The p16-cyclin D-pRB-E2F pathway is frequently deregulated in human tumors. This 
critical regulatory pathway controls the G1/S transition of the mammalian cell 
cycle by positive and negative regulation of E2F-responsive genes required for 
DNA replication. To assess the value of the transcription factors E2Fs as 
targets for antiproliferative strategies, we have initiated a program aiming to 
develop inhibitors targeting specifically these proteins in vitro and in vivo. 
The cellular activity of E2F is the result of the heterodimeric association of 
two families of proteins, E2Fs and DPs, which then bind DNA. Here, we use a two 
hybrid approach to isolate from combinatorial libraries peptide aptamers that 
specifically interact with E2Fs DNA binding and dimerization domains. One of 
these is a potent inhibitor of E2F binding activity in vitro and in mammalian 
fibroblasts, blocks cells in G1, and the free variable region from this aptamer 
has the same effect. Our experiments argue that the variable region of this 
aptamer is structured, and that it functions by binding E2F with a motif that 
resembles a DP heterodimerization region, and blocking E2F's association with 
DP. These results show that cell proliferation can be inhibited using 
genetically-selected synthetic peptides that specifically target protein-protein 
interaction motifs within cell cycle regulators. These results also emphasize 
the critical role of the E2F pathway for cell proliferation and might allow the 
design of novel antiproliferative agents targeting the cyclin/CDK-pRB-E2F 
pathway.