FLT3 is a member of the Class III Receptor Tyrosine Kinase Family, which also includes FMS, KIT, PDGFRA and PDGFRB. It binds the cytokine FLT3LG (Hannum et al. 1994), which regulates differentiation, proliferation and survival of hematopoietic progenitor cells and dendritic cells.
FLT3LG is probably dimeric. Binding to monomeric FLT3 induces receptor dimerization (Verstraete et al. 2011, Grafone et al. 2012), which promotes phosphorylation of the tyrosine kinase domain, activating the receptor and consequently the downstream effectors. Early studies of FLT3 using a chimeric receptor composed of the extracellular domain of human FMS and the transmembrane and cytoplasmic domains of FLT3 demonstrated the activation of PLCG1, RASA1, SHC, GRB2, VAV, FYN, and SRC pathways. PLCG1, SHC, GRB2, and FYN were found to directly associate with the cytoplasmic domain of FLT3 (Dosil et al. 1993). Later studes using the full-length human receptor identified that FLT3LG binding to FLT3 leads to FLT3 autophosphorylation, association of FLT3 with GRB2, tyrosine phosphorylation of SHC and CBL, formation of a complex that includes CBL, the p85 subunit of PI3K and GAB2, and tyrosine phosphorylation of GAB1 and GAB2 and their association with PTPN11 (SHP-2) and GRB2. PTPN11 (SHP-2), but not PTPN6 (SHP-1) binds GRB2 directly and becomes tyrosine-phosphorylated in response to FLT3LG stimulation. INPP5D (SHIP) also becomes tyrosine-phosphorylated after FLT3LG stimulation but binds to SHC. GAB1 and GAB2 are rapidly tyrosine phosphorylated after FLT3LG stimulation of cells, interacting with tyrosine-phosphorylated PTPN11, p85 subunit of PI3K, GRB2, and SHC (Zhang & Broxmeyer 2000). GAB may mediate the downstream activation of PTPN11, PI3K and thereby PDK1 and AKt which activate the mTOR pathway (Grafone et al. 2012), and possibly the Ras/Raf/MAPK pathway. (Zhang et al. 1999, Marchetto et al. 1999, Zhang e& Broxmeyer 2000). Activation of FLT3 leads to limited activation of STAT5A via a JAK-independent mechanism (Zhang et al. 2000).
FLT3 is mutated in about 1/3 of acute myeloid leukemia (AML) patients, either by internal tandem duplications (ITD) of the juxtamembrane domain or by point mutations usually involving the kinase domain (KD). Both types of mutation constitutively activate FLT3 (Small 2006).