A fragment of human TrpRS as a potent antagonist of ocular angiogenesis

Inhibition of tumor angiogenesis by a natural fragment of a tRNA synthetase

Human tyrosyl- and tryptophanyl-tRNA synthetases (TyrRS and TrpRS, respectively) link protein synthesis to signal-transduction pathways, including angiogenesis. Fragments of TyrRS stimulate angiogenesis, whereas those of TrpRS (T2-TrpRS) inhibit angiogenesis. Thus, these two synthetases acquired opposing activities during evolution, possibly as a coordinated mechanism for regulating angiogenesis. The recent identification of the cellular target of T2-TrpRS sheds light into the mechanism of angiogenesis inhibition. This mechanism provides a molecular basis for the lack of effect of T2-TrpRS on the normal vasculature. With these features, we suggest that this fragment of a tRNA synthetase might safely be used to arrest neovascularization of tumors. In particular, an anti-angiogenesis agent that stops the growth of tumor vessels without affecting normal vessels might serve as an adjunct to cytotoxic therapy.

Functional expansion of aminoacyl-tRNA synthetases and their interacting factors: new perspectives on housekeepers

Aminoacyl-tRNA synthetases (ARSs) are essential enzymes that join amino acids to tRNAs, thereby linking the genetic code to specific amino acids. Once considered a class of 'housekeeping' enzymes, ARSs are now known to participate in a wide variety of functions, including transcription, translation, splicing, inflammation, angiogenesis and apoptosis. Three nonenzymatic proteins--ARS-interacting multi-functional proteins (AIMPs)--associate with ARSs in a multi-synthetase complex of higher eukaryotes. Similarly to ARSs, AIMPs have novel functions unrelated to their support role in protein synthesis, acting as a cytokine to control angiogenesis, immune response and wound repair, and as a crucial regulator for cell proliferation and DNA repair. Evaluation of the functional roles of individual ARSs and AIMPs might help to elucidate why these proteins as a whole contribute such varied functions and interactions in complex systems.

Tryptophanyl-tRNA synthetase gene polymorphisms and risk of incident myocardial infarction

Tryptophanyl-tRNA synthetase (WARS) gene polymorphisms have been associated with the patho-physiology of vascular angiogenesis and homeostasis. Data from a recent genome-wide linkage analysis suggested a potential role of WARS in the risk of myocardial infarction. However, no genetic-epidemiological data are available. From a prospective cohort of 14,916 initially healthy American men, we evaluated five common polymorphisms within or close to the WARS locus, all with a minor allele frequency >0.10, amongst 386 individuals who subsequently developed myocardial infarction and 386 matched individuals who remained free of reported cardiovascular events during follow-up. The polymorphisms were: a G > C substitution in the 5'-flanking region (dbSNP rs2273804), an A > G substitution in intron 1 (dbSNP rs941931), a 335T > C substitution in exon 10 (dbSNP rs9453), a C > T substitution in intron 10 (dbSNP rs1570305), and a C > T substitution in the C14orf68 region (dbSNP rs3736951). The observed genotypes were in Hardy-Weinberg equilibrium in the control group. Genotype- and haplotype-frequency distributions were similar between cases and controls. Further investigation using a haplotype-based matched logistic regression analysis, adjusting for age, smoking, randomized treatment-assignment (likelihood ratio test: chi(3)(2) = 3.20, p = 0.36) or with additional adjustment for BMI, hypertension, and diabetes (likelihood ratio test: chi(3)(2) = 2.38, p = 0.50) yielded similar null findings. An alternative haplotype analysis based on evolutionary arguments again yielded null results. In conclusion, we found no evidence for an association between the common polymorphisms or haplotypes of the tryptophanyl-tRNA synthetase gene tested and risk of myocardial infarction.

Aminoacyl-tRNA synthetase complexes: beyond translation

Although aminoacyl-tRNA synthetases (ARSs) are housekeeping enzymes essential for protein synthesis, they can play non-catalytic roles in diverse biological processes. Some ARSs are capable of forming complexes with each other and additional proteins. This characteristic is most pronounced in mammals, which produce a macromolecular complex comprising nine different ARSs and three additional factors: p43, p38 and p18. We have been aware of the existence of this complex for a long time, but its structure and function have not been well understood. The only apparent distinction between the complex-forming ARSs and those that do not form complexes is their ability to interact with the three non-enzymatic factors. These factors are required not only for the catalytic activity and stability of the associated ARSs, such as isoleucyl-, methionyl-, and arginyl-tRNA synthetase, but also for diverse signal transduction pathways. They may thus have joined the ARS community to coordinate protein synthesis with other biological processes.

VE-cadherin links tRNA synthetase cytokine to anti-angiogenic function

A natural fragment of an enzyme that catalyzes the first step of protein synthesis-human tryptophanyl-tRNA synthetase (T2-TrpRS) has potent anti-angiogenic activity. A cellular receptor through which T2-TrpRS exerts its anti-angiogenic activity has not previously been identified. Here T2-TrpRS was shown to bind at intercellular junctions of endothelial cells (ECs). Using genetic knock-outs, binding was established to depend on VE-cadherin, a calcium-dependent adhesion molecule, which is selectively expressed in ECs, concentrated at adherens junctions, and is essential for normal vascular development. In contrast, T2-TrpRS binding to EC junctions was not dependent on platelet endothelial cell adhesion molecule type-1, another adhesion molecule found at EC junctions. Pull-down assays confirmed direct complex formation between T2-TrpRS and VE-cadherin. Binding of T2-TrpRS inhibited VEGF-induced ERK activation and EC migration. Thus, a VE-cadherin-dependent pathway is proposed to link T2-TrpRS to inhibition of new blood vessel formation.

VEGF165b, an inhibitory vascular endothelial growth factor splice variant: mechanism of action, in vivo effect on angiogenesis and endogenous protein expression

Growth of new blood vessels (angiogenesis), required for all tumor growth, is stimulated by the expression of vascular endothelial growth factor (VEGF). VEGF is up-regulated in all known solid tumors but also in atherosclerosis, diabetic retinopathy, arthritis, and many other conditions. Conventional VEGF isoforms have been universally described as proangiogenic cytokines. Here, we show that an endogenous splice variant, VEGF(165)b, is expressed as protein in normal cells and tissues and is circulating in human plasma. We also present evidence for a sister family of presumably inhibitory splice variants. Moreover, these isoforms are down-regulated in prostate cancer. We also show that VEGF(165)b binds VEGF receptor 2 with the same affinity as VEGF(165) but does not activate it or stimulate downstream signaling pathways. Moreover, it prevents VEGF(165)-mediated VEGF receptor 2 phosphorylation and signaling in cultured cells. Furthermore, we show, with two different in vivo angiogenesis models, that VEGF(165)b is not angiogenic and that it inhibits VEGF(165)-mediated angiogenesis in rabbit cornea and rat mesentery. Finally, we show that VEGF(165)b expressing tumors grow significantly more slowly than VEGF(165)-expressing tumors, indicating that a switch in splicing from VEGF(165) to VEGF(165)b can inhibit tumor growth. These results suggest that regulation of VEGF splicing may be a critical switch from an antiangiogenic to a proangiogenic phenotype.

Noncanonical function of glutamyl-prolyl-tRNA synthetase: gene-specific silencing of translation

Aminoacyl tRNA synthetases (ARS) catalyze the ligation of amino acids to cognate tRNAs. Chordate ARSs have evolved distinctive features absent from ancestral forms, including compartmentalization in a multisynthetase complex (MSC), noncatalytic peptide appendages, and ancillary functions unrelated to aminoacylation. Here, we show that glutamyl-prolyl-tRNA synthetase (GluProRS), a bifunctional ARS of the MSC, has a regulated, noncanonical activity that blocks synthesis of a specific protein. GluProRS was identified as a component of the interferon (IFN)-gamma-activated inhibitor of translation (GAIT) complex by RNA affinity chromatography using the ceruloplasmin (Cp) GAIT element as ligand. In response to IFN-gamma, GluProRS is phosphorylated and released from the MSC, binds the Cp 3'-untranslated region in an mRNP containing three additional proteins, and silences Cp mRNA translation. Thus, GluProRS has divergent functions in protein synthesis: in the MSC, its aminoacylation activity supports global translation, but translocation of GluProRS to an inflammation-responsive mRNP causes gene-specific translational silencing.

Adult bone marrow-derived stem cells use R-cadherin to target sites of neovascularization in the developing retina

Adult bone marrow contains a population of hematopoietic stem cells (HSCs) that can give rise to cells capable of targeting sites of neovascularization in the peripheral or retinal vasculature. However, relatively little is known about the mechanism of targeting of these cells to sites of neovascularization. We have analyzed subpopulations of HSCs for the expression of a variety of cell surface adhesion molecules and found that R-cadherin, a calcium-dependent cell-cell adhesion molecule important for normal retinal endothelial cell guidance, was preferentially expressed by functionally targeting HSCs. Preincubation of HSCs with function-blocking anti-R-cadherin antibodies or novel R-cadherin-specific peptide antagonists effectively prevented targeting of bone marrow-derived cells to the developing retinal vasculature in vivo. Whereas control-injected HSCs targeted to all 3 normal developing retinal vascular layers, blocking R-cadherin-mediated adhesion resulted in mistargeting of the HSCs to the normally avascular outer retina. Our results suggest that vascular targeting of bone marrow-derived HSCs is dependent on mechanisms similar to those used by endogenous retinal vascular endothelial cells. Thus, R-cadherin antagonists may be useful in the treatment of neovascular diseases in which circulating HSCs contribute to abnormal angiogenesis. (Blood. 2004;103:3420-3427)

A domain for editing by an archaebacterial tRNA synthetase

The rules of the genetic code are established by aminoacylations of transfer RNAs by aminoacyl tRNA synthetases. New codon assignments, and the introduction of new kinds of amino acids, are blocked by vigorous tRNA-dependent editing reactions occurring at hydrolytic sites embedded within specialized domains in the synthetases. For some synthetases, these domains were present at the time of the last common ancestor and were fixed in evolution through all three of the kingdoms of life. Significantly, a well characterized domain for editing found in bacterial and eukaryotic threonyl- and all alanyl-tRNA synthetases is missing from archaebacterial threonine enzymes. Here we show that the archaebacterial Methanosarcina mazei ThrRS efficiently misactivates serine, but does not fuse serine to tRNA. Consistent with this observation, the enzyme cleared serine that was linked to threonine-specific tRNAs. M. mazei and most other archaebacterial ThrRSs have a domain, N2(A), fused to the N terminus and not found in bacterial or eukaryotic orthologs. Mutations at conserved residues in this domain led to an inability to clear threonine-specific tRNA mischarged with serine. Thus, these results demonstrate a domain for editing that is distinct from all others, is restricted to just one branch of the tree of life, and was most likely added to archaebacterial ThrRSs after the eukaryote/archaebacteria split.

Evidence for the involvement of SDF-1 and CXCR4 in the disruption of endothelial cell-branching morphogenesis and angiogenesis by TNF-alpha and IFN-gamma

Vigorous inflammatory responses are associated with tissue damage, particularly when toxic levels of inflammatory cytokines are produced. Despite proangiogenic factors being present early at sites of inflammation, vascular repair occurs toward the end of the inflammatory response, suggesting modulation of the proangiogenic response. Endogenous inhibitors of angiogenesis induced during acute inflammation are poorly characterized. Here, we looked for endothelial cell-derived modulators of angiogenesis that may account for delayed neovascularization during inflammation. Gene profiling of endothelial cells showed that the inflammatory cytokines tumor necrosis factor alpha (TNF-alpha) and interferon-gamma (IFN-gamma) selectively promote expression of the antiangiogenic molecules, IFN-inducible protein-10, monokine induced by IFN-gamma, tryptophanyl-tRNA synthetase, and tissue inhibitor of metalmetalloproteinase-1, and inhibit expression of the proangiogenic molecules, platelet-endothelial cell adhesion molecule-1, vascular endothelial growth factor receptor-2, stromal cell-derived factor-1 (SDF-1), collagen type IV, endothelial cell growth factor-1, and carcinoembryonic antigen-related cell adhesion molecule-1. Reduced endothelial cell expression of SDF-1 protein by TNF-alpha and IFN-gamma disrupts extracellular matrix-dependent endothelial cell tube formation, an in vitro morphogenic process that recapitulates critical steps in angiogenesis. Replacement of SDF-1 onto the endothelial cell surface reconstitutes this morphogenic process. In vivo, TNF-alpha and IFN-gamma inhibit growth factor-induced angiogenesis and SDF-1 expression in endothelial cells. These results demonstrate that SDF-1/CXC chemokine receptor-4 constitutes a TNF-alpha- and IFN-gamma-regulated signaling system that plays a critical role in mediating angiogenesis inhibition by these inflammatory cytokines.

A new gamma-interferon-inducible promoter and splice variants of an anti-angiogenic human tRNA synthetase

Two forms of human tryptophanyl-tRNA synthetase (TrpRS) are produced in vivo through alternative mRNA splicing. The two forms, full-length TrpRS and mini TrpRS, are catalytically active, but are distinguished by the striking anti-proliferative and anti-angiogenic activity specific to mini TrpRS. Here we describe two new splice variants of human TrpRS mRNA. Their production was strongly regulated by gamma-interferon (IFN-gamma), an anti-proliferative cytokine known to stimulate the expression of other anti-angiogenic factors. A new IFN-gamma-sensitive promoter was demonstrated to drive production of these splice variants. In human endothelial cells, both the newly discovered and a previously reported promoter were shown to respond specifically to IFN-gamma and not to other cytokines such as tumor necrosis factor-alpha, transforming growth factor-beta, interleukin-4 or erythropoietin. In addition, both promoters were stimulated by the 'downstream' interferon regulatory factor 1 that, in turn, is known to be regulated by the 'upstream' signal transducer and activator of transcription 1alpha subunit. Thus, the tandem promoters provide a dual system to regulate expression and alternative splicing of human TrpRS in vivo.

A short peptide insertion crucial for angiostatic activity of human tryptophanyl-tRNA synthetase

Human tryptophanyl-tRNA synthetase (TrpRS) is secreted into the extracellular region of vascular endothelial cells. The splice variant form (mini TrpRS) functions in vascular endothelial cell apoptosis as an angiostatic cytokine. In contrast, the closely related human tyrosyl-tRNA synthetase (TyrRS) functions as an angiogenic cytokine in its truncated form (mini TyrRS). Here, we determined the crystal structure of human mini TrpRS at a resolution of 2.3 A and compared the structure with those of prokaryotic TrpRS and human mini TyrRS. Deletion of the tRNA anticodon-binding (TAB) domain insertion, consisting of eight residues in the human TrpRS, abolished the enzyme's apoptotic activity for endothelial cells, whereas its translational catalysis and cell-binding activities remained unchanged. Thus, we have identified the inserted peptide motif that activates the angiostatic signaling.

Crystal structure of human tryptophanyl-tRNA synthetase catalytic fragment: insights into substrate recognition, tRNA binding, and angiogenesis activity

Human tryptophanyl-tRNA synthetase (hTrpRS) produces a full-length and three N terminus-truncated forms through alternative splicing and proteolysis. The shortest fragment that contains the aminoacylation catalytic fragment (T2-hTrpRS) exhibits the most potent angiostatic activity. We report here the crystal structure of T2-hTrpRS at 2.5 A resolution, which was solved using the multi-wavelength anomalous diffraction method. T2-hTrpRS shares a very low sequence homology of 22% with Bacillus stearothermophilus TrpRS (bTrpRS); however, their overall structures are strikingly similar. Structural comparison of T2-hTrpRS with bTrpRS reveals substantial structural differences in the substrate-binding pocket and at the entrance to the pocket that play important roles in substrate binding and tRNA binding. T2-hTrpRS has a wide opening to the active site and adopts a compact conformation similar to the closed conformation of bTrpRS. These results suggest that mammalian and bacterial TrpRSs might use different mechanisms to recognize the substrate. Modeling studies indicate that tRNA binds with the dimeric enzyme and interacts primarily with the connective polypeptide 1 of hTrpRS via its acceptor arm and the alpha-helical domain of hTrpRS via its anticodon loop. Our results also suggest that the angiostatic activity is likely located at the alpha-helical domain, which resembles the short chain cytokines.

Biologically active fragment of a human tRNA synthetase inhibits fluid shear stress-activated responses of endothelial cells

Human tryptophanyl-tRNA synthetase (TrpRS) is active in translation and angiogenesis. In particular, an N-terminally truncated fragment, T2-TrpRS, that is closely related to a natural splice variant is a potent antagonist of vascular endothelial growth factor-induced angiogenesis in several in vivo models. In contrast, full-length native TrpRS is inactive in the same models. However, vascular endothelial growth factor stimulation is only one of many physiological and pathophysiological stimuli to which the vascular endothelium responds. To investigate more broadly the role of T2-TrpRS in vascular homeostasis and pathophysiology, the effect of T2-TrpRS on well characterized endothelial cell (EC) responses to flow-induced fluid shear stress was studied. T2-TrpRS inhibited activation by flow of protein kinase B (Akt), extracellular signal-regulated kinase 1/2, and EC NO synthase and prevented transcription of several shear stress-responsive genes. In addition, T2-TrpRS interfered with the unique ability of ECs to align in the direction of fluid flow. In all of these assays, native TrpRS was inactive, demonstrating that angiogenesis-related activity requires fragment production. These results demonstrate that T2-TrpRS can regulate extracellular signal-activated protein kinase, Akt, and EC NO synthase activation pathways that are associated with angiogenesis, cytoskeletal reorganization, and shear stress-responsive gene expression. Thus, this biological fragment of TrpRS may have a role in the maintenance of vascular homeostasis.

Expression, localization and alternative function of cytoplasmic asparaginyl-tRNA synthetase in Brugia malayi

Aminoacyl-tRNA synthetases (AARS) are a family of enzymes that exhibit primary and various secondary functions in different species. In Brugia malayi, the gene for asparaginyl-tRNA synthetase (AsnRS), a class II AARS, previously has been identified as a multicopy gene encoding an immunodominant antigen in the serum of humans with lymphatic filariasis. However, the relative level of expression and alternative functions of AARS in nematode parasites is not well understood. We searched the Filarial Genome Project database to identify the number and amino acid specificity of B. malayi AARS cDNAs to gain insight into the role of different AARS in filaria. These data showed that cytoplasmic AsnRS was present in five gene clusters, and is the most frequently represented member of the aminoacyl-tRNA synthetase family in adult B. malayi. The relative level of AsnRS transcribed in adult female B. malayi was compared to the levels of a low abundance and medium abundance AARS by quantitative real-time RT-PCR. By this method, AsnRS cDNA was 11 times greater than arginyl-tRNA synthetase and methionyl-tRNA synthetase cDNA. In situ hybridization using a B. malayi AsnRS-specific oligonucleotide probe identified abundant cytoplasmic mRNA, particularly in the hypodermis of adult B. malayi. In the absence of tRNA, AsnRS synthesizes diadenosine triphosphate, a potent regulator of cell growth in other eukaryotes. These data support the hypothesis that all AARS are not equally expressed in B. malayi and that these enzymes may demonstrate important alternative functions in filaria.

Crystal structure of a human aminoacyl-tRNA synthetase cytokine

The 20 aminoacyl-tRNA synthetases catalyze the first step of protein synthesis and establish the rules of the genetic code through aminoacylation reactions. Biological fragments of two human enzymes, tyrosyl-tRNA synthetase (TyrRS) and tryptophanyl-tRNA synthetase, connect protein synthesis to cell-signaling pathways including angiogenesis. Alternative splicing or proteolysis produces these fragments. The proangiogenic N-terminal fragment mini-TyrRS has IL-8-like cytokine activity that, like other CXC cytokines, depends on a Glu-Leu-Arg motif. Point mutations in this motif abolish cytokine activity. The full-length native TyrRS lacks cytokine activity. No structure has been available for any mammalian tRNA synthetase that, in turn, might give insight into why mini-TyrRS and not TyrRS has cytokine activities. Here, the structure of human mini-TyrRS, which contains both the catalytic and the anticodon recognition domain, is reported to a resolution of 1.18 A. The critical Glu-Leu-Arg motif is located on an internal alpha-helix of the catalytic domain, where the guanidino side chain of R is part of a hydrogen-bonding network tethering the anticodon-recognition domain back to the catalytic site. Whereas the catalytic domains of the human and bacterial enzymes superimpose, the spatial disposition of the anticodon recognition domain relative to the catalytic domain is unique in mini-TyrRS relative to the bacterial orthologs. This unique orientation of the anticodon-recognition domain can explain why the fragment mini-TyrRS, and not full-length native TyrRS, is active in cytokine-signaling pathways.

Aminoacyl-tRNA synthetases: versatile players in the changing theater of translation

Aminoacyl-tRNA synthetases attach amino acids to the 3' termini of cognate tRNAs to establish the specificity of protein synthesis. A recent Asilomar conference (California, January 13-18, 2002) discussed new research into the structure-function relationship of these crucial enzymes, as well as a multitude of novel functions, including participation in amino acid biosynthesis, cell cycle control, RNA splicing, and export of tRNAs from nucleus to cytoplasm in eukaryotic cells. Together with the discovery of their role in the cellular synthesis of proteins to incorporate selenocysteine and pyrrolysine, these diverse functions of aminoacyl-tRNA synthetases underscore the flexibility and adaptability of these ancient enzymes and stimulate the development of new concepts and methods for expanding the genetic code.

Histidyl-tRNA synthetase and asparaginyl-tRNA synthetase, autoantigens in myositis, activate chemokine receptors on T lymphocytes and immature dendritic cells

Autoantibodies to histidyl-tRNA synthetase (HisRS) or to alanyl-, asparaginyl-, glycyl-, isoleucyl-, or threonyl-tRNA synthetase occur in approximately 25% of patients with polymyositis or dermatomyositis. We tested the ability of several aminoacyl-tRNA synthetases to induce leukocyte migration. HisRS induced CD4(+) and CD8(+) lymphocytes, interleukin (IL)-2-activated monocytes, and immature dendritic cells (iDCs) to migrate, but not neutrophils, mature DCs, or unstimulated monocytes. An NH(2)-terminal domain, 1-48 HisRS, was chemotactic for lymphocytes and activated monocytes, whereas a deletion mutant, HisRS-M, was inactive. HisRS selectively activated CC chemokine receptor (CCR)5-transfected HEK-293 cells, inducing migration by interacting with extracellular domain three. Furthermore, monoclonal anti-CCR5 blocked HisRS-induced chemotaxis and conversely, HisRS blocked anti-CCR5 binding. Asparaginyl-tRNA synthetase induced migration of lymphocytes, activated monocytes, iDCs, and CCR3-transfected HEK-293 cells. Seryl-tRNA synthetase induced migration of CCR3-transfected cells but not iDCs. Nonautoantigenic aspartyl-tRNA and lysyl-tRNA synthetases were not chemotactic. Thus, autoantigenic aminoacyl-tRNA synthetases, perhaps liberated from damaged muscle cells, may perpetuate the development of myositis by recruiting mononuclear cells that induce innate and adaptive immune responses. Therefore, the selection of a self-molecule as a target for an autoantibody response may be a consequence of the proinflammatory properties of the molecule itself.

Bone marrow-derived stem cells target retinal astrocytes and can promote or inhibit retinal angiogenesis

Adult bone marrow (BM) contains cells capable of differentiating along hematopoietic (Lin(+)) or non-hematopoietic (Lin(-)) lineages. Lin(-) hematopoietic stem cells (HSCs) have recently been shown to contain a population of endothelial precursor cells (EPCs) capable of forming blood vessels. Here we show that intravitreally injected Lin(-) BM cells selectively target retinal astrocytes, cells that serve as a template for both developmental and injury-associated retinal angiogenesis. When Lin(-) BM cells were injected into neonatal mouse eyes, they extensively and stably incorporated into forming retinal vasculature. When EPC-enriched HSCs were injected into the eyes of neonatal rd/rd mice, whose vasculature ordinarily degenerates with age, they rescued and maintained a normal vasculature. In contrast, normal retinal angiogenesis was inhibited when EPCs expressing a potent angiostatic protein were injected. We have demonstrated that Lin(-) BM cells and astrocytes specifically interact with one another during normal angiogenesis and pathological vascular degeneration in the retina. Selective targeting with Lin(-) HSC may be a useful therapeutic approach for the treatment of many ocular diseases.

Tales of the cryptic: unveiling more angiogenesis inhibitors

Over the past few years, there has been a dramatic increase in the identification of anti-angiogenic fragments from larger proteins with unrelated functions. These cryptic anti-angiogenic molecules are largely derived from matrix components such as collagen and fibronectin, as well as from circulating proteins and some intracellular proteins. Here, we discuss the relevance of these developments in terms of their physiological roles and possible therapeutic applications.

Induction of angiogenesis by a fragment of human tyrosyl-tRNA synthetase

The first step of protein synthesis is catalyzed by aminoacyl-tRNA synthetases. In addition, certain mammalian tRNA synthetases link protein synthesis to cytokine signaling pathways. In particular, human tyrosyl-tRNA synthetase (TyrRS) can be split by proteolysis into two fragments having distinct cytokine activities. One of the TyrRS fragments (mini TyrRS) contains features identical to those in CXC chemokines (like interleukin-8) that also act as angiogenic factors. Here mini TyrRS (but not full-length TyrRS) is shown to stimulate chemotaxis of endothelial cells in vitro and stimulate angiogenesis in each of two in vivo animal models. The angiogenic activity of mini TyrRS can be opposed by anti-angiogenic chemokines like IP-10. Thus, a biological fragment of human tyrosyl-tRNA synthetase links protein synthesis to regulation of angiogenesis.

A human aminoacyl-tRNA synthetase as a regulator of angiogenesis

Aminoacyl-tRNA synthetases catalyze the first step of protein synthesis. It was shown recently that human tyrosyl-tRNA synthetase (TyrRS) can be split into two fragments having distinct cytokine activities, thereby linking protein synthesis to cytokine signaling pathways. Tryptophanyl-tRNA synthetase (TrpRS) is a close homologue of TyrRS. A natural fragment, herein designated as mini TrpRS, was shown by others to be produced by alternative splicing. Production of this fragment is reported to be stimulated by IFN-gamma, a cytokine that also stimulates production of angiostatic factors. Mini TrpRS is shown here to be angiostatic in a mammalian cell culture system, the chicken embryo, and two independent angiogenesis assays in the mouse. The full-length enzyme is inactive in the same assays. Thus, protein synthesis may be linked to the regulation of angiogenesis by a natural fragment of TrpRS.