A small-molecule enhancer of signal transducer and activator of transcription 1 transcriptional activity accentuates the antiproliferative effects of IFN-gamma in human cancer cells.

The transcription factor signal transducer and activator of transcription (STAT) 1 can mediate antiproliferative and proapoptotic effects in cancer cells, and a number of mechanisms have been found whereby STAT1 signaling is attenuated in tumors thereby increasing their malignant behavior. Thus, enhancing gene transcription mediated by STAT1 may be an effective approach to cancer therapy. A high-throughput screen was developed to identify molecules that could enhance STAT1-dependent gene expression. Through this approach, it was found that 2-(1,8-naphthyridin-2-yl)phenol (2-NP) caused a 2-fold increase in STAT1-dependent reporter gene expression compared with that seen with maximally effective concentrations of IFN-gamma alone. This effect was specific to STAT1 because 2-NP had no effect on unrelated transcription factors such as nuclear factor (NF) kappaB or the highly homologous transcription factor STAT3. STAT1-dependent gene activation was enhanced by this compound in a variety of human and murine cell lines and was independent of the stimulus used. Furthermore, 2-NP enhanced the expression of the bona fide endogenous STAT1 target gene interferon regulatory factor 1. 2-NP increased the duration of STAT1 tyrosine phosphorylation in response to IFN-gamma, and this may underlie its enhancement of STAT1-dependent transcription. Reflecting the fact that STAT1 can exert tumor-suppressive effects, 2-NP enhanced the ability of IFN-gamma to inhibit the proliferation of human breast cancer and fibrosarcoma cells. Tumor cells lacking STAT1 were unaffected by either IFN-gamma or 2-NP. These findings indicate that enhancement of STAT1 transcriptional activity may have utility in anticancer therapies, and that cell-based screens for modulators of transcription factor function can be a useful approach for drug discovery.