Expression Analysis of ATP-Binding Cassette Transporters ABCB11 and ABCB4 in Primary Sclerosing Cholangitis and Variety of Pediatric and Adult Cholestatic and Noncholestatic Liver Diseases

Analysis of various ATP-binding cassette transporters revealed quantification of ABCB4 as a potential diagnostic tool in primary sclerosing cholangitis (PSC)

AIMS

ATP-binding cassette transporters are important proteins in regulating bile constituent transport between hepatocytes and the bile canalicular system. Dysfunctional transporters lead to accumulation of toxic bile components within hepatocytes or the biliary system, known as cholestasis, resulting in liver damage. It has been previously reported that two particular ATP-binding cassette transporters, ABCB4 and ABCB11, have altered expression in patients with primary sclerosing cholangitis (PSC). Interested in further analysis of expression patterns of ATP-binding cassette transporters in PSC patients, we investigated liver samples from 201 patients, including 43 patients with PSC and 51 patients with primary biliary cholangitis patients (PBC). In addition to ABCB4 and ABCB11, we also included other ATP-binding cassette transporters, to determine if upregulation of ABCB4 and ABCB11 is specifically found in the liver of patients with PSC.

METHODS AND RESULTS

Retrospectively, formalin-fixed and paraffin-embedded liver biopsies, resections, and explants were selected to investigate the expression of ABCB1, ABCB4, ABCB11, ABCG5/8, and FXR1 using nanoString nCounter and immunohistochemistry for validation of differently expressed transporters seen in PSC liver samples in comparison to non-PSC liver specimens. Strikingly, ABCB4 was the only ATP-binding cassette transporter showing increased gene and protein expression in hepatocytes of PSC livers when compared to non-PSC liver specimens. Furthermore, ABCB4 protein expression also correlated with disease stage in PSC.

CONCLUSION

Our study concluded that altered ABCB4 expression is specifically seen in liver specimens of PSC patients. Therefore, quantitative ABCB4 analysis may be an additional useful tool for the histopathological diagnosis of PSC to distinguish this entity from other cholangiopathies.

Postoperative jaundice related to UGT1A1 and ABCB11 gene mutations: A case report and literature review

BACKGROUND

Patients with obstructive jaundice caused by intrahepatic bile duct stones can be effectively managed by surgery. However, some patients may develop postoperative complications, liver failure, and other life-threatening situations. Here, we report a patient with mutations in the uridine 5’-diphospho-glucuronosyltransferase 1A1 (UGT1A1) and bile salt export pump (adenosine triphosphate-binding cassette subfamily B member 11, ABCB11) genes who presented multiple intrahepatic bile duct stones and cholestasis, and the jaundice of the patient increased after partial hepatectomy.

CASE SUMMARY

A 52-year-old male patient admitted to the hospital on October 23, 2021, with a progressive exacerbation of jaundice, was found to have multiple intrahepatic bile duct stones with the diagnoses of obstructive jaundice and acute cholecystitis. Subsequently, the patient underwent left hepatectomy with biliary exploration, stone extraction, T-tube drainage, and cholecystectomy without developing any intraoperative complications. The patient had a dark urine color with worsening jaundice postoperatively and did not respond well to plasma exchange and other symptomatic and supportive treatments. Since the progressive increase in postoperative bilirubin could not be clinically explained with any potential reason, including, if not at all, viral infection, cholangitis, autoimmune liver disease, and other causes, the patient underwent whole-exon screening for any genetic diseases, which surprisingly identified UGT1A1 and ABCB11 gene mutations related to glucuronidation of indirect bilirubin as well as bile acid transport in hepatocytes, respectively. Thus, we hypothesized that postoperative refractory cholestasis might result from UGT1A1 and ABCB11 gene mutations and further recommended liver transplantation to the patient, who eventually declined it and died from liver failure six months later.

CONCLUSION

Surgery may aggravate cholestasis in patients with multiple intrahepatic bile duct stones and cholestasis associated with UGT1A1 and ABCB11 gene mutations. A liver transplant may be the best option if active medical treatment fails.

The Pathological Mechanisms of Estrogen-Induced Cholestasis: Current Perspectives

Estrogens are steroid hormones with a wide range of biological activities. The excess of estrogens can lead to decreased bile flow, toxic bile acid (BA) accumulation, subsequently causing intrahepatic cholestasis. Estrogen-induced cholestasis (EIC) may have increased incidence during pregnancy, and within women taking oral contraception and postmenopausal hormone replacement therapy, and result in liver injury, preterm birth, meconium-stained amniotic fluid, and intrauterine fetal death in pregnant women. The main pathogenic mechanisms of EIC may include deregulation of BA synthetic or metabolic enzymes, and BA transporters. In addition, impaired cell membrane fluidity, inflammatory responses and change of hepatocyte tight junctions are also involved in the pathogenesis of EIC. In this article, we review the role of estrogens in intrahepatic cholestasis, and outlined the mechanisms of EIC, providing a greater understanding of this disease.

miR-199a-5p inhibits the Expression of ABCB11 in Obstructive Cholestasis

ATP-binding cassette, subfamily B member 11 (ABCB11) is an efflux transporter for bile acids on the liver canalicular membrane. The expression of this transporter is reduced in cholestasis; however, the mechanisms contributing to this reduction are unclear. In this study, we sought to determine whether miR-199a-5p contributes to the depletion of ABCB11/Abcb11 in cholestasis in mice. In a microRNA (miRNA) screen of mouse liver after common bile duct ligation (CBDL), we found that miR-199a-5p was significantly upregulated by approximately fourfold. In silico analysis predicted that miR-199a-5p would target the 3′-untranslated region (3′-UTR) of ABCB11/Abcb11 mRNA. The expression of ABCB11-3′-UTR luciferase construct in Huh-7 cells was markedly inhibited by cotransfection of a miRNA-199a-5p mimic, which was reversed by an miRNA-199a-5p mimic inhibitor. We also show treatment of mice after CBDL with the potent nuclear receptor FXR agonist obeticholic acid (OCA) significantly increased Abcb11 mRNA and protein and decreased miR-199a-5p expression. Computational mapping revealed a well-conserved FXR-binding site (FXRE) in the promoter of the gene encoding miR-199a-5, termed miR199a-2. Electromobility shift, chromatin immunoprecipitation, and miR199a-2 promoter-luciferase assays confirmed that this binding site was functional. Finally, CBDL in mice led to depletion of nuclear repressor NcoR1 binding at the miR199a-2 promoter, which facilitates transcription of miR199a-2. In CBDL mice treated with OCA, NcoR1 recruitment to the miR199a-2 FXRE was maintained at levels found in sham-operated mice. In conclusion, we demonstrate that miR-199a-5p is involved in regulating ABCB11/Abcb11 expression, is aberrantly upregulated in obstructive cholestasis, and is downregulated by the FXR agonist OCA.

Glycosyltransferase POMGNT1 deficiency strengthens N-cadherin-mediated cell-cell adhesion

Defects in protein O-mannosylation lead to severe congenital muscular dystrophies collectively known as α-dystroglycanopathy. A hallmark of these diseases is the loss of the O-mannose-bound matriglycan on α-dystroglycan, which reduces cell adhesion to the extracellular matrix. Mutations in protein O-mannose β1,2-N-acetylglucosaminyltransferase 1 (POMGNT1), which is crucial for the elongation of O-mannosyl glycans, have mainly been associated with muscle-eye-brain (MEB) disease. In addition to defects in cell-extracellular matrix adhesion, aberrant cell-cell adhesion has occasionally been observed in response to defects in POMGNT1. However, specific molecular consequences of POMGNT1 deficiency on cell-cell adhesion are largely unknown. We used POMGNT1 knockout HEK293T cells and fibroblasts from an MEB patient to gain deeper insight into the molecular changes in POMGNT1 deficiency. Biochemical and molecular biological techniques combined with proteomics, glycoproteomics, and glycomics revealed that a lack of POMGNT1 activity strengthens cell-cell adhesion. We demonstrate that the altered intrinsic adhesion properties are due to an increased abundance of N-cadherin (N-Cdh). In addition, site-specific changes in the N-glycan structures in the extracellular domain of N-Cdh were detected, which positively impact on homotypic interactions. Moreover, in POMGNT1-deficient cells, ERK1/2 and p38 signaling pathways are activated and transcriptional changes that are comparable with the epithelial-mesenchymal transition (EMT) are triggered, defining a possible molecular mechanism underlying the observed phenotype. Our study indicates that changes in cadherin-mediated cell-cell adhesion and other EMT-related processes may contribute to the complex clinical symptoms of MEB or α-dystroglycanopathy in general and suggests that the impact of changes in O-mannosylation on N-glycosylation has been underestimated.