MYC synergizes with activated BRAFV600E in mouse lung tumor development by suppressing senescence

The dual role of cellular senescence in human tumor progression and therapy

Cellular senescence, one of the hallmarks of cancer, is characterized by cell cycle arrest and the loss of most normal cellular functions while acquiring a hypersecretory, proinflammatory phenotype. The function of senescent cells in cancer cells varies depending on the cellular conditions. Before the occurrence of cancer, senescent cells act as a barrier to prevent its development. But once cancer has occurred, senescent cells play a procancer role. However, few of the current studies have adequately explained the diversity of cellular senescence across cancers. Herein, we concluded the latest intrinsic mechanisms of cellular senescence in detail and emphasized the senescence-associated secretory phenotype as a key contributor to heterogeneity of senescent cells in tumor. We also discussed five kinds of inducers of cellular senescence and the advancement of senolytics in cancer, which are drugs that tend to clear senescent cells. Finally, we summarized the various effects of senescent cells in different cancers and manifested that their functions may be diametrically opposed under different circumstances. In short, this paper contributes to the understanding of the diversity of cellular senescence in cancers and provides novel insight for tumor therapy.

MYC and KRAS cooperation: from historical challenges to therapeutic opportunities in cancer

RAS and MYC rank amongst the most commonly altered oncogenes in cancer, with RAS being the most frequently mutated and MYC the most amplified. The cooperative interplay between RAS and MYC constitutes a complex and multifaceted phenomenon, profoundly influencing tumor development. Together and individually, these two oncogenes regulate most, if not all, hallmarks of cancer, including cell death escape, replicative immortality, tumor-associated angiogenesis, cell invasion and metastasis, metabolic adaptation, and immune evasion. Due to their frequent alteration and role in tumorigenesis, MYC and RAS emerge as highly appealing targets in cancer therapy. However, due to their complex nature, both oncogenes have been long considered "undruggable" and, until recently, no drugs directly targeting them had reached the clinic. This review aims to shed light on their complex partnership, with special attention to their active collaboration in fostering an immunosuppressive milieu and driving immunotherapeutic resistance in cancer. Within this review, we also present an update on the different inhibitors targeting RAS and MYC currently undergoing clinical trials, along with their clinical outcomes and the different combination strategies being explored to overcome drug resistance. This recent clinical development suggests a paradigm shift in the long-standing belief of RAS and MYC "undruggability", hinting at a new era in their therapeutic targeting.

MYC the oncogene from hell: Novel opportunities for cancer therapy

Cancer comprises a heterogeneous disease, characterized by diverse features such as constitutive expression of oncogenes and/or downregulation of tumor suppressor genes. MYC constitutes a master transcriptional regulator, involved in many cellular functions and is aberrantly expressed in more than 70 % of human cancers. The Myc protein belongs to a family of transcription factors whose structural pattern is referred to as basic helix-loop-helix-leucine zipper. Myc binds to its partner, a smaller protein called Max, forming an Myc:Max heterodimeric complex that interacts with specific DNA recognition sequences (E-boxes) and regulates the expression of downstream target genes. Myc protein plays a fundamental role for the life of a cell, as it is involved in many physiological functions such as proliferation, growth and development since it controls the expression of a very large percentage of genes (∼15 %). However, despite the strict control of MYC expression in normal cells, MYC is often deregulated in cancer, exhibiting a key role in stimulating oncogenic process affecting features such as aberrant proliferation, differentiation, angiogenesis, genomic instability and oncogenic transformation. In this review we aim to meticulously describe the fundamental role of MYC in tumorigenesis and highlight its importance as an anticancer drug target. We focus mainly on the different categories of novel small molecules that act as inhibitors of Myc function in diverse ways hence offering great opportunities for an efficient cancer therapy. This knowledge will provide significant information for the development of novel Myc inhibitors and assist to the design of treatments that would effectively act against Myc-dependent cancers.

Myc Supports Self-Renewal of Basal Cells in the Esophageal Epithelium

It is widely believed that cellular senescence plays a critical role in both aging and cancer, and that senescence is a fundamental, permanent growth arrest that somatic cells cannot avoid. Here we show that Myc plays an important role in self-renewal of esophageal epithelial cells, contributing to their resistance to cellular senescence. Myc is homogeneously expressed in basal cells of the esophageal epithelium and Myc positively regulates their self-renewal by maintaining their undifferentiated state. Indeed, Myc knockout induced a loss of the undifferentiated state of esophageal epithelial cells resulting in cellular senescence while forced MYC expression promoted oncogenic cell proliferation. A superoxide scavenger counteracted Myc knockout-induced senescence, therefore suggesting that a mitochondrial superoxide takes part in inducing senescence. Taken together, these analyses reveal extremely low levels of cellular senescence and senescence-associated phenotypes in the esophageal epithelium, as well as a critical role for Myc in self-renewal of basal cells in this organ. This provides new avenues for studying and understanding the links between stemness and resistance to cellular senescence.

Capitalizing on Synthetic Lethality of MYC to Treat Cancer in the Digital Age

Deregulation of MYC is among the most frequent oncogenic drivers of cancer. Developing targeted therapies against MYC is, therefore, one of the most critical unmet needs of cancer therapy. Unfortunately, MYC has been labelled as undruggable due to the lack of success in developing clinically relevant MYC-targeted therapies. Synthetic lethality is a promising approach that targets MYC-dependent vulnerabilities in cancer. However, translating the synthetic lethality targets to the clinics is still challenging due to the complex nature of cancers. This review highlights the most promising mechanisms of MYC synthetic lethality and how these discoveries are currently translated into the clinic. Finally, we discuss how in silico computational platforms can improve clinical success of synthetic lethality-based therapy.

Methods to Study Myc-Regulated Cellular Senescence: An Update

Cellular senescence plays a role in several physiological processes including aging, embryonic development, tissue remodeling, and wound healing and is considered one of the main barriers against tumor development. Studies of normal and tumor cells both in culture and in vivo suggest that MYC plays an important role in regulating senescence, thereby contributing to tumor development. We have previously described different common methods to measure senescence in cell cultures and in tissues. Unfortunately, there is no unique marker that unambiguously defines a senescent state, and it is therefore necessary to combine measurements of several different markers in order to assure the correct identification of senescent cells. Here we describe protocols for simultaneous detection of multiple senescence markers in situ, a quantitative fluorogenic method to measure senescence-associated β-galactosidase activity (SA-β-gal), and a new method to detect senescent cells based on the Sudan Black B (SBB) analogue GL13, which is applicable to formalin-fixed paraffin-embedded tissues. The application of these methods in various systems will hopefully shed further light on the role of MYC in regulation of senescence, and how that impacts normal physiological processes as well as diseases and in particular cancer development.

Pharmacological inactivation of CDK2 inhibits MYC/BCL-XL-driven leukemia in vivo through induction of cellular senescence

Deregulated expression of the MYC oncogene is a frequent event during tumorigenesis and generally correlates with aggressive disease and poor prognosis. While MYC is a potent inducer of apoptosis, it often suppresses cellular senescence, which together with apoptosis is an important barrier against tumor development. For this latter function, MYC is dependent on cyclin-dependent kinase 2 (CDK2). Here, we utilized a MYC/BCL-X_L-driven mouse model of acute myeloblastic leukemia (AML) to investigate whether pharmacological inhibition of CDK2 can inhibit MYC-driven tumorigenesis through induction of senescence. Purified mouse hematopoietic stem cells transduced with MYC and BCL-X_L were transplanted into lethally irradiated mice, leading to the development of massive leukemia and subsequent death 15–17 days after transplantation. Upon disease onset, mice were treated with the selective CDK2 inhibitor CVT2584 or vehicle either by daily intraperitoneal injections or continuous delivery via mini-pumps. CVT2584 treatment delayed disease onset and moderately but significantly improved survival of mice. Flow cytometry revealed a significant decrease in tumor load in the spleen, liver and bone marrow of CVT2584-treated compared to vehicle-treated mice. This was correlated with induced senescence evidenced by reduced cell proliferation, increased senescence-associated β-galactosidase activity and heterochromatin foci, expression of p19^ARF and p21^CIP1, and reduced phosphorylation (activation) of pRb, while very few apoptotic cells were observed. In addition, phosphorylation of MYC at Ser-62 was decreased. In summary, inhibition of CDK2 delayed MYC/BCL-X_L-driven AML linked to senescence induction. Our results suggest that CDK2 is a promising target for pro-senescence cancer therapy, in particular for MYC-driven tumors, including leukemia.

MYC, MYCL, and MYCN as therapeutic targets in lung cancer

Introduction: Lung cancer is the leading cause of cancer-related mortality globally. Despite recent advances with personalized therapies and immunotherapy, the prognosis remains dire and recurrence is frequent. Myc is an oncogene deregulated in human cancers, including lung cancer, where it supports tumorigenic processes and progression. Elevated Myc levels have also been associated with resistance to therapy.Areas covered: This article summarizes the genomic and transcriptomic studies that compile evidence for (i) MYC, MYCN, and MYCL amplification and overexpression in lung cancer patients, and (ii) their prognostic significance. We collected the most recent literature regarding the development of Myc inhibitors where the emphasis is on those inhibitors tested in lung cancer experimental models and their potential for future clinical application.Expert opinion: The targeting of Myc in lung cancer is potentially an unprecedented opportunity for inhibiting a key player in tumor progression and maintenance and therapeutic resistance. Myc inhibitory strategies are on the path to their clinical application but further work is necessary for the assessment of their use in combination with standard treatment approaches. Given the role of Myc in immune suppression, a significant opportunity may exist in the combination of Myc inhibitors with immunotherapies.

Inhibition of lung cancer cells and Ras/Raf/MEK/ERK signal transduction by ectonucleoside triphosphate phosphohydrolase-7 (ENTPD7)

Background

The aim of this study was to investigate the effects and mechanisms of ectonucleoside triphosphate phosphohydrolase-7 (ENTPD7) on lung cancer cells.

Methods

The expression characteristics of ENTPD7 and its effect on the survival of lung cancer patients were analyzed by referring to The Cancer Genome Atlas (TCGA). Streptavidin-peroxidase (SP) staining was performed to detect the ENTPD7 protein in tumor tissues and adjacent tissues. Plasmid transfection technology was also applied to silence ENTPD7 gene. Crystal violet staining and flow cytometry were performed to determine cell proliferation and apoptosis. Tumor-bearing nude mice model was established to investigate the effect of sh-ENTPD7 on tumors.

Results

The results showed that patients with low levels of ENTPD7 had higher survival rates. ENTPD7 was up-regulated in lung cancer tissues and cells. Down-regulation of the expression of ENTPD7 inhibited proliferation but promoted apoptosis of lung cancer cell. Silencing ENTPD7 also inhibited the expression levels of Ras and Raf proteins and the phosphorylation of mitogen-activated protein kinase (MEK) and extracellular signal-regulated kinase (ERK). Tumor-bearing nude mice experiments showed that silencing ENTPD7 had an inhibitory effect on lung cancer cells.

Conclusions

ENTPD7 was overexpressed in lung cancer cells. Down-regulating ENTPD7 could inhibit lung cancer cell proliferation and promote apoptosis via inhibiting the Ras/Raf/MEK/ERK pathway.

MYC and RAS are unable to cooperate in overcoming cellular senescence and apoptosis in normal human fibroblasts

The MYC and RAS oncogenes are sufficient for transformation of normal rodent cells. This cooperativity is at least in part based on suppression of RAS-induced cellular senescence by MYC and block of MYC-induced apoptosis by RAS - thereby canceling out two main barriers against tumor development. However, it remains unclear whether MYC and RAS cooperate in this way in human cells, where MYC and RAS are not sufficient for transformation. To address this question, we established a combined Tet-inducible H-RAS^V12 and hydroxytamoxifen-inducible MycER system in normal human BJ fibroblasts. We show here that activation of RAS alone induced senescence while activation of MYC alone or together with RAS triggered DNA damage, induction of p53 and massive apoptosis, suggesting that RAS cannot rescue MYC-induced apoptosis in this system. Although coexpression with MYC reduced certain RAS-induced senescence markers (histone H3 lysine 9 trimethylation and senescence-associated β-GAL activity), the induction of the senescence marker p16INK4A was further enhanced and the culture ceased to proliferate within a few days, revealing that MYC could not fully suppress RAS-induced senescence. Furthermore, depletion of p53, which enhanced proliferation and rescued the cells from RAS-induced senescence, did not abrogate MYC-induced apoptosis. We conclude that MYC and RAS are unable to cooperate in overcoming senescence and apoptosis in normal human fibroblasts even after depletion of p53, indicating that additional oncogenic events are required to abrogate these fail-safe mechanisms and pave the way for cellular transformation. These findings have implications for our understanding of the transformation process in human cells. Abbreviations and acronyms: CDK: Cyclin-dependent kinase; DDR: DNA damage response; DOX: Doxycycline; EdU: 5-ethynyl-2'-deoxyuridine; FACS: Fluorescence Activated Cell Sorting; MycER: MYC-estrogen receptor; OHT: 4-hydroxytamoxifen; OIS: Oncogene-induced senescence; PP2A: Protein phosphatase 2A; ROS: Reactive oxygen species; SA-β-GAL: Senescence-associated β-galactosidase; SAHF: Senescence-associated heterochromatin foci; shRNA: Short hairpin RNA; YFP: Yellow fluorescent protein.

A selective high affinity MYC-binding compound inhibits MYC:MAX interaction and MYC-dependent tumor cell proliferation

MYC is a key player in tumor development, but unfortunately no specific MYC-targeting drugs are clinically available. MYC is strictly dependent on heterodimerization with MAX for transcription activation. Aiming at targeting this interaction, we identified MYCMI-6 in a cell-based protein interaction screen for small inhibitory molecules. MYCMI-6 exhibits strong selective inhibition of MYC:MAX interaction in cells and in vitro at single-digit micromolar concentrations, as validated by split Gaussia luciferase, in situ proximity ligation, microscale thermophoresis and surface plasmon resonance (SPR) assays. Further, MYCMI-6 blocks MYC-driven transcription and binds selectively to the MYC bHLHZip domain with a KD of 1.6 ± 0.5 μM as demonstrated by SPR. MYCMI-6 inhibits tumor cell growth in a MYC-dependent manner with IC50 concentrations as low as 0.5 μM, while sparing normal cells. The response to MYCMI-6 correlates with MYC expression based on data from 60 human tumor cell lines and is abrogated by MYC depletion. Further, it inhibits MYC:MAX interaction, reduces proliferation and induces massive apoptosis in tumor tissue from a MYC-driven xenograft tumor model without severe side effects. Since MYCMI-6 does not affect MYC expression, it is a unique molecular tool to specifically target MYC:MAX pharmacologically and it has good potential for drug development.

Role of PLK1 signaling pathway genes in gastrointestinal stromal tumors

In previous studies by the authors, aurora kinase A (AURKA) was demonstrated as an independent poor prognostic marker for the recurrence of localized gastrointestinal stromal tumors (GISTs) and for the progression of advanced GISTs. In the present study, the prognostic effect of genes involved in cell cycle regulation in GISTs was further examined. Leading edge analysis in gene set enrichment analysis was used to identify the most common genes in the top 10 enriched gene sets of high-risk patients with GISTs in a Japanese study. The obtained gene list was uploaded to the Pathway Interaction Database to search for critical pathways. Selected genes within the pathway were subsequently verified through immunohistochemistry (IHC) in another cohort of patients. A total of 5 genes in 'PLK1 signaling events,' namely AURKA, polo-like kinase 1 (PLK1), cell division cycle 25C (CDC25C), budding uninhibited by benzimidazoles (BUB1), and targeting protein for Xklp2 (TPX2), were identified for subsequent study. Among the Japanese cohort, all 5 genes, except BUB1, were significant prognostic factors for poor recurrence-free survival (RFS). Among 141 patients enrolled for the IHC study, all 5 genes exhibited variable expression patterns. In the association study, only AURKA exhibited significant overexpression in non-gastric tumors. Although all 5 genes were considered as risk factors for poor RFS based on a univariate analysis, only the mitotic count and expression levels of CDC25C, BUB1, and TPX2 retained prognostic effects in the multivariate analysis. The PLK1 signaling pathway is crucial in the disease progression of GISTs. Genes within this pathway may serve as predictive markers for adjuvant therapy.

MEK1 is required for the development of NRAS-driven leukemia

The dual-specificity kinases MEK1 and MEK2 act downstream of RAS/RAF to induce ERK activation, which is generally considered protumorigenic. Activating MEK mutations have not been discovered in leukemia, in which pathway activation is caused by mutations in upstream components such as RAS or Flt3. The anti-leukemic potential of MEK inhibitors is being tested in clinical trials; however, downregulation of MEK1 promotes Eμ-Myc-driven lymphomagenesis and MEK1 ablation induces myeloproliferative disease in mice, raising the concern that MEK inhibitors may be inefficient or counterproductive in this context. We investigated the role of MEK1 in the proliferation of human leukemic cell lines and in retroviral models of leukemia. Our data show that MEK1 suppression via RNA interference and genomic engineering does not affect the proliferation of human leukemic cell lines in culture; similarly, MEK1 ablation does not impact the development of MYC-driven leukemia in vivo. In contrast, MEK1 ablation significantly reduces tumorigenesis driven by Nras alone or in combination with Myc. Thus, while MEK1 restricts proliferation and tumorigenesis in some cellular and genetic contexts, it cannot be considered a tumor suppressor in the context of leukemogenesis. On the contrary, its role in NRAS-driven leukemogenesis advocates the use of MEK inhibitors, particularly in combination with PI3K/AKT inhibitors, in hematopoietic malignancies involving RAS activation.

p53 loss does not permit escape from BrafV600E-induced senescence in a mouse model of lung cancer

Lung cancer arises through the acquisition of a number of genetic lesions, with a preponderance of activating mutations in the canonical mitogen-activated protein kinase (MAPK) cascade (RTK-RAS-RAF-MEK). Braf^V600E expression induces benign lung adenomas that fail to progress to adenocarcinoma because of oncogene-induced senescence (OIS). Braf^V600E expression, coupled with simultaneous p53 ablation, permits bypass of senescence and progression to lung adenocarcinoma. However, spontaneous human tumors sustain mutations in a temporally separated manner. Here, we use a mouse lung cancer model where oncogene activation (Braf^V600E expression) and tumor suppressor loss (p53 ablation) are independently controlled through the actions of Flp and Cre recombinase, respectively. We show that p53 loss before OIS is permissive for the transition from lung adenoma to adenocarcinoma. In contrast, p53 loss after senescence is established fails to enable escape from senescence and disease progression. This study demonstrates that Braf^V600E induced senescence is irreversible in vivo and suggests that therapy-induced senescence would halt further tumor progression.

MYC and RAF: Key Effectors in Cellular Signaling and Major Drivers in Human Cancer

The prototypes of the human MYC and RAF gene families are orthologs of animal proto-oncogenes that were originally identified as transduced alleles in the genomes of highly oncogenic retroviruses. MYC and RAF genes are now established as key regulatory elements in normal cellular physiology, but also as major cancer driver genes. Although the predominantly nuclear MYC proteins and the cytoplasmic RAF proteins have different biochemical functions, they are functionally linked in pivotal signaling cascades and circuits. The MYC protein is a transcription factor and together with its dimerization partner MAX holds a central position in a regulatory network of bHLH-LZ proteins. MYC regulates transcription conducted by all RNA polymerases and controls virtually the entire transcriptome. Fundamental cellular processes including distinct catabolic and anabolic branches of metabolism, cell cycle regulation, cell growth and proliferation, differentiation, stem cell regulation, and apoptosis are under MYC control. Deregulation of MYC expression by rearrangement or amplification of the MYC locus or by defects in kinase-mediated upstream signaling, accompanied by loss of apoptotic checkpoints, leads to tumorigenesis and is a hallmark of most human cancers. The critically controlled serine/threonine RAF kinases are central nodes of the cytoplasmic MAPK signaling cascade transducing converted extracellular signals to the nucleus for reshaping transcription factor controlled gene expression profiles. Specific mutations of RAF kinases, such as the prevalent BRAF(V600E) mutation in melanoma, or defects in upstream signaling or feedback loops cause decoupled kinase activities which lead to tumorigenesis. Different strategies for pharmacological interference with MYC- or RAF-induced tumorigenesis are being developed and several RAF kinase inhibitors are already in clinical use.

BRG1 and BRM loss selectively impacts RB and P53, respectively: BRG1 and BRM have differential functions in vivo

The SWI/SNF complex is an important regulator of gene expression that functions by interacting with a diverse array of cellular proteins. The catalytic subunits of SWI/SNF, BRG1 and BRM, are frequently lost alone or concomitantly in a range of different cancer types. This loss abrogates SWI/SNF complex function as well as the functions of proteins that are required for SWI/SNF function, such as RB1 and TP53. Yet while both proteins are known to be dependent on SWI/SNF, we found that BRG1, but not BRM, is functionally linked to RB1, such that loss of BRG1 can directly or indirectly inactivate the RB1 pathway. This newly discovered dependence of RB1 on BRG1 is important because it explains why BRG1 loss can blunt the growth-inhibitory effect of tyrosine kinase inhibitors (TKIs). We also observed that selection for Trp53 mutations occurred in Brm-positive tumors but did not occur in Brm-negative tumors. Hence, these data indicate that, during cancer development, Trp53 is functionally dependent on Brm but not Brg1. Our findings show for the first time the key differences in Brm- and Brg1-specific SWI/SNF complexes and help explain why concomitant loss of Brg1 and Brm frequently occurs in cancer, as well as how their loss impacts cancer development.

Decoding c-Myc networks of cell cycle and apoptosis regulated genes in a transgenic mouse model of papillary lung adenocarcinomas

The c-Myc gene codes for a basic-helix-loop-helix-leucine zipper transcription factor protein and is reported to be frequently over-expressed in human cancers. Given that c-Myc plays an essential role in neoplastic transformation we wished to define its activity in lung cancer and therefore studied its targeted expression to respiratory epithelium in a transgenic mouse disease model. Using histological well-defined tumors, transcriptome analysis identified novel c-Myc responsive cell cycle and apoptosis genes that were validated as direct c-Myc targets using EMSA, Western blotting, gene reporter and ChIP assays.Through computational analyses c-Myc cooperating transcription factors emerged for repressed and up-regulated genes in cancer samples, namely Klf7, Gata3, Sox18, p53 and Elf5 and Cebpα, respectively. Conversely, at promoters of genes regulated in transgenic but non-carcinomatous lung tissue enriched binding sites for c-Myc, Hbp1, Hif1 were observed. Bioinformatic analysis of tumor transcriptomic data revealed regulatory gene networks and highlighted mortalin and moesin as master regulators while gene reporter and ChIP assays in the H1299 lung cancer cell line as well as cross-examination of published ChIP-sequence data of 7 human and 2 mouse cell lines provided strong evidence for the identified genes to be c-Myc targets. The clinical significance of findings was established by evaluating expression of orthologous proteins in human lung cancer. Taken collectively, a molecular circuit for c-Myc-dependent cellular transformation was identified and the network analysis broadened the perspective for molecularly targeted therapies.