Análisis del Contexto Actual y Esperado

The E2F family of transcription factors regulates many functions including: proliferation, apoptosis, differentiation, metabolism and DNA repair. In mammals, the E2F family is composed of E2F 1-8 and three dimerization partners (DP), DP1, -2 and -4 (Figure 1.7). E2F1 through -6 have a DNA binding domain, a leucine zipper, a marked box domain,

Figure 1.7: The mammalian E2F family of a transcription factor.

E2F family of transcription factors are comprised of ten members and three polypeptide DP1, -2, and -4 to form a heterodimer. E2F family members share several homologous regions including the DNA binding domain and a dimerization domain. E2F1 through -3 have a nuclear localization sequence and a CDK-cyclin binding domain in the N- terminus; and a transcriptional activation domain and Rb proteins binding site (specifically interacts with pRb, p107 or p130) at the C-terminus. E2F4 and -5 lack a CDK-cyclin binding sequence but still contain a pocket protein binding domain (preferentially interact with the other Rb family proteins, p107 and p130). E2F4 and -5 are exported out of the nucleus via their nuclear export signal. E2F6 shares only the DNA binding domain and the dimerization domain with the other E2F family members. Both E2F7 and -8 contain two conserved DNA binding domains allowing it to form heterodimer without DP proteins. E2F6 through -8 lack pRb-binding and a transcriptional activation domain. DP1, -2, and -4 proteins lack transcriptional activation domains and pRb-binding regions.

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and a dimerization domain which associates with DP proteins to form a functional heterodimer complex. In E2F1, the marked box domain interacts with c-Jun activation domain-binding protein 1 (Jab1) to enhance E2F1-induced apoptosis and not proliferation (Hallstrom and Nevins, 2006). E2F1 through -5 have a transactivation domain and an Rb protein-binding region. Rb family proteins can modulate the transcriptional activity of E2F proteins. E2F1 through -3 associate specifically with pRb, E2F4 preferentially interacts with p107 or p130 and E2F5 binds p130 (Beijersbergen et al., 1994; Hijmans et al., 1995; Ivey-Hoyle et al., 1993; Lees et al., 1993; Shan et al., 1992). The interaction between E2F and Rb family proteins represses the transcriptional activity of E2F1 through -5 (Figure 1.8). E2F6 does not have a transactivation domain or an Rb-binding region. E2F1 through -3 are structurally similar and have a cyclin A-binding domain, which is not present on E2F4 and -5. E2F7 and -8 have two DNA binding domains but lack a dimerization domain. Thus, they are not dependent on DP proteins for functional activity. E2F7 and -8 can interact with each other to form homodimer complexes (Li et al., 2008).

Binding of E2F family of proteins to hypophosphorylated Rb family proteins recruits histone deacetylases and DNA methyltransferases, such as HDAC1, suppressor of variegation 3-9 homolog 1 (Suv39H1) and SWItch/Sucrose Non-Fermentable (SWI/SNF) (Figure 1.8). This complex functions to inhibit transcription of E2F-responsive genes (Magnaghi-Jaulin et al., 1998; Nagl et al., 2007; Vandel et al., 2001). In an actively proliferating cell, Rb family proteins are sequentially phosphorylated by CDK4/6-cyclin D and CDK2-cyclin E complexes resulting in dissociation from E2F, rendering E2F activators transcriptionally active to target genes required for G1/S phase progression

Figure 1.8: Cell cycle regulation of E2F family of transcription factors.

The activity of different E2F-DP-Rb protein complexes during each phase of the cell cycle are shown. E2F1 through -3 function as activators during late G1 to initiation S phase progression and G2 phase to promote activation of genes required for mitosis. E2F6 through -8 are involved in repressing E2F-target genes during S phase. E2F1 through -5 are involved in repressing E2F-dependent transcription during cell cycle arrest (G0 phase). pRb is preferentially phosphorylated by CDK4/6-cyclin D during late G1 phase which leads to subsequent phosphorylation at multiple sites on pRb by CDK2-Cyclin E causing full inactivation of pRb and dissociation with E2F1 through -3 to promote G1 to S phase transition. P, phosphorylation.

(Ezhevsky et al., 1997; Lundberg and Weinberg, 1998). The E2F family members function as both transcriptional repressors and activators, depending on cellular context. E2F1 through -3 can switch from being transcriptional activators in proliferating cells to repressors in cells committed to differentiate and in response to DNA damage (Chong et al., 2009a; Ianari et al., 2009).

The best-characterized function of E2F1 through -3 is the activation of gene transcription to promote G1/S phase transition leading to proper cellular proliferation (Wu et al., 2001). Constitutive expression of E2F1 through -3 induce S phase entry of quiescent cells and triple knockout lead to defects in proliferation and increased cell death (Chong et al., 2009a; Johnson et al., 1993; Wu et al., 2001). During G2 phase, E2F1 through -3 activate transcription of genes required for G2/M progression (Zhu et al., 2004). The E2F3 locus encodes two gene products, E2f3a and E2f3b, through the use of alternate promoters. While E2F3a expression is highest during S phase, E2F3b is expressed constitutively throughout the cell cycle (Leone et al., 2000). Both E2F3 isoforms can function as activators or repressors of E2F target genes (Chong et al., 2009b).

E2F4 and -5 function as transcriptional repressors by forming a complex with p107 or p130, which recruits chromatin remodeling proteins to repress E2F target genes and prevent G1/S phase transition (Takahashi et al., 2000). E2F6 acts as a transcriptional repressor to inactivate E2F-responsive genes and inhibit S phase entry by forming a repressor complex with enhancer of polycomb (Attwooll et al., 2005; Cartwright et al., 1998; Trimarchi et al., 1998). E2F7 and -8 delay cell cycle progression during S phase functioning as transcriptional repressors (Logan et al., 2005). E2F7 and -8 repress the

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expression of E2F1 and their expression is dependent on E2F1 (Moon and Dyson, 2008). This negative feedback loop serves to maintain E2F1 balance during cell cycle progression.