Polycyclic hydrocarbons, enzyme induction

Nebert, D.W. and Bausserman, L.L. (1970). Fate of inducer during induction of aryl hydrocarbon hydroxylase activity in mammalian cell culture. II. Levels of intracellular content of polycyclic hydrocarbon during enzyme induction and decay ) Mol. Pharmacol 6,304. 

Induction of the microsomal enzymes may also have other effects as well as the increased production of particular enzymes and isozymes. Here again the different types of inducer vary. Thus, the barbiturate type of inducer differs significantly from the polycyclic hydrocarbon type as can be seen from the list below. Some of the characteristic changes caused by the barbiturate type of inducer are... 

Figure 5.33 Mechanism of the receptor-mediated induction of CYP1A1 by a polycyclic hydrocarbon such as TCDD. The inducer-receptor (AhR) complex enters the nucleus, binds with ARNT, and the complex binds to the CYP1A1 gene in the DNA. This induces the production of CYP1A1 mRNA, which leads to the production of CYP1A1 protein and functional enzyme. Abbreviations AIP, AhR interacting protein AhR, aryl hydrocarbon receptor HSP, heat shock protein 90 ARNT, AhR nuclear translocator E.R., endoplasmic reticulum.

It is clear that the effects of induction or inhibition of the metabolism will be complex because of the large number of possible metabolic pathways through which benzofalpyrene may be metabolized. For instance, the microsomal enzyme inducer 5,6-benzoflavone inhibits the carcinogenicity of benzofalpyrene in mouse lung and skin, whereas inhibitors such as SKF 525A may increase the tumor production from certain polycyclic hydrocarbons. 

As well as detoxication via reaction with GSH, the reactive 3,4-epoxide can be removed by hydration to form the dihydrodiol, a reaction that is catalyzed by epoxide hydrolase (also known as epoxide hydratase). This enzyme is induced by pretreatment of animals with the polycyclic hydrocarbon 3-methylcholanthrene, as can be seen from the increased excretion of 4-bromophenyldihydrodiol (Table 7.5). This induction of a detoxication pathway offers a partial explanation for the decreased hepatotoxicity of bromobenzene observed in such animals. A further explanation, also apparent from the urinary metabolites, is the induction of the form of cytochrome P-450 that catalyzes the formation of the 2,3-epoxide. This potentially reactive metabolite readily rearranges to 2-bromophenol, and hence there is increased excretion of 2-bromophenol in these pretreated animals (Table 7.5). 

Ah receptor a protein, which binds polycyclic hydrocarbons such as dioxin (TCDD). Binding to this receptor is part of the process of induction of xenobiotic metabolizing enzymes. 

In the early 1960s, during investigations on the N-demethylation of aminoazo dyes, it was observed that pretreatment of mammals with the substrate or, more remarkably, with other xenobiotics, caused an increase in the ability of the animal to metabolize these dyes. It was subsequently shown that this effect was due to an increase in the microsomal enzymes involved. A symposium in 1965 and a landmark review by Conney in 1967 established the importance of induction in xenobiotic interactions. Since then, it has become clear that this phenomenon is widespread and nonspecific. Several hundred compounds of diverse chemical structure have been shown to induce monooxygenases and other enzymes. These compounds include drugs, insecticides, polycyclic hydrocarbons, and many others the only obvious common denominator... 

Substances carcinogenic to animals (polycyclic hydrocarbons and nicotine-derived N-nitrosamines) have been identified in tobacco smoke condensates from cigarettes, cigars and pipes. Polycyclic hydrocarbons are responsible for the hepatic enzyme induction that occurs in smokers. 

THE JOYS OF DISCOVERY ENZYME INDUCTION BY POLYCYCLIC AROMATIC HYDROCARBONS... 

In 1966-1967, we demonstrated that treatment of rats with certain polycyclic hydrocarbons or aromatic azo derivatives stimulated the in vitro and in vivo metabolism of 7,12-dimethylbenz[a]anthracene (9,10), which provided a mechanistic explanation for earlier research indicating an inhibitory effect of these compounds on 7,12-dimethylbenz[a]anthracene-induced mammary cancer and adrenal toxicity in rats (11-15). These early studies on mechanisms of inhibition of azo dye and polycyclic hydrocarbon carcinogenesis as well as subsequent studies suggested that induction of carcinogen detoxifying enzymes may be a useful strategy for cancer chemoprevention [reviewed in Refs. (16-18)]. 

Theophylline and polycyclic hydrocarbons in tobacco smoke (3-methylcholanthrene) represent a second type of enzyme inducer with broad induction effects. They induce CyP lA in which no change in P4go reductase occurs and a different terminal oxidase appears. After this type of induction, the clearance of tlieophylline but not that of antipyrine is increased. These substances have served as prototypes for the classification of enzyme inducers. Obviously, when patients are on a drug with a narrow therapeutic index, their dosing regimen would need to be adjusted should a known enzyme-inducing drug be added to or deleted from their therapy. 

The polycyclic hydrocarbon type of inducer does not have such major effects, only causing slight liver enlargement and having no effect on liver blood or bile flow. The increase in cytochromes P-450 is not confined to the centrilobular area of the liver, protein synthesis is only slightly increased and there is no increase in phospholipid synthesis. Other enzymes than cytochromes P-450 are also induced by polycyclic hydrocarbons, although generally to a lesser extent than with barbiturate induction. NADPH cytochrome P-450 reductase is not induced by polycyclic hydrocarbons however. 

With the clofibrate type of inducer other changes are also apparent. Thus, there is a proliferation in the number of peroxisomes (an intracellular organelle), as well as induction of a particular form of cytochrome P-450 involved in fatty acid metabolism. A number of other enzymes associated with the role of this organelle in fatty acid metabolism are also increased, such as carnitine acyltransferase and catalase. This phenomenon is discussed in more detail in Chapter 6. The onset of the inductive response is in the order of a few hours (3-6 h after polycyclic hydrocarbons, 8-12 h after barbiturates), is maximal after 3-5 days with barbiturates (24-48 h with polycyclic hydrocarbons) and lasts for at least 5 days (somewhat longer with polycyclic hydrocarbon induction). The magnitude of the inductive effect may depend on the size and duration of dosing with the inducer, and will also be influenced by the sex, species, strain of animal and the tissue exposed. 

The induction of OU resistance occurs by a mutation that decreases the binding of OU to the Na /K ATPase but leaves the enzyme functional. As expected for such a restricted mutation, OU resistance is induced at a frequency that is approximately 1% of that for 6TG resistance. We hypothesize that nearly all mutations to OU resistance will be of the base-substitution type. Thus, we conjecture that the ratio of mutation to OU resistance and mutation to 6TG resistance will indicate the relative importance of base substitution to the overall mutation caused by a chemical. The ratio of OU /6TG for the alkylating agent EMS is 6 X 10", while the ratio for the polycyclic hydrocarbon FA is approximately 4 X 10 . This observation supports the hypothesis that OU is induced only by base substitution. A better test of this hypothesis would compare the mutagenicity of base analogues, such as BUdR, and frameshift mutagens, such as ICR-191. 

---