The ability of cells to sense and respond to changes in oxygen tension is critical for many developmental, physiological, and pathological processes, including angiogenesis, control of blood flow and tissue repair. In many oxygen sensitive tissues, hypoxic stress induces the expression of genes whose products act in concert to facilitate the supply of metabolic energy. Among these are genes coding for erythropoietin (EPO), vasoactive endothelial growth factor (VEGF), the glucose transporters, nitric oxide synthetase and tyrosine hydroxylase. In skeletal tissue, intramembranous and endochondral ossification occur in close association and proximity to capillary in-growth and angiogenesis. In addition, disruption of normal afferent blood supply occurs following fracture and leads to hypoxia of adjacent tissue. Based on these observations, we reasoned that cells of mesenchymal origin including osteoblasts were ideally positioned in bone to sense and respond to fluctuations in oxygen and nutrient supply. Consistent with this concept, osteoblasts and osteocytes respond to hypoxia by elevating the level of the hypoxia-inducible factor 1 (HIF-1), an evolutionarily conserved master regulator of the hypoxic response, which in turn, transactivates VEGF and other HIF-1 target genes. We therefore hypothesize that osteoblasts use the HIF-1alpha pathway to sense reduced oxygen tension and transmit signals that impinge on angiogenic and osteogenic gene programs. To test this hypothesis we will use genetically altered mice to determine the cellular and molecular impact of gain of HIF-1alpha function through targeted overexpression of a stable HIF-1 molecule or loss of HIF-1alpha by conditional mutagenesis in osteoblasts during bone development and following bone injury. Complementary in vitro studies in primary osteoblasts will attempt to determine whether HIF-1alpha impacts osteoblast growth and differentiation independent of the vasculature.
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