Chung S. Yang

Modulation of cytochrome P450 isozymes in human liver, by ethanol and drug intake

Abstract. Cytochromes P450 (P450) are a family of isozymes which play an important role in xenobiotic metabolism. The concentration of three P450 isozymes, namely P450‐IIEl(Alc),‐HIA(NF) and ‐IIC8‐10(MP) has been measured in human liver biopsies of patients with different alcohol and drug intake status. All these three P450s were expressed in all subjects. Ethanol intake increased P450‐IIEl(Alc) content with no effect on the content of the two other P450s. Drug intake (barbiturates) increased both P450‐IIIA(NF) and ‐IIC8‐10(MP) content without any effect on P450‐IIEl(Alc). This paper brought, at protein level, further evidence of the importance of environmental conditions on P450 isozyme pattern, and therefore, on drug metabolizing capacity of human liver.

Inhibitory effects of tea extracts and (−)-epigallocatechin gallate on DNA synthesis and proliferation of hepatoma and erythroleukemia cells

Polyphenols extracted from green or black tea with ethyl acetate were strongly inhibitory for DNA synthesis in HTC rat hepatoma cells and DS19 mouse erythroleukemia cells at concentrations of 0.1-0.2 mg/ml. There was less inhibition with a subsequent black tea fraction extracted with butanol and with the residual water-soluble fraction. Although cell proliferation was inhibited by (-)-epigallocatechin gallate and the tea extracts, there were only marginal effects on differentiation of DS19 cells as judged by hemoglobin synthesis.

Protective effects of diallyl sulfide on acetaminophen-induced toxicities.

Diallyl sulfide (DAS), a major flavour component of garlic, is known to modulate drug metabolism and may protect animals from chemically induced toxicity and carcinogenesis. In this study the effects of DAS on the oxidative metabolism and hepatotoxicity induced by acetaminophen (APAP) in rats were investigated. In the hepatotoxicity evaluation of Fischer 344 rats there was a dose-dependent increase in the odds of mortality rate by APAP (P = 0.009); DAS treatment significantly protected rats from APAP-related mortality (P = 0.026). Liver toxicity determined by lactate dehydrogenase activity was significantly increased by APAP treatment (0.75 g/kg). Pretreatment with DAS protected animals from APAP-induced liver toxicity in a time- and dose-dependent fashion. Treatment of DAS (50 mg/kg) 3 hr after APAP dosing significantly (P < 0.05) protected rats from APAP-induced liver toxicity. The metabolism of APAP (50 microM) in vitro was significantly inhibited by DAS (0.3-1 mM) in liver microsomes isolated from F344 rats. As the effect of DAS on APAP-induced hepatotoxicity in vivo was observed only when DAS was administered before or shortly after (< 3 hr) APAP dosing, data suggested that the protective effect of DAS is mainly at the metabolic activation step of APAP. However, the possibility that DAS may also have effects on other drug metabolism systems, such as glutathione (GSH) and glutathione S-transferases, cannot be ruled out.

Mouse renal cytochrome p450IIE1: Immunocytochemical localization, sex-related difference and regulation by testosterone☆

Cytochrome P450IIE1 is responsible for the metabolic activation of N-nitrosodimethylamine and a variety of other chemicals. Renal P450IIE1 was shown previously to be regulated by testosterone in C3H/HeJ and BALB/c mice. The present study investigated the distribution of cytochrome P450IIE1 in the kidneys of C3H/HeJ and BALB/c mice. The amount of P450IIE1 was immunotitrated by immunohistochemistry using polyclonal antibodies against rat P450IIE1. Strong immunoreactivity was identified mainly in the cortical tubules, including proximal tubules and some tubules. Weak immunoreactivity was also observed in the outer medulla when higher concentrations of antibodies were used. Much higher immunostaining was observed in male mice than in female mice when identical antibody dilutions were used. The renal P450IIE1 level in females was elevated to the same level as that in males 24 hr after administration of testosterone. The results showed a specific cellular localization of cytochrome P450IIE1 in mouse kidney. The findings may lead to a better understanding of the site-specific renal toxicity and carcinogenesis due to the activation of chemicals by cytochrome P450IIE1.

Glucocorticoid hormones prevent the induction of γ-glutamyl transpeptidase by ethanol in a rat hepatoma cell line

The increase in serum gamma-glutamyl transpeptidase (GGT) is a well known marker of chronic alcoholism in man. We have previously shown that ethanol (180 mM) induces GGT activity 2-3-fold in the C2 rat hepatoma cell line. In this study, we have analyzed the interaction of ethanol with steroid hormones and drugs in this well defined cell culture system. Dexamethasone (100 nM), a synthetic glucocorticoid agonist, completely prevented the induction of GGT by ethanol, but had no effect when added alone. This inhibitory effect was also observed with other corticosteroids, but not with sex steroids; it was prevented by RU 486, a glucocorticoid antagonist. These observations suggest that dexamethasone acts through a high affinity glucocorticoid receptor. Conversely, ethanol did not interfere with the glucocorticoid induction of alanine aminotransferase in the same cell. We have analyzed the metabolism of ethanol in the C2 cells. These cells lack significant alcohol dehydrogenase activity as well as any cytochrome P-450 Alc immunoreactivity. Dexamethasone did not modify the disappearance of ethanol in the culture medium of those cells. We conclude that glucocorticoid hormones interact with ethanol at the cellular level, and that this interaction does not involve a modification of alcohol metabolism.

Binding of metabolically activated benzo(a)pyrene to DNA and histones of rat liver, lung and regenerating liver.

Abstract Binding of metabolically activated benzo(a)pyrene (BP) to the DNA and histones of nuclei isolated from rat liver, lung, and regenerating liver was examined. Separation of enzymically degraded DNA by Sephadex LH-20 chromatography resulted in several peaks of radioactivity and the major peak was probably derived from the binding of 7,8-diol-9,10-epoxide of BP with DNA. A high level of BP metabolites was also bound to the Hl histone fraction. The patterns of BP binding with the components of nuclei from the different cell types were similar. Implications of these observations as related to carcinogenic tissue susceptibility are discussed.

Metabolism and activation of nitrosamines catalysed by cytochrome P-450 isoenzymes

Nitrosamines are a group of nitrogenous compounds that are widely distributed in the environment and can also be synthesized endogenously (Bartsch and Montesano, 1984). Many of these compounds are potent carcinogens showing remarkable tissue and species specificity. Since the discovery of the carcinogenicity of NDMA, the metabolism of nitrosamines has been studied extensively (Magee and Barnes, 1967; Lai and Arcos, 1980). Although α-hydroxylation has been shown many years ago to be a key step in the activation of nitrosamines the enzymatic mechanisms of the metabolic activation of some of these compounds are not clearly understood. The lack of such information has hindered our progress in understanding the tissue and species specific carcinogenicity of nitrosamines. In this review, the enzymes and mechanisms involved in the activation of NDMA and other nitrosamines are discussed.