Small molecule probes of MYC stability and function intumorigenesis

ABSTRACT
MYC oncoproteins (including c-MYC, L-MYC and N-MYC) play critical roles in the initiation, progression and
recurrence of many human malignancies. Extensive studies indicate that MYC is required to maintain tumor cell
survival and proliferation. We have recently used a novel approach that combined computer-aided modeling with
a rapid in vivo screen to develop a new series of direct small molecule inhibitors (MYCi’s) that show excellent
selectivity, potency and tolerability in multiple MYC-driven cancer models. These compounds demonstrate a
dual mechanism of action. First, direct binding of MYCi to MYC in the basic helix-loop-helix (bHLH) region
disrupts complex formation with MYC which is required for MYC transcriptional activity. Secondly, binding of
MYCi enhances MYC phosphorylation on threonine-58 (pT58) which promotes MYC degradation via the
ubiquitin-proteasome pathway. However the key downstream effectors of these events and how they might
impact cellular function are unknown. Reduction of MYC protein and enhanced pT58MYC may be expected to
have profound effects on MYC family protein interactions with each other and with chromatin. In this regard, we
have observed in preliminary studies that MYCi leads to selective loss of MYC at genomic loci enriched for
master chromatin regulators (CTCF and FOX), suggesting disruption of the 3D architecture of the MYC-bound
genome in response to MYCi. Additionally, unfolded MYC due to MYCi binding and/or enhanced MYC
degradation may provoke a cellular stress response. Using unbiased ATAC-seq and RNA-seq approaches, we
found that MYCi treatment activates the ATF4/CHOP stress response pathway. Importantly, activation of
ATF4/CHOP by MYCi is an on-target, MYC-dependent effect. ATF4 mediates MYCi antitumor activity as ATF4
depletion partially ameliorates the antitumor effects of MYCi. Furthermore, we propose that MYCi-induced ATF4
cytokines modulate the tumor microenvironment. Activation of the ATF4 pathway by MYCi exposes potential
therapeutic vulnerabilities for rational combination approaches, such as combination of MYCi with proteasome
inhibitors that activates ATF4. Based on the preliminary findings, our central hypotheses is that MYCi inhibits
MYC-dependent tumorigenesis by a dual-pronged mode of action. First, MYCi affects MYC family target gene
expression by disrupting MYC/MAX interaction and by promoting MYC degradation. Secondly, binding of MYCi
to MYC and/or MYC degradation activates an ATF4/CHOP stress response pathway that suppresses tumor cell
viability. We propose the following specific aims to test these hypotheses: Aim 1). To investigate the mechanisms
by which MYC inhibitor modulates MYC transcriptional activity and the epigenetic landscape. We will investigate
the consequences of MYCi treatment on the recruitment of MYC, pT58MYC, and associated factors to chromatin;
changes to 3D chromatin architecture; as well as the effects on MYC-driven transcriptional output in tumor cells
vitro and in vivo. Aim 2). To define the mechanisms and functional consequences of ATF4/CHOP pathway
activation by MYCi. We will determine mechanism of ARF4 upregulation by MYCi; define the role of MYCi-
induced ATF4 in regulating target gene expression, cell viability and tumorigenicity; and assess strategies that
exploit the consequences of ATF4 activation as a means of enhancing MYCi anti-tumor efficacy.
These studies are significant as MYC is implicated in the majority of human cancers. The studies advance the
use of MYCi as chemical probes to unmask distinct biology that complements the knowledge derived from
genetic manipulations of MYC proteins. The findings will contribute to the efforts aimed at developing small
molecule MYCi as potential therapeutics. Specifically, this work indicates that small-molecule MYC inhibitors
have an additional anti-tumor effect due to the activation of the ATF4 pathway beyond the antitumor effects of
suppressing MYC function. Finally, understanding this on-target ATF4 response provoked by small-molecule
MYCi will provide rational strategies for combination therapy to enhance MYCi efficacy.

Grant Number: 5R01CA257258-05
NIH Institute/Center: NIH
Principal Investigator: Sarki Abdulkadir