Abstract LB-113: Antihormone-induced expression of BCL3 in estrogen receptor-positive breast cancer drives resistant cell growth

Introduction: The development of resistance to antihormones in the treatment of oestrogen receptor-positive (ER+) breast cancer is a major clinical problem. There is emerging evidence that antihormones can induce expression of genes/proteins that allow a cohort of cells to evade the growth-inhibitory impact of these therapies during initial drug response and ultimately aid acquisition of resistant cell growth. In the present study we have examined antihormone-induced expression of the pro-survival/pro-proliferative gene BCL3 and its potential role in the development of resistance. Methods: ER+ MCF-7, T47D, BT474 and MDAMB361 cells were exposed to antihormone treatment and effects on BCL3 expression were examined. BCL3 expression was also examined in cell models of resistance to tamoxifen (TAMR) and oestrogen deprivation (XR). BCL3 siRNA knockdown studies were performed on wild-type MCF-7, TAMR and XR cell models and impact on p50 expression and EGFR/HER2/MAPK signalling was assessed by Western blotting. Cell survival and growth was also assessed using FACS analysis and cell counting. Cellular localisation and association of BCL3 and p50 in antihormone-responsive and -resistant MCF-7 cells was also determined by immunofluorescence and immunoprecipitation. Results: BCL3 mRNA and protein expression was markedly increased in response to all antihormonal treatments in our panel of four ER+ breast cancer cell lines and elevated BCL3 levels were also observed in the TAMR and XR cell models. Surprisingly, transient BCL3 knockdown by siRNA in TAMR cells had no effect on cell survival but reduced cell proliferation. This growth inhibitory effect of BCL3 knockdown was also observed to a lesser degree in XR but not in wild-type MCF-7 cells. BCL3 knockdown was without effect on previously identified growth regulatory signalling proteins in our TAMR cells but was associated with loss of p50 expression, suggesting the identification of a novel NF-κB-mediated resistant growth regulatory pathway. Immunofluorescence revealed nuclear expression of BCL3 and p50 during resistance, however, during response BCL3 was located in the nucleus and p50 in the cytoplasm. In support of these findings, immunoprecipitation studies identified an association between BCL3 and p50 during resistance which was absent during antihormone response. Conclusion: These results demonstrate that BCL3 is induced by antihormones during response and induction is maintained through to resistance. Interestingly, BCL3 appears to have a growth regulatory role in antihormone-resistant but not wild-type MCF-7 cells possibly due to its association with p50 in resistance, thus potentially activating transcription of target genes involved in cell proliferation. These findings suggest that targeting BCL3 may provide a novel therapeutic strategy to treat resistant disease; however its role in response remains to be elucidated. Citation Format: Jessica Davis, Janice Knowlden, Chris Pepper, Julia Gee, Iain Hutcheson. Antihormone-induced expression of BCL3 in estrogen receptor-positive breast cancer drives resistant cell growth. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-113.