Connective tissue biomatrix: its isolation and utilization for long-term cultures of normal rat hepatocytes.

A new procedure is introduced for the isolation of connective tissue fibers, called biomatrix, containing a significant portion of the extracellular matrix (basement membrane components and components of the ground substance). Biomatrix isolated from normal rat liver contains >90% of the tissue's collagens and all of the known collagen types, including types I and III and basement membrane collagens. The purified collagenous fibers are associated with noncollagenous acidic proteins (including fibronectins and possibly small amounts of glycosaminoglycans). Procedures are also described for preparing tissue culture substrates with these fibers by either smearing tissue culture dishes with frozen sections or by shredding the biomatrix into small fibrils with a homogenizer. The biomatrix as a substrate has a remarkable ability to sustain normal rat hepatocytes long-term in culture. The hepatocytes, which on tissue culture plastic or on type I collagen gels do not survive more than a few weeks, have been maintained for more than 5 mo in vitro when cultured on biomatrix. These cells cultured on rat liver biomatrix show increased attachment and survival efficiencies, long-term survival (months) and retention of some hepatocyte-specific functions.

Electrophysiological properties of gap junctions between dissociated pairs of rat hepatocytes.

Physiological properties of isolated pairs of rat hepatocytes were examined within 5 h after dissociation. These cells become round when separated, but cell pairs still display membrane specializations. Most notably, canaliculi are often present at appositional membranes which are flanked by abundant gap and tight junctions. These cell pairs are strongly dye-coupled; Lucifer Yellow CH injected into one cell rapidly diffuses to the other. Pairs of hepatocytes are closely coupled electrically. Conductance of the junctional membrane is not voltage sensitive: voltage clamp studies demonstrate that gj is constant in response to long (5 s) transjunctional voltage steps of either polarity (to greater than +/- 40 mV from rest). Junctional conductance (gj) between hepatocyte pairs is reduced by exposure to octanol (0.1 mM) and by intracellular acidification. Normal intracellular pH (pHi), measured with a liquid ion exchange microelectrode, was generally 7.1-7.4, and superfusion with saline equilibrated with 100% CO2 reduced pHi to 6.0-6.5. In the pHi range 7.5-6.6, gj was constant. Below pH 6.6, gj steeply decreased and at 6.1 coupling was undetectable. pHi recovered when cells were rinsed with normal saline; in most cases gj recovered in parallel so that gj values were similar for pHs obtained during acidification or recovery. The low apparent pK and very steep pHi-gj relation of the liver gap junction contrast with higher pKs and more gradually rising curves in other tissues. If H+ ions act directly on the junctional molecules, the channels that are presumably homologous in different tissues must differ with respect to reactive sites or their environment.

Regulation of growth and differentiation of a rat hepatoma cell line by the synergistic interactions of hormones and collagenous substrata.

Serum-free, hormonally defined media have been developed for optimal growth of a rat hepatoma cell line. The cells' hormonal requirements for growth are dramatically altered both qualitatively and quantitatively by whether they were plated onto tissue culture plastic or collagenous substrata. On collagenous substrata, the cells required insulin, glucagon, growth hormone, prolactin, and linoleic acid (bound to BSA), and zinc, copper, and selenium. For growth on tissue culture plastic, the cells required the above factors at higher concentrations plus several additional factors: transferrin, hydrocortisone, and triiodothyronine. To ascertain the relative influence of hormones versus substratum on the growth and differentiation of rat hepatoma cells, various parameters of growth and of liver-specific and housekeeping functions were compared in cells grown in serum-free, hormonally supplemented, or serum-supplemented medium and on either tissue culture plastic or type I collagen gels. The substratum was found to be the primary determinant of attachment and survival of the cells. Even in serum-free media, the cells showed attachment and survival efficiencies of 40-50% at low seeding densities and even higher efficiencies at high seeding densities when the cells were plated onto collagenous substrata. However, optimal attachment and survival efficiencies of the cells on collagenous substrata still required either serum or hormonal supplements. On tissue culture plastic, there was no survival of the cells at any seeding density without either serum or hormonal supplements added to the medium. A defined medium designed for cells plated on tissue culture plastic, containing increased levels of hormones plus additional factors over those in the defined medium designed for cells on collagenous substrata, was found to permit attachment and survival of the cells plated into serum-free medium and onto tissue culture plastic. Growth of the cells was influenced by both substrata and hormones. When plated onto collagen gel substrata as compared with tissue culture plastic, the cells required fewer hormones and growth factors in the serum-free, hormone-supplemented media to achieve optimal growth rates. Growth rates of the cells at low and high seeding densities were equivalent in the hormonally and serum-supplemented media as long as comparisons were made on the same substratum and the hormonally supplemented medium used was the one designed for that substratum. For a given medium, either serum or hormonally supplemented, the saturation densities were highest for tissue culture plastic as compared with collagen gels.(ABSTRACT TRUNCATED AT 400 WORDS)

Two hepatic cytoplasmic protein fractions, Y and Z, and their possible role in the hepatic uptake of bilirubin, sulfobromophthalein, and other anions.

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.

Two distinct mechanisms for bilirubin glucuronide transport by rat bile canalicular membrane vesicles. Demonstration of defective ATP-dependent transport in rats (TR-) with inherited conjugated hyperbilirubinemia.

Bilirubin is conjugated with glucuronic acid in hepatocytes and subsequently secreted in bile. The major conjugate is bilirubin diglucuronide. Using sealed vesicles which are primarily derived from the canalicular (CMV) and sinusoidal (SMV) membrane vesicle domains of the plasma membrane of hepatocytes, we demonstrated that bilirubin glucuronides are transported by CMV by both ATP- and membrane potential-dependent transport systems. In CMV from normal rats, these processes are additive. In CMV from TR- rats, which have an autosomal recessively inherited defect in biliary secretion of nonbile acid organic anions, ATP-dependent transport of bilirubin diglucuronide was absent whereas the membrane potential driven system was retained. Other canalicular ATP-dependent transport systems, which were previously described for organic cations and bile acids, are functionally retained in TR- rats. Our study indicates that bilirubin glucuronides are primarily secreted into the bile canaliculus by an ATP-dependent mechanism which is defective in an animal model of the human Dubin-Johnson syndrome.

Studies of Y and Z, two hepatic cytoplasmic organic anion-binding proteins: effect of drugs, chemicals, hormones, and cholestasis.

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.

Circular dichroism analysis of the secondary structure of Z protein and its complexes with bilirubin and other organic anions.

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 role of thiols in ATP-dependent transport of S-(2,4-dinitrophenyl)glutathione by rat liver plasma membrane vesicles.

The effect of thiol/disulfide exchange on ATP-dependent S-(2,4-dinitrophenyl)glutathione (GS-DNP) transport was studied in sodium nitrate treated rat liver plasma membrane vesicles. Transport followed Michaelis-Menten kinetics with an apparent Km of 9.6 microM for GS-DNP and 124 microM for ATP. 5,5'-Dithiobis(2-nitrobenzoate) (DTNB) and N-ethylmaleimide (NEM) efficiently inactivated GS-DNP transport activity in a dose- and time-dependent manner. Half-maximal inactivation occurred in 10 min at 40 microM for DTNB and 550 microM for NEM. Inactivation by DTNB was reversed by dithiothreitol. S-(N-Ethyl)maleimyl glutathione and/or ATP-Mg2+, but neither S-(N-ethyl)maleimyl cysteinylglycine nor oxidized glutathione could protect transport activity from inactivation by NEM or cystamine. These results suggest that reactive thiols are located near the active site of the transporter and that S-alkyl and the gamma-glutamyl residues of glutathione are important for protection. Biological disulfides which were tested included cystine, oxidized glutathione, oxidized Coenzyme-A, oxidized lipoic acid, and oxidized lipoamide; cystamine was the most potent reversible inactivator. Molecular oxygen also inactivated transport activity, which was recovered on addition of dithiothreitol, suggesting intramolecular disulfide formation by vicinal thiols. We interpret these results to indicate that the ATP-dependent GS-DNP transporter contains two or more thiols which are necessary for the maintenance of transport activity. The reversible inactivation of the activity by biological disulfides suggests that the transporter may be regulated by thiol/disulfide exchange in vivo.

Benzquinamide inhibits P-glycoprotein mediated drug efflux and potentiates anticancer agent cytotoxicity in multidrug resistant cells.

We have previously shown that efflux of cytotoxic drugs from multidrug resistant (MDR) cell lines can be quantitated at the single cell level using a sensitive fluorescence microscopy technique. Based on the structure of compounds which inhibited the efflux of Rhodamine-123 (Rho-123) using this methodology, we hypothesized that the antiemetic, antihistaminic agent benzquinamide (BZQ) would interfere with P-glycoprotein (P-gp) mediated drug transport and potentiate the effects of anticancer agents in MDR cell lines. We show that BZQ interferes with P-gp mediated drug efflux and increases drug accumulation in MDR cells using Rho-123 as a fluorescent probe. BZQ increases the cytotoxicity of chemotherapeutic agents to both human and hamster MDR cell lines in vitro. A slight increase in cytotoxicity to chemotherapeutic agents is also observed in the parental cell lines with BZQ. BZQ increases [3H]daunorubicin accumulation and inhibits the binding of [125I]iodoaryl azidoprazosin to the P-gp in MDR cells. BZQ is a new agent to increase the cytotoxic effects of anticancer agents in MDR cells and may ultimately prove useful as an adjunct in cancer chemotherapy.

The function of Gp170, the multidrug resistance gene product, in rat liver canalicular membrane vesicles.

Gp170 (also known as P-glycoprotein) is a transmembrane glycoprotein which is overexpressed in multidrug-resistant tumor cells and is also found in the apical plasma membrane domain of several normal human and animal tissues. Gp170 has been postulated to function as an energy-dependent efflux pump for cytotoxic drugs. In rat liver, Gp170 is restricted to the bile canalicular domain of the plasma membrane. Canalicular membrane vesicles (CMV), but not sinusoidal membrane vesicles, contained a approximately 160-kDa protein which reacts with anti-Gp170 monoclonal antibody and manifest ATP-dependent [3H]daunomycin transport which is temperature dependent, osmotically sensitive, and saturable. Among several nucleotides, ATP was a potent stimulator of transport whereas non- or slowly hydrolyzable analogues (adenosin-5-O-(3-thiotriphosphate, adenyl-5-yl-imidodiphosphate) were ineffective. ATP-dependent daunomycin transport was inhibited by cytotoxic drugs (vinblastine, vincristine, and adriamycin) and other drugs, such as verapamil and quinidine, which restore anti-cancer drug sensitivity in resistant cells. Inside-out CMV were separated from right side-out CMV by antibody-induced affinity density perturbation. Only inside-out CMV manifested ATP-dependent daunomycin transport. These results suggest that Gp170 is an ATP-dependent efflux pump which is responsible for the undirectional, energy-dependent transport of daunomycin and other drugs by rat liver into the bile.

Regulation and translocation of ATP-dependent apical membrane proteins in rat liver.

The bile canalicular membrane contains four specific ATP-dependent transport processes that are involved in secretion of bile acids, non-bile acid organic anions (mrp1), phospholipids (mdr2), and organic cations (mdr3). The aim of this study was to determine how the canalicular presence of these transport proteins is regulated. Canalicular membrane vesicles (CMV) were prepared from livers of rats treated with taurocholate (TC) and/or dibutyryl-adenosine 3',5'-cycle monophosphate (DBcAMP) with and without pretreatment with colchicine. Transport studies were performed with radiolabeled substrates. Changes in the relative amounts of transport proteins were determined by Western blots. Compared with controls, the specific activity of each of the transport processes was enhanced 1.5- and 3-fold with TC and DBcAMP treatments, respectively. Western blots revealed the same increases with mdr2 and mdr3. Pretreatment of rats with colchicine prevented these responses fully with TC treatment, whereas only partial prevention was obtained with DBcAMP treatment. Besides the ATP-dependent transporters, the relative specific activities of the canalicular membrane ectoenzyme markers, leucine aminopeptidase and gamma-glutamyltranspeptidase, were also affected the same way. These results suggest that TC and DBcAMP increase the specific activity of the canalicular ATP-dependent transport proteins and some canalicular membrane ectoenzymes by stimulating an increase in the relative amounts of these proteins in the membrane.

ATP-dependent organic anion transport system in normal and TR- rat liver canalicular membranes.

Many non-bile acid organic anions enter hepatocytes, where they are converted into polar conjugates and preferentially secreted into bile. ATP-dependent and membrane potential-dependent transport of non-bile acid organic anions across the plasma membrane of rat hepatocytes was studied in canalicular (CMV) or sinusoidal membrane vesicles (SMV) using sulfobromophthalein (BSP), a representative non-bile acid organic anion. Temperature-dependent, osmotically sensitive, saturable, ATP-dependent, and unidirectional (inside to outside) BSP transport was observed in CMV but not in SMV. Apparent Michaelis constants for ATP and BSP were 0.26 mM and 31 microM, respectively. ATP-dependent BSP transport was inhibited by oxidized glutathione, dinitrophenyl-glutathione (GSDNP), BSP glutathione, and bilirubin diglucuronide but not by daunomycin, taurocholate, and reduced glutathione. Inhibition by GSDNP and bilirubin diglucuronide was competitive, with apparent inhibitor constants of 41 and 4.2 microM, respectively. CMV from normal rats demonstrated ATP-dependent and membrane potential-dependent BSP transport that was additive. CMV from TR- rats lacked ATP-dependent BSP transport but retained membrane potential-dependent transport of non-bile acid organic anions as well as ATP-dependent transport mechanisms for organic cations (i.e., daunomycin) and bile acids (i.e., taurocholate).

Ectonucleotidases, purine nucleoside transporter, and function of the bile canalicular plasma membrane of the hepatocyte.

Ectonucleotidases are enzymes that degrade extracellular nucleotides. Extracellular nucleotides (especially ATP) and their degradation products (particularly adenosine) have multiple effects on cell functions by acting through purinergic receptors. Adenosine nucleotides are present in bile, which suggests that hepatocytes may release nucleotides into the canaliculus where they are promptly degraded into adenosine by ecto-ATPase and 5'-nucleotidase, which have been identified in the canalicular plasma membrane. Adenosine is then transported into hepatocytes by a Na+-dependent nucleoside transporter that is present in the canalicular plasma membrane. Purification and molecular cloning of ecto-ATPase and other canalicular proteins are complicated by an abundant canalicular plasma membrane protein, cCAM 105. However, the recent cloning of an ecto-ATPase (apyrase) from potato tubers provides a new opportunity to identify the canalicular ecto-ATPase. The canalicular Na+-dependent purine nucleoside transporter has been cloned from rat liver. Study of its expression during development and other physiological circumstances suggests that the transporter may play an important role in maintaining hepatic purine levels that are essential for the liver to serve as a major source of purines for tissues (i.e., brain, muscle) that lack pathways for de novo purine biosynthesis.

ATP-dependent phosphatidylcholine translocation in rat liver canalicular plasma membrane vesicles.

Phosphatidylcholine (PC) translocation was studied in rat liver canalicular plasma membrane vesicles using a fluorescent PC analogue that permitted the quantitation of asymmetric PC distribution in the outer and inner leaflet of the vesicles. PC translocation to the outer leaflet of the canalicular membrane was stimulated by ATP and an ATP-regenerating system in a time and temperature-dependent manner resulting in 200 pmol PC translocated/mg protein per 30 min. A non-hydrolyzable ATP analogue did not support translocation. Translocating activity was observed with PC but not with phosphatidylethanolamine and was specific for inside-out oriented canalicular membrane vesicles. Addition of taurocholate (10 microM), a micelle-forming bile acid, enhanced ATP-dependent PC translocation 1.5 +/- 0.1-fold, whereas addition of taurodehydrocholate (10 microM), a non-micelle-forming bile acid, did not. These results indicate the presence of an ATP-dependent transporter that "flips" phosphatidylcholine from the inner to the outer leaflet of the rat bile canalicular plasma membrane from where it can become associated with bile acids in the canalicular lumen, thereby enhancing ATP-dependent flipping activity. Several lines of evidence suggest that the transporter is Mdr2 P-glycoprotein.

Ligandin retains and albumin loses bilirubin binding capacity in liver cytosol.

Circular dichroism methods were used to detect bilirubin-ligandin interactions in rat liver cytosol and fractions obtained at various stages during purification of ligandin. Ligandin retained its capacity to bind bilirubin in the presence of components of liver supernatant, but albumin, which binds bilirubin in serum, lost the capacity to bind bilirubin in liver supernatant. This was attributed to a greater binding specificity exhibited by ligandin. In their respective physiological milieus, albumin and ligandin are structurally adapted to bind ligands: albumin in serum, and ligandin in the cytosol of the liver cell. These studies are consistent with the hypothesis that the concentration of ligandin within the liver could regulate the net flux of certain organic anions from plasma into the liver.

Rat liver canalicular membrane vesicles contain an ATP-dependent bile acid transport system.

The secretion of bile by the liver is primarily determined by the ability of the hepatocyte to transport bile acids into the bile canaliculus. A carrier-mediated process for the transport of taurocholate, the major bile acid in humans and rats, was previously demonstrated in canalicular membrane vesicles from rat liver. This process is driven by an outside-positive membrane potential that is, however, insufficient to explain the large bile acid concentration gradient between the hepatocyte and bile. In this study, we describe an ATP-dependent transport system for taurocholate in inside-out canalicular membrane vesicles from rat liver. The transport system is saturable, temperature-dependent, osmotically sensitive, specifically requires ATP, and does not function in sinusoidal membrane vesicles and right side-out canalicular membrane vesicles. Transport was inhibited by other bile acids but not by substrates for the previously demonstrated ATP-dependent canalicular transport systems for organic cations or nonbile acid organic anions. Defects in ATP-dependent canalicular transport of bile acids may contribute to reduced bile secretion (cholestasis) in various developmental, inheritable, and acquired disorders.

Extracellular ATP, intracellular calcium and canalicular contraction in rat hepatocyte doublets.

Bile-canaliculus contraction in rat hepatocyte doublets is postulated to involve activation of an actin-myosin system. We examined this hypothesis by determining the relationship between canalicular contraction and cystolic free Ca2+ ([Ca2+]i) concentration after extracellular addition of ATP or microdialysis of myosin light chain kinase or its Ca(2+)-independent fragment, which retains catalytic activity. After incubation of doublets with 200 mumol/L ATP in the absence of extracellular Ca2+, [Ca2+]i peaked at 40 sec and 71% of canaliculi contracted within 4 min. Decreasing effects were observed with equimolar ADP, AMP and nonhydrolyzable ATP, but no effect was observed with adenosine. The effect of extracellular ATP on [Ca2+]i and canalicular contraction was dose dependent. Addition of extracellular Ca2+ and ATP resulted in a plateau level of [Ca2+]i. Cytochalasin D, which depolymerizes actin filaments, inhibited ATP-induced canalicular contraction, but not the increase in [Ca2+]i. Microdialysis of myosin light chain kinase and its Ca(2+)-independent fragment (but not the heat-denatured fragment, albumin, trypsin plus soybean inhibitor or buffer) into one hepatocyte of a doublet resulted in canalicular contraction in 86% of doublets. Injection of myosin light chain kinase or its Ca(2+)-independent fragment did not increase [Ca2+]i within 5 min. These results indicate that (a) the basolateral plasma membrane of hepatocytes has a P2Y-class purinoceptor, (b) increased [Ca2+]i after incubation with ATP is initially due to mobilization from internal sites and (c) canalicular contraction is directly related to [Ca2+]i and activation of an actin-myosin system. The physiological role of extracellular ATP in canalicular contraction is uncertain.