Chromosome analyses and anchorage-independent growth of SV40-induced morphologically transformed epithelial cells from amniotic fluids

Spontaneous cell transformation: karyoplasts derived from multinucleated cells produce new cell growth in senescent human epithelial cell cultures

Previously, it was shown that SV40-induced cell transformation of human diploid (2N), epithelial cells was a dynamic process of nuclear and cellular events. In this process, nuclei of polyploid (above 2N) cells broke down into multinucleated cells (MNCs) by amitotic division. An induced mass karyoplast (i.e., small cell with reduced amount of cytoplasm) budding process from the MNCs produced transformed cells with extended life span (EL) and altered morphology. In this study, without the use of SV40 and no induction of karyoplast budding, the same sequence of cellular events was found to occur spontaneously for the same type of cells at replicative senescence (no mitosis). These cell transformation events were followed by phase-contrast photography of living cell cultures. Primary, diploid, epithelial cell cultures grew for two to three passages and then entered senescence. Cells remaining in the cultures after widespread cell death (mortality stage 1; M1) developed the typical large, flat-cell morphology of senescence with increased cytoplasmic volume. Some of these cells were MNCs, mostly with two to four nuclei. Cytokinesis in MNCs and spontaneous karyoplast budding from MNCs were observed, and new, limited EL cell growth was present either in foci of cells or as prolonged cell growth over one to two passages. At the end of their replicative phase, the EL cells entered another death crisis (M2) from which no cells survived. In M2-crisis, rarely transformed cells appear with immortal cell growth characteristics (i.e., cell lines). Numerous examples of fragmentation or amitosis of polyploid nuclei in the production of multinucleated cells (MNCs) are presented. Such nuclear divisions produced nuclei with unequal sizes, which suggest unbalanced chromosomal segregations. The nuclear and cellular events in cell transformation are compared with a natural (no induction) occurrence of MNC-offspring cells in mammalian placentas. The possibility of a connection between these two processes is discussed. And finally the difference in the duration of EL cell growth from SV40-MNCs versus from senescent-MNCs is ascribed to increased mutational load in SV40-induced MNCs as compared with that in senescence MNCs.

The origin of transformed cells. studies of spontaneous and induced cell transformation in cell cultures from marsupials, a snail, and human amniocytes

Transformation of cells in culture is a model system for carcinogenesis, and two major theories (i.e., mutagenesis and aneuploidy) have emerged from in vitro and in vivo studies. A third view is presented here on the initial steps in the change of primary cells to extended life cells, and their change to immortalized cells. Both changes involve identical, microscopically visible cell abnormalities hitherto dismissed as cell degenerative characteristics. The major cell changes (i.e., giant cells, nuclear fragmentation to form multinucleated cells [MNC]) translated into genetic terms begin with the creation of polyploidy by DNA endoreduplication, followed by amitotic division of these giant cells to produce MNC. Individual nuclei, surrounded by a cell membrane, bud from the surface of the MNC, and represent the origin of the transformed cells. Induced budding by a protease treatment of MNC suggests that the extracellular matrix is an inhibitor of the budding process from human MNC. The production of the MNC is a genetic process determined by two abnormal events (i.e., overproduction of DNA and amitotic chromosomal segregation) during which there are possibilities for different genetic mechanisms to produce inherited variability within and between MNC. These concepts are discussed in regard to carcinogenesis, and by extension its possible prevention by use of the special cytopathic cell changes in carcinogen testing and in clinical screening programs.

'Spontaneous' transformation as aberrant epigenesis

The NIH 3T3 line of cells has particular advantages for studying the dynamics of change in cellular phenotype in response to environmental conditions. Similar to stem cell growth during development, the cell line changes its phenotype under growth constraints that elicit differentiation or, alternatively, it maintains its original state over many replication cycles when grown without constraint. Unlike many cell types which respond by undergoing terminal differentiation, the NIH 3T3 cells continue to multiply indefinitely following an induced alteration in phenotype; the heritability of this change may thus be analyzed under stringent conditions of cell culture. During the course of over two years of frequent passage at low density in high calf serum (CS) concentration, a new subline developed which exhibited a consistent capacity to respond quickly and pervasively to growth constraints with an increase in saturation density, development of transformed foci in confluent cultures and altered appearance of isolated colonies. A retrospective study was undertaken, with cells from cryopreserved samples, of the course of changes in responsiveness of the cells to growth constraint leading up to the highly responsive state. Three stages were discerned, the first with an initially high capacity of a small fraction of cells to produce diffuse foci, but with a rapid decline in this capacity with frequent low density passages; the second stage, extending over more than 200 passages, of refractoriness to transformation; and the third stage (which probably arose by mutation) in which there is a consistent transformation-related response by the entire population to growth constraint, a response which has remained relatively constant over some 100 passages. A striking and novel feature of the third stage is seen on cloning the cells. Almost all the colonies obtained by cloning cells from post-confluent, growth-inhibited cultures are distinctly different in morphology from those obtained by cloning cells from the frequent low density passages. The pervasiveness of this morphological change among the clones is unmistakable evidence for a heritable adaptive response to growth constraint by most if not all of the cells in the population. The population-wide response of the cells of the third stage offers the opportunity for a rigorous, quantitative analysis of the nature of this type of persistent cellular change. Although cells of the third stage may be of mutational origin, their pervasive heritable response once the variant population is established supports the concept of progressive state selection which postulates that transformation can arise by the continuous fluctuation of growth states within cells, accompanied by the progressive selection of those states best suited to function under the selecting constraint. Relevance of the concept to the process of differentiation under growth constraint is considered.

Establishment of transformed swine fibroblast cell lines using SV40 large T antigen

Swine testicle cell lines were established by transformation of primary swine testicle (PST) cells with an SV40 plasmid (pSV3-neo), which contains genes conferring resistance to neomycin and expressing SV40 large T antigen. Plasmid DNA was transfected into PST cells using a lipofection system. Two related plasmids, pSV2-neo and pSV5-neo, failed to induce transformed cells. Cells transformed with pSV3-neo formed single colonies that were resistant to the antibiotic, G418, and expressed large T antigen. Upon two cycles of cloning by endpoint dilution method, three transformed clones, designated transformed swine testicle (tST)-3, tST-14 and tST-18, were selected and characterized in regards to cell replication and susceptibility to swine viruses. The resultant clones were compared with a counterpart non-transformed ST cell line (ATCC-ST). The three tST cell lines showed longer or the same doubling times and higher saturation densities compared to ATCC-ST cells. These cells were free from a range of adventitious agents and supported the replication of porcine parvovirus (PPV), pseudorabies virus (PRV) and transmissible gastroenteritis virus (TGEV), comparable to ATCC-ST cells. All three cell lines have been maintained in continuous cultures for over 60 passages with no changes in growth characteristics. These findings indicate that lipofection with pSV3-neo is an efficient means for the introduction of exogenous DNA into porcine cells and for establishment of transformed immortalized cell lines.

Preferential sites for viral integration on mammalian genome

Chromosomal localization of human papillomavirus (HPV) 16 and 18 on human cervical carcinomas and epithelial cell lines obtained after HPV transfection has uncovered a nonrandom association of viral integration and specific genome sites. Fragile sites appear to be preferential targets for viral integration because of their structural and functional characteristics through which chromosomal anomalies, alterations in protooncogene activity, and gene amplification can occur. Individually or in association, such changes lead to the acquisition of an unlimited cell growth potential but not tumorigenicity. Genetic instability and uncontrolled cell division resulting from HPV integration increase the cell's susceptibility to other exogenous carcinogenic factors that may complete the process of neoplastic development.

Chromosome analyses of human mammary epithelial cells at stages of chemical-induced transformation progression to immortality

Benzo(a)pyrene induced extended life (EL) (i.e., a longer than normal proliferative lifespan before senescence) of human breast cells in culture. From many EL cell cultures immortalized cells emerged only once in each of two separate experiments. The original EL cells were mostly normal diploid with only a small percentage of tetraploid cells. The two immortalized cell lines, however, were near diploid, each containing a set of chromosomal aberrations that were present in all the cells analyzed, confirming the clonal origin of both cell lines. For cell line 184A1 the aberrations consisted of deficiencies only, whereas a combination of deficiencies and duplications characterized the 184B5 line. None of the individual aberrations of each set were shared by both cell lines. Both sets of aberrations have remained stable for over 150 population doublings, while some of the other chromosomes showed breakage and reunions. These data are discussed in regard to types of mutations in the sequence of changes from primary to immortalized cells, and it is concluded that the sets of aberrations most likely originated as multiple events in a single cell.

Chromosome instability in cell lineages of amniocyte clones morphologically transformed by Simian virus 40

Seven clonally derived human amniocyte cell lines from independent Simian virus-40 transformation events were analyzed for chromosomal aberrations in the precrisis stage. The distribution of chromosomal aberrations over the haploid complement was nonrandom. With increasing passage number the number of involved chromosomes increased, but the type of aberrations were the same: dicentric chromosomes and translocations due, respectively, to fusion of telomeres and breakage in the centromere regions. Interruptions to telomeres were also evident from extrachromosomal material terminally on the p arms, especially, of chromosomes #7 and #11. The cytogenetic similarities between the present results and those in transfection experiments are mentioned. All seven clones became highly heterogeneous over 10 to 20 passages (i.e., 30-60 population doublings) when, in general, transformed amniocytes enter crisis. The chromosomes which were nonrandomly involved in aberrations exhibited great diversity in types of rearrangements. Thus, for each clone multiple different rearrangements for specific chromosomes were observed in different cells. Most of these aberrations had a single step origin, which implies that breakage occurred repeatedly for the same chromosome in different cells, suggesting that the specific chromosome had become unstable in an earlier cell generation. Thus, we observed development of clonal chromosome instability, a phenomenon not reported for viral transformed cells.

Induction of tumorigenesis and chromosomal abnormalities in human amniocytes infected with simian virus 40 and Kirsten sarcoma virus

Cell cultures of epithelial-like human amniocytes were infected with simian virus 40 (SV40) and Kirsten sarcoma virus (KSV) in various sequential orders, and tested for agar growth, chromosome abnormalities, and tumorigenesis in the nude mouse assay. We observed that regardless of the order in which the viruses were introduced, the doubly infected cells always exhibited the typical SV40 premalignantly transformed phenotype before changing to the malignant phenotype. All doubly transformed cells from different cell donors produced tumors in adult and suckling nude athymic mice, classified as poorly differentiated sarcomas. Infection with SV40 alone conferred extended life span and accelerated growth without the malignant capability of tumor production. Kirsten sarcoma virus alone produced only focal cell alterations with no change in cell longevity or tumorigenesis. Chromosome studies of the premalignant and malignant cells from one cell donor did not reveal any significant clonal development for marker chromosomes in either cell line. Chromosome 12, which carries the homologous cellular oncogene to KSV, had no increase in aberrations in the malignant cells. Chromosome 8 was most often involved in aberrations, and the most frequent aberration for both series was dicentric chromosomes due to telomere fusion. For other translocations the breakpoints were almost exclusively in the centromere regions. The vulnerability of telomere and centromere regions to the free virus present in these precrisis cells is discussed, and similarities in regard to types of aberrations in transfection experiments are noted.

Induction of transformation and continuous cell lines from normal human mammary epithelial cells after exposure to benzo[a]pyrene

Rapidly growing primary cultures of normal human mammary epithelial cells (HMEC) were exposed to 1 microgram of benzo[a]pyrene (B[a]P) per ml for two or three 24-hr periods. The B[a]P-treated populations consistently contained cells displaying a longer period of active growth in culture compared to the untreated control cells. Widespread heterogeneity in morphology and growth patterns was evidenced in these "extended life" (EL) cultures, with multiple sequential changes in these parameters occurring during the course of their life in culture. Two apparently immortal continuous cell lines have thus far emerged from these EL cultures. These lines have been characterized to be of human mammary epithelial origin and derived from the originally treated HMEC specimen. The continuous lines do not appear to be malignantly transformed as they do not cause tumor formation in nude mice and show little or no anchorage-independent growth. Nonetheless, they have acquired several properties characteristic of tumor-derived HMEC, which distinguish them from their normal progenitors. These cell lines, as well as the EL strains, may provide useful substrates for studies to determine what agents can induce further transforming events. Additionally, analysis of the multiple steps occurring in the El cultures, as well as in the emergence of the continuous cell lines, could potentially elucidate the processes occurring during human epithelial cell carcinogenesis and escape from senescence.

Cytogenetic analysis of SV40-transformed human breast epithelial cells

The SV40-transformed breast epithelial cell lines established by Chang et al. [1] were shown to be hypotetraploid and characterized by six chromosome markers: M1 i(1q), M2 del(1)(q21), M3 i(6p), M4 del(1)(q11), M5 t(8p;12q), and M6 dir dup(11)(p12 leads to pter). The presence of common chromosome markers indicates that these cell lines are probably derived from the same original transformed cell.

Anchorage independent growth of SV40 transformed human epithelial cells from amniotic fluids: differences within and among cell donors

Cells from seven individuals were cultured separately for the induction of morphological transformation by SV40. Sixty-three transformed colonies were tested for anchorage independent growth in soft agar at various passage levels. Colony formation was consistent for all clones of respective cell donors. Four donors yielded clones that grew in soft agar in the first passage. Clones from three donors were similar to controls and formed no colonies. The size of the agar colonies was constant in the early passages. Size differences were observed in later passages and for negative clones that gained anchorage independence during time in culture. The early passage positive type of anchorage independence is expressed concomitantly with morphological transformation. Considering that the clonal isolates were genetically homogeneous within cell donors and heterogeneous among cell donors, it is concluded that the phenotype of anchorage independence is determined by at least two genetic mechanisms; namely, the genotype of the cell donor (the hereditary type) or by culturally derived new genetic variability, or both. Family history on cancer incidence showed that one grandparent for each of the four positive donors for the hereditary type of anchorage independence had cancer, whereas the grandparents of the three negative donors were asymptomatic. The incidence of cancer did not appear to be age related. Chromosome analyses of two morphologically transformed colonies from each of the cell donors by the in situ technique, showed diploid and tetraploid cells and a small number of cells with rearrangements. It is concluded as previously that the progenitor transformed cell to the colony of cells is normal diploid.