Abstract 428: Decreased RXRα dampened APC-independent proteosomal targeting of β-catenin to cause increased intestinal tumorigenesis after 56-Fe radiation.

Purpose: Radiation exposure is a risk factor for colorectal cancer (CRC) and high linear energy transfer radiation (high-LET) radiation such as 56-Fe in outer space is expected to pose greater risk of CRC to space travelers. Earlier, we have reported increased intestinal tumorigenesis in heavy ion radiation (HZE) exposed APCMin/+ mice, a well studied model of CRC, and higher tumor incidence was due to increased stabilization and activation of β-catenin signaling after 56-Fe radiation exposure. However the molecular mechanism behind β-catenin activation is not yet explored. Levels of β-catenin are tightly regulated through its targeting to proteasomal degradation mediated via adenomatous polyposis coli (APC) dependent and independent pathways. The APC independent pathway of β-catenin degradation is facilitated via retinoid-X receptor alpha (RXRα). The present study aimed to investigate alterations in RXRα mediated proteasomal pathways in intestinal tumors induced by 56Fe relative to γ radiation. Materials and Methods: Six to eight weeks old female APCMin/+ mice (n=6 mice/ group) were irradiated with 5 Gy of gamma and isotoxic doses of 56Fe (4 Gy, energy: 1 GeV/nucleon; LET: 148 keV/μ) radiation. Mice were euthanized between 100 and 110 days after radiation and intestinal tumor samples were collected. Tumor samples were analyzed to deliniate the 56-Fe-induced changes in RXRα-proteasomal pathways using activity assays, Western blot and immunohistochemistry (IHC). Results: Higher proteasomal activity was observed in 56Fe-induced tumors accompanied with increased expression of proteasomal subunit 20S-β and increased expression/activation of proteosomal activator-200 (PA200) compared to γ radiation. However, we found decreased phosphorylation and consequent reduction in ubiquitination of β-catenin, leading to its increased cellular level in 56Fe relative to gamma radiation-induced tumors. We also show both by Western blot as well as IHC that exposure to 56-Fe radiation led to decreased RXRα expression, a key element in the APC-independent β-catenin degradation pathway. Conclusions: Although 56Fe radiation led to elevated proteasomal activity via overexpression of proteasomal subunit and its activator, we observed increased rather than decreased active β-catenin. We propose that increased β-catenin activity and consequent higher frequency of intestinal tumors is due to the inability of not only the APC-dependent but also the APC-independent RXRα-mediated pathway to facilitate ubiquitination of β-catenin and its subsequent proteasomal degradation. Taken together our study shows for the first time that 56-Fe radiation exposure modulates β-catenin stability in an APC-independent manner to promote intestinal tumorigenesis. Citation Format: Shubhankar Suman, Kamal Datta, Albert J. Fornace. Decreased RXRα dampened APC-independent proteosomal targeting of β-catenin to cause increased intestinal tumorigenesis after 56-Fe radiation. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 428. doi:10.1158/1538-7445.AM2013-428