Colorectal carcinogenesis is a multistep process involving genetic mutations and alterations in rigorously controlled signaling pathways and gene expression that control intestinal epithelial cell proliferation, differentiation, and apoptosis. Cyclooxygenase-2 (COX-2) is aberrantly expressed in premalignant adenomatous polyps and colorectal carcinomas and is associated with increased epithelial cell proliferation, decreased apoptosis, and increased cell invasiveness. Currently, knowledge of the regulation of expression of COX-2 by endogenous cell-surface receptors is inadequate. Recently, in a non-transformed rat intestinal epithelial cell line (IEC-18), we showed induction of cell proliferation and DNA synthesis by angiotensin II (Ang II) via the endogenous Ang II type 1 receptor (Chiu, T., Santiskulvong, C., and Rozengurt, E. (2003) Am. J. Physiol. 285, G1-G11). We report that Ang II potently stimulated expression of COX-2 mRNA and protein as an immediate-early gene response through the Ang II type 1 receptor, correlating with an increase in prostaglandin I2 production. Ang II induced Cdc42 activation and filopodial formation. COX-2 expression was induced by epidermal growth factor (EGF), which activated Rac with lamellipodial formation. Inhibition of small GTPases by Clostridium difficile toxin B blocked COX-2 expression by Ang II and EGF. Inhibition of ERK activation by U0126 or PD98059 significantly decreased EGF-dependent COX-2 expression, but did not affect Ang II-dependent COX-2 expression. Conversely, inhibition of p38MAPK by SB202190 or PD169316 inhibited COX-2 expression by Ang II, but did not block COX-2 induction by EGF. Ang II caused Ca2+ mobilization. Inhibition of Ca2+ signaling by 2-aminobiphenyl borate blocked Ang II-dependent COX-2 expression. EGF did not induce Ca2+ mobilization, and 2-aminobiphenyl borate did not inhibit EGF-dependent COX-2 expression. Inhibition of COX-2 expression correlated with inhibition of prostaglandin I2 production. Luciferase promoter assays showed that Ang II-dependent transcriptional activation of the COX-2 promoter was dependent on activation of small GTPases and p38(MAPK) and on Ca2+ signaling via the cAMP-responsive element/activating transcription factor cis-acting element.