Colorectal carcinogenesis is a complex, multistep process involving genetic alterations and progressive changes in signaling pathways regulating intestinal epithelial cell proliferation, differentiation, and apoptosis. Although cyclooxygenase-2 (COX-2), gastrin-releasing peptide (GRP), and its receptor, GRP-R, are not normally expressed by the epithelial cells lining the human colon, the levels of all three proteins are aberrantly overexpressed in premalignant adenomatous polyps and colorectal carcinomas of humans. Overexpression of these proteins is associated with altered epithelial cell growth, adhesion, and tumor cell invasiveness, both in vitro and in vivo; however, a mechanistic link between GRP-R-mediated signaling pathways and increased COX-2 overexpression has not been established. We report that bombesin, a homolog of GRP, potently stimulates the expression of COX-2 mRNA and protein as well as the release of prostaglandin E 2 from a rat intestinal epithelial cell line engineered to express GRP-R. Bombesin stimulation of COX-2 expression requires an increase in [Ca 2؉ ] i , activation of extracellular signal-regulated kinase (ERK)-1 and -2 and p38 MAPK , and increased activation and expression of the transcription factors Elk-1, ATF-2, c-Fos, and c-Jun. These data suggest that the expression of GRP-R in intestinal epithelial cells may play a role in carcinogenesis by stimulating COX-2 overexpression through an activator protein-1-dependent pathway.Colorectal cancers are the third leading cause of cancer deaths in the United States (1). One in 20 Americans is at risk of developing this disease during their lifetime. Considerable experimental data have accumulated indicating an important role for cyclooxygenase-2 (COX-2) 1 in colorectal carcinogenesis. COX-2 is a key enzyme in the biosynthesis of prostaglandins from arachidonic acid and is overexpressed in 85-90% of human colon cancers and 40 -50% of premalignant adenomas (2). Several large epidemiological studies have shown that mortality from colorectal cancers decreases (40 -50%) in persons who regularly take aspirin or other nonsteroidal antiinflammatory drugs (3), which inhibit COX activity. Additionally, experiments with adenomatous polyposis coli (APC) gene-deficient mice (Min mice) revealed that inhibition of COX activity with nonsteroidal antiinflammatory drugs resulted in a reduction in the number and multiplicity of spontaneously formed tumors (4 -6), and APC ⌬716 /COX-2 double-knockout mice showed reduction in both the neoplastic growth and number of intestinal tumors (7). Although mounting evidence supports an important role for COX-2 in colorectal carcinogenesis, the molecular mechanisms leading to COX-2 overexpression in intestinal epithelial cells are not completely understood.Like COX-2, the mammalian homologue of bombesin (BBS), gastrin-releasing peptide (GRP) and its cognate G-protein-coupled receptor, GRP receptor (GRP-R), are aberrantly overexpressed in premalignant adenomatous polyps and colorectal cancers. Preston et al. (8) showed...