Ryan Elizabeth P, Holz Jonathan D, Mulcahey Mary, Sheu Tzong-Jen, Gasiewicz Thomas A, Puzas J Edward
Department of Environmental Medicine, University of Rochester School of Medicine and Dentistry, Rochester, New York 14642, USA.
J Bone Miner Res. 2007 Oct;22(10):1571-80. doi: 10.1359/jbmr.070615.
The AHR mediates many of the toxicological effects of aromatic hydrocarbons. We show that AHR expression in osteoblasts parallels the induction of early bone-specific genes involved in maturation. The AHR may not only mediate the effects of toxicants, but with an as yet unidentified ligand, be involved in the differentiation pathways of osteoblasts.
Metabolic bone diseases arise as a result of an imbalance in bone cell activities. Recent evidence suggests that environmental toxicants may be contributing factors altering these activities. One candidate molecule implicated in mediating the toxic effects of exogenous compounds is the aryl hydrocarbon receptor (AHR).
Osteoblasts isolated from neonatal rat calvaria were analyzed for AHR expression by quantitative PCR, Western blot, and immunohistochemistry. In addition, AHR activation was evaluated by electromobility gel shift assay and fluorescence microscopy.
Our findings showed AHR expression in mature osteoblasts in vivo. The pattern of AHR expression peaks after alkaline phosphatase and before induction of osteocalcin. We first show that AHR functions as a transactivating receptor in osteoblasts, as evidenced by its ligand-dependent migration to the nucleus and its association with known dioxin response elements. AHR activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) mediated the induction of cytochrome p450 1A1 and cycloxygenase-2 protein levels. This effect could be inhibited by the potent AHR antagonist, 3'4 methoxynitroflavone. Furthermore, lead treatment of osteoblasts upregulates the expression of AHR mRNA and protein levels, supporting a novel mechanism whereby lead in the skeleton may increase the sensitivity of bone cells to toxicant exposure.
These data imply that the AHR mediates the effects of aromatic toxicants on bone and that AHR expression is regulated during osteoblast differentiation.
芳烃受体（AHR）介导了许多芳香烃的毒理学效应。我们发现，成骨细胞中的AHR表达与参与成熟过程的早期骨特异性基因的诱导平行。AHR不仅可能介导毒物的效应，还可能通过一种尚未确定的配体参与成骨细胞的分化途径。
代谢性骨病是由于骨细胞活动失衡所致。最近的证据表明，环境毒物可能是改变这些活动的促成因素。一种被认为介导外源性化合物毒性作用的候选分子是芳烃受体（AHR）。
通过定量PCR、蛋白质印迹和免疫组织化学分析从新生大鼠颅骨分离的成骨细胞中的AHR表达。此外，通过电泳迁移率凝胶移位分析和荧光显微镜评估AHR激活情况。
我们的研究结果显示，体内成熟成骨细胞中有AHR表达。AHR表达模式在碱性磷酸酶之后、骨钙素诱导之前达到峰值。我们首次表明，AHR在成骨细胞中作为一种反式激活受体发挥作用，这可通过其依赖配体向细胞核的迁移以及与已知二噁英反应元件的结合得到证明。2,3,7,8-四氯二苯并对二噁英（TCDD）激活AHR介导了细胞色素p450 1A1和环氧化酶-2蛋白水平的诱导。这种效应可被强效AHR拮抗剂3',4'-甲氧基硝基黄酮抑制。此外，铅处理成骨细胞会上调AHR mRNA和蛋白水平的表达，这支持了一种新机制，即骨骼中的铅可能会增加骨细胞对毒物暴露的敏感性。
这些数据表明，AHR介导了芳香族毒物对骨骼的影响，并且AHR表达在成骨细胞分化过程中受到调控。
