Background: Prostaglandin E2 plays an important role in inflammation and cancer. Prostaglandin E2 acts on the PGE receptors (EP), including EP1, EP2, EP3, and EP4. EP4 is expressed in the colorectal epithelium. Recent studies showed that EP4 agonists ameliorate colitis in experimental murine models by inhibiting the production of chemokines and cytokines from immune cells. Furthermore, in a clinical trial, administration of EP4 agonist was effective in treating patients with mild-to-moderate ulcerative colitis (UC) who were refractory to 5-AZA therapy. Therefore, administration of EP4 agonists may be a therapeutic strategy for the management of UC. However, the functional role of intestinal EP4 in homeostasis of the colorectal epithelium in a normal condition is not fully understood. The aim of this study is to elucidate the functional role of intestinal EP4 in homeostasis of the colon.
Methods: We generated Villin-Cre mice (control) and Villin-Cre; EP4 flox/flox mice (EP4 cKO) and assessed the colorectal epithelium at the age of 8–9 weeks. In EP4 cKO mice, EP4 was deleted exclusively in the intestinal epithelium from an embryonic stage. Colonic epithelium was evaluated in terms of morphology, apoptosis, cell proliferation, differentiation, and signaling pathways by hematoxylin and eosin staining, immunohistochemistry and quantitative RT-PCR analyses. Furthermore, to induce experimental colitis, 2% of DSS was administered for 7 days and mice were analyzed histologically.
Results: EP4 cKO mice were born according to the Mendelian frequencies. The length of crypts in the distal colon was significantly shorter in EP4 cKO mice relative to control mice. Apoptosis and cell proliferation were markedly increased within the crypts in EP4 cKO mice. In terms of differentiation, secretary lineages including goblet cells, tuft cells, and entero-endocrine cells were significantly decreased in EP4 cKO mice. Furthermore, ectopic Paneth cells were observed in EP4 cKO colons, whereas they were not observed in controls. Moreover, colonic epithelial cells expressing Muc2 and Cdx2 were markedly decreased in EP4 cKO mice. Notably, instead of decreased number of cells expressing Muc2 and Cdx2, colonic epithelial cells expressing Notch1IC, Hes1 and Sox9 were significantly increased and distribution of these cells was extended from the bottom to the more upper regions of the crypts in EP4 cKO mice, suggesting that Notch signaling was more broadly activated in EP4 cKO mice. Additionally, in a DSS-colitis model, colitis was more severely exacerbated in EP4 cKO mice.
Conclusions: Intestinal EP4 receptor is required for homeostasis of the colon in mice, which appears to be mediated by Notch signaling pathway.