Microsomal prostaglandin E 2 synthase-1 (mPGES-1) is a stimulus-inducible enzyme that functions downstream of cyclooxygenase (COX)-2 in the PGE 2 -biosynthetic pathway. Given the accumulating evidence that COX-2-derived PGE 2 participates in the development of various tumors, including colorectal cancer, we herein examined the potential involvement of mPGES-1 in tumorigenesis. Immunohistochemical analyses demonstrated the expression of both COX-2 and mPGES-1 in human colon cancer tissues. HCA-7, a human colorectal adenocarcinoma cell line that displays COX-2-and PGE 2 -dependent proliferation, expressed both COX-2 and mPGES-1 constitutively. Treatment of HCA-7 cells with an mPGES-1 inhibitor or antisense oligonucleotide attenuated, whereas overexpression of mPGES-1 accelerated, PGE 2 production and cell proliferation. Moreover, cotransfection of COX-2 and mPGES-1 into HEK293 cells resulted in cellular transformation manifested by colony formation in soft agar culture and tumor formation when implanted subcutaneously into nude mice. cDNA array analyses revealed that this mPGES-1-directed cellular transformation was accompanied by changes in the expression of a variety of genes related to proliferation, morphology, adhesion, and the cell cycle. These results collectively suggest that aberrant expression of mPGES-1 in combination with COX-2 can contribute to tumorigenesis.Clinical, genetic, and biochemical evidence has suggested that prostaglandin (PG) 1 E 2 produced via the cyclooxygenase (COX)-2-dependent pathway plays a crucial role in the development of colorectal cancer and possibly other cancers (1). Non-steroidal anti-inflammatory drugs, which inhibit COX-2, reduce the incidence of colorectal cancer (2-4). The major prostanoid produced by several types of cancer is PGE 2 , which is produced by three biosynthetic reactions involving phospholipase A 2 (PLA 2 ), COX, and terminal PGE 2 synthase (PGES). PGE 2 promotes survival and motility of colon cancer cells in vitro and promotes tumorigenesis and angiogenesis in vivo (5-7). High levels of constitutive expression of COX-2 have been found in various cancer cells and tissues (8,9), and studies employing overexpression, antisense suppression, and specific inhibitors of COX-2 have demonstrated that COX-2 contributes to the progression of several types of cancer (10 -12).More direct evidence for the role of COX-2 and its product PGE 2 in colorectal tumorigenesis has been provided by gene targeting studies. Gene disruption of either COX-2 (13) or the PGE receptor EP2 (14) results in reduction of the number and size of intestinal polyps in Apc mutant mice, a model for human familial adenomatous polyposis. In another model, disruption of the genes for the PGE receptors EP1 (15) or EP4 (16) suppresses the development of colorectal cancer induced by carcinogen. Moreover, gene knockout of cytosolic PLA 2 ␣ (cPLA 2 ␣), which supplies the substrate arachidonic acid to COX-2, also leads to reduced polyposis in Apc mutant mice (17,18).PGES catalyzes the conversion of PGH 2 ...