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Synergistic stimulation of early events and DNA synthesis by phorbol esters, polypeptide growth factors, and retinoids in cultured fibroblasts.

12-O-Tetradecanoyl-phorbol-13-acetate (TPA), in the absence of serum, acts synergistically with a range of polypeptide growth factors to stimulate DNA synthesis in quiescent Swiss 3T3 cells. These growth factors include epidermal growth factor (EGF), insulin, and the peptide produced by BHK cells transformed by SV-40 virus (fibroblast-derived growth factor, FDGF). Retinoids also show mitogenic synergism with TPA or polypeptide growth factors. The spectrum of mitogenic synergisms displayed by TPA are similar to those of vasopressin, a pituitary peptide. However, TPA and vasopressin do not synergistically interact to stimulate DNA synthesis in quiescent 3T3 cells. This suggests that TPA and vasopressin act via an identical biochemical pathway. Several lines of evidence suggest rapid postreceptor convergence of the mitogenic mechanisms of action of the hormone and the tumor promotor. Thus, vasopressin and TPA both inhibit EGF binding to cellular receptors. Furthermore, TPA and vasopressin induce a similar array of early events in quiescent cells--most strikingly, identical stimulation of Rb+ influx. Stimulation of ion flux is suggested as the possible convergence point of the pathway by which TPA and vasopressin act as mitogens.