Abstract P3-05-24: Adaptation to AI therapy in breast cancer can induce dynamic alterations in ER activity resulting in estrogen independent metastatic tumours

Abstract

Abstract Acquired resistance to aromatase inhibitor therapy is a major clinical problem in the treatment of breast cancer. The detailed mechanisms of how tumour cells develop this resistance remain unclear. Here, discovery of pro-survival adaptations to the estrogen receptor (ER) signalling pathway in response to AI treatment is reported. Global ChIPseq analysis shows that the ER retains binding activity in AI resistant cells under steroid-depleted conditions. In AI treated patients, evidence of steroid independent adaptive ER signalling was demonstrated by the ER target gene Early Growth Response 3 (EGR3). Expression of EGR3 decreased initially upon AI treatment but subsequently recovered following as little as 12 weeks of AI treatment. In vitro data indicates that this increased expression of EGR3 may enhance cell growth and motility of tumour cells. Finally, evidence from established metastatic tumours suggests that the ER signalling network may undergo further adaptations with disease progression as EGR3 expression is routinely lost in the established metastatic tumour. Overall, these data provide evidence of a dynamic ER response to AI treatment which may provide vital clues for overcoming the clinical issue of AI resistance. Citation Format: Damir Vareslija, Jean McBryan, Ailis Fagan, Aisling Redmond, Yuan Hao, Arnold DK Hill, Leonie S Young. Adaptation to AI therapy in breast cancer can induce dynamic alterations in ER activity resulting in estrogen independent metastatic tumours [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P3-05-24.