Investigation of the HIF-PHD System in Nucleus Pulposus Cells of the Intervertebral Disc
Low back pain is a taxing condition, both on the individual and on society. Although low back pain has several etiologies, intervertebral disc degeneration is considered a likely cause of a large percentage of cases. The intervertebral disc contains a central proteoglycan-rich nucleus pulposus, surrounded circumferentially by a fibrocartilagenous annulus fibrosus. The nucleus pulposus is avascular and physiologically hypoxic, and the cells residing within this niche have adapted to survive through expression and unique regulation of the transcription factor hypoxia-inducible factor 1α (HIF-1α). Whereas in other cell types, the regulation of HIF-1α by members of the prolyl hydroxylase (PHD) family has been well characterized, previous work has shown that the HIF-PHD system in the nucleus pulposus is fairly unique. The purpose of this research was to more closely examine this circuit and investigate how HIF-1α expression and activity are maintained in the nucleus pulposus to facilitate proper tissue homeostasis and function. The specific aims of this work were twofold. The first was to examine the role of histone deacetylases (HDACs) in regulating HIF-1α through PHD2-dependent and -independent mechanisms. The second was to investigate how PHD3 promotes transcriptional activity of HIF-1α. We show that multiple Class I and Class IIa HDACs promote HIF-1α stability in the nucleus pulposus through regulating the HIF-PHD2-VHL axis. This HDAC-PHD2 relationship is a novel observation that contrasts what has been reported in other cell types. Furthermore, we show that HDAC6 independently promotes activity of HIF-1α through important cofactors Hsp90 and p300. We also show that PHD3 serves as a transcriptional cofactor for HIF-1 in the nucleus pulposus, and activates HIF-1α C-TAD activity through a p300-dependent mechanism. PHD3 is necessary for maintenance of hypoxic expression of HIF-1 target genes important for cell function and survival. Importantly, we show that PHD3 cofactor function is independent of the PKM2-JMJD5 axis, underscoring an important cell-specific feature of nucleus pulposus cells. Taken together, these results expand our current understanding of the unique HIF-PHD circuit in nucleus pulposus cells, and highlight specific characteristics of which we could potentially take advantage for developing therapeutics targeted at treating disc degeneration.
Schoepflin, Zachary, "Investigation of the HIF-PHD System in Nucleus Pulposus Cells of the Intervertebral Disc" (2016). ETD Collection for Thomas Jefferson University. AAI10252930.