Search

ATP-binding cassette (ABC)-type proteins are essential for bile formation in vertebrate liver. BSEP, MDR1, MDR2, and MRP2 ABC transporters are targeted to the apical (canalicular) membrane of hepatocytes where they execute ATP-dependent transport of bile acids, drugs, amphipathic cations, phospholipids, and conjugated organic anions, respectively. Changes in activity and abundance of transporters in the canalicular membrane regulate bile flow; however, little is known regarding cellular proteins that bind ABC transporters and regulate their trafficking. A yeast two-hybrid screen identified HAX-1 as a binding partner for BSEP, MDR1, and MDR2. The interactions were validated biochemically by glutathione S-transferase pull-down and co-immunoprecipitation assays. BSEP and HAX-1 were over-represented in rat liver subcellular fractions enriched for canalicular membrane vesicles, microsomes, and clathrin-coated vesicles. HAX-1 was bound to BSEP, MDR1, and MDR2 in canalicular membrane vesicles and co-localized with BSEP and MDR1 in the apical membrane of Madin-Darby canine kidney (MDCK) cells. RNA interference of HAX-1 increased BSEP levels in the apical membrane of MDCK cells by 71%. Pulse-chase studies indicated that HAX-1 depletion did not affect BSEP translation, post-translational modification, delivery to the plasma membrane, or half-life. HAX-1 depletion resulted in an increased peak of metabolically labeled apical membrane BSEP at 4 h and enhanced retention at 6 and 9 h. HAX-1 also interacts with cortactin. Expression of dominant negative cortactin increased steady state levels of BSEP 2-fold in the apical membrane of MDCK cells, as did expression of dominant negative EPS15. These findings suggest that HAX-1 and cortactin participate in BSEP internalization from the apical membrane.

Fatty acids radioactivity was bound to Z protein in liver after administration of['3H]oleate to rats or to a perfused rat liver preparation. Pretreatment withflavaspidic acid (340 mumol/kg), a potent inhibitor of fatty acid binding to hepatic Zprotein in vitri, effectively reduced oleate radioactivity bound to Z by 90.2 plusor minus 4.3% and 85.0 plus or minus 6.2% in the intact rat and perfused liver, respectively. In spite of this effect, pretreatment of rats with flavaspidic acid did notalter plasma clearance, hepatic uptake, and esterification of ['3H]oleate. In contrast, in the perfused liver preparation, infusion of flavaspidic acid (340 mumol/kg)or bromosulphalein (360 mumol/kg) increased uptake of ['3H]oleate at least twofold,and oleate esterification was decreased by 15-30%. These results suggest that the binding of long-chain fatty acids to Z protein is not an obligatory step in their uptakeby the liver and that Z protein may be involved in fatty acid esterification.

Twenty min after i.p. administration of 3'-[14C]methyl-N,N-dimethyl-4-aminoazobenzene in corn oil to rats, 0.73% of administered radioactivity was present in the liver. Only 0.45% of radioactivity present in liver was recovered in the nuclear fraction, whereas 25% was present in the cytosol fraction. Twenty-seven % of cytosolic radioactivity was trichloroacetic acid precipitable, and 2% was immunoprecipitable with monospecific anti-rat liver ligandin immunoglobulin G. After 3 hr of administration, 3.2% of administered radioactivity was present in the liver, 40% of which was in the cytosol. Although 59% of radioactivity present in liver cytosol was trichloroacetic acid precipitable as compared to 27% at 20 min, the radioactivity precipitated by anti-ligandin immunoglobulin G was still 2%. When liver cytosol obtained from rats after 20 min of 3'-[14C]methyl-N,N-dimethyl-4-aminoazobenzene administration was fractionated on a Sephadex G-75 column, three peaks of radioactivity were observed. When cytosol was subjected to sodium dodecyl sulfate gel electrophoresis and fluorography, radioactivity was mainly associated with 5 proteins with molecular weights of 88,000, 47,000, 41,000, 31,000, and 22,000. When the immunoprecipitate obtained from cytosol with anti-ligandin immunoglobulin G was subjected to sodium dodecyl sulfate gel electrophoresis and fluorography, radioactivity was exclusively associated with the subunit of ligandin with a molecular weight of 22,000. Approximately 90% of the radioactivity in the immunoprecipitate was covalently associated with this subunit. These studies reveal that 3'-methyl-N,N-dimethyl-4-aminoazobenzene or its metabolites are selectively bound to the subunit of ligandin with a molecular weight of 22,000 and four other cytosol proteins in vivo.

Sephadex gel filtration of the 1000,000 g supernate of homogenates of rat kidney revealed binding of various organic anions (penicillin, Bromsulphalein [BSP], bilirubin, phenolsulfonphthalein [PSP], phlorizin, glutathione [GSH], p-amino hippurate (PAH), probenecid, conjugated bilirubin, and BSP-GSH) to a nonalbumin-containing protein fraction (Y), which precipated on addition of monospecific anti-rat liver ligandin (Y protein)-IgG, but not control IgG. Quantitatively similar organic anion binding was observed in vivo after injection of BSP, BSP-GSH, phlorizin, probenecid, conjugated bilirubin, PAH, or penicillin. The binding protein was purified to apparent homogeneity and is a basic protein (pI 8.9) of 44,000 daltons with two apparently identical subunits of 22,000 daltons. Monospecific antibody was produced against the renal protein. The results of binding studies in vivo and in vitro and phsicochemical, immunologic, structural, and binding site investigations indicate that the renal protein is identical to hepatic ligandin. Immunofluorescent studies utilizing anti-ligandin IgG previously localized ligandin in the kidney to all proximal tubular cells. By quantitative radial immunodiffusion, the concentration of renal ligandin was 31.2 plus or minus 2.2 mug/mg supernatant protein and was increased 160% above basal values by pretreatment of rats with tetrachloro-dibenzo-p-dioxin. Pretreatment with phenobarbital, DDT, or pregnene-16alpha-carbonitrile did not increase renal ligandin concentration but doubled hepatic ligandin concentration. Circular dichroism studies of renal ligandin revealed percent helical structure similar to hepatic ligandin and primary association contrasts were derived for BSP (10-6 M-1) and PAH, probenecid, and penicillin (10-3 M-1). Administration of BSP or probenecid simultaneously with [C14] penicillin resulted in increased plasma retention and reduced kidney and urinary bladder content of [14C] penicillin and a correlation coefficient of -0.8 between total kidney/plasma radioactivity and percent of protein-bound radioactivity bound to ligandin in the kidney. These studies indicate that renal and hepatic ligandin are identical. Their response to drugs and chemicals varies. Competitive binding between several organic anions for ligandin correlated with their renal uptake from plasma, which suggests that ligandin may function in the proximal tubular cell as a component of the renal organic anion transport system.

Circular dichroism (CD) methods were employed to study the conformation of Z protein and characterize its complexes with bilirubin and other organic anions. Z protein-bilirubin complexes exhibited a spectrum with overlapping ellipticity bands of opposite sign in the bilirubin absorption region. These results were compared with those obtained with ligandin, the other major organic anion binding protein of liver. Secondary structural differences between the two proteins were easily demonstrated since ligandin is predominantly an alpha-helical protein and Z features mainly beta-structure. Furthermore, the optical activity pattern generated by bilirubin binding to Z was virtually a mirror image of that of the ligandin bilirubin system. CD experiments were designed to study the direct transfer of bilirubin between Z protein and ligandin, and it was shown that both proteins have almost equal affinities for bilirubin. The bilirubin on Z was readily displaced by oleic acid and displaced to a lesser extent by sulfobromophthalein,

The process by which various anions, including bilirubin and several dyes, drugs, hormones and their metabolites, are transferred from plasma into the liver cell is poorly understood. Two hepatic cytoplasmic proteins, Y and Z, that bind various organic anions in vivo and in vitro have been postulated to be involved in this process. The concentration of Y, the major organic anion-binding protein, increases in rat liver after administration of phenobarbital in association with enhanced organic anion transfer from plasma into liver as determined by initial plasma disappearance rate (K(1)) and hepatic dye content for sulfobromophthalein (BSP) and indocyanine green (ICG), as well as increased relative hepatic storage of BSP. Acute bile duct ligation failed to alter plasma disappearance or hepatic content of BSP in normal or phenobarbital-treated rats. Other drugs and chemicals which cause proliferation of hepatic smooth endoplasmic reticulum and enhancement of drug metabolism, such as allylisopropylacetamide, dieldrin, DDT, 3-methylcholanthrene, and benzpyrene increased Y and BSP K(1) and, where studied, hepatic BSP content. Alcohol feeding had no effect on Y, Z, or K(1) for BSP. Hypophysectomy and thyroidectomy increased Y but decreased K(1) and, where studied, hepatic content of BSP. Of several hormones studied, only thyroxine restored Y and K(1) to normal in hypophysectomized or thyroidectomized rats. Mice with congenital pituitary insufficiency also manifested increased Y which returned to normal after thyroxine administration. In hormone-deficient rats and mice, phenobarbital administration produced a further increase in Y suggesting that different mechanisms may be responsible for the change in Y resulting from drug administration and hormonal deprivation. Thyroxine, testosterone, or hydrocortisone did not alter BSP K(1) or Y in normal rats.Cholestasis produced by ethinyl estradiol administration or biliary obstruction reduced Y, Z, BSP K(1) and hepatic BSP content. These results support the hypothesis that Y and Z are involved in the transfer of BSP, ICG, and possibly other organic anions from plasma into the liver. The concentration of Y increased after administration of various drugs and chemicals as well as in thyroid deficiency. Thyroid hormone appears to be important in regulation of the intracellular concentration of Y. Because thyroid deficiency increased Y but decreased BSP K(1) and hepatic BSP content, other factors beside Y and Z influence hepatic organic anion uptake.

Two hepatic cytoplasmic protein fractions, designated Y and Z, which bind sulfobromophthalein (BSP), bilirubin, and other organic anions, have been separated by G75 Sephadex gel filtration. The physiologic role of these protein fractions has been investigated. They are present in the 110,000 g supernatant fraction from the livers of all the species tested (rats, mice, guinea pigs, Rhesus monkeys, sheep, and man). Tissues which do not preferentially extract BSP or bilirubin from plasma do not contain these fractions, with the exception of small intestinal mucosa which contains Z. Anion binding by Y and Z fractions is not due to contamination with albumin. These fractions are responsible for the cytoplasmic localization of bilirubin in Gunn rats, and the fractions bind bilirubin, BSP, or indocyanine green (ICG), whether given in vivo or added in vitro to liver supernate from normal rats. Flavaspidic acid-N-methylglucaminate, bunamiodyl, and iodipamide, drugs known to interfere with the hepatic uptake mechanism, compete with bilirubin and BSP for binding to Z. These proteins appear to be important in the transfer of organic anions from plasma into the liver and provide a tool for the investigation of hepatic uptake mechanisms.