Generation of metastatic variants in populations of mutator and amplificator mutants

The cellular basis of tumor progression

Variability in disease presentation and course is a hallmark of cancer. Variability is seen among similarly diagnosed cancers in different patients or animal hosts and in the same cancer at different periods of time. This latter type of variability, termed "tumor progression," was defined by Foulds in a series of six rules that describe the independent behavior of individual cancers and the independent evolution of different cancer characteristics. Tumor progression is believed to result from variability among subpopulations of tumor cells within individual cancers and from selection of these subpopulations by conditions within the cancer environment, such that different subpopulations come to prominence over the course of cancer development and growth. Interactions among subpopulations, however, modulate tumor behavior as well as tumor evolution. The leading hypothesis for the origin of tumor subpopulations is the genetic instability of cancer cells. There are a number of possible mechanisms of genetic instability, some internal to cancer cells (mutation, amplification, mutator phenotypes, DNA repair deficiencies) and some present in the tumor microenvironment (endogenous mutagens). There are also potential epigenetic mechanisms of variability, including alterations in gene regulation, differentiation, adaptation, and cell fusion. Regardless of mechanism, the heterogeneity of tumor subpopulations poses a number of challenges to the practice of cancer research, including the design of reproducible and meaningful experiments. Tumor heterogeneity also has significant consequences for the clinical assessment of tumor prognosis and the development of effective treatment regimens.

NIH3T3 transfectant containing human K-ras oncogene shows enhanced metastatic activity after in vivo tumor growth or co-culture with fibroblasts

A clone of NIH3T3 transformant (H-3), obtained by transfecting genomic DNA of a human colon carcinoma cell line, contains human K-ras oncogene and yields metastatic pulmonary nodules after intravenous injection of the cells into nude mice. This metastatic ability was enhanced remarkably after in vivo tumor growth (subcutaneous tumor formation in nude mice) accompanied by increased mRNA expression and gene amplification of the human-derived K-ras oncogene, while it declined gradually as the passage number increased in vitro, with corresponding decreases of gene amplification and mRNA expression. Six subclones were randomly selected from H-3 cells which had been subcultured to passage 22. All of the clones in culture showed almost the same low level of metastatic ability and exhibited little K-ras oncogene amplification with correspondingly low mRNA expression. However, after they formed tumors in nude mice, every clone acquired high metastatic ability and the gene amplification increased, with elevated mRNA expression. These experimental facts indicated that acquisition of metastatic ability coupled with the function of K-ras oncogene was conditional in nature, being strongly affected by in vivo tumor circumstances. The low metastatic and G-418-resistant H-3 cells were co-cultured with BALB/c3T3 fibroblasts for 2-4 weeks. After removal of fibroblasts by exposure to G-418, the tumor cells exhibited increased metastatic ability and human K-ras oncogene mRNA, suggesting an intimate interaction between H-3 cells and fibroblasts influencing the function of transfected human K-ras oncogene. Fibroblasts of the host animal may thus have an important role in generating enhanced metastatic activity of H-3 cells.

Comparison of spontaneous mutagenesis in early-passage human mammary cells from normal and malignant tissues

Spontaneous mutant frequencies were determined in early-passage epithelial-cell strains derived from either normal or malignant human breast tissues, as well as a non-tumorigenic immortalized human mammary epithelial cell line (184B5) derived from normal cells. Mutations at the hypoxanthine-guanine phophoribosyltransferase (HPRT) locus were quantified by determining the 6-thioguanine resistance. Mutant frequencies in human mammary epithelial cells (HMEC) from 4 normal and 5 carcinoma tissue samples did not differ significantly. In contrast, the mutant frequency in the immortalized HMEC was approximately 10 times higher than in average normal HMEC and normal non-immortalized cells from early-passage cultures of the same cell lineage. Our data suggest that the progression of normal breast cells to invasive carcinoma cells does not necessarily involve the establishment of a general genetic instability during this progression.

Drug sensitivity and metastatic ability in B16 melanoma cells

We have previously reported that highly metastatic cell lines derived from KHT fibrosarcoma (KHT 35L1) and B16 melanoma (B16F10) are more resistant to N-phosphonacetyl-L-aspartate (PALA) and methotrexate (MTX) than the parental cell lines. This correlation between drug resistance and metastatic ability suggested the possibility that both phenotypes might have arisen in parallel as a result of a similar mechanism. In this study, we examined this possibility by reproducing the selection procedure for B16F10 cells (by serial passage of B16F1 cells as lung nodules) and testing the cells at each passage for changes in resistance to PALA and MTX. The results confirm that serial passage of B16F1 cells as lung nodules (LP) selects for cells with increasing metastatic ability (100-fold after seven passages), but these cells did not develop increased resistance to PALA and became more sensitive to MTX. For comparison B16F1 cells were also serially passaged (six passages) as leg tumors (LT). These cells became slightly more metastatic (3-fold) than B16F1 cells maintained in tissue culture, and demonstrated a small increase in sensitivity to MTX, as in the LP lines. There was also an apparent increase in resistance to PALA. In no instance was there a parallel increase in drug resistance and metastatic ability indicating that these two phenotypes do not necessarily arise in parallel in this cell line.

Modulation of clonal progression in B16F1 melanoma cells

We have examined the effects of the microenvironment on the frequency and generation of metastatic variant cells in both parental B16F1 melanoma cells and nascent clones. The metastatic abilities of cultured B16F1 cells were tested after a period of growth in the presence or absence of a second cell population separated from each other by a transwell membrane (0.45 micron pore size). The first population is defined as the 'responder' cells and the second as the 'stimulator' cells. We found that the presence of 10(5) B16F1 stimulator cells during the growth of responder B16F1 cells from approximately 10(4) to approximately 10(6) cells resulted in cells with an increased metastatic phenotype (greater than 8-fold increase in median number of lung tumors relative to untreated B16F1 parental cells). The presence of stimulator cells also increased the metastatic phenotype of nascent clones, which were grown to a population size of less than 10(6) cells, suggesting that the rate of generation of metastatic variants of the responder B16F1 clones was affected by the stimulator cells. Other cell lines, including highly metastatic B16F10 and BL6 melanoma cells, and KHT35-L1 fibrosarcoma cells, were effective stimulator cells when as few as 10(4) cells were added to transwells. In addition, normal immortalized NIH 3T3 cells were effective stimulator cells only at 10(5) cells/transwell. The cell density at which untreated parental B16F1 cells were harvested (3 x 10(3)-3 x 10(5) cells/cm2) did not affect the median number of lung tumors significantly. These results suggest that factors released from both tumor and immortalized normal cells can modulate epigenetic changes in the metastatic phenotype of B16F1 melanoma cells.

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.