thiobenzamide and Carcinoma--Hepatocellular

Nitric oxide (NO) modifies the functions of a variety of proteins containing cysteine thiols or transition-metal centers, particularly by S-nitrosylation. In inflamed liver, NO is overproduced and hepatic drug-metabolizing enzymes, the flavin-containing monooxygenases (FMOs) and cytochrome P450s (CYPs), are suppressed. However, the NO-related mechanisms underlying the loss of these activities are not well understood, particularly for FMOs. In this study, we suggest that FMO3, the major FMO in human liver, is modified post-translationally by NO. This hypothesis is based on the imbalance observed between the decrease in FMO3 expression (40.7% of controls) and FMO3-specific ranitidine N-oxidation activity (15.1%), and on the partial or complete reversibility of FMO inhibition by sulfhydryl-reducing regents such as DTT (effective on both S-S and S-NO adducts) and ascorbate (effective on S-NO only). Furthermore, NO donors (SNP, SNAP, and Sin-1), including the pure NO donor DEA/NO, directly suppressed in vitro FMO activity (N- or S-oxidation of ranitidine, trimethylamine, and thiobenzamide) in human liver microsomal proteins and recombinant human FMO3. These activities were restored completely after treatment with DTT or ascorbate. These results suggest that NO-mediated S-nitrosylation is involved in the rigorous inhibition of FMO activity in vitro and in vivo, resulting in the suppression of FMO-based drug metabolism or detoxification.

Topics: Adult; Ascorbic Acid; Carcinoma, Hepatocellular; Dithiothreitol; Enzyme Inhibitors; Hepatitis B, Chronic; Humans; Liver; Liver Cirrhosis; Liver Neoplasms; Methylamines; Microsomes, Liver; Middle Aged; Nitric Oxide; Nitric Oxide Donors; Nitrosation; Oxygenases; Ranitidine; Recombinant Proteins; Thioamides

Thiobenzamide (TB), a thiono-containing compound, was administered for 38 weeks to female Sprague-Dawley rats at a dose of 1 g/kg standard diet; the resulting liver pathology was followed up to 8 months after withdrawal of the compound from the diet. TB administration induced the appearance of biliary cirrhosis. In the first weeks of intoxication the progressive distortion of the liver architecture was mainly due to significant proliferation of the bile ductules. Later, the liver assumed a macronodular appearance. In addition to regenerative and degenerative changes of the hepatocytes, preneoplastic lesions were also detected, and some enzymic markers of the mixed-function monooxygenase system were decreased. Cholangiofibrotic areas were evident, and many biliary tubules within them showed mucous metaplasia. At the end of the intoxication period, as well as 4 months after drug suspension, large portions of the liver or entire lobes were substituted by connective tissue surrounding nests of bile ductules and atrophied hepatocellular nodules. Four months later, in the virtual absence of cirrhotic changes, each animal harboured one or more tumors (mainly cholangiomas).

Topics: Adenoma, Bile Duct; Amides; Animals; Body Weight; Carcinoma, Hepatocellular; Diet; Female; Liver; Liver Cirrhosis, Biliary; Liver Neoplasms; Organ Size; Rats; Rats, Inbred Strains; Thioamides