Temperature-sensitive variants for saturation density and anchorage dependencey of a simian virus 40-transformed human X mouse hybrid cell line.

Several variant clones temperature sensitive for some parameters of transformation from a hybrid cell line containing a stable diploid mouse genome and two human chromosomes 7 carrying an integrated defective simian virus 40 have been isolated. Like the wild-type (wt) cells, at the permissive temperature the temperature-sensitive (ts) clones grew to high saturation densities and readily formed colonies in methyl cellulose. In contrast to the wt cells, at the nonpermissive temperature they had variable but always lower saturation densities and were unable to form colonies in methyl cellulose. The fraction of cells that synthesized DNA decreased at both temperatures when the cells reached saturation density, but it represented always at least 20% of the total population. All of the ts clones so far tested had a near triploid chromosome number, contained from one to three human chromosomes 7, and were T-antigen-positive both at the permissive and nonpermissive temperatures. The ts clones maintained low saturation density at the nonpermissive temperature because of a decrease in the number of cells that were actively proliferating and because of the shedding of cells into the medium. Temperature downshifts and refeeding allowed for expression of the permissive phenotype. In most of the isolated clones anchorage independency was not correlated with unrestricted cell proliferation. These variant clones may provide useful systems for a better understanding of the number of interdependent pathways involved in the expression of the phenotype of a transformed cell and elucidate the molecular mechanism required for the maintenance of a normal state in a cell population.