Abstract 2231: 15-Keto prostaglandin E2 inhibits STAT3 signaling in H- Ras transformed mammary epithelial cells

Prostaglandin E2 (PGE2) is a potent lipid mediator that plays a key role in inflammation and carcinogenesis. The intracellular level of PGE2 is regulated by NAD+-dependent 15-hydroxyprostaglandin dehydrogenase (15-PGDH). This enzyme catalyzes the oxidation of the 15(S)-hydroxyl group of PGE2 to generate 15-keto PGE2. Down-regulation of 15-PGDH was observed in various malignancies and overexpression of 15-PGDH has been known to suppress the carcinogenic processes. However, mechanisms underlying anti-carcinogenic effects and tumor suppressive functions of 15-keto PGE2 are poorly understood in various cancer. H-Ras transformed mammary epithelial MCF10A-ras cells show enhanced expression of cyclooxygenase-2 and activation of signal transducer and activator of transcription 3 (STAT3) and down-regulation of 15-PGDH compared to normal mammary epithelial MCF10A cells. STAT 3 plays a key role in growth of cancer cells as well as inflammation-associated carcinogenesis. This prompted us to investigate the effect of 15-keto PGE2 on STAT3 activation in MCF10A-ras cells. 15-Keto PGE2 suppressed the phosphorylation of STAT3 in MCF10A-ras cells in concentration- and time-dependent manners as determined by Western blot and immunocytochemical analyses. 15-Keto PGE2 also inhibited the dimerization of STAT3 which led to suppression of nuclear translocation and transcriptional activity of STAT3 as determined by Western blot analysis and the luciferase assay, respectively. 15-keto PGE2 suppressed the expression of cyclin D1. In addition, 15-keto PGE2 inhibited the growth of MCF10A-ras cells as determined by anchorage-independent growth and colony forming assays. However, a non-electrophilic analogue 13,14-dihydro-15-keto PGE2 which lacks the α,β-unsaturated carbonyl moiety failed to inhibit the phosphorylation, dimerization, and nuclear translocation of STAT3. Moreover, 13,14-dihydro-15-keto PGE2 did not affect the growth of MCF10A ras cells. These findings suggest that the α,β-unsaturated carbonyl moiety of 15-keto PGE2 is essential for its suppression of STAT3 signaling. STAT3 has several cysteine residues in its activation sites. We observed that reducing agents N-acetyl-L-cysteine and dithiothreitol abrogated the suppressive effect of 15-keto PGE2 on STAT3 phosphorylation, suggesting that the involvement of the thiol modification. Biotinylated 15-keto PGE2 covalently bound to STAT3 as determined by immunoprecipitation with STAT3 followed by immunoblotted with streptavidin.