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E2F1 binds to E2F binding sites on the genome activating the synthesis of the target proteins. For annotation purposes, the reactions regulated by E2F1 are grouped under this pathway and information about the target genes alone are displayed for annotation purposes.
Cellular targets for activation by E2F1 include thymidylate synthase (TYMS) (DeGregori et al. 1995), Rir2 (RRM2) (DeGregori et al. 1995, Giangrande et al. 2004), Dihydrofolate reductase (DHFR) (DeGregori et al. 1995, Wells et al. 1997, Darbinian et al. 1999), Cdc2 (CDK1) (Furukawa et al. 1994, DeGregori et al. 1995, Zhu et al. 2004), Cyclin A1 (CCNA1) (DeGregori et al. 1995, Liu et al. 1998), CDC6 (DeGregori et al. 1995, Yan et al. 1998; Ohtani et al. 1998), CDT1 (Yoshida and Inoue 2004), CDC45 (Arata et al. 2000), Cyclin E (CCNE1) (Ohtani et al. 1995), Emi1 (FBXO5) (Hsu et al. 2002), and ORC1 (Ohtani et al. 1996, Ohtani et al. 1998). The activation of TK1 (Dnk1) (Dou et al. 1994, DeGregori et al. 1995, Giangrande et al. 2004) and CDC25A (DeGregori et al. 1995, Vigo et al. 1999) by E2F1 is conserved in Drosophila (Duronio and O'Farrell 1994, Reis and Edgar 2004).
RRM2 protein is involved in dNTP level regulation and activation of this enzyme results in higher levels of dNTPs in anticipation of S phase. E2F activation of RRM2 has been shown also in Drosophila by Duronio and O'Farrell (1994). E2F1 activation of CDC45 is shown in mouse cells by using human E2F1 construct (Arata et al. 2000). Cyclin E is also transcriptionally regulated by E2F1. Cyclin E protein plays important role in the transition of G1 in S phase by associating with CDK2 (Ohtani et al. 1996). E2F1-mediated activation of PCNA has been demonstrated in Drosophila (Duronio and O'Farrell 1994) and in some human cells by using recombinant adenovirus constructs (DeGregori et al. 1995). E2F1-mediated activation of the DNA polymerase alpha subunit p180 (POLA1) has been demonstrated in some human cells. It has also been demonstrated in Drosophila by Ohtani and Nevins (1994). It has been observed in Drosophila that E2F1 induced expression of Orc1 stimulates ORC1 6 complex formation and binding to the origin of replication (Asano and Wharton 1999). ORC1 6 recruit CDC6 and CDT1 that are required to recruit the MCM2 7 replication helicases. E2F1 regulation incorporates a feedback mechanism wherein Geminin (GMNN) can inhibit MCM2 7 recruitment of ORC1 6 complex by interacting with CDC6/CDT1. The activation of CDC25A and TK1 (Dnk1) by E2F1 has been inferred from similar events in Drosophila (Duronio RJ and O'Farrell 1994; Reis and Edgar 2004). E2F1 activates string (CDC25) that in turn activates the complex of Cyclin B and CDK1. A similar phenomenon has been observed in mouse NIH 3T3 cells and in Rat1 cells.
E2f6 knockout mice are viable and embryonic fibroblasts derived from these mice proliferate normally. Although E2f6 knockout mice appear healthy, they are affected by homeotic transformations of the axial skeleton, involving vertebrae and ribs. Similar skeletal defects have been reported in mice harboring mutations in polycomb genes, suggesting that E2F6 may function in recruitment of polycomb repressor complex(es) to target promoters (Storre et al. 2002).
E2F6 mediates repression of E2F responsive genes. While E2F6 was suggested to maintain G0 state in quiescent cells (Gaubatz et al. 1998, Ogawa et al. 2002), this finding has been challenged (Giangrande et al. 2004, Bertoli et al. 2013, Bertoli et al. 2016). Instead, E2F6-mediated gene repression in proliferating (non-quiescent) cells is thought to repress E2F targets involved in G1/S transition during S phase of the cell cycle. E2F6 does not affect E2F targets involved in G2/M transition (Oberley et al. 2003, Giangrande et al. 2004, Attwooll et al. 2005, Trojer et al. 2011, Bertoli et al. 2013). In the context of the E2F6.com-1 complex, E2F6 was shown to bind to promoters of E2F1, MYC, CDC25A and TK1 genes (Ogawa et al. 2002). E2F6 also binds the promoters of CDC6, RRM1 (RR1), PCNA and TYMS (TS) genes (Giangrande et al. 2004), as well as the promoter of the DHFR gene (Gaubatz et al. 1998). While transcriptional repression by the E2F6.com 1 complex may be associated with histone methyltransferase activity (Ogawa et al. 2002), E2F6 can also repress transcription independently of H3K9 methylation (Oberley et al. 2003).
During S phase, E2F6 is involved in the DNA replication stress checkpoint (Bertoli et al. 2013, Bertoli et al. 2016). Under replication stress, CHEK1-mediated phosphorylation prevents association of E2F6 with its target promoters, allowing transcription of E2F target genes whose expression is needed for resolution of stalled replication forks and restart of DNA synthesis. Inability to induce transcription of E2F target genes (due to CHEK1 inhibition or E2F6 overexpression) leads to replication stress induced DNA damage (Bertoli et al. 2013, Bertoli et al. 2016). E2F6 represses transcription of a number of E2F targets involved in DNA synthesis and repair, such as RRM2, RAD51, BRCA1, and RBBP8 (Oberley et al. 2003, Bertoli et al. 2013).