Background
Myc is the most frequent amplified oncogene in human cancers and its alteration is observed in a wide range of tumors, including breast, lung and prostate cancer [
1]. Deregulated expression of Myc in cancers occurs through gene amplification, chromosomal translocation, focal enhancer amplification, germline enhancer polymorphism or, commonly, through constitutive activation of upstream signaling pathways [
2]. The link between Myc and cancer was greatly strengthened by the discovery that avian leukosis virus (ALV)-induced B-cell lymphomas consistently contained retroviral insertions in the vicinity of the Myc gene [
3]. This means that the oncogenic properties of Myc are not only manifested by the retroviral-transduced v-myc, but can also occur as a consequence of viral perturbation of cellular Myc. It was also clear that Myc can be complicit in neoplasms that lack any retroviral involvement [
4]. In 1985, Adams et al. demonstrated that Myc is crucial for the genesis of B-cell lymphomas through the generation of transgenic mice carrying an Ig-enhancer linked to Myc, this consolidating the notion of a strong involvement of Myc in hematological tumors [
5]. Thus, all three vertebrate Myc family members (c-Myc, MYCN and MYCL1) are involved in the etiology of human cancers [
4]. c-Myc is a rapidly degraded protein with half-life of 20-30 minutes [
6] and a variety of different proteins interact with c-Myc to control its stability and transcriptional activity. The oncogenic potential of c-Myc stems from its function as transcriptional regulator that binds DNA on heterodimerization with myc-associated factor X (MAX) [
7]. The carboxyl terminus of c-Myc encodes a 100-residue basic helix-loop-helix-leucine-zipper (bHLH-LZ) DNA-binding domain. The leucine zipper forms a coiled-coil heterodimer with a homologous region on the transcriptional repressor MAX, which together engage E-box DNA-binding sites [
4]. Localization of the heterodimer to either promoter or enhancer regions positively regulates transcription of proliferation-associated genes through control of transcription elongation [
8]. In addition to its canonical function as transcriptional activator, c-Myc induction causes transcriptional repression of target genes [
9]. The discordance in c-Myc-dependent genomic binding and expression analysis suggests that target gene expression after binding to DNA is highly regulated by the presence of specific cofactors. Indeed structural studies indicate that the Myc-MAX dimeric region presents a large solvent-accessible surface area forming a platform for binding by other factors [
2]. These can act as molecular switches to mediate c-Myc-induced proliferation and tumorigenesis, suggesting that dynamic complexes of cofactors can differentially regulate the transcriptional activity and target gene selection of c-Myc to mediate diverse biological outcomes [
10,
11]. The sequence DNA-binding of c-Myc is specific for E-boxes and can occur only following recognition of open chromatin context. When overexpressed, the level of c-Myc that is bound to E-boxes-containing promoters increases, with more promoters becoming occupied, and c-Myc starts binding larger numbers of distal sites [
10]. On the other hand, promoters of repressed genes are poorly enriched in E-boxes, suggesting that other factors recruit c-Myc to those promoters, including the molecular complex deriving from dimerization with MAX [
12,
13]. Among c-Myc-induced genes, the functional categories that recur most consistently in independent studies are cell growth, cell cycle control, energy production, anabolic metabolism and DNA replication [
14]. The mechanism of action of c-Myc is still not clear and two hypothesis are still competing. One proposes a model in which c-Myc functions as a direct activator or amplifier of transcription at all active loci [
10]. In an alternative scenario, c-Myc activates and represses selected target genes, with RNA amplification occurring only as secondary consequence [
15].
Regardless of its specific mechanism of action, c-Myc remains one of the targets for effective antineoplastic therapy, due to its deregulation in numerous tumors. Unfortunately, c-Myc presents specific, significant obstacles to develop a strategy for its direct inhibition. Indeed, c-Myc lacks enzymatic activity, this limiting those approaches that require its direct inhibition. Rather, c-Myc activity is exerted by protein-protein interactions, which remains a technical barrier impeding organized efforts in drug discovery. The biological behavior of c-Myc in physiology and disease must still be fully elucidated, requiring comprehensive mapping of its target genes and the importance of c-Myc cofactors. These molecules function, at least in part, by affecting chromatin structure through their intrinsic enzymatic activities, including ATPase/helicases, histone acetyl-transferase (HATS) and histone deacetylase (HDACs). Therefore, a possible model of targeting c-Myc could involve the inhibition of these coactivator proteins, critical to c-Myc-specific initiation and elongation.
One of the first c-Myc cofactors was discovered by Peukert K et al in 1997. The authors identificated a protein that interacts with the carboxy-terminal HLH domain of Myc, Miz-1 (
Myc-
interacting
Zn finger protein-1). It belongs to the BTB/POZ family of zinc finger proteins and interacts with DNA in a sequence-specific manner. Both Max and Miz-1 interact with the HLH domain of Myc suggesting that Max and Miz-1 may form alternate complexes with Myc. In particular Miz-1 is involved in the c-Myc-dependent mechanism of repression of particular genes like Cyclin D1 [
16]. In addition, only recently it has been demonstrated that the interaction of Myc with Miz1 is critical for the development of G3 MBs (Medulloblastoma) and distinguishes G3 from other MB subgroups [
17].
McMahon et al, in 1998, showed that inhibition of TRRAP synthesis or function blocks c-Myc-mediated oncogenic activity. TRRAP with TIP49 and TIP48 is involved in chromatin modifying complexes. In particular, ATPase/helicase motifs contained in TIP49 and TIP48, when mutated, create a dominant inhibitor of c-Myc oncogenic activity [
18]. Subsequently, the co-activator CBP was identified as a novel c-Myc interaction partner. These findings showed that CBP interacts directly with c-Myc and stimulates its function. Furthermore, in association with p300, CBP is recruited to c-Myc-regulated genes [
19]. Fujii M et al. in 2006 demonstrated that SNIP1 functions as a regulator of c-Myc activity and that it enhances the transcriptional activity of c-Myc both stabilizing it against proteasomal degradation and bridging the c-Myc/p300 complex [
20]. Then, a new model was proposed, where, in a direct feedback mechanism, ARF binds with c-Myc to inhibit canonical c-Myc target gene induction and proliferation, while inducing non-canonical expression of Egr1 and EGR1-mediated apoptosis [
21]. The heterodimerization with Max is also necessary for c-Myc to recruit pTEFb, the positive transcription factor that phosphorylates the carboxy-terminal domain of RNA polymerase II, at target genes [
22]. Furthermore, it is known that c-Myc requires SP1 in order to participate in the regulation of survivin promoter in controlling tumor drug resistance [
23].
Recently, numerous additional c-Myc interactors have been described, further characterizing the functions of this protein and suggesting possible new therapeutic targets. In this review, we update these more recent findings about c-Myc cofactors active in tumorigenesis, with the aim to develop, through the comparison of their mechanisms of action, either novel therapy strategies or identification of selective biomarkers for diagnosis.