Mutagenic analysis of the roles of SH2 and SH3 domains in regulation of the Abl tyrosine kinase

Future Approaches for Treating Chronic Myeloid Leukemia: CRISPR Therapy

Simple Summary

In the last two decades, the therapeutic landscape of several tumors have changed profoundly with the introduction of drugs against proteins encoded by oncogenes. Oncogenes play an essential role in human cancer and when their encoded proteins are inhibited by specific drugs, the tumoral process can be reverted or stopped. An example of this is the case of the chronic myeloid leukemia, in which all the pathological features can be attributed by a single oncogene. Most patients with this disease now have a normal life expectancy thanks to a rationality designed inhibitor. However, the drug only blocks the protein, the oncogene continues unaffected and treatment discontinuation is only an option for a small subset of patients. With the advent of genome-editing nucleases and, especially, the CRISPR/Cas9 system, the possibilities to destroy oncogenes now is feasible. A novel therapeutic tool has been developed with unimaginable limits in cancer treatment. Recent studies support that CRISPR/Cas9 system could be a definitive therapeutic option in chronic myeloid leukemia. This work reviews the biology of chronic myeloid leukemia, the emergence of the CRISPR system, and its ability as a specific tool for this disease.

Abstract

The constitutively active tyrosine-kinase BCR/ABL1 oncogene plays a key role in human chronic myeloid leukemia development and disease maintenance, and determines most of the features of this leukemia. For this reason, tyrosine-kinase inhibitors are the first-line treatment, offering most patients a life expectancy like that of an equivalent healthy person. However, since the oncogene stays intact, lifelong oral medication is essential, even though this triggers adverse effects in many patients. Furthermore, leukemic stem cells remain quiescent and resistance is observed in approximately 25% of patients. Thus, new therapeutic alternatives are still needed. In this scenario, the interruption/deletion of the oncogenic sequence might be an effective therapeutic option. The emergence of CRISPR (clustered regularly interspaced short palindromic repeats) technology can offer a definitive treatment based on its capacity to induce a specific DNA double strand break. Besides, it has the advantage of providing complete and permanent oncogene knockout, while tyrosine kinase inhibitors (TKIs) only ensure that BCR-ABL1 oncoprotein is inactivated during treatment. CRISPR/Cas9 cuts DNA in a sequence-specific manner making it possible to turn oncogenes off in a way that was not previously feasible in humans. This review describes chronic myeloid leukemia (CML) disease and the main advances in the genome-editing field by which it may be treated in the future.

Structure, Function, and Regulation of the SRMS Tyrosine Kinase

Src-related kinase lacking C-terminal regulatory tyrosine and N-terminal myristoylation sites (SRMS) is a tyrosine kinase that was discovered in 1994. It is a member of a family of nonreceptor tyrosine kinases that also includes Brk (PTK6) and Frk. Compared with other tyrosine kinases, there is relatively little information about the structure, function, and regulation of SRMS. In this review, we summarize the current state of knowledge regarding SRMS, including recent results aimed at identifying downstream signaling partners. We also present a structural model for the enzyme and discuss the potential involvement of SRMS in cancer cell signaling.

Targeting ABL1 or ARG Tyrosine Kinases to Restrict HIV-1 Infection in Primary CD4+ T-Cells or in Humanized NSG Mice

BACKGROUND

Previous studies support dasatinib as a potent inhibitor of HIV-1 replication. However, a functional distinction between 2 kinase targets of the drug, ABL1 and ARG, has not been assessed.

SETTING

We used primary CD4 T-cells, CD8-depleted peripheral blood mononuclear cells (PBMCs) from a treatment naïve HIV-1 patient, and a humanized mouse model of HIV-1 infection. We assessed the roles of ABL1 and ARG during HIV-1 infection and use of dasatinib as a potential antiviral against HIV-1 in humanized mice.

METHODS

Primary CD4 T-cells were administered siRNA targeting ABL1 or ARG, then infected with HIV-1 containing luciferase reporter viruses. Quantitative polymerase chain reaction of viral integration of 4 HIV-1 strains was also assessed. CD8-depleted PBMCs were treated for 3 weeks with dasatinib. NSG mice were engrafted with CD34 pluripotent stem cells from human fetal cord blood, and infected with Ba-L virus after 19 weeks. Mice were treated daily with dasatinib starting 5 weeks after infection.

RESULTS

siRNA knockdown of ABL1 or ARG had no effect on viral reverse transcripts, but increased 2-LTR circles 2- to 4-fold and reduced viral integration 2- to 12-fold. siRNA knockdown of ARG increased SAMHD1 activation, whereas knockdown of either kinase reduced RNA polymerase II activation. Treating CD8-depleted PBMCs from a treatment-naïve patient with 50 nM of dasatinib for 3 weeks reduced p24 levels by 99.8%. Ba-L (R5)-infected mice injected daily with dasatinib showed a 95.1% reduction in plasma viral load after 2 weeks of treatment.

CONCLUSIONS

We demonstrate a novel nuclear role for ABL1 and ARG in ex vivo infection experiments, and proof-of-principle use of dasatinib in a humanized mouse model of HIV-1 infection.

MAPK15 mediates BCR-ABL1-induced autophagy and regulates oncogene-dependent cell proliferation and tumor formation

A reciprocal translocation of the ABL1 gene to the BCR gene results in the expression of the oncogenic BCR-ABL1 fusion protein, which characterizes human chronic myeloid leukemia (CML), a myeloproliferative disorder considered invariably fatal until the introduction of the imatinib family of tyrosine kinase inhibitors (TKI). Nonetheless, insensitivity of CML stem cells to TKI treatment and intrinsic or acquired resistance are still frequent causes for disease persistence and blastic phase progression experienced in patients after initial successful therapies. Here, we investigated a possible role for the MAPK15/ERK8 kinase in BCR-ABL1-dependent autophagy, a key process for oncogene-induced leukemogenesis. In this context, we showed the ability of MAPK15 to physically recruit the oncogene to autophagic vesicles, confirming our hypothesis of a biologically relevant role for this MAP kinase in signal transduction by this oncogene. Indeed, by modeling BCR-ABL1 signaling in HeLa cells and taking advantage of a physiologically relevant model for human CML, i.e. K562 cells, we demonstrated that BCR-ABL1-induced autophagy is mediated by MAPK15 through its ability to interact with LC3-family proteins, in a LIR-dependent manner. Interestingly, we were also able to interfere with BCR-ABL1-induced autophagy by a pharmacological approach aimed at inhibiting MAPK15, opening the possibility of acting on this kinase to affect autophagy and diseases depending on this cellular function. Indeed, to support the feasibility of this approach, we demonstrated that depletion of endogenous MAPK15 expression inhibited BCR-ABL1-dependent cell proliferation, in vitro, and tumor formation, in vivo, therefore providing a novel "druggable" link between BCR-ABL1 and human CML.

Truncated Cables1 causes agenesis of the corpus callosum in mice

Agenesis of the corpus callosum (ACC) is a congenital abnormality of the brain structure. More than 60 genes are known to be involved in corpus callosum development. However, the molecular mechanisms underlying ACC are not fully understood. Previously, we produced a novel transgenic mouse strain, TAS, carrying genes of the tetracycline-inducible expression system that are not involved in brain development, and inherited ACC was observed in the brains of all homozygous TAS mice. Although ACC was probably induced by transgene insertion mutation, the causative gene and the molecular mechanism of its pathogenesis remain unclear. Here, we first performed interphase three-color fluorescence in situ hybridization (FISH) analysis to determine the genomic insertion site. Transgenes were inserted into chromosome 18 ∼12.0 Mb from the centromere. Gene expression analysis and genomic PCR walking showed that the genomic region containing exon 4 of Cables1 was deleted by transgene insertion and the other exons of Cables1 were intact. The mutant allele was designated as Cables1(TAS). Interestingly, Cables1(TAS) mRNA consisted of exons 1-3 of Cables1 and part of the transgene that encoded a novel truncated Cables1 protein. Homozygous TAS mice exhibited mRNA expression of Cables1(TAS) in the fetal cerebrum, but not that of wild-type Cables1. To investigate whether a dominant negative effect of Cables1(TAS) or complete loss of function of Cables1 gives rise to ACC, we produced Cables1-null mutant mice. ACC was not observed in Cables1-null mutant mice, suggesting that a dominant negative effect of Cables1(TAS) impairs callosal formation. Moreover, ACC frequency in Cables1(+/TAS) mice was significantly lower than that in Cables1(-/TAS) mice, indicating that wild-type Cables1 interfered with the dominant negative effect of Cables1(TAS). This study indicated that truncated Cables1 causes ACC and wild-type Cables1 contributes to callosal formation.

c-Abl Kinase Is a Regulator of αvβ3 Integrin Mediated Melanoma A375 Cell Migration

Integrins are heterodimeric transmembrane receptors that physically link the extracellular matrix (ECM) to the intracellular actin cytoskeleton, and are also signaling molecules that transduce signals bi-directionally across the plasma membrane. Integrin regulation is essential for tumor cell migration in response to growth factors. c-Abl kinase is a nonreceptor tyrosine kinase and is critical for signaling transduction from various receptors. Here we show that c-Abl kinase is involved in A375 cell migration mediated by α_vβ₃ integrin in response to PDGF stimulation. c-Abl kinase colocalizes with α_vβ₃ integrin dynamically and affects α_vβ₃ integrin affinity by regulating its cluster. The interaction between c-Abl kinase and α_vβ₃ integrin was dependent on the activity of c-Abl kinase induced by PDGF stimulation, but was not dependent on the binding of α_vβ₃ integrin with its ligands, suggesting that c-Abl kinase is not involved in the outside-in signaling of α_vβ₃ integrin. Talin head domain was required for the interaction between c-Abl kinase and α_vβ₃ integrin, and the SH3 domain of c-Abl kinase was involved in its interaction with talin and α_vβ₃ integrin. Taken together, we have uncovered a novel and critical role of c-Abl kinase in α_vβ₃ integrin mediated melanoma cell migration.

The role of activation-induced deaminase in antibody diversification and genomic instability

More than a decade ago, activation-induced deaminase (AID) was identified as the initiator for somatic hypermutation (SHM) and class switch recombination (CSR). Since then, tremendous progress has been achieved toward elucidating how AID functions. AID targets the highly repetitive switch regions of the immunoglobulin heavy chain (IgH) locus to induce DNA double-strand breaks (DSBs), which can be rejoined, leading to switch of constant regions of antibody. When targeting to variable region exons of IgH and IgL loci, AID predominantly induces point mutations, termed SHM, resulting in increased affinity of antibody for antigen. While SHM and CSR enhance antibody diversity, AID-initiated DSBs and mutations may predispose B cells to carcinogenesis. This review focuses on the mechanisms that provide the specificity of AID targeting to Ig loci and the role of AID in genomic instability.

Mechanisms and impacts of chromosomal translocations in cancers

Chromosomal aberrations have been associated with cancer development since their discovery more than a hundred years ago. Chromosomal translocations, a type of particular structural changes involving heterologous chromosomes, have made a critical impact on diagnosis, prognosis and treatment of cancers. For example, the discovery of translocation between chromosomes 9 and 22 and the subsequent success of targeting the fusion product BCR-ABL transformed the therapy for chronic myelogenous leukemia. In the past few decades, tremendous progress has been achieved towards elucidating the mechanism causing chromosomal translocations. This review focuses on the basic mechanisms underlying the generation of chromosomal translocations. In particular, the contribution of frequency of DNA double strand breaks and spatial proximity of translocating loci is discussed.

Abl kinase constructs expressed in bacteria: facilitation of structural and functional studies including segmental labeling by expressed protein ligation

A great portion of tyrosine kinases are involved in cell development and their structural alteration is intimately involved in associated pathologies of development and oncology. These kinases are one of the major groups of targets under investigation for molecular therapeutics. To carry out biochemical and structural biological studies on these kinases, economical production of their purified forms is highly desirable. However over-expressing tyrosine kinases as recombinant forms in bacterial systems and their purification is a significant challenge. Abelson kinase (Abl) has previously been expressed on a large scale to facilitate X-ray crystallography and NMR structure studies mainly in baculovirus infected insect cells. Even though success has been achieved in expression of soluble tyrosine kinases in E. coli with chaperones to improve correct folding, low expression levels of kinases are intrinsic in such systems because of diversion of resources to produce chaperones. Here we present a straightforward method to express and purify isolated Abl kinase domain and SH3-SH2-kinase multi-domain structures. The expressed Abl protein retains its correct folding and biological function. The yield of soluble protein is in a several mg L(-1) range in minimal media. Furthermore we demonstrate that segmental isotopic labelling using expressed protein ligation can be achieved using bacterial expressed Abl kinase domain constructs, which is especially useful in NMR structure-activity studies.

Functional mechanisms and roles of adaptor proteins in abl-regulated cytoskeletal actin dynamics

Abl is a nonreceptor tyrosine kinase and plays an essential role in the modeling and remodeling of F-actin by transducing extracellular signals. Abl and its paralog, Arg, are unique among the tyrosine kinase family in that they contain an unusual extended C-terminal half consisting of multiple functional domains. This structural characteristic may underlie the role of Abl as a mediator of upstream signals to downstream signaling machineries involved in actin dynamics. Indeed, a group of SH3-containing accessory proteins, or adaptor proteins, have been identified that bind to a proline-rich domain of the C-terminal portion of Abl and modulate its kinase activity, substrate recognition, and intracellular localization. Moreover, the existence of signaling cascade and biological outcomes unique to each adaptor protein has been demonstrated. In this paper, we summarize functional roles and mechanisms of adaptor proteins in Abl-regulated actin dynamics, mainly focusing on a family of adaptor proteins, Abi. The mechanism of Abl's activation and downstream signaling mediated by Abi is described in comparison with those by another adaptor protein, Crk.

The mouse B cell-specific mb-1 gene encodes an immunoreceptor tyrosine-based activation motif (ITAM) protein that may be evolutionarily conserved in diverse species by purifying selection

The B-lymphocyte accessory molecule Ig-alpha (Ig-α) is encoded by the mouse B cell-specific gene (mb-1), and along with the Ig-beta (Ig-β) molecule and a membrane bound immunoglobulin (mIg) makes up the B-cell receptor (BCR). Ig-α and Ig-β form a heterodimer structure that upon antigen binding and receptor clustering primarily initiates and controls BCR intracellular signaling via a phosphorylation cascade, ultimately triggering an effector response. The signaling capacity of Ig-α is contained within its immunoreceptor tyrosine-based activation motif (ITAM), which is also a key component for intracellular signaling initiation in other immune cell-specific receptors. Although numerous studies have been devoted to the mb-1 gene product, Ig-α, and its signaling mechanism, an evolutionary analysis of the mb-1 gene has been lacking until now. In this study, mb-1 coding sequences from 19 species were compared using Bayesian inference. Analysis revealed a gene phylogeny consistent with an expected species divergence pattern, clustering species from the primate order separate from lower mammals and other species. In addition, an overall comparison of non-synonymous and synonymous nucleotide mutational changes suggests that the mb-1 gene has undergone purifying selection throughout its evolution.

Assay development and high-throughput screening of small molecular c-Abl kinase activators

A 2-step kinase assay was developed and used in a high-throughput screen (HTS) of more than 1 million compounds in an effort to identify c-Abl tyrosine kinase activators. This assay employed a 2-step phosphorylation reaction: in the first step, purified recombinant c-Abl was activated by incubating with compound in the presence of adenosine triphosphate (ATP). In the second step, the TAMRA-labeled IMAP Abltide substrate was added to allow phosphorylation of the substrate to occur. The assay was calibrated such that inactive c-Abl protein was activated by ATP alone to a degree that it not only demonstrated a measurable c-Abl activity but also maintained a robust assay window for screening. The screen resulted in 8624 primary hits with >30% response. Further analysis showed that 1024 had EC(50) <10 µM with a max % response of >50%. These hits were structurally and chemically diverse with possibly different mechanisms for activating c-Abl. In addition, selective hits were shown to be cell permeable and were able to induce c-Abl activation as determined by In-Cell Western (ICW) analysis of HEK-MSRII cells transduced with BacMam virus expressing full-length c-Abl.

c-Abl kinase is required for beta 2 integrin-mediated neutrophil adhesion

Integrin regulation in neutrophil adhesion is essential for innate immune response. c-Abl kinase is a nonreceptor tyrosine kinase and is critical for signaling transduction from various receptors in leukocytes. Using neutrophils and dHL-60 (neutrophil-like differentiation of HL-60) cells, we show that c-Abl kinase is activated by beta(2) integrin engagement and is required for beta(2) integrin-dependent neutrophil sustained adhesion and spreading. The expression of beta(2) integrin on neutrophils induced by TNF-alpha is not affected by c-Abl kinase inhibitor STI571, suggesting that c-Abl kinase is not involved in TNF-alpha-induced integrin activation. The recruitment of c-Abl kinase to beta(2) integrin is dependent on talin head domain, which constitutively interacts with beta(2) integrin cytoplasmic domain. After activated, c-Abl kinase increases the tyrosine phosphorylation of Vav. The SH3 domain of c-Abl kinase is involved in its interaction with talin and Vav. Thus, c-Abl kinase plays an essential role in the activation of Vav induced by beta(2) integrin ligation and in regulating neutrophil-sustained adhesion and spreading.

Reciprocal regulation of Abl and receptor tyrosine kinases

Previously, we showed that Abl kinases (c-Abl, Arg) are activated downstream of PDGF in a manner dependent on Src kinases and PLC-gamma1, and promote PDGF-mediated proliferation and migration of fibroblasts. We additionally demonstrated that Abl kinases bind directly to PDGFR-beta via their SH2 domains.In this study, we extend these findings by demonstrating that Abl kinases also are activated downstream of aPDGF autocrine growth loop in glioblastoma cells, indicating that the PDGFR-Abl signaling pathway also is likely to be important in glioblastoma development and/or progression.We recently showed that Abl kinases are highly active in many breast cancer cell lines, and the Her-2 receptor tyrosine kinase contributes to c-Abl and Arg kinase activation. In this study, we show that Abl kinase SH2 domains bind directly to Her-2, and like PDGFR-beta , Her-2 directly phosphorylates c-Abl. Previously, we demonstrated that PDGFR-beta directly phosphorylates Abl kinases in vitro, and Abl kinases reciprocally phosphorylate PDGFR-beta . Here, we show that PDGFR-beta-phosphorylation of Abl kinases has functional consequences as PDGFR-beta phosphorylates Abl kinases on Y245 and Y412, sites known to be required for activation of Abl kinases. Moreover, PDGFR-beta phosphorylates Arg on two additional unique sites whose function is unknown. Importantly, we also show that Abl-dependent phosphorylation of PDGFR-beta has functional and biological significances. c-Abl phosphorylates three tyrosine residues on PDGFR-beta (Y686, Y934, Y970), while Arg only phosphorylatesY686. Y686 and Y934 reside in PDGFR-beta catalytic domains, while Y970 is in the C-terminal tail. Using site-directed mutagenesis, we show that Abl-dependent phosphorylation of PDGFR-beta activates PDGFR-beta activity, in vitro, but serves to downregulate PDGFR-mediated chemotaxis. These data are exciting as they indicate that Abl kinases not only are activated by PDGFR and promote PDGFR-mediated proliferation and migration,but also act in an intricate negative feedback loop to turn-off PDGFR-mediated chemotaxis.

Tyrosine phosphorylation in the SH3 domain disrupts negative regulatory interactions within the c-Abl kinase core

Recent studies have shown that trans-phosphorylation of the Abl SH3 domain at Tyr89 by Src-family kinases is required for the full transforming activity of Bcr-Abl. Tyr89 localizes to a binding surface of the SH3 domain that engages the SH2-kinase linker in the crystal structure of the c-Abl core. Displacement of SH3 from the linker is likely to influence efficient downregulation of c-Abl. Hydrogen-deuterium exchange (HX) and mass spectrometry (MS) were used to investigate whether Tyr89 phosphorylation affects the ability of the SH3 domain to interact intramolecularly with the SH2-kinase linker in cis as well as other peptide ligands in trans. HX MS analysis of SH3 binding showed that when various Abl constructs were phosphorylated at Tyr89 by the Src-family kinase Hck, SH3 was unable to engage a high-affinity ligand in trans and that interaction with the linker in cis was reduced dramatically in a construct containing the SH3 and SH2 domains plus the linker. Phosphorylation of the Abl SH3 domain on Tyr89 also interfered with binding to the negative regulatory protein Abi-1 in trans. Site-directed mutagenesis of Tyr89 and Tyr245, another tyrosine phosphorylation site located in the linker that may also influence SH3 binding, implicated Tyr89 as the key residue necessary for disrupting regulation after phosphorylation. These results imply that phosphorylation at Tyr89 by Src-family kinases prevents engagement of the Abl SH3 domain with its intramolecular binding partner leading to enhanced Abl kinase activity and cellular signaling.

c-Abl is required for the signaling transduction induced by L-selectin ligation

Lymphocyte recruitment onto inflamed tissues requires cells tethering to and rolling on vascular surfaces under flow. L-selectin is constitutively expressed on leukocytes to mediate the leukocytes' initial capture and subsequent rolling along the vessel. Apart from its adhesive function, engagement of L-selectin also results in cell activation, which is related to the completed signaling transduction. Here we show that ligation of L-selectin with its mAb increases c-Abl kinase activity, and that the activated c-Abl kinase can be recruited to and phosphorylate the cytoplasmic domain of L-selectin. In addition, the activated c-Abl kinase can regulate Zap70 kinase by increasing the phosphorylation of the Y319 site of Zap70 kinase and connect with Zap70 kinase through its SH2 domain. These results indicate that c-Abl kinase plays an important role in accepting and transferring the upstream activation events induced by L-selectin ligation.

Phosphoinositide, phosphopeptide and pyridone interactions of the Abl SH2 domain

Signaling proteins are localized and regulated by Src homology 2 domains which recognize phosphotyrosine-containing sequences. Recently, noncanonical ligands have been proposed for Src homology 2 domains including that of Abl and its breakpoint cluster region fusion, which causes chronic myelogenous leukemia. Here, the Abl Src homology 2 domain's binding sites and affinities for phosphotyrosine- and phosphoserine-containing motifs, phosphoinositides as well as a pyridone-based peptidomimetic inhibitor were determined using nuclear magnetic resonance spectroscopy in order to define their roles. The cognate Crk peptide ligand was bound with an affinity of 69 microM and, like the higher affinity peptidomimetic, engages the phosphotyrosine and +3 hydrophobic pockets while putative phosphoserine-containing breakpoint cluster region ligands are ruled out. Surprisingly, phosphatidylinositol 4, 5 bisphosphate interacts with an overlapping site through an electrostatic mechanism that does not appear to involve hydrophobic insertion into micelles. The conserved Arg36 residue in the FLVRES motif is required for both phosphotyrosine binding and for localization to phosphatidylinositol 4, 5 bisphosphate-containing liposomes, while Arg59 in the betaD strand is necessary for the phosphoinositide interaction. Thus the Src homology 2 domain of Abl, a myristoylated and membrane-localized protein, is able to interact directly with phosphoinositides through a multifunctional basic site that overlaps the phosphotyrosine pocket.

Altered gene expression profiles by sodium/iodide symporter gene transfection in a human anaplastic thyroid carcinoma cell line using a radioactive complementary DNA microarray

BACKGROUND

The sodium/iodide symporter (NIS) is a membrane glycoprotein that mediates active 131I uptake during the treatment of cancer of the thyroid gland and extrathyroidal tissues. NIS gene transfection, a gene-therapy modality, has been introduced in many types of cancer, such as prostate cancer and breast cancer, and has demonstrated a high potential for the treatment of non-thyroidal cancers.

AIM

To investigate the pattern of NIS gene expression and provide evidence of its beneficial effects in human anaplastic cancer ARO cells by using a radioactive complementary DNA (cDNA) microarray.

METHODS

For cDNA microarray data analysis, superimposed images and clustergrams were prepared from basic radioactivity data obtained using a phosphoimager system. Gene expression profiles were constructed using the Z-transformed values of genes related to cancer biology.

RESULTS

Radioactive cDNA microarray studies showed that 11 genes were upregulated (Z ratio > 1.5) and 31 genes were downregulated (Z ratio < -1.5) in response to NIS gene transfection. Of these differentially expressed genes, 33% were related to cell proliferation and apoptosis. Moreover, NIS gene transfection into an anaplastic thyroid cancer cell line affected the expression of the protein tyrosine phosphatase (PTP) family and Ras oncogene family, including Ras, Rac and Rab.

CONCLUSION

The identification of changes in the patterns of gene expression may provide a better understanding of the response of molecular mechanisms to NIS gene transfection.

c-Yes response to growth factor activation

Transmembrane receptors link the extracellular environment to the internal control elements of the cell. This signaling influences cell division, differentiation, survival, motility, adhesion, spreading and vesicular transport. Central to this signaling is the Src family of nonreceptor tyrosine kinases. The most studied kinase of this nine member family, c-Src, shares a similar structure, as well as a similar expression pattern to that of another Src family protein, c-Yes. Despite high conservation in sequence, molecular studies demonstrate that the functional domains of these kinases can contribute to specificity in signaling. At the cellular level, analysis of tight junction formation also serves as a model to differentiate c-Yes and c-Src signaling. Results suggest that c-Yes promotes formation of the tight junction by phosphorylating occludin, while c-Src signaling downregulates occludin formation in a Raf-1 dependent manner. In addition, pp62c-Yes knockout mice exhibit a specific physiological function phenotype that is distinct from c-src-/- mice. In these studies, c-yes-/- mice exhibit decreased transcytosis of pIgA from the blood to the bile, while c-src-/- mice exhibit deficits in osteoclasts function and bone resorption. Of particular interest in this review are receptor signals that specifically influence the actions of c-Yes. Growth factors that influence many Src family proteins include the PDGF-R, CSF-1 receptor and others. Since these receptors interact with various Src-family kinases, it is predicted that specific signaling is generated by differential recruitment to the cell membrane and/or differentiated interactions with substrates and binding partners. This review provides an overview of c-Yes interactions with specific receptor signaling pathways and how this interaction potentially influences the known physiological roles of c-Yes.

The SCF ubiquitin ligase: insights into a molecular machine

Ubiquitin ligases are well suited to regulate molecular networks that operate on a post-translational timescale. The F-box family of proteins - which are the substrate-recognition components of the Skp1-Cul1-F-box-protein (SCF) ubiquitin ligase - are important players in many mammalian functions. Here we explore a unifying and structurally detailed view of SCF-mediated proteolytic control of cellular processes that has been revealed by recent studies.

Phosphorylation of DNA topoisomerase I by the c-Abl tyrosine kinase confers camptothecin sensitivity

DNA topoisomerase I (topo I) is involved in the regulation of DNA supercoiling, gene transcription, recombination, and DNA repair. The anticancer agent camptothecin specifically targets topo I. The mechanisms responsible for the regulation of topo I in cells, however, are not known. This study demonstrates that c-Abl-dependent phosphorylation up-regulates topo I activity. The c-Abl SH3 domain bound directly to the N-terminal region of topo I. The results demonstrate that c-Abl phosphorylated topo I at Tyr268 in core subdomain II. c-Abl-mediated phosphorylation of topo I Tyr268 in vitro and in cells conferred activation of the topo I isomerase function. Moreover, activation of c-Abl by treatment of cells with ionizing radiation was associated with c-Abl-dependent phosphorylation of topo I and induction of topo I activity. The functional significance of the c-Abl/topo I interaction is supported by the findings that (i) mutant topo I(Y268F) exhibited loss of c-Abl-induced topo I activity, and (ii) c-Abl-/- cells were deficient in the accumulation of protein-linked DNA breaks. In addition, loss of topo I phosphorylation in c-Abl-deficient cells conferred resistance to camptothecin-induced apoptosis. These findings collectively support a model in which c-Abl-mediated phosphorylation of topo I is functionally important to topo I activity and sensitivity to topo I poisons.

Biology of chronic myelogenous leukemia--signaling pathways of initiation and transformation

Chronic myeloid leukemia (CML) is caused by the Bcr-Abl oncoprotein,the product of the t(9;22) chromosomal translocation that generates the Philadelphia chromosome. Different disease phenotypes are associated with each of the three Bcr-Abl isoforms: p190Bcr-Abl, p210Bcr-Abl, and p230Bcr-Abl all of which have a constitutively activated tyrosine kinase. Mechanisms associated with malignant transformation include altered cellular adhesion, activation of mitogenic signaling pathways, inhibition of apoptosis, and proteasomal degradation of physiologically important cellular proteins.CML is subject to an inexorable progression from an "indolent" chronic phase to a terminal blast crisis. Disease progression is presumed to be associated with the phenomenon of genomic instability.

Abl-dependent tyrosine phosphorylation of Sos-1 mediates growth-factor-induced Rac activation

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. Here, we elucidate the molecular mechanisms through which Abl signals to Rac in RTK-activated pathways. We show that 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) resulted in a marked decrease in Rac activation induced by physiological doses of growth factors. Thus, our data identify the molecular connections of a pathway RTKs-Abl-Sos-1-Rac that is involved in signal transduction and actin remodelling.

Mechanisms and implications of imatinib resistance mutations in BCR-ABL

PURPOSE OF REVIEW

Aside from bone marrow transplantation, a definitive cure for Philadelphia (Ph) chromosome-positive chronic myeloid leukemia (CML) has yet to be developed. Although Imatinib, the first molecularly targeted drug developed for CML has achieved a remarkable success, the emergence of resistance to this agent mitigates the prospect of a cure for this leukemia. Though a variety of resistance mechanisms can arise, in the majority of patients resistance coincides with reactivation of the tyrosine kinase activity of the BCR-ABL fusion oncoprotein. This can result from gene amplification and, more importantly, point mutations that disrupt the bind of imatinib to BCR-ABL itself. In this review, we aim to define and illuminate mechanisms of resistance and describe how drug resistance is shedding new light on kinase domain regulation.

RECENT FINDINGS

In light of recent studies and publications, it is now clear that Imatinib exerts its inhibitory action by stabilizing the inactive non ATP-binding conformation of BCR-ABL and that mutations even outside the kinase domain can lead to enhanced autophosphorylation of the kinase, thereby stabilizing the active conformation that resists imatinib binding. So far, 25 different substitutions of 21 amino acid residues of BCR-ABL have been detected in CML patients. In addition, it has been recently illustrated that mutations preexist the onset of treatment and that some confer a more aggressive disease phenotype. Finally it has been shown that molecular remission is almost never reached through Imatinib therapy.

SUMMARY

The most common mechanism of relapse for CML patients treated with Imatinib is the appearance of point mutations in the BCR-ABL oncogene that confer resistance to this drug. Insights into the emerging problem of resistance should promote the rational development of alternative, synergistic, and potentially curative treatment strategies.

Specific targeted therapy of chronic myelogenous leukemia with imatinib

Chronic myeloid leukemia (CML) is characterized by the Philadelphia translocation that fuses BCR sequences from chromosome 22 upstream of the ABL gene on chromosome 9. The chimerical Bcr-Abl protein expressed by CML cells has constitutive tyrosine kinase activity, which is essential for the pathogenesis of the disease. Imatinib, an ATP-competitive selective inhibitor of Bcr-Abl, has unprecedented efficacy for the treatment of CML. Most patients with early stage disease achieve durable complete hematological and complete cytogenetic remissions, with minimal toxicity. In contrast, responses are less stable in patients with advanced CML. This review highlights the pathogenesis of CML, its clinical features, and the development of imatinib as a specific molecularly targeted therapy. Aspects of disease monitoring and side effects are covered as well as resistance to imatinib and strategies to overcome resistance, such as alternative signal transduction inhibitors and drug combinations. Perspectives for further development are also discussed.

Regulation of F-actin-dependent processes by the Abl family of tyrosine kinases

The F-actin cytoskeleton is a fundamental component of all eukaryotic cells. It provides force and stability and plays an integral role in a diverse array of cellular processes. The spatiotemporal regulation of F-actin dynamics is essential for proper biological output. The basic molecular machinery underlying the assembly and disassembly of filamentous actin is conserved in all eukaryotic cells. Additionally, protein tyrosine kinases, found only in multicellular eukaryotes, provide links between extracellular signals and F-actin-dependent cellular processes. Among the tyrosine kinases, c-Abl and its relative Arg are unique in binding directly to F-actin. Recent results have demonstrated a role for c-Abl in membrane ruffling, cell spreading, cell migration, and neurite extension in response to growth factor and extracellular matrix signals. c-Abl appears to regulate the assembly of F-actin polymers into different structures, depending on the extracellular signal. Interestingly, c-Abl contains nuclear import and export signals, and the nuclear c-Abl inhibits differentiation and promotes apoptosis in response to genotoxic stress. The modular structure and the nuclear-cytoplasmic shuttling of c-Abl suggest that it integrates multiple signals to coordinate F-actin dynamics with the cellular decision to differentiate or to die.

Chronic myelogenous leukemia as a paradigm of early cancer and possible curative strategies

The chronological history of the important discoveries leading to our present understanding of the essential clinical, biological, biochemical, and molecular features of chronic myelogenous leukemia (CML) are first reviewed, focusing in particular on abnormalities that are responsible for the massive myeloid expansion. CML is an excellent target for the development of selective treatment because of its highly consistent genetic abnormality and qualitatively different fusion gene product, p210(bcr-abl). It is likely that the multiple signaling pathways dysregulated by p210(bcr-abl) are sufficient to explain all the initial manifestations of the chronic phase of the disease, although understanding of the circuitry is still very incomplete. Evidence is presented that the signaling pathways that are constitutively activated in CML stem cells and primitive progenitors cooperate with cytokines to increase the proportion of stem cells that are activated and thereby increase recruitment into the committed progenitor cell pool, and that this increased activation is probably the primary cause of the massive myeloid expansion in CML. The cooperative interactions between Bcr-Abl and cytokine-activated pathways interfere with the synergistic interactions between multiple cytokines that are normally required for the activation of stem cells, while at the same time causing numerous subtle biochemical and functional abnormalities in the later progenitors and precursor cells. The committed CML progenitors have discordant maturation and reduced proliferative capacity compared to normal committed progenitors, and like them, are destined to die after a limited number of divisions. Thus, the primary goal of any curative strategy must be to eliminate all Philadelphia positive (Ph+) primitive cells that are capable of symmetric division and thereby able to expand the Ph+ stem cell pool and recreate the disease. Several highly potent and moderately selective inhibitors of Bcr-Abl kinase have recently been discovered that are capable of killing the majority of actively proliferating early CML progenitors with minimal effects on normal progenitors. However, like their normal counterparts, most of the CML primitive stem cells are quiescent at any given time and are relatively invulnerable to the Bcr-Abl kinase inhibitors as well as other drugs. We propose that survival of dormant Ph+ stem cells may be the most important reason for the inability to cure the disease during initial treatment, while resistance to the inhibitors and other drugs becomes increasingly important later. An outline of a possible curative strategy is presented that attempts to take advantage of the subtle differences in the proliferative behavior of normal and Ph+ stem cells and the newly discovered selective inhibitors of Bcr-Abl. Leukemia (2003) 17, 1211-1262. doi:10.1038/sj.leu.2402912

Abl interactor 1 promotes tyrosine 296 phosphorylation of mammalian enabled (Mena) by c-Abl kinase

Mammalian Enabled (Mena) is a mammalian homologue of Drosophila Enabled (Ena), which genetically interacts with Drosophila Abl tyrosine kinase. The signaling pathway involving c-Abl and Mena (Ena) is not fully understood. To find molecules that participate in the c-Abl/Mena pathway, we searched for Mena-binding proteins using a yeast two-hybrid system. We identified Abl interactor 1 (Abi-1), which is known to interact with c-Abl, as a binding protein for Mena. Binding analysis revealed that the Ena/Vasp homology 1 domain of Mena and the polyproline structure of Abi-1 are necessary for the interaction. The interaction between Mena and Abi-1 was also observed in a mammalian expression system. Importantly, Abi-1 dramatically promoted c-Abl-mediated tyrosine phosphorylation of Mena but not other substrates such as c-Cbl. Mutational analysis demonstrated that the phosphorylation site of Mena is Tyr-296. Our results suggest that Abi-1 regulates c-Abl-mediated phosphorylation of Mena by interacting with both proteins.

A new link between the c-Abl tyrosine kinase and phosphoinositide signalling through PLC-gamma1

The c-Abl tyrosine (Tyr) kinase is activated after platelet-derived-growth factor receptor (PDGFR) stimulation in a manner that is partially dependent on Src kinase activity. However, the activity of Src kinases alone is not sufficient for activation of c-Abl by PDGFR. Here we show that functional phospholipase C-gamma1 (PLC-gamma1) is required for c-Abl activation by PDGFR. Decreasing cellular levels of phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) by PLC-gamma1-mediated hydrolysis or dephosphorylation by an inositol polyphosphate 5-phosphatase (Inp54) results in increased Abl kinase activity. c-Abl functions downstream of PLC-gamma1, as expression of kinase-inactive c-Abl blocks PLC-gamma1-induced chemotaxis towards PDGF-BB. PLC-gamma1 and c-Abl form a complex in cells that is enhanced by PDGF stimulation. After activation, c-Abl phosphorylates PLC-gamma1 and negatively modulates its function in vivo. These findings uncover a newly discovered functional interdependence between non-receptor Tyr kinase and lipid signalling pathways.

Mechanisms of autoinhibition and STI-571/imatinib resistance revealed by mutagenesis of BCR-ABL

The Bcr-Abl fusion protein kinase causes chronic myeloid leukemia and is targeted by the signal transduction inhibitor STI-571/Gleevec/imatinib (STI-571). Sequencing of the BCR-ABL gene in patients who have relapsed after STI-571 chemotherapy has revealed a limited set of kinase domain mutations that mediate drug resistance. To obtain a more comprehensive survey of the amino acid substitutions that confer STI-571 resistance, we performed an in vitro screen of randomly mutagenized BCR-ABL and recovered all of the major mutations previously identified in patients and numerous others that illuminate novel mechanisms of acquired drug resistance. Structural modeling implies that a novel class of variants acts allosterically to destabilize the autoinhibited conformation of the ABL kinase to which STI-571 preferentially binds. This screening strategy is a paradigm applicable to a growing list of target-directed anti-cancer agents and provides a means of anticipating the drug-resistant amino acid substitutions that are likely to be clinically problematic.

Regulation of ionizing radiation-induced Rad52 nuclear foci formation by c-Abl-mediated phosphorylation

The RAD52 epistasis group of proteins, including Rad51, Rad52, and Rad54, plays an important role in the homologous recombination repair of double strand breaks. A well characterized feature associated with the ability of these proteins to repair double strand breaks is inducible nuclear foci formation at the sites of damage. How the process is functionally regulated in response to DNA damage, however, remains elusive. We show here that c-Abl tyrosine kinase associates with and phosphorylates Rad52 on tyrosine 104. Importantly, the very same site of Rad52 is phosphorylated on exposure of cells to ionizing radiation (IR). The functional significance of c-Abl-dependent phosphorylation of Rad52 is underscored by our findings that cells that express the phosphorylation-resistant Rad52 mutant, in which tyrosine 104 is replaced by phenylalanine, exhibit compromised nuclear foci formation in response to IR. Furthermore, IR-induced Rad52 nuclear foci formation is markedly suppressed by the expression of dominant-negative c-Abl. Together our data support a mode of post-translational regulation of Rad52 mediated by the c-Abl tyrosine kinase.

BCR/ABL genes and leukemic phenotype: from molecular mechanisms to clinical correlations

The Philadelphia chromosome (Ph), a minute chromosome that derives from the balanced translocation between chromosomes 9 and 22, was first described in 1960 and was for a long time the only genetic lesion consistently associated with human cancer. This chromosomal translocation results in the fusion between the 5' part of BCR gene, normally located on chromosome 22, and the 3' part of the ABL gene on chromosome 9 giving origin to a BCR/ABL fusion gene which is transcribed and then translated into a hybrid protein. Three main variants of the BCR/ABL gene have been described, that, depending on the length of the sequence of the BCR gene included, encode for the p190(BCR/ABL), P210(BCR/ABL), and P230(BCR/ABL) proteins. These three main variants are associated with distinct clinical types of human leukemias. Herein we review the data on the correlations between the type of BCR/ABL gene and the corresponding leukemic clinical features. Lastly, drawing on experimental data, we provide insight into the different transforming power of the three hybrid BCR/ABL proteins.

UCS15A, a novel small molecule, SH3 domain-mediated protein-protein interaction blocking drug

Protein-protein interactions play critical regulatory roles in mediating signal transduction. Previous studies have identified an unconventional, small-molecule, Src signal transduction inhibitor, UCS15A. UCS15A differed from conventional Src-inhibitors in that it did not alter the levels or the tyrosine kinase activity of Src. Our studies suggested that UCS15A exerted its Src-inhibitory effects by a novel mechanism that involved the disruption of protein-protein interactions mediated by Src. In the present study we have examined the ability of UCS15A to disrupt the interaction of Src-SH3 with Sam68, both in vivo and in vitro. This ability of UCS15A was not restricted to Src-SH3 mediated protein-protein interactions, since the drug was capable of disrupting the in vivo interactions of Sam68 with other SH3 domain containing proteins such as Grb2 and PLCgamma. In addition, UCS15A was capable of disrupting other typical SH3-mediated protein-protein interactions such as Grb2-Sos1, cortactin-ZO1, as well as atypical SH3-mediated protein-protein interactions such as Grb2-Gab1. However, UCS15A was unable to disrupt the non-SH3-mediated protein-protein interactions of beta-catenin, with E-cadherin and alpha-catenin. In addition, UCS15A had no effect on the SH2-mediated interaction between Grb2 and activated Epidermal Growth Factor receptor. Thus, the ability of UCS15A, to disrupt protein-protein interactions appeared to be restricted to SH3-mediated protein-protein interactions. In this regard, UCS15A represents the first example of a non-peptide, small molecule agent capable of disrupting SH3-mediated protein-protein interactions. In vitro analyses suggested that UCS15A did not bind to the SH3 domain itself but rather may interact directly with the target proline-rich domains.

Regulation of p73 by c-Abl through the p38 MAP kinase pathway

p73 is a novel member of the p53 family of tumor suppressor proteins which is involved in cellular differentiation, tumor suppression, and the response to genotoxic stress. The molecular mechanisms regulating p73 activity are still poorly understood. Recently, p73 was found to be a target of the enzymatic activity of c-Abl, a non-receptor tyrosine kinase that potently activated in response to DNA damage. Here, we present evidence that c-Abl induces the phosphorylation of p73 in threonine residues adjacent to prolines, and that the p38 MAP kinase pathway mediates this response. Furthermore, we found that activation of p38 is sufficient to enhance the stability of p73, and that the transcriptional activation of p73 by c-Abl requires the activity of p38. These findings indicate that members of the MAP kinases superfamily of signaling molecules can regulate p73, and support a role for the p38 MAP kinase in a novel biochemical pathway by which c-Abl regulates this p53-related molecule.

Autoinhibition of c-Abl

Despite years of investigation, the molecular mechanism responsible for regulation of the c-Abl tyrosine kinase has remained elusive. We now report inhibition of the catalytic activity of purified c-Abl in vitro, demonstrating that regulation is an intrinsic property of the molecule. We show that the interaction of the N-terminal 80 residues with the rest of the protein mediates autoregulation. This N-terminal "cap" is required to achieve and maintain inhibition, and its loss turns c-Abl into an oncogenic protein and contributes to deregulation of BCR-Abl.

Mutational analysis of the regulatory function of the c-Abl Src homology 3 domain

The catalytic activity of the c-Abl tyrosine kinase is tightly regulated by its Src homology 3 (SH3) domain through a complex mechanism that may involve intramolecular binding to Pro242 in the linker region between the SH2 and catalytic domains as well as interactions with a trans-inhibitor. We analysed the effect of mutation or replacement of SH3 on c-Abl tyrosine kinase activity and transformation. Random mutagenesis of SH3 identified several novel point mutations that dysregulated c-Abl kinase activity in vivo, but the RT loop was insensitive to mutational activation. Activating SH3 mutations abolished binding of proline-rich SH3 ligands in vitro, while mutations at Ser140 in the connector between the SH3 and SH2 domains activated Abl kinase activity in vivo and in vitro but did not impair SH3 ligand-binding. Abl was regulated efficiently when its SH3 domain was replaced with a heterologous SH3 from c-Src that binds a different spectrum of proline-rich ligands, but not by substitution of a modular WW domain with similar ligand-binding specificity. These results suggest that the SH3 domain regulates Abl principally by binding to the atypical intramolecular ligand Pro242 rather than a canonical PxxP ligand. Coordination between the SH3 and SH2 domains mediated by the connector region may be required for regulation of Abl even in the absence of SH2 ligand binding.

Processive phosphorylation of p130Cas by Src depends on SH3-polyproline interactions

Many in vivo substrates of Src family tyrosine kinases possess sequences conforming to Src homology 2 and 3 (SH2 and SH3) domain-binding motifs. One such substrate is p130Cas, a protein that is hyperphosphorylated in v-Src transformed cells. Cas contains a substrate domain consisting of 15 potential tyrosine phosphorylation sites, C- and N-terminal polyproline regions fitting the consensus sequence for SH3 domain ligands, and a YDYV motif that binds the Src SH2 domain when phosphorylated. In an effort to understand the mechanisms of processive phosphorylation, we have explored the regions of Cas necessary for interaction with Src using the yeast two-hybrid system. Mutations in the SH2 domain-binding region of Cas or the Src SH2 domain have little effect in Cas-Src complex formation or phosphorylation. However, disruption of the C-terminal polyproline region of Cas completely abolishes interaction between the two proteins and results in impaired phosphorylation of Cas. Kinetic analyses using purified proteins indicated that multisite phosphorylation of Cas by Src follows a processive rather than a distributive mechanism. Furthermore, the kinetic studies show that there are two properties of the polyproline region of Cas that are important in enhancing substrate phosphorylation. First, the C-terminal polyproline serves to activate Src kinases through the process of SH3 domain displacement. Second, this region aids in anchoring the kinase to Cas to facilitate processive phosphorylation of the substrate domain. The two processes combine to ensure phosphorylation of Cas with high efficiency.

Regulation of Cbl phosphorylation by the Abl tyrosine kinase and the Nck SH2/SH3 adaptor

The Cbl proto-oncogene product is tyrosine phosphorylated in response to a wide variety of stimuli. Cbl and the Abl nonreceptor tyrosine kinase both bind to SH3 domains from the SH2/SH3 adaptor Nck, and are candidate effectors for Nck function. Numerous additional SH2- and SH3-domain-mediated interactions are also possible between Cbl, Abl, and Nck. We find that these three signaling proteins associate when overexpressed in mammalian cells and can regulate each other's activity. Co-expression of wt Cbl together with c-Abl, the activity of which is normally repressed in vivo, led to extensive Abl-dependent phosphorylation of Cbl. The major proline-rich region of Cbl was required for its phosphorylation by c-Abl, but not by a constitutively activated Abl mutant, suggesting Cbl activates c-Abl by engaging its SH3 domain. Efficient phosphorylation of Cbl and its stable association with Abl required the SH2 domain of Abl, suggesting that SH2-phosphotyrosine interactions prevent dissociation of active Abl from Cbl. We also show that overexpression of Nck could repress the phosphorylation of Cbl by Abl in vivo. Studies with Nck mutants suggested that the Nck SH2 domain is responsible for inhibiting the activity of Abl toward both Cbl and Nck itself, most likely by competing with the Abl SH2 for tyrosine-phosphorylated binding sites.

Crk family adaptor proteins trans-activate c-Abl kinase

BACKGROUND

c-Abl kinase is activated in response to a variety of biological stimuli. Crk family adaptor proteins can interact physically with c-Abl and be involved in the activation of c-Abl kinase.

RESULTS

We report that the Crk family of adaptor proteins act as trans-acting activators of c-Abl kinase. The interaction of the amino-terminal Src-homology (SH) 3 domain of c-Crk and the proline-rich motifs of c-Abl is an essential step for the phosphorylation of c-Crk by c-Abl, as well as the activation of c-Abl by c-Crk. The activation of c-Abl by c-Crk is negatively regulated by phosphorylation of the tyrosine 221 of c-Crk. Our data suggest that, in the absence of phosphorylation of the tyrosine Y221, the SH2 domain of c-Crk becomes free to bind to target molecules while the carboxyl-terminal SH3 domain of c-Crk binds to the proline-rich region of c-Abl, inducing the activation of c-Abl by c-Crk.

CONCLUSION

This study suggests that the Crk family functions as trans-acting activators of c-Abl kinase. The phosphorylation of c-Crk may regulate c-Abl kinase.

Protein tyrosine kinase structure and function

Tyrosine phosphorylation is one of the key covalent modifications that occurs in multicellular organisms as a result of intercellular communication during embryogenesis and maintenance of adult tissues. The enzymes that carry out this modification are the protein tyrosine kinases (PTKs), which catalyze the transfer of the phosphate of ATP to tyrosine residues on protein substrates. Phosphorylation of tyrosine residues modulates enzymatic activity and creates binding sites for the recruitment of downstream signaling proteins. Two classes of PTKs are present in cells: the transmembrane receptor PTKs and the nonreceptor PTKs. Because PTKs are critical components of cellular signaling pathways, their catalytic activity is strictly regulated. Over the past several years, high-resolution structural studies of PTKs have provided a molecular basis for understanding the mechanisms by which receptor and nonreceptor PTKs are regulated. This review will highlight the important results that have emerged from these structural studies.

c-Abl has high intrinsic tyrosine kinase activity that is stimulated by mutation of the Src homology 3 domain and by autophosphorylation at two distinct regulatory tyrosines

Using the specific Abl tyrosine kinase inhibitor STI 571, we purified unphosphorylated murine type IV c-Abl and measured the kinetic parameters of c-Abl tyrosine kinase activity in a solution with a peptide-based assay. Unphosphorylated c-Abl exhibited substantial peptide kinase activity with K(m) of 204 microm and V(max) of 33 pmol min(-1). Contrary to previous observations using immune complex kinase assays, we found that a transforming c-Abl mutant with a Src homology 3 domain point mutation (P131L) had significantly (about 6-fold) higher intrinsic kinase activity than wild-type c-Abl (K(m) = 91 microm, V(max) = 112 pmol min(-1)). Autophosphorylation stimulated the activity of wild-type c-Abl about 18-fold and c-Abl P131L about 3.6-fold, resulting in highly active kinases with similar catalytic rates. The autophosphorylation rate was dependent on Abl protein concentration consistent with an intermolecular reaction. A tyrosine to phenylalanine mutation (Y412F) at the c-Abl residue homologous to the c-Src catalytic domain autophosphorylation site impaired the activation of wild-type c-Abl by 90% but reduced activation of c-Abl P131L by only 45%. Mutation of a tyrosine (Tyr-245) in the linker region between the Src homology 2 and catalytic domains that is conserved among the Abl family inhibited the autophosphorylation-induced activation of wild-type c-Abl by 50%, whereas the c-Abl Y245F/Y412F double mutant was minimally activated by autophosphorylation. These results support a model where c-Abl is inhibited in part through an intramolecular Src homology 3-linker interaction and stimulated to full catalytic activity by sequential phosphorylation at Tyr-412 and Tyr-245.

The substrate specificity of the catalytic domain of Abl plays an important role in directing phosphorylation of the adaptor protein Crk

c-Abl preferentially phosphorylates peptide substrates that contain proline at the P+3 site (relative to the phosphorylated tyrosine). We previously described a mutant form of the Abl catalytic domain (Y569W) with altered substrate specificity at the P+3 position, as measured using synthetic peptides. In this study, we examine the phosphorylation of Crk, a protein substrate of Abl that is phosphorylated in the sequence Tyr221-Ala-Gln-Pro. In vitro, phosphorylation of Crk by Y569W Abl is greatly reduced relative to wild-type Abl. Overexpression of Y569W mutant Abl in 293T kidney cells produces a similar overall pattern of tyrosine phosphorylation as wild-type Abl, indicating that not all cellular proteins depend on Pro at P+3 for Abl recognition. However, phosphorylation of Crk by Y569W Abl in these cells is markedly reduced relative to wild-type Abl. A truncated form of Abl lacking the C-terminal polyproline region is not able to phosphorylate Crk in these assay conditions. Thus, proper phosphorylation of Crk by Abl depends not only on the interaction of the Crk SH3 domain with the Abl polyproline region, but also on the recognition of amino acids surrounding tyrosine by the Abl catalytic domain.

Phosphorylation of c-Crk II on the negative regulatory Tyr222 mediates nerve growth factor-induced cell spreading and morphogenesis

The Crk family of adaptor proteins participate in diverse signaling pathways that regulate growth factor-induced proliferation, anchorage-dependent DNA synthesis, and cytoskeletal reorganization, important for cell adhesion and motility. Using kidney epithelial 293T cells for transient co-transfection studies and the nerve growth factor (NGF)-responsive PC12 cell line as a model system for neuronal morphogenesis, we demonstrate that the non-receptor tyrosine kinase c-Abl is an intermediary for NGF-inducible c-Crk II phosphorylation on the negative regulatory Tyr(222). Transient expression of a c-Crk II Tyr(222) point mutant (c-Crk Y222F) in 293T cells induces hyperphosphorylation of paxillin on Tyr(31) and enhances complex formation between c-Crk Y222F and paxillin as well as c-Crk Y222F and c-Abl, suggesting that c-Crk II Tyr(222) phosphorylation induces both the dissociation of the Crk SH2 domain from paxillin and the Crk SH3 domain from c-Abl. Interestingly, examination of the early kinetics of NGF stimulation in PC12 cells showed that c-Crk II Tyr(222) phosphorylation preceded paxillin Tyr(31) phosphorylation, followed by a transient initial dissociation of the c-Crk II paxillin complex. PC12 cells overexpressing c-Crk Y222F manifested a defect in cellular adhesion and neuritogenesis that led to detachment of cells from the extracellular matrix, thus demonstrating the biological significance of c-Crk II tyrosine phosphorylation in NGF-dependent morphogenesis. Whereas previous studies have shown that Crk SH2 binding to paxillin is critical for cell adhesion and migration, our data show that the phosphorylation cycle of c-Crk II determines its dynamic interaction with paxillin, thereby regulating turnover of multiprotein complexes, a critical aspect of cytoskeletal plasticity and actin dynamics.

SH3 domains and drug design: ligands, structure, and biological function

The ligand binding preferences, structural features, and biological function of SH3 (Src homology 3) domains are discussed. SH3 domains bind "core" Pro-rich peptide ligands (7-9 amino acids in length) in a polyproline II helical conformation in a highly conserved aromatic rich patch on the protein surface (approximately 390 A2). The ligands can interact with the protein in one of two orientations, depending on the position (N- vs C-terminal) of ligand residues binding to the SH3 selectivity pocket. Core SH3 ligands are characterized by relatively weak interactions (KD = 5-100 microM) that show little binding selectivity within SH3 families. Higher affinity, more selective contiguous ligands require additional flanking residues that bind to less conserved portions of the SH3 surface, with corresponding increase in ligand size and complexity. In contrast to peptide ligands, protein ligands of SH3 domains can exploit multiple discontiguous interactions to enhance affinity and selectivity. A protein-SH3 interaction that utilizes unique interactions may permit the design of small high affinity SH3 ligands. At present, the extended nature of the binding site and homologous nature of the core binding region among SH3 domains present key challenges for structure-based drug design.