Hypoxic downregulation of cellular proliferation and loss of phenotype stability in human osteoblasts is mediated by HIF-1α
Issue title: Selected Proceedings of the 16th Conference of the European Society for Clinical Hemorheology and Microcirculation (ESCHM), 18–21 June, 2011, Munich, Germany
Affiliations: Department of Orthopedic Surgery, University of Regensburg, Asklepios Klinikum Bad Abbach, Bad Abbach, Germany | Department of Trauma and Plastic Surgery, University of Regensburg, Regensburg, Germany
Note: [] Corresponding author: Dr. med. Philipp Lechler, Klinik und Poliklinik für Orthopädie und Orthopädische Chirurgie, für die Universität Regensburg, Asklepios Klinikum Bad Abbach, Kaiser Karl V Allee 3, 93077 Bad Abbach, Germany. Tel.: +49 9405 18 4885; Fax: +49 9405 18 2925; E-mail: p.lechler@asklepios.com
Abstract: Both, skeletal development and fracture healing depend on an orchestrated sequence of cellular growth and differentiation processes. Regional changes in tissue oxygen tension were proposed as key regulators of osteoblast proliferation and phenotype. Hypoxia results in the stabilization of hypoxia-inducible factor-1α (HIF-1α), thus influencing expression of a multitude of genes required for cellular adaptation. In the present study we dissected the effects of HIF-1α on cellular proliferation and gene expression of primary human osteoblasts. Primary human osteoblasts were studied by transfecting siRNA and plasmids coding for human HIF-1α. Gene expression was analyzed by western blot and quantitative PCR. Functional assays were performed to study HIF-1α function, i.e. proliferation and cell cycle analysis. As previously reported exposure to hypoxia led to a stabilization of HIF-1α on protein level and resulted in reduced rates of proliferation and osteocalcin expression. Furthermore, the expression of the proproliferative gene survivin was significantly reduced (p < 0.01). Knock down of HIF-1α attenuated hypoxic downregulation of proliferation (p < 0.05), and osteocalcin (p < 0.05) as well as survivin (p < 0.05) expression significantly. Importantly, the isolated overexpression of HIF-1α impaired proliferative activity and led to significantly reduced rates of expression of osteocalcin (p < 0.05) and survivin (p < 0.01). The present study shows that HIF-1α might reduce proliferation and survivin expression in primary human osteoblasts independently from cellular hypoxia. Furthermore, HIF-1α promoted the loss of the characteristic osteoblastic marker, osteocalcin in vitro. These findings underline the important role of HIF-1α in bone physiology and pathophysiology. Modulating HIF-1α function in hypoxic environments could be of value for future therapeutic approaches.
