Singhal Rohit, Badger Thomas M, Ronis Martin J
Department of Pharmacology and Toxicology, University of Arkansas for Medical Sciences, Little Rock, AR 72205, USA.
Toxicol Appl Pharmacol. 2008 Mar 1;227(2):275-83. doi: 10.1016/j.taap.2007.11.009. Epub 2007 Nov 21.
Consumption of soy diets has been found to reduce cancer incidence in animals and is associated with reduced cancer risk in humans. Previously, we have demonstrated that female Sprague-Dawley rats fed purified AIN-93G diets with soy protein isolate (SPI) as the sole protein source had reduced CYP1A1 induction and basal aryl hydrocarbon receptor (AhR) levels relative to those fed the same diet containing casein (CAS). In the present study, the molecular mechanisms underlying reduced AhR expression have been studied. The SPI-effect on AhR was not observed after feeding diets containing the purified soy isoflavones genistein or daidzein. Rat hepatoma FGC-4 cells were treated with the serum obtained from rats fed CAS- or SPI-containing diets. Reduced AhR levels (P<0.05) were observed after 24 h exposure to SPI-serum without any changes in the overall expression of chaperone proteins--HSP90 and XAP2. SPI-serum-stimulated AhR degradation was inhibited by treating the cells with the proteasome inhibitor, MG132, and was observed to be preceded by ubiquitination of the receptor. A reduced association of XAP2 with the immunoprecipitated AhR complex was observed. SPI-serum-mediated AhR degradation was preceded by nuclear translocation of the receptor. However, the translocated receptor was found to be unable to heterodimerize with ARNT or to bind to XRE elements on the CYP1A1 enhancer. These data suggest that feeding SPI-containing diets antagonizes AhR signaling by a novel mechanism which differs from those established for known AhR antagonists.
已发现食用大豆饮食可降低动物的癌症发病率，并与降低人类患癌风险相关。此前，我们已证明，以大豆分离蛋白（SPI）作为唯一蛋白质来源，喂食纯化AIN-93G饮食的雌性斯普拉格-道利大鼠，相对于喂食含酪蛋白（CAS）的相同饮食的大鼠，其CYP1A1诱导和基础芳烃受体（AhR）水平降低。在本研究中，对AhR表达降低的分子机制进行了研究。在喂食含有纯化大豆异黄酮染料木黄酮或大豆苷元的饮食后，未观察到SPI对AhR的影响。用喂食含CAS或SPI饮食的大鼠血清处理大鼠肝癌FGC-4细胞。暴露于SPI血清24小时后，观察到AhR水平降低（P<0.05），而伴侣蛋白HSP90和XAP2的整体表达无任何变化。用蛋白酶体抑制剂MG132处理细胞可抑制SPI血清刺激的AhR降解，且观察到该降解发生在受体泛素化之前。观察到XAP2与免疫沉淀的AhR复合物的结合减少。SPI血清介导的AhR降解发生在受体核转位之前。然而，发现转位的受体无法与ARNT异源二聚化或与CYP1A增强子上的XRE元件结合。这些数据表明，喂食含SPI的饮食通过一种不同于已知AhR拮抗剂的新机制拮抗AhR信号传导。
