Phenotypic changes associated with cellular senescence

When a cell becomes senescent, changes at the genetic level occur which subsequently has an effect on both cell behaviour and morphology. Microarray analysis of senescent dermal fibroblasts, retinal pigment epithelial cells and vascular endothelial cells demonstrate overlap in gene expression changes but overall display cell-type specific changes (Shelton et al, 1999). Similar studies were carried out looking at human dermal fibroblasts and oral keratinocytes (Yoon et al, 2004, and Kang et al, 2003). These studies found transcriptional changes in genes associated with inflammation, regulation of cell cycle, cytoskeletal genes and extracellular matrix (ECM) genes. More recently, microarray analysis of primary human lung fibroblasts (IMR-90) and primary skin fibroblasts (Detroit 551) reported that out of the of the 4183 genes analysed, 165 were down-regulated and 191 up-regulated in senescent IMR-90 cells and 154 down-regulated and 76 up-regulated in senescent Detroit 551 cells compared with their growing counterparts (Chen et al, 2004). This degree of alteration to the transcriptome is akin to that seen when cells are induced to differentiate (Truckenmiller et al, 2001; Gerhold et al, 2002).

Impairment in cell mobility, secretion of matrix degrading proteins, secretion of growth factors and pro-inflammatory cytokines are considered as significant changes associated with cellular senescence. All these factors have the potential to cause detrimental damage to tissues.

A number of papers have reported that the ability of senescent cells to migrate is severely reduced (Schneider and Mitsui, 1976; Sandeman et al, 2000; Reed et al, 2001). This decline in the ability to migrate may be related to changes which occur to the cytoskeleton during cellular senescence (Nishio and Inoue, 2005). Actin is an important component of the cytoskeleton required for cellular migration. However, in senescent fibroblasts for example it has been shown that vimentin is produced in place of actin which is down-regulated. This migration deficit has important implications during wound healing since cells are stimulated to migrate into the wound, proliferate and construct the new extra-cellular matrix (ECM). Also, since senescent cells tend to secrete proteins which degrade the matrix, wound repair would be impaired.

Matrix metalloproteases (MMPs) are also commonly up-regulated in senescent cells (Sandeman et al, 2001, Campisi, 2005). In normal tissue processes, MMPs are required for fertilization, cellular adhesion, development, neurogenesis, and metastasis (Page-McCaw et al, 2007). However, MMP secretion by senescent cells has also been suggested to play a role in the progression of disease such as in the pathogenesis of coronary heart disease (CAD) (Nanni et al, 2007). MMPs have also been implicated in the progression of osteoporosis, since MMPs play important roles in bone resorption (Logar et al, 2007). One study has also shown that the secretion of MMPs by senescent chondrocytes may contribute to the development or progression of osteoarthritis (Price et al, 2002).

Abnormal secretion of some growth factors has been shown to be another general characteristic of senescent cells. Work on human fibroblasts found that vascular endothelial growth factor (VEGF) secretion is elevated in senescent cell cultures (Coppe et al, 2006). Since growth factors are capable of stimulating cellular proliferation it has been suggested that while initially cellular senescence may be a mechanism to suppress tumourigenesis early in life it may promote cancer in aged organisms (Campisi, 1997). Human senescent fibroblasts for example have been shown to stimulate premalignant and malignant, but not normal epithelial cells to proliferate in culture and form tumours in mice (Krtolica et al, 2001, Krtolica and Campisi 2002). Another study sought to characterise the molecular alterations that occur during prostate fibroblast senescence to identify factors which may be capable of promoting the proliferation and potentially the neoplastic progression of prostate epithelium (Bavik et al, 2006). Fibroblast growth factor 7 (FGF7), hepatocyte growth factor and amphiregulin (AREG) were found to be elevated in the extracellular environment of senescent prostate fibroblasts. Direct co-culture and conditioned medium from senescent prostate fibroblasts stimulated epithelial cell proliferation 3-fold and 2-fold respectively. These results suggest that senescent cells may contribute to the progression of prostate neoplasia by altering the prostate microenvironment.

Probably the most potentially detrimental changes which can occur when cells become senescent is that of secreted cytokines since they not only effect local tissue but can have much wider impacts throughout the organism. Enhanced inflammation during ageing is thought to contribute to many of the diseases of ageing.

Vascular smooth muscle cells (VSMC) that have become senescent due to the activation of ras have been shown to drastically increase the expression of pro-inflammatory cytokines (Minamino et al, 2003). IL1α was shown to be up-regulated 11-fold, IL1β 50-fold, IL-6 12-fold and IL-8 77-fold. With such dramatic changes, it was suggested that this proinflammatory phenotype may contribute to the progression of atherosclerosis.

Senescent T-cells in vivo have been shown to produce high levels of two cytokines, IL6 and TNFα (Effros, 2004). Interestingly, the up-regulation of TNFα by T-cells in the bone marrow has been implicated as a causal mechanism in bone loss (Roggia et al, 2001).

Replicative senescence of human hepatic stellate cells (a major cell type involved in liver fibrosis) in culture also display a higher expression of inflammation genes (Schnabl et al, 2003). Interleukin-8 is among the cytokines up-regulated in senescent stellate cells (SC) which correlates with increase expression observed with disease activity in human alcoholic liver fibrosis (Sheron et al, 1993). Interleukin-6 is a known fibrogenic cytokine which was also shown to be up-regulated in SC senescent cultures. In normal conditions, chronic tissue damage results in the activation of SC characterised by proliferation, motility, contractility and synthesis of extracellular matrix (Gutiérrez-Ru, 2002). Since SC are stimulated to proliferate in response to tissue damage, the replicative capacity of these cells will be reduced and the accumulation of senescent cells accelerated. This activation of SC in response to tissue damage is regulated by cytokines and growth factors. Therefore, unregulated secretion of pro-inflammatory cytokines and growth factors from senescent SC within the liver may cause further damage. One study has shown for example that replicative senescence does have a significant impact in the long-term progression of fibrosis (Trak-Smayra et al, 2004).

This pro-inflammatory phenotype may partly be due to the up-regulation of intercellular adhesion molecule-1 (ICAM-1), a molecule known to be involved in inflammatory response and is over-expressed in senescent cells and aged tissues (Gorgoulis et al, 2005). One study has shown that p53 can directly activate the expression of ICAM-1 (Kletsas et al, 2004) and since p53 is activated and up-regulated during cellular senescence, it may activate ICAM-1, thereby contributing to the pro-inflammatory phenotype of senescent cells.

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