Abstract P3-04-12: Both spliced and unspliced XBP1 regulates breast cancer cell fate response to antiestrogen via NFkappaB signaling

Unfolded protein response (UPR), a stress-induced survival mechanism, may be used by cancer cells to avoid cell death. Antiestrogen therapy, widely applied in the treatment of estrogen receptor-positive (ER+) breast cancer, induces endoplasmic reticulum stress (EnR stress) that leads to activation of each of the three arms of the UPR. One critical prosurvival activator that is regulated by two arms of the UPR is the transcription factor X-box binding protein 1 (XBP1). XBP1 exists in two isoforms, the transcriptionally inactive unspliced XBP1(U) and the spliced, active XBP1(S). Overexpression of XBP1(S) confers estrogen independence and antiestrogen resistance in ER+ breast cancer cells and XBP1(S) expression correlates with poor clinical responsiveness to Tamoxifen in ER+ breast tumors. However, the underlying signaling mechanisms regulated by XBP1, which may mediate its effects on antiestrogen resistance, are unknown. We show that depletion of endogenous XBP-1 by siRNA increases apoptosis, decreases autophagy, requires down-regulation of p65/RelA, a component of the pro-survival NFκB complex. Using novel spliced and non-spliceable forms of XBP1, we show that XBP1(U) and XBP1(S) both regulate NFκB activity via ERα signaling in breast cancer cells. XBP1(S), but not XBP1(U), also can regulate p65/RelA expression independent of ERα. Antiestrogen resistance as conferred by XBP1 overexpression in MCF-7 cells requires the activation of NFκB signaling; inhibition of NFκB signaling by either the small molecule NFκB inhibitor Parthenolide or p65/RelA siRNA sensitizes XBP1 overexpressing cells to Tamoxifen. The activation of XBP1 and the downstream NFκB signaling is likely to contribute to the TMEM33 overepxression induced apoptosis, as both NFκB signaling and XBP1(S) are elevated in TMEM33 overexpressed MCF7 cells. Thus, we have identified a critical regulatory link between the UPR/XBP1 pathway and pro-survival NFκB signaling that is a major contributor to endocrine responsiveness in ER positive breast cancer. Citation Format: Clarke R, Hu R, Warri A, Jin L, Zwart A, Riggins R, Fang H-b. Both spliced and unspliced XBP1 regulates breast cancer cell fate response to antiestrogen via NFkappaB signaling [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P3-04-12.