Research Papers: Pathology:
Aquaporin 1 and 5 expression decreases during human intervertebral disc degeneration: novel HIF-1-mediated regulation of aquaporins in NP cells
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Zariel I. Johnson1,*, Shilpa S. Gogate1,*, Rebecca Day2, Abbie Binch2, Dessislava Z. Markova1, Neil Chiverton3, Ashley Cole3, Matt Conner2, Irving M. Shapiro1, Christine L. Le Maitre2, Makarand V. Risbud1
1 Department of Orthopaedic Surgery and Graduate Program in Cell and Developmental Biology, Thomas Jefferson University, Philadelphia, PA, USA
2 Biomedical Research Centre, Sheffield Hallam University, Sheffield, UK
3 Sheffield Teaching Hospitals NHS Foundation Trust, Sheffield, UK
* These authors have contributed equally to this work
Makarand V. Risbud, email:
Keywords: intervertebral disc, nucleus pulposus, aquaporin 1, aquaporin 5, HIF-1
Received: January 23, 2015 Accepted: March 05, 2015 Published: March 20, 2015
Objectives of this study were to investigate whether AQP1 and AQP5 expression is altered during intervertebral disc degeneration and if hypoxia and HIF-1 regulate their expression in NP cells. AQP expression was measured in human tissues from different degenerative grades; regulation by hypoxia and HIF-1 was studied using promoter analysis and gain- and loss-of-function experiments. We show that both AQPs are expressed in the disc and that mRNA and protein levels decline with human disease severity. Bioinformatic analyses of AQP promoters showed multiple evolutionarily conserved HREs. Surprisingly, hypoxia failed to induce promoter activity or expression of either AQP. While genomic chromatin immunoprecipitation showed limited binding of HIF-1α to conserved HREs, their mutation did not suppress promoter activities. Stable HIF-1α suppression significantly decreased mRNA and protein levels of both AQPs, but HIF-1α failed to induce AQP levels following accumulation. Together, our results demonstrate that AQP1 and AQP5 expression is sensitive to human disc degeneration and that HIF-1α uniquely maintains basal expression of both AQPs in NP cells, independent of oxemic tension and HIF-1 binding to promoter HREs. Diminished HIF-1 activity during degeneration may suppress AQP levels in NP cells, compromising their ability to respond to extracellular osmolarity changes.
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