Perifused monolayer cultures of rat hepatocytes as an improved in vitro system for studies on ureogenesis

Three-dimensional (3D) liver cell models - a tool for bridging the gap between animal studies and clinical trials when screening liver accumulation and toxicity of nanobiomaterials

Despite the exciting properties and wide-reaching applications of nanobiomaterials (NBMs) in human health and medicine, their translation from bench to bedside is slow, with a predominant issue being liver accumulation and toxicity following systemic administration. In vitro 2D cell-based assays and in vivo testing are the most popular and widely used methods for assessing liver toxicity at pre-clinical stages; however, these fall short in predicting toxicity for NBMs. Focusing on in vitro and in vivo assessment, the accurate prediction of human-specific hepatotoxicity is still a significant challenge to researchers. This review describes the relationship between NBMs and the liver, and the methods for assessing toxicity, focusing on the limitations they bring in the assessment of NBM hepatotoxicity as one of the reasons defining the poor translation for NBMs. We will then present some of the most recent advances towards the development of more biologically relevant in vitro liver methods based on tissue-mimetic 3D cell models and how these could facilitate the translation of NBMs going forward. Finally, we also discuss the low public acceptance and limited uptake of tissue-mimetic 3D models in pre-clinical assessment, despite the demonstrated technical and ethical advantages associated with them.

Primary Hepatocytes Cultured on a Fiber-Embedded PDMS Chip to Study Drug Metabolism

In vitro drug screening using reliable and predictable liver models remains a challenge. The identification of an ideal biological substrate is essential to maintain hepatocyte functions during in vitro culture. Here, we developed a fiber-embedded polydimethylsiloxane (PDMS) chip to culture hepatocytes. Hepatocyte spheroids formed in this device were subjected to different flow rates, of which a flow rate of 50 μL/min provided the optimal microenvironment for spheroid formation, maintained significantly higher rates of albumin and urea synthesis, yielded higher CYP3A1 (cytochrome P450 3A1) and CYP2C11 (cytochrome P450 2C11) enzyme activities for metabolism, and demonstrated higher expression levels of liver-specific genes. In vitro metabolism tests on tolbutamide and testosterone by hepatocytes indicated predicted clearance rates of 1.98 ± 0.43 and 40.80 ± 10.13 mL/min/kg, respectively, which showed a good in vitro⁻in vivo correspondence. These results indicate that this system provides a strategy for the construction of functional engineered liver tissue that can be used to study drug metabolism.

Multi-cellular 3D human primary liver cell culture elevates metabolic activity under fluidic flow

We have developed a low-cost liver cell culture device that creates fluidic flow over a 3D primary liver cell culture that consists of multiple liver cell types, including hepatocytes and non-parenchymal cells (fibroblasts, stellate cells, and Kupffer cells). We tested the performance of the cell culture under fluidic flow for 14 days, finding that hepatocytes produced albumin and urea at elevated levels compared to static cultures. Hepatocytes also responded with induction of P450 (CYP1A1 and CYP3A4) enzyme activity when challenged with P450 inducers, although we did not find significant differences between static and fluidic cultures. Non-parenchymal cells were similarly responsive, producing interleukin 8 (IL-8) when challenged with 10 μM bacterial lipoprotein (LPS). To create the fluidic flow in an inexpensive manner, we used a rocking platform that tilts the cell culture devices at angles between ±12°, resulting in a periodically changing hydrostatic pressure drop between reservoirs and the accompanying periodically changing fluidic flow (average flow rate of 650 μL min(-1), and a maximum shear stress of 0.64 dyne cm(-2)). The increase in metabolic activity is consistent with the hypothesis that, similar to unidirectional fluidic flow, primary liver cell cultures increase their metabolic activity in response to fluidic flow periodically changes direction. Since fluidic flow that changes direction periodically drastically changes the behavior of other cells types that are shear sensitive, our findings support the theory that the increase in hepatic metabolic activity associated with fluidic flow is either activated by mechanisms other than shear sensing (for example increased opportunities for gas and metabolite exchange), or that it follows a shear sensing mechanism that does not depend on the direction of shear. Our mode of device operation allows us to evaluate drugs under fluidic cell culture conditions and at low device manufacturing and operation costs.

Microengineered cell and tissue systems for drug screening and toxicology applications: Evolution of in-vitro liver technologies

The liver performs many key functions, the most prominent of which is serving as the metabolic hub of the body. For this reason, the liver is the focal point of many investigations aimed at understanding an organism's toxicological response to endogenous and exogenous challenges. Because so many drug failures have involved direct liver toxicity or other organ toxicity from liver generated metabolites, the pharmaceutical industry has constantly sought superior, predictive in-vitro models that can more quickly and efficiently identify problematic drug candidates before they incur major development costs, and certainly before they are released to the public. In this broad review, we present a survey and critical comparison of in-vitro liver technologies along a broad spectrum, but focus on the current renewed push to develop "organs-on-a-chip". One prominent set of conclusions from this review is that while a large body of recent work has steered the field towards an ever more comprehensive understanding of what is needed, the field remains in great need of several key advances, including establishment of standard characterization methods, enhanced technologies that mimic the in-vivo cellular environment, and better computational approaches to bridge the gap between the in-vitro and in-vivo results.

Bioreactor technologies to support liver function in vitro

Liver is a central nexus integrating metabolic and immunologic homeostasis in the human body, and the direct or indirect target of most molecular therapeutics. A wide spectrum of therapeutic and technological needs drives efforts to capture liver physiology and pathophysiology in vitro, ranging from prediction of metabolism and toxicity of small molecule drugs, to understanding off-target effects of proteins, nucleic acid therapies, and targeted therapeutics, to serving as disease models for drug development. Here we provide perspective on the evolving landscape of bioreactor-based models to meet old and new challenges in drug discovery and development, emphasizing design challenges in maintaining long-term liver-specific function and how emerging technologies in biomaterials and microdevices are providing new experimental models.

Organs-on-a-chip: a new tool for drug discovery

INTRODUCTION

The development of emerging in vitro tissue culture platforms can be useful for predicting human response to new compounds, which has been traditionally challenging in the field of drug discovery. Recently, several in vitro tissue-like microsystems, also known as 'organs-on-a-chip', have emerged to provide new tools for better evaluating the effects of various chemicals on human tissue.

AREAS COVERED

The aim of this article is to provide an overview of the organs-on-a-chip systems that have been recently developed. First, the authors introduce single-organ platforms, focusing on the most studied organs such as liver, heart, blood vessels and lung. Later, the authors briefly describe tumor-on-a-chip platforms and highlight their application for testing anti-cancer drugs. Finally, the article reports a few examples of other organs integrated in microfluidic chips along with preliminary multiple-organs-on-a-chip examples. The article also highlights key fabrication points as well as the main application areas of these devices.

EXPERT OPINION

This field is still at an early stage and major challenges need to be addressed prior to the embracement of these technologies by the pharmaceutical industry. To produce predictive drug screening platforms, several organs have to be integrated into a single microfluidic system representative of a humanoid. The routine production of metabolic biomarkers of the organ constructs, as well as their physical environment, have to be monitored prior to and during the delivery of compounds of interest to be able to translate the findings into useful discoveries.

Three-dimensional perfused cell culture

Compelling evidence suggests the limitation and shortcomings of the current and well established cell culture method using multi-well plates, flasks and Petri dishes. These are particularly important when cell functions are sensitive to the local microenvironment, cell-cell and cell-extracellular matrix interactions. There is a clear need for advanced cell culture systems which mimic in vivo and more physiological conditions. This review summarises and analyses recent progress in three dimensional (3D) cell culture with perfusion as the next generation cell culture tools, while excluding engineered tissue culture where three dimensional scaffold has to be used for structural support and perfusion for overcoming mass transfer control. Apart from research activities in academic community, product development in industry is also included in this review.

Spheroid-based three-dimensional liver-on-a-chip to investigate hepatocyte-hepatic stellate cell interactions and flow effects

We have developed a three-dimensional (3D) liver-on-a-chip to investigate the interaction of hepatocytes and hepatic stellate cells (HSCs) in which primary 3D hepatocyte spheroids and HSCs are co-cultured without direct cell-cell contact. Here, we show that the 3D liver chip offers substantial advantages for the formation and harvesting of spheroids. The most important feature of this liver chip is that it enables continuous flow of medium to the cells through osmotic pumping, and thus requires only minimal handling and no external power source. We also demonstrate that flow assists the formation and long-term maintenance of spheroids. Additionally, we quantitatively and qualitatively investigated the paracrine effects of HSCs, demonstrating that HSCs assist in the maintenance of hepatocyte spheroids and play an important role in the formation of tight cell-cell contacts, thereby improving liver-specific function. Spheroids derived from co-cultures exhibited improved albumin and urea secretion rates compared to mono-cultured spheroids after 9 days. Immunostaining for cytochrome P450 revealed that the enzymatic activity of spheroids co-cultured for 8 days was greater than that of mono-cultured spheroids. These results indicate that this system has the potential for further development as a unique model for studying cellular interactions or as a tool that can be incorporated into other models aimed at creating hepatic structure and prolonging hepatocyte function in culture.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

A continuous perfusion microplate for cell culture

We describe a 96-well microplate with fluidically connected wells that enables the continuous fluid perfusion between wells without the need for external pumping. A single unit in such a perfusion microplate consists of three wells: a source well, a sample (cell culture) well in the middle and a waste well. Fluid perfusion is achieved using a combination of the hydrostatic pressure generated by different liquid levels in the wells and the fluid wicking through narrow strips of a cellulose membrane connecting the wells. There is an excellent correspondence between the observed perfusion flow dynamics and the flow simulations based on Darcy's Law. Hepatocytes (C3A cells) cultured for 4 days in the perfusion microplate with no media exchange in the cell culture well had the same viability as hepatocytes exposed to a daily exchange of media. EOC 20 cells that require media conditioned by LADMAC cells were shown to be equally viable in the adjacent cell culture well of the perfusion microplate with LADMAC cells cultured in the source well. Tegafur, a prodrug, when added to primary human hepatocytes in the source well, was metabolized into a cytotoxic metabolite that kills colon cancer cells (HCT 116) cultured in the adjacent cell culture well; no toxicity was observed when only medium was in the source well. These results suggest that the perfusion microplate is a useful tool for a variety of cell culture applications with benefits ranging from labor savings to enabling in vivo-like toxicity studies.

Organotypic liver culture models: meeting current challenges in toxicity testing

Prediction of chemical-induced hepatotoxicity in humans from in vitro data continues to be a significant challenge for the pharmaceutical and chemical industries. Generally, conventional in vitro hepatic model systems (i.e. 2-D static monocultures of primary or immortalized hepatocytes) are limited by their inability to maintain histotypic and phenotypic characteristics over time in culture, including stable expression of clearance and bioactivation pathways, as well as complex adaptive responses to chemical exposure. These systems are less than ideal for longer-term toxicity evaluations and elucidation of key cellular and molecular events involved in primary and secondary adaptation to chemical exposure, or for identification of important mediators of inflammation, proliferation and apoptosis. Progress in implementing a more effective strategy for in vitro-in vivo extrapolation and human risk assessment depends on significant advances in tissue culture technology and increasing their level of biological complexity. This article describes the current and ongoing need for more relevant, organotypic in vitro surrogate systems of human liver and recent efforts to recreate the multicellular architecture and hemodynamic properties of the liver using novel culture platforms. As these systems become more widely used for chemical and drug toxicity testing, there will be a corresponding need to establish standardized testing conditions, endpoint analyses and acceptance criteria. In the future, a balanced approach between sample throughput and biological relevance should provide better in vitro tools that are complementary with animal testing and assist in conducting more predictive human risk assessment.

Microfluidic platforms for hepatocyte cell culture: new technologies and applications

In this article, we summarize the key elements of microfluidic platforms for mimicking in vivo hepatocyte cell culture and the major recent advances in this area. Specifically, we will give brief background and rationale for key design requirements for mimicking in vivo hepatocyte cell culture, and then summarize findings, applications, and limitations from microfluidic platforms that addressed these design requirements. Although no ideal microfluidic platform has so far been developed for fully mimicking in vivo hepatocyte cell culture, some approaches and designs have demonstrated great potential in this area.

Oxygen uptake rates in cultured rat hepatocytes

One potential treatment of acute liver failure involves the use of an extracorporeal device composed of functional hepatocytes. A major issue in the design of such a large-scale device is providing the hepatocytes with a sufficient supply of oxygen and other nutrients. In this study, we have designed and characterized a simple perfusion system hepatocytes using this system. The OUR of hepatocytes was determined during the first day after seeding on a single collagen gel and during the long-term stable culture after the addition of a top layer of collagen. The OUR increased to 20.7 +/- 0.57 pmol/sec/microg DNA during the first 13 hours of culture on a single collagen gel, while during the next 11 hours, the OUR declined to 10.6 +/- 1.5 pmol/sec/microg DNA. In parallel with the increase in OUR during the first 10 hours, we observed significant cell spreading, suggesting that the oxygen supply to the cells may be critical for the spreading and adaptation of the anchorage-dependent hepatocytes following isolation. Addition of a top layer of collagen to hepatocyte cultures for 24 hours of culture on a single collagen layer resulted in a stable OUR for 15 days. These results indicate that OUR of hepatocytes in culture may vary depending on the phase of culture (i.e., early vs. late) and on the extracellular environment.

New hepatocyte in vitro systems for drug metabolism: metabolic capacity and recommendations for application in basic research and drug development, standard operation procedures

Primary hepatocytes represent a well-accepted in vitro cell culture system for studies of drug metabolism, enzyme induction, transplantation, viral hepatitis, and hepatocyte regeneration. Recently, a multicentric research program has been initiated to optimize and standardize new in vitro systems with hepatocytes. In this article, we discuss five of these in vitro systems: hepatocytes in suspension, perifusion culture systems, liver slices, co-culture systems of hepatocytes with intestinal bacteria, and 96-well plate bioreactors. From a technical point of view, freshly isolated or cryopreserved hepatocytes in suspension represent a readily available and easy-to-handle in vitro system that can be used to characterize the metabolism of test substances. Hepatocytes in suspension correctly predict interspecies differences in drug metabolism, which is demonstrated with pantoprazole and propafenone. A limitation of the hepatocyte suspensions is the length of the incubation period, which should not exceed 4hr. This incubation period is sufficiently long to determine the metabolic stability and to allow identification of the main metabolites of a test substance, but may be too short to allow generation of some minor, particularly phase II metabolites, that contribute less than 3% to total metabolism. To achieve longer incubation periods, hepatocyte culture systems or bioreactors are used. In this research program, two bioreactor systems have been optimized: the perifusion culture system and 96-well plate bioreactors. The perifusion culture system consists of collagen-coated slides allowing the continuous superfusion of a hepatocyte monolayer with culture medium as well as establishment of a constant atmosphere of 13% oxygen, 82% nitrogen, and 5% CO2. This system is stable for at least 2 weeks and guarantees a remarkable sensitivity to enzyme induction, even if weak inducers are tested. A particular advantage of this systemis that the same bioreactor can be perfused with different concentrations of a test substance in a sequential manner. The 96-well plate bioreactor runs 96 modules in parallel for pharmacokinetic testing under aerobic culture conditions. This system combines the advantages of a three-dimensional culture system in collagen gel, controlled oxygen supply, and constant culture medium conditions, with the possibility of high throughput and automatization. A newly developed co-culture system of hepatocytes with intestinal bacteria offers the possibility to study the metabolic interaction between liver and intestinal microflora. It consists of two chambers separated by a permeable polycarbonate membrane, where hepatocytes are cultured under aerobic and intestinal bacteria in anaerobic conditions. Test substances are added to the aerobic side to allow their initial metabolism by the hepatocytes, followed by the metabolism by intestinal bacteria at the anaerobic side. Precision-cut slices represent an alternative to isolated hepatocytes and have been used fo the investigation of hepatic metabolism, hepatotoxicity, and enzyme induction. A specific advantage of liver slices is the possibility to study toxic effects on hepatocytes that are mediated or modified by nonparenchymal cells (e.g., by cytokine release from Kupffer cells) because the physiological liver microarchitecture is maintained in cultured slices. For all these in vitro systems, a prevalidation has been performed using standard assays for phase I and II enzymes. Representative results with test substances and recommendations for application of these in vitro systems, as well as standard operation procedures are given.

Perifusion of co-cultured hepatocytes: optimization of studies on drug metabolism and cytotoxicity in vitro

The combination of co-cultivation of hepatocytes and epithelial cell lines with a newly developed perifusion system was used for in vitro studies on drug metabolism and cytotoxicity. This approach improved the viability and enhanced the induction of the biotransforming capacity of the hepatocytes. As demonstrated for the induction of 7-ethoxyresorufin O-deethylase activity by 3-methylcholanthrene or benzanthracene, co-cultured hepatocytes in the perifusion system responded more sensitively to these inducers than without perifusion, most likely owing to stable (steady-state) concentrations of the inducers under the former conditions and rapidly declining concentrations under the latter conditions. The perifusion approach rendered it possible to determine the kinetics of drug metabolism during single or sequential incubations. After induction with 3-methylcholanthrene and phenobarbital, phase I metabolism of lonazolac to the monohydroxylated product in perifused co-cultures closely (87%) approached the values reported for the in vivo production, whereas in stationary co-cultures only 52% could be reached. Likewise, cytotoxic effects could be detected more precisely in the perifused co-cultures. If cells were pretreated with 0.2 mmol/L galactosamine for 3 h, perifusion with increasing concentrations of menadione differentially killed epithelial RL-ET-14 cells and hepatocytes at low and high concentrations, respectively, while in stationary co-cultures no differential effect was observed and only the higher concentrations were cytotoxic for both cells. Prevention by incubation with S-adenosylmethionine of menadione cytotoxicity up to a menadione concentration of 250 micromol/L was seen only in the perifused co-cultures, whereas in stationary cultures only a slight shift of the cytotoxic concentration exerting 50% cell damage to higher values was noted. These results demonstrate the versatile application of perifused co-cultures for studies on drug metabolism including induction of cytochrome P450-dependent enzymes and steady-state kinetics of biotransformation, as well as cytotoxic and protective effects of different drugs.

Constitutive expression and inducibility of O6-methylguanine-DNA methyltransferase and N-methylpurine-DNA glycosylase in rat liver cells exhibiting different status of differentiation

We have analyzed the expression of the DNA repair genes O6-methylguanine-DNA methyltransferase (MGMT) and N-methylpurine-DNA glycosylase (MPG) at RNA and protein activity level in primary rat hepatocytes in vitro and various rat hepatoma cell lines exhibiting different status of differentiation. The basal level of MGMT mRNA and activity correlated well with the degree of differentiation, as measured by tyrosine aminotransferase (TAT) mRNA expression. Induction of MGMT mRNA and protein activity by X-ray and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) treatment was most pronounced in the well-differentiated hepatocytes and in various differentiated hepatoma cell lines (up to 6-fold). There was virtually no induction in H5 hepatoma cells which exhibited the lowest degree of differentiation and expressed only low amounts of MGMT. For the other hepatoma cell lines tested, MGMT induction did not clearly correlate with TAT expression. Thus, Fao cells which exhibited a high degree of differentiation responded only very weakly with respect to induction. The results indicate that the basal level of MGMT mRNA expression is dependent on liver-specific regulatory factors, whereas the inducibility is a more complex phenomenon not solely dependent on them. Contrary to MGMT, MPG was constitutively expressed at relatively high amounts in all cell lines tested and no correlation was apparent with the degree of differentiation. MPG activity was significantly induced by mutagen treatment only in H4IIE cells. The tumor promoter phenobarbital induced MGMT, but not MPG mRNA in hepatocytes. The results indicate that MGMT and MPG are not co-regulated. Hepatoma cells with low MGMT level were most sensitive to MNNG-induced cytotoxicity. On the other hand, no correlation was apparent between MPG activity and sensitivity of the cell lines to methylating agents indicating that the MPG level is not predictive for alkylating drug resistance.

A perifusion system for cultured hepatocytes

A perifusion system for primary cultures of hepatocytes is described. The system accommodates 20 rotated petri dishes (60 mm) and allows individual medium composition and sampling for each dish. Cell number and insulin (15 pM to 7.7 nM) were stable in the system for at least 24 h. The dose-response relationship for induction by insulin of glucokinase and pyruvate kinase was shifted to the left by a factor of 9 and 5, respectively, as compared to conventional, stationary cultures. The system is useful for studies at low and/or constant concentrations of substrates, hormones, growth factors, etc., with monolayers of cells having a high metabolic capacity.

Membrane modifications in the course of hepatocyte isolation

A transmission E/M, scanning E/M and freeze fracture ultrastructural study has been performed on the rat hepatocyte in the course of isolation from the liver parenchyma. The cell submicroscopic aspect indicates a good morpho-functional preservation from the liver perfusion to the final stages of cell isolation. The freeze fracture membrane analysis evidentiates the constant presence of gap junctions and tight junctions, characterized by particular structural alterations, probably due to progressive functional uncoupling. The persistence of these cell differentiations until complete cell isolation may be considered a further morphological expression of the maintenance of the differentiated stage of the hepatocyte. Fragments of membranes from adjacent cells, still adherent to isolated hepatocyte surfaces, can also be occasionally detected by freeze-fracture techniques.

Influence of hormones and drugs on glutathione-S-transferase levels in primary culture of adult rat hepatocytes

GST activities against 1-Chloro-2,4-dinitrobenzene (CDNB) and 1,2-dichloro-4-nitrobenzene (DCNB) were measured in isolated and cultured adult rat hepatocytes. Within 24 h in culture, both GST activities decreased to about 70% and either stabilized at this level (CDNB) or recovered (DCNB) to the initial level. Use of hyaluronidase in addition to collagenase during the isolation of the cells strongly reduced both activities and its stimulation by various drugs for up to 168 h. The hormones insulin, glucagon, triiodothyronine, estradiol, testosterone, and progesterone did not affect GST activity, while dexamethasone showed some interference. In the presence of dexamethasone the activity against CDNB was mainly stimulated by the combination of methylcholanthrene (MC) and phenobarbital (PB) to about 260% within 168 h. The activity against DCNB was stimulated predominantly by MC alone reaching 170% after 168 h. Quantification of the GST subunits Ya, Yb1 and Yp by an ELISA technique revealed a strong decrease of Ya, a transient increase of Yb1 after 24 h followed by a moderate decrease, and a stable low level of the transformation marker Yp during cultivation. The level of Ya was markedly induced by PB, particularly in combination with MC. The level of Yb1 was equally induced by MC or PB with no synergistic effect. Yp was not affected by these drugs. None of the hormones affected the level of these GST subunits. These results indicate that the physiological type of regulation of the GSTs is maintained during primary culture and no signs of dedifferentiation or transformation are observed. Furthermore, they demonstrate that the interaction of drugs and hormones and their inducing potential can be efficiently studied in the cultured hepatocytes.

Spermidine level and protein synthesis are coregulated in nonproliferating hepatocytes

The relationship between polyamines and the rate of protein synthesis was investigated in non-proliferating cells: primary cultures of adult rat hepatocytes maintained in serum-free media, and treated with dexamethasone or dexamethasone + insulin. During the second day of culture, polyamine biosynthesis became induced along with the rate of protein synthesis. While the activity of ornithine decarboxylase and the intracellular concentration of putrescine increased only transiently and that of spermine declined, the rise of the protein synthetic rate was paralleled by that of the intracellular spermidine concentration. The polyamine analogue diamino-propanol specifically decreased spermidine content and the protein synthetic rate. The intracellular concentration of spermidine was found subject to tight homeostatic regulation, e.g. not being altered by the addition of up to 1 mM of this polyamine to the culture medium. In contrast, addition of putrescine or spermine led to an increase in their respective intracellular concentrations. These findings indicate that spermidine specifically of the polyamines is involved in protein synthesis in the intact hepatocyte. Moreover, spermidine may mediate part of the trophic action of dexamethasone and insulin upon cultured hepatocytes.

Alterations of hepatic enzyme levels and of the acinar distribution of glutamine synthetase in response to experimental liver injury in the rat

Glutamine synthetase shows a striking heterogeneous distribution in normal rat liver as consistently revealed by immunohistochemistry using a specific antiserum against the rat liver enzyme or a cross-reacting antiserum. The effects of zonal liver injury induced by allylformate or CCl4 on this distribution and on the activity of glutamine synthetase as well as of enzymes with different acinar distribution were investigated. Treatment with allylformate or CCl4 at appropriate concentrations led to severe hepatocyte necrosis in the periportal and perivenous zone, respectively, as revealed by histological examination and by the levels of serum marker enzymes. Exposure to allylformate (50 to 100 microliter per kg) for less than 1 day did not change the distribution and activity of glutamine synthetase but reduced the specific activities of the urea cycle enzymes. In contrast, treatment with CCl4 (1,000 microliter per kg) strongly reduced the activity and the acinar region covered by glutamine synthetase but not, for instance, the activities of the urea cycle enzymes. These results in conjunction with the data obtained for other enzymes indicate that a short exposure to these hepatotoxins affects different enzyme activities in close accord with their preferential acinar localization. During prolonged exposure this initial response was often modified due to adaptation. In the case of glutamine synthetase, however, no adaptive appearance of glutamine synthetase in other parts of the acinus could be detected even if the cell population originally expressing this phenotype was destroyed. This extremely inflexible distribution suggests that glutamine synthetase expression is a matter of cell differentiation rather than of modulation by nutritional and hormonal factors (or their acinar gradients) as found for many other hepatic enzymes.

Regulation of synthesis and secretion of major rat acute-phase proteins by recombinant human interleukin-6 (BSF-2/IL-6) in hepatocyte primary cultures

The regulation of the three major acute-phase proteins alpha 2-macroglobulin, cysteine proteinase inhibitor and alpha 1-antitrypsin by recombinant human interleukin-1 beta, recombinant human interleukin-6 and recombinant human tumor necrosis factor alpha was studied in rat hepatocyte primary cultures. Synthesis and secretion of the acute-phase proteins was measured after labeling with [35S]methionine and immunoprecipitation. Incubation of hepatocytes with interleukin-6 led to dose-dependent and time-dependent changes in the synthesis of the three major acute-phase proteins and albumin, similar to those occurring in vivo during experimental inflammation. alpha 2-Macroglobulin and cysteine proteinase inhibitor synthesis was induced 54-fold and 8-fold, respectively, 24 h after the addition of 100 units/ml interleukin-6. At the same time synthesis of the negative acute-phase protein albumin was reduced to 30% of controls. Half-maximal effects were achieved with 4 units interleukin-6/ml. Interleukin-1 beta had only a partial effect on the regulation of the four patients studied: only a twofold stimulation of alpha 2-macroglobulin and a 60% reduction of albumin synthesis were observed. Tumor necrosis factor alpha did not alter the synthesis of acute-phase proteins. The stimulation of alpha 2-macroglobulin and cysteine proteinase inhibitor synthesis by interleukin-6 was inhibited by interleukin-1 beta in a dose-dependent manner. In pulse-chase experiments the effect of interleukin-1 beta, interleukin-6 and tumor necrosis factor alpha on the secretion of acute-phase proteins was examined. Interleukin-6 markedly accelerated the secretion of total proteins and alpha 2-macroglobulin, whereas the secretion of cysteine proteinase inhibitor, alpha 1-antitrypsin and albumin was not affected. The inhibition of N-glycosylation by tunicamycin abolished the effect of interleukin-6 on the secretion of alpha 2-macroglobulin, indicating a possible role of interleukin-6 on N-glycosylation.

Factors influencing the uptake of iron and plutonium into cells

Uptake of 59Fe as well as 125I-labelled Fe-transferrin into HeLa cells points to the existence of a limited number of specific binding sites. This is in contrast to hepatocytes and hepatoma cells (Hep G2) where metal uptake from transferrin is very low, not saturable and cannot be prevented by an excess of the protein. Iron uptake into these cells is much higher from the citrate complex. The same is true for plutonium uptake into rat hepatocytes, while the uptake of this metal into Hep G2 cells is very small regardless of the ligand. In contrast to iron, plutonium presented as citrate is taken up into HeLa cells much better than plutonium presented as transferrin. The uptake of both metals from the citrate complex requires a high activation energy and can be prevented only by inhibition of oxidative phosphorylation. Other processes such as endocytosis, intactness of microtubuli, assembly of microfilaments or pH of the lysosomes do not seem to be of importance. Metal uptake from the citrate complex can be prevented only by the presence of other chelating agents and/or by transferrin. It can be assumed, therefore, that the metals react directly with constituents of the cell membrane, a process in which chelating agents can successfully compete if they form strong enough complexes with the metals.

Differences in the uptake of transferrin bound 239Pu and 59Fe into multicellular spheroids of hepatocytes from adult male rats

Hepatocytes were cultured as monolayers and multicellular spheroids, respectively. The uptake of both transferrin-bound metals, iron and plutonium, differed significantly between these two culture systems. The uptake into the multicellular spheroids for plutonium was about 30 times greater, and for iron about 4 times greater, than in monolayer-cultured hepatocytes, which is not a consequence of proliferation and/or de-differentiation of the hepatocytes in the multicellular spheroid culture system. A comparison of the iron and plutonium uptake showed that plutonium was delivered to the cells to an 8-fold greater extent than iron if the hepatocytes were cultured as spheroids. Additionally, the binding of plutonium was not inhibited by preincubation of the spheroids with the iron-transferrin complex. Therefore, we propose that there are two different binding sites for iron and plutonium on hepatocyte membranes.

Cultured Ito cells of rat liver express the alpha 2-macroglobulin gene

Ito cells were isolated from rat liver and kept in culture for up to 13 days. The capability of the Ito cells to synthesize alpha 2-macroglobulin was analyzed at different times after isolation and by pulse-chase experiments. Newly synthesized alpha 2-macroglobulin was determined by immunoprecipitation followed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis and fluorography. alpha 2-Macroglobulin synthesis was hardly detectable in Ito cells and their media 3 days after plating. However, 5-11 days after the isolation of the cells, increasing amounts of alpha 2-macroglobulin were synthesized. The results of pulse-chase experiments performed on day 7 showed that radioactively labeled alpha 2-macroglobulin decreased in the intracellular compartment and increased in the culture medium. alpha 2-Macroglobulin was identified by immunoprecipitation and sodium dodecyl sulfate/polyacrylamide gel electrophoresis under reducing and non-reducing conditions. Furthermore, when unlabeled alpha 2-macroglobulin was added during the immunoprecipitation, a competition was observed. Incubation of pancreatic elastase with culture medium of rat Ito cells or rat hepatocytes led to the same cleavage products as found with alpha 2-macroglobulin. alpha 2-Macroglobulin-specific mRNA could be demonstrated by Northern blot analysis of total RNA extracted from rat Ito cells. Under the conditions where alpha 2-macroglobulin was synthesized in Ito cells, no synthesis of alpha 1-macroglobulin, alpha 1-inhibitor 3, alpha 1-proteinase inhibitor, alpha 1-acid glycoprotein, alpha 1-acute-phase globulin (T-kininogen) and albumin could be demonstrated. It is concluded that alpha 2-macroglobulin is a true secretory protein of rat Ito cells in culture. This could be of importance for collagen metabolism in liver diseases.

Discrimination of hepatocyte-stimulating activity from human recombinant tumor necrosis factor alpha

The involvement of tumor necrosis factor alpha (TNF alpha) in the regulation of acute-phase protein synthesis is currently under discussion. In this study the effect of human recombinant TNF alpha on the regulation of the 4 acute-phase proteins alpha 2-macroglobulin, albumin, alpha 1-proteinase inhibitor and alpha 1-acute-phase globulin was investigated in rat hepatocyte primary cultures. No changes in synthesis of any of the 4 proteins were observed. However, an acute-phase response similar to that in vivo could be generated by conditioned media from human monocytes containing natural TNF alpha. This response remained unchanged after neutralizing TNF alpha activity by the addition of a specific antibody to TNF alpha. It is concluded that the hepatocyte-stimulating activity synthesized by human monocytes is different from TNF alpha.

Biosynthesis and regulation of rat alpha 1-inhibitor3, a negative acute-phase reactant of the macroglobulin family

The biosynthesis of rat alpha 1-inhibitor3, a negative acute-phase reactant specifically found in rodents, was studied in vitro in a cell-free translation system from rabbit reticulocytes, in rat hepatocyte primary cultures and in vivo by immunocytochemistry using normal and turpentine-injected rats. By sucrose-gradient centrifugation and subsequent translation of the fractionated RNA in vitro it was found that the mRNA coding for alpha 1-inhibitor3 exhibited a size of about 28S. For the alpha 1-inhibitor3 translated in vitro an apparent Mr of 155,000 was determined. A continuous decrease in the level of alpha 1-inhibitor3 in serum during experimental inflammation induced by turpentine injection was demonstrated by means of quantitative 'rocket' immunoelectrophoresis. This result agrees with the observation by immunocytochemistry of a drastic decrease in alpha 1-inhibitor3 levels in hepatocytes 24 h after turpentine injection. At that time alpha 1-inhibitor3 is mainly located in the Golgi apparatus, whereas it is also present in the membranes of the rough and smooth endoplasmic reticulum when normal liver is used. All hepatocytes, but no other hepatic cells, contain alpha 1-inhibitor3. When hepatocyte primary cultures were labelled with [35S]methionine and alpha 1-inhibitor3 was immunoprecipitated from the hepatocyte medium and the supernatant of homogenized cells, two different forms of alpha 1-inhibitor3 were found. The intracellular form of alpha 1-inhibitor3, with an apparent Mr of 173,000, is characterized by oligosaccharide side chains of the high-mannose type. The form of alpha 1-inhibitor3 in the medium exhibited an Mr of 186,000 and carried carbohydrate side chains of the complex type. After labelling hepatocytes with radioactive sugars, [3H]mannose was found in both forms of alpha 1-inhibitor3, whereas [3H]fucose and [3H]galactose were incorporated only into the form found in the medium. In the presence of tunicamycin an unglycosylated alpha 1-inhibitor3 with an apparent Mr of 154,000 was found in cells and in the medium. In a pulse-chase experiment it was shown that inhibition of glycosylation by tunicamycin resulted in a marked delay of secretion of alpha 1-inhibitor3. Thus the oligosaccharide side chains of alpha 1-inhibitor3 play an important role during its transport into the medium.

Hormonal regulation of amino acid transport system N in primary cultures of rat hepatocytes

The transport of histidine and glutamine via system N in cultured hepatocytes was found to be subject to hormonal control. This long-term regulation showed the following characteristics. The transport capacity for histidine and glutamine (system N) increased slowly in response to the combination of dexamethasone and insulin to about 4-fold that of controls after 18-30 h. A similar time course was found for the stimulation of system N (2.5-fold) by dexamethasone and glucagon. In contrast the uptake of alpha-aminoisobutyric acid (system A) was rapidly stimulated 3-fold by dexamethasone and insulin and 5-fold by dexamethasone and glucagon within 3-6 h but decreased towards control rates after 24 h of cultivation in minimal essential medium. Dexamethasone, insulin and glucagon each stimulated glutamine uptake about 2-fold in cultures maintained in W/AB 77 medium, while the combination of dexamethasone with either glucagon or insulin resulted in a 3-4-fold increase. Dexamethasone was most effective at about 0.1 microM. Higher concentrations were less efficient. Insulin reached its optimal effect at concentrations above 1 microM. Kinetic analysis revealed that the increased capacity of glutamine transport in response to hormones was due to an increase in Vmax, while Km was essentially unchanged. The hormone-induced stimulation of system N was prevented by cycloheximide. The induced uptake of glutamine was inhibited by excess amounts of asparagine and histidine but not of alpha-methylaminoisobutyric acid or cysteine. These results clearly differentiate the hormonal regulation of system N from that of system A.

Selective loss of hormonal induction of glutamate transport in primary cultures of hepatocytes from rats treated with CCl4

A protocol for the mass isolation and successful cultivation of hepatocytes from the acinar zones 1 and 2 is described. The hepatocyte suspensions isolated from rats pretreated with CCl4 contain less than one-tenth of control levels of the perivenous marker enzyme glutamine synthetase, as judged by immunocytochemistry and enzyme activity excluding contamination by hepatocytes from zone 3. In culture these hepatocytes form morphologically and functionally intact monolayers. They synthesize urea and glucose at rates above those of controls, but are unable to produce glutamine. These results demonstrate the suitability of this culture system for the investigation of hepatic functions that are characteristic of cells from the periportal and midzonal part of the acinus. An interesting feature of these cultures is the failure of dexamethasone to induce the uptake of glutamate while the hormonal induction of the amino acid transport systems A and N and of tyrosine aminotransferase is not affected.

The inflammation mediators interleukin 1 and hepatocyte-stimulating factor are differently regulated in human monocytes

Human peripheral monocytes can be induced by bacterial lipopolysaccharide to produce the inflammatory mediators interleukin 1 (IL 1) and hepatocyte-stimulating (HS) activity. IL1 and HS activities were separated by gel permeation chromatography. It is also shown that the two monokines are differently regulated. Evidence for this stems from the finding that monocytes cultured for 24 h lose their ability to produce IL1 in response to lipopolysaccharide, while synthesis of HS activity remains essentially unaffected.

Differential effect of growth factors on growth stimulation and phenotypic stability of glutamine-synthetase-positive and -negative hepatocytes in primary culture

In rat liver parenchyma, two subpopulations of hepatocytes can be distinguished by the absence or presence of the marker enzyme, glutamine synthetase (GS). Hepatocytes in the perivenous zone immediately adjacent to the hepatic venules in the liver acinus are positive for GS. Using autoradiography in combination with immunocytochemistry, the response of these two hepatocyte populations (GS positive and GS negative) to a variety of growth factors (defined compounds or complex stimuli) was investigated in vitro. Irrespective of the individual growth-promoting activity (which varied considerably), all stimuli led to much higher labeling indices in GS-negative cells as compared to GS-positive cells. In GS-negative cells, the strongest effect was exerted by serum obtained from partially hepatectomized rats (labeling index, 67%) and the conditioned media of JM1 and JM2 hepatoma cells (63%-82%), followed by a combination of insulin and either norepinephrine (46%) or epidermal growth factor (EGF; 42%). In contrast, serum had the weakest influence on GS-positive cells (0.3%), while the other potent stimuli enhanced the labeling index of these cells by between 6% and 15% within 48 h. The percentage of labeled nuclei was higher in mononucleated than in binucleated GS-positive hepatocytes. The time course of thymidine incorporation was also different for the two subpopulations. Under all growth-promoting conditions, the stimulation of GS-negative cells peaked between 72 and 96 h, while it increased continuously in GS-positive cells for at least 120 h, particularly in the case of serum. In proliferating cultures, both the absolute and the relative number of GS-positive hepatocytes decreased, while no such effect was found in various nonproliferating control cultures maintained at low and high cell density. Similar results were found for GS activity. In contrast, the hormonal induction of tyrosine aminotransferase (TAT) was not affected. It is suggested that these differences in the growth response of GS-positive and -negative cells contribute to the acinar gradient in hepatocyte proliferation that occurs during liver regeneration. Furthermore, the striking phenotypic instability of GS-positive cells that have undergone DNA synthesis and mitosis supports the hypothesis that cellular reprogramming depends on passage through the cell cycle.

Different effects of the glucosidase inhibitors 1-deoxynojirimycin, N-methyl-1-deoxynojirimycin and castanospermine on the glycosylation of rat alpha 1-proteinase inhibitor and alpha 1-acid glycoprotein

The glucosidase inhibitors 1-deoxynojirimycin, N-methyl-1-deoxynojirimycin and castanospermine were used to inhibit oligosaccharide processing in primary cultures of rat hepatocytes. Their effect on the glycosylation of alpha 1-proteinase inhibitor (alpha 1PI) and alpha 1-acid glycoprotein (alpha 1AGP) was studied. Of the three glucosidase inhibitors examined, 1-deoxynojirimycin inhibited not only oligosaccharide trimming but also glycosylation de novo of newly synthesized proteins, resulting in the formation of alpha 1PI with two and three (normally carrying three) and alpha 1AGP with two to five (normally carrying six) oligosaccharide side chains. In the presence of the glucosidase inhibitors, glucosylated high-mannose-type oligosaccharides accumulated. Whereas most of the endoglucosaminidase-H-sensitive oligosaccharides formed in the presence of 1-deoxynojirimycin contained only one glucose residue, N-methyl-1-deoxynojirimycin and castanospermine led mainly to the formation of oligosaccharides with three glucose residues. None of the three glucosidase inhibitors completely prevented the formation of complex-type oligosaccharides. Thus, in their presence, alpha 1PI and alpha 1AGP with a mixture of both high-mannose and complex-type oligosaccharides were secreted.

Amino acid transport in established adult rat liver epithelial cell lines

The capacities of Na+-dependent transport of alpha-aminoisobutyrate, glutamine and glutamate in four established and three transformed rat liver epithelial cell lines were found to be considerably higher than those of isolated and cultured hepatocytes. At least for transport systems A and G- this seemed to be due to elevated values of Vmax, whereas the values for Km were quite comparable to those of hepatocytes. In contrast to hepatocytes, however, no significant hormonal stimulation of amino acid uptake could be detected in the cell lines. Each normal cell line expressed a distinct pattern of transport capacities with respect to the three systems measured and this was not altered by chemical transformation of the lines. The individual patterns of the lines showed no similarity to presumptive patterns of subpopulations of liver parenchymal cells. In particular, there was no evidence for a direct relationship of one of the cell lines with a small subpopulation of parenchymal cells located adjacent to hepatic venules as revealed by additional measurements of glutamine synthetase, a marker enzyme for this particular subpopulation. It is concluded that established rat liver epithelial cell lines express features characteristic of normal hepatocytes with respect to amino acid transport, but have developed a distinct phenotype adapted to a rapid, hormone-independent growth in vitro. Alteration of their phenotype by transformation is not coupled with a further increase in amino acid transport capacity.

Content and accessibility of sialic acid on the surface of rat hepatocytes during primary culture

The content and accessibility of terminal sialic acid and galactose residues of rat hepatocytes in primary culture were determined by in situ labeling using either periodate or sialidase/galactose oxidase treatment followed by sodium borotritiide reduction. Rat erythrocytes which were used for comparison showed a strongly enhanced tritium incorporation into galactose after sialidase treatment. In contrast, with freshly prepared rat hepatocytes only a small amount of galactose labeling was achieved after sialidase treatment. The amount of galactose labeled following sialidase treatment increased with time in culture up to day 6 and roughly paralleled the increase of the total sialic acid content. Major changes of sialic acid-containing glycoconjugates were restricted to the gangliosides. There was a transient drop in surface labeling of ganglioside-associated sialic acid on the first day in culture. The specific radioactivity of the in situ-tritiated ganglioside-sialic acid also fell by 50% in this period. Between day 2 and 4, there was an increase in gangliosidesialic acid labeling but the specific radioactivity of the sialic acid remained constant. This indicates that newly synthesized gangliosides but not the preexisting ones were accessible to periodate oxidation. The data allow conclusions about turnover and topology of the sialic acid-containing glycolipids.

Primary culture of rat hepatocytes as a model system of canalicular development, biliary secretion, and intrahepatic cholestasis. V. Disturbance of the cellular membrane and bile canalicular ultrastructure induced by chlorpromazine

In the present paper rat hepatocytes in primary monolayer culture were used to investigate the adverse effects of chlorpromazine (CPZ) at the cellular level. As revealed by thin sectioning many of the ultrastructural alterations were comparable to those described for the isolated perfused rat liver under the influence of CPZ. Alterations comprised short-term effects, such as dilation of the rough endoplasmic reticulum and the nuclear envelope, and long-term effects including huge accumulations of myeloid bodies within the cytoplasm as well as dilation and diverticulation of bile canaliculi. Freeze-fracturing revealed the dislocation of intramembrane particles in the sinusoidal plasma membrane which could be detected as early as 30 min after exposure to CPZ. As judged from filipin cytochemistry, alterations in the cholesterol content seems to play a minor role in the process of membrane damage except at the sinusoidal surface where a reduction of cholesterol content may contribute to the impairment of membrane functions. It is concluded that CPZ exerts its cholestatic effect primarily by a rapid disturbance of the membrane architecture of the sinusoidal surface and secondarily by other interactions with the bile secretory apparatus.

Uptake of 59Fe and 239Pu by rat liver cells and human hepatoma cells

The accumulation of 59Fe and 239Pu was studied in rat hepatocytes in primary culture and in human hepatoma cells (Hep-G2 cells) and was compared with the uptake in isolated perfused rat liver and in rat liver in vivo. With respect to iron uptake from citrate both cell types react similarly: time and concentration dependence as well as the influence of temperature point to a non-specific but energy-dependent uptake mechanism. Pu shows similar behaviour except that a relatively large fraction remains bound to the cell membrane and uptake is lower. This is much more pronounced in Hep-G2 cells than in hepatocytes. If the metals are bound to transferrin, uptake into both cell lines, as well as into isolated perfused rat liver, is very low. After intravenous injection of 239Pu-citrate, accumulation in the rat liver is very rapid during the first 10 min and much slower after that. The role of citrate in metal uptake is discussed.

Amino acid environment determines expression of carbamoylphosphate synthetase and phosphoenolpyruvate carboxykinase in embryonic rat hepatocytes

A completely defined medium (EHM-1), which reflects the amino acid composition of fetal rat serum and contains albumin as the sole proteinaceous compound, allows the accumulation of carbamoylphosphate synthetase and phosphoenolpyruvate carboxykinase in the presence of dexamethasone, dibutyryl cyclic AMP, and triiodothyronine to approximately twice the level attained in a standard culture medium (RPMI 1640) supplemented with 10% fetal bovine serum (and hormones). Using the EHM-1 medium we could show that the capacity of hepatocytes to synthesize phosphoenolpyruvate carboxykinase in the presence of hormones is manifest as soon as the cells differentiate from the embryonic foregut (embryonic Day 11). Furthermore we could show that embryonic hepatocytes can become binuclear or polyploid when cultured in the presence of thyroid hormone.

Secretion of high-mannose-type alpha 1-proteinase inhibitor and alpha 1-acid glycoprotein by primary cultures of rat hepatocytes in the presence of the mannosidase I inhibitor 1-deoxymannojirimycin

Two different forms of alpha 1-proteinase inhibitor and alpha 1-acid glycoprotein were found in primary cultures of rat hepatocytes. After a 2.5-h labeling period with [35S]methionine the high-mannose-type precursor of alpha 1-proteinase inhibitor (Mr 49000) and alpha 1-acid glycoprotein (Mr 39 000) and the mature-complex-type alpha 1-proteinase inhibitor (Mr 54 000) and alpha 1-acid glycoprotein (Mr 43 000-60 000) could be immunoprecipitated from the cells, but only the complex-type forms of the two glycoproteins were secreted into the hepatocyte media. When hepatocytes were incubated with the mannosidase I inhibitor 1-deoxymannojirimycin at a concentration of 4 mM, the 49 000-Mr form of alpha 1-proteinase inhibitor and the 39 000-Mr form of alpha 1-acid glycoprotein could be detected in the cells as well as in their media. Neither the secretion of alpha 1-proteinase inhibitor nor that of alpha 1-acid glycoprotein was impaired by 1-deoxymannojirimycin. While alpha 1-proteinase inhibitor and alpha 1-acid glycoprotein, secreted by control cells, were resistant to endoglucosaminidase H, alpha 1-proteinase inhibitor and alpha 1-acid glycoprotein, secreted by hepatocytes treated with 4 mM 1-deoxymannojirimycin, could be deglycosylated by endoglucosaminidase H. When the [3H]mannose-labeled oligosaccharides of alpha 1-proteinase inhibitor, secreted by 1-deoxymannojirimycin-treated hepatocytes, were cleaved off by endoglucosaminidase H and analyzed by Bio-Gel P-4 chromatography, they eluted at the position of Man9GlcNAc, indicating that mannosidase I had been efficiently inhibited. 1-Deoxymannojirimycin did not inhibit the synthesis or the cotranslational N-glycosylation of alpha 1-proteinase inhibitor or alpha 1-acid glycoprotein.

Dexamethasone restores hormonal inducibility of ornithine decarboxylase in primary cultures of rat hepatocytes

Induction of ornithine decarboxylase by various hormones was studied in quiescent primary cultures of adult rat hepatocytes maintained in a chemically defined medium. The following results were obtained: Enzyme activity rose transiently during the first day of cultivation in hormone-untreated cells. During this phase, insulin increased ornithine decarboxylase activity. Inducibility by insulin was maintained for more than 40 h only after pretreatment with 0.1 microM dexamethasone. Enzyme activity could be induced by 1 nM insulin and peaked after 7 h. Inducibility by glucagon and growth hormone required pretreatment with the glucocorticoid hormone. Ornithine decarboxylase activity was maximal 5 h after glucagon addition. Concentrations down to 0.1 nM were effective. Pretreatment with dexamethasone was most effective, when the hormone was present during the first 20 h of cultivation. The effect of the glucocorticoid during the pretreatment phase was diminished by colchicine and to a lesser extent by cytochalasine B. We suggest that part of the permissive effect of dexamethasone could be mediated by changes in the cytoskeleton and the function of hormone receptors. The fact that induction of ornithine decarboxylase was exerted by several hormones despite the absence of cell proliferation and DNA synthesis may indicate that polyamine biosynthesis has an important role in the quiescent hepatocyte.

Toxicity of D-galactosamine for rat hepatocytes in monolayer culture

Hepatocellular injury was induced by exposure of primary cultures of rat hepatocytes to 4 mM D-galactosamine. The cell damage was very similar to that seen in vivo and in the isolated perfused rat liver, both in biochemical and in structural terms. The severity of the lesions caused by D-galactosamine was dependent on the age of the culture being treated. Less severe damage was found with older cultures. Since the primary metabolic effects of D-galactosamine were age-independent, the reduction in cell damage seems to be due to progressive cell dedifferentiation. Dexamethasone (1 microM) suppressed the full development of the injury, while 1 microM triiodo-L-thyronine enhanced it. A protection of hepatocytes by alpha 2-macroglobulin against the effects of D-galactosamine could be observed neither in vivo nor in vitro. Direct cytotoxic effects of endotoxin from Salmonella minnesota R 595 could be demonstrated only on hepatocytes in the early phases of primary culture using rather high doses of the purified lipopolysaccharide. It is unlikely that they play a major role in the hepatocellular injury seen following endotoxinemia in vivo. Lowering of extracellular Ca2+ concentration and additions of calcium/calmodulin inhibitors did not prevent cell injury after treatment with D-galactosamine. The results suggest that cell death is not due to an increased influx of Ca2+ into the cells.

Biosynthesis and secretion of alpha 1 acute-phase globulin in primary cultures of rat hepatocytes

Experimental inflammation in rats led to a sevenfold increase in serum levels of alpha 1 acute-phase globulin. This increase is correlated with elevated levels of translatable mRNA for alpha 1 acute-phase globulin in the liver. Biosynthesis and secretion of alpha 1 acute-phase globulin were studied in rat hepatocyte primary cultures. An intracellular form of alpha 1 acute-phase globulin with an apparent relative molecular mass of 63 500 and a secreted form of 68 000 were found. The intracellular form of alpha 1 acute-phase globulin could be deglycosylated by endoglucosaminidase H treatment indicating that its oligosaccharide chains were of the high-mannose type. The secreted form of alpha 1 acute-phase globulin was not sensitive to endoglucosaminidase H, but was susceptible to the action of sialidase reflecting carbohydrate side-chains of the complex type. Pulse-chase experiments revealed a precursor-product relationship for the high-mannose and the complex type alpha 1 acute-phase globulin. In the hepatocyte medium newly synthesized alpha 1 acute-phase globulin was detected 30 min after the pulse. Unglycosylated alpha 1 acute-phase globulin was found in the cells as well as in the medium when the transfer of oligosaccharide chains onto the polypeptide chains was blocked by tunicamycin. Tunicamycin led to a marked delay in alpha 1 acute-phase globulin secretion.

Inducibility of carbamoylphosphate synthetase (ammonia) in cultures of embryonic hepatocytes: ontogenesis of the responsiveness to hormones

Glucocorticosteroids and cyclic AMP induce carbamoylphosphate synthetase (ammonia) (CPS) in rat hepatocytes. Using an enzyme immunoassay applied to hepatocyte cultures fixed in situ, it has been demonstrated that the capacity of hepatocytes to synthesize CPS in the presence of both hormones is present as soon as the cells become recognizable as hepatocytes. Immunochemical staining of the cultures shows that hepatocytes do not acquire or express the capacity to accumulate CPS at high rates synchronously. The average levels of CPS per hepatocyte that are observed upon hormone treatment are approx 50-fold lower in embryonic than in adult hepatocytes, corresponding with an approx 10-fold lower synthetic capacity (per gram hepatocytes) and an approx 5-fold smaller size of embryonic compared to adult hepatocytes. Carbamoylphosphate synthetase levels are therefore a good parameter in studies that aim to establish the mechanisms that underly the ontogenesis of the hepatic phenotype.