Beyond gene expression: how MYC relieves transcription stress

The protein deacetylase HDAC10 controls DNA replication in malignant lymphoid cells

Histone deacetylases (HDACs) comprise a family of 18 epigenetic modifiers. The biologically relevant functions of HDAC10 in leukemia cells are enigmatic. We demonstrate that human cultured and primary acute B cell/T cell leukemia and lymphoma cells require the catalytic activity of HDAC10 for their survival. In such cells, HDAC10 controls a MYC-dependent transcriptional induction of the DNA polymerase subunit POLD1. Consequently, pharmacological inhibition of HDAC10 causes DNA breaks and an accumulation of poly-ADP-ribose chains. These processes culminate in caspase-dependent apoptosis. PZ48 does not damage resting and proliferating human normal blood cells. The in vivo activity of PZ48 against ALL cells is verified in a Danio rerio model. These data reveal a nuclear function for HDAC10. HDAC10 controls the MYC-POLD1 axis to maintain the processivity of DNA replication and genome integrity. This mechanistically defined "HDAC10ness" may be exploited as treatment option for lymphoid malignancies.

HJURP modulates cell proliferation and chemoresistance via the MYC/TOP2A transcriptional axis in gastric cancer

Background

The histone chaperone Holliday Junction Recognition Protein (HJURP) has been associated with multiple types of cancers, but its role in GC is not yet fully understood. Considering its functions in centromere stability and DNA repair, investigating HJURP's role in GC may offer novel therapeutic perspectives.

Methods

HJURP expression was examined in a dataset comprising TCGA-STAD samples and an internal group of GC patients, utilizing RNA sequencing and Western blot techniques. Functional experiments were carried out on the AGS and HGC-27 GC cell lines. The expression levels of HJURP, MYC, and Topoisomerase II alpha (TOP2A) were assessed via quantitative real-time PCR and Western blot. Proliferation rates of the cells were determined through EdU, CCK-8, and colony formation assays.

Results

Compared to adjacent normal tissues, HJURP expression was notably increased in GC tissues, a finding consistent across both the TCGA-STAD database and our internal patient group. Silencing HJURP markedly reduced GC cell growth and chemoresistance. Mechanistically, HJURP enhanced MYC stability, which in turn promoted TOP2A transcription. Rescue experiments confirmed that overexpression of TOP2A alters proliferation and chemoresistance of GC cells with HJURP knockdown, indicating the dependency of this axis on MYC activity.

Conclusion

Our study demonstrates that HJURP is critical for promoting GC proliferation and chemoresistance through the regulation of the MYC/TOP2A transcriptional network. Targeting HJURP might offer a novel therapeutic avenue for GC, necessitating further exploration of its clinical potential. This work underscores the value of investigating histone chaperones as potential targets in cancer treatment.

Pharmacologically induced proteolysis of histone deacetylase-6 attenuates influenza virus replication despite limited anti-tumor effects

The protein deacetylase HDAC6 has been controversially linked to cancer cell proliferation and viral propagation. We analyzed whether a pharmacological depletion of HDAC6 with a recent proteolysis-targeting chimera (PROTAC) kills tumor cells. We show that low micromolar doses of the cereblon-based PROTAC TH170, but not its inactive analog TH170E, induce proteasomal degradation of HDAC6. The elimination of HDAC6 by TH170 does not compromise leukemia cell growth and survival, DNA integrity, and the stability of selected cancer-relevant proteins. Increasing the doses of TH170 generates a hook-effect on HDAC6 degradation and the apoptosis-associated fragmentation of HDAC6. Unlike the specific elimination of HDAC6 that low doses of TH170 evoke, this fragmentation of HDAC6 is linked to apoptosis and an accumulation of acetylated histones. Thus, like HDAC6 inhibitors, pharmacological degraders of HDAC6 do not induce leukemic cell death unless they are used in non-selective concentrations. Bioinformatic analyses of 91 lymphoid, 37 myeloid, and 125 lung cancer cells in which HDAC6 was deleted by CRISPR-Cas9 corroborate these data. HDAC6 is expressed in various bronchus and lung cell types. In a human lung cell model, TH170 reduces influenza A virus replication dependent on the strain and without compromising cell vitality. These data suggest that pharmacologically amenable kinase-independent functions of HDAC6 control viral replication. Eliminating HDAC6 could be a promising anti-viral strategy with a benign impact on host cells.

Targeting oncogenic transcriptional factor c-myc by oligonucleotide PROTAC for the treatment of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the leading causes of cancer-related death, but effective therapeutic strategies are limited. Transcriptional factor c-Myc plays an oncogenic role in tumorigenesis and is an attractive target for HCC treatment. However, targeted therapy against c-Myc remains challenging. Herein, by conjugating VH032 with an optimized DNA sequence that recognized c-Myc complex, we discovered oligonucleotide-based proteolysis targeting chimeras (PROTACs), termed as MP-16 and MP-17, which effectively induced degradation of c-Myc. Mechanically, MP-16 or MP-17 directly interacted with c-Myc complex to form VHL/PROTAC/c-Myc ternary complex, and triggered c-Myc degradation by recruiting ubiquitin-proteasome system, suppressing cell proliferation of HCC cells. In mice model, MP-16 or MP-17 significantly inhibited HCC tumor growth and exhibited promising drug safety. This work provided novel oligonucleotide PROTACs that degraded c-Myc, giving a new lead structure for HCC therapy.

Excessive MYC-topoisome activity triggers acute DNA damage, MYC degradation, and replacement by a p53-topoisome

Hyperproliferation driven by the protooncogene MYC may lead to tumor suppressor p53 activating DNA damage that has been presumed to derive from hypertranscription and over-replication. Here, we report that excessive MYC-topoisome (MYC/topoisomerase 1/topoisomerase 2) activity acutely damages DNA-activating pATM and p53. In turn, MYC is shut off and degraded, releasing TOP1 and TOP2A from MYC topoisomes in vitro and in vivo. To manage the topological and torsional stress generated at its target genes, p53 assembles a separate topoisome. Because topoisomerase activity is intrinsically DNA damaging, p53 topoisomes provoke an initial burst of DNA damage. Because p53, unlike MYC, upregulates the DNA-damage response (DDR) and activates tyrosyl-DNA-phosphodiesterase (TDP) 1 and TDP2, it suppresses further topoisome-mediated damage. The physical coupling and activation of TOP1 and TOP2 by p53 creates a tool that supports p53-target expression while braking MYC-driven proliferation in mammalian cells.

Immune evasion: An imperative and consequence of MYC deregulation

MYC has been implicated in the pathogenesis of a wide range of human tumors and has been described for many years as a transcription factor that regulates genes with pleiotropic functions to promote tumorigenic growth. However, despite extensive efforts to identify specific target genes of MYC that alone could be responsible for promoting tumorigenesis, the field is yet to reach a consensus whether this is the crucial function of MYC. Recent work shifts the view on MYC's function from being a gene-specific transcription factor to an essential stress resilience factor. In highly proliferating cells, MYC preserves cell integrity by promoting DNA repair at core promoters, protecting stalled replication forks, and/or preventing transcription-replication conflicts. Furthermore, an increasing body of evidence demonstrates that MYC not only promotes tumorigenesis by driving cell-autonomous growth, but also enables tumors to evade the host's immune system. In this review, we summarize our current understanding of how MYC impairs antitumor immunity and why this function is evolutionarily hard-wired to the biology of the MYC protein family. We show why the cell-autonomous and immune evasive functions of MYC are mutually dependent and discuss ways to target MYC proteins in cancer therapy.

Association with TFIIIC limits MYCN localisation in hubs of active promoters and chromatin accumulation of non-phosphorylated RNA polymerase II

MYC family oncoproteins regulate the expression of a large number of genes and broadly stimulate elongation by RNA polymerase II (RNAPII). While the factors that control the chromatin association of MYC proteins are well understood, much less is known about how interacting proteins mediate MYC's effects on transcription. Here, we show that TFIIIC, an architectural protein complex that controls the three-dimensional chromatin organisation at its target sites, binds directly to the amino-terminal transcriptional regulatory domain of MYCN. Surprisingly, TFIIIC has no discernible role in MYCN-dependent gene expression and transcription elongation. Instead, MYCN and TFIIIC preferentially bind to promoters with paused RNAPII and globally limit the accumulation of non-phosphorylated RNAPII at promoters. Consistent with its ubiquitous role in transcription, MYCN broadly participates in hubs of active promoters. Depletion of TFIIIC further increases MYCN localisation to these hubs. This increase correlates with a failure of the nuclear exosome and BRCA1, both of which are involved in nascent RNA degradation, to localise to active promoters. Our data suggest that MYCN and TFIIIC exert an censoring function in early transcription that limits promoter accumulation of inactive RNAPII and facilitates promoter-proximal degradation of nascent RNA.

The MYCN oncoprotein is an RNA-binding accessory factor of the nuclear exosome targeting complex

The MYCN oncoprotein binds active promoters in a heterodimer with its partner protein MAX. MYCN also interacts with the nuclear exosome, a 3'-5' exoribonuclease complex, suggesting a function in RNA metabolism. Here, we show that MYCN forms stable high-molecular-weight complexes with the exosome and multiple RNA-binding proteins. MYCN binds RNA in vitro and in cells via a conserved sequence termed MYCBoxI. In cells, MYCN associates with thousands of intronic transcripts together with the ZCCHC8 subunit of the nuclear exosome targeting complex and enhances their processing. Perturbing exosome function results in global re-localization of MYCN from promoters to intronic RNAs. On chromatin, MYCN is then replaced by the MNT(MXD6) repressor protein, inhibiting MYCN-dependent transcription. RNA-binding-deficient alleles show that RNA-binding limits MYCN's ability to activate cell growth-related genes but is required for MYCN's ability to promote progression through S phase and enhance the stress resilience of neuroblastoma cells.

PAF1c links S-phase progression to immune evasion and MYC function in pancreatic carcinoma

In pancreatic ductal adenocarcinoma (PDAC), endogenous MYC is required for S-phase progression and escape from immune surveillance. Here we show that MYC in PDAC cells is needed for the recruitment of the PAF1c transcription elongation complex to RNA polymerase and that depletion of CTR9, a PAF1c subunit, enables long-term survival of PDAC-bearing mice. PAF1c is largely dispensable for normal proliferation and regulation of MYC target genes. Instead, PAF1c limits DNA damage associated with S-phase progression by being essential for the expression of long genes involved in replication and DNA repair. Surprisingly, the survival benefit conferred by CTR9 depletion is not due to DNA damage, but to T-cell activation and restoration of immune surveillance. This is because CTR9 depletion releases RNA polymerase and elongation factors from the body of long genes and promotes the transcription of short genes, including MHC class I genes. The data argue that functionally distinct gene sets compete for elongation factors and directly link MYC-driven S-phase progression to tumor immune evasion.