Schwab (ed.), Encyclopedia of Cancer, 2015
The myc gene was originally identified in the avian myelocytomatosis retrovirus as the oncogene responsible for inducing myeloid leukemia in birds. The cellular homologue, c-MYC, was discovered as a promoter insertion in chicken, and the human gene was later identified in the B-cell tumor Burkitt lymphoma. The related MYCN and MYCL genes were found amplified in neuroblastoma and in small cell lung cancer, respectively. The myc genes are highly conserved through evolution and are found in all animals examined with the exception of the nematode Caenorhabditis elegans that seems to lack a myc ortholog. In vertebrates, c-myc and MYCN are essential during development. Two additional myc genes, S-myc and B-myc, are expressed exclusively in rodents and are not as well characterized. The gene product Myc is a multifunctional protein with the ability to regulate the cell cycle, cell growth and metabolism, differentiation, apoptosis, angiogenesis, and transformation.
The MYC proto-oncogene encodes a nuclear transcription factor of the basic region-helix-loophelix-leucine zipper (bHLH-Zip) family, belonging to the Myc/Max/Mad(Mxd) network (Fig. 1). The expression of MYC is regulated by external signals such as growth factors and extracellular matrix contacts and through internal cell cycle control. Myc has an important role in driving proliferation and is expressed during all stages of the cell cycle during division. When quiescent cells are stimulated to reenter the cell cycle, Myc levels increase dramatically and then return to steady-state levels once cells start to cycle. In contrast, Myc is normally downregulated during differentiation, while ectopic myc expression inhibits this process.
Myc forms a complex with Max, and these heterodimers bind specifically to 50-CACGTG-30 or noncanonical E-box sequences and regulate transcription through recruitment of histone acetyltransferases (HATs) and chromatin remodeling proteins (Fig. 2). These proteins will acetylate Myc target gene promoters and open up the chromatin to permit access for transcription factors and co-activators. Complexes containing the HATs GCN5 or TIP60 are recruited to the N-terminal of the Myc protein via the adaptor transformation/transcription domain-associated protein (TRRAP). Increasing evidence suggests that Myc regulates general acetylation of the genome and not only of specific targets. Another important role of Myc is the transcriptional activation of ribosomal DNA (rDNA), promoted through interaction with Max in nucleoli.
Fig. 1. Schematic presentation of the Myc network proteins, including chromosomal localization of the human genes encoding these proteins. In the c-Myc protein, the transcriptional initiation sites for the three different isoforms are indicated. MB Myc homology box, NTD N-terminal domain, TAD transcriptional activation domain, CTD C-terminal domain, b basic region, HLH helix-loop-helix, Zip leucine zipper, NLS nuclear localization signal, SID Sin3-interacting domain, HR homology region
Fig. 2. Transcriptional regulation by Myc network proteins upon binding to the 50-CACGTG-30 E-box sequence. During quiescence or differentiation, Mxd/Max represses transcription by recruiting the repressor complex containing histone deacetylases to promote a closed chromatin conformation. In proliferating cells, Myc/Max promotes transcription through activator complexes containing histone acetyltransferases and cofactors resulting in an open and accessible chromatin configuration. Mnt is expressed at all stages of the cell cycle and confers transcriptional repression in a similar manner as the Mxd proteins. Indicated below are some of the biological effects of these transcription factors. SID Sin3-interacting domain, MBII Myc homology box II
In addition, Myc possesses the capacity to repress transcription through interaction with transcriptional activators such as Miz-1. Miz-1 promotes transcription when bound to initiator elements (Inr) in promoters of target genes. By forming a ternary complex with Max and Miz-1 at the Inr, Myc quenches gene activation. To date, several thousand Myc targets have been identified, some of which are activated and some which are repressed. Cyclin D2, cyclin-dependent kinase 4 (CDK4), carbamoyl-transferase-dihydroorotase (cad), ornithine decarboxylase (ODC), and telomerase reverse transcriptase (hTERT) are examples of target genes activated by c-Myc, and genes repressed by Myc include those encoding the CDK inhibitors p15 INK4b and p21 and the transcriptional repressor Mxd4.
Структура и функция Myc
The Myc protein can be divided into three major parts: the C-terminal domain harboring the basic region and the HLH-Zip motifs, the central region, and the N-terminal domain that is also referred to as the transcriptional activation domain (TAD) (Fig. 1). Myc forms a complex with Max through the HLH-Zip motif, and the basic regions of the Myc/Max heterodimer confer DNA binding to the E-box. Most of the functions of Myc have been ascribed to four highly conserved regions within the protein, called Myc homology boxes (MB). MBI and MBII within the TAD are the best characterized ones, while MBIII and MBIV in the central region were more recently identified. Collectively, the Myc boxes are important for proliferation, oncogene co-operation to induce transformation, Myc-induced apoptosis, and blockage of differentiation.
Posttranslational modifications and regulation of Myc turnover
Myc is a highly unstable protein with a short halflife (15–30 min). Multiple phosphorylation sites have been described for Myc protein. The phosphorylation of the N-terminus residues S62 and T58 is the most extensively studied. Phosphorylation and dephosphorylation of highly conserved residues within the TAD of the protein dictate whether Myc will be ubiquitinated for degradation or stabilized.
These events are in large monitored by the RAS-activated pathways, Raf/MEK/ERK and PI3K/AKT, controlling stabilization and de-stabilization of the Myc protein. The phosphorylation of S62 by mitogen-stimulated kinases promotes Myc protein stabilization, while subsequent phosphorylation of T58 by glycogen synthase kinase 3-β triggers degradation of the protein by the ubiquitin/ proteasome system, where ubiquitin-tagged molecules are recognized and degraded by the proteasome (Fig. 3). SUMOylation has also been described to induce ubiquitination and proteasomal degradation of Myc. To date, four E3 ubiquitin ligases that regulate Myc protein turnover and/or activity have been identified. Ubiquitination by the F-box proteins Skp2, Fbw7, and FBXO32 targets the protein for degradation. In addition, Skp2-mediated ubiquitination seems to be required for Myc-induced transcriptional activation. Similarly, the HectH9 ligase facilitates Myc target gene activation but forms an ubiquitin chain that does not induce protein degradation. Acetylation of Myc will also influence its stability by affecting its subcellular localization and turnover.
Other members of the Max-interacting network
The small phosphoprotein Max is in the center of the network (Fig. 1) and a prerequisite for transcriptional activity of the other network proteins. Myc, as well as the transcriptional repressors Mad(Mxd) and Mnt, needs to dimerize with Max in order to bind E-box DNA (Fig. 2). Max is ubiquitously expressed at levels that remain constant throughout the cell cycle. Transcriptional repression by Mxd and Mnt is mediated through association with the adaptor protein Sin3 that in turn recruits histone deacetylase complexes (HDACs) to target genes (Fig. 2).
Removal of acetyl groups results in a closed chromatin conformation, inaccessible for the transcription machinery. The expression pattern of Mxd proteins is mainly the opposite to that of Myc, being upregulated during differentiation and barely detectable in proliferating cells. The exception is Mxd3 that is expressed primarily during S-phase of the cell cycle. Of the four Mxd proteins identified, Mxd1 and Mxd2 are the best characterized (Fig. 1). Mnt is ubiquitously expressed throughout the cell cycle and has been suggested to be a modulator of Myc function. This notion is supported by the finding that mnt-deficient mouse fibroblasts show a disrupted cell cycle control and can be transformed by the oncoprotein RAS alone, traits resembling Myc-overexpressing cells. The network also includes the larger transcriptional repressor Mga as well as Mlx that heterodimerizes with Mxd1 and Mxd4.
Fig. 3. Regulation of Myc stability by phosphorylation. Activation of the RAS pathway promotes stabilization of Myc protein by promoting the phosphorylation of S62. Subsequently, glycogen synthase kinase 3-β (GSK3b) phosphorylates MYC at T58. This second phosphorylation allows the protein to be a substrate of protein phosphatase 2A (PP2A), which removes the phosphorylation of S62. The single phosphorylation of Myc on T58 marks the protein to be polyubiquitinated by ubiquitin ligases (UL), followed by proteasomal degradation. TKR tyrosin kinase receptor
Effects of Myc activation
The cellular response to activated Myc will mainly depend on the immediate cellular environment and include cell cycle progression, blockage of differentiation, apoptosis, cell growth and division, acquisition and maintenance of pluripotency, as well as transformation. Myc is required to drive cellular proliferation, a function that is carried out primarily during the G1/S transition of the cell cycle by directly upregulating cyclin D2 and CDK4. When in complex, these proteins will both initiate phosphorylation of the retinoblastoma protein (pRb) to enable release of E2F transcription factors and sequester the CDK2 inhibitor p27KIP1 to release the cyclin E-CDK2 complex for continued pRb phosphorylation. In addition, Myc regulates expression of CDK inhibitors, either by repressing the expression p15INK4b and p21CIP1 through inhibition of Miz-1-mediated transcription, or by promoting ubiquitin-mediated degradation of p27KIP1. The fact that Myc also induces expression of E2Fs reveals that Myc is acting at several different levels to promote proliferation.
Another important function of Myc is potentiation of apoptosis in response to different cellular stress signals such as activation of the Fas death receptor and growth factor deprivation. The ability of Myc to enhance the apoptotic effects of many mechanistically distinct inducers indicates that Myc acts in a common control and/or execution pathway of apoptosis. It has been suggested that proliferation of cells is essentially tied to apoptosis as a built-in safety mechanism to defend against inappropriate proliferation. Thus sensitization to apoptosis is a normal function of Myc, and the suicide signal can be rescued by specific survival factors such as insulin-like growth factor (IGF) or platelet-derived growth factor (PDGF). According to this model, the availability of apoptosis inhibitors determines whether cells divide or die.
In vitro experiments in rat cell lines have shown that cyclin A and ODC are potential mediators of Myc-induced apoptosis. Blockage of ODC inhibited apoptosis in Myc-overexpressing cells and forced expression of cyclin A was sufficient to induce apoptosis under low serum conditions. Ectopic expression of cyclin A could also restore drug-induced apoptosis in c-myc null cells. In addition to the Fas receptor and its ligand, induction of apoptosis by Myc has also been correlated to proteins in the BCL-2 family, normally active at or in proximity to mitochondria. The antiapoptotic proteins Bcl-2 and Bcl-XL are repressed by Myc, while expression and/or activity of proapoptotic members such as Bim or Bax is induced. In particular, the proapoptotic Bax molecule appears to be essential for triggering c-Myc-induced apoptosis by permeabilizing the mitochondrial membrane to release additional proapoptotic factors such as cytochrome c that will initiate the apoptosis cascade.
The tumor suppressor protein p53 acts as a sensor for intrinsic cellular damage signals by inducing cell cycle arrest to enable DNA repair or, if this fails, to induce apoptosis. Myc has been reported to potentiate apoptosis both through p53-dependent and p53-independent mechanisms.
Consequences of Myc deregulation
Myc activation triggers apoptosis in normal cells. However, if the apoptotic pathway is affected by mutations, there will be an imbalance between proliferation and cell death with predominance for the former. This results in uncontrolled cell proliferation and, as the pool of proliferating cells has an increased risk of secondary mutations contributing to tumor development, thereby facilitates Myc-driven tumor development. Oncogenic activation of MYC is induced by events such as point mutations, gene amplification, translocation (see Chromosomal Translocations), overexpression, enhanced translation, or increased protein stability. This in turn results in immortalization, induction of genomic destabilization, and angiogenesis and metabolic remodeling. Myc proteins are also able to induce the expression of the miR-17-92 microRNA cluster, which potentiates oncogenic signaling in several cancer types. Myc overexpression in cancer cells is related to general transcriptional amplification of already active promoters, together with the direct regulation of specific target genes.
Deregulation of the myc gene alone is not sufficient for induction of cellular transformation of mouse cells in vitro, but requires co-operation with other oncogenes, such as RAS or bcl-2. This is also manifested in human malignancies since many tumors with deregulated c-MYC overexpress BCL-2 or harbor RAS mutations.
Tumors associated with Myc
MYC is activated in a number of different tumors such as small cell lung cancer, breast carcinoma, osteosarcoma, glioblastoma, cervix carcinoma, myeloid myeloma, acute lymphoblastic leukemia, Burkitt lymphoma, and neuroblastoma.
Chromosomal translocation is the most common aberration in hematological malignancies (see Hematological Malignancies, Leukemias and Lymphomas). In Burkitt lymphoma, the c-MYC gene is translocated to one of the immunoglobulin loci, resulting in constitutively high Myc levels. In contrast, the most common MYC aberration in solid tumors is gene amplification. In childhood neuroblastoma, amplification of the MYCN oncogene, occurring in 40–50 % of high-risk neuroblastoma cases, is one of the key predictors of poor outcome.
Targeting Myc as an approach to treat cancer
The high frequency of tumors with deregulated MYC expression is the main reason why Myc is an attractive target for cancer therapy. Thus, a Myc-targeting drug has the potential to provide an efficient treatment of a broad range of human cancers. However, since the protein is expressed in all proliferating cells, it will be important to consider the potential damage inflicted on the gastrointestinal tract, the reproductive system, and the hematopoietic system if such a drug is administered systemically.
It has been shown that Myc-induced tumorigenesis can be reversible in conditional myc-driven transgenic models. In most of the cases, inactivation of myc resulted in induction of proliferation arrest or differentiation and/or apoptosis depending on the type of cancer as well as on the tumor-specific genetic events. However, tumor cells can also develop compensatory mechanisms to evade the dependence on Myc. In addition, some tumors may contain cancer stem cells that remain dormant when myc is inactivated but recover their tumorigenic properties upon myc reactivation. In other cases, even brief inactivation of Myc is sufficient for induction of sustained tumor regression.
Given the fact that MYC is one of the most frequently deregulated oncogenes in human cancer, much would be gained from specifically targeting Myc or the Myc pathway in tumor cells. For this purpose, one strategy would be to identify molecules that can reactivate the intrinsic response to Myc overexpression by reversing the acquired resistance to Myc-driven apoptosis. It is also possible to design or identify drugs directly targeting the Myc protein, stimulating its degradation or preventing its interaction with Max. With these approaches, the efficacy of conventional chemotherapy would be enhanced and enable the use of lower drug doses to reduce adverse effects. Several direct and indirect small molecule inhibitors of Myc have been described to the moment. Some of them are already in clinical trials for several kinds of cancer, including the Aurora kinase A inhibitor alisertib and several bromodomain inhibitors as OTX015.
Myc is a key regulator of cell cycle, cell growth and metabolism, differentiation, apoptosis, angiogenesis, and transformation. In spite of the efforts and progress that have been made in understanding Myc function, much remains to be resolved. Unraveling the complexity of the Myc protein and its many roles may provide insights for design and development of novel therapies for treatment of human cancer.
Cole MD, Henriksson M (eds) (2006) 25 years of the c-Myc oncogene. Semin Cancer Biol 16:241–330
Felsher DW (2003) Cancer revoked: oncogenes as therapeutic targets. Nat Rev Cancer 3:375–380
Kress TR, Sabo` A, Amati B (2015) MYC: connecting selective transcriptional control to global RNA production. Nat Rev Cancer 15:593–607
McKeown MR, Bradner JE (2014) Therapeutic strategies to inhibit MYC. Cold Spring Harb Perspect Med 4: pii: a014266
Nilsson JA, Cleveland JL (2003) Myc pathways provoking cell suicide and cancer. Oncogene 22:9007–9021
Oster SK, Ho CS, Soucie EL et al (2002) The myc oncogene: MarvelouslY Complex. Adv Cancer Res 84:81–154
Pelengaris S, Khan M, Evan G (2002) c-MYC: more than just a matter of life and death. Nat Rev Cancer 2:764–776
Stine ZE, Walton ZE, Altman BJ, Hsieh AL, Dang CV (2015) MYC, metabolism and cancer. Cancer Discov 5:1024–1039