肿瘤鞘脂代谢异常致维甲酸核受体表达缺失与功能失活的研究

The activity of retinoids is mainly mediated by the expression of nuclear receptors, including retinotic acid receptors (RARs) and rexinoid X recepyors (RXRs) etc. However, the loss of nuclear retinoid receptors and function is the common event during development of cancer growth. This status of nuclear receptors does not provide the work base to retinoids for exerting their activities of the inhibition of cancer growth. Until now, the mechanisms of loss of nuclear retinoid receptors and function have not been understood. In our previous studies, we found that sphingosine 1-phosphate (S1P) antagonized the effect of all-trans retinoic acid (ATRA) in a colon cancer cell line. Accordingly, we suggest that the loss of nuclear receptor expression might associate with its high levels of S1P and Sphks (sphingosine kinase) in cancer cells. In this study, we aim to establish the relationship between the abnormal metabolism of sphingolipids and the loss of nuclear retinoid receptors expression and its function. The levels of RARs and RXRs will be examined by Western blot assay. Reporter gene assay will be employed to analyze the transcription activity of RA-responsive element (RARE). After degradation by MG132, the transient transfection of flag-RXRα and flag-RARβ will employ to analyze the activities of acetylation and ubiquitylation in the nuclear receptors by S1P. Further assays will perform to study the mechanism of ubiquitination or acetylation that is mediated by the linkage of TRAF, RNF8 and HDAC1 with RAR or RXR. The assays of siRNA and transfection will be used to study the roles of Sphk subtypes in the antagonistic effects of S1P. Overall, the mechanism of loss of nuclear retinoid receptors expression and function will be understand and the association with the abnormal metabolism of sphingolipids will be established. We will clarify the mechanism that S1P and Sphks increase the ligand-dependent degradation of RXRα and RARβ and the modification effects during the treatment of retinoids. These studies will provide a helpful clue to find the new methods of overcoming the resistance against retinoids in cancer cells. This study will also find the useful targets to provide for designing new derivatives of retinoids.

维甲酸类药物活性主要基于核内维甲酸受体（RARs）和维甲酸X受体（RXRs）表达而实现，而维甲酸受体表达缺失是癌发生与发展中的常发事件。在发现鞘脂代谢物S1P拮抗全反式维甲酸ATRA抗癌活性基础上，我们认定这可能与高表达的S1和Sphks促进核受体配体依赖性降解和修饰作用有关。拟研究癌细胞鞘脂代谢异常与核受体表达缺失和功能失活之间相关性，阐明S1P介导的受体配体依赖性降解机制。以Western blot等分析RARs和RXRs表达水平，荧光素酶报告基因分析RXR/RAR异二聚体识别元件转录活性，转染flag-RXRα和flag-RARβ分析S1P介导核受体降解的泛素化或乙酰化修饰机制，进一步阐明该机制与具有泛素连接酶活性蛋白和乙酰化修饰相关蛋白介导有关。以转染和siRNA法分析Sphks亚型在核受体缺失中的角色。该研究为逆转肿瘤对维甲酸类药物耐药性提出思路，为新药靶点设计提供生物学信息。

维甲酸类药物活性主要基于核受体RARs和X受体RXRs正常表达而实现,而维甲酸核受体缺失是癌症发生发展过程中的常发事件，其机制不清。我们发现，SphK2或S1P过表达导致结癌细胞对维甲酸类药物耐药性，其机制与其促进受体配体依赖性降解RARs和RXRs有关，致使RARs/RXRs异二聚体或RARE形成受阻。.1. 发现Sphk2及产物S1P促进RAR乙酰化，引起RXR/RAR二聚体降解： .（1）Sphk2及S1P能够拮抗ATRA对肿瘤细胞的抑制活性。.（2）S1P拮抗ATRA诱导RARβ表达：ATRA能使肿瘤细胞RARβ表达量增加，而S1P则抑制ATRA对RARβ诱导作用。以荧光素酶报告基因还发现，S1P明显抑制ATRA诱导RARE转录。.（3）Sphk2能够与RXRα结合并抑制ATRA和LGD1069对RXR/RAR激活：在HT-29细胞转染HA-Sphk2和GFP-RXRα均位于核内，在TPA刺激下进入胞质。以免疫共沉淀实验发现Sphk2与RXRα结合，进一步显示Sphk2抑制了ATRA和LGD1069诱导RARβ转录活性。.（4）Sphk2产物S1P促进维甲酸受体配体依赖性降解，其机制与其促进核受体乙酰化修饰有关：在过表达Sphk2细胞，以抗乙酰化赖氨酸抗体分析发现，以ATRA治疗时发现其核受体RARβ发生乙酰化修饰，致使产生RARβ受体配体依赖性降解。总之，高表达Sphk2和S1P拮抗维甲酸类药物ATRA对肿瘤抑制活性。高表达Sphk2和S1P通过受体配体依赖性乙酰化修饰降解RARs和RXRs，致使RARs/RXRs异二聚体或RARE形成受阻。.2. 过表达SphK2对结肠癌核受体RARs和RXRs及RXRs-RARs heterodimers功能影响：构建细胞系HCT-116SphK2，体内外实验证实SphK2介导对ATRA产生耐药性。当ATRA与HCT-116SphK2细胞接触时诱导RXRα由核内向胞质转移并产生降解，其降解过程发生在翻译后修饰水平。深入研究发现，SphK2能够促使在K48和K63赖氨酸连接位点发生多聚泛素化修饰，其中与K48赖氨酸连接位点相连RXRα通过泛素蛋白酶体途径降解。SphK2过表达还能促进RXRα与K63赖氨酸连接位点结合，通过多聚泛素化修饰介导自噬降解，因而SphK2还能通过自噬降解机制促进RXRα降解。