Tumor promoters inhibit morphological differentiation in cultured mouse neuroblastoma cells

When added to mouse neuroblastoma cultures, the potent tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) inhibits spontaneous neurite formation as well as that induced in response to serum deprivation, prostaglandin E1, 5-bromo-2'-deoxyuridine, and papaverine. Other tumor-promoting macrocyclic plant diterpenes also inhibit neurite formation, whereas nonpromoting diterpenes do not. Inhibition by TPA was reversible and was unrelated to toxicity.

Induction of plasminogen activator in cultured cells by macrocyclic plant diterpene esters and other agents related to tumor promotion

In vitro systems that are responsive to tumor-promoting agents may facilitate the identification of such agents and the analysis of their mode of action. We have previously reported that the potent tumor promoter phorbol-12-myristate-13-acetate induces the synthesis of the enzyme plasminogen activator in cultured chick embryo fibroblasts. We have, therefore, tested various compounds for their ability to induce plasminogen activator in chicken embryo fibroblasts. Among these, phorbol esters and other macrocyclic diterpene esters isolated from species of the families Euphorbiaceae and Thymelaeaceae were potent inducers of plasminogen activator. These compounds maximally induced enzyme to the same levels, although they differed in their relative molar potencies. Structural requirements for in vitro activity paralleled the requirements for activity in vivo. These results indicate that induction of plasminogen activator is a useful marker for the biologically active macrocyclic diterpene esters. On the other hand, tumor-promoting agents such as anthralin, cantharidin, Tween 60, and tobacco leaf extract failed to induce plasminogen activator.

Uptake of the tumor promoting agent 12-0-tetradecanoyl-phorbol-13-acetate by HeLa cells

The uptake of 12-0-tetradecanoyl-phorbal-13-acetate (TPA) by HeLa cells, cells of a line responsive to the action of this tumor promoting agent, was studied. Our results indicate that most of the uptake is due to a physical nonsaturable process which is not grossly affected by inhibitors of energy metabolism or macromolecular synthesis. Uptake occurs mainly in membranous fractions of the cell, is largely reversible, and is temperature dependent. These results suggest that 3H-TPA uptake is largely due to partitioning of this lipophilic substance into the lipid phase of the cell membranes.

Interactions between adenovirus, a tumor promoter, and chemical carcinogens in transformation of rat embryo cell cultures

The tumor-promoting agent 12-O-tetradecanoyl-phorbol-13-acetate (TPA) caused a 2- to 3-fold enhancement of transformation of secondary rat embryo cells that had been injected with a temperature-sensitive mutant of adenovirus type 5(H5ts 125). In addition, transformed foci appeared earlier and were larger in cultures grown in the presence of TPA. The addition of TPA could be delayed until up to 7 days after viral injection and still enhancement was observed. Exposure of the cells to 7,12-dimethylbenz[a]pyrene prior to H5ts125 infection also resulted in a 2- to 4-fold enhancement of transformation, and this enhancement was further augmented 2- to 3-fold when cells were grown in TPA after virus infection. Whereas TPA did not enhance the cloning efficiency of normal rat embryo cells, it did enhance the cloning efficiency of isolated colonies of adenovirus-transformed cells when these were grown alone or cocultured with a 100-fold excess of normal rat embryo cells. The enhancement of adenovirus transformation by TPA appears to be due to its ability to facilitate expression of the transformed state rather than an effect on virus uptake or integration.

Conformation of dinucleoside monophosphates modified with benzo[a]pyrene-7,8-dihydrodiol 9,10-oxide as measured by circular dichroism

The conformational properties of GpU modified with the reactive derivative of benzo[a]pyrene, (+/-)-7beta,8alpha-dihydroxy-9alpha,10alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, has been investigated utilizing circular dichroism spectroscopy. Binding of this carcinogen to the N2 of G residues in GpU resulted in the formation of four compounds (I to IV) representing two pairs of diastereoisomers. The molar ellipticity values of the modified dimers were approximately twofold higher than those of the modified guanosine monomers. These values were decreased appreciably when the spectra of the dimers were obtained at 80 degrees C or in methanol rather than at 25 degrees C in water, suggesting that under the latter conditions there is a stacking interaction between the carcinogen and the neighboring uridine residue. Based on these results, a conformation is proposed for modified GpU. It includes insertion of the benzo[a]pyrene moiety, by rotation of the modified guanine residue about its glycoside bond, coplanar to the neighboring uridine and perpendicular to the phosphodiester backbone.

Current concepts on mechanisms of chemical carcinogenesis

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Cell and microsome mediated binding of 7,12-dimethylbenz(a)anthracene to DNA studied by fluorescence spectroscopy

Fluorescence spectra of DNA isolated from hamster embryo cells incubated with 7,12-dimethylbenz(a)anthracene, or DNA modified in a microsomal system by reaction with this carcinogen or its 7-hydroxymethyl derivative, were compared to various model compounds. The spectra indicate that the DMBA derivative bound to DNA, in all 3 cases, has a 9,10-dimethylanthracene-like chromophore. They also provide the first evidence of the similarity in structure of the DNA-bound products between 7,12-dimethylbenz(a)anthracene and its 7-hydroxymethyl derivative. Our results are consistent with an activation mechanism that involves saturation of the 1,2,3,4-ring positions.

Test for malignant transformation of rat liver cells in culture: cytology, growth in soft agar, and production of plasminogen activator

Three parameters were evaluated as diagnostic of the malignant potential of cultured rat liver epithelial cells: cytology, growth in soft agar, and production of extracellular plasminogen activator. A total of 22 tumorigenic and nontumorigenic cultures from 15 cell lines were sent coded from their originators to two different laboratories for the evaluation of these three parameters. Cytologic diagnosis and growth in soft agar were reliable means of determining the malignant potential of the cultured cells. However, the production of extracellular plasminogen activator showed little correlation with tumorigenicity. Of cytologic properties evaluated, the two that correlated best with malignant potential were increased cytoplasmic basophilia and and increased nuclear:cytoplasmic ratio.

Glycopeptides from epithelial cell mutants: temperature sensitive for the transformation phenotype

Fucose-labelled glycopeptides obtained from the cell surfaces of normal and transformed epithelial cells were compared by co-chromatography on Sephadex G-50. The material obtained from epithelial cells transformed in vitro or from hepatoma cells in culture elutes earlier than the fucose-containing glycopeptides obtained from normal rat epithelial cells. A mutant (TS 223) of a transformed epithelial cell that is temperature-sensitive for maintenance of the transformed phenotype, varies in its Sephadex G-50 profile of cell surface glycopeptides when grown at the permissive (36 degrees C) or the non-permissive temperature (40 degrees C). When grown and labelled at 36 degrees C the gel filtration profile of the glycopeptides resembles that of transformed cells. At 40 degrees C there is an enrichment of later eluting glycopeptides. These differences are more striking in confluent-phase cultures than in log-phase culture. The differences are reversible following upward or downward shifts in growth temperature although there appears to be a lag of at least 6 h before the alteration can be demonstrated by these procedures.

Differential excision from DNA of the C-8 and N2 guanosine adducts of N-acetyl-2-aminofluorene by single strand-specific endonucleases

Purified duck reticulocyte DNA was reacted in vitro with [9-14C]-N-acetoxy-N-acetyl-2-aminofluorene. Hydrolysis of the [14C]-N-acetyl-2-aminofluorene-modified DNA followed by Sephadex LH-20 column chromatography showed that 85% of the DNA-bound [14C]-N-acetyl-2-aminofluorene was N-(deoxyguanosin-8-yl)-N-acetyl-2-aminofluorene and 15% was 3-(deoxyguanosin-N2-yl)-N-acetyl-2-aminofluorene. When this modified DNA was incubated with the single strand-specific nuclease, S1, and the undigested fraction of the DNA was analyzed, there was preferential loss of the quanosine C-8 adduct from the DNA. Moreover, analysis of the nucleosides released by exposure of N-acetyl-2-aminofluorene-modified DNA to a single strand-specific nuclease from Neurospora crassa showed only the guanosine C-8 adduct in the supernatant fraction. These results suggest that, whereas the N-(deoxyguanosin-8-yl)-N-acetyl-2-aminofluorene adduct in DNA causes major conformational changes in the double-stranded helix and localized regions of denaturation, the 3-(deoxyguanosin-N2-yl)-N-acetyl-2-aminofluorene adduct does not cause major distortions of the native DNA structure.

Tumor promoters inhibit spontaneous and induced differentiation of murine erythroleukemia cells in culture

Addition of the potent tumor promoter, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), to murine erythroleukemia cell lines in suspension cultures inhibited both spontaneous differentiation and differentiation induced by hexamethylene bisacetamide (HMBA), dimethyl sulfoxide, or butyric acid. Inhibition was unrelated to cytotoxicity and was reversible. When several plant diterpenes were tested, there was a positive correlation between tumor-promoting activity and inhibition of differentiation. TPA inhibited HMBA-induced differentiation only if added prior to the time of commitment to differentiation, as assayed by scoring for differentiation after transfer of cells from HMBA to fresh medium without HMBA. TPA-mediated inhibition of differentiation was associated with a decrease in globin mRNA accumulation.

Template activity of calf thymus DNA modified by a dihydrodiol epoxide derivative of benzo[a]pyrene

The purpose of the present study was to determine the effects of covalent binding to DNA of a reactive derivative of benzo[a]pyrene on template activity during in vitro transcription with RNA polymerase. Calf thymus deoxyribonucleic acid, modified by reaction with (+/-)-7beta,8alpha-dihydroxy-9alpha, 10alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, was transcribed with Escherichia coli DNA-dependent RNA polymerase. With increasing levels of modification, there was a progressive inhibition of transcription. The inhibition was much greater under conditions where continuous reinitiation of transcription occurred than under conditions where only one RNA chain was synthesized per initiation site. This suggested that the modified sites block the movement of polymerase along the template and prevent recycling of the enzyme. Consistent with this interpretation were analyses of RNA transcripts on sucrose density gradients which showed a progressive decrease in average RNA chain length as the extent of template modification increased. In contrast to the inhibitory effect on chain elongation, evidence was obtained that the modified DNA had an increase in the number of initiation sites for transcription. These results are consistent with separate physical studies indicating that modification of DNA by this benzo[a]pyrene derivative can induce small localized regions of denaturation.

Conformation of DNA modified with a dihydrodiol epoxide derivative of benzo[a]pyrene

The conformation of calf thymus DNA modified by reaction with (+/-)-7beta,8alpha-dihydroxy-9alpha,10alpha-epoxy7,8,9,10-tetrahydrobenzo[a]pyrene, which binds covalently mainly to the 2-amino group of guanosine residues, was studied. With samples in which 1.5 or 2.2% of the bases were modified, there was a slight decrease in Tm during heat denaturation and a slight increase in susceptibility to the single strand specific nuclease S1. In a DNA sample in which 4.5% of the bases were modified, there was an appreciable decrease in Tm and a marked increase in susceptibility to S1 nuclease. The kinetics of the reaction of the modified DNAs with formaldehyde provided evidence for locally destabilized regions ranging from 1 to 7 base plates, depending on the extent of modification. Alkaline and neutral sucrose gradient analyses revealed no evidence for strand breakage in the 1.5 and 2.2% modified samples, although single-strand breaks were found in the 4.5% modified samples. Taken together, these results suggest that DNA molecules containing a covalently bound benzo[a]pyrene derivative have an altered conformation characterized by small localized regions which are destabilized and easily denatured. The conformational changes associated with the covalent binding of the benzo[a]pyrene derivative to native DNA appear to be different from, and less marked, than those associated with the covalent binding of N-2-acetylaminofluorene to native DNA.

Effects of antiinflammatory agents on mouse skin tumor promotion, epidermal DNA synthesis, phorbol ester-induced cellular proliferation, and production of plasminogen activator

The antinflammatory ateroids fluocinoine acetonide, fluocinonide, and fluclorolone acetonide were found to be very effectiveinhibitory agents of mouse skin tumor promotion. These steroids also drastically inhibited epidermal DNA synthesis and epidermal cellular proliferation induced by a phorbal ester tumor promoter. In addition, these compounds were potent inhibitors, of plasminogen activator production in tumor cell cultures. The clinically used non-steroidal antiinflammatory agents oxyphenbutazone, indomethacin, and Seclazone also inhibite tumor promotion but were much less effective. Although these agents are useful against inflammatory disorders in general when given p.o., in our studies they had little effect on inflammation and epidermal cellular proliferation induced by a phorbol ester tumor promoter when given topically. The afore mentioned nonsteroidal antiinflammatory agents also had little effect on epidermal DNA synthesis. Oxyphenbutazone and indomethacin were less potent inhibitors of plasminogen activator production in tumor cells than were the antiinflammatory steroids, and Seclazone produced a negligible inhibition. There is, therefore, a general correlation in the potencies of a series of steroidal antiinflammatory agents for inhibition of tumor promotion and their ability to inhibit plasminogen activator production by tumor cell cultures and epidermal DNA synthesis.

Nucleoside adducts from the in vitro reaction of benzo[a]pyrene-7,8-dihydrodiol 9,10-oxide or benzo[a]pyrene 4,5-oxide with nucleic acids

The covalent binding of benzo[a]pyrene 4,5-oxide and benzo[a]pyrene-7,8-dihydrodiol 9,10-oxide isomer I and isomer II to nucleic acids in aqueous acetone solution has been investigated. Benzo[a]pyrene 4,5-oxide reacted preferentially with guanosine residues. On the other hand, benzo[a]pyrene-7,8-dihydrodiol 9,10-oxide isomer I and II reacted extensively with guanosine, adenosine, and cytidine residues. Time course studies showed that the reactivity of isomer I or isomer II with homopolyribonucleotides followed the order poly(G) greater than poly(A) greater than poly(C). Alkaline or enzymatic hydrolysis of the modified nucleic acids and subsequent chromatography on Sephadex LH-20 columns yielded benzo[a]pyrene-nucleotide adducts. These were enzymatically converted to the corresponding nucleosides which were resolved into several distinct components by high-pressure liquid chromatography. Evidence was obtained for the presence of multiple nucleoside adducts of guanosine, adenosine, cytidine, deoxyguanosine, deoxyadenosine, and deoxycytidine. The HPLC profiles of adducts formed with isomer I were different from the corresponding profiles of adducts formed with isomer II. Structural aspects of these nucleoside adducts are discussed.

Effect of N-2-acetylaminofluorene modification on the structure and template activity of DNA and reconstituted chromatin

This study compares the effects of in vitro modification of native duck reticulocyte DNA by [14C]-N-acetoxy-2-acetylaminofluorene in terms of alterations in DNA secondary structure, ability to reconstitute nucleosome structures in chromatin, and template activity for in vitro transcription. In contrast to the control native DNA, the carcinogen-modified DNA was susceptible to partial digestion by the single-strand-specific endonuclease S1. Depending on the particular conditions, for every [14C]-N-2-acetylaminofluorene residue released, about 5 to 35 base pairs of DNA were also released during the S1 nuclease digestion. Chromatin was reconstituted in vitro utilizing [14C]-N-2-acetylaminofluorene-modified DNA and unmodified chromatin-associated proteins. This reconstituted chromatin showed the same kinetics and extent of digestion by staphylococcal nuclease and similar nucleosome profiles on sucrose gradient density centrifugation as those obtained with native chromatin or chromatin reconstituted with unmodified DNA. The carcinogen-modified DNA and also chromatin reconstituted from this DNA showed, however, marked reductions in their abilities to serve as templates for transcription with Escherichia coli RNA polymerase. These results suggest that the covalent binding of N-2-acetylaminofluorene to DNA produces localized regions of denaturation in the DNA and that this is associated with a marked impairment in template activity during transcription. This modification, however, does not grossly affect the ability of the DNA to interact with chromosomal proteins to form apparently normal nucleosome structures.

Benzo(a)pyrene diol epoxides as intermediates in nucleic acid binding in vitro and in vivo

Evidence has been obtained that a specific isomer of a diol epoxide derivative of benzo(a)pyrene, (+/-)-7 beta,8alpha-dihydroxy-9alpha, 10alpha-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene, is an intermediate in the binding of benzo(a)pyrene to RNA in cultured bovine bronchial mucosa. An adduct is formed between position 10 of this derivative and the 2-amino group of guanine.

Structure of 7,12-dimethylbenz(a)anthracene-guanosine adducts

Arene oxides have been proposed as the reactive intermediates in the process of carcinogenesis induced by polycyclic aromatic hydrocarbons. The present study defines the structures of four guanosine adducts formed by the reaction of 7,12-dimethylbenz[a]anthracene-5,6-oxide with polyguanylic acid. The modified polymer was hydrolyzed to nucleotides and the hydrophobic guanosine adducts separated from unmodified guanosine by LH-20 column chromatograhy. The adducts were further resolved into four components (I-IV) by reverse phase high pressure liquid chromatography. Analysis of the ultraviolet, circular dichroism, mass, and proton magnetic resonance spectra of these compounds, or their acetate and free base derivatives, indicates that in all four compounds the aromatic hydrocarbon is present on the 2 amino group of guanine. Compounds I and IV, and II and III constitute diastereoisomeric pairs, respectively. In the I and IV pair, the adducts result from addition at the 6 position of the original dimethylbenz[a]anthracene oxide, whereas in the II and III pair, the addition occurs at the 5 position. Indirect evidence suggests that trans opening of the oxide occurred in all cases but this remains to be established.

Binding of K- and non-K-region arene oxides and phenols of polycyclic hydrocarbons to polyguanylic acid

Arene oxide derivatives of carcinogenic polycyclic hydrocarbons have been postulated as the reactive intermediates responsible for the in vivo binding of the parent hydrocarbon to cellular nucleic acids. In this study the reaction of 12 different K- and non-K-region arene oxides and 7 benzo(a)pyrene phenols with polyguanylic acid in aqueous acetone solutions has been investigated. The extent of binding of the polycyclic hydrocarbon was monitored by changes in the ultraviolet absorption and fluorescence spectra of the reisolated polyguanylic acid. The most reactive compound was the K-region arene oxide of 7,12-dimethylbenz(a)anthracene. A lower but significant level of binding was detected with the K-region arene oxides of benz(a)anthracene, benzo(a)pyrene, and 3-methylcholanthrene. Very low or negligible binding was detected with the K-region arene oxides of pyrene and phenanthrene; the non-K-region arene oxides of benzo(a)pyrene, phenanthrene, and naphthalene; and all of the benzo(a)pyrene phenols. Significant differences in the fluorescence spectra of polyguanylic acid modified with three different benzo(a)-pyrene arene oxides were observed.

Preferential inhibition of the synthesis of 45S ribosomal rna precursor by N-acetoxyacetylaminofluorene in rat liver epithelial cultures

After exposing a line of rat liver epithelial cells to a single dose of the carcinogen N-acetoxy-2-acetylaminofluorene (N-acetoxy-AAF), a dose-dependent decrease in (3H) uridine incorporation into total cellular RNA was found. Approx. 50% inhibition occurred with 0.5 mug/ml of the compound. The kinetics of the response, the effects of actinomycin D, and the fractionation of the newly synthesized RNA by polyacrylamide gel electrophoresis indicated preferential inhibition of the synthesis of 45S ribosomal RNA precursor and relative sparing of the synthesis of heterogeneous nuclear RNA.

Binding of [3H]benzo(a)pyrene to natural and synthetic nucleic acids in a subcellular microsomal system

Several carcinogens are bound covalently to cellular nucleic acids. This is also the case with polycyclic hydrocarbon carcinogens, but their precise mechanism of in vivo activation to reactive forms and the structure(s) of the nucleic acid adducts are not known. This study demonstrates that in the presence of rat liver microsomes and reduced nicotinamide adenine dinucleotide phosphate there is covalent attachment of tritiated benzo(a)pyrene (BP) to transfer RNA, DNA, certain synthetic polyribonucleotides, and an RNA species endogenous to the microsomal fraction. Evidence has been obtained that the binding occurs mainly to guanine and, to a lesser extent, adenine residues and is not simple an artifact of tritium exchange. The microsomal-mediated binding of [3H]BP to nucleic acids requires reduced nicotinamide adenine dinucleotide phosphate and in inhibited by 7,8-benzoflavone, glutathione, and magnesium. It is enhanced somewhat by the addition of styrene oxide, cyclohexene oxide, and trichloropropylene oxide. These results provide the first evidence that: (a) the microsome-mediated binding of [3H]BP to nucleic acids is not just due to tritium exchange; (b) a derivative of the hydrocarbon is covalently bound to the nucleic acid, and not simply intercalated; (c) there is a preferential binding to guanine residues; and (d) in addition to binding to exogenous nucleic acids, [3H]BP is bound to an RNA species present in the microsomes. Our data are consistent with but do not prove that nucleic acid binding of this polycyclic hydrocarbon proceeds via an epoxide intermediate.

Products obtained after in vitro reaction of 7,12-dimethylbenz[alpha]anthracene 5,6-oxide with nucleic acids

Several lines of evidence suggest that oxide derivatives of carcinogenic polycyclic hydrocarbons are the reactive intermediates for in vivo binding to cellular nucleic acids. In the present study the covalent binding of 7,12-dimethylbenz[alpha]anthracene 5,6-oxide to synthetic homopolymers and nucleic acids in aqueous-acetone solutions has been investigated. Poly(G) was found to be the most reactive nucleic acid and underwent approximately 7-10% modification. Alkaline hydrolysis of the poly(G)-dimethylbenzathracene conjugate yielded chromatographically distinct polycyclic hydrocarbon-modified nucleotides which were further characterized by spectral analyses and enzymatic and chemical degradation. When the oxide was allowed to react with GMP or dGMP, at least two products were obtained in about 1% yield. Acid hydrolysis of the dGMP-dimethylbenzanthracene conjugates liberated the corresponding guanine-dimethylbenzathracene products. Mass spectral analysis of the modified bases provided direct evidence that we had obtained covalent binding of the poly-cyclic hydrocarbon to guanine. The mass spectral cleavage pattern suggest that one of these products is a hydroxydihydro derivative of dimethylbenzanthracene bound to guanine and the other is a dimethylbenzanthracene-guanine conjugate. Additional structural aspects of these guanine derivatives are discussed.

Use of epithelial cell cultures for studies on the mechanism of transformation by chemical carcinogens

Evidence is reviewed for and against four major theories of chemical carcinogenesis. The development of several normal and transformed epithelial cell lines which should be useful for the analysis of this problem is described. The detection of RNA viral particles in cells transformed with chemical carcinogens is a recurrent finding in studies from our own and other laboratories, but the significance of these particles in terms of the mechanism of chemical carcinogenesis remains to be determined. Finally, we have described the first mutants of chemically transformed epithelial cells which are temperature sensitive in the maintenance of the transformed phenotype. These mutants should be particularly useful for detecting the critical biochemical changes that distinguish a chemically induced tumor cell from its normal counterpart.

Temperature-sensitive mutants of chemically transformed epithelial cells

The first temperature-sensitive mutants of epithelial cells transformed with chemical carcinogens have been isolated. Like the wild-type transformed parental cells, the mutants readily grow in agar suspension at 36 degrees, but in contrast to the wild type, they do not do so at 40 degrees. Detailed studies of one of these mutants, TS-223, indicate that at high temperature it also has reduced cloning efficiency in monolayer culture and a lower saturation density. Scanning electron microscopy revealed that at 40 degrees confluent cultures of TS-223 consist of a monolayer of generally flat polygonal cells, whereas 36 degrees cultures contain many patches of piled-up cells that are spherical and have rougher surface membranes. All of these cellular changes are reversible with upward or downward temperature shifts. The temperature-sensitive lesion appears to reside in a host cell gene which modulates expression of the transformed cell phenotype. These mutants may provide a useful system for elucidating the minimal biochemical changes required for expression of the transformed phenotype in epithelial cells.

Growth and structural properties of epithelial cell cultures established from normal rat liver and chemically induced hepatomas

Epithelial cultures established from adult rat liver and from rat hepatomas induced in vivo with aromatic amine carcinogens have been compared by light and electron microscopy and by growth properties in liquid medium and in agar. The morphology and growth patterns of all of these cultures indicate that they have characteristics of epithelial rather than fibroblast cells. The criteria generally used to score for transformation of fibroblasts were not satisfactory for distinguishing normal epithelial cells from hepatoma cells in culture. Growth in agar, however, provides a simple and objective method of scoring for transformed epithelial cells, because only the tumorigenic cells grow in agar. Since none of the normal cultures had hydrocortisone-inducible tryosine aminotransferase, we lack definitive evidence that they are derived from liver parenchymal cells. The outstanding feature in the ultrastructure of the hepatoma cells in culture was the presence of type A and C viral particles. Whereas five hepatoma cultures and a spontaneously transformed normal liver cell line were positive for these particles, five independently isolated cell cultures from normal adult rat liver were negative. Evidence is presented that the viral particles seen in hepatoma cultures are due to activation of latent viruses rather than to in vitro contamination. The possible significance of these particles in hepatocarcinogenesis is discussed.

Type C virus from cell cultures of chemically induced rat hepatomas

Type C RNA viruses are present in cell cultures from transplantable and primary hepatomas induced by aromatic amine carcinogens. Virus yield was markedly enhanced by treating the cells with bromodeoxyuridine. Preparations of rat hepatoma-associated virus obtained from cultures treated with this compound were deficient in DNA polymerase activity.