María José Gómez-Lechón

Interleukin-6 is the major regulator of acute phase protein synthesis in adult human hepatocytes

The three monokines interleukin‐1β (IL‐1β), tumor necrosis factor α (TNFα), and interleukin‐6 (IL‐6) modulate acute phase plasma protein synthesis in adult human hepatocytes. Only IL‐6 stimulates the synthesis of the full spectrum of acute phase proteins as seen in inflammatory states in humans, i.e. synthesis and secretion of C‐reactive protein, serum amyloid A, fibrinogen, α1‐antitrypsin, α1‐antichymotrypsin and haptoglobin are increased while albumin, transferrin and fibronectin are decreased. IL‐1β as well as TNFα, although having a moderate effect on the positive acute phase proteins and inhibiting the synthesis of fibrinogen, albumin and transferrin, fail to induce serum amyloid A and C‐reactive protein. These data suggest that IL‐6 plays the key role in the regulation of acute phase protein synthesis in human hepatocytes.

Recombinant human interleukin‐6 (IL‐6/BSF‐2/HSF) regulates the synthesis of acute phase proteins in human hepatocytes

Recombinant human IL‐6 (rhIL‐6) is a potent inducer of the synthesis of acute phase proteins in adult human hepatocytes. A wide spectrum of acute phase proteins is regulated by this mediator. After labeling of rhIL‐6 stimulated human hepatocytes with [35S]methionine acute phase protein synthesis was measured by immunoprecipitation. Serum amyloid A, C‐reactive protein, haptoglobin, α1‐antichymotrypsin and fibrinogen were strongly induced (26‐, 23‐, 8.6‐, 4.6‐ and 3.8‐fold increases, respectively). Moderate increases were found for α1‐antitrypsin (2.7‐fold) and α1‐acid glycoprotein (2.7‐fold). RhIL‐6 had no effect on α1‐macroglobulin, whereas fibronectin, albumin and transferrin decreased to 64, 56 and 55% of controls. In the cases of serum amyloid A, haptoglobin, α1‐antichymotrypsin, α1‐antitrypsin and α1‐acid glycoprotein, dexamethasone enhanced the action of rhIL‐6. We conclude that rhIL‐6 controls the acute phase response in human liver cells.

Human Hepatocytes in Primary Culture: The Choice to Investigate Drug Metabolism in Man

Different types of hepatic tissue, including whole or split livers from organ donors or waste liver from therapeutic liver resections, are used to prepare human hepatocyte cultures. Characteristics of liver samples from different origins (gender, age, healthy/pathological status, xenobiotic treatment) as sources of human hepatocytes are key factors which notably determine viability and functionality of hepatocytes. The characterisation of the CYP system can be assessed in terms of activity (using specific substrates/inhibitors), protein (antibody analysis) and molecular biology-based mRNA amplification techniques (PCR technology and DNA microarrays). It could reasonably be considered that human hepatocytes reflect the heterogeneity of CYP expression in human liver and is a suitable model for drug metabolism studies. Several key issues need to be addressed at the early stages of drug development to better select drug candidates (metabolic profile and rate, identification of CYPs involved, drug-drug interactions due to enzyme induction/inhibition). The metabolic stability and metabolite profile of new chemicals can be easily investigated by incubating the drugs with fully competent metabolic models like hepatocyte suspensions or 24 h-cultured hepatocytes. CYP inhibitory effects are usually screened in recombinant CYP enzymes or microsomes, however, the actual concentration of substrate and inhibitor available to the CYP enzyme depends on processes missing in subcellular models (transport mechanisms, cytosolic enzymes, binding to intracellular proteins). Since intact cells more closely reflect the environment to which drugs are exposed in the liver, cultured hepatocytes constitute a more predictive model for drug-drug interactions. Screening of CYP inducers cannot be done in microsomes as it requires a cellular system fully capable of expressing CYP genes. Primary hepatocytes are still the unique in vitro model for global examination of inductive potential of drugs (monitored as increases in mRNA content or activity).

Cell Lines: A Tool for In Vitro Drug Metabolism Studies

Primary cultured hepatocytes are a valuable in vitro model for drug metabolism studies. However, their widespread use is greatly hindered by the scarcity of suitable human liver samples. Moreover, the well-known in vitro phenotypic instability of hepatocytes, the irregular availability of fresh human liver for cell harvesting purposes, and the high batch-to-batch functional variability of hepatocyte preparations obtained from different human liver donors, seriously complicate their use in routine testing. To overcome these limitations, different cell line models have been proposed for drug metabolism screening. Human liver-derived cell lines would be ideal models for this purpose given their availability, unlimited life-span, stable phenotype, and the fact that they are easy to handle. However, the human hepatoma cells currently used (i.e. HepG2, Mz-Hep-1) show negligible levels of drug-metabolizing and do not constitute a real alternative to primary hepatocytes. Different strategies have been proposed to generate metabolically competent immortalized hepatocytes (transformation of human hepatocytes with plasmids encoding immortalizing genes, hepatocyte-like cells derived from stem cells, cell lines generated from transgenic animals, hepatocyte/hepatoma hydrid cells). Moreover, recombinant models heterologously expressing P450 enzymes in different host cells have been developed and successfully used in drug metabolism testing. In addition, new strategies have recently been explored to upregulate the expression of drug-metabolizing enzymes in cell lines of a human origin (i.e. transfection with expression vectors encoding key hepatic transcription factors). Among metabolic-based drug-drug interactions, P450 inhibition seems to be the most important. A major application of recombinant models expressing a single P450 is the screening of potential enzyme inhibitors. Therefore, pharmaceutical companies increasingly make use of cell lines to speed up the selection of new drugs with favourable pharmacokinetic and metabolic properties.

Human Hepatocytes as a Tool for Studying Toxicity and Drug Metabolism

Drugs are usually biotransformed into new chemical species that may have either toxic or therapeutic effects. Drug metabolism studies are routinely performed in laboratory animals but, due to metabolic interspecies differences when compared to man, they are not accurate enough to anticipate the metabolic profile of a drug in humans. Human hepatocytes in primary culture provide the closest in vitro model to human liver and the only model that can produce a metabolic profile of a given drug that is very similar to that found in vivo. However their availability is limited due to the restricted access to suitable tissue samples. The scarcity of human liver has led to optimising the cryopreservation of adult hepatocytes for long-term storage and regular supply. Human hepatocytes in primary culture express typical hepatic functions and express drug metabolising enzymes. Moreover, qualitative and quantitative similarities between in vitro and in vivo metabolism of drugs were observed. Different strategies have been envisaged to prolong cell survival and delay the spontaneous decay of the differentiated phenotype during culture. Thus, hepatocytes represent the most appropriate model for the evaluation of integrated drug metabolism, toxicity/metabolism correlations, mechanisms of hepatotoxicity, and the interactions (inhibition and induction) of xenobiotics and drug-metabolising enzymes. However, in view of limitations of primary hepatocytes, efforts are made to develop alternative cellular models (i.e. metabolic competent CYP-engineered cells stably expressing individual CYPs and transient expression of CYPs by transduction of hepatoma cells with recombinant adenoviruses). In summary, several cellular tools are available to address key issues at the earliest stages of drug development for a better candidate selection and hepatotoxicity risk assessment.

Long‐term expression of differentiated functions in hepatocytes cultured in three‐dimensional collagen matrix

Hepatocytes entrapped in collagen gel and cultured in serum‐free conditions survived longer than cells cultured on plastic (5 days vs. 3 weeks), showed fewer signs of early cell senescence (no increase in c‐fos oncoprotein expression), and maintained the expression of differentiated hepatic metabolic functions over a longer period of time. Cells cultured in collagen gels retained their ability to respond to hormones. The insulin‐stimulated glycogen synthesis rate remained fairly constant during 18 days in culture (between 5.4 ± 0.37 and 9 ± 2.7 nmol glucose/h/μg DNA). Collagen‐cultured hepatocytes recovered glycogen stores to levels similar to those found in liver, or in hepatocytes isolated from fed rats. Urea synthesis from ammonia remained stable for more than 2 weeks (average value, 23 ± 4 nmol urea/h/μg DNA). The rate of albumin synthesis in collagen‐entrapped cells was maintained above the day‐1 level during 18 days in culture. Cells showed high levels of glutathione (GSH) (1,278 ± 152 pmol/μg DNA). Biotransformation activities CYP4501A1, CYP4502A2, CYP4502B1, and CYP4503A1 remained fairly stable in collagen‐cultured hepatocytes. CYP4502E1 and CYP4502C11 decreased but were still measurable after 18 days. After 4 days in culture, GST activity returned to levels observed in isolated hepatocytes. In contrast with plastic cultures, cells responded to CYP450 inducers (methylcholanthrene for CYP4501A1, CYP4501A2, and gluthatione‐transferase, and ethanol for CYP4502E1) for more than 2 weeks. CYP4501A1, CYP4501A2, and glutathione‐transferase A2 (GST A2) induction was preceded by an increase in specific mRNA, while the effects on CYP4502E1 seemed to be at a posttranslational level. Analysis of the expression of relevant hepatic genes by reverse Northern and semiquantitative reverse transcriptase‐polymerase chain reaction (RT‐PCR) revealed that culturing hepatocytes in collagen gels results in a sustained higher expression of key liver transcription factor genes DBP, C/EBP‐α and ‐β, and HNF‐1 and ‐4, as well as specific liver enzyme genes (phosphoenol pyryvate carboxykinase, and carbamoylphosphate‐synthetase I). J Cell Physiol 177:553–562, 1998.

PHOTODYNAMIC LIPID PEROXIDATION BY THE PHOTOSENSITIZING NONSTEROIDAL ANTIINFLAMMATORY DRUGS SUPROFEN AND TIAPROFENIC ACID

Abstract The photochemistry of the photosensitizing nonsteroidal antiinflammatory drugs tiaprofenic acid and suprofen involves the intermediacy of short‐lived species (i.e. radicals). The data obtained in the present work strongly suggest that such intermediates may be responsible for the phototoxicity of 2‐arylpropionic acids by inducing photodynamic lipid peroxidation at drug concentrations likely to be reached in the skin. This has been investigated using linoleic acid as a model lipid and determining the amount of hydroperoxides by measuring the spectrophotometric absorption at 233 nm, associated with the formation of dienic hydroperoxides. The major photoproducts of tiaprofenic acid and suprofen are derivatives bearing an ethyl side chain. Photoproducts of this type, due to the lack of polar moieties, are highly lipophilic and likely to accumulate in the lipid bilayer of cell membranes. Taking into account their ability to induce photodynamic lipid peroxidation and their marked photostability, it is conceivable that such photoproducts can participate in many catalytic cycles, playing a significant role in the mechanism of photosensitizatinn by tiprofenic acid and suprofen.

An update on metabolism studies using human hepatocytes in primary culture

Background: Cultured human hepatocytes are the closest in vitro model to human liver and constitute a very predictive model for drug metabolism in vivo. The variability observed in human hepatocytes reflects the existing phenotypic heterogeneity of cytochrome P450 expression in human liver. Objectives: As drug metabolism is the major source of pharmacokinetic variability in human beings, the main areas of current drug metabolism research in human hepatocytes are reviewed. Methods: To speed up the selection of drug candidates, the evaluation of metabolic stability, metabolite profiling and identification, and drug–drug interaction potential are key issues in drug development. Results/conclusion: In vitro drug metabolism studies, which are inexpensive and readily carried out, serve as an adequate screening mechanism to characterize drug metabolites, elucidate their pathways, and make suggestions for further in vivo testing.

Sequential Hepatogenic Transdifferentiation of Adipose Tissue-Derived Stem Cells: Relevance of Different Extracellular Signaling Molecules, Transcription Factors Involved, and Expression of New Key Marker Genes:

Adipose tissue contains a mesenchymal stem cell (MSC) population known as adipose-derived stem cells (ASCs) capable of differentiating into different cell types. Our aim was to induce hepatic transdifferentiation of ASCs by sequential exposure to several combinations of cytokines, growth factors, and hormones. The most efficient hepatogenic protocol includes fibroblastic growth factors (FGF) 2 and 4 and epidermal growth factor (EGF) (step 1), hepatocyte growth factor (HGF), FGF2, FGF4, and nicotinamide (Nic) (step 2), and oncostatin M (OSM), dexamethasone (Dex), and insulin-tranferrin-selenium (step 3). This protocol activated transcription factors [GATA6, Hex, CCAAT/enhancer binding protein α and β (CEBPα and β), peroxisome proliferator-activated receptor-γ, coactivator 1 α (PGC1α), and hepatocyte nuclear factor 4 α (HNF4α)], which promoted a characteristic hepatic phenotype, as assessed by new informative markers for the step-by-step hepatic transdifferentiation of hMSC [early markers: albumin (ALB), α-2-macroglobuline (α2M), complement protein C3 (C3), and selenoprotein P1 (SEPP1); late markers: cytochrome P450 3A4 (CYP3A4), apolipoprotein E (APOE), acyl-CoA synthetase long-chain family member 1 (ACSL1), and angiotensin II receptor, type 1 (AGTR1)]. The loss of adipose adult stem cell phenotype was detected by losing expression of Thy1 and inhibitor of DNA binding 3 (Id3). The reexpression of phosphoenolpyruvate corboxykinase (PEPCK), apolipoprotein C3 (APOCIII), aldolase B (ALDOB), and cytochrome P450 1A2 (CYP1A2) was achieved by transduction with a recombinant adenovirus for HNF4α and finally hepatic functionality was also assessed by analyzing specific biochemical markers. We conclude that ASCs could represent an alternative tool in clinical therapy for liver dysfunction and regenerative medicine.

Interactions of polyphenols with the P450 system: possible implications on human therapeutics.

Polyphenols are a family of natural compounds with many biological properties. This review focuses on their potential interaction on the cytochrome P450 system. Effects of phenolic acids, anthocyanins, stilbenes, catechins and other flavonoids on the drug metabolising function are revised. Their daily intake and presence in herbal medicines justify the study of potential drug-interaction to prevent undesirable clinical consequences.