Mechanism of synergistic induction of DNA synthesis by epidermal growth factor and tumor promoters

Evidence is presented to support our previously proposed hypothesis that the hyperplastic effect of tumor promoters is related to their ability to alter existing physiological levels of growth factors in target tissues. Epidermal growth factor and phorbol ester tumor promoters acted synergistically at low (0.001–0.05 ng/ml) but not high (> 0.1 ng/ml) EGF concentrations to induce DNA synthesis in cultured Rat‐1 fibroblast cells. The degree of synergism correlated with the tumor‐promoting ability of the compound. The tumor promoters decreased ¹²⁵I‐EGF binding to cellular receptors in a dose‐dependent manner that also correlated with the tumor‐promoting ability of the compound. The inhibition of EGF binding by phorbolester compounds resulted in a decrease in the amount of EGF degraded as compared to control cultures. At limiting EGF concentrations, the sparing of EGF degradation resulted in an increase in the amount of EGF remaining in the culture medium after 12 h of incubation and a concomitant increase in the amount of EGF bound to phorbol ester‐treated cells at this time as compared to control cultures. The ability of a phorbol ester compound to alter EGF degradation and to stimulate DNA synthesis synergistically with EGF correlated with the tumor‐promoting ability of the compound and occurred only at low EGF concentrations.