近日，国际著名杂志PLoS One在线刊登了厦门大学药学院的研究人员的最新研究成果“Antagonist Effect of Triptolide on AKT Activation by Truncated Retinoid X Receptor-alpha，”，文章中，研究者从传统中草药雷公藤分离出活性成分雷公藤甲素（triptolide），并在体内外实验中证实其可以调控tRXRα介导的癌细胞存活信号通路。
厦门大学的曾锦章教授和张晓坤教授为这篇文章的共同通讯作者。陆娜和刘金星两位硕士研究生是该文章的共同第一作者。
从天然产物中寻找抗肿瘤活性成分是发展抗肿瘤药物的重要策略之一，雷公藤是我国一种资源比较丰富的传统中草药，其结构多样的有效成分具有明显的抗炎、免疫抑制和抗肿瘤等作用，是开发治疗药物的一个宝藏，但雷公藤具有很大的毒性，分子靶点和作用机制不清楚，是迄今制约雷公藤发展成真正治疗药物的关键。
在这篇文章中，研究人员从中药雷公藤中分离出活性成分雷公藤甲素，并证实其调控了tRXRα介导的癌细胞存活信号通路。研究结果显示雷公藤甲素依赖于细胞内的tRXRα表达水平强有力地诱导了癌细胞凋亡，证实了tRXRα是雷公藤甲素一个重要的细胞内靶点。
tRXRα是核受体视黄醇X受体-α（RXRα）的一种在其N-端截短的突变体，普遍存在于各种肿瘤组织中，与其全长RXRα?主要定位于细胞核不同，tRXRα?通常转位于细胞质，通过与p85相互作用激活PI3K/AKT信号转导通路，是肿瘤微环境中大量表达的肿瘤坏死因子TNFα?所依赖的生存通道，tRXRα的高度表达对于肿瘤的生长有重要的促进作用。
研究人员证实雷公藤甲素选择性地诱导了tRXRα降解，并抑制了tRXRα依赖的AKT活性，但却没有影响全长的RXRα。研究人员还证实雷公藤甲素靶向tRXRα强有力地激活了TNFα死亡信号，并促进了其他化疗的抗癌活性。
新研究确定了雷公藤甲素是tRXRα依赖的存活信号通路的一个新的调控物质，从而提供了关于雷公藤甲素作用诱导癌细胞凋亡机制的新见解。雷公藤甲素代表了具有不良副效应的传统中草药天然产物最有希望的一个治疗先导物。新研究发现为开发出用于癌症治疗的改良雷公藤甲素类似物提供了分子基础和新方向。（生物谷Bioon.com）
doi:10.1371/journal.pone.0035722
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Antagonist Effect of Triptolide on AKT Activation by Truncated Retinoid X Receptor-alpha
Na Lu1#, Jinxing Liu1#, Jie Liu1, Chunyun Zhang1, Fuquan Jiang1, Hua Wu1, Liqun Chen1, Wenjun Zeng1, Xihua Cao2, Tingdong Yan1, Guanghui Wang1, Hu Zhou2, Bingzhen Lin2, Xiaomei Yan3, Xiao-kun Zhang1,2*, Jin-Zhang Zeng1*
Background Retinoid X receptor-alpha (RXRα) is a key member of the nuclear receptor superfamily. We recently demonstrated that proteolytic cleavage of RXRα resulted in production of a truncated product, tRXRα, which promotes cancer cell survival by activating phosphatidylinositol-3-OH kinase (PI3K)/AKT pathway. However, how the tRXRα-mediated signaling pathway in cancer cells is regulated remains elusive. Methodology/Principal Findings We screened a natural product library for tRXRα targeting leads and identified that triptolide, an active component isolated from traditional Chinese herb Trypterygium wilfordii Hook F, could modulate tRXRα-mediated cancer cell survival pathway in vitro and in animals. Our results reveal that triptolide strongly induces cancer cell apoptosis dependent on intracellular tRXRα expression levels, demonstrating that tRXRα serves as an important intracellular target of triptolide. We show that triptolide selectively induces tRXRα degradation and inhibits tRXRα-dependent AKT activity without affecting the full-length RXRα. Interestingly, such effects of triptolide are due to its activation of p38. Although triptolide also activates Erk1/2 and MAPK pathways, the effects of triptolide on tRXRα degradation and AKT activity are only reversed by p38 siRNA and p38 inhibitor. In addition, the p38 inhibitor potently inhibits tRXRα interaction with p85α leading to AKT inactivation. Our results demonstrate an interesting novel signaling interplay between p38 and AKT through tRXRα mediation. We finally show that targeting tRXRα by triptolide strongly activates TNFα death signaling and enhances the anticancer activity of other chemotherapies Conclusions/Significance Our results identify triptolide as a new xenobiotic regulator of the tRXRα-dependent survival pathway and provide new insight into the mechanism by which triptolide acts to induce apoptosis of cancer cells. Triptolide represents one of the most promising therapeutic leads of natural products of traditional Chinese medicine with unfortunate side-effects. Our findings will offer new strategies to develop improved triptolide analogs for cancer therapy.
