RIG -I regulates myeloid differentiation by promoting TRIM25-mediated ISGylation

Retinoic acid -inducible gene I (RIG -I) is up -regulated during granu- locytic differentiation of acute promyelocytic leukemia (APL) cells induced by all - trans retinoic acid (ATRA). It has been reported that RIG -I recognizes virus -specific 5 ?-ppp-double-stranded RNA (dsRNA) and activates the type I interferons signaling pathways in innate immunity. However, the functions of RIG -I in hematopoiesis re- main unclear, especially regarding its possible interaction with endogenous RNAs and the associated pathways that could con- tribute to the cellular differentiation and maturation. Herein, we identified a number of RIG-I -binding endogenous RNAs in APL cells following ATRA treatment, including the tripartite motif - containing protein 25 ( TRIM25 ) messenger RNA (mRNA). TRIM25 encodes the protein known as an E3 ligase for ubiquitin/interferon (IFN)-induced 15-kDa protein (ISG15) that is involved in RIG- I -mediated antiviral signaling. We show that RIG -I could bind TRIM25 mRNA via its helicase domain and C -terminal regulatory domain, enhancing the stability of TRIM25 transcripts. RIG -I could increase the transcriptional expression of TRIM25 by caspase re- cruitment domain (CARD) domain through an IFN-stimulated re- sponse element. In addition, RIG -I activated other key genes in the ISGylation pathway by activating signal transducer and acti- vator of transcription 1 (STAT1), including the modifier ISG15 and several enzymes responsible for the conjugation of ISG15 to pro- tein substrates. RIG -I cooperated with STAT1/2 and interferon reg- ulatory factor 1 (IRF1) to promote the activation of the ISGylation pathway. The integrity of ISGylation in ATRA or RIG-I -induced cell differentiation was essential given that knockdown of TRIM25 or ISG15 resulted in significant inhibition of this process. Our results provide insight into the role of the RIG-I-TRIM25-ISGylation axis in myeloid differentiation.