PROTEIN-KINASE C-INDEPENDENT ACTIVATION OF MITOGEN-ACTIVATED PROTEIN-KINASE BY EPIDERMAL GROWTH-FACTOR IN SKIN FIBROBLASTS

In this study, we demonstrated that epidermal growth factor (EGF) stim ulated the phosphorylation of myelin basic protein (MBP), a mitogen-ac tivated protein kinase (MAPK) substrate, in crude extracts of human de rmal fibroblasts. Moreover, using a selective protein kinase C inhibit or, GF 109203X indol-3-yl]-4-(1H-indol-3-yl)-1H-pyrrole-2,5-dione mono hydrochloride), we observed that protein kinase C was partially involv ed in the total MBP phosphorylation. To determine the role of protein kinase C in the MBP phosphorylation, we separated, using fast protein liquid chromatography, the proteins present in the fibroblast crude ex tracts; we thus detected two distinct MBP kinase activities. The first one was stimulated by EGF and corresponded to p42(mapk) and p44(mapk) isoforms; this stimulation was not modified by GF 109203X. The second MBP kinase activity was not stimulated by EGF and was due to two prot ein kinase C isoforms reacting with an anti-protein kinase C zeta anti body. These results show that, in human dermal fibroblasts, EGF stimul ates p42(mapk) and p44(mapk) isoforms in a protein kinase C-independen t manner.