Toxicology investigations with cell culture systems

This review concerns some of the cell culture systems that are most frequently used in toxicology investigations. In particular, it sets out to evaluate the effectiveness of these cell culture systems in assessing the toxic potential of chemicals. Metabolic studies and general and specific toxicology investigations are highlighted. Specific toxicology investigations relate to the effects of the tests substances on the highly specialized functions typical of the cell systems chosen. The general toxicology investigations include most of the other studies where differentiated or undifferentiated cells have been used to evaluate the effects of the tested substances on common basic biochemical processes essential for life. Lastly, we have attempted to focus attention on the most promising applications of cell cultures in toxicology studies for the near future and to identify those areas where further research is needed. Because of the several excellent reviews that already exist, we have decided not to consider cell cultures utilized in screening potential mutagens and carcinogens. We have also excluded investigations of drug therapeutic effects and action mechanisms of drugs.

Connective tissue biomatrix: its isolation and utilization for long-term cultures of normal rat hepatocytes

A new procedure is introduced for the isolation of connective tissue fibers, called biomatrix, containing a significant portion of the extracellular matrix (basement membrane components and components of the ground substance). Biomatrix isolated from normal rat liver contains >90% of the tissue's collagens and all of the known collagen types, including types I and III and basement membrane collagens. The purified collagenous fibers are associated with noncollagenous acidic proteins (including fibronectins and possibly small amounts of glycosaminoglycans). Procedures are also described for preparing tissue culture substrates with these fibers by either smearing tissue culture dishes with frozen sections or by shredding the biomatrix into small fibrils with a homogenizer. The biomatrix as a substrate has a remarkable ability to sustain normal rat hepatocytes long-term in culture. The hepatocytes, which on tissue culture plastic or on type I collagen gels do not survive more than a few weeks, have been maintained for more than 5 mo in vitro when cultured on biomatrix. These cells cultured on rat liver biomatrix show increased attachment and survival efficiencies, long-term survival (months) and retention of some hepatocyte-specific functions.

Taurocholate inhibits the glucocorticoid-induced rise of 3-hydroxy-3-methylglutaryl-CoA reductase in primary culture of hepatocytes

The influence of taurocholate, the major bile acid of the rat, on 3-hydroxy-3-methylglutaryl-CoA reductase [mevalonate: NADP+ oxidoreductase (acylating CoA); EC 1.1.1.34], the regulatory enzyme of cholesterol synthesis, was studied in primary cultures of rat hepatocytes. The basal activity of the enzyme was not altered by adding up to 10 microM taurocholate to the culture medium. On the contrary, 1nM to 10 microM taurocholate caused a dose-dependent inhibition of enzyme activity within 6 h if added simultaneously with 10 microM dexamethasone. Because this glucocorticoid causes a cycloheximide-sensitive rise of 3-hydroxy-3-methylglutaryl-CoA reductase activity in this system the results are taken as evidence that bile salts inhibit the synthesis of the enzyme. The induction of tyrosine transaminase (L-tyrosine:2-oxoglutarate aminotransferase; EC 2.6.1.5) by dexamethasone was not influenced by taurocholate, which demonstrates that the glucocorticoid sensitivity of the cells was not impaired by the bile salt. It is concluded that there is a direct control of hepatic cholesterol synthesis by bile salts.

Ducts of the rat pancreas in a agarose matrix culture

Interlobular and intralobular ducts isolated from the pancreas of the rat by digestion with collagenase and chymotrypsin were cultured in an agarose matrix containing CMRL-1066 supplemented with insulin, dexamethasone, L-glutamine, soybean trypsin inhibitor, antibiotics, and fetal bovine serum. The cut ends of most interlobular ducts sealed to create enclosed lumina. Some ducts retained their original cylindrical organization; others enlarged to varying degrees, resulting in structures that ranged from cylindrical to spherical in shape. The duct walls consisted of viable epithelium and connective tissue, although the amount of connective tissue declined with age. Both epithelial and connective tissue cells became flattened in the enlarged ducts. Intralobular and small interlobular ducts often remained associated with the larger interlobular ducts. These duct fragments have been cultured for as long as 6 weeks.

Regulation of gene expression in primary cultures of adult rat hepatocytes on collagen gels

The studies described in this paper demonstrate rather conclusively the efficacy of the study of the regulation of gene expression in primary cultures of adult rat hepatocytes. The utilization of these cells in completely defined medium allows one to determine the exact environmental conditions for the regulation of the expression of specific genes. In the studies described in this work, we have demonstrated that the regulation of glucokinase involved three hormones, insulin, corticosteroids, and T3. In contrast, the regulation of an enzyme involved primarily in fatty acid metabolism, ATP-citrate lyase, required only insulin and T3 for its full expression. Cyclic GMP appeared to be involved in the regulation of glucokinase, but not ATP-citrate lyase, a fact that would be extremely difficult to demonstrate clearly in vivo. The regulation of the gluconeogenic enzyme, ornithine aminotransferase, in vitro involved only a single hormone, glucagon, the inhibition of induction by corticoid steroids demonstrable in vivo being absent in cell culture. However, the repressive effect of glucose on the induction of this enzyme was quite comparable to that seen in vivo and was not mediated through cyclic AMP or insulin, based on findings in cell culture. Thus, the requirements for and the mechanisms involved in enzyme induction and repression by hormones and glucose may be much more easily studied in primary cultures of rat hepatocytes than in vivo, or even in hepatoma cell lines, where relatively few genes are expressed as compared with adult liver. In addition to the regulation of enzyme levels, the characteristics of protein secretion may be investigated in primary cultures of rat hepatocytes and compared with the biochemical and physiological parameters in the whole organism. This was exemplified by the study of the synthesis and secretion of alpha 2u-globulin that was secreted into the culture medium in both glycosylated and nonglycosylated forms but was maintained in the circulation in vivo, principally as the glycosylated form. Furthermore, the function of glycosylation in this particular instance may be deduced from a combination of the in vivo and in vitro approaches. The advantages of the use of primary hepatocyte cultures for the study of the regulation of gene expression in mammalian tissue has only recently been explored. Future investigations of the regulation of a variety of enzymes in these cultures as well as a study of the regulation of the synthesis of their messenger RNA are now possible and should provide an exciting system in which to understand at a molecular level the regulation of the expression of a number of genes.

Comparison of aryl hydrocarbon hydroxylase and epoxide hydratase. Induction in primary fetal rat liver cell culture

The activity of aryl hydrocarbon hydroxylase (AHH) and/or epoxide hydratase (EH) is induced in primary fetal rat liver cell culture by benz-[alpha]anthracene (BA), phenobarbital (PB), cigarette smoke condensate (CSC), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and trans-stilbene oxide (TSO). The response of the two enzymes to the different chemicals varies as follows: (a) AHH is induced by lower concentrations of BA, PB and CSC than those required to significantly induce EH; (b) AHH is selectively induced by TCDD and by low BA concentrations; (c) the kinetics of AHH induction by BA, PB and CSC is faster than that of EH; (d) TSO is a selective inducer of EH. As described earlier for AHH, RNA and protein synthesis and the continuous presence of the inducer are required in the early phases of EH induction. Later when the EH activity has reached a plateau, intact RNA and protein synthesis is not necessary to maintain the enzyme at its optimal value. The removal of the inducer determines a decay of the EH activity, allowing the estimation of a biological tau 1/2 of about 72 h. TSO prevents the AHH induction by PB, but not that mediated by BA and CSC. Added together with PB, BA, CSC or PB plus BA, TSO induces the EH activity in a more than additive manner. This effect is only seen after 6 days of continuous treatment. These results indicate that in this tissue culture model, the mechanism of AHH and EH induction can clearly be dissociated.

Morphology and metabolism of adult rat hepatocytes in primary culture

Isolated hepatocytes obtained by collagenase perfusion of adult rat livers were seeded on collagen gels and kept in a chemically defined culture medium (except for the first 6 h of culture where 10% fetal calf serum was added). Cells adopted an epitheloid shape within 4 h and arranged themselves in a trabeculae-like pattern during the first 20 h of culture. In the electron microscope numerous tight junctions and bile capillaries were observed at sites of cell-to-cell contact. From metabolite analyses in the culture medium the following conclusions can be drawn: The cells continued to synthesize urea and ketone bodies for 5 days of culture. The cytosolic and mitochondrial redox states of the nicotinamide adenine nucleotide systsm were as in the liver in vivo and the oxygen supply of hepatocytes was sufficient under the culture conditions. Maximal velocities of ketogenesis from octanoate and of urea formation from ornithine plus ammonium chloride were stable during a 120 h culture period and compared well with rates found in the isolated perfused rat liver.

The effects of dexamethasone on metabolic activity of hepatocytes in primary monolayer culture

The effects of dexamethasone on multiple metabolic functions of adult rat hepatocytes in monolayer culture were studied. Adult rat liver parenchymal cells were isolated by collagenase perfusion and cultured as a primary monolayer in HI/WO/BA, a serum free, completely defined, synthetic culture medium. Cells inoculated into the culture medium formed a monolayer within 24 hr. Electron microscopy showed that the cells in primary culture had a fine structure identical to liver parenchymal cells in vivo, including the observation of desmosomes and bile canaliculi in intercellular space. There was significant gluconeogenesis by the cells 24 hr postinoculation but it had decreased markedly by 48 hr. There was a marked induction of tyrosine aminotransferase (TAT) by dexamethasone, which was maintained for up to 72 hr postinoculation of cells. The transport of alpha-aminoisobutyric acid into the cells in monolayer culture was stimulated by dexamethasone and was dependent on the concentration of dexamethasone. Albumin synthesis and secretion by the cells was measured by a quantitative electroimmunoassay. Albumin production was shown to increase linearly over an incubation period of 24 to 48 hr postinoculation. Dexamethasone depressed the albumin synthesis. The effects of dexamethasone are slow, and at times require more than 6 hr to show variation from the control, indicating that dexamethasone is not a single controlling hormone. Possibly it functions in a cooperative and coordinating role in the regulation of cell metabolism.

Transformation of isolated rat hepatocytes with simian virus 40

Rat hepatocytes were transformed by simian virus 40 (SV40). Hepatocytes from two different strains of rats and a temperature-sensitive mutant (SV40tsA 1609), as well as wild-type virus were used. In all cases, transformed cells arose from approximately 50% of the cultures containing hepatocytes on collagen gels or a collagen gel-nylon mesh substratum. Cells did not proliferate in mock-infected cultures. SV40-transformed hepatocytes were epithelial in morphology, retained large numbers of mitochondria, acquired an increased nucleus to cytoplasm ratio, and contained cytoplasmic vacuoles. Evidence that these cells were transformed by SV40 came from the findings that transformants were 100% positive for SV40 tumor antigen expression, and that SV40 was rescued when transformed hepatocytes were fused with monkey cells. All SV40-transformed cell lines tested formed clones in soft agarose. Several cell lines transformed by SV40tsA 1609 were temperature dependent for colony formation on plastic dishes. Transformants were diverse in the expression of characteristic liver gene functions. Of eight cell lines tested, one secreted 24% of total protein as albumin, which was comparable to albumin production by freshly plated hepatocytes; two other cell lines produced 4.2 and 5.7%, respectively. Tyrosine aminotransferase activity was present in five cell lines tested but was inducible by dexamethasone treatment in only two. We conclude from these studies that adult, nonproliferating rat hepatocytes are competent for virus transformation.

Primary culture of mouse mammary tumor epithelial cells embedded in collagen gels

Mammary tumor epithelial cells from BALB/cfC3H mice were dispersely embedded inside the collagen gels in Ham's F-12 medium containing horse serum. A sustained cell growth leading to a 5- to 10-fold increase in cell number over initial level was observed in less than 2 weeks. The extent of this growth was found to be dependent on serum concentration. However, addition of various protein and steroid hormones, both singly and in combination, to low-serum-containing medium failed to achieve a comparable level of growth to that promoted by higher serum concentration. Mammary tumor cells can now be consistently propagated in primary culture.

Sustained growth in primary culture of normal mammary epithelial cells embedded in collagen gels

Normal mammary epithelial cells from BALB/cfC3H midpregnant mice were freed from stromal cell types by Percoll density gradient centrifugation after collagenase digestion and were then embedded within collagen gels. Sustained growth leading to an increase in cell number was accomplished in response to cholera toxin and high concentrations of horse serum. The extent of growth was found to be dependent on the horse serum concentration, the maximum growth being attained at 50%. A serum concentration of 12.5% horse serum and 2.5% fetal calf serum, along with cholera toxin at 0.01 mug/ml, allowed maintenance but failed to cause any significant increase in cell number during the experimental period of 2 weeks. This same maintenance medium was used to determine the effects of various exogenously added steroids, protein hormones, and organ extracts on the proliferation of mammary epithelial cells in culture. Hormones failed to elicit any proliferative response, but extracts of kidney, brain, uterus, and spleen produced proliferative responses equal to or greater than the response obtained with 50% horse serum and cholera toxin. Kidney extracts prepared from midpregnant mice, virgin mice, and virgin mice given pituitary isografts all showed comparable activities, suggesting that the concentration of stimulatory factor(s) was not influenced by the hormonal status of the donor. Normal mammary epithelial cells that had undergone a 10- to 15-fold increase in cell number over initial values during 2-3 weeks in culture were passaged to secondary gel cultures. Outgrowth similar to those seen in primary culture were seen again in secondary culture. The present system provides a method for sustaining growth in culture of primary mammary epithelial cells from normal tissues.

Primary monolayer culture of liver parenchymal cells and kidney cortical tubules as a useful new model for biochemical pharmacology and experimental toxicology. Studies in vitro on hepatic membrane transport, induction of liver enzymes, and adaptive changes in renal cortical enzymes

Freshly isolated liver parenchymal cells were maintained in either short-term monolayer, suspension of long-term monolayer culture. Rapidly occurring processes through hepatocellular membrane, e.g., the enhanced amino acid transport and the concomitantly increased potassium influx following progressive starvation, were kinetically evaluated best in short-term monolayer culture. The inducibility of tyrosine aminotransferase by glucagon, dexamethasone, and a combination of both was compared in suspension and in monolayer culture. The induction of slowly inducible foreign compound-metabolizing enzymes, (e.g., ethoxycoumarin-O-dealkylase, p-nitroanisole-O-demethylase, and UDP-glucuronyltransferase) by phenobarbital, 3-methylcholanthrene, and dexamethasone were studied in long-term monolayer culture. The latter system was also used to maintain isolated kidney cortical tubules for the investigation of renal enzyme adaptation during progressive time in culture.

Monooxygenase and UDP-glucuronyltransferase activities in established cell culture

Cells in culture were investigated for the expression of monooxygenase and UDP-glucuronyltransferase activities as representatives of activating and inactivating pathways of drug metabolism. Most established cell lines express monooxygenase activity that is detected by the oxygenation of polycyclic hydrocarbons and appears to be a function of cytochrome P-448-dependent enzyme forms (Wiebel et al., 1977). In the hepatoma cell line, H-4-II-3, dexamethasone is capable of increasing the level of benzo(a)pyrene-monooxygenation about 10-fold and of potentiating its induction by benzo(a)anthracene. The enzyme activities elicited by dexamethasone and the polycyclic hydrocarbon did not significantly differ in their response to 7,8-benzoflavone, an inhibitor of cytochrome P-448-dependent monooxygenases. Observations on the pattern of benzo(a)pyrene metabolites formed in benz(a)anthracene-treated H-4-II-E and BHK21(C13) cells indicate that established cell cultures may contain different forms of monooxygenases of the cytochrome P-448 type. The majority of cell lines tested express UDP-glucuronyltransferase activity directed toward the substrate, 3-hydroxybenzo(a)pyrene. No clear correlation appears to exist between the presence and level of monooxygenase and glucuronyltransferase activities in the various cell lines, i.e., the cultures may express any one or both enzymes. The ratio of the two enzyme levels can be modified by selective induction. Thus, at present there is a choice of established cells in culture exhibiting widely differing ratios of activating and inactivating enzymes to analyse the metabolic pathways of selected classes of foreign compounds, such as polycyclic hydrocarbons, and to determine their toxicological significance. Further efforts are likely to yield cell lines that express the enzymic functions lacking in the cultures currently used and will be suitable to study the full spectrum of foreign compounds.

Comparative induction of unscheduled DNA synthesis by physical and chemical agents in non-proliferating primary cultures of rat hepatocytes

The incorporation of [3H]thymidine into DNA due to unscheduled DNA synthesis (UDS) induced by N-OH-2-acetylaminofluorene (N-OH-AAF), aflatoxin B1 (AFB1), ethyl methanesulfonate (EMS) and ultra-violet light was quantitated by autoradiography and by scintillation spectrometry on acid precipitable macromolecules or DNA insolated by isopycnic banding in cesium chloride (CsCl). Dose-dependent increases in UDS due to N-OH-AAF and AFB1 treatment were found. Only 2-fold increases at the highest dose levels were found, however, when incorporated [3H]thymidine was quantitated by scintillation spectrometry. Seven, 11, and 25-fold increases in UDS induced by AFB1, N-OH-AAF and ultra-violet light, respectively, were found when incorporated [3H]thymidine was quantitated by autoradiography, indicating a high sensitivity for detecting 'long patch' repair by this technique. Scintillation spectrometry was completely ineffective in detecting EMS-induced UDS, whereas autoradiography demonstrated a small, but significant induction in [3H]thymidine incorporation at high dose levels. The non-proliferative nature of the primary hepatocyte prohibits the uniform radioactive prelabeling of DNA, necessary in other techniques, for the detection of 'short patch' repair induced by compounds such as EMS. Therefore, the sensitivity of the primary cultured rat hepatocyte in conjunction with UDS for detecting DNA damage caused by mutagens and carcinogens which induce 'short patch' repair may be limited to the autoradiographic analysis of the unscheduled incorporation of [3H]thymidine.

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.