8-Chloro-adenosine-induced E2F1 promotes p14ARF gene activation in H1299 cells through displacing Sp1 from multiple overlapping E2F1/Sp1 sites.

The regulation of p14ARF gene by E2F transcription factor, which differs from that of classical E2F targets, has recently been attributed to a variant E2F-response element. However, promoter assays suggest multiple elements present in the p14ARF promoter and argue against the idea that the ARF promoter has a unique ability to distinguish between aberrant and physiological levels of E2F1. Therefore, the functional characterization of the promoter still needs to be done. We demonstrate that at least two overlapping E2F1/Sp1 binding sites are present in the p14ARF promoter, and E2F1 activates the promoter through displacing constitutive Sp1 from the overlapping sites. We found that 8-chloro-adenosine (a metabolite of 8-Cl-cAMP) exposure induced the p14ARF gene in human lung cancer H1299 cells, followed by increased expression of E2F1 and constitutive expression of Sp1. The combination of cotransfection and electrophoretic mobility shift assay (EMSA) indicated that constitutive binding of Sp1 to the overlapping sites contributed to a constitutive expression of the ARF gene in unexposed H1299, whereas displacing Sp1 from the overlapping sites by E2F1 promoted the gene activation after exposure. EMSA and chromatin immunoprecipitation revealed increased association of E2F1 with the overlapping sites in the active promoter in 8-Cl-Ado-exposed cells. Together, these data suggest that the overlapping E2F1/Sp1 site, being present in multiple copies in the p14ARF promoter, may serve as the targets for both E2F1 and Sp1, thereby playing a crucial role in response to some oncogenic signals and stimulators, which activate the ARF gene through inducing E2F in the cell.