Hepatocellular expression of glucose-6-phosphatase is unaltered during hepatic regeneration.

Gluconeogenesis and glycogenolysis are essential hepatic functions required for glucose homeostasis. During the initial phase of hepatic regeneration, the immediate-early genes (IEG) are rapidly expressed, and the IEG RL-1 encodes for glucose-6-phosphatase (G-6-Pase). G-6-Pase is a microsomal enzyme essential for gluconeogenesis and glycogenolysis. This study employs a partial-hepatectomy model to examine the expression and activity of G-6-Pase. After partial hepatectomy, rat hepatic G-6-Pase gene expression is transcriptionally regulated, and mRNA levels are increased approximately 30-fold. However, in contrast to this rapid gene induction, microsomal enzyme activity is unchanged after partial hepatectomy. Western blotting demonstrates that microsomal G-6-Pase protein expression is also unchanged after partial hepatectomy, and similar results are also noted in whole liver homogenate. Thus, despite marked induction in gene expression of the IEG G-6-Pase after partial hepatectomy, protein expression and enzyme activity remain unchanged. These data indicate that, although this hepatocyte IEG is transcriptionally regulated, the physiologically important level of regulation is posttranscriptional. This highlights the importance of correlating gene expression of IEG with protein expression and physiological function.

Regulation of hepatocyte bile salt transporters during hepatic regeneration.

Bile formation is an essential liver-specific function, and the hepatic regeneration that occurs in response to hepatocellular injury is often associated with cholestasis. We have employed a partial hepatectomy model to examine the effect of hepatic regeneration on tissue-specific bile salt transporters and on Na(+)-K(+)-adenosinetriphosphatase (ATPase). Liver-specific sodium-dependent taurocholate uptake by basolateral plasma membrane vesicles was undetectable 24 h after hepatectomy. Basolateral membrane protein expression of the sodium-taurocholate cotransporter and gene expression of Ntcp were decreased by > 90% 24 h after partial hepatectomy. In vitro transcription assays demonstrated that Ntcp gene transcription was also markedly reduced. In contrast, hepatic Na(+)-K(+)-ATPase activity, protein expression, and gene expression were unaffected by partial hepatectomy. Similarly, protein and gene expression of the ectoATPase, a putative canalicular bile salt transporter, and canalicular ATP-dependent taurocholate uptake remained unchanged. Partial hepatectomy results in a marked reduction in the gene transcription and expression of the liver-specific Ntcp, as well as a decrease in protein expression and loss of transport activity. These changes provide a potential mechanism for the decrease in hepatocellular bile salt transport that is associated with hepatic regeneration.

Cloning and characterization of a mammalian proton-coupled metal-ion transporter.

Metal ions are essential cofactors for a wealth of biological processes, including oxidative phosphorylation, gene regulation and free-radical homeostasis. Failure to maintain appropriate levels of metal ions in humans is a feature of hereditary haemochromatosis, disorders of metal-ion deficiency, and certain neurodegenerative diseases. Despite their pivotal physiological roles, however, there is no molecular information on how metal ions are actively absorbed by mammalian cells. We have now identified a new metal-ion transporter in the rat, DCT1, which has an unusually broad substrate range that includes Fe2+, Zn2+, Mn2+, Co2+, Cd2+, Cu2+, Ni2+ and Pb2+. DCT1 mediates active transport that is proton-coupled and depends on the cell membrane potential. It is a 561-amino-acid protein with 12 putative membrane-spanning domains and is ubiquitously expressed, most notably in the proximal duodenum. DCT1 is upregulated by dietary iron deficiency, and may represent a key mediator of intestinal iron absorption. DCT1 is a member of the 'natural-resistance-associated macrophage protein' (Nramp) family and thus its properties provide insight into how these proteins confer resistance to pathogens.

Performance characteristics and results of a large-scale screening program for viral hepatitis and risk factors associated with exposure to viral hepatitis B and C: results of the National Hepatitis Screening Survey. National Hepatitis Surveillance Group.

Chronic viral hepatitis frequently goes undetected until cirrhosis develops. Although the effect of interferon on the natural history of hepatitis B virus (HBV) or hepatitis C virus (HCV) infection in asymptomatic persons is unknown, treatment may modify the course of the infection, producing cures in some. In September 1992, screening for HBV and HCV was offered in 40 centers throughout the United States. Demographic features, potential risk factors, and symptoms were studied. Blood samples were obtained for the determination of serum alanine aminotransferase levels and for markers of HBV and HCV infection. Thirteen thousand nine hundred ninety seven subjects were screened. The prevalence of infection with HBV or HCV was 24.8% (HBV 17.8%; HCV 7.0%; and both 2.8%). Hepatitis B and C disease was present in 0.7% and 4.4% of the population, respectively. Risk factors for HBV and HCV infection were similar in: blood transfusions, hemodialysis, IV drug use, and sex with an IV drug user. For HBV infection, sex with multiple partners, increasing age, and birth in South East Asia or Africa were additional risk factors. The cost to find a case of HCV infection is less than the costs for finding many other treatable diseases. Screening for HBV, though more costly, is reasonably efficient, and simultaneous screening for HBV and HCV provides greater efficiency. It is practical to consider screening for HBV and HCV in the United States, particularly if any risk factor is present. Improved treatment strategies will make screening even more cost effective.

Hepatic regeneration is associated with preservation of microsomal glucuronidation.

Significant controversy exists regarding the regulation of glucuronidation during the process of hepatic regeneration. We used a partial hepatectomy rat model to elucidate the effects of hepatic regeneration on the various components of the microsomal glucuronidation system. Hepatic microsomes were prepared by standard sucrose density centrifugation, coupled with a modified technique involving Percoll centrifugation. Microsomal uridine diphosphate (UDP)-glucuronosyltransferase (UGT) protein expression and UGT messenger RNA (mRNA) levels were measured by Western and Northern blotting. UGT enzyme activity was determined toward two prototypical aglycones, p-nitrophenol and estrone, in intact and digitonin-treated microsomes. Microsomal uptake of the cosubstrate for all glucuronidation reactions, UDP-glucuronic acid (UDP-GlcUA), was determined using a rapid-filtration assay. Microsomal enrichment after hepatectomy was preserved only when the Percoll method was used. Microsomal UGT protein expression and UGT mRNA levels were unaltered after hepatectomy. UGT enzyme activity toward estrone was unchanged 1 day posthepatectomy compared with sham laparotomy controls. Similarly, p-nitrophenol glucuronide formation was unaffected by hepatic regeneration 1, 2, and 5 days posthepatectomy when digitonin-treated microsomes were used. Glucuronidation of p-nitrophenol in intact microsomes was increased in partial hepatectomy compared with sham-operated controls at 1 and 2 days. This increase was not attributable to changes in microsomal UDP-GlcUA uptake, which was comparable in both groups. We conclude that microsomal glucuronidation, in contrast to other well characterized hepatic metabolic functions, is highly preserved during liver regeneration.

Two kinetically-distinct components of UDP-glucuronic acid transport in rat liver endoplasmic reticulum.

Previous studies have documented the presence of protein-mediated transport of UDP-glucuronic acid (UDP-GlcUA) in rat liver endoplasmic reticulum (ER). Measurement of uptake at varying concentrations of high specific activity [beta-32P]UDP-GlcUA has revealed the presence of a two component UDP-GlcUA transporting system. Transport at low substrate concentrations occurred predominantly via a high affinity component (K(m) = 1.6 microM), whereas a low affinity component (K(m) = 38 microM) predominated at high substrate concentrations. The K(m) for the high affinity system is in agreement with that previously published, while the low affinity component is a new finding. The uptake of UDP-GlcUA was temperature-sensitive, time dependent, and saturable for both components. The high affinity transport was affected by trans-stimulation and cis-inhibition by UDP-N-acetylglucosamine (UDP-GlcNAc); however, the same concentrations of UDP-GlcNAc had less effect on the low affinity system. In order to further study the two transport components, various inhibitors of anion transport carriers were tested. The high affinity component was strongly inhibited by 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) and furosemide, while the low affinity system was less sensitive to these reagents. Dose-dependent inhibition by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) was found for both transport systems. Probenecid was found to be a weak inhibitor of both components of the UDP-GlcUA uptake. Finally, the major metabolite of 3'-azido-3'-deoxythymidine, 3'-azido-3'-deoxythymidine monophosphate (AZTMP), was able to inhibit the uptake of UDP-GlcUA by both components. The results indicate the presence of two carrier-mediated UDP-glucuronic acid transporting components in rat liver ER.

Genetic hemochromatosis: pathogenesis, diagnosis, and therapy.

Genetic hemochromatosis is an autosomal recessive disease characterized by increased intestinal iron absorption and consequent tissue iron overload. The hemochromatosis gene has been localized on the short arm of chromosome 6, in close proximity to the HLA locus, but has yet to be identified. Neither the gene product nor the pathogenetic defect have been characterized. Clinical manifestations vary according to the degree of iron overload, ranging from the asymptomatic state to the features of cirrhosis and hepatocellular carcinoma. Early diagnosis remains essential, since the survival of patients without established cirrhosis is comparable to that of the general population. Transferrin saturation and ferritin levels are suggestive of the diagnosis, but measurement of the hepatic iron concentration still remains the gold standard, despite the utilization of computerized tomography and magnetic resonance imaging. Routine phlebotomies constitute the principal therapeutic option, despite the recent preliminary data on oral iron chelators.

Regulation of hepatocyte bile salt transporters by endotoxin and inflammatory cytokines in rodents.

BACKGROUND & AIMS: Pathophysiological conditions such as sepsis and hepatitis are mediated by inflammatory cytokines and frequently are associated with cholestasis. The aim of this study was to determine the effect of endotoxin (lipopolysaccharide [LPS]) and cytokine administration on hepatocellular transporters involved in bile salt transport. METHODS: LPS and cytokines were administered to Sprague-Dawley rats or C57BL/6 mice, and the expression and function of hepatocyte transporters involved in bile salt secretion were examined. RESULTS: LPS caused gene expression of the hepatocyte basolateral sodium-dependent taurocholate cotransporter (Ntcp) to decrease by more than 90%. Tumor necrosis factor alpha (TNF-alpha) or interleukin (IL) 1beta also produced a time-dependent decrease in Ntcp messenger RNA levels, whereas IL-6 had no effect. LPS administration resulted in a concordant 90% reduction of basolateral protein expression of the hepatocyte sodium taurocholate cotransporter and markedly diminished sodium-dependent taurocholate uptake. Activity of the hepatocyte basolateral Na+,K+-adenosine triphosphatase (ATPase) was also decreased by 50% in a posttranslational manner after endotoxin treatment. CONCLUSIONS: Endotoxin inhibits hepatocellular sodium-dependent bile salt uptake by decreasing both expression of Ntcp and activity of the Na+,K-ATPase. The effects on Ntcp are mediated via TNF-alpha and IL-1beta. Alterations of these transporters may contribute to the cholestasis of sepsis and inflammation.

Intracellular transport of small hydrophobic compounds by the hepatocyte.

The liver is responsible for the detoxification and biliary excretion of a variety of endogenous and xenobiotic compounds and, therefore, is capable of responding to rapid fluctuations in metabolic demand. In order to accomplish this function, the hepatocyte must efficiently transport a host of substrates with wide-ranging physical-chemical properties to intracellular sites for biotransformation and subsequent secretion into the bile. The trafficking of substrates and metabolites within the liver cell is a complex process, involving the coordinated action of cellular proteins, organellar membranes, the cytoskeleton, vesicular transport pathways, and bulk convective cytoplasmic flow. This review summarizes recent developments in the field of intracellular transport, with particular reference to the metabolism of small hydrophobic and amphipathic molecular species (e.g., bilirubin, bile salts, fatty acids) by the hepatocyte.

A new voyage of discovery: transport through the hepatocyte.

In summary, hepatocellular membranes likely play an essential role in the binding and directed trafficking of unconjugated bilirubin, and potentially of a variety of other small hydrophobic molecules. Targeting of these substrates to the endoplasmic reticulum is determined by membrane cholesterol content, surface area and integral protein binding and enzyme activity. The rate of intracellular transport potentially may be modulated by the concentration of cytosolic binding proteins, but, at least for ligandin, this protein does not appear to function primarily as an intracellular bilirubin transporter.

Tumor necrosis factor-alpha decreases hepatocyte bile salt uptake and mediates endotoxin-induced cholestasis.

Tumor necrosis factor-alpha (TNF alpha), a cytokine that is produced in a variety of inflammatory diseases associated with cholestasis, is believed to be the primary mediator of the systemic effects of endotoxin. Thus, we have investigated the role of TNF alpha in the pathogenesis of endotoxin-induced cholestasis in intact animals, and in the uptake of taurocholate by cultured hepatocytes. Male Sprague-Dawley rats received either intravenous (IV) endotoxin (7.5 mg/kg) or monoclonal anti-TNF alpha antibody followed by endotoxin. Basal bile flow and bile salt excretion were measured for a 2-hour period, after which all animals received an IV bolus of taurocholate (10 mumol/100 g body weight). Endotoxin decreased basal bile flow by 41% and bile salt stimulated bile flow by 38% (n = 12; P < .01). Basal bile salt excretion was decreased 86% after endotoxin administration. Passive immunization with anti-TNF alpha antibody blocked this endotoxin-associated cholestasis. In addition, rat hepatocytes were isolated and cultured in the presence of either endotoxin (10 micrograms/mL) or TNF alpha (100 ng/mL) for 24 hours. These primary hepatocyte cultures exhibited a dose- and time-dependent, noncompetitive, inhibition of taurocholate uptake. We postulate that TNF alpha is an important mediator of the cholestasis of sepsis.

Influence of glutathione S-transferase B (ligandin) on the intermembrane transfer of bilirubin. Implications for the intracellular transport of nonsubstrate ligands in hepatocytes.

To examine the hypothesis that glutathione S-transferases (GST) play an important role in the hepatocellular transport of hydrophobic organic anions, the kinetics of the spontaneous transfer of unconjugated bilirubin between membrane vesicles and rat liver glutathione S-transferase B (ligandin) was studied, using stopped-flow fluorometry. Bilirubin transfer from glutathione S-transferase B to phosphatidylcholine vesicles was best described by a single exponential function, with a rate constant of 8.0 +/- 0.7 s-1 (+/- SD) at 25 degrees C. The variations in transfer rate with respect to acceptor phospholipid concentration provide strong evidence for aqueous diffusion of free bilirubin. This finding was verified using rhodamine-labeled microsomal membranes as acceptors. Bilirubin transfer from phospholipid vesicles to GST also exhibited diffusional kinetics. Thermodynamic parameters for bilirubin dissociation from GST were similar to those for human serum albumin. The rate of bilirubin transfer from rat liver basolateral plasma membranes to acceptor vesicles in the presence of glutathione S-transferase B declined asymptotically with increasing GST concentration. These data suggest that glutathione S-transferase B does not function as an intracellular bilirubin transporter, although expression of this protein may serve to regulate the delivery of bilirubin, and other nonsubstrate ligands, to sites of metabolism within the cell.

Evidence for a low Km transporter for non-transferrin-bound iron in isolated rat hepatocytes.

Non-transferrin-bound iron (NTBI) plays an important role in the hepatocellular injury induced by iron overload. However, the mechanism responsible for NTBI uptake into hepatocytes remains poorly defined. The purpose of this study was to define the kinetics of NTBI uptake by isolated rat hepatocytes and to characterize the uptake process. NTBI uptake was time and temperature dependent, exhibited a Michaelis-Menten constant (Km) value of 1.25 microM and maximum uptake of 241 pmol.10(6) cells-1.min-1, and 55Fe was incorporated in part into intracellular ferritin. Uptake was Ca2+ dependent, exhibiting 15 and 80% of maximal uptake in the presence of 0.6 and 0.75 mM CaCl2, respectively. The putative NTBI transporter was highly specific; divalent (Zn2+, Mn2+, Cd2+, and Co2+) or trivalent (La3+) cations did not inhibit Fe3+ uptake. Reduction from Fe3+ to Fe2+ was not essential for uptake or the process occurred deep within the membrane bilayer, since the Fe2+ chelator ferrozine did not influence 55Fe uptake. These data provide evidence for a low Km plasma membrane transporter for NTBI, which should be functional at physiological serum concentrations and saturated in iron-overload diseases, such as hemochromatosis.

Ryanodine-induced calcium release from hepatic microsomes and permeabilized hepatocytes.

Inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] is a second messenger that releases Ca2+ from hepatocyte microsomes. The toxic alkaloid ryanodine modulates Ca2+ release via a receptor (RyR) identified in a variety of cell systems, but its regulation and functional significance in liver are undefined. Similarly, the role in hepatocyte Ca2+ regulation of adenosine 5'-cyclic diphosphate-ribose (cADPR), which is the putative endogenous ligand for RyR in other cell systems, has not been defined. Utilizing microsomes and permeabilized cells, we have investigated Ca2+ regulation in hepatocytes and, in particular, effects of ryanodine, cADPR, and other putative modulators on Ca2+ release and compared these with Ins(1,4,5)P3-induced Ca2+ release. Ryanodine at > or = 50 microM released 20% of microsomal Ca2+, and, in contrast to Ins(1,4,5)P3, no potentiation was observed with guanosine 5'-triphosphate and polyethylene glycol. Ins(1,4,5)P3-induced Ca2+ release was demonstrable after maximal ryanodine-induced Ca2+ release, suggesting that distinct Ca2+ stores are involved. cADPR (5 microM) did not induce Ca2+ release, alone or in combination with calmodulin or hepatic cytosol, nor did it influence ryanodine-induced release, in microsomes or permeabilized hepatocytes (in which ryanodine released 25% of the sequestered Ca2+). Ryanodine-induced Ca2+ release in microsomes was not influenced by 20 mM caffeine, which itself did not mobilize Ca2+, but was prevented by 500 microM tetracaine, which was shown to induce Ca2+ release. We conclude that ryanodine is capable of mobilizing Ca2+ in the hepatocyte from microsomal stores that are distinct from those that can be regulated by Ins(1,4,5)P3 but that cADPR has no such effect. These data suggest that cADPR does not serve as the endogenous ligand for RyR in liver cells or that the site of action of ryanodine in hepatocyte microsomes is distinct from that in other cell types.

Membrane translocation and regulation of uridine diphosphate-glucuronic acid uptake in rat liver microsomal vesicles.

BACKGROUND/AIMS: Hepatic glucuronidation is quantitatively the most important conjugation reaction by which an array of endogenous compounds and xenobiotics undergo biotransformation and detoxification. The active site of the uridine diphosphate (UDP) glucuronosyltransferases, which catalyze glucuronidation reactions, has been postulated to reside in the lumen of the endoplasmic reticulum. The aim of this study was to characterize the process whereby UDP glucuronic acid (UDP-GlcUA), the cosubstrate for all glucuronidation reactions, is transported into microsomal vesicles. METHODS: The uptake process was analyzed using rapid filtration techniques, radiolabeled UDP-GlcUA, and rat liver microsomes. RESULTS: Uptake was saturable with respect to time and concentration, inhibited by 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid and 4-acetamido-4'-isothio-cyanatostilbene-2-2'-disulfonic acid, and was osmotically sensitive. Transport was stimulated by Mg2+ and guanosine triphosphate (50 mumol/L) but not guanosine 5'-O-(3-thiotriphosphate) or adenosine triphosphate. Luminal UDP-N-acetylglucosamine (1 mmol/L) produced enhanced uptake of UDP-GlcUA (trans stimulation). In contrast to nucleotide sugar transport in the Golgi apparatus, trans uridine monophosphate and UDP did not alter UDP-GlcUA transport in microsomes, indicating distinct processes. CONCLUSIONS: These data provide unambiguous evidence for the existence of a unique, substrate-specific, regulated, carrier-mediated process that transports UDP-GlcUA into the lumen of hepatocyte microsomes. This transporter may regulate glucuronidation in vivo.

Kinetics of bilirubin transfer between serum albumin and membrane vesicles. Insight into the mechanism of organic anion delivery to the hepatocyte plasma membrane.

Unconjugated bilirubin is transported in the plasma bound primarily to serum albumin, from which it is taken up and metabolized by the liver. To better characterize the mechanism of bilirubin delivery to the hepatocyte, stopped-flow techniques were utilized to study the kinetics of bilirubin transfer between serum albumin and both model phospholipid and native hepatocyte plasma membrane vesicles. The transfer process was best described by a single exponential function, with rate constants of 0.93 +/- 0.04, 0.61 +/- 0.03, and 0.10 +/- 0.01 s-1 (+/- S.D.) at 25 degrees C for human, rat, and bovine serum albumins, respectively. The observed variations in rate with respect to donor and acceptor concentrations provide strong evidence for the diffusional transfer of free bilirubin. Thermodynamic analysis suggests that the binding site on bovine serum albumin demonstrates higher specificity for the bilirubin molecule than that on human or rat serum albumin, which exhibit similar binding characteristics. Kinetic analysis of bilirubin transfer from rat serum albumin to isolated rat basolateral liver plasma membranes indicates that the delivery of albumin-bound bilirubin to the hepatocyte surface occurs via aqueous diffusion, rather than a collisional process, thereby mitigating against the presence of an "albumin receptor."

Interleukin-6 inhibits hepatocyte taurocholate uptake and sodium-potassium-adenosinetriphosphatase activity.

The potential effects of cytokines on hepatocellular transport functions remain undefined. Interleukin-6 (IL-6) is a cytokine that is produced in sepsis, hepatitis, and other inflammatory conditions often associated with cholestasis. Using cultured rat hepatocytes, we have investigated the effects of IL-6 on hepatocellular bile salt uptake. Because hepatocyte Na(+)-K(+)-adenosinetriphosphatase (ATPase) produces the electrochemical gradient that drives sodium-dependent bile salt contransport, we also examined the effects of IL-6 on Na(+)-K(+)-ATPase activity. Hepatocytes cultured for 20 h in media containing IL-6 exhibited a dose-dependent noncompetitive inhibition of [3H]taurocholate uptake, which was maximal at an IL-6 dose of 100 U/ml. IL-6 treatment had no effect on hepatocyte sodium-independent taurocholate uptake. Northern blotting of RNA from cultured hepatocytes revealed that IL-6 had no effect on steady-state RNA levels of the Na(+)-taurocholate transporter (Ntcp). Hepatocytes incubated with IL-6 for 20 h, however, exhibited a 55% decrease in hepatocyte Na(+)-K(+)-ATPase activity. This effect also was dose dependent, with maximal inhibition occurring at an IL-6 dose of 100 U/ml. Similar treatment with IL-6 did not influence hepatocyte Mg(2+)-ATPase activity. The inhibition of Na(+)-K(+)-ATPase activity induced by IL-6 provides a putative mechanism for the observed inhibition of sodium-dependent taurocholate uptake. Since modulation of bile salt transport and Na(+)-K(+)-ATPase activity occurred at IL-6 concentrations comparable to the serum levels observed in patients with severe inflammatory states, these findings have potential pathophysiological relevance for the cholestasis of sepsis and other inflammatory disorders.