The mitogenic response to EGF of rat hepatocytes cultured on laminin-rich gels (EHS) is blocked downstream of receptor tyrosine-phosphorylation

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.

Hepatic Sinusoidal Endothelial Cells Promote Hepatocyte Proliferation Early after Partial Hepatectomy in Rats

BACKGROUND

We undertook this study in rats to investigate the role of hepatic sinusoidal endothelial cells (SECs) in hepatocyte proliferation early after partial hepatectomy and the regulatory mechanisms involved.

METHODS

The animal model of 70% hepatectomy was made. Hepatic SECs and hepatocytes were isolated and cultured according to the method of Braet et al. with some modifications. Levels of nitric oxide (NO), interleukin-6 (IL-6), and hepatic growth factor (HGF) in the supernatants of hepatic SEC cultures were measured, and the expression of HGF mRNA by hepatic SECs was analyzed. The relationship between the supernatants of hepatic SEC cultures and hepatocyte proliferation was probed. (3)H-thymidine incorporation and the proliferating cell nuclear antigen (PCNA) labeling index of hepatocytes were used as signs of hepatocyte proliferation.

RESULTS

Levels of NO, IL-6, and HGF in the supernatants of hepatic SECs cultures were increased markedly 6 and 24 h after hepatectomy and then were decreased gradually. The expression of HGF mRNA by cultured SECs was increased markedly 6 and 24 h after hepatectomy, with a peak 6 h after hepatectomy. The PCNA labeling index and (3)H-thymidine incorporation of hepatocytes started to increase 6 h after hepatectomy, with a peak at 24 h. Hepatic SECs were isolated from rats 24 h after partial hepatectomy and cultured for 24 h, and the culture supernatants were obtained. The supernatants not only significantly enhanced the PCNA labeling index and (3)H-thymidine incorporation of proliferating hepatocytes isolated from rats after partial hepatectomy but also obviously increased the DNA synthesis of quiescent hepatocytes from the control rats. The extent of hepatocyte proliferation was closely related to the amount of the SEC culture supernatants added in both rats after partial hepatectomy and control rats.

CONCLUSIONS

These results suggest that cytokines (such as IL-6, HGF and NO) secreted by SECs play important roles in liver regeneration early after partial hepatectomy. We speculate that activated hepatic SECs secrete some substances that induce or trigger liver regeneration after partial hepatectomy.

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.

The phosphoinositide 3-kinase/Akt-signal pathway mediates proliferation and secretory function of hepatic sinusoidal endothelial cells in rats after partial hepatectomy

OBJECTIVE

To investigate the role of AKT signaling pathway in hepatic sinusoidal endothelial cells (SECs) early after partial hepatectomy in rats and the regulatory mechanisms involved.

METHODS

The animal model of 70% hepatectomy was made. Hepatic SECs were isolated and cultured according to Braet et al.'s method with some modifications. The cultured hepatic SECs were divided into two groups: 70% partial hepatectomy groups and LY294002 group (LY). We observed the expressions of AKT and NF-kappaB in cultured hepatic SECs by Western blot, measured the levels of NO, NOs, IL-6, and HGF in the supernatants of hepatic SEC cultures and [3H]thymidine incorporation, and analyzed cell cycle of cultured hepatic SECs by flow cytometer. The relationship of the Akt pathway with secretions and proliferation of hepatic SECs after partial hepatectomy was probed.

RESULTS

The levels of Akt protein expression increased significantly after partial hepatectomy in OG group and with a peak at 24 h post operation. Meanwhile, there was a markedly increase in phosphorylated Akt protein during 2-72 h after operation. But the expression and activity of Akt protein did not change significantly after partial hepatectomy in the LY group. So, partial hepatectomy can marked induce Akt expression and result in rapid and marked phosphorylation of Akt from 2 to 72 h thereafter. The changes of NF-kappaB expression in cultured hepatic SECs were similar to those of Akt expression after operation. The concentrations of HGF and IL-6 in the supernatants of cultured hepatic SECs were relatively low in the LY group, and were markedly increased after partial hepatectomy, with a peak at 24 h in the OG group. There were significant differences between the OG and LY groups at 6 and 24 h (P < 0.05). Both NO and NOS secretion was increased in the OG group compared to the LY group within 24 h after partial hepatectomy. But the secretion of NO and NOS was increased more markedly in the LY group than that in the OG beyond 24 h. These findings suggest that the secretion of the cytokines by hepatic SECs is mediated by Akt signaling. Akt signaling pathway in relationship with proliferation of hepatic SECs and suppression of apoptosis. In OG group, the hepatic SECs in S and G2/M obviously increased. The proliferative index of hepatic SECs in OG group had significant differences with that in LY group at 6, 24, and 72 h, P < 0.05. Meanwhile, the cells of apoptosis in OG group were very low, and the cells in LY group gradually increased.

CONCLUSIONS

These results suggest that AKT signaling pathway plays a crucial role in mediating proliferating and secreted signals in hepatic SECs. AKT has been suggested to play a pivotal role in early liver regeneration involved in the induction of secreted cytokines and proliferation of hepatic SECs.

Phosphorylation of hepatocyte growth factor receptor and epidermal growth factor receptor of human hepatocytes can be maintained in a (3D) collagen sandwich culture system

In vitro culture models that employ human liver cells could be potent tools for predictive studies on drug toxicity and metabolism in the pharmaceutical industry. However, an adequate receptor responsiveness is necessary to allow intracellular signalling and metabolic activity. We tested the ability of three-dimensionally arranged human hepatocytes to respond to the growth factors hepatocyte growth factor (HGF) or epidermal growth factor (EGF). Isolated adult human hepatocytes were cultivated within a three-dimensional collagen gel (sandwich) or on a two-dimensional collagen matrix. Cells were treated with HGF or EGF and expression and phosphorylative activity of HGF receptors (HGFr, c-met) or EGF receptors (EGFr) were measured by flow cytometry and Western blot. Increasing HGFr and EGFr levels were detected in hepatocytes growing two-dimensionally. However, both receptors were not activated in presence of growth factors. In contrast, when hepatocytes were plated within a three-dimensional matrix, HGFr and EGFr levels remained constantly low. However, both receptors became strongly phosphorylated by soluble HGF or EGF. We conclude that cultivation of human hepatocytes in a three-dimensionally arranged in vitro system allows the maintenance of specific functional activities. The necessity of cell dimensionality for HGFr and EGFr function should be considered when an adequate in vitro system has to be introduced for drug testing.

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.

Effect of bile acids on the proliferative activity and apoptosis of rat hepatocytes

Bile acids are known to have damaging as well as protective effects on liver cells. A likely candidate for bile acid-mediated hepatocellular injury during cholestasis is glycochenodeoxycholic acid (GCDCA), a hydrophobic bile acid with a direct cytotoxic effect on hepatocytes. In contrast, ursodeoxycholic acid was shown to exhibit protective effects. Our aim was to determine the effect of GCDCA on proliferation, synthesis and secretion of proteins and death processes in cultured rat hepatocytes. Furthermore, it should be studied whether the hydrophilic bile acid tauroursodeoxycholic acid (TUDCA) might be able to protect cells from the damaging effect of GCDCA. Our results demonstrate that GCDCA decreased dose-dependently hepatocellular proliferation, synthesis and secretion of newly synthesized proteins and, at low concentration, induced apoptosis or, at high doses, cytolysis of cultured hepatocytes. TUDCA did not exert cytotoxic effects on the isolated hepatocytes at a wide range of concentrations. However, TUDCA coincubated with GCDCA protected the cells from the damaging effect of GCDCA at all measured parameters except the secretion of newly synthesized protein.

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.

Chronic liver injury alters basal and stimulated nitric oxide production and 3H-thymidine incorporation in cultured sinusoidal endothelial cells from rats

BACKGROUND/AIM

Under pathological conditions the nitric oxide synthase (NOS)-mediated nitric oxide production of sinusoidal endothelial cells might be altered. Therefore, studies were performed to evaluate the nitrite formation by cultured sinusoidal endothelial cells from rat livers chronically injured by thioacetamide and the effect of endogenously or exogenously generated nitric oxide on their proliferative activity.

METHODS

Basal and stimulated nitrite formation, expression of NOS and DNA synthesis were examined in sinusoidal endothelial cells isolated and cultivated from livers with incipient or advanced chemically-induced cirrhosis.

RESULTS

Cultured sinusoidal endothelial cells from injured livers exhibited a reduced basal and an increased lipopolysaccharide-stimulated nitrite production when compared with controls. Western blot analysis revealed a markedly reduced protein expression of endothelial NOS (eNOS) and inducible NOS (iNOS) in sinusoidal endothelial cells from both experimental groups when compared with controls. Lipopolysaccharide stimulated iNOS expression in sinusoidal endothelial cells from control livers only marginally, and from those with cirrhosis more strongly. There was no clear correlation between the amount of enzyme and nitrite formation. Cultured sinusoidal endothelial cells from livers with incipient cirrhosis showed a higher proliferative activity than controls. Endogenously-produced nitric oxide inhibited DNA synthesis in all groups in a cGMP-independent way. Exogenously-generated nitric oxide affected DNA synthesis differently in sinusoidal endothelial cells from controls and injured livers.

CONCLUSION

The results provide evidence that cultured sinusoidal endothelial cells from controls and livers with incipient or advanced cirrhosis differ with respect to basal and lipopolysaccharide-stimulated nitrite production. The data can be taken as evidence that in sinusoidal endothelial cells from livers chronically injured by thioacetamide, eNOS and iNOS are aberrantly expressed and differently regulated.

EGF-Induced receptor autophosphorylation in primary hepatocytes isolated from phenobarbitone-treated mice

Phenobarbitone (PB) treatment of mice causes a decrease in the growth factor responsiveness of hepatocytes. Here, epidermal growth factor receptor (EGFR) expression and receptor autophosphorylation was determined in hepatocytes isolated from control and PB-treated mice. There was a decrease in the level of EGFR expression in hepatocytes isolated from mice following PB administration when compared to controls. EGF caused an approximate 20-fold increase of the 170 kD phosphotyrosine band in control hepatocytes, which was inhibited by the EGFR specific tyrosine kinase inhibitor 4, 5-dianilinopthalamide. Following PB treatment, the degree of basal receptor phosphorylation (in the absence of EGF) was significantly greater and therefore the fold rise in EGFR phosphorylation in isolated hepatocytes was lower than in controls. However, the overall extent of EGF-induced receptor phosphorylation was not diminished in hepatocytes isolated from PB-treated mice. Therefore the reduction in responsiveness to growth factors seen in hepatocytes ex vivo or the cessation of proliferation observed in vivo following PB administration is unlikely to be attributed to a decrease in ligand binding and subsequent receptor autophosphorylation.

The effects of cell density, attachment substratum and dexamethasone on spontaneous apoptosis of rat hepatocytes in primary culture

The rates of spontaneous cell detachment, cell viability, and apoptosis in primary cultures of rat hepatocytes plated at high and low density were compared. Apoptosis was frequent in detached cells, and the rates of cell detachment and apoptosis were greater in high-density than in low-density cultures. Among attached cells, more cells had condensed or fragmented nuclei in high-density than in low-density cultures. Further, ladder-like DNA fragmentation was not seen in low-cell-density cultures but was clearly evident in high-density cultures. Bax was more highly expressed in cells cultured at high density, and on collagen vs. matrigel, whereas changes of Bcl-2 and Fas expression observed in culture appeared unrelated to the rate of apoptosis. The rate of hepatocyte apoptosis appeared to be identical in low-density cultures on collagen 1 and matrigel, but when cells were cultured at high density, matrigel suppressed apoptosis by more than 50% at 36 h. In hepatocytes cultured on collagen 1, dexamethasone (0.1 microM) suppressed apoptosis in both low- and high-density cultures; higher doses had no further effects. In high density cultures, aurintricarboxylic acid (10 microM) suppressed apoptosis and this improved cell attachment at 48 h. It is concluded that cell viability in primary cultures of rat hepatocytes grown on collagen I is dependent on optimal culture density and that the cell population is regulated, at least in part, by apoptosis. Corticosteroids suppress spontaneous apoptosis of cultured hepatocytes in a non-dose-dependent manner, whereas matrigel abolishes apoptosis induced by increasing cell density. Bax may be an important protein in the cell density and cell matrix-dependent regulation of apoptosis in cultured hepatocytes.