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Biomedical subjects

J L Boyer

Publications and source records attributed to J L Boyer.

At least 91 records · Page 5Linked to original sources
PubMed1997

The rat canalicular conjugate export pump (Mrp2) is down-regulated in intrahepatic and obstructive cholestasis.

BACKGROUND & AIMS: The excretion of various organic anions into bile is mediated by an adenosine triphosphate-dependent conjugate export pump, which has been identified as the canalicular isoform of the multidrug resistance protein (Mrp2). Mrp2 function is impaired in various experimental models of intrahepatic and obstructive cholestasis, but the underlying molecular mechanisms are unclear. The aim of this study was to investigate these molecular mechanisms. METHODS: The effects of endotoxin, ethinylestradiol, and common bile duct ligation (CBDL) on Mrp2 protein, messenger RNA (mRNA) expression, and Mrp2 tissue localization were determined in rat livers by Northern blotting, Western analysis, and tissue immunofluorescence. To assess whether changes were specific for Mrp2, we also examined the expression of canalicular ecto-adenosine triphosphatase (ecto-ATPase) and mdr P-glycoproteins (P-gp). RESULTS: All three cholestatic models resulted in a marked decrease in Mrp2 protein (P < 0.01) and its tissue localization at the canalicular membrane. Mrp2 mRNA levels diminished profoundly after endotoxin (P < 0.0005) and CBDL (P < 0.05), but did not change after ethinylestradiol. In contrast to Mrp2, protein expression of ecto-ATPase and P-gp remained unchanged in endotoxin- and ethinylestradiol-treated animals, whereas P-gp levels increased after CBDL (P < 0.05). CONCLUSIONS: Down-regulation of Mrp2 expression may explain impaired biliary excretion of amphiphilic anionic conjugates in these models of cholestasis.

ATP-Binding Cassette Transporters↗
PubMed1997

Bombesin stimulates bicarbonate secretion from rat cholangiocytes: implications for neural regulation of bile secretion.

BACKGROUND & AIMS: Bombesin is a neuropeptide with many biological functions and is known to stimulate bile secretion. The aim of this study was to determine the role of bombesin in bile secretion and its site of action. METHODS: The effects of bombesin on bile secretion were examined using isolated perfused rat livers, hepatocyte couplets, and isolated bile duct units (IBDU) from rat liver. RESULTS: Bombesin (100 nmol/L) increased bile pH, bicarbonate concentration, and output in isolated perfused rat livers from both normal and 2-week bile duct-ligated rats, although bile flow increased only in the latter model. Bombesin (10-100 nmol/L) also had no effect on canalicular bile secretion in isolated hepatocyte couplets. However, bombesin produced a dose-dependent increase in secretion in IBDU, which was inhibited almost completely by a specific bombesin receptor inhibitor, [Tyr4, D-Phe12]-bombesin (1 micromol/L). This bombesin (10 nmol/L)-stimulated secretion in IBDU was accompanied by an increase in luminal pH and was dependent on bicarbonate and chloride in the medium. Somatostatin but not substance P inhibited the bombesin response. CONCLUSIONS: Neuropeptides such as bombesin can directly stimulate fluid and bicarbonate secretion at the level of cholangiocytes, suggesting that neuropeptides play an important regulatory role in biliary transport and secretion.

Animals↗
PubMed1997

Isolated hepatocyte couplets and bile duct units--novel preparations for the in vitro study of bile secretory function.

Studies of the mechanisms of bile formation have been facilitated by the development of in-vitro models that enable the transport functions of hepatocytes and bile duct epithelial cells to be analyzed in isolation. Isolated hepatocyte couplets represent a primary bile secretory unit that retain secretory polarity. Isolated bile duct units are intact polarized segments of interlobular bile duct segments. Both preparations can be maintained in short term culture, respond appropriately to choleretic agonists and facilitate a variety of functional applications.

Animals↗
PubMed1997

Fidelity in functional coupling of the rat P2Y1 receptor to phospholipase C.

1. The rat homologue of the P2Y1 receptor has been heterologously expressed in 1321N1 human astrocytoma cells and in C6 rat glioma cells. 2. As has been shown previously for the turkey and human P2Y1 receptors, the rat P2Y1 receptor expressed in either cell type responded to 2MeSATP with increases in inositol phosphate accumulation that were competitively blocked by the antagonist PPADS. Neither of the wild type cell lines exhibited inositol phosphate responses to P2Y1 receptor agonists. 3. Expression of the rat P2Y1 receptor did not confer a capacity of 2MeSATP to inhibit adenylyl cyclase activity in 1321N1 cells. Moreover, the inhibition of adenylyl cyclase mediated by an endogenous P2Y receptor of C6 glioma cells was not enhanced by expression of the rat P2Y1 receptor. The P2Y receptor-mediated inhibition of adenylyl cyclase in C6 glioma cells expressing both the endogenous P2Y receptor and the rat P2Y1 receptor remained unaffected by PPADS. 4. Since the P2Y receptor responsible for inhibition of adenylyl cyclase in C6 glioma cells does not share the pharmacological or functional properties of the P2Y1 receptor, even when both receptors originate from the same species and are simultaneously expressed in the same cell line, it is concluded that the P2Y1 receptor is distinct from an endogenous P2Y receptor in C6 cells that couples to inhibition of adenylyl cyclase.

Adenosine Triphosphate↗
PubMed1997

Molecular cloning and expression of an avian G protein-coupled P2Y receptor.

A family of G protein-coupled P2Y receptors that are activated by adenine and uridine nucleotides has been identified recently. Degenerate primers based on conserved sequences in these P2Y receptors were used to amplify turkey DNA, which was used to isolate the complete coding sequence of a cDNA that encodes a novel G protein-coupled receptor. Stable expression of this avian cDNA in 1321N1 human astrocytoma cells resulted in the conveyance of marked inositol phosphate responses to various nucleotides. Although this cloned avian receptor exhibited its highest homology to the previously cloned mammalian P2Y4 receptor, its pharmacological selectivity was not consistent with the avian receptor's being a species homologue of the P2Y4 receptor. That is, whereas the P2Y4 receptor is selectively activated by UTP and is not activated by ATP or Ap4A, the novel avian receptor was potently activated by ATP and Ap4A as well as by UTP. Taken together, these results describe the identification of an avian phospholipase C-coupled P2Y receptor that, like the mammalian P2Y2 receptor, is activated by both adenine and uridine nucleotides.

Adenine Nucleotides↗
PubMed1997

Role of kinases and phosphatases in the regulation of fluid secretion and Cl-/HCO3- exchange in cholangiocytes.

The role of protein kinase A (PKA), protein kinase C (PKC), and protein phosphatases in the process of secretin stimulation of fluid and bicarbonate secretion from biliary epithelium was examined using a novel isolated bile duct unit (IBDU) model from rat liver. Sp-adenosine 3',5'-cyclic monophosphothiolate (Sp-cAMPS), 100 microM, a PKA-specific agonist, significantly increased secretion during a 30-min perfusion (+61%, P < 0.01). In contrast, preincubation and perfusion of Rp-cAMPS, 100 microM, a specific PKA inhibitor, reduced the ability of secretin to stimulate both fluid secretion (111 vs. 25%; P < 0.01) and Cl-/HCO3- exchanger activity (80 vs. 28%). Neither the PKC agonist phorbol 12-myristate 13-acetate, 10 microM, nor the PKC antagonist staurosporine showed any effect on either basal or secretin-stimulated fluid secretion or Cl-/HCO3- exchange activity in IBDU. Okadaic acid, a specific inhibitor of protein phosphatases 1 and 2A, also had no effect on basal fluid secretion or on the basal activity of the Cl-/HCO3- exchanger. However, okadaic acid resulted in persistence of secretion after removal of secretin, in contrast to the reduction in secretion observed in controls. These findings indicate that PKA but not PKC is involved in the signal transduction of secretin-stimulated fluid secretion and Cl-/HCO3- exchange activity in rat bile duct epithelium, a process inactivated by dephosphorylation by protein phosphatases 1 and/or 2A.

Animals↗
PubMed1997

Enterohepatic circulation of scymnol sulfate in an elasmobranch, the little skate (Raja erinacea).

The sulfated bile alcohol scymnol sulfate (ScyS), 3 alpha,7 alpha,12 alpha,24 xi, 26,27-hexahydroxy-5 beta-cholestane-26(27)-sulfate, is the major bile salt in bile of an elasmobranch, the little skate. To investigate hepatic transport of bile alcohols in skate liver, [3H]ScyS and a potential precursor, 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestane (chtriol), were used as model compounds. Their transport into isolated hepatocytes was partially saturable, temperature sensitive, and Na+ independent. The uptake of ScyS was inhibited by cholyltaurine, and uptake of cholyltaurine was inhibited by ScyS in a competitive manner. In contrast, uptake of chtriol was not inhibited by cholyltaurine, suggesting separate transport systems. ScyS and chtriol showed a choleretic effect in isolated perfused livers. When ScyS was added to the perfusate of isolated perfused livers, > 25% was found in bile within 7 h. When chtriol was added to the perfusate, 10% of the dose was secreted into the bile mainly in the form of polar metabolites, whereas only nonmetabolized chtriol remained in the livers. The slow bile flow of 40-50 microliters/h and the high recovery in the liver suggest that metabolism may be the rate-limiting step in the hepatic elimination of chtriol. The major metabolites secreted into bile were identified by mass spectrometry and chromatography as scymnol and ScyS. To study the enterohepatic circulation, [3H]ScyS or [3H]chtriol was administered into the duodenum of free-swimming skates, and bile was collected through exteriorized indwelling cannulas over a 4-day period. More than 90% of the radioactivity was recovered from bile, indicating that there was a highly effective absorption in the intestinal epithelium, as well as specific transport mechanisms for hepatic uptake and biliary secretion of these compounds. This is the first direct demonstration of an enterohepatic circulation for a bile alcohol sulfate in fish liver.

Animals↗
PubMed1996

Molecular cloning, expression and regulatory activity of G alpha 11- and beta gamma-subunit-stimulated phospholipase C-beta from avian erythrocytes.

A turkey erythrocyte phospholipase C (PLC) has been instrumental in delineating the role of G-proteins in receptor-regulated inositol lipid signalling. This isoenzyme is uniquely regulated both by alpha-subunits of the Gq family and by G-protein beta gamma-subunits. A 4819 bp cDNA encoding this PLC has been cloned from a turkey erythrocyte cDNA library. The open reading frame of this cDNA encodes a 1211-amino-acid protein (calculated molecular mass 139050 Da) that contains amino acid sequences of 16 peptides sequenced from the turkey erythrocyte PLC. The predicted sequence of the turkey PLC shows considerable similarity with the sequences of previously cloned members of the PLC-beta family, with the highest identity (71%) shared with PLC-beta 2 and lesser identities observed with PLC-beta 1 (49%), PLC-beta 3 (46%) and PLC-beta 4 (37%). The largest differences in sequence between the turkey PLC-beta and other PLC-beta isoenzymes occur in the C-terminal domain and in the region between the X- and Y-domains. The turkey isoenzyme and PLC-beta 2, which differ in their regulation by G-protein alpha-subunits, are only 44% similar across the approx. 400 amino acid residues of the C-terminal domain that has been implicated in alpha q activation of these proteins. Recombinant turkey PLC-beta was purified to homogeneity following expression from a recombinant baculovirus in Sf9 insect cells. The immunoreactivity and mobility on SDS/PAGE of the recombinant enzyme were the same as observed with native turkey erythrocyte PLC-beta. Moreover, the catalytic activities of the recombinant enzyme were indistinguishable from those of native turkey erythrocyte PLC-beta in assays carried out in the presence of cholate and Ca2+, or in assays of activity after reconstitution with G alpha 11 or G-protein beta gamma-subunits. The turkey PLC-beta was more sensitive to activation by G alpha 11 than was PLC-beta 2, and was more sensitive to activation by beta gamma-subunits than either PLC-beta 2 or PLC-beta 1.

Amino Acid Sequence↗
PubMed1996

Disruption of cell volume regulation by mercuric chloride is mediated by an increase in sodium permeability and inhibition of an osmolyte channel in skate hepatocytes.

The mechanism by which mercury leads to cell swelling and impairs the normal regulatory volume decrease (RVD) in cells swollen in hypotonic media was examined in hepatocytes isolated from the little skate, Raja erinacea, an osmoconforming marine elasmobranch. Skate hepatocytes treated with 50 microM HgCl2 in isotonic medium swelled to volumes double those of control cells, and this was associated with an increase in Na+ and K+ permeability. The gain in intracellular Na+ exceeded the K+ loss by 0.27 microEq/mg protein, accounting in large part for the observed cell swelling. The effects of mercury were blunted when hepatocytes were incubated in medium in which the Na+ was replaced with K+, and were essentially absent when Na+ was replaced with choline+, indicating an important role of Na+ influx in mediating mercury's effects on cell volume regulation. The inhibition of RVD by mercury was prevented if the metal was administered as a mercaptide with dithiothreitol or glutathione. However, when these chelating agents were added after the mercury, only the membrane permeant dithiothreitol was able to reverse the inhibition of RVD, suggesting an intracellular site of action. Mercuric chloride also produced a concentration-dependent inhibition of the ATP-sensitive volume-regulatory osmolyte channel in skate hepatocytes, as assessed by inhibition of swelling-activated [14C]taurine efflux. [14C]Taurine efflux was inhibited at mercury concentrations (20-40 microM) that had no effect on intracellular ATP levels or ATP/ADP ratios, consistent with a direct interaction with the channel. These findings indicate that mercury impairs cell volume regulation in skate hepatocytes at multiple sites, including the volume-regulatory osmolyte channels, and Na+ and K+ permeability pathways. The combined effects of increased Na+ influx and the inability to extrude organic osmolytes may account for the inhibition of RVD.

Animals↗
PubMed1996

Down-regulation of expression and function of the rat liver Na+/bile acid cotransporter in extrahepatic cholestasis.

BACKGROUND & AIMS: The molecular regulation of hepatic bile acid transporters during cholestasis is largely unknown. Cloning of complementary DNAs for the sinusoidal sodium-dependent taurocholate cotransporting polypeptide (ntcp), the cytosolic bile acid-binding protein 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD), and a putative canalicular bile acid transporter Ca2+, Mg(2+)-ecto-adenosine triphosphatase, now facilitates such studies. METHODS: Protein mass, steady-state messenger RNA (mRNA) levels, and gene transcription were assessed in rat livers after common bile duct ligation (CBDL) from 1-7 days, and taurocholate uptake was determined in isolated hepatocytes. RESULTS: After CBDL, Na(+)-dependent taurocholate uptake (Vmax) declined by 70%. The levels of ntcp protein were reduced by more than 90%, and 3 alpha-HSD levels decreased by 66% by 7 days. Expression and canalicular localization of the ecto-adenosine triphosphatase remained unchanged. mRNA levels for both ntcp and 3 alpha-HSD diminished by about 60% 1 day after CBDL and remained unchanged up to 7 days. Transcriptional activity was decreased 1 day after CBDL only for ntcp. CONCLUSIONS: Extrahepatic cholestasis results in rapid down-regulation of Na(+)-dependent taurocholate uptake, ntcp transcription, and posttranscriptional regulation of both ntcp and 3 alpha-HSD mRNA. This selective decline of ntcp may represent a protective feedback mechanism in cholestasis to diminish uptake of potentially hepatotoxic bile acids.

3-Hydroxysteroid Dehydrogenases↗
PubMed1996

Tauroursodeoxycholic acid activates protein kinase C in isolated rat hepatocytes.

BACKGROUND & AIMS: Ursodeoxycholic acid (UDCA) improves liver function in patients with chronic cholestatic liver diseases by an unknown mechanism. UDCA is conjugated to taurine in vivo, and tauroursodeoxycholic acid (TUDCA) is a potent hepatocellular Ca2+ agonist and stimulates biliary exocytosis and hepatocellular Ca2+ influx, both of which are defective in experimental cholestasis. Protein kinase C (PKC) mediates stimulation of exocytosis in the liver. The aim of this study was to determine the effects of TUDCA on PKC in isolated hepatocytes. METHODS: The effect of TUDCA on the distribution of PKC isoenzymes within the hepatocyte was studied using immunoblotting and immunofluorescence techniques. In addition, the effect of TUDCA on the accummulation of sn-1,2-diacylglycerol (DAG), the intracellular activator of PKC, and hepatocellular PKC activity was studied using radioenzymatic techniques. RESULTS: Immunoblotting studies showed the presence of four isoenzymes (alpha, delta, epsilon, and zeta). The phorbol ester phorbol 12-myristate 13-acetate (1 mumol/L) induced translocation of alpha-PKC, delta-PKC, and epsilon-PKC from cytosol to a particulate membrane fraction, a key step for activation of PKC. TUDCA, but not taurocholic acid, selectively induced translocation of the alpha-PKC isoenzyme from cytosol to the membranes. In addition, TUDCA induced a significant increase in hepatocellular DAG mass and stimulated membrane-associated PKC activity. CONCLUSIONS: TUDCA might stimulate Ca(2+)-dependent hepatocellular exocytosis into bile in part by activation of alpha-PKC.

Animals↗
PubMed1996

Second messenger cascade specificity and pharmacological selectivity of the human P2Y1-purinoceptor.

1. The coding sequence of the P2Y1-purinoceptor was cloned from a human genomic library. 2. The open reading frame encodes a protein of 373 amino acids that is 83% identical to the previously cloned chick and turkey P2Y1-purinoceptor and is > or = 95% homologous to the recently cloned rat, mouse, and bovine P2Y1-purinoceptors. 3. The human P2Y1-purinoceptor was stably expressed in 1321N1 human astrocytoma cells using a retroviral vector. Although the P2Y1-purinoceptor agonist, 2MeSATP, had no effect on inositol phosphate accumulation in cells infected with the P2Y1-purinoceptor virus. No effect of 2MeSATP on cyclic AMP accumulation was observed in P2Y1-receptor-expressing 1321N1 cells. 4. The pharmacological selectively of 18 purinoceptor agonists was established for the expressed human P2Y1-purinoceptor. 2MeSATp was more potent than ATP but less potent than 2MeSADP. ADP also was more potent than ATP. A similar maximal effect was observed with most agonists tested. However, alpha, beta-MeATP had no effect and 3'-NH2-3'-deoxyATP and A2P4 were partial agonists. The order of potency of agonists for activation of the turkey P2Y1-purinoceptor, also stably expressed in 1321N1 cells, was identical to that observed for the human P2Y1-purinoceptor. 5. C6 glioma cells express a P2Y-purinoceptor that inhibits adenylyl cyclase but does not activate phospholipase C. Expression of the human P2Y1-purinoceptor in C6 cells conferred 2MeSATP-stimulated inositol lipid hydrolysis to these cells. The phospholipase C-activating human P2Y1-purinoceptor could be delineated from the endogenous P2Y-purinoceptor of C6 glioma cells by use of the P2-purinoceptor antagonist, PPADS, which blocks the P2Y1-purinoceptor but does not block the endogenous P2Y-purinoceptor of C6 cells. P2-purinoceptor agonists also exhibited differential selectivities for activation of these two P2Y-purinoceptors.

Amino Acid Sequence↗
PubMed1996

Identification of potent P2Y-purinoceptor agonists that are derivatives of adenosine 5'-monophosphate.

1. A series of chain-extended 2-thioether derivatives of adenosine monophosphate were synthesized and tested as agonists for activation of the phospholipase C-linked P2Y-purinoceptor of turkey erythrocyte membranes, the adenylyl cyclase-linked P2Y-purinoceptor of C6 rat glioma cells, and the cloned human P2U-receptor stably expressed in 1321N1 human astrocytoma cells. 2. Although adenosine monophosphate itself was not an agonist in the two P2Y-purinoceptor test systems, eleven different 2-thioether-substituted adenosine monophosphate analogues were full agonists. The most potent of these agonists, 2-hexylthio AMP, exhibited an EC50 value of 0.2 nM for activation of the C6 cell receptor. This potency was 16,000 fold greater than that of ATP and was only 10 fold less than the potency of 2-hexylthio ATP in the same system. 2-hexylthio adenosine was inactive. 3. Monophosphate analogues that were the most potent activators of the C6 cell P2Y-purinoceptor were also the most potent activators of the turkey erythrocyte P2Y-purinoceptor. However, agonists were in general more potent at the C6 cell receptor, and potency differences varied between 10 fold and 300 fold between the two receptors. 4. Although 2-thioether derivatives of adenosine monophosphate were potent P2Y-purinoceptor agonists no effect of these analogues on the human P2U-purinoceptor were observed. 5. These results support the view that a single monophosphate is sufficient and necessary for full agonist activity at P2Y-purinoceptors, and provide insight for strategies for development of novel P2Y-purinoceptor agonists of high potency and selectivity.

Adenosine Monophosphate↗
PubMed1996

Swelling-activated anion conductance in skate hepatocytes: regulation by cell Cl- and ATP.

Cell swelling activates an outwardly rectifying anion conductance in mammalian cells. The channel responsible for this conductance mediates volume-regulatory efflux of organic osmolytes such as taurine. We observed a similar conductance in hepatocytes from the skate Raja erinacea. Whole cell Cl- conductance was increased > 100-fold by a 2-fold increase in hepatocyte volume. The conductance was outwardly rectifying and had a relative cation permeability of approximately 0.2. Cation permeability was increased by reductions in patch pipette CsCl concentration, suggesting that the channel pore contains saturable anion and cation binding sites with different anion and cation affinities. The conductance had a broad anion selectivity and a relative taurine permeability of 0.17. Activation of the conductance required intracellular ATP or a nonhydrolyzable ATP analogue. Elevation of intracellular Cl- from 20 to 155 mM reduced current activation while the rate and extent of cell swelling were unaffected. Reduction of intracellular Cl- concentration to 5-10 mM caused spontaneous current activation without cell swelling. These results suggest that increases in cell Cl- levels increase the volume set point of the channel. We propose that the main function of the outwardly rectifying anion channel is nonselective transport of organic solutes.

Adenosine Triphosphate↗
PubMed1996

Bile duct epithelium: frontiers in transport physiology.

The bile duct epithelium is known to modify hepatic bile by both secretory and absorptive processes. However, identification and characterization of the transport systems that carry out these physiological functions at the cellular and molecular level have been slow compared with progress in understanding hepatic bile production. Recently, techniques have been developed that enable bile duct cells to be isolated in substantial number and purity and as intact polarized units that can be studied in vitro. These newer preparations have enabled classic physiological approaches to be performed directly with the bile duct epithelial tissues. Progress in this important area of biliary physiology is now rapid and forms the basis for this review.

Animals↗
PubMed1996

Active microtubule-dependent secretion of a fluorescent bile salt derivative in skate hepatocyte clusters.

Fluorescence microscopy and video image analysis were used to study the transport of a fluorescent bile acid derivative [N-[7-(4-nitrobenzo-2-oxa-1,3-diazol)]-7 beta-amino-3 alpha, 12 alpha-dihydroxy-5 beta-cholan-24-oyl-2-aminoethanesulfonate (NBD-TC)] in isolated clusters of hepatocytes from the little skate Raja erinacea. Analysis of images of hepatocyte clusters that were incubated in medium with 0.5-1 microM NBD-TC showed that the fluorescent derivative accumulated in the cells and that the clusters retained a patent canalicular lumen as well as the ability to actively transport the bile acid derivative from the cells into the lumen; i.e., the lumen-to-cell fluorescence ratio greatly exceeded unity. NBD-TC uptake by hepatocytes was inhibited by several organic anions, of which taurocholate was the most effective. Uptake was also blocked by metabolic inhibitors and by incubation in the cold. Neither Na replacement nor increased medium K, which depolarizes the membrane electrical potential [potential difference (PD)], affected NBD-TC accumulation by hepatocytes. Transport of NBD-TC into the canalicular lumen was inhibited by incubation in the cold and was substantially reduced by high-K medium; these blocks were removed by warming and transfer to normal-K medium, respectively. Treatment of hepatocytes with 20-40 microM nocodazole, a drug that reversibly depolymerizes microtubules, reduced cellular NBD-TC accumulation and blocked its secretion into the canalicular lumen; nocodazole effects were reversed by washing the hepatocyte clusters in drug-free medium. Thus uptake of NBD-TC by skate hepatocytes is active and carrier mediated but not dependent on the PD or Na gradient. NBD-TC secretion from cell to canalicular lumen also appears to be active and carrier mediated. Canalicular secretion appears to be driven at least in part by the PD and is highly dependent on an intact microtubular system in this marine species.

Alkanesulfonates↗