Lyn Is an Important Component of the Signal Transduction Pathway Specific to FLT3/ITD and Can Be a Therapeutic Target in the Treatment of AML with FLT3/ITD.

Fms-like tyrosine kinase 3 (FLT3) is expressed in hematopoietic progenitor cells. An internal tandem duplication (ITD) of the FLT3 juxtamembrane domain (FLT3/ITD) is the most frequent mutation in human adult acute myeloid leukemia (AML). Being transfected into IL-3 dependent cell line, 32D, FLT3/ITD causes constitutive activation of FLT3 itself and its downstream signal components, which leads to IL-3 independent cell growth. In particular, STAT5 activation is thought to be a key signaling event since it is induced only by FLT3/ITD not by wild type FLT3. However the signal component which mediates FLT3/ITD to STAT5 is yet to be clarified. In the present study, we showed the mutant- specific association of FLT3/ITD with Lyn, which led to the phosphorylation of Lyn in vivo. We also demonstrated that Lyn directly bound to FLT3 through its SH2 domain in vitro and that the association depended on tyrosyl-phosphorylation at juxtamembrane domain of FLT3. We revealed the critical role of Lyn for STAT5 activation and the autonomous cell growth in FLT3/ITD transfectant of 32D by using anti-Lyn siRNA and the Src family kinase inhibitor PP2. More importantly, we demonstrated successful treatment of FLT3/ITD induced tumors with PP2 in the mice model. These results demonstrate that Lyn is a critical component of the signal transduction pathway specific to FLT3/ITD and can be a therapeutic target in the treatment of AML with FLT3/ITD.