Page 127 - 82_02
P. 127
The new levels of redox regulation of S-adenosylmethionine synthesis
redox stress and to reduce the GSH/GSSG ratio. Although groups, which are essential for MAT I and MAT III
the mechanism by which D-galactosamine exerts this function, and hence additional studies are required to
effect seems rather indirect, this is not the case for address this aspect.
paracetamol, whose detoxification requires GSH
consumption, thus leading to altered GSH/GSSG ratios 5. INTEGRATED FOR MATs AND FUTURE
(94). As mentioned previously, my own work RESEARCH MODEL OF REDOX REGULATION
demonstrated in 1992 that MAT I and MAT III activities
were modulated by the GSH/GSSG ratios and, at extreme Altogether the available data concerning redox
concentrations of GSSG, even the association state was regulation of MATs expand from expression to subcellular
modified (41). Therefore, we decided to explore whether localization and were summarized in a hypothesis recently
glutathione ratios exerted an additional role in the control proposed by our group (97) (Figure 6). This new theory
expands a previous proposal that we made back in 1992
of MATa1 subcellular distribution using a cellular system, (98), by adding the accumulated knowledge obtained in
which allowed multiple combinations of drugs. Available these years. Since the purpose of this review is not to
hepatic cell lines include hepatomas and iPS-hepatocytes revisit these hypotheses, only a few data will be
with low or no MAT1A expression (79,95), and primary highlighted.
cultures of hepatocytes rapidly loose expression of this
gene (40,96). Therefore, transfection was needed to Most efforts in the last decades have been concentrated
in understanding the regulation of MAT expression,
explore MATa1 behavior in any of these systems, among rendering information from transcription factors acting on
which we chose the hepatoma H35 cell line for
the promoter (i.e. NF?B), the different gene methylation
overexpression of tagged-MATa1 (93). D-galactosamine levels according to the liver state, stabilization of the
treatment in this system reproduced the changes detected mRNAs (i.e. AUF1 and HuR binding), etc. Our own work
in the rat model: i) reduction in the GSH/GSSG ratio; ii) has been concentrated however in the proteins themselves,
our results concerning from the structure (i.e. intrasubunit
decreased cytosolic levels of MATa1; iii) accumulation of disulfide bond), to cofactor regulation (i.e. NADP+
MATa1 in the nucleus; and iv) increase of the me3K27H3 binding) and, more recently, subcellular distribution of
signal. Agents that serve as sources of cysteine for
glutathione synthesis, N-acetylcysteine (NAC) and SAM, MATa1 and its relationship with disease. Briefly, the
were then combined with D-galactosamine treatment and proposal indicates that the high SAM levels found in
their preventive effects evaluated (Figure 5). Among them, normal liver depend on: 1) low methylation levels of the
only NAC was effective in preventing all the changes MAT1A promoter allowing gene expression; 2) high
induced by the drug (93). MAT2A promoter methylation levels, together with
decreased MAT2A mRNA stability by binding of meHuR;
In this same cellular system the effects induced by 3) the normal ratios of GSH/GSSG preserving high MAT
paracetamol in the rat model were also reproduced,
namely: i) GSH depletion with the concomitant decrease in I/III activity in the cytosol and keeping MATa1 transport
to the nucleus at low ratios, therefore maintaining adequate
the GSH/GSSG ratio; ii) nuclear accumulation of MATa1; methylation levels of histones and DNA. In contrast, the
and iii) increased levels of the me3K27H3 repression low SAM levels found in damaged liver are due to the
mark. However, in this case the combination of combination of: 1) an inversion of the promoter
paracetamol with NAC only prevented effects on methylation levels, which become high for MAT1A and
low for MAT2A promoters; 2) enhanced MAT2A and
glutathione levels and MATa1 localization, whereas MAT2B transcription by the action of redox regulated
me3K27H3 levels remained elevated (93). Further transcription factors; 3) destabilization of MAT1A mRNA
confirmation of the role of GSH/GSSG ratio in the by AUF binding, together with stabilization of MAT2A
transcripts by HuR binding; 4) increased oxidative stress
regulation of MATa1 subcellular distribution was leading to higher levels of NADP+ and reduced
obtained using BSO, a compound that inhibits glutathione GSH/GSSG ratios, which together favor cytosolic MAT II
synthesis. Again, BSO treatment reduced the GSH/GSSG accumulation, the low Vmax MAT; and 5) reduced
GSH/GSSG ratios induce nuclear accumulation of
ratio and nuclear MATa1 accumulation was induced in the
cells, these effects being prevented by addition of MATa1 and MAT I providing higher nuclear SAM
glutathione ethyl ester (93). Differences between D- concentrations for epigenetic mechanisms.
galactosamine and paracetamol intoxications may relay not
only in their effects on glutathione levels, but also in the
ability of the paracetamol intermediate NAPQI for protein
modification. Such modification involves again sulfhydryl
@Real Academia Nacional de Farmacia. Spain 241
