The role of HIF proteins in maintaining the metabolic health of the intervertebral disc

Abstract

The physiologically hypoxic intervertebral disc and cartilage rely on the hypoxia-inducible factor (HIF) family of transcription factors to mediate cellular responses to changes in oxygen tension. During homeostatic development, oxygen-dependent prolyl hydroxylases, circadian clock proteins and metabolic intermediates control the activities of HIF1 and HIF2 in these tissues. Mechanistically, HIF1 is the master regulator of glycolytic metabolism and cytosolic lactate levels. In addition, HIF1 regulates mitochondrial metabolism by promoting flux through the tricarboxylic acid cycle, inhibiting downsteam oxidative phosphorylation and controlling mitochondrial health through modulation of the mitophagic pathway. Accumulation of metabolic intermediates from HIF-dependent processes contribute to intracellular pH regulation in the disc and cartilage. Namely, to prevent changes in intracellular pH that could lead to cell death, HIF1 orchestrates a bicarbonate buffering system in the disc, controlled by carbonic anhydrase 9 (CA9) and CA12, sodium bicarbonate cotransporters and an intracellular H+/lactate efflux mechanism. In contrast to HIF1, the role of HIF2 remains elusive; in disorders of the disc and cartilage, its function has been linked to both anabolic and catabolic pathways. The current knowledge of hypoxic cell metabolism and regulation of HIF1 activity provides a strong basis for the development of future therapies designed to repair the degenerative disc. The intervertebral disc (IVD) is a hypoxic environment, and the hypoxia-inducible factor (HIF) family of transcription factors enable cells of the disc to adapt to these conditions. Understanding HIF-related mechanisms could help in the generation of therapies for IVD degeneration. Loss of control of hypoxia-inducible factor 1 (HIF1) and HIF-dependent metabolic pathways can lead to intervertebral disc degeneration, whereas loss of HIF2 function is implicated in osteoarthritis. In nucleus pulposus cells, HIF1 and HIF2 are uniquely regulated by both oxygen-dependent and oxygen-independent mechanisms involving prolyl hydroxylase domain-containing proteins (PHDs) and circadian clock genes. Cells of the intervertebral disc possess functional mitochondria and, in nucleus pulposus cells, mitochondria undergo HIF-dependent mitophagy and fragmentation. HIF1 maintains glycolytic and tricarboxylic acid cycle flux while simultaneously inhibiting oxidative phosphorylation in nucleus pulposus cells. HIF1 controls intracellular H+/lactate levels via monocarboxylate transporter 4 (MCT4); conversely, the accumulated lactate is capable of stabilizing HIF proteins by inhibiting PHD function as well as controlling transcriptional programmes. In addition to the well-studied proton extrusion mechanisms, the intracellular pH in nucleus pulposus cells is maintained by a HIF-dependent bicarbonate buffering mechanism controlled by various components including carbonic anhydrases. Loss of control of hypoxia-inducible factor 1 (HIF1) and HIF-dependent metabolic pathways can lead to intervertebral disc degeneration, whereas loss of HIF2 function is implicated in osteoarthritis. In nucleus pulposus cells, HIF1 and HIF2 are uniquely regulated by both oxygen-dependent and oxygen-independent mechanisms involving prolyl hydroxylase domain-containing proteins (PHDs) and circadian clock genes. Cells of the intervertebral disc possess functional mitochondria and, in nucleus pulposus cells, mitochondria undergo HIF-dependent mitophagy and fragmentation. HIF1 maintains glycolytic and tricarboxylic acid cycle flux while simultaneously inhibiting oxidative phosphorylation in nucleus pulposus cells. HIF1 controls intracellular H+/lactate levels via monocarboxylate transporter 4 (MCT4); conversely, the accumulated lactate is capable of stabilizing HIF proteins by inhibiting PHD function as well as controlling transcriptional programmes. In addition to the well-studied proton extrusion mechanisms, the intracellular pH in nucleus pulposus cells is maintained by a HIF-dependent bicarbonate buffering mechanism controlled by various components including carbonic anhydrases.