Leucine motif-dependent tyrosine autophosphorylation of type III receptor tyrosine kinases

Phylogenetic and transcriptomic characterization of insulin and growth factor receptor tyrosine kinases in crustaceans

Receptor tyrosine kinases (RTKs) mediate the actions of growth factors in metazoans. In decapod crustaceans, RTKs are implicated in various physiological processes, such molting and growth, limb regeneration, reproduction and sexual differentiation, and innate immunity. RTKs are organized into two main types: insulin receptors (InsRs) and growth factor receptors, which include epidermal growth factor receptor (EGFR), fibroblast growth factor receptor (FGFR), vascular endothelial growth factor receptor (VEGFR), and platelet-derived growth factor receptor (PDGFR). The identities of crustacean RTK genes are incomplete. A phylogenetic analysis of the CrusTome transcriptome database, which included all major crustacean taxa, showed that RTK sequences segregated into receptor clades representing InsR (72 sequences), EGFR (228 sequences), FGFR (129 sequences), and PDGFR/VEGFR (PVR; 235 sequences). These four receptor families were distinguished by the domain organization of the extracellular N-terminal region and motif sequences in the protein kinase catalytic domain in the C-terminus or the ligand-binding domain in the N-terminus. EGFR1 formed a single monophyletic group, while the other RTK sequences were divided into subclades, designated InsR1-3, FGFR1-3, and PVR1-2. In decapods, isoforms within the RTK subclades were common. InsRs were characterized by leucine-rich repeat, furin-like cysteine-rich, and fibronectin type 3 domains in the N-terminus. EGFRs had leucine-rich repeat, furin-like cysteine-rich, and growth factor IV domains. N-terminal regions of FGFR1 had one to three immunoglobulin-like domains, whereas FGFR2 had a cadherin tandem repeat domain. PVRs had between two and five immunoglobulin-like domains. A classification nomenclature of the four RTK classes, based on phylogenetic analysis and multiple sequence alignments, is proposed.

N-Glycosylation regulates ligand-dependent activation and signaling of vascular endothelial growth factor receptor 2 (VEGFR2)

The tumor microenvironment and proinflammatory signals significantly alter glycosylation of cell-surface proteins on endothelial cells. By altering the N-glycosylation machinery in the endoplasmic reticulum and Golgi, proinflammatory cytokines promote the modification of endothelial glycoproteins such as vascular endothelial growth factor receptor 2 (VEGFR2) with sialic acid-capped N-glycans. VEGFR2 is a highly N-glycosylated receptor tyrosine kinase involved in pro-angiogenic signaling in physiological and pathological contexts, including cancer. Here, using glycoside hydrolase and kinase assays and immunoprecipitation and MS-based analyses, we demonstrate that N-linked glycans at the Asn-247 site in VEGFR2 hinder VEGF ligand-mediated receptor activation and signaling in endothelial cells. We provide evidence that cell surface-associated VEGFR2 displays sialylated N-glycans at Asn-247 and, in contrast, that the nearby sites Asn-145 and Asn-160 contain lower levels of sialylated N-glycans and higher levels of high-mannose N-glycans, respectively. Furthermore, we report that VEGFR2 Asn-247-linked glycans capped with sialic acid oppose ligand-mediated VEGFR2 activation, whereas the uncapped asialo-glycans favor activation of this receptor. We propose that N-glycosylation, specifically the capping of N-glycans at Asn-247 by sialic acid, tunes ligand-dependent activation and signaling of VEGFR2 in endothelial cells.

Insights into the anti-angiogenic properties of phosphaplatins

Phosphaplatins are platinum-based antitumor compounds that, unlike other clinically utilized platinum drugs (i.e. cisplatin, carboplatin, and oxaliplatin), appear to target proteins rather than DNA. Because of their unique mode of action, phosphaplatins are promising drug candidates for cisplatin-resistant cancers. In this study, we discovered that Pt(II) and Pt(IV) phosphaplatins possess diverse antitumor properties. In addition to targeting apoptosis antigen (FAS) and proapoptotic gene products as described previously, phosphaplatins also target angiogenesis. We demonstrate that phosphaplatins inhibit human umbilical vein endothelial cell (HUVEC) migration and tube formation in vitro and suppress tumor angiogenesis and growth in immunodeficient mice that were inoculated with A2780 ovarian cancer cells in vivo. To provide insight into this novel antitumor mechanism, phosphaplatin-treated HUVECs were found to exhibit lower gene expression levels of vascular endothelial growth factors (VEGFs) and the VEGFR-2 receptor compared to untreated cells. Kinase inhibition studies suggest that phosphaplatins are inhibitors of VEGFR-2. In ligand exchange experiments using both Pt atomic absorption and ³¹P NMR spectroscopies, we show that phosphaplatins most likely bind to VEGFR-2 through metal-ligand coordination rather than electrostatic interactions. These studies enhance our understanding of the diverse and novel mechanisms of action of the phosphaplatin antitumor agents, which could potentially be used as chemotherapeutic agents against cisplatin-resistant cancers.

Identification of IGPR-1 as a novel adhesion molecule involved in angiogenesis

IGPR-1 is a novel adhesion molecule that regulates cell–cell interaction. IGPR-1 associates with several SH3-containing proteins, including SPIN90/WISH, and regulates capillary tube formation of primary endothelial cells.

Angiogenesis—the growth of new blood vessels from preexisting vessels—is an important physiological process and is considered to play a key role in tumor growth and metastasis. We identified the immunoglobulin-containing and proline-rich receptor-1 (IGPR-1, also called TMIGD2) gene as a novel cell adhesion receptor that is expressed in various human organs and tissues, mainly in cells with epithelium and endothelium origins. IGPR-1 regulates cellular morphology, homophilic cell aggregation, and cell–cell interaction. IGPR-1 activity also modulates actin stress fiber formation and focal adhesion and reduces cell migration. Silencing of expression of IGPR-1 by small interfering RNA (siRNA) and by ectopic overexpression in endothelial cells showed that IGPR-1 regulates capillary tube formation in vitro, and B16F melanoma cells engineered to express IGPR-1 displayed extensive angiogenesis in the mouse Matrigel angiogenesis model. Moreover, IGPR-1, through its proline-rich cytoplasmic domain, associates with multiple Src homology 3 (SH3)–containing signaling proteins, including SH3 protein interacting with Nck (SPIN90/WISH), bullous pemphigoid antigen-1, and calcium channel β2. Silencing of expression of SPIN90/WISH by siRNA in endothelial cells showed that SPIN90/WISH is required for capillary tube formation. These features of IGPR-1 suggest that IGPR-1 is a novel receptor that plays an important role in cell–cell interaction, cell migration, and angiogenesis.

Identification of critical residues in Gap3 of Streptococcus parasanguinis involved in Fap1 glycosylation, fimbrial formation and in vitro adhesion

Background

Streptococcus parasanguinis is a primary colonizer of human tooth surfaces and plays an important role in dental plaque formation. Bacterial adhesion and biofilm formation are mediated by long peritrichous fimbriae that are composed of a 200 kDa serine rich glycoprotein named Fap1 (fimbriae-associated protein). Glycosylation and biogenesis of Fap1 are modulated by a gene cluster downstream of the fap1 locus. A gene encoding a glycosylation-associated protein, Gap3, was found to be important for Fap1 glycosylation, long fimbrial formation and Fap1-mediated biofilm formation.

Results

Deletion and site-directed mutagenesis were employed to dissect the regions within Gap3 that were important for its function in Fap1 glycosylation and biogenesis. A deletion of 6 consecutive amino acids, PDLPIL, eliminated the production of the mature 200 kDa Fap1 protein and gave rise instead to a 470 kDa Fap1 intermediate that was only partially glycosylated. Site-directed mutagenesis of the 6 amino acids revealed that only three of these amino acids were required. Mutants in these amino acids (L64R, P65R and L67T) produced the premature 470 kDa Fap1 intermediate. Mutants in the remaining amino acids produced the mature form of Fap1. Cell surface expression of the Fap1 precursor among L64R, P65R and L67T mutants was reduced to levels consistent with that of a gap3 insertional mutant. Electron micrographs showed that these 3 mutants lost their long peritrichous fimbriae. Furthermore, their in vitro adhesion ability to saliva-coated hydroxylapatite (SHA) was inhibited.

Conclusion

Our data suggest that 3 highly conserved, hydrophobic residues L64, P65 and L67 in Gap3 are essential for Gap3 function and are important for complete glycosylation of Fap1, fimbrial formation and bacterial adhesion.