P143 EPITHELIAL-DERIVED SELENOPROTEIN P PROTECTS FROM COLITIS-ASSOCIATED CARCINOGENESIS
Authors: Short Sarah , Reddy Vishruth , Hendren Jared , Haberman Yael , Pilat Jennifer , Marsh Benjamin , Washington Mary , Hyams Jeffrey , Denson Ted , Rosen Michael , Williams Christopher 
Published: January 2020

Abstract
Patients with inflammatory bowel disease demonstrate selenium nutritional deficiencies and are at increased risk for colon cancer. Previously, we determined that global loss of the selenium-containing antioxidant protein, Selenoprotein P (SELENOP), exacerbates experimental colitis-associated cancer (CAC). Using a variety of tissue-specific knockout mouse lines, we further determined that epithelial-derived SELENOP served as the primary mediator of disease severity, and Villin-Cre; Selenop f/f (Selenop ΔIEC) mice have increased tumor incidence, number, size, and dysplasia as compared to WT cohorts.
We next aimed to establish the mechanism by which epithelial-derived SELENOP modifies inflammatory tumorigenesis. We first investigated SELENOP’s effect on tumor promotion, and although Selenop ΔIEC tumors were larger, tumor cell proliferation was unchanged. Because global loss of SELENOP modified immune cell recruitment, we next characterized tumoral immunoinfiltrates by immunohistochemistry and seq-ImmuCC analysis of RNA-sequencing results. Neither metric identified significant changes in immune cell populations in either tumor or adjacent normal tissues. Further, histological endpoint analysis noted no change in degree of inflammation in Selenop ΔIEC colons at the end of the azoxymethane/dextran sodium sulfate protocol. Thus, epithelial-derived SELENOP likely does not augment inflammatory tumorigenesis via tumor promotion or inflammatory means by the measured parameters.
We next hypothesized that loss of epithelial SELENOP modifies tumor initiation, and in support of this, analysis of endoscopy videos identified increased tumor numbers at early stages of experimental carcinogenesis. Tumors from Selenop ΔIEC mice also showed increased staining for markers of apoptosis and DNA damage, suggesting earlier tumor initiation may be due to increased oxidative stress. Further, myeloid-specific Selenop knockout, which did not alter inflammatory tumorigenesis, upregulated other antioxidant selenoproteins which may compensate for SELENOP loss, but this increase was not observed in the Selenop ΔIEC model. Interestingly, query of the Predicting Response to Standardized Pediatric Colitis Therapy (PROTECT) cohort indicated that SELENOP is downregulated in UC tissues, with greater downregulation in severe disease, but levels of other key antioxidant selenoproteins were again unchanged. Finally, to more clearly investigate SELENOP loss in epithelial cells, SELENOP was knocked down in human adult ulcerative colitis organoids. Here, knockdown induced a pronounced growth defect, increased apoptosis, and increased oxidative stress as measured by CellRox staining intensity. Taken together, these results suggest a model in which epithelial SELENOP protects from CAC by buffering oxidative stress in colonic epithelial cells in the setting of an inflammatory microenvironment.