Project Summary Vitamin D has shown cancer preventive benefit in colorectal cancer (CRC). Despite some positive findings, conflicting data is also evident in human intervention trials that further study is warranted. An important observation linking the limited effects of vitamin D is a significant reduction in the levels of vitamin D receptor (VDR) activity. Our recent study in ApcΔ14/+ mice showed a loss of VDR expression in intestinal polyps, most likely accounting for the lack of cancer protection afforded by high-dose vitamin D3. Further analysis in this study and results of others suggested that compromised VDR expression may have influence on the regulation of β- catenin activation. Based on these observations, we hypothesize that forced re-expression of VDR within intestinal tumors can restore β-catenin control and potentially promote tumor regression in ApcΔ14/+ mice. To test this hypothesis, we will use our recently developed mouse model, Scd3iCreER/+, in which the expression of a floxed transgene can be driven specifically to intestinal tumors. To control VDR expression, we will utilize a floxed- stop system, in which Vdr expression is restricted by a STOP sequence until tamoxifen-controlled Cre recombinase is expressed. The successful creation of this conditional mouse model will allow us to directly test the influence of timed VDR expression on tumor regression in the intestine. In Aim 1, we plan to generate ROSA26LSL-Vdr-GFP mice, and then a compound mutant mice, ApcΔ14/+:ROSA26LSL-Vdr-GFP:Scd3iCreER/+. These mouse models will be thoroughly characterized to confirm the forced expression of VDR within intestinal tumors. In Aim 2, using the ApcΔ14/+:ROSA26LSL-Vdr-GFP:Scd3iCreER/+ mice, we will examine the impact of exogenous VDR reactivation on intestinal tumor development in a spatio-temporal manner. We will also profile the gene expression changes associated with the re-expression of VDR, focusing on the interplay between VDR and Wnt/β-catenin signaling pathways. Aim 3 will investigate the effect of exogenous expression of VDR on immune cell trafficking within the tumor microenvironment using cyTOF analysis. These proposed exploratory studies will provide a clear understanding of the anticancer activities of VDR, and ultimately enable us to uncover new therapeutic strategies for the reactivation of repressed VDR in CRC.