A requirement of STAT3 DNA binding precludes Th-1 immunostimulatory gene expression by NF-κB in tumors

Both STAT3 and NF-κB are persistently activated in diverse cancers and promote tumor cell proliferation, survival, angiogenesis, and metastasis through transcriptional activation of multiple common genes. Paradoxically, STAT3 also suppresses many NF-κB-inducible genes involved in innate and adaptive antitumor immunity in spite of elevated levels of NF-κB in tumors. In this study, we show that expression of many NF-κB downstream target genes in tumors depends on STAT3 DNA binding. When STAT3 is elevated in tumor cells and tumor-infiltrating immune cells, persistently activated NF-κB interacts with STAT3 and preferentially binds to genes with STAT3-binding site(s) in promoters. A large number of NF-κB downstream genes associated with oncogenesis and chronic inflammation contain STAT3 DNA-binding site(s). However, in contrast, many genes frequently associated with antitumor immunity lack STAT3 DNA-binding site(s) and can only be activated by NF-κB when STAT3 is inhibited in tumors. The introduction of STAT3 DNA-binding sequences by site-specific mutagenesis in an immunostimulatory gene promoter allows its transcriptional activation by NF-κB in tumor cells. Furthermore, STAT3 facilitates NF-κB binding to genes that are important for tumor growth while inhibiting its binding to Th-1 immunostimulatory genes in growing tumors, including in tumor-infiltrating immune cells. The results of this study provide insight into how some of the oncogenic/inflammatory and Th-1 immunostimulatory genes are differentially regulated in cancer.