Epithelial splicing regulatory protein 1 (ESRP1) is an RNA binding protein that governs the alternative splicing events related to epithelial phenotypes. ESRP1 contributes significantly at different stages of cancer progression. ESRP1 expression is substantially elevated in carcinoma in situ compared to the normal epithelium, whereas it is drastically ablated in cancer cells within hypoxic niches, which promotes epithelial to mesenchymal transition (EMT). Although a considerable body of research sought to understand the EMT-associated ESRP1 downregulation, the regulatory mechanisms underlying ESRP1 upregulation in primary tumors remained largely uncharted. This study seeks to unveil the regulatory mechanisms that spatiotemporally fine-tune the ESRP1 expression during breast carcinogenesis. Our results reveal that an elevated expression of transcription factor E2F1 and increased CpG hydroxymethylation of the E2F1 binding motif conjointly induce ESRP1 expression in breast carcinoma. However, E2F1 fails to upregulate ESRP1 despite its abundance in oxygen-deprived breast cancer cells. Mechanistically, impelled by the hypoxia-driven reduction of tet methylcytosine dioxygenase 3 (TET3) activity, CpG sites across the E2F1 binding motif lose the hydroxymethylation marks while gaining the de novo methyltransferase-elicited methylation marks. These two oxygen-sensitive epigenetic events work in concert to repel E2F1 from the ESRP1 promoter, thereby diminishing ESRP1 expression under hypoxia. Furthermore, E2F1 skews the cancer spliceome by upregulating splicing factor SRSF7 in hypoxic breast cancer cells. Our findings provide previously unreported mechanistic insights into the plastic nature of ESRP1 expression and insinuate important implications in therapeutics targeting breast cancer progression.

上皮剪接调节蛋白 1 (ESRP1) 是一种 RNA 结合蛋白，控制与上皮表型相关的选择性剪接事件。ESRP1 在癌症进展的不同阶段有显着贡献。与正常上皮相比，原位癌中 ESRP1 的表达显着升高，而在缺氧壁龛内的癌细胞中却被彻底消融，从而促进上皮间质转化 (EMT)。尽管大量研究试图了解与 EMT 相关的 ESRP1 下调，但原发性肿瘤中 ESRP1 上调的潜在调控机制在很大程度上仍然未知。本研究旨在揭示在乳腺癌发生过程中时空微调 ESRP1 表达的调控机制。我们的结果表明，转录因子 E2F1 的表达升高和 E2F1 结合基序的 CpG 羟甲基化增加共同诱导乳腺癌中 ESRP1 的表达。然而，尽管 E2F1 在缺氧乳腺癌细胞中含量丰富，但它未能上调 ESRP1。从机制上讲，在缺氧驱动的 tet 甲基胞嘧啶双加氧酶 3 (TET3) 活性降低的推动下，跨 E2F1 结合基序的 CpG 位点失去了羟甲基化标记，同时获得了从头甲基转移酶引发的甲基化标记。这两个氧敏感的表观遗传事件协同作用从 ESRP1 启动子中排斥 E2F1，从而减少缺氧条件下 ESRP1 的表达。此外，E2F1 通过上调缺氧乳腺癌细胞中的剪接因子 SRSF7 来扭曲癌症剪接组。