The role of polypeptide growth factors in phenotypic transformation of normal rat kidney cells.

A serum-free assay has been established for studying the role of polypeptide growth factors in inducing loss of density-dependent inhibition of growth of normal rat kidney (NRK) cells. The process has been characterized by measuring the time course of [3H]thymidine incorporation into confluent, quiescent NRK cultures stimulated by defined polypeptide growth factors, in combination with cell counting studies, increases in DNA content, and cell cycle analysis by means of a fluorescence-activated cell sorter. It is shown that none of the growth factors tested (epidermal growth factor, platelet-derived growth factor, transforming growth factor-beta, and retinoic acid) is able to induce loss of density-dependent inhibition of growth by itself, but strong synergism was observed when combinations of growth factors were tested. None of the above factors was found to be essential, however, since any combination of three of the above four growth factors strongly induced the process. Strong parallels were observed between the growth factor requirements for inducing loss of density-dependent inhibition of growth under serum-free conditions and the requirements for induction of anchorage-independent proliferation under growth factor-defined assay conditions. This indicates that most likely the same cellular processes underlie these two aspects of phenotypic transformation, although data indicate that anchorage-independent proliferation may be a more restricted property of phenotypic transformation than loss of density dependence of proliferation. It is concluded that phenotypic transformation of NRK cells does not require specific polypeptide growth factors, but reflects the ability of these cells to respond to multiple growth factors.