Advanced glycation end products suppress osteoblastic differentiation of stromal cells by activating endoplasmic reticulum stress

Advanced glycation end products (AGEs) are involved in bone quality deterioration in diabetes mellitus. We previously showed that AGE2 or AGE3 inhibited osteoblastic differentiation and mineralization of mouse stromal ST2 cells, and also induced apoptosis and decreased cell growth. Although quality management for synthesized proteins in endoplasmic reticulum (ER) is crucial for the maturation of osteoblasts, the effects of AGEs on ER stress in osteoblast lineage are unknown. We thus examined roles of ER stress in AGE2- or AGE3-induced suppression of osteoblastogenesis of ST2 cells. An ER stress inducer, thapsigargin (TG), induced osteoblastic differentiation of ST2 cells by increasing the levels of Osterix, type 1 collagen (Col1), alkaline phosphatase (ALP) and osteocalcin (OCN) mRNA. AGE2 or AGE3 suppressed the levels of ER stress sensors such as IRE1α, ATF6 and OASIS, while they increased the levels of PERK and its downstream molecules, ATF4. A reduction in PERK level by siRNA did not affect the AGEs-induced suppression of the levels of Osterix, Col1 and OCN mRNA. In conclusion, AGEs inhibited the osteoblastic differentiation of stromal cells by suppressing ER stress sensors and accumulating abnormal proteins in the cells. This process might accelerate AGEs-induced suppression of bone formation found in diabetes mellitus.     

ER stress is the initial response to polyglutamine toxicity in PC12 cells

Persistent endoplasmic reticulum (ER) stress and impairment of the ubiquitin-proteasome system (UPS) cause neuronal cell death. However, the relationship between these two phenomena remains controversial. In our current study, we have utilized an expanded polyglutamine fusion protein (polyQ81) expression system in PC12 cells to further examine the involvement of ER stress and UPS impairment in cell death. The expression of polyQ81-induced ER stress and cell death. PolyQ81 also induced the activation of c-Jun N-terminal kinase (JNK) and caspase-3 and an increase in polyubiquitin immunoreactivity, suggesting UPS impairment. ER stress was induced prior to the accumulation of polyubiquitinated proteins. Low doses of lactacystin had almost similar effects on cell viability and on the activation of JNK and caspase-3 between normal cells and polyQ81-expressing cells. These results suggest that ER stress mediates polyglutamine toxicity prior to UPS impairment during the initial stages of these toxic effects.     

TRAM1 is involved in disposal of ER membrane degradation substrates

ER quality control consists of monitoring protein folding and targeting misfolded proteins for proteasomal degradation. ER stress results in an unfolded protein response (UPR) that selectively upregulates proteins involved in protein degradation, ER expansion, and protein folding. Given the efficiency in which misfolded proteins are degraded, there likely exist cellular factors that enhance the export of proteins across the ER membrane. We have reported that translocating chain-associated membrane protein 1 (TRAM1), an ER-resident membrane protein, participates in HCMV US2- and US11-mediated dislocation of MHC class I heavy chains (Oresic, K., Ng, C.L., and Tortorella, D. 2009). Consistent with the hypothesis that TRAM1 is involved in the disposal of misfolded ER proteins, cells lacking TRAM1 experienced a heightened UPR upon acute ER stress, as evidenced by increased activation of unfolded protein response elements (UPRE) and elevated levels of NF-κB activity. We have also extended the involvement of TRAM1 in the selective degradation of misfolded ER membrane proteins Cln6^M241T and US2, but not the soluble degradation substrate α₁-antitrypsin null_HK. These degradation model systems support the paradigm that TRAM1 is a selective factor that can enhance the dislocation of ER membrane proteins.     

The role of MAPK signalling pathways in the response to endoplasmic reticulum stress

Perturbations in endoplasmic reticulum (ER) homeostasis, including depletion of Ca^2 + or altered redox status, induce ER stress due to protein accumulation, misfolding and oxidation. This activates the unfolded protein response (UPR) to re-establish the balance between ER protein folding capacity and protein load, resulting in cell survival or, following chronic ER stress, promotes cell death. The mechanisms for the transition between adaptation to ER stress and ER stress-induced cell death are still being understood. However, the identification of numerous points of cross-talk between the UPR and mitogen-activated protein kinase (MAPK) signalling pathways may contribute to our understanding of the consequences of ER stress. Indeed, the MAPK signalling network is known to regulate cell cycle progression and cell survival or death responses following a variety of stresses. In this article, we review UPR signalling and the activation of MAPK signalling pathways in response to ER stress. In addition, we highlight components of the UPR that are modulated in response to MAPK signalling and the consequences of this cross-talk. We also describe several diseases, including cancer, type II diabetes and retinal degeneration, where activation of the UPR and MAPK signalling contribute to disease progression and highlight potential avenues for therapeutic intervention. This article is part of a Special Issue entitled: Calcium Signaling In Health and Disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.     

Effect of Liraglutide on endoplasmic reticulum stress in diabetes

Endoplasmic reticulum (ER) stress is associated with the development of diabetes. The present study sought to investigate the effect of Liraglutide, a glucagon like peptide 1 analogue, on ER stress in β-cells. We found that Liraglutide protected the pancreatic INS-1 cells from thapsigargin-induced ER stress and the ER stress associated cell apoptosis, mainly by suppressing the PERK and IRE1 pathways. We further tested the effects of Liraglutide in the Akita mouse, an ER-stress induced type 1 diabetes model. After administration of Liraglutide for 8 weeks, p-eIF2α and p-JNK were significantly decreased in the pancreas of the Akita mouse, while the treatment showed no significant impact on the levels of insulin of INS-cells. Taken together, our findings suggest that Liraglutide may protect pancreatic cells from ER stress and its related cell death.     

Peroxisome deficiency-induced ER stress and SREBP-2 pathway activation in the liver of newborn PEX2 knock-out mice

Disruption of the Pex2 gene leads to peroxisome deficiency and widespread metabolic dysfunction. We previously demonstrated that peroxisomes are critical for maintaining cholesterol homeostasis, using peroxisome-deficient Pex2^−/− mice on a hybrid Swiss Webster × 129S6/SvEv (SW/129) genetic background. Peroxisome deficiency activates hepatic endoplasmic reticulum (ER) stress pathways, leading to dysregulation of the endogenous sterol response mechanism. Herein, we demonstrate a more profound dysregulation of cholesterol homeostasis in newborn Pex2^−/− mice congenic on a 129S6/SvEv (129) genetic background, and substantial differences between newborn versus postnatal Pex2^−/− mice in factors that activate ER stress. These differences extend to relationships between activation of genes regulated by SREBP-2 versus PPARα. The SREBP-2 pathway is induced in neonatal Pex2^−/− livers from 129 and SW/129 strains, despite normal hepatic cholesterol levels. ER stress markers are increased in newborn 129 Pex2^−/− livers, which occurs in the absence of hepatic steatosis or accumulation of peroxins in the ER. Moreover, the induction of SREBP-2 and ER stress pathways is independent of PPARα activation in livers of newborn 129 and SW/129 Pex2^−/− mice. Two-week-old wild-type mice treated with the peroxisome proliferator WY-14,643 show strong induction of PPARα-regulated genes and decreased expression of SREBP-2 and its target genes, further demonstrating that SREBP-2 pathway induction is not dependent on PPARα activation. Lastly, there is no activation of either SREBP-2 or ER stress pathways in kidney and lung of newborn Pex2^−/− mice, suggesting a parallel induction of these pathways in peroxisome-deficient mice. These findings establish novel associations between SREBP-2, ER stress and PPARα pathway inductions.     

Endoplasmic reticulum stress associated responses in cancer

The endoplasmic reticulum (ER) is responsible for many housekeeping functions within the cell and is an important site for pathways that regulates its state of homeostasis. When cellular states perturb ER functions, a phenomenon termed “ER stress” activates a number of pathways to counteract the associated damages; these pathways are together called the unfolded protein response (UPR). The UPR has a dualistic function; it exists to alleviate damage associated with ER stress, however, if this is not possible, then it signals for cell death through apoptosis. Cancer cells are shown to be very resilient under extreme environmental stress and an increasing number of studies have indicated that this may be largely due to an altered state of the UPR. The role of ER stress and the UPR in cancer is still not clear, however many components are involved and may prove to be promising targets in future anti-cancer therapy. This article is part of a Special Issue entitled: Calcium Signaling in Health and Disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.     

Endoplasmic reticulum stress induces inverse regulations of major functions in portal myofibroblasts during liver fibrosis progression

Portal myofibroblasts (PMF) form a sub-population of highly proliferative and proangiogenic liver myofibroblasts that derive from portal mesenchymal progenitors. Endoplasmic reticulum (ER) stress was previously shown to modulate fibrogenesis, notably in the liver. Our aim was to determine if ER stress occurred in PMF and affected their functions. PMF were obtained after their expansion in vivo from bile duct-ligated (BDL) rats and referred to as BDL PMF. Compared to standard PMF obtained from normal rats, BDL PMF were more myofibroblastic, as assessed by higher alpha-smooth muscle actin expression and collagen 1 production. Their proangiogenic properties were also higher, whereas their proliferative and migratory capacities were lower. CHOP expression was detected in the liver of BDL rats, at the leading edge of portal fibrosis where PMF accumulate. BDL PMF displayed ER dilatation and an overexpression of the PERK pathway downstream targets, Chop, Gadd34 and Trb3, in comparison with standard PMF. In vitro, the induction of ER stress by tunicamycin in standard PMF, caused a decrease in their proliferative and migratory activity, and an increase in their proangiogenic activity, without affecting their myofibroblastic differentiation. Conversely, the treatment of BDL PMF with the PERK inhibitor GSK2656157 reduced ER stress, which caused a decrease in their angiogenic properties, and restored their proliferative and migratory capacity. In conclusion, PMF develop ER stress as they expand with the progression of fibrosis, which further increases their proangiogenic activity, but also inhibits their proliferation and migration. This phenotypic switch may restrict PMF expansion while they support angiogenesis.     

Molecular and structural analysis of C4-specific PEPC isoform from Pennisetum glaucum plays a role in stress adaptation

As an activator of adenylate cyclase, the neuropeptide Pituitary Adenylate Cyclase Activating Peptide (PACAP) impacts levels of cyclic AMP, a key second messenger available in brain cells. PACAP is involved in certain adult behaviors. To elucidate PACAP interactions, a compendium of microarrays representing mRNA expression in the adult mouse whole brain was pooled from the Phenogen database for analysis. A regulatory network was computed based on mutual information between gene pairs using gene expression data across the compendium. Clusters among genes directly linked to PACAP, and probable interactions between corresponding proteins were computed. Database “experts” affirmed some of the inferred relationships. The findings suggest ADCY7 is probably the adenylate cyclase isoform most relevant to PACAP's action. They also support intervening roles for kinases including GSK3B, PI 3-kinase, SGK3 and AMPK. Other high-confidence interactions are hypothesized for future testing. This new information has implications for certain behavioral and other disorders.     

Inositol 1,4,5-trisphosphate receptor-isoform diversity in cell death and survival

Cell-death and -survival decisions are critically controlled by intracellular Ca^2 + homeostasis and dynamics at the level of the endoplasmic reticulum (ER). Inositol 1,4,5-trisphosphate (IP₃) receptors (IP₃Rs) play a pivotal role in these processes by mediating Ca^2 + flux from the ER into the cytosol and mitochondria. Hence, it is clear that many pro-survival and pro-death signaling pathways and proteins affect Ca^2 + signaling by directly targeting IP₃R channels, which can happen in an IP₃R-isoform-dependent manner. In this review, we will focus on how the different IP₃R isoforms (IP₃R1, IP₃R2 and IP₃R3) control cell death and survival. First, we will present an overview of the isoform-specific regulation of IP₃Rs by cellular factors like IP₃, Ca^2 +, Ca^2 +-binding proteins, adenosine triphosphate (ATP), thiol modification, phosphorylation and interacting proteins, and of IP₃R-isoform specific expression patterns. Second, we will discuss the role of the ER as a Ca^2 + store in cell death and survival and how IP₃Rs and pro-survival/pro-death proteins can modulate the basal ER Ca^2 + leak. Third, we will review the regulation of the Ca^2 +-flux properties of the IP₃R isoforms by the ER-resident and by the cytoplasmic proteins involved in cell death and survival as well as by redox regulation. Hence, we aim to highlight the specific roles of the various IP₃R isoforms in cell-death and -survival signaling. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.