Culture matrix configuration and composition in the maintenance of hepatocyte polarity and function

Synthetic sandwich culture of 3D hepatocyte monolayer

The sandwich culture of hepatocytes, between double layers of extra-cellular matrix (ECM), is a well-established in vitro model for re-establishing hepatic polarity and maintaining differentiated functions. Applications of the ECM-based sandwich culture are limited by the mass transfer barriers induced by the top gelled ECM layer, complex molecular composition of ECM with batch-to-batch variation and uncontrollable coating of the ECM double layers. We have addressed these limitations of the ECM-based sandwich culture by developing an 'ECM-free' synthetic sandwich culture, which is constructed by sandwiching a 3D hepatocyte monolayer between a glycine-arginine-glycine-aspatic acid-serine (GRGDS)-modified polyethylene terephthalate (PET) track-etched membrane (top support) and a galactosylated PET film (bottom substratum). The bioactive top support and bottom substratum in the synthetic sandwich culture substituted for the functionalities of the ECM in the ECM-based sandwich culture with further improvement in mass transfer and optimal material properties. The 3D hepatocyte monolayer in the synthetic sandwich culture exhibited a similar process of hepatic polarity formation, better cell-cell interaction and improved differentiated functions over 14-day culture compared to the hepatocytes in collagen sandwich culture. The novel 3D hepatocyte monolayer sandwich culture using bioactive synthetic materials may readily replace the ECM-based sandwich culture for liver tissue engineering applications, such as drug metabolism/toxicity testing and hepatocyte-based bioreactors.

Utility and control of proteoglycans in tissue engineering

This review addresses various methods of integrating proteoglycans (PGs) into the design of engineered tissues and provides insight for designing tissue-engineered disease models that leverage current knowledge of PG biology. Even though PGs show immense possibilities in tissue-engineering applications, they have seldom been used to their full potential. The most common tissue-engineering application of PGs has been in scaffolds (matrigels and collagen-chondroitin sulfate matrices), in which PGs or their glycosaminoglycan (GAG) chains are incorporated into the scaffold to promote cell growth, tissue remodeling, and intracellular signaling. In addition, many studies have reported the total amount of PGs synthesized within engineered tissues but have not delineated which specific PGs or GAG classes are involved in engineered tissue development. In native tissues, various PGs are dynamically and differentially regulated to achieve specific biophysical and biological functions, such as compressibility and transparency. Therefore, the targeted modulation of specific PGs (via exogenous addition, endogenous stimulation with growth factors, or mechanical stimulation) may help engineered tissues to achieve native tissue properties. The PG composition of engineered tissues could also be modified to achieve disease models in vitro and thus provide a way to study the effect of external agents on PG-related disease mechanisms.

Gene expression profiling and differentiation assessment in primary human hepatocyte cultures, established hepatoma cell lines, and human liver tissues

Frequently, primary hepatocytes are used as an in vitro model for the liver in vivo. However, the culture conditions reported vary considerably, with associated variability in performance. In this study, we characterized the differentiation character of primary human hepatocytes cultured using a highly defined, serum-free two-dimensional sandwich system, one that configures hepatocytes with collagen I as the substratum together with a dilute extracellular matrix (Matrigeltrade mark) overlay combined with a defined serum-free medium containing nanomolar levels of dexamethasone. Gap junctional communication, indicated by immunochemical detection of connexin 32 protein, was markedly enhanced in hepatocytes cultured in the Matrigel sandwich configuration. Whole genome expression profiling enabled direct comparison of liver tissues to hepatocytes and to the hepatoma-derived cell lines, HepG2 and Huh7. PANTHER database analyses were used to identify biological processes that were comparatively over-represented among probe sets expressed in the in vitro systems. The robustness of the primary hepatocyte cultures was reflected by the extent of unchanged expression character when compared directly to liver, with more than 77% of the probe sets unchanged in each of the over-represented categories, representing such genes as C/EBPalpha, HNF4alpha, CYP2D6, and ABCB1. In contrast, HepG2 and Huh7 cells were unchanged from the liver tissues for fewer than 48% and 55% of these probe sets, respectively. Further, hierarchical clustering of the hepatocytes, but not the cell lines, shifted from donor-specific to treatment-specific when the probe sets were filtered to focus on phenobarbital-inducible genes, indicative of the highly differentiated nature of the hepatocytes when cultured in a highly defined two-dimensional sandwich system.

Gene expression profiling of extracellular matrix as an effector of human hepatocyte phenotype in primary cell culture

Previously, we demonstrated that primary cultures of rat hepatocytes evidence higher levels of differentiated function when cultured in the presence of a dilute overlay of extracellular matrix (Matrigel). In this investigation, we used DNA microarrays, quantitative RT-PCR, immunoblotting, and cell morphology analyses to evaluate the biological responses imparted by Matrigel overlays on primary cultures of human hepatocytes from five independent donors. Although interindividual variability in responses was evident, our results demonstrated that Matrigel additions typically improved hepatocyte morphology and differentiation character. Results from RNA-profiling experiments indicated that Matrigel additions enhanced hepatocyte RNA expression levels associated with a battery of differentiated features, to levels comparable to those seen in vivo, for genes such as the cytochrome P450s, solute carrier family members, sulfotransferases, certain nuclear transcription factors, and other liver-specific markers, such as albumin, transferrin, and response to the inducer, phenobarbital. In contrast, Matrigel additions were generally associated with reduced RNA expression levels for several cytokeratins, integrins, and a number of stress-related pathways. Decreases in integrin protein expression were similarly detected, although enhanced levels of the gap junction-associated protein, connexin 32, were detected in Matrigel-treated cultures. These data support the concept that ECM functions mechanistically to augment the differentiation character of primary human hepatocytes in culture by mediating a reduction in cellular stress response signaling and by enhancing gap junctional cell-cell communication.

Assessment of hepatocellular function within PEG hydrogels

Tissue-engineered therapies for liver failure offer the potential to augment or replace whole organ transplantation; however, fabrication of hepatic tissue poses unique challenges largely stemming from the complexity of liver structure and function. In this study, we illustrate the utility of highly-tunable, photopolymerizable poly(ethylene glycol) (PEG) hydrogels for 3D encapsulation of hepatic cells and highlight a range of techniques important for examining hepatocellular function in this platform. Owing to our long-term interest in incorporating proliferative progenitor cell types (e.g. hepatoblasts, oval cells, or cells derived from embryonic stem cells) and maintaining the phenotype of differentiated cells, we explored the behavior of bipotential mouse embryonic liver (BMEL) cells as a model progenitor cell and mature, fully differentiated, primary hepatocytes in this biomaterial system. We demonstrated the importance of cell-cell and cell-matrix interactions in the survival and function of these cell types, and the capacity to influence encapsulated cell phenotypes through modulation of hydrogel characteristics or gene silencing. Additionally, we demonstrated imaging techniques critical for the in situ assessment of encapsulated hepatocyte function combined with the ability to control cellular organization and overall architecture through microscale patterning technologies. Further analysis of liver progenitor as well as mature hepatocyte processes within the versatile PEG hydrogel platform will aid in the development of tissue engineered implantable liver systems.

Control of hepatic differentiation via cellular aggregation in an alginate microenvironment

Integral to the development of embryonic stem cell therapeutic strategies for hepatic disorders is the identification and establishment of a controllable hepatic differentiation strategy. In order to address this issue we have established an alginate microencapsulation approach which provides a means to modulate the differentiation process through changes in key encapsulation parameters. We report that a wide array of hepatocyte specific markers is expressed by cells differentiated during a 23-day period within an alginate bead microenvironment. These include urea and albumin secretion, glycogen storage, and cytochrome P450 transcription factor activity. In addition, we demonstrate that cellular aggregation is integral to the control of differentiation within the bead environment and this process is mediated by the E-cadherin protein. The temporal expression of surface E-cadherin and hepatocyte functional expression occur concomitantly and both cellular aggregation and albumin synthesis are blocked in the presence of anti E-cadherin immunoglobulin. Furthermore, by establishing a compartmental model of differentiation, which incorporates this aggregation phenomenon, we can optimize key encapsulation parameters.

Polyelectrolyte nano-scaffolds for the design of layered cellular architectures

The design of in vitro multilayered cellular architectures that resemble the stratified, lattice-like structure in tissues poses a significant challenge for tissue engineering. There is currently no generally applicable methodology to design multilayered cellular constructs that mimic the structure of tissues in vivo. We report a novel and generalizable approach to create multilayered cellular constructs that addresses these issues. These in vitro constructs comprise alternating layers of cells and nano-scale biocompatible polyelectrolyte (PE) scaffolds. We apply this methodology to address two specific problems in hepatic tissue engineering: the design of in vitro liver sinusoidal structures and the critical need to increase viable cell mass in extracorporeal liver-assist devices. We assembled ultrathin polymer scaffolds on the top of a confluent monolayer of cells by the sequential deposition of oppositely charged PEs. The thickness of the PE scaffold lies in the nanometer range. The PE scaffold plays a dual role. First, it is a technique to culture hepatocytes in vitro that maintains their morphology, cytoskeletal structure, and liver-specific functions. Second, the nano-scaffold provides a cell-adhesive surface on which a second layer of cells can be cultured, resulting in layered architectures. We have used this approach to design layered three-dimensional hepatocyte-PE-hepatocyte constructs, hepatocyte-PE-endothelial cell constructs, and hepatocyte-PE-fibroblast constructs. As a result of its versatility, this approach can, in principle, be used to design layered cellular constructs of any tissue type, and therefore has potentially wide applications in tissue engineering, bioreactor devices, and in drug delivery. This methodology has the potential to generate realistic in vitro constructs of any tissue type.

Integration of technologies for hepatic tissue engineering

The liver is the largest internal organ in the body, responsible for over 500 metabolic, regulatory, and immune functions. Loss of liver function leads to liver failure which causes over 25,000 deaths/year in the United States. Efforts in the field of hepatic tissue engineering include the design of bioartificial liver systems to prolong patient's lives during liver failure, for drug toxicity screening and for the study of liver regeneration, ischemia/reperfusion injury, fibrosis, viral infection, and inflammation. This chapter will overview the current state-of-the-art in hepatology including isolated perfused liver, culture of liver slices and tissue explants, hepatocyte culture on collagen "sandwich" and spheroids, coculture of hepatocytes with non-parenchymal cells, and the integration of these culture techniques with microfluidics and reactor design. This work will discuss the role of oxygen and medium composition in hepatocyte culture and present promising new technologies for hepatocyte proliferation and function. We will also discuss liver development, architecture, and function as they relate to these culture techniques. Finally, we will review current opportunities and major challenges in integrating cell culture, bioreactor design, and microtechnology to develop new systems for novel applications.

Tissue assembly guided via substrate biophysics: applications to hepatocellular engineering

The biophysical nature of the cellular microenvironment, in combination with its biochemical properties, can critically modulate the outcome of three-dimensional (3-D) multicellular morphogenesis. This phenomenon is particularly relevant for the design of materials suitable for supporting hepatocellular cultures, where cellular morphology is known to be intimately linked to the functional output of the cells. This review summarizes recent work describing biophysical regulation of hepatocellular morphogenesis and function and focuses on the manner by which biochemical cues can concomitantly augment this responsiveness. In particular, two distinct design parameters of the substrate biophysics are examined--microtopography and mechanical compliance. Substrate microtopography, introduced in the form of increasing pore size on collagen sponges and poly(glycolic acid) (PGLA) foams, was demonstrated to restrict the evolution of cellular morphogenesis to two dimensions (subcellular and cellular void sizes) or induce 3-D cellular assembly (supercellular void size). These patterns of morphogenesis were additionally governed by the biochemical nature of the substrate and were highly correlated to resultant levels of cell function. Substrate mechanical compliance, introduced via increased chemical crosslinking of the basement membrane, Matrigel, and polyacrylamide gel substrates, also was shown to be able to induce active two-dimensional (2-D, rigid substrates) or 3-D (malleable substrates) cellular reorganization. The extent of morphogenesis and the ensuing levels of cell function were highly dependent on the biochemical nature of the cellular microenvironment, including the presence of increasing extracellular matrix (ECM) ligand and growth-factor concentrations. Collectively, these studies highlight not only the ability of substrate biophysics to control hepatocellular morphogenesis but also the ability of biochemical cues to further enhance these effects. In particular, results of these studies reveal novel means by which hepatocellular morphogenesis and assembly can be rationally manipulated leading to the strategic control of the expression of liver-specific functions for hepatic tissue-engineering applications.

Rho kinase, myosin-II, and p42/44 MAPK control extracellular matrix-mediated apical bile canalicular lumen morphogenesis in HepG2 cells

The molecular mechanisms that regulate multicellular architecture and the development of extended apical bile canalicular lumens in hepatocytes are poorly understood. Here, we show that hepatic HepG2 cells cultured on glass coverslips first develop intercellular apical lumens typically formed by a pair of cells. Prolonged cell culture results in extensive organizational changes, including cell clustering, multilayering, and apical lumen morphogenesis. The latter includes the development of large acinar structures and subsequent elongated canalicular lumens that span multiple cells. These morphological changes closely resemble the early organizational pattern during development, regeneration, and neoplasia of the liver and are rapidly induced when cells are cultured on predeposited extracellular matrix (ECM). Inhibition of Rho kinase or its target myosin-II ATPase in cells cultured on glass coverslips mimics the morphogenic response to ECM. Consistently, stimulation of Rho kinase and subsequent myosin-II ATPase activity by lipoxygenase-controlled eicosatetranoic acid metabolism inhibits ECM-mediated cell multilayering and apical lumen morphogenesis but not initial apical lumen formation. Furthermore, apical lumen remodeling but not cell multilayering requires basal p42/44 MAPK activity. Together, the data suggest a role for hepatocyte-derived ECM in the spatial organization of hepatocytes and apical lumen morphogenesis and identify Rho kinase, myosin-II, and MAPK as potentially important players in different aspects of bile canalicular lumen morphogenesis.

Characterization of the secreted proteome of rat hepatocytes cultured in collagen sandwiches

Analysis of proteins in biological samples opens up the possibility of discovering new markers of toxicity. The liver is one of the primary targets of drug-induced toxicity, and it also secretes many plasma proteins, which can be measured clinically. Most of the plasma proteins produced by the liver are secreted by hepatocytes, but there is little information regarding the protein profile secreted by these cells. The purpose of this study was to analyze the secreted proteome of primary rat hepatocytes in a collagen gel sandwich configuration by a gel-LC-MS/MS procedure. We identified over 600 peptides corresponding to more than 200 proteins. The protein profile included over 50 plasma proteins, suggesting that the cultured hepatocytes secrete many of the proteins that they produce in vivo. Our data also suggests that the hepatocytes are actively remodeling their environment, since we identified several structural extracellular matrix proteins as well as some proteins known to be secreted specifically during liver regeneration. We also identified two proteins, alpha1-antitrypsin and alpha2-macroglobulin, whose secretions appear to be down-regulated in cells exposed to aflatoxin B1. It was noted that a 15 nM dose of aflatoxin B1 led to substantially diminished levels of these proteins and that day 6 of incubation was the ideal time point for medium collection. These data suggest that proteins in the conditioned medium of hepatocyte sandwich culture might lead to the discovery of biomarkers for drug-induced chemical toxicity.

Cytomimetic engineering of hepatocyte morphogenesis and function by substrate-based presentation of acellular E-cadherin

Although cadherin-mediated intercellular contacts can be integral to the maintenance of functionally competent hepatocytes in vitro, the ability to engineer hepatocellular differentiated function via acellular E-cadherin has yet to be thoroughly explored. To investigate the potential of substrate-presented, acellular E-cadherin to modulate hepatocellular self-assembly and functional fate, rat hepatocytes were cultured at sparse densities on surfaces designed to display recombinant E-cadherin/Fc chimeras. On these substrates, hepatocytes were observed to recognize microdisplayed E-cadherin/Fc and responded by modulating the spatial distribution of the intracellular cadherin-complexing protein beta-catenin. Substrate-presented E-cadherin/Fc was also found to markedly alter patterns of hepatocyte morphogenesis, as cellular spreading and two-dimensional reorganization were significantly inhibited under these conditions, leading to multicellular aggregates that were considerably more three-dimensional in nature. Increasing cadherin exposure was also associated with elevated levels of albumin and urea secretion, two markers of hepatocyte differentiation, over control cultures. This suggested that cell-substrate cadherin engagement established more functionally competent hepatocellular phenotypes, coinciding with the notion that E-cadherin is a differentiation-inducing ligand for these cells. The morphogenetic and function-promoting effects of substrate-bound E-cadherin/Fc were further enhanced under conditions in which protein A was utilized as an anchoring molecule to present cadherin molecules, suggesting that ligand mobility may play an important role in the effective establishment of cell-to-substrate cadherin interactions. Interestingly, the percent increase in function detected for conditions of high cadherin exposure versus control cultures was found to be substantially higher at extremely low cell densities. This observation indicated that hepatocytes respond to substrate-presented E-cadherin even in the absence of native intercellular interactions and associated juxtacrine signaling. The incorporation of acellular E-cadherin on biomaterial substrates may thus potentially present a means to prevent hepatocellular dedifferentiation by maintaining liver-specific function in otherwise severely functionally repressive culture conditions.

Regulation of hepatocyte cell cycle progression and differentiation by type I collagen structure

Cell behavior is strongly influenced by the extracellular matrix (ECM) to which cells adhere. Both chemical determinants within ECM molecules and mechanical properties of the ECM network regulate cellular response, including proliferation, differentiation, and apoptosis. Type I collagen is the most abundant ECM protein in the body with a complex structure that can be altered in vivo by proteolysis, cross-linking, and other processes. Because of collagen's complex and dynamic nature, it is important to define the changes in cell response to different collagen structures and its underlying mechanisms. This chapter reviews current knowledge of potential mechanisms by which type I collagen affects cell behavior, and it presents data that elucidate specific intracellular signaling pathways by which changes in type I collagen structure differentially regulate hepatocyte cell cycle progression and differentiation. A network of polymerized fibrillar type I collagen (collagen gel) induces a highly differentiated but growth-arrested phenotype in primary hepatocytes, whereas a film of monomeric collagen adsorbed to a rigid dish promotes cell cycle progression and dedifferentiation. Studies presented here demonstrate that protein kinase A (PKA) activity is significantly elevated in hepatocytes on type I collagen gel relative to collagen film, and inhibition of this elevated PKA activity can promote hepatocyte cell cycle progression on collagen gel. Additional studies are presented that examine changes in hepatocyte cell cycle progression and differentiation in response to increased rigidity of polymerized collagen gel by fiber cross-linking. Potential mechanisms underlying these cellular responses and their implications are discussed.

Galactosylated poly(vinylidene difluoride) hollow fiber bioreactor for hepatocyte culture

To overcome the limitations of long-term expression of highly differentiated hepatocyte functions, we have developed a novel bioreactor in which hepatocytes are seeded in a ligand-immobilized hollow fiber cartridge. Galactosylated Pluronic polymer is immobilized on poly(vinylidene difluoride) (PVDF) hollow fiber surface through an adsorption scheme yielding a substrate with hepatocyte-specific ligand and a hydrophilic surface layer, which can resist nonspecific protein adsorption and facilitate cell binding to the galactose ligand. Interestingly, the galactosylated PVDF hollow fiber shows enhanced serum albumin diffusion across the membrane. Freshly isolated rat hepatocytes were seeded and cultured in the extralumenal space of the hollow fiber cartridge for 18 days in a continuously circulated system. Albumin secretion function of the seeded hepatocytes was monitored by analyzing circulating medium by enzyme-linked immunosorbent assay. Urea synthesis and P-450 function (7-ethoxycoumarin dealkylase activity) were measured periodically by doping the circulating medium with NH4Cl and 7-ethoxycoumarin, respectively. Hepatocytes cultured on galactosylated PVDF hollow fibers maintained better albumin secretion and P-450 functions than on unmodified and serum-coated PVDF hollow fibers when cultured in serum-containing medium. Morphological examination by scanning electron microscopy showed that hepatocytes cultured on galactosylated PVDF hollow fibers developed significant aggregation, in contrast to those cultured on unmodified PVDF fibers or on serum-coated PVDF fibers. Transmission electron microscopy images revealed that tight junctions and canaliculus-like structures formed in these aggregates. These results suggest the potential application of this galactosylated PVDF hollow fiber cartridge for the design of a bioartificial liver assist device.

Liver endothelial cells promote LDL-R expression and the uptake of HCV-like particles in primary rat and human hepatocytes

Low-density lipoprotein (LDL) is an important carrier of plasma cholesterol and triglycerides whose concentration is regulated by the liver parenchymal cells. Abnormal LDL regulation is thought to cause atherosclerosis, while viral binding to LDL has been suggested to facilitate hepatitis C infection. Primary hepatocytes quickly lose the ability to clear LDL during in vitro culture. Here we show that the coculture of hepatocytes with liver sinusoidal endothelial cells (LSEC) significantly increases the ability of hepatocytes to uptake LDL in vitro. LDL uptake does not increase when hepatocytes are cocultured with other cell types such as fibroblasts or umbilical vein endothelial cells. We find that LSECs induce the hepatic expression of the LDL receptor and the epidermal growth factor receptor. In addition, while hepatocytes in single culture did not take up hepatitis C virus (HCV)-like particles, the hepatocytes cocultured with LSECs showed a high level of HCV-like particle uptake. We suggest that coculture with LSECs induces the emergence of a sinusoidal surface in primary hepatocytes conducive to the uptake of HCV-like particles. In conclusion, our findings describe a novel model of polarized hepatocytes in vitro that can be used for the study of LDL metabolism and hepatitis C infection.

Schwann cell behavior in three-dimensional collagen gels: Evidence for differential mechano-transduction and the influence of TGF-beta 1 in morphological polarization and differentiation

Schwann cells (SCs) cultured on and within magnetically aligned collagen gels were examined for their abilities to spread and exhibit contact guidance, two functions that are relevant to their potential enhancement of neurite migration and regeneration in entubulation repair of transection-type nerve injuries. Cells seeded at or near the surfaces of gels abandoned their initially spherical shapes, adopting spread morphologies rapidly compared to cells within the gels. Those few cells within the gels that did spread exhibited marked contact guidance responses, aligning strongly with the aligned collagen fibrils. Spreading of cells in gels could not be induced by varied cell concentration, collagen density, mitogen presence, inclusion of soluble laminin, or use of fibrin gel in lieu of collagen. However, cells that settled at the interface between collagen gel layers during gellation of the top layer above a preformed bottom layer were highly spread. This suggests that a differential mechanical interaction across the cell at an interface, where at least one surface presents constituents of the basal lamina, permits the Schwann cell to rapidly revert to a spread, differentiated phenotype. Unlike other reagents, TGF-beta1 was able to induce significant SC spreading as early as 4 h post-seeding. Consistent with the differential-mechanical cue mechanism, TGF-beta1 appears to facilitate this response, at least in part, by upregulating beta1 integrin expression, thereby enabling the SC to more acutely detect these local cues in the mechanical environment.

Three-dimensional co-culture of rat hepatocyte spheroids and NIH/3T3 fibroblasts enhances hepatocyte functional maintenance

Functional maintenance of primary hepatocytes in culture can be improved by several distinct approaches involving optimization of the extracellular matrix microenvironment, media composition and cell-cell interactions, both homotypic and heterotypic. Using a galactose-decorated surface, we have developed a method to combine these two approaches by co-culturing rat primary hepatocyte spheroids with NIH/3T3 mouse fibroblast cells. Spheroids were performed by culturing hepatocytes for 3 days on galactosylated poly(vinylidene difluoride) membrane; NIH/3T3 cells were subsequently seeded and co-cultured with the spheroids. Results showed that although NIH/3T3 cells alone responded poorly to the galactosylated PVDF surface and displayed limited attachment, NIH/3T3 fibroblasts attached to the periphery of the hepatocyte spheroids and proliferated around them. Co-cultured hepatocyte spheroids exhibited significantly higher liver-specific functions as compared to spheroids cultured alone. Albumin secretion level in this co-culture system peaked on day 11, which was 1.8- and 2.9-times higher than the peak expression level in spheroid homo-culture control in serum-free (day 3) and serum-containing media (day 4), respectively. The albumin secretion function was maintained for at least two weeks; it was 5.1 (in serum-free medium) and 17.8 (in serum-containing medium) times higher than spheroid homo-culture on day 13. Similarly, the co-culture system also expressed approximately 5.5- and 3.1-times higher 3-methylcholanthrene-induced cytochrome P450 enzymatic activity on day 14 as compared to the homo-culture control in serum-free and serum-containing medium, respectively. In conclusion, this unique co-culture system demonstrated the synergistic roles of homotypic cell-cell interaction, heterotypic cell-cell interaction, cell-substrate interaction and soluble stimuli in hepatocyte functional maintenance.

Three-dimensional polarization sensitizes hepatocytes to Fas/CD95 apoptotic signalling

Maintenance of epithelial cell shape and polarity determines many vital cell functions, including the appropriate response to external stimuli. Murine hepatocytes cultured in a three-dimensional Matrigel matrix formed highly polarized organoids characterized by specific localization of an ERM (ezrin/radixin/moesin) protein, radixin, at microvillus-lined membrane domains. These apical domains surrounded a lumen and were bordered by tight junctions. The hepatocyte organoids were functional as judged by the high level of albumin secretion and accumulation of bilirubin. Stimulation of the Fas/CD95 death receptor, which is highly hepatotoxic in vivo, was a strong inducer of apoptosis in the polarized organoids. This was in sharp contrast to the monolayer hepatocyte cultures, which were protected from death by exacerbated NF-κB signalling following engagement of the death receptors. Thus, hepatocytes in polarized, functional organoids modulate an intracellular signal transduction pathway, allowing the recapitulation of their physiological response to an apoptotic stimulus.

Optimization of 3-D hepatocyte culture by controlling the physical and chemical properties of the extra-cellular matrices

Hepatocytes are anchorage-dependent cells sensitive to microenvironment; the control of the physicochemical properties of the extra-cellular matrices may be useful to the maintenance of hepatocyte functions in vitro for various applications. In a microcapsule-based 3-D hepatocyte culture microenvironment, we could control the physical properties of the collagen nano-fibres by fine-tuning the complex-coacervation reaction between methylated collagen and terpolymer of hydroxylethyl methacrylate-methyl methacrylate-methylacrylic acid. The physical properties of the nano-fibres were quantitatively characterized using back-scattering confocal microscopy to help optimize the physical support for hepatocyte functions. We further enhanced the chemical properties of the collagen nano-fibres by incorporating galactose onto collagen, which can specifically interact with the asialoglycoprotein receptor on hepatocytes. By correlating a range of collagen nano-fibres of different physicochemical properties with hepatocyte functions, we have identified a specific combination of methylated and galactosylated collagen nano-fibres optimal for maintaining hepatocyte functions in vitro. A model of how the physical and chemical supports interplay to maintain hepatocyte functions is discussed.

Assessing porcine liver-derived biomatrix for hepatic tissue engineering

Acellular, biologically derived matrices such as small intestinal submucosa have been extensively utilized to induce tissue regeneration and remodeling of connective tissue, vascular grafts, and urinary bladder; however, decellularized scaffolds have not been explored for their potential utility in hepatic tissue engineering. In the case of both extracorporeal hepatocyte-based devices and implantable hepatocyte-scaffold tissue-engineered constructs, maintenance of hepatocellular function is of prime importance. In this study, we specifically explored decellularized, porcine, liver-derived biomatrix (LBM) as a bioresorbable scaffold for primary hepatocytes. Primary rat hepatocytes were cultured on LBM and compared with well-characterized hepatocyte culture models--double-gel cultures that promote maintenance of liver-specific functions for many weeks, and adsorbed collagen monolayers that lead to the rapid decline of hepatocellular function and viability. Hepatocytes were maintained for up to 45 days on LBM and liver-specific functions such as albumin synthesis, urea production, and P-450 IA1 activity were found to be significantly improved over adsorbed collagen cultures. Our data indicate that LBM may be a favorable alternative to existing scaffolds for tissue engineering in that it is bioresorbable, can be easily manipulated, and supports long-term hepatocellular functions in vitro.

Characterization of chemically induced hepatotoxicity in collagen sandwiches of rat hepatocytes

It has been shown that hepatocytes cultured in a collagen sandwich configuration maintain cell viability, morphology, and drug metabolizing activities for several weeks. The purpose of this study was to characterize chemically induced general toxicity in this system by exposing hepatocytes to eight different hepatotoxic compounds. Cell function and viability was measured by analyzing the secretions of urea and albumin and the release of lactate dehydrogenase. Significant decreases in urea and albumin secretions were detected after treatments with 32 nM aflatoxin B(1) and 1 mM doses of cadmium and the alkylating agents N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and methyl methane sulfonate (MMS). However, no significant toxicity could be measured following exposures to 5 mM carbon tetrachloride, 1 mM N, N-dimethylformamide (DMF), 1 mM vinyl acetate, and 1 mM acetaminophen. Western blots of cell lysates showed that hepatocytes maintained CYP1A, 2B, 3A2 but gradually lost CYP2E1, which is the main metabolic enzyme for acetaminophen, carbon tetrachloride, and DMF. The metabolites of acetaminophen were identified using liquid chromatography and electrospray mass spectrometry. It was determined that the hepatocytes converted most of the acetaminophen to the glucuronide and sulfate metabolites and only formed a small amount of the glutathione adduct. This research shows that the collagen sandwich culture system can only be used selectively for detecting hepatotoxicity and for identifying major metabolites of xenobiotic compounds.

Engineering the hepatocyte differentiation-proliferation balance by acellular cadherin micropresentation

The successful development of bioartificial and cell-based liver support systems relies on the identification of molecular mechanisms controlling the balance between hepatocellular proliferation and differentiation. Although a definitive function-inductive role for the cell-cell adhesion molecule, E-cadherin, was established through lateral cadherin-cadherin engagement in hepatocyte cocultures (Brevia, T.A., and Moghe, P.V. Biotechnol. Bioeng. 76, 295, 2001), the roles of other modes of cadherin presentation are not well understood. Further, alternative cadherin display configurations promoting cell growth/proliferative pathways, a major requisite for sustainable engineered tissues, remain to be identified. In this report, we employed protein A-functionalized polymeric microsphere substrates that specifically bound self-dimerizing cadherin-IgG/Fc fusion chimeras via their Fc regions, thereby orienting them outward for active adhesion, and presented the E-cadherin chimeras basally to cultured rat hepatocytes to study the effects of cadherin display on cell proliferative potential and differentiated function. In contrast to the previously documented function-inductive roles of laterally expressed cadherin, basal acellular cadherin presentation resulted in an increase in hepatocyte DNA synthesis and cell divisions, accompanied by a decrease in the expression of a key marker of liver-specific function, albumin message levels. Next, we probed the relative effect of basal exogenous display of acellular cadherins on the inductive phase of differentiation within hepatocyte cocultures with cadherin-expressing L929 cells. When acellular cadherins were applied to hepatocyte cocultures involving chaperone cell-mediated cadherin presentation, the previously reported function-inductive effects of cadherins on hepatocyte function were reversed, resulting in lower levels of albumin and urea secretion indicating a dominance of acellular cadherin effects. Our results demonstrate that cadherins are important regulators of the balance between hepatocyte differentiation and proliferation, and furthermore that the direction of balance shift may be dependent on the method of cadherin presentation. Thus, the geometric display of cadherin could be a potential parameter to switch hepatocyte functional-proliferative balance and may aid in customizing scaffolds for regulating hepatic tissue dynamics.

Supramolecular assemblies of adsorbed collagen affect the adhesion of endothelial cells

The behavior of endothelial cells (HUVECs) in contact with thin collagen films presenting different supramolecular organizations was investigated. Collagen was adsorbed on polystyrene (PS) and plasma-oxidized PS (PSox) in conditions ensuring the formation of continuous layers presenting an increasing density of fibrillar structures. Discontinuous collagen layers were also prepared on PS by adsorption followed by dewetting. The morphology of the obtained collagen films was checked by using atomic force microscopy. HUVECs adhesion was evaluated in terms of cell number, cell area, cell shape, and actin structure after 4 h of contact with the prepared collagen layers. In the presence of serum, no adhesion was observed on PS, whereas a substantial adhesion was found on PSox. This is explained by the competition for adsorption, which turns in favor of adhesive proteins secreted by the cells on the hydrophilic PSox, but turns in favor of serum albumin on the hydrophobic PS. The progressive coating of PS by smooth collagen films increased cell adhesion and spreading. However, cell spreading and cytoskeleton organization were adversely affected by the appearance of a high density of collagen fibrillar structures. This latter trend was similarly observed on PSox. On the other hand, HUVECs spreading and cytoskeleton organization were clearly enhanced on discontinuous collagen layers compared with continuous ones. A possible explanation for these observations lies in the modification of exposure and/or spatial distribution of recognition sequences due to spontaneous collagen self-assembly on fibril formation or to collagen aggregation on dewetting.

INVESTIGATION OF A HEPATOTOXICITY SCREENING SYSTEM IN PRIMARY CELL CULTURES-"WHAT BIOMARKERS WOULD NEED TO BE ADDRESSED TO ESTIMATE TOXICITY IN CONVENTIONAL AND NEW APPROACHES?"-

High throughput toxicological estimation is required for safety evaluation in the early stage of drug discovery. In this context, establishment of an in vitro screening system reflecting in vivo toxicity is demanded for earlier safety assessment. We investigated LDH release and mitochondrial respiration (WST-1 reduction assay; WST-1) to detect cytotoxicity, morphological evaluation, and proteomics for estimating the reliable and sensitive biomarkers by using rat primary hepatocytes exposed to the compounds (acetaminophen, amiodarone, tetracycline and carbon tetrachloride) that are known to induce hepatotoxicity. In LDH release, no significant difference was detected between the control and compound exposed cells after exposure for 3 or 6 hr, but a dose-dependent increase was observed after exposure for 24 hr. Regarding the WST-1 assay, a dose-dependent reduction was detected after exposure for 6 and 24 hr to all of the compounds evaluated. In the proteomics analysis, 31 candidate proteins were identified from among the 103 demonstrating altered expression spots after exposure to acetaminophen. It was concluded that the cytotoxicity was detected earlier by measuring WST-1 than by measuring LDH release because the reduction of mitochondrial respiration is an expressions of earlier toxicity for cellular function, while the measured increase in the LDH release occurs after the failure of the cell membrane. Mitochondrial respiration ability was a useful parameter for cytotoxicity in in vitro hepato-toxicity screening, as cytotoxicity can be detected during the early stage of exposure. In addition to the conventional biomarkers, several protein biomarkers which relate to oxidative stress and metabolism-regulation were detected. Further comprehensive analysis of defined proteins would be necessary to estimate the more sensitive toxicology biomarker.

Physiologically based pharmacokinetic (PBPK) modeling of disposition of epiroprim in humans

The objective of this study was to use in synergy physiologically based and empirical approaches to estimate the drug-specific input parameters of PBPK models of disposition to simulate the plasma concentration-time profile of epiroprim in human. The estimated input parameters were the tissue:plasma partition coefficients (Pt:p) for distribution and the blood clearance (CL) for the in vivo conditions. Epiroprim represents a challenge for such methods, because it shows large interspecies differences in its pharmacokinetic properties. Two approaches were used to predict the human Pt:p values: the tissue composition model (TCM) and the "Arundel approach" based on the volume of distribution at steady state (Vdss) determined in vivo in the rat. CL in human was predicted by (1) conventional allometric scaling of in vivo animal clearances (CAS), (2) physiologically based direct scaling up of in vitro hepatocyte data (DSU), and (3) allometric scaling of animal intrinsic in vivo blood CL normalized by the ratios of animal:human intrinsic clearances determined in vitro with hepatocytes (NAS). The performance of prediction was assessed by comparing separately the above pharmacokinetic parameters (Vdss estimated from the Pt:p values and blood CL) with the corresponding in vivo data obtained from the plasma kinetic profiles. These input parameters were used in PBPK models, and the resulting plasma concentration-time profiles of epiroprim were compared with those observed in rat and human. Previously to the construction of the human PBPK model, a model for the rat was also developed to gain more confidence on the model structure and assumptions. Overall, using the TCM and the NAS for the parameterization of the distribution and clearance, respectively, the PBPK model gave the more accurate predictions of epiroprim's disposition in human. This study represents therefore an attractive approach, which may potentially help the clinical candidate selection.

Designing a hepatocellular microenvironment with protein microarraying and poly(ethylene glycol) photolithography

In this study, robotic protein printing was employed as a method for designing a cellular microenvironment. Protein printing proved to be an effective strategy for creating micropatterned co-cultures of primary rat hepatocytes and 3T3 fibroblasts. Collagen spots (ca. 170 microm in diameter) were printed onto amino-silane- and glutaraldehyde-modified glass slides. Groups of 15-20 hepatocytes attached to collagen regions in a highly selective manner forming cell clusters corresponding in size to the printed collagen domains. Fibroblasts, seeded onto the same surface, adhered and spread around arrays of hepatocyte islands creating a heterotypic environment. The co-cultured hepatocytes produced and maintained high levels of liver-specific biomarkers, albumin and urea, over the course of 2 weeks. In addition, protein printing was combined with poly(ethylene glycol) photolithography to define intercellular contacts within the clusters of hepatocytes residing on individual collagen islands. Glass slides, treated with 3-acryloxypropyl trichlorosilane and imprinted with 170 m diameter collagen spots, were micropatterned with a high-density array of 30 microm x 30 microm poly(ethylene glycol) (PEG) wells. As a result, discrete groups of ca. 9 PEG microwells became functionalized with the cell-adhesive ligand. When exposed to micropatterned surfaces, hepatocytes interacted exclusively with collagen-modified regions, attaching and becoming confined at a single-cell level within the hydrogel wells. Micropatterning strategies proposed here will lead to greater insights into hepatocellular behavior and will benefit the fields of hepatic tissue engineering and liver biology.

Phenobarbital responsiveness as a uniquely sensitive indicator of hepatocyte differentiation status: requirement of dexamethasone and extracellular matrix in establishing the functional integrity of cultured primary rat hepatocytes

We used a serum-free, highly defined primary hepatocyte culture model to investigate the mechanisms whereby dexamethasone (Dex) and extracellular matrix (ECM) coordinate cell differentiation and transcriptional responsiveness to the inducer, phenobarbital (PB). Low nanomolar levels of Dex and dilute concentrations of ECM overlay were essential in the maintenance of normal hepatocyte physiology, as assessed by cell morphology, LDH release, expression of the hepatic nuclear factors C/EBPalpha, -beta, -gamma, HNF-1alpha, -1beta, -4alpha, and RXRalpha, expression of prototypical hepatic marker genes, including albumin and transferrin, and ultimately, cellular capacity to respond to PB. The loss of hepatocyte integrity produced by deficiency of these components correlated with the activation of several stress signaling pathways including the MAPK, SAPK/JNK, and c-Jun signaling pathways, with resulting nuclear recruitment of the activated protein-1 (AP-1) complex. In Dex-deficient cultures, normal cellular function, including the PB induction response, was largely restored in a dose-dependent manner by reintroduction of nanomolar additions of the hormone, in the presence of ECM. Our results demonstrate critical and cooperative roles for Dex and ECM in establishing hepatocyte integrity and in the coordination of an array of liver-specific functions. These studies further establish the PB gene induction response as an exceptionally sensitive indicator of hepatocyte differentiation status.

Long-term function of cryopreserved rat hepatocytes in a coculture system

The goal of this study was to investigate postpreservation long-term function of cryopreserved primary rat hepatocytes using the hepatocyte/3T3-J2 fibroblast coculture system. The long-term function of thawed hepatocytes cocultured with fibroblasts was evaluated and compared with hepatocytes cultured without fibroblasts. Fresh isolated primary rat hepatocytes were frozen at a controlled rate (-1 degrees C/min) up to -80 degrees C, and then stored in liquid nitrogen for up to 90 days. Thawed hepatocytes were thereafter cocultured with 3T3-J2 murine fibroblasts and cocultivation was monitored for 14 days. The viability of fresh isolated hepatocytes was 91.4%, and that of cryopreserved hepatocytes was 82.1%. Cellular morphology and polarity, which were determined by the localization of actin filaments and connexin-32, were successfully maintained in cryopreserved hepatocytes following cryopreservation. Albumin and urea synthesis reached the maximum level and became stable after day 7 in coculture in both fresh and cryopreserved hepatocytes. Urea synthesis of cryopreserved hepatocytes was maintained 89.0% of nonfrozen fresh control, and albumin production of cryopreserved hepatocytes was 63.7% of control in coculture. Cytochrome P450 activity, which was measured by deethylation of ethoxyresorufin, was also maintained in cryopreserved hepatocytes at 88.6% of nonfrozen fresh control in coculture. The retention of synthetic and detoxification activities was verified to be well preserved during extended low-temperature storage (90 days). Both fresh control and cryopreserved hepatocytes cultured without fibroblast did not retain their synthetic and detoxification functions in long-term culture. These data illustrate that, through the utilization of our cryopreservation procedure, primary hepatocyte function was successfully maintained when placed into coculture configuration following thawing.

Immobilization of galactose ligands on acrylic acid graft-copolymerized poly(ethylene terephthalate) film and its application to hepatocyte culture

Surface modification of argon-plasma-pretreated poly(ethylene terephthalate) (PET) films via UV-induced graft copolymerization with acrylic acid (AAc) was carried out. Galactosylated surfaces were then obtained by coupling a galactose derivative (1-O-(6'-aminohexyl)-D-galactopyranoside) to the AAc graft chains with the aid of a water-soluble carbodiimide (WSC) and N-hydroxysulfosuccinimide (sulfo-NHS). The modified PET films were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and water contact-angle measurements. The galactosylated PET films were used as substrates for hepatocyte culture. The effects of surface carboxyl group concentration on the extent of galactose ligand immobilization, the extent of hepatocyte attachment, and the surface morphology were investigated. The amount of the galactose ligands immobilized on the PET surface increased with the AAc polymer graft concentration. AFM images revealed that the surface roughness of the PET film increased after graft copolymerization with AAc, but did not change appreciably with the subsequent immobilization of the galactose ligands. At the surface carboxyl group concentration of about 0.56 micromol/cm(2) or galactose ligand concentration of about 0.51 micromol/cm(2), the hepatocyte culture on the galactosylated surface exhibited the optimum concentration and physiological functions and formed aggregates or spheroids after just 1 day of culture. The albumin and urea synthesis functions of these hepatocytes were comparable to or higher than those of the hepatocytes cultured on the collagen-modified PET substrates.

Biology on a chip: microfabrication for studying the behavior of cultured cells

The ability to culture cells in vitro has revolutionized hypothesis testing in basic cell and molecular biology research and has become a standard methodology in drug screening and toxicology assays. However, the traditional cell culture methodology--consisting essentially of the immersion of a large population of cells in a homogeneous fluid medium--has become increasingly limiting, both from a fundamental point of view (cells in vivo are surrounded by complex spatiotemporal microenvironments) and from a practical perspective (scaling up the number of fluid handling steps and cell manipulations for high-throughput studies in vitro is prohibitively expensive). Microfabrication technologies have enabled researchers to design, with micrometer control, the biochemical composition and topology of the substrate, the medium composition, as well as the type of neighboring cells surrounding the microenvironment of the cell. In addition, microtechnology is conceptually well suited for the development of fast, low-cost in vitro systems that allow for high-throughput culturing and analysis of cells under large numbers of conditions. Here we review a variety of applications of microfabrication in cell culture studies, with an emphasis on the biology of various cell types.

Evaluation of a galactose-carrying gelatin sponge for hepatocytes culture and transplantation

This study proposes a new three-dimensional culture of mouse hepatocytes in a porous galactose-carrying modified gelatin sponge matrix. The modification of gelatin using galactose residues significantly increased the attachment of hepatocytes on the substrate. A modified gelatin sponge with lactobionic acid (MGLA) was prepared to increase the specific interaction between the hepatocytes and the matrix. Hepatocytes cultured in a three-dimensional MGLA sponge released much less lactate dehydrogenase than those cultured on a collagen Type I-coated monolayer. Moreover, the survival rate of hepatocytes cultured on an MGLA sponge was longer than the survival rate of hepatocytes cultured on a collagen Type I-coated monolayer. Hepatic specific metabolic functions, namely, the secretion of serum albumin and the synthesis of urea, were well maintained and promoted by spheroidal hepatocytes formed in the MGLA sponge.

Novel approach for achieving double-layered cell sheets co-culture: overlaying endothelial cell sheets onto monolayer hepatocytes utilizing temperature-responsive culture dishes

Confluent human aortic endothelial cells (HAECs) cultured on thermo-responsive culture dish grafted with poly (N-isopropylacrylamide) were recovered as a contiguous cell sheet. The double-layered co-culture was achieved by placing the recovered HAEC sheet onto the rat hepatocyte layer directly. The double-layered structure of HAEC and hepatocytes remained in tight contact during culture. Hepatocytes in the layered co-culture system with the HAEC sheet maintained the differentiated cell shape and the albumin expression for over 41 days of culture, whereas the functions disappeared within 10 days of culture in control hepatocytes without the HAEC sheet. The layered co-culture of hepatocytes and the HAEC sheets, which allows for the expression of differentiated functions of hepatocyte continuously, such as liver lobule, offers a major advancement in liver tissue engineering.

Influence of isolation procedure, extracellular matrix and dexamethasone on the regulation of membrane transporters gene expression in rat hepatocytes

The influence of the isolation procedure of hepatocytes, extracellular matrix (ECM) configuration and incubation medium supplementation by dexamethasone (DEX) on the cell morphology and on the gene expression of membrane transporters was examined in rat hepatocytes. The mRNA levels were determined using oligonucleotide microarrays, in liver, in suspension and in primary culture in monolayer (CPC), and in collagen gels sandwich (SPC) in absence and presence of DEX (100 and 1000 nM). The results indicated pronounced morphological differences between CPC and SPC in response to DEX demonstrating that the hepatocytes re-formed, as in vivo, multicellular arrays with extensive bile canalicular network only in SPC in presence of DEX. The mRNA levels of membrane transporters were not affected significantly during isolation procedure. However, plating hepatocytes in CPC resulted in a decrease of major basolateral transporters mRNA level whereas mRNA levels of mdr1b and mrp3 were increased (>100-fold). Similar observations were made in SPC in the absence of DEX demonstrating that the ECM configuration alone did not play a critical role in the regulation of membrane transporters. However, adding DEX to the incubation medium in SPC resulted in an up-regulation of mdr2, oatp2 and mrp2 in a concentration-dependent way for the two latter genes, whereas mdr1b and mrp3 expression were maintained to their baseline liver levels. These data suggested therefore that the combination of ECM and DEX supplementation is essential for the formation of the bile canalicular network and is a determinant factor in the regulation of membrane transporters in cultured rat hepatocytes.

Optimised conditions for the production of hepatitis B virus from cell culture

In chronically infected patients, hepatitis B virus (HBV) particles reach numbers as large as >10(9) genome equivalents (GE)/ml of serum. However, expression of infectious HBV particles in cell culture only yields 10(5)-10(6) GE/ml, which is insufficient for many studies. HBV transcription and possibly replication is dependent on hepatocyte-specific differentiation. Thus, we tested several cell culture parameters that have been reported to enhance the expression of hepatocyte-specific markers, such as growth on different extracellular matrices, different cell culture media, low concentrations of fetal calf serum (FCS) and the addition of dimethyl sulfoxide (DMSO) to the medium. Lower concentrations of FCS, growth on collagen and inclusion of DMSO in the medium only moderately enhanced HBV production in vitro when applied individually. However, combinations of these parameters optimised cell culture conditions and reproducibly increased the release of HBV particles about 100-fold to titres >10(8) GE/ml of culture medium.

Structural polarity and functional bile canaliculi in rat hepatocyte spheroids

Primary hepatocytes self-assemble into spheroids that possess tight junctions and microvilli-lined channels. We hypothesized that polarity develops gradually and that the channels structurally and functionally resemble bile canaliculi. Immunofluorescence labeling of apical and basolateral proteins demonstrated reorganization of the membrane proteins into a polarized distribution during spheroid culture. By means of fluorescent dextran diffusion and confocal microscopy, an extensive network of channels was revealed in the interior of the spheroids. These channels connected over several planes and opened to pores on the surface. To examine the content of apical proteins in the channel membranes, the bile canalicular enzyme dipeptidyl peptidase IV (DPPIV) was localized using a fluorogenic substrate, Ala-Pro-cresyl violet. The results show that DPPIV activity is heterogeneously distributed in spheroids and localized in part to channels. Bile acid excretion was then investigated to demonstrate functional polarity. A fluorescent bile acid analogue, fluorescein isothiocyanate-labeled glycocholate, was taken up into the spheroids and excreted into bile canalicular channels. Due to the structural polarity of spheroids and their ability to excrete bile into channels, they are a unique three-dimensional model of in vitro liver tissue self-assembly. (Videoanimations of some results are available at http://hugroup.cems.umn.edu/research_movies).

Galactosylated alginate as a scaffold for hepatocytes entrapment

Galactose moieties were covalently coupled with alginate through ethylenediamine as the spacer for enhancing the interaction of hepatocytes with alginate. Adhesion of hepatocytes onto the galactosylated alginate (GA)-coated polystyrene (PS) surface showed an 18-fold increase as compared with that of the alginate-coated surface and it increased with an increase in the concentration of GA. The morphologies of attached hepatocytes were observed to spread out at the 0.15 wt% GA-coated PS surface while round cells were observed at the 0.5 wt% GA-coated PS surface. Inhibition of hepatocytes attachment onto the galactose-carrying PS-coated surface occurred with the addition of the GA into the hepatocyte suspension, indicating the binding of GA with hepatocytes via the patch of asialoglycoprotein receptors. Primary hepatocytes were entrapped in the GA/Ca2+ capsules (GAC). Higher cell viability and more spheroid formation of hepatocytes were obtained in the GAC than in the alginate/Ca2+ capsules (AC). Moreover, liver functions of the hepatocytes such as albumin secretion and urea synthesis in the GAC were improved in comparison with those in the AC.

Engineering hepatocyte functional fate through growth factor dynamics: the role of cell morphologic priming

We have reported previously that cellular stimulation induced by variable mechanochemical properties of the extracellular microenvironment can significantly alter liver-specific function in cultured hepatocytes (Semler et al., Biotech Bioeng 69:359-369, 2000). Cell activation via time-invariant presentation of biochemical growth factors was found to either enhance or repress cellular differentiation of cultured hepatocytes depending on the mechanical properties of the underlying substrate. In this work, we investigated the effects of dynamic growth factor stimulation on the cell growth and differentiation behavior of hepatocytes cultured on either compliant or rigid substrates. Specifically, hepatotrophic growth factors (epidermal and hepatocyte) were either temporally added or withdrawn from hepatocyte cultures on Matrigel that was crosslinked to yield differential degrees of mechanical compliance. We determined that the functional responsiveness of hepatocytes to fluctuations in GF stimulation is substrate specific but only in conditions in which the initial mechanochemical environment induced significant cell morphogenesis. Our studies indicate that in conditions under which hepatocytes adopted a "rounded" phenotype, they exhibited increased levels of differentiated function upon soluble stimulation and markedly decreased function upon the depletion of GF stimulation. In contrast, hepatocytes that assumed a "spread" phenotype exhibited slightly increased function upon the depletion of GF stimulation. By examining the functional responsiveness of hepatocytes of differential morphology to varied fluctuations in GF activation, insights into the ability of cell shape to "prime" hepatocyte behavior in dynamic microenvironments were elucidated. We report on the possibility of uncoupling and, thus, selectively manipulating, the concerted contributions of GF-induced cellular activation and substrate- and GF-induced cell morphogenesis toward induction of cell function.

Three-dimensional cell-scaffold constructs promote efficient gene transfection: implications for cell-based gene therapy

To date, introduction of gene-modified cells in vivo is still a critical limitation for cell-based gene therapy. In this study, based on tissue engineering techniques, we developed a three-dimensional (3-D) transfection system to be cell-based gene delivery vehicle. Human trophoblast-like ED(27) and fibroblastic NIH3T3 cells were used as model cell lines. Cells were seeded onto PET fibrous matrices and plated on polyethylene terephathalate (PET) films as 2-D transfection control. The cell-matrices and cell-films were transfected with pCMV-betagal and pEGFP (green fluorescent protein) reporter gene vectors using LipofectAmine reagent. Gene expression on 3-D versus 2-D growth surface were investigated. The effects of seeding method, seeding density, porosity of the PET matrix, and culturing time of the cell-matrix complex on cDNA transfection and expression in the 3-D cell-matrix complex were also investigated. The beta-gal assay and GFP detection showed that 3-D transfection promoted a higher gene expression level and longer expression time as compared to 2-D transfection. There existed an optimal initial cell seeding density for gene transfection of 3-D cell-matrix complex. Cells seeded on PET matrices with a lower porosity ( approximately 87%) had higher gene expression activities than cells in the matrices with a higher porosity ( approximately 90%). Also, Higher gene expression levels of beta-gal were obtained for the more uniformly seeded matrices that were seeded with a depth-filtration method. The results from this study demonstrate the potential utility of cells seeded onto 3-D fibrous matrices as cell-based gene delivery vehicle for in vitro study of gene expression or in vivo gene therapy.

Hepatocytes entrapped in alginate gel beads and cultured in bioreactor: rapid repolarization and reconstitution of adhesion areas

The maintenance of the differentiated hepatocyte phenotype and its specific physiological properties is known to depend on several factors, such as chemical signals, cell-cell and extracellular matrix molecular interactions, as well as the use of three-dimensional matrices. The entrapment of hepatocytes within Ca-alginate at high cell density and the culture under continuous flow favour the development of three-dimensional organization and promote expression of the differentiated hepatic phenotype. This system could represent an improvement in hepatocyte cultivation for basic studies of liver physiology and metabolism; it could also be applicable in toxicology, hepatocyte transplantation or development of bioartificial organs. This report describes the effect of alginate entrapment and culture in a bioreactor on hepatocyte aggregate formation, with particular attention to the re-establishment of cell polarity, cell junctions and three-dimensional re-organization of the cytoskeleton. Oxygen supply and cell oxygen consumption rate were monitored in order to evaluate possible changes in hepatocyte energy requirement. Our data show that after only 6 h of perfusion in the bioreactor, actin and cytokeratin localize along the adhesion areas of the plasma membrane, in which reconstituted bile canaliculi were also observed. Moreover, the presence of connexin at the level of joined membranes of neighbouring cells suggests the establishment of gap junctions between hepatocytes. After the first 30 min of perfusion the oxygen consumption rate remained constant throughout the experimental period.

Hepatocyte transplantation using biodegradable matrices in ascorbic acid-deficient rats: comparison with heterotopically transplanted liver grafts

BACKGROUND

Hepatocyte transplantation using polymeric matrices is under investigation as an alternative therapy for metabolic liver diseases. Long-term engraftment of hepatocytes in polymers has been demonstrated. However, the metabolic activity of hepatocytes in such devices has never been assessed in direct comparison with liver grafts.

METHODS

Hepatocyte and partial liver transplantation were evaluated in the scurvy-prone osteogenic disorder Shionogi rat model. Biodegradable poly glycolic acid matrices seeded with hepatocytes equivalent to 20% of the recipient's liver mass, or 20% liver grafts were heterotopically transplanted into ascorbic acid- (AsA) deficient recipients. Recipients of cell-free matrices or AsA-deficient liver grafts served as controls. Recipients were set on AsA-free diet after transplantation. Plasma AsA levels, AsA concentrations in liver and adrenal gland tissue, and body weight ratios were assessed and H&E histology was performed.

RESULTS

Recipients from the control groups showed symptoms of scurvy at 1 month after cessation of AsA supply. Hepatocyte transplantation and auxiliary liver transplantation prevented symptoms of scurvy and increased plasma and tissue AsA levels and body weight ratios. AsA levels in recipients of 20% liver grafts were comparable to normal control animals.

CONCLUSIONS

Hepatocytes transplanted in polymeric matrices are able to compensate for liver-based metabolic deficiencies. Hepatocyte transplantation improves plasma AsA levels in AsA-deficient recipients. However, auxiliary liver grafts are superior to hepatocyte grafts in improving metabolic parameters. Further research work is needed to increase the efficiency of liver cell transplantation with regard to a clinical application.

Spatiotemporal expression of catenins, ZO-1, and occludin during early polarization of hepatic WIF-B9 cells

WIF-B9 is a suitable model for in vitro studies of hepatocyte polarity. To better understand polarity establishment, we have localized key proteins of the adhesion system, cytoskeleton, and tight junctions soon after plating, when most cells are isolated or in doublets. In isolated attached cells, only cytoskeletal proteins (tubulin, cytokeratins) displayed a precise localization. As soon as two cells formed a doublet, E-cadherin, alpha-, beta-, and gamma-catenins, and p120 protein were present at the doublet contiguous membrane. Actin, ezrin, and zonula occludens-1 (ZO-1) colocalized at this membrane, but not in all doublets: ezrin was present only at contiguous membrane expressing ZO-1, and ZO-1 was present only at membrane expressing actin. In contrast, occludin was spread throughout the doublet cytoplasm. With time in culture, these proteins localized transiently, as in cells expressing simple epithelial polarity, and finally, as in hepatocytes. We conclude that during WIF-B9 early polarization, key proteins are settled according to a hierarchy, as has been shown for Madin-Darby canine kidney cells. Cytoplasmic complexes of E-cadherin-catenin were detected during the whole polarization process; they were more abundant in fully polarized cells.

The role of actin filaments and microtubules in hepatocyte spheroid self-assembly

Cultured rat hepatocytes self-assemble into three-dimensional structures or spheroids that exhibit ultrastructural characteristics of native hepatic tissue and enhanced liver-specific functions. The spheroid formation process involves cell translocation and changes in cell shape, indicative of the reorganization of the cytoskeletal elements. To elucidate the function of the cytoskeleton, hepatocytes undergoing spheroid formation were treated with drugs that disrupt the different cytoskeletal components. Cytochalasin D, which targets the actin filaments, caused inhibition of spheroid formation. The role of microtubules in this process was assessed by incubating the cells with taxol or nocodazole. Perturbation of microtubules had minimal effects on spheroid assembly. Scanning electron micrographs showed no morphological differences between spheroids formed in control cultures and those formed in the presence of taxol or nocodazole. In addition, the effects of those agents on hepatocyte functions were investigated. Albumin secretion and cytochrome P450 2B1/2 activities of hepatocytes were comparable in spheroids formed in the presence of taxol or nocodazole to those formed in control cultures. The levels of these liver-specific activities were lower in cytochalasin D--treated cultures where only dispersed cells or cell clumps were found but spheroids had not found. Thus, hepatocytes require an intact actin network to self-assemble efficiently into functional tissue-like structures. Perturbation of the microtubule lattice does not impair the formation process. Events that transpire during hepatocyte spheroid self-assembly exhibit striking similarities to processes commonly observed in tissue morphogenesis. The results provide insight into the mechanisms that cells employ to organize into tissues and can contribute to our understanding of how to control the cellular assembly in tissue engineering and clinical applications.

Human hepatocyte cell lines proliferating as cohesive spheroid colonies in alginate markedly upregulate both synthetic and detoxificatory liver function

BACKGROUND/AIMS

Bio-artificial liver support systems for treatment of hepatic failure require maintained expression of hepatocyte function in vitro. We studied cultures of human hepatocyte cell-lines proliferating within alginate beads, investigating the hypothesis that 3-dimensional cohesive colonies of hepatocyte cell-lines would achieve polarity and cell-to-cell contact resulting in upregulation of function.

METHODS

HepG2 and HHY41 human cell lines in alginate beads were cultured for >20 days.

RESULTS

Proliferation was maintained for 20 days. Production of albumin, prothrombin, fibrinogen, alpha-1-acid glycoprotein and alpha-1-antitrypsin was maintained throughout, maximal at days 8-10, when upregulation was 300-1100% compared with monolayer cultures at similar cell number per unit volume. Detoxificatory functions: ethoxyresorufin deethylase activity, androstenedione metabolism, and urea synthesis from arginine was also increased several-fold. Function returned to pre-freezing levels within 18 h of thawing after cryopreservation of cells in alginate. Electron microscopy revealed spherical colonies of cells of cuboidal shape, with cell-to-cell contact via desmosomes and junctional complexes, abundant microvilli, and cytoplasmic appearances suggesting transcriptionally active hepatocytes.

CONCLUSION

Hepatocyte cell-lines, proliferating in alginate express a range of liver-specific functions at levels approaching those found in vivo, relevant to their use in liver support systems.

Effects of L-proline on phase I and phase II xenobiotic biotransformation capacities of rat and human hepatocytes in long-term collagen gel cultures

L-Proline supplementation of the medium for collagen gel cultures of hepatocytes has been shown to improve albumin secretion. A study was made as to whether L-proline is also essential for the maintenance of xenobiotic biotransformation capacities in collagen gel sandwich and immobilisation cultures of rat and human hepatocytes. Key phase I (cytochrome P450-dependent monooxygenase [CYP)] and microsomal epoxide hydrase [mEH]) and phase II (glutathione S-transferase [GST]) biotransformation enzyme activities and the secretion of albumin in the culture medium were assessed in the absence and presence of L-proline. CYP and mEH activities were not affected by the addition of L-proline, whereas phase II alpha-Class GST activity of rat hepatocytes in collagen cultures was decreased. Species differences were demonstrated, as human hepatocytes showed a better maintenance of GST activities than their rat counterparts in the presence of L-proline. Albumin secretion, often considered to be a marker for differentiated cell function, does not parallel the biotransformation capacities of the hepatocytes in culture. Additional results demonstrated an L-proline-mediated enhancement of the proliferation rate of contaminating stellate cells in conventional monolayer culture. Transdifferentiation of stellate cells to proliferating myofibroblasts, along with an increased albumin secretion and collagen synthesis, are characteristic of fibrotic liver. Since the last two phenomena have been observed in L-proline-supplemented collagen gel cultures, it can be concluded that when stable collagen gel cultures of rat hepatocytes are needed for long-term pharmacotoxicological studies, it is preferable to use an L-proline-free culture medium. Further studies on medium optimisation are required for hepatocytes from species other than rat.

Staurosporine promotion of formation of continuous monolayers of primary rat hepatocytes by improving attachment and spreading

Primary rat hepatocytes form discontinuous monolayers even at their maximum density. Here, we show that staurosporine promotes attachment and spreading of hepatocytes onto culture substrates, so that hepatocytes form a close, continuous monolayer. This treatment did not attenuate major hepatic functions. Therefore, this technique is promising for making seamless cell-sheet structures, which will be applicable for cell-polarity experiments or artificial liver construction.

Mechanochemical manipulation of hepatocyte aggregation can selectively induce or repress liver-specific function

Controlled activation of hepatocyte aggregation is critical to three-dimensional (3D) multicellular morphogenesis during native regeneration of liver as well as tissue reconstruction therapies. In this work, we quantify the stimulatory effects of two model hepatotrophic activators, epidermal growth factor (EGF) and hepatocyte growth factor (HGF), on the aggregation kinetics and liver-specific function of hepatocytes cultured on organotypic substrates with differing mechanical resistivity. Substrate-specific morphogenesis of cultured hepatocytes is induced on a tissue basement membrane extract, Matrigel, formulated at two distinct levels of mechanical compliance (storage modulus G', at oscillatory shear rate 1 rad/s, was 34 Pa for basal Matrigel and 118 Pa for crosslinked Matrigel). Overall, we report that growth factor stimulation selectively promotes the kinetics of aggregation in the form of two-dimensional corded aggregates on basal Matrigel and three-dimensional spheroidal aggregates on crosslinked Matrigel. Our analysis also indicates that costimulation with EGF and HGF (20 ng/mL each) cooperatively maximizes the kinetics of aggregation in a substrate-specific manner. In addition, we show that the role of growth factor stimulation on hepatocyte function is sensitively governed by the mechanical compliance of the substrate. In particular, on matrices with high compliance, costimulatory aggregation is shown to elicit a marked increase in albumin secretion rate, whereas on matrices with low compliance aggregation results in effective functional repression to basal, unstimulated levels. Thus, our studies highlight a novel interplay of physicochemical parameters of the culture microenvironment, leading to selective enhancement or repression of differentiated functions of hepatocytes, in concert with the activation of cellular morphogenesis.

Control of hepatocyte function on collagen foams: sizing matrix pores toward selective induction of 2-D and 3-D cellular morphogenesis

While microporous biopolymer matrices are being widely tested as cell culture substrates in hepatic tissue engineering, the microstructural basis for their control of cell differentiation is not well understood. In this paper, we studied the role of void size of collagen foams in directing the induction of liver-specific differentiated morphology and secretory activities of cultured rat hepatocytes. Hepatocytes cultured on collagen foams with subcellular sized pore diameters of 10 microm assumed a compact, cuboidal cell morphology, rapidly achieving monolayer coverage, and secreted albumin at the rate of 40 +/- 8 pg/cell/d. Increasing the pore size to 18 microm elicited a distinctly spread cellular phenotype, with discontinuous surface coverage, and albumin secretion rates declined precipitiously to 3.6 +/- 0.8 pg/cell/d. However, when collagen foams with an even higher average void size of 82 microm were used, hepatocytes exhibited high degree of spreading within an extensive three-dimensional cellular network, and exhibited high albumin secretory activity (26 +/- 0.6 pg/cell/d). The effect of void geometry on cellular ultrastructral polarity was further analyzed for the three void size configurations employed. The distribution of the cell-cell adhesion protein, E-cadherin, was primarily restricted to cell-cell contacts on the 10 microm foams, but was found to be depolarized to all membrane regions in cells cultured on the 18 and 82 microm foams. Vinculin-enriched focal adhesions were found to be peripherally clustered on cells cultured on 10 microm foams, but were found to redistribute to the entire ventral surface of cells cultured on the 18 and 82 microm foams. Overall, we demonstrate the significance of designing pore sizes of highly adhesive substrates like collagen toward selective cell morphogenesis in 2-D or 3-D. Subcellular and supercellular ranges of pore size promote hepatocellular differentiation by limiting 2-D cell spreading or effecting 3-D intercellular contacts, while intermediate range of pore sizes repress differentiation by promoting 2-D cell spreading.

Hepatocyte behavior within three-dimensional porous alginate scaffolds

A potential approach to facilitate the performance of implanted hepatocytes is to enable their aggregation and re-expression of their differentiated function prior to implantation. Here we examined the behavior of freshly isolated rat adult hepatocytes seeded within a novel three-dimensional (3-D) scaffold based on alginate. The attractive features of this scaffold include a highly porous structure (sponge-like) with interconnecting pores, and pore sizes with diameters of 100-150 microm. Due to their hydrophilic nature, seeding hepatocytes onto the alginate sponges was efficient. DNA measurements showed that the total cell number within the sponges did not change over 2 weeks, indicating that hepatocytes do not proliferate under these culture conditions. Nearly all seeded cells maintained viability, according to the MTT assay. Within 24 h post-seeding, small clusters of viable cells, were seen scattered within the sponge. More than 90% of the seeded cells participated in the aggregation; the high efficiency is attributed to the non-adherent nature of alginate. The spheroids had smooth boundaries and by day 4 in culture reached an average diameter of 100 microm, which is at the same magnitude of the sponge pore size. The cells appeared to synthesize fibronectin which was deposited on the spheroids. No laminin or collagen type IV were detected in the deposit. The 3-D arrangement of hepatocytes within the alginate sponges promoted their functional expression; within a week the cells secreted the maximal albumin secretion rate of 60 microg albumin/10(6) cells/day. Urea secretion rate did not depend on cell aggregation and was similar to that obtained when hepatocytes were cultured on collagen type I coated dishes (100 microg/10(6) cells/day). Our studies show that alginate sponges can provide a conducive environment to facilitate the performance of cultured hepatocytes by enhancing their aggregation.