As the primary cause of cancer-related deaths, cancer metastasis is a complicated process in which multiple signal pathways are orchestrated to foster the success of multi-steps involving local invasion, intravasation, transport, extravasation and colonization in distant vital organs. Breast cancer (BC) is one of the most common cancers among women worldwide [1], among which triple-negative breast cancer (TNBC) is the most malignant subtype with high metastatic potential and recurrence as well as limited response to current therapies [2]. TNBC cells and tumor specimens has been shown to exhibit an activated epithelial-mesenchymal transition (EMT) program, which endows cancer cells with invasive properties and critically contributes to malignant progression [3,4].

Snail1, as a key EMT-Transcription factor, is pivotal to repress E-cadherin, trigger EMT and metastasis in BC [[5], [6], [7]]. And the frequent overexpression of Snail1 in TNBC has been demonstrated to drive EMT and metastasis of TNBC by various xenograft and patient-derived xenograft (PDX) mouse models, suggesting that blocking EMT and EMT dependent malignancies by targeting Snail1 is a promising strategy in the management of TNBC. However, the development of inhibitors to directly target Snail1 could be a big challenge due to the absence of specific binding pockets and drugs or compounds to directly target Snail1 in cancers are not yet conceivable. Thus, to identify more druggable targets for Snail1-based therapy is urgently needed.

Emerging evidence demonstrates that Snail1 undergoes several different post-translational modifications. For instance, GSK-3β phosphorylates Snail1 at two consensus motifs. The first phosphorylation motif (Ser96, Ser100, and Ser104) leads to the ubiquitination of Snail1 by β-Trcp whose destruction box DSGXXS overlaps with the first GSK-3β phosphorylation motif. The second phosphorylation motif (Ser107, Ser111, Ser115 and Ser119) causes Snail1 to localize in the cytoplasm [8]. Meanwhile, PAK1 phosphorylates Snail1 at Ser246, which could cause the accumulation of Snail1 in the nucleus and consequently promote its repressor activity on E-cadherin in BC [9]. Snail1 was also reported to be acetylated in various cancers. CREB-binding protein (CBP) can acetylate Snail1 at K146 and K187 to prevent the assembly of the repressor complex and increase the stability of Snail1 [10]. In addition, Snail1 can be ubiquitinated and targeted to proteasome-dependent degradation by several E3 ligases such as β-Trcp, FBXL14 and FBXO45 [[11], [12], [13]]. Recent studies have reported the involvement of deubiquitinating enzymes in regulating the stabilization of Snail1 in multiple cancers [[14], [15], [16], [17]].

In this study, we identified OTUD4 as a bona fide deubiquitinase of Snail1. OTUD4 directly deubiquitinated and stabilized Snail1, thereby promoting TNBC cell migration and invasion abilities in vitro as well as lung metastasis in vivo. Overall, our study establishes the OTUD4‐Snail1 axis as an important regulatory mechanism of BC metastasis and provides a rationale for potential therapeutic interventions in the treatment of TNBC.