Mutations dislocate caspase-12 from the endoplasmatic reticulum to the cytosol

Diet-Induced Obesity Mice Execute Pulmonary Cell Apoptosis via Death Receptor and ER-Stress Pathways after E. coli Infection

Obesity has developed into a considerable health problem in the whole world. Escherichia coli (E. coli) can cause nosocomial pneumonia and induce cell apoptosis during injury and infection. Normal (lean) and diet-induced obesity mice (DIO, fed with high-fat diet) were chosen to perform nasal instillation with E. coli to establish a nonfatal acute pneumonia model. At 0 h, 12 h, 24 h, and 72 h postinfection, lung tissues were obtained to measure cell apoptosis. As shown in this study, both lean and DIO mice exhibited histopathological lesions of acute pneumonia and increased cell apoptosis in the lung infected with E. coli. Interestingly, the relative mRNA and protein expressions associated with either endoplasmic reticulum stress or death receptor apoptotic pathway were all dramatically increased in the DIO mice after infection, while only significant upregulation of death receptor apoptotic pathway in the lean mice at 72 h. These results indicated that the DIO mice executed excess cell apoptosis in the nonfatal acute pneumonia induced by E. coli infection through endoplasmic reticulum stress and death receptor apoptotic pathway.

Cleavage of caspase-12 at Asp94, mediated by endoplasmic reticulum stress (ERS), contributes to stretch-induced apoptosis of myoblasts

Mechanical overloading can lead to skeletal muscle damage instead of remodeling. This is attributed to the excessive apoptosis of myoblasts, mechanism of which remains to be elucidated. The present study aimed to investigate the involvement of endoplasmic reticulum stress (ERS) and caspase-12 in mediating the stretch-induced apoptosis of myoblasts. Myoblast apoptosis was evaluated by Hoechst staining, DNA fragmentation assay, Annexin V binding, and propidium iodide staining, as well as caspase-3 and poly-ADP-ribose polymerase 1 cleavage. First, our results showed that apoptosis was elevated in a time-dependent manner when myoblasts were subjected to cyclic mechanical stretch (CMS) for 12, 24, and 36 hr. Concomitantly, CMS triggered the ERS and caspase-12 cleavage; ERS inhibitor GSK 2606414 suppressed the CMS-induced cleavage of caspase-12 and myoblast apoptosis. Silencing caspase-12 attenuated the apoptosis of myoblasts under CMS. Furthermore, CMS-induced myoblast apoptosis was partially recovered by overexpressing wild-type caspase-12 in caspase-12-silenced myoblasts. In contrast, overexpressing mutant caspase-12 (D94N), which cannot be cleaved into the active caspase-12 fragments, failed to accomplish the same effect. Finally, C2C12 overexpressing truncated caspase-12 segment (TC-casp12-D94), which starts from Asp94 and ends at Asn419, underwent apoptosis under both static and stretched conditions. Interestingly, C2C12 myoblasts seemed to be resistant to stretch-induced apoptosis upon low-serum-induced differentiation. In conclusion, our study provided evidence that caspase-12 cleavage at Asp94, induced by ERS under mechanical stimuli, is the key molecule in initiating the stretch-triggered apoptosis of myoblasts.

Caspase-4 is essential for saikosaponin a-induced apoptosis acting upstream of caspase-2 and γ-H2AX in colon cancer cells

Saikosaponin a (SSa), a bioactive phytochemical from Bupleurum, triggers sequential caspase-2 and caspase-8 activation, and thereby induces caspase-mediated apoptosis in human colon carcinoma (HCC) cells. However, the upstream mechanism of caspase-2 activation remains unknown. Therefore, we investigated the signaling mechanisms underlying SSa-induced caspase activation and apoptosis in HCC cells. SSa treatment triggered marked antitumor effects, especially in HCC cells, in a cell culture model and a mouse xenograft model. SSa also induced the activation of several endoplasmic reticulum (ER) stress signals. Specifically, caspase-4, a critical regulator of ER stress-induced apoptosis, was activated significantly after SSa treatment. Mechanistically, selective inhibition of caspase-4 suppressed SSa-induced apoptosis, colony inhibition, and the activation of caspase-3, -8, and -2, but not vice versa. Consistent with the important role of caspase-2 in the DNA damage response, SSa induced DNA damage, as evidenced by a cytokinesis-block micronucleus assay, single-cell gel electrophoresis, and an increase in the levels of γ-H2AX, a DNA damage marker. Moreover, inhibition of caspase-4 activation inhibited SSa-induced histone H2AX phosphorylation. Taken together, these results suggest that caspase-4 is an upstream regulator of SSa-induced DNA damage and caspase activation in HCC cells. Given that SSa-induced apoptosis appeared to be specific to certain cell types including HCC cells, SSa may be a promising cancer therapy agent in certain types of cancer.

ER Stress-Mediated Signaling: Action Potential and Ca(2+) as Key Players

The proper functioning of the endoplasmic reticulum (ER) is crucial for multiple cellular activities and survival. Disturbances in the normal ER functions lead to the accumulation and aggregation of unfolded proteins, which initiates an adaptive response, the unfolded protein response (UPR), in order to regain normal ER functions. Failure to activate the adaptive response initiates the process of programmed cell death or apoptosis. Apoptosis plays an important role in cell elimination, which is essential for embryogenesis, development, and tissue homeostasis. Impaired apoptosis can lead to the development of various pathological conditions, such as neurodegenerative and autoimmune diseases, cancer, or acquired immune deficiency syndrome (AIDS). Calcium (Ca(2+)) is one of the key regulators of cell survival and it can induce ER stress-mediated apoptosis in response to various conditions. Ca(2+) regulates cell death both at the early and late stages of apoptosis. Severe Ca(2+) dysregulation can promote cell death through apoptosis. Action potential, an electrical signal transmitted along the neurons and muscle fibers, is important for conveying information to, from, and within the brain. Upon the initiation of the action potential, increased levels of cytosolic Ca(2+) (depolarization) lead to the activation of the ER stress response involved in the initiation of apoptosis. In this review, we discuss the involvement of Ca(2+) and action potential in ER stress-mediated apoptosis.

Constitutive protein content of procaspases in murine tissue

Caspases are proteases most notably involved in apoptosis and inflammation. Although mRNA content is better described, the constitutive protein content of procaspases between tissue types is not well documented. Since mRNA and protein content do not necessarily correlate, we aimed to discern protein content differences between various tissues. Protein content of procaspase-1, -8, -9, and -12 was assessed in gastrocnemius, heart, liver, and kidney. Since highly expressed in skeletal muscle, content of procaspase-12 was also analyzed in muscles with different fiber type compositions to discern any fiber type differences. Furthermore, Western analysis for procaspase-12 revealed prominent bands of ∼40 kDa and ∼30 kDa under basal conditions, in addition to the 50 kDa band corresponding to the full-length procaspase. Therefore, the content of these caspase-12 related species in the tissue and muscle types is also described. Results show protein content of procaspase-1,-8, -9, and -12 and caspase-12 related species differs between tissue types and do not necessarily correlate with mRNA content reported in the published literature. Procaspase-12 content in skeletal muscle may be fiber-type dependent with higher expression in more oxidative fibers. Furthermore, the 40 kDa species of caspase-12 was the dominant form of the protein in most tissues analyzed.

Endoplasmic reticulum stress is involved in granulosa cell apoptosis during follicular atresia in goat ovaries

Follicular atresia is primarily induced by granulosa cell apoptosis, but description of the apoptotic pathway in granulosa cells is incomplete. In this study, we explored the possibility that endoplasmic reticulum (ER) stress could be involved in granulosa cell apoptosis during goat follicular atresia. Immunohistochemical analysis revealed that DNA damage-inducible transcript 3 (DDIT3) and glucose-regulated protein 78 (Grp78) were observed in scattered apoptotic granulosa cells of atretic follicles. Grp78 and DDIT3 mRNA and protein were upregulated in granulosa cells during follicular atresia, although DDIT3 was not significantly different between early atretic and progressed atretic follicles. Spontaneous apoptosis was also observed in vitro in granulosa cells induced by serum deprivation or by the ER stress agent tunicamycin, both inducing similar increases in DDIT3 mRNA. Activating transcription factor-6 (ATF6) and ATF4 mRNAs were significantly increased during granulosa cell apoptosis in vivo; in contrast to ATF6, ATF4 mRNA was attenuated after 16 hr of culture despite the persistence of ER stress. Taken together, ER stress-dependent DDIT3 pathways may play an important role in the regulation of selective granulosa cell apoptosis in goat ovaries during early follicular atresia. Serum deprivation could also increase apoptosis of cultured granulosa cells through the ER stress pathway as both ATF6 and PERK/eIF2α/ATF4 signaling have been implicated in the granulosa cell apoptosis of atretic follicles.

Β-adrenergic receptor stimulation induces endoplasmic reticulum stress in adult cardiac myocytes: role in apoptosis

Accumulation of misfolded proteins and alterations in calcium homeostasis induces endoplasmic reticulum (ER) stress, leading to apoptosis. In this study, we tested the hypothesis that β-AR stimulation induces ER stress, and induction of ER stress plays a pro-apoptotic role in cardiac myocytes. Using thapsigargin and brefeldin A, we demonstrate that ER stress induces apoptosis in adult rat ventricular myocytes (ARVMs). β-AR-stimulation (isoproterenol; 3h) significantly increased expression of ER stress proteins, such as GRP-78, Gadd-153, and Gadd-34, while activating caspase-12 in ARVMs. In most parts, these effects were mimicked by thapsigargin. β-AR stimulation for 15 min increased PERK and eIF-2α phosphorylation. PERK phosphorylation remained higher, while eIF-2α phosphorylation declined thereafter, reaching to ~50% below basal levels at 3 h after β-AR stimulation. This decline in eIF-2α phosphorylation was prevented by β1-AR, not by β2-AR antagonist. Forskolin, adenylyl cyclase activator, simulated the effects of ISO on eIF-2α phosphorylation. Salubrinal (SAL), an ER stress inhibitor, maintained eIF-2α phosphorylation and inhibited β-AR-stimulated apoptosis. Furthermore, inhibition of caspase-12 using z-ATAD inhibited β-AR-stimulated and thapsigargin-induced apoptosis. In vivo, β-AR stimulation induced ER stress in the mouse heart as evidenced by increased expression of GRP-78 and Gadd-153, activation of caspase-12, and dephosphorylation of eIF-2α. SAL maintained phosphorylation of eIF-2α, inhibited activation of caspase-12, and decreased β-AR-stimulated apoptosis in the heart. Thus, β-AR stimulation induces ER stress in cardiac myocytes and in the heart, and induction of ER stress plays a pro-apoptotic role.

Calpain and caspase processing of caspase-12 contribute to the ER stress-induced cell death pathway in differentiated PC12 cells

Neuronal cell death after traumatic brain injury, Alzheimer's disease and ischemic stroke may in part be mediated through endoplasmic reticulum (ER) stress and unfolded protein response (UPR). UPR results in induction of molecular chaperone GRP78 and the ER-resident caspase-12, whose activation has been proposed to be mediated by calpain and caspase processing, although their relative contribution remains unclear. In this study we induced ER stress with thapsigargin (TG), and determined the activation profile of calpain-2, caspase-3, caspase-7, and caspase-12 by analyses of protein levels, corresponding substrates and breakdown products (BDP). Specific calpain and caspase activity was assessed by analysis of αII-spectrin BDP of 145 kDa (SBDP145), BDP of 150 kDa (SBDP150) and BDP of 120 kDa (SBDP120). Decrease in pro-calpain-2 protein and increased SBDP145 levels by 3 h after TG treatment indicated early calpain activity. Active caspase-7 (p20) increase occurred after 8 h, followed by concomitant up-regulation of active caspase-3 and SBDP120 after 24 h. In vitro digestion experiments supported that SBDP120 was exclusively generated by active caspase-3 and validated that kinectin and co-chaperone p23 were calpain and caspase-7 substrates, respectively. Pro-caspase-12 protein processing by the specific action of calpain and caspase-3/7 was observed in a time-dependent manner. N-terminal pro-domain processing of pro-caspase-12 by calpain generated a 38 kDa fragment, while caspase-3/7 generated a 35 kDa fragment. Antibody developed specifically against the caspase-3/7 C-terminal cleavage site D(341) detected the presence of large subunit (p20) containing 23 kDa fragment that increased after 24 h of TG treatment. Significant caspase-12 enzyme activity was only detected after 24 h of TG treatment and was completely inhibited by caspase 3/7 inhibitor DEVD-fmk and partially by calpain inhibitor SNJ-1945. ER-stress-induced cell death pathway in TG-treated PC12 cells was characterized by up-regulation of GRP-78 and processing and activation of caspase-12 by the orchestrated proteolytic activity of calpain-2 and caspase-3/7.

Involvement of 4E-BP1 in the protection induced by HDLs on pancreatic beta-cells

High-density lipoproteins (HDLs) protect pancreatic beta-cells against apoptosis. This property might relate to the increased risk to develop diabetes in patients with low HDL blood levels. However, the mechanisms by which HDLs protect beta-cells are poorly characterized. Here we used a transcriptomic approach to identify genes differentially modulated by HDLs in beta-cells subjected to apoptotic stimuli. The transcript encoding 4E-binding protein (4E-BP)1 was up-regulated by serum starvation, and HDLs blocked this increase. 4E-BP1 inhibits cap-dependent translation in its non- or hypophosphorylated state but it loses this ability when hyperphosphorylated. At the protein level, 4E-BP1 was also up-regulated in response to starvation and IL-1beta, and this was blunted by HDLs. Whereas an ectopic increase of 4E-BP1 expression induced beta-cell death, silencing 4E-BP1 increase with short hairpin RNAs inhibited the apoptotic-inducing capacities of starvation. HDLs can therefore protect beta-cells by blocking 4E-BP1 protein expression, but this is not the sole protective mechanism activated by HDLs. Indeed, HDLs blocked apoptosis induced by endoplasmic reticulum stress with no associated decrease in total 4E-BP1 induction. Although, HDLs favored the phosphorylation, and hence the inactivation of 4E-BP1 in these conditions, this appeared not to be required for HDL protection. Our results indicate that HDLs can protect beta-cells through modulation of 4E-BP1 depending on the type of stress stimuli.

Serum-withdrawal-dependent apoptosis of hippocampal neuroblasts involves Ca++ release by endoplasmic reticulum and caspase-12 activation

Apoptotic death caused by diseases or toxic insults is preceded and determined by endoplasmic reticulum dysfunction and altered intraluminar calcium homeostasis in many different cell types. With the present study we have explored the possibility that the ER stress could be involved also in apoptotic death induced by serum deprivation in neuronal cells. We have chosen as a model of study the cell line HN9.10e, constituted by immortalized hippocampal neuroblasts. The Ca(++) concentration in the lumen of the ER has been evaluated by using the low affinity Ca(++) probe Mag-fluo-4. We show that serum deprivation lowers the ER Ca(++) concentration with a time course closely related to the increase of apoptosis incidence. Serum deprivation also enhances the expression of a well-known marker of ER stress, the glucose-regulated protein-78 (GRP-78), a member of the heat shock/stress response protein family. Moreover, in serum-deprived neuroblasts, following GRP-78 up-regulation, the ER-associated procaspase-12 is cleaved with a time course which parallels the ER calcium loss while activation of caspase-3 is a later event. Depletion of ER Ca(++) by thapsigargin, a specific inhibitor of the ER-associated Ca(++) ATPase, also produces caspase-12 processing and apoptotic cell death, whereas agents capable of reducing the ER calcium loss protect the cells from serum-deprivation-induced apoptosis. These findings indicate that, in hippocampal neuroblasts, Ca(++) mobilization from ER and caspase-12 activation are components of the molecular pathway that leads to apoptosis triggered by serum deprivation and may constitute an amplifying loop of the mitochondrial pathway.

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

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.

Norepinephrine-induced oxidative stress causes PC-12 cell apoptosis by both endoplasmic reticulum stress and mitochondrial intrinsic pathway: inhibition of phosphatidylinositol 3-kinase survival pathway

Norepinephrine (NE) induces endoplasmic reticulum (ER) unfolded protein response and reduces maturation and translocation of NE transporter to cell membrane via enhanced formation of reactive oxygen species in PC-12 cells. In the present study, we investigated whether ER stress is also implicated in the proapoptotic effect of NE. We found that the apoptotic effect of NE was associated with increased processing of ER-resident pro-caspase-12, cleavage of caspase-9 and -3, and mitochondrial release of cytochrome c. ER stress was evidenced by upregulation of ER chaperone GRP78 and transcription factor CHOP and the translocation of XBP-1 from the ER to the nucleus by NE. NE also reduced phospho-Akt (Ser473), indicating suppression of the phosphatidylinositol 3-kinase (PI3-kinase)/Akt survival pathway. Similar results were produced by thapsigargin. NGF, which promotes the PI3-kinase/Akt activity, reduced the effects of NE and thapsigargin on apoptosis and activation of caspase-12 and -3. However, the effects of NE, but not of thapsigargin, were abolished by pretreatment with SOD and catalase. In contrast, the PI3-kinase inhibitors LY-294002 and wortmannin abolished the protective effects of both SOD/catalase and NGF on NE-induced apoptosis. The functional importance of caspase-12 activation was supported by the use of Z-ATAD-FMK, which reduced the NE-induced processing of caspase-12 and cell apoptosis, but the caspase-12, -9, and -3 inhibitors had no effects on the increase in cytosolic cytochrome c produced by NE. In contrast, the release of mitochondrial cytochrome c was abolished by SOD/catalase and NGF. These results indicate that NE induced cell apoptosis by both ER stress and a mitochondrial death pathway and that the effects of NE were mediated via oxidative stress and inhibition of the PI3-kinase/Akt survival pathway.