The knockout of miR-143 and -145 alters smooth muscle cell maintenance and vascular homeostasis in mice: correlates with human disease.

Mechanisms controlling vascular smooth muscle cell (VSMC) plasticity and renewal still remain to be elucidated completely. A class of small RNAs called microRNAs (miRs) regulate gene expression at the post-transcriptional level. Here, we show a critical role of the miR-143/145 cluster in SMC differentiation and vascular pathogenesis, also through the generation of a mouse model of miR-143 and -145 knockout (KO). We determined that the expression of miR-143 and -145 is decreased in acute and chronic vascular stress (transverse aortic constriction and in aortas of the ApoE KO mouse). In human aortic aneurysms, the expression of miR-143 and -145 was significantly decreased compared with control aortas. In addition, overexpression of miR-143 and -145 decreased neointimal formation in a rat model of acute vascular injury. An in-depth analysis of the miR-143/145 KO mouse model showed that this miR cluster is expressed mostly in the SMC compartment, both during development and postnatally, in vessels and SMC-containing organs. Loss of miR-143 and miR-145 expression induces structural modifications of the aorta, because of an incomplete differentiation of VSMCs. In conclusion, our results show that the miR-143/145 gene cluster has a critical role during SMC differentiation and strongly suggest its involvement in the reversion of the VSMC differentiation phenotype that occurs during vascular disease.

The SRC substrate Tks5, podosomes (invadopodia), and cancer cell invasion.

Some years ago, we employed a screen of phage cDNA expression libraries to identify novel substrates of the protein tyrosine kinase Src. One of these, Tks5 (previously known as Fish), is a large scaffolding protein with an amino-terminal PX domain and five SH3 domains. In normal fibroblasts, Tks5 is cytoplasmic, but the protein is found in podosomes when the cells are transformed with Src. Using short interfering RNA technology, we have shown that Tks5 is required for podosome formation. Furthermore, cells with reduced Tks5 expression are poorly invasive through Matrigel. Tks5 is expressed and localized to podosomes in invasive human cancer cell lines and in tumor tissue, particularly breast cancers and melanomas. In these cells too, Tks5 is required for invasion. Our future work will focus on the identification of the binding partners of Tks5 that are responsible for podosome formation and invasion, and on determining the role of Tks5 in animal models of metastasis.

Isolation of novel Src substrates.

We have established and used a method to rapidly isolate tyrosine kinase substrates. The method entails inserting mammalian cDNA libraries into phage vectors. Protein production is induced, then plaque proteins are transferred to nitrocellulose and phosphorylated by the kinase of interest. Proof of principle for this technique was established by the isolation of a number of known Src substrates. We also implicated other known proteins as substrates for Src by this approach, and isolated a number of novel genes. Several of these are indeed Src substrates in mammalian cells. We have characterized further one of these novel substrates, Fish, which is a multi-domain adaptor protein.

Stat3-mediated Myc expression is required for Src transformation and PDGF-induced mitogenesis.

Signal transducer and activator of transcription (STAT) proteins perform key roles in mediating signaling by cytokines and growth factors, including platelet-derived growth factor (PDGF). In addition, Src family kinases activate STAT signaling and are required for PDGF-induced mitogenesis in normal cells. One STAT family member, Stat3, has been shown to have an essential role in cell transformation by the Src oncoprotein. However, the mechanisms by which STAT-signaling pathways contribute to mitogenesis and transformation are not fully defined. We show here that disruption of Stat3 signaling by using dominant-negative Stat3beta protein in NIH 3T3 fibroblasts suppresses c-Myc expression concomitant with inhibition of v-Src-induced transformation. Ectopic expression of c-Myc is able to partially reverse this inhibition, suggesting that c-Myc is a downstream effector of Stat3 signaling in v-Src transformation. Furthermore, c-myc gene knockout fibroblasts are refractory to transformation by v-Src, consistent with a requirement for c-Myc protein in v-Src transformation. In normal NIH 3T3 cells, disruption of Stat3 signaling with dominant-negative Stat3beta protein inhibits PDGF-induced mitogenesis in a manner that is reversed by ectopic c-Myc expression. Moreover, inhibition of Src family kinases with the pharmacologic agent, SU6656, blocks Stat3 activation by PDGF. These findings, combined together, delineate the signaling pathway, PDGF --> Src --> Stat3 --> Myc, that is important in normal PDGF-induced mitogenesis and subverted in Src transformation.

SU6656, a selective src family kinase inhibitor, used to probe growth factor signaling.

The use of small-molecule inhibitors to study molecular components of cellular signal transduction pathways provides a means of analysis complementary to currently used techniques, such as antisense, dominant-negative (interfering) mutants and constitutively activated mutants. We have identified and characterized a small-molecule inhibitor, SU6656, which exhibits selectivity for Src and other members of the Src family. A related inhibitor, SU6657, inhibits many kinases, including Src and the platelet-derived growth factor (PDGF) receptor. The use of SU6656 confirmed our previous findings that Src family kinases are required for both Myc induction and DNA synthesis in response to PDGF stimulation of NIH 3T3 fibroblasts. By comparing PDGF-stimulated tyrosine phosphorylation events in untreated and SU6656-treated cells, we found that some substrates (for example, c-Cbl, and protein kinase C delta) were Src family substrates whereas others (for example, phospholipase C-gamma) were not. One protein, the adaptor Shc, was a substrate for both Src family kinases (on tyrosines 239 and 240) and a distinct tyrosine kinase (on tyrosine 317, which is perhaps phosphorylated by the PDGF receptor itself). Microinjection experiments demonstrated that a Shc molecule carrying mutations of tyrosines 239 and 240, in conjunction with an SH2 domain mutation, interfered with PDGF-stimulated DNA synthesis. Deletion of the phosphotyrosine-binding domain also inhibited synthesis. These inhibitions were overcome by heterologous expression of Myc, supporting the hypothesis that Shc functions in the Src pathway. SU6656 should prove a useful additional tool for further dissecting the role of Src kinases in this and other signal transduction pathways.

No requirement for src family kinases for PDGF signaling in fibroblasts expressing SV40 large T antigen.

A growing body of literature suggests that the ubiquitously expressed Src family kinases (Src, Fyn and Yes) are required for agents such as platelet-derived growth factor (PDGF) to stimulate DNA synthesis. Yet Klinghoffer and colleagues recently presented evidence that fibroblasts derived from mice null for Src, Fyn and Yes responded normally to PDGF (Klinghoffer et al., 1999, EMBO J., 18: 2459 - 2471). What is the reason for this discrepancy? We noted that Klinghoffer et al. (1999) used SV40 large T antigen (largeT) to facilitate derivation of cell lines from the embryos. We therefore tested the effect of largeT on PDGF receptor signaling. We found that expression of largeT overcame the inhibitory effects of interfering forms of both Ras (N17Ras) and Src (SrcK-). Furthermore, injection of SrcK- or the cst.1 antibody (which inhibits Src, Fyn and Yes) failed to inhibit PDGF-stimulated DNA synthesis in NIH3T3 cells expressing dominant negative p53, and fibroblasts derived from p53 null embryos. These data suggest firstly that caution should be used in interpretation of experiments conducted in cell lines expressing largeT, and secondly that the role of Src family kinases in growth factor signaling may be to oppose the effects of negative growth regulators such as p53. Oncogene (2000) 19, 2867 - 2869

Diaphanous-related formins bridge Rho GTPase and Src tyrosine kinase signaling.

We have examined the role of the mouse Diaphanous-related formin (DRF) Rho GTPase binding proteins, mDia1 and mDia2, in cell regulation. The DRFs are required for cytokinesis, stress fiber formation, and transcriptional activation of the serum response factor (SRF). 'Activated' mDia1 and mDia2 variants, lacking their GTPase binding domains, cooperated with Rho-kinase or ROCK to form stress fibers but independently activated SRF. Src tyrosine kinase associated and co-localized with the DRFs in endosomes and in mid-bodies of dividing cells. Inhibition of Src also blocked cytokinesis, SRF induction by activated DRFs, and cooperative stress fiber formation with active ROCK. Our results show that the DRF proteins couple Rho and Src during signaling and the regulation of actin dynamics.

The proto-oncogene c-Cbl is a negative regulator of DNA synthesis initiated by both receptor and cytoplasmic tyrosine kinases.

In C. elegans, genetic and biochemical data indicate that the Cbl homolog Sli-1 attenuates Let-23 (EGFR) signaling. To investigate whether c-Cbhl might have a role in mammalian growth factor-mediated mitogenic signaling, we microinjected NIH3T3 mouse fibroblasts with expression plasmids encoding wt and G306ECbl (a 'loss of function' mutant identified in C. elegans). We observed inhibition of PDGF BB- and EGF-induced DNA synthesis by wt Cbl but not the mutant. Microinjection of two different affinity purified polyclonal antisera against Cbl boosted a suboptimal PDGF-stimulated mitogenic response. The inhibition of both PDGF BB- and EGF-induced DNA synthesis by wt Cbl was reversed by co-expression with Myc but not with Fos. DNA synthesis initiated by constitutively activated Src was also blocked by Cbl expression, but curiously by the G306E mutant as well. These data are all consistent with the proposition that Cbl negatively affects mitogenic signaling in mammalian fibroblasts.

Src promotes PKCdelta degradation.

Platelet-derived growth factor BB (PDGF) stimulates DNA synthesis through a mechanism that is at least partially dependent upon Src family tyrosine kinases, although the signal transduction pathway downstream of Src is poorly understood. We have studied the signaling between Src and different protein kinase C (PKC) isoforms and its possible role in the regulation of PDGF-stimulated DNA synthesis. We found that Src promoted the tyrosine phosphorylation of PKCdelta, and its subsequent degradation. Enforced expression of PKCdelta inhibited PDGF-stimulated DNA synthesis, whereas expression of PKCalpha and PKCepsilon did not, a finding consistent with a model in which PKCdelta negatively regulates G1-to-S-phase progression. We used mutagenesis to map a critical Src phosphorylation site on PKCdelta to tyrosine 311. A mutant form of PKCdelta in which tyrosine 311 was replaced with phenylalanine (Y311F) was more stable in the presence of Src, suggesting that Src-induced degradation was a direct result of PKCdelta tyrosine phosphorylation. We conclude that PKCdelta is downstream of Src but is unlikely to play a positive role in the signaling pathway by which Src promotes DNA synthesis.

A function for phosphatidylinositol 3-kinase beta (p85alpha-p110beta) in fibroblasts during mitogenesis: requirement for insulin- and lysophosphatidic acid-mediated signal transduction.

We have previously shown that phosphatidylinositol 3-kinase alpha (PI 3-Kalpha) (p85alpha-p110alpha) is required for DNA synthesis induced by various growth factors (S. Roche, M. Koegl, and S. A. Courtneidge, Proc. Natl. Acad. Sci. USA 91:9185-9189, 1994) in fibroblasts. In the present study, we have investigated the function of PI 3-Kbeta (p85alpha-p110beta) during mitogenesis. By using antibodies specific to p110beta we showed that PI 3-Kbeta is expressed in NIH 3T3 cells. PI 3-Kbeta and PI 3-Kalpha have common features: PI 3-Kbeta is tightly associated with a protein serine kinase that phosphorylates p85alpha, it interacts with the Src-middle T antigen complex and the activated platelet-derived growth factor (PDGF) receptor in fibroblasts in vivo, and it becomes tyrosine phosphorylated after PDGF stimulation. PI 3-Kbeta was also activated in Swiss 3T3 and Cos7 cells stimulated with lysophosphatidic acid (LPA), a mitogen that interacts with a heterotrimeric G protein-coupled receptor. In contrast PI 3-Kalpha was activated to a lesser extent in these cells. Microinjection of neutralizing antibodies specific for p110beta into quiescent fibroblasts inhibited DNA synthesis induced by both insulin and LPA but poorly affected PDGF receptor signaling. Therefore, PI 3-Kbeta plays an important role in transmitting the mitogenic response induced by some, but not all, growth factors. Finally, we show that while oncogenic V12Ras interacts with type I PI 3-Ks, it could induce DNA synthesis in the absence of active PI 3-Kalpha and PI 3-Kbeta, suggesting that Ras uses other effectors for DNA synthesis.

Src-like adaptor protein (Slap) is a negative regulator of mitogenesis.

The Src-like adaptor protein (Slap) is a recently identified adaptor protein containing Src homology 3 (SH3) and SH2 domains. Slap is found in a wide range of cell types and was shown to interact with the Eck receptor tyrosine kinase in a yeast two-hybrid interaction screen [1]. Here, we found that Slap is expressed in NIH3T3 cells and could associate with the activated platelet-derived growth factor (PDGF) receptor. Using mutated versions of the PDGF receptor and phosphopeptide competition experiments, we determined that Slap has the highest affinity for the Src-binding site of the PDGF receptor. Our inability to produce cell lines that stably expressed Slap suggested that Slap inhibited cell growth. We further investigated this issue by transiently expressing Slap by microinjection. Overexpression of Slap by this method inhibited DNA synthesis induced by PDGF and serum, whereas overexpression of the adaptor proteins Grb2 and Shc did not. Finally, microinjection of a Slap antibody into NIH3T3 cells that had been stimulated with suboptimal doses of growth factors potentiated the effects of the growth factors. These data suggest that, unlike other adaptor proteins, Slap is a negative regulator of signalling initiated by growth factors.

A new method for isolating tyrosine kinase substrates used to identify fish, an SH3 and PX domain-containing protein, and Src substrate.

We describe a method for identifying tyrosine kinase substrates using anti-phosphotyrosine antibodies to screen tyrosine-phosphorylated cDNA expression libraries. Several potential Src substrates were identified including Fish, which has five SH3 domains and a recently discovered phox homology (PX) domain. Fish is tyrosine-phosphorylated in Src-transformed fibroblasts (suggesting that it is a target of Src in vivo) and in normal cells following treatment with several growth factors. Treatment of cells with cytochalasin D also resulted in rapid tyrosine phosphorylation of Fish, concomitant with activation of Src. These data suggest that Fish is involved in signalling by tyrosine kinases, and imply a specialized role in the actin cytoskeleton.

The discovery and validation of new drug targets in cancer.

Advances in our understanding of the signal transduction pathways involved in cellular growth control have provided several new strategies for cancer therapy. Recent advances now make it possible to develop selective inhibitors targeting genomic instability, the growth, survival, and invasion of the tumor, and its nourishment through the growth of new blood vessels.

Src regulated by C-terminal phosphorylation is monomeric.

The activity of the c-Src protein tyrosine kinase is regulated by phosphorylation of a tyrosine residue (Tyr-527) in the C-terminal tail of the molecule. Phosphorylation of Tyr-527 promotes association of the tail with the SH2 domain and a concomitant reduction of the enzymatic activity of Src. We asked the question whether regulation by C-terminal phosphorylation was accompanied by a change in the quaternary structure of the enzyme or if it occurred within a monomeric form of Src. For this purpose we purified to homogeneity a chicken Src form lacking the unique domain from Schizosaccharomyces pombe cells. The cells were engineered to express Src along with Csk, a protein kinase able to phosphorylate Tyr-527 efficiently. Mass spectrometric analysis showed that purified Src was homogeneously phosphorylated at Tyr-527. The enzyme was in the regulated form, because it could be activated by a phosphorylated peptide able to bind the SH2 domain with high affinity. Using gel filtration chromatography, dynamic light scattering, and ultracentrifugation, we found that the regulated form of Src was a monomer. We have obtained crystals diffracting to 2.4 A with space group P2(1)2(1)2(1) and one molecule per asymmetric unit, in agreement with the monomeric state. These results indicate that the structural rearrangements of regulated Src are of an intramolecular nature.

The 2.35 A crystal structure of the inactivated form of chicken Src: a dynamic molecule with multiple regulatory interactions.

The Src protein tyrosine kinase plays a critical role in a variety of signal transduction pathways. Strict regulation of its activity is necessary for proper signalling. We present here the crystal structure of chicken Src which is phosphorylated at Tyr527 and represents its least active form. Our structure, similar to the recently reported human Hck and Src structures, contains the SH3, SH2 and the kinase domains and the C-terminal regulatory tail but not the N-terminal unique domain. The SH3 domain uses its hydrophobic surface to coordinate the SH2-kinase linker such that residues Gln251 and Leu255 specifically interact with side chains in the beta2-beta3 and the alphaC-beta4 loops of the N-terminal lobe opposite of the kinase active site. This position of the SH3 domain and the coordination of the SH2-kinase linker also optimally places the SH2 domain such that the phosphorylated Tyr527 in the C-terminal tail interacts with the SH2 binding pocket. Analogous to Cdk2 kinase, the position of the Src alphaC-helix in the N-terminal lobe is swung out disrupting the position of the active site residues. Superposition of other protein kinases including human Hck and Src onto chicken Src indicate that the alphaC-helix position is affected by the relative position of the N-terminal lobe with respect to the C-terminal lobe of the kinase and that the presence of the SH3/SH2-kinase linker/N-terminal lobe interactions restricts the kinase lobes and alphaC-helix access to the active conformation. These superpositions also suggest that the highly conserved alphaC-beta4 loop restricts the conformational freedom of the N-terminal lobe by anchoring it to the C-terminal lobe. Finally, based on sequence alignments and conservation of hydrophobic residues in the Src SH2-kinase linker as well as in the alphaC-beta4 and beta2-beta3 loops, we propose that the Src-related kinases, Abl, Btk and Csk, share the same quaternary structure.

PDGF-induced phosphorylation of Tyr28 in the N-terminus of Fyn affects Fyn activation.

Binding of platelet-derived growth factor (PDGF) to its receptors leads to the activation of members of the Src family of protein tyrosine kinases. We show here that Fyn, a member of the Src family, is phosphorylated on Tyr28 in the unique N-terminal part of the molecule after interaction with the intracellular domain of the PDGF beta-receptor. Activated Fyn furthermore undergoes autophosphorylation on Tyr30, Tyr39 and Tyr420. When Fyn mutants with Tyr28, Tyr30 or Tyr39 replaced with phenylalanine residues were transfected into NIH3T3 cells a decreased activation after PDGF stimulation was seen, suggesting a functional importance of the N-terminal tyrosine phosphorylation of Fyn.

A role for Pyk2 and Src in linking G-protein-coupled receptors with MAP kinase activation.

The mechanisms by which mitogenic G-protein-coupled receptors activate the MAP kinase signalling pathway are poorly understood. Candidate protein tyrosine kinases that link G-protein-coupled receptors with MAP kinase include Src family kinases, the epidermal growth factor receptor, Lyn and Syk. Here we show that lysophosphatidic acid (LPA) and bradykinin induce tyrosine phosphorylation of Pyk2 and complex formation between Pyk2 and activated Src. Moreover, tyrosine phosphorylation of Pyk2 leads to binding of the SH2 domain of Src to tyrosine 402 of Pyk2 and activation of Src. Transient overexpression of a dominant interfering mutant of Pyk2 or the protein tyrosine kinase Csk reduces LPA- or bradykinin-induced activation of MAP kinase. LPA- or bradykinin-induced MAP kinase activation was also inhibited by overexpression of dominant interfering mutants of Grb2 and Sos. We propose that Pyk2 acts with Src to link Gi- and Gq-coupled receptors with Grb2 and Sos to activate the MAP kinase signalling pathway in PC12 cells.

The purification and characterization of the catalytic domain of Src expressed in Schizosaccharomyces pombe. Comparison of unphosphorylated and tyrosine phosphorylated species.

The catalytic domain of chicken Src including the C-terminal tail (Src-CD), has been expressed in Schizosaccharomyces pombe and purified to homogeneity. The expressed protein is a mixture of unphosphorylated (80%) and mono-phosphorylated (20%) species, that can be separated from each other by Mono Q chromatography. By a novel mass spectrometric method that utilizes parent ion scans of unseparated peptide mixtures, we found that the mono-phosphorylated form is phosphorylated either at Tyr416 or at Tyr436. The stability of Src-CD is comparable to the wild-type protein. Src-CD auto-phosphorylates and efficiently phosphorylates substrate peptides and proteins. Auto-phosphorylation occurs by an intermolecular mechanism and is completely inhibited by an excess of substrate peptide. Kinetic measurements for two exogenous substrates, the Src substrate peptide (AEEEIYGEFEAKKKK) and denatured enolase, showed that the overall activity (kcat) of the Src-CD molecule is about 10 times higher than that of wild-type Src. The kcat values for phosphorylation of the Src substrate peptide are similar for the unphosphorylated and monophosphorylated Src-CD (50 min-1), but the apparent K(m) values differ significantly (approximately 3 microM and 10 microM, respectively). Therefore, at low substrate concentrations in vitro the mono-phosphorylated form is more active, in agreement with the importance of Tyr416 for in vivo activity. The apparent K(m) values of the mono-phosphorylated Src-CD and wild-type Src for the Src substrate peptide and enolase are similar, indicating that, under these conditions, the kinase domain is mainly responsible for substrate binding.

Requirement of phospholipase C gamma, the tyrosine phosphatase Syp and the adaptor proteins Shc and Nck for PDGF-induced DNA synthesis: evidence for the existence of Ras-dependent and Ras-independent pathways.

We have investigated the roles of the phosphotyrosine phosphatase Syp (also called SH-PTP2), phospholipase C (PLC) gamma1, rasGTPase Activating Protein (rasGAP) and the adapter molecules Nck and Shc in the mitogenic response induced by PDGF in fibroblasts. Two separate approaches were used to inhibit the biological activity of these signalling proteins in vivo. Either glutathione S-transferase (GST) fusion proteins containing the SH2 domains of these proteins, or antibodies specific for these polypeptides, were microinjected into cells. GST-SH2 fusion proteins are expected to act as dominant inhibitors by competing for physiological SH2-mediated interactions, while microinjected antibodies can directly block protein functions. Inhibition of PLCgamma, Syp, Shc and Nck signals blocked PDGF-stimulated cells in G1 showing a requirement for these proteins for S-phase entry. Inhibition of rasGAP, in contrast, had no effect on S-phase entry. We next examined which of these signals were required for PDGF-induced cFos expression, a Ras-dependent event important for signalling. By using the same approaches with cells expressing beta-galactosidase under the control of a c-fos promoter, we showed that PLCgamma, Syp and Shc were necessary for ligand-induced cFos expression whereas Nck and phosphatidylinositol 3-kinase alpha were not. From these results we concluded that PDGF generates Ras-dependent and Ras-independent pathways important for DNA synthesis.