Contrasting effects of acute and chronic dietary exposure to 2-amino-3-methyl-imidazo[4,5-f]quinoline (IQ) on xenobiotic metabolising enzymes in the male Fischer 344 rat: implications for chemoprevention studies.
BACKGROUND
2-Amino-3-methyl-imidazo[4,5-f]quinoline (IQ) is a mutagen produced in cooked food. It is commonly present in the human diet and often used as a (pro)carcinogen for chemoprevention studies. Many foodstuffs act as chemopreventers by altering xenobiotic metabolising enzyme expression in favour of detoxication over bioactivation pathways. However, IQ itself can also affect enzyme expression, which may be a confounding factor in chemoprevention studies.


OBJECTIVE
Chronic low dose IQ exposure is intuitively closest to the human dietary situation. The aim was to investigate the effects of chronic dietary exposure to IQ on the expression of enzymes involved in the bioactivation and detoxification of xenobiotics and to compare this with acute exposure, often used in chemoprevention studies.


METHODS
Male Fischer rats received IQ (300 ppm) in the diet (AIN-76) for 52 weeks or were given IQ (20 mg.kg-1) orally for 3 days. Animals were killed, livers removed and subcellular fractions prepared. A range of enzymes was selected to allow investigation of several cellular mechanisms. Enzyme expression and activity were determined by Western blotting and the use of selective probe substrates as appropriate.


RESULTS
Chronic exposure to IQ led to an increase in phase II detoxifying enzymes. Both the activity and expression of glutathione S-transferase (GST-A1/2) were increased, as were NADPH: Quinone oxidoreductase (NQO), UDP-glucuronosyl transferase (UGT) and beta-glucuronidase activities. There were no statistically significant changes in the potential for bioactivation by three cytochrome P450s. In contrast, acute IQ exposure significantly increased the expression and activity of some cytochrome P450 (CYP1A1 and CYP1A2), UGT and beta-glucuronidase, but significantly decreased glutathione S-transferase expression and activity. There was a non-significant decrease in NQO but no change in CYP3A2 and CYP2E1 activities.


CONCLUSIONS
The changes after acute exposure suggest an interaction through the Ah receptor and xenobiotic response element, modified by the glucocorticoid response element. In contrast, the pattern of effects after chronic exposure suggests activation of the antioxidant response element (ARE). Although the acute model is more practically convenient for short-term chemoprevention screening, the data suggest that an entirely new mechanism is being invoked that completely masks effects of the ARE that occur during chronic exposure. There is a danger that chemopreventive strategies developed using acute models may be misleading, since the mechanism is unlikely to occur during human dietary exposures.
McPherson
R A
RA
Division of Pharmacology and Clinical Pharmacology, University of Auckland, Private Bag 92019, Auckland 1000, New Zealand.
Tingle
M D
MD
Ferguson
L R
LR
D000818:Animals; D015153:Blotting, Western; D018890:Chemoprevention; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D005982:Glutathione Transferase / Q000378:metabolism*; D008099:Liver / Q000187:drug effects / Q000201:enzymology*; D008297:Male; D009153:Mutagens / Q000008:administration & dosage* / Q000378:metabolism; D009249:NADP; D011804:Quinolines / Q000008:administration & dosage* / Q000378:metabolism; D051381:Rats; D011916:Rats, Inbred F344; D013997:Time Factors; D015262:Xenobiotics / Q000378:metabolism*
D009153:Mutagens; D011804:Quinolines; D015262:Xenobiotics; C029216:2-amino-3-methylimidazo(4,5-f)quinoline; D009249:NADP; D003577:Cytochrome P-450 Enzyme System; D005982:Glutathione Transferase