Lessons learned from Myc/Max/Mad knockout mice

PKP1 and MYC create a feedforward loop linking transcription and translation in squamous cell lung cancer

PURPOSE

Plakophilin 1 (PKP1) is well-known as an important component of the desmosome, a cell structure specialized in spot-like cell-to-cell adhesion. Although desmosomes have generally been associated with tumor suppressor functions, we recently found that PKP1 is recurrently overexpressed in squamous cell lung cancer (SqCLC) to exert an oncogenic role by enhancing the translation of MYC (c-Myc), a major oncogene. In this study, we aim to further characterize the functional relationship between PKP1 and MYC.

METHODS

To determine the functional relationship between PKP1 and MYC, we performed correlation analyses between PKP1 and MYC mRNA expression levels, gain/loss of function models, chromatin immunoprecipitation (ChIP) and promoter mutagenesis followed by luciferase assays.

RESULTS

We found a significant correlation between the mRNA levels of MYC and PKP1 in SqCLC primary tumor samples. In addition, we found that MYC is a direct transcription factor of PKP1 and binds to specific sequences within its promoter. In agreement with this, we found that MYC knockdown reduced PKP1 protein expression in different SqCLC models, which may explain the PKP1-MYC correlation that we found. Conversely, we found that PKP1 knockdown reduced MYC protein expression, while PKP1 overexpression enhanced MYC expression in these models.

CONCLUSIONS

Based on these results, we propose a feedforward functional relationship in which PKP1 enhances MYC translation in conjunction with the translation initiation complex by binding to the 5'-UTR of MYC mRNA, whereas MYC promotes PKP1 transcription by binding to its promoter. These results suggest that PKP1 may serve as a therapeutic target for SqCLC.

Targeting Myc Interacting Proteins as a Winding Path in Cancer Therapy

MYC, as a well-known oncogene, plays essential roles in promoting tumor occurrence, development, invasion and metastasis in many kinds of solid tumors and hematologic neoplasms. In tumors, the low expression and the short half-life of Myc are reversed, cause tumorigenesis. And proteins that directly interact with different Myc domains have exerted a significant impact in the process of Myc-driven carcinogenesis. Apart from affecting the transcription of Myc target genes, Myc interaction proteins also regulate the stability of Myc through acetylation, methylation, phosphorylation and other post-translational modifications, as well as competitive combination with Myc. In this review, we summarize a series of Myc interacting proteins and recent advances in the related inhibitors, hoping that can provide new opportunities for Myc-driven cancer treatment.

Reciprocal Induction of MDM2 and MYCN in Neural and Neuroendocrine Cancers

MYC family oncoproteins MYC, MYCN, and MYCL are deregulated in diverse cancers and via diverse mechanisms. Recent studies established a novel form of MYCN regulation in MYCN-overexpressing retinoblastoma and neuroblastoma cells in which the MDM2 oncoprotein promotes MYCN translation and MYCN-dependent proliferation via a p53-independent mechanism. However, it is unclear if MDM2 also promotes expression of other MYC family members and has similar effects in other cancers. Conversely, MYCN has been shown to induce MDM2 expression in neuroblastoma cells, yet it is unclear if MYC shares this ability, if MYC family proteins upregulate MDM2 in other malignancies, and if this regulation occurs during tumorigenesis as well as in cancer cell lines. Here, we report that intrinsically high MDM2 expression is required for high-level expression of MYCN, but not for expression of MYC, in retinoblastoma, neuroblastoma, small cell lung cancer, and medulloblastoma cells. Conversely, ectopic overexpression of MYC as well as MYCN induced high-level MDM2 expression and gave rise to rapidly proliferating and MDM2-dependent cone-precursor-derived masses in a cultured retinoblastoma genesis model. These findings reveal a highly specific collaboration between the MDM2 and MYCN oncoproteins and demonstrate the origin of their oncogenic positive feedback circuit within a normal neuronal tissue.

Plakophilin 1 enhances MYC translation, promoting squamous cell lung cancer

Plakophilin 1 (PKP1) is a member of the arm-repeat (armadillo) and plakophilin gene families and it is an essential component of the desmosomes. Although desmosomes have generally been associated with tumor suppressor functions, we have consistently observed that PKP1 is among the top overexpressed proteins in squamous cell lung cancer. To explore this paradox, we developed in vivo and in vitro functional models of PKP1 gain/loss in squamous cell lung cancer. CRISPR-Cas9 PKP1 knockout severely impaired cell proliferation, but it increased cell dissemination. In addition, PKP1 overexpression increased cell proliferation, cell survival, and in vivo xenograft engraftment. We further investigated the molecular mechanism of the mainly oncogenic function of PKP1 by combining transcriptomics, proteomics, and protein-nucleic acid interaction assays. Interestingly, we found that PKP1 enhances MYC translation in collaboration with the translation initiation complex by binding to the 5'-UTR of MYC mRNA. We propose PKP1 as an oncogene in SqCLC and a novel posttranscriptional regulator of MYC. PKP1 may be a valuable diagnostic biomarker and potential therapeutic target for SqCLC. Importantly, PKP1 inhibition may indirectly target MYC, a primary anticancer target.

Blood feeding activates the vitellogenic stage of oogenesis in the mosquito Aedes aegypti through inhibition of glycogen synthase kinase 3 by the insulin and TOR pathways

Most mosquitoes, including Aedes aegypti, only produce eggs after blood feeding on a vertebrate host. Oogenesis in A. aegypti consists of a pre-vitellogenic stage before blood feeding and a vitellogenic stage after blood feeding. Primary egg chambers remain developmentally arrested during the pre-vitellogenic stage but complete oogenesis to form mature eggs during the vitellogenic stage. In contrast, the signaling factors that maintain primary egg chambers in pre-vitellogenic arrest or that activate vitellogenic growth are largely unclear. Prior studies showed that A. aegypti females release insulin-like peptide 3 (ILP3) and ovary ecdysteroidogenic hormone (OEH) from brain neurosecretory cells after blood feeding. Here, we report that primary egg chambers exit pre-vitellogenic arrest by 8 h post-blood meal as evidenced by proliferation of follicle cells, endoreplication of nurse cells, and formation of cytoophidia. Ex vivo assays showed that ILP3 and OEH stimulate primary egg chambers to exit pre-vitellogenic arrest in the presence of nutrients but not in their absence. Characterization of associated pathways indicated that activation of insulin/insulin growth factor signaling (IIS) by ILP3 or OEH inactivated glycogen synthase kinase 3 (GSK3) via phosphorylation by phosphorylated Akt. GSK3 inactivation correlated with accumulation of the basic helix-loop-helix transcription factor Max and primary egg chambers exiting pre-vitellogenic arrest. Direct inhibition of GSK3 by CHIR-99021 also stimulated Myc/Max accumulation and primary egg chambers exiting pre-vitellogenic arrest. Collectively, our results identify GSK3 as a key factor in regulating the pre- and vitellogenic stages of oogenesis in A. aegypti.

A balance of Mad and Myc expression dictates larval cell apoptosis and adult stem cell development during Xenopus intestinal metamorphosis

The Myc/Mad/Max network has long been shown to be an important factor in regulating cell proliferation, death and differentiation in diverse cell types. In general, Myc–Max heterodimers activate target gene expression to promote cell proliferation, although excess of c-Myc can also induce apoptosis. In contrast, Mad competes against Myc to form Mad–Max heterodimers that bind to the same target genes to repress their expression and promote differentiation. The role of the Myc/Mad/Max network during vertebrate development, especially, the so-called postembryonic development, a period around birth in mammals, is unclear. Using thyroid hormone (T3)-dependent Xenopus metamorphosis as a model, we show here that Mad1 is induced by T3 in the intestine during metamorphosis when larval epithelial cell death and adult epithelial stem cell development take place. More importantly, we demonstrate that Mad1 is expressed in the larval cells undergoing apoptosis, whereas c-Myc is expressed in the proliferating adult stem cells during intestinal metamorphosis, suggesting that Mad1 may have a role in cell death during development. By using transcription activator-like effector nuclease-mediated gene-editing technology, we have generated Mad1 knockout Xenopus animals. This has revealed that Mad1 is not essential for embryogenesis or metamorphosis. On the other hand, consistent with its spatiotemporal expression profile, Mad1 knockout leads to reduced larval epithelial apoptosis but surprisingly also results in increased adult stem cell proliferation. These findings not only reveal a novel role of Mad1 in regulating developmental cell death but also suggest that a balance of Mad and Myc controls cell fate determination during adult organ development.

Functional interactions among members of the MAX and MLX transcriptional network during oncogenesis

The transcription factor MYC and its related family members MYCN and MYCL have been implicated in the etiology of a wide spectrum of human cancers. Compared to other oncoproteins, such as RAS or SRC, MYC is unique because its protein coding region is rarely mutated. Instead, MYC's oncogenic properties are unleashed by regulatory mutations leading to unconstrained high levels of expression. Under both normal and pathological conditions MYC regulates multiple aspects of cellular physiology including proliferation, differentiation, apoptosis, growth and metabolism by controlling the expression of thousands of genes. How a single transcription factor exerts such broad effects remains a fascinating puzzle. Notably, MYC is part of a network of bHLHLZ proteins centered on the MYC heterodimeric partner MAX and its counterpart, the MAX-like protein MLX. This network includes MXD1-4, MNT, MGA, MONDOA and MONDOB proteins. With some exceptions, MXD proteins have been functionally linked to cell cycle arrest and differentiation, while MONDO proteins control cellular metabolism. Although the temporal expression patterns of many of these proteins can differ markedly they are frequently expressed simultaneously in the same cellular context, and potentially bind to the same, or similar DNA consensus sequence. Here we review the activities and interactions among these proteins and propose that the broad spectrum of phenotypes elicited by MYC deregulation is intimately connected to the functions and regulation of the other network members. Furthermore, we provide a meta-analysis of TCGA data suggesting that the coordinate regulation of the network is important in MYC driven tumorigenesis. This article is part of a Special Issue entitled: Myc proteins in cell biology and pathology.

MYC proteins promote neuronal differentiation by controlling the mode of progenitor cell division

The role of MYC proteins in somatic stem and progenitor cells during development is poorly understood. We have taken advantage of a chick in vivo model to examine their role in progenitor cells of the developing neural tube. Our results show that depletion of endogenous MYC in radial glial precursors (RGPs) is incompatible with differentiation and conversely, that overexpression of MYC induces neurogenesis independently of premature or upregulated expression of proneural gene programs. Unexpectedly, the neurogenic function of MYC depends on the integrity of the polarized neural tissue, in contrast to the situation in dissociated RGPs where MYC is mitogenic. Within the polarized RGPs of the neural tube, MYC drives differentiation by inhibiting Notch signaling and by increasing neurogenic cell division, eventually resulting in a depletion of progenitor cells. These results reveal an unexpected role of MYC in the control of stemness versus differentiation of neural stem cells in vivo.

The myc road to hearing restoration

Current treatments for hearing loss, the most common neurosensory disorder, do not restore perfect hearing. Regeneration of lost organ of Corti hair cells through forced cell cycle re-entry of supporting cells or through manipulation of stem cells, both avenues towards a permanent cure, require a more complete understanding of normal inner ear development, specifically the balance of proliferation and differentiation required to form and to maintain hair cells. Direct successful alterations to the cell cycle result in cell death whereas regulation of upstream genes is insufficient to permanently alter cell cycle dynamics. The Myc gene family is uniquely situated to synergize upstream pathways into downstream cell cycle control. There are three Mycs that are embedded within the Myc/Max/Mad network to regulate proliferation. The function of the two ear expressed Mycs, N-Myc and L-Myc were unknown less than two years ago and their therapeutic potentials remain speculative. In this review, we discuss the roles the Mycs play in the body and what led us to choose them to be our candidate gene for inner ear therapies. We will summarize the recently published work describing the early and late effects of N-Myc and L-Myc on hair cell formation and maintenance. Lastly, we detail the translational significance of our findings and what future work must be performed to make the ultimate hearing aid: the regeneration of the organ of Corti.

The transcription factor Mxd4 controls the proliferation of the first blood precursors at the onset of hematopoietic development in vitro

OBJECTIVE

The balance between proliferation and differentiation during hematopoietic development in the embryo is a complex process, the detailed molecular mechanisms of which remain to be fully characterized. The transcription factor Mxd4, a member of the Myc-Max-Mad network, was identified in a global gene expression profiling screen as being tightly regulated at the onset of hematopoietic lineage specification upon in vitro differentiation of mouse embryonic stem cells. Our study investigated the Mxd4 expression pattern at the onset of hematopoiesis and the biological relevance of its sharp and transient downregulation.

MATERIALS AND METHODS

To study the expression pattern and role of Mxd4 at the onset of hematopoiesis, the in vitro differentiation of embryonic stem cells was used as a model system. Gain of function assays were performed using a doxycycline-inducible embryonic stem cell system.

RESULTS

We show here that Mxd4 expression is transiently downregulated at an early stage of commitment to the hematopoietic lineage. Enforced expression of Mxd4 at this period of differentiation results in a defect in hematopoietic progenitor development, with impaired development of both primitive and definitive blood lineages. This effect is due to a severe decrease in cell proliferation, with an increased frequency of cells in the G(0)/G(1) phase of the cell cycle, alongside a reduced frequency of cells in the S phase.

CONCLUSIONS

Together our results indicate that during embryonic hematopoietic differentiation Mxd4 is an important player in the regulation of blood progenitor proliferation, and suggest that downregulation of its expression might be required for a proliferative burst preceding lineage specification.

Drosophila Myc interacts with host cell factor (dHCF) to activate transcription and control growth

The Myc proto-oncoproteins are transcription factors that recognize numerous target genes through hexameric DNA sequences called E-boxes. The mechanism by which they then activate the expression of these targets is still under debate. Here, we use an RNAi screen in Drosophila S2 cells to identify Drosophila host cell factor (dHCF) as a novel co-factor for Myc that is functionally required for the activation of a Myc-dependent reporter construct. dHCF is also essential for the full activation of endogenous Myc target genes in S2 cells, and for the ability of Myc to promote growth in vivo. Myc and dHCF physically interact, and they colocalize on common target genes. Furthermore, down-regulation of dHCF-associated histone acetyltransferase and histone methyltransferase complexes in vivo interferes with the Myc biological activities. We therefore propose that dHCF recruits such chromatin-modifying complexes and thereby contributes to the expression of Myc targets and hence to the execution of Myc biological activities.

[Progress of studies on family members and functions of animal bHLH transcription factors]

bHLH transcription factors play essential roles in the regulation of eukaryotic growth and development. Animal bHLH transcription factors comprise of 45 families. They are involved in regulating biological processes such as neurogenesis, myogenesis, gut development and response to environmental toxins. In the past two decades, extensive studies had been conducted on identification of bHLH family members and their biological functions in animals. Based on introduction of origin of the 45 animal bHLH family names, this article reviewed the progresses of studies on bHLH family members and functions of three model animals namely mouse, fruit fly and nematode. There are 114, 59 and 42 bHLH proteins in mouse, fruit fly and nematode, respectively. Among them, the functions of 108 mouse, 47 fruit fly and 20 nematode bHLH proteins have been well characterized. Among the 22 nematode bHLH proteins of unknown functions, 15 have not yet been assigned into certain families. This article also explained misused names of several bHLH family members, thus providing clear and overall background information for relevant researchers to conduct in-depth studies on structures and functions of bHLH transcription factors.

The conserved Myc box 2 and Myc box 3 regions are important, but not essential, for Myc function in vivo

Myc proto-oncoproteins are important regulators of growth and proliferation in development. Their functions have been evolutionarily conserved from insects to vertebrates, although the sequence conservation is limited to a few short domains. Here, we analyze the requirement for the most highly conserved domains, called Myc boxes 2 and 3 (MB2 and MB3), and for the weakly conserved N-terminus for the biological activity of the single Drosophila Myc protein in the animal in vivo. We find that a Myc mutant lacking the N-terminus retains very little activity, whereas Myc transgenes carrying a deletion of MB3 have a moderately increased ability to promote growth and apoptosis; mutation of MB2 reduces transcriptional output and the biological activities of Myc. Surprisingly though, Myc without MB2 retains enough activity to partially rescue the lethality of a Myc null mutation. Thus, although MB2 and MB3 are highly conserved in evolution, loss of either domain has comparatively mild consequences on Myc activity in vivo.

Cell-restricted immortalization by human papillomavirus correlates with telomerase activation and engagement of the hTERT promoter by Myc

The high-risk human papillomaviruses (HPVs) are the causative agents of nearly all cervical cancers and are etiologically linked to additional human cancers, including those of anal, oral, and laryngeal origin. The main transforming genes of the high-risk HPVs are E6 and E7. E6, in addition to its role in p53 degradation, induces hTERT mRNA transcription in genital keratinocytes via interactions with Myc protein, thereby increasing cellular telomerase activity. While the HPV type 16 E6 and E7 genes efficiently immortalize human keratinocytes, they appear to only prolong the life span of human fibroblasts. To examine the molecular basis for this cell-type dependency, we examined the correlation between the ability of E6 to transactivate endogenous and exogenous hTERT promoters and to immortalize genital keratinocytes and fibroblasts. Confirming earlier studies, the E6 and E7 genes were incapable of immortalizing human fibroblasts but did delay senescence. Despite the lack of immortalization, E6 was functional in the fibroblasts, mediating p53 degradation and strongly transactivating an exogenous hTERT promoter. However, E6 failed to transactivate the endogenous hTERT promoter. Coordinately with this failure, we observed that Myc protein was not associated with the endogenous hTERT promoter, most likely due to the extremely low level of Myc expression in these cells and/or to differences in chromatin structure, in contrast with hTERT promoters that we found to be activated by E6 (i.e., the endogenous hTERT promoter in primary keratinoctyes and the exogenous hTERT core promoter in fibroblasts), where Myc is associated with the promoter in either a quiescent or an E6-induced state. These findings are consistent with those of our previous studies on mutagenesis and the knockdown of small interfering RNA, which demonstrated a requirement for Myc in the induction of the hTERT promoter by E6 and suggested that occupancy of the promoter by Myc determines the responsiveness of E6 and the downstream induction of telomerase and cell immortalization.

Transcription-independent functions of MYC: regulation of translation and DNA replication

MYC is a potent oncogene that drives unrestrained cell growth and proliferation. Shortly after its discovery as an oncogene, the MYC protein was recognized as a sequence-specific transcription factor. Since that time, MYC oncogene research has focused on the mechanism of MYC-induced transcription and on the identification of MYC transcriptional target genes. Recently, MYC was shown to control protein expression through mRNA translation and to directly regulate DNA replication, thus initiating exciting new areas of oncogene research.

The oncogene c-Myc coordinates regulation of metabolic networks to enable rapid cell cycle entry

The c-myc proto-oncogene is rapidly activated by serum and regulates genes involved in metabolism and cell cycle progression. This gene is thereby uniquely poised to coordinate both the metabolic and cell cycle regulatory events required for cell cycle entry. However, this function of Myc has not been evaluated. Using a rat fibroblast model of isogenic cell lines, myc(-/-), myc(+/-), myc(+/+) and myc(-/-) cells with an inducible c-myc transgene (mycER), we show that the Myc protein programs cells to utilize both oxidative phosphorylation and glycolysis to drive cell cycle progression. We demonstrate this coordinate regulation of metabolic networks is essential, as specific inhibitors of these pathways block Myc-induced proliferation. Metabolic events temporally correlated with cell cycle entry include increased oxygen consumption, mitochondrial function, pyruvate and lactate production, and ATP generation. Treatment of normal cells with inhibitors of oxidative phosphorylation recapitulates the myc(-/-) phenotype, resulting in impaired cell cycle entry and reduced metabolism. Combined with a kinetic expression profiling analysis of genes linked to mitochondrial function, our study indicates that Myc's ability to coordinately regulate the mitochondrial metabolic network transcriptome is required for rapid cell cycle entry. This function of Myc may underlie the pervasive presence of Myc in many human cancers.

A C. elegans Myc-like network cooperates with semaphorin and Wnt signaling pathways to control cell migration

Myc and Mondo proteins are key regulators of cell growth, proliferation, and energy metabolism, yet often overlooked is their vital role in cell migration. Complex networks of protein-protein and protein-DNA interactions control the transcriptional activity of Myc and MondoA confounding their functional analysis in higher eukaryotes. Here we report the identification of the transcriptional activation arm of a simplified Myc-like network in Caenorhabditis elegans. This network comprises an Mlx ortholog, named MXL-2 for Max-like 2, and a protein that has sequence features of both Myc and Mondo proteins, named MML-1 for Myc and Mondo-like 1. MML-1/MXL-2 complexes have a primary function in regulating migration of the ray 1 precursor cells in the male tail. MML-1/MXL-2 complexes control expression of ECM components in the non-migratory epidermis, which we propose contributes to the substratum required for migration of the neighboring ray 1 precursor cells. Furthermore, we show that pro-migratory Wnt/beta-catenin and semaphorin signaling pathways interact genetically with MML-1/MXL-2 to determine ray 1 position. This first functional analysis of the Myc superfamily in C. elegans suggests that MondoA and Myc may have more predominant roles in cell migration than previously appreciated, and their cooperation with other pro-migratory pathways provides a more integrated view of their role in cell migration.