Modulation of the epidermal growth factor receptor by platelet-derived growth factor and choleragen: effects on mitogenesis.

The addition of fresh medium supplemented with partially purified platelet-derived growth factor (PDGF) to quiescent density-arrested cultures of BALB/c-3T3 cells decreases the subsequent binding of radiolabeled epidermal growth factor (EGF). The decrease in EGF binding can be observed 1 hr after the addition of PDGF. This effect is maximal in 2-3 hr, and binding remains diminished for at least 6 hr. These effects can be accounted for by a decrease in the number of EGF receptors with no change in receptor affinity. The action of PDGF is concentration dependent, but even at very high concentrations of PDGF the reduction in EGF binding is never more than 50%. Similar decreases in EGF binding are produced by other treatments that render BALB/c-3T3 cells competent, such as the addition of fibroblast growth factor or medium previously exposed to the macrophage-like cell line P388D(1). Cholera toxin (choleragen), which alone had no effect on EGF binding, dramatically potentiated the ability of PDGF to down regulate EGF receptors. Two to three hours after the addition of PDGF and choleragen, EGF binding was reduced by 80-90% compared with control values. The ability of PDGF and choleragen together to decrease EGF binding was substantially inhibited by cycloheximide. Autoradiography of [(3)H]thymidine-labeled cells shows that choleragen potentiates the action of PDGF; lower concentrations of PDGF are required to make cells competent after choleragen treatment. Furthermore, cells treated with PDGF and choleragen no longer require EGF for traverse of G(1) phase and initiation of DNA synthesis in defined medium. The reduction in receptor number produced by choleragen and PDGF, which may be due to internalization of the EGF receptor, may mimic the action of EGF and thereby remove the EGF requirement for DNA synthesis.