Signal transducer and activator of transcription 3 (STAT3) is a transcription factor that regulates the expression of genes related to cell cycle, cell survival, and immune response associated with cancer progression and malignancy in a number of cancer types. Once activated, STAT3 forms a homodimer and translocates to the nucleus where it binds DNA promoting the translation of target genes associated with antiapoptosis, angiogenesis, and invasion/migration. In normal cells, levels of activated STAT3 remain transient; however, STAT3 remains constitutively active in approximately 70% of human solid tumors. The pivotal role of STAT3 in tumor progression has promoted a campaign in drug discovery to identify small molecules that disrupt the function of STAT3. A range of approaches have been used to identify novel small molecule inhibitors of STAT3, including high-throughput screening of chemical libraries, computational-based virtual screening, and fragment-based design strategies. The most common approaches in targeting STAT3 activity are either via the inhibition of tyrosine kinases capable of phosphorylating and thereby activating STAT3 or by preventing the formation of functional STAT3 dimers through disruption of the SH2 domains. However, the targeting of the STAT3 DNA-binding domain and disruption of binding of STAT3 to its DNA promoter have not been thoroughly examined, mainly due to the lack of adequate assay systems. This review summarizes the development of STAT3 inhibitors organized by the approach used to inhibit STAT3, the current inhibitors of each class, and the assay systems used to evaluate STAT3 inhibition and offers an insight into future approaches for small molecule STAT3 inhibitor development.

Ependymoma (EPN) in childhood is a brain tumor with substantial mortality. Inflammatory response has been identified as a molecular signature of high-risk Group A EPN. To better understand the biology of this phenotype and aid therapeutic development, transcriptomic data from Group A and B EPN patient tumor samples, and additional malignant and normal brain data, were analyzed to identify the mechanism underlying EPN Group A inflammation. Enrichment of interleukin-6 (IL6) and STAT3 pathway genes were found to distinguish Group A EPN from Group B EPN and other brain tumors, implicating an IL6 activation of STAT3 mechanism. EPN tumor cell growth was shown to be dependent on STAT3 activity, as demonstrated using shRNA knockdown and pharmacologic inhibition of STAT3 that blocked proliferation and induced apoptosis. The inflammatory factors secreted by EPN tumor cells were shown to reprogram myeloid cells and this paracrine effect was characterized by a significant increase in pSTAT3 and IL8 secretion. Myeloid polarization was shown to be dependent on tumor secretion of IL6, and these effects could be reversed using IL6 neutralizing antibody or IL6 receptor-targeted therapeutic antibody tocilizumab. Polarized myeloid cell production of IL8 drove un-polarized myeloid cells to up-regulate CD163 and to produce a number of pro-inflammatory cytokines. Collectively, these findings indicate that constitutive IL6/STAT3 pathway activation is important in driving tumor growth and inflammatory crosstalk with myeloid cells within the Group A EPN microenvironment. Effective design of Group A-targeted therapy for children with EPN may require reversal of this potentially immunosuppressive and pro-tumor pathway.

AML is characterized by the presence of leukemia stem cells (LSCs), and failure to fully eradicate this population contributes to disease persistence/relapse. Prior studies have characterized metabolic vulnerabilities of LSCs, which demonstrate preferential reliance on oxidative phosphorylation (OXPHOS) for energy metabolism and survival. In the present study, using both genetic and pharmacologic strategies in primary human AML specimens, we show that signal transducer and activator of transcription 3 (STAT3) mediates OXPHOS in LSCs. STAT3 regulates AML-specific expression of MYC, which in turn controls transcription of the neutral amino acid transporter SLC1A5. We show that genetic inhibition of MYC or SLC1A5 acts to phenocopy the impairment of OXPHOS observed with STAT3 inhibition, thereby establishing this axis as a regulatory mechanism linking STAT3 to energy metabolism. Inhibition of SLC1A5 reduces intracellular levels of glutamine, glutathione and multiple TCA metabolites, leading to reduced TCA cycle activity and inhibition of OXPHOS. Based on these findings, we used a novel small molecule STAT3 inhibitor, that binds STAT3 and disrupts STAT3-DNA, to evaluate the biological role of STAT3. We show that STAT3 inhibition selectively leads to cell death in AML stem and progenitor cells derived from newly diagnosed and relapsed patients, while sparing normal hematopoietic cells. Together, these findings establish a STAT3-mediated mechanism that controls energy metabolism and survival in primitive AML cells.