Hypoxia-Inducible Factors 1 alpha and 2 alpha 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 HIF alpha subunits in osteoblasts produce abundant VEGF and develop extremely dense, highly vascularized long bones. In this study, we investigated the individual contributions of Hif-1 alpha and Hif-2 alpha in angiogenesis and osteogenesis by individually disrupting each Hif alpha gene in osteoblasts using the Crc-loxP method. Mice lacking Hif-1 alpha demonstrated markedly decreased trabecular bone volume, reduced bone formation rate, and altered cortical bone architecture. By contrast, mice lacking Hif-2 alpha 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 Delta Hif-1 alpha and Delta Hif-2 alpha mice suggesting a common role for these Hif alpha Subunits in skeletal angiogenesis. In agreement with this idea, osteoblasts lacking either Hif-1 alpha or Hif-2 alpha had profound reductions in VEGF mRNA expression but only the loss of Hif-1 alpha impaired osteoblast proliferation. These findings indicate that expression of both Hif-1 alpha and Hif-2 alpha by osteoblasts is required for long bone development. We propose that both Hif-1 alpha and Hif-2 alpha function through cell non-autonomous modes to promote vascularization of bone and that Hif-1 alpha also promotes bone formation by exerting direct actions on the osteoblast. J. Cell. Biochem. 109: 196-204, 2010. (C) 2009 Wiley-Liss, Inc.