Metabolic oxidation phenotypes as markers for susceptibility to lung cancer

The application of immunoassays and fluorometry to the detection of polycyclic hydrocarbon-macromolecular adducts and anti-adduct antibodies in humans

The metabolic activation of polycyclic aromatic hydrocarbons (PAH) to chemical species that form covalent adducts with cellular macromolecules (DNA and protein) is central to theories of carcinogenesis. Assays are currently being developed that will accurately reflect human macromolecular exposure to these carcinogens. Immunoassays are capable of detecting low levels of PAH-DNA adducts and antibodies directed against these adducts in humans and HPLC/spectrophotofluorimetry allows the detection of carcinogen-DNA or carcinogen-protein adducts in human peripheral blood. Both types of method have inherent advantages and disadvantages, and the use of more than one type of corroborative assay is a feature in our work. Simplified but highly specific synchronous fluorescence spectra have been obtained for BP-tetrols after mild acid hydrolysis and HPLC of sample materials. When using a wavelength difference of 34 nm to drive the excitation and emission monochromators simultaneously, the pyrene fluorophore, when present, has a signature peak at 345 nm (excitation). The results of immunoassays demonstrate human exposure to PAH as a class of carcinogen, since serological cross-reactivity of antibodies does not limit detection in this system to a single chemical compound. In addition the formation in humans of anti-PAH-DNA antibodies has been shown, presumably in response to both past and present exposure to the parent compounds. In summary the results of each assay can indicate human exposure to PAH and have the potential for molecular dosimetry of this exposure.

Stereoselective aspects of the metabolic activation of benzo[a]pyrene by human skin in vitro

Benzo[a]pyrene (BP) is activated within tissues in both a regio- and a stereoselective manner and, since human skin is sensitive to tumour induction by polycyclic aromatic hydrocarbons (PAH), the steroselective metabolism of BP in this tissue has been investigated. Samples of skin from eleven individuals were treated with [3H]BP in short-term organ culture. Two samples were also treated with mixtures of [14C](+)- and (-)-trans-7,8-dihydro-7,8-dihydroxybenzo[a]pyrene (BP-7,8-dihydrodiol) in varying proportions. Following application of [3H] BP, more 7,8-dihydrodiol was recovered from the skin itself than from the culture fluid in ten cases; no 7.8-dihydrodiol was detected in extracts from the eleventh. The 7,8-dihydrodiol metabolite was extracted predominantly (range 74-greater than 99%) as the (-)-enantiomer in nine of these ten patients, although proportionately more (+)-enantiomer was recovered from the culture fluid than from the skin in each case. The relative proportions of [3H]BP tetrols derived from syn- and anti-7,8-dihydroxy-9,10-oxy-7,8,9,10-tetrahydroxybenzo[a]pyrene (BPDE) detected in these extracts was more variable. When skin samples were treated with [14C]BP-7,8-dihydrodiol, more anti- than syn-BPDE-derived tetrols were extracted, irrespective of the optical purity of the dihydrodiol applied. These findings provide evidence for interindividual variations in the stereoselective metabolism of BP, which may be of some importance in determining individual susceptibility to PAH-induced skin carcinogenesis.

Determination of debrisoquine metabolic ratio from hourly urine collections in healthy volunteers

The possibility of simplifying the regimen for the collection of urine samples in the determination of the debrisoquine metabolic ratio (DMR) was explored in 15 normal subjects. In the extensive metaboliser subgroup (EM; n = 11), there was a close correlation between the DMR as determined by an 8 h urine collection and the debrisoquine/4-hydroxydebrisoquine ratio (D/4-OHD) in the hourly samples (excluding the first hour). In the poor metabolisers (PM; n = 4) the phenotype could be identified, but it was not possible to estimate the DMR reliably.

Enzymatic basis of the debrisoquine/sparteine-type genetic polymorphism of drug oxidation. Characterization of bufuralol 1'-hydroxylation in liver microsomes of in vivo phenotyped carriers of the genetic deficiency

The genetically controlled polymorphic oxidation of debrisoquine and sparteine is caused by the absence or functional deficiency of a cytochrome P-450 isozyme. In order to elucidate the mechanisms underlying the differences in cytochrome P-450 function we have studied the 1'-hydroxylation of the prototype drug bufuralol in human liver microsomes of individuals phenotyped in vivo as extensive metabolizers (EM, N = 10), poor metabolizers (PM, N = 5) and in subjects with an intermediate rate of metabolism (IM, N = 4). PM- as compared to EM-microsomes were characterized by a decreased Vmax for (+)-bufuralol 1'-hydroxylation (7.51 +/- 2.03 nmol X mg-1 X hr-1 vs 11.95 +/- 4.80 nmol X mg-1 X hr-1) but not for (-)-bufuralol 1'-hydroxylation (4.72 +/- 0.87 nmol X mg-1 X hr-1 vs 5.55 +/- 1.49 nmol X mg-1 X hr-1). The apparent Km for (+)-bufuralol 1'-hydroxylation was increased in PM microsomes (118 +/- 84.9 microM vs 17.9 +/- 6.30 microM). Inhibition of bufuralol 1'-hydroxylation by quinidine was biphasic in EM microsomes, providing further support for the involvement of at least two cytochrome P-450 isozymes. Quinidine acted as a competitive inhibitor of only the high affinity/stereoselectivity component of the reaction. Our data suggest that the debrisoquine/sparteine type of oxidation polymorphism is caused by an almost complete loss of a minor cytochrome P-450 isozyme which has a high affinity and stereoselectivity for (+)-bufuralol and a high sensitivity to inhibition by quinidine.

Passively inhaled tobacco smoke: a challenge to toxicology and preventive medicine

The difficulties in defining the exposure of a passive smoker might explain the controversial results regarding an association between passive smoking on one hand and lung cancer, tumors of all sites and ischemic heart diseases on the other. The plausibility of these epidemiological observations will be discussed in the light of analytical, toxicological, biochemical and oncological data. The minute amounts of nicotine and particulate matter, even the much higher concentrations of volatile substances, such as nitrosamines, NOx, acroleine and formaldehyde, present in diluted sidestream compared to mainstream smoke and breathed by involuntarily smoking people, cannot explain their relatively high cancer risk. It is plausible if one considers the high capacity of cigarette smoke to induce drug metabolizing enzymes. Diluted sidestream smoke, however, lacks compounds which induce several iso-enzymes of cyt. P-450 monooxygenase in the tissues. The best evidence is the up to 100-fold increase in placental enzymes if pregnant women smoke, whereas passively inhaled tobacco smoke is ineffective as inducer. The small amounts of paternal smoke inhaled by pregnant women, containing teratogenic and carcinogenic compounds, which are supposedly not detoxified in the placenta, seem to explain the higher risk for malformations of the fetus and the same or even increased risk for perinatal mortality, compared with the outcome of pregnancy if the mother smoked. The induction of placental enzymes very probably protects the fetus against the much higher amounts of toxic agents inhaled by the smoking mother. The increased activity of placental enzymes seems to be a model for the probably greater capacity of certain cyt. P-450 iso-enzymes in the lung and other tissues to convert carcinogens to inactive metabolites when the individual smokes actively. It is well known that concomitant administration of carcinogens with inducing agents inhibits tumor growth in animals because of a shift in the metabolism which favours the formation of ineffective substances. The negligible amounts of nicotine and CO in passively inhaled tobacco smoke cannot be responsible for the surprisingly high risk for ischemic heart diseases of passive smokers. A plausible explanation is offered by experiments with doves and chicken, which develop atherosclerotic lesions due to the action of carcinogens which are metabolized by certain inducible cyt. P-450 iso-enzymes in the aortic wall. Much circumstantial evidence will be presented, indicating that PAHs, contrary to the propagated opinion, play a minor role for the initiation of cancer in active smokers.(ABSTRACT TRUNCATED AT 400 WORDS)

Biochemical and molecular epidemiology of human cancer: indicators of carcinogen exposure, DNA damage, and genetic predisposition

The primary goal of biochemical and molecular epidemiology is to identify individuals at high cancer risk by obtaining evidence of high exposure to carcinogens, leading to pathobiological lesions in target cells, and/or increased oncogenic susceptibility due to either inherited or acquired host factors. This emerging and multidisciplinary area of cancer research combines epidemiological and laboratory approaches. Because DNA is considered to be an important target for modification by mutagens and carcinogens, damage to DNA can be used as an internal, molecular dosimeter of carcinogen exposure. The reactive species of these carcinogens may directly bind to DNA to form adducts and may indirectly cause secondary DNA lesions, e.g., via induction of free radicals and aldehydes. Highly sensitive and specific methods have been developed to measure the minute amounts of DNA lesions and DNA repair products found in biological specimens from humans exposed to carcinogens. For example, DNA adducts have been measured in cells and tissues from people occupationally exposed to carcinogenic polycyclic aromatic hydrocarbons. Antibodies recognizing carcinogen-DNA adducts have also been detected in human sera. Inherited predisposition to cancer has been revealed by recent advances in molecular genetics, including restriction-fragment-length polymorphism. For example, the hypothesis that rare alleles of the Ha-ras proto-oncogene are associated with an increased risk of lung cancer is currently being tested. These approaches afford the potential of biochemical and molecular epidemiology to predict disease risk for individual persons, instead of for populations, and before the onset of clinically evident disease.

m-Xylene inhalation destroys cytochrome P-450 in rat lung at low exposure

Rats were exposed to 0, 75, 150 or 300 ppm (1 ppm = 1 cm3/m3 = 4.35 mg/m3) m-xylene for 24 h and then killed. In the lungs, the cytochrome P-450 decreased to 45, 13 and 20% of the control value with the increasing exposure intensity and the activity of 7-ethoxycoumarin O-deethylase to 70, 27 and 14%, respectively. The activity of epoxide hydrolase increased slightly after exposures both at 150 (1.6-fold) and 300 cm3/m3 (1.4-fold), while the other measured drug-metabolizing enzyme activities showed no consistent changes. The non-protein sulfhydryl group content of the lungs was not affected. The concentrations of m-xylene in blood indicated that the solvent uptake increased in the different exposure groups more than expected, based on atmospheric concentrations alone. Morphologic studies of the lungs with scanning electron microscopy showed no apparent changes after exposure to 300 cm3/m3 or after a high oral dose (2 ml/kg/day, 3 days). Inhalation exposure to m-xylene for 5 weeks (7 h/day, 4 days/week) at a concentration of 300 ppm lowered the contents of cytochrome P-450 in rat lungs to 65% and the activity of 7-ethoxycoumarin O-deethylase to 41% without any other marked effects on the other drug-metabolizing enzymes or on the levels of non-protein sulfhydryl groups. In this study, the selective destruction of cytochrome P-450 in rat lung could be shown both after acute and subacute exposures and at concentrations low enough to warrant occupational concern.

Aryl hydrocarbon hydroxylase in lymphocytes and lung tissue from lung cancer patients and controls

The inducibility of aryl hydrocarbon hydroxylase (AHH) activity in peripheral mitogen-treated lymphocytes, and of AHH and other monoxygenase activities in lung samples, was studied in 41 patients--34 with pulmonary carcinoma, 4 with a benign lung tumour and 3 with chronic obstructive pulmonary disease. Lymphocyte AHH induction alone was studied in 43 non-smoking and 37 smoking surgical patients. Absolute induced and non-induced AHH activities were at about the same level in the lymphocytes from the lung cancer patients as in those from the non-smoking controls, whereas the activities in smoking controls were about 100% higher. The mean inducibility ratios were very similar in all groups, ranging from 4.4 in the benign tumour patients to 5.4 in both control groups. Thymidine incorporation was on average about 40% lower in the lymphocytes from the lung cancer patients. AHH activity was detectable in all the peripheral lung samples, both normal or tumorous tissue, and its inter-individual variation was more than 67-fold. ECDE activity was also detectable in all the samples studied and its correlation with AHH activity was statistically significant (r = 0.888), suggesting that the same enzyme metabolizes both substrates. ERDE was detectable only in the samples with the highest AHH and ECDE activities. There was no correlation between basal or induced lymphocyte AHH activities and lung tissue AHH activity, but there were statistically significant correlations between lung AHH activity and the inducibility ratio with (r = 0.618) or without correction by thymidine incorporation (r = 0.442). These correlations suggest that there are common regulatory factors for AHH inducibility in different tissues. No significant difference in any drug metabolism parameter measured was observed between the lung cancer patients and the controls.

Chromosomal assignment of human cytochrome P-450 (debrisoquine/sparteine type) to chromosome 22

In order to determine on which chromosome the gene controlling human cytochrome P-450 (debrisoquine/sparteine type) is located a linkage study of polymorphic sparteine oxidation (PSO) to various polymorphic markers was carried out. Positive information for linkage between PSO and the P1 blood group was obtained with a maximal LOD-score of LOD = 3.35 for both male and female recombination fraction estimates of theta m = theta f = 0.0. The P1 blood group has been recently mapped to the long arm of chromosome 22. Thus it can be concluded that the gene controlling human cytochrome P-450 (debrisoquine/sparteine) is situated on the long arm of chromosome 22 in close vicinity to P1.

Polymorphism of human cytochrome P-450

The cytochrome P-450 forms involved in debrisoquine 4-hydroxylation (P-450DB), phenacetin O-deethylation (P-450PA), S-mephenytoin 4-hydroxylation (P-450MP), and nifedipine 1,4-oxidation (P-450NF) have been purified to electrophoretic homogeneity from human liver microsomes. All of these reactions show in vivo polymorphism in humans. Evidence for the roles of the purified proteins in these processes comes from in vitro reconstitution and immunoinhibition studies. The rat orthologs of these enzymes are as follows--P-450DB: P-450UT-H; P-450PA: P-450ISF-G; P-450MP: P-450UT-I; P-450NF: P-450PCN-E. Only in the case of P-450UT-H is the primary rat ortholog the same cytochrome P-450 which catalyses the catalytic reaction under consideration. Reconstitution and immunochemical studies establish that the following reactions are catalysed by the individual P-450s--P-450DB: debrisoquine 4-hydroxylation, sparteine delta 5-oxidation, bufuralol 1'-hydroxylation, encainide O-demethylation, and propanolol 4-hydroxylation; P-450PA: phenacetin O-deethylation; P-450MP: S-mephenytoin 4-hydroxylation and tolbutamide methyl hydroxylation; P-450NF: oxidation of nifedipine and 16 other substituted dihydropyridines, estradiol 2- and 4-hydroxylation, aldrin epoxidation, benzphetamine N-demethylation and 6 beta-hydroxylation of testosterone, androstenedione and cortisol. A cDNA clone has been isolated that corresponds to rat P-450UT-H, as shown by a number of criteria. Studies with this probe establish that the sex and strain variation in debrisoquine 4-hydroxylase and related activities is related to differences in the levels of a 2.0 kb length mRNA present.(ABSTRACT TRUNCATED AT 250 WORDS)

Altered distribution of debrisoquine oxidation phenotypes in patients with systemic lupus erythematosus

Oxidative metabolism in patients with systemic lupus erythematosus (SLE) was studied using the antihypertensive drug, debrisoquine. The metabolism of this drug to its principal metabolite, 4-hydroxydebrisoquine, is catalyzed by a discrete isozyme of cytochrome P-450. The extent of this reaction exhibits genetic polymorphism, with 2 phenotypes, "poor metabolizers" and "extensive metabolizers," discernible in the normal population. We observed the poor metabolizer debrisoquine phenotype in 9 of 42 patients with idiopathic SLE (21%), in contrast with 12 of 147 healthy volunteers (8%), which is a significant difference in frequency (P less than 0.04). These data provide further evidence for altered oxidative metabolism in SLE and support the concept that genetic differences in oxidative metabolism of endogenous compounds, such as sex steroid hormones, or of xenobiotics might influence susceptibility to SLE.

Genetics of drug transformation

Biotransformations of drugs are controlled or strongly affected by genetic factors. During the past few years several genetic deficiencies of drug-metabolizing reactions catalyzed by members of the family of cytochrome P-450 were observed. Choice of the appropriate drug to study and attention to urinary metabolites have been the essential ingredients for the recent discovery of genetic deficiencies of drug metabolism in man which include recessive deficiency of debrisoquine/sparteine metabolism and of mephenytoin metabolism. The clinical significance of these defects is discussed. Ethanol after metabolism to acetaldehyde is further metabolized to acetic acid by aldehyde dehydrogenase. Numerous isozymes of aldehyde dehydrogenase exist, one of which possesses a high affinity for acetaldehyde. Approximately 40% of the Oriental population lack this high affinity isozyme so that in these individuals who may have symptoms of flushing and other unpleasant effects the acetaldehyde formed is destroyed only at high plasma concentrations.

Inter-individual variations in carcinogen metabolism

Explant cultures of human tissues metabolized chemical carcinogens into the ultimate carcinogenic form as measured by binding to cellular DNA. A wide inter-individual variation in the binding level of many chemical carcinogens to target cell DNA was observed, whereas lesser inter-individual variation was observed in primary epithelial cell cultures derived from the explants. The binding levels in the explants showed a unimodal distribution without any sub-populations being present. The binding level of benzo(a)pyrene to human bronchial DNA has been analyzed with respect to genetic and environmental factors such as family history of cancer and smoking history.

Identification of human cytochromes P-450 analogous to forms induced by phenobarbital and 3-methylcholanthrene in the rat

Antibodies to four rat liver forms of cytochrome P-450, two phenobarbital-inducible (PB1 and PB2) and two 3-methylcholanthrene-inducible (MC1 and MC2) proteins, have been used to make a structural and functional comparison of rat and human cytochromes P-450. Proteins from both species were identified on Western blots by their reaction with these antibodies. In the human liver preparations, structurally related proteins to PB1 and to PB2 were identified in all the samples tested with apparent Mr values of 51 800 and 54 800 for PB1 and 53 600 and 57 200 for PB2. Considerable variation in the content of the lower-Mr proteins was measured between samples and, as with the rat enzymes, samples which reacted well with anti-PB1 also reacted with anti-PB2, indicating that these proteins are regulated at least to some degree, co-ordinately. The apparent Mr values of the major human proteins identified with anti-MC1 and anti-MC2 were 54 400 and 57 000 respectively. Only six (of 31) human samples contained significant amounts of these proteins. The same six samples which reacted with anti-MC1 also reacted with anti-MC2, again indicating co-ordinate regulation of these two proteins. Antibody inhibition of microsomal 7-ethoxycoumarin and 7-ethoxyresorufin metabolism demonstrated a degree of conservation of substrate specificity related to specific P-450 isoenzymes between the species. However, the contributions of the different P-450 isoenzymes to the human microsomal activity were not always related to the rat enzyme with the highest activity towards these substrates.

Implications for risk assessment of host factors causing large pharmacokinetic variations

Normal human subjects vary widely in their capacity to eliminate many drugs and environmental chemicals. These variations range in magnitude from fourfold to fortyfold depending on the drug and the population studied. Pharmacogenetics deals with only one of many host factors responsible for these large pharmacokinetic differences. Age, sex, diet and exposure to other drugs and chemicals, including oral contraceptives, ethanol and cigarette smoking, can alter the genetically determined rate at which a particular subject eliminates drugs and environmental chemicals. These elimination rates, therefore, are dynamic and change even in the same subject with time and condition. Regulatory legislation has only recently begun to recognize this very broad spectrum of human susceptibility and the existence of multiple special subgroups of particularly sensitive subjects. In setting standards for environmental chemicals, EPA and NIOSH have attempted to protect the most sensitive humans and should be encouraged to continue this policy. For some drugs and environmental chemicals, the commonly used safety factor of 100 may be too low; for these chemicals large, interindividual pharmacokinetic variations produced by pharmacogenetic and other host factors may make a safety factor of 400 or 500 more adequate.

Benzo(a)pyrene metabolism in hepatic microsomes from female DA rats with a genetic sparteine oxidation deficiency

The impact of the genetic hydroxylation deficiency described for several drugs such as sparteine, debrisoquine, and phenformin, has been studied with respect to benzo(a)pyrene (BP) metabolite formation. 14C-BP (80 microM) was incubated with liver microsomes from female DA rats deficient in sparteine oxidation; microsomes from female Sprague-Dawley rats served as controls. BP metabolites were separated by high pressure liquid chromatography (HPLC). No significant differences were detected in the overall formation rate of 9,10-,4,5-, and 7,8-dihydrodiol-BP, of 4 quinones, or of 9-OH- and 3-OH-BP. Thus, the study suggested no association between genetic hydroxylation polymorphism (debrisoquine/sparteine type) and the formation of at least 6 BP metabolites.

Comparative metabolism of debrisoquine, 7-ethoxyresorufin and benzo(a)pyrene in liver microsomes from humans, and from rats treated with cytochrome P-450 inducers

The metabolism of debrisoquine, 7-ethoxyresorufin and benzo(a)pyrene has been studied in human liver microsomes. There was a significant correlation (r = 0.70, P less than 0.05) between debrisoquine hydroxylation and 7-ethoxyresorufin 0-deethylation among various livers, and debrisoquine inhibited 7-ethoxyresorufin deethylation competitively. These results suggest that debrisoquine and 7-ethoxyresorufin may be metabolised by a common P-450 form in human liver. The effect of cytochrome P-450 inducers on the metabolism of the three substrates was also examined in rat liver. Debrisoquine hydroxylation was not enhanced by phenobarbitone, beta-naphthoflavone or isosafrole.