The modulation of tamoxifen response induced by splice variant BQ323636.1 in estrogen receptor positive breast cancer

Abstract

Breast cancer is the most common cancer in women. Among different types of breast cancer, estrogen receptor positive (ER+) breast cancer is the most frequently occurring subtype. Activation of ER signaling plays a critical role in ER+ tumor progression. Therefore, targeting ER is an efficient approach to treat ER+ breast cancer. Tamoxifen, an antagonist of ER, has long been used as first-line adjuvant treatment for ER+ breast cancer. However, resistance to tamoxifen compromises its clinical efficacy. The mechanism of tamoxifen resistance has been studied for decades, but still remains unclear. Our group previously identified a novel splice variant of nuclear receptor co-repressor 2 (NCOR2) called BQ323636.1 associated with tamoxifen resistance. BQ323636.1 is a truncated product of NCOR2, which only retains the first repression domain of NCOR2 wild-type protein. NCOR2 is a transcriptional co-repressor for estrogen receptor (ER). Wild-type NCOR2 recruits other proteins to form a functional co-repressor complex, leading to suppressed ER signaling. However, BQ323636.1 overexpression by competing with NCOR2 compromises its repressive function, resulting in the activation of ER signaling and tamoxifen resistance. My study aimed to investigate the downstream mechanism of BQ323636.1 overexpression in tamoxifen resistant breast cancer. Apart from ER, NCOR2 is also a co-repressor for androgen receptor (AR). Thus BQ323636.1 overexpression by compromising in the repressor function of NCOR2 would also result in the activated AR signaling. Therefore, I hypothesized that BQ323636.1 employs both AR and ER targeted genes to confer tamoxifen resistance in ER+ breast cancer. Based on this hypothesis, I identified 22 candidate genes that contain both androgen responsive element (ARE) and estrogen responsive element (ERE) by in silico analysis. After validation of ARE and ERE activity by RT-qPCR analysis, I confirmed that interleukin-8 (IL-8) had both functional ARE and ERE activity and demonstrated that overexpression of BQ323636.1 could enhance expression of IL-8 in the presence of AR in ER+ breast cancer cell lines. The effect of IL-8 signaling on BQ323636.1 induced tamoxifen resistance was further studied by CXCR1 inhibitor repertaxin. Inhibition of IL-8 signaling by CXCR1 inhibitor could recover response of BQ323636.1 overexpressed ER+ breast cancer cells to tamoxifen. Combined treatment of repertaxin and tamoxifen could significantly reduce BQ323636.1 overexpressed tumor growth in vivo mouse model. I further studied activities of PI3K/AKT and MAPK/ERK mediated by IL-8. In BQ323636.1 overexpressed ER+ breast cell lines, inhibition of CXCR1 significantly reduced MAPK/ERK and PI3K/AKT activities. In summary, my results suggested that BQ323636.1 overexpression in ER+ breast cancer could enhance IL-8 mediated signaling to modulate tamoxifen resistance. Targeting IL-8 signaling might be a promising approach to overcome tamoxifen resistance in ER+ breast cancer.