Igarashi Mamoru, Yogiashi Yoshiko, Mihara Masatomo, Takada Ichiro, Kitagawa Hirochika, Kato Shigeaki
The Institute of Molecular and Cellular Biosciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan.
Mol Cell Biol. 2007 Nov;27(22):7947-54. doi: 10.1128/MCB.00813-07. Epub 2007 Sep 17.
Vitamin K is a fat-soluble vitamin that serves as a coenzyme for vitamin K-dependent carboxylase. Besides its canonical action, vitamin K binds to the steroid and xenobiotic receptor (SXR)/pregnane X receptor (PXR) and modulates gene transcription. To determine if the osteoprotective action of vitamin K is the result of the PXR/SXR pathway, we screened by two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis the PXR/SXR target genes in an osteoblastic cell line (MC3T3-E1) treated with a vitamin K2 (menaquinone 4 [MK4]). Osteoblastic differentiation of MC3T3-E1 cells was induced by MK4. Msx2, an osteoblastogenic transcription factor, was identified as an MK4-induced gene. Functional analysis of the Msx2 gene promoter mapped a vitamin K-responsive element (PXR-responsive element [PXRE]) that was directly bound by a PXR/retinoid X receptor alpha heterodimer. In a chromatin immunoprecipitation analysis, PXR was recruited together with a coactivator, p300, to the PXRE in the Msx2 promoter. MK4-bound PXR cooperated with estrogen-bound estrogen receptor alpha to control transcription at the Msx2 promoter. Knockdown of either PXR or Msx2 attenuated the effect of MK4 on osteoblastic differentiation. Thus, the present study suggests that Msx2 is a target gene for PXR activated by vitamin K and suggests that the osteoprotective action of MK4 in the human mediates, at least in part, a genomic pathway of vitamin K signaling.
维生素K是一种脂溶性维生素，作为维生素K依赖性羧化酶的辅酶。除了其经典作用外，维生素K还与类固醇和外源性物质受体（SXR）/孕烷X受体（PXR）结合并调节基因转录。为了确定维生素K的骨保护作用是否是PXR/SXR途径的结果，我们通过二维十二烷基硫酸钠-聚丙烯酰胺凝胶电泳筛选了用维生素K2（甲萘醌4 [MK4]）处理的成骨细胞系（MC3T3-E1）中的PXR/SXR靶基因。MK4诱导了MC3T3-E1细胞的成骨分化。Msx2是一种成骨转录因子，被鉴定为MK4诱导的基因。对Msx2基因启动子的功能分析确定了一个维生素K反应元件（PXR反应元件[PXRE]），该元件直接与PXR/视黄酸X受体α异二聚体结合。在染色质免疫沉淀分析中，PXR与共激活因子p300一起被募集到Msx2启动子中的PXRE。与雌激素结合的雌激素受体α结合的MK4-PXR协同控制Msx2启动子处的转录。敲低PXR或Msx2会减弱MK4对成骨分化的作用。因此，本研究表明Msx2是维生素K激活的PXR的靶基因，并表明MK4在人体中的骨保护作用至少部分介导了维生素K信号的基因组途径。
