Disruption of Multiple Overlapping Functions Following Stepwise Inactivation of the Extended Myc Network

Myc, a member of the "Myc Network" of bHLH-ZIP transcription factors, supervises proliferation, metabolism, and translation. It also engages in crosstalk with the related "Mlx Network" to co-regulate overlapping genes and functions. We investigated the consequences of stepwise conditional inactivation of Myc and Mlx in primary and SV40 T-antigen-immortalized murine embryonic fibroblasts (MEFs). Myc-knockout (MycKO) and Myc × Mlx "double KO" (DKO)-but not MlxKO-primary MEFs showed rapid growth arrest and displayed features of accelerated aging and senescence. However, DKO MEFs soon resumed proliferating, indicating that durable growth arrest requires an intact Mlx network. All three KO MEF groups deregulated multiple genes and functions pertaining to aging, senescence, and DNA damage recognition/repair. Immortalized KO MEFs proliferated in Myc's absence while demonstrating variable degrees of widespread genomic instability and sensitivity to genotoxic agents. Finally, compared to primary MycKO MEFs, DKO MEFs selectively downregulated numerous gene sets associated with the p53 and retinoblastoma (Rb) pathways and G₂/M arrest. Thus, the reversal of primary MycKO MEF growth arrest by either Mlx loss or SV40 T-antigen immortalization appears to involve inactivation of the p53 and/or Rb pathways.

Regulation of Normal and Neoplastic Proliferation and Metabolism by the Extended Myc Network

The Myc Network, comprising a small assemblage of bHLH-ZIP transcription factors, regulates many hundreds to thousands of genes involved in proliferation, energy metabolism, translation and other activities. A structurally and functionally related set of factors known as the Mlx Network also supervises some of these same functions via the regulation of a more limited but overlapping transcriptional repertoire. Target gene co-regulation by these two Networks is the result of their sharing of three members that suppress target gene expression as well as by the ability of both Network's members to cross-bind one another's consensus DNA sites. The two Networks also differ in that the Mlx Network's control over transcription is positively regulated by several glycolytic pathway intermediates and other metabolites. These distinctive properties, functions and tissue expression patterns potentially allow for sensitive control of gene regulation in ways that are differentially responsive to environmental and metabolic cues while allowing for them to be both rapid and of limited duration. This review explores how such control might occur. It further discusses how the actual functional dependencies of the Myc and Mlx Networks rely upon cellular context and how they may differ between normal and neoplastic cells. Finally, consideration is given to how future studies may permit a more refined understanding of the functional interrelationships between the two Networks.

Normal and Neoplastic Growth Suppression by the Extended Myc Network

Among the first discovered and most prominent cellular oncogenes is MYC, which encodes a bHLH-ZIP transcription factor (Myc) that both activates and suppresses numerous genes involved in proliferation, energy production, metabolism and translation. Myc belongs to a small group of bHLH-ZIP transcriptional regulators (the Myc Network) that includes its obligate heterodimerization partner Max and six "Mxd proteins" (Mxd1-4, Mnt and Mga), each of which heterodimerizes with Max and largely opposes Myc's functions. More recently, a second group of bHLH-ZIP proteins (the Mlx Network) has emerged that bears many parallels with the Myc Network. It is comprised of the Myc-like factors ChREBP and MondoA, which, in association with the Max-like member Mlx, regulate smaller and more functionally restricted repertoires of target genes, some of which are shared with Myc. Opposing ChREBP and MondoA are heterodimers comprised of Mlx and Mxd1, Mxd4 and Mnt, which also structurally and operationally link the two Networks. We discuss here the functions of these "Extended Myc Network" members, with particular emphasis on their roles in suppressing normal and neoplastic growth. These roles are complex due to the temporal- and tissue-restricted expression of Extended Myc Network proteins in normal cells, their regulation of both common and unique target genes and, in some cases, their functional redundancy.

Genes conserved in bilaterians but jointly lost with Myc during nematode evolution are enriched in cell proliferation and cell migration functions

Animals use a stereotypical set of developmental genes to build body architectures of varying sizes and organizational complexity. Some genes are critical to developmental patterning, while other genes are important to physiological control of growth. However, growth regulator genes may not be as important in small-bodied “micro-metazoans” such as nematodes. Nematodes use a simplified developmental strategy of lineage-based cell fate specifications to produce an adult bilaterian body composed of a few hundreds of cells. Nematodes also lost the MYC proto-oncogenic regulator of cell proliferation. To identify additional regulators of cell proliferation that were lost with MYC, we computationally screened and determined 839 high-confidence genes that are conserved in bilaterians/lost in nematodes (CIBLIN genes). We find that 30 % of all CIBLIN genes encode transcriptional regulators of cell proliferation, epithelial-to-mesenchyme transitions, and other processes. Over 50 % of CIBLIN genes are unnamed genes in Drosophila, suggesting that there are many understudied genes. Interestingly, CIBLIN genes include many Myc synthetic lethal (MycSL) hits from recent screens. CIBLIN genes include key regulators of heparan sulfate proteoglycan (HSPG) sulfation patterns, and lysyl oxidases involved in cross-linking and modification of the extracellular matrix (ECM). These genes and others suggest the CIBLIN repertoire services critical functions in ECM remodeling and cell migration in large-bodied bilaterians. Correspondingly, CIBLIN genes are co-expressed with Myc in cancer transcriptomes, and include a preponderance of known determinants of cancer progression and tumor aggression. We propose that CIBLIN gene research can improve our understanding of regulatory control of cellular growth in metazoans.

Chromatin dynamics at the hTERT promoter during transcriptional activation and repression by c-Myc and Mnt in Xenopus leavis oocytes

The transcription factors c-Myc and Mnt regulate gene expression through dimerization with Max and binding to E-boxes in target genes. While c-Myc activates gene expression via recruitment of histone modifying complexes, Mnt acts as a transcriptional repressor. Here, we used the Xenopus leavis oocyte system to address the effect of c-Myc and Mnt on transcription and chromatin remodeling over the E-box region in the human telomerase reverse transcriptase (hTERT) promoter. As expected we found elevated and decreased levels of hTERT transcription upon exogenously expressed c-Myc/Max and Mnt/Max, respectively. In addition, we confirmed binding of these heterodimers to both E-boxes already enriched with H3K9ac and H4K16ac. These chromatin marks were further enhanced upon c-Myc/Max binding followed by increased DNA accessibility in the E-box region. In contrast, Mnt/Max inhibited Myc-induced transcription and mediated repression through complete chromatin condensation and deacetylation of H3K9 and H4K16 across the E-box region. Importantly, Mnt was able to counteract c-Myc mediated activation even when expressed at low levels, suggesting Mnt to act as a strong repressor by closing the chromatin structure. Collectively our data demonstrate that the balance between c-Myc and Mnt activity determines the transcriptional outcome of the hTERT promoter by modulation of the chromatin architecture.