The altered expression of glucose-regulated proteins 78 in different phase of streptozotocin-affected pancreatic beta-cells

Potential Effects of Hyperglycemia on SARS-CoV-2 Entry Mechanisms in Pancreatic Beta Cells

The COVID-19 pandemic has revealed a bidirectional relationship between SARS-CoV-2 infection and diabetes mellitus. Existing evidence strongly suggests hyperglycemia as an independent risk factor for severe COVID-19, resulting in increased morbidity and mortality. Conversely, recent studies have reported new-onset diabetes following SARS-CoV-2 infection, hinting at a potential direct viral attack on pancreatic beta cells. In this review, we explore how hyperglycemia, a hallmark of diabetes, might influence SARS-CoV-2 entry and accessory proteins in pancreatic β-cells. We examine how the virus may enter and manipulate such cells, focusing on the role of the spike protein and its interaction with host receptors. Additionally, we analyze potential effects on endosomal processing and accessory proteins involved in viral infection. Our analysis suggests a complex interplay between hyperglycemia and SARS-CoV-2 in pancreatic β-cells. Understanding these mechanisms may help unlock urgent therapeutic strategies to mitigate the detrimental effects of COVID-19 in diabetic patients and unveil if the virus itself can trigger diabetes onset.

Secreted glucose regulated protein78 ameliorates DSS-induced mouse colitis

The secreted form of 78-kDa glucose-regulated protein (sGRP78) has been widely reported for its property in aiding resolution of inflammatory. However, little is known on its potential in the treatment of colitis. To investigate the expression pattern and functional outcome of GRP78 in ulcerative colitis, its expression was measured in human and murine colitis samples. It was found that GRP78 was spontaneously secreted to a high level in gut, which is a physiological site of immune tolerance. During the active phase of DSS-induced colitis, the sGRP78 level was significantly reduced but rebounded quickly during resolving phase, making it a potential candidate for the treatment of colitis. In the following experiments, the administration of sGRP78 was proved to decrease susceptibility to experimental colitis, as indicated by an overall improvement of intestinal symptoms, restoration of TJ integrity, decreased infiltration of immune cells and impaired production of inflammatory cytokines. And specific cleavage of endogenous sGRP78 could aggravate DSS colitis. Adoptive transfer of sGRP78-conditioned BMDMs reduced inflammation in the gut. We linked sGRP78 treatment with altered macrophage biology and skewed macrophage polarization by inhibiting the TLR4-dependent MAP-kinases and NF-κB pathways. Based on these studies, as a naturally occurring immunomodulatory molecule, sGRP78 might be an attractive novel therapeutic agent for acute intestinal inflammation.

Going viral in the islet: mediators of SARS-CoV-2 entry beyond ACE2

Coronavirus disease 2019 (COVID-19) is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Following initial infection of airway epithelia, SARS-CoV-2 invades a wide range of cells in multiple organs, including pancreatic islet cells. Diabetes is now recognised as a risk factor for severe COVID-19 outcomes, including hospitalisation and death. Additionally, COVID-19 is associated with a higher risk of new-onset diabetes and metabolic complications of diabetes. One mechanism by which these deleterious outcomes may occur is via the destruction of insulin-producing islet β cells, either directly by SARS-CoV-2 entry into β cells or indirectly due to inflammation and fibrosis in the surrounding microenvironment. While the canonical pathway of viral entry via angiotensin-converting enzyme 2 (ACE2) has been established as a major route of SARS-CoV-2 infection in the lung, it may not be solely responsible for viral entry into the endocrine pancreas. This is likely due to the divergent expression of viral entry factors among different tissues. For example, expression of ACE2 has not been unequivocally demonstrated in β cells. Thus, it is important to understand how other proteins known to be highly expressed in pancreatic endocrine cells may be involved in SARS-CoV-2 entry, with the view that these could be targeted to prevent the demise of the β cell in COVID-19. To that end, this review discusses alternate receptors of SARS-CoV-2 (CD147 and GRP78), as well as mediators (furin, TMPRSS2, cathepsin L, ADAM17, neuropilin-1, and heparan sulphate) that may facilitate SARS-CoV-2 entry into pancreatic islets independent of or in conjunction with ACE2.

Tauroursodeoxycholic acid alleviates secondary injury in the spinal cord via up-regulation of CIBZ gene

Spinal cord injury (SCI) is generally divided into primary and secondary injuries, and apoptosis is an important event of the secondary injury. As an endogenous bile acid and recognized endoplasmic reticulum (ER) stress inhibitor, tauroursodeoxycholic acid (TUDCA) administration has been reported to have a potentially therapeutic effect on neurodegenerative diseases, but its real mechanism is still unclear. In this study, we evaluated whether TUDCA could alleviate traumatic damage of the spinal cord and improve locomotion function in a mouse model of SCI. Traumatic SCI mice were intraperitoneally injected with TUDCA, and the effects were evaluated based on motor function assessment, histopathology, apoptosis detection, qRT-PCR, and western blot at different time periods. TUDCA administration can improve motor function and reduce secondary injury and lesion area after SCI. Furthermore, the apoptotic ratios were significantly reduced; Grp78, Erdj4, and CHOP were attenuated by the treatment. Unexpectedly, the levels of CIBZ, a novel therapeutic target for SCI, were specifically up-regulated. Taken together, it is suggested that TUDCA effectively suppressed ER stress through targeted up-regulation of CIBZ. This study also provides a new strategy for relieving secondary damage by inhibiting apoptosis in the early treatment of spinal cord injury.

Quercetin improves endothelial function in diabetic rats through inhibition of endoplasmic reticulum stress-mediated oxidative stress

Endoplasmic reticulum (ER) stress attributes a crucial role in diabetes-induced endothelial dysfunction. The present study investigated the effects of quercetin, a potent antioxidant on the attenuation of ER stress-modulated endothelial dysfunction in streptozotocin (STZ)-induced diabetic rats. Oral administration of quercetin for six weeks to diabetic rats dose-dependently reduced the blood glucose levels and improved insulin secretion. Histopathological examination of pancreatic tissues in diabetic rats showed pathological changes such as shrunken islets, reduction in islet area and distorted β-cells, which were found to be restored by quercetin treatment. In addition, quercetin reduced the pancreatic ER stress-induced endothelial dysfunction as assessed by immunohistochemical analysis of C/ERB homologous protein (CHOP) and endothelin-1 (ET-1). Moreover, quercetin administration progressively increased the expression of vascular endothelial growth factor (VEGF) and its receptor, VEGFR2 in diabetes rats. Quercetin-mediated decrease in the nitric oxide (NO∙) and cyclic 3',5'- guanosine monophosphate (cGMP) levels were also observed in the diabetic rats. Quercetin treatment reduced the lipid peroxidation in the diabetic rats, meanwhile increased the total antioxidant capacity in the pancreas from diabetic rats. Altogether, these results demonstrated the vasoprotective effect of quercetin against STZ-induced ER stress in the pancreas of diabetic rats.

Glucose-Regulated Protein 78-Induced Myeloid Antigen-Presenting Cells Maintained Tolerogenic Signature upon LPS Stimulation

The 78-kDa glucose-regulated protein (Grp78) is stress-inducible chaperone that mostly reside in the endoplasmic reticulum. Grp78 has been described to be released at times of cellular stress and as having extracellular properties that are anti-inflammatory or favor the resolution of inflammation. As antigen-presenting cells (APCs) play a critical role in both the priming of adaptive immune responses and the induction of self-tolerance, herein, we investigated the effect of Grp78 on the maturation of murine myeloid APCs (CD11c⁺ cells). Results showed that CD11c⁺ cells could be bound by AF488-labeled Grp78 and that Grp78 treatment induced a tolerogenic phenotype comparable to immature cells. Furthermore, when exposed to lipopolysaccharide, Grp78-treated CD11c⁺ cells (DC_Grp78) did not adopt a mature dendritic cell phenotype. DC_Grp78-primed T cells exhibited reduced proliferation along with a concomitant expansion of CD4⁺CD25⁺FoxP3⁺ cells in pancreaticoduodenal lymph nodes and induction of T cell apoptosis in vitro and ex vivo. The above work suggests that Grp78 is an immunomodulatory molecule that could aid resolution of inflammation. It may thus contribute to induce durable tolerance to be of potential therapeutic benefit in transplanted allogeneic grafts and autoimmune diseases such as type I diabetes.

Glucosamine-induced Sp1 O-GlcNAcylation ameliorates hypoxia-induced SGLT dysfunction in primary cultured renal proximal tubule cells

The aim of this study is to determine whether GlcN could recover the endoplasmic reticulum (ER) stress-induced dysfunction of Na(+) /glucose cotransporter (SGLT) in renal proximal tubule cells (PTCs) under hypoxia. With the rabbit model, the renal ischemia induced tubulointerstitial abnormalities and decreased SGLTs expression in tubular brush-border, which were recovered by GlcN. Thus, the protective mechanism of GlcN against renal ischemia was being examined by using PTCs. Hypoxia decreased the level of protein O-GlcNAc and the expression of O-GlcNAc transferase (OGT) while increased O-GlcNAcase (OGA) and these were reversed by GlcN. Hypoxia also decreased the expression of SGLTs (SGLT1 and 2) and [(14) C]-α-methyl-D-glucopyranoside (α-MG) uptake which were recovered by GlcN and PUGNAc (OGA inhibitor). Hypoxia enhanced reactive oxygen species (ROS) and then ER stress proteins, glucose-regulated protein 78 (GRP78), and C/EBP-homologous protein (CHOP). However, the expression of GRP78 increased till 6 h and then decreased whereas CHOP increased gradually. Moreover, decreased GRP78 and increased CHOP were reversed by NAC (antioxidant) and GlcN. GlcN ameliorated hypoxia-induced decrease of O-GlcNAc modification of Sp1 but OGT or Sp1 siRNAs blocked the recovery effect of GlcN on SGLT expression and α-MG uptake. In addition, hypoxia-decreased GRP78 and HIF-1α expression was reversed by GlcN but OGT siRNA or Sp1 siRNA ameliorated the effect of GlcN. When PTCs were transfected with GRP78 siRNA or HIF-1α siRNA, SGLT expression and α-MG uptake was decreased. Taken together, these data suggest that GlcN-induced O-GlcNAc modified Sp1 with stimulating GRP78 and HIF-1α activity ameliorate hypoxia-induced SGLT dysfunction in renal PTCs. J. Cell. Physiol. 229: 1557-1568, 2014. © 2014 Wiley Periodicals, Inc.

Impact of exposure to low concentrations of nitric oxide on protein profile in murine and human pancreatic islet cells

Homeostatic levels of nitric oxide (NO) protect efficiently against apoptotic death in both human and rodent pancreatic β cells, but the protein profile of this action remains to be determined. We have applied a 2 dimensional LC-MS-MALDI-TOF/TOF-based analysis to study the impact of protective NO in rat insulin-producing RINm5F cell line and in mouse and human pancreatic islets (HPI) exposed to serum deprivation condition. 24 proteins in RINm5F and 22 in HPI were identified to undergo changes in at least one experimental condition. These include stress response mitochondrial proteins (UQCRC2, VDAC1, ATP5C1, ATP5A1) in RINm5F cells and stress response endoplasmic reticulum proteins (HSPA5, PDIA6, VCP, GANAB) in HPI. In addition, metabolic and structural proteins, oxidoreductases and chaperones related with protein metabolism are also regulated by NO treatment. Network analysis of differentially expressed proteins shows their interaction in glucocorticoid receptor and NRF2-mediated oxidative stress response pathways and eNOS signaling. The results indicate that exposure to exogenous NO counteracts the impact of serum deprivation on pancreatic β cell proteome. Species differences in the proteins involved are apparent.

Modulation of endoplasmic reticulum chaperone GRP78 by high glucose in hippocampus of streptozotocin-induced diabetic mice and C6 astrocytic cells

Diabetes mellitus is known to increase the risk of neurodegeneration, and both diseases are reported to be linked to dysfunction of endoplasmic reticulum (ER). Astrocytes are important in the defense mechanism of central nervous system (CNS), with great ability of tolerating accumulation of toxic substances and sensitivity in Ca(2+) homeostasis which are two key functions of ER. Here, we investigated the modulation of the glucose-regulated protein 78 (GRP78) in streptozotocin (STZ)-induced diabetic mice and C6 cells cultured in high glucose condition. Our results showed that more reactive astrocytes were presented in the hippocampus of STZ-induced diabetic mice. Simultaneously, decrease of GRP78 expression was found in the astrocytes of diabetic mice hippocampus. In in vitro study, C6 cells were treated with high glucose to investigate the role of high glucose in GRP78 modulation in astrocytic cells. GRP78 as well as other chaperones like GRP94, calreticulin and calnexin, transcription levels were down-regulated after high glucose treatment. Also C6 cells challenged with 48h high glucose were activated, as indicated by increased level of glial fibrillary acidic protein (GFAP). Activated C6 cells simultaneously exhibited significant decrease of GRP78 level and was followed by reduced phosphorylation of Akt. Moreover, unfolded protein response was induced as an early event, which was marked by the induction of CHOP with high glucose treatment, followed by the reduction of GRP78 after 48h. Finally, the upsurge of ROS production was found in high glucose treated C6 cells and chelation of ROS could partially restore the GRP78 expression. Taken together, these data provide evidences that high glucose induced astrocytic activation in both in vivo and in vitro diabetic models, in which modulation of GRP78 would be an important event in this activation.

Association between promoter polymorphisms of the GRP78 gene and risk of type 2 diabetes in a Chinese Han population

There are large amounts of unfolding or misfolding protein accumulation in the endoplasmic reticulum in patients with type 2 diabetes (T2D), which in turn induces the expression of the glucose-regulated protein 78 (GRP78) that plays a key role in influencing insulin secretion and maintaining glucose homeostasis in pancreatic beta cells. The aim in the study is to analyze the potential association between single-nucleotide polymorphisms (SNPs) of GRP78 and the risk of T2D. To assess the association between GRP78 polymorphisms and T2D, a case-control study was conducted among 1058 consecutive unrelated subjects. Of the 1058 subjects, 523 of them were diagnosed with T2D and 535 of them were healthy controls. Four SNPs with R(2)>0.8 and the minor allele frequency>0.05 (rs391957, rs17840761, rs17840762, and rs11355458) in the GRP78 gene promoter were analyzed. Overall, no associations of GRP78 polymorphisms with T2D were observed in genotypic analyses. In addition, haplotypes combining those SNPs in the promoter in high linkage disequilibrium were also not associated with a T2D risk. However, the levels of fasting plasma glucose and HbA1c in patients with the -415AA/-180GG genotype were significantly lower than those of the patients with -415GG/-180deldel and -415AG/-180Gdel genotypes, and the level of fasting insulin in patients with the -415AA/-180GG genotype was significantly lower than that of the patients with -415GG/-180deldel. The study does not support a role for promoter polymorphisms of GRP78 in T2D in a Chinese Han population, but it does provide a clue for association between low levels of fasting plasma glucose, HbA1c and fasting insulin, and the -415AA/-180GG model.

Endoplasmic reticulum chaperone GRP78 suppresses the aggregation of proteins containing expanded polyglutamine tract

Polyglutamine (polyQ) diseases are inherited neurodegenerative diseases characterized by the aggregation of proteins containing expanded polyQ tract. It has been shown that expanded polyQ tract-containing proteins impair the functions of other cellular proteins. However, quantitative changes of cellular proteins in cells expressing expanded polyQ tract-containing proteins have not been performed. Here, we performed proteomic analysis of cells expressing expanded polyQ tract-containing proteins, and showed that GRP78, the endoplasmic reticulum (ER) chaperone, was significantly decreased in the cells expressing enhanced green fluorescent protein with a pathological-length polyQ tract (EGFP-polyQ97), but not with a non-pathological-length polyQ tract (EGFP-polyQ24). In addition, we revealed that down-regulation of GRP78 expression resulted in increase of the aggregation of EGFP-polyQ97. Conversely, the aggregation of EGFP-polyQ97 was suppressed by the overexpression of GRP78 in the cells. Furthermore, it seemed that the decreased GRP78 expression in the cells expressing EGFP-polyQ97 was due to the enhanced protein degradation of GRP78 through the ubiquitin-proteasome pathway. These findings indicated that GRP78, which has an inhibitory effect on the aggregation of proteins containing expanded polyQ tract, may be an effective target for the treatment of polyQ diseases.

Co-silencing of Birc5 (survivin) and Hspa5 (Grp78) induces apoptosis in hepatoma cells more efficiently than single gene interference

Birc5 (previously known as survivin) is a cancer-specific protein. Due to the upregulation of its expression in various human malignancies and its key role in apoptosis, proliferation and angiogenesis, Birc5 has attracted attention as a target for anticancer therapies. In this study, when Birc5 was silenced in HepG2 cells, 29.7±3.3% cells underwent apoptosis as expected. It was found that the expression levels of glucose-regulated protein 78 (Hspa5, previously known as Grp78) was increased by almost 3-fold in Birc5-silenced HepG2 cells. Hspa5, a master regulator of the anti-apoptotic unfolded protein response signalling network, can also promote tumor proliferation, survival and metastasis. Hence, we hypothesized that the co-silencing of Birc5 and Hspa5 may exert a stronger apoptosis-inducing effect than single gene interference. To verify this, the expression levels of Birc5 and Hspa5 in human hepatocellular carcinoma tissues were determined. Immunohistochemical staining showed that the expression of Birc5 and Hspa5 was elevated in 28 out of 31 samples. Additionally, plasmid-based siRNA against Birc5 and/or Hspa5 were constructed and transfected into the human hepatocellular liver carcinoma cell line, HepG2. Compared with the HepG2 cells, in which Birc5 or Hspa5 were silenced alone, only 44.2±3.4% of the co-silenced cells proliferated, and 40.3±3.7% co-silenced cells underwent apoptosis (p<0.05). Furthermore, tumor formation from inoculated subcutaneous co-silenced cells in nude mice was inhibited significantly. The current study suggests that Birc5 and Hspa5 could be important survival factors for hepatoma carcinoma cells and that the simultaneous knockdown of Birc5 and Hspa5 is more effective in inducing apoptosis in HepG2 cells than the knockdown of Birc5 or Hspa5 alone. The co-silencing of Birc5 and Hspa5 could be warranted for cancer therapy.

Effect of Environmental Contaminants on Beta Cell Function

There is an increasing concern that chemicals in the environment are contributing to the global rise in the prevalence of type 2 diabetes (T2D). However, there is limited evidence for direct effects of these chemicals on beta cell function. Therefore, the goals of this study were (1) to test the hypothesis that environmental contaminants can directly affect beta cell function and (2) examine mechanistic pathways by which these contaminants could affect beta cell function. Using mouse beta TC-6 cells, we examined the acute effects of 6 substances (benzo[a]pyrene, bisphenol A [BPA], propylparaben, methylparaben, perfluorooctanoic acid, and perfluorooctyl sulfone) on insulin secretion. Only BPA treatment directly affected insulin secretion. Furthermore, chronic exposure to BPA altered the expression of key proteins in the cellular and endoplasmic reticulum stress response. These data suggest that long-term BPA exposure may be detrimental to beta cell function and ultimately be an important contributor to the etiology of T2D.

Cannabinoid receptor 1 mediates high glucose-induced apoptosis via endoplasmic reticulum stress in primary cultured rat mesangial cells

The endocannabinoid system in animals and humans is involved in the onset of diverse diseases, including obesity and diabetic nephropathy, which is a major end-stage renal disease characterized by high glucose (HG)-induced apoptosis of mesangial cells. Endocannabinoids induce physiological and behavioral effects by activating two specific receptors, cannabinoid receptor 1 (CB(1)R) and cannabinoid receptor 2 (CB(2)R). However, the pathophysiology of CB(1)R in diabetic nephropathy has not been elucidated. We investigated the effects of HG on CB(1)R expression and its signaling pathways in primary cultured rat mesangial cells. HG significantly increased CB(1)R mRNA and protein levels in a time-dependent manner and induced CB(1)R internalization. NF-κB and cPLA(2) were involved in the HG-induced increase in CB(1)R levels. Using a CB(1)R antagonist (AM251) and CB(1) siRNA transfection, we showed that HG-induced CB(1)R is linked to apoptosis. Specifically, HG inhibited the expression of GRP78, but induced increases in endoplasmic reticulum (ER) stress proteins, including phosphorylated (p)-protein kinase-like ER-associated kinase, p-eukaryotic initiation factor 2α, p-activating transcription factor-4, and C/EBP homologous protein. In addition, HG increased the Bax/Bcl-2 ratio and increased the amounts of cleaved poly(ADP-ribose) polymerase and caspase-3. These apoptotic effects were prevented by AM251 and by the downregulation of CB(1)R expression by small interfering RNA. We propose a mechanism by which blockade of CB(1)R attenuates HG-induced apoptosis in rat mesangial cells. Our findings suggest that blockade of CB(1)R may be a potential therapy in diabetic nephropathy.

Cannabinoid receptor 1 mediates palmitic acid-induced apoptosis via endoplasmic reticulum stress in human renal proximal tubular cells

The endocannabinoid system (ECS) is activated at the onset of obesity and diverse metabolic diseases. Endocannabinoids mediate their physiological and behavioral effects by activating specific cannabinoid receptors, mainly cannabinoid receptor 1 (CB(1)R). Diabetic nephropathy (DN) is induced by hyperlipidemia, and renal proximal tubule cells are an important site for the onset of DN. However, the pathophysiology of CB(1)R, especially in the hyperlipidemia of DN, has not been elucidated. Therefore, we examined the effect of palmitic acid (PA) on CB(1)R expression and its related signal pathways in human renal proximal tubular cells (HK-2 cells). PA significantly increased CB(1)R mRNA and protein levels and induced CB(1)R internalization. PA-induced activation of CB(1)R is prevented by the treatment of AACOCF(3) (a cPLA(2) inhibitor), indomethacin and NS398 (a COX 2 inhibitors). Indeed, PA increased cPLA(2), and COX-2 but not COX-1. We also investigated whether the PA-induced activation of CB(1)R is linked to apoptosis. As a result, AM251 (a CB(1)R antagonist) attenuated PA-mediated apoptosis in a concentration-dependent manner. Furthermore, PA decreased GRP78 expression and induced increases in the endoplasmic reticulum (ER) stress signaling pathways p-PERK, p-eIF2α, p-ATF4, and CHOP, which were blocked by AM251 treatment. Moreover, PA increased the Bax/Bcl-2 ratio, cleaved PARP, and caspase-3 levels. The PA-induced apoptotic effects were decreased with CB(1)R-specific antagonist (AM251) treatment and CB1 si-RNA transfection. In conclusion, PA induced apoptosis through ER stress via CB(1)R expression in human proximal tubule cells. Our results provide evidence that CB(1)R blockade may be a potential anti-diabetic therapy for the treatment of DN.