Role of the Rb/E2F pathway in cell growth control

The understanding of mechanisms regulating cell the accumulation of individual E2F activities in the absence of other aspects of a growth stimulus. By so growth has progressed rapidly through the combined genetic and biochemical analyses afforded by yeast and doing, we have shown that a variety of genes encoding proteins important for DNA replication are activated other systems. Likewise, the study of human cancer cells has provided many important insights into the by E2F. These studies also show that the individual E2F proteins display distinct specificities in the activamolecular mechanisms regulating cell growth. In particular, a large body of evidence now points to the regution of the target genes. In large part, the transcription activation capacity of E2F is limited to the E2F1, E2F2, latory pathway involving the G1 cyclin-dependent kinases, the Rb tumor suppressor family, and the E2F and E2F3 proteins, although E2F4 does activate a limited number of target genes. This distinction between family of transcription factors as critical for the passage of cells through G1 and into S phase (Fig. 1). A number E2F1, E2F2, and E2F3 vs. E2F4, and E2F5 reflects the evolutionary conservation of the DNA sequence encodof studies have shown that the growth suppressing action of the retinoblastoma tumor suppressor protein ing the proteins (Fig. 2), the unique protein interaction properties of E2F4 and E2F5 with respect to binding (Rb) and other Rb family members is dependent on their ability to regulate the E2F family of transcription to the Rb-related p107 and p130 proteins (10,11,29), and the ability of cyclin A/cdk2 to regulate the DNA factors (1–3). E2F was originally identified as a cellular transcription factor which could bind to and activate binding activity of the E2F1, E2F2, and E2F3 proteins through a direct interaction with a conserved domain the adenovirus E2 promoter (4), but is now known to comprise a family of at least five distinct DNA binding in the amino terminal portions of the proteins (30–32). activities, composed of the products of five distinct E2F CAN INDUCE S PHASE genes designated E2F1–5, together with heterodimeric partner proteins termed DP1 and DP2 (5–15). E2F acA distinction in the functional capacity of the E2F tivity appears to play a critical role in cell growth reguproteins is also seen in the capacity of the various prolation through the activation of a variety of genes that teins to induce otherwise quiescent cells to enter S encode proteins important for DNA replication in S phase. Production of the individual E2F activities in phase, as well as various genes that encode cell cycle either G1 arrested or quiescent cells has revealed an regulatory proteins that facilitate the transition ability of E2F1, E2F2, and E2F3 but not E2F4 or E2F5 through the cell cycle (16–19). That E2F activity is to induce S phase, as seen either as bromodeoxyuridine indeed critical for cell cycle progression has been shown (BrdU) incorporation or actual genome replication indiby the requirement for E2F function for cell cycle procated by fluorescence activated cell sorting (FACS) gression in Drosophila (20) and mammalian cells analysis. (21,22), as well as the fact that overexpression of the That the ability of E2F1, E2F2, or E2F3 to induce S Drosophila E2F gene (23,24) or the mammalian E2F1 phase represents a critical role in the G1 cdk pathway gene stimulates quiescent cells to enter S phase (25– has been shown by the fact that E2F1 can induce S 28). As such, these studies point to a critical role for phase in the absence of G1 cdk activity (33). In addition, this transcriptional activity in controlling the process we have also shown that cells infected with a recombiof cell growth. nant adenovirus containing the p21 cyclin kinase gene Our recent work has aimed to establish the imporarrest in G1 and lack cyclin D1, cyclin E, cyclin A, or tance of accumulation of E2F activity as an event regucdk2-associated kinase activities. Coinfection with an lated by the G1 cyclin-dependent kinases. This work E2F1-expressing adenovirus bypasses the p21-medisuggests that E2F and cyclin E/cdk2 collaborate for the ated block and leads to an accumulation of cells with a induction of S phase but also shows that E2F overproG2 content of DNA as well as the appearance of mitotic duction can suppress the normal requirement for kifigures. Importantly, the E2F1-mediated S phase innase activity. duction is not accompanied by a restoration of G1 cyclin-dependent kinase activity; nor is there a restoraPOSITIVE AND NEGATIVE TRANSCRIPTION CONTROL BY THE E2F FAMILY