NOVEL VITAMIN D TARGET GENES 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）发挥作用，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1，25（OH）2D3诱导的几个新基因。该提案重点关注两个1，25（OH）2D3诱导的影响成骨细胞生物学的基因，即Semaphorin 3B（SEMA3B）和CCAAT/增强子结合蛋白β（C/EBPb）。SEMA3B是抑制细胞生长和迁移的分泌型粘附分子。初步数据显示，SEMA3B在体内成骨细胞中表达，并在体外原代成骨细胞中受1，25（OH）2D3调节。成像研究揭示了从在成骨细胞中表达人SEMA3B的转基因小鼠（Ob-SEMA3B）获得的胫骨的骨小梁密度的显著降低。这些研究是第一个解决SEMA3B在骨骼稳态中的功能。C/EBPb是成骨细胞中被1，25（OH）2D3显著上调的转录因子，其与VDR协同作用以激活成骨细胞基因表达。初步数据显示，C/EBPb激活成骨细胞中的SEMA3B启动子活性。在这个提议中测试的一个主要假设是，VDR和C/EBPb是SEMA3B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。我们建议研究维生素D内分泌系统在骨骼动态平衡的意义，重点是继续表征这些新的小鼠模型。具体而言，我们建议：1）鉴定Ob-SEMA3B转基因小鼠中矿化缺陷的潜在基础; 2）开发成骨细胞选择性敲除小鼠以测试SEMA3B在体内的生理作用; 3）在细胞和分子水平上检查VDR和C/EBP对SEMA3B的调节。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。

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 two 1,25(OH)2D3-induced genes that impact osteoblast biology, namely Semaphorin 3B (SEMA3B) and CCAAT/enhancer binding protein beta (C/EBPb). SEMA3B is a secreted adhesion molecule that inhibits cell growth and migration. Preliminary data show that SEMA3B is expressed in osteoblasts in vivo and is regulated by 1,25(OH)2D3 in primary osteoblasts in vitro. Imaging studies reveal a dramatic decrease in trabecular bone density of tibias obtained from a transgenic mouse expressing human SEMA3B in osteoblasts (Ob-SEMA3B). These studies are the first to address SEMA3B function in skeletal homeostasis. C/EBPb is transcription factor that is dramatically upregulated by 1,25(OH)2D3 in osteoblastic cells and it acts cooperatively with VDR to activate osteoblast gene expression. Preliminary data show that C/EBPb activates SEMA3B promoter activity in osteoblasts. A major hypothesis tested in this proposal is that VDR and C/EBPb are potent regulators of SEMA3B 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 the skeletal homeostasis by focusing on the continued characterization of these new mouse models. Specifically, we propose to: 1) identify the underlying basis for the mineralization defect in the Ob- SEMA3B transgenic mouse; 2) develop an osteoblast selective knockout mouse to test the physiological role of SEMA3B in vivo; 3) examine the regulation of SEMA3B by VDR and C/EBPs at the cell and molecular levels. These studies will provide new insight into the direct actions of the vitamin D endocrine system in skeletal homeostasis.