Journal of Hepatology
Volume 34, Issue 2 , Pages 339-341, February 2001

Cholesterol and cholestasis: a lesson from the Mdr2 (−/−) mouse

  • Michael Fuchs

      Affiliations

    • Division of Gastroenterology, Department of Medicine I, University Hospital of Lübeck, Ratzeburger Allee 160, D-23538 Lübeck, Germany
    • Corresponding Author InformationCorresponding author. Tel.: +49-451-500-6255; fax: +49-451-500-3645
    • ,
  • Eduard F Stange

      Affiliations

    • Department of Gastroenterology and Endocrinology, Robert Bosch Hospital, D-70376 Stuttgart, Germany

See Article, pages 202-209

Article Outline

 

Proper bile formation and bile flow is essential for a number of important physiological functions, e.g. fat digestion and assimilation of lipid-soluble nutrients in the small intestine and cholesterol homeostasis. In order to fulfil these needs, a concerted adequate functioning of membrane transport systems in liver, bile ducts and the intestine is required. In 1993, the landmark discovery of Smit and colleagues [1] showed that homozygous disruption of the Mdr2 gene in mice produced bile virtually free of phospholipids and cholesterol being dramatically reduced. Subsequent studies demonstrated not only the role of Mdr2 for bile acid metabolism, but also reported reduced cholesterol absorption, low HDL cholesterol levels and increased fecal cholesterol excretion in mice lacking Mdr2 expression [2], [3], [4].

In this issue of the Journal of Hepatology, Voshol et al. [5] present a study in Mdr2 (−/−) mice designed to investigate whether dietary supplementation of phospholipids and/or cholesterol is capable to restore normal HDL cholesterol levels and whether the liver can handle excess dietary cholesterol despite a dramatically lowered biliary cholesterol secretion. Much to their surprise, the investigators noticed that plasma of Mdr2 (−/−) mice became jaundiced within 3 days of cholesterol feeding and this was associated with elevated plasma bile salt and bilirubin levels. Based on this finding one would have expected cholestasis, however, the puzzling finding was that neither bile flow or bile salt secretion was decreased by dietary cholesterol. How can we explain this amazing observation in the Mdr2 (−/−) mouse?

The physiologist defines cholestasis as cessation or impairment of bile flow. Therefore, cholesterol fed Mdr2 (−/−) mice with elevated plasma bile salt and bilirubin levels despite a normal bile flow are not cholestatic in a strict physiological sense. Interestingly, a similar observation has been made in a mouse model of erythropoietic protoporphyria [6]. By increasing bile flow, the liver most likely aims at keeping bile salt concentrations low to prevent damage to the biliary tree [7], [8]. Indeed, such a mechanism appears to occur in guinea pigs with low biliary phospholipids and a bile acid pool that is more hydrophobic than that of mice [9]. Because bile salt secretion is not impaired in cholesterol-fed Mdr2 (−/−) mice, transporting bile salts from bile back to the systemic circulation by means of cholehepatic shunting may prevent further damage to the biliary tree. If this holds true, one would expect a higher choleretic activity of tauroursodeoxycholate in Mdr2 (−/−) mice, and an increased expression of the intestinal bile salt transporter on cholangiocytes [10] would provide the molecular basis. As suggested by the work of Voshol et al. [5], decreased expression of the sodium dependent taurocholate cotransporter Ntcp in chow-fed Mdr2 (−/−) mice [4] may be attenuated by cholesterol feeding mediated through putative bile salt and sterol regulatory elements in the Ntcp promoter [11].

Because in the Mdr2 (−/−) mouse unconjugated and conjugated bilirubin levels were elevated, hemolysis due to elevated plasma bile salt levels can not be the only explanation. Biliary excretion of conjugated bilirubin requires Mrp2, evident by hyperbilirubinemia in patients with Dubin–Johnson syndrome and mutations in the MRP2 gene [12]. Post-transcriptional regulation of Mrp2 in cholesterol-fed Mdr2 (−/−) mice would be in line with Mrp2 mRNA expression data presented by Voshol et al. [5] and further supported by analysis of the MRP2 promoter apparently lacking a sterol regulatory element binding protein recognition consensus site [13]. As observed during hepatic inflammation [14], altered Mrp2 insertion and retrieval into and from the canalicular membrane may have contributed to some extent to elevated plasma bilirubin levels in mice with homozygous disruption of the Mdr2 gene. Studies with isolated canalicular liver plasma membranes or, more elegantly, by immunoelectronmicroscopy should provide clarification. Furthermore, increased expression of Mrp3 at the basolateral surface of hepatocytes [15] may also contribute to elevated conjugated bilirubin levels in plasma of cholesterol-fed Mdr2 (−/−) mice.

The recent finding of heterozygous mutations in the MDR3 gene, the human orthologue of Mdr2, in female patients with intrahepatic cholestasis of pregnancy (reviewed in [16]) suggest that mutations in hepatobiliary transport proteins may predispose to this disease, probably due to a modulatory effect of progesterone on MDR3 activity [17]. It is tempting to speculate that estrogens may also alter MDR3 function and this may be accompanied by decreased NTCP gene expression as has been reported for pregnant rats [18], [19]. No data are currently available for bile flow and biliary lipid secretion rates in pregnant women. Nevertheless, the Mdr2 (+/−) mouse may represent a suitable animal model to study the pathophysiology of intrahepatic cholestasis of pregnancy.

The study by Voshol et al. [5] also demonstrated that dietary cholesterol is not capable to increase the low HDL cholesterol levels observed in Mdr2(−/−) mice. At first hand, the lack of hepatic up-regulation of the HDL receptor Srb1 [20] appears to exclude a contribution of Srb1 and the reverse cholesterol transport pathway to the low HDL cholesterol levels observed. However, altered HDL kinetics have not been ruled out and impaired chylomicron production by enterocytes of Mdr2 (−/−) mice [21] may also be of relevance. Furthermore, desquamation of enterocytes as a result of lipid-free bile together with elevated intestinal cholesterol synthesis due to the lack of biliary cholesterol and increased intestinal cholesterol excretion through the action of Abc1 [22] may provide another potential explanation.

Despite the principal capacity of bile salts to extract cholesterol from plasma membranes of Mdr2 (−/−) mice [2], dietary cholesterol is not capable of inducing biliary cholesterol secretion in Mdr2(−/−) mice. This finding together with elevated hepatic cholesterol levels may reflect an altered biliary cholesterol secretory process. One possible explanation may be impairment of free cholesterol delivery to the canalicular membrane which is thought to involve sterol carrier protein 2 [23], fatty acid binding protein of liver [24], [25] or other yet unidentified transport processes. Alternatively, a decreased bile canalicular membrane fluidity due to cholesterol and sphingomyelin accumulation with a reduced phosphatidylcholine content of the outer membrane leaflet may come into play.

In conclusion, the Mdr2 (−/−) mouse once again provided exciting findings with relevance to biliary physiology. Future studies should bring us not only another step closer towards understanding the molecular mechanisms of biliary cholesterol secretion, but also towards identification of the inter-relationship between hepatobiliary and intestinal transport processes.

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PII: S0168-8278(00)00111-2

Journal of Hepatology
Volume 34, Issue 2 , Pages 339-341, February 2001

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