Cytosolic DNA Promotes Signal Transducer and Activator of Transcription 3 (STAT3) Phosphorylation by TANK-binding Kinase 1 (TBK1) to Restrain STAT3 Activity

Cytosolic DNA can elicit beneficial as well as undesirable immune responses. For example, viral or microbial DNA triggers cell-intrinsic immune responses to defend against infections, whereas aberrant cytosolic accumulation of self-DNA results in pathological conditions, such as autoimmunity. Given the importance of these DNA-provoked responses, a better understanding of their molecular mechanisms is needed. Cytosolic DNA engages stimulator of interferon genes (STING) to activate TANK-binding kinase 1 (TBK1), which subsequently phosphorylates the transcription factor interferon regulatory factor 3 (IRF3) to promote interferon expression. Recent studies have reported that additional transcription factors, including nuclear factor kappa B (NF-kappa B) and signal transducer and activator of transcription 6 (STAT6), are also activated by cytosolic DNA, suggesting that cytosolic DNA-induced gene expression is orchestrated by multiple factors. Here we show that cytosolic DNA activates STAT3, another member of the STAT family, via an autocrine mechanism involving interferon beta (IFN beta) and IL-6. Additionally, we observed a novel cytosolic DNA-induced phosphorylation at serine 754 in the transactivation domain of STAT3. Upon cytosolic DNA stimulation, Ser(754) is directly phosphorylated by TBK1 in a STING-dependent manner. Moreover, Ser(754) phosphorylation inhibits cytosolic DNA-induced STAT3 transcriptional activity and selectively reduces STAT3 target genes that are up-regulated in response to cytosolic DNA. Taken together, our results suggest that cytosolic DNA-induced STAT3 activation via IFN beta and IL-6 is restrained by Ser(754) phosphorylation of STAT3. Our findings reveal a new signaling axis downstream of the cytosolic DNA pathway and suggest potential interactions between innate immune responses and STAT3-driven oncogenic pathways.