SCF specifically activates MEK and p44/p42MAPK in a time-dependent manner. Hematopoietic progenitor cells cultured in EPO-containing medium for 6 days prior to the addition of SCF. Cellular protein was analyzed by Western blotting at 0, 10, and 30 minutes after the addition of SCF (EPO+SCF) and compared with protein extracts from matched controls (EPO). (A) Phosphorylated (Phos) and total MEK1 (lower bands) as well as (B) phosphorylated and total p44/p42MAPK were studied. (C) Hematopoietic progenitor cells cultured in medium containing EPO and varying concentrations of PD98059 (PD; 0.8-100 μM). Cell protein extracts were probed 10 minutes after the addition of SCF. Phosphorylated and total MEK as well as (D) phosphorylated and total p44/p42MAPK bands are shown for comparison.