Identifying Therapeutic Vulnerabilities in Cancers Driven by the Oncogenic Transcription Factor STAT3

Abstract

Cancer is characterized by aberrant gene expression patterns that alter cellular function. Such alterations are commonly caused by the inappropriate activation of transcription factors. In particular, STAT3 is a key transcriptional regulator of many pro-tumorigenic processes, including survival, proliferation, and differentiation, and is persistently activated in many types of human cancer. However, like many transcription factors, STAT3 has proven difficult to target clinically. To address this unmet clinical need, we developed a cell-based assay of STAT3 transcriptional activity and performed an unbiased, high-throughput screen of a library of 1,120 small molecules known to be biologically active in humans. We identified the antimicrobial drug pyrimethamine as a novel and specific inhibitor of STAT3 transcriptional activity at concentrations known to be safely achieved in humans. Intriguingly, at concentrations sufficient to inhibit STAT3 transcriptional activity, pyrimethamine does not affect STAT3 phosphorylation, nuclear translocation, or DNA binding. Accordingly, we hypothesized that pyrimethamine inhibits STAT3 transcriptional activity via a unique mechanism. To evaluate that possibility, we first developed and characterized a pyrimethamine-resistant cell line to reveal potential mechanisms of resistance that may shed light on pyrimethamine-mediated STAT3 inhibition. We found that loss of the transcriptional regulator BCL6 leads to pyrimethamine resistance, suggesting that pyrimethamine disrupts a functional interaction between BCL6 and STAT3 required for STAT3 transcriptional activity. Second, we performed a new quantitative profiling approach called the proteome integral stability alteration (PISA) assay to identify direct molecular targets of pyrimethamine that may play a role in regulating STAT3 transcriptional activity. We identified human dihydrofolate reductase (DHFR) as one of the targets responsible for the STAT3-inhibitory effects of pyrimethamine, implicating folate metabolism in the regulation of STAT3 transcriptional activity. Together, these findings reveal two regulatory nodes of STAT3 activation that may be important for the development of novel strategies to treat STAT3-driven cancers.