L-744,832 blunts MAP kinase activation in response to hematopoietic growth factors. Mononuclear cells collected from recipients engrafted with either Nf1+/+ (left 6 lanes) orNf1−/− (right 9 lanes) cells were incubated at 37°C overnight with medium alone (−), medium plus 25 μmol/L L-744,832 (L) or medium and 100 μmol/L compactin (C). The cultures were then split and either not stimulated with any growth factors (−) or stimulated with 10 ng/mL of either GM-CSF (G) or IL-3 (I). The cells were lysed and MAP kinase activity was measured. The top graph shows a phosphoimager plot (counts per minute [CPM]-background) for each condition over the raw data from the autoradiograph. Loss ofNf1 is associated with constitutive activation of MAP kinase (compare lane 1 with 7) that is unaffected by L-744,832 (compare lane 7 with 10) but is inhibited by compactin (compare lane 7 with 13). In this experiment, L-744,832 blunted MAP kinase activation induced by GM-CSF by 31% in Nf1+/+ cells (compare lane 2 with 4) and by 30% in Nf1−/− cells (compare lane 8 with 11). Similarly, FTI treatment decreased the kinase activity measured inNf1−/− cells stimulated with IL-3 by 37% (compare lane 9 with 12). In contrast, compactin completely abrogated this induction of MAP kinase activity (compare lane 2 with 6, lane 8 with 15, and lane 9 with 16).