Prostaglandin (PG) E(2), a cyclooxygenase (COX) product, and angiotensin II are endogenous and have physiological roles in the body. On the other hand, an inducible isoform of COX (COX-2), insulin-like growth factor (IGF) II, and IGF-I receptor (IGF-IR) are up-regulated in colon carcinoma and might have crucial roles in tumor growth and invasion. The aim of the present study was to investigate the effects of COX-2 inhibitor and drugs blocking the biological activities of angiotensin II [angiotensin-converting enzyme (ACE) inhibitors or angiotensin II receptor blockers (ARBs)] on IGF-IR expression and tumor growth in vivo. We also investigated the effects of PGE(2), a major COX-2 product, in cancer cells and the effects of angiotensin II on IGF-IR expression and the underlying mechanism of action. In in vivo studies, tumor growth and IGF-IR expression were investigated in Colon 26 cells inoculated into BALB/c mice. In in vitro studies, the effects of nonsteroidal anti-inflammatory drugs (NSAIDs) on IGF-IR expression were analyzed in three colon cancer cell lines (Colon 26, HCA-7, and LS174T). IGF-II-induced cell growth and invasion were analyzed in Colon 26 cells in the presence and absence of NSAIDs (indomethacin and celecoxib) and angiotensin II. Celecoxib at the lowest effective dose for suppression of PG production (3 mg/kg) or an ACE inhibitor/ARB alone did not have a significant effect as compared with controls, although a high dose of celecoxib (>20 mg/kg) suppressed tumor growth. On the other hand, combination therapy with these two categories of drugs significantly reduced tumor growth in vivo. Treatment with both celecoxib and an ACE inhibitor/ARB decreased IGF-IR expression levels in inoculated tumor cells. In in vitro studies, NSAIDs reduced IGF-IR expression in a dose-dependent manner in all three cell lines. NSAIDs also inhibited IGF-II-stimulated growth and invasion in a dose-dependent manner. PGE(2) or angiotensin II treatment reversed the NSAID-induced down-regulation of IGF-IR expression, growth, and invasion. PGE(2) and angiotensin II induced Akt phosphorylation, and LY294002 or wortmannin inhibited PGE(2)- or angiotensin II-induced IGF-IR expression, indicating that PGE(2) and angiotensin II both regulate IGF-IR expression by the same Akt/phosphatidylinositol-3 pathway. Thus, combination therapy with NSAIDs and ACE inhibitors targeting IGF-IR might be a novel and potentially promising strategy for the chemoprevention of colon cancer.