15-Hydroxyprostaglandin dehydrogenase (PGDH) is an aberrant tumor suppressor in human breast cancer

Abstract

10550 Background: Prostaglandin E2 plays a growth stimulatory role in breast cancer; and the rate-limiting enzyme in its synthesis, cyclooxygenase-2 (COX-2), is over-expressed in breast cancers. However, little is known about the expression pattern and activities of the key prostaglandin catabolic enzyme PGDH, in breast cancer pathogenesis. Methods: 5-aza-2’-deoxycytidine and suberoyl anilide bishydroxamide (SAHA) were used to unmask epigenetically silenced genes. DNA and RNA were extracted from tumors and cell lines using standard methods, and bisulfite sequencing was used for methylation analysis. Western blotting and real time PCR were used for expression analysis, and PGDH expression vector and PGDH-directed siRNA were used for overexpression and downregulation of PGDH, respectively. Athymic mice were utilized for xenograft models. Results: Using a pharmacologically based, genome-wide screen for epigenetically silenced genes, we found low levels of PGDH in MDA-MB-231 cells (estrogen receptor- (ER) and HER2-negative), but high levels in MCF-7 cells (ER-positive, HER2-negative), and observed upregulation of this enzyme following demethylation treatment. Methylation analysis revealed methylation of the PGDH promoter in one breast cancer cell line and in 3 of 10 primary tumors. Analysis of PGDH expression demonstrated low levels in 40% of primary breast tumors and identified a correlation between PGDH and ER expression. Upregulation of PGDH levels in MDA-MB-231 cells decreased their clonal growth and reduced their ability to form tumors in athymic mice. In contrast, silencing of PGDH in MCF-7 cells increased proliferation and enhanced in-vivo tumorigenicity. Further analysis revealed upregulation of aromatase following silencing of PGDH and inhibition of the ER pathway by forced expression of the enzyme. PGDH levels were downregulated by estrogen but upregulated by the tumor suppressor gene C/EBPα. Conclusions: Our results identify for the first time, that PGDH, the COX-2 antagonist, is aberrantly silenced in breast cancer. Activation of this enzyme, particularly in ER- and HER2-negative tumors, now becomes an interesting target for clinical studies. No significant financial relationships to disclose.