Vitamin D Receptor Coactivators in Osteoblasts

描述（由申请人提供）：1,25-二羟基维生素D3 （1,25(OH)2D3）是维生素D内分泌系统的生物活性激素。它通过维生素D受体（VDR）通过增加肠道钙吸收和直接影响成骨细胞/破骨细胞活性来影响骨骼。本研究计划的总体目标是研究1,25(OH)2D3和VDR在成骨细胞中的直接转录作用。为了实现这一目标，表达阵列鉴定了成骨细胞中由125 (OH)2D3诱导的几个新基因。本研究的重点是一个影响成骨细胞生物学的1,25(OH) 2d3诱导基因，即脑膜瘤-1或MN1。MN1是一种转录因子，在核受体介导的转录中起辅助激活作用。小鼠中MN1基因的缺失会显著损害膜内骨化，导致颅骨发育缺陷。这些数据表明MN1在骨骼稳态中具有重要的生物学作用。我们的初步数据表明，MN1在许多成骨细胞系中表达，其表达受体外和体内成骨细胞中1,25(OH)2D3的调控。成骨细胞中MN1异位表达抑制成骨细胞增殖，shRNA介导的MN1表达下调可降低成骨细胞分化标志物。这些研究首次探讨了MN1在骨骼稳态中的功能。本研究的一个主要假设是，VDR和125 (OH)2D3是MN1基因表达和维持正常骨骼稳态所需的其他基因的有效调节剂。我们建议通过关注MN1在成骨细胞功能中的持续表征来研究维生素D内分泌系统在骨骼稳态中的意义。具体而言，我们建议：1)确定MN1在尾骨和轴骨发育和维持中的意义；2)确定成骨细胞中1,25（OH)2D3转录调控MN1基因的分子机制；3)确定MN1在成骨细胞中1,25(OH)2D3激活转录中的功能意义；4）确定MN1在控制成骨细胞增殖和分化中的作用。这些研究将为维生素D内分泌系统在骨骼稳态中的直接作用提供新的见解，并提高对维持骨骼完整性所需的关键因素的理解。公共卫生相关性：维生素D对骨骼和成骨细胞影响的分子细节在很大程度上是未知的。我们的工作假设表明，维生素D控制MN1辅激活蛋白的表达并影响成骨细胞的功能。本研究的主要目的是确定成骨细胞中VDR-RXR异源二聚体的新靶基因。

DESCRIPTION (provided by applicant): 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is the bioactive hormone of the vitamin D endocrine system. It functions via the vitamin D receptor (VDR) to impact bone by increasing intestinal calcium absorption and by directly affecting osteoblast/osteoclast activity. The overall goal of this research proposal is to examine direct transcriptional effects of 1,25(OH)2D3 and VDR in osteoblasts. Toward this goal, expression arrays identified several novel genes that are induced by 1,25(OH)2D3 in osteoblastic cells. This proposal focuses on one 1,25(OH)2D3-induced gene that impact osteoblast biology, namely Meningioma-1 or MN1. MN1 is a transcription factor that serves as a coactivator in nuclear receptor- mediated transcription. Deletion of the MN1 gene in mice markedly impairs intramembranous ossification leading to defects in cranial skeletal development. These data demonstrate an important biological role for MN1 in skeletal homeostasis. Our preliminary data show that MN1 is expressed in numerous osteoblastic cell lines and its expression is regulated by 1,25(OH)2D3 in osteoblastic cells in vitro and in vivo. Ectopic expression of MN1 in osteoblasts inhibits osteoblast proliferation and shRNA- mediated knock-down of MN1 expression reduces osteoblast differentiation markers in vitro. These studies are the first to address MN1 function in skeletal homeostasis. A major hypothesis tested in this proposal is that VDR and 1,25(OH)2D3 are potent regulators of MN1 gene expression and of other genes that are required to maintain normal skeletal homeostasis. We propose to examine the significance of the vitamin D endocrine system in skeletal homeostasis by focusing on the continued characterization of MN1 in osteoblast function. Specifically, we propose to: 1) determine the significance of MN1 in development and maintenance of the appendicular and axial skeletons, 2) determine the molecular mechanisms involved in the transcriptional regulation of the MN1 gene by 1,25(OH)2D3 in osteoblastic cells, 3) establish the functional significance of MN1 in 1,25(OH)2D3-activated transcription in osteoblastic cells, and 4) determine the role of MN1 in controlling osteoblast proliferation and differentiation. These studies will provide new insight into the direct actions of the vitamin D endocrine system in skeletal homeostasis and an improved understanding of key factors that are needed to maintain skeletal integrity. PUBLIC HEALTH RELEVANCE: The molecular details involved in vitamin D effects in bone and osteoblasts are largely unknown. Our working hypothesis states that vitamin D controls the expression of the MN1 coactivator protein and impacts osteoblast function. The main goal of this proposal is to define novel target genes for VDR-RXR heterodimers in osteoblasts.