F U L L T E X T ( . P D F ) : Willey Online Library
Abstract
Cellular senescence and mitochondrial dysfunction have both been defined as classical hallmarks of the ageing process. Here, we review the intricate relationship between the two. In the context of ageing, it is now well regarded that cellular senescence is a key driver in both ageing and the onset of a number of age-related pathologies. Emerging evidence has pinpointed mitochondria as one of the key modulators in the development of the senescence phenotype, particularly the pro-inflammatory senescence associated secretory phenotype (SASP). This review focuses on the contribution of homeostatic mechanisms, as well as of reactive oxygen species (ROS) and mitochondrial metabolites in the senescence programme. Furthermore, we discuss emerging pathways and mitochondrial-mediated mechanisms that may be influencing the SASP and, subsequently, explore how these may be exploited to open up new therapeutic avenues.
1. Introduction
Cellular senescence was first described in cultured normal human fibroblasts when it was observed that they ceased to proliferate following a finite number of cell divisions[1]. It has since been established that this initial observation reflects a specific type of senescence that occurs in response to telomere attrition [2]. It is now well established that senescence is a biological programme that can occur in response to a number of stresses and is a feature in multiple physiological and pathological processes. It has been demonstrated that senescence can occur as a consequence of oxidative stress, oncogene activation, or chromatin modifications, amongst other forms of stress [3]. Most notably, it has been suggested that senescence has evolved as a cellular process to avert cell replication on a damaged DNA template, therefore acting as an effective mechanism to prevent tumour progression[4]. Furthermore, recent evidence has emerged that senescence also plays a key role inthe healing of wounds, tissue repair, and during embryonic development [5, 6]. Transient senescence, whereby damaged cells are eliminated, clearly has beneficial effects for an organism. However, persistence ofsenescent cells leads paradoxically to deleterious effects. There is now substantial evidence that an accumulation of senescent cells is a driver of the ageing process[7].
Senescent cells can be identified by several notable markers and morphological changes that distinguish them from proliferating cells and other non-dividing cells. However no single marker that unequivocally identifies senescent cellshas been identified so far. The most prominent feature of senescence is the cell cycle arrest; therefore senescent cells exhibit an upregulation of genes that enforce the cell cycle arrest, such as p16, ARF, p21and p53[8].
The genes which are upregulated during senescence generally reflect the senescence-inducing stimuli, for example, the upregulation of p16 and ARF occurs predominantly in response to mitogenic stress [9]. By contrast, p21 and p53 are usually upregulated following the activation of a DNA damage response (DDR). The DDR is a key feature of senescent cells and is observed following a number of stimuli such as genotoxic stress, replicative exhaustion and telomere attrition. Senescent cells have been shown to display a variety of markers of a DDRsuch as the presence of γH2AX (phosphorylated Histone 2AX) and 53BP1 foci (p53-binding protein-1)[10]. In addition, senescent cells display an absence of proliferative markers such as Ki67 and incorporation of BrdU (5-bromodeoxyuridine) [10]. Furthermore, the presence of increased lysosomal content in senescent cells allows the detection of β-galactosidase activity at pH 6.0, referred to as senescence-associated β-galactosidase [11]. Morphologically, senescent cells in culture have an enhanced intracellular volume which is associated with a flattening of the cytoplasm, as well as a vacuolized appearance [12]. Senescent cells also display a unique and complex secretory phenotype referred to as the Senescence-associated Secretory Phenotype (SASP). This SASP consists of an array of pro-inflammatory cytokinesand chemokines, as well as growth factors and various proteins responsible for degrading the extracellular matrix [13]. Alongside the SASP, senescent cells produce exacerbated levels of mitochondrial derived reactive oxygen species (ROS)[14]. Collectively, the SASP in conjunction with enhanced ROS production functions to reinforce the cell cycle arrest in an autocrine manner [15, 16], but can also contribute to senescence in a paracrine manner [17, 18]
2. Cellular senescence is a driver of ageing and age-related disease
Several groups have observedthat senescent cells accumulatewith age in different tissuesand species[19-22]. Furthermore, senescent cells have been observed in a variety of age-related diseases[11, 19, 23, 24].
To address the fundamental question regarding whether senescence plays a causal role in ageing, mouse models were recently developed which allow the specific elimination of senescent cells. Studies have demonstrated that clearance of p16positive senescent cells alleviates the onset of a number of age-related pathologies in progeroid and wild type mice [7, 25-27]. Following this initial evidence of causality, the role of senescence in the context of various age-related diseases was investigated by various groups. To date there is evidence using mouse models that senescence plays a causal role in multiple age-related conditions. These include: neurodegeneration [28-30], osteoarthritis [31, 32], osteroporosis [33], atherosclerosis [34, 35], fatty liver disease [36], myocardial infarction [37], diabetes [38, 39], pulmonary fibrosis [40-42], neuropsychiatric disorders [43] amongst others [44-49] (Table 1).
A key question for the field is to understand specifically how senescent cells exert their detrimental effects and thus contribute to the ageing process. A popular hypothesis is that senescent cells are major contributors to the chronic inflammation which is observed during ageing, via the SASP.
The SASP is thought to signal the immune system to mediate the clearance of senescent cells, however, it is believed that with advancing age the capacity for the immune system to effectively remove senescent cells becomes diminished, therefore contributing to their accumulation [50-52]. As senescent cells accumulate partly due to reduced immune clearance, the associated SASP acts in a paracrine fashion to induce senescence in surrounding cells [18]. This process can therefore further exacerbate the accumulation of senescent cells and thus the associated chronic inflammation (Figure 1). In addition, it has been demonstrated that the SASP can be tumourigenic by promoting proliferation of malignant cells, as well as promoting epithelial-mesenchymal transition whereby epithelial cells acquire mesenchymal like properties allowing them to become invasive and promote malignant transformation [13, 53, 54]. At this stage, there is not sufficient evidence to directly conclude that the SASP is the maindriver of ageing. However, a number of studies have demonstrated that inhibition of pathways which suppress the SASP have beneficial effects on both health span and lifespan [55-60]. Therefore, from a clinical perspective there is great interest in understanding the biological pathways which regulate senescence and the SASP in order to develop drugs which either eliminate senescent cells (senolytics) or alleviate the SASP but do not remove the tumour suppressive ability of senescent cells (senostatic) [61]. So far, several drugs with senolytic and senostatic properties have been identified and shown to attenuate age-related disorders in mice. The clinical potential of potential senolytic drugs has been recently reviewed in detail by Kirkland and colleagues [62].Several clinical trials are under way and the first pilot study where senolytic cocktail dasatinib and quercetin were administered to Idiopathic pulmonary fibrosis (IPF) patients was recently published [63].
3. Mitochondrial dysfunction is a feature of cellular senescence
From a cellular perspective, the importance of mitochondria as the primary source of energy has been well defined for some time, however, mitochondria are also important in a myriad of other cellular processes such as cell cycle control, apoptosis and regulating cellular metabolism [64]. Interestingly both mitochondrial dysfunction and cellular senescence are consideredkey hallmarks of ageing, however, it is only work in the past decade that has begun to uncover the relationship between the two [65].
Cumulative evidence indicates that senescent cells exhibit a variety of changes in terms of the structure, dynamics and function of mitochondria.
Senescent fibroblasts show an increase in mitochondrial mass and abundance of tricarboxylic acid (TCA) cycle metabolites [15, 66, 67]. However, while mitochondria are more abundant in senescent cells they are less functional. Mitochondria from senescent cells show a decreased mitochondrial membrane potential, increased proton leak and increased generation of ROS [14, 15]. Recently, it was also reported that mitochondria from senescent cells have decreased fatty acid oxidation, which results in increased accumulation of lipids [36].
Senescent cells also show dramatic changes in mitochondrial morphology. Mitochondria from senescent cells exist in a state of hyper-fusion as opposed to a healthy mitochondrial network which continuously undergoes fission and fusion events to meet metabolic demands and allow the removal of dysfunctional entities. This fused state occurs in response to reduced expression of mediators of the fission process; Drp1(dynamin related protein 1) and Fis1 (mitochondrial fission protein 1), as well as an overall reduction in the frequency of fusion and fission events [68, 69].
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