Apoptosis mitochondrial-independent pathway

The essential role of cytochrome c release from injured mitochondria in the activation of caspase 9 has been alluded to above. This pathway is especially important in proapoptotic stimuli that are not initiated by surface receptors for apoptosis, such as UV irradiation, and may involve mitochondrial dependent pathways [83]. Continued respiration in the presence of an open mitochondrial pore may result in the generation of reactive oxygen species. Release of cytochrome c may be mediated by the opening of the mitochondrial FT pore, a non-selective channel whose composition is only partially defined [84]. Inhibitors of FT pore opening, such as cyclosporine, which binds to the adenine nucleotide translocator (ANT), a component of the FT pore, and bongkrekic acid, as well as Bcl-2, prevent cytochrome c release and inhibit apoptosis [85] whereas activators of the FT pore, such as atractyloside and Bax induce it [86]. Oxidants can rupture the outer membrane of mitochondria and release caspase-activating proteins [87]. Some studies have shown cytochrome c release before collapse of the mitochondrial membrane potential [83] suggesting alternate control of the FT pore. Many, but not all, of the members of the Bd-2 family of proteins reside in the inner mitochondrial membrane, form ionic channels in hpid membranes and increase rates of proton extrusion in mitochondria [88] and thus may control the FT pore. The antiapoptotic and mitochondrial affects of Bd-2 are independent of caspase activity as they occur in the presence of caspase inhibitors and also in yeast that lack caspases [86]. 

In tumor cells isolated from solid tumors, NO via p53 negatively regulates Bcl-2 expression, mRNA and protein synthesis [115, 116] and positively regulates Bax protein expression [117]. Otherwise, NO acts more downstream on caspases such as caspase 1 which activates caspase 3 which inactives itself poly-ADP-ribose-polymerase (PARP), an enzyme participating in DNA repair. NO, when activating caspase 1 and/or 2, inactivates PARP and allows apoptosis [118]. Meanwhile, NO can induce apoptosis of cancer cells by a caspase independent pathway but controlled by Bcl-2 [119]. One pathway or the other is reinforced when NO inhibits DNA-alkyltransferase, an enzyme that participates to DNA repair. In an other hand, apoptosis includes modifications affecting mitochondria, such as the reduction of membrane potential which is an early event in apoptosis. NO induces mitochondrial disfunctions [120] and cytochrome c release [118] that seem to activate caspases. 

In summary, treatment with 13-MTD induced no change in activity and expression of caspase-3. Furthermore, 13-MTD induced no mobilization of cellular calcium. However, 13-MTD altered mitochondrial transmembrane potential and induced AIF translocation from the mitochondria to the nucleus after 4 h of treatment. These results support the view that incorporation of 13-MTD into cellular lipids triggers apoptosis via a caspase-independent pathway. 

As well as activating the classical intrinsic pathway involving activation of caspases-9 and -3, Cd " can also utilize caspase-independent pathways, in a context dependent manner. Our laboratory demonstrated calpain activation at early time points (3-6 hours) whereas mitochondrial damage, cytochrome c and AIF release and subsequent caspase activation was seen only after 24 hours with 10 pM Cd " [522]. Other smdies have reported similar observations multiple apoptosis signaling pathways can be affected by Cd " to induce apoptotic cell death [523,524]. 

Bcl-2 is a key apoptosis regnlatory protein of the mitochondrial death pathway whose function is dependent on its expression levels. NO prevented Bcl-2 cleavage and suppressed cytochrome c release in TNF-a and actinomycin D-treated adenocarcinoma (MCF-7) cells exposed to SNAP (Kim et al. 1998). This level is regulated by a ubiqutination-proteasome degradation system. There was inhibition of NO production by the NO scavenger. The NO donors DPTA/NONO and sodium nitro-prusside effectively upregulated Bcl-2 S-nitrosylation, decreased its ubiquitination, and inhibited apoptotic cell death induced by chromium. The effect of NO on Bcl-2 stability was shown to be independent of its dephosphorylation (Azad et al. 2006). 

Suliman, A., Lam, A., Datta, R., and Srivastava, R.K. (2001). Intracellular mechanisms of trail Apoptosis through mitochondrial-dependent and -independent pathways. Oncogene 20, 2122-2133. 

Hou, D. X., Uto, T., Tong, X., Takeshita, T., Tanigawa, S., Imamura, I., Ose, T., and Fujii, M. (2004). Involvement of reactive oxygen species-independent mitochondrial pathway in gossypol-induced apoptosis. Arch. Biochem. Biophys. 428,179-187. 

Cytoplasmic acidification has been shown to be an important feature in apoptotic cells and may be necessary for optimal activation of caspases and execution of the apoptotic pathway. In some systems, concurrent alkinalization of mitochondria is identified, suggesting PT-opening with diffusion of protons into the cytoplasm (18). On the other hand, in somatostatin-induced apoptosis of MCF-7 cells, a nonmitochondrial source of H+ was implicated in the development of caspase 8-independent cytoplasmic acidification and this acidification was shown to be necessary for the induction of mitochondrial alterations (39). SNARF-1 is the dye most commonly employed in cells to assess alterations in pH in the near-physiologic range (7.0-8.0). It can be excited by the 488 nm line of the argon ion laser and the... 

Fig. 3. Possible pathways of qftokine-induced hepatocyte death. Tumor necrosis factor receptor (,TNF-R) and CD95 stimulated by independent signals (1,2) activate signaling proteases such as caspase-8. This step is experimentally inhibited by caspase inhibitors such as z-VAD-fink which protect hepatocytes. An irreversible step may include mitochondrial alterations and release of downstream signals such as cytochrome-c or apoptosis-inducing factor that would cause the activation of caspase-3 and/or other proteases. Completion of all downstream steps results in apoptosis and failure to do so would result in necrosis or in cell death of intermediate morphology...

In HL60 (promyelocytic leukemia cell fine), RAR and RXR binding also stimulates apoptosis (Bollag and Holdener, 1992) the same occurs in medulloblastoma cell fines, at least partially, through caspase-3 activation (Gumireddy et ai, 2003). Similarly, CD437, a synthetic retinoid, induces the initial stages of apoptosis in human respiratory cells via a mitochondrial pathway independent... 

Based on this model, moderate NADVATP depletion or the (ADP-ribose)n polymer itself triggers the apoptotic pathway. In addition, p53-induced mediators, as well as HI.2 released from the nucleus into the cytoplasm, act synergistically to induce apoptosis via mitochondrial dysfunction. At high intensity DNA damage, PARP overactivarion and PARP-independent mediators may also tri er mitochondrial alterations but downstream apoptotic signaling is incapacitated by the lack of ATP/NAD. ... 

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