KITD814V-induced ligand-independent and SCF-stimulated growth of hematopoietic progenitors is sensitive to the Rac-GTPase inhibitor, NSC23766, and the mTor inhibitor, rapamycin. (A) KITD814V-expressing wild-type or Rac2^−/− bone marrow progenitors were treated with the indicated concentrations of NSC23766 and subjected to a thymidine incorporation assay in the absence of cytokine stimulation (NO GF, left panel) or in the presence of SCF 50 ng/mL (right panel). After 48 hours of culture, cells were pulsed with [³H] thymidine for an additional 6 hours. Bars denote the mean thymidine incorporation (cpm ± S.D). *P < .05, KITD814V-expressing wild-type cells treated with 50, 100, or 200 μM NSC23766 versus vehicle (water). (B) WT KIT- or KITD814V-expressing cells were treated with vehicle (ethanol), SCF 50 ng/mL, or the indicated concentrations of rapamycin and subjected to a thymidine incorporation assay. WT KIT-expressing cells treated with rapamycin were also treated with SCF 50 ng/mL. After 24 hours of culture, cells were pulsed with [³H] thymidine for an additional 6 hours. Bars denote the mean thymidine incorporation (cpm ± SD) from 1 of 2 independent experiments performed in quadruplicate. *P < .05, KITD814V-expressing cells treated with 1 nM or 2 nM rapamycin versus vehicle (ethanol).