Spontaneous mutation rates in cloned murine tumors do not correlate with metastatic potential, whereas the prevalence of karyotypic abnormalities in the parental tumors does

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.

Differences in the rates of gene amplification in nontumorigenic and tumorigenic cell lines as measured by Luria-Delbrück fluctuation analysis

It has been hypothesized that genomic fluidity is an important component of tumorigenesis. Previous studies described the relationship between tumorigenicity and one marker for genomic fluidity, gene amplification. In this report, these studies are extended with the rat liver epithelial cell lines to show that: (i) the amplification in these cells arises in a spontaneous fashion in the population (i.e., the variants detected are not preexisting in the population), and (ii) the rate of spontaneous amplification (mutation), as measured by Luria-Delbrück fluctuation analysis, is significantly lower in the nontumorigenic cells than in the tumorigenic cells. The rate was estimated by using the Po method and the method of means. The rate of spontaneous amplification of the gene encoding the multifunctional protein CAD (containing the enzymatic activities carbamoyl-phosphate synthase, aspartate transcarbamylase, and dihydroorotase) in the highly tumorigenic cells was significantly greater than that for the nontumorigenic cells, reaching almost 1 x 10(-4) events per cell per generation. The rate of this mutagenic event is high compared to the rate of point mutations usually reported in mammalian cells, and its potential contribution to the tumorigenic process will be discussed.

Rates of generation of methotrexate-resistant variants in cells temperature-sensitive for malignant transformation

We used tsLA23-NRK cells, in which malignant transformation is under the control of a temperature-sensitive (ts) src oncogene, to examine the relationship between genetic instability and malignant transformation. Cells were maintained in vitro as either transformed cells (at 36 degrees C) or as normal cells (at 39 degrees C). Genetic instability was assessed in these two cell populations by determining (1) the frequencies of methotrexate (MTX)-resistant variants present in these populations, and (2) the rates of generation of MTX-resistant variants, as determined from Luria-Delbrück fluctuation analyses. We observed no significant differences, in either of these parameters of genetic instability, that could be attributed to transformation status. We conclude that in this defined cellular system there is no obligatory relationship between malignant transformation and increased genetic instability, as assessed by either frequencies or rates of generation of variants resistant to MTX.

In adequate tumor surgery of chemically induced soft tissue sarcomas--an experimental approach for induction of metastasis formation?

In preceding experiments with nude mice bearing xenotransplanted soft tissue sarcomas after inoculation of cultured sarcoma cells we attained a high percentage of metastatic dissemination by repeated inadequate tumor surgery. In the study presented here we used methylcholanthrene induced sarcomas (rhabdomyosarcomas and undifferentiated sarcomas in part showing the picture of storiform/pleomorphic malignant fibrous histiocytomas) which were produced in NMRI mice and examined (1) whether or not metastasis formation could be triggered or enhanced by repeated inadequate tumor surgery in these animals, and (2) the efficacy of the same surgical procedure following xenotransplantation of the tumors into nude mice. We did not reach an increasing frequency with both experimental arrangements. After discussing several factors which could be in general responsible for inducing metastasis formation and after comparing the results of our several experiments it is suggested that the methylcholanthrene induced soft tissue tumors may not contain cell clones able to metastasize.

Protection of cynomolgus monkeys against infection by human T-cell leukemia virus type-I by immunization with viral env gene products produced in Escherichia coli

Protection against human T-cell leukemia virus type-I (HTLV-I) infection in cynomolgus monkeys, achieved by immunizing the animals with env gene products of HTLV-I produced in Escherichia coli, was evaluated. Four monkeys that had been immunized with the env product produced antibody against HTLV-I gp68 and gp46, and their sera were found to cause strong inhibition of syncytium formation of a cat fibroblast cell line induced by HTLV-I. Immunized and non-immunized monkeys were challenged with live MT-2 cells, a high HTLV-I-producer cell line. After challenge, all the control non-immunized monkeys were infected with HTLV-I, as judged by the frequent detection of HTLV-I-antigens in cultures of their peripheral blood mononuclear cells (PBMC), whereas no antigens were recovered from PBMC of immunized monkeys. These results indicate that humoral immunity against HTLV-I-envelope protein elicited by immunization with the polypeptides synthesized in bacteria protected the monkeys against primary infection with HTLV-I.