Ubiquitination in Cancer Metastasis: Emerging Functions,Underlying Mechanisms,and Clinical Implications

Cancer metastasis refers to the dissemination of primary cancer cells into secondary organs. Metastasis is the primary cause of cancer-associated deaths worldwide and remains the most complicated, devastating, and mysterious aspect of cancer. It is a cascade of several steps involving epithelial-mesenchymal transition (EMT), invasion, migration, entry of cancer cells into circulation (intravasation), survival in the circulation, exit from circulation (extravasation), and colonization in the distant secondary organ. The involvement of different posttranslational modifications of proteins in cancer metastasis is widely explored. Ubiquitination is one of the major posttranslational modifications of proteins where the target proteins are tagged with a small protein ubiquitin. Deubiquitination, on the other hand, is the process of removal of ubiquitin from the proteins. Ubiquitination and deubiquitination are mainly regulated by E3 ubiquitin ligases and deubiquitinases (DUBs) respectively. Ubiquitination plays crucial roles in diverse physiological processes by regulating protein localization, stability, activity, and protein-protein interactions. Therefore, deregulation of ubiquitin signaling leads to several pathological conditions including cancer. In the past few decades, a growing body of research has shown that E3 ligases and DUBs are dramatically dysregulated during the advancement of metastatic disease which is strongly associated with a worse patient prognosis as reviewed by Gallo et al. ¹

Numerous studies reveal that E3 ligases and DUBs regulate several steps of the metastatic cascade (majorly EMT, invasion and migration) via diverse mechanisms. E3 ligases function as either metastasis suppressor or promoter by targeting several metastasis-associated proteins for proteasomal degradation as reviewed by Han and colleagues. ² For instance, SKP1-Cullin1-F-box (SCF) E3 ligase FBXW2 suppresses cancer metastasis by targeting β-catenin and EGFR for polyubiquitination and proteasomal degradation.^3,4 On the other hand, tripartite motif-containing protein 65 (TRIM65) promotes colorectal cancer metastasis by targeting ARHGAP35 for polyubiquitination and degradation. ⁵ Some E3 ligases promote metastasis by directing non-proteolytic polyubiquitination of target oncoproteins. For example, E3 ligase TNF receptor-associated factor 4 (TRAF4) overexpressed in metastatic prostate cancer catalyzes the non-proteolytic K27- and K29-linked polyubiquitination of TrkA (a receptor tyrosine kinase) leading to TrkA activation and prostate cancer metastasis. ⁶ Really interesting new gene (RING) E3 ligase RNF2 represses E-cadherin transcription via histone monoubiquitination and thereby promotes hepatocellular carcinoma metastasis indicating that monoubiquitination also plays an important role in metastasis. ⁷ Except a handful DUBs which act as metastasis suppressors, ⁸ most DUBs known till date operate as metastasis promoters by deubiquitinating and thereby stabilizing multiple oncoproteins. ² It is interesting to note that E3 ligases govern the stability of DUBs and vice versa to control cancer spread. For instance, E3 ligase carboxyl terminus of Hsc70-interacting protein (CHIP) polyubiquitinates Ovarian tumor domain-containing protein 3 (OTUD3) (a DUB) and promotes its proteasomal degradation, thereby suppressing lung cancer metastasis. ⁹ Several studies highlight that ubiquitin conjugating E2 enzymes also play a significant role in EMT and metastasis and are closely linked to poor patient prognosis.^10,11

Proteolysis-targeting chimeras (PROTACs) are the hetero-bifunctional small molecules which bring target proteins to the close vicinity of E3 ligases leading to target protein degradation. Recently, the use of PROTACs in a series of clinical studies raises the possibility that oncoproteins (such as KRAS/c-Myc) that were once thought to be “undruggable” may now be targeted, leading to tumor regression and better patient survival. ¹² Deubiquitinase-targeting chimaeras (DUBTACs) that stabilize tumor suppressors are also being developed. ¹³ In this context, recently developed PROTACs cause the degradation of a number of oncoproteins that promote cancer metastasis such as FAK, S100A4, PYK2, and PTK6.^14–16 In addition, molecular glues induce proximity-induced ubiquitination and degradation of oncoproteins leading to metastasis suppression. ¹⁷ Moreover, it has been demonstrated that suppressing metastasis in various tumors can be accomplished by directly or indirectly targeting E2 enzymes, E3 ligases, and DUBs with small-molecule inhibitors.^18,19

It is fair to state that ubiquitination research in cancer metastasis is still in its infancy. In the near future, several important areas need to be investigated. For example, it is yet unclear whether ubiquitination of important metastasis-associated proteins might initiate the metastatic cascade and promote metastatic colonization, which can be used for therapeutic intervention. Furthermore, how ubiquitination affects crucial metastatic processes such as intravasation, extravasation, circulation, mesenchymal-epithelial transition (MET), and metastatic dormancy remains elusive. Intriguingly, humans have more than 600 E3 ligases, about 100 DUBs, and 40 E2 enzymes; nevertheless, the role of only a few is understood, necessitating more study on the other members. The tumor microenvironment (TME) plays a critical role in tumor metastasis. ²⁰ A few studies have demonstrated the role of E3 ligases such UBR5 in TME and metastasis. ²¹ So, it would be interesting to explore whether and how ubiquitination associated factors are significant in this context. In order to investigate these aspects, appropriate genetically modified organism models and clinical patient samples should be used. Collectively, ubiquitination is essential for the complex process of metastasis, and a thorough knowledge of the underlying molecular mechanism would be extremely helpful in developing a better therapeutic strategy to stop cancer from spreading and improve patient survival.