A joint research group led by Assistant Professor Yoshimi Kawamura and Professor Kyoko Miura of the Department of Aging and Longevity Research at the Faculty of Life Sciences of Kumamoto University, in collaboration with Keio University, Hiroshima University, Kyoto University, Hoshi University, and the National Institute of Infectious Diseases (NIID), announced their discovery that senescent cells undergo cell death in naked mole rats (NMRs), a rodent that is resistant to senescence and cancer. Cellular senescence (aging-related cell deterioration) was induced in fibroblasts for NMRs analysis. The research group clarified that this cell death process is related to the regulation of serotonin metabolism and vulnerability to hydrogen peroxide (H2O2), both mechanisms of which are unique to NMRs. The results are expected to lead to the development of safer senolytic drugs and were published in the 11 July 2023 issue of The EMBO Journal, an international academic publication.
In mammals, including humans and mice, cellular senescence is induced by different kinds of senescent stress, such as DNA damage, and the accumulation of cellular senescence is thought to accelerate the process and contribute to carcinogenesis. NMRs are small (8−10 cm in length), long-lived rodents that exist underground in Africa. Much like honeybees, they are eusocial; meaning that they live in cooperative groups consisting of a queen, a king, and workers, a rare characteristic in mammals.
Aside from having a long lifespan (up to 37 years confirmed), NMRs rarely form tumors spontaneously (they are cancer resistant), and their physical functions are less likely to decline with age (they are senescence resistant). Because of these characteristics, NMRs have been drawing attention as a research model for the development of new anti-aging and cancer prevention methods. However, little was known about the mechanisms involved in their cancer and senescence resistance.
Miura's laboratory is the only one in Japan that breeds this rodent species and is promoting research on their resistance to aging and carcinogenesis. So far, the researchers have clarified the existence of a tumor resistance mechanism and DNA damage resistance in this species.
In this study, the research group analyzed the behavior of senescent cells in NMRs to uncover the mechanism of resistance to senescence and carcinogenesis. First, cellular senescence was induced by adding low concentrations of DNA-damaging agents to the skin fibroblasts of NMRs and mice, and the effects were compared. Although cellular senescence was induced in the fibroblasts of both species, cell death increased markedly over time only in those from the NMRs. Furthermore, in the NMR fibroblasts, the timing of increased expression of INK4a (a cyclin-dependent kinase inhibitor involved in cellular senescence) coincided with the timing of increased cell death.
Next, INK4a was overexpressed in cells of both animal species to investigate its involvement in cell death. The results showed that INK4a overexpression only increased cell death in the NMR cells. INK4a activates the retinoblastoma protein (RB) gene, a tumor suppressor gene that acts downstream of the INK4a gene. In general, cell death is known to involve p53 and INK4a, which are tumor suppressor genes. Experiments to inhibit each of these genes revealed that NMR cell death was caused by INK4a-mediated RB gene activation in a p53-independent manner.
To investigate this mechanism in detail, the research group searched for common genes that are upregulated when INK4a is forcibly expressed in fibroblasts derived from several different NMR individuals.
As a result, increased expression of genes involved in the metabolism of H2O2 was confirmed. Additionally, a comprehensive analysis of metabolites (metabolomic analysis) produced in both INK4a-introduced species revealed that, unlike mice, NRMs have a large amount of serotonin under normal conditions, and the induction of cellular senescence through the introduction of INK4a increases the level of 5-hydroxyindoleacetic acid (5-HIAA), a metabolite of serotonin. Serotonin is metabolized to 5-HIAA by monoamine oxidase (MAO), which simultaneously produces large amounts of H2O2.
It is also known that NMR cells show much higher vulnerability to H2O2 than mouse cells. To verify whether this vulnerability to H2O2 is related to cell death, the research group conducted an experiment in which antioxidants and other agents were added after the induction of cellular senescence.
The results showed that cell death was suppressed in the treated senescent cells, indicating that MAO plays an important role in cell death.
Further analysis was undertaken into whether a similar mechanism exists in vivo, by inducing cellular senescence via administration of DNA-damaging agents to the lungs of NMRs and mice. Then, cellular senescence and cell death were examined on day 21 in two treatment groups: one in which MAO inhibitors were continuously administered for 5 days starting on day 16 of DNA-damaging agent administration, and another in which no MAO inhibitors were administered over the indicated period.
The results of this indicated that cell death in NMRs is dependent on MAO and important for suppressing the accumulation of senescent cells. In this study, the research group conducted an analysis using NMR workers (other than the queen and king). The results suggest that a similar mechanism exists in the workers, even though they have a shorter lifespan than the king and queen. Although they live long lives, the cause of death is often unknown, which also needs to be studied.
Miura commented, "Currently, senolytic drugs that remove senescent cells are being developed worldwide for anti-aging and anticancer purposes, but we also know that senescent cells maintain homeostasis, or good function, in a living body. By analyzing and researching when, how, and in what organ senescent cells are removed in NMRs, which have acquired the ability to eliminate senescent cells during the course of evolution, we will be able to contribute to the development of safer senolytic drugs."
Publication: The EMBO Journal
Title: Cellular senescence induction leads to progressive cell death via the INK4a-RB pathway in naked mole-rats
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