Estrogen-Dependent Cyclin E-cdk2 Activation through p21 Redistribution

Abstract

In order to elucidate the mechanisms by which estrogens and antiestrogens modulate the growth of breast cancer cells, we have characterized the changes induced by estradiol that occur during the G 1 phase of the cell cycle of MCF-7 human mammary carcinoma cells. Addition of estradiol relieves the cell cycle block created by tamoxifen treatment, leading to marked activation of cyclin E-cdk2 complexes and phosphorylation of the retinoblastoma protein within 6 h. Cyclin D1 levels increase significantly while the levels of cyclin E, cdk2, and the p21 and p27 cdk inhibitors are relatively constant. However, the p21 cdk inhibitor shifts from its association with cyclin E-cdk2 to cyclin D1-cdk4, providing an explanation for the observed activation of the cyclin E-cdk2 complexes. These results support the notion that cyclin D1 has an important role in steroiddependent cell proliferation and that estrogen, by regulating the activities of G 1 cyclin-dependent kinases, can control the proliferation of breast cancer cells.A variety of models have been proposed to explain how estrogen drives the proliferation of normal mammary epithelial cells and breast cancer cells (8). By acting through the estrogen receptor (ER), estrogen can regulate the transcription of a cohort of responsive genes and in this way appears to regulate cell cycle progression. Even in the face of still-incomplete mechanistic insight into how estrogen regulates growth, effective antitumor therapies directed against the ER have been developed around the use of antiestrogens such as tamoxifen (20). Many studies of the effects of tamoxifen have indicated that tamoxifen acts in a cytostatic fashion on breast cancer cells, causing them to arrest in the G 0 /G 1 phases of their growth cycle (36,49). For these reasons, it is important to understand how estrogens and antiestrogens control G 1 progression.The central regulator of this process is the cell cycle clock apparatus, which operates in the cell nucleus and is assembled from an array of cyclins and cyclin-dependent kinases (cdks) (45). The activities of the cdks are positively controlled by their association with cyclins and restrained by cdk inhibitors. Included among the latter are p21, p27, and p57, which can inhibit a wide range of cyclin-cdk complexes, and the INK4 family (p15, p16, p18, and p19), which specifically inhibits cdk4 and cdk6 (46). Extracellular signals such as those conveyed by growth factors affect the activity of cyclins and cdks largely during the G 1 phase of the cell cycle. The most important components of the cell cycle clock apparatus during this period are (i) the D-type cyclins together with their catalytic partners cdk4 and cdk6 and (ii) cyclin E, which interacts with cdk2. Both classes of G 1 cyclin-cdk complexes are known to drive the phosphorylation of the retinoblastoma protein (pRb) (18,19,21,42). This phosphorylation represents a key event in G 1 progression (53). Hypophosphorylated pRb is active in mediating G 1 arrest while hyperphosphorylated pRb appears to be inactive ...

RETRACTED ARTICLE: Regulation of estrogen receptor signaling in breast carcinogenesis and breast cancer therapy

“…Up-regulation of cyclin D1 gene expression in response to E2 promotes G1 to S transition by activating CDK4 through cyclin D1 induction [54]. Therefore, the treatment of breast cancer cells with anti-estrogens is often associated with an acute decline in cyclin D1 mRNA and protein expression accompanied by a decline in cyclin D1-CDK4 activity and decreased phosphorylation of retinoblastoma [55,56]. Cyclin D1 can also interact with ERα in a CDK-independent manner through the cAMP/protein kinase A (PKA)-mediated pathway [57].…”

Therapeutic CDK4/6 inhibition in breast cancer: key mechanisms of response and failure

“…Such mitogenic signaling is modulated through a variety of mechanisms that attenuate CDK4/6 activity to elicit the activation of Rb, p107, and p130 (Watts et al, 1995;Yu et al, 2006). Correspondingly, there are a myriad of mechanisms integrated within these mitogenic signaling cascades, which mediate the downstream inhibition of CDK2/cyclin A/E activity (p27 Kip1 and p21 Cip1 ) (Planas-Silva and Weinberg, 1997;Swarbrick et al, 2000;Abukhdeir and Park, 2008). These cyclin-dependant kinase inhibitors (CKIs) function to facilitate CDK4/6/cyclin D complex formation while conversely inhibiting CDK2/cyclin A/E formation (Poon et al, 1995;LaBaer et al, 1997;Sherr and Roberts, 1999;Abukhdeir and Park, 2008).…”

Pre-clinical evaluation of cyclin-dependent kinase 2 and 1 inhibition in anti-estrogen-sensitive and resistant breast cancer cells

“…Studies in breast cancer have highlighted CDK2 as an essential regulator of estrogen-mediated G1/S transition (Planas-Silva and Weinberg, 1997;Cariou et al, 2000); however, recent studies in non-breast cancer cell lines and in knockout mice have questioned the role of CDK2 in cancer cell proliferation (Berthet et al, 2003;Ortega et al, 2003;Tetsu and McCormick, 2003). To date, the effects of CDK2 knockdown in breast cancer cell lines have not been determined.…”

“…Phosphorylation of pRb relieves transcriptional repression by the pRb-E2F complex and disrupts the binding of pRb to E2F, allowing E2F activation and transcription of genes necessary for S-phase entry and progression. In anti-estrogen-responsive breast cancers, tamoxifen causes a reduction in cyclin D1 expression, reducing CDK4/6 activity and promoting p27 Kip1 /p21 Waf1/Cip1 inhibition of CDK2, resulting in decreased pRb phosphorylation and G1 cell cycle arrest (Planas-Silva and Weinberg, 1997).…”