Islet amyloid toxicity: From genesis to counteracting mechanisms.

Islet amyloid polypeptide (IAPP or amylin) is a hormone co-secreted with insulin by pancreatic ß-cells and is the major component of islet amyloid. Islet amyloid is found in the pancreas of patients with type 2 diabetes (T2D) and may be involved in ß-cell dysfunction and death, observed in this disease. Thus, investigating the aspects related to amyloid formation is relevant to the development of strategies towards ß-cell protection. In this sense, IAPP misprocessing, IAPP overproduction, and disturbances in intra- and extracellular environments seem to be decisive for IAPP to form islet amyloid. Islet amyloid toxicity in ß-cells may be triggered in intra- and/or extracellular sites by membrane damage, endoplasmic reticulum stress, autophagy disruption, mitochondrial dysfunction, inflammation, and apoptosis. Importantly, different approaches have been suggested to prevent islet amyloid cytotoxicity, from inhibition of IAPP aggregation to attenuation of cell death mechanisms. Such approaches have improved ß-cell function and prevented the development of hyperglycemia in animals. Therefore, counteracting islet amyloid may be a promising therapy for T2D treatment.

Paternal Exercise Improves the Metabolic Health of Offspring via Epigenetic Modulation of the Germline.

BACKGROUND/AIMS: Epigenetic regulation is considered the main molecular mechanism underlying the developmental origin of health and disease's (DOHAD) hypothesis. Previous studies that have investigated the role of paternal exercise on the metabolic health of the offspring did not control for the amount and intensity of the training or possible effects of adaptation to exercise and produced conflicting results regarding the benefits of parental exercise to the next generation. We employed a precisely regulated exercise regimen to study the transgenerational inheritance of improved metabolic health. METHODS: We subjected male mice to a well-controlled exercise -training program to investigate the effects of paternal exercise on glucose tolerance and insulin sensitivity in their adult progeny. To investigate the molecular mechanisms of epigenetic inheritance, we determined chromatin markers in the skeletal muscle of the offspring and the paternal sperm. RESULTS: Offspring of trained male mice exhibited improved glucose homeostasis and insulin sensitivity. Paternal exercise modulated the DNA methylation profile of PI3Kca and the imprinted H19/Igf2 locus at specific differentially methylated regions (DMRs) in the skeletal muscle of the offspring, which affected their gene expression. Remarkably, a similar DNA methylation profile at the PI3Kca, H19, and Igf2 genes was present in the progenitor sperm indicating that exercise-induced epigenetic changes that occurred during germ cell development contributed to transgenerational transmission. CONCLUSION: Paternal exercise might be considered as a strategy that could promote metabolic health in the offspring as the benefits can be inherited transgenerationally.

Mice born to females with oocyte-specific deletion of mitofusin 2 have increased weight gain and impaired glucose homeostasis.

Offspring born to obese and diabetic mothers are prone to metabolic diseases, a phenotype that has been linked to mitochondrial dysfunction and endoplasmic reticulum (ER) stress in oocytes. In addition, metabolic diseases impact the architecture and function of mitochondria-ER contact sites (MERCs), changes which associate with mitofusin 2 (MFN2) repression in muscle, liver and hypothalamic neurons. MFN2 is a potent modulator of mitochondrial metabolism and insulin signaling, with a key role in mitochondrial dynamics and tethering with the ER. Here, we investigated whether offspring born to mice with MFN2-deficient oocytes are prone to obesity and diabetes. Deletion of Mfn2 in oocytes resulted in a profound transcriptomic change, with evidence of impaired mitochondrial and ER function. Moreover, offspring born to females with oocyte-specific deletion of Mfn2 presented increased weight gain and glucose intolerance. This abnormal phenotype was linked to decreased insulinemia and defective insulin signaling, but not mitochondrial and ER defects in offspring liver and skeletal muscle. In conclusion, this study suggests a link between disrupted mitochondrial/ER function in oocytes and increased risk of metabolic diseases in the progeny. Future studies should determine whether MERC architecture and function are altered in oocytes from obese females, which might contribute toward transgenerational transmission of metabolic diseases.

Swimming training reduces glucose-amplifying pathway and cholinergic responses in islets from lean- and MSG-obese rats.

Here, we investigate the effects of exercise training on glucose- and cholinergic-induced insulin secretion in pancreatic islets from obese and lean rats. Male Wistar rats were treated with monosodium glutamate (MSG) for the first 5 days of life, while control (CON) rats received saline. At 21 days, the rats were divided into exercised (EXE) and sedentary (SED) groups. The EXE rats swam for 30 minutes, three times/week, for 10 weeks. After this, MSG-SED rats showed hyperglycaemia, hypertriglyceridaemia and hyperinsulinaemia. Besides, islets from MSG-SED rats exhibited increased glucose-stimulated insulin secretion (GSIS), followed by impaired glucose sensitivity, absence of glucose-amplifying pathway and weak cholinergic response. In contrast, adiposity, hyperinsulinaemia and hypertriglyceridaemia were reduced in MSG-EXE rats. Moreover, islets from MSG-EXE rats exhibited lower GSIS and improved islet glucose sensitivity, without restoration of the glucose-amplifying pathway or alteration in the weak cholinergic effect of these islets. In islets from CON-EXE rats we also observed reduced GSIS and absence of glucose-amplifying effects and an accentuated reduction in cholinergic insulinotropic responses, without effect on glucose sensitivity in pancreatic islets from this group. Neither obesity nor exercise modified Muscarinic Receptor 3 (M3R) immunocontent or its downstream pathways (PKC and PKA). Moreover, only CON-EXE showed increased GSIS in the presence of calcium blocker, Thapsigargin. In conclusion, swimming training reduces GSIS and cholinergic responsiveness in isolated pancreatic islets from lean and hypothalamic obese rats, which could be due to the inhibition of glucose-amplifying pathways.

Hyperinsulinemia is associated with increasing insulin secretion but not with decreasing insulin clearance in an age-related metabolic dysfunction mice model.

Human life expectancy is increasing faster lately and, consequently, the number of patients with age-related diseases such as type 2 diabetes (T2D) is rising every year. Cases of hyperinsulinemia have been extensively reported in elderly subjects and this alteration in blood insulin concentration is postulated to be a cause of insulin resistance, which in some cases triggers T2D onset. Thus, it is important to know the underlying mechanisms of age-dependent hyperinsulinemia to find new strategies to prevent T2D in elderly subjects. Two processes control blood insulin concentration: Insulin secretion by the endocrine portion of the pancreas and insulin clearance, which occurs mainly in the liver by the action of the insulin-degrading enzyme (IDE). Here, we demonstrated that 10-month-old mice (old) display increased body and fat pad weight, compared with 3-month-old mice (control), and these alterations were accompanied by glucose and insulin intolerance. We also confirm hyperinsulinemia in the old mice, which was related to increased insulin secretion but not to reduced insulin clearance. Although no changes in insulin clearance were observed, IDE activity was lower in the liver of old compared with the control mice. However, this decreased IDE activity was compensated by increased expression of IDE protein in the liver, thus explaining the similar insulin clearance observed in both groups. In conclusion, at the beginning of aging, 10-month-old mice do not display any alterations in insulin clearance. Therefore, hyperinsulinemia is initiated primarily due to a higher insulin secretion in the age-related metabolic dysfunction in mice.

Glucose intolerance in monosodium glutamate obesity is linked to hyperglucagonemia and insulin resistance in α cells.

Obesity predisposes to glucose intolerance and type 2 diabetes (T2D). This disease is often characterized by insulin resistance, changes in insulin clearance, and ß-cell dysfunction. However, studies indicate that, for T2D development, disruptions in glucagon physiology also occur. Herein, we investigated the involvement of glucagon in impaired glycemia control in monosodium glutamate (MSG)-obese mice. Male Swiss mice were subcutaneously injected daily, during the first 5 days after birth, with MSG (4 mg/g body weight [BW]) or saline (1.25 mg/g BW). At 90 days of age, MSG-obese mice were hyperglycemic, hyperinsulinemic, and hyperglucagonemic and had lost the capacity to increase their insulin/glucagon ratio when transitioning from the fasting to fed state, exacerbating hepatic glucose output. Furthermore, hepatic protein expressions of phosphorylated (p)-protein kinase A (PKA) and cAMP response element-binding protein (pCREB), and of phosphoenolpyruvate carboxykinase (PEPCK) enzyme were higher in fed MSG, before and after glucagon stimulation. Increased pPKA and phosphorylated hormone-sensitive lipase content were also observed in white fat of MSG. MSG islets hypersecreted glucagon in response to 11.1 and 0.5 mmol/L glucose, a phenomenon that persisted in the presence of insulin. Additionally, MSG α cells were hypertrophic displaying increased α-cell mass and immunoreactivity to phosphorylated mammalian target of rapamycin (pmTOR) protein. Therefore, severe glucose intolerance in MSG-obese mice was associated with increased hepatic glucose output, in association with hyperglucagonemia, caused by the refractory actions of glucose and insulin in α cells and via an effect that may be due to enhanced mTOR activation.

Exercise training protects human and rodent ß cells against endoplasmic reticulum stress and apoptosis.

Prolonged exercise has positive metabolic effects in obese or diabetic individuals. These effects are usually ascribed to improvements in insulin sensitivity. We evaluated whether exercise also generates circulating signals that protect human and rodent ß cells against endoplasmic reticulum (ER) stress and apoptosis. For this purpose, we obtained serum from humans or mice before and after an 8 wk training period. Exposure of human islets or mouse or rat ß cells to human or rodent sera, respectively, obtained from trained individuals reduced cytokine (IL-1ß+IFN-γ)- or chemical ER stressor-induced ß-cell ER stress and apoptosis, at least in part via activation of the transcription factor STAT3. These findings indicate that exercise training improves human and rodent ß-cell survival under diabetogenic conditions and support lifestyle interventions as a protective approach for both type 1 and 2 diabetes.-Paula, F. M. M., Leite, N. C., Borck, P. C., Freitas-Dias, R., Cnop, M., Chacon-Mikahil, M. P. T., Cavaglieri, C. R., Marchetti, P., Boschero, A. C., Zoppi, C. C., Eizirik, D. L. Exercise training protects human and rodent ß cells against endoplasmic reticulum stress and apoptosis.

ARHGAP21 as a master regulator of multiple cellular processes.

The cellular cytoskeleton is involved with multiple biological processes and is tightly regulated by multiple proteins and effectors. Among these, the RhoGTPases family is one of the most important players. RhoGTPAses are, in turn, regulated by many other elements. In the past decade, one of those regulators, the RhoGAP Rho GTPase Activating Protein 21 (ARHGAP21), has been overlooked, despite being implied as having an important role on many of those processes. In this paper, we aimed to review the available literature regarding ARHGAP21 to highlight its importance and the mechanisms of action that have been found so far for this still unknown protein involved with cell adhesion, migration, Golgi regulation, cell trafficking, and even insulin secretion.

Whole body ARHGAP21 reduction improves glucose homeostasis in high-fat diet obese mice.

GTPase activating proteins (GAPs) are ubiquitously expressed, and their role in cellular adhesion and membrane traffic processes have been well described. TBC1D1, which is a Rab-GAP, is necessary for adequate glucose uptake by muscle cells, whereas increased TCGAP, which is a Rho-GAP, decreases GLUT4 translocation, and consequently glucose uptake in adipocytes. Here, we assessed the possible involvement of ARHGAP21, a Rho-GAP protein, in glucose homeostasis. For this purpose, wild type mice and ARHGAP21 transgenic whole-body gene-deficiency mice (heterozygous mice, expressing approximately 50% of ARHGAP21) were fed either chow (Ctl and Het) or high-fat diet (Ctl-HFD and Het-HFD). Het-HFD mice showed a reduction in white fat storage, reflected in a lower body weight gain. These mice also displayed an improvement in insulin sensitivity and glucose tolerance, which likely contributed to reduced insulin secretion and pancreatic beta cell area. The reduction of body weight was also observed in Het mice and this phenomenon was associated with an increase in brown adipose tissue and reduced muscle weight, without alteration in glucose-insulin homeostasis. In conclusion, the whole body ARHGAP21 reduction improved glucose homeostasis and protected against diet-induced obesity specifically in Het-HFD mice. However, the mechanism by which ARHGAP21 leads to these outcomes requires further investigation.

Reduced glucose-induced insulin secretion in low-protein-fed rats is associated with altered pancreatic islets redox status.

In the present study, we investigated the relationship between early life protein malnutrition-induced redox imbalance, and reduced glucose-stimulated insulin secretion. After weaning, male Wistar rats were submitted to a normal-protein-diet (17%-protein, NP) or to a low-protein-diet (6%-protein, LP) for 60 days. Pancreatic islets were isolated and hydrogen peroxide (H2 O2 ), oxidized (GSSG) and reduced (GSH) glutathione content, CuZn-superoxide dismutase (SOD1), glutathione peroxidase (GPx1) and catalase (CAT) gene expression, as well as enzymatic antioxidant activities were quantified. Islets that were pre-incubated with H2 O2 and/or N-acetylcysteine, were subsequently incubated with glucose for insulin secretion measurement. Protein malnutrition increased CAT mRNA content by 100%. LP group SOD1 and CAT activities were 50% increased and reduced, respectively. H2 O2 production was more than 50% increased whereas GSH/GSSG ratio was near 60% lower in LP group. Insulin secretion was, in most conditions, approximately 50% lower in LP rat islets. When islets were pre-incubated with H2 O2 (100 µM), and incubated with glucose (33 mM), LP rats showed significant decrease of insulin secretion. This effect was attenuated when LP islets were exposed to N-acetylcysteine.

Role of microRNAs on the Regulation of Mitochondrial Biogenesis and Insulin Signaling in Skeletal Muscle.

Mitochondria play a critical role in several cellular processes and cellular homeostasis. Mitochondrion dysfunction has been correlated with numerous metabolic diseases such as obesity and type 2 diabetes. MicroRNAs are non-coding RNAs that have emerged as key regulators of cell metabolism. The microRNAs act as central regulators of metabolic gene networks by leading to the degradation of their target messenger RNA or repression of protein translation. In addition, vesicular and non-vesicular circulating miRNAs exhibit a potential role as mediators of the cross-talk between the skeletal muscle and other tissues/organs. In this review, we will focus on the emerging knowledge of miRNAs controlling mitochondrial function and insulin signaling in skeletal muscle cells. J. Cell. Physiol. 232: 958-966, 2017. © 2016 Wiley Periodicals, Inc.

Benefits of L-alanine or L-arginine supplementation against adiposity and glucose intolerance in monosodium glutamate-induced obesity.

PURPOSE: L-alanine (Ala) and L-arginine (Arg) have been reported to regulate pancreatic ß-cell physiology and to prevent body fat accumulation in diet-induced obesity. Here, we assessed growth and adiposity parameters, glucose tolerance, insulin secretion and the expression of insulin and nutrient-regulated proteins in monosodium glutamate (MSG)-obese mice supplemented with either Ala or Arg. METHODS: Male newborn C57Bl/6 mice received a daily subcutaneous injection of MSG or saline solution (CTL group), during the first 6 days of life. From 30 to 90 days of age, MSG and CTL mice received or not 2.55 % Ala (CAla or MArg groups) or 1.51 % Arg-HCl (CArg or MArg groups) in their drinking water. RESULTS: Adult MSG mice displayed higher adiposity associated with lower phosphorylation of the adipogenic enzyme, ACC, in adipose tissue. Glucose intolerance in MSG mice was linked to lower insulin secretion and to lower expression of IRß in adipose tissue, as well as AS160 phosphorylation in skeletal muscle. Perigonadal fat depots were smaller in Ala and Arg mice, while retroperitoneal fat pads were decreased by Ala supplementation only. Both Ala and Arg improved fed-state glycemia as well as IRß and pAS160 content, but only Ala led to improved glucose tolerance and insulin secretion. Adipostatic signals were increased in MAla mice, as indicated by enhanced AMPK phosphorylation and pACC content in fat depots. CONCLUSIONS: Ala supplementation led to more pronounced metabolic improvements compared to Arg, possibly due to suppression of lipogenesis through activation of the AMPK/ACC pathway.

Exercise increases pancreatic ß-cell viability in a model of type 1 diabetes through IL-6 signaling.

Type 1 diabetes (T1D) is provoked by an autoimmune assault against pancreatic ß cells. Exercise training enhances ß-cell mass in T1D. Here, we investigated how exercise signals ß cells in T1D condition. For this, we used several approaches. Wild-type and IL-6 knockout (KO) C57BL/6 mice were exercised. Afterward, islets from control and trained mice were exposed to inflammatory cytokines (IL-1ß plus IFN-γ). Islets from control mice and ß-cell lines (INS-1E and MIN6) were incubated with serum from control or trained mice or medium obtained from 5-aminoimidazole-4 carboxamide1-ß-d-ribofuranoside (AICAR)-treated C2C12 skeletal muscle cells. Subsequently, islets and ß cells were exposed to IL-1ß plus IFN-γ. Proteins were assessed by immunoblotting, apoptosis was determined by DNA-binding dye propidium iodide fluorescence, and NO(•) was estimated by nitrite. Exercise reduced 25, 75, and 50% of the IL-1ß plus IFN-γ-induced iNOS, nitrite, and cleaved caspase-3 content, respectively, in pancreatic islets. Serum from trained mice and medium from AICAR-treated C2C12 cells reduced ß-cell death, induced by IL-1ß plus IFN-γ treatment, in 15 and 38%, respectively. This effect was lost in samples treated with IL-6 inhibitor or with serum from exercised IL-6 KO mice. In conclusion, muscle contraction signals ß-cell survival in T1D through IL-6.

Taurine supplementation ameliorates glucose homeostasis, prevents insulin and glucagon hypersecretion, and controls ß, α, and δ-cell masses in genetic obese mice.

Taurine (Tau) regulates ß-cell function and glucose homeostasis under normal and diabetic conditions. Here, we assessed the effects of Tau supplementation upon glucose homeostasis and the morphophysiology of endocrine pancreas, in leptin-deficient obese (ob) mice. From weaning until 90-day-old, C57Bl/6 and ob mice received, or not, 5% Tau in drinking water (C, CT, ob and obT). Obese mice were hyperglycemic, glucose intolerant, insulin resistant, and exhibited higher hepatic glucose output. Tau supplementation did not prevent obesity, but ameliorated glucose homeostasis in obT. Islets from ob mice presented a higher glucose-induced intracellular Ca(2+) influx, NAD(P)H production and insulin release. Furthermore, α-cells from ob islets displayed a higher oscillatory Ca(2+) profile at low glucose concentrations, in association with glucagon hypersecretion. In Tau-supplemented ob mice, insulin and glucagon secretion was attenuated, while Ca(2+) influx tended to be normalized in ß-cells and Ca(2+) oscillations were increased in α-cells. Tau normalized the inhibitory action of somatostatin (SST) upon insulin release in the obT group. In these islets, expression of the glucagon, GLUT-2 and TRPM5 genes was also restored. Tau also enhanced MafA, Ngn3 and NeuroD mRNA levels in obT islets. Morphometric analysis demonstrated that the hypertrophy of ob islets tends to be normalized by Tau with reductions in islet and ß-cell masses, but enhanced δ-cell mass in obT. Our results indicate that Tau improves glucose homeostasis, regulating ß-, α-, and δ-cell morphophysiology in ob mice, indicating that Tau may be a potential therapeutic tool for the preservation of endocrine pancreatic function in obesity and diabetes.

Taurine supplementation preserves hypothalamic leptin action in normal and protein-restricted mice fed on a high-fat diet.

Malnutrition programs the neuroendocrine axis by disruption of food-intake control, leading to obesity. Taurine (Tau) is neuroprotective and improves anorexigenic actions in the hypothalamus. We evaluated the hypothalamic gene-expression profile and food-intake control in protein-restricted mice submitted to a high-fat diet (HFD) and Tau supplementation. Mice were fed on a control (14 % protein-C) or a protein-restricted diet (6 % protein-R) for 6 weeks. Thereafter, mice received, or not, HFD for 8 weeks (CH and RH) with or without 5 % Tau supplementation (CHT and RHT). Protein restriction led to higher food intake, but calories were matched to controls. Excessive calorie intake occurred in HFD mice and this was prevented by Tau supplementation only in the CH group. Additionally, RH and CH mice developed hypothalamic leptin resistance, which was prevented by Tau. Global alterations in the expressions of genes involved in hypothalamic metabolism, cellular defense, apoptosis and endoplasmic reticulum stress pathways were induced by dietary manipulations and Tau treatment. The orexigenic peptides NPY and AgRP were increased by protein restriction and lowered by the HFD. The anorexigenic peptide Pomc was increased by HFD, and this was prevented by Tau only in CH mice. Thus, food intake was disrupted by dietary protein restriction and obesity. HFD-induced alterations were not enhanced by previous protein deficiency, but the some beneficial effects of Tau supplementation upon food intake were blunted by protein restriction. Tau effects upon feeding behavior control are complex and involve interactions with a vast gene network, preventing hypothalamic leptin resistance.

Leucine supplementation does not affect protein turnover and impairs the beneficial effects of endurance training on glucose homeostasis in healthy mice.

Endurance exercise training as well as leucine supplementation modulates glucose homeostasis and protein turnover in mammals. Here, we analyze whether leucine supplementation alters the effects of endurance exercise on these parameters in healthy mice. Mice were distributed into sedentary (C) and exercise (T) groups. The exercise group performed a 12-week swimming protocol. Half of the C and T mice, designated as the CL and TL groups, were supplemented with leucine (1.5 % dissolved in the drinking water) throughout the experiment. As well known, endurance exercise training reduced body weight and the retroperitoneal fat pad, increased soleus mass, increased VO2max, decreased muscle proteolysis, and ameliorated peripheral insulin sensitivity. Leucine supplementation had no effect on any of these parameters and worsened glucose tolerance in both CL and TL mice. In the soleus muscle of the T group, AS-160(Thr-642) (AKT substrate of 160 kDa) and AMPK(Thr-172) (AMP-Activated Protein Kinase) phosphorylation was increased by exercise in both basal and insulin-stimulated conditions, but it was reduced in TL mice with insulin stimulation compared with the T group. Akt phosphorylation was not affected by exercise but was lower in the CL group compared with the other groups. Leucine supplementation increased mTOR phosphorylation at basal conditions, whereas exercise reduced it in the presence of insulin, despite no alterations in protein synthesis. In trained groups, the total FoxO3a protein content and the mRNA for the specific isoforms E2 and E3 ligases were reduced. In conclusion, leucine supplementation did not potentiate the effects of endurance training on protein turnover, and it also reduced its positive effects on glucose homeostasis.

Short-term low-protein diet during pregnancy alters islet area and protein content of phosphatidylinositol 3-kinase pathway in rats.

The phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways mediate ß cell growth, proliferation, survival and death. We investigated whether protein restriction during pregnancy alters islet morphometry or the expression and phosphorylation of several proteins involved in the phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways. As controls, adult pregnant and non-pregnant rats were fed a normal-protein diet (17%). Pregnant and non-pregnant rats in the experimental groups were fed a low-protein diet (6%) for 15 days. Low protein diet during pregnancy increased serum prolactin level, reduced serum corticosterone concentration and the expression of both protein kinase B/AKT1 (AKT1) and p70 ribosomal protein S6 kinase (p70S6K), as well as the islets area, but did not alter the insulin content of pancreatic islets. Pregnancy increased the expression of the Src homology/collagen (SHC) protein and the extracellular signal-regulated kinases 1/2 (ERK1/2) independent of diet. ERK1/2 phosphorylation (pERK1/2) was similar in islets from pregnant and non-pregnant rats fed a low-protein diet, and was higher in islets from pregnant rats than in islets from non-pregnant rats fed a normal-protein diet. Thus, a short-term, low-protein diet during pregnancy was sufficient to reduce the levels of proteins in the phosphatidylinositol 3-kinase pathway and affect islet morphometry.

Impaired compensatory beta-cell function and growth in response to high-fat diet in LDL receptor knockout mice.

In this study, we investigated the effect of low density lipoprotein receptor (LDLr) deficiency on gap junctional connexin 36 (Cx36) islet content and on the functional and growth response of pancreatic beta-cells in C57BL/6 mice fed a high-fat (HF) diet. After 60 days on regular or HF diet, the metabolic state and morphometric islet parameters of wild-type (WT) and LDLr-/- mice were assessed. HF diet-fed WT animals became obese and hypercholesterolaemic as well as hyperglycaemic, hyperinsulinaemic, glucose intolerant and insulin resistant, characterizing them as prediabetic. Also they showed a significant decrease in beta-cell secretory response to glucose. Overall, LDLr-/- mice displayed greater susceptibility to HF diet as judged by their marked cholesterolaemia, intolerance to glucose and pronounced decrease in glucose-stimulated insulin secretion. HF diet induced similarly in WT and LDLr-/- mice, a significant decrease in Cx36 beta-cell content as revealed by immunoblotting. Prediabetic WT mice displayed marked increase in beta-cell mass mainly due to beta-cell hypertrophy/replication. Nevertheless, HF diet-fed LDLr-/- mice showed no significant changes in beta-cell mass, but lower islet-duct association (neogenesis) and higher beta-cell apoptosis index were seen as compared to controls. The higher metabolic susceptibility to HF diet of LDLr-/- mice may be explained by a deficiency in insulin secretory response to glucose associated with lack of compensatory beta-cell expansion.

Taurine supplementation increases K(ATP) channel protein content, improving Ca2+ handling and insulin secretion in islets from malnourished mice fed on a high-fat diet.

Pancreatic ß-cells are highly sensitive to suboptimal or excess nutrients, as occurs in protein-malnutrition and obesity. Taurine (Tau) improves insulin secretion in response to nutrients and depolarizing agents. Here, we assessed the expression and function of Cav and KATP channels in islets from malnourished mice fed on a high-fat diet (HFD) and supplemented with Tau. Weaned mice received a normal (C) or a low-protein diet (R) for 6 weeks. Half of each group were fed a HFD for 8 weeks without (CH, RH) or with 5% Tau since weaning (CHT, RHT). Isolated islets from R mice showed lower insulin release with glucose and depolarizing stimuli. In CH islets, insulin secretion was increased and this was associated with enhanced KATP inhibition and Cav activity. RH islets secreted less insulin at high K(+) concentration and showed enhanced KATP activity. Tau supplementation normalized K(+)-induced secretion and enhanced glucose-induced Ca(2+) influx in RHT islets. R islets presented lower Ca(2+) influx in response to tolbutamide, and higher protein content and activity of the Kir6.2 subunit of the KATP. Tau increased the protein content of the α1.2 subunit of the Cav channels and the SNARE proteins SNAP-25 and Synt-1 in CHT islets, whereas in RHT, Kir6.2 and Synt-1 proteins were increased. In conclusion, impaired islet function in R islets is related to higher content and activity of the KATP channels. Tau treatment enhanced RHT islet secretory capacity by improving the protein expression and inhibition of the KATP channels and enhancing Synt-1 islet content.

Taurine-induced insulin signalling improvement of obese malnourished mice is associated with redox balance and protein phosphatases activity modulation.

BACKGROUND & AIMS: Obese protein malnourished mice display liver insulin resistance and taurine (TAU) seems to attenuate this effect. The association between early-life malnutrition and hepatic redox balance in diet-induced insulin resistance is unknown. We investigated TAU supplementation effects upon liver redox state and insulin signalling in obese protein malnourished mice. METHODS: Weaned male C57BL-6 mice were fed a control (14% protein - C) or a protein-restricted diet (6% protein - R) for 6 weeks. Afterwards, mice received a high-fat diet (34% fat - HFD) for 8 weeks (CH - RH). Half of the HFD-mice were supplemented with TAU (5%) throughout the treatment (CHT - RHT). Body and tissues' weight, respiratory quotient (RQ), glucose tolerance and insulin sensitivity, hepatic oxidant and antioxidant markers and insulin cascade proteins were assessed. RESULTS: Protein restriction leads to typical features whereas HFD was able to induce a catch-up growth in RH. HFD-groups showed higher energy intake and adiposity, lower energy expenditure and altered RQ. Glucose tolerance and insulin sensitivity were impaired in HFD-groups and TAU attenuated these effects. H2 O2 content was increased in CHT and RHT despite no differences in antioxidant enzymes and GSH concentration. AKT and PTEN phosphorylation were significantly increased in CHT but not in RHT. CONCLUSION: Our data provide evidence for an association between TAU-induced improved glycaemic control because of PTEN inactivation and higher AKT phosphorylation. These effects seem to be related with altered hepatic redox balance in obese mice, and this effect is impaired by protein malnutrition.