Reciprocal modulation of growth and differentiated functions of mature rat hepatocytes in primary culture by cell--cell contact and cell membranes

Membrane dips over nuclei correlate with DNA synthesis in spreading hepatocytes

Spreading of hepatocytes on different supports was examined using scanning electron microscopy. Positively charged Primaria plates gave a uniform morphology in 2 h. The spreading was rapid and the surface of the cells showed early prominent dips. The hepatocytes had one or two of these structures corresponding with nuclearity of the cells. The nuclear origin of the dips was confirmed after 6 h. The indentations contained solid structures the number, size, and shape of which were identical to the nucleoli seen by light microscopy. The spreading on the other supports was less uniform. Nuclear dips appeared more slowly and were less marked initially in their depths. The nuclear dipping was independent of cell density and took place under conditions under which the cells undergo phenotypic changes during culture. Individual phenotypic changes occur at different times and rates so that the initial signal for their onset cannot be determined with any certainty. However, the appearance of the dips was accompanied by DNA synthesis in the normally quiescent cells. The process stopped when the dipping was completed. The unavoidable change in nuclear morphology in spread cells may explain why maintenance of a spherical shape circumvents inappropriate DNA synthesis and maintains hepatocyte differentiation in vitro.

Fifty-seven-kDa Protein in Royal Jelly Enhances Proliferation of Primary Cultured Rat Hepatocytes and Increases Albumin Production in the Absence of Serum

We have previously shown that 57-kDa protein in royal jelly (RJ) was specifically degraded in proportion to both storage temperature and storage period, and we suggested that it could be useful as a marker of freshness of RJ (Kamakura, M., Fukuda, T., Fukushima, M. and Yonekura, M. (2001) Biosci. Biotechnol. Biochem. 65, 277-284.). Here, we investigated the physiological effects of 57-kDa protein on primary cultured rat hepatocytes in the absence of serum. The 57-kDa protein and RJ significantly stimulated hepatocyte DNA synthesis, whereas bovine serum albumin and RJ stored at 40 degrees C for 7 days, which lacks 57-kDa protein, did not. The mitogenic activity of 57-kDa protein was lost after treatment with trypsin. These results indicate that 57-kDa protein acts as a mitogen. The stimulatory effect of 57-kDa protein was dose-dependent and was more potent at lower than at higher cell densities. The 57-kDa protein also prolonged the cell proliferation of primary cultured rat hepatocytes, with an enhancement of albumin production compared to untreated cells. Therefore, 57-kDa protein is likely to promote liver regeneration and may have a cytoprotective action on hepatocytes.

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.

Cell density-dependent regulation of hepatic development by a gp130-independent pathway

We previously demonstrated that oncostatin M (OSM) promotes hepatic development in concert with glucocorticoid. The livers from mice deficient for gp130, a signaling subunit of the OSM receptor, displayed reduced expression of hepatic differentiation marker and defective glycogenic function. However, these phenotypes were not completely abolished in gp130(-/-) mice, suggesting that there is an alternative pathway regulating hepatic development in vivo. To test this possibility, we cultured gp130(-/-) fetal hepatic cells and investigated a signal that induces hepatic differentiation. When hepatocytes were forced to interact with each other by inoculating cells at high densities, hepatic differentiation was induced even in the absence of gp130. Moreover, cells stimulated with OSM and/or cultured at a high density possess many other metabolic functions. These observations suggest that fetal hepatic cells acquire multiple characteristics of differentiated hepatocytes in response to the signals generated by cell-cell contacts as well as by OSM.

Differential expression of two CYP1A genes in rainbow trout (Oncorhynchys mykiss)

Differential expression of two rainbow trout CYP1A genes was measured in vivo and in vitro in response to treatment with the model CYP1A inducers beta-naphthoflavone (BNF), 3-methylcholanthrene (3-MC), isosafrole (ISF), and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, only in vitro). Originally described by Berndtson and Chen (Arch. Biochem. Biophys. 310, 187-195, 1994) as CYP1A1 and CYP1A2, these genes were renamed CYP1A3 and CYP1A1, respectively, by the P450 nomenclature committee. A significant, differential, inducer-dependent induction of the two CYP1A mRNAs, as measured by RNase protection assay, was observed in vivo. CYP1A3 and CYP1A1 mRNA levels in liver were significantly induced 50- and 18-fold, respectively, following ip injection with BNF. Conversely, CYP1A3 and CYP1A1 mRNA levels were significantly induced 5- and 66-fold, respectively, following ip injection with 3-MC. Isosafrole had no significant effect on in vivo induction of CYP1A mRNA levels. In primary cultures of hepatocytes, BNF, 3-MC, ISF, as well as TCDD all significantly induced CYP1A3 and CYP1A1 mRNA levels compared to controls. The differential induction of the two CYP1A genes was not as evident in vitro as in vivo. In addition, reanalysis and sequence comparison of the these two trout CYP1A genes with the first trout CYP1A cDNA described by Heilmann et al. (DNA 7, 379-387, 1988) indicate that the Heilmann cDNA is a hybrid of the two trout genes. The 5' portion of the cDNA sequence (212 bp) was determined by sequencing of a genomic clone and is 100% identical to the trout CYP1A3 gene. The majority of the cDNA sequence (2377 bp), however, was sequenced from a partial cDNA clone and is 99.2% identical to trout CYP1A1. Although the nomenclature of these two trout CYP1A genes is undergoing revision, these results demonstrate a differential, inducer-dependent response to model mammalian CYP1A inducers.

Responsiveness to thyroid hormone is enhanced in rat hepatocytes cultured as spheroids compared with that in monolayers: altered responsiveness to thyroid hormone possibly involves complex formed on thyroid hormone response elements

We previously reported that the expression of type I iodothyronine 5'-deiodinase (5'DI) gene was increased by 3,3,',5-triiodothyronine (T3) in isolated rat hepatocytes when cultured as spherical aggregates (spheroids), whereas this effect was greatly attenuated in conventional monolayer cultures. In the current study, we examined whether the enhanced T3 responsiveness in spheroid cultures extends to other T3-responsive genes. As observed for 5'DI, T3 increased spot 14, malic enzyme and fibronectin messenger RNAs (mRNAs) by fourfold to fivefold in spheroid cultures, while the effect in monolayer cultures was blunted. This difference in T3 responsiveness was also observed when T3-responsive reporters consisting of the luciferase gene under the control of triiodothyronine response element (TRE) were introduced into hepatocytes using a replication-defective adenovirus vector. These results suggest that the factors required for T3-dependent transcriptional activation are preserved in spheroid cultures and that they must exert their effect by interacting with TRE. Maximal binding capacity of nuclear T3 receptor was not different between monolayer and spheroid cultures while the expression of retinoid X receptor-alpha (RXR alpha) mRNA was higher in spheroid cultures compared with that in monolayers. The difference in RXR alpha mRNA expression, together with enhanced proteolytic cleavage in monolayers that we demonstrated recently, may account for the difference in T3 responsiveness between the two hepatocyte culture systems.

Growth factors and gene expression in cultured rat hepatocytes

BACKGROUND/AIM

The aim of this study was to evaluate the effect of replication on function variables in cultured hepatocytes.

METHODS

Isolated rat hepatocytes were cultured in HCM medium and plated on collagen-coated dishes at cell densities from 0.2 x 10(5) (subconfluent) to 1.0 x 10(5) x cm(-2) (confluent) with and without addition of hepatocyte growth factor, epidermal growth factor and insulin-like growth factor-I. The synthesis rate was measured for DNA, albumin, urea, and glucose together with mRNA levels (Northern blots) for albumin, urea cycle enzymes, and acute phase and "house-keeping" proteins.

RESULTS

In subconfluent culture the synthesis of DNA and urea was higher (118% and 112%, respectively), and of albumin and glucose lower (40% and 67%, respectively) than in confluent culture. The mRNA levels of carbamoylphosphate synthase, argininosuccinate synthetase, argininosuccinate lyase, arginase, a2-macroglobulin, beta-fibrinogen, and albumin were lower (23%, 58%, 77%, 33%, 12%, 50%, and 51%, respectively) in subconfluent culture compared with confluent culture. Relatively increased levels were found for beta-actin (109%) and alpha-tubulin (136%). In subconfluent culture hepatocyte growth factor increased the DNA synthesis rate 6-fold, epidermal growth factor 3-fold, and insulin-like growth factor-I 2-fold; that of albumin, urea and glucose was not increased significantly. In confluent culture the effect of growth factors on synthesis rates was not significant, and the growth factors had little influence on mRNA levels.

CONCLUSIONS

Hepatocytes produce urea at the same rate in subconfluent as in confluent culture in spite of a lower mRNA level of urea cycle enzymes. Hepatocyte growth factor and epidermal growth factor increase DNA synthesis markedly in subconfluent culture only, without significantly changing the ratio between subconfluent and confluent culture of other variables. This suggests that active replication is not an important cause of the relatively low liver-specific function of hepatocytes in subconfluent culture.

Glutathione S-conjugate transport in hepatocytes entering the cell cycle is preserved by a switch in expression from the apical MRP2 to the basolateral MRP1 transporting protein

The multidrug resistance protein MRP1 and its isoform MRP2 are involved in ATP-dependent glutathione S-conjugate transport and have similar substrate specificities. MRP2 mediates hepatic organic anion transport into bile. The physiological function of MRP1 in hepatocytes is unknown. Previous results show that MRP1 expression is low in quiescent hepatocytes but increased after SV40 large T antigen immortalization, suggesting a relationship with cell proliferation. Therefore, we determined mrp1 and mrp2 expression in rat hepatocytes in relation to the cell cycle. By varying cell density we obtained cultures that are mainly in G1 (high density) or have progressed into the S-phase or beyond (low density). In both cultures mrp1 mRNA and protein levels are increased, concomitantly with the disappearance of mrp2. This switch from mrp2 to mrp1 occurs in the G1 phase of the cell cycle and is associated with a decreased cell polarity. Mrp1 is located on lateral membranes or on intracellular vesicles, depending on whether cell-cell contact is established. In both locations mrp1 contributes to cellular glutathione S-conjugate efflux and protects against oxidative stress-inducing quinones. We conclude that a switch in expression from the apically located mrp2 to the basolaterally located mrp1 preserves glutathione S-conjugate transport in hepatocytes entering the cell cycle and protects against certain cytotoxic agents.

Gap junctional communication and regulation of the glycogenic response to insulin by cell density and glucocorticoids in cultured fetal rat hepatocytes

Cell culture studies have revealed that metabolic functions of the adult hepatocyte are related to cell density. Development of the glycogenic response to insulin under glucocorticoid control was investigated in 15- and 18-day-old fetal rat hepatocytes plated at different cell densities. After culturing for 48 hours with glucocorticoids, the stimulatory effect of insulin on [14C]glucose incorporation into glycogen after 3 hours progressed from weak response (less than 1.4-fold) in sparse cultures to a maximal response in dense ones (3.0- to 4.5-fold), depending on the fetal stage. The response was always no more than 2.0-fold in the absence of glucocorticoids, even with dense cultures. Such a dual regulation pattern was not found for the glycogenolytic effect of glucagon similarly expressed regardless of culture conditions. When cells were clustered in limited circular regions of the dish, the insulin response was higher than for sparse cultures for a similar number of cells per culture. Using the scrape-loading technique with Lucifer Yellow CH, a positive dye transfer was obtained in clustered cultures providing that they were grown in the presence of glucocorticoids; insulin as well as glucagon stimulated twofold intercellular communication. Connexin32 (Cx32) and connexin26 (Cx26) protein levels were assayed by Western immunoblotting and developed according to age and exposure to glucocorticoids. Thus, glucocorticoids through development of gap junctions enabled establishment of intercellular communication that could be stimulated by insulin and glucagon in cultured fetal hepatocytes. Gap junction functioning and the biologic effect of insulin correlated closely.

The cytoskeletal network controls c-Jun expression and glucocorticoid receptor transcriptional activity in an antagonistic and cell-type-specific manner

The physical and functional link between adhesion molecules and the cytoskeletal network suggests that the cytoskeleton might mediate the transduction of cell-to-cell contact signals, which often regulate growth and differentiation in an antagonistic manner. Depolymerization of the cytoskeleton in confluent cell cultures is reportedly sufficient to initiate DNA synthesis. Here we show that depolymerization of the cytoskeleton is also sufficient to repress differentiation-specific gene expression. Glutamine synthetase is a glia-specific differentiation marker gene whose expression in the retinal tissue is regulated by glucocorticoids and is ultimately dependent on glia-neuron cell contacts. Depolymerization of the actin or microtubule network in cells of the intact retina mimics the effects of cell separation, repressing glutamine synthetase induction by a mechanism that involves induction of c-Jun and inhibition of glucocorticoid receptor transcriptional activity. Depolymerization of the cytoskeleton activates JNK and p38 mitogen-activated protein kinase and induces c-Jun expression by a signaling pathway that depends on tyrosine kinase activity. Induction of c-Jun expression is restricted to Müller glial cells, the only cells in the tissue that express glutamine synthetase and maintain the ability to proliferate upon cell separation. Our results suggest that the cytoskeletal network might play a part in the transduction of cell contact signals to the nucleus.

Hepatocyte growth factor leads to recovery from alcohol-induced fatty liver in rats

A fatty liver is characterized by the hyperaccumulation of lipids within hepatocytes and is often caused by excessive alcohol intake. Rats fed ethanol-containing diets for 37 days showed remarkable increase in hepatic lipids and lipid droplet accumulation in the hepatocytes, indicating the onset of alcoholic fatty liver. Administration of hepatocyte growth factor (HGF) for the last seven days of ethanol treatment markedly decreased hepatic lipids to a level lower than that seen before HGF treatment. In contrast, serum levels of lipids and lipoproteins increased with HGF administration. Primary cultured hepatocytes prepared from the fatty liver retained lipid droplets during a 48-hour culture. However, when cultured in the presence of HGF, intracellular lipid concentrations decreased and lipid secretion was enhanced. Consistent with these events, HGF stimulated the rate of protein synthesis of apolipoprotein B (apoB) and enhanced subsequent mobilization of lipids into the medium. These results indicate that HGF administration induced recovery from the fatty liver, at least in part, by enhancing apoB synthesis and the subsequent mobilization of lipids from hepatocytes with fatty change. The possibility that HGF can be therapeutic for subjects with an alcohol-related fatty liver warrants further attention.

Behavior of a cell line derived from normal human hepatocytes on non-physiological and physiological-type substrates: evidence for enhancement of secretion of liver-specific proteins by a three-dimensional growth pattern

The behavior of a recently described cell line, HH25, derived from normal human hepatocytes, has been investigated on several different substrates--tissue-culture plastic, glass, a thin layer of rat-tail collagen I, and thin layers or thick gels of extracellular matrix derived from the Engelbreth-Holm-Swarm murine sarcoma (EHS matrix). Cellular morphology, proliferation, and secretion of three hepatocyte-specific proteins (albumin, alpha1 acid glycoprotein, and alpha1 antitrypsin) have been examined. There were no differences in morphology, proliferation, or differentiated function in the cells on either plastic, glass, collagen, I, or a thin layer of EHS matrix, but on a thick EHS matrix gel the cells altered their morphology (forming three-dimensional colonies with canalicular-like structures) and their production of albumin and alpha1 acid glycoprotein was enhanced. This suggests that the enhanced differentiated function is associated with the morphological change (occurring only on the thick EHS gel) rather than with receptor-mediated cell-matrix interactions (which can also occur on the thin layer of EHS matrix). This cell line is therefore a good in vitro cellular model for the investigation of the roles of morphological changes and of cell-cell and cell-matrix interactions in the control of human hepatocyte behavior without the need for an extensive source of primary tissue.

Adenovirus-transferred HNF-3 gamma conserves some liver functions in primary cultured hepatocytes of adult rats

Hepatocyte nuclear factor-3 (HNF-3) isoforms are key factors for regulation of gene expression and differentiation in hepatocytes. HNF-3gamma is abundantly expressed in the mature liver, but down-regulated in primary cultured hepatocytes, in which some other hepatic gene expressions are also decreased. In this study, the primary hepatocytes were infected with the recombinant adenovirus carrying HNF-3gamma gene (AxCAHNF3gamma), and this led to marked induction of the HNF-3gamma gene. As a result, the expressions of albumin, catalase, and ornithine transcarbamylase (OTC) genes were also recovered to significant levels in the AxCAHNF3gamma-infected hepatocytes. Moreover, hepatocyte proliferation stimulated by epidermal growth factor (EGF) and insulin was also inhibited by AxCAHNF3gamma infection. Our results demonstrate that the enforced expression of HNF-3gamma gene can lead to conservation of some original liver functions in the primary cultured hepatocytes accompanied by morphological differentiation and growth inhibition.

Zonal location of compensatory hepatocyte proliferation following chemically induced hepatotoxicity in rats and humans

Hepatocyte proliferation stimulated by partial hepatectomy occurs first in periportal cells, with midlobular and then perivenous cell division occurring later. We have previously shown that this pattern of compensatory cell proliferation also occurs following the hepatotoxicity of N-nitrosodimethylamine. We examined the generality of this pattern in livers of rats given a minimally toxic dose of an hepatotoxin and in liver biopsy samples from patients who had taken overdoses of acetaminophen. Proliferating hepatocytes were detected immunohistochemically (5'-bromodeoxyuridine or Ki-67 antigens). The perivenous necrogens N-nitrosodiethylamine, carbon tetrachloride (CCl4), bromobenzene, and acetaminophen all induced periportal hepatocyte proliferation. With CCl4, bromobenzene, and acetaminophen, the initial appearance of proliferating periportal hepatocytes was followed 12-24 hr later by division in the midlobular region, with a few cells dividing adjacent to the perivenous region of necrosis. The periportal necrogen allyl alcohol also induced periportal cell division. In the human studies, perivenous necrosis was accompanied by periportal and midlobular hepatocyte proliferation. These results suggest that regardless of its lobular location chemically induced hepatotoxicity stimulates cell proliferation that begins in the periportal zone and then moves in an orchestrated response into the midlobular and perivenous zones.

Effects of insulin-like growth factor I and II on DNA synthesis and proliferation in primary cultures of adult rat hepatocytes

We compared the effects of insulin-like growth factor I (IGF-I) and II (IGF-II) on DNA synthesis and proliferation and investigated various signal transduction mechanisms involved in insulin-like growth factor-induced mitogenesis in primary cultures of adult rat hepatocytes. IGF-I stimulated hepatocyte DNA synthesis and proliferation with an EC50 of 75 ng/ml within 4 h of culture. These effects were sensitive to the IGF-I concentration and cell density. Hepatocyte proliferation induced by IGF-I was potentiated by metaproterenol (10(-6) M) as well as by 8-bromo-cAMP, phorbol 12-myristate 13-acetate (PMA; 10(-8) M) and was inhibited by U-73122 (1-(-[[17beta-3-methoxyestra-1,3,5(10)-triene-17-yl]amino]hexyl]-+ ++1Hpyrrol-2,5-dione)), genistein, wortmannin, PD98059 (2'-amino-3'-methoxyflavone) and rapamycin. The IGF-I effect was independent of pertussis toxin (100 ng/ml). IGF-II also dose dependently stimulated hepatocyte DNA synthesis and proliferation with an EC50 of 0.75 ng/ml within 4 h of culture. However, these effects were not dependent on the initial plating density. The stimulatory effects of IGF-II were potentiated by UK-14304 (5-bromo-6-[2-imidazolin-2-ylamino]-quinoxaline) (10(-5) M) and inhibited by phenylephrine, PMA, metaproterenol, 8-bromo-cAMP, PD98059, rapamycin, and pertussis toxin. The IGF-II effects were not affected by genistein, U-73122, and wortmannin. These results suggest that IGF-I and IGF-II rapidly stimulate the DNA synthesis and proliferation of adult rat hepatocytes by separate mechanisms.

Insulin-like growth factor binding protein-4 accumulation is negatively correlated with growth rate in TM-3 cells

Cellular growth is controlled by multiple regulators, including the insulin-like growth factors (IGFs). In some cells, the IGF binding proteins (IGFBPs) are thought to be inhibitory molecules for cell growth and may be related to the process of contact inhibition. In the TM-3 (mouse Leydig) cell line, IGFBP-4 is the major IGFBP secreted into conditioned media (CM), as we have reported. In this study, we investigated cell growth, the peptide levels of IGFBP-4 in CM, and the inverse relationship between IGFBP-4 accumulation and cell growth rate. Quantification of TM-3 growth in serum-containing media demonstrated that TM-3 cell number gradually rose after plating, and plateaued when cells became confluent. The rate of cell growth fell gradually, and net cell growth stopped when cells reached confluency. IGFBP-4 peptide levels in CM, as measured by Western ligand blot, rose gradually during the culture period and plateaued when cells reached confluency. The amount of IGFBP-4 peptide level in CM correlated for cell number (IGFBP-4 accumulation rate) also rose gradually during the course of culture and plateaued. The IGFBP-4 accumulation rate was strongly negatively correlated with the rate of cell growth (r = 0.98, P < 0.001). In conclusion, our data suggest that in TM-3 cells, cell growth is related to IGFBP-4 accumulation. The negative correlation between IGFBP-4 accumulation and the rate of cell growth suggests that IGFBP-4 may be a primary regulator of TM-3 cell growth and possibly participate in the process of contact inhibition.

Hepatocyte culture utilizing porous polyvinyl formal resin maintains long-term stable albumin secretion activity

To investigate the effects of culture conditions on the maintenance of metabolic functions of cultured hepatocytes, long-term hepatocyte culture lasting 20 days was performed under two different culture conditions, i.e. stationary cultures utilizing porous polymer (polyvinyl formal (PVF) resin) as a substratum and conventional monolayer dish cultures without PVF. Metabolic activities specific to hepatocytes were evaluated in terms of ammonia metabolism, urea synthesis, and albumin secretion. Concerning ammonia metabolic and urea synthetic activities, no significant differences in maintenance of these activities were found between the two culture conditions, and these activities rapidly decreased with the elapse of the culture period, especially during the early stage of the experiments. However, after day 10, these activities in the stationary cultures were maintained at a slightly more favorable level than in the monolayer cultures. On the other hand, compared with ammonia metabolism and urea synthesis, stable and well-maintained albumin secretion of hepatocytes (60% of the activity in day 1) was exhibited in the stationary culture experiments, despite that this particular activity under the monolayer culture condition gradually reduced to a very low level (5.7% of that on day 1) at the end of the culture. From the morphological observations, hepatocytes immobilized in the PVF resin revealed individual spherical shapes without forming multicellular aggregation, and it was suggested that this characteristic structure contributed to good albumin secretion of hepatocytes. In conclusion, the advantages of the hepatocyte culture technique utilizing PVF resin over the conventional dish culture in maintaining some representative metabolic function specific to hepatocytes were clarified.

Homotypic cell contact-dependent inhibition of astrocyte proliferation

The normal adult vertebrate nervous system is a relative quiescent tissue in terms of cell proliferation. However, astrocytes in many regions of the central nervous system (CNS) retain the capacity to undergo cell division. To examine the mechanisms that regulate the proliferation of astrocytes in the CNS we have utilized an in vitro assay in which astrocyte density and cellular environment could be regulated. We demonstrate that type 1 astrocytes derived from the cerebral cortex of developing rats exhibit a profound density-dependent inhibition of proliferation. This inhibition of proliferation was cell type specific, but not restricted to type 1 astrocytes. NIH 3T3 cells but not smooth muscle cells inhibited astrocyte proliferation, while contact-inhibited astrocytes did not inhibit oligodendrocyte proliferation. Co-culture of type 1 astrocytes with neurons from a variety of sources resulted in induction of a process-bearing astrocyte morphology and promoted glial cell proliferation. Thus, induction of a process-bearing astrocyte morphology does not lead to a cessation of proliferation. The inhibition of astrocyte proliferation did not appear to be mediated through the release or sequestration of soluble factors but rather could be induced by membrane-associated factors.

Induction of tyrosine aminotransferase of primary cultured rat hepatocytes depends on the organization of microtubules

We investigated the relationship between the expression of tyrosine aminotransferase (TAT) and cytoskeletal systems of cultured rat hepatocytes by using serum-free culture conditions and changing three factors: 1) the concentration of calcium, 2) the dish-coating material, and 3) the cell-plating density. In hepatocytes in low-calcium medium, induction of TAT by dexamethasone and glucagon was maintained, although cell-cell adhesion was lost. Hepatocytes on Matrigel formed a nonspreading, spherical shape that provided them with the full extent of TAT activity without cell-cell adhesion. Hepatocytes plated on collagen at low cell density spread and changed shape, and the induction of TAT activity was markedly reduced. By using confocal laser-scanning microscopy, we analyzed the three-dimensional organization of cytoplasmic microtubules of hepatocytes maintaining the ability of TAT induction. Hepatocytes plated on collagen at low cell density possessed the radial filamentous structure of cytoplasmic microtubules. When the spherical shape of hepatocytes was maintained by cultivating cells on Matrigel, a ring-like structure of cytoplasmic microtubules beneath the plasma membrane was dominant. Moreover, the induction of TAT activity of hepatocytes in a standard culture system was strongly inhibited by the addition of 1 microM colchicine. These studies suggest that the organization of cytoplasmic microtubules may participate in the shape-related regulation of cell function.

Long-term cultures of human fetal liver cells: a three-dimensional experimental model for monitoring liver tissue development

BACKGROUND/AIMS

The present study describes an embryonic-fetal liver culture system which allows morphogenetic interactions consistent with the development of the hepatocellular function.

METHODS

Intact livers from 8-12-week embryos were soaked in an extracellular matrix at 4 degrees C and gently dissociated without any enzymatic treatment. The resulting spherical hepatic units were cultured in a chemically defined serum-free medium and seeded into an extracellular matrix layer. Adherent three-dimensional tissue specimens were examined at various times by light and electron microscopy to evaluate the maintenance of hepatocyte morphology.

RESULTS

The liver cells were viable for over 4 months; erythropoietic burst colonies were detected for longer than 6 weeks. Parallel detection of bile salt production in the medium by high performance liquid chromatography proved liver tissue functionality. Bile salt composition revealed predominance of taurine-conjugates rather than glycine. Maximum bile salt concentration (approximately 3 months) coincided with structural and ultrastructural observations indicating a marked decline in hematopoiesis, well-defined biliary canaliculi and formation of an organ-like structure.

CONCLUSIONS

This three-dimensional culture system recapitulates fetal liver development with: (i) initial proliferation of both fetal erythropoietic and hepatic cells and (ii) subsequent shut-off of erythropoiesis and a shift to a more advanced stage of hepatocyte function, such as bile salt secretion.

Proliferation of adult rat hepatocytes in primary cultures induced by platelet-derived growth factor is potentiated by phenylephrine

We investigated whether or not proliferation of adult rat hepatocytes induced by platelet-derived growth factor (PDGF) is affected by alpha1-adrenoceptor agonists such as phenylephrine during the early and late phases of primary culture. Adult rat hepatocytes underwent significant DNA synthesis after culture with 10 ng/ml of PDGF for 2 hr at a low cell density (3.3 x 10(4) cells/cm2). Under these culture conditions, the number of nuclei increased significantly during the 3.5-hr culture period. Hepatocyte DNA synthesis and proliferation induced by 10 ng/ml of PDGF decreased slightly as a result of increasing the initial plating density. An alpha1-adrenoceptor agonist, phenylephrine (10(-6) and 10(-5) M), alone did not affect hepatocyte DNA synthesis and proliferation, but markedly potentiated PDGF-induced hepatocyte DNA synthesis and proliferation. The phenylephrine effect was mimicked by phorbol myristate acetate (10(-7) M), but not by ionomycin (10(-5) M). The mitogenic effects of PDGF were almost completely blocked by treating hepatocytes with genistein (5 x 10(-6) M), U-73122 (3 x 10(-6) M), sphingosine (10(-5) M), wortmannin (10(-7) M) and rapamycin (10 ng/ml). These results demonstrate that PDGF can induce the proliferation of adult rat hepatocytes rapidly in primary culture, regardless of the initial plating density. The present results also suggest that following stimulation with PDGF, activation of tyrosine kinase, phospholipase C, phosphatidylinositol 3-kinase, protein kinase C (PKC) and p70 ribosomal protein S6 kinase is essential for the proliferation of adult rat hepatocytes. The co-mitogenic effects of phenylephrine may involve PKC activation.

Inhibition of lipopolysaccharide and cytokine mixture-mediated hepatocyte nitric oxide synthesis by dimethyl sulfoxide

Treatment and pretreatment of hepatocytes with 2% dimethyl sulfoxide (DMSO) inhibited lipopolysaccharide and cytokine mixture (LPS/CM)-mediated NO synthesis in hepatocytes without any obvious effects on cell viability. DMSO at concentrations of 0.5-4% stimulated DNA replication and increased albumin secretion in LPS/CM-treated hepatocytes. Genisein, a inhibitor of protein tyrosine kinase (PTK), inhibited LPS/CM-mediated NO synthesis in hepatocytes. These results suggest that PTK is critical for hepatocyte NO synthesis, and DMSO-inhibited NO synthesis may be associated with prevention of LPS/CM-induced PTK activation in hepatocytes.

Modulation of hepatocyte function by changing the cell shape in primary culture

To study the role of cell shape in control of hepatocyte function, we have developed a system that can quantitatively control the spreading of cultured rat hepatocytes using poly[2-hydroxyethyl methacrylate]. When hepatocytes were cultured in a dish coated with high concentration of poly[2-hydroxyethyl methacrylate] solution, formation of stress fibers were suppressed and they continued to have a compact shape. In the compact-shaped hepatocytes, the ability to induce tyrosine aminotransferase with dexamethasone remained high for longer periods of time, as compared to the hepatocytes that spread following culture in the polystyrene dish. Conversely, the hepatocytes showed more active DNA synthesis when they assumed a flat shape as a result of spreading. When the hepatocytes that had spread following long-term culture in the polystyrene dishes were treated with cytochalasin to induce depolymerization of F-actin, the ability of the cells to induce tyrosine aminotransferase upon stimulation with dexamethasone improved markedly. This effect was not altered by treatment with actinomycin D but was completely suppressed by cycloheximide, suggesting that microfilaments are involved in the post-transcriptional process of tyrosine aminotransferase induction. Thus, there is a possibility that F-actin rather than cell shape might regulate cellular function in primary cultured hepatocytes.

Proliferation of adult rat hepatocytes in primary culture induced by insulin is potentiated by cAMP-elevating agents

We investigated whether or not insulin and cAMP-elevating agents induce the proliferation of adult rat hepatocytes during the early and late phases of primary culture. Adult rat hepatocytes synthesized a significant amount of DNA when cultured in the presence of 10(-7) M insulin for 3 h. Under these conditions, the number of nuclei increased within 4 h. Hepatocyte DNA synthesis and proliferation were not essentially affected by the initial plating densities. Other cAMP-elevating agents, such as glucagon, forskolin and dibutyryl cAMP, as well as beta-adrenoceptor agonists (i.e., metaproterenol and isoproterenol) alone had no effect on either hepatocyte DNA synthesis or proliferation in primary culture. In contrast, these agents potentiated both processes at concentrations as low as 10(-7) M when cultured in combination with 10(-7) M insulin. The stimulatory effects of beta-adrenoceptor agonists and other cAMP-elevating agents were significantly blocked by the cAMP-dependent protein kinase inhibitor, H-89 (N-[2-(p-(bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride; 10(-7) M). The mitogenic effect of insulin upon hepatocytes was almost completely suppressed by genistein (5 x 10(-6) M), wortmannin (10(-7) M) and by rapamycin (10 ng/ml). These results show that insulin rapidly induced the proliferation of adult rat hepatocytes in primary culture. The mitogenic effects of insulin were potentiated by beta-adrenoceptor agonists and cAMP-elevating agents. The effects of beta-adrenoceptor agonists and cAMP-elevating agents may be mediated through cAMP-dependent protein kinase. In addition, the activation of receptor tyrosine kinase, phosphoinositide 3-kinase and p70 ribosomal protein S6 kinase may be involved in the insulin signal transduction pathway.

Density-dependent proliferation of adult rat hepatocytes in primary culture induced by epidermal growth factor is potentiated by cAMP-elevating agents

We investigated whether or not epidermal growth factor (EGF) and cAMP-elevating agents induce the proliferation of adult rat hepatocytes during the early (4 h after adding EGF) and late phases (21 h after adding EGF) of primary cultures. Adult rat hepatocytes did not significantly proliferate after culture with 20 ng/ml EGF for 4 h at a density of 1 X 10(5) cells/cm2. In contrast, when the density was decreased by about one-third to 3.3 X 10(4) cells/cm2, the number of nuclei increased about 1.2-fold after culture with 10-20 ng/ml EGF for 4 h. Under these culture conditions, DNA synthesis began within 2-4 h of exposure to 20 ng/ml of EGF, although at the high cell density, DNA was not synthesized during this period. The beta-adrenoceptor agonists, metaproterenol and isoproterenol, and other cAMP-elevating agents, such as glucagon, forskolin, and dibutyryl cAMP, potentiated both hepatocyte DNA synthesis and proliferation about 1.4-fold when cultured in combination with 20 ng/ml EGF. The stimulatory effects of metaproterenol and other cAMP-elevating agents were specifically blocked by the cAMP-dependent protein kinase inhibitor, H-89 (10(-7) M). The effect of EGF was almost completely suppressed by genistein (5 X 10(-6) M) and rapamycin (10 ng/ml), but it was unaffected by wortmannin (10(-7) M). These results demonstrate that mature rat hepatocytes can proliferate very rapidly in low-density cultures with EGF, the effects of which were potentiated by beta-adrenoceptor agonists and cAMP-elevating agents. In addition, the activation of receptor tyrosine kinase and p70 ribosomal protein S6 kinase may be involved in EGF-induced hepatocyte DNA synthesis and proliferation.

Cell density-dependent mitogenic effect and -independent cellular handling of epidermal growth factor in primary cultured rat hepatocytes

AIMS

Mitogenic effect and cellular handling of epidermal growth factor (EGF) were analyzed in primary cultured rat hepatocytes at several cell densities.

METHODS/RESULTS

DNA synthesis, assessed by the incorporation of 125I-deoxyuridine, was accelerated by EGF at a low cell density while that stimulated by EGF was relatively low at the highest cell density, suggesting a cell density-dependent regulation of mitogenic response to EGF. An equilibrium binding study of 125I-EGF in the presence of various concentrations of unlabeled EGF at 0 degree C revealed that the dissociation constant (Kd) was 0.47-0.88 nM while the specific binding capacity (n) was 86-96 fmol/mg protein at each cell density. No significant difference was observed in the time profiles of the surface-bound, internalized, and degradation products of 125I-EGF, assessed per mg protein, between different cell densities. Based on a kinetic analysis of the time-profiles, the internalization rate constant and the degradation rate constant were found to be independent of cell density.

CONCLUSIONS

These results indicate that the cellular binding and disposition of EGF are not regulated by cell density, and that the cell density-dependence of the mitogenic effect cannot be attributed to differences in the affinity or capacity of the EGF receptor, internalization, or degradation of EGF. We speculate that the cell density-dependent mitogenic response may be accounted for by the difference in other factors such as the signal transduction processes induced by the receptor binding of EGF, or the translocation of a small fraction of the total EGF to hepatocyte nuclei.

Transforming growth factor beta increases the activity of phosphatidate phosphohydrolase-1 in rat hepatocytes

Phosphatidic acid (PA) is a potent second messenger arising from growth factor-induced stimulation of phospholipase D which hydrolyses phosphatidylcholine. PA is hydrolysed to diacylglycerol by PA phosphohydrolase (PAP) which exists in two forms: PAP-1 and PAP-2. In rat hepatocyte cultures, overnight (20h) incubation with transforming growth factor (TGF) beta (1 ng/ml) increased PAP-1 activity two-fold. This effect was concentration and time dependent and was greatest at low cell density. The TGFbeta effect on PAP-1 was additive to stimulation induced by dexamethasone but not by glucagon and it reversed the inhibition by insulin. Epidermal growth factor had no effect on PAP-1 activity. None of the above hormones or growth factors affected the subcellular distribution of PAP-1. Stimulation of PAP-1 by TGFbeta may be involved in mediating some of its biological effects.

Expression of the asialoglycoprotein receptor in cultured rat hepatocytes is modulated by cell density

The influence of cell density on expression of the asialoglycoprotein receptor system in primary cultures of rat hepatocytes was evaluated by measuring the level of the receptor specific mRNA. When the hepatocytes are cultured at high cellular density and are not in a proliferative condition, the transcript molecules of the receptor appear increased about 50% with respect to the low plating density, indicating a modulation of asialoglycoprotein receptor expression at transcriptional level. Such control may be dependent on surface molecules involved in cell specific reassociation, since it is well known that cell contacts play a significant regulatory role in differentiated cells.

Correlation of expression of connexin mRNA isoforms with degree of cellular differentiation

Examination of rat hepatic cell lines has revealed a correlation between the differentiated state of the cells and the gap junctional proteins, or connexins, they express. The cell lines RLC (Gershenson et al, 1970) and FTO.2B (Killary et al, 1984) were examined and compared to primary adult hepatocytes for expression of fetal and adult hepatic antigens under various tissue culture conditions. Maximal expression of fetal antigens was observed in cells grown in serum-supplemented medium, on either tissue culture plastic or type IV collagen. Maximal expression of adult specific antigens was seen in cells grown in a hormonally defined medium containing heparin, on type I or type IV collagen. The cell line RLC strongly expressed fetal antigens, while FTO.2B expressed both fetal and adult antigens. These cell lines and another poorly differentiated hepatic cell line, WB-F344 (Tsao et al., 1984) were used to assess the developmental profile of mRNAs encoding isoforms of gap junctions: connexins 26, 32, and 43. The cell lines each transcribed mRNAs of all three connexins, as determined by transcriptional elongation analysis. By contrast, only certain of the connexin mRNAs could be detected in specific cell lines by Northern analysis: RLC expressed only connexin 43 mRNA; WB-F344 expressed connexin 32 and 43 mRNAs. Selection among the connexin mRNAs appears to occur post-transcriptionally. Culture of the cell lines in hormonally defined medium vs. serum supplemented medium did not affect the patterns of connexin mRNA abundance. When the cell lines were cultured in hormonally defined medium containing heparin, however, the level of connexin mRNAs did vary: Connexin 26 mRNA increased in WB-F344 cells, and connexins 32 and 43 mRNAs increased in FTO.2B, but connexin 43 mRNA decreased in WB-F344 and RLC. The abundance of connexin mRNAs also varied when the cell lines were analyzed at different cell densities: connexin 43 mRNA increased with cell density in RLC and WB-F344, and connexin 26 mRNA peaked at an intermediate density and fell at higher cell densities in WB-F344. The differences in connexin mRNA expression among cell lines characteristic of different stages of hepatic differentiation, and the differences in regulation of connexin mRNAs in the hepatic cell lines, suggest distinct biological roles of the highly homologous proteins. Moreover, connexin gene expression may be a marker of hepatic development: as hepatocytes differentiate the proportions of connexin 43 then 26 mRNAs decrease while that of connexin 32 mRNA increases.

Factors affecting bovine corneal endothelial cell density in vitro

AIMS

To examine factors influencing the density and contact inhibition of bovine corneal endothelial cells cultured in vitro.

METHODS

Cell counts were performed on bovine corneal endothelial cells cultured for various times in the presence of 10% fetal calf serum, with or without varying concentrations of growth factors, 5% dextran T-500, or 2% chondroitin sulphate, at 32 degrees C or 37 degrees C, and after treatment with beta galactosidase.

RESULTS

Both basic fibroblast growth factor (FGFb) and retinal crude extract (RCE), but neither epidermal growth factor (EGF) nor acidic fibroblast growth factor (FGFa), increased endothelial cell density in vitro (p < 0.05). Continuous exposure to RCE resulted in a higher cell density than did a 24 hour pulse (p < 0.01), and higher cell densities were achieved at 37 degrees C than at 32 degrees C (p < 0.0001). In the absence of RCE, dextran T-500 increased cell density modestly (p < 0.05); in the presence of RCE, the addition of dextran T-500 had no effect on final cell density, whereas chondroitin sulphate significantly decreased final cell density (p < 0.01). In the absence of exogenous growth factors, beta galactosidase treatment resulted in a 50% increase in final cell density compared with controls (p < 0.0001).

CONCLUSIONS

Bovine corneal endothelial cell growth can be augmented under conditions different from those used in corneal preservation systems. The final cell density in a confluent monolayer can be increased by treatment with beta galactosidase, suggesting that corneal endothelial cells may be contact inhibited through a beta galactosidase sensitive receptor system.

The effect of partial hepatectomy on tumor growth in rats: in vivo and in vitro studies

Residual tumor in the remnant liver after partial hepatectomy (PH) for colorectal liver metastases is a serious clinical problem. This fact is reflected by the high number of recurrences after potentially curative liver resections. Liver regeneration, it appears, might influence the growth of remaining micrometastases in the liver. Using rats, we demonstrated enhancement of growth of a syngeneic colon carcinoma (CC 531) in the remnant liver after 70% PH. Fourteen days after PH, tumor weights in the liver were twice as high as those of sham-operated rats. This difference in tumor weight was not found in extrahepatic tumors. In vitro experiments did not show stimulation of cultured CC 531 cells by portal or systemic serum withdrawn 24 hours or 14 days after hepatectomy as compared with sera obtained after sham operation. Co-cultures of CC 531 cells and hepatocytes (in ratios of 1:10 or 1:1) demonstrated a higher 3H-thymidine incorporation than was the case in separately cultured cells. In co-cultures, bromodeoxyuridine (BrdU) incorporation in DNA was found primarily in CC 531 cells and rarely in hepatocytes. Cell density appeared to be of influence on 3H-thymidine incorporation in co-cultures. Hepatocytes were found to have a stimulating effect on CC 531 cells in low-density cultures, whereas high-density cultures exhibited an inhibiting effect after a culture time of 120 hours. These results show that, depending on cell density in co-cultures, a paracrine stimulating influence of hepatocytes on this type of colon carcinoma cells (CC 531) might be responsible for the increased tumor growth in vivo.

Reciprocal regulation of prothrombin secretion and tyrosine aminotransferase induction in hepatocytes

Multicellular spheroids of hepatocytes are known to maintain liver functions for a long period. Rat hepatocytes were isolated to form spheroids by rotation culture and immobilized within calcium alginate. Immobilized spheroids had a much higher extent of tyrosine aminotransferase induction, which is one of the liver-specific differentiated functions, than immobilized non-aggregated cells, while the spheroids secreted significantly less prothrombin than non-aggregated cells. Co-culture of hepatocytes and non-parenchymal liver cells in a monolayer enhanced tyrosine aminotransferase induction and suppressed prothrombin secretion, while conditioned medium prepared from non-parenchymal cells greatly stimulated tyrosine aminotransferase induction and suppressed the prothrombin secretion and DNA synthesis in monolayer-cultured hepatocytes. Prothrombin secretion in hepatocytes was subjected to cell-density-dependent regulation. In a similar manner to other growth-related functions, prothrombin secretion was stimulated at low cell density. It has been reported that thrombin activates the zymogen of hepatocyte growth factor activator [Shimomura, T., Kondo, J., Ochiai, M., Naka, D., Miyazawa, K., Morimoto, Y. & Kitamura, N. (1993) J. Biol. Chem. 268, 22,927-22,932]. Therefore, prothrombin secretion could be one of the growth-related functions and involved in wound healing and liver regeneration.

Receptor-mediated regulation of differentiation and proliferation of hepatocytes by synthetic polymer model of asialoglycoprotein

Morphology and responses of hepatocytes are investigated using an artificial asialoglycoprotein model polymer--lactose-carrying polystyrene (PVLA) as a culture substratum, especially in focusing on the effect of the surface density of the PVLA substratum. The surface density of PVLA on polystyrene dishes was determined using fluorescein-labeled PVLA as a probe under a fluorescence laser microscope. PVLA-coated surfaces were observed by scanning electron microscope and atomic force microscopies under air and water, which showed that PVLA molecules were adsorbed patchily on low density surfaces and uniformly concentrated all over the dish on high density surfaces. It is suggested from the requirement of the Ca2+ ion, inhibition of galactosyl substances, and localization of receptors that the adhesion of hepatocytes to both low and high PVLA-density surfaces is mediated by galactose-specific interactions between PVLA and asialoglycoprotein receptors. At low PVLA densities (0.07 micrograms cm-2), the hepatocytes were flat and expressed high levels of 3H-thymidine uptake and low levels of bile acid secretion. Contrastingly, at high PVLA densities (1.08 micrograms cm-2), they were round and expressed a low level of 3H-thymidine uptake and a high level of bile acid secretion. The shapes, proliferation, and differentiation of hepatocytes could be regulated by varying the densities of PVLA adsorbed to polystyrene dishes. We assume that there are two recognition mechanisms operating between PVLA and hepatocytes: (1) adhesion through highly concentrated or clustered galactose-specific interaction; and (2) responses in shape, proliferation, and differentiation by PVLA-coating densities.

TGF beta 1 suppresses EGF-induced increase in nuclear type 1 protein phosphatase activity at the G1/S transition of hepatocyte proliferation

Nuclear type 1 protein phosphatase (PP1) activity in primary culture of EGF-stimulated hepatocytes was significantly and transiently increased at the G1/S transition, being about 2.5-fold, while that in non-stimulated hepatocytes showed almost no change. On the other hand, non-nuclear PP1 activity was gradually increased until the G1/S transition, but the activity showed no difference between EGF-stimulated and non-stimulated hepatocytes. Under growth-inhibited conditions in the presence of TGF beta 1, the increase in nuclear PP1 activity was completely suppressed, whereas non-nuclear PP1 activity was little affected. Such close correlation between nuclear PP1 activity and growth factor-induced positive or negative growth signaling strongly suggests an involvement of PP1 in progression from G1 to S phase of hepatocytes. On Western immunoblotting using antisera for PP1 alpha, PP1 gamma 1, and PP1 delta, no isoform showed any change in amount under these conditions. Mechanism(s) of growth-associated alterations in nuclear PP1 activity is discussed.

Regulation of Mel-CAM/MUC18 expression on melanocytes of different stages of tumor progression by normal keratinocytes

The cell-cell adhesion receptor, Mel-CAM/MUC18, is highly expressed on metastatic melanoma cells and is also detectable on primary melanomas but not on normal melanocytes. Previous studies have shown that increased Mel-CAM/MUC18 expression correlates with tumor thickness and metastatic potential. We show here that normal melanocytes and nevus cells in culture express Mel-CAM/MUC18, but expression is down-regulated when cells are co-cultured with keratinocytes. Such keratinocyte-mediated regulation of Mel-CAM/MUC18 expression on melanocytes, nevus cells, and early melanomas can also be demonstrated in situ in patients' specimens. On the other hand, melanoma cells from primary and metastatic lesions constitutively express Mel-CAM/MUC18, and keratinocytes have no modulatory effect. These results suggest that contact between keratinocytes and human melanocytic cells modulates Mel-CAM/MUC18 expression, raising the possibility that escape from keratinocyte control during melanoma development leads to expression of antigens that contribute to the malignant phenotype.

Switching from alpha 1- to beta-subtypes in adrenergic response during primary culture of adult-rat hepatocytes as affected by the cell-to-cell interaction through plasma membranes

The alpha 1-adrenergic response was predominant over the beta-adrenergic one in adult rat hepatocytes, when the responses were measured as the agonist-induced generations of Ins(1,4,5)P3 and cyclic AMP, respectively. During primary culture of the adult rat hepatocytes, the beta-adrenergic response developed rapidly, whereas the alpha 1-response decreased gradually. Such receptor-subtype switching did not occur unless the cells were cultured under conditions favourable for cell growth, i.e. at low cell density (10(4) cells/cm2). The switching was prevented progressively as the cell culture density was increased up to 20-fold or the low-density culture was achieved by addition of increasing amounts of liver plasma membranes. The gradual decrease in alpha 1-response was accounted for by a concurrent decrease in the receptor site density, whereas rapid development of the beta-response definitely preceded the increase in beta-ligand binding sites during the culture. This rapid development of the beta-response reflected enhanced coupling of the receptor to G-protein during the early stage of culture, as evidenced by the progressively developed ability of GTP to lower the affinity of beta-agonist binding to membranes prepared from these short-time-cultured hepatocytes.

Collagenase pretreatment and the mitogenic effects of hepatocyte growth factor and transforming growth factor-alpha in adult rat liver

Hepatocyte growth factor and transforming growth factor-alpha are two well-known hepatomitogens for primary hepatocyte cultures. Here we report that these two growth factors also stimulate in vivo DNA syntheses in normal, unoperated, adult rat liver after 24-hr continuous intraportal infusion. As determined by an immunohistochemical staining technique, 5-bromo-2'-deoxyuridine incorporation was increased in a dose-dependent fashion after infusion of up to 10 micrograms of growth factor/100 gm body weight in the rat. Stimulation of DNA synthesis was seen in the periportal area. Pretreatment using intraportal infusion of collagenase (1 U/kg body weight) for 4 hr before administration of growth factor increased the labeling by 2- to 4-fold to a labeling index range of 48% to 52%. These results suggest that collagenases and possibly other proteases are involved in making hepatocytes competent to respond to growth factors at very early stages of liver regeneration.

Inhibition of glycogen synthesis by epidermal growth factor in hepatocytes. The role of cell density and pertussis toxin-sensitive GTP-binding proteins

Epidermal growth factor (EGF) counteracts the stimulation of glycogen synthesis by insulin in hepatocytes, but it is not known whether this is due to inhibition of glycogen synthesis or to inhibition of the insulin-signalling mechanism. This study investigates the mechanisms by which EGF affects the basal rate and the insulin stimulation of glycogen synthesis. The basal rate of glycogen synthesis is higher at low than at high cell density. EGF inhibits the basal rate of glycogen synthesis at low cell density but not in confluent cultures and abolishes the difference due to density. However, EGF inhibits the stimulation of glycogen synthesis by insulin irrespective of cell density. Increasing glycogen synthesis by increasing the [glucose] does not abolish the difference in rates of glycogen synthesis due to cell density, neither does it induce responsiveness to EGF at high cell density, establishing that responsiveness to EGF is a function of cell density and not of the basal rate and that inhibition of the insulin stimulation also cannot be accounted for by the higher rate of glycogen synthesis. Cytochalasin D and phalloidin, which alter cell morphology through interactions with the microfilament cytoskeleton, mimic the cell-density-dependent inhibition of glycogen synthesis by EGF. The inhibition of glycogen synthesis by EGF and cytochalasin D is additive and cytochalasin D potentiates the inhibition of glycogen synthesis by EGF, suggesting involvement of a cytoskeletal mechanism. Exogenous phospholipase C inhibits glycogen synthesis at both low and high cell density and the inhibition at low cell density is not additive with that caused by either EGF or cytochalasin D, suggesting that these agonists inhibit glycogen synthesis through changes in Ca2+ and/or diacylglycerol. The inhibition of glycogen synthesis by EGF in the absence of insulin stimulation is blocked by neomycin, which inhibits Ca2+ release from intracellular stores but not by antagonists of protein kinase C. It was also inhibited by pertussis toxin (50%), suggesting that it may involve GTP-binding-protein-mediated release of Ca2+ from intracellular stores. The inhibition of the stimulation of glycogen synthesis by insulin was not affected by neomycin and was only marginally inhibited by pertussis toxin or guanosine 5'-O-[3-thio]triphosphate (GTP[S]). We infer from these findings that the inhibition by EGF of the basal rate of glycogen synthesis and of the insulin stimulation are mediated by different mechanisms. The latter is pertussis toxin insensitive and independent of cell density, whereas the former is expressed only at low cell density, it is potentiated by cytochalasin D and inhibited by pertussis toxin.

Suppression of albumin enhancer activity by H-ras and AP-1 in hepatocyte cell lines

We demonstrated, using a transient transfection assay, that the albumin enhancer increased the expression of the albumin promoter in a highly differentiated, simian virus 40 (SV40)-immortalized hepatocyte cell line, CWSV1, but was not functional in two ras-transformed cell lines (NR3 and NR4) derived from CWSV1 by stable transfection with the T24ras oncogene. A transient cotransfection assay showed that T24ras and normal c-Ha-ras were each able to inhibit the activity of the albumin enhancer in an immortal hepatocyte cell line. DNase I footprinting and gel mobility shift assays demonstrated that the DNA binding activities specific to the albumin enhancer were not decreased in the ras-transformed cells. ras also did not diminish the expression of HNF1 alpha, C/EBP alpha, HNF3 alpha, HNF3 beta, or HNF3 gamma but did significantly increase AP-1 binding activity. Three AP-1 binding sites were identified within the albumin enhancer, and DNA binding activities specific to these AP-1 sites were induced in the ras-transformed hepatocytes. Subsequent functional assays showed that overexpression of c-jun and c-fos inhibited the activity of the albumin enhancer. Site-directed mutagenesis of the AP-1 binding sites in the albumin enhancer partially abrogated the suppressing effect of ras and c-jun/c-fos on the enhancer. These functional studies therefore supported the results of the structural studies with AP-1. We conclude that the activity of the albumin enhancer is subject to regulation by ras signaling pathways and that the effect of ras on the albumin enhancer activity may be mediated by AP-1.

Suppression of albumin enhancer activity by H-ras and AP-1 in hepatocyte cell lines

We demonstrated, using a transient transfection assay, that the albumin enhancer increased the expression of the albumin promoter in a highly differentiated, simian virus 40 (SV40)-immortalized hepatocyte cell line, CWSV1, but was not functional in two ras-transformed cell lines (NR3 and NR4) derived from CWSV1 by stable transfection with the T24ras oncogene. A transient cotransfection assay showed that T24ras and normal c-Ha-ras were each able to inhibit the activity of the albumin enhancer in an immortal hepatocyte cell line. DNase I footprinting and gel mobility shift assays demonstrated that the DNA binding activities specific to the albumin enhancer were not decreased in the ras-transformed cells. ras also did not diminish the expression of HNF1 alpha, C/EBP alpha, HNF3 alpha, HNF3 beta, or HNF3 gamma but did significantly increase AP-1 binding activity. Three AP-1 binding sites were identified within the albumin enhancer, and DNA binding activities specific to these AP-1 sites were induced in the ras-transformed hepatocytes. Subsequent functional assays showed that overexpression of c-jun and c-fos inhibited the activity of the albumin enhancer. Site-directed mutagenesis of the AP-1 binding sites in the albumin enhancer partially abrogated the suppressing effect of ras and c-jun/c-fos on the enhancer. These functional studies therefore supported the results of the structural studies with AP-1. We conclude that the activity of the albumin enhancer is subject to regulation by ras signaling pathways and that the effect of ras on the albumin enhancer activity may be mediated by AP-1.

Gap junctional intercellular communication of cultured rat liver parenchymal cells is stabilized by epithelial cells and their isolated plasma membranes

The gap junctional intercellular communication (GJIC) determined by measuring dye coupling with Lucifer yellow, decreased within 3 d from 66% to 28% in monocultures of rat liver parenchymal cells. Coculturing of the parenchymal cells with a nonparenchymal epithelial cell line from rat liver resulted in increased and stabilized intercellular communication (83% after 3 d). The presence of isolated plasma membrane vesicles of the nonparenchymal epithelial cells also stabilized the intercellular communication between the liver parenchymal cells (70% after 3 d). When liver parenchymal cells were cocultured with a rat liver fibroblast cell line the gap junctional communication between the parenchymal cells was not stabilized (43% after 3 d), and isolated plasma membrane vesicles of the fibroblast were also unable to support the GJIC in parenchymal cells (35% after 3 d). It is concluded that plasma membrane constituents of the nonparenchymal epithelial cells were responsible for the stabilization of the GJIC between parenchymal cells. A heterotypic gap junctional communication between parenchymal and nonparenchymal cells was not observed.

Changes in DNA strand breaks in non parenchymal cells following hepatocyte regeneration in CCl4-induced rat liver injury

DNA strand breaks (nicks) in non-parenchymal cells (NPCs) in CCl4-induced acute or chronic liver injury in rats were detected using an in situ nick translation method; their dynamic changes were analysed in relation to the proliferation pattern of hepatocytes and NPCs, as revealed by bromodeoxyuridine (BrdU)-uptake. In acute injury, hepatocyte proliferation started before centrilobular necrosis had occurred, whereas BrdU-labeled sinusoidal NPCs markedly increased only after centrilobular necrosis was apparent. DNA breakages in NPCs paralleled the proliferation pattern of these cells, suggesting that nicks are physiological, and reflect proliferation and activated gene expression. In chronic injury, liver cirrhosis developed after 9 weeks, but BrdU-labeling of hepatocytes was almost the same level as that in untreated liver. The number of BrdU-labeled NPCs showed only a slight increase, while those with DNA breakages were much more frequent in the cirrhotic stage, suggesting a significant role for NPCs in the fibrotic process. These results indicate that DNA strand breaks in NPCs act as a marker for activation states such as proliferation, differentiation and/or activated gene expression.

Redifferentiation of proliferated rat hepatocytes cultured in L15 medium supplemented with EGF and DMSO

Primary adult rat hepatocytes were cultured in serum-free L15 medium supplemented with 20 mM NaHCO3 and 10 ng/ml epidermal growth factor in a 5% CO2:95% air incubator. The number of cells increased and reached about 180% of the initial value by Day 4, and after 2% dimethyl sulfoxide (DMSO) was added to the culture medium at Day 4, the cells continued to proliferate until Day 6. The number of cells reached about 210% at Day 6 and they were well maintained until Day 18. The cell number gradually decreased with time in culture, but many cells remained for more than 2 mo. On the other hand, without 2% DMSO, the cells proliferated until Day 5, but thereafter they rapidly decreased. After DMSO addition, albumin and transferrin were secreted into the medium and the production of both proteins continued for more than 2 mo. Immunocytochemically both proteins were strongly stained in the cells treated with 2% DMSO. Although the expression of G6Pase in the cells disappeared at Day 6 without DMSO, the cells treated with 2% DMSO recovered G6Pase activity at Day 16. In addition, induction of peroxisomes by 2 mM sodium clofibric acid was clearly shown in the hepatocytes at Day 14 and Day 25 using enzyme-cytochemistry. Ultrastructurally, DMSO-treated hepatocytes had many mitochondria and large peroxisomes with a crystalline nucleoid, and both gap junctions and desmosomes were well developed between the cells even at Day 40. Thus, the number of cells doubled, some differentiated functions of the primary hepatocytes were well restored by the use of 2% DMSO, and these functions were maintained for more than 2 mo.