Src activity increases and Yes activity decreases during mitosis of human colon carcinoma cells

Using BioPAX-Parser (BiP) to enrich lists of genes or proteins with pathway data

BACKGROUND

Pathway enrichment analysis (PEA) is a well-established methodology for interpreting a list of genes and proteins of interest related to a condition under investigation. This paper aims to extend our previous work in which we introduced a preliminary comparative analysis of pathway enrichment analysis tools. We extended the earlier work by providing more case studies, comparing BiP enrichment performance with other well-known PEA software tools.

METHODS

PEA uses pathway information to discover connections between a list of genes and proteins as well as biological mechanisms, helping researchers to overcome the problem of explaining biological entity lists of interest disconnected from the biological context.

RESULTS

We compared the results of BiP with some existing pathway enrichment analysis tools comprising Centrality-based Pathway Enrichment, pathDIP, and Signaling Pathway Impact Analysis, considering three cancer types (colorectal, endometrial, and thyroid), for a total of six datasets (that is, two datasets per cancer type) obtained from the The Cancer Genome Atlas and Gene Expression Omnibus databases. We measured the similarities between the overlap of the enrichment results obtained using each couple of cancer datasets related to the same cancer.

CONCLUSION

As a result, BiP identified some well-known pathways related to the investigated cancer type, validated by the available literature. We also used the Jaccard and meet-min indices to evaluate the stability and the similarity between the enrichment results obtained from each couple of cancer datasets. The obtained results show that BiP provides more stable enrichment results than other tools.

Tyrosine phosphorylation of lamin A by Src promotes disassembly of nuclear lamina in interphase

Lamins form the nuclear lamina, which is important for nuclear structure and activity. Although posttranslational modifications, in particular serine phosphorylation, have been shown to be important for structural properties and functions of lamins, little is known about the role of tyrosine phosphorylation in this regard. In this study, we found that the constitutively active Src Y527F mutant caused the disassembly of lamin A/C. We demonstrate that Src directly phosphorylates lamin A mainly at Tyr45 both in vitro and in intact cells. The phosphomimetic Y45D mutant was diffusively distributed in the nucleoplasm and failed to assemble into the nuclear lamina. Depletion of lamin A/C in HeLa cells induced nuclear dysmorphia and genomic instability as well as increased nuclear plasticity for cell migration, all of which were partially restored by re-expression of lamin A, but further promoted by the Y45D mutant. Together, our results reveal a novel mechanism for regulating the assembly of nuclear lamina through Src and suggest that aberrant phosphorylation of lamin A by Src may contribute to nuclear dysmorphia, genomic instability, and nuclear plasticity.

Linkage disequilibrium and haplotype analysis of Src and Yes1 genes in thyroid cancer

Purpose: This study was planned to examine the effects of Src and Yes1 single nucleotide polymorphism (SNPs) on the risk of thyroid cancer in 499 patients and 500 controls. Materials & methods: Three SNPs of Src gene and three SNPs of Yes1 gene were analyzed using Tetra-primer ARMS-PCR followed by sequencing. Results: rs121913314 of Src gene genotype TT showed 32-fold increased risk of thyroid cancer and rs2305994 of Yes1 genotypes TT and CT showed 2.7-fold and 16-fold increased risk in thyroid cancer (p < 0.0001). Haplotype analysis revealed that CATGCC, CATGCT, CATGTC, CATGTT, TATGCC and TATGTTA haplotypes are associated with thyroid cancer risk. Conclusion: Results showed that genotypes and allele distribution of Src and Yes1 genes are significantly linked with increased risk of thyroid cancer.

1,4-dihydroxy quininib attenuates growth of colorectal cancer cells and xenografts and regulates the TIE-2 signaling pathway in patient tumours

Colorectal cancer (CRC) is the second leading cause of cancer associated deaths in developed countries. Cancer progression and metastatic spread is reliant on new blood vasculature, or angiogenesis. Tumour-related angiogenesis is regulated by pro- and anti-angiogenic factors secreted from malignant tissue in a stepwise process. Previously we structurally modified the small anti-angiogenic molecule quininib and discovered a more potent anti-angiogenic compound 1, 4 dihydroxy quininib (Q8), an antagonist of cysteinyl leukotriene receptor-1 with VEGF-independent bioactivity. Here, Q8, quininib (Q1) and five structural analogues were assayed for anti-tumorigenic effects in pre-clinical cancer models. Q8 reduced clone formation of the human colorectal cancer cell line HT29-Luc2. Gene silencing of CysLT₁ in HT29-Luc2 cells significantly reduced expression of calpain-2. In human ex vivo colorectal cancer tumour explants, Q8 significantly decreased the secretion of both TIE-2 and VCAM-1 expression. In vivo Q8 was well tolerated up to 50 mg/kg by Balb/C mice and significantly more effective at reducing tumour volume in colorectal tumour xenografts compared to the parent drug quininib. In tumour xenografts, Q8 significantly reduced expression of the angiogenic marker calpain-2. In summary, we propose Q8 may act on the TIE-2-Angiopoietin signalling pathway to significantly inhibit the process of tumour angiogenesis in colorectal cancer.

Tyrosine Phosphorylation of Caspase-8 Abrogates Its Apoptotic Activity and Promotes Activation of c-Src

Src family tyrosine kinases (SFKs) phosphorylate caspase-8A at tyrosine (Y) 397 resulting in suppression of apoptosis. In addition, the phosphorylation of caspase-8A at other sites including Y465 has been implicated in the regulation of caspase-8 activity. However, the functional consequences of these modifications on caspase-8 processing/activity have not been elucidated. Moreover, various Src substrates are known to act as potent Src regulators, but no such role has been explored for caspase-8. We asked whether the newly identified caspase-8 phosphorylation sites might regulate caspase-8 activation and conversely, whether caspase-8 phosphorylation might affect Src activity. Here we show that Src phosphorylates caspase-8A at multiple tyrosine sites; of these, we have focused on Y397 within the linker region and Y465 within the p12 subunit of caspase-8A. We show that phosphomimetic mutation of caspase-8A at Y465 prevents its cleavage and the subsequent activation of caspase-3 and suppresses apoptosis. Furthermore, simultaneous phosphomimetic mutation of caspase-8A at Y397 and Y465 promotes the phosphorylation of c-Src at Y416 and increases c-Src activity. Finally, we demonstrate that caspase-8 activity prevents its own tyrosine phosphorylation by Src. Together these data reveal that dual phosphorylation converts caspase-8 from a pro-apoptotic to a pro-survival mediator. Specifically, tyrosine phosphorylation by Src renders caspase-8 uncleavable and thereby inactive, and at the same time converts it to a Src activator. This novel dynamic interplay between Src and caspase-8 likely acts as a potent signal-integrating switch directing the cell towards apoptosis or survival.

Functional activation of Src family kinase yes protein is essential for the enhanced malignant properties of human melanoma cells expressing ganglioside GD3

The possible roles of Src family kinases in the enhanced malignant properties of melanomas related to GD3 expression were analyzed. Among Src family kinases only Yes, not Fyn or Src, was functionally involved in the increased cell proliferation and invasion of GD3-expressing transfectant cells (GD3+). Yes was located upstream of p130Cas and paxillin and at an equivalent level to focal adhesion kinase. Yes underwent autophosphorylation even before serum treatment and showed stronger kinase activity in GD3+ cells than in GD3- cells following serum treatment. Coimmunoprecipitation experiments revealed that Yes bound to focal adhesion kinase or p130Cas more strongly in GD3+ cells than in GD3- cells. As a possible mechanism for the enhancing effects of GD3 on cellular phenotypes, it was shown that majority of Yes was localized in glycolipid-enriched microdomain/rafts in GD3+ cells even before serum treatment, whereas it was scarcely detected in glycolipid-enriched microdomain/rafts in GD3- cells. An in vitro kinase assay of Yes revealed that coexistence of GD3 with Yes in membranous environments enhances the kinase activity of GD3- cell-derived Yes toward enolase, p125, and Yes itself. Knockdown of GD3 synthase resulted in the alleviation of tumor phenotypes and reduced activation levels of Yes. Taken together, these results suggest a role of GD3 in the regulation of Src family kinases.

Cross talk between receptor guanylyl cyclase C and c-src tyrosine kinase regulates colon cancer cell cytostasis

Increased activation of c-src seen in colorectal cancer is an indicator of a poor clinical prognosis, suggesting that identification of downstream effectors of c-src may lead to new avenues of therapy. Guanylyl cyclase C (GC-C) is a receptor for the gastrointestinal hormones guanylin and uroguanylin and the bacterial heat-stable enterotoxin. Though activation of GC-C by its ligands elevates intracellular cyclic GMP (cGMP) levels and inhibits cell proliferation, its persistent expression in colorectal carcinomas and occult metastases makes it a marker for malignancy. We show here that GC-C is a substrate for inhibitory phosphorylation by c-src, resulting in reduced ligand-mediated cGMP production. Consequently, active c-src in colonic cells can overcome GC-C-mediated control of the cell cycle. Furthermore, docking of the c-src SH2 domain to phosphorylated GC-C results in colocalization and further activation of c-src. We therefore propose a novel feed-forward mechanism of activation of c-src that is induced by cross talk between a receptor GC and a tyrosine kinase. Our findings have important implications in understanding the molecular mechanisms involved in the progression and treatment of colorectal cancer.

The transmembrane src substrate Trask is an epithelial protein that signals during anchorage deprivation

The roles of epithelial cells encompass both cellular- and tissue-level functions that involve numerous cell-cell and cell-matrix interactions, which ultimately mediate the highly structured arrangement of cells on a basement membrane. Although maintaining this basic structure is critical for preserving tissue integrity, plasticity in epithelial cell behavior is also critical for processes such as cell migration during development or wound repair, mitotic cell detachment, and physiological shedding. The mechanisms that mediate epithelial cell plasticity are only beginning to be understood. We previously identified Trask, a transmembrane protein that is phosphorylated by src kinases during mitosis. In this study, we report that the phosphorylation of Trask is associated with anchorage loss in epithelial cells. Phosphorylation of Trask is seen during the cell-detachment phase of mitosis, in experimentally induced interphase detachment, and during cell migration in experimental epithelial models. An analysis of human tissues shows that Trask is widely expressed in many epithelial tissues but not in most tissues of mesenchymal origin, except for a subset of early hematopoietic cells. Trask is not phosphorylated in epithelial tissues in vivo; however, its phosphorylation is seen in epithelial cells undergoing mitosis or physiological shedding. Trask is a novel epithelial membrane protein that is phosphorylated by src kinases when epithelial cells disengage from their tissue framework, identifying an important new regulator of epithelial tissue dynamics.

Potential molecular mechanism for c-Src kinase-mediated regulation of intestinal cell migration

The ubiquitously expressed Src tyrosine kinases (c-Src, c-Yes, and c-Fyn) regulate intestinal cell growth and differentiation. Src activity is also elevated in the majority of malignant and premalignant tumors of the colon. The development of fibroblasts with the three ubiquitously expressed kinases deleted (SYF cells) has identified the role of Src proteins in the regulation of actin dynamics associated with increased cell migration and invasion. Despite this, unexpectedly nothing is known about the role of the individual Src kinases on intestinal cell cytoskeleton and/or cell migration. We have previously reported that villin, an epithelial cell-specific actin-modifying protein that regulates actin reorganization, cell morphology, cell migration, cell invasion, and apoptosis, is tyrosine-phosphorylated. In this report using the SYF cells reconstituted individually with c-Src, c-Yes, c-Fyn, and wild type or phosphorylation site mutants of villin, we demonstrate for the first time the absolute requirement for c-Src in villin-induced regulation of cell migration. The other major finding of our study is that contrary to previous reports, the nonreceptor tyrosine kinase, Jak3 (Janus kinase 3), does not regulate phosphorylation of villin or villin-induced cell migration and is, in fact, not expressed in intestinal epithelial cells. Further, we identify SHP-2 and PTP-PEST (protein-tyrosine phosphatase proline-, glutamate-, serine-, and threonine-rich sequence) as negative regulators of c-Src kinase and demonstrate a new function for these phosphatases in intestinal cell migration. Together, these data suggest that in colorectal carcinogenesis, elevation of c-Src or down-regulation of SHP-2 and/or PTP-PEST may promote cancer metastases and invasion by regulating villin-induced cell migration and cell invasion.

Increases in c-Yes expression level and activity promote motility but not proliferation of human colorectal carcinoma cells

Increases in the levels and/or activity of nonreceptor tyrosine kinases c-Src and c-Yes are often associated with colorectal carcinogenesis. The physiological consequences of increased c-Yes activity during the early and late stages of tumorigenesis, in addition to the degree of redundancy between c-Yes and c-Src in colorectal cancer cells, remain elusive. To study the consequences of increases in c-Yes levels and activity in later stages of colorectal carcinogenesis, we developed human colorectal cancer cell lines in which c-Yes levels and activity can be inducibly increased by a tightly controlled expression of wild-type c-Yes or by constitutively active mutants of c-Yes, c-YesY537F, and c-Yes Delta t6aa. c-Yes induction resulted in increased cell motility but did not promote proliferation either in vitro or in vivo. These results suggest that in later stages of colorectal carcinogenesis, elevations in c-Yes levels/activity may promote cancer spread and metastasis rather than tumor growth.

Increases in c-Src expression level and activity do not promote the growth of human colorectal carcinoma cells in vitro and in vivo

The levels and activity of c-Src in colorectal cancer cells increase steadily during the course of colorectal carcinogenesis and are most highly elevated in advanced metastatic disease. However, the effects of increases in c-Src activity on the proliferation of colorectal cancer cells during early and late stages of tumorigenesis remain elusive. To study the consequences of increases in c-Src levels and activity on the growth of colorectal cancer cells in later stages of colorectal carcinogenesis, we developed human colorectal cancer cell lines in which c-Src levels and activity could be inducibly increased by a tightly controlled expression of wild-type c-Src or of the constitutively active mutant of c-Src, c-SrcY527F. Src induction activated multiple signaling pathways (often associated with a proliferative response) but promoted neither cell proliferation in vitro nor tumor growth in a xenograft model in vivo. These results indicate that, in more advanced stages of colorectal carcinogenesis, increases in c-Src levels and activity are likely to have functions other than the direct promotion of tumor growth.

RACK1 inhibits colonic cell growth by regulating Src activity at cell cycle checkpoints

Previously, we showed that Src tyrosine kinases are activated early in the development of human colon cancer and are suppressed as intestinal cells differentiate. We identified RACK1 as an endogenous substrate, binding partner and inhibitor of Src. Here we show (by overexpressing RACK1, depleting Src or RACK1 and utilizing cell-permeable peptides that perturb RACK1's interaction with Src) that RACK1 regulates growth of colon cells by suppressing Src activity at G(1) and mitotic checkpoints, and consequently delaying cell cycle progression. Activated Src rescues RACK1-inhibited growth of HT-29 cells. Conversely, inhibiting Src abolishes growth promoted by RACK1 depletion in normal cells. Two potential mechanisms whereby RACK1 regulates mitotic exit are identified: suppression of Src-mediated Sam68 phosphorylation and maintenance of the cyclin-dependent kinase (CDK) 1-cyclin B complex in an active state. Our results reveal novel mechanisms of cell cycle control in G(1) and mitosis of colon cells. The significance of this work lies in the discovery of a mechanism by which the growth of colon cancer cells can be slowed, by RACK1 suppression of an oncogenic kinase at critical cell cycle checkpoints. Small molecules that mimic RACK1 function may provide a powerful new approach to the treatment of colon cancer.

Tyrosine phosphorylated Par3 regulates epithelial tight junction assembly promoted by EGFR signaling

The conserved polarity complex, comprising the partitioning-defective (Par) proteins Par3 and Par6, and the atypical protein kinase C, functions in various cell-polarization events and asymmetric cell divisions. However, little is known about whether and how external stimuli-induced signals may regulate Par3 function in epithelial cell polarity. Here, we found that Par3 was tyrosine phosphorylated through phosphoproteomic profiling of pervanadate-induced phosphotyrosine proteins. We also demonstrated that the tyrosine phosphorylation event induced by multiple growth factors including epidermal growth factor (EGF) was dependent on activation of Src family kinase (SFK) members c-Src and c-Yes. The tyrosine residue 1127 (Y1127) of Par3 was identified as the major EGF-induced phosphorylation site. Moreover, we found that Y1127 phosphorylation reduced the association of Par3 with LIM kinase 2 (LIMK2), thus enabling LIMK2 to regulate cofilin phosphorylation dynamics. Substitution of Y1127 for phenylalanine impaired the EGF-induced Par3 and LIMK2 dissociation and delayed epithelial tight junction (TJ) assembly considerably. Collectively, these data suggest a novel, phosphotyrosine-dependent fine-tuning mechanism of Par3 in epithelial TJ assembly controlled by the EGF receptor-SFK signaling pathway.

Involvement of the SH3 domain in Ca2+-mediated regulation of Src family kinases

When cells are treated with Ca(2+) and Ca(2+)-ionophore, c-Src kinase activity increases, whereas c-Yes kinase activity decreases. This opposite modulation can be reproduced in an in vitro reconstitution assay and is dependent on Ca(2+) and on soluble factors present in cell lysates. Since c-Src and c-Yes share a high degree of homology, with the exception of their N-terminal "unique" domains, their activity was thought to be coordinately regulated. To assess the mechanism of regulation we generated stable cell lines expressing eight different constructs containing wild type c-Src and c-Yes, as well as swaps of the unique domain alone, unique and Src homology 3 (SH3) domains together and the SH3 domain alone. Swapping of the unique domains was not sufficient to reverse the regulation of the chimeric molecules. On the other hand, chimeras containing swaps of the unique plus the SH3 domains displayed reverse regulation, implicating both domains in the regulation of kinase activity by Ca(2+). To rule out the participation of the unique domain, we used chimeric molecules with swapped SH3 domains only and found that the SH3 domain is necessary and sufficient to confer Ca(2+)-mediated regulation of Src and Yes tyrosine kinases.

The c-Yes 3'-UTR contains adenine/uridine-rich elements that bind AUF1 and HuR involved in mRNA decay in breast cancer cells

c-Yes is a member of the c-Src family of tyrosine kinases and has been implicated in intracellular signaling, cell morphology, and adhesion. Changes in its expression have also been associated with the aggressiveness of human breast and colon cancer cells. In MDA-MB-231 human breast cancer cells, overexpression of the small heat shock protein 27 (hsp27) results in a downregulation of c-Yes levels, concomitant with increased in vitro invasiveness and in vivo metastatic behavior. Very little is known, however, about the mechanisms regulating c-Yes expression. Here, we demonstrate that hsp27-induced c-Yes downregulation is not due to a reduction in transcriptional activity. However, the 3'-untranslated region (3'-UTR) of the c-Yes gene may be involved in its own regulation, since this region affects heterologous reporter gene activity in transactivation assays. This down-regulatory effect maps to three adenine/uridine-rich elements (AREs) that bind to cellular HuR and AUF1 (hnRNP D), two ARE-binding proteins (ARE-BPs) implicated in accelerated mRNA degradation. Our results suggest that the c-Yes 3'-UTR contains at least three newly identified AREs which are bound specifically by ARE-BPs, and provide a structural basis for post-transcriptional regulation of c-Yes expression.

Adhesion signaling by a novel mitotic substrate of src kinases

Src kinases are activated and relocalize to the cytoplasm during mitosis, but their mitotic function has remained elusive. We describe here a novel mitotic substrate of src kinases. Trask (transmembrane and associated with src kinases) is a 140 kDa type I transmembrane glycoprotein unrelated to currently known protein families. Src kinases phosphorylate Trask in vitro and mediate its mitotic hyperphosphorylation in vivo. Trask associates with both yes and src, is localized to the cell membrane during interphase, and undergoes cytoplasmic relocalization during mitosis. Overexpression of Trask leads to cell rounding and a loss of adhesion phenotype. Consistent with a function in cell adhesion, Trask interacts with a number of adhesion and matrix proteins including cadherins, syndecans, and the membrane-type serine protease 1 (MT-SP1), and is proteolytically cleaved by MT-SP1. Trask is unique among cell adhesion molecules in that it is under cell cycle regulation and thus links src kinases with the mitotic regulation of cell adhesion. This suggests a potential pathway by which hyperactive src kinases in tumors can deregulate adhesion signaling and mediate the metastatic phenotype.

RACK1 regulates G1/S progression by suppressing Src kinase activity

Cancer genes exert their greatest influence on the cell cycle by targeting regulators of a critical checkpoint in late G(1). Once cells pass this checkpoint, they are fated to replicate DNA and divide. Cancer cells subvert controls at work at this restriction point and remain in cycle. Previously, we showed that RACK1 inhibits the oncogenic Src tyrosine kinase and NIH 3T3 cell growth. RACK1 inhibits cell growth, in part, by prolonging G(0)/G(1). Here we show that RACK1 overexpression induces a partial G(1) arrest by suppressing Src activity at the G(1) checkpoint. RACK1 works through Src to inhibit Vav2, Rho GTPases, Stat3, and Myc. Consequently, cyclin D1 and cyclin-dependent kinases 4 and 2 (CDK4 and CDK2, respectively) are suppressed, CDK inhibitor p27 and retinoblastoma protein are activated, E2F1 is sequestered, and G(1)/S progression is delayed. Conversely, downregulation of RACK1 by short interference RNA activates Src-mediated signaling, induces Myc and cyclin D1, and accelerates G(1)/S progression. RACK1 suppresses Src- but not mitogen-activated protein kinase-dependent platelet-derived growth factor signaling. We also show that Stat3 is required for Rac1 induction of Myc. Our results reveal a novel mechanism of cell cycle control in late G(1) that works via an endogenous inhibitor of the Src kinase.

pp60c-Src Kinase mediates growth effects of the full-length precursor progastrin1-80 peptide on rat intestinal epithelial cells, in vitro

Growth factor effects of precursor forms of gastrins have become evident in recent years. However, intracellular pathways that mediate growth effects of the precursor molecules are not known. In previous studies, we reported an increase in Tyr phosphorylation of pp60(c-Src) in intestinal epithelial cells (IEC) in response to the fully processed form of gastrin [gastrin(1-17) (G17)]. We have now examined whether c-Src kinase is similarly phosphorylated and activated in response to the full-length precursor molecule, progastrin (PG)(1-80), (recombinant human PG) in IEC cells. We found a significant increase in pp60(c-Src) kinase activity in response to both G17 and PG (0.1-1.0 nM), suggesting that growth effects of both the precursor and fully processed gastrin molecules may be mediated via similar pathways. On the other hand, pp62(c-Yes) was not phosphorylated or activated in response to either G17 or PG. To examine whether c-Src kinase mediates proliferative effects of PG, IEC cells were microinjected with anti-Src-IgG and (3)H-thymidine ((3)H-Tdr) uptake of the cells measured. Control cells received nonimmune IgG. The (3)H-Tdr uptake of cells stimulated with 1.0 nM PG was significantly reduced in cells microinjected with anti-c-Src-IgG; control IgG had no effect. In cells stimulated with 1.0% fetal calf serum, microinjection with c-Src-IgG had no effect on (3)H-Tdr uptake. The specificity of the effect was further confirmed by blocking the inhibitory effect of anti-c-Src-IgG with antigenic Src peptide. These results suggest that activation of c-Src kinase likely represents a critical step in mediating proliferative effects of both the precursor and fully processed forms of gastrins on IEC.

Elevated c-Src is linked to altered cell-matrix adhesion rather than proliferation in KM12C human colorectal cancer cells

Elevated expression and/or activity of c-Src, the prototype of the Src family of protein tyrosine kinases, is associated with the development of human colon cancer. However, despite the known pleiotropic effects of these kinases in promoting (a) cell growth downstream of growth factor receptors, and (b) the dynamic regulation of integrin adhesions in fibroblast model systems, their precise role in epithelial cancer cells is unknown. Here we addressed whether elevated expression and activity of cellular Src alters cell proliferation and/or cell-matrix adhesion in cancer cells from the Fidler model of colorectal metastasis. Although elevated Src correlates with ability to metastasise to the liver after intrasplenic injection, we found that this was not linked to enhanced growth, either in vitro or in vivo as sub-cutaneous tumours. However, elevated Src was associated with enhanced attachment to extracellular matrix. In addition, adhesion to fibronectin, was suppressed by agents that inhibited Src activity, while enforced elevation of Src in non-metastatic cells was sufficient to stimulate adhesion to fibronectin and enhanced assembly of adhesion complexes, without influencing cell growth. Thus, we conclude that one role of elevated Src in human colon cancer cells is to modulate integrin-dependent cell-matrix attachment and formation of adhesion structures, which may, in turn, influence cell motility and integrin-dependent cellular responses.

RACK1: a novel substrate for the Src protein-tyrosine kinase

RACK1 is one of a group of PKC-interacting proteins collectively called RACKs (Receptors for Activated C-Kinases). Previously, we showed that RACK1 also interacts with the Src tyrosine kinase, and is an inhibitor of Src activity and cell growth. PKC activation induces the intracellular movement and co-localization of RACK1 and Src, and the tyrosine phosphorylation of RACK1. To determine whether RACK1 is a Src substrate, we assessed phosphorylation of RACK1 by various tyrosine kinases in vitro, and by kinase-active and inactive mutants of Src in vivo. We found that RACK1 is a Src substrate. Moreover, Src activity is necessary for both the tyrosine phosphorylation of RACK1 and the binding of RACK1 to Src's SH2 domain that occur following PKC activation. To identify the tyrosine(s) on RACK1 that is phosphorylated by Src, we generated and tested a series of RACK1 mutants. We found that Src phosphorylates RACK1 on Tyr 228 and/or Tyr 246, highly-conserved tyrosines located in the sixth WD repeat that interact with Src's SH2 domain. We think that RACK1 is an important Src substrate that signals downstream of growth factor receptor tyrosine kinases and is involved in the regulation of Src function and cell growth.

Gut-enriched Kruppel-like factor represses ornithine decarboxylase gene expression and functions as checkpoint regulator in colonic cancer cells

Gut-enriched Krüppel-like factor (GKLF, KLF4) is an epithelial-specific transcription factor that expresses in the gastrointestinal tract and mediates growth arrest of colonic epithelium. The molecular mechanisms governing its growth inhibitory effect have not been fully elucidated. In the present study, we showed that induction of GKLF mRNA and protein expression by interferon-gamma treatment was associated with reduction of ornithine decarboxylase (ODC) gene expression and enzyme activity in colon cancer HT-29 cells. Overexpression of GKLF in HT-29 cells significantly reduced ODC mRNA and protein levels as well as enzyme activity and resulted in growth arrest, indicating that ODC might be a downstream target of GKLF. This conclusion was further supported by data showing that GKLF mRNA and protein concentrations were the highest at the G(1)/S boundary of the cell cycle, where ODC mRNA and protein levels were the lowest and that overexpression of GKLF resulted in cell arrested at the G(1) phase. Reporter gene transfection studies and electrophoretic mobility gel shift assays demonstrated that GKLF repressed ODC promoter activity and that these effects appeared to be mediated through interaction with a GC box in the proximal portion of the promoter. Transfection studies using reporter constructs and chromatin immunoprecipitation assays also demonstrated that GKLF inhibited transactivation of the ODC gene by interfering with the binding of Sp1 to the ODC promoter. These results indicate that GKLF may function as a G(1)/S checkpoint regulator and exert its growth arrest effect through down-regulation of ODC gene expression. Furthermore, GKLF is a transcriptional repressor of the ODC gene, and these effects are mediated by interaction with the GC-rich region on the promoter.

Src in cancer: deregulation and consequences for cell behaviour

Considerable evidence now implicates elevated expression and/or activity of Src in cancer development. In cells, endogenous Src is switched from an inactive to an active state by a variety of mechanisms that simultaneously relieve constraints on the kinase and protein-interacting Src homology (SH) domains. As a result, Src is translocated to the cell periphery, often to sites of cell adhesion, where myristylation mediates attachment to the inner surface of the plasma membrane. From these peripheral sites, Src's catalytic activity initiates intracellular signal transduction pathways that influence cell growth and adhesion strength, the latter contributing to control of cell migration. De-regulation in cancer cells may therefore enhance tumour growth and/or stimulate migratory or invasive potential in cells that would normally be relatively non-motile. Evidence now exists to suggest that Src may also influence the life or death decisions that cells make during many biological processes. Thus, Src modulation in cancer cells can alter cell responses that are often perturbed in cancer. Consequently, there is optimism that drugs which inhibit Src's kinase activity, or the activity of its downstream effectors, might have profound effects on cancer cell behaviour and be useful therapeutic agents.

The interaction of Src and RACK1 is enhanced by activation of protein kinase C and tyrosine phosphorylation of RACK1

RACK1 is an intracellular receptor for the serine/ threonine protein kinase C. Previously, we demonstrated that RACK1 also interacts with the Src protein-tyrosine kinase. RACK1, via its association with these protein kinases, may play a key role in signal transduction. To further characterize the Src-RACK1 interaction and to analyze mechanisms by which cross-talk occurs between the two RACK1-linked signaling kinases, we identified sites on Src and RACK1 that mediate their binding, and factors that regulate their interaction. We found that the interaction of Src and RACK1 is mediated, in part, by the SH2 domain of Src and by phosphotyrosines in the sixth WD repeat of RACK1, and is enhanced by serum or platelet-derived growth factor stimulation, protein kinase C activation, and tyrosine phosphorylation of RACK1. To the best of our knowledge, this is the first report of tyrosine phosphorylation of a member of the WD repeat family of proteins. We think that tyrosine phosphorylation of these proteins is an important mechanism of signal transduction in cells.

Role of Src expression and activation in human cancer

Since the original identification of a transmissible agent responsible for the development of tumors in chickens, now known to be a retrovirus encoding the v-src gene, significant progress has been made in defining the potential functions of its human homolog, SRC. The product of the human SRC gene, c-Src, is found to be over-expressed and highly activated in a wide variety of human cancers. The relationship between Src activation and cancer progression appears to be significant. Moreover, Src may have an influence on the development of the metastatic phenotype. This review discusses the data supporting a role for c-Src as a critical component of the signal transduction pathways that control cancer cell development and growth, and provides the rationale for targeting Src in drug discovery efforts.

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.

A functional significance for codon third bases

Most amino acids are specified by more than one trinucleotide codon. Here we show that amino acids of differing functional importance may be distinguished by the pattern of synonymous codon usage. GC-rich genes tend to be of a greater transcriptional (p<0.01) and mitogenic (p<0.0001) significance than AT-rich genes, consistent with GC-->AT mutational drift in methylated genomic regions. Third-base GC retention also identifies critical amino acids within individual proteins, as indicated by non-random patterns of codon variation between gene homologs and also by differential sequelae of site-directed mutagenesis. Sequence analysis of human receptor tyrosine kinase genes confirms that functionally important transmembrane hydrophobic amino acids are specified by codons containing GC third bases more often than are transmembrane neutral amino acids (chi(2)=134.2). Amino acids encoded by GC third bases thus appear more tightly linked to cell function and survival than are those encoded by AT third bases.

Kinase inhibitors in cancer therapy: a look ahead

The most essential kinases involved in cell membrane receptor activation, signal transduction and cell cycle control or programmed cell death and their interconnections are reviewed. In tumours, the genes of many of those kinases are mutated or amplified or the proteins are overexpressed. The use of key kinases offers the possibility to screen in vitro for synthetic small molecule kinase inhibitors. In view of the many interconnections of cellular kinases, their role in preventing or inducing programmed cell death and the possibility that a considerable number of signal transducing proteins are still unknown, cellular test systems are recommended in which the respective key kinase or one of its main partner molecules are overexpressed.

The SH3 and SH2 domains are capable of directing specificity in protein interactions between the non-receptor tyrosine kinases cSrc and cYes

The c-src and c-yes proto-oncogenes encode 60 000 and 62 000 Dalton non-receptor tyrosine kinases of the Src family, pp60c-src and pp62c-yes, respectively. These kinases are over 80% homologous outside of their unique amino termini, yet several studies suggest that differences exist in the regulation, activation, and function of cSrc and cYes. The determinants of specificity in signaling between these proteins, however, remain unclear. In order to investigate the roles of the Src Homology (SH) 3 and 2 domains in mediating signaling specificity between cSrc and cYes, chimeras were created in which the SH3 and/or SH2 domains of cSrc or the fully activated variant Src527F were replaced by the corresponding domains of cYes. These constructs were used to assess the effects of the Yes SH3 and SH2 domains on the ability of Src to form stable complexes with and induce tyrosine phosphorylation of Src SH3 and SH2 domain binding partners in vivo. Both the Yes SH3 and SH2 domains were found to alter the capacity of Src to form stable associations with heterologous proteins. The Yes SH3 domain was unable to affinity absorb the Src SH3/SH2 binding partner AFAP-110 from COS-1 cell lysates, and chimeric constructs of Src527F containing the cYes SH3 domain were unable to efficiently co-immunoprecipitate with AFAP-110 from chicken embryo fibroblasts. Interactions with the Src SH2 domain binding partner pp130cas were unaffected. Additionally, only chimeras containing the cYes SH2 domain were able to co-immunoprecipitate with an unidentified 87 kDa tyrosine-phosphorylated protein. These results indicate that the SH3 and SH2 domains are capable of directing specificity in substrate binding between Src and Yes, suggesting potential mechanisms for generating specificity in signaling between these two highly related non-receptor tyrosine kinases.

The Shp-2 tyrosine phosphatase activates the Src tyrosine kinase by a non-enzymatic mechanism

Previously, we demonstrated that the Src tyrosine kinase interacts with the Shp-2 tyrosine phosphatase. To determine whether Shp-2 regulates Src kinase activity, we measured Src activity in cells overexpressing wild-type or catalytically-inactive C463S Shp-2. We observed a 2-3-fold increase in the specific activity of Src in both cell types and the increase did not appear to be due to dephosphorylation of Tyr 527 or phosphorylation of Tyr 416 on Src. Conversely, we observed a 2-3-fold decrease in the specific activity of Src when Shp-2 expression was inhibited. Using glutathione S-transferase-fusion proteins, we demonstrated that Shp-2 binds to the SH3 domain of Src. Our findings reveal that the Shp-2 tyrosine phosphatase can regulate the Src tyrosine kinase by a non-enzymatic mechanism. We also found that the phosphatase activity of Shp-2 immunoprecipitates is downregulated in cells transformed by Src or other proteins, and that Shp-2 preferentially associates with the membrane fraction of transformed cells. We suggest that membrane-association of Shp-2 is important for regulating Shp-2 activity.

RACK1, a receptor for activated C kinase and a homolog of the beta subunit of G proteins, inhibits activity of src tyrosine kinases and growth of NIH 3T3 cells

To isolate and characterize proteins that interact with the unique domain and SH3 and SH2 domains of Src and potentially regulate Src activity, we used the yeast two-hybrid assay to screen a human lung fibroblast cDNA library. We identified RACK1, a receptor for activated C kinase and a homolog of the beta subunit of G proteins, as a Src-binding protein. Using GST-Src fusion proteins, we determined that RACK1 binds to the SH2 domain of Src. Coimmunoprecipitation of Src and RACK1 was demonstrated with NIH 3T3 cells. Purified GST-RACK1 inhibited the in vitro kinase activity of Src in a concentration-dependent manner. GST-RACK1 (2 microM) inhibited the activities of purified Src and Lck tyrosine kinases by 40 to 50% but did not inhibit the activities of three serine/threonine kinases that we tested. Tyrosine phosphorylation on many cellular proteins decreased in 293T cells that transiently overexpressed RACK1. Src activity and cell growth rates decreased by 40 to 50% in NIH 3T3 cells that stably overexpressed RACK1. Flow cytometric analyses revealed that RACK1-overexpressing cells do not show an increased rate of necrosis or apoptosis but do spend significantly more time in G0/G1 than do wild-type cells. Prolongation of G0/G1 could account for the increased doubling time of RACK1-overexpressing cells. We suggest that RACK1 exerts its effect on the NIH 3T3 cell cycle in part by inhibiting Src activity.

Overexpression and activation of the tyrosine kinase Src in human pancreatic carcinoma

Src family tyrosine kinases participate in the regulation of cell adhesion, cell growth and differentiation. Here, we examine for the first time the potential role of Src for growth regulation of human pancreatic carcinoma cells. By immunohistochemical analysis, Src was overexpressed in 13/13 pancreatic carcinoma tissue but not in 6 normal pancreatic tissue specimen. In Western blots of total cellular extracts, Src protein expression was elevated in 14/17 carcinoma cell lines as compared to normal pancreas or cultured human pancreatic duct cells. Kinase activity was only detectable in cancer cells and did not correlate with the amount of kinase protein or with the expression of the regulatory kinase Csk, indicating that Src is not regulated through protein expression or through expression of Csk. The Src-specific tyrosine kinase inhibitor herbimycin A decreased cell growth in a dose-dependent manner. We suggest that Src family kinases participate in growth regulation of pancreatic cancer cells.

Expression, purification, and initial characterization of human Yes protein tyrosine kinase from a bacterial expression system

Protein tyrosine kinase Yes is a cellular homolog of v-Yes, the oncogenic protein product of avian sarcoma virus Y73. Yes is a member of the Src family and its activation has been associated with several types of human cancer. Human Yes has not been previously characterized enzymatically. To carry out biochemical characterizations of this enzyme, we expressed it as a fusion protein with glutathione S-transferase in Escherichia coli, to allow purification in a single step. The affinity-purified GST-Yes has a specific activity of 1.3 nmol min-1 mg-1 with polyE4Y as substrate and Km values of 100 microg ml-1 for polyE4Y and 70 microM for ATP-Mg. The enzyme has a preference for magnesium over manganese ion for maximal activity. The divalent metal cation serves two essential functions for the activity of Yes: one as a part of the phosphate-donating substrate ATP-Mg and the other as an essential activator. The enzyme undergoes autophosphorylation without apparent activation. Finally, we show that the enzyme is inactivated by incubation with protein tyrosine kinase Csk in an ATP-Mg-dependent manner, indicating that cellular Yes can be regulated by Csk phosphorylation. These represent the first biochemical characterization of human Yes protein tyrosine kinase.

Activation of pp60c-src is necessary for human vascular smooth muscle cell migration

BACKGROUND

The most widely distributed nonreceptor tyrosine kinase is pp60c-src (src), yet the role of this intracellular signaling protein in cell migration has not been defined. Given that smooth muscle cell (SMC) migration is essential for the development of intimal hyperplasia, we investigated the importance of src in locomotion of human vascular SMC.

METHODS

SMC migration was evaluated using a microchemotaxis chamber assay and videomicroscopy. Src kinase activity was determined by measuring phosphorylation of a synthetic derivative of p34cdc2, a specific substrate for src. Blocking antibodies to src were introduced using a cytoplasmic microinjection technique.

RESULTS

Stimulation of SMC with platelet-derived growth factor (PDGF)-BB and AB resulted in an increase in src activation, whereas PDGF-AA did not consistently enhance src activity. These findings correlated with the ability of the PDGF isotypes to stimulate SMC chemotaxis; PDGF-BB and AB produced 7.4 +/- 0.3- and 5.3 +/- 0.5-fold increases in SMC chemotaxis, whereas PDGF-AA inhibited chemotaxis. SMC migration in response to PDGF-BB and serum was significantly inhibited by intracellular injection of a blocking antibody.

CONCLUSIONS

Our findings reveal an association between agonist-induced src activation and chemotaxis. Moreover, an antibody that inhibits src activation dramatically inhibits migration of individual SMC. We conclude that activation of src is necessary for SMC migration. Because of its importance in SMC migration, either molecular or pharmacologic inhibitors of src may be useful in the control of intimal hyperplasia.

Molecular pathogenesis of colorectal cancer

The enormous progress made in the identification of genes that are involved in colon carcinogenesis has provided the foundation for further understanding the biology of both normal and cancer cells and for targeted therapeutic strategies. In one sense, the genes described in this review are only the building blocks of a larger puzzle that constitutes the integrated metabolic function of a cell. The current challenge is to understand the functional role of these genes in normal cellular physiology and make the connections between pathways that knit together integrated cellular homeostasis. A complete understanding of the regulatory pathways, and the synthesis and modifications of the proteins involved, will provide novel targets for therapeutic agents.

Adhesion of HT-29 colon carcinoma cells to E-selectin results in increased tyrosine phosphorylation and decreased activity of c-src

Adhesion of metastatic cancer cells at secondary sites is known to be regulated by several families of adhesion proteins, including selectins and integrins. Colon carcinoma cells have been shown to tether to and roll on both stimulated endothelial cells and purified E-selectin. We have demonstrated that HT-29 human colon carcinoma cells adhere specifically to an E-selectin-IgG chimera. Upon adhesion to E-selectin, the amount of tyrosine phosphorylation of several proteins in HT-29 cell lysates increases compared with cells in bovine serum albumin-coated wells on phosphotyrosine Western blots; this increase is statistically significant. This effect is specific for adhesion to E-selectin, since addition of an E-selectin blocking monoclonal antibody (MAb), E3, to the wells causes a statistically significant decrease in tyrosine phosphorylation relative to E-selectin alone on phosphotyrosine Western blots. One protein that is affected this way has been identified as c-src. Kinase assays show a dose-dependent and statistically significant decrease in c-src activity upon adhesion to E-selectin, which correlates with an increase in phosphorylation of Tyr 527, the negative regulatory tyrosine. CnBr digestion of 32P-labeled c-src shows an increase in phosphorylation of tyrosine 527 after adhesion to E-selectin. Our results may identify a signaling pathway involving the E-selectin ligand on HT-29 cells and c-src.

Endothelin-1 induces tyrosine phosphorylation in human blood monocytes

Mediators including the neuropeptide endothelin-1 (ET-1), which are released in response to injury, modulate the expression of cell adhesion molecules on leukocytes and endothelial cells. The mechanisms underlying this process are not clear. In this study we investigated the effect of endothelin-1 on the expression of tyrosine phosphorylated proteins in human blood monocytes. Endothelin-1 caused an increase in tyrosine phosphorylated proteins in monocytes in a time-dependent and dose-dependent manner, the Mr 60, 80 and 110 kDa proteins being the most prominent. This effect was blocked by pre-incubating the monocytes with the selective tyrosine kinase inhibitors genistein or herbimycin A. Endothelin-1 induced upregulation of tyrosine phosphorylated proteins appears to be mediated by the ETA receptor. Unlike our previously reported studies in endothelial cells, immunoprecipitation with anti-src or anti-JAK antibodies followed by immunoblotting with PY20 in human blood monocytes revealed that these proteins of Mr 60, 80 and 110 kDa were not related to src or JAK kinases. These findings suggest that ET- exerts its effect on monocytes by a pathway involving tyrosine kinases other than src or JAK kinases.