The relationship between Rap1 proteins and cell proliferation was assessed by investigating the effect of isoforms AA and BB of platelet-derived growth factor (PDGF) on Rap1 protein and mRNA expression throughout the smooth muscle cell cycle. Firstly, PDGF BB-induced cell cycle traverse was studied, thus demonstrating entry into S phase at 18 to 20 h. Western blotting carried out on total Rap1 proteins showed that 5 ng/ml of PDGF BB instigated a biphasic induction of total Rap1 proteins during the cell cycle. This involved a 2.1 +/- 0.4-fold increase at 6 h (early G1) and a 2.8 +/- 0.6-fold increase at 20 to 24 h (G1/S transition). Such an up-regulation was abolished by addition of 1 ng/ml of transforming growth factor-beta 1 (TGF-beta 1), which inhibited up to 80% of the PDGF BB-induced entry into S phase. Comparative RT-PCR of both rap1a and rap1b mRNAs throughout the cell cycle allowed us to differentiate between the two rap1a and rap1b species. PDGF BB induced a 1.9 +/- 0.3-fold increase at 4 h and a 2.4 +/- 0.2-fold relative increase at 16 h for rap1b mRNA, whereas a unique 1.9 +/- 0.5-fold increase in rap1a mRNA was observed at 14 h. Again, this induction of rap1a and rap1b mRNAs by PDGF BB was totally abolished by TGF-beta 1. We conclude that the differential up-regulation of Rap1a and Rap1b proteins during the smooth muscle cell cycle is directly linked to cell proliferation.