Exploring the Physiological Roles of Osteoblastic EPO and Osteoblastic EPOR

 说明(申请人提供)：促红细胞生成素(EPO)是一种激素，通过激活红系祖细胞中存在的其受体(EPOR)来刺激红细胞生成。在成人中，超过90%的EPO是由肾小管周围间质成纤维细胞亚群产生的。EPO表达的关键调控机制是通过激活低氧诱导因子(HIF)途径的低氧。贫血通过诱导局部缺氧来刺激EPO的产生。EPO和EPOR在多种非造血组织中也有表达，这意味着EPO的功能可能超出了对红细胞生成的调控。例如，EPO在骨量控制中的作用已被提出，尽管EPO是否真的调控骨发育和/或动态平衡，以及这些假定的EPO对骨的影响是否至少部分归因于成骨细胞系细胞中EPOR的激活，仍存在很大争议。我们最近发现：1)成骨细胞系的细胞可以产生和分泌功能性的EPO；2)成骨细胞中HIF-2的活性增加刺激成骨细胞EPO的表达并导致红细胞增多症；3)相同细胞中的HIF-2失活会减少成骨细胞EPO的表达并减少骨髓红系祖细胞的数量。综上所述，这些发现表明成骨细胞促红细胞生成素可能与生理相关。沿着这些思路，我们还提供了EPOR在成骨细胞中表达的证据(初步数据)。鉴于所有这些新的发现，我们不建议探讨成骨细胞EPO和成骨细胞EPOR的生理作用。为此，在AIM I中，我们将分析成骨细胞系细胞中缺乏EPO的骨骼。此外，在相同的目的中，我们将确定急性贫血是否可以调节成骨细胞EPO的表达水平，以确定贫血是否是成骨细胞EPO表达的生理性刺激。在AIM II中，我们将研究成骨细胞系细胞中缺乏EPOR的骨。在骨骼中缺乏EPOR的突变小鼠也将接受药理剂量的外源性EPO治疗，以确定全身性EPO对骨骼的作用是否至少部分地由成骨细胞EPOR介导。这些实验的成功完成可能导致成骨细胞EPO和/或成骨细胞EPOR被确认为骨发育和/或内稳态的新调节因子。

 DESCRIPTION (provided by applicant): Erythropoietin (EPO) is the hormone that stimulates erythropoiesis via activation of its receptor (EPOR) present in erythroid progenitor cells. In adults, more than 90% of EPO is produced in the kidney by a subset of peritubular interstitial fibroblasts. The key regulatory mechanism of EPO expression is hypoxia via activation of the Hypoxia-inducible factor (HIF) pathway. Anemia stimulates EPO production by inducing local hypoxia. Both EPO and EPOR are also expressed in a variety of non-hematopoietic tissues, which implies that EPO may have functions that go beyond regulation of erythropoiesis. For example, a role for EPO in the control of bone mass has been proposed, though it is still highly controversial whether EPO does, indeed, modulate bone development and/or homeostasis, and whether these putative EPO effects on bone are due, at least in part, to activation of EPOR in cells of the osteoblast lineage. We recently showed: 1) cells of the osteoblast lineage can produce and secrete functional EPO; 2) increased activity of HIF-2 in osteoblastic cells stimulates expression of osteoblastic EPO and causes polycythemia; 3) inactivation of HIF-2 in the same cells reduces expression of osteoblastic EPO and decreases number of bone marrow erythroid progenitors. Taken together, these findings indicate that osteoblastic EPO is likely to be physiologically relevant. Along these lines, we also provided evidence that EPOR is expressed in osteoblastic cells (preliminary data). In light of all these novel findings, we are no proposing to explore the physiologic roles of osteoblastic EPO and osteoblastic EPOR. For this purpose, in AIM I we will analyze bones lacking EPO in cells of the osteoblast lineage. In addition, in the same AIM, we will determine whether levels of expression of osteoblastic EPO can be modulated by acute anemia, in order to establish whether anemia is a physiological stimulus for EPO expression in osteoblastic cells. In AIM II, we will study bones lacking EPOR in cells of the osteoblast lineage. Mutant mice lacking EPOR in bone will also be treated with pharmacological doses of exogenous EPO in order to determine whether the actions of systemic EPO on bone are mediated, at least in part, by osteoblastic EPOR. The successful accomplishment of the proposed experiments could lead to the identification of osteoblastic EPO and/or osteoblastic EPOR as novel regulators of bone development and/or homeostasis.