The homologous to E6AP C-terminus (HECT) domain and RCC1-like domain-containing (HERC) proteins can function as tumour suppressors and as oncogenes, depending on the cancer type. However, the expression patterns of HERCs in colorectal cancer (CRC) cells are unclear. Here, we show that only HERC1 and HERC5 are downregulated in CRC tumours, and we focus our study on revealing HERC5-mediating signalling because the change in downregulation is much more obvious for HERC5 than for HERC1. We demonstrate that HERC5 recruits an adaptor protein, CREB-binding protein, to ubiquitinate C-terminal binding protein 1 (CtBP1) in non-cancerous colon cells. The downregulation of HERC5 in CRC cells attenuates the ubiquitination of CtBP1, which then accumulates and assembles into a transcriptional complex with histone deacetylase 1 and a transcription factor c-MYC. This transcriptional complex binds to the promoters of three proapoptotic genes, Bcl2 associated X (BAX), Bcl2 interacting killer (BIK) and p53upregulated modulator of apoptosis (PUMA), and inhibits their expression, thereby suppressing apoptotic signalling and promoting tumourigenesis. Overexpression of HERC5, downregulation of CtBP1 or blocking of the CtBP1 function with its inhibitors (NSC95397 and 4-methylthio-2-oxobutyric acid [MTOB]) significantly prevents CRC cell proliferation in vitro and tumour growth in vivo. Combining NSC95397 (or MTOB) with chemotherapeutic drugs (oxaliplatin or capecitabine) gives a much stronger inhibition of cell proliferation and tumour growth compared with their single treatments. Collectively, our results reveal that downregulation of HERC5 E3 ligase attenuates the ubiquitination of CtBP1 to inhibit apoptosis. Therefore, CtBP1 may be a promising target in CRC chemotherapy.

中文翻译：

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HERC5 E3 连接酶的下调减弱了 CtBP1 的泛素化以抑制结直肠癌细胞的凋亡。

根据癌症类型，与 E6AP C 端 (HECT) 结构域和 RCC1 样结构域 (HERC) 同源的蛋白质可以作为肿瘤抑制基因和致癌基因发挥作用。然而，HERCs 在结直肠癌 (CRC) 细胞中的表达模式尚不清楚。在这里，我们表明只有 HERC1 和 HERC5 在 CRC 肿瘤中被下调，我们的研究重点是揭示 HERC5 介导的信号传导，因为 HERC5 的下调变化比 HERC1 更明显。我们证明 HERC5 在非癌性结肠细胞中招募一种接头蛋白 CREB ​​结合蛋白来泛素化 C 端结合蛋白 1 (CtBP1)。CRC 细胞中 HERC5 的下调减弱了 CtBP1 的泛素化，然后它与组蛋白脱乙酰酶 1 和转录因子 c-MYC 积累并组装成转录复合物。这种转录复合物与三个促凋亡基因的启动子结合，即 Bcl2 相关 X (BAX)、Bcl2 相互作用杀手 (BIK) 和 p53 凋亡上调调节剂 (PUMA)，并抑制它们的表达，从而抑制凋亡信号传导并促进肿瘤发生。HERC5 的过表达、CtBP1 的下调或用其抑制剂（NSC95397 和 4-甲硫基-2-氧代丁酸 [MTOB]）阻断 CtBP1 的功能可显着阻止体外 CRC 细胞增殖和体内肿瘤生长。与单一治疗相比，将 NSC95397（或 MTOB）与化疗药物（奥沙利铂或卡培他滨）结合使用可更有效地抑制细胞增殖和肿瘤生长。总的来说，我们的结果表明 HERC5 E3 连接酶的下调减弱了 CtBP1 的泛素化以抑制细胞凋亡。