The Promise of Senolytics
As the body ages increasing amounts of your cells fall into a state of senescence. By the time people reach old age significant numbers of these senescent cells have accumulated in the body. Senescent cells do not divide or support the tissue they are a part of, but instead emit a range of potentially harmful chemical signals, these encourage other nearby cells to also enter the same senescent state. Their presence causes many problems: they degrade tissue function, increase levels of chronic inflammation, and can even eventually raise the risk of cancer.
Senescent cells normally self destruct via a process called Apoptosis or they are also removed by the immune system, however the immune system weakens with age and increasing numbers of these senescent cells escape this process and build up.
These senescent “death resistant” cells comprise a small number of total cells in the body but they secrete pro-inflammatory cytokines, chemokines, and extracellular matrix proteases, which together form the senescence-associated secretory phenotype or SASP. The resulting SASP is thought to significantly contribute to aging (Freund, Campisi, et al, 2010) and cancer (Coppé, Campisi, et al, 2010) and thus removal of senescent cells and SASP is a potential strategy for promoting health and longevity.
Indeed, the health and lifespan of mice have been demonstrated to improve by the removal of senescent cells using a transgenic suicide gene (Darren et al., 2011) and later experiments showed the same could be achieved using small molecules called Senolytics. They represent a relatively new class of drugs that focuses on the removal of senescent cells.
It was discovered through transcript analysis that senescent cells have increased expression of pro-survival genes, consistent with their resistance to apoptosis (Zuh et al., 2015). Drugs targeting these pro-survival factors selectively killed senescent cells. Two such drugs were Dasatinib and Quercetin which were both able to remove senescent cells but were better in differing tissue types. However it was discovered that a combination of the two drugs formed a synergy that was significantly more effective at removing some senescent cell types (Zuh et al., 2015).
In other studies whilst only removing thirty percent of senescent cells there were improvements to age related decline. These results suggest the feasibility of selectively ablating senescent cells and the efficacy of senolytics for alleviating symptoms of aging and promoting healthy longevity (Tchkonia et al., 2013; Kirkland et al., 2014; Kirkland and Tchkonia, 2015).
Even more recently a further study demonstrated the benefits of senolytics for certain aspects of vascular aging (Roos, Zhu, Tchkonia, Kirkland et al, 2016). This is the first study to confirm that clearance of senescent cells improves aspects of vascular aging and chronic hypercholesterolemia, and may be a viable therapeutic to reduce morbidity and mortality from cardiovascular diseases - which contributes the most to mortality in old age.
Tchkonia T, Zhu Y, van Deursen J, Campisi J, Kirkland JL. (2013) Cellular senescence and the senescent secretory phenotype: therapeutic opportunities. J Clin Invest. 2013 Mar;123(3):966-72.
Zhu Y, Tchkonia T, Pirtskhalava T, Gower AC, Ding H, Giorgadze N, Palmer AK, Ikeno Y, Hubbard GB, Lenburg M, O'Hara SP, LaRusso NF, Miller JD, Roos CM, Verzosa GC, LeBrasseur NK, Wren JD, Farr JN, Khosla S, Stout MB, McGowan SJ, Fuhrmann-Stroissnig H, Gurkar AU, Zhao J, Colangelo D, Dorronsoro A, Ling YY, Barghouthy AS, Navarro DC, Sano T, Robbins PD, Niedernhofer LJ, Kirkland JL. (2015) The Achilles' heel of senescent cells: from transcriptome to senolytic drugs. Aging Cell. Aug;14(4):644-58.
Coppé, J.-P., Desprez, P.-Y., Krtolica, A., & Campisi, J. (2010). The Senescence-Associated Secretory Phenotype: The Dark Side of Tumor Suppression. Annual Review of Pathology, 5, 99–118.
Freund, A., Orjalo, A. V., Desprez, P.-Y., & Campisi, J. (2010). Inflammatory Networks during Cellular Senescence: Causes and Consequences. Trends in Molecular Medicine, 16(5), 238–246.
Baker,van Deursen Kirkland et al (2011) Clearance of p16Ink4a-positive senescent cells delays ageing-associated disorders, Nature 479, 232–236
Baker, van Deursen et al (2016) Naturally occurring p16Ink4a-positive cells shorten healthy lifespan nature 16932
Roos, Zhu, Tchkonia, Kirkland et al (2016) Chronic senolytic treatment alleviates established vasomotor dysfunction in aged or atherosclerotic mice DOI: 10.1111/acel.12458