The oxidative stress: endoplasmic reticulum stress axis in cadmium toxicity

Cadmium preferentially accumulates in the kidney, the major target for cadmium-related toxicity. Several underlying mechanisms are postulated, and reactive oxygen species (ROS) have been considered as crucial mediators for tissue injuries. In addition to oxidative stress, we recently disclosed that endoplasmic reticulum (ER) stress also plays a critical role. Cadmium causes ER stress in vitro and in vivo and mediates induction of apoptosis in target tissues. In this article, we describe a role for ER stress and involvement of particular branches of the unfolded protein response (UPR) in cadmium-triggered tissue injury, especially nephrotoxicity. We also discuss relationship between oxidative stress and ER stress, and involvement of selective ROS in the induction of pro-apoptotic branches of the UPR.     

Taurine protection of PC12 cells against endoplasmic reticulum stress induced by oxidative stress

Background

Taurine is a free amino acid present in high concentrations in a variety of organs of mammalians. As an antioxidant, taurine has been found to protect cells against oxidative stress, but the underlying mechanism is still unclear.

Methods

In this report, we present evidence to support the conclusion that taurine exerts a protective function against endoplasmic reticulum (ER) stress induced by H₂O₂ in PC 12 cells. Oxidative stress was introduced by exposure of PC 12 cells to 250 uM H₂O₂ for 4 hours.

Results

It was found that the cell viability of PC 12 cells decreased with an increase of H₂O₂ concentration ranging from approximately 76% cell viability at 100 uM H₂O₂ down to 18% at 500 uM H₂O₂. At 250 uM H₂O₂, cell viability was restored to 80% by taurine at 25 mM. Furthermore, H₂O₂ treatment also caused a marked reduction in the expression of Bcl-2 while no significant change of Bax was observed. Treatment with taurine restored the reduced expression of Bcl-2 close to the control level without any obvious effect on Bax. Furthermore, taurine was also found to suppress up-regulation of GRP78, GADD153/CHOP and Bim induced by H₂O₂, suggesting that taurine may also exert a protective function against oxidative stress by reducing the ER stress.

Conclusion

In summary, taurine was shown to protect PC12 cells against oxidative stress induced by H₂O₂. ER stress was induced by oxidative stress and can be suppressed by taurine.

     

Calnexin is involved in apoptosis induced by endoplasmic reticulum stress in the fission yeast

Stress conditions affecting the functions of the endoplasmic reticulum (ER) cause the accumulation of unfolded proteins. ER stress is counteracted by the unfolded-protein response (UPR). However, under prolonged stress the UPR initiates a proapoptotic response. Mounting evidence indicate that the ER chaperone calnexin is involved in apoptosis caused by ER stress. Here, we report that overexpression of calnexin in Schizosaccharomyces pombe induces cell death with apoptosis markers. Cell death was partially dependent on the Ire1p ER-stress transducer. Apoptotic death caused by calnexin overexpression required its transmembrane domain (TM), and involved sequences on either side of the ER membrane. Apoptotic death caused by tunicamycin was dramatically reduced in a strain expressing endogenous levels of calnexin lacking its TM and cytosolic tail. This demonstrates the involvement of calnexin in apoptosis triggered by ER stress. A genetic screen identified the S. pombe homologue of the human antiapoptotic protein HMGB1 as a suppressor of apoptotic death due to calnexin overexpression. Remarkably, overexpression of human calnexin in S. pombe also provoked apoptotic death. Our results argue for the conservation of the role of calnexin in apoptosis triggered by ER stress, and validate S. pombe as a model to elucidate the mechanisms of calnexin-mediated cell death.     

Cellular responses to endoplasmic reticulum stress and apoptosis

The endoplasmic reticulum (ER) is the cell organelle where secretory and membrane proteins are synthesized and folded. Correctly folded proteins exit the ER and are transported to the Golgi and other destinations within the cell, but proteins that fail to fold properly—misfolded proteins—are retained in the ER and their accumulation may constitute a form of stress to the cell—ER stress. Several signaling pathways, collectively known as unfolded protein response (UPR), have evolved to detect the accumulation of misfolded proteins in the ER and activate a cellular response that attempts to maintain homeostasis and a normal flux of proteins in the ER. In certain severe situations of ER stress, however, the protective mechanisms activated by the UPR are not sufficient to restore normal ER function and cells die by apoptosis. Most research on the UPR used yeast or mammalian model systems and only recently Drosophila has emerged as a system to study the molecular and cellular mechanisms of the UPR. Here, we review recent advances in Drosophila UPR research, in the broad context of mammalian and yeast literature.     

Integrating the mechanisms of apoptosis induced by endoplasmic reticulum stress

The ability to respond to perturbations in endoplasmic reticulum (ER) function is a fundamentally important property of all cells, but ER stress can also lead to apoptosis. In settings of chronic ER stress, the associated apoptosis may contribute to pathophysiological processes involved in a number of prevalent diseases, including neurodegenerative diseases, diabetes, atherosclerosis and renal disease. The molecular mechanisms linking ER stress to apoptosis are the topic of this review, with emphases on relevance to pathophysiology and integration and complementation among the various apoptotic pathways induced by ER stress.     

Depletion of oxidative and endoplasmic reticulum stress regulators in Pick disease

Both oxidative and endoplasmic reticulum (ER) stress is associated with multiple neurodegenerative, age-related diseases. The rare disorder Pick disease (PiD) shares some pathological hallmarks of other neurodegenerative diseases that may be related to oxidative stress. Importantly, activation of an ER stress response, which is also involved in aging, has not yet been investigated in PiD. In this study, we assessed the implication of ER stress associated with oxidative stress in PiD as a potential mechanism involved in its pathogenesis. Samples from morphologically affected frontal cortex and apparently pathologically preserved occipital cortex showed region-dependent increases in different protein oxidative damage pathways. The oxidative modifications targeted antioxidant enzymes, proteases, heat shock proteins, and synaptic proteins. These effects were associated with compromised proteasomal function and ER stress in frontal cortex samples. In addition, we observed a depletion in ER chaperones (glucose-regulated proteins Grp78/BiP and glucose-regulated protein 94) and differences in tissue content and distribution of nuclear factor-erythroid 2 p45-related respiratory 2, required for cell survival during the unfolded protein response. These results demonstrate increased region-specific protein oxidative damage in PiD, with proteasomal alteration and dysfunctional ER stress response. We suggest this was caused by complete and specific depletion of Grp78/BiP, contributing to the pathophysiology of this neurodegenerative disease.     

Genetic instability induced by overexpression of DNA ligase I in budding yeast

Recombination and microsatellite mutation in humans contribute to disorders including cancer and trinucleotide repeat (TNR) disease. TNR expansions in wild-type yeast may arise by flap ligation during lagging-strand replication. Here we show that overexpression of DNA ligase I (CDC9) increases the rates of TNR expansion, of TNR contraction, and of mitotic recombination. Surprisingly, this effect is observed with catalytically inactive forms of Cdc9p protein, but only if they possess a functional PCNA-binding site. Furthermore, in vitro analysis indicates that the interaction of PCNA with Cdc9p and Rad27p (Fen1) is mutually exclusive. Together our genetic and biochemical analysis suggests that, although DNA ligase I seals DNA nicks during replication, repair, and recombination, higher than normal levels can yield genetic instability by disrupting the normal interplay of PCNA with other proteins such as Fen1.     

Apoptosis induced by endoplasmic reticulum stress involved in diabetic kidney disease

Endoplasmic reticulum stress has been suggested to play a crucial role in the pathogenesis of diabetic complications. However, whether it is involved in the renal injury of diabetic nephropathy is still not known. We investigated the involvement of ER-associated apoptosis in kidney disease of streptozocin (STZ)-induced diabetic rats. We used albuminuria examination, hematoxylin & eosin (H&E) staining and TUNEL analysis to identify the existence of diabetic nephropathy and enhanced apoptosis. We performed immunohistochemistry, Western blot, and real-time PCR to analyze indicators of ER molecule chaperone and ER-associated apoptosis. GRP78, the ER chaperone, was up-regulated significantly in diabetic kidney compared to control. Furthermore, three hallmarks of ER-associated apoptosis, C/EBP homologous protein (CHOP), c-JUN NH2-terminal kinase (JNK) and caspase-12, were found to have activated in the diabetic kidney. Taken together, those results suggested that apoptosis induced by ER stress occurred in diabetic kidney, which may contribute to the development of diabetic nephropathy.     

内质网应激的细胞效应分子机制

内质网应激（endoplasmic reticulum stress,ERs）是内质网腔内错误折叠蛋白聚积的一种适应性反应,适度ERs通过激活未折叠蛋白反应起适应性的细胞保护作用,而过高和持久的ERs则通过诱导转录因子CHOP表达、激活caspase-12和c—Jun氨基末端激酶（JNK）等导致细胞凋亡。近年来,越来越多的研究提示内质网应激是神经退行性病变、2型糖尿病以及肥胖等疾病发生过程中的重要环节。对内质网应激的细胞效应分子机制进行综述。随着对ERs机制理解的深入,有可能会发现新的分子标志物或新的诊疗策略。     

Environmentally induced changes in mitochondria and endoplasmic reticulum of Saccharomyces carlsbergensis yeast

The effects of culture environment on the volume density and surface density of mitochondria and endoplasmic reticulum in a facultative yeast were studied. When compared with cells grown aerobically on a nonrepressive substrate, cells grown in the absence of oxygen showed a sharp reduction in both volume density of mitochondria and surface density of the inner mitochondrial membrane (imm) in the remaining mitochondrial profiles. Use of fermentable (repressive) substrates under aerobic conditions restricted the volume density of mitochondria to a much greater extent than the surface density of imm. The range of mitochondrial volume densities in these experiments was 4-11%. Surface density of endoplasmic reticulum (ER) was sensitive to growth rate and in particular to changes in oxygen tension, showing large fluctuations during both anaerobic and aerobic adaptation. These fluctuations in ER are discussed in relation to the known role of this organelle in lipid metabolism.     

HDLs inhibit endoplasmic reticulum stress and autophagic response induced by oxidized LDLs

The apoptotic effect of oxidized LDLs (oxLDLs) is mediated through a complex sequence of signaling events involving a deregulation of the cytosolic Ca(2+) homeostasis. OxLDLs also trigger ER stress that may lead to cellular dysfunction and apoptosis, through the activation of the IRE1α/c-Jun N-terminal kinase pathway. Moreover, ER stress and oxidized lipids have been shown to trigger autophagy. The antiatherogenic high-density lipoproteins (HDLs) display protective effects against oxLDLs toxicity. To more deeply investigate the mechanisms mediating the protective effects of HDLs, we examined whether ER stress and autophagy were implicated in oxLDLs-induced apoptosis and whether HDLs prevented these stress processes. We report that, in human endothelial cells, HDLs prevent the oxLDL-induced activation of the ER stress sensors IRE1α, eIF2α and ATF6 and subsequent activation of the proapoptotic mediators JNK and CHOP. OxLDLs also trigger the activation of autophagy, as assessed by LC3 processing and Beclin-1 expression. The autophagic process is independent of the proapoptotic arms of ER stress, but Beclin-1 contributes to PS exposure and subsequent phagocytosis of oxLDLs exposed cells. Induction of autophagy and PS exposure by oxLDLs is prevented by HDLs. Finally, the cytosolic Ca(2+) deregulation triggered by oxLDLs is a common signaling pathway that mediates ER stress-induced cell death and autophagy, all these events being blocked by HDLs.     

Perturbation of sphingolipid metabolism induces endoplasmic reticulum stress‐mediated mitochondrial apoptosis in budding yeast

Sphingolipids are a class of membrane lipids conserved from yeast to mammals which determine whether a cell dies or survives. Perturbations in sphingolipid metabolism cause apoptotic cell death. Recent studies indicate that reduced sphingolipid levels trigger the cell death, but little is known about the mechanisms. In the budding yeast Saccharomyces cerevisiae, we show that reduction in complex sphingolipid levels causes loss of viability, most likely due to the induction of mitochondria‐dependent apoptotic cell death pathway, accompanied by changes in mitochondrial and endoplasmic reticulum morphology and endoplasmic reticulum stress. Elevated cytosolic free calcium is required for the loss of viability. These results indicate that complex sphingolipids are essential for maintaining endoplasmic reticulum homeostasis and suggest that perturbation in complex sphingolipid levels activates an endoplasmic reticulum stress‐mediated and calcium‐dependent pathway to propagate apoptotic signals to the mitochondria.     

Intermittent selective clamping improves rat liver regeneration by attenuating oxidative and endoplasmic reticulum stress

Intermittent clamping of the portal trial is an effective method to avoid excessive blood loss during hepatic resection, but this procedure may cause ischemic damage to liver. Intermittent selective clamping of the lobes to be resected may represent a good alternative as it exposes the remnant liver only to the reperfusion stress. We compared the effect of intermittent total or selective clamping on hepatocellular injury and liver regeneration. Entire hepatic lobes or only lobes to be resected were subjected twice to 10 min of ischemia followed by 5 min of reperfusion before hepatectomy. We provided evidence that the effect of intermittent clamping can be damaging or beneficial depending to its mode of application. Although transaminase levels were similar in all groups, intermittent total clamping impaired liver regeneration and increased apoptosis. In contrast, intermittent selective clamping improved liver protein secretion and hepatocyte proliferation when compared with standard hepatectomy. This beneficial effect was linked to better adenosine-5′-triphosphate (ATP) recovery, nitric oxide production, antioxidant activities and endoplasmic reticulum adaptation leading to limit mitochondrial damage and apoptosis. Interestingly, transient and early chaperone inductions resulted in a controlled activation of the unfolded protein response concomitantly to endothelial nitric oxide synthase, extracellular signal-regulated kinase-1/2 (ERK1/2) and p38 MAPK activation that favors liver regeneration. Endoplasmic reticulum stress is a central target through which intermittent selective clamping exerts its cytoprotective effect and improves liver regeneration. This procedure could be applied as a powerful protective modality in the field of living donor liver transplantation and liver surgery.