Association of platelet-derived growth factor-induced protein with nuclear material

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Abstract
Platelet-derived growth factor (PDGF) has been proposed to initiate the cell-cycle traverse of density-arrested BALB/c-3T3 cells by rendering quiescent cells ‘competent’ to respond to ‘progression’ factors contained in platelet-poor plasma (PPP)1. PDGF-treated cells remain competent for many hours following PDGF removal; subsequent addition of PPP triggers G0–G1 traversal and entry into S phase1,2. Numerous observations suggest that the competent state reflects the existence of a stable PDGF-induced ‘second signal’ as opposed to a persistent association of PDGF with cells1–3. Several unique proteins have been shown to be synthesized in response to PDGF; of these, the dose-dependent production of ‘pI’ (molecular weight 29,000) was found to correlate closely with the induction of competence4. We report here the further characterization of pI in terms of its time-dependent synthesis, intracellular location and stability. Electrophoretic analysis of nuclear and non-nuclear extracts of PDGF-treated BALB/C-3T3 cells demonstrated that pI is synthesized during the first 6 h of G0–G1 is associated with nuclear material and is stable for 9–12 h. These findings are consistent with the proposed role of pI as the cellular mediator of PDGF.