cAMP synergizes with IL-3 to activate ERK2 in 32D/CSF-1R and FDC-P1 cells. Cells were treated as described in the legend to Fig2, except that they were stimulated with murine rIL-3. (A) Cells were pretreated or not with forskolin/IBMX and stimulated with IL-3 for 15 minutes. ERK (top) and MEK1 (bottom) assays were performed as described in Fig 2. (B) Time dependence of ERK2 stimulation by CSF-1 (top panel) or IL-3 (bottom panel) in the presence (•) or absence (▪) of forskolin (10 μmol/L) and IBMX (0.1 mmol/L) pretreatment. Cells were starved in medium containing 10% FBS but no IL-3 for 18 hours and CSF-1 or IL-3 was added. Aliquots were taken at the indicated time points (0, 5 minutes, 30 minutes, 1 hour, 3 hours, 5 hours, 7 hours, 9 hours, 12 hours, and 22 hours) and processed for ERK activity. The experiments shown in (A) and (B) have been repeated twice with very similar results.