Senescence and the SASP: many therapeutic avenues

Kulkarni A.S., Gubbi S., Barzilai N.

Biological aging involves an interplay of conserved and targetable molecular mechanisms, summarized as the hallmarks of aging. Metformin, a biguanide that combats age-related disorders and improves health span, is the first drug to be tested for its age-targeting effects in the large clinical trial-TAME (targeting aging by metformin). This review focuses on metformin's mechanisms in attenuating hallmarks of aging and their interconnectivity, by improving nutrient sensing, enhancing autophagy and intercellular communication, protecting against macromolecular damage, delaying stem cell aging, modulating mitochondrial function, regulating transcription, and lowering telomere attrition and senescence. These characteristics make metformin an attractive gerotherapeutic to translate to human trials.

Grosse L., Wagner N., Emelyanov A., Molina C., Lacas-Gervais S., Wagner K., Bulavin D.V.

Amor C., Feucht J., Leibold J., Ho Y., Zhu C., Alonso-Curbelo D., Mansilla-Soto J., Boyer J.A., Li X., Giavridis T., Kulick A., Houlihan S., Peerschke E., Friedman S.L., Ponomarev V., et. al.

Cellular senescence is characterized by stable cell-cycle arrest and a secretory program that modulates the tissue microenvironment1,2. Physiologically, senescence serves as a tumour-suppressive mechanism that prevents the expansion of premalignant cells3,4 and has a beneficial role in wound-healing responses5,6. Pathologically, the aberrant accumulation of senescent cells generates an inflammatory milieu that leads to chronic tissue damage and contributes to diseases such as liver and lung fibrosis, atherosclerosis, diabetes and osteoarthritis1,7. Accordingly, eliminating senescent cells from damaged tissues in mice ameliorates the symptoms of these pathologies and even promotes longevity1,2,8–10. Here we test the therapeutic concept that chimeric antigen receptor (CAR) T cells that target senescent cells can be effective senolytic agents. We identify the urokinase-type plasminogen activator receptor (uPAR)11 as a cell-surface protein that is broadly induced during senescence and show that uPAR-specific CAR T cells efficiently ablate senescent cells in vitro and in vivo. CAR T cells that target uPAR extend the survival of mice with lung adenocarcinoma that are treated with a senescence-inducing combination of drugs, and restore tissue homeostasis in mice in which liver fibrosis is induced chemically or by diet. These results establish the therapeutic potential of senolytic CAR T cells for senescence-associated diseases. Chimeric antigen receptor (CAR) T cells targeting uPAR, a cell-surface protein that is upregulated on senescent cells, eliminate senescent cells in vitro and in vivo and reduce liver fibrosis in mice.

Wagner V., Gil J.

Cyclin-dependent kinases 4 and 6 (CDK4/6) phosphorylate and inhibit retinoblastoma (RB) family proteins. Hyperphosphorylated RB releases E2F transcription factors, activating a transcriptional program that initiates S phase. Due to the critical role that this pathway has in regulating cell cycle progression, inhibiting CDK4/6 is an attractive therapeutic strategy. Indeed, CDK4/6 inhibitors in combination with antiestrogens produce a significant benefit in patients with ER+/HER2− breast cancer. Clinical trials are currently investigating if the use of CDK4/6 inhibitors alone or in combination can be extended to other cancer types. Inhibition of CDK4/6 can result in different cell fates such as quiescence, senescence, or apoptosis. Senescence is a stress response that can be induced by stimuli that include oncogenic activation, chemotherapy, irradiation, and targeted therapies such as CDK4/6 inhibitors. Senescent cells undergo a stable cell cycle arrest and produce a bioactive secretome that remodels their microenvironment and engages the immune system. In this review, we analyze the therapeutic relevance of senescence induction by CDK4/6 inhibitors. We also discuss how different therapies, including checkpoint inhibitors and drugs targeting MEK or PI3K, can be used in combination with CDK4/6 inhibitors to reinforce or exploit senescence. Recently, a lot of effort has been put into identifying compounds that selectively kill senescent cells (termed senolytics). Thus, sequential treatment with senolytics might be an additional strategy to potentiate the antitumor effects of CDK4/6 inhibitors.

He Y., Zhang X., Chang J., Kim H., Zhang P., Wang Y., Khan S., Liu X., Zhang X., Lv D., Song L., Li W., Thummuri D., Yuan Y., Wiegand J.S., et. al.

Small molecules that selectively kill senescent cells (SCs), termed senolytics, have the potential to prevent and treat various age-related diseases and extend healthspan. The use of Bcl-xl inhibitors as senolytics is largely limited by their on-target and dose-limiting platelet toxicity. Here, we report the use of proteolysis-targeting chimera (PROTAC) technology to reduce the platelet toxicity of navitoclax (also known as ABT263), a Bcl-2 and Bcl-xl dual inhibitor, by converting it into PZ15227 (PZ), a Bcl-xl PROTAC, which targets Bcl-xl to the cereblon (CRBN) E3 ligase for degradation. Compared to ABT263, PZ is less toxic to platelets, but equally or slightly more potent against SCs because CRBN is poorly expressed in platelets. PZ effectively clears SCs and rejuvenates tissue stem and progenitor cells in naturally aged mice without causing severe thrombocytopenia. With further improvement, Bcl-xl PROTACs have the potential to become safer and more potent senolytic agents than Bcl-xl inhibitors. Senolytics have the potential to extend healthspan by selectively killing senescent cells (SCs), but senolytics that target Bcl-xl may cause platelet toxicity. Here, the authors generated a Bcl-xl proteolysis-targeting chimera (PROTAC) senolytic, which effectively clears SCs and rejuvenates tissue stem and progenitor cells in naturally aged mice without causing severe thrombocytopenia.

Ruscetti M., Morris J.P., Mezzadra R., Russell J., Leibold J., Romesser P.B., Simon J., Kulick A., Ho Y., Fennell M., Li J., Norgard R.J., Wilkinson J.E., Alonso-Curbelo D., Sridharan R., et. al.

KRAS mutant pancreatic ductal adenocarcinoma (PDAC) is characterized by a desmoplastic response that promotes hypovascularity, immunosuppression, and resistance to chemo- and immunotherapies. We show that a combination of MEK and CDK4/6 inhibitors that target KRAS-directed oncogenic signaling can suppress PDAC proliferation through induction of retinoblastoma (RB) protein-mediated senescence. In preclinical mouse models of PDAC, this senescence-inducing therapy produces a senescence-associated secretory phenotype (SASP) that includes pro-angiogenic factors that promote tumor vascularization, which in turn enhances drug delivery and efficacy of cytotoxic gemcitabine chemotherapy. In addition, SASP-mediated endothelial cell activation stimulates the accumulation of CD8+ T cells into otherwise immunologically "cold" tumors, sensitizing tumors to PD-1 checkpoint blockade. Therefore, in PDAC models, therapy-induced senescence can establish emergent susceptibilities to otherwise ineffective chemo- and immunotherapies through SASP-dependent effects on the tumor vasculature and immune system.

González‐Gualda E., Pàez‐Ribes M., Lozano‐Torres B., Macias D., Wilson J.R., González‐López C., Ou H., Mirón‐Barroso S., Zhang Z., Lérida‐Viso A., Blandez J.F., Bernardos A., Sancenón F., Rovira M., Fruk L., et. al.

Pharmacologically active compounds with preferential cytotoxic activity for senescent cells, known as senolytics, can ameliorate or even revert pathological manifestations of senescence in numerous preclinical mouse disease models, including cancer models. However, translation of senolytic therapies to human disease is hampered by their suboptimal specificity for senescent cells and important toxicities that narrow their therapeutic windows. We have previously shown that the high levels of senescence-associated lysosomal β-galactosidase (SA-β-gal) found within senescent cells can be exploited to specifically release tracers and cytotoxic cargoes from galactose-encapsulated nanoparticles within these cells. Here, we show that galacto-conjugation of the BCL-2 family inhibitor Navitoclax results in a potent senolytic prodrug (Nav-Gal), that can be preferentially activated by SA-β-gal activity in a wide range of cell types. Nav-Gal selectively induces senescent cell apoptosis and has a higher senolytic index than Navitoclax (through reduced activation in nonsenescent cells). Nav-Gal enhances the cytotoxicity of standard senescence-inducing chemotherapy (cisplatin) in human A549 lung cancer cells. Concomitant treatment with cisplatin and Nav-Gal in vivo results in the eradication of senescent lung cancer cells and significantly reduces tumour growth. Importantly, galacto-conjugation reduces Navitoclax-induced platelet apoptosis in human and murine blood samples treated ex vivo, and thrombocytopenia at therapeutically effective concentrations in murine lung cancer models. Taken together, we provide a potentially versatile strategy for generating effective senolytic prodrugs with reduced toxicities.

Guerrero A., Guiho R., Herranz N., Uren A., Withers D.J., Martínez‐Barbera J.P., Tietze L.F., Gil J.

Senescence is a stable growth arrest that impairs the replication of damaged, old or preneoplastic cells, therefore contributing to tissue homeostasis. Senescent cells accumulate during ageing and are associated with cancer, fibrosis and many age-related pathologies. Recent evidence suggests that the selective elimination of senescent cells can be effective on the treatment of many of these senescence-associated diseases. A universal characteristic of senescent cells is that they display elevated activity of the lysosomal β-galactosidase, and this has been exploited as a marker for senescence (senescence-associated β-galactosidase activity). Consequently, we hypothesized that galactose-modified cytotoxic prodrugs will be preferentially processed by senescent cells, resulting in their selective killing. Here, we show that different galactose-modified duocarmycin (GMD) derivatives preferentially kill senescent cells. GMD prodrugs induce selective apoptosis of senescent cells in a lysosomal β-galactosidase (GLB1)-dependent manner. GMD prodrugs can eliminate a broad range of senescent cells in culture, and treatment with a GMD prodrug enhances the elimination of bystander senescent cells that accumulate upon whole-body irradiation treatment of mice. Moreover, taking advantage of a mouse model of adamantinomatous craniopharyngioma (ACP), we show that treatment with a GMD prodrug selectively reduced the number of β-catenin-positive preneoplastic senescent cells. In summary, the above results make a case for testing the potential of galactose-modified duocarmycin prodrugs to treat senescence-related pathologies.

Vizioli M.G., Liu T., Miller K.N., Robertson N.A., Gilroy K., Lagnado A.B., Perez-Garcia A., Kiourtis C., Dasgupta N., Lei X., Kruger P.J., Nixon C., Clark W., Jurk D., Bird T.G., et. al.

Cellular senescence is a potent tumor suppressor mechanism but also contributes to aging and aging-related diseases. Senescence is characterized by a stable cell cycle arrest and a complex proinflammatory secretome, termed the senescence-associated secretory phenotype (SASP). We recently discovered that cytoplasmic chromatin fragments (CCFs), extruded from the nucleus of senescent cells, trigger the SASP through activation of the innate immunity cytosolic DNA sensing cGAS–STING pathway. However, the upstream signaling events that instigate CCF formation remain unknown. Here, we show that dysfunctional mitochondria, linked to down-regulation of nuclear-encoded mitochondrial oxidative phosphorylation genes, trigger a ROS–JNK retrograde signaling pathway that drives CCF formation and hence the SASP. JNK links to 53BP1, a nuclear protein that negatively regulates DNA double-strand break (DSB) end resection and CCF formation. Importantly, we show that low-dose HDAC inhibitors restore expression of most nuclear-encoded mitochondrial oxidative phosphorylation genes, improve mitochondrial function, and suppress CCFs and the SASP in senescent cells. In mouse models, HDAC inhibitors also suppress oxidative stress, CCF, inflammation, and tissue damage caused by senescence-inducing irradiation and/or acetaminophen-induced mitochondria dysfunction. Overall, our findings outline an extended mitochondria-to-nucleus retrograde signaling pathway that initiates formation of CCF during senescence and is a potential target for drug-based interventions to inhibit the proaging SASP.

Basisty N., Kale A., Jeon O.H., Kuehnemann C., Payne T., Rao C., Holtz A., Shah S., Sharma V., Ferrucci L., Campisi J., Schilling B.

The senescence-associated secretory phenotype (SASP) has recently emerged as a driver of and promising therapeutic target for multiple age-related conditions, ranging from neurodegeneration to cancer. The complexity of the SASP, typically assessed by a few dozen secreted proteins, has been greatly underestimated, and a small set of factors cannot explain the diverse phenotypes it produces in vivo. Here, we present the “SASP Atlas,” a comprehensive proteomic database of soluble proteins and exosomal cargo SASP factors originating from multiple senescence inducers and cell types. Each profile consists of hundreds of largely distinct proteins but also includes a subset of proteins elevated in all SASPs. Our analyses identify several candidate biomarkers of cellular senescence that overlap with aging markers in human plasma, including Growth/differentiation factor 15 (GDF15), stanniocalcin 1 (STC1), and serine protease inhibitors (SERPINs), which significantly correlated with age in plasma from a human cohort, the Baltimore Longitudinal Study of Aging (BLSA). Our findings will facilitate the identification of proteins characteristic of senescence-associated phenotypes and catalog potential senescence biomarkers to assess the burden, originating stimulus, and tissue of origin of senescent cells in vivo.

Zahid A., Li B., Kombe A.J., Jin T., Tao J.

Inflammasomes play a crucial role in innate immunity by serving as signaling platforms which deal with a plethora of pathogenic products and cellular products associated with stress and damage. By far, the best studied and most characterized inflammasome is NLRP3 inflammasome, which consists of NLRP3 (nucleotide-binding domain leucine-rich repeat (NLR) and pyrin domain containing receptor 3), ASC (apoptosis-associated speck-like protein containing a caspase recruitment domain), and procaspase-1. Activation of NLRP3 inflammasome is mediated by highly diverse stimuli. Upon activation, NLRP3 protein recruits the adapter ASC protein, which recruits the procaspase-1 resulting in its cleavage and activation, inducing the maturation, and secretion of inflammatory cytokines and pyroptosis. However, aberrant activation of the NLRP3 inflammasome is implicated in various diseases including diabetes, atherosclerosis, metabolic syndrome, cardiovascular, and neurodegenerative diseases; raising a tremendous clinical interest in exploring the potential inhibitors of NLRP3 inflammasome. Recent investigations have disclosed various inhibitors of the NLRP3 inflammasome pathway which were validated through in vitro studies and in vivo experiments in animal models of NLRP3-associated disorders. Some of these inhibitors directly target the NLRP3 protein whereas some are aimed at other components and products of the inflammasome. Direct targeting of NLRP3 protein can be a better choice because it can prevent off target immunosuppressive effects, thus restrain tissue destruction. This paper will review the various pharmacological inhibitors of the NLRP3 inflammasome and will also discuss their mechanism of action.

Victorelli S., Lagnado A., Halim J., Moore W., Talbot D., Barrett K., Chapman J., Birch J., Ogrodnik M., Meves A., Pawlikowski J.S., Jurk D., Adams P.D., Heemst D., Beekman M., et. al.

Triana-Martínez F., Picallos-Rabina P., Da Silva-Álvarez S., Pietrocola F., Llanos S., Rodilla V., Soprano E., Pedrosa P., Ferreirós A., Barradas M., Hernández-González F., Lalinde M., Prats N., Bernadó C., González P., et. al.

Compounds with specific cytotoxic activity in senescent cells, or senolytics, support the causal involvement of senescence in aging and offer therapeutic interventions. Here we report the identification of Cardiac Glycosides (CGs) as a family of compounds with senolytic activity. CGs, by targeting the Na+/K+ATPase pump, cause a disbalanced electrochemical gradient within the cell causing depolarization and acidification. Senescent cells present a slightly depolarized plasma membrane and higher concentrations of H+, making them more susceptible to the action of CGs. These vulnerabilities can be exploited for therapeutic purposes as evidenced by the in vivo eradication of tumors xenografted in mice after treatment with the combination of a senogenic and a senolytic drug. The senolytic effect of CGs is also effective in the elimination of senescence-induced lung fibrosis. This experimental approach allows the identification of compounds with senolytic activity that could potentially be used to develop effective treatments against age-related diseases. Senolytic compounds have the ability to eliminate senescent cells from tissues and have been shown to be beneficial in various animal models of age-related diseases. Here the authors show that cardiac glycosides commonly used for heart diseases have senolytic properties in humanized mouse models of tumorigenesis and lung fibrosis.

Guerrero A., Herranz N., Sun B., Wagner V., Gallage S., Guiho R., Wolter K., Pombo J., Irvine E.E., Innes A.J., Birch J., Glegola J., Manshaei S., Heide D., Dharmalingam G., et. al.

Senescence is a cellular stress response that results in the stable arrest of old, damaged or pre-neoplastic cells. Oncogene-induced senescence is tumour suppressive but can also exacerbate tumorigenesis through the secretion of proinflammatory factors from senescent cells. Drugs that selectively kill senescent cells, termed ‘senolytics’, have proved beneficial in animal models of many age-associated diseases. In the present study, we show that the cardiac glycoside ouabain is a senolytic agent with broad activity. Senescent cells are sensitized to ouabain-induced apoptosis, a process mediated in part by induction of the proapoptotic Bcl-2 family protein NOXA. We demonstrate that cardiac glycosides synergize with anti-cancer drugs to kill tumour cells and eliminate senescent cells that accumulate after irradiation or in old mice. Ouabain also eliminates senescent pre-neoplastic cells. The findings of the present study suggest that cardiac glycosides may be effective anti-cancer drugs by acting through multiple mechanisms. Given the broad range of senescent cells targeted by cardiac glycosides, their use against age-related diseases warrants further exploration. In addition to having direct anti-cancer effects, the cardiac glycoside ouabain is shown to kill a broad range of senescent cells, thus suggesting that cardiac glycosides represent a novel class of senolytics.

Wang C., Vegna S., Jin H., Benedict B., Lieftink C., Ramirez C., de Oliveira R.L., Morris B., Gadiot J., Wang W., du Chatinier A., Wang L., Gao D., Evers B., Jin G., et. al.

Liver cancer remains difficult to treat, owing to a paucity of drugs that target critical dependencies1,2; broad-spectrum kinase inhibitors such as sorafenib provide only a modest benefit to patients with hepatocellular carcinoma3. The induction of senescence may represent a strategy for the treatment of cancer, especially when combined with a second drug that selectively eliminates senescent cancer cells (senolysis)4,5. Here, using a kinome-focused genetic screen, we show that pharmacological inhibition of the DNA-replication kinase CDC7 induces senescence selectively in liver cancer cells with mutations in TP53. A follow-up chemical screen identified the antidepressant sertraline as an agent that kills hepatocellular carcinoma cells that have been rendered senescent by inhibition of CDC7. Sertraline suppressed mTOR signalling, and selective drugs that target this pathway were highly effective in causing the apoptotic cell death of hepatocellular carcinoma cells treated with a CDC7 inhibitor. The feedback reactivation of mTOR signalling after its inhibition6 is blocked in cells that have been treated with a CDC7 inhibitor, which leads to the sustained inhibition of mTOR and cell death. Using multiple in vivo mouse models of liver cancer, we show that treatment with combined inhibition of of CDC7 and mTOR results in a marked reduction of tumour growth. Our data indicate that exploiting an induced vulnerability could be an effective treatment for liver cancer. CDC7 inhibition selectively induces senescence in hepatocellular carcinoma cells with TP53 mutations, which enables the selective apoptotic cell death of these senescent cells using inhibitors of mTOR signalling.

Wang T., Shi X., Xu X., Zhang J., Ma Z., Meng C., Jiao D., Wang Y., Chen Y., He Z., Zhu Y., Liu H., Zhang T., Jiang Q.

Jing F., Mu J., Liu J., Hu C., Wu F., Gao Q.

Ling L., Li M., Lu J., Zhang F., Pan F., Su Y., Myatt L., Wang W., Sun K., Ying H.

Ameling S., Van der Auwera S., Holtfreter S., Wiechert A., Michalik S., Friedrich N., Hammer E., Völzke H., Nauck M., Grabe H.J., Bröker B.M., Völker U.

Brito-de-Sousa J.P., Lima-Silva M.L., Costa-Rocha I.A., Campi-Azevedo A.C., Mambrini J.V., Faria A.M., Lima-Costa M.F., Peixoto S.V., Teixeira-Carvalho A., Torres K.C., Martins-Filho O.A.

Kang D., Sung J.Y., Hwang H.J., Baek Y., Kim M., Lim G., Kim Y., Cha J., Lee J.

Cao B., Zhao X., Lu Z., Zhang H.

You L., Wu Q.

Xu G., Wang Y.

Wang X., Huang Z., Sui C., Xu M., Sun X.

Zhang J., Zhang S., Cheng C., Zhu C., Wang T., Tang L., Lou J., Li X., Wang H., Hu F., Sun M., Zhang K., Yu F.

Chen H., Ren B., Wang J., Liu X., Yi X., Goltzman D., Miao D.

Kouroumalis E., Tsomidis I., Voumvouraki A.

The pathogenesis of primary biliary cholangitis (PBC) is not fully understood. Despite recent progress, many aspects require further clarification. Thus, PBC is regarded as an autoimmune disease, but immunosuppressive treatment, which is effective in other autoimmune diseases, is not working in the case of PBC. Moreover, there are controversies over the pathogenetic role of anti-mitochondrial antibodies as mitochondria are present in all cells but only cholangiocytes are damaged. In this review, all the proposed models and factors that have been involved in the pathogenesis of PBC are presented. They include mechanisms such as dysregulated autophagy, senescence, apoptosis, impairment of the protective bicarbonate umbrella, immunological abnormalities, the dysbiosis of gut microbiota, and the role of bile acids. Genetics of PBC and epigenetic transcriptional modifications are also presented. Data supporting molecular mimicry and the viral etiology of PBC are analyzed. Finally, an integrated model is proposed based on interactions of the factors that may participate in PBC pathogenesis. Therefore, the purpose of this review is to provide a unifying presentation of the various aspects of PBC pathophysiology, which will allow for a better understanding of this multifaceted disease. New treatment targets may also be identified in such a holistic model.

Lu T., Qin Z., Wu J.

Bar S., Hilsabeck T.A., Pattavina B., López-Domínguez J.A., Basisty N., Bons J., Watson M., Schilling B., Campisi J., Kapahi P., Sharma A.