Melanoma chondroitin sulphate proteoglycan regulates cell spreading through Cdc42, Ack-1 and p130cas

Regulation of high molecular weight-melanoma associated antigen (HMW-MAA) gene expression by promoter DNA methylation in human melanoma cells

The human high molecular weight-melanoma associated antigen (HMW-MAA) is a membrane-bound chondroitin sulfate proteoglycan that is variably expressed in a high percentage of melanoma cell lines and tumors. Since the mechanism(s) regulating HMW-MAA expression has(ve) not been defined, in this study, we have examined whether promoter DNA methylation regulates the level of HMW-MAA expression. In melanoma cell lines, the level of HMW-MAA mRNA and protein expression is coordinately regulated, implicating a transcriptional control mechanism. Consistent with a role for regulation by DNA methylation, we have found that a dense CpG island flanks the human HMW-MAA gene transcriptional start site. Methylation-specific PCR and sodium bisulfite DNA sequencing analyses indicate that the HMW-MAA promoter is heavily methylated in melanoma cell lines, melanoma lesions and normal lymphocytes that do not express HMW-MAA; in contrast, the HMW-MAA promoter is not methylated in melanoma cell lines and tumors that express this antigen. In addition, HMW-MAA expression is markedly induced in HMW-MAA-negative melanoma cell lines by incubation with the DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine. In summary, our results establish DNA methylation as a key regulator of HMW-MAA expression by human melanoma cells. This information represents a useful background to optimize immunotherapeutic strategies targeting HMW-MAA.

Activation of the nonreceptor protein tyrosine kinase Ack by multiple extracellular stimuli

Ack/Ack1 is a nonreceptor protein tyrosine kinase that comprises a tyrosine kinase core, an SH3 domain, a Cdc42-binding region, a Ralt homology region, and a proline-rich region. Here we describe a detailed characterization of the Ack protein as well as the chromosomal localization of human Ack (chromosome 3q29) and the primary structure of murine Ack. We demonstrate that Ack is ubiquitously expressed, with highest expression seen in thymus, spleen, and brain. Activation of integrins by cell adhesion on fibronectin leads to strong tyrosine phosphorylation and activation of Ack. Upon cell stimulation with EGF or PDGF, Ack is tyrosine-phosphorylated and recruited to activated EGF or PDGF receptors, respectively. A pool of endogenous Ack molecules is constitutively tyrosine-phosphorylated, even in starved cells. Moreover, tyrosine-phosphorylated Ack forms a stable complex with the adapter protein Nck via its SH2 domain. Finally, we have characterized a membrane-targeting sterile alpha motif-like domain in the amino terminus of Ack. Using several Ack mutants, we show that the amino-terminal and CRIB domains are necessary for Ack autophosphorylation, whereas the SH3 domain appears to have an autoinhibitory role. These experiments suggest a functional role for Ack as an early transducer of multiple extracellular stimuli.

The genetics of Fraser syndrome and the blebs mouse mutants

Fraser syndrome is a recessive multisystem disorder characterized by embryonic epidermal blistering, cryptophthalmos, syndactyly, renal defects and a range of other developmental abnormalities. More than 17 years ago, the family of four mapped mouse blebs mutants was proposed as models of this disorder, given their striking phenotypic overlaps. In the last few years, these loci have been cloned, uncovering a family of three large extracellular matrix proteins and an intracellular adapter protein which are required for normal epidermal adhesion early in development. The proteins have also been shown to play a crucial role in the development and homeostasis of the kidney. We review the cloning and characterization of these genes and explore the consequences of their loss.

NG2-expressing cells in the nervous system: role of the proteoglycan in migration and glial-neuron interaction

The NG2 glycoprotein is a type I membrane protein expressed in the developing and adult central nervous system (CNS) by subpopulations of glia including oligodendroglial precursor cells (OPCs), and in the developing CNS additionally by pericytes. In the mouse CNS, expression of NG2 protein is already observed at embryonic day 13 and peaks between postnatal days 8 and 12. NG2+ cells persist in grey and white matter in adult mouse brain: cells in the developing and adult brain show clear differences in migration, cell-cycle length and lineage restriction. Several groups have provided evidence that subpopulations of NG2+ cells can generate neurons in vivo. Neuronal stimulation in the developing and adult hippocampus leads to Ca2+ signals in apposing NG2+ glia, suggesting that these cells may modulate synaptic activity, and NG2+ cells often ensheath synapses. The structure of the protein with two N-terminal LamininG/Neurexin/Sex-hormone-binding globulin domains suggests a role in adhesion. The C-terminal PSD-95/DiscsLarge/Zona Occludens-1 (PDZ)-binding motif has been found to associate with several PDZ proteins including the Glutamate Receptor Interacting Protein GRIP: NG2 may thus act to position AMPA receptors on glia towards sites of neuronal glutamate release. Furthermore, the NG2 proteoglycan plays a role in cell migration and spreading and associates with actin-containing cytoskeletal structures.

Targeting Melanoma Cells with Human High Molecular Weight-Melanoma Associated Antigen-Specific Antibodies Elicited by a Peptide Mimotope: Functional Effects

Human high molecular weight-melanoma associated Ag (HMW-MAA) mimics have been shown to elicit HMW-MAA-specific humoral immune responses that appear to be clinically beneficial. This finding has stimulated interest in characterizing the mechanism(s) underlying the ability of the elicited Abs to exert an anti-tumor effect. To address this question, in the present study, we have generated HMW-MAA-specific Abs by sequentially immunizing rabbits with the peptide P763.74, which mimics the HMW-MAA determinant recognized by mAb 763.74, and with HMW-MAA(+) melanoma cells. HMW-MAA-specific Abs isolated from immunized rabbits mediated cell-dependent cytotoxicity but did not mediate complement-dependent cytotoxicity of HMW-MAA(+) melanoma cells. These Abs also effectively inhibited spreading, migration and Matrigel invasion of HMW-MAA(+) melanoma cells. Besides contributing to our understanding of the role of HMW-MAA in the biology of melanoma cells, these results suggest that both immunological and nonimmunological mechanisms underlie the beneficial clinical effects associated with the induction of HMW-MAA-specific Abs in melanoma patients immunized with a HMW-MAA mimic.

Immunohistochemical study of NG2 chondroitin sulfate proteoglycan expression in the small and large intestines

The intestinal subepithelial myofibroblasts (ISEMFs) are located in the lamina propria under the epithelial cells. ISEMFs are thought to have an important role in protecting and maintaining the integrity of the epithelial cell layer and also in the process of wound healing. In this study, we report that the membrane-bound proteoglycan NG2 is abundantly distributed in the ISEMF layer of the mouse and human intestines. NG2 immunostaining in this layer is distributed with similar intensity from the crypt to villi. NG2 is also immunolocalized along the membranes of smooth muscle cells in the intestinal muscle layer. However, skeletal and cardiac muscles are not immunostained for NG2, demonstrating selective expression of the proteoglycan by smooth muscle cells. Using electron microscopy, NG2 immunoreactivity was strongly observed along the cell membranes of ISEMF, with weak diffusion into the neighboring matrix, indicative of the presence of some "shed" NG2. This first report of NG2 proteoglycan expression by ISEMF provides insights into the nature of the interaction of these cells with extracellular matrix and/or intestinal epithelial cells.

Altered expression of the MCSP/NG2 chondroitin sulfate proteoglycan in collagen VI deficiency

NG2, the rat homologue of the human melanoma chondroitin sulfate proteoglycan (MCSP), is a ligand for collagen VI (COL6). We have examined skeletal muscles of patients affected by Ullrich scleroatonic muscular dystrophy (UCMD), an inherited syndrome caused by COL6 genes mutations. A significant decrease of NG2 immunolabeling was found in UCMD myofibers, as well as in skeletal muscle and cornea of COL6 null-mice. In UCMD muscles, truncated NG2 core protein isoforms were detected. However, real-time RT-PCR analysis revealed marked increase in NG2 mRNA content in UCMD muscle compared to controls. We hypothesize that NG2 immunohistochemical and biochemical behavior may be compromised owing to the absence of its physiological ligand. MCSP/NG2 proteoglycan may be considered an important receptor mediating COL6-sarcolemma interactions, a relationship that is disrupted by the pathogenesis of UCMD muscle.

Protein kinase activity is associated with CD63 in melanoma cells

BACKGROUND

The tetraspan protein CD63, originally described as a stage-specific melanoma antigen but also present in a number of normal cells, regulates melanoma cell growth in nude mice, motility in serum containing media, and adhesion to several extracellular matrix proteins. CD63 has been reported to associate with beta1 and beta2 integrins, but the mechanism of signal transduction by CD63 is not clear. This study examined whether CD63 is associated with protein kinase and can transmit signals in melanoma cells.

METHODS

Immunoprecipitation and radiolabeling were used to test for association of protein kinase activity with CD63. Adhesion of cells to monoclonal antibodies immobilized to microtiter plates was used to examine the ability of CD63 to transmit signals.

RESULTS

CD63 was capable of transmitting a signal in melanoma cells that required extracellular calcium. In the absence of extracellular calcium at the time of binding to the CD63 mAb, the cell was no longer responsive to stimulation by CD63. Immunoprecipitation studies demonstrated protein kinase activity associated with CD63, and phosphoamino acid analysis revealed that most of this protein kinase activity was due to serine kinase activity.

CONCLUSION

The current study suggests that serine protein kinase activity associated with CD63 may play a role in signaling by CD63 in melanoma cells.

Activated Tyrosine Kinase Ack1 Promotes Prostate Tumorigenesis: Role of Ack1 in Polyubiquitination of Tumor Suppressor Wwox

Aberrant activation of tyrosine kinases is linked causally to human cancers. Activated Cdc42-associated kinase (Ack1), an intracellular tyrosine kinase, has primarily been studied for its signaling properties but has not been linked to specific pathologic conditions. Herein, we report that expression of activated Ack1 in LNCaP cells, while minimally increasing growth in culture, enhanced anchorage-independent growth in vitro and dramatically accelerated tumorigenesis in nude mice. Molecular chaperone heat shock protein 90beta (Hsp90beta)-bound Ack1 and treatment of cells with geldanamycin, a Hsp90 inhibitor, inhibited Ack1 kinase activity and suppressed tumorigenesis. Further, we identify the tumor suppressor WW domain containing oxidoreductase (Wwox) as an Ack1-interacting protein. Activated Ack1 tyrosine phosphorylated Wwox, leading to rapid dissociation of the Ack1-Wwox complex and concomitant Wwox polyubiquitination followed by degradation. Tyrosine phosphorylation of Wwox was critical for its degradation, as splice variant WwoxDelta5-8 that was not phosphorylated by Ack1 failed to undergo polyubiquitination and degradation. It has been reported that phosphorylation of Wwox at Tyr33 stimulated its proapoptotic activity. We observed that Y33F Wwox mutant was still tyrosine phosphorylated and polyubiquitinated by Ack1 action. Site-directed mutagenesis revealed that activated Ack1 primarily phosphorylated Wwox at Tyr287, suggesting that phosphorylation of distinct tyrosine residues activate or degrade Wwox. Primary androgen-independent prostate tumors but not benign prostate showed increased tyrosine-phosphorylated Ack1 and decreased Wwox. Taken together, these data indicate that Ack1 stimulated prostate tumorigenesis in part by negatively regulating the proapoptotic tumor suppressor, Wwox. Further, these findings suggest that Ack1 could be a novel therapeutic target for prostate cancer.

Rho GTPase expression in tumourigenesis: evidence for a significant link

Rho proteins belong to the small GTPases superfamily. They function as molecular switches that, in response to diverse stimuli, control key signaling and structural aspects of the cell. Although early studies proposed a role for Rho GTPases in cellular transformation, this effect was underestimated due to the fact that no genetic mutations affecting Rho-encoding genes were found in tumors. Recently, it has become evident that Rho GTPases participate in the carcinogenic process by either overexpression of some of the members of the family with oncogenic activity, downmodulation of other members with suggested tumor suppressor activity, or by alteration of upstream modulators or downstream effectors. Thus, alteration of the levels of expression of different members of the family of Rho GTPases has been detected in many types of human tumors leading to a great interest in the cellular effects elicited by these oncoproteins. This essay reviews the current evidence of dysregulation of Rho signaling by overexpression in human tumors.

Expression, synaptic localization, and developmental regulation of Ack1/Pyk1, a cytoplasmic tyrosine kinase highly expressed in the developing and adult brain

Cytosolic tyrosine kinases play a critical role both in neural development and in adult brain function and plasticity. Here we isolated a cDNA with high homology to human Ack1 and mouse Tnk2. This cDNA directs the expression of a 125-kD protein that can be autophosphorylated in tyrosines. Initially, this clone was named Pyk1 for proline-rich tyrosine kinase (Lev et al., 1995); however, since it corresponds to the mouse homolog of Ack1, here we called it Ack1/Pyk1. In this study we show that Ack1/Pyk1 mRNA and protein is highly expressed in the developing and adult brain. The highest levels of Ack1/Pyk1 expression were detected in the hippocampus, neocortex, and cerebellum. Electron microscopy studies showed that Ack1/Pyk1 protein is expressed in these regions both at dendritic spines and presynaptic axon terminals, indicating a role in synaptic function. Furthermore, we demonstrate that Ack1/Pyk1 mRNA levels are strongly upregulated by increased neural activity, produced by intraperitoneal kainate injections. During development, Ack1/Pyk1 was also expressed in the proliferative ventricular zones and in postmitotic maturing neurons. In neuronal cultures, Ack1/Pyk1 was detected in developing dendrites and axons, including dendritic tips and growth cones. Moreover, Ack1/Pyk1 colocalized with Cdc42 GTPase in neuronal cultures and coimmunoprecipitated with Cdc42 in HEK 293T cells. Altogether, our findings indicate that Ack1/Pyk1 tyrosine kinase may be involved both in adult synaptic function and plasticity and in brain development.

Metastatic properties and genomic amplification of the tyrosine kinase gene ACK1

Metastasis of primary tumors leads to a very poor prognosis for patients suffering from cancer. Although it is well established that not every tumor will eventually metastasize, it is less clear whether primary tumors acquire genetic alterations in a stochastic process at a late stage, which make them invasive, or whether genetic alterations acquired early in the process of tumor development drive primary tumor growth and determine whether this tumor is going to be metastatic. To address this issue, we tested genes identified in a large-scale comparative genomic hybridization analysis of primary tumor for their ability to confer metastatic properties on a cancer cell. We identified amplification of the ACK1 gene in primary tumors, which correlates with poor prognosis. We further show that overexpression of Ack1 in cancer cell lines can increase the invasive phenotype of these cells both in vitro and in vivo and leads to increased mortality in a mouse model of metastasis. Biochemical studies show that Ack1 is involved in extracellular matrix-induced integrin signaling, ultimately activating signaling processes like the activation of the small GTPase Rac. Taken together, this study supports a theory from Bernards and Weinberg [Bernards, R. & Weinberg, R. A. (2002) Nature 418, 823], which postulates that the tendency to metastasize is largely predetermined.

Proteoglycan control of cell movement during wound healing and cancer spreading

By virtue of their multifunctional nature, proteoglycans (PGs) are thought to govern the process of cell movement in numerous physiological and pathological contexts, spanning from early embryonic development to tumour invasion and metastasis. The precise mode by which they influence this process is still fragmentary, but evidence is accruing that they may affect it in a multifaceted manner. PGs bound to the plasma membrane mediate the polyvalent interaction of the cell with matrix constituents and with molecules of the neighbouring cells' surfaces; they modulate the activity of receptors implicated in the recognition of these components; and they participate in the perception and convergence of growth- and motility-promoting cues contributed by soluble factors. Through some of these interactions several PGs transduce to pro-motile cells crucial intracellular signals that are likely to be essential for their mobility. A regulated shedding of certain membrane-intercalated PGs seems to provide an additional level of control of cell movement. Coincidentally, matrix-associated PGs may govern cell migration by structuring permissive and non-permissive migratory paths and, when directly secreted by the moving cells, may alternatively create favourable or hostile microenvironments. To exert this latter, indirect effect on cell movement, matrix PGs strongly rely upon their primary molecular partners, such as hyaluronan, link proteins, tenascins, collagens and low-affinity cell surface receptors, whereas a further finer control is provided by a highly regulated proteolytic processing of the PGs accounted by both the migrating cells themselves and cells of their surrounding tissues. Overall, PGs seem to play an important role in determining the migratory phenotype of a cell by initiating, directing and terminating cell movement in a spatio-temporally controlled fashion. This implies that the "anti-adhesive and/or "anti-migratory" properties that have previously been assigned to certain PGs may be re-interpreted as being a means by which these macromolecules elaborate haptotaxis-like mechanisms imposing directionality upon the moving cells. Since these conditions would allow cells to be led to given tissue locations and become immobilized at these sites, a primary function may be ascribed to PGs in the dictation of a "stop or go" choice of the migrating cells.

High-molecular-weight melanoma-associated antigen mimotope immunizations induce antibodies recognizing melanoma cells

Size and posttranslational modifications are obstacles in the recombinant expression of high-molecular-weight melanoma-associated antigen (HMW-MAA). Creating a tumor antigen mimic via the phage display technology may be a means to overcome this problem for vaccine design. In this study, we aimed to generate an immunogenic epitope mimic of HMW-MAA. Therefore we screened a linear 9mer phage display peptide library, using the anti-HMW-MAA monoclonal antibody (mAb) 225.28S. This antibody mediates antibody-dependent cellular cytotoxicity (ADCC) and has already been used for anti-idiotype therapy trials. Fifteen peptides were selected by mAb 225.28S in the biopanning procedure. They share a consensus sequence, but show only partial homology to the amino acid sequence of the HMW-MAA core protein, indicating mimicry with a conformational epitope. One mimotope was chosen to be fused to albumin binding protein (ABP) as an immunogenic carrier. Immunoassays with 225.28S indicated that the mimotope fusion protein was folded correctly. Subsequently, the fusion protein was tested for immunogenicity in BALB/c mice. The induced anti-mimotope antibodies recognized HMW-MAA of 518A2 human melanoma cells, whereas sera of mice immunized with the carrier ABP alone showed no reactivity. These anti-mimotope antibodies were capable of inducing specific lysis of 518A2 melanoma cells in ADCC assays with murine effector cells. In conclusion, the presented data indicate that mimotopes fused to an immunogenic carrier are suitable tools to elicit epitope-specific anti-melanoma immune responses.

The 1.1 A resolution crystal structure of the p130cas SH3 domain and ramifications for ligand selectivity

The Crk-associated tyrosine kinase substrate p130cas (CAS) is a docking protein containing an SH3 domain near its N terminus, followed by a short proline-rich segment, a large central substrate domain composed of 15 repeats of the four amino acid sequence YxxP, a serine-rich region and a carboxy-terminal domain, which possesses consensus binding sites for the SH2 and SH3 domains of Src (YDYV and RPLPSPP, respectively). The SH3 domain of CAS mediates its interaction with several proteins involved in signaling pathways such as focal adhesion kinase (FAK), tyrosine phosphatases PTP1B and PTP-PEST, and the guanine nucleotide exchange factor C3G. As a homolog of the corresponding Src docking domain, the CAS SH3 domain binds to proline-rich sequences (PxxP) of its interacting partners that can adopt a polyproline type II helix. We have determined a high-resolution X-ray structure of the recombinant human CAS SH3 domain. The domain, residues 1-69, crystallized in two related space groups, P2(1) and C222(1), that provided diffraction data to 1.1 A and 2.1 A, respectively. The crystal structure shows, in addition to the conserved SH3 domain architecture, the way in which the CAS characteristic amino acids form an atypically charged ligand-binding surface. This arrangement provides a rationale for the unusual ligand recognition motif exhibited by the CAS SH3 domain. The structure enables modelling of the docking interactions to its ligands, for example from focal adhesion kinase, and supports structure-based drug design of inhibitors of the CAS-FAK interaction.

NG2-positive cells in CNS function and the pathological role of antibodies against NG2 in demyelinating diseases

NG2 is expressed by a variety of immature glia in the CNS including oligodendrocyte progenitor cells, paranodal astrocytes and perisynaptic glia. The protein has a large extracellular domain with two LNS/Lam G domains at the N-terminus and a short intracellular tail with a PDZ-recognition domain at the C-terminus. Experiments suggest that the protein plays a role in migration. The PDZ protein GRIP was identified as an intracellular binding partner of NG2 in immature glial cells. A complex is formed between GRIP, NG2 and the AMPA class of glutamate receptors: this may position these glial receptors towards sites of neuronal glutamate release at synapses and during myelination. Identification of neuronal receptors and links to the cytoskeleton of NG2 is of critical importance. Some Multiple Sclerosis patients have autoantibodies to NG2 in the cerebral spinal fluid: such antibodies could interfere with remyelination by lysing oligodendrocyte progenitor cells or blocking their migration but may also cause pathology by disrupting glial-neuronal signalling at synapses and paranodes.

The prognostic value of BCAR1 in patients with primary breast cancer

PURPOSE

BCAR1, the human homologue of the rat p130Cas protein, was identified in a functional screen for human breast cancer cell proliferation resistant to antiestrogen drugs. Here, we study the prognostic value of quantitative BCAR1 levels in a large series of breast cancer specimens.

EXPERIMENTAL DESIGN

A specific ELISA was developed to measure BCAR1 protein levels in 2593 primary breast tumor cytosols. Tumor levels of BCAR1 were correlated with relapse-free survival (RFS) and overall survival (OS) and compared with collected data on urokinase-type plasminogen activator (uPA) and plasminogen activator inhibitor 1 (PAI-1).

RESULTS

In tumor cytosols, BCAR1 protein levels varied between 0.02 and 23 ng/mg protein. BCAR1 levels exhibited a positive correlation with steroid hormone receptor levels, age and menopausal status, and uPA and PAI-1 levels. The level of BCAR1 (continuous or categorized as low, intermediate, or high) was inversely related with RFS and OS time. Multivariate analysis showed that BCAR1 levels contributed independently to a base model containing the traditional prognostic factors for both RFS and OS (both P < 0.0001). When added together with uPA and PAI-1 in the multivariate model, BCAR1 contributed independently of PAI-1 and was favored over uPA. Interaction tests allowed for additional analyses of BCAR1 protein levels in clinically relevant subgroups stratified by nodal and menopausal status.

CONCLUSIONS

The quantitative BCAR1 protein level represents a prognostic factor for RFS and OS in primary breast cancer, independent of the traditional prognostic factors and the other novel marker PAI-1.

Suppression of human melanoma tumor growth in SCID mice by a human high molecular weight-melanoma associated antigen (HMW-MAA) specific monoclonal antibody

The lack of efficacy of available therapies for the treatment of malignant melanoma has emphasized the need to develop novel therapeutic strategies to prevent melanoma growth. We have tested whether the anti-HMW-MAA mAb 225.28S is able to inhibit human melanoma tumor growth in SCID mice because in vitro data suggested that this antigen plays a role in spreading, migration and invasion of melanoma cells. Tumors were established by subcutaneous injection of the human melanoma cell line 518A2 into SCID mice. When tumors reached a size of 5 mm, the mAb 225.28S was administered intravenously 4 times in 3 day intervals at 100 microg/injection. Within 14 days after the first administration of the mAb 225.28S, tumor growth was reduced by 52% as compared to control mice. Three hundred and seven genes of >20,000 genes contained on the GeneChip were changed in their expression level at least 2-fold after administration of the mAb 225.28S. The encoded proteins were mostly components or modifiers of the extracellular matrix, tumor suppressors, and melanogenesis associated proteins. Surprisingly, the administration of the control mAb that did not lead to a significant tumor growth inhibition in vivo resulted in the modulation of two-thirds of these genes. This is the first report of suppression of human melanoma tumor growth in SCID mice by the mAb 225.28S. Our results suggest that anti-HMW-MAA mAbs may represent useful reagents to apply passive immunotherapy to patients with malignant melanoma.

Phosphorylation of NG2 proteoglycan by protein kinase C-alpha regulates polarized membrane distribution and cell motility

Protein kinase C (PKC)-alpha phosphorylation of recombinant NG2 cytoplasmic domain and phorbol ester-induced PKC-dependent phosphorylation of full-length NG2 expressed in U251 cells are both blocked by mutation of Thr(2256), identifying this residue as a primary phosphorylation site. In untreated U251/NG2 cells, NG2 is present along with ezrin and alpha(3)beta(1) integrin in apical cell surface protrusions. Phorbol ester treatment causes redistribution of all three components to lamellipodia, accompanied by increased cell motility. U251 cells expressing NG2 with a valine substitution at position 2256 are resistant to phorbol ester treatment: NG2 remains in membrane protrusions and cell motility is unchanged. In contrast, NG2 with a glutamic acid substitution at position 2256 redistributes to lamellipodia even without phorbol ester treatment, rendering transfected U251 cells spontaneously motile. PKC-alpha-mediated NG2 phosphorylation at Thr(2256) is therefore a key step for initiating cell polarization and motility.

Melanoma chondroitin sulfate proteoglycan enhances FAK and ERK activation by distinct mechanisms

Melanoma chondroitin sulfate proteoglycan (MCSP) is an early cell surface melanoma progression marker implicated in stimulating tumor cell proliferation, migration, and invasion. Focal adhesion kinase (FAK) plays a pivotal role in integrating growth factor and adhesion-related signaling pathways, facilitating cell spreading and migration. Extracellular signal–regulated kinase (ERK) 1 and 2, implicated in tumor growth and survival, has also been linked to clinical melanoma progression. We have cloned the MCSP core protein and expressed it in the MCSP-negative melanoma cell line WM1552C. Expression of MCSP enhances integrin-mediated cell spreading, FAK phosphorylation, and activation of ERK1/2. MCSP transfectants exhibit extensive MCSP-rich microspikes on adherent cells, where it also colocalizes with α4 integrin. Enhanced activation of FAK and ERK1/2 by MCSP appears to involve independent mechanisms because inhibition of FAK activation had no effect on ERK1/2 phosphorylation. These results indicate that MCSP may facilitate primary melanoma progression by enhancing the activation of key signaling pathways important for tumor invasion and growth.

Myelin-, reactive glia-, and scar-derived CNS axon growth inhibitors: expression, receptor signaling, and correlation with axon regeneration

Axon regeneration is arrested in the injured central nervous system (CNS) by axon growth-inhibitory ligands expressed in oligodendrocytes/myelin, NG2-glia, and reactive astrocytes in the lesion and degenerating tracts, and by fibroblasts in scar tissue. Growth cone receptors (Rc) bind inhibitory ligands, activating a Rho-family GTPase intracellular signaling pathway that disrupts the actin cytoskeleton inducing growth cone collapse/repulsion. The known inhibitory ligands include the chondroitin sulfate proteoglycans (CSPG) Neurocan, Brevican, Phosphacan, Tenascin, and NG2, as either membrane-bound or secreted molecules; Ephrins expressed on astrocyte/fibroblast membranes; the myelin/oligodendrocyte-derived growth inhibitors Nogo, MAG, and OMgp; and membrane-bound semaphorins (Sema) produced by meningeal fibroblasts invading the scar. No definitive CSPG Rc have been identified, although intracellular signaling through the Rho family of G-proteins is probably common to all the inhibitory ligands. Ephrins bind to signalling Ephs. The ligand-binding Rc for all the myelin inhibitors is NgR and requires p75(NTR) for transmembrane signaling. The neuropilin (NP)/plexin (Plex) Rc complex binds Sema. Strategies for promoting axon growth after CNS injury are thwarted by the plethora of inhibitory ligands and the ligand promiscuity of some of their Rc. There is also paradoxical reciprocal expression of many of the inhibitory ligands/Rc in normal and damaged neurons, and NgR expression is restricted to a limited number of neuronal populations. All these factors, together with an incomplete understanding of the normal functions of many of these molecules in the intact CNS, presently confound interpretive acumen in regenerative studies.

Identification of the region in Cdc42 that confers the binding specificity to activated Cdc42-associated kinase

The Rho family small G-protein Cdc42 has been implicated in a diversity of biological functions. Multiple downstream effectors have been identified. How Cdc42 discriminates the interaction with its multiple downstream effectors is not known. Activated Cdc42-associated tyrosine kinase (ACK) is a very specific effector of Cdc42. To delineate the Cdc42 signaling pathway mediated by ACK, we set about to identify the specific ACK-binding region in Cdc42. We utilized TC10, another member of the Rho family of G-proteins that is 66.7% identical to Cdc42, to construct TC10/Cdc42 chimeras for screening the specific ACK-binding region in Cdc42. A region between switch I and switch II has been identified as the specific ACK-binding (AB) region. The replacement of the AB region with the corresponding region in TC10 resulted in the complete loss of ACK-binding ability but did not affect the binding to WASP, suggesting that the AB region confers the binding specificity to ACK. On the other hand, replacement of the corresponding region of TC10 with the AB region enabled TC10 to acquire ACK-binding ability. Eight residues are different between the AB region and the corresponding region of TC10. The mutational analysis indicated that all eight residues contribute to the binding to ACK2. The assays for the Cdc42-mediated activation of ACK2 indicated that the AB region is essential for Cdc42 to activate ACK2 in cells. Thus, our studies have defined a specific ACK-binding region in Cdc42 and have provided a molecular basis for generating ACK binding-defective mutants of Cdc42 to delineate ACK-mediated signaling pathway.

The integrin beta1 subunit transmembrane domain regulates phosphatidylinositol 3-kinase-dependent tyrosine phosphorylation of Crk-associated substrate

Our previous studies on the transmembrane domain of human integrin subunits have shown that a conserved basic amino acid in both subunits of integrin heterodimers is positioned in the plasma membrane in the absence of interacting proteins. To investigate the possible functional role of the lipid-embedded lysine in the mouse integrin beta1 subunit, this amino acid was replaced with leucine, and the mutated beta1 subunit (beta1A(K756L)) was stably expressed in beta1-deficient GD25 cells. The extracellular domain of beta1A(K756L) integrins possesses a competent conformation for ligand binding as determined by the ability to mediate cell adhesion, and by the presence of the monoclonal antibody 9EG7 epitope. However, the spreading of GD25-beta1A(K756L) cells on fibronectin and laminin-1 was impaired, and the rate of migration of GD25-beta1A(K756L) cells on fibronectin was reduced compared with GD25-beta1A cells. Phosphorylation of tyrosines in focal adhesion kinase (FAK) and the Y416 in c-Src in response to beta1A(K756L)-mediated adhesion was similar to that induced by wild-type beta1. The tyrosine phosphorylation level of paxillin, a downstream target of FAK/Src, was unaffected by the beta1 mutation, whereas tyrosine phosphorylation of CAS was strongly reduced. The results demonstrate that CAS is a target for phosphorylation both by FAK-dependent and -independent pathways after integrin ligation. The latter pathway was inhibited by wortmannin and LY294002, implicating that it required an active phosphatidylinositol 3-kinase. Furthermore, the K756L mutation in the beta1 subunit was found to interfere with beta1-induced activation of Akt. The results from this study identify phosphatidylinositol 3-kinase as an early component of a FAK-independent integrin signaling pathway triggered by the membrane proximal part of the beta1 subunit.

Epidermal and Hair Follicle Progenitor Cells Express Melanoma-Associated Chondroitin Sulfate Proteoglycan Core Protein

Basal keratinocytes in the epidermis and hair follicle are biologically heterogeneous but must include a stable subpopulation of epidermal stem cells. In animal models these can be identified by their retention of radioactive label due to their slow cycle (label-retaining cells) but human studies largely depend on in vitro characterization of colony forming efficiency and clonogenicity. Differential integrin expression has been used to detect cells of increased proliferative potential but further stem cell markers are urgently required for in vivo and in vitro characterization. Using LHM2, a monoclonal antibody reacting with a high molecular weight melanoma-associated proteoglycan core protein, a subset of basal keratinocytes in both the interfollicular epidermis and the hair follicle has been identified. Coexpression of melanoma-associated chondroitin sulfate proteoglycan with keratins 15 and 19 as well as beta 1 and alpha 6 integrins has been examined in adult and fetal human skin from hair bearing, nonhair bearing, and palmoplantar regions. Although melanoma-associated chondroitin sulfate proteoglycan coexpression with a subset of beta 1 integrin bright basal keratinocytes within the epidermis suggests that melanoma-associated chondroitin sulfate proteoglycan colocalizes with epidermal stem cells, melanoma-associated chondroitin sulfate proteoglycan expression within the hair follicle was more complex and multiple subpopulations of basal outer root sheath keratinocytes are described. These data suggest that epithelial compartmentalization of the outer root sheath is more complex than interfollicular epidermis and further supports the hypothesis that more than one hair follicle stem cell compartment may exist.

Cdc42-dependent nuclear translocation of non-receptor tyrosine kinase, ACK

Ras signals for the transformation of mammalian cells are apparently transduced through Rho GTPases. The Rho GTPase family member Cdc42 generates independent signals that regulate the rearrangement of the actin cytoskeleton and the transcription of genes. However, the molecular mechanism of signal transduction from Cdc42 to the nucleus remains to be understood. The non-receptor tyrosine kinases ACK-1 and ACK-2 have been found to bind specifically to Cdc42. In this paper we studied whether ACKs transduce Cdc42 signals to the nucleus directly, or through other cytoplasmic proteins. Using immunocytochemistry and Western blot analysis, we found a nuclear localization of ACKs in semi-confluent glioblastoma (U251) cells, as opposed to a cytosolic localization in confluent cells. In agreement with the nuclear localization, a putative nuclear export signal was identified in ACK-1 and ACK-2. Furthermore, the interaction of Cdc42 with ACKs was shown to be essential for the nuclear localization of ACKs. Overexpression of ACK42 (a Cdc42 binding domain of ACK) inhibited cell growth and movement, indicating that Cdc42 signals are transduced to the nucleus through ACKs. This is the first report providing evidence of a novel role for ACKs in transducing Cdc42 signals directly to the nucleus.

Role of melanoma chondroitin sulphate proteoglycan in patterning stem cells in human interfollicular epidermis

Human interfollicular epidermis is renewed by stem cells that are clustered in the basal layer in a patterned, non-random distribution. Stem cells can be distinguished from other keratinocytes by high expression of β1 integrins and lack of expression of terminal differentiation markers; they divide infrequently in vivo but form actively growing colonies in culture. In a search for additional stem cell markers, we observed heterogeneous epidermal expression of melanoma chondroitin sulphate proteoglycan (MCSP). MCSP was expressed by those keratinocytes with the highest β1 integrin levels. In interfollicular epidermis, expression was confined to non-cycling cells and,in culture, to self-renewing clones. However, fluorescence-activated cell sorting on the basis of MCSP and β1 integrin expression gave no more enrichment for clonogenic keratinocytes than sorting for β1 integrins alone. To interfere with endogenous MCSP, we retrovirally infected keratinocytes with a chimera of the CD8 extracellular domain and the MCSP cytoplasmic domain. CD8/MCSP did not affect keratinocyte proliferation or differentiation but the cohesiveness of keratinocytes in isolated clones or reconstituted epidermal sheets was greatly reduced. CD8/MCSP caused stem cell progeny to scatter without differentiating. CD8/MCSP did not alter keratinocyte motility but disturbed cadherin-mediated cell-cell adhesion and the cortical actin cytoskeleton, effects that could be mimicked by inhibiting Rho. We conclude that MCSP is a novel marker for epidermal stem cells that contributes to their patterned distribution by promoting stem cell clustering.

Expression of NG2 proteoglycan during endochondral and intramembranous ossification

We have used immunohistochemistry to study the distribution of the NG2 proteoglycan during bone development in the mouse. At embryonic day 15.5, NG2 was strongly detected in the immature cartilage of developing limbs. After transient down-regulation in mature chondrocytes, NG2 was up-regulated during primary ossification, colocalizing with alkaline phosphatase and tenascin C. In the epiphyseal growth plates of newborn mouse tibia, NG2 and alkaline phosphatase exhibited overlapping patterns of expression by hypertrophic chondrocytes and by osteoblasts surrounding newly formed bone trabeculae. NG2 was down-regulated after puberty, being only faintly detectable in the tibial growth plates of 3-month-old mice. In cranial sutures, NG2 was strongly labeled in osteogenic bone fronts and in the suture matrix. Our results indicate that NG2 expression is up-regulated during both endochondral and intramembranous ossification, but is down-regulated as ossification is completed.

Syndecan-1-mediated cell spreading requires signaling by alphavbeta3 integrins in human breast carcinoma cells

Syndecans are cell surface heparan sulfate proteoglycans with regulatory roles in cell adhesion, proliferation, and differentiation [Annu. Rev. Biochem. 68 (1999) 729]. While the syndecan heparan sulfate chains are essential for matrix binding, less is known about the signaling role of their core proteins. To mimic syndecan-specific adhesion, MDA-MB-231 mammary carcinoma cells were plated on antibodies against syndecan-4 or syndecan-1. While cells adherent via syndecan-4 spread, cells adherent via syndecan-1 do not. However, cells adherent via syndecan-1 can be induced to spread by Mn(2+), suggesting that activation of a beta(1) or beta(3) integrin partner is required. Surprisingly, pretreatment of cells with a function-activating beta(1) antibody does not induce spreading, whereas function-blocking beta(1) integrin antibodies do, suggesting involvement of a beta(1)-to-beta(3) integrin cross-talk. Indeed, blockade of beta(1) integrin activation induces alpha(v)beta(3) integrin activation detectable by soluble fibrinogen binding. Spreading in response to syndecan-1 is independent of integrin-ligand binding. Furthermore, competition with soluble murine syndecan-1 ectodomain, which does not disrupt cell adhesion, nonetheless blocks the spreading mechanism. These data suggest that the ectodomain of the syndecan-1 core protein directly participates in the formation of a signaling complex that signals in cooperation with alpha(v)beta(3) integrins; signaling via this complex is negatively regulated by beta(1) integrins.

A synthetic peptide from the heparin-binding domain III (repeats III4-5) of fibronectin promotes stress-fibre and focal-adhesion formation in melanoma cells

Cell adhesion to fibronectin results in formation of actin stress fibres and focal adhesions. In fibroblasts, this response requires two co-operative signals provided by interactions of the RGD sequence with alpha5beta1 integrin and the heparin-binding domain II (Hep II) domain with syndecan-4. Within Hep II, this activity was mapped to repeat III13 and to the peptide FN-C/H-V(WQPPRARITGY, repeat III14). We previously described that the synthetic heparin-binding peptide/III5 (HBP/III5) (WTPPRAQITGYRLTVGLTRR, repeat III5) binds heparin and mediates cell adhesion via chondroitin sulphate proteoglycans. We have now studied whether HBP/III5 co-operates with alpha5beta1 and drives a full cytoskeletal response in melanoma cells. SKMEL-178 cells attached and spread on the RGD-containing FNIII7-FNIII10 (FNIII7-10) fragment, but did not form stress fibres or focal adhesions. Co-immobilization of HBP/III5 with FNIII7-10 or adding soluble HBP/III5 to cells prespread on FNIII7-10, effectively induced these structures. Cell transfection with dominant-negative N19RhoA, a member of the small GTPase family, abolished the HBP/III5 effect. Both chondroitinase and heparitinase diminished focal adhesions, indicating that both types of proteoglycans bound HBP/III5 in melanoma cells. We have mapped the active sequence of HBP/III5 to YRLTVGLTRR, which is a novel sequence in fibronectin with focal-adhesion-promoting activity. The last two arginine (R) residues of this sequence are required for activity, since their replacement by alanine completely abrogated the HBP/III5 cytoskeletal effect. Moreover, this sequence is also active in the context of large fibronectin fragments. Our results establish that the Hep III region provides co-operative signals to alpha5beta1 for the progression of the cytoskeletal response and that these include activation of RhoA.

Expression of the proteoglycans versican and mel-CSPG in dysplastic nevi

Nevi with architectural disorder and cytologic atypia of melanocytes (NAD) (also called dysplastic nevi) have been controversial with regard to their relationship with melanoma risk and to their gradation in 3 degrees of atypia. Versican and the melanoma-associated proteoglycan (mel-CSPG) are 2 major proteoglycans expressed by malignant melanoma, and they have a role in the regulation of cell adhesion, migration, and differentiation. We evaluated the differences in versican and mel-CSPG expression in nevi, NAD with several degrees of atypia, and primary malignant melanoma. Immunoreactivity for versican was negative in benign melanocytic nevi, positive in NAD (ranging from weakly to intensely positive), and intensely positive in malignant melanoma. Immunostaining for mel-CSPG was negative in benign melanocytic nevi and mild to moderately positive in NAD and melanoma. Our results suggest that versican expression may be of value for distinguishing NAD from benign melanocytic nevi and for distinguishing severe NAD from mild and moderate NAD.

NG2 proteoglycan mediates beta1 integrin-independent cell adhesion and spreading on collagen VI

Collagens V and VI have been previously identified as specific extracellular matrix (ECM) ligands for the NG2 proteoglycan. In order to study the functional consequences of NG2/collagen interactions, we have utilized the GD25 cell line, which does not express the major collagen-binding beta(1) integrin heterodimers. Use of these cells has allowed us to study beta(1) integrin-independent phenomena that are mediated by binding of NG2 to collagens V and VI. Heterologous expression of NG2 in the GD25 line endows these cells with the capability of attaching to surfaces coated with collagens V and VI. The specificity of this effect is emphasized by the failure of NG2-positive GD25 cells to attach to other collagens or to laminin-1. More importantly, NG2-positive GD25 cells spread extensively on collagen VI. beta(1) integrin-independent extension of ruffling lamellipodia demonstrates that engagement of NG2 by the collagen VI substratum triggers signaling events that lead to rearrangement of the actin cytoskeleton. In contrast, even though collagens V and VI each bind to the central segment of the NG2 ectodomain, collagen V engagement of NG2 does not trigger cell spreading. The distinct morphological consequences of NG2/collagen VI and NG2/collagen V interaction indicate that closely-related ECM ligands for NG2 differ in their ability to initiate transmembrane signaling via engagement of the proteoglycan.

Engagement of the NG2 proteoglycan triggers cell spreading via rac and p130cas

Cells that express the NG2 proteoglycan will spread on surfaces coated with monoclonal antibodies against this membrane-spanning protein. On surfaces coated with the N143 monoclonal antibody, this cell spreading occurs by extension of lamellipodia, suggesting that activation of the small GTPase rac is involved in the observed morphological change. Support for this hypothesis comes from the finding of increased levels of GTP-bound rac in cells spreading on N143-coated surfaces. Furthermore, lamellipodia extension is blocked by transfection of cells with the dominant negative rac construct N17rac, but not by transfection with N17cdc42. Formation of lamellipodia on the N143-coated surfaces is also inhibited by transfection of the dominant negative CasdeltaSD construct. This result implicates p130cas as an additional functional player in NG2-mediated cell spreading.

A novel repeat in the melanoma-associated chondroitin sulfate proteoglycan defines a new protein family

The human melanoma-associated chondroitin sulfate proteoglycan (MCSP) and its rat ortholog NG2 are thought to play important roles in angiogenesis-dependent processes like wound healing and tumor growth. Based on electron microscopy studies, the highly glycosylated ectodomain of NG2 has been subdivided into the globular N-terminus, a flexible rod-like central region and a C-terminal portion in globular conformation. We identified a novel repeat named CSPG in the central ectodomain of NG2, MCSP and other proteins from fly, worm, human, sea urchin and a cyanobacterium which shows similarity to cadherin repeats. As earlier electron microscopy studies indicate, the folding of the tandem repeats compresses the length of the proposed repeat region by a factor of approximately 10 compared to the fully extended peptide chain. We identified two conserved negatively charged residues which might govern the binding properties of CSPG repeats. The phyletic distribution of CSPG repeats suggests that horizontal gene transfer contributed to their evolutionary history.

Monitoring gene expression profile changes in bladder transitional cell carcinoma using cDNA microarray

PURPOSE

Differential gene expression profiles between normal bladder mucosas and bladder transitional cell carcinomas TCC were detected.

MATERIALS AND METHODS

cDNA microarrays were prepared by spotting PCR products of 12,800 human genes onto specially treated glass slides. The cDNA probes were prepared by labeling normal bladder mucosa mRNA and TCC tissue mRNA with Cy3-dUTP and Cy5-dUTP respectively through reverse transcription. The arrays were then hybridized against the cDNA probe mixture and the fluorescent signals were scanned. The ratios of Cy5/Cy3 were computed. Northern analysis was used to confirm the results of microarray hybridization.

RESULTS

Eighty-three genes (0.65%), whose ratios of Cy5/Cy3 were greater than 4.0 or less than 0.25, were screened out after 10 groups of hybridization. In the cancerous tissues 28 of them showed higher expression and 55 lower. Twenty-three genes are unregistered in GenBank. These differentially expressed genes are always involved in the physiological processes such as signal transduction, apoptosis and cell cycle, etc.

CONCLUSIONS

This technique provides a powerful method for quantitative analysis of the expression levels of thousands of genes in parallel, and is used to identify genes involved in TCC carcinogenesis. The data obtained by this means are comparable to those obtained by other methods. Using cDNA microarrays to define alterations in gene expression associated with a specific cancer may be an efficient way to uncover the clues to specific molecular derangements that account for its pathogenesis and thus identify potential targets for therapeutic intervention.

Effect of transforming growth factor-beta1, insulin-like growth factor-I, and hepatocyte growth factor on proteoglycan production and regulation in canine melanoma cell lines

OBJECTIVE

To identify extracellular proteoglycans produced by canine melanoma cell lines and analyze the effect of transforming growth factor-beta1 (TGF-beta1), insulin-like growth factor-I (IGF-I), and hepatocyte growth factor (HGF) on these proteoglycans.

SAMPLE POPULATION

3 canine melanoma cell lines (ie, CML-1, CML-6M, and CML-10c2).

PROCEDURE

Extracellular proteoglycans were analyzed by use of metabolic labeling and western immunoblot analysis. The effect of TGF-beta1 on cell proliferation was determined by incorporation of 5-bromo-2'-deoxyuridine.

RESULTS

The CML-1 and CML-6M melanoma cell lines produced 2 main extracellular proteoglycans. One of them was identified as versican, a proteoglycan found in undifferentiated human melanoma cell lines. The CML-10c2 cells produced a small amount of extracellular proteoglycans. Addition of TGF-beta1 (1.25 to 6.25 ng/ml) increased the release of sulfated proteoglycans into the medium. The TGF-beta1 had mainly a posttranslational effect, because it increased the molecular mass of the sulfated bands. Addition of IGF-I (50 ng/ml) slightly increased production of proteoglycans in the CML-6M cell line, whereas HGF (50 ng/ml) did not have any effect on proteoglycan production.

CONCLUSIONS AND CLINICAL RELEVANCE

The proteoglycan content and response toTGF-beta1 treatment for CML-1 and CML-6M canine melanoma cell lines are similar to that for undifferentiated human melanoma cell lines. In contrast, CML-10c2 cells produced a low amount of proteoglycans with high molecular weight. Because these extracellular proteoglycans are involved in the control of cell adhesion, proliferation, and migration, they may play an important role in the progression of melanomas in dogs.

Proteoglycans and tumor progression: Janus-faced molecules with contradictory functions in cancer

Understanding the details of the molecular mechanism of tumor dissemination revealed that several proteoglycan species are involved in the process but their role can be described as Janus-faced. One level of proteoglycan alterations is at the expression of their genes coding for the core protein. Characteristically, in progressing tumors two patterns emerged: loss or neoexpression of surface proteoglycans (PG) depending on the initial expression pattern of the cell type of origin. The situation is similarly complex concerning the changes of glycosaminoglycan (GAG) of the PG during tumor progression. This is due to the fact that the majority of PGs involved is hybrid molecule meaning that their core protein can be glycanated both with chondroitin and heparan sulfate. However, such an alteration in glycanation of PG may fundamentally change the function of the molecule, especially the one operating at the cell surface. Among the extracellular PGs, decorin emerged as inhibitor of progression while perlecan as a promoter of the process. Analysis of the available data indicate that during metastatization tumor cells must express at least one cell surface HSPG species from the syndecan-glypican-CD44v3 group. Furthermore, the HS-chain of these proteoglycan(s) carry important molecular signatures (suphution or epimerization patterns). Experimental data suggest that tumor cell surface heparan sulfate (PG) may provide a target for specific anti-metastatic interventions.

ACK family tyrosine kinase activity is a component of Dcdc42 signaling during dorsal closure in Drosophila melanogaster

We have characterized Drosophila melanogaster ACK (DACK), one of two members of the ACK family of nonreceptor tyrosine kinases in Drosophila. The ACKs are likely effectors for the small GTPase Cdc42, but signaling by these proteins remains poorly defined. ACK family tyrosine kinase activity functions downstream of Drosophila Cdc42 during dorsal closure of the embryo, as overexpression of DACK can rescue the dorsal closure defects caused by dominant-negative Dcdc42. Similar to known participants in dorsal closure, DACK is enriched in the leading edge cells of the advancing epidermis, but it does not signal through activation of the Jun amino-terminal kinase cascade operating in these cells. Transcription of DACK is responsive to changes in Dcdc42 signaling specifically at the leading edge and in the amnioserosa, two tissues involved in dorsal closure. Unlike other members of the ACK family, DACK does not contain a conserved Cdc42-binding motif, and transcriptional regulation may be one route by which Dcdc42 can affect DACK function. Expression of wild-type and kinase-dead DACK transgenes in embryos, and in the developing wing and eye, reveals that ACK family tyrosine kinase activity is involved in a range of developmental events similar to that of Dcdc42.

Modulation of HeLa cells spreading by the non-receptor tyrosine kinase ACK-2

The CDC42 regulated non-receptor tyrosine kinase ACK-2 has been associated with integrin signaling. In this report, the effect of ACK-2 on the modulation of cell spreading and motility was examined. HeLa cells expressing epitope-tagged wild type ACK-2 showed a slower rate of spreading on fibronectin when compared with untransfected cells. An ACK-2 protein lacking its SH3 domain was still capable of modulating HeLa cell spreading suggesting that its tyrosine kinase activity is sufficient to induce the observed phenotype. The ACK-2 effect on the rate of cell spreading did not involve inhibition of integrin-mediated activation of PI-3K signaling, since it did not alter membrane translocation of a GFP-PH-AKT domain (AKT pleckstrin homology domain) used as a reporter for PI-3K products induced by cell adhesion. The ACK-2 effect appears to be upstream from the adapter protein CrkII, since co-expression of CrkII and ACK-2 results in a neutralization of ACK-2 mediated effects on HeLa cell spreading. Similarly, co-expression of p130Cas, which interacts with the adapter protein CrkII, with ACK-2, also results in a partial reversion of the ACK-2 effects on cell spreading. CrkII mediated reversal of the ACK-2 induced phenotype requires the activity of the small GTPase, Rap1. Co-expression of ACK-2 and CrkII with a dominant negative form of Rap1 reverses the neutralization by CrkII suggesting that CrkII mediated activation of Rap1 is required. However, an active form of Rap1 is not sufficient to reverse the ACK-2 phenotype by itself. A role for Rac1 in ACK-2 effects was also established. An activated Rac1 protein neutralized the ACK-2 mediated inhibition of cell spreading. A direct measurement of cell motility by either a modified Boyden chamber or wounding assay demonstrates that ACK-2 overexpression increases the motility of the cells. These results suggest that ACK-2 modulates HeLa cells spreading upstream of pathways regulated by CrkII and that ACK-2 may regulate cell motility by controlling the activation of small GTPases such as Rap1 and Rac1.

Alternative splicing of the IIICS domain in fibronectin governs the role of the heparin II domain in fibrillogenesis and cell spreading

The Heparin (Hep) II-binding domain of fibronectin regulates the formation of focal adhesions and actin stress fibers and hence plays an important role in cell spreading, migration, and fibronectin fibrillogenesis. Using human skin fibroblast cultures, we demonstrate that alternative splicing of the neighboring IIICS domain may regulate the activities of the Hep II domain in cell spreading and fibronectin fibrillogenesis. Recombinant Hep II domains, adjacent to either the IIICS domain or the H89 splice variant that contains the amino-terminal sequence of the IIICS domain, blocked fibronectin fibrillogenesis and required sulfated proteoglycans to mediate cell spreading. If the Hep II domain was adjacent to either the H0 or H95 splice variants, which both lack the amino terminus of the IIICS domain, fibrillogenesis was not inhibited and cell spreading was independent of a sulfated proteoglycan-mediated mechanism. The effect of the splice variants on the Hep II domain could be mimicked using a Hep II domain that contained only 6 amino acids from the III(15) repeat or 10 amino acids from the IIICS domain suggesting that sequences proximal to the III(14) repeat determined the role of the Hep II domain in these processes. We propose that alternative splicing of the IIICS domain modulates interactions between heparan sulfate proteoglycans and the Hep II domain and that this serves as a mechanism to control the biological activities of fibronectin.

Signal transduction by cell adhesion receptors and the cytoskeleton: functions of integrins, cadherins, selectins, and immunoglobulin-superfamily members

Cellular interactions with the extracellular matrix and with neighboring cells profoundly influence a variety of signaling events including those involved in mitogenesis, survival, and differentiation. Recent advances have provided insights into mechanisms underlying the ability of integrins, cadherins, selectins, and other cell adhesion molecules to regulate signal transduction cascades. These mechanisms often involve the ability of cell adhesion molecules to initiate the formation of organized structures or scaffolds that permit the efficient flow of information in signaling pathways.

HSH2: a novel SH2 domain-containing adapter protein involved in tyrosine kinase signaling in hematopoietic cells

We isolated a cDNA clone encoding a novel Src homology (SH)2 domain-containing protein of 47 kDa from a human cDNA library. As its transcript was predominantly expressed in hematopoietic cells, this gene was termed HSH2 for hematopoietic SH2 protein. This protein contains several putative protein-binding motifs, SH3-binding proline-rich regions, and phosphotyrosine sites, but lacks enzymatic motifs. In a yeast two-hybrid screen, we identified a cytokine-regulated tyrosine kinase c-FES and an activated Cdc42-associated tyrosine kinase ACK1 as HSH2 interactors. HSH2 bound c-FES via its C-terminal region as well as its N-terminal region including the SH2 domain, whereas it bound ACK1 via its N-terminal proline-rich region. Furthermore, these two kinases bound and tyrosine-phosphorylated HSH2 in mammalian cells. Hence, we postulate that HSH2 functions as an adapter protein involved in tyrosine kinase signaling, and possibly regulates cytokine signaling and cytoskeletal reorganization, in hematopoietic cells.

Functions of the adapter protein Cas: signal convergence and the determination of cellular responses

Since Cas was first identified as a highly phosphorylated 130 kilodalton protein that associated with the v-Src and v-Crk-oncoproteins, considerable effort has been made to determine its function. Its predicted role as a scaffolding molecule based on its domain structure has been largely confirmed. Through its ability to undergo rapid changes in phosphorylation, subcellular localization and association with heterologous proteins, Cas may spatially and temporally regulate the function of its binding partners. Numerous proteins have been identified that bind to Cas in vitro and/or in vivo, but in only a few cases is there an understanding of how Cas may function in these protein complexes. To date, Cas-Crk and Cas-Src complexes have been most frequently implicated in Cas function, particularly in regards to processes involving regulation of the actin cytoskeleton and proliferation. These and other Cas protein complexes contribute to the critical role of Cas in cell adhesion, migration, proliferation and survival of normal cycling cells. However, under conditions in which these processes are deregulated, Cas appears to play a role in oncogenic transformation and perhaps metastasis. Therefore, in its capacity as an adapter protein, Cas serves as a point of convergence for many distinct signaling inputs, ultimately contributing to the generation of specific cellular responses.

Occurrence of oligosialic acids on integrin alpha 5 subunit and their involvement in cell adhesion to fibronectin

Integrin alpha(5)beta(1), a major fibronectin receptor, functions in a wide variety of biological phenomena. We have found that alpha 2-8-linked oligosialic acids with 5 < or = degree of polymerization (DP) < or = 7 occur on integrin alpha(5) subunit of the human melanoma cell line G361. The integrin alpha(5) subunit immunoprecipitated with anti-integrin alpha(5) antibody reacted with the monoclonal antibody 12E3, which recognizes oligo/polysialic acid with DP > or = 5 but not with the polyclonal antibody H.46 recognizing oligo/polysialic acid with DP > or = 8. The occurrence of oligosialic acids was further demonstrated by fluorometric C(7)/C(9) analysis on the immunopurified integrin alpha(5) subunit. Oligosialic acids were also found in the alpha(5) subunit of several other human cells such as foreskin fibroblast and chronic erythroleukemia K562 cells. These results suggest the ubiquitous modification with unique oligosialic acids occurs on the alpha(5) subunit of integrin alpha(5)beta(1). The adhesion of human melanoma G361 cells to fibronectin was mainly mediated by integrin alpha(5)beta(1). Treatment of cells with sialidase from Arthrobacter ureafaciens cleaving alpha 2-3-, alpha 2-6-, and alpha 2-8-linked sialic acids inhibited adhesion to fibronectin. On the other hand, N-acetylneuraminidase II, which cleaves alpha 2-3 and alpha 2-6 but not alpha 2-8 linkages, showed no inhibitory activity. After the loss of oligosialic acids, integrin alpha(5)beta(1) failed to bind to fibronectin-conjugated Sepharose, indicating that the oligosialic acid on the alpha(5) subunit of integrin alpha(5)beta(1) plays important roles in cell adhesion to fibronectin.

Chondroitin sulfate and cytoplasmic domain-dependent membrane targeting of the NG2 proteoglycan promotes retraction fiber formation and cell polarization

Targeting of the NG2 proteoglycan to cellular retraction fibers was studied by expressing mutant NG2 molecules lacking specific structural elements of the proteoglycan. Both the cytoplasmic domain and the chondroitin sulfate chain of NG2 appear to have roles in sorting NG2 to subcellular microdomains destined to become retraction fibers. Neither of these structural features alone is sufficient to allow optimal targeting of NG2 to retraction fibers, but together they promote efficient localization of the proteoglycan to these sites. This pattern of NG2 sorting seems to be necessary for optimal retraction fiber formation, as cells expressing poorly targeted NG2 mutants are noticeably deficient in their ability to extend retraction fibers. Furthermore, retraction fiber formation correlates strongly with the tendency of cells to assume a polarized morphology with NG2-positive retraction fibers at one pole of the cell and actin-rich lamellipodia at the other. This polarization can be triggered either through engagement of NG2 by the substratum or by exposure to lysophosphatidic acid, a potent activator of the rho GTPase. These results suggest a possible role for NG2 in regulating rho-dependent mechanisms in the trailing processes of motile cells.

Epidermal growth factor stimulation of the ACK1/Dbl pathway in a Cdc42 and Grb2-dependent manner

The tyrosine kinase ACK1 phosphorylates and activates the guanine nucleotide exchange factor Dbl, which in turn directs the Rho family GTP-binding proteins. However, the regulatory mechanism of ACK1/Dbl signaling in response to extracellular stimuli remains poorly understood. Here we describe that epidermal growth factor stimulates the ACK1/Dbl pathway, leading to actin cytoskeletal rearrangements. The role of the two ACK1-binding proteins Cdc42 and Grb2 was assessed by overexpression of the Cdc42/Rac interactive binding domain and a dominant-negative Grb2 mutant, respectively. Specific inhibition of the interaction of ACK1 with Cdc42 or Grb2 by the use of these constructs diminished tyrosine phosphorylation of both ACK1 and Dbl in response to EGF. Therefore, the activation of ACK1 and subsequent downstream signaling require both Cdc42-dependent and Grb2-dependent processes within the cell. In addition, we show that EGF transiently induces formation of the focal complex and stress fibers when ACK1 was ectopically expressed. The induction of these structures was totally sensitive to the action of botulinum toxin C from Clostridium botulinum, suggesting a pivotal role of Rho. These results provide evidence that ACK1 acts as a mediator of EGF signals to Rho family GTP-binding proteins through phosphorylation and activation of GEFs such as Dbl.

The AN2 protein is a novel marker for the Schwann cell lineage expressed by immature and nonmyelinating Schwann cells

The expression of the 330 kDa AN2 glycoprotein was studied in the rodent peripheral nervous system. AN2 is expressed by immature Schwann cells in vitro and in vivo and downregulated as the cells upregulate myelin genes. A subpopulation of nonmyelinating Schwann cells in the adult sciatic nerve retains expression of AN2. In rat sciatic nerve crushes, where Schwann cell numbers increase after initial axonal loss and markers of immature Schwann cells show an upregulation, no increased expression of AN2 was observed. In contrast, AN2 expression was upregulated in nerves from peripheral myelin protein-22-transgenic rats, where immature Schwann cells expand without axonal loss. Furthermore, coculture with neurons upregulated AN2 expression on Schwann cells in vitro. Polyclonal antibodies against AN2 inhibited the migration of an immortalized Schwann cell clone in an in vitro migration assay, and the purified AN2 protein was shown to be neither inhibitory nor permissive for outgrowing dorsal root ganglion neurites. AN2 is thus a novel marker for the Schwann cell lineage. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis of purified AN2 from early postnatal mouse brain demonstrated that AN2 is the murine homolog of the rat NG2 proteoglycan.

Melanoma chondroitin sulfate proteoglycan regulates matrix metalloproteinase-dependent human melanoma invasion into type I collagen

Tumor cell adhesion and proteolysis of the extracellular matrix proteins surrounding the cells are tightly linked processes in tumor invasion. In this study, we sought to identify components of the cell surface of a vertical growth phase melanoma cell line, WM1341D, that mediate invasive cellular behavior. We determined by antisense inhibition that melanoma chondroitin sulfate proteoglycan (MCSP) and membrane-type 3 matrix metalloproteinase (MT3-MMP) expressed on WM1341D are required for invasion of type I collagen and degradation of type I gelatin. MT3-MMP co-immunoprecipitated with MCSP in WM1341D melanoma cells cultured on type I collagen or laminin. The association between MT3-MMP and MCSP was largely disrupted by removing chondroitin sulfate glycosaminoglycan (CS) from the cell surface, suggesting CS could mediate the association between the two cell surface core proteins. Recombinant MT3-MMP and MT3-MMP from whole cell lysates of WM1341D cells were specifically eluted from CS- conjugated affinity columns. The results indicate that MT3-MMP possesses the potential to promote melanoma invasion and proteolysis and that the formation of a complex between MT3-MMP and MCSP may be a crucial step in activating these processes.

The tyrosine kinase ACK1 associates with clathrin-coated vesicles through a binding motif shared by arrestin and other adaptors

One target for the small GTPase Cdc42 is the nonreceptor tyrosine kinase activated Cdc42-associated kinase (ACK), which binds selectively to Cdc42.GTP. We report that ACK1 can associate directly with the heavy chain of clathrin. A central region in ACK1 containing a conserved motif behaves as a clathrin adaptor and competes with beta-arrestin for a common binding site on the clathrin N-terminal head domain. Overexpressed ACK1 perturbs clathrin distribution, an activity dependent on the presence of C-terminal "adaptor" sequences that are also present in the related nonkinase gene 33. ACK1 interacts with the adaptor Nck via SH3 interactions but does not form a trimeric complex with p21-activated serine/threonine kinase, which also binds Nck. Stable low level expression of green fluorescent protein-ACK1 in NIH 3T3 cells has been used to localize ACK1 to clathrin-containing vesicles. The co-localization of ACK1 in vivo with clathrin and AP-2 indicates that it participates in trafficking, underlying an ability to increase receptor-mediated transferrin uptake.

Mammalian mitogen-activated protein kinase signal transduction pathways activated by stress and inflammation

The molecular details of mammalian stress-activated signal transduction pathways have only begun to be dissected. This, despite the fact that the impact of these pathways on the pathology of chronic inflammation, heart disease, stroke, the debilitating effects of diabetes mellitus, and the side effects of cancer therapy, not to mention embryonic development, innate and acquired immunity, is profound. Cardiovascular disease and diabetes alone represent the most significant health care problems in the developed world. Thus it is not surprising that understanding these pathways has attracted wide interest, and in the past 10 years, dramatic progress has been made. Accordingly, it is now becoming possible to envisage the transition of these findings to the development of novel treatment strategies. This review focuses on the biochemical components and regulation of mammalian stress-regulated mitogen-activated protein kinase (MAPK) pathways. The nuclear factor-kappa B pathway, a second stress signaling paradigm, has been the subject of several excellent recent reviews (258, 260).