Regulation of apoptosis by endoplasmic reticulum pathways

The small leucine-rich proteoglycan lumican inhibits melanoma progression

Lumican is a member of the small leucine-rich proteoglycan (SLRP) family. It contributes to the organisation of the collagen network and plays an important role in cell migration and tissue repair. The present study aimed to determine the influence of lumican expression on adhesion, anchorage-dependent and -independent growth, migration, in vitro invasion and in vivo melanoma growth. For that purpose, B16F1 mouse melanoma cells were stably transfected with an expression plasmid containing the complete lumican cDNA. Lumican expression by tumor cells did not change the proliferative activity of mouse melanoma cells in monolayer culture and did not influence either cell adhesion to extracellular matrix gel or type I collagen or cell spreading on these substrates. In contrast, lumican-transfected cells were characterized by a strong reduction of their anchorage-independent proliferation in agarose gel and capacity to invade extracellular matrix gel. After subcutaneous injections of transfected B16F1 cells in syngenic mice, lumican expression significantly decreased subcutaneous tumor formation in vivo, with a concomitant decrease of cyclin D1 expression. Lumican induced and/or increased the apoptosis of B16F1 cells. The results suggest that lumican is involved in the control of melanoma growth and invasion and may be considered, like decorin, as an anti-tumor factor from the extracellular matrix.

Involvement of BNIP1 in apoptosis and endoplasmic reticulum membrane fusion

BNIP1, a member of the BH3-only protein family, was first discovered as one of the proteins that are capable of interacting with the antiapoptotic adenovirus E1B 19-kDa protein. Here we disclose a totally unexpected finding that BNIP1 is a component of the complex comprising syntaxin 18, an endoplasmic reticulum (ER)-located soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptor (SNARE). Functional analysis revealed that BNIP1 participates in the formation of the ER network structure, but not in membrane trafficking between the ER and Golgi. Notably, a highly conserved leucine residue in the BH3 domain of BNIP1 plays an important role not only in the induction of apoptosis but also in the binding of alpha-SNAP, an adaptor that serves as a link between the chaperone ATPase NSF and SNAREs. This predicts that alpha-SNAP may suppress apoptosis by competing with antiapoptotic proteins for the BH3 domain of BNIP1. Indeed, overexpression of alpha-SNAP markedly delayed staurosporine-induced apoptosis. Our results shed light on possible crosstalk between apparently independent cellular events, apoptosis and ER membrane fusion.

Melanoma differentiation associated gene-7/interleukin-24 promotes tumor cell-specific apoptosis through both secretory and nonsecretory pathways

Melanoma differentiation associated gene-7/interleukin-24 (Mda-7/IL-24), a novel member of the IL-10 family of cytokines, uniquely displays cancer-specific apoptosis-inducing activity. Positive results in ongoing phase I/II clinical trials have strengthened the possibility of its utilization as a cancer gene therapeutic. Previous studies document that signaling events leading to Ad.mda-7-induced transformed cell apoptosis are tyrosine kinase-independent. These results suggest that mda-7/IL-24 cancer cell-specific activity could occur through mechanisms independent of binding to its currently recognized cognate receptors and might even occur independent of receptor function. An adenovirus vector expressing a nonsecreted version of MDA-7/IL-24 protein was generated via deletion of its signal peptide. This nonsecreted protein was as effective as wild-type secreted MDA-7/IL-24 in inducing apoptosis in prostate carcinoma cell lines and displayed transformed cell specificity and localization of MDA-7/IL-24 in the Golgi/endoplasmic reticulum compartments. Our results indicate that mda-7/IL-24-mediated apoptosis can be triggered through a combination of intracellular as well as secretory mechanisms and can occur efficiently in the absence of protein secretion.

Endoplasmic reticulum (ER) stress induced by a neurovirulent mouse retrovirus is associated with prolonged BiP binding and retention of a viral protein in the ER

Some murine retroviruses cause a spongiform neurodegenerative disease exhibiting pathology resembling that observed in transmissible spongiform encephalopathies. The neurovirulence of these "spongiogenic retroviruses" is determined by the sequence of their respective envelope proteins, although the mechanisms of neurotoxicity are not understood. We have studied a highly neurovirulent virus called FrCasE that causes a rapidly progressive form of this disease. Recently, transcriptional markers of endoplasmic reticulum (ER) stress were detected during the early preclinical period in the brains of FrCasE-infected mice. In contrast, ER stress was not observed in mice infected with an avirulent virus, F43, which carries a different envelope gene, suggesting a role for ER stress in disease pathogenesis. Here we have examined in NIH 3T3 cells the cause of this cellular stress response. The envelope protein of F43 bound BiP, a major ER chaperone, transiently and was processed normally through the secretory pathway. In contrast, the envelope protein of FrCasE bound to BiP for a prolonged period, was retained in the ER, and was degraded by the proteasome. Furthermore, engagement of the FrCasE envelope protein by ER quality control pathways resulted in decreased steady-state levels of this protein, relative to that of F43, both in NIH 3T3 cells and in the brains of infected mice. Thus, the ER stress induced by FrCasE appears to be initiated by inefficient folding of its viral envelope protein, suggesting that the neurodegenerative disease caused by this virus represents a protein misfolding disorder.

Edaravone Protects against Hypoxia/Ischemia-Induced Endoplasmic Reticulum Dysfunction

Endoplasmic reticulum (ER) stress-induced cell death plays an important role in cerebral ischemia. In the present study, we investigated whether edaravone (3-methyl-1-phenyl-pyrazolin-5-one), a free radical scavenger, can protect against ER damage induced by cerebral ischemia. In a mouse model of hypoxia/ischemia, treatment with edaravone reduced edema-corrected infarction volume, attenuated hemispheric swelling, and improved neurological status. Moreover, edaravone suppressed ER stress-mediated apoptosis by inhibiting eukaryotic initiation factor alpha phosphorylation, C/EBP homologous protein (CHOP) induction, and caspase-12 activation. In mouse primary cultured glial cells, edaravone attenuated ER stress as evidenced by inhibition of the induction of glucose regulated protein 78 and CHOP and XBP-1 splicing under treatment with tunicamycin (Tm), which induces ER stress. Tm did not induce the production of reactive oxygen species in primary cultured glial cells. In addition, the free radical scavengers N-acetyl-l-cysteine and ebselen [2-phenyl-1,2-benzisoselenazol-3(2H)-one] did not affect ER stress response caused by Tm. These results demonstrated a novel action of edaravone that can protect against ER dysfunction in cerebral ischemia.

Bcl-2 and calcium: controversy beneath the surface

The function of Bcl-2 family members on the endoplasmic reticulum has received increasing attention in recent years. The endoplasmic reticulum is the major organelle involved in intracellular calcium homeostasis and calcium signaling, including calcium signals that mediate apoptosis induction by anticancer drugs. But currently a controversy exists regarding reported effects of Bcl-2 on the calcium concentration within the lumen of the endoplasmic reticulum. Although several prominent reports indicate that Bcl-2 overexpression is associated with a decrease in luminal calcium, there are a large number of reports indicating that Bcl-2 either does not decrease luminal calcium or actually increases luminal calcium. This review summarizes this vast array of conflicting findings, and analyses potential reasons why different conclusions have been reached by different laboratories. Future directions are emphasized that might bring clarity to this important area of apoptosis biology.

Co-expression of MG29 and ryanodine receptor leads to apoptotic cell death: effect mediated by intracellular Ca2+ release

Perturbation of intracellular Ca2+ homeostasis has been shown to regulate the process of cell proliferation and apoptosis. Our previous studies show that mitsugumin 29 (MG29), a synaptophysin-related protein localized in the triad junction of skeletal muscle, serves an essential role in muscle Ca2+ signaling by regulating the process of store-operated Ca2+ entry. Here we report a functional interaction between MG29 and the ryanodine receptor (RyR)/Ca2+ release channel. The purified MG29 protein enhances activity of the RyR/Ca2+ release channel incorporated into the lipid bilayer membrane. Co-expression of MG29 and RyR in Chinese hamster ovary cells leads to apoptotic cell death resulting from depletion of intracellular Ca2+ stores, despite neither protein expression alone exhibits any significant effect on cell viability. In transient expression studies, the presence of RyR in the endoplasmic reticulum leads to retention of MG29 from the plasma membrane into the intracellular organelles. This functional interaction between MG29 and RyR could have important implications in the Ca2+ signaling processes of muscle cells. Our data also show that perturbation of intracellular Ca2+ homeostasis can serve as a key signal in the initiation of apoptosis.

Over-Expressed Bcl-2 Cannot Suppress Apoptosis via the Mitochondria in Buprenorphine Hydrochloride-Treated NG108-15 Cells

We previously reported that the morphine alkaloid derivative buprenorphine hydrochloride (Bph) induces rapid apoptosis in NG108-15 nerve cells accompanied by the activation of caspase-3. Here, we found this kind of apoptosis was also accompanied by rapid loss of the mitochondrial membrane potential, followed by the efflux of cytochrome c from the mitochondria to the cytosol and the activation of caspases-9 and -3. Together, these results strongly suggested the Bph death signal was routed through the mitochondrial pathway in NG108-15 cells. In these cells, serum-starvation induces a different apoptosis, which we exploited to investigate Bcl-2's role as an apoptosis inhibitor. We made an NG108-15 transfectant, Bcl-2(P2), that stably expressed human Bcl-2, and used it to test Bcl-2's effect on the serum-starvation-induced apoptosis in NG108-15 cells. Cell viability, DNA-ladder formation, and efflux of cytochrome c from the mitochondria were all detected, showing that the human Bcl-2 functioned normally in the Bcl-2(P2) cells. Although the apoptotic events tested were identical in the parental cells and transformants, Bcl-2 expression completely failed to inhibit Bph-induced apoptosis in the Bcl-2(P2) cells.

The Bcl-2 family: roles in cell survival and oncogenesis

Apoptosis, the cell-suicide programme executed by caspases, is critical for maintaining tissue homeostasis, and impaired apoptosis is now recognized to be a key step in tumorigenesis. Whether a cell should live or die is largely determined by the Bcl-2 family of anti- and proapoptotic regulators. These proteins respond to cues from various forms of intracellular stress, such as DNA damage or cytokine deprivation, and interact with opposing family members to determine whether or not the caspase proteolytic cascade should be unleashed. This review summarizes current views of how these proteins sense stress, interact with their relatives, perturb organelles such as the mitochondrion and endoplasmic reticulum and govern pathways to caspase activation. It briefly explores how family members influence cell-cycle entry and outlines the evidence for their involvement in tumour development, both as oncoproteins and tumour suppressors. Finally, it discusses the promise of novel anticancer therapeutics that target these vital regulators.