The sodium taurocholate cotransporting polypeptide (Ntcp, Slc10a1) is the major uptake system for bile acids into liver cells. This study investigated the degradation of rat Ntcp and human NTCP by ...the ubiquitin-proteasome system (UPS). In stably transfected HepG2 cells, rat Ntcp was complex-glycosylated and localized at the plasma membrane. Inhibition of proteasomes by MG-132 or lactacystin led to the accumulation of intracellular Ntcp, a process dependent on de novo protein synthesis. Intracellular Ntcp was core-glycosylated, indicating an endoplasmic reticulum (ER) origin. Core-glycosylated Ntcp was found in cytosolic, detergent-insoluble deposits with characteristics of aggresomes: they co-localized with ubiquitin at the microtubule organization center and Ntcp from these deposits was polyubiquitinated. Transient transfections of Ntcp/NTCP induced intracellular deposits that co-localized with ubiquitin, even in the absence of proteasome inhibitors. Similarly, in livers of patients with progressive familial intrahepatic cholestasis, NTCP could be detected co-localized with ubiquitin in hepatocytes. We conclude that maturing Ntcp/NTCP is degraded by the ubiquitin-proteasome system at the level of ER-associated degradation (ERAD). An imbalance in the synthesis and degradation of NTCP at the level of the ER or alterations in the ERAD machinery might be the cause of intracellular NTCP deposits in transient transfections and in cholestatic livers.
The transport systems involved in hepatobiliary transport have been cloned and characterized at the molecular level and it is becoming clear that mutations and polymorphisms of individual transporter ...molecules underlie a variety of liver diseases. This book provides surveys on the structure and function of transport molecules involved in hepatobiliary transport, on the role of different bile acids receptors in various organs and their function in health and disease. The book will be of interest for biochemists, structural chemists, biologists and clinicians. Dieter Häussinger, Heinrich-Heine Universität, Düsseldorf, Germany.Ralf Kubitz, Heinrich-Heine Universität, Düsseldorf, Germany.Verena Keitel, Heinrich-Heine Universität, Düsseldorf, Germany.
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•149 patients with at least one disease-causing mutation in FIC1, BSEP or MDR3 were identified.•154 different genetic variants detected, of which 25 have not been described yet.•All ...13 novel missense mutations were predicted to be disease-causing.•Risk alleles of common BSEP SNPs were overrepresented in the patient cohort, underscoring a contribution to cholestasis.
The bile salt export pump (BSEP, ABCB11), multidrug resistance protein 3 (MDR3, ABCB4) and the ATPase familial intrahepatic cholestasis 1 (FIC1, ATP8B1) mediate bile formation. This study aimed to determine the contribution of mutations and common variants in the FIC1, BSEP and MDR3 genes to cholestatic disorders of differing disease onset and severity.
Coding exons with flanking intron regions of ATP8B1, ABCB11, and ABCB4 were sequenced in cholestatic patients with assumed genetic cause. The effects of new variants were evaluated by bioinformatic tools and 3D protein modeling.
In 427 patients with suspected inherited cholestasis, 149 patients carried at least one disease-causing mutation in FIC1, BSEP or MDR3, respectively. Overall, 154 different mutations were identified, of which 25 were novel. All 13 novel missense mutations were disease-causing according to bioinformatics analyses and homology modeling. Eighty-two percent of patients with at least one disease-causing mutation in either of the three genes were children. One or more common polymorphism(s) were found in FIC1 in 35.3%, BSEP in 64.3% and MDR3 in 72.6% of patients without disease-causing mutations in the respective gene. Minor allele frequencies of common polymorphisms in BSEP and MDR3 varied in our cohort compared to the general population, as described by gnomAD. However, differences in ethnic background may contribute to this effect.
In a large cohort of patients, 154 different variants were detected in FIC1, BSEP, and MDR3, 25 of which were novel. In our cohort, frequencies for risk alleles of BSEP (p.V444A) and MDR3 (p.I237I) polymorphisms were significantly overrepresented in patients without disease-causing mutation in the respective gene, indicating that these common variants can contribute to a cholestatic phenotype.
FIC1, BSEP, and MDR3 represent hepatobiliary transport proteins essential for bile formation. Genetic variants in these transporters underlie a broad spectrum of cholestatic liver diseases. To confirm a genetic contribution to the patients’ phenotypes, gene sequencing of these three major cholestasis-related genes was performed in 427 patients and revealed 154 different variants of which 25 have not been previously reported in a database. In patients without a disease-causing mutation, common genetic variants were detected in a high number of cases, indicating that these common variants may contribute to cholestasis development.
The human multidrug resistance protein 3 (MDR3/ABCB4) belongs to the ubiquitous family of ATP-binding cassette (ABC) transporters and is located in the canalicular membrane of hepatocytes. There it ...flops the phospholipids of the phosphatidylcholine (PC) family from the inner to the outer leaflet. Here, we report the characterization of wild type MDR3 and the Q1174E mutant, which was identified previously in a patient with progressive familial intrahepatic cholestasis type 3 (PFIC-3). We expressed different variants of MDR3 in the yeast Pichia pastoris, purified the proteins via tandem affinity chromatography, and determined MDR3-specific ATPase activity in the presence or absence of phospholipids. The ATPase activity of wild type MDR3 was stimulated 2-fold by liver PC or 1,2-dioleoyl-sn-glycero-3-phosphatidylethanolamine lipids. Furthermore, the cross-linking of MDR3 with a thiol-reactive fluorophore blocked ATP hydrolysis and exhibited no PC stimulation. Similarly, phosphatidylethanolamine, phosphatidylserine, and sphingomyelin lipids did not induce an increase of wild type MDR3 ATPase activity. The phosphate analogues beryllium fluoride and aluminum fluoride led to complete inhibition of ATPase activity, whereas orthovanadate inhibited exclusively the PC-stimulated ATPase activity of MDR3. The Q1174E mutation is located in the nucleotide-binding domain in direct proximity of the leucine of the ABC signature motif and extended the X loop, which is found in ABC exporters. Our data on the Q1174E mutant demonstrated basal ATPase activity, but PC lipids were incapable of stimulating ATPase activity highlighting the role of the extended X loop in the cross-talk of the nucleotide-binding domain and the transmembrane domain.
Background: A mutation of the extended X loop of MDR3 caused hereditary liver cholestasis.
Results: Wild type MDR3 exhibited PC-induced ATPase activity, but the Q1174E mutant displayed no stimulation.
Conclusion: The glutamine preceding the ABC signature motif communicates substrate binding within the TMD to the extended X loop of the NBD.
Significance: This study provides evidence for a transmission interface coupling ATP hydrolysis to substrate transport.
Progressive familial intrahepatic cholestasis type 2 (PFIC‐2) is caused by mutations of the bile salt export pump (BSEP ABCB11), an ATP‐binding cassette (ABC)‐transporter exclusively expressed at the ...canalicular membrane of hepatocytes. An absence of BSEP from the canalicular membrane causes cholestasis and leads to liver cirrhosis, which may necessitate liver transplantation in early childhood. We report on the first case of a child with PFIC‐2 suffering from repeated posttransplant recurrence of progressive intrahepatic cholestasis due to autoantibodies against BSEP. These antibodies occurred after transplantation and were detected in the patient's serum and at the canalicular membrane of two consecutive liver transplants. The antibodies were reactive toward the first extracellular loop of BSEP, were of high affinity, and inhibited transport activity of BSEP, thus causing severe cholestasis. The patient had three homozygous, missense changes in the BSEP gene. Their combination resulted in the complete absence of BSEP, which explains the lack of tolerance, a prerequisite of autoantibody formation toward BSEP. The findings illustrate a novel disease mechanism due to a new class of functionally relevant autoantibodies resulting in cholestasis and subsequent liver failure. (HEPATOLOGY 2009;50:510–517.)
A recent genome‐wide study revealed an association between variation in the PNPLA3 gene and liver fat content. In addition, the PNPLA3 single‐nucleotide polymorphism rs738409 (M148I) was reported to ...be associated with advanced alcoholic liver disease in alcohol‐dependent individuals of Mestizo descent. We therefore evaluated the impact of rs738409 on the manifestation of alcoholic liver disease in two independent German cohorts. Genotype and allele frequencies of rs738409 (M148I) were determined in 1,043 alcoholic patients with or without alcoholic liver injury and in 376 at‐risk drinkers from a population‐based cohort. Relative to alcoholic patients without liver damage (n = 439), rs738409 genotype GG was strongly overrepresented in patients with alcoholic liver cirrhosis (n = 210; OR 2.79; Pgenotype = 1.2 × 10−5; Pallelic = 1.6 × 10−6) and in alcoholic patients without cirrhosis but with elevated alanine aminotransferase levels (n = 219; OR 2.33; Pgenotype = 0.0085; Pallelic = 0.0042). The latter, biochemically defined association was confirmed in an independent population‐based cohort of at‐risk drinkers with a median alcohol intake of 300 g/week (OR 4.75; Pgenotype = 0.040; Pallelic = 0.022), and for aspartate aminotransferase (AST) levels. Frequencies of allele PNPLA3 rs738409(G) in individuals with steatosis and normal alanine aminotransferase (ALT) and AST levels were lower than in alcoholics without steatosis and normal ALT/AST (Pcombined = 0.03). The population attributable risk of cirrhosis in alcoholic carriers of allele PNPLA3 rs738409(G) was estimated at 26.6%. Conclusion: Genotype PNPLA3 rs738409(GG) is associated with alcoholic liver cirrhosis and elevated aminotransferase levels in alcoholic Caucasians. (HEPATOLOGY 2011)
Bile acids perform vital functions in the human liver and are the essential component of bile. It is therefore not surprising that the biology of bile acids is extremely complex, regulated on ...different levels, and involves soluble and membrane receptors as well as transporters. Hereditary disorders of these proteins manifest in different pathophysiological processes that result in liver diseases of varying severity. In this review, we summarize our current knowledge of the physiology and pathophysiology of bile acids with an emphasis on recently established analytical approaches as well as the molecular mechanisms that underlie signaling and transport of bile acids. In this review, we will focus on ABC transporters of the canalicular membrane and their associated diseases. As the G protein-coupled receptor, TGR5, receives increasing attention, we have included aspects of this receptor and its interaction with bile acids.
Mutations in the gene encoding the canalicular bile salt export pump (BSEP) can result in progressive familial intrahepatic cholestasis type 2 (PFIC-2). Treatment options are limited, and PFIC-2 ...often necessitates liver transplantation. We report on a young woman and a boy who clinically presented with PFIC-2 phenotypes and dramatically improved with steroid treatment. Gene sequencing of ABCB11 encoding for BSEP revealed 2 relevant mutations in both patients. The young woman was compound heterozygous for p.T919del and p.R1235X. At the age of 5 years, partial biliary diversion was performed and rescued liver function but left serum bile salt levels elevated. At age 23 she developed systemic lupus erythematosus. Unexpectedly, steroid therapy normalized serum bile salt levels, with a strong correlation with the steroid dose. She is currently in clinical remission. The boy was compound heterozygous for the ABCB11 mutations c.150+3A>C and p.R832C and presented with intractable pruritus. When he developed colitis, he was treated with steroids. The pruritus completely disappeared and relapsed when steroids were withdrawn. To date, with low-dose budesonide, the boy has been symptom-free for >3 years. In conclusion, the clinical courses suggest that patients with BSEP deficiency and residual BSEP activity may benefit from steroid-based therapy, which represents a new treatment option.
The influence of aniso-osmolarity on the activity of the MAP kinases Erk-1 and Erk-2 was studied in C6 glioma cells. Hypo-osmotic treatment (205 mosmol/l) led to an increased activity of Erk-1 and ...Erk-2 within 3 min, which became maximal at 10 min and returned to basal level within 120 min. In contrast, Erk activity was reduced under hyper-osmotic conditions (405 mosmol/l), compared to the normo-osmotic control (305 mosmol/l). Erk activation was accompanied by a mobility shift of Raf-1. Hypo-osmotic exposure increased the cytosolic Ca
2+ concentration (Ca
2+
i). Absence of extracellular Ca
2+ largely abolished the Ca
2+
i response to hypo-osmolarity, whereas Erk activation following hypo-osmotic stimulation remained unaffected, suggesting a Ca
2+ independence of the osmosignalling pathway to the MAP kinases. Both the Ca
2+ response as well as the Erk activation following hypo-osmotic exposure were maintained in the presence of the phospholipase C inhibitor U73122. Application of 8-CPT cAMP, forskolin/isobutylmethylxanthine or isoproterenol blocked Erk activation following hypo-osmotic treatment of the cells, suggesting a role of the Ras/Raf pathway upstream from Erk-1 and Erk-2. Protein kinase C (PKC) is unlikely to play a role in the hypo-osmolarity- induced signalling towards MAP kinases, as revealed by inhibition of PKC with Gö6850. Inhibition of pertussis- or cholera toxin-sensitive G-proteins as well as inhibition of tyrosine kinases with genistein and of PI3 kinase by wortmannin had no effect on the Erk response to hypo-osmolarity. It is concluded that osmosignalling in C6 glioma cells differs upstream of the MAP kinases from that observed in primary rat astrocytes, H4IIE rat hepatoma cells and isolated rat hepatocytes.
Recurrent bile salt export pump (rBSEP) disease has been reported in progressive familial intrahepatic cholestasis type 2 (PFIC2) patients following liver transplantation (LT) and is often refractory ...to standard anti-cellular rejection immunosuppressants. The mechanism of rBSEP disease is proposed to be a form of type II hypersensitivity reaction with de novo anti-BSEP antibodies blocking the function of allograft BSEP. Utilization of C4d has not been evaluated in rBSEP. We describe a girl with 3 episodes of rBSEP with severe pruritus at 8.9, 10.3, and 11.0 years post-LT, respectively. Patient's serum reacted with normal liver canaliculi by indirect immunofluorescence (IF), whereas patient's liver showed canalicular immunoglobulin G deposition. The histologic features of all 3 liver biopsies recapitulate PFIC2 with cholestatic giant cell hepatitis. Canalicular BSEP expression was not detected in areas of feathery degeneration by immunohistochemistry, but was retained in morphologically normal liver. By direct IF, C4d showed diffuse sinusoidal staining in the third biopsy. Patient responded well to rituximab with or without intravenous immunoglobulin with subsiding symptoms and normalization of serum bile acid levels. In conclusion, rBSEP disease should be considered in the differential diagnosis when evaluating for rejection in a PFIC2 patient post-LT presenting with pruritus. A portion of liver core may be snap frozen in OCT medium for possible direct IF for C4d, that can serve as a surrogate marker for complement activation and antibody-mediated graft dysfunction.
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