R e s e a R c h a R t i c l e3 4 0 7 jci.org Volume 124 Number 8 August 2014 IntroductionThe intestine is made up of repetitive units that consist of a differentiated compartment (the villus) and a proliferative compartment (the crypt). Intestinal stem cells are located in the crypt (1, 2) and produce rapidly proliferating daughter cells, the transit amplifying cells, which subsequently differentiate into 2 main epithelial lineages. The absorptive lineage is composed of all enterocytes, while the secretory lineage is composed of goblet cells (secreting protective mucins), enteroendocrine cells (secreting hormones like serotonin or secretin), and Paneth cells (3, 4). Whether transit amplifying cells differentiate along an absorptive or a secretory lineage is regulated by the Notch pathway (5). Engagement of Notch receptors by Notch ligands, such as Delta or Jagged, induces proteolytic cleavage of the receptor by γ-secretase. The cleaved NOTCH1 receptor (NICD1) translocates into the nucleus, resulting in the formation of an active transcriptional complex composed of RBPJκ (also known as CSL or CBF1) and NICD1. Notch signal activation induces hairy/enhancer of split (Hes) gene expression. Ablation of Notch signaling via genetic deletion of Rbpj results in secretory cell expansion (6). Conversely, in transgenic mice overexpressing NICD1, goblet cells are absent, and the proliferative compartment is expanded (7).The Wnt signaling pathway is a key regulator of intestinal stem cell homeostasis (8), and 2 of the target genes induced by Wnt signaling, c-MYC and c-JUN, encode transcription factors that have oncogenic potential (9, 10). Consequently, aberrant activation of the adenomatous polyposis coli/β-catenin/T cell factor (APC/ β-catenin/TCF) pathway is an initiating event in the majority of human colorectal cancers (11).c-MYC is a transcription factor with key functions in cell differentiation and cancer development (12)(13)(14). In the intestine, c-MYC is required for the altered proliferation and differentiation induced by APC inactivation (15-18). c-MYC is a highly labile protein, and at least 2 ubiquitin ligases, SKP2 and FBW7, can target it for proteasomal degradation (19-21). c-MYC ubiquitination is antagonized by the deubiquitinase USP28, which "piggybacks" on FBW7 and stabilizes c-MYC protein (22). Thus, an E3 ubiquitin ligase and a deubiquitinase, FBW7 and USP28, are together recruited to substrates (22), and a cycle of deubiquitination and ubiquitination controls c-MYC stability.Genomic data from human cancers suggest that most colorectal cancer mutations converge on c-MYC misregulation (23). Due to its key role in tumorigenesis, much recent research has been directed to finding ways to target c-MYC function (24-29). Dominant-negative approaches targeting c-MYC function impair intestinal tumor formation, and c-Myc heterozygous mice show reduced tumor development in the Apc min/+ model (16,17). Transgenic expression of a dominant-negative allele of Myc, OmoMyc, has provided proof of principle that targeting ...

The SCFFbw7 ubiquitin ligase mediates growth-factor-regulated turnover of the Myc oncoprotein. Here we show that SCFβ-TrCP binds to Myc by means of a characteristic phosphodegron and ubiquitylates Myc; this results in enhanced Myc stability. SCFFbw7 and SCFβ-TrCP can exert these differential effects through polyubiquitylation of the amino terminus of Myc. Whereas SCFFbw7 with the Cdc34 ubiquitin-conjugating enzyme specifically requires lysine 48 (K48) of ubiquitin, SCFβ-TrCP uses the UbcH5 ubiquitin-conjugating enzyme to form heterotypic polyubiquitin chains on Myc. Ubiquitylation of Myc by SCFβ-TrCP is required for Myc-dependent acceleration of cell cycle progression after release from an arrest in S phase. Therefore, alternative ubiquitylation events at the N terminus can lead to the ubiquitylation-dependent stabilization of Myc.

MYC is an unstable protein, and its turnover is controlled by the ubiquitin system. Ubiquitination enhances MYC-dependent transactivation, but the underlying mechanism remains unresolved. Here we show that MYC proteasomal turnover is dispensable for loading of RNA polymerase II (RNAPII). In contrast, MYC turnover is essential for recruitment of TRRAP, histone acetylation, and binding of BRD4 and P-TEFb to target promoters, leading to phosphorylation of RNAPII and transcriptional elongation. In the absence of histone acetylation and P-TEFb recruitment, MYC associates with the PAF1 complex (PAF1C) through a conserved domain in the MYC amino terminus ("MYC box I"). Depletion of the PAF1C subunit CDC73 enhances expression of MYC target genes, suggesting that the MYC/PAF1C complex can inhibit transcription. Because several ubiquitin ligases bind to MYC via the same domain ("MYC box II") that interacts with TRRAP, we propose that degradation of MYC limits the accumulation of MYC/PAF1C complexes during transcriptional activation.

In several developmental lineages, an increase in MYC expression drives the transition from quiescent stem cells to transit-amplifying cells. We show that MYC activates a stereotypic transcriptional program of genes involved in cell growth in mammary epithelial cells. This change in gene expression indirectly inhibits the YAP/TAZ co-activators, which maintain the clonogenic potential of these cells. We identify a phospholipase of the mitochondrial outer membrane, PLD6, as the mediator of MYC activity. MYC-dependent growth strains cellular energy resources and stimulates AMP-activated kinase (AMPK). PLD6 alters mitochondrial fusion and fission dynamics downstream of MYC. This change activates AMPK, which in turn inhibits YAP/TAZ. Mouse models and human pathological data show that MYC enhances AMPK and suppresses YAP/TAZ activity in mammary tumors.

Deregulated expression of MYC is a driver of colorectal carcinogenesis, necessitating novel strategies to inhibit MYC function. The ubiquitin ligase HUWE1 (HECTH9, ARF-BP1, MULE) associates with both MYC and the MYC-associated protein MIZ1. We show here that HUWE1 is required for growth of colorectal cancer cells in culture and in orthotopic xenograft models. Using high-throughput screening, we identify small molecule inhibitors of HUWE1, which inhibit MYC-dependent transactivation in colorectal cancer cells, but not in stem and normal colon epithelial cells. Inhibition of HUWE1 stabilizes MIZ1. MIZ1 globally accumulates on MYC target genes and contributes to repression of MYC-activated target genes upon HUWE1 inhibition. Our data show that transcriptional activation by MYC in colon cancer cells requires the continuous degradation of MIZ1 and identify a novel principle that allows for inhibition of MYC function in tumor cells.See also: FX Schaub & JL Cleveland (December 2014)