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

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). E2F1-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 E2F1(1-374), E2F1(390-1)DF(delta mdm2), and E2F1(406-415)(delta Rb) 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 E2F1(132) and E2F1(132)(1-374) failed to induce apoptosis. Therefore Rb may inhibit E2F1-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 E2F1(390-1)DF(delta mdm2), it failed to inhibit the apoptosis induced by mutants E2F1(1-374) and E2F1(delta 406-415)(delta 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.