Biomedical Engineering Reference
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type information regulation 2 homolog 1 [SIRT1]), the mammalian homologue
of the yeast silencing information regulator (SIR)-2, physically interacts with p65
and inhibits transcription by deacetylating p65 [57]. The association of NF-
κ
B
with HDAC1 and HDAC2 inhibits the expression of many NF-
B-regulated genes,
including the IL-8 gene [56]. In contrast to HDAC3 and SIRT1, HDAC1 and
HDAC2 appear to operate solely by promoting changes in chromatin structure
rather than through deacetylation of NF-
κ
B. Silencing mediator for retinoid and
thyroid hormone receptor (SMRT) and the nuclear corepressors (NCoRs) also
bind to NF-
κ
B and may in fact form a bridge for the binding of HDAC3 and
SIRT1 [58,59,60,61].
As noted, coactivators and corepressors regulate gene expression by modifying
the N-terminal tails of histones through their HAT or HDAC activities [62]. Acety-
lated histones are associated with transcriptionally active regions of chromatin, while
deacetylated histones are associated with repressive chromatin [63,64,65]. In addi-
tion to targeting histones for modification, the HATs and HDACs also directly
regulate the acetylation status of a variety of transcription factors, including p53,
GATA-1, MyoD, Stat3, and E2F. Acetylation has been implicated in the regulation
of many functions of these transcription factors, including dimerization, subcellular
localization, assembly with DNA, interaction with other cofactors, and overall tran-
scriptional activity [66,67,68,69,70].
κ
5.3.1
A
CETYLATION
OF
p65
B. Acetylation
of p65 has now been demonstrated both
in vivo
and
in vitro
[38,52]. Endogenous
p65 is acetylated in a stimulus-coupled manner — after activation of cells with
TNF-
Acetylation plays a key role in modulating the nuclear action of NF-
κ
or PMA, for example. The p300/CBP and PCAF acetyltransferases are of
particular importance in the acetylation of p65 both
in vivo
and
in vitro
[38,52,71].
Like the histone proteins, acetylated p65 is subject to deacetylation by HDACs. In
the case of p65, HDAC3 and SIRT1 mediate this function in association with the
NCoR/SMRT complex [52,57,61,71].
Three major acetylation sites have been identified within p65: lysines 218, 221,
α
and 310 (
Figure 5.2
)
[72]. Modification of these three lysines induces specific
changes in the action of NF-
B. For example, acetylation of lysine 221 enhances
the DNA binding properties of NF-
κ
α
in conjuction with the acetylation of lysine 218. Conversely, acetylation of lysine
310 is required for full transcriptional activity of p65 but exerts no effect on its DNA
binding or I
κ
B and impairs the assembly of p65 with I
κ
B
assembly properties. More importantly, acetylation of lysine 310
is required to activate endogenous NF-
κ
B
α
-induced
expression of the E-selectin gene is markedly lower in p65-deficient cells reconsti-
tuted with p65-K310R than in cells reconstituted with wild-type p65 [38]. Further-
more, deacetylation of lysine 310 by SIRT1 correlates with a loss of NF-
κ
B genes. For example, TNF-
α
κ
B-regulated
gene expression and sensitization of cells to TNF-
-induced apoptosis [57].
Lysines
122 and 123 are also subject to acetylation by p300/CBP and PCAF, although these
modifications reduce both NF-
α
B-
enhancer DNA (Figure 5.2) [71]. It is interesting to consider the possibility that
κ
B-mediated transcription and binding of p65 to
κ
