Department of Biochemistry & Microbiology, University of Victoria, Victoria, British Columbia, Canada.
PLoS One. 2011;6(12):e28658. doi: 10.1371/journal.pone.0028658. Epub 2011 Dec 5.
INhibitor of Growth (ING) proteins belong to a large family of plant homeodomain finger-containing proteins important in epigenetic regulation and carcinogenesis. We have previously shown that ING1 and ING2 expression is regulated by thyroid hormone (TH) during metamorphosis of the Xenopus laevis tadpole. The present study investigates the possibility that ING proteins modulate TH action.
METHODOLOGY/PRINCIPAL FINDINGS: Tadpoles expressing a Xenopus ING2 transgene (Trans(ING2)) were significantly smaller than tadpoles not expressing the transgene (Trans(GFP)). When exposed to 10 nM 3,5,3'-triiodothyronine (T(3)), premetamorphic Trans(ING2) tadpoles exhibited a greater reduction in tail, head, and brain areas, and a protrusion of the lower jaw than T(3)-treated Trans(GFP) tadpoles. Quantitative real time polymerase chain reaction (QPCR) demonstrated elevated TH receptor β (TRβ) and TH/bZIP transcript levels in Trans(ING2) tadpole tails compared to Trans(GFP) tadpoles while TRα mRNAs were unaffected. In contrast, no difference in TRα, TRβ or insulin-like growth factor (IGF2) mRNA abundance was observed in the brain between Trans(ING2) and Trans(GFP) tadpoles. All of these transcripts, except for TRα mRNA in the brain, were inducible by the hormone in both tissues. Oocyte transcription assays indicated that ING proteins enhanced TR-dependent, T(3)-induced TRβ gene promoter activity. Examination of endogenous T(3)-responsive promoters (TRβ and TH/bZIP) in the tail by chromatin immunoprecipitation assays showed that ING proteins were recruited to TRE-containing regions in T(3)-dependent and independent ways, respectively. Moreover, ING and TR proteins coimmunoprecipitated from tail protein homogenates derived from metamorphic climax animals.
CONCLUSIONS/SIGNIFICANCE: We show for the first time that ING proteins modulate TH-dependent responses, thus revealing a novel role for ING proteins in hormone signaling. This has important implications for understanding hormone influenced disease states and suggests that the induction of ING proteins may facilitate TR function during metamorphosis in a tissue-specific manner.
生长抑制因子（ING）蛋白属于植物同源结构域指状蛋白家族，在表观遗传调控和致癌作用中具有重要作用。我们之前已经表明，在非洲爪蟾蝌蚪变态过程中，甲状腺激素（TH）调节 ING1 和 ING2 的表达。本研究探讨了 ING 蛋白是否调节 TH 作用的可能性。
方法/主要发现：表达非洲爪蟾 ING2 转基因（Trans（ING2））的蝌蚪明显小于未表达转基因的蝌蚪（Trans（GFP））。当暴露于 10 nM 3,5,3'-三碘甲状腺原氨酸（T（3））时，前变态期 Trans（ING2）蝌蚪的尾巴、头部和大脑区域减少更大，下颌突出比 T（3）处理的 Trans（GFP）蝌蚪。定量实时聚合酶链反应（QPCR）显示，与 Trans（GFP）蝌蚪相比，Trans（ING2）蝌蚪尾巴中的 TH 受体 β（TRβ）和 TH/bZIP 转录物水平升高，而 TRα mRNA 不受影响。相反，在 Trans（ING2）和 Trans（GFP）蝌蚪的大脑中，没有观察到 TRα、TRβ 或胰岛素样生长因子（IGF2）mRNA 丰度的差异。除了大脑中的 TRα mRNA 外，这些转录物在两种组织中均被激素诱导。卵母细胞转录测定表明，ING 蛋白增强了 TR 依赖性、T（3）诱导的 TRβ 基因启动子活性。通过染色质免疫沉淀测定检查尾部内源性 T（3）反应启动子（TRβ 和 TH/bZIP），结果表明 ING 蛋白分别以依赖于 TRE 和非依赖于 TRE 的方式募集到 T（3）依赖性和非依赖性区域。此外，从变态高峰期动物的尾部蛋白匀浆中免疫沉淀 ING 和 TR 蛋白。
结论/意义：我们首次表明，ING 蛋白调节 TH 依赖性反应，从而揭示了 ING 蛋白在激素信号中的新作用。这对于理解受激素影响的疾病状态具有重要意义，并表明在组织特异性方式下，ING 蛋白的诱导可能有助于 TR 在变态过程中的功能。
