c-Myc target genes involved in cell growth, apoptosis, and metabolism

MBP-1 physically associates with histone deacetylase for transcriptional repression

MBP-1, a c-myc promoter binding protein, is a mammalian transcription factor with intriguing properties including transcriptional repression of cellular genes. Recently, we have identified and characterized two different repressor domains of MBP-1. In this report, we have demonstrated that MBP-1 physically associates with histone deacetylase (HDAC), thus promoting formation of neucleosomes that inhibit transcription. Trichostatin A, an inhibitor of histone deacetylase, significantly reduces MBP-1-mediated transcriptional repression. However, MBP-1-mediated repression on c-myc promoter is resistant to histone deacetylase activity. Our results suggest that MBP-1 represses transcription by recruiting histone deacetylase as one of the mechanisms, whereas the other mechanism is resistant to HDAC activity and probably related to direct binding of promoter sequences or interaction through yet unidentified factor.

Mechanisms of apoptosis by c-Myc

Much recent research on c-Myc has focused on how it drives apoptosis. c-Myc is widely known as a crucial regulator of cell proliferation in normal and neoplastic cells, but until relatively recently its apoptotic properties, which appear to be intrinsic, were not fully appreciated. Its death-dealing aspects have gained wide attention in part because of their potential therapeutic utility in advanced malignancy, where c-Myc is frequently deregulated and where novel modalities are badly needed. Although its exact function remains obscure, c-Myc is a transcription factor and advances have been made in characterizing target genes which may mediate its apoptotic properties. Candidate regulators and effectors are also emerging. Among recent findings are connections to the CD95/Fas and TNF pathways and roles for the tumor suppressor p19ARF and the c-Myc-interacting adaptor protein Binl in mediating cell death. In this review I summarize the data establishing a role for c-Myc in apoptosis in diverse settings and present a modified dual signal model for c-Myc function. It is proposed that c-Myc induces apoptosis through separate 'death priming' and 'death triggering' mechanisms in which 'death priming' and mitogenic signals are coordinated. Investigation of the mechanisms that underlie the triggering steps may offer new therapeutic opportunities.

The basic region/helix-loop-helix/leucine zipper domain of Myc proto-oncoproteins: function and regulation

A large body of evidence has been accumulated that demonstrates dominant effects of Myc proto-oncoproteins on different aspects of cellular growth. Myc is one of the few proteins that is sufficient to drive resting cells into the cell cycle and promote DNA synthesis. In line with this finding is that the constitutive expression of Myc in cells blocks their differentiation. These growth stimulating properties are most likely responsible for Myc's ability to initiate and promote tumor formation. Interestingly Myc can also sensitize cells to apoptosis, suggesting that this protein is part of a life-and-death switch. Molecularly Myc functions as a transcriptional regulator that needs to heterodimerize with Max to exert the biological activities described above and to regulate gene transcription. Myc and Max are just two members of a growing family of proteins referred to as the Myc/Max/Mad network. A hallmark of these proteins is that they possess a C-terminal basic region/helix-loop-helix/leucine zipper domain (bHLHZip). The bHLHZip domain specifies dimerization within the network and determines sequence specific DNA binding. Importantly this domain together with the N-terminal transactivation domain is essential for Myc biology. Here we have summarized the structural, functional, and regulatory aspects of the bHLHZip domain of Myc proteins.

The Myc oncoprotein: a critical evaluation of transactivation and target gene regulation

Mutations which disrupt the regulation or expression level of the c-myc gene are among the most common found in human and animal cancers (reviewed in ref. Cole, 1986; Henriksson and Luscher, 1996; Marcu et al., 1992). Ectopic expression studies define numerous biological activities of the c-myc gene, including transformation, immortalization, blockage of cell differentiation and induction of apoptosis (Askew et al., 1991; Cole, 1986; Evan and Littlewood, 1993; Freytag et al., 1990; Henriksson and Luscher, 1996; Marcu et al., 1992). Furthermore, c-myc is required for efficient progression through the cell cycle (Goruppi et al., 1994; Prochownik et al., 1988; Yokoyama and Imamoto, 1987), although recent studies indicate that it is not absolutely essential (Mateyak et al., 1997). This fascinating array of biological activities makes the c-myc gene one of the most intriguing oncogenes and presents the challenging question of how a single gene can manifest so many different effects. The c-Myc protein exhibits sequence-specific DNA binding when dimerized with its partner Max, and DNA binding is mediated through the basic region, which recognizes the core sequence CACGTG (Berberich et al., 1992; Blackwell et al., 1993; Blackwood and Eisenman, 1991; Prendergast and Ziff, 1991; Prendergast et al., 1991), but exhibits somewhat higher affinity for the more extended sequence ACCACGTGGT (Berberich et al., 1992; Blackwell et al., 1993; Halazonetis and Kandil, 1991). There are three closely related Myc family proteins (c-Myc, N-Myc and L-Myc), each with documented oncogenic potential (Birrer et al., 1988; Schwab et al., 1985; Yancopoulos et al., 1985) and similar DNA binding properties (Mukherjee et al., 1992). For simplicity, we will use the term Myc to refer to all three proteins, but delineate any distinct activities where they apply. The goal of this review is to discuss Myc as a transcriptional activator and critically evaluate the evidence for the transactivation of specific target genes as direct downstream effectors. Since excellent comprehensive reviews on Myc have been published recently (Facchini and Penn, 1998; Henriksson and Luscher, 1996), we will focus on the latest observations that offer mechanistic insight into transactivation and oncogenic transformation.

MYC oncogenes and human neoplastic disease

c-myc, N-myc and L-myc are the three members of the myc oncoprotein family whose role in the pathogenesis of many human neoplastic diseases has received wide empirical support. In this review, we first summarize data, derived mainly from non-clinical studies, indicating that these oncoproteins actually serve quite different roles in vivo. This concept necessarily lies at the heart of the basis for the observation that the deregulated expression of each MYC gene is reproducibly associated with only certain naturally occurring malignancies in humans and that these genes are not interchangeable with respect to their aberrant functional consequences. We also review evidence implicating each of the above MYC genes in specific neoplastic diseases and have attempted to identify unresolved questions which deserve further basic or clinical investigation. We have made every attempt to review those diseases for which significant and confirmatory evidence, based on studies with primary tumor material, exists to implicate MYC members in their causation and/or progression.

New Myc-interacting proteins: a second Myc network emerges

Despite its intensive investigation for almost two decades, c-Myc remains a fascinating and enigmatic subject. A large and compelling body of evidence indicates that c-Myc is a transcription factor with central roles in the regulation of cell proliferation, differentiation, and apoptosis, but its exact function has remained elusive. In this review we survey recent advances in the identification and analysis of c-Myc-binding proteins, which suggest insights into the transcriptional roles of c-Myc but which also extend the existing functional paradigms. The C-terminal domain (CTD) of c-Myc mediates interaction with Max and physiological recognition of DNA target sequences, events needed for all biological actions. Recently described interactions between the CTD and other cellular proteins, including YY-1, AP-2, BRCA-1, TFII-I, and Miz-1, suggest levels of regulatory complexity beyond Max in controlling DNA recognition by c-Myc. The N-terminal domain (NTD), which includes the evolutionarily conserved and functionally crucial Myc Box sequences (MB1 and MB2), contains the transcription activation domain (TAD) of c-Myc as well as regions required for transcriptional repression, cell cycle regulation, transformation, and apoptosis. In addition to interaction with the retinoblastoma family protein p107, the NTD has been shown to interact with alpha-tubulin and the novel adaptor proteins Binl, MM-1, Pam, TRRAP, and AMY-1. The structure of these proteins and their effects on c-Myc actions suggest links to the transcriptional regulatory machinery as well as to cell cycle regulation, chromatin modeling, and apoptosis. Investigations of this emerging NTD-based network may reveal how c-Myc is regulated and how it affects cell fate, as well as providing tools to distinguish the physiological roles of various Myc target genes.

The tmp gene, encoding a membrane protein, is a c-Myc target with a tumorigenic activity

The c-Myc oncoprotein induces cell proliferation and transformation through its activity as a transcription factor. Uncovering the genes regulated by c-Myc is an essential step for understanding these processes. We recently isolated the tumor-associated membrane protein gene, Tmp, from a c-myc-induced mouse brain tumor. Here we show that Tmp is specifically highly expressed in mammary tumors and T-cell lymphomas which develop in c-myc transgenic mice, suggesting that Tmp expression is a general characteristic of c-Myc-induced tumors. In addition, Tmp expression is induced upon serum stimulation of fibroblasts as shown in a time course closely correlated with c-myc expression. We have isolated the Tmp promoter region and identified a putative c-Myc binding element, CACGTG, located in the first intron of the gene. We show here that constructs containing the Tmp regulatory region fused to a reporter gene are activated by c-Myc through this CACGTG element and that the c-Myc-Max protein complex can bind to this element. Moreover, an inducible form of c-Myc, the MycER fusion protein, can activate the endogenous Tmp gene. We also show that Tmp-overexpressing fibroblasts induce rapidly growing tumors when injected into nude mice, suggesting that Tmp may possess a tumorigenic activity. Thus, TMP, a member of a novel family of membrane glycoproteins with a suggested role in cellular contact, is a c-Myc target and is possibly involved in c-Myc-induced transformation.

Cancer genetics: tumor suppressor meets oncogene

The adenomatous polyposis coli (APC) tumor suppressor protein is inactivated by mutations in the majority of colorectal cancers. A recent study has revealed that alterations in the APC signaling pathway can result in the transcriptional activation of the c-MYC gene.