Biomedical Engineering Reference
In-Depth Information
While site-specific phosphorylation and acetylation of p65 regulate many dif-
ferent functions of NF-
κ
B, acetylation of p65 is, in fact, regulated by prior phos-
phorylation of NF-
B. How is this linkage established? Phosphorylation of p65 on
either serines 276 or 536 enhances the interaction of p65 with p300/CBP [38,51],
providing a potential link between phosphorylation and acetylation. Indeed, blockade
of phosphorylation of serine 276 or 536 of p65 reduces acetylation of lysine 310
[38]. Although p65 is phosphorylated both in the cytoplasm and in the nucleus, it
appears to be acetylated only in the nucleus, likely reflecting the principally nuclear
localization of the relevant acetyltransferases. Consistent with a contingent relation-
ship between phosphorylation and acetylation, phosphorylation of p65 is detected
within 5 minutes after TNF-
κ
stimulation, while acetylation is not detected until 10
minutes after stimulation. Whether phosphorylation of p65 on other serines similarly
regulates acetylation is uncertain. However, phosphorylation of serine 311 is asso-
ciated with improved binding of p300/CBP to p65 [17], and thus a link to subsequent
acetylation of p65 appears likely.
α
5.3.2
A
CETYLATION
OF
p50
B undergoes acetylation at three sites: lysines 431, 440, and
441. As with p65, these acetylations are stimulus-coupled. For example, addition of
TNF-
The p50 subunit of NF-
κ
or LPS consistently induces the acetylation of p50 [77,78]. These posttrans-
lational modifications lead to improved DNA binding by p50 and enhanced transcrip-
tional activity. For example, acetylation of p50 increases NF-
α
B-dependent transcrip-
tion from the HIV LTR and enhances the expression of several NF-
κ
κ
B target genes,
including cyclooxygenase and nitric oxide synthase in TNF-
or LPS-stimulated cells
[77,78,79]. It is currently unknown if other members of the NF-
α
B/Rel family are
regulated by acetylation. Interestingly, except for lysine 310 in p65, the other lysine
residues targeted for acetylation in p50 and p65 are highly conserved in all NF-
κ
B/Rel
family members, including the Dorsal protein of
Drosophila
. Thus, acetylation may
play a more general role in regulating the activity of the entire family of Rel proteins.
κ
5.4
HISTONE ACETYLATION REGULATES NF-
κ
B
GENE ACTIVATION
In addition to direct posttranslational modification of p65, stimulus-coupled acety-
lation of histones present in chromatin surrounding NF-
B target genes is likely
important in governing the overall transcriptional response. In eukaryotic cells,
DNA is packaged into chromatin with varying degrees of compaction. The funda-
mental structural units of chromatin, the nucleosomes, are composed of histone
octamers, with a central heterotetramer of histones H3 and H4 and two het-
erodimers of histones H2A and H2B. Genes residing in densely compacted het-
erochromatin are maintained in a transcriptionally inactive state, likely because
the promoters are inaccessible to RNA polymerase II (polII) and other cofactors
[80]. To promote increased accessibility of various promoters to components of
the cell's transcription and replication machinery, chromatin remodeling must
κ
