Biomedical Engineering Reference
In-Depth Information
5.5
HISTONE PHOSPHORYLATION ACTIVATES NF-
κ
B
TARGET GENES
Phosphorylation is an important posttranslational modification of histones [91]. Like
histone acetylation, histone phosphorylation is associated with transcriptional acti-
vation. Indeed, phosphorylation of serine 10 of histone H3 is a strong predictor of
inducible gene expression [92,93,94,95,96]. For example, in mammalian cells, his-
tone H3 surrounding the c-fos promoter undergoes rapid phosphorylation during the
immediate-early response elicited by epidermal growth factor [95,97].
A strong link also exists between the phosphorylation of H3 and the activation
of NF-
B also
induce the phosphorylation of histone H3 on serine 10 [98]. Often, specific sites are
phosphorylated and acetylated in tandem. For example, LPS and TNF-
κ
B target genes. Many proinflammatory stimuli that activate NF-
κ
induce both
the phosphorylation of serine 10 in the N-terminal tail of histone H3 and the
acetylation of the adjacent lysine 14 in the chromatin surrounding various cytokine
and chemokine genes. Histone H3 phosphorylation similarly correlates with the
effective recruitment of NF-
α
B enhancers residing within the upstream
regulatory regions of the IL-6, IL-8, monocyte chemotactic protein (MCP), and
IL-12p40 genes [98]. Thus, NF-
κ
B to the
κ
B activation is associated with the effective recruit-
ment of both histone kinases and HATs to the promoter regions of target genes. The
resulting phosphorylation and acetylation of the surrounding histone tails creates an
environment that is favorable for the recruitment of RNA polII to these promoters
[98]. However, phosphorylation of histone H3 is not required for activation of the
promoter of every NF-
κ
κ
B target gene; activation of the NF-
κ
B-responsive TNF-
α
and MIP-1
genes, for example, is not associated with H3 phosphorylation [98].
Phosphorylation of histone H3 serine 10 may be correlated with the activation
of the p38 mitogen-activated protein kinase; however, the effect of p38 appears
indirect. Specifically, p38 is not recruited to the promoters of NF-
α
B target genes
and requires the subsequent activity of downstream kinases [98]; IKK
κ
α
may be one
of these kinases. IKK
is readily detected in the nucleus and promotes the phos-
phorylation of serine 10 of histone H3 both
in vivo
and
in vitro
[99,100]. Further,
the kinetics of serine 10 phosphorylation closely correlate with the recruitment of
IKK
α
α
likely functions as part of a larger nuclear complex containing p65 and CBP. All
three of these proteins are recruited to the IL-6 promoter as a unit after TNF-
α
to the promoter, as revealed in chromatin immunoprecipitation assays. IKK
α
stimulation. P65 may play a key nucleating role in the assembly of this complex
and in recruiting IKK
and CBP to specific promoters [99,100]. These results
suggest a novel nuclear function for IKK
α
in modifying histone function through
the phosphorylation of serine 10 in histone H3, which is critical for the activation
of many NF-
α
B-dependent target genes.
However, some redundancy in this phosphorylation event appears likely. Loss
of IKK
κ
expression is not associated with defective phosphorylation of serine 10
in histone H3 in the cIAP-2 and IL-8 promoter after treatment with laminin [61].
In addition, some NF-
α
κ
B target genes, such as I
κ
B
α
, are activated normally after
LPS stimulation in the absence of IKK
recruitment to the promoter [86]. Further-
more, cells from “knock-in” mice expressing a catalytically inactive form of IKK
α
α
