Never-ageing cellular senescence

Immunomodulatory and Antiaging Mechanisms of Resveratrol, Rapamycin, and Metformin: Focus on mTOR and AMPK Signaling Networks

Aging results from the progressive dysregulation of several molecular pathways and mTOR and AMPK signaling have been suggested to play a role in the complex changes in key biological networks involved in cellular senescence. Moreover, multiple factors, including poor nutritional balance, drive immunosenescence progression, one of the meaningful aspects of aging. Unsurprisingly, nutraceutical and pharmacological interventions could help maintain an optimal biological response by providing essential bioactive micronutrients required for the development, maintenance, and the expression of the immune response at all stages of life. In this regard, many studies have provided evidence of potential antiaging properties of resveratrol, as well as rapamycin and metformin. Indeed, in vitro and in vivo models have demonstrated for these molecules a number of positive effects associated with healthy aging. The current review focuses on the mechanisms of action of these three important compounds and their suggested use for the clinical treatment of immunosenescence and aging.

Senotherapeutics: Targeting senescent cells for the main age-related diseases

The review aims to summarize and discuss the current knowledge on targeting senescent cells to reduce the risk of age-related diseases in animal models and human studies. The role of cellular senescence in aging and the major age-related diseases -including Alzheimer's disease, atherosclerosis, and type 2 diabetes- as well as the use of senotherapeutic strategies in both experimental and preclinical studies, will be described. A large number of molecules, including synthetic agents and natural compounds, have been proposed for anti-senescence activities. Research on senotherapeutics, which includes senolytic and senomorphic, has a growing interest, and their safety and reliability as anti-aging drugs have been tested in clinical trials. Initial findings suggest that the senotherapeutic approach may be translatable to humans. Due to the lack of evidence, caution must be used against senolytic agents due to their potential side-effects. In this context, natural senolytic compounds should have the advantage of low toxicity and potentially more useful in humans, although the mechanisms of action need to be defined.

Stiffness and Aging in Cardiovascular Diseases: The Dangerous Relationship between Force and Senescence

Biological aging is a process associated with a gradual decline in tissues' homeostasis based on the progressive inability of the cells to self-renew. Cellular senescence is one of the hallmarks of the aging process, characterized by an irreversible cell cycle arrest due to reactive oxygen species (ROS) production, telomeres shortening, chronic inflammatory activation, and chromatin modifications. In this review, we will describe the effects of senescence on tissue structure, extracellular matrix (ECM) organization, and nucleus architecture, and see how these changes affect (are affected by) mechano-transduction. In our view, this is essential for a deeper understanding of the progressive pathological evolution of the cardiovascular system and its relationship with the detrimental effects of risk factors, known to act at an epigenetic level.

A mechanistic evaluation of the Syrian hamster embryo cell transformation assay (pH 6.7) and molecular events leading to senescence bypass in SHE cells

The implementation of the Syrian hamster embryo cell transformation assay (SHE CTA) into test batteries and its relevance in predicting carcinogenicity has been long debated. Despite prevalidation studies to ensure reproducibility and minimise the subjective nature of the assay's endpoint, an underlying mechanistic and molecular basis supporting morphological transformation (MT) as an indicator of carcinogenesis is still missing. We found that only 20% of benzo(a)pyrene-induced MT clones immortalised suggesting that, alone, the MT phenotype is insufficient for senescence bypass. From a total of 12 B(a)P- immortalised MT lines, inactivating p53 mutations were identified in 30% of clones, and the majority of these were consistent with the potent carcinogen's mode of action. Expression of p16 was commonly silenced or markedly reduced with extensive promoter methylation observed in 45% of MT clones, while Bmi1 was strongly upregulated in 25% of clones. In instances where secondary events to MT appeared necessary for senescence bypass, as evidenced by a transient cellular crisis, clonal growth correlated with monoallelic deletion of the CDKN2A/B locus. The findings further implicate the importance of p16 and p53 pathways in regulating senescence while providing a molecular evaluation of SHE CTA -derived variant MT clones induced by benzo(a)pyrene.

ATM regulates insulin-like growth factor 1-secretory clusterin (IGF-1-sCLU) expression that protects cells against senescence

Downstream factors that regulate the decision between senescence and cell death have not been elucidated. Cells undergo senescence through three pathways, replicative senescence (RS), stress-induced premature senescence (SIPS) and oncogene-induced senescence. Recent studies suggest that the ataxia telangiectasia mutant (ATM) kinase is not only a key protein mediating cellular responses to DNA damage, but also regulates cellular senescence induced by telomere end exposure (in RS) or persistent DNA damage (in SIPS). Here, we show that expression of secretory clusterin (sCLU), a known pro-survival extracellular chaperone, is transcriptionally up-regulated during both RS and SIPS, but not in oncogene-induced senescence, consistent with a DNA damage-inducible mechanism. We demonstrate that ATM plays an important role in insulin-like growth factor 1 (IGF-1) expression, that in turn, regulates downstream sCLU induction during senescence. Loss of ATM activity, either by genomic mutation (ATM-deficient fibroblasts from an ataxia telangiectasia patient) or by administration of a chemical inhibitor (AAI, an inhibitor of ATM and ATR), blocks IGF-1-sCLU expression in senescent cells. Downstream, sCLU induction during senescence is mediated by IGF-1R/MAPK/Egr-1 signaling, identical to its induction after DNA damage. In contrast, administration of an IGF-1 inhibitor caused apoptosis of senescent cells. Thus, IGF-1 signaling is required for survival, whereas sCLU appears to protect cells from premature senescence, as IMR-90 cells with sCLU knockdown undergo senescence faster than control cells. Thus, the ATM-IGF-1-sCLU pathway protects cells from lethality and suspends senescence.

Oncogene-induced reactive oxygen species fuel hyperproliferation and DNA damage response activation

Oncogene-induced reactive oxygen species (ROS) have been proposed to be signaling molecules that mediate proliferative cues. However, ROS may also cause DNA damage and proliferative arrest. How these apparently opposite roles can be reconciled, especially in the context of oncogene-induced cellular senescence, which is associated both with aberrant mitogenic signaling and DNA damage response (DDR)-mediated arrest, is unclear. Here, we show that ROS are indeed mitogenic signaling molecules that fuel oncogene-driven aberrant cell proliferation. However, by their very same ability to mediate cell hyperproliferation, ROS eventually cause DDR activation. We also show that oncogenic Ras-induced ROS are produced in a Rac1 and NADPH oxidase (Nox4)-dependent manner. In addition, we show that Ras-induced ROS can be detected and modulated in a living transparent animal: the zebrafish. Finally, in cancer we show that Nox4 is increased in both human tumors and a mouse model of pancreatic cancer and specific Nox4 small-molecule inhibitors act synergistically with existing chemotherapic agents.

Oncogene-induced reactive oxygen species fuel hyperproliferation and DNA damage response activation

Oncogene-induced reactive oxygen species (ROS) have been proposed to be signaling molecules that mediate proliferative cues. However, ROS may also cause DNA damage and proliferative arrest. How these apparently opposite roles can be reconciled, especially in the context of oncogene-induced cellular senescence, which is associated both with aberrant mitogenic signaling and DNA damage response (DDR)-mediated arrest, is unclear. Here, we show that ROS are indeed mitogenic signaling molecules that fuel oncogene-driven aberrant cell proliferation. However, by their very same ability to mediate cell hyperproliferation, ROS eventually cause DDR activation. We also show that oncogenic Ras-induced ROS are produced in a Rac1 and NADPH oxidase (Nox4)-dependent manner. In addition, we show that Ras-induced ROS can be detected and modulated in a living transparent animal: the zebrafish. Finally, in cancer we show that Nox4 is increased in both human tumors and a mouse model of pancreatic cancer and specific Nox4 small-molecule inhibitors act synergistically with existing chemotherapic agents.

Telomeric noncoding RNA: telomeric repeat-containing RNA in telomere biology

Telomeres are nucleoprotein structures that cap the ends of eukaryotic chromosomes, protecting them from degradation and activation of DNA damage response. For this reason, functional telomeres are vital to genome stability. For years, telomeres were assumed to be transcriptionally silent, because of their heterochromatic state. It was only recently shown that, in several organisms, telomeres are transcribed, giving rise to a long noncoding RNA (lncRNA) called telomeric repeat-containing RNA (TERRA). Several lines of evidence now indicate that TERRA molecules play crucial roles in telomere homeostasis and telomere functions. Recent studies have shown that the expression and regulation of TERRA are dynamically controlled by each chromosome end. TERRA has been involved in the regulation of telomere length, telomerase activity, and heterochromatin formation at telomeres. The correct regulation of the telomeric transcripts may be essential to genome stability, and altered TERRA levels associate with tumorigenesis and cellular senescence. Thus, the study of the molecular mechanisms of TERRA biogenesis and function may advance the understanding of telomere-related diseases, including cancer and aging.

The transcription factor FOXM1 (Forkhead box M1): proliferation-specific expression, transcription factor function, target genes, mouse models, and normal biological roles

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor, which stimulates cell proliferation and exhibits a proliferation-specific expression pattern. Accordingly, both the expression and the transcriptional activity of FOXM1 are increased by proliferation signals, but decreased by antiproliferation signals, including the positive and negative regulation by protooncoproteins or tumor suppressors, respectively. FOXM1 stimulates cell cycle progression by promoting the entry into S-phase and M-phase. Moreover, FOXM1 is required for proper execution of mitosis. Accordingly, FOXM1 regulates the expression of genes, whose products control G1/S-transition, S-phase progression, G2/M-transition, and M-phase progression. Additionally, FOXM1 target genes encode proteins with functions in the execution of DNA replication and mitosis. FOXM1 is a transcriptional activator with a forkhead domain as DNA binding domain and with a very strong acidic transactivation domain. However, wild-type FOXM1 is (almost) inactive because the transactivation domain is repressed by three inhibitory domains. Inactive FOXM1 can be converted into a very potent transactivator by activating signals, which release the transactivation domain from its inhibition by the inhibitory domains. FOXM1 is essential for embryonic development and the foxm1 knockout is embryonically lethal. In adults, FOXM1 is important for tissue repair after injury. FOXM1 prevents premature senescence and interferes with contact inhibition. FOXM1 plays a role for maintenance of stem cell pluripotency and for self-renewal capacity of stem cells. The functions of FOXM1 in prevention of polyploidy and aneuploidy and in homologous recombination repair of DNA-double-strand breaks suggest an importance of FOXM1 for the maintenance of genomic stability and chromosomal integrity.

CBX8 suppresses Sirtinol-induced premature senescence in human breast cancer cells via cooperation with SIRT1

Stress-induced premature senescence (SIPS) has been implicated in the suppression of carcinogenesis. We identified chromodomain protein 8 (CBX8), a Polycomb group (PcG) protein, as a novel binding partner of SIRT1. The interaction between CBX8 and SIRT1 was demonstrated by immunoprecipitation, GST pull-down, fluorescence microscopy, and cooperation for transcriptional repression. Like SIRT1, CBX8 repressed premature senescence and growth arrest induced by the SIRT1 inhibitor Sirtinol in MCF7 cells, which was reversed by depleting CBX8. CBX8 cooperated with SIRT1 for suppressing p53 acetylation induced by Sirtinol and etoposide/TSA. Upon ectopic expression, CBX8 or SIRT1 repressed the expression of p21(WAF1) by inhibiting p53 binding to the promoter. We provide the first evidence that CBX8 plays a potential role in regulating premature senescence in human breast cancer cells through cooperation with SIRT1.

FOXM1 (Forkhead box M1) in tumorigenesis: overexpression in human cancer, implication in tumorigenesis, oncogenic functions, tumor-suppressive properties, and target of anticancer therapy

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor and is also intimately involved in tumorigenesis. FOXM1 stimulates cell proliferation and cell cycle progression by promoting the entry into S-phase and M-phase. Additionally, FOXM1 is required for proper execution of mitosis. In accordance with its role in stimulation of cell proliferation, FOXM1 exhibits a proliferation-specific expression pattern and its expression is regulated by proliferation and anti-proliferation signals as well as by proto-oncoproteins and tumor suppressors. Since these factors are often mutated, overexpressed, or lost in human cancer, the normal control of the foxm1 expression by them provides the basis for deregulated FOXM1 expression in tumors. Accordingly, FOXM1 is overexpressed in many types of human cancer. FOXM1 is intimately involved in tumorigenesis, because it contributes to oncogenic transformation and participates in tumor initiation, growth, and progression, including positive effects on angiogenesis, migration, invasion, epithelial-mesenchymal transition, metastasis, recruitment of tumor-associated macrophages, tumor-associated lung inflammation, self-renewal capacity of cancer cells, prevention of premature cellular senescence, and chemotherapeutic drug resistance. However, in the context of urethane-induced lung tumorigenesis, FOXM1 has an unexpected tumor suppressor role in endothelial cells because it limits pulmonary inflammation and canonical Wnt signaling in epithelial lung cells, thereby restricting carcinogenesis. Accordingly, FOXM1 plays a role in homologous recombination repair of DNA double-strand breaks and maintenance of genomic stability, that is, prevention of polyploidy and aneuploidy. The implication of FOXM1 in tumorigenesis makes it an attractive target for anticancer therapy, and several antitumor drugs have been reported to decrease FOXM1 expression.

Functional comparison of chronological and in vitro aging: differential role of the cytoskeleton and mitochondria in mesenchymal stromal cells

Mesenchymal stromal cells (MSCs) are of high relevance for the regeneration of mesenchymal tissues such as bone and cartilage. The promising role of MSCs in cell-based therapies and tissue engineering appears to be limited due to a decline of their regenerative potential with increasing donor age, their limited availability in human tissues and the need of in vitro expansion prior to treatment. We therefore aimed to determine to which degree in vitro aging and chronological aging may be similar processes or if in vitro culture-related changes at the cellular and molecular level are at least altered as a function of donor age. For that purpose we established MSCs cultures from young (yMSCs) and aged (aMSCs) rats that were cultured for more than 100 passages. These long-term MSCs cultures were non-tumorigenic and exhibited similar surface marker patterns as primary MSCs of passage 2. During in vitro expansion, but not during chronological aging, MSCs progressively lose their progenitor characteristics, e.g., complete loss of osteogenic differentiation potential, diminished adipogenic differentiation, altered cell morphology and increased susceptibility towards senescence. Transcriptome analysis revealed that long-term in vitro MSCs cultivation leads to down-regulation of genes involved in cell differentiation, focal adhesion organization, cytoskeleton turnover and mitochondria function. Accordingly, functional analysis demonstrated altered mitochondrial morphology, decreased antioxidant capacities and elevated ROS levels in long-term cultivated yMSCs as well as aMSCs. Notably, only the MSC migration potential and their antioxidative capacity were altered by in vitro as well as chronological aging. Based on specific differences observed between the impact of chronological and in vitro MSC aging we conclude that both are distinct processes.

Crosstalk between chromatin state and DNA damage response in cellular senescence and cancer

The generation of DNA lesions and the resulting activation of DNA damage response (DDR) pathways are both affected by the chromatin status at the site of damaged DNA. In turn, DDR activation affects the chromatin at both the damaged site and across the whole genome. Cellular senescence and cancer are associated with the engagement of the DDR pathways and with profound chromatin changes. In this Opinion article, we discuss the interplay between chromatin and DDR factors in the context of cellular senescence that is induced by oncogenes and in cancer.

Oxydative stress alters nuclear shape through lamins dysregulation: a route to senescence

Progeroid phenotypes are mainly encountered in 2 types of syndromes: in laminopathies, which are characterized by nuclear shape abnormalities due to lamin A alteration, and in DNA damage response defect syndromes. Because lamin A dysregulation leads to DNA damages, it has been proposed that senescence occurs in both types of syndromes through the accumulation of damages. We recently showed that elevated oxidative stress is responsible for lamin B1 accumulation, nuclear shape alteration and senescence in the DDR syndrome, ataxia telangiectasia (A-T). Interestingly, overexpression of lamin B1 in wild type cells is sufficient to induce senescence without the induction of DNA damages. Here, we will discuss the importance of controlling the lamins level in order for maintenance nuclear architecture and we will comment the relationships of lamins with other senescence mechanisms. Finally, we will describe emerging data reporting redox control by lamins, leading us to propose a general mechanism by which reactive oxygen species can induce senescence through lamin dysregulation and NSA.

Translational research in immune senescence: assessing the relevance of current models

Advancing age is accompanied by profound changes in immune function; some are induced by the loss of critical niches that support development of naïve cells (e.g. thymic involution), others by the intrinsic physiology of long-lived cells attempting to maintain homeostasis, still others by extrinsic effects such as oxidative stress or long-term exposure to antigen due to persistent viral infections. Once compensatory mechanisms can no longer maintain a youthful phenotype the end result is the immune senescent milieu - one characterized by chronic, low grade, systemic inflammation and impaired responses to immune challenge, particularly when encountering new antigens. This state is associated with progression of chronic illnesses like atherosclerosis and dementia, and an increased risk of acute illness, disability and death in older adults. The complex interaction between immune senescence and chronic illness provides an ideal landscape for translational research with the potential to greatly affect human health. However, current animal models and even human investigative strategies for immune senescence have marked limitations, and the reductionist paradigm itself may be poorly suited to meet these challenges. A new paradigm, one that embraces complexity as a core feature of research in older adults is required to address the critical health issues facing the burgeoning senior population, the group that consumes the majority of healthcare resources. In this review, we outline the major advantages and limitations of current models and offer suggestions for how to move forward.

Telomeres and disease: enter TERRA

Telomere function is tightly regulated in order to maintain chromosomal stability. When telomeres become dysfunctional, the replicative capacity of cells diminishes and cellular senescence ensues. This can lead to impaired tissue replenishment and eventually degenerative disorders, referred to as telomere syndromes. Cancer can also develop as a result of the genomic instability associated with telomere dysfunction. TERRA (TElomeric Repeat containing RNA) is a long non-coding transcript that stems from sub-telomeric regions and continues into the telomeric tract and is therefore a hybrid of both sub-telomeric and telomeric sequence. In general, increased TERRA transcription is associated with telomere shortening and compromised telomere function. Here we will briefly outline the general principles behind telomere dysfunction-associated diseases. Furthermore, we will discuss the few known links that exist between telomere transcription (TERRA) and disease. Finally, we will speculate on how the understanding, and eventual manipulation, of TERRA transcription could potentially be used in terms of therapeutic strategies.