Fretz Jackie A, Zella Lee A, Kim Sungtae, Shevde Nirupama K, Pike J Wesley
Department of Biochemistry, University of Wisconsin-Madison, 433 Babcock Drive, Madison, Wisconsin 53706, USA.
Mol Endocrinol. 2006 Sep;20(9):2215-30. doi: 10.1210/me.2006-0102. Epub 2006 Apr 13.
The skeleton is a direct target of vitamin D action, where the hormone modulates the proliferation of osteoblast precursors, their differentiation into mature osteoblasts, and their functional activity. Some of these effects of vitamin D are reminiscent of those orchestrated by the Wnt signaling pathway wherein stimulation of the membrane receptor Frizzled and its coreceptor LRP5 leads to activation of beta-catenin and subsequent transcription-mediated changes in osteoblast biology. Indeed, LRP5 is now known to play a particularly important role in bone formation such that the loss of this component results in a reduction in osteoblast number, a delay in mineralization, and a reduction in peak bone mineral density. Interestingly, we discovered during the course of a vitamin D receptor (VDR) chromatin immunoprecipitation/DNA microarray analysis that 1,25-(OH)2D3 could induce binding of the VDR to sites within the Lrp5 gene locus. VDR and retinoid X receptor binding was evident both in primary osteoblasts as well as in osteoblasts of cell line origin. Importantly, this interaction between 1,25-(OH)2D3-activated VDR and the Lrp5 gene led to both a modification in chromatin structure within the Lrp5 locus and the induction of Lrp5 mRNA transcripts in vivo as well as in vitro. One of these sites within the Lrp5 locus was discovered to confer vitamin D response to a heterologous promoter when introduced into osteoblastic cells, permitting both the identification and characterization of the vitamin D response element located within. Interestingly, additional studies revealed that whereas the regulatory region in the mouse Lrp5 gene was highly conserved in the human genome, the vitamin D response element was not. Our studies show that 1,25-(OH)2D3 can enhance the expression of a critical component of the Wnt signaling pathway that is known to impact osteogenesis.
骨骼是维生素D作用的直接靶点，该激素可调节成骨细胞前体的增殖、其向成熟成骨细胞的分化及其功能活性。维生素D的这些作用有些让人联想到由Wnt信号通路所调控的作用，其中膜受体卷曲蛋白（Frizzled）及其共受体低密度脂蛋白受体相关蛋白5（LRP5）的激活会导致β-连环蛋白的活化以及随后成骨细胞生物学中转录介导的变化。事实上，现在已知LRP5在骨形成中起特别重要的作用，以至于该成分的缺失会导致成骨细胞数量减少、矿化延迟以及骨矿物质密度峰值降低。有趣的是，我们在维生素D受体（VDR）染色质免疫沉淀/DNA微阵列分析过程中发现，1,25-二羟基维生素D3（1,25-(OH)2D3）可诱导VDR与Lrp5基因座内的位点结合。VDR和视黄酸X受体的结合在原代成骨细胞以及细胞系来源的成骨细胞中均很明显。重要的是，1,25-(OH)2D3激活的VDR与Lrp5基因之间的这种相互作用导致Lrp5基因座内染色质结构的改变以及体内和体外Lrp5 mRNA转录本的诱导。当将Lrp5基因座内的这些位点之一引入成骨细胞时，发现其可赋予异源启动子维生素D反应性，从而能够鉴定和表征位于其中的维生素D反应元件。有趣的是，进一步的研究表明，虽然小鼠Lrp5基因中的调控区域在人类基因组中高度保守，但维生素D反应元件并非如此。我们的研究表明，1,25-(OH)2D3可增强已知影响骨生成的Wnt信号通路关键成分的表达。
