Studies on stimulation of DNA synthesis with epidermal growth factor and insulin-like growth factor-I in cultured human keratinocytes.

Epidermal growth factor (EGF) and insulin are the most widely used growth factors (GFs) in human keratinocyte cultures. Insulin is supposed to exert its growth-promoting activity in this system through the insulin-like growth factor-I (IGF-I) receptor. In order to obtain more information about the contribution of EGF and IGF-I to the proliferation of keratinocytes, the effect of each factor on DNA synthesis was studied with 3H thymidine incorporation in an otherwise GF-free system. Both factors are able to initiate DNA synthesis, DNA synthesis peaks 18-20 h after the addition of each factor, and neither factor influences significantly the binding of the other to the respective receptor. IGF-I is the more potent growth factor, and IGF-I-stimulated cells enter the S-phase regularly approximately 3 h earlier than EGF-stimulated keratinocytes (7-9 h vs 10-12 h). However, IGF-I-stimulated DNA synthesis can be completely turned off by the addition of the monoclonal antibody (mAb) to the EGF receptor LA1. This inhibition cannot be reversed by higher IGF-I concentrations, but only by the addition of EGF to the culture medium. These results may suggest the presence of two keratinocyte populations, one responding to EGF and one to IGF-I, with an additional signal from the EGF receptor, or they may be explained on the basis of only one cell population for which EGF acts as a "competence' factor and IGF-I as a "progression' factor.