Signal transducer and activator of transcription 3 (STAT3) plays crucial roles in multiple biological processes, such as cell proliferation, migration, and inflammation. Elevated STAT3 activity is found in many types of cancer. In a screen for non-translational targets of threonyl-tRNA synthetase (ThrRS), we found in HEK293 cells STAT3 phosphorylation at Y705, both at steady-state and stimulated by IL-6, to be reduced by ThrRS knockdown. Conversely, overexpression of ThrRS enhanced STAT3 phosphorylation. Consistently, nuclear translocation and transcriptional activity of STAT3 were also inhibited by ThrRS knockdown and activated by ThrRS overexpression. This function of ThrRS was independent of its canonical activity of aminoacylation. In searching for a molecular mechanism of this novel regulation, we found that ThrRS physically interacted with both STAT3 and JAK1/2, and that knockdown of ThrRS dampened the interaction between STAT3 and JAK1/2. Furthermore, STAT3 and JAK1/2 interacted with the aminoacylation domain and the editing domain of ThrRS, respectively. Taken together, our observations suggest that ThrRS may serve to recruit STAT3 to JAK for activation. Finally, elevated ThrRS expression was found to accompany hyperactivity of STAT3 in breast cancer cells, and knockdown of ThrRS reduced STAT3 phosphorylation and suppressed cancer cell proliferation. In conclusion, we have identified ThrRS as a non-canonical regulator of STAT3 activation and a potential anti-cancer therapeutic target.
