Evidence for carrier-mediated transport of unconjugated bilirubin across plasma membrane vesicles from human placental trophoblast.

Unconjugated bilirubin (UCB) is currently believed to cross the placenta only by passive diffusion. To assess whether carrier-mediated transport might be involved, the uptake of [(3)H]-UCB by basal (bTPM) and apical (aTPM) plasma membrane vesicles from human placental trophoblast at term was investigated. In both types of vesicles, the uptake of [(3)H]-UCB into an osmotically sensitive space was temperature-dependent, independent of the presence of Na(+), and not affected by changes in membrane potential. The uptake of [(3)H]-UCB by aTPM, but not bTPM, was activated by ATP hydrolysis and inhibited by vanadate. Thus, the exact contribution of both inside out and right-side out bTPM to UCB uptake could not be distinguished, while only inverted aTPM were expected to carry out ATP-dependent UCB uptake. In bTPM and aTPM, uptake of free (unbound) [(3)H]-UCB (B(f)) consisted of a dominant, saturable, presumably carrier-mediated process and a diffusional component that became predominant only at B(f) near or above aqueous solubility limit for UCB (70 nM ). For bTPM, K(m)=7.2 nM; V(max)=9.8 pmol/20s/mg protein; and diffusion coefficient (K(D))=0.14 ml/20s/mg protein. For aTPM in the presence of 9.5m M ATP, K(m)=18 n M; V(max)=131 pmol/20s/mg protein; and K(D)=0.47 ml/20s/mg protein. The uptake of [(3)H]-UCB by bTPM was cis-inhibited by estrone-3-sulfate and estradiol-17 beta-glucuronide and trans-stimulated by unlabelled UCB and bromosulphopthalein. ATP-dependent UCB uptake by aTPM was cis-inhibited by doxorubicin, cholic acid, methotrexate and pronenecid. These findings suggest the presence of distinct transporters in the two domains of human placental trophoblast that could cooperate to transfer UCB from the foetus to the maternal circulation.

The products of YCF1 and YLL015w (BPT1) cooperate for the ATP-dependent vacuolar transport of unconjugated bilirubin in Saccharomyces cerevisiae.

Since bilirubin-like pigments are present in the environment as degradation products of heme-containing proteins, yeast could have developed a detoxifying system to transport these compounds into their vacuoles. Vacuoles from Saccharomyces cerevisiae showed an ATP-dependent, saturative transport of unconjugated bilirubin (UCB) that was reduced by 60% and 40% in YCF1 and YLL015w-deleted cells, respectively; the double deletant showed no UCB uptake. Conversely, the transport of bile acids (taurocholate) was comparable in wild and deleted stains. These data identify YCF1 and YLL015w, named BPT1 (Bile Pigment Transporter), as the genes responsible for ATP-dependent UCB transport in yeast. Since YCF1 and YLL015w are rather homologous with multidrug resistant proteins (MRPs), they also suggest the involvement of this class of transporters in the ATP-dependent transport of unconjugated bilirubin.

ATP-dependent transport of unconjugated bilirubin by rat liver canalicular plasma membrane vesicles.

The transport of highly purified 3H-labelled unconjugated bilirubin (UCB) was investigated in rat liver plasma membrane vesicles enriched in the canalicular domain and found to be stimulated (more than 5-fold) by the addition of ATP. Other nucleotides, such as AMP, ADP, GTP and a non-hydrolysable ATP analogue (adenosine 5'-[alpha, beta-methylene] triphosphate), did not stimulate [3H]UCB transport, indicating that ATP hydrolysis was necessary for the stimulatory effect. [3H]UCB uptake occurred into an osmotically sensitive space. At an unbound bilirubin concentration ([Bf]) below saturation of the aqueous phase (no more than 70 nM UCB), the ATP-dependent transport followed saturation kinetics with respect to [Bf], with a Km of 26+/-8 nM and a Vmax of 117+/-11 pmol per 15 s per mg of protein. Unlabelled UCB inhibited the uptake of [3H]UCB, indicating that UCB was the transported species. Inhibitors of ATPase activity such as vanadate or diethyl pyrocarbonate decreased the ATP effect (59+/-11% and 100% respectively). Daunomycin, a known substrate for multidrug resistance protein-1, and taurocholate did not inhibit the ATP-dependent [3H]UCB transport, suggesting that neither mdr-1 nor the canalicular bile acid transporter is involved in the canalicular transport of UCB. [3H]UCB uptake (both with and without ATP) in canalicular vesicles obtained from TR- rats was comparable to that in vesicles obtained from Wistar rats, indicating that the canalicular multispecific organic anion transporter, cMOAT, does not account for UCB transport. These results indicate that UCB is transported across the canalicular membrane of the liver cell by an ATP-dependent mechanism involving an as yet unidentified transporter.

APF/CBP, the small, amphipathic, anionic protein(s) in bile and gallstones, consists of lipid-binding and calcium-binding forms.

Two very similar small anionic, amphipathic proteins, a phospholipid-binding apoprotein (anionic polypeptide fraction [APF]) and a calcium-binding polypeptide (CBP), are found abundantly in bile and all types of gallstones. The often disparate properties among various preparations of APF/CBP could reflect different sources and separation procedures, leading to partly degraded and/or denatured protein and varied association of bile salts, lipids, bile pigments, and detergents. The present study presents new methods for isolation and purification of APF/CBP, and characterizes the preparations thus obtained. It was found that isolation by selective precipitation of proteins from fresh T-tube bile by added calcium chloride, followed by demineralization with ethylenediaminetetraacetic acid (EDTA), removal of salts, lipids, and some pigment by Sephadex LH-20, and serial ultrafiltration yields the purest preparations. Though free of lipids, bile salts, detergents, and most pigments, these new preparations all show the same 7-kd and 12-kd bands on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), the same major peaks on hydrophobic high-performance liquid chromatography (HPLC), and retain the self-associative, lipid- and calcium-binding functions, typical of older preparations obtained by potentially denaturative procedures. The varied properties among APF/CBP preparations are thus apparently related mainly to their content of different proportions of two major components, lipid-binding APF and calcium-binding CBP. Immunologic cross-reactions indicate common epitopes, and amino acid analyses are also similar, suggesting that APF and CBP may have the same polypeptide backbone, but differ because of posttranslational modification(s). Sufficiently pure APF and CBP have now been obtained to permit possible structural identification by sequencing and molecular biological techniques, though such attempts have thus far been unsuccessful.

New concepts in bilirubin and jaundice: report of the Third International Bilirubin Workshop, April 6-8, 1995, Trieste, Italy.

The workshop covered three major areas: Unconjugated bilirubin (UCB) chemistry and physical chemistry; UCB transport and intracellular trafficking; and evaluation and therapy of neonatal and congenital hyperbilirubinemias. Findings of studies in the chemistry and physical chemistry area were as follows. (1) Nuclear magnetic resonance (NMR) studies of highly enriched 13COOH mesobilirubin in water-dimethyl sulfoxide systems indicated that the pKa values of the carboxyl groups are 4.2 and 4.9, respectively. This finding differs from some reports that suggest that the two pKa values in aqueous systems are near or above pH 7.0. (2) Contrasting views of the hydrophobic interactions of UCB with bile salts were presented: one suggested that multiple bile salt monomers bind to one UCB molecule; the other suggested that UCB binds to the nonpolar surface of helical bile salt micelles. (3) Structures were proposed for the varied calcium and copper bilirubinate salts formed at various pH values and cation/UCB ratios. (4) Studies of binding of UCB to human serum albumin (HSA) showed marked diminution of UCB-binding affinity as albumin and chloride concentrations increased. (5) A unique UCB derivative, bilirubin-C10-sulfonic acid, was identified as the major bile pigment in bullfrog bile. (6) New methods were presented for removal of impurities from preparations of bile salts and UCB. Findings of studies in the transport area were as follows. (1) Four putative basolateral and two putative canalicular hepatocytic transporters of UCB and related organic anions were described. Special emphasis was given to the adenosine triphosphate (ATP)-dependent canalicular multi-specific organic anion transporter that is defective in three strains of mutant rats with congenital conjugated hyperbilirubinemia. (2) The roles of the classical and newer molecular biological approaches to identification of these transporters were contrasted, and their limitations were discussed. (3) The relative roles of the multiple carriers in UCB transport under different conditions and substrate concentrations were discussed. (4) Cytosolic UCB-binding proteins (e.g., ligandin) were shown to promote transcellular movement of UCB by solubilizing and transporting the pigment in the aqueous phase while limiting binding of UCB to the relatively immobile membranes of cell organelles. (5) Mechanisms were presented for translocation of UDP-glucuronic acid (UDPGA) into the lumenal location of UDPGA transferase in the endoplasmic reticulum, as well as the enhancement of this process by N-acetyl-glucosamine. Studies in the neonatal and congenital jaundice area were as follows. (1) Criteria were reviewed for initiating treatment of neonatal jaundice, emphasizing the primacy of serum bilirubin levels, gestational age, and hemolysis as risk factors for kernicterus. (2) New methods were presented for frequent, automated monitoring of serum bilirubin levels and breath CO levels as an index of rates of formation of UCB from heme. (3) The current status and limitations of new approaches to treatment of severe unconjugated hyperbilirubinemia were discussed: hepatocyte transplantation and gene therapy, still in the stage of development in animal models, have provided only partial and temporary relief of hyperbilirubinemia; extracorporeal liver assist devices have had some success in initial human studies; and inhibition of heme oxygenase (HO) with metalloporphyrins, especially tin mesoporphyrin, which markedly decreases bilirubin production for prolonged periods, is a new alternative to phototherapy. (4) The ontogeny of the two HO isozymes was contrasted in the liver, spleen, kidney, and lung.

Epitope mapping of the human biliary amphipathic, anionic polypeptide: similarity with a calcium-binding protein isolated from gallstones and bile, and immunologic cross-reactivity with apolipoprotein A-I.

Biliary amphipathic anionic polypeptide (APF) the major protein of the pigment-lipoprotein complex in bile, and calcium-binding protein (CBP) from gallstones are both small (less than 10 kDa), highly acidic, amphipathic proteins present in bile and closely associated also with pigmented areas in human gallstones. Polyclonal antibodies against APF have shown cross-reactivity with plasma high density lipoproteins (HDL). This study examines the hypothesis that APF and CBP might be closely related or even identical, and might also share common epitopes with the larger apoA-I (23 kDa). To assess this, immunoreactivity of the three delipidated, highly purified proteins was determined against a panel of 12 monoclonal antibodies (MAbs) prepared against APF and a panel of 4 MAbs against apoA-I. APF was isolated from bile by zonal ultracentrifugation. CBP was isolated from proteins precipitated from bile by CaCl2, as well as from the calcium bilirubinate shells of cholesterol gallstones, by extraction successively with methyl-t-butyl ether, methanol, and Na2EDTA, followed by Sephadex G-25 chromatography and two-stage preparative SDS-PAGE. ApoA-I was prepared by two types of chromatography: Sephacryl S200 chromatography and heparin-chromatographic immunoaffinity. Specific polyclonal antibodies to APF and apoA-I were prepared from immunized rabbits. MAbs to APF and apoA-I were prepared by immunization of mice, using standard hybridoma technique. Western blotting of APF and CBP in 15% SDS-PAGE yielded one band with an apparent molecular weight of 6.5 kDa, which, along with apoA-I, was immunostained by polyclonal antibodies to APF and apoA-I. Using 12 MAbs against APF with three types of ELISA (direct antigen binding, competitive antigen displacement, and epitope competition between antibodies), it was shown that APF and delipidated apoA-I shared six epitopes, three of which were detected also on the surface of intact HDL particles. Six other epitopes were present in APF but not apoA-I, four of which were exposed on the surface of HDL. Four MAbs against apoA-I reacted with APF and CBP. Amino acid analyses of APF and CBP were similar with 20-23% acidic and 7-11% basic amino acids and low contents of cysteine, methionine, and tyrosine; both differed from apoA-I in containing isoleucine and cysteine. Using ELISA and one MAb (no. 32) against APF, this polypeptide was detected in human plasma HDL, the pigment-lipoprotein complex in the bile of humans, dogs, and rats, and in both pigment and cholesterol gallstones.(ABSTRACT TRUNCATED AT 400 WORDS)

Natural history and significance of esophageal squamous cell dysplasia.

Balloon-mesh cytologic screening for esophageal cancer done in 255 asymptomatic high-risk United States veterans (age greater than 40 years, ethanol abuse for greater than 20 years, and cigarette smoking greater than 20 pack years) identified 37 patients with squamous cell dysplasia. Of the 37 patients with dysplasia, 28 were re-evaluated prospectively at 6-month intervals for up to 36 months by balloon-mesh cytology, esophagoscopy with vital staining and biopsies, chest radiographs, oropharyngeal examination, and indirect laryngoscopy. During prospective follow-up evaluation, cytology specimens were repetitively normal in 16 patients (57%), showed inflammatory changes in eight patients (29%), persisted as dysplasia in two patients (7%) (both had endoscopic and histologic evidence of esophagitis), and progressed to carcinoma in two patients (7%) (one esophageal, one laryngeal). Although histologic findings concurred with the resolution of dysplasia, biopsy specimens were characterized by a similar difficulty in distinguishing dysplasia from inflammation. Erroneous histologic diagnoses of carcinoma in situ were made in two patients with reflux esophagitis evident endoscopically and confirmed during the course of a 24-36 month follow-up period. The authors conclude that squamous cell dysplasia detected by balloon-mesh cytology is seldom a precursor of esophageal cancer in the high-risk U.S. population but, rather, is often related to esophagitis. Thus, balloon-mesh cytology has limited use as a screening method for the early detection of esophageal cancer in the United States.

Reliability of balloon-mesh cytology in detecting esophageal carcinoma in a population of US veterans.

A catheter, equipped with a terminal balloon covered with nylon mesh, was developed to study the reliability of abrasive cytology for the diagnosis of esophageal carcinoma. Eighty-seven balloon cytology analyses were attempted in 82 subjects. Four patients were unable to swallow the balloon. In the 78 successful attempts, the initial diagnoses were: esophagitis (34) and esophageal carcinoma (13), established by endoscopic examination and histologic sampling; and normal esophagus (31) confirmed histologically in 25. The remaining 6 controls were younger than 40 years old, without any significant history of smoking, drinking and esophageal symptoms. For esophageal carcinoma, the sensitivity of balloon cytology was 91% and the specificity was 94% with four false-positives. Balloon cytology was generally well-tolerated and easily performed. This method is now being tested for screening high-risk patients for esophageal carcinoma.

New concepts in phototherapy: photoisomerization of bilirubin IX alpha and potential toxic effects of light.

New information is summarized, indicating that configurational photoisomerization of bilirubin at the 5 and 15 carbon bridges is the major mechanism of bilirubin photocatabolism in vivo, and that singlet oxygen photooxidation plays only a minor role. The literature is reviewed concerning potentially damaging photodynamic reactions that are observed in vitro with vitamins, proteins, lipids, and nucleic acids, and their possible relationships to the limited number of toxic side-effects that have been detected with clinical phototherapy of neonatal jaundice. Secondary toxic effects, mediated by bilirubin photoderivatives or by retina-neuroendocrine pathways are also considered. Areas requiring further investigations are delineated.

Pigment gallstones.

Pigment gallstones are defined as any dark brown-to-black stone, consisting of calcium salts of bilirubin, phosphate, carbonate and other anions, and can be separated into carbonate- and noncarbonate-containing groups. Pigment stones predominate in the rural Orient, in cirrhosis, and in elderly United States patients undergoing cholecystectomy. Clinical associations include bile duct obstruction, stasis, and possibly hemolysis. Of pigment stones, 50% are radioopaque and account for two-thirds of all opaque stones. The concentrations of bile salts, phospholipids,, cholesterol, and total bilirubin in bile are similar to normal levels, but the concentration of unconjugated bilirubin is increased in the bile of some patients. Increased unconjugated bilirubin in bile may be caused by increased hydrolysis of excreted conjugated bilirubin. Unconjugated bilirubin is solubilized by bile salts, but the interaction is primarily nonmicellar. Ionized calcium and pH are important determinants of solubility. Sulfated glycoproteins, excreted in increased amounts in patients with cholelithiasis, may be the site of pigment stone precipitation because these compounds bind calcium salts tightly. E coli is frequently cultured from pigment stones in Japan but not in the United States; thus, bacterial beta-glucuronidase may be important in stone formation in Japan but probably not in the West. Stasis leads to increased calcium secretion and to increases in the concentration of sparingly soluble compounds that may then precipitate. Incomplete emptying of the gallbladder may result in the same concentration process. Unsaturated fats and chronic vagal stimulation cause pigment stone formation in animals. At present, surgery is the only treatment for pigment lithiasis.