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      Hypoxia-Induced Changes in Extracellular Matrix Metabolism in Renal Cells

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          The mechanisms underlying the progressive fibrosis that characterises end-stage renal disease in vivo remain to be established but hypoxia, as a result of microvascular injury and loss, has been suggested to play an important role. In support of this hypothesis, in vitro studies show that hypoxia (1% O<sub>2</sub>) induces a fibrogenic phenotype in human renal tubular endothelia, interstitial fibroblasts and microvascular endothelial cells, simultaneously increasing extracellular matrix (ECM) production and decreasing turnover via effectors on matrix-degrading enzymes and their inhibitors. The effects of hypoxia on ECM metabolism are independent of hypoxia-induced growth factors and are mediated by a haem-protein sensor and activation of both protein kinase C- and tyrosine kinase-mediated signal transduction pathways. De novo gene transcription is regulated by both hypoxia-inducible factor-1-dependent and -independent mechanisms. Further understanding of the molecular mechanisms by which decreased oxygen alters expression of genes involved in ECM metabolism in renal cells may provide new insights into the pathogenesis of fibrosis and identify novel avenues for intervention.

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          Most cited references 10

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          Transforming growth factor beta in tissue fibrosis.

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            Hypoxia-inducible factor-1 mediates transcriptional activation of the heme oxygenase-1 gene in response to hypoxia.

            Exposure of rats to hypoxia (7% O2) markedly increased the level of heme oxygenase-1 (HO-1) mRNA in several tissues. Accumulation of HO-1 transcripts was also observed after exposure of rat aortic vascular smooth muscle (VSM) cells to 1% O2, and this induction was dependent on gene transcription. Activation of the mouse HO-1 gene by all agents thus far tested is mediated by two 5'-enhancer sequences, SX2 and AB1, but neither fragment was responsive to hypoxia in VSM cells. Hypoxia-dependent induction of the chloramphenicol acetyltransferase (CAT) reporter gene was mediated by a 163-bp fragment located approximately 9.5 kilobases upstream of the transcription start site. This fragment contains two potential binding sites for hypoxia-inducible factor 1 (HIF-1). A role for HIF-1 in HO-1 gene regulation was established by the following observations: 1) HIF-1 specifically bound to an oligonucleotide spanning these sequences, 2) mutation of these sequences abolished HIF-1 binding and hypoxia-dependent gene activation in VSM cells, 3) hypoxia increased HIF-1alpha and HIF-1beta protein levels in VSM cells, and 4) hypoxia-dependent HO-1 mRNA accumulation was not observed in mutant hepatoma cells lacking HIF-1 DNA-binding activity. Taken together, these data demonstrate that hypoxia induces HO-1 expression in animal tissues and cell cultures and implicate HIF-1 in this response.
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              Hypoxic stabilization of vascular endothelial growth factor mRNA by the RNA-binding protein HuR.

              Vascular endothelial growth factor (VEGF) is a potent angiogenic factor whose expression is dramatically induced by hypoxia due in large part to an increase in the stability of its mRNA. Here we show that HuR binds with high affinity and specificity to the element that regulates VEGF mRNA stability by hypoxia. Inhibition of HuR expression abrogates the hypoxia-mediated increase in VEGF mRNA stability. Overexpression of HuR increases the stability of VEGF mRNA. However, this only occurs efficiently in hypoxic cells. We further show that the stabilization of VEGF mRNA can be recapitulated in vitro. Using an S-100 extract, we show that the addition of recombinant HuR stabilizes VEGF mRNA markedly. These data support the critical role of HuR in mediating the hypoxic stabilization of VEGF mRNA by hypoxia.

                Author and article information

                Nephron Exp Nephrol
                Cardiorenal Medicine
                S. Karger AG
                December 1999
                28 October 1999
                : 7
                : 5-6
                : 463-469
                Department of Medicine, University College London Medical School, Rayne Institute, London, UK
                20625 Exp Nephrol 1999;7:463–469
                © 1999 S. Karger AG, Basel

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                Page count
                Figures: 1, Tables: 1, References: 56, Pages: 7
                Self URI (application/pdf): https://www.karger.com/Article/Pdf/20625


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