Differential inhibition in proliferation of G1E-ER2 cells by ERK inhibitor and stromal cells expressing different isoforms of SCF.

G1E-ER2 cells were cocultured for 48 hours with mitomycin C–treated stable stromal cell transfectants expressing S-SCF, MA-SCF, or MR-SCF in the absence or presence of indicated concentrations of the ERK inhibitor PD98059. (A) Proliferation was measured by thymidine incorporation assay. Bars denote the mean inhibition in proliferation (± SEM) of 2 independent experiments performed in replicates of 6. *P < .05 MA-SCF and MR-SCF (12.5 μM and 50 μM) versus S-SCF (12.5 μM and 50 μM). **P < .05 MR-SCF versus S-SCF (25 μM). (B) Level of ERK inhibition in G1E-ER2 cells treated with 12.5 μM PD98059 and stimulated with soluble SCF (top panel) or MR-SCF (bottom panel). Lanes 1 and 4: unstimulated G1E-ER2 cells. Lanes 2 and 5: G1E-ER2 cells stimulated with soluble SCF and MR-SCF, respectively. Lanes 3 and 6: PD98059 pretreated G1E-ER2 cells stimulated with soluble and MR-SCF, respectively. Arrows indicate the phosphorylated and total ERK1/2 protein.