Visit our Expo - Redox and Inflammation signaling 2012

Signaling pathways leading to the activation of the interferon antiviral response
John Hiscott, Molecular Oncology Group
Lady Davis Institute –McGill University, Montreal Canada H3T 1E2
IRF-3 and IRF-7 transcription factors are essential for the induction of type I interferon (IFN)
and development of the innate antiviral response. Retinoic acid inducible gene I (RIG-I)
contributes to virus-induced IFN production independent of the Toll-like receptor pathways in
response to RNA viruses and dsRNA. We recently demonstrated that the NF-kB inducible,
anti-apoptotic protein A20 functioned as a negative regulator of RIG-I and blocked RIG-Imediated
NF-kB and IRF gene activation, but only weakly interfered with TLR-3 mediated
IFN activation. Expression of A20 completely blocked CARD domain containing DRIG-Iinduced
IRF-3 Ser396 phosphorylation, homodimerization and DNA binding. The level of
A20 inhibition was upstream of the TBK1/IKKe kinases that phosphorylate IRF3 and IRF7
but downstream of RIG-I itself, since RIG-I was not a target for the ubiquitin-editing
functions of A20.
To identify the A20 target, a BLAST search was performed that identified an uncharacterized
family of proteins – including the prototypical KIAA1271 – that contains a single CARD-like
domain at the N-terminus. Coexpression of KIAA1271 strongly activated IRF and NF-kB
dependent promoters and appeared to be the adapter that links RIG-I sensing to downstream
signaling through the TBK1/IKKe kinases. Four independent groups demonstrated that
MAVS/VISA/IPS-1/CARDIF acts downstream of RIG-I and stimulates the expression of
IFNb through the activation of NF-kB and IRF-3. MAVS localizes to the mitochondria via a
C-terminal transmembrane domain that is required for its function. Within the context of
hepatitis C virus infection, KIAA1271 appears to be the direct target of the HCV NS3/4A
protease; NS3/4A strongly inhibited KIAA1271-mediated activation of IFNB promoter and a
mutated form of KIAA1271, KIAA1271(C508A) which altered the site of NS3/4A proteolytic
cleavage was completely resistant to cleavage. Cleavage of KIAA1271 by NS3/4A disrupts
the C-terminal sequence involved in mitochondrial localization of the adapter. The
characterization of MAVS/KIAA1271 provides the first link between mitochondria and the
innate immune response.
How cross-talk between the mitochondrial apoptotic pathways and the innate response is
achieved remains unknown. Using vesicular stomatitis virus as a model, we demonstrated that
the intrinsic apoptotic cascade, specifically the BAX-BCL-2-Caspase-9 cascade, was the
primary pathway of VSV-induced apoptosis. Cell death was significantly reduced in BaxBak -/-
murine embryonic fibroblasts (MEFs). Conversely, virus replication was enhanced in the
BaxBak -/- MEFs compared to wild-type cells. Accompanying increased viral replication,
expression of antiviral and cytokine genes was also attenuated in infected BaxBak -/- MEFs.
BAX but not BAK was required for IRF-3 phosphorylation and DNA binding, thus
substantiating a role for apoptotic signaling in the activation of innate immune response.
Therefore, virus-induced apoptosis through a BAX-dependent mitochondrial pathway
represents an early signal for the initiation of IFN antiviral response.
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