The Road to Elucidating the Mechanism of Manganese-Bilirubin-Induced Cholestasis

The article highlighted in this issue is “Synergistic Role of 3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase and Cholesterol 7-Hydroxylase in the Pathogenesis of Manganese-Bilirubin– Induced Cholestasis in Rats,” by Marie-Yvonne Akoume, Shahid Perwaiz, Ibrahim M. Yousef, and Gabriel L. Plaa (pp. 331–338). A major function of the liver is the formation of bile and hepatobiliary excretion of endogenous substances and xenobiotics, such as chemicals, drugs, and their metabolites. Cholestasis is a general condition of multiple etiologies, hereditary and acquired, in which bile excretion from the liver is attenuated or blocked. In some forms of cholestasis, cholesterol metabolism is perturbed, resulting in cholesterol accumulation in liver bile canalicular membranes (BCM). Intrahepatic cholestasis can be caused by drugs, sepsis, total parenteral nutrition, lymphomas, tuberculosis, sarcoidosis, and amyloidosis. Other causes of this form of the disorder include primary biliary cirrhosis, primary sclerosing cholangitis, viral hepatitis, alcoholic liver disease, pregnancy, and inborn errors or mutations of bile acid metabolism. Extrahepatic cholestasis can be caused by bile duct tumors, strictures, cysts, diverticula, and various forms of injury. Potential causes for this specific form also include stones in the common bile duct, pancreatitis and pancreatic tumor, primary sclerosing cholangitis, and compression due to a mass or tumor on a nearby organ. The clinical use or abuse of drugs, including gold salts, nitrofurantoin, anabolic steroids, estrogens and oral contraceptives, chlorpromazine, prochlorperazine, sulindac, cholesterollowering “statins,” cimetidine, erythromycin, tobutamide, imipramine, and some penicillin-based antibiotics and herbal remedies, can cause cholestasis (Chitturi and Farrell, 2001). Exposure to these agents may reduce or block bile flow and precipitate liver injury, in part due to the toxicity of bile acids and other bile constituents. Thus, there is signficant clinical relevance for conducting studies examining mechanisms or modes of action of chemical-induced cholestasis. Cholestasis and perturbations in cholesterol metabolism have been the subjects of an exciting and intriguing path of inquiry undertaken by investigators from the laboratory that produced the highlighted paper in this issue (Akoume et al., 2003). In this article, the authors have extended their earlier work defining mechanisms associated with the manganesebilirubin (Mn-BR) model of experimentally-induced intrahepatic cholestasis. The authors have been using this model for over 20 years because of the similarities in the pathophysiological changes in BCM to those observed in cases of cholestasis in humans. In the Mn-BR model, cholestasis is achieved through the sequential intravenous injection of Mn and BR, neither of which is cholestatic when administered alone. The onset of cholestasis is remarkably rapid—Mn is injected followed by injection of BR 15 min later, and maximum decreases in bile flow are observed 30 min after BR injection. Through a series of investigations, including the highlighted paper, the authors have contributed a body of work that illustrates a logical and rational approach to elucidate the mechanisms involved in Mn-Br–induced cholestasis. In previous studies, the authors provided evidence that accumulation of cholesterol in the BCM and cytosol, increases in the BCM cholesterol/phospholipids ratio, and changes in BCM membrane fluidity play a role in the pathogenesis of Mn-BR cholestasis (Duguay et al., 1998). While narrowing the focus of action to the BCM, an unresolved question from that study was: What is the source of cholesterol that accumulates in the BCM? Or, in other words, is the elevated cholesterol derived from the existing cellular pool or from an increase in the de novo synthesis of cholesterol? This question was answered in a subsequent investigation (Duguay et al., 2000) in which the authors employed an experimental design utilizing two radiolabeled compounds that would discriminate between the existing cellular pool of cholesterol and cholesterol synthesized de novo. 3 H-cholesterol was injected to label the existing intracellular cholesterol pool followed by subsequent injection of