Abl tyrosine kinase


Abl and Ras

Bcr/Abl oncoproteins were tested for interactions with cytoplasmic proteins that mediate Ras activation. Such polypeptides include Grb2, which comprises a single Src homology 2 (SH2) domain flanked by two SH3 domains, and the 66, 52 and 46 kDa Shc proteins, which all possess an SH2 domain in their carboxy-terminus. Grb2 associates with tyrosine phosphorylated proteins through its SH2 domain, and with the Ras guanine nucleotide releasing protein mSos1 through its SH3 domains. mSos1 stimulates conversion of the inactive GDP-bound form of Ras to the active GTP-bound state. In bcr-abl-transformed cells, Grb2 and mSos1 form a physical complex with Bcr/Abl. In vitro, the Grb2 SH2 domain binds Bcr/Abl through recognition of a tyrosine phosphorylation site within the amino-terminal bcr-encoded sequence (p.Tyr177-Val-Asn-Val) common to both Bcr/Abl proteins. These results suggest that autophosphorylation within the Bcr element of Bcr/Abl creates a direct physical link to Grb2-mSos1, and potentially to the Ras pathway, and thereby modifies the target specificity of the Abl tyrosine kinase (Puil, 1994).

A 62 kDa protein is highly phosphorylated in many cells containing activated tyrosine kinases. This protein, characterized mainly by its avid association with rasGAP, has proved difficult to identify. In this study, anti-phosphotyrosine antibody was used to purify p62. Based on its peptide sequence, molecular cloning reveals a cDNA encoding a novel protein, p62dok, with little homology to other proteins but with a prominent set of tyrosines and nearby sequences suggestive of SH2 binding sites. v-Abl tyrosine kinase binds and strongly phosphorylates p62dok, which then binds rasGAP. 2C4A, a monoclonal antibody to the rasGAP-associated p62, reacts with p62dok. Thus, p62dok appears to be the long-sought major substrate of many tyrosine kinases (Yamanashi, 1997).

Ras is a necessary downstream element of Abelson murine leukemia virus (Ab-MLV)-mediated transformation. It can be activated by the phosphotyrosine-dependent association of Shc with the Grb2-mSos complex. Shc is tyrosine-phosphorylated and associates with Grb2 in v-Abl-transformed cells, whereas Shc in NIH3T3 cells is phosphorylated solely on serine and is not Grb2-associated. Shc coprecipitates with P120 v-Abl and with P70 v-Abl (which lacks the carboxyl terminus). Surprisingly, a kinase-defective mutant of P120 also binds Shc, demonstrating that Shc/v-Abl association is a phosphotyrosine-independent interaction. Shc from both NIH3T3 and v-Abl-transformed cells binds to the Abl SH2 domain; P120 v-Abl binds to a region in the amino terminus of Shc. Consistent with these data, a v-Abl mutant encoding only the Gag and SH2 regions is able to bind Shc in vivo. The unique non-phosphotyrosine-mediated binding of Shc may allow direct tyrosine phosphorylation of Shc by v-Abl and subsequent activation of the Ras pathway through assembly of a signaling complex with Grb2-mSos (Raffel, 1996).

BCR/ABL is a chimaeric oncogene generated by translocation of sequences from the c-ABL protein-tyrosine kinase gene on chromosome 9 into the BCR (breakpoint cluster region) gene on chromosome 22. Alternative chimeric proteins, p210(BCR/ABL) and p190(BCR/ABL), are produced that are characteristic of chronic myelogenous leukemia and acute lymphoblastic leukemia, respectively. Their role in the etiology of human leukemia remains to be defined. The tumorigenic effect of BCR/ABL oncogenes is mediated by Bcl-2. Bcl-2 is a protein essential for transformation by BCR/ABL. However, it is not known how Bcl-2 and Ras fit together in cell transformation by BCR/ABL. The data presented here establish that Bcl-2 is a downstream target gene of the Ras signaling pathway in cells transformed by BCR/ABL, and that constitutive Ras activation results in constitutive expression of the gene. Conversely, a truncated form of the BCR/ABL, which lacks a critical BCR region required for activation of the Ras signaling pathway, fails to induce Bcl-2 expression. These results indicate that BCR/ABL prevents apoptosis by inducing Bcl-2 through a signaling pathway involving Ras and links constitutive Ras activation and Bcl-2 gene regulation. Hence, these results further imply that Ras is involved in both mitogenic signals and survival signals (Sanchez-Garcia, 1997).

v-Abl induces transcription of c-myc, and c-Myc function is a necessary but not sufficient component of the v-Abl transformation program. The E2F site in the c-myc promoter is a v-Abl response element. v-Abl appears to induce c-myc by initiating a phosphorylation cascade that ultimately activates E2F-binding proteins. The Ras GTPase and Raf1 serine/threonine kinase are required in this pathway. However, in contrast to other aspects of v-Abl signaling, induction of c-myc transcription is independent of the Rac GTPase. These results also establish a requirement for activated cyclin-dependent kinases (cdks), as v-Abl-dependent induction of c-myc transcription is blocked by cdk inhibitor p21 and induction of c-myc is accompanied by activation of cdk2 and cdk4. v-Abl-dependent induction of c-myc is accompanied by hyperphosphorylation of pRb, p107, and p130 (Zou, 1997).

Drosophila disabled interacts genetically with Abl oncogene, Disabled-2 (Dab2), a mammalian structural homolog of Drosophila Disabled is a mitogen-responsive phosphoprotein. It has been speculated to be a negative regulator of growth since its expression is lost in ovarian carcinomas. Dab2 contains a C-terminal proline-rich domain with sequences similar to those found in Sos, a guanine nucleotide exchange factor for Ras (see Drosophila Ras). The proline-rich sequences of Sos mediate the interaction of Sos with Grb2, an adaptor protein which coupled tyrosine kinase receptors to Sos. The possibility that Dab2 interacts with Grb2 has been investigated. In experiments of co-immunoprecipitation from BAC1.2F5 macrophage cell lysates, significant quantities of Grb2 are associated with both Sos and Dab2, although Dab2 and Sos were not present in the same complex. Transfection of Dab2 into a Dab2-negative cell line (293 cells) decreases the amount of Grb2 associated with Sos, suggesting that Dab2 competes with Sos for binding to Grb2. Proline-rich peptides corresponding to Dab2 (#661-669) and to Sos (#1146-1161) inhibit the binding of Dab2 to Grb2, but are less effective in disrupting the Grb2-Sos complex. The expressed proline-rich domain of Dab2 (#600-730) binds Grb2, but other regions of Dab2 fail to bind Grb2. Both of the individual SH3 domains of Grb2 bind to Sos (the N-terminal SH3 domain of Grb2 has a greated affinity than C-terminal SH3 domain), but binding to Dab2 requires the intact Grb2, suggesting cooperative binding using both SH3 domains of Grb2. These data indicate that Dab2 binds to the SH3 domains of Grb2 via its C-terminal proline-rich sequences. Dab2 may modulate growth factor/Ras pathways by competing with Sos for binding to Grb2. While the N-terminal region of Dab2 and Drosophila Dab show significant homology to one another (both contain the PID domain), the C-terminal proline-rich domain of Dab2 differes significantly from that of Drosophila Dab. However, the C-terminal half of Drosophila Dab also contains several stretches of proline-rich sequences, which might function analgously to those in Dab2. Thus, it is possible that Drosophila Dab binds to a homologous SH2-containing Drosophila proteins, such as the Grb2 homolog Drk2 (Xu, 1998).

Abl and Rac

The phenotype of hematopoietic cells transformed by the BCR/ABL oncoprotein of the Philadelphia chromosome is characterized by growth factor-independent proliferation, reduced susceptibility to apoptosis, and altered adhesion and motility. The mechanisms underlying this phenotype are not fully understood, but there is evidence that some of the properties of BCR/ABL-expressing cells are dependent on the activation of downstream effector molecules such as RAS, PI-3k, and bcl-2. It is shown that the small GTP-binding protein Rac is activated by BCR/ABL in a tyrosine kinase-dependent manner. Upon transfection with a vector carrying the dominant-negative N17Rac, BCR/ABL-expressing myeloid precursor 32Dcl3 cells retain the resistance to growth factor deprivation-induced apoptosis but show a decrease in proliferative potential in the absence of interleukin-3 (IL-3) and markedly reduced invasive properties. Moreover, compared with BCR/ABL-expressing cells, fewer BCR/ABL plus N17Rac double transfectants are capable of homing to bone marrow and spleen. Consistent with these findings, survival of SCID mice injected with the BCR/ABL plus N17Rac double transfectants is markedly prolonged, when compared with that of mice injected with BCR/ABL-expressing cells. Together, these data support the important role of a Rac-dependent pathway(s) controlling motility in BCR/ABL-mediated leukemogenesis (Skorski, 1998).

The non-receptor tyrosine kinase Abl participates in receptor tyrosine kinase (RTK)-induced actin cytoskeleton remodelling, a signalling pathway in which the function of Rac is pivotal. More importantly, the activity of Rac is indispensable for the leukaemogenic ability of the BCR-Abl oncoprotein. Thus, Rac might function downstream of Abl and be activated by it. This study elucidates the molecular mechanisms through which Abl signals to Rac in RTK-activated pathways. Sos-1, a dual guanine nucleotide-exchange factor (GEF), is phosphorylated on tyrosine, after activation of RTKs, in an Abl-dependent manner. Sos-1 and Abl interact in vivo, and Abl-induced tyrosine phosphorylation of Sos-1 is sufficient to elicit its Rac-GEF activity in vitro. Genetic or pharmacological interference with Abl (and the related kinase Arg) results in a marked decrease in Rac activation induced by physiological doses of growth factors. Thus, these data identify the molecular connections of a pathway RTKs-Abl-Sos-1-Rac that is involved in signal transduction and actin remodelling (Sini, 2004).

The JIP1 scaffold protein regulates axonal development in cortical neurons: interaction with c-Abl

The development of neuronal polarity is essential for the determination of neuron connectivity and for correct brain function. The c-Jun N-terminal kinase (JNK)-interacting protein-1 (JIP1) is highly expressed in neurons and has previously been characterized as a regulator of JNK signaling. JIP1 has been shown to localize to neurites in various neuronal models, but the functional significance of this localization is not fully understood. JIP1 is also a cargo of the motor protein kinesin-1, which is important for axonal transport. This study demonstrated that before primary cortical neurons become polarized, JIP1 specifically localizes to a single neurite and that after axonal specification, it accumulates in the emerging axon. JIP1 is necessary for normal axonal development and promotes axonal growth dependent upon its binding to kinesin-1 and via a newly described interaction with the c-Abl tyrosine kinase. JIP1 associates with and is phosphorylated by c-Abl, and the mutation of the c-Abl phosphorylation site on JIP1 abrogates its ability to promote axonal growth. The kinesin-1-dependent localization of JIP1 to developing axonal growth cones and its colocalization with dynamic microtubules and exploratory processes emanating from the growth cone suggest that it may be important for the regulation of cytoskeletal structures. c-Abl is a well-established regulator of cytoskeletal dynamics, and JIP1 may be an important downstream effector of c-Abl in cytoskeletal reorganization. The JIP1 scaffold protein, therefore, has the potential to act as a crucial link between extracellular signals and the regulation of cytoskeletal dynamics that lead to axonal development (Dajas-Bailador, 2008).

Abl is activated by growth factors

The c-Abl tyrosine kinase localizes to the cytoplasm and plasma membrane in addition to the nucleus. However, there is little information regarding a role for c-Abl in the cytoplasm/plasma membrane compartments. A membrane pool of c-Abl is activated by the growth factors PDGF and EGF in fibroblasts. The pattern and kinetics of activation are similar to growth factor activation of Src family kinases. To determine whether a link exists between activation of c-Abl and members of the Src family, c-Abl kinase activity was examined in cells that express oncogenic Src proteins. c-Abl kinase activity is increased by 10- to 20-fold in these cells, and Src and Fyn kinases are shown to directly phosphorylate c-Abl in vitro. Furthermore, overexpression of wild-type Src potentiates c-Abl activation by growth factors, and a kinase-inactive form of Src reduces this activation, showing that Abl activation by growth factors occurs at least in part via activation of Src kinases. Significantly, c-Abl has a functional role in the morphological response to PDGF. Whereas PDGF treatment of serum-starved wild-type mouse embryo fibroblasts results in distinct linear or circular/dorsal membrane ruffling, c-Abl-null cells demonstrate dramatically reduced ruffling in response to PDGF, which is rescued by physiological re-expression of c-Abl. These data identify c-Abl as a downstream target of activated receptor tyrosine kinases and Src family kinases, and show for the first time that c-Abl functions in the cellular response to growth factors (Plattner, 1999).

c-Abl, Lamellipodin, and Ena/VASP proteins cooperate in dorsal ruffling of fibroblasts and axonal morphogenesis

Tight regulation of cell motility is essential for many physiological processes, such as formation of a functional nervous system and wound healing. Drosophila Abl negatively regulates the actin cytoskeleton effector protein Ena during neuronal development in flies, and it has been postulated that this may occur through an unknown intermediary. Lamellipodin (Lpd) regulates cell motility and recruits Ena/VASP proteins (Ena, Mena, VASP, EVL) to the leading edge of cells. However, the regulation of this recruitment has remained unsolved. This study shows that Lpd is a substrate of Abl kinases and binds to the Abl SH2 domain. Phosphorylation of Lpd positively regulates the interaction between Lpd and Ena/VASP proteins. Consistently, efficient recruitment of Mena and EVL to Lpd at the leading edge requires Abl kinases. Furthermore, transient Lpd phosphorylation by Abl kinases upon netrin-1 stimulation of primary cortical neurons positively correlates with an increase in Lpd-Mena coprecipitation. Lpd is also transiently phosphorylated by Abl kinases upon platelet-derived growth factor (PDGF) stimulation, regulates PDGF-induced dorsal ruffling of fibroblasts and axonal morphogenesis, and cooperates with c-Abl in an Ena/VASP-dependent manner. These findings suggest that Abl kinases positively regulate Lpd-Ena/VASP interaction, Ena/VASP recruitment to Lpd at the leading edge, and Lpd-Ena/VASP function in axonal morphogenesis and in PDGF-induced dorsal ruffling. These data do not support the suggested negative regulatory role of Abl for Ena. Instead, it is proposed that Lpd is the hitherto unknown intermediary between Abl and Ena/VASP proteins (Michael, 2010).

Drosophila ena was originally identified as a suppressor of lethality induced by mutations in Drosophlia abl, and it was postulated that abl and ena negatively regulate each other. Both the Abl tyrosine kinase family (Drosophila Abl, vertebrate c-Abl and Arg) and the Ena/VASP family (Ena, vertebrate Mena, VASP, and EVL) act downstream of the netrin-1 axon guidance receptor DCC and regulate cell motility. Abl kinases regulate platelet-derived growth factor (PDGF)-induced dorsal ruffling of fibroblasts, but it is not known whether Lamellipodin (Lpd) or Ena/VASP proteins function in this pathway (Michael, 2010).

Ena/VASP proteins play a crucial role in cell motility by antagonizing actin filament capping. This alters the geometry of the actin network toward longer, less-branched filaments, thereby changing the speed and persistence of lamellipodia. Ena/VASP proteins are recruited to the leading edge through interactions between their EVH1 domain and FP4 motifs within Lpd, a member of the MRL family of Ras effector proteins, which includes C. elegans MIG-10, vertebrate RIAM and Lpd, and Drosophila Pico (Lyulcheva, 2008). Lpd contains a proline-rich region harboring potential SH3-binding sites and a PH domain that mediates membrane targeting. It has been shown that Lpd is required for lamellipodia formation and that Lpd overexpression increases the speed of lamellipodial protrusion in an Ena/VASP-dependent manner (Michael, 2010).

Lpd-dependent recruitment of Ena/VASP proteins to the leading edge needs to be tightly regulated in order to precisely control lamellipodia formation, but it is not known how this is achieved. Studies in Drosophila have suggested that Abl regulates Ena localization. Yet how the localization of Ena/VASP proteins is controlled, and whether Abl plays a role in this process in vertebrates, remains unclear (Michael, 2010).

This study shows that phosphorylation of Lpd by c-Abl increases its interaction with Ena/VASP proteins. Consistently, efficient recruitment of Mena and EVL to Lpd-positive lamellipodia requires Abl kinases. Evidence is provided that Lpd and Ena/VASP proteins regulate dorsal ruffling of fibroblasts upon PDGF treatment and that Lpd function in this process is controlled by Abl kinases and mediated by Ena/VASP proteins. Furthermore, it was demonstrated that both Lpd and c-Abl cooperate during axonal morphogenesis in an Ena/VASP-dependent manner. The data do not support the suggested antagonistic roles of Abl and Ena, and an alternative hypothesis is proposed that Abl kinases, via Lpd, positively regulate Ena/VASP proteins (Michael, 2010).

Abl and T cell receptor signaling

The c-Abl and Arg proteins comprise a unique family of nonreceptor tyrosine kinases that have been implicated in the regulation of cell proliferation and survival, cytoskeletal reorganization, cell migration, and the response to oxidative stress and DNA damage. Targeted deletion or mutation of c-Abl in mice results in a variety of immune system phenotypes, including splenic and thymic atrophy, lymphopenia, and an increased susceptibility to infection. However, despite the generation of these mice over a decade ago, little is known regarding the mechanisms responsible for these phenotypes or the immune-related consequences of ablation of both the c-Abl and Arg kinases, which are coexpressed in lymphoid tissues. T cell receptor (TCR) stimulation is shown to result in activation of the endogenous Abl kinases. Zap70 and the transmembrane adaptor linker for activation of T cells (LAT) are targets of the Abl kinases, and loss of Abl kinase activity reduces TCR-induced Zap70 phosphorylation at tyrosine 319. This correlates with diminished LAT tyrosine phosphorylation, as well as reduced tyrosine phosphorylation and recruitment of phospholipase Cγ1 to LAT. Significantly, Abl kinase activity is required for maximal signaling leading to transcription of the IL-2 promoter, as well as TCR-induced IL-2 production and proliferation of primary T cells. It is concluded that the Abl kinases have a role in the regulation of TCR-mediated signal transduction leading to IL-2 production and cell proliferation (Zipfel, 2004).

Early developmental role of Abl

Fertilization causes tyrosine phosphorylation of several sea urchin egg proteins within minutes of sperm-egg binding, although the identity of the kinase(s) involved and the mechanism of regulation is not known. Antibodies against a homolog of the ABL family of protein tyrosine kinases expressed in the sea urchin egg identify a 220-kDa protein kinase, highly enriched in the egg cortex, where it is tightly associated with detergent-insoluble cytoskeletal elements. The enzyme is capable of phosphorylating synthetic peptide substrates that were used to characterize the kinase activity in an immune-complex assay. Measurement of the protein tyrosine kinase activity immunoprecipitates at different times after fertilization, revealing that the level of kinase activity is transiently elevated during the first few minutes postinsemination. The amount of the 220-kDa protein does not increase significantly during this period, so the increased kinase activity probably results from activation of the enzyme. These in vitro studies indicate that the 220-kDa Abl-related kinase is one of the protein kinases activated during fertilization and suggest that it may play a role in the egg activation process (Moore, 1994).

The Abl and Arg (the Abl-related gene) tyrosine kinases play fundamental roles in the development and function of the central nervous system. Arg is most abundant in adult mouse brain, especially in synapse-rich regions. arg(-/-) mice develop normally but exhibit multiple behavioral abnormalities, suggesting that arg(-/-) brains suffer from defects in neuronal function. Embryos deficient in both Abl and Arg suffer from defects in neurulation and die before 11 days postcoitum. Although they divide normally, abl(-/-);arg(-/-) double mutant neuroepithelial cells display gross alterations in their actin cytoskeleton. Abl and Arg colocalize with one another and with actin microfilaments at the apical surface of the developing neuroepithelium. Thus, Abl and Arg play essential roles in neurulation and can regulate the structure of the actin cytoskeleton (Koleske, 1998).

The role of adhesion in c-Abl function

Cell adhesion regulates the kinase activity and subcellular localization of c-Abl. When fibroblastic cells are detached from the extracellular matrix, kinase activity of both cytoplasmic and nuclear c-Abl decreases, but there is no detectable alteration in the subcellular distribution. Upon adhesion to the extracellular matrix protein fibronectin, a transient recruitment of a subset of c-Abl to early focal contacts is observed coincident with the export of c-Abl from the nucleus to the cytoplasm. The cytoplasmic pool of c-Abl is reactivated within 5 min of adhesion, but the nuclear c-Abl is reactivated after 30 min, correlating closely with its return to the nucleus and suggesting that the active nuclear c-Abl originates in the cytoplasm. In quiescent cells where nuclear c-Abl activity is low, the cytoplasmic c-Abl is similarly regulated by adhesion but the nuclear c-Abl is not activated upon cell attachment. These results show that c-Abl activation requires cell adhesion and that this tyrosine kinase can transmit integrin signals to the nucleus where it may function to integrate adhesion and cell cycle signals (Lewis, 1996).

Abl interactions with the cytoskeleton

The myristoylated form of c-Abl protein, as well as the P210bcr/abl protein, have been shown to associate with F-actin stress fibers in fibroblasts. The domain responsible for this interaction maps to the extreme COOH-terminus of Abl. The actin-binding domain is localized to a 58 amino acid region, including a charged motif at the extreme COOH-terminus that is important for efficient binding. F-actin binding by Abl is calcium independent; Abl competes with gelsolin for binding to F-actin. In addition to the F-actin binding domain, the COOH-terminus of Abl contains a proline-rich region that mediates binding and sequestration of G-actin. The Abl F- and G-actin binding domains cooperate to bundle F-actin filaments in vitro. The COOH terminus of Abl thus confers several novel localizing functions upon the protein, including actin binding, nuclear localization, and DNA binding. Abl may modify and receive signals from the F-actin cytoskeleton in vivo, and is an ideal candidate to mediate signal transduction from the cell surface and cytoskeleton to the nucleus (Van Etten, 1994).

BCR/ABL, is localized to the actin cytoskeleton. One of the major tyrosine phosphoproteins in cells transformed by p210BCR/ABL is the proto-oncoprotein p120c-Cbl. p210BCR/ABL induces formation of a multimeric complex of proteins, including p120c-Cbl, phosphotidylinositol-3' kinase, and p210BCR/ABL itself. Certain focal adhesion proteins are also part of this complex, including paxillin and talin. The sites in paxillin required for binding to p120c-Cbl in this complex have been partially mapped. The interaction of pl20c-Cbl with paxillin is specific, since other focal adhesion proteins, such as p125FAK, vinculin, and alpha-actinin, are not in this complex. The binding of p120c-Cbl to the focal adhesion protein paxillin could contribute to the known adhesive defects of CML cells (Salgia, 1996).

Proper regulation of cell morphogenesis and migration by adhesion and growth-factor receptors requires Abl-family tyrosine kinases. Several substrates of Abl-family kinase have been identified, but they are unlikely to mediate all of the downstream actions of these kinases on cytoskeletal structure. A human protein microarray was used to identify the actin-regulatory protein cortactin as a novel substrate of the Abl and Abl-related gene (Arg) nonreceptor tyrosine kinases. Cortactin stimulates cell motility, and its upregulation in several cancers correlates with poor prognosis. Even though cortactin can be tyrosine phosphorylated by Src-family kinases in vitro, Abl and Arg are more adept at binding and phosphorylating cortactin. Importantly, platelet-derived growth-factor (PDGF)-induced cortactin phosphorylation on three tyrosine residues requires Abl or Arg. Cortactin triggers F-actin-dependent dorsal waves in fibroblasts after PDGF treatment and thus results in actin reorganization and lamellipodial protrusion. Evidence is provided that Abl/Arg-mediated phosphorylation of cortactin is required for this PDGF-induced dorsal-wave response. The results reveal that Abl-family kinases target cortactin as an effector of cytoskeletal rearrangements in response to PDGF (Boyle, 2007).

Nuclear activities of Abl

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Abl tyrosine kinase: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

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