The rat homologue of a mitochondrial ATP-dependent protease Lon was cloned from cultured astrocytes exposed to hypoxia. Expression of Lon was enhanced in vitro by hypoxia or ER stress, and in vivo by brain ischemia. These observations suggested that changes in nuclear gene expression (Lon) triggered by ER stress had the potential to impact important mitochondrial processes such as assembly and/or degradation of cytochrome c oxidase (COX). In fact, steady-state levels of nuclear-encoded COX IV and V were reduced, and mitochondrial-encoded subunit II was rapidly degraded under ER stress. Treatment of cells with cycloheximide caused a similar imbalance in the accumulation of COX subunits, and enhanced mRNA for Lon and Yme1, the latter another mitochondrial ATP-dependent protease. Furthermore, induction of Lon or GRP75/mtHSP70 by ER stress was inhibited in PERK (−/−) cells. Transfection studies revealed that overexpression of wild-type or proteolytically inactive Lon promoted assembly of COX II into a COX I–containing complex, and partially prevented mitochondrial dysfunction caused by brefeldin A or hypoxia. These observations demonstrated that suppression of protein synthesis due to ER stress has a complex effect on the synthesis of mitochondrial-associated proteins, both COX subunits and ATP-dependent proteases and/or chaperones contributing to assembly of the COX complex.

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Aims/hypothesis The WFS1 gene encodes an endoplasmic reticulum (ER) membrane-embedded protein called Wolfram syndrome 1 protein, homozygous mutations of which cause selective beta cell loss in humans. The function(s) of this protein and the mechanism by which the mutations of this gene cause beta cell death are still not fully understood. We hypothesised that increased insulin demand as a result of obesity/insulin resistance causes ER stress in pancreatic beta cells, thereby promoting beta cell death. Methods We studied the effect of breeding Wfs1 −/− mice on a C57BL/6J background with mild obesity and insulin resistance, by introducing the agouti lethal yellow mutation (A y /a). We also treated the mice with pioglitazone.

Stress-associated endoplasmic reticulum (ER) protein 1 (SERP1), also known as ribosome-associated membrane protein 4 (RAMP4), is a Sec61-associated polypeptide that is induced by ER stress. SERP1؊/؊ mice, made by targeted gene disruption, demonstrated growth retardation, increased mortality, and impaired glucose tolerance. Consistent with high levels of SERP1 expression in pancreas, pancreatic islets from SERP1 ؊/؊ mice failed to rapidly synthesize proinsulin in response to a glucose load. In addition, reduced size and enhanced ER stress were observed in the anterior pituitary of SERP1 ؊/؊ mice, and growth hormone production was slowed in SERP1 ؊/؊ pituitary after insulin stimulation. Experiments using pancreatic microsomes revealed aberrant association of ribosomes and the Sec61 complex and enhanced ER stress in SERP1 ؊/؊ pancreas. In basal conditions, the Sec61 complex in SERP1 ؊/؊ microsomes was more cofractionated with ribosomes, compared with SERP1 ؉/؉ counterparts, in high-salt conditions. In contrast, after glucose stimulation, the complex showed less cofractionation at an early phase (45 min) but more at a later phase (120 min). Although intracellular insulin/proinsulin levels were not significantly changed in both genotypes, these results suggest that subtle changes in translocation efficiency play an important role in the regulation of ER stress and rapid polypeptide synthesis.Secretory proteins undergo posttranslational processing, including correct folding and oligomerization, in the endoplasmic reticulum (ER). In order to effectively produce and secrete mature proteins, cellular mechanisms for monitoring the ER environment are essential. In mammalian cells, at least three different mechanisms contribute to this surveillance system: regulated induction of transcription, attenuation of translation, and degradation (11). Exposure of cells to conditions promoting accumulation of unfolded proteins in the ER (ER stress) induces molecular chaperones, folding catalysts, and subunits of the translocation machinery (Sec61 complex), a process known as the unfolded protein response. Attenuation of protein synthesis in response to ER stress provides another point of regulation, in this case serving to reduce the load of proteins entering the ER. The latter pathway requires activation of the ER-resident membrane protein PERK and phosphorylation of eukaryotic translation initiation factor 2␣ (eIF2␣) (5).SERP1 (stress-associated ER protein 1) was identified because of its induction in response to hypoxia or ER stress (20) and found to be identical to RAMP4 (ribosome-associate membrane protein 4). The latter was originally recognized because it was copurified with the Sec61 complex (3, 19) and then found to be a genuine and evolutionarily conserved part of the ER translocon. SERP1/RAMP4 controls glycosylation of major histocompatibility complex class II-associated invariant chains by a translocational pausing mechanism (16), and its overexpression stabilizes newly synthesized membrane proteins under ER stress by ass...

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