Increased calcium intake regulates colonic CaSR expression and inhibits induced colitis and Wnt/β-catenin signaling
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One of the major complications in patients with long standing inflammatory bowel disease (IBD) is colitis-associated colon cancer. The signaling pathways that promote the transformation of colitis to colorectal cancer (CRC) are unknown but one candidate is the prolonged interaction between tumor necrosis factor alpha (TNF-α) and its receptor, TNFR1. This interaction may stimulate a defective Wnt signaling pathway to evoke epithelial cells to hyperproliferate. This crypt hyperproliferation can lead to the development CRC. Activation of Extracellular Calcium-sensing receptor (CaSR) a G-protein coupled receptor, on sub-epithelial myfibroblasts has been found to stimulate the expression of Wnt5a, a member of Wnt family, while CaSR activation on the epithelial cells stimulates Wnt5a receptor, Ror2, a member of receptor tyrosine kinase family. Wnt5a/Ror2 (non canonical signaling) interaction has been suggested to decrease TNFR1 expression and inhibit Wnt/β-catenin signaling to regulate cellular growth. The aims of this thesis were to determine whether increased dietary calcium will increase colonic CaSR expression, characterize the colons of “rescued” CaSR/PTH double homozygous knockout and their response to an acute model of colitis, and assess whether CaSR activation influenced the status of Low-density lipoprotein receptor-related protein 6 (pLRP6), co-receptor for Wnt, in cell culture models. Our results demonstrated that CaSR expression is higher in the colon of the NIH-Swiss Webster mice after feeding them 2% calcium alone or in conjunction with vitamin D for four weeks compared to two weeks, suggesting that dietary calcium alone or together with vitamin D can regulate colonic CaSR expression. We found the CaSR-/PTH- colons had a significant reduction in the non canonical signaling and enhanced Wnt/β-catenin signaling and the amount of inflammation was increased, suggesting that CaSR can inhibit Wnt/β-catenin signaling in the colonic crypt and acts as an anti-inflammatory signal. Our results demonstrated that CaSR activation alone in CaSR-HEK and RKO cells reduced Wnt-stimulated LRP6 phosphorylation suggesting a crucial role of CaSR in the inhibition of LRP6 phosphorylation. The observations seen in this study strongly suggest a role for CaSR in inhibiting the defective Wnt/β-catenin signaling pathway, induced colitis, and LRP6 phosphorylation to regulate cellular growth.