CtIP Activates Its Own and Cyclin D1 Promoters via the E2F/RB Pathway during G₁/S Progression

Abstract

Cell cycle progression from G₁ to S phase is mainly controlled by E2F transcription factors and RB family proteins. Previously we showed that the presence of CtIP is essential for G₁/S transition in primary mouse blastocysts, as well as in NIH 3T3 cells. However, how CtIP executes this function remains to be elucidated. Here we show that in NIH 3T3 cells the expression of CtIP is regulated by the E2F/RB pathway during late G₁ and S phases. The presence of wild-type CtIP, but not the E157K mutant form, which failed to interact with RB, enhanced its own promoter activity. Chromatin immunoprecipitation analysis indicated that the recruitment of CtIP to its promoter occurs concomitantly with TFIIB, a component of the RNA polymerase II complex, and with dissociation of RB from the promoter during late G₁ and G₁/S transition. Similar positive regulation of cyclin D1 expression by CtIP was also observed. Consistently, cells expressing the CtIP(E157K) protein alone exhibited growth retardation, an increase in the G₁ population, and a decrease in the S-phase population. Taken together, these results suggest that, contrary to the postulated universal corepressor role, CtIP activates a subset of E2F-responsive promoters by releasing RB-imposed repression and therefore promotes G₁/S progression.

We thank Richard Pestell for the cyclin D1 promoter reporter construct. We also thank Saori Furuta for critical reading of the manuscript.

This research was supported by a grant from the NIH (CA 94170 to W.H.L.). Feng Liu is supported by a Susan G. Komen breast cancer postdoctoral fellowship.