Abundant expression of 150-kDa oxygen-regulated protein in mouse pancreatic beta cells is correlated with insulin secretion

Effects of Edible Insect Tenebrio molitor Larva Fermentation Extract as a Substitute Protein on Hepatosteatogenesis and Proteomic Changes in Obese Mice Induced by High-Fat Diet

Mealworms (Tenebrio molitor larva) are an edible insect and a protein-rich food; however, research on mealworms as a substitute protein is insufficient. In this study, mealworm fermentation extract (TMP) was assessed as a replacement for soy protein (SP) in a control diet (CON) or a high-fat diet (HFD) of mice for 12 weeks. TMP substitution reduced body weight, body weight gain, body fat mass (perirenal and mesenteric), fat size, glucose intolerance, and insulin resistance compared to the HFD-SP group. TMP alleviated hepatic steatosis (lipid contents and lipid droplets) in high-fat-fed mice and down-regulated the PPARγ, CD36, and DGAT2 gene levels. Proteomic analysis showed that a HFD for 12 weeks up-regulated 20 proteins and down-regulated 17 proteins in mice fed SP. On the other hand, TMP reversed the protein profiles. TMP significantly down-regulated KHK, GLO1, ATP5H, SOD, and DDAH1 and up-regulated DLD, Mup1, CPS1, Ces3b, PDI, and HYOU1 compared to the HFD-SP group. These proteins are involved in the glucose, lipid, and amino acid metabolism, as well as in oxidative stress and endoplasmic reticulum stress. Thus, substituting SP for TMP helped improve HFD-induced obesity, steatosis, and insulin resistance in mice. These results suggest that TMP is a potential substitute for commonly used protein sources.

Misfolded proinsulin in the endoplasmic reticulum during development of beta cell failure in diabetes

The endoplasmic reticulum (ER) is broadly distributed throughout the cytoplasm of pancreatic beta cells, and this is where all proinsulin is initially made. Healthy beta cells can synthesize 6000 proinsulin molecules per second. Ordinarily, nascent proinsulin entering the ER rapidly folds via the formation of three evolutionarily conserved disulfide bonds (B7-A7, B19-A20, and A6-A11). A modest amount of proinsulin misfolding, including both intramolecular disulfide mispairing and intermolecular disulfide-linked protein complexes, is a natural by-product of proinsulin biosynthesis, as is the case for many proteins. The steady-state level of misfolded proinsulin-a potential ER stressor-is linked to (1) production rate, (2) ER environment, (3) presence or absence of naturally occurring (mutational) defects in proinsulin, and (4) clearance of misfolded proinsulin molecules. Accumulation of misfolded proinsulin beyond a certain threshold begins to interfere with the normal intracellular transport of bystander proinsulin, leading to diminished insulin production and hyperglycemia, as well as exacerbating ER stress. This is most obvious in mutant INS gene-induced Diabetes of Youth (MIDY; an autosomal dominant disease) but also likely to occur in type 2 diabetes owing to dysregulation in proinsulin synthesis, ER folding environment, or clearance.

Unfolding the Role of Large Heat Shock Proteins: New Insights and Therapeutic Implications

Heat shock proteins (HSPs) of eukaryotes are evolutionarily conserved molecules present in all the major intracellular organelles. They mainly function as molecular chaperones and participate in maintenance of protein homeostasis in physiological state and under stressful conditions. Despite their relative abundance, the large HSPs, i.e., Hsp110 and glucose-regulated protein 170 (Grp170), have received less attention compared to other conventional HSPs. These proteins are distantly related to the Hsp70 and belong to Hsp70 superfamily. Increased sizes of Hsp110 and Grp170, due to the presence of a loop structure, result in their exceptional capability in binding to polypeptide substrates or non-protein ligands, such as pathogen-associated molecules. These interactions that occur in the extracellular environment during tissue injury or microbial infection may lead to amplification of an immune response engaging both innate and adaptive immune components. Here, we review the current advances in understanding these large HSPs as molecular chaperones in proteostasis control and immune modulation as well as their therapeutic implications in treatment of cancer and neurodegeneration. Given their unique immunoregulatory activities, we also discuss the emerging evidence of their potential involvement in inflammatory and immune-related diseases.

BiP and its nucleotide exchange factors Grp170 and Sil1: mechanisms of action and biological functions

BiP (immunoglobulin heavy-chain binding protein) is the endoplasmic reticulum (ER) orthologue of the Hsp70 family of molecular chaperones and is intricately involved in most functions of this organelle through its interactions with a variety of substrates and regulatory proteins. Like all Hsp70 family members, the ability of BiP to bind and release unfolded proteins is tightly regulated by a cycle of ATP binding, hydrolysis, and nucleotide exchange. As a characteristic of the Hsp70 family, multiple DnaJ-like co-factors can target substrates to BiP and stimulate its ATPase activity to stabilize the binding of BiP to substrates. However, only in the past decade have nucleotide exchange factors for BiP been identified, which has shed light not only on the mechanism of BiP-assisted folding in the ER but also on Hsp70 family members that reside throughout the cell. We will review the current understanding of the ATPase cycle of BiP in the unique environment of the ER and how it is regulated by the nucleotide exchange factors, Grp170 (glucose-regulated protein of 170kDa) and Sil1, both of which perform unanticipated roles in various biological functions and disease states.

Alterations of pancreatic islet structure, metabolism and gene expression in diet-induced obese C57BL/6J mice

The reduction of functional β cell mass is a key feature of type 2 diabetes. Here, we studied metabolic functions and islet gene expression profiles of C57BL/6J mice with naturally occurring nicotinamide nucleotide transhydrogenase (NNT) deletion mutation, a widely used model of diet-induced obesity and diabetes. On high fat diet (HF), the mice developed obesity and hyperinsulinemia, while blood glucose levels were only mildly elevated indicating a substantial capacity to compensate for insulin resistance. The basal serum insulin levels were elevated in HF mice, but insulin secretion in response to glucose load was significantly blunted. Hyperinsulinemia in HF fed mice was associated with an increase in islet mass and size along with higher BrdU incorporation to β cells. The temporal profiles of glucose-stimulated insulin secretion (GSIS) of isolated islets were comparable in HF and normal chow fed mice. Islets isolated from HF fed mice had elevated basal oxygen consumption per islet but failed to increase oxygen consumption further in response to glucose or carbonyl cyanide-4-trifluoromethoxyphenylhydrazone (FCCP). To obtain an unbiased assessment of metabolic pathways in islets, we performed microarray analysis comparing gene expression in islets from HF to normal chow-fed mice. A few genes, for example, those genes involved in the protection against oxidative stress (hypoxia upregulated protein 1) and Pgc1α were up-regulated in HF islets. In contrast, several genes in extracellular matrix and other pathways were suppressed in HF islets. These results indicate that islets from C57BL/6J mice with NNT deletion mutation develop structural, metabolic and gene expression features consistent with compensation and decompensation in response to HF diet.

Vacuolar degeneration of skeletal muscle in transgenic mice overexpressing ORP150

ORP150 is a hypoxic stress-induced protein located in the endoplasmic reticulum. Transgenic mice overexpressing ORP150 (ORP-Tg) exhibit vacuolar degeneration in the heart. To determine whether vacuolization is present in skeletal muscle, we pathologically examined ORP-Tg mice. After 60 days of age, severe vacuolization was found in the soleus muscles of the hind legs of the ORP-Tg mice. Immunohistochemical staining of ORP150 revealed co-localization of ORP150 and vacuolization in the affected cells. Electron microscopy revealed a marked increase in the number of rough-surfaced endoplasmic reticula (rER) and distention of the cisterna. These findings suggest that overexpression of ORP150 causes accumulation of ORP150 in the rER, resulting in vacuolar degeneration in the skeletal muscle of ORP-Tg mice.

The role for endoplasmic reticulum stress in diabetes mellitus

Accumulating evidence suggests that endoplasmic reticulum (ER) stress plays a role in the pathogenesis of diabetes, contributing to pancreatic beta-cell loss and insulin resistance. Components of the unfolded protein response (UPR) play a dual role in beta-cells, acting as beneficial regulators under physiological conditions or as triggers of beta-cell dysfunction and apoptosis under situations of chronic stress. Novel findings suggest that "what makes a beta-cell a beta-cell", i.e., its enormous capacity to synthesize and secrete insulin, is also its Achilles heel, rendering it vulnerable to chronic high glucose and fatty acid exposure, agents that contribute to beta-cell failure in type 2 diabetes. In this review, we address the transition from physiology to pathology, namely how and why the physiological UPR evolves to a proapoptotic ER stress response and which defenses are triggered by beta-cells against these challenges. ER stress may also link obesity and insulin resistance in type 2 diabetes. High fat feeding and obesity induce ER stress in liver, which suppresses insulin signaling via c-Jun N-terminal kinase activation. In vitro data suggest that ER stress may also contribute to cytokine-induced beta-cell death. Thus, the cytokines IL-1beta and interferon-gamma, putative mediators of beta-cell loss in type 1 diabetes, induce severe ER stress through, respectively, NO-mediated depletion of ER calcium and inhibition of ER chaperones, thus hampering beta-cell defenses and amplifying the proapoptotic pathways. A better understanding of the pathways regulating ER stress in beta-cells may be instrumental for the design of novel therapies to prevent beta-cell loss in diabetes.

Targeting of the molecular chaperone oxygen-regulated protein 150 (ORP150) to mitochondria and its induction by cellular stress

Oxygen-regulated protein 150 (ORP150) is an inducible endoplasmic reticulum (ER) chaperone molecule that is upregulated after numerous cellular insults and has a cytoprotective role in renal, neural, and cardiac models of ischemia-reperfusion injury. ORP150 also has been shown to play a role in cellular Ca(2+) homeostasis, and in turn, regulating calpain activity. In this study, we identified ORP150 in whole rat renal cortical mitochondria and matrix fractions, demonstrated the targeting of an ORP150-GFP construct to the mitochondria of NIH-3T3 cells, and showed that the NH(2)-terminal 13 amino acids of ORP150 are sufficient for this translocation. ORP150 expression was found to be regulated by the anti-C/enhancer-binding protein homologous protein (CHOP)/GADD153 transcription factor and ORP150 levels increased in the mitochondria and ER of COS-7 cells after diverse stresses, including hypoxia, serum starvation, prolyl hydroxylase inhibition with dimethyloxaloylglycine, and exposure to tunicamycin, ethidium, bromide, and 2-deoxyglucose. Induction of the mitochondrial specific stress response in COS-7 cells through expression of an ornithine transcarbamylase mutant (Delta OTC) increased mitochondrial ORP150 levels and mitochondrial calpain activity. To determine whether mitochondrial ORP150 and mitochondrial calpain 10 interact, rat cortical mitochondria exposed to Ca(2+) resulted in ORP150 cleavage in a calpain inhibitor-dependent manner, revealing that ORP150 is a substrate and may be regulated by calpain 10. These data reveal a novel cellular localization for ORP150 and that mitochondrial ORP150 is upregulated by CHOP/GADD153 in response to mitochondrial and ER stress. Our data also reveal that ORP150 is a substrate for mitochondrial calpain 10.

Interferon-gamma potentiates endoplasmic reticulum stress-induced death by reducing pancreatic beta cell defence mechanisms

AIMS/HYPOTHESIS

A tight control of endoplasmic reticulum homeostasis is crucial for beta cell function and survival. We recently described that IL-1beta plus IFN-gamma deplete endoplasmic reticulum Ca2+ stores in beta cells, leading to endoplasmic reticulum stress and apoptosis. IL-1beta alone induced endoplasmic reticulum stress but failed to induce beta cell death, while IFN-gamma alone neither caused endoplasmic reticulum stress nor induced beta cell death. This suggests that IFN-gamma aggravates endoplasmic reticulum stress induced by IL-1beta, eventually triggering apoptosis. Here we tested this hypothesis and the mechanisms involved, by investigating the effects of IFN-gamma on endoplasmic reticulum-stress-induced beta cell apoptosis caused by a specific blocker of the sarcoendoplasmic-reticulum pump Ca2+-ATPase (SERCA).

MATERIALS AND METHODS

INS-1E cells or beta cells were pretreated with IFN-gamma and then exposed to the SERCA blocker cyclopiazonic acid (CPA) for induction of endoplasmic reticulum stress. Cell death was evaluated by Hoechst 342 and propidium iodide staining. Expression of genes related to endoplasmic reticulum stress was determined by real-time RT-PCR, while activation of the endoplasmic reticulum stress response was determined by analysing X-box binding protein-1 (Xbp1) splicing and using a reporter construct containing five copies of the unfolded protein response element (UPRE).

RESULTS

CPA induces endoplasmic reticulum stress and apoptosis in insulin-producing cells. Pretreatment with IFN-gamma decreased the basal level of spliced Xbp1 mRNA, the basal and CPA-induced activity of the UPRE reporter, and the mRNA expression of several endoplasmic reticulum chaperones (Bip, Grp94 and Orp 150) and Sec61a. Furthermore, CPA-induced Chop mRNA expression and beta cell apoptosis were potentiated in cells that had been pretreated with IFN-gamma.

CONCLUSIONS/INTERPRETATION

CPA-induced endoplasmic reticulum stress and apoptosis is enhanced in IFN-gamma-treated beta cells. These effects are mediated via downregulation of the expression of genes involved in beta cell defence against endoplasmic reticulum stress.

150-kD oxygen-regulated protein is an essential factor for insulin release

OBJECTIVE

This study was designed to show the induction and function of 150-kD oxygen-regulated protein (ORP150) in insulin secretion in vitro.

METHODS

A mouse beta-cell line, MIN6 cells, was cultured in medium containing potassium channel openers or various concentrations of glucose (2-25 mmol/L). ORP150 expression was studied by reporter assay, Western blot, or Northern blot analysis, concomitantly with insulin secretion. In addition, MIN6 cells infected with ORP150 recombinant adenovirus were adopted to show the function of ORP150 in insulin release.

RESULTS

ORP150 expression in MIN6 cells was suppressed dose-dependently by the potassium channel opener diazoxide. Both low glucose (<2 mmol/L) and high glucose concentrations (25 mmol/L glucose) significantly induced more ORP150 expression compared with 10 mmol/L glucose. The treatment with diazoxide or infection of ORP150 antisense adenovirus suppressed ORP150 expression, and glucose-stimulated insulin secretion was effectively prevented.

CONCLUSION

These findings show the involvement of ORP150 in insulin secretion in MIN6 cells.

The endoplasmic reticulum chaperone improves insulin resistance in type 2 diabetes

To determine the role of the endoplasmic reticulum (ER) in diabetes, Akita mice, a mouse model of type 2 diabetes, were mated with either heterozygous knockout mice or two types of transgenic mice of 150-kDa oxygen-regulated protein (ORP150), a molecular chaperone located in the ER. Systemic expression of ORP150 in Akita mice improves insulin intolerance, whereas the exclusive overexpression of ORP150 in pancreatic beta-cells of Akita mice did not change their glucose tolerance. Both an insulin tolerance test and hyperinsulinemic-euglycemic clamp revealed that ORP150 enhanced glucose uptake, accompanied by suppression of oxidized protein. Furthermore, ORP150 enhanced the insulin sensitivity of myoblast cells treated with hydrogen peroxide. These data suggest that ORP150 plays an important role in insulin sensitivity and is a potential target for the treatment of diabetes.

The endoplasmic reticulum stress response is stimulated through the continuous activation of transcription factors ATF6 and XBP1 in Ins2+/Akita pancreatic beta cells

The dominant C96Y mutation of one of the two murine insulin genes, Ins2, causes diabetes mellitus in 'Akita' mice. Here we established pancreatic islet beta cell lines from heterozygous mice (Ins2+/Akita). Western blot analysis of endoplasmic reticulum (ER) molecular chaperones indicated that Grp78, Grp94 and Orp150 are significantly increased in Ins2+/Akita cells compared with wild-type (Ins2+/+) cells. Reporter gene assays using the human GRP78 promoter with or without the ER stress response element (ERSE) showed that Ins2+/Akita cells exhibit significantly stronger ERSE-dependent transcriptional activity than Ins2+/+ cells. Transient over-expression of the Ins2 C96Y mutant in wild-type beta cells induces a stronger ERSE-dependent stress response than does wild-type Ins2 over-expression. The ERSE-binding transcription factor ATF6 is strongly activated in Ins2+/Akita cells. The activity of a reporter containing the specific binding sequence of another ERSE-binding transcription factor, XBP1, is also enhanced in Ins2+/Akita cells. Levels of active forms of XBP1 mRNA and protein are both markedly elevated in Ins2+/Akita cells. These results indicate that this cell line is subject to continuous ER stress and that the Ins2 C96Y mutation induces the expression of ER chaperones through the activation of ATF6 and XBP1.

Does hyperoxia affect glucose regulation and transport in the newborn?

OBJECTIVE

Hyperglycemia has been found to occur in children placed on cardiopulmonary bypass. Our laboratory demonstrated that hyperoxia plays a role in this hyperglycemic response and also occurs in the absence of cardiopulmonary bypass. The purpose of this study was to elucidate potential mechanisms underlying the hyperoxic-induced hyperglycemia by examining glucagon, insulin, and epinephrine, which are important in glucose regulation and skeletal and cardiac glucose transporters (GLUT1 and GLUT4), which facilitate glucose entry.

METHODS

Three-day-old piglets were anesthetized, intubated, and ventilated to normoxia. Animals were then randomly allocated to either 5 hours of normoxia (n = 4) or hyperoxia (n = 6). Measurements of oxygen, blood glucose, plasma glucagon, insulin, and epinephrine levels were made. Total GLUT1 and GLUT4 content in cardiac and skeletal muscle was measured using Western blotting analysis.

RESULTS

A sustained hyperglycemic response (P <.001) was seen throughout the 5-hour ventilatory period. A significant twofold elevation in glucagon levels (P <.001) and a threefold elevation (P <.003) in plasma insulin levels occurred, despite no significant changes in plasma epinephrine. Total GLUT1 and GLUT4 content were significantly reduced in skeletal muscle by 66% and 59%, respectively, while no significant changes occurred in cardiac muscle.

CONCLUSION

This study demonstrates that significant elevations in glucagon and insulin and reductions in total skeletal muscle GLUT1 and GLUT4 content all contribute to hyperoxia-induced hyperglycemia seen in newborns. To optimize postoperative recovery of newborns, consideration should be given to the levels of oxygen used to avoid the potential development of insulin resistance and subsequent decrease in glucose entry.

Enforced expression of oxygen-regulated protein, ORP150, induces vacuolar degeneration in mouse myocardium

Although the 150 kDa oxygen-regulated protein (ORP150) is known as a protein induced by low oxygen tension or ischemical insult, its possible role has not been fully investigated in vivo. To investigate the intracellular function of this protein, we generated the ORP150 over-expressing transgenic mice (ORP-Tg mice) under beta-actin promoter, and established three independent lines of the transgene expressed mice. All lines invariably showed growth retardation. Over-expression of ORP150 was confirmed by western blotting in heart, brain, spleen, skeletal muscle, pancreas, lung, thymus, and kidney. To ascertain the relationship between the over-expression of the ORP150 and the growth retardation in the transgenic mice, we examined pathological changes in the transgenics. In the ORP-Tg mice, vacuolar degeneration appeared in the heart. The degeneration in the myocytes became conspicuous with advancing age. Immunostaining demonstrated ORP150 in the vacuoles of degenerating myocytes. Electron microscopical findings revealed striking development of intracellular membrane system, for example, rough endoplasmic reticula (rER), vacuoles and Golgi bodies, swelling of sarcoplasmic reticulum, and lysis of myofibrils and mitochondria. These findings indicate that ORP150 may locate in the rER and other outer compartment of ER, and that constitutive over-expression of ORP150 in the heart induces vacuolar degeneration in myocytes, resulting in growth retardation of the transgenics.

Polymorphisms in the oxygen-regulated protein 150 gene (ORP150) are associated with insulin resistance in Pima Indians

The ORP150 gene that encodes the human oxygen-regulated protein (150 kDa) maps to chromosome 11q23, a region previously reported to be linked to type 2 diabetes and obesity in Pima Indians. This gene was also found to be differentially expressed in global gene expression studies comparing muscle mRNA from insulin-resistant versus insulin-sensitive subjects. Therefore, ORP150 was analyzed as a candidate gene for susceptibility to diabetes. Twelve variants were identified, and three unique representative polymorphisms were genotyped in 1,338 Pima Indians. None of these polymorphisms were associated with diabetes, but two polymorphisms were significantly associated with measures of insulin resistance. These data indicate that ORP150 has a role in insulin action but does not have a major role in determining susceptibility to type 2 diabetes in Pima Indians.

The increment of anti-ORP150 autoantibody in initial stages of atheroma in high-fat diet fed mice

Expression of 150 kda oxygen-regulated protein, ORP150, was examined in the atheromatous lesions on aortic valves in high-fat diet fed mice. Immunohistochemical staining revealed that ORP150 was expressed on the surface of plaque and was co-localized with phagocytes bearing Mac-3, a mouse macrophage differentiation antigen. These findings suggest that ORP150 is involved in the development of the atheromatous plaque. Titer of autoantibody against ORP150 was gradually elevated in parallel with the length of period of high-fat diet feeding. These results suggest that the deposition of immunocomplex toward ORP150 antigen is involved in atheromatous plaque progression.