Glucose concentration and AMP-dependent kinase activation regulate expression of insulin receptor family members in rat islets and INS-1E beta cells

Analysis of miRNA and mRNA expression in the dysregulation of insulin secretion in MIN6 cells exposed to microcystin-leucine-arginine

Microcystin -leucine-arginine (MC-LR), produced by freshwater cyanobacteria, is a potential pancreatic β-cell toxin. In this study, the function of the mouse pancreatic β-cell line, MIN6, was evaluated after MC-LR exposure, and the underlying molecular mechanisms were explored. Exposure to MC-LR for 24 h was found to inhibit cell viability and impair insulin secretion. Such findings indicate that β-cell function would be impaired following MC-LR treatment. The microarray results revealed altered miRNA and mRNA expression profiles that might be responsible for the abnormal function of MIN6 cells. Further, miRNA-gene network analysis demonstrated that miR-29b-3p, miR-6967-5p, miR-3473, miR-7061-5p, Xkr4, Tmem178b, Scp2, Ypel2, and Kcnj11 are key miRNAs and genes in the MC-LR-induced MIN6-cell toxicity. The altered expression levels of several miRNAs (e.g., miR-320-5p, miR-770-5p, miR-99a-3p, and miR-375-5p) and genes (e.g., Pklr and Gpd2) involved in insulin secretion or the onset of diabetes were also identified in MIN6 cells after treatment with MC-LR. Collectively, these findings provide evidence of the toxic effects of MC-LR on β-cells and the underlying molecular mechanisms of its glycometabolism toxicity. MCs may thus possibly play an important role in the development of diabetes mellitus in humans.

Identification of functional candidate variants and genes for feed efficiency in Holstein and Jersey cattle breeds using RNA-sequencing

The identification of functional genetic variants and associated candidate genes linked to feed efficiency may help improve selection for feed efficiency in dairy cattle, providing economic and environmental benefits for the dairy industry. This study used RNA-sequencing data obtained from liver tissue from 9 Holstein cows [n = 5 low residual feed intake (RFI), n = 4 high RFI] and 10 Jersey cows (n = 5 low RFI, n = 5 high RFI), which were selected from a single population of 200 animals. Using RNA-sequencing, 3 analyses were performed to identify: (1) variants within low or high RFI Holstein cattle; (2) variants within low or high RFI Jersey cattle; and (3) variants within low or high RFI groups, which are common across both Holstein and Jersey cattle breeds. From each analysis, all variants were filtered for moderate, modifier, or high functional effect, and co-localized quantitative trait loci (QTL) classes, enriched biological processes, and co-localized genes related to these variants, were identified. The overlapping of the resulting genes co-localized with functional SNP from each analysis in both breeds for low or high RFI groups were compared. For the first two analyses, the total number of candidate genes associated with moderate, modifier, or high functional effect variants fixed within low or high RFI groups were 2,810 and 3,390 for Holstein and Jersey breeds, respectively. The major QTL classes co-localized with these variants included milk and reproduction QTL for the Holstein breed, and milk, production, and reproduction QTL for the Jersey breed. For the third analysis, the common variants across both Holstein and Jersey breeds, uniquely fixed within low or high RFI groups were identified, revealing a total of 86,209 and 111,126 functional variants in low and high RFI groups, respectively. Across all 3 analyses for low and high RFI cattle, 12 and 31 co-localized genes were overlapping, respectively. Among the overlapping genes across breeds, 9 were commonly detected in both the low and high RFI groups (INSRR, CSK, DYNC1H1, GAB1, KAT2B, RXRA, SHC1, TRRAP, PIK3CB), which are known to play a key role in the regulation of biological processes that have high metabolic demand and are related to cell growth and regeneration, metabolism, and immune function. The genes identified and their associated functional variants may serve as candidate genetic markers and can be implemented into breeding programs to help improve the selection for feed efficiency in dairy cattle.

Microwave-assisted synthesis of chitosan biguanidine hydrochloride and its regulation on InsR and GLUT2 in insulin resistant HepG2 cells

CSGH increased glucose consumption of HepG2 cells by accelerating the expression of InsR and inhibiting the high-insulin-induced over-expression of GLUT2.

Role of AMPK α in skeletal muscle glycometabolism regulation and adaptation in relation to sepsis

Background. AMP-activated protein kinase (AMPK) and the translocation of glucose transporter 4 (GLUT4) protein always involve disturbance of carbohydrate metabolism. Objective. To determine whether the change of blood glucose in the early stage of septic rat is associated with the alteration of AMPKα protein expression and GLUT4 protein translocation expression. Methods. Animal models of sepsis were induced by tail vein injection of LPS in Wistar rats. The dynamic values of blood glucose within 2 hours after injection of LPS were observed. AMPKα protein and GLUT4 protein translocation in different tissues (such as soleus muscle and extensor digitorum longus) were assessed by western blot. Results. Blood glucose levels appeared to rise at 0.5 h after injection of LPS, arrived the peak value at 1 h, then fell at 1.5 h and 2 h Animals in LPS group experienced the increase of phos-AMPKα protein and GLUT4 protein translocation expression in soleus muscle and extensor digitorum longus. Conclusion. The dynamic change of blood glucose, represented in a form of initiative increase and subsequent decrease in the early stage of sepsis, may be related to glycometabolism disorder in the skeletal muscle, coming down to enhancement of GLUT4 translocation expression promoted by activation of AMPKα.

Distinct cellular origins for serotonin-expressing and enterochromaffin-like cells in the gastric corpus

BACKGROUND & AIMS

The alimentary tract contains a diffuse endocrine system comprising enteroendocrine cells that secrete peptides or biogenic amines to regulate digestion, insulin secretion, food intake, and energy homeostasis. Lineage analysis in the stomach revealed that a significant fraction of endocrine cells in the gastric corpus did not arise from Neurogenin3 (Neurog3)-expressing cells, unlike enteroendocrine cells elsewhere in the digestive tract. We aimed to isolate enriched serotonin-secreting and enterochromaffin-like (ECL) cells from the stomach and to clarify their cellular origin.

METHODS

We used Neurogenic differentiation 1 (NeuroD1) and Neurog3 lineage analysis and examined the differentiation of serotonin-producing and ECL cells in stomach tissues of NeuroD1-cre;ROSA(tdTom), tryptophan hydroxylase 1 (Tph1)-cyan fluorescent protein (CFP), c-Kit(wsh/wsh), and Neurog3Cre;ROSA(tdTom) mice by immunohistochemistry. We used fluorescence-activated cell sorting to isolate each cell type for gene expression analysis. We also performed RNA sequencing analysis of ECL cells.

RESULTS

Neither serotonin-secreting nor ECL cells of the corpus arose from cells expressing NeuroD1. Serotonin-secreting cells expressed a number of mast cell genes but not genes associated with endocrine differentiation; they did not develop in c-Kit(wsh/wsh) mice and were labeled with transplanted bone marrow cells. RNA sequencing analysis of ECL cells revealed high expression levels of many genes common to endocrine cells, including transcription factors, hormones, ion channels, and solute transporters but not markers of bone marrow cells.

CONCLUSIONS

Serotonin-expressing cells of the gastric corpus of mice appear to be bone marrow-derived mucosal mast cells. Gene expression analysis of ECL cells indicated that they are endocrine cells of epithelial origin that do not express the same transcription factors as their intestinal enteroendocrine cell counterparts.

Vaspin inhibits kallikrein 7 by serpin mechanism

The molecular target of the adipokine vaspin (visceral adipose tissue-derived serpin; serpinA12) and its mode of action are unknown. Here, we provide the vaspin crystal structure and identify human kallikrein 7 (hK7) as a first protease target of vaspin inhibited by classical serpin mechanism with high specificity in vitro. We detect vaspin-hK7 complexes in human plasma and find co-expression of both proteins in murine pancreatic β-cells. We further demonstrate that hK7 cleaves human insulin in the A- and B-chain. Vaspin treatment of isolated pancreatic islets leads to increased insulin concentration in the media upon glucose stimulation without influencing insulin secretion. By application of vaspin and generated inactive mutants, we find the significantly improved glucose tolerance in C57BL/6NTac and db/db mice treated with recombinant vaspin fully dependent on the vaspin serpin activity and not related to vaspin-mediated changes in insulin sensitivity as determined by euglycemic-hyperinsulinemic clamp studies. Improved glucose metabolism could be mediated by increased insulin plasma concentrations 150 min after a glucose challenge in db/db mice, supporting the hypothesis that vaspin may inhibit insulin degradation by hK7 in the circulation. In conclusion, we demonstrate the inhibitory serpin nature and the first protease target of the adipose tissue-derived serpin vaspin, and our findings suggest hK7 inhibition by vaspin as an underlying physiological mechanism for its compensatory actions on obesity-induced insulin resistance.

4-Methlycatechol prevents NGF/p75(NTR)-mediated apoptosis via NGF/TrkA system in pancreatic β cells

In this study, it was aimed to investigate whether 4-methylcatechol (4-MC) could serve as an autocrine antiapoptotic agent by increasing nerve growth factor (NGF) in β cells of hyperglycemic rats. Rats were divided into four groups: the first group was given citrate buffer and saline, the second group was administered 4-MC, the third group received streptozotocin (STZ), and the fourth group was given both 4-MC and STZ. 4-MC (10 μg/kg) was administered by daily intraperitoneal injection for 10 days before the animals were rendered hyperglycemic by administration of STZ (75 mg/kg). With 4-MC pretreatment on hyperglycemic rats the following results were noted: (i) Increase in plasma glucose, β cell apoptosis and caspase-8 activation was prevented. (ii) Reduction of NGF+ and tyrosine receptor kinase A (TrkA)+ β cell number was blocked. (iii) p75 neurotrophin receptor (p75(NTR))+ β cell number was increased. These data suggest that 4-MC might exert its antiapoptotic actions through NGF/TrkA system which may block NGF/p75(NTR) activation in pancreatic β cells of hyperglycemic rats.

Activation of Erk1/2 phosphorylation but not of Akt/Pkb through an inducible CSF1R/IRR-receptor construct in INS-1E beta-cells

CONTEXT

The insulin receptor-related receptor (IRR) is an orphan receptor belonging to the insulin receptor (IR) family. Despite its unknown function, the specific tissue expression and the high sequence homology with the IR and the insulin-like growth factor 1 receptor (IGF1R) suggest a biological role in beta-cells.

OBJECTIVES

In this study we investigated the influence of a stimulatable IRR-tyrosine kinase on major IR/IGF1R signaling pathways and on proliferation and apoptosis of INS-1E beta-cells.

METHODS

INS-1E cells were stably transfected with a colony stimulating factor 1 receptor (CSF1R)/IRR construct activated by a macrophage colony stimulating factor.

RESULTS AND CONCLUSION

After stimulation the construct showed time and dose dependent autophosphorylation and transient extracellular signal regulated kinase 1/2 activation. Protein kinase b was not phosphorylated and also an effect on proliferation and apoptosis of INS-1E could not be demonstrated. Thus, no obvious biologic function of the IRR is present in INS-1E beta-cells.

AMP-activated protein kinase and pancreatic/duodenal homeobox-1 involved in insulin secretion under high leucine exposure in rat insulinoma beta-cells

The effect of leucine on glucose-stimulated insulin secretion (GSIS) in pancreatic beta-cells is quite controversial, and mechanism involved in the effect has not been elucidated yet. Consequently, we aimed to investigate effect of leucine on GSIS and its mechanism focusing on contribution of AMP-activated protein kinase (AMPK) and pancreatic/duodenal homeobox-1 (PDX-1). Rat insulinoma beta-cells (INS-1, RIN m5F, DN-PDX-1#28 and PDX-1#6) were cultured with or without leucine, AICAR (AMPK agonist) or compound C (AMPK antagonist) for 48 hrs. In contrast to control, AICAR treatment decreased GSIS at high glucose and insulin content, also impaired protein and mRNA expression of PDX-1 and its downstream targets, glucokinase (GCK) and glucose transporter 2 (GLUT2). Compound C treatment had the opposite effects. We observed that neither AICAR nor compound C could affect expression of GCK and GLUT2 when PDX-1 expression was absent. Chronic leucine exposure inhibited GSIS at high glucose and insulin content in a dose-dependent manner, concomitant with an increase in AMPK and a decrease in PDX-1, GCK and GLUT2. The inhibitory effects of leucine was potentiated by AICAR treatment and rescued by compound C treatment. Finally, the inhibition of PDX-1 could potentiate the impaired effects induced by leucine whereas overexpression of PDX-1 could protect the cell from impairment induced by leucine. The study indicated that chronic leucine might result in an increase in AMPK and then a decrease in PDX-l, in turn to depress GCK and GLUT2 resulting in decreased GSIS at high glucose and insulin content.

Glycerolipid metabolism and signaling in health and disease

Maintenance of body temperature is achieved partly by modulating lipolysis by a network of complex regulatory mechanisms. Lipolysis is an integral part of the glycerolipid/free fatty acid (GL/FFA) cycle, which is the focus of this review, and we discuss the significance of this pathway in the regulation of many physiological processes besides thermogenesis. GL/FFA cycle is referred to as a "futile" cycle because it involves continuous formation and hydrolysis of GL with the release of heat, at the expense of ATP. However, we present evidence underscoring the "vital" cellular signaling roles of the GL/FFA cycle for many biological processes. Probably because of its importance in many cellular functions, GL/FFA cycling is under stringent control and is organized as several composite short substrate/product cycles where forward and backward reactions are catalyzed by separate enzymes. We believe that the renaissance of the GL/FFA cycle is timely, considering the emerging view that many of the neutral lipids are in fact key signaling molecules whose production is closely linked to GL/FFA cycling processes. The evidence supporting the view that alterations in GL/FFA cycling are involved in the pathogenesis of "fatal" conditions such as obesity, type 2 diabetes, and cancer is discussed. We also review the different enzymatic and transport steps that encompass the GL/FFA cycle leading to the generation of several metabolic signals possibly implicated in the regulation of biological processes ranging from energy homeostasis, insulin secretion and appetite control to aging and longevity. Finally, we present a perspective of the possible therapeutic implications of targeting this cycling.

Mutated ATP synthase induces oxidative stress and impaired insulin secretion in beta-cells of female BHE/cdb rats

BACKGROUND

Adenosine triphosphate (ATP) is a critical determinant of beta-cell insulin secretion in response to glucose. BHE/cdb rats have a mutation in ATP synthase that limits ATP production, yet develop mild diabetes only with ageing. We investigated the cellular basis for reduced insulin secretion and compensatory mechanisms that mitigate the effects of the ATP synthase mutation.

METHODS

In vitro beta-cell function in isolated islets and expression of key regulatory genes was compared with in vivo oral glucose tolerance and insulin sensitivity in BHE/cdb and control rats.

RESULTS

BHE/cdb rat islets had reduced responsiveness to glucose stimulation and ATP content was 35% lower than in control islets. Oral glucose tolerance was impaired at both 21 and 43 weeks of age because of a reduction in glucose-stimulated insulin secretion (GSIS). An increase in inducible nitric oxide synthase (INOS, 3-fold) and manganese superoxide dismutase (MnSOD, 1.6-fold), detection of nitrotyrosine, beta-cell apoptosis, and nucleocytoplasmic translocation of pancreas duodenum homeobox-1 (PDX-1) in beta-cells indicated increased oxygen radical formation. However, BHE/cdb rats partially compensated for low glucose responsiveness by increasing the number of small islets and beta-cell hypertrophy. There was also an increase in the proportion of mature insulin relative to proinsulin (PI) detected within beta-cell granules. Increased activation of AMP-dependent kinase (AMPK)-regulated pathways was consistent with increased oxidative stress and with induction of apoptosis and reduction of preproinsulin gene transcription.

CONCLUSIONS

The findings are consistent with impaired but partially compensated mechanisms of insulin secretion early in life, but progressive non-compensated impairments due to oxidative stress occurs by age 43 weeks.

Crosstalk between the AMP-activated kinase and insulin signaling pathways rescues murine blastocyst cells from insulin resistance

Maternal insulin resistance results in poor pregnancy outcomes. In vivo and in vitro exposure of the murine blastocyst to high insulin or IGF1 results in the down-regulation of the IGF1 receptor (IGF1R). This in turn leads to decreased glucose uptake, increased apoptosis, as well as pregnancy resorption and growth restriction. Recent studies have shown that blastocyst activation of AMP-activated protein kinase (AMPK) reverses these detrimental effects; however, the mechanism was not clear. The objective of this study was to determine how AMPK activation rescues the insulin-resistant blastocyst. Using trophoblast stem (TS) cells derived from the blastocyst, insulin resistance was recreated by transfecting with siRNA to Igf1r and down-regulating expression of the protein. These cells were then exposed to AMPK activators 5-aminoimidazole-4-carboxamide riboside and phenformin, and evaluated for apoptosis, insulin-stimulated 2-deoxyglucose uptake, PI3-kinase activity, and levels of phospho-AKT, phospho-mTor, and phospho-70S6K. Surprisingly, disrupted insulin signaling led to decreased AMPK activity in TS cells. Activators reversed these effects by increasing the AMP/ATP ratio. Moreover, this treatment increased insulin-stimulated 2-deoxyglucose transport and cell survival, and led to an increase in PI3-kinase activity, as well as increased P-mTOR and p70S6K levels. This study is the first to demonstrate significant crosstalk between the AMPK and insulin signaling pathways in embryonic cells, specifically the enhanced response of PI3K/AKT/mTOR to AMPK activation. Decreased insulin signaling also resulted in decreased AMPK activation. These findings provide mechanistic targets in the AMPK signaling pathway that may be essential for improved pregnancy success in insulin-resistant states.

The role of AMPK and mTOR in nutrient sensing in pancreatic beta-cells

The AMP-activated protein kinase (AMPK) is a central regulator of the energy status of the cell, based on its unique ability to respond directly to fluctuations in the ratio of AMP:ATP. Because glucose and amino acids stimulate insulin release from pancreatic beta-cells by the regulation of metabolic intermediates, AMPK represents an attractive candidate for control of beta-cell function. Here, we show that inhibition of AMPK in beta-cells by high glucose inversely correlates with activation of the mammalian Target of Rapamycin (mTOR) pathway, another cellular sensor for nutritional conditions. Forced activation of AMPK by AICAR, phenformin, or oligomycin significantly blocked phosphorylation of p70S6K, a downstream target of mTOR, in response to the combination of glucose and amino acids. Amino acids also suppressed the activity of AMPK, and this at a minimum required the presence of leucine and glutamine. It is unlikely that the ability of AMPK to sense both glucose and amino acids plays a role in regulation of insulin secretion, as inhibition of AMPK by amino acids did not influence insulin secretion. Moreover, activation of AMPK by AICAR or phenformin did not antagonize glucose-stimulated insulin secretion, and insulin secretion was also unaffected in response to suppression of AMPK activity by expression of a dominant negative AMPK construct (K45R). Taken together, these results suggest that the inhibition of AMPK activity by glucose and amino acids might be an important component of the mechanism for nutrient-stimulated mTOR activity but not insulin secretion in the beta-cell.

Chronic hexosamine flux stimulates fatty acid oxidation by activating AMP-activated protein kinase in adipocytes

The hexosamine biosynthesis pathway (HBP) serves as a nutrient sensor and has been implicated in the development of type 2 diabetes. We previously demonstrated that fatty acid oxidation was enhanced in transgenic mouse adipocytes, wherein the rate-limiting enzyme of the HBP, glutamine:fructose-6-phosphate amidotransferase (GFA), was overexpressed. To explore the molecular mechanism of the HBP-induced fatty acid oxidation in adipocytes, we studied AMP-activated protein kinase (AMPK), an energy sensor that stimulates fatty acid oxidation by regulating acetyl-CoA carboxylase (ACC) activity. Phosphorylation and activity of AMPK were increased in transgenic fat pads and in 3T3L1 adipocytes treated with glucosamine to stimulate hexosamine flux. Glucosamine also stimulated phosphorylation of ACC and fatty acid oxidation in 3T3L1 adipocytes, and these stimulatory effects were diminished by adenovirus-mediated expression of a dominant negative AMPK in 3T3L1 adipocytes. Conversely, blocking the HBP with a GFA inhibitor reduced AMPK activity, ACC phosphorylation, and fatty acid oxidation. These changes are not explained by alterations in the cellular AMP/ATP ratio. Further demonstrating that AMPK is regulated by the HBP, we found that AMPK was recognized by succinylated wheat germ agglutinin, which specifically binds O-GlcNAc. The levels of AMPK in succinylated wheat germ agglutinin precipitates correlated with hexosamine flux in mouse fat pads and 3T3L1 adipocytes. Moreover, removal of O-GlcNAc by hexosaminidase reduced AMPK activity. We conclude that chronically high hexosamine flux stimulates fatty acid oxidation by activating AMPK in adipocytes, in part through O-linked glycosylation.

AICAR potentiates ROS production induced by chronic high glucose: roles of AMPK in pancreatic beta-cell apoptosis

We previously demonstrated that chronic high glucose (33.3 mM) induced beta-cell dysfunction and apoptosis through glucokinase (GCK) downregulation, but the exact mechanisms involved remain unclear. Here, we show that prolonged exposure of 5-aminoimidazole-4-carboxamide (AICA)-riboside potentiated apoptosis induced by high glucose in MIN6N8 pancreatic beta-cells, correlating with enhanced GCK downregulation and decreased production of ATP and insulin. These events are potentiated in AMPK-overexpressing cells, but are prevented in cells transfected with mutant dominant-negative AMPK (AMPK-K45R). Furthermore, AMPK activation increases production of reactive oxygen species (ROS) and loss of mitochondria membrane potential induced by high glucose, which is significantly inhibited by treatment with compound C or by AMPK-K45R overexpression. Overexpression of GCK prevents apoptosis; decreased cellular ATP and insulin secretion, and ROS production enhanced by AICAR, but does not affect AMPK activation. Similar results are obtained using isolated primary islet cells. Collectively, these data demonstrate that AMPK activation potentiates beta-cell apoptosis induced by chronic high glucose through augmented GCK downregulation mediated by enhanced ROS production.

Expanding role of AMPK in endocrinology

Adenosine 5' monophosphate-activated protein kinase (AMPK) is a regulator of cellular and systemic energy homeostasis. It mediates some of the effects of peripheral hormones such as leptin, ghrelin and adiponectin, and it is involved in the insulin-sensitizing role of the antidiabetic drug metformin. There is increasing evidence that AMPK has a central role in mediating the appetite-modulating and metabolic effects of many other hormones and substances, including the cannabinoids. Recent studies have illustrated the interaction between hormones and AMPK, and highlighted AMPK as a potential target for the development of tissue-specific AMPK modulators in the treatment of obesity and the metabolic syndrome.

Glucose regulates expression of the nerve growth factor (NGF) receptors TrkA and p75NTR in rat islets and INS-1E beta-cells

BACKGROUND

The function and survival of pancreatic beta-cells strongly depend on glucose concentration and on autocrine secretion of peptide growth factors. NGF and its specific receptors TrkA and p75NTR play a pivotal role in islet survival and glucose-dependent insulin secretion. We therefore investigated whether or not glucose concentration influences expression of TrkA and p75NTR in rat islets and in INS-1E beta-cells at the mRNA and protein level (INS-1E).

METHODS

Gene expression of the NGF receptors TrkA and p75NTR but also of the metabolic gene liver-type pyruvate kinase (L-PK) and the neurotrophin receptors TrkB and TrkC was studied by semi-quantitative PCR and by real-time PCR in islets and INS-1E beta-cells.

RESULTS

In rat islets, high glucose exposure (25 mmol/l) increased gene expression of TrkA, p75NTR and L-PK. Expression of TrkA, p75NTR and L-PK reflected insulin secretion at the respective glucose concentration. In rat INS-1E insulinoma cells, expression of L-PK and p75NTR was suppressed by low glucose as in the islets, while expression of TrkA was strongly increased by low glucose levels and thus was regulated differently than in islets. Expression of TrkB and TrkC was not regulated by glucose concentration at all. TrkA protein was regulated in the same fashion as its mRNA expression, while p75NTR protein was not significantly regulated within 24 h.

CONCLUSION

Glucose interacts with gene expression of TrkA and p75NTR that are strongly involved in beta-cell growth and glucose-dependent insulin secretion. The fact that TrkA expression is regulated the opposite way in islets and in INS-1E beta-cells might reflect their specific grade of differentiation and tendency to proliferate.