Cellular senescence in ageing: from mechanisms to therapeutic opportunities

Abstract

Cellular senescence, first described in vitro in 1961, has become a focus for biotech companies that target it to ameliorate a variety of human conditions. Eminently characterized by a permanent proliferation arrest, cellular senescence occurs in response to endogenous and exogenous stresses, including telomere dysfunction, oncogene activation and persistent DNA damage. Cellular senescence can also be a controlled programme occurring in diverse biological processes, including embryonic development. Senescent cell extrinsic activities, broadly related to the activation of a senescence-associated secretory phenotype, amplify the impact of cell-intrinsic proliferative arrest and contribute to impaired tissue regeneration, chronic age-associated diseases and organismal ageing. This Review discusses the mechanisms and modulators of cellular senescence establishment and induction of a senescence-associated secretory phenotype, and provides an overview of cellular senescence as an emerging opportunity to intervene through senolytic and senomorphic therapies in ageing and ageing-associated diseases.

Fig. 1 |. Senescence drivers and phenotypes.

Nuclear DNA damage is often causatively associated with senescence establishment. DNA damage activates a signalling cascade defined as DNA damage response (DDR), characterized by phosphorylated histone H2AX (γH2AX), 53BP1 and MDC1, the apical kinases ataxia telangiectasia mutated (ATM) and ATR and the downstream kinases CHK2 and CHK1. Signals ultimately converge on p53 activation, which in turn elicits cell cycle arrest. Prolonged DDR activation triggers senescence. One or a few DDR signalling telomeres, the ends of chromosomes, are sufficient to trigger replicative cell senescence. Oncogene activation is also a powerful senescence trigger. Specifically, most activated oncogenes, partly via reactive oxygen species (ROS) production, induce hyperproliferation and altered DNA replication patterns that ultimately result in replication stress and DNA damage accumulation at fragile sites, which include telomeres. Besides prolonged DDR activation, senescence features include cell cycle arrest (by upregulation of p21 and p16 cell cycle inhibitors), oxidative damage (as detected by increased ROS levels), upregulation of the BCL-2 family of antiapoptotic proteins, which induce resistance to apoptosis, metabolic changes (including senescence-associated-β-galactosidase (SA-β-gal) accumulation), senescence-associated heterochromatin foci (SAHF) and a senescence-associated secretory phenotype (SASP).

Fig. 2 |. SASP regulation.

Senescence-associated secretory phenotype (SASP) activation is a dynamic process that accompanies cell cycle exit initiated by senescence triggers. A core SASP programme comprises mainly proinflammatory interleukin-6 (IL-6), IL-8 and monocyte chemoattractant protein 1 (MCP1), regulated in an IL-1-dependent manner, and enzymes involved in extracellular matrix (ECM) remodelling, such as matrix metalloproteinases (MMPs), serine/cysteine proteinase inhibitors (SERPINs) and tissue inhibitors of metalloproteinases (TIMPs). More recently, additional core SASP effectors released as soluble molecules or in exosomes were identified, including GDF15, STC1 and MMP1. DNA damage response factors, including the upstream DNA damage response kinase induce SASP genes via nuclear factor-κB (NF-κB). The mitogen-activated protein kinase p38 also induces SASP genes by increasing the activity of NF-κB. Activation of several transcription factors and chromatin regulators has been implicated in SASP activation and regulation. NF-κB the transcription factor CCAAT/enhancer-binding protein-β (C/EBPβ) bind promoters of SASP genes and regulate their activation. GATA4 regulates NF-κB and SASP genes indirectly via IL-1 production. The mammalian target of rapamycin (mTOR) pathway also promotes SASP production through increased translation of subsets of mRNAs, including that encoding for IL-1α. In concert with transcription factors, the epigenetic reader bromodomain-containing protein 4 (BRD4), an acetylated histone-binding protein involved in oncogenesis, is recruited to superenhancers adjacent to SASP genes, thus contributing to the proper execution of cellular senescence. BRD4 binds acetylated histone H3 Lys27 (H3K27), thus competing with Polycomb repressor complex 2 (PRC2), which methylates the same histone residue (to give trimethylated H3K27) for transcriptional repression. Consistent with this, PRC2 inhibits SASP genes in senescent cells. More recently, the DNA sensor cyclic GMP–AMP synthase (cGAS) and the adaptor stimulator of interferon genes (STING) have been reported to be major regulators of the SASP programme across species and senescence modes, presumably by activating NF-κB and interferon response factor IRF3 on recognition of cytosol DNA and cytosolic chromatin fragments (CCFs). Aberrant activation of the cGAS–STING pathway could be linked to the downregulation of DNases (for example, DNase 2 and TREX1), enzymes normally involved in cytoplasmic DNA degradation. ATM, ataxia telangiectasia mutated; cGAMP, cyclic GMP–AMP; IL-1R, interleukin-1 receptor.

Fig. 3 |. Biological consequences of cell senescence.

Senescent cells execute distinct biological functions, which can have deleterious or beneficial consequences in a context-dependent manner. As beneficial functions, senescent cells guide tissue regeneration and embryonic development in the embryo in transient structures by secretion of FGF4 and FGF8 and shape the placenta structure and function with matrix metalloproteinase 2 and 9 (MMP2 and MMP9). Senescent cells also limit tissue damage by limiting excessive proliferation of cells and promote wound healing in part by secretion of PDGF-AA. One of the most prominent functions of senescence is tumour suppression. Senescent cells limit tumour development by cell-autonomous block of cell cycle progression via upregulation of p53, p16 and p21 and in a cell-non-autonomous manner by promoting senescence in neighbouring cells through secretion of interleukin-6 (IL-6) and IL-8. As deleterious functions, senescent cells can promote a proinflammatory microenvironment and therefore support tumour development in their proximity through multiple senescence-associated secretory phenotype (SASP) components. Similarly, senescent cells promote sterile chronic inflammation during ageing and during multiple age-related diseases. SASP factors, including IL-6, IL-1 receptor antagonist (IL-1RA), GROα and interferon-γ (IFNγ), are the main mediators of this effect. Additional SASP factors, including MMPs, might further damage tissue architecture and promote inflammation and tumorigenesis. When stem or progenitor cells enter senescence due to upregulation of the cell cycle inhibitory proteins, such as p16 and p21, they can no longer perform their function in supporting tissues by providing new cells, thus limiting tissue regenerative potential. Senescent cells also promote reprograming to an embryonic state, at least partially through IL-6. The reprograming, on one hand, can support tissue regeneration and, on the other hand, favours tumour development.

Fig. 4 |. Senolytic therapeutic interventions.

The sensitivities of senescent cells to pharmacological treatments that can promote their death are diverse. A number of known mechanisms of senolytic action are indicated; the various specific compounds that hit these nodes are indicated. Impacting tyrosine kinase (TK) through the use of dasatinib (when used either alone or in combination with the flavonoid quercetin) is capable of initiating death of certain senescent cell types. Quercetin and fisetin are natural flavonoids that impact mammalian target of rapamycin (mTOR) signalling. Inhibitors of the antiapoptotic members of the BCL-2 family are capable of inducing death through mitochondrial-mediated mechanisms, which can also be elicited by the action of cardiac glycosides such as ouabain. Inhibitors of HSP90 or histone deacetylases (HDAC) have also been suggested to promote selective apoptosis of senescent cells. Additionally, disruption of binding of forkhead box protein O4 (FOXO4) to p53, which occurs in senescent cells, through the use of a small peptide liberates p53 to activate apoptosis. Galactose-conjugated senolytic prodrugs (GAL-prodrugs) are processed by senescence-associated-β-galactosidase (SA-β-gal) to exert selective senescence targeting. RTK, receptor tyrosine kinase.

Fig. 5 |. Senomorphic therapeutic interventions.

As an alternative to active killing of senescent cells, senomorphic approaches try to limit the detrimental impacts of these cells, largely through modulation of the senescence-associated secretory phenotype (SASP). As for senolytics, a number of common nodes have been identified that may be unique opportunities for intervention. Rapamycin, a well-characterized inhibitor of mammalian target of rapamycin (mTOR), has been shown to increase lifespan of laboratory mice. Additionally, rapamycin decreases production of the SASP, which may explain the beneficial impacts on life. Nuclear factor-κB (NF-κB) is a critical component for SASP production, and inhibition of NF-κB activity decreases the ability of cells to be proinflammatory. Additionally, inhibition of HSP90 is able to modulate SASP production. Similarly, Janus kinase (JAK)/signal transducer and activator of transcription (STAT) inhibitors and blocking of interleukin-6 (IL-6), IL-1 and tumour necrosis factor (TNF). Taken together, these molecules are beginning to elucidate ways that proinflammatory signalling from senescent cells can be attenuated in the hope of decreasing the consequences of senescent cell accumulation in tissues. ASO, antisense oligonucleotides; DDR, DNA damage response; DDRNA, DNA damage response RNA; dilncRNA, damage-induced long non-coding RNA; NDGA, nordihydroguaiaretic acid; RTK, receptor tyrosine kinase.