The Arf tumor suppressor protein inhibits Miz1 to suppress cell adhesion and induce apoptosis

Expanding Roles of the E2F-RB-p53 Pathway in Tumor Suppression

The transcription factor E2F links the RB pathway to the p53 pathway upon loss of function of pRB, thereby playing a pivotal role in the suppression of tumorigenesis. E2F fulfills a major role in cell proliferation by controlling a variety of growth-associated genes. The activity of E2F is controlled by the tumor suppressor pRB, which binds to E2F and actively suppresses target gene expression, thereby restraining cell proliferation. Signaling pathways originating from growth stimulative and growth suppressive signals converge on pRB (the RB pathway) to regulate E2F activity. In most cancers, the function of pRB is compromised by oncogenic mutations, and E2F activity is enhanced, thereby facilitating cell proliferation to promote tumorigenesis. Upon such events, E2F activates the Arf tumor suppressor gene, leading to activation of the tumor suppressor p53 to protect cells from tumorigenesis. ARF inactivates MDM2, which facilitates degradation of p53 through proteasome by ubiquitination (the p53 pathway). P53 suppresses tumorigenesis by inducing cellular senescence or apoptosis. Hence, in almost all cancers, the p53 pathway is also disabled. Here we will introduce the canonical functions of the RB-E2F-p53 pathway first and then the non-classical functions of each component, which may be relevant to cancer biology.

Poly(ADP-Ribose) Polymerase-1 Lacking Enzymatic Activity Is Not Compatible with Mouse Development

Poly(ADP-ribose) polymerase-1 (PARP1) binds DNA lesions to catalyse poly(ADP-ribosyl)ation (PARylation) using NAD+ as a substrate. PARP1 plays multiple roles in cellular activities, including DNA repair, transcription, cell death, and chromatin remodelling. However, whether these functions are governed by the enzymatic activity or scaffolding function of PARP1 remains elusive. In this study, we inactivated in mice the enzymatic activity of PARP1 by truncating its C-terminus that is essential for ART catalysis (PARP1ΔC/ΔC, designated as PARP1-ΔC). The mutation caused embryonic lethality between embryonic day E8.5 and E13.5, in stark contrast to PARP1 complete knockout (PARP1-/-) mice, which are viable. Embryonic stem (ES) cell lines can be derived from PARP1ΔC/ΔC blastocysts, and these mutant ES cells can differentiate into all three germ layers, yet, with a high degree of cystic structures, indicating defects in epithelial cells. Intriguingly, PARP1-ΔC protein is expressed at very low levels compared to its full-length counterpart, suggesting a selective advantage for cell survival. Noticeably, PARP2 is particularly elevated and permanently present at the chromatin in PARP1-ΔC cells, indicating an engagement of PARP2 by non-enzymatic PARP1 protein at the chromatin. Surprisingly, the introduction of PARP1-ΔC mutation in adult mice did not impair their viability; yet, these mutant mice are hypersensitive to alkylating agents, similar to PARP1-/- mutant mice. Our study demonstrates that the catalytically inactive mutant of PARP1 causes the developmental block, plausibly involving PARP2 trapping.

It’s Getting Complicated—A Fresh Look at p53-MDM2-ARF Triangle in Tumorigenesis and Cancer Therapy

Anti-tumorigenic mechanisms mediated by the tumor suppressor p53, upon oncogenic stresses, are our bodies’ greatest weapons to battle against cancer onset and development. Consequently, factors that possess significant p53-regulating activities have been subjects of serious interest from the cancer research community. Among them, MDM2 and ARF are considered the most influential p53 regulators due to their abilities to inhibit and activate p53 functions, respectively. MDM2 inhibits p53 by promoting ubiquitination and proteasome-mediated degradation of p53, while ARF activates p53 by physically interacting with MDM2 to block its access to p53. This conventional understanding of p53-MDM2-ARF functional triangle have guided the direction of p53 research, as well as the development of p53-based therapeutic strategies for the last 30 years. Our increasing knowledge of this triangle during this time, especially through identification of p53-independent functions of MDM2 and ARF, have uncovered many under-appreciated molecular mechanisms connecting these three proteins. Through recognizing both antagonizing and synergizing relationships among them, our consideration for harnessing these relationships to develop effective cancer therapies needs an update accordingly. In this review, we will re-visit the conventional wisdom regarding p53-MDM2-ARF tumor-regulating mechanisms, highlight impactful studies contributing to the modern look of their relationships, and summarize ongoing efforts to target this pathway for effective cancer treatments. A refreshed appreciation of p53-MDM2-ARF network can bring innovative approaches to develop new generations of genetically-informed and clinically-effective cancer therapies.

MXD/MIZ1 transcription regulatory complexes activate the expression of MYC-repressed genes

MXDs are transcription repressors that antagonize MYC-mediated gene activation. MYC, when associated with MIZ1, acts also as a repressor of a subset of genes, including p15 and p21. A role for MXDs in regulation of MYC-repressed genes is not known. We report that MXDs activate transcription of p15 and p21 in U2OS cells. This activation required DNA binding by MXDs and their interaction with MIZ1. MXD mutants deficient in MIZ1 binding interacted with the MYC-binding partner MAX and were active as repressors of MYC-activated genes but failed to activate MYC-repressed genes. Mutant MXDs with reduced DNA-binding affinity interacted with MAX and MIZ1 but neither repressed nor activated transcription. Our data show that MXDs and MYC have a reciprocally antagonistic potential to regulate transcription of target genes.

Dual Role of the Alternative Reading Frame ARF Protein in Cancer

The CDKN2a/ARF locus expresses two partially overlapping transcripts that encode two distinct proteins, namely p14ARF (p19Arf in mouse) and p16INK4a, which present no sequence identity. Initial data obtained in mice showed that both proteins are potent tumor suppressors. In line with a tumor-suppressive role, ARF-deficient mice develop lymphomas, sarcomas, and adenocarcinomas, with a median survival rate of one year of age. In humans, the importance of ARF inactivation in cancer is less clear whereas a more obvious role has been documented for p16INK4a. Indeed, many alterations in human tumors result in the elimination of the entire locus, while the majority of point mutations affect p16INK4a. Nevertheless, specific mutations of p14ARF have been described in different types of human cancers such as colorectal and gastric carcinomas, melanoma and glioblastoma. The activity of the tumor suppressor ARF has been shown to rely on both p53-dependent and independent functions. However, novel data collected in the last years has challenged the traditional and established role of this protein as a tumor suppressor. In particular, tumors retaining ARF expression evolve to metastatic and invasive phenotypes and in humans are associated with a poor prognosis. In this review, the recent evidence and the molecular mechanisms of a novel role played by ARF will be presented and discussed, both in pathological and physiological contexts.

Increased expression of Myc-interacting zinc finger protein 1 in APP/PS1 mice

Myc-interacting zinc-finger protein 1 (Miz1) is a member of the poxvirus and zinc-finger domain/zinc finger transcription factor family. Its transcription activation and repression functions in the nucleus are well elucidated; however its cytoplasmic inflammation function is poorly understood and may be associated with the pathogenesis of Alzheimer's disease (AD). The aim of the present study was to investigate the association between AD and Miz1 expression. In the present study, the expression and distribution of Miz1 in wild-type (WT) and amyloid precursor protein/presenelin-1 (AD) mice was studied using reverse transcription-quantitative polymerase chain reaction, western blot analysis, and immunohistochemical and immunofluorescence staining. The results indicated that Miz1 was significantly upregulated in the cortex of AD mice (P<0.05). Double immunofluorescence labeling revealed that Miz1 protein was predominantly expressed in neurons and astrocytes, as evidenced by co-localization with the dendritic markers microtubule associated protein 2 and glial fibrillary acidic protein, respectively. The results of the present study suggest that the expression of Miz1 in the brain tissue of AD mice may serve an important role in AD pathogenesis.

The evolution of Sex-linked barring alleles in chickens involves both regulatory and coding changes in CDKN2A

Sex-linked barring is a fascinating plumage pattern in chickens recently shown to be associated with two non-coding and two missense mutations affecting the ARF transcript at the CDKN2A tumor suppressor locus. It however remained a mystery whether all four mutations are indeed causative and how they contribute to the barring phenotype. Here, we show that Sex-linked barring is genetically heterogeneous, and that the mutations form three functionally different variant alleles. The B0 allele carries only the two non-coding changes and is associated with the most dilute barring pattern, whereas the B1 and B2 alleles carry both the two non-coding changes and one each of the two missense mutations causing the Sex-linked barring and Sex-linked dilution phenotypes, respectively. The data are consistent with evolution of alleles where the non-coding changes occurred first followed by the two missense mutations that resulted in a phenotype more appealing to humans. We show that one or both of the non-coding changes are cis-regulatory mutations causing a higher CDKN2A expression, whereas the missense mutations reduce the ability of ARF to interact with MDM2. Caspase assays for all genotypes revealed no apoptotic events and our results are consistent with a recent study indicating that the loss of melanocyte progenitors in Sex-linked barring in chicken is caused by premature differentiation and not apoptosis. Our results show that CDKN2A is a major locus driving the differentiation of avian melanocytes in a temporal and spatial manner.

p19ARF is a critical mediator of both cellular senescence and an innate immune response associated with MYC inactivation in mouse model of acute leukemia

MYC-induced T-ALL exhibit oncogene addiction. Addiction to MYC is a consequence of both cell-autonomous mechanisms, such as proliferative arrest, cellular senescence, and apoptosis, as well as non-cell autonomous mechanisms, such as shutdown of angiogenesis, and recruitment of immune effectors. Here, we show, using transgenic mouse models of MYC-induced T-ALL, that the loss of either p19ARF or p53 abrogates the ability of MYC inactivation to induce sustained tumor regression. Loss of p53 or p19ARF, influenced the ability of MYC inactivation to elicit the shutdown of angiogenesis; however the loss of p19ARF, but not p53, impeded cellular senescence, as measured by SA-beta-galactosidase staining, increased expression of p16INK4A, and specific histone modifications. Moreover, comparative gene expression analysis suggested that a multitude of genes involved in the innate immune response were expressed in p19ARF wild-type, but not null, tumors upon MYC inactivation. Indeed, the loss of p19ARF, but not p53, impeded the in situ recruitment of macrophages to the tumor microenvironment. Finally, p19ARF null-associated gene signature prognosticated relapse-free survival in human patients with ALL. Therefore, p19ARF appears to be important to regulating cellular senescence and innate immune response that may contribute to the therapeutic response of ALL.

Repression of SRF target genes is critical for Myc-dependent apoptosis of epithelial cells

Oncogenic levels of Myc expression sensitize cells to multiple apoptotic stimuli, and this protects long-lived organisms from cancer development. How cells discriminate physiological from supraphysiological levels of Myc is largely unknown. Here, we show that induction of apoptosis by Myc in breast epithelial cells requires association of Myc with Miz1. Gene expression and ChIP-Sequencing experiments show that high levels of Myc invade target sites that lack consensus E-boxes in a complex with Miz1 and repress transcription. Myc/Miz1-repressed genes encode proteins involved in cell adhesion and migration and include several integrins. Promoters of repressed genes are enriched for binding sites of the serum-response factor (SRF). Restoring SRF activity antagonizes Myc repression of SRF target genes, attenuates Myc-induced apoptosis, and reverts a Myc-dependent decrease in Akt phosphorylation and activity, a well-characterized suppressor of Myc-induced apoptosis. We propose that high levels of Myc engage Miz1 in repressive DNA binding complexes and suppress an SRF-dependent transcriptional program that supports survival of epithelial cells.

Myc-interacting zinc-finger protein 1 positively regulates Wnt signalling by protecting Dishevelled from Dapper1-mediated degradation

Wnt signalling regulates embryonic development and tissue homoeostasis by modulating cell proliferation, differentiation and migration. Dapper1 (Dpr1) has been shown to be an important key negative regulator of Wnt signalling by promoting Dishevelled (Dvl) degradation. In the present study, we found that Myc-interacting zinc-finger protein 1 (MIZ1) interacts with Dpr1 and this interaction attenuates the ability of Dpr1 to induce Dvl2 degradation, thus enhancing Wnt signalling. Mechanistically, MIZ1 is translocated from the nucleus to the cytoplasm upon Wnt3a stimulation or overexpression of Dpr1 and Dvl2, disrupting the interaction between Dpr1 and Dvl2. Furthermore, MIZ1 can promote the proliferation of breast cancer MDA-MB-231 and BT-549 cells through Wnt signalling and reverse the anti-proliferative effect of Dpr1 on colorectal cancer Caco-2. Together, our findings establish a novel layer of Wnt signalling regulation via the MIZ1-Dpr1-Dvl axis.

Smg6/Est1 licenses embryonic stem cell differentiation via nonsense-mediated mRNA decay

Nonsense-mediated mRNA decay (NMD) is a post-transcriptional mechanism that targets aberrant transcripts and regulates the cellular RNA reservoir. Genetic modulation in vertebrates suggests that NMD is critical for cellular and tissue homeostasis, although the underlying mechanism remains elusive. Here, we generate knockout mice lacking Smg6/Est1, a key nuclease in NMD and a telomerase cofactor. While the complete loss of Smg6 causes mouse lethality at the blastocyst stage, inducible deletion of Smg6 is compatible with embryonic stem cell (ESC) proliferation despite the absence of telomere maintenance and functional NMD. Differentiation of Smg6-deficient ESCs is blocked due to sustained expression of pluripotency genes, normally repressed by NMD, and forced down-regulation of one such target, c-Myc, relieves the differentiation block. Smg6-null embryonic fibroblasts are viable as well, but are refractory to cellular reprograming into induced pluripotent stem cells (iPSCs). Finally, depletion of all major NMD factors compromises ESC differentiation, thus identifying NMD as a licensing factor for the switch of cell identity in the process of stem cell differentiation and somatic cell reprograming.

Tumor cell-specific inhibition of MYC function using small molecule inhibitors of the HUWE1 ubiquitin ligase

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.

Taming of the beast: shaping Myc-dependent amplification

Myc deregulation is a hallmark oncogenic event where overexpression of the transcription factor gives rise to numerous tumorigenic phenotypes. The complex consequences of Myc deregulation have prevented clear mechanistic interpretations of its function. A synthesis of recent experimental observations offers a consensus on the direct transcriptional function of Myc: when overexpressed, Myc broadly engages the established euchromatic cis-regulatory landscape of the cell, where the factor generally amplifies transcription. The level of Myc binding at target genes and the transcriptional output are differentially modulated by additional regulators, including Miz1. Targeting Myc oncogenic activity will require an understanding of whether amplification promotes tumorigenesis and the consequences of amplification in tumors adapted to oncogenic Myc.

Epstein-Barr virus nuclear antigen 3A protein regulates CDKN2B transcription via interaction with MIZ-1

The Epstein-Barr virus (EBV) nuclear antigen 3 family of protein is critical for the EBV-induced primary B-cell growth transformation process. Using a yeast two-hybrid screen we identified 22 novel cellular partners of the EBNA3s. Most importantly, among the newly identified partners, five are known to play direct and important roles in transcriptional regulation. Of these, the Myc-interacting zinc finger protein-1 (MIZ-1) is a transcription factor initially characterized as a binding partner of MYC. MIZ-1 activates the transcription of a number of target genes including the cell cycle inhibitor CDKN2B. Focusing on the EBNA3A/MIZ-1 interaction we demonstrate that binding occurs in EBV-infected cells expressing both proteins at endogenous physiological levels and that in the presence of EBNA3A, a significant fraction of MIZ-1 translocates from the cytoplasm to the nucleus. Moreover, we show that a trimeric complex composed of a MIZ-1 recognition DNA element, MIZ-1 and EBNA3A can be formed, and that interaction of MIZ-1 with nucleophosmin (NPM), one of its coactivator, is prevented by EBNA3A. Finally, we show that, in the presence of EBNA3A, expression of the MIZ-1 target gene, CDKN2B, is downregulated and repressive H3K27 marks are established on its promoter region suggesting that EBNA3A directly counteracts the growth inhibitory action of MIZ-1.

MYC cofactors: molecular switches controlling diverse biological outcomes

The transcription factor MYC has fundamental roles in proliferation, apoptosis, tumorigenesis, and stem cell pluripotency. Over the last 30 years extensive information has been gathered on the numerous cofactors that interact with MYC and the target genes that are regulated by MYC as a means of understanding the molecular mechanisms controlling its diverse roles. Despite significant advances and perhaps because the amount of information learned about MYC is overwhelming, there has been little consensus on the molecular functions of MYC that mediate its critical biological roles. In this perspective, the major MYC cofactors that regulate the various transcriptional activities of MYC, including canonical and noncanonical transactivation and transcriptional repression, will be reviewed and a model of how these transcriptional mechanisms control MYC-mediated proliferation, apoptosis, and tumorigenesis will be presented. The basis of the model is that a variety of cofactors form dynamic MYC transcriptional complexes that can switch the molecular and biological functions of MYC to yield a diverse range of outcomes in a cell-type- and context-dependent fashion.

Miz1 deficiency in the mammary gland causes a lactation defect by attenuated Stat5 expression and phosphorylation

Miz1 is a zinc finger transcription factor with an N-terminal POZ domain. Complexes with Myc, Bcl-6 or Gfi-1 repress expression of genes like Cdkn2b (p15^Ink4) or Cdkn1a (p21^Cip1). The role of Miz1 in normal mammary gland development has not been addressed so far. Conditional knockout of the Miz1 POZ domain in luminal cells during pregnancy caused a lactation defect with a transient reduction of glandular tissue, reduced proliferation and attenuated differentiation. This was recapitulated in vitro using mouse mammary gland derived HC11 cells. Further analysis revealed decreased Stat5 activity in Miz1ΔPOZ mammary glands and an attenuated expression of Stat5 targets. Gene expression of the Prolactin receptor (PrlR) and ErbB4, both critical for Stat5 phosphorylation (pStat5) or pStat5 nuclear translocation, was decreased in Miz1ΔPOZ females. Microarray, ChIP-Seq and gene set enrichment analysis revealed a down-regulation of Miz1 target genes being involved in vesicular transport processes. Our data suggest that deranged intracellular transport and localization of PrlR and ErbB4 disrupt the Stat5 signalling pathway in mutant glands and cause the observed lactation phenotype.

Biphasic influence of Miz1 on neural crest development by regulating cell survival and apical adhesion complex formation in the developing neural tube

Myc interacting zinc finger protein-1 (Miz1) is a transcription factor known to regulate cell cycle- and cell adhesion-related genes in cancer. Here we show that Miz1 also plays a critical role in neural crest development. In the chick, Miz1 is expressed throughout the neural plate and closing neural tube. Its morpholino-mediated knockdown affects neural crest precursor survival, leading to reduction of neural plate border and neural crest specifier genes Msx-1, Pax7, FoxD3, and Sox10. Of interest, Miz1 loss also causes marked reduction of adhesion molecules (N-cadherin, cadherin6B, and α1-catenin) with a concomitant increase of E-cadherin in the neural folds, likely leading to delayed and decreased neural crest emigration. Conversely, Miz1 overexpression results in up-regulation of cadherin6B and FoxD3 expression in the neural folds/neural tube, leading to premature neural crest emigration and increased number of migratory crest cells. Although Miz1 loss effects cell survival and proliferation throughout the neural plate, the neural progenitor marker Sox2 was unaffected, suggesting a neural crest-selective effect. The results suggest that Miz1 is important not only for survival of neural crest precursors, but also for maintenance of integrity of the neural folds and tube, via correct formation of the apical adhesion complex therein.

The role of MIZ-1 in MYC-dependent tumorigenesis

A hallmark of MYC-transformed cells is their aberrant response to antimitogenic signals. Key examples include the inability of MYC-transformed cells to arrest proliferation in response to antimitogenic signals such as TGF-β or DNA damage and their inability to differentiate into adipocytes in response to hormonal stimuli. Given the plethora of antimitogenic signals to which a tumor cell is exposed, it is likely that the ability to confer resistance to these signals is central to the transforming properties of MYC in vivo. At the same time, the inability of MYC-transformed cells to halt cell-cycle progression on stress may establish a dependence on mutations that impair or disable apoptosis. We propose that the interaction of MYC with the zinc finger protein MIZ-1 mediates resistance to antimitogenic signals. In contrast to other interactions of MYC, there is currently little evidence that MIZ-1 associates with MYC in normal, unperturbed cells. The functional interaction of both proteins becomes apparent at oncogenic expression levels of MYC and association with MIZ-1 mediates both oncogenic functions of MYC as well as tumor-suppressive responses to oncogenic levels of MYC.

Anti-metastatic and differential effects on protein expression of epigallocatechin-3-gallate in HCCLM6 hepatocellular carcinoma cells

Hepatocellular carcinoma (HCC) is the fifth most common cancer worldwide and the third highest cause of cancer-related mortality in humans. Epigallocatechin-3-gallate (EGCG) has been shown to inhibit the metastatic activity of certain cancer cells. The aim of this study was to determine the effects and molecular mechanism(s) of action of EGCG in human HCC cells. A migration and invasion assay for the metastatic behavior of HCCLM6 cells was performed. The anti-metastatic effects of EGCG were investigated by RT-PCR and gelatin zymography. A total cellular protein profile was obtained using 2-dimensional gel electrophoresis (2-DE), followed by matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF-MS) analyses of proteins with significant differences in expression following treatment with EGCG. The results revealed that EGCG induced apoptosis and inhibited the metastasis of HCCLM6 cells. The anti-metastatic effects of EGCG were associated with the inhibition of matrix metalloproteinase (MMP)-2 and MMP-9 activity. The expression levels of far upstream element (FUSE) binding protein 1 (FUBP1), heat shock protein beta 1 (HSPB1), heat shock 60 kDa protein 1 (chaperonin) (CH60) and nucleophosmin (NPM) proteins, which are associated with metastasis, were significantly altered in the EGCG-treated HCCLM6 cells. The data from the present study suggest that EGCG has potential as a therapeutic agent for the treatment of HCC.

Phosphorylation of KRAB-associated protein 1 (KAP1) at Tyr-449, Tyr-458, and Tyr-517 by nuclear tyrosine kinases inhibits the association of KAP1 and heterochromatin protein 1α (HP1α) with heterochromatin

Protein tyrosine phosphorylation regulates a wide range of cellular processes at the plasma membrane. Recently, we showed that nuclear tyrosine phosphorylation by Src family kinases (SFKs) induces chromatin structural changes. In this study, we identify KRAB-associated protein 1 (KAP1/TIF1β/TRIM28), a component of heterochromatin, as a nuclear tyrosine-phosphorylated protein. Tyrosine phosphorylation of KAP1 is induced by several tyrosine kinases, such as Src, Lyn, Abl, and Brk. Among SFKs, Src strongly induces tyrosine phosphorylation of KAP1. Nucleus-targeted Lyn potentiates tyrosine phosphorylation of KAP1 compared with intact Lyn, but neither intact Fyn nor nucleus-targeted Fyn phosphorylates KAP1. Substitution of the three tyrosine residues Tyr-449/Tyr-458/Tyr-517, located close to the HP1 binding-motif, into phenylalanine ablates tyrosine phosphorylation of KAP1. Immunostaining and chromatin fractionation show that Src and Lyn decrease the association of KAP1 with heterochromatin in a kinase activity-dependent manner. KAP1 knockdown impairs the association of HP1α with heterochromatin, because HP1α associates with KAP1 in heterochromatin. Intriguingly, tyrosine phosphorylation of KAP1 decreases the association of HP1α with heterochromatin, which is inhibited by replacement of endogenous KAP1 with its phenylalanine mutant (KAP1-Y449F/Y458F/Y517F, KAP1-3YF). In DNA damage, KAP1-3YF repressed transcription of p21. These results suggest that nucleus-localized tyrosine kinases, including SFKs, phosphorylate KAP1 at Tyr-449/Tyr-458/Tyr-517 and inhibit the association of KAP1 and HP1α with heterochromatin.

Mimicking p14ARF phosphorylation influences its ability to restrain cell proliferation

The INK4a/ARF locus on the short arm of chromosome 9 is one of the most frequently altered loci in human cancer. It is generally accepted that ARF is involved in oncogenic checkpoint pathways by sensitizing incipient cancer cells to undergo growth arrest or apoptosis through both p53-dependent and independent pathways. While intensive studies have been focused on ARF activation at the transcriptional level, only recently mechanisms governing ARF turnover have been identified. Here, we show for the first time that p14ARF is a PKC target. Prediction analysis showed many potential phosphorylation sites in PKC consensus sequences within ARF protein, and, among them, the threonine at position 8 was the most conserved. Substitution of this threonine influences both ARF stability and localization. Furthermore, a phosphomimetic ARF mutation reduces the ability to arrest cell growth although the ability to bind MDM2 and stabilize p53 result unaffected. Thus we propose that phosphorylation of ARF in both immortalized and tumor cell lines could be a mechanism to escape ARF surveillance following proliferative and oncogenic stress.

MYCN is recruited to the RASSF1A promoter but is not critical for DNA hypermethylation in neuroblastoma

Tumor suppressor genes such as RASSF1A are often epigenetically repressed by DNA hypermethylation in neuroblastoma, where the MYCN proto-oncogene is frequently amplified. MYC has been shown to associate with DNA methyltransferases, thereby inducing transcriptional repression of target genes, which suggested that MYCN might play a similar mechanistic role in the hypermethylation of tumor suppressor genes in neuroblastoma. This study tested that hypothesis by using co-immunoprecipitation and ChIP to investigate MYCN-DNA methyltransferase interactions, together with MYCN knock-down and over-expression systems to examine the effect of MYCN expression changes on gene methylation, employing both candidate gene and genome-wide assays. We show that MYCN interacts with DNA methyltransferases and is recruited to the promoter region of RASSF1A. However, using four model systems, we showed that long-term silencing of MYCN induces only a small loss of DNA methylation at the RASSF1A promoter in MYCN amplified neuroblastoma cell lines and over-expression of MYCN does not induce any DNA methylation, suggesting that MYCN is not critical for DNA hypermethylation in neuroblastoma.

NMI mediates transcription-independent ARF regulation in response to cellular stresses

ETOC: NMI is a novel ARF-interacting protein identified in a yeast two-hybrid screen. NMI inhibits ULF-induced ubiquitin degradation of ARF protein. It mediates transcription-independent ARF regulation and is required for the stabilization and up-regulation of ARF in response to cellular stresses.

The ARF tumor suppressor is a product of the INK4a/ARF locus, which is frequently mutated in human cancer. The expression of ARF is up-regulated in response to certain types of DNA damage, oncogene activation, and interferon stimuli. Through interaction with the p53 negative regulator MDM2, ARF controls a well-described p53/MDM2-dependent checkpoint. However, the mechanism of ARF induction is poorly understood. Using a yeast two-hybrid screen, we identify a novel ARF-interacting protein, N-Myc and STATs interactor (NMI). Previously, NMI was known to be a c-Myc–interacting protein. Here we demonstrate that through competitive binding to the ARF ubiquitin E3 ligase (ubiquitin ligase for ARF [ULF]), NMI protects ARF from ULF-mediated ubiquitin degradation. In response to cellular stresses, NMI is induced, and a fraction of NMI is translocated to the nucleus to stabilize ARF. Thus our work reveals a novel NMI-mediated, transcription-independent ARF induction pathway in response to cellular stresses.

Miz1 is a critical repressor of cdkn1a during skin tumorigenesis

The transcription factor Miz1 forms repressive DNA-binding complexes with the Myc, Gfi-1 and Bcl-6 oncoproteins. Known target genes of these complexes encode the cyclin-dependent kinase inhibitors (CKIs) cdkn2b (p15^Ink4), cdkn1a (p21^Cip1), and cdkn1c (p57^Kip2). Whether Miz1-mediated repression is important for control of cell proliferation in vivo and for tumor formation is unknown. Here we show that deletion of the Miz1 POZ domain, which is critical for Miz1 function, restrains the development of skin tumors in a model of chemically-induced, Ras-dependent tumorigenesis. While the stem cell compartment appears unaffected, interfollicular keratinocytes lacking functional Miz1 exhibit a reduced proliferation and an accelerated differentiation of the epidermis in response to the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). Tumorigenesis, proliferation and normal differentiation are restored in animals lacking cdkn1a, but not in those lacking cdkn2b. Our data demonstrate that Miz1-mediated attenuation of cell cycle arrest pathways via repression of cdkn1a has a critical role during tumorigenesis in the skin.

The human papillomavirus type 16 E7 oncoprotein targets Myc-interacting zinc-finger protein-1

We demonstrate that HPV-16 E7 forms a complex with Miz-1. UV-induced expression of the CDK-inhibitor p21^Cip1 and subsequent cell cycle arrest depends upon endogenous Miz-1 in HPV-negative C33A cervical cancer cells containing mutated p53. Transient expression of E7 in C33A inhibits UV-induced expression of p21^Cip1 and overcomes Miz-1-induced G1-phase arrest. The C-terminal E7Δ79LEDLL83-mutant with reduced Miz-1-binding capacity was impaired in its capability to repress p21^Cip1 expression; whereas the pRB-binding-deficient E7C24G-mutant inhibited p21^Cip1 expression similar to wild-type E7. Using ChIP, we demonstrate that endogenous E7 is bound to the endogenous p21^Cip1 core-promoter in CaSki cells and RNAi-mediated knock down of Miz-1 abrogates E7-binding to the p21^Cip1 promoter. Co-expression of E7 with Miz-1 inhibited Miz-1-induced p21^Cip1 expression from the minimal-promoter via Miz-1 DNA-binding sites. Co-expression of E7Δ79LEDLL83 did not inhibit Miz-1-induced p21^Cip1 expression. E7C24G retained E7-wild-type capability to inhibit Miz-1-dependent transactivation. These findings suggest that HPV-16 E7 can repress Miz-1-induced p21^Cip1 gene expression.

Transcriptional repression: the dark side of myc

The (c-)Myc oncoprotein and its cousins, the N-Myc and L-Myc proteins, show all hallmarks of transcriptional activator proteins: Myc carries a carboxy-terminal DNA binding domain, which mediates sequence-specific binding to DNA. At its amino-terminus, Myc carries a transcriptional regulatory domain that strongly activates transcription when fused to an ectopic DNA binding domain; moreover, the strength of activation of different members of the Myc family correlates with their ability to transform rodent cells. Furthermore, activation of conditional alleles of Myc, either tetracycline or estrogen inducible, upregulates expression of a large number of genes, both in tissue culture and in transgenic animals. Indeed, many of these genes have essential roles in cell proliferation, cell growth, and metabolism; two of them, odc, encoding ornithine decarboxylase, a rate-limiting enzyme of polyamine biosynthesis, and rpl24, encoding a constituent of the large ribosomal subunit, are haploinsufficient for Myc-induced lymphomagenesis but not for normal development, arguing very strongly that upregulation of both genes is critical for Myc-dependent tumor formation. Undoubtedly, therefore, Myc exerts part of its biological activities via transcriptional upregulation of a large number of target genes. One of the key issues in the field is whether there are additional biochemical activities of the Myc protein and, if so, whether and how they contribute to Myc biology. This review summarizes evidence demonstrating that Myc has the ability to repress transcription and that this may be an important function during oncogenic transformation.

The role of the transcription factor Miz-1 in lymphocyte development and lymphomagenesis-Binding Myc makes the difference

The Myc interacting zinc finger protein 1 (Miz-1) is a BTB/POZ domain containing transcription factor that can function as an activator or repressor depending on its binding partners. In a complex with co-factors such as nuclophosmin or p300, Miz-1 stimulates transcription of genes that encode regulators of cell cycle progression such as p21(Cip1) or p15(Ink4b) or inhibitors of apoptosis such as Bcl-2. In contrast, Miz-1 becomes a transcriptional repressor when it binds to c-Myc or Bcl-6, which replace nucleophosmin or p300. During lymphocyte development, Miz-1 functions as a regulator of the IL-7 signaling pathway at very early steps in the bone marrow and thymus. When the IL-7 receptor (IL-7R) recognizes its cognate cytokine, a cascade of events is initiated that involves the recruitment of janus kinases (JAK) to the cytoplasmic part of the IL-7R, the phosphorylation of Stat5, its dimerization and relocation to the nucleus, enabling a transcriptional programming that governs commitment, survival and proliferation of lymphoid lineage cells. Miz-1 is critical in this signal transduction pathway, since it controls the expression of Socs1, an inhibitor of JAKs and thus of Stat5 activation and Bcl-2 expression. A lack of Miz-1 blocks IL-7 mediated signaling, which is detrimental for early B- and T-lymphoid development. These functions of Miz-1 during early lymphocyte development are c-Myc-independent. In contrast, when c-Myc is constitutively over-expressed, for instance during c-Myc induced lymphomagenesis, the interaction between Miz-1 and c-Myc becomes important and critical for the initiation and maintenance of c-Myc-dependent lymphoid malignancies.

Addicted to Myc--but why?

Many tumors continuously depend on the initiating oncogenes, but whether this extends to their downstream targets is unclear. In this issue of Genes & Development, Sodir and colleagues (pp. 907-916) demonstrate an essential role for endogenous Myc proteins in maintaining the tumor microenvironment, providing an unexpected molecular explanation for addiction to Myc.

The MK5/PRAK kinase and Myc form a negative feedback loop that is disrupted during colorectal tumorigenesis

Expression of the Myc oncoprotein is downregulated in response to stress signals to allow cells to cease proliferation and escape apoptosis, but the mechanisms involved in this process are poorly understood. Cell cycle arrest in response to DNA damage requires downregulation of Myc via a p53-independent signaling pathway. Here we have used siRNA screening of the human kinome to identify MAPKAPK5 (MK5, PRAK) as a negative regulator of Myc expression. MK5 regulates translation of Myc, since it is required for expression of miR-34b and miR-34c that bind to the 3'UTR of MYC. MK5 activates miR-34b/c expression via phosphorylation of FoxO3a, thereby promoting nuclear localization of FoxO3a and enabling it to induce miR-34b/c expression and arrest proliferation. Expression of MK5 in turn is directly activated by Myc, forming a negative feedback loop. MK5 is downregulated in colon carcinomas, arguing that this feedback loop is disrupted during colorectal tumorigenesis.

Bidirectional autoregulatory mechanism of metastasis-associated protein 1-alternative reading frame pathway in oncogenesis

Although metastasis-associated protein 1 (MTA1), a component of the nucleosome remodeling and histone deacetylation complex, is widely up-regulated in human cancers and correlates with tumor metastasis, its regulatory mechanism and related signaling pathways remain unknown. Here, we report a previously unrecognized bidirectional autoregulatory loop between MTA1 and tumor suppressor alternative reading frame (ARF). MTA1 transactivates ARF transcription by recruiting the transcription factor c-Jun onto the ARF promoter in a p53-independent manner. ARF, in turn, negatively regulates MTA1 expression independently of p53 and c-Myc. In this context, ARF interacts with transcription factor specificity protein 1 (SP1) and promotes its proteasomal degradation by enhancing its interaction with proteasome subunit regulatory particle ATPase 6, thereby abrogating the ability of SP1 to stimulate MTA1 transcription. ARF also physically associates with MTA1 and affects its protein stability. Thus, MTA1-mediated activation of ARF and ARF-mediated functional inhibition of MTA1 represent a p53-independent bidirectional autoregulatory mechanism in which these two opposites act in concert to regulate cell homeostasis and oncogenesis, depending on the cellular context and the environment.

Degringolade, a SUMO-targeted ubiquitin ligase, inhibits Hairy/Groucho-mediated repression

Transcriptional cofactors are essential for proper embryonic development. One such cofactor in Drosophila, Degringolade (Dgrn), encodes a RING finger/E3 ubiquitin ligase. Dgrn and its mammalian ortholog RNF4 are SUMO-targeted ubiquitin ligases (STUbLs). STUbLs bind to SUMOylated proteins via their SUMO interaction motif (SIM) domains and facilitate substrate ubiquitylation. In this study, we show that Dgrn is a negative regulator of the repressor Hairy and its corepressor Groucho (Gro/transducin-like enhancer (TLE)) during embryonic segmentation and neurogenesis, as dgrn heterozygosity suppresses Hairy mutant phenotypes and embryonic lethality. Mechanistically Dgrn functions as a molecular selector: it targets Hairy for SUMO-independent ubiquitylation that inhibits the recruitment of its corepressor Gro, without affecting the recruitment of its other cofactors or the stability of Hairy. Concomitantly, Dgrn specifically targets SUMOylated Gro for sequestration and antagonizes Gro functions in vivo. Our findings suggest that by targeting SUMOylated Gro, Dgrn serves as a molecular switch that regulates cofactor recruitment and function during development. As Gro/TLE proteins are conserved universal corepressors, this may be a general paradigm used to regulate the Gro/TLE corepressors in other developmental processes.

The interaction between Myc and Miz1 is required to antagonize TGFbeta-dependent autocrine signaling during lymphoma formation and maintenance

The Myc protein suppresses the transcription of several cyclin-dependent kinase inhibitors (CKIs) via binding to Miz1; whether this interaction is important for Myc's ability to induce or maintain tumorigenesis is not known. Here we show that the oncogenic potential of a point mutant of Myc (MycV394D) that is selectively deficient in binding to Miz1 is greatly attenuated. Binding of Myc to Miz1 is continuously required to repress CKI expression and inhibit accumulation of trimethylated histone H3 at Lys 9 (H3K9triMe), a hallmark of cellular senescence, in T-cell lymphomas. Lymphomas that arise express high amounts of transforming growth factor beta-2 (TGFbeta-2) and TGFbeta-3. Upon Myc suppression, TGFbeta signaling is required to induce CKI expression and cellular senescence and suppress tumor recurrence. Binding of Myc to Miz1 is required to antagonize growth suppression and induction of senescence by TGFbeta. We demonstrate that, since lymphomas express high levels of TGFbeta, they are poised to elicit an autocrine program of senescence upon Myc inactivation, demonstrating that TGFbeta is a key factor that establishes oncogene addiction of T-cell lymphomas.