Increased amounts of PGE₂ have been detected in the inflamed mucosa of patients with inflammatory bowel disease (IBD). This increase has been attributed to enhanced synthesis rather than reduced catabolism of PGE₂. 15-Hydroxyprostaglandin dehydrogenase (15-PGDH) plays a major role in the catabolism of PGE₂. In this study, we investigated whether amounts of 15-PGDH were altered in inflamed mucosa from patients with IBD. Amounts of 15-PGDH protein and mRNA were markedly reduced in inflamed mucosa from patients with Crohn's disease and ulcerative colitis. In situ hybridization demonstrated that 15-PGDH was expressed in normal colonic epithelium but was virtually absent in inflamed colonic mucosa from IBD patients. Because of the importance of TNF-α in IBD, we also determined the effects of TNF-α on the expression of 15-PGDH in vitro. Treatment with TNF-α suppressed the transcription of 15-PGDH in human colonocytes, resulting in reduced amounts of 15-PGDH mRNA and protein and enzyme activity. In contrast, TNF-α induced two enzymes (cyclooxygenase-2 and microsomal prostaglandin E synthase-1) that contribute to increased synthesis of PGE₂. Overexpressing 15-PGDH blocked the increase in PGE₂ production mediated by TNF-α. Taken together, these results suggest that reduced expression of 15-PGDH contributes to the elevated levels of PGE₂ found in inflamed mucosa of IBD patients. The decrease in amounts of 15-PGDH in inflamed mucosa can be explained at least, in part, by TNF-α-mediated suppression of 15-PGDH transcription.