[Stimulating action of epidermal growth factor and insulin on uridine phosphorylation in 3T3 and 3T6 cells]

The influence of epidermal growth factor (EGF) and insulin on uridine phosphorilation was investigated in cell cultures Swiss 3T3 and 3T6, arrested in the medium with serum content of 0.5%. It is shown that following 5-10 minutes the addition of EGF into the culture medium in concentrations from 0.15 to 51 nM results in the increase in the rate of uridine phosphorilation which reaches the maximum value, similar to that of the stimulating effect of 10% serum. Insulin in the 4-85 nM concentrations also enhanced the rate of uridine phosphorilation and exerted a potential influence on EGF effect only in high concentrations. The investigation of the dynamics of binding and internalization of 125I-EGF showed that the number of EGF molecules on membrane increase during 10 minutes after addition of EGF into the medium and then begin to decrease. The binding of EGF with not more than 2% of the total number of cell receptors was shown by approximal estimation to be enough for stimulation of uridine phosphorilation. A conclusion is drawn that the presense of single growth factor EGF in enough for maximum stimulation of early reaction of cells on the proliferation stimulus realized at the posttranscriptional level, while both the additional factors and higher concentrations of EGF are necessary for the maximum induction of DNA synthesis.