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Normal mammary epithelial cells are rapidly induced to G 1 arrest by the widely expressed cytokine, transforming growth factor beta (TGF-β1). Studies in established breast cancer cell lines that express the estrogen receptor alpha (ERα) have demonstrated loss of this responsiveness. This inverse correlation suggests interpathway signaling important to cell growth and regulation. The adenocarcinoma breast cell line BT474, which was not growth arrested by TGF-β1, was used as a model of estrogen-inducible growth to explore interpathway crosstalk. Although BT474 cells were not growth-arrested by TGF-β1 as determined by flow cytometry analysis and 5′-bromo-3′-deoxyuridine incorporation into DNA, estrogen receptor protein levels were attenuated by 100 pM TGF-β1 after 6 h. This decrease in ERα reached 50% of untreated control levels by 24 h of treatment and was further supported by a 50% decrease in estrogen-inducible DNA synthesis. Inspection of ERα transcripts suggested that this decrease was primarily the result of altered ERα protein stability or availability. Use of the proteasome inhibitor, MG132, abolished all effects on ERα by TGF-β1. Collectively, this data supports a role for TGF-β1 in regulating the growth of otherwise insensitive breast cancer cells through modulation of ERα stability.Keywords breast cancer; estrogen receptor alpha; transforming growth factor beta; ubiquitin; proteasome; cell proliferation Breast cancer is a leading cause of death among women worldwide and an estimated 203,500 new cases will be diagnosed in the US alone in 2002 [American Cancer Society, 2001]. Systemic and locally produced estrogens are intimately associated with increased risk for breast cancer development. Early menarche and late menopause increase the lifetime exposure to estrogens and as much as a 20% decrease in breast cancer risk is observed for each year this exposure is shortened [Kelsey, 1979]. By the same token, increased and sustained physical activity can reduce the total lifetime ovulatory period and has been shown to confer a significant decrease in breast cancer risk [Bernstein et al., 1994]. Numerous epidemiological studies of breast cancer risk in postmenopausal women have shown higher mean serum estrogen concentrations than disease-free control subjects [Thomas et al., 1997]. These and additional risk factors share a common theme, i.e., increased lifetime tissue exposure to estrogens. Thus, estrogen exposure is a central and reoccurring phenomenon in breast cancer etiology.Although many risk factors have been identified, the molecular mechanisms underlying the initiation and progression of breast cancer are still elusive. Estrogens such as 17β-estradiol (E 2 ) play a key role in the development and maintenance of the mature mammary gland. The most well characterized cellular receptor for this steroid hormone is the estrogen receptor alpha (ERα) [Moggs and Orphanides, 2001]. This receptor is expressed in a tissue specific manner and regulates growth and differentiation in response to hormonal stimul...

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