Chemistry Reference
In-Depth Information
methylation, targeting either arginine (R) or lysine (K)
residues. Generally, arginine methylation is involved
in gene activation, and histone methyltransferases
(HMTs) are recruited to promoters as coactivators. By
comparison, methylation of the lysine residue can have
multiple effects on chromatin function, depending
on the specifi c lysine and the level of modifi cation
(mono-, di-, or trimethylation of a single lysine). For
instance, H3-K9 dimethylation and H3-K27 trimeth-
ylation are both largely associated with gene silencing
and heterochromatin formation, whereas methylation
of H3-K4, H3-K36, or H3-K79 is associated with active
chromatin (Maison and Almouzni, 2004; Peterson and
Laniel, 2004).
2004a). It has been shown that the levels of uH2A were
higher in transformed human fi broblasts and keratino-
cytes than in their normal counterparts (Vassilev
et al.
,
1995).
5.3 Impacts of Metal Compounds
on Epigenetics
5.3.1 As
Arsenite (As) has been shown to reduce global lev-
els of DNA methylation, as well as cause both DNA
hypomethylation and hypermethylation of specifi c
genes or DNA fragments. The status of histone acetyla-
tion and phosphorylation is also changed on As expo-
sure and has been associated with the activation of
genes.
It has been shown that chronic exposure of TRL 1215
(a rat liver epithelial cell line) to As (0.125-0.500
5.2.2.3 Histone Phosphorylation
The correlation between mitotic chromosome con-
densation and phosphorylation of histones H1 and
H3 is well established, although the specifi c function
of this modifi cation in chromosomes remains to be
determined. Physical studies have shown that phos-
phorylation disrupted histone-DNA interactions and
destabilized chromatin structure (Chadee
et al.
, 1995;
Hendzel
et al.
, 1997). Phosphorylation of a histone H2A
variant, H2A.X at serine (S) 139, increases during the
early stages of DNA fragmentation in apoptosis and
correlates with double-stranded DNA breaks (Gurley
et al.
, 1973). Recently, it has been reported that phos-
phorylation of histone H2B at S14 correlates with cells
undergoing apoptosis in vertebrates, which may serve
as a marker for apoptosis (Cheung
et al.
, 2003).
M
for more than 18 weeks) resulted in the depletion of
S-adenosyl-methionine (SAM), global DNA hypometh-
ylation, and malignant transformation. As-induced
DNA hypomethylation was a function of dose and
exposure duration and remained constant even after
withdrawal of As (Zhao
et al.
, 1997). Overexpression
of several oncogenes and genes regulating cell pro-
liferation were detected in the As-transformed cells,
including the protooncogene
c-myc
(Chen
et al.
, 2001).
Although the DNA methylation status on the promoter
of the
c-myc
gene was not examined, the transcription
of
c-myc
gene was increased by As in a dose- and time-
dependent manner and strongly correlated with levels
of global genomic DNA hypomethylation. Because As
is detoxifi ed by methylation using a methyltransferase
and SAM as the methyl donor, it is conceivable that
As metabolism may deplete intracellular methyl group
storage and thereby lead to DNA hypomethylation.
This, in turn, may facilitate aberrant gene expression
and play a role in As carcinogenesis (Chen
et al.
, 2001).
However, the data concerning DNA methylation altera-
tions after As exposure are ambiguous. By using quan-
titative PCR/
Hpa
II restriction site analysis to examine
DNA methylation status, it has been shown that expo-
sure of human lung adenocarcinoma A549 cells to
sodium arsenite (As III) (0.08-2
µ
5.2.2.4 Histone Ubiquitination
The most prevalent ubiquitinated histones are
monoubiquitinated-H2A and H2B (Nickel and Davie,
1989). Approximately 10% of total H2A and to a lesser
extent, 1-1.5% of H2B is ubiquitinated in eukaryotic
cells, although ubiquitinated H2A (uH2A) has not been
reported in the budding yeast
Saccharomyces cerevisiae
(Goldknopf
et al.
, 1977). In higher eukaryotic organ-
isms, the ubiquitin molecule is linked to K119 of H2A
and K120 of H2B (K123 in yeast). Physiological obser-
vations suggest that ubiquitinated histones may have
multiple functions and structural effects rather than
playing a role in the degradation by the 26S protea-
some (Nickel and Davie, 1989). Reports on the roles of
histone ubiquitination in chromatin fractions are con-
troversial and, in many cases, contradictory. Neverthe-
less, the current view favors that the ubiquitination of
H2B is associated with gene transcription (Davie and
Murphy, 1990; Krogan
et al.
, 2003). It is also required
for methylation of H3 K4 and K79 in yeast (Dover
et al.
, 2002). In contrast, the ubiquitination of H2A has
been linked to polycomb gene silencing (Wang
et al.
,
µ
M
) or sodium arse-
nate (As VI) (30-300
M
) for 2 weeks produced signifi -
cant dose-dependent DNA hypermethylation within
the promoter region of p53 (Mass and Wang, 1997). In
addition, by use of methylation-sensitive arbitrarily
primed PCR, both hypomethylation and hypermeth-
ylation of genomic DNA fragments (300-500 bp) were
detected with As treatment in both human kidney cells
(UOK) and in A549 cells. Although the affected genes
have not yet been identifi ed, this result suggested that
µ
