More than 50 years ago, Hayflick and Moorhead revealed that normal cells enter irreversible growth arrest upon the exhaustion of their proliferative potential after a certain number of cell divisions1. This phenomenon is now known as replicative senescence and is believed to strongly correlate with organismal aging2. Although the progressive erosion of telomeres is considered a major cause of replicative senescence, various cellular stresses, such as DNA damage, oncogenic activation, and oxidative stress, have been reported to induce another type of cellular senescence called "premature senescence" or "stress-induced senescence". Interestingly, premature senescence plays a potent tumor-suppressive role upon the activation of oncogenes such as H-Ras and BRAF. Studies of mouse models and human tissues have produced strong evidence that biomarkers of cell senescence were predominantly present in premalignant lesions where oncogenic Ras and BRAF are activated but were diminished in malignant cancers that developed from these lesions3,4,5. Beyond its role in aging and tumor suppression, cellular senescence has been shown in previous studies to play a role in various physiological processes, including wound healing, tissue repair, immune surveillance, and embryonic development6.
Although growth arrest has been extensively studied as a hallmark of cellular senescence7, a significant body of evidence suggests that intracellular reactive oxygen species (ROS) also contributes to cellular senescence8. The elevation of ROS levels during various types of cellular senescence, including replicative senescence and oncogene-induced senescence (OIS), was originally reported decades ago9,10. A more directly, exogenous treatment with a sublethal dose of H2O2 induces senescence11,12. The inhibition of ROS-scavenging enzymes, such as SOD1, also causes premature senescence13. In contrast, low ambient oxygen conditions and increasing ROS scavenging delay the onset of senescence10,14,15. These results undoubtedly indicate that ROS are important mediators or determinants of cellular senescence induction. However, how ROS contribute to the induction of cellular senescence and how ROS levels are elevated during cellular senescence require further investigation.
Recent studies have revealed that senescent cells have potent paracrine activities on neighboring cells and tissues through an SASP16,17. In aged tissue, senescent cells promote age-related tissue dysfunctions via many pathways through SASP in addition to an autonomous depletion of proliferative cells. Various proinflammatory factors, such as IL-6, IL-8, TGFβ, and matrix metalloproteinases (MMPs), secreted by senescent cells, cause age-related tissue dysfunctions through the impairment of tissue homeostasis, destruction of the tissue architecture, senescence of neighboring cells, and sterile inflammation18,19. However, SASPs can have beneficial effects depending on the biological context. In addition, the heterogenetic nature of SASPs depends on the senescent cell type and the cell stage, emphasizing the need for further research19.
Here, we describe rapid and sensitive cytometry-based techniques for assessing intracellular ROS levels during OIS. In addition, methods for the analysis of SASP factors using quantitative real-time polymerase chain reaction (qPCR) and ELISA are introduced.