Abstract
The platelet‐derived growth factor (PDGF), which is found in serum but not in plasma, has been purified to homogeneity; it stimulates replication at a concentration of 10^−10^M. Brief treatment with PDGF causes densityinhibited Balb/c‐3T3 cells to become competent to synthesize DNA; pituitary fibroblast growth factor (FGF) or precipitates of calcium phosphate also induce competence. Continuous treatment with plasma allows competent, but not incompetent, cells to synthesize DNA. A critical component of plasma is somatomedin, a group of hormones with insulin‐like activity; multiplication‐stimulating activity (MSA) or insulin replace plasma somatomedin in promoting DNA synthesis.
We have studied the molecular correlates of competence and the role of SV40 gene A products in regulating DNA synthesis. Treatment of quiescent cells with pure PDGF or FGF causes the preferential synthesis of five cytoplasmic proteins (approximate molecular weight 29,000, 35,000, 45,000, 60,000, and 72,000 detected by SDS‐PAGE under reducing conditions). Two of these competence‐associated proteins (29,000 and 35,000 daltons) are found within 40 min of PDGF addition; they are not induced by plasma, insulin, or epidermal growth factor (EGF), PDGF, FGF, or calcium phosphate induce an ultrastructure change within the centriole of 3T3 cells; this ultrastructural modification of the centriole is detectable by immunofluorescence within 2 h of PDGF treatment. Plasma, EGF, or MSA do not modify the centriole. SV40 induces replicative DNA synthesis in growth‐arrested 3T3 cells but does not cause this alteration in centriole structure.
Gene A variants of SV40, including a mutant with temperature‐sensitive (ts) T‐antigen (ts A209), a deletion in t‐antigen (dl 884), and several ts A209 strains containing t‐antigen deletions were used to induce DNA synthesis in Balb/c‐3T3 cells. Like wild type SV40, all strains induced DNA synthesis equally well under permissive or nonpermissive conditions. Addition of PDGF or plasma had little effect on SV40‐induced DNA synthesis. Thus, the viral function that induces replicative DNA synthesis in Balb/c‐3T3 cells is not t and is not temperature sensitive. This SV40 gene function overrides the cellular requirement for hormonal growth factors. It does not induce transient centriole deciliation, a hormonally regulated event.