Cartilage and Disc Biology
Hypoxia-inducible factor regulation of ANK expression in nucleus pulposus cells: Possible implications in controlling dystrophic mineralization in the intervertebral disc
Article first published online: 21 MAY 2010
Copyright © 2010 by the American College of Rheumatology
Arthritis & Rheumatism
Volume 62, Issue 9, pages 2707–2715, September 2010
How to Cite
Skubutyte, R., Markova, D., Freeman, T. A., Anderson, D. G., Dion, A. S., Williams, C. J., Shapiro, I. M. and Risbud, M. V. (2010), Hypoxia-inducible factor regulation of ANK expression in nucleus pulposus cells: Possible implications in controlling dystrophic mineralization in the intervertebral disc. Arthritis & Rheumatism, 62: 2707–2715. doi: 10.1002/art.27558
- Issue published online: 31 AUG 2010
- Article first published online: 21 MAY 2010
- Manuscript Accepted: 6 MAY 2010
- Manuscript Received: 23 DEC 2009
- NIH. Grant Numbers: R01-AR-052619, R01-AR-050087, R01-AR-055655
Since nucleus pulposus cells reside under conditions of hypoxia, we determined if the expression of ANK, a pyrophosphate transporter, is regulated by the hypoxia-inducible factor (HIF) proteins.
Quantitative reverse transcription–polymerase chain reaction and Western blot analyses were used to measure ANK expression in nucleus pulposus cells from rats and humans. Transfections were performed to determine the effect of HIF-1/2 on ANK promoter activity.
ANK was expressed in embryonic and mature rat discs. Oxygen-dependent changes in ANK expression in nucleus pulposus cells were minimal. However, silencing of HIF-1α and HIF-2α resulted in increased ANK expression and up-regulation of promoter activity. HIF-mediated suppression of ANK was validated by measuring promoter activity in HIF-1β–null embryonic fibroblasts. Under conditions of hypoxia, there was induction of promoter activity in the null cells as compared with the wild-type cells. Overexpression of HIF-1α and HIF-2α in nucleus pulposus cells resulted in a significant suppression of ANK promoter activity. Since the ANK promoter contains 2 hypoxia-responsive elements (HREs), we performed site-directed mutagenesis and measured promoter activity. We found that HIF-1 can bind to either of the HREs and can suppress promoter activity; in contrast, HIF-2 was required to bind to both HREs in order to suppress activity. Finally, analysis of human nucleus pulposus tissue showed that while ANK was expressed in normal tissue, there was increased expression of ANK along with alkaline phosphatase in the degenerated state.
Both HIF-1 and HIF-2 serve as negative regulators of ANK expression in the disc. We propose that baseline expression of ANK in the disc serves to prevent mineral formation under physiologic conditions.