Natural Lta Tumor Cell Resistance To Malignant Conversion By Oncogenesras Andmyc

A tumorigenic, non-metastatic murine fibroblast cell line (LTA) was here used to study aspects of natural cellular resistance to induction of progression to malignancy. The nature of LTA cell growth was established by testing both in vitro and in vivo. In vitro growth showed some but not all characteristic features of transformation (eg. cells showed little contact inhibition of cell growth and were able to multilayer, but were not serum independent). In vivo tumor growth was that of a benign (non-metastatic) fibrosarcoma. Attempts were made to induce LTA cell progression to malignancy by both epigenetic and genetic means. Treatment of these cells with phorbol ester tumor promoter (TPA) or 5-azacytidine at doses known to exert phenotypic effects on other tumor cell lines did not induce metastatic ability of LTA cells. Transfection with known oncogenes ras (T24-H-ras) and myc (v-myc) alone or in combination, also did not induce malignant conversion of LTA cells, even when these oncogenes were expressed at the high levels known to induce malignancy in other murine fibroblast cell lines. Expression of a group of genes known or suspected to be involved in ras-induced progression to malignancy ( metastasis-associated genes) was examined in LTA cells in the presence and absence of transfected ras. Pattern and ras-responsiveness of expression of these genes was also tested for NIH 3T3 cells, which are known to be transformed to metastatic ability upon transfection and expression of activated ras. Distinct differences were observed both in basal levels of expression and in ras-responsiveness of several of these genes between LTA- and NIH 3T3-derived cell lines. These differences could potentially account for differences in malignancy and ras-sensitivity between the cell lines. Somatic cell hybrids between LTA and NIH 3T3 cells showed complementation of phenotype such that isolated hybrid clones were consistantly much more metastatic than either parent. Somatic cell hybrids between LTA- and NIH 3T3-derived cell lines also showed ras-sensitivity as they were more metastatic in the presence of high levels of ras expression. Expression of ras-responsive and metastasis-associated genes in these hybrids also reflected their level of malignancy. It thus appears that the NIH 3T3 parent supplied some factor(s) essential to the progression and ras-responsiveness of LTA cells