Tumor Suppressor p14ARF Enhances IFN-γ-Activated Immune Response by Inhibiting PIAS1 via SUMOylation

The ability of cells to induce the appropriate transcriptional response to inflammatory stimuli is crucial for the timely induction of host defense mechanisms. Although a role for tumor suppressor p14ARF (ARF) in the innate immune response was previously demonstrated, the underlying mechanism is still unclear. ARF is a potent upregulator of protein SUMOylation; however, no association of this function with the immune system has been made. In this study, we show the unique role of ARF in IFN-gamma-induced immune response using human cell lines. Through a systematic search of proteins SUMOylated by ARF, we identified PIAS1, an inhibitor of IFN-activated transcription factor STAT1, as a novel ARF-binding partner and SUMOylation target. In response to IFN-gamma treatment, ARF promoted PIAS1 SUMOylation to inhibit the ability of PIAS1 to attenuate IFN-gamma response. Wild-type, but not ARF mutants unable to enhance PIAS1 SUMOylation, prevented the PIAS1-mediated inhibition of IFN-gamma response. Conversely, the SUMO-deconjugase SENP1 deSUMOylated PIAS1 to reactivate PIAS1 that was inhibited by ARF. These findings suggest that PIAS1 function is negatively modulated by SUMO modification and that SUMOylation by ARF is required to inhibit PIAS1 activity and restore IFN-gamma-induced transcription. In the presence of ARF, in which case PIAS1 is inhibited, depletion of PIAS1 did not have an additive effect on IFN-gamma response, suggesting that ARF-mediated enhancement of IFN-gamma response is mainly due to PIAS1 inhibition. Our findings reveal a novel function of ARF to inhibit PIAS1 by enhancing SUMOylation to promote the robust induction of IFN-gamma response.