Generation of useful cell culture data.

As the need for viable and interpretable cell culture systems increases, it is not sufficient to simply be successful at growing cells in vitro. Rather, vigilance is required to obtain repeatable data from these systems, especially if mechanistic or developmental experimental designs are attempted. We suggest that all aspects of basic cell culture are as important as growing cells. We offer the papers of this issue to help the cell scientist scrutinize and identify problems in many of these important areas, including obtaining tissue, eliminating microbial contamination, formulating a defined medium, isolating specific cell types from tissue and then using them for in vitro studies, cloning cells, selecting and developing methods for cell culture analyses, and recognizing abnormal cell culture activity.

Fluorometric quantitation of broth-cultured mycoplasmas by using alkaline ethidium bromide.

We developed a fluorometric system which does for broth-grown mycoplasmas what turbidimetric analysis does for broth-grown bacteria. It allows one to monitor the growth of broth-grown mycoplasmas at any interval desired. The entire procedure is quick, taking not more than 20 min. The fluorometric readings correlate with colonial growth on agar, making it possible, for the first time, to take readings which closely estimate the CFU present in the culture at a given moment in time. We show that this system can be used to assess the effectiveness of an antimycoplasmal antibiotic and to optimize medium components and that fluorometer readings taken during the logarithmic phase of growth correlate with the DNA content of the viable cells. Use of this methodology will permit investigators to know absolutely the phase of the growth cycle of the culture concomitant with the growth of the culture itself, and since fluorometer readings of culture aliquots can be converted to DNA equivalents, the standardization of mycoplasmal cultures within and between laboratories will be a possibility.

Analysis of Mycoplasma hyorhinis DNA in the presence of host cells without growing the mycoplasma axenically.

Mycoplasma hyorhinis coisolates with the mitochondria of the cells in which it is carried as an infection. Since both mitochondria and mycoplasmas synthesize DNA by using the prokaryotic DNA polymerase gamma, the use of aphidicolin, which inhibits eukaryotic DNA polymerase alpha, allows for selective synthesis of mycoplasmal and mitochondrial DNA. The restriction patterns of mitochondria and mycoplasmas can easily be differentiated from each other in mixtures of both DNAs. Thus, it is possible to study the molecular biology of this noncultivable mycoplasma in situ rather than after growth in artificial media, with its potential genetic consequences during adjustment to axenic growth.

A system employing a minimal defined medium for the selection of tumorigenic cells.

Populations of normal and tumorigenic epithelial cells were plated in normal (serum supplemented) and a minimal defined medium. The ability of cells to grow in the minimal defined medium vs. the normal medium was related to their tumorigenic ability. All of the clones that were capable of growth in the minimal defined medium demonstrated tumorigenic ability. None of the normal clones survived in the minimal defined medium. As expected, some of the tumorigenic clones could not grow in the minimal medium. However, at no time did a clone that grew in the minimal defined medium fail to demonstrate tumorigenic ability. This validated that this system can be an animal-free means for selecting tumorigenic epithelial cells out of a mixed population.

Cytoplasmic mediation of malignancy.

The relative roles of nucleus and cytoplasm in the induction and maintenance of the malignant state were studied. Cytoplasmic hybrid (cybrid) clones, derived from the fusion of cytoplasts from malignantly transformed cells to normal whole cells, produced tumors in 17% of the animals injected with them. Nuclear/cytoplasmic hybrid (reconstituted cell) clones, derived by fusion of cytoplasts from malignant cells with karyoplasts of normal cells, produced tumors in 97% of the animals injected. A unique aspect of this study is the fact that all of the cells utilized, both normal and malignantly transformed, were derived from an original cloned cell.

Interaction of the 4 S polycyclic aromatic hydrocarbon-binding protein with the cytochrome P-450c gene.

The induction of cytochrome P-450c, the isozyme most closely associated with aryl hydrocarbon hydroxylase activity in the rat, is mediated through a cytosolic polycyclic aromatic hydrocarbon (PAH)-binding protein(s). We have reported on the purification and characterization of a 4 S protein that interacts in a specific and saturable manner with [3H]benzo[a]pyrene and other PAHs. (W. H. Houser et al. (1985) Biochemistry 24, 7839-7845). We have also reported on the specific and saturable interaction of the 4 S protein with a plasmid containing 1.9 kbp of cloned rat P-450c sequence including exon 1, the 5' half of intron 1, and approximately 882 bp upstream information. Our investigations now show that incubation of this protein with a portion of the rat P-450c gene, followed by digestion with either lambda exonuclease or exonuclease III, tentatively identified two protected regions at -225 and -455 bp 5' from exon 1. To further study the significance of these protected regions, a 3.4-kbp fragment containing cytochrome P-450c promoter and 5'-upstream sequences (-882 to +2545) was fused to the chloramphenicol acetyl transferase (CAT) reporter gene and transfected into either rat epithelial RL-PR-C cells or rat hepatoma H-4-II-E cells. Both cell lines expressed CAT activity in response to induction by 3-methylcholanthrene (3MC), indicating that important regulatory regions responsive to 3MC are present in these constructs. However, neither cell line expressed CAT activity in response to 3MC when transfected with plasmids containing deletions (-95 to -724 or -240 to -720) in the regions shown to be protected by our footprinting studies. These results corroborate previous studies which indicated that the 4 S PAH-binding protein interacts in a specific manner with regions of the rat cytochrome P-450c gene. We conclude that the 4 S protein may play an important role in the regulation of expression of cytochrome P-450c in the rat.

Cytoplasmic suppression of malignancy.

Using both normal and transformed rat liver epithelial cells to prepare cytoplasmic hybrids (cybrids) we have found evidence to support the theory that the cytoplasm from a normal cell can suppress tumorigenicity. A unique aspect of this study is that all of the cells utilized, both normal and malignantly transformed, were derived from an original cloned cell. We found that fusing cytoplasts from normal cells to malignantly transformed whole cells resulted in cybrid clones which, when injected into newborn rat pups, isogenic with those from which the cell culture was initiated, yielded tumors in 51% of the animals injected compared to 92% of the animals injected with the tumorigenic parent. Those animals that did develop tumors from the cybrid cells survived longer than those injected with cells from the tumorigenic parent. Thus, the cybrid, formed of cytoplasm from both parents, was less tumorigenic than the malignantly transformed parent cell. When reconstituted cells were prepared by fusing cytoplasts from normal cells with karyoplasts from malignantly transformed cells, a situation in which essentially all of the cytoplasm of the reconstituted cell is derived from normal cells, the tumorigenic phenotype was extinguished.

The selection of malignantly transformed rat liver cells using a hormonally defined medium.

A hormonally defined medium consisting of a 1:1 mixture of Dulbecco's modified Eagle's medium and nutrient mixture F12 supplemented with 50 micrograms/ml each of insulin and transferrin was found to grow late passage malignantly transformed cells of the RL-PR-C rat liver cell culture. The same medium formulation would not grow the early passage, normal, diploid counterpart of the RL-PR-C cell culture. When mixtures of the early and late passage cells were made, only the late passage cells would grow, thus providing a selection system for the late passage cells.

Correlation of induction of aryl hydrocarbon hydroxylase in cultured rat hepatocytes with saturable high-affinity binding of 3-methylcholanthrene to a 4S cytoplasmic protein.

The binding of 3-methylcholanthrene (3-MC), a potent inducer of aryl hydrocarbon hydroxylase activity, to cytoplasmic proteins of a cloned rat hepatocyte culture, RL-PR-C, was studied by sucrose gradient centrifugation. Time course and dose-binding experiments performed on late-passage aryl hydrocarbon hydroxylase-inducible cultures indicate the presence of a saturable pool of high-affinity (average Kd, 3.6 nm) binding sites in the cytosol of these cells. The number of binding sites varied from 20,000 to 80,000 per late-passage hepatocyte with a total capacity of approximately 2.2 pmol of 3-MC bound per mg of cytosolic protein. The complex sedimented at 4.0 +/- 0.2S regardless of the ionic strength of the homogenization buffer or gradient solutions. It was sensitive to denaturation by sodium dodecyl sulfate and trypsin but not by DNase I, RNase A, or the nonionic detergent Nonidet P-40. The binding of 3-MC to the protein was inhibited by 1,2-benzanthracene, benzo(a)pyrene, 5,6-benzoflavone, and 7,8-benzoflavone but not by a series of steroids, aflatoxin B1, phenobarbital, or Aroclor 1254. Elevating the temperature of cultures cells to 37 degrees after the standard ligand-binding incubation at 4 degrees resulted in a rapid decrease in cytoplasmic saturable binding and a concomitant increase in nuclear- and chromatin-associated ligand. A portion of this nuclear-associated ligand was extractable with 400 mM KCl. Adsorption of the [3H]-3-MC binding complex by nuclei in vitro suggested that the 4S binding protein facilitated the entry of 3-MC into the nucleus. The presence of the 4S binding species correlated with the level of inducibility of aryl hydrocarbon hydroxylase throughout its development in RL-PR-C and therefore may be involved in the process of induction of this enzyme.

Malignant transformation of cultured rat hepatocytes by (+)- and (-)-trans-7,8-dihydro-7,8-dihydroxybenzo(a)pyrene.

Rat hepatocyte cultures (RL-PR-C) were tested with the (+)- or (-)-trans-7,8-dihydrodiol isomers of benzo(a)pyrene [B(a)P] and were assessed for growth inhibition, chromosomal damage, growth in soft agar, and tumor formation. Because early-passage cells have a noninducible low specific activity, aryl hydrocarbon hydroxylase parallel inhibition studies were performed on aryl hydrocarbon hydroxylase-inducible late-passage cultures. Early-passage cells exhibited little inhibition in the presence of B(a)P or either isomer. Comparable later-passage cells demonstrated inhibitory effects with B(a)P and the (+)- and (-)-trans-7,8-dihydrodiol metabolites. No treated cultures grew in soft agar, and the modal number of chromosomes was unaffected by carcinogen treatment. However, both (+) and (-) isomer-treated early-passage cells formed tumors in isogeneic animals, the (+) isomer being more efficient in this regard. These results indicate that noninhibitory doses of either the (+)- or (-)-trans-7,8-dihydrodiol isomer of B(a)P are nonetheless capable of malignantly transforming hepatocyte cells in vitro.

The long-term culture of a diploid rat hepatocyte cell strain.

We have developed a cell culture system of cloned rat hepatocytes which is named RL-PR-C. This culture has been grown in culture for over 150 passages (560 p.d.). It remains diploid by modal number count for over 50 passages (183 p.d.) after which there is a variation in the modal number between 41 and 42. The culture becomes intrinsically (spontaneously) malignantly transformed at about this time whereas cells treated with aflatoxin B1, at subacute and subtoxic doses, from Passage 12(65 p.d.) were malignantly transformed at Passage 35(135 p.d.). Evidence indicates that not all cells in the population are malignantly transformed. Back injection of mixed population of malignantly transformed cells into isogenic rat pups, yield tumors which are histologically classified as mixed. That is, areas of the same tissue slice exhibit both carcinomas and sarcoma characteristics. Data obtained by injection of clones derived from malignantly transformed cells suggest that the cells injected are multipotent with respect to the type of tumor formed. Further investigations are continuing in this regard. This cultural system will be used to study the evolution and maintenance of the transformed state and to develop a test system for putative carcinogens and mutagens.

Sensitivity of cultured rat hepatocytes to aflatoxin B1 and benzo(a)pyrene.

We have tested the sensitivity of a cloned rat hepatocyte line, RL-PR-C, to aflatoxin B1 and benzo(a)pyrene as a function of population-doubling level. The cells were much more sensitive to the cytotoxic action of these agents subsequent to 230 population doublings. This sensitivity corresponded to the enhanced inducibility of aryl hydrocarbon hydroxylase activity by 3-methylcholanthrene.

A diploid rat liver cell culture. IV. Malignant transformation by aflatoxin B1.

Chronic exposure of a cloned rat hepatocyte culture (RL-PR-C) to a subtoxic, sublethal dose of aflatoxin B1 resulted in malignant transformation. Continuous exposure to aflatoxin B1 caused increasing tumorigenic potential as tested by back injection into isogenic animals. Control cultures exhibited spontaneous transformation, although approximately 20 passages beyond the chemically induced event. Neither aflatoxin-treated nor control cultures exhibited cytopathological morphology, formation of cell foci, growth in soft agar, or irregular fibroblast-like growth patterns that could be specifically related to the onset of tumorigenic potential. In general, those parameters commonly used to monitor fibroblast cultures for transformation in vitro were not applicable for assessing the tumorigenic potential of these epithelial cells. Karyotypic analyses revealed no specific chromosomal aberrations associated with aflatoxin treatment; however, chromosomal instability was a property of the tumorigenic cell populations. Injection of both aflatoxin-treated and control cultures at passage 56 resulted in tumors indicative of both carcinoma and sarcoma indicating to us the multipotency of these epithelial cells transformed in vitro.

Efficient detection of soft agar grown colonies using a tetrazolium salt.

The use of 2-(p-iodophenyl)-3-(p-nitrophenul)-5-phenyl tetrazolium chloride at concentrations of from 0.25-1.0 mg/ml, when added to plates containing soft agar grown colonies, colors the cell clusters a deep brick red against a colorless background. This greatly facilitates the quantitation of such colonies since clusters of cells containing as few as 16-32 cells are visible macroscopically. Incubation for 20 h is optimal although coloration takes place as early as 6-8 h.