Oncogenic activation loop KIT mutations are found in acute myeloid leukemia (AML) and systemic mastocytosis (SM); however unlike the KIT juxtamembrane mutants the activation loop mutants are insensitive to imatinib mesylate. mast cell progenitors (MCps) induces constitutive KIT autophosphorylation supports ligand-independent hyperproliferation and promotes promiscuous cooperation with multiple cytokines. Genetic disruption of p85α the regulatory subunit of class IA lipid kinase phosphoinositol-3-kinase (PI3K) but not of p85β or genetic disruption of the hematopoietic cell-specific Rho GTPase Rac2 normalizes KITD814V-induced ligand-independent hyperproliferation. Additionally deficiency of p85α or Rac2 corrects the promiscuous hyperproliferation observed in response to multiple cytokines in both KITD814V-expressing HSC/Ps GS-9190 and MCps. Treatment of KITD814V-expressing HSC/Ps with a Rac inhibitor (NC23766) or with rapamycin showed a dose-dependent GS-9190 suppression in ligand-independent growth. Taken together our results identify p85α and Rac2 as potential novel therapeutic targets for the treatment of KITD814V-bearing AML and SM. Introduction Stem cell factor (SCF) is a unique cytokine with important functional roles in melanocytes germ cells interstitial cells of Cajal mast cells and hematopoietic stem cells.1 Consistent with the importance of SCF signaling within EPOR GS-9190 these defined tissues activating mutations of activation loop mutant mutations are also observed in core binding factor-acute myeloid leukemia (CBF-AML) leukemias that bear either the t(8;21) or inv(16) cytogenetic abnormality generating the fusion genes or and disrupting mutant in CBF-AML carrying t(8;21) worsens the prognosis based on several clinical indices.9-12 Oncogenic KIT is constitutively phosphorylated suggesting that signals emanating from this receptor are not regulated by ligand stimulation 13 14 and consistently cell lines expressing oncogenic KIT demonstrate ligand-independent proliferation.13 15 16 KIT contains an extracellular portion containing 5 immunoglobulin-like repeats a transmembrane domain a juxtamembrane domain and a cytoplasmic tyrosine kinase domain that is split by an insert sequence. Activating mutations within the juxtamembrane region are commonly found in GISTs and are sensitive to inhibition by the tyrosine kinase inhibitor imatinib mesylate (Gleevec); however mutations within the carboxy-terminal lobe of the cytoplasmic tyrosine kinase domain (TK2) such as activation loop mutants including SM and CBF-AML.17-19 Accordingly experimental tyrosine kinase inhibitors have been examined for efficacy in inhibiting the proliferation or promoting the apoptosis of as well as the mutations and Ba/F3 cells bearing activation loop mutants in relevant major cells. It’s been hypothesized that activation loop mutants including (individual) and or (murine) alter the specificity of Package substrate reputation and usage.14 Because of this the nonspecific signals emanating from oncogenic KIT are promiscuous in nature and induce aberrant signals not normally regulated by wild-type KIT including the activation of signal transducer and activator of transcription 3 (STAT3)24 25 and the degradation of KIT-signaling inhibitory molecules such as Shp-1.14 Therefore an alternative therapeutic approach to directly targeting KIT is to target KIT effector molecules that contribute to the transformation of oncogenic KIT-bearing cells. Previous functional and pharmacologic studies using cell line models and wortmannin respectively have demonstrated that this lipid kinase phosphoinositol-3-kinase (PI3K) may contribute to the transforming ability of D816V (murine D814V).15 26 Although informative these studies do not provide information regarding the extent to which PI3K contributes to the transforming ability of D816V. In addition conclusions drawn from these studies GS-9190 are limited as in some cases the cell types used normally do not express KIT and thus it is likely that this substrate availability within these cells differs from that of primary KIT-expressing hematopoietic progenitor/stem cells (HSC/Ps) and mast cell progenitors (MCps). Additionally the class IA PI3Ks are a group of heterodimeric lipid kinases composed of a p85 regulatory subunit (p85α p55α p50α p85β or p85γ) and a p110 catalytic subunit (p110α p110β or p110δ)29 30 and are all nonspecifically inhibited by.