Significance

Dysregulated expression of master transcription factor c-MYC has been shown to promote proliferation and cell survival programs in cancer cells to mediate resistance to anticancer therapies and promote metastasis. Development of pharmacological agents to inhibit c-MYC as an anticancer therapy is a longstanding but elusive goal in the cancer field. Our study provides a potential widely applicable pharmacological strategy for targeting c-MYC–driven oncogenic activity by inhibiting cIAP1 E3 ubiquitin ligase activity as a treatment for cancers. Furthermore, we demonstrate the pharmacological interference in the dynamic interaction of an E3 ubiquitin ligase with its E2s as a strategy for inhibiting ubiquitination reactions.

Abstract

Protooncogene c-MYC, a master transcription factor, is a major driver of human tumorigenesis. Development of pharmacological agents for inhibiting c-MYC as an anticancer therapy has been a longstanding but elusive goal in the cancer field. E3 ubiquitin ligase cIAP1 has been shown to mediate the activation of c-MYC by destabilizing MAD1, a key antagonist of c-MYC. Here we developed a high-throughput assay for cIAP1 ubiquitination and identified D19, a small-molecule inhibitor of E3 ligase activity of cIAP1. We show that D19 binds to the RING domain of cIAP1 and inhibits the E3 ligase activity of cIAP1 by interfering with the dynamics of its interaction with E2. Blocking cIAP1 with D19 antagonizes c-MYC by stabilizing MAD1 protein in cells. Furthermore, we show that D19 and an improved analog (D19-14) promote c-MYC degradation and inhibit the oncogenic function of c-MYC in cells and xenograft animal models. In contrast, we show that activating E3 ubiquitin ligase activity of cIAP1 by Smac mimetics destabilizes MAD1, the antagonist of MYC, and increases the protein levels of c-MYC. Our study provides an interesting example using chemical biological approaches for determining distinct biological consequences from inhibiting vs. activating an E3 ubiquitin ligase and suggests a potential broad therapeutic strategy for targeting c-MYC in cancer treatment by pharmacologically modulating cIAP1 E3 ligase activity.

Footnotes

• ?¹To whom correspondence may be addressed. Email: junying_yuan@hms.harvard.edu or zhujd@sioc.ac.cn.

• Author contributions: J.Y. and J.Z. designed research; H.L., Y.F., C.N., H.C., T.M., and H.Z. performed research; J.Y. and J.Z. analyzed data; and J.Y. and J.Z. wrote the paper.

• Reviewers: J.D.A., NIH; and E.W., Rutgers Cancer Institute of New Jersey.

• Conflict of interest statement: J.Y. and Eileen White are coauthors of a 2016 paper on guidelines for autophagy.

• This article contains supporting information online at www./lookup/suppl/doi:10.1073/pnas.1807711115/-/DCSupplemental.

Published under the PNAS license.