Overexpression of E2F5/p130, but not E2F5 alone, can inhibit E2F-induced cell cycle entry in transgenic mice

PURPOSE The retinoblastoma (Rb) gene family member p130 binds preferentially to the E2F5 transcription factor and forms a repressive E2F5/p130 complex that inhibits cell cycle progression and tumor growth. It is unclear whether E2F5, either alone or in combination with p130, can interfere with the transcriptional activity of other E2F family members, such as E2F1 and E2F3a, in vivo. In this study, we used transgenic mice to test whether overexpression of E2F5 with/without p130 would be sufficient to inhibit E2F1 or E2F3a induced cell cycle reentry. METHODS Transgenic mice were generated by microinjection of constructs containing different E2F cDNAs (E2F1, E2F3a, and E2F5) or the p130 cDNA linked to the mouse alphaA-crystallin promoter. The E2F5 single and E2F5/p130 double transgenic mice were cross-mated with E2F1 or E2F3a transgenic mice. The resulting double or triple transgenic mouse embryos were characterized by histology, in situ hybridization, immunohistochemistry, and BrdU incorporation assays. RESULTS Overexpression of E2F5 alone was not sufficient to inhibit E2F1 or E2F3a induced cell cycle reentry in lens fiber cells. Transgenic mice coexpressing E2F5 and p130 in lens fiber cells did not show lens defects. However, coexpression of E2F5/p130 with E2F1 or E2F3a in lens fiber cells decreased the number of BrdU positive fiber cells induced by the E2F1 or E2F3a alone. Therefore, overexpression of E2F5/p130, but not E2F5 alone, can inhibit activator E2F-mediated cell proliferation in vivo, confirming that p130 plays a critical role in the repressive activity of E2F5/p130 complex. CONCLUSIONS Overexpression of E2F5/p130 in post-mitotic lens fiber cells does not affect their normal differentiation program, but can inhibit inappropriate cell cycle reentry induced by the activator E2Fs. Since E2F5 alone cannot interfere with these E2F activities, we conclude that p130 is a key player in the inhibitory process.