Activation of the pp60c-src kinase by middle T antigen binding or by dephosphorylation

TLR4-Mediated Recognition of Mouse Polyomavirus Promotes Cancer-Associated Fibroblast-Like Phenotype and Cell Invasiveness

The tumorigenic potential of mouse polyomavirus (MPyV) has been studied for decades in cell culture models and has been mainly attributed to nonstructural middle T antigen (MT), which acts as a scaffold signal adaptor, activates Src tyrosine kinases, and possesses transforming ability. We hypothesized that MPyV could also transform mouse cells independent of MT via a Toll-like receptor 4 (TLR4)-mediated inflammatory mechanism. To this end, we investigated the interaction of MPyV with TLR4 in mouse embryonic fibroblasts (MEFs) and 3T6 cells, resulting in secretion of interleukin 6 (IL-6), independent of active viral replication. TLR4 colocalized with MPyV capsid protein VP1 in MEFs. Neither TLR4 activation nor recombinant IL-6 inhibited MPyV replication in MEFs and 3T6 cells. MPyV induced STAT3 phosphorylation through both direct and MT-dependent and indirect and TLR4/IL-6-dependent mechanisms. We demonstrate that uninfected mouse fibroblasts exposed to the cytokine environment from MPyV-infected fibroblasts upregulated the expressions of MCP-1, CCL-5, and α-SMA. Moreover, the cytokine microenvironment increased the invasiveness of MEFs and CT26 carcinoma cells. Collectively, TLR4 recognition of MPyV induces a cytokine environment that promotes the cancer-associated fibroblast (CAF)-like phenotype in noninfected fibroblasts and increases cell invasiveness.

Gain-of-function mutations in a member of the Src family kinases cause autoinflammatory bone disease in mice and humans

Chronic recurrent multifocal osteomyelitis (CRMO) is an autoinflammatory bone disease that presents with bone destruction occurring primarily in children. In a mouse ENU mutagenesis screen, the Ali18 strain was isolated because of spontaneous inflammation in the joints and bones. Sequencing candidate genes in the Ali18 critical region identified a missense mutation in the C-terminal regulatory region of the Src family kinase (SFK) member, Fgr. Genome editing revealed Fgr dependency of the inflammatory phenotype in Ali18 mice. Further, whole exome sequencing in our CRMO cohort identified two patients with missense mutations in FGR. In vitro functional assays confirm altered protein function. This work identifies FGR as a CRMO susceptibility gene and suggests that targeting SFKs may be useful in its treatment.

Autoinflammatory syndromes are characterized by dysregulation of the innate immune response with subsequent episodes of acute spontaneous inflammation. Chronic recurrent multifocal osteomyelitis (CRMO) is an autoinflammatory bone disorder that presents with bone pain and localized swelling. Ali18 mice, isolated from a mutagenesis screen, exhibit a spontaneous inflammatory paw phenotype that includes sterile osteomyelitis and systemic reduced bone mineral density. To elucidate the molecular basis of the disease, positional cloning of the causative gene for Ali18 was attempted. Using a candidate gene approach, a missense mutation in the C-terminal region of Fgr, a member of Src family tyrosine kinases (SFKs), was identified. For functional confirmation, additional mutations at the N terminus of Fgr were introduced in Ali18 mice by CRISPR/Cas9-mediated genome editing. N-terminal deleterious mutations of Fgr abolished the inflammatory phenotype in Ali18 mice, but in-frame and missense mutations in the same region continue to exhibit the phenotype. The fact that Fgr null mutant mice are morphologically normal suggests that the inflammation in this model depends on Fgr products. Furthermore, the levels of C-terminal negative regulatory phosphorylation of Fgr^Ali18 are distinctly reduced compared with that of wild-type Fgr. In addition, whole-exome sequencing of 99 CRMO patients including 88 trios (proband and parents) identified 13 patients with heterozygous coding sequence variants in FGR, including two missense mutant proteins that affect kinase activity. Our results strongly indicate that gain-of-function mutations in Fgr are involved in sterile osteomyelitis, and thus targeting SFKs using specific inhibitors may allow for efficient treatment of the disease.

Small size, big impact: how studies of small DNA tumour viruses revolutionized biology

Intense study of three families of small tumour viruses with double-stranded DNA genomes, carried out over 50 years, has had a profound impact on biology. The polyomaviruses and papillomaviruses have circular DNA genomes of approximately 5000 and approximately 8000 base-pairs, respectively, and thus encode only a handful of proteins. Adenoviruses have a 32 000-base-pair linear DNA genome, still far smaller than the three billion-base-pair human genome. Members of all three virus families can transform cultured cells to tumorigenicity and cause tumours in experimental animals. Several human papillomaviruses (HPV) and at least one polyomavirus are oncogenic in humans. Early analysis of these viruses, particularly the polyomavirus SV40, led to the development of many powerful experimental tools, including restriction mapping, site-directed mutagenesis, gene transfer, genome-wide sequencing and recombinant DNA. These tools have since been refined and used to study cellular genes, revolutionizing our understanding of biology. These tools were also applied to the viruses themselves. Analysis of the virus life cycle and the effect of these viruses on cells yielded important new insights into many aspects of gene expression, DNA replication, cell biology and carcinogenesis. These studies have also led to vaccination strategies to prevent infection and cancer in humans. This article is part of the theme issue 'Silent cancer agents: multi-disciplinary modelling of human DNA oncoviruses'.

A switch in nucleotide affinity governs activation of the Src and Tec family kinases

The Tec kinases, closely related to Src family kinases, are essential for lymphocyte function in the adaptive immune system. Whilst the Src and Abl kinases are regulated by tail phosphorylation and N-terminal myristoylation respectively, the Tec kinases are notable for the absence of either regulatory element. We have found that the inactive conformations of the Tec kinase Itk and Src preferentially bind ADP over ATP, stabilising both proteins. We demonstrate that Itk adopts the same conformation as Src and that the autoinhibited conformation of Src is independent of its C-terminal tail. Allosteric activation of both Itk and Src depends critically on the disruption of a conserved hydrophobic stack that accompanies regulatory domain displacement. We show that a conformational switch permits the exchange of ADP for ATP, leading to efficient autophosphorylation and full activation. In summary, we propose a universal mechanism for the activation and autoinhibition of the Src and Tec kinases.

The nonreceptor tyrosine kinase c-Src attenuates SCF(β-TrCP) E3-ligase activity abrogating Taz proteasomal degradation

The polyomavirus middle T antigen (PyMT) oncogene activates the cellular nonreceptor tyrosine kinase c-Src and recruits the Hippo pathway effectors, Yap (yes-associated protein) and Taz (transcriptional coactivator with PDZ-binding motif), as key steps in oncogenesis. Yap and Taz are transcription coactivators shuttling from the cytoplasm to the nucleus. The Hippo pathway kinase Lats1/2 (large tumor suppressor homolog) reduces Yap/Taz nuclear localization and minimizes their cytoplasmic levels by facilitating their ubiquitination by the E3 ligase SCF(β-TrCP). In contrast, PyMT increases the cytoplasmic Taz level. Here we show that this unique PyMT behavior is mediated by Src. We demonstrate that PyMT-induced Src activation inhibits degradation of both wild-type and tyrosine-less Taz, ruling out Taz modification as a mechanism of escaping degradation. Instead, we found that Src attenuates the SCF(β-TrCP) E3-ligase activity in blunting Taz proteasomal degradation. The role of Src in rescuing Taz from TrCP-mediated degradation gives rise to higher cell proliferation under dense cell culture. Finally, IkB (NF-kappa-B inhibitor), a known substrate of β-TrCP, was rescued by Src, suggesting a wider effect of Src on β-TrCP substrates. These findings introduce the Src tyrosine kinase as a regulator of SCF(β-TrCP).

Application of Gene Expression Profiling for Validating Models of Human Breast Cancer

While classical histopathologic approaches are invaluable in classifying tumors and understanding aspects of cellular interactions, genomic approaches provide a means to molecularly dissect tumorigenesis. The relationship of gene expression to the development of neoplasia remains an area of intensive research. With the advent of large-scale genomic platforms, alterations in gene expression can be related to the morphological development of cancer. The feasibility of using large-scale genomic analysis platforms has dramatically changed the landscape of biological sciences, as cellular processes must be considered in the context of complex networks. Alterations in gene expression must now be understood in a systems approach in which the relationships between genes expression changes are studied by considering the interplay of multiple regulatory networks. Ultimately, such changes must be understood at the protein level. We have begun to apply this technology to determine changes in gene expression that differentiate various types of mammary cancers that arise in mouse models that have been initiated by different genetic alterations. Ultimately, a molecular catalogue of similarities and differences between rodent and human tumors can be created which will serve to validate or credential particular models for specific experimental purposes, such as preclinical testing. These approaches have led to new insights into molecular pathways involved in oncogenesis, new classifications of human breast cancer, and the identification of new genes that may be relevant to understanding and treating human cancer.

Redox circuitries driving Src regulation

SIGNIFICANCE

Here, we review recent advances with regard to the role of Src kinase in the regulation of cytoskeleton organization, cell adhesion, and motility, focusing on redox circuitries engaging this kinase for anchorage and motility, control of cell survival to anoikis, as well as metabolic deregulation, all features belonging to the new hallmarks of cancer.

RECENT ADVANCES

Several recent insights have reported that, alongside the well-known phosphorylation/dephosphorylation control, cysteine oxidation is a further mechanism of enzyme activation for both c-Src kinase and its oncogenic counterparts. Indeed, mounting evidence portrays redox regulation of Src kinase as a compulsory outcome in growth factors/cytokines signaling, integrin engagement, motility and invasiveness of tissues, receptor cross-talking at plasmamembrane, as well as during carcinogenesis and progression toward tumor malignancy or fibrotic disease. In addition, the kinase is an upstream regulator of NADPH oxidase-driven oxidants, a critical step for invadopodia formation and metastatic spread.

CRITICAL ISSUES

Not satisfactorily unraveled yet, the exact role of Src kinase in redox cancer biology needs to be implemented with studies that are aimed at clarifying (i) the exact hierarchy between oxidants sources, Src redox-dependent activation and the regulation of cell motility, and (ii) the actual susceptibility of invading cells to redox-based treatments, owing to the well-recognized ability of cancer cells to find new strategies to adapt to new environments.

FUTURE DIRECTIONS

Once these critical issues are addressed, redox circuitries involving Src kinase should potentially be used as both biomarkers and targets for personalized therapies in the fight against cancer or fibrotic diseases.

In vitro membrane reconstitution of the T-cell receptor proximal signaling network

T-cell receptor (TCR) phosphorylation is controlled by a complex network that includes Lck, a Src family kinase (SFK), the tyrosine phosphatase CD45, and the Lck-inhibitory kinase Csk. How these competing phosphorylation and dephosphorylation reactions are modulated to produce T-cell triggering is not fully understood. Here we reconstituted this signaling network using purified enzymes on liposomes, recapitulating the membrane environment in which they normally interact. We demonstrate that Lck's enzymatic activity can be regulated over a ~10-fold range by controlling its phosphorylation state. By varying kinase and phosphatase concentrations, we constructed phase diagrams that reveal ultrasensitivity in the transition from the quiescent to the phosphorylated state and demonstrate that coclustering TCR-Lck or detaching Csk from the membrane can trigger TCR phosphorylation. Our results provide insight into the mechanism of TCR signaling as well as other signaling pathways involving SFKs.

Intracellular trafficking of integrins in cancer cells

Integrins are heterodimeric cell surface receptors, which principally mediate the interaction between cells and their extracellular microenvironments. Because of their pivotal roles in cancer proliferation, survival, invasion and metastasis, integrins have been recognized as promising targets for cancer treatment. As is the case with other receptors, the localization of integrins on the cell surface has provided opportunities to block their functions by various inhibitory monoclonal antibodies. A number of small molecule agents blocking integrin-ligand binding have also been established, and some such agents are currently on the market or in clinical trials for some diseases including cancer. This review exclusively focuses on another strategy for cancer therapy, which comes from the obligate localization of integrins on the cell surface; targeting the intracellular trafficking of integrins. A number of studies have shown the essential roles of integrin trafficking in hallmarks of cancer, such as activation of oncogenic signaling pathways as well as acquisition of invasiveness. Recent findings have shown that increased integrin recycling activity is associated with some types of gain-of-function mutations of p53, a common feature of diverse types of cancers, which also indicates that targeting integrin recycling could be widely applicable and effective against many cancers. We also discuss possible therapeutic contexts where integrin trafficking can be effectively targeted, and what molecular interfaces may hopefully be druggable.

Distinct transcriptional regulation of human large conductance voltage- and calcium-activated K+ channel gene (hSlo1) by activated estrogen receptor alpha and c-Src tyrosine kinase

Estrogen receptor α (ERα) regulates gene transcription via "genomic" (binding directly or indirectly, typically via Sp1 or AP-1 sites, to target genes) and/or "nongenomic" (signaling) mechanisms. ERα activation by estrogen up-regulates the murine Ca(2+)-activated K(+) channel α subunit gene (mSlo1) via genomic mechanisms. Here, we investigated whether ERα also drives transcription of the human (hSlo1) gene. Consistent with this view, estrogen increased hSlo1 transcript levels in primary human smooth muscle cells. Promoter studies revealed that estrogen/hERα-mediated hSlo1 transcription was nearly 6-fold more efficient than for mSlo1 (EC(50), 0.07 versus 0.4 nM). Unlike the genomic transcriptional mechanism employed by mSlo1, hSlo1 exhibits a nongenomic hERα-mediated regulatory mechanism. This is supported by the following: 1) efficient hSlo1 transcription after disruption of the DNA-binding domain of hERα or knockdown of Sp1, and 2) lack of AP-1 sites in the hSlo1 promoter. Three nongenomic signaling pathways were explored: Src, Rho, and PI3K. Inhibition of Src with 10 μM PP2, and reported downstream ERK with 25 μM PD98059 did not prevent estrogen action but caused an increase in hSlo1 basal transcription; conversely, constitutively active c-Src (Y527F) decreased hSlo1 basal transcription even preventing its estrogen/hERα-mediated transcriptional activation. Rho inhibition by coexpressed Clostridium botulinum C3 transferase did not alter estrogen action. In contrast, inhibition of PI3K activity with 10 μM LY294002 decreased estrogen-stimulated hSlo1 transcription by ∼40%. These results indicate that the nongenomic PI3K signaling pathway plays a role in estrogen/hERα-stimulated hSlo1 gene expression; whereas c-Src activity leads to hSlo1 gene tonic repression independently of estrogen, likely through ERK activation.

α-Synuclein, leucine-rich repeat kinase-2, and manganese in the pathogenesis of Parkinson disease

Parkinson disease (PD) is the most common movement disorder. It is characterized by bradykinesia, postural instability, resting tremor, and rigidity associated with the progressive loss of dopaminergic neurons in the substantia nigra pars compacta. Another pathological hallmark of PD is the presence of α-synuclein proteiniacous inclusions, known as Lewy bodies and Lewy neurites, in some of the remaining dopaminergic neurons. Mounting evidence indicates that both genetic and environmental factors contribute to the etiology of PD. For example, genetic mutations (duplications, triplications or missense mutations) in the α-synuclein gene can lead to PD, but even in these patients, age-dependent physiological changes or environmental exposures appear to be involved in disease presentation. Several additional alterations in many other genes have been established to either cause or increase the risk of parkinson disease. More specifically, autosomal dominant missense mutations in the gene for leucine-rich repeat kinase 2 (LRRK2/PARK8) are the most common known cause of PD. Recently it was shown that G2019S, the most common diseasing-causing mutant of LRRK2, has dramatic effects on the kinase activity of LRRK2: while activity of wild-type LRRK2 is inhibited by manganese, the G2019S mutation abrogates this inhibition. Based on the in vitro kinetic properties of LRRK2 in the presence of manganese, we proposed that LRRK2 may be a sensor of cytoplasmic manganese levels and that the G2019S mutant has lost this function. This finding, alongside a growing number of studies demonstrating an interaction between PD-associated proteins and manganese, suggest that dysregulation of neuronal manganese homeostasis over a lifetime can play an important role in the etiology of PD.

Herpes simplex virus requires VP11/12 to activate Src family kinase-phosphoinositide 3-kinase-Akt signaling

We previously showed that the herpes simplex virus 1 (HSV-1) tegument protein VP11/12 activates the lymphocyte-specific Src family kinase (SFK) Lck and is tyrosine phosphorylated in an Lck-dependent manner during T cell infection. We now extend these findings to show that ectopic expression of Lck induces robust tyrosine phosphorylation of VP11/12 in Vero cells, strongly suggesting that VP11/12 participates in an Lck-mediated signaling pathway as a substrate of Lck or a kinase activated by Lck. We sought to elucidate signaling events downstream of VP11/12-SFK interactions. SFKs lie upstream of the canonical phosphoinositide 3-kinase (PI3K)-Akt pathway in signaling emanating from immune receptors, growth factor receptors, and polyomavirus middle T antigen. Here, we show that VP11/12 is required for virus-induced activation of PI3K-Akt signaling in HSV-infected Jurkat T cells and primary fibroblasts. VP11/12 interacts with PI3K or PI3K signaling complexes during infection, suggesting that VP11/12 activates PI3K directly. SFK activity is required for tyrosine phosphorylation of VP11/12, VP11/12-PI3K interactions, and Akt activation in infected fibroblasts, suggesting that SFK-dependent phosphorylation of VP11/12 is required for interactions with downstream signaling effectors. Akt controls many biological functions, including cell survival, cell motility, and translation, but it is currently unclear which Akt targets are modulated by VP11/12 during infection. Although the Akt target mTORC1 is activated during HSV-1 infection, VP11/12 is not required for this effect, implying that one or more additional viral proteins regulate this pathway. Further studies are therefore required to determine which Akt targets and associated biological functions are uniquely modulated by VP11/12.

The G2019S pathogenic mutation disrupts sensitivity of leucine-rich repeat kinase 2 to manganese kinase inhibition

Mutations in leucine-rich repeat kinase-2 (LRRK2) are the most common cause of late-onset Parkinson disease. Previously, we showed that the G2019S pathogenic mutation can cause a dramatic increase (approximately 10-fold) in kinase activity, far above other published studies. A notable experimental difference was the use of Mn-ATP as a substrate. Therefore, the effects of metal cation-ATP cofactors on LRRK2 kinase activity were investigated. It is shown, using several divalent metal cations, that only Mg(2+) or Mn(2+) can support LRRK2 kinase activity. However, for wild-type, I2020T, and R1441C LRRK2, Mn(2+) was significantly less effective at supporting kinase activity. In sharp contrast, both Mn(2+) and Mg(2+) were effective at supporting the activity of G2019S LRRK2. These divergent effects associated with divalent cation usage and the G2019S mutation were predominantly because of differences in catalytic rates. However, LRRK2 was shown to have much lower (approximately 40-fold) ATP K(m) for Mn-ATP compared with Mg-ATP. Consequently, sub-stoichiometric concentrations of Mn(2+) can act to inhibit the kinase activity of wild-type, but not G2019S LRRK2 in the presence of Mg(2+) . From these findings, a new model is proposed for a possible function of LRRK2 and the consequence of the G2019S LRRK2 pathogenic mutation.

Lessons in signaling and tumorigenesis from polyomavirus middle T antigen

The small DNA tumor viruses have provided a very long-lived source of insights into many aspects of the life cycle of eukaryotic cells. In recent years, the emphasis has been on cancer-related signaling. Here we review murine polyomavirus middle T antigen, its mechanisms, and its downstream pathways of transformation. We concentrate on the MMTV-PyMT transgenic mouse, one of the most studied models of breast cancer, which permits the examination of in situ tumor progression from hyperplasia to metastasis.

Cellular transformation by Simian Virus 40 and Murine Polyoma Virus T antigens

Simian Virus 40 (SV40) and Mouse Polyoma Virus (PY) are small DNA tumor viruses that have been used extensively to study cellular transformation. The SV40 early region encodes three tumor antigens, large T (LT), small T (ST) and 17KT that contribute to cellular transformation. While PY also encodes LT and ST, the unique middle T (MT) generates most of the transforming activity. SV40 LT mediated transformation requires binding to the tumor suppressor proteins Rb and p53 in the nucleus and ST binding to the protein phosphatase PP2A in the cytoplasm. SV40 LT also binds to several additional cellular proteins including p300, CBP, Cul7, IRS1, Bub1, Nbs1 and Fbxw7 that contribute to viral transformation. PY MT transformation is dependent on binding to PP2A and the Src family protein tyrosine kinases (PTK) and assembly of a signaling complex on cell membranes that leads to transformation in a manner similar to Her2/neu. Phosphorylation of MT tyrosine residues activates key signaling molecules including Shc/Grb2, PI3K and PLCgamma1. The unique contributions of SV40 LT and ST and PY MT to cellular transformation have provided significant insights into our understanding of tumor suppressors, oncogenes and the process of oncogenesis.

Spatiotemporal regulation of intracellular trafficking of Toll-like receptor 9 by an inhibitory receptor, Ly49Q

Toll-like receptor (TLR) 9 recognizes unmethylated microorganismal cytosine guanine dinucleotide (CpG) DNA and elicits innate immune responses. However, the regulatory mechanisms of the TLR signaling remain elusive. We recently reported that Ly49Q, an immunoreceptor tyrosine-based inhibitory motif-bearing inhibitory receptor belonging to the natural killer receptor family, is crucial for TLR9-mediated type I interferon production by plasmacytoid dendritic cells. Ly49Q is expressed in plasmacytoid dendritic cells, macrophages, and neutrophils, but not natural killer cells. In this study, we showed that Ly49Q regulates TLR9 signaling by affecting endosome/lysosome behavior. Ly49Q colocalized with CpG in endosome/lysosome compartments. Cells lacking Ly49Q showed a disturbed redistribution of TLR9 and CpG. In particular, CpG-induced tubular endolysosomal extension was impaired in the absence of Ly49Q. Consistent with these findings, cells lacking Ly49Q showed impaired cytokine production in response to CpG-oligodeoxynucleotide. Our data highlight a novel mechanism by which TLR9 signaling is controlled through the spatiotemporal regulation of membrane trafficking by the immunoreceptor tyrosine-based inhibitory motif-bearing receptor Ly49Q.

Lessons from polyoma middle T antigen on signaling and transformation: A DNA tumor virus contribution to the war on cancer

Middle T antigen (MT) is the principal oncogene of murine polyomavirus. Its study has led to the discovery of the roles of tyrosine kinase and phosphoinositide 3-kinase (PI3K) signaling in mammalian growth control and transformation. MT is necessary for viral transformation in tissue culture cells and tumorigenesis in animals. When expressed alone as a transgene, MT causes tumors in a wide variety of tissues. It has no known catalytic activity, but rather acts by assembling cellular signal transduction molecules. Protein phosphatase 2A, protein tyrosine kinases of the src family, PI3K, phospholipase Cgamma1 as well as the Shc/Grb2 adaptors are all assembled on MT. Their activation sets off a series of signaling cascades. Analyses of virus mutants as well as transgenic animals have demonstrated that the effects of a given signal depend not only tissue type, but on the genetic background of the host animal. There remain many opportunities as we seek a full molecular understanding of MT and apply some of its lessons to human cancer.

Protein phosphatase 2A regulatory subunits and cancer

The serine/threonine protein phosphatase (PP2A) is a trimeric holoenzyme that plays an integral role in the regulation of a number of major signaling pathways whose deregulation can contribute to cancer. The specificity and activity of PP2A are highly regulated through the interaction of a family of regulatory B subunits with the substrates. Accumulating evidence indicates that PP2A acts as a tumor suppressor. In this review we summarize the known effects of specific PP2A holoenzymes and their roles in cancer relevant pathways. In particular we highlight PP2A function in the regulation of MAPK and Wnt signaling.

A RNA interference screen identifies the protein phosphatase 2A subunit PR55gamma as a stress-sensitive inhibitor of c-SRC

Protein Phosphatase type 2A (PP2A) represents a family of holoenzyme complexes with diverse biological activities. Specific holoenzyme complexes are thought to be deregulated during oncogenic transformation and oncogene-induced signaling. Since most studies on the role of this phosphatase family have relied on the use of generic PP2A inhibitors, the contribution of individual PP2A holoenzyme complexes in PP2A-controlled signaling pathways is largely unclear. To gain insight into this, we have constructed a set of shRNA vectors targeting the individual PP2A regulatory subunits for suppression by RNA interference. Here, we identify PR55γ and PR55δ as inhibitors of c-Jun NH₂-terminal kinase (JNK) activation by UV irradiation. We show that PR55γ binds c-SRC and modulates the phosphorylation of serine 12 of c-SRC, a residue we demonstrate to be required for JNK activation by c-SRC. We also find that the physical interaction between PR55γ and c-SRC is sensitive to UV irradiation. Our data reveal a novel mechanism of c-SRC regulation whereby in response to stress c-SRC activity is regulated, at least in part, through loss of the interaction with its inhibitor, PR55γ.

Protein Phosphatase type 2A (PP2A) represent a family of holoenzyme complexes involved in wide range of activities such as growth, differentiation, and cell death. The PP2A holoenzyme complex is made up of a catalytic, a structural, and one of various “B” subunits. These “B” subunits are thought to provide the substrate specificity required for PP2A activity. Previous work on PP2A has mostly been derived by inhibiting the catalytic subunit through chemical inhibition, as such inhibiting all of the pathways associated with PP2A. To identify individual “B” subunits involved in specific cellular processes we have generated a “B” subunit gene knockdown library, which allows us to inhibit each of the known “B” subunits individually. One of the many pathways regulated by PP2A is the c-Jun NH₂-terminal kinase (JNK) kinase pathway, which, depending on stimulus, can affect either cell survival or cell proliferation. Here we report that the “B” subunit PR55γ acts as a negative regulator of JNK activity and cell death. We show that PR55γ influences JNK activity by inhibiting one of its upstream regulators, the proto-oncogene c-SRC, through dephosphorylation at one of the key residues on c-SRC, a site we show to be critical for c-SRC activation following cell stress. Overall our work describes the novel function of a specific PP2A subunit involved in cell survival and identifies a novel mechanism of c-SRC regulation.

Identification of a critical tyrosine residue in caspase 8 that promotes cell migration

Caspase 8 is a critical upstream initiator of programmed cell death but, paradoxically, has also been shown to promote cell migration. Here, we show that tyrosine 380 in the linker loop of human caspase 8 is a critical switch determining caspase 8 function. Our studies show that, in addition to its cytosolic distribution, caspase 8 is recruited to lamella of migrating cells. Although the catalytic domain of caspase 8 is sufficient for recruitment and promotion of cell migration, catalytic activity per se is not required. Instead, we find that integrin-mediated adhesion promotes caspase 8 phosphorylation on tyrosine 380. Accordingly, mutation of this site compromises localization to the periphery and the potentiation of cell migration. Mechanistically, this linker region of caspase 8 acts as a Src homology 2 binding site. In particular, tyrosine 380 is critical for interaction with Src homology 2 domains. The results identify a novel mechanism by which caspase 8 is recruited to the lamella of a migrating cell, promoting cell migration independent of its protease activity.

Treatment for chronic myelogenous leukemia: the long road to imatinib

The scientists of today have become accustomed to the extremely rapid pace of progress in the biomedical sciences spurred on by the discovery of recombinant DNA and the advent of automated DNA sequencing and PCR, with progress usually being measured in months or years at most. What is often forgotten, however, are the many prior advances that were needed to reach our present state of knowledge. Here I illustrate this by discussing the scientific discoveries made over the course of the past century and a half that ultimately led to the recent successful development of drugs, particularly imatinib mesylate, to treat chronic myelogenous leukemia.

Label-Free Electrical Sensing of Small-Molecule Inhibition on Tyrosine Phosphorylation

Protein tyrosine kinases (PTKs) play a central role in human carcinogenesis and have emerged as the promising new targets. Small-molecule inhibitors of PTKs have shown impressive anticancer effects and are rapidly entering the clinic. PTK assays allow for high-throughput identification of small-molecule inhibitors. However, current methods of detecting kinase activity require the use of radioisotopes or expensive reagents; such as fluorescently labeled antibodies. We have developed a novel label-free approach for the quantitative detection of peptide tyrosine (Tyr) phosphorylation using the electrochemical oxidation current signal of Tyr. When the phosphorylation is achieved, the phosphorylated Tyr (Tyr-P) cannot be oxidized at approximately 0.65 V. However, when the phosphorylation is successfully inhibited using a small molecule, Tyr can be oxidized and result in a high current response on a multiwalled carbon nanotube-modified screen-printed carbon electrode. We determined the activity of cellular-sarcoma (c-Src) nonreceptor PTK, p60(c-Src), in combination with its highly specific substrate peptide, Raytide. Tyr kinase reactions were also performed in the presence of a well-defined small-molecule inhibitor, 4-amino-5-(4-chlorophenyl)-7- (tert-butyl)pyrazolo[3,4-d]pyrimidine (PP2). Based on the dependency of Tyr oxidation signal on inhibitor concentration, IC50 value, half-maximal inhibition of the inhibitor, was estimated as 5 nM for PP2. Our label-free electrochemical method is a promising candidate for pharmaceutical research and development in screening small-molecule inhibitors of PTKs.

PRL3 promotes cell invasion and proliferation by down-regulation of Csk leading to Src activation

Phosphatase of regenerating liver 3 (PRL3) is overexpressed in a variety of tumors, and high levels of PRL3 expression are associated with tumorigenesis and metastasis. Consistent with an oncogenic role for PRL3, we show that ectopic PRL3 expression promotes cell proliferation and invasion. However, little is known about the molecular basis for PRL3 function. Obtaining this knowledge is vital for understanding PRL3-mediated disease processes and for the development of novel anticancer therapies targeted to PRL3. Here we report that up-regulation of PRL3 activates the Src kinase, which initiates a number of signal pathways culminating in the phosphorylation of ERK1/2, STAT3, and p130(Cas). The activation of these pathways likely contributes to the increased cell growth and motility of PRL3 cells. We provide evidence that PRL3 induces Src activation through down-regulation of Csk, a negative regulator of Src. Importantly, Src activation and Csk down-regulation are also observed in colon cancer cells expressing a higher level of PRL3. Thus, we have revealed a biochemical mechanism for the PRL3-mediated cell invasion and proliferation in which elevated PRL3 expression causes a reduction in Csk level, leading to Src activation.

The adaptor protein Tom1L1 is a negative regulator of Src mitogenic signaling induced by growth factors

The Src family of protein-tyrosine kinases (SFK) play important roles in mitogenesis and morphological changes induced by growth factors. The involved substrates are, however, ill defined. Using an antiphosphotyrosine antibody to screen tyrosine-phosphorylated cDNA expression library, we have identified Tom1L1, an adaptor protein of the Tom1 family and a novel substrate and activator of the SFK. Surprisingly, we found that Tom1L1 does not promote DNA synthesis induced by Src. Furthermore, we report that Tom1L1 negatively regulates SFK mitogenic signaling induced by platelet-derived growth factor (PDGF) through modulation of SFK-receptor association: (i) Tom1L1 inhibits DNA synthesis induced by PDGF; (ii) inhibition is overcome by c-myc expression or p53 inactivation, two regulators of SFK mitogenic function; (iii) Src or Fyn coexpression overrides Tom1L1 mitogenic activity; (iv) overexpression of the adaptor reduces Src association with the receptor; and (v) protein inactivation potentiates receptor complex formation, allowing increased SFK activation and DNA synthesis. However, Tom1L1 affects neither DNA synthesis induced by the constitutively active allele SrcY527F nor SFK-regulated actin assembly induced by PDGF. Finally, overexpressed Tom1 and Tom1L2 also associate with Src and affected mitogenic signaling in agreement with some redundancy among members of the Tom1 family. We concluded that Tom1L1 defines a novel mechanism for regulation of SFK mitogenic signaling induced by growth factors.

Cbp deficiency alters Csk localization in lipid rafts but does not affect T-cell development

The ubiquitously expressed transmembrane adaptor Csk-binding protein (Cbp) recruits Csk to lipid rafts, where the latter exerts its negative regulatory effect on the Src family of protein tyrosine kinases. We have inactivated Cbp in the mouse germ line. In contrast to Csk gene inactivation, which leads to embryonic lethality and impaired T-cell development, Cbp-deficient mice were viable and exhibited normal T-cell development but with an increased thymocyte population. In the absence of Cbp, the amount of Csk that localizes to the lipid rafts was greatly reduced. Interestingly, this altered lipid raft localization of Csk did not lead to any detectable biochemical or functional defect in T cells. The T-cell receptor-induced intracellular calcium flux, cell proliferation, and cytokine secretion were not affected by the absence of Cbp. Peripheral T-cell tolerance to superantigen SEB was also largely intact in Cbp-deficient mice. Thus, Cbp is dispensable for T-cell development and activation.

In silico activation of Src tyrosine kinase reveals the molecular basis for intramolecular autophosphorylation

Structural data suggest that important hinge-bending motions of the two lobes that shape the catalytic domain of Src tyrosine kinase, together with reorganization of an alpha helix (helix C), are needed for the activation loop to adopt the catalytically competent conformation. The phosphorylation of a Tyr residue (Tyr-416) in this loop also seems to be essential for enzyme activation. However, no information is available about the dynamics of this activation process. By comparing the inactive and active forms of the catalytic domains of Src and Lck, another member of the Src family, we first identified a short stretch that can act as a hinge for the interlobe motion. The opening of the lobes was then simulated using a targeted molecular dynamics approach. The results obtained suggested that pulling the two lobes apart is not enough to induce the required conformational change in the activation loop. Rather unexpectedly, however, swinging of the lobes situated Tyr-416 in a suitable position for intramolecular autophosphorylation, and further simulation of Tyr-416-phosphorylated Src in the presence of ADP did then result in a conformational change that placed the activation loop in a position similar to that found in the active open conformation of Lck. Taken together, our results establish a physical link between intramolecular autophosphorylation and loop activation.

SRC gene expression in human cancer: the role of transcriptional activation

Human pp60c-Src (or c-Src) is a 60 kDa nonreceptor tyrosine kinase encoded by the SRC gene and is the cellular homologue to the potent transforming v-Src viral oncogene. c-Src functions at the hub of a vast array of signal transduction cascades that influence cellular proliferation, differentiation, motility, and survival. c-Src activation has been documented in upwards of 50% of tumors derived from the colon, liver, lung, breast, and pancreas. Therefore, a major focus has been to understand the mechanisms of c-Src activation in human cancer. Early studies concentrated on post-translational mechanisms that lead to increased c-Src kinase activity, which often correlated with overexpression of c-Src protein. More recently, the discovery of an activating SRC mutation in a small subset of advanced colon tumors has been reported. In addition, elevated SRC transcription has been identified as yet another mechanism contributing significantly to c-Src activation in a subset of human colon cancer cell lines. Interestingly, histone deacetylase (HDAC) inhibitors, agents with well-documented anti-cancer activity, repress SRC transcription in a wide variety of human cancer cell lines. Analysis of the mechanisms behind HDAC inhibitor mediated repression could be utilized in the future to specifically inhibit SRC gene expression in human cancer.

pp60c-src activation in lung adenocarcinoma

Nine src family members are known including c-Src, c-Yes, c-Lck, c-Fyn, c-Hck, c-Lyn, c-Blk, c-Fgr and c-Yrk. They encode proteins with molecular weights of 55-62 kilodaltons (kDa), which are either cytoplasmic or membrane-associated protein tyrosine kinases. A close correlation exists between an elevated pp60c-src tyrosine kinase activity and cell transformation. However, the level of activation of pp60c-src in non-small cell lung cancers (NSCLC) remains obscure. The aim of this study was to examine the level of activity of pp60c-src in NSCLC. pp60c-src expression and in vitro protein tyrosine kinase activity in lung cancer tissue samples were measured by western blotting and in vitro kinase assays and compared with those in the surrounding non-tumour lung tissue from the same patient. pp60c-src phosphorylation was assessed by two-dimensional tryptic phosphopeptide mapping. The kinase activity of pp60c-src was significantly activated in NSCLC, especially in adenocarcinomas. In addition, the pp60c-src kinase activity increased with the size of the adenocarcinoma. Two-dimensional tryptic phosphopeptide mapping showed dephosphorylation of pp60c-src at Tyr 530 in adenocarcinomas. The proto-oncogene product, pp60c-src, was activated in NSCLC, especially in adenocarcinomas, in part through the dephosphorylation of Tyr 530. Our results suggest that activation of pp60c-src might play an important role in the progression of lung adenocarcinomas.

Protein kinase A intersects SRC signaling in membrane microdomains

Regulation of Src kinase activity is tightly coupled to the phosphorylation status of the C-terminal regulatory tyrosine Tyr(527), which, when phosphorylated by Csk, represses Src. Here, we demonstrate that activation of Csk through a prostaglandin E(2)-cAMP-protein kinase A (PKA) pathway inhibits Src. This inhibitory pathway is operative in detergent-resistant membrane fractions where cAMP-elevating agents activate Csk, resulting in a concomitant decrease in Src activity. The inhibitory effect on Src depends on a detergent-resistant membrane-anchored Csk and co-localization of all components of the inhibitory pathway in membrane microdomains. Furthermore, epidermal growth factor-induced activation of Src and phosphorylation of the Src substrates Cbl and focal adhesion kinase are inhibited by activation of the cAMP-PKA-Csk pathway. We propose a novel mechanism whereby G protein-coupled receptors inhibit Src signaling by activation of Csk in a cAMP-PKA-dependent manner.

Polyoma virus middle T antigen and its role in identifying cancer-related molecules

Most cancer researchers take for granted some of the basic concepts about the molecular changes that underlie tumorigenesis. These include the principles that tyrosine kinases and the phosphorylation of phosphatidylinositol by phosphatidylinositol 3-kinases are important in the signalling pathways that control proliferation and apoptosis, and hence cancer formation. However, how many know that a small DNA mouse virus was crucial in establishing both of these tenets?

v-Src's hold over actin and cell adhesions

The oncoprotein v-Src and its cellular homologue (c-Src) are tyrosine kinases that modulate the actin cytoskeleton and cell adhesions. Through the concerted action of their protein-interaction and kinase domains, they are targeted to cell matrix integrin adhesions or cadherin-dependent junctions between epithelial cells, where they phosphorylate substrates that induce adhesion turnover and actin re-modelling. Recent experiments have defined some of the key targets and effector pathways that mediate the pleiotropic oncogenic effects of v-Src.

Hic-5-reduced cell spreading on fibronectin: competitive effects between paxillin and Hic-5 through interaction with focal adhesion kinase

Hic-5 is a paxillin homologue that is localized to focal adhesion complexes. Hic-5 and paxillin share structural homology and interacting factors such as focal adhesion kinase (FAK), Pyk2/CAKbeta/RAFTK, and PTP-PEST. Here, we showed that Hic-5 inhibits integrin-mediated cell spreading on fibronectin in a competitive manner with paxillin in NIH 3T3 cells. The overexpression of Hic-5 sequestered FAK from paxillin, reduced tyrosine phosphorylation of paxillin and FAK, and prevented paxillin-Crk complex formation. In addition, Hic-5-mediated inhibition of spreading was not observed in mouse embryo fibroblasts (MEFs) derived from FAK(-/-) mice. The activity of c-Src following fibronectin stimulation was decreased by about 30% in Hic-5-expressing cells, and the effect of Hic-5 was restored by the overexpression of FAK and the constitutively active forms of Rho-family GTPases, Rac1 V12 and Cdc42 V12, but not RhoA V14. These observations suggested that Hic-5 inhibits cell spreading through competition with paxillin for FAK and subsequent prevention of downstream signal transduction. Moreover, expression of antisense Hic-5 increased spreading in primary MEFs. These results suggested that the counterbalance of paxillin and Hic-5 expression may be a novel mechanism regulating integrin-mediated signal transduction.

Attenuation of adhesion-dependent signaling and cell spreading in transformed fibroblasts lacking protein tyrosine phosphatase-1B

Previous biochemical evidence has yielded conflicting models for the role of protein tyrosine phosphatase-1B (PTP-1B) in the regulation of integrin signaling. Thus, to establish the physiological relevance for such a role, we employed a genetic approach by generating embryonic fibroblasts from PTP-1B knockout mice. Both primary fibroblasts and their derived cell lines were used in this study. Immortalization of wild-type primary cells with the SV40 Large T antigen resulted in a dramatic increase in the endogenous expression of PTP-1B, suggesting a role during transformation. Moreover, the absence of PTP-1B in the transformed cell lines led to a more pronounced effect on different pathways of fibronectin-mediated signaling compared with the untransformed state. Specifically, p130(Cas) phosphorylation, Erk activation as well as cell spreading were delayed in PTP-1B-deficient cells, compared with their wild-type counterparts. Interestingly, this attenuation in integrin-mediated events closely resembles that of Src-deficient fibroblasts. Indeed, PTP-1B deficient, transformed fibroblasts held in suspension do exhibit a hyperphosphorylation of the inhibitory site (Tyr-527) of Src, compared with their wild-type counterparts. These results establish PTP-1B as a positive physiological regulator of integrin signaling in transformed cells, acting upstream of Src Tyr-527 dephosphorylation that leads to several adhesion-dependent events.

Reduced C-terminal Src kinase activity is correlated inversely with pp60(c-src) activity in colorectal carcinoma

BACKGROUND

Specific kinase activity of the proto-oncogene product pp60(c-src) is reported to be elevated in patients with carcinoma of the colon, and a novel cytoplasmic protein-tyrosine kinase, C-terminal Src kinase (Csk), has been found to inactivate the members of the Src family protein-tyrosine kinase. In this study, Csk activity and pp60(c-src) activity were examined in colorectal tumors as well as in colon carcinoma cell lines.

METHODS

Colorectal carcinoma tissue and adjacent nonneoplastic tissue from 24 patients, from 8 colon carcinoma cell lines, and from 1 normal colon cell line were used. The levels of pp60(c-src) and Csk in colorectal tissue and cell lines were analyzed by Western and/or Northern blot analysis, and their kinase activity levels were measured by in-gel kinase assay.

RESULTS

In the samples from 24 patients with colorectal carcinoma, pp60(c-src) kinase activity and protein levels were increased by 7.8 +/- 0.55 and 2.6 +/- 0.13 times the control levels, respectively. Conversely, the Csk protein level and its kinase activity were reduced by 0.53 +/- 0.08 and 0.53 +/- 0.09 times the control levels, respectively. pp60(c-src) kinase activity was correlated inversely with Csk activity (correlation coefficient = -0.71; P < 0.0001). Of the cell lines, pp60(c-src) kinase activity and protein levels, respectively, were 7.4 +/- 1.22 and 1.86 +/- 0.28 times greater than normal control levels. Csk protein level and kinase activity, respectively, were 0.54 +/- 0.13 and 0.52 +/- 0.11 times less normal control levels and were correlated with mRNA amount.

CONCLUSIONS

Csk mRNA, protein, and its kinase activity were reduced in colorectal carcinoma and were correlated with pp60(c-src) kinase activity level. The reduced activity of Csk may be involved in the transformation of a subset of colorectal carcinoma.

The hunting of the Src

The non-receptor tyrosine kinase Src is important for many aspects of cell physiology. The viral src gene was the first retroviral oncogene to be identified, and its cellular counterpart was the first proto-oncogene to be discovered in the vertebrate genome. Src has been important, not only as an object of study in itself, but also as an entry point into the molecular genetics of cancer.

Natural biology of polyomavirus middle T antigen

"It has been commented by someone that 'polyoma' is an adjective composed of a prefix and suffix, with no root between--a meatless linguistic sandwich" (C. J. Dawe). The very name "polyomavirus" is a vague mantel: a name given before our understanding of these viral agents was clear but implying a clear tumor life-style, as noted by the late C. J. Dawe. However, polyomavirus are not by nature tumor-inducing agents. Since it is the purpose of this review to consider the natural function of middle T antigen (MT), encoded by one of the seemingly crucial transforming genes of polyomavirus, we will reconsider and redefine the virus and its MT gene in the context of its natural biology and function. This review was motivated by our recent in vivo analysis of MT function. Using intranasal inoculation of adult SCID mice, we have shown that polyomavirus can replicate with an MT lacking all functions associated with transformation to similar levels to wild-type virus. These observations, along with an almost indistinguishable replication of all MT mutants with respect to wild-type viruses in adult competent mice, illustrate that MT can have a play subtle role in acute replication and persistence. The most notable effect of MT mutants was in infections of newborns, indicating that polyomavirus may be highly adapted to replication in newborn lungs. It is from this context that our current understanding of this well-studied virus and gene is presented.

Protein-tyrosine phosphatase D1, a potential regulator and effector for Tec family kinases

Etk, also named Bmx, is a member of the Tec tyrosine kinase family, which is characterized by a multimodular structure including a pleckstrin homology (PH) domain, an SH3 domain, an SH2 domain, and a catalytic domain. The signaling mechanisms regulating Etk kinase activity remain largely unknown. To identify factor(s) regulating Etk activity, we used the PH domain and a linker region of Etk as a bait for a yeast two-hybrid screen. Three independent clones encoding protein-tyrosine phosphatase D1 (PTPD1) fragments were isolated. The binding of PTPD1 to Etk is specific since PTPD1 cannot associate with either the Akt PH domain or lamin. In vitro and in vivo binding studies demonstrated that PTPD1 can interact with Etk and that residues 726-848 of PTPD1 are essential for this interaction. Deletion analysis of Etk indicated that the PH domain is essential for PTPD1 interaction. Furthermore, the Etk-PTPD1 interaction stimulated the kinase activity of Etk, resulting in an increased phosphotyrosine content in both factors. The Etk-PTPD1 interaction also increased Stat3 activation. The effect of PTPD1 on Etk activation is specific since PTPD1 cannot potentiate Jak2 activity upon Stat3 activation. In addition, Tec (but not Btk) kinase can also be activated by PTPD1. Taken together, these findings indicate that PTPD1 can selectively associate with and stimulate Tec family kinases and modulate Stat3 activation.

VIP enhances the differentiation of retinal pigment epithelium in culture: from cAMP and pp60(c-src) to melanogenesis and development of fluid transport capacity

The retinal pigment epithelium (RPE) is a single cell layer juxtaposed between the neural retina and the choroid and functions as a blood-retina barrier. The RPE performs functions essential for photoreceptor (PR) survival. Although the regulation of these functions has remained unknown, it is a distinct possibility that the RPE is under constant regulation by signaling molecules coming from the choroid and the retina. Vasoactive intestinal peptide (VIP), a 28-amino acid neuropeptide present in the retina and in the choroid, has been shown to promote the growth and differentiation of a variety of cells in tissue and organ cultures. In cultured RPE cells, VIP is the one most effective stimulator of the cAMP signaling pathway among a long list of neurotransmitters and modulators tested. For example, VIP, at 1 microM, stimulates the intracellular cAMP to 80-100- and 20-fold in 3 min in RPE cells cultured from chick embryos and adult human donor eyes, respectively. In cultured chick embryonic RPE, VIP is also shown to be a potent and effective modulator of pp60(c-src), the non-receptor tyrosine kinase present in differentiating and terminally differentiated cells. VIP stimulates both overall phosphorylation at unknown sites and phosphotyrosine dephosphorylation in pp60(c-src). A 190-kDa microtubule-associated protein is known to be one of the downstream targets in VIP-modulated signaling pathways. At the cellular level, VIP stimulates cell proliferation modestly and melanogenesis pronouncedly in growing chick embryonic RPE cultures. Ultimately, the differentiation goal of RPE cells in vivo is to perform functions that are essential for photoreceptor survival. On bare permeable supports (that is, without biological material coating), the chick embryonic RPE cells grow to become RPE sheets with a cytoarchitecture that allows the display of two of the RPE functions. These cultures demonstrate structural polarity and are functionally polarized, allowing for proper macromolecule secretion and fluid transport. VIP is shown to stimulate macromolecule secretion at the apical surface (retina facing) and the development of the capacity for fluid transport from the apical to the basal surface of the RPE sheet. In conclusion, studies in our laboratory indicate that VIP is a differentiation promotor during the development of a functional RPE. Recent advances in the molecular biology of melanogenesis and the fluid transport-linked Na-K-2Cl cotransporter in other cells will allow future studies of VIP modulated events in the RPE at the molecular level. Finally, identification of RPE differentiation factors may prove essential for the ultimate success of RPE transplantation, thus promoting the rescue of photoreceptor cells in retinal degeneration.

Disruption of cell-substrate adhesion activates the protein tyrosine kinase pp60(c-src)

Treatment of confluent chicken embryo fibroblasts (CEFs) with trypsin results in a dose- and time-dependent increase in c-Src protein tyrosine kinase (PTK) activity. A similar, but less marked, increase in c-Src PTK activity occurs upon incubation of CEFs in calcium-free phosphate-buffered saline, which also causes a decrease in cell-substrate adhesion. The increase in c-Src PTK activity following disruption of cell-substrate adhesion correlates with a decrease in the phosphorylation of c-Src at the regulatory site, Tyr527. The phosphotyrosine phosphatase inhibitor phenylarsine oxide blocks the increase in c-Src PTK activity seen following treatment with trypsin and the morphological changes associated with the disruption of cell-substrate adhesion. In contrast, disruption of cell-substrate adhesion causes a decrease in FAK PTK activity that rapidly returns to control levels when the cells are plated on fibronection-coated dishes. Treatment of cells with cytochalasin D, which disrupts actin filaments but not cell-substrate adhesion, causes only a slight increase in c-Src PTK activity. Thus, these studies demonstrate a ligand-independent mechanism for the activation of c-Src that is consistent with its role in both cell adhesion and cell motility. Furthermore, these data suggest that similar to adhesion, loss of adhesion is not a passive process but can activate specific signaling pathways that may have significant effects on cellular function.

A specific intermolecular association between the regulatory domains of a Tec family kinase

Interleukin-2 tyrosine kinase (Itk), is a T-cell specific tyrosine kinase of the Tec family. We have examined a novel intermolecular interaction between the SH3 and SH2 domains of Itk. In addition to the interaction between the isolated domains, we have found that the dual SH3/SH2 domain-containing fragment of Itk self-associates in a specific manner in solution. Tec family members contain the SH3, SH2 and catalytic domains common to many kinase families but are distinguished by a unique amino-terminal sequence, which contains a proline-rich stretch. Previous work has identified an intramolecular regulatory association between the proline-rich region and the adjacent SH3 domain of Itk. The intermolecular interaction between the SH3 and SH2 domains of Itk that we describe provides a possible mechanism for displacement of this intramolecular regulatory sequence, a step that may be required for full Tec kinase activation. Additionally, localization of the interacting surfaces on both the SH3 and SH2 domains by chemical shift mapping has provided information about the molecular details of this recognition event. The interaction involves the conserved aromatic binding pocket of the SH3 domain and a newly defined binding surface on the SH2 domain. The interacting residues on the SH2 domain do not conform to the consensus motif for an SH3 proline-rich ligand. Interestingly, we note a striking correlation between the SH2 residues that mediate this interaction and those residues that, when mutated in the Tec family member Btk, cause the hereditary immune disorder, X-linked agamaglobulinemia.

Adenovirus E4 open reading frame 4-induced apoptosis involves dysregulation of Src family kinases

The adenoviral early region 4 open reading frame 4 (E4orf4) death factor induces p53-independent apoptosis in many cell types and appears to kill selectively transformed cells. Here we show that expression of E4orf4 in transformed epithelial cells results in early caspase-independent membrane blebbing, associated with changes in the organization of focal adhesions and actin cytoskeleton. Evidence that E4orf4 can associate with and modulate Src family kinase activity, inhibiting Src-dependent phosphorylation of focal adhesion kinase (FAK) and paxillin while increasing phosphorylation of cortactin and some other cellular proteins, is presented. Furthermore, E4orf4 dramatically inhibited the ability of FAK and c-src to cooperate in induction of tyrosine phosphorylation of cellular substrates, suggesting that E4orf4 can interfere with the formation of a signaling complex at focal adhesion sites. Consistent with a functional role for E4orf4-Src interaction, overexpression of activated c-src dramatically potentiated E4orf4-induced membrane blebbing and apoptosis, whereas kinase dead c-src constructs inhibited E4orf4 effects on cell morphology and death. Moreover treatment of E4orf4-expressing cells with PP2, a selective Src kinase inhibitor, led to inhibition of E4orf4-dependent membrane blebbing and later to a marked decrease in E4orf4-induced nuclear condensation. Taken together, these observations indicate that expression of adenovirus 2 E4orf4 can initiate caspase-independent extranuclear manifestations of apoptosis through a modulation of Src family kinases and that these are involved in signaling E4orf4-dependent apoptosis. This study also suggests that Src family kinases are likely to play a role in the cytoplasmic execution of apoptotic programs.

Nonreceptor tyrosine protein kinase pp60c-src in spatial learning: synapse-specific changes in its gene expression, tyrosine phosphorylation, and protein-protein interactions

c-src is a nonreceptor tyrosine protein kinase that is highly concentrated in synaptic regions, including synaptic vesicles and growth cones. Here, we report that the mRNA signal of pp60c-src is widely distributed in the rat brain with particularly high concentrations in the hippocampus. After spatial maze learning, up-regulation of c-src mRNA was observed in the CA3 region of the hippocampus, which was accompanied by increases in pp60c-src protein in hippocampal synaptosomal preparations. Training also triggered an increase in c-src protein tyrosine kinase activity that was correlated with its tyrosine dephosphorylation in the synaptic membrane fraction. After training, pp60c-src from hippocampus showed enhanced interactions with synaptic proteins such as synapsin I, synaptophysin, and the type 2 N-methyl-d-aspartate receptor, as well as the cytoskeletal protein actin. The association of pp60c-src with insulin receptor in the synaptic membrane fraction, however, was temporally decreased after training. Furthermore, in vitro results showed that Ca(2+) and protein kinase C might be involved in the regulation of protein-protein interactions of pp60c-src. These results suggest, therefore, that pp60c-src participates in the regulation of hippocampal synaptic activity during learning and memory.

Overexpression of C-terminal Src kinase blocks 14, 15-epoxyeicosatrienoic acid-induced tyrosine phosphorylation and mitogenesis

We have previously reported that 14,15-epoxyeicosatrienoic acid (14, 15-EET) is a potent mitogen for the renal epithelial cell line, LLCPKcl4. This mitogenic effect is dependent upon activation of a protein-tyrosine kinase cascade that results in activation of mitogen-activated protein kinase and phosphatidylinositol 3-kinase. Because of suggestive evidence that 14,15-EET also activated Src in these cells, we stably transfected LLCPKcl4 with an expression construct of the C-terminal Src kinase (CSK), which inhibits Src family kinase activity. In vitro Src kinase activity assays confirmed that in empty vector-transfected cells (Vector cells), 14, 15-EET increased Src kinase activity, while in clones overexpressing CSK mRNA and immunoreactive protein (CSK cells), 14,15-EET-induced activation of Src was almost completely blocked (94% inhibition). Of interest, epidermal growth factor (EGF) and fetal bovine serum (FBS) also increased Src activity in Vector cells, but not in CSK cells, further confirming the ability of CSK overexpression to prevent Src activation. CSK cells failed to increase [(3)H]thymidine incorporation in response to exogenous 14,15-EET. In contrast, both EGF and FBS significantly increased [(3)H]thymidine incorporation in CSK cells. Immunoprecipitation with anti-phosphotyrosine antibodies and immunoblotting with an antibody against extracellular signal-regulated kinase (ERK) indicated that in CSK cells, 14,15-EET failed to activate ERK1 and ERK2; however, EGF- and FBS-induced activation of ERKs was not different from that seen in Vector cells. In Vector cells, the 14,15-EET-stimulated tyrosine phosphorylation of ERKs was blocked by pretreatment with 1 microm PP2, a selective inhibitor of Src kinases. The present study demonstrates that 14, 15-EET exerts its mitogenic effects predominantly through a Src kinase-mediated pathway, which is the most upstream signaling step determined to date in the 14,15-EET-activated tyrosine kinase cascade in renal epithelial cells.

beta-arrestin1 interacts with the catalytic domain of the tyrosine kinase c-SRC. Role of beta-arrestin1-dependent targeting of c-SRC in receptor endocytosis

beta-Arrestins can act as adapter molecules, coupling G-protein-coupled receptors to proteins involved in mitogenic as well as endocytic pathways. We have previously identified c-SRC as a molecule that is rapidly recruited to the beta2-adrenergic receptor in a beta-arrestin1-dependent manner. Recruitment of c-SRC to the receptor appears to be involved in pathways leading to receptor internalization and mitogen-activated protein kinase activation. This recruitment of c-SRC to the receptor involves an interaction between the amino-terminal proline-rich region of beta-arrestin1 and the Src homology 3 (SH3) domain of c-SRC, but deletion of the proline-rich domain does not totally ablate the interaction. We have found that a major interaction also exists between beta-arrestin1 and the catalytic or kinase domain (SH1) of c-SRC. We therefore hypothesized that a catalytically inactive mutant of the isolated catalytic subunit, SH1(kinase dead) (SH1(KD)), would specifically block those cellular actions of c-SRC that are mediated by beta-arrestin1 recruitment to the G-protein-coupled receptor. In contrast, the majority of cellular phosphorylations catalyzed by c-SRC, which do not involve interaction with the SH1 domain, would be predicted to be unaffected. The SH1(KD) mutant did indeed block beta2-adrenergic receptor internalization and receptor-stimulated tyrosine phosphorylation of dynamin, actions previously shown to be c-SRC-dependent. In contrast, SAM-68 and whole cell tyrosine phosphorylation by c-SRC was unaffected, indicating that the SH1(KD) mutant did not inhibit c-SRC tyrosine kinase activity in general. These results not only clarify the nature of the beta-arrestin1/c-SRC interaction but also implicate beta-arrestin1 as an important mediator of receptor internalization by recruiting tyrosine kinase activity to the cell surface to phosphorylate key endocytic intermediates, such as dynamin.

No evidence of correlation between mutation at codon 531 of src and the risk of colon cancer in Chinese

The protein tyrosine kinase activity of c-src proto-oncogene product, pp60(c-src), is elevated in a number of human cancers, including colon cancer. Phosphorylation of human pp60(c-src) carboxy-terminal tyrosine 530 suppresses its kinase activity. A recent report suggested that the risk of colon cancer is higher for those who carry a C-->T transition mutation on codon 531 (Gln-531-->Amber-531) of src gene. This mutation caused a prematured translation termination and up-regulated the kinase activity. To examine whether this mutation could be a risk factor for colon carcinoma in the Chinese population, we used the same PCR-based assay to analyze src genotypes of 131 colon cancers and other various types of carcinoma. No mutation was detected in all specimens that were screened in this study. Thus, mutation at Gln-531 of src gene does not seem to be involved in the development of colon cancer in Chinese ethnicity.

Virocrine transformation: the intersection between viral transforming proteins and cellular signal transduction pathways

This review describes a mechanism of viral transformation involving activation of cellular signaling pathways. We focus on four viral oncoproteins: the E5 protein of bovine papillomavirus, which activates the platelet-derived growth factor beta receptor; gp55 of spleen focus forming virus, which activates the erythropoietin receptor; polyoma virus middle T antigen, which resembles an activated receptor tyrosine kinase; and LMP-1 of Epstein-Barr virus, which mimics an activated tumor necrosis factor receptor. These examples indicate that diverse viruses induce cell transformation by activating cellular signal transduction pathways. Study of this mechanism of viral transformation will provide new insights into viral tumorigenesis and cellular signal transduction.