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.

Burton (2009) cellular senescence, ageing and disease

Replicative lifespan of fibroblasts in ageing studies

A recent paper by Maier and Westendorp (2009) focused on the replicative capacity of fibroblasts from patients with accelerated ageing syndromes, patients with age-related diseases and donors of varying chronological age. Their findings were as follows:

(1) Fibroblasts from patients with accelerated ageing syndromes are lower when compared with strains from age-matched controls.

(2) No difference in replicative capacity was found in fibroblasts from patients with age-related diseases when compared to age-matched controls.

(3) No relationship between replicative capacity of fibroblasts and donor age.

It is probably not surprising that there is a lower replicative capacity in skin fibroblasts taken from patients with Werner- and Hutchinson-Gilford syndrome patients as the mechanisms underlying these syndromes are probably universally found throughout all the somatic cells in these patients. For example, Werner syndrome is caused by a mutation in the WRN gene and is associated with short telomeres and accelerated cellular senescence (Cox and Faragher, 2007). This mutation is going to be present in all cell types, therefore it does not matter which cell type is investigated, the result of a reduced replicative capacity is likely to be the same. However, the same result is unlikely to be true when investigating the replicative capacity of skin fibroblasts in subjects suffering from diseases associated with a completly different cell type.

Maier and Westendorp investigated the replicative capacity of skin fibroblasts in patients with age-related disease. However, some of the diseases classed as age related in this instance are not. These include cystic fibrosis and familial Alzheimer’s disease. This is not the main point in question. It is not surprising that there is no relationship between the replicative capacity of skin fibroblasts in patients suffering from say cardiovascular disease or diabetes because this cell type has no involvement in the development or progression of those particular diseases. If they looked at cell types related to a particular disease such as vascular endothelial cells in cardiovascular disease (Minamino et al, 2002), microglial cells in Alzheimer’s (Streit et al 2007) or pancreatic beta cells in diabetes (Sone and Kagawa, 2005) they would most likely see a decline in replicative capacity compared to age-matched controls. This was the case for lung fibroblasts in lung emphysema, demonstrated in this investigation.

Different cell types have different replicative capacities, have different functions, are maintained within different environments and thus undergo varying degrees of stresses. In addition to this, there are risk factors such as sun exposure, smoking and diet which have the potential to accelerate cellular ageing. As such, different tissues age at different rates. Therefore, the presence of disease in one tissue is not necessarily going to reflect the biological condition of another. The replicative capacity of skin fibroblasts is not necessarily going to be influenced by the presence of disease in other tissues.

A theoretical scenario where a particular disease may impact on the replicative capacity of skin fibroblasts, is if the presence of disease uses up the stem cell/progenitor cell reserve needed for cellular repair and replacement, or somehow impacts on the functioning of stem cell/progenitor cells. In this instance, damaged or lost skin cells can no longer be replaced by the stem cell/progenitor cell reserve, causing local cells to divide and replace instead. This in turn reduces the replicative capacity of those cells. This may occur in advanced stages of a disease where constant cell replacement has been undertaken. This may explain results of studies investigated in this paper which demonstrated that the replicative capacity of fibroblasts in patients with severe diabetes was diminished when compared with controls, but was insignificantly decreased in patients with mild to moderate diabetes. Also, Kuki et al (2006) has demonstrated that endothelial progenitor cells (EPCs) cultured under high glucose levels (associated with diabetes) undergo accelerated senescence. The presence of elevated oxidised low density lipoproteins (ox-LDL) observed in diabetics has also been shown to reduce the number and impair function of circulating EPCs. In addition to this, it is known that stem cells lose the capacity for self renewal when removed from the stem cell niche, suggesting that the local environment plays a crucial role in determining stem cell behaviour (Boyle et al, 2007). Therefore, the presence of diseases in advanced stages, especially those associated with inflammation, may alter the environment of stem cell niches and thus impacting on their ability to function. In this scenario, the presence of disease has the potential to impact other tissues by impairing the function of stem/progenitor cells needed for repair and maintenance.

It has often been shown that a decline in the replicative capacity of fibroblasts is correlated with an increase in chronological age of a donor. However, if the health state of donors is taken into consideration and only “healthy” subjects are investigated in this regard, there appears to be no correlation (Cristofalo et al, 1998). This suggests that the replicative capacity of a tissue only reflects biological age and not chronological age. Of course it is true, that a longer a person lives, the increased likelihood that cells become damaged, lost and replaced and this in turn would reduce the replicative capacity of those cells. However, if factors which result in cellular damage/loss such as the presence of disease (not necessarily age-related), infection or environmental factors such as smoking and sun exposure are reduced, then damage/loss of cells is reduced and the replicative capacity of those cells remains high.

Maier and Westendorp suggest an alternative explanation for the lack of relationship between donor age and replicative lifespan of skin fibroblasts: “The overall replicative capacity might decline with age but rare fibroblasts clones with extended replicative potential continue to be present at old age but do not nessesarily reflect the properties of the overall population. Therefore, the replicative capacity in vitro reflects only the expansive propagation of the longest surviving clone, which seems to have comparable in vitro characteristics when obtained from young and old individuals.”

Data on the replicative capacity of cells in regard to ageing and age-related disease is only important because the shorter the replicative capacity of a tissue, the increased likelihood that senescent cells will appear or are present. The presence of senescent cells in tissues is thought to play a role in ageing and age-related disease. Thus, it is more important to investigate the distribution and frequency of senescent cells in tissues associated with accelerated ageing syndromes, age-related diseases and chronological age.