ER stress-induced apoptosis and caspase-12 activation occurs downstream of mitochondrial apoptosis involving Apaf-1

Abstract

Accumulation of unfolded proteins induces endoplasmic reticulum (ER) stress. Excessive and prolonged stresses lead cells to apoptosis. However, the precise molecular mechanisms of ER stress-induced apoptosis have not been fully elucidated. We investigated the involvement of the apoptosome in ER stress-induced cell death pathway using mouse embryonic fibroblasts (MEFs) and mice deficient for Apaf-1. Apaf-1-deficient MEFs showed more resistance to ER stress-inducing reagents as compared with wild type cells. Despite comparable induction of ER stress in both wild type and Apaf-1-deficient cells, activation of caspase-3 was only observed in wild type, but not Apaf-1-deficient, MEFs. Under ER stress conditions, BAX translocated to mitochondria and cytochrome c was released from mitochondria. We also demonstrated that caspase-12 was processed downstream of Apaf-1 and caspase-3, and neither overexpression nor knockdown of caspase-12 affected susceptibility of the cells to ER stress-induced cell death. Furthermore, in the kidneys of Apaf-1-deficient mice, apoptosis induced by in vivo administration of tunicamycin was remarkably suppressed as compared with wild type mice. These data collectively demonstrated that Apaf-1 and the mitochondrial pathway of apoptosis play significant roles in ER stress-induced apoptosis.

Endoplasmic reticulum stress-induced apoptosis requires bax for commitment and Apaf-1 for execution in primary neurons

“…14 Additionally, we find that a zVAD-fmk inhibitable caspase, such as caspase 12, 9 does not act upstream of the mitochondrial release of cytochrome c during ER stressinduced apoptosis, as addition of zVAD-fmk did not block cytochrome c release in response to TU treatment (Figure 5a). In agreement with this finding, recent studies have indicated that caspase 12 is a downstream target of caspases 3 38 and that caspase 12 and human caspase 4 are dispensable for ER stress-induced apoptosis. 39 Therefore, caspase 12 is not likely to play an essential role as a link between the ER and mitochondrial to release cytochrome c in sympathetic neurons.…”

Glycogen synthase kinase-3β plays a pro-apoptotic role in β-adrenergic receptor-stimulated apoptosis in adult rat ventricular myocytes: Role of β1 integrins

“…Overexpression of Gadd153 resulted in the downregulation of Bcl-2 expression, depletion of glutathione levels and exaggerated production of reactive oxygen species, leading to sensitization of cells to endoplasmic reticulum (ER)-stress induced apoptosis [34]. In mouse embryonic fibroblasts, ER-stressinduced apoptosis and activation of caspase-12 is shown to occur downstream of mitochondrial apoptosis involving Apaf-1 [35].…”

Bax Inhibitor 1 in apoptosis and disease

“…The ER is associated with the synthesis, initial post-translational modification, folding and maturation of proteins and is a major regulator of intracellular Ca 2 þ homeostasis (Shiraishi et al, 2006). Improper protein maturation, unfavourable environmental conditions, protein redox and glycosylation status alterations or nutrient and ATP deprivation, disrupt ER function inducing ER stress and the unfolded protein response (UPR).…”

“…ER stress signalling is mediated by three transmembrane proteins ATF6 (activating transcription factor 6), PERK (PKR-like ER kinase) and IRE1 (inositolrequiring-1), which are kept inactive through interaction (Imaizumi et al, 2001;Shiraishi et al, 2006). Initially, adaptation to ER stress induces cleavage of ATF6 releasing its cytosolic fraction, which translocates to the nucleus inducing the expression of ER chaperones and the ER associated degradation machinery (ERAD) genes.…”

“…The main cellular store for Ca 2 þ is the ER; release of Ca 2 þ from the ER will determine the sensitivity of cells to apoptosis by determining mitochondrial permeability (Shiraishi et al, 2006). Due to the negative (inside) membrane potential of mitochondria, they can efficiently accumulate Ca 2 þ , which acts to increase their metabolic activity and permeability.…”