Regulation of the E2F-associated phosphoprotein promoter by GC-box binding proteins.

The E2F-associated phosphoprotein (EAPP) is a ubiquitous nuclear protein that interacts with the activating members of the E2F family of transcription factors and increases the activity of several cell-cycle regulated promoters in an E2F-dependent manner. Our previous studies also showed that EAPP levels are elevated in most transformed human cells. To examine the molecular basis of this increase of EAPP we isolated and studied the nucleotide sequence at the 5' end of the EAPP gene. In silico analysis revealed a TATA-less promoter with several putative binding sites for transcription factors, the most probable ones being Sp1, Sp3 and Egr-1. We could confirm the binding of these factors in vitro by electrophoretic mobility shift assays, supershift experiments and competition assays. Additionally we could validate the binding in vivo by chromatin-immunoprecipitation assays. To analyse the function of these transcription factors in the expression of EAPP, we performed reporter-assays with the promoter and truncations thereof. We found that Sp1 and Egr-1 stimulate the EAPP promoter, whereas Sp3 acts as a repressor that could even overcome the positive effect of the activators. Increasing the amounts of Sp3 also caused a strong reduction of EAPP, but the overexpression of Sp1 or Egr-1 resulted in only marginally higher EAPP levels. Our results suggest that the elevated EAPP levels in transformed cells can be caused by reduced Sp3 activity, but higher Sp1 activity might also play a role.