title: Long-term estradiol deprivation in breast cancer cells up-regulates growth factor signaling and enhances estrogen sensitivity.
abstract: Deprivation of estrogen causes breast tumors in women to adapt and develop enhanced sensitivity to this steroid. Accordingly, women relapsing after treatment with oophorectomy, which substantially lowers estradiol for a prolonged period, respond secondarily to aromatase inhibitors with tumor regression. We have utilized in vitro and in vivo model systems to examine the biologic processes whereby long-term estradiol deprivation (LTED) causes cells to adapt and develop hypersensitivity to estradiol. Several mechanisms are associated with this response, including up-regulation of estrogen receptor-alpha (ERalpha) and the MAP kinase, phosphoinositol 3 kinase (PI3-K) and mammalian target of rapamycin (mTOR) growth factor pathways. ERalpha is four- to tenfold up-regulated and co-opts a classical growth factor pathway using Shc, Grb-2 and Sos. This induces rapid non-genomic effects which are enhanced in LTED cells. The molecules involved in the non-genomic signaling process have been identified. Estradiol binds to cell membrane-associated ERalpha, which physically associates with the adaptor protein Shc, and induces its phosphorylation. In turn, Shc binds Grb-2 and Sos, which result in the rapid activation of MAP kinase. These non-genomic effects of estradiol produce biologic effects as evidenced by Elk-1 activation and by morphologic changes in cell membranes. Additional effects include activation of the PI3-K and mTOR pathways through estradiol-induced binding of ERalpha to the IGF-I and epidermal growth factor receptors. A major question is how ERalpha locates in the plasma membrane since it does not contain an inherent membrane localization signal. We have provided evidence that the IGF-I receptor serves as an anchor for ERalpha in the plasma membrane. Estradiol causes phosphorylation of the adaptor protein, Shc and the IGF-I receptor itself. Shc, after binding to ERalpha, serves as the 'bus' which carries ERalpha to Shc-binding sites on the activated IGF-I receptors. Use of small inhibitor (si) RNA methodology to knockdown Shc allows the conclusion that Shc is needed for ERalpha to localize in the plasma membrane. In order to abrogate growth factor-induced hypersensitivity, we have utilized a drug, farnesylthiosalicylic acid, which blocks the binding of GTP-Ras to its membrane acceptor protein, galectin 1, and reduces the activation of MAP kinase. We have also shown that this drug is a potent inhibitor of mTOR as an additional mechanism of inhibition of cell proliferation. The concept of 'adaptive hypersensitivity' and the mechanisms responsible for this phenomenon have important clinical implications. The efficacy of aromatase inhibitors in patients relapsing on tamoxifen could be explained by this mechanism and inhibitors of growth factor pathways should reverse the hypersensitivity phenomenon and result in prolongation of the efficacy of hormonal therapy for breast cancer.
authors.affiliation: Department of Medicine, University of Virginia Health System, 450 Ray C Hunt Dr, Charlottesville, VA 22903, USA. rjs5y@virginia.edu
authors.forename: R J
authors.initials: RJ
authors.lastname: Santen
authors.forename: R X
authors.initials: RX
authors.lastname: Song
authors.forename: Z
authors.initials: Z
authors.lastname: Zhang
authors.forename: R
authors.initials: R
authors.lastname: Kumar
authors.forename: M-H
authors.initials: MH
authors.lastname: Jeng
authors.forename: A
authors.initials: A
authors.lastname: Masamura
authors.forename: J
authors.initials: J
authors.lastname: Lawrence
authors.forename: L
authors.initials: L
authors.lastname: Berstein
authors.forename: W
authors.initials: W
authors.lastname: Yue
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics / Q000378:metabolism; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000378:metabolism; D049109:Cell Proliferation; D019008:Drug Resistance, Neoplasm / Q000502:physiology; D004958:Estradiol / Q000172:deficiency* / Q000494:pharmacology*; D047628:Estrogen Receptor alpha / Q000378:metabolism; D005260:Female; D006133:Growth Substances / Q000378:metabolism*; D006801:Humans; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D011494:Protein Kinases / Q000378:metabolism; D034741:RNA, Small Interfering / Q000235:genetics / Q000494:pharmacology; D055765:Shc Signaling Adaptor Proteins; D015398:Signal Transduction / Q000187:drug effects; D000071425:Src Homology 2 Domain-Containing, Transforming Protein 1; D058570:TOR Serine-Threonine Kinases; D014407:Tumor Cells, Cultured; D015854:Up-Regulation
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D004958:Estradiol; D047628:Estrogen Receptor alpha; D006133:Growth Substances; D020929:Mitogen-Activated Protein Kinase Kinases; D019869:Phosphatidylinositol 3-Kinases; D011494:Protein Kinases; D034741:RNA, Small Interfering; D055765:Shc Signaling Adaptor Proteins; D000071425:Src Homology 2 Domain-Containing, Transforming Protein 1; D058570:TOR Serine-Threonine Kinases; C528024:SHC1 protein, human; C546842:MTOR protein, human