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

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). PDGF-treated cells remain competent for many hours following PDGF removal; subsequent addition of PPP triggers G0-G1 traversal and entry into S phase. 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 cells. 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 competence. 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.