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-a (ERa) and the MAP kinase, phosphoinositol 3 kinase (PI3-K) and mammalian target of rapamycin (mTOR) growth factor pathways. ERa 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 ERa, 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 estradiolinduced binding of ERa to the IGF-I and epidermal growth factor receptors. A major question is how ERa 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 ERa in the plasma membrane. Estradiol causes phosphorylation of the adaptor protein, Shc and the IGF-I receptor itself. Shc, after binding to ERa, serves as the 'bus' which carries ERa 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 ERa to localize in the plasma membrane. In order to abrogate growth factorinduced 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.

Transient over-expression of estrogen receptor-α in breast cancer cells promotes cell survival and estrogen-independent growth

“…These findings indicate that crosstalk between ERα, AIB1 and growth factor receptor pathways are important in determining response and resistance to tamoxifen. HER2 expression and P-MAPK levels are elevated in the LTED cells [36], similar results being obtained in other LTED cells [37].…”

Mechanisms of endocrine resistance and novel therapeutic strategies in breast cancer

“…The majority of breast cancer patients have been treated up to now with tamoxifen. Breast cancer cells that acquire resistance to tamoxifen show phenotypic characteristics that are similar to the initial adaptive hypersensitivity described by Santen et al (2003) and Dowsett (2003). In these cells growth factor signaling is increased and tamoxifen might behave as an agonist.…”

Biological reprogramming in acquired resistance to endocrine therapy of breast cancer

“…As supported by the molecular data presented above, acquired resistance may be mediated by constitutive activation of ERa and growth factor signaling pathway cross-talk (Martin et al, 2003;Jelovac et al, 2005;Sabnis et al, 2005;Santen et al, 2005;Masri et al, 2008). Thus, the identities of genes differentially expressed during MCF7-LTED adaptation may support the important role of the non-genomic function of ERa.…”

“…Current literature supports the hypothesis that acquired resistance is mainly mediated by molecular events that-particularly in the case of resistance to AIs-lead to constitutive activation of ERa and growth factor signaling pathway cross-talk (Clarke et al, 2003;Martin et al, 2003;Yue et al, 2003;Jelovac et al, 2005;Normanno et al, 2005;Sabnis et al, 2005;Santen et al, 2005;Masri et al, 2008). On the basis of these studies, several clinical trials have attempted to overcome resistance through combination with growth factor signaling inhibitors Massarweh and Schiff, 2006).…”