Hormonal regulation of discrete portions of the cell cycle: Commitment to DNA synthesis is commitment to cellular division

Abstract

Density‐arrested human fibroblasts were stimulated to traverse G₀/G₁ and initiate DNA synthesis by the addition of medium containing either serum or a combination of platelet‐derived growth factor and platelet‐poor plasma. Medium containing a combination of epidermal growth factor and high concentrations of insulin also stimulated DNA synthesis in platelet factor‐treated quiescent cells. Platelet factor was required only to initiate proliferation. Epidermal growth factor and insulin then allowed G₁ traverse and commitment to DNA synthesis. Cells could complete S, G₂, and M in unsupplemented medium lacking peptide growth factors.