Expression, localization, and inducibility by bile acids of hepatobiliary transporters in the new polarized rat hepatic cell lines, Can 3−1 and Can 10

Functional rescue of an ABCB11 mutant by ivacaftor: A new targeted pharmacotherapy approach in bile salt export pump deficiency

BACKGROUND & AIM

The canalicular bile salt export pump (BSEP/ABCB11) of hepatocytes is the main adenosine triphosphate (ATP)-binding cassette (ABC) transporter responsible for bile acid secretion. Mutations in ABCB11 cause several cholestatic diseases, including progressive familial intrahepatic cholestasis type 2 (PFIC2) often lethal in absence of liver transplantation. We investigated in vitro the effect and potential rescue of a BSEP mutation by ivacaftor, a clinically approved cystic fibrosis transmembrane conductance regulator (CFTR/ABCC7) potentiator.

METHODS

The p.T463I mutation, identified in a PFIC2 patient and located in a highly conserved ABC transporter motif, was studied by 3D structure modelling. The mutation was reproduced in a plasmid encoding a rat Bsep-green fluorescent protein. After transfection, mutant expression was studied in Can 10 cells. Taurocholate transport activity and ivacaftor effect were studied in Madin-Darby canine kidney (MDCK) clones co-expressing the rat sodium-taurocholate co-transporting polypeptide (Ntcp/Slc10A1).

RESULTS

As the wild-type protein, Bsep^T463I was normally targeted to the canalicular membrane of Can 10 cells. As predicted by 3D structure modelling, taurocholate transport activity was dramatically low in MDCK clones expressing Bsep^T463I . Ivacaftor treatment increased by 1.7-fold taurocholate transport activity of Bsep^T463I (P < .0001), reaching 95% of Bsep^wt activity. These data suggest that the p.T463I mutation impairs ATP-binding, resulting in Bsep dysfunction that can be rescued by ivacaftor.

CONCLUSION

These results provide experimental evidence of ivacaftor therapeutic potential for selected patients with PFIC2 caused by ABCB11 missense mutations affecting BSEP function. This could represent a significant step forward for the care of patients with BSEP deficiency.

Successful Treatment with Rituximab and Immunoadsorption for an Auto-Antibody Induced Bile Salt Export Pump Deficiency in a Liver Transplanted Patient

We present an 8 years old girl who was diagnosed at 6 months of age of Progressive Familial Intrahepatic Cholestasis type 2. Although liver transplantation (LT) was classically considered curative for these patients, cholestasis recurrence with normal gamma-glutamyl transpeptidase (GGT), mediated by anti-bile salt export pump (BSEP) antibodies after LT (auto-antibody Induced BSEP Deficiency, AIBD) has been recently reported. Our patient underwent LT at 14 months. During her evolution, patient presented three episodes of acute rejection. Seven years after the LT, the patient presented pruritus with cholestasis and elevation of liver enzymes with persistent normal GGT. Liver biopsy showed intrahepatic cholestasis and giant-cell transformation with very low BSEP activity. Auto-antibodies against BSEP were detected therefore an AIBD was diagnosed. She was treated with Rituximab and immunoadsorption with resolution of the AIBD. As a complication of the treatment she developed a pneumocystis infection successfully treated with corticoids, cotrimoxazol and anidulafungin.

The P2X4 purinergic receptor impacts liver regeneration after partial hepatectomy in mice through the regulation of biliary homeostasis

Many regulatory pathways are involved in liver regeneration after partial hepatectomy (PH), to initiate growth, protect liver cells, and sustain remnant liver functions. Extracellular adenosine triphosphate rises in blood and bile after PH and contributes to liver regeneration, although purinergic receptors and mechanisms remain to be precisely explored. In this work we analyzed during regeneration after PH the involvement of P2X4 purinergic receptors, highly expressed in the liver. P2X4 receptor expression in the liver, liver histology, hepatocyte proliferation, plasma bile acid concentration, bile flow and composition, and lysosome distribution in hepatocytes were studied in wild-type and P2X4 knockout (KO) mice, before and after PH. P2X4 receptors were expressed in hepatocytes and Kupffer cells; in hepatocytes, P2X4 was concentrated in subcanalicular areas closely costained with lysosomal markers. After PH, delayed regeneration, hepatocyte necrosis, and cholestasis were observed in P2X4-KO mice. In P2X4-KO mice, post-PH biliary adaptation was impaired with a smaller increase in bile flow and HCO3 (-) biliary output, as well as altered biliary composition with reduced adenosine triphosphate and lysosomal enzyme release. In line with these data, lysosome distribution and biogenesis were altered in P2X4-KO compared with wild-type mice.

CONCLUSION

During liver regeneration after PH, P2X4 contributes to the complex control of biliary homeostasis through mechanisms involving pericanalicular lysosomes, with a resulting impact on hepatocyte protection and proliferation. (Hepatology 2016;64:941-953).

Basolateral sorting and transcytosis define the Cu+-regulated translocation of ATP7B to the bile canaliculus

The Cu+ pump ATP7B plays an irreplaceable role in the elimination of excess Cu+ by the hepatocyte into the bile. The traffic and site of ATP7B action is a subject of controversy. One current proposal is that an increase in intracellular Cu+ results in the translocation of ATP7B to the lysosomes and excretion of excess Cu+ by lysosomal mediated exocytosis at the bile canaliculus. Here we show that ATP7B is transported from the trans-Golgi network to the bile canaliculus by basolateral sorting and endocytosis, and microtubule mediated transcytosis through the subapical compartment. Trafficking ATP7B is not incorporated into lysosomes and addition of Cu+ does not cause relocalization of lysosomes and the appearance of lysosome markers in the bile canaliculus. Our data describes the pathway of the Cu+-mediated transport of ATP7B from the TGN to the bile canaliculus and indicates that the bile canaliculus is the prime site of ATP7B action in the elimination of excess Cu+.

Notch inhibition promotes fetal liver stem/progenitor cells differentiation into hepatocytes via the inhibition of HNF-1β

In a previous study, the Notch pathway inhibited with N-[N-(3,5-difluorophenacetyl)-L-alanyl]-S-phenylglycine t-butyl ester (also called DAPT) was shown to promote the differentiation of fetal liver stem/progenitor cells (FLSPCs) into hepatocytes and to impair cholangiocyte differentiation. The precise mechanism for this, however, was not elucidated. Two mechanisms are possible: Notch inhibition might directly up-regulate hepatocyte differentiation via HGF (hepatocyte growth factor) and HNF (hepatocyte nuclear factor)-4α or might impair cholangiocyte differentiation thereby indirectly rendering hepatocyte differentiation as the dominant state. In this study, HGF and HNF expression was detected after the Notch pathway was inhibited. Although our initial investigation indicated that the inhibition of Notch induced hepatocyte differentiation with an efficiency similar to the induction via HGF, the results of this study demonstrate that Notch inhibition does not induce significant up-regulation of HGF or HNF-4α in FLSPCs. This suggests that Notch inhibition induces hepatocyte differentiation without the influence of HGF or HNF-4α. Moreover, significant down-regulation of HNF-1β was observed, presumably dependent on an impairment of cholangiocyte differentiation. To confirm this presumption, HNF-1β was blocked in FLSPCs and was followed by hepatocyte differentiation. The expression of markers of mature cholangiocyte was impaired and hepatocyte markers were elevated significantly. The data thus demonstrate that the inhibition of cholangiocyte differentiation spontaneously induces hepatocyte differentiation and further suggest that hepatocyte differentiation from FLSPCs occurs at the expense of the impairment of cholangiocyte differentiation, probably being enhanced partially via HNF-1β down-regulation or Notch inhibition.

Role of drug-independent stress factors in liver injury associated with diclofenac intake

Although a basic understanding of the chemical and biological events leading to idiosyncratic drug toxicity is still lacking, it appears that drug-independent risk factors that increase reactive metabolite formation or alter cellular stress and immune response may be critical determinants in the response to an otherwise non-toxic drug. Thus, we were interested to determine the impact of various drug-independent stress factors - lipopolysaccharide (LPS), poly I:C (PIC) or glutathione depletion via buthionine sulfoximine (BSO) - on the toxicity of diclofenac (Dcl), a model drug associated with rare but significant cases of serious hepatotoxicity, and to understand if enhanced toxicity occurs through alterations of drug metabolism and/or modulation of stress response pathways. Co-treatment of rats repeatedly given therapeutic doses of Dcl for 7 days with a single dose of LPS 2h before the last Dcl dose resulted in severe liver toxicity. Neither LPS nor diclofenac alone or in combination with PIC or BSO had such an effect. While it is thought that bioactivation to reactive Dcl acyl glucuronides (AG) and subsequent protein adduct formation contribute to Dcl induced liver injury, LC-MS/MS analyses did not reveal increased formation of 4'- and 5-hydroxy-Dcl, Dcl-AG or Dcl-AG dependent protein adducts in animals treated with LPS/Dcl. Hepatic gene expression analysis suggested enhanced activation of NFκB and MAPK pathways and up-regulation of co-stimulatory molecules (IL-1β, TNF-α, CINC-1) by LPS/Dcl and PIC/Dcl, while protective factors (HSPs, SOD2) were down-regulated. LPS/Dcl led to extensive release of pro-inflammatory cytokines (IL-1β, IL-6, IFN-γ, TNF-α) and factors thought to constitute danger signals (HMGB1, CINC-1) into plasma. Taken together, our results show that Dcl enhanced the inflammatory response induced by LPS - and to a lesser extent by PIC - through up-regulation of pro-inflammatory molecules and down-regulation of protective factors. This suggests sensitization of cells to cellular stress mediated by non-drug-related risk factors by therapeutic doses of Dcl, rather than potentiation of Dcl toxicity by the stress factors.

Build them up and break them down: Tight junctions of cell lines expressing typical hepatocyte polarity with a varied repertoire of claudins

Tight junctions (TJs) of cells expressing simple epithelial polarity have been extensively studied, but less is known about TJs of cells expressing complex polarity. In this paper we analyzed, TJs of four different lines, that form bile canaliculi (BC) and express typical hepatocyte polarity; WIF-B9, 11–3, Can 3–1, Can 10. Striking differences were observed in claudin expression. None of the cell lines produced claudin-1. WIF-B9 and 11–3 expressed only claudin-2 while Can 3–1 and Can 10 expressed claudin-2,-3,-4,-5. TJs of these two classes of lines differed in their ultra-stucture, paracellular permeability, and robustness. Lines expressing a large claudin repertoire, especially Can 10, had complex and efficient TJs, that were maintained when cells were depleted in calcium. Inversely, TJs of WIF-B9 and 11–3 were leaky, permissive and dismantled by calcium depletion. Interestingly, we found that during the polarization process, TJ proteins expressed by all lines were sequentially settled in a specific order: first occludin, ZO-1 and cingulin, then JAM-A and ZO-2, finally claudin-2. Claudins expressed only in Can lines were also sequentially settled: claudin-3 was the first settled. Inhibition of claudin-3 expression delayed BC formation in Can10 and induced the expression of simple epithelial polarity. These results highlight the role of claudins in the settlement and the efficiency of TJs in lines expressing typical hepatocyte polarity. Can 10 seems to be the most promising of these lines because of its claudin repertoire near that of hepatocytes and its capacity to form extended tubular BC sealed by efficient TJs.

Liver metabolic/oxidative stress induces hepatic and extrahepatic changes in the expression of the vitamin C transporters SVCT1 and SVCT2

PURPOSE

Owing to its ability to inactivate harmful radicals, vitamin C plays a key role in antioxidant defense. The bioavailability of this vitamin depends upon the nutritional intake and its uptake by cells, mainly through the sodium-dependent transporters SVCT1/Svct1 and SVCT2/Svct2 (human/rat). Here, we investigated the effect of liver metabolic/oxidative stress on the expression of these transporters in extrahepatic tissues.

METHODS AND RESULTS

In Zucker rats, used here as a model of liver steatosis, Svct1-2 mRNA levels were similar in obese and lean animals, except for lung tissue, where Svct2 was up-regulated. Diabetes mellitus, developed by streptozotocin administration, was accompanied by a down-regulation of Svct1 in liver and kidney, together with a down-regulation of Svct2 in kidney and brain. Complete obstructive cholestasis due to bile duct ligation for 1 week induced a significant down-regulation of both Svct1 and Svct2 in ileum, whereas Svct2 was up-regulated in liver, and no significant changes in the expression of either transporter were found in kidney, brain or lung. In rat hepatoma Can-10 cells, bile acids, but not the FXR agonist GW4064, induced an up-regulation of Svct1 and Svct2. In human hepatoma Alexander cells transfected with FXR/RXRα/OATP1B1, neither GW4064 nor unconjugated or glycine-/taurine-conjugated major bile acids were able to up-regulate either SVCT1 or SVCT2.

CONCLUSIONS

Pathological circumstances characterized by the presence of metabolic/oxidative stress in the liver induce different responses in the expression of ascorbic acid transporters in intrahepatic and extrahepatic tissues, which may affect the overall bioavailability and cellular uptake of this vitamin.

Successful mutation-specific chaperone therapy with 4-phenylbutyrate in a child with progressive familial intrahepatic cholestasis type 2

BACKGROUND & AIMS

Progressive familial intrahepatic cholestasis type 2 (PFIC2) is due to mutations in ABCB11 encoding the canalicular bile salt export pump (BSEP) of hepatocyte. Liver transplantation is usually required. 4-phenylbutyrate (4-PB) has been shown in vitro to retarget some selected mutated apical transporters. After an in vitro study in a hepatocellular polarized line, we tested 4-PB treatment in a child with a homozygous p.T1210P BSEP mutation.

METHODS

Can 10 cells were transfected with plasmids encoding wild type Bsep (Bsep(wt)) and mutated p.T1210P Bsep (Bsep(T1210P)), both tagged with GFP. Then, cells were treated with 4-PB at 37 or 27°C, immunostained and analyzed using confocal microscopy. The child received 4-PB orally in two divided doses and BSEP liver immunostaining was performed before and after 4-PB as well as bile analysis.

RESULTS

In Can 10 cells, in contrast to Bsep(wt)-GFP, Bsep(T1210P)-GFP was not detected at the canalicular membrane but in the endoplasmic reticulum. 4-PB as well as incubation at 27°C partially corrected Bsep(T1210P)-GFP targeting to the canalicular membrane, while combined treatments resulted in normal canalicular localization. In the child, we showed that 4-PB improved clinical and biological parameters of cholestasis and liver function. Also, canalicular expression of p.T1210P BSEP mutant was partially corrected as was biliary bile acid excretion.

CONCLUSIONS

The results illustrate for the first time the therapeutic potential of a clinically approved chaperone drug in a selected patient with PFIC2 and support that bile secretion improvement might be due to the ability of 4-PB to retarget mutated BSEP.

Claudin-1 involved in neonatal ichthyosis sclerosing cholangitis syndrome regulates hepatic paracellular permeability

Neonatal ichthyosis and sclerosing cholangitis (NISCH) syndrome is a liver disease caused by mutations of CLDN1 encoding Claudin-1, a tight-junction (TJ) protein. In this syndrome, it is speculated that cholestasis is caused by Claudin-1 absence, leading to increased paracellular permeability and liver injuries secondary to paracellular bile regurgitation. We studied the role of claudin-1 in hepatic paracellular permeability. A NISCH liver and polarized rat cell lines forming TJs, the hepatocellular Can 10 and the cholangiocellular normal rat choloangiocyte (NRC), were used. In contrast to NRC, Can 10 does not express claudin-1. Can 10 cells were transfected with a plasmid encoding Claudin-1, and stable Claudin-1-expressing clones were isolated. Claudin-1 expression was silenced by transfection with short interfering RNA in Can 10 clones and with short hairpin RNA in NRC. Claudin-1 expression was evaluated by quantitative reverse-transcriptase polymerase chain reaction, immunoblotting, and immunolocalization. Paracellular permeability was assessed by fluorescein isothiocyanate-dextran passage in both lines and by transepithelial resistance measurements in NRC. In the NISCH liver, Claudin-1 was not detected in hepatocytes or cholangiocytes. In Claudin-1 expressing Can 10 clones, Claudin-1 was localized at the TJ and paracellular permeability was decreased, compared to parental Can 10 cells, this decrease correlating with claudin-1 levels. Silencing of Claudin-1 in Can 10 clones increased paracellular permeability to a level similar to that of parental cells. Similarly, we observed an increase of paracellular permeability in NRC cells silenced for claudin-1 expression.

CONCLUSION

Defect in claudin-1 expression increases paracellular permeability in polarized hepatic cell lines, supporting the hypothesis that paracellular bile leakage through deficient TJs is involved in liver pathology observed in NISCH syndrome.

Regulation of hepatic ABCC transporters by xenobiotics and in disease states

The subfamily of ABCC transporters consists of 13 members in mammals, including the multidrug resistance-associated proteins (MRPs), sulfonylurea receptors (SURs), and the cystic fibrosis transmembrane conductance regulator (CFTR). These proteins play roles in chemical detoxification, disposition, and normal cell physiology. ABCC transporters are expressed differentially in the liver and are regulated at the transcription and translation level. Their expression and function are also controlled by post-translational modification and membrane-trafficking events. These processes are tightly regulated. Information about alterations in the expression of hepatobiliary ABCC transporters could provide important insights into the pathogenesis of diseases and disposition of xenobiotics. In this review, we describe the regulation of hepatic ABCC transporters in humans and rodents by a variety of xenobiotics, under disease states and in genetically modified animal models deficient in transcription factors, transporters, and cell-signaling molecules.

NMR studies reveal the role of biomembranes in modulating ligand binding and release by intracellular bile acid binding proteins

Bile acid molecules are transferred vectorially between basolateral and apical membranes of hepatocytes and enterocytes in the context of the enterohepatic circulation, a process regulating whole body lipid homeostasis. This work addresses the role of the cytosolic lipid binding proteins in the intracellular transfer of bile acids between different membrane compartments. We present nuclear magnetic resonance (NMR) data describing the ternary system composed of the bile acid binding protein, bile acids, and membrane mimetic systems, such as anionic liposomes. This work provides evidence that the investigated liver bile acid binding protein undergoes association with the anionic membrane and binding-induced partial unfolding. The addition of the physiological ligand to the protein-liposome mixture is capable of modulating this interaction, shifting the equilibrium towards the free folded holo protein. An ensemble of NMR titration experiments, based on nitrogen-15 protein and ligand observation, confirm that the membrane and the ligand establish competing binding equilibria, modulating the cytoplasmic permeability of bile acids. These results support a mechanism of ligand binding and release controlled by the onset of a bile salt concentration gradient within the polarized cell. The location of a specific protein region interacting with liposomes is highlighted.

Which in vitro models could be best used to study hepatocyte polarity?

The correct functioning of the liver is ensured by the setting and the maintenance of hepatocyte polarity. The complex polarity of the hepatocyte is characterized by the existence of several basolateral and apical poles per cell. Many in vitro models are available for studying hepatocyte polarity, but which are the more suitable? To answer this question, we aimed to identify criteria which determine the typical hepatocyte polarity. Therefore, we compiled a range of protein markers of membrane domains in rat hepatocytes and investigated their involvement in hepatocytic functions. Then, we focused on the relationship between hepatic functions and the cytoskeleton, Golgi apparatus and endoplasmic reticulum. Subsequently, we compared different cell lines expressing hepatocyte polarity. Finally, to demonstrate the usefulness of some of these lines, we presented new data on endoplasmic reticulum organization in relation to polarity.

ATP7B copper-regulated traffic and association with the tight junctions: copper excretion into the bile

BACKGROUND & AIMS

The copper transporter ATP7B plays a central role in the elimination of excess copper by the liver into the bile, yet the site of its action remains controversial. The studies reported here examine the correspondence between the site of ATP7B action and distribution and the pathways of copper disposal by the liver.

METHODS

Microscopy and cell fractionation studies of polarized Can 10 cells forming long-branched bile canaliculi have been used to study the cellular distribution of ATP7B. Copper excretion into the bile was studied in perfused rat liver.

RESULTS

Copper excess provokes a massive download of the ATP7B retained in the trans-Golgi network into the bile canalicular membrane. Furthermore, a stable ATP7B pool is localized to the tight junctions that seal the bile canaliculi. The profile of Cu(64) excretion into the bile by isolated rat livers perfused under one-pass conditions provides evidence of copper excretion by 2 separate mechanisms, transcytosis across the hepatocyte and paracellular transport throughout the tight junctions.

CONCLUSIONS

Whereas the ATP7B retained in the trans-Golgi-network is massively translocated to the bile canalicular membrane in response to increased copper levels, a pool of ATP7B associated with the tight junctions remains stable. In situ studies indicate that copper is excreted into the bile by 2 separate pathways. The results are discussed in the frame of the normal and impeded excretion of copper into the bile.