Longevity research is a rapidly growing field that aims to understand the fundamental biological mechanisms of aging and to develop new interventions to promote healthy aging and extend lifespan. One such mechanism is cellular senescence, a permanent phase of cellular arrest in which cells remain metabolically active but no longer proliferate or undergo apoptosis. With increasing age senescent cells accumulate. Studies have shown that senescence contributes to aging and the development of age-related diseases. Cellular senescence is a double-edged sword. On the one hand, it prevents the proliferation of damaged cells, which plays a crucial role in tissue repair and cancer attenuation. On the other hand, senescent cells can also secrete pro-inflammatory cytokines that may promote chronic inflammation and malignancy.

Senolytics are a group of small molecules that specifically target senescent cells by interfering with their anti-apoptotic signaling pathways and inducing apoptosis. Studies have shown that senolytic treatments can selectively remove senescent cells, reduce proinflammatory cytokines and prolong the survival of mice.

Rationale behind our study

Most of the in vitro studies performed to date have been based on modelling the senescence system by artificially inducing DNA damage through radiation or inflammatory cytokines to show that senolytics have the potential to eliminate these cells. There is a lack of studies to quantify and understand senolytic treatment in primary blood samples and especially in healthy aging humans. Therefore, we hypothesized that targeting these cells with senolytic drugs may have the potential to reduce epigenetic aging and other markers associated with senescence.

Key Findings

Epigenetic age reduction: A three-day senolytic treatment of human peripheral blood mononuclear cells (PBMCs) in vitro showed a reduction in epigenetic age as measured by DNA methylation changes in age-associated regions.

Elimination of senescent cells: Senolytic treatment resulted in a decrease in senescent markers such as senescence-associated β-galactosidase with C12FDG and p16INK4a expression, as well as a shift within blood cell composition.

Effective senolytic compounds: Four drugs showed an overall effect on both epigenetic age and cellular senescence: JQ1 is an effective inhibitor of the BET family that targets the DNA repair pathway of the non-homologous end joining. The other three drugs, RG7112, AMG232 and nutlin-3a, are MDM2 inhibitors that disrupt with the interaction between p53 and MDM2, leading to a stabilized p53 and promoting apoptosis in senescent cells.

Limitations of our work

Our results show that the percentage of senescent cells increases with age, as it would be expected that the treatment would have a stronger effect on the old samples. However, the observed age reductions during treatment are independent of the age of the sample. It would be interesting to investigate whether it would be beneficial to adjust the drug dosage for each individual in order to treat them efficiently.

The composition of blood cells changes with age, and the decision on the fate of senescence may be more favorable for one cell type than another due to proliferation stress. In our study, the treatment led to a relative change in the composition of the blood cells. It is not yet known whether the senolytics are susceptible to a particular cell type due to the change in cytokine profiles or whether the percentage of senescent cells is higher in a particular cell type composition than in another.

A substantial challenge in detecting senescent cells is that there is no specific marker that can be applied to a large variety of cell types and senescence types. In addition, measuring the state of cellular senescence with either a single marker or at a single time point can lead to incorrect conclusions.

Open questions and future outlook

Many cells go through DNA damage, but what causes senescent cells to remain in this permanent cell arrest phase, and what causes the other cells to repair themselves and escape the cell arrest? Additionally, what happens to the plasticity and identity of the cells when they become senescent?

Senolytics seem to be a rightful path to eliminate the senescent cells without disrupting the beneficial mechanism, although their broader effects need to be further studied.

It is important to note that epigenetic aging signatures have not been specifically trained for the state of cellular senescence, but rather for chronological age. Studies have shown that epigenetic aging and cellular senescence are independent mechanisms, but in our study, both are interlinked. Further studies on DNA methylation changes in both mechanisms are needed to further elucidate the overlapping pathways.

The use of senolytics to selectively remove non-functional aged cells from a heterogeneous cell population reduces SASP-induced inflammation and creates a microenvironment that favors healthy functioning cells. This approach opens new possibilities for healthy aging and delaying or preventing the onset of the disease. It needs to be further studied how the functionality of a cell and the effects on the stem cell population change after treatment.

The underlying age-related discrepancies in blood composition may reflect a bias in the hematopoietic stem cell hierarchy, such as lineage shift or clonal dominance. Senescence can be used as one of the mechanisms to study these discrepancies that influence the perturbations in stem cell fate decisions and may eventually lead to disease.

Take home message

Senolytic treatment can selectively eliminate senescent cells and reduce epigenetic aging, which could potentially result in youthful blood. This study highlights the use of epigenetic aging in drug screening and emphasizes the need for further research to confirm the effects in vivo and explore the potential to promote healthy aging.