Hypoxia-inducible factors 1α and 2α exert both distinct and overlapping functions in long bone development

The hypoxia-inducible factors have recently been identified as critical regulators of angiogenic-osteogenic coupling. Mice overexpressing HIFα subunits in osteoblasts produce abundant VEGF and develop extremely dense, highly vascularized long bones. In this study, we investigated the individual contributions of Hif-1α and Hif-2α in angiogenesis and osteogenesis by individually disrupting each Hifα gene in osteoblasts using the Cre-loxP method. Mice lacking Hif-1α demonstrated markedly decreased trabecular bone volume, reduced bone formation rate, and altered cortical bone architecture. By contrast, mice lacking Hif-2α had only a modest decrease in trabecular bone volume. Interestingly, long bone blood vessel development measured by angiography was decreased by a similar degree in both DHif-1α and DHif-2α mice suggesting a common role for these Hifa subunits in skeletal angiogenesis. In agreement with this idea, osteoblasts lacking either Hif-1α or Hif-2α had profound reductions in VEGF mRNA expression but only the loss of Hif-1α impaired osteoblast proliferation. These findings indicate that expression of both Hif-1α and Hif-2α by osteoblasts is required for long bone development. We propose that both Hif-1α and Hif-2α function through cell non-autonomous modes to promote vascularization of bone and that Hif-1α also promotes bone formation by exerting direct actions on the osteoblast.