The majority of breast tumours at primary diagnosis are estrogen receptor positive (ER). Estrogen (E) mediates its effects by binding to the ER. E-bound ER associates classically with estrogen response elements (EREs) on target genes controlling proliferation and cell survival. ER's ability to regulate gene transcription is also influenced by the activity of co-regulatory proteins such as AIB1. Therapies targeting the estrogenic stimulation of tumour growth have been a major success in reducing mortality from ER-positive breast cancer. However, resistance remains a major clinical problem. To identify the molecular mechanisms associated with resistance to E-deprivation, we assessed the temporal changes in gene expression during adaptation to long-term culture of MCF7 human breast cancer cells in the absence of E2 (LTED), modelling resistance to an aromatase inhibitor (AI). Analyses of canonical signalling pathways showed that platelet-derived growth factor (PDGF)/Abl was significantly elevated as early as one-week post E-deprivation (p=1.94 E-04). Of note this became the top adaptive pathway at the point of resistance (p=1.15 E-07). Clinical data from 88 patients treated with an AI in the neoadjuvant setting, showed increases in PDGFRβ and PDGF expression after two weeks (1.25 fold, p=0.003 and 1.43 fold, p=4.4E-06). Low PDGFRβ at pre-treatment was associated with a better response. Assessment of the level of protein expression and activation status in a panel of cell lines modelling both endocrine sensitive and resistant breast cancer showed PDGFRβ was elevated in both LTED and tamoxifen resistant MCF7 cells (TAMR) compared to wt-MCF7. Moreover, the level of receptor phosphorylation was increased in the resistant cell lines. PDGFRβ expression was also evident in SKBR3 (ER-/ERBB2+) but not in BT474 (ER+/ERBB2+) cells. Of note both LTED and TAMR cells were sensitive to the inhibitory effect of the PDGFR/Abl tyrosine kinase inhibitor nilotinib, in the presence or absence of E. In contrast wt-MCF7 cells showed limited sensitivity in the presence of E. Both SKBR3 and BT474 cells were insensitive. These data suggested that PDGFRβ was associated with the endocrine resistant phenotype. Assessment using ER/ERE linked reporter assays showed nilotinib suppressed ER-mediated transcription and inhibited expression of two endogenous E-regulated genes TFF1 and PGR. Chromatin immunoprecipitation indicated that nilotinib reduced the recruitment of AIB1 and CBP to the TFF1 promoter. Furthermore we were able to demonstrate that nilotinib in combination with tamoxifen resensitised TAMR cells to the inhibitory effect of tamoxifen. These data for the first time reveal cross talk between ER and PDGF/Abl, strongly suggesting that the PDGF signalling pathway merits clinical evaluation as a therapeutic target in endocrine resistant breast cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2920.