Differential Dependence on Beclin 1 for the Regulation of Pro-Survival Autophagy by Bcl-2 and Bcl-xL in HCT116 Colorectal Cancer Cells

Abstract

Autophagy is described to be involved in homeostasis, development and disease, both as a survival and a death process. Its involvement in cell death proceeds from interrelationships with the apoptotic pathway. We focused on survival autophagy and investigated its interplays with the apoptotic machinery. We found that while Mcl-1 remained ineffective, Bcl-2 and Bcl-xL were required for starved cells to display a fully functional autophagic pathway as shown by proteolysis activity and detection of autophagic vesicles. Such pro-autophagic functions of Bcl-2 and Bcl-xL were independent of Bax. However they appeared to operate through non redundant mechanisms as Bcl-xL wielded a tighter control than Bcl-2 over the regulation of autophagy: unlike Bcl-2, Bcl-xL and Atg7 manipulation yielded identical phenotypes suggesting they could be components of the same signalling pathway; Bcl-xL subcellular localisation was modified upon starvation, and importantly Bcl-xL acted independently of Beclin 1. Still an intact BH3-binding site was required for Bcl-xL to stimulate a fully functional autophagic pathway. This study highlights that, in addition to their well-established anti-death function during apoptosis, Bcl-2 and Bcl-xL have a broader role in cell survival. Should Bcl-2 and Bcl-xL stand at the cross-roads between pro-survival and pro-death autophagy, this study introduces the new concept that the regulation of autophagy by Bcl-2 and Bcl-xL is adjusted according to its survival or death outcome.

The anti-apoptotic Bcl-B protein inhibits BECN1-dependent autophagic cell death

“…[8][9][10][11] It is known that not all anti-apoptotic Bcl-2 family members are capable of antagonizing autophagy by binding to BECN1. 24,25 We show, in this study, that Bcl-B interacts with BECN1 and suppresses autophagy. We also established that the BECN1/Bcl-B interaction requirements were similar to those of Bcl-2/Bcl-X L, requiring the BH1 and BH3 domains of Bcl-B and BECN1, respectively.…”

“…These results suggest that the molecular basis of autophagy regulation by Bcl-B protein is similar to that of Bcl-2 and Bcl-X L . Bcl-W and, to a lesser extent, Mcl-1 have also been reported to bind to BECN1, 24 but this observation was not linked to autophagy inhibition 24,25 (and unpublished data). These observations imply that the binding of anti-apoptotic Bcl-2 members to BECN1 is necessary but not sufficient to inhibit autophagy and that further intrinsic properties shared only by Bcl-2, Bcl-X L and Bcl-B are also required.…”

“…Conversely, under autophagic conditions, BECN1/Vps34 interaction is stabilized at the expense of Bcl-2/ BECN1 binding. This model is, however, controversial because other studies also reported that Bcl-2 is unable to displace the BECN1/Vps34 complex 25,32,33 and because the BECN1/Bcl-2 complex is not disrupted under autophagic conditions. 25 Further work is needed to elucidate the anti-autophagic mechanism of Bcl-B as pertains to BECN1.…”

“…28 For example, in contrast to several studies that reported that silencing or knocking out certain autophagy genes, such as BECN1, reduces nonapoptotic cell death, 29 Priault et al showed that, in colorectal cancer cells, Bcl-2 and Bcl-X L are required for starved cells to display a fully functional autophagic pathway. 25 They concluded that Bcl-2 and Bcl-X L aid cell survival by assisting with the formation of autophagosomes. In our study, Bcl-B silencing apparently induced autophagic cell death, inasmuch as siRNA-mediated silencing of LC3 or BECN1 improved cell survival when Bcl-B is depleted.…”

Placental autophagy regulation by the BOK-MCL1 rheostat

“…31 Our immunoprecipitation data suggests that recruitment of MCL1 by BOK interferes with MCL1-BECN1 interactions, thereby freeing BECN1 that subsequently induces autophagy. Alternatively, similarly to BCL2 and BCL2L1, which induce cytoprotective-and cell death-promoting autophagy via a mechanism independent of BECN1, 32 BOK alone may directly trigger autophagy (Fig. 8).…”