Modern solutions that rely on biomedical advancements to intervene age-related diseases may yield lifespan extension, but potentially at the cost of elevated late life morbidity. Physical dysfunction and incapability to respond to environmental stimuli become increasingly prevalent toward the end of life, and is associated with considerable morbidity, including reduced mobility, loss of independence, hospital admissions and clinical mortality (1). However, the cell level pathogenesis of age-related physical dysfunction and various pathologies has not been thoroughly addressed, with mechanism-based interventions to improve physical function in the elderly highly desired for decades (2).

Cellular senescence represents a cell fate that involves extensive changes in gene expression, cytoplasmic signaling and metabolic activities. Induced by a myriad of stressors such as DNA damage, oncogenic mutation, telomeric attrition, metabolic and mitochondrial dysfunction, cellular senescence occurs progressively with organismal ageing. Senescent cells actively secrete a range of pro-inflammatory cytokines, chemokines, growth factor and proteases, a phenomenon termed the senescence-associated secretory phenotype (SASP) (3). Despite the fact that senescence protects against carcinogenesis and limits most types of fibrosis, the aberrant accumulation of senescent cells during ageing and disease is largely detrimental (4). Numerous studies have demonstrated the consequence of the SASP in vivo and suggested the contribution of SASP to an increasing list of age-related disorders (5). This negative role of senescent cells in ageing was first demonstrated with animal models by selective ablation of senescent cells via genetic manipulation (6, 7). Importantly, clearance of senescent cells from progeroid or naturally ageing mice improves healthspan, extends lifespan and benefits an array of pathologies including but not limited to atherosclerosis, glaucoma, osteoporosis, osteoarthritis and neurodegenerative symptoms (8).

Recent success in genetic studies prompted a search for drugs that can selectively remove senescent cells, namely ‘senolytics’. A handful of senolytic compounds have been identified, including dasatinib and quercetin, piperlongumine, HSP90 suppressors, Bcl-2 family inhibitors such as ABT-263 (navitoclax) and ABT-737, and cardiac glycosides (9). To date, Bcl-2 family antagonists may be the most widely used senolytics, displaying efficacy in killing a range of senescent cells and reproducing in vivo effects observed in transgenic mice. Originally developed as therapies for malignancies particularly lymphoma, Bcl-2 inhibitors have low solubility and oral bioavailability, and in some cases, causes severe thrombocytopenia (2). Given the unnegligible side effects associated with reported senolytics, there is a substantial need to identify safer compounds with senolytic properties.

To this end, we performed a large scale drug screening with a natural product library of plant-derived medicinal agents (PDMA) (Figure 1). As a result, grape seed extract (GSE) was identified as a competent antiageing product and selected for further investigation due to its leading capacity in targeting senescent cells. We found procyanidin C1 (PCC1), a polyphenolic component, plays a critical role in mediating the observed effects of GSE. Specifically, PCC1 blocks the SASP expression when used at low concentrations, while selectively killing senescent cells at higher concentrations, mainly by enhancing production of reactive oxygen species (ROS) and disturbing mitochondrial membrane potential, processes accompanied by upregulation of Bcl-2 family pro-apoptotic factors Puma and Noxa in senescent cells. PCC1 depletes senescent cells in treatment-damaged tumour microenvironment (TME) and enhances therapeutic efficacy when combined with chemotherapy in preclinical assays. Intermittent administration of PCC1 to whole-body radiated, senescent cell-implanted or naturally aged mice alleviated physical dysfunction and prolonged post-treatment survival, thus providing substantial benefits in the late life stage. Together, our study identifies PCC1 as a distinct natural senolytic agent, which may be exploited to delay ageing and control age-related pathologies in future medicine (10) (Figure 2).

Figure 1.  Schematic workflow of screening candidate senolytics

Figure 2. Landscape presentation of the signaling network engaged in PCC1-mediated senolysis

We further demonstrated that intermittent PCC1 treatment reduced the number of senescent cells in aged mice. In that context, PCC1 not only targeted senescent cells, but also provided a broader range of benefits reflected in improved metabolic parameters and physical fitness. Of note, PCC1-induced reduction of senescent cell burden prevented chronic inflammation and reversed organ dysfunction observed in old animals. These effects seem to be confined to ageing-related frailty as PCC1 treatment did not impact into the cause and number of diseases (including tumours) at the late stage of lifespan. Mice receiving PCC1 treatment starting at 24-27 months of age (equivalent to 75-90 years of age in human) had more than 60% longer median post-treatment lifespan (or approximately 10% longer overall lifespan), with 65.0% lower mortality hazard. These data do provide first-line evidence of PCC1-conferred health benefits by eliminating senescent cells from aged tissues and organs.

Despite the long-standing suggestions for novel clinical indications of PCC1 beyond health conditions such as atherosclerosis, periodontitis, insomnia and local inflammation, a factor limiting its use has been the potential for side effects. Although data from our preclinical investigations indicated limited systemic cytotoxicity of PCC1 in experimental animals, the safety of PCC1 administration in human patients needs to be warranted by future clinical trials. Further, senolytic action of PCC1 may require intermittent, rather than continuous usage and, and depending on the indication, administration could be local rather than systemic. The present study suggests that the use of PCC1 as a senolytic agent could be safer than initially thought. Given that procyanidins have a long history of use in clinics and its side effects can be monitored and managed, our study supports that PCC1 should be trialled as a novel senolytic in the context of geriatric medicine and beyond.

References

1. Xu M, Pirtskhalava T, Farr JN, Weigand BM, Palmer AK, Weivoda MM, et al. Senolytics improve physical function and increase lifespan in old age. Nat Med. 2018;24(8):1246-56.
2. Song S, Lam EW, Tchkonia T, Kirkland JL, Sun Y. Senescent Cells: Emerging Targets for Human Aging and Age-Related Diseases. Trends Biochem Sci. 2020;45(7):578-92.
3. Coppe JP, Patil CK, Rodier F, Sun Y, Munoz DP, Goldstein J, et al. Senescence-associated secretory phenotypes reveal cell-nonautonomous functions of oncogenic RAS and the p53 tumor suppressor. PLoS Biol. 2008;6(12):2853-68.
4. Munoz-Espin D, Serrano M. Cellular senescence: from physiology to pathology. Nat Rev Mol Cell Biol. 2014;15(7):482-96.
5. Gorgoulis V, Adams PD, Alimonti A, Bennett DC, Bischof O, Bishop C, et al. Cellular Senescence: Defining a Path Forward. Cell. 2019;179(4):813-27.
6. Baker DJ, Wijshake T, Tchkonia T, LeBrasseur NK, Childs BG, van de Sluis B, et al. Clearance of p16Ink4a-positive senescent cells delays ageing-associated disorders. Nature. 2011;479(7372):232-6.
7. Demaria M, Ohtani N, Youssef SA, Rodier F, Toussaint W, Mitchell JR, et al. An essential role for senescent cells in optimal wound healing through secretion of PDGF-AA. Dev Cell. 2014;31(6):722-33.
8. Song S, Tchkonia T, Jiang J, Kirkland JL, Sun Y. Targeting Senescent Cells for a Healthier Aging: Challenges and Opportunities. Adv Sci (Weinh). 2020;7(23):2002611.
9. Borghesan M, Hoogaars WMH, Varela-Eirin M, Talma N, Demaria M. A Senescence-Centric View of Aging: Implications for Longevity and Disease. Trends Cell Biol. 2020;30(10):777-91.
10. Xu Q, Fu Q, Li Z, Liu H, Wang Y, Lin X, et al. The flavonoid procyanidin C1 has senotherapeutic activity and increases lifespan in mice. Nat Metab. 2021. DOI: 10.1038/s42255-021-00491-8