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      Angiotensin II Evokes Angiogenic Signals within Skeletal Muscle through Co-ordinated Effects on Skeletal Myocytes and Endothelial Cells

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          Abstract

          Skeletal muscle overload induces the expression of angiogenic factors such as vascular endothelial growth factor (VEGF) and matrix metalloproteinase (MMP)-2, leading to new capillary growth. We found that the overload-induced increase in angiogenesis, as well as increases in VEGF, MMP-2 and MT1-MMP transcripts were abrogated in muscle VEGF KO mice, highlighting the critical role of myocyte-derived VEGF in controlling this process. The upstream mediators that contribute to overload-induced expression of VEGF have yet to be ascertained. We found that muscle overload increased angiotensinogen expression, a precursor of angiotensin (Ang) II, and that Ang II signaling played an important role in basal VEGF production in C2C12 cells. Furthermore, matrix-bound VEGF released from myoblasts induced the activation of endothelial cells, as evidenced by elevated endothelial cell phospho-p38 levels. We also found that exogenous Ang II elevates VEGF expression, as well as MMP-2 transcript levels in C2C12 myotubes. Interestingly, these responses also were observed in skeletal muscle endothelial cells in response to Ang II treatment, indicating that these cells also can respond directly to the stimulus. The involvement of Ang II in muscle overload-induced angiogenesis was assessed. We found that blockade of AT1R-dependent Ang II signaling using losartan did not attenuate capillary growth. Surprisingly, increased levels of VEGF protein were detected in overloaded muscle from losartan-treated rats. Similarly, we observed elevated VEGF production in cultured endothelial cells treated with losartan alone or in combination with Ang II. These studies conclusively establish the requirement for muscle derived VEGF in overload-induced angiogenesis and highlight a role for Ang II in basal VEGF production in skeletal muscle. However, while Ang II signaling is activated following overload and plays a role in muscle VEGF production, inhibition of this pathway is not sufficient to halt overload-induced angiogenesis, indicating that AT1-independent signals maintain VEGF production in losartan-treated muscle.

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          HIF-independent regulation of VEGF and angiogenesis by the transcriptional coactivator PGC-1alpha.

          Ischaemia of the heart, brain and limbs is a leading cause of morbidity and mortality worldwide. Hypoxia stimulates the secretion of vascular endothelial growth factor (VEGF) and other angiogenic factors, leading to neovascularization and protection against ischaemic injury. Here we show that the transcriptional coactivator PGC-1alpha (peroxisome-proliferator-activated receptor-gamma coactivator-1alpha), a potent metabolic sensor and regulator, is induced by a lack of nutrients and oxygen, and PGC-1alpha powerfully regulates VEGF expression and angiogenesis in cultured muscle cells and skeletal muscle in vivo. PGC-1alpha-/- mice show a striking failure to reconstitute blood flow in a normal manner to the limb after an ischaemic insult, whereas transgenic expression of PGC-1alpha in skeletal muscle is protective. Surprisingly, the induction of VEGF by PGC-1alpha does not involve the canonical hypoxia response pathway and hypoxia inducible factor (HIF). Instead, PGC-1alpha coactivates the orphan nuclear receptor ERR-alpha (oestrogen-related receptor-alpha) on conserved binding sites found in the promoter and in a cluster within the first intron of the VEGF gene. Thus, PGC-1alpha and ERR-alpha, major regulators of mitochondrial function in response to exercise and other stimuli, also control a novel angiogenic pathway that delivers needed oxygen and substrates. PGC-1alpha may provide a novel therapeutic target for treating ischaemic diseases.
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            Autocrine VEGF signaling is required for vascular homeostasis.

            Vascular endothelial growth factor (VEGF) is essential for developmental and pathological angiogenesis. Here we show that in the absence of any pathological insult, autocrine VEGF is required for the homeostasis of blood vessels in the adult. Genetic deletion of vegf specifically in the endothelial lineage leads to progressive endothelial degeneration and sudden death in 55% of mutant mice by 25 weeks of age. The phenotype is manifested without detectable changes in the total levels of VEGF mRNA or protein, indicating that paracrine VEGF could not compensate for the absence of endothelial VEGF. Furthermore, wild-type, but not VEGF null, endothelial cells showed phosphorylation of VEGFR2 in the absence of exogenous VEGF. Activation of the receptor in wild-type cells was suppressed by small molecule antagonists but not by extracellular blockade of VEGF. These results reveal a cell-autonomous VEGF signaling pathway that holds significance for vascular homeostasis but is dispensable for the angiogenic cascade.
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              The vascular endothelial growth factor (VEGF) isoforms: differential deposition into the subepithelial extracellular matrix and bioactivity of extracellular matrix-bound VEGF.

              Vascular endothelial growth factor (VEGF)mRNA undergoes alternative splicing events that generate four different homodimeric isoforms, VEGF121, VEGF165, VEGF189, or VEGF206. VEGF121 is a nonheparin-binding acidic protein, which is freely diffusible. The longer forms, VEGF189 or VEGF206, are highly basic proteins tightly bound to extracellular heparin-containing proteoglycans. VEGF165 has intermediate properties. To determine the localization of VEGF isoforms, transfected human embryonic kidney CEN4 cells expressing VEGF165, VEGF189, or VEGF206 were stained by immunofluorescence with a specific monoclonal antibody. The staining was found in patches and streaks suggestive of extracellular matrix (ECM). VEGF165 was observed largely in Golgi apparatus-like structures. Immunogold labeling of cells expressing VEGF189 or VEGF206 revealed that the staining was localized to the subepithelial ECM. VEGF associated with the ECM was bioactive, because endothelial cells cultured on ECM derived from cells expressing VEGF189 or VEGF206 were markedly stimulated to proliferate. In addition, ECM-bound VEGF can be released into a soluble and bioactive form by heparin or plasmin. ECM-bound VEGF189 and VEGF206 have molecular masses consistent with the intact polypeptides. The ECM may represent an important source of VEGF and angiogenic potential.
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                Author and article information

                Contributors
                Role: Editor
                Journal
                PLoS One
                PLoS ONE
                plos
                plosone
                PLoS ONE
                Public Library of Science (San Francisco, USA )
                1932-6203
                2014
                9 January 2014
                : 9
                : 1
                : e85537
                Affiliations
                [1 ]School of Kinesiology and Health Science, Angiogenesis Research Group and Muscle Health Research Centre, York University, Toronto, Ontario, Canada
                [2 ]West Virginia University School of Medicine, Center for Cardiovascular and Respiratory Sciences, Division of Exercise Physiology, Morgantown, West Virginia, United States of America
                University of Sassari, Italy
                Author notes

                Competing Interests: The authors have declared that no competing interests exist.

                Conceived and designed the experiments: JLG TLH. Performed the experiments: JLG STKL DS KS AH. Analyzed the data: JLG STKL DS TLH. Contributed reagents/materials/analysis tools: SO IMO. Wrote the paper: JLG TLH.

                Article
                PONE-D-13-15052
                10.1371/journal.pone.0085537
                3887063
                24416421
                26ef325f-4080-4655-acb3-ca8413ae1e73
                Copyright @ 2014

                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                History
                : 11 April 2013
                : 4 December 2013
                Page count
                Pages: 11
                Funding
                Funding for this project to TLH from Natural Sciences and Engineering Research Council of Canada. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Categories
                Research Article
                Biology
                Model Organisms
                Animal Models
                Mouse
                Molecular Cell Biology
                Cellular Types
                Endothelial Cells
                Myocytes
                Signal Transduction
                Signaling in Cellular Processes
                Signaling Pathways
                Extracellular Matrix
                Gene Expression
                Mathematics
                Statistics
                Biostatistics
                Medicine
                Anatomy and Physiology
                Cardiovascular System
                Circulatory Physiology
                Musculoskeletal System
                Muscle
                Cell Physiology
                Cardiovascular
                Vascular Biology

                Uncategorized
                Uncategorized

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