Impaired recruitment of bone-marrow-derived endothelial and hematopoietic precursor cells blocks tumor angiogenesis and growth

The role of bone marrow (BM)-derived precursor cells in tumor angiogenesis is not known. We demonstrate here that tumor angiogenesis is associated with recruitment of hematopoietic and circulating endothelial precursor cells (CEPs). We used the angiogenic defective, tumor resistant Id-mutant mice to show that transplantation of wild-type BM or vascular endothelial growth factor (VEGF)-mobilized stem cells restore tumor angiogenesis and growth. We detected donor-derived CEPs throughout the neovessels of tumors and Matrigel-plugs in an Id1+/-Id3-/- host, which were associated with VEGF-receptor-1-positive (VEGFR1+) myeloid cells. The angiogenic defect in Id-mutant mice was due to impaired VEGF-driven mobilization of VEGFR2+ CEPs and impaired proliferation and incorporation of VEGFR1+ cells. Although targeting of either VEGFR1 or VEGFR2 alone partially blocks the growth of tumors, inhibition of both VEGFR1 and VEGFR2 was necessary to completely ablate tumor growth. These data demonstrate that recruitment of VEGF-responsive BM-derived precursors is necessary and sufficient for tumor angiogenesis and suggest new clinical strategies to block tumor growth.

Tyrosine phosphorylation of the beta 4 integrin cytoplasmic domain mediates Shc signaling to extracellular signal-regulated kinase and antagonizes formation of hemidesmosomes

Ligation of the alpha(6)beta(4) integrin induces tyrosine phosphorylation of the beta(4) cytoplasmic domain, followed by recruitment of the adaptor protein Shc and activation of mitogen-activated protein kinase cascades. We have used Far Western analysis and phosphopeptide competition assays to map the sites in the cytoplasmic domain of beta(4) that are required for interaction with Shc. Our results indicate that, upon phosphorylation, Tyr(1440), or secondarily Tyr(1422), interacts with the SH2 domain of Shc, whereas Tyr(1526), or secondarily Tyr(1642), interacts with its phosphotyrosine binding (PTB) domain. An inactivating mutation in the PTB domain of Shc, but not one in its SH2 domain, suppresses the activation of Shc by alpha(6)beta(4). In addition, mutation of beta(4) Tyr(1526), which binds to the PTB domain of Shc, but not of Tyr(1422) and Tyr(1440), which interact with its SH2 domain, abolishes the activation of ERK by alpha(6)beta(4). Phenylalanine substitution of the beta(4) tyrosines able to interact with the SH2 or PTB domain of Shc does not affect incorporation of alpha(6)beta(4) in the hemidesmosomes of 804G cells. Exposure to the tyrosine phosphatase inhibitor orthovanadate increases tyrosine phosphorylation of beta4 and disrupts the hemidesmosomes of 804G cells expressing recombinant wild type beta(4). This treatment, however, exerts a decreasing degree of inhibition on the hemidesmosomes of cells expressing versions of beta(4) containing phenylalanine substitutions at Tyr(1422) and Tyr(1440), at Tyr(1526) and Tyr(1642), or at all four tyrosine phosphorylation sites. These results suggest that beta(4) Tyr(1526) interacts in a phosphorylation-dependent manner with the PTB domain of Shc. This event is required for subsequent tyrosine phosphorylation of Shc and signaling to ERK but not formation of hemidesmosomes.

Role of tyrosine residues and protein interaction domains of SHC adaptor in VEGF receptor 3 signaling

The VEGFR3/FLT4 receptor, which is involved in vasculogenesis and angiogenesis, binds and phosphorylates SHC proteins on tyrosine residues. SHC contains two phosphotyrosine interaction domains: a PTB (Phosphotyrosine Binding) and a SH2 (Src Homology 2) domain. Previous studies have shown that SHC proteins are phosphorylated on Y239/Y240 and Y313 (Y317 in humans) by tyrosine kinases such as the EGF and IL3 receptors. We have investigated which of the SHC tyrosine residues are targeted by the VEGFR3/ FLT4 kinase and the role of the SHC PTB and SH2 domains in this process. Our results show that Y239/ Y240 and Y313 are simultaneously phosphorylated by the kinase, creating GRB2 binding sites. Mutation of SHC PTB, but not SH2, domain interferes with the SHC phosphorylation by VEGFR3/FLT4. Soft agar assay experiments revealed that the VEGFR3/FLT4 transforming capacity is increased by the mutation of Y239/Y240 to phenylalanines in SHC, suggesting that these two residues mediate an inhibitory signal for cell growth. Mutation of the two phosphorylation sites increases this effect, suggesting that they have a synergistic role.

Cell cycle and adhesion defects in mice carrying a targeted deletion of the integrin beta4 cytoplasmic domain

The cytoplasmic domain of the integrin beta4 subunit mediates both association with the hemidesmosomal cytoskeleton and recruitment of the signaling adaptor protein Shc. To examine the significance of these interactions during development, we have generated mice carrying a targeted deletion of the beta4 cytoplasmic domain. Analysis of homozygous mutant mice indicates that the tail-less alpha6beta4 binds efficiently to laminin 5, but is unable to integrate with the cytoskeleton. Accordingly, these mice display extensive epidermal detachment at birth and die immmediately thereafter from a syndrome resembling the human disease junctional epidermolysis bullosa with pyloric atresia (PA-JEB). In addition, we find a significant proliferative defect. Specifically, the number of precursor cells in the intestinal epithelium, which remains adherent to the basement membrane, and in intact areas of the skin is reduced, and post-mitotic enterocytes display increased levels of the cyclin-dependent kinase inhibitor p27(Kip). These findings indicate that the interactions mediated by the beta4 tail are crucial for stable adhesion of stratified epithelia to the basement membrane and for proper cell-cycle control in the proliferative compartments of both stratified and simple epithelia.

The 66-kDa Shc isoform is a negative regulator of the epidermal growth factor-stimulated mitogen-activated protein kinase pathway

In addition to tyrosine phosphorylation of the 66-, 52-, and 46-kDa Shc isoforms, epidermal growth factor (EGF) treatment of Chinese hamster ovary cells expressing the human EGF receptor also resulted in the serine/threonine phosphorylation of approximately 50% of the 66-kDa Shc proteins. The serine/threonine phosphorylation occurred subsequent to tyrosine phosphorylation and was prevented by pretreatment of the cells with the MEK-specific inhibitor PD98059. Surprisingly, only the gel-shifted 66-kDa Shc isoform (serine/threonine phosphorylated) was tyrosine phosphorylated and associated with Grb2. In contrast, only the non-serine/threonine-phosphorylated fraction of 66-kDa Shc was associated with the EGF receptor. To assess the relationship between the three Shc isoforms in EGF-stimulated signaling, the cDNA encoding the 66-kDa Shc species was cloned from a 16-day-old mouse embryo library. Sequence alignment confirmed that the 66-kDa Shc cDNA resulted from alternative splicing of the primary Shc transcript generating a 110-amino acid extension at the amino terminus. Co-immunoprecipitation of Shc and Grb2 from cells overexpressing the 52/46-kDa Shc isoforms versus the 66-kDa Shc species directly demonstrated a competition of binding for a limited pool of Grb2 proteins. Furthermore, expression of the 66-kDa Shc isoform markedly accelerated the inactivation of ERK following EGF stimulation. Together, these data indicate that the serine/threonine phosphorylation of 66-kDa Shc impairs its ability to associate with the tyrosine-phosphorylated EGF receptor and can function in a dominant-interfering manner by inhibiting EGF receptor downstream signaling pathways.

Interaction between the phosphotyrosine binding domain of Shc and the insulin receptor is required for Shc phosphorylation by insulin in vivo

Stimulation of the insulin receptor (IR) results in tyrosine phosphorylation of the intermediate molecules insulin receptor substrate-1 (IRS-1), IRS-2, and Shc, which then couple the IR to downstream signaling pathways by serving as binding sites for signaling molecules with SH2 domains. It has been proposed that direct binding of IRS-1, IRS-2, and Shc to an NPX-Tyr(P) motif in the juxtamembrane region of the IR is required for tyrosine phosphorylation of these molecules by the IR. In this regard, Shc and IRS-1 contain domains that are distinct from SH2 domains, referred to as the phosphotyrosine binding (PTB) or phosphotyrosine interaction (PI) domains, which bind phosphotyrosine in the context of an NPX-Tyr(P) motif. To further clarify the role of the Shc PTB/PI domain, we identified a mutation in this domain that abrogated binding of Shc to the IR in vitro. Interestingly, this mutation completely abolished Shc phosphorylation by the IR in vivo whereas mutation of the arginine in the FLVRES motif of the Shc SH2 domain did not affect Shc phosphorylation by insulin. In addition, we identified specific amino acids on the IR that are required for the IR to stimulate Shc but not IRS-1 phosphorylation in vivo. As with the PTB/PI domain Shc mutant, the ability of these mutant receptors to phosphorylate Shc correlates with the binding of the PTB/PI domain of Shc to similar sequences in vitro. These findings support a model in which binding of the PTB/PI domain of Shc directly to the NPX-Tyr(P) motif on the IR mediates Shc phosphorylation by insulin.

Identification of residues within the SHC phosphotyrosine binding/phosphotyrosine interaction domain crucial for phosphopeptide interaction

Shc is an Src homology 2 (SH2) domain protein thought to be an important component of the signaling pathway leading from cell surface receptors to Ras. A new phosphotyrosine interaction (PI) domain (also known as the phosphotyrosine binding (PTB) domain) has been described in the amino terminus of Shc. The Shc PI domain binding specificity is dependent on residues lying amino-terminal to the phosphotyrosine rather than carboxyl-terminal as is seen with SH2 domains. We randomly mutagenized the Shc PTB/PI domain in an effort to identify residues in the domain crucial for interaction with phosphotyrosine-containing peptides. We then screened the mutants for binding to the tyrosine-phosphorylated carboxyl-terminal tail of the epidermal growth factor (EGF) receptor. Most striking were mutations that altered a phenylalanine residue in block 4 of the domain severely impairing PI domain function. This phenylalanine residue is conserved in all but one subfamily of PI domains that have been identified to date. Reconstitution of this phenylalanine mutation into full-length Shc created a protein unable to interact with the EGF receptor in living cells.

The phosphotyrosine interaction domain of Shc binds an LXNPXY motif on the epidermal growth factor receptor

Shc is an SH2 domain protein that is tyrosine phosphorylated in cells stimulated with a variety of growth factors and cytokines. Once phosphorylated, Shc binds the Grb2-Sos complex, leading to Ras activation. Shc can interact with tyrosine-phosphorylated proteins by binding to phosphotyrosine in the context of an NPXpY motif, where pY is a phosphotyrosine. This is an unusual binding site for an SH2 domain protein whose binding specificity is usually controlled by residues carboxy terminal, not amino terminal, to the phosphotyrosine. Recently we identified a second region in Shc, named the phosphotyrosine interaction (PI) domain, and we have found it to be present in a variety of other cellular proteins. In this study we used a dephosphorylation protection assay, competition analysis with phosphotyrosine-containing synthetic peptides, and epidermal growth factor receptor (EGFR) mutants to determine the binding sites of the PI domain of Shc on the EGFR. We demonstrate that the PI domain of Shc binds the LXNPXpY motif that encompasses Y-1148 of the activated EGFR. We conclude that the PI domain imparts to Shc its ability to bind the NPXpY motif.

Shc binding to nerve growth factor receptor is mediated by the phosphotyrosine interaction domain

Shc is an adaptor protein that contains two phosphotyrosine-binding domains, a Src homology 2 (SH2) domain and the newly described phosphotyrosine interaction (PI) domain. Shc interacts with several tyrosine-phosphorylated proteins and is itself tyrosine-phosphorylated in cells stimulated with a variety of growth factors and cytokines. Upon phosphorylation, Shc binds to the Grb2.Sos complex leading to the activation of the Ras signaling pathway. Mutational analysis of the nerve growth factor (NGF) receptor (TrkA) suggested that the binding of Shc to the activated receptor is required for NGF-induced neuronal differentiation of PC12 cells. Here we report that the PI domain of Shc directly binds to tyrosine 490 on the autophosphorylated NGF receptor. The PI domain specifically recognizes an I/LXN-PXpY motif (where p indicates phosphorylation) as determined by phosphopeptide competition assay. In addition, the PI domain is able to efficiently compete for binding of full-length Shc proteins to the NGF receptor. In PC12 cells, the Shc SH2 domain interacts with an unidentified tyrosine-phosphorylated protein of 115 kDa but not with the activated NGF receptor. The ability of Shc to interact with different tyrosine-phosphorylated proteins via its PI and SH2 domains may allow Shc to play a unique role in tyrosine kinase signal transduction pathways.

A region in Shc distinct from the SH2 domain can bind tyrosine-phosphorylated growth factor receptors

Shc is a ubiquitously expressed Src homology 2 (SH2) domain protein that can transform fibroblasts and differentiate PC12 cells in a Ras-dependent fashion. Shc binds a variety of tyrosine-phosphorylated growth factor receptors presumably via its carboxyl-terminal SH2 domain. We cloned a fragment of Shc when screening a bacterial expression library with tyrosine-phosphorylated epidermal growth factor (EGF) receptor. Surprisingly, this fragment encodes the amino terminus of Shc, a region that has no significant similarity to an SH2 domain. When expressed as a glutathione S-transferase fusion protein, this amino-terminal domain binds to autophosphorylated EGF receptor, as well as HER2/neu and TrkA receptors. This fragment acts like an SH2 domain in that it does not bind non-phosphorylated EGF receptor or EGF receptor with all tyrosine phosphorylation sites mutated or deleted. Our data define a novel domain in Shc that has the potential to interact with growth factor receptors and other tyrosine-phosphorylated proteins.