Chromosomes of primary carcinomas induced by 7,12-dimethylbenz(a)anthracene in the rat

7,12-DMBA-induced rat leukemia: a review with insights into future research

7,12-Dimethylbenz[a]anthracene (DMBA) elicits leukemia in Long-Evans rats (LE). This leukemia is mostly erythroblastic and 30% of leukemias have total and partial trisomy of #2 chromosome and the rest have diploid karyotype. The common duplication site is in 2q26-q34 and N-ras gene is located in 2q34. 7,8,12-Trimethylbenz[a]anthracene (TMBA) also induces similar leukemias. These leukemias reveal a highly specific mutation of N-ras gene as in human leukemias. N-ras mutation is induced 48h after DMBA treatment. Wild type N-ras allele is frequently lost in diploid leukemias but not in trisomy type. Therefore, a gene dosage problem related to the mutant N-ras gene is involved in development of leukemia. Some secondary genetic rearrangements involving abl and H-ras are also observed in cultured leukemia cells. DMBA-induced chromosome aberrations as well as leukemia are enhanced by erythropoietin and blocked by Sudan III given prior to DMBA treatment. This leukemia will provide an important tool for chemical carcinogenesis and leukemia studies.

Ras gene mutations in 7,12-dimethylbenz[a]anthracene (DMBA)-induced rat sarcomas

Tumor induction in rats by 7,12-dimethylbenz[a]anthracene (DMBA) will generate malignancies that display reproducible chromosomal abnormalities involving rat chromosome (RNO) 2. Thus, it has been reported that rat DMBA erythroleukemias display RNO2 abnormalities, which in this case were closely correlated to mutations in the Nras oncogene located in RNO2q34. Our cytogenetic analysis in a series of 17 DMBA-induced rat sarcomas showed that 11 (65%) tumors had a significant increase in RNO2 copy number. Furthermore, the incidence of point mutations in codons 12, 13 and 61 of Hras, Kras, and Nras was examined in the same set of sarcomas, and mutations were detected in three (18%) tumors, in codon 61 of Kras (CAA-->CAT) (1 of 17) and Nras (CAA-->CTA) (2 of 17). We conclude that the high frequency of RNO2 gain was in accordance with previous studies of DMBA-induced rat neoplasms, supporting the idea of a significant role of RNO2 in DMBA carcinogenesis. However, there was no clear-cut relationship between activated Nras and gain of RNO2 material, implying that mutational activation of Nras is not the causative factor underlying the gain of RNO2 copy number in rat DMBA sarcomas, in contrast to what has been suggested for DMBA-induced erythroleukemias.

Aneuploidy precedes and segregates with chemical carcinogenesis

A century ago, Boveri proposed that cancer is caused by aneuploidy, an abnormal balance of chromosomes, because aneuploidy correlates with cancer and because experimental aneuploidy generates "pathological" phenotypes. Half a century later, when cancers were found to be nonclonal for aneuploidy, but clonal for somatic gene mutations, this hypothesis was abandoned. As a result, aneuploidy is now generally viewed as a consequence, and mutated genes as a cause of cancer. However, we have recently proposed a two-stage mechanism of carcinogenesis that resolves the discrepancy between clonal mutation and nonclonal karyotypes. The proposal is as follows: in stage 1, a carcinogen "initiates" carcinogenesis by generating a preneoplastic aneuploidy; in stage 2, aneuploidy causes asymmetric mitosis because it biases balance-sensitive spindle and chromosomal proteins and alters centrosomes both numerically and structurally (in proportion to the degree of aneuploidy). Therefore, the karyotype of an initiated cell evolves autocatalytically, generating ever-new chromosome combinations, including neoplastic ones. Accordingly, the heterogeneous karyotypes of "clonal" cancers are an inevitable consequence of the karyotypic instability of aneuploid cells. The notorious long latent periods, of months to decades, from carcinogen to carcinogenesis, would reflect the low probability of evolving by chance karyotypes that compete favorably with normal cells, in principle analagous to natural evolution. Here, we have confirmed experimentally five predictions of the aneuploidy hypothesis: (1) the carcinogens dimethylbenzanthracene and cytosine arabinoside induced aneuploidy in a fraction of treated Chinese hamster embryo cells; (2) aneuploidy preceded malignant transformation; (3) transformation of carcinogen-treated cells occurred only months after carcinogen treatment, i.e., autocatalytically; (4) preneoplastic aneuploidy segregated with malignant transformation in vitro and with 14 of 14 tumors in animals; and (5) karyotypes of tumors were heterogeneous. We conclude that, with the carcinogens studied, aneuploidy precedes cancer and is necessary for carcinogenesis.

Aneuploidy correlated 100% with chemical transformation of Chinese hamster cells

Aneuploidy or chromosome imbalance is the most massive genetic abnormality of cancer cells. It used to be considered the cause of cancer when it was discovered more than 100 years ago. Since the discovery of the gene, the aneuploidy hypothesis has lost ground to the hypothesis that mutation of cellular genes causes cancer. According to this hypothesis, cancers are diploid and aneuploidy is secondary or nonessential. Here we reexamine the aneuploidy hypothesis in view of the fact that nearly all solid cancers are aneuploid, that many carcinogens are nongenotoxic, and that mutated genes from cancer cells do not transform diploid human or animal cells. By regrouping the gene pool-as in speciation-aneuploidy inevitably will alter many genetic programs. This genetic revolution can explain the numerous unique properties of cancer cells, such as invasiveness, dedifferentiation, distinct morphology, and specific surface antigens, much better than gene mutation, which is limited by the conservation of the existing chromosome structure. To determine whether aneuploidy is a cause or a consequence of transformation, we have analyzed the chromosomes of Chinese hamster embryo (CHE) cells transformed in vitro. This system allows (i) detection of transformation within 2 months and thus about 5 months sooner than carcinogenesis and (ii) the generation of many more transformants per cost than carcinogenesis. To minimize mutation of cellular genes, we have used nongenotoxic carcinogens. It was found that 44 out of 44 colonies of CHE cells transformed by benz[a]pyrene, methylcholanthrene, dimethylbenzanthracene, and colcemid, or spontaneously were between 50 and 100% aneuploid. Thus, aneuploidy originated with transformation. Two of two chemically transformed colonies tested were tumorigenic 2 months after inoculation into hamsters. The cells of transformed colonies were heterogeneous in chromosome number, consistent with the hypothesis that aneuploidy can perpetually destabilize the chromosome number because it unbalances the elements of the mitotic apparatus. Considering that all 44 transformed colonies analyzed were aneuploid, and the early association between aneuploidy, transformation, and tumorigenicity, we conclude that aneuploidy is the cause rather than a consequence of transformation.

Establishment of immortalized alveolar type II epithelial cell lines from adult rats

We developed methodology to isolate and culture rat alveolar Type II cells under conditions that preserved their proliferative capacity, and applied lipofection to introduce an immortalizing gene into the cells. Briefly, the alveolar Type II cells were isolated from male F344 rats using airway perfusion with a pronase solution followed by incubation for 30 min at 37 degrees C. Cells obtained by pronase digestion were predominantly epithelial in morphology and were positive for Papanicolaou and alkaline phosphatase staining. These cells could be maintained on an extracellular matrix of fibronectin and Type IV collagen in a low serum, insulin-supplemented Ham's F12 growth medium for four to five passages. Rat alveolar epithelial cells obtained by this method were transformed with the SV40-T antigen gene and two immortalized cell lines (RLE-6T and RLE-6TN) were obtained. The RLE-6T line exhibits positive nuclear immunostaining for the SV40-T antigen and the RLE-6TN line does not. PCR analysis of genomic DNA from the RLE-6T and RLE-6TN cells demonstrated the T-antigen gene was present only in the RLE-6T line indicating the RLE-6TN line is likely derived from a spontaneous transformant. After more than 50 population doublings, the RLE-6T cells stained positive for cytokeratin, possessed alkaline phosphatase activity, and contained lipid-containing inclusion bodies (phosphine 3R staining); all characteristics of alveolar Type II cells. The RLE-6TN cells exhibited similar characteristics except they did not express alkaline phosphatase activity. Early passage RLE-6T and 6TN cells showed a near diploid chromosome number.(ABSTRACT TRUNCATED AT 250 WORDS)

Karyotype instability and altered differentiation of rat sarcoma cells after retroviral infection

The karyotypic and phenotypic stability of cultured rat fibrosarcoma cells was challenged by infection with Moloney murine sarcoma virus (MoMuSV). After transformation, the spindle-like morphology of the parental HH-16 cl.2/1 cells had altered to a rounded phenotype, which was maintained in tumors produced by inoculating transformed cells into congenic animals. In contrast to the parental cells, transformed cells lacked cables of cytokeratins 14-16 and 19 and showed reduction of the mesenchymal marker protein vimentin. Additionally, the morphologically altered cell clones tf-1 to tf-3 had lost growth arrest in the presence of dexamethasone. The DNA of the transformed cells contained between four and six randomly integrated proviral copies. Karyotypic alterations were manifested by reduction of morphologically intact chromosomes in the MoMuSV-transformed cells together with increase of structural aberrations. Three additional markers were identified in the virus-transformed cell clones. Karyotypic instability induced by MoMuSV infection appeared closely related to reduction of the cellular differentiation status, although only cells of clone tf-1 had increased metastatic potential.

Nonrandom involvement of chromosome 4 in the progression of rat prostatic cancer

Owing to progression of the original spontaneous Dunning R-3327 rat prostatic cancer, a large series of transplantable prostatic tumors have been isolated that differ widely in their histological degree of differentiation, growth rate, androgen sensitivity, and metastatic ability. Using these parameters as criteria, the full spectrum of disease progression is represented within this Dunning system of rat prostatic cancers, ranging from slow-growing, well-differentiated, androgen-sensitive, nonmetastatic forms to fast-growing, anaplastic, androgen-independent, highly metastatic forms. Cytogenetic analysis of the two least progressionally advanced Dunning cancers (i.e., histologically well-differentiated, slow-growing, nonmetastatic variants) demonstrated no structural or numerical chromosomal aberration, suggesting that the initial development of prostatic cancer may not require detectable cytogenetic changes. In contrast, all 16 of the progressionally more advanced Dunning variants analyzed had a series of characteristic structural and/or numerical chromosomal aberrations that minimally involved chromosome 4. This nonrandom involvement of chromosome 4 was consistently observed regardless of whether the karyotype of the cancer was near-diploid or hyperaneuploid, suggesting that chromosome 4 aberrations are specifically involved in the progression of rat prostatic cancer. In addition, all four variants that were highly metastatic had, besides aberration of chromosome 4, structural aberrations involving chromosomes 1, 2, and 11. Of the 14 variants that did not have a high metastatic ability, only two had a similar aberrations involving chromosomes 1, 2, 4, and 11, suggesting that these specific chromosomal aberrations may be necessary, albeit not sufficient, for a high metastatic ability of rat prostatic cancers.

Chemically induced aneuploidy in mammalian cells: mechanisms and biological significance in cancer

A growing body of evidence from human and animal cancer cytogenetics indicates that aneuploidy is an important chromosome change in carcinogenesis. Aneuploidy may be associated with a primary event of carcinogenesis in some cancers and a later change in other tumors. Evidence from in vitro cell transformation studies supports the idea that aneuploidy has a direct effect on the conversion of a normal cell to a preneoplastic or malignant cell. Induction of an aneuploid state in a preneoplastic or neoplastic cell could have any of the following four biological effects: a change in gene dosage, a change in gene balance, expression of a recessive mutation, or a change in genetic instability (which could secondarily lead to neoplasia). To understand the role of aneuploidy in carcinogenesis, cellular and molecular studies coupled with the cytogenetic studies will be required. There are a number of possible mechanisms by which chemicals might induce aneuploidy, including effects on microtubules, damage to essential elements for chromosome function (ie, centromeres, origins of replication, and telomeres), reduction in chromosome condensation or pairing, induction of chromosome interchanges, unresolved recombination structures, increased chromosome stickiness, damage to centrioles, impairment of chromosome alignment, ionic alterations during mitosis, damage to the nuclear membrane, and a physical disruption of chromosome segregation. Therefore, a number of different targets exist for chemically induced aneuploidy. Because the ability of certain chemicals to induce aneuploidy differs between mammalian cells and lower eukaryotic cells, it is important to study the mechanisms of aneuploidy induction in mammalian cells and to use mammalian cells in assays for potential aneuploidogens (chemicals that induce aneuploidy). Despite the wide use of mammalian cells for studying chemically induced mutagenesis and chromosome breakage, aneuploidy studies with mammalian cells are limited. The lack of a genetic assay with mammalian cells for aneuploidy is a serious limitation in these studies.

Cellular DNA regions involved in the induction of rat thymic lymphomas (Mlvi-1, Mlvi-2, Mlvi-3, and c-myc) represent independent loci as determined by their chromosomal map location in the rat

The induction of thymic lymphomas by Moloney murine leukemia virus in the rat is linked to provirus integration in at least four independent cellular DNA regions (Mlvi-1, Mlvi-2, Mlvi-3, and c-myc). Because sequences homologous to at least three of these regions (Mlvi-1, Mlvi-2, and c-myc) map to chromosome 15 in the mouse, the question was raised whether they are closely linked in the rat genome and whether provirus integration in any one of these regions affects the same functional domain in rat DNA. In this study, we identified the chromosomal map location of Mlvi-1, Mlvi-2, and Mlvi-3 in the rat by using mouse-rat somatic cell hybrids that lose the rat chromosomes. The results showed that Mlvi-1 maps similarly to c-myc to chromosome 7, and Mlvi-2 maps to chromosome 2. Mlvi-3 probably maps to chromosome 15. We conclude that Mlvi-1, Mlvi-2, and Mlvi-3 are separate and independent genetic loci. Although Mlvi-1 and c-myc map to the same chromosome, they are not related, as determined by hybridization and restriction endonuclease mapping. The chromosomal map location of Mlvi-1 to chromosome 7 and Mlvi-2 to chromosome 2 is interesting, since chromosomal aberrations involving these two chromosomes are reproducibly observed in rat neoplasias induced by a variety of agents.

Chromosome and DNA analyses of rat 13762NF mammary adenocarcinoma cell lines and clones of different metastatic potentials

Chromosome morphologies revealed by Giemsa-banded karyotypes and chromosome numbers were compared between parental tumor-, lymph node- and lung metastasis-derived rat 13762NF mammary adenocarcinoma cell lines and clones having different spontaneous metastatic potentials. Although chromosome numbers in the cell lines and clones generally correlated with DNA content by flow cytometry, ploidy did not correlate with spontaneous metastatic potentials. Chromosome number and DNA content drifted during prolonged in vitro growth in each of the cell lines and clones. Common chromosome rearrangements were found, confirming a common origin for all the cell lines and clones, and the frequency and appearance of the individual marker chromosomes fluctuated during in vitro growth. Karyotypic analyses revealed that the markers coinciding with phenotypic drift in spontaneous metastatic potential and other biological properties of parental tumor-derived clones MTC and MTF7 and lung metastasis-derived clone MTLn3 involved chromosomes 3, 4, 5, 6, and 8. Clone MTC exhibited a shift in several markers and an increase in metastatic potential at passage T20, while clone MTF7 displayed a lesser spontaneous metastatic potential at high passage (T34) concomitant with an increase in the frequency of certain marker chromosomes. Lung metastasis-derived clone MTLn3 also exhibited a shift in some marker chromosomes, colonization preference and metastatic potential to lung and lymph nodes at high tissue culture passages. The changes in marker chromosomes during in vitro passage of clones MTC and MTLn3 suggested the presence of at least two cell subpopulations which could be responsible for the observed shift in spontaneous metastatic properties. Karyotypic features of the 13762NF cell lines and clones indicate that subtle cytogenetic changes, in contrast to gross chromosomal abnormalities, may be more important in determining metastatic phenotype.

Acid phosphatase-producing androgen-independent subline of rat prostatic adenocarcinoma (Dunning R3327 tumor) in cell culture

Establishment of a cell line derived from the androgen-independent subline of rat prostatic adenocarcinoma (Dunning R3327 tumor) is reported. Cells of this line produced acid phosphatase. When the cultured cells were transplanted to Copenhagen rats, solid tumors were formed. Histologically, the tumor consisted of spindle-shaped, large and bizarre polygonal cells; this feature was almost identical to that of the original tumor. Chromosomes were in the triploid range with seven frequently appearing marker chromosomes.

Chromosome changes in rat embryo cell lines transformed by temperature-sensitive mutants and sheared DNA of herpes simplex virus

The chromosomes of six rat embryo cell lines transformed with herpes simplex virus (HSV) temperature-sensitive (ts) mutants were examined at different passages of in vitro cultivation. Two cell lines were predominantly diploid, one cell line was hyperdiploid, one cell line was pseudodiploid, and two cell lines were hypotetraploid. In near-diploid cell lines chromosome No. 9 was most frequently involved in chromosome changes. All three cell lines derived from tumors obtained after one transplantation of HSV-transformed cells into baby rats were pseudodiploid, but each had different marker chromosomes. Chromosome No. 15 was involved in the formation of two out of four marker chromosomes. Four cell lines derived from tumors developing after two and three transplantations were hypodiploid and showed large chromosome variation. The occurrence of 25 marker chromosomes in three tumor-derived cell lines resulted in gains in parts from chromosomes No. 2, 6, and 7. One marker chromosome had a homogeneously faintly stained region. Chromosomes No. 2, 3, 7, and 12 were more frequently involved in the formation of marker chromosomes. No chromosome change was found to be specifically associated with HSV-induced transformation of rat cells, but chromosome changes in tumor-derived cell lines may provide selective advantage for survival and autonomous growth in the host animal.

Chromosome and cell surface marker studies in 1-propyl-1-nitrosourea-induced thymic lymphomas of the rat

Immunological cell surface markers were studied in seven transplantable 1-propyl-1-nitrosourea-induced thymic lymphoma lines in F344 rats by reactivity to anti-Thy-1.1, anti-rat Ig (anti-Ig), and anti-rat T-cell (anti-T) sera, and by the capacity to form rosettes with guinea pig red blood cells. All the tumor lines were estimated to be sensitive to anti-Thy-1.1 but insensitive to anti-Ig serum in the presence of complement. The differences in reactivity to anti-T serum and rosette-forming capacity (RFC) allowed classification of the lines into three types. In type I, three lines were highly sensitive to anti-T serum but low in RFC, indicating that these lymphomas probably originated from relatively mature intrathymic T cells. In type II, two lines were moderately sensitive to anti-T serum and relatively high in RFC, indicating that these lymphomas derived from intrathymic T cells. In type III lymphomas, the remaining two lines were not only insensitive to anti-T serum but also low in RFC, suggesting that these lymphomas might have arisen from immature precursors of T and/or B cells. The chromosome study revealed that type I lymphomas were diploid, with slight numerical and structural variations. Type II lymphomas were pseudodiploid or hypotetraploid, with considerable variation in the number and morphology of chromosomes. Type III lymphomas had a diploid or hyperdiploid constitution, with a moderate degree of karyotypic variation. Neither consistent nor common karyotypic alterations among the seven lines were found, although the karyotypic instability seemed to be related to the immunological types of the lymphoma lines, possibly reflecting the differentiation process of the target cells involved in the malignant transformation.

Sex chromosome aberrations involving loss and translocation of tumor-inducing loci in Xiphophorus

Karyotypic changes involving a deletion and a translocation of certain sex-linked tumor-inducing loci of the platyfish, Platypoecilus (Xiphophorus) maculatus, have been investigated. The effects of these chromosome aberrations on tumor formation and viability of the fish are discussed.

Vulnerability of specific rat chromosomes to in vitro chemically induced damage

The treatment of rat embryo secondary cultures with DMBA or DMBA-3H for 5, 9, or 24 h resulted in chromosome damage consisting mainly of chromatid type aberrations. There was an increase in the percentage of labelled nuclei and metaphases with increasing length of exposure. In terms of incidence of chromatid lesions, the largest telocentric chromosome (No. 2) was the most susceptible of the autosomes. Banding pattern analysis demonstrated that the region associated with negative band 2q24 of the No. 2 chromosome had the highest number of lesions. An increased accumulation of DMBA-3H label occurred in approximately the same chromatid area of a small fraction of cells exposed for either 5 or 9 h prior to mitosis. The complete loss of DMBA-3H chromosomal labelling after DNAse treatment suggests that the visible grains represent carcinogen-bound DNA. After DMBA and BrdUrd, there was an increase in the number of sister chromatid exchanges compared to controls treated with BrdUrd only; the location of the exchange points on chromosome No. 2 was similar in samples treated with either DMBA and BrdUrd or BrdUrd alone. Additional experiments with thymidine-3H showed that the non-random chromatid lesions on chromosome No. 2 may result from endogenous radiation from the incorporated tritium. These studies demonstrate that a specific chromosome may be affected by diverse agents and that chromatid lesions frequently occur at the site of sister chromatid exchanges.