Polyvinylalcohol three-dimensional matrices for improved long-term dynamic culture of hepatocytes

Design and Characterization of a Bioinspired Polyvinyl Alcohol Matrix with Structural Foam-Wall Microarchitectures for Potential Tissue Engineering Applications

Traditional medical soft matrix used in a surgical treatment or in wound management was not good enough in both the structural support and interconnectivity to be applied in tissue engineering as a scaffold. Avian skeleton and feather rachises might be good reference objects to mimic in designing a scaffold material with good structural support and high interconnectivity because of its structural foam-wall microarchitectures and structural pneumaticity. In this study, a biomimetic airstream pore-foaming process was built up and the corresponding new medical soft matrix derived from polyvinyl alcohol matrix (PVAM) with air cavities inspired by avian skeleton and feather rachises was prepared. Furthermore, the resulting medical soft matrix and bovine Achilles tendon type I collagen could be employed to prepare a new collagen-containing composite matrix. Characterization, thermal stability and cell morphology of the bioinspired PVA matrix and the corresponding collagen-modified PVA composite matrix with open-cell foam-wall microarchitectures were studied for evaluation of potential tissue engineering applications. TGA, DTG, DSC, SEM and FTIR results of new bioinspired PVA matrix were employed to build up the effective system identification approach for biomimetic structure, stability, purity, and safety of target soft matrix. The bioinspired PVA matrix and the corresponding collagen-modified PVA composite matrix would be conductive to human hepatoblastoma HepG2 cell proliferation, migration, and expression which might serve as a promising liver cell culture carrier to be used in the biological artificial liver reactor.

Pullulan/Poly(Vinyl Alcohol) Composite Hydrogels for Adipose Tissue Engineering

Composite hydrogels based on pullulan (HP) and poly(vinyl alcohol) (PVA) were both prepared by simple chemical crosslinking with sodium trimethaphosphate (STMP) or by dual crosslinking (simultaneously chemical crosslinking with STMP and physical crosslinking by freeze-thaw technique). The resulting hydrogels and cryogels were designed for tissue engineering applications. PVA, with two different molecular weights (47,000 and 125,000 g/mol; PVA₄₇ and PVA₁₂₅, respectively), as well as different P/PVA weight ratios were tested. The physico-chemical characterization of the hydrogels was performed by FTIR spectroscopy and scanning electron microscopy (SEM). The swelling kinetics, dissolution behavior, and degradation profiles in simulated physiological conditions (phosphate buffer at pH 7.4) were investigated. Pullulan concentration and the crosslinking method had significant effects on the pore size, swelling ratio, and degradation profiles. Cryogels exhibit lower swelling capacities than the conventional hydrogels but have better stability against hydrolitic degradation. Biocompatibility of the hydrogels was also investigated by both MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) and LDH (lactaten dehydrogenase) assay. The MTT and LDH assays proved that dual crosslinked HP/PVA₁₂₅ (75:25, w/w) scaffolds are more biocompatible and promote to a greater extent the adhesion and proliferation of L929 murine fibroblast cells than chemically crosslinked HP/PVA₄₇ (50/50, w/w) scaffolds. Moreover, the HP/PVA₁₂₅ cryogel had the best ability for the adipogenic differentiation of cells. The overall results demonstrated that the HP/PVA composite hydrogels or cryogels are suitable biomaterials for tissue engineering applications.

Biomimetically Reinforced Polyvinyl Alcohol-Based Hybrid Scaffolds for Cartilage Tissue Engineering

Articular cartilage has a very limited regeneration capacity. Therefore, injury or degeneration of articular cartilage results in an inferior mechanical stability, load-bearing capacity, and lubrication capability. Here, we developed a biomimetic scaffold consisting of macroporous polyvinyl alcohol (PVA) sponges as a platform material for the incorporation of cell-embedded photocrosslinkable poly(ethylene glycol) diacrylate (PEGDA), PEGDA-methacrylated chondroitin sulfate (PEGDA-MeCS; PCS), or PEGDA-methacrylated hyaluronic acid (PEGDA-MeHA; PHA) within its pores to improve in vitro chondrocyte functions and subsequent in vivo ectopic cartilage tissue formation. Our findings demonstrated that chondrocytes encapsulated in PCS or PHA and loaded into macroporous PVA hybrid scaffolds maintained their physiological phenotypes during in vitro culture, as shown by the upregulation of various chondrogenic genes. Further, the cell-secreted extracellular matrix (ECM) improved the mechanical properties of the PVA-PCS and PVA-PHA hybrid scaffolds by 83.30% and 73.76%, respectively, compared to their acellular counterparts. After subcutaneous transplantation in vivo, chondrocytes on both PVA-PCS and PVA-PHA hybrid scaffolds significantly promoted ectopic cartilage tissue formation, which was confirmed by detecting cells positively stained with Safranin-O and for type II collagen. Consequently, the mechanical properties of the hybrid scaffolds were biomimetically reinforced by 80.53% and 210.74%, respectively, compared to their acellular counterparts. By enabling the recapitulation of biomimetically relevant structural and functional properties of articular cartilage and the regulation of in vivo mechanical reinforcement mediated by cell⁻matrix interactions, this biomimetic material offers an opportunity to control the desired mechanical properties of cell-laden scaffolds for cartilage tissue regeneration.

3D Cultivation Techniques for Primary Human Hepatocytes

One of the main challenges in drug development is the prediction of in vivo toxicity based on in vitro data. The standard cultivation system for primary human hepatocytes is based on monolayer cultures, even if it is known that these conditions result in a loss of hepatocyte morphology and of liver-specific functions, such as drug-metabolizing enzymes and transporters. As it has been demonstrated that hepatocytes embedded between two sheets of collagen maintain their function, various hydrogels and scaffolds for the 3D cultivation of hepatocytes have been developed. To further improve or maintain hepatic functions, 3D cultivation has been combined with perfusion. In this manuscript, we discuss the benefits and drawbacks of different 3D microfluidic devices. For most systems that are currently available, the main issues are the requirement of large cell numbers, the low throughput, and expensive equipment, which render these devices unattractive for research and the drug-developing industry. A higher acceptance of these devices could be achieved by their simplification and their compatibility with high-throughput, as both aspects are of major importance for a user-friendly device.

Long-term culture of primary hepatocytes: new matrices and microfluidic devices

Prediction of in vivo drug-induced hepatotoxicity by in vitro cell culture systems is still one of the main challenges in drug development. To date, most in vitro approaches are based on monolayer cultures of primary hepatocytes, although it is known that they rapidly lose their morphology and liver-specific functions, such as activities of drug-metabolizing enzymes and transporters. Hepatocyte dedifferentiation can be delayed by culturing cells in a 3D environment. Combination with continuous medium flow, which creates a more physiological situation, further improves the maintenance of hepatic functions. Here, we present recently developed hydrogels and scaffolds for 3D culture of hepatocytes, which aim at preserving hepatic morphology and functionality for up to 4 weeks in culture. Furthermore, major benefits and drawbacks of microfluidic devices for in vitro hepatotoxicity screening are discussed. Although promising advances have been made regarding the preservation of hepatic functions in 3D flow culture, major issues, such as expensive equipment, large cell numbers and low throughput, are still hampering their use in drug toxicity screening. For these devices to be applied and accepted in the drug-developing industry, it is necessary to combine easily accessible matrices that highly preserve the activities of drug-metabolizing enzymes with a user-friendly microfluidic platform, thereby finding the right balance between reflecting the in vivo situation and enabling satisfying throughput for drug candidate screening.

Fabrication and characterization of porous alginate/polyvinyl alcohol hybrid scaffolds for 3D cell culture

Porous alginate/polyvinyl alcohol (PVA) hybrid scaffolds as bioartificial cell scaffolds were fabricated to improve cell compatibility as well as flexibility of the scaffolds. The alginate/PVA hybrid scaffolds with different PVA compositions up to 50 wt% were fabricated by a modified freeze-drying method including the physical cross-linking of PVA and the following chemical cross-linking of alginate. The prepared alginate/PVA hybrid scaffolds were characterized by morphology observations using scanning electron microscopy (SEM), the measurements of porosity and average pore sizes and the measurements of compressive strength and modulus. The scaffolds exhibited highly porous, open-cellular pore structures with almost the same surface and cross-sectional porosities (total porosities about 85%, regardless of PVA composition) and the pore sizes from about 290 microm to about 190 microm with increasing PVA composition. The alginate/PVA hybrid scaffolds were more soft and elastic than the control alginate scaffold without significant changes of mechanical strength. The scaffolds were examined for their in vitro cell compatibility by the culture of chondrocytes (human chondrocyte cell line) in the scaffolds and the following analyses by MTT assay and SEM observation. It was observed that the alginate/PVA scaffolds had better cell adhesion and faster growth than the control alginate scaffold. It seems that 30 wt% addition of PVA to alginate in the fabrication of the hybrid scaffolds is desirable for improving their flexibility and cell compatibility.

Treatment of acute liver failure in mice by hepatocyte xenotransplantation

Liver diseases still have a high mortality even though liver transplantation has become a standard treatment. Currently, hepatocyte transplantation has been proposed as another promising strategy. One limitation is the availability of human livers as a source of hepatocytes. Because of an unlimited supply, the use of porcine hepatocytes might address this problem. Regardless of the source, once isolated hepatocytes lose specific functionality due to the loss of the natural microenvironment. For this reason, we tested the ability of a self-assembling peptide nanofiber (SAPNF) to provide a provisional three-dimensional (3D) support to interact with cells to control their function in vivo. Isolated porcine hepatocytes were embedded in SAPNF, or collagen type I and transplanted by direct injection into the splenic pulp of SCID mice suffering from acute liver failure (ALF) by 90% hepatectomy. SAPNF porcine hepatocyte transplantation produced engraftment that was far superior to that obtained using collagen and prolonged the survival of mice with ALF, in contrast with controls. An ultrastructural evaluation using transmission electron microscopy indicated extensive cell-cell communication and preservation of hepatocyte architecture. The transplanted SAPNF hepatocytes showed higher expression of albumin and PAS and lower apoptotic events assessed by TUNEL staining. Hepatocytes culture in a truly 3D network allows in vivo maintaining of differentiated functions, and once transplanted between widely divergent species can function to correct acute liver failure in mice and prolong their survival.

Self-assembling peptide nanofiber as a novel culture system for isolated porcine hepatocytes

Freshly isolated porcine hepatocytes are a very attractive cell source in the cell-based therapies to treat liver failure because of unlimited availability. However, due to the loss of hepatocyte functions in vitro, there is a need to develop a functional culture system to keep the cells metabolically active. Here we compared the effect of a self-assembling peptide nanofiber (SAPNF) as an extracellular matrix (ECM) with collagen type I on hepatocyte metabolic and secretion activities following hepatocyte isolation. Isolated porcine hepatocytes were cultured in SAPNF and collagen type I. Morphological assessment at different time points was performed by using SEM and phase contrast microscope. Metabolic and secretion activities were comparatively performed in the groups, by means of ammonia, lidocaine, and diazepam as well as albumin. Hepatocytes cultured on SAPNF revealed a three-dimensional spheroidal formation, thus maintaining cell differentiation status during 2 weeks of culture. On the other hand, hepatocytes in collagen revealed a spread shape, and by day 14 no hepatocyte-like cells were observed, but cells with long shape were present, thus revealing a degree of dedifferentiation in collagen culture. Hepatocytes in SAPNF were capable of drug-metabolizing activities and albumin secretion in higher ratio than those cultured on collagen. The present work clearly demonstrates the usefulness of SAPNF for maintaining differentiated functions of porcine hepatocytes in culture.

Galactose-carrying polymers as extracellular matrices for liver tissue engineering

Extracellular matrix (ECM) plays important roles in tissue engineering because cellular growth and differentiation, in the two-dimensional cell culture as well as in the three-dimensional space of the developing organism, require ECM with which the cells can interact. Especially, the bioartificial liver-assist device or regeneration of the liver-tissue substitutes for liver tissue engineering requires a suitable ECM for hepatocyte culture because hepatocytes are anchorage-dependent cells and are highly sensitive to the ECM milieu for the maintenance of their viability and differentiated functions. Galactose-carrying synthetic ECMs derived from synthetic polymers and natural polymers bind hepatocytes through a receptor-mediated mechanism, resulting in enhanced hepatocyte functions. Attachment and functions of hepatocytes were affected by physico-chemical properties including ECM geometry as well as the type, density and orientation of galactose. Also, cellular environment, medium composition and dynamic culture system influenced liver-specific functions of hepatocytes beside ECM.

Albumin and urea production by hepatocytes cultured on extracellular matrix proteins-conjugated poly(vinyl alcohol) membranes

Production of albumin and urea by mouse hepatocytes on poly(vinylalcohol-co-ethylamine) (PVA-EA) membranes containing immobilized extracellular matrix (ECM) proteins was investigated for 7 days. The amount of ECM proteins (collagen, vitronectin and laminin) immobilized on PVA-EA and PVA-ECM membranes was determined to range from 1.09 microg/cm2 to 1.60 microg/cm2. Hepatocytes cultured on PVA-ECM membranes in serum-free media showed higher albumin production than those cultured on PVA-EA membranes after a 7-day incubation under the conditions in this study. Urea production by hepatocytes on PVA-ECM membranes was also determined to be higher than that on PVA-EA membranes up until day 5 of incubation in serum-free media, whereas no difference of urea production by hepatocytes on different PVA-ECM membranes and PVA-EA membranes was observed at 7 days of incubation. The effect of ECM proteins in PVA-ECM membranes on hepatocyte function (such as albumin and urea production) was observed in hepatocytes cultured in serum-free media up to day 5 of incubation. The ECM proteins immobilized on the PVA-ECM membranes contributed not only to the long-term stable production of albumin and urea by hepatocytes, but also the improved surviVal (viability) of hepatocytes on PVA-ECM membranes.

Effects of plasma from patients with fulminant hepatic failure on function of primary rat hepatocytes in three-dimensional culture

BACKGROUND/AIM

As biotechnology continues to advance, a bioartificial liver is expected to be developed for the treatment of patients with fulminant hepatic failure (FHF) whose liver dysfunction is potentially reversible or for providing liver support as a bridge to liver transplantation. While monolayer-cultured hepatocytes rapidly lose their capacity to express many liver-specific functions over time when cultured, spherical-shaped hepatocytes in three-dimensional culture with the use of extracellular matrix components sustain long-term survival by maintaining differentiated hepatocyte functions. The aim of this study was to investigate whether sufficient functions of viable spherical-shaped hepatocytes could be maintained in plasma of patients with FHF in order to use these cells in an extracorporeal system.

METHODS

Hepatocyte functions were evaluated under monolayer or three-dimensional culture in FHF plasma.

RESULTS

Primary rat hepatocytes on poly-N-p-vinylbenzyl-D-lactonamide (PVLA) formed spheroids even in FHF plasma and maintained their spherical shapes in FHF plasma as long as in medium. Spherical-shaped hepatocytes on PVLA cultured in FHF plasma showed higher activity in albumin secretion, urea formation, and gluconeogenesis than those in normal human plasma or medium. As being cultured in medium, hepatocytes on PVLA cultured in plasma were also superior to cells on collagen in regard to albumin secretion, amino acid metabolism, and gluconeogenesis.

CONCLUSIONS

These findings demonstrated that FHF plasma is not toxic to rat hepatocyte spheroids and that hepatocyte spheroids have potential use in the development of a bioartificial liver.