During the last project period, a great deal has been learned about toll-like receptor (TLR) signaling. The work we have performed on the present project entitled [unreadable]Toll-like receptor complex in intestinal epithelial cells[unreadable] has helped to define the role of TLR signaling in the intestinal epithelium. Based on our own research and recent work by others, we wish to test the hypothesis that TLR4 links innate immune signaling to APCdependent sporadic colon cancer. Specifically, we plan to focus on this rather new pathway in both the colonic epithelial cell (Aims 1 and 2) and the intestinal macrophage (Aim 3) because of the close interaction between these two important cell types. The work proposed is a direct extension of our work in the previous period which dealt with the gut innate immune system, but which now connects these pathways to understand mechanisms of colorectal neoplasia. A great deal of research has elucidated the molecular pathways that result in colon cancer. Mutations in the adenomatous polyposis coli (APC) gene and/or the Wnt signaling pathway are found in 85% of sporadic colon cancers. Part of the mechanism by which APC mutations result in colon cancer is through induction of COX-2 and activation of epidermal growth factor receptor (EGFR) signaling. We have shown that both of these pathways are activated by TLR4 in intestinal epithelial cells (IEC) 2. The APC pathway to cancer seems to be particularly relevant in the colon, as distinct from other epithelia, but the reason why APC mutations disproportionately culminate in colon cancer are unknown. Bacteria and TLR signaling have been implicated in the development of colorectal cancer. In preliminary data, we show that TLR4 is over-expressed in most sporadic colon cancers. These compelling findings force us to understand how TLR4 participates in the development of sporadic colon cancer. Based on our preliminary data, we hypothesize that TLR4 signaling supports the development of APC-dependent colorectal cancers through several distinct mechanisms. First, TLR4 may interact with APC to activate target genes such as COX-2. Second, TLR4 is both a target of EGFR signaling as well as capable of activating EGFR resulting in dysregulated proliferation. Finally, TLR4 expression by the epithelium leads to the recruitment of tumor-associated macrophages, which are required to promote tumor growth. Studying TLR4 provides a framework to understand how the innate immune system in the gut may inadvertently promote the development of cancer as an extension of its normal role in repair. These studies offer an additional avenue by which to prevent colon cancer in susceptible hosts and treat patients with colon cancer. This proposal is a natural extension of the work performed during the four and a half years of our grant.