Progressive familial intrahepatic cholestasis (PFIC) refers to heterogeneous group of autosomal recessive disorders of childhood that disrupt bile formation and present with cholestasis of hepatocellular origin. The exact prevalence remains unknown, but the estimated incidence varies between 1/50,000 and 1/100,000 births.
Three types of PFIC have been identified and related to mutations in hepatocellular transport system genes involved in bile formation. PFIC1 and PFIC2 usually appear in the first months of life, whereas onset of PFIC3 may also occur later in infancy, in childhood or even during young adulthood. Main clinical manifestations include cholestasis, pruritus and jaundice. PFIC patients usually develop fibrosis and end-stage liver disease before adulthood. Serum gamma-glutamyltransferase (GGT) activity is normal in PFIC1 and PFIC2 patients, but is elevated in PFIC3 patients. Both PFIC1 and PFIC2 are caused by impaired bile salt secretion due respectively to defects in ATP8B1 encoding the FIC1 protein, and in ABCB11 encoding the bile salt export pump protein (BSEP). Defects in ABCB4, encoding the multi-drug resistant 3 protein (MDR3), impair biliary phospholipid secretion resulting in PFIC3.
Diagnosis is based on clinical manifestations, liver ultrasonography, cholangiography and liver histology, as well as on specific tests for excluding other causes of childhood cholestasis. MDR3 and BSEP liver immunostaining, and analysis of biliary lipid composition should help to select PFIC candidates in whom genotyping could be proposed to confirm the diagnosis. Antenatal diagnosis can be proposed for affected families in which a mutation has been identified. Ursodeoxycholic acid (UDCA) therapy should be initiated in all patients to prevent liver damage. In some PFIC1 or PFIC2 patients, biliary diversion can also relieve pruritus and slow disease progression. However, most PFIC patients are ultimately candidates for liver transplantation. Monitoring of hepatocellular carcinoma, especially in PFIC2 patients, should be offered from the first year of life. Hepatocyte transplantation, gene therapy or specific targeted pharmacotherapy may represent alternative treatments in the future.
Progressive familial intrahepatic cholestasis (PFIC) is a group of rare disorders which are caused by defect in bile secretion and present with intrahepatic cholestasis, usually in infancy and childhood. These are autosomal recessive in inheritance. The estimated incidence is about 1 per 50,000 to 1 per 100,000 births, although exact prevalence is not known. These diseases affect both the genders equally and have been reported from all geographical areas. Based on clinical presentation, laboratory findings, liver histology and genetic defect, these are broadly divided into three types—PFIC type 1, PFIC type 2 and PFIC type 3. The defect is in ATP8B1 gene encoding the FIC1 protein, ABCB 11 gene encoding BSEP protein and ABCB4 gene encoding MDR3 protein in PFIC1, 2 and 3 respectively. The basic defect is impaired bile salt secretion in PFIC1/2 whereas in PFIC3, it is reduced biliary phospholipid secretion. The main clinical presentation is in the form of cholestatic jaundice and pruritus. Serum gamma glutamyl transpeptidase (GGT) is normal in patients with PFIC1/2 while it is raised in patients with PFIC3. Treatment includes nutritional support (adequate calories, supplementation of fat soluble vitamins and medium chain triglycerides) and use of medications to relieve pruritus as initial therapy followed by biliary diversion procedures in selected patients. Ultimately liver transplantation is needed in most patients as they develop progressive liver fibrosis, cirrhosis and end stage liver disease. Due to the high risk of developing liver tumors in PFIC2 patients, monitoring is recommended from infancy. Mutation targeted pharmacotherapy, gene therapy and hepatocyte transplantation are being explored as future therapeutic options.
cholestasis; familial; bile secretion; pruritus; children; ABC, ATP binding cassette; ASBT, apical sodium bile salt transporter; ATP, adenosine triphosphate; ATPase, adenosine triphosphatase; BRIC, benign recurrent intrahepatic cholestasis; BSEP, bile salt exporter protein; CFTR, cystic fibrosis transmembrane conductance regulator; CYP, cytochrome P; DNA, deoxyribonucleic acid; ERAD, endoplasmic reticulum associated degradation; ESLD, end stage liver disease; FIC1, familial intrahepatic cholestasis protein 1; FXR, farnesoid X receptor; HCC, hepatocellular carcinoma; IB, ileal bypass; ICP, intrahepatic cholestasis of pregnancy; LT, liver transplant; MARS, Molecular Adsorbent Recirculating System; MDR, multidrug resistance protein; MRCP, magnetic resonance cholangiopancreaticography; mRNA, messenger ribonucleic acid; PBD, partial biliary drainage; PEBD, partial external biliary drainage; PFIC, progressive familial intrahepatic cholestasis; PIBD, partial internal biliary drainage; pGp, p-glycoprotein; PPAR, peroxisome proliferator activator receptor; UDCA, ursodeoxycholic acid
Progressive familial intrahepatic cholestasis, type 2 (PFIC2), characterized by cholestasis in infancy that may progress to cirrhosis, is caused by mutation in ABCB11, which encodes bile salt export pump (BSEP). We correlated histopathologic, immunohistochemical, and ultrastructural features in PFIC2 with specific mutations and clinical course. Twelve patients with clinical PFIC2 and ABCB11 mutations were identified, and 22 liver biopsy and explant specimens were assessed. All had hepatocellular cholestasis; most had canalicular bile plugs. At least 1 specimen from every patient had centrizonal/sinusoidal fibrosis, often with periportal fibrosis. Neonatal hepatitis-like features (inflammation, giant cells, necrosis) varied. In 2 of the 5 patients with paired specimens obtained > 6 months apart, lobular and portal fibrosis worsened. Transmission electron microscopy (EM) in all 9 patients studied showed canalicular dilatation, microvilli loss, abnormal mitochondrial internal structure, and varying intra-canalicular accumulation of finely granular bile. Canalicular staining for BSEP was absent in 10 patients and present in 2 patients, 1 of whom had intermittent symptoms. ABCB11 sequencing of all patients identified 6 novel and 10 previously described mutations, with nonsense, missense, and/or noncoding mutations in the 10 patients without immunohistochemically demonstrable BSEP. Missense and/or noncoding mutations were identified in the 2 patients with demonstrable BSEP, whose clinical course was more indolent. Mutations ending ABCB11 transcription appear linked, through hepatocellular necrosis and fibrosis, to worse outcome. In conclusion, light microscopy and electron microscopy findings in clinical PFIC2 can support diagnosis, but are variable and nonspecific. Therefore, no correlation between specific mutations and histopathology is yet possible.
PFIC2; progressive familial intrahepatic cholestasis; bile salt export pump; BSEP; ABCB11; neonatal hepatitis
PFIC II is a subtype of progressive familial intrahepatic cholestasis (PFIC) that is associated with mutations in the ABCB11 gene encoding the bile salt export pump (BSEP). However it is not known how these mutations cause this disease. To evaluate these mechanisms, we introduced seven PFIC II–associated missense mutations into rat Bsep and assessed their effects on Bsep membrane localization and transport function in MDCK and Sf9 cells, respectively. Five mutations, G238V, E297G, G982R, R1153C, and R1268Q, prevented the protein from trafficking to the apical membrane, and E297G, G982R, R1153C, and R1268Q also abolished taurocholate transport activity, possibly by causing Bsep to misfold. Mutation C336S affected neither Bsep transport activity nor the apical trafficking of Bsep, suggesting that this mutation alone may not cause this disease. D482G did not affect the apical expression but partially decreased the transport activity of Bsep. Mutant G238V was rapidly degraded in both MDCK and Sf9 cells, and proteasome inhibitor resulted in intracellular accumulation of this and other mutants, suggesting proteasome-mediated degradation plays an important role in expression of these PFIC II mutants. Our studies highlight the heterogeneous nature of PFIC II mutations and illustrate the significance of these mutations in the function and expression of Bsep.
Background & Aims
Progressive Familial Intrahepatic Cholestasis 1 (PFIC1) results from mutations in ATP8B1 (also known as FIC1), a putative aminophospholipid flippase. However conflicting hypotheses have been proposed for the pathogenesis of PFIC1. The aim of this study was to determine whether ATP8B1-deficiency produces cholestasis by altering the activity of the nuclear receptor FXR or by impairing the structure of the canalicular membrane
ATP8B1/Atp8b1 was knocked down in human and rat hepatocytes, and Caco2 cells using adenoviral and oligonucleotide siRNAs.
ATP8B1 mRNA and protein expression was greatly reduced in human and rat hepatocytes and Caco2 cells. In contrast, FXR expression and several FXR dependent membrane transporters (BSEP, MRP2) were unchanged at mRNA or protein levels in ATP8B1-deficient cells, whereas Mrp3 and Mrp4 were up-regulated in rat hepatocytes. FXR activity remained intact in these cells as evidenced by 6-ECDCA mediated induction of SHP, BSEP and MDR3/Mdr2. Fluorescent substrate excretion assays indicate that Bsep function was significantly reduced in Atp8b1-deficient rat hepatocytes although Bsep remained localized to the canalicular membrane. Exposure to the hydrophobic bile acid, CDCA resulted in focal areas of canalicular membrane disruption by electron microscopy and luminal accumulation of NBD-phosphatidylserine consistent with Atp8b1’s function as an aminophospholipid flippase.
ATP8B1- deficiency predisposes to cholestasis by favoring bile acid-induced injury in the canalicular membrane, but does not directly affect FXR expression, which may occur in PFIC1 as a secondary phenomenon associated to bile acid accumulation.
Prior loss-of-function analyses revealed that ATP8B1 (FIC1) post-translationally activated the Farnesoid X-Receptor (FXR).
Mechanisms underlying this regulation are elaborated upon by these gain-of-function studies in UPS cells, which lack endogenous FIC1 expression. FXR function was assayed in response to wild type and mutated FIC1 expression constructs using a human bile salt export pump (BSEP) promoter and a variety of cellular localization techniques.
FIC1 overexpression led to enhanced phosphorylation and nuclear localization of FXR that was associated with FXR-dependent activation of the BSEP promoter. The FIC1 effect was lost after mutation of the FXR response element in the BSEP promoter. Despite similar levels of FIC1 protein expression, Byler-disease FIC1 mutants did not activate BSEP, while benign recurrent intrahepatic cholestasis mutants partially activated BSEP. The FIC1 effect was dependent upon the presence of the FXR ligand, chenodeoxycholic acid. The FIC1 effect on FXR phosphorylation and nuclear localization and its effects on BSEP promoter activity could be blocked with protein kinase C (PKC) ζ inhibitors (pseudosubstrate or siRNA silencing). Recombinant PKCζ directly phosphorylated immunoprecipitated FXR. Mutation of threonine 442 of FXR to alanine yielded a dominant negative protein, while the phosphomimetic conversion to glutamate resulted in FXR with enhanced activity and nuclear localization. Inhibition of PKCζ in Caco-2 cells resulted in activation of the human apical sodium dependent bile acid transporter promoter.
These results demonstrate that FIC1 signals to FXR via PKCζ. FIC1-related liver disease is likely related to downstream effects of FXR on bile acid homeostasis. BRIC emanates from a partially functional FIC1 protein. Phosphorylation of FXR is an important mechanism for regulating its activity.
nuclear receptor; cholestasis; liver; ileum; bile acid
Progressive familial intrahepatic cholestasis type 1 (PFIC1), an inherited liver disease caused by mutations in ATP8B1, progresses to severe cholestasis with a sustained intractable itch. Currently, no effective therapy has been established for PFIC1. Decreased function of the bile salt export pump (BSEP) in hepatocytes is suggested to be responsible for the severe cholestasis observed in PFIC1. We found a previously unidentified pharmacological effect of 4-phenylbutyrate (4PB) that increases the expression and function of BSEP. Here, we tested 4PB therapy in three patients with PFIC1.
The therapeutic potency of 4PB in these patients was tested by oral administration of this drug with gradually increasing dosage (200, 350, and 500 mg/kg/day) for 6 months. Biochemical, histological, and clinical data were collected.
4PB therapy had no beneficial effect on the patients’ liver functions, as assessed by biochemical and histological analyses, despite an increase in hepatic BSEP expression. However, therapy with 4PB at a dosage of 350 or 500 mg/kg/day significantly relieved the intractable itch. Serum levels of potential pruritogens in cholestasis were much higher than the reference ranges during the 4PB therapy.
4PB therapy may be a new medication for patients with intractable cholestatic pruritus and may improve quality of life for patients and their families.
Pediatric liver disease; Cholestasis; PFIC1; Pruritus; 4PB
Partial External Biliary Diversion (PEBD) is a surgical intervention to treat children with Progressive Familial Intrahepatic Cholestasis (PFIC) and Alagille syndrome (AGS). PEBD can reduce disease progression, and examining the alterations in biliary lipid composition may be a prognostic factor for outcome.
Biliary lipid composition and the clinical course of AGS and PFIC patients were examined before and after PEBD.
Pre-PEBD bile from AGS patients had greater chenodeoxycholic/cholic acid (CDCA/CA), bile salt, cholesterol and phospholipid concentrations than PFIC patients. AGS patients, and PFIC patients with familial intrahepatic cholestasis 1 (FIC1) genotype, responded better to PEBD than PFIC patients with bile salt export protein (BSEP) genotype. After successful PEBD, AGS patients have higher biliary lipid concentrations than PFIC patients and PEBD also increases biliary phospholipid concentrations in FIC1 patients.
Both AGS and FIC1 patients can benefit from PEBD, and preserved biliary phospholipid concentrations may be associated with better outcomes post-PEBD.
Familial intrahepatic cholestases (FICs) are a heterogeneous group of autosomal recessive disorders of childhood that disrupt bile formation and present with cholestasis of hepatocellular origin. Three distinct forms are described: FIC1 and FIC2, associated with low/normal GGT level in serum, which are caused by impaired bile salt secretion due to defects in ATP8B1 encoding the FIC1 protein and defects in ABCB11 encoding bile salt export pump protein, respectively; FIC3, linked to high GGT level, involves impaired biliary phospholipid secretion due to defects in ABCB4, encoding multidrug resistance 3 protein. Different mutations in these genes may cause either a progressive familial intrahepatic cholestasis (PFIC) or a benign recurrent intrahepatic cholestasis (BRIC). For the purposes of the present study we genotyped 27 children with intrahepatic cholestasis, diagnosed on either a clinical or histological basis. Two BRIC, 23 PFIC and 2 BRIC/PFIC were identified. Thirty-four different mutations were found of which 11 were novel. One was a 2Mb deletion (5’UTR- exon 18) in ATP8B1. In another case microsatellite analysis of chromosome 2, including ABCB11, showed uniparental disomy. Two cases were compound heterozygous for BRIC/PFIC2 mutations. Our results highlight the importance of the pathogenic role of novel mutations in the three genes and unusual modes of their transmission.
The liver specific bile salt export pump (BSEP) is crucial for bile-acid dependent bile flow at the apical membrane. BSEP, a member of the family of structurally related ATP-Binding Cassette (ABC) proteins, is composed of 12 transmembrane segments (TMS) and 2 large cytoplasmic nucleotide binding domains (NBD). The regulation of trafficking of BSEP to and from the cell surface is not well understood, but is believed to play an important role in cholestatic liver diseases such as primary familial intrahepatic cholestasis type 2 (PFIC2). To address this issue, BSEP endocytosis was studied by immunofluorescence and a cell surface ELISA endocytosis reporter system using a chimera of the interleukin 2 receptor α (previously referred to as Tac) and the C-terminal tail of BSEP (TacCterm). An autonomous endocytosis motif in the carboxyl cytoplasmic terminus of BSEP was identified. We define this endocytic motif by site-directed mutagenesis as a canonical tyrosine-based motif 1310YYKLV1314 (Yxx∅). When expressed in HEK293T cells TacCterm is constitutively internalized via a dynamin- and clathrin-dependent pathway. Mutation of the Y1310Y1311 amino acids in TacCterm and in full length human BSEP blocks the internalization. Subsequent sequence analysis reveals this motif to be highly conserved between the closely related ABCB subfamily members that mediate ATP-dependent transport of broad substrate specificity.
Our results indicate constitutive internalization of BSEP is clathrin-mediated and dependent on the tyrosine-based endocytic motif at the C-terminal end of BSEP.
bile acid transporter; trafficking motif
As a canalicular bile acid effluxer, bile salt export pump (BSEP) plays a vital role in maintaining bile acid homeostasis. BSEP deficiency leads to severe cholestasis and hepatocellular carcinoma (HCC) in young children. Regardless of the etiology, chronic inflammation is the common pathological process for HCC development. Clinical studies showed that bile acid homeostasis is disrupted in HCC patients with elevated serum bile acid level as a proposed marker for HCC. However, the underlying mechanisms remain largely unknown. In this study, we found that BSEP expression was severely diminished in HCC tissues and markedly reduced in adjacent non-tumor tissues. In contrast to mouse, human BSEP was regulated by farnesoid x receptor (FXR) in an isoform-dependent manner. FXRα2 exhibited a much more potent activity than FXRα1 in transactivating human BSEP in vitro and in vivo. The decreased BSEP expression in HCC was associated with altered relative expression of FXRα1 and FXRα2. The FXRα1/FXRα2 ratios were significantly increased with undetectable FXRα2 expression in one third of the HCC tumor samples. Similar correlation between BSEP and FXR isoform expression was confirmed in hepatoma Huh 7 and HepG2 cells. Further studies showed that intrahepatic proinflammatory cytokines interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α) were significantly elevated in HCC tissues. Treatment of Huh 7 cells with IL-6 and TNF-α resulted in a marked increase in the FXRα1/FXRα2 ratio concurrent with a significant decrease in BSEP expression. In conclusion, BSEP expression was severely diminished in HCC patients associated with alteration of FXR isoform expression induced by inflammation, and the restoration of BSEP expression through suppressing inflammation in the liver may re-establish the bile acid homeostasis.
BSEP; FXR; HCC; Bile acids; Gene regulation
Multiple respiratory chain deficiencies represent a common cause of mitochondrial diseases and often result in hepatic failure. There is no gold-standard test for diagnosing mitochondrial disease, and the current diagnosis relies on establishing a consistent pattern of evidence from clinical data, neuroimaging, tissue biopsy, and biochemical investigations. In some patients, the mitochondrial respiratory chain defect (MRCD) diagnosis is confirmed by genetic investigations. In most cases, genetic investigations are not informative and a number of cases remain unexplained.
Here, we report on two children presenting with liver disease in whom first investigations suggested MRCD, due to decreased liver respiratory chain activities and decreased mitochondrial DNA copy number. However, sequencing of the genes known to be associated with mitochondrial DNA instability did not identify any pathogenic mutations. Further investigations including exome analysis, biliary bile salt analysis, and/or BSEP immunostaining detected a defect in the bile salt export pump (BSEP). Diagnosis of progressive familial intrahepatic cholestasis type 2 (PFIC2), a hereditary disorder in bile formation due to BSEP deficiency was confirmed by ABCB11 gene sequencing. Deleterious mutations were identified in both patients: one harboring compound heterozygous mutations (p.Arg470*/c.1308+2T>A) and the other homozygous nonsense mutation (p.Tyr354*). This report increases awareness of a possible secondary mitochondrial respiratory chain defect in the liver tissue associated with other underlying causes such as PFIC2.
The primary transporter responsible for bile salt secretion is the bile salt export pump (BSEP, ABCB11), a member of the ATP-binding cassette (ABC) superfamily, which is located at the bile canalicular apical domain of hepatocytes. In humans, BSEP deficiency results in several different genetic forms of cholestasis, which include progressive familial intrahepatic cholestasis type 2 (PFIC2), benign recurrent intrahepatic cholestasis type 2 (BRIC2), as well as other acquired forms of cholestasis such as drug-induced cholestasis (DIC) and intrahepatic cholestasis of pregnancy (ICP). Because bile salts play a pivotal role in a wide range of physiologic and pathophysiologic processes, regulation of BSEP expression has been a subject of intense research. The authors briefly describe the molecular characteristics of BSEP and then summarize what is known about its role in the pathogenesis of genetic and acquired cholestatic disorders, emphasizing experimental observations from animal models and cell culture in vitro systems.
Cholestasis; progressive familial intrahepatic cholestasis; bile salt export pump (BSEP) mutations; polymorphisms; ATP-binding cassette (ABC) transporters; trafficking; recycling; ubiquitination
Progressive familial intrahepatic cholestasis (PFIC) type 2 is caused by mutations in ABCB11, which encodes bile salt export pump (BSEP). We report a Thai female infant who presented with progressive cholestatic jaundice since 1 mo of age, with normal serum γ-glutamyltransferase. Immunohistochemical staining of the liver did not demonstrate BSEP along the canaliculi, while multidrug resistance protein 3 was expressed adequately. Novel mutations in ABCB11, a four-nucleotide deletion in exon 3, c.90_93delGAAA, and a single-nucleotide insertion in exon 5, c.249_250insT, were identified, with confirmation in her parents. These mutations were predicted to lead to synthesis of truncated forms of BSEP. Immunostaining and mutation analysis thus established the diagnosis of PFIC type 2.
ABCB11; Bile salt export pump; Progressive familial intrahepatic cholestasis
The bile salt export pump (BSEP/ABCB11) is the primary transporter for the excretion of bile acids from hepatocytes into bile. In human, inhibition of BSEP by drugs has been related to drug-induced cholestasis and subsequent cytotoxic effects. The role of BSEP in canine and feline liver diseases has not been studied in detail, but the same mechanism of inhibition by drugs as in humans could play a role in veterinary medicine. The aim of this study was to investigate the functional characteristics of feline Bsep in comparison with canine and human Bsep/BSEP with respect to substrate affinities and inhibitory potential of model drugs. Orthologs of all three species were cloned and cell membrane vesicles overexpressing feline, canine and human Bsep/BSEP were prepared for functional analyses.
The cDNA sequences of the open reading frames of feline, canine and human Bsep/BSEP showed a high similarity between the species. Functional studies demonstrated for all species a tendency to a higher affinity of BSEP/Bsep for the conjugated bile acid taurocholic acid (TCA) than glycocholic acid (GCA), and a higher affinity for GCA than for the unconjugated cholic acid (CA). The inhibitory potency of the model inhibitors cyclosporine A, troglitazone and ketoconazole was characterized against TCA uptake into BSEP/Bsep containing membrane vesicles. All three substances potently inhibited TCA uptake without significant species differences.
Structure and functional characteristics of cat, dog and human Bsep/BSEP appeared to be very similar, indicating that the properties of this transporter have been highly preserved among the different species. Therefore, inhibition of BSEP by drugs could also be a mechanism in cholestasis and liver disease in veterinary relevant animal species. This model could be used to predict drug-induced liver injury caused by BSEP inhibition at an early stage in veterinary drug development.
BSEP; ABCB11; Transporter; Cat; Dog; Liver; Drugs; Toxicity; Bile acids; Inhibitor
The bile salt export pump (Bsep) represents the major bile salt transport system at the canalicular membrane of hepatocytes. When examined in model cell lines, genetic mutations in the BSEP gene impair its targeting and transport function, contributing to the pathogenesis of PFIC II. PFIC II mutations are known to lead to a deficiency of BSEP in human hepatocytes, suggesting that PFIC II mutants are unstable and degraded in the cell. To investigate this further, we have characterized the impact of several PFIC II mutations on the processing and stability of rat Bsep. G238V, D482G, G982R, R1153C and R1286Q all retain Bsep to the endoplasmic reticulum (ER) to different extents. Except for R1153C, the PFIC II mutants are degraded with varying half-lives. G238V and D482G are partially misfolded and can be stabilized by low temperature and glycerol. The proteasome provides the major degradation pathway for the PFIC II mutants, while the lysosome also contributes to the degradation of D482G. The PFIC II mutants appear to be more heavily ubiquitinated compared with the wild-type (wt) Bsep, and their ubiquitination is increased by the proteasome inhibitors. Overexpression of several E3 ubiquitin ligases, which are involved in ER-associated degradation (ERAD), lead to the decrease of both mutant and wt Bsep. Gene knockdown studies revealed that the ERAD E3s Rma1 and TEB4 contribute to the degradation of G238V, while HRD1 contributes to the degradation of a mutant lacking the lumenal glycosylation domain (ΔGly). Furthermore we present evidence that G982R weakly associates with various components of the ER quality control system. These data together demonstrate that the PFIC II mutants and ΔGly are degraded by the ERAD pathway.
The bile salt export pump (Bsep) mediates the hepatic excretion of bile acids, and its deficiency causes progressive familial intrahepatic cholestasis. The current study aimed to induce bile acid stress in Bsep−/− mice and to test the efficacy of hepatocyte transplantation in this disease model. We fed Bsep−/− and wild-type mice cholic acid (CA) or ursodeoxycholic acid (UDCA). Both CA and UDCA caused cholestasis and apoptosis in the Bsep−/− mouse liver. Wild-type mice had minimal liver injury and apoptosis when fed CA or UDCA, yet had increased proliferative activity. On the basis of the differential cytotoxicity of bile acids on the livers of wild-type and Bsep−/− mice, we transplanted wild-type hepatocytes into the liver of Bsep−/− mice fed CA or CA + UDCA. After 1–6 weeks, the donor cell repopulation and canalicular Bsep distribution were documented. An improved repopulation efficiency in the CA + UDCA-supplemented group was found at 2 weeks (4.76 ± 5.93% vs. 1.32 ± 1.48%, P = 0.0026) and at 4–6 weeks (12.09 ± 14.67% vs. 1.55 ± 1.28%, P < 0.001) compared with the CA-supplemented group. Normal-appearing hepatocytes with prominent nuclear staining for FXR were noted in the repopulated donor nodules. After hepatocyte transplantation, biliary total bile acids increased from 24% to 82% of the wild-type levels, among which trihydroxylated bile acids increased from 41% to 79% in the Bsep−/− mice. We conclude that bile acid stress triggers differential injury responses in the Bsep−/− and wild-type hepatocytes. This strategy changed the balance of the donor–recipient growth capacities and was critical for successful donor repopulation.
cholestasis; cell therapy; spgp (sister of p-glycoprotein); ATP-binding cassette transporters; bile acids; hepatocyte transplantation
Bile salt export pump (BSEP) is responsible for biliary secretion of bile acids, a rate limiting step in the enterohepatic circulation of bile acids and transactivated by nuclear receptor farnesoid x receptor (FXR). Intrahepatic cholestasis of pregnancy (ICP) is the most prevalent disorder among diseases unique to pregnancy and primarily occurs in the third trimester of pregnancy with a hallmark of elevated serum bile acids. Currently, the transcriptional regulation of BSEP during pregnancy and its underlying mechanisms and involvement in ICP are not fully understood. In this study, the dynamics of BSEP transcription in vivo in the same group of pregnant mice before, during and after gestation were established with in vivo imaging system (IVIS). BSEP transcription was markedly repressed in the later stages of pregnancy and immediately recovered after parturition, resembling the clinical course of ICP in human. The transcriptional dynamics of BSEP was inversely correlated with serum 17β-estradiol (E2) levels before, during and after gestation. Further studies showed that E2 repressed BSEP expression in human primary hepatocytes, Huh 7 cells and in vivo in mice. Such transrepression of BSEP by E2 in vitro and in vivo required estrogen receptor α (ERα). Mechanistic studies with chromatin immunoprecipitation (ChIP), protein co-immunoprecipitation (Co-IP) and bimolecular fluorescence complementation (BiFC) assays demonstrated that ERα directly interacted with FXR in living cells and in vivo in mice. In conclusion, BSEP expression was repressed by E2 in the late stages of pregnancy through a non-classical E2/ERα transrepressive pathway, directly interacting with FXR. E2-mediated repression of BSEP expression represents an etiological contributing factor to ICP and therapies targeting the ERα/FXR interaction may be developed for prevention and treatment of ICP.
BSEP; Bile acids; 17β-estradiol; FXR; ERα
The exact molecular mechanism(s) of the disease that results from defects in the ATPase Class I Type 8B Member 1 gene remains controversial. Prior investigations of human ileum and in intestinal and ovarian cell lines have suggested that Familial Intrahepatic Cholestasis 1 (FIC1) activates the Farnesoid X-Receptor (FXR) via a pathway involving Protein Kinase C ζ (PKCζ). Translational investigations of human liver from individuals with FIC1 disease have been confounded by secondary affects of progressive cholestatic liver disease and limited numbers of samples for analysis. These studies, performed in primarily derived human hepatocytes, circumvent this issue. The canalicular bile salt export pump (BSEP) served as a downstream target of FXR. siRNA mediated silencing of FIC1 in human hepatocytes led to a reduction in both human BSEP promoter activity and BSEP protein expression, which correlated with a reduction in FXR expression and redistribution of its localization from the nucleus to the cytoplasm. These changes in BSEP expression could be reproduced by altering the expression of PKCζ; with a positive correlation of PKCζ activity and BSEP expression. Overall, these findings support the hypothesis that FIC1 enhances FXR signaling via a PKCζ dependent signaling pathway.
The bile salt export pump BSEP mediates bile formation. Over 150 BSEP mutations are associated with progressive familial intrahepatic cholestasis type 2 (PFIC-2), with few characterised specifically. We examined liver tissues from two PFIC-2 patients compound heterozygous for the splice-site mutation c.150 + 3A > C and either c.2783_2787dup5 resulting in a frameshift with a premature termination codon (child 1) or p.R832C (child 2). Splicing was analysed with a minigene system and mRNA sequencing from patients’ livers. Protein expression was shown by immunofluorescence. Using the minigene, c.150 + 3A > C causes complete skipping of exon 3. In liver tissue of child 1, c.2783_2787dup5 was found on DNA but not on mRNA level, implying nonsense-mediated mRNA decay (NMD) when c.2783_2787dup5 is present. Still, BSEP protein as well as mRNA with and without exon 3 were detectable and can be assigned to the c.150 + 3A > C allele. Correctly spliced transcripts despite c.150 + 3A > C were also confirmed in liver of child 2. In conclusion, we provide evidence (1) for effective NMD due to a BSEP frameshift mutation and (2) partial exon-skipping due to c.150 + 3A > C. The results illustrate that the extent of exon-skipping depends on the genomic and cellular context and that regulation of splicing may have therapeutic potential.
Background and aims: The aim of this study was to investigate the genetic aetiology of intrahepatic cholestasis of pregnancy (ICP) and the impact of known cholestasis genes (BSEP, FIC1, and MDR3) on the development of this disease.
Patients and methods: Sixty nine Finnish ICP patients were prospectively interviewed for a family history of ICP, and clinical features were compared in patients with familial ICP (patients with a positive family history, n=11) and sporadic patients (patients with no known family history of ICP, n=58). For molecular genetic analysis, 16 individuals from two independently ascertained Finnish ICP families were genotyped for the flanking markers for BSEP, FIC1, and MDR3.
Results: The pedigree structures in 16% (11/69) of patients suggested dominant inheritance. Patients with familial ICP had higher serum aminotransferase levels and a higher recurrence risk (92% v 40%). Both segregation of haplotypes and multipoint linkage analysis excluded BSEP, FIC1, and MDR3 genes in the studied pedigrees. Additionally, the MDR3 gene, previously shown to harbour mutations in ICP patients, was negative for mutations when sequenced in four affected individuals from the two families.
Conclusions: These results support the hypothesis that the aetiology of ICP is heterogeneous and that ICP is due to a genetic predisposition in a proportion of patients. The results of molecular genetic analysis further suggest that the previously identified three cholestasis genes are not likely to be implicated in these Finnish ICP families with dominant inheritance.
intrahepatic cholestasis of pregnancy; obstetric cholestasis; linkage analysis
The bile salt export pump (BSEP, ABCB11) is the primary transporter of bile acids from the hepatocyte to the biliary system. This rate-limiting step in bile formation is essential to the formation of bile salt dependent bile flow, the enterohepatic circulation of bile acids, and the digestion of dietary fats. Mutations in BSEP are associated with cholestatic diseases such as progressive familial intrahepatic cholestasis type 2 (PFIC2), benign recurrent intrahepatic cholestasis type 2 (BRIC2), drug-induced cholestasis, and intrahepatic cholestasis of pregnancy. Development of clinical therapies for these conditions necessitates a clear understanding of the cell biology of biosynthesis, trafficking, and transcriptional and translational regulation of BSEP. This chapter will focus on the molecular and cell biological aspects of this critical hepatic membrane transporter.
Bile salt secretion; ATP-binding cassette transporter; Cholestasis
The human liver ATP-binding cassette (ABC) transporters bile salt export pump (BSEP/ABCB11) and the multidrug resistance protein 3 (MDR3/ABCB4) fulfill the translocation of bile salts and phosphatidylcholine across the apical membrane of hepatocytes. In concert with ABCG5/G8, these two transporters are responsible for the formation of bile and mutations within these transporters can lead to severe hereditary diseases. In this study, we report the heterologous overexpression and purification of human BSEP and MDR3 as well as the expression of the corresponding C-terminal GFP-fusion proteins in the yeast Pichia pastoris. Confocal laser scanning microscopy revealed that BSEP-GFP and MDR3-GFP are localized in the plasma membrane of P. pastoris. Furthermore, we demonstrate the first purification of human BSEP and MDR3 yielding ∼1 mg and ∼6 mg per 100 g of wet cell weight, respectively. By screening over 100 detergents using a dot blot technique, we found that only zwitterionic, lipid-like detergents such as Fos-cholines or Cyclofos were able to extract both transporters in sufficient amounts for subsequent functional analysis. For MDR3, fluorescence-detection size exclusion chromatography (FSEC) screens revealed that increasing the acyl chain length of Fos-Cholines improved monodispersity. BSEP purified in n-dodecyl-β-D-maltoside or Cymal-5 after solubilization with Fos-choline 16 from P. pastoris membranes showed binding to ATP-agarose. Furthermore, detergent-solubilized and purified MDR3 showed a substrate-inducible ATPase activity upon addition of phosphatidylcholine lipids. These results form the basis for further biochemical analysis of human BSEP and MDR3 to elucidate the function of these clinically relevant ABC transporters.
The bile salt export pump (BSEP, ABCB11) plays an essential role in the formation of bile. In hepatocytes, BSEP is localized within the apical (canalicular) membrane and a deficiency of canalicular BSEP function is associated with severe forms of cholestasis. Regulation of correct trafficking to the canalicular membrane and of activity is essential to ensure BSEP functionality and thus normal bile flow. However, little is known about the identity of interaction partners regulating function and localization of BSEP. In our study, interaction partners of BSEP were identified in a complementary approach: Firstly, BSEP interaction partners were co-immunoprecipitated from human liver samples and identified by mass spectrometry (MS). Secondly, a membrane yeast two-hybrid (MYTH) assay was used to determine protein interaction partners using a human liver cDNA library. A selection of interaction partners identified both by MYTH and MS were verified by in vitro interaction studies using purified proteins. By these complementary approaches, a set of ten novel BSEP interaction partners was identified. With the exception of radixin, all other interaction partners were integral or membrane-associated proteins including proteins of the early secretory pathway and the bile acyl-CoA synthetase, the second to last, ER-associated enzyme of bile salt synthesis.
Background. Primary biliary cirrhosis (PBC) is a chronic and progressive cholestasis liver disease. Bile salt export pump (BSEP) is the predominant bile salt efflux system of hepatocytes. BSEP gene has been attached great importance in the susceptibility of PBC and the response rate of ursodeoxycholic acid (UDCA) treatment of PBC patients. Methods. In this study, TaqMan assay was used to genotype four variants of BSEP, and the Barcelona criteria were used for evaluating the response rate of UDCA treatment. Results. Variant A allele of BSEP rs473351 (dominant model, OR = 2.063; 95% CI, 1.254–3.393; P = 0.004) was highly associated with PBC susceptibility. On the contrary, variant A allele of BSEP rs2287618 (dominant model, OR = 0.617; 95% CI, 0.411–0.928; P = 0.020) provided a protective role and Barcelona evaluation criterion indicated that the frequency of variant allele at BSEP rs2287618 was significantly decreased in UDCA-responsive PBC patients (P = 0.021). Conclusion. These results suggested that BSEP rs473351 was closely associated with the susceptibility of PBC and if people with BSEP rs2287618 were diagnosed as PBC, the UDCA treatment was not satisfactory. Larger studies with mixed ethnicity subjects and stratified by clinical and subclinical characteristics are needed to validate our findings.