Hypoxic regulation of nucleus pulposus cell survival: from niche to notch

Abstract

This minireview examines the role of hypoxia, and hypoxia inducible factors (HIF-1 and HIF-2), in regulating the metabolism, function, and fate of cells of the nucleus pulposus in the intervertebral disk. We focus on the mechanisms by which both these hypoxia-sensitive transcription factors influence energy metabolism, radical dismutation, and expression of survival proteins. In addition, we discuss how cells of the nucleus respond to a number of hypoxia-sensitive proteins, including galectin-3, Akt, and VEGF. Where applicable, these discussions are extended to include the impact of these molecules and hypoxia on degenerating resident cells in the intervertebral niche. Finally, because the notch signaling pathway is responsive to hypoxia, we speculate that in the intervertebral niche, notch proteins participate in the regulation of disk precursor cell proliferation and differentiation. We predict that knowledge of each of these interactive proteins within the disk niche could be used to enhance renewal and promote differentiation and function of cells of the nucleus pulposus.

Figure 1

A: A cartoon showing the structure of the intervertebral disk. Tissue is avascular except for small capillaries in the outer one third of annulus fibrosus and in cartilaginous endplates. The nucleus pulposus, which is completely devoid of any blood supply, receives its nutrition and O₂ by diffusion through the endplates and is hypoxic. B: Functional activity of HIF target genes. Critical functions include energy metabolism, angiogenesis, cell survival, autophagy and apoptosis, matrix synthesis, proliferation, self-renewal and differentiation, radical dismutation, and pH regulation. Many of these functions are critical for survival and functioning of the nucleus pulposus cells in the avascular niche of the intervertebral disk. Hypoxia/HIF–sensitive proteins that are identified in the nucleus pulposus cells are shown in parentheses.

Figure 2

O₂-dependent metabolism of HIF-1α. A: Normoxic regulation of HIF-1α. Under normoxic conditions, the α subunit of HIF-1 is hydroxylated at specific proline residues by prolyl hydroxylases (PHD1-3) that require Fe²⁺, 2-oxoglutarate (2-OG), and ascorbate. Of these, PHD1 plays a major role in HIF-1α regulation. Once hydroxylated, HIF-1α is recognized and bound by von Hippel Lindau, an E3 ubiquitin ligase; it is subsequently ubiquitinated and rapidly degraded through the 26s proteasomal pathway. B: Hypoxic regulation of HIF-1α. Under hypoxic conditions, HIF-1α is stabilized, translocates to the nucleus, and dimerizes with a constitutively expressed HIF-1β subunit. The dimer associates with several coactivators, including p300/CBP and the DNA polymerase II (Pol II) complex. This protein complex then binds to an enhancer, the Hypoxia-Response Element (HRE), in HIF-1 target genes to initiate gene transcription. Of note, in nucleus pulposus cells, HIF-1α is stabilized even in normoxia.