FLT3 activity is negatively regulated by ubiquitin-mediated internalization (Sargin et al, 2007; Reindl et al, 2009; reviewed in Kazi and Ronnstrand, 2019). Several E3 ubiquitin ligases are implicated in the downregulation of active FLT3 including CBL, SOCS2 and SOCS6 (Sargin et al, 2007, Reindl et al, 2009; Kazi and Ronnstrand, 2013; Kazi et al, 2012).
Ubiquitination of human FLT3 in COS-7 cells is abrogated by the expression of a dominant negative form of CBL, implicating CBL as a major E3 ubiquitin ligase for the FLT3 receptor (Sargin et al, 2007). While direct ubiquitination of FLT3 by SOCS2 and SOCS6 has not been demonstrated, overexpression of these E3 ligases induces FLT3 ubiquitination and internalization in cell lines (Kazi and Ronnstrand, 2012; Kazi and Ronnstrand, 2013; reviewed in Kazi and Ronnstrand, 2019).

E3 ligase- mediated ubiquitination of FLT3 leads to its internalization to the endosomal compartment (Sargin et al, 2007; Reindl et al, 2009; reviewed in Kazi and Ronnstrand, 2019).

FLT3 activation leads to tyrosine phosphorylation of CBL (Sargin et al, 2007; Reindl et al, 2009). Phosphorylation of CBL is abolished in FLT3 Y589 and Y599 mutants (Heiss et al, 2006).

GRB10 is phosphorylated in response to treatment of cells with FL (Kazi and Ronnstrand, 2013).

Feline McDonough Sarcoma-like tyrosine kinase (FLT3) (also known as FLK2 (fetal liver tyrosine kinase 2), STK-1 (stem cell tyrosine kinase 1) or CD135) is a member of the class III receptor tyrosine kinase family involved in the differentiation, proliferation and survival of hematopoietic progenitor cells and of dendritic cells. Upon FLT3 ligand (FL) binding, the receptor forms dimers and is phosphorylated. Consequently, adapter and signaling molecules bind with the active receptor and trigger the activation of various pathways downstream including PI3K/Akt and MAPK cascades (Grafone T et al. 2012).

FLT3 is a type III receptor tyrosine kinase (RTK). The extracellular domain consists of 5 immunoglobulin (Ig) domains that contribute to dimerization and ligand binding. The intracellular region has a juxtamembrane domain that plays a role in autoinhibiting the receptor in the absence of ligand, and a bi-lobed kinase region with an activation loop and the catalytic cleft (reviewed in Klug et al, 2018). Signaling through FLT3 occurs after ligand-induced dimerization and transautophosphorylation, and promotes signaling through the MAP kinase, PI3K and STAT5 pathways, among others. FLT3 signaling promotes cellular proliferation and differentiation and contributes to haematopoeisis. FLT3 is mutated in up to 30% of acute myeloid leukemias. ~25% of the FLT3 mutations in AML cases occur as internal tandem duplications (ITDs) either in the juxtamembrane domain region encoded by exon 14 or the tyrosine kinase domain (TKD), while ~7-10% of AML cases contain FLT3 missense mutations in the TKD (reviewed in Klug et al, 2018; Daver et al, 2019). These mutations all support ligand-independent activation of the receptor and result in constitutive activation and signaling (Zheng et al, 2004; reviewed in Klug et al, 2018; Kazi and Roonstrand, 2019). In rare cases, the FLT3 locus is also subject to translocations that generate constitutively active fusion proteins (reviewed in Kazi and Roonstrand, 2019). Oncogenic FLT3 activity can be targeted with tyrosine kinase inhibitors, although resistance often arises due to secondary mutations or activation of bypass pathways (reviewed in Staudt et al, 2018; Daver et al, 2019).

FLT3 activity is negatively regulated through several mechanisms including dephosphorylation, interaction with protein partners that limit downstream signaling pathways, and by ubiquitin-mediated internalization and degradation (reviewed in Kazi and Roonstrand, 2019).

Pexidartinib is a type II tyrosine kinase inhibitor that is active against FLT3, including the quizartinib-resistant gatekeeper mutation F691L. Activating mutations at TKD aspartic acid residue 835 are resistant to pexidartinib-mediated inhibition, however (Smith et al, 2015a, b).

A Novartis tyrosine kinase inhibitor with activity against multiple tyrosine kinase receptors including FGFRs, VEGFRs, PDGFRs, KIT, FLT3 and CSFR. TKI258 is in Phase II clinical trials for advanced breast cancer in patients with and without FGFR1 amplification (NCT00958971), for endometrial cancer with WT or activated FGFR2 mutants (NCT01379534), for relapsed myeloma with and without the t4:14 FGFR3 translocation/amplification (NCT01058434), and in bladder cancer in cases where archived material is available to check for correlation with FGFR3 mutation status (NCT00790426).

Receptor-type tyrosine-protein kinase FLT3

Graphic representation of FLT3 fusion proteins

Fms-related tyrosine kinase 3 ligand

Gilteritinib is an orally bioavailable inhibitor of the receptor tyrosine kinases (RTKs) FMS-related tyrosine kinase 3 (FLT3), AXL and anaplastic lymphoma kinase (ALK), with clinical antineoplastic activity. Gilteritinib inhibits the activity of FLT3-activating mutations which are one of the most common genetic alterations in acute myeloid leukemia (AML).