Identification of Immune-Responsive Gene 1 (IRG1) as a Target of A20

A20
is a negative regulator of NF-κB signaling; it controls
inflammatory responses and ensures tissue homeostasis. A20 is thought
to restrict NF-κB activation both by its ubiquitin-editing activity
as well as by its nonenzymatic activities. Besides its role in NF-κB
signaling, A20 also acts as a protective factor inhibiting apoptosis
and necroptosis. Because of the ability of A20 to both ubiquitinate
and deubiquitinate substrates, and its involvement in many cellular
processes, we hypothesized that deletion of A20 might generally impact
on protein levels, thereby disrupting cellular signaling. We performed
a differential proteomics study on bone marrow-derived macrophages
(BMDMs) from control and myeloid-specific A20 knockout mice, both
in untreated conditions and after LPS or TNF treatment, and demonstrated
A20-dependent changes in protein expression. Several inflammatory
proteins were found up-regulated in the absence of A20, even without
an inflammatory stimulus, but, depending on the treatment and the
treatment time, more proteins were found regulated. Together these
protein changes may affect normal signaling events, which may disturb
tissue homeostasis and induce (autoimmune) inflammation, in agreement
with A20s proposed identity as a susceptibility gene for inflammatory
disease. We further verify that immune-responsive gene 1 (IRG1) is
up-regulated in the absence of A20 and that its levels are transcriptionally
regulated.