Ronald P. J. Oude Elferink

Congenital Jaundice in Rats with a Mutation in a Multidrug Resistance-Associated Protein Gene

The human Dubin-Johnson syndrome and its animal model, the TR− rat, are characterized by a chronic conjugated hyperbilirubinemia. TR− rats are defective in the canalicular multispecific organic anion transporter (cMOAT), which mediates hepatobiliary excretion of numerous organic anions. The complementary DNA for rat cmoat, a homolog of the human multidrug resistance gene (hMRP1), was isolated and shown to be expressed in the canalicular membrane of hepatocytes. In the TR− rat, a single-nucleotide deletion in this gene resulted in a reduced messenger RNA level and absence of the protein. It is likely that this mutation accounts for the TR− phenotype.

The breast cancer resistance protein protects against a major chlorophyll-derived dietary phototoxin and protoporphyria

The breast cancer resistance protein (BCRP/ABCG2) is a member of the ATP-binding cassette family of drug transporters and confers resistance to various anticancer drugs. We show here that mice lacking Bcrp1/Abcg2 become extremely sensitive to the dietary chlorophyll-breakdown product pheophorbide a, resulting in severe, sometimes lethal phototoxic lesions on light-exposed skin. Pheophorbide a occurs in various plant-derived foods and food supplements. Bcrp1 transports pheophorbide a and is highly efficient in limiting its uptake from ingested food. Bcrp1−/− mice also displayed a previously unknown type of protoporphyria. Erythrocyte levels of the heme precursor and phototoxin protoporphyrin IX, which is structurally related to pheophorbide a, were increased 10-fold. Transplantation with wild-type bone marrow cured the protoporphyria and reduced the phototoxin sensitivity of Bcrp1−/− mice. These results indicate that humans or animals with low or absent BCRP activity may be at increased risk for developing protoporphyria and diet-dependent phototoxicity and provide a striking illustration of the importance of drug transporters in protection from toxicity of normal food constituents.

Tissue Distribution and Induction of Human Multidrug Resistant Protein 3

The multidrug resistance protein (MRP) family consists of several members and, for some of these transporter proteins, distinct roles in multidrug resistance and normal tissue functions have been well established (MRP1 and MRP2) or are still under investigation (MRP3). MRP3 expression studies in human tissues have been largely restricted to the mRNA level. In this report we extended these studies and further explored MRP3 expression at the protein level. Western blot and immunohistochemistry with two MRP3-specific monoclonal antibodies, M3II-9 and M3II-21, showed MRP3 protein to be present in adrenal gland, and kidney and in tissues of the intestinal tract: colon, pancreas, gallbladder, and liver. In epithelia, MRP3 was found to be located at the basolateral sides of cell membranes. In normal liver, MRP3 was detected at lower levels than anticipated from the mRNA data and was found present mainly in the bile ducts. In livers from patients with various forms of cholestasis, MRP3 levels were frequently increased in the proliferative cholangiocytes, with sometimes additional staining of the basolateral membranes of the hepatocytes. This was especially evident in patients with type 3 progressive familial intrahepatic cholestasis. The present results support the view that MRP3 plays a role in the cholehepatic and enterohepatic circulation of bile and in protection within the biliary tree and tissues along the bile circulation route against toxic bile constituents. The possible functional roles for MRP3 in the adrenal gland and in the kidney remain as yet unknown. In a panel of 34 tumor samples of various histogenetic origins, distinct amounts of MRP3 were detected in a limited number of cases, including lung, ovarian, and pancreatic cancers. These findings may be of potential clinical relevance when considering the drug treatment regimens for these tumor types.

Lysophosphatidic Acid Is a Potential Mediator of Cholestatic Pruritus

BACKGROUND & AIMS Pruritus is a common and disabling symptom in cholestatic disorders. However, its causes remain unknown. We hypothesized that potential pruritogens accumulate in the circulation of cholestatic patients and activate sensory neurons. METHODS Cytosolic free calcium ([Ca(2+)](i)) was measured in neuronal cell lines by ratiometric fluorometry upon exposure to serum samples from pruritic patients with intrahepatic cholestasis of pregnancy (ICP), primary biliary cirrhosis (PBC), other cholestatic disorders, and pregnant, healthy, and nonpruritic disease controls. Putative [Ca(2+)](i)-inducing factors in pruritic serum were explored by analytical techniques, including quantification by high-performance liquid chromatography/mass spectroscopy. In mice, scratch activity after intradermal pruritogen injection was quantified using a magnetic device. RESULTS Transient increases in neuronal [Ca(2+)](i) induced by pruritic PBC and ICP sera were higher than corresponding controls. Lysophosphatidic acid (LPA) could be identified as a major [Ca(2+)](i) agonist in pruritic sera, and LPA concentrations were increased in cholestatic patients with pruritus. LPA injected intradermally into mice induced scratch responses. Autotaxin, the serum enzyme converting lysophosphatidylcholine into LPA, was markedly increased in patients with ICP versus pregnant controls (P < .0001) and cholestatic patients with versus without pruritus (P < .0001). Autotaxin activity correlated with intensity of pruritus (P < .0001), which was not the case for serum bile salts, histamine, tryptase, substance P, or mu-opioids. In patients with PBC who underwent temporary nasobiliary drainage, both itch intensity and autotaxin activity markedly decreased during drainage and returned to preexistent levels after drain removal. CONCLUSIONS We suggest that LPA and autotaxin play a critical role in cholestatic pruritus and may serve as potential targets for future therapeutic interventions.

Function and pathophysiological importance of ABCB4 (MDR3 P-glycoprotein)

Like several other ATP-binding cassette (ABC) transporters, ABCB4 is a lipid translocator. It translocates phosphatidylcholine (PC) from the inner to the outer leaflet of the canalicular membrane of the hepatocyte. Its function is quite crucial as evidenced by a severe liver disease, progressive familial intrahepatic cholestasis type 3, which develops in persons with ABCB4 deficiency. Translocation of PC makes the phospholipid available for extraction into the canalicular lumen by bile salts. The primary function of biliary phospholipid excretion is to protect the membranes of cells facing the biliary tree against these bile salts: the uptake of PC in bile salt micelles reduces the detergent activity of these micelles. In this review, we will discuss the functional aspects of ABCB4 and the regulation of its expression. Furthermore, we will describe the clinical and biochemical consequences of complete and partial deficiency of ABCB4 function.

Complete OATP1B1 and OATP1B3 deficiency causes human Rotor syndrome by interrupting conjugated bilirubin reuptake into the liver

Bilirubin, a breakdown product of heme, is normally glucuronidated and excreted by the liver into bile. Failure of this system can lead to a buildup of conjugated bilirubin in the blood, resulting in jaundice. The mechanistic basis of bilirubin excretion and hyperbilirubinemia syndromes is largely understood, but that of Rotor syndrome, an autosomal recessive disorder characterized by conjugated hyperbilirubinemia, coproporphyrinuria, and near-absent hepatic uptake of anionic diagnostics, has remained enigmatic. Here, we analyzed 8 Rotor-syndrome families and found that Rotor syndrome was linked to mutations predicted to cause complete and simultaneous deficiencies of the organic anion transporting polypeptides OATP1B1 and OATP1B3. These important detoxification-limiting proteins mediate uptake and clearance of countless drugs and drug conjugates across the sinusoidal hepatocyte membrane. OATP1B1 polymorphisms have previously been linked to drug hypersensitivities. Using mice deficient in Oatp1a/1b and in the multispecific sinusoidal export pump Abcc3, we found that Abcc3 secretes bilirubin conjugates into the blood, while Oatp1a/1b transporters mediate their hepatic reuptake. Transgenic expression of human OATP1B1 or OATP1B3 restored the function of this detoxification-enhancing liver-blood shuttle in Oatp1a/1b-deficient mice. Within liver lobules, this shuttle may allow flexible transfer of bilirubin conjugates (and probably also drug conjugates) formed in upstream hepatocytes to downstream hepatocytes, thereby preventing local saturation of further detoxification processes and hepatocyte toxic injury. Thus, disruption of hepatic reuptake of bilirubin glucuronide due to coexisting OATP1B1 and OATP1B3 deficiencies explains Rotor-type hyperbilirubinemia. Moreover, OATP1B1 and OATP1B3 null mutations may confer substantial drug toxicity risks.

The biliary HCO3− umbrella: A unifying hypothesis on pathogenetic and therapeutic aspects of fibrosing cholangiopathies

This review focuses on the hypothesis that biliary HCO  3− secretion in humans serves to maintain an alkaline pH near the apical surface of hepatocytes and cholangiocytes to prevent the uncontrolled membrane permeation of protonated glycine‐conjugated bile acids. Functional impairment of this biliary HCO  3− umbrella or its regulation may lead to enhanced vulnerability of cholangiocytes and periportal hepatocytes toward the attack of apolar hydrophobic bile acids. An intact interplay of hepatocellular and cholangiocellular adenosine triphosphate (ATP) secretion, ATP/P2Y‐ and bile salt/TGR5‐mediated Cl−/ HCO  3− exchange and HCO  3− secretion, and alkaline phosphatase–mediated ATP breakdown may guarantee a stable biliary HCO  3− umbrella under physiological conditions. Genetic and acquired functional defects leading to destabilization of the biliary HCO  3− umbrella may contribute to development and progression of various forms of fibrosing/sclerosing cholangitis. (HEPATOLOGY 2010)

ATP8B1 requires an accessory protein for endoplasmic reticulum exit and plasma membrane lipid flippase activity

Mutations in ATP8B1 cause progressive familial intrahepatic cholestasis type 1 and benign recurrent intrahepatic cholestasis type 1. Previously, we have shown in mice that Atp8b1 deficiency leads to enhanced biliary excretion of phosphatidylserine, and we hypothesized that ATP8B1 is a flippase for phosphatidylserine. However, direct evidence for this function is still lacking. In Saccharomyces cerevisiae, members of the Cdc50p/Lem3p family are essential for proper function of the ATP8B1 homologs. We have studied the role of two human members of this family, CDC50A and CDC50B, in the routing and activity of ATP8B1. When only ATP8B1 was expressed in Chinese hamster ovary cells, the protein localized to the endoplasmic reticulum. Coexpression with CDC50 proteins resulted in relocalization of ATP8B1 from the endoplasmic reticulum to the plasma membrane. Only when ATP8B1 was coexpressed with CDC50 proteins was a 250%‐500% increase in the translocation of fluorescently labeled phosphatidylserine observed. Importantly, natural phosphatidylserine exposure in the outer leaflet of the plasma membrane was reduced by 17%‐25% in cells coexpressing ATP8B1 and CDC50 proteins in comparison with cells expressing ATP8B1 alone. The coexpression of ATP8B1 and CDC50A in WIF‐B9 cells resulted in colocalization of both proteins in the canalicular membrane. Conclusion: Our data indicate that CDC50 proteins are pivotal factors in the trafficking of ATP8B1 to the plasma membrane and thus may be essential determinants of ATP8B1‐related disease. In the plasma membrane, ATP8B1 functions as a flippase for phosphatidylserine. Finally, CDC50A may be the potential β‐subunit or chaperone for ATP8B1 in hepatocytes. (HEPATOLOGY 2007.)

Ae2a,b-Deficient Mice Develop Antimitochondrial Antibodies and Other Features Resembling Primary Biliary Cirrhosis

BACKGROUND & AIMS Cl(-)/HCO(3)(-) anion exchanger 2 (AE2) is involved in intracellular pH (pH(i)) regulation and transepithelial acid-base transport, including secretin-stimulated biliary bicarbonate excretion. AE2 gene expression was found to be reduced in liver biopsy specimens and blood mononuclear cells from patients with primary biliary cirrhosis (PBC), a disease characterized by chronic nonsuppurative cholangitis associated with antimitochondrial antibodies (AMA) and other autoimmune phenomena. In mice with widespread Ae2 gene disruption, we previously reported altered spermiogenesis and reduced gastric acid secretion. We now describe the hepatobiliary and immunologic changes observed in these Ae2(a.b)-deficient mice. METHODS In this murine model, splenocyte pH(i) and T-cell populations were studied by flow cytometry. CD3-stimulated cytokine secretion was estimated using cytokine arrays. AMA were evaluated by immunoblotting and proteomics. Hepatobiliary changes were assessed by immunohistopathology, flow cytometry, and serum biochemistry. Cholangiocyte gene expression was analyzed by real-time polymerase chain reaction. RESULTS Ae2(a,b)(-/-) mice exhibit splenomegaly, elevated pH(i) in splenocytes, increased production of interleukin-12p70 and interferon gamma, expanded CD8(+) T-cell population, and under represented CD4(+)FoxP3(+)/regulatory T cells. Most Ae2(a,b)(-/-) mice tested positively for AMA, showing increased serum levels of immunoglobulin M and G, and liver-specific alkaline phosphatase. About one third of Ae2(a,b)(-/-) mice had extensive portal inflammation with CD8(+) and CD4(+) T lymphocytes surrounding damaged bile ducts. Cholangiocytes isolated from Ae2(a,b)(-/-) mice showed gene expression changes compatible with oxidative stress and increased antigen presentation. CONCLUSIONS Ae2 deficiency alters pH(i) homeostasis in immunocytes and gene expression profile in cholangiocytes, leading to immunologic and hepatobiliary changes that resemble PBC.

A biliary HCO3− umbrella constitutes a protective mechanism against bile acid‐induced injury in human cholangiocytes

Human cholangiocytes are continuously exposed to millimolar levels of hydrophobic bile salt monomers. We recently hypothesized that an apical biliary HCO  3− umbrella might prevent the protonation of biliary glycine‐conjugated bile salts and uncontrolled cell entry of the corresponding bile acids, and that defects in this biliary HCO  3− umbrella might predispose to chronic cholangiopathies. Here, we tested in vitro whether human cholangiocyte integrity in the presence of millimolar bile salt monomers is dependent on (1) pH, (2) adequate expression of the key HCO  3− exporter, anion exchanger 2 (AE2), and (3) an intact cholangiocyte glycocalyx. To address these questions, human immortalized cholangiocytes and cholangiocarcinoma cells were exposed to chenodeoxycholate and its glycine/taurine conjugates at different pH levels. Bile acid uptake was determined radiochemically. Cell viability and apoptosis were measured enzymatically. AE2 was knocked down by lentiviral short hairpin RNA. A cholangiocyte glycocalyx was identified by electron microscopy, was enzymatically desialylated, and sialylation was quantified by flow cytometry. We found that bile acid uptake and toxicity in human immortalized cholangiocytes and cholangiocarcinoma cell lines in vitro were pH and AE2 dependent, with the highest rates at low pH and when AE2 expression was defective. An apical glycocalyx was identified on cholangiocytes in vitro by electron microscopic techniques. Desialylation of this protective layer increased cholangiocellular vulnerability in a pH‐dependent manner. Conclusion: A biliary HCO  3− umbrella protects human cholangiocytes against damage by bile acid monomers. An intact glycocalyx and adequate AE2 expression are crucial in this process. Defects of the biliary HCO  3− umbrella may lead to the development of chronic cholangiopathies. (HEPATOLOGY 2012;55:173–183)