Conditional expression of the transformed phenotype in an “epithelial-like” cell line

Abstract

A subclone of ts 14/MNU/2, derived from a heteroploid (epithelial) monkey kidney cell line of BSC-1 and transformed in vitro by methylnitrosourea, expressed the transformed phenotype only at the elevated temperature of 39.5 °C. Compared to the growth characteristics at 33 °C, the transformed property was exhibited in the (1) high efficiency of plating at low cell densities, (2) colony morphology, (3) growth in absence of serum and (4) alterations in the quality and the quantity of chromosomal proteins in presence of low concentrations of serum. Such experimental systems may provide an effective control for studying molecular mechanisms related to transformation and neoplasia. All the early in vitro transformation studies involved fibroblast cultures (Berwald and Sachs, 1962; Heidelburger and Iype, 1967) which on malignant transformation produced sarcomas in suitable hosts. Since carcinomas are more important clinically, investigations involving epithelial cells have been initiated in many laboratories. Problems have arisen concerning the criteria employed to describe transformation in vitro since these were established for fibroblast cultures, but their relevance to epithelial cultures is now questioned. Cells initially characterized as being malignantly transformed using the criteria of piling up and loss of orientation, have failed to grow on inoculation in vivo ( Sanford et al., 1974). In other instances, morphologically transformed cells will only produce tumours after prolonged culture in vitro ( Eagle et al., 1970). Tumour induction in vivo requires varying lengths of time for the inoculated cells to become established; initial detection can be from two weeks to nine months in rodents. We have described before ( Naha, 1975; Naha and Ashworth, 1974) the use of temperature-sensitive (ts) variant cell lines, carrying biochemical lesions in thymidine metabolism, which when induced by the carcinogen methylnitrosourea showed a high frequency of transformation at the restricted temperature; the variants at the permissive temperature of the parental cell line showed no detectable change in the cellular morphology. These experiments suggested that transformation in the ts variants was selective in nature and raised the possibility of “clonal selection” during carcinogenic transformation. Derived from a heteroploid monkey kidney cell line ( Naha, 1975), the morphology and growth characteristics of the transformed clones exhibited (a) altered colony morphology; (b) loss of temperature sensitivity; (c) increased efficiency of plating at low cell densities (< 40 cells/cm 2); (d) ability to grow on soft agar (for a limited division only). The results presented in this paper describe experiments on a sub-clone of one of these: ts 14/MNU/2, which expressed the transformed phenotype only at 39.5 °C, and which was tumorigenic in nude mice (to be published). Epithelial nature of these cells has not yet been properly assessed.