Glucolipotoxicity of the pancreatic beta cell

Degradable Nanoparticles Restore Lysosomal pH and Autophagic Flux in Lipotoxic Pancreatic Beta Cells

Chronic exposure to high levels of fatty acids (lipotoxicity) in pancreatic beta cells (β-cells) decreases lysosomal acidity and inhibits autophagic flux. Today, there are a lack of approaches to modify lysosomal acidity to determine whether impaired lysosomal acidification is causally inhibiting autophagic flux and cellular functions. Biodegradable poly(lactic-co-glycolic acid) (PLGA) nanoparticles (NPs) with diameters of ≈100 nm localize to lysosomes and serve as an ideal method to deliver lactic and glycolic acid to lysosomes upon NP polymer degradation. In this study, the ability of PLGA NPs to lower lysosomal pH and restore autophagic flux is investigated in pancreatic insulin secreting (INS1) β-cells. PLGA NPs display a concentration dependent performance with higher concentrations of PLGA NPs, lowering lysosomal pH, as well as restoring autophagic flux and insulin secretion in pancreatic β-cells. These results document that acidifying the lysosome, via an external perturbation, in lipotoxic pancreatic β-cells affords a specific biological outcome of improved cellular degradative activity.

Visualization of Endogenous Mitophagy Complexes In Situ in Human Pancreatic Beta Cells Utilizing Proximity Ligation Assay

Mitophagy is an essential mitochondrial quality control pathway, which is crucial for pancreatic islet beta cell bioenergetics to fuel glucose-stimulated insulin release. Assessment of mitophagy is challenging and often requires genetic reporters or multiple complementary techniques not easily utilized in tissue samples, such as primary human pancreatic islets. Here we demonstrate a robust approach to visualize and quantify formation of key endogenous mitophagy complexes in primary human pancreatic islets. Utilizing the sensitive proximity ligation assay technique to detect interaction of the mitophagy regulators NRDP1 and USP8, we are able to specifically quantify formation of essential mitophagy complexes in situ. By coupling this approach to counterstaining for the transcription factor PDX1, we can quantify mitophagy complexes, and the factors that can impair mitophagy, specifically within beta cells. The methodology we describe overcomes the need for large quantities of cellular extracts required for other protein-protein interaction studies, such as immunoprecipitation (IP) or mass spectrometry, and is ideal for precious human islet samples generally not available in sufficient quantities for these approaches. Further, this methodology obviates the need for flow sorting techniques to purify beta cells from a heterogeneous islet population for downstream protein applications. Thus, we describe a valuable protocol for visualization of mitophagy highly compatible for use in heterogeneous and limited cell populations.

Asprosin impairs insulin secretion in response to glucose and viability through TLR4/JNK-mediated inflammation

Severe inflammation in the islets is observed in obese patients with type 2 diabetes. Inflammation in the islets is caused by obesity-induced serum free fatty acids. Asprosin is a fasting-induced adipokine, which contributes to hepatic glucose production. However, the effects of asprosin on inflammation and cellular dysfunction in pancreatic β-cells remain to be elucidated. Here, we demonstrated that treatment of mouse insulinoma MIN6 cells and human primary islets containing β-cells with palmitate increased asprosin expression and secretion. Treatment of MIN6 cells and human primary islets with palmitate increased phosphorylation of the inflammatory marker nuclear factor-kappa B (NFκB) and the release of pro-inflammatory cytokines including TNF and MCP-1 and decreased glucose-stimulated insulin secretion and cell viability. However, siRNA-mediated suppression of asprosin reversed these changes. Recombinant asprosin treatment of MIN6 cells and human primary islets augmented the inflammation response, cellular dysfunction, and apoptosis in a dose-dependent manner. Asprosin induced toll-like receptor (TLR) 4 expression and JNK phosphorylation. siRNA for TLR4 or JNK mitigated the effects of asprosin on inflammation and cellular dysfunction. These results suggest that palmitate-derived asprosin secretion from β-cells results in their inflammation and dysfunction through a TLR4/JNK-mediated pathway. This report suggests asprosin as a novel therapeutic target for the treatment of type 2 diabetes through preservation of β-cell function.

Associations Between Pancreatic Lipids and β-Cell Function in Black African and White European Men With Type 2 Diabetes

CONTEXT

Intrapancreatic lipid (IPL) has been linked to β-cell dysfunction. Black populations disproportionately develop type 2 diabetes (T2D) and show distinctions in β-cell function compared with white populations.

OBJECTIVE

We quantified IPL in white European (WE) and black West African (BWA) men with early T2D and investigated the relationships between IPL and β-cell insulin secretory function (ISF).

DESIGN, SETTING, AND PARTICIPANTS

We performed a cross-sectional assessment of 18 WE and 19 BWA middle-age men with early T2D as part of the South London Diabetes and Ethnicity Phenotyping study.

MAIN OUTCOME MEASURES

The participants underwent Dixon MRI to determine IPL in the pancreatic head, body, and tail and subcutaneous and visceral adipose tissue volumes. Modeled first- and second-phase ISFs were comprehensively determined using C-peptide measurements during a 3-hour meal tolerance test and a 2-hour hyperglycemic clamp test.

RESULTS

The WE men had greater mean IPL levels compared with BWA men (P = 0.029), mainly owing to greater IPL levels in the pancreatic head (P = 0.009). The mean IPL level was inversely associated with orally stimulated first-phase ISF in WE but not BWA men (WE, r = -0.554, P = 0.026; BWA, r = -0.183, P = 0.468). No association was found with orally stimulated second-phase ISF in either WE or BWA men. No associations were found between the mean IPL level and intravenously stimulated ISF.

CONCLUSIONS

The IPL levels were lower in BWA than WE men with early T2D, and the lack of inverse association with first-phase ISF in BWA men indicates that IPL might be a less important determinant of the development of T2D in BWA than in WE men.

DETERMINING INSULIN DOSE AT THE TIME OF DISCHARGE IN A HIGH-RISK POPULATION: IS THERE ROOM FOR IMPROVEMENT?

OBJECTIVE

To evaluate the adequacy of the insulin dose prescribed at hospital discharge in a high-risk population and assess patient characteristics that influence insulin dose requirement in the immediate postdischarge period.

METHODS

This was a retrospective study conducted at Parkland Health System. We included all patients admitted to a medical floor who received an insulin prescription at discharge and had at least one follow-up visit within 6 months of discharge. All data were extracted by a detailed manual review of each electronic medical record.

RESULTS

At the postdischarge follow-up (N = 797, median 33 days from discharge), 60% of patients required an insulin dose adjustment; 47% of the patients required a dose decrease. Significant predictors of a decrease insulin requirement postdischarge included (multiple regression beta coefficient [95% confidence interval]): newly diagnosed diabetes, -12.7 (-17.7, -7.7); ketosis-prone diabetes, -8.4 (-15, -1.8); glycated hemoglobin A1c (HbA1c), <10% (86 mmol/mol) -7.0 (-11.4, -2.6); discharge insulin total daily dose, -5.3 (-9.3, -1.3); and metformin prescription, -4.9 (-8.4, -1.3).

CONCLUSION

An insulin dose adjustment (most commonly a decrease) was necessary shortly after discharge in more than half of our patients. A better model to estimate insulin dose at discharge is needed along with short-term follow-up after discharge for insulin titration. A pre-emptive insulin dose reduction at discharge should be considered for patients with newly diagnosed diabetes, ketosis-prone diabetes, metformin prescription, and those with HbA1c <10% at presentation.

ABBREVIATIONS

DKA = diabetic ketoacidosis; HbA1c = glycated hemoglobin A1c; KPDM = ketosis-prone diabetes mellitus; TDD = total daily dose.

Pharmacologic or genetic activation of SIRT1 attenuates the fat-induced decrease in beta-cell function in vivo

Background

There is evidence that sirtuin 1 (SIRT1), a key regulator of nutrient metabolism, increases β-cell secretory function. Excess circulating fat, as seen in obesity, has been shown to decrease β-cell function, an effect that may involve decreased SIRT1 activity. Consequently, SIRT1 activation may increase β-cell function in conditions of elevated plasma-free fatty acid levels. Here we attempted to attenuate the lipid-induced decrease in β-cell function in vivo using pharmacological and genetic models of SIRT1 activation.

Methods

Our pharmacologic model involved 48 h intravenous infusion of Wistar rats with either saline or oleate with or without the SIRT1 activator resveratrol. Additionally, we used β-cell-specific SIRT1 overexpressing (BESTO) mice and wild-type littermates infused for 48 h intravenously with either saline or oleate. In both models, the infusion period was followed by assessment of β-cell function using the hyperglycemic clamp method.

Results

Lipid infusion resulted in a significant decrease in β-cell function as expected in both rats (p < 0.05) and mice (p < 0.001). Both models of SIRT1 activation, which did not alter β-cell function in the absence of fat, resulted in partial protection from the fat-induced decrease in β-cell function (NS vs. control).

Conclusion

These results suggest that SIRT1 is a therapeutic target in decreased β-cell function specifically induced by fat.

Lipotoxicity and β Cell Maintenance in Obesity and Type 2 Diabetes

Obesity and diabetes are often associated with lipotoxic conditions in multiple tissues. The insulin-producing β cells are susceptible to elevated lipid levels and the ensuing lipotoxicity. The preservation of β cell mass and function is one of the main goals of diabetes management under these metabolically stressful conditions. However, the adverse effects from the adaptive signaling pathways that β cells use to counteract lipotoxic stress have secondary negative effects in their own right. Antilipotoxic signaling cascades in β cells can contribute to their eventual failure. Such dual roles are seen for many other biological adaptive processes as well.

Diabetic encephalopathy: beneficial effects of supplementation with fatty acids ω3 and nordihydroguaiaretic acid in a spontaneous diabetes rat model

Background

Diabetic encephalopathy is a chronic complications of diabetes mellitus that affects the central nervous system. We evaluated the effect of ω3 and ω6 polyunsaturated fatty acids (PUFAs) supplementation plus the antioxidant agent nordihydroguaiaretic acid (NDGA) on the etiopathology of diabetic encephalopathy in eSS rats, a spontaneous model of type 2 diabetes.

Methods

One hundred twenty spontaneous diabetic eSS male rats and 38 non-diabetic Wistar, used as healthy control, received monthly by intraperitoneal route, ω3 or ω6 PUFA (6.25 mg/kg) alone or plus NDGA (1.19 mg/kg) for 12 months. Diabetic rats had a worse performance in behavioural Hole-Board test. Histopathological analysis confirmed lesions in diabetic rats brain tissues. We also detected low expression of synaptophysin, a protein linked to release of neurotransmitters, by immunohistochemically techniques in eSS rats brain. Biochemical and histopathological studies of brain were performed at 12th month. Biochemical analysis showed altered parameters related to metabolism. High levels of markers of oxidative stress and inflammation were detected in plasma and brain tissues. Data were analysed by ANOVA test and paired t test was used by comparison of measurements of the same parameter at different times.

Results

The data obtained in this work showed that behavioural, biochemical and morphological alterations observed in eSS rats are compatible with previously reported indices in diabetic encephalopathy and are associated with increased glucolipotoxicity, chronic low-grade inflammation and oxidative stress burden. Experimental treatments assayed modulated the values of studied parameters.

Conclusions

The treatments tested with ω3 or ω3 plus NDGA showed improvement in the values of the studied parameters in eSS diabetic rats. These observations may form the basis to help in prevent and manage the diabetic encephalopathy.

Electronic supplementary material

The online version of this article (10.1186/s12944-018-0938-7) contains supplementary material, which is available to authorized users.

Negative Effects of Cyclic Palmitate Treatment on Glucose Responsiveness and Insulin Production in Mouse Insulinoma Min6 Cells Are Reversible

Pancreatic β-cell failure is characterized by compromised insulin secretion in response to glucose, which ultimately results in hyperglycemia, the clinical hallmark of type 2 diabetes mellitus (T2DM). Acute exposure to plasma free fatty acids (FFAs) potentiates glucose stimulated insulin secretion (GSIS), while chronic exposure impairs GSIS, and the latter has been associated with the mechanism of β cell failure in obesity linked T2DM. By contrast, growth hormone (GH) signaling has been linked positively to GSIS in β cells. Numerous studies have examined chronic exposure of β cells to elevated FFAs both with in vivo cohorts and in vitro models. Little attention, however, has been given to the fluctuation of plasma FFA levels due to rhythmic effects that are affected by daily diet and fat intake. Mouse insulinoma Min6 cells were exposed to cyclic/daily palmitate treatment over 2 and 3 days to assess effects on GSIS. Cyclic/daily palmitate treatment with a period of recovery negatively affected GSIS in a dose-dependent manner. Removal of palmitate after two cycles/day resulted in reversal of the effect on GSIS, which was also reflected by relative gene expression involved in insulin biosynthesis (Ins1, Ins2, Pdx1, and MafA) and GSIS (glucose 2 transporter and glucokinase). Modest positive effects on GSIS and glucokinase transcript levels were also observed when Min6 cells were cotreated with human GH and palmitate. These observations indicate that like continuous palmitate treatment, cyclic exposure to palmitate can acutely impair GSIS over 48 and 72 h. However, they also suggest that the negative effects of short periods of exposure to FFAs on β cell function remain reversible.

Interleukin-1 Gene Variability and Plasma Levels in Czech Patients with Chronic Periodontitis and Diabetes Mellitus

Recent studies have suggested a bidirectional relationship between chronic periodontitis (CP) and diabetes mellitus (DM). Immunoregulatory factors such as cytokines play an important role in etiopathogenesis of both diseases. The aim of this study was to analyze variability in interleukin-1 (IL-1) gene cluster and IL-1β plasma levels in patients with CP, DM, and a combination of both diseases. A total of 1016 individuals participating in this case-control study—225 healthy controls, 264 patients with CP, 132 with type 1 diabetes (T1DM), and 395 patients with type 2 diabetes (T2DM)—were genotyped using methods based on polymerase chain reaction for IL-1 gene polymorphisms (IL-1A (−889C/T, rs1800587), IL-1B (+3953C/T, rs1143634), and IL-1RN (gene for IL-1 receptor antagonist, IL-1RA, 86 bp tandem repeats in intron 2)). Levels of IL-1β were measured by Luminex methods in subgroups of controls, CP, T1DM + CP, and T2DM + CP subjects. Although no significant associations were found in the genotype and allele frequencies of IL-1A (−889C/T), significant differences in the allele frequencies of IL-1B (+3953C/T) were observed between controls and CP patients (P < 0.05). In T1DM patients, IL-1RN^∗S “short” allele and IL-1RN 12 genotype were significantly less frequent than those in controls (P < 0.01). In haplotype analysis, TTL haplotype decreased the risk of CP development (P < 0.01), whereas CCS and CTL haplotypes (P < 0.01 and P < 0.05) were associated with T1DM. Although IL-1β levels were measured significantly higher in mononuclear cells after stimulation by mitogens, HSP70, or selected periodontal bacteria than in unstimulated cells, IL-1 genotypes did not correlate with circulating IL-1β levels. In the Czech population, significant associations between the IL-1B polymorphism with CP and the IL-1RN variant with T1DM were found. Haplotype analysis suggests that variability in IL-1 gene cluster may be one of the factors in the CP and T1DM pathogenesis, although single variants of these polymorphisms are not substantial for protein production.

ATF6α downregulation of PPARα promotes lipotoxicity-induced tubulointerstitial fibrosis

Tubulointerstitial fibrosis is a strong predictor of progression in patients with chronic kidney disease, and is often accompanied by lipid accumulation in renal tubules. However, the molecular mechanisms modulating the relationship between lipotoxicity and tubulointerstitial fibrosis remain obscure. ATF6α, a transcription factor of the unfolded protein response, is reported to be an upstream regulator of fatty acid metabolism. Owing to their high energy demand, proximal tubular cells (PTCs) use fatty acids as their main energy source. We therefore hypothesized that ATF6α regulates PTC fatty acid metabolism, contributing to lipotoxicity-induced tubulointerstitial fibrosis. Overexpression of activated ATF6α transcriptionally downregulated peroxisome proliferator-activated receptor-α (PPARα), the master regulator of lipid metabolism, leading to reduced activity of fatty acid β-oxidation and cytosolic accumulation of lipid droplets in a human PTC line (HK-2). ATF6α-induced lipid accumulation caused mitochondrial dysfunction, enhanced apoptosis, and increased expression of connective tissue growth factor (CTGF), as well as reduced cell viability. Atf6α-/- mice had sustained expression of PPARα and less tubular lipid accumulation following unilateral ischemia-reperfusion injury (uIRI), resulting in the amelioration of apoptosis; reduced expression of CTGF, α-smooth muscle actin, and collagen I; and less tubulointerstitial fibrosis. Administration of fenofibrate, a PPARα agonist, reduced lipid accumulation and tubulointerstitial fibrosis in the uIRI model. Taken together, these findings suggest that ATF6α deranges fatty acid metabolism in PTCs, which leads to lipotoxicity-mediated apoptosis and CTGF upregulation, both of which promote tubulointerstitial fibrosis.

The role of Cox-2 and prostaglandin E2 receptor EP3 in pancreatic β-cell death

Elevated levels of lipids, in particular saturated fatty acids, are known to be associated with type 2 diabetes (T2D) and to have a negative effect on β-cell function and survival. We bring new evidence indicating that palmitate up-regulates cyclooxygenase-2 (COX-2) expression levels in human islets and in MIN6 β cells, and that it is elevated in islets isolated from T2D donors. Both small interfering specific cyclooxygenase-2 small interfering RNA (siRNA) or the COX-2 inhibitor celecoxib significantly inhibited apoptosis induced by palmitate. Prostaglandin E₂ (PGE₂), the predominant product of COX-2 enzymatic activity, activates membrane receptors, which are members of the GPCR-family (EP1-EP4). In the present study, elevated expression of the PGE₂ receptor subtype 3 (EP3) receptor was observed in β cells exposed to palmitate and in islets from individuals with T2D. Down-regulation of the pathway using EP3 siRNA or the specific L-798,106 antagonist markedly decreased the levels of palmitate-induced apoptosis. Altogether, our data put forward the COX-2-PGE₂-EP3 pathway as one of the mediators of palmitate-induced apoptosis in β-cells.-Amior, L., Srivastava, R., Nano, R., Bertuzzi, F., Melloul, D. The role of Cox-2 and prostaglandin E₂ receptor EP3 in pancreatic β-cell death.

Calorie restriction for long-term remission of type 2 diabetes

Starting with a hypothesis which postulated a simple explanation arising from the basic cause of type 2 diabetes, a series of studies has introduced a paradigm shift in our understanding of the condition. Gradual accumulation of fat in the liver and pancreas leads eventually to beta cell dedifferentiation and loss of specialised function. The consequent hyperglycaemia can be returned to normal by removing the excess fat from liver and pancreas. At present this can be achieved only by substantial weight loss, and a simple practical and efficacious method for this has been developed and applied in a series of studies. For those people who used to have type 2 diabetes, the state of post-diabetes can be long term provided that weight regain is avoided. The implications for personal health and for national health economics are considerable.

Higher levels of thioredoxin interacting protein (TXNIP) in patients with prediabetes compared to obese normoglycemic subjects

BACKGROUND

Thioredoxin interacting protein (TXNIP) is one of the mediators of oxidative stress induced beta-cell glucotoxisity. TXNIP might play a key role in impaired glucose homeostasis preceding overt T2DM. The aim of the present study was to compare TXNIP levels between patients with prediabetes and obese normoglycemic controls and to evaluate the link between TXNIP and metabolic risk factors.

PATIENTS AND METHODS

In the present study we included 79 patients with mean age 50.3 ± 10.6 years, divided into two age and BMI matched groups -control group with obesity without glycemic disturbances (NGT) (n = 40) and prediabetes (n = 39).

RESULTS

We found significantly higher levels of TXNIP in patients with prediabetes compared to normoglycemic obese controls (54.2 ± 69.9 vs. 23.9 ± 47.1 pg/ml; p = 0.03). The levels of TXNIP gradually increased from normal glucose tolerance trough IFG/IGT only to IFG + IGT (27,1; 44.0; 49.9 and 95.7 pg/ml respectively; p = 0.025 between NGT and IFG + IGT). TXNIP levels correlated weakly only with fasting blood glucose (r = 0.235; p = 0.04) but not with glucose during OGTT or the markers of insulin resistance.

CONCLUSIONS

The levels of TXNIP are higher in patients with prediabetes compared to normoglycemic controls as they increase gradually from NGT trough IFG/IGT only to IFG + IGT.

The Expression of Aldolase B in Islets Is Negatively Associated With Insulin Secretion in Humans

CONTEXT

Reduced β-cell mass, impaired islet function, and dedifferentiation are considered causal to development of hyperglycemia and type 2 diabetes. In human cohort studies, changes of islet cell-specific expression patterns have been associated with diabetes but not directly with in vivo insulin secretion.

OBJECTIVE

This study investigates alterations of islet gene expression and corresponding gene variants in the context of in vivo glycemic traits from the same patients.

METHODS

Fasting blood was collected before surgery, and pancreatic tissue was frozen after resection from 18 patients undergoing pancreatectomy. Islet tissue was isolated by laser capture microdissection. Islet transcriptome was analyzed using microarray and quantitative RT-PCR. Proteins were examined by immunohistochemistry and western blotting. The association of gene variants with insulin secretion was investigated with oral glucose tolerance test (OGTT)-derived insulin secretion measured in a large cohort of subjects at increased risk of type 2 diabetes and with hyperglycemic clamp in a subset.

RESULTS

Differential gene expression between islets from normoglycemic and hyperglycemic patients was prominent for the glycolytic enzyme ALDOB and the obesity-associated gene FAIM2. The mRNA levels of both genes correlated negatively with insulin secretion and positively with HbA1c. Islets of hyperglycemic patients displayed increased ALDOB immunoreactivity in insulin-positive cells, whereas α- and δ-cells were negative. Exposure of isolated islets to hyperglycemia augmented ALDOB expression. The minor allele of the ALDOB variant rs550915 associated with significantly higher levels of C-peptide and insulin during OGTT and hyperglycemic clamp, respectively.

CONCLUSION

Our analyses suggest that increased ALDOB expression in human islets is associated with lower insulin secretion.

High-fat-diet-induced remission of diabetes in a subset of KATP -GOF insulin-secretory-deficient mice

AIMS

To examine the effects of a high-fat-diet (HFD) on monogenic neonatal diabetes, without the confounding effects of compensatory hyperinsulinaemia.

METHODS

Mice expressing K_ATP channel gain-of-function (K_ATP -GOF) mutations, which models human neonatal diabetes, were fed an HFD.

RESULTS

Surprisingly, K_ATP -GOF mice exhibited resistance to HFD-induced obesity, accompanied by markedly divergent blood glucose control, with some K_ATP -GOF mice showing persistent diabetes (K_ATP -GOF-non-remitter [NR] mice) and others showing remission of diabetes (K_ATP -GOF-remitter [R] mice). Compared with the severely diabetic and insulin-resistant K_ATP -GOF-NR mice, HFD-fed K_ATP -GOF-R mice had lower blood glucose, improved insulin sensitivity, and increased circulating plasma insulin and glucagon-like peptide-1 concentrations. Strikingly, while HFD-fed K_ATP -GOF-NR mice showed increased food intake and decreased physical activity, reduced whole body fat mass and increased plasma lipids, K_ATP -GOF-R mice showed similar features to those of control littermates. Importantly, K_ATP -GOF-R mice had restored insulin content and β-cell mass compared with the marked loss observed in both HFD-fed K_ATP -GOF-NR and chow-fed K_ATP -GOF mice.

CONCLUSION

Together, our results suggest that restriction of dietary carbohydrates and caloric replacement by fat can induce metabolic changes that are beneficial in reducing glucotoxicity and secondary consequences of diabetes in a mouse model of insulin-secretory deficiency.

Sheng-jiang powder ameliorates obesity-induced pancreatic inflammatory injury via stimulating activation of the AMPK signalling pathway in rats

AIM

To investigate the mechanisms by which Sheng-jiang powder (SJP) ameliorates obesity-induced pancreatic inflammatory injury.

METHODS

Sprague-Dawley rats were randomized into three groups: normal group (NG), obese group (HLG), or SJP treatment group (HSG). Obesity was induced by feeding a high-fat diet in the HLG and HSG, while the NG received standard chow. Rats were euthanized after 12 wk, and blood and pancreatic tissues were collected for histopathological analyses. Nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and transforming growth factor beta (TGF-β) expression, serum triglyceride and adiponectin levels, and apoptosis in pancreatic acinar cells were assessed. A high-fat AR42J acinar cell injury model was established using very low-density lipoprotein (VLDL). AR42J acinar cell culture supernatant, treated with different interventions, was applied to seven groups of pancreatic stellate cells (PSCs). The proliferation of PSCs and the expression of fibronectin and type I collagenase were assessed.

RESULTS

Compared with the NG, we found higher pathological scores for pancreatic tissues, lower serum adiponectin levels, higher expression levels of NF-κB in pancreatic tissues and TGF-β in pancreatic inflammatory cells, and increased apoptosis among pancreatic acinar cells for the HLG (P < 0.05). Compared with the HLG, we found reduced body weight, Lee’s index scores, serum triglyceride levels, and pathological scores for pancreatic tissues; higher serum adiponectin levels; and lower expression levels of NF-κB, in pancreatic tissue and TGF-β in pancreatic inflammatory cells for the HSG (P < 0.05). The in vitro studies showed enhanced PSC activation and increased expression levels of fibronectin and type I collagenase after SJP treatment. An adenosine 5‘-monophosphate-activated protein kinase (AMPK) inhibitor inhibited PSC activation.

CONCLUSION

SJP may ameliorate obesity-induced pancreatic inflammatory injury in rats by regulating key molecules of the adiponectin-AMPK signalling pathway.

MicroRNA-182-5p contributes to the protective effects of thrombospondin 1 against lipotoxicity in INS-1 cells

The dysfunction of beta cells serves an important role in the pathogenesis of type 2 diabetes mellitus (T2DM). An improved understanding of the molecular mechanisms underlying beta cell mass and failure will be useful for identifying novel approaches toward preventing and treating this disease. Recent studies have indicated that free fatty acids (FFAs) can cause beta cell dysfunction. In the present study, palmitate (Pal) was used as a FFA and its functions on cell viability and apoptosis were detected. MTT assay and flow cytometry were used and the results revealed that incubation of INS-1 cells with Pal significantly decreased cell viability and increased cell apoptosis. However, a co-incubation with thrombospondin 1 (THBS-1) protected the cells against Pal-induced toxicity. Numerous studies have demonstrated that microRNAs (miRs) are involved in fatty acid-induced beta cell dysfunction. Various studies have reported that miR-182-5p is associated with a number of diseases, including cancer, heart disease, and leukemia. However, to the best of our knowledge miR-182-5p has never been reported to be associated with diabetes. In the present study, miR-182-5p, which is predicted to target the 3'-untranslated region (UTR) of THBS-1, was detected using reverse transcription-quantitative polymerase chain reaction in INS-1 cells in response to Pal. miR-182-5p was significantly increased in Pal-treated cells compared with the control cells. Furthermore, miR-182-5p mimics significantly decreased cell viability and increased Pal-induced apoptosis in INS-1 cells. However, cell viability was increased and Pal-induced apoptosis was decreased in cells that were treated with miR-182-5p inhibitors. The present findings also revealed that overexpression of THBS-1 counteracted the effect of miR-182-5p on cell viability and apoptosis. These results suggested that miR-182-5p is involved in the mechanism of THBS 1 on the modulation of beta cell survival.

Remission of Human Type 2 Diabetes Requires Decrease in Liver and Pancreas Fat Content but Is Dependent upon Capacity for β Cell Recovery

The Diabetes Remission Clinical Trial reported return and persistence of non-diabetic blood glucose control in 46% of people with type 2 diabetes of up to 6 years duration. Detailed metabolic studies were performed on a subgroup (intervention, n = 64; control, n = 26). In the intervention group, liver fat content decreased (16.0% ± 1.3% to 3.1% ± 0.5%, p < 0.0001) immediately after weight loss. Similarly, plasma triglyceride and pancreas fat content decreased whether or not glucose control normalized. Recovery of first-phase insulin response (0.04[-0.05-0.32] to 0.11[0.0005-0.51] nmol/min/m², p < 0.0001) defined those who returned to non-diabetic glucose control and this was durable at 12 months (0.11[0.005-0.81] nmol/min/m², p = 0.0001). Responders were similar to non-responders at baseline but had shorter diabetes duration (2.7 ± 0.3 versus 3.8 ± 0.4 years; p = 0.02). This study demonstrates that β cell ability to recover long-term function persists after diagnosis, changing the previous paradigm of irreversible loss of β cell function in type 2 diabetes.

Deficiency of Sphingosine-1-Phosphate Reduces the Expression of Prohibitin and Causes β-Cell Impairment via Mitochondrial Dysregulation

BACKGROUND

Emerging evidence suggests that sphingolipids may be involved in type 2 diabetes. However, the exact signaling defect through which disordered sphingolipid metabolism induces β-cell dysfunction remains unknown. The current study demonstrated that sphingosine-1-phosphate (S1P), the product of sphingosine kinase (SphK), is an essential factor for maintaining β-cell function and survival via regulation of mitochondrial action, as mediated by prohibitin (PHB).

METHODS

We examined β-cell function and viability, as measured by mitochondrial function, in mouse insulinoma 6 (MIN6) cells in response to manipulation of cellular S1P and PHB levels.

RESULTS

Lack of S1P induced by sphingosine kinase inhibitor (SphKi) treatment caused β-cell dysfunction and apoptosis, with repression of mitochondrial function shown by decreases in cellular adenosine triphosphate content, the oxygen consumption rate, the expression of oxidative phosphorylation complexes, the mitochondrial membrane potential, and the expression of key regulators of mitochondrial dynamics (mitochondrial dynamin-like GTPase [OPA1] and mitofusin 1 [MFN1]). Supplementation of S1P led to the recovery of mitochondrial function and greatly improved β-cell function and viability. Knockdown of SphK2 using small interfering RNA induced mitochondrial dysfunction, decreased glucose-stimulated insulin secretion (GSIS), and reduced the expression of PHB, an essential regulator of mitochondrial metabolism. PHB deficiency significantly reduced GSIS and induced mitochondrial dysfunction, and co-treatment with S1P did not reverse these trends.

CONCLUSION

Altogether, these data suggest that S1P is an essential factor in the maintenance of β-cell function and survival through its regulation of mitochondrial action and PHB expression.

Pro-inflammatory cytokines attenuate glucose-stimulated insulin secretion from INS-1E insulinoma cells by restricting mitochondrial pyruvate oxidation capacity - Novel mechanistic insight from real-time analysis of oxidative phosphorylation

Pro-inflammatory cytokines cause pancreatic beta cell failure during the development of type 2 diabetes. This beta cell failure associates with mitochondrial dysfunction, but the precise effects of cytokines on mitochondrial respiration remain unclear. To test the hypothesis that pro-inflammatory cytokines impair glucose-stimulated insulin secretion (GSIS) by inhibiting oxidative ATP synthesis, we probed insulin release and real-time mitochondrial respiration in rat INS-1E insulinoma cells that were exposed to a combination of 2 ng/mL interleukin-1-beta and 50 ng/mL interferon-gamma. We show that 24-h exposure to these cytokines dampens both glucose- and pyruvate-stimulated insulin secretion (P < 0.0001 and P < 0.05, respectively), but does not affect KCl-induced insulin release. Mirroring secretory defects, glucose- and pyruvate-stimulated mitochondrial respiration are lowered after cytokine exposure (P < 0.01). Further analysis confirms that cytokine-induced mitochondrial respiratory defects occur irrespective of whether fuel oxidation is coupled to, or uncoupled from, ATP synthesis. These observations demonstrate that pro-inflammatory cytokines attenuate GSIS by restricting mitochondrial pyruvate oxidation capacity. Interleukin-1-beta and interferon-gamma also increase mitochondrial superoxide levels (P < 0.05), which may reinforce the inhibition of pyruvate oxidation, and cause a modest (20%) but significant (P < 0.01) loss of INS-1E cells. Cytokine-induced INS-1E cell failure is insensitive to palmitoleate and linoleate, which is at odds with the cytoprotection offered by unsaturated fatty acids against harm caused by nutrient excess. Our data disclose a mitochondrial mechanism for cytokine-impaired GSIS in INS-1E cells, and suggest that inflammatory and nutrient-related beta cell failure emerge, at least partly, through distinct paths.

Endoplasmic Reticulum Stress in Metabolic Disorders

Metabolic disorders have become among the most serious threats to human health, leading to severe chronic diseases such as obesity, type 2 diabetes, and non-alcoholic fatty liver disease, as well as cardiovascular diseases. Interestingly, despite the fact that each of these diseases has different physiological and clinical symptoms, they appear to share certain pathological traits such as intracellular stress and inflammation induced by metabolic disturbance stemmed from over nutrition frequently aggravated by a modern, sedentary life style. These modern ways of living inundate cells and organs with saturating levels of sugar and fat, leading to glycotoxicity and lipotoxicity that induce intracellular stress signaling ranging from oxidative to ER stress response to cope with the metabolic insults (Mukherjee, et al., 2015). In this review, we discuss the roles played by cellular stress and its responses in shaping metabolic disorders. We have summarized here current mechanistic insights explaining the pathogenesis of these disorders. These are followed by a discussion of the latest therapies targeting the stress response pathways.

Skeletal muscle ceramides and daily fat oxidation in obesity and diabetes

BACKGROUND/OBJECTIVES

Ectopic accumulation of lipids in skeletal muscle and the formation of deleterious lipid intermediates is thought to contribute to the development of insulin resistance and type 2 diabetes mellitus (T2DM). Similarly, impaired fat oxidation (metabolic inflexibility) are predictors of weight gain and the development of T2DM; however, no study has investigated the relation between muscle ceramide accumulation and 24-hour macronutrient oxidation. The purpose of this study was to retrospectively explore the relationships between whole body fat oxidation and skeletal muscle ceramide accumulation in obese non-diabetic individuals (ND) and in people with obesity and T2DM.

METHODS

Daily substrate oxidation was measured in a respiratory chamber and skeletal muscle ceramides were measured using liquid chromatographyelectrospray ionization tandem-mass spectrometry.

RESULTS

After adjusting for sex, age, and BMI, no differences existed between the groups for fat oxidation or 24-h RQ. However, ceramides C18:1, C:20, C22, C24 and C24:1 were significantly higher in people with T2DM compared to ND whereas no differences existed for C16 and C18. Despite low amounts of muscle ceramides, fat oxidation rates were positively associated with ceramide species concentration in ND only. Our data suggests that ceramides do not interfere with whole-body fat oxidation in ND individuals whereas a persistent lipid oversupply results in excessive ceramide muscle accumulation in people with T2DM.

Acute pancreatitis secondary to hypertriglyceridemia precipitated by diabetic ketoacidosis in a previously undiagnosed ketosis-prone patient with diabetes mellitus

Diabetic ketoacidosis is a potentially fatal complication of diabetes mellitus that may result in hypertriglyceridemia. Rarely, the resulting hypertriglyceridemia may precipitate acute pancreatitis. We report a case of acute pancreatitis secondary to hypertriglyceridemia precipitated by diabetic ketoacidosis and postulate that this unusual presentation is due to the patient being prone to ketosis.

The pathogenetic role of β-cell mitochondria in type 2 diabetes

Mitochondrial metabolism is a major determinant of insulin secretion from pancreatic β-cells. Type 2 diabetes evolves when β-cells fail to release appropriate amounts of insulin in response to glucose. This results in hyperglycemia and metabolic dysregulation. Evidence has recently been mounting that mitochondrial dysfunction plays an important role in these processes. Monogenic dysfunction of mitochondria is a rare condition but causes a type 2 diabetes-like syndrome owing to β-cell failure. Here, we describe novel advances in research on mitochondrial dysfunction in the β-cell in type 2 diabetes, with a focus on human studies. Relevant studies in animal and cell models of the disease are described. Transcriptional and translational regulation in mitochondria are particularly emphasized. The role of metabolic enzymes and pathways and their impact on β-cell function in type 2 diabetes pathophysiology are discussed. The role of genetic variation in mitochondrial function leading to type 2 diabetes is highlighted. We argue that alterations in mitochondria may be a culprit in the pathogenetic processes culminating in type 2 diabetes.

Pioglitazone Attenuates Palmitate-Induced Inflammation and Endoplasmic Reticulum Stress in Pancreatic β-Cells

BACKGROUND

The nuclear receptor peroxisome proliferator-activator gamma (PPARγ) is a useful therapeutic target for obesity and diabetes, but its role in protecting β-cell function and viability is unclear.

METHODS

To identify the potential functions of PPARγ in β-cells, we treated mouse insulinoma 6 (MIN6) cells with the PPARγ agonist pioglitazone in conditions of lipotoxicity, endoplasmic reticulum (ER) stress, and inflammation.

RESULTS

Palmitate-treated cells incubated with pioglitazone exhibited significant improvements in glucose-stimulated insulin secretion and the repression of apoptosis, as shown by decreased caspase-3 cleavage and poly (adenosine diphosphate [ADP]-ribose) polymerase activity. Pioglitazone also reversed the palmitate-induced expression of inflammatory cytokines (tumor necrosis factor α, interleukin 6 [IL-6], and IL-1β) and ER stress markers (phosphor-eukaryotic translation initiation factor 2α, glucose-regulated protein 78 [GRP78], cleaved-activating transcription factor 6 [ATF6], and C/EBP homologous protein [CHOP]), and pioglitazone significantly attenuated inflammation and ER stress in lipopolysaccharide- or tunicamycin-treated MIN6 cells. The protective effect of pioglitazone was also tested in pancreatic islets from high-fat-fed KK-Ay mice administered 0.02% (wt/wt) pioglitazone or vehicle for 6 weeks. Pioglitazone remarkably reduced the expression of ATF6α, GRP78, and monocyte chemoattractant protein-1, prevented α-cell infiltration into the pancreatic islets, and upregulated glucose transporter 2 (Glut2) expression in β-cells. Moreover, the preservation of β-cells by pioglitazone was accompanied by a significant reduction of blood glucose levels.

CONCLUSION

Altogether, these results support the proposal that PPARγ agonists not only suppress insulin resistance, but also prevent β-cell impairment via protection against ER stress and inflammation. The activation of PPARγ might be a new therapeutic approach for improving β-cell survival and insulin secretion in patients with diabetes mellitus.

Intracellular lipid metabolism impairs β cell compensation during diet-induced obesity

The compensatory proliferation of insulin-producing β cells is critical to maintaining glucose homeostasis at the early stage of type 2 diabetes. Failure of β cells to proliferate results in hyperglycemia and insulin dependence in patients. To understand the effect of the interplay between β cell compensation and lipid metabolism upon obesity and peripheral insulin resistance, we eliminated LDL receptor-related protein 1 (LRP1), a pleiotropic mediator of cholesterol, insulin, energy metabolism, and other cellular processes, in β cells. Upon high-fat diet exposure, LRP1 ablation significantly impaired insulin secretion and proliferation of β cells. The diminished insulin signaling was partly contributed to by the hypersensitivity to glucose-induced, Ca2+-dependent activation of Erk and the mTORC1 effector p85 S6K1. Surprisingly, in LRP1-deficient islets, lipotoxic sphingolipids were mitigated by improved lipid metabolism, mediated at least in part by the master transcriptional regulator PPARγ2. Acute overexpression of PPARγ2 in β cells impaired insulin signaling and insulin secretion. Elimination of Apbb2, a functional regulator of LRP1 cytoplasmic domain, also impaired β cell function in a similar fashion. In summary, our results uncover the double-edged effects of intracellular lipid metabolism on β cell function and viability in obesity and type 2 diabetes and highlight LRP1 as an essential regulator of these processes.

The EndoC-βH1 cell line is a valid model of human beta cells and applicable for screenings to identify novel drug target candidates

OBJECTIVE

To characterize the EndoC-βH1 cell line as a model for human beta cells and evaluate its beta cell functionality, focusing on insulin secretion, proliferation, apoptosis and ER stress, with the objective to assess its potential as a screening platform for identification of novel anti-diabetic drug candidates.

METHODS

EndoC-βH1 was transplanted into mice for validation of in vivo functionality. Insulin secretion was evaluated in cells cultured as monolayer and as pseudoislets, as well as in diabetic mice. Cytokine induced apoptosis, glucolipotoxicity, and ER stress responses were assessed. Beta cell relevant mRNA and protein expression were investigated by qPCR and antibody staining. Hundreds of proteins or peptides were tested for their effect on insulin secretion and proliferation.

RESULTS

Transplantation of EndoC-βH1 cells restored normoglycemia in streptozotocin induced diabetic mice. Both in vitro and in vivo, we observed a clear insulin response to glucose, and, in vitro, we found a significant increase in insulin secretion from EndoC-βH1 pseudoislets compared to monolayer cultures for both glucose and incretins. Apoptosis and ER stress were inducible in the cells and caspase 3/7 activity was elevated in response to cytokines, but not affected by the saturated fatty acid palmitate. By screening of various proteins and peptides, we found Bombesin (BB) receptor agonists and Pituitary Adenylate Cyclase-Activating Polypeptides (PACAP) to significantly induce insulin secretion and the proteins SerpinA6, STC1, and APOH to significantly stimulate proliferation. ER stress was readily induced by Tunicamycin and resulted in a reduction of insulin mRNA. Somatostatin (SST) was found to be expressed by 1% of the cells and manipulation of the SST receptors was found to significantly affect insulin secretion.

CONCLUSIONS

Overall, the EndoC-βH1 cells strongly resemble human islet beta cells in terms of glucose and incretin stimulated insulin secretion capabilities. The cell line has an active cytokine induced caspase 3/7 apoptotic pathway and is responsive to ER stress initiation factors. The cells' ability to proliferate can be further increased by already known compounds as well as by novel peptides and proteins. Based on its robust performance during the functionality assessment assays, the EndoC-βH1 cell line was successfully used as a screening platform for identification of novel anti-diabetic drug candidates.

Type 2 Diabetes Genetic Risk Scores Are Associated With Increased Type 2 Diabetes Risk Among African Americans by Cardiometabolic Status

The relationship between genetic risk variants associated with glucose homeostasis and type 2 diabetes risk has yet to be fully explored in African American populations. We pooled data from 4 prospective studies including 4622 African Americans to assess whether β-cell dysfunction (BCD) and/or insulin resistance (IR) genetic variants were associated with increased type 2 diabetes risk. The BCD genetic risk score (GRS) and combined BCD/IR GRS were significantly associated with increased type 2 diabetes risk. In cardiometabolic-stratified models, the BCD and IR GRS were associated with increased type 2 diabetes risk among 5 cardiometabolic strata: 3 clinically healthy strata and 2 clinically unhealthy strata. Genetic risk scores related to BCD and IR were associated with increased risk of type 2 diabetes in African Americans. Notably, the GRSs were significant predictors of type 2 diabetes among individuals in clinically normal ranges of cardiometabolic traits.

Fatty Acid-Induced Lipotoxicity in Pancreatic Beta-Cells During Development of Type 2 Diabetes

Type 2 diabetes is caused by chronic insulin resistance and progressive decline in beta-cell function. Optimal beta-cell function and mass is essential for glucose homeostasis and beta-cell impairment leads to the development of diabetes. Elevated levels of circulating fatty acids (FAs) and disturbances in lipid metabolism regulation are associated with obesity, and they are major factors influencing the increase in the incidence of type 2 diabetes. Chronic free FA (FFA) treatment induces insulin resistance and beta-cell dysfunction; therefore, reduction of elevated plasma FFA levels might be an important therapeutic target in obesity and type 2 diabetes. Lipid signals via receptors, and intracellular mechanisms are involved in FFA-induced apoptosis. In this paper, we discuss lipid actions in beta cells, including effects on metabolic pathways and stress responses, to help further understand the molecular mechanisms of lipotoxicity-induced type 2 diabetes.

Role of Lipotoxicity and Contribution of the Renin-Angiotensin System in the Development of Polycystic Ovary Syndrome

Polycystic ovary syndrome (PCOS) is a common and significant condition associated with hyperandrogenism, infertility, low quality of life, and metabolic comorbidities. One possible explanation of PCOS development is cellular dysfunction induced by nonesterified fatty acids (NEFAs), that is, lipotoxicity, which could explain both the hyperandrogenemia and insulin resistance that characterize women with PCOS. The literature suggests that androgen biosynthesis may be induced by overexposure of androgen-secreting tissues to NEFA and/or defective NEFA metabolism, leading to lipotoxic effects. Indeed, lipotoxicity could trigger androgenic hyperresponsiveness to insulin, LH, and ACTH. In most PCOS women, lipotoxicity also causes insulin resistance, inducing compensatory hyperinsulinemia, and may thus further increase hyperandrogenemia. Many approaches aimed at insulin sensitization also reduce lipotoxicity and have been shown to treat PCOS hyperandrogenemia. Furthermore, our group and others found that angiotensin II type 2 receptor (AT2R) activation is able to improve lipotoxicity. We provided evidence, using C21/M24, that AT2R activation improves adipocytes' size and insulin sensitivity in an insulin-resistant rat model, as well as androgen levels in a PCOS obese rat model. Taken together, these findings point toward the important role of lipotoxicity in PCOS development and of the RAS system as a new target for the treatment of PCOS.

Elevated Markers of Death Receptor-Activated Apoptosis are Associated with Increased Risk for Development of Diabetes and Cardiovascular Disease

Background

An increased rate of cell death by apoptosis has been implicated in both diabetes and atherosclerosis. Apoptosis can be induced through activation of the death receptors TNF receptor 1 (TNFR-1), TRAIL receptor 2 (TRAILR-2) and Fas. Soluble forms of these receptors are found in plasma. The objective of this study was to determine if soluble death receptors are markers of receptor-activated apoptosis and predict risk for development of diabetes and cardiovascular events.

Methods

Fas ligand was used to induce apoptosis in peripheral blood mononuclear cells and INS-1 pancreatic β-cells and release of TNFR-1, TRAILR-2 and Fas measured by ELISA. Proximity Extension Assay was used to analyze plasma levels of TNFR-1, TRAILR-2 and Fas in baseline samples of 4742 subjects in the Malmö Diet and Cancer Study and related to development of diabetes and cardiovascular events during 19.2 years of follow-up.

Results

Activation of apoptosis by Fas ligand was associated with release of soluble Fas, TNFR-1 and TRAILR-2 in both cell types. Circulating levels of all three receptors were higher in subjects with diabetes and correlated with markers of impaired glucose metabolism in non-diabetic subjects. Among the latter, those in the highest tertile of soluble Fas, TNFR-1 and TRAILR-2 had increased risk for development of diabetes and cardiovascular events. These associations became weaker when adjusting for cardiovascular risk factors in Cox regression models, but remained significant for TRAILR-2 with incident diabetes, cardiovascular mortality, myocardial infarction and ischemic stroke, and for TNFR-1 with myocardial infarction.

Conclusion

The present study demonstrates an association between several cardiovascular risk factors and elevated levels of circulating markers of apoptotic cell death. It also shows that subjects with high levels of these biomarkers have increased risk of diabetes and CVD. This implies that soluble death receptors are markers of β-cell and vascular injury and potentially could be used as surrogate markers of therapeutic efficiency in risk factor interventions.

•Receptor-activated apoptosis is associated with release of soluble death receptors that act as biomarkers of apoptosis •Several cardiovascular risk factors including markers of impaired glucose metabolism associate with elevated plasma levels of death receptors •Subjects with high plasma levels of death receptors have an increased risk of diabetes and cardiovascular disease

Atherosclerosis has been proposed to develop in response to chronic arterial injury caused by cardiovascular risk factors. The present study provides clinical evidence for this hypothesis by demonstrating an association between several cardiovascular risk factors and elevated levels of circulating markers of apoptotic cell death and that subjects with high levels of these biomarkers have increased risk of cardiovascular mortality, MI and stroke. These observations point to the possibility that the plasma level of soluble death receptors can be used as surrogate markers of arterial injury and atherosclerotic disease activity in cardiovascular interventions. Finally, our findings imply that soluble death receptors also may serve as biomarkers of the damage caused by metabolic stress to β-cells and risk for development of type 2 diabetes.

Autophagy protects pancreatic beta cell mass and function in the setting of a high-fat and high-glucose diet

Autophagy is a major regulator of pancreatic beta cell homeostasis. Altered autophagic activity has been implicated in the beta cells of patients with type 2 diabetes, and in the beta cells of obese diabetic rodents. Here, we show that autophagy was induced in beta cells by either a high-fat diet or a combined high-fat and high-glucose diet, but not by high-glucose alone. However, a high-glucose intake alone did increase beta cell mass and insulin secretion moderately. Depletion of Atg7, a necessary component of the autophagy pathway, in beta cells by pancreatic intra-ductal AAV8-shAtg7 infusion in C57BL/6 mice, resulted in decreased beta cell mass, impaired glucose tolerance, defective insulin secretion, and increased apoptosis when a combined high-fat and high-glucose diet was given, seemingly due to suppression of autophagy. Taken together, our findings suggest that the autophagy pathway may act as a protective mechanism in pancreatic beta cells during a high-calorie diet.

Fatty Acids, Antioxidants and Physical Activity in Brain Aging

Polyunsaturated fatty acids and antioxidants are important mediators in the central nervous system. Lipid derivatives may control the production of proinflammatory agents and regulate NF-κB activity, microglial activation, and fatty acid oxidation; on the other hand, antioxidants, such as glutathione and ascorbate, have been shown to signal through transmitter receptors and protect against acute and chronic oxidative stress, modulating the activity of different signaling pathways. Several authors have investigated the role of these nutrients in the brains of the young and the aged in degenerative diseases such as Alzheimer’s and Parkinson’s, and during brain aging due to adiposity- and physical inactivity-mediated metabolic disturbances, chronic inflammation, and oxidative stress. Through a literature review, we aimed to highlight recent data on the role of adiposity, fatty acids, antioxidants, and physical inactivity in the pathophysiology of the brain and in the molecular mechanisms of senescence. Data indicate the complexity and necessity of endogenous/dietary antioxidants for the maintenance of redox status and the control of neuroglial signaling under stress. Recent studies also indicate that omega-3 and -6 fatty acids act in a competitive manner to generate mediators for energy metabolism, influencing feeding behavior, neural plasticity, and memory during aging. Finding pharmacological or dietary resources that mitigate or prevent neurodegenerative affections continues to be a great challenge and requires additional effort from researchers, clinicians, and nutritionists in the field.

The Regulatory Roles of Mitogen-Activated Protein Kinase (MAPK) Pathways in Health and Diabetes: Lessons Learned from the Pancreatic β-Cell

BACKGROUND

Glucose-stimulated insulin secretion (GSIS) from the pancreatic β-cell involves several intracellular metabolic events which lead to the translocation of insulin granules towards the membrane for fusion and release. It is well established that loss of β-cell function and decreased GSIS underlie the pathogenesis of diabetes. Evidence from several laboratories, including our own, demonstrated requisite roles of Rac1 and phagocyte-like NADPH oxidase (Nox2)-derived reactive oxygen species (ROS) in optimal function of the pancreatic β-cell, including GSIS. However, it is becoming increasingly clear that prolonged exposure of β-cells to hyperglycemic conditions, leads to sustained activation of Rac1-Nox2 signaling axis culminating in excessive generation of intracellular ROS (oxidative stress) and β-cell dysregulation and demise. Such "cytotoxic" effects of ROS appear to be mediated via the stress-activated protein kinases/mitogen-activated protein kinases (SAPK/MAPK) signaling pathways.

OBJECTIVE

This review discusses our current understanding of regulation and functions of the conventional MAPKs, namely, ERK1/2, JNK1/2 and p38MAPK.

CONCLUSION

The MAPK pathways are activated in the presence of various stress stimuli including intracellular ROS, via distinct signaling cascades. Once activated, MAPKs participate in specific intracellular signaling processes via interaction with several downstream kinases including the MAPKactivated protein kinases (MAPKAPKs) and transcription factors including c-jun and p53. We have provided an overview of existing evidence in the islet β-cell on the regulatory roles of these MAPKs in mediating cellular responses to alterations in intracellularly generated ROS, which is mediated by the Rac1-Nox2 signaling module. Additionally, we enlisted recent patents developed to improve β-cell function in diabetes and novel pharmacological agents that target oxidative stress and MAPK pathways.

Metabolic crosstalk between fatty pancreas and fatty liver: effects on local inflammation and insulin secretion

AIMS/HYPOTHESIS

Obesity-linked ectopic fat accumulation is associated with the development of type 2 diabetes. Whether pancreatic and liver steatosis impairs insulin secretion is controversial. We examined the crosstalk of human pancreatic fat cells with islets and the role of diabetogenic factors, i.e. palmitate and fetuin-A, a hepatokine released from fatty liver.

METHODS

Human pancreatic resections were immunohistochemically stained for insulin, glucagon, somatostatin and the macrophage/monocyte marker CD68. Pancreatic adipocytes were identified by Oil Red O and adiponectin staining. Primary pancreatic pre-adipocytes and differentiated adipocytes were co-cultured with human islets isolated from organ donors and the metabolic crosstalk between fatty liver and fatty pancreas was mimicked by the addition of palmitate and fetuin-A. Insulin secretion was evaluated by ELISA and RIA. Cytokine expression and secretion were assessed by RT-PCR and multiplex assay, respectively. Subcellular distribution of proteins was examined by confocal microscopy and protein phosphorylation by western blotting.

RESULTS

In human pancreatic parenchyma, highly differentiated adipocytes were detected in the proximity of islets with normal architecture and hormone distribution. Infiltration of adipocytes was associated with an increased number of CD68-positive cells within islets. In isolated primary pancreatic pre-adipocytes and differentiated adipocytes, palmitate and fetuin-A induced IL6, CXCL8 and CCL2 mRNA expression. Cytokine production was toll-like receptor 4 (TLR4)-dependent and further accentuated in pre-adipocytes when co-cultured with islets. In islets, IL6 and CXCL8 mRNA levels were also increased by fetuin-A and palmitate. Only in macrophages within the isolated islets, palmitate and fetuin-A stimulated the production of the cytotoxic cytokine IL-1β. Palmitate, but not fetuin-A, exerted pro-apoptotic effects in islet cells. Instead, fetuin-A impaired glucose-induced insulin secretion in a TLR4-independent, but c-Jun N-terminal kinase- and Ca²⁺-dependent, manner.

CONCLUSIONS/INTERPRETATION

These results provide the first evidence that fetuin-A-mediated metabolic crosstalk of fatty liver with islets may contribute to obesity-linked glucose blindness of beta cells, while fatty pancreas may exacerbate local inflammation.

Oleate attenuates palmitate-induced endoplasmic reticulum stress and apoptosis in placental trophoblasts

Pre-pregnancy obesity is increasingly common and predisposes pregnant women and offspring to gestational diabetes, pre-eclampsia, fetal growth abnormalities and stillbirth. Obese women exhibit elevated levels of the two most common dietary fatty acids, palmitate and oleate, and the maternal blood containing these nutrients bathes the surface of trophoblasts of placental villi in vivo We test the hypothesis that the composition and concentration of free fatty acids modulate viability and function of primary human villous trophoblasts in culture. We found that palmitate increases syncytiotrophoblast death, specifically by caspase-mediated apoptosis, whereas oleate does not cause enhanced cell death. Importantly, exposure to both fatty acids in equimolar amounts yielded no increase in death or apoptosis, suggesting that oleate can protect syncytiotrophoblasts from palmitate-induced death. We further found that palmitate, but not oleate or oleate with palmitate, increases endoplasmic reticulum (ER) stress, signaling through the unfolded protein response, and yielding CHOP-mediated induction of apoptosis. Finally, we show that oleate or oleate plus palmitate both lead to increased lipid droplets in syncytiotrophoblasts, whereas palmitate does not. The data show palmitate is toxic to human syncytiotrophoblasts, through the induction of ER stress and apoptosis mediated by CHOP, whereas oleate is not toxic, abrogates palmitate toxicity and induces fat accumulation. We speculate that our in vitro results offer pathways by which the metabolic milieu of the obese pregnant woman can yield villous trophoblast dysfunction and sub-optimal placental function.

Fatty Acids and NLRP3 Inflammasome-Mediated Inflammation in Metabolic Tissues

Worldwide obesity rates have reached epidemic proportions and significantly contribute to the growing prevalence of metabolic diseases. Chronic low-grade inflammation, a hallmark of obesity, involves immune cell infiltration into expanding adipose tissue. In turn, obesity-associated inflammation can lead to complications in other metabolic tissues (e.g., liver, skeletal muscle, pancreas) through lipotoxicity and inflammatory signaling networks. Importantly, although numerous signaling pathways are known to integrate metabolic and inflammatory processes, the nucleotide-binding and oligomerization domain-like receptor, leucine-rich repeat and pyrin domain-containing 3 (NLRP3) inflammasome is now noted to be a key regulator of metabolic inflammation. The NLRP3 inflammasome can be influenced by various metabolites, including fatty acids. Specifically, although saturated fatty acids may promote NLRP3 inflammasome activation, monounsaturated fatty acids and polyunsaturated fatty acids have recently been shown to impede NLRP3 activity. Therefore, the NLRP3 inflammasome and associated metabolic inflammation have key roles in the relationships among fatty acids, metabolites, and metabolic disease. This review focuses on the ability of fatty acids to influence inflammation and the NLRP3 inflammasome across numerous metabolic tissues in the body. In addition, we explore some perspectives for the future, wherein recent work in the immunology field clearly demonstrates that metabolic reprogramming defines immune cell functionality. Although there is a paucity of information about how diet and fatty acids modulate this process, it is possible that this will open up a new avenue of research relating to nutrient-sensitive metabolic inflammation.

Fructose replacement of glucose or sucrose in food or beverages lowers postprandial glucose and insulin without raising triglycerides: a systematic review and meta-analysis

Background: Conflicting evidence exists on the effects of fructose consumption in people with type 1 and type 2 diabetes mellitus. No systematic review has addressed the effect of isoenergetic fructose replacement of glucose or sucrose on peak postprandial glucose, insulin, and triglyceride concentrations.Objective: The objective of this study was to review the evidence for postprandial glycemic and insulinemic responses after isoenergetic replacement of either glucose or sucrose in foods or beverages with fructose.Design: We searched the Cochrane Library, MEDLINE, EMBASE, the WHO International Clinical Trials Registry Platform Search Portal, and clinicaltrials.gov The date of the last search was 26 April 2016. We included randomized controlled trials measuring peak postprandial glycemia after isoenergetic replacement of glucose, sucrose, or both with fructose in healthy adults or children with or without diabetes. The main outcomes analyzed were peak postprandial blood glucose, insulin, and triglyceride concentrations.Results: Replacement of either glucose or sucrose by fructose resulted in significantly lowered peak postprandial blood glucose, particularly in people with prediabetes and type 1 and type 2 diabetes. Similar results were obtained for insulin. Peak postprandial blood triglyceride concentrations did not significantly increase.Conclusions: Strong evidence exists that substituting fructose for glucose or sucrose in food or beverages lowers peak postprandial blood glucose and insulin concentrations. Isoenergetic replacement does not result in a substantial increase in blood triglyceride concentrations.

TRPM2-mediated rise in mitochondrial Zn2+ promotes palmitate-induced mitochondrial fission and pancreatic β-cell death in rodents

Rise in plasma free fatty acids (FFAs) represents a major risk factor for obesity-induced type 2 diabetes. Saturated FFAs cause a progressive decline in insulin secretion by promoting pancreatic β-cell death through increased production of reactive oxygen species (ROS). Recent studies have demonstrated that palmitate (a C₁₆-FFA)-induced rise in ROS causes β-cell death by triggering mitochondrial fragmentation, but the underlying mechanisms are unclear. Using the INS1-832/13 β-cell line, here we demonstrate that palmitate generates the ROS required for mitochondrial fission by activating NOX (NADPH oxidase)-2. More importantly, we show that chemical inhibition, RNAi-mediated silencing and knockout of ROS-sensitive TRPM (transient receptor potential melastatin)-2 channels prevent palmitate-induced mitochondrial fission. Although TRPM2 activation affects the intracellular dynamics of Ca²⁺ and Zn²⁺, chelation of Zn²⁺ alone was sufficient to prevent mitochondrial fission. Consistent with the role of Zn²⁺, palmitate caused a rise in mitochondrial Zn²⁺, leading to Zn²⁺-dependent mitochondrial recruitment of Drp-1 (a protein that catalyses mitochondrial fission) and loss of mitochondrial membrane potential. In agreement with the previous reports, Ca²⁺ caused Drp-1 recruitment, but it failed to induce mitochondrial fission in the absence of Zn²⁺. These results indicate a novel role for Zn²⁺ in mitochondrial dynamics. Inhibition or knockout of TRPM2 channels in mouse islets and RNAi-mediated silencing of TRPM2 expression in human islets prevented FFA/cytokine-induced β-cell death, findings that are consistent with the role of abnormal mitochondrial fission in cell death. To conclude, our results reveal a novel, potentially druggable signalling pathway for FFA-induced β-cell death. The cascade involves NOX-2-dependent production of ROS, activation of TRPM2 channels, rise in mitochondrial Zn²⁺, Drp-1 recruitment and abnormal mitochondrial fission.

Upregulation of UCP2 in beta-cells confers partial protection against both oxidative stress and glucotoxicity

Deterioration of pancreatic beta-cells plays a critical role in the development of type 2 diabetes. Among the various stressors contributing to these deleterious effects, glucotoxicity and superoxides have been proposed as major players. In this context, the mitochondrial uncoupling protein UCP2 is regularly associated with the stress response. In the present study, we tested the effects of UCP2 upregulation in mouse islets with beta-cell specific overexpression of UCP2 (RIP-UCP2). Islets were subjected to both chronic glucotoxicity (7 days at 30 mM glucose) and acute oxidative stress (200 µM H₂O₂ for 10 min). Increased UCP2 expression did not alter mitochondrial potential and ATP generation but protected against glucotoxic effects. Glucose-stimulated insulin secretion was altered by both glucotoxicity and oxidative stress, in particular through higher basal insulin release at non-stimulatory glucose concentrations. The secretory response to glucose stimulation was partially preserved in beta-cells overexpressing UCP2. The higher rate of cell death induced by chronic high glucose exposure was lower in RIP-UCP2 islets. Finally, superoxide production was reduced by high glucose, both under acute and chronic conditions, and not modified by UCP2 overexpression. In conclusion, upregulation of UCP2 conferred protective effects to the stressed beta-cell through mechanisms not directly associated with superoxide production.

•

UCP2 upregulation protects pancreatic ß-cells against glucotoxicity.

•

High glucose reduces superoxide production in pancreatic islets.

•

UCP2 upregulation does not change superoxide production.

•

UCP2 upregulation protects ß-cells against oxidative stress.

Protein kinase STK25 aggravates the severity of non-alcoholic fatty pancreas disease in mice

Characterising the molecular networks that negatively regulate pancreatic β-cell function is essential for understanding the underlying pathogenesis and developing new treatment strategies for type 2 diabetes. We recently identified serine/threonine protein kinase 25 (STK25) as a critical regulator of ectopic fat storage, meta-inflammation, and fibrosis in liver and skeletal muscle. Here, we assessed the role of STK25 in control of progression of non-alcoholic fatty pancreas disease in the context of chronic exposure to dietary lipids in mice. We found that overexpression of STK25 in high-fat-fed transgenic mice aggravated diet-induced lipid storage in the pancreas compared with that of wild-type controls, which was accompanied by exacerbated pancreatic inflammatory cell infiltration, stellate cell activation, fibrosis and apoptosis. Pancreas of Stk25 transgenic mice also displayed a marked decrease in islet β/α-cell ratio and alteration in the islet architecture with an increased presence of α-cells within the islet core, whereas islet size remained similar between genotypes. After a continued challenge with a high-fat diet, lower levels of fasting plasma insulin and C-peptide, and higher levels of plasma leptin, were detected in Stk25 transgenic vs wild-type mice. Furthermore, the glucose-stimulated insulin secretion was impaired in high-fat-fed Stk25 transgenic mice during glucose tolerance test, in spite of higher net change in blood glucose concentrations compared with wild-type controls, suggesting islet β-cell dysfunction. In summary, this study unravels a role for STK25 in determining the susceptibility to diet-induced non-alcoholic fatty pancreas disease in mice in connection to obesity. Our findings highlight STK25 as a potential drug target for metabolic disease.

Lysosome acidification by photoactivated nanoparticles restores autophagy under lipotoxicity

Lipotoxicity is frequently associated with alkalinization of lysosomes and impaired autophagic flux but the relationship between the two is unclear. Trudeau et al. show that autophagic flux can be restored in β islet cells chronically exposed to fatty acids by re-acidification of lysosomes with photoactivatable nanoparticles.

In pancreatic β-cells, liver hepatocytes, and cardiomyocytes, chronic exposure to high levels of fatty acids (lipotoxicity) inhibits autophagic flux and concomitantly decreases lysosomal acidity. Whether impaired lysosomal acidification is causally inhibiting autophagic flux and cellular functions could not, up to the present, be determined because of the lack of an approach to modify lysosomal acidity. To address this question, lysosome-localizing nanoparticles are described that, upon UV photoactivation, enable controlled acidification of impaired lysosomes. The photoactivatable, acidifying nanoparticles (paNPs) demonstrate lysosomal uptake in INS1 and mouse β-cells. Photoactivation of paNPs in fatty acid–treated INS1 cells enhances lysosomal acidity and function while decreasing p62 and LC3-II levels, indicating rescue of autophagic flux upon acute lysosomal acidification. Furthermore, paNPs improve glucose-stimulated insulin secretion that is reduced under lipotoxicity in INS1 cells and mouse islets. These results establish a causative role for impaired lysosomal acidification in the deregulation of autophagy and β-cell function under lipotoxicity.

Impact of non-alcoholic fatty pancreas disease on glucose metabolism

With the increasing global epidemic of obesity, the clinical importance of non-alcoholic fatty pancreas disease (NAFPD) has grown. Even though the pancreas might be more susceptible to ectopic fat deposition compared with the liver, NAFPD is rarely discussed because of the limitation of detection techniques. In the past, NAFPD was considered as an innocent condition or just part of clinical manifestations during the course of obesity. Recently, a growing body of research suggests that NAFPD might be associated with β-cell dysfunction, insulin resistance and inflammation, which possibly lead to the development of diabetes and metabolic syndrome. The present review summarized the current literature on the epidemiology, potential pathophysiology, diagnostic techniques, impact of NAFPD on β-cell function and insulin resistance, and the clinical relevance of the interplay between NAFPD and glucometabolic disorders.

Hepcidin links gluco-toxicity to pancreatic beta cell dysfunction by inhibiting Pdx-1 expression

OBJECTIVE

Gluco-toxicity is a term used to convey the detrimental effect of hyperglycemia on β-cell function through impaired insulin synthesis. Although it is known that the expression and activity of several key insulin transcription regulators is inhibited, other molecular mechanisms that mediate gluco-toxicity are poorly defined. Our objective was to explore the role of hepcidin in β-cell gluco-toxicity.

DESIGN

We first confirmed that high glucose levels inhibited hepcidin expression in the mouse insulinoma cell line, MIN6. The downregulation of hepcidin decreased Pdx-1 expression, which reduced insulin synthesis.

METHODS

MIN6 cells were exposed to high glucose concentrations (33.3 mmol/L). Glucose-stimulated insulin secretion (GSIS) and serum hepcidin levels were measured by ELISA. The mRNA levels of insulin1, insulin2, Pdx-1 and hepcidin were measured by real-time polymerase chain reaction. Western blot analysis was used to detect the changes in PDX-1 expression. Transient overexpression with hepcidin was used to reverse the downregulation of Pdx-1 and insulin synthesis induced by gluco-toxicity.

RESULTS

Exposure of MIN6 cells to high glucose significantly decreased GSIS and inhibited insulin synthesis as well as Pdx-1 transcriptional activity and expression at both the mRNA and protein levels. High glucose also decreased hepcidin expression and secretion. Hepcidin overexpression in MIN6 cells partially reversed the gluco-toxicity-induced downregulation of Pdx-1 and insulin expression and improved GSIS. The restoration of insulin synthesis by transfection of a hepcidin overexpression plasmid confirmed the role of hepcidin in mediating the gluco-toxic inhibition of insulin synthesis.

CONCLUSIONS

Our observations suggest that hepcidin is associated with gluco-toxicity-reduced pancreatic β-cell insulin synthesis in type 2 diabetes by inhibiting Pdx-1 expression.

Evidence for Loss in Identity, De-Differentiation, and Trans-Differentiation of Islet β-Cells in Type 2 Diabetes

The two main types of diabetes mellitus have distinct etiologies, yet a similar outcome: loss of islet β-cell function that is solely responsible for the secretion of the insulin hormone to reduce elevated plasma glucose toward euglycemic levels. Type 1 diabetes (T1D) has traditionally been characterized by autoimmune-mediated β-cell death leading to insulin-dependence, whereas type 2 diabetes (T2D) has hallmarks of peripheral insulin resistance, β-cell dysfunction, and cell death. However, a growing body of evidence suggests that, especially during T2D, key components of β-cell failure involves: (1) loss of cell identity, specifically proteins associated with mature cell function (e.g., insulin and transcription factors like MAFA, PDX1, and NKX6.1), as well as (2) de-differentiation, defined by regression to a progenitor or stem cell-like state. New technologies have allowed the field to compare islet cell characteristics from normal human donors to those under pathophysiological conditions by single cell RNA-Sequencing and through epigenetic analysis. This has revealed a remarkable level of heterogeneity among histologically defined "insulin-positive" β-cells. These results not only suggest that these β-cell subsets have different responses to insulin secretagogues, but that defining their unique gene expression and epigenetic modification profiles will offer opportunities to develop cellular therapeutics to enrich/maintain certain subsets for correcting pathological glucose levels. In this review, we will summarize the recent literature describing how β-cell heterogeneity and plasticity may be influenced in T2D, and various possible avenues of therapeutic intervention.