Caspase-mediated cleavage of Beclin-1 inactivates Beclin-1-induced autophagy and enhances apoptosis by promoting the release of proapoptotic factors from mitochondria

Abstract

Autophagy and apoptosis are two important and interconnected stress-response mechanisms. However, the molecular interplay between these two pathways is not fully understood. To study the fate and function of autophagic proteins at the onset of apoptosis, we used a cellular model system in which autophagy precedes apoptosis. IL-3 depletion of Ba/F3 cells caused caspase (casp)-mediated cleavage of Beclin-1 and PI3KC3, two crucial components of the autophagy-inducing complex. We identified two casp cleavage sites in Beclin-1, TDVD133 and DQLD149, cleavage at which yields fragments lacking the autophagy-inducing capacity. Noteworthy, the C-terminal fragment, Beclin-1-C, localized predominantly at the mitochondria and sensitized the cells to apoptosis. Moreover, on isolated mitochondria, recombinant Beclin-1-C was able to induce the release of proapoptotic factors. These findings point to a mechanism by which casp-dependent generation of Beclin-1-C creates an amplifying loop enhancing apoptosis upon growth factor withdrawal.

Cross talk between apoptosis and autophagy by caspase-mediated cleavage of Beclin 1

“…More importantly, Wirawan et al (2010) proved that Beclin 1, once cleaved by caspases, can acquire a new apoptosis-promoting function. Indeed, Beclin 1-C translocated from the cytosol to mitochondria and amplified apoptosis induced by interleukin-3 deprivation.…”

“…And do apoptosis and pro-apoptotic molecules modulate the function of Beclin 1? Wirawan et al (2010) examined these burning questions in a cellular model system in which the withdrawal of the obligatory growth factor interleukin-3 induces apoptotic cell death that is preceded by a phase of increased autophagy. They showed that Beclin 1 and class III phosphatidyl inositol-3-kinase, two components of the autophagy-inducing complex, are direct substrates of caspases.…”

“…Our own unpublished data confirm that nutrient deprivation and oxidative stress can trigger caspasedependent cleavage of Beclin 1. Wirawan et al (2010) reported that the N-terminal (Beclin 1-N, aa 1-133) and C-terminal (Beclin 1-C, aa 150-450) cleavage products of Beclin 1 failed to induce autophagy in conditions, in which the positive control, full-length Beclin 1 clearly stimulated an autophagic response. Moreover, analysis of the subcellular localization of Beclin 1 and its cleavage products revealed the nuclear accumulation of the N-terminal Beclin 1 fragment, contrasting with the mitochondrial localization of its C-terminal fragment and the diffuse cytoplasmic distribution of full-length Beclin 1.…”

“…The most important finding of the study by Wirawan et al (2010) is the novel pro-apoptotic function of Beclin 1 that becomes apparent after its cleavage by caspases. Thus, besides its tumor-suppressor function, Beclin 1 might sensitize cells to pro-apoptotic inducing chemotherapies.…”

PAWR-mediated suppression of BCL2 promotes switching of 3-azido withaferin A (3-AWA)-induced autophagy to apoptosis in prostate cancer cells

“…[44][45][46] Because quite a few other autophagy proteins are cleaved during apoptosis, it is hypothesized that cleavage of BECN1 serves to inhibit cytoprotective autophagy in cells that have committed to apoptotic cell death. Similarly, the results shown here demonstrating that knockdown of endogenous PAWR by siRNA prolonged autophagy by 3-AWA even after 24 h of treatment, might be due to lack of caspase interactions and ectopic overexpression of DS-Red-BECN1 delayed apoptosis until 24 h in the presence of higher concentration of 3-AWA.…”