Acute myeloid leukemia (AML) is a heterogeneous disease of the myeloid lineage. About 35% of AML patients carry an oncogenic FLT3 mutant making FLT3 an attractive target for treatment of AML. Major problems in the development of FLT3 inhibitors include lack of specificity, poor response and development of a resistant phenotype upon treatment. Further understanding of FLT3 signaling and discovery of novel regulators will therefore help to determine additional pharmacological targets in FLT3-driven AML. In this report, we identified BEX1 as a novel regulator of oncogenic FLT3-ITD-driven AML. We showed that BEX1 expression was down-regulated in a group of AML patients carrying FLT3-ITD. Loss of BEX1 expression resulted in poor overall survival (hazard ratio, HR = 2.242, p = 0.0011). Overexpression of BEX1 in mouse pro-B and myeloid cells resulted in decreased FLT3-ITD-dependent cell proliferation, colony and tumor formation, and in increased apoptosis in vitro and in vivo. BEX1 localized to the cytosolic compartment of cells and significantly decreased FLT3-ITD-induced AKT phosphorylation without affecting ERK1/2 or STAT5 phosphorylation. Our data suggest that the loss of BEX1 expression in FLT3-ITD driven AML potentiates oncogenic signaling and leads to decreased overall survival of the patients.

Aberrant activation of anaplastic lymphoma kinase (ALK) can cause sporadic and familial neuroblastoma. Using a proteomics approach, we identified Bruton's tyrosine kinase (BTK) as a novel ALK interaction partner, and the physical interaction was confirmed by co-immunoprecipitation. BTK is expressed in neuroblastoma cell lines and tumor tissues. Its high expression correlates with poor relapse-free survival probability of neuroblastoma patients. Mechanistically, we demonstrated that BTK potentiates ALK-mediated signaling in neuroblastoma, and increases ALK stability by reducing ALK ubiquitination. Both ALK and ALK can induce BTK phosphorylation and higher capacity of ALK is observed. Furthermore, the BTK inhibitor ibrutinib can effectively inhibit the growth of neuroblastoma xenograft in nude mice, and the combination of ibrutinib and the ALK inhibitor crizotinib further enhances the inhibition. Our study provides strong rationale for clinical trial of ALK-positive neuroblastoma using ibrutinib or the combination of ibrutinib and ALK inhibitors.

Fms-like tyrosine kinase (FLT3) is a frequently mutated oncogene in acute myeloid leukemia (AML). FLT3 inhibitors display promising results in a clinical setting, but patients relapse after short-term treatment due to the development of resistant disease. Therefore, a better understanding of FLT3 downstream signal transduction pathways will help to identify an alternative target for the treatment of AML patients carrying oncogenic FLT3. Activation of FLT3 results in phosphorylation of FLT3 on several tyrosine residues that recruit SH2 domain-containing signaling proteins. We screened a panel of SH2 domain-containing proteins and identified SLAP2 as a potent interacting partner of FLT3. We demonstrated that interaction occurs when FLT3 is activated, and also, an intact SH2 domain of SLAP2 is required for binding. SLAP2 binding sites in FLT3 mainly overlap with those of SRC. SLAP2 over expression in murine proB cells or myeloid cells inhibited oncogenic FLT3-ITD-mediated cell proliferation and colony formation in vitro, and tumor formation in vivo. Microarray analysis suggests that higher SLAP2 expression correlates with a gene signature similar to that of loss of oncogene function. Furthermore, FLT3-ITD positive AML patients with higher SLAP2 expression displayed better prognosis compared to those with lower expression of SLAP2. Expression of SLAP2 blocked FLT3 downstream signaling cascades including AKT, ERK, p38 and STAT5. Finally, SLAP2 accelerated FLT3 degradation through enhanced ubiquitination. Collectively, our data suggest that SLAP2 acts as a negative regulator of FLT3 signaling and therefore, modulation of SLAP2 expression levels may provide an alternative therapeutic approach for FLT3-ITD positive AML.

The type III receptor tyrosine kinase FLT3 is frequently mutated in acute myeloid leukemia. Oncogenic FLT3 mutants display constitutive activity leading to aberrant cell proliferation and survival. Phosphorylation on several critical tyrosine residues is known to be essential for FLT3 signaling. Among these tyrosine residues, Y842 is located in the so-called activation loop. The position of this tyrosine residue is well conserved in all receptor tyrosine kinases. It has been reported that phosphorylation of the activation loop tyrosine is critical for catalytic activity for some but not all receptor tyrosine kinases. The role of Y842 residue in FLT3 signaling has not yet been studied. In this report, we show that Y842 is not important for FLT3 activation or ubiquitination but plays a critical role in regulating signaling downstream of the receptor as well as controlling receptor stability. We found that mutation of Y842 in the FLT3-ITD oncogenic mutant background reduced cell viability and increased apoptosis. Furthermore, the introduction of the Y842 mutation in the FLT3-ITD background led to a dramatic reduction in in vitro colony forming capacity. Additionally, mice injected with cells expressing FLT3-ITD/Y842F displayed a significant delay in tumor formation, compared to FLT3-ITD expressing cells. Microarray analysis comparing gene expression regulated by FLT3-ITD versus FLT3-ITD/Y842F demonstrated that mutation of Y842 causes suppression of anti-apoptotic genes. Furthermore, we showed that cells expressing FLT3-ITD/Y842F display impaired activity of the RAS/ERK pathway due to reduced interaction between FLT3 and SHP2 leading to reduced SHP2 activation. Thus, we suggest that Y842 is critical for FLT3-mediated RAS/ERK signaling and cellular transformation.Electronic supplementary materialThe online version of this article (doi:10.1007/s00018-017-2494-0) contains supplementary material, which is available to authorized users.

D816V is the most often occurred mutation of type III receptor tyrosine kinase KIT in mastocytosis and CBF-AML. In attempting to identify specific downstream signaling pathways of KIT/D816V, we found that KIT/D816V but not wild-type KIT can bind to Xkr5 that was not studied previously. In addition to the association with KIT/D816V, Xkr5 is phosphorylated at Tyr 369, Tyr487 and Tyr 543 by KIT/D816V, and the phosphorylation is mediated by only mutations of Asp 816 in KIT but not by other KIT mutations that occurred in gastrointestinal stromal tumors and melanoma. Furthermore, the tyrosine phosphorylation of Xkr5 is not dependent on Src family kinases that play a crucial role in the activation of wild-type KIT. In KIT/D816V expressing mast cells, phosphorylation of Xkr5 inhibits KIT/D816V downstream signaling molecules Akt, Erk and p38. As a consequence, cell proliferation and colony formation are inhibited by the tyrosine phosphorylation of Xkr5 as well, indicating that Xkr5 is a negative regulator of KIT/D816V signaling. Citation Format: Jianmin Sun, Tine Thingholm, Tianfeng Li, Julhash U. Kazi, Hui Zhao, Lars Rönnstrand. Xkr5 negatively regulates KIT/D816V signaling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2372. doi:10.1158/1538-7445.AM2017-2372