Abstract 2292: Absent E2 liganded AhR inhibits tumor inhibition by ERα in Min mice

Abstract

Abstract Previously, we showed that E2 and ERα prevent intestinal tumor formation in the Min mouse. Others showed that Aryl hydrocarbon Receptor (AhR) associates with ubiquitin ligase to down-regulate β-catenin and Wnt signaling. AhR activity also may affect ERα. AhR has endogenous, dietary, and xenobiotic ligands. We hypothesized that the tumor inhibiting function of ERα promotes Wnt signaling in vivo and is antagonized by liganded-AhR in the absence of E2. In groups of 10, ovariectomized Min (OvxMin) mice were treated w/wo AhR ligand 5, 6-benzoflavone (BF; 2,000 ppm) by diet for 17 weeks. Treated mice developed more tumors in the small intestine (54.7±11.1) than controls (38.3±4.5 P<0.0001). Protein expression was evaluated in non-tumor ileum of study mice by immunohistochemistry, immunoprecipitation (IP), and immunoblotting (IB). ERα and β-catenin nuclear expression in crypt base cells, total β-catenin and ERα protein amounts, and Wnt signaling-dependent Paneth cell markers (Sox9 and Lysozyme) were significantly reduced in BF-treated OvxMin compared to control mice. Reciprocal IP/IBs using total cell lysates from study mice and antibodies for β-catenin and ubiquitin (Ub) showed increased expression of ubiquitylated β-catenin in BF-treated OvxMin mice relative to controls. Wnt signaling-dependent cell proliferation (Ki67+ nuclei/crypt) was reduced in BF-treated OvxMin (11.4±2.5) relative to controls (16.2±3.0) and untreated intact Min (20.7±3.2) mice (P<0.01). AhR was expressed throughout the crypt-villus unit of OvxMin ileum; its activity was assessed by induction of Cyp1A1. A minimal amount of Cyp1A1, but elevated levels were found in untreated mucosa of intact Min vs. OvxMin mice w/wo BF, respectively. Lower expression of the AhR negative regulator, AhR Repressor, was present in ileum of OvxMin mice regardless of treatment relative to untreated intact Min mice. In study mice mucosa, other xenobiotic responsive regulators (Pxr, Nrf2, Car1/2) were minimally expressed, not affected by BF, hence not implicated in Cyp1A1 induction. In conclusion, these results indicate that E2 loss reduces AhR feedback inhibition and Wnt signaling in Min intestine. Since liganded-AhR promoted β-catenin turnover and reduced ERα expression absent E2, our data suggest that deficient Wnt signaling in the non-tumor mucosa of Min mice provides a selective pressure for Apc functional loss in intestinal stem cells and tumor initiation. 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 2292. doi:10.1158/1538-7445.AM2011-2292