Platelet-derived growth factor, epidermal growth factor, and insulin-like growth factor I regulate specific cell-cycle parameters of human diploid fibroblasts in serum-free culture

The growth regulation of human diploid fibroblasts by platelet-derived growth factor (PDGF), epidermal growth factor (EGF), insulin-like growth factor I (IGF-I) (somatomedin C), dexamethasone, and transferrin was investigated in a serum-free, chemically defined culture system. Cell-cycle kinetic parameters were determined using 5'-bromodeoxyuridine (BrdU) incorporation and flow cytometric analysis with the DNA-specific dye Hoechst 33258. We found that PDGF and EGF regulate the proportion of cells capable of entering the cell cycle from the quiescent state, with smaller effects upon the rate of cell transition from G1 into S phase. IGF-1, on the other hand, regulates the rate of cell exit from G1 without affecting the cycling fraction. Transferrin and dexamethasone showed less effect upon the cell-cycle kinetics under these culture conditions. The data provide functional evidence that PDGF and EGF regulate similar cell-kinetic parameters in human fibroblast cultures. IGF-I is functionally distinct from both PDGF and EGF in its role of regulating G1 exit rate without affecting the cycling fraction. These observations made by BrdU-Hoechst flow cytometric techniques provide a novel perspective on the regulatory effects exerted by different classes of growth factors, and suggest a mode of interdependence of these mitogens in regulating the net growth rate which could be a feature of growth regulation in vivo. These data also provide a different perspective on the regulation of the growth of fibroblast-like cells than that of the "competence/progression" cell-cycle model.