Downregulation of gap junction expression and function by endoplasmic reticulum stress

Astragaloside IV attenuated TGF-β1- induced epithelial-mesenchymal transition of renal tubular epithelial cells via connexin 43 and Akt/mTOR signaling pathway

INTRODUCTION

The objective of the study was to observe whether connexin 43 (Cx43) could regulate epithelial mesenchymal transformation (EMT) of renal tubular epithelial cells (RTECs) by influencing Akt/mTOR signaling pathway, and whether ASV could inhibit the development of renal interstitial fibrosis by regulating Cx43.

METHODS

Lentivirus infection was transfected into RTECs with the final concentration of 50 ×PFU/ cell to regulate the expression of Cx43. And RTECs were intervened by different doses of Astragaloside IV (ASV). After synchronous culture of RTECs in each group，the expression levels of EMT-related indicators and Cx43 were detected by fluorescence microscope and Western-Blotting (WB), even the protein expressions and phosphorylation levels of AKT and mTOR in different groups were detected by WB.

RESULTS

When the expression of Cx43 in RTECs was regulated by lentivirus infection, the degree of EMT induced by TGF‑β1 and the phosphorylation level of Akt and mTOR were changed accordingly, indicating that Akt/mTOR pathway might be a downstream molecular mechanism by which Cx43 could regulate EMT. After intervention with different doses of ASV, the expression level of Cx43 increased with obvious concentration dependence, and the expression levels of p-Akt and p- mTOR were significantly altered, suggesting that ASV could effectively increase the protein expressions of TGF‑β1-induced Cx43 in RTECs and inhibit the phosphorylation levels of Akt and mTOR.

CONCLUSION

Cx43 were the main material basis of RTECs' injury, and ASV could inhibit TGF-β1- induced RTECs' transdifferentiation. In-depth study of the mechanism might provide a broad application prospect for the treatment of renal interstitial fibrosis.

Advances of Traditional Chinese Medicine Regulating Connexin43 in the Prevention and Treatment of Myocardial Infarction

Gap junctions are the main form of interaction between cardiomyocytes, through which the electrochemical activities between cardiomyocytes can be synchronized to maintain the normal function of the heart. Connexins are the basis of gap junctions. Changes in the expression, structural changes (e.g., phosphorylation and dephosphorylation), and distribution of connexins can affect the normal electrophysiological activities of the heart. Myocardial infarction (MI) and concurrent arrhythmia, shock, or heart failure can endanger life. The structural and functional damage of connexin (Cx) 43 in cardiomyocytes is a central part of the pathological progression of MI and is one of the main pathological mechanisms of arrhythmia after MI. Therefore, increasing Cx43 expression has become one of the main measures to prevent MI. Also, intervention in Cx43 expression can improve the structural and electrical remodeling of the myocardium to improve MI prognosis. Here, research progress of Cx43 in MI and its prevention and treatment using Traditional Chinese Medicine formulations is reviewed.

MicroRNA-30/Cx43 axis contributes to podocyte injury by regulating ER stress in diabetic nephropathy

Background

The microRNA-30 family plays a critical role in the pathogenesis of podocyte injury. Cx43 plays an essential role in intercellular communication, which is essential for coordinated kidney function. This study was conducted to explore the function of microRNA-30s/Cx43 in podocyte injury in diabetic nephropathy (DN), both in vivo and in vitro.

Methods

SD rats were given streptozotocin (STZ) injections to induce DN. Podocytes were incubated in the medium in the presence or absence of high glucose (HG). The effects of the microRNA-30/Cx43 axis on DN and its underlying mechanisms were investigated by TUNEL assay, PAS, immunohistochemical staining, immunofluorescence staining, Western blot, RT-qPCR, RNA interference, and luciferase reporter assay. Podocytes were transfected with microRNA-30 family mimics, microRNA-30 family inhibitors, Cx43 siRNA, and negative controls to detect the effect of the microRNA-30/Cx43 axis. MicroRNA-30 family mimic AAVs, and microRNA-30 family inhibitor AAVs applied to regulate microRNA-30 family expression in the kidneys of the STZ-induced DN model rats to reveal the underlying mechanisms of the microRNA-30/Cx43 axis in DN.

Results

MicroRNA-30 family member expression was downregulated in HG-treated podocytes and the glomeruli of STZ-induced DN rats. Luciferase reporter assays confirmed Cx43 is a directed target of microRNA-30s. The overexpression of microRNA-30 family members attenuated the HG-induced podocyte injury and protected against podocyte apoptosis and endoplasmic reticulum stress (ERS) both in vivo and in vitro. Also, silencing Cx43 expression eased podocyte apoptosis, injury, and ERS induced by a HG+microRNA-30 family inhibitor. Double-immunofluorescence staining assays proved the co-localization of caspase12 and Cx43.

Conclusions

The overexpression of microRNA-30 family members prevents HG-induced podocyte injury and attenuates ERS by modulating Cx43 expression. The microRNA-30/Cx43/ERS axis might be a potential therapeutic target to treat DN.

Connexin43 Contributes to Inflammasome Activation and Lipopolysaccharide-Initiated Acute Renal Injury via Modulation of Intracellular Oxidative Status

Aims: Inflammasome activation plays a pivotal role in many inflammatory diseases. Given that connexin (Cx) channels regulate numerous cellular events leading to inflammasome activation, we determined whether and how connexin affected inflammasome activation and inflammatory cell injury. Results: Exposure of mouse peritoneal macrophages (PMs) to lipopolysaccharide (LPS) plus ATP caused NLRP3 inflammasome activation, together with an increased connexin43 (Cx43). Inhibition of Cx43 blunted inflammasome activation. Consistently, PMs from the Cx43 heterozygous mouse (Cx43^+/-) exhibited weak inflammasome activation, in comparison with those from the Cx43^+/+ mouse. Further analysis revealed that inflammasome activation was preceded by an increased reactive oxygen species (ROS) production, nicotinamide adenine dinucleotide phosphate hydrogen (NADPH) oxidase 2 (NOX2), protein carbonylation, and mitogen-activated protein kinase (MAPK) activation. Suppression of ROS with antioxidant, downregulation of NOX2 with small interfering RNA (siRNA), or inhibition of NADPH oxidase or MAPKs with inhibitors blocked Cx43 elevation and inflammasome activation. Intriguingly, suppression of Cx43 also blunted NOX2 expression, protein carbonylation, p38 phosphorylation, and inflammasome activation. In a model of acute renal injury induced by LPS, the Cx43^+/- mouse exhibited a significantly lower level of blood interleukin-1β (IL-1β), blood urea nitrogen, and urinary protein, together with milder renal pathological changes and renal expression of NLRP3 and NOX4, as compared with the Cx43^+/+ mouse. Moreover, inhibition of gap junctions suppressed IL-1β- and tumor necrosis factor-α-induced expression of NOX4 in glomerular podocytes and tubular epithelial cells. Innovation and Conclusion: Our study indicates that Cx43 contributes to inflammasome activation and the progression of renal inflammatory cell injury through modulation of intracellular redox status. Cx43 could be a novel target for the treatment of certain inflammatory diseases.

Loss of Cx43-Mediated Functional Gap Junction Communication in Meningeal Fibroblasts Following Mouse Hepatitis Virus Infection

Mouse hepatitis virus (MHV) infection causes meningoencephalitis by disrupting the neuro-glial and glial-pial homeostasis. Recent studies suggest that MHV infection alters gap junction protein connexin 43 (Cx43)-mediated intercellular communication in brain and primary cultured astrocytes. In addition to astrocytes, meningeal fibroblasts also express high levels of Cx43. Fibroblasts in the meninges together with the basal lamina and the astrocyte endfeet forms the glial limitans superficialis as part of the blood-brain barrier (BBB). Alteration of glial-pial gap junction intercellular communication (GJIC) in MHV infection has the potential to affect the integrity of BBB. Till date, it is not known if viral infection can modulate Cx43 expression and function in cells of the brain meninges and thus affect BBB permeability. In the present study, we have investigated the effect of MHV infection on Cx43 localization and function in mouse brain meningeal cells and primary meningeal fibroblasts. Our results show that MHV infection reduces total Cx43 levels and causes its intracellular retention in the perinuclear compartments reducing its surface expression. Reduced trafficking of Cx43 to the cell surface in MHV-infected cells is associated with loss functional GJIC. Together, these data suggest that MHV infection can directly affect expression and cellular distribution of Cx43 resulting in loss of Cx43-mediated GJIC in meningeal fibroblasts, which may be associated with altered BBB function observed in acute infection.

Regulation of tumor-stroma interactions by the unfolded protein response

The unfolded protein response (UPR) is a conserved adaptive pathway that helps cells cope with the protein misfolding burden within the endoplasmic reticulum (ER). Imbalance between protein folding demand and capacity in the ER leads to a situation called ER stress that is often observed in highly proliferative and secretory tumor cells. As such, activation of the UPR signaling has emerged as a key adaptive mechanism promoting cancer progression. It is becoming widely acknowledged that, in addition to its intrinsic effect on tumor biology, the UPR can also regulate tumor microenvironment. In this review, we discuss how the UPR coordinates the crosstalk between tumor and stromal cells, such as endothelial cells, normal parenchymal cells, and immune cells. In addition, we further describe the involvement of ER stress signaling in the response to current treatments as well as its impact on antitumor immunity mainly driven by immunogenic cell death. Finally, in this context, we discuss the relevance of targeting ER stress/UPR signaling as a potential anticancer approach.

Asymmetric dimethylarginine (ADMA) treatment induces apoptosis in cultured rat mesangial cells via endoplasmic reticulum stress activation

Asymmetric dimethylarginine (ADMA), a high risk factor for endothelial dysfunction and cardiovascular disease (CVD), has been reported to promote cellular dysfunction via endoplasmic reticulum (ER) stress activation in various cells. Additionally, increased serum ADMA levels have been observed in incipient kidney diseases. Previously, we reported that activated ER stress is associated with mesangial cell apoptosis, observed mainly in overt nephropathy or chronic kidney disease (CKD). However, the effect of ADMA on mesangial cell apoptosis is unknown. Thus, we investigated the effects of ADMA on mesangial cell apoptosis and ER stress signaling. ADMA treatment increased caspase-3 activity and activated three branches of ER stress signaling (PERK, IRE1, and ATF6) that induce mesangial cell apoptosis. Pharmacological inhibitors of ER stress (inhibitors of PERK, IRE1, and S1P) attenuated ADMA-induced cleavage of caspase-3 and induced a decrease in the mitochondrial membrane potential. Furthermore, these inhibitors diminished the number of apoptotic cells induced by ADMA treatment. Taken together, our results indicated that ADMA treatment induces mesangial cell apoptosis via ER stress signaling. These results suggest that ADMA-induced mesangial cell apoptosis could contribute to the progression of overt nephropathy and CKD.

Subtilase cytotoxin produced by locus of enterocyte effacement-negative Shiga-toxigenic Escherichia coli induces stress granule formation

Subtilase cytotoxin (SubAB) is mainly produced by locus of enterocyte effacement (LEE)-negative strains of Shiga-toxigenic Escherichia coli (STEC). SubAB cleaves an endoplasmic reticulum (ER) chaperone, BiP/Grp78, leading to induction of ER stress. This stress causes activation of ER stress sensor proteins and induction of caspase-dependent apoptosis. We found that SubAB induces stress granules (SG) in various cells. Aim of this study was to explore the mechanism by which SubAB induced SG formation. Here, we show that SubAB-induced SG formation is regulated by activation of double-stranded RNA-activated protein kinase (PKR)-like endoplasmic reticulum kinase (PERK). The culture supernatant of STEC O113:H21 dramatically induced SG in Caco2 cells, although subAB knockout STEC O113:H21 culture supernatant did not. Treatment with phorbol 12-myristate 13-acetate (PMA), a protein kinase C (PKC) activator, and lysosomal inhibitors, NH4 Cl and chloroquine, suppressed SubAB-induced SG formation, which was enhanced by PKC and PKD inhibitors. SubAB attenuated the level of PKD1 phosphorylation. Depletion of PKCδ and PKD1 by siRNA promoted SG formation in response to SubAB. Furthermore, death-associated protein 1 (DAP1) knockdown increased basal phospho-PKD1(S916) and suppressed SG formation by SubAB. However, SG formation by an ER stress inducer, Thapsigargin, was not inhibited in PMA-treated cells. Our findings show that SubAB-induced SG formation is regulated by the PERK/DAP1 signalling pathway, which may be modulated by PKCδ/PKD1, and different from the signal transduction pathway that results in Thapsigargin-induced SG formation.

Bortezomib alleviates drug-induced liver injury by regulating CYP2E1 gene transcription

Acute liver failure, i.e., the fatal deterioration of liver function, is the most common indication that emergency liver transplantation is necessary. Moreover, in the USA, drug-induced liver injury (DILI), including acetaminophen (APAP)-induced hepatotoxicity, is the main cause of acute liver failure. Matching a donor for liver transplantation is extremely difficult, and thus the development of a novel therapy for DILI is urgently needed. Following recent approval by the FDA of the proteasomal inhibitor bortezomib, its therapeutic effects on various human diseases, including solid and hematologic malignancies, have been validated. However, the specific action of proteasomal inhibition in cases of DILI had not been elucidated prior to this study. To examine the effects of proteasomal inhibition in DILI experimentally, male C56Bl/6 mice were injected with 1 mg bortezomib/kg before APAP treatment. Bortezomib not only alleviated APAP-induced hepatotoxicity in a time- and dose-dependent manner, it also alleviated CCl4- and thioacetamide-induced hepatotoxicity. We also noted that bortezomib significantly reduced cytochrome P450 2E1 (CYP2E1) expression and activity in the liver, which was accompanied by the induction of endoplasmic reticulum (ER) stress. In addition, bortezomib decreased hepatocyte nuclear factor‑1α-induced promoter activation of CYP2E1 in Hep3B cells. By contrast, another proteasome inhibitor, MG132, did not cause ER stress and did not markedly affect CYP2E1 enzyme activity. Liver injury induced by APAP was aggravated by MG132, possibly via elevation of connexin 32 expression. This study suggests that proteasome inhibition has different effects in cases of DILI depending on the specific inhibitor being used. Furthermore, results from the mouse model indicated that bortezomib, but not MG132, was effective in alleviating DILI. ER stress induced by proteasome inhibition has previously been shown to exert various effects on DILI patients, and thus each available proteasomal inhibitor should be evaluated individually in order to determine its potential for clinical application.

Mouse Hepatitis Virus Infection Remodels Connexin43-Mediated Gap Junction Intercellular Communication In Vitro and In Vivo

Gap junctions (GJs) form intercellular channels which directly connect the cytoplasm between neighboring cells to facilitate the transfer of ions and small molecules. GJs play a major role in the pathogenesis of infection-associated inflammation. Mutations of gap junction proteins, connexins (Cxs), cause dysmyelination and leukoencephalopathy. In multiple sclerosis (MS) patients and its animal model experimental autoimmune encephalitis (EAE), Cx43 was shown to be modulated in the central nervous system (CNS). The mechanism behind Cx43 alteration and its role in MS remains unexplored. Mouse hepatitis virus (MHV) infection-induced demyelination is one of the best-studied experimental animal models for MS. Our studies demonstrated that MHV infection downregulated Cx43 expression at protein and mRNA levels in vitro in primary astrocytes obtained from neonatal mouse brains. After infection, a significant amount of Cx43 was retained in endoplasmic reticulum/endoplasmic reticulum Golgi intermediate complex (ER/ERGIC) and GJ plaque formation was impaired at the cell surface, as evidenced by a reduction of the Triton X-100 insoluble fraction of Cx43. Altered trafficking and impairment of GJ plaque formation may cause the loss of functional channel formation in MHV-infected primary astrocytes, as demonstrated by a reduced number of dye-coupled cells after a scrape-loading Lucifer yellow dye transfer assay. Upon MHV infection, a significant downregulation of Cx43 was observed in the virus-infected mouse brain. This study demonstrates that astrocytic Cx43 expression and function can be modulated due to virus stress and can be an appropriate model to understand the basis of cellular mechanisms involved in the alteration of gap junction intercellular communication (GJIC) in CNS neuroinflammation.

IMPORTANCE

We found that MHV infection leads to the downregulation of Cx43 in vivo in the CNS. In addition, results show that MHV infection impairs Cx43 expression in addition to gap junction communication in primary astrocytes. After infection, Cx43 did not traffic normally to the membrane to form gap junction plaques, and that could be the basis of reduced functional gap junction coupling between astrocytes. This is an important first step toward understanding how viruses affect Cx43 expression and trafficking at the cellular level. This may provide a basis for understanding how structural alterations of astrocytic gap junctions can disrupt gap junction communication between other CNS cells in altered CNS environments due to infection and inflammation. More specifically, alteration of Cx43 may be the basis of the destabilization of Cx47 in oligodendrocytes seen in and around inflammatory demyelinating plaques in MS patients.

The Renal Connexome and Possible Roles of Connexins in Kidney Diseases

Connexins are membrane-spanning proteins that allow for the formation of cell-to-cell channels and cell-to-extracellular space hemichannels. Many connexin subtypes are expressed in kidney cells. Some mutations in connexin genes have been linked to various human pathologies, including cardiovascular, neurodegenerative, lung, and skin diseases, but the exact role of connexins in kidney disease remains unclear. Some hypotheses about a connection between genetic mutations, endoplasmic reticulum (ER) stress, and the unfolded protein response (UPR) in kidney pathology have been explored. The potential relationship of kidney disease to abnormal production of connexin proteins, mutations in their genes together with ER stress, or the UPR is still a matter of debate. In this scenario, it is tantalizing to speculate about a possible role of connexins in the setting of kidney pathologies that are thought to be caused by a deregulated podocyte protein expression, the so-called podocytopathies. In this article, we give examples of the roles of connexins in kidney (patho)physiology and propose avenues for further research concerning connexins, ER stress, and UPR in podocytopathies that may ultimately help refine drug treatment.

DAP1, a negative regulator of autophagy, controls SubAB-mediated apoptosis and autophagy

Autophagy and apoptosis play critical roles in cellular homeostasis and survival. Subtilase cytotoxin (SubAB), produced by non-O157 type Shiga-toxigenic Escherichia coli (STEC), is an important virulence factor in disease. SubAB, a protease, cleaves a specific site on the endoplasmic reticulum (ER) chaperone protein BiP/GRP78, leading to ER stress, and induces apoptosis. Here we report that in HeLa cells, activation of a PERK (RNA-dependent protein kinase [PKR]-like ER kinase)-eIF2α (α subunit of eukaryotic initiation factor 2)-dependent pathway by SubAB-mediated BiP cleavage negatively regulates autophagy and induces apoptosis through death-associated protein 1 (DAP1). We found that SubAB treatment decreased the amounts of autophagy markers LC3-II and p62 as well as those of mTOR (mammalian target of rapamycin) signaling proteins ULK1 and S6K. These proteins showed increased expression levels in PERK knockdown or DAP1 knockdown cells. In addition, depletion of DAP1 in HeLa cells dramatically inhibited the SubAB-stimulated apoptotic pathway: SubAB-induced Bax/Bak conformational changes, Bax/Bak oligomerization, cytochrome c release, activation of caspases, and poly(ADP-ribose) polymerase (PARP) cleavage. These results show that DAP1 is a key regulator, through PERK-eIF2α-dependent pathways, of the induction of apoptosis and reduction of autophagy by SubAB.

Subtilase cytotoxin enhances Escherichia coli survival in macrophages by suppression of nitric oxide production through the inhibition of NF-κB activation

Subtilase cytotoxin (SubAB), which is produced by certain strains of Shiga-toxigenic Escherichia coli (STEC), cleaves an endoplasmic reticulum (ER) chaperone, BiP/Grp78, leading to induction of ER stress and caspase-dependent apoptosis. SubAB alters the innate immune response. SubAB pretreatment of macrophages inhibited lipopolysaccharide (LPS)-induced production of both monocyte chemoattractant protein 1 (MCP-1) and tumor necrosis factor α (TNF-α). We investigated here the mechanism by which SubAB inhibits nitric oxide (NO) production by mouse macrophages. SubAB suppressed LPS-induced NO production through inhibition of inducible NO synthase (iNOS) mRNA and protein expression. Further, SubAB inhibited LPS-induced IκB-α phosphorylation and nuclear localization of the nuclear factor-κB (NF-κB) p65/p50 heterodimer. Reporter gene and chromatin immunoprecipitation (ChIP) assays revealed that SubAB reduced LPS-induced NF-κB p65/p50 heterodimer binding to an NF-κB binding site on the iNOS promoter. In contrast to the native toxin, a catalytically inactivated SubAB mutant slightly enhanced LPS-induced iNOS expression and binding of NF-κB subunits to the iNOS promoter. The SubAB effect on LPS-induced iNOS expression was significantly reduced in macrophages from NF-κB1 (p50)-deficient mice, which lacked a DNA-binding subunit of the p65/p50 heterodimer, suggesting that p50 was involved in SubAB-mediated inhibition of iNOS expression. Treatment of macrophages with an NOS inhibitor or expression of SubAB by E. coli increased E. coli survival in macrophages, suggesting that NO generated by macrophages resulted in efficient killing of the bacteria and SubAB contributed to E. coli survival in macrophages. Thus, we hypothesize that SubAB might represent a novel bacterial strategy to circumvent host defense during STEC infection.

Connexin 43 and metabolic effect of fatty acids in stressed endothelial cells

Changes in the inner mitochondrial membrane potential (∆ψ) may lead either to apoptosis or to protective autophagy. Connexin 43 (Cx43), a gap junction protein, is suggested to affect mitochondrial membrane permeability. The aim of our study was to analyze Cx43 gene expression, Cx43 protein localization and mitochondrial function in the human endothelial cells stressed by dietary-free fatty acids (FFA) and TNFα. Human endothelial cells (HUVECs) were incubated with (10–30 uM) palmitic (PA), oleic (OA), eicosapentaenoic (EPA) or arachidonic (AA) acids for 24 h. TNFα (5 ng/ml) was added at the last 4 h of incubation. The Cx43 gene expression was analyzed by the quantitative real-time PCR. The Cx43 protein concentrations in whole cells and in the isolated mitochondria were measured. Changes in ∆ψ and Cx43 localization were analyzed by flow cytometry or fluorescence microscopy. Generated ATP was measured by a luminescence assay. TNFα, PA and OA significantly decreased ∆ψ, while AA (P = 0.047) and EPA (P = 0.004) increased ∆ψ value. Preincubation with EPA or AA partially prevented the TNFα-induced decrease of ∆ψ. Incubation with AA resulted in up-regulation of the Cx43 gene expression. AA or PA significantly increased Cx43 protein content; however, presence of TNFα in general aggravated the negative effect of FFA. Only EPA was found to increase ATP generation in HUVECs. The fatty acid-specific induction of changes in Cx43 expression and protein concentration as well as the normalization of ∆ψ and increase of ATP generation seem to be the separate, independent mechanisms of FFA-mediated modulatory effect in the human endothelial cells pathology.

Expression of connexin 43 and E-cadherin in choroidal melanoma

AIM

To investigate the expression of connexin 43 and epithelial cadherin (E-cadherin) in choroidal melanoma, to explore the clinical and pathological implications of expression of these proteins, and to determine their relations with malignant features.

METHODS

The expression of connexin 43 and E-cadherin in choroidal melanoma were detected by immunohistochemistry and correlated with clinicopathological features.

RESULTS

Positive rates of connexin 43 in choroidal melanomas and benign pigmented nevus tissues were 75% and 40% respectively with significant differences between the two groups (χ(2)=5.607, P=0.009). Positive rates of E-cadherin in choroidal melanomas and benign pigmented nevus tissues were 40% and 75% respectively with significant differences between the two groups (χ(2)=5.214, P=0.010). Significant overexpression of connexin 43 and reduction of E-cadherin expression was associated with the invasion to the sclera, and there were respectively significant differences between without and with scleral invasion groups (χ(2)=2.880, P=0.040; χ(2)=2.778, P=0.046). Overexpression of connexin 43 were correlated with tumor cell types and the expression of connexin 43 and E-cadherin may be correlated with each other.

CONCLUSION

The increased expression of connexin 43 and the decreased expression of E-cadherin may be involved in the process of invasion of choroidal melanoma. The overepression of connexin 43 and reduction of E-cadherin may contribute to the development of choroidal melanoma.

Fatal hemorrhage induced by subtilase cytotoxin from Shiga-toxigenic Escherichia coli

Subtilase cytotoxin (SubAB) is an AB(5) type toxin produced by a subset of Shiga-toxigenic Escherichia coli. The A subunit is a subtilase-like serine protease and cleaves an endoplasmic reticulum chaperone BiP. The B subunit binds to a receptor on the cell surface. Although SubAB is lethal for mice, the cause of death is not clear. In this study, we demonstrate in mice that SubAB induced small bowel hemorrhage and a coagulopathy characterized by thrombocytopenia, prolonged prothrombin time and activated partial thromboplastin time. SubAB also induced inflammatory changes in the small intestine as detected by ¹⁸F-fluoro-2-deoxy-d-glucose positron emission tomography imaging and histochemical analysis. Using RT-PCR and ELISA, SubAB was shown to increase interleukin-6 in a time-dependent manner. Thus, our results indicate that death in SubAB-treated mice may be associated with severe inflammatory response and hemorrhage of the small intestine, accompanied by coagulopathy and IL6 production.

Identification of subtilase cytotoxin (SubAB) receptors whose signaling, in association with SubAB-induced BiP cleavage, is responsible for apoptosis in HeLa cells

Subtilase cytotoxin (SubAB), which is produced by certain strains of Shiga-toxigenic Escherichia coli (STEC), causes the 78-kDa glucose-regulated protein (GRP78/BiP) cleavage, followed by induction of endoplasmic reticulum (ER) stress, leading to caspase-dependent apoptosis via mitochondrial membrane damage by Bax/Bak activation. The purpose of the present study was to identify SubAB receptors responsible for HeLa cell death. Four proteins, NG2, α2β1 integrin (ITG), L1 cell adhesion molecule (L1CAM), and hepatocyte growth factor receptor (Met), were identified to be SubAB-binding proteins by immunoprecipitation and purification, followed by liquid chromatography-tandem mass spectrometry analysis. SubAB-induced Bax conformational change, Bax/Bak complex formation, caspase activation, and cell death were decreased in β1 ITG, NG2, and L1CAM small interfering RNA-transfected cells, but unexpectedly, BiP cleavage was still observed. Pretreatment of cells with a function-blocking β1 ITG antibody (monoclonal antibody [MAb] P5D2) enhanced SubAB-induced caspase activation; MAb P5D2 alone had no effect on caspase activation. Furthermore, we found that SubAB induced focal adhesion kinase fragmentation, which was mediated by a proteasome-dependent pathway, and caspase activation was suppressed in the presence of proteasome inhibitor. Thus, β1 ITG serves as a SubAB-binding protein and may interact with SubAB-signaling pathways, leading to cell death. Our results raise the possibility that although BiP cleavage is necessary for SubAB-induced apoptotic cell death, signaling pathways associated with functional SubAB receptors may be required for activation of SubAB-dependent apoptotic pathways.

Suppression of expression of endoplasmic reticulum chaperones by Helicobacter pylori and its role in exacerbation of non-steroidal anti-inflammatory drug-induced gastric lesions

Both the use of non-steroidal anti-inflammatory drugs (NSAIDs), such as indomethacin, and infection with Helicobacter pylori are major causes of gastric ulcers. Although some clinical studies suggest that infection with H. pylori increases the risk of developing NSAID-induced gastric lesions, the molecular mechanism governing this effect is unknown. We recently found that in cultured gastric cells, expression of endoplasmic reticulum (ER) chaperones (such as 150-kDa oxygen-regulated protein (ORP150) and glucose-regulated protein 78 (GRP78)) is induced by NSAIDs and confers protection against NSAID-induced apoptosis, which is important in the development of NSAID-induced gastric lesions. In this study we have found that co-culture of gastric cells with H. pylori suppresses the expression of ER chaperones. This suppression was regulated at the level of transcription and accompanied by a reduction in the level of activating transcription factor 6 (ATF6), one of the transcription factors for ER chaperone genes. In vivo, inoculation of mice with H. pylori suppressed the expression of ER chaperones at gastric mucosa both with and without administration of indomethacin. Inoculation with H. pylori also stimulated formation of indomethacin-induced gastric lesions and mucosal cell death. In addition, we found that heterozygous ORP150-deficient mice are sensitive to the development of indomethacin-induced gastric lesions and mucosal cell death. The results of this study suggest that H. pylori exacerbates NSAID-induced gastric lesions through suppression of expression of ER chaperones, which stimulates NSAID-induced mucosal cell death.

Expression and significance of Cx43 and E-cadherin in gastric cancer and metastatic lymph nodes

Connexin 43(Cx43) and E-cadherin are concurrently expressed in many tumors and were ever classified as tumor suppressors in primary tumors (PT), whereas recent studies showed that these two proteins played specific roles in tumor metastasis. The aim of our study is to determine the expression of Cx43 and E-cadherin in primary gastric tumors (PTs) and matched metastatic lymph nodes (MLNs) and to explore the clinical and pathological implications of expression of these proteins. Immunohistochemical assay was conducted to detect the expression of Cx43 and E-cadherin in PTs and MLNs, and the clinical and pathological implications were analyzed by statistical methods. In PTs, the expression of Cx43 and E-cadherin was significantly reduced, compared to adjacent normal tissues (P < 0.01). The expression of Cx43 and E-cadherin was significantly increased in MLNs compared with PTs (P < 0.01 and P < 0.01, for Cx43 and E-cadherin, respectively), and some Cx43 and E-cadherin-negative PTs developed Cx43 and E-cadherin-positive MLNs. Furthermore, reduced expression of both Cx43 and E-cadherin significantly correlated with poor differentiation, advanced TNM stage, and lymph note metastasis of gastric cancers. Cx43 and E-cadherin expression significantly correlated with each other. We concluded that concurrent reduction in Cx43 and E-cadherin may contribute to the occurrence of gastric cancer. However, concurrent increased expression of Cx43 and E-cadherin may contribute to the efficient metastasis of gastric cancer to the lymph nodes.

Structure, biological functions and applications of the AB5 toxins

AB(5) toxins are important virulence factors for several major bacterial pathogens, including Bordetella pertussis, Vibrio cholerae, Shigella dysenteriae and at least two distinct pathotypes of Escherichia coli. The AB(5) toxins are so named because they comprise a catalytic A-subunit, which is responsible for disruption of essential host functions, and a pentameric B-subunit that binds to specific glycan receptors on the target cell surface. The molecular mechanisms by which the AB(5) toxins cause disease have been largely unravelled, including recent insights into a novel AB(5) toxin family, subtilase cytotoxin (SubAB). Furthermore, AB(5) toxins have become a valuable tool for studying fundamental cellular functions, and are now being investigated for potential applications in the clinical treatment of human diseases.

Gap junctions sensitize cancer cells to proteasome inhibitor MG132-induced apoptosis

Proteasome inhibition is a promising approach for cancer therapy. However, the mechanisms involved have not been fully elucidated. Gap junctions play important roles in the regulation of tumor cell phenotypes and mediation of the bystander effect in cancer therapy. Because the degradation of gap junction proteins involves the proteasome, we speculated that altered gap junctions might contribute to the antitumor activities of proteasome inhibition. Incubation of Hepa-1c1c7 cells with the proteasome inhibitor MG132 elevated the levels of gap junction protein connexin 43 (Cx43) and promoted gap junctional intercellular communication. This was associated with a marked accumulation of ubiquitylated Cx43 and a significantly decreased rate of Cx43 degradation. The elevated Cx43 contributed to MG132-induced cell apoptosis. This is shown by the observations that: (i) overexpression of Cx43 in the gap junction-deficient LLC-PK1 cells rendered them vulnerable to MG132-elicited cell injury; (ii) fibroblasts derived from Cx43-null mice were more resistant to MG-132 compared with Cx43 wild-type control; and (iii) the gap junction inhibitor flufenamic acid significantly attenuated cell damage caused by MG132 in Hepa-1c1c7 cells. Further studies demonstrated that MG132 activates endoplasmic reticulum stress. Exposure of cells to the endoplasmic reticulum stress inducers thapsigargin and tunicamycin also led to cell apoptosis, which was modulated by Cx43 levels in a way similar to MG132. These results suggested that elevated Cx43 sensitizes cells to MG132-induced cell apoptosis. Regulation of gap junctions could be an important mechanism behind the antitumor activities of proteasome inhibitors.

Escherichia coli Subtilase Cytotoxin

Subtilase cytotoxin (SubAB) is the prototype of a new AB(5) toxin family produced by a subset of Shiga toxigenic Escherichia coli (STEC) strains. Its A subunit is a subtilase-like serine protease and cytotoxicity for eukaryotic cells is due to a highly specific, single-site cleavage of BiP/GRP78, an essential Hsp70 family chaperone located in the endoplasmic reticulum (ER). This cleavage triggers a severe and unresolved ER stress response, ultimately triggering apoptosis. The B subunit has specificity for glycans terminating in the sialic acid N-glycolylneuraminic acid. Although its actual role in human disease pathogenesis is yet to be established, SubAB is lethal for mice and induces pathological features overlapping those seen in the haemolytic uraemic syndrome, a life-threatening complication of STEC infection. The toxin is also proving to be a useful tool for probing the role of BiP and ER stress in a variety of cellular functions.