Retinoic Acid-inducible Gene-I Increases Interferon-stimulated Gene 15 Expression in Human Renal Proximal Tubule...

Viral infections in the kidney activate innate immunity via double-stranded RNA (dsRNA) sensors such as retinoic acid-inducible gene-I (RIG-I). This induces the expression of interferons and interferon-stimulated genes (ISGs), including ISG15. Similarly to ubiquitin, ISG15 functions by binding to target proteins and exerting antiviral effects through ISGylation. ISG15 is secreted extracellularly and exerts antiviral effects. Ubiquitin-like modifier-activating enzyme 7 (UBA7) initiates ISGylation, whereas ubiquitin-specific protease 18 (USP18) removes ISG15 from conjugated proteins. Both RIG-I and ISG15 are involved in antiviral responses and renal fibrosis. However, their interaction during kidney inflammation remains unclear.

Primary human renal proximal tubule epithelial cells (hRPTECs) were stimulated with polyinosinic polycytidylic acid [poly(I:C)] to mimic viral dsRNA. The mRNA and protein levels were analyzed using RT-qPCR, western blotting, or ELISA.

Poly(I:C) upregulated the mRNA and protein expression of RIG-I, ISG15, UBA7, and USP18. RIG-I, ISG15, and UBA7 levels increased over time, whereas USP18 levels decreased rapidly. UBA7 knockdown reduced ISGylation, whereas USP18 knockdown enhanced it. Silencing RIG-I decreased ISG15 conjugates, extracellular ISG15, and protein levels of UBA7 and USP18.

RIG-I promotes ISGylation by modulating UBA7, USP18, and ISG15 in renal proximal tubule epithelial cells. RIG-I may help maintain ISGylation homeostasis by balancing the activity of these molecules and preventing the excessive accumulation of free ISG15 or ISGylated proteins. These findings highlight the dual role of RIG-I in antiviral defense and its potential contribution to renal fibrosis, thereby providing insights into therapeutic strategies to balance immunity and kidney protection.