Atherosclerosis was once considered to be predominantly a lipid storage disease but mounting evidence suggests that inflammation is critical at every stage, from initiation to progression and eventually plaque rupture (Paoletti et al 2004).
Inflamed endothelial cells in the lining of arteries release pro-inflammatory cytokines which provide a chemotactic stimulus to adhere leukocytes and monocytes, directing their migration into the intima (Boisvert, 2004). These inflammatory cells release pro-inflammatory mediators responsible for differentiating monocytes to lipid-laden macrophages, foam cells (Frostegard et al 1999). These foam cells also secrete proinflammatory cytokines that amplify the local inflammatory response in the lesion (Libby, 2002). The secretion of cytokines and growth factors stimulate the migration and proliferation of SMC. These cytokines also stimulate the secretion of matrix degrading proteins from SMC which permits the penetration of SMC through the elastic laminae and extracellular matrix (ECM) of the growing plaque. Inflammatory mediators can inhibit ECM protein synthesis and increase expression of matrix degrading proteins by foam cells within the intimal lesion (Libby, 2002). Since the strength of the plaques fibrous cap is due to the extracellular matrix, its degradation would result in loss of strength and increased chance of rupture.
Cellular senescence and atherosclerosis
It has been suggested that injury to endothelial cells results in endothelial dysfunction which may lead to the development of atherosclerotic plaques (Kitamoto and Egashira, 2004). How this initial damage to endothelial cells occurs is currently speculative, but there is increasing evidence to postulate that this initial endothelial dysfunction may be the result of cellular senescence.
Early histological studies of advanced human atherosclerotic lesions suggested the presence of senescent endothelial cells (Burrig et al, 1991). Endothelial cells exhibiting the morphological features of senescence were frequently found on the plaque surface. The presence of senescent cells within plaques was also found in studies of vascular cells in culture, derived from human atherosclerotic plaques (Bennett et al 1998). VSMC derived from atherosclerotic plaques were shown to have lower rates of proliferation and underwent senescence earlier than cells derived from normal vessels. With the emergence of a biomarker (SA-β-Gal) which could detect senescent cells in vivo, a more direct approach for investigating cellular senescence in diseased tissue was undertaken (Fenton et al, 2001). This study sought to detect the presence of senescent cells in injured rabbit carotid arteries. Results indicated the accumulation of senescent cells in the neointima and media of all injured vessels, in contrast to the near absence of such cells in control vessels. Similar investigations have also been carried out on human atherosclerotic plaques (Vasile et al 2001, Minamino et al 2002). Both these studies demonstrated the presence of senescent vascular endothelial cells in vivo at sites of atherosclerotic plaque formation as detected by SA- β-Gal.
More recently due to advances in molecular biology, there have been numerous investigations involving the biology of telomeres in atherosclerosis. One such study examined telomere length in cells from atherosclerotic plaques and normal vessels and demonstrated that VSMC from plaques had markedly shorter telomeres compared with normal VSMC (Matthews et al 2006). This shortening was found to be closely associated with increasing severity of atherosclerosis. As with previously mentioned studies, these VSMC demonstrate morphological features of senescence when cultured in vitro. A similar study investigated telomere lengths of endothelial cells (EC) from coronary artery disease (CAD) and also found that telomeres were significantly shorter in CAD compared with normal arteries (Ogami et al, 2004). Since both VSMC and EC of atherosclerotic plaques have been shown to have senescent cells present and cellular senescence is generally attributed to the attrition of telomeres, the presence of cells with shorter telomeres in these tissues is therefore not surprising.
The above studies provide evidence for the presence of senescent cells in atherosclerotic plaques, but provide little explanation for their occurrence. This is less true for senescent SMC, since their presence can be explained by stimulated proliferation and migration observed in atherosclerosis. SMC have a finite replicative capacity, most likely as a result of telomere shortening, therefore, constant rounds of cell division would eventually result in the cell becoming senescent. The presence of senescent SMC would therefore only be observed in late stage plaque development, since this is when SMC are stimulated to migrate and proliferate.
Since the initiation of plaque development begins at EC, an explanation for why there may be senescent cells present is harder to explain. One possibility is that senescent EC cells in atherosclerotic plaques is most likely due to proliferative exhaustion as a result of replacing lost and damaged cells. As previously discussed, plaque formation is commonly seen within arteries at areas of high shear stress. It is therefore possible that such high shear stress could lead to the loss of ECs in these areas, which subsequently need to be replaced. This would result in an increase in cell turnover at those sites and consequently the occurrence of senescent EC. Since senescent cells in general can be classed as dysfunctional, it may be the presence of senescent EC within the endothelium which is the initiating factor in plaque formation. Further evidence for this may be provided if the expression profile of senescent vascular ECs were compared with vascular ECs of lesion-prone sites within arteries.
Since the recruitment and accumulation of leukocytes and monocytes is an important step in the development of atherosclerosis, the expression of proteins such as intracellular adhesion molecule-1 (ICAM1) and vascular cell adhesion molecular-1 (VCAM-1), important mediators of leukocyte and monocyte adherence have been investigated. One study looked at the expression of VCAM-1 and ICAM-1 at lesion-prone sites on the endothelium in ApoE-deficient mice (which are more prone to lesion development) (Nakashima et al, 1998). Staining for VCAM-1 showed localised staining at lesion-prone sites in ApoE-/- mice and only weak staining limited to sites of altered blood flow in control mice. ICAM-1 was the most prominent adhesion molecule in lesion prone sites and was up-regulated in ApoE -/- mice and control mice. If ICAM-1 is being up-regulated as a result of senescent cell formation, it is not surprising that it is found up-regulated in both ApoE -/- and control mice, since the same high shear stress is most likely occurring in both mice. Another study specifically investigated whether endothelial dysfunction was the result of endothelial cell senescence by inducing senescence in human aortic endothelial cells (HAECs) and examining the expression of ICAM-1 and endothelial nitric oxide synthase activity (eNOS) (Minamino et al, 2002). Results showed that ICAM-1 expression was increased and eNOS activity decreased in senescent HAECs. There are numerous studies that have shown eNOS activity to be decreased during endothelial dysfunction and this decrease is thought to play a critical role in the development and progression of atherosclerosis (Yang et al, 2006). Up-regulation of ICAM-1 and a decrease in eNOS activity in both senescent endothelial cells and at lesion-prone sites, strongly suggests that senescent cells are a significant contributing factor in initiation of atherosclerotic plaque formation.