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Background and Purpose: Endoplasmic reticulum (ER) stress triggers an adaptive response in tumours which fosters cell survival and resilience to stress. Activation of the ER stress response, through its PERK branch, promotes phosphorylation of the α‐subunit of the translation initiation factor eIF2, thereby repressing general protein translation and augmenting the translation of ATF4 with the downstream CHOP transcription factor and the protein disulfide oxidase, ERO1‐alpha Experimental Approach: Here, we show that ISRIB, a small molecule that inhibits the action of phosphorylated eIF2alpha, activating protein translation, synergistically interacts with the genetic deficiency of protein disulfide oxidase ERO1‐alpha, enfeebling breast tumour growth and spread. Key Results: ISRIB represses the CHOP signal, but does not inhibit ERO1. Mechanistically, ISRIB increases the ER protein load with a marked perturbing effect on ERO1‐deficient triple‐negative breast cancer cells, which display impaired proteostasis and have adapted to a low client protein load in hypoxia, and ERO1 deficiency impairs VEGF‐dependent angiogenesis. ERO1‐deficient triple‐negative breast cancer xenografts have an augmented ER stress response and its PERK branch. ISRIB acts synergistically with ERO1 deficiency, inhibiting the growth of triple‐negative breast cancer xenografts by impairing proliferation and angiogenesis. Conclusion and Implications: These results demonstrate that ISRIB together with ERO1 deficiency synergistically shatter the PERK‐dependent adaptive ER stress response, by restarting protein synthesis in the setting of impaired proteostasis, finally promoting tumour cytotoxicity. Our findings suggest two surprising features in breast tumours: ERO1 is not regulated via CHOP under hypoxic conditions, and ISRIB offers a therapeutic option to efficiently inhibit tumour progression in conditions of impaired proteostasis. [ABSTRACT FROM AUTHOR]

Arsenite induces many critical effects associated with the formation of reactive oxygen species (ROS) through different mechanisms. We focused on Ca2+-dependent mitochondrial superoxide (mitoO 2 -.) formation and addressed questions on the effects of low concentrations of arsenite on the mobilization of the cation from the endoplasmic reticulum and the resulting mitochondrial accumulation. Using various differentiated and undifferentiated cell types uniquely expressing the inositol-1, 4, 5-triphosphate receptor (IP 3 R), or both the IP 3 R and the ryanodine receptor (RyR), we determined that expression of this second Ca2+ channel is an absolute requirement for mitoO 2 -. formation and for the ensuing mitochondrial dysfunction and downstream apoptosis. In arsenite-treated cells, RyR was recruited after IP 3 R stimulation and agonist studies provided an indirect indication for a close apposition between RyR and mitochondria. It was also interesting to observe that arsenite fails to promote mitochondrial Ca2+ accumulation, mitoO 2 -. formation and mitochondrial toxicity in RyR-devoid cells, in which the IP 3 R is in close contact with the mitochondria. We therefore conclude that low dose arsenite-induced mitoO 2 - formation, and the resulting mitochondrial dysfunction and toxicity, are prerequisite of cell types expressing the RyR in close apposition with mitochondria. [Display omitted] • Arsenite stimulates the release of Ca2+ from the IP 3 R in different cell types. • Stimulation of Ca2+ release from the IP 3 R causes further Ca2+ release from the RyR. • Mitochondrial Ca2+ accumulation takes place only in RyR expressing cells. • Ca2+-dependent mitochondrial O 2 −. formation is restricted to RyR expressing cells. • Arsenite causes mitochondrial toxicity only in RyR expressing cells. [ABSTRACT FROM AUTHOR]

Selenoprotein N (SELENON) is an endoplasmic reticulum (ER) protein whose loss of function leads to a congenital myopathy associated with insulin resistance (SEPN1-related myopathy). The exact cause of the insulin resistance in patients with SELENON loss of function is not known. Skeletal muscle is the main contributor to insulin-mediated glucose uptake, and a defect in this muscle-related mechanism triggers insulin resistance and glucose intolerance. We have studied the chain of events that connect the loss of SELENON with defects in insulin-mediated glucose uptake in muscle cells and the effects of this on muscle performance. Here, we show that saturated fatty acids are more lipotoxic in SELENON-devoid cells, and blunt the insulin-mediated glucose uptake of SELENON-devoid myotubes by increasing ER stress and mounting a maladaptive ER stress response. Furthermore, the hind limb skeletal muscles of SELENON KO mice fed a high-fat diet mirrors the features of saturated fatty acid-treated myotubes, and show signs of myopathy with a compromised force production. These findings suggest that the absence of SELENON together with a high-fat dietary regimen increases susceptibility to insulin resistance by triggering a chronic ER stress in skeletal muscle and muscle weakness. Importantly, our findings suggest that environmental cues eliciting ER stress in skeletal muscle (such as a high-fat diet) affect the pathological phenotype of SEPN1-related myopathy and can therefore contribute to the assessment of prognosis beyond simple genotype-phenotype correlations., (Copyright © 2019. Published by Elsevier B.V.)

Selenoprotein N (SELENON) is an endoplasmic reticulum (ER) protein whose loss of function leads to human SELENON-related myopathies. SelenoN knockout (KO) mouse limb muscles, however, are protected from the disease, and display no major alterations in muscle histology or contractile properties. Interestingly, we find that the highly active diaphragm muscle shows impaired force production, in line with the human phenotype. In addition, after repeated stimulation with a protocol which induces muscle fatigue, also hind limb muscles show altered relaxation times. Mechanistically, muscle SELENON loss alters activity-dependent calcium handling selectively impinging on the Ca 2+ uptake of the sarcoplasmic reticulum and elicits an ER stress response, including the expression of the maladaptive CHOP-induced ERO1. In SELENON-devoid models, ERO1 shifts ER redox to a more oxidised poise, and further affects Ca 2+ uptake. Importantly, CHOP ablation in SelenoN KO mice completely prevents diaphragm dysfunction, the prolonged limb muscle relaxation after fatigue, and restores Ca 2+ uptake by attenuating the induction of ERO1. These findings suggest that SELENON is part of an ER stress-dependent antioxidant response and that the CHOP/ERO1 branch of the ER stress response is a novel pathogenic mechanism underlying SELENON-related myopathies., (Copyright © 2018 The Author. Published by Elsevier B.V. All rights reserved.)