FLT3 SIGNALING IN HEMATOPOIETIC-CELLS INVOLVES CBL, SHC AND AN UNKNOWN P115 AS PROMINENT TYROSINE-PHOSPHORYLATED SUBSTRATES

Proliferation and survival of hematopoietic progenitors are partially dependent on the interaction between the FLT3 receptor tyrosine kinase (RTK) and its ligand, FL. This biological function depends primarily on tyrosine phosphorylation of cellular targets that initiate several transduction cascades. These events return to their basal levels upon activation of specific phosphatases. We analyzed tyrosine phosphorylat ion events in response to FL, in human cell lines of different hematop oietic origins that express endogenous FLT3, namely the myelomonocytic , monocytic, pre-B and pro-B lineages, This study aimed at determining (I) the identity of FLT3 downstream substrates in physiologically rel evant cells and (2) distinct substrate involvement in myeloid or early B cells. The two prominent tyrosine-phosphorylated proteins are p52(S HC) and p115(CBL) in myeloid cell lines and p52(SHC) and an uncharacte rized p115 in early B cell lines. Following FL stimulation, a concomit ant increase in both CBL phosphorylation and complex formation with p8 5 subunit of phosphatidylinositol 3' kinase is observed. In contrast, the GRB2/CBL association observed in unstimulated cells is not modifie d after stimulation, and SHC is never detected in anti-CBL immunopreci pitates, FL-inducible binding of CBL to the CRKII adaptor molecule is also demonstrated, This study presents a picture of the signaling even ts triggered by activation of endogenous FLT3 receptor in human hemato poietic cells, including the existence of a B cell-specific FLT3 subst rate.