How Antioxidants Can Selectively Remove Some Senescent Cells

In Aging Cell, researchers have described the way that antioxidants work against senescence in muscle cells by altering mTOR signaling.

Senescent cells don’t sense nutrients properly

Deregulated Nutrient Sensing

The four pathways of nutrient-sensing regulate metabolism and influence aging. The four associated key protein groups are IGF-1, mTOR, sirtuins, and AMPK. We call these proteins “nutrient-sensing” because nutrient levels influence their activity.
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Oxidative stress and mTOR signaling, which determines how cells respond to nutrient availability, are among the most well-researched and well-known facets of aging biology; mTOR, the mechanistic target of rapamycin, gets its name from a similarly well-researched compound. Specifically, complex 1 of mTOR (mTORC1) is one of the principal regulators of how cells conduct metabolism [1], and dysregulated mTORC1 signaling has been found to induce age-related muscle loss (sarcopenia) [2] and regularly appears in senescent cells [3].

When muscle-producing stem cells (myoblasts) become senescent instead of proliferating, they become incapable of restoring muscle tissue after injury [4] and emit the senescence-associated secretory phenotype (SASP), which itself has been found to be linked to mTOR signaling [5]. However, these researchers lamented that most research into mTOR has been focused on fibroblasts rather than on myoblasts, and so they conducted their own investigation to fill that gap.

Senescent cells handle mTORC1 differently

In their first experiment, the researchers used etoposide to chemically drive a population of myoblasts into senescence. Noting that mTORC1 dysfunction can only be properly analyzed after manipulating its activators [6], they analyzed the behavior of these cells in a growth-promoting medium and in a nutrient-poor medium that lacked amino acids. As expected, regardless of nutrient availability, the senescent cells demonstrated upregulated expression of proteins that were downstream of mTORC1.

mTORC1 is related to nutrient sensing from both external and internal sources: the cell’s consumption of its own organelles through autophagy can activate mTORC1. However, autophagy was found to be unrelated to senescence-related increased mTORC1 signaling in these cells; the researchers found that mTORC1 is not co-located with lysosomes in senescent myoblasts the way that it is in other cell types, and hindering autophagy in senescent myoblasts did not diminish the downstream proteins of mTORC1. Instead, Akt and insulin/growth factor signaling were found to drive the increase in mTORC1 in these cells.

Oxidative stress directly affects mTOR

The researchers then investigated SASP components to determine how they affect mTORC1. Intestingly, blunting the effects of SASP cytokines through a JAK inhibitor had no effects on mTORC1 in senescent cells. Instead, the antioxidants Tiron and N-acetylcysteine (NAC) were found to significantly inhibit mTORC1, preventing its overexpression when nutrients were scarce. This increase in oxidative stress was found to be due to the increased mitochondrial count within senescent cells. Directly exposing non-senescent cells to hydrogen peroxide, a well-known oxidant, increased mTORC1 activity under starvation conditions as well.

Possibly the researchers’ most critical finding was that antioxidant treatment can be fatal to senescent cells. Under nutrient starvation conditions, senescent cells treated with NAC expressed greater markers of DNA damage and membrane stress. Continuing to treat these starved senescent cells with antioxidants decreased the number of these cells and decreased the surviving cells’ proteins. Cleaved caspase-3 expression confirmed that more of these cells were being killed off by antioxidant exposure.

These effects were specific to senescent cells; treating proliferating myoblasts with NAC bolstered their nucleic membranes while weakening the nucleic membranes of senescent myoblasts. This is in line with research suggesting that existing senolytics work through mTOR signaling [7], which this study has found to be strongly affected by antioxidants. Giving antioxidants to senescent myoblasts also diminished their SASP production and increased their ability to differentiate, which was nearly eliminated with senescence.

This was an exploratory cellular study; mice were not involved, and the researchers conducted only limited gene expression analyses. However, these results must be taken seriously, as they point to a potentially concerning possibility: to maximize the effects of antioxidants, and possibly some senolytics, against senescent cells, caloric or other dietary restriction may be required as well. Further work is required to determine if this is indeed true and if this study’s results hold up in vivo.

Literature

[1] Liu, G. Y., & Sabatini, D. M. (2020). mTOR at the nexus of nutrition, growth, ageing and disease. Nature reviews Molecular cell biology, 21(4), 183-203.

[2] Tang, H., Inoki, K., Brooks, S. V., Okazawa, H., Lee, M., Wang, J., … & Shrager, J. B. (2019). mTORC1 underlies age‐related muscle fiber damage and loss by inducing oxidative stress and catabolism. Aging cell, 18(3), e12943.

[3] Carroll, B., Nelson, G., Rabanal-Ruiz, Y., Kucheryavenko, O., Dunhill-Turner, N. A., Chesterman, C. C., … & Korolchuk, V. I. (2017). Persistent mTORC1 signaling in cell senescence results from defects in amino acid and growth factor sensing. Journal of Cell Biology, 216(7), 1949-1957.

[4] Sousa-Victor, P., Gutarra, S., García-Prat, L., Rodriguez-Ubreva, J., Ortet, L., Ruiz-Bonilla, V., … & Muñoz-Cánoves, P. (2014). Geriatric muscle stem cells switch reversible quiescence into senescence. Nature, 506(7488), 316-321.

[5] Herranz, N., Gallage, S., Mellone, M., Wuestefeld, T., Klotz, S., Hanley, C. J., … & Gil, J. (2015). mTOR regulates MAPKAPK2 translation to control the senescence-associated secretory phenotype. Nature cell biology, 17(9), 1205-1217.

[6] Carroll, B., Nelson, G., Rabanal-Ruiz, Y., Kucheryavenko, O., Dunhill-Turner, N. A., Chesterman, C. C., … & Korolchuk, V. I. (2017). Persistent mTORC1 signaling in cell senescence results from defects in amino acid and growth factor sensing. Journal of Cell Biology, 216(7), 1949-1957.

[7] Di Micco, R., Krizhanovsky, V., Baker, D., & d’Adda di Fagagna, F. (2021). Cellular senescence in ageing: from mechanisms to therapeutic opportunities. Nature reviews Molecular cell biology, 22(2), 75-95.