Up-regulation of GRP78 and antiapoptotic signaling in murine peritoneal macrophages exposed to insulin

Endocrine and exocrine pancreas pathologies crosstalk: Insulin regulates the unfolded protein response in pancreatic exocrine acinar cells

Exocrine pancreas insufficiency is common in diabetic mellitus (DM) patients. Cellular stress is a prerequisite in the development of pancreatic pathologies such as acute pancreatitis (AP). The molecular mechanisms underlying exocrine pancreatic ER-stress in DM are largely unknown. We studied the effects of insulin and glucose (related to DM) alone and in combination with cerulein (CER)-induced stress (mimicking AP) on ER-stress unfolded protein response (UPR) in pancreatic acinar cells. Exocrine pancreas cells (AR42J) were exposed to high glucose (Glu, 25 mM) and insulin (Ins, 100 nM) levels with or without CER (10 nM). ER-stress UPR activation was analyzed at the transcript, protein, immunocytochemistry, western blotting, quantitative RT-PCR and XBP1 splicing, including; XBP1, sXBP1, ATF6, cleaved ATF6, IRE1-p, CHOP, Caspase-12 and Bax. Exocrine acinar cells exposed to high Ins or Ins+Glu concentrations (but not Glu alone) exhibited ER-stress UPR, demonstrated by significant increase of transcript and protein levels of downstream markers in the ATF6 and IRE1 transduction arms, including: sXBP1, cleaved ATF6, XBP1, CHOP, IRE1-p and caspase-12. UPR activation resulted in IRE1-p aggregation and nuclear trans-localization of cleaved activated ATF6 and sXBP1. Ins further aggravated UPR when cells were co-challenged with CER-induced stress, exacerbating the effects of CER alone. High Ins levels, typical to type-2-DM, activate the ER-stress UPR in pancreatic acinar cells, through the ATF6 and IRE1 pathways. This effect of Ins in naïve acinar cells further augments CER-induced UPR. Our data highlight molecular pathways through which DM enhances exocrine pancreas pathologies.

BRG1 and BRM SWI/SNF ATPases redundantly maintain cardiomyocyte homeostasis by regulating cardiomyocyte mitophagy and mitochondrial dynamics in vivo

There has been an increasing recognition that mitochondrial perturbations play a central role in human heart failure. Mitochondrial networks, whose function is to maintain the regulation of mitochondrial biogenesis, autophagy ('mitophagy') and mitochondrial fusion/fission, are new potential therapeutic targets. Yet our understanding of the molecular underpinning of these processes is just emerging. We recently identified a role of the SWI/SNF ATP-dependent chromatin remodeling complexes in the metabolic homeostasis of the adult cardiomyocyte using cardiomyocyte-specific and inducible deletion of the SWI/SNF ATPases BRG1 and BRM in adult mice (Brg1/Brm double mutant mice). To build upon these observations in early altered metabolism, the present study looks at the subsequent alterations in mitochondrial quality control mechanisms in the impaired adult cardiomyocyte. We identified that Brg1/Brm double-mutant mice exhibited increased mitochondrial biogenesis, increases in 'mitophagy', and alterations in mitochondrial fission and fusion that led to small, fragmented mitochondria. Mechanistically, increases in the autophagy and mitophagy-regulated proteins Beclin1 and Bnip3 were identified, paralleling changes seen in human heart failure. Evidence for perturbed cardiac mitochondrial dynamics included decreased mitochondria size, reduced numbers of mitochondria, and an altered expression of genes regulating fusion (Mfn1, Opa1) and fission (Drp1). We also identified cardiac protein amyloid accumulation (aggregated fibrils) during disease progression along with an increase in pre-amyloid oligomers and an upregulated unfolded protein response including increased GRP78, CHOP, and IRE-1 signaling. Together, these findings described a role for BRG1 and BRM in mitochondrial quality control, by regulating mitochondrial number, mitophagy, and mitochondrial dynamics not previously recognized in the adult cardiomyocyte. As critical to the pathogenesis of heart failure, epigenetic mechanisms like SWI/SNF chromatin remodeling seem more intimately linked to cardiac function and mitochondrial quality control mechanisms than previously realized.

Insulin resistance contributes to multidrug resistance in HepG2 cells via activation of the PERK signaling pathway and upregulation of Bcl-2 and P-gp

Liver tumorigenesis frequently causes insulin resistance which may be used as an independent risk factor for evaluation of survival and post-surgery relapse of liver cancer patients. In the present study, HepG2/IR, an insulin resistant HepG2 cell line, was established by exposing HepG2 cells to 0.5 µmol/l of insulin for 72 h, and comparison of HepG2/IR with the parental HepG2 cells indicated that the HepG2/IR cells showed significantly enhanced resistance to the most frequently used chemotherapeutics for solid tumors, such as cisplatin, 5-fluorouracil, vincristine and mitomycin. Flow cytometric analysis of cisplatin-treated HepG2/IR cells showed a significantly decreased hypodiploid peak and a significantly downregulated expression level of pro-apoptotic protein caspase-3 compared with the parental HepG2 cells. Our data further showed swollen endoplasmic reticulum (ER) in the cisplatin-treated HepG2/IR cells with significantly increased levels of glucose-regulated protein 78 (GRP78), phosphorylated protein kinase R-like ER kinase (p-PERK) and P-glycoprotein (P-gp). There was also an upregulated expression of anti-apoptotic protein B-cell lymphoma 2 (Bcl-2) whereas no significant change was observed for CCAAT-enhancer-binding protein homologous protein (CHOP), which is known to be induced by ER stress and to mediate apoptosis. Our results demonstrated that insulin resistance in HepG2 cells promoted a protective unfolded protein response and upregulated the expression of ER chaperone protein GRP78, which resulted in the phosphorylation of PERK kinase to activate the PERK-mediated ER stress signal transduction pathway and the upregulation of Bcl-2 and P-gp, leading to the inhibition of the caspase-3-dependent apoptosis pathway and to the survival of liver tumor cells.

Molecular mechanism of ER stress-induced gene expression of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) in macrophages

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a member of the tumor necrosis factor superfamily, whose members are capable of inducing apoptosis and inflammation. Endoplasmic reticulum stress (ERS) plays a key role in immune surveillance in macrophages. TRAIL mRNA and protein expression have previously been detected in macrophages; however, whether ERS has any effects on TRAIL expression in macrophages has not yet been determined. Here, we demonstrate that thapsigargin (TG) and tunicamycin (TM), two ERS inducers activated macrophages were able to increase TRAIL mRNA and protein expression in RAW264.7 macrophages, the culture supernatant of THP-1 cells, and mouse peritoneal macrophages, indicating that ERS as a potent inducer of TRAIL transcription and expression in macrophages. This effect was blocked by the specific JNK inhibitor SP600125 and transcription factor AP-1 inhibitor SR 1130. Interestingly, at the molecular level, regulation of TRAIL expression by ERS was accompanied by a significant decrease in cytokine signaling suppressor 3 (SOCS3). SOCS3 siRNA clearly increased the expression of TRAIL mRNA and protein under ERS by activating the AP-1 components phosphorylated c-Jun and phosphorylated c-Fos in RAW264.7 cells. In contrast, over-expression of SOCS3 reversed ERS-induced TRAIL expression. These findings provide in vitro evidence that SOCS3 plays a critical negative role in the regulation of ERS-induced TRAIL expression via the Jun N-terminal kinase/AP-1 signaling pathway in macrophages.

Autoantibodies against cell surface GRP78 promote tumor growth in a murine model of melanoma

Autoantibodies that react with GRP78 expressed on the cell-surface of many tumor cell lines occur in the sera of patients with prostate cancer, melanoma, and ovarian cancer. These autoantibodies are a negative prognostic factor in prostate cancer and, when purified, stimulate tumor cell proliferation in vitro. It is unclear, however, whether these immunoglobulin Gs are merely a biomarker, or whether they actually promote the tumor growth in vivo. We immunized C57Bl/6 mice with recombinant GRP78 and then implanted the B16F1 murine melanoma cell line as flank tumors. We used the antisera from these mice for in-vitro cell signaling and proliferation assays. The immunodominant epitope in patients with cancer was well represented in the antibody repertoire of these immunized mice. We observed significantly accelerated tumor growth, and shortened survival in GRP78-immunized mice compared with controls. Furthermore, antisera from these mice, and purified anti-GRP78 immunoglobulin G from similarly immunized mice, stimulate Akt phosphorylation and proliferation in B16F1 and human DM6 melanoma cells in culture. These studies show a causal link between a humoral response to GRP78 and the progression of cancer in a murine melanoma model. They support the hypothesis that such autoantibodies are involved in the progression of human cancers and are not simply a biomarker. As GRP78 is present on the surface of many types of cancer cells, this hypothesis has broad clinical and therapeutic implications.

Paeoniflorin protects cells from GalN/TNF-α-induced apoptosis via ER stress and mitochondria-dependent pathways in human L02 hepatocytes

Paeoniflorin (PF) is one of the main effective components extracted from the root of Paeonia lactiflora, which has been used clinically to treat hepatitis in traditional Chinese medicine, but the details of the underlying mechanism remain unknown. The present study was designed to investigate the mechanism of protective effect of PF on d-galactosamine (GalN) and tumor necrosis factor-α (TNF-α)-induced cell apoptosis using human L02 hepatocytes. Our results confirmed that PF could attenuate GalN/TNF-α-induced apoptotic cell death in a dose-dependent manner. The disruption of mitochondrial membrane potential and the disturbance of intracellular Ca(2+) concentration were also recovered by PF. Western blot analysis revealed that GalN/TNF-α induced the activation of a number of signature endoplasmic reticulum (ER) stress and mitochondrial markers, while PF pre-treatment had a marked dose-dependent suppression on them. Additionally, the anti-apoptotic effect of PF was further evidenced by the inhibition of caspase-3/9 activities in L02 cells. These findings suggest that PF can effectively inhibit hepatocyte apoptosis and the underlying mechanism is related to the regulating mediators in ER stress and mitochondria-dependent pathways.

Insulin regulates the unfolded protein response in human adipose tissue

Endoplasmic reticulum (ER) stress is increased in obesity and is postulated to be a major contributor to many obesity-related pathologies. Little is known about what causes ER stress in obese people. Here, we show that insulin upregulated the unfolded protein response (UPR), an adaptive reaction to ER stress, in vitro in 3T3-L1 adipocytes and in vivo, in subcutaneous (sc) adipose tissue of nondiabetic subjects, where it increased the UPR dose dependently over the entire physiologic insulin range (from ∼ 35 to ∼ 1,450 pmol/L). The insulin-induced UPR was not due to increased glucose uptake/metabolism and oxidative stress. It was associated, however, with increased protein synthesis, with accumulation of ubiquitination associated proteins, and with multiple posttranslational protein modifications (acetylations, methylations, nitrosylations, succinylation, and ubiquitinations), some of which are potential causes for ER stress. These results reveal a new physiologic role of insulin and provide a putative mechanism for the development of ER stress in obesity. They may also have clinical and therapeutic implications, e.g., in diabetic patients treated with high doses of insulin.

A murine monoclonal antibody directed against the carboxyl-terminal domain of GRP78 suppresses melanoma growth in mice

The HSP70 family member GRP78 is a selective tumor marker upregulated on the surface of many tumor cell types, including melanoma, where it acts as a growth factor receptor-like protein. Receptor-recognized forms of the proteinase inhibitor α2-macroglobulin (α2M*) are the best-characterized ligands for GRP78, but in melanoma and other cancer patients, autoantibodies arise against the NH2-terminal domain of GRP78 that react with tumor cell-surface GRP78. This causes the activation of signaling cascades that are proproliferative and antiapoptotic. Antibodies directed against the COOH-terminal domain of GRP78, however, upregulate p53-mediated proapoptotic signaling, leading to cell death. Here, we describe the binding characteristics, cell signaling properties, and downstream cellular effects of three novel murine monoclonal antibodies. The NH2-terminal domain-reactive antibody, N88, mimics α2M* as a ligand and drives PI 3-kinase-dependent activation of Akt and the subsequent stimulation of cellular proliferation in vitro. The COOH-terminal domain-reactive antibody, C38, acts as an antagonist of both α2M* and N88, whereas another, C107, directly induces apoptosis in vitro. In a murine B16F1 melanoma flank tumor model, we demonstrate the acceleration of tumor growth by treatment with N88, whereas C107 significantly slowed tumor growth whether administered before (P<0.005) or after (P<0.05) tumor implantation.

Sub-proteomic study on macrophage response to Candida albicans unravels new proteins involved in the host defense against the fungus

In previous proteomic studies on the response of murine macrophages against Candida albicans, many differentially expressed proteins involved in processes like inflammation, cytoskeletal rearrangement, stress response and metabolism were identified. In order to look for proteins important for the macrophage response, but in a lower concentration in the cell, 3 sub-cellular extracts were analyzed: cytosol, organelle/membrane and nucleus enriched fractions from RAW 264.7 macrophages exposed or not to C. albicans SC5314 for 3 h. The samples were studied using DIGE technology, and 17 new differentially expressed proteins were identified. This sub-cellular fractionation permitted the identification of 2 mitochondrion proteins, a membrane receptor, Galectin-3, and some ER related proteins, that are not easily detected in total cell extracts. Besides, the study of different fractions allowed us to detect, not only total increase in Galectin-3 protein amount, but its distinct allocation along the interaction. The identified proteins are involved in the pro-inflammatory and oxidative responses, immune response, unfolded protein response and apoptosis. Some of these processes increase the host response and others could be the effect of C. albicans resistance to phagocytosis. Thus, the sub-proteomic approach has been a very useful tool to identify new proteins involved in macrophage-fungus interaction. This article is part of a Special Issue entitled: Translational Proteomics.

The role of endoplasmic reticulum stress in the early stage of diabetic retinopathy

The aim of this study was to evaluate the role of endoplasmic reticulum (ER) stress in diabetic retinopathy (DR) using in vitro and in vivo models. For the in vivo studies, diabetes was induced in rats, and retinal expression of glucose-regulated protein 78 (GRP78), activating transcription factor 4 (ATF4), C/EBP homologous protein (CHOP), and vascular endothelial growth factor (VEGF) in groups of rats at 1, 3, and 6 months was assessed. For the in vitro studies, human retinal capillary endothelial cells (HRCECs) were cultured in the presence of varying glucose concentrations, and the expression of mRNA and protein for GRP78, ATF4, CHOP, and VEGF was assessed. The chosen glucose concentrations were reflective of those apparent in diabetic patients. Expression of VEGF and CHOP mRNA levels were significantly increased in diabetic rats compared to control rats at 1, 3, and 6 months (P < 0.05). In HRCECs cultured in the presence of high as well as variable glucose concentrations, CHOP expression and apoptosis were significantly increased (P < 0.05). However, GRP78 and ATF4 expression levels were unchanged. Our findings suggest that early progression of DR may be mediated by ER stress, but probably does not involve changes in ATF4 or GRP78.

Modulation of the unfolded protein response by GRP78 in prostate cancer

The unfolded protein response (UPR) is an adaptive survival mechanism through which cells can weather the stress of misfolded protein accumulation induced by a wide variety of pathophysiologic and pharmacologic insults. The ER chaperone GRP78 is a central modulator of the UPR both through its protein-binding capacity and its direct regulation of the UPR signaling molecules IRE1α, PERK, and ATF6. Recent reports have revealed the presence of GRP78 on the surface of cancer cells. Biological roles for cell-surface GRP78 include competing NH(2)-domain and COOH-domain agonist receptor activities that induce opposite effects on proliferation and apoptosis. Modulation of the UPR impacts both of these processes directly and indirectly. Here, we outline methods that we use to investigate UPR modulation via direct ligation of cell-surface GRP78. Specifically, we review methods of cell culture, cell-signaling analysis with emphasis on UPR components, and ultimately, the impact that these have on cell proliferation, survival, and apoptosis.

Induction of the unfolded protein response after monocyte to macrophage differentiation augments cell survival in early atherosclerotic lesions

Endoplasmic reticulum (ER) stress causes macrophage cell death within advanced atherosclerotic lesions, thereby contributing to necrotic core formation and increasing the risk of atherothrombotic disease. However, unlike in advanced lesions, the appearance of dead/apoptotic macrophages in early lesions is less prominent. Given that activation of the unfolded protein response (UPR) is detected in early lesion-resident macrophages and can enhance cell survival against ER stress, we investigated whether UPR activation occurs after monocyte to macrophage differentiation and confers a cytoprotective advantage to the macrophage. Human peripheral blood monocytes were treated with monocyte colony-stimulating factor to induce macrophage differentiation, as assessed by changes in ultrastructure and scavenger receptor expression. UPR markers, including GRP78, GRP94, and spliced XBP-1, were induced after macrophage differentiation and occurred after a significant increase in de novo protein synthesis. UPR activation after differentiation reduced macrophage cell death by ER stress-inducing agents. Further, GRP78 overexpression in macrophages was sufficient to reduce ER stress-induced cell death. Consistent with these in vitro findings, UPR activation was observed in viable lesion-resident macrophages from human carotid arteries and from the aortas of apoE(-/-) mice. However, no evidence of apoptosis was observed in early lesion-resident macrophages from the aortas of apoE(-/-) mice. Thus, our findings that UPR activation occurs during macrophage differentiation and is cytoprotective against ER stress-inducing agents suggest an important cellular mechanism for macrophage survival within early atherosclerotic lesions.

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.

Corticosterone exerts immunostimulatory effects on macrophages via endoplasmic reticulum stress

BACKGROUND

: Glucocorticoids are the central effector hormones for the hypothalamic-pituitary-adrenal axis. However, the effects of endogenous glucocorticoids on the immune system are not understood completely.

METHODS

: Macrophage function (adherence, chemotaxis and cytokine production) was assessed in the presence of increasing concentrations of corticosterone. The role of endoplasmic reticulum (ER) stress in corticosterone immunoregulation was determined with thapsigargin and plasmid pGCL-GFP-siXBP1. Mifepristone was used to determine the role of glucocorticoid receptor in the corticosterone-induced ER stress response.

RESULTS

: Corticosterone exerted immunostimulatory effects on macrophage function at low concentrations. No effects were observed at high concentrations in the absence of immunological stimulation. Low-dose corticosterone induced ER stress, which was correlated to the corticosterone immunostimulatory activities. Expression of X box-binding protein (XBP) 1, but not activating transcription factor 6, was significantly increased at both mRNA and protein levels only in the presence of low-dose corticosterone. Inhibition of XBP1 expression with small interfering RNA significantly inhibited the corticosterone immunostimulatory effects. In addition, pretreatment of macrophages with mifepristone significantly inhibited the expression of glucose response protein 78 and XBP1 in macrophages by low-dose corticosterone.

CONCLUSION

: At low concentrations, endogenous glucocorticoids exert immunostimulatory actions on macrophages. The underlying mechanisms may be correlated to ER stress via the glucocorticoid receptor, in which XBP1 plays an important role.

Effect of p38MAPK on immunofunction of spleen macrophages from heat stressed rats

AIM: To investigate the role of p38MAPK in Bip protein-mediated functional changes of mild heat stressed rat splenic macrophages in vitro.

METHODS: Rat splenic macrophages were pretreated with p38MAPK inhibitor and placed into 41 ℃ incubator for mild heat stress. One hour later, temperature was restored to 37 ℃ in inhibition group. Non stressed rat spleen macrophages were assigned to the control group, and macrophages which was heat stressed at 41 ℃ for 1 h (stress group) were used as controls, too. Three groups were detected for macrophage phagocytosis, cytotoxicity and chemotaxis. At the same time p38MAPK protein and Bip protein expressions were detected.

RESULTS: p38MAPK inhibitor pretreated rat splenic macrophages, when compared with the stress group, their phagocytosis, cytotoxicity and chemotaxis were significantly lowered after mild heat stress (0.17 ± 0.01 vs 0.74 ± 0.03, 33.32 ± 3.55 vs 82.07 ± 5.17, 24.20% ± 2.39% vs 60.80% ± 4.02%, all P < 0.01). In stress group p38MAPK protein expressions were significantly increased; compared with the stress group, p38MAPK protein expressions were significantly inhibited after p38MAPK inhibitor pretreatment in inhibition group (p38/β-actin: 2.863 ± 0.794 vs 4.752 ± 1.386, P < 0.01). p38MAPK inhibitor pretreatment also caused changes in Bip protein expressions (Bip/β-actin) in the stress group from 1.2702 ± 0.5345 dropped to 1.0281 ± 1.0614 in inhibition group (P < 0.05).

CONCLUSION: p38 inhibitors can significantly inhibit mild heat stressed rat splenic macrophage phagocytosis, cytotoxicity and chemotaxis, which inhibit p38MAPK and Bip protein expressions.

Glycolytic control of adjuvant-induced macrophage survival: role of PI3K, MEK1/2, and Bcl-2

Uptake by macrophages forms an important part of the mode of action of particulate adjuvants such as oil-in-water emulsions and alum. We have found previously that such adjuvants promote macrophage survival and suggested that this response may contribute to their efficacy. To explore this adjuvant activity further, we have investigated whether oil-in-water emulsion stimulates glucose uptake in macrophages and whether such uptake is relevant to the promotion of survival. We found that oil-in-water emulsion stimulated glucose uptake in a biphasic manner. The first acute phase was independent of mRNA and protein synthesis but appeared to require PI3K activity. In contrast, the second chronic phase was dependent on mRNA and protein synthesis. Importantly, the second phase of glucose uptake required MEK1/2 as well as PI3K activity, indicating that the MEK1/2 pathway can also contribute to cellular glucose uptake. The increased glucose transporter 1 expression during the second phase and long-term survival also appeared to be dependent on PI3K and MEK1/2 signaling pathways. Metabolism of the glucose was required for the emulsion-stimulated survival as well as the increase of prosurvival Bcl-2 transcript levels and maintenance of Bcl-2 protein expression. As transgenic overexpression of Bcl-2 enhances the survival of macrophages in the absence of growth factor, the glycolytic control of Bcl-2 levels may play a central role in emulsion-stimulated macrophage survival. Enhanced glucose uptake by macrophages may therefore be critical to the action of particulate adjuvants.

Activation of the unfolded protein response is required for defenses against bacterial pore-forming toxin in vivo

Pore-forming toxins (PFTs) constitute the single largest class of proteinaceous bacterial virulence factors and are made by many of the most important bacterial pathogens. Host responses to these toxins are complex and poorly understood. We find that the endoplasmic reticulum unfolded protein response (UPR) is activated upon exposure to PFTs both in Caenorhabditis elegans and in mammalian cells. Activation of the UPR is protective in vivo against PFTs since animals that lack either the ire-1-xbp-1 or the atf-6 arms of the UPR are more sensitive to PFT than wild-type animals. The UPR acts directly in the cells targeted by the PFT. Loss of the UPR leads to a normal response against unrelated toxins or a pathogenic bacterium, indicating its PFT-protective role is specific. The p38 mitogen-activated protein (MAPK) kinase pathway has been previously shown to be important for cellular defenses against PFTs. We find here that the UPR is one of the key downstream targets of the p38 MAPK pathway in response to PFT since loss of a functional p38 MAPK pathway leads to a failure of PFT to properly activate the ire-1-xbp-1 arm of the UPR. The UPR-mediated activation and response to PFTs is distinct from the canonical UPR-mediated response to unfolded proteins both in terms of its activation and functional sensitivities. These data demonstrate that the UPR, a fundamental intracellular pathway, can operate in intrinsic cellular defenses against bacterial attack.

Pore-forming toxins (PFTs) are bacterial toxins that form holes at the plasma membrane of cells and play an important role in the pathogenesis of many important human pathogens. Although PFTs comprise an important and the single largest class of bacterial protein virulence factors, how cells respond to these toxins has been understudied. We describe here the surprising discovery that a fundamental pathway of eukaryotic cell biology, the endoplasmic reticulum unfolded protein response (UPR), is activated by pore-forming toxins in Caenorhabditis elegans and mammalian cells. We find that this activation is functionally important since loss of either of two of the three arms of UPR leads to hypersensitivity of the nematode to attack by PFTs. The response of the UPR to PFTs can be separated from its response to unfolded proteins both at the level of activation and functional relevance. The response of the UPR to PFTs is dependent on a central pathway of cellular immunity, the p38 MAPK pathway. Our data show that the response of cells to bacterial attack can reveal unanticipated uses and connections between fundamental cell biological pathways.

Insulin induces chaperone and CHOP gene expressions in adipocytes

Adipocyte secretes bioactive proteins called adipocytokines, and biosynthesis of secretory proteins requires molecular chaperones and folding enzymes in endoplasmic reticulum (ER). ER chaperones are known to be induced by unfolded protein response (UPR) and growth factors, however, it has not been determined how ER chaperones expression is regulated in adipocytes. Here we show that insulin treatment induced GRP78 and ERO1L mRNA levels in 3T3-L1 adipocytes. Insulin also upregulated CHOP mRNA levels, but did not induce phosphorylation of eIF2alpha. Pretreatment with insulin protected 3T3-L1 adipocytes against thapsigargin-mediated phosphorylation of eIF2alpha but did not against DTT-mediated one. In vivo mice study showed that GRP78 and CHOP expressions were regulated by feeding conditions. These results suggest that insulin signaling is important to induce mRNA expressions of GRP78 and CHOP, and may have a protective role against UPR.

Plasminogen structural domains exhibit different functions when associated with cell surface GRP78 or the voltage-dependent anion channel

Both the voltage-dependent anion channel and the glucose-regulated protein 78 have been identified as plasminogen kringle 5 receptors on endothelial cells. In this study, we demonstrate that kringle 5 binds to a region localized in the N-terminal domain of the glucose-regulated protein 78, whereas microplasminogen does so through the C-terminal domain of the glucose-regulated protein 78. Both plasminogen fragments induce Ca(2+) signaling cascades; however, kringle 5 acts through voltage-dependent anion channel and microplasminogen does so via the glucose-regulated protein 78. Because trafficking of voltage-dependent anion channel to the cell surface is associated with heat shock proteins, we investigated a possible association between voltage-dependent anion channel and glucose-regulated protein 78 on the surface of 1-LN human prostate tumor cells. We demonstrate that these proteins co-localize, and changes in the expression of the glucoseregulated protein 78 affect the expression of voltage-dependent anion channel. To differentiate the functions of these receptor proteins, either when acting singly or as a complex, we employed human hexokinase I as a specific ligand for voltage-dependent anion channel, in addition to kringle 5. We show that kringle 5 inhibits 1-LN cell proliferation and promotes caspase-7 activity by a mechanism that requires binding to cell surface voltage-dependent anion channel and is inhibited by human hexokinase I.

Soft substrate induces apoptosis by the disturbance of Ca2+ homeostasis in renal epithelial LLC-PK1 cells

Different rigidities of adhesive collagen substrate affect cellular functions with unclear mechanisms. Here, we cultured a renal epithelial cell line (LLC-PK1) and a tumor cell line (HeLa) on substrates of different rigidities and compared the cell type-specific responses. The culture dish was coated with a very thin layer of collagen gel (control group) or overlaid with collagen gel (soft substrate). LLC-PK1 cells contracted as they grew on collagen gel and the apoptotic bodies obviously appeared with time. The protein levels of procaspase-12 and its downstream target procaspase-3 were decreased when LLC-PK1 cells cultured on collagen gel. Mu-calpain was activated on collagen gel. Collage gel also induced the cleavage of alpha-spectrin which resulted in the disorganization of actin cytoskeleton. In contrast, there was no significant change in cytochrome c revelation, mitochondrial membrane potential, and the protein levels of procaspase-8 and procaspase-9. Moreover, soft substrate caused elevated cytosolic Ca(2+), Ca(2+) overload in ER and upregulation of capacitative calcium entry. Ca(2+) chelator or channel blocker partially rescued the collagen-gel induced apoptosis by inhibiting mu-calpain activation. In contrast, for HeLa cells cultured either on collagen gel or on gel-coated dish, there was no significant change in positive Annexin V staining, no activation of procaspase-12 and no cleavage of mu-calpain. Thus, soft substrate induces apoptosis in LLC-PK1 cells by the disturbance of Ca(2+) homeostasis.

Insulin protects liver cells from saturated fatty acid-induced apoptosis via inhibition of c-Jun NH2 terminal kinase activity

Hepatocyte apoptosis is increased in patients with nonalcoholic steatohepatitis and correlates with disease severity. Long-chain saturated fatty acids, such as palmitate and stearate, induce apoptosis in liver cells. The present study examined insulin-mediated protection against saturated fatty acid-induced apoptosis in the rat hepatoma cell line, H4IIE, and primary rat hepatocytes. Cells were provided a control media (no fatty acids) or the same media containing 250 micromol/liter of albumin-bound oleate or palmitate for 16 h. Insulin concentrations were 0, 1, 10, or 100 nmol/liter (n=4-6/treatment). Palmitate, but not oleate, activated caspase-3 and induced DNA fragmentation in the absence of insulin. Insulin reduced palmitate-mediated activation of caspase-3 and DNA fragmentation in a dose-dependent manner. Phosphatidylinositol 3-kinase inhibitors abolished these effects of insulin. Insulin-mediated inhibition of palmitate-induced apoptosis was not due to an augmentation in the unfolded protein response or increased expression of genes encoding the inhibitor of apoptosis proteins, inhibitor of apoptosis protein-2 and X-linked mammalian inhibitor of apoptosis protein. Palmitate, but not oleate, increased c-Jun NH2 terminal kinase activity in the absence of insulin. Insulin or SP600125, a chemical inhibitor of c-Jun NH2 terminal kinase, blocked palmitate-mediated activation of c-Jun NH2 terminal kinase and reduced apoptosis. These data suggest that insulin is an important determinant of saturated fatty acid-induced apoptosis in liver cells and may have implications for fatty acid-mediated liver cell injury in insulin-deficient and/or -resistant states.

Induction of endoplasmic reticulum stress in retinal pericytes by glucose deprivation

Diabetic retinopathy is one of the major microvascular complications associated with diabetes mellitus, and the selective degeneration of retinal capillary pericytes is considered to be a hallmark of early retinopathy. Because glucose fluctuations commonly occur in diabetes, we hypothesized that these fluctuations will increase the endoplasmic reticulum (ER) stress and induce the unfolded protein response (UPR) in retinal pericytes. To study whether ER stress and the UPR can be induced in retinal pericytes, rat retinal capillary pericytes were cultured in different concentrations of glucose. Hypoglycemia but not hyperglycemia was found to activate UPR-specific enzymes in pericytes. Strong UPR activation leading to apoptosis was also observed when pericytes were cultured in glucose concentrations that were reduced from high to low or no glucose. These results indicate that induction of UPR is related not only to absolute concentrations but also to a shifting from higher to lower concentrations of glucose.

ATrypanosoma cruzi antigen signals CD11b+ cells to secrete cytokines that promote polyclonal B cell proliferation and differentiation into antibody-secreting cells

Microbial-induced polyclonal activation of B cells is a common event in several forms of infections, and is believed to play a crucial role both for enhancing the production of specific antibodies and for maintenance of B cell memory. Therefore, a major challenge in biomedical research is the identification of pathogen-derived products capable of rapidly mounting B cell expansion and differentiation. Here we report that glutamate dehydrogenase (GDH) stimulates polyclonal proliferation and differentiation of naive B cells. This stimulation was found to be T cell independent, but to absolutely require CD11b(+) cells. Moreover, we demonstrate that stimulation of CD11b(+) cells by GDH leads to the production of IL-6, IL-10 and B cell-activating factor (BAFF), all of which combine to powerfully induce B cell expansion. Importantly, IL-6 and IL-10 further drive B cell terminal differentiation into plasma cells by up-regulating critical transcription factors and immunoglobulin secretion. Our data provide the first evidence that a protozoan antigen can induce BAFF production by accessory cells, which in concert with other cytokines trigger polyclonal B cell activation.

Differential protein expression of murine macrophages upon interaction with Candida albicans

Numerous studies highlight the importance of macrophages for optimal host protection against systemic Candida albicans infections. We chose the murine macrophage cell line RAW 264.7 and the wild-type strain C. albicans SC5314 to study of the induced expression/repression of proteins in macrophages when they are in contact with C. albicans, based on 2-DE, comparison between different gels and protein identification. RAW 264.7 cells were allowed to interact with C. albicans cells for 45 min, and a significant differential protein expression was observed in these macrophages compared to controls. Gels were stained with SYPRO Ruby, allowing a better quantification of the intensity of the protein spots. Fifteen spots were up-regulated, whereas 32 were down-regulated; 60 spots appeared and 49 disappeared. Among them, we identified 11 proteins: annexin I, LyGDI (GDID4), Hspa5 (Grp78, Bip), tropomyosin 5 and L-plastin, that augment; and Eif3s5, Hsp60, Hspa9a, Grp58 (ER75), and Hspa8a (Hsc70), that decrease. The translation elongation factor (Eef2p) is modified in some of its different protein species. Many processes seem to be affected: cytoskeletal organisation, oxidative responses (superoxide and nitric oxide production) and protein biosynthesis and refolding.

Activation and cross-talk between Akt, NF-kappaB, and unfolded protein response signaling in 1-LN prostate cancer cells consequent to ligation of cell surface-associated GRP78

Binding of activated forms of the proteinase inhibitor alpha2-macroglobulin (alpha2M*) to cell surface-associated GRP78 on 1-LN human prostate cancer cells causes their proliferation. We have now examined the interplay between Akt activation, regulation of apoptosis, the unfolded protein response, and activation of NF-kappaB in alpha2M*-induced proliferation of 1-LN cells. Exposure of cells to alpha2M* (50 pM) induced phosphatidylinositol 3-kinase-dependent activation of Akt by phosphorylation at Thr-308 and Ser-473 with a concomitant 60-80% increase in Akt-associated kinase activity. ERK1/2 and p38 MAPK were also activated, but there was only a marginal effect on JNK activation. Treatment of 1-LN cells with alpha2M* down-regulated apoptosis and promoted NF-kappaB activation as shown by increases of Bcl-2, p-Bad(Ser-136), p-FOXO1(Ser-253), p-GSK3beta(Ser-9), XIAP, NF-kappaB, cyclin D1, GADD45beta, p-ASK1(Ser-83), and TRAF2 in a time of incubation-dependent manner. alpha2M* treatment of 1-LN cells, however, showed no increase in the activation of caspase -3, -9, or -12. Under these conditions, we observed increased unfolded protein response signaling as evidenced by elevated levels of GRP78, IRE1alpha, XBP-1, ATF4, ATF6, p-PERK, p-eIF2alpha, and GADD34 and reduced levels of GADD153. Silencing of GRP78 gene expression by RNAi suppressed activation of Akt(Thr-308), Akt(Ser-473), and IkappaB kinase alpha kinase. The effects of alpha2M* on the NF-kappaB activation, antiapoptotic signaling, unfolded protein response signaling, and proapoptotic signaling were also reversed by this treatment. In conclusion, alpha2M* promotes cellular proliferation of 1-LN prostate cancer cells by activating MAPK and Akt-dependent signaling, down-regulating apoptotic signaling, and activating unfolded protein response signaling.