HIF-2alpha, a Novel Regulator of Osteoblastogenesis

摘要 增进我们对控制骨量和成骨细胞分化机制的理解至关重要 揭示骨骼疾病的发病机制并确定治疗方法。 成骨细胞在低氧（缺氧）环境中运作。缺氧诱导转录因子 因子 1a (HIF1) 和 HIF2 是细胞对缺氧反应的关键介质。两种转录因子 在成骨细胞谱系的细胞中表达。据报道，HIF1 是骨形成的正调节因子 和成骨细胞分化。相反，HIF2 在控制骨量和成骨细胞生物学方面的作用是 仍然知之甚少。我们最近培育了具有功能丧失和功能获得的突变小鼠 使用 PRX1-Cre 观察肢芽间充质祖细胞中 HIF2 的突变。初步分析这些 突变小鼠表明，HIF2 通过以下方式成为成骨细胞生成和骨形成的负调节因子： 直接作用于成骨细胞谱系的细胞。从机制上讲，我们收集了 Sox9 的初步证据， 它正在成为成骨细胞分化的负调节因子，可能介导 HIF2 依赖性 成骨细胞生成受损。我们的发现构成了 HIF 信号激活的范式转变 途径与成骨细胞活性增加而不是减少有关。此外，它们还暗示 与其他细胞类型一样，HIF1 和 HIF2 在成骨细胞中具有相反的功能。另外，我们初步 数据显示，肢芽间充质祖细胞中 HIF2 的缺失会增加两者的骨量 小梁骨和皮质骨。 HIF2可以被小分子选择性抑制，其中一些目前已被研究 在患有与高水平 HIF2 活性相关的疾病（例如透明细胞）的患者的临床试验中 肾癌。因此，确定 HIF2 是否以及如何控制成骨细胞生成和骨量 不仅将扩大和加深我们对缺氧信号通路在骨骼中的作用的理解 但也可以为治疗慢性疾病、骨质疏松症中出现的低骨量提供新的靶点 并随着衰老。因此，在本提案中，我们试图证明成骨细胞 HIF2 在 骨骼发育和成年期（目标 1）。此外，我们将确定成骨细胞 HIF2 是否控制 通过直接作用于间充质祖细胞并以 Sox9 依赖性方式形成成骨细胞（目标 2）。 ！

ABSTRACT Advancing our understanding of the mechanisms that control bone mass and osteoblast differentiation is crucial to unveil the pathogenesis of skeletal diseases and identify therapeutical approaches for their treatment. Osteoblastic cells operate in a low oxygen (hypoxic) environment. The transcription factors Hypoxia Inducible Factor-1a (HIF1) and HIF2 are critical mediators of the cellular response to hypoxia. Both transcription factors are expressed in cells of the osteoblast lineage. HIF1 was reported to be a positive regulator of bone formation and osteoblast differentiation. Conversely, the role of HIF2 in the control of bone mass and osteoblast biology is still poorly understood. We recently generated mutant mice carrying loss-of-function and gain-of-function mutations of HIF2 in mesenchymal progenitors of the limb bud by using PRX1-Cre. Preliminary analysis of these mutant mice suggested that HIF2 is a negative regulator of osteoblastogenesis and bone formation through a direct action on cells on the osteoblast lineage. Mechanistically, we gathered preliminary evidence that Sox9, which is emerging as a negative regulator of osteoblast differentiation, is likely to mediate the HIF2-dependent impairment of osteoblastogenesis. Our findings constitute a paradigm shift as activation of the HIF signaling pathway has been associated with increased, rather than decreased, osteoblast activity. Moreover, they imply that, as in other cell types, HIF1 and HIF2 have opposing functions in osteoblastic cells. Also, our preliminary data showed that loss of HIF2 in mesenchymal progenitors of the limb bud increases bone mass in both trabecular and cortical bone. HIF2 can be selectively inhibited by small molecules, some of which are currently in clinical trials in patients carrying pathologies associated with high levels of HIF2 activity such as clear cell renal carcinoma. Therefore, determining whether and how HIF2 controls osteoblastogenesis and bone mass not only will expand and deepen our understanding of the role of the hypoxia signaling pathway in the skeleton but could also provide a novel target for the treatment of low bone mass seen in chronic diseases, osteoporosis and with aging. In this proposal, we thus seek to demonstrate that osteoblastic HIF2 regulates bone mass during skeletal development and in adulthood (Aim 1). Moreover, we will establish whether osteoblastic HIF2 controls osteoblastogenesis through a direct action on mesenchymal progenitors and in a Sox9-dependent manner (Aim 2). !