Hypoxia-inducible factor-1α restricts the anabolic actions of parathyroid hormone

The hypoxia inducible factors (Hifs) are evolutionarily conserved transcriptional factors that control homeostatic responses to low oxygen. In developing bone, Hif-1 generated signals induce angiogenesis necessary for osteoblast specification, but in mature bone, loss of Hif-1 in osteoblasts resulted in a more rapid accumulation of bone. These findings suggested that Hif-1 exerts distinct developmental functions and acts as a negative regulator of bone formation. To investigate the function of Hif-1 alpha in osteoanabolic signaling, we assessed the effect of Hif-1 alpha loss-of-function on bone formation in response to intermittent parathyroid hormone (PTH). Mice lacking Hif-1 alpha in osteoblasts and osteocytes form more bone in response to PTH, likely through a larger increase in osteoblast activity and increased sensitivity to the hormone. Consistent with this effect, exposure of primary mouse osteoblasts to PTH resulted in the rapid induction of Hif-1 alpha protein levels via a post-transcriptional mechanism. The enhanced anabolic response appears to result from the removal of Hif-1 alpha-mediated suppression of beta-catenin transcriptional activity. Together, these data indicate that Hif-1 alpha functions in the mature skeleton to restrict osteoanabolic signaling. The availability of pharmacological agents that reduce Hif-1 alpha function suggests the value in further exploration of this pathway to optimize the therapeutic benefits of PTH.