Increased proteasome‐dependent degradation of estrogen receptor‐alpha by TGF‐β1 in breast cancer cell lines

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Abstract
Normal mammary epithelial cells are rapidly induced to G1 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.