Incorporation of radioactive amino acids into protein in isolated rat hepatocytes

Involvement of diclofenac acyl-β-d-glucuronide in diclofenac-induced cytotoxicity in glutathione-depleted isolated murine hepatocytes co-cultured with peritoneal macrophages

Direct hepatotoxic effects of drugs can occur when a parent drug and/or its reactive metabolites induces the formation of reactive oxygen species. Reactive metabolites of diclofenac (DIC) such as DIC acyl-β-d-glucuronide (DIC-AG) bind covalently to proteins, potentially decreasing protein function or inducing an immune response. However, it is unclear whether the macrophages and GSH depletion participate in DIC-induced cytotoxicity. Mouse hepatocytes (Hep) co-cultured with peritoneal macrophages (PMs) were used to clarify the effects of presence of PM with GSH depletion on DIC-induced cytotoxicity in Hep. DIC-AG but not hydroxy-DIC concentrations in medium were significantly increased in Hep co-cultured with PM with GSH depletion. Depletion of GSH resulted in significantly higher LDH leakage. Interestingly, LDH leakage in Hep/PM (1:0.4) with GSH depletion was significantly higher than in Hep/PM (1:0 and 1:0.1) with BSO. It is likely that macrophages with GSH depletion could facilitate DIC-induced cytotoxicity.

Survival and Metabolic Function of Freshly Isolated Rat Hepatocytes Exposed First to a Heat Shock and Then to an Oxidative Stress

The formation of heat shock proteins (hsp) leading to thermotolerance has been extensively reported in many cell types. In freshly isolated rat hepatocytes, hsp were synthesized after 60 minutes of incubation at 42°C. Cell survival was not modified by such a treatment, but protein synthesis, secretion of triglycerides as lipoproteins, and the maintenance of both ATP and glycogen levels were significantly impaired. When exposed to an oxidative stress, heat-shocked hepatocytes were not more resistant than cells always kept at 37°C. Conversely, the addition of tert-butyl hydroperoxide (tBOOH) resulted, in general, in an increased lactate dehydrogenase leakage. The metabolism of tBOOH, as estimated by the reduced glutathione (GSH) content and GSH peroxidase activity, was similar in both control and heat-shocked hepatocytes. Despite the synthesis of hsp in rat hepatocytes, the lack of resistance to a subsequent oxidant injury may be due to the metabolic impairment caused by the heat shock.

Hepatocyte Heparan Sulfate Is Required for Adeno-Associated Virus 2 but Dispensable for Adenovirus 5 Liver Transduction In Vivo

Adeno-associated virus 2 (AAV2) and adenovirus 5 (Ad5) are promising gene therapy vectors. Both display liver tropism and are currently thought to enter hepatocytes in vivo through cell surface heparan sulfate proteoglycans (HSPGs). To test directly this hypothesis, we created mice that lack Ext1, an enzyme required for heparan sulfate biosynthesis, in hepatocytes. Ext1(HEP) mutant mice exhibit an 8-fold reduction of heparan sulfate in primary hepatocytes and a 5-fold reduction of heparan sulfate in whole liver tissue. Conditional hepatocyte Ext1 gene deletion greatly reduced AAV2 liver transduction following intravenous injection. Ad5 transduction requires blood coagulation factor X (FX); FX binds to the Ad5 capsid hexon protein and bridges the virus to HSPGs on the cell surface. Ad5.FX transduction was abrogated in primary hepatocytes from Ext1(HEP) mice. However, in contrast to the case with AAV2, Ad5 transduction was not significantly reduced in the livers of Ext1(HEP) mice. FX remained essential for Ad5 transduction in vivo in Ext1(HEP) mice. We conclude that while AAV2 requires HSPGs for entry into mouse hepatocytes, HSPGs are dispensable for Ad5 hepatocyte transduction in vivo. This study reopens the question of how adenovirus enters cells in vivo.

IMPORTANCE

Our understanding of how viruses enter cells, and how they can be used as therapeutic vectors to manage disease, begins with identification of the cell surface receptors to which viruses bind and which mediate viral entry. Both adeno-associated virus 2 and adenovirus 5 are currently thought to enter hepatocytes in vivo through heparan sulfate proteoglycans (HSPGs). However, direct evidence for these conclusions is lacking. Experiments presented herein, in which hepatic heparan sulfate synthesis was genetically abolished, demonstrated that HSPGs are not likely to function as hepatocyte Ad5 receptors in vivo. The data also demonstrate that HSPGs are required for hepatocyte transduction by AAV2. These results reopen the question of the identity of the Ad5 receptor in vivo and emphasize the necessity of demonstrating the nature of the receptor by genetic means, both for understanding Ad5 entry into cells in vivo and for optimization of Ad5 vectors as therapeutic agents.

Leishmanial lipid affords protection against oxidative stress induced hepatic injury by regulating inflammatory mediators and confining apoptosis progress

Persistence of liver injury alters the internal milieu, promotes deregulation of inflammatory factors, and leads to dysplastic lesions like fibrosis, cirrhosis to hepatocellular carcinoma. Our previous study revealed that leishmanial lipid (pLLD) exerts potential anti-inflammatory activity in sepsis associated hepatic injury. We now show that pLLD gives protection against chemical induced hepatotoxicity in murine system. The beneficial effect of treatment with pLLD on such hepatic injury in mice was analyzed using different assays including ELISA, FACS, western blot and immunohistochemical analysis. pLLD significantly suppressed serum enzymes and rectified the histopathological alteration to induce the antioxidant level in CCl4 intoxicated liver. Levels of several growth factors including TGF-β, HGF, and EGF were significantly improved in serum and hepatic tissue with consequent reduction of caspase activities and expressions of Bad, Bax, p53, and NF-κBp65. Moreover, pLLD modulated inflammatory responses by decreasing the production of several cytokines and chemokines, thus preventing the infiltration of immune cells to the damaged area. It accelerated the repair process in liver damage with modulation of signalling cascade via alteration of apoptotic factors. Our experimental approaches suggest that pLLD effectively prevents liver injury mainly through down regulation of oxidative stress and inflammatory response towards anti-apoptotic changes.

Ezetimibe decreases SREBP-1c expression in liver and reverses hepatic insulin resistance in mice fed a high-fat diet

Ezetimibe inhibits intestinal cholesterol absorption, thereby reducing serum cholesterol. Recent studies suggest that ezetimibe affects liver steatosis and insulin resistance. We investigated the impact of ezetimibe on insulin sensitivity and glucose metabolism in C57BL/6 mice. We analyzed 4 mouse groups fed the following diets: normal chow (4% fat) for 12 weeks, normal chow for 10 weeks followed by normal chow plus ezetimibe for 2 weeks, high-fat chow (32% fat) for 12 weeks, and high-fat chow for 10 weeks followed by high-fat chow plus ezetimibe for 2 weeks. In the normal chow + ezetimibe group, ezetimibe had no impact on body weight, fat mass, lipid metabolism, liver steatosis, glucose tolerance, or insulin sensitivity. In the high-fat chow + ezetimibe group, ezetimibe had no impact on body weight or fat mass but significantly decreased serum low-density lipoprotein cholesterol, triglyceride, and glutamate pyruvate transaminase levels; liver weight; hepatic triglyceride content; and hepatic cholesterol content and increased the hepatic total bile acid content. In association with increases in IRS-2 and Akt phosphorylation, ezetimibe ameliorated hepatic insulin resistance in the high-fat chow + ezetimibe group, but had no effect on insulin sensitivity in primary cultured hepatocytes. A DNA microarray and Taqman polymerase chain reaction revealed that ezetimibe up-regulated hepatic SREBP2 and SHP expression and down-regulated hepatic SREBP-1c expression. SHP silencing mainly in the liver worsened insulin resistance, and ezetimibe protected against insulin resistance induced by down-regulation of SHP. Ezetimibe down-regulated SREBP-1c in the liver and reversed hepatic insulin resistance in mice fed a high-fat diet.

An in vitro comparative study with furyl-1,4-quinones endowed with anticancer activities

We describe the biological activity of some furylbenzo- and naphthoquinones (furylquinones) on hepatocarcinoma cells and healthy rat liver slices. The effects of furylquinones on cancer cells (Transplantable Liver Tumor, TLT) were assessed by measuring cell death (membrane cell lysis); intracellular contents of ATP and GSH and the activity of caspase-3 were used to determine the type of cell death. Most of the furylquinones tested (at a concentration of 25 μg/ml) induced caspase-independent cell death but compound 4 had no cytotoxic effects. The levels of both ATP and GSH were severely affected by quinones 1, 2 and 5, while no effect was observed with compound 4. These cytotoxic properties of quinones are associated with physico-chemical properties as shown by the LUMO energies and lipophilicity. Interestingly, no cytotoxic effects of furylquinones were detected when the in vitro model of precision-cut liver slices (PCLS) was used. Indeed, although CYP2E1 activity was slightly affected, ATP and GSH levels as well as protein synthesis were not modified by furylquinones. Paracetamol, a well-known hepatotoxicant, reduced these parameters by more than 80% compared to control conditions. Taking into account the considerable incidence of adverse-effects induced by most current anticancer drugs, the selective cytotoxicity shown by compounds 1, 2 and 5, in particular that of 1, represents a safety factor that encourages the further development of these quinones as new drugs in cancer therapy.

A rapid and easy to handle thermoluminescence based technique for evaluation of carbon tetrachloride-induced oxidative stress on rat hepatocytes

Oxidative stress has become one of the most intensively studied topics in biomedical research and is an often observed mechanism of non-genotoxic carcinogens like carbon tetrachloride. To monitor the oxidative stress status in in vitro hepatocytes, we compared thermoluminescence (TL) measurements with biochemical standard methods for oxidative stress markers. In contrast to biochemical analysis, TL measurements can be performed without any time-consuming extraction procedures by using directly collected cell material. After incubation with CCl(4) (24 h), thermo-induced light emission increased with rising concentration of CCl(4) up to eightfold at 10 mM CCl(4). Simultaneously, we determined the content of different secondary oxidative stress products, like thiobarbituric acid reactive substances and malondialdehyde. The rise of all biochemical markers complied with the increasing concentration of CCl(4). Finally, we could show that the CCl(4)-induced increase of oxidative stress markers determined by time-consuming biochemical methods perfectly correlates with the increase of high temperature bands in rapid TL measurements.

Hyperthyroidism induces apoptosis in rat liver through activation of death receptor-mediated pathways

BACKGROUND/AIMS

The molecular basis of hepatic dysfunction in thyrotoxicosis is not fully understood. Here, we investigated the effect of altered thyroidal status on death receptor pathways including p75 neurotrophin receptor (p75NTR), a member of tumor necrosis factor (TNF) receptor superfamily, in rat liver.

METHODS

Hyperthyroidism was induced in Sprague-Dawley rats by daily injections of triiodothyronine in a dose of 12.5 microg/100 g body weight for 10 days.

RESULTS

Terminal deoxynucleotide-transferase-mediated dUTP nick end labeling assay and caspase-3 activation data confirmed apoptosis in hyperthyroid rat liver. We observed the elevated levels of death ligands, TNF-alpha, Fas ligand and their cognate receptors, TNF-receptor-1 and Fas, and 8-fold increase in caspase-8 activation in hyperthyroid rat liver (p<0.001). We demonstrated for the first time that hyperthyroidism elevates p75NTR levels and its ligands, pro-nerve growth factor and pro-brain-derived neurotrophic factor, in rat liver. Further we showed that most of the apoptotic cells in hyperthyroid liver express p75NTR. We also demonstrated that triiodothyronine administration to rats causes NF-kappaB activation, but persistent exposure (10 days) to triiodothyronine deactivates NF-kappaB leading to sustained c-Jun N-terminal kinase (JNK) activation.

CONCLUSIONS

This study showed that hyperthyroidism-induced apoptosis in rat liver involves the activation of death receptor-mediated pathways, including p75NTR.

Evaluation of Felbamate and Other Antiepileptic Drug Toxicity Potential Based on Hepatic Protein Covalent Binding and Gene Expression

Felbamate is an antiepileptic drug that is associated with minimal toxicity in preclinical species such as rat and dog but has an unacceptable incidence of serious idiosyncratic reactions in man. Idiosyncratic reactions account for over half of toxicity-related drug failures in the marketplace, and improving the preclinical detection of idiosyncratic toxicities is thus of paramount importance to the pharmaceutical industry. The formation of reactive metabolites is common among most drugs associated with idiosyncratic drug reactions and may cause deleterious effects through covalent binding and/or oxidative stress. In the present study, felbamate was compared to several other antiepileptic drugs (valproic acid, carbamazepine, phenobarbital, and phenytoin), using covalent binding of radiolabeled drugs and hepatic gene expression responses to evaluate oxidative stress/reactive metabolite potential. Despite causing only very mild effects on covalent binding parameters, felbamate produced robust effects on a previously established oxidative stress/reactive metabolite gene expression signature. The other antiepileptic drugs and acetaminophen are known hepatotoxicants at high doses in the rat, and all increased covalent binding to liver proteins in vivo and/or to liver microsomes from human and rat. With the exception of acetaminophen, valproic acid exhibited the highest covalent binding in vivo, whereas carbamazepine exhibited the highest levels in vitro. Pronounced effects on oxidative stress/reactive metabolite-responsive gene expression were observed after carbamazepine, phenobarbital, and phenytoin administration. Valproic acid had only minor effects on the oxidative stress/reactive metabolite indicator genes. The relative ease of detection of felbamate based on gene expression results in rat liver as having potential oxidative stressor/reactive metabolites indicates that this approach may be useful in screening for potential idiosyncratic toxicity. Together, measurements of gene expression along with covalent binding should improve the safety assessment of candidate drugs.

The use of precision-cut liver slices from male Wistar rats as a tool to study age related changes in CYP3A induction and in formation of paracetamol conjugates

Precision-cut liver slices (PCLS) offer a lot of advantages because all heterogeneity and cell-cell interactions within the original tissue matrix are maintained. This in vitro model was used to study the effect of ageing on certain aspects of drug metabolism and liver function in young (3 months), adult (9 months) and old (24 months) Wistar male rats. Protein synthesis, an important liver function, was not modified in young, adult and old rats, suggesting that ageing does not impair liver functionality but it affects some specific targets. Among them, a decrease in total P450 in liver microsomes and the loss of CYP3A23 inducibility in PCLS were clearly observed in old rats as compared to adult rats. Finally, the amount of total paracetamol conjugates was not modified between 9 and 24 months but in old rats, sulfoconjugation of paracetamol, its major route of elimination, was decreased.

Role of temperature on protein and mRNA cytochrome P450 3A (CYP3A) isozymes expression and midazolam oxidation by cultured rat precision-cut liver slices

The cytochrome P450 3A (CYP3A)-mediated midazolam oxidation was studied in rat precision-cut liver slices (PCLS) maintained for 20hr at 4, 20 and 37 degrees, and further incubated for 8hr at 37 degrees. Either at 4 or 20 degrees, midazolam was oxidised by PCLS at similar rates to that observed in freshly cut slices. Moreover, PCLS kept a regioselectivity since 4-hydroxylation was more important than 1'-hydroxylation. Conversely, PCLS totally lost their capacity to oxidise midazolam after 20hr at 37 degrees, and both CYP3A2 protein and mRNA were not detected. CYP3A1 protein was unaffected by a temperature of 37 degrees but its mRNA was totally lost. By blocking transcription with actinomycin D, the decay of both CYP3A mRNAs followed the same profile at either 20 or 37 degrees, indicating that temperature affected the CYP3A2 protein stability. Cell functionality was not involved in such an impairment since the low values of ATP, GSH and protein synthesis rates observed at 4 and 20 degrees were rapidly restored, when PCLS were further incubated at 37 degrees. The use of rat supersomes expressing either CYP3A1 or CYP3A2, strongly supported the hypothesis that 4-hydroxymidazolam was mainly formed by CYP3A2. These results suggest that: (1) CYP3A1 protein is constitutive and largely expressed in rat liver slices; (2) regioselective midazolam oxidation appears to be mainly CYP3A2 dependent; and (3) since CYP3A isoforms have similar half-lives (about 10-14hr), the loss of CYP3A2 protein at 37 degrees might be due to a selective targeting (phosphorylation ?) leading to proteolytic disposal by the proteasome.

Ischemic preconditioning in a rodent hepatocyte model of liver hypothermic preservation injury

Ischemic preconditioning (IPC) is a phenomenon of protection in various tissues from normothermic ischemic injury by previous exposure to short cycles of ischemia-reperfusion. The ability of IPC to protect hepatocytes from a model of hypothermic transplant preservation injury was tested in this study. Rat hepatocytes were subjected to 30min of warm ischemia (37 degrees C) followed by 24 or 48h of hypothermic (4 degrees C) storage in UW solution and subsequent re-oxygenation at normothermia for 1h. Studies were performed with untreated control cells and cells treated with IPC (10min anoxia followed by 10min re-oxygenation, 1 cycle). Hepatocytes exposed to IPC prior to warm ischemia released significantly less LDH and had higher ATP concentrations, relative to untreated ischemic hepatocytes. IPC significantly reduced LDH release after 24h of cold storage before reperfusion and after 48h of cold storage and after 60min of warm re-oxygenation, relative to the corresponding untreated hepatocytes. ATP levels were also significantly higher when IPC was used prior to the warm and cold ischemia-re-oxygenation protocols. In parallel studies, IPC increased new protein synthesis and lactate after cold storage and reperfusion compared to untreated cells but no differences in the patterns of protein banding were detected on electrophoresis between the groups. In conclusion, IPC significantly improves hepatocyte viability and energy metabolism in a model of hypothermic preservation injury preceded by normothermic ischemia. These protective effects on viability may be related to enhanced protein and ATP synthesis at reperfusion.

Hypoxia causes downregulation of protein and RNA synthesis in noncontracting Mammalian cardiomyocytes

The aim was to identify energy-consuming processes, other than contraction, downregulated during moderate hypoxia ( approximately 5 micromol/L, 0.5% O(2)) and severe hypoxia (<0.5 micromol/L, <0.05% O(2)) in isolated neonatal cardiomyocytes. The metabolic response of cardiomyocytes to moderate and severe hypoxia was assessed by measuring rates of energy consumption and energetic status of cells maintained under these conditions. We found that the rates of energy production were decreased during both forms of hypoxia. Decreased rates of energy production under moderate hypoxia were associated with reduced energy wastage through a downregulation of proton leak in the mitochondria. Cellular protein synthesis and RNA synthesis, major energy-consuming pathways, were downregulated only during severe hypoxia, when oxygen concentrations were low enough to induce energetic stress (quantitatively defined as being any situation in which phosphocreatine concentrations had fallen by > or = 40%). Our results suggest that energetic stress is the signal responsible for this downregulation.

Effects of bacterial endotoxin (lipopolysaccharides) on survival and metabolism of cultured precision-cut rat liver slices

The effect of bacterial endotoxin (lipopolysaccharides from Escherichia coli, LPS) on cellular metabolism and drug biotransformation was studied in precision-cut rat liver slices (PCLS). Xenobiotic metabolism by PCLS was assessed by measuring phase I (midazolam hydroxylation) and phase II (paracetamol conjugates) enzyme activities. Nitrites formation was used as an indirect way to assess LPS-mediated activation of nitric oxide synthase (iNOS, type 2). PCLS incubation with various LPS doses results in a dose-dependent formation of nitrites. Such a nitrite formation is decreased by dexamethasone. After incubation of PCLS for 24 h LPS addition did not increase the basal nitrite formation, indicating that cells are not responsive any more. Paracetamol conjugation was unaffected by LPS treatment but midazolam hydroxylation was reduced by more than 50%. Such a loss is not due to cell impairment since neither survival (LDH leakage) nor cellular metabolism (protein synthesis or ATP content) were modified by LPS. Indeed, under defined conditions, namely Williams' medium E and O(2)/CO(2) (95:5), PCLS maintained both ATP and GSH levels and the capacity of hepatocytes to synthesize proteins. In conclusion, the in vitro model of PCLS reproduces the inhibitory effect of LPS on a CYP3A-dependent activity, allowing a mechanistic approach to study cell-cell interactions.

Reoxygenation after cold hypoxic storage of cultured precision-cut rat liver slices: effects on cellular metabolism and drug biotransformation

Cultured rat precision-cut liver slices (PCLS) were used to study the influence of hypothermic preservation and reoxygenation at 37 degrees C on cellular metabolism and drug biotransformation. Cold hypoxic storage caused a depressed metabolism in rat liver slices, but reoxygenation for 8 h at 37 degrees C partially restored the levels of both ATP and GSH and totally restored the capacity to synthesize proteins. Metabolism of midazolam (CYP3A-dependent oxidation) by cold preserved liver slices was decreased by 30% but no further affected by reoxygenation, showing the same profile as freshly cut slices. Such a reoxygenation at 37 degrees C is accompanied by a dramatic loss of CYP3A2 protein while CYP3A1 protein was unaffected. These results suggest that CYP3A2 did not play a major role in midazolam oxidation. Such results are not consistent with a putative reoxygenation injury but rather with cold hypoxic damage. Since cold preserved liver slices did not respond to bacterial endotoxin stimulation (lipopolysaccharides), a minor role of non-parenchymal cells is suggested as mediators for deleterious effects developed during the cold storage.

Role of ATP and glycogen reserves in both paracetamol sulfation and glucuronidation by cultured precision-cut rat liver slices

Precision-cut rat liver slices (PCLS) were used to investigate the formation of paracetamol conjugates. The time course of biochemical markers such as ATP and GSH content, glycogen levels and protein synthesis rates was recorded over a period of time of 26 h and taken as index of slices viability. Low values of ATP (3.6 nmol/mg prot), GSH (7.1 nmol/mg prot) and protein synthesis rates (94.1 pmol leu/mg prot x min(-1)) were initially observed. Thereafter, they gradually recovered up to 6 h but decreased values were seen after 20 h. Glycogen, however, dropped rapidly during the first 6 h, being no longer detected after 20 h of incubation. The reincubation of PCLS in a fresh medium for 6 h allowed a strong recovery of GSH, ATP and protein synthesis rates, but no gluconeogenesis was observed. Meanwhile, paracetamol sulfate formation was fairly constant (about 3 microg/mg protein) while glucuronide gradually disappeared. The amount of both UGT1A1 and ST1A1 did not correlate with their respective enzymatic activities. We suggest that loss of glycogen impair glucuronide conjugation by decreasing the availability of UDPGA, and that low values of ATP are largely enough to support sulfotransferase activity.

Protein S-thiolation can mediate the inhibition of protein synthesis induced by tert-butyl hydroperoxide in isolated rat hepatocytes

A rapid inhibition of protein synthesis is observed when isolated rat hepatocytes are incubated in the presence of 0.25-0.5 mM of tert-butyl hydroperoxide (tBOOH). Such an inhibition occurs in the absence of a cytolytic effect by tBOOH. Iron chelators (o-phenanthroline and desferrioxiamine), protected against oxidative cell death, but they did not modify the inhibition of protein synthesis caused by tBOOH (0.5 mM), suggesting that free radicals are less implicated in such an impairment. Electron micrographs of hepatocytes under oxidative stress show disaggregation of polyribosomes but not oxidative alterations, such as blebs or mitochondrial swelling. Protein synthesis inhibition is accompanied by a decrease in reduced glutathione (GSH) and an increase in glutathione disulfide (GSSG) and the level of protein S-thiolation (protein mixed disulfides formation). Such an increase of GSSG appears as a critical event since diethylmaleate (DEM) at 0.2 mM reduced GSH content by more than 50% but did not affect either GSSG content or protein synthesis. The addition of exogenous GSH and N-acetylcysteine (NAC) to tBOOH-treated hepatocytes significantly reduced the formation of protein mixed disulfides and restored the depressed protein synthesis either completely or partially. We suggest that S-thiolation of some key proteins may be involved in protein synthesis inhibition by tBOOH.

Biphasic mechanism for hypothermic induced loss of protein synthesis in hepatocytes

BACKGROUND

A complication in liver transplantation is increased clotting times due to inhibition of protein synthesis resulting from prolonged hypothermic preservation. Protein synthesis is also blocked in cold preserved hepatocytes. In this study, the mechanism of inhibition of protein synthesis in cold preserved hepatocytes was investigated.

METHODS

Hepatocytes prepared from rat liver were cold preserved in University of Wisconsin solution for 4, 24, and 48 hr. Protein synthesis was measured as incorporation of radiolabeled leucine into acid precipitable proteins. Hepatocytes were treated with antioxidants (dithiothreitol, trolox or deferoxamine, nitric oxide synthase inhibitor (N(G)-monomethyl-L-arginine monoacetate), steroids (dexamethasone or methylprednisolone), methods to keep adenosine triphosphate high (aerobic storage), and cytoskeletal disrupting agents (cytochalasin D or colchicine).

RESULTS

There was a 26% decrease in protein synthesis after only 4 hr of cold storage and a further 25% decrease at 24 hr. Antioxidants, elevated adenosine triphosphate, and N(G)-monomethyl-L-arginine monoacetate did not affect the rate of loss of protein synthesis. Protein synthesis was not due to inhibition of amino acid transport or lack of amino acids in the storage medium. Steroid pretreatment of hepatocytes had no effect on the loss of protein synthesis occurring in the first 4 hr of storage but did suppress the loss occurring during the next 44 hr of storage. Cytoskeletal disrupting agents, added to freshly isolated cells, inhibited protein synthesis.

CONCLUSION

The mechanism of loss of protein synthesis in cold preserved liver cells is not mediated by: (1) oxygen free radical generation or improved by antioxidant therapy, (2) nitric oxide generation in hepatocytes, (3) an adenosine triphosphate-sensitive destruction of cell viability, and (4) decreased permeability of amino acids or loss of amino acids from the cells. Loss of protein synthesis due to hypothermic storage appears biphasic. The first phase, occurring within 4 hr of storage, may be the result of the effects of hypothermia on the cell cytoskeletal system and may be untreatable. The second phase, which occurs during the next 24 to 48 hr is sensitive to steroid pretreatment. This phase may be amenable to improved preservation methodology. Improved preservation of the liver may require the use of steroids to conserve protein synthetic capabilities.

Hypoxia increases the association of 4E-binding protein 1 with the initiation factor 4E in isolated rat hepatocytes

Incubation of hepatocytes under hypoxia increases binding of translation initiation factor eIF-4E to its inhibitory regulator 4E-BP1, and this correlates with dephosphorylation of 4E-BP1. Rapamycin induced the same effect in aerobic cells but no additive effect was observed when hypoxic cells were treated with rapamycin. This enhanced association of 4E-BP1 with eIF-4E might be mediated by mTOR. Nevertheless, only hypoxia produces a rapid inhibition of protein synthesis. Although hypoxia might be signalling via the rapamycin-sensitive pathway by changing eIF-4E availability, such a pathway is unlikely to be responsible for the depression in overall protein synthesis under hypoxia.

Role of protein-phosphorylation events in the anoxia signal-transduction pathway leading to the inhibition of total protein synthesis in isolated hepatocytes

Incubation of isolated hepatocytes under N2/CO2 (no O2) produced a rapid and strong inhibition of overall polypeptide biosynthesis, which was neither related to cell death nor to the appearance of specific stress proteins. Treatment of the cells with the tyrosine-kinase inhibitor genistein or with the serine/threonine-protein-kinase inhibitor H7 did not modify the impairment of protein synthesis induced by oxygen deprivation, indicating that such signal-transduction pathways are probably not involved in the anoxia-mediated effect. Okadaic acid (100 nM) and Na3VO4 (1 mM) reduced the incorporation of [14C]Leu into proteins of hepatocytes maintained under aerobic conditions (93.3 kPa O2). The effects of oxygen deprivation and okadaic acid were additive, whereas sodium vanadate did not enhance the impairment of protein synthesis induced by anoxia. This observation suggests that a common mechanism, involving the net phosphorylation of protein tyrosine residues, that is insensitive to genistein might participate in the negative control of the translation induced by oxygen deprivation. The effect of anoxia on the synthesis of proteins was fully and rapidly reversible upon the restoration of oxygen supply, thus indicating that hepatocytes are able to sense O2. Although high concentrations of cobalt chloride partially mimic the effect of oxygen deprivation on protein biosynthesis, the nature of such an oxygen sensor remains unknown, and appears unlikely to be a part of a classic haem protein.

Protective effect of fructose on survival and metabolic capacities of hepatocytes kept overnight under cold hypoxia before normothermic reoxygenation

The protective effect of fructose with regard to hypoxia-induced cell injury in overnight cold preserved hepatocytes (20 hr at 4 degrees C) was investigated. The addition of fructose (at 10 and 20 mM) resulted in an improved survival of hepatocytes during their normothermic (37 degrees C) reoxygenation, irrespective of the time of fructose addition before the onset of hypoxia (i.e. 10, 20 or 30 min). Such a protective effect was even higher than that observed when hepatocytes were incubated in the University of Wisconsin solution (UW). Moreover, neither Desferal (an iron chelator) nor adenosine (an ATP precursor), nor other carbohydrates (glucose, galactose and the antioxidant mannitol) were able to protect cells against such an hypoxia-mediated injury. The intracellular ATP content was lower in both adenosine- and fructose-treated hepatocytes than in control untreated cells. However, the cellular metabolic capacities such as protein synthesis and gluconeogenesis from lactate recovered faster during reoxygenation of previously hypoxic fructose-treated cells compared with both control and adenosine-treated cells.

Effects of gemfibrozil and clofibric acid on the uptake of taurocholate by isolated rat hepatocytes

Clinical use of fibrate hypolipidaemic agents has been associated with an increased incidence of hepatobiliary dysfunction including increased bile lithogenicity, gallstone formation, and cholestasis. The hepatic transport of bile acids plays an important role in bile formation and flow, and interference with the hepatocellular transport of bile acids may result in hepatobiliary dysfunction. The aim of this study was to investigate the effects of gemfibrozil and clofibric acid on the uptake of taurocholate by rat isolated hepatocytes. In control hepatocyte preparations (N = 5) at 37 degrees, the uptake of taurocholate was described by saturable Michaelis-Menten kinetics with a mean (+/-SD) Km of 44.1 +/- 10.2 microM and Vmax of 62.0 +/- 23.0 nmol/10(6) cells/min. In the presence of 200 microM clofibric acid, there was no significant change in the kinetics of taurocholate uptake. However, in the presence of 200 microM gemfibrozil there was a statistically significant (P < 0.05) decrease in the Vmax of taurocholate uptake (32.0 +/- 18.2 nmol/10(6) cells/min, N = 5) and no change (P > 0.05) in Km (48.5 +/- 29.5 microM, N = 5). Gemfibrozil behaved as a non-competitive inhibitor of taurocholate uptake, with a Ki of 144 microM, which is approximately 50 times higher than the unbound gemfibrozil concentrations achieved clinically in humans. Thus, gemfibrozil and clofibric acid did not appear to directly alter the hepatic uptake of taurocholate at clinically relevant concentrations.

Homocysteine enhances the inhibitory effect of extracellular adenosine on the synthesis of proteins in isolated rat hepatocytes

Previous work has shown that extracellular adenosine inhibits the incorporation of radiolabelled leucine into proteins in isolated rat hepatocytes [Tinton, Lefebvre, Cousin and Buc Calderon (1993) Biochim. Biophys. Acta 1176, 1-6]. In this study, we investigated whether its metabolism into adenine nucleotides, inosine or S-adenosylhomocysteine (AdoHcy) is required to induce such an impairment. Incubation of isolated hepatocytes in the presence of adenosine at 0.5 or 1 mM reduces the synthesis of proteins by about 45% after 120 min of incubation. Such an inhibition occurred without cell lysis and was not modified by adding the adenosine kinase inhibitor 5-iodotubercidin (15 microM) or the adenosine deaminase inhibitor coformycin (0.1 microM). It is therefore unlikely that the anabolic and catabolic pathways of adenosine are involved in the inhibition of protein synthesis. Adenosine (1 mM) increased the level of AdoHcy and S-adenosylmethionine by 20- and 5-fold respectively after 60 min of incubation and reduced the methylation index. These events as well as the inhibition of protein synthesis were strongly enhanced in the presence of L-homocysteine (2 mM). It is therefore concluded that the metabolism of adenosine into AdoHcy, which is known to be a potent inhibitor of cellular methylation reactions, may play an important role in the control of translation.

Inhibition of protein synthesis induced by adenine nucleotides requires their metabolism into adenosine

Adenine nucleotides and adenosine inhibit the incorporation of radiolabelled leucine into proteins of isolated hepatocytes. Impairment occurred with nucleotides which can be converted into 9-beta-D-ribofuranosyladenine (adenosine) but was not observed after treatment with adenine or AMPCPP (the alpha, beta-methylene analogue of ATP). Metabolism into adenosine was further suggested by the increase in cellular ATP levels following treatment of hepatocytes with ATP, adenosine or AMPPCP (the beta, gamma-methylene ATP analogue) while AMPCPP was without any significant effect. The inhibition of protein synthesis caused by adenosine was not due to a lytic effect nor to a general disturbance in hepatic functions and was reversed when the cells were washed and transferred to a nucleoside-free medium. This impairment, however, was not coupled to the activation of adenylate cyclase, as preincubation of hepatocytes with P1 purinoceptor antagonists failed to prevent protein synthesis inhibition. In contrast, L-homocysteine enhanced the inhibitory effect of adenosine on the incorporation of radiolabelled leucine into proteins. Our results thus suggest that the inhibition of protein synthesis caused by adenine nucleotides requires their conversion into adenosine. They also indicate that the inhibitory effect of adenosine does not involve a receptor-mediated effect but may be related to an increase in S-adenosylhomocysteine content and a subsequent low level of macromolecule methylation.

Microcystin-LR induced an inhibition of protein synthesis in isolated rat hepatocytes

The effect of microcystin-LR, an inhibitor of protein phosphatases PP1 and PP2A, was studied on protein synthesis by measuring the incorporation of labelled amino acid into protein in isolated rat hepatocytes. Microcystin-LR inhibited protein synthesis in the first minutes of the incubation period, and half-maximum effect was obtained at about 60 nM. Such an inhibition was also observed in the presence of different protein phosphatase inhibitors, i.e. okadaic acid, calyculin A and microcystin-RR. This effect was observed in whole hepatocytes, in the supernatant of the post-mitochondrial fraction and in the microsomal fraction. It was independent of a substrate supply and of the labelled amino acid used. Furthermore, this inhibition preceded the previously reported glucose-6-phosphatase activation induced by microcystin-LR [Claeyssens, Chédeville and Lavoinne (1993) FEBS Lett. 315, 7-10].

Fructose metabolism and cell survival in freshly isolated rat hepatocytes incubated under hypoxic conditions: proposals for potential clinical use

The protective effect of fructose with regard to hypoxia-induced cell injury was investigated. The addition of fructose (2 to 20 mmol/L) protected hepatocytes against hypoxia-mediated cell lysis in a concentration-dependent way. The intracellular ATP content was initially decreased as a result of fructose-1-phosphate formation, but it remained constant during the hypoxic incubation. Conversely, high initial ATP values observed at low fructose concentrations progressively declined. Cellular protection was observed only when fructose was added before (and not after) the start of hypoxia. In addition, a sufficient amount of fructose-1-phosphate rapidly accumulated before the induction of hypoxia, and the linear production of lactate, during hypoxic incubation, indicated that cells synthesized ATP continuously. The lack of cell protection by fructose added after the onset of the hypoxia may be explained by a lesser fructose-1-phosphate formation and a subsequently low accumulation leading to insufficient glycolytic ATP production. Under aerobic conditions, both glycolysis (lactate formation) and gluconeogenesis (glucose formation) were carried out in fructose-1-phosphate-loaded cells with the same initial rates, whereas under hypoxic conditions glycolysis was the main metabolic event. The fact that protein synthesis activity recovered faster during reoxygenation of previously hypoxic fructose-treated cells than in glucose-treated cells led us to hypothesize that in situ perfusion of liver with fructose, before its removal, would improve its metabolic capacity during the hypoxic cold preservation and subsequent transplantation.

Cytolytic effects and biochemical changes induced by extracellular ATP to isolated hepatocytes

Cell death, as estimated by the release of lactate dehydrogenase (LDH), was induced by incubating isolated hepatocytes for 60 min in the presence of extracellular ATP (ecATP), while AMP, adenosine, GTP and UTP were without any significant effects, even when tested at 3 mM (final concentration). At such a concentration, the release of LDH induced by ecATP, but also by ecADP, reached almost 50% and 30%, respectively. Since UTP and GTP (which have no lytic effects) were able to activate phosphorylase a at the same rate as ATP, we excluded the possibility that an increase of free cytosolic Ca2+ triggers the onset of a process leading to cell lysis. Moreover, such a lytic ability of ecATP (1.7 mM) can not be the result of a previous complexation of ionic iron (making it catalytically available for a Fenton reaction), because Desferal, a strong iron chelator, did not modify the cytolytic effect of the ecATP observed after 60 min of incubation. A major cellular function such as protein synthesis was impaired in a dose-dependent way by incubating hepatocytes during 60 min in the presence of ecATP. The inhibition was already observed at 0.1 mM ecATP, a dose without any effect on cell viability. The biological relevance of such metabolic impairment, however, remains to be elucidated.

Cell death and lipid peroxidation in isolated hepatocytes incubated in the presence of hydrogen peroxide and iron salts

The incubation of isolated hepatocytes in the presence of glucose plus glucose oxidase, a H2O2-generating system, resulted in extensive loss of cell viability, as expressed by the release of lactate dehydrogenase (LDH). Disturbance of metabolic functions such as glycogen and protein synthesis was also caused by H2O2, but in no case was malondialdehyde (MDA)-like products detected. The lytic effect of H2O2 was significantly enhanced by incubating hepatocytes in the presence of iron salts. Under these conditions, MDA-like products were detected, but lipid peroxidation and cell injury did not correlate. Iron chelators modulated the cytotoxicity of H2O2 in different (and opposite) ways: when iron was complexed with ADP, increased cell lysis was observed compared to uncomplexed iron plus H2O2. Iron-DTPA, on the contrary, decreased such a lytic effect. The preincubation of hepatocytes with desferrioxamine mesylate (Desferal; a strong iron chelator) abolished the cytolytic effects produced by the association of iron salts and H2O2, as well as the membrane oxidative injury due to H2O2 alone, thus suggesting the existence of an intracellular source of iron. This kind of mechanism (metal chelation rather than radical scavenging) is supported by the absence of any protective effect by some free radical scavengers against the oxidative injury induced by the association iron H2O2. Nevertheless, the glycogenolytic effects observed in the presence of H2O2 were not modified by Desferal. In our opinion, the cytotoxicity of the association H2O2 plus iron salts involves at least two different and independent mechanisms.

The effects of ibuprofen enantiomers on hepatocyte intermediary metabolism and mitochondrial respiration

In vivo and in vitro (-)R-ibuprofen is inverted to the (+)S antipode via stereoselective formation of an R-ibuprofenyl-CoA intermediate. In this study the effects of (-)R- and (+)S-ibuprofen on metabolism and respiration were studied using isolated rat hepatocytes and mitochondria. R-Ibuprofen significantly increased the lactate to pyruvate ratio, perturbed mitochondrial ketogenesis as evidenced by alterations in the beta-hydroxybutyrate to acetoacetate ratio and uncoupled mitochondrial oxidative phosphorylation. In addition, substantial dose- and time-dependent sequestration of reduced CoA (CoASH) occurred in the presence of the R enantiomer. Similarly, S-ibuprofen altered both the cytosolic and mitochondrial redox states although the magnitude of the effect was substantially less than that observed with the R enantiomer. In contrast to R-ibuprofen, S-ibuprofen did not uncouple oxidative phosphorylation or sequester hepatocyte CoASH. It is proposed that the perturbations observed in hepatocyte intermediary metabolism and mitochondrial function are attributable to a combination of the direct effects of R-ibuprofen per se and the sequestration of CoASH as R-ibuprofenyl-CoA during the process of chiral inversion. On the basis of these results, R-ibuprofen should be considered more in terms of metabolism to a reactive acyl-CoA intermediate rather than as a pro-drug for the pharmacologically active S-enantiomer.

Characterization of cryopreserved rat liver parenchymal cells by metabolism of diagnostic substrates and activities of related enzymes

The metabolism of testosterone and benzo(a)pyrene (BaP) which is mediated by diverse enzymes was determined in cryopreserved rat liver parenchymal cells and compared with that found in freshly isolated cells. In addition, the activities of single xenobiotic-metabolizing enzymes were measured by using specific substrates. The cytochrome P450 (P450)-mediated total metabolic conversion of testosterone was reduced to 55% in cryopreserved cells. The metabolite profile, i.e. the formation of single metabolites compared with total metabolic conversion, was however unchanged when compared with freshly isolated cells. A concomitant reduction in the activities of the involved P450 isoenzymes can therefore be postulated. The amount of detected phase I-metabolites of BaP was unaffected by the cryopreservation method. The formation of phase II-metabolites and total metabolic conversion of BaP in cryopreserved cells was however reduced to about 50-60%. The reduced glutathione S-transferase and more obviously phenol sulfotransferase activities measured in cryopreserved cells, may explain the impaired conjugation of BaP. The ratio between phase I- and phase II-metabolites was thus changed by cryopreservation. Density separation on Percoll yielded cryopreserved cells with a viability and metabolic capacity not measurably different from freshly isolated cells. To this extent, cryopreserved, Percoll-purified liver parenchymal cells are a useful in vitro system for drug metabolism studies. However due to the extensive loss in cell number during this procedure (recovery = 22% of freshly isolated cells) the application of this system is limited.

Urea and protein synthesis in cold-preserved isolated rat hepatocytes

We used an isolated-hepatocyte model to study how hypothermic storage (simulating liver preservation) affects metabolism after prolonged preservation. Rat hepatocytes were stored in the University of Wisconsin solution for up to 72 hr. After each day of storage, protein synthesis, urea synthesis, ATP content and lactate dehydrogenase release were determined in rewarmed (37 degrees C) and oxygenated hepatocytes. Protein synthesis ([3H]-leucine incorporation into protein) was depressed by 16% +/- 4%, 54% +/- 6% and 69% +/- 4% after 24 hr, 48 hr and 72 hr, respectively. Urea synthesis, ATP synthesis and lactate dehydrogenase release were similar to those in control hepatocytes (no preservation). Fasting of the rats before isolation of hepatocytes caused more rapid loss of protein-synthesis capabilities (59% in 24 hr) with no significant loss of lactate dehydrogenase, urea synthesis or ATP synthesis. Hepatocyte viability (lactate dehydrogenase release) as judged by membrane permeability, ATP synthesis and potassium content can be maintained after up to 6 days of cold storage. However, protein synthesis is depressed after only 48 hr of cold storage. Thus hypothermic storage of the liver causes a change in the metabolic capabilities of the hepatocytes, and the timing of the loss of protein synthesis is similar to the limits of successful cold storage of the whole liver (48 hr). Thus a limit to long-term storage of the liver may be related to loss of protein synthesis. In liver transplantation, one indication of poor preservation is a decrease in serum albumin and clotting factors with increased tissue edema and bleeding diathesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical changes in isolated hepatocytes exposed to tert-butyl hydroperoxide. Implications for its cytotoxicity

When isolated hepatocytes were exposed to tert-butyl hydroperoxide (tBOOH) they lost their cellular membrane integrity. Decreased levels of GSH, increased phosphorylase a activity (an indirect index of the amount of free cytosolic Ca2+), and increase in the formation of malondialdehyde (MDA)-like products (an index of lipid peroxidation) preceded the release into the culture medium of the cytosolic enzyme lactate dehydrogenase (LDH), indicating that this later process was the consequence of the former intracellular events. While ATP levels were not modified during the incubation of cells with increasing concentrations of tBOOH, protein synthesis was decreased in a concentration-dependent manner. The glycogen content decreased at the same time as the increase in LDH leakage. The addition of promethazine (PMZ) an antioxidant molecule, prevented the lipid peroxidation, but did not protect cells against the oxidative effects of tBOOH, including loss of membrane integrity. Nevertheless, the addition of GSH to cell suspensions incubated with tBOOH, decreased the formation of MDA-like products, restored the protein synthesis rate, prevented partially the activation of phosphorylase a and preserved cell viability. On the basis of these results, we postulate that both GSH depletion and modification in phosphorylase a activity (Ca2+ levels) were the most relevant intracellular events to explain the cytotoxicity of tBOOH.

Interaction between aflatoxin B1 and oxytetracycline in isolated rat hepatocytes

Isolated rat hepatocytes were used as an in vitro model to investigate a possible interaction between oxytetracycline (OXT) and aflatoxin B1 (AFB1). LDH leakage, RNA and protein synthesis and glycogen accumulation were measured in the presence of both drugs, either separately or in combination. The evolution of LDH leakage during the incubation was identical in untreated and treated cells. AFB1 inhibited RNA and protein synthesis at a concentration of 10(-7) M and 10(-6) M, respectively, and higher, whereas OXT did not influence RNA synthesis but inhibited protein synthesis at the highest tested concentration, 10(-3) M. As far as glycogen is concerned, rats were injected with glucagon before sacrifice in order to obtain a constant synthesis rate in isolated hepatocytes. AFB1 inhibited the accumulation of glycogen from 10(-6) M upward. This effect was never observed before 90 min of incubation. OXT had no effect on glycogen synthesis. In the presence of both drugs, no interaction was demonstrated as far as RNA and protein synthesis were concerned, but OXT opposed the inhibition induced by AFB1 on glycogen accumulation. If the "in vivo" protection, provided by OXT against AFB1-induced toxicity, is due to a direct interference in the toxic mechanisms of the mycotoxin, these results show that OXT does not influence the AFB1-inhibition of RNA and protein synthesis. The latter are early and sensitive parameters inhibited by AFB1. On the contrary, taking into consideration the results on glycogen accumulation, it seems more interesting to investigate further this metabolism.

Regio- and stereoselective regulation of monooxygenase activities by isoenzyme-selective phosphorylation of cytochrome P450

The phosphorylation of the two major phenobarbital-inducible cytochrome P450 isoenzymes IIB1 and IIB2 was increased in hepatocytes by the action of the membrane permeating cAMP derivatives N6-dibutyryl-cAMP and 8-thiomethyl-cAMP. Under these conditions the dealkylation of 7-pentoxyresorufin, a selective substrate of cytochrome P450IIB1 and P450IIB2 was markedly reduced. 16 beta-Hydroxylation of testosterone which is catalyzed specifically only by cytochrome P450IIB1 and IIB2 was strongly reduced; for 16 alpha-hydroxylation which is also catalyzed by cytochrome P450IIB1 and IIB2 but additionally by 3 further cytochrome P450 isoenzymes, this reduction was less pronounced; for the oxidation of the 17 beta-hydroxyl group which besides cytochromes P450IIB1 and IIB2 is additionally catalyzed not only by other cytochromes P450 but also by 17 beta-hydroxysteroid dehydrogenase there was a clear tendency of reduction which, however, no longer reached statistical significance. Hydroxylation at other positions of testosterone which are catalyzed by other cytochrome P450 isoenzymes were not significantly changed. Hence isoenzyme-selective phosphorylation of cytochrome P450 leads to a corresponding isoenzyme-selective modulation of monooxygenase activity which holds promise to be especially important as a fast regulation of the control of genotoxic metabolites.

Identification of the inhibitor of the plasminogen activator as the major protein secreted by endothelial rat liver cells

Freshly isolated Kupffer and endothelial liver cells exhibit a rate of 'de novo' protein synthesis which is twice as high per mg cell protein as that of parenchymal liver cells and contribute significantly (7.5% and 5.9%, respectively) to total liver protein secretion. In parenchymal cells the main secretory protein is a 68 kDa protein (containing 19% fo the secreted radioactivity, presumably albumin). In Kupffer cells a 49 kDa protein contains 8% of the secreted radioactivity, while in endothelial liver cells a 55 kDa protein is the most prominent secretory protein (containing 11% of the secreted radioactivity). By aid of a specific antibody the 55 kDa protein was identified as the inhibitor of the plasminogen activator and in the liver this protein was only secreted by the endothelial cells.

Investigation into the mechanism of tetracycline-induced steatosis: study in isolated hepatocytes

Tetracycline is known to cause hepatic dysfunction in humans by inducing steatosis. Accumulation of fat in the liver could result from biochemical effects at various levels in the sequence from protein and triglyceride synthesis to lipoprotein secretion. The effects of tetracycline on the synthesis and secretion of triglycerides and proteins were studied in isolated rat hepatocytes surviving in suspension for up to 2.5 hr. Interpretation of the results obtained for tetracycline was made by comparison with results obtained, under the same experimental conditions, for the well-known steatogenic compounds, cycloheximide and colchicine. The data indicate that tetracycline produces a concentration-dependent inhibition of 14C-triglyceride secretion without affecting triglyceride synthesis. This inhibition explains the intracellular triglyceride accumulation. However, tetracycline does not affect protein secretion. Furthermore, it was demonstrated that the effect of tetracycline on protein synthesis was not related to inhibition of triglyceride release. In conclusion, it is proposed that the effect of tetracycline could be at the level of the association between triglycerides and apoproteins to form lipoproteins.

Distribution and induction of cytochrome P-450 and two cytochrome P-450-dependent monooxygenase activities in rat liver parenchymal cell subpopulations separated by centrifugal elutriation

Liver parenchymal cells from the periportal and centrilobular zones differ in their morphological, biochemical and functional characteristics. In an effort to obtain fractions enriched in either periportal or centrilobular cells, isolated rat liver parenchymal cells were separated into five subpopulations by centrifugal elutriation. The mean diameters of the cells present in fractions I-V were 19.6, 21.1, 21.8, 22.7 and 23.5 micron, respectively. The content of cytochrome P-450 as well as benzphetamine N-demethylase and 7-ethoxyresorufin O-deethylase activities were higher in the larger parenchymal cells than in the smaller ones. After administration of phenobarbital the content of cytochrome P-450 was approximately two-fold greater in the cells present in fractions 3-5, when compared to the same subpopulations isolated from untreated rats; the activity of benzphetamine N-demethylase was enhanced to a similar extent in all five fractions. 3-Methylcholanthrene treatment resulted in a significant increase of cytochrome P-450 content and 7-ethoxyresorufin O-deethylase activity in all five fractions: both parameters were slightly higher in fractions 4 and 5 than in fractions 1 and 2. In conclusion, the elutriated liver parenchymal cells seem to preserve the biochemical heterogeneity observed in the intact liver; the potential enrichment of periportal and centrilobular cells in the different fractions by centrifugal elutriation is discussed.

The stereospecific incorporation of fenoprofen into rat hepatocyte and adipocyte triacylglycerols

The formation of triacylglycerols containing fenoprofen was studied in rat isolated adipocytes and hepatocytes incubated with [3H]glycerol and R or S fenoprofen. In both hepatocytes and adipocytes there was a high-affinity enzymatic process for the synthesis of triacylglycerol containing fenoprofen which was stereospecific for the R enantiomer. The apparent Km values for R fenoprofen were 1.0 microM in adipocytes and 2.8 microM in hepatocytes. These results are consistent with the proposed stereospecific formation of R-2-arylpropionyl-CoA thioesters resulting in the stereospecific formation of R-tri-acylglycerol at clinically relevant unbound fenoprofen concentrations. In isolated hepatocytes, but not adipocytes, a second low-affinity enzymatic process for the synthesis of triacylglycerol containing fenoprofen was also observed. However, this process (Km = 3780 microM) occurred at concentrations much higher than those found in man with usual doses.

A radioisotopic assay of picomolar concentrations of coenzyme A in liver tissue

A single-step enzyme assay using [14C]palmitic acid and bacterial acyl-coenzyme A synthetase (EC 6.2.1.3) is described for the determination of reduced coenzyme A (CoASH) levels in liver samples. Use of this technique provides a rapid and accurate determination of CoASH in the range 1-250 pmol. Application of the method to the quantitation of CoASH in samples of human liver tissue and rat liver homogenate, isolated hepatocytes, and mitochondria is described.

Effect of tryptophan on isolated hepatocytes of rats

The addition of tryptophan to adult rat hepatocyte cultures stimulated DNA synthesis. The increase in DNA synthesis as measured by 3H-thymidine incorporation into DNA was observed on treatment of the cultures with tryptophan for 48 h but also as short as for 6 h in comparison with control cultures. An increase was also apparent at 30 h which was maintained for up to 48 h post treatment with tryptophan. The increase in DNA synthesis by tryptophan cannot be attributed to cell injury or to increased DNA degradation. Of the degradative enzymes added after harvesting the hepatocytes, only DNase decreased incorporation of 3H-thymidine. The observed effect was specific for tryptophan since treatment with kynurenine, isoleucine, methionine or serine failed to show a significant effect. Pretreatment of cultured hepatocytes with hydroxyurea prevented the tryptophan stimulated increase in DNA synthesis suggesting that the latter was due to replicative and not to reparative DNA synthesis. Experiments performed with the addition of diethylnitrosamine also alluded to tryptophan's role in replicative DNA synthesis. The mechanism of tryptophan-induced DNA synthesis is discussed.

High-performance liquid chromatographic quantitation of triacylglycerols containing fenoprofen from biological samples

A normal-phase high-performance liquid chromatographic (HPLC) method has been developed for the quantitation of radiolabelled triacylglycerols containing fenoprofen, synthesized from [3H]glycerol by isolated hepatocytes and adipocytes. The assay consists of extracting the lipids into diethyl ether, separating triacylglycerols from polar endogenous lipids using silica Sep-Pak cartridges and quantitating endogenous triacylglycerols and triacylglycerols containing fenoprofen by HPLC resolution and scintillation counting. HPLC separation is achieved in less than 10 min. Using [14C]tripalmitin as internal standard the assay has a linear relationship between added triacylglycerol and measured endogenous triacylglycerols and triacylglycerols containing fenoprofen with regression coefficients of 0.997 and 0.998, respectively.

Isolation of hepatocytes from postnatal mice

A method for the isolation of hepatocytes from postnatal (1- to 3-week-old) mice has been developed. Cell isolation was carried out by retrograde perfusion of the liver with a collagenase-containing bicarbonate buffer. Viable cells were separated by selective adsorption onto collagen membranes. Cell viability was assessed by measuring ATP and glutathione content, lactate:pyruvate ratio, and the rate of protein synthesis. Comparisons of these parameters were made with those in cells isolated from adult mice and with values in whole liver. These hepatocytes were capable of metabolizing acetaminophen to its known hepatic conjugates and were susceptible to acetaminophen toxicity. This procedure for isolating mouse hepatocytes should be useful for the study of hepatic drug metabolism and its relationship to toxicity in the postnatal mouse.

Comparison of hepatic protein synthesis in vivo versus in vitro in the tumor-bearing rat

We have used the model of the freshly isolated hepatocyte to study liver metabolism in a rat tumor model and have reported elevations in the rates of protein synthesis, gluconeogenesis, alanine transport, and oxygen utilization by the tumor-influenced hepatocyte. While the metabolic activity of the isolated hepatocyte has been taken to reflect metabolism in the in vivo state, this assumption has not been validated. In the present study, we measure hepatic protein synthesis in vivo using a flooding dose of tracer amino acid, and in vitro in hepatocytes freshly isolated from tumor-bearing (TB) and pair-fed control rats. Increased protein synthesis was observed for the TB rats using both methods of analysis. However, the degree of stimulation seen in the TB animals was much greater in the in vitro assay than in the in vivo approach, suggesting that absolute protein synthetic rates in vitro must be interpreted with caution when extrapolating to the in vivo state.