Signaling are not well known. In this study, we performed a

Signaling are not well known. In this study, we performed a series of experiments to study the effect of HIF-1a on the extracellular Wnt antagonist Sost. We provide evidences toHIF-1a Activates Sost Gene Expressiondemonstrate that HIF-1a activates Sost expression, a novel mechanism of HIF-1a inhibitory effect on Wnt signaling pathway in osteoblasts. Wnt signaling is known to have the major impact at different stages of bone formation and bone metabolism [9,10]. Wnt signaling-mediated gene Title Loaded From File expression can promote osteoblast proliferation and differentiation. Some studies investigated the role of Wnt/b-catenin signaling in nonunion and osteoporosis, suggesting Wnt signaling could possibly have potential to become a target of pharmacological intervention to increase bone formation [32,33]. Sost is one of the Wnt antagonists. The Sost loss-of-function mutations in human cause the autosomal recessive bone dysplasias Sclerosteosis and Van Buchem disease, which are characterized by progressive bone overgrowth throughout life, enlargement of the jaw and facial bones, and increased bone formation [20,34]. HIF-1a is the crucial mediator of the adaptive response of cells to hypoxia. The oxygen dependent degradation of HIF-1a is controlled by a family of HIF prolyl hydroxylases. Under normoxic conditions, HIF-1a is hydroxylated by prolyl hydroxylases that act as oxygen sensors. Hydroxylation of specific proline residues on HIF-1a is followed by proteasomal degradation. Under hypoxic conditions, HIF-1a is stabilized, translocated to the nucleus, and forms a Title Loaded From File heterodimer with HIF-1b to regulate target genes. These target genes are involved in a variety of cellular processes including angiogenesis, energy metabolism, cell proliferation and survival, vasomotor control, and matrix metabolism [35]. It has been shown that constitutive activation of the HIF-1a pathway in mice promotes robust bone modeling and acquisition in long bones, and conversely, loss of HIF-1a in osteoblasts results in narrow, less vascularized bones [7]. These results suggest thatHIF-1a is critical for coupling angiogenesis to osteogenesis during long bone formation. Osteoblasts reside on the nascent bone surface and sense reduced oxygen or nutrient levels, and HIF-1a is an important mediator in this process. The current study addresses possible mechanisms for hypoxia/ HIF-1a to inhibit Wnt pathway. This study indicates that HIF-1amediated Sost activation is one of possible mechanisms for hypoxia/HIF-1a to inhibit Wnt pathway. This is supported by several evidences: 1) quantitative RT-PCR results showed that Sost gene was upregulated along with HIF-1a under hypoxia; 2) the treatment of HIF-1a activator DFO further enhanced the expression of Sost gene; 3) the inhibition of HIF-1a by siRNA in osteoblasts led to the expression decrease of Sost gene; 4) our transfection assay showed that HIF-1a activated Sost promoter reporter activity. A rescue experiment on cell growth by overexpressing Sost in HIF-1a knockdown cells could help to address the function activity further in the future. However, our study cannot rule out other possible mechanisms of the inhibitory effect of hypoxia/HIF-1a on Wnt signaling pathway. In summary, HIF-1a activates the expression of Wnt antagonist Sost gene. This provides a novel mechanism through which HIF1a inhibits Wnt signaling pathway in osteoblasts. Elucidation of HIF-1a inhibition of Wnt signaling will help to better understand the molecular mechanism of.Signaling are not well known. In this study, we performed a series of experiments to study the effect of HIF-1a on the extracellular Wnt antagonist Sost. We provide evidences toHIF-1a Activates Sost Gene Expressiondemonstrate that HIF-1a activates Sost expression, a novel mechanism of HIF-1a inhibitory effect on Wnt signaling pathway in osteoblasts. Wnt signaling is known to have the major impact at different stages of bone formation and bone metabolism [9,10]. Wnt signaling-mediated gene expression can promote osteoblast proliferation and differentiation. Some studies investigated the role of Wnt/b-catenin signaling in nonunion and osteoporosis, suggesting Wnt signaling could possibly have potential to become a target of pharmacological intervention to increase bone formation [32,33]. Sost is one of the Wnt antagonists. The Sost loss-of-function mutations in human cause the autosomal recessive bone dysplasias Sclerosteosis and Van Buchem disease, which are characterized by progressive bone overgrowth throughout life, enlargement of the jaw and facial bones, and increased bone formation [20,34]. HIF-1a is the crucial mediator of the adaptive response of cells to hypoxia. The oxygen dependent degradation of HIF-1a is controlled by a family of HIF prolyl hydroxylases. Under normoxic conditions, HIF-1a is hydroxylated by prolyl hydroxylases that act as oxygen sensors. Hydroxylation of specific proline residues on HIF-1a is followed by proteasomal degradation. Under hypoxic conditions, HIF-1a is stabilized, translocated to the nucleus, and forms a heterodimer with HIF-1b to regulate target genes. These target genes are involved in a variety of cellular processes including angiogenesis, energy metabolism, cell proliferation and survival, vasomotor control, and matrix metabolism [35]. It has been shown that constitutive activation of the HIF-1a pathway in mice promotes robust bone modeling and acquisition in long bones, and conversely, loss of HIF-1a in osteoblasts results in narrow, less vascularized bones [7]. These results suggest thatHIF-1a is critical for coupling angiogenesis to osteogenesis during long bone formation. Osteoblasts reside on the nascent bone surface and sense reduced oxygen or nutrient levels, and HIF-1a is an important mediator in this process. The current study addresses possible mechanisms for hypoxia/ HIF-1a to inhibit Wnt pathway. This study indicates that HIF-1amediated Sost activation is one of possible mechanisms for hypoxia/HIF-1a to inhibit Wnt pathway. This is supported by several evidences: 1) quantitative RT-PCR results showed that Sost gene was upregulated along with HIF-1a under hypoxia; 2) the treatment of HIF-1a activator DFO further enhanced the expression of Sost gene; 3) the inhibition of HIF-1a by siRNA in osteoblasts led to the expression decrease of Sost gene; 4) our transfection assay showed that HIF-1a activated Sost promoter reporter activity. A rescue experiment on cell growth by overexpressing Sost in HIF-1a knockdown cells could help to address the function activity further in the future. However, our study cannot rule out other possible mechanisms of the inhibitory effect of hypoxia/HIF-1a on Wnt signaling pathway. In summary, HIF-1a activates the expression of Wnt antagonist Sost gene. This provides a novel mechanism through which HIF1a inhibits Wnt signaling pathway in osteoblasts. Elucidation of HIF-1a inhibition of Wnt signaling will help to better understand the molecular mechanism of.