Abl kinases
The Abelson (Abl) kinase family members include Abl1 and Abl2 (Abl-related gene, Arg), which are encoded by both ABL1 and ABL2 genes. This is one of the most conserved branches among the TKs. Human Abl1 and Abl2 proteins are ubiquitously expressed and needed for normal development. Cytoplasmic c-Abl is activated by various growth factors such as PDGF, EGFR, transforming growth factor β and angiotensin subtype 1 receptors [
28]. Abl kinases links distinct extracellular stimuli to signaling cascades that regulate cell multiplication and survival, response to DNA damage and stress, dynamics of actin, cell migration, invasion and adhesion [
29].
Abl1 and Abl2 kinases have a central SH3-SH2-SH1 (tyrosine kinase) domain unit, with more than 90% sequence similarity among them, and is also shared within the majority of other cytoplasmic kinases. Both have an amino terminal “cap” region and a unique long carboxy terminal tail with various protein-protein interaction sites for proteins such as p53, ATM, etc. This includes a common filamentous actin binding domain (F-BD), Abl1 specific DNA binding domain and globular domain binding with actin upstream of F-BD, and Abl2 specific second F-BD and a domain which is micro tubule binding, upstream of the F-BD. The Abl kinases have a unique cluster of three PXXP motifs, enabling interaction with other SH3 domain containing adaptor proteins such as Abi, Crk, and Nck [
30]. Abl1 contains three signal motifs with nuclear localization and in the c-terminal region a nuclear export signal, which regulates its nuclear-cytoplasmic shuttling, while the Abl2 is mainly localized to F-actin rich regions within the cytoplasm and other cellular organelles owing to the lack of any nuclear localization signals [
31,
32].
Abl1 was initially thought to be the oncogene vital for the generation of leukemia’s triggered by the Abelson murine leukemia virus. Later identification of the fusion oncoprotein BCR–ABL1 formed by chromosome translocation, t(9;22)(q34.1;q11.2), commonly identified as Philadelphia chromosome (Ph) confirmed the role of Abl family in cancers such as acute myeloid leukemia (AML), chronic myeloid leukemia (CML), and acute lymphoblastic leukemia (ALL), all of which are hematological malignancies. The various malignant Abl fusion gene products encode for constitutively activated Abl kinases that can lead to cellular transformation. In solid tumors, chromosome translocation leading to activation of ABL kinases rarely exists, but is mainly regulated by its over-expression, presence of upstream oncogenic TKs or other chemokine receptors, inactivation of negative regulatory proteins and/or oxidative stress [
33,
34].
Numerous intramolecular interactions influencing the SH1 kinase domain can lead to auto inhibition of the catalytic function of Abl kinases. Both SH3 and SH2 domains are involved in the regulation of auto-inhibition. Interactions amid the SH3 domain and the SH2-SH1 linker sequence as well as SH2 domain and the SH1 C-terminal lobe can lead to the formation of SH3-SH2-SH1 clamp structure, which is the auto inhibited conformation. Even a partial disruption of auto-inhibitory constraints results in oncogenic transformation. Inhibition of Abl kinases can also be achieved by interactions with lipids such as phosphatidylinositol 4,5-bisphosphate and myristoylation of amino terminal cap region. The cap region can bind intramolecularly to stabilize the inactive conformation and is required to achieve and maintain inhibition [
35]. The abnormal BCR-ABL oncogenic proteins lacks auto-inhibitory cap region and the reintroduction of Abl inhibitory effects upon the reintroduction of cap region conveys the importance of the region in maintaining normal functioning.
Activation of Abl involves extensive domain rearrangements; primarily disruption of SH2 interaction with SH1 c-terminal lobe and in turn binding with the amino terminal lobe of the SH1 domain, leading to allosteric activation that is independent of the ability to bind phosphotyrosine residues. Abl activation can occur through tyrosine phosphorylation in trans by autophosphorylation, SRC family kinases induced phosphorylation and RTKs like PDGFR. Tyrosine phosphorylation of Tyr
412 in Abl1 / Tyr
439 in Abl2 positioned inside the activation loop of kinase domain and Tyr
245 in Abl1 / Tyr
272 in Abl2 located within the SH2-kinase linker domain is essential to start the kinase activity. Trans-phosphorylation of Abl1 Tyr
89 located within the binding surface of SH3 domain by Src-family kinases disrupts SH3 domain–based autoinhibition leading to enhanced kinase activity and is obligatory for the full transforming activity of BCR-ABL [
36]. Abl1 mediated Tyr
261 phosphorylation of Abl2 increases protein stability of Abl2 [
37], while phosphorylation of Tyr
147 in the SH3-SH2 connector region of BCR-ABL protein by Src family kinases (Hck, Lyn, and Fyn) modulate BCR-ABL protein conformation and transforming activity [
38].
Chronic myelogenous leukemia, a clonal bone marrow stem cell malignancy, is the first human cancer to be correlated with a certain genetic abnormality. CML accounts for 15% - 20% of adult leukemia’s with a frequency of 1–2 cases per 100,000 individuals. It is more common in men and is rarely seen in children. Disruption of the auto-inhibitory intramolecular interactions due chromosome translocation leads to the formation of constitutively active chimeric BCR-ABL1 fusion oncoproteins that drives CML pathogenesis.
Depending on the length of BCR sequence involved during these translocations, 3 distinct BCR-ABL variants may be created, namely p185, p210, and p230. The most common variant in CML is p210 which is observed in hematopoietic cells of CML patients in stable phase, and in ALL and AML [
39]. The p230 form is associated with acute leukemia’s, neutrophilic-CML and rare cases of CML. The p185 form is found in about 20–30% of affected adults and about 3–5% of children with B-cell acute lymphocytic leukemia [
40]. The loss of cap region inhibition together with the formation of a coiled-coil domain at N terminus of BCR-ABL oncoproteins causes oligomerization and subsequent proximity of numerous kinase domains leading to transphosphorylation of the critical tyrosine residues in the activation loop and other sites contributing to kinase activation [
41]. BCR-ABL oncoprotein is the target of the first tyrosine kinase inhibitor (TKI), imatinib mesylate also known as STI571 which is sold under brand name of Gleevec. Majority of the FDA approved kinase inhibitors are currently in clinical use to target BCR-ABL [
42]. Imatinib mesylate is an orally available ATP-competitive inhibitor that works by stabilizing the inactive ABL kinase–domain conformation. Nilotinib, Dasatinib, Bosutinib and Ponatinib are second generation TKI used for imatinib mesylate resistant cases.
While BCR-ABL is the most common chromosomal translocation, several other chromosomal abnormalities lead to the expression of various fusion proteins, but there are no activating point mutations identified in the ABL1/ABL2 genes. Various Abl1 fusion proteins involved in hematological malignancies includes BCR-ABL1 (p210), BCR-ABL1 (p185), BCR-ABL1 (p230), NUP214-ABL1, EML1-ABL1, ETV6-ABL1, ZMIZ1-ABL1, RCSD1-ABL1, SFPQ-ABL1, FOXP1-ABL1, SNX2-ABL1, RANBP2-ABL1; while ETV6-ABL2, RCSD1-ABL2, PAG1-ABL2 and ZC3HAV1-ABL2 are originating from Abl2. A large number of signaling pathways are activated by BCR-ABL, but the pathways that are critical for BCR-ABL–dependent transformation includes Gab2, Myc, CrkL and STAT5 [
43].
Presence of BCR-ABL oncoprotein is the most frequent genetic abnormality found in adult ALL patients. Nearly 3–5% childhood and 25–40% adult cases of ALL have Philadelphia chromosome, the presence of which confers a worst prognosis and most of these cases present with an aggressive leukemia. First generation tyrosine kinase inhibitor, imatinib mesylate monotherapy can lead to complete remission rates (90%–100%) and combining imatinib mesylate with standard chemotherapy also increases the overall long-term disease-free survival in both adults and children. Imatinib mesylate based induction and consolidation regimens followed by hematopoietic stem cell transplantation significantly improved the outcome Ph
+ ALL [
44].
Approximately 1% of newly diagnosed AML cases show a consistent association with the Ph chromosome [
45]. The presentation of cases with CML in myeloid blast crisis and Ph
+ AML needs stringent criteria to differentiate. Characteristic of Ph
+ AML includes co-occurrence of typical metaphase chromosome alongside Ph
+ metaphases during diagnosis, less probability for additional copies of Ph and trisomy 8. Ph
+ AML patients will have a poor prognosis with standard chemotherapy regimen, and would benefit from combination therapy with imatinib mesylate [
46].
Feline Sarcoma (FES) kinases
FEline Sarcoma (FES) and FEs Related (FER) are members of a separate class of NRTKs called FES kinase family. These kinases are homologous to the viral oncogenes; feline
v-fes (Feline sarcoma) and avian
v-fps (Fujinami poultry sarcoma) which are responsible for cancerous transformation. Fes, a 93KDa proto-oncogene, is predominantly present in the myeloid lineage of hematopoietic cells, epithelial, neuronal and vascular endothelial cells, while Fer is ubiquitously expressed. Human c-Fes has been linked to multiple cell surface growth factor and cytokine receptors (ex, interleukin 3 & 4 and GM-CSF receptors) which are involved in cell survival and migration, release of inflammatory mediator and innate immune responses. In addition, it might play a direct part in myeloid differentiation and angiogenesis [
47].
Recent findings show that both kinases remain activated in primary AML blasts as well as cell lines. Fes has been reported to have a role in phosphorylation/activation of STAT family of transcription factors, and signaling proteins such as phosphatidylinositol-4,5-bisphosphate 3-kinase, mitogen-activated protein kinases and extracellular signal–regulated kinases [
48]. Fes is essential for downstream signaling of the mutated oncogenic KIT receptor. Both Fes and Fer are involved in regulation of vital functions downstream of internal tandem duplication containing FLT3. Fer kinase is necessary for cell cycle progression, while Fes is needed for D816V mutated KIT dependent cell survival.
FES kinases have a unique amino terminal FCH (Fes/Fer/Cdc-42-interacting protein homology) domain, three coiled-coil motifs that facilitate oligomerization, a central SH2 domain for various protein-protein interactions and a kinase domain in the carboxy terminal region. FCH domain together with the first coiled-coil motif is called F-BAR (FCH-Bin–Amphiphysin–Rvs) domain [
49]. The biological activity of Fes is tightly regulated, with a tight packing between SH2 and kinase domain to maintain a catalytically repressed state, so that kinase activity is regulated despite the absence of a negative regulatory SH3 domain. Activation of Fes kinase needs active phosphorylation of Tyr
713 located inside the activation loop. Tyr
811 is another critical phosphorylation site for the activation of Fes.
Aberrant activation of Fes is not connected with human cancers. Regardless, studies show that hyper-activation of Fes kinase is critical in maintaining the deregulated proliferation in human lymphoid malignancies elicited by constitutively active forms of mutated surface receptors (internal tandem duplication containing FLT3 and KIT D816V) [
50]. Four somatic mutations within the kinase domain of Fes was reported in colorectal cancers, but none of them are gain-of-function mutations [
51]. Similarly Fer mutations in small cell lung cancer has been reported [
52] Over expression of human c-fps/fes using retroviral vector can transform fibroblasts and other established mouse cells [
53] and it requires Ras, Rac, and Cdc42 [
47].
JAK kinases
JAK family of tyrosine kinases consist of four members that includes JAK1, JAK2, JAK3, and Tyk2 [
54]. All members of JAKs family contain a similar protein structure; a carboxy terminal kinase domain flanked by a catalytically inactive JH2 (Janus homology domain 2), pseudokinase domain which possesses a kinase-regulatory activity via a SH2 domain. There is also a FERM domain that regulates the binding to the membrane-proximal part of the cytokine receptors [
55,
56]. Following binding of the ligand (usually cytokines, such as interferon α/β/γ, interleukins, GPCR ligands and growth factors) to specific receptor, these kinases are activated [
57] via tyrosine phosphorylation of the cytoplasmic domains of cytokine receptors [
58]. Activated JAKs then subsequently phosphorylate cytoplasmic domain of the receptor [
59]. The resulting complex of receptor then recruits and phosphorylates the cytoplasmic STAT family members [
60,
61]. STAT family members are major downstream targets of JAK kinases in the pathogenesis oh hematological malignancies [
62]. STAT phosphorylation is followed by dimerization and translocation from cytoplasm to the nucleus, where it regulates the manifestation of various target genes [
54,
63].
Constitutive activation of JAKs has been reported in many cancers including various hematological malignancies. Deregulated JAK activity arises by numerous means, including aberrant cytokine production via autocrine/paracrine mechanism, activating point mutations within JAKs or any other oncogene upstream of signaling cascade.
Through past several years, a number of JAK mutations that lead to activation of constitutively active or hyperactive JAK activity have been identified [
64]. The genetic alteration of JAK family has been reported in all members. It is a well known fact that JAK mutations is linked with development of hematological malignancies [
59,
65]. The majority of these alterations are point mutations [
59]. JAK2V617F mutation is one of the most studies genetic alteration in JAK family [
59]. JAK2V617F mutation is mainly found in primary myelofibrosis or essential thrombocythemia patients. These patients have an incidence of 50% to 60% JAK2V617F mutational freqency and majority (95%) reported polycythemia vera [
66]. Another point JAK1 mutation, A634D has been reported in the pseudokinase domain [
67]. This mutation has been shown to cause a prominent effect on signaling functions [
67]. JAK1 mutation has been found to involve in the development of AML [
68] JAK1 mutations are commonly found in T-cell ALL (18%) and with a lesser frequency in B-cell ALL (B-ALL). Constitutive activation of STAT5 has been linked with mutation of JAK1 [
65,
69,
70]. JAK1 mutation-mediated activation of STAT5 is also reported in AML patients. JAK3 member of JAK family is found only in hematopoietic lineage. Point mutations leading to aberrant activation of JAK3 have been reported in various leukemia/lymphomas [
71]. Juvenile myelomonocytic leukemia (JMML) patients with secondary mutations in
JAK3 have poor prognosis and clinical outcome. In JMML 12% of JAK3 gene has been found to be mutated [
72]. Mutation of JAK3 is reported in 15% acute megakaryoblastic leukemia [
73]. T-cell lymphoma patients (Extranodal nasal-type natural killer) (21%) were reported to have JAK3 mutations (A573V or V722I) in the pseudokinase domain [
74]. These mutations can lead to constitutive JAK3 activation conferring invasive growth and survival advantages. In aggressive T-ALL, JAK3 mutation has been found to be significantly associated [
75]. Mutation inTYK2 kinase have been reported in T-ALL (21%) and play a role in promote cell survival via activation of STAT1 as well BCL2 upregulation expression [
76].
JAK2 amplification via telomeric segment translocation (9p24) leading to increased JAK2 expression and kinase activity has been described in Hodgkin lymphoma and primary mediastinal B-cell lymphoma [
77‐
79].
ACK kinases
Acks also known as Activated Cdc42 kinases (Acks) are the important components of signal transduction pathways which comes under the category of non-receptor tyrosine kinases. There are seven different types of Acks viz., Ack1/Tnk2, Ack2, DACK, TNK1, ARK1, DPR2 and Kos1 [
16]. Most of the members of the Acks are evolutionary conserved and consists of both N-terminal and C-terminal domains such as a SH3 domain and a kinase domain with key difference in the protein’s c-terminal region [
16,
80]. Presence of C terminal kinase domain followed by a SH3 domain along with (CRIB) makes them unique NTRKs [
16,
80].
Ack1 (ACK, TNK2, or activated Cdc42 kinase) is one of the most widely studied and first well known members of the Acks. Ack1, a ubiquitous 140KDa protein located on the chromosome 3q, was first cloned in hippocampus of the human brain that binds to active form of CdC42 i.e., in its GTP bound form [
80,
81]. Presence of multiple structural domains (N-terminal; SAM domain, tyrosine kinase catalytic domain, SH3 domain, CRIB domain, and C terminal; proline rich domain, ubiquitin associated domain) makes ACK1 distinct from other NRTKs and also provides strong force for its functional diverseness [
16,
82].
ACK1 play vital role in cell survival, migration, cell growth and proliferation via acting as an integral cytosolic signal transducer for the array of receptor tyrosine kinases (MERTK, EGFR, PDGFR, IR etc.) to different intracellular effectors which includes both cytosolic as well as nuclear [
81]. Furthermore, Ack1 is also an important epigenetic regulator with negative regulatory effect on tumor suppressors [
81‐
86].
Considerable number of reports has revealed crucial role of ACK in the carcinogenesis of various types of neoplasm. Abnormal overexpression, amplification, or mutation of ACK1 has been well documented in many forms of human cancers, including gastric, breast, ovarian, pancratic, colorectal, head and neck squamous cell carcinomas, osteosarcoma, hepatocellular carcinoma, and prostate cancers [
81,
85‐
90]. Recently, Xu et al., revealed that ACK1 promotes development of gastric tumors by p53 ubiquitination degradation via upregulating ecdysoneless homolog, a cell cycle regulator [
86] and also reported earlier that ACK regulates the expression of about 147 proteins which are closely associated with cell survival [
91].
A number of underlying mechanisms have been documented for ACK1 mediated cancer development. Recently, Maxon et al. reported that mutations in ACK1/TNK2 gene is the main oncogenic cause for AML and chronic myelomonocytic leukemia and that these mutations were sensitive to inhibitors of ACK1 [
92]. Furthermore, in the case of chronic neutrophilic leukemia and atypical CML, ACK1 plays critical role in growth by inducing JAK and SRC machinery [
17]. In acute leukemia patients harboring NRAS mutation, ACK1 along with other survival proteins have been identified as important therapeutic targets [
93]. Diverse crucial role of ACK1 implicated in carcinogenesis including the stimulatory effect on the array of signaling molecules related to cancer development such as AKT, AR, and also by down regulation of tumor suppressors entails its therapeutic importance and prompts the community to look for potential inhibitors.
TNK1 (thirty-eight-negative kinase 1), another important member of ACK family NRTKs of size about 72 KDa, was first reported in blood stem cells of human umbilical cord and murine embryonic cells [
16,
94]. Available literature reveals that TNK1 has both tumor suppressing and oncogenic potential as it can mitigate the growth of tumor cells by dowenregulating Ras-Raf1-MAPK pathway [
95], induce apoptosis through NF-κB inhibition [
96], activate cellular transformation and growth of neoplastic cells [
97,
98]. TNK1 sorted out as an important kinase with oncogenic potential implicated in hematological carcinogenesis such as in AML and Hodgkin’s Lymphoma which suggests that targeted intervention of TNK1 may open new platform for therapy.
TEC kinases
Tec family kinases, the second largest subfamily of the NRTKs, consist of five members, including BTK (Bruton’s tyrosine kinase), ITK/EMT/TSK (interleukin 2-inducible T-cell kinase), RLK/TXK (tyrosine-protein kinase), BMX/ETK (bone marrow tyrosine kinase on chromosome) and Tec (tyrosine kinase expressed in hepatocellular carcinoma) [
107]. One of the main features of Tec is the presence of an amino terminal pleckstrin homology (PH) and Btk-type zinc finger (BTK) motif followed by a SH3 and SH2 domains and a carboxy terminal kinase domain in their protein structure. As PH domain can bind phosphoinositides, Tec family kinases are assumed to act as the connection between phosphotyrosine-mediated and phospholipid-mediated signaling pathways. Tec kinases are associated with cellular signaling pathways of cytokine receptors, RTKs, lymphocyte surface antigens, G-protein-coupled receptors and integrins [
18]. Tec are abundantly expressed in hematopoietic cells and contribute towards their growth and differentiation [
18].
Mutations found in the gene BTK, essential for B-lymphocyte development, differentiation, and signaling [
108], have been shown to trigger the human B-cell immunodeficiency, X-linked agammaglobulinemia and X chromosome-linked immunodeficiency in mice. This not only proved that BTK activity is required for B-cell development, but supports the presumption that Tec family proteins are crucial for growth and maturation of blood cells [
18]. Previously, the majority of indolent B-cell lymphoma patients did not enter complete remission with treatment and inevitably relapsed [
109]. Over the past 10 years, innovative immunochemotherapies have increasingly improved disease control rates but not survival. Therefore, the development of novel agents were urgently needed, which targeted dysregulated pathways in hematological malignancies. In addition, recent preclinical data has illustrated that BTK is present in specific tumor subtypes and in other relevant cells contributing to the tumor microenvironment, e.g. dendritic cells, macrophages, myeloid derived suppressor cells and endothelial cells. BTK inhibitors against hematological malignancies [
110] have hence been developed, most notably Ibrutinib (PCI-32765), a first in class covalent inhibitor of BTK. Ibrutinib is an orally available small molecule approved for the treatment of patients with some hematological malignancies and It has been proposed that Ibrutinib may also display antitumor activity in solid neoplasms [
111]. Ibrutinib is claimed to be a “breakthrough therapy” by the FDA [
109] and overall has changed the future outlook of therapy for lymphoma.
ITKs, the predominant and highly expressed Tec kinase in T cells, act as vital signaling mediators in normal as well as malignant T-cells and natural killer cells [
112]. Thus, playing an important role in autoimmune inflammatory diseases [
113]. ITK is involved in a variety of downstream signaling from T-cell and NK cell surface receptors and RTKs, mainly the T-cell receptor and Fc receptor [
114‐
116]. ITK mediates signaling by activating phospholipase Cγ1, resulting in activation of downstream targets such as, nuclear factor of activated T-cells (NFAT), NFκB, and mitogen-activated protein kinase pathway [
117]. ITK inhibitors could hence have therapeutic potential in several autoimmune, inflammatory, and malignant diseases. For example, in a recent study by Zhong et al. [
112], using the novel ITK/RLK inhibitor PRN694, ex vivo assays reported inhibitory activity against T-cell prolymphocytic leukemia cells.
TXK expression is mainly detected in some myeloid cell lines and T-cells. Moreover, TXK is expressed in T-cell subsets (e.g. Th1/Th0 cells), and was reported to act as a Th1 cell-specific transcription factor, regulating IFN-γ gene expression via binding to its promoter region, increasing transcriptional activity [
118]. An increasing amount of interest has focused on T-cell subsets, which have been characterized, based on their array of cytokine production, e.g. Th1 cells have been found to secrete IL-2, IFN-γ, and lymphotoxin, supporting cell-mediated response [
118‐
122].
BTK, ITK, and TXK have shown selective expression in bone marrow cells [
123]; however BMX and TEC have displayed a much broader expression, even extending to normal somatic cells (e.g. cardiac endothelium) [
107]. BMX has been reported to be expressed in myeloid lineage hematopoietic cells (e.g. granulocytes and monocytes), endothelial cells, and numerous types of oncologic disorders [
107]. Over the past decade, there was significant progress made in this area of research, which has suggested a prominent role of BMX in cell survival, differentiation and motility, and as such, a key player in inflammation and cancer [
107,
124].
TEC is expressed in hematopoietic cells like myeloid lineage cells, B and T cells, as well as neutrophils and has been reported to be involved in the stabilization of lymphocytes (B and T), T and B cell receptor signaling, and in the nuclear factor activation of activated T-cells [
125]. The overexpression of TEC has been found to be associated with tumorigenesis and liver cancer progression [
126]. Inhibiting TEC or degrading the phosphorylation of TEC may therefore have a direct affect on the progression and development of liver cancer. This was supported by an investigation carried out by Chen et al. [
127] exploring TEC protein expression in hepatocellular carcinoma and TEC phosphorylation in 200 specimens of cancerous and non-cancerous liver tissue. A more recent study by Vanova et al. [
128] with interest in the expression of TEC in hepatocellcular carcinoma, identified TEC as a regulator in controlling pluripotent cell fate in human pluripotent stem cells, acting through the regulation of fibroblast growth factor-2 secretion. Such studies provide further support and evidence of the broad activities and roles of tyrosine kinase preferentially expressed in hepatocellular carcinoma.
Src kinases
The Src family of tyrosine kinases (SFKs) are membrane-associated NRTKs active as key mediators of biological signal transduction pathways. This family includes 11 related kinases: Blk, Fgr, Fyn, Hck, Lck, Lyn, c-Src, c-Yes, Yrk, Frk (also known as Rak) and Srm [
134,
135].
SFK members share a highly conserved structure, comprising of a membrane-targeting myristoylated or palmitoylated SH4 domain in the amino terminal region, trailed by SH3, SH2 and a kinase domains, and a short carboxy terminal tail with an auto-inhibitory phosphorylation site [
134]. Furthermore, each member of SFKs has a specific domain of 50–70 residues that is consecutive to the SH4 region and divergent among the different family members [
136].
Five members of the SKFs (Blk, Fgr, Hck, Lck and Lyn) are expressed predominantly in hematopoietic cells. However, c-Src, c-Yes, Yrk and Fyn, are expressed ubiquitously with high levels in platelets, neurons and some epithelial tissues [
134,
137]. Moreover, Srm is present in keratinocytes, and Frk expressed primarily in bladder, breast, brain, colon, and lymphoid cells [
135].
SFKs have a major role in a variety of cellular signaling pathways activated through various RTKs (PDGF-R, EGF-R, FGF-R, IGF1-R, CSF-R) [
138] and G-protein coupled receptors, regulating cell survival, DNA synthesis and division, actin cytoskeleton rearrangements and motility [
137,
139]. Src family member exerts its full catalytic activity upon phosphorylation of a critical residue (Tyr
419) within the activation loop. Phosphorylation of the auto-inhibitory site Tyr
530 within the carboxy terminal tail forms a closed auto-inhibited inactive conformation via the association of the SH2, SH3, and kinase domains by intramolecular interactions. Many factors, including specific cellular signals, or transforming mutations, could break these interactions and produce an active open kinase [
140]. Several protein tyrosine phosphatases can dephosphorylate Tyr
530 and thus regulate its kinase activity.
SFKs associate with PDGF-R via an interaction of their SH2 domain with Tyr579 of the ligand bound activated receptor. This association will release the auto-inhibitory intra-molecular interface between the SH2 domain and the carboxy terminal tail, subsequently permitting the formation of catalytically active conformation. SFKs in turn modulate RTK activation and are involved in promoting mitogenesis.
SFKs might have a part in cancer development due to their implication in the regulation of cell–cell adhesion. This regulatory pathway involves different molecules such as p120-catenin protein, a substrate of Src [
141]. SFK, particularly Src, might also be involved in tumorigenesis by activation of STAT transcription factors which regulate cytokine signaling in hematopoietic cells [
142]. Moreover, SFKs like focal adhesion kinase, paxillin and p130CAS have been implicated in monitoring of signaling pathways mediated by integrin. Alterations in integrin activity have been described in several tumor types [
143]. Src is also thought to have a role in the progression of CML, AML, CLL, and ALL through activation of STAT pathways and regulation of RAS/RAF/MEK/ERK MAPK and VEGF pathways. Other oncogenic pathways of C-Src include translocation of B-catenin, promotion of ERK and Cbl phosphorylation and increase in anti-apoptotic Bcl2 in cancer cells [
144‐
146].
SFKs also play a role in the development and signaling of T and B cells. Indeed, SFKs, particularly Lck, appear necessary for T cell receptor-based signaling essential for various phases of T-cell development [
134,
147]. In addition, Lyn, have a major role in B-cell lineage development and maturation, activation as well as inhibition [
148].
A consistent number of studies point the role of SFKs in human tumors since they are often overexpressed and/or constitutively hyper-activated in several cancers [
137]. Activation of SFKs could arise either after a mutation of Src allele leading to disrupted negative regulatory network or to binding of SFKs to activating partners such as growth factor receptors (Her2/Neu, PDGF, EGFR, c-kit), adaptor proteins and other NRTKs (focal adhesion kinase, Bcr-Abl) [
149]. Various SFKs members have been implicated in the development of hematopoietic malignancies such as leukemia and lymphomas (AML, ALL, CML, Burkitt’s lymphoma, etc.) [
150]. However, oncogenic mutations of SFKs are rarely observed in hematological malignancies [
151]. Therefore, the progression of leukemia and lymphoma malignancies is mainly associated with the constitutive activation of SFKs and to amplification of anti-apoptotic and oncogenic downstream signaling pathways [
149,
150].
In cancer cells, multiple mechanisms are able to disrupt the inactive conformation of SFKs including binding of SH2 to activated receptors such as flt3 (in AML) and to oncogenic protein kinase such as BCR-ABL (in CML and ALL) [
152]. Furthermore, in cancer cells, the SFKs inhibitory signaling pathways such as C-terminal Src kinase have shown to be suppressed thus leading to a stabilization of SFK activated conformation [
151]. Activation of SFKs promotes multiple downstream signal transduction cascades implicated in apoptosis and oncogenesis (STAT3 and STAT5, MEPK, EGFR, PDGFR, PI3K/AKT and VEGFR) [
146,
149,
150,
153].
Moreover, it has been shown that SFKs promote cancer cells resistance to chemotherapy and radiation as well as targeted RTK therapies [
154,
155]. Donato et al. have demonstrated that Lyn and Hck, were upregulated in imatinib mesylate resistant cell line and in specimens of advanced CML and ALL from patients who relapsed to imatinib mesylate [
149,
156]. Indeed, SFKs members, particularly Hck and Lyn, interact with the oncogenic BCR-ABL fusion protein and promote resistance to imatinib mesylate treatment [
157].
Given the importance of SFKs in multiple aspects of tumor development, such as proliferation, migration, resistance to apoptosis, and angiogenesis, these proteins can be considered as attractive targets for future anti-cancer therapeutics. Moreover, inhibition of SFKs in combination with standard anti-cancer therapies has been suggested as a promising treatment strategy with a clinical potential in overcoming resistance to current regimens and preventing metastatic recurrence [
154].
The viral encoded Src (v-Src) is constitutively active and highly transforming, where as c-Src over expression does not induce transformation. v-Src transformed cells, but not c-Src over expressing cells, have the ability to form tumors in nude mice [
158]. But mutant form of c-Src created by single amino acid changes (Thr to Ile at position 338/Glu to Gly at position 378/Phe to Ile at position 441) or by fragment of c-src (Gly-63, Arg-95, and Thr-96) with a corresponding fragment of v-src (Asp-63, Trp-95, and Ile-96) is oncogenic and induce transformation ([
159,
160].
Fyn has been found to be overexpressed in various types of cancers including hematological malignancies [
15,
161,
162]. Fyn is involved in development and activation of T cells [
15]. Activated Fyn is proven to play a role in the pathogenesis of multiple human carcinomas via influencing cell growth, transformation ability of cells and apoptosis [
15]. Fyn has been also found to participate in generation of mitogenic signaling, initiation of cell cycle and cell to cell adhesion [
163]. Fyn also play a critical role in aggressiveness of CLL.
Lyn is aberrantly expressed and highly activated in many cancer cells [
164,
165]. Association of Lyn kinase with dysregulated signaling pathways in various hematopoietic as well solid tumors implicates that it might be an important target for the treatment of cancer. Dysregulation of Lyn have an important role in progression of CLL via regulation of apoptotic signaling pathway [
166]. A number of substrates has been identified in CLL including SYK, PI3K, HS1, procaspase-8, and PP2A [
167‐
169].
C-terminal Src kinases
C-terminal Src kinases (Csk) and Csk-homologous kinase (Chk) are the two members of this family of NRTKs. Csk is a 50 kDa protein with an amino terminal SH3 domain followed by a SH2 domain and a carboxy terminal kinase domain. A characteristic feature of Csk is the absence of a site in the activation loop for auto-phosphorylation. The active conformation is stabilized by the binding of SH2-kinase and SH2-SH3 linkers to the amino terminal lobe of the kinase domain.
CSKs phosphorylates the auto-inhibitory tyrosine residues in the Src-family kinases’s C-terminal tail which stabilizes SFKs in a closed inactive conformation and thus functions as the major endogenous negative regulators of SFKs. Chk can engage a complementary mechanism to inhibit SFKs by direct binding to SFKs, which is also called as non-catalytic inhibitory mechanism. Several other signaling proteins such as paxillin, P2X3 receptor, c-Jun and Lats can also serves as substrates of Csk, but the physiological relevance of it is not yet known [
151,
170].
Csk is ubiquitously expressed in all cells, however, Chk is mainly expressed in the brain, haematopoietic cells, colon tissue and smooth muscle cells [
170]. Csk is primarily present in cytosol as it does not have a transmembrane domain or any fatty acyl modifications. As the substrate molecules (SFKs) are attached to the membrane, the mobility of Csk to the membrane by means of numerous scaffolding proteins (caveolin-1, paxillin, Dab2, VE-cadherin, IGF-1R, IR, LIME, and SIT1), is a crucial step in the regulation of Csk activity [
151].
They have an important role in the regulation of cell functions like growth, migration, differentiation, and immune response. Recent studies suggest that Csk can have a function as tumor suppressor through the inhibition of SFKs oncogenic activity.