Androgen receptor promotes tamoxifen agonist activity by activation of EGFR in ERα-positive breast cancer

Abstract

Purpose Tamoxifen (Tam) resistance represents a significant clinical problem in estrogen receptor (ER) -positive breast cancer. We previously showed that decreased expression of Rho guanine nucleotide dissociation inhibitor (Rho GDI), a negative regulator of the Rho GTPase pathway, is associated with Tam resistance. We now discover that androgen receptor (AR) is overexpressed in cells with decreased Rho GDI and seek to determine AR’s contribution to resistance. Methods We engineered ER -positive cell lines with stable knock-down (KD) of Rho GDI (KD cells). Resistance mechanisms were examined using microarray profiling, protein-interaction studies, growth and reporter gene assays, and Western blot analysis combined with a specific AR antagonist and other signaling inhibitors. Results Tam-resistant tumors and cell lines with low Rho GDI levels exhibited upregulated AR expression. Microarray of Rho GDI KD cells indicated that activation of EGFR and ER was associated with Tam treatment. When AR levels were elevated interaction between AR and EGFR was detected. Constitutive and Tam-induced phosphorylation of EGFR and ERK1/2 was blocked by the AR antagonist Enzalutamide, suggesting that AR-mediated EGFR activation was a mechanism of resistance in these cells. Constitutive ERα phosphorylation and transcriptional activity was inhibited by Enzalutamide and the EGFR inhibitor gefitinib, demonstrating that AR-mediated EGFR signaling activated ER. Tam exhibited agonist activity in AR over-expressing cells, stimulating ERα transcriptional activity and proliferation, which was blocked by Enzalutamide and gefitinib. Conclusions We describe a novel model of AR-mediated Tam resistance through activation of EGFR signaling leading to ER activation in ER -positive cells with low expression of Rho GDI.

Emerging functional roles of nuclear receptors in breast cancer

“…However, AR has also been reported to contribute to therapy resistance in ER+ breast cancer cells overexpressing AR, with increase in AR-to-ESR1 ratio associated with worse outcome for tamoxifen-treated patients (Cochrane et al 2014) and increased agonist activity of Tam in AR overexpressing cells. In this context, AR reportedly increased tamoxifen agonist activity via the activation of EGFR in ER+ breast cancer, and this was blocked by dual treatment with the anti-androgen enzalutamide and EGFR inhibitor gefitinib (Ciupek et al 2015). In addition, AR appears to promote proliferation of molecular apocrine breast cancer; a subset of TNBC expressing AR, through activation of genes that are normally regulated by ESR1 in ER+ cancer (Robinson et al 2011).…”

Tamoxifen Resistance: Emerging Molecular Targets

“…Moreover, increased AR expression has been observed in TAM-resistant breast cancer models in vitro and in vivo [126,127]. In fact, it has recently been reported that TAM-resistant tumors express higher levels of AR than TAM-sensitive tumors.…”

Steroid Hormone Receptor Positive Breast Cancer Patient-Derived Xenografts

“…Our one HER2+ER− PDX line is AR+ and, where measured, 2/6 TN PDX lines are AR+ (Figure 1D). Each of these types of PDX models will be valuable as AR is being investigated as a putative target in ER+ and ER− tumor subtypes [63–65]. Unlike ER and PR which reside predominantly in the nucleus in both the absence or presence of ligand, AR is usually cytoplasmic and is shuttled to the nucleus with addition of an androgenic ligand.…”