Functional specialization of stable and dynamic microtubules in protein traffic in WIF-B cells

We found that the magnesium salt of ilimaquinone, named 201-F, specifically disassembled dynamically unstable microtubules in fibroblasts and various epithelial cell lines. Unlike classical tubulin- interacting drugs such as nocodazole or colchicine which affect all classes of microtubules, 201-F did not depolymerize stable microtubules. In WIF-B-polarized hepatic cells, 201-F disrupted the Golgi complex and inhibited albumin and alpha1-antitrypsin secretion to the same extent as nocodazole. By contrast, 201-F did not impair the transport of membrane proteins to the basolateral surface, which was only affected by the total disassembly of cellular microtubules. Transcytosis of two apical membrane proteins-the alkaline phosphodiesterase B10 and dipeptidyl peptidase IV-was affected to the same extent by 201-F and nocodazole. Taken together, these results indicate that only dynamically unstable microtubules are involved in the transport of secretory proteins to the plasma membrane, and in the transcytosis of membrane proteins to the apical surface. By contrast, stable microtubules, which are not functionally affected by 201-F treatment, are involved in the transport of membrane proteins to the basolateral surface. By specifically disassembling highly dynamic microtubules, 201-F is an invaluable tool with which to study the functional specialization of stable and dynamic microtubules in living cells.

Inverse targeting of retroviral vectors: selective gene transfer in a mixed population of hematopoietic and nonhematopoietic cells

We previously reported that retroviral vectors displaying epidermal growth factor (EGF) as part of a chimeric envelope glycoprotein are sequestered upon binding to EGF receptor (EGFR)-positive target cells, leading to loss of infectivity. In the current study, we have displayed stem cell factor (SCF) on beta-galactosidase-transducing ecotropic and amphotropic retroviral vector particles as a factor Xa protease-cleavable N-terminal extension of the envelope glycoprotein. Viral incorporation of the SCF chimeric envelopes was demonstrated by immunoblotting of pelleted virions and their specific attachment to Kit receptors was demonstrated by flow cytometry. Gene transfer studies showed that when SCF was displayed on an amphotropic envelope, the infectivity of the SCF-displaying vectors was selectively inhibited on Kit-expressing cells, but could be restored by adding soluble SCF to block the Kit receptors or by cleaving the displayed SCF domain from the vector particles with factor Xa protease. The host range properties of EGF-displaying and SCF-displaying vectors were then compared in cell mixing experiments. When EGFR-positive cancer cells and Kit-positive hematopoietic cells were mixed and exposed to the different engineered vector particles, the cancer cells were selectively transduced by the SCF-displaying vector and the hematopoietic cells were selectively transduced by the EGF-displaying vector. Retroviral display of polypeptide growth factors can therefore provide the basis for a novel inverse targeting strategy with potential use for selective transduction of hematopoietic or nonhematopoietic cells (eg, cancer cells) in a mixed cell population.

In vivo stimulation of polymeric Ig receptor transcytosis by circulating polymeric IgA in rat liver

Binding of human polymeric IgA ligand to its epithelial cell polymeric Ig receptor, pIgR, has been shown to stimulate pIgR apical transcytosis in an in vitro system, based on polarized confluent MDCK cells expressing rabbit pIgR. The present study aimed at testing whether such a stimulation also occurs in vivo. Transcytosis of pIgR was monitored by rat liver output of total secretory component (SC) into bile, measured by radial immunodiffusion as the sum of free SC and pIgA-bound SC. Whereas in the perfused rat liver system addition of pIgA to the perfusate showed no effect, i.v. injection of human and rat pIgA, but not of monomeric IgA nor PBS, in living rats significantly increased total bile SC output for more than 1 h. Furthermore, depletion of the normal pIgA level circulating in the liver before injecting more pIgA was not required to show the stimulation. Our data thus strongly suggest that stimulation of liver pIgR transcytosis by pIgA ligand binding is physiologically relevant, helping to quickly adjust pIgA transport into bile to increase circulating pIgA levels, without need for increased SC/pIgR synthesis.

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.

Ecto-phosphodiesterase/pyrophosphatase of lymphocytes and non-lymphoid cells: structure and function of the PC-1 family

Many developmentally regulated membrane proteins of lymphocytes are ecto-enzymes, with their active sites on the external surface of the cell. These enzymes commonly have peptidase, phosphodiesterase or nucleotidase activity. Their biological roles are just beginning to be discovered. Although their expression is usually associated with particular stages of lymphoid differentiation, the same gene products are often expressed on the surface of certain non-lymphoid cell types outside the immune system, indicating that their functions cannot be unique to lymphocytes, nor can they be ubiquitous. The plasma cell membrane protein PC-1 (phosphodiesterase I; EC 3.1.4.1/nucleotide pyrophosphatase; EC 3.6.1.9), which was one of the first serological markers for lymphocyte subsets to be discovered, is a typical example. Within the immune system, PC-1 is confined to plasma cells, which represent about 0.1% of lymphocytes. However, PC-1 is also expressed on cells of the distal convoluted tubule of the kidney, chondrocytes, osteoblasts, epididymis and hepatocytes. Recent work has shown that PC-1 is a member of a multigene family of ecto-phosphodiesterases that currently has two other members, PD-1 alpha (autotaxin) and PD-1 beta (B10). Within this family, the extracellular domains are highly conserved, especially around the active site. In contrast, the transmembrane and cytoplasmic domains are highly divergent. Individual members of the eco-phosphodiesterase family have distinct patterns of distribution in different cell types, and even within the same cell. For example, PC-1 is present only on the basolateral surface of hepatocytes, while B10 (PD-1 beta) is confined to the apical surface. Analysis of conservation and differences in the sequence of their cytoplasmic tails may illuminate intracellular targetting signals. Ecto-phosphodiesterases may play a part in diverse activities in different tissues, including recycling of nucleotides. They may also regulate the concentration of pharmacologically active extracellular compounds such as adenosine or its derivatives and cell motility. Some members may modulate local concentrations of pyrophosphate, and hence influence calcification in bone and cartilage.

Rotavirus is released from the apical surface of cultured human intestinal cells through nonconventional vesicular transport that bypasses the Golgi apparatus

Rotaviruses are nonenveloped viruses that infect enterocytes of the small intestine and cause severe infantile gastroenteritis. It was previously thought that rotavirus exits cells by lysis, but this behavior does not match the local pathogenesis of the virus. In this study, we have investigated the release of the simian rotavirus strain (RRV) from the polarized intestinal Caco-2 cells. We found that RRV is released almost exclusively from the apical pole of Caco-2 cells before any cells lyse. Using confocal laser scanning microscopy and drugs that inhibit vesicular transport, we studied the RRV transport route from the endoplasmic reticulum (ER) to the apical side of intestinal cells. We demonstrated that RRV exits from the ER through a carbonyl cyanide m-chlorophenylhydrazone-sensitive vesicular transport. RRV staining was never found within the Golgi apparatus or lysosomes, suggesting that the RRV intracellular pathway does not involve these organelles. This finding was confirmed by treatment with monensin or NH4Cl, which do not affect release of RRV. Electron microscopic analysis revealed RRV containing small smooth vesicles in the apical area and free virions outside the cell in the brush border, consistent with a vesicular vectorial transport of virus. These results may provide, for the first time, a cellular explanation of the pathogenesis of rotavirus.

Biochemical and molecular identification of distinct forms of alkaline phosphodiesterase I expressed on the apical and basolateral plasma membrane surfaces of rat hepatocytes

We have identified B10, a plasma membrane protein previously defined by a monoclonal antibody, as an alkaline phosphodiesterase I (APDE) expressed in the plasma membrane of rat hepatocytes and enterocytes, with a restricted apical distribution. B10 complementary DNA (cDNA) was cloned from a rat intestinal library screened with a polyclonal antibody directed to the hepatic protein. Two distinct B10 clones with an open reading frame of 2,625 bp were obtained that differed only by 12 bases in the coding region. One B10 clone had a single base difference with gp130RB13-6 cDNA, which was recently cloned in rat fetal brain. B10/gp130RB13-6 had 50% identity at the amino acid level with the plasma cell antigen PC-1, an APDE cloned in the mouse and in human. Anti-B10 antibodies immunoprecipitated 34% of the APDE activity in liver plasma membranes and over 95% of the APDE activity in intestinal cells. Most of the remaining activity in hepatocytes (44%) could be immunoprecipitated by antibodies directed to PC-1. APDE activity immunoprecipitated with anti-B10 antibodies was found in the apical rat liver plasma membrane fractions on a sucrose gradient whereas most of the remaining APDE activity was associated with the basolateral fractions, which contained PC-1. By immunofluorescence, B10 was localized to the apical surfaces of hepatocytes and enterocytes whereas PC-1 was present on the basolateral surfaces of hepatocytes. B10/gp130RB13-6 and rat PC-1 are a unique example of distinct molecules having similar enzymatic activity but different apical/basolateral location, and possibly different functions.

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.

Traveller's loiasis in Zimbabwe: a case report

A case of loiasis diagnosed at Wankie Colliery Hospital is presented. The disease was suspected by the history of the patient and the presenting clinical signs, and it was confirmed by identification of Microfilariae loa loa in peripheral blood. The patient was successfully treated with a course of diethylcarbamazine. The paper gives a brief account of the clinical aspects of loiasis and emphasizes the importance of the laboratory methods to differentiate microfilariae. The case is discussed against the background of important diseases in Zimbabwe.

Immortalization of rat hepatocytes by fusion with hepatoma cells. I. Cloning of a hepatocytoma cell line with bile canaliculi

Hepatocytoma (HPCT) hybrid cells were obtained by fusion of cultured rat hepatocytes with Fao Reuber hepatoma cells H35 by polyethylene glycol treatment. Surviving cells were cloned in HAT (hypoxanthine-aminopterine-thymidine)/ouabain medium and propagated in cell lines over 80 passages. Morphological criteria were chosen to allow differentiation of the clones into two types of cells: 1) Type I cells which formed irregular cell layers, lacked contact inhibition and resembled the parental Fao hepatoma cells and 2) type II cells, which proliferated in monolayer cultures, exhibited contact inhibition during growth in culture plates and formed bile canaliculi thereby resembling cultured hepatocytes by phenotype. Bile canaliculi were absent in type I clones and Fao cells. One particular type II clone 1E3 was studied in detail. These cells formed bile canaliculi sealed by tight junctions and were comparably polarized as cultured hepatocytes. They expressed canalicular antigen B10, canalicular aminopeptidase N, and even secreted the fluorescent bile acid derivative NBD-cholate into the canalicular lumen. This type of HPCT cells lacked malignancy by tests in vivo and in vitro, and contained 110 +/- 5 chromosomes. The cells were considered to represent an immortalized hepatocyte-like cell line.

Biosynthesis and intracellular transport of a bile canalicular plasma membrane protein: studies in vivo and in the perfused rat liver

B10 is an integral glycoprotein of the plasma membrane that is exclusively localized to the canalicular (apical) domain in normal rat hepatocytes but may be expressed on the basolateral (sinusoidal and lateral) membrane in pathophysiological situations. To understand how B10 may be localized to the basolateral surface, we studied the biosynthesis and transport of this apical protein. In vivo pulse-chase experiments, followed by subcellular fractionation of the liver and immunoprecipitation, showed that B10 is first synthesized as a high-mannose form of 123 kD and then matured to a complex glycosylated form of 130 kD, which peaks in the Golgi apparatus after 15 min of chase and reaches the plasma membrane with a half-time of 30 to 45 min. Analysis of the protein in plasma membrane domain fractions showed that most of the newly synthesized molecule was localized in basolateral fractions after 30 min of chase and subsequently appeared in apical fractions. After 90 min of chase, most of the radiolabeled protein had reached its steady-state apical distribution. The same experiments performed in the perfused rat liver, in which the chase can be improved, gave similar results, except that the apical distribution of the radioactive molecule was attained more quickly. Thus B10, like all apical plasma membrane proteins studied so far in hepatocytes, is first transported to the basolateral surface and then reaches the membrane of the bile canaliculi. Alterations of the transcytotic step from the basolateral to the apical surfaces may result in abnormal basolateral localization.

Effect of tauroursodeoxycholate on actin filament alteration induced by cholestatic agents. A study in isolated rat hepatocyte couplets

The mechanism of the protective effect of ursodeoxycholic acid in cholestatic liver diseases remains unclear. Since there is evidence that alterations in the pericanalicular actin microfilament network play a major role in cholestasis, the aims of this study were (a) to determine the effect of the cholestatic agents, taurolithocholate (TLC) and erythromycin estolate (ERY), on F-actin distribution in isolated rat hepatocyte couplets and (b) to assess the effect of tauroursodeoxycholate (TUDC) and taurocholate on the modifications induced by these two compounds. F-actin was stained with fluorescein-isothiocyanate phalloidin and fluorimetric measurements were performed using a scanning laser cytometer ACAS 570. F-actin distribution was assessed in the couplets by the ratio of the pericanalicular area fluorescence/total couplet fluorescence (CF/TF). At non-cytotoxic concentrations, TLC (50, 100 microM) and ERY (10, 50, 100 microM) induced a significant accumulation of F-actin around the bile canaliculus as indicated by increased fluorescence in the pericanalicular area and by the increased CF/TF ratio compared with the controls. Electron microscopy studies showed significant alterations in bile canaliculi microvilli in couplets treated with 100 microM TLC. Only a few canaliculi showed an increase in pericanalicular microfilaments after treatment with 100 microM ERY. As assessed by scanning laser cytometry, TUDC prevented changes in F-actin distribution when it was added to the medium with taurolithocholate or erythromycin estolate at equimolar concentrations. However, the morphological changes observed by electron microscopy after treatment with TLC were not modified by co-treatment with TUDC. Taurocholate was ineffective. We conclude that (a) abnormalities of pericanalicular F-actin microfilaments occur in two different models of cholestasis, (b) tauroursodeoxycholate prevents the accumulation of pericanalicular F-actin detected by scanning laser cytometry but not the morphological changes of the canaliculus observed by electronic microscopy. Therefore, in these experimental conditions, the protective effect of TUDC appears to be partial.

Cytochromes P-450 in human hepatocyte plasma membrane: recognition by several autoantibodies

BACKGROUND

Anti-cytochrome P-450 autoantibodies are present in several forms of autoimmune hepatitis. The possibility that cytochromes P-450 are present in the plasma membrane of human hepatocytes was examined.

METHODS

(1) Plasma membranes with microsomal contamination < 1%, as judged from the activities of glucose-6-phosphatase and NADH-cytochrome c reductase, were prepared. (2) After exposure of uncut, fixed hepatocytes to antibodies, immunofluorescence and immunoperoxidase studies were performed.

RESULTS

(1) The specific content of cytochrome P-450 in plasma membrane was 9% of that in microsomes. Plasma membranes showed NADPH-cytochrome c reductase and mono-oxygenase activities; immunoblots showed the presence of cytochromes P-450 1A2, 2C, 2D6, 2E1, and 3A4; cytochromes P-450 1A2, 2D6, and 2C were also recognized by anti-liver microsome and anti-liver/kidney microsome type 1 and type 2 autoantibodies, respectively. (2) Immunofluorescence and immunoperoxidase labeling of the plasma membrane was observed with the three auto-antibodies and with anti-cytochrome P-450 1A2, 2C, 2E1, or 3A4.

CONCLUSIONS

It is concluded that cytochromes P-450 are present and functional in the plasma membrane of human hepatocytes and that anti-cytochrome P-450 autoantibodies recognize epitopes expressed on the outer surface.

5'nucleotidase is sorted to the apical domain of hepatocytes via an indirect route

In hepatocytes, all newly synthesized plasma membrane (PM) proteins so far studied arrive first at the basolateral domain; apically destined proteins are subsequently endocytosed and sorted to the apical domain via transcytosis. A mechanism for the sorting of newly synthesized glycophosphatidylinositol (GPI)-linked proteins has been proposed whereby they associate in lipid microdomains in the trans-Golgi network and then arrive at the apical domain directly. Such a mechanism poses a potential exception to the hepatocyte rule. We have used pulse-chase techniques in conjunction with subcellular fractionation to compare the trafficking of 5' nucleotidase (5NT), an endogenous GPI-anchored protein of hepatocytes, with two transmembrane proteins. Using a one-step fractionation technique to separate a highly enriched fraction of Golgi-derived membranes from ER and PM, we find that both 5NT and the polymeric IgA receptor (pIgAR) traverse the ER and Golgi apparatus with high efficiency. Using a method that resolves PM vesicles derived from the apical and basolateral domains, we find that 5NT first appears at the basolateral domain as early as 30 min of chase. However the subsequent redistribution to the apical domain requires > 3.5 h of chase to reach steady state. This rate of transcytosis is much slower than that observed for dipeptidylpeptidase IV, an apical protein anchored via a single transmembrane domain. We propose that the slow rate of transcytosis is related to the fact that GPI-linked proteins are excluded from clathrin-coated pits/vesicles, and instead must be endocytosed via a slower nonclathrin pathway.

Williams syndrome in adults

There are few published reports of adults with Williams syndrome (WS). We have evaluated ten adult WS patients. The patients in our study were very variable in clinical presentation, ranging from severely affected patients with complicated medical histories to mildly affected patients who are generally in good health. Cardiovascular anomalies and hypertension were frequent. Supravalvular aortic stenosis was seen in four patients, mitral valve prolapse in three, bicuspid aortic valve in one, valvular aortic stenosis in one, and pulmonary stenosis with right ventricular hypertrophy in one. Typical facial features included stellate irides, prominent cheeks, full lips, and micrognathia. Mental retardation was seen in all patients. Verbal skills were better developed than motor skills. All patients in our study lead active lives, and most are involved in sports. Some hold supervised jobs. Eight of our patients live with their parents and two in group homes. Independent living is restricted by their mental and adaptive limitations.

Effects of imidazole derivatives on cytochromes P450 from human hepatocytes in primary culture

The expression of several forms of cytochrome P450 including P450 1A2, 2D6, 2E1, and 3A was investigated in human hepatocytes maintained in primary culture for 96 h in the absence or presence of 50 microM of various imidazole derivatives. These included ketoconazole, clotrimazole, miconazole, fluconazole, secnidazole and metronidazole. In addition, the typical inducers rifampicin and beta-naphthoflavone were used for comparison. Western and Northern blot analysis of microsomes and RNA prepared from these cultures as well as de novo synthesis experiments revealed that, among the imidazole derivatives tested, only clotrimazole was a strong rifampicin-like inducer of P450 3A. The expression of the other forms of P450 tested was not affected by the treatments. Analysis of the inhibition of 13 monoxygenase activities, including ethoxyresorufin and phenacetin O-deethylases, coumarin 7 alpha-, lauric acid 11- and 12-, mephenytoin 4-, debrisoquin 4-, and aniline hydroxylases, benzphetamine, aminopyrine, mephenytoin and erythromycin demethylases, and cyclosporin oxidase (representative of 10 different forms of P450 in human liver microsomes) revealed that ketoconazole was a strong and selective in vitro inhibitor of P450 3A (cyclosporin oxidase) with a Ki less than 1 microM. Clotrimazole and miconazole were also strong inhibitors of P450 3A-mediated activities in contrast to the other imidazole derivatives.

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.

Isolation and characterization of a cytochrome P450 of the IIA subfamily from human liver microsomes

Antibodies raised against cytochrome P450, which is overexpressed in mouse hepatic tumors, (P450tu) crossreact with two human liver microsomal proteins (49 kDa and 52 kDa). We have quantified these proteins in 60 human liver samples and found great interindividual variability in both of them. The concentration of the 49-kDa protein varies up to 144 fold in the various samples and represents typically 10% of the total mincrosomal P450 content. Its immunologically determined concentration correlates well (R = 0.78) with the microsomal coumarin-7-hydroylase (COH) activity. This activity is strongly and completely inhibited by anti-P450tu antibody (IC50 = 0.13 mg IgG/mg microsomal protein). The crossreacting 49-kDa protein shows an unusually high substrate specificity towards coumarin; it presents all human COH and part of 7-ethoxycoumarin O-deethylase (ECOD). Besides these two activities, we did not find any activity with other typical P450 substrates. In primary cultures of human hepatocytes, it is inducible by phenobarbital and dexamethasone, but not by pyrazole and beta-naphthoflavone. We isolated this protein from human liver microsomes and purified it to homogeneity by a combination of aminooctyl-amino-Sepharose chromatography and immunoaffinity chromatography. The protein was identified as a cytochrome P450 of the IIA subfamily. Its N-terminal amino-acid sequence was identical with the first 20 residues deduced from the nucleotide sequence of P450IIA6.

Presence of functional cytochrome P-450 on isolated rat hepatocyte plasma membrane

Antibodies against cytochrome P-450 are found in some children with autoimmune hepatitis (antiliver/kidney microsome 1) and in patients with ticrynafen hepatitis (antiliver/kidney microsome 2). For an immune reaction against cytochrome P-450 to possibly destroy the hepatocytes, one must assume that cytochrome P-450 is present on the plasma membrane surface of hepatocytes. In a first series of experiments, plasma membranes were prepared with a technique based on the electrostatic attachment of isolated hepatocytes to polyethyleneimine-coated beads. After vortexing, beads were coated with a very pure plasma membrane fraction. Microsomal contamination, judged from the specific activities of glucose-6-phosphatase or NADH-cytochrome c reductase, was less than 1%. Nevertheless, the specific content (per milligram of protein) of CO-binding cytochrome P-450 was 20% of that in microsomes; the specific benzo(a)pyrene hydroxylase activity was 25%, and ethoxycoumarin deethylase 11%. Immunoblots showed the presence of cytochromes P-450 UT-A, UT-H, PB-B, ISF-G and PCN-E, the last three isoenzymes being inducible by, respectively, phenobarbital, 3-methylcholanthrene and dexamethasone. In a second series of experiments, nonpermeabilized isolated hepatocytes from untreated rats were incubated with anticytochrome P-450 antibodies. Immunofluorescence and immunoperoxidase staining confirmed the presence of cytochromes P-450 UT-A, PB-B and ISF-G on the membrane. In a last series of experiments, human antiliver-kidney microsomal 1 antibodies were found to react specifically with rat liver plasma membrane cytochrome P-450 UT-H (IID subfamily). We conclude that several cytochrome P-450 isoenzymes are present, active and inducible on the plasma membrane surface of hepatocytes. It is therefore conceivable that immunization against plasma membrane cytochrome P-450 might lead to the immunological destruction of hepatocytes in some patients.

Detection of a biliary cell membrane glycoprotein in the serum of cholangiocarcinoma-bearing rats. Possible relevance to the membrane proteins used as serum tumor markers in humans

Certain markers used in the diagnosis and monitoring of human adenocarcinomas, such as carcinoembryonic antigen are membrane glycoproteins normally absent from the serum. How those proteins may reach the blood after the neoplastic transformation remains debated. In this work, we show that cholangiocarcinoma induced by 3'-methyl-4-dimethylaminoazobenzene in the rat provides some insight into the mechanisms implicated in this process. We observed that the extracellular matrix of all cholangiocarcinomas tested contained large amounts of a glycoprotein identified by a monoclonal antibody termed B10, and previously characterized as an integral membrane protein normally restricted to the apical domain of epithelial cell plasma membrane. The extracellular deposition of the B10-binding glycoprotein in cholangiocarcinomas was associated with the appearance of detectable levels of the protein in the serum, and an abnormal membrane expression of the protein, which was detected on both apical and basal plasma membrane domains of neoplastic biliary cells. We postulate that neoplastic transformation of biliary cells leads to an inappropriate membrane expression of the B10-binding protein, which in turn, results in the extracellular release of the protein and in its diffusion into the blood. The characteristic desmoplastic stroma of cholangiocarcinoma offers the opportunity to easily visualize the release process. A comparable mechanism is likely to explain how certain membrane glycoproteins used as serum markers in human malignancy may reach the blood.

Permeability of the rat biliary tree to ursodeoxycholic acid

The permeability of the biliary epithelium to [14C]ursodeoxycholic acid (UDCA), a hypercholeretic bile acid, was compared to that of the 14C-labeled nonhypercholeretic bile acids cholic acid (CA), taurocholic acid (TCA), and tauroursodeoxycholic acid (TUDCA) by means of anterograde intrabiliary infusions and retrograde intrabiliary injections in the anesthetized rat. Anterograde intrabiliary infusions were performed by perfusing an isolated segment of common bile duct in vivo. After anterograde intrabiliary infusions, the fraction of unrecovered UDCA (that had presumably been absorbed from the biliary lumen) was 11.03 +/- 1.03 (SE)% (n = 6) of the administered dose. It was significantly higher than that of TUDCA (1.25 +/- 0.27%; n = 5; P less than 0.01), CA (2.62 +/- 0.43%; n = 4; P less than 0.01), and TCA (2.57 +/- 0.79%; n = 6; P less than 0.01). In separate experiments, bile was collected from the common bile duct and from the left hepatic duct. UDCA recovered from the left hepatic duct was found in the conjugated form, indicating that, after absorption in the common bile duct, it had been conjugated by the hepatocyte and secreted into bile. After retrograde intrabiliary injections of UDCA and CA, the cumulative percentages of recovered radioactivity were not significantly different (84.50 +/- 2.65 and 87.33 +/- 1.80%, respectively); however, peak recovery of UDCA was significantly delayed compared with that of CA. Moreover, UDCA was recovered mostly in the conjugated form, while CA was recovered mostly in the unconjugated form. These results suggest that, in the rat, UDCA is significantly more absorbed by the biliary tree than CA, TUDCA, and TCA. They support the hypothesis that UDCA undergoes a cholehepatic circulation.

Expression of rat hepatocyte plasma membrane antigens in hybrid clones: assignment of genes coding for two antigens of the basolateral domain to chromosomes 11 and 13

Expression of three rat hepatocyte plasma membrane antigens defined by monoclonal antibodies (mAbs) was examined by immunofluorescence in mouse hepatoma x rat hepatocyte hybrid clones segregating rat chromosomes. The antigen defined by mAb B1, a marker of the lateral domain of the hepatocyte plasma membrane in vivo, was expressed in hybrids retaining the rat chromosome 11. The antigen defined by mAb A39, mainly located on the sinusoidal (basal) domain of the plasma membrane in vivo, was expressed when chromosome 13 was present. The genes coding for these two antigens were thus assigned to chromosomes 11 and 13, respectively. The antigen defined by mAb B10, exclusively located on the canalicular (apical) domain of the plasma membrane in vivo, was not expressed in most hybrid clones, and the chromosome location of the gene could not be determined.