Cited2 modulates hypoxia-inducible factor-dependent expression of vascular endothelial growth factor in nucleus pulposus cells of the rat intervertebral disc.

Objective: To determine whether nucleus pulposus cells of the intervertebral disc express hypoxia-inducible factor 2alpha (HIF-2alpha), and to assess the role of HIF-1 and HIF-2 in controlling cited2 and vascular endothelial growth factor (VEGF) expression.

Methods: Rat cells were cultured under normoxic (21% O2) or hypoxic (2% O2) conditions, and expression and promoter activity of HIF-2 target genes were evaluated. Gain- or loss-of-function experiments were performed to investigate the contribution of HIF isoforms to cited2 activity as well as the role of cited2 in regulating VEGF expression.

Results: We found that HIF-2alpha protein was expressed in vivo and that protein and messenger RNA expression were similar under both normoxic and hypoxic conditions. However, there was a significant increase in HIF-2alpha transactivation under hypoxic conditions. With respect to functional activity, unlike the case in most other tissues, HIF-2 failed to increase the transcriptional activities of superoxide dismutase 2 and frataxin, 2 common target genes involved in radical dismutation. However, under hypoxic conditions, HIF-2 preferentially regulated the expression and promoter activity of cited2, a p300 binding protein. When HIF-2alpha or HIF-1alpha was suppressed, cited2 promoter activity was inhibited. Finally, we showed that forced expression or suppression of cited2 resulted in corresponding changes in expression of VEGF, a common target gene for HIF-1 and HIF-2 in the nucleus pulposus cells.

Conclusion: Results of this study indicate that in nucleus pulposus cells, HIF-2 and HIF-1 modulate their own transcriptional activity through cited2. We suggest that the 2 arms of the regulatory circuit serve to maintain survival activities and inhibit angiogenesis in the healthy disc.