A molecular model for the mechanism of acquired tamoxifen resistance in breast cancer

Abstract

Purpose: Estrogen (E2)-stimulated growth re-emerges after a c-Src inhibitor blocking E2-induced apoptosis. A resulting cell line, MCF-7:PF, is selected with features of functional estrogen receptor (ER) and over-expression of insulin-like growth factor-1 receptor beta (IGF-1Rβ). We addressed the question of whether the selective ER modulator (SERM), 4-hydroxytamoxifen (4-OHT) or other SERMs could target ER to prevent E2-stimulated growth in MCF-7:PF cells. Methods: Protein levels of receptors and signaling pathways were examined by immunoblotting. Expression of mRNA was measured through real-time RT-PCR. Recruitment of ER or nuclear receptor coactivator 3 (SRC3) to the promoter of ER-target gene was detected by chromatin-immunoprecipitation (ChIP). Results: 4-OHT and other SERMs stimulated cell growth in an ER-dependent manner. However, unlike E2, 4-OHT suppressed classical ER-target genes as does the pure antiestrogen ICI 182,780 (ICI). ChIP assay indicated that 4-OHT did not recruit ER or SRC3 to the promoter of ER-target gene, pS2. Paradoxically, 4-OHT reduced total IGF-1Rβ but increased phosphorylation of IGF-1Rβ. Mechanistic studies revealed that 4-OHT functioned as an agonist to enhance the non-genomic activity of ER and activate focal adhesion molecules to further increase phosphorylation of IGF-1Rβ. Disruption of membrane-associated signaling, IGF-1R and focal adhesion kinase (FAK), completely abolished 4-OHT-stimulated cell growth. Conclusions: This study is the first to recapitulate a cellular model in vitro of acquired tamoxifen resistance developed in athymic mice in vivo. Importantly, it provides a rationale that membrane-associated pathways may be valuable therapeutic targets for tamoxifen resistant patients in clinic.

Effect of single mutagenesis on the binding pocket of canine estrogen receptor alpha: Structure and binding affinity

“…It should be kept in mind that different algorithms could give rise to different structural prediction and possibly different binding affinities [21,[24][25][26] . The acquired SERM resistance in breast cancer cells has been studied, especially with respect to long-term treatment [27][28][29][30][31][32] . Interestingly, ER expression is maintained in the acquired resistant tumors [29] .…”

The function of membrane-associated molecules in acquired resistance to antiestrogens in breast cancer

“…A two month period of selection pressure was chosen, as this is the time period used clinically to evaluate tumor response to therapy [10] . The cell populations (MCF-7: PF) that grow out under the pressure of E2 plus the c-Src inhibitor is particularly interesting as, for the first time, it replicates phase I acquired resistance to SERMs in vitro [10,11] . Our recent publication "A molecular model for the mechanism of acquired tamoxifen resistance in breast cancer" [11] demonstrates that MCF-7: PF cells grow robustly with E2 but also SERMs will stimulate growth in vitro based on their individual intrinsic estrogenic efficacy as partial agonists [11] .…”

“…The cell populations (MCF-7: PF) that grow out under the pressure of E2 plus the c-Src inhibitor is particularly interesting as, for the first time, it replicates phase I acquired resistance to SERMs in vitro [10,11] . Our recent publication "A molecular model for the mechanism of acquired tamoxifen resistance in breast cancer" [11] demonstrates that MCF-7: PF cells grow robustly with E2 but also SERMs will stimulate growth in vitro based on their individual intrinsic estrogenic efficacy as partial agonists [11] . Further investigation suggests that ER is a major driver of growth utilized by both E2 and SERMs in resistant models in vivo [1,3] and in vitro [11] .…”

“…Our recent publication "A molecular model for the mechanism of acquired tamoxifen resistance in breast cancer" [11] demonstrates that MCF-7: PF cells grow robustly with E2 but also SERMs will stimulate growth in vitro based on their individual intrinsic estrogenic efficacy as partial agonists [11] . Further investigation suggests that ER is a major driver of growth utilized by both E2 and SERMs in resistant models in vivo [1,3] and in vitro [11] . The finding in laboratory model is consistent with the clinical observation that aromatase inhibitors (AIs) or fulvestrant (ICI) are equally effective in the treatment of acquired tamoxifen resistant patients [12,13] .…”

“…The deregulation of these genes by E2 or 4-OHT is able to be blocked by the pure antiestrogen ICI [14] . In contrast to E2 that activates classical ER-target genes, SERMs continue to act as effective antiestrogens to inhibit classical ER-target genes, even at the time of growth stimulation [11,14] . This result is supported by our previous finding in vivo [15] that growth of tamoxifen or fulvestrant resistant tumors does not rely on classical ER transcriptional pathways, which is evidenced by suppression of E2-responsive genes [15] .…”