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  • Title: Enhanced bone regeneration associated with decreased apoptosis in mice with partial HIF-1alpha deficiency.
    Author: Komatsu DE, Bosch-Marce M, Semenza GL, Hadjiargyrou M.
    Journal: J Bone Miner Res; 2007 Mar; 22(3):366-74. PubMed ID: 17181398.
    Abstract:
    UNLABELLED: HIF-1alpha activates genes under hypoxia and was hypothesized to regulate bone regeneration. Surprisingly, HIF-1alpha+/- fracture calluses are larger, stronger, and stiffer than HIF-1alpha+/+ calluses because of decreased apoptosis. These data identify apoptosis inhibition as a means to enhance bone regeneration. INTRODUCTION: Bone regeneration subsequent to fracture involves the synergistic activation of multiple signaling pathways. Localized hypoxia after fracture activates hypoxia-inducible factor 1alpha (HIF-1alpha), leading to increased expression of HIF-1 target genes. We therefore hypothesized that HIF-1alpha is a key regulator of bone regeneration. MATERIALS AND METHODS: Fixed femoral fractures were generated in mice with partial HIF-1alpha deficiency (HIF-1alpha+/-) and wildtype littermates (HIF-1alpha+/+). Fracture calluses and intact contralateral femurs from postfracture days (PFDs) 21 and 28 (N=5-10) were subjected to microCT evaluation and four-point bending to assess morphometric and mechanical properties. Molecular analyses were carried out on PFD 7, 10, and 14 samples (N=3) to determine differential gene expression at both mRNA and protein levels. Finally, TUNEL staining was performed on PFD 14 samples (N=2) to elucidate differential apoptosis. RESULTS: Surprisingly, fracture calluses from HIF-1alpha+/- mice exhibited greater mineralization and were larger, stronger, and stiffer. Microarray analyses focused on hypoxia-induced genes revealed differential expression (between genotypes) of several genes associated with the apoptotic pathway. Real-time PCR confirmed these results, showing higher expression of proapoptotic protein phosphatase 2a (PP2A) and lower expression of anti-apoptotic B-cell leukemia/lymphoma 2 (BCL2) in HIF-1alpha+/+ calluses. Subsequent TUNEL staining showed that HIF-1alpha+/+ calluses contained larger numbers of TUNEL+ chondrocytes and osteoblasts than HIF-1alpha+/- calluses. CONCLUSIONS: We conclude that partial HIF-1alpha deficiency results in decreased chondrocytic and osteoblastic apoptosis, thereby allowing the development of larger, stiffer calluses and enhancing bone regeneration. Furthermore, apoptosis inhibition may be a promising target for developing new treatments to accelerate bone regeneration.
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