Prostaglandin E₂ plays a growth-stimulatory role in breast cancer, and the rate-limiting enzyme in its synthesis, cyclooxygenase-2, is often overexpressed in these cancers. Little is known about the role of the key prostaglandin catabolic enzyme 15-hydroxyprostaglandin dehydrogenase (15-PGDH) in breast cancer pathogenesis. Using a pharmacologically based screen for epigenetically silenced genes, we found low levels of 15-PGDH in MDA-MB-231 cells [estrogen receptor (ER) negative] but high levels in MCF-7 cells (ER positive) and observed its up-regulation following demethylation treatment. Further analysis revealed methylation of the 15-PGDH promoter in one breast cancer cell line and 30% of primary tumors. Analysis of 15-PGDH expression revealed low levels in 40% of primary breast tumors and identified a correlation between 15-PGDH and ER expression. Transfection assays showed that transient up-regulation of 15-PGDH levels in MDA-MB-231 cells resulted in a decreased clonal growth, and stable up-regulation significantly decreased the ability of these cells to form tumors in athymic mice. In contrast, transient silencing of 15-PGDH in MCF-7 cells resulted in their enhanced proliferation, and a stable silencing in these cells enhanced cell cycle entry in vitro and tumorigenicity in vivo. Forced expression of 15-PGDH inhibited the ER pathway and silencing of 15-PGDH up-regulated expression of aromatase. In addition, 15-PGDH levels were down-regulated by estrogen but up-regulated by the tumor suppressor gene CAAT/enhancer binding protein α. Our results indicate for the first time that 15-PGDH may be a novel tumor suppressor gene in breast cancer, and suggest that this enzyme can modulate the ER pathway. (Cancer Res 2006; 66(15): 7818-23)