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Molecular and Cellular Biology, February 2001, p. 794-810, Vol. 21, No. 3
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.3.794-810.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Multifaceted Regulation of Cell Cycle Progression by Estrogen: Regulation of Cdk Inhibitors and Cdc25A Independent of Cyclin D1-Cdk4 Function

James S. Foster,1 Donald C. Henley,1 Antonin Bukovsky,1 Prem Seth,2 and Jay Wimalasena1,*

Department of Obstetrics and Gynecology, Graduate School of Medicine, University of Tennessee Medical Center, Knoxville, Tennessee 37920,1 The Medicine Branch, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 208922

Received 18 July 2000/Returned for modification 25 August 2000/Accepted 9 November 2000

Estrogens induce proliferation of estrogen receptor (ER)-positive MCF-7 breast cancer cells by stimulating G1/S transition associated with increased cyclin D1 expression, activation of cyclin-dependent kinases (Cdks), and phosphorylation of the retinoblastoma protein (pRb). We have utilized blockade of cyclin D1-Cdk4 complex formation through adenovirus-mediated expression of p16INK4a to demonstrate that estrogen regulates Cdk inhibitor expression and expression of the Cdk-activating phosphatase Cdc25A independent of cyclin D1-Cdk4 function and cell cycle progression. Expression of p16INK4a inhibited G1/S transition induced in MCF-7 cells by 17-beta -estradiol (E2) with associated inhibition of both Cdk4- and Cdk2-associated kinase activities. Inhibition of Cdk2 activity was associated with delayed removal of Cdk-inhibitory activity in early G1 and decreased cyclin A expression. Cdk-inhibitory activity and expression of both p21Cip1 and p27Kip1 was decreased, however, in both control and p16INK4a-expressing cells 20 h after estrogen treatment. Expression of Cdc25A mRNA and protein was induced by E2 in control and p16INK4a-expressing MCF-7 cells; however, functional activity of Cdc25A was inhibited in cells expressing p16INK4a. Inhibition of Cdc25A activity in p16INK4a-expressing cells was associated with depressed Cdk2 activity and was reversed in vivo and in vitro by active Cdk2. Transfection of MCF-7 cells with a dominant-negative Cdk2 construct inhibited the E2-dependent activation of ectopic Cdc25A. Supporting a role for Cdc25A in estrogen action, antisense CDC25A oligonucleotides inhibited estrogen-induced Cdk2 activation and DNA synthesis. In addition, inactive cyclin E-Cdk2 complexes from p16INK4a-expressing, estrogen-treated cells were activated in vitro by treatment with recombinant Cdc25A and in vivo in cells overexpressing Cdc25A. The results demonstrate that functional association of cyclin D1-Cdk4 complexes is required for Cdk2 activation in MCF-7 cells and that Cdk2 activity is, in turn, required for the in vivo activation of Cdc25A. These studies establish Cdc25A as a growth-promoting target of estrogen action and further indicate that estrogens independently regulate multiple components of the cell cycle machinery, including expression of p21Cip1 and p27Kip1.

* Corresponding author. Mailing address: Department of Obstetrics and Gynecology, University of Tennessee Medical Center, 1924 Alcoa Highway, Knoxville, TN 37920. Phone: (865) 544-8960. Fax: (423) 544-6863. E-mail: mcf7{at}msn.com.

Molecular and Cellular Biology, February 2001, p. 794-810, Vol. 21, No. 3
0270-7306/01/$04.00+0   DOI: 10.1128/MCB.21.3.794-810.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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