Potentiation of Human Estrogen Receptor α Transcriptional Activation through Phosphorylation of Serines 104 and 106 by the Cyclin A-CDK2 Complex

Abstract

Both estradiol binding and phosphorylation regulate transcriptional activation by the human estrogen receptor ␣ (ER). We have previously shown that activation of the cyclin A-CDK2 complex by overexpression of cyclin A leads to enhanced ER-dependent transcriptional activation and that the cyclin A-CDK2 complex phosphorylates the ER N-terminal activation function-1 (AF-1) between residues 82 and 121. Within ER AF-1, serines 104, 106, and 118 represent potential CDK phosphorylation sites, and in this current study, we ascertain their importance in mediating cyclin A-CDK2-dependent enhancement of ER transcriptional activity. Cyclin A overexpression does not enhance transcriptional activation by an ER derivative bearing serine-to-alanine changes at residues 104, 106, and 118. Likewise, the cyclin A-CDK2 complex does not phosphorylate this triple-mutated derivative in vitro. Individual serine-to-alanine mutations at residues 104 and 106, but not 118, decrease ER-dependent transcriptional enhancement in response to cyclin A. The same relationship holds for ER phosphorylation by cyclin A-CDK2 in vitro. Finally, enhancement of ER transcriptional activation by cyclin A is evident in the absence and presence of estradiol, as well as in the presence of tamoxifen, suggesting that the effect of the cyclin A-CDK2 on ER transcriptional activation is AF-2-independent. These results indicate that the enhancement of ER transcriptional activation by the cyclin A-CDK2 complex is mediated via the AF-1 domain by phosphorylation of serines 104 and 106. We propose that these residues control ER AF-1 activity in response to signals that affect cyclin A-CDK2 function.

Cyclin A is a prognostic indicator in early stage breast cancer with and without tamoxifen treatment

“…Only specific p53 mutations, determined by sequencing of the p53 gene, are most informative in predicting response to systemic therapy of advanced breast cancer (Berns et al, 2000), whereas we examined early stage breast cancer by p53 immunohistochemistry, which is less accurate in detecting mutations. We hypothesized that cyclin D1 and/or cyclin A might affect outcome of tamoxifen treatment, since both cyclins are hormone-independent activators of OR-a in in vitro experiments (Zwijsen et al, 1997;Rogatsky et al, 1999). Cyclin D1 does so by binding to OR-a and thereby enhances the interaction between OR-a and SRC (steroid receptor coactivator)-1 (Zwijsen et al, 1999), whereas cyclin A-cdk2 phosphorylates cdk2 at serines 104/106 (Rogatsky et al, 1999), and thereby activates OR-a tran- Adjusted for lymph node status, age, performance status, T-classification, grade, prior surgery, oestrogen receptor status and tamoxifen use.…”

“…We hypothesized that cyclin D1 and/or cyclin A might affect outcome of tamoxifen treatment, since both cyclins are hormone-independent activators of OR-a in in vitro experiments (Zwijsen et al, 1997;Rogatsky et al, 1999). Cyclin D1 does so by binding to OR-a and thereby enhances the interaction between OR-a and SRC (steroid receptor coactivator)-1 (Zwijsen et al, 1999), whereas cyclin A-cdk2 phosphorylates cdk2 at serines 104/106 (Rogatsky et al, 1999), and thereby activates OR-a tran- Adjusted for lymph node status, age, performance status, T-classification, grade, prior surgery, oestrogen receptor status and tamoxifen use. HR below 1 indicates a better prognosis (for increased expression of the marker).…”

“…OR-a signalling activity is not only induced by oestrogen, but OR-a is also activated as a transcriptional transactivator in a hormone-independent manner by phosphorylation of serines 104/ 106 by cyclin A-cdk2 (Rogatsky et al, 1999) and by direct binding of OR-a to cyclin D1 (Zwijsen et al, 1997). Both of these modulations affect the association between OR-a and co-activators of ORa (Rogatsky et al, 1999;Zwijsen et al, 1999) and may thereby affect the ability of anti-oestrogens to inhibit OR-a activity.…”

“…Both of these modulations affect the association between OR-a and co-activators of ORa (Rogatsky et al, 1999;Zwijsen et al, 1999) and may thereby affect the ability of anti-oestrogens to inhibit OR-a activity. It has, therefore, been hypothesized that tamoxifen treatment of breast cancer may be inefficient in tumour cases that harbour genetic lesions which result in elevated levels of cyclin D1 or A, or that lead to reduced levels of cdk inhibitors p21 or p27 (Michalides, 1999;Cariou et al, 2000).…”

Cyclin-dependent kinase antagonizes promyelocytic leukemia zinc-finger through phosphorylation

“…CDK2 activity plays an important role in G 1 /S progression by association with cyclin E in early G 1 and then cyclin A in late G 1 and early S phase (Sherr and Roberts, 1999;Malumbres and Barbacid, 2001). Several transcription factors are among the known substrates of CDK2/cyclin complexes and this CDK-mediated phosphorylation can have diverse effects on their activity (Deed et al, 1997;Johnson et al, 1999;Rogatsky et al, 1999). Given the dramatic effects of PLZF expression on cell cycle progression and its known modification by phosphorylation, we sought to determine whether PLZF is a target of CDK/cyclin activity and whether phosphorylation can impact on PLZF function.…”