Regulation of E2F1-induced apoptosis by poly(ADP-ribosyl)ation.

The transcription factor adenovirus E2 promoter-binding factor (E2F)-1 normally enhances cell-cycle progression, but it also induces apoptosis under certain conditions, including DNA damage and serum deprivation. Although DNA damage facilitates the phosphorylation and stabilization of E2F1 to trigger apoptosis, how serum starvation renders cells vulnerable to E2F1-induced apoptosis remains unclear. Because poly(ADP-ribose) polymerase 1 (PARP1), a nuclear enzyme essential for genomic stability and chromatin remodeling, interacts directly with E2F1, we investigated the effects of PARP1 on E2F1-mediated functions in the presence and absence of serum. PARP1 attenuation, which increased E2F1 transactivation, induced G2/M cell-cycle arrest under normal growth conditions, but enhanced E2F1-induced apoptosis in serum-starved cells. Interestingly, basal PARP1 activity was sufficient to modify E2F1 by poly(ADP-ribosyl)ation, which stabilized the interaction between E2F1 and the BIN1 tumor suppressor in the nucleus. Accordingly, BIN1 acted as an RB1-independent E2F1 corepressor. Because E2F1 directly activates the BIN1 gene promoter, BIN1 curbed E2F1 activity through a negative-feedback mechanism. Conversely, when the BIN1-E2F1 interaction was abolished by PARP1 suppression, E2F1 continuously increased BIN1 levels. This is functionally germane, as PARP1-depletion-associated G2/M arrest was reversed by the transfection of BIN1 siRNA. Moreover, PARP-inhibitor-associated anti-transformation activity was compromised by the coexpression of dominant-negative BIN1. Because serum starvation massively reduced the E2F1 poly(ADP-ribosyl)ation, we conclude that the release of BIN1 from hypo-poly(ADP-ribosyl)ated E2F1 is a mechanism by which serum starvation promotes E2F1-induced apoptosis.

The c-MYC-interacting proapoptotic tumor suppressor BIN1 is a transcriptional target for E2F1 in response to DNA damage.

The E2F1 transcription factor, which was originally identified as a cell-cycle initiator, mediates apoptosis in response to DNA damage. As E2F1-induced apoptosis is an attractive mechanism for cancer therapy, it is critical to fully elucidate its effector pathways. Here, we show that the c-MYC-interacting proapoptotic tumor suppressor, BIN1, is transcriptionally activated by E2F1 and mediates E2F1-induced apoptosis in response to DNA damage. Acting through the DNA-binding and transactivation domains, ectopically expressed E2F1 activated the human BIN1 promoter, which contains canonical E2F-recognition sites. Conversely, depletion of E2F1 by small interfering RNA or germline deletion led to BIN1 deficiency. DNA-damaging agents (which included etoposide) increased BIN1 levels, unless E2F1 was deficient. Moreover, endogenous E2F1 protein interacted directly with the BIN1 gene promoter in chromatin, particularly after etoposide treatment. Notably, suppression of BIN1 expression using an antisense (AS) technique attenuated the cell death mediated by E2F1 and etoposide. Although the p53 tumor suppressor, its sibling protein p73, and caspases are well-known E2F1 effectors for DNA damage-induced apoptosis, AS-BIN1 did not compromise their apoptotic functions. Our results collectively suggest that BIN1 is a novel transcriptional target of E2F1 that triggers a unique mode of cell death in response to DNA damage.