Contrasting roles for C/EBPα and Notch in irradiation-induced multipotent hematopoietic progenitor cell defects

Inflammation promotes aging-associated oncogenesis in the lung

Background

Lung cancer is the leading cause of cancer death in the world. While cigarette smoking is the major preventable factor for cancers in general and lung cancer in particular, old age is also a major risk factor. Aging-related chronic, low-level inflammation, termed inflammaging, has been widely documented; however, it remains unclear how inflammaging contributes to increased lung cancer incidence. Aim: To establish connections between aging-associated changes in the lungs and cancer risk.

Methods

We analyzed public databases of gene expression for normal and cancerous human lungs and used mouse models to understand which changes were dependent on inflammation, as well as to assess the impact on oncogenesis.

Results

Analyses of GTEx and TCGA databases comparing gene expression profiles from normal lungs, lung adenocarcinoma, lung squamous cell carcinoma of subjects across age groups revealed upregulated pathways such as inflammatory response, TNFA signaling via NFκB, and interferon-gamma response. Similar pathways were identified comparing the gene expression profiles of young and old mouse lungs. Transgenic expression of alpha 1 antitrypsin (AAT) partially reverses increases in markers of aging-associated inflammation and immune deregulation. Using an orthotopic model of lung cancer using cells derived from EML4-ALK fusion-induced adenomas, we demonstrated an increased tumor outgrowth in lungs of old mice while NLRP3 knockout in old mice decreased tumor volumes, suggesting that inflammation contributes to increased lung cancer development in aging organisms.

Conclusions

These studies reveal how expression of an anti-inflammatory mediator (AAT) can reduce some but not all aging-associated changes in mRNA and protein expression in the lungs. We further show that aging is associated with increased tumor outgrowth in the lungs, which may relate to an increased inflammatory microenvironment.

NF-κB signaling controls H3K9me3 levels at intronic LINE-1 and hematopoietic stem cell genes in cis

Ionizing radiations (IR) alter hematopoietic stem cell (HSC) function on the long term, but the mechanisms underlying these effects are still poorly understood. We recently showed that IR induces the derepression of L1Md, the mouse young subfamilies of LINE-1/L1 retroelements. L1 contributes to gene regulatory networks. However, how L1Md are derepressed and impact HSC gene expression are not known. Here, we show that IR triggers genome-wide H3K9me3 decrease that occurs mainly at L1Md. Loss of H3K9me3 at intronic L1Md harboring NF-κB binding sites motifs but not at promoters is associated with the repression of HSC-specific genes. This is correlated with reduced NFKB1 repressor expression. TNF-α treatment rescued all these effects and prevented IR-induced HSC loss of function in vivo. This TNF-α/NF-κB/H3K9me3/L1Md axis might be important to maintain HSCs while allowing expression of immune genes during myeloid regeneration or damage-induced bone marrow ablation.

Chronic interleukin-1 exposure triggers selection for Cebpa-knockout multipotent hematopoietic progenitors

The early events that drive myeloid oncogenesis are not well understood. Most studies focus on the cell-intrinsic genetic changes and how they impact cell fate decisions. We consider how chronic exposure to the proinflammatory cytokine, interleukin-1β (IL-1β), impacts Cebpa-knockout hematopoietic stem and progenitor cells (HSPCs) in competitive settings. Surprisingly, we found that Cebpa loss did not confer a hematopoietic cell–intrinsic competitive advantage; rather chronic IL-1β exposure engendered potent selection for Cebpa loss. Chronic IL-1β augments myeloid lineage output by activating differentiation and repressing stem cell gene expression programs in a Cebpa-dependent manner. As a result, Cebpa-knockout HSPCs are resistant to the prodifferentiative effects of chronic IL-1β, and competitively expand. We further show that ectopic CEBPA expression reduces the fitness of established human acute myeloid leukemias, coinciding with increased differentiation. These findings have important implications for the earliest events that drive hematologic disorders, suggesting that chronic inflammation could be an important driver of leukemogenesis and a potential target for intervention.

Cancer as a disease of old age: changing mutational and microenvironmental landscapes

Why do we get cancer mostly when we are old? According to current paradigms, the answer is simple: mutations accumulate in our tissues throughout life, and some of these mutations contribute to cancers. Although mutations are necessary for cancer development, a number of studies shed light on roles for ageing and exposure-dependent changes in tissue landscapes that determine the impact of oncogenic mutations on cellular fitness, placing carcinogenesis into an evolutionary framework. Natural selection has invested in somatic maintenance to maximise reproductive success. Tissue maintenance not only ensures functional robustness but also prevents the occurrence of cancer through periods of likely reproduction by limiting selection for oncogenic events in our cells. Indeed, studies in organisms ranging from flies to humans are revealing conserved mechanisms to eliminate damaged or oncogenically initiated cells from tissues. Reports of the existence of striking numbers of oncogenically initiated clones in normal tissues and of how this clonal architecture changes with age or external exposure to noxious substances provide critical insight into the early stages of cancer development. A major challenge for cancer biology will be the integration of these studies with epidemiology data into an evolutionary theory of carcinogenesis, which could have a large impact on addressing cancer risk and treatment.

Roles of C/EBP class bZip proteins in the growth and cell competition of Rp ('Minute') mutants in Drosophila

Reduced copy number of ribosomal protein (Rp) genes adversely affects both flies and mammals. Xrp1 encodes a reportedly Drosophila-specific AT-hook, bZIP protein responsible for many of the effects including the elimination of Rp mutant cells by competition with wild type cells. Irbp18, an evolutionarily conserved bZIP gene, heterodimerizes with Xrp1 and with another bZip protein, dATF4. We show that Irbp18 is required for the effects of Xrp1, whereas dATF4 does not share the same phenotype, indicating that Xrp1/Irbp18 is the complex active in Rp mutant cells, independently of other complexes that share Irbp18. Xrp1 and Irbp18 transcripts and proteins are upregulated in Rp mutant cells by auto-regulatory expression that depends on the Xrp1 DNA binding domains and is necessary for cell competition. We show that Xrp1 is conserved beyond Drosophila, although under positive selection for rapid evolution, and that at least one human bZip protein can similarly affect Drosophila development.

Studying Cancer Evolution and Therapeutic Responses in Different Organs: The Pros and Cons of a Broad Focus

Cellular adaptation brought upon by insults such as old age and therapeutic exposure is a complex phenomenon in which cells undergo adaptive phenotypic changes. Our lab has focused on understanding the mechanisms underlying adaptation during the evolution of cancer, from the early stages of development to the ability of cancer cells to escape therapeutic challenges. Our studies span hematopoietic and lung systems. Herein, we discuss the advantages and disadvantages involved in studying two vastly different organ systems. Through the use of these organ/cancer model systems, we hope to develop interventions to limit oncogenic adaptation leading to cancer development and to prevent adaptation of cancers following treatment leading to cancer relapse.

Thrombopoietin protects hematopoietic stem cells from retrotransposon-mediated damage by promoting an antiviral response

Maintenance of genomic integrity is crucial for the preservation of hematopoietic stem cell (HSC) potential. Retrotransposons, spreading in the genome through an RNA intermediate, have been associated with loss of self-renewal, aging, and DNA damage. However, their role in HSCs has not been addressed. Here, we show that mouse HSCs express various retroelements (REs), including long interspersed element-1 (L1) recent family members that further increase upon irradiation. Using mice expressing an engineered human L1 retrotransposition reporter cassette and reverse transcription inhibitors, we demonstrate that L1 retransposition occurs in vivo and is involved in irradiation-induced persistent γH2AX foci and HSC loss of function. Thus, RE represents an important intrinsic HSC threat. Furthermore, we show that RE activity is restrained by thrombopoietin, a critical HSC maintenance factor, through its ability to promote a potent interferon-like, antiviral gene response in HSCs. This uncovers a novel mechanism allowing HSCs to minimize irradiation-induced injury and reinforces the links between DNA damage, REs, and antiviral immunity.

Dose-dependent effects of gamma radiation on the early zebrafish development and gene expression

Ionizing radiation from natural sources or anthropogenic activity has the potential to cause oxidative stress or genetic damage in living organisms, through the ionization and excitation of molecules and the subsequent production of free radicals and reactive oxygen species (ROS). The present work focuses on radiation-induced biological effects using the zebrafish (Danio rerio) vertebrate model. Changes in developmental traits and gene expression in zebrafish were assessed after continuous external gamma irradiation (0.4, 3.9, 15 and 38 mGy/h) with corresponding controls, starting at 2.5 hours post fertilization (hpf) and lasting through embryogenesis and the early larval stage. The lowest dose rate corresponded to recommended benchmarks at which adverse effects are not expected to occur in aquatic ecosystems (2-10 mGy/day). The survival observed at 96 hours post fertilization (hpf) in the 38 mGy/h group was significantly lower, while other groups showed no significant difference compared to controls. The total hatching was significantly lower from controls in the 15 mGy/h group and a delay in hatching onset in the 0.4 mGy/h group was observed. The deformity frequency was significantly increased by prolonged exposure duration at dose rates ≥ 0.4 mGy/h. Molecular responses analyzed by RNA-seq at gastrulation (5.5 hpf transcriptome) indicate that the radiation induced adverse effects occurred during the earliest stages of development. A dose-response relationship was found in the numbers of differentially regulated genes in exposure groups compared to controls at a total dose as low as 1.62 mGy. Ingenuity Pathway Analysis identified retinoic acid receptor activation, apoptosis, and glutathione mediated detoxification signaling as the most affected pathways in the lower dose rate (0.54 mGy/h), while eif2 and mTOR, i.e., involved in the modulation of angiogenesis, were most affected in higher dose rates (5.4 and 10.9 mGy/h). By comparing gene expression data, myc was found to be the most significant upstream regulator, followed by tp53, TNF, hnf4a, TGFb1 and cebpa, while crabp2b and vegfab were identified as most frequent downstream target genes. These genes are associated with various developmental processes. The present findings show that continuous gamma irradiation (≥ 0.54 mGy/h) during early gastrula causes gene expression changes that are linked to developmental defects in zebrafish embryos.

DNA-damage response in hematopoietic stem cells: an evolutionary trade-off between blood regeneration and leukemia suppression

Self-renewing and multipotent hematopoietic stem cells (HSCs) maintain lifelong hematopoiesis. Their enormous regenerative potential coupled with lifetime persistence in the body, in contrast with the Progenitors, demand tight control of HSCs genome stability. Indeed, failure to accurately repair DNA damage in HSCs is associated with bone marrow failure and accelerated leukemogenesis. Recent observations exposed remarkable differences in several DNA-damage response (DDR) aspects between HSCs and Progenitors, especially in their DNA-repair capacities and susceptibility to apoptosis. Human HSCs in comparison with Progenitors exhibit delayed DNA double-strand break rejoining, persistent DDR signaling activation, higher sensitivity to the cytotoxic effects of ionizing radiation and attenuated expression of DNA-repair genes. Importantly, the distinct DDR of HSCs was also documented in mouse models. Nevertheless, physiological significance and the molecular basis of the HSCs-specific DDR features are only partially understood. Taking radiation-induced DDR as a paradigm, this review will focus on the current advances in understanding the role of cell-intrinsic DDR regulators and the cellular microenvironment in balancing stemness with genome stability. Pre-leukemia HSCs and clonal hematopoiesis evolvement will be discussed as an evolutionary compromise between the need for lifelong blood regeneration and DDR. Uniquely for this review, we outline the differences in HSCs-related DDR as highlighted by various experimental systems and attempt to provide their critical analysis.

Changing mutational and adaptive landscapes and the genesis of cancer

By the time the process of oncogenesis has produced an advanced cancer, tumor cells have undergone extensive evolution. The cellular phenotypes resulting from this evolution have been well studied, and include accelerated growth rates, apoptosis resistance, immortality, invasiveness, and immune evasion. Yet with all of our current knowledge of tumor biology, the details of early oncogenesis have been difficult to observe and understand. Where different oncogenic mutations may work together to enhance the survival of a tumor cell, in isolation they are often pro-apoptotic, pro-differentiative or pro-senescent, and therefore often, somewhat paradoxically, disadvantageous to a cell. It is also becoming clear that somatic mutations, including those in known oncogenic drivers, are common in tissues starting at a young age. These observations raise the question: how do we largely avoid cancer for most of our lives? Here we propose that evolutionary forces can help explain this paradox. As humans and other organisms age or experience external insults such as radiation or smoking, the structure and function of tissues progressively degrade, resulting in altered stem cell niche microenvironments. As tissue integrity declines, it becomes less capable of supporting and maintaining resident stem cells. These stem cells then find themselves in a microenvironment to which they are poorly adapted, providing a competitive advantage to those cells that can restore their functionality and fitness through mutations or epigenetic changes. The resulting oncogenic clonal expansions then increase the odds of further cancer progression. Understanding how the causes of cancer, such as aging or smoking, affect tissue microenvironments to control the impact of mutations on somatic cell fitness can help reconcile the discrepancy between marked mutation accumulation starting early in life and the somatic evolution that leads to cancer at advanced ages or following carcinogenic insults. This article is part of a Special Issue entitled: Evolutionary principles - heterogeneity in cancer?, edited by Dr. Robert A. Gatenby.

Long-term genotoxic effects in the hematopoietic system of prenatally X-irradiated mice

PURPOSE

To investigate the genotoxic effects of prenatal X-irradiation in mice and the possible presence of late genomic instability.

MATERIALS AND METHODS

Pregnant mice were exposed to 0, 1 or 2 Gy at embryonic day 11.5. Blood smears were obtained from pups at birth and on post-natal day 11, 21, 42 and 140. Hematological data (diameter of erythrocytes, percentage of reticulocytes and Granulocyte-to-Lymphocyte ratio [GLR]) and genotoxicity (micronucleated erythrocytes, micronucleated reticulocytes, CREST-positive and negative micronuclei) were assessed.

RESULTS

Prenatal irradiation caused perinatal reticulocytosis (which ended on postnatal day 11) and a dose-dependent increase of GLR (indicative of myeloid skewing) on postnatal days 42 and 140. Two temporally distinct genotoxic effects were observed: an early, acute damage (still detectable at birth and soon after) and a late, long-term damage.

CONCLUSIONS

Increases in micronuclei frequencies and GLR observed from day 42 on are both ascribable to DNA damage. Time of appearance of this late effect may be linked to the shift of hematopoiesis from spleen to bone marrow and to cell-extrinsic factor such as the microenvironment. This study confirms that ionizing radiation can induce long-term genotoxic effects in the hematopoietic system and shows that prenatal irradiation determines genomic instability in blood-forming tissues of adult mice.

The evolution of lifespan and age-dependent cancer risk

The Armitage-Doll multi-stage model of carcinogenesis tremendously refocused cancer science by postulating that carcinogenesis is driven by a sequence of genetic changes in cells. Age-dependent cancer incidence thus has been explained in terms of the time necessary for oncogenic mutations to occur. While the multi-step nature of cancer evolution is well-supported by evidence, the mutation-centric theory is unable to explain a number of phenomena, such as the disproportion between cancer frequency and animal body size or the scaling of cancer incidence to animal lifespan. In this paper, we present a theoretical review of the current paradigm and discuss some fundamental evolutionary theory postulates that explain why cancer incidence is a function of lifespan and physiological, not chronological, aging.

Toward an evolutionary model of cancer: Considering the mechanisms that govern the fate of somatic mutations

Our understanding of cancer has greatly advanced since Nordling [Nordling CO (1953) Br J Cancer 7(1):68-72] and Armitage and Doll [Armitage P, Doll R (1954) Br J Cancer 8(1):1-12] put forth the multistage model of carcinogenesis. However, a number of observations remain poorly understood from the standpoint of this paradigm in its contemporary state. These observations include the similar age-dependent exponential rise in incidence of cancers originating from stem/progenitor pools differing drastically in size, age-dependent cell division profiles, and compartmentalization. This common incidence pattern is characteristic of cancers requiring different numbers of oncogenic mutations, and it scales to very divergent life spans of mammalian species. Also, bigger mammals with larger underlying stem cell pools are not proportionally more prone to cancer, an observation known as Peto's paradox. Here, we present a number of factors beyond the occurrence of oncogenic mutations that are unaccounted for in the current model of cancer development but should have significant impacts on cancer incidence. Furthermore, we propose a revision of the current understanding for how oncogenic and other functional somatic mutations affect cellular fitness. We present evidence, substantiated by evolutionary theory, demonstrating that fitness is a dynamic environment-dependent property of a phenotype and that oncogenic mutations should have vastly different fitness effects on somatic cells dependent on the tissue microenvironment in an age-dependent manner. Combined, this evidence provides a firm basis for understanding the age-dependent incidence of cancers as driven by age-altered systemic processes regulated above the cell level.

Evolved Cellular Mechanisms to Respond to Genotoxic Insults: Implications for Radiation-Induced Hematologic Malignancies

Human exposure to ionizing radiation is highly associated with adverse health effects, including reduced hematopoietic cell function and increased risk of carcinogenesis. The hematopoietic deficits manifest across blood cell types and persist for years after radiation exposure, suggesting a long-lived and multi-potent cellular reservoir for radiation-induced effects. As such, research has focused on identifying both the immediate and latent hematopoietic stem cell responses to radiation exposure. Radiation-associated effects on hematopoietic function and malignancy development have generally been attributed to the direct induction of mutations resulting from radiation-induced DNA damage. Other studies have illuminated the role of cellular programs that both limit and enhance radiation-induced tissue phenotypes and carcinogenesis. In this review, distinct but collaborative cellular responses to genotoxic insults are highlighted, with an emphasis on how these programmed responses impact hematopoietic cellular fitness and competition. These radiation-induced cellular programs include apoptosis, senescence and impaired self-renewal within the hematopoietic stem cell (HSC) pool. In the context of sporadic DNA damage to a cell, these cellular responses act in concert to restore tissue function and prevent selection for adaptive oncogenic mutations. But in the contexts of whole-tissue exposure or whole-body exposure to genotoxins, such as radiotherapy or chemotherapy, we propose that these programs can contribute to long-lasting tissue impairment and increased carcinogenesis.