Autorepression of c-myc requires both initiator and E2F-binding site elements and cooperation with the p107 gene product

Transcriptional suppression of the miR-15/16 family by c-Myc in malignant pleural mesothelioma

MicroRNA downregulation is frequent in malignant pleural mesothelioma (MPM), but the mechanisms responsible for loss of miR-15/16 and miR-193a are yet to be elucidated and were investigated in this study. Copy Number Variation (CNV) of microRNA-coding genes was analyzed in MPM cells by digital droplet PCR (ddPCR) and revealed heterozygous loss of miR-193a and miR-15a/16-1, but no change in miR-15b/16-2. Epigenetic control of microRNA expression was inferred following decitabine and Trichostatin A (TSA) treatment which did not substantially affect microRNA expression. Knockdown of c-Myc expression led to upregulation of SMC4, miR-15b and 16, and to a lesser extent DLEU2 and miR-15a, whereas c-Myc overexpression repressed microRNA expression. Chromatin immunoprecipitation (ChIP) assays confirmed the interaction of c-Myc with the DLEU2 and SMC4 promoters. Tumor microRNA expression was determined in samples from MPM patients, with samples of pleura from cardiac surgery patients used as controls. In tumor samples, a strong correlation was observed between the expression of miR-15b and 16 (R²=0.793), but not miR-15a and 16. Our data suggest that in MPM, the downregulation of miR-15/16 is due to transcriptional repression by c-Myc, primarily via control of the miR-15b/16-2 locus, while miR-193a-3p loss is due to genomic deletion.

Proliferation and Survival of Embryonic Sympathetic Neuroblasts by MYCN and Activated ALK Signaling

Neuroblastoma (NB) is a childhood tumor that arises from the sympathoadrenal lineage. MYCN amplification is the most reliable marker for poor prognosis and MYCN overexpression in embryonic mouse sympathetic ganglia results in NB-like tumors. MYCN cooperates with mutational activation of anaplastic lymphoma kinase (ALK), which promotes progression to NB, but the role of MYCN and ALK in tumorigenesis is still poorly understood. Here, we use chick sympathetic neuroblasts to examine the normal function of MYCN and MYC in the control of neuroblast proliferation, as well as effects of overexpression of MYCN, MYC, and activated ALK, alone and in combination. We demonstrate that MYC is more strongly expressed than MYCN during neurogenesis and is important for in vitro neuroblast proliferation. MYC and MYCN overexpression elicits increased proliferation but does not sustain neuroblast survival. Unexpectedly, long-term expression of activated ALK^F1174L leads to cell-cycle arrest and promotes differentiation and survival of postmitotic neurons. ALK^F1174L induces NEFM, RET, and VACHT and results in decreased expression of proapototic (BMF, BIM), adrenergic (TH), and cell-cycle genes (e.g., CDC25A, CDK1). In contrast, neuroblast proliferation is maintained when MYCN and ALK^F1174L are coexpressed. Proliferating MYCN/ALK^F1174L neuroblasts display a differentiated phenotype but differ from ALK-expressing neurons by the upregulation of SKP2, CCNA2, E2F8, and DKC1 Inhibition of the ubiquitin ligase SKP2 (S-phase kinase-associated protein 2), which targets the CDK inhibitor p27 for degradation, reduces neuroblast proliferation, implicating SKP2 in the maintained proliferation of MYCN/ALK^F1174L neuroblasts. Together, our results characterize MYCN/ALK cooperation leading to neuroblast proliferation and survival that may represent initial steps toward NB development.

SIGNIFICANCE STATEMENT

MYCN overexpression combined with activated anaplastic lymphoma kinase (ALK) is sufficient to induce neuroblastoma (NB) in mouse sympathoadrenal cells. To address cellular and molecular effects elicited by MYCN/ALK cooperation, we used cultures of chick sympathetic neuroblasts. We demonstrate that MYCN increases proliferation but not survival, whereas long-term expression of ALK^F1174L elicits cell-cycle exit, differentiation, and survival of postmitotic neurons. Combined MYCN/ALK^F1174L expression allows long-term proliferation and survival of neuroblasts with differentiated characteristics. In the presence of ALK^F1174L signaling, MYCN induces the expression of the ubiquitin ligase SKP2 (S-phase kinase-associated protein 2), which targets p27 for degradation and is also upregulated in high-risk NB. SKP2 inhibition supports a function for SKP2 in the maintained neuroblast proliferation downstream of MYCN/ALK, which may represent an early step toward tumorigenesis.

pVHL-mediated transcriptional repression of c-Myc by recruitment of histone deacetylases

The biological functions of Myc are to regulate cell growth,apoptosis, cell differentiation and stem-cell self-renewal. Abnormal accumulation of c-Myc is able to induce excessive proliferation of normal cells. von Hippel-Lindau protein(pVHL) is a key regulator of hypoxia-inducible factor 1α(HIF1α), thus accumulation and hyperactivation of HIF1α is the most prominent feature of VHL-mutated renal cell carcinoma. Interestingly, the Myc pathway is reported to be activated in renal cell carcinoma even though the precise molecular mechanism still remains to be established. Here, we demonstrated that pVHL locates at the c-Myc promoter region through physical interaction with Myc. Furthermore, pVHL reinforces HDAC1/2 recruitment to the Myc promoter, which leads to the auto-suppression of Myc. Therefore, one possible mechanism of Myc auto-suppression by pVHL entails removing histone acetylation. Our study identifies a novel mechanism for pVHL-mediated negative regulation of c-Myc transcription.

Interferon-inducible IFI16, a negative regulator of cell growth, down-regulates expression of human telomerase reverse transcriptase (hTERT) gene

Background

Increased levels of interferon (IFN)-inducible IFI16 protein (encoded by the IFI16 gene located at 1q22) in human normal prostate epithelial cells and diploid fibroblasts (HDFs) are associated with the onset of cellular senescence. However, the molecular mechanisms by which the IFI16 protein contributes to cellular senescence-associated cell growth arrest remain to be elucidated. Here, we report that increased levels of IFI16 protein in normal HDFs and in HeLa cells negatively regulate the expression of human telomerase reverse transcriptase (hTERT) gene.

Methodology/Principal Findings

We optimized conditions for real-time PCR, immunoblotting, and telomere repeat amplification protocol (TRAP) assays to detect relatively low levels of hTERT mRNA, protein, and telomerase activity that are found in HDFs. Using the optimized conditions, we report that treatment of HDFs with inhibitors of cell cycle progression, such as aphidicolin or CGK1026, which resulted in reduced steady-state levels of IFI16 mRNA and protein, was associated with increases in hTERT mRNA and protein levels and telomerase activity. In contrast, knockdown of IFI16 expression in cells increased the expression of c-Myc, a positive regulator of hTERT expression. Additionally, over-expression of IFI16 protein in cells inhibited the c-Myc-mediated stimulation of the activity of hTERT-luc-reporter and reduced the steady-state levels of c-Myc and hTERT.

Conclusions/Significance

These data demonstrated that increased levels of IFI16 protein in HDFs down-regulate the expression of hTERT gene. Our observations will serve basis to understand how increased cellular levels of the IFI16 protein may contribute to certain aging-dependent diseases.

In vivo GSH depletion induces c-myc expression by modulation of chromatin protein complexes

We hypothesize that glutathione (GSH) fluctuations could have a prominent role in the modulation of c-myc expression through a mechanism affecting chromatin remodeling complexes. This could lead to an open chromatin structure accessible to transcription factors. We studied the in vivo effect of GSH depletion on these complexes bound to the c-myc promoter in the liver of l-buthionine-(S,R)-sulfoximine (BSO)-treated rats. Using chromatin immunoprecipitation we found that 3 h after BSO treatment the repressing complexes Id2 and Sin3A (part of a histone-deacetylase complex) were released from the c-myc promoter. STAT3 was phosphorylated and associated with its coactivator p300 with intrinsic acetyltransferase activity. Consequently, STAT3 was acetylated and bound to the c-myc promoter and histone H3 became hyperacetylated. At the same time, the RNApol II paused on the c-myc promoter was released, and the gene was overexpressed. After 6 h of BSO treatment, Id2/Sin3A returned to the c-myc promoter and the gene expression was down-regulated. Moreover, we observed a second peak of c-myc expression 48 h after BSO treatment, although at this time histone H3 was hypoacetylated and RNApol II paused, suggesting that this second peak was not subject to transcriptional control, but to posttranscriptional modulation. On the whole, our experiments suggest a novel mechanism for the effect of GSH on gene expression involving chromatin changes from a repressive to an open structure accessible to transcription factors such as STAT3.

Differential effects of Mxi1-SRalpha and Mxi1-SRbeta in Myc antagonism

Mxi1 belongs to the Myc-Max-Mad transcription factor network. Two Mxi1 protein isoforms, Mxi1-SRalpha and Mxi1-SRbeta, have been described as sharing many biological properties. Here, we assign differential functions to these isoforms with respect to two distinct levels of Myc antagonism. Unlike Mxi1-SRbeta, Mxi1-SRalpha is not a potent suppressor of the cellular transformation activity of Myc. Furthermore, although Mxi1-SRbeta exhibits a repressive effect on the MYC promoter in transient expression assays, Mxi1-SRalpha activates this promoter. A specific domain of Mxi1-SRalpha contributes to these differences. Moreover, glyceraldehyde-3-phosphate dehydrogenase interacts with Mxi1-SRalpha and enhances its ability to activate the Myc promoter. Our findings suggest that Mxi1 gains functional complexity by encoding isoforms with shared and distinct activities.

BCL2 is a downstream effector of MIZ-1 essential for blocking c-MYC-induced apoptosis

The c-MYC oncoprotein is among the most potent transforming agents in human cells. Ironically, c-MYC is also capable of inducing massive apoptosis under certain conditions. A clear understanding of the distinct pathways activated by c-MYC during apoptosis induction and transformation is crucial to the design of therapeutic strategies aimed at selectively reactivating the apoptotic potential of c-MYC in cancer cells. We recently demonstrated that apoptosis induction in primary human cells strictly requires that c-MYC bind and inactivate the transcription factor MIZ-1. This presumably blocked the ability of MIZ-1 to activate the transcription of an unidentified pro-survival gene. Here we report that MIZ-1 activates the transcription of BCL2. More importantly, inhibition of the MIZ-1/BCL2 signal is an essential event during the apoptotic response. Furthermore, targeting BCL2 with short hairpin RNA or small molecule inhibitors restores the apoptotic potential of a c-MYC mutant that is defective for MIZ-1 inhibition. These observations suggest that repression of BCL2 transcription is the single essential consequence of targeting the MIZ-1 pathway during apoptosis induction. These data define a genetic pathway that helps to explain historical observations documenting cooperation between c-MYC and BCL2 overexpression in human cancer.

KetogenicHMGCS2Is a c-Myc Target Gene Expressed in Differentiated Cells of Human Colonic Epithelium and Down-Regulated in Colon Cancer

HMGCS2, the gene that regulates ketone body production, is expressed in liver and several extrahepatic tissues, such as the colon. In CaCo-2 colonic epithelial cells, the expression of this gene increases with cell differentiation. Accordingly, immunohistochemistry with specific antibodies shows that HMGCS2 is expressed mainly in differentiated cells of human colonic epithelium. Here, we used a chromatin immunoprecipitation assay to study the molecular mechanism responsible for this expression pattern. The assay revealed that HMGCS2 is a direct target of c-Myc, which represses HMGCS2 transcriptional activity. c-Myc transrepression is mediated by blockade of the transactivating activity of Miz-1, which occurs mainly through a Sp1-binding site in the proximal promoter of the gene. Accordingly, the expression of human HMGCS2 is down-regulated in 90% of Myc-dependent colon and rectum tumors. HMGCS2 protein expression is down-regulated preferentially in moderately and poorly differentiated carcinomas. In addition, it is also down-regulated in 80% of small intestine Myc-independent tumors. Based on these findings, we propose that ketogenesis is an undesirable metabolic characteristic of the proliferating cell, which is down-regulated through c-Myc-mediated repression of the key metabolic gene HMGCS2.

Tumor suppressor SMAR1 mediates cyclin D1 repression by recruitment of the SIN3/histone deacetylase 1 complex

Matrix attachment region binding proteins have been shown to play an important role in gene regulation by altering chromatin in a stage- and tissue-specific manner. Our previous studies report that SMAR1, a matrix-associated protein, regresses B16-F1-induced tumors in mice. Here we show SMAR1 targets the cyclin D1 promoter, a gene product whose dysregulation is attributed to breast malignancies. Our studies reveal that SMAR1 represses cyclin D1 gene expression, which can be reversed by small interfering RNA specific to SMAR1. We demonstrate that SMAR1 interacts with histone deacetylation complex 1, SIN3, and pocket retinoblastomas to form a multiprotein repressor complex. This interaction is mediated by the SMAR1(160-350) domain. Our data suggest SMAR1 recruits a repressor complex to the cyclin D1 promoter that results in deacetylation of chromatin at that locus, which spreads to a distance of at least the 5 kb studied upstream of the cyclin D1 promoter. Interestingly, we find that the high induction of cyclin D1 in breast cancer cell lines can be correlated to the decreased levels of SMAR1 in these lines. Our results establish the molecular mechanism exhibited by SMAR1 to regulate cyclin D1 by modification of chromatin.

p53-Dependent transcriptional repression of c-myc is required for G1 cell cycle arrest

The ability of p53 to promote apoptosis and cell cycle arrest is believed to be important for its tumor suppression function. Besides activating the expression of cell cycle arrest and proapoptotic genes, p53 also represses a number of genes. Previous studies have shown an association between p53 activation and down-regulation of c-myc expression. However, the mechanism and physiological significance of p53-mediated c-myc repression remain unclear. Here, we show that c-myc is repressed in a p53-dependent manner in various mouse and human cell lines and mouse tissues. Furthermore, c-myc repression is not dependent on the expression of p21(WAF1). Abrogating the repression of c-myc by ectopic c-myc expression interferes with the ability of p53 to induce G(1) cell cycle arrest and differentiation but enhances the ability of p53 to promote apoptosis. We propose that p53-dependent cell cycle arrest is dependent not only on the transactivation of cell cycle arrest genes but also on the transrepression of c-myc. Chromatin immunoprecipitation assays indicate that p53 is bound to the c-myc promoter in vivo. We report that trichostatin A, an inhibitor of histone deacetylases, abrogates the ability of p53 to repress c-myc transcription. We also show that p53-mediated transcriptional repression of c-myc is accompanied by a decrease in the level of acetylated histone H4 at the c-myc promoter and by recruitment of the corepressor mSin3a. These data suggest that p53 represses c-myc transcription through a mechanism that involves histone deacetylation.

G1/S transcriptional networks modulated by the HOX11/TLX1 oncogene of T-cell acute lymphoblastic leukemia

The HOX11/TLX1 homeobox gene is aberrantly expressed in a subset of T-cell acute lymphoblastic leukemia (T-ALL). Here, we employed oligonucleotide microarrays to compare the expression profiles of the K3P and Sil leukemic cell lines originating from patients with HOX11+ T-ALL to that of Jurkat cells, which originated from a distinct subtype of T-ALL (TAL1+). To distinguish potential HOX11 target genes from those characteristic of the stage of HOX11 leukemic arrest, we also performed gene expression analysis on Jurkat cells, genetically engineered to express exogenous HOX11. The resulting HOX11 gene expression signature, which was validated for representative signaling pathways by transient transfection of reporter constructs, was characterized by elevated expression of transcriptional programs involved in cell proliferation, including those regulated by E2F, c-Myc and cAMP response element-binding protein. We subsequently showed that ectopic HOX11 expression resulted in hyperphosphorylation of the retinoblastoma protein (Rb), which correlated with inhibition of the major Rb serine/threonine phosphatase PP1. HOX11 also inhibited PP2A serine/threonine phosphatase activity concomitant with stimulation of the AKT/PKB signaling cascade. These results suggest that transcriptional deregulation of G1/S growth-control genes, mediated in large part through blockade of PP1/PP2A phosphatase activity, plays an important role in HOX11 pathobiology.

C-Myc-independent restoration of multiple phenotypes by two C-Myc target genes with overlapping functions

C-MYC, a transforming oncogene that is frequently overexpressed in many human cancers, regulates a variety of normal functions including cell cycle progression, apoptosis, and maintenance of cell size, morphology, and genomic integrity. Many target genes are modulated by c-Myc, and some can recapitulate a limited number of the above functions. Because most of these have been assessed in cells which also express endogenous c-Myc, however, it is not clear to what extent its proper regulation is also required. We show here that, in c-Myc nullizygous cells, two direct target genes, MT-MC1 and HMG-I, could each recapitulate multiple c-Myc phenotypes. Although these differ somewhat for the two genes, substantial overlap and cooperativity exist. The enforced expression of these two genes was also associated with the differential deregulation of some previously described c-Myc target genes, indicating the presence of a complex molecular circuitry. These observations argue that, despite the great diversity of gene regulation by c-Myc, many, although not all, of its functions can be phenocopied by a small subset of key downstream target genes. The approach described here should permit the identification of other target genes capable of further c-Myc-independent complementation.

Autoregulation of the N-myc gene is operative in neuroblastoma and involves histone deacetylase 2

BACKGROUND

Autoregulation of the myc gene family is a negative feedback mechanism known to occur at high levels of Myc expression. Loss of this mechanism and associated Myc overexpression has been observed in human tumors, thereby contributing to tumorigenesis. The childhood tumor neuroblastoma is characterized by N-myc amplification in aggressive and highly proliferative tumors that occur in a subset of patients. The precise molecular mechanism of autoregulation is unknown, and previous observations indicated that N-myc autoregulation was intact only in single-copy neuroblastoma cell lines.

METHODS

Transient reporter assays and trichostatin A (TSA) experiments were performed to evaluate several candidate genes, including Mxi1, c-myc promoter binding protein 1 (MBP-1), Miz, and histone deacetylase 2 (HDAC2), for their involvement in N-myc autoregulation. Mxi1 and HDAC2 were examined further for their expression levels and effects on endogenous N-myc levels. Finally, their recruitments to the N-myc promoter were investigated by chromatin immunoprecipitation (ChIP).

RESULTS

The autoregulatory circuit was operative, even in amplified cell lines. Mxi1 consistently showed a modest effect in down-regulating N-myc in transient reporter assays. Overexpression of the c-myc, Mxi1, and mHDAC2 genes resulted in a threefold to fourfold decrease in endogenous N-myc levels. Mxi1 and HDAC2 were up-regulated by N-Myc in an myc-inducible cell line and in N-myc-expressing cell lines. In addition, down-regulation of the N-myc promoter was relieved in the presence of TSA. Increased association of HDAC2 with the autoregulatory region within the N-myc promoter by ChIP was observed upon down-regulation of endogenous N-myc.

CONCLUSIONS

The autoregulatory circuit was intact in both amplified and single-copy neuroblastoma cell lines. Furthermore, myc gene autoregulation occurred through histone deacetylation.