15-Keto prostaglandin E2 suppresses STAT3 signaling and inhibits breast cancer cell growth and progression

Overproduction of prostaglandin E-2 (PGE(2)) has been linked to enhanced tumor cell proliferation, invasiveness and metastasis as well as resistance to apoptosis. 15-Keto prostaglandin E-2 (15-keto PGE(2)), a product formed from 15-hydroxyprostaglandin dehydrogenase-catalyzed oxidation of PGE(2,) has recently been shown to have anti-inflammatory and anticarcinogenic activities. In this study, we observed that 15-keto PGE(2) suppressed the phosphorylation, dimerization and nuclear translocation of signal transducer and activator of transcription 3 (STAT3) in human mammary epithelial cells transfected with H-ras (MCF10A-ras). 15-Keto PGE(2) inhibited the migration and clonogenicity of MCF10A-ras cells. In addition, subcutaneous injection of 15-keto PGE(2) attenuated xenograft tumor growth and phosphorylation of STAT3 induced by breast cancer MDA-MB-231 cells. However, a non-electrophilic analogue, 13,14-dihydro-15-keto PGE(2) failed to inhibit STAT3 signaling and was unable to suppress the growth and transformation of MCF10A-ras cells. These findings suggest that the alpha,beta-unsaturated carbonyl moiety of 15-keto PGE(2) is essential for its suppression of STAT3 signaling. We observed that the thiol reducing agent, dithiothreitol abrogated 15-keto PGE(2)-induced STAT3 inactivation and disrupted the direct interaction between 15-keto PGE(2) and STAT3. Furthermore, a molecular docking analysis suggested that Cys251 and Cys259 residues of STAT3 could be preferential binding sites for this lipid mediator. Mass spectral analysis revealed the covalent modification of recombinant STAT3 by 15-keto PGE(2) at Cys259. Taken together, thiol modification of STAT3 by 15-keto PGE(2) inactivates STAT3 which may account for its suppression of breast cancer cell proliferation and progression.