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.

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 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).

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.

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.

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.

Molecular and cellular characteristics of ABCA3 mutations associated with diffuse parenchymal lung diseases in children

ABCA3 (ATP-binding cassette subfamily A, member 3) is expressed in the lamellar bodies of alveolar type II cells and is crucial to pulmonary surfactant storage and homeostasis. ABCA3 gene mutations have been associated with neonatal respiratory distress (NRD) and pediatric interstitial lung disease (ILD). The objective of this study was to look for ABCA3 gene mutations in patients with severe NRD and/or ILD. The 30 ABCA3 coding exons were screened in 47 patients with severe NRD and/or ILD. ABCA3 mutations were identified in 10 out of 47 patients, including 2 homozygous, 5 compound heterozygous and 3 heterozygous patients. SP-B and SP-C expression patterns varied across patients. Among patients with ABCA3 mutations, five died shortly after birth and five developed ILD (including one without NRD). Functional studies of p.D253H and p.T1173R mutations revealed that p.D253H and p.T1173R induced abnormal lamellar bodies. Additionally, p.T1173R increased IL-8 secretion in vitro. In conclusion, we identified new ABCA3 mutations in patients with life-threatening NRD and/or ILD. Two mutations associated with ILD acted via different pathophysiological mechanisms despite similar clinical phenotypes.

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

Progressive familial intrahepatic cholestasis type 3 (PFIC3) is a rare liver disease characterized by early onset of cholestasis that leads to cirrhosis and liver failure before adulthood. PFIC3 may be improved by chronic administration of ursodeoxycholic acid, although in many cases liver transplantation is the only therapy. The disease is caused by mutations of the adenosine triphosphate (ATP)-binding cassette, sub-family B, member 4 (ABCB4) [multidrug resistance 3 (MDR3)] gene encoding a specific hepatocellular canalicular transporter involved in biliary phosphatidylcholine secretion. Several mutations have been reported; however, the effect of individual mutations has not been investigated. ABCB4 is highly homologous to ATP-binding cassette, sub-family B, member 1 (ABCB1) (MDR1), the multidrug transporter responsible for drug resistance of cancer cells. We have studied the effect of mutation I541F localized to the first nucleotide-binding domain, which is highly conserved between ABCB4 and ABCB1. Plasmids encoding the wild-type human ABCB4 or rat ABCB1-green fluorescing protein (GFP) construct, and corresponding I541F-mutants, were expressed in hepatocellular carcinoma, human (HepG2) and Madin-Darby canine kidney (MDCK) cells. Expression studies showed that ABCB4 was localized at the bile canalicular membrane in HepG2 cells and at the apical surface in MDCK cells, whereas the I541F mutant was intracellular. In MDCK cells, ABCB1-I541F also accumulated intracellularly in compartments, which were identified as the endoplasmic reticulum and cis-Golgi, and remained partially endoH-sensitive. After shifting cells to 27 degrees C, ABCB1-I541F was expressed at the apical cell surface in a mature and active form. Similarly, ABCB4 was significantly trafficked to the membrane of bile canaliculi in HepG2 cells.

CONCLUSION

Mutation I541F causes mislocalization of both ABCB4 and ABCB1. Intracellular retention of ABCB4-I541F can explain the disease in PFIC3 patients bearing this mutation. The observation that plasma membrane expression and activity can be rescued by low temperature opens perspectives to develop novel therapies for the treatment of PFIC3.

The secreted form of dengue virus nonstructural protein NS1 is endocytosed by hepatocytes and accumulates in late endosomes: implications for viral infectivity

The flavivirus nonstructural protein NS1 is expressed as three discrete species in infected mammalian cells: an intracellular, membrane-associated form essential for viral replication, a cell surface-associated form that may be involved in signal transduction, and a secreted form (sNS1), the biological properties of which remain elusive. To determine the distribution of the dengue virus (DEN) sNS1 protein in vivo, we have analyzed by immunohistological means the tissue tropism of purified DEN sNS1 injected intravenously into adult mice. The sNS1 protein was found predominantly associated with the liver, where hepatocytes appeared to represent a major target cell. We further showed that sNS1 could be efficiently endocytosed by human Huh7 and HepG2 hepatocytes in vitro. After its internalization, the protein was detected intracellularly for at least 48 h without being substantially degraded. Colocalization studies of sNS1 with markers of the endolysosomal compartments revealed that the protein was specifically targeted to lysobisphosphatidic acid-rich structures reminiscent of late endosomes, as confirmed by electron microscopy. Intracellular accumulation of sNS1 in Huh7 cells enhanced the fluid phase uptake of rhodamine-labeled dextran. Furthermore, preincubation of Huh7 cells with sNS1 increased dengue virus production after infection with the homologous strain of DEN-1 virus. Our results demonstrate that the accumulation of DEN sNS1 in the late endosomal compartment of hepatocytes potentializes subsequent dengue virus infection in vitro, raising the possibility that sNS1 may contribute to viral propagation in vivo.

Spike protein VP4 assembly with maturing rotavirus requires a postendoplasmic reticulum event in polarized caco-2 cells

Rotavirus assembly is a multistep process that requires the successive association of four major structural proteins in three concentric layers. It has been assumed until now that VP4, the most external viral protein that forms the spikes of mature virions, associates with double-layer particles within the endoplasmic reticulum (ER) in conjunction with VP7 and with the help of a nonstructural protein, NSP4. VP7 and NSP4 are two glycosylated proteins. However, we recently described a strong association of VP4 with raft-type membrane microdomains, a result that makes the ER a highly questionable site for the final assembly of rotavirus, since rafts are thought to be absent from this compartment. In this study, we used tunicamycin (TM), a drug known to block the first step of protein N glycosylation, as a tool to dissect rotavirus assembly. We show that, as expected, TM blocks viral protein glycosylation and also decreases virus infectivity. In the meantime, viral particles were blocked as enveloped particles in the ER. Interestingly, TM does not prevent the targeting of VP4 to the cell surface nor its association with raft membranes, whereas the infectivity associated with the raft fractions strongly decreased. VP4 does not colocalize with the ER marker protein disulfide-isomerase even when viral particles were blocked by TM in this compartment. These results strongly support a primary role for raft membranes in rotavirus final assembly and the fact that VP4 assembly with the rest of the particle is an extrareticular event.

Early ultrastructural injuries in the thyroid of the normal rat radioinduced by diagnostic and/or therapeutic amounts of iodine-131

After irradiation, two principal mechanisms of cytolytic cell death can be involved: apoptosis and necrosis. By using morphological criteria, cells undergoing apoptosis can be distinguished from cells dying by necrosis. In nuclear medicine 131I is used to ablate thyroid remnants or to treat well differentiated thyroid carcinoma. The aim of study was to describe the progressive morphological thyroid changes induced by a diagnostic and/or therapeutic amounts of 131I in the rat using electron microscopy, in an attempt to determine which is the cell death pathway and to analyse "stunned" thyroid tissue to elucidate this effect. Tissular and ultrastructural examinations show that damages induced by 131I irradiation of the normal thyroid gland are heterogeneous. Thyroid cells die by necrosis after this metabolic irradiation, and no signs of apoptosis were observed by electron microscopy. In the other hand, stunning effect did not seem to impair the effectiveness of 131I treatment.

Potentiation of Smad transactivation by Jun proteins during a combined treatment with epidermal growth factor and transforming growth factor-beta in rat hepatocytes. role of phosphatidylinositol 3-kinase-induced AP-1 activation

Cross-talk between Smad and mitogen-activated protein kinase pathways has been described recently, and evidence for Smad cooperation with AP-1 is emerging. Here we report that epidermal growth factor (EGF) potentializes transforming growth factor beta (TGF-beta)-induced Smad3 transactivation in rat hepatocytes, an effect abrogated by TAM-67, a dominant negative mutant of AP-1. Antisense transfection experiments indicated that c-Jun and JunB were involved in the synergistic effect, and endogenous c-Jun physically associated with Smad3 during a combined EGF/TGF-beta treatment. We next investigated which signaling pathway transduced by EGF was responsible for the Jun-induced synergism. Whereas inhibition of JNK had no effect, inhibition of the phosphatidylinositol-3' kinase (PI3-kinase) pathway by LY294002 or by expression of a dominant negative mutant of PI3-kinase reduced EGF/TGF-beta-induced Smad3 transcriptional activity. Transfection of an activated Ras with a mutation enabling the activation of the PI3-kinase pathway alone mimicked the EGF/TGF-beta potentiation of Smad3 transactivation, and TAM-67 abolished this effect, suggesting that the PI3-kinase pathway stimulates Smad3 via AP-1 stimulation. The EGF/TGF-beta-induced activation of Smad3 correlated with PI3-kinase and p38-dependent but not JNK-dependent phosphorylation of c-Jun. Since potentiation of a Smad-binding element-driven gene was also induced by EGF/TGF-beta treatment, this novel mechanism of Jun/Smad cooperation might be crucial for diversifying TGF-beta responses.

Opening of the mitochondrial permeability transition pore causes matrix expansion and outer membrane rupture in Fas-mediated hepatic apoptosis in mice

Although Fas stimulation has been reported to cause outer mitochondrial membrane rupture in Jurkat cells, the mechanism of this effect is debated, and it is not known if outer membrane rupture also occurs in hepatocyte mitochondria. We studied the in vivo effects of Fas stimulation on ultrastructural lesions and mitochondrial function in mice. Four hours after administration of an agonistic anti-Fas antibody (8 microg/animal), caspase activity increased 5.4-fold. Nuclear DNA showed internucleosomal fragmentation, whereas supercoiled mitochondrial DNA was replaced by circular and linear forms. Mitochondrial cytochrome c was partly released into the cytosol. Ultrastructurally, mitochondrial lesions were observed in both apoptotic hepatocytes (with nuclear chromatin condensation/fragmentation) and nonapoptotic hepatocytes (without nuclear changes). In nonapoptotic cells, outer mitochondrial membrane rupture allowed herniation of the inner membrane and matrix through the outer membrane gap. In apoptotic hepatocytes, the matrix became electron-lucent and no longer protruded through the outer membrane gap. Mitochondria clustered around the nucleus, whereas rough endoplasmic reticulum cisternae became peripheral. In liver mitochondria isolated after Fas stimulation, the membrane potential decreased, whereas basal respiration increased. Pretreatment with either z-VAD-fmk (an inhibitor of caspases) or cyclosporin A (a permeability transition inhibitor) totally or mostly prevented mitochondrial outer membrane rupture, membrane potential decrease, cytochrome c release, and apoptosis. In conclusion, in vivo Fas stimulation causes caspase activation, mitochondrial permeability transition (decreasing the membrane potential and increasing basal respiration), mitochondrial matrix expansion (as shown by matrix herniation), outer mitochondrial membrane rupture, and cytochrome c release.

Hepatocyte growth factor activates the AP-1 complex: a comparison between normal and transformed rat hepatocytes

BACKGROUND/AIMS

Stimulation of activator protein-1 (AP-1), a Fos/Jun complex, is a key event in the cell response to growth factors. We have investigated whether hepatocyte growth factor (HGF) induces differential AP-1 responses in normal and transformed rat hepatocytes, the 7777 cells.

METHODS

Primocultures of isolated hepatocytes or 7777 cells were stimulated with HGF. Gene expression was evaluated by ribonuclease protection assay and Western blot analysis. AP-1 DNA binding activity was measured by electrophoretic mobility shift assay. Identification of the proteins bound to the probes was made by supershift assays with specific antibodies. Cells were electroporated with plasmids containing an AP-1-dependent chloramphenicol acetyl transferase (CAT) gene, and CAT activity was measured 24 h after treatment with medium alone or HGF.

RESULTS

In both cell types, HGF triggered the same program of jun family mRNA activation, but distinct Fos/Jun proteins accumulated in the nucleus. HGF increased DNA-binding activity to the phorbol 12-O-tetradecanoate-13-acetate responsive element (TRE) in both cell types, but distinct TRE-binding proteins were recruited in the AP-1 dimers. HGF also increased consistently binding to a cAMP responsive element (CRE) in hepatocytes only. Finally, HGF triggered TRE- and CRE-dependent gene activations in hepatocytes but TRE-dependent gene activation alone in 7777 cells.

CONCLUSIONS

HGF-induced AP-1 activation leads to the formation of distinct dimers with different functional capacities in normal and transformed hepatocytes. These data suggest the importance of qualitative abnormalities of the AP-1 complex for the establishment or maintainance of a transformed phenotype.

Identification of B10, an alkaline phosphodiesterase of the apical plasma membrane of hepatocytes and biliary cells, in rat serum: increased levels following bile duct ligation and during the development of cholangiocarcinoma

Alkaline phosphodiesterase (APDE) is associated with the cellular plasma membrane of many organs. Several isoforms are also detected in normal human serum and their respective amounts vary in liver diseases but their significance is unknown. The aims of this study were: 1) to identify a serum form of B10, an APDE exclusively localized at the apical pole of the plasma membrane of rat hepatocytes and biliary cells; 2) to gain insight into its origin; and 3) to investigate its behavior, in two liver diseases in which an abnormal membrane expression of B10 has been reported, namely cholestasis and cholangiocarcinoma. A soluble form of B10 was immunoprecipitated from normal rat serum, which amounted to 13% of total serum APDE activity. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the size of the serum enzyme was 125 kd, which is slightly lower than that found in the plasma membrane (130 kd). In bile, a 120-kd and a 130-kd form was found. A sixfold and fivefold increase of B10 APDE activity was observed in the serum of bile duct-ligated rats and in the Long-Evans Cinnamon (LEC) rats which spontaneously develop cholangiocarcinoma. The molecular size of the form present in serum was unchanged. A threefold increase was also observed in LEC rats which had not yet developed a cholangiocarcinoma. In conclusion, we identified a soluble form of B10 in normal rat serum. The increase in serum B10 in the experimental and pathological conditions investigated does not seem to result from passage of the biliary form to the serum but seems to be caused by increased cleavage of the membrane form. Its rise early during the onset of cholangiocarcinoma suggests that B10 in the serum might be a marker of carcinogenesis and/or be involved in the development of cholangiocarcinoma.

Presence of distinct AP-1 dimers in normal and transformed rat hepatocytes under basal conditions and after epidermal growth factor stimulation

Activation of the transcriptional regulator AP-1, a dimeric complex formed of various combinations of Fos and Jun proteins, is a key step in the cellular response to mitogens. Because different dimers are believed to display different regulatory functions, we hypothesized that transformed cells that lack normal growth constraints might display AP-1 dimers that are different from those of normal cells. We therefore compared in primary and transformed rat hepatocytes (1) the composition of AP-1 dimers under basal conditions and (2) AP-1 induction by epidermal growth factor (EGF). Under basal conditions, AP-1 contained predominantly Jun homodimers in both cell types. However, whereas normal hepatocytes contained only JunD, both JunD and JunB were present in the AP-1 complex of 7777 cells. EGF treatment triggered almost identical programs of fos and jun gene activation at the messenger RNA (mRNA) level in both cell types, with an early accumulation of c-fos, c-jun, and junB mRNAs, but no change in junD mRNA levels. In both cell types, c-Fos and Fra-1 proteins increased after EGF treatment, but differences in the induction of Jun proteins were noted, with an increase of c-Jun in hepatocytes and an increase of JunB in 7777 cells. In both cell types, activation of AP-1 DNA binding activity by EGF was accompanied by the recruitment of Fra-1 into AP-1, detected earlier in 7777 cells than in hepatocytes, and with the transient appearance of c-Fos in 7777 cells only. Finally, EGF activated AP-1-dependent transcription in 7777 cells but not in hepatocytes. These data indicate important differences in the functional activity of AP-1 in transformed hepatocytes.

The transcytotic pathway of an apical plasma membrane protein (B10) in hepatocytes is similar to that of IgA and occurs via a tubular pericentriolar compartment

In hepatocytes, newly synthesized apical plasma membrane proteins are first delivered to the basolateral surface and are supposed to reach the apical surface by transcytosis. The transcytotic pathway of apical membrane proteins and its relationship with other endosomal pathways has not been demonstrated morphologically. We compared the intracellular route of an apical plasma membrane protein, B10, with that of polymeric IgA (pIgA), which is transcytosed, transferrin (Tf) which is recycled, and asialoorosomucoid (ASOR) which is delivered to lysosomes. Ligands and anti-B10 monoclonal IgG were linked to fluorochromes or with peroxidase. The fate of each ligand was followed by confocal and electron microscopy in polarized primary monolayers of rat hepatocytes. When fluorescent anti-B10 IgG and fluorescent pIgA were simultaneously endocytosed for 15–30 minutes, they both uniformly labelled a juxtanuclear compartment. By 30–60 minutes, they reached the bile canaliculi. Tf and ASOR were also routed to the juxtanuclear area, but their fluorescence patterns were more punctate. Microtubule disruption prevented all ligands from reaching the juxtanuclear area. This area corresponded, at least partially, to the localization of the mannose 6-phosphate receptor, an endosomal marker. By electron microscopy, the juxtanuclear compartment was made up of anastomosing tubules connected to vacuoles, and was organized around the centrioles. B10 and pIgA were mainly found in the tubules, whereas ASOR was segregated inside the vacuolar elements and Tf within thinner, recycling tubules. In conclusion, transcytosis of the apical membrane protein B10 occurs inside tubules similar to those carrying pIgA, and involves passage via the pericentriolar area. In the pericentriolar area, the transcytotic tubules appear to maintain connections with other endosomal elements where sorting between recycled and degraded ligands occurs.

Uncoupling of rat and human mitochondria: a possible explanation for tacrine-induced liver dysfunction

BACKGROUND & AIMS

Tacrine administration (1-3 mg/kg) may lead to sinusoidal concentrations in the micromolar range and produce liver dysfunction in 50% of recipients. The aim of this study was to determine the cellular effects of tacrine that account for liver dysfunction.

METHODS

The effects of tacrine on mitochondrial function were determined in isolated rat liver mitochondria, cultured rat hepatocytes, and isolated human lymphocytes.

RESULTS

In vitro, tacrine was taken up by rat liver mitochondria, decreased their membrane potential, and stimulated their respiration. Ex vivo, respiration was increased in rat mitochondria isolated 30 minutes after the administration of 2 mg of tacrine per kilogram. After 7 days of culture, tacrine (2.5 mumol/L) decreased rat hepatocyte adenosine triphosphate levels. Ten micromolar decreased 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium reduction and neutral red uptake without modifying cell glutathione, the morphology of the endoplasmic reticulum, or protein synthesis. Tacrine (1.25 mumol/L) decreased levels of adenosine triphosphate in human lymphocytes.

CONCLUSIONS

The weak base tacrine exerts a protonophoric effect in mitochondria that wastes energy and decreases levels of adenosine triphosphate in rat and human cells. These effects are visible after clinically relevant doses of tacrine and might be involved in tacrine-induced liver dysfunction.

Cytochrome P4502B follows a vesicular route to the plasma membrane in cultured rat hepatocytes

BACKGROUND/AIMS

Autoantibodies against cytochrome P450 are found in some forms of autoimmune hepatitis. Cytochrome P450 is synthesized and mainly located in the endoplasmic reticulum but may also be expressed on the plasma membrane of hepatocytes. Vesicles migrate from the endoplasmic reticulum to the Golgi apparatus and then to the plasma membrane along microtubules. We determined the route followed by cytochrome P4502B to reach the plasma membrane.

METHODS

Rat hepatocytes were cultured for 2 hours after plating with various inhibitors of cellular trafficking. Detached, uncut, nonpermeabilized hepatocytes were then exposed to a monoclonal antibody specific for cytochrome P4502B and studied by flow cytometry and confocal microscopy.

RESULTS

The plasma membrane expression of cytochrome P4502B was markedly decreased after 2 hours of culture with cycloheximide (an inhibitor of protein synthesis), caffeine at 20 degrees C (conditions that decrease vesicular transport from the endoplasmic reticulum to the Golgi apparatus), brefeldin A (which redistributes Golgi components back to the endoplasmic reticulum), monensin (an inhibitor of Golgi functions), and colchicine, vinblastine, or nocodazole (three microtubule inhibitors).

CONCLUSIONS

Part of cytochrome P4502B follows a microtubule-dependent vesicular route from the endoplasmic reticulum to the plasma membrane in cultured rat hepatocytes.

Behaviour of the small GTP-binding protein rab6 in the liver of normal rats and rats presenting an acute inflammatory reaction

While it is known that the small GTP-binding protein rab6 is localized in vitro to the Golgi apparatus of several mammalian cells, its behaviour in vivo has not yet been investigated. The aim of this work was to compare by immunocytochemistry and immunoblotting the distribution of rab6 in hepatocytes from normal rats and from rats with an acute inflammatory reaction, a circumstance where the synthesis and secretion of plasma proteins by the hepatocytes is increased and which is accompanied by several changes in the Golgi apparatus. Our results show that in normal rats, rab6 was present in all hepatocytes irrespective of the location of the cell in the hepatic lobule. At the ultrastructural level, rab6 was mainly visible on the three Golgi saccules, but in some cells it appeared to be absent in saccules corresponding to the cis or the trans saccule. The inflammatory reaction was accompanied by an increase of the immunocytochemical labelling at the light and electron microscopy levels. However, by immunoblotting, no differences in the total amount of rab6, nor in its subcellular distribution were found in liver cells after acute inflammatory reaction. These results demonstrate that rab6 is restricted in vivo to the Golgi apparatus and that no significant redistribution occurs during an acute inflammatory reaction.

Effects of phalloidin and colchicine on diethylmaleate-induced choleresis and ultrastructural appearance of rat hepatocytes

Diethylmaleate is used as a model compound whose glutathione conjugates are secreted into bile, and which induce choleresis and the formation of Golgi-derived vesicles in hepatocytes. This study was performed to test the assumption that these vesicles are involved in the bile canalicular secretion of diethylmaleate. We reasoned that phalloidin and colchicine, two drugs acting on microfilaments and microtubules, respectively, can modify the movements of diethylmaleate-induced vesicles towards the bile canaliculus. Phalloidin induced the formation of a thick microfilamentous network around the bile canalicular plasma membrane domain. A significant decrease in diethylmaleate-stimulated choleresis was observed, associated with a striking accumulation of pericanalicular vesicles, which were confirmed by morphometric analysis. In contrast, in rats pretreated with colchicine, after diethylmaleate administration, only a few vesicles were observed around the bile canaliculus, while diethylmaleate-induced choleresis also decreased. These results suggest that: a) the thick microfilament network induced by phalloidin prevents diethylmaleate-associated vesicles reaching the bile canalicular plasma membrane; and b) colchicine produces a dispersion of these vesicles in the cytoplasm of hepatocytes by inhibiting the polymerization of microtubules. These observations support a role of vesicles in the transport of diethylmaleate by hepatocyte into bile, and are consistent with the existence of a vesicular pathway for the biliary secretion of diethylmaleate and possibly other organic anions.

Decreased mitochondrial oxidation of fatty acids in pregnant mice: possible relevance to development of acute fatty liver of pregnancy

Severe impairment of the beta-oxidation of fatty acids, as a consequence of a single factor or a combination of different causes, leads to microvesicular steatosis of the liver. In an effort to understand the mechanism(s) leading to the development of acute fatty liver of pregnancy in some women, we determined the effects of pregnancy on the mitochondrial oxidation of fatty acids in mice. In vivo, the rate of oxidation of the whole fatty-acid chain length was determined by measuring the rate of exhalation of [14C]CO2 after intragastric administration of a tracer dose of [U-14C]palmitic acid. [14C]CO2 exhalation was not significantly decreased at 14 days of gestation, but it had declined by 40% at 18 days of gestation (i.e., 24 to 48 hr before delivery). The rate of first beta-oxidation cycle was assessed by measuring the rate of [14C]CO2 exhalation after administration of [1-14C]octanoic acid, [1-14C]butyric acid or [1-14C]palmitic acid. [14C]CO2 exhalation had declined by 60%, 46%, and 24% after administration of [1-14C]octanoic acid, [1-14C]butyric acid and [1-14C]palmitic acid, respectively, in 18-day-pregnant mice. Total hepatic lipids and triglycerides, expressed per gram of liver, remained unchanged in 18-day-pregnant mice. In vitro, the rate of mitochondrial beta-oxidation (expressed per milligram of protein) had decreased by 47% at 18 days' gestation with [U-14C]palmitic acid as substrate and by 33% with [1-14C]octanoic acid but remained unchanged with [1-14C]palmitic acid. The activity of the tricarboxylic acid cycle, assessed by the formation of [14C]CO2 from [1-14C]acetic acid, had decreased by 24%. We conclude that the mitochondrial oxidation of fatty acids decreased during late-term pregnancy in mice as a consequence of both decreased mitochondrial beta-oxidation of medium-chain fatty acids, and decreased activity of the tricarboxylic acid cycle. We suggest that this effect, in combination with other factors, may contribute to the development of fatty liver of pregnancy in some pregnant women.

Drug-induced prolonged cholestasis in adults: a histological semiquantitative study demonstrating progressive ductopenia

Drug-induced acute hepatitis may be followed by prolonged cholestasis despite the withdrawal of the drug. Eight patients suffering from prolonged cholestasis caused by several drugs were investigated with sequential liver biopsies. At the early stage, lesions of acute cholangitis were observed in most patients; at the chronic phase, ductopenia, defined by the absence of interlobular bile ducts in at least 50% of small portal tracts, was demonstrated in all patients. Ductopenia might be the consequence of acute cholangitis; the degree of ductopenia and the chronicity of the disease might be directly related to the severity of the early acute damage of bile ducts. Consequently, in patients with severe cholestasis related to drugs, research of early morphological signs of acute cholangitis and then of ductopenia seems to be important.

Microtubule disruption interferes with the structural and functional integrity of the apical pole in primary cultures of rat hepatocytes

The effect of microtubule disruption on the development and maintenance of cell polarity was studied in rat hepatocytes cultured as primary monolayers in the presence of colchicine or nocodazole. Addition of colchicine immediately after plating did not inhibit the generation of bile canaliculi (the apical pole) after 1 day of culture, as judged by electron microscopic examination, and did not allow penetration of Ruthenium Red through the tight junctions. However, the bile canaliculi developed in the presence of colchicine or nocodazole were not fully normal since they were not able to concentrate fluorescein diacetate in their lumina, and did not enrich with proteins of the apical plasma membrane domain, as control cells did. When the drugs were added after 1 or 2 days of culture, the new bile canaliculi appeared to be unaffected when examined by electron microscopy, but many of them did not concentrate fluorescein and were not enriched with apical membrane proteins within 4 to 24 h after drug addition. Whenever the drugs were added, the proteins that would normally concentrate on the membrane of the bile canaliculi accumulated intracellularly in endocytic vesicles after 2 to 4 h of drug treatment, and in vacuoles resembling lysosomes when the drugs were maintained for 24 h or more. These results show that microtubule disruption does not inhibit the structural reconstitution of bile canaliculi, but impairs their normal function and the transport of proteins of the apical plasma membrane domain.

Inhibition by colchicine of biliary secretion of diethylmaleate in the rat: evidence for microtubule-dependent vesicular transport

It has been proposed that a microtubule-dependent transport of vesicles derived from the Golgi apparatus may play a role in biliary secretion of bile salts and other cholephilic anions. To test this hypothesis, we examined the influence of colchicine and vinblastine, two microtubule inhibitors, on diethylmaleate-induced bile flow and on the biliary secretion of diethylmaleate, an organic anion whose glutathione conjugates may be secreted into bile through the Golgi apparatus and Golgi-derived vesicles. Rats were pretreated with colchicine or vinblastine, and diethylmaleate was injected intraperitoneally at doses of 28 to 400 mumol/100 gm body wt. Basal bile flow was unaffected by colchicine or vinblastine. In contrast, diethylmaleate-induced bile flow and the secretion into bile of diethylmaleate conjugates (estimated by the cation-anion gap in bile) were significantly lower in colchicine-treated and vinblastine-treated animals than in controls. Diethylmaleate-induced bile flow was reduced in proportion to diethylmaleate conjugate secretion. A linear relationship was seen between bile flow and biliary output of diethylmaleate conjugates: this relationship was similar in colchicine-treated or vinblastine-treated animals and in controls. At electron microscopy, diethylmaleate had induced distension of the Golgi saccules of the hepatocytes. In conclusion, colchicine and vinblastine inhibited the secretion into bile of diethylmaleate conjugates and diethylmaleate-induced bile flow. These results support the view that microtubule-dependent transport of Golgi-derived vesicles is involved in the biliary secretion of diethylmaleate and, perhaps, other cholephilic organic anions.

Peliosis hepatis and sinusoidal dilation during infection by the human immunodeficiency virus (HIV). An ultrastructural study

The description of hepatic sinusoidal lesions in a significant number of acquired immunodeficiency syndrome (AIDS) patients prompted the authors to undertake an ultrastructural study of the sinusoidal barrier abnormalities during human immunodeficiency virus (HIV) infection, in order to compare these lesions with those described in other conditions and to discuss their possible origin. In a series of 29 patients with serologic evidence of HIV infection and liver abnormalities, 8 (28%) had sinusoidal lesions. Peliosis hepatis was present in 2 cases, and sinusoidal dilatation in 6. These patients were classified as follows: 3 AIDS, 4 AIDS-related complex, 1 unclassifiable. Ultrastructural lesions of the sinusoidal barrier were observed in all the cases. They closely mimicked the changes previously reported in peliotic and peliotic-like changes of various origins. A striking particularity was, however, the presence of numerous and hyperplastic sinusoidal macrophages. This work suggests that an injury of the endothelial cells, directly or indirectly related to the presence of HIV, may be incriminated in the pathogenesis of sinusoidal lesions during HIV infection.

Effect of colchicine and phalloidin on the distribution of three plasma membrane antigens in rat hepatocytes: comparison with bile duct ligation

The hepatocyte plasma membrane presents a morphological and functional regionalization into three domains: the sinusoidal; the lateral, and the canalicular. The mechanisms responsible for the biogenesis and maintenance of this regionalization are poorly understood. In this work, we have used colchicine and phalloidin, two drugs known to interfere with the secretory processes in hepatocytes, to study whether they also affect the transport of membrane proteins. The localization of three plasma membrane antigens was studied by light and electron microscopy using monoclonal antibodies identifying either the sinusoidal (A39) or the lateral (B1) or the canalicular (B10) domains in normal hepatocytes. In rats injected with colchicine (0.25 mg per 100 gm), A39 moved from the sinusoidal membrane to the lateral and canalicular ones, whereas B10 was displaced from the canalicular to the sinusoidal and lateral membranes, resulting after 8 hr in an almost equal labeling of the three domains with both antibodies. In rats injected daily for 7 days with phalloidin (50 micrograms per 100 gm), A 39 became mainly localized on the bile canalicular membrane instead of the sinusoidal one; B10 predominated on the canalicular membrane as in controls but in places it labeled the sinusoidal and lateral domains as well. In bile duct-ligated rats studied for comparison for 4, 10 or 21 days, A39 and B10 localizations evolved as after phalloidin, but the changes were more marked. B1 was not affected by any of the treatments. In conclusion, colchicine, phalloidin and bile duct ligation do not seem to hinder the antigens in reaching the plasma membrane, but induce a redistribution of two of them, suggesting a disturbance in the biogenesis and/or control of the plasma membrane regionalization. Such an abnormal distribution could be involved in--or contribute to--the initiation of cholestasis.

The hepatic sinusoid in hairy cell leukemia: an ultrastructural study of 12 cases

Ultrastructural lesions of the liver were studied in 12 cases of hairy cell leukemia, with the alterations of the sinusoidal barrier receiving special emphasis. Portal and sinusoidal tumoral infiltration was observed in all cases. It was associated with angiomatous lesions of the sinusoids in eight cases; these lesions consisted of randomly distributed cavities lined by hairy cells and containing hairy cells and erythrocytes. In addition to the attachment of hairy cells to the sinusoidal wall, other striking electron microscopic abnormalities of the sinusoids included 1) wide areas of communication between the sinusoidal lumen and Disse's space, allowing extravasation of blood cells; 2) focal disruption of the sinusoidal wall; and 3) replacement of the sinusoidal cell lining by tumor cells in close contact with hepatocytes. Most of these changes closely resembled those observed in peliosis hepatis. As in peliosis, sinusoidal alterations in hairy cell leukemia might be due to the destruction of the sinusoidal wall, and tumor cells could play a role in the pathogenesis of the lesions. The pattern of liver involvement in hairy cell leukemia, which is peculiar among hepatic localizations of blood malignancies, might reflect the unique phenotype of the tumor cells.

Serial transmission of a human non A-non B hepatitis viral strain to HBV-protected chimpanzees: successive histological and ultrastructural studies

A NANB agent of human origin was inoculated in HBV-immunized chimpanzees. Infection was proven in two animals and serially passed to two others. The absence of anti-HBc in serum and the absence of HBsAg and HbcAg in liver are arguments against the HBV nature of the transmitted infection. Moreover, the reproducible appearance of the NANBcAg/Ab system at each passage from man to chimpanzee and from chimpanzee to chimpanzee, a response not elicited in control animals, suggests that this reaction may be a specific immunologic marker for the strain. NANB infection was transmitted in all chimpanzees. Distinctive hepatic morphologic features were obtained in the liver biopsies of the human donor and the inoculated chimpanzees: eosinophilic alterations of hepatocytes and numerous inflammatory cells. Inflammation was more prominent than necrosis, appearing earlier and lasting longer, but was not topographically close to the eosinophilic changes. On electron microscopy, particles characteristic of NANB agent were observed in the cytoplasm of the hepatocytes. No particles were demonstrated in the nucleus of these cells.

Characterization of rat hepatocyte plasma membrane domains by monoclonal antibodies

The hepatocyte plasma membrane consists of three morphologically and functionally distinct domains, the sinusoidal, the lateral and the canalicular. To study the distribution of antigenic determinants among these domains, we prepared monoclonal antibodies by immunizing mice with a crude, plasma membrane-enriched liver fraction. Four monoclonal antibodies were obtained that recognized various parts of the rat hepatocyte plasma membrane when tested by indirect immunofluorescence and immunoperoxidase assay performed on formaldehyde-fixed liver tissue. Each antibody gave a different staining pattern when analyzed by light and electron microscopy. A59 exclusively labelled the part of the sinusoidal membrane facing the sinusoids. A39 mainly labelled the sinusoidal membrane. B1 mainly labelled the lateral membrane, while the labelling by B10 was almost completely limited to the canalicular membrane. Immunoblotting showed that the antibody B1 recognized an antigen of approximately 100 kilodaltons and that B10 recognized an antigen of approximately 125 to 130 kilodaltons. These antibodies allow us to distinguish the three domains of the hepatocyte plasma membrane.