Metastatic clones selected from an RSV-induced mouse sarcoma share a common marker chromosome

High Chemotactic Motility and Growth in Hard Agar of a Variant of RSV-Transformed Fibroblasts are Lost in Late Passages

Cloning efficiency in hard agar (0.6%) and high chemotactic migration toward fibroblast conditioned medium have been shown to characterize metastatic tumor cells. We studied growth in 0.6% agar and chemotaxis of two lines of Rous Sarcoma virus-transformed Balb/ c3T3 cells, B77/3T3 (low metastatic) and AA12 (high metastatic), and compared them to their non-transformed counterpart, in order to verify whether these properties were maintained during several subcultivations. Cells were cryopreserved at early passages and thawed for experiments. Both assays were performed on freshly thawed cells (4-6 weeks in culture) and on cells which had been cultured 15-20 weeks after thawing. B77/3T3, which are tumorigenic but low metastatic and which have a very low cloning efficiency in hard agar (0.1-1%), showed a chemotactic response to Balb/c3T3 conditioned medium about two-fold higher than control Balb/c3T3. This response did not change with time in culture. AA12 cells, a genetic unstable variant of B77/3T3 selected for its growth in hard agar (0.6%), had a high cloning efficiency in hard agar and showed a high chemotactic motility (three-fold the controls). High growth in 0.6 % agar and high chemotaxis of AA12 were lost in late passages, where cells behaved as the controls. It seems that besides the already reported variation in anchorage-independent growth, genetically unstable tumor cells can also have important variations in chemotactic motility during subcultivations.

Increasing metastatic potential is associated with induced chromosome 14 translocations in a previously nonmetastatic murine melanoma cell line

The purpose of these studies was to demonstrate causal effects of abnormalities induced in mouse chromosome 14 on tumorigenicity and metastasis using the K-1735 murine melanoma cell line. Because anomalies in chromosome 14 have previously been associated with increases in metastatic potential, we induced chromosome 14 anomalies in a nonmetastatic K-1735 clone 10 cells initially containing two normal copies of chromosome 14 by treatment with mitomycin C. Clone 10-M1, in which a small population of cells (approximately 4%) contained translocations involving chromosome 14, was isolated and injected into athymic nude mice. Unlike the parental C-10 cells, C-10 M1 cells produced experimental lung metastases. Chromosomal analysis of cell cultures established from both subcutaneous tumors and lung metastases indicated that approximately 35% of the cell population contained chromosome 14 anomalies suggesting that this chromosome may play a role in tumor growth and metastasis.

Chromosome 14 alteration is associated with increased collagenase expression and the metastatic potential of murine melanomas

The purpose of this study was to correlate abnormalities in chromosome 14 with the invasive metastatic phenotype of K-1735 murine melanoma cells. Low metastatic K-1735 clone 10 and clone 23 cells were transfected with either basic fibroblast growth factor (bFGF), Kaposi's fibroblast growth factor (kFGF), or c-H-ras gene. A high number of bFGF- and H-ras-transfected cells exhibited chromosome 14 rearrangements. These cells also had increased expression of collagenase IV. The kFGF-transfected cells were highly metastatic but did not have increased expression of collagenase type IV. The kFGF-transfected cells were highly metastatic but did not have increased expression of collagenase type IV, nor abnormalities in chromosome 14. The data imply that karyotypic changes in chromosome 14 are associated with increase expression of collagenase type IV.

Expression of V-SRC and chromosome analysis of a newly established cell line from rat sarcoma induced by an avian retroviral strain

In the newly established rat sarcoma cell line LSR-SF (SR) expression of pp60v-src was detected. Karyotype analyses revealed various chromosome aberrations during prolonged passaging of the tumor cells in vitro. Polyploidy was found to be a characteristic feature of the line studied. A large metacentric chromosome persistently present in the cells was accepted as a line marker.

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.

Molecular oncogenetics of metastasis

Metastasis is a complex non-stochastic process that is most likely the result of genetic and epigenetic interactions of a wide variety of genes. The search for a single gene which can encompass such a pleiotropic response as to account for the observed phenotypic characteristics of metastatic tumour populations has been unsuccessful. Particular studies involving gene transfection, subtractive hybridisation and cell fusion are beginning to identify specific genes which contribute to metastasis in some cell types. However, such analyses are complicated by the inherent genetic instability and phenotypic heterogeneity present in tumour populations. A more detailed understanding of the metastatic process may require an abandoning of current generalised approaches to metastasis in favour of concentrating on key components of the metastatic cascade such as adhesion and invasion.

Involvement of unstable chromosomal regions containing C-heterochromatin and fragile sites in the integration of amplified dhfr domains

A 10(-3) M methotrexate (MTX)-resistant variant (H2), selected from the murine fibrosarcoma line B77-3T3/AA12, was characterized after 5 (H2 MTXRes I) and 9 (H2 MTXRes II) months of in vitro propagation in the presence of the drug. Southern blot hybridization of wild-type and H2 MTXRes DNAs confirmed amplification of the dhfr gene without apparent rearrangements in its structure. Cytogenetic analysis revealed that double minutes (DMs) predominated in H2 MTXRes I, whereas homogeneously staining regions (HSRs) were the main feature of H2 MTXRes II cells. HSRs, shown to contain dhfr sequences by in situ chromosome hybridization, were localized within two rearranged chromosomes, designated as m1 and m2 because of their derivation from the marker chromosome m of AA12 cells. This chromosome, characterized by two interstitial C bands adjacent to two nonstaining gaps, was no longer observed in H2 MTXRes II cells. A role for nonrandom involvement of chromosome m in the integration of amplified DNA is suggested by the finding of another HSR-chromosome, m3, derived from m, in an independent MTXRes clone (B1). Rearrangement in one of the unstable C-band/gap regions of chromosome m is proposed as the unifying mechanism that may account for the outcome of the three HSR chromosomes observed.

Rous sarcoma virus-transformed fibroblasts adhere primarily at discrete protrusions of the ventral membrane called podosomes

Rous sarcoma virus-transformed BHK cells (RSV/B4-BHK) adhere to a fibronectin-coated substratum primarily at specific dot-shaped sites. Such sites contain actin and vinculin and represent close contacts with the substratum as revealed by interference reflection microscopy. Only a few adhesion plaques and actin filament bundles can be detected in these cells as compared to untransformed parental fibroblasts. In thin sections examined with transmission electron microscopy (TEM) these adhesion sites correspond to short protrusions of the ventral cell surface that contact the substratum at their apical portion. These structures, which may represent cellular feet, are therefore called podosomes. By screening a number of different transformed fibroblasts plated on a fibronectin-coated substratum we find that podosomes are common to mammalian and avian cell lines transformed either by Rous sarcoma virus (RSV) or by Fujinami avian sarcoma virus (FSV), whose oncogenes encode specific tyrosine kinases. Using antibodies reacting with phosphotyrosine in immunofluorescence experiments, we show that phosphotyrosine-containing molecules are concentrated in podosomes. Podosomes are not detected in fibroblasts transformed by other retroviruses (Snyder-Theilen sarcoma virus, Abelson leukemia virus and Kirsten sarcoma virus) or by DNA tumor viruses (polyoma, SV40), indicating that podosome-mediated adhesion in transformed fibroblasts is related to the peculiar properties of some oncoproteins and possibly to their tropism for adhesion systems. Podosomes and adhesion plaques, although similar in cytoskeletal protein composition, have different mechanisms and kinetics of formation. Assembly of podosomes, in fact (i) does not require fetal calf serum (FCS) in the adhesion medium, that is necessary for the organization of adhesion plaques; (ii) does not require protein synthesis; and (iii) is insensitive to the ionophore monensin, that prevents adhesion plaque formation. Moreover, during attachment to fibronectin-coated dishes, podosomes appear in the initial phase (60 min) of attachment, while adhesion plaques require a minimum of 180 min. In conclusion podosomes of RSV- and FSV-transformed fibroblasts represent a phenotypic variant of adhesion structures.

Role of heterochromatin variation in the instability of a marker chromosome during tumor progression

Karyotypic evolution of the poorly metastasizing tumorigenic RSV-transformed B77-3T3 fibroblast line was investigated both in highly metastasizing clones (selected by growth in hard agar) and in spontaneous metastases. Analysis of structural chromosome aberrations associated with the transition from the nonmetastatic to the metastatic phenotype was focused on a readily identifiable marker chromosome (A), displaying an extracentromeric heterochromatic region as a main feature promoting genetic instability. Well-defined changes in the structure of this marker were observed, both in vitro and in vivo, and invariably involved C-heterochromatic variation. In the metastatic clones, a specific rearrangement of the A chromosome was selected. This structural variant (B) showed two extracentromeric C-positive regions and probably originated from duplication of the segment of A included between the centromere and the internal C-band. On the other hand, selection of a modified form of chromosome A, not displaying the interpolated C-heterochromatin, had occurred in the extremely rare B77-3T3 spontaneous metastases. The connection among heterochromatin variants, genetic instability, and chromosome aberrations is discussed.

Organization of cytoskeleton and fibronectin matrix in Rous sarcoma virus (RSV)-transformed fibroblast lines with different metastatic potential

Metastatic clones growing in 0.6% 'hard' agar were selected from the non-metastatic Rous sarcoma virus (RSV)-transformed tumorigenic B77-3T3 mouse fibroblast line. The incidence of spontaneous lung metastases varied among clones around 100%, while it was lower than 5% in the parental tumor line. The organization of microfilaments, microtubules and intermediate filaments as well as the pattern of extracellular fibronectin matrix were analyzed by immunofluorescence in two representative clones (B77-AA6 and B77-AA12) and was compared with the structural features displayed by a highly metastasizing RSV-induced mouse sarcoma line (SR-BALB). In the metastatic clones studied microtubules and intermediate filaments were similarly organized in a pattern not significantly different from that of the non-metastatic parental cell line. The major finding was a marked concentration of actin-containing structures in the periphery of cells and notably at the level of surface protrusions, suggesting a high surface motility. In the same lines the production of fibronectin and its distribution in the cell layer and culture medium were analyzed. Metabolic labelling and immunofluorescence experiments indicated that the nonmetastasizing cells (B77-3T3) retain higher amounts of fibronectin in the cell layer and organize this molecule in extracellular fibers, while the metastatic clones (B77-AA6 and B77-AA12) as well as the metastatic line (SR-BALB) are unable to retain and organize fibronectin at their surface. This paper shows that the progression of tumorigenic cell lines toward a metastatic phenotype involves a redistribution of cytoskeletal actin and a loss of organized fibronectin matrix.

Somatic cell fusion as a source of genetic rearrangement leading to metastatic variants

Tumor cell populations displaying metastatic properties often have higher gene dosage than their less malignant progenitor tumors, as shown by increased ploidy levels, chromosome duplication and gene amplification. The acquisition by tumor cells of high chromosome numbers may be due to endoreduplication or somatic hybridization either between tumor cells or between tumor and host cells. All such mechanisms increase genetic variability and instability in tumor cells since they trigger a polyploidization-segregation cycle. Among the wide variety of segregants which may emerge from high-ploidy cells, variants with increased malignancy can be positively selected in vivo. Evidence for in vivo fusion of tumor and normal host cells has been reported in different tumor systems. However the attainment by tumor-host hybrids of a higher degree of malignancy has only been observed following substantial chromosome segregation. The involvement of a cell of bone marrow origin as preferential host partner in the fusion process has been proved both by studies on tumor-host hybrids in bone marrow radiation chimeras and in vitro hybridization experiments between non-metastatic tumors and normal lymphoreticular cells which have led to the establishment of metastatic variants. Several different segregational mechanisms may bring about homozygosity or hemizygosity of recessive alleles in tumor-host hybrids, leading to their expression. The marked chromosome dynamics of tumor-host hybrids are also responsible for extensive chromosome rearrangements. At the molecular level these may represent mechanisms causing altered oncogene activity. The activation of new oncogenes by transposition or amplification as well as the amplification of previously activated oncogenes are the mechanisms most likely to be responsible for transition from low to high malignancy, occurring through ploidy changes, such as those produced by somatic mating.

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.