Time resolved quantitative phospho-tyrosine analysis reveals Bruton's Tyrosine kinase mediated signaling downstream of the mutated granulocyte-colony stimulating factor receptors

Granulocyte-colony stimulating factor receptor (G-CSFR) controls myeloid progenitor proliferation and differentiation to neutrophils. Mutations in CSF3R (encoding G-CSFR) have been reported in patients with chronic neutrophilic leukemia (CNL) and acute myeloid leukemia (AML); however, despite years of research, the malignant downstream signaling of the mutated G-CSFRs is not well understood. Here, we used a quantitative phospho-tyrosine analysis to generate a comprehensive signaling map of G-CSF induced tyrosine phosphorylation in the normal versus mutated (proximal: T618I and truncated: Q741x) G-CSFRs. Unbiased clustering and kinase enrichment analysis identified rapid induction of phospho-proteins associated with endocytosis by the wild type G-CSFR only; while G-CSFR mutants showed abnormal kinetics of canonical Stat3, Stat5, and Mapk phosphorylation, and aberrant activation of Bruton's Tyrosine Kinase (Btk). Mutant-G-CSFR-expressing cells displayed enhanced sensitivity (3-5-fold lower IC50) for ibrutinib-based chemical inhibition of Btk. Primary murine progenitor cells from G-CSFR-Q741x knock-in mice validated activation of Btk by the mutant receptor and retrovirally transduced human CD34⁺ umbilical cord blood cells expressing mutant receptors displayed enhanced sensitivity to Ibrutinib. A significantly lower clonogenic potential was displayed by both murine and human primary cells expressing mutated receptors upon ibrutinib treatment. Finally, a dramatic synergy was observed between ibrutinib and ruxolinitib at lower dose of the individual drug. Altogether, these data demonstrate the strength of unsupervised proteomics analyses in dissecting oncogenic pathways, and suggest repositioning Ibrutinib for therapy of myeloid leukemia bearing CSF3R mutations. Phospho-tyrosine data are available via ProteomeXchange with identifier PXD009662.

Molecular basis for the recognition of phosphorylated STAT1 by importin alpha5

Interferon-gamma stimulation triggers tyrosine phosphorylation of the transcription factor STAT1 at position 701, which is associated with switching from carrier-independent nucleocytoplasmic shuttling to carrier-mediated nuclear import. Unlike most substrates that carry a classical nuclear localization signal (NLS) and bind to importin alpha1, STAT1 possesses a nonclassical NLS recognized by the isoform importin alpha5. In the present study, we have analyzed the mechanisms by which importin alpha5 binds phosphorylated STAT1 (pSTAT1). We found that a homodimer of pSTAT1 is recognized by one equivalent of importin alpha5 with K(d)=191+/-20 nM. Whereas tyrosine phosphorylation at position 701 is essential to assemble a pSTAT1-importin alpha5 complex, the phosphate moiety is not a direct binding determinant for importin alpha5. In contrast to classical NLS substrates, pSTAT1 binding to importin alpha5 is not displaced by the N-terminal importin beta binding domain and requires the importin alpha5 C-terminal acidic tail (505-EEDD-508). A local unfolding of importin alpha5 Armadillo (ARM) repeat 10 accompanies high-affinity binding to pSTAT1. This unfolding is mediated by a single conserved tyrosine at position 476 of importin alpha5, which is inserted between ARM repeat 10 helices H1-H2-H3, thereby preventing intramolecular helical stacking essential to stabilize the folding conformation of ARM 10. Introducing a glycine at this position, as in importin alpha1, disrupts high-affinity binding to pSTAT1, suggesting that pSTAT1 recognition is dependent on the intrinsic flexibility of ARM 10. Using the quantitative stoichiometry and binding data presented in this article, together with mutational information available in the literature, we propose that importin alpha5 binds between two STAT1 monomers, with two major binding determinants in the SH2 and DNA binding domains. In vitro, this model is supported by the observation that a 38-mer DNA oligonucleotide containing two tandem cfosM67 promoters can displace importin alpha5 from pSTAT1, suggesting a possible role for DNA in releasing activated STAT1 in the cell nucleus.