SHC and SHIP phosphorylation and interaction in response to activation of the FLT3 receptor

The FLT3 receptor tyrosine kinase and its ligand, FL, regulate the developm ent of hematopoietic stem cells and early B lymphoid progenitors. FL has a strong capacity to boost production of dendritic and natural killer cells i n vivo, thereby providing a new and promising tool for anti-cancer immunoth erapy. Intracellular FLT3 signaling involves tyrosine phosphorylation of se veral cytoplasmic proteins including SHC. We have found that upon FLT3 acti vation SHC phosphorylation occurs at tyrosine 239/240 and 313. SHC possesse s two phosphotyrosine-binding domains: an amino-terminal phosphotyrosine bi nding (PTB) and a carboxy-terminal Src Homology 2 (SH2) domain. Neither is required for SHC phosphorylation, but the PTB domain is necessary and suffi cient for SHC binding to the SH2 containing inositol phosphatase (SHIP). Ov erexpression of SHC increases the level of SHIP phosphorylation on tyrosine s in response to FLT3 activation, suggesting that SHC availability is a lim iting step for SHIP phosphorylation. This effect is observed only if the SH C PTB domain is functional. Interestingly, SHC overexpression in FLT3-activ atable Ba/F3 cells limits FLT3-dependent cell growth and this effect requir es tyrosine 313. Taken together, the present data show that SHC can antagon ize cell proliferation induced by FLT3 stimulation and regulate phosphoryla tion of the SHIP negative regulator. In addition, our study provides the st ructural bases for SHC phosphorylation and formation of the SHC/SHIP comple x.