Fig. 4. Forskolin regulates cyclin D1 in PDGF-primed Schwann cells. A, Continous exposure to PDGF (P) and/or forskolin (F). PDGF (30 ng/ml), forskolin (5 μm), or PDGF + forskolin were added to Schwann cells that had been growth-arrested in PDGF-free medium. At timed intervals (6, 12, 18, 24, and 30 hr), samples were harvested, and cyclin D1 protein was monitored by immunoblot.NT, No treatment. B, PDGF priming followed by immediate addition of forskolin as above, except that Schwann cells were primed with PDGF for 1 hr. After removal of PDGF, the medium was supplemented immediately with forskolin or left untreated. C, PDGF priming followed by delayed addition of forskolin as above, except that Schwann cells were primed with PDGF for 1 hr or not primed (NT). After the removal of PDGF, all cells were incubated in serum-free DMEM for 15 hr before the addition of forskolin.