As we age, cells that have stopped dividing but have not died accumulate in tissues. They release inflammatory substances and interfere with organ function. Scientists have long been looking for ways to get rid of such cells. Nearby in the archive are studies on rejuvenation after transplantation, on how cells listen to rhythm, and on how a tumor regrows after an electric shock. All of them deal with the fate of cells, but this paper stands apart: it tests whether a senescent cell can be killed without touching a healthy one, and it does so in human muscle cells.
Human muscle cells of the LHCN-M2 line were grown in nutrient medium. Senescence was induced by a two-day treatment with doxorubicin at a concentration of 200 nanomolar, after which the cells rested for eight days. Then, for three days, different concentrations of the antioxidant N-acetylcysteine (NAC) or the solvent DMSO were added. Some experiments were carried out in a depleted medium with less amino acids, glucose, and serum. Live and dead cells were distinguished by staining: live ones glowed green, dead ones blue. Counting was done manually from images using the ImageJ program.
It turned out that senescent cells are more sensitive to high doses of the antioxidant. At an NAC concentration of 20 millimolar in depleted medium, the proportion of dead senescent cells nearly doubled compared to the control, whereas normal cells were almost unaffected. At the same time, low concentrations of NAC (up to 5 millimolar) slightly protected both cell types. This biphasic dependence resembles the behavior of the flavonoid fisetin, a known senolytic. The authors suggest that it is reductive stress, rather than inhibition of the PI3K/Akt/mTORC1 signaling pathway, that is the key mechanism of senescent cell death under the action of antioxidants.
The work offers a new explanation for why certain natural compounds kill senescent cells. Instead of the usual inhibition of a signaling pathway, the idea of reductive stress is put forward. This changes the way we look for senolytics: now it is important not only to remove oxidants but also not to overload the cell with reductants. The paper stands out among its neighbors: it shifts the focus from signaling pathways to redox balance and uses human cells rather than mouse ones.
If reductive stress really destroys senescent cells, we can look for safer ways to induce it. For example, by delivering antioxidants directly into senescent cells using their surface markers. It is also worth checking how universal this mechanism is across different cell and tissue types. Perhaps new senolytics acting through redox imbalance will emerge.
The results could influence the development of drugs against aging and age-related diseases. They are also important for understanding the toxicity of high doses of antioxidants.
Test the effect of NAC on other types of senescent cells, including skin and fat tissue cells. Study how reductive stress triggers death specifically in senescent cells.
In the biology of aging, it remains an open question why senescent cells are resistant to ordinary death but vulnerable to certain influences. This work links that vulnerability to a disruption of redox balance, which may explain the heterogeneity of the response to senolytics.
N-acetylcysteine is often sold as a dietary supplement to support the liver and lungs. In this work, it was used at concentrations tens of times higher than those reached in the blood after taking the supplement.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.