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Research Articles: Therapeutics, Targets, and Development

Cooperative action of tamoxifen and c-Src inhibition in preventing the growth of estrogen receptor–positive human breast cancer cells

¹ Breast Center, Baylor College of Medicine, One Baylor Plaza, Houston, Texas; and ² AstraZeneca, Cheshire, United Kingdom

Requests for reprints: Suzanne A.W. Fuqua, Breast Center, Baylor College of Medicine, One Baylor Plaza, Mailstop 600, Houston, TX 77030. Phone: 713-798-1671; Fax: 713-798-1673. E-mail: sfuqua{at}bcm.edu

Abstract

It has long been appreciated that estrogenic signaling contributes to breast cancer progression. c-Src is also required for a number of processes involved in tumor progression and metastasis. We have previously identified the K303R mutant estrogen receptor (ER) that confers hypersensitivity to low levels of estrogen. Because ER and c-Src have been shown to interact in a number of different systems, we wanted to evaluate the role of c-Src kinase in estrogen-stimulated growth and survival of ER-positive breast cancer cells. MCF-7 cells stably expressing the mutant receptor showed increased c-Src kinase activity and c-Src tyrosine phosphorylation when compared with wild-type ER-expressing cells. A c-Src inhibitor, AZD0530, was used to analyze the biological effects of pharmacologically inhibiting c-Src kinase activity. MCF-7 cells showed an anchorage-dependent growth IC₅₀ of 0.47 µmol/L, which was increased 4-fold in the presence of estrogen. In contrast, cells stably expressing the mutant ER had an elevated IC₅₀ that was only increased 1.4-fold by estrogen stimulation. The c-Src inhibitor effectively inhibited the anchorage-independent growth of both of these cells, and estrogen was able to reverse these effects. When cells were treated with suboptimal concentrations of c-Src inhibitor and tamoxifen, synergistic inhibition was observed, suggesting a cooperative interaction between c-Src and ER. These data clearly show an important role for ER and estrogen signaling in c-Src–mediated breast cancer cell growth and survival. Here, we show that c-Src inhibition is blocked by estrogen signaling; thus, the therapeutic use of c-Src inhibitors may require inhibition of ER in estrogen-dependent breast cancer. [Mol Cancer Ther 2006;5(12):3023–31]

Grant support: DAMD17-03-1-0417 (M.H. Herynk), NIH/National Cancer Institute grants CA58183 and CA72038, and an AstraZeneca grant (S.A.W. Fuqua).

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 7/ 7/06; revised 9/29/06; accepted 10/30/06.