A great deal of research into the phenomenon of cellular senescence is taking place these days; an explosion of effort and funding in comparison to the start of the decade, a time at which it was next to impossible to make any progress in this part of the field. The turning point was philanthropy, a gift of the necessary funds to run the first animal study that provided direct evidence for targeted removal of senescent cells to slow the aging process. For decades prior to this point, compelling indirect evidence existed for senescent cells to be a contributing cause of degenerative aging, but despite the growing advocacy of groups like Methuselah Foundation and SENS Research Foundation, it was still very hard to find that money. Now the tipping point has passed, all sorts of findings are being made, however: direct links between cellular senescence and age-related disease, findings that could have been made ten or twenty years ago were there the will and the funding at that time, albeit at greater cost and effort. This is something we should all bear in mind as we look at other areas of the SENS rejuvenation research agenda and ask why it is not progressing as rapidly as we’d like. All it takes is that one study and suddenly everyone in the research community, all the people who wouldn’t give you the time of day last year, agree that you were right all along – and then forget your name in the rush to append their own to the newly growing field. Such is the way the world works. It isn’t fair, it isn’t efficient, but it is what it is, and we do our best to change it.
The open access paper linked below is one of many examples to illustrate two trends in the more energetic recent research of cellular senescence: firstly, to find more and better biomarkers that distinguish senescent cells from their peers, and secondly to find ways to minimize the harms done by senescent cells without destroying them. The first sounds like a great idea, as the presently established state of the art in senescent cell assays and markers is more or less the same as it was fifteen years ago – good enough for laboratory research after the old model, but not a sound basis for the clinical therapies and more discriminating research of the years ahead. It seems evident that something better is possible in this age of accelerating growth in the capabilities of biotechnology. The second course of action, on the other hand, strikes me as a tough road in comparison to the more direct approach of destroying senescent cells. That destruction seems unambiguously beneficial in mice, even when all such cells are constantly removed throughout life, via genetic engineering approaches. In human therapies, at least at the outset, removal would only occur every so often, during a treatment. The transient roles for senescent cells would continue as they were, such as in wound healing and suppression of potentially cancerous cells.
So the argument made in this paper, and elsewhere, that we should be cautious and leave senescent cells in place, doesn’t seem like one with a lot of support given the evidence to date. Those researchers making it are asking for the community to give up the short path to effective therapies in exchange for a long path to worse therapies. Removal of senescent cells could be carried out quite infrequently, perhaps every few years or every decade. Suppression of senescent cells on the other hand would mean constant medication, and the struggle to safely adjust very complex cellular behavior that is still incompletely cataloged. Each form of damage and misbehavior created by the senescence-associated secretory phenotype (SASP) would have to be mapped and then drugs designed to impact it; it could take decades, even under optimistic estimates of future capabilities of the industry. Destruction of these cells, on the other hand, can be done now in the lab, and is only a few years away from the clinic. Time matters in the treatment of aging, as we don’t have an infinite amount of it.
Cellular senescence is characterized by a proliferative arrest induced to prevent the propagation of damaged cells in a tissue. This arrest is mainly driven by the activation of two important pathways, p53/p21CIP and RB/p16INK4A. The senescence program can be triggered by a number of stressors, like the activation of oncogenes, drug treatment, or deregulation of Polycomb Repressive Complex 1 (PRC1) proteins, including the polycomb protein chromobox 7 (CBX7). Although arrested, senescent cells are metabolically and transcriptionally functional, and they actively communicate with their surroundings. In fact, senescent cells secrete an array of inflammatory proteins, growth factors, and metalloproteases that collectively constitute the SASP (senescence-associated secretory phenotype). The SASP recruits the immune system in order to eliminate senescent cells and induces changes in the extracellular matrix (ECM), thus facilitating tissue homeostasis and regeneration. The presence of senescent cells has been found in vivo in preneoplastic lesions, in wound healing, during embryonic development, and in different tissues throughout aging. Interestingly, a recent study has demonstrated that p16INK4A-positive cells accumulate during aging and contribute to age-related dysfunctions in different tissues. Thus, the elimination of senescent cells reverses the aging phenotype and stimulates tissue regeneration, demonstrating that the activation of senescence is a direct cause of aging and opening avenues for targeting senescent cells as a therapy to extend healthy lifespan.
Intercellular communication is an important feature to maintain tissue homeostasis, where the activation of cellular senescence plays a crucial role. In fact, previous reports have found ECM remodeling to regulate fibrosis by activating the senescence program. Apart from inflammation and ECM remodeling, cells can communicate via the secretion of extracellular vesicles, cell-cell contact, or intercellular protein transfer. Here, we provide evidence that the integrin β3 subunit plays a role in senescence through activation of the TGF-β pathway. A great deal of information exists regarding the biological function of integrins and their regulation of the microenvironment, but relatively little is known about the transcriptional regulation of integrins themselves. We show that β3 subunit expression accelerates the onset of senescence in human primary fibroblasts, which is dependent on the activation of the p21CIP/p53 pathways. Our results also show a robust expression of β3 upon senescence activation induced by a variety of stimuli, while interference with its expression levels disrupts the senescence phenotype. Furthermore, mice lacking β3 accelerate wound-healing closure, which could be by restricting the induction of senescence.
Cellular adhesion is a key feature of senescence. In agreement with our results, several reports have found differential expression of integrins during cellular senescence activation. Analysis of published datasets show that the “cellular adhesion” pathway and integrins are differentially expressed during senescence activation. Likewise, a number of studies have found that TGF-β ligands are part of the SASP and play an important role in senescence through p21CIP regulation, in agreement with our data. The TGF-β superfamily controls numerous cellular and biological processes, such as development, regeneration, fibrosis, and cancer. Accumulating evidence indicates that a cross-talk between integrins and TGF-β exists, in particular to regulate fibrosis, wound healing, and cancer. However, even if senescence is known to regulate all these biological processes, none of these studies have reported the existence of a cross-talk between integrins and TGF-β in senescence or aging. Our data show that β3 regulates senescence by activating TGF-β via cell-autonomous and non-cell-autonomous mechanisms. The use of small molecule inhibitors, RNAi technology, and the analysis of the expression levels of various members of the TGF-β pathway authenticate a role for TGF-β during senescence induced by β3 expression.
Our data show an increase in the expression levels of β3 mRNA concomitant with an increase in different markers of senescence in tissue from old mice. Upregulation of β3 and senescence/aging markers, including TGF-β members, was further observed in fibroblasts from old human donors. This is in accordance with previous reports, which have found that p16INK4A levels correlate with chronological age in most tissues analyzed, both in mice and in humans. Interestingly, knockdown of β3 mRNA partially reversed the aging phenotype of fibroblasts derived from old human donors. However, the αvβ3 antagonist, cilengitide, could not reverse aging, suggesting that the role for β3 in this cellular system is independent of its ligand-binding activity. Our data show that cilengitide has a diverse effect on the SASP and on the senescence growth arrest. As senescent cells accumulate during aging, causing chronic inflammation, cilengitide could be a potential therapeutic route to block inflammation without affecting proliferation in aging. In summary, here, we provide evidence for the β3 subunit being a marker and regulator of senescence, and identify integrins as potential therapeutic targets to promote healthy aging.