Target genes and cellular regulators of the Myc transcription complex

B-myc: N-terminal recognition of myc binding proteins

B-Myc is an endogenous, N-terminal homologue of transcription factor c-Myc that lacks the C-terminal DNA binding and protein dimerization domain of c-Myc. Clinical mutations in the c-Myc N-terminal region, and the subsequent misregulation of Myc, are implicated in the development of numerous human cancers. Myc functions to both activate and repress transcription by associating with multiple binding partners. We investigated the structural and dynamical properties of B-Myc, free or associated with the transactivation inhibitor, MM-1, and the activator, TBP, using NMR spectroscopy. B-Myc has no persistent tertiary structure, yet regions corresponding to Myc homology boxes 1 and 2 (MBI and MBII, respectively) have molten globule-like characteristics. B-Myc binds to MM-1 in a specific manner without becoming highly structured. The local regions of B-Myc involved in binding differ for MM-1 and TBP, and regions not identified by mutagenesis are found to be involved in MM-1 binding. The results provide new insights into Myc N-terminal protein-protein interactions. We propose a model for Myc regulation through differential involvement of MBI and MBII in the binding of Myc interacting proteins.

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

Myc proteins are transcriptional activators, but also repress transcription through initiator (Inr) elements. Repression requires the conserved Myc Box II, but the cis-acting element(s) required for c-myc autorepression have eluded definition. Since the gene has a candidate Inr at the P2 promoter, we tested whether Myc autorepression operates through the Inr/BoxII mechanism. Overexpression of c-Myc but not a Box II deletion mutation represses both c-myc P2 reporter genes and endogenous c-myc, as does Mxi1 expression. Only 45 nucleotides surrounding the P2 start suffice to mediate autorepression, but Myc and Mxi1 also downregulate P2 Inr mutations, suggesting other core promoter sequence requirements for autorepression. We tested the importance of conserved E2F sites, based on known Myc interaction with the pRb-related p107 and on the transrepressive effects of Rb family proteins. Myc, Mxi1, and p107 repress c-myc somewhat less well in the absence of E2F binding sites, while an E2F+Inr double mutation is not repressed at all by these gene products. Further, Myc repression at the c-myc P2 core promoter is augmented by p107, but not by pRb or p130, nor by p107 lacking the conserved pocket domain. Our data suggest that Myc autorepression requires both the c-myc Inr and E2F sites in cis, as well as p107 in trans. Consistent with this, we found that retrovirally transduced c-Myc cannot downregulate endogenous c-myc in p107-null fibroblasts, and show evidence that both Myc and p107 are present in a complex assembled at the c-myc P2 core promoter.

The growth-inhibitory Ndrg1 gene is a Myc negative target in human neuroblastomas and other cell types with overexpressed N- or c-myc

A major prognostic marker for neuroblastoma (Nb) is N-myc gene amplification, which predicts a poor clinical outcome. We sought genes differentially expressed on a consistent basis between multiple human Nb cell lines bearing normal versus amplified N-myc, in hopes of finding target genes that might clarify how N-myc overexpression translates into poor clinical prognosis. Using differential display, we find the previously described growth-inhibitory gene Ndrg1 is strongly repressed in all tested Nb cell lines bearing N-myc amplification, as well as in a neuroepithelioma line with amplified c-myc. Overexpression of N-myc in non-amplified Nb cells leads to repression of Ndrg1, as does activation of an inducible c-myc transgene in fibroblasts. Conversely, N-myc downregulation in N-myc-amplified Nb cells results in re-expression of the Ndrg1, and stimuli known to induce Ndrg1 do so in Nb cells while simultaneously down-regulating N-myc. Relevant to these results, we demonstrate an in vitro interaction of Myc protein with the Ndrg1 core promoter. We also find that Ndrg1 levels increase dramatically during in vitro differentiation of two cell lines modeling neural and glial development, while c- and N-myc levels decline. Our results combined with previous information on the Ndrg1 gene product suggest that downregulation of this gene is an important component of N-Myc effects in neuroblastomas with poor clinical outcome. In support of this notion, we find that re-expression of Ndrg1 in high-Myc Nb cells results in smaller cells with reduced colony size in soft-agar assays, further underscoring the functional significance of this gene in human neuroblastoma cells.

Transcription from the tartrate-resistant acid phosphatase promoter is negatively regulated by the Myc oncoprotein

TRAP, a characteristic marker of osteoclast differentiation, is an enzyme that plays an active role in the process of bone resorption. Despite the importance of TRAP in osteoclast biology, the components involved in the transcriptional regulation of this gene are largely unknown. This study investigated the regulation of TRAP transcription by the Myc oncoprotein in three different cell types. A series of nested TRAP promoter deletion constructs were cotransfected into P388D1 murine macrophages and C3H10T1/2 murine embryonic fibroblasts along with a backbone plasmid control or expression plasmids containing v-Myc, c-Myc, or an inactive v-Myc protein construct (delta84/NLS). Both v-Myc and c-Myc negatively regulated transcription from the TRAP promoter in P388D1 and C3H10T1/2 cells, 90% and 50%, respective to cell type and amount of endogenous Myc protein, and delta84/NLS had no effect. The functional Myc-responsive element(s) within the TRAP promoter was localized to a region between -436 and +1 bp, which contains two putative Myc-inhibitory binding sites coincident with an initiator element (Inr) at -116 bp and -18 bp. Conversely, in the HD-11EM chicken v-Myc transformed preosteoclast cell line, the full-length TRAP promoter transcription was increased when endogenous v-Myc levels were decreased in response to pretreatment of these cells with 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3]. This report provides the first evidence of the specific regulation of TRAP at the transcriptional level by Myc, a transcription factor that is normally expressed at relatively high levels in preosteoclasts and other myelomonocytic cells and suggests that Myc plays an active role in suppressing the transcription of a mature osteoclast selective gene.

The myc oncogene: MarvelouslY Complex

The activated product of the myc oncogene deregulates both cell growth and death check points and, in a permissive environment, rapidly accelerates the affected clone through the carcinogenic process. Advances in understanding the molecular mechanism of Myc action are highlighted in this review. With the revolutionary developments in molecular diagnostic technology, we have witnessed an unprecedented advance in detecting activated myc in its deregulated, oncogenic form in primary human cancers. These improvements provide new opportunities to appreciate the tumor subtypes harboring deregulated Myc expression, to identify the essential cooperating lesions, and to realize the therapeutic potential of targeting Myc. Knowledge of both the breadth and depth of the numerous biological activities controlled by Myc has also been an area of progress. Myc is a multifunctional protein that can regulate cell cycle, cell growth, differentiation, apoptosis, transformation, genomic instability, and angiogenesis. New insights into Myc's role in regulating these diverse activities are discussed. In addition, breakthroughs in understanding Myc as a regulator of gene transcription have revealed multiple mechanisms of Myc activation and repression of target genes. Moreover, the number of reported Myc regulated genes has expanded in the past few years, inspiring a need to focus on classifying and segregating bona fide targets. Finally, the identity of Myc-binding proteins has been difficult, yet has exploded in the past few years with a plethora of novel interactors. Their characterization and potential impact on Myc function are discussed. The rapidity and magnitude of recent progress in the Myc field strongly suggests that this marvelously complex molecule will soon be unmasked.

What retroviruses teach us about the involvement of c-Myc in leukemias and lymphomas

Overexpression of the cellular oncogene c-Myc frequently occurs during induction of leukemias and lymphomas in many species. Retroviruses have enhanced our understanding of the role of c-Myc in such tumors. Leukemias and lymphomas induced by retroviruses activate c-Myc by: (1) use of virally specified proteins that increase c-Myc transcription, (2) transduction and modification of c-Myc to generate a virally encoded form of the gene, v-Myc, and (3) proviral integration in or near c-Myc. Proviral integrations elevate transcription by insertion of retroviral enhancers found in the long terminal repeat (LTR). Studies of the LTR enhancer elements from these retroviruses have revealed the importance of these elements for c-Mycactivation in several cell types. Retroviruses also have been used to identify genes that collaborate with c-Myc during development and progression of leukemias and lymphomas. In these experiments, animals that are transgenic for c-Mycoverexpression (often in combination with the overexpression or deletion of known proto-oncogenes) have been infected with retroviruses that then insertionally activate novel co-operating cellular genes. The retrovirus then acts as a molecular 'tag' for cloning of these genes. This review covers several aspects of c-Myc involvement in retrovirally induced leukemias and lymphomas.

The genomic structure of human olfactory receptor genes

The genomic and cDNA structures were studied for eight human olfactory receptor (OR) genes within the chromosome 17p13.3 cluster. A common gene structure was revealed, which included an approximately 1-kb intronless coding region terminated by a signal for polyadenylation and a variable number of upstream noncoding exons. The latter were found to be alternatively spliced, giving rise to different isoforms of OR mRNA. While the initial exons mostly agreed with previous computer predictions and were conserved within OR subfamilies, other upstream exons were novel and idiosyncratic. In some cases, repetitive sequences were involved in the generation of splice sites and putative transcription control elements. Such gene structure is consistent with early repertoire enhancement by retrogene generation, which was likely followed by extensive genomic duplication. Each OR gene had a unique signature of transcription factor elements, consistent with a combinatorial expression control mechanism.

TCR-independent T cell development mediated by gain-of-oncogene function or loss-of-tumor-suppressor gene function

The mechanisms that govern differentiation of T cell precursors during intrathymic development bridge an interdisciplinary research field of immunology, oncology and developmental biology. Critical checkpoints controlling early thymic T cell development and homeostasis are set by the proper signaling function of the IL-7 receptor, c-Kit receptor, and the pre-T cell antigen receptor (pre-TCR). Given the intimate link between cell cycle control and differentiation in T cell development, proto-oncogenes and tumor suppressors participate as physiological effectors downstream of these receptors not only to influence the cell cycle but also to determine differentiation and survival. Gain- or loss-of-function mutations of these downstream effectors uncouples partially or completely T cell precursors from these checkpoints, providing a selective advantage and enabling aberrant development. These effectors can be identified by provirus tagging in normal mice and more readily by complementation tagging in mice with a predefined block in T cell differentiation.

Mlx, a new Max-like bHLHZip family member: the center stage of a novel transcription factors regulatory pathway?

The Myc proto-oncogene family members have been identified as the cellular homologs of the transforming oncogene of avian retroviruses. They encode central regulators of mammalian cell proliferation and apoptosis, and they associate with the bHLHZip protein Max to bind specific DNA sequences and regulate the expression of genes important for cell cycle progression. The other family members, Mad1, Mxi1, Mad3, Mad4 and Rox (Mnt) antagonize their activities. The Mads and Rox compete with Myc in heterodimerizing with Max and in binding to the same specific target sequences. These Mads:Max and Rox:Max dimers repress transcription through binding to the mSIN3 corepressor protein and by tethering histone deacetylase-containing complexes to the DNA. In a screen for Rox interactors we isolated Mlx, a bHLHZip protein previously identified in a screen for Mad1 interactors. In the present work we extend the known dimerization partners of Mlx by demonstrating its ability to interact with Rox. Moreover, we show that contrary to previous reports Mlx is able to homodimerize and to bind E-box sequences at low concentration levels. The possible role of Mlx in an emerging regulatory pathway and acting parallel to the Max driven network is discussed.

The essential cofactor TRRAP recruits the histone acetyltransferase hGCN5 to c-Myc

The c-Myc protein functions as a transcription factor to facilitate oncogenic transformation; however, the biochemical and genetic pathways leading to transformation remain undefined. We demonstrate here that the recently described c-Myc cofactor TRRAP recruits histone acetylase activity, which is catalyzed by the human GCN5 protein. Since c-Myc function is inhibited by recruitment of histone deacetylase activity through Mad family proteins, these opposing biochemical activities are likely to be responsible for the antagonistic biological effects of c-Myc and Mad on target genes and ultimately on cellular transformation.

The Adrenomedullin gene is a target for negative regulation by the Myc transcription complex

The Myc family of transcription factors plays a central role in vertebrate growth and development although relatively few genetic targets of the Myc transcription complex have been identified. In this study, we used mRNA differential display to investigate gene expression changes induced by the overexpression of the MC29 v-Myc oncoprotein in C3H10T1/2 mouse fibroblasts. We identified the transcript of the adrenomedullin gene (AM) as an mRNA that is specifically down-regulated in v-Myc overexpressing C3H10T1/2 cell lines as well as in a Rat 1a cell line inducible for c-Myc. Nucleotide sequence analysis of the mouse AM promoter reveals the presence of consensus CAAT and TATA boxes as well as an initiator element (INR) with significant sequence similarity to the INR responsible for Myc-mediated repression of the adenovirus major late promoter (AdMLP). Reporter gene assays confirm that the region of the AM promoter containing the INR is the target of Myc-mediated repression. Exogenous application of AM peptide to quiescent C3H10T1/2 cultures does not stimulate growth, and constitutive expression of AM mRNA in C3H10T1/2 cells correlates with a reduced potential of the cells to be cotransformed by v-Myc and oncogenic Ras p21. Additional studies showing that AM mRNA is underrepresented in C3H10T1/2 cell lines stably transformed by Ras p21 or adenovirus E1A suggest that AM gene expression is incompatible with deregulated growth in this cell line. We propose a model in which the repression of AM gene expression by Myc is important to the role of this oncoprotein as a potentiator of cellular transformation in C3H10T1/2 and perhaps other cell lines.