The consequences of environmental conditions for antagonistic pleiotropic effects of cellular
senescence
van Vliet, Thijmen
DOI:
10.33612/diss.156836397
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from
it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
2021
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
van Vliet, T. (2021). The consequences of environmental conditions for antagonistic pleiotropic effects of
cellular senescence. University of Groningen. https://doi.org/10.33612/diss.156836397
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum.
554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet Processed on: 13-1-2021 Processed on: 13-1-2021 Processed on: 13-1-2021
Processed on: 13-1-2021 PDF page: 11PDF page: 11PDF page: 11PDF page: 11
10 11
Chapter 2
Consequences of senotherapies for tissue repair and reprogramming
Thijmen van Vliet 1,*, Jaskaren Kohli 1,*, Marco Demaria1
*equal contribution
1European Research Institute for the Biology of Aging, University Medical Center Groningen, University
of Groningen, Groningen, The Netherlands
554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet Processed on: 13-1-2021 Processed on: 13-1-2021 Processed on: 13-1-2021
Processed on: 13-1-2021 PDF page: 12PDF page: 12PDF page: 12PDF page: 12
12 Introduction
Cellular senescence is a complex stress response where cells undergo an irreversible cell cycle arrest in response to DNA damage and mitochondrial dysfunction. Although no exclusive and unique markers of senescence exist, senescent cells can be identified by a persistent DNA damage, expression of cyclin dependent kinase inhibitors p16 and p21, resistance to apoptotic stimuli, increased lysosomal content, ER stress, accumulation of mitochondria, and alterations in compositions of the plasma and nuclear membranes1. In addition, senescent cells secrete various factors including pro-inflammatory cytokines,
growth factors, chemokines and matrix metalloproteinases (MMP’s), collectively named the senescence-associated secretory phenotype (SASP)1. Senescent cells are found in many age-associated
disorders, where their detrimental effects are influenced by the SASP. Clearing naturally occurring senescent cells delays the onset of several of these diseases and increases median lifespan in mice2.
Accumulating evidence suggests that modulating the SASP can also improve healthspan and reduce frailty3. Owing to these exciting findings, compounds which selectively eliminate senescent cells
(senolytics) or modulate the SASP (senomorphics) are under current investigation for their effectiveness to alleviate age-associated disorders4.
Resistance to apoptotic stimuli occurs through upregulation of several members of the Bcl-2 family of anti-apoptotic proteins, namely Bcl-xL and Bcl-w. ABT-263 and ABT-737 are two confirmed senolytic agents which sensitize senescent cells to undergo intrinsic apoptosis via inhibition of these proteins. In vivo these compounds have been demonstrated to eliminate senescent cells induced by ionizing radiation or chemotherapeutic agents, as well as naturally occurring senescent cells in aged mice, in various tissues such as the lung, skin and hematopoietic system5–7. ABT-263 has also been shown to
kill senescent cells in atherosclerotic lesions, reducing the overall disease burden8. More recently,
ABT-263 has been shown to reduce senescence markers in the cortex and hippocampus and attenuate tau phosphorylation in a tau-dependent neurodegenerative mouse model9.
Other compounds with senolytic activity include Dasatanib (D), an inhibitor of multiple tyrosine kinases already used in cancer treatment, and a natural flavanol called Quercetin (Q). When combined (D+Q), these drugs rescued cardiovascular function and frailty in aged mice, extended health span in progeroid ERCC1-/Δ mice and ameliorated obesity associated loss of neurogenesis and anxiety- like
behavior10,11,12. However, the mechanism as to how D+Q induce cell death specifically in senescent
cells is currently unknown.
HSP90 inhibitors such as 17-DMAG have been demonstrated to eliminate senescent cells through downregulation of the anti-apoptotic PI3K-AKT pathway and, similarly to D+Q treatment, to extend healthspan in ERCC1-/Δ mice13.
13
Recent studies have also shown that molecules which modulate p53 activity and/or localization can act as senolytics. FOXO4 is required for maintaining viability in senescent cells, but a peptide which interferes with the binding of this protein to p53, resulted in p53 nuclear exclusion and apoptosis in senescent cells. This peptide improved healthspan in progeroid XPDTTD/TTD and naturally aged mice, and
rescued chemotoxicity in doxorubicin-treated mice14. Another compound, termed UBX0101, functions
as a p53/MDM2 inhibitor and has been demonstrated to kill senescent cells in osteoarthritic lesions in mice and decrease overall disease phenotypes15.
The NF-kB (nuclear factor-kB) transcription factor acts as a master regulator of SASP factor expression16. The NF-kB pathway during senescence is modulated by the mammalian target of
rapamycin (mTOR)17 and mitogen-activated protein kinase (MAPK) cascades18. mTOR stimulates IL-1α
translation, which leads to activation of IL1R signaling in an autocrine manner to trigger NF-ĸB mediated transcription of pro-inflammatory SASP factors17. mTOR also regulates translation of MK2,
which in turn induces downstream phosphorylation of ZFP36L1. This results in a suppression in the protein’s ability to degrade transcripts of various SASP factors during senescence19. Interfering with
the mTOR pathway could therefore also be an approach to ameliorate detrimental effects of the SASP and improve mammalian healthspan. Rapamycin is a specific suppressor of mTORC1 and its administration has been shown to alleviate senescence-associated inflammation and increase lifespan in mice17,20. Specific MAPK inhibitors SB203580, UR13756 and BIRB 796 significantly inhibited SASP
expression in senescent cells and decreased their effect in promoting breast cancer cell invasion in vitro18,21. Inhibition of MK2 was shown to have similar effects in dampening the SASP21. Glucocorticoids
have also been discovered to inhibit selected components of the SASP through blockage of the IL1α-NF-kB signaling axis22.
Accumulating evidence suggests that caloric restriction (CR) is also an effective method to diminish the SASP in vivo and promote health- and lifespan in a wide variety of organisms23. Markers of senescence
including p16 and p21 are downregulated in colons from mouse and humans which underwent CR24.
Therefore it is likely that CR inhibits the natural occurrence of senescent cells although the mechanism is currently unknown. In addition, drugs that are believed to be ‘caloric restriction mimetics’ such as resveratrol and metformin were shown to have similar effects on the SASP. Metformin administrations in mice resulted in an increase in both health- and lifespan, presumably through activation of the Nrf2/ARE antioxidant pathway25. This would possibly result in an overall reduction in oxidative damage
and decrease in both NF-Kb activation and chronic inflammation25. Metformin also directly inhibited
NF-Kb signaling by preventing phosphorylation of IKK/B and nuclear translocation of RELA and RELB26.
554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet Processed on: 13-1-2021 Processed on: 13-1-2021 Processed on: 13-1-2021
Processed on: 13-1-2021 PDF page: 13PDF page: 13PDF page: 13PDF page: 13
2
12 Introduction
Cellular senescence is a complex stress response where cells undergo an irreversible cell cycle arrest in response to DNA damage and mitochondrial dysfunction. Although no exclusive and unique markers of senescence exist, senescent cells can be identified by a persistent DNA damage, expression of cyclin dependent kinase inhibitors p16 and p21, resistance to apoptotic stimuli, increased lysosomal content, ER stress, accumulation of mitochondria, and alterations in compositions of the plasma and nuclear membranes1. In addition, senescent cells secrete various factors including pro-inflammatory cytokines,
growth factors, chemokines and matrix metalloproteinases (MMP’s), collectively named the senescence-associated secretory phenotype (SASP)1. Senescent cells are found in many age-associated
disorders, where their detrimental effects are influenced by the SASP. Clearing naturally occurring senescent cells delays the onset of several of these diseases and increases median lifespan in mice2.
Accumulating evidence suggests that modulating the SASP can also improve healthspan and reduce frailty3. Owing to these exciting findings, compounds which selectively eliminate senescent cells
(senolytics) or modulate the SASP (senomorphics) are under current investigation for their effectiveness to alleviate age-associated disorders4.
Resistance to apoptotic stimuli occurs through upregulation of several members of the Bcl-2 family of anti-apoptotic proteins, namely Bcl-xL and Bcl-w. ABT-263 and ABT-737 are two confirmed senolytic agents which sensitize senescent cells to undergo intrinsic apoptosis via inhibition of these proteins. In vivo these compounds have been demonstrated to eliminate senescent cells induced by ionizing radiation or chemotherapeutic agents, as well as naturally occurring senescent cells in aged mice, in various tissues such as the lung, skin and hematopoietic system5–7. ABT-263 has also been shown to
kill senescent cells in atherosclerotic lesions, reducing the overall disease burden8. More recently,
ABT-263 has been shown to reduce senescence markers in the cortex and hippocampus and attenuate tau phosphorylation in a tau-dependent neurodegenerative mouse model9.
Other compounds with senolytic activity include Dasatanib (D), an inhibitor of multiple tyrosine kinases already used in cancer treatment, and a natural flavanol called Quercetin (Q). When combined (D+Q), these drugs rescued cardiovascular function and frailty in aged mice, extended health span in progeroid ERCC1-/Δ mice and ameliorated obesity associated loss of neurogenesis and anxiety- like
behavior10,11,12. However, the mechanism as to how D+Q induce cell death specifically in senescent
cells is currently unknown.
HSP90 inhibitors such as 17-DMAG have been demonstrated to eliminate senescent cells through downregulation of the anti-apoptotic PI3K-AKT pathway and, similarly to D+Q treatment, to extend healthspan in ERCC1-/Δ mice13.
13
Recent studies have also shown that molecules which modulate p53 activity and/or localization can act as senolytics. FOXO4 is required for maintaining viability in senescent cells, but a peptide which interferes with the binding of this protein to p53, resulted in p53 nuclear exclusion and apoptosis in senescent cells. This peptide improved healthspan in progeroid XPDTTD/TTD and naturally aged mice, and
rescued chemotoxicity in doxorubicin-treated mice14. Another compound, termed UBX0101, functions
as a p53/MDM2 inhibitor and has been demonstrated to kill senescent cells in osteoarthritic lesions in mice and decrease overall disease phenotypes15.
The NF-kB (nuclear factor-kB) transcription factor acts as a master regulator of SASP factor expression16. The NF-kB pathway during senescence is modulated by the mammalian target of
rapamycin (mTOR)17 and mitogen-activated protein kinase (MAPK) cascades18. mTOR stimulates IL-1α
translation, which leads to activation of IL1R signaling in an autocrine manner to trigger NF-ĸB mediated transcription of pro-inflammatory SASP factors17. mTOR also regulates translation of MK2,
which in turn induces downstream phosphorylation of ZFP36L1. This results in a suppression in the protein’s ability to degrade transcripts of various SASP factors during senescence19. Interfering with
the mTOR pathway could therefore also be an approach to ameliorate detrimental effects of the SASP and improve mammalian healthspan. Rapamycin is a specific suppressor of mTORC1 and its administration has been shown to alleviate senescence-associated inflammation and increase lifespan in mice17,20. Specific MAPK inhibitors SB203580, UR13756 and BIRB 796 significantly inhibited SASP
expression in senescent cells and decreased their effect in promoting breast cancer cell invasion in vitro18,21. Inhibition of MK2 was shown to have similar effects in dampening the SASP21. Glucocorticoids
have also been discovered to inhibit selected components of the SASP through blockage of the IL1α-NF-kB signaling axis22.
Accumulating evidence suggests that caloric restriction (CR) is also an effective method to diminish the SASP in vivo and promote health- and lifespan in a wide variety of organisms23. Markers of senescence
including p16 and p21 are downregulated in colons from mouse and humans which underwent CR24.
Therefore it is likely that CR inhibits the natural occurrence of senescent cells although the mechanism is currently unknown. In addition, drugs that are believed to be ‘caloric restriction mimetics’ such as resveratrol and metformin were shown to have similar effects on the SASP. Metformin administrations in mice resulted in an increase in both health- and lifespan, presumably through activation of the Nrf2/ARE antioxidant pathway25. This would possibly result in an overall reduction in oxidative damage
and decrease in both NF-Kb activation and chronic inflammation25. Metformin also directly inhibited
NF-Kb signaling by preventing phosphorylation of IKK/B and nuclear translocation of RELA and RELB26.
554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet Processed on: 13-1-2021 Processed on: 13-1-2021 Processed on: 13-1-2021
Processed on: 13-1-2021 PDF page: 14PDF page: 14PDF page: 14PDF page: 14
14
SIRT1, a mammalian sirtuin that normally suppresses the SASP epigenetically27. Kaempferol and
apigenin are also natural flavonoids which inhibit the SASP but function by inhibiting NF-Kb activity28.
Although senotherapies appear promising as a therapeutic intervention to treat age-associated disorders, there are some limitations with their use due to intrinsic and extrinsic toxicities. ABT-263 and ABT-737 can cause thrombocytopenia or neutropenia owing to the importance of Bcl-2 anti-apoptotic proteins in platelet and neutrophil survival4. Rapamycin is an immunosuppressant as mTOR
suppression can inhibit T and B cell activation and proliferation29. Importantly, senescent cells have
also been reported to play beneficial roles in tissue repair and remodeling, which further complicates the development of senotherapies with minimal adverse effects.
Regulation of tissue repair, reprogramming and regeneration by cellular senescence
In recent years, beneficial roles for senescent cells in promoting wound healing and limiting excessive fibrosis in response to tissue damage have been described. At the site of a wound, senescent cells transiently appear in a coordinated manner to promote fibroblast to myofibroblast differentiation through secretion of platelet derived growth factor AA (PDGF-AA). Myofibroblasts drive wound contraction and promote optimal repair. Clearing senescent cells in engineered mouse models (p16-3MR and INK-ATTAC) resulted in a delay in wound closure30,31. In humans, fibroblast cultures from
venous ulcers and pressure ulcer beds show an increase in senescence markers such as SA-β-gal positivity and shorter replicative lifespan, compared to normal skin fibroblasts32,33. Together, these
data suggest that dysregulation of the senescent response during wound healing has the potential to alter wound healing kinetics in both humans and mice.
A fibrotic response is initiated during wound healing where excessive extracellular matrix (ECM) components are deposited at the site of injury. Although this typically results in scar formation, in extreme cases it can lead to organ failure. Liver fibrosis can be induced in mice through CCl4
administration, where hepatic stellate cells (HSCs) are activated into senescent HSCs34. The
matricellular protein CCN1 is a key senescence regulator in these cells35,36. CCN1 activates Ras-related
C3 botulinium toxin substrate 1 (RAC1), leading to downstream generation of reactive oxygen species (ROS) via NADPH oxidase 1 (NOX1) activation. ROS activity leads to upregulation of p16 via ERK and p38 MAPK, and p53 via a DNA damage response. Senescent HSCs transiently accumulate along fibrotic scars where they secrete ECM degrading enzymes such as matrix metalloproteinases (MMPs) to prevent further scaring. CCL4 treated livers from p53-/-;INK4A/ARF-/- mice showed significant decreased
numbers of senescent cells yet severe cirrhosis, confirming an essential beneficial role for senescent cells34.
15
CCN1 has also been reported to limit fibrosis in other tissue contexts by regulating senescence via RAC1-NOX1. CCN1 induces fibroblast senescence in cutaneous wounds in mice. These cells also secrete antifibrotic SASP factors to prevent excess collagen deposition in the skin36. Interestingly clearing
senescent cells in cutaneous wounds in the p16-3MR mouse model also resulted in excess skin fibrosis, most likely due to the resultant absence of proteases and MMPs30. Exogenous expression of CCN1 in
infarcted hearts in mice resolved fibrosis and rescued heart function, suggesting also an essential role for restricting myocardial fibrosis37.
A breakthrough in regenerative medicine has been the discovery that forced expression of only four factors (now called Yamanaka factors) was sufficient to reprogram differentiated cells into a pluripotent state38. Cellular senescence has been reported to positively influence this process in vivo
by creating a favorable microenvironment for reprogramming to occur. Genetically engineered mice that express the four Yamanaka factors upon doxycycline administration (i4F) display NANOG positive cells and teratomas in several tissues, indicative of successful reprogramming39. Remarkably, this
process does not occur in INK4A/ARF heterozygous mice, suggesting an intact INK4A/ARF locus is required40. Further work has demonstrated that INK4A, rather than ARF, is specifically required for in vivo reprogramming in mice41. SA-β-gal positive cells were found in close proximity to NANOG positive
cells in wild-type i4F mice. These senescent cells promoted reprogramming through secretion of IL6, which functions in a paracrine manner to activate PIM1 and induce cellular plasticity in reprogrammed cells40. Senescent cells also favors reprogramming of tissues which are normally less susceptible to the
process. Inducing tissue damage in skeletal muscle in i4F mice gave rise to senescent muscle cells. Doxycycline treatment post injury resulted in generation of pluripotent muscle stem cells. This did not occur in uninjured mice owing to the absence of a reprogrammable niche42. These findings may help
generate new approaches in regenerative medicine for difficult to reprogram tissues.
As well as mediating transcription of pro-inflammatory SASP components, NF-kB has also been recently discovered to regulate transcription of stemness genes in senescent cells, providing further evidence of their pro-regenerative capabilities. Engraftment of newborn keratinocytes transiently exposed to the SASP from HRASV12-expressing keratinocytes resulted in an increased number of hair follicles and
hair growth in nude mice. Interestingly, when cells were exposed to the SASP for prolonged periods they undergo cell-cycle arrest and paracrine induced senescence, possibly to prevent tumorigenesis in response to persistent regenerative stimuli43.
In contrast to mammals, salamanders have the ability to regenerate a variety of complex body parts including limbs. Senescent cells appear at the regenerating site in a defined spatial-temporal manner. Interference with macrophage mediated clearance of senescent cells, before or during limb
554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet Processed on: 13-1-2021 Processed on: 13-1-2021 Processed on: 13-1-2021
Processed on: 13-1-2021 PDF page: 15PDF page: 15PDF page: 15PDF page: 15
2
14
SIRT1, a mammalian sirtuin that normally suppresses the SASP epigenetically27. Kaempferol and
apigenin are also natural flavonoids which inhibit the SASP but function by inhibiting NF-Kb activity28.
Although senotherapies appear promising as a therapeutic intervention to treat age-associated disorders, there are some limitations with their use due to intrinsic and extrinsic toxicities. ABT-263 and ABT-737 can cause thrombocytopenia or neutropenia owing to the importance of Bcl-2 anti-apoptotic proteins in platelet and neutrophil survival4. Rapamycin is an immunosuppressant as mTOR
suppression can inhibit T and B cell activation and proliferation29. Importantly, senescent cells have
also been reported to play beneficial roles in tissue repair and remodeling, which further complicates the development of senotherapies with minimal adverse effects.
Regulation of tissue repair, reprogramming and regeneration by cellular senescence
In recent years, beneficial roles for senescent cells in promoting wound healing and limiting excessive fibrosis in response to tissue damage have been described. At the site of a wound, senescent cells transiently appear in a coordinated manner to promote fibroblast to myofibroblast differentiation through secretion of platelet derived growth factor AA (PDGF-AA). Myofibroblasts drive wound contraction and promote optimal repair. Clearing senescent cells in engineered mouse models (p16-3MR and INK-ATTAC) resulted in a delay in wound closure30,31. In humans, fibroblast cultures from
venous ulcers and pressure ulcer beds show an increase in senescence markers such as SA-β-gal positivity and shorter replicative lifespan, compared to normal skin fibroblasts32,33. Together, these
data suggest that dysregulation of the senescent response during wound healing has the potential to alter wound healing kinetics in both humans and mice.
A fibrotic response is initiated during wound healing where excessive extracellular matrix (ECM) components are deposited at the site of injury. Although this typically results in scar formation, in extreme cases it can lead to organ failure. Liver fibrosis can be induced in mice through CCl4
administration, where hepatic stellate cells (HSCs) are activated into senescent HSCs34. The
matricellular protein CCN1 is a key senescence regulator in these cells35,36. CCN1 activates Ras-related
C3 botulinium toxin substrate 1 (RAC1), leading to downstream generation of reactive oxygen species (ROS) via NADPH oxidase 1 (NOX1) activation. ROS activity leads to upregulation of p16 via ERK and p38 MAPK, and p53 via a DNA damage response. Senescent HSCs transiently accumulate along fibrotic scars where they secrete ECM degrading enzymes such as matrix metalloproteinases (MMPs) to prevent further scaring. CCL4 treated livers from p53-/-;INK4A/ARF-/- mice showed significant decreased
numbers of senescent cells yet severe cirrhosis, confirming an essential beneficial role for senescent cells34.
15
CCN1 has also been reported to limit fibrosis in other tissue contexts by regulating senescence via RAC1-NOX1. CCN1 induces fibroblast senescence in cutaneous wounds in mice. These cells also secrete antifibrotic SASP factors to prevent excess collagen deposition in the skin36. Interestingly clearing
senescent cells in cutaneous wounds in the p16-3MR mouse model also resulted in excess skin fibrosis, most likely due to the resultant absence of proteases and MMPs30. Exogenous expression of CCN1 in
infarcted hearts in mice resolved fibrosis and rescued heart function, suggesting also an essential role for restricting myocardial fibrosis37.
A breakthrough in regenerative medicine has been the discovery that forced expression of only four factors (now called Yamanaka factors) was sufficient to reprogram differentiated cells into a pluripotent state38. Cellular senescence has been reported to positively influence this process in vivo
by creating a favorable microenvironment for reprogramming to occur. Genetically engineered mice that express the four Yamanaka factors upon doxycycline administration (i4F) display NANOG positive cells and teratomas in several tissues, indicative of successful reprogramming39. Remarkably, this
process does not occur in INK4A/ARF heterozygous mice, suggesting an intact INK4A/ARF locus is required40. Further work has demonstrated that INK4A, rather than ARF, is specifically required for in vivo reprogramming in mice41. SA-β-gal positive cells were found in close proximity to NANOG positive
cells in wild-type i4F mice. These senescent cells promoted reprogramming through secretion of IL6, which functions in a paracrine manner to activate PIM1 and induce cellular plasticity in reprogrammed cells40. Senescent cells also favors reprogramming of tissues which are normally less susceptible to the
process. Inducing tissue damage in skeletal muscle in i4F mice gave rise to senescent muscle cells. Doxycycline treatment post injury resulted in generation of pluripotent muscle stem cells. This did not occur in uninjured mice owing to the absence of a reprogrammable niche42. These findings may help
generate new approaches in regenerative medicine for difficult to reprogram tissues.
As well as mediating transcription of pro-inflammatory SASP components, NF-kB has also been recently discovered to regulate transcription of stemness genes in senescent cells, providing further evidence of their pro-regenerative capabilities. Engraftment of newborn keratinocytes transiently exposed to the SASP from HRASV12-expressing keratinocytes resulted in an increased number of hair follicles and
hair growth in nude mice. Interestingly, when cells were exposed to the SASP for prolonged periods they undergo cell-cycle arrest and paracrine induced senescence, possibly to prevent tumorigenesis in response to persistent regenerative stimuli43.
In contrast to mammals, salamanders have the ability to regenerate a variety of complex body parts including limbs. Senescent cells appear at the regenerating site in a defined spatial-temporal manner. Interference with macrophage mediated clearance of senescent cells, before or during limb
554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet Processed on: 13-1-2021 Processed on: 13-1-2021 Processed on: 13-1-2021
Processed on: 13-1-2021 PDF page: 16PDF page: 16PDF page: 16PDF page: 16
16
amputation, led to complete loss of regenerative capacity. Therefore macrophage-mediated clearance of senescent cells is a critical step for proper limb regeneration in salamanders, although it is not yet known why44.
Senescence has recently been discovered to be involved in embryonic development. SA-β-gal+ cells are
visible in the apical ectodermal ridge (AER), neural roof plate, mesonephros and endolymphatic sac where they contribute to ensure correct patterning and limb formation45,46. Interestingly, embryonic
senescence is p21-dependent but p16- and p53-independent. p21 is instead activated via TGF-β/SMAD and PI3K/FOXO pathways. Embryos that are p21-deficient only showed mild patterning and limb formation defects, suggesting compensatory pathways such as apoptosis exist.
Possible toxicities of senotherapies Senolytics
Cellular senescence limits liver fibrosis by initially halting the proliferation of HSCs to prevent further tissue damage, and by also increasing expression of fibrolytic factors to facilitate repair34. Therefore
there is a risk that patients with unresolved liver fibrosis develop cirrhosis if senolytics are administered systemically. In a similar manner, senolytics could lead to heart failure in patients with unresolved myocardial fibrosis. As cellular senescence also plays beneficial roles in limiting skin fibrosis36 and
promoting cutaneous wound healing30, there is a risk that senolytic administrations could lead to
ineffective tissue repair and exacerbated fibrosis, which should especially be considered if patients have undergone surgery. Local senolytic administrations could be considered to bypass these possible toxicities if possible. For example, UBX0101 can be delivered via intra-articular injection to clear senescent cells in osteoarthritic lesions, and this administration is currently under evaluation in a Phase I clinical trial15. Drug-eluting stents which release senolytic molecules could be considered for
atherosclerosis treatment. As senescent cells are present in eyes of glaucoma patients, intraocular administrations could be utilized to eliminate them47.
Secreted HSP90 is present throughout the time course of acute and diabetic wound healing in the extracellular space where it promotes dermal fibroblast and keratinocyte migration and recruitment48– 50. Inhibition of secreted HSP90 activity completely blocks dermal fibroblasts migration and slowed
down wound healing kinetics in mice48,50. Intriguingly, application of recombinant HSP90 or a 115aa
sized part of HSP90alpha to the wound accelerates wound closure kinetics in mice49,50.Mechanistically,
HSP90 secretion is promoted by Hif-1alpha and TNF alpha. Hif-1alpha is induced by low oxygen supply after wounding50 and can induce TNF-alpha in renal cancers51. Whether Hif-1alpha and TNF-alpha are
causally linked in wound healing is unclear. These data indicate that the elimination of senescent cells
17
by HSP90 inhibitors might lead to impaired wound healing capacity. Topical treatment of peptides that resemble part of the HSP90 protein may be an effective treatment to avoid these negative effects. Senescent cells have been shown to promote in vivo reprogramming in neighboring somatic cells40.
Therefore senescent cells are likely to play important roles in future therapies which utilize techniques in regenerative medicine. Studies show remarkable advances in in vivo reprogramming in mammals. For example, pancreatic exocrine cells could be converted into functional B-cells (care of diabetes)52,
cardiac fibroblasts into cardiomyocytes (treatment after heart infarcts)53 and astrocytes into
proliferative neuroblasts (in neurodegenerative diseases)54 through in vivo reprogramming.
Interestingly, some of these approaches are less efficient in vitro, possible owing to a lack of senescent cells found in vivo. It has already been demonstrated that ABT-263 prevented reprogramming in doxycycline fed i4F mice40,42. In possible future settings, patients who undergo regenerative medicine
should avoid senolytic administrations in order for tissue regeneration to be effective.
Depletion of senescent cells during embryonic development in mice leads to mild patterning and limb formation defects45,46. Therefore, senotherapies should be avoided, similar to most drugs, during
pregnancies to prevent possible developmental abnormalities and miscarriages. Senomorphics
Rapamycin is an approved immunosuppressant drug to prevent acute graft rejection, but complications involving wound healing are associated with its use55. Additionally, accumulating
evidence suggests that mTOR promotes wound healing kinetics in rodents possibly through the SASP56– 58. VEGF (Vascular Endothelial Growth Factor) is a SASP factor that transiently populate cutaneous
wounds and is needed for angiogenesis in wound healing30,59. In rats, rapamycin treatment delayed
wound healing kinetics and reduced the number of VEGF expressing cells compared to placebo treated control rats after incision of the skin56,58. Rapamycin treatment significantly lowered the number of
ECM producing myofibroblasts leading to decreased tension strength of wounds after injury. Epithelial cell specific disruption of PTEN or Tsc1, key suppressors of the PI3K/Akt/mTOR signaling axis, accelerated wound closure57. These data suggest that the use of rapamycin as a SASP modulator to
alleviate age-related phenotypes may be detrimental for wound healing. Local stimulation of mTOR through targeting PTEN or Tsc1 may help to prevent this.
Similar to rapamycin, glucocorticoids also alleviate the SASP through IL1α-NF-kB inhibition22. The
membranous glucocorticoid receptor inhibits keratinocyte migration and wound closure through activation of the Wnt-like phospholipase (PLC)/protein kinase C (PKC) signaling cascade60,61. Activation
of this pathway leads to the induction of known biomarkers for non-healing wounds including β-catenin and c-myc. In addition, genetically modified mice that express a defective form of endogenous
554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet Processed on: 13-1-2021 Processed on: 13-1-2021 Processed on: 13-1-2021
Processed on: 13-1-2021 PDF page: 17PDF page: 17PDF page: 17PDF page: 17
2
16
amputation, led to complete loss of regenerative capacity. Therefore macrophage-mediated clearance of senescent cells is a critical step for proper limb regeneration in salamanders, although it is not yet known why44.
Senescence has recently been discovered to be involved in embryonic development. SA-β-gal+ cells are
visible in the apical ectodermal ridge (AER), neural roof plate, mesonephros and endolymphatic sac where they contribute to ensure correct patterning and limb formation45,46. Interestingly, embryonic
senescence is p21-dependent but p16- and p53-independent. p21 is instead activated via TGF-β/SMAD and PI3K/FOXO pathways. Embryos that are p21-deficient only showed mild patterning and limb formation defects, suggesting compensatory pathways such as apoptosis exist.
Possible toxicities of senotherapies Senolytics
Cellular senescence limits liver fibrosis by initially halting the proliferation of HSCs to prevent further tissue damage, and by also increasing expression of fibrolytic factors to facilitate repair34. Therefore
there is a risk that patients with unresolved liver fibrosis develop cirrhosis if senolytics are administered systemically. In a similar manner, senolytics could lead to heart failure in patients with unresolved myocardial fibrosis. As cellular senescence also plays beneficial roles in limiting skin fibrosis36 and
promoting cutaneous wound healing30, there is a risk that senolytic administrations could lead to
ineffective tissue repair and exacerbated fibrosis, which should especially be considered if patients have undergone surgery. Local senolytic administrations could be considered to bypass these possible toxicities if possible. For example, UBX0101 can be delivered via intra-articular injection to clear senescent cells in osteoarthritic lesions, and this administration is currently under evaluation in a Phase I clinical trial15. Drug-eluting stents which release senolytic molecules could be considered for
atherosclerosis treatment. As senescent cells are present in eyes of glaucoma patients, intraocular administrations could be utilized to eliminate them47.
Secreted HSP90 is present throughout the time course of acute and diabetic wound healing in the extracellular space where it promotes dermal fibroblast and keratinocyte migration and recruitment48– 50. Inhibition of secreted HSP90 activity completely blocks dermal fibroblasts migration and slowed
down wound healing kinetics in mice48,50. Intriguingly, application of recombinant HSP90 or a 115aa
sized part of HSP90alpha to the wound accelerates wound closure kinetics in mice49,50.Mechanistically,
HSP90 secretion is promoted by Hif-1alpha and TNF alpha. Hif-1alpha is induced by low oxygen supply after wounding50 and can induce TNF-alpha in renal cancers51. Whether Hif-1alpha and TNF-alpha are
causally linked in wound healing is unclear. These data indicate that the elimination of senescent cells
17
by HSP90 inhibitors might lead to impaired wound healing capacity. Topical treatment of peptides that resemble part of the HSP90 protein may be an effective treatment to avoid these negative effects. Senescent cells have been shown to promote in vivo reprogramming in neighboring somatic cells40.
Therefore senescent cells are likely to play important roles in future therapies which utilize techniques in regenerative medicine. Studies show remarkable advances in in vivo reprogramming in mammals. For example, pancreatic exocrine cells could be converted into functional B-cells (care of diabetes)52,
cardiac fibroblasts into cardiomyocytes (treatment after heart infarcts)53 and astrocytes into
proliferative neuroblasts (in neurodegenerative diseases)54 through in vivo reprogramming.
Interestingly, some of these approaches are less efficient in vitro, possible owing to a lack of senescent cells found in vivo. It has already been demonstrated that ABT-263 prevented reprogramming in doxycycline fed i4F mice40,42. In possible future settings, patients who undergo regenerative medicine
should avoid senolytic administrations in order for tissue regeneration to be effective.
Depletion of senescent cells during embryonic development in mice leads to mild patterning and limb formation defects45,46. Therefore, senotherapies should be avoided, similar to most drugs, during
pregnancies to prevent possible developmental abnormalities and miscarriages. Senomorphics
Rapamycin is an approved immunosuppressant drug to prevent acute graft rejection, but complications involving wound healing are associated with its use55. Additionally, accumulating
evidence suggests that mTOR promotes wound healing kinetics in rodents possibly through the SASP56– 58. VEGF (Vascular Endothelial Growth Factor) is a SASP factor that transiently populate cutaneous
wounds and is needed for angiogenesis in wound healing30,59. In rats, rapamycin treatment delayed
wound healing kinetics and reduced the number of VEGF expressing cells compared to placebo treated control rats after incision of the skin56,58. Rapamycin treatment significantly lowered the number of
ECM producing myofibroblasts leading to decreased tension strength of wounds after injury. Epithelial cell specific disruption of PTEN or Tsc1, key suppressors of the PI3K/Akt/mTOR signaling axis, accelerated wound closure57. These data suggest that the use of rapamycin as a SASP modulator to
alleviate age-related phenotypes may be detrimental for wound healing. Local stimulation of mTOR through targeting PTEN or Tsc1 may help to prevent this.
Similar to rapamycin, glucocorticoids also alleviate the SASP through IL1α-NF-kB inhibition22. The
membranous glucocorticoid receptor inhibits keratinocyte migration and wound closure through activation of the Wnt-like phospholipase (PLC)/protein kinase C (PKC) signaling cascade60,61. Activation
of this pathway leads to the induction of known biomarkers for non-healing wounds including β-catenin and c-myc. In addition, genetically modified mice that express a defective form of endogenous
554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet Processed on: 13-1-2021 Processed on: 13-1-2021 Processed on: 13-1-2021
Processed on: 13-1-2021 PDF page: 18PDF page: 18PDF page: 18PDF page: 18
18
glucocorticoids showed remarkably enlarged granulation tissue after cutaneous wounding62. These
data show the use of glucocorticoids should be avoided during tissue repair, limiting the potential use of glucocorticoids to alleviate age-related disorders.
Accumulating evidence suggest that the MAPK pathway is important for wound healing. Mitogen-activated protein kinase (MEK) kinase 1 (MEKK1) is expressed in cutaneous wounds and is required for ECM homeostasis through MMP expression, epithelial cell migration and wound epithelialization. Interference with MEKK1 during cutaneous wound healing significantly slowed down the wound healing rate63. Specific inhibition of MAPK by SB203580 and subsequent reduction of ERK-1 activity led
to decreased epithelial cell proliferation and migration, resulting in delayed corneal wound closure64.
These data suggest that inhibition of the MAPK pathway to alleviate the SASP may impact on wound healing.
Systemic metformin administration in rats decreases wound closure rates through inhibition of keratinocyte proliferation65. Wounds of metformin treated rats showed significantly more redness and
scarring, indicating inhibitory effects of metformin on tissue remodeling. Metformin is frequently used as an antidiabetic drug. Diabetic patients treated with metformin have foot ulcers with significantly bigger diameters. Contrarily, local administration of metformin in the wound beds of rats increased wound vascularization through activation of AMPK and subsequent expression of VEGF66. This suggests
that metformin act on different phases of wound healing and the effect of metformin is dose-dependent. Thus, the systemic chronic intake of metformin to alleviate SASP mediated detrimental effects on health may have a negative impact on tissue repair and remodeling in the skin.
Caloric restriction in rodents show decreased wound healing rates, ablated synthesis of collagen and decreased number of endothelial cells and fibroblasts in the wound compared to refed control animals possibly due to impaired IGF-1 signaling67,68. This suggests that caloric restriction should be avoided
during wound healing which limits the applicability of this intervention to alleviate aging. Whether caloric restriction influences wound healing through senescent cells remains to be investigated. A major challenge in the senescence field is the lack of a uniform definition of senescence supported by specific molecular markers1. A combination of individually non-specific markers should be used to
define senescence and to evaluate the efficacy of senotherapies1,69. Currently, the
senescence-associated markers used for testing senotherapies are often variable and inconsistent, making cross-validations between different labs and the comparison among different compounds extremely challenging (Table 1).
19 Table 1.
Type Compound/treatment Model Marker Cell type/Tissue References
Senolytic
ABT 263 In vitro: Ionizing
Radiation, Chemotherapy In vivo: ionizing Radiation, chemotherapy, Atherosclerosis, Tau dependent neurodegenerative disease (MAPTp301S PS19) SA-β-gal, p16, p21, p19,SASP BCL-2, BCL-xL, BAK, BAX , In vitro: WI38 (lung), human renal epithelial cells (REC),Mouse embryonic fibroblasts (MEF) In vivo: hematopoietic system, atherosclerotic plaques, brain cortex, hippocampus. Chang et al., 2016 Demaria et al., 2016, Childs et al., 2016 Bussian, T.J., 2018
ABT 737 In vitro: ionizing
radiation, Oncogene induced senescence, replicative exhaustion In vivo: skin senescence (K5-rtTA/tet-p14), ionizing radiation SA-β-gal, p16, p21, p53, Ƴ-H2AX, BCL-W In vitro: IMR90 (Lung),
In vivo: Lung, skin
Yosef, et al., 2015
D + Q In vitro: ionizing
radiation
In vivo: progeria (ERCC
-/Δ), ionizing radiation,
natural aging, obesity
SA-β-gal, p16, p21, Telomere associated DNA damage foci (TAFS), SASP, BCL-XL, In vitro: Pre-adipocytes, HUVEC vascular endothelial cells, In vivo: Adipose tissue Ex vivo: Human adipose tissues Zhu et al., 2015 Xu et al, 2018 Orgodnik, M. et al., 2019
17-DMAG In vitro: Oxidative
stress
In vivo: Progeria model (ERCC -/Δ) SA-β-gal,p16, p21, SASP, Ƴ-H2AX, In vitro: Mouse embryonic fibroblasts (MEF), Mesenchymal stem cells (MSC), IMR90(Lung), Wi38 (Lung), Human umbilical vein endothelial cell (HUVEC) In vivo: kidney, liver Fuhrmann-Stroissnigg, H., et al., 2017
Foxo4 -DRI In vitro: ionizing
radiation, chemotherapy, In vivo: progeria (XPDTTD/TTD), natural aging, chemotherapy SA-β-gal, p16,
SASP, LMNB1 In vitro: IMR90(Lung)
In vivo: Kidney
Baar, M.P., et al., 2017
UBX0101 In vivo: Post-traumatic
osteoarthritis (OA) SA-β-gal, p16, p21, SASP, Ki67, HMBGB1 In vivo: synovium and cartilage of the joint Ex vivo: Osteoarthritis Chondrocytes Jeon O.H., et al., 2017
554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet Processed on: 13-1-2021 Processed on: 13-1-2021 Processed on: 13-1-2021
Processed on: 13-1-2021 PDF page: 19PDF page: 19PDF page: 19PDF page: 19
2
18
glucocorticoids showed remarkably enlarged granulation tissue after cutaneous wounding62. These
data show the use of glucocorticoids should be avoided during tissue repair, limiting the potential use of glucocorticoids to alleviate age-related disorders.
Accumulating evidence suggest that the MAPK pathway is important for wound healing. Mitogen-activated protein kinase (MEK) kinase 1 (MEKK1) is expressed in cutaneous wounds and is required for ECM homeostasis through MMP expression, epithelial cell migration and wound epithelialization. Interference with MEKK1 during cutaneous wound healing significantly slowed down the wound healing rate63. Specific inhibition of MAPK by SB203580 and subsequent reduction of ERK-1 activity led
to decreased epithelial cell proliferation and migration, resulting in delayed corneal wound closure64.
These data suggest that inhibition of the MAPK pathway to alleviate the SASP may impact on wound healing.
Systemic metformin administration in rats decreases wound closure rates through inhibition of keratinocyte proliferation65. Wounds of metformin treated rats showed significantly more redness and
scarring, indicating inhibitory effects of metformin on tissue remodeling. Metformin is frequently used as an antidiabetic drug. Diabetic patients treated with metformin have foot ulcers with significantly bigger diameters. Contrarily, local administration of metformin in the wound beds of rats increased wound vascularization through activation of AMPK and subsequent expression of VEGF66. This suggests
that metformin act on different phases of wound healing and the effect of metformin is dose-dependent. Thus, the systemic chronic intake of metformin to alleviate SASP mediated detrimental effects on health may have a negative impact on tissue repair and remodeling in the skin.
Caloric restriction in rodents show decreased wound healing rates, ablated synthesis of collagen and decreased number of endothelial cells and fibroblasts in the wound compared to refed control animals possibly due to impaired IGF-1 signaling67,68. This suggests that caloric restriction should be avoided
during wound healing which limits the applicability of this intervention to alleviate aging. Whether caloric restriction influences wound healing through senescent cells remains to be investigated. A major challenge in the senescence field is the lack of a uniform definition of senescence supported by specific molecular markers1. A combination of individually non-specific markers should be used to
define senescence and to evaluate the efficacy of senotherapies1,69. Currently, the
senescence-associated markers used for testing senotherapies are often variable and inconsistent, making cross-validations between different labs and the comparison among different compounds extremely challenging (Table 1).
19 Table 1.
Type Compound/treatment Model Marker Cell type/Tissue References
Senolytic
ABT 263 In vitro: Ionizing
Radiation, Chemotherapy In vivo: ionizing Radiation, chemotherapy, Atherosclerosis, Tau dependent neurodegenerative disease (MAPTp301S PS19) SA-β-gal, p16, p21, p19,SASP BCL-2, BCL-xL, BAK, BAX , In vitro: WI38 (lung), human renal epithelial cells (REC),Mouse embryonic fibroblasts (MEF) In vivo: hematopoietic system, atherosclerotic plaques, brain cortex, hippocampus. Chang et al., 2016 Demaria et al., 2016, Childs et al., 2016 Bussian, T.J., 2018
ABT 737 In vitro: ionizing
radiation, Oncogene induced senescence, replicative exhaustion In vivo: skin senescence (K5-rtTA/tet-p14), ionizing radiation SA-β-gal, p16, p21, p53, Ƴ-H2AX, BCL-W In vitro: IMR90 (Lung),
In vivo: Lung, skin
Yosef, et al., 2015
D + Q In vitro: ionizing
radiation
In vivo: progeria (ERCC
-/Δ), ionizing radiation,
natural aging, obesity
SA-β-gal, p16, p21, Telomere associated DNA damage foci (TAFS), SASP, BCL-XL, In vitro: Pre-adipocytes, HUVEC vascular endothelial cells, In vivo: Adipose tissue Ex vivo: Human adipose tissues Zhu et al., 2015 Xu et al, 2018 Orgodnik, M. et al., 2019
17-DMAG In vitro: Oxidative
stress
In vivo: Progeria model (ERCC -/Δ) SA-β-gal,p16, p21, SASP, Ƴ-H2AX, In vitro: Mouse embryonic fibroblasts (MEF), Mesenchymal stem cells (MSC), IMR90(Lung), Wi38 (Lung), Human umbilical vein endothelial cell (HUVEC) In vivo: kidney, liver Fuhrmann-Stroissnigg, H., et al., 2017
Foxo4 -DRI In vitro: ionizing
radiation, chemotherapy, In vivo: progeria (XPDTTD/TTD), natural aging, chemotherapy SA-β-gal, p16,
SASP, LMNB1 In vitro: IMR90(Lung)
In vivo: Kidney
Baar, M.P., et al., 2017
UBX0101 In vivo: Post-traumatic
osteoarthritis (OA) SA-β-gal, p16, p21, SASP, Ki67, HMBGB1 In vivo: synovium and cartilage of the joint Ex vivo: Osteoarthritis Chondrocytes Jeon O.H., et al., 2017
554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet Processed on: 13-1-2021 Processed on: 13-1-2021 Processed on: 13-1-2021
Processed on: 13-1-2021 PDF page: 20PDF page: 20PDF page: 20PDF page: 20
20
Senomorphics
Rapamycin In vitro: Ionizing
radiation, chemotherapy, oncogenic activation, replicative exhaustion In vivo: Senescence promoted tumor growth SA-β-gal, SASP,NF-κB, BrdU, In vitro: HCA2 (skin), PSC27 (human prostate), PC3 (Prostate cancer) Laberge, R.M., et al., 2015 MAPK Inhibitors (SB203580, UR13756,BIRB 796) In vitro: ionizing radiation, oncogenic activation SA-β-gal, SASP, NF-κB, BrdU.
HCA2 (skin) Freund, A., et al., 2011 Alimbetov, D., et al., 2016
Glucocorticoids In vitro: ionizing
radiation SA-β-gal, SASP, NF-κB, HCA2 (Skin) Laberge, R-M., et al., 2012
Caloric restriction In vivo: caloric
restriction p16,p21, SASP In vivo: colon Fontana, L., et al., 2018 Caloric restriction mimetics (resveratrol, metformin, Kaempferol and apigenin) In vitro: ionizing radiation, chemotherapy, oncogenic activation
In vivo: natural aging
SA-β-gal, SASP, NF-κB, Ki67 In vitro: IMR90 (lung), Mouse embryonic fibroblasts (MEF), Macrophages BJ (Skin). In vivo: kidney Mooiseeva O.et al., 2013 Hayakawa, T., et al., 2015 Lim, H., et al., 2015 4) Concluding remarks
Accumulating evidence finds detrimental roles for senescent cells in many age-associated disorders. Senotherapies are attractive options to alleviate a number of these diseases at the same time, especially with an ever increasing aging population. However, as senescent cells have been shown to also influence beneficial physiological roles, care must be taken with possible side effects. These can include delayed wound healing post-surgery, cirrhosis in patients with liver fibrosis or developmental abnormalities in embryos. No study to our knowledge has investigated whether senotherapies do negatively affect these outcomes, especially ones focused on the impact on senescent cells. Local senolytic/senomorphic administrations could counter these possible effects, although this may depend on disease context and on compound chemistry. Future studies addressing whether local administrations are effective at alleviating disease burden and not associated with toxic side effects should be performed.
Moreover, more studies should be performed to identify whether it is possible to discriminate between detrimental and beneficial senescence programs. This understanding could provide novel targets to develop therapeutic approaches aimed at interfering solely with deleterious senescent cells. Finally, head-to-head comparisons of senotherapies in different disease models should be done to provide insights in compound-specific toxicities. In the case of systemic application, the identification of delivering systems to preferentially target senescent cells and/or specific types of senescent cells might reduce off-target effects of senotherapies, an approach which has been recently shown to be possible70
21
Figure 1: Beneficial effects of senescent cells during tissue repair and potential detrimental effects of senotherapies.
References
1. Hernandez-Segura,A., Nehme, J., Demaria, M. Hallmarks of Cellular senescence. Trends in Cellbiology 28, 436-453 (2018).
2. Baker, D.J., Wijshake, T., Tchkonia, T., LeBrasseur, N.K., Childs, B.G., van de Sluis B., Kirkland, J.L., van Deursen, J. M. Clearance of p16ink4a-positive senescent cells delays ageing-associated disorders. Nature 479, 232–236 (2011).
3. Xu, M., et al. JAK inhibition alleviates the cellular senescence-associated secretory phenotype and frailty in old age. PNAS 112, 6301–6310 (2015).
4. Soto-Gamez, A., Demaria, M. Therapeutic interventions for aging: the case of cellular senescence. Drug Discov. Today 22, 786–795 (2017).
5. Yosef, R., et al. Directed elimination of senescent cells by inhibition of BCL-W and BCL-XL. Nat. Commun. 7, 11190 (2015).
6. Chang, J., et al. Clearance of senescent cells by ABT 263 rejuvenates aged hematopoetic stem cells in mice. Nat. Med. 22, 78–83 (2016).
7. Demaria, M. et al. Cellular senescence promotes adverse effects of chemotherapy and cancer relapse. Cancer Discov. 7, 165–176 (2017).
554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet Processed on: 13-1-2021 Processed on: 13-1-2021 Processed on: 13-1-2021
Processed on: 13-1-2021 PDF page: 21PDF page: 21PDF page: 21PDF page: 21
2
20
Senomorphics
Rapamycin In vitro: Ionizing
radiation, chemotherapy, oncogenic activation, replicative exhaustion In vivo: Senescence promoted tumor growth SA-β-gal, SASP,NF-κB, BrdU, In vitro: HCA2 (skin), PSC27 (human prostate), PC3 (Prostate cancer) Laberge, R.M., et al., 2015 MAPK Inhibitors (SB203580, UR13756,BIRB 796) In vitro: ionizing radiation, oncogenic activation SA-β-gal, SASP, NF-κB, BrdU.
HCA2 (skin) Freund, A., et al., 2011 Alimbetov, D., et al., 2016
Glucocorticoids In vitro: ionizing
radiation SA-β-gal, SASP, NF-κB, HCA2 (Skin) Laberge, R-M., et al., 2012
Caloric restriction In vivo: caloric
restriction p16,p21, SASP In vivo: colon Fontana, L., et al., 2018 Caloric restriction mimetics (resveratrol, metformin, Kaempferol and apigenin) In vitro: ionizing radiation, chemotherapy, oncogenic activation
In vivo: natural aging
SA-β-gal, SASP, NF-κB, Ki67 In vitro: IMR90 (lung), Mouse embryonic fibroblasts (MEF), Macrophages BJ (Skin). In vivo: kidney Mooiseeva O.et al., 2013 Hayakawa, T., et al., 2015 Lim, H., et al., 2015 4) Concluding remarks
Accumulating evidence finds detrimental roles for senescent cells in many age-associated disorders. Senotherapies are attractive options to alleviate a number of these diseases at the same time, especially with an ever increasing aging population. However, as senescent cells have been shown to also influence beneficial physiological roles, care must be taken with possible side effects. These can include delayed wound healing post-surgery, cirrhosis in patients with liver fibrosis or developmental abnormalities in embryos. No study to our knowledge has investigated whether senotherapies do negatively affect these outcomes, especially ones focused on the impact on senescent cells. Local senolytic/senomorphic administrations could counter these possible effects, although this may depend on disease context and on compound chemistry. Future studies addressing whether local administrations are effective at alleviating disease burden and not associated with toxic side effects should be performed.
Moreover, more studies should be performed to identify whether it is possible to discriminate between detrimental and beneficial senescence programs. This understanding could provide novel targets to develop therapeutic approaches aimed at interfering solely with deleterious senescent cells. Finally, head-to-head comparisons of senotherapies in different disease models should be done to provide insights in compound-specific toxicities. In the case of systemic application, the identification of delivering systems to preferentially target senescent cells and/or specific types of senescent cells might reduce off-target effects of senotherapies, an approach which has been recently shown to be possible70
21
Figure 1: Beneficial effects of senescent cells during tissue repair and potential detrimental effects of senotherapies.
References
1. Hernandez-Segura,A., Nehme, J., Demaria, M. Hallmarks of Cellular senescence. Trends in Cellbiology 28, 436-453 (2018).
2. Baker, D.J., Wijshake, T., Tchkonia, T., LeBrasseur, N.K., Childs, B.G., van de Sluis B., Kirkland, J.L., van Deursen, J. M. Clearance of p16ink4a-positive senescent cells delays ageing-associated disorders. Nature 479, 232–236 (2011).
3. Xu, M., et al. JAK inhibition alleviates the cellular senescence-associated secretory phenotype and frailty in old age. PNAS 112, 6301–6310 (2015).
4. Soto-Gamez, A., Demaria, M. Therapeutic interventions for aging: the case of cellular senescence. Drug Discov. Today 22, 786–795 (2017).
5. Yosef, R., et al. Directed elimination of senescent cells by inhibition of BCL-W and BCL-XL. Nat. Commun. 7, 11190 (2015).
6. Chang, J., et al. Clearance of senescent cells by ABT 263 rejuvenates aged hematopoetic stem cells in mice. Nat. Med. 22, 78–83 (2016).
7. Demaria, M. et al. Cellular senescence promotes adverse effects of chemotherapy and cancer relapse. Cancer Discov. 7, 165–176 (2017).
554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet 554157-L-bw-Vliet Processed on: 13-1-2021 Processed on: 13-1-2021 Processed on: 13-1-2021
Processed on: 13-1-2021 PDF page: 22PDF page: 22PDF page: 22PDF page: 22
22 Science (80-. ). 354, 472–477 (2016).
9. Bussian, T. J. Clearance of senescent glial cells prevents tau-dependent pathology and cognitive decline. Nature 562, 578–582.
10. Zhu, Y. et al. The Achilles’ heel of senescent cells: from transcriptome to senolytic drugs. Aging Cell 14, 644–658 (2015).
11. Xu, M., et al. Senolytics improve physical function and increase lifespan in old age. Nat. Med. 24, 1246–1256 (2018).
12. Ogrodnik et al. Obesity-Induced Cellular Senescence Drives Anxiety and Impairs Neurogenesis. Cell Metab. 29, 1–17 (2019).
13. Fuhrmann-stroissnigg, H. Identification of HSP90 inhibitors as a novel class of senolytics. Nat. Commun. 8, 422.
14. Baar, M., et al. Targeted Apoptosis of Senescent Cells Restores Tissue Homeostasis in Response to Chemotoxicity and Aging. Cell 169, 132–147 (2017).
15. Jeon, O. H. Local clearance of senescent cells attenuates the development of post-traumatic osteoarthritis and creates a pro-regenerative environment. Nat. Cell Biol. 23, 775–781 (2017). 16. Chien, Y. Control of the senescence-associated secretory phenotype by NF-κB promotes
senescence and enhances chemosensitivity. Genes Dev. 25, 2125–2136. (2011).
17. Laberge, R.-M. et al. MTOR regulates the pro-tumorigenic senescence-associated secretory phenotype by promoting IL1A translation. Nat. Cell Biol. 27, 1049–1061 (2015).
18. Freund, A. et al. p38MAPK is a novel DNA damage response-independent regulator of the senescence-associated secretory phenotype. EMBO J. 30, 1536–1548 (2011).
19. Herranz., N. et al. mTOR regulates MAPKAPK2 translation to control the senescence-associated secretory phenotype. Nat. Cell Biol. 17, 1205–1207 (2015).
20. Harrison, D. E. Rapamycin fed late in life extends lifespan in genetically heterogeneous mice. Nature 460, 392–395.
21. Alimbetov, D., et al. Suppression of the senescence-associated secretory phenotype (SASP) in human fibroblasts using small molecule inhibitors of p38 MAP kinase and MK2.
Biogerontology 17, 305–315 (2016).
22. Laberge, R.-M. et al. Glucocorticoids suppress selected components ofthe senescence-associated secretory phenotype. Aging Cell 11, 569–578 (2012).
23. Fontana, L., Partridge, L. Promoting Health and Longevity through Diet: from Model Organisms to Humans. Cell 161, 106–118 (2015).
24. Fontana, L., et al. The effects of graded caloric restriction: XII. Comparison ofmouse to human impact on cellular senescence in the colon. Aging Cell e12746. (2018).
25. Martin-Montalvo, A. Metformin improves healthspan and lifespan in mice. Nat. Commun. 4, 2192 (2013).
26. Mooiseeva, O. Metformin inhibits the senescence-associated secretoryphenotype by interfering with IKK/NF-jB activation. Aging Cell 12, 489–498 (2013).
23
27. Hayakawa, T. et al. SIRT1 Suppresses the Senescence-Associated Secretory Phenotype through Epigenetic Gene Regulation. PLoS One 10, e0116480 (2015).
28. Lim, H. Effects of flavonoids on senescence-associated secretory phenotype formation from bleomycin-induced senescence in BJ fibroblasts. Biochem. Pharmacol. 96, 337–348 (2015). 29. Law, B. K. Rapamycin: an anti-cancer immunosuppressant? Crit. Rev. Oncol. Hematol. 56, 47–
60.
30. Demaria, M., Ohtani, N., Youssef, S.A., Rodier, F., Tousssaint, W., Mitchell, J.R., Laberge, R.M., Vijg, J., van Steeg, H., Dolle, M.E.T., Hoeijmakers, J.H.J., de Bruin, A., Hara, E., Campisi, J. An essential role for senescent cells in optimal woundhealing throuhg secretion of PDGF-AA. Dev. Cell 31, 722–733 (2014).
31. D.J., Baker, et al. Naturally occurring p16Ink4a-positive cells shorten healthy lifespan. Nature 530, 184–189 (2016).
32. van de Berg, J. S. Cultured pressure ulcer fibroblasts show replicative senescence with elevated production of plasmin, plasminogen activator inhibitor-1, and transforming growth factor-b1. Wound repair Regen. 13, 76–83 (2003).
33. Mendez, M.V., et al. Fibroblasts cultured from venous ulcers display cellular characteristics of senescence. J. Vasc. Surg. 28, 876–883 (1998).
34. Krizhanovsky, V. et al. Senescence of Activated Stellate Cells Limits Liver Fibrosis. Cell 134, 657–667 (2008).
35. Kim, K.-H., Chen, C.-C., Monzon, R. I. & Lau, L. F. Matricellular Protein CCN1 Promotes Regression of Liver Fibrosis through Induction of Cellular Senescence in Hepatic Myofibroblasts. Mol. Cell. Biol. 33, 2078–2090 (2013).
36. Jun, J.-I. & Lau, L. F. The matricellular protein CCN1 induces fibroblast senescence and restricts fibrosis in cutaneous wound healing. Nat. Cell Biol. 12, 676–685 (2010).
37. Meyer, K., Hodwin, B., Ramanujam, D., Engelhardt, S. & Sarikas, A. Essential Role for
Premature Senescence of Myofibroblasts in Myocardial Fibrosis. J. Am. Coll. Cardiol. 67, 2018– 2028 (2016).
38. Takahashi, K. & Yamanaka, S. Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors. Cell 126, 663–76 (2006).
39. Abad, M. et al. Reprogramming in vivoproduces teratomas and iPS cells with totipotency features. Nature 502, 540–545 (2013).
40. Mosteiro, L. et al. Tissue damage and senescence provide critical signals for cellular reprogramming in vivo. Science (80-. ). 354, aaf4445 (2016).
41. Mostreiro, L. Senescence promotes in vivo reprogramming through p16 INK 4a and IL-6. Aging Cell 7, e12711 (2017).
42. Chiche, A. et al. Injury-Induced Senescence Enables In Vivo Reprogramming in Skeletal Muscle. Cell Stem Cell 20, 407-414.e4 (2017).
43. Ritschka, B., Storer, M., Mas, A., Heinzmann, F., Ortells, M.C., Morton, J.P., Sansom, O.J., Zender, L., Keyes, W. M. The Senescence-associated secretory phenotype induces cellular plasticity adn tissue regeneration. Genes Dev. 31, 172–183 (2017).
