Cellular senescence (CS) is one of hallmarks of aging and accumulation of senescent cells (SCs) with age contributes to tissue or organismal aging, as well as the pathophysiologies of diverse age-related diseases (ARDs). Genetic ablation of SCs in tissues lengthened health span and reduced the risk of age-related pathologies in a mouse model, suggesting a direct link between SCs, longevity, and ARDs. Therefore, senotherapeutics, medicines targeting SCs, might be an emerging strategy for the extension of health span, and prevention or treatment of ARDs. Senotherapeutics are classified as senolytics which kills SCs selectively; senomorphics which modulate functions and morphology of SCs to those of young cells, or delays the progression of young cells to SCs in tissues; and immune-system mediators of the clearance of SCs. Some senolytics and senomorphics have been proven to markedly prevent or treat ARDs in animal models. This review will present the current status of the development of senotherapeutics, in relation to aging itself and ARDs. Finally, future directions and opportunities for senotherapeutics use will discussed. This knowledge will provide information that can be used to develop novel senotherapeutics for health span and ARDs.
Cellular senescence (CS) is initially defined as an irreversible growth arrest of normal somatic cells, and has been proposed to contribute to tissue and organismal aging itself, and to be an intrinsic safeguard against tumor progression (1). SCs have been shown to have diverse phenotypes, including cellular flattening and hypertrophy, senescence-associated β-galactosidase activity (SAβG), senescence-associated secretory phenotype (SASP), resistance to apoptotic cell death, alterations in nuclear structure and senescence-associated heterochromatic foci (SAHF), mitochondrial expansion, and signaling events, including upregulation of cell cycle inhibitors and -pro-survival effectors (2). SCs accumulated in various tissues in mice with age, and comprised 5–40% of the total cells, depending on the tissue types (3). Although their number are relatively small in tissues, SCs can cause extensive dysfunction of the microenvironment and damage to surrounding cells and tissues, due to their pro-inflammatory senescence-associated secretory phenotype (SASP). Accumulating evidence suggests that CS plays an important role in diverse biological processes, such as embryonic development, diabetes, host immunity, wound healing, tissue renewal, as well as fibrosis, cardiovascular diseases, and cancer (4).
Since aging is one of major risk factors of human diseases, a variety of aging interventions have been developed to extend health span and to prevent or treat ARDs in
Although CS is one of hallmarks of aging (17), and accumulation of SCs with age has been suggested to be associated with aging and ARDs (18), direct evidence of a causal relationship between CS and aging or ARDs has only recently been validated in rodent models. Furthermore, senotherapeutics, have been implicated as novel strategies for aging intervention in applications designed to extend healthy aging and to prevent or treat ARDs.
Since elimination of SCs using genetic approaches mitigated aging and ARDs, pharmacological intervention targeting SCs, named as senotherapeutics, has been proposed. Senotherapeutics are classified as senolytics, which selectively kill SCs; senomorphics which modulate SCs by blocking SASP; and senoinflammation, the immune system-mediated clearance of SCs (Fig. 1).
In 2015, Zhu
Until now, seven classes of senolytics have been reported, including kinase inhibitors, a Bcl-2 family inhibitors, natural compounds, a p53 binding inhibitor, heat shock protein 90 (HSP90) inhibitors, UBX0101, and a histone deacetylase (HDAC) inhibitor (30) (Table 1). The combined administration of dasatinib and quercetin improved the pathologies of diverse ARDs, including cardiac aging (29), atherosclerosis (31), osteoporosis (23), pulmonary fibrosis (32), hepatic steatosis (22), and Alzheimer’s disease (24) by clearing SCs in the tissues. Recently, the combination of dasatinib and quercetin was reported to enhance physical function and to lengthen health span and lifespan in old mice (28).
Resistance of SCs to apoptotic cell death, due to upregulation of Bcl-2 and Bcl-xL suggested inhibitors of these anti-apoptotic proteins as useful candidates for senolytics. In this regard, ABT-263, ABT-737, A1331852, and A1155463 were reported as senolytic candidates in
Inhibitors of HSP90, 17-N-allylamino-17-demethoxygeldanamycin (17-AAG) and geldanamycin, were suggested as a new class of senolytic candidates from the screening of 97 autophagy regulators in oxidative stress-induced senescent
In addition to quercetin, natural phytochemicals, piperlongumine (PL) and fisetin, were reported to have senolytic activities in
UBX0101, a small molecule inhibitor of the MDM2/p53 protein interaction, was revealed to be a potent senolytic candidate (26). Intra-articular injection of UBX0101 eliminated SCs accumulated in the articular cartilage and synovium, and resulted in attenuation of the development of post-traumatic osteoarthritis (OA) in aged mice (26). UBX0101 also cleared SCs by inducing apoptosis and improved the cartilage-forming ability of chondrocytes from human OA tissue (26). UBX0101 is the first senolytic under Phase 1 clinical trial in patients with OA of the knee (https://clinicaltrials.gov/ct2/show/NCT03513016).
In addition to primary cells, senolytics targeting cancer cells have been also proposed. Panobinostat, an FDA approved HDAC inhibitor, was identified to have senolytic activity in chemotherapy-induced senescence of cancer cells, non-small cell lung cancer, and head and neck squamous cell carcinoma cell lines (41). During standard chemotherapy, senescence-associated events, such as decreased histone acetylation, enhancement of Bcl-2 family proteins, and SASP production occurred in cancer cells (42, 43). The latter events are a biomarker of senescence and a target of senotherapeutics. Chemotherapy-induced senescence has reported to be involved in the increased invasion of cancer cells, due to epithelial-to-mesenchymal transition in thyroid cancer (42), as well as acquisition of stem-cell-related properties in B-cell lymphoma (43). Thus, senolytics targeting senescent cancer cells might be a new strategy for treatment cancer metastasis and cancer stemness.
Forkhead box protein O4 (FOXO4) was identified as a pivotal protein involved in the viability of SCs. Using bioinformatic analysis of RNA sequence data from IR-induced senescent IMR90 cells, FOXO4-D-retro-inverso (FOXO4-DRI) peptide was demonstrated to be a new class of senolytic candidates (44). FOXO4-DRI peptide blocked the interaction of FOXO4 and p53 in senescent cells, which induced apoptosis of senescent IMR90 cells and HUVECs. Administration of the peptide improved fitness, hair density, and kidney function in both
Based on the countable lists of senolytics candidates, research on senolytics is still in its infancy, but growing rapidly. Since most senolytics target known proteins, such as p53, a tyrosine kinase, HSP90, Bcl-2, and Bcl-xL, elucidation of novel targets specific to CS is necessary for further development of senolytics. In addition, senolytics demonstrated cell-specific effects in killing SCs, suggesting CS might be differentially regulated depending on cell or tissue-specific factors. Further unbiased exploration of senolytics using chemical libraries or drug repositioning might contribute to the development of novel senolytics, as well as to the elucidation of novel mechanisms of CS.
Senomorphics is a wide range of agents that can modulate the phenotypes of SCs to those of young cells through interfering with senoinflammation/inflammaging, senescence-related signal pathways, and SASP, without induction of SC apoptosis (Table 2). Senomorphics include previous anti-aging or anti-senescence compounds, such as telomerase activator (45), CRMs (7), caloric restriction diets (8), sirtuin activators (46), mTOR inhibitors (47), antioxidants (48), anti-inflammatory agents targeting senoinflammation or inflammaging (49), autophagy activators (12), and proteasome activators (50). This review will not cover the aforementioned anti-aging or anti-senescence agents as senomophics. Instead, we have included recent results on agents exhibiting senomorphic activity, based on selective markers of cellular senescence, such as SASP and SAβG in
Senoinflammation or inflammaging refers to chronic, sterile, low-grade, and unresolved inflammation characterized upon aging (51, 52). Nuclear factor-kappa B (NF-κB) activated in aging and ARDs, is a key transcription factor mediating senoinflammation or inflammaging and its activation has been linked to known regulators of the aging process, such as insulin/insulin-like growth factor (IGF)-1, FOXO, SIRT, mTOR and DNA damage (53). Therefore, inhibition of NF-κB has been suggested as a potential target of senomorphics. A peptide inhibitor of IKK, the NF-κB–activating kinase, reduced cellular senescence
Ataxia-telangiectasia mutated (ATM) kinase, a serine/threonine protein kinase activated by DNA double strand breaks, functions in the regulation of CS (2). Therefore, ATM kinase inhibitor, KU-60019, was identified as a potential senomorphic candidate. KU-60019 induced the functional recovery of the lysosome/autophagy system, coupled with mitochondrial functional recovery and metabolic reprogramming (56).
Progerin, a truncated form of lamin A protein involved in Hutchitson-Gilford progeria syndrome (HGPS), accumulated with progressive telomere shortening during CS of normal human fibroblasts, and suggested a causative role in CS (57). A small molecule, JH4, interfering binding of progerin and lamin A/C, has been shown to be a senomorphic candidate. JH4 alleviated nuclear deformation and reversed senescence markers, such as growth arrest and SAβG activity, in HGPS and aged cells. Administration of JH4 markedly mitigated several age-related pathologies and extended lifespan in HGPS-progeroid mice (58).
Natural compounds derived from plants, including juglanin (59), quercetin-3-
Embryonic stem cells (ESCs) and ESC conditioned medium (CM) have been suggested as a source of anti-aging interventions that modulate the phenotypes of SCs. ESC-CM and ESC-CM derived factors, such as growth factor and miRNAs, reportedly might also be senomorphic candidates. ESC-CM markedly improved the phenotypes of SCs in HDFs and HUVECs, through a PDGF-FGF mediated pathway and accelerated wound healing in an
SCs in tissues can activate both innate and adaptive immune responses, and are eliminated by the immune system to maintain tissue homeostasis mainly, through the DNA damage response (65). Clearance of SCs contributed to the regulation of tissue homeostasis, such as wound healing (66), hepatic fibrosis (67), and embryonic/placenta development (68). However, inefficient clearance of SCs due to aging of immune system or senescence immunosurveillence was shown to facilitate their accumulation in various tissues with age, and led to aging and ARDs (65). SCs revealed upregulation of MHC class I polypeptide-related sequence A (MICA) and UL16 binding protein 2 (ULBP2), ligands of an activating natural killer (NK) cell receptor (NKG2D), which induced NK-mediated clearance of SCs and decreased liver fibrosis (69). Dipeptidyl peptidase 4 (DPP4) was selectively upregulated in the cell membrane of senescent fibroblasts, but not in proliferating cells or DPP4-positive SCs preferentially eliminated by antibody-mediated NK cell-mediated cytotoxicity, and suggested a possible immunotherapeutic strategy for targeting SCs (70) (Fig. 1). In addition to NK cells, CD4+ T cells and macrophages were also involved in clearance of SCs, and prevented tumor development (71) and their contribution to embryonic development (68). Since NKG2D ligands are also expressed in many cancer cells, strategies to improve immunosurveillance already being developed as cancer therapy, might be repurposed to target SCs for the alleviation of aging and ARDs (18, 72, 73) (Table 3). T cells engineered to express the NKG2D chimeric antigen receptor (CAR), which recognizes NKG2D ligands on the surface of SCs, may be used to target SCs (65). CD9 is upregulated in the cell membrane of SCs (74), and CD9 antibody conjugated nanoparticles or liposomes preferentially delivered drugs to SCs and inhibited phenotypes of SCs in HDFs (75, 76). Therefore, antibody-mediated targeted drug delivery to SCs represents another immunotherapeutic strategy against SCs. Although some cell surface proteins preferentially upregulated in SCs have been shown to be involved in immune-system mediated clearance of SCs, more cell membrane proteins specific to diverse SCs should be investigated. Their identification would contribute to the development of senescence vaccines, targeting senescence-specific antigens, and promote development of novel strategies for immune-system mediated clearance of SCs.
Recent accumulating evidence has suggested a causal linkage between CS, and aging and ARDs, and has led to the development of senotherapeutics targeting SCs. These senotherapeutics are classified as senolytics, senomorphics, and immune-system mediators of the clearance of SCs, and represent an emerging strategy of aging intervention for healthy aging, and prevention and treatment of ARDs. Similar to chemotherapeutics, the development of novel senotherapeutics must be more active and promote aging research on the mechanisms of CS regulation to identify novel targets. However, additional crucial issues must be overcome before senotherapeutics are applied in clinical practice. Although preclinical animal experiments revealed no significant side effects of some senolytics, further in-depth and careful analyses on potential adverse ef fects of long-term administration of senotherapeutics is essential for its successful application in healthy aging and ARDs of humans. Occasionally, imagination becomes realty. In 1997, GATTACA, a science fiction (SF) film directed by Andrew Niccol, highlighted a eugenics issue in genetically modified babies in a not-too-distant future society. In 2010, he released other SF film,
This research was supported by Medical Research Center Program (2015R1A5A2009124) and Basic Science Research Program (218C000399), through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT.
The authors have no conflicting interests.
|Agents||Target/Pathway||Effective senescent cells||Outcomes in vivo||Ref.|
|Dasatinib + Quercetin||Pan-receptor tyrosine kinase/Multiple pathways||HUVECs||↓Atherosclerosis||22|
|ABT-263||Bcl-2 family (Bcl-2, BCL-XL, Bcl-W)||HUVECs||↑Hematopoietic and muscle stem cell function||33|
|IMR90||↓Atherosclerotic lesion formation||34|
|ABT-737||Bcl-2 family (Bcl-2, Bcl-xL, Bcl-W)||IMR90||↑Hair follicle stem cell function||35|
|↓IR-induced lung injury|
|A1331852||Bcl-2 family (Bcl-xL)||HUVECs||↓Liver fibrosis||37|
|17-AAG Geldanamycin||HSP90||MEFs||↓Age-related symptoms||38|
|Piperlongumine||Multiple pathways||WI-38||Not reported||39|
|Quercetin-3-D-galactose||Multiple pathways||HCAECs||Not reported||40|
|Panobinostat||HDAC||NSCLC cell lines||Not reported||41|
|HNSCC cell lines|
|FOXO4-DRI peptide||p53/p21/serpine||IMR90||↓Liver toxicity induced by doxorubicin||44|
|NBD peptide||IKK/NFB pathway||Delaying aging symptoms and chronic diseases in
|JAK inhibitor (ruxolitinib)||JAK (Janus kinase) pathway||Alleviation of the SASP and frailty in old mice||55|
|KU-60019||ATM kinase||Functional recovery of senescent fibroblasts through lysosomal-mitochondrial axis
|JH4||Progerin/lamin A/C||Alleviation of nuclear deformation and reduction of senescence markers in HGPS and aged cells||58|
|Mitigation of age-related pathologies and extension of lifespan in the HGPS-progeroid mice|
|Juglanin||Not reported||Inhibition of doxorubicin-induced senescence of HDFs and HUVECs and replicative cellular senescence of HDFs||59|
||Not reported||Inhibition of doxorubicin-induced and replicative senescence of HDFs and HUVECs||60|
|(−)-Loliolide||Not reported||Inhibition of doxorubicin-induced and replicative senescence of HDFs||61|
|Quercetagetin 3,4′-dimethyl ether||Not reported||Inhibition of doxorubicin-induced and replicative senescence of HUVECs||62|
|ESC-CM||PDGF/FGF pathway||Decreases in senescence phenotypes of HDFs and HUVECs
Acceleration of the skin wound healing in mice
|Mmu-miR-291a-3p||TGFBR2/p21 pathway||Inhibition of senescence phenotypes of HDFs and HUVECs
Acceleration of the excisional skin wound healing in aged mice
Immune-system mediated clearance of SCs
|Surface proteins||Expression||Therapeutic cells||Ref.|
|MICA, ULBP2||Replicative, oncogene-induced, and DNA damage-induced senescent fibroblasts||Clearance of SCs by NK cells||69|
|DPP4||Senescent fibroblasts||Clearance of SCs by NK cells||70|
|CD9||Replicative and doxorubicin-induced senescent HUVECs and HDFs||Delivery of senomorphics by CD9-antibody conjugated nanoparticles and liposomes||75