Cell-type restricted activity of hnRNPM promotes breast cancer metastasis via regulating alternative splicing

Tumor metastasis remains the major cause of cancer-related death, but its molecular basis is still not well understood. Here we uncovered a splicing-mediated pathway that is essential for breast cancer metastasis. We show that the RNA-binding protein hnRNPM promotes breast cancer metastasis by activating the switch of alternative splicing that occurs during epithelial-mesenchymal transition (EMT). Genome-wide deep sequencing analysis suggests that hnRNPM potentiates TGFb signaling and identifies CD44 as a key downstream target of hnRNPM. hnRNPM ablation prevents TGFb-induced EMT and inhibits breast cancer metastasis in mice, whereas enforced expression of the specific CD44s splice isoform overrides the loss of hnRNPM and permits EMT and metastasis. Mechanistically, we demonstrate that the ubiquitously expressed hnRNPM acts in a mesenchymal-specific manner to precisely control CD44 splice isoform switching during EMT. This restricted cell-type activity of hnRNPM is achieved by competition with ESRP1, an epithelial-splicing regulator that binds to the same cis-regulatory RNA elements and is repressed during EMT. Importantly, hnRNPM is associated with aggressive breast cancer and correlates with increased CD44s in patient specimens. These findings demonstrate a novel molecular mechanism through which tumor metastasis is endowed by the hnRNPM-mediated splicing program. RNAseq for control, hnRNPM siRNA treated lung metastatic LM2 clonal line, derived from the mesenchymal MDA-MB-231 cells

肿瘤转移仍是癌症相关死亡的首要诱因，但其分子机制尚未完全阐明。本研究揭示了一条对乳腺癌转移至关重要的剪接介导通路。研究发现，RNA结合蛋白hnRNPM通过激活上皮间质转化（EMT）过程中发生的可变剪接转换，促进乳腺癌转移。全基因组深度测序分析显示，hnRNPM可增强转化生长因子β（TGFβ）信号通路，并确定CD44是hnRNPM的关键下游靶标。敲除hnRNPM可阻断TGFβ诱导的EMT过程，并抑制小鼠体内的乳腺癌转移；而特异性过表达CD44s剪接异构体，可逆转hnRNPM缺失所带来的影响，恢复EMT过程与转移能力。机制层面上，本研究证实，广泛表达的hnRNPM以间质细胞特异性的方式，精准调控EMT过程中CD44剪接异构体的转换。hnRNPM的这种细胞类型限制性活性，是通过与ESRP1竞争结合相同的顺式调控RNA元件实现的；ESRP1是一种上皮剪接调控因子，在EMT过程中其表达会被抑制。值得注意的是，hnRNPM的表达与侵袭性乳腺癌密切相关，并与患者标本中CD44s的水平升高呈正相关。上述研究发现揭示了一种由hnRNPM介导的剪接程序赋予肿瘤转移能力的全新分子机制。本研究的RNA测序样本涵盖对照组、经hnRNPM小干扰RNA（siRNA）处理的肺转移LM2克隆株，该克隆株源自间质型MDA-MB-231细胞。