The activation of pro-inflammatory gene programs by
nuclear factor-kappaB (
NF-kappaB) is primarily regulated through cytoplasmic sequestration of
NF-kappaB by the inhibitor of kappaB (IkappaB) family of
proteins.
IkappaBbeta, a major
isoform of IkappaB, can sequester
NF-kappaB in the cytoplasm, although its
biological role remains unclear. Although cells lacking
IkappaBbeta have been reported, in vivo studies have been limited and suggested redundancy between
IkappaBalpha and
IkappaBbeta. Like
IkappaBalpha,
IkappaBbeta is also inducibly degraded; however, upon stimulation by
lipopolysaccharide (LPS), it is degraded slowly and re-synthesized as a hypophosphorylated form that can be detected in the nucleus. The crystal structure of
IkappaBbeta bound to p65 suggested this complex might bind
DNA. In vitro, hypophosphorylated
IkappaBbeta can bind
DNA with p65 and c-Rel, and the
DNA-bound
NF-kappaB:
IkappaBbeta complexes are resistant to
IkappaBalpha, suggesting hypophosphorylated, nuclear
IkappaBbeta may prolong the expression of certain genes. Here we report that in vivo
IkappaBbeta serves both to inhibit and facilitate the inflammatory response.
IkappaBbeta degradation releases
NF-kappaB dimers which upregulate pro-inflammatory target genes such as tumour
necrosis factor-alpha (
TNF-alpha). Surprisingly, absence of
IkappaBbeta results in a dramatic reduction of
TNF-alpha in response to LPS even though activation of
NF-kappaB is normal. The inhibition of
TNF-alpha messenger RNA (
mRNA) expression correlates with the absence of nuclear, hypophosphorylated-
IkappaBbeta bound to p65:c-Rel heterodimers at a specific kappaB site on the
TNF-alpha promoter. Therefore
IkappaBbeta acts through p65:c-Rel dimers to maintain prolonged expression of
TNF-alpha. As a result,
IkappaBbeta(-/-) mice are resistant to LPS-induced
septic shock and
collagen-induced arthritis. Blocking
IkappaBbeta might be a promising new strategy for selectively inhibiting the chronic phase of
TNF-alpha production during the inflammatory response.