therapeutic target. The present review also focuses on other amyloid forming proteins from molecular to physiological and physiopathological points of view that could help to better decipher hIAPP-induced β-cell death mechanisms and to prevent hIAPP fibril formation.     

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.     

Interaction of β2-glycoprotein I with lipopolysaccharide leads to Toll-like receptor 4 (TLR4)-dependent activation of macrophages

β(2)-Glycoprotein I (β(2)GPI) is an abundant plasma protein that binds to the surface of cells and particles expressing negatively charged lipids, but its physiological role remains unknown. Antibodies to β(2)GPI are found in patients with anti-phospholipid syndrome, a systemic autoimmune disease associated with vascular thrombosis and pregnancy morbidity. Although it has been suggested that anti-β(2)GPI antibodies activate endothelial cells and monocytes by signaling through TLR4, it is unclear how anti-β(2)GPI antibodies and/or β(2)GPI interact with TLR4. A number of mammalian proteins (termed "endogenous Toll-like receptor (TLR) ligands") have been reported to bind to TLR4, but, in most cases, subsequent studies have shown that LPS interaction with these proteins is responsible for TLR activation. We hypothesized that, like other endogenous TLR ligands, β(2)GPI interacts specifically with LPS and that this interaction is responsible for apparent TLR4 activation by β(2)GPI. Here, we show that both LPS and TLR4 are required for β(2)GPI to bind to and activate macrophages. Untreated β(2)GPI stimulated TNF-α production in TLR4-sufficient (but not TLR4-deficient) macrophages. In contrast, neither polymyxin B-treated nor delipidated β(2)GPI stimulated TNF-α production. Furthermore, β(2)GPI bound to LPS in a specific and dose-dependent manner. Finally, untreated β(2)GPI bound to the surface of TLR4-sufficient (but not TLR4-deficient) macrophages. Polymyxin B treatment of β(2)GPI abolished macrophage binding. Our findings suggest a potential new biological activity for β(2)GPI as a protein that interacts specifically with LPS and point to the need to evaluate newly discovered endogenous TLR ligands for potential interactions with LPS.     

On the controversial questions of the taxonomy of Bivalvia (Mollusca): Too many species or too few characters?

Problems emerging in the course of taxonomic studies and species diagnostics of freshwater bivalves are discussed by the example of one of the bivalve groups (the family Unionidae). It is shown that one of the causes of the current, diametrically opposing views on specific and generic systematics of Bivalvia is the fact that researchers revising taxonomic groups ignore complex analysis of several independent characters (conchological, anatomical, biochemical, genetic, etc.).     

The β5-Loop and Lid Domain Contribute to the Substrate Specificity of Pancreatic Lipase-related Protein 2 (PNLIPRP2)

Pancreatic triglyceride lipase (PNLIP) is essential for dietary fat digestion in children and adults, whereas a homolog, pancreatic lipase-related protein 2 (PNLIPRP2), is critical in newborns. The two lipases are structurally similar, yet they have different substrate specificities. PNLIP only cleaves neutral fats. PNLIPRP2 cleaves neutral and polar fats. To test the hypothesis that the differences in activity between PNLIP and PNLIPRP2 are governed by surface loops around the active site, we created multiple chimeras of both lipases by exchanging the surface loops singly or in combination. The chimeras were expressed, purified, and tested for activity against various substrates. The structural determinants of PNLIPRP2 galactolipase activity were contained in the N-terminal domain. Of the surface loops tested, the lid domain and the β5-loop influenced activity against triglycerides and galactolipids. Any chimera on PNLIP with the PNLIPRP2 lid domain or β5-loop had decreased triglyceride lipase activity similar to that of PNLIPRP2. The corresponding chimeras of PNLIPRP2 did not increase activity against neutral lipids. Galactolipase activity was abolished by the PNLIP β5-loop and decreased by the PNLIP lid domain. The source of the β9-loop had minimal effect on activity. We conclude that the lid domain and β5-loop contribute to substrate specificity but do not completely account for the differing activities of PNLIP and PNLIPRP2. Other regions in the N-terminal domain must contribute to the galactolipase activity of PNLIPRP2 through direct interactions with the substrate or by altering the conformation of the residues surrounding the hydrophilic cavity in PNLIPRP2.     

Circulating Reg1α proteins and autoantibodies to Reg1α proteins as biomarkers of β-cell regeneration and damage in type 1 diabetes

Type 1 diabetes is an autoimmune disease where β-cells are in a constant process of death and renewal. Reg genes play a role in β-cells regeneration. Reg proteins may be target of an autoimmune response in type 1 diabetes with consequent production of autoantibodies and failure of regeneration. The objective of this work was to characterize the role of Reg1α proteins and anti-Reg1α antibodies as biomarkers of β-cell regeneration and damage. Serum levels of Reg1α protein were investigated in 87 type 1 diabetic subjects (31 newly diagnosed and 56 long standing), 63 type 2 diabetic subjects, 39 subjects with systemic lupus erythematosus (SLE), a nonpancreatic autoimmune disorder, and 64 healthy subjects. The presence of anti-Reg1α antibodies and correlation with metabolic, immune, and genetic parameters were analyzed in diabetic subjects. Increased levels of Reg1α protein were observed in newly diagnosed (p=0.002), and long standing (p=0.001) type 1 diabetes patients and type 2 diabetic subjects (p<0.001). Anti-Reg1α antibodies were found in 47% of patients with type 1 diabetes. No correlation was found with metabolic, immune, and genetic parameters. Patients with SLE showed no increase in Reg1α protein. We report here for the first time raised serum Reg1α protein in type 1 and type 2 diabetes and anti-Reg1α antibodies in type 1 diabetes. Reg1α levels appear not to be influenced by genetic or metabolic control. These findings allow considering future studies on Reg1α protein and autoantibody as new tools in the evaluation and monitoring of β-cells regeneration and autoimmunity.     

Inositol pyrophosphates and Akt/PKB: Is the pancreatic β-cell the exception to the rule?

The inositol pyrophosphate, diphosphoinositol pentakisphosphate (IP₇), is thought to negatively regulate the critical insulin signaling protein Akt/PKB. Knockdown of the IP₇-generating inositol hexakisphosphate kinase 1 (IP6K1) results in a concomitant increase in signaling through Akt/PKB in most cell types so far examined. Total in vivo knockout of IP6K1 is associated with a phenotype resistant to high-fat diet, due to enhanced Akt/PKB signaling in classic insulin regulated tissues, counteracting insulin resistance. In contrast, we have shown an important positive role for IP6K1 in insulin exocytosis in the pancreatic β-cell. These cells also possess functional insulin receptors and the feedback loop following insulin secretion is a key aspect of their normal function. Thus we examined the effect of silencing IP6K1 on the activation of Akt/PKB in β-cells. Silencing reduced the glucose-stimulated increase in Akt/PKB phosphorylation on T308 and S473. These effects were reproduced with the selective pan-IP6K inhibitor TNP. The likely explanation for IP₇ reduction decreasing rather than increasing Akt/PKB phosphorylation is that IP₇ is responsible for generating the insulin signal, which is the main source of Akt/PKB activation. In agreement, insulin receptor activation was compromised in TNP treated cells. To test whether the mechanism of IP₇ inhibition of Akt/PKB still exists in β-cells, we treated them at basal glucose with an insulin concentration equivalent to that reached during glucose stimulation. TNP potentiated the Akt/PKB phosphorylation of T308 induced by exogenous insulin. Thus, the IP₇ regulation of β-cell Akt/PKB is determined by two opposing forces, direct inhibition of Akt/PKB versus indirect stimulation via secreted insulin. The latter mechanism is dominant, masking the inhibitory effect. Consequently, pharmacological strategies to knock down IP6K activity might not have the same positive output in the β-cell as in other insulin regulated tissues.