Lysine methylation promotes VEGFR-2 activation and angiogenesis

Inactivation of Minar2 in mice hyperactivates mTOR signaling and results in obesity

OBJECTIVE

Obesity is a complex disorder and is linked to chronic diseases such as type 2 diabetes. Major intrinsically disordered NOTCH2-associated receptor2 (MINAR2) is an understudied protein with an unknown role in obesity and metabolism. The purpose of this study was to determine the impact of Minar2 on adipose tissues and obesity.

METHOD

We generated Minar2 knockout (KO) mice and used various molecular, proteomic, biochemical, histopathology, and cell culture studies to determine the pathophysiological role of Minar2 in adipocytes.

RESULTS

We demonstrated that the inactivation of Minar2 results in increased body fat with hypertrophic adipocytes. Minar2 KO mice on a high-fat diet develop obesity and impaired glucose tolerance and metabolism. Mechanistically, Minar2 interacts with Raptor, a specific and essential component of mammalian TOR complex 1 (mTORC1) and inhibits mTOR activation. mTOR is hyperactivated in the adipocytes deficient for Minar2 and over-expression of Minar2 in HEK-293 cells inhibited mTOR activation and phosphorylation of mTORC1 substrates, including S6 kinase, and 4E-BP1.

CONCLUSION

Our findings identified Minar2 as a novel physiological negative regulator of mTORC1 with a key role in obesity and metabolic disorders. Impaired expression or activation of MINAR2 could lead to obesity and obesity-associated diseases.

cKMT1 is a new lysine methyltransferase that methylates the ferredoxin-NADP(+) oxidoreductase (FNR) and regulates energy transfer in cyanobacteria

Lysine methylation is a conserved and dynamic regulatory post-translational modification performed by lysine methyltransferases (KMTs). KMTs catalyze the transfer of mono-, di-, or tri-methyl groups to substrate proteins and play a critical regulatory role in all domains of life. To date, only one KMT has been identified in cyanobacteria. Here, we tested all of the predicted KMTs in the cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis), and we biochemically characterized sll1526 that we termed cKMT1 (cyanobacterial lysine methyltransferase 1), and determined that it can catalyze lysine methylation both in vivo and in vitro. Loss of cKMT1 alters photosynthetic electron transfer in Synechocystis. We analyzed cKMT1-regulated methylation sites in Synechocystis using a timsTOF Pro instrument. We identified 305 class I lysine methylation sites within 232 proteins, and of these, 80 methylation sites in 58 proteins were hypomethylated in ΔcKMT1 cells. We further demonstrated that cKMT1 could methylate ferredoxin-NADP(+) oxidoreductase (FNR) and its potential sites of action on FNR were identified. Amino acid residues H118 and Y219 were identified as key residues in the putative active site of cKMT1 as indicated by structure simulation, site-directed mutagenesis, and KMT activity measurement. Using mutations that mimic the unmethylated forms of FNR, we demonstrated that the inability to methylate K139 residues results in a decrease in the redox activity of FNR and affects energy transfer in Synechocystis. Together, our study identified a new KMT in Synechocystis and elucidated a methylation-mediated molecular mechanism catalyzed by cKMT1 for the regulation of energy transfer in cyanobacteria.

PRMT4-mediated arginine methylation promotes tyrosine phosphorylation of VEGFR-2 and regulates filopodia protrusions

Through tightly controlled multilayer mechanisms, vascular endothelial growth factor receptor-2 (VEGFR-2) activation and its downstream signal transduction govern vasculogenesis and pathological angiogenesis, such as tumor angiogenesis. Therefore, it is critical to understand the molecular mechanisms governing VEGFR-2 signal transduction. We report that protein arginine methyltransferase 4 (PRMT4) via its highly conserved EVH1 and PH domain-like N-terminal domain binds to VEGFR-2 and mediates methylation of the juxtamembrane arginine 817 (R817) on VEGFR-2. Methylation of R817 selectively increases phosphorylation of tyrosine 820 (Y820). Phosphorylation of Y820 facilitates the c-Src binding with VEGFR-2 via Src homology domain 2 (SH2). Interfering with the methylation of R817 or phosphorylation of Y820 inhibits VEGFR-2-induced filopodia protrusions, a process that is critical for the core angiogenic responses of VEGFR-2. Methylation of R817 is an important previously unrecognized mechanism of the angiogenic signaling of VEGFR-2, with implications for the development of novel-targeted VEGFR-2 inhibitors.

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Arginine 817 methylation regulates phosphorylation of Y820 on VEGFR-2

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Phosphorylation of Y820 recruits c-Src kinase to VEGFR-2, leading to its activation

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VEGFR-2/c-Src axis mediates filopodia protrusions in endothelial cells

VEGFR endocytosis: Implications for angiogenesis

The binding of vascular endothelial growth factor (VEGF) superfamily to VEGF receptor tyrosine kinases (VEGFRs) and co-receptors regulates vasculogenesis, angiogenesis and lymphangiogenesis. A recurring theme is that dysfunction in VEGF signaling promotes pathological angiogenesis, an important feature of cancer and pro-inflammatory disease states. Endocytosis of basal (resting) or activated VEGFRs facilitates signal attenuation and endothelial quiescence. However, increasing evidence suggest that activated VEGFRs can continue to signal from intracellular compartments such as endosomes. In this chapter, we focus on the evolving link between VEGFR endocytosis, signaling and turnover and the implications for angiogenesis. There is much interest in how such understanding of VEGFR dynamics can be harnessed therapeutically for a wide range of human disease states.

Extracellular vimentin is an attachment factor that facilitates SARS-CoV-2 entry into human endothelial cells

SARS-CoV-2 entry into host cells is a crucial step for virus tropism, transmission, and pathogenesis. Angiotensin-converting enzyme 2 (ACE2) has been identified as the primary entry receptor for SARS-CoV-2; however, the possible involvement of other cellular components in the viral entry has not yet been fully elucidated. Here we describe the identification of vimentin (VIM), an intermediate filament protein widely expressed in cells of mesenchymal origin, as an important attachment factor for SARS-CoV-2 on human endothelial cells. Using liquid chromatography-tandem mass spectrometry, we identified VIM as a protein that binds to the SARS-CoV-2 spike (S) protein. We showed that the S-protein receptor binding domain (RBD) is sufficient for S-protein interaction with VIM. Further analysis revealed that extracellular VIM binds to SARS-CoV-2 S-protein and facilitates SARS-CoV-2 infection, as determined by entry assays performed with pseudotyped viruses expressing S and with infectious SARS-CoV-2. Coexpression of VIM with ACE2 increased SARS-CoV-2 entry in HEK-293 cells, and shRNA-mediated knockdown of VIM significantly reduced SARS-CoV-2 infection of human endothelial cells. Moreover, incubation of A549 cells expressing ACE2 with purified VIM increased pseudotyped SARS-CoV-2-S entry. CR3022 antibody, which recognizes a distinct epitope on SARS-CoV-2-S-RBD without interfering with the binding of the spike with ACE2, inhibited the binding of VIM with CoV-2 S-RBD, and neutralized viral entry in human endothelial cells, suggesting a key role for VIM in SARS-CoV-2 infection of endothelial cells. This work provides insight into the pathogenesis of COVID-19 linked to the vascular system, with implications for the development of therapeutics and vaccines.

Drug design targeting active posttranslational modification protein isoforms

Modern drug design aims to discover novel lead compounds with attractable chemical profiles to enable further exploration of the intersection of chemical space and biological space. Identification of small molecules with good ligand efficiency, high activity, and selectivity is crucial toward developing effective and safe drugs. However, the intersection is one of the most challenging tasks in the pharmaceutical industry, as chemical space is almost infinity and continuous, whereas the biological space is very limited and discrete. This bottleneck potentially limits the discovery of molecules with desirable properties for lead optimization. Herein, we present a new direction leveraging posttranslational modification (PTM) protein isoforms target space to inspire drug design termed as "Post-translational Modification Inspired Drug Design (PTMI-DD)." PTMI-DD aims to extend the intersections of chemical space and biological space. We further rationalized and highlighted the importance of PTM protein isoforms and their roles in various diseases and biological functions. We then laid out a few directions to elaborate the PTMI-DD in drug design including discovering covalent binding inhibitors mimicking PTMs, targeting PTM protein isoforms with distinctive binding sites from that of wild-type counterpart, targeting protein-protein interactions involving PTMs, and hijacking protein degeneration by ubiquitination for PTM protein isoforms. These directions will lead to a significant expansion of the biological space and/or increase the tractability of compounds, primarily due to precisely targeting PTM protein isoforms or complexes which are highly relevant to biological functions. Importantly, this new avenue will further enrich the personalized treatment opportunity through precision medicine targeting PTM isoforms.

Drug-encapsulated blend of PLGA-PEG microspheres: in vitro and in vivo study of the effects of localized/targeted drug delivery on the treatment of triple-negative breast cancer

Triple-negative breast cancer (TNBC) is more aggressive and difficult to treat using conventional bulk chemotherapy that is often associated with increased toxicity and side effects. In this study, we encapsulated targeted drugs [A bacteria-synthesized anticancer drug (prodigiosin) and paclitaxel] using single solvent evaporation technique with a blend of FDA-approved poly lactic-co-glycolic acid-polyethylene glycol (PLGA_PEG) polymer microspheres. These drugs were functionalized with Luteinizing Hormone-Releasing hormone (LHRH) ligands whose receptors are shown to overexpressed on surfaces of TNBC. The physicochemical, structural, morphological and thermal properties of the drug-loaded microspheres were then characterized using Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Dynamic Light Scattering (DLS), Nuclear Magnetic Resonance Spectroscopy (NMR), Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). Results obtained from in vitro kinetics drug release at human body temperature (37 °C) and hyperthermic temperatures (41 and 44 °C) reveal a non-Fickian sustained drug release that is well-characterized by Korsmeyer-Peppas model with thermodynamically non-spontaneous release of drug. Clearly, the in vitro and in vivo drug release from conjugated drug-loaded microspheres (PLGA-PEG_PGS-LHRH, PLGA-PEG_PTX-LHRH) is shown to result in greater reductions of cell/tissue viability in the treatment of TNBC. The in vivo animal studies also showed that all the drug-loaded PLGA-PEG microspheres for the localized and targeted treatment of TNBC did not caused any noticeable toxicity and thus significantly extended the survival of the treated mice post tumor resection. The implications of this work are discussed for developing targeted drug systems to treat and prevent local recurred triple negative breast tumors after surgical resection.

Basic and Therapeutic Aspects of Angiogenesis Updated

All organisms growing beyond the oxygen diffusion limit critically depend on a functional vasculature for survival. Yet blood vessels are far more than passive, uniform conduits for oxygen and nutrient supply. A remarkable organotypic heterogeneity is brought about by tissue-specific differentiated endothelial cells (lining the blood vessels' lumen) and allows blood vessels to deal with organ-specific demands for homeostasis. On the flip side, when blood vessels go awry, they promote life-threatening diseases characterized by endothelial cells inappropriately adopting an angiogenic state (eg, tumor vascularization) or becoming dysfunctional (eg, diabetic microvasculopathies), calling respectively for antiangiogenic therapies and proangiogenic/vascular regenerative strategies. In solid tumors, despite initial enthusiasm, growth factor-based (mostly anti-VEGF [vascular endothelial growth factor]) antiangiogenic therapies do not sufficiently live up to the expectations in terms of efficiency and patient survival, in part, due to intrinsic and acquired therapy resistance. Tumors cunningly deploy alternative growth factors than the ones targeted by the antiangiogenic therapies to reinstigate angiogenesis or revert to other ways of securing blood flow, independently of the targeted growth factors. In trying to alleviate tissue ischemia and to repair dysfunctional or damaged endothelium, local in-tissue administration of (genes encoding) proangiogenic factors or endothelial (stem) cells harnessing regenerative potential have been explored. Notwithstanding evaluation in clinical trials, these approaches are often hampered by dosing issues and limited half-life or local retention of the administered agents. Here, without intending to provide an all-encompassing historical overview, we focus on some recent advances in understanding endothelial cell behavior in health and disease and identify novel molecular players and concepts that could eventually be considered for therapeutic targeting.

Epigenetic regulation of TLR2-mediated periapical inflammation

AIM

To determine the methylation pattern of TLR2 gene promoter and its association with the transcriptional regulation of periapical inflammatory and angiogenic responses in symptomatic and asymptomatic forms of apical periodontitis.

METHODOLOGY

In this cross-sectional study, apical lesions were obtained from volunteers with asymptomatic apical periodontitis (AAP) (n = 17) and symptomatic apical periodontitis (SAP) (n = 17) scheduled for tooth extraction, and both total RNA and DNA were extracted. DNA was bisulfite-treated, a region of CpG island within the TLR2 gene was amplified by qPCR and the products were sequenced. Additionally, the mRNA expression of TLR2, TLR4, IL-6, IL-12, TNFalpha, IL-23, IL-10, TGFbeta, VEGFA and CDH5 was analysed by qPCR. The data were analysed with chi-square tests, Mann-Whitney or unpaired t-tests, and Spearman´s correlation; variable adjustments were performed using multiple linear regression (P < 0.05).

RESULTS

TLR2 depicted a hypomethylated DNA profile at the CpG island in SAP when compared with AAP, along with upregulated expression of TLR2, with pro-inflammatory cytokines IL-6 and IL-23, and the angiogenesis marker CDH5 (P < 0.05). TLR2 methylation percentage negatively correlated with mRNA levels of IL-23 and CDH5 in apical periodontitis. Lower methylation frequencies of single CpG dinucleotides -8 and -10 localized in close proximity to nuclear factor κB (NFκB) binding within the TLR2 promoter were identified in SAP versus AAP (P < 0.05). Finally, unmethylated -10 and -8 single sites demonstrated up-regulation of IL-23, IL-10 and CDH5 transcripts compared to their methylated counterparts (P < 0.05).

CONCLUSIONS

TLR2 gene promoter hypomethylation was linked to transcriptional activity of pro-inflammatory cytokines and angiogenic markers in exacerbated periapical inflammation. Moreover, unmethylated single sites in close proximity to NFκB binding were involved in active transcription of IL-23, IL-10 and CDH5.

MINAR1 is a Notch2-binding protein that inhibits angiogenesis and breast cancer growth

Intrinsically disordered proteins (IDPs)/intrinsically unstructured proteins are characterized by the lack of fixed or stable tertiary structure, and are increasingly recognized as an important class of proteins with major roles in signal transduction and transcriptional regulation. In this study, we report the identification and functional characterization of a previously uncharacterized protein (UPF0258/KIAA1024), major intrinsically disordered Notch2-associated receptor 1 (MINAR1). While MINAR1 carries a single transmembrane domain and a short cytoplasmic domain, it has a large extracellular domain that shares no similarity with known protein sequences. Uncharacteristically, MINAR1 is a highly IDP with nearly 70% of its amino acids sequences unstructured. We demonstrate that MINAR1 physically interacts with Notch2 and its binding to Notch2 increases its stability and function. MINAR1 is widely expressed in various tissues including the epithelial cells of the breast and endothelial cells of blood vessels. MINAR1 plays a negative role in angiogenesis as it inhibits angiogenesis in cell culture and in mouse matrigel plug and zebrafish angiogenesis models. Furthermore, while MINAR1 is highly expressed in the normal human breast, its expression is significantly downregulated in advanced human breast cancer and its re-expression in breast cancer cells inhibited tumor growth. Our study demonstrates that MINAR1 is an IDP that negatively regulates angiogenesis and growth of breast cancer cells.

Context-dependent regulation of receptor tyrosine kinases: Insights from systems biology approaches

Receptor tyrosine kinases (RTKs) are cell membrane proteins that provide cells with the ability to sense proteins in their environments. Many RTKs are essential to development and organ growth. Derangement of RTKs-by mutation or by overexpression-is central to several developmental and adult disorders including cancer, short stature, and vascular pathologies. The mechanism of action of RTKs is complex and is regulated by contextual components, including the existence of multiple competing ligands and receptors in many families, the intracellular location of the RTK, the dynamic and cell-specific coexpression of other RTKs, and the commonality of downstream signaling pathways. This means that both the state of the cell and the microenvironment outside the cell play a role, which makes sense given the pivotal location of RTKs as the nexus linking the extracellular milieu to intracellular signaling and modification of cell behavior. In this review, we describe these different contextual components through the lens of systems biology, in which both computational modeling and experimental "omics" approaches have been used to better understand RTK networks. The complexity of these networks is such that using these systems biology approaches is necessary to get a handle on the mechanisms of pathology and the design of therapeutics targeting RTKs. In particular, we describe in detail three concrete examples (involving ErbB3, VEGFR2, and AXL) that illustrate how systems approaches can reveal key mechanistic and therapeutic insights. This article is categorized under: Biological Mechanisms > Cell Signaling Models of Systems Properties and Processes > Mechanistic Models Translational, Genomic, and Systems Medicine > Therapeutic Methods.

Pan-Cancer Analysis Reveals the Functional Importance of Protein Lysine Modification in Cancer Development

Large-scale tumor genome sequencing projects have revealed a complex landscape of genomic mutations in multiple cancer types. A major goal of these projects is to characterize somatic mutations and discover cancer drivers, thereby providing important clues to uncover diagnostic or therapeutic targets for clinical treatment. However, distinguishing only a few somatic mutations from the majority of passenger mutations is still a major challenge facing the biological community. Fortunately, combining other functional features with mutations to predict cancer driver genes is an effective approach to solve the above problem. Protein lysine modifications are an important functional feature that regulates the development of cancer. Therefore, in this work, we have systematically analyzed somatic mutations on seven protein lysine modifications and identified several important drivers that are responsible for tumorigenesis. From published literature, we first collected more than 100,000 lysine modification sites for analysis. Another 1 million non-synonymous single nucleotide variants (SNVs) were then downloaded from TCGA and mapped to our collected lysine modification sites. To identify driver proteins that significantly altered lysine modifications, we further developed a hierarchical Bayesian model and applied the Markov Chain Monte Carlo (MCMC) method for testing. Strikingly, the coding sequences of 473 proteins were found to carry a higher mutation rate in lysine modification sites compared to other background regions. Hypergeometric tests also revealed that these gene products were enriched in known cancer drivers. Functional analysis suggested that mutations within the lysine modification regions possessed higher evolutionary conservation and deleteriousness. Furthermore, pathway enrichment showed that mutations on lysine modification sites mainly affected cancer related processes, such as cell cycle and RNA transport. Moreover, clinical studies also suggested that the driver proteins were significantly associated with patient survival, implying an opportunity to use lysine modifications as molecular markers in cancer diagnosis or treatment. By searching within protein-protein interaction networks using a random walk with restart (RWR) algorithm, we further identified a series of potential treatment agents and therapeutic targets for cancer related to lysine modifications. Collectively, this study reveals the functional importance of lysine modifications in cancer development and may benefit the discovery of novel mechanisms for cancer treatment.

c-Cbl Expression Correlates with Human Colorectal Cancer Survival and Its Wnt/β-Catenin Suppressor Function Is Regulated by Tyr371 Phosphorylation

The proto-oncogene β-catenin drives colorectal cancer (CRC) tumorigenesis. Casitas B-lineage lymphoma (c-Cbl) inhibits CRC tumor growth through targeting nuclear β-catenin by a poorly understood mechanism. In addition, the role of c-Cbl in human CRC remains largely underexplored. Using a novel quantitative histopathologic technique, we demonstrate that patients with high c-Cbl-expressing tumors had significantly better median survival (3.7 years) compared with low c-Cbl-expressing tumors (1.8 years; P = 0.0026) and were more than twice as likely to be alive at 3 years compared with low c-Cbl tumors (P = 0.0171). Our data further demonstrate that c-Cbl regulation of nuclear β-catenin requires phosphorylation of c-Cbl Tyr371 because its mutation compromises its ability to target β-catenin. The tyrosine 371 (Y371H) mutant interacted with but failed to ubiquitinate nuclear β-catenin. The nuclear localization of the c-Cbl-Y371H mutant contributed to its dominant negative effect on nuclear β-catenin. The biological importance of c-Cbl-Y371H was demonstrated in various systems, including a transgenic Wnt-8 zebrafish model. c-Cbl-Y371H mutant showed augmented Wnt/β-catenin signaling, increased Wnt target genes, angiogenesis, and CRC tumor growth. This study demonstrates a strong link between c-Cbl and overall survival of patients with CRC and provides new insights into a possible role of Tyr371 phosphorylation in Wnt/β-catenin regulation, which has important implications in tumor growth and angiogenesis in CRC.

Clinical Presentation of Preeclampsia and the Diagnostic Value of Proteins and Their Methylation Products as Biomarkers in Pregnant Women with Preeclampsia and Their Newborns

Preeclampsia (PE) is a disorder which affects 1-10% of pregnant women worldwide. It is characterised by hypertension and proteinuria in the later stages of gestation and can lead to maternal and perinatal morbidity and mortality. Other than the delivery of the foetus and the removal of the placenta, to date there are no therapeutic approaches to treat or prevent PE. It is thus only possible to reduce PE-related mortality through early detection, careful monitoring, and treatment of the symptoms. For these reasons the search for noninvasive, blood-borne, or urinary biochemical markers that could be used for the screening, presymptomatic diagnosis, and prediction of the development of PE is of great urgency. So far, a number of biomarkers have been proposed for predicting PE, based on pathophysiological observations, but these have mostly proven to be unreliable and inconsistent between different studies. The clinical presentation of PE and data gathered for the biochemical markers placental growth factor (PlGF), soluble Feline McDonough Sarcoma- (fms-) like tyrosine kinase-1 (sFlt-1), asymmetric dimethylarginine (ADMA), and methyl-lysine is being reviewed with the aim of providing both a clinical and biochemical understanding of how these biomarkers might assist in the diagnosis of PE or indicate its severity.

Defenders and Challengers of Endothelial Barrier Function

Regulated vascular permeability is an essential feature of normal physiology and its dysfunction is associated with major human diseases ranging from cancer to inflammation and ischemic heart diseases. Integrity of endothelial cells also play a prominent role in the outcome of surgical procedures and organ transplant. Endothelial barrier function and integrity are regulated by a plethora of highly specialized transmembrane receptors, including claudin family proteins, occludin, junctional adhesion molecules (JAMs), vascular endothelial (VE)-cadherin, and the newly identified immunoglobulin (Ig) and proline-rich receptor-1 (IGPR-1) through various distinct mechanisms and signaling. On the other hand, vascular endothelial growth factor (VEGF) and its tyrosine kinase receptor, VEGF receptor-2, play a central role in the destabilization of endothelial barrier function. While claudins and occludin regulate cell-cell junction via recruitment of zonula occludens (ZO), cadherins via catenin proteins, and JAMs via ZO and afadin, IGPR-1 recruits bullous pemphigoid antigen 1 [also called dystonin (DST) and SH3 protein interacting with Nck90/WISH (SH3 protein interacting with Nck)]. Endothelial barrier function is moderated by the function of transmembrane receptors and signaling events that act to defend or destabilize it. Here, I highlight recent advances that have provided new insights into endothelial barrier function and mechanisms involved. Further investigation of these mechanisms could lead to the discovery of novel therapeutic targets for human diseases associated with endothelial dysfunction.

Angiodysplasia in embryo lacking protein arginine methyltransferase 1 in vascular endothelial cells

Protein arginine methyltransferase 1 (PRMT1) is involved in multiple cellular functions including proliferation and differentiation. Although PRMT1 is expressed in vascular endothelial cells (ECs), which are responsible for angiogenesis during embryonic development, its role has remained elusive. In this study, we generated endothelial-specific prmt1-knockout (Prmt1-ECKO) mice, and found that they died before embryonic day 15. The superficial temporal arteries in these embryos were poorly perfused with blood, and whole-mount 3D imaging revealed dilated and segmentalized luminal structures in Prmt1-ECKO fetuses in comparison with those of controls. Our findings provide evidence that PRMT1 is important for embryonic vascular formation.

Site-Specific N-Glycosylation of Endothelial Cell Receptor Tyrosine Kinase VEGFR-2

Vascular endothelial growth factor receptor-2 (VEGFR-2) is an important receptor tyrosine kinase (RTK) that plays critical roles in both physiologic and pathologic angiogenesis. The extracellular domain of VEGFR-2 is composed of seven immunoglobulin-like domains, each with multiple potential N-glycosylation sites (sequons). N-glycosylation plays a central role in RTK ligand binding, trafficking, and stability. However, despite its importance, the functional role of N-glycosylation of VEGFR-2 remains poorly understood. The objectives of the present study were to characterize N-glycosylation sites in VEGFR-2 via enzymatic release of the glycans and concomitant incorporation of ¹⁸O into formerly N-glycosylated sites followed by tandem mass spectrometry (MS/MS) analysis to determine N-glycosylation site occupancy and the site-specific N-glycan heterogeneity of VEGFR-2 glycopeptides. The data demonstrated that all seven VEGFR-2 immunoglobulin-like domains have at least one occupied N-glycosylation site. MS/MS analyses of glycopeptides and deamidated, deglycosylated (PNGase F-treated) peptides from ectopically expressed VEGFR-2 in porcine aortic endothelial (PAE) cells identified N-glycans at the majority of the 17 potential N-glycosylation sites on VEGFR-2 in a site-specific manner. The data presented here provide direct evidence for site-specific, heterogeneous N-glycosylation and N-glycosylation site occupancy on VEGFR-2. The study has important implications for the therapeutic targeting of VEGFR-2, ligand binding, trafficking, and signaling.

IGPR-1 Is Required for Endothelial Cell-Cell Adhesion and Barrier Function

Endothelial cell (EC) barrier function plays a prevalent regulatory mechanism for the integrity and homeostasis of blood vessels and modulates angiogenesis and immune responses. Cell adhesion molecules (CAMs) play a central role in the barrier function of ECs. Although Ig-containing and proline-rich receptor-1(IGPR-1) was recently identified as a novel CAM expressed in ECs, the molecular mechanisms underlying the function of IGPR-1 in ECs remain uncharacterized. In this report, we investigated the role of IGPR-1 in EC barrier function and the molecular mechanism of its activation in ECs. We demonstrate that IGPR-1 is localized to endothelial adherens junctions and, through trans-homophilic dimerization, regulates endothelial cell-cell adhesion and barrier function. Trans-homophilic dimerization of IGPR-1 stimulates the phosphorylation of serine 220 (Ser220), which is required for IGPR-1 to regulate endothelial barrier function and angiogenesis. Moreover, IGPR-1 chimera, which mimics the trans-homophilic dimerization of IGPR-1, induced a sustained phosphorylation of Ser220 upon stimulation with a ligand. Coordinated dimerization of IGPR-1 and its homophilic interaction modulates its adhesive function and Ser220 phosphorylation. This adhesive function of IGPR-1 contributes to the barrier function of ECs.

Photoactivatable Caged Prodrugs of VEGFR-2 Kinase Inhibitors

In this study, we report on the design, synthesis, photokinetic properties and in vitro evaluation of photoactivatable caged prodrugs for the receptor tyrosine kinase VEGFR-2. Highly potent VEGFR-2 inhibitors 1 and 3 were caged by introduction of a photoremovable protecting group (PPG) to yield the caged prodrugs 4 and 5. As expected, enzymatic and cellular proliferation assays showed dramatically diminished efficacy of caged prodrugs in vitro. Upon ultraviolet (UV) irradiation of the prodrugs original inhibitory activity was completely restored and even distinctly reinforced, as was the case for the prodrug 4. The presented results are a further evidence for caging technique being an interesting approach in the protein kinase field. It could enable spatial and temporal control for the inhibition of VEGFR-2. The described photoactivatable prodrugs might be highly useful as biological probes for studying the VEGFR-2 signal transduction.

RNF121 Inhibits Angiogenic Growth Factor Signaling by Restricting Cell Surface Expression of VEGFR-2

Ligand stimulation promotes downregulation of RTKs, a mechanism by which RTKs, through the ubiquitination pathway are removed from the cell surface, causing a temporary termination of RTK signaling. The molecular mechanisms governing RTK trafficking and maturation in the endoplasmic reticulum (ER)/Golgi compartments are poorly understood. Vascular endothelial growth factor receptor-2 (VEGFR-2) is a prototypic RTK that plays a critical role in physiologic and pathologic angiogenesis. Here we demonstrate that Ring Finger Protein 121 (RNF121), an ER ubiquitin E3 ligase, is expressed in endothelial cells and regulates maturation of VEGFR-2. RNF121 recognizes newly synthesized VEGFR-2 in the ER and controls its trafficking and maturation. Over-expression of RNF121 promoted ubiquitination of VEGFR-2, inhibited its maturation and resulted a significantly reduced VEGFR-2 presence at the cell surface. Conversely, the shRNA-mediated knockdown of RNF121 in primary endothelial cells reduced VEGFR-2 ubiquitination and increased its cell surface level. The RING Finger domain of RNF121 is required for its activity toward VEGFR-2, as its deletion significantly reduced the effect of RNF121 on VEGFR-2. Additionally, RNF121 inhibited VEGF-induced endothelial cell proliferation and angiogenesis. Taken together, these data identify RNF121 as a key determinant of angiogenic signaling that restricts VEGFR-2 cell surface presence and its angiogenic signaling.

Hypoxia-induced expression of phosducin-like 3 regulates expression of VEGFR-2 and promotes angiogenesis

Expression and activation of vascular endothelial growth factor receptor 2 (VEGFR-2) by VEGF ligands are the main events in the stimulation of pathological angiogenesis. VEGFR-2 expression is generally low in the healthy adult blood vessels, but its expression is markedly increased in the pathological angiogenesis. In this report, we demonstrate that phosducin-like 3 (PDCL3), a recently identified chaperone protein involved in the regulation of VEGFR-2 expression, is required for angiogenesis in zebrafish and mouse. PDCL3 undergoes N-terminal methionine acetylation, and this modification affects PDCL3 expression and its interaction with VEGFR-2. Expression of PDCL3 is regulated by hypoxia, the known stimulator of angiogenesis. The mutant PDCL3 that is unable to undergo N-terminal methionine acetylation was refractory to the effect of hypoxia. The siRNA-mediated silencing of PDCL3 decreased VEGFR-2 expression resulting in a decrease in VEGF-induced VEGFR-2 phosphorylation, whereas PDCL3 over-expression increased VEGFR-2 protein. Furthermore, we show that PDCL3 protects VEGFR-2 from misfolding and aggregation. The data provide new insights for the chaperone function of PDCL3 in angiogenesis and the roles of hypoxia and N-terminal methionine acetylation in PDCL3 expression and its effect on VEGFR-2.

VEGF-mediated cell survival in non-small-cell lung cancer: implications for epigenetic targeting of VEGF receptors as a therapeutic approach

Aims: To evaluate the potential therapeutic utility of histone deacetylase inhibitors (HDACi) in targeting VEGF receptors in non-small-cell lung cancer. Materials & methods: Non-small-cell lung cancer cells were screened for the VEGF receptors at the mRNA and protein levels, while cellular responses to various HDACi were examined. Results: Significant effects on the regulation of the VEGF receptors were observed in response to HDACi. These were associated with decreased secretion of VEGF, decreased cellular proliferation and increased apoptosis which could not be rescued by addition of exogenous recombinant VEGF. Direct remodeling of the VEGFR1 and VEGFR2 promoters was observed. In contrast, HDACi treatments resulted in significant downregulation of the Neuropilin receptors. Conclusion: Epigenetic targeting of the Neuropilin receptors may offer an effective treatment for lung cancer patients in the clinical setting.

The cellular response to vascular endothelial growth factors requires co-ordinated signal transduction, trafficking and proteolysis

VEGFs (vascular endothelial growth factors) are a family of conserved disulfide-linked soluble secretory glycoproteins found in higher eukaryotes. VEGFs mediate a wide range of responses in different tissues including metabolic homoeostasis, cell proliferation, migration and tubulogenesis. Such responses are initiated by VEGF binding to soluble and membrane-bound VEGFRs (VEGF receptor tyrosine kinases) and co-receptors. VEGF and receptor splice isoform diversity further enhances complexity of membrane protein assembly and function in signal transduction pathways that control multiple cellular responses. Different signal transduction pathways are simultaneously activated by VEGFR-VEGF complexes with membrane trafficking along the endosome-lysosome network further modulating signal output from multiple enzymatic events associated with such pathways. Balancing VEGFR-VEGF signal transduction with trafficking and proteolysis is essential in controlling the intensity and duration of different intracellular signalling events. Dysfunction in VEGF-regulated signal transduction is important in chronic disease states including cancer, atherosclerosis and blindness. This family of growth factors and receptors is an important model system for understanding human disease pathology and developing new therapeutics for treating such ailments.

Systems biology of the microvasculature

The vascular network carries blood throughout the body, delivering oxygen to tissues and providing a pathway for communication between distant organs. The network is hierarchical and structured, but also dynamic, especially at the smaller scales. Remodeling of the microvasculature occurs in response to local changes in oxygen, gene expression, cell-cell communication, and chemical and mechanical stimuli from the microenvironment. These local changes occur as a result of physiological processes such as growth and exercise, as well as acute and chronic diseases including stroke, cancer, and diabetes, and pharmacological intervention. While the vasculature is an important therapeutic target in many diseases, drugs designed to inhibit vascular growth have achieved only limited success, and no drug has yet been approved to promote therapeutic vascular remodeling. This highlights the challenges involved in identifying appropriate therapeutic targets in a system as complex as the vasculature. Systems biology approaches provide a means to bridge current understanding of the vascular system, from detailed signaling dynamics measured in vitro and pre-clinical animal models of vascular disease, to a more complete picture of vascular regulation in vivo. This will translate to an improved ability to identify multi-component biomarkers for diagnosis, prognosis, and monitoring of therapy that are easy to measure in vivo, as well as better drug targets for specific disease states. In this review, we summarize systems biology approaches that have advanced our understanding of vascular function and dysfunction in vivo, with a focus on computational modeling.

The c-Cbl ubiquitin ligase regulates nuclear β-catenin and angiogenesis by its tyrosine phosphorylation mediated through the Wnt signaling pathway

Wnt signaling plays important roles in both the tumor-induced angiogenesis and tumorigenesis through the transcriptionally active nuclear β-catenin. Recently, c-Cbl was identified as a unique E3 ubiquitin ligase targeting the active nuclear β-catenin. However, little is known about the molecular mechanisms by which c-Cbl regulates ubiquitination and degradation of active β-catenin. Here, we demonstrate that Wnt activation promotes the phosphorylation of c-Cbl at tyrosine 731(Tyr-731), which increases c-Cbl dimerization and binding to β-catenin. Tyr-731 phosphorylation and dimerization mediate c-Cbl nuclear translocation and lead to the degradation of nuclearly active β-catenin in the Wnt-on phase. c-Cbl activation also inhibits expression of the pro-angiogenic Wnt targets, IL-8 and VEGF. Phospho-Tyr-731-inactive mutant c-Cbl (Y731F) enhances and phosphomimetic mutant c-Cbl (Y731E) suppresses angiogenesis in zebrafish. Taken together, we have identified a novel mechanism for the regulation of active nuclear β-catenin by c-Cbl and its critical role in angiogenesis. This mechanism can be further explored to modulate both the pathological angiogenesis and the tumorigenesis.

Non-histone protein methylation as a regulator of cellular signalling and function

Methylation of Lys and Arg residues on non-histone proteins has emerged as a prevalent post-translational modification and as an important regulator of cellular signal transduction mediated by the MAPK, WNT, BMP, Hippo and JAK-STAT signalling pathways. Crosstalk between methylation and other types of post-translational modifications, and between histone and non-histone protein methylation frequently occurs and affects cellular functions such as chromatin remodelling, gene transcription, protein synthesis, signal transduction and DNA repair. With recent advances in proteomic techniques, in particular mass spectrometry, the stage is now set to decode the methylproteome and define its functions in health and disease.

Emerging roles of post-translational modifications in signal transduction and angiogenesis

The vascular endothelial growth factor receptor-2 (VEGFR-2) belongs to the family of receptor tyrosine kinases and is a key player in vasculogenesis and pathological angiogenesis. An emerging picture of PTMs of VEGFR-2 suggests that they play central roles in generating a highly dynamic and complex signaling system that regulates key angiogenic responses ranging from endothelial cell differentiation, proliferation, migration to permeability. Recent MS analysis of VEGFR-2 uncovered previously unrecognized PTMs on VEGFR-2 with a distinct function. The ligand binding extracellular domain of VEGFR-2 is composed of seven immunoglobulin-like domains highly decorated with N-glycosylation, while its cytoplasmic domain is subject to multiple PTMs including Tyr, Ser/Thr phosphorylation, Arg and Lys methylation, acetylation and ubiquitination. Here we review the PTMs on VEGFR-2, their importance in angiogenic signaling relays and possible novel therapeutic potentials.

Lysine methylation in cancer: SMYD3-MAP3K2 teaches us new lessons in the Ras-ERK pathway

Lysine methylation has been traditionally associated with histones and epigenetics. Recently, lysine methyltransferases and demethylases - which are involved in methylation of non-histone substrates - have been frequently found deregulated in human tumours. In this realm, a new discovery has unveiled the methyltransferase SMYD3 as an enhancer of Ras-driven cancer. SMYD3 is up-regulated in different types of tumours. SMYD3-mediated methylation of MAP3K2 increases mutant K-Ras-induced activation of ERK1/2. Methylation of MAP3K2 prevents it from binding to the phosphatase PP2A, thereby impeding the impact of this negative regulator on Ras-ERK1/2 signals, leading to the formation of lung and pancreatic adenocarcinomas. Furthermore, depletion of SMYD3 synergises with a MEK inhibitor, currently in clinical trials, to block Ras-driven pancreatic neoplasia. These results underscore the importance of lysine methylation in the regulation of signalling pathways relevant for tumourigenesis and endorse the development of drugs targeting unregulated lysine methylation as therapeutic agents in the struggle against cancer.