During estrogen-induced proliferation, c-Myc and cyclin D1 initiate independent pathways that activate cyclin E1-Cdk2 by sequestration and/or downregulation of the CDK inhibitor p21Waf1/Cip1 , without significant increases in cyclin E1 protein levels. In contrast, cyclin E2 undergoes a marked increase in expression, which occurs within 9 to 12 h of estrogen treatment of antiestrogen-pretreated MCF-7 breast cancer cells. Both E cyclins are important to estrogen action, as small interfering RNA (siRNA)-mediated knockdown of either cyclin E1 or cyclin E2 attenuated estrogen-mediated proliferation. Inducible expression of cyclin D1 upregulated cyclin E2, while siRNA-mediated knockdown of cyclin D1 attenuated estrogen effects on cyclin E2. However, manipulation of c-Myc levels did not profoundly affect cyclin E2. Cyclin E2 induction by estrogen was accompanied by recruitment of E2F1 to the cyclin E1 and E2 promoters, and cyclin D1 induction was sufficient for E2F1 recruitment. siRNA-mediated knockdown of the chromatin remodelling factor CHD8 prevented cyclin E2 upregulation. Together, these data indicate that cyclin E2-Cdk2 activation by estrogen occurs via E2F-and CHD8-mediated transcription of cyclin E2 downstream of cyclin D1. This contrasts with the predominant regulation of cyclin E1-Cdk2 activity via CDK inhibitor association downstream of both c-Myc and cyclin D1 and indicates that cyclins E1 and E2 are not always coordinately regulated.In mammalian cells, there are two E-type cyclins, cyclins E1 and E2 (collectively referred to as cyclin E), that activate Cdk2 in late G 1 phase. The E-type cyclins are encoded by separate genes, located at chromosomes 19q12 (CCNE1) and 8q22.1 (CCNE2) in humans, but share substantial sequence identity and functional redundancy. The predominant function of cyclin E is believed to be the activation of Cdk2 and consequent effects on cell cycle progression and DNA replication, since both E-type cyclins accelerate the G 1 -to-S-phase transition and most cyclin E-Cdk2 substrates identified to date have roles in cell cycle progression and DNA replication (34). Cyclin E also has crucial, and possibly CDK-independent, functions in the initiation of DNA replication and centrosome duplication (25,26,42), and its deregulation may promote oncogenesis via genomic instability (63) arising through centrosome amplification (35,49,63) or alterations in replication complex assembly (19,45).Overexpression of cyclin E1 in the mouse mammary gland leads to tumor formation at a low frequency (ϳ12%) and long latency (8 to 13 months) (4), although tumorigenesis may be augmented by cooperation with other oncogenic events, particularly p53 inactivation (61). High expression of cyclin E1 protein in breast cancers is strongly correlated with proliferative markers such as elevated Ki67 levels and an elevated mitotic index (6), suggesting that cyclin E1 promotes tumor cell proliferation. This provides good evidence for an association between cyclin E1 and breast oncogenesis, but cyclin E2 has not been indepen...