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 α subunit (HIF‐α), of which there are two transcriptionally active isoforms (HIF‐1α and HIF‐2α), and a constitutively expressed β subunit (HIFβ). Under normoxic conditions, HIF‐α 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‐α stabilization and induction of target transcriptional changes. Our previous studies showed that Vhl deletion in osteocytes ( Dmp1‐cre; Vhl f/f ) resulted in HIF‐α stabilization and generation of a high bone mass (HBM) phenotype. The skeletal impact of HIF‐1α accumulation has been well characterized; however, the unique skeletal impacts of HIF‐2α remain understudied. Because osteocytes orchestrate skeletal development and homeostasis, we investigated the role of osteocytic HIF‐α isoforms in driving HBM phenotypes via osteocyte‐specific loss‐of‐function and gain‐of‐function HIF‐1α and HIF‐2α mutations in C57BL/6 female mice. Deletion of Hif1a or Hif2a in osteocytes showed no effect on skeletal microarchitecture. Constitutively stable, degradation‐resistant HIF‐2α (HIF‐2α cDR), but not HIF‐1α 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‐2α in driving HBM phenotypes that can potentially be harnessed pharmacologically to improve bone mass and reduce fracture risk. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Hypoxia‐inducible factors (HIFs) are oxygen‐dependent heterodimeric transcription factors that mediate molecular responses to reductions in cellular oxygen (hypoxia). HIF signaling involves stable HIF‐β subunits and labile, oxygen‐sensitive HIF‐α subunits. Under hypoxic conditions, the HIF‐α subunit is stabilized, complexes with nucleus‐confined HIF‐β subunit, and transcriptionally regulates hypoxia‐adaptive genes. Transcriptional responses to hypoxia include altered energy metabolism, angiogenesis, erythropoiesis, and cell fate. Three isoforms of HIF‐α—HIF‐1α, HIF‐2α, and HIF‐3α—are found in diverse cell types. HIF‐1α and HIF‐2α serve as transcriptional activators, whereas HIF‐3α restricts HIF‐1α and HIF‐2α. The structure and isoform‐specific functions of HIF‐1α in mediating molecular responses to hypoxia are well established across a wide range of cell and tissue types. The contributions of HIF‐2α to hypoxic adaptation are often unconsidered if not outrightly attributed to HIF‐1α. This review establishes what is currently known about the diverse roles of HIF‐2α in mediating the hypoxic response in skeletal tissues, with specific focus on development and maintenance of skeletal fitness. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

No abstract

No abstract