Degradation-Resistant Hypoxia Inducible Factor-2 alpha in Murine Osteocytes Promotes a High Bone Mass Phenotype

Molecular oxygen levels vary during development and disease. Adaptations to decreased oxygen bioavailability (hypoxia) are mediated by hypoxia-inducible factor (HIF) transcription factors. HIFs are composed of an oxygen-dependent alpha subunit (HIF-alpha), of which there are two transcriptionally active isoforms (HIF-1 alpha and HIF-2 alpha), and a constitutively expressed beta subunit (HIF beta). Under normoxic conditions, HIF-alpha is hydroxylated via prolyl hydroxylase domain (PHD) proteins and targeted for degradation via Von Hippel-Lindau (VHL). Under hypoxic conditions, hydroxylation via PHD is inhibited, allowing for HIF-alpha stabilization and induction of target transcriptional changes. Our previous studies showed that Vhl deletion in osteocytes (Dmp1-cre; Vhl(f/f)) resulted in HIF-alpha stabilization and generation of a high bone mass (HBM) phenotype. The skeletal impact of HIF-1 alpha accumulation has been well characterized; however, the unique skeletal impacts of HIF-2 alpha remain understudied. Because osteocytes orchestrate skeletal development and homeostasis, we investigated the role of osteocytic HIF-alpha isoforms in driving HBM phenotypes via osteocyte-specific loss-of-function and gain-of-function HIF-1 alpha and HIF-2 alpha mutations in C57BL/6 female mice. Deletion of Hif1a or Hif2a in osteocytes showed no effect on skeletal microarchitecture. Constitutively stable, degradation-resistant HIF-2 alpha (HIF-2 alpha cDR), but not HIF-1 alpha cDR, generated dramatic increases in bone mass, enhanced osteoclast activity, and expansion of metaphyseal marrow stromal tissue at the expense of hematopoietic tissue. Our studies reveal a novel influence of osteocytic HIF-2 alpha in driving HBM phenotypes that can potentially be harnessed pharmacologically to improve bone mass and reduce fracture risk. (c) 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.