SUMOylation of the Forkhead Transcription Factor FOXL2 Promotes Its Stabilization/Activation through Transient Recruitment to PML Bodies

BackgroundFOXL2 is a transcription factor essential for ovarian development and maintenance. It is mutated in the genetic condition called Blepharophimosis Ptosis Epicantus inversus Syndrome (BPES) and in cases of isolated premature ovarian failure. We and others have previously shown that FOXL2 undergoes several post-translational modifications.
Methods and Principal FindingsHere, using cells in culture, we show that interference with FOXL2 SUMOylation leads to a robust inhibition of its transactivation ability, which correlates with a decreased stability. Interestingly, FOXL2 SUMOylation promotes its transient recruitment to subnuclear structures that we demonstrate to be PML (Promyelocytic Leukemia) Nuclear Bodies. Since PML bodies are known to be sites where post-translational modifications of nuclear factors take place, we used tandem mass spectrometry to identify new post-translational modifications of FOXL2. Specifically, we detected four phosphorylated, one sulfated and three acetylated sites.
ConclusionsBy analogy with other transcription factors, we propose that PML Nuclear Bodies might transiently recruit FOXL2 to the vicinity of locally concentrated enzymes that could be involved in the post-translational maturation of FOXL2. FOXL2 acetylation, sulfation, phosphorylation as well as other modifications yet to be discovered might alter the transactivation capacity of FOXL2 and/or its stability, thus modulating its global intracellular activity.

研究背景
FOXL2是一种对卵巢发育与维持至关重要的转录因子（transcription factor）。其突变可引发名为睑裂狭小-上睑下垂-倒向内眦赘皮综合征（Blepharophimosis Ptosis Epicantus inversus Syndrome, BPES）的遗传性疾病，也可见于单纯性卵巢早衰病例。本团队及其他研究组此前已证实，FOXL2存在多种翻译后修饰（post-translational modifications）。

研究方法与主要发现
本文利用培养细胞开展实验，结果显示，干扰FOXL2的SUMO化修饰（SUMOylation）会显著抑制其反式激活（transactivation）能力，且该效应与其稳定性降低呈相关性。值得注意的是，FOXL2的SUMO化修饰可促进其暂时性招募至核内亚结构，经证实该结构为早幼粒细胞白血病（Promyelocytic Leukemia, PML）核体（Nuclear Bodies）。鉴于PML核体是核因子发生翻译后修饰的重要位点，我们采用串联质谱（tandem mass spectrometry）技术对FOXL2的新型翻译后修饰位点进行了鉴定，最终检测到4个磷酸化位点、1个硫酸化位点及3个乙酰化位点。

研究结论
类比其他转录因子，我们提出PML核体可能暂时性将FOXL2招募至局部高浓度酶类的富集区域，而这类酶可能参与FOXL2的翻译后成熟过程。FOXL2的乙酰化、硫酸化、磷酸化及尚未被发现的其他修饰，或可改变其反式激活能力和/或稳定性，进而调控其整体细胞内活性。