Biochemical evidence that Na+,K+-ATPase is located at the lateral region of the hepatocyte surface membrane

Hepatic glutamate transport and glutamine synthesis capacities are decreased in finished vs. growing beef steers, concomitant with increased GTRAP3-18 content

Hepatic glutamate uptake and conversion to glutamine is critical for whole-body N metabolism, but how this process is regulated during growth is poorly described. The hepatic glutamate uptake activities, protein content of system [Formula: see text] transporters (EAAC1, GLT-1) and regulatory proteins (GTRAP3-18, ARL6IP1), glutamine synthetase (GS) activity and content, and glutathione (GSH) content, were compared in liver tissue of weaned Angus steers randomly assigned (n = 8) to predominantly lean (growing) or predominantly lipid (finished) growth regimens. Steers were fed a cotton seed hull-based diet to achieve final body weights of 301 or 576 kg, respectively, at a constant rate of growth. Liver tissue was collected at slaughter and hepatic membranes fractionated. Total (75%), Na+-dependent (90%), system [Formula: see text]-dependent (abolished) glutamate uptake activity, and EAAC1 content (36%) in canalicular membrane-enriched vesicles decreased as steers developed from growing (n = 6) to finished (n = 4) stages, whereas Na+-independent uptake did not change. In basolateral membrane-enriched vesicles, total (60%), Na+-dependent (60%), and Na+-independent (56%) activities decreased, whereas neither system [Formula: see text]-dependent uptake nor protein content changed. EAAC1 protein content in liver homogenates (n = 8) decreased in finished vs. growing steers, whereas GTRAP3-18 and ARL6IP1 content increased and GLT-1 content did not change. Concomitantly, hepatic GS activity decreased (32%) as steers fattened, whereas GS and GSH contents did not differ. We conclude that hepatic glutamate uptake and GS synthesis capacities are reduced in livers of finished versus growing beef steers, and that hepatic system [Formula: see text] transporter activity/EAAC1 content is inversely proportional to GTRAP3-18 content.

[Cholestasis and cholestatic liver diseases]

The main determinant of bile formation is an osmotic filtration process resulting from active transport of bile acids and other osmotic solutes (glutathion). Most of the membrane transporters ensuring bile formation have now been identified. The expression of these membrane transporters is regulated through transcriptional and post-traductional mechanisms. Transcriptional regulation is under the control of nuclear receptors activated by ligands such as bile acids, which act as endogenous steroids synthesized from cholesterol in hepatocytes. Cholestatic liver diseases comprise genetic diseases resulting from the complex interaction between genetic and environmental factors. Monogenic cholestatic diseases recently identified illustrate the key role of membrane transporters in biliary function. Bile acids and inflammatory mediators are potent modulators of transporters and nuclear receptor genes and thus trigger an adaptative response to cholestasis. The extent of this adaptative response could explain the compelling phenotypic variability of cholestatic diseases in childhood and adults. The first-line medical treatment is currently ursodeoxycholic acid and in case of failure of this medical treatment, liver transplantation is required. Recent progress in the molecular pathogenesis of bile formation and cholestatic liver diseases is expected to provide the design of drugs targeted to the molecular abnormalities typical of cholestatic diseases.

Highly purified bile-canalicular vesicles and lateral plasma membranes isolated from rat liver on Nycodenz gradients. Biochemical and immunolocalization studies

1. A liver canalicular plasma-membrane fraction enriched 115-155-fold in five marker enzymes relative to the tissue homogenate was obtained by sonication of liver plasma membranes followed by fractionation in iso-osmotic Nycodenz gradients. 2. Two lateral-plasma membrane fractions were also collected by this procedure; the lighter-density fraction was still associated with canalicular membranes, as assessed by enzymic and polypeptide analysis. 3. The polypeptide composition of the domain-defined plasma-membrane fractions was evaluated. It was demonstrated by immunoblotting that the 41 kDa alpha-subunit of the inhibitory G-protein, associated in high relative amounts with canalicular plasma-membrane fractions, was partially lost in the last stage of purification; however, this subunit was retained by lateral plasma membranes. 4. Antibodies to the proteins of bile-canalicular vesicles were shown to localize to the hepatocyte surface in thin liver sections examined by immunofluorescent and immuno-gold electron microscopy. Two subsets of antigens were identified, one present on both sinusoidal and canalicular plasma-membrane domains and another, by using antisera pre-absorbed with sinusoidal plasma membranes, that was confined to the bile-canalicular domain.

Cryptic Na+,K(+)-ATPase activity in rat liver canalicular plasma membranes: evidence for its basolateral origin

Controversy exists concerning the localization of the enzyme Na+,K(+)-ATPase to canalicular membranes in hepatocytes. Most studies find enzyme activity only at the basolateral plasma membrane domain of the hepatocyte. However, Na+,K(+)-ATPase activity has been detected recently in a canalicular membrane fraction prepared by Mg++ precipitation, suggesting that differences in membrane domain fluidity account for these discrepancies. To reinvestigate this question, we used free-flow electrophoresis to further purify canalicular liver plasma membranes originally separated by sucrose density centrifugation. With this technique, canalicular membranes devoid of Na+,K(+)-ATPase activity by routine assay were separated into six subfractions. More than 80% of the activities of canalicular marker enzymes was recovered in two subfractions closest to the anode, which were totally devoid of Na+,K(+)-ATPase activity. However, Na+,K(+)-ATPase activity could now be detected in the four other fractions that contained only small amounts of canalicular marker enzymes. The basolateral marker enzyme, glucagon-stimulated adenyl cyclase, comigrated with this cryptic Na+,K(+)-ATPase activity. Furthermore, addition of 6 mumol/L [12-(2-methoxyethoxy)-ethyl-8-(cis-2-n-octylcyclopropyl)-octanoate ], a membrane-fluidizing agent, to the original canalicular membrane preparation and to all subfractions did not stimulate or unmask latent Na+,K(+)-ATPase activity. Finally, when canalicular membranes isolated by Mg++ precipitation were subjected to free-flow electrophoresis, they could not be separated from the more positively charged Na+,K(+)-ATPase-containing fractions, probably because of alterations in surface charge. Together these findings suggest that Na+,K(+)-ATPase is a basolateral enzyme, that represents a small contaminant when present in canalicular liver plasma membranes and that methodological differences may account for previous discrepancies.

Biochemical localization of hepatic surface-membrane Na+,K+-ATPase activity depends on membrane lipid fluidity

Membrane proteins of transporting epithelia are often distributed between apical and basolateral surfaces to produce a functionally polarized cell. The distribution of Na+,K+-ATPase [ATP phosphohydrolase (Na+/K+-transporting), EC 3.6.1.37] between apical and basolateral membranes of hepatocytes has been controversial. Because Na+,K+-ATPase activity is fluidity dependent and the physiochemical properties of the apical membrane reduces its fluidity, we investigated whether altering membrane fluidity might uncover cryptic Na+,K+-ATPase in bile canalicular (apical) surface fractions free of detectable Na+,K+-ATPase and glucagon-stimulated adenylate cyclase activities. Apical fractions exhibited higher diphenylhexatriene-fluorescence polarization values when compared with sinusoidal (basolateral) membrane fractions. When 2-(2-methoxyethoxy)ethyl 8-(cis-2-n-octylcyclopropyl)octanoate (A2C) was added to each fraction, Na+,K+-ATPase, but not glucagon-stimulated adenylate cyclase activity, was activated in the apical fraction. In contrast, further activation of both enzymes was not seen in sinusoidal fractions. The A2C-induced increase in apical Na+,K+-ATPase approached 75% of the sinusoidal level. Parallel increases in apical Na+,K+-ATPase were produced by benzyl alcohol and Triton WR-1339. All three fluidizing agents decreased the order component of membrane fluidity. Na+,K+-ATPase activity in each subfraction was identically inhibited by the monoclonal antibody 9-A5, a specific inhibitor of this enzyme. These findings suggest that hepatic Na+,K+-ATPase is distributed in both surface membranes but functions more efficiently and, perhaps, specifically in the sinusoidal membranes because of their higher bulk lipid fluidity.

Localization of Na+,K+-ATPase alpha-subunit to the sinusoidal and lateral but not canalicular membranes of rat hepatocytes

Controversy has recently developed over the surface distribution of Na+,K+-ATPase in hepatic parenchymal cells. We have reexamined this issue using several independent techniques. A monoclonal antibody specific for the endodomain of alpha-subunit was used to examine Na+,K+-ATPase distribution at the light and electron microscope levels. When cryostat sections of rat liver were incubated with the monoclonal antibody, followed by either rhodamine or horseradish peroxidase-conjugated goat anti-mouse secondary, fluorescent staining or horseradish peroxidase reaction product was observed at the basolateral surfaces of hepatocytes from the space of Disse to the tight junctions bordering bile canaliculi. No labeling of the canalicular plasma membrane was detected. In contrast, when hepatocytes were dissociated by collagenase digestion, Na+,K+-ATPase alpha-subunit was localized to the entire plasma membrane. Na+,K+-ATPase was quantitated in isolated rat liver plasma membrane fractions by Western blots using a polyclonal antibody against Na+,K+-ATPase alpha-subunit. Plasma membranes from the basolateral domain of hepatocytes possessed essentially all of the cell's estimated Na+,K+-ATPase catalytic activity and contained a 96-kD alpha-subunit band. Canalicular plasma membrane fractions, defined by their enrichment in alkaline phosphatase, 5' nucleotidase, gamma-glutamyl transferase, and leucine aminopeptidase had no detectable Na+,K+-ATPase activity and no alpha-subunit band could be detected in Western blots of these fractions. We conclude that Na+,K+-ATPase is limited to the sinusoidal and lateral domains of hepatocyte plasma membrane in intact liver. This basolateral distribution is consistent with its topology in other ion-transporting epithelia.

Solubilization of lipids from hamster bile-canalicular and contiguous membranes and from human erythrocyte membranes by conjugated bile salts

We have demonstrated in vitro the efficacy of the taurine-conjugated dihydroxy bile salts deoxycholate and chenodeoxycholate in solubilizing both cholesterol and phospholipid from hamster liver bile-canalicular and contiguous membranes and from human erythrocyte membrane. On the other hand, the dihydroxy bile salt ursodeoxycholate and the trihydroxy bile salt cholate solubilize much less lipid. The lipid solubilization by the four bile salts correlated well with their hydrophobicity: glycochenodeoxycolate, which is more hydrophobic than the tauro derivative, also solubilized more lipid. All the dihydroxy bile salts have a threshold concentration above which lipid solubilization increases rapidly; this correlates approximately with the critical micellar concentration. The non-micelle-forming bile salt dehydrocholate solubilized no lipid at all up to 32 mM. All the dihydroxy bile acids are much more efficient at solubilizing phospholipid than cholesterol. Cholate does not show such a pronounced discrimination. Lipid solubilization by chenodeoxycholate was essentially complete within 1 min, whereas that by cholate was linear up to 5 min. Maximal lipid solubilization with chenodeoxycholate occurred at 8-12 mM; solubilization by cholate was linear up to 32 mM. Ursodeoxycholate was the only dihydroxy bile salt which was able to solubilize phospholipid (although not cholesterol) below the critical micellar concentration. This similarity between cholate and ursodeoxycholate may reflect their ability to form a more extensive liquid-crystal system. Membrane specificity was demonstrated only inasmuch as the lower the cholesterol/phospholipid ratio in the membrane, the greater the fractional solubilization of cholesterol by bile salts, i.e. the total amount of cholesterol solubilized depended only on the bile-salt concentration. On the other hand, the total amount of phospholipid solubilized decreased with increasing cholesterol/phospholipid ratio in the membrane.

Subfractionation of hepatic endosomes in Nycodenz gradients and by free-flow electrophoresis. Separation of ligand-transporting and receptor-enriched membranes

The complexity of rat liver endosome fractions containing internalized radioiodinated asialotransferrin, asialo-(alkaline phosphatase), insulin and prolactin was investigated by using free-flow electrophoresis and isopycnic centrifugation in Nycodenz gradients. Two subfractions were separated by free-flow electrophoresis. Both subfractions contained receptors for asialoglycoprotein and insulin. Glycosyltransferase activities were associated with the more electronegative vesicles, whereas 5'-nucleotidase and alkaline phosphodiesterase activities were associated with the less electronegative vesicles. Three subfractions were separated on Nycodenz gradients. Two subfractions, previously shown to become acidified in vitro, contained the ligands. At short intervals after uptake (1-2 min), ligands were mainly in subfraction DN-2 (density 1.115 g/cm3), but movement into subfraction DN-1 (density 1.090 g/cm3) had occurred 10-15 min after internalization. Low amounts of glycosyltransferase activities were associated with subfraction DN-2, and 5'-nucleotidase and alkaline phosphodiesterase activities were mainly located in subfraction DN-1. The binding sites for asialoglycoproteins and insulin were distributed towards the higher density range in the Nycodenz gradients, thus indicating a segregation of receptor-enriched vesicles and those vesicles containing the various ligands 10-15 min after internalization. Electron microscopy of the subfractions separated on Nycodenz gradients indicated that whereas the ligand-transporting fractions consisted mainly of empty vesicles (average diameter 100-150 nm), the receptor-enriched component was more granular and smaller (average diameter 70-95 nm). The properties of the endosome subfraction are used to assign their origin to the regions of the endocytic compartment where ligand-receptor dissociation and separation occur.

Effect of lithocholic acid on cholesterol synthesis and transport in the rat liver

The cellular origin of cholesterol which accumulates in liver cell plasma membrane fractions enriched in bile canalicular structures after lithocholic acid treatment was determined in vivo. Rats were given [3H]cholesterol followed 16 h later by [2-14C]mevalonic acid, [2-14C]acetic acid or lithocholic acid. Lithocholic acid injection enhanced the de novo synthesis of cholesterol in the microsomes and both compounds were transported to the bile canalicular membranes. However, in vitro studies demonstrated that lithocholic acid is capable of stripping cholesterol from microsomal membranes even in the absence of increased de novo synthesis. This suggests that transfer of cholesterol from subcellular organelles (microsomes) to bile canalicular membranes may be the initial step in the development of lithocholic acid-induced cholestasis.

Quantitative immunoferritin localization of [Na+,K+]ATPase on canine hepatocyte cell surface

Distribution of [Na+,K+]ATPase on the cell surface of canine hepatocytes was investigated quantitatively by incubating prefixed and dissociated liver cells with ferritin antibody conjugates against canine kidney holo[Na+,K+]ATPase. We found that [Na+,K+]-ATPase exists bilaterally both on the bile canalicular and sinusoid-lateral surfaces. The particle density on the bile canalicular surface was much higher (approximately 2.5 times) than that on the sinusoid-lateral surface. In the latter region, the enzyme was detected almost equally both on the sinusoidal and lateral surfaces. On all the surfaces, the distribution of the enzyme was homogeneous and no clustering of the enzyme was detected. Total number of the enzyme on the sinusoid-lateral surface was, however, approximately three times higher than that on the bile canalicular region, because the sinusoid-lateral surface represents approximately 87% of the total cell surface of a hepatocyte. We suggest that the [Na+, K+]ATPase on the bile canalicular surface is responsible for the bile acid-independent bile flow and the other transport processes on the bile canalicular cell surface, while that on the sinusoid-lateral surface is responsible not only for the active transport of Na+ but also for the secondary active transport of various substances in this region.

Characterization of the epidermal-growth-factor-dependent phosphorylation system from normal mouse-liver sinusoidal plasma membranes

Blood sinusoidal plasma membrane subfractions were isolated from normal mouse liver in the presence of the proteinase inhibitors PhMeSO2F and iodoacetamide. They were purified from smooth microsomal and Golgi vesicle contaminants. The phosphorylation reaction was studied at 33 degrees C, in the presence of 2 mM MnCl2. Addition of epidermal growth factor (EGF) to the preparations stimulated 32P incorporation from [gamma-32P]ATP or [gamma-32P]GTP essentially into one 170 000 Mr protein. Some incorporation was observed in a minor 120 000-Mr component which appears to be a degradation product of the 170 000-Mr component. No EGF-dependent phosphorylation of other membrane proteins or various exogenous proteins could be detected in vitro. The dephosphorylation of the 170 000-Mr component was observed after 4 min of incubation at 33 degrees C. This dephosphorylation reaction was inhibited by addition of 5 mM p-nitrophenyl phosphate but not by addition of micromolar Zn2+, Be2+ or orthovanadate. The 170 000-Mr protein specifically bound 125I-labeled EGF and thus appeared to be the hepatic EGF receptor. The EGF stimulatable kinase activity considerably enhances incorporation of 32P into tyrosine residues of the 170 000-Mr EGF receptor at 33 degrees C. Tryptic peptide maps of the 32P-labeled 170 000-Mr protein revealed a multiplicity of phosphorylated sites. Seven 32P-labeled phosphopeptides were observed after EGF stimulation, three of them being largely prominent. Tryptic peptide maps of the 170 000-Mr protein after it was covalently linked to 125I-labeled EGF showed only one 125I-labeled peptide, the migration of which appeared different from that of 32P-labeled phosphopeptides. These findings were confirmed by V8 protease unidimensional peptide mapping of the 170 000-Mr protein, labeled with 32P or 125I-EGF.

Biochemical separation of Na+,K+-ATPase from a "purified" light density, "canalicular"-enriched plasma membrane fraction from rat liver

Cytochemical studies suggest that Na+,K+-ATPase is localized to sinusoidal and lateral portions of the hepatocyte plasma membrane whereas Mg++-ATPase and alkaline phosphatase are luminal or canalicular membrane markers. To validate further these cytochemical findings, we have isolated from the nuclear pellet of rat liver homogenates a liver plasma membrane (LPM) fraction enriched in all three enzyme markers, as previously described (Biochimica et Biophysica Acta 1975; 401:59-52). Following tight Dounce homogenization, the vesiculated membrane preparation was further separated on a multiple-step discontinuous sucrose density gradient (d 1.12 to 1.22). Na+,K+-ATPase activity "dissociated" from Mg++-ATPase activity, sedimenting at densities of 1.14 and greater. Further studies were carried out in two-step discontinuous sucrose gradients (1.13 and d greater than 1.13), and a light density fraction (d 1.13) was further characterized in calcium-free media (since addition of calcium increased contamination with intracellular membranes). Electron microscopy demonstrated a homogeneous vesicular membrane population in contrast to the heavy density fraction (d greater than 1.13) which contained membrane sheets and junction complexes as well as vesicles. The light density fraction was highly enriched n Mg++-ATPase (42.1 x homogenate specific activity) and alkaline phosphatase (64.6 x homogenate), 3 to 4 times their activities in the original LPM. In contrast, Na+,K+-ATPase activity in the light density fraction, diminished 16-fold from values in the original unfractionated LPM. All but 15% of total Na+,K+-ATPase activity in the original LPM could be accounted for in unwashed preparations. Neither cholesterol/phospholipid ratios nor an analysis of peptides on sodium dodecyl sulfate gel electrophoresis demonstrated differences in the composition of the light vs. heavy density subfractions, although there were relative increases in several peptide bands in the light density subfraction. These studies provide further supporting biochemical evidence for the concept that Na+,K+-ATPase resides on different membrane domains than does Mg++-ATPase and alkaline phosphatase and further characterizes a vesiculated membrane preparation highly enriched in putative "canalicular" enzyme markers.

Orientation of rat-liver plasma membrane vesicles. A biochemical and ultrastructural study

Using both biochemical and morphological methods, the membrane orientation of plasma membrane vesicles from rat liver which are capable of catalysing the active transport of amino acids was investigated. In intact vesicles, the plasma membrane enzyme (Na+ + K+)-ATPase displays only a minor portion of its total activity which is greatly increased upon vesicle disruption. The same intact vesicles show an almost maximal binding of ouabain, which binds only to the extracellular side of the plasma membrane. A freeze-fracture analysis of the vesicles shows that a distinct population of relatively large vesicles have predominantly the in vivo membrane orientation. These large vesicles are labelled with numerous filipin-sterol complexes following exposure to the cholesterol probe, filipin, and are therefore assumed to be plasma membrane vesicles. A population of smaller vesicles with mainly an inside-out orientation were not labelled with filipin and are probably microsomes. The data obtained with both biochemical and ultrastructural techniques indicate that the plasma membrane vesicles isolated from rat liver for transport studies are mostly (at least 70%) orientated as in vivo, i.e. inside-in.

Insulin-induced choleresis in relation to insulin concentrations in plasma and bile in the cat

In experiments on fasting, chloralose-anesthetized cats the concentrations of insulin and glucagon in arterial or portal plasma and bile were studied during insulin-induced choleresis. The administration of insulin in single doses (0.1-1.0 U x kg-1) and infusions (0.001-0.01 U x kg-1 x min-1) resulted in parallel increases in bile flow and 14C-erythritol clearances of 17%-96%, whereas the net ductular fluid transport and the biliary excretion rate of bile acids remained unchanged. Insulin therefore stimulated the bile acid-independent fraction of the canalicular bile formation. When corrected for the hepatic transit time of insulin (25 min) the initial phase of insulin choleresis was closely related in time to the plasma (30-5000 microunits x ml-1) and biliary (40-210 microunits x ml-1) concentrations of insulin. Bile flow was stimulated even at plasma insulin concentrations similar to those seen postprandially. It is concluded that insulin-induced choleresis to a great extent can be explained by direct, hepatic actions of the hormone, but a delayed release of glucagon may be of importance for the terminal phase of insulin choleresis. The results also demonstrate that insulin crosses the liver by an intercellular pathway. This mode of transport could be of importance for its action on bile production.

Sodium ion-coupled uptake of taurocholate by rat-liver plasma membrane vesicles

Uptake of taurocholate into plasma membrane vesicles isolated from rat liver was investigated. In the presence of an extra- to intravesicular gradient of Na+ ions, a typical "overshoot" phenomenon in the accumulation pattern was observed. Osmotic manipulation of the incubation medium indicated that the transport of this bile acid occurs into an osmotically active intravesicular space. Uptake of taurocholate as measured after 1 min was specifically stimulated by Na+ ions: NaNO3 and NaCl were capable of supporting accumulation, whereas KNO3 was not. Na+-coupled uptake of taurocholate showed saturation kinetics and was inhibited by other bile acids or by preloading the vesicles with Na+. Our observations support the idea of a carrier-mediated bile-acid uptake system, as suggested previously for the intact rat liver and isolated rat hepatocytes. When the electrical potential difference across the vesicle membrane was changed by inducing different diffusion potentials (anion replacement), a more negative potential inside stimulated Na+-dependent taurocholate transport. The results demonstrate that rat-liver plasma membrane vesicles possess an electrogenic Na+-coupled transport system for taurocholate.

Biogenesis of hepatocyte plasma-membrane domains. Incorporation of (3H)fucose into plasma-membrane and golgi-apparatus glycoproteins

1. Rats were injected intracaudally with [3H]fucose and its rate of incorporation into the fucoproteins of serum, Golgi and plasma-membrane subfractions was followed for up tp 2h. 2. Incorporation into the Golgi dictyosome and secretory-vesicular fractions reached a maximum at 15 min or less, but most of the radioactivity was associated with classes of secretory glycoproteins. Incorporation into sinusoidal plasma-membrane fractions reached a maximum at 30 min, coinciding with the maximum release of fucoproteins into the serum. Contiguous and canalicular plasma-membrane fractions were labelled slightly later and at a lower rate and specific radioactivity. 3. Fluorography of fucoproteins separated by polyacrylamide-gel electrophoresis helped to distinguish between the major secretory and membrane-bound glycoproteins. The results show that a major biogenetic sequence is probably from Golgi dictyosomes to Golgi secretory elements to a sinusoidal plasma membrane. 4. The kinetics of incorporation make it unlikely that there is rapid and direct insertion of glycoproteins into the bile-canalicular plasma membrane. A route involving direct transfer of glycoproteins via a membrane-mediated intracellular path from the blood sinusoidal to the bile-canalicular plasma membranes is proposed.