Phorbol Diester and Epidermal Growth Factor Receptors in 12-O-Tetradecanoylphorbol-13-Acetate-Resistant and -Sensitive Mouse Epidermal Cells

Several cell variants have been isolated from promotable mouse JB6 epidermal cells which are resistant either to mitogenic stimulation at quiescence or to promotion of anchorage independence by 12-O-tetradecanoylphorbol-13-acetate (TPA). Such resistant variants would be expected to lack one or more steps in the TPA response pathway leading to mitogenesis or promotion of tumor cell phenotype. This report is concerned with determining whether resistance is attributable to lack of receptors for phorbol diesters or epidermal growth factor (EGF, a potential mediator) or to absence of receptor down modulation following ligand binding. The results show that neither lack of phorbol diester receptors nor absence of down modulation can be demonstrated in the TPA-resistant variants. The phorbol ester binding affinity is also not altered in the resistant variants. The presence of EGF receptors cannot be an absolute requirement for TPA promotion sensitivity since three of the TPA-promotable cell lines lack available EGF receptors. Lack of EGF receptors may account for TPA mitogen resistance in at least three of four resistant variants. The TPA-induced EGF binding decrease occurs in both sensitive and resistant variants. Thus, phorbol diester binding and receptor down modulation remain as possible required events in mitogenic and promotion responses to TPA. EGF receptors are clearly not necessary for TPA promotion of anchorage independence in JB6 cells but may mediate mitogenic stimulation of these cells by TPA.