In most age-related diseases, normal cellular/tissue functions fail and thus, most age- related pathologies are degenerative in nature. Cancer in contrast requires the cell to assume a completely new phenotype and can hardly be considered a degenerative process.
There is increasing evidence that cellular senescence contributes to ageing and age-related diseases other than cancer.
Among the more compelling evidence that senescent cells can drive degenerative ageing pathologies are the phenotypes of transgenic mice with hyperactive p53. Two landmark papers described mouse models with induced, chronically elevated p53 activity (99;100).
These mice were exceptionally cancer-free (as expected) since p53 is a critical tumour suppressor. What was surprising was their shortened life span and premature ageing.
Notably, cells from these mice underwent rapid senescence in culture (99). Moreover, tissues from these mice rapidly accumulated senescent cells, and, in lymphoid tissue, the p53 response shifted from primarily apoptotic to primarily senescent in vivo (101). Thus, there was a strong correlation between excessive cellular senescence and premature ageing phenotypes.
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In all these (and other) models of both accelerated and normal ageing, it is important to note that the crucial roles for the p53 and/or pRB/p16INK4a pathways are not singular.
There is mounting evidence that these pathways interact and modulate each other (102- 105).
Although these mouse models and other findings indicate a strong association between ageing phenotypes, certain pathologies and cellular senescence, other processes undoubtedly also contribute to ageing and age-related disease. One such process is cell death. In addition, some cells in ageing organisms simply lose functionality, which certainly also contributes to ageing phenotypes. Neurons, for example, lose the ability to form synapses, despite cell bodies remaining viable, which is an important component of many neurodegenerative pathologies (106). Likewise, cardiomyocytes lose synchronicity of gene expression, which almost certainly affects heart function (107).
So how is it that senescent cells promote age related pathologies? and in particular relevance to renal transplantation, how is it that senescence contributes to impaired renal function several months or years after implantation?
There are currently three theories that may explain this phenomenon:
Firstly, as suggested by Liu et al. (98), cellular senescence can deplete tissues of stem or progenitor cells. This depletion will compromise tissue repair, regeneration, and normal turnover, leading to functional decrements (108). Secondly, the factors that senescent cells secrete affect vital processes, such as cell growth and migration, tissue architecture, blood vessel formation and differentiation, so are tightly regulated. The inappropriate presence of these factors can disrupt tissue structure and function. Thirdly, the SASP includes several potent inflammatory cytokines (109). Low-level, chronic, “sterile”inflammation is a hallmark of ageing that initiates or promotes most, if not all, major age-related diseases (110;111). Chronic inflammation can destroy cells and tissues because some immune cells produce strong oxidants. Also, immune cells secrete factors that further alter and remodel the tissue environment, which can cause cell/tissue dysfunction and impair stem cell niches. As will be shown in the results section to this thesis, increased donor chronological age and extended criteria kidneys are associated with lower white cell counts at six, twelve and twenty four months post transplant. The reason behind this most probably attributed to increasing doses of immunosuppression administered to counteract clinical or subclinical
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rejection episodes, as a result of transplanted organs with increased background inflammatory oxidative damage.
Concrete evidence that senescence drives the ageing process remains contentious however.
Studies have shown that organisms in which cells fail to undergo senescence do not live longer; rather, they die prematurely of cancer (81). Several other studies are ongoing to elucidate cause and effect in this particular field (112).
Surprisingly, it has recently been shown that the senescence response may have a role in tissue repair. The SASP is associated with the secretion of growth factors and proteases that participate in wound healing, attractants for immune cells that kill pathogens, and proteins that mobilize stem or progenitor cells. Thus, the SASP may serve to communicate cellular damage/dysfunction to the surrounding tissue and stimulate repair, if needed (113;114). It could be that this new senescence associated function in tissue repair is suggesting that the growth arrest was selected during evolution to suppress tumourigenesis, and possibly excessive cell proliferation or matrix deposition during wound repair.
In summary therefore, cellular senescence seems to be a part of four complex processes (tumour suppression, tumour promotion, ageing, and tissue repair), some of which have apparently opposing effects. Upon experiencing a potentially oncogenic insult, cells assess the stress and must “decide” whether to attempt repair and recovery, or undergo senescence. After an interval or “decision period”, the length of which is imprecisely known, the senescence growth arrest becomes essentially permanent, effectively suppressing the ability of the stressed cell to form a malignant tumour.
One early manifestation of the senescent phenotype is the expression of cell surface–bound IL-1α (115). This cytokine acts in a juxtacrine manner to bind the cell surface–bound IL-1 receptor, which initiates a signalling cascade that activates transcription factors (NF-κB, C/EBPβ). The transcription factors subsequently stimulate the expression of many secreted (SASP) proteins, (68;71;109) including increasing the expression of IL-1α and inducing expression of the inflammatory cytokines IL-6 and IL-8. These positive cytokine feedback loops intensify the SASP until it reaches levels found in senescent cells. SASP components such as IL-6, IL-8, and Matrix Metalloproteinases (MMPs) can promote tissue repair, but also cancer progression. Some SASP proteins, in conjunction with cell surface ligands and adhesion molecules expressed by senescent cells, eventually attract immune cells that kill and clear senescent cells. A late manifestation of the senescent phenotype is the expression
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of microRNAs (mir-146a and mir-146b), which tune down the expression IL-6, IL-8, and possibly other SASP proteins. MicroRNAs (miRNA) are non-coding, single-stranded RNA molecules that are involved in the regulation of a variety of biological processes, including embryogenesis, differentiation, and senescence. The CDKN2 locus is complex, comprising a series of developmentally and epigenetically regulated transcript isoforms. We have demonstrated that transcriptional regulation of CDKN2 isoforms by certain miRNAs are able to predict the functional status of renal allografts up to six months post transplant. We have also demonstrated an association with rejection episodes and have proved an association between certain miRNA levels and increasing cold ischaemic time (CIT) (McGuinness et al, Sci Trans Med. In submission). The data from this research indicates that miRNA profiling has clear potential to be used for pre transplant assessment of post transplant allograft function.
The reason for which certain miRNAs tune down the expression of senescence associated interleukins is not really understood but is primarily believed to prevent the SASP from generating a persistent acute inflammatory response (116). Despite this dampening effect, the SASP can nonetheless continue to generate low level chronic inflammation.
The accumulation of senescent cells that either escape or outpace immune clearance and express a SASP at chronic low levels is hypothesized to drive ageing phenotypes. Thus, senescent cells, over time, develop a phenotype that becomes increasingly complex, with both beneficial (tumour suppression and tissue repair) and deleterious (tumour promotion and ageing) effects on the health of the organism.