Introduction

Colorectal cancer associated to chronic colitis (CAC) has a different pathogenesis compared to sporadic or familial colorectal cancer (CRC) and represents the major complication of inflammatory bowel disease (IBD). It is now well-established that IL-33 and its receptor, ST2, are important factors in the pathogenesis of IBD. Emerging evidence also suggests its critical role in epithelial proliferation and the potential contribution to inflammation-driven tumorigenesis that can lead to CRC.

Aims & Methods

The aim of our study was to characterize the precise contribution of IL-33/ST2 axis in the azoxymethane (AOM)/dextran sodium sulfate (DSS) model of colitis-associated CRC. C57/BL6 wild-type (WT), Il33^-/-, T1/St2^-/- and Nt5e^-/- mice were given a single dose of AOM (10 mg/kg) followed by two cycles of 3% DSS for 7d in drinking water. Vehicle-treated WT mice served as controls and were sacrificed at the same time points. Another group of WT followed the same protocol and received CD73 inhibitor i.p. treatment or vehicle. Disease Activity Index (DAI), as well as endoscopic, stereomicroscopic and histological evaluations of colons were performed. IHC, immunofluorescence (IF), qPCR, and ELISA were done on full-thickness colons of WT for IL-33 and ST2 localization and identification, as well as mRNA and protein expression, respectively. RNA-Seq was performed on whole tissues from AOM/DSS treated WT, Il33^-/- and T1/St2^-/-. qPCR analysis was done on isolated polyps from WT, Il33^-/- and T1/St2^-/- for Nt5e (CD73) and adenosine pathway targets.

Results

Il33, Ilrl1(ST2L), and Ilrl1(sST2) mRNA transcripts, as well as IL-33 and total ST2 proteins were dramatically elevated in AOM/DSS-treated WT mice vs. controls. IHC and IF of treated WT mice revealed localization of IL-33 to the colonic epithelium and to cells within the polyp LP morphologically consistent with stromal and mast cells. Little to no staining for IL-33 was present in controls. Using IF, IL-33 co-localized with sub-epithelial myofibroblast markers Actin and Vimentin, or with mast cell markers Tryptase and MCPT1. AOM/DSS treatment in Il33^-/- and T1/St2^-/- mice resulted in a significant decreased polyp number and size vs. WT, with colonoscopy revealing the development of protruding lesions with abnormal vascular patterns, suggesting tumorous lesions in WT mice, while all deficient mice showed their absence with a more impressive mucosal inflammation, likely due to reduced epithelial proliferation and repair caused by the deficiency of IL-33 signaling. RNA-Seq identified a significant reduction of Nt5e and adenosine pathway targets in Il33^-/- and T1/St2^-/- vs. WT. qPCR on isolated polyps confirmed this observation. AOM/DSS-treated Nt5e^-/- showed a significant decreased polyp number and size vs. controls. Therapeutic inhibition of CD73 produced similar results.

Conclusion

Our results suggest that the IL-33/ST2 axis promotes tumorigenesis in colitis-associated CRC through the activation of CD73. Further studies are underway to determine mechanisms of action that support these findings.