USP15 antagonizes CRL4 ^CRBN -mediated ubiquitylation of glutamine synthetase and neosubstrates

Abstract

Targeted protein degradation by the ubiquitin–proteasome system represents a new strategy to destroy pathogenic proteins in human diseases, including cancer and neurodegenerative diseases. The immunomodulatory drugs (IMiDs) thalidomide, lenalidomide, and pomalidomide have revolutionized the treatment of patients with multiple myeloma (MM) and other hematologic malignancies, but almost all patients eventually develop resistance to IMiDs. CRBN, a substrate receptor of CUL4-RBX1-DDB1-CRBN (CRL4CRBN) E3 ubiquitin ligase, is a direct target for thalidomide teratogenicity and antitumor activity of IMiDs (now known as Cereblon E3 ligase modulators: CELMoDs). Despite recent advances in developing potent CELMoDs and CRBN-based proteolysis-targeting chimeras (PROTACs), many questions apart from clinical efficacy remain unanswered. CRBN is required for the action of IMiDs, but its protein expression levels do not correlate with intrinsic resistance to IMiDs in MM cells, suggesting other factors involved in regulating resistance to IMiDs. Our recent work revealed that the CRL4CRBN-p97 pathway is required for degradation of natural substrate glutamine synthetase (GS) and neosubstrates. Here, I show that USP15 is a key regulator of the CRL4CRBN-p97 pathway to control stability of GS and neosubstrates IKZF1, IKZF3, CK1-α, RNF166, GSPT1, and BRD4, all of which are crucial drug targets in different types of cancer. USP15 antagonizes ubiquitylation of CRL4CRBN target proteins, thereby preventing their degradation. Notably, USP15 is highly expressed in IMiD-resistant cells, and depletion of USP15 sensitizes these cells to lenalidomide. Inhibition of USP15 represents a valuable therapeutic opportunity to potentiate CELMoD and CRBN-based PROTAC therapies for the treatment of cancer.

Overcoming Cancer Drug Resistance Utilizing PROTAC Technology

“…These results also suggest that patients with prior exposure to IMiD treatments may have limited response to CRBN-based PROTACs. In another study, it was found that upregulation of USP15, a deubiquitinating enzyme (DUB), can antagonize IMiDs and CRBN-based PROTAC-induced ubiquitination, thereby preventing degradation, suggesting DUB overexpression is another potential resistance mechanism to PROTACs ( Nguyen 2021 ).…”

A Regulatory Network Analysis of the Importance of USP15 in Breast Cancer Metastasis and Prognosis

“…Western blot experiments then verified the expression of USP15. We found that the expression of USP15 in the two cell lines with the strongest metastatic and invasive abilities, MDA-MB-231 and SUM159; this result suggests that USP15 may affect breast cancer prognosis by altering the ability of tumor metastasis and invasion [ 55 ]. In addition, GO enrichment analysis and KEGG pathway analysis via the GESA database were performed on USP15.…”

The importance of cellular degradation kinetics for understanding mechanisms in targeted protein degradation

“…Their specific involvement and impact on degrader efficacy has been limited, but an excellent example of USP15 counteracting CRBN molecular glue activity on particular neosubstrates has been shown. 71 The diversity of DUB:target specificities, and the inability to selectively inhibit DUBs makes them difficult to study in this context. In cases where degraders fail to efficiently ubiquitinate their targets, it is possible that the rate of deubiquitination by DUBs predominates (Fig.…”