ACTIVATION OF THE E2F TRANSCRIPTION FACTOR BY CYCLIN D1 IS BLOCKED BYP16(INK4), THE PRODUCT OF THE PUTATIVE TUMOR-SUPPRESSOR GENE MTS1

The oncoprotein cyclin D1 binds to and activates cyclin-dependent kina se 4 (cdk4), whose activity is inhibited by p16(INK4), the product of the putative tumor suppressor gene MTS1. Cyclin D1 controls the timing of S phase onset in mammalian cells. We show that cyclin D1 acts as a positive regulator of the transcription factor E2F. In particular, cy clin D1 overexpression leads to the activation of the dihydrofolate re ductase (DHFR) gene promoter. Activation depends on the E2F binding si te in the DHFR promoter, known to mediate its activation at the G1/S t ransition in vivo. Cyclin D1 can also activate the adenovirus E2 promo ter via E2F. Both promoters are repressed by p16(INK4) and this repres sion can be released by overexpression of cdk4. The data reported here support a direct role for cyclin D1 and its associated kinase in cell cycle regulation of E2F activity and S phase-specific gene expression . In addition, we show that both E2F sites bind complexes containing t he retinoblastoma protein (pRB) and that in RB-deficient cell lines ov erexpression of cyclin D1 fails to activate E2F-dependent transcriptio n, indicating that pRB may be involved in promoter activation.