Osteocalcin and osteopontin are noncollagenous proteins secreted by osteoblasts and regulated by a complex interplay of systemic and locally produced factors, including growth factors and steroid hormones. We investigated the mechanism by which transforming growth factor-beta (TGF beta) inhibits 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3)-enhanced expression of the osteocalcin (OC) and osteopontin (OP) genes. ROS 17/2.8 cells, in which both genes are expressed, were transfected with reporter constructs driven by native (i.e. wild-type) rat OC and mouse OP promoters. TGF beta abrogated the 1,25-(OH)2D3 enhanced transcription of both the OC and OP genes. The inhibitory TGF beta response for each requires vitamin D response element (VDRE) sequences, although there are additional contributions from proximal basal regulatory elements. These transcriptional effects were further investigated for contribution of the trans-activating factors, which interact with OC and OP VDREs, involving the vitamin D receptor (VDR) and retinoid X receptor (RXR). Gel mobility shift assays show that TGF beta significantly reduces induction of the heterodimers VDR/RXR complexes in 1,25-(OH)2D3-treated ROS 17/2.8 cells. However, Western blot and ligand binding analysis reveal that TGF beta does not affect nuclear availability of the VDR. We also show that activator protein-1 activity is up-regulated by TGF beta; thus, activator protein-1 binding sites in the OC promoter may potentially contribute to inhibitory effects of TGF beta on basal transcription. Our studies demonstrate that the inhibitory action of TGF beta on the 1,25-(OH)2D3 enhancement of OC and OP transcription in osteoblastic cells results from modulations of protein-DNA interactions at the OC and OP VDRE, which cannot be accounted for by changes in VDR protein levels. As OC and OP participate in bone turnover, our results provide insight into the contributions of TGF beta and 1,25-(OH)2D3 to VDR-mediated gene regulatory mechanism operative in bone formation and/or resorption events.

译文

骨钙素和骨桥蛋白是由成骨细胞分泌的非胶原蛋白,受全身和局部产生的因子 (包括生长因子和类固醇激素) 的复杂相互作用调节。我们研究了转化生长因子 β (TGF β) 抑制1,25-二羟基维生素D3 (1,25-(OH)2D3) 增强的骨钙素 (OC) 和骨桥蛋白 (OP) 基因表达的机制。用由天然 (即野生型) 大鼠OC和小鼠OP启动子驱动的报告构建体转染两个基因均表达的ROS 17/2.8细胞。TGF β 废除了1,25-(OH)2D3增强了OC和OP基因的转录。尽管近端基础调节元件还有其他贡献,但每种药物的抑制性TGF β 反应都需要维生素d反应元件 (VDRE) 序列。进一步研究了这些转录作用的反式激活因子的作用,这些反式激活因子与OC和OP VDREs相互作用,涉及维生素d受体 (VDR) 和类维生素a X受体 (RXR)。凝胶迁移率变化测定显示TGF β 显著降低在1,25-(OH) 2d3处理的ROS 17/2.8细胞中对异二聚体VDR/RXR复合物的诱导。然而,蛋白质印迹和配体结合分析表明,TGF β 不会影响VDR的核可用性。我们还显示TGF β 上调了激活蛋白1的活性; 因此,OC启动子中的激活蛋白1结合位点可能潜在地有助于TGF β 对基础转录的抑制作用。我们的研究表明,TGF β 对成骨细胞中OC和OP转录的1,25-(OH)2D3增强的抑制作用是由OC和OP VDRE处蛋白质-DNA相互作用的调节引起的,这不能解释VDR蛋白水平的变化。当OC和OP参与骨转换时,我们的结果提供了对TGF β 和1,25-(OH)2D3对VDR介导的在骨形成和/或吸收事件中起作用的基因调节机制的贡献的见解。

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