GENETIC DELETION OF FAT REDUCES MUSCULARIS PROPRIA THICKNESS AND MUSCLE CELL PROLIFERATION IN CHRONIC DSS COLITIS-A POTENTIAL THERAPEUTIC TARGET FOR STRICTURING CROHN’S DISEASE

Abstract BACKGROUND Mesenteric fat wrapping around the bowel wall, or ‘creeping fat’ (CF), is spatially associated with stricture formation in Crohn’s disease (CD). Intestinal muscularis propria (MP) smooth muscle cell hyperplasia is a major contributor to luminal narrowing in stricturing CD. We investigated the effect of mesenteric fat on intestinal muscularis propria thickness and smooth muscle cell hyperplasia in in vivo mouse models. METHODS We generated the FAT-ATTAC mouse, in which administration of the dimerizer AP20187 activates caspase-8, deleting adipocytes through apoptosis. All the mice were exposed to two-rounds of 1.5% dextran sodium sulfate (DSS) treatment to induce chronic colitis. Two strategies were employed to investigate the preventative and therapeutic role of fat deletion in chronic DSS colitis, consisting of AP20187 administration before DSS treatment and after the first round of DSS treatment, respectively. RESULTS EchoMRI analysis confirmed a robust 70% reduction in body fat mass in FAT-ATTAC mice compared to baseline within 2 weeks following administration of the dimerizer AP20187. In preventative DSS colitis, there was no significant difference in weight loss, colon length, clinical colitis score, inflammation or fibrosis scores in the DSS treated animals (FAT-ATTAC compared to wildtype). As expected, in DSS treated WT mice the MP underlying the mesentery thickened, whereas FAT-ATTAC mice did not show any increase in MP thickening. Concordant with these findings, DSS treated WT animals showed an increase in proportion of Ki67+ cells in the intestinal MP underlying the mesentery and an increase in colonic smooth muscle cell (Sm22 and aSMA) gene expression, which was reduced in the DSS treated FAT-ATTAC mice. No difference of inflammation or fibrosis gene expression was observed when comparing DSS treated WT and FAT-ATTAC mice. In therapeutic DSS colitis, there was a significant difference in weight loss and clinical colitis score between DSS treated WT and FAT-ATTAC mice, largely due to fat deletion inducing weight loss in FAT-ATTAC mice. No difference of colon length, inflammation or fibrosis score was observed when comparing DSS treated WT and FAT-ATTAC mice. Importantly, the DSS treated FAT-ATTAC mice showed reduction in MP thickening and reduced proportion of Ki67+ cells in the intestinal MP underlying the mesentery. No significant difference of inflammation, fibrosis or smooth muscle cell associated gene expression was observed when comparing DSS treated WT and FAT-ATTAC mice. CONCLUSION Deletion of fat in chronic DSS colitis reduced the MP thickness and smooth muscle cell proliferation underlying mesenteric fat. This suggests that presence of mesenteric fat is required for MP thickening underlying the mesentery in experimental colitis. These results point to CF as a novel contributor to stricture formation in CD.