Activation mechanisms of STAT5 by oncogenic Flt3-ITD

Mutations in the receptor tyrosine kinase FIt3 represent a very common genetic lesion in acute myeloid leukemia (AML). Internal tandem duplication (ITD) mutations clustered in the juxtamembrane domain are the most frequent and best characterized mutations found in FIt3. Oncogenic activation of FIt3 by ITD mutations is known to activate aberrant signaling including activation of STAT5 and repression of myeloid transcription factors Pu.1 and c/EBP-alpha. However, the mechanisms of STAT5 activation by Flt3-ITD remain unclear. Using small molecule inhibitors and cell lines deficient for Src family kinases or Jak2 or Tyk2, here we show that Fit3-ITD-induced STAT5 activation is independent of Src or Jak kinases. Also, overexpression of SOCS1, an inhibitor of Jak kinases, inhibited IL-3- but not FIt3-ITD-mediated STAT5 activation. Furthermore, in vitro kinase assays revealed that STAT5 is a direct target of FIt3. Taken together, our data provide the mechanistic basis of STAT5 activation by FIt3-ITD.