A missense mutation in ABCB4 gene involved in progressive familial intrahepatic cholestasis type 3 leads to a folding defect that can be rescued by low temperature

Clinical, genetic and functional perspectives on ATP-binding cassette subfamily B member 4 variants in five cholestasis adults

BACKGROUND

ATP-binding cassette subfamily B member 4 (ABCB4) deficiency is associated with cholestatic liver disease primarily because of missense mutations, and many variants remain unidentified. Here, we validate the pathogenicity and mechanism of ABCB4 variants in clinical and in vitro trials, hypothesizing that these variants are responsible for impaired biliary function and contribute to the development of cholestatic liver diseases.

AIM

To clarify the functional features and pathogenicity of ABCB4 variants.

METHODS

Clinical data were collected from five patients with cholestatic liver disease that was initially not detected by routine examinations. Later, whole-exome sequencing confirmed ABCB4 variants and the patients were treated from January 2017 to December 2023. Pathogenic mechanisms were analyzed using bioinformatics tools, and a cell model in vitro was established to investigate ABCB4 mRNA expression, multidrug resistance protein 3 (MDR3) expression, cellular localization, and phosphatidylcholine secretion. Results were compared using Student's t-tests.

RESULTS

Five missense variants (c.1757T>A, c.1865G>A, c.2362C>T, c.2777C>T and c.3250C>T), one intron variant (c.537-32G>T), and one synonymous (c.C504T) variant were identified. Three of the five patients had various degrees of cholestasis, two presented with liver cirrhosis, and all had elevated gamma-glutamyl transferase. Three of the four patients who underwent a liver biopsy had bile duct dilation, and one had gallstones. Two of the four patients had normal and reduced MDR3 immunohistochemical levels. Bioinformatic analysis indicated that these variants were likely pathogenic except c.C504T variant. None of the missense variants influenced subcellular MDR3 Localization in vitro. However, the c.1865G>A variant significantly decreased ABCB4 mRNA values, and all missense variants down-regulated phosphatidylcholine secretion.

CONCLUSION

This study uncovered new ABCB4 variants and emphasized the pathogenic potential of specific variants. The findings from five patients provided insight into the pathogenic mechanisms underlying ABCB4-related diseases.

ABC transporters involved in respiratory and cholestatic diseases: From rare to very rare monogenic diseases

ATP-binding cassette (ABC) transporters constitute a 49-member superfamily in humans. These proteins, most of them being transmembrane, allow the active transport of an important variety of substrates across biological membranes, using ATP hydrolysis as an energy source. For an important proportion of these ABC transporters, genetic variations of the loci encoding them have been correlated with rare genetic diseases, including cystic fibrosis and interstitial lung disease (variations in CFTR/ABCC7 and ABCA3) as well as cholestatic liver diseases (variations in ABCB4 and ABCB11). In this review, we first describe these ABC transporters and how their molecular dysfunction may lead to human diseases. Then, we propose a classification of the genetic variants according to their molecular defect (expression, traffic, function and/or stability), which may be considered as a general guideline for all ABC transporters' variants. Finally, we discuss recent progress in the field of targeted pharmacotherapy, which aim to correct specific molecular defects using small molecules. In conclusion, we are opening the path to treatment repurposing for diseases involving similar deficiencies in other ABC transporters.

Identification of new correctors for traffic-defective ABCB4 variants by a high-content screening approach

ABCB4 is located at the canalicular membrane of hepatocytes and is responsible for the secretion of phosphatidylcholine into bile. Genetic variations of this transporter are correlated with rare cholestatic liver diseases, the most severe being progressive familial intrahepatic cholestasis type 3 (PFIC3). PFIC3 patients most often require liver transplantation. In this context of unmet medical need, we developed a high-content screening approach to identify small molecules able to correct ABCB4 molecular defects. Intracellularly-retained variants of ABCB4 were expressed in cell models and their maturation, cellular localization and function were analyzed after treatment with the molecules identified by high-content screening. In total, six hits were identified by high-content screening. Three of them were able to correct the maturation and canalicular localization of two distinct intracellularly-retained ABCB4 variants; one molecule was able to significantly restore the function of two ABCB4 variants. In addition, in silico molecular docking calculations suggest that the identified hits may interact with wild type ABCB4 residues involved in ATP binding/hydrolysis. Our results pave the way for their optimization in order to provide new drug candidates as potential alternative to liver transplantation for patients with severe forms of ABCB4-related diseases, including PFIC3.

Genetic alterations and molecular mechanisms underlying hereditary intrahepatic cholestasis

Hereditary cholestatic liver disease caused by a class of autosomal gene mutations results in jaundice, which involves the abnormality of the synthesis, secretion, and other disorders of bile acids metabolism. Due to the existence of a variety of gene mutations, the clinical manifestations of children are also diverse. There is no unified standard for diagnosis and single detection method, which seriously hinders the development of clinical treatment. Therefore, the mutated genes of hereditary intrahepatic cholestasis were systematically described in this review.

Ivacaftor-Mediated Potentiation of ABCB4 Missense Mutations Affecting Critical Motifs of the NBDs: Repositioning Perspectives for Hepatobiliary Diseases

ABCB4 (ATP-binding cassette subfamily B member 4) is a hepatocanalicular floppase involved in biliary phosphatidylcholine (PC) secretion. Variations in the ABCB4 gene give rise to several biliary diseases, including progressive familial intrahepatic cholestasis type 3 (PFIC3), an autosomal recessive disease that can be lethal in the absence of liver transplantation. In this study, we investigated the effect and potential rescue of ten ABCB4 missense variations in NBD1:NBD2 homologous positions (Y403H/Y1043H, K435M/K1075M, E558K/E1200A, D564G/D1206G and H589Y/H1231Y) all localized at the conserved and functionally critical motifs of ABC transporters, six of which are mutated in patients. By combining structure analysis and in vitro studies, we found that all ten mutants were normally processed and localized at the canalicular membrane of HepG2 cells, but showed dramatically impaired PC transport activity that was significantly rescued by treatment with the clinically approved CFTR potentiator ivacaftor. Our results provide evidence that functional ABCB4 mutations are rescued by ivacaftor, paving the way for the repositioning of this potentiator for the treatment of selected patients with PFIC3 caused by mutations in the ATP-binding sites of ABCB4.

MRCK-Alpha and Its Effector Myosin II Regulatory Light Chain Bind ABCB4 and Regulate Its Membrane Expression

ABCB4, is an adenosine triphosphate-binding cassette (ABC) transporter localized at the canalicular membrane of hepatocytes, where it mediates phosphatidylcholine secretion into bile. Gene variations of ABCB4 cause different types of liver diseases, including progressive familial intrahepatic cholestasis type 3 (PFIC3). The molecular mechanisms underlying the trafficking of ABCB4 to and from the canalicular membrane are still unknown. We identified the serine/threonine kinase Myotonic dystrophy kinase-related Cdc42-binding kinase isoform α (MRCKα) as a novel partner of ABCB4. The role of MRCKα was explored, either by expression of dominant negative mutant or by gene silencing using the specific RNAi and CRISPR-cas9 strategy in cell models. The expression of a dominant-negative mutant of MRCKα and MRCKα inhibition by chelerythrine both caused a significant increase in ABCB4 steady-state expression in primary human hepatocytes and HEK-293 cells. RNA interference and CRISPR-Cas9 knockout of MRCKα also caused a significant increase in the amount of ABCB4 protein expression. We demonstrated that the effect of MRCKα was mediated by its downstream effector, the myosin II regulatory light chain (MRLC), which was shown to also bind ABCB4. Our findings provide evidence that MRCKα and MRLC bind to ABCB4 and regulate its cell surface expression.

RAB10 Interacts with ABCB4 and Regulates Its Intracellular Traffic

ABCB4 (ATP-binding cassette subfamily B member 4) is an ABC transporter expressed at the canalicular membrane of hepatocytes where it ensures phosphatidylcholine secretion into bile. Genetic variations of ABCB4 are associated with several rare cholestatic diseases. The available treatments are not efficient for a significant proportion of patients with ABCB4-related diseases and liver transplantation is often required. The development of novel therapies requires a deep understanding of the molecular mechanisms regulating ABCB4 expression, intracellular traffic, and function. Using an immunoprecipitation approach combined with mass spectrometry analyses, we have identified the small GTPase RAB10 as a novel molecular partner of ABCB4. Our results indicate that the overexpression of wild type RAB10 or its dominant-active mutant significantly increases the amount of ABCB4 at the plasma membrane expression and its phosphatidylcholine floppase function. Contrariwise, RAB10 silencing induces the intracellular retention of ABCB4 and then indirectly diminishes its secretory function. Taken together, our findings suggest that RAB10 regulates the plasma membrane targeting of ABCB4 and consequently its capacity to mediate phosphatidylcholine secretion.

Effect of CFTR correctors on the traffic and the function of intracellularly retained ABCB4 variants

BACKGROUND & AIM

ABCB4 is expressed at the canalicular membrane of hepatocytes. This ATP-binding cassette (ABC) transporter is responsible for the secretion of phosphatidylcholine into bile canaliculi. Missense genetic variations of ABCB4 are correlated with several rare cholestatic liver diseases, the most severe being progressive familial intrahepatic cholestasis type 3 (PFIC3). In a repurposing strategy to correct intracellularly retained ABCB4 variants, we tested 16 compounds previously validated as cystic fibrosis transmembrane conductance regulator (CFTR) correctors.

METHODS

The maturation, intracellular localization and activity of intracellularly retained ABCB4 variants were analyzed in cell models after treatment with CFTR correctors. In addition, in silico molecular docking calculations were performed to test the potential interaction of CFTR correctors with ABCB4.

RESULTS

We observed that the correctors C10, C13, and C17, as well as the combinations of C3 + C18 and C4 + C18, allowed the rescue of maturation and canalicular localization of four distinct traffic-defective ABCB4 variants. However, such treatments did not permit a rescue of the phosphatidylcholine secretion activity of these defective variants and were also inhibitory of the activity of wild type ABCB4. In silico molecular docking analyses suggest that these CFTR correctors might directly interact with transmembrane domains and/or ATP-binding sites of the transporter.

CONCLUSION

Our results illustrate the uncoupling between the traffic and the activity of ABCB4 because the same molecules can rescue the traffic of defective variants while they inhibit the secretion activity of the transporter. We expect that this study will help to design new pharmacological tools with potential clinical interest.

ABCB4/MDR3 in health and disease – at the crossroads of biochemistry and medicine

Several ABC transporters of the human liver are responsible for the secretion of bile salts, lipids and cholesterol. Their interplay protects the biliary tree from the harsh detergent activity of bile salts. Among these transporters, ABCB4 is essential for the translocation of phosphatidylcholine (PC) lipids from the inner to the outer leaflet of the canalicular membrane of hepatocytes. ABCB4 deficiency can result in altered PC to bile salt ratios, which led to intrahepatic cholestasis of pregnancy, low phospholipid associated cholelithiasis, drug induced liver injury or even progressive familial intrahepatic cholestasis type 3. Although PC lipids only account for 30–40% of the lipids in the canalicular membrane, 95% of all phospholipids in bile are PC lipids. We discuss this discrepancy in the light of PC synthesis and bile salts favoring certain lipids. Nevertheless, the in vivo extraction of PC lipids from the outer leaflet of the canalicular membrane by bile salts should be considered as a separate step in bile formation. Therefore, methods to characterize disease causing ABCB4 mutations should be considered carefully, but such an analysis represents a crucial point in understanding the currently unknown transport mechanism of this ABC transporter.

Chaperoning Endoplasmic Reticulum-Associated Degradation (ERAD) and Protein Conformational Diseases

Misfolded proteins compromise cellular homeostasis. This is especially problematic in the endoplasmic reticulum (ER), which is a high-capacity protein-folding compartment and whose function requires stringent protein quality-control systems. Multiprotein complexes in the ER are able to identify, remove, ubiquitinate, and deliver misfolded proteins to the 26S proteasome for degradation in the cytosol, and these events are collectively termed ER-associated degradation, or ERAD. Several steps in the ERAD pathway are facilitated by molecular chaperone networks, and the importance of ERAD is highlighted by the fact that this pathway is linked to numerous protein conformational diseases. In this review, we discuss the factors that constitute the ERAD machinery and detail how each step in the pathway occurs. We then highlight the underlying pathophysiology of protein conformational diseases associated with ERAD.

Structural analogues of roscovitine rescue the intracellular traffic and the function of ER-retained ABCB4 variants in cell models

Adenosine triphosphate binding cassette transporter, subfamily B member 4 (ABCB4) is the transporter of phosphatidylcholine at the canalicular membrane of hepatocytes. ABCB4 deficiency, due to genetic variations, is responsible for progressive familial intrahepatic cholestasis type 3 (PFIC3) and other rare biliary diseases. Roscovitine is a molecule in clinical trial that was shown to correct the F508del variant of cystic fibrosis transmembrane conductance regulator (CFTR), another ABC transporter. In the present study, we hypothesized that roscovitine could act as a corrector of ABCB4 traffic-defective variants. Using HEK and HepG2 cells, we showed that roscovitine corrected the traffic and localisation at the plasma membrane of ABCB4-I541F, a prototypical intracellularly retained variant. However, roscovitine caused cytotoxicity, which urged us to synthesize non-toxic structural analogues. Roscovitine analogues were able to correct the intracellular traffic of ABCB4-I541F in HepG2 cells. Importantly, the phospholipid secretion activity of this variant was substantially rescued by three analogues (MRT2-235, MRT2-237 and MRT2-243) in HEK cells. We showed that these analogues also triggered the rescue of intracellular traffic and function of two other intracellularly retained ABCB4 variants, i.e. I490T and L556R. Our results indicate that structural analogues of roscovitine can rescue genetic variations altering the intracellular traffic of ABCB4 and should be considered as therapeutic means for severe biliary diseases caused by this class of variations.

Functional rescue of misfolding ABCA3 mutations by small molecular correctors

Adenosine triphosphate (ATP)-binding cassette subfamily A member 3 (ABCA3), a phospholipid transporter in lung lamellar bodies (LBs), is essential for the assembly of pulmonary surfactant and LB biogenesis. Mutations in the ABCA3 gene are an important genetic cause for respiratory distress syndrome in neonates and interstitial lung disease in children and adults, for which there is currently no cure. The aim of this study was to prove that disease causing misfolding ABCA3 mutations can be corrected in vitro and to investigate available options for correction. We stably expressed hemagglutinin (HA)-tagged wild-type ABCA3 or variants p.Q215K, p.M760R, p.A1046E, p.K1388N or p.G1421R in A549 cells and assessed correction by quantitation of ABCA3 processing products, their intracellular localization, resembling LB morphological integrity and analysis of functional transport activity. We showed that all mutant proteins except for M760R ABCA3 were rescued by the bithiazole correctors C13 and C17. These variants were also corrected by the chemical chaperone trimethylamine N-oxide and by low temperature. The identification of lead molecules C13 and C17 is an important step toward pharmacotherapy of ABCA3 misfolding-induced lung disease.

Thyroid hormone receptor β1 stimulates ABCB4 to increase biliary phosphatidylcholine excretion in mice

The ATP-binding cassette transporter ABCB4/MDR3 is critical for biliary phosphatidylcholine (PC) excretion at the canalicular membrane of hepatocytes. Defective ABCB4 gene expression and protein function result in various cholestatic liver and bile duct injuries. Thyroid hormone receptor (THR) is a major regulator of hepatic lipid metabolism; we explored its potential role in ABCB4 regulation. Thyroid hormone T3 stimulation to human hepatocyte models showed direct transcriptional activation of ABCB4 in a dose- and time-dependent manner. To determine whether THRβ1 (the main THR isoform of the liver) is involved in regulation, we tested THRβ1-specific agonists (e.g., GC-1, KB-141); these agonists resulted in greater stimulation than the native hormone. KB-141 activated hepatic ABCB44 expression in mice, which enhanced biliary PC secretion in vivo. We also identified THR response elements 6 kb upstream of the ABCB4 locus that were conserved in humans and mice. Thus, T3-via THRβ1 as a novel transcriptional activator regulates ABCB4 to increase ABCB4 protein levels at the canalicular membrane and promote PC secretion into bile. These findings may have important implications for understanding thyroid hormone function as a potential modifier of bile duct homeostasis and provide pharmacologic opportunities to improve liver function in hepatobiliary diseases caused by low ABCB4 expression.

Folding correction of ABC-transporter ABCB1 by pharmacological chaperones: a mechanistic concept

Point mutations of ATP‐binding cassette (ABC) proteins are a common cause of human diseases. Available crystal structures indicate a similarity in the architecture of several members of this protein family. Their molecular architecture makes these proteins vulnerable to mutation, when critical structural elements are affected. The latter preferentially involve the two transmembrane domain (TMD)/nucleotide‐binding domain (NBD) interfaces (transmission interfaces), formation of which requires engagement of coupling helices of intracellular loops with NBDs. Both, formation of the active sites and engagement of the coupling helices, are contingent on correct positioning of ICLs 2 and 4 and thus an important prerequisite for proper folding. Here, we show that active site compounds are capable of rescuing P‐glycoprotein (P‐gp) mutants ∆Y490 and ∆Y1133 in a concentration‐dependent manner. These trafficking deficient mutations are located at the transmission interface in pseudosymmetric position to each other. In addition, the ability of propafenone analogs to correct folding correlates with their ability to inhibit transport of model substrates. This finding indicates that folding correction and transport inhibition by propafenone analogs are brought about by binding to the active sites. Furthermore, this study demonstrates an asymmetry in folding correction with cis‐flupentixol, which reflects the asymmetric binding properties of this modulator to P‐gp. Our results suggest a mechanistic model for corrector action in a model ABC transporter based on insights into the molecular architecture of these transporters.

Current and future therapies for inherited cholestatic liver diseases

Familial intrahepatic cholestasis (FIC) comprises a group of rare cholestatic liver diseases associated with canalicular transport defects resulting predominantly from mutations in ATP8B1, ABCB11 and ABCB4. Phenotypes range from benign recurrent intrahepatic cholestasis (BRIC), associated with recurrent cholestatic attacks, to progressive FIC (PFIC). Patients often suffer from severe pruritus and eventually progressive cholestasis results in liver failure. Currently, first-line treatment includes ursodeoxycholic acid in patients with ABCB4 deficiency (PFIC3) and partial biliary diversion in patients with ATP8B1 or ABCB11 deficiency (PFIC1 and PFIC2). When treatment fails, liver transplantation is needed which is associated with complications like rejection, post-transplant hepatic steatosis and recurrence of disease. Therefore, the need for more and better therapies for this group of chronic diseases remains. Here, we discuss new symptomatic treatment options like total biliary diversion, pharmacological diversion of bile acids and hepatocyte transplantation. Furthermore, we focus on emerging mutation-targeted therapeutic strategies, providing an outlook for future personalized treatment for inherited cholestatic liver diseases.

Functional defect of variants in the adenosine triphosphate-binding sites of ABCB4 and their rescue by the cystic fibrosis transmembrane conductance regulator potentiator, ivacaftor (VX-770)

ABCB4 (MDR3) is an adenosine triphosphate (ATP)-binding cassette (ABC) transporter expressed at the canalicular membrane of hepatocytes, where it mediates phosphatidylcholine (PC) secretion. Variations in the ABCB4 gene are responsible for several biliary diseases, including progressive familial intrahepatic cholestasis type 3 (PFIC3), a rare disease that can be lethal in the absence of liver transplantation. In this study, we investigated the effect and potential rescue of ABCB4 missense variations that reside in the highly conserved motifs of ABC transporters, involved in ATP binding. Five disease-causing variations in these motifs have been identified in ABCB4 (G535D, G536R, S1076C, S1176L, and G1178S), three of which are homologous to the gating mutations of cystic fibrosis transmembrane conductance regulator (CFTR or ABCC7; i.e., G551D, S1251N, and G1349D), that were previously shown to be function defective and corrected by ivacaftor (VX-770; Kalydeco), a clinically approved CFTR potentiator. Three-dimensional structural modeling predicted that all five ABCB4 variants would disrupt critical interactions in the binding of ATP and thereby impair ATP-induced nucleotide-binding domain dimerization and ABCB4 function. This prediction was confirmed by expression in cell models, which showed that the ABCB4 mutants were normally processed and targeted to the plasma membrane, whereas their PC secretion activity was dramatically decreased. As also hypothesized on the basis of molecular modeling, PC secretion activity of the mutants was rescued by the CFTR potentiator, ivacaftor (VX-770).

CONCLUSION

Disease-causing variations in the ATP-binding sites of ABCB4 cause defects in PC secretion, which can be rescued by ivacaftor. These results provide the first experimental evidence that ivacaftor is a potential therapy for selected patients who harbor mutations in the ATP-binding sites of ABCB4. (Hepatology 2017;65:560-570).

In silico investigation of the impact of synonymous variants in ABCB4 gene on mRNA stability/structure, splicing accuracy and codon usage: Potential contribution to PFIC3 disease

Progressive Familial Intrahepatic Cholestasis type 3 (PFIC3) is an autosomal-recessive liver disease due to mutations in the ABCB4 gene encoding for the MDR3 protein. In the present study, we performed molecular and bioinformatic analyses in PFIC3 patients in order to understand the molecular basis of the disease. The three studied patients with PFIC3 were screened by PCR amplification followed by direct sequencing of the 27 coding exons of ABCB4. In silico analysis was performed by bioinformatic programs. We revealed three synonymous polymorphisms c.175C>T, c.504C>T, c.711A>T respectively in exon 4, 6, 8 and an intronic c.3487-16T>C variation in intron 26. The computational study of these polymorphic variants using Human Splicing Finder, ex-skip, Mfold and kineFold tools showed the putative impact on the composition of the cis-acting regulatory elements of splicing as well as on the mRNA structure and stability. Moreover, the protein level was affected by codon usage changes estimated by the calculation of ΔRSCU and ΔLog Ratio of codon frequencies interfering as consequence with the accurate folding of the MDR3 protein. As the first initiative of the mutational study of ABCB4 genes in Tunisia, our results are suggestive of a potential downstream molecular effect for the described polymorphisms on the expression pattern of the ABCB4 underlining the importance of synonymous variants.

The characteristics of activated portal fibroblasts/myofibroblasts in liver fibrosis

Liver fibrosis results from chronic injury of hepatocytes and activation of Collagen Type I producing myofibroblasts that produce fibrous scar in liver fibrosis. Myofibroblasts are not present in the normal liver but rapidly appear early in experimental and clinical liver injury. The origin of the myofibroblast in liver fibrosis is still unresolved. The possibilities include activation of liver resident cells including portal fibroblasts, hepatic stellate cells, mesenchymal progenitor cells, and fibrocytes recruited from the bone marrow. It is considered that hepatic stellate cells and portal fibroblasts are the major source of hepatic myofibroblasts. In fact, the origin of myofibroblasts differs significantly for chronic liver diseases of different etiologies, such as cholestatic liver disease or hepatotoxic liver disease. Depending on etiology of hepatic injury, the fibrogenic foci might initiate within the hepatic lobule as seen in chronic hepatitis, or primarily affect the portal areas as in most biliary diseases. It has been suggested that activated portal fibroblasts/myofibroblasts work as "myofibroblasts for cholangiocytes" while hepatic stellate cells work as "myofibroblast for hepatocytes". This review will focus on our current understanding of the activated portal fibroblasts/myofibroblasts in cholestatic liver fibrosis.

Functional characterization of ABCB4 mutations found in progressive familial intrahepatic cholestasis type 3

Multidrug resistance 3 (MDR3), encoded by the ATP-binding cassette, subfamily B, member 4 gene (ABCB4), localizes to the canalicular membrane of hepatocytes and translocates phosphatidylcholine from the inner leaflet to the outer leaflet of the canalicular membrane. Progressive familial intrahepatic cholestasis type 3 (PFIC3) is a rare hepatic disease caused by genetic mutations of ABCB4. In this study, we characterized 8 ABCB4 mutations found in PFIC3 patients, using in vitro molecular assays. First, we examined the transport activity of each mutant by measuring its ATPase activity using paclitaxel or phosphatidylcholine. Then, the pathogenic mechanisms by which these mutations affect MDR3 were examined through immunoblotting, cell surface biotinylation, and immunofluorescence. As a result, three ABCB4 mutants showed significantly reduced transport activity. Among these mutants, one mutation A364V, located in intracellular domains, markedly decreased MDR3 expression on the plasma membrane, while the others did not affect the expression. The expression of MDR3 on the plasma membrane and transport activity of A364V was rescued by a pharmacological chaperone, cyclosporin A. Our study provides the molecular mechanisms of ABCB4 mutations and may contribute to the understanding of PFIC3 pathogenesis and the development of a mutation-specific targeted treatment for PFIC3.

A functional classification of ABCB4 variations causing progressive familial intrahepatic cholestasis type 3

Progressive familial intrahepatic cholestasis type 3 is caused by biallelic variations of ABCB4, most often (≥70%) missense. In this study, we examined the effects of 12 missense variations identified in progressive familial intrahepatic cholestasis type 3 patients. We classified these variations on the basis of the defects thus identified and explored potential rescue of trafficking-defective mutants by pharmacological means. Variations were reproduced in the ABCB4 complementary DNA and the mutants, thus obtained, expressed in HepG2 and HEK293 cells. Three mutants were either fully (I541F and L556R) or largely (Q855L) retained in the endoplasmic reticulum, in an immature form. Rescue of the defect, i.e., increase in the mature form at the bile canaliculi, was obtained by cell treatments with cyclosporin A or C and, to a lesser extent, B, D, or H. Five mutations with little or no effect on ABCB4 expression at the bile canaliculi caused a decrease (F357L, T775M, and G954S) or almost absence (S346I and P726L) of phosphatidylcholine secretion. Two mutants (T424A and N510S) were normally processed and expressed at the bile canaliculi, but their stability was reduced. We found no defect of the T175A mutant or of R652G, previously described as a polymorphism. In patients, the most severe phenotypes appreciated by the duration of transplant-free survival were caused by ABCB4 variants that were markedly retained in the endoplasmic reticulum and expressed in a homozygous status.

CONCLUSION

ABCB4 variations can be classified as follows: nonsense variations (I) and, on the basis of current findings, missense variations that primarily affect the maturation (II), activity (III), or stability (IV) of the protein or have no detectable effect (V); this classification provides a strong basis for the development of genotype-based therapies.

Functional Rescue of Trafficking-Impaired ABCB4 Mutants by Chemical Chaperones

Multidrug resistance protein 3 (MDR3, ABCB4) is a hepatocellular membrane protein that mediates biliary secretion of phosphatidylcholine. Null mutations in ABCB4 gene give rise to severe early-onset cholestatic liver disease. We have previously shown that the disease-associated mutations p.G68R, p.G228R, p.D459H, and p.A934T resulted in retention of ABCB4 in the endoplasmic reticulum, thus failing to target the plasma membrane. In the present study, we tested the ability of two compounds with chaperone-like activity, 4-phenylbutyrate and curcumin, to rescue these ABCB4 mutants by assessing their effects on subcellular localization, protein maturation, and phospholipid efflux capability. Incubation of transfected cells at a reduced temperature (30°C) or exposure to pharmacological doses of either 4-PBA or curcumin restored cell surface expression of mutants G228R and A934T. The delivery of these mutants to the plasma membrane was accompanied by a switch in the ratio of mature to inmature protein forms, leading to a predominant expression of the mature protein. This effect was due to an improvement in the maturation rate and not to the stabilization of the mature forms. Both mutants were also functionally rescued, displaying bile salt-dependent phospholipid efflux activity after addition of 4-PBA or curcumin. Drug-induced rescue was mutant specific, given neither 4-PBA nor curcumin had an effect on the ABCB4 mutants G68R and A934T. Collectively, these data indicate that the functionality of selected trafficking-defective ABCB4 mutants can be recovered by chemical chaperones through restoration of membrane localization, suggesting a potential treatment for patients carrying such mutations.

Heterozygous ABCB4 mutations in children with cholestatic liver disease

BACKGROUND & AIMS

Monoallelic defects in ABCB4, which encodes the canalicular floppase for phosphatidylcholine MDR3, have been encountered in association with a variety of hepatobiliary disorders, particularly in adult subjects. In this study, we examined the presence of heterozygous ABCB4 variants in a cohort of children with chronic cholestasis and assessed the pathogenicity of the missense changes identified.

METHODS

Sixty-seven children with chronic liver dysfunction were studied by the sequencing of ABCB4 and multiplex ligation-dependent probe amplification analysis. The molecular defects arising from missense variants were analysed in MDCK-II and AD-293 cells.

RESULTS

Defects in a single allele of ABCB4 were identified in nine subjects. They included one small insertion (p.I1242Nfs), one nonsense mutation (p.R144X) and six missense changes (p.T175A, p.G228R, p.A250T, p.S320F, p.P352L and p.A934T). In four children, these defects in ABCB4 co-existed with various medical conditions. In vitro phenotyping of the six missense variants revealed that four (T175A, G228R, S320F and A934T) led to reduced MDR3 protein levels. Two mutations (G228R and A934T) resulted in trapping of the protein in the endoplasmic reticulum. Phosphatidylcholine efflux activity was decreased to 56-18% of reference levels for MDR3 mutants T175A, A250T and S320F. The G228R, P352L and A934T mutants were found to be non-functional.

CONCLUSIONS

These results illustrate the varying effects of ABCB4 missense mutations and suggest that even a modest reduction in MDR3 activity may contribute or predispose to the onset of cholestatic liver disease in the paediatric age.

A PDZ-Like Motif in the Biliary Transporter ABCB4 Interacts with the Scaffold Protein EBP50 and Regulates ABCB4 Cell Surface Expression

ABCB4/MDR3, a member of the ABC superfamily, is an ATP-dependent phosphatidylcholine translocator expressed at the canalicular membrane of hepatocytes. Defects in the ABCB4 gene are associated with rare biliary diseases. It is essential to understand the mechanisms of its canalicular membrane expression in particular for the development of new therapies. The stability of several ABC transporters is regulated through their binding to PDZ (PSD95/DglA/ZO-1) domain-containing proteins. ABCB4 protein ends by the sequence glutamine-asparagine-leucine (QNL), which shows some similarity to PDZ-binding motifs. The aim of our study was to assess the potential role of the QNL motif on the surface expression of ABCB4 and to determine if PDZ domain-containing proteins are involved. We found that truncation of the QNL motif decreased the stability of ABCB4 in HepG2-transfected cells. The deleted mutant ABCB4-ΔQNL also displayed accelerated endocytosis. EBP50, a PDZ protein highly expressed in the liver, strongly colocalized and coimmunoprecipitated with ABCB4, and this interaction required the QNL motif. Down-regulation of EBP50 by siRNA or by expression of an EBP50 dominant-negative mutant caused a significant decrease in the level of ABCB4 protein expression, and in the amount of ABCB4 localized at the canalicular membrane. Interaction of ABCB4 with EBP50 through its PDZ-like motif plays a critical role in the regulation of ABCB4 expression and stability at the canalicular plasma membrane.

Targeted pharmacotherapy in progressive familial intrahepatic cholestasis type 2: Evidence for improvement of cholestasis with 4-phenylbutyrate

Progressive familial intrahepatic cholestasis type 2 (PFIC2) is a result of mutations in ABCB11 encoding bile salt export pump (BSEP), the canalicular bile salt export pump of hepatocyte. In some PFIC2 patients with missense mutations, BSEP is not detected at the canaliculus owing to mistrafficking of BSEP mutants. In vitro, chaperone drugs, such as 4-phenylbutyrate (4-PB), have been shown to partially correct mistrafficking. Four PFIC2 patients harboring at least one missense mutation (p.G982R, p.R1128C, and p.T1210P) were treated orally with 4-PB and followed prospectively. Patient mutations were reproduced in a Bsep/green fluorescent protein plasmid. Cellular localization of the resulting Bsep mutants was studied in a hepatocellular line (Can 10), and effects of treatment with 4-PB and/or ursodeoxycholic acid (UDCA) were assessed. In Can 10 cells, Bsep mutants were detected in the endoplasmic reticulum instead of at the canalicular membrane. Treatment with 4-PB and UDCA partially corrected Bsep mutant targeting. With 4-PB, we observed, in all patients, a decrease of pruritus and serum bile acid concentration (BAC) as well as an improvement of serum liver tests. Pathological liver injuries improved, and BSEP, which was not detected at the canalicular membrane before treatment, appeared at the canalicular membrane. Bile analyses showed an increase in BAC with 4-PB. Patient conditions remained stable with a median follow-up of 40 months (range, 3-53), and treatment tolerance was good.

CONCLUSION

4-PB therapy may be efficient in selected patients with PFIC2 owing to ABCB11 missense mutations affecting BSEP canalicular targeting. Bile secretion improvement may be a result of the ability of 4-PB to retarget mutated BSEP.

Promoting expression of transporters for treatment of progressive familial intrahepatic cholestasis disease

Progressive familial intrahepatic cholestasis (PFIC) is a heterogeneous group of autosomal recessive genetic diseases with a major clinical manifestation of severe intrahepatic cholestasis and an incidence rate of 1/10000 to 1/5000. PFIC is usually first diagnosed in infancy or childhood and eventually develops into liver failure and death. Based on clinical manifestations, laboratory tests, and genetic defects in liver tissue, PFIC is roughly divided into three types: PFIC-1, PFIC-2 and PFIC-3. Studies have demonstrated that all three types of PFIC are associated with the mutations of bile transport system genes in the liver. Promoting transporter expression has important clinical significance for the treatment of PFIC. In this paper, we summarize the etiology and treatment status of PFIC and discuss the treatment of PFIC by promoting the expression of transporters.

Functional analysis of ABCB4 mutations relates clinical outcomes of progressive familial intrahepatic cholestasis type 3 to the degree of MDR3 floppase activity

OBJECTIVE

Progressive familial intrahepatic cholestasis type 3 (PFIC3) is a potentially lethal autosomal recessive liver disease associated with mutations in ABCB4, the gene encoding the canalicular translocator of phosphatidylcholine MDR3. While some affected children benefit from ursodeoxycholic acid (UDCA) therapy, others evolve to end-stage liver disease. We aimed to evaluate whether these different outcomes are related to the impact of ABCB4 mutations.

DESIGN

Six children with PFIC3 were investigated by sequencing of ABCB4 exons and flanking intron-exon boundaries and by immunohistochemistry. ABCB4 missense mutations were phenotyped in vitro by assessing their effects on MDR3 expression, subcellular localisation, and phosphatidylcholine-translocating activity. The resulting data were contrasted with the clinical outcomes.

RESULTS

Eight distinct ABCB4 mutations were identified: one nonsense, one splicing and six missense mutations, four of which (G68R, T201M, P479L, D459H) affected MDR3 expression level. G68R and D459H also led to retention of the protein in endoplasmic reticulum. Phosphatidylcholine efflux assays indicated that T201M, P479L, S978P and E1118K mutations impaired MDR3 activity to variable degrees. Three children with mutations that caused a total loss of MDR3 expression/function manifested progressive liver disease refractory to UDCA treatment. This was also the case in a patient carrying two different mutations that, in combination, resulted in a 90% reduction in total MDR3 activity. A favourable response to UDCA was achieved in two patients with estimated MDR3 activities of 50% and 33%, respectively.

CONCLUSIONS

These data provide experimental evidence of the correlation between the degree of MDR3 floppase activity and the clinical outcomes of PFIC3.

ABCB4: Insights from pathobiology into therapy

Adenosine triphosphate (ATP)-binding cassette, sub-family B, member 4 (ABCB4), also called multidrug resistance 3 (MDR3), is a member of the ATP-binding cassette transporter superfamily, which is localized at the canalicular membrane of hepatocytes, and mediates the translocation of phosphatidylcholine into bile. Phosphatidylcholine secretion is crucial to ensure solubilization of cholesterol into mixed micelles and to prevent bile acid toxicity towards hepatobiliary epithelia. Genetic defects of ABCB4 may cause progressive familial intrahepatic cholestasis type 3 (PFIC3), a rare autosomic recessive disease occurring early in childhood that may be lethal in the absence of liver transplantation, and other cholestatic or cholelithiasic diseases in heterozygous adults. Development of therapies for these conditions requires understanding of the biology of this transporter and how gene variations may cause disease. This review focuses on our current knowledge on the regulation of ABCB4 expression, trafficking and function, and presents recent advances in fundamental research with promising therapeutic perspectives.

Sizing up surfactant synthesis

Phosphatidylcholine is generated through de novo synthesis and remodeling involving a lysophospholipid. In this issue of Cell Metabolism, research from the Shimizu lab (Harayama et al., 2014) demonstrates the highly selective enzymatic behavior of lysophospholipid acyltransferases. The authors present an enzymatic model for phosphatidylcholine molecular species diversification that impacts surfactant formation.

Phosphorylation of ABCB4 impacts its function: insights from disease-causing mutations

The ABCB4 transporter mediates phosphatidylcholine (PC) secretion at the canalicular membrane of hepatocytes and its genetic defects cause biliary diseases. Whereas ABCB4 shares high sequence identity with the multidrug transporter, ABCB1, its N-terminal domain is poorly conserved, leading us to hypothesize a functional specificity of this domain. A database of ABCB4 genotyping in a large series of patients was screened for variations altering residues of the N-terminal domain. Identified variants were then expressed in cell models to investigate their biological consequences. Two missense variations, T34M and R47G, were identified in patients with low-phospholipid-associated cholelithiasis or intrahepatic cholestasis of pregnancy. The T34M and R47G mutated proteins showed no or minor defect, respectively, in maturation and targeting to the apical membrane, in polarized Madin-Darby Canine Kidney and HepG2 cells, whereas their stability was similar to that of wild-type (WT) ABCB4. By contrast, the PC secretion activity of both mutants was markedly decreased. In silico analysis indicated that the identified variants were likely to affect ABCB4 phosphorylation. Mass spectrometry analyses confirmed that the N-terminal domain of WT ABCB4 could undergo phosphorylation in vitro and revealed that the T34M and R47G mutations impaired such phosphorylation. ABCB4-mediated PC secretion was also increased by pharmacological activation of protein kinases A or C and decreased by inhibition of these kinases. Furthermore, secretion activity of the T34M and R47G mutants was less responsive than that of WT ABCB4 to protein kinase modulators.

CONCLUSION

We identified disease-associated variants of ABCB4 involved in the phosphorylation of its N-terminal domain and leading to decreased PC secretion. Our results also indicate that ABCB4 activity is regulated by phosphorylation, in particular, of N-terminal residues.

Molecular mechanistic explanation for the spectrum of cholestatic disease caused by the S320F variant of ABCB4

ABCB4 flops phosphatidylcholine into the bile canaliculus to protect the biliary tree from the detergent activity of bile salts. Homozygous-null ABCB4 mutations cause the childhood liver disease, progressive familial intrahepatic cholestasis, but cause and effect is less clear, with many missense mutations linked to less severe cholestatic diseases. ABCB4(S320F), in particular, is described in 13 patients, including in heterozygosity with ABCB4(A286V), ABCB4(A953D), and null mutants, whose symptoms cover the spectrum of cholestatic disease. We sought to define the impact of these mutations on the floppase, explain the link with multiple conditions at the molecular level, and investigate the potential for reversal. ABCB4(S320F), ABCB4(A286V), and ABCB4(A953D) expression was engineered in naïve cultured cells. Floppase expression, localization, and activity were measured by western blot, confocal microscopy, and lipid transport assays, respectively. ABCB4(S320F) was fully active for floppase activity but expression at the plasma membrane was reduced to 50%. ABCB4(A286V) expressed and trafficked efficiently but could not flop lipid, and ABCB4(A953D) expressed poorly and was impaired in floppase activity. Proteasome inhibition stabilized nascent ABCB4(S320F) and ABCB4(A953D) but did not improve plasma membrane localization. Cyclosporin-A improved plasma membrane localization of both ABCB4(S320F) and ABCB4(A953D), but inhibited floppase activity.

CONCLUSION

The level of ABCB4 functionality correlates with, and is the primary determinant of, cholestatic disease severity in these patients. ABCB4(S320F) homozygosity, with half the normal level of ABCB4, is the tipping point between more benign and potentially fatal cholestasis and makes these patients more acutely sensitive to environmental effects. Cyclosporin-A increased expression of ABCB4(S320F) and ABCB4(A953D), suggesting that chemical chaperones could be exploited for therapeutic benefit to usher in a new era of personalized medicine for patients with ABCB4-dependent cholestatic disease.

Lack of association of ABCB4 insertion mutation with gallbladder mucoceles in dogs

The etiology of canine gallbladder mucocele (GBM) has not yet been identified. However, several studies have linked GBM in dogs to particular breeds (Shetland Sheepdogs are commonly implicated), concurrent endocrine disease (hyperadrenocorticism and/or hypothyroidism), and a mutation in the canine ABCB4 gene (ABCB4 1583_1584G), particularly in Shetland Sheepdogs. The current study assessed ABCB4 1583_1584G, in a wider sample of dogs with GBM compared with age and breed-matched controls. ABCB4 1583_1584G was identified in 4 of 8 Shetland Sheepdogs and 13 of 28 other breeds with GBM. ABCB4 1583_1584G was also detected in 9 of 12 Shetland Sheepdogs and 23 of 37 other breeds that did not have GBM. No statistically significant association existed between ABCB4 1583_1584G and the presence of GBM for all dogs combined or for Shetland Sheepdogs alone. In contrast to previously reported findings, the current study did not identify a strong association between ABCB4 1583_1584G and GBM in Shetland Sheepdogs or other breeds.

The role of canalicular ABC transporters in cholestasis

Cholestasis, a hallmark feature of hepatobiliary disease, is characterized by the retention of biliary constituents. Some of these constituents, such as bile acids, inflict damage to hepatocytes and bile duct cells. This damage may lead to inflammation, fibrosis, cirrhosis, and eventually carcinogenesis, sequelae that aggravate the underlying disease and deteriorate clinical outcome. Canalicular ATP-binding cassette (ABC) transporters, which mediate the excretion of individual bile constituents, play a key role in bile formation and cholestasis. The study of these transporters and their regulatory nuclear receptors has revolutionized our understanding of cholestatic disease. This knowledge has served as a template to develop novel treatment strategies, some of which are currently already undergoing phase III clinical trials. In this review we aim to provide an overview of the structure, function, and regulation of canalicular ABC transporters. In addition, we will focus on the role of these transporters in the pathogenesis and treatment of cholestatic bile duct and liver diseases.

Progressive familial intrahepatic cholestasis

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.

Functional Characterization of ABCB4 Mutations Found in Low Phospholipid-Associated Cholelithiasis (LPAC)

Multidrug resistance 3 (MDR3) is expressed on the canalicular membrane of the hepatocytes and plays an important role in protecting the liver from bile acids. Altered ABCB4 gene expression can lead to a rare hepatic disease, low phospholipid-associated cholelithiasis (LPAC). In this study, we characterized 3 ABCB4 mutations in LPAC patients using various in vitro assay systems. We first measured the ability of each mutant to transport paclitaxel and then the mechanisms by which these mutations might change MDR3 transport activity were determined using immunoblotting, cell surface protein biotinylation, and immunofluorescence. Through a membrane vesicular transport assay, we observed that the uptake of paclitaxel was significantly reduced in membrane vesicles expressing 2 ABCB4 mutations, F165I and S320F. Both mutants showed significantly decreased total and cell surface MDR3 expression. These data suggest two missense mutations of ABCB4 may alter function of MDR3 and ultimately can be determined as LPAC-causing mutations.

Defining the blanks--pharmacochaperoning of SLC6 transporters and ABC transporters

SLC6 family members and ABC transporters represent two extremes: SLC6 transporters are confined to the membrane proper and only expose small segments to the hydrophilic milieu. In ABC transporters the hydrophobic core is connected to a large intracellular (eponymous) ATP binding domain that is comprised of two discontiguous repeats. Accordingly, their folding problem is fundamentally different. This can be gauged from mutations that impair the folding of the encoded protein and give rise to clinically relevant disease phenotypes: in SLC6 transporters, these cluster at the protein–lipid interface on the membrane exposed surface. Mutations in ABC-transporters map to the interface between nucleotide binding domains and the coupling helices, which provide the connection to the hydrophobic core. Folding of these mutated ABC-transporters can be corrected with ligands/substrates that bind to the hydrophobic core. This highlights a pivotal role of the coupling helices in the folding trajectory. In contrast, insights into pharmacochaperoning of SLC6 transporters are limited to monoamine transporters – in particular the serotonin transporter (SERT) – because of their rich pharmacology. Only ligands that stabilize the inward facing conformation act as effective pharmacochaperones. This indicates that the folding trajectory of SERT proceeds via the inward facing conformation. Mutations that impair folding of SLC6 family members can be transmitted as dominant or recessive alleles. The dominant phenotype of the mutation can be rationalized, because SLC6 transporters are exported in oligomeric form from the endoplasmic reticulum (ER). Recessive transmission requires shielding of the unaffected gene product from the mutated transporter in the ER. This can be accounted for by a chaperone-COPII (coatomer protein II) exchange model, where proteinaceous ER-resident chaperones engage various intermediates prior to formation of the oligomeric state and subsequent export from the ER. It is likely that the action of pharmacochaperones is contingent on and modulated by these chaperones.

Bile formation and secretion

Bile is a unique and vital aqueous secretion of the liver that is formed by the hepatocyte and modified down stream by absorptive and secretory properties of the bile duct epithelium. Approximately 5% of bile consists of organic and inorganic solutes of considerable complexity. The bile-secretory unit consists of a canalicular network which is formed by the apical membrane of adjacent hepatocytes and sealed by tight junctions. The bile canaliculi (∼1 μm in diameter) conduct the flow of bile countercurrent to the direction of portal blood flow and connect with the canal of Hering and bile ducts which progressively increase in diameter and complexity prior to the entry of bile into the gallbladder, common bile duct, and intestine. Canalicular bile secretion is determined by both bile salt-dependent and independent transport systems which are localized at the apical membrane of the hepatocyte and largely consist of a series of adenosine triphosphate-binding cassette transport proteins that function as export pumps for bile salts and other organic solutes. These transporters create osmotic gradients within the bile canalicular lumen that provide the driving force for movement of fluid into the lumen via aquaporins. Species vary with respect to the relative amounts of bile salt-dependent and independent canalicular flow and cholangiocyte secretion which is highly regulated by hormones, second messengers, and signal transduction pathways. Most determinants of bile secretion are now characterized at the molecular level in animal models and in man. Genetic mutations serve to illuminate many of their functions.

Two ABCB4 point mutations of strategic NBD-motifs do not prevent protein targeting to the plasma membrane but promote MDR3 dysfunction

The ABCB4 gene encodes for MDR3, a protein that translocates phosphatidylcholine from the inner to the outer leaflet of the hepatocanalicular membrane; its deficiency favors the formation of 'toxic bile'. Several forms of hepatobiliary diseases have been associated with ABCB4 mutations, but the detrimental effects of most mutations on the encoded protein needs to be clarified. Among subjects with cholangiopathies who were screened for mutations in ABCB4 by direct sequencing, we identified the new mutation p.(L481R) in three brothers. According to our model of tertiary structure, this mutation affects the Q-loop, whereas the p.(Y403H) mutation, that we already described in two other families, involves the A-loop. This study was aimed at analyzing the functional relevance of these two ABCB4 mutations: MDR3 expression and lipid content in the culture supernatant were evaluated in cell lines stably transfected with the ABCB4 wild-type clone and corresponding mutants. No differences of expression were observed between wild-type and mutant gene products. Instead, both mutations caused a reduction of phosphatidylcholine secretion compared with the wild-type transfected cell lines. On the contrary, cholesterol (Chol) release, after 1 and 3 mM sodium taurocholate stimulation, was higher in the mutant-transfected cell lines than that in the wild-type and was particularly enhanced in cells transfected with the p.Y403H-construct.In summary, our data show that both mutations do not seem to affect protein expression, but are able to reduce the efflux of phosphatidylcholine associated with increase of Chol, thereby promoting the formation of toxic bile.

Functional characterization of genetic variations in the MDR3 promoter

Multidrug resistance 3 (MDR3) is present on the canalicular membrane of the hepatocyte and plays an important role in protecting the liver from bile acids. In this study, we characterized the transcriptional effects of four common haplotypes and four polymorphic variants in the promoter region of MDR3 that were identified in 126 DNA samples from Koreans. We measured the luciferase activities of the four MDR3 promoter haplotypes using in vitro reporter assays. Among them, two haplotypes showed a significant decrease in reporter activity compared to the reference. One of the mechanisms by which these haplotypes might decrease MDR3 transcriptional activity was determined: one of the polymorphisms that are present in haplotype 3, was associated with a significant reduction in the promoter activity of MDR3, and the transcription factor NF-Y was predicted to bind to the promoter in the region of g.-1584C>T. Electrophoretic mobility shift assays showed that the g.-1584C allele exhibited greater binding to NF-Y than did the g.-1584T allele. Through the measurement of promoter activity after the overexpression of NF-Y, we found that NF-Y can act as a transcriptional activator of MDR3. These data suggest that the reduced transcriptional activity of g.-1584C>T results from a reduction in the binding affinity of the activator NF-Y to the MDR3 promoter region. Our study suggests that two common haplotypes of MDR3 can regulate the transcriptional rate of MDR3 and that NF-Y may be one of the transcriptional factors involved in this regulation.

Progressive familial intrahepatic cholestasis

Progressive familial intrahepatic cholestasis (PFIC) refers to a 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 associated with 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 arise later in infancy, in childhood or even during young adulthood. The 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 to defects in ATP8B1 encoding the FIC1 protein and in ABCB11 encoding bile salt export pump (BSEP) protein, respectively. Defects in ABCB4, encoding multidrug resistance 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 to exclude other causes of childhood cholestasis. MDR3 and BSEP liver immunostaining, and analysis of biliary lipid composition should help to select PFIC candidates for whom genotyping could be proposed to confirm the diagnosis. Antenatal diagnosis may 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 and PFIC2 patients, biliary diversion may also relieve pruritus and slow disease progression. However, most PFIC patients are ultimately candidates for liver transplantation. Monitoring of liver tumors, especially in PFIC2 patients, should be offered from the first year of life. Hepatocyte transplantation, gene therapy and specific targeted pharmacotherapy may represent alternative treatments in the future.

Effects of cellular, chemical, and pharmacological chaperones on the rescue of a trafficking-defective mutant of the ATP-binding cassette transporter proteins ABCB1/ABCB4

The ATP-binding cassette transporter ABCB4 is a phosphatidylcholine translocator specifically expressed at the bile canalicular membrane in hepatocytes, highly homologous to the multidrug transporter ABCB1. Variations in the ABCB4 gene sequence cause progressive familial intrahepatic cholestasis type 3. We have shown previously that the I541F mutation, when reproduced either in ABCB1 or in ABCB4, led to retention in the endoplasmic reticulum (ER)/Golgi. Here, Madin-Darby canine kidney cells expressing ABCB1-GFP were used as a model to investigate this mutant. We show that ABCB1-I541F is not properly folded and is more susceptible to in situ protease degradation. It colocalizes and coprecipitates with the ER chaperone calnexin and coprecipitates with the cytosolic chaperone Hsc/Hsp70. Silencing of calnexin or overexpression of Hsp70 have no effect on maturation of the mutant. We also tested potential rescue by chemical and pharmacological chaperones. Thapsigargin and sodium 4-phenyl butyrate were inefficient. Glycerol improved maturation and exit of the mutant from the ER. Cyclosporin A, a competitive substrate for ABCB1, restored maturation, plasma membrane expression, and activity of ABCB1-I541F. Cyclosporin A also improved maturation of ABCB4-I541F in Madin-Darby canine kidney cells. In HepG(2) cells transfected with ABCB4-I541F cDNA, cyclosporin A allowed a significant amount of the mutant protein to reach the membrane of bile canaliculi. These results show that the best strategy to rescue conformation-defective ABCB4 mutants is provided by pharmacological chaperones that specifically target the protein. They identify cyclosporin A as a potential novel therapeutic tool for progressive familial intrahepatic cholestasis type 3 patients.

Complementary functions of the flippase ATP8B1 and the floppase ABCB4 in maintaining canalicular membrane integrity

BACKGROUND & AIMS

Progressive familial intrahepatic cholestasis can be caused by mutations in ABCB4 or ATP8B1; each encodes a protein that translocates phospholipids, but in opposite directions. ABCB4 flops phosphatidylcholine from the inner to the outer leaflet, where it is extracted by bile salts. ATP8B1, in complex with the accessory protein CDC50A, flips phosphatidylserine in the reverse direction. Abcb4(-/-) mice lack biliary secretion of phosphatidylcholine, whereas Atp8b1-deficient mice have increased excretion of phosphatidylserine into bile. Each system is thought to have a role protecting the canalicular membrane from bile salts.

METHODS

To investigate the relationship between the mechanisms of ABCB4 and ATP8B1, we expressed the transporters separately and together in cultured cells and studied viability and phospholipid transport. We also created mice with disruptions in ABCB4 and ATP8B1 (double knockouts) and studied bile formation and hepatic damage in mice fed bile salts.

RESULTS

Overexpression of ABCB4 was toxic to HEK293T cells; the toxicity was counteracted by coexpression of the ATP8B1-CDC50A complex. In Atp8b1-deficient mice, bile salts induced extraction of phosphatidylserine and ectoenzymes from the canalicular membrane; this process was not observed in the double-knockout mice.

CONCLUSIONS

ATP8B1 is required for hepatocyte function, particularly in the presence of ABCB4. This is most likely because the phosphatidylserine flippase complex of ATP8B1-CDC50A counteracts the destabilization of the membrane that occurs when ABCB4 flops phosphatidylcholine. Lipid asymmetry is therefore important for the integrity of the canalicular membrane; ABCB4 and ATP8B1 cooperate to protect hepatocytes from bile salts.

Intracellular α(2C)-adrenoceptors: storage depot, stunted development or signaling domain?

G-protein coupled receptors (GPCRs) are generally considered to function as cell surface signaling structures that respond to extracellular mediators, many of which do not readily access the cell's interior. Indeed, most GPCRs are preferentially targeted to the plasma membrane. However, some receptors, including α(2C)-Adrenoceptors, challenge conventional concepts of GPCR activity by being preferentially retained and localized within intracellular organelles. This review will address the issues associated with this unusual GPCR localization and discuss whether it represents a novel sub-cellular niche for GPCR signaling, whether these receptors are being stored for rapid deployment to the cell surface, or whether they represent immature or incomplete receptor systems.

Ubiquitin-mediated proteasomal degradation of ABC transporters: a new aspect of genetic polymorphisms and clinical impacts

The interindividual variation in the rate of drug metabolism and disposition has been known for many years. Pharmacogenomics dealing with heredity and response to drugs is a part of science that attempts to explain variability of drug responses and to search for the genetic basis of such variations or differences. Genetic polymorphisms of drug metabolizing enzymes and drug transporters have been found to play a significant role in the patients' responses to medication. Accumulating evidence demonstrates that certain nonsynonymous polymorphisms have great impacts on the protein stability and degradation, as well as the function of drug metabolizing enzymes and transporters. The aim of this review article is to address a new aspect of protein quality control in the endoplasmic reticulum and to present examples regarding the impact of nonsynonymous single-nucleotide polymorphisms on the protein stability of thiopurine S-methyltransferase as well as ATP-binding cassette (ABC) transporters including ABCC4, cystic fibrosis transmembrane conductance regulator (CFTR, ABCC7), ABCC11, and ABCG2. Furthermore, we will discuss the molecular mechanisms underlying posttranslational modifications (intramolecular and intermolecular disulfide bond formation and N-linked glycosylation) and ubiquitin-mediated proteasomal degradation of ABCG2, one of the major drug transporter proteins in humans.

Intragenic suppressing mutations correct the folding and intracellular traffic of misfolded mutants of Yor1p, a eukaryotic drug transporter

ATP-binding cassette (ABC) transporters play pivotal physiological roles in substrate transport across membranes, and defective assembly of these proteins can cause severe disease associated with improper drug or ion flux. The yeast protein Yor1p is a useful model to study the biogenesis of ABC transporters; deletion of a phenylalanine residue in the first nucleotide-binding domain (NBD1) causes misassembly and retention in the endoplasmic reticulum (ER) of the resulting protein Yor1p-ΔF670, similar to the predominant disease-causing allele in humans, CFTR-ΔF508. Here we describe two novel Yor1p mutants, G278R and I1084P, which fail to assemble and traffic similar to Yor1p-ΔF670. These mutations are located in the two intracellular loops (ICLs) that interface directly with NBD1, and thus disrupt a functionally important structural module. We isolated 2 second-site mutations, F270S and R1168M, which partially correct the folding injuries associated with the G278R, I1084P, and ΔF670 mutants and reinstate their trafficking. The position of both corrective mutations at the cytoplasmic face of a transmembrane helix suggests that they restore biogenesis by influencing the behavior of the transmembrane domains rather than by direct restoration of the ICL1-ICL4-NBD1 structural module. Given the conserved topology of many ABC transporters, our findings provide new understanding of functionally important inter-domain interactions and suggest new potential avenues for correcting folding defects caused by abrogation of those domain interfaces.

Familial cholestasis: progressive familial intrahepatic cholestasis, benign recurrent intrahepatic cholestasis and intrahepatic cholestasis of pregnancy

Progressive familial intrahepatic cholestasis (PFIC) type 1, 2 and 3 are due to mutations in ATP8B1, ABCB11 and ABCB4, respectively. Each of these genes encodes a hepatocanalicular transporter, which is essential for the proper formation of bile. Mutations in ABCB4 can result in progressive cholestatic disease, while mutations in ATP8B1 and ABCB11 can result both in episodic cholestasis, referred to as benign recurrent intrahepatic cholestasis (BRIC) type 1 and 2, as well as in progressive cholestatic disease. This suggests a clinical continuum and these diseases are therefore preferably referred to as ATP8B1 deficiency and ABCB11 deficiency. Similarly PFIC type 3 is designated as ABCB4 deficiency. Heterozygous mutations in each of these transporters can also be associated with intrahepatic cholestasis of pregnancy. This review summarizes the pathophysiology, clinical features and current as well as future therapeutic options for progressive familial- and benign recurrent intrahepatic cholestasis as well as intrahepatic cholestasis of pregnancy.