The Ha‐ras‐induced transformed phenotype of Rat‐1 cells can be suppressed in hybrids with rat embryonic fibroblasts

Somatic cell hybrids were isolated from fusions of diploid embryonic rat fibro‐blasts with transformed Rat‐1 cells which contained 4 to 5 copies of the transforming human Ha‐ras 1 gene. In contrast to their transformed parental cells four hybrid clones showed normal morphology, long latency periods of tumorigenicity in newborn rats, anchorage requirement of proliferation, and an eightfold‐reduced amount of secreted transforming growth factor activity. Thus these hybrids are called suppressed with regard to expression of the Ha‐ras‐induced transformed phenotype. Tumorigenic derivatives of the suppressed hybrids that had segregated chromosomes were isolated. Since two of the tumorigenic hybrid clones showed the similar low level of secreted transforming growth factors as the suppressed hybrids, decreased production of transforming growth factor activity is unlikely to be a sufficient criterion for suppression of malignancy. Whereas one of the suppressed hybrids expressed the transforming gene product p21 at a level similar to that of the transformed parental cells, other suppressed hybrids expressed less p21. This suggests that the suppressed phenotype can be regulated at the posttranslational level of p21 but that additional controls of expression of p21 are likely to exist. DNA of the suppressed hybrids transformed Rat‐1 cells to proliferation in the presence of semisolid agar. Thus the activated human Ha‐ras gene in the suppressed hybrids retained its biological activity even though it did not transform these cells to tumorigenicity.