Oncogenic signal-induced ability to enter S phase in the absence of anchorage is the mechanism for the growth of transformed NRK cells in soft agar.

Upon neoplastic transformation, cells acquire the ability to grow in soft agar. We investigated how this occurs by cell cycle analysis of a rat cell line NRK-49F and its transformation-deficient mutants. Rapidly growing NRK and mutants arrest in G1 when deprived of anchorage by suspending in methylcellulose. Addition of epidermal growth factor (EGF) together with transforming growth factor-beta (TGF-beta), which is highly oncogenic to NRK, induces the rapid progression of G1-arrested NRK cells into S phase. The time course and the extent of synchronization are very similar to the cell cycle progression in the presence of anchorage. EGF alone, which is highly mitogenic but only slightly oncogenic, fails to induce such progression. Both mutants remain arrested in G1. These data indicate that oncogenic signals confer on NRK the ability to enter S phase in the absence of anchorage and that this is the principal mechanism for its ability to grow in soft agar.