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

Immune response to cancer cells (and maybe senescent cells)

Since any immune response to the presence of senescent cells may possibly be similar to that of cancer cells, the following is a brief outline describing the key points in the removal process (Ullrich et al, 2007, Vulink et al, 2008, Wesa and Storkus, 2008, Chan and Housseau, 2008).

Dendritic cells are antigen-presenting cells found in all tissues of the body and are crucial for stimulating a naïve T-lymphocyte response in the removal of tumour cells.

In the presence of tumour cells, dendritic cells capture (by engulfing portions of the tumour cell) and process tumour-specific molecules (antigens) so that they become presented on their cell surface. Dendritic cells start to mature as they migrate to the lymph nodes, a process which enables dendritic cells to present the tumour information. The maturation process is needed as additional co-stimulatory molecules are required so that they can be recognised by other immune cells. When mature dendritic cells reach the lymph nodes, they interact with cytotoxic T-lymphocytes (CTLs), and pass on the tumour information, causing CTLs to become activated and consequently proliferate. The large numbers of CTLs then circulate the body, recognising the tumour-specific antigens, binding to them and destroying tumour cells by the release of enzymes.

If a cancer cell (or a senescent cell) is not removed by the immune system, then something isn’t working as it should. From the brief overview above, there are a number of points between cell recognition and removal that could have failed. These are:

(1) Dendritic cells did not recognise the cancer/senescent cell.
(2) The dendritic cells did not display cancer/senescent specific markers on it’s surface.
(3) Lymphocytes were not activated in response to dendritic cells.

The changes which occur as we age which may have an impact on the removal of tumour cells/senescent cells will be discussed next.

.

.

Review Paper: Physiological and pathological consequences of cellular senescence

Why do senescent cells accumulate in tissues?

If the accumulation of senescent cells are so detrimental to the tissues in which they reside, why haven’t we evolved mechanisms to remove them? The answer is that we probably have, but the mechanism which removes them from the tissues becomes impaired as with age.

To understand how this removal system may work, we need to look at the phenotype of senescent cells. Although a large number of the changes which occur during cellular senescence may be cell specific, there appears to be features which are common to the majority of senescent cell types. These include the secretion of growth factors, matrix degrading proteins (MMPs) and the production of cytokines. Since these factors are a common feature, it is likely that they have a common function and are not just a random consequence of the changes which occur during senescence.

One possibility is that senescent cells are removed by the immune system. Senescent cells secrete cytokines to attract immune cells to their location (for their removal), secrete matrix degrading proteins to allow the immune cells easy access and secrete growth factors to stimulate the proliferation of surrounding cells for its replacement once the cell is removed. However, since the immune system itself is governed by ageing mechanisms, its ability to remove senescent cells gradually decreases, therefore the accumulation of senescent cells gradually increases.

The majority of the work on the immune clearance of unwanted cells has been carried out in cancer research. The prevalence of cancer as we all know increases with age, and this may be due to an ageing immune system, consequently resulting in an impaired ability to remove cancer cells as they appear. Over the past several years it has become clear that the immune system plays a crucial role in preventing cancer. As a consequence, there has been a great deal of interest in using our bodies own immune system to recognise and destroy cancer cells (FDA, cancer research uk), a process which could potentially be used to target and destroy senescent cells in ageing tissues.

Publication

DGA Burton (2008) Cellular senescence, ageing and disease. AGE