HCV NS5A and IRF9 compete for CypA binding

Abstract

Background Cyclophilin A (CypA) is vital for HCV replication. Cyp inhibitors successfully decrease viral loads in HCV-infected patients. However, their mechanisms of action remain unknown. Since interferon (IFN) can also suppress HCV replication, we asked whether a link between CypA and the IFN response exists. Methods We used cellular and recombinant pulldown approaches to investigate the possibility of a specific association of CypA with host ligands. Results We found for the first time that CypA binds to a major component of the IFN response – the IFN regulatory factor 9 (IRF9). IRF9 is the DNA-binding component of the transcriptional IFN-stimulated gene factor 3 (ISGF3). CypA binds directly IRF9 via its peptidyl-prolyl isomerase (PPIase) pocket. Cyp inhibitors such as cyclosporine A (CsA) or non-immunosuppressive derivates such as alisporivir and SCY-635, prevent IRF9-CypA complex formation. CypA binds to the C-terminal IRF-association-domain (IAD), but not to the DNA-binding or linker domains of IRF9. Remarkably, CypA associates with the multimeric ISGF3 complex. We also obtained evidence that CypA neutralization enhances IFN-induced transcription. Interestingly, the hepatitis C virus (HCV) nonstructural 5A (NS5A) protein, which is known to modulate the IFN response, competes with IRF9 for CypA binding and can prevent the formation of IRF9-CypA complexes. Conclusions This study demonstrates for the first time that CypA binds specifically to a component of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway - IRF9. This study also reveals a novel opportunity of HCV to modulate the IFN response via NS5A.

Interferon Regulatory Factor 9 Protects Against Cardiac Hypertrophy by Targeting Myocardin

“…The N terminus of IRF9 is the DNA-binding domain, which is characterized by 5 tryptophan repeats and is highly conserved within the IRF family. 30 The pro-domain corresponds to a linker region, 13,40 which is reported to be the PPARα-binding domain. 13 Only the TAD of myocardin contributes to the association with IRF9.…”

Profile of alisporivir and its potential in the treatment of hepatitis C

“…Previous work has demonstrated that NS5A possesses the ability to counteract the innate response to permit establishment of HCV replication 47. A very recent study demonstrated that cyclophilin A binds both NS5A and the IFN-regulatory factors, especially IRF9, and that alisporivir blocks all these interactions 48. Thus, one can envisage that contact between cyclophilin A and NS5A and/or specific IFN-regulatory factors modulates the innate response.…”

Cyclophilins as Modulators of Viral Replication

“…These results demonstrated that CyPA indeed plays an additional role at the assembly step of HCV life cycle. A recent report showed that CyPA could also interact with the interferon (IFN) regulatory factor 9 (IRF9) [108] in vitro . However, because previous studies have demonstrated that CsA functions through an IFN-independent mechanism [95,99], the significance of the CyPA-IRF9 interaction remains unclear.…”