Distinct roles for the hypoxia-inducible transcription factors HIF-1α and HIF-2α in human osteoclast formation and function

Bone homeostasis is maintained by a balance between osteoblast-mediated bone formation and osteoclast-driven bone resorption. Hypoxia modulates this relationship partially via direct and indirect effects of the hypoxia-inducible factor-1 alpha (HIF-1 alpha) transcription factor on osteoclast formation and bone resorption. Little data is available on the role(s) of the HIF-2 alpha isoform of HIF in osteoclast biology. Here we describe induction of HIF-1 alpha and HIF-2 alpha during the differentiation of human CD14+ monocytes into osteoclasts. Knockdown of HIF-1 alpha did not affect osteoclast differentiation but prevented the increase in bone resorption that occurs under hypoxic conditions. HIF-2 alpha knockdown did not affect bone resorption but moderately inhibited osteoclast formation. Growth of osteoclasts in 3D gels reversed the effect of HIF-2 alpha knockdown; HIF-2 alpha siRNA increasing osteoclast formation in 3D. Glycolysis is the main HIF-regulated pathway that drives bone resorption. HIF knockdown only affected glucose uptake and bone resorption in hypoxic conditions. Inhibition of glycolysis with 2-deoxy-D-glucose (2-DG) reduced osteoclast formation and activity under both basal and hypoxic conditions, emphasising the importance of glycolytic metabolism in osteoclast biology. In summary, HIF-1 alpha and HIF-2 alpha play different but overlapping roles in osteoclast biology, highlighting the importance of the HIF pathway as a potential therapeutic target in osteolytic disease.