The MYC family oncogene is deregulated in >50% of human cancers, and this deregulation is frequently associated with poor prognosis and unfavorable patient survival. Myc has a central role in almost every aspect of the oncogenic process, orchestrating proliferation, apoptosis, differentiation, and metabolism. Although Myc inhibition would be a powerful approach for the treatment of many types of cancers, direct targeting of Myc has been a challenge for decades owing to its “undruggable” protein structure. Hence, alternatives to Myc blockade have been widely explored to achieve desirable anti-tumor effects, including Myc/Max complex disruption, MYC transcription and/or translation inhibition, and Myc destabilization as well as the synthetic lethality associated with Myc overexpression. In this review, we summarize the latest advances in targeting oncogenic Myc, particularly for cancer therapeutic purposes.

We recently demonstrated that the major decapping activity in mammalian cells involves DcpS, a scavenger pyrophosphatase that hydrolyzes the residual cap structure following 3¢ to 5¢ decay of an mRNA. The association of DcpS with 3¢ to 5¢ exonuclease exosome components suggests that these two activities are linked and there is a coupled exonucleolytic decaydependent decapping pathway. We puri®ed DcpS from mammalian cells and identi®ed the cDNA encoding a novel 40 kDa protein possessing DcpS activity. Consistent with puri®ed DcpS, the recombinant protein speci®cally hydrolyzed methylated cap analog but did not hydrolyze unmethylated cap analog nor did it function on intact capped RNA. Sequence alignments of DcpS from different organisms revealed the presence of a conserved hexapeptide, containing a histidine triad (HIT) sequence with three histidines separated by hydrophobic residues. Mutagenesis analysis revealed that the central histidine within the DcpS HIT motif is critical for decapping activity and de®nes the HIT motif as a new mRNA decapping domain, making DcpS the ®rst member of the HIT family of proteins with a de®ned biological function.

MicroRNAs (miRNAs) are endogenous single-stranded RNA molecules of about 21 nucleotides in length that are fundamental post-transcriptional regulators of gene expression. Although the transcriptional and processing events involved in the generation of miRNAs have been extensively studied, very little is known pertaining to components that regulate the stability of individual miRNAs. All RNAs have distinct inherent half-lives that dictate their level of accumulation and miRNAs would be expected to follow a similar principle. Here we demonstrate that although most miRNA appear to be stable, like mRNAs, miRNAs possess differential stability in human cells. In particular, we found that miR-382, a miRNA that contributes to HIV-1 provirus latency, is unstable in cells. To determine the region of miR-382 responsible for its rapid decay, we developed a cell-free system that recapitulated the observed cell-based-regulated miR-382 turnover. The system utilizes in vitro-processed mature miRNA derived from pre-miRNA and follows the decay of the processed miRNA. Using this system, we demonstrate that instability of miR-382 is driven by sequences outside its seed region and required the 39 terminal seven nucleotides where mutations in this region increased the stability of the RNA. Moreover, the exosome 39-59 exoribonuclease complex was identified as the primary nuclease involved in miR-382 decay with a more modest contribution by the Xrn1 and no detectable contribution by Xrn2. These studies provide evidence for an miRNA element essential for rapid miRNA decay and implicate the exosome in this process. The development of a biochemically amendable system to analyze the mechanism of differential miRNA stability provides an important step in efforts to regulate gene expression by modulating miRNA stability.

Complete removal of residual N-7 guanine cap from degraded messenger RNA is necessary to prevent accumulation of intermediates that might interfere with RNA processing, export, and translation. The human scavenger decapping enzyme, DcpS, catalyzes residual cap hydrolysis following mRNA degradation, releasing N-7 methyl guanosine monophosphate and 5'-diphosphate terminated cap or mRNA products. DcpS structures bound to m(7)GpppG or m(7)GpppA reveal an asymmetric DcpS dimer that simultaneously creates an open nonproductive DcpS-cap complex and a closed productive DcpS-cap complex that alternate via 30 A domain movements. Structural and biochemical analysis suggests an autoregulatory mechanism whereby premature decapping mRNA is prevented by blocking the conformational changes that are required to form a closed productive active site capable of cap hydrolysis.

Most tumor cells exhibit obligatory demands for essential amino acids (EAAs), but the regulatory mechanisms whereby tumor cells take up EAAs and EAAs promote malignant transformation remain to be determined. Here, we show that oncogenic MYC, solute carrier family (SLC) 7 member 5 (SLC7A5), and SLC43A1 constitute a feedforward activation loop to promote EAA transport and tumorigenesis. MYC selectively activates Slc7a5 and Slc43a1 transcription through direct binding to specific E box elements within both genes, enabling effective EAA import. Elevated EAAs, in turn, stimulate Myc mRNA translation, in part through attenuation of the GCN2-eIF2α-ATF4 amino acid stress response pathway, leading to MYC-dependent transcriptional amplification. SLC7A5/SLC43A1 depletion inhibits MYC expression, metabolic reprogramming, and tumor cell growth in vitro and in vivo. These findings thus reveal a MYC-SLC7A5/SLC43A1 signaling circuit that underlies EAA metabolism, MYC deregulation, and tumorigenesis.

Cancer cells must rewire cellular metabolism to satisfy the demands of unbridled growth and proliferation. As such, most human cancers differ from normal counterpart tissues by a plethora of energetic and metabolic reprogramming. Transcription factors of the MYC family are deregulated in up to 70% of all human cancers through a variety of mechanisms. Oncogenic levels of MYC regulates almost every aspect of cellular metabolism, a recently revisited hallmark of cancer development. Meanwhile, unrestrained growth in response to oncogenic MYC expression creates dependency on MYC-driven metabolic pathways, which in principle provides novel targets for development of effective cancer therapeutics. In the current review, we summarize the significant progress made toward understanding how MYC deregulation fuels metabolic rewiring in malignant transformation.

MYCN amplification in human cancers predicts poor prognosis and resistance to therapy. However, pharmacological strategies that directly target N-Myc, the protein encoded by MYCN, remain elusive. Here, we identify a molecular mechanism responsible for reciprocal activation between Polo-like kinase-1 (PLK1) and N-Myc. PLK1 specifically binds to the SCF ubiquitin ligase, phosphorylates it, and promotes its autopolyubiquitination and proteasomal degradation, counteracting Fbw7-mediated degradation of N-Myc and additional substrates, including cyclin E and Mcl1. Stabilized N-Myc in turn directly activates PLK1 transcription, constituting a positive feedforward regulatory loop that reinforces Myc-regulated oncogenic programs. Inhibitors of PLK1 preferentially induce potent apoptosis of MYCN-amplified tumor cells from neuroblastoma and small cell lung cancer and synergistically potentiate the therapeutic efficacies of Bcl2 antagonists. These findings reveal a PLK1-Fbw7-Myc signaling circuit that underlies tumorigenesis and validate PLK1 inhibitors, alone or with Bcl2 antagonists, as potential effective therapeutics for MYC-overexpressing cancers.

Eukaryotic cells primarily utilize exoribonucleases and decapping enzymes to degrade their mRNA. Two major decapping enzymes have been identified. The hDcp2 protein catalyzes hydrolysis of the 5 cap linked to an RNA moiety, whereas the scavenger decapping enzyme, DcpS, functions on a cap structure lacking the RNA moiety. DcpS is a member of the histidine triad (HIT) family of hydrolases and catalyzes the cleavage of m 7 GpppN. HIT proteins are homodimeric and contain two conserved 100-amino-acid HIT fold domains with independent active sites that are each sufficient to bind and hydrolyze cognate substrates. We carried out a functional characterization of the DcpS enzyme and demonstrate that unlike previously described HIT proteins, DcpS is a modular protein that requires both the core HIT fold at the carboxyl-terminus and sequences at the amino-terminus of the protein for cap binding and hydrolysis. Interestingly, DcpS can efficiently compete for and hydrolyze the cap structure even in the presence of excess eIF4E, implying that DcpS could function to alleviate the accumulation of complexes between eIF4E and cap structure that would otherwise accumulate following mRNA decay. Using immunofluorescence microscopy, we demonstrate that DcpS is predominantly a nuclear protein, with low levels of detected protein in the cytoplasm. Furthermore, analysis of the endogenous hDcp2 protein reveals that in addition to the cytoplasmic foci, it is also present in the nucleus. These data reveal that both decapping enzymes are contained in the nuclear compartment, indicating that they may fulfill a greater function in the nucleus than previously appreciated.