Abstract 1200: BP1 regulates ERα activity in MCF-7 cells

Abstract Introduction. BP1 is a member of the homeobox gene family of transcription factors. Our recent studies have shown that BP1 may play a role in breast cancer cell survival, aggressiveness and metastasis. BP1 protein (pBP1) is expressed in 80% of invasive ductal breast tumors, and is associated with estrogen receptor negativity – 100% of ERα negative (hereafter called ER-) tumors expressed BP1, while 73% of ER positive (ER+) tumors expressed BP1; this difference was statistically significant (p=0.03). Initially most tumors are hormone dependent and are sensitive to antiestrogens. Over time, many tumors progress to hormone-independence. One possible mechanism by which this might occur is reduction of ER protein. Here we present data indicating that high BP1 expression decreases ER protein levels in ER+ cells grown in the absence of serum. Since it is not currently understood what factors may cause ER+ cells to become more ER negative, it is important to determine the role of BP1 in reducing ER protein levels. Results. Our data suggest that MCF-7 ER+ cells engineered to overexpress BP1, when grown in the absence of serum, exhibit decreased ER protein compared with control cells containing an empty vector and grown under the same conditions. This is not a transcriptional effect since there was no change in ER mRNA levels in the presence of high levels of BP1. There was also a decrease in ER immunostaining in cells overexpressing BP1 grown without serum. Decreased ER protein levels correlated with decreased expression of pS2, a known downstream target of ER, demonstrating that there is less functional ER protein in cells overexpressing BP1 when grown in the absence of serum. We speculate that regulation of ER levels by BP1 is post-translational. Several pathways that control ER protein stability are being evaluated in cells grown with or without serum. Phosphorylation of Akt on Ser473, the site that is known to phosphorylate and stabilize ER protein, is being investigated to determine whether BP1 regulates phospho Akt levels. MAPK signaling can result in activation of ER in the absence of estrogen, but we did not see any evidence that BP1 is involved in the MAPK pathway. p300 is known to acetylate and stabilize ER protein; experiments are underway to determine whether BP1 directly regulates p300 expression since there are two potential BP1 binding sites in the p300 gene. Conclusions. We hypothesize that BP1 overexpression leads to reduced ER protein in ER positive cells. This could result in decreased sensitivity to antiestrogens in those cells. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1200. doi:10.1158/1538-7445.AM2011-1200