The presence of DNA damage and subsequent up-regulation of p16(INK4a) in quiescent cells in vivo may induce a pre-senescent state that converts to a full senescent state when cells are stimulated to proliferate. This suggests that DNA replication is required to induce a persistent DDR associated with cell senescence. For example, damage to skeletal muscle in normal young mice causes the activation of quiescent satellite cells (adult stem cells), which proliferate and undergo myogenic differentiation required for muscle repair. However, a recent study has shown that in geriatric mice (28-32 months of age), satellite cell activation is impaired and satellite cells instead convert from a pre-senescent state (quiescent cells with high p16(INK4a) expression) to a full senescent state (including a DDR) when stimulated to proliferate in response to injury. As such, the induction of senescent satellite cells with age can impair satellite muscle regeneration. This study suggests that senescent cells may accumulate in late life due to a conversion from quiescence to senescence (termed geroconversion) in response to a requirement for cells to replicate over time to regenerate tissue. In this model, more and more quiescent cells are likely to accumulate DNA damage over the life-time of an organism and are therefore more likely to become senescent when induced to proliferate later in life. Therefore, if quiescent cells inflicted with DNA damage convert to senescence when stimulated to proliferate, then eliminating such damage may prevent this conversion.
A blog for those interested in the physiological and pathological impact of senescent cells.
Showing posts with label anti-ageing. Show all posts
Showing posts with label anti-ageing. Show all posts
Quiescent to Senescent Conversion
Physiological and pathological consequences of cellular senescence
Abstract
Cellular senescence, a permanent state of cell cycle arrest accompanied by a complex phenotype, is an essential mechanism that limits tumorigenesis and tissue damage. In physiological conditions, senescent cells can be removed by the immune system, facilitating tumor suppression and wound healing. However, as we age, senescent cells accumulate in tissues, either because an aging immune system fails to remove them, the rate of senescent cell formation is elevated, or both. If senescent cells persist in tissues, they have the potential to paradoxically promote pathological conditions. Cellular senescence is associated with an enhanced pro-survival phenotype, which most likely promotes persistence of senescent cells in vivo. This phenotype may have evolved to favor facilitation of a short-term wound healing, followed by the elimination of senescent cells by the immune system. In this review, we provide a perspective on the triggers, mechanisms and physiological as well as pathological consequences of senescent cells.
LINK: Burton and Krizhanovsky (2014) Physiological and pathological consequences of cellular senescence
The removal of senescent cells using therapeutic agents
As discussed in the previous blog, one of the strategies for overcoming the detrimental effects of senescent cells is to remove them as they appear through the use of therapeutic agents. At present, no drug-based system exists which can specifically identify senescent cells and remove them. However, there is currently great interest in the development of drugs which specifically target and remove cancer cells. The problem with current cancer treatments (such as drugs used in chemotherapy) is that they are non-specific and as such can cause damage and undesirable changes to non-cancerous cells, causing side-effects. The development of cell-specific drug targeting is greatly needed and such research could be adapted to target senescent cells. Cell-specific drug targeting requires a carrier molecule containing a targeting agent which specifically recognises and binds to a specific receptor or binding site on the surface membrane of target cells and a therapeutic agent which could trigger programmed cell death, apoptosis. The following are crucial factors in determining the success of drug-targeting systems (Beljaars et al, 2001, Petrak 2005).
(1) Cellular specificity: For a drug to exert its desired effect it needs to be in physical contact with its physiological target, such as a receptor.
(2) Rate of elimination of the drug-carrier conjugate: It is essential that the drug-carrier conjugate is not removed too rapidly from the circulation. If it is eliminated from systemic circulation more rapidly than it is delivered to the target site, the amount of conjugate at the target site might never be enough to provide the required concentration of free (unbound) drug.
(3) Rate of release of free drug at the non-target site: Depending on the amount of drug, the release of drug away from the target site could nullify any benefits that might potentially come from delivering the drug to the target site.
(4) Rate of delivery of drug-carrier conjugate to the target site: If the drug conjugate reaches the target site too slowly, the supply of free drug might never be sufficient to generate the concentration required to elicit the desired therapeutic effect at the site of action.
(5) Rate of release of free drug at target site: The capacity of the system selected for the release of free drug from the conjugate should be considered. It needs to be suitable for processing the entirety of the drug-carrier conjugate arriving at the target site, doing so at a rate that also ensures drug accumulation at this site.
(6) Rate of removal of free drug from the target site: Drugs that benefit most from target-selective delivery are those that are retained at the site while acting on their target of action.
(7) Rate of elimination of the drug-carrier conjugate and free drug from the body: For optimal targeting, elimination of the complete drug-carrier system should be minimal.
One promising area of research in the development of drug delivery systems incorporates the use of nanotechnology (http://nano.cancer.gov/). Such technology has been used to create dendrimers, spheroid or globular nanostructures which are highly branched (Alexis et al, 2008). The branched regions of these dendrimers can be used to attach molecules such as targeting and therapeutic agents (Gillies and Frechet 2005). To test this nano-delivery system, invesitgators at the University of Michigan attached a targeting agent, a therapeutic agent and an imaging agent to the surface of dendrimers (Majoros et al, 2006, Shi et al 2007). The investigators chose folic acid as the tumour-targeting agent (a molecule which binds to a high-affinity receptor found on many types of tumour cells), paclitaxel as the therapeutic agent (a drug which triggers programmed cell death, apoptosis) and the fluorescent dye known as fluorescein isothiocyanate as the imaging agent. This nano-dilivery system was then tested on two sets of cancer cells in vitro: one that expresses the folic acid receptor and one that does not. Only the cells containing the folic acid receptor took up the dendrimer, visualised by the presence of the imaging agent. The dendrimer construct was highly toxic to these cells but had no effect on cells without the folic acid receptor. When both of these cells were exposed to dendrimers containing the targeting and imaging agent but no paclitaxel, no detrimental effects were observed.
These promising initial results thus call for tests to be carried out on animals with tumours that overexpress folic acid receptors. It is research like this that could one day be adapted to specifically target senescent cells. For this to be the case, a target agent is required that specifically recognises senescent cells. For this to be achieved, a deeper understanding of the changes which occur when a cell becomes senescent is required. Ideally a universally expressed senescent membrane receptor would be ideal, but at present no such receptor is known. If it did, it would also make a useful biomarker for detecting senescent cells in tissues.
Cellular Senescence in Anti-Ageing Research
Introduction
The accumulation of senescent cells (cells which have undergone permanent growth arrest) in tissues is thought to contribute to the development/progression of age-related disease and disability. Why? Partly because when cells become senescent, their gene expression becomes radically altered and as a result secrete proteins that damages the body. Growth-competent cells can become senescent as a result of telomere shortening. Telomeres are a region of repetitive DNA at the end of chromosomes, important in chromosome stability. Every time a cell divides, telomeres gradually become shorter and shorter until they trigger a response which causes them to enter senescence. This is known as replicative senescence. However, an enzyme known as telomerase can lengthen telomeres and thus prevent a cell from becoming senescent.
Telomerase is an enzyme which consists of an RNA molecule and a catalytic component known as hTERT. It is a reverse transcriptase which uses its RNA component as a template to reverse transcribes DNA back to the ends of chromosomes. Telomerase activity is repressed in most somatic cells and reactivated in ~90% of human cancers (Artandi, 2006). Introduction of telomerase into normal somatic cells has been shown to extend replicative life-span (Bodnar et al, 1998) and not induce changes associated with a malignant phenotype (Jiang et al, 1999).
Since senescent cells are potentially detrimental to the tissues in which they reside, anti-ageing research has three main aims for dealing with this problem:
(1) Prevention: prevent cells from becoming senescent.
(2) Removal: remove senescent cells as they appear.
(3) Replacement: replacement of cells which have naturally or artificially been removed.
PREVENTION: Telomerase Therapy
Telomerase therapy is aimed at preventing the appearance of senescent cells in tissues by lengthening telomeres in somatic cells. At present, this is not possible. It is possible to get cells to express telomerase in culture by insertion of the hTERT gene (Bodnar et al, 1998), but there is currently no technology which can insert the hTERT gene into every cell in the body. Since every cell in the body already has the gene for hTERT (it is just not activated) a better alternative approach is the development of drugs which “turn on” the hTERT gene. This is the main focus for companies like Sierra Sciences.
Problems associated with Telomerase Therapy
Apart from the problem of turning on telomerase expression in all the cells of the body, there are a number of other issues that need to be questioned.
(1) Not all cells enter senescence as a result of telomeres shortening: Some cell types, such as keratinocytes (Darbro and Klingelhutz, 2004), and possibly astrocytes and corneal endothelial cells (unpublished) enter senescence by a mechanism independent of telomere shortening. As such, cellular senescence cannot be prevented by the addition of telomerase.
(2) Cellular senescence can be triggered as a response to DNA damage: Even if telomeres are elongated, cells can still become senescent as a result of DNA damage. It is not known what fraction of senescent cells in tissues is due to replicative senescence or the result of DNA damage.
(3) Cancer risk: The risk of cancer is likely to be great if telomerase is constantly being expressed in cells, but if telomerase expression is transiently expressed by drugs then this risk would be minimised.
REMOVAL: Therapeutic agents and/or the use of the Immune System.
All three of the above problems associated with telomerase therapy could be eliminated if senescent cells were removed as they appeared in tissues. Prevention therapies should therefore be applied along side removal strategies. Two possible approaches for removing senescent cells are:
(1) The use of therapeutic agents (drugs) to specifically target and destroy senescent cells.
(2) The use of our own immune system to remove senescent cells.
Use of Therapeutic Agents
Therapeutic agents have the potential to specifically target senescent cells and induce programmed cell death (apoptosis). At present, no such drug is available. However, drugs that are being developed to specifically target cancer cells could one day be adapted to target senescent cells. For this to be made possible, a cell surface marker specific to all senescent cells needs to be identified. A drug can then be developed which specifically identifies that marker, binds to it and induces apoptosis. A more detailed review of cell specific drug targeting will be presented at a later date.
Use of the bodies own immune system
Cancer cells (and possibly senescent cells) may persist in tissues in later life because the immune system fails to remove them (see here). Why? Because the immune system is also governed by ageing mechanisms, and as we age the immune systems ability to remove cancer and senescent cells is gradually impaired. An understanding of the mechanisms which lead to functional decline in the immune system is thus needed for the development of anti-ageing therapies. This is discussed in more detail at a later date.
The use of these two removal strategies without the use of telomerase therapy could be more harmful than good. The removal of one cell only promotes the division of another, thereby reducing the replicative capacity of cells and increasing the appearance of senescent cells. However, if cell removal strategies are used in conjunction with telomerase therapy (at least in some cell types), the negative impact normally observed with cell replacement may not be seen.
REPLACEMENT
If a senescent cell is removed from tissue without the use of telomerase therapy, surrounding cells will divide to replace it, thus decreasing the replicative capacity of those cells and increasing the appearance of senescent cells. Replacement strategies focus on the use of stem cells to replace lost and damaged cells. Stem cells naturally replace lost cells in tissues but it is not known to what extent both stem cells and the surrounding somatic cells play in this process. Also, the functional ability of stem cells has been shown to decline with age in tissues (Sharpless and DePinho, 2007), so the addition of functional stem cells into tissues would be beneficial. Interestingly, it may the the presence of senescent cells that is having a detrimental impact on the functional ability of stem cells. The microenvironment of stem cell niches is important for the normal functioning of these cells (Boyle et al, 2007). Therfore, the presence of senescent cells with their altered secretome may alter the environment of the stem cell niche, thus altering their ability to function properly. The removal of senescent cells alone may therefore partly prevent the age-related decline in stem cell function, providing a stronger repair process.
Conclusion
Like all anti-ageing research, telomerase therapy, senescent cell removal and cell replacement are at their infancy. Only with time, money, a deeper understanding of the ageing process and a motivation to succeed, will we begin to see the inevitable benefits of anti-ageing research.
Anti-Ageing Research Blog
Soon to be up and running is my new blog, anti-ageing research, which focuses NOT on the biology of ageing, but the ideas and research aimed at preventing or treating the effects of the ageing process. Check it out.
Cheers
Dom
http://anti-ageing-research.blogspot.com/
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