Sections were then washed with PBS and incubated with biotinylated anti-rat secondary antibody

lated 150 kDa and weak glycosylated 130 kDa FLT3 species. The FLT3 protein was present in all cell lines, with equal actin controls. We noted that a strong signal for the 150 kDa-sized FLT3 protein could be detected in FLT3-WT, FLT3-V592A and FLT3-I867S expressing cells in comparison to FLT3-ITD mutants which show a higher proportion of the 130 kDa-sized form. The remaining FLT3-TKD mutation expressing cells fall in between those two extremes. In order to evaluate the distribution of the FLT3 receptor on the cell surface, the Ba/F3 cell lines were stained with a CD135 antibody and evaluated using flow cytometry. Difference of geometric mean fluorescence to isotype control antibody showed no significant difference between FLT3-WT and -I867S expressing cells. Interestingly, the cell surface expression of the FLT3 receptor was significantly reduced by a factor of approximately two between FLT3-WT and FLT3-V592A expressing cells. The cell surface expression of the FLT3 receptor decreased further for FLT3-TKD and FLT3-ITD cell lines FLT3 Mutants Show a Broad Spectrum of Activating Phenotypes in Ba/F3 Cells To clarify the potency to induce aberrant activation and signaling we analyzed eight different FLT3 mutations: Three different FLT3-ITD constructs, Vorapaxar web FLT3-JM mutation V592A, common FLT3-TKD mutations D835Y and D835V as well as D839G and I867S in the second TKD. FLT3-D839G and -I867S were recently found in AML patients by our group during routine diagnostics but have not been functionally characterized before. The corresponding remission samples did not express these mutations showing that they are not rare germline variants. Proliferation experiments revealed significant growth advantages for all FLT3 mutants relative to FLT3-WT expressing cells. FLT3-ITD expressing cells grew completely PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19631915 IL-3 independent, regardless of length and insertion site. Cell lines expressing FLT3-V592A, -I867S, -D839G or -D835V/Y mutations showed a clear gain-of-function phenotype, but differed Oncogenic Potential of FLT3 Mutations . Reduced cell surface expression of FLT3 was directly correlated with the 150/130 kDa ratio. FLT3-ITD Mutants Induce STAT5 and FLT3 Tyr591 Phosphorylation To gain insight into the signaling properties of the various FLT3 mutants, we analyzed STAT5 phosphorylation by Western blot. STAT5 was only activated in FLT3-ITD expressing cells, but not in the cell lines expressing FLT3 point PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19632868 mutations. Ba/F3 cells expressing FLT3-ITD1 showed the weakest STAT5 activation in the group of ITD mutations. To further confirm the role of STAT5 activation in FLT3 mutants, an ELISA detecting phosphorylation of the FLT3 receptor at Tyr591 was performed. Tyr591 has been described to be critical for the regulative activity of STAT5 and becomes accessible through the conformational changes induced by ITD. All FLT3-ITD mutants induced considerable phosphorylation of Tyr591. Probably due to the small number of values, significance compared to FLT3-WT could be verified for FLT3ITD2 and -ITD3, but not for FLT3-ITD1 expressing cells. FLT3-I867S, -D839G and -D835Y expressing cells exceeded phosphorylation levels of Tyr591 of FLT3-WT expressing cells significantly . In Contrast to FLT3-ITD Mutants FLT3 Point Mutation Expressing Cells Rely on AKT and MAPK Signaling and are Inhibited by AKT Inhibitor MK 2206 In Western blot analysis, activation level of AKT for FLT3I867S, D839G and -D835V expressing cell lines were significantly stronger than in FLT3-WT expressing cells