A recent study reported that the effectiveness of the senotherapeutic drug ABT-263 depends on the cell’s DNA content, which is based on the cell cycle phase at which the senescent cell was arrested [1].
A personalized approach
Last week, we discussed clinical trials of senolytics and what we can learn from the data obtained so far. The authors of that paper discussed how it might be possible to personalize senolytic treatments to achieve better results.
This recent study published in Aging discusses similar ideas. The authors examine how cell population diversity impacts responses to senolytics and what differences in senescent cells drive those different responses.
Not all senescent cells are created equal
Even though senescent cells generally share many common characteristics, there is a significant variability among those cells. Such variability was reported in previous studies investigating the gene expression profiles of senescent cells; however, there is a scarcity of information about functional differences. Those researchers investigated those functional differences using high-content imaging, a technique that allows for measuring several protein markers at the single-cell level.
In the initial experiment, the researchers used a high-content image analysis to measure the expression of several senescence-associated markers following senescence induction through ionizing radiation (IR) in primary human endothelial cells and fibroblasts.
At the population level, their data confirmed IR-induced senescence. They noted that even in this high-level analysis, they identified differences in the levels of senescence markers between the two cell lines they used, suggesting cell-type-dependent differences in senescent cells.
Further analysis showed even more differences. In the following steps, the researchers investigated the senescent cell diversity at the single-cell level, and they noted two populations of cells that differ in senescence marker expression. They hypothesized that those populations might be associated with “the phase of the cell cycle at which senescent cells were growth-arrested.”
The tale of two cell phases
The cell cycle consists of two main phases: interphase and mitosis (cell division). Interphase is further divided into the G1 phase, where cell growth happens, the S phase, during which DNA is replicated so that it can be divided into two cells later, and the G2 phase, during which cells grow further and prepare for cell division.
The researchers analyzed the DNA content of the cells, as cells in the G1 and G2 phases have either low or high DNA content. G2-arrested cells expressed more senescent markers than G1-arrested cells, and the cells within each subgroup were roughly uniform in their expression of these markers. Identification of these two subgroups led to further testing of the differences between them.
In the next experiment, the researchers prepared cells so that each sample was enriched in either G1 or G2-phase arrested cells, irradiated them to induce senescence, and compared the secretion of IL-6, a SASP component that is associated with inflammation. IL-6 secretion was increased in the G2 group compared to the G1 group.
Most importantly, the researchers investigated the cells’ response to senolytics. Specifically, they tested ABT263, a senolytic that induces cell death (apoptosis) by inhibiting the anti-apoptotic proteins BCL-2 and BCL-xL. G2-arrested cells were more sensitive to ABT263 treatment than G1-arrested cells at the different concentrations tested.
The researchers noted that similar effects were obtained during cancer drug investigation; the cytotoxic effect of some drugs was impacted by DNA content and cell cycle phase, “with some drugs preferentially targeting cells in G1 and others in G2.” [2]
A piece of a bigger puzzle
This small study adds another piece of evidence to the idea that many cellular-level factors and interactions impact the efficacy of senotherapeutics. Those observations are essential in developing future clinical trials or therapies based on senolytics, as they will help to create personalized therapies that would be best tailored for particular patients.
The authors noted that much more needs to be explored in this topic. For example, this study was limited to only two cell lines and one mechanism of senescence induction; therefore, future investigation should expand to different cell types and senescence-inducing mechanisms. Furthermore, there is a need to investigate the diversity of senescent cells in living organisms and whether the effectiveness of senomorphics, which can reduce SASP factors and alleviate senescence-related tissue dysfunction instead of eliminating senescent cells, is similarly impacted.
Additionally, while this study reported on senolytics having different effects in various senescent subpopulations, it did not investigate the mechanism behind this observation. Similarly, these researchers examined only one senotherapeutic drug; it is highly possible that similar mechanisms can also be applied to different senotherapeutics, but this remains to be explored.
Literature
[1] Neri, F., Zheng, S., Watson, M. A., Desprez, P. Y., Gerencser, A. A., Campisi, J., Wirtz, D., Wu, P. H., & Schilling, B. (2025). Senescent cell heterogeneity and responses to senolytic treatment are related to cell cycle status during senescence induction. Aging, 17(8), 2063–2078.
[2] Johnson, T. I., Minteer, C. J., Kottmann, D., Dunlop, C. R., Fernández, S. B. Q., Carnevalli, L. S., Wallez, Y., Lau, A., Richards, F. M., & Jodrell, D. I. (2021). Quantifying cell cycle-dependent drug sensitivities in cancer using a high throughput synchronisation and screening approach. EBioMedicine, 68, 103396.
View the article at lifespan.io
