Elucidating the molecular mechanism(s) by which aspirin use reduces the risk and mortality of colorectal cancer (CRC) could lead to major breakthroughs in the field of cancer chemoprevention and treatment. The most compelling evidence to date indicates that the anti-tumor effects of aspirin and other NSAIDs are due to reduction of pro-inflammatory prostaglandin E2 (PGE2) production via inhibiting cyclooxygenase enzymatic activity. Our preliminary data supports this hypothesis by showing that aspirin reduced polyp numbers along with a decrease of prostaglandin production in tumors. However, no direct evidence has been developed that aspirin inhibits CRC initiation, progression, and metastasis by reduction of PGE2 production via targeting COX enzymes. In addition, previous studies have focused on the roles of NSAIDs in eliminating tumor epithelial cells and suppressing tumor-associated angiogenesis. Little is known about the impact of aspirin on CRC immune evasion. Our observations, never before reported, indicate that aspirin restores host immunosurveillance by inhibition of myeloid-derived suppressor cells (MDSCs) and PGE2 induces an infiltration of MDSCs into the intestinal tumor and mucosa via induction of CXCR2 ligand expression prompted us to postulate that aspirin might inhibit tumor initiation, progression, and metastasis by suppressing recruitment of MDSCs via targeting a novel COX-PGE2-CXCR2 pathway. Aim 1 is designed to test this hypothesis. The results from this aim could not only identify key mechanisms responsible for anti-tumor effects of aspirin, but also may provide a rationale for development of new therapeutic approaches to subvert APC mutation- and tumor-induced immunosuppression by using CXCR2 antagonists and/or CXCR2 neutralizing antibodies. Moreover, our preliminary studies revealed for the first time that primarytumor induced immunosuppression in pre-metastatic organs, whereas treatment with a COXIB attenuated the effects of the primary tumor on pre-metastatic niche formation in the liver. Thus, it is conceivable to hypothesize that aspirin inhibits metastasis by blocking the formation of pre-metastatic niches via targeting the COX-PGE2-CXCR2 pathway. Aim 2 is designed to examine this postulation. The results from this aim should provide a rationale for developing novel therapeutic approaches to inhibit CRC metastasis by blocking the pre-metastatic niche formation. Finally, one potential explanation for the cancer-preventive effects of aspirin could bedue to inhibition of COX-1 activity in platelets. However, there is no clear evidence supporting this idea. We will test this role of platelet COX-1 in Aim 3. The results from this aim will provid a rationale for development of new therapeutic approaches to target platelet COX-1 in future cancer prevention and treatment efforts. Collectively, the mechanisms we identify in this proposal might be applicable for other solid cancers in general and can certainly be tested in other systems. In addition, targeting host immunosurveillance or platelets may also represent a novel therapeutic approach for CRC patients.