Editors' ChoiceAUTOPHAGY

Bcl-2, the Antideath Protein

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Science's STKE  11 Oct 2005:
Vol. 2005, Issue 305, pp. tw352
DOI: 10.1126/stke.3052005tw352

Cells can undergo cell death using at least two pathways: apoptotic cell death involving caspases and autophagic cell death involving lysosomes. Bcl-2 is well known for its role as an antiapoptotic protein, and Pattingre et al. now show that Bcl-2 also inhibits autophagy through its interactions with beclin 1. Using a yeast autophagy reconstitution assay and two transfected mammalian cell systems, Bcl-2 was found to inhibit beclin 1-mediated formation of autophagic structures through a mechanism that required the functional binding sites mediating the interaction between these two proteins. The interaction between beclin 1 and Bcl-2 was confirmed in HT-29 cells overexpressing Bcl-2 by coimmmunoprecipitation, and beclin 1 was identified on endoplasmic and mitochondrial membranes (the locations of Bcl-2) based on cell fractionation and confocal microscopy. In HT-29 cells overexpressing Bcl-2, the formation of the beclin and hVps34 complex, which has class III phosphoinositide 3-kinase (PI3K) activity, was inhibited, as was the colocalization of beclin 1 with PI3P (phosphatidylinositol 3-phosphate), suggesting that Bcl-2 inhibited the PI3K activity of the beclin 1-hVSP34 complex. In HeLa cells, which endogenously express both beclin 1 and Bcl-2, Bcl-2 and beclin 1 coimmunoprecipitated under nutrient-rich conditions, and this interaction was decreased when the cells were starved of nutrients. Knockdown of Bcl-2 in HeLa cells with siRNA resulted in an increase in the number of autophagic structures under starvation conditions compared with those observed in starved cells with wild-type levels of Bcl-2. Forced expression of Bcl-2 in cardiac muscle of transgenic mice decreased the number of autophagic structures that appeared in mice after 48 hours of starvation. In MCF7 cells (which do not have wild-type beclin 1), expression of mutant forms of beclin 1 that could not bind Bcl-2 caused an increase in basal, as well as starvation-induced, autophagic structures. In addition, these cells were more susceptible to cell death under normal and starvation conditions. This cell death was inhibited by knockdown of the autophagy protein ATG5 by RNAi. Thus, Bcl-2 appears to be a more general inhibitor of cell death than previously appreciated.

S. Pattingre, A. Tassa, X. Qu, R. Garuti, X. H. Liang, N. Mizushima, M. Packer, M. D. Schneider, B. Levine, Bcl-2 antiapoptotic proteins inhibit beclin 1-dependent autophagy. Cell 122, 927-939 (2005). [PubMed]