Concurrent changes in sinusoidal expression of laminin and affinity of hepatocytes to laminin during rat liver regeneration

Chemically Modified Dipeptide Based Hydrogel Supports Three-Dimensional Growth and Functions of Primary Hepatocytes

A huge shortage of organ donors, particularly in the case of liver, has necessitated the development of alternative therapeutic strategies. Primary hepatocytes (pHCs) transplantation has made a considerable transition from bench to bedside, but the short-term viability and functionality of pHCs in in vitro limit their use for clinical applications. Different cell culture strategies are required to maintain the proliferation of pHCs for extended periods. Here, we described the formation of a hybrid scaffold based on a modified dipeptide for the culture of pHCs. First, the dipeptide (Dp), isoleucine-α,β-dehydrophenylalanine (IΔF) was synthesized, purified, and fully characterized. IΔF readily formed a highly stable hydrogel, which was also characterized by CD, TEM, and thioflavin T assay. The addition of soluble liver extracellular matrix (sLEM) to the dipeptide readily formed a hybrid scaffold that was characterized by TEM, and its mechanical strength was determined by rheology experiments. The hybrid scaffold was translucent, biocompatible, and proteolytically stable and, with its mechanical strength, closely mimicked that of the native liver. LEM1-Dp matrix exhibited high biocompatibility in the readily available adherent liver cell line Huh7 and primary rat hepatocyte cells (pHCs). pHCs cultured on LEM1-Dp matrix also maintained significantly higher cell viability and an escalated expression of markers related to the hepatocytes such as albumin as compared to that observed in cells cultured on collagen type I (Col I)-coated substrate plate (col-TCTP). Z-stacking of confocal laser microscopy's volume view clearly indicated pHCs seeded on top of the hydrogel matrix migrated toward the Z direction showing 3D growth. Our results indicated that low molecular weight dipeptide hydrogel along with sLEM can resemble biomimetic 3D-like microenvironments for improved pHCs proliferation, differentiation, and function. This hybrid scaffold is also easy to scale up, which makes it suitable for several downstream applications of hepatocytes, including drug development, pHCs transplantation, and liver regeneration.

Maintenance of human hepatocyte function in vitro by liver-derived extracellular matrix gels

Tissue engineering and regenerative medicine (TE&RM) approaches to treating liver disease have the potential to provide temporary support with biohybrid-liver-assist devices or long-term therapy by replacing the diseased liver with functional constructs. A rate-limiting step for TE&RM strategies has been the loss of hepatocyte-specific functions after hepatocytes are isolated from their highly specialized in vivo microenvironment and placed in in vitro culture systems. The identification of a biologic substrate that can maintain a functional hepatocyte differentiation profile during in vitro culture would advance potential TE&RM therapeutic strategies. The present study compared two different biologic substrates for their ability to support human hepatocyte function in vitro: porcine-liver-derived extracellular matrix (PLECM) or Matrigel. Because Matrigel has been shown to be the most useful matrix for static, traditional hepatocyte culture, we directly compared PLECM with Matrigel in each experiment. Albumin secretion, hepatic transport activity, and ammonia metabolism were used to determine hepatocyte function. Hepatocytes cultured between two layers of PLECM or Matrigel showed equally high levels of albumin expression and secretion, ammonia metabolism, and hepatic transporter expression and function. We conclude that like Matrigel, PLECM represents a favorable substrate for in vitro culture of human hepatocytes.

Hepatic non-parenchymal cells and extracellular matrix participate in oval cell-mediated liver regeneration

AIM: To elucidate the interaction between non-parenchymal cells, extracellular matrix and oval cells during the restituting process of liver injury induced by partial hepatectomy (PH).

METHODS: We examined the localization of oval cells, non-parenchymal cells, and the extracellular matrix components using immunohistochemical and double immunofluorescent analysis during the proliferation and differentiation of oval cells in N-2-acetylaminofluorene (2-AAF)/PH rat model.

RESULTS: By day 2 after PH, small oval cells began to proliferate around the portal area. Most of stellate cells and laminin were present along the hepatic sinusoids in the periportal area. Kupffer cells and fibronectin markedly increased in the whole hepatic lobule. From day 4 to 9, oval cells spread further into hepatic parenchyma, closely associated with stellate cells, fibronectin and laminin. Kupffer cells admixed with oval cells by day 6 and then decreased in the periportal zone. From day 12 to 15, most of hepatic stellate cells (HSCs), laminin and fibronectin located around the small hepatocyte nodus, and minority of them appeared in the nodus. Kupffer cells were mainly limited in the pericentral sinusoids. After day 18, the normal liver lobule structures began to recover.

CONCLUSION: Local hepatic microenvironment may participate in the oval cell-mediated liver regeneration through the cell-cell and cell-matrix interactions.

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.

Transient expression of laminin ?1 chain in regenerating murine liver: Restricted localization of laminin chains and nidogen-1

Most interstitia between epithelial and endothelial cells contain basal laminae (BLs), as defined by electron microscopy. However, in liver, the sinusoidal interstitium (called space of Disse) between hepatocytes and sinusoidal endothelial cells (SECs) lacks BLs. Because laminins are major components of BLs throughout the body, whether laminins exist in sinusoids has been a controversial issue. Despite recent advances, the distribution and expression of laminin chains have not been well defined in mammalian liver. Here, using a panel of antibodies, we examined laminins in normal and regenerating mouse livers. Of alpha chains, alpha5 was widely observed in all BLs except for sinusoids, while the other alpha chains were variously expressed in Glisson's sheath and central veins. Laminin gamma1 was also distributed to all BLs except for sinusoids. Although the beta2 chain was observed in all BLs and sinusoids, the expression of beta1 chain was restricted to Glisson's sheath. Detailed analysis of regenerating liver revealed that alpha1 and gamma1 chains appeared in sinusoids and were produced by stellate cells. The staining of alpha1 and gamma1 chains reached its maximum intensity at 6 days after two-thirds partial hepatectomy (PHx). Moreover, in vitro studies showed that alpha1-containing laminin promoted spreading of sinusoidal endothelial cells (SECs) isolated from normal liver, but not other hepatic cells. In addition, SECs isolated from regenerating liver elongated pseudopodia on alpha1-containing laminin more so than did cells from normal liver. The transient expression of laminin alpha1 may promote formation of sinusoids after PHx.

Time course of primary liver cell reorganization in three-dimensional high-density bioreactors for extracorporeal liver support: an immunohistochemical and ultrastructural study

To enable extracorporeal liver support based on the use of primary liver cells, culture models supporting the maintenance of cell integrity and function in vitro are required. In this study the cell organization and ultrastructure of primary porcine hepatocytes cocultured with nonparenchymal cells in three-dimensional high-density bioreactors were analyzed after 10, 20, and 30 days of culture by immunohistochemistry and transmission electron microscopy. Biochemical data showed that metabolic activity of the cells in the system was relatively stable over at least 20 days. Immunohistochemical studies were performed in comparison with donor organ biopsies. They showed that hepatocytes and nonparenchymal cells reaggregated in bioreactors, forming structures partly resembling natural liver parenchyma. Bile duct-like structures characterized by cytokeratin 7 (CK-7) immunoreactivity (IR) were regularly detected. Nonparenchymal cells (vimentin IR) formed sinusoidal-like structures within parenchymal cell aggregates. Proliferative activity (Ki-67 IR) increased over time. The detection of collagen I and laminin indicated the production of extracellular matrix components within bioreactors. The results showed that primary liver cell reorganization and long-term maintenance of their differentiated state were achieved within the bioreactors The findings on cell proliferation indicated that the culture model is also of interest for further in vitro studies on cell regeneration and tissue formation.

The changes in glycosylation after partial hepatectomy enhance collagen binding of vitronectin in plasma

Vitronectin is a multifunctional glycoprotein present in the extracellular matrix and plasma. Changes in rat vitronectin were studied during liver regeneration after partial hepatectomy. Carbohydrate concentrations of vitronectin decreased to 2/3 of sham-operated rats at 24 h after partial hepatectomy. Carbohydrate composition and lectin reactivity indicated that N-glycosylation and sialylation of vitronectin changed markedly after partial hepatectomy, while amino acid composition did not change significantly. We previously showed that deN-glycosylation of vitronectin in vitro affects collagen binding among various ligands (Yoneda et al., Biochemistry (1998) 37, 6351-6360). Vitronectins from partially hepatectomized rats at 24 h were found to exhibit markedly enhanced binding to type I collagen. The effect of sialylation on collagen binding was further examined using enzymatically deglycosylated vitronectin of nonoperated rats. Collagen binding increased by 1.2 times after deN-glycosylation of vitronectin, while it increased more than 2.9 times after desialylation. Various glycosyltransferases in liver are known to change after partial hepatectomy, including the attenuation of N-oligosaccharide transferase. The findings therefore suggest that the collagen binding of vitronectin is modulated by the alteration of peptide glycosylation caused by postoperative physiological changes of glycosyltransferases and that the change may contribute to tissue remodeling processes.

Cationic colloidal silica membrane perturbation as a means of examining changes at the sinusoidal surface during liver regeneration

By employing the cationic colloidal silica membrane density perturbation technique, we examined growth factor receptor and extracellular matrix (ECM) changes at the sinusoidal surface during rat liver regeneration 72 hours after 70% partial hepatectomy (PHx). At this time after PHx, hepatocyte division has mostly subsided, while sinusoidal endothelial cell (SEC) proliferation is initiating, resulting in avascular hepatocyte islands. Because of the discontinuous nature of the surface of liver SEC, ECM proteins underlying the SEC, as well as SEC luminal membrane proteins, are available to absorption to the charged silica beads when the liver is perfused with the colloid. Subsequent liver homogenization and density centrifugation yield two separate fractions, enriched in SECs as well as hepatocyte basolateral membrane-specific proteins up to 50-fold over whole liver lysates. This technique facilitates examination of changes in protein composition that influence or occur as a result of SEC mitogenesis and migration during regeneration of the liver. When ECM and receptor proteins from SEC-enriched fractions were examined by Western immunoblotting, urokinase plasminogen activator receptor, fibronectin, and plasmin increased at the SEC surface 72 hours after PHx. Epidermal growth factor receptor, plasminogen, SPARC (secreted protein, acidic and rich in cysteine, also called osteonectin or BM40), and collagen IV decreased, and fibrinogen subunits and c-Met expression remained constant 72 hours after PHx when compared to control liver. These results display the usefulness of the cationic colloidal silica membrane isolation protocol. They also show considerable modulation of surface components that may regulate angiogenic processes at the end stage of liver regeneration during the reformation of sinusoids.

Dedifferentiation of human hepatocytes by extracellular matrix proteins in vitro: quantitative and qualitative investigation of cytokeratin 7, 8, 18, 19 and vimentin filaments

BACKGROUND/AIMS

Liver cirrhosis and carcinogenesis are accompanied by an alteration in extracellular matrix material. Histological studies reveal upregulation of the intermediate filaments cytokeratins 8 and 18 and de novo synthesis of vimentin, and cytokeratin 7 or 19 in hepatocytes. The aim of this study was to investigate how these two processes are linked.

METHODS

Human hepatocytes were seeded: (i) on the matrix components collagen I, IV, laminin, or fibronectin; (ii) on stoichiometrically different complete matrices, derived from human placenta (matrix I) or the Englebreth-Holm-Swarm tumor (matrix II), and (iii) inside a three-dimensional collagen I sandwich. Filament expression and assembly were measured by cytofluor analysis or confocal laserscan microscopy.

RESULTS

The matrix components or complete matrices triggered enhancement of cytokeratins 8 and 18 and de novo synthesis of cytokeratins 7, 19 and vimentin in a characteristic way. Confocal images demonstrated a dense and uniform network of cytokeratin 18 in freshly isolated cells, which was "replaced" by a few, thick protein bundles within 20 days. Interestingly, newly synthesized cytokeratin 19 structurally resembled the cytokeratin 19 organization in biliary epithelial cells. Marked cytokeratin alterations could be partially prevented when hepatocytes were grown in a three-dimensional collagen sandwich.

CONCLUSIONS

Pathological alterations to the chemical composition, molecular structure, or spatial arrangement of the liver matrix lead to specific changes in the intermediate filament pattern in human hepatocytes. We assume that degradation of the matrix results in pathological alterations to the hepatocyte-receptor matrix-ligand ratio, followed by a switch from physiological to pathological cell-activation.

Synergistic enhancement of EGF, but not HGF, stimulated hepatocyte motility by TGF-beta 1 in vitro

The ability of TGF-beta 1 (transforming growth factor-beta 1) to suppress growth factor induced proliferation of many cell types in vitro is well documented; however, TGF-beta 1 increases within a similar time frame as the hepatocyte mitogens HGF (hepatocyte growth factor), EGF (epidermal growth factor), and TGF-alpha (transforming growth factor-alpha) prior to hepatocyte proliferation during liver regeneration. This has raised the issue that TGF-beta 1 may have effects on hepatocytes additional to mito-inhibition and that these effects may be relevant to the regenerative process. To this end, we examined the effect of TGF-beta 1 on both the mitogenesis and the motility of growth factor stimulated primary rat hepatocytes and the hepatoblastoma cell line HepG2 in vitro. TGF-beta 1 significantly enhanced the chemotactic motility of EGF or TGF-alpha, and not HGF, stimulated hepatocytes on a collagen I substratum. TGF-beta 1 was not chemotactic when added alone and decreased the DNA synthesis of all hepatocyte cultures to near control levels. HepG2 cells were chemotactic toward HGF, EGF, and TGF-beta 1 alone and displayed an additive chemotactic response when TGF-beta 1 was added to either HGF or EGF. Additionally, HepG2 cells were refractory to the growth stimulatory effects of HGF or EGF and the growth inhibitory effects of TGF-beta 1. Hepatocytes plated onto other collagen-containing substrates (collagen IV, Matrigel, or ECL, an entactin-collagen IV-laminin matrix), but not on fibronectin or laminin alone, also displayed enhanced EGF stimulated motility by TGF-beta 1. The data indicate that an additional, novel role for TGF-beta 1 during liver tissue remodeling following PHx may include the synergistic enhancement EGF stimulated hepatocyte motility responses, and this enhancement is observed only on collagen-containing extracellular matrices.

Cytoskeletal filament assembly and the control of cell spreading and function by extracellular matrix

This study was undertaken to analyze how cell binding to extracellular matrix produces changes in cell shape. We focused on the initial process of cell spreading that follows cell attachment to matrix and, thus, cell ‘shape’ changes are defined here in terms of alterations in projected cell areas, as determined by computerized image analysis. Cell spreading kinetics and changes in microtubule and actin microfilament mass were simultaneously quantitated in hepatocytes plated on different extracellular matrix substrata. The initial rate of cell spreading was highly dependent on the matrix coating density and decreased from 740 microns 2/h to 50 microns 2/h as the coating density was lowered from 1000 to 1 ng/cm2. At approximately 4 to 6 hours after plating, this initial rapid spreading rate slowed and became independent of the matrix density regardless of whether laminin, fibronectin, type I collagen or type IV collagen was used for cell attachment. Analysis of F-actin mass revealed that cell adhesion to extracellular matrix resulted in a 20-fold increase in polymerized actin within 30 minutes after plating, before any significant change in cell shape was observed. This was followed by a phase of actin microfilament disassembly which correlated with the most rapid phase of cell extension and ended at about 6 hours; F-actin mass remained relatively constant during the slow matrix-independent spreading phase. Microtubule mass increased more slowly in spreading cells, peaking at 4 hours, the time at which the transition between rapid and slow spreading rates was observed. However, inhibition of this early rise in microtubule mass using either nocodazole or cycloheximide did not prevent this transition. Use of cytochalasin D revealed that microfilament integrity was absolutely required for hepatocyte spreading whereas interference with microtubule assembly (using nocodazole or taxol) or protein synthesis (using cycloheximide) only partially suppressed cell extension. In contrast, cell spreading could be completely inhibited by combining suboptimal doses of cytochalasin D and nocodazole, suggesting that intact microtubules can stabilize cell form when the microfilament lattice is partially compromised. The physiological relevance of the cytoskeleton and cell shape in hepatocyte physiology was highlighted by the finding that a short exposure (6 hour) of cells to nocodazole resulted in production of smaller cells 42 hours later that exhibited enhanced production of a liver-specific product (albumin). These data demonstrate that spreading and flattening of the entire cell body is not driven directly by net polymerization of either microfilaments or microtubules.(ABSTRACT TRUNCATED AT 400 WORDS)