Growth factor‐induced proliferation of human fibroblasts in serum‐free culture depends on cell density and extracellular calcium concentration

Human neonatal skin fibroblasts plated sparsely in MCDB 105 traversed a complete cell cycle in the absense of serum or serum‐derived proteins. Addition of pure PDGF did not significantly increase entrance into S phase as revealed by 3 H‐thymidine labeling index or clonal growth on palladium islands. In subphysiologic Ca 2+ concentrations or in the presence of a calmodulin inhibitor, W7, proliferation in the absence of growth factors ceased and PDGF became mitogenic. In contrast, confluent fibroblast cultures were stimulated by PDGF in physiologic Ca 2+ concentrations. This was also the case with sparse adult skin fibroblast cultures while a fetal strain entered S in the absence of PDGF even in low extracellular Ca 2+ concentrations. EGF gave similar results as PDGF in all experiments performed. This proposes a similar role for the two growth factors in the cell cycle. However, a difference in the mechanisms of action of PDGF and EGF is indicated by the fact that PDGF and EGF were additive at optimal concentrations when maximal growth response by a single growth factor was restricted by a subphysiologic extracellular Ca 2+ concentration.