Intestinal stem cell overproliferation resulting from inactivation of the APC tumor suppressor requires the transcription cofactors Earthbound and Erect wing

Wnt/β-catenin signal transduction directs intestinal stem cell (ISC) proliferation during homeostasis. Hyperactivation of Wnt signaling initiates colorectal cancer, which most frequently results from truncation of the tumor suppressor Adenomatous polyposis coli (APC). The β-catenin-TCF transcription complex activates both the physiological expression of Wnt target genes in the normal intestinal epithelium and their aberrantly increased expression in colorectal tumors. Whether mechanistic differences in the Wnt transcription machinery drive these distinct levels of target gene activation in physiological versus pathological states remains uncertain, but is relevant for the design of new therapeutic strategies. Here, using a Drosophila model, we demonstrate that two evolutionarily conserved transcription cofactors, Earthbound (Ebd) and Erect wing (Ewg), are essential for all major consequences of Apc1 inactivation in the intestine: the hyperactivation of Wnt target gene expression, excess number of ISCs, and hyperplasia of the epithelium. In contrast, only Ebd, but not Ewg, mediates the Wnt-dependent regulation of ISC proliferation during homeostasis. Therefore, in the adult intestine, Ebd acts independently of Ewg in physiological Wnt signaling, but cooperates with Ewg to induce the hyperactivation of Wnt target gene expression following Apc1 loss. These findings have relevance for human tumorigenesis, as Jerky (JRK/JH8), the human Ebd homolog, promotes Wnt pathway hyperactivation and is overexpressed in colorectal, breast, and ovarian cancers. Together, our findings reveal distinct requirements for Ebd and Ewg in physiological Wnt pathway activation versus oncogenic Wnt pathway hyperactivation following Apc1 loss. Such differentially utilized transcription cofactors may offer new opportunities for the selective targeting of Wnt-driven cancers.

Wnt/β-连环蛋白信号转导（Wnt/β-catenin signal transduction）在机体稳态过程中调控肠道干细胞（intestinal stem cell, ISC）的增殖。Wnt信号通路过度激活可诱发结直肠癌，其最常见的分子机制为肿瘤抑制因子腺瘤性结肠息肉病蛋白（Adenomatous polyposis coli, APC）发生截短突变。β-连环蛋白-TCF转录复合物（β-catenin-TCF transcription complex）既能介导正常肠上皮中Wnt靶基因（Wnt target gene）的生理性表达，也可在结直肠肿瘤中驱动其异常高表达。目前尚不明确Wnt转录机器的机制差异是否会导致生理与病理状态下靶基因激活水平的显著分化，但该问题对于新型治疗策略的开发具有重要参考价值。本研究以果蝇（Drosophila）为实验模型，证实两个进化保守的转录辅因子（transcription cofactor）——地球结合蛋白（Earthbound, Ebd）与直立翅蛋白（Erect wing, Ewg）——对于肠道中Apc1失活引发的全部主要表型均不可或缺：包括Wnt靶基因表达的过度激活、肠道干细胞数量异常增多以及上皮组织增生。与之形成鲜明对比的是，在机体稳态过程中，仅Ebd（而非Ewg）参与Wnt信号通路对肠道干细胞增殖的调控。由此可见，在成年肠道中，Ebd在生理性Wnt信号转导过程中不依赖于Ewg发挥功能，但在Apc1缺失后，Ebd会与Ewg协同诱导Wnt靶基因的过度激活。上述研究结果与人类肿瘤发生（tumorigenesis）密切相关，人类Ebd的同源蛋白Jerky（JRK/JH8）可促进Wnt通路过度激活，且在结直肠癌、乳腺癌及卵巢癌中呈现高表达状态。综上，本研究揭示了生理性Wnt通路激活与Apc1缺失后致癌性Wnt通路过度激活这两种过程中，Ebd与Ewg的不同功能需求模式。这类具有差异化调控功能的转录辅因子，有望为靶向Wnt通路驱动的癌症提供全新的治疗机遇。