Activating E2Fs mediate transcriptional regulation of human E2F6 repressor.
E2F6 is believed to repress E2F-responsive genes and therefore serve a role in cell cycle regulation. Analysis of the human E2F6 promoter region revealed the presence of two putative E2F binding sites, both of which were found to be functionally critical because deletion or mutations of these sites abolished promoter activity. Ectopic expression of E2F1 protein was found to increase E2F6 mRNA levels and significantly upregulate E2F6 promoter activity. Deletion or mutation of the putative E2F binding sites nullified the effects of E2F1 on the E2F6 promoter activity. Studies on the temporal induction of E2F family members demonstrated that the activating E2Fs, and most notably E2F1, were upregulated before E2F6 during cell cycle progression at the G1/S phase, and this coincided with the time course of induction experienced by the E2F6 promoter during the course of the cell cycle. EMSAs indicated the specific binding of nuclear complexes to the E2F6 promoter that contained E2F1-related species whose binding was specifically competed by the consensus E2F binding site. Chromatin immunoprecipitation assays with anti-E2Fs demonstrated the association of E2F family members with the E2F6 promoter in vivo. These data indicate that the expression of the E2F6 repressor is influenced at the transcriptional level by E2F family members and suggest that interplay among these transcriptional regulators, especially E2F1, may be critical for cell cycle regulation.
