Discussion

Abstract

The induction of a number of drug-metabolizing enzymes is among the best-understood biochemical effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related agonists of the aryl hydrocarbon receptor (AhR). Among the cytochrome P450s (CYPs), the genes encoding CYP1A1, 1A2, and 1B1 are responsive to AhR agonists, i.e. their expression is inducible in various mammalian tissues and organs as well as in many types of cell lines and primary cells in culture. In addition, an aldehyde dehydrogenase, an NADPH-quinone-oxidoreductase, and the phase II conjugating enzymes glutathione-S-transferase (GST) Ya and UDP-glucuronosyltransferase 1A1 have been identified as responsive to AhR agonists. Induced expression of these members of the AhR gene battery is thought to be aimed at an improved elimination of the inducing agent and its metabolites. However, the identity of the physiological ligand(s) of the AhR is still obscure. The consequences of induced expression of AhR-regulated genes encoding drug-metabolizing enzymes have been investigated in human populations, e.g. in smokers, and in various experimental models. A prominent example of increased adverse effects due to the induction of CYP1A isozymes is the metabolic activation of carcinogenic aromatic amines and polycyclic aromatic hydrocarbons. An increasing amount of data is also available on the impact of dioxin-type induction on the metabolism of drugs, food constituents, and endogenous substrates. For example, the hepatic clearance of the drug theophylline, which is widely used in asthma therapy, is enhanced significantly in smokers. Increased glucuronidation of thyroxine in rats treated with TCDD or other potent AhR agonists is thought to result in hypothyroxinemia and its biological consequences, such as sustained hyperplasia of the thyroid, bearing a higher risk of thyroid cancer. The relevance of these observations for humans exposed to dioxin-type inducers is discussed.