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Department of Oncology (G.E.S., A.W., H.-J.L., R.R.,M.B.M., A.S.), Lombardi Cancer Center, Georgetown University, Washington, DC 20007; Department of Pharmacology (T.F.F.), Columbia University, New York, New York 10032; Department of Pharmacology and Toxicology (F.C.), Philipps University, School of Medicine, 35033 Marburg, Germany; and School of Nursing and Health Studies (E.M., A.S.), Georgetown University, Washington, DC 20007
Address all correspondence and requests for reprints to: Adriana Stoica, Ph.D., E411 Research Building, 3970 Reservoir Road NW, Washington, DC 20007. E-mail: stoicaa{at}georgetown.edu.
Previously, we have demonstrated that the two mitogenic growth factors epidermal growth factor and IGF-I can activate Akt and estrogen receptor-
(ER) in the hormone-dependent breast cancer cell line, MCF-7. In this report we now show that estradiol can also rapidly activate phosphatidylinositol 3-kinase (PI 3-K)/Akt and that this effect is mediated by the ErbB2 signaling pathway. Treatment of cells with estradiol resulted in phosphorylation of Akt and a 9-fold increase in Akt activity in 10 min. Akt activation was blocked by wortmannin and LY 294,002, two inhibitors of PI 3-K; by genistein, a protein tyrosine kinase inhibitor and an ER agonist; by AG825, a selective ErbB2 inhibitor; and by the antiestrogens ICI 182,780 and 4-hydroxy-tamoxifen; but not by rapamycin, an inhibitor of the ribosomal protein kinase p70S6K; nor by AG30, a selective epidermal growth factor receptor inhibitor. Akt activation by estradiol was abrogated by an arginine-to-cysteine mutation in the pleckstrin homology domain of Akt (R25C). Growth factors also activated Akt in the ER-negative variant of MCF-7, MCF-7/ADR, but estradiol did not induce Akt activity in these cells. Transient transfection of ER into these cells restored Akt activation by estradiol, suggesting that estradiol activation of Akt requires the ER. Estradiol did not activate Akt in MCF-7 cells stably transfected with an anti-ErbB2-targeted ribozyme, further confirming a role for ErbB2. In vitro kinase assays using immunoprecipitation and anti-Akt1, -Akt2, and -Akt3-specific antibodies demonstrated that Akt1 is activated by estradiol in MCF-7 cells whereas Akt3 is the activated isoform in ER-negative MDA-MB231 cells, implying that selective activation of Akt subtypes plays a role in the actions of estradiol. Taken together, our data suggest that estradiol, bound to membrane ER, interacts with and activates an ErbB dimer containing ErbB2, inducing activation of PI 3-K/Akt.
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