Inhibition of Myc transcriptional activity by a mini‐protein based upon Mxd1

Abstract

Myc, a transcription factor with oncogenic activity, is upregulated by amplification, translocation, and mutation of the cellular pathways that regulate its stability. Inhibition of the Myc oncogene by various modalities has had limited success. One Myc inhibitor, Omomyc, has limited cellular and in vivo activity. Here, we report a mini‐protein, referred to as Mad, which is derived from the cellular Myc antagonist Mxd1. Mad localizes to the nucleus in cells and is 10‐fold more potent than Omomyc in inhibiting Myc‐driven cell proliferation. Similar to Mxd1, Mad also interacts with Max, the binding partner of Myc, and with the nucleolar upstream binding factor. Mad binds to E‐Box DNA in the promoters of Myc target genes and represses Myc‐mediated transcription to a greater extent than Omomyc. Overall, Mad appears to be more potent than Omomyc both in vitro and in cells.

MAGI2‐AS3 suppresses MYC signaling to inhibit cell proliferation and migration in ovarian cancer through targeting miR‐525‐5p/MXD1 axis

“…Previous studies confirmed that MXD1 could compete with MYC for the binding of MAX and thereby inhibited the transcriptional function of MYC 18 . Thus, we supposed MAGI2‐AS3 could regulate MYC via modulating MXD1.…”

Blocking Myc to Treat Cancer: Reflecting on Two Decades of Omomyc

“…In this same study, Mad affected Myc target gene expression between 1.5 and 2 times more than Omomyc, despite having the same affinity for E-boxes [90]. However, when validating this reduction in selected genes by qPCR, the reduction in expression caused by Mad and Omomyc was the same.…”

“…Surprisingly, treatment with Mad was 10 times more potent than Omomyc in reducing cell viability in the two cell lines tested (HCT116 and Ramos). This effect seemed to be dependent on Myc, since low Myc HDMYZ cells responded only to much higher concentrations of both Mad and Omomyc [90]. Validation in other cancer types and, importantly, efficacy, PK, and toxicity data in vivo will be key in understanding the translation potential of this protein to the clinic.…”

“…A recent publication has reported a recombinant 146 amino acid cell-penetrating mini-protein derived from the Myc antagonist MXD1 named Mad, which, even if not directly related to Omomyc, was compared to it and reported to be more potent [90]. Mad is composed of the mSin3a binding domain and the bHLHLZ domain of MXD1, with a S145A mutation to prevent phosphorylation and consequent ubiquitination.…”

“…Mad binds E-boxes in the promoters of Myc targets, blunting Myc binding to these sites and blocking Myc target gene expression. It also interacts with UBF, thereby repressing transcription of rRNA genes [90].…”