Interferon-Stimulated Gene 15, a Type I Interferon-Dependent Transcript, Is Involved in a Negative Feedback Loop in Innate Immune Reactions in Human Mesangial Cells

Background: Since innate immunity plays a pivotal role in the pathogenesis of glomerulonephritis, the activation of toll-like receptor (TLR) 3/type I interferon (IFN) cascades is important in glomerular inflammation. However, the role of IFN-stimulated gene 15 (ISG15), a type IFN-dependent transcript, in glomerular inflammation is unclear. We, therefore, examined the role of ISG15 in innate immune reactions induced by TLR3 signaling in cultured human mesangial cells (MCs).

Methods: We treated MCs with polyinosinic-polycytidylic acid (poly IC), an authentic double-stranded RNA, and analyzed the ISG15 expression by reverse transcription-polymerase chain reaction and western blotting. To examine the regulation of ISG15 expression, we subjected MCs to RNA interference (siRNA) against TLR3, IFN-β, ISG56, and melanoma differentiation-associated gene 5 (MDA5).

Results: ISG15 expression induced by poly IC in MCs was inhibited by siRNA against TLR3 and IFN-β, whereas silencing of ISG56 or MDA5 had no effect. A knockdown of ISG15 upregulated the expression of ISG56, MDA5, CXCL10 and phosphorylated signal transducers and activators of transcription protein 1 (P-STAT1), while a knockdown of ubiquitin-like modifier activating enzyme 7, a key enzyme that conjugates ISG15 to target proteins, did not affect the expression. Knockdown of ubiquitin specific protease 18, an ISG15 isopeptidase, also upregulated P-STAT1, ISG56, MDA5 and CXCL10.

Conclusion: Since unconjugated free ISG15 negatively regulates the phosphorylation of STAT1 and its downstream reactions, ISG15 dysregulation may be involved in the pathogenesis of glomerular inflammation. We believe that suitable interventions in these innate immune cascades is desirable for the future therapeutic strategies for glomerulonephritis.