Interaction of cimetidine with P450 in a mouse model of hepatocarcinogenesis initiation.

Cytochrome P450 (P450) plays a key role in the oxidative metabolism of drugs and xenobiotics, many of which are lipophilic and to be excreted they should be transformed into more polar water soluble molecules by the system of hepatic mono-oxygenases (Lim and Lu, 1998). P450 can be influenced by a number of exogenous and endogenous factors (Whitlock and Denison, 1995) and its induction and inhibition is of the utmost interest in carcinogenesis (Toussaint et al, 1993). We have earlier proposed a mechanism for the onset of hepatocarcinogenesis involving an activating status of the whole liver which reflected an important and sustained increase in P450 levels, leading to biochemical aberrations in haem metabolism which in turn would lead to the tumorigenic process (Gerez et al, 1997). We have also suggested that reactive oxygen species (ROS), produced during carcinogenesis chemically induced by p-dimethylaminoazobenzene (DAB), would be involved in the generation of hepatic lesions (Gerez et al, 1998a,b) and the so triggered peroxidative damage would be implicated in the initiation step of hepatocarcinogenesis. The development of cancer is a dynamic process of de-regulation of gene function. Accumulation of damage alters gene function and clonal expansion of mutated cells (Cerutti, 1994). During drug metabolism, generated reactive oxygen species (ROS) play a key role in several stages of carcinogenesis (Halliwell, 1999; Caballero et al, 2001). Cimetidine (CIM) is a H2-histamine receptor antagonist clinically used in the treatment of peptic ulcers and other gastric acid-related disorders (Chang et al, 1992a). It inhibits hepatic mixed-function oxidase activity (Baird et al, 1987) and it appears that CIM is a more potent inhibitor of hepatic P450 when administered in vivo than when it is added to microsomes in vitro (Chang et al, 1992b). The presence of a high affinity binding site for CIM on P450 in liver microsomes, with both the imidazole and cyano positions of CIM interacting with the hemin iron is well documented. Ranitidine, a structurally dissimilar H2-histamine antagonist, not inhibiting hepatic mixed-function oxidases, has not a binding site on P450, suggesting therefore that CIM would alter the oxidative metabolism of some compounds by having a direct inhibitory effect on P450. If CIM exerts a significant general effect upon haem biosynthetic and/or degradative pathway, we would expect these effects to be manifest in both P450 and in other haem containing proteins when animals are treated with CIM (Baird et al, 1987). Since elucidation of a specific form of microsomal P450 exclusively associated with azoreduction remains elusive (Zbaida, 1995), we have shown that administration of DAB to mice, remarkably increases total P450 and that these changes are associated with hepatoxicity and lipid peroxidation, leading to the carcinogenesis onset. Considering that in our experimental model, the sustained P450 induction provoked as a consequence of the carcinogen metabolism, is responsible for the liver injury and the peroxidative damage, our aim was to investigate if diminution of P450 caused by CIM could avoid the biochemical aberrations associated with the initiation stage of hepatocarcinogenesis (Gerez et al, 1998a,b; Caballero et al, 2001).