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| 8: Chem Biol Interact 1998 Apr 24;111-112:51-67 |
Hayes JD, Pulford DJ, Ellis EM, McLeod R, James RF, Seidegard J, Mosialou E,
Jernstrom B, Neal GE.
Biomedical Research Centre, Ninewells Hospital and Medical School, University of
Dundee, Scotland, UK.
The rat can be protected against aflatoxin B1 (AFB1) hepatocarcinogenesis by
being fed on a diet containing the synthetic antioxidant ethoxyquin. Evidence
suggests that chemoprotection against AFB1 is due to increased detoxification of
the mycotoxin by one or more inducible drug-metabolising enzymes. The
glutathione S-transferase (GST) isoenzymes in rat liver that contribute to
ethoxyquin-induced chemoprotection against AFB1 have been identified by protein
purification. This approach resulted in the isolation of several heterodimeric
class alpha GST, all of which contained the A5 subunit and possessed at least
50-fold greater activity towards AFB1-8,9-epoxide than previously studied
transferases. Molecular cloning and heterologous expression of rat GSTA5-5 has
led to the demonstration that it exhibits substantially greater activity for
AFB1-8,9-epoxide than other rat transferases. The A5 homodimer can also catalyse
the conjugation of glutathione with other epoxides, such as trans-stilbene oxide
and 1,2-epoxy-3-(4'-nitrophenoxy)propane, and possesses high catalytic activity
for the reactive aldehyde 4-hydroxynonenal. Western blotting has shown that the
A5 subunit is not only induced by ethoxyquin but that it is also induced by
other cancer chemopreventive agents, such as butylated hydroxyanisole, oltipraz,
benzyl isothiocyanate, indole-3-carbinol and coumarin. In addition to GSTA5, we
have identified a novel aflatoxin-aldehyde reductase (AFAR) that is similarly
induced by ethoxyquin. However, immunoblotting has shown that GSTA5 and AFAR are
not always co-ordinately regulated by chemoprotectors. In order to gain a better
understanding of the mechanisms responsible for the induction of GSTA5 protein,
the GSTA5 gene has been cloned. It was isolated on two overlapping bacteriophage
lambda clones and found to be approximately 12 kb in length. The transcriptional
start site of GSTA5 has been identified 228 bp upstream from the ATG
translational initiation codon. Computer-assisted analysis of the upstream
sequence has indicated the presence of a putative antioxidant responsive element
(located between -421 and -429 bp) which may be responsible for the induction of
GSTA5 by chemopreventive agents.
Publication Types:
PMID: 9679543
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