Direct binding of activated c-Src to the beta 3-adrenergic receptor is required for MAP kinase activation

Both beta(2)- and beta(3)-adrenergic receptors (ARs) are able to activate the extracellular signal-regulated kinase (ERK) pathway. We previously showed that c-Src is required for ERK activation by beta(2)AR and that it is recruited to activated beta(2)AR through binding of the Src homology 3 (SH3) domain to proline-rich regions of the adapter protein beta-arrestin1. Despite the absence of sites for phosphorylation and beta-arrestin binding, ERK activation by beta(3)AR still requires c-Src. Agonist activation of beta(2)AR, but not beta(3)AR, led to redistribution of green fluorescent protein-tagged beta-arrestin to the plasma membrane. In beta-arrestin-deficient COS-7 cells, beta-agonist-dependent co-precipitation of c-Src with the beta(2)AR required exogenous beta-arrestin, but activated beta(3)AR co-precipitated c-Src in the absence or presence of beta-arrestin. ERK activation and Src co-precipitation with beta(3)AR also occurred in adipocytes in an agonist-dependent and pertussis toxin-sensitive manner. Protein interaction studies show that the beta(3)AR interacts directly with the SH3 domain of Src through proline-rich motifs (PXXP) in the third intracellular loop and the carboxyl terminus. ERK activation and Src co-precipitation were abolished in cells expressing point mutations in these PXXP motifs. Together, these data describe a novel mechanism of ERK activation by a G protein-coupled receptor in which the intracellular domains directly recruit c-Src.     

CrkL is recruited through its SH2 domain to the erythropoietin receptor and plays a role in Lyn-mediated receptor signaling.

The erythropoietin (Epo) receptor transduces its signals by activating physically associated tyrosine kinases, mainly Jak2 and Lyn, and thereby inducing tyrosine phosphorylation of various substrates including the Epo receptor (EpoR) itself. We previously demonstrated that, in Epo-stimulated cells, an adapter protein, CrkL, becomes tyrosine-phosphorylated, physically associates with Shc, SHP-2, and Cbl, and plays a role in activation of the Ras/Erk signaling pathway. Here, we demonstrate that Epo induces binding of CrkL to the tyrosine-phosphorylated EpoR and SHIP1 in 32D/EpoR-Wt cells overexpressing CrkL. In vitro binding studies showed that the CrkL SH2 domain directly mediates the EpoR binding, which was specifically inhibited by a synthetic phosphopeptide corresponding to the amino acid sequences at Tyr(460) in the cytoplasmic domain of EpoR. The CrkL SH2 domain was also required for tyrosine phosphorylation of CrkL in Epo-stimulated cells. Overexpression of Lyn induced constitutive phosphorylation of CrkL and activation of Erk, whereas that of a Lyn mutant lacking the tyrosine kinase domain attenuated the Epo-induced phosphorylation of CrkL and activation of Erk. Furthermore, Lyn, but not Jak2, phosphorylated CrkL on tyrosine in in vitro kinase assays. Together, the present study suggests that, upon Epo stimulation, CrkL is recruited to the EpoR through interaction between the CrkL SH2 domain and phosphorylated Tyr(460) in the EpoR cytoplasmic domain and undergoes tyrosine phosphorylation by receptor-associated Lyn to activate the downstream signaling pathway leading to the activation of Erk and Elk-1.     

Integrin engagement, the actin cytoskeleton, and c-Src are required for the calcitonin-induced tyrosine phosphorylation of paxillin and HEF1, but not for calcitonin-induced Erk1/2 phosphorylation

We have previously shown that in a HEK-293 cell line that overexpresses the C1a isoform of the calcitonin receptor (C1a-HEK), calcitonin induces the tyrosine phosphorylation of the focal adhesion-associated proteins HEF1 (a p130(Cas)-like docking protein), paxillin, and focal adhesion kinase and that it also stimulates the phosphorylation and activation of Erk1 and Erk2. We report here that cell attachment to the extracellular matrix, an intact actin cytoskeleton, and c-Src are absolutely required for the calcitonin-induced phosphorylation of focal adhesion-associated proteins. In contrast to the phosphorylation of paxillin and HEF1 in cells attached to fibronectin-coated dishes, calcitonin failed to stimulate the phosphorylation of paxillin and HEF1 in suspended cells, in cells attached to poly-d-lysine-coated dishes, and in attached cells pretreated with the RGD-containing peptide GRGDS. Overexpression of wild-type c-Src increased calcitonin-induced paxillin and HEF1 phosphorylation, whereas overexpression of kinase-dead Src or Src lacking a functional SH2 domain inhibited the calcitonin-stimulated tyrosine phosphorylation of these proteins. Overexpression of Src lacking the SH3 domain did not affect the calcitonin-induced phosphorylation of paxillin and HEF1. In contrast to the regulation of paxillin and HEF1 phosphorylation, the calcitonin-induced phosphorylation of Erk1 and Erk2 did not appear to involve c-Src and was only partially dependent on cell adhesion to the extracellular matrix and an intact actin cytoskeleton. Furthermore, inhibition of Erk1 and Erk2 phosphorylation had no effect on the calcitonin-induced phosphorylation of paxillin and HEF1. Thus, in C1a-HEK cells, the calcitonin receptor is coupled to the tyrosine phosphorylation of focal adhesion-associated proteins and to Erk1/2 phosphorylation by mechanisms that are in large part independent.     

Uncoupling ligand-dependent and -independent mechanisms for mitogen-activated protein kinase activation by the murine Ron receptor tyrosine kinase

Receptor tyrosine kinases (RTKs) activate downstream signaling through cognate growth factor receptor-induced dimerization and autophosphorylation. Overexpression of RTKs can lead to constitutive activation due to increased dimerization in the absence of ligand, and downstream signals are presumed to be the same as the ligand-induced signals. We have shown that the murine Ron (mRon) receptor tyrosine kinase exhibits constitutive activation of the MAP kinase pathway that is independent of the two docking site tyrosines, whereas activation of this pathway in response to ligand (macrophage-stimulating protein) is abolished in the absence of these tyrosines. Furthermore, we identified three tyrosines (Tyr-1175, Tyr-1265, and Tyr-1294) within the kinase domain that play critical but overlapping roles in controlling constitutive Erk activation by mRon. Phenylalanine mutations at these three tyrosines results in a receptor that fails to constitutively activate the Erk pathway but retains the ability to induce Erk phosphorylation in response to ligand stimulation. The ability of mRon to activate the MAP kinase pathway is dependent on c-Src activity, and we have shown that c-Src co-immunoprecipitates with mRon. c-Src fails to interact with mRon when the three tyrosines required for MAP kinase activation are mutated, whereas the presence of any one of these tyrosines alone restores Erk phosphorylation and recruitment of c-Src. Thus, the ligand-dependent and -independent activity of mRon can be uncoupled through the alteration of selective sets of tyrosines.     

Steroid-induced androgen receptor-oestradiol receptor beta-Src complex triggers prostate cancer cell proliferation

Treatment of human prostate carcinoma-derived LNCaP cells with androgen or oestradiol triggers simultaneous association of androgen receptor and oestradiol receptor beta with Src, activates the Src/Raf-1/Erk-2 pathway and stimulates cell proliferation. Surprisingly, either androgen or oestradiol action on each of these steps is inhibited by both anti-androgens and anti-oestrogens. Similar findings for oestradiol receptor alpha were observed in MCF-7 or T47D cells stimulated by either oestradiol or androgens. Microinjection of LNCaP, MCF-7 and T47D cells with SrcK(-) abolishes steroid-stimulated S-phase entry. Data from transfected Cos cells confirm and extend the findings from these cells. Hormone-stimulated Src interaction with the androgen receptor and oestradiol receptor alpha or beta is detected using glutathione S:-transferase fusion constructs. Src SH2 interacts with phosphotyrosine 537 of oestradiol receptor alpha and the Src SH3 domain with a proline-rich stretch of the androgen receptor. The role of this phosphotyrosine is stressed by its requirement for association of oestradiol receptor alpha with Src and consequent activation of Src in intact Cos cells.     

Erk/Src phosphorylation of cortactin acts as a switch on-switch off mechanism that controls its ability to activate N-WASP

The Arp2/3 complex can be independently activated to initiate actin polymerization by the VCA domain of WASP family members and by the acidic N-terminal and F-actin-binding repeat region of cortactin, which possesses a C-terminal SH3 domain. Cortactin is a target for phosphorylation by Src tyrosine kinases and by serine/threonine kinases that include Erk. Here we demonstrate that cortactin binds N-WASP and WASP via its SH3 domain, induces in vitro N-WASP-mediated actin polymerization, and colocalizes with N-WASP and WASP at sites of active actin polymerization. Erk phosphorylation and a mimicking S405,418D double mutation enhanced cortactin binding and activation of N-WASP. In contrast, Src phosphorylation inhibited the ability of cortactin previously phosphorylated by Erk, and that of S405,418D double mutant cortactin, to bind and activate N-WASP. Furthermore, Y-->D mutation of three tyrosine residues targeted by Src (Y421, Y466, and Y482) inhibited the ability of S405,418D cortactin to activate N-WASP. We propose that Erk phosphorylation liberates the SH3 domain of cortactin from intramolecular interactions with proline-rich regions, causing it to synergize with WASP and N-WASP in activating the Arp2/3 complex, and that Src phosphorylation terminates cortactin activation of N-WASP and WASP.     

Csk inhibition of c-Src activity requires both the SH2 and SH3 domains of Src.

The protein tyrosine kinase c-Src is negatively regulated by phosphorylation of Tyr527 in its carboxy-terminal tail. A kinase that phosphorylates Tyr527, called Csk, has recently been identified. We expressed c-Src in yeast to test the role of the SH2 and SH3 domains of Src in the negative regulation exerted by Tyr527 phosphorylation. Inducible expression of c-Src in Schizosaccharomyces pombe caused cell death. Co-expression of Csk counteracted this effect. Src proteins mutated in either the SH2 or SH3 domain were as lethal as wild type c-Src, but were insensitive to Csk, even though they were substrates for Csk in vivo. Peptide binding experiments revealed that Src proteins with mutant SH3 domains adopted a conformation in which the SH2 domain was not interacting with the tail. These data support the model of an SH2 domain-phosphorylated tail interaction repressing c-Src activity, but expand it to include a role for the SH3 domain. We propose that the SH3 domain contributes to the maintenance of the folded, inactive configuration of the Src molecule by stabilizing the SH2 domain-phosphorylated tail interaction. Moreover, the system we describe here allows for further study of the regulation of tyrosine kinases in a neutral background and in an organism amenable to genetic analysis.     

Agonist-specific transactivation of phosphoinositide 3-kinase signaling pathway mediated by the dopamine D2 receptor

Bromocriptine, acting through the dopamine D2 receptor, provides robust protection against apoptosis induced by oxidative stress in PC12-D2R and immortalized nigral dopamine cells. We now report the characterization of the D2 receptor signaling pathways mediating the cytoprotection. Bromocriptine caused protein kinase B (Akt) activation in PC12-D2R cells and the inhibition of either phosphoinositide (PI) 3-kinase, epidermal growth factor receptor (EGFR), or c-Src eliminated the Akt activation and the cytoprotective effects of bromocriptine against oxidative stress. Co-immunoprecipitation studies showed that the D2 receptor forms a complex with the EGFR and c-Src that was augmented by bromocriptine, suggesting a cross-talk between these proteins in mediating the activation of Akt. EGFR repression by inhibitor or by RNA interference eliminated the activation of Akt by bromocriptine. D2 receptor stimulation by bromocriptine induced c-Src tyrosine 418 phosphorylation and EGFR phosphorylation specifically at tyrosine 845, a known substrate of Src kinase. Furthermore, Src tyrosine kinase inhibitor or dominant negative Src interfered with Akt translocation and phosphorylation. Thus, the predominant signaling cascade mediating cytoprotection by the D2 receptor involves c-Src/EGFR transactivation by D2 receptor, activating PI 3-kinase and Akt. We also found that the agonist pramipexole failed to stimulate activation of Akt in PC12-D2R cells, providing an explanation for our previous observations that, despite efficiently activating G-protein signaling, this agonist had little cytoprotective activity in this experimental system. These results support the hypothesis that specific dopamine agonists stabilize distinct conformations of the D2 receptor that differ in their coupling to G-proteins and to a cytoprotective c-Src/EGFR-mediated PI-3 kinase/Akt pathway.     

Indirect recruitment of the signalling adaptor Shc to the fibroblast growth factor receptor 2 (FGFR2)

The adaptor protein Shc (Src homology and collagen-containing protein) plays an important role in the activation of signalling pathways downstream of RTKs (receptor tyrosine kinases) regulating diverse cellular functions, such as differentiation, adhesion, migration and mitogenesis. Despite being phosphorylated downstream of members of the FGFR (fibroblast growth factor receptor) family, a direct interaction of Shc with this receptor family has not been described to date. Various studies have suggested potential binding sites for the Shc PTB domain (phosphotyrosine-binding domain) and/or the SH2 (Src homology 2) domain on FGFR1, but no interaction of full-length Shc with these sites has been reported in vivo. In the present study, we investigated the importance of the SH2 domain and the PTB domain in recruitment of Shc to FGFR2(IIIc) to characterize the interaction of these two proteins. Confocal microscopy revealed extensive co-localization of Shc with FGFR2. The PTB domain was identified as the critical component of Shc which mediates membrane localization. Results from FLIM (fluorescence lifetime imaging microscopy) revealed that the interaction between Shc and FGFR2 is indirect, suggesting that the adaptor protein forms part of a signalling complex containing the receptor. We identified the non-RTK Src as a protein which potentially mediates the formation of such a ternary complex. Although an interaction between Src and Shc has been described previously, in the present study we implicate the Shc SH2 domain as a novel mediator of this association. The recruitment of Shc to FGFR2 via an indirect mechanism provides new insight into the regulation of protein assembly and activation of various signalling pathways downstream of this RTK.     

The C terminus of c-Src inhibits breast tumor cell growth by a kinase-independent mechanism

Overexpression or increased activity of cellular Src (c-Src) is frequently detected in human breast cancer, implicating involvement of c-Src in the etiology of breast carcinomas. Curiously, overexpression of c-Src in tissue culture cells results in a weakly or non-transforming phenotype, indicating that it alone is not sufficient for oncogenesis. However, the protein has been demonstrated to potentiate mitogenic signals from transmembrane receptors. This report investigates the requirement for c-Src in breast cancer as a transducer and integrator of anchorage-dependent and -independent growth signals by utilizing the Src family pharmacological inhibitors, PP1 and PP2, or stable overexpression of the catalytically inactive c-Src mutant (K- c-Src). Both methods of inhibiting endogenous c-Src diminished formation of soft agar colonies and tumors in nude mice. The majority of the dominant-negative activity of K- c-Src was mapped to the Src homology 2 (SH2) domain and C-terminal half of the molecule, but not to the Unique domain, Src homology 3 (SH3) domain, or the N-terminal half of K- c-Src. Further analysis of the C terminus revealed that its ability to inhibit growth localized to the N-terminal lobe (N-lobe) of the catalytic region. These results underscore the requirement for c-Src to maintain the oncogenic phenotype of breast cancer cells and suggest that c-Src may be manipulated to inhibit cell growth by the direct disruption of its catalytic activity or the introduction of either the SH2 domain or the N-lobe of K- c-Src.