Ethacrynic acid inhibits STAT3 activity through the modulation of SHP2 and PTP1B tyrosine phosphatases in DU145 prostate carcinoma cells.

To identify signal transducer and activator of transcription factor 3 (STAT3) inhibitors, we generated STAT3-dependent gene expression signature by analyzing gene expression profiles of DU145 cancer cells treated with STAT3 inhibitor, piperlongumine and 2-hydroxycinnamaldehyde. Then we explored gene expression signature-based strategies using a connectivity map database and identified several STAT3 inhibitors, including ethacrynic acid (EA). EA is currently used as a diuretic drug. EA inhibited STAT3 activation in DU145 prostate cancer cells and consequently decreased the levels of STAT3 target genes such as cyclin A and MCL-1. Furthermore, EA treatment inhibited tumor growth in mice xenografted with DU145 cells and decreased p-STAT3 expression in tumor tissues. Knockdown of Src homology region 2 domain-containing phosphatase-2 (SHP2) or Protein tyrosine phosphatase 1B (PTP1B) gene expression by siRNA suppressed the ability of EA to inhibit STAT3 activation. When EA was combined with an activator of SHP2 or PTP1B, p-STAT3 expression was synergistically decreased; when EA was combined with an inhibitor of SHP2 or PTP1B, p-STAT3 expression was rescued. By using an affinity pulldown assay with biotinyl-EA, EA was shown to associate with SHP2 and PTP1B in vitro. Additionally, the drug affinity responsive target stability (DARTS) assay confirmed the direct binding of EA to SHP2 and PTP1B. SHP2 is activated by EA through active phosphorylation at Y580 and direct binding to SHP2. Collectively, our results suggest that EA inhibits STAT3 activity through the modulation of phosphatases such as SHP2 and PTP1B and may be a potential anticancer drug to target STAT3 in cancer progression.