Growth control of differentiated fetal rat hepatocytes in primary monolayer culture. IX. Specific inhibition of DNA synthesis initiation by very low density lipoprotein and possible significance to the problem of liver regeneration

N-Acetyl-2-Aminofluorene (AAF) Processing in Adult Rat Hepatocytes in Primary Culture Occurs by High-Affinity Low-Velocity and Low-Affinity High-Velocity AAF Metabolite-Forming Systems

N-acetyl-2-aminofluorene (AAF) is a procarcinogen used widely in physiological investigations of chemical hepatocarcinogenesis. Its metabolic pathways have been described extensively, yet little is known about its biochemical processing, growth cycle expression, and pharmacological properties inside living hepatocytes-the principal cellular targets of this hepatocarcinogen. In this report, primary monolayer adult rat hepatocyte cultures and high specific-activity [ring G-3 H]-N-acetyl-2-aminofluorene were used to extend previous observations of metabolic activation of AAF by highly differentiated, proliferation-competent hepatocytes in long-term cultures. AAF metabolism proceeded by zero-order kinetics. Hepatocytes processed significant amounts of procarcinogen (≈12 μg AAF/106 cells/day). Five ring-hydroxylated and one deacetylated species of AAF were secreted into the culture media. Extracellular metabolite levels varied during the growth cycle (days 0-13), but their rank quantitative order was time invariant: 5-OH-AAF > 7-OH-AAF > 3-OH-AAF > N-OH-AAF > aminofluorene (AF) > 1-OH-AAF. Lineweaver-Burk analyses revealed two principal classes of metabolism: System I (high-affinity and low-velocity), Km[APPARENT] = 1.64 × 10-7  M and VMAX[APPARENT] = 0.1 nmol/106 cells/day and System II (low-affinity and high-velocity), Km[APPARENT] = 3.25 × 10-5  M and VMAX[APPARENT] = 1000 nmol/106 cells/day. A third system of metabolism of AAF to AF, with Km[APPARENT] and VMAX[APPARENT] constants of 9.6 × 10-5  M and 4.7 nmol/106 cells/day, was also observed. Evidence provided in this report and its companion paper suggests selective roles and intracellular locations for System I- and System II-mediated AAF metabolite formation during hepatocarcinogenesis, although some of the molecules and mechanisms responsible for multi-system processing remain to be fully defined.

Proliferation‑inhibiting pathways in liver regeneration (Review)

Liver regeneration, an orchestrated process, is the primary compensatory mechanism following liver injury caused by various factors. The process of liver regeneration consists of three stages: Initiation, proliferation and termination. Proliferation‑promoting factors, which stimulate the recovery of mitosis in quiescent hepatocytes, are essential in the initiation and proliferation steps of liver regeneration. Proliferation‑promoting factors act as the 'motor' of liver regeneration, whereas proliferation inhibitors arrest cell proliferation when the remnant liver reaches a suitable size. Certain proliferation inhibitors are also expressed and activated in the first two steps of liver regeneration. Anti‑proliferation factors, acting as a 'brake', control the speed of proliferation and determine the terminal point of liver regeneration. Furthermore, anti‑proliferation factors function as a 'steering‑wheel', ensuring that the regeneration process proceeds in the right direction by preventing proliferation in the wrong direction, as occurs in oncogenesis. Therefore, proliferation inhibitors to ensure safe and stable liver regeneration are as important as proliferation‑promoting factors. Cytokines, including transforming growth factor‑β and interleukin‑1, and tumor suppressor genes, including p53 and p21, are important members of the proliferation inhibitor family in liver regeneration. Certain anti‑proliferation factors are involved in the process of gene expression and protein modification. The suppression of liver regeneration led by metabolism, hormone activity and pathological performance have been reviewed previously. However, less is known regarding the proliferation inhibitors of liver regeneration and further investigations are required. Detailed information regarding the majority of known anti‑proliferation signaling pathways also remains fragmented. The present review aimed to understand the signalling pathways that inhbit proliferation in the process of liver regeneration.

Hormonal factors concerned with liver regeneration

Hepatic regeneration in partially hepatectomized, eviscerated rats, and survival in mice infected with lethal doses of murine hepatitis virus, are both strikingly promoted by combined administration of insulin and glucagon. These two hormones, although potent promotors, fail as initiators of hepatocyte proliferation in animals with intact liver, which suggests a requirement for additional factors, probably derived from non-portal-splanchnic organs. We now find that continous intraperitoneal infusion of epidermal growth factor (EGF) initiates DNA synthesis, as determined by incorporation of [3H] thymidine, in livers of adult rats in vivo. The rise in DNA labelling, which is small with EGF alone, is augmented by addition to the infusion of either glucagon or insulin. This is in agreement with reports on adult hepatocytes in culture. Whether EGF has a physiological role in regulating liver growth under normal conditions in vivo remains to be determined.

Proliferation of hepatocytes

Hepatocyte proliferation may be controlled by reversible patterns of endocrine changes, monitored by the liver, involving known hormones and their receptors. A two-programme model of related interactions among nutrients, specific lipoproteins, and highly phosphorylated nucleotides is postulated. This hypothesis stems from in vitro studies of rat hepatocyte proliferation under chemically defined conditions and from in vivo studies using partially hepatectomized, hormone-infused, developing and lipotrope-deficient rats. Certain findings are discussed with regard to receptor systems which show negatively cooperative properties; to problems of proliferative specificity; and to novel approaches for defined studies of chemical hepatocarcinogenesis.

Hepatotrophic effects of pancreatic and gastrointestinal hormones in the rat in vivo and in vitro

The role of gastrointestinal and pancreatic hormones in regulating liver growth was evaluated by measuring their effect on DNA synthesis in the normal and regenerating liver of rats in vivo and in maintenance cultures of adult rat hepatocytes in vitro. After partial liver resection DNA synthesis reached peak levels after 24 hours while serum concentrations of immunoreactive insulin in portal and peripheral blood at this time were still suppressed. Increase of endogenous insulin levels by intravenous glucose infusion or portal infusion of insulin, glucagon or both together with glucose did not change DNA synthesis in normal or regenerating rat liver. After acute carbon tetrachloride poisoning of rats, survival rate and degree of liver necrosis was not changed by intraperitoneal infusion of glucagon and insulin with glucose. In vitro, insulin, glucagon and somatostatin synergistically stimulated the specific thymidine uptake in seven-day-old maintenance cultures of rat hepatocytes. The hormones did not cause cell multiplication but enhanced cell survival, probably by improving the uptake and utilization of nutrients. Gastrin G-17, secretin and cholecystokinin (contaminated with gastric inhibitory polypeptide) had no effect. It is concluded that the results do not support the contention that liver regeneration is regulated by the known pancreatic hormones. However, a trophic effect of pancreatic hormones on liver cells in vitro could be demonstrated. Gastrointestinal hormones had no such effect.

Activin A: an autocrine inhibitor of initiation of DNA synthesis in rat hepatocytes

The present study was conducted to examine the effect of activin A on growth of rat hepatocytes. EGF induced a 10-fold increase in DNA synthesis as assessed by [3H]thymidine incorporation in cultured hepatocytes. When activin A was added together with EGF, DNA synthesis induced by EGF was markedly inhibited. Inhibition was detected at a concentration of 10(-10) M, and 5 x 10(-9) M activin A almost completely blocked EGF-mediated DNA synthesis. Similarly, activin A completely blocked DNA synthesis induced by hepatocyte growth factor/scatter factor. Activin A was capable of inhibiting EGF-mediated DNA synthesis, even when added 36 h after the addition of EGF. With the same time interval, TGF-beta also blocked EGF-induced DNA synthesis. Although both activin A and TGF-beta inhibited growth of hepatocytes in a similar manner, either activin A or TGF-beta did not compete with each other in their binding when assessed by competitive binding using an iodinated ligand. When hepatocytes were incubated with EGF, release of bioactivity of activin A into culture medium was detected after 48 h or later. Activity of activin A was released from parenchymal cells but not from nonparenchymal cells. mRNA for beta A subunit of activin was detected only slightly in unstimulated hepatocytes, but markedly increased at 48 h after the addition of EGF. To determine whether endogenously produced activin A affects DNA synthesis, we examined the effect of follistatin, an activin-binding protein that blocks the action of activin A. An addition of follistatin significantly enhanced EGF-induced DNA synthesis. Finally, in partial hepatectomized rat, expression of mRNA for beta A subunit in liver was markedly increased 24 h after the partial hepatectomy. These results indicate that activin A inhibits initiation of DNA synthesis in hepatocytes by acting on its own receptor and that activin A acts as an autocrine inhibitor of DNA synthesis in rat hepatocytes.

Activin A: an autocrine inhibitor of initiation of DNA synthesis in rat hepatocytes

The present study was conducted to examine the effect of activin A on growth of rat hepatocytes. EGF induced a 10-fold increase in DNA synthesis as assessed by [3H]thymidine incorporation in cultured hepatocytes. When activin A was added together with EGF, DNA synthesis induced by EGF was markedly inhibited. Inhibition was detected at a concentration of 10(-10) M, and 5 x 10(-9) M activin A almost completely blocked EGF-mediated DNA synthesis. Similarly, activin A completely blocked DNA synthesis induced by hepatocyte growth factor/scatter factor. Activin A was capable of inhibiting EGF-mediated DNA synthesis, even when added 36 h after the addition of EGF. With the same time interval, TGF-beta also blocked EGF-induced DNA synthesis. Although both activin A and TGF-beta inhibited growth of hepatocytes in a similar manner, either activin A or TGF-beta did not compete with each other in their binding when assessed by competitive binding using an iodinated ligand. When hepatocytes were incubated with EGF, release of bioactivity of activin A into culture medium was detected after 48 h or later. Activity of activin A was released from parenchymal cells but not from nonparenchymal cells. mRNA for beta A subunit of activin was detected only slightly in unstimulated hepatocytes, but markedly increased at 48 h after the addition of EGF. To determine whether endogenously produced activin A affects DNA synthesis, we examined the effect of follistatin, an activin-binding protein that blocks the action of activin A. An addition of follistatin significantly enhanced EGF-induced DNA synthesis. Finally, in partial hepatectomized rat, expression of mRNA for beta A subunit in liver was markedly increased 24 h after the partial hepatectomy. These results indicate that activin A inhibits initiation of DNA synthesis in hepatocytes by acting on its own receptor and that activin A acts as an autocrine inhibitor of DNA synthesis in rat hepatocytes.

Glucocorticoid and thyroid hormones inhibit proliferation of serum-free mouse embryo (SFME) cells

Mouse embryo cells derived in a serum-free medium formulation (SFME cells) do not exhibit growth crisis or chromosomal abnormalities and are nontumorigenic in vivo; these cells are also reversibly growth inhibited by serum or platelet-free plasma (Loo et al.; Science, 236:200-202, 1987). A portion of the inhibitory activity of serum could be extracted by charcoal, a procedure that removes steroid and thyroid hormones. Both L-3,5,3'-triiodothyronine (T3) and hydrocortisone inhibited growth of SFME cells in a reversible manner. The inhibitory activity of serum also was partially removed by treatment with anion exchange resin in a procedure designed to deplete serum of thyroid hormone. However, the effect of serum on untransformed SFME cells could not be prevented by addition of the antiglucocorticoid RU38486, and ras-transformed clones of SFME cells, which are capable of growing in serum-containing medium, retained inhibitory responses to glucocorticoid and, with some clonal variability, to T3. These results suggest that glucocorticoid or thyroid hormones may contribute to the inhibitory activity of serum on SFME cells, but additional factors are also involved.

Growth-inhibitory protein present in rabbit serum, which is more effective on tumorigenic rat liver epithelial cells than on non-tumorigenic ones: its species, and mode of existence

We have previously reported that in culture, rabbit serum inhibits the growth of the epithelial cell line from Buffalo rat liver (BRL) lower than that of the tumorigenic one transformed by Rous sarcoma virus (RSV-BRL). Here, the serum was fractionated by several different methods. The findings are: 1) the growth inhibitor present (GI) existed as large complexes with non-inhibitory proteins; 2) the complexes were dissociated by 1 M NaCl plus 6 M urea; 3) the dissociated GI did not pass through membrane filter with Mr cutoff 10k; 4) it was stable in 8.5 M urea and 1 M acetic acid (pH 2.5), but labile against either dithiothreitol and trypsin; 5) it was separable into two species with pI 7.5 and 9.5; 6) both species were more effective on RSV-BRL than on BRL.

Toxicity of oxidized low-density lipoprotein to cultured fibroblasts is selective for S phase of the cell cycle

Oxidized LDL (o-LDL) is toxic to a variety of cultured cells. Preliminary results suggested that susceptibility is enhanced by cell proliferation. As a step toward determining the mechanism of cytotoxicity, we chose to identify the cell cycle phase(s) during which exposure of cultured human fibroblasts to o-LDL leads to death. Cytochalasin B, which blocks cell migration and proliferation, and irradiation, which prevents mitosis but not migration, both blocked cytotoxicity. Colchicine, which arrests cells in mitosis but does not inhibit DNA synthesis, did not block cytotoxicity. Treatment of cells with hydroxyurea, which blocks cells prior to S phase, prevented cell death. Addition of o-LDL to cells immediately after S phase allowed mitosis without death. The above results coupled with results using cells synchronized by three different means indicate that cell death is selective for proliferating cells and occurs after exposure to o-LDL during S phase. Understanding the mechanism of o-LDL-induced death may have implications for tissue damage in vivo in the numerous instances of pathology in which oxidized lipoproteins or lipids are present.

Inhibitory activity of the serum from patients with fulminant hepatitis against liver regeneration

The effect of sera from 8 patients with fulminant hepatitis, including 2 survival cases, on DNA and protein synthesis in primary cultured rat hepatocytes was studied. The serum from patients at an early stage or within 10 days after onset tended to intensify DNA synthesis in isolated hepatocytes, whereas the serum from patients with a history of over 50 days distinctly inhibited synthesis. When the serum was fractionated by gel filtration or free-flow electrophoresis, only the albumin fraction inhibited DNA synthesis in cultured hepatocytes. The suppressive effect of the albumin fraction was demonstrated even in patients suffering for only a short period of time. The inhibitory activity against DNA and protein synthesis in cultured hepatocytes was demonstrated in a substance extracted with a chloroform and methanol mixture from the albumin fraction of patients with fulminant hepatitis. The extract from the patients' sera also inhibited acceleration of DNA synthesis by epidermal growth factor (EGF) in the same cells.

Characterization of a growth-inhibiting protein present in rat serum that exerts a differential effect on in vitro growth of nonmalignant rat liver cells when compared with Rous sarcoma virus-transformed rat liver cells

We have previously reported the transformation by Rous sarcoma virus of a cloned epithelial cell line (BRL) established from Buffalo rat liver by H. Coon. The nontransformed (BRL) and transformed (RSV-BRL) cells grew at comparable rates in culture, whereas only the transformed cells were tumorigenic in vivo. We report here on the existence in rat and mouse sera of a growth inhibitor for the nontransformed BRL cells. The transformed BRL cells (RSV-BRL) were insensitive to this inhibitor. The inhibitory activity was not prominent in sera from other species of animals tested except for rabbit; this serum inhibited the growth of RSV-BRL cells more strongly than that of BRL cells. The growth inhibitor was partially purified from rat serum. It is a protein free of lipid and has a molecular weight of about 220 000. The inhibitor could be separated into three components of pI 4.6, 5.2 (major) and 5.6 by isoelectric electrophoresis.

Purification of a growth inhibitor for Ehrlich ascites mammary carcinoma cells from bovine mammary gland

A growth inhibitor for Ehrlich ascites mammary carcinoma cells in vitro has been purified from bovine mammary gland. The purification procedure involving homogenization and differential centrifugation under hypotonic conditions, ammonium sulfate precipitation, ultrafiltration, gel chromatography and preparative polyacrylamide gel electrophoresis (PAGE) yielded an inhibitor showing half-maximal inhibition of cell proliferation in concentrations of 1-3 ng protein per ml. Upon 125I labelling and analysis by SDS gel electrophoresis, most purified preparations revealed a single band of 12-14 kD, likely to be representative for the inhibitory protein. The inhibitor was shown to affect resumption of proliferation of stationary cells; however, it was inactive towards cells stimulated by incubation with medium before adding the inhibitor. The inhibitor is heat-labile, does not act by exhausting essential components of culture medium, and its action is antagonized by insulin.

Alternative hypotheses for the role of promotion in chemical carcinogenesis

A new protocol for carcinogenesis in rat liver is described in order that confirmatory experiments might be undertaken concurrently. The basic protocol, designated IPI (initiator + promoter + initiator), is presented in several alternative forms, including the possible use of X-irradiation as the initiator. The rationale is discussed in terms of the two-hit somatic mutation theory of Armitage and Doll, with an initial hit produced by the first dose of initiator and expansion of single cells to sizable clones by promotion thereby increasing the probability of a second hit by the second dose of initiator. The question of relevant mutations was taken up and it was proposed that genes for chalones (C) and for chalone receptors (R) are logical targets for consideration in a two-mutation sequence. Alternative hypotheses pertaining to promoter action were described in terms of possible mechanisms by which nonelectrophilic promoters might simulate a second mutation by increasing or decreasing the levels of a nonchromosomal replicating particle in target cells.

Reversible inhibition by human serum lipoproteins of cell proliferation

Normal human serum or plasma was studied for the presence of inhibitors of cell proliferation by assaying inhibition of incorporation of labeled thymidine into acid-insoluble fraction using human FL cells. Lipoprotein fraction obtained by gel filtration through Sepharose 4B and by KBr density gradient centrifugation was found to play a major part of the inhibitory activity of the serum. It was also shown that the inhibitory activity resides in low-density lipoprotein (LDL). The addition of the lipoprotein fraction to growing FL cells caused an early decrease in the transport of uridine and thymidine across the membrane. This change in the permeability of membrane was followed by the preferential inhibition of DNA synthesis and a reduction in the percentage of mitotic cells in the cell population. The inhibition of the growth was reversible and was observed in various types of cells irrespective of species.

Stimulation of DNA synthesis in primary cultures of adult rat hepatocytes by rat platelet-associated substance(s)

Experiments in whole animals have shown that normally quiescent adult rat hepatocytes are induced to proliferate by blood borne substances, which we are now probing in primary monolayer cultures. Under our conditions, freshly isolated adult hepatocytes do not proliferate actively in a defined medium, but are stimulated to synthesize DNA--an essential first step--by either serum or an EGF-hormone combination. Stimulation of [3H]thymidine incorporation into hepatocyte DNA by addition of dialyzed mouse, human, horse, or bovine (fetal, newborn, or calf) serum, whose activities are all similar, is regularly surpassed by an EGF-insulin mixture without serum. This, in turn, is exceeded by dialyzed normal rat serum, which is several times more potent than the other sera tested. Removal of blood platelets reduces the activity of normal rat serum by over 50%. Heat inactivation (56 degrees C) causes a similar loss, but heat treatment of platelet-poor serum fails to cause further reduction. The activity of mouse and human serum is not reduced by platelet removal. Serum from partially hepatectomized rats is not significantly more stimulatory than normal rat serum, and its activity is depressed in the same way by platelet deprivation and heat inactivation. Lack of enhancement by partial hepatectomy is not consonant with whole animal studies and requires further investigation. The heat-labile portion of the DNA synthesis-stimulating activity of rat serum appears to derive from platelets. This activity differs from the well-characterized heat-stable human PDGF. Its relation to other reported platelet-associated growth factors is still undetermined.

Long-term culture of epithelial cells from the normal rat colon

Serial passage cultures of colonic epithelial cells from young rats have been maintained for more than 6 months in Eagle's minimum essential medium buffered with HEPES (25 mM) and supplemented with 2.5% fetal bovine serum, 0.5 micrograms/ml insulin, 5.0 micrograms/ml transferrin, and antibiotics. The cells proliferated in this medium with a population doubling time of approximately 53 h. The cells retained differentiated morphology as evidenced by secretory activity and the presence of secretory granules, microvilli, tonofilaments, and desmosomal junctions. Further, cells at the fourth passage had normal karyotypes with 42 chromosomes and exhibited anchorage dependent growth. High concentrations of fetal bovine serum (10 to 15%) exerted toxic effects on the colonic epithelial cell cultures.

A selective inhibitor of cell proliferation from normal serum

A factor in normal serum that selectively and reversibly inhibits proliferation of cells in culture has been enriched 160-fold from calf serum by sequential ammonium sulfate precipitation, gel filtration, and lectin-affinity chromatography. DNA synthesis of normal (but not transformed) rat hepatocytes, human lymphoblast lines, and mitogen-stimulated murine spleen cells is inhibited by greater than 90%, and Vero, murine myeloma, MELC, and a human colon carcinoma cell line to a lesser extent. Growth of other cell lines tested was not affected. Responsive cells are arrested apparently in G1 by this inhibitor, the effect of which is maximal by 24 hr and is spontaneously reversible thereafter unless it is renewed. The active fraction is a protein that migrates with the alpha 2-globulins; it is not a lipoprotein, and it is of high apparent molecular weight.

Increased sodium ion influx is necessary to initiate rat hepatocyte proliferation

Serum-free media containing 10-50 ng insulin, glucagon and epidermal growth factor (EGF) ml-1 stimulate adult rat hepatocyte proliferation in 10-15 day old primary liver cell cultures. The kinetics of this response simulate hepatocellular transitions that accompnay liver regeneration after 67% hepatectomy. Amiloride, a Na+ influx inhibitor, reversibly blocks these transitions in vitro (ID50 approximately 0.02 mM) and in vivo (ID50 approximately 25 mg kg-1). Inhibition is observed with other cation flux modulators, including ouabain (ID50 approximately 0.2 mM), 0.2 microM monensin and 0.2 microM nigericin, but not with 0.3 mM furosemide or tetrodotoxin. The prereplicative interval in culture (0-12 hr) is characterized by preferential cellular responsiveness to EGF (0-3 hr) followed by insulin plus glucagon (3-12 hr). Parallel culture and animal studies show that the amiloride-sensitive and prereplicative intervals coincide. In culture, a "burst" of 22Na+ influx, stimulated by peptide-supplemented media within 1 min but decreased later at 12 hr, is retarded by amiloride. This drug also blocks delayed prereplicative events involving increased amino acid "A" transport system function at 4-8 hr, and 3H-uridine and 3H-leucine incorporation into RNA and protein, respectively, at 8-12 hr. These findings suggest that at least two time-ordered processes are necessary to initiate hepatic growth fully: first, activation of Na+ flux systems by peptides similar or identical to EGF; and second, potentiation of these and subsequent cellular events by the combined action of insulin plus glucagon. [Amiloride: N-amidino-3,5-diamino-6-chloropyrazinecarboxamide; furosemide: 4-chloro-N-furfuryl-5-sulfamoylanthranilic acid; AIB: alpha-aminoisobutyric acid; ID50: administered dose giving 50% inhibition of a maximal response; dFBS: dialyzed fetal bovine serum; L.I.: 3H-dT nuclear labeling index.]

Inhibition of lymphocyte proliferation stimulated by lectins and allogeneic cells by normal plasma lipoproteins

Lipoproteins, isolated by sequential flotation at densities 1.006, 1.019, 1.063, and 1.21, were examined for their ability to inhibit human lymphocytes stimulated by allogeneic cells and by lectins (phytohemagglutinin-P and concanavalin A). All the classes of normal plasma lipoproteins inhibited lymphoproliferation when peripheral blood lymphocytes were cultured in autologous, heterologous, or lipoprotein-deficient plasma (d greater than 1.21). The rank order of inhibitory potency was intermediate density lipoprotein (IDL) greater than very low density lipoproteins (VLDL) greater than low density lipoproteins (LDL) greater than high density lipoproteins (HDL), regardless of the mode of stimulation. The concentrations of IDL, VLDL, and LDL required for complete inhibition of stimulated lymphoproliferation were considerably below the levels of each of these lipoproteins normally found in human plasma. In addition, the concentration of HDL required for 50-90% inhibition was in the range of HDL levels normally found in human plasma. Moreover, at relatively higher concentrations, lipoproteins suppressed the incorporation of [3H]thymidine into DNA below the levels seen with reseting, unstimulated lymphocytes. The results suggest that circulating lymphocytes may normally be highly suppressed by the combined effects of all the endogenous lipoproteins and that the lipoproteins may play important roles in vivo in modulating lymphocyte functions and responses.