UDP-glucuronosyltransferase activity in rat-and hairless mouse skin-microsomes

Xenobiotica-metabolizing enzymes in the skin of rat, mouse, pig, guinea pig, man, and in human skin models

Studies on the metabolic fate of medical drugs, skin care products, cosmetics and other chemicals intentionally or accidently applied to the human skin have become increasingly important in order to ascertain pharmacological effectiveness and to avoid toxicities. The use of freshly excised human skin for experimental investigations meets with ethical and practical limitations. Hence information on xenobiotic-metabolizing enzymes (XME) in the experimental systems available for pertinent studies compared with native human skin has become crucial. This review collects available information of which—taken with great caution because of the still very limited data—the most salient points are: in the skin of all animal species and skin-derived in vitro systems considered in this review cytochrome P450 (CYP)-dependent monooxygenase activities (largely responsible for initiating xenobiotica metabolism in the organ which provides most of the xenobiotica metabolism of the mammalian organism, the liver) are very low to undetectable. Quite likely other oxidative enzymes [e.g. flavin monooxygenase, COX (cooxidation by prostaglandin synthase)] will turn out to be much more important for the oxidative xenobiotic metabolism in the skin. Moreover, conjugating enzyme activities such as glutathione transferases and glucuronosyltransferases are much higher than the oxidative CYP activities. Since these conjugating enzymes are predominantly detoxifying, the skin appears to be predominantly protected against CYP-generated reactive metabolites. The following recommendations for the use of experimental animal species or human skin in vitro models may tentatively be derived from the information available to date: for dermal absorption and for skin irritation esterase activity is of special importance which in pig skin, some human cell lines and reconstructed skin models appears reasonably close to native human skin. With respect to genotoxicity and sensitization reactive-metabolite-reducing XME in primary human keratinocytes and several reconstructed human skin models appear reasonably close to human skin. For a more detailed delineation and discussion of the severe limitations see the Conclusions section in the end of this review.

Xenobiotic-metabolizing enzymes in the skin of rat, mouse, pig, guinea pig, man, and in human skin models

The exposure of the skin to medical drugs, skin care products, cosmetics, and other chemicals renders information on xenobiotic-metabolizing enzymes (XME) in the skin highly interesting. Since the use of freshly excised human skin for experimental investigations meets with ethical and practical limitations, information on XME in models comes in the focus including non-human mammalian species and in vitro skin models. This review attempts to summarize the information available in the open scientific literature on XME in the skin of human, rat, mouse, guinea pig, and pig as well as human primary skin cells, human cell lines, and reconstructed human skin models. The most salient outcome is that much more research on cutaneous XME is needed for solid metabolism-dependent efficacy and safety predictions, and the cutaneous metabolism comparisons have to be viewed with caution. Keeping this fully in mind at least with respect to some cutaneous XME, some models may tentatively be considered to approximate reasonable closeness to human skin. For dermal absorption and for skin irritation among many contributing XME, esterase activity is of special importance, which in pig skin, some human cell lines, and reconstructed skin models appears reasonably close to human skin. With respect to genotoxicity and sensitization, activating XME are not yet judgeable, but reactive metabolite-reducing XME in primary human keratinocytes and several reconstructed human skin models appear reasonably close to human skin. For a more detailed delineation and discussion of the severe limitations see the “Overview and Conclusions” section in the end of this review.

Drug-metabolizing enzymes in the skin of man, rat, and pig

The mammalian skin has long been considered to be poor in drug metabolism. However, many reports clearly show that most drug metabolizing enzymes also occur in the mammalian skin albeit at relatively low specific activities. This review summarizes the current state of knowledge on drug metabolizing enzymes in the skin of human, rat, and pig, the latter, because it is often taken as a model for human skin on grounds of anatomical similarities. However only little is known about drug metabolizing enzymes in pig skin. Interestingly, some cytochromes P450 (CYP) have been observed in the rat skin which are not expressed in the rat liver, such as CYP 2B12 and CYP2D4. As far as investigated most drug metabolizing enzymes occur in the suprabasal (i.e. differentiating) layers of the epidermis, but the rat CYP1A1 rather in the basal layer and human UDP-glucuronosyltransferase rather in the stratum corneum. The pattern of drug metabolizing enzymes and their localization will impact not only the beneficial as well as detrimental properties of drugs for the skin but also dictate whether a drug reaches the blood flow unchanged or as activated or inactivated metabolite(s).

Xanthine oxidase-catalyzed metabolism of 2-nitrofluorene, a carcinogenic air pollutant, in rat skin

The reductive metabolism of 2-nitrofluorene, a carcinogenic air pollutant, in rat skin microsomes and cytosol was investigated. 2-Nitrofluorene was reduced to the corresponding amine by the microsomes with NADPH and by the cytosol with 2-hydroxypyrimidine or 4-hydroxypyrimidine under anaerobic conditions. The cytosolic activity was much higher than that of skin microsomes. The 2- or 4-hydroxypyrimidine-linked nitroreductase activity was inhibited by oxypurinol and (+/-)-8-(3-methoxy-4-phenylsulfinylphenyl) pyrazolo[1,5-a]-1,3,5-triazine-4(1H)-one (BOF-4272), inhibitors of xanthine oxidase, but not by menadione, chlorpromazine and isovanillin, inhibitors of aldehyde oxidase. When skin cytosol was applied to a DEAE-cellulose column, the fractions containing xanthine oxidase exhibited a marked 2-hydroxypyrimidine-linked nitroreductase activity. In contrast, the aldehyde oxidase fraction showed little activity. Nitroreductase fractions obtained by ion exchange chromatography showed a band in Western blotting analysis using anti-rat xanthine oxidase. Moreover, the xanthine oxidase fraction exhibited a significant nitroreductase activity in the presence of 2-hydroxypyrimidine, 4-hydroxypyrimidine or hypoxanthine, and these activities were inhibited by inhibitors of xanthine oxidase. These results indicated that reduction of 2-nitrofluorene in the skin was mainly catalyzed by xanthine oxidase.

Carcinogen metabolizing enzymes in nude mice

Hepatic and cutaneous microsomal hydroxylating and conjugating enzymes from female and male NMRI nu/nu mice were analyzed, and the response of these enzymes to repeated exposures with 3-methylcholanthrene were studied. Sex differences were observed in basal activities of hydroxylating enzymes. These differences were not the same in the liver as in the skin and were reversed in female and male. Activity ratios of hepatic and cutaneous hydroxylating enzymes were between 40-200. The ratios between hydroxylating and conjugating enzymes were much lower in the skin than in liver. Furthermore the ratios were depending on hydroxylating enzymes. 3-Methylcholanthrene treatment increased both hydroxylating and conjugating enzymes in the liver and in the skin. Again, there were sex differences in the induction pattern, and also the induction in the liver was unrelated to that in the skin. There was no correlation in induction between hydroxylating and conjugating enzymes.

Induction of UDP-glucuronyltransferase and arylhydrocarbon hydroxylase activity in mouse skin and in normal and transformed skin cells in culture

Methods have been developed which allow quantitative determination of UDP-glucuronyltransferase (UDPGT) and arylhydrocarbon hydroxylase (AHH) activities in unfractionated mouse skin. These methods were used for comparative studies of basal and induced enzyme activities in whole skin and cultured skin cells. After topical application of Aroclor 1254 to the skin UDPGT activities towards 1-naphthol, 3-hydroxybenzo[a]pyrene and benzo[a]pyrene-7,8-dihydrodiol were increased 3-fold and AHH activity was increased 15-fold. Topical application of the inducer also led to a marked increase of these enzyme activities in liver. UDPGT activity towards 1-naphthol was comparable in whole skin and in cultured keratinocytes and fibroblasts. In contrast, AHH activity was higher in cultured keratinocytes than in skin. In transformed epithelial cell lines the pattern of drug metabolizing enzymes was altered: UDPGT activity was increased 4- to 6-fold whereas AHH activity was decreased. However, AHH activity was still inducible by benz[a]anthracene or 7,12-dimethylbenz[a]anthracene in cultured cells. The altered pattern of AHH and UDPGT in transformed epithelial cell lines is consistent with toxin-resistance of initiated cells, similar to the toxin-resistance phenotype characterized in liver after initiation of hepatocarcinogenesis.

Oxidation of 7-ethoxycoumarin and conjugation of umbelliferone by intact, viable epidermal cells from the hairless mouse

Intact, viable (greater than 80%) epidermal cells were isolated from the hairless mouse. These cells metabolized 7-ethoxycoumarin (7-EC) to umbelliferone ( UMB ) (3 pmol/min/10(6) cells) and UMB to the sulfate and glucuronide conjugates (1 pmol/min/10(6) cells). The rate of oxidation in intact cells compared well with that in disrupted cells with added NADPH, but conjugation proceeded more rapidly in disrupted cells with added cofactors, due to a combination of "activation" of the UDP-glucuronosyltransferase, and to a limitation of activity by the concentration of UDP-glucuronic acid in the intact cells. Pretreatment of the animals with 5,6-benzoflavone resulted in a 5-fold increase in the rate of oxidation, and a 2-fold increase in both the rate of conjugation and the intracellular concentration of UDP-glucuronic acid. UDP-glucuronic acid concentration in isolated cells increased during incubation with glucose, and was regenerated to a steady-state concentration on incubation of cells with UMB . Pretreatment of animals with 5,6-benzoflavone decreased the percentage of metabolite conjugated (from 30% to 15%), whereas adding an inhibitor of oxidation, ellipticine, to cells isolated from pretreated animals, increased the percentage of metabolite conjugated (from 15% to 40%). Sulfation of UMB was almost undetectable, except at very low concentrations (less than 10 nM) of substrate. Thus, glucuronidation of UMB in epidermal cells may be limited by UDP-glucuronic acid availability; sulfation in the epidermis may contribute little to the conjugation of UMB ; and greater than 70% of the products of 7-EC oxidation in the skin may remain unconjugated.