E2F1-induced apoptosis requires DNA binding but not transactivation and is inhibited by the retinoblastoma protein through direct interaction.

Abstract

E2F1 overexpression has been shown to induce apoptosis in cooperation with p53. Using Saos-2 cells, which are null for p53 and lack functional Rb, we have demonstrated that E2F1 overexpression can also induce apoptosis in the absence of p53 and retinoblastoma protein (Rb). E2Fl-induced apoptosis can be specifically inhibited by Rb but not mdm2, which is known for its ability to inhibit p53-induced apoptosis. Through the study of the apoptotic function of a set of E2F1 mutants, it was clear that the transactivation and the apoptotic function of E2F1 are uncoupled. The transactivation-defective E2F1 mutants E2Fl(1-374), E2Fl(390-1)DF(Amdm2), and E2Fl(406-415)(ARb) can induce apoptosis as effectively as wild-type E2F1. In contrast to E2F1 transactivation, the DNA-binding activity of E2F1 was proven to be essential for its apoptotic function, as the DNA-binding-defective mutants E2Fl(132) and E2Fl(132)(1-374) failed to induce apoptosis. Therefore Rb may inhibit E2Fl-induced apoptosis by mechanisms other than the suppression of the transactivation of E2F1. This hypothesis was supported by our observation that although Rb overexpression can specifically repress the apoptosis induced by wild-type E2F1 and a Rb-binding-competent E2F1 mutant E2Fl(390-1)DF(Amdm2), it failed to inhibit the apoptosis induced by mutants E2Fl(1-374) and E2Fl(~406-415)(~Rb), which are defective or reduced in Rb binding and transactivation. All of these points argue for a novel function for E2F1 and Rb in controlling apoptosis. The results also indicate that transcriptional repression rather than the transactivation function of E2F1 may be involved in its apoptotic function. The results presented here may provide us some physiological implication of the repression function of the Rb-E2F1 complex.

MIF loss impairs Myc-induced lymphomagenesis

“…Consistent with this view, DNA binding is required for E2F1-responsive apoptosis, whereas transactivation is largely dispensable. 65,66 Strikingly, this does not seem to be the case in MIF deficiency, as our data with DbE2F1 clearly indicate that the growth-and apoptosis-promoting E2F1 activity in MIF null cells depends on the presence of its C-terminal Rb-binding/ transactivation domain. Because Rb and its family proteins are largely inactive in Myc-or E2F-overexpressing cells, 49,50,56 we speculate that an Rb-independent inhibitor of E2F1 is lost in MIF-deficient cells, explaining why ectopic E2F1 has more effect in MIF-KO than WT fibroblasts.…”

“…25,36,62 Moreover, Myc-induced p53-dependent apoptosis is increased in the epithelium of mice lacking E2F1, 63 while the role of E2F1 in mediating tumorigenesis can be explained by its ability to suppress rather than induce apoptosis. [64][65][66] Collectively, these data imply that E2Fs play complex roles in gene regulation and that apoptotic target genes of E2F1, such as p73, Apaf-1, and procaspases, may be downregulated (rather than upregulated) by E2F1 in vivo, while p53 acts to relieve this expression block. Consistent with this view, DNA binding is required for E2F1-responsive apoptosis, whereas transactivation is largely dispensable.…”

Induction of S phase and apoptosis by the human papillomavirus type 16 E7 protein are separable events in immortalized rodent fibroblasts

“…Next we determined the ability of the E7 proteins to trigger apoptosis. Analogously to other inducers of apoptosis, such as c-myc, Adenovirus E1A or E2F-1 (Evan et al, 1992;Hsieh et al, 1997;White et al, 1994), E7-induced apoptosis is more pronounced when the cells are deprived of serum survival factors . Therefore, NIH3T3 cells expressing the di erent E7 genes were cultured in medium containing 0.5% calf serum and the percentage of apoptotic cells was determined using time-lapse videomicroscopy.…”

Apoptosis induced by the myelodysplastic syndrome-associated NPM-MLF1 chimeric protein

“…Other apoptotic inducers such as c-Myc and E2F-1 also promote S phase entry (Evan et al, 1992;Askew et al, 1991;Wu and Levine, 1994;Qin et al, 1994;Shan and Lee, 1994), but accelerated initiation of DNA replication itself does not trigger apoptotic pathways (Hsieh et al, 1997;Phillips et al, 1997). For example, other E2F family members (E2F-2 and -3) induce S phase entry of serum-starved ®broblasts without evoking apoptosis (DeGregori et al, 1997).…”