Tumor necrosis factor alpha stimulates invasion of Src-activated intestinal cells

BACKGROUND & AIMS

Src activation is correlated with progression of colorectal cancer (CRC). CRCs accompanied by ulcerative colitis, chronic inflammation in the colon, often have elevated Src activity, and ulcerative colitis-related CRCs are more likely to become invasive, whereas Ras activation is rarely associated with this disease. The aim of this study was to investigate the effects of a proinflammatory cytokine, tumor necrosis factor alpha (TNF-alpha), on the invasive properties of epithelial cells constitutively expressing activated Ras or Src.

METHODS

A cell line derived from intestinal epithelia was transfected with a v-src- or v-H-ras-expressing vector. The effect of TNF-alpha on morphologic changes in colonies cultured in soft agar was determined. Src protein kinase activity, peroxide production, E-cadherin expression levels, and the phosphorylation status of beta-catenin and E-cadherin were determined. The invasive potential of these cells was determined by measuring cell motility and using an in vitro invasion assay.

RESULTS

TNF-alpha altered the colony morphology of src-, but not ras-expressing cells. TNF-alpha increased peroxide production, leading to Src protein expression as well as Src activity in src transfectants. Activation of Src by TNF-alpha led to reduced E-cadherin levels and enhanced invasion of src transfectants. Pyrrolidine dithiocarbamate and herbimycin A inhibited these effects.

CONCLUSION

These results indicate that Src kinase activation enhances the response of epithelial cells to TNF-alpha leading to increased invasion through mechanisms that involve production of reactive oxygen intermediates.

PSTPIP is a substrate of PTP-PEST and serves as a scaffold guiding PTP-PEST toward a specific dephosphorylation of WASP

PSTPIP is a tyrosine-phosphorylated protein involved in the organization of the cytoskeleton. Its ectopic expression induces filipodial-like membrane extensions in NIH 3T3 cells. We previously observed a defect in cytokinesis and an increase in the tyrosine phosphorylation of PSTPIP in PTP-PEST-deficient fibroblasts. In this article, we demonstrate that PTP-PEST and PSTPIP are found in the same complexes in vivo and that they interact directly through the CTH domain of PTP-PEST and the coiled-coil domain of PSTPIP. We tested pathways that could regulate the tyrosine phosphorylation of PSTPIP. We found that the activation of the epidermal growth factor and platelet-derived growth factor receptors can induce PSTPIP phosphorylation. With the use of the PP2 inhibitor, we demonstrate that Src kinases are not involved in the epidermal growth factor-mediated phosphorylation of PSTPIP. Together with previous results, this suggests that c-Abl is the critical tyrosine kinase downstream of growth factor receptors responsible for PSTPIP phosphorylation. We also demonstrate that PTP-PEST dephosphorylates PSTPIP at tyrosine 344. Importantly, we identified tyrosine 344 as the main phosphorylation site of PSTPIP by performing tryptic phosphopeptide maps. This is an important finding since tyrosine 367 of PSTPIP was also proposed as a candidate phosphorylation site involved in the negative regulation of the association between PSTPIP and WASP. In this respect, we observed that the PSTPIP.WASP complex is stable in vivo and is not affected by the phosphorylation of PSTPIP. Furthermore, we demonstrate that PSTPIP serves as a scaffold protein between PTP-PEST and WASP and allows PTP-PEST to dephosphorylate WASP. This finding suggests a possible mechanism for PTP-PEST to directly modulate actin remodeling through the PSTPIP-WASP interaction.

Increased activity of c-Src and Csk in fibroblasts transformed by v-src oncogene

When c-Src and v-Src were immunoprecipitated together from hamster fibroblasts transformed by Rous sarcoma virus containing v-src oncogene, the total Src activity was almost threefold higher compared to c-Src activity in the control cells. The activity of v-Src immunoprecipitated separately, however, accounting for only 40% of the total Src activity, indicating that c-Src is activated upon transformation. An increased activity of Csk was also found in RSV-transformed cells. It decreased upon serum stimulation in parallel with an increase in Src kinase activity. In nontransformed cells, serum stimulation induced an enhanced Csk activity, but no changes in c-Src activity were observed. This may suggest that Csk may have more functions in hamster fibroblasts, in addition to its inhibitory effect on c-Src.

A chemical genetic screen for direct v-Src substrates reveals ordered assembly of a retrograde signaling pathway

Using an ATP analog that is a specific substrate for an analog-specific allele of v-Src, we identified several novel cytoskeletal substrates that control actin assembly processes. A screen for less abundant v-Src substrates revealed the scaffolding protein Dok-1 as a direct substrate of v-Src. Further studies suggest that v-Src phosphorylation sites on Dok-1 are critical for its binding to RasGAP and Csk, negative regulators of Src signaling. This results in the downregulation of growth-promoting signals of the Src family kinases and the Ras pathway. Identification of the direct substrates of v-Src leads to a model for the precise order of assembly of a retrograde signaling pathway in v-Src-transformed cells and has provided new insight into the balance between those signals that promote cell transformation mediated by v-Src catalyzed tyrosine phosphorylation and those that inhibit it.

v-Src-induced modulation of the calpain-calpastatin proteolytic system regulates transformation

v-Src-induced oncogenic transformation is characterized by alterations in cell morphology, adhesion, motility, survival, and proliferation. To further elucidate some of the signaling pathways downstream of v-Src that are responsible for the transformed cell phenotype, we have investigated the role that the calpain-calpastatin proteolytic system plays during oncogenic transformation induced by v-Src. We recently reported that v-Src-induced transformation of chicken embryo fibroblasts is accompanied by calpain-mediated proteolytic cleavage of the focal adhesion kinase (FAK) and disassembly of the focal adhesion complex. In this study we have characterized a positive feedback loop whereby activation of v-Src increases protein synthesis of calpain II, resulting in degradation of its endogenous inhibitor calpastatin. Reconstitution of calpastatin levels by overexpression of exogenous calpastatin suppresses proteolytic cleavage of FAK, morphological transformation, and anchorage-independent growth. Furthermore, calpastatin overexpression represses progression of v-Src-transformed cells through the G(1) stage of the cell cycle, which correlates with decreased pRb phosphorylation and decreased levels of cyclins A and D and cyclin-dependent kinase 2. Calpain 4 knockout fibroblasts also exhibit impaired v-Src-induced morphological transformation and anchorage-independent growth. Thus, modulation of the calpain-calpastatin proteolytic system plays an important role in focal adhesion disassembly, morphological transformation, and cell cycle progression during v-Src-induced cell transformation.

Regulation of extracellular signal-regulated kinase activity by p120 RasGAP does not involve its pleckstrin homology or calcium-dependent lipid binding domains but does require these domains to regulate cell proliferation

The gene encoding for p120 RasGAP, has been disrupted in mice (M. Henkemeyer et al., Nature (Lond.), 377: 695-701, 1995). In this study, using fibroblasts derived from these mouse embryos (Gap-/-; P. van der Geer et al., Mol. Cell Biol., 17: 1840-1847, 1997), we demonstrate that mitogen-activated protein kinase (MAPK) activation is prolonged after epidermal growth factor (EGF), but not lysophosphatidic acid, stimulation as compared with wild-type cells. Furthermore, these cells exhibited a moderate increase in their proliferative rate and saturation density, as well as a limited ability to form colonies in soft agar. Stable cell lines expressing full-length p120GAP not only restored the ability to down-regulate MAPK after EGF stimulation but also lowered their saturation densities. Similarly, expression of p120GAP, missing either its pleckstrin homology (PH) or its calcium-dependent lipid binding (CaLB)/C2 domain, restored MAPK down-regulation and retained the ability to associate with p190 RhoGAP and to be phosphorylated by v-src but exhibited higher saturation densities similar to Gap-/- cells. Our results, therefore, suggest that p120GAP functions not only by down-regulating the Ras/MAPK pathway after growth factor stimulation but is also important in regulating cell proliferation that involves its PH and CaLB domains.

Transformation of chicken embryo fibroblasts by v-src uncouples beta1 integrin-mediated outside-in but not inside-out signaling

Adhesion of cells to extracellular matrix is mediated by integrin family receptors. The process of receptor-ligand binding is dependent on metabolic energy and is regulated by intracellular signals, termed inside-out signals. The strength of the initial alpha5beta1-mediated adhesion of v-src-transformed chicken embryo fibroblasts (v-srcCEF) was similar to that of normal CEF. A chemically cross-linked fibronectin substrate was able to restore cell spreading and the ability of v-srcCEF to assemble a fibronectin matrix. Over time, v-srcCEF showed decreased adhesion due to the reduction of alpha5beta1-fibronectin bonds consequent on the reduction of substrate-bound fibronectin due to the secretion of proteases by v-srcCEF. Excess synthesis of hyaluronic acid by v-srcCEF also reduced the alpha5beta1-fibronectin bonds and contributed to cell detachment at later times in culture. Thus, the adhesion defects were not due to a failure of alpha5beta1 function and adhesion of the v-srcCEF was alpha5beta1 dependent. Integrin-mediated adhesion also produces signals that affect cell proliferation and cell differentiation. An early consequence of these "outside-in" signals was the phosphorylation of FAK Y397 in direct proportion to the number of alpha5beta1-fibronectin bonds formed. In contrast, v-srcCEF had an increased level of phosphorylation on five different tyrosines in FAK, and none of these phosphorylation levels were sensitive to the number of alpha5beta1-fibronectin bonds. In the absence of serum, CEF proliferation was sensitive to changes in alpha5beta1-mediated adhesion levels. Transformation by v-src increased the serum-free proliferation rate and made it insensitive to alpha5beta1-mediated adhesion. Thus, the v-srcCEF were insensitive to the normal outside-in signals from alpha5beta1 integrin.

Anticarcinogenic effect and enhancement of metastatic potential of BALB/c 3T3 cells by ginsenoside Rh(2)

It has been reported that ginsenoside Rh(2), a purified ginseng saponin with a dammarane skeleton, has anticarcinogenic effects on mammalian cells. To determine the significance of these effects on multistage carcinogenesis, we utilized the BALB / c 3T3 cell system. In an in vitro two-stage neoplastic transformation assay, the initiating activity of 3-methylcholanthrene (3-MCA) was suppressed by Rh(2) (>or= 1 x 10(-5) M) in both BALB / c 3T3 A31-1-1 cells and the more carcinogen-susceptible variant A31-1-13 cells. The suppressive effects in this concentration range were thought to be caused by suppression of DNA replication via indirect Cdk2 inhibition. On the other hand, the promotion steps of both the target cells were not affected by Rh(2) even if the transformation frequency was enhanced by a tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA). To examine the other effects of Rh(2) on carcinogenesis, we turned our attention to the metastatic phenotype. Using metastatic src-transformed A31-1-1 cells, we found that Rh(2) augmented the metastatic potential in an experimental metastasis assay. These data indicate that Rh(2) has diverse effects on the expression of the transformed phenotype in BALB / c 3T3 cells, but support the idea that growth suppression is likely to be a major mechanism of the anticarcinogenic effects of Rh(2).

Protein phosphatase 2A interacts with the Src kinase substrate p130(CAS)

In this study, we report that the Src substrate Cas (p130 Crk-associated substrate) associates with protein phosphatase 2A (PP2A), a serine/threonine phosphatase. We investigated this interaction in cells expressing a temperature-sensitive mutant form of v-Src. v-Src activation (by shifting cells from the nonpermissive to the permissive temperature) led to an increase in the tyrosine phosphorylation of v-Src and Cas, as well as in the association between v-Src and Cas. v-Src has previously been shown to bind to PP2A and to phosphorylate the catalytic subunit of PP2A, resulting in inhibition of phosphatase activity. We found that the association between v-Src and PP2A decreased as cells were shifted to the permissive temperature. In contrast, the levels of PP2A that co-immunoprecipitated with Cas increased when v-Src was activated. We obtained similar results in pull-down experiments with immobilized Microcystin, a PP2A inhibitor. Serine/threonine phosphorylation of Cas has previously been shown to occur in a cell cycle regulated matter. Treatment of NIH3T3 cells with okadaic acid, a PP2A inhibitor, augments the serine/threonine phosphorylation of Cas that occurs at mitosis. Furthermore, PP2A dephosphorylates serine residues on Cas in vitro. Taken together, our results suggest that PP2A may be involved in the cell cycle-specific dephosphorylation of Cas.

A novel group IIA phospholipase A2 interacts with v-Src oncoprotein from RSV-transformed hamster cells

We have isolated a novel isoform of phospholipase A(2). This enzyme was designated srPLA(2) because it was discovered while analyzing the proteins interacting with different forms of the v-Src oncoproteins isolated from Rous sarcoma virus-transformed hamster cells. It contains all the functional regions of the PLA(2) group IIA proteins but differs at its C-terminal end where there is an additional stretch of 8 amino acids. The SrPLA(2) isoform was detected as a 17-kDa precursor in cells and as a mature 14-kDa form secreted in culture medium. A direct interaction of the 17-kDa precursor with the Src protein was observed in lysates of transformed cells. Both the 17- and 14-kDa forms were found to be phosphorylated on tyrosine. To our knowledge, this is the first report of a PLA(2) group II protein that is tyrosine phosphorylated. We surmise that srPLA(2) interacts with the Src protein at the cell membrane during the process of its maturation.

Evaluation of site-directed spin labeling for characterizing protein-ligand complexes using simulated restraints

Simulation studies have been performed to evaluate the utility of site-directed spin labeling for determining the structures of protein-ligand complexes, given a known protein structure. Two protein-ligand complexes were used as model systems for these studies: a 1.9-A-resolution x-ray structure of a dihydrofolate reductase mutant complexed with methotrexate, and a 1.5-A-resolution x-ray structure of the V-Src tyrosine kinase SH2 domain complexed with a five-residue phosphopeptide. Nitroxide spin labels were modeled at five dihydrofolate reductase residue positions and at four SH2 domain residue positions. For both systems, after energy minimization, conformational ensembles of the spin-labeled residues were generated by simulated annealing while holding the remainder of the protein-ligand complex fixed. Effective distances, simulating those that could be obtained from (1)H-NMR relaxation measurements, were calculated between ligand protons and the spin labels. These were converted to restraints with several different levels of precision. Restrained simulated annealing calculations were then performed with the aim of reproducing target ligand-binding modes. The effects of incorporating a few supplementary short-range (< or =5.0 A) distance restraints were also examined. For the dihydrofolate reductase-methotrexate complex, the ligand-binding mode was reproduced reasonably well using relatively tight spin-label restraints, but methotrexate was poorly localized using loose spin-label restraints. Short-range and spin-label restraints proved to be complementary. For the SH2 domain-phosphopeptide complex without the short-range restraints, the peptide did not localize to the correct depth in the binding groove; nevertheless, the orientation and internal conformation of the peptide was reproduced moderately well. Use of the spin-label restraints in conjunction with the short-range restraints resulted in relatively well defined structural ensembles. These results indicate that restraints derived from site-directed spin labeling can contribute significantly to defining the orientations and conformations of bound ligands. Accurate ligand localization appears to require either a few supplementary short-range distance restraints, or relatively tight spin-label restraints, with at least one spin label positioned so that some of the restraints draw the ligand into the binding pocket in the latter case.

Regulation of Akt/PKB activation by tyrosine phosphorylation

Activation of Akt/PKB by growth factors requires multiple phosphorylation events. Phosphorylation of Thr(308) and Ser(473) of Akt by its upstream kinase(s) or autophosphorylation is critical for optimal activation of its kinase activity. Here, we present evidence that tyrosine phosphorylation is required for Akt activation. Epidermal growth factor treatment induces tyrosine phosphorylation of Akt in COS1 and PC3M cells, which is abrogated by PP2, a selective inhibitor for Src family tyrosine kinases. Elevated Akt activity is observed in v-Src transformed NIH3T3 cells, which is accompanied with increased tyrosine phosphorylation of Akt. Akt activity induced by growth factors is significantly reduced in SYF cells lacking Src, Yes, and Fyn, which can be restored by introducing c-Src, but not the kinase-inactive Src, back to these cells. Furthermore, we have identified two tyrosine residues near the activation loop of Akt that are important for its activation. Substitution of these residues with phenylalanine abolishes Akt kinase activity stimulated by growth factors. These two YF mutants fail to block Forkhead transcription factor activity in 293 cells and are unable to prevent apoptosis induced by matrix detachment. Our data suggest that, in addition to phosphorylation of Thr(308) and Ser(473), tyrosine phosphorylation of Akt may be essential for its biological function.

Soluble Jagged 1 represses the function of its transmembrane form to induce the formation of the Src-dependent chord-like phenotype

We have previously demonstrated that the expression of the soluble extracellular domain of the transmembrane ligand for Notch receptors, Jagged 1 (sJ1), in NIH 3T3 cells results in the formation of a matrix-dependent chord-like phenotype, the loss of contact inhibition of growth, and an inhibition of pro-alpha 1(I) collagen expression. In an effort to define the mechanism by which sJ1 induces this phenotype, we report that sJ1 transfectants display biochemical and cytoskeletal alterations consistent with the activation of Src. Indeed, cotransfection of sJ1 transfectants with a dominant-negative mutant of Src resulted in the loss of matrix-dependent chord formation and correlated with the restoration of type I collagen expression and contact inhibition of growth. We also report that the sJ1-mediated induction of Src activity and related phenotypes, including chord formation, may result from the inhibition of endogenous Jagged 1-mediated Notch signaling since it was not possible to detect an sJ1-dependent induction of CSL-dependent transcription in these cells. Interestingly, NIH 3T3 cells transfected with dominant-negative (but not constitutively active) mutants of either Notch 1 or Notch 2 displayed a similar Src-related phenotype as the sJ1 transfectants. These data suggest that the ability of sJ1 to mediate chord formation is Src-dependent and requires the repression of endogenous Jagged 1-mediated Notch signaling, which is tolerant to the destabilization of the actin cytoskeleton, a mediator of cell migration.

v-Src phosphorylation of connexin 43 on Tyr247 and Tyr265 disrupts gap junctional communication

The mechanism by which v-Src disrupts connexin (Cx)43 intercellular gap junctional communication (GJC) is not clear. In this study, we determined that Tyr247 (Y247) and the previously identified Tyr265 (Y265) site of Cx43 were the primary phosphorylation targets for activated Src in vitro. We established an in vivo experimental system by stably expressing v-Src and wild-type (wt) Cx43, or Y247F, Y265F, or Y247F/Y265F Cx43 mutants in a Cx43 knockout mouse cell line. Wt and mutant Cx43 localized to the plasma membrane in the absence or presence of v-Src. When coexpressed with v-Src, the Y247F, Y265F, and Y247F/Y265F Cx43 mutants exhibited significantly reduced levels of tyrosine phosphorylation compared with wt Cx43, indicating that Y247 and Y265 were phosphorylation targets of v-Src in vivo. Most importantly, GJC established by the Y247F, Y265F, and Y247F/Y265F Cx43 mutants was resistant to disruption by v-Src. Furthermore, we did not find evidence for a role for mitogen-activated protein kinase in mediating the disruption of GJC by v-Src. We conclude that phosphorylation on Y247 and Y265 of Cx43 is responsible for disrupting GJC in these mammalian cells expressing v-Src.

Discovery of thioazepinone ligands for Src SH2: from non-specific to specific binding

The structure-based design and synthesis of new thioazepinones as ligands for Src SH2 protein is presented. From benzothioazepinones, ligands with somewhat unspecific binding properties, simpler thioazepinones were designed, the best ones demonstrated nanomolar affinity for Src SH2. A few of these new ligands were crystallized with the protein and demonstrated a specific binding mode with the protein.

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

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

Src is an important mediator of extracellular signal-regulated kinase 1/2-dependent growth signaling by angiotensin II in smooth muscle cells from resistance arteries of hypertensive patients

The role of c-Src in growth signaling by angiotensin (Ang) II was investigated in vascular smooth muscle cells (VSMCs) from arteries of hypertensive patients. c-Src and extracellular signal-regulated kinase 1/2 (ERK1/2) activity, proto-oncogene expression, activating protein-1 (AP-1) DNA-binding activity, and DNA and protein synthesis were studied in Ang II-stimulated VSMCs derived from small peripheral resistance arteries of normotensive subjects (NTs, n=5) and age-matched untreated hypertensive patients (HTs, n=10). Ang II type 1 (AT(1)) and type 2 (AT(2)) receptor status was also assessed. Ang II dose-dependently increased the synthesis of DNA and protein, with enhanced effects in VSMCs from HTs. PD 098,059, a selective inhibitor of the ERK1/2 pathway, attenuated Ang II-stimulated growth in HTs. The effects of PD 098,059 were greater in HTs than in NTs. In NTs, Ang II transiently increased ERK1/2 phosphorylation, whereas in HTs, Ang II-stimulated actions were augmented and sustained. PP2, a selective Src inhibitor, reduced ERK1/2 activity and normalized ERK1/2 responses in HTs. Ang II-induced c-Src phosphorylation was 2- to 3-fold greater in HTs than in NTs. In HTs but not NTs, kinase activation was followed by overexpression of c-fos and enhanced AP-1 DNA-binding activity. PD 098,059 and PP2 attenuated these responses. AT(1) receptor expression was similar in NTs and HTs. In HT cells transfected with c-fos antisense oligodeoxynucleotide, Ang II-stimulated growth was reduced compared with sense oligodeoxynucleotide. Our findings suggest that augmented Ang II-stimulated VSMC growth is mediated via hyperactivation of c-Src-regulated ERK1/2-dependent pathways, leading to overexpression of c-fos mRNA and enhanced AP-1 DNA-binding activity. Because AT(1) receptor expression was unaltered in HTs, increased Ang II signaling may be a postreceptor phenomenon. These data define a signal transduction pathway whereby Ang II mediates exaggerated growth in VSMCs from HTs.

Hyaluronan activates cell motility of v-Src-transformed cells via Ras-mitogen-activated protein kinase and phosphoinositide 3-kinase-Akt in a tumor-specific manner

We investigated the production of hyaluronan (HA) and its effect on cell motility in cells expressing the v-src mutants. Transformation of 3Y1 by v-src virtually activated HA secretion, whereas G2A v-src, a nonmyristoylated form of v-src defective in cell transformation, had no effect. In cells expressing the temperature-sensitive mutant of v-Src, HA secretion was temperature dependent. In addition, HA as small as 1 nM, on the other side, activated cell motility in a tumor-specific manner. HA treatment strongly activated the motility of v-Src-transformed 3Y1, whereas it showed no effect on 3Y1- and 3Y1-expressing G2A v-src. HA-dependent cell locomotion was strongly blocked by either expression of dominant-negative Ras or treatment with a Ras farnesyltransferase inhibitor. Similarly, both the MEK1 inhibitor and the kinase inhibitor clearly inhibited HA-dependent cell locomotion. In contrast, cells transformed with an active MEK1 did not respond to the HA. Finally, an anti-CD44-neutralizing antibody could block the activation of cell motility by HA as well as the HA-dependent phosphorylation of mitogen-activated protein kinase and Akt. Taken together, these results suggest that simultaneous activation of the Ras-mitogen-activated protein kinase pathway and the phosphoinositide 3-kinase pathway by the HA-CD44 interaction is required for the activation of HA-dependent cell locomotion in v-Src-transformed cells.

Tyrosine-phosphorylated low density lipoprotein receptor-related protein 1 (Lrp1) associates with the adaptor protein SHC in SRC-transformed cells

v-Src transforms fibroblasts in vitro and causes tumor formation in the animal by tyrosine phosphorylation of critical cellular substrates. Exactly how v-Src interacts with these substrates remains unknown. One of its substrates, the adaptor protein Shc, is thought to play a crucial role during cellular transformation by v-Src by linking v-Src to Ras. We used Shc proteins with mutations in either the phosphotyrosine binding (PTB) or Src homology 2 domain to determine that phosphorylation of Shc in v-Src-expressing cells depends on the presence of a functional PTB domain. We purified a 100-kDa Shc PTB-binding protein from Src-transformed cells that was identified as the beta chain of the low density lipoprotein receptor-related protein LRP1. LRP1 acts as an import receptor for a variety of proteins and is involved in clearance of the beta-amyloid precursor protein. This study shows that LRP1 is tyrosine-phosphorylated in v-Src-transformed cells and that tyrosine-phosphorylated LRP1 binds in vivo and in vitro to Shc. The association between Shc and LRP1 may provide a mechanism for recruitment of Shc to the plasma membrane where it is phosphorylated by v-Src. It is at the membrane that Shc is thought to be involved in Ras activation. These observations further suggest that LRP1 could function as a signaling receptor and may provide new avenues to investigate its possible role during embryonal development and the onset of Alzheimer's disease.

RalA Mediates v-Src, v-Ras, and v-Raf Regulation of CD44 and Fibronectin Expression in NIH3T3 Fibroblasts

Oncogenic transformation of fibroblasts by v-Src and v-Ras is often associated with downregulation of fibronectin (FN) and increased expression of CD44, a receptor for hyaluronan. Both v-Src and v-Ras as well as v-Raf activate phospholipase D through the small GTPase, RalA, an important mediator of transformation and tumorigenesis in vivo. We have therefore investigated whether RalA is involved in the downregulation of FN and overproduction of CD44 upon oncogenic transformation. We report here that compared to untransfected cells NIH3T3 cells transformed by v-Src, v-Ras, or v-Raf have reduced levels of FN and increased levels of CD44. Moreover, the ability to form extracellular FN fibrils was significantly reduced in the oncogene-transformed cells compared to parental controls. Coexpression of the dominant negative S28N-RalA mutant restored the levels of CD44 and FN and the capacity of v-Src-, v-Ras-, and v-Raf-expressing cells to form extracellular FN fibrils, to those observed in NIH3T3 cells. The data presented here show a novel regulatory role for RalA, which is required for tumor formation in transformed NIH3T3 cells, in mediating the signal transduction pathway activated by v-Src, v-Ras, and v-Raf, that leads to FN downregulation and CD44 overexpression.

Identification of mouse NMDA receptor subunit NR2A C-terminal tyrosine sites phosphorylated by coexpression with v-Src

The protein tyrosine kinase Src is known to regulate NMDA receptors in native neurons. While NR2A, NR2B and NR2D are known to be phosphorylated on tyrosine residues, the exact sites have remained unidentified. Immunoprecipitation of NMDA receptor subunits followed by western blotting was used to analyze the state of tyrosine phosphorylation of recombinant NMDA receptor subunits expressed in HEK293 cells. Using antiphosphotyrosine antibody PY20, we find that on expression in HEK cells, v-Src and Fyn cause detectable tyrosine phosphorylation only of NR2A. Because a stronger signal was produced by the constitutively active v-Src, the general region of v-Src phosphorylation was delimited by expression of a series of truncation mutants of NR2A. Site-directed mutagenesis on candidate sites within the likely region allowed identification of three sites, Y1292, Y1325, and Y1387 that account for a significant fraction of the total PY20 signal. Two of these sites, Y1292 and Y1387, were suggested to control current modulation by Src in previous studies of HEK cells expressing NR1/NR2A. One of these sites, Y1325, has not yet been evaluated for effects on receptor current. A unique tyrosine site, Y1267, was shown not to be a site of detectable phosphorylation, in accordance with its Src-independent regulation of receptor currents.

G protein coupled receptor signaling through the Src and Stat3 pathway: role in proliferation and transformation

Extracellular signals when routed through signaling pathways that use heterotrimeric G proteins can engage multiple signaling pathways leading to diverse biological consequences. One locus at which signal sorting occurs is at the level of G proteins. G protein alpha-subunits appear to be capable of interacting with different effectors leading to engagement of distinct signaling pathways. Regulation of different pathways in turn leads to different biological outcomes. The process of neoplastic transformation is controlled to a large extent through the activation and inhibition of signaling pathways. Signaling pathways such as the Ras-MAPK, v-Src-Stat3 pathways are activated in the process of transformation. Expression of activated Galpha subunits have been shown to cause transformation of cells. While activation of the MAPK 1,2 pathway by various Galpha subunits has been reported for several years, recent studies show the activation and involvement of Src and Stat3 pathways in Galphao and Galphai mediated transformation of cells. Recent studies also suggest that both Galphai and Galphas may be able to interact with and activate Src. The activation of Src and Stat3 by G proteins has also been demonstrated by ligand-induced activation of G protein receptors. So increasingly it is becoming clear that the Src and Stat3 pathways are potential effectors for G proteins and that they may play a role in G protein function.

Constitutively activated Stat3 protects fibroblasts from serum withdrawal and UV-induced apoptosis and antagonizes the proapoptotic effects of activated Stat1

Stats1 and 3 (signal transducers and activators of transcription) can be activated simultaneously, although not necessarily to the same degree or duration, by the interaction of cells with the same polypeptide ligand (EGF, PDGF, or high concentrations of IL-6, for example). However, these two Stat proteins can mediate opposing effects on cell growth and survival. Stat1 activation slows growth and promotes apoptosis. In contrast, activated Stat3 can protect cells from apoptosis. Furthermore, a constitutively active form of Stat3, Stat3-C (bridged by S-S linkages between cysteines instead of phosphotyrosines) can induce cellular transformation of fibroblasts. We have determined that fibroblasts transformed by Stat3-C are more resistant to proapoptotic stimuli than nontransformed cells. Also, to examine the potential opposing roles in apoptosis of Stat1 and Stat3, we studied the cervical carcinoma-derived cell line, Me180, which undergoes Stat1-dependent, IFN gamma-induced apoptosis. Me180 cells that express Stat3-C are protected against IFN gamma-mediated apoptosis.

RING finger mutations that abolish c-Cbl-directed polyubiquitination and downregulation of the EGF receptor are insufficient for cell transformation

The c-Cbl protooncogene can function as a negative regulator of receptor protein tyrosine kinases (RPTKs) by targeting activated receptors for polyubiquitination and downregulation. This function requires its tyrosine kinase binding (TKB) domain for targeting RPTKs and RING finger domain to recruit E2 ubiquitin-conjugating enzymes. It has therefore been proposed that oncogenic Cbl proteins act in a dominant-negative manner to block this c-Cbl activity. In testing this hypothesis, we found that although mutations spanning the RING finger abolish c-Cbl-directed polyubiquitination and downregulation of RPTKs, they do not induce transformation. In contrast, it is mutations within a highly conserved alpha-helical structure linking the SH2 and RING finger domains that render Cbl proteins oncogenic. Thus, Cbl transformation involves effects additional to polyubiquitination of RPTKs that are independent of the RING finger and its ability to recruit E2-conjugating enzymes.