"Cell Senescence" verses "Cell Ageing": What is the Difference?

First published @ Senescent CELL blog

In recent times, scientific findings on cellular senescence have made headlines.  The majority of these highly publicized articles are concerned with the potential health benefits of removing senescent cells from our bodies. Destroying senescent cells in mice can reverse aspects of ageing and prevent side effects in response to chemotherapy.  

In an attempt to simplify the term "cell senescence" for public consumption, the media incorrectly use words such as "old", "aged" and "elderly" to describe such cells.  This is understandable since "senescence" means "to grow old". 

The term "senescence" regarding cells was first used over fifty years ago to describe cells that could no longer proliferate after extended time in culture. Without our current understandings, this inability to proliferate was thought to be due to processes related to cell ageing and so such cells were labelled as "senescent".  Although now inaccurate, this labeling is still in use today.

So what is the difference between cell "ageing" and cell "senescence"?

Cell ageing results from the accumulation of random damage leading to impairment in cell function with time.  Cell ageing may result from the build up of damage to cellular lipids (i.e. peroxidation), damage to proteins leading to altered protein folding and aggregation, damage to the mitochondria resulting in abnormal metabolic processes and changes (epigenetic) to DNA causing alterations in gene expression.

In contrast to cell ageing, cell senescence is a programmed change in cell state often initiated by persistent damage to DNA.  

Although the initial factors which trigger DNA damage in cells may itself be random, the accompanying cellular changes associated with cell senescence are not random.  In an orchestrated response, cells permanently stop dividing, they secrete molecules that can attract immune cells and express immune proteins on their cell surface.  As such, cell senescence can be considered as a mechanism to eliminate unwanted cells by the immune system.

Part of the reason why senescent cells stay in our bodies and promote ageing may be due to a failure in the ability of an aged immune system to kill senescent cells.  The molecules that were once beneficial in attracting immune cells now become destructive over time.

Further scientific evidence demonstrating that senescent cells are not "aged" cells but rather a programmed change in cell state, comes from studies on embryonic development.  

Two back to back publications in Cell from 2013 demonstrated that the change in cell state associated with senescent cells may be beneficial during embryonic development.  If this indeed the case, it is highly unlikely that such cells suddenly become "aged" or "old" to carry out their function.  Embryonic development is highly a regulated process.  What is more likely is that such senescent cells are indeed programmed and like programmed cell death (apoptosis), play an important role in tissue remodeling during embryonic development. 

Telomere shortening: adding further to the confusion

The vast majority of the early studies on cell senescence were focused on cells which stopped dividing after extended proliferation in culture.  This was later shown to be due to telomere shortening.

Every time a cell divides it loses a portion of DNA at the end of its chromosomes called telomeres- long repeats of non-coding DNA. Telomeres protect the ends of our DNA, but when they become too short, they can no longer perform this task. This causes the cell to recognize unprotected DNA-ends as damage. A result of this DNA damage signal is induction of cell senescence.  

Throughout our lives, cells divide and our telomeres gradually become shorter. There have been numerous studies investigating the correlation between telomere length and chronological age.  In parallel to telomere shortening, gradual random alterations associated with cell ageing will also occur.

This relationship between age, telomere length and cell senescence is likely another explanation for why senescent cells are often thought of as "aged" or "old" cells.  But even in this instance, cell senescence does not occur gradually over time like ageing cells, but suddenly in response to a very short telomere. 

There is little or no evidence suggesting that telomere shortening per se causes ageing.  Cells likely function perfectly well with progressively shorter telomeres.  Problems only arise when a telomere eventually becomes too short. As such, telomere shortening increases our risk of age-related conditions as cells are more likely to become senescent.  

Drugs which could extend the length of telomeres by activating an enzyme called telomerase (which adds lost telomeres back to DNA) could prevent cells from becoming senescent and help prevent ageing.  

In summary, cell ageing can be considered as a unprogrammed, random process leading to a gradual decline in cell function.  Cell ageing is detrimental to the function of normal biological processes.  Cell senescence can be induced randomly, but is a programmed change in cell state that can occur independent of age. Cell senescence can be both detrimental and beneficial depending on the biological context.  

Killing 'zombie' cells to improve health in old age

 Image 20170316 10913 xvyi5c

Dominick Burton, Research Fellow, Aston University

This article was originally published on The Conversation. Read the original article.

 Imagine a world where you could take just a single pill for the treatment or prevention of several age-related diseases. Although still in the realms of science fiction, accumulating scientific data now suggests that despite their biological differences a variety of these diseases share a common cause: senescent cells. This has led scientists to find drugs that can destroy these cells.

When cells become damaged, they either self-destruct (apoptosis) or they lose their ability to grow and remain stuck within the body. These are the non-growing senescent cells that no longer carry out their tasks properly. They spew out chemicals that cause damage to cells nearby, sometimes turning them into “zombies” – hence why they are sometimes referred to as “zombie cells”. Eventually, the damage builds up so much that the function of bodily organs and tissues, such as skin and muscle, becomes impaired. At this point, we identify the changes as disease.

Depending on where these senescent cells gather within the body will determine which disease will develop. Senescent cells have now been shown to be linked to several diseases, including cardiovascular disease, type 2 diabetes, osteoarthritis and cancer.

In 2011 and in 2016, researchers at the Mayo Clinic in the US showed, through the use of genetically engineered (transgenic) mice, that the removal of senescent cells reduced cancer formation, delayed ageing and protected the mice against age-related diseases. The mice also lived 25% longer, on average. A similar result in humans would mean an increase in life expectancy from 80 years to 100 years. It was proof-of-principle studies like these that laid the groundwork and inspired other researchers to build on these findings.

  I’ll live how much longer? Kirill Kurashov/Shutterstock.com

Killing a few to save the many

It is not known how many senescent cells need to be present to cause damage to the body, but the harmful effects of the chemicals they release can spread quickly. A few zombie cells may have a huge impact. Drugs for specifically killing senescent cells in order to extinguish their destructive force have recently been revealed and tested on mice. The collective term for these drugs is “senolytics”.

In 2016, two research groups independently published findings on the discovery of two new senolytic drugs which target proteins responsible for protecting senescent cells from cell death. Research lead by scientists from the University of Arkansas, US, showed that the drug ABT-263 (Navitoclax) could selectively kill senescent cells in mice, making aged tissues young again. And scientists from the Weizmann Institute of Science in Israel used the drug ABT-737 to kill senescent cells in the lungs and skin of mice.

There has also been a lot of interest in the role of senescent cells in pulmonary diseases caused by damage to the lungs. Among the risk factors, smoking is known to speed up lung ageing and disease, partly by attacking healthy cells with toxic chemicals from cigarette smoke which can result in cells becoming senescent.

In late 2016, Japanese scientists showed that the removal of senescent cells using genetically engineered mice greatly restored lung function in old mice. A more recent study, lead by scientists at the Mayo Clinic in the US, showed that idiopathic pulmonary fibrosis (scarring of the lungs) was linked to an increase in the number of senescent cells and the damaging effects of the chemicals they release. The killing of senescent cells using genetically engineered mice again greatly improved lung function. In the same study, this group also reported the possible use of a combination of drugs, dasatinib and quercetin, to destroy senescent cells.

A study published earlier this month from the University of Arkansas, extended their previous findings on the drug ABT-263 to pulmonary fibrosis. They found that ABT-263 treatment reduced the problems caused by senescent cells and reversed the disease in mice.

There’s money in senolytics

In light of these accumulating and highly promising findings, a number of start-up biotechnology companies have been created to exploit the health benefits of targeting senescent cells.  Probably the most well funded is Unity Biotechnology in the US which raised US$116m for research and development.

It will likely be several years before we see senolytic drugs being tested on humans. If you can’t wait that long, exercise may be the answer. A study published in March 2016 by the Mayo Clinic showed that exercise prevented the accumulation of senescent cells caused by a high-fat diet in mice. So if the regular health benefits of exercise were not enough to get you off the sofa, maybe the anti-ageing benefits will be.

Original Article: The Conversation

Targeting Cellular Senescence: One Drug for Many Diseases?

Cellular senescence is an altered cell state associated with permanent cell cycle arrest and an immunogenic, pro-inflammatory secretome that can contribute to the development and progression of age-related diseases.  Because cellular senescence can occur in different cell types (i.e. pancreatic beta cells, vascular smooth muscle cells, astrocytes) that undertake different biological functions, then their appearance can manifest differently and we refer to these manifestations as different diseases.  These include, diabetes, cardiovascular disease, COPD and cancer.  The mechanisms by which senescent cells can cause disease include:

        (1)  Loss of cellular regenerative capacity.  
        (2) Loss of normal cell function.  
        (3) Persistent pro-inflammatory tissue damage.
        (4) Altering the behaviour of neighbouring cells.
        (5) Protease-mediated degradation of extracellular structural proteins.

Although scientists researching cell senescence have long suspected that senescent cells play an important role in ageing and age-related disease, convincing evidence had not been provided until 2011 when Scientists from the Mayo Clinic in the US published their findings on the elimination of senescent cells in mice.  The elimination of senescent cells using transgenic (genetically engineered) mice delayed the onset of disease, thereby increasing healthspan.  However, likely owing to the use of an accelerated ageing mouse model, no life extension was observed in this instance.  However, a follow-up study by the same group using naturally aged mice lead to delayed tumorigenesis and reduced age-related decline leading to significant increase (up to 35%) in lifespan.  Studies like these thus provide a convincing rationale for developing therapeutic approaches for targeting senescent cells, so-called “senotherapeutics”.  These may include:

  1.        Specifically inducing cell death in senescent cells (i.e. small-molecule compounds).
  2.        Inhibiting the senescent secretome (i.e. inhibitors of inflammation).
  3.        Preventing senescence induction (i.e. telomerase activators, geroprotectors).
  4.        Boost immune response towards senescent cells (i.e. immunotherapy).

A few recent studies have published findings regarding the elimination of senescent cells by small-molecule compounds.  Wang et al (2016) identified the compound ABT-263 as a potent inducer of cell death in senescent cells leading to rejuvenation of aged tissue stem cells.  In another study, Yosef et al (2016) identified ABT-737 which through the elimination of senescent cells from the epidermis of the skin of mice lead to increased hair-follicle stem cell proliferation.  Both ABT-263 and ABT-737 inhibit proteins (BCL-2 family) known to play a role in cell survival.

Studies focused on the elimination of senescent cells are only beginning to emerge and will no doubt gain momentum as they show tremendous potential for improving health and wellbeing.  One intriguing notion that may arise from this research is concerned with the question of whether it may one day be possible to treat many diseases with a single drug.  If senescent cells play a role in the development of many different diseases, then a drug that can eliminate senescent cells in all cell types could act as both a preventative and a treatment for many diseases.

One of the obstacles preventing research into senotherapeutics from advancing and ultimately becoming translational to help increase healthspan of individuals within the general public, is funding.  However, this has not discouraged some researchers who were determined enough to acquire funding through the help of crowdfunding.  The Major Mouse Testing Programme (MMTP) raised over $50,000 towards research focused on eliminating senescent cells and is still ongoing (Click Here). 

A new start-up company, CellAge (click here) is also interested in targeting senescent cells and has recently announced a crowdfunding campaign (click here) to raise funds for their ongoing research.  CellAge aims are to “Increase human healthspan and reduce the incidence of age-related diseases by helping the human body eliminate senescent cells.  Our breakthrough technology concept harvests the promise of synthetic biology and recent findings in ageing research to deliver novel products and therapies to enable people to live healthier longer lives”

So if you are interested in stimulating research in this field for the benefit of all, then please make a donation (link here).

NKG2D ligands mediate immunosurveillance of senescent cells


Cellular senescence is a stress response mechanism that limits tumorigenesis and tissue damage. Induction of cellular senescence commonly coincides with an immunogenic phenotype that promotes self-elimination by components of the immune system, thereby facilitating tumor suppression and limiting excess fibrosis during wound repair. The mechanisms by which senescent cells regulate their immune surveillance are not completely understood. Here we show that ligands of an activating Natural Killer (NK) cell receptor (NKG2D), MICA and ULBP2 are consistently up-regulated following induction of replicative senescence, oncogene-induced senescence and DNA damage - induced senescence. MICA and ULBP2 proteins are necessary for efficient NK-mediated cytotoxicity towards senescent fibroblasts. The mechanisms regulating the initial expression of NKG2D ligands in senescent cells are dependent on a DNA damage response, whilst continuous expression of these ligands is regulated by the ERK signaling pathway. In liver fibrosis, the accumulation of senescent activated stellate cells is increased in mice lacking NKG2D receptor leading to increased fibrosis.  Overall, our results provide new insights into the mechanisms regulating the expression of immune ligands in senescent cells and reveal the importance of NKG2D receptor-ligand interaction in protecting against liver fibrosis.

Senescent cells communicate via intercellular protein transfer


Mammalian cells mostly rely on extracellular molecules to transfer signals to other cells. However, in stress conditions, more robust mechanisms might be necessary to facilitate cell–cell communications. Cellular senescence, a stress response associated with permanent exit from the cell cycle and the development of an immunogenic phenotype, limits both tumorigenesis and tissue damage. Paradoxically, the long-term presence of senescent cells can promote tissue damage and aging within their microenvironment. Soluble factors secreted from senescent cells mediate some of these cell-nonautonomous effects. However, it is unknown whether senescent cells impact neighboring cells by other mechanisms. Here we show that senescent cells directly transfer proteins to neighboring cells and that this process facilitates immune surveillance of senescent cells by natural killer (NK) cells. We found that transfer of proteins to NK and T cells is increased in the murine preneoplastic pancreas, a site where senescent cells are present in vivo. Proteomic analysis and functional studies of the transferred proteins revealed that the transfer is strictly dependent on cell–cell contact and CDC42-regulated actin polymerization and is mediated at least partially by cytoplasmic bridges. These findings reveal a novel mode of intercellular communication by which senescent cells regulate their immune surveillance and might impact tumorigenesis and tissue aging.

The senescent phenotype and promiscuous gene expression

Senescent cells are often associated with changes in gene expression that appear to occur independent of the regulated gene expression linked to aspects of the senescent phenotype such as cell cycle arrest, the secretory response and apoptosis resistance. This phenomenon has been termed promiscuous gene expression (pGE) (Burton and Krizhanovsky, 2014) and can be more specifically defined as gene expression that is uncoupled from tissue or developmental regulation. 

pGE can be observed in microarray analysis by comparing the gene expression profiles of different senescent cell types and lines. Zhang et al (2003) has demonstrated that the up-regulation of genes in senescent fibroblasts was associated with gene clustering (150 of the 376 gene up-regulated), whereas the down-regulation of genes (313) was not. 48.1% of the up-regulated genes were designated as membrane-associated proteins, 10.5% related to apoptosis and 15.8% to transport, whereas 17.9% of the down-regulated genes are involved in cell cycle regulation. Gene expression changes in senescent human mammary epithelial cells (HMECs) were shown to be drastically different than that of the fibroblasts, despite both undergoing senescence induced by telomere attrition. Only five genes up-regulated and seven genes down-regulated in HMECs showed similar regulation in fibroblasts. However, like senescent fibroblasts, HMECs also demonstrated gene clustering associated with up-regulated genes only. Zhang et al postulated at the time, that if senescence is a response to DNA damage, then the observed differences in gene expression between senescent fibroblasts and HMECs imply that the effects of DNA damage must vary according to cell type and line. This study also suggested that processes occurring during senescence may lead to localized alteration in chromatin and the consequent up-regulation of groups of genes within “opened” domains. 

Shelton et al (1999) also demonstrated that senescence-mediated gene expression between different cell lineages varies greatly. BJ fibroblasts, HUVECs and retinal pigment epithelial cells (RPE340) that underwent replicative senescence demonstrated substantial variation in gene expression. A genomic comparison of three different senescent fibroblasts strains also demonstrated significant differences in gene expression, but also shared trends were apparent. If indeed pGE is uncoupled from tissue or developmental regulation, then stochastic processes that alter chromatin structure could be at play and the different response between cell types and cell strains could reflect differences in cell-specific chromatin architecture important for cell-specific gene expression. Elevated levels of oxidative stress, a feature of senescent cells could be one such stochastic process. 

Bahar et al demonstrated that although gene expression levels varied among cardiomyocytes taken from hearts of young mice, the heterogeneity is elevated with age (Bahar et al. 2006). This increased stochastic gene expression with age was suggested to be the result of genomic damage, as mouse embryonic fibroblasts treated with hydrogen peroxide in culture resulted in significant cell-cell variation in gene expression in conjunction with these cells showing morphological signs of cellular senescence (Bahar et al. 2006). 

So how could DNA damage induced by oxidative stress result in stochastic changes in gene expression? When cells sustain DNA damage, chromatin undergoes remodeling to facilitate DNA repair (Price and D’Andrea, 2013, House et al. 2014). This remodeling or “opening” of tightly packed DNA could allow transcription factors access to previously inaccessible genes. Therefore, persistent DNA damage and consequently continuous chromatin remodeling may facilitate pGE. While the induction of DNA damage is likely a stochastic process, the sites of DNA damage may not be completely random, as certain areas of the genome may be more or less prone to genomic insults (Ma et al. 2012). The clustering phenomenon reported by Zhang et al may be the result of these DNA damage prone sites (Zhang et al. 2003). If this were indeed the case, while there may be substantial differences in gene expression at a cell-cell comparison, an overall comparison between cell cultures would likely demonstrate consistent gene alterations resulting from an average expression of all cells within a culture. 

In addition to oxidative stress, a number of other possible mechanisms may exist for generating pGE. Senescent fibroblasts are known to undergo methylation changes (Cruickshanks et al. 2013) and these alterations may lead to epigenetic alterations that promote stochastic changes in gene expression. Alternatively, it has been suggested that DNA damage may modulate gene expression by altering the binding capacity of transcription factors (Rose et al. 2012). 

Interestingly, the reprogramming of fibroblasts into induced pluripotent stem cells (iPSCs) via the addition of OCT4, SOX2, KLF4 and MYC (OSKM) requires a long stochastic phase of gene activation associated with changes in histone modifications at somatic genes and activation of DNA repair and RNA processing (Buganim et al. 2013). This stochastic gene expression may be the result of “promiscuous binding” by OCT4, SOX2 and KLF4, where they occupy accessible chromatin and bind to promoters of genes that are active or repressed (Buganim et al. 2013). It is possible that pGE in senescent cells partly mimics stochastic gene activation associated with cellular reprogramming. However, whether pGE in senescent cells is associated with factors that can undergo “promiscuous binding” has yet to be determined. 

Whether pGE plays a functional role in cell senescence has yet to be determined. However, it can be speculated that pGE may function to generate an array of tissue-restricted proteins that can subsequently be processed into peptides by autophagic proteases for presentation on MHC molecules (Dengjel et al. 2005). Similar to the presentation of tumour-associated antigens (Reuschenbach et al. 2009), senescent cells may also present antigens that can be recognized by immune cells, thereby becoming antigen-presenting cells (APCs). Although the up-regulation of MHC molecules on senescent cells have yet to be fully evaluated, the up-regulation of MHC class I but not MHC class II in response to DNA damage in fibroblasts has been reported (Tang et al. 2014). It remains to be determined whether pGE is a component of immunogenic conversion.

Atypical senescent states: Experimental induction of cyclin-dependent kinase inhibitors (e.g. p16, p21)

For many researchers, irreversible cell cycle arrest is the canonical trait of senescent cells.   Such growth arrest can be induced experimentally by the up-regulation or over-expression of cyclin dependent kinase inhibitors (CDKi).  Thus valuable models are, at least potentially, available in which to study the physiological effect of growth arrest distinct from the DDR or any other upstream response.   Unfortunately there has been little characterization of the phenotype of cells rendered ‘senescent’ by this means.

Blagosklonny and co-workers (Korotchkina et al. 2009) used an isopropyl-thio-galactosidase (IPTG)-inducible p21 expression construct to induce a senescence-like state in an HT1080-derived cell line (HT-p21-9).   Characterisation of the phenotype of these cells does not appear to have been attempted beyond observing irreversible growth arrest and the presence of increased SA-β-Gal activity.  Given that HT1080 is a highly tumorigenic fibrosarcoma carrying an activated N-ras oncogene (Benedict et al. 1984), it probably represents a poor genetic background in which to assess whether markers of immunogenic conversion or resistance to cell death can be induced by CDKi overexpression alone.  However, the basic principle of using such a construct for that purpose is sound.

Tokarsky-Amiel et al (2013) showed that overexpression of p14ARF in the epidermis of the skin of mice (using a tetracyclin-inducible construct) resulted in mass apoptosis and cell cycle arrest.  As measured by SA-β-Gal activity, the p14ARF transgene drove senescence in up to 8% of the surviving cells in the epithelium by a p53-dependent mechanism (demonstrated by ablation of p53 through co-expression of a specific shRNA directed against it).  These senescent cells were viable within the epidermis for several weeks consistent with lack of clearance.  Unfortunately, minimal analysis of their phenotype was conducted (beyond assessment of the message levels for the senescence-associated genes Pai-1 and Dcr2).  Thus, the immune state of the p14ARF-senescent cells is currently unclear and the picture is complicated by the fact that senescent rodent cells do not display a senescent secretome under some conditions.  However, given that alopecia and follical stem cell dysfunction were observed in the animals, it is clear that cells rendered ‘senescent’ in this manner can exert phenotypic effects.  Thus, there is some evidence that cell cycle arrest alone may be sufficient to cause problems in highly mitotic tissues such as the epidermis, but large amounts of work remain to be done.  

CDKi overexpression systems clearly have the potential to be valuable tools.  However the extent to which these are physiologically reflective can legitimately be challenged.  This can be understood in two ways (i) the mechanism by which the growth arrest is induced has not been reported in vivo and (ii) cells do not become senescent en mass but gradually as a result of tissue turnover throughout life.  Thus, findings made with these systems could be considered ‘artefactual’

By way of addressing these concerns, it is worth remembering that for many years replicative senescence was dismissed as a ‘tissue culture artefact’ because senescent cells had not been observed in vivo (evidence for their existence in tissue remained severely limited until the late 1990s).  By the same token, elevation of CDKi alone in cells in vivo is not impossible.  Absence of evidence is never evidence of absence.   Similarly, many over-expression systems model systems can be said to be non-physiological.  However, valuable data is routinely gathered using them and in this instance could allow researchers to gage the maximum physiological impact that irreversible growth arrest can have on tissue function.  Thus, if these limits are recognized, such models are potentially utile, especially when combined with detailed analysis of phenotypes known to exist in other ‘senescent cells’ (e.g. apoptosis resistance, immune ligand presentation and the secretory response) 

The main focus of ageing research is to prevent/combat age-related disease and disability, allowing everyone to live healthier lives for longer.