Apoptosis Associated with Deregulated E2F Activity Is Dependent on E2F1 and Atm/Nbs1/Chk2

Abstract

The retinoblastoma protein (Rb)/E2F pathway links cellular proliferation control to apoptosis and is critical for normal development and cancer prevention. Here we define a transcription-mediated pathway in which deregulation of E2F1 by ectopic E2F expression or Rb inactivation by E7 of human papillomavirus type 16 signals apoptosis by inducing the expression of Chk2, a component of the DNA damage response. E2F1-and E7-mediated apoptosis are compromised in cells from patients with the related disorders ataxia telangiectasia and Nijmegen breakage syndrome lacking functional Atm and Nbs1 gene products, respectively. Both Atm and Nbs1 contribute to Chk2 activation and p53 phosphorylation following deregulation of normal Rb growth control. E2F2, a related E2F family member that does not induce apoptosis, also activates Atm, resulting in phosphorylation of p53. However, we found that the key commitment step in apoptosis induction is the ability of E2F1, and not E2F2, to upregulate Chk2 expression. Our results suggest that E2F1 plays a central role in signaling disturbances in the Rb growth control pathway and, by upregulation of Chk2, may sensitize cells to undergo apoptosis.

Novel link between E2F and p53: proapoptotic cofactors of p53 are transcriptionally upregulated by E2F

“…E2F1 can induce phosphorylation of p53 on residues that are phosphorylated in response to DNA damage 11,15,16 and, in agreement with these observations, E2F1 transcriptionally regulates the expression of the DNA damage-responsive kinases ATM and Chk2 that phosphorylate p53. 15,17 Consistent with the ability of both ARF and the DNA damage-induced kinases to cause p53 stabilization, ectopic expression of E2F1 indeed induces p53 accumulation. 5,18 p53 regulates the transcription of many genes that induce cell cycle arrest or apoptosis in response to DNA damage or oncogenic stress.…”

“…One mechanism involves E2F-induced activation of genes whose protein products positively regulate p53 stability and transcriptional activity; this group is constantly growing and presently includes ARF, ATM, Chk2 and PIN1. [15][16][17][30][31][32][33][34][35][36] After its stabilization and activation, p53 can elicit a number of different cellular responses including, most notably, growth arrest and apoptosis. The E2F-regulated genes listed above, while contributing to p53 activation, do not explain how E2F1 directs p53 to favor apoptosis over growth arrest.…”

“…11,15,16 E2F transcriptionally activates the kinases ATM and Chk2 and this most probably contributes to E2F-induced p53 phosphorylation on these residues. 15,17 It remains to be determined whether E2F also regulates the expression of kinases such as HIPK2, which mediates p53 phosphorylation on serine 46. 39,40 As shown here, E2F1-induced phosphorylation of p53 on serine 46 is required for E2F1-p53 cooperation in apoptosis, as evidenced by the fact that a mutant p53 incapable of undergoing phosphorylation at serine 46 fails to exhibit such cooperation.…”

“…EcotR-expressing H1299 cells were cultured in RPMI 1640 plus 5% FCS. Early passage (passage [16][17][18][19][20] WI38 human embryonic lung fibroblasts and EcotR-expressing WI38 fibroblasts were grown in MEM plus 15% FCS, 2 mM L-glutamine, 1 mM sodium pyruvate and nonessential amino acids. Cells were maintained at 371C in a humidified 8% CO 2 -containing atmosphere.…”

DNA-binding independent cell death from a minimal proapoptotic region of E2F-1

“…p14ARF inhibits this effect causing stabilization of p53 and, in some situations, results in programmed cell death (Pomerantz et al, 1998;Stott et al, 1998;Zhang et al, 1998;Honda and Yasuda, 1999). E2F-1 has also been shown to induce apoptosis in a p53-dependant manner in mice and cells that are lacking p14ARF (Russell et al, 2002;Tolbert et al, 2002;Tsai et al, 2002;Rogoff et al, 2004). For example, E2F-1 can transactivate several DNA damage responsive kinases, such as ATM and Chk2, which phosphorylate p53 and alleviate its Mdm2-mediated degradation (Rogoff et al, 2002(Rogoff et al, , 2004Berkovich and Ginsberg, 2003).…”