Oncogenic stimulation recruits cyclin-dependent kinase in the cell cycle start in rat fibroblast

The rat fibroblast NRK cells are transformed reversibly by a combination of growth factors. When stimulated with serum, NRK cells rely on cyclin-depen dent kinase 4 (Cdk4) for their 5 phase entry. However, when stimulated with serum containing oncogenic growth factors, they come to rely on either Cdk 4 or Cdk6, and their S phase entry cannot be blocked unless both Cdk4 and C dk6 are immunodepleted. Such change of dependence does not occur in the NRK cell mutants defective in an oncogenic signal pathway and, therefore, defi cient in anchorage-independent cell cycle start ability, correlating Cdk6 d ependence with this remarkable, cancer-associated phenotype, However, both Cdk4 and Cdk6 are activated upon serum stimulation, and neither the amounts of Cdk6, Cdk4, cyclin D1, and cyclin-dependent kinase inhibitors nor the a ctivities or subcellular localization of Cdk6 and Cdk4 are significantly in fluenced by oncogenic stimulation. Thus, oncogenic stimulation invokes Cdk6 to participate in a critical step of the cell cycle start in a rat fibrobl ast, but by a mechanism seemingly unrelated to the regulation of the kinase . Given that many hematopoietic cells employ predominantly Cdk6 for the cel l cycle start and perform anchorage-independent growth by nature, our resul ts raise the possibility that the oncogenic: stimulation-induced anchorage- independent cell cycle start of NRK is elicited by a mechanism similar to t he one used for hematopoietic cell proliferation.