Source Data LabZhang

Background
Fork-head box protein M1 (FOXM1) plays critical roles in development and progression of multiple cancers, including hepatocellular carcinoma (HCC). However, the exact regulatory hierarchy of FOXM1 remains unclear. Here, a genome-wide screen is performed to identify intranuclear proteins that promote FOXM1 transcription activity via liquid-liquid phase separation (LLPS).
Results
Abnormal spindle-like microcephaly associated (ASPM) is identified to interact with FOXM1 protein via LLPS and enhance its stability by preventing proteasome-mediated degradation. ChIP-sequencing data show ASPM and FOXM1 co-occupy the promoters of multiple genes to promote their transcription, enhancing FOXM1-driven oncogenic progression. In functional experiments, inhibition of ASPM suppresses tumor growth of HCC cells in vivo and in vitro, while overexpression of ASPM has opposite effects. Importantly, reconstitution of FOXM1 partially compensates for the weakened proliferative capacity of HCC cells caused by ASPM silencing. Intriguingly, FOXM1 binds to the promoter region of ASPM and transcriptionally activates ASPM expression in HCC cells. Furthermore, we find that a higher co-expression of ASPM and FOXM1 significantly correlates with poor prognosis in HCC patients. It indicates a double positive feedback loop between ASPM and FOXM1 which coordinately promotes the aggressive progression of HCC.
Conclusions
Collectively, we demonstrate that LLPS and transcriptional regulation form an oncogenic double positive feedback loop between ASPM and FOXM1. This provides a rationale strategy to treat HCC by targeting this mechanism.

研究背景
叉头框蛋白M1（Fork-head box protein M1, FOXM1）在包括肝细胞癌（hepatocellular carcinoma, HCC）在内的多种癌症的发生与进展中发挥关键作用。然而，FOXM1的确切调控层级仍不明确。本研究通过全基因组筛选，鉴定出通过液-液相分离（liquid-liquid phase separation, LLPS）调控FOXM1转录活性的核内蛋白质。
研究结果
本研究发现异常纺锤样小头畸形相关蛋白（Abnormal spindle-like microcephaly associated, ASPM）通过液-液相分离与FOXM1蛋白相互作用，并通过抑制蛋白酶体介导的降解增强FOXM1的稳定性。染色质免疫沉淀测序（ChIP-sequencing, ChIP-seq）数据显示，ASPM与FOXM1共同结合于多个基因的启动子区域以促进其转录，增强FOXM1驱动的肿瘤进展。功能实验表明，抑制ASPM可在体内外抑制肝癌细胞的肿瘤生长，而过表达ASPM则产生相反效应。值得注意的是，重新表达FOXM1可部分补偿ASPM沉默导致的肝癌细胞增殖能力减弱。有趣的是，FOXM1可结合ASPM的启动子区域，并在肝癌细胞中转录激活ASPM的表达。此外，研究发现ASPM与FOXM1的高共表达与肝癌患者的不良预后显著相关，提示二者之间存在双正向反馈环路，协同促进肝癌的侵袭性进展。
研究结论
综上，本研究证实液-液相分离与转录调控共同构成了ASPM与FOXM1之间的致癌性双正向反馈环路，为靶向该机制治疗肝细胞癌（HCC）提供了理论依据。