Deciphering the FLT3 Signaling Pathway: Unveiling the Key Players in Hematopoietic Regulation

What is FLT3?

FLT3 (FMS-Like Tyrosine Kinase 3) is a receptor tyrosine kinase that belongs to the tyrosine kinase family. It plays an important role in normal cells and participates in the regulation of cell growth, differentiation and survival. FLT3 is normally expressed in the hematopoietic system and is essential for normal hematopoiesis and immune function.

The Structure of FLT3

FLT3, also recognized as fetal liver kinase-2 (Flk-2) or stem cell tyrosine kinase-1 (STK-1), was initially documented in 1991 by two distinct research teams[1]. The human FLT3 gene is positioned on chromosome 13q12 and encompasses 24 exons. The mRNA in its full form extends approximately 3 kb, coding for a 993-amino acid protein weighing 110 kDa[2]. Upon glycosylation with intricate carbohydrates, the active and membrane-bound version attains a size of 158 kDa[3]. In mice, Flt3 resides on chromosome 5 and translates into a 1000-amino acid protein, exhibiting an 85% resemblance to the human FLT3[4].

An alternative splice variation has been identified, involving the omission of two exons within the extracellular domain-encoding section, resulting in the loss of the fifth immunoglobulin-like domain[5]. However, this particular isoform is present in lower abundance, and its precise physiological role remains undisclosed.

The structure of FLT3 aligns with other members of the class III receptor tyrosine kinase family, including platelet-derived growth factor receptor (PDGFR), stem cell factor receptor (SCFR, c-KIT), and colony-stimulating factor receptor (CSF1R, c-FMS)[6]. What sets them apart is an extracellular ligand-binding domain constituted of five immunoglobulin-like domains, a regulatory juxtamembrane domain (JMD), and a cytoplasmic tyrosine kinase domain (TKD) partitioned into two segments by a kinase-insert domain.

The Expression of FLT3

The expression pattern of FLT3 is predominantly confined to the early progenitor cells during the process of hematopoiesis[8-9]. In murine models, both long-term and short-term hematopoietic stem cells exhibit an absence of Flt3 expression[10-11]. Meanwhile, the multipotent progenitor cells (MPP) house the most primal fraction of FLT3-positive progenitors[12].

The presence of Flt3 expression defines a specific subset of progenitor cells known as lymphoid-primed multipotent progenitor cells (LMPPs). These LMPPs demonstrate the capacity to differentiate into granulocyte-monocyte (GM) lineages, while their differentiation potential into megakaryocyte-erythrocyte (MkE) lineages remains restricted[13].

As hematopoiesis progresses into later stages, Flt3 expression diminishes once the granulocyte-monocyte (GM) progenitor cell stage is reached. Notably, Flt3 expression is not detectable in progenitors leading to the megakaryocyte-erythrocyte (MkE) lineages[14]. However, a noteworthy exception exists in the form of mature dendritic cells (DCs), as these cells maintain Flt3 expression even in their fully differentiated states[15].

The Impact of FLT3 Mutations on AML

Acute myeloid leukemia (AML) is a rapidly progressing blood cancer characterized by the abnormal proliferation of myeloid progenitor cells (blasts)[16]. Despite advances, the 5-year survival rate for AML patients (2009-2015 data) stands at 28.3%[17]. Prognosis hinges on factors like mutation profile and patient age, with younger patients (under 60) having 40-50% survival chances, while those over 60 face a graver prognosis, with only 10-20% surviving. This disparity owes partially to the older cohort's elevated occurrence of unfavorable mutations and inability to tolerate intense chemotherapy. AML emerges from genetic changes in hematopoietic stem cells due to aging or previous treatments like radiation or specific drug exposure[18].

Wild-type FLT3 (WT-FLT3) is overexpressed in 93% of AML cases and 80-90% of B- and T-cell acute lymphoblastic leukemia (B- and T-ALL)[19-20]. Additionally, FLT3 is the most commonly mutated gene in AML, with mutations detected in roughly 30% of all AML cases and 70% of patients with a normal chromosomal makeup. Two predominant FLT3 mutation types exist: (i) internal tandem duplication (ITD), prevalent in the JM domain of the receptor in about 25% of AML cases, and (ii) point mutations in the tyrosine kinase domain (TKD mutations), occurring in approximately 7% of cases[21].

A groundbreaking moment occurred in 1996 with the discovery of FLT3-ITD mutation[22], underscoring FLT3's relevance in AML. Within AML, ITD often arises in exons 14 and 15 of FLT3, duplicating various base pairs in multiples of three, preserving the reading frame[23]. The exact cause of FLT3-ITD mutation isn't fully elucidated, though one theory posits a DNA replication error due to the palindromic sequence in the duplication-prone zone – the tyrosine-rich area of the JM domain (codons 589-599). This scenario suggests impaired DNA mismatch repair contributing to ITD occurrence[24].

FLT3-ITD mutation leads to ligand-independent receptor dimerization and self-phosphorylation, activating the receptor autonomously. This arises from a change in the JM domain's structure, nullifying its inhibitory role by disconnecting from the kinase domain. Unlike the wild-type FLT3, where ligand-triggered auto-phosphorylation of tyrosine residues in the JM region is necessary to relieve the A-loop's inhibitory impact, the extra tyrosine residues due to duplication don't partake in receptor constitutive activation[25].

The FLT3 Signaling Pathway: Unveiling Cellular Regulation

The FLT3 signaling pathway plays a pivotal role in regulating a multitude of hematopoietic cell processes. These encompass crucial aspects like phospholipid metabolism, transcription, proliferation, apoptosis, and even its entwinement with the RAS pathway. This intricate network also holds relevance in leukemogenesis[26-28]. Activation of FLT3-WT primarily engages signal transduction networks involving phosphatidylinositol-3-kinase (PI3K) and the cascading RAS pathway. This activation facilitates the subsequent activation of key players including AKT (protein kinase B, PKB), signal transducer and activator of transcription (STAT), and extracellular-signal regulated kinase 1 and 2 (ERK1/2)[29].

Activated FLT3 forms an alliance with growth factor receptor bound protein-2 (GRB2), a linker protein with a penchant for binding an array of signaling proteins. This association is mediated through SHC (Src homology 2 containing protein) by means of the SH2 domain. An additional feather in the cap of the adapter protein GRB2 is its SH3 domain, which binds to proline-rich residues found in other proteins like SOS (guanine nucleotide exchange factor). This interaction triggers the release of GDP, paving the way for GTP binding to RAS. This cascade subsequently activates RAS, setting in motion a sequence that stimulates downstream effectors including RAF, MAPK/ERK kinase, and RSK (90-kDa ribosomal protein S6 kinase). This domino effect culminates in the activation of CREB (cyclic adenosine monophosphate response element-binding protein), ELK, and STAT. The end result? The transcription of genes crucial for cell proliferation.

Of equal importance are the PI3K/AKT and RAS/ERK pathways, which often function in tandem. They likely cross paths with several other anti-apoptotic and cell cycle proteins such as WAF1, KIP1, and BRCA1. Activated FLT3 propagates the signal by partnering with and phosphorylating various cytoplasmic proteins. Among them are PLCγ1 (phospholipase Cγ1), a regulatory player in phosphatidylinositol metabolism, as well as VAV and FYN, which contribute to the intricate dance of cellular regulation.

FLT3 Protein

Recombinant Human Receptor-Type Tyrosine-Protein Kinase Flt3 (FLT3) Protein (His)

Click here for more FLT3

Synonym:FLK2,STK1,CD135

References:

 

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