Signal transducer and activator of transcription 3 (STAT3), which is activated by cytokines and growth factors, mediates biological actions in many physiological processes. In a previous study, we found that Y14, a core component of the exon-junction complex (EJC) bound to STAT3 and upregulated the transcriptional activity of STAT3 by influencing its DNA-binding activity. In the present study, we demonstrate that STAT3 endogenously interacts with Y14. In addition, we found that MAGOH, a Y14 partner in the EJC, inhibits the STAT3-Y14 complex formation.Furthermore, small-interfering RNA-mediated reduction of MAGOH expression enhanced interleukin-6-induced gene expression. These results indicate that MAGOH regulates the transcriptional activation of STAT3 by interfering complex formation between STAT3 and Y14.

Although Y14 is known to be a component of the exon junction complex, we previously reported that Y14 regulates IL-6–induced STAT3 activation. In this study, we showed that endogenous Y14 positively regulated TNF-α–induced IL-6 expression in HeLa cells. Small interfering RNA–mediated Y14-knockdown reduced TNF-α–induced and NF-κB–mediated transcriptional activity, phosphorylation/degradation of IκBα, and nuclear localization of NF-κB/p65. As in the case of IL-6 stimuli, Y14 enhanced TNF-α–induced STAT3 phosphorylation, which is important for its nuclear retention. However, our manipulation of Y14 expression indicated that it is involved in TNF-α–induced IL-6 expression via both STAT3-dependent and -independent mechanisms. We screened signaling molecules in the TNF-α–NF-κB pathway and found that Y14 endogenously associated with receptor-interacting protein 1 (RIP1) and TNFR-associated death domain (TRADD). Overexpression of RIP1, but not TRADD, restored TNF-α–induced NF-κB activation in Y14-knockdown cells, and Y14 overexpression restored TNF-α–induced NF-κB activation in TRADD-knockdown cells, but not in RIP1-knockdown cells, indicating that Y14 lies downstream of TRADD and upstream of RIP1. Of importance, Y14 significantly enhanced the binding between RIP1 and TRADD, and this is a possible new mechanism for Y14-mediated modification of TNF-α signals. Although Y14 associates with MAGOH in the exon junction complex, Y14’s actions in the TNF-α–NF-κB pathway are unlikely to require MAGOH. Therefore, Y14 positively regulates signals for TNF-α–induced IL-6 production at multiple steps beyond an exon junction complex protein.