The sodium taurocholate cotransporting polypeptide (Ntcp) is the major bile salt uptake transporter at the sinusoidal membrane of hepatocytes. Short‐term feedback regulation of Ntcp by primary bile ...salts has not yet been investigated in vivo. Subcellular localization of Ntcp was analyzed in Ntcp‐transfected HepG2‐cells by flow cytometry and in immunofluorescence images from tissue sections by a new automated image analysis method. Net bile salt uptake was investigated in perfused rat liver by a pulse chase technique. In Flag‐Ntcp‐EGFP (enhanced green fluorescent protein) expressing HepG2‐cells, taurochenodeoxycholate (TCDC), but not taurocholate (TC), induced endocytosis of Ntcp. TCDC, but not TC, caused significant internalization of Ntcp in perfused rat livers, as shown by an increase in intracellular Ntcp immunoreactivity, whereas Bsep distribution remained unchanged. These results correlate with functional studies. Rat livers were continuously perfused with 100 μmol/L of TC. 25 μmol/L of TCDC, taurodeoxycholate (TDC), tauroursodeoxycholate (TUDC), or TC were added for 30 minutes, washed out, followed by a pulse of 3H‐TC. TCDC, but not TDC, TUDC, or TC significantly increased the amount of 3H‐TC in the effluent, indicating a reduced sinusoidal net TC uptake. This effect was sensitive to chelerythrine (protein kinase C inhibitor) and cypermethrin (protein phosphatase 2B inhibitor). Phosphoinositide 3‐kinase (PI3K) inhibitors had an additive effect, whereas Erk1/2 (extracellular signal activated kinase 1/2), p38MAPK, protein phosphatase 1/2A (PP1/2A), and reactive oxygen species (ROS) were not involved. Conclusion: TCDC regulates bile salt transport at the sinusoidal membrane by protein kinase C‐ and protein phosphatase 2B‐mediated retrieval of Ntcp from the plasma membrane. During increased portal bile salt load this mechanism may adjust bile salt uptake along the acinus and protect periportal hepatocytes from harmful bile salt concentrations. (HEPATOLOGY 2012;56:2142–2153)
Experimental evidence has been provided that a histidine-loop within the nucleotide binding domain of ABC transporter is essential for efficient function of this class of transporter proteins. Here ...we report the first patient with a mutation of the putative histidine‐loop of a human ABC transporter, the multi drug resistance protein 3 (MDR3). The patient presented at the age of 4years with a history of severe pruritus, elevated serum gamma-glutamyltransferase and bile acid levels since several years suggesting the diagnosis of progressive familial intrahepatic cholestasis type 3 (PFIC-3) due to defects in MDR3. Liver biopsy demonstrated an apparently normal MDR3 expression, however, genetic analysis revealed a novel homozygous mutation in the ABCB4 gene (c.3691C>T) in the patient. This mutation was associated with a change of histidine to tyrosine at amino acid position 1231 of MDR3 (p.H1231Y). As shown by sequence alignment, this amino acid corresponds to the highly conserved histidine of the “H-loop”, which is critical for ATP-hydrolysis, suggesting an essential role of histidine 1231 of human MDR3.
► A new disease-causing mutation of the phospholipid transporter MDR3 is reported. ► This is the first mutation of the histidine-loop identified in a human ABC transporter. ► The findings approve the importance of the histidine-loop of ABC transporter. ► Ursodeoxycholic acid is an appropriate treatment for this particular mutation of MDR3.
Genetic variants in the adenosine triphosphate‐binding cassette subfamily B member 4 (ABCB4) gene, which encodes hepatocanalicular phosphatidylcholine floppase, can lead to different phenotypes, such ...as progressive familial intrahepatic cholestasis (PFIC) type 3, low phospholipid‐associated cholelithiasis, and intrahepatic cholestasis of pregnancy. The aim of this multicenter project was to collect information on onset and progression of this entity in different age groups and to assess the relevance of this disease for the differential diagnosis of chronic liver disease. Clinical and laboratory data of 38 patients (17 males, 21 females, from 29 families) with homozygous or (compound) heterozygous ABCB4 mutations were retrospectively collected. For further analysis, patients were grouped according to the age at clinical diagnosis of ABCB4‐associated liver disease into younger age (<18 years) or adult age (≥18 years). All 26 patients diagnosed in childhood presented with pruritus (median age 1 year). Hepatomegaly and splenomegaly were present in 85% and 96% of these patients, respectively, followed by jaundice (62%) and portal hypertension (69%). Initial symptoms preceded diagnosis by 1 year, and 13 patients received a liver transplant (median age 6.9 years). Of note, 9 patients were misdiagnosed as biliary atresia, Alagille syndrome, or PFIC type 1. In the 12 patients with diagnosis in adulthood, the clinical phenotype was generally less severe, including intrahepatic cholestasis of pregnancy, low phospholipid‐associated cholelithiasis, or (non)cirrhotic PFIC3. Conclusion: ABCB4 deficiency with onset in younger patients caused a more severe PFIC type 3 phenotype with the need for liver transplantation in half the children. Patients with milder phenotypes are often not diagnosed before adulthood. One third of the children with PFIC type 3 were initially misdiagnosed, indicating the need for better diagnostic tools and medical education. (Hepatology Communications 2018;2:504‐514)
Activation of hepatic stellate cells (HSCs) results in multiple alterations of cell function, but nothing is known about organic osmolytes in these cells. Organic osmolyte transport and transporter ...messenger RNA (mRNA) expression was studied in quiescent rat HSCs and after their transformation into α1–smooth muscle actin–positive myofibroblastlike cells. Quiescent stellate cells expressed in an osmosensitive manner the mRNA levels of the transporters for taurine (TAUT) and myoinositol (SMIT), whereas that for betaine was not detectable. However, these cells showed osmosensitive uptake not only of taurine and myoinositol but also of betaine. Osmosensitive betaine uptake was mediated by amino acid transport system A. After transformation into myofibroblasts, taurine and myoinositol uptake increased 5.5‐fold and 4.5‐fold, respectively, together with the respective transporter mRNA levels. Betaine uptake increased twofold because of osmosensitive induction of BGT1 expression. In both quiescent and activated HSCs, hypoosmotic cell swelling induced a rapid and 4,4′‐diisothiocyanatostilbene‐2,2′‐disulphonic acid–sensitive osmolyte efflux. In quiescent HSCs, hyperosmotic exposure increased the messenger RNA (mRNA) level of cyclooxygenase‐2, which was counteracted by taurine but not by betaine or myoinositol. The study identifies taurine, myoinositol, and betaine as osmolytes in HSCs. Transformation of HSCs is accompanied by enhanced osmolyte transport activity and induction of the BGT1 transporter, which may be another activation marker of HSCs.
Cholestatic liver disease (CLD) is a major cause of progressive liver damage and liver failure. Several forms of biliary cirrhosis are caused by mutations in specific genes. We sought to identify a ...genetic defect in a family with CLD impossible to assign to a distinct pathogenic entity. Clinical and histopathological characterization of the family members, microarray‐based single‐nucleotide polymorphism genotyping, and analysis of candidate genes were performed. Among six of 11 siblings severely affected by idiopathic CLD in a family from a population isolate in Transylvania, three died of cirrhosis (aged 5, 7, and 43 years) and three had adult‐onset disease with small duct cholangiopathy, including ductopenia. Others were mildly affected and experienced intrahepatic cholestasis of pregnancy, miscarriages, or stillbirth. Pedigree studies revealed distant parental consanguinity. Genome‐wide linkage analysis and autozygosity mapping yielded a single maximal lod‐score of 3.88 on chromosome 7q21.1‐7q22, excluding other genomic loci. Sequencing of ABCB4 at this locus revealed a novel missense mutation c.2362C>T (p.Arg788Trp) which cosegregated with severity of disease. Bile from a mutation homozygote showed a reduced phosphatidylcholine/bile acid ratio, consistent with reduced ABCB4 phosphatidylcholine transport activity. Conclusion: We show that a missense mutation in ABCB4 is a cause for ductopenic CLD in adulthood. Allelic status correlated with severity of liver disease ranging from intrahepatic cholestasis of pregnancy through fibrosis to cirrhosis and death in childhood and adulthood. Mutational analysis of ABCB4 should be generally considered in all patients with cholestatic liver disease of unknown etiology regardless of age and onset of disease. (HEPATOLOGY 2008.)
PFIC due to BSEP mutations (PFIC type 2) often necessitates OLT. It has recently been recognized that some PFIC‐2 patients develop phenotypic disease recurrence post‐OLT due to the appearance of ...anti‐BSEP antibodies. Here, we describe a boy who became cholestatic four yr after OLT during modification of immunosuppression. Canalicular antibody deposits were detected in biopsies of the transplant and antibodies specifically reacting with BSEP were identified at high titers in his serum. These antibodies bound extracellular epitopes of BSEP and inhibited BS transport and were assumed to cause disease recurrence. Consequently, anti‐BSEP antibody depletion was pursued by IA and B‐cell depletion by anti‐CD20 antibodies (rituximab) along with a switch of immunosuppression. This treatment resulted in prolonged relief of symptoms. Depletion of pathogenic anti‐BSEP antibodies causing AIBD after OLT in PFIC‐2 patients should be considered as a central therapeutic goal.
A young patient with recurrent attacks of intrahepatic cholestasis is described. On the basis of clinical presentation, laboratory findings and genetic analysis, the diagnosis of benign recurrent ...intrahepatic cholestasis type 2 (BRIC-2) was established. By the use of BSEP-specific antibodies, almost complete absence of BSEP from the canalicular membrane of liver cells was detected in the patient. Two different BSEP mutations were found. One mutation (E186G) had been described in one BRIC-2 case; the second mutation (V444A) is more frequent and has been linked to intrahepatic cholestasis of pregnancy. It is concluded that this form of compound heterozygosity of the BSEP gene reduces the amount of BSEP protein due to protein instability or mis-targeting, which is the underlying reason for reduced bile salt excretion and cholemia.
Background & Aims
The bile salt export pump (BSEP, ABCB11) is essential for bile salt secretion at the canalicular membrane of liver cells. Clinical phenotypes associated with BSEP mutations are ...commonly categorized as benign recurrent intrahepatic cholestasis (BRIC‐2) or progressive familial intrahepatic cholestasis (PFIC‐2).
Methods
The molecular basis of BSEP‐associated liver disease in a sibling pair was characterized by immunostaining, gene sequencing, bile salt analysis and recombinant expression in mammalian cells and yeast for localization and in vitro activity studies respectively.
Results
Benign recurrent intrahepatic cholestasis was considered in a brother and sister who both suffered from intermittent cholestasis since childhood. Gene sequencing of ABCB11 identified the novel missense mutation p.G374S, which is localized in the putative sixth transmembrane helix of BSEP. Liver fibrosis was present in the brother at the age of 18 with progression to cirrhosis within 3 years. Immunofluorescence of liver tissue showed clear canalicular BSEP expression; however, biliary concentration of bile salts was drastically reduced. In line with these in vivo findings, HEK293 cells showed regular membrane targeting of human BSEPG374S, whereas in vitro transport measurements revealed a strongly reduced transport activity.
Conclusions
The novel mutation p.G374S impairs transport function without disabling membrane localization of BSEP. While all other known BSEP mutations within transmembrane helices are associated with PFIC‐2, the new p.G374S mutation causes a transitional phenotype between BRIC‐2 and PFIC‐2.
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