Proline is required for the stimulation of DNA synthesis in hepatocyte cultures by EGF

Epidermal growth factor (EGF) has been shown to stimulate DNA synthesis in rat parenchymal hepatocytes both in vivo in vitro. We report here that this response in vitro is dependent on the amino acids present in the media. Of all the amino acids, proline has the strongest effect. The response to EGF is absent without proline and none of the other amino acids can substitute for it. Added proline (1 mM) to the media caused the labeling index to increase from 11% to 55% in the presence of 50 ng/ml EGF and insulin. In the presence of proline, small additional increases of the EGF effect on DNA synthesis were stimulated by phenylalanine and tyrosine.

Inhibition of DNA synthesis by chemical carcinogens in cultures of initiated and normal proliferating rat hepatocytes

Rat hepatocytes in primary culture can be stimulated to replicate under the influence of rat serum and sparse plating conditions. Higher replication rates are induced by serum from two-thirds partially hepatectomized rats (Michalopoulos, G., Cianciulli, H. D., Novotny, A. R., Kligerman, A. D., Strom, S. C., and Jirtle, R. L. Cancer Res., 42: 4673-4682, 1982). The effects of carcinogens and noncarcinogens on the ability of hepatocytes to synthesize DNA were examined by measuring the incorporation of [3H]thymidine by liquid scintillation counting and autoradiography. Hepatocyte DNA synthesis was not decreased by ethanol or dimethyl sulfoxide at concentrations less than 0.5%. No effect was observed when 0.1 mM ketamine, Nembutal, hypoxanthine, sucrose, ascorbic acid, or benzo(e)pyrene was added to cultures of replicating hepatocytes. Estrogen, testosterone, tryptophan, and vitamin E inhibited DNA synthesis by approximately 50% at 0.1 mM, a concentration at which toxicity was noticeable. Several carcinogens requiring metabolic activation as well as the direct-acting carcinogen N-methyl-N'-nitro-N-nitrosoguanidine interfered with DNA synthesis. Aflatoxin B1 inhibited DNA synthesis by 50% (ID50) at concentrations between 1 X 10(-8) and 1 X 10(-7) M. The ID50 for 2-acetylaminofluorene was between 1 X 10(-7) and 1 X 10(-6) M. Benzo(a)pyrene and 3'-methyl-4-dimethylaminoazobenzene inhibited DNA synthesis 50% between 1 X 10(-5) and 1 X 10(-4) M. Diethylnitrosamine and dimethylnitrosamine (ID50 between 1 X 10(-4) and 5 X 10(-4) M) and 1- and 2-naphthylamine (ID50 between 1 X 10(-5) and 5 X 10(-4) M) caused inhibition of DNA synthesis at concentrations which overlapped with concentrations that caused measurable toxicity. The ability of hepatocytes to activate 2-acetylaminofluorene to reactive intermediates capable of binding to DNA and inhibiting new DNA synthesis decreased as a function of time in culture. gamma-Glutamyl-transferase-positive hepatocytes from diethylnitrosamine-treated rats were observed to be relatively insensitive to carcinogen inhibition of DNA synthesis.

Control of hepatocyte replication by two serum factors

Serum proteins from hepatectomized or control rats were separated by gel permeation chromatography and assayed for stimulation of hepatocyte proliferation in primary cultures of hepatocytes. Two peaks of activity were seen in the areas of large (greater than 120,000) and small (less than 3,000) molecular weight. These activities are different from insulin, epidermal growth factor, or vasopressin and are empirically termed hepatopoietin A and B, respectively. The two activities interact in a synergistic manner to stimulate hepatocyte proliferation at rates comparable to that of the whole serum.

Lack of genotoxic activity of di(2-ethylhexyl)phthalate (DEHP) in rat and human hepatocytes

Di(2-ethylhexyl)phthalate (DEHP) is a widely used plasticizer which has been reported to induce a statistically significant increase in the incidence of hepatocellular carcinomas in female Fischer-344 rats (8/50) when administered in the diet at 12 000 p.p.m. for two years. Numerous studies with cells in culture have failed to show any genotoxic activity associated with DEHP. Because DEHP induces multiple changes in the liver, such as peroxisomal proliferation, it was possible that these alterations could result in genotoxic effects in the treated whole animal that would not be seen in cells in culture. Accordingly, the ability of DEHP to induce DNA damage or repair was examined in rat hepatocytes in vivo and in vitro and in human hepatocytes in vitro. Unscheduled DNA synthesis was measured by incorporation of [3H]thymidine into primary hepatocyte cultures immediately isolated from treated animals or hepatocyte cultures incubated directly with DEHP. DNA damage was measured by alkaline elution of cellular DNA from the same cultures. In vivo-in vitro treatment regimens were: (i) female rats, 12 000 p.p.m. DEHP in the diet for 30 days; (ii) female rats, 12 000 p.p.m. in the diet for 30 days, followed by 500 mg/kg DEHP by gavage 2 h before sacrifice; (iii) male rats, 500 mg/kg DEHP by gavage 2, 12, 24, or 48 h before sacrifice; and (iv) male rats, 150 mg/kg/day by gavage for 14 days. In vitro conditions were 0.1, 1.0 and 10.0 mM DEHP in the cultures for 18 h. Primary cultures of human hepatocytes were prepared from freshly discarded surgical material and exposed to the same concentration of DEHP. Concentrations up to 0.5 mM mono(2-ethylhexyl)phthalate, a principal metabolite of DEHP, were also examined in the human hepatocyte assay. No chemically induced DNA damage or repair was observed in vivo or in vitro in rat or human hepatocytes under any of the conditions employed. However, an increase in the percentage of cells in S-phase in the animals given DEHP was observed. These data indicate that DEHP does not exhibit direct genotoxic activity in the animals even with a treatment regimen which eventually produced tumors in a long term bioassay, and that both rat and human hepatocytes are similar in their lack of a genotoxic response to DEHP exposure in culture.

Response of hepatocytes transplanted into syngeneic hosts and heterotransplanted into athymic nude mice to peroxisome proliferators

The development of a transplantation system by which rat hepatocytes can be implanted and remain viable in the dorsal fascia of two-thirds hepatectomized syngeneic hosts provides an opportunity to examine whether such transplanted hepatocytes retain the capacity to recognize and respond to the peroxisome proliferators 2-[4-(2,2- dichlorocyclopropyl )phenoxy]-2- methylpropionic acid (ciprofibrate), a hypolipidemic drug, and di-(2-ethylhexyl)phthalate (DEHP), an industrial plasticizer. Male F344 rats with transplanted rat hepatocytes were fed a control diet or a diet containing either 0.05% ciprofibrate (w/w) or 2% DEHP (v/w). After 4 weeks, the animals were sacrificed, and transplanted hepatocytes revealed a significant increase in the numerical density of peroxisomes in both ciprofibrate- and DEHP-fed rats. The volume density of peroxisomes in transplanted hepatocytes increased 9.2- and 5.3-fold, respectively, in ciprofibrate- and DEHP-fed rats, whereas the volume density of mitochondria remained essentially unchanged. The magnitude of increase in peroxisome volume density in transplanted hepatocytes was comparable to increases in the volume density of these organelles in the liver parenchymal cells of syngeneic hosts. The present study also demonstrates that hepatocytes isolated from cat liver and heterotransplanted into partially hepatectomized athymic nude mice retain their biological integrity and respond to the peroxisome proliferative effect of ciprofibrate. This observation suggests the possibility that hepatocytes obtained from small segments of liver of humans, primates, and other species and heterotransplanted into nude mice might provide a valuable model system for toxicological evaluation of chemicals. These studies suggest that hepatocytes, irrespective of their location in the body, recognize the peroxisome proliferator or its active metabolite(s), which stimulates the expression of peroxisome-specific genes in these cells.

Hepatocyte receptors for antithrombin III-proteinase complexes

The in vivo clearance of antithrombin III-proteinase complexes occurs via a specific and saturable pathway located on hepatocytes. We now report studies of the catabolism of antithrombin III-proteinase complexes in vitro using rat hepatocytes in primary culture. Antithrombin III-thrombin and trypsin complexes were prepared and purified to homogeneity. Ligand uptake by hepatocytes was concentration, temperature, and time dependent. Initial rate studies were performed to characterize the maximum rate of uptake, V, and apparent Michaelis constant Kapp. These studies yielded a V of 12.8 fmol/mg cell protein/min and a Kapp of 144 nM for antithrombin-trypsin complexes. Competition experiments with antithrombin III, antithrombin III-proteinase complexes, alpha 2-macroglobulin-methylamine, asialoorosomucoid and the neoglycoproteins, fucosyl-bovine serum albumin (BSA), N-acetylglucosaminyl-BSA, and mannosyl-BSA indicated that only antithrombin III-proteinase complexes were recognized by the hepatocyte receptor. Uptake studies were performed at 37 degrees C with 125I-antithrombin III-trypsin and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) in conjunction with autoradiography. These studies demonstrate time-dependent uptake and degradation of the ligand to low molecular weight peptides. In addition, there was a time-dependent accumulation of a high molecular weight complex of ligand and a cellular protein. This complex disappeared when gels were performed under reducing conditions.

The survival of parenchymal hepatocytes irradiated with low and high LET radiation

We have developed an in vivo clonogenic assay system for parenchymal hepatocytes which has enabled us to investigate the reproductive survival and repair capacity of liver cells exposed to genotoxic agents. In this report we present our results with hepatocytes irradiated 60Co and neutrons. The survival curve for parenchymal hepatocytes enzymatically dispersed 30 min after exposure to 60Co has a D0 value of 2.7 Gy and an extrapolation number insignificantly different from unity. However, when the hepatocytes were allowed to remain in situ for 24 h before being assayed for survival, the extrapolation number significantly increased to 2.3, whereas the D0 value remained unchanged. Therefore, normal parenchymal hepatocytes in G0 are able to repair potentially lethal damage (PLD) after exposure to 60Co and this repair phenomenon is expressed solely as an increase in the n value. In contrast, with hepatocytes exposed to 14.3 MeV neutrons, the survival curve is exponential with a D0 value of 1.7 Gy regardless of whether the cells remained in situ for 30 min or 24 h before the assessment of their reproductive survival. A comparison of the 60Co and neutron survival curves, where 24 h is allowed for PLD repair to occur, demonstrates that the RBE of neutrons for hepatocytes increases with decreasing dose and equals 4.2 at 50 cGy. The radioprotective agent WR-2721 was shown to act as a dose modifying agent with a DMF of 2.1, implying that it may be of potential clinical value as a radiation hepato-protective drug.

Hepatocyte uptake of alpha 1-proteinase inhibitor-trypsin complexes in vitro: evidence for a shared uptake mechanism for proteinase complexes of alpha 1-proteinase inhibitor and antithrombin III

In vivo clearance studies have indicated that the clearance of proteinase complexes of the homologous serine proteinase inhibitors alpha 1-proteinase inhibitor and antithrombin III occurs via a specific and saturable pathway located on hepatocytes. In vitro hepatocyte-uptake studies with antithrombin III-proteinase complexes confirmed the hepatocyte uptake and degradation of these complexes, and demonstrated the formation of a disulfide interchange product between the ligand and a cellular protein. We now report the results of in vitro hepatocyte uptake studies with alpha 1-proteinase inhibitor-trypsin complexes. Trypsin complexes of alpha 1-proteinase inhibitor were prepared and purified to homogeneity. Uptake of these complexes by hepatocytes was time and concentration-dependent. Competition experiments with alpha 1-proteinase inhibitor, alpha 1-proteinase inhibitor-trypsin, and antithrombin III-thrombin indicated that the proteinase complexes of these two inhibitors are recognized by the same uptake mechanism, whereas the native inhibitor is not. Uptake studies were performed at 37 degrees C with 125I-alpha 1-proteinase inhibitor-trypsin and analyzed by sodium dodecyl sulfate-gel electrophoresis in conjunction with autoradiography. These studies demonstrated time-dependent uptake and degradation of the ligand to low molecular weight peptides. In addition, there was a time-dependent accumulation of a high molecular weight complex of ligand and a cellular protein. This complex disappeared when gels were performed under reducing conditions. The sole cysteine residue in alpha 1-proteinase inhibitor was reduced and alkylated with iodoacetamide. Trypsin complexes of the modified inhibitor were prepared and purified to homogeneity. Uptake and degradation studies demonstrated no differences in the results obtained with this modified complex as compared to unmodified alpha 1-proteinase inhibitor-trypsin complex. In addition, the high molecular weight disulfide interchange product was still present on sodium dodecyl sulfate-polyacrylamide gel electrophoresis of solubilized cells. Clearance and clearance competition studies with alpha 1-proteinase inhibitor-trypsin, alkylated alpha 1-proteinase inhibitor-trypsin, antithrombin III-thrombin, and anti-thrombin III-factor IXa further demonstrated the shared hepatocyte uptake mechanism for all these complexes.

Induction of DNA repair in human and rat hepatocytes by 1,6-dinitropyrene

1,6-Dinitropyrene (DNP) was found to be an extremely potent genotoxicant in metabolically competent primary cultures of human and rat hepatocytes. Dose-dependent increases in DNA repair as measured by unscheduled DNA synthesis (UDS) were observed in the range from 0.05 to 5 microM 1,6-DNP for both species, indicating that the rat-hepatocyte assay is an appropriate model for assessing genotoxic potential in human hepatocytes for this class of compound. Unlike some nitroaromatic compounds, 1,6-DNP did not require gut flora for metabolic activation. No DNA repair was observed in hepatocytes isolated from rats treated with 50 mg/kg 1,6-DNP in corn oil by gavage 2, 12 or 24 h previously. The reason for the lack of a response in vivo is not known, but may relate to detoxification or distribution of the compound in the animal.

gamma-Glutamyl transpeptidase activity in benign and malignant human mammary epithelial lesions. Histochemical evaluation

gamma-Glutamyl transpeptidase (GGT) has been identified in many tissues. The enzyme is present in normal human serum and is accepted widely as an indicator of liver disease. gamma-Glutamyl transpeptidase has been characterized as a putative hepatocellular carcinoma marker in rats and humans and a similar role is suggested in carcinomas of other tissues, including breast, colon, and ovary. Using a histochemical technique to demonstrate GGT activity, we have defined characteristic staining patterns for 67 normal or benign, and 108 overtly malignant human breast lesions. Transitions between benign, atypical, and malignant tumors were observed as were differences between intraductal and infiltrating components of a tumor. Normal breast epithelium and nondysplastic proliferative lesions were characterized by concentrated apical and luminal staining with weak cytoplasmic activity. Malignant lesions exhibited diffuse cytoplasmic staining when staining was observed.

Genotoxic effects of 2-acetylaminofluorene on rat and human hepatocytes

Isolated rat and human hepatocytes in primary culture were shown to metabolize AAF to reactive intermediates which damaged hepatocyte DNA. A significant increase in unscheduled DNA synthesis was detectable by autoradiography in rat and human hepatocytes exposed to concentrations of AAF as low as 1 microM. When rat hepatocytes were plated over confluent monolayers of human fibroblasts and exposed to 3H-AAF, significant binding of AAF to the DNA of the fibroblasts as well as the hepatocytes was measured. In other experiments with hepatocyte-fibroblast cocultures, nonradioactive AAF, at concentrations greater than 40 microM, induced a significant increase in the HPRT- mutation frequency in the human fibroblasts. These results demonstrate that hepatocytes can be used to assess genotoxicity of carcinogenic compounds and are useful for interspecies comparisons in chemical carcinogenesis.

Establishment of two rat hepatoma cell strains produced by a carcinogen initiation, phenobarbital promotion protocol

Two Fischer 344 rat hepatoma cell strains, JM1 and JM2, have been isolated from a primary hepatocellular carcinoma. Primary tumor formation was induced in a two-thirds partially hepatectomized rat by a single low dose (70 mg/kg of diethylnitrosamine followed by chronic phenobarbital administration (0.1 g/100 ml drinking water). The primary tumors were passed three times by subcutaneous implantation of tumor fragments into the inguinal region of syngeneic recipients. The fourth pass was by injection of tumor cells directly into the livers of recipient rats. Several weeks later, the tumor-containing rat livers were subjected to collagenase perfusion. Two cell lines emerged from tissue culture of the cells isolated by perfusion. Each cell line was cloned by serial dilution. Cells JM1 and JM2 were tumorigenic when injected into syngeneic rats. The tumors, which arose from injected cell strains, exhibited several characteristics of hepatocellular carcinoma. Morphology was examined by light and electron microscopy. Histochemical studies of JM1 and JM2 cells grown in vitro and in vivo were done. The levels of tyrosine aminotransferase and three microsomal enzymes of importance to drug and carcinogen metabolism were investigated. To our knowledge, this is the first report of cell strains derived from an initiation promotion protocol in rats.

Human hepatocyte-mediated mutagenesis and DNA repair activity

Combined cultures of human hepatocytes and human fibroblasts constitute a system composed entirely of normal human cells that can be used to investigate the mutagenicity of chemicals requiring metabolic activation. Addition of diethylnitrosamine (DEN) to this system resulted in mutations at the hypoxanthine-guanine phosphoribosyltransferase locus of the human fibroblasts. In separate experiments with cultures of hepatocytes alone, DEN induced unscheduled DNA synthesis (UDS) in the human hepatocytes. A comparative analysis of UDS and hepatocyte-mediated mutagensis indicates a great degree of similarity between the human and previously studied rat hepatocytes in their response to DEN in vitro.

Liver regeneration studies with rat hepatocytes in primary culture

Adult rat parenchymal hepatocytes in primary culture can be induced to enter into DNA synthesis and mitosis. The optimal conditions for hepatocyte replication are low plating density (less than 10,000 cells/sq cm) and 50% serum from two-thirds partially hepatectomized rats (48 hr after hepatectomy). Approximately 80% of the hepatocytes enter the cell cycle, and most of these cells go through mitosis. The replicating hepatocytes remain positive for glucose-6-phosphatase and negative for gamma-glutamyl transpeptidase, and they accumulate fat, in analogy to regenerating liver. Most of the replicating hepatocytes enter into multiple consecutive rounds of DNA synthesis. Dose-response studies between control animal serum and hepatocyte labeling index indicate that in unoperated animals the serum contains substances stimulatory as well as inhibitory for hepatic growth, with the inhibitory effect prevailing at high concentrations. After partial hepatectomy, the inhibitory activity disappears whereas the hepatopoietin activity reaches almost 90% of maximal biological effectiveness at 25% serum concentration. Addition of hormones to the system shows that the hepatopoietin activity is not identical to epidermal growth factor, platelet-derived growth factor, thyroxine, glucagon, or hydrocortisone. Norepinephrine abolishes the difference between control and hepatectomized serum but does not restore hepatopoietin activity when added to heat-inactivated serum. The results show that this system of replicating hepatocytes can be used to investigate the trophic factors that control growth of normal and neoplastic hepatocytes.

Repair of radiation damage in noncycling parenchymal hepatocytes

The purpose of this study was to determine whether parenchymal hepatocytes in G0 are able to repair potentially lethal damage. The survival curve for hepatocytes assayed immediately after X irradiation was found to have an extrapolation number, n, which was not significantly different from unit, and a D0 equal to 2.7 Gy. In contrast, when irradiated hepatocytes were allowed to remain in situ for 24 h before being assayed for their reproductive survival , the n value significantly increased to 2.3, whereas the D0 value remains unchanged. But parenchymal hepatocytes are able to repair potentially lethal damage and it is expressed exclusively as an increase in the extrapolation number.

Mutagenesis and DNA binding of benzo(a)pyrene in cocultures of rat hepatocytes and human fibroblasts

The genotoxicity of benzo(a)pyrene (BP) was investigated in combined cultures of rat hepatocytes and human diploid fibroblasts. Freshly isolated rat hepatocytes were shown to activate BP to a species which bound to and damaged hepatocyte and fibroblast DNA. A significant increase in the hypoxanthine-guanine phosphoribosyltransferase mutation frequency was induced when 10 to 100 microM BP was added to the cocultures. A comparative analysis of the binding of BP metabolites to hepatocyte and fibroblast DNA revealed that approximately 4 times more [3H]BP metabolites were bound to the fibroblast DNA than were bound to the hepatocyte DNA (per microgram DNA). Activation of BP by the fibroblasts themselves was shown not to be the cause of the relatively greater binding of BP to fibroblast DNA than to the hepatocyte DNA. These results suggest that proximate and/or ultimately carcinogenic metabolites of BP are readily released from isolated hepatocytes and that the metabolites are sufficiently stable and long lived so as to bind to the DNA of an adjacent cell. The relative protection of the hepatocytic DNA from BP metabolites that generated in the cytoplasm of the hepatocyte may be significant in view of the observations that the liver is not under normal conditions a target of BP carcinogenicity in vivo.

Effect of hydrocortisone and nicotinamide on gamma glutamyltransferase in primary cultures of rat hepatocytes

Isolated rat hepatocytes cultured on collagen coated plates exhibit a gradual fetal phenotypic change during time in culture. The fetal liver marker gamma glutamyltransferase (GGT) was used to follow this change. Inasmuch as a significant overgrowth of nonparenchymal liver derived cells is seen frequently in primary cultures of hepatocytes, a technique was utilized that corrects for the presence of nonparenchymal cells. In media supplemented with either hydrocortisone (10(-5) M) or nicotinamide (25 mM) the original epithelial morphology of hepatocytes was preserved for a longer period of time than in unsupplemented media. Hepatocytes in unsupplemented media exhibited an increase in GGT specific activity over time. Hydrocortisone (10(-5) M) induced an increase in GGT activity compared to controls. Nicotinamide (25 mM) inhibited the increase in GGT activity compared to the unsupplemented hepatocytes. Our results indicate that GGT is regulated by hydrocortisone and nicotinamide.

Isolation, culture, and transplantation of human hepatocytes

The in situ two-step collagenase perfusion technique used for the isolation of hepatocytes from rat liver was adapted into a procedure applicable to pieces of human liver obtainable from surgical procedures. Human hepatocytes obtained by this method were maintained in primary culture for 10 days. The cellular changes observed at the light microscopic and electron microscopic levels are described. The changes in microsomal enzymes as a function of the age of the cultures were also measured. Exposure of the human hepatocytes to procarcinogens known to be metabolized by rodent liver resulted in unscheduled DNA synthesis. The isolated hepatocytes were also transplanted into two-thirds partially hepatectomized athymic nude mice. The transplanted cells formed nodules with characteristic hepatic architecture. These studies demonstrate that hepatocytes obtained from human liver by the described modified collagenase technique can be used for in vitro studies in chemical carcinogenesis.

Cryopreservation of isolated rat hepatocytes

Isolated parenchymal hepatocytes from adult rats were frozen in media containing 10% glycerol, 10% dimethylsulfoxide (DMSO), 20% DMSO. Thus microsome-associated functions were compared in nonfrozen cells and cells frozen in each of the above cryoprotectant solutions. Freezing in DMSO maintains cytochromes P-450 and b5 and NADPH-cytochrome C reductase at levels nearer to control values than does freezing in glycerol. Cells frozen and subsequently thawed and cultured for 24 h lose a greater amount of cytochrome P-450 than do nonfrozen cultured cells. The levels of cytochrome b5 and reductase in frozen-thawed cells remain close to control values. Cell viability (trypan blue dye exclusion and percentage of attached cells) after freezing is maintained better using DMSO as a cryoprotectant. Dimethylsulfoxide protects the hepatocytes from freeze-induced damage to the extent that many viable cells attach to collagen-coated petri dishes, survive for at least 24 h, and still maintain significant levels of enzymes of importance to drug and carcinogen metabolism.

Transplantation system for determining the clonogenic survival of parenchymal hepatocytes exposed to ionizing radiation

An in vivo transplantation technique has been developed which can be utilized to determine the reproductive survival of parenchymal hepatocytes exposed either in vitro or in vivo to both physical and chemical genotoxic agents. We have used this assay system to determine the survival of liver cells exposed to sparsely ionizing radiation. The D0 value of the survival curve was 249 rads and the extrapolation number was 1.2. These results indicate that hepatocytes irradiated while in the G0 phase are unable to accumulate sublethal damage to an appreciable extent if they are stimulated to undergo replication within 24 hr after the infliction of the damage.

Mutagenesis induced by procarcinogens at the hypoxanthine-guanine phosphoribosyl transferase locus of human fibroblasts cocultured with rat hepatocytes

The addition of diethylnitrosamine or cyclophosphamide in cultures of hepatocytes overlaid on confluent diploid human fibroblasts resulted in a dose-dependent increase in the frequency of hypoxanthine-guanine phosphoribosyl transferase human fibroblast mutants with both chemicals. Different toxicity patterns for the two cell types were seen. Diethylnitrosamine was more toxic to the hepatocytes, whereas cyclophosphamide was more toxic to the fibroblasts. These data open the possibility of using strains of human fibroblasts as in vitro screens for mutagenicity of procarcinogens.

Comparisons of the effects of chemical carcinogens in mixed cultures of rat hepatocytes and human fibroblasts

Unscheduled DNA synthesis (UDS) was measured simultaneously in rat hepatocytes and human fibroblasts when combined cultures of the 2 cell types were exposed to procarcinogens. Human fibroblasts were preincubated with 5-bromo-2'-deoxyuridine (BRdU) to substitute for thymidine in the DNA. Hepatocyte DNA was separated from the heavier BRdU-substituted fibroblast DNA by isopycnic centrifugations in neutral cesium chloride and the specific activities of the DNA's were determined. In the presence of hepatocytes, benzo - [a]pyrene (BP) induced more UDS in the fibroblasts than in the hepatocytes. BP induced no UDS in the fibroblasts in the absence of hepatocytes. Diethylnitrosamine (DEN) and 2-acetylaminofluorene (AAF) stimulated a significant amount of UDS only in the hepatocytes. Thus, the co-cultures of hepatocytes and fibroblasts responded with UDS to these chemical carcinogens in a manner that parallels the tissue specificity of the carcinogenicity of these chemicals in vivo. That is, the known hepatocarcinogens DEN and AAF, only stimulated significant UDS in the hepatocytes, whereas the non-hepatocarcinogen, BP, though activated in these cultures mainly by the hepatocytes, stimulated more UDS in the fibroblasts than in the hepatocytes. The amount of [3H]BP bound to DNA was investigated in the co-cultures and in cultures of fibroblasts alone. When co-cultures were exposed to [3H]BP the fibroblast DNA had approximately 3 times more BP bound to it (per microgram DNA) than did the hepatocyte DNA. The amount of [3H]BP bound to the DNA of cultures of fibroblasts alone was 29% of the amount bound to the DNA of the fibroblasts from the co-cultures. Thus, although the hepatocytes were mainly responsible for the activation of BP, more [3H]BP was bound to the fibroblast DNA. It is suggested that the intercellular distribution of carcinogenic metabolites may be a significant determinant of the carcinogenic effect.

Morphologic and histochemical analysis of hepatocytes transplanted into syngeneic hosts

Suspensions of freshly isolated hepatocytes were prepared by collagenase perfusion of livers of adult Fischer 344 female rats. The cells were injected into the dorsal fascia of 2/3 partially hepatectomized syngeneic hosts (10(6) cells per injection site) and were monitored from 3 days to 3 months after injection. Brown nodules developed at the transplantation site. Histologic examination of the nodules revealed that the hepatocytes were arranged in cords and clusters surrounded by fibrovascular connective tissue. Bile ductules were also seen. Hepatocytes were positive for glucose-6-phosphatase. Staining for gamma-glutamyltranspeptidase showed that the parenchymal hepatocytes were mostly (approximately 95%) negative, whereas bile ductules were positive. These histochemical findings were seen in hepatocytes up to 3 months after transplantation and did not vary with the age of the transplants. Electron-microscopic examination of the transplanted nodules demonstrated that the cells maintained the characteristics of hepatocellular cytoplasmic structure. The relationship between the bile canaliculi and the stromal vessels was found to be similar to the bile canaliculi and hepatic sinusoid polarity seen in the normal liver. Autoradiographic analysis showed that a fraction of the transplanted cells was active in DNA synthesis. This system may become a tool in the study of survival and neoplastic transformation of hepatocytes as a result of exposure to X-irradiation and chemical carcinogens.

Sister chromatid exchange studies in human fibroblast-rat hepatocyte co-cultures: a new in vitro system to study SCEs

To approximate better the metabolic reactions that take place in vivo yet maintain the simplicity and reproducibility of in vitro systems, we have developed a co-culture system making use of freshly isolated rat hepatocytes and confluent human fibroblasts for the study of SCE induction by genotoxic agents. Hepatocytes were obtained from male rats by reverse collagenase perfusion and plated over low-passage male human fibroblasts. Preliminary studies demonstrated that although the number of hepatocytes plated was not critical, the best attachment to, and coverage of, the fibroblasts occurred when between 10 x 10(6) and 20 x 10(6) hepatocytes were plated/100 mm tissue culture dish. Results with the promutagen cyclophosphamide showed that the hepatocytes could metabolize the compound and deliver active moieties to the fibroblasts resulting in a linear dose-dependent increase in SCE frequencies. Control fibroblast cultures lacking hepatocytes displayed no increase in SEC frequencies. Control fibroblast cultures lacking hepatocytes displayed no increase in SEC frequencies following cyclophosphamide administration.