Abstract
Insulin‐like growth factor 1 and insulin reduced the binding of 125I‐labelled epidermal growth factor (125I‐EGF) to Swiss 3T3 cells by 15–20% at 37°C, but not at 4°C. Scatchard analysis indicated that IGF‐1 and insulin affected the higher‐affinity component of EGF binding, an effect previously associated with the activation of protein kinase C. However, the inhibition of 125I‐EGF binding by IGF‐1 and insulin was increased, not reduced, when the cells were treated with high concentrations of phorbol esters to down‐modulate protein kinase C. We suggest that IGF‐1 and insulin activate a protein kinase with similar or overlapping specificity to that of protein kinase C.

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