Inflammation Impacts the Development of Colorectal Cancer Through Changes in Microbial Composition and Associated Genotoxic Capacity

Abstract

Background: Selenoproteins, many of which protect against oxidative injury, have been postulated to affect the development of GI inflammation and cancer. Glutathione peroxidase3 (Gpx3) is an extracellular selenoprotein that is elevated in the plasma of patients with inflammatory bowel disease. Also, mice treated with Dextran Sodium Sulfate (DSS), a model of murine colitis, have increased plasma Gpx3. Based on these observations we hypothesized that Gpx3 could modify inflammatory carcinogenesis (CAC) in the colon. Methods: We selected the azoxymethane (AOM)/cyclic DSS rodent model for our studies as this is a robust model for interrogating genetic modifiers of CAC. 11 Gpx3-/and 12 WT mice were injected with 12 mg/kg AOM followed by four cycles of 3% DSS ad lib. An additional cohort of 18 Gpx3-/and 13 WT mice were subjected to cyclic DSS therapy without AOM initiation. At necropsy, colons were isolated and tumor burden and distribution were scored. TUNEL staining to evaluate apoptosis and IHC for BrdU and β-catenin was performed on colon sections. Results: When treated with AOM and cyclic DSS ad lib, both Gpx3-/and WT mice developed polyps and had mucosal injury localized to the distal colon. However, Gpx3-/mice had increased polyposis (21.1 ± 1.6 vs 10.8 ± 1.5 polyps per colon, P<0.0001), without a difference in polyp size (7.9 ± 1.1 vs 6.4 ± 0.3, P=0.20). There appeared to be an increase in polyp burden in DSS only treated Gpx3-/mice, although this did not reach statistical significance (1.0 ± 0.3 vs 0.4 ± 0.2, P=0.15). Polyp sizes were similar between both groups of mice. Histologic injury was more severe in the Gpx3-/mice (16.2 ± 2.5 vs 7.8 ±0.64, P<0.01, combined histologic injury score). Overall, there was no difference in grade of lesions, with high-grade dysplasia present in both groups of mice. While there was no difference in intra-tumoral apoptosis observed, there was significantly higher proliferation in Gpx3-/tumors (164.0 ± 46.7 vs 73.9 ±54.4 Ki67+ cells/HPF, P<0.01). Consistent with this we observed increased cytoplasmic and nuclear β-catenin staining in Gpx3-/tumors (P<0.001). Conclusions: These studies demonstrate that Gpx3 modifies tumor initiation in inflammatory carcinogenesis. This suggests that Gpx3may serve a protective role in inflammation associated colitis perhaps via decreasing levels of H2O2 and hydroperoxides in the colonic mucosa, thus reducing oxidative DNA damage.