SENESCENT CELLS: AN IMPORTANT CAUSE OF CELLULAR AGING
Systemic cellular aging occurs because of the failure of a few basic health maintenance mechanisms, which collectively are known as the hallmarks of aging.1 These are an interconnected set of cellular processes that determine how organs and systems age. One of the hallmarks of aging is cellular senescence; senescent cells are alive and metabolically active, but nonproliferative. Importantly, they demonstrate differential functions to their native counterparts. Predominant among these new characteristics is the secretion of the senescence-associated secretory phenotype (SASP), a collection of proinflammatory cytokines and tissue remodeling proteins.2 In young and healthy tissues, senescent cells and their associated SASP have an important role in normal biology, with roles in wound healing, cancer prevention, and embryonic development.3 In aging systems, however, the unresolved clearance of even small numbers of senescent cells and their associated SASP response can result in profound changes to the tissues and organs that are characteristic of aging.4,5 Selective ablation of senescent cells in transgenic animal models indicated that the removal of senescent cells was able to delay several age-associated diseases.6 Follow on work has since demonstrated that removal of senescent cells yields improvements in renal, cardiac, motor, and cognitive functions in animal models.7 Senescent cells thus comprise a tractable and emerging target for new therapies aiming to attenuate aging phenotypes.
SENESCENCE IN THE CONTEXT OF SKIN AGING
The hallmarks of aging act on skin, as they do on other organs. DNA damage induced by sun exposure can cause the characteristic aesthetic signs of aging, as can epigenetic changes resulting from exposure to pollutants and other damaging chemicals. Inflammation, arising from dysfunctional cell communication can lead to skin reddening, changes to the extracellular matrix, and inflammatory infiltration. Stem cell exhaustion also means that skin tissues may lose their ability to repopulate and differentiate following the loss of cells through damage or senescence. Senescent cells accumulate in the cells of the epidermis and dermis, as well as in the subdermal adipose tissue depots (Fig. 1), as they do in all tissues and organs. The secretion of the SASP may also drive aberrant tissue remodeling and extracellular matrix dysfunction, causing changes in collagen composition and structure, destruction of elastin as well as inflammatory infiltration, fibrotic changes, and atrophy of fat tissues. Collectively, these phenomena lead to the characteristic aesthetic changes associated with aging, including rhytids, pigmentation changes, skin thinning, and deterioration of the underlying skin substructure. Removal or rejuvenation of senescent cells therefore has the potential to remove the negative effects of the SASP, leading to normalization of the extracellular matrix, renewed differentiation of new adipocytes, reduction of overt inflammation, and restoration of the skin substructure (Fig. 2). Interventions designed to reduce the senescent cell load of aged skin are amenable for topical delivery to treat the most external layers of the skin to ameliorate pigmentation changes and surface skin quality, as well as having positive effects on skin substructure if delivered by injection to the dermis or the adipose tissue depots.
SENOLYTICS AND SENOMORPHICS: ALTERNATIVE APPROACHES FOR THE REMOVAL OF SENESCENT CELLS
There are two basic approaches for the removal of senescent cells. These are senolytic approaches whereby senescent cells are killed selectively, or senomorphic approaches whereby they are rejuvenated. There may be benefits and drawbacks to both approaches. Rejuvenated senescent cells may require repeated treatment to maintain their renewed status, and will of course retain some features of age, similar to nonsenescent cells present in the host. Senolytic approaches, however, may not take account of findings that senescent cells comprise several subtypes, some of which may be beneficial.8 Removal of senescent cells by selective apoptosis will likely affect both subtypes without discrimination. The necrotic factors and other cell signals released upon cell death associated with proinflammatory mediators and immune responses may also cause tissue damage and contribute to disease pathogenesis.9 Furthermore, some disease indications may involve tissues that are cell poor, and may not tolerate cell removal. It is likely therefore that the choice of senotherapeutic modality that is most appropriate in any particular instance will depend on the therapeutic aim. Potential senolytic and senomorphic candidates are given in Table 1.
Table 1. -
Some Examples of Senolytic and Senomorphic Drugs*
||Innate defence regulatory peptide
*The molecular targets and mode of intervention are provided for a nonexhaustive list of senolytic and senomorphic compounds. These approaches, although primarily experimental at present, are under exploration for clinical use in some cases.
NEW MODALITIES FOR THE REMOVAL OR REJUVENATION OF SENESCENT CELLS
To date, the majority of senotherapeutic approaches have been based on small molecule candidates. There are, however, some emerging novel modalities for targeting senescent cells. These include approaches to harness the immune system for clearance of senescent cells. T cells engineered to express chimeric antigen receptors (CAR T therapies) have emerged as a new potential means to clear senescent cells. For example, CAR T cells engineered with the urokinase-type plasminogen activator receptor have been demonstrated to reverse senescence-associated pathologies in animal models.35 Other approaches have involved the use of proteolysis-targeting chimeras, whereby a ligand to a target of interest is conjugated to an E3 ubiquitin ligase, which brings about proteolytic degradation of targets. A proteolysis-targeting chimera targeted to BRD4 demonstrated good senolytic activity in cultured cells and animal models.27 Other approaches target the unique characteristics of senescent cells for senotherapeutic purposes. One such property is the very high levels of lysosomal beta galactosidase that are present in senescent cells. Drugs with known senolytic or senomorphic properties can be galactose modified, and can thus be used to produce a prodrug that is only processed to its active form in senescent cells.36
TARGETING RNA PROCESSING FOR SENOMORPHIC EFFECT
RNA processing is the collection of events that are necessary to allow the production of multiple mRNAs from a gene, in a process known as alternative splicing (AS). AS is a prerequisite to the plastic and adaptable transcriptome necessary for avoidance of cellular senescence. The decision as to which alternative RNA is expressed in any given situation is made by the combinatorial binding of a group of proteins called splicing factors.37 Dysregulation of AS has emerged as a new, and therapeutically tractable, hallmark of aging,38 and disruption to this is associated with cellular senescence and adverse aging outcomes in vitro and in vivo.39–42 Splicing factor expression declines with age as a result of repeated and constitutive activation of the AKT and ERK signaling pathways, and their effector genes FOXO1 and ETV6.17 A promising new senomorphic strategy for cellular rejuvenation involves the rescue of splicing factor expression by genetic or small molecule means, and restoration of more youthful splicing patterns. Splicing factor expression can be restored by naturally occurring small molecules such as polyphenols13 or donors of the gasotransmitter hydrogen sulfide,15 or by inhibition of their upstream negative regulators AKT and ERK.17 These interventions result in the rejuvenation of senescent cells and the attenuation of the SASP, with or without rebuilding of telomeres and resumption of cell cycle, depending on the intervention. Importantly, these interventions would not need to be applied daily for reversal of senescence; treatment with polyphenols was shown to provoke a measureable effect on senescent cell load in human primary dermal fibroblasts 4 weeks after initial treatment,43 whereas treatment with H2S donors was able to retard senescence in human endothelial cell models.15 This raises the possibility of prophylactic application for skin aging phenotypes.
OLIGONUCLEOTIDE THERAPIES: FUTURE PRECISION MEDICINE FOR CELLULAR SENESCENCE
The majority of senotherapeutic candidates available at present have potential off-target effects. Manipulation of signaling pathways such as p53, JAK-STAT, ATM, or AKT will yield effects on many other downstream targets in addition to those intended. Similarly, small molecules such as fisetin, dasatinib, or digoxin may produce unforeseen effects on other cell types or organ systems. Our discovery of the pivotal role of disrupted splicing in cellular senescence raises the possibility of targeting individual splicing events in a very precise manner, which may allow us to pinpoint and target the exact molecular causes of cellular senescence in the future. Splicing patterns can be modified by the use of splice switching oligonucleotide biologics, which bind to the pre-RNA sequences that define splice sites and promote or forbid their usage.44 By these technologies, we can either restore the expression of individual splicing factors (since splicing factors are themselves regulated by AS,45 or force the expression of youthful patterns of splicing for key senescence genes). These emerging approaches should replicate the natural regulatory relationships that maintain homeostasis and molecular stress resilience in young cells, and may form the basis for long-term rejuvenation of aged cells, tissues, and organs.
PROGRESS TOWARD THE CLINIC AND FUTURE OUTLOOK
Although evaluation of senotherapeutics is at present in the preclinical phase for the majority of indications, several are now entering trials for aging phenotypes; NCT02848131 (dasatinib and quercetin for the treatment of chronic kidney disease), NCT0367524 (fisetin in the context of frailty), NCT029151898, and NCT 03451006 (metformin, also as an intervention for frailty).46 The use of senotherapeutics for skin phenotypes is, however, in its infancy, despite the observation that the skin may represent an early human in vivo proof of concept for these approaches, due to its accessibility compared with other organ systems. At the time of writing, the only senotherapeutic intervention on the market specifically for skin aesthetics is IDR-1018, a proprietary innate defense regulatory peptide which has been reported to exhibit some senomodulatory effect.32 The long-lasting effect of the novel modalities we describe here opens doors to the development of a new range of clinician-dispensed skincare products that could be applied weekly or more frequently in the case of topical application, or at longer intervals by subdermal injection in a clinical dermatology setting.
Cellular senescence is emerging as one of the most tractable intervention points for cellular and organismal aging. Although harnessing these interventions for systemic clinical benefit for the diseases of aging is some way from the clinic at present, topical or injected application for skin aging is rather nearer term. Topical application negates many of the barriers associated with systemic toxicity or difficulty in delivery to target organs, while providing early human in vivo proof of principle for later endeavors. Senotherapeutic approaches to remove or rejuvenate senescent cells offer an “inside out” approach to ameliorate the aesthetic effects of the aging process, treating the cause of cellular aging at its root, rather than managing the effects of the passage of time.
OFF-LABEL USE/UNAPPROVED DRUGS OR PRODUCTS
Table 1 contains reference to the potential unlabeled repurposing of drugs for senotherapy. We would like to declare that these drugs are still investigational.
1. López-Otín C, Blasco MA, Partridge L, et al. The hallmarks of aging. Cell. 2013;153:1194–1217.
2. Acosta JC, Banito A, Wuestefeld T, et al. A complex secretory program orchestrated by the inflammasome controls paracrine senescence. Nat Cell Biol. 2013;15:978–990.
3. Demaria M, Ohtani N, Youssef SA, et al. An essential role for senescent cells in optimal wound healing through secretion of PDGF-AA. Dev Cell. 2014;31:722–733.
4. Campisi J, d’Adda di Fagagna F. Cellular senescence: when bad things happen to good cells. Nat Rev Mol Cell Biol. 2007;8:729–740.
5. Wang C, Jurk D, Maddick M, Nelson G, et al. DNA damage response and cellular senescence in tissues of aging mice. Aging Cell. 2009;8:311–323.
6. Baker DJ, Wijshake T, Tchkonia T, et al. Clearance of p16Ink4a-positive senescent cells delays ageing-associated disorders. Nature. 2011;479:232–236.
7. Childs BG, Gluscevic M, Baker DJ, et al. Senescent cells: an emerging target for diseases of ageing. Nat Rev Drug Discov. 2017;16:718–735.
8. Attaallah A, Lenzi M, Marchionni S, et al. A pro longevity role for cellular senescence. Geroscience. 2020;42:867–879.
9. Rock KL, Kono H. The inflammatory response to cell death. Annu Rev Pathol. 2008;3:99–126.
10. He Y, Zhang X, Chang J, et al. Using proteolysis-targeting chimera technology to reduce navitoclax platelet toxicity and improve its senolytic activity. Nat Commun. 2020;11:1996.
11. Chang J, Wang Y, Shao L, et al. Clearance of senescent cells by ABT263 rejuvenates aged hematopoietic stem cells in mice. Nat Med. 2016;22:78–83.
12. De Cecco M, Ito T, Petrashen AP, et al. L1 drives IFN in senescent cells and promotes age-associated inflammation. Nature. 2019;566:73–78.
13. Latorre E, Birar VC, Sheerin AN, et al. Small molecule modulation of splicing factor expression is associated with rescue from cellular senescence. BMC Cell Biol. 2017;18:31.
14. Fuhrmann-Stroissnigg H, Ling YY, Zhao J, et al. Identification of HSP90 inhibitors as a novel class of senolytics. Nat Commun. 2017;8:422.
15. Latorre E, Torregrossa R, Wood ME, et al. Mitochondria-targeted hydrogen sulfide attenuates endothelial senescence by selective induction of splicing factors HNRNPD and SRSF2. Aging (Albany NY). 2018;10:1666–1681.
16. Jeon OH, Kim C, Laberge RM, et al. Local clearance of senescent cells attenuates the development of post-traumatic osteoarthritis and creates a pro-regenerative environment. Nat Med. 2017;23:775–781.
17. Latorre E, Ostler EL, Faragher RGA, et al. FOXO1 and ETV6 genes may represent novel regulators of splicing factor expression in cellular senescence. FASEB J. 2019;33:1086–1097.
18. Baar MP, Brandt RMC, Putavet DA, et al. Targeted apoptosis of senescent cells restores tissue homeostasis in response to chemotoxicity and aging. Cell. 2017;169:132–147.e16.
19. He Y, Li W, Lv D, et al. Inhibition of USP7 activity selectively eliminates senescent cells in part via restoration of p53 activity. Aging Cell. 2020;19:e13117.
20. Tilstra JS, Robinson AR, Wang J, et al. NF-κB inhibition delays DNA damage-induced senescence and aging in mice. J Clin Invest. 2012;122:2601–2612.
21. Zhang X, Zhang S, Liu X, et al. Oxidation resistance 1 is a novel senolytic target. Aging Cell. 2018;17:e12780.
22. Xu M, Tchkonia T, Ding H, et al. JAK inhibition alleviates the cellular senescence-associated secretory phenotype and frailty in old age. Proc Natl Acad Sci U S A. 2015;112:E6301–E6310.
23. Zhu Y, Tchkonia T, Pirtskhalava T, et al. The Achilles’ heel of senescent cells: from transcriptome to senolytic drugs. Aging Cell. 2015;14:644–658.
24. Lee SJ, Jung YS, Yoon MH, et al. Interruption of progerin-lamin A/C binding ameliorates Hutchinson-Gilford progeria syndrome phenotype. J Clin Invest. 2016;126:3879–3893.
25. Guerrero A, Herranz N, Sun B, et al. Cardiac glycosides are broad-spectrum senolytics. Nat Metab. 2019;1:1074–1088.
26. Bae YU, Choi JH, Nagy A, et al. Antisenescence effect of mouse embryonic stem cell conditioned medium through a PDGF/FGF pathway. FASEB J. 2016;30:1276–1286.
27. Wakita M, Takahashi A, Sano O, et al. A BET family protein degrader provokes senolysis by targeting NHEJ and autophagy in senescent cells. Nat Commun. 2020;11:1935.
28. Bae YU, Son Y, Kim CH, et al. Embryonic stem cell-derived mmu-miR-291a-3p inhibits cellular senescence in human dermal fibroblasts through the TGF-β receptor 2 pathway. J Gerontol A Biol Sci Med Sci. 2019;74:1359–1367.
29. Johmura Y, Yamanaka T, Omori S, et al. Senolysis by glutaminolysis inhibition ameliorates various age-associated disorders. Science. 2021;371:265–270.
30. Kang HT, Park JT, Choi K, et al. Chemical screening identifies ATM as a target for alleviating senescence. Nat Chem Biol. 2017;13:616–623.
31. Zhu Y, Doornebal EJ, Pirtskhalava T, et al. New agents that target senescent cells: the flavone, fisetin, and the BCL-XL inhibitors, A1331852 and A1155463. Aging (Albany NY). 2017;9:955–963.
32. Alencar-Silva T, Zonari A, Foyt D, et al. IDR-1018 induces cell proliferation, migration, and reparative gene expression in 2D culture and 3D human skin equivalents. J Tissue Eng Regen Med. 2019;13:2018–2030.
33. Samaraweera L, Adomako A, Rodriguez-Gabin A, et al. A Novel indication for panobinostat as a senolytic drug in NSCLC and HNSCC. Sci Rep. 2017;7:1900.
34. Hwang HV, Tran DT, Rebuffatti MN, et al. Investigation of quercetin and hyperoside as senolytics in adult human endothelial cells. PLoS One. 2018;13:e0190374.
35. Amor C, Feucht J, Leibold J, et al. Senolytic CAR T cells reverse senescence-associated pathologies. Nature. 2020;583:127–132.
36. Cai Y, Zhou H, Zhu Y, et al. Elimination of senescent cells by β-galactosidase-targeted prodrug attenuates inflammation and restores physical function in aged mice. Cell Res. 2020;30:574–589.
37. Smith CW, Valcárcel J. Alternative pre-mRNA splicing: the logic of combinatorial control. Trends Biochem Sci. 2000;25:381–388.
38. Harries LW, Hernandez D, Henley W, et al. Human aging is characterized by focused changes in gene expression and deregulation of alternative splicing. Aging Cell. 2011;10:868–878.
39. Holly AC, Melzer D, Pilling LC, et al. Changes in splicing factor expression are associated with advancing age in man. Mech Ageing Dev. 2013;134:356–366.
40. Lee BP, Pilling LC, Bandinelli S, et al. The transcript expression levels of HNRNPM, HNRNPA0 and AKAP17A splicing factors may be predictively associated with ageing phenotypes in human peripheral blood. Biogerontology. 2019;20:649–663.
41. Lye J, Latorre E, Lee BP, et al. Astrocyte senescence may drive alterations in GFAPa, CDKN2A p14ARF and TAU3 transcript expression and contribute to cognitive decline. Geroscience. 2019;42:1–13.
42. Latorre E, Pilling LC, Lee BP, et al. The VEGFA156b isoform is dysregulated in senescent endothelial cells and may be associated with prevalent and incident coronary heart disease. Clin Sci (Lond). 2018;132:313–325.
43. Latorre E, Birar VC, Sheerin AN, et al. Small molecule modulation of splicing factor expression is associated with rescue from cellular senescence. BMC Cell Biol. 2017;18:31.
44. Havens MA, Hastings ML. Splice-switching antisense oligonucleotides as therapeutic drugs. Nucleic Acids Res. 2016;44:6549–6563.
45. Lareau LF, Inada M, Green RE, et al. Unproductive splicing of SR genes associated with highly conserved and ultraconserved DNA elements. Nature. 2007;446:926–929.
46. Docherty MH, Baird DP, Hughes J, et al. Cellular senescence and senotherapies in the kidney: current evidence and future directions. Front Pharmacol. 2020;11:755.