The stimulation by epidermal growth factor (urogastrone) of the growth of neonatal rat hepatocytes in primary tissue culture and its modulation by serum and associated pancreatic hormones

Nerve Growth Factor-Laden Anisotropic Silk Nanofiber Hydrogels to Regulate Neuronal/Astroglial Differentiation for Scarless Spinal Cord Repair

Scarless spinal cord regeneration remains a challenge due to the complicated microenvironment at lesion sites. In this study, the nerve growth factor (NGF) was immobilized in silk protein nanofiber hydrogels with hierarchical anisotropic microstructures to fabricate bioactive systems that provide multiple physical and biological cues to address spinal cord injury (SCI). The NGF maintained bioactivity inside the hydrogels and regulated the neuronal/astroglial differentiation of neural stem cells. The aligned microstructures facilitated the migration and orientation of cells, which further stimulated angiogenesis and neuron extensions both in vitro and in vivo. In a severe rat long-span hemisection SCI model, these hydrogel matrices reduced scar formation and achieved the scarless repair of the spinal cord and effective recovery of motor functions. Histological analysis confirmed the directional regenerated neuronal tissues, with a similar morphology to that of the normal spinal cord. The in vitro and in vivo results showed promising utility for these NGF-laden silk hydrogels for spinal cord regeneration while also demonstrating the feasibility of cell-free bioactive matrices with multiple cues to regulate endogenous cell responses.

EGFR Signaling in Liver Diseases

The epidermal growth factor receptor (EGFR) is a transmembrane receptor tyrosine kinase that is activated by several ligands leading to the activation of diverse signaling pathways controlling mainly proliferation, differentiation, and survival. The EGFR signaling axis has been shown to play a key role during liver regeneration following acute and chronic liver damage, as well as in cirrhosis and hepatocellular carcinoma (HCC) highlighting the importance of the EGFR in the development of liver diseases. Despite the frequent overexpression of EGFR in human HCC, clinical studies with EGFR inhibitors have so far shown only modest results. Interestingly, a recent study has shown that in human HCC and in mouse HCC models the EGFR is upregulated in liver macrophages where it plays a tumor-promoting function. Thus, the role of EGFR in liver diseases appears to be more complex than what anticipated. Further studies are needed to improve the molecular understanding of the cell-specific signaling pathways that control disease development and progression to be able to develop better therapies targeting major components of the EGFR signaling network in selected cell types. In this review, we compiled the current knowledge of EGFR signaling in different models of liver damage and diseases, mainly derived from the analysis of HCC cell lines and genetically engineered mouse models (GEMMs).

EGFR: A Master Piece in G1/S Phase Transition of Liver Regeneration

Unraveling the molecular clues of liver proliferation has become conceivable thanks to the model of two-third hepatectomy. The synchronicity and the well-scheduled aspect of this process allow scientists to slowly decipher this mystery. During this phenomenon, quiescent hepatocytes of the remnant lobes are able to reenter into the cell cycle initiating the G1-S progression synchronously before completing the cell cycle. The major role played by this step of the cell cycle has been emphasized by loss-of-function studies showing a delay or a lack of coordination in the hepatocytes G1-S progression. Two growth factor receptors, c-Met and EGFR, tightly drive this transition. Due to the level of complexity surrounding EGFR signaling, involving numerous ligands, highly controlled regulations and multiple downstream pathways, we chose to focus on the EGFR pathway for this paper. We will first describe the EGFR pathway in its integrity and then address its essential role in the G1/S phase transition for hepatocyte proliferation. Recently, other levels of control have been discovered to monitor this pathway, which will lead us to discuss regulations of the EGFR pathway and highlight the potential effect of misregulations in pathologies.

Growth factor- and cytokine-driven pathways governing liver stemness and differentiation

Liver is unique in its capacity to regenerate in response to injury or tissue loss. Hepatocytes and other liver cells are able to proliferate and repopulate the liver. However, when this response is impaired, the contribution of hepatic progenitors becomes very relevant. Here, we present an update of recent studies on growth factors and cytokine-driven intracellular pathways that govern liver stem/progenitor cell expansion and differentiation, and the relevance of these signals in liver development, regeneration and carcinogenesis. Tyrosine kinase receptor signaling, in particular, c-Met, epidermal growth factor receptors or fibroblast growth factor receptors, contribute to proliferation, survival and differentiation of liver stem/progenitor cells. Different evidence suggests a dual role for the transforming growth factor (TGF)-β signaling pathway in liver stemness and differentiation. On the one hand, TGF-β mediates progression of differentiation from a progenitor stage, but on the other hand, it contributes to the expansion of liver stem cells. Hedgehog family ligands are necessary to promote hepatoblast proliferation but need to be shut off to permit subsequent hepatoblast differentiation. In the same line, the Wnt family and β-catenin/T-cell factor pathway is clearly involved in the maintenance of liver stemness phenotype, and its repression is necessary for liver differentiation during development. Collectively, data indicate that liver stem/progenitor cells follow their own rules and regulations. The same signals that are essential for their activation, expansion and differentiation are good candidates to contribute, under adequate conditions, to the paradigm of transformation from a pro-regenerative to a pro-tumorigenic role. From a clinical perspective, this is a fundamental issue for liver stem/progenitor cell-based therapies.

Mechanisms of hepatic regeneration following portal vein embolization and partial hepatectomy: a review

BACKGROUND

Portal vein embolization (PVE) improves outcome following major hepatectomy, and basic studies have presented evidence related to the mechanisms responsible for hepatic regeneration. Hemodynamic changes following PVE are similar to, but slightly different from, those of partial hepatectomy (PH) because arterial flow to the embolized lobe is preserved. However, the process of hepatic regeneration is essentially the same after both PVE and PH. A number of mediators are involved in PVE or PH-induced hepatic regeneration. These include inflammatory cytokines, vasoregulators, growth factors, eicosanoids, and various hormones. These mediators activate a complex network of signal transduction that promotes hepatic regeneration. A variety of conditions have been shown to modulate the function of these mediators and inhibit regeneration. These include biliary obstruction, diabetes, chronic ethanol consumption, malnutrition, gender, aging, and infection.

CONCLUSION

Optimizing these factors, where possible, before PVE or PH, is essential to maximize hypertrophy of the liver. A fuller understanding of hepatic physiology and pathophysiology following PVE or PH may lead to greater functional capacity of the remaining liver and extend the indications for hepatectomy in patients who require large liver volume resection.

Mechanism of impaired hepatic regeneration in cholestatic liver

The regenerative capacity of the liver is an important factor following liver surgery. The dramatic change in portal venous flow, due to either portal vein embolization or partial hepatectomy, induces a rapid change in liver volume. In response to these stresses, hepatocytes are primed, through the release of inflammatory cytokines, to increase the expression of immediate early genes and increase the activation of transcriptional factors. The primed hepatocytes then respond to growth factors, including hepatocyte growth factor, epidermal growth factor, and transforming growth factor-alpha. Several pathologic conditions have been shown to inhibit hepatic regeneration. These include diabetes mellitus, malnutrition, aging, infection, chronic ethanol consumption, and biliary obstruction. Impaired hepatic regeneration in the setting of biliary obstruction is an especially serious problem because it can be a major determinant in not considering surgical treatment. The mechanism responsible for impaired hepatic regeneration in patients with biliary obstruction includes decreased portal venous flow, attenuated production of liver proliferation-associated factors, an increased rate of apoptosis, and lack of enterohepatic circulation. Restoring these factors may lead to an improvement in regeneration in a cholestatic liver following portal vein embolization or partial hepatectomy. This review article summarizes the current understanding of the mechanism of hepatic regeneration, with particular emphasis on that in the cholestatic liver.

Differential Ca2+ signaling in neonatal and adult rat hepatocyte doublets

BACKGROUND/AIMS

Intracellular Ca2+ ([Ca2+]i) is important in various cellular functions, including cellular proliferation and differentiation. To elucidate the relationship between [Ca2+]i oscillations and physiological hepatocyte proliferation, phenylephrine-evoked [Ca2+]i responses were sequentially investigated using short-term cultured hepatocyte doublets obtained from 1-, 3-, 6- and 8-week-old rats.

METHODS/RESULTS

DNA synthesis in hepatocytes, determined by BrdU incorporation, was approximately 20% in 1-week-old rats, and decreased to <1% as the rats aged. Correspondingly, [Ca2+]i responses evoked by 10 micromol/l phenylephrine in hepatocyte doublets shifted from transient to sinusoidal-type [Ca2+]i oscillations and then to a sustained increase in [Ca2+]i, followed by a gradual return to baseline. The incidence of [Ca2+]i oscillations was 100+/-0.0%, 83.3+/-16.7%, 38.7+/-0.6% and 5.5+/-5.0% in 1-, 3-, 6- and 8-week-old rats, respectively. Removal of extracellular Ca2+ did not abolish [Ca2+]i oscillations, indicating that [Ca2+]i oscillations were caused primarily by Ca2+ mobilization from internal sites of the cells. The [Ca2+]i level in each of the adjacent cells was synchronous in sustained increase in [Ca2+]i, but asynchronous in [Ca2+]i oscillations. In proliferating doublets obtained from 1-week-old rats, the frequency of oscillations increased in a dose-dependent manner for phenylephrine concentrations of 1 to 100 micromol/l.

CONCLUSIONS

Phenylephrine-evoked [Ca2+]i oscillations were directly related to hepatocyte proliferation and were mediated by frequency modulation. These results suggest that phenylephrine-evoked [Ca2+]i oscillations may contribute to cell-cycle progression of hepatocytes in physiological liver growth.

Inhibition of DNA synthesis in primary rat hepatocyte cultures by malachite green: a new liver tumor promoter

Malachite green (MG), a triarylmethane dye is highly cytotoxic to mammalian cells and also acts as a liver tumour-enhancing agent. In view of its industrial importance and possible exposure to individuals, MG may pose a potential environmental health hazard. The mechanism(s) by which MG enhances DEN-induced liver carcinogenesis is still unknown even though growth modulatory effects appear to be important in its action. Therefore, we have studied the effect of MG on DNA synthesis in primary cultures of normal adult rat hepatocytes maintained under fully defined conditions. The rate of DNA synthesis in both untreated and hepatocytes treated with epidermal growth factor (10 ng/ml) were inhibited by MG at concentrations of 0.025-0.4 micrograms/ml. These inhibitory effects were concomitant with an extensive release of lactate dehydrogenase which began after 24 h. MG inhibited DNA synthesis when added after only 16 h in hepatocytes either primed or not primed with EGF, suggesting that the target site may be other than the EGF-receptor or EGF-mediated early events involving signal transduction. The present study indicates that cytotoxic and mitoinhibitory properties of MG possibly play an important role during tumour promotion.

Extracellular calcium modulates prereplicative cyclic AMP surges in EGF-stimulated primary neonatal rat hepatocytes

The cells in nearly pure (96-98%) primary cultures of hepatocytes from neonatal rat liver in high (1.0 mM)-Ca2+, serum-free, synthetic HiWo5Ba2000 medium initiated DNA synthesis and entered mitosis between 11 and 30 h after the addition of 10 ng/ml EGF. During the 10-h prereplicative period, the cultured hepatocytes, like regenerating rat liver cells, generated two large cyclic AMP transients, one peaking between 30 min and 2 h and the other around 6 h. Hepatocytes stimulated by the same concentration of EGF in low (0.02 mM)-Ca2+ medium increased cyclic AMP synthesis as much as the EGF-treated hepatocytes in high-Ca2+ medium, but they released the additional cyclic AMP into the medium and could not generate prereplicative internal cyclic AMP surges, initiate DNA replication, or enter mitosis. These results suggest that one of the ways external Ca2+ controls prereplicative development of hepatocytes is to restrain the release of cyclic AMP and thus enable the cell to accumulate enough internal cyclic AMP to stimulate events required to initiate DNA replication.

An assessment of the in vitro hepatocyte micronucleus assay

The in vitro hepatocyte micronucleus assay was tested for its practicability and its usefulness in detecting mutagens. The assay protocol developed by Alati et al. (1989) was shown to give reproducible levels of proliferating hepatocytes and the formation of micronuclei could be readily assessed by fluorescence microscopy. Epidermal growth factor and insulin were used as mitogens, yielding mitotic indices of 2.4 +/- 0.74% after 72 h of culture. The high number of 8.0 +/- 3.33% micronucleated hepatocytes in control cultures at that time, typically for in vitro stimulated hepatocytes, is probably due to disordered mitoses frequently leading to chromosome loss. The direct acting mutagen N-methyl-N'-nitro-N-nitrosoguanidine and the clastogens cyclophosphamide and retrorsine, which require metabolic activation, induced dose dependent increases in the frequencies of micronucleated hepatocytes. The carcinogen 2-AAF also yielded significantly enhanced rates of micronuclei. The non-mutagen KCl as well as the peroxisome proliferator clofibrate, which is considered to be a non-genotoxic hepatocarcinogen, yielded consistently negative results. Problems occurred when chemicals exerting strong cytotoxic effects were tested in this assay. The mutagen and hepatocarcinogen aflatoxin B1 did not enhance the number of micronucleated hepatocytes. Rather a reduction of micronuclei and of mitoses was observed at AFB1 concentrations considered positive in other genotoxicity assays. Hepatocyte proliferation seems to be highly susceptible to the cytotoxic action of chemicals. A decrease in the proliferating activity of hepatocytes can obviously prevent the detection of mutagenic effects. Further studies on the in vitro hepatocyte micronucleus assay are necessary to clarify its role in mutagenicity testing.

Studies on the growth-stimulatory activity of pigeon milk--comparison and synergistic effects with serum

Pigeon milk, a nutritive secretion from the crop of breeding pigeons, was tested (on v/v basis) for growth factor activity either separately or in combination with other growth supplements. Synthesis of DNA in confluent monolayers of quiescent Chinese hamster ovary cells was enhanced by the homogenates of pigeon milk in the presence of both fetal bovine serum and bovine serum albumin, although the response with fetal bovine serum was greater than that with bovine serum albumin. The in vitro growth stimulation by pigeon milk was also reflected in the increase in cell number. Specific activity of pigeon milk growth factor, measured against both Chinese hamster ovary cells and mouse embryo fibroblasts, was found to be higher than that of fetal calf serum, fetal bovine serum, and goat, horse, pig and human serum. The growth-stimulatory property of pigeon milk did not change in the first 5 days of its secretion.

Transforming growth factor alpha in developing rats

Transforming growth factor-alpha (TGF-alpha) concentrations were measured in lung, brain, liver, and kidney of rats at three different ages (20 days gestation and 9 and 50 days postnatal). TGF-alpha concentrations were maximal in the lung and brain by 20 days of gestation and showed minimal changes during nursing (day 9) and young adulthood (day 50). The liver, which also showed maximal TGF-alpha concentration by 20 days of gestation, demonstrated a progressive reduction with age to nadir values in the young adult. In contrast to the pattern in other tissues, kidney had the lowest concentration of TGF-alpha in late gestation and showed an increase by 50 days of age. As TGF-alpha acts via the epidermal growth factor (EGF) receptor, its function in development may be analogous to that of EGF. Thus TGF-alpha may have a role in lung maturation and postinjury repair, liver repair and regeneration, and neuronal cell growth.

Arrest of epidermal growth factor-dependent growth in fetal hepatocytes after ethanol exposure

Exposure of the fetal rat hepatocyte to ethanol in vitro blocks epidermal growth factor (EGF)-dependent cell replication. To define possible mechanisms for this growth arrest, we determined the effects of ethanol on EGF binding and EGF receptor (EGF-R) levels. During a 24-h exposure to ethanol (1.7 mg/ml, 31 mM), cell replication was completely blocked while EGF binding per cell doubled. This effect was no specific for EGF, with variable degrees of increased binding noted for insulin, transferrin, and glucagon. Significantly increased EGF binding was seen after 6 h of ethanol exposure, and both growth arrest and enhanced EGF binding were reversed within 12 h of ethanol withdrawal. Increases in both "high" and "low" affinity sites were seen, with no changes in the apparent Kd's. Total RNA, beta-actin mRNA, and EGF-R mRNA were increased 50-70% in ethanol exposed cells. However, direct measurements of EGF-R synthesis rates by [35S]methionine incorporation revealed no differences between control and ethanol exposed cells. Internalization of EGF-R was significantly altered by ethanol exposure. A 2-h incubation resulted in the internalization of 57% of the ligand in control cells, while only 31% of bound EGF was internalized in the ethanol exposed cells. Thus, the enhanced EGF binding may be due to decreased efficiency of internalization.

Decrease in the number of receptors for epidermal growth factor in the liver of D-galactosamine-intoxicated rats

Hepatic transport of epidermal growth factor (EGF) was studied in D-galactosamine-intoxicated rats by the multiple-indicator dilution (MID) method. The extraction ratio of 125I-labeled EGF in the intoxicated rats, obtained from a model-independent analysis of the dilution curves, decreased to 45% of the control values. A distributed two-compartment model was fitted to the dilution data by nonlinear least-squares regression, and the kinetic parameters, kon.PT (product of on-rate constant and receptor density), koff (off-rate constant) and ks (sequestration rate constant) were determined. The values of kon.PT and ks in the intoxicated rats decreased to approximately one-half and one-third of those in the control rats respectively. Similar decreases in the kon.PT and ks values in the intoxicated rats were also observed for the transport of 125I-labeled insulin, a positive control, into the liver. The 125I-labeled EGF binding experiment at equilibrium using liver homogenates revealed that the intoxication reduced the receptor density (PT) to one-third of the control values, whereas the equilibrium dissociation constant (kd) did not change significantly. The activities of Na+,K+-ATPase, cytochrome P-450 and glutathione S-transferase decreased in the intoxicated rats to 70-80% of the control values. The number of nuclei per unit area of tissue slices was also reduced to 70% of the control. Thus, the extent to which the enzyme activities and the number of nuclei decreased in the intoxicated liver was smaller than that of the number of EGF receptors. It is concluded that the reduction of EGF receptors cannot be explained by the "intact hepatocyte hypothesis" but rather by the functional change of hepatocytes induced by the administration of D-galactosamine.

Proliferation of fetal rat hepatocytes in response to growth factors and hormones in primary culture

Fetal rat hepatocytes (day 19 of gestation) multiply in primary culture in arginine-free, hydrocortisone-containing chemically defined medium MX-82 supplemented either with epidermal growth factor (EGF) or insulin or both. In contrast, hepatocytes did not multiply under similar culture conditions using Dulbecco's minimum essential medium (DMEM). Cells underwent two divisions within 10 days in cultures maintained in MX-82 medium without a medium change, and cells grew to increased final cell densities when the medium was renewed every third day. When the medium MX-82 was enriched by the addition of lipids, intermediary metabolites, and trace metals (medium MX-83), cells grew to higher densities. In the absence of the growth factors, cells became quiescent and subsequently could be induced to synthesize DNA in response to EGF. With the increasing numbers of cells per dish, the growth response of the hepatocytes diminished. Levels of hepatocyte-specific albumin and alpha-fetoprotein mRNAs at day 0 were similar to those observed at day 10 in primary fetal rat hepatocyte cultures and were maintained at higher levels in medium MX-83 than in medium MX-82.

Distribution of i.v. administered epidermal growth factor in the rat

The distribution of i.v. injected 125I-labeled epidermal growth factor (EGF) was examined in the rat. The uptake of radioactivity was examined for the following tissues: liver, kidney, skin, stomach, small intestine, colon, brain, submandibular gland, lung, spleen, and testis. 125I-EGF was cleared from the circulation within minutes. At 2.5 min after the injection only 7% of the label was left in the blood. Most of the label was found in the liver (52%), the kidneys (14%), the small intestine (11%) and the skin (7%). The other organs examined contained 1% or less of the radioactivity. The uptake of 125I-EGF per g tissue was markedly higher for the liver and kidneys than for the rest of the organs. By autoradiography 125I-EGF was found in the peripheral parts of the classical liver lobule, in the proximal tubules of the kidneys, in the surface epithelium of the stomach, and in the surface epithelium of the villi in the small intestine. In conclusion the present study showed that small doses of homologous EGF was cleared from the circulation of rats within minutes, mainly by the liver, the kidneys, and the small intestine.

Ontogenesis of epidermal growth factor in liver of BALB mice

To characterize the ontogenesis of hepatic epidermal growth factor (EGF) metabolism in normal BALB mice, we measured serum and liver concentrations of EGF and liver concentrations of pre-pro EGF mRNA. Female and male animals were studied at 1, 2, 5, 7, and 10 wk of life. After death, body weight and length were measured, and serum and liver tissues were collected for EGF determinations. Immunoreactive serum EGF (means +/- SE) increased at 7 and 10 wk and was significantly higher (P less than 0.05) in males (465 +/- 58 and 683 +/- 120 pg/ml) than females (188 +/- 52 and 295 +/- 64 pg/ml). Liver EGF concentrations were low at 1, 2, and 5 wk, significantly increasing (P less than 0.01) at 10 wk to 179 +/- 36 vs. 268 +/- 49 pg/mg protein for females and males, respectively (female and male values were significantly different, P less than 0.01). Pre-pro EGF mRNA was examined at 1, 2, 3, 5, 7 and 10 wk. EGF message increased in liver to highest values at 10 wk in both males and females. There was a high correlation between serum and liver EGF concentrations during the first 10 wk (r = 0.97 and 0.85 for males and females, respectively) and a twofold increase in liver EGF mRNA between 3 and 10 wk of postnatal life. These results suggest that liver may be an important source of circulating EGF in developing BALB mice.

Epidermal growth factor (urogastrone)-stimulated gluconeogenesis in isolated mouse hepatocytes

In freshly isolated mouse hepatocytes obtained from fasted animals, we have studied the receptors for epidermal growth factor urogastrone (EGF-URO) in terms of the electrophoretic profile, ligand affinity, and numbers of EGF-URO receptors present on the cells, and also in terms of the ability of EGF-URO to stimulate gluconeogenesis, as reflected by the increased incorporation of [3-14C]pyruvate into glucose. The effects of EGF-URO were compared with those of glucagon. Ligand-binding studies revealed that the mouse hepatocytes possess an unusually high number of EGF-URO receptors (about 3 X 10(6) binding sites/cell), with a ligand dissociation constant of 4.4 nM. The binding of EGF-URO by mouse hepatocytes was more than 10-fold higher than the previously measured binding of EGF-URO by rat hepatocytes. Crosslink-labeling studies, coupled with gel electrophoretic analysis, demonstrated the presence of intact EGF-URO receptors, although some receptor processing had occurred during the isolation procedure. EGF-URO was able to stimulate the incorporation of 3-14C-labeled pyruvate into glucose; glucagon was unable to do so. In contrast, in rat hepatocytes isolated and assayed under identical conditions, glucagon (10 nM) caused a marked (250%) stimulation of the incorporation of pyruvate into glucose. Maximally, EGF-URO caused a 34% increase in the incorporation of [3-14C]pyruvate into glucose; a half-maximal effect was observed at a concentration of 2.5 nM EGF-URO. The stimulatory effect of EGF-URO was not dependent on the concentration of pyruvate, lactate, glucose, or calcium in the incubation medium. Although raising the concentration of pyruvate in the incubation medium increased the incorporation of [3-14C]pyruvate into glycogen, EGF-URO did not cause any change in the incorporation of radioactivity into glycogen. Overall, our data point to marked differences between rat and mouse liver preparations, in terms of the hormonal regulation of glucose metabolism, and our work documents a potential role for the remarkably high number of mouse hepatocyte EGF-URO receptors in terms of the modulation of gluconeogenesis in the mouse.

Studies on the mechanisms by which tumor promoters stimulate the growth of primary neonatal rat hepatocytes

A single exposure to a low concentration (10(-10) mol/L) of several tumor promoters, namely 12-O-tetradecanoylphorbol-13-acetate (TPA), phenobarbital (PB), nafenopin, saccharin, teleocidin, benzoyl peroxide, butylated hydroxytoluene (BHT), dichlorodiphenyltrichloroethane (DDT), lindane, clofibrate, and melittin significantly stimulated DNA synthesis of neonatal rat hepatocytes in 4-day-old primary cultures. These cultures were kept in low-calcium (0.01 mmol/L) HiWoBa2000 synthetic medium, thereby evoking a neoplastic phenotype in otherwise normal (i.e., non-initiated) cells. The simultaneous addition of a single dose of alpha-tocopherol (10(-4) mol/L) or selenous acid (10(-5) mol/L), just as that of exogenous superoxide dismutase (SOD) (4), together with each of the above agents fully suppressed the stimulation of hepatocytic DNA synthesis by the xenobiotics. Hence, these findings strengthen the view that superoxide anions (or some other oxidizing compounds) act as the common mediators of the mitogenic effects of various tumor promoters in hepatocytes. Inhibition kinetics studies, in which TPA in a single dose (10(-10) mol/L) was used as the paradigmatic compound together with several kinds of inhibitors of its activity showed that the early mitogenic effects of TPA, i.e., the commitment of quiescent (G0) hepatocytes and the reentry into active cycling of hepatocytes spontaneously poised at the G1/S boundary, required oxidizing compounds, arachidonate metabolism derivatives, and plasmalemmal calcium-binding sites and transmembrane calcium fluxes. Instead, a later TPAs effect, the flow into DNA synthesis of hepatocytes previously committed to cycle, was shown to be controlled by retinoid-modulable activities, by some product(s) of the lipoxygenase pathway, and again by plasmalemmal calcium-binding sites and transmembrane calcium fluxes. Such results reveal that in the neonatal rat hepatocyte the ability to answer to a single mitogenic stimulus and the metabolic pathways by which this answer is enacted depend upon the mitotic cycle setting of the hepatocytes at the moment of the experimental treatment.

Glucagon enhances growth of cultured human colorectal cancer cells in vitro

Early passage in vitro cultures of colorectal adenocarcinoma cells were used to determine if glucagon exerts a direct effect on growth of human colon cancer cells. Growth response assays indicated that glucagon generally stimulated growth between 2 and 10 micrograms/ml, with peak responses at 5 to 10 micrograms/ml. When glucagon-treated and control cultures were compared, 12 of the 14 cultures (86 percent) were stimulated by glucagon, with an increase in cells from 41 to 100 percent. The other two cultures did not respond to glucagon. These in vitro results suggest that glucagon may enhance growth of most colon cancer cells. Further studies on responsiveness to glucagon may help elucidate mechanisms of oncogenesis and suggest new therapeutic protocols for patients with colorectal cancer.

Effect of calf and rat serum on the induction of DNA synthesis and mitosis in primary cultures of adult rat hepatocytes by cyproterone acetate and epidermal growth factor

Induction of hepatocyte DNA synthesis in culture by cyproterone acetate (CPA), a potent hepatomitogen in vivo, was studied. Adult rat hepatocytes were grown on collagen gels in primary culture for 3 to 10 d. Epidermal growth factor (EGF) was used as a model inducer to establish appropriate culture conditions. (a) In serum-free medium EGF stimulated a wave of DNA synthesis in 10 to 30% of the hepatocytes. CPA had only a weak effect. (b) Increasing concentrations of newborn bovine serum (NBS) at 5 to 95% progressively inhibited the stimulatory effect of EGF. A similar inhibition was obtained by adding bovine serum albumin; 20% NBS, however, had a slightly stimulatory effect on the induction of DNA synthesis by CPA. (c) Portal rat serum (RS) at concentration of 5 to 95% markedly stimulated DNA synthesis, a plateau being reached between 20 and 95%. EGF had a distinct enhancing effect on DNA synthesis in the presence of 5 and 20% RS but not at 50 and 95%. CPA stimulated DNA synthesis in the presence of 20, 50, and 95% RS in a synergistic way. (d) Mitoses were found after treatment with EGF or with CPA. These results show that CPA can induce DNA synthesis in cultured hepatocytes and that RS contains factors facilitating the response to CPA.

Differential responsiveness of cultured suckling and adult rat hepatocytes to growth-promoting factors: entry into S phase and mitosis

The capacity of suckling and adult rat hepatocytes in culture to enter into S phase and mitosis in response to EGF, insulin, and glucagon was measured. Both cell types were isolated in high yield and purity and cultured in the absence of serum under identical conditions. At the time of isolation, suckling rat hepatocytes were all diploid and in the G1 phase of the cell cycle. Adult rat hepatocytes constituted a population of mixed ploidy level, as shown by flow cytometry. Upon stimulation, both suckling and adult rate hepatocytes entered S phase after a minimum lag period of 24 h. For suckling rat hepatocytes EGF was required, but its stimulating action was dependent on insulin and/or glucagon. In contrast, adult rat hepatocytes entered into S phase in response to EGF alone; insulin and glucagon did not significantly potentiate its effect. Under optimal hormonal stimulation for entry into S phase a large proportion of suckling rat hepatocytes underwent mitosis, whereas only a few mitoses were observed in the case of adult rat hepatocytes. Therefore, there is a differential response of suckling and adult rat hepatocytes to growth factors which correlates with ploidy level, and this difference may be associated with the degree of maturation.

The tumor promoters TPA, phenobarbital, and nafenopin and the prostaglandins of A, E, and F series overcome the G1/S block imposed by extracellular calcium deprivation on neonatal rat hepatocytes

A single exposure to a low concentration (10(-9) mole/l) of exogenous arachidonic acid or of prostaglandins of A, E, and F series significantly stimulated primary neonatal rat hepatocytes to enter S phase irrespective of whether the extracellular calcium concentration was high (i.e., 1.8 mmole/l) or markedly reduced (i.e., 0.01 mmole/l). Similarly, a single treatment with an even smaller (10(-10) mole/l) dose of the known tumor promoters 12-O-tetradecanoylphorbol-13-acetate (TPA), phenobarbital, and nafenopin enhanced hepatocytic DNA synthesis when the environmental calcium level was both high and low. By contrast, a single application of a small concentration (10(-11)-10(-10) mole/l) of hormones such as epidermal growth factor (EGF), glucagon, and insulin, and of drugs such as imidazole and indomethacin only increased the hepatocytic flow into DNA synthesis when the extracellular calcium was high. These findings reveal that the mechanisms of physiological or pharmacological, calcium-dependent stimulation of hepatocellular growth are likely to be different from those of pathological, calcium-independent stimulation, as the latter, but not the former, would involve prostaglandin-mediated metabolic processes.

Prostaglandins of the F series are extremely powerful growth factors for primary neonatal rat hepatocytes

At low concentrations (i.e., 10(-12)-10(-9) mol/1), PGF1 alpha and PGF2 alpha very intensely stimulated both the DNA-synthetic and mitotic activities of hepatocytes in 4-day-old primary cultures of neonatal rat liver. DNA replication was more intensely enhanced by PGF2 alpha than by PGF1 alpha, whereas mitotic activity was nearly equally affected by the two prostaglandins. On the whole, the growth-promoting activity of PGF1 alpha used by itself or in equimolar mixtures with other prostaglandins (e.g., A1, E1, etc.) mimicked that of arachidonic acid we previously reported (1). On a molar basis, PGF2 alpha by itself stimulated hepatocytes' DNA synthesis more powerfully than arachidonate did, and when used in equimolar mixtures with other prostaglandins was at least as potent as arachidonic acid. These observations establish prostaglandins of the F series as quite powerful commitment factors and, though by a lesser degree, also intracycle regulators for neonatal rat hepatocytes in primary culture. However, the understanding of the role(s) of prostaglandins of F and other series in the physiological control of hepatocytes' proliferative activation must await the clarification of their interaction(s) with other arachidonate derivative(s) and polypeptide growth factor(s) which also may be involved in the process.

The calcium-dependence of the stimulation of neonatal rat hepatocyte DNA synthesis and division by epidermal growth factor, glucagon and insulin

A low concentration (10(-11) mol/l) of epidermal growth factor (EGF) and/or an equimolar (10(-14) mol/l) mixture of glucagon and insulin stimulated DNA synthesis in hepatocytes in 4-day-old primary cultures of neonatal rat liver. EGF seems to have acted by inducing quiescent hepatocytes to begin cycling, while the glucagon-insulin combination seems to have acted mainly by shortening the cell cycle time. Incubation in low calcium medium blocked untreated hepatocytes in the G1 phase of their cycle and prevented EGF and the glucagon-insulin mixture from stimulating DNA synthesis. Nevertheless, hepatocytes in calcium-deficient medium did respond to these agents, as they reached a late stage of prereplicative development before being blocked: in fact, they initiated DNA synthesis soon after the addition of calcium. EGF, but not the glucagon-insulin combination, also enabled the already cycling hepatocytes (but not the newly activated ones) to overcome the block imposed by the extracellular calcium deficiency after a delay of several hours.

Hepatic sequestration and biliary secretion of epidermal growth factor: evidence for a high-capacity uptake system

Epidermal growth factor (EGF) promotes hepatocyte growth and is bound in the liver by specific receptors. We have determined hepatic uptake of EGF in intact rats after an intravenous or intraportal injection of a bolus of 125I-labeled EGF. Ninety-nine percent of the intraportal dose was taken up by the liver in 3 min, whereas only 58% of the intravenous dose appeared in the liver in 10 min. Uptake was inhibited by simultaneous treatment with an excess of unlabeled EGF. At time zero, uptake appeared to be complete. Disappearance from the liver followed first-order kinetics. Within 90 min of an intraportal injection, an average of 19% of the injected radioisotope appeared in bile, of which approximately one-fifth was shown to be immunoprecipitable with a specific anti-EGF antiserum. Light microscopic autoradiography demonstrated a very steep portal-to-central lobular concentration gradient consistent with a high-capacity uptake system. After intraportal injection or after incubation with cultured hepatocytes, labeled EGF was shown to be bound to its hepatic receptors. The main receptor-ligand complex had a Mr of approximately equal to 160,000-170,000, determined by NaDodSO4/polyacrylamide gel electrophoresis.

Effects of growth factors on hormonal stimulation of amino acid transport in primary cultures of rat hepatocytes

In primary cultures of rat hepatocytes, epidermal growth factor (EGF), platelet-derived growth factor (PDGF) and foetal-calf serum (FCS) prevented the stimulation of amino acid transport by glucagon (cyclic AMP-dependent) and by catecholamines (cyclic AMP-independent), but not by insulin. The insulin effect, as well as the effect of other hormones, were totally inhibited by thrombin through a mechanism independent of its proteolytic activity. The inhibitory effect of growth factors, not found in freshly isolated hepatocytes, was expressed very early in culture (4h). Induction of tyrosine aminotransferase by glucagon or dexamethasone, which, like stimulation of transport, represents a late hormonal effect, was not affected by EGF, PDGF or FCS, but was inhibited by thrombin. In contrast, none of the rapid changes in protein phosphorylation caused by hormones was altered by growth factors. Thus the inhibition by growth factors of hormonal stimulation of transport presumably involves late step(s) in the cascade of events implicated in this hormonal effect.

Stimulation of embryonic mouse limb bud mesenchymal cell growth by peptide growth factors

The possible role of peptide growth factors in mammalian intrauterine cell growth has been investigated using primary cultures of undifferentiated mesenchymal cells from 11-day mouse embryo limb buds. When grown as monolayer cultures, proliferation is greatly favored by high cell densities. In medium containing 0.2% serum, purified epidermal growth factor (EGF), fibroblast growth factor (FGF), multiplication stimulating activity (MSA), insulin, and somatomedin-C (Sm-C) do not increase cell growth, but a 30-40,000 molecular weight component of mouse fetal liver conditioned medium is stimulatory. On the other hand, when limb bud cells are grown as high density or micromass cultures, a method which better approximates in vivo growth conditions, all of the purified growth factors tested stimulate cell growth significantly. These growth factors have additive effects when used in combination, the best stimulation being observed with liver medium (10% v/v), EGF (10 ng/ml), FGF (200 ng/ml), and either insulin (1 microgram/ml) or Sm-C (20 ng/ml). We conclude that the response of limb bud cells to growth stimulation is influenced by the manner in which the cells are cultured and that at least four different growth factors are required for optimal in vitro proliferation. One of these, the active component of liver medium, appears to be a previously uncharacterized growth factor.

The calcium-dependent stimulation of the proliferation of neonatal rat hepatocytes by imidazole and indomethacin

Low concentrations (e.g. 10(-12) and 10(-11) mol/l) of imidazole and indomethacin strongly stimulated DNA synthesis and mitosis of hepatocytes in 4-day-old primary cultures of neonatal rat liver. These agents seem to have acted by inducing quiescent hepatocytes to begin cycling rather than by affecting already cycling cells, because they did not shorten the total cell cycle time. Neither compound stimulated DNA synthesis by hepatocytes cultured in low (0.010 mol/l) calcium medium. Nevertheless, hepatocytes in calcium-deficient medium must have been mitogenically activated by these compounds and, hence, been able to reach a late stage of prereplicative development because they did initiate DNA synthesis very soon after the addition of calcium.