Role of Bcl-2 family proteins in a non-apoptotic programmed cell death dependent on autophagy genes

Abstract

Programmed cell death can be divided into several categories including type I (apoptosis) and type II (autophagic death). The Bcl-2 family of proteins are well-characterized regulators of apoptosis, and the multidomain pro-apoptotic members of this family, such as Bax and Bak, act as a mitochondrial gateway where a variety of apoptotic signals converge. Although embryonic fibroblasts from Bax/Bak double knockout mice are resistant to apoptosis, we found that these cells still underwent a non-apoptotic death after death stimulation. Electron microscopic and biochemical studies revealed that double knockout cell death was associated with autophagosomes/autolysosomes. This non-apoptotic death of double knockout cells was suppressed by inhibitors of autophagy, including 3-methyl adenine, was dependent on autophagic proteins APG5 and Beclin 1 (capable of binding to Bcl-2/Bcl-x(L)), and was also modulated by Bcl-x(L). These results indicate that the Bcl-2 family of proteins not only regulates apoptosis, but also controls non-apoptotic programmed cell death that depends on the autophagy genes.

Clearance of dying autophagic cells of different origin by professional and non-professional phagocytes

“…14,18,19,26,27 It has been suggested that autophagy as a mechanism of cell death may have been developed because in some forms of PCD the availability of engulfing cells is insufficient for the clearance of dead cells and autophagy has been evoked for elimination of excess cells by their own lysosomes. 13 In the present study we clearly demonstrate this is not the case: autophagic MCF-7 cells of different origin are recognized and eaten by both differentiated macrophages and non-dying MCF-7 cells. As almost all engulfed cells were MDC þ and all were PI þ , we may conclude that the eaten cells were dead or dying autophagic cells, and not cells that use autophagy to survive.…”

“…12 In embryonic fibroblasts with homozygous deletion of both bax and bak, RNA interference against atg5 and beclin could prevent autophagic cell death. 13 The fate of dead cells depends on their localization within the body. Macrophages acting as professional phagocytes are capable of engulfing apoptotic cells undergoing type I PCD without inducing inflammatory response.…”

Apoptosis and autophagy: regulatory connections between two supposedly different processes

“…2) instead of apoptosis; it is not known if both these mechanisms apply at the same time or if one is more important than the other in different contexts. Added complexity comes from the fact that under other circumstances -in cells that have lost both Bax and Bak and are therefore essentially completely resistant to the intrinsic apoptosis pathway-Bcl-2 (and Bcl-xL) appears to have the opposite effect because increasing their expression stimulates autophagic cell death while Bcl-xL knockdown reduces autophagy (25). Thus while the connections between Bcl family of apoptosis regulators and autophagy appear to be complicated it is clear that this family of proteins are intimately involved in regulating the two processes and that it may be where the proteins are-i.e.…”

“…Unlike apoptosis, which relies upon the activation of caspases that cleave hundreds of target proteins (24), autophagic cell death is usually thought of as caspase-independent (21). Indeed, two of the best examples of autophagic cell death where the demise of the cell was shown to require the autophagy machinery took place in cells with profound defects in the apoptosis machinery (25) or in the presence of caspase inhibitors (26). Cells undergoing autophagic cell death or apoptosis look different (27); the characteristic cellular morphology associated with apoptosis is usually due to caspase cleavage of cytoskeletal and other structural proteins (24) so that apoptotic cells show early degradation of the cytoskeleton but preserve organelles until fairly late in the process.…”

Alternate functions of viral regulators of cell death

“…Viral and cellular proteins are reported to regulate autophagy, a recycling process first delineated in yeast that promotes cell survival when nutrients are limiting. [93][94][95] In a yeast two-hybrid screen, cellular Bcl-2 pulled out Beclin, the mammalian homolog of yeast ATG6 that regulates autophagy. 96 Beclin also protects mice from a fatal Sindbis virus infection.…”