Emerging evidence is revealing that exosomes contribute to many aspects of physiology and disease through intercellular communication. However, the biological roles of exosome secretion in exosome-secreting cells have remained largely unexplored. Here we show that exosome secretion plays a crucial role in maintaining cellular homeostasis in exosome-secreting cells. The inhibition of exosome secretion results in the accumulation of nuclear DNA in the cytoplasm, thereby causing the activation of cytoplasmic DNA sensing machinery. This event provokes the innate immune response, leading to reactive oxygen species (ROS)-dependent DNA damage response and thus induce senescence-like cell-cycle arrest or apoptosis in normal human cells. These results, in conjunction with observations that exosomes contain various lengths of chromosomal DNA fragments, indicate that exosome secretion maintains cellular homeostasis by removing harmful cytoplasmic DNA from cells. Together, these findings enhance our understanding of exosome biology, and provide valuable new insights into the control of cellular homeostasis.

Cellular senescence prevents the proliferation of cells at risk for neoplastic transformation. However, the altered secretome of senescent cells can promote the growth of the surrounding cancer cells. Although extracellular vesicles (EVs) have emerged as new players in intercellular communication, their role in the function of senescent cell secretome has been largely unexplored. Here, we show that exosome-like small EVs (sEVs) are important mediators of the pro-tumorigenic function of senescent cells. sEV-associated EphA2 secreted from senescent cells binds to ephrin-A1, that is, highly expressed in several types of cancer cells and promotes cell proliferation through EphA2/ephrin-A1 reverse signalling. sEV sorting of EphA2 is increased in senescent cells because of its enhanced phosphorylation resulting from oxidative inactivation of PTP1B phosphatase. Our results demonstrate a novel mechanism of reactive oxygen species (ROS)-regulated cargo sorting into sEVs, which is critical for the potentially deleterious growth-promoting effect of the senescent cell secretome.

“Cellular senescence” is a state in which cells undergo irreversible cell cycle arrest in response to a variety of cellular stresses. Once cells senesce, they are strongly resistant to any mitogens, including oncogenic stimuli. Therefore, cellular senescence has been assumed to be a potent anticancer mechanism. Although irreversible cell‐cycle arrest is traditionally considered the major characteristic of senescent cells, recent studies have revealed some additional functions. Most noteworthy is the increased secretion of various secretory proteins, such as inflammatory cytokines, chemokines, growth factors, and MMPs, into the surrounding extracellular fluid. These newly recognized senescent phenotypes, termed senescence‐associated secretory phenotypes (SASPs), reportedly contribute to tumor suppression, wound healing, embryonic development, and even tumorigenesis promotion. Thus, SASPs appear to be beneficial or deleterious, depending on the biological context. As senescent cells are known to accumulate during the aging process in vivo, it is quite possible that their accumulation in aged tissues promotes age‐associated functional decline and various diseases, including cancers, at least to some extent. Here, we focus on and discuss the functional and regulatory network of SASPs toward opening up new possibilities for controlling aging and aging‐associated diseases.

Accumulating evidence indicates that the senescence-associated secretory phenotype (SASP) contributes to many aspects of physiology and disease. Thus, controlling the SASP will have tremendous impacts on our health. However, our understanding of SASP regulation is far from complete. Here, we show that cytoplasmic accumulation of nuclear DNA plays key roles in the onset of SASP. Although both DNase2 and TREX1 rapidly remove the cytoplasmic DNA fragments emanating from the nucleus in pre-senescent cells, the expression of these DNases is downregulated in senescent cells, resulting in the cytoplasmic accumulation of nuclear DNA. This causes the aberrant activation of cGAS-STING cytoplasmic DNA sensors, provoking SASP through induction of interferon-β. Notably, the blockage of this pathway prevents SASP in senescent hepatic stellate cells, accompanied by a decline of obesity-associated hepatocellular carcinoma development in mice. These findings provide valuable new insights into the roles and mechanisms of SASP and possibilities for their control.

DNA and protein modifications create a new surface for interaction with target molecules (1-5). Cytosine methylation is involved in gene regulation and the formation of transcriptionally inactive chromatin, together with methyl-CpG binding domain proteins (MBD 1 proteins) (6 -7). There are at least five mammalian MBD proteins, namely MeCP2, MBD1, MBD2, and MBD3 for transcriptional repression, and MBD4 (also known as MED1) for mismatch repair as a thymine glycosylase. In addition, posttranslational modifications of amino termini of the core histones cooperatively produce epigenetic codes for genome dynamics in the nucleus. These include acetylation, phosphorylation, and methylation of the histone molecules.Recent studies of cytosine hypomethylation mutants have revealed that both DNA methylation and histone modifications share a common pathway in chromatin organization (8). The Neurospora crassa DIM-5 gene product is a histone H3 methylase containing a SET domain, which specifically methylates lysine 9 of histone H3, abbreviated throughout as H3 (Lys-9). The mutant dim-5 completely abolished cytosine methylation of the genome. Furthermore, Arabidopsis thaliana DDM1, which shows amino acid similarity to a helicase of the SWI2/ SNF2 family, is also required for the maintenance of genomic methylation (9), and disruption of the homolog Lsh gene caused demethylation of the mouse genome (10). These suggest that DNA methylation may depend on methyl-H3 (Lys-9) and unique chromatin structure. However, interrelations between cytosine methylation, MBD proteins, and the histone-modifying system remain to be elucidated (1,3,11).MBD1 is known to act as a transcriptional repressor through the cooperation of MBD, cysteine-rich CXXC domains, and a C-terminal transcriptional repression domain (TRD) (12-15). The conserved CXXC sequence was originally found in DNMT1 and the trithorax group protein ALL-1, but its precise role is unknown (12, 15). MBD1 produces an active transcriptional repression that was partially reversed by the addition of histone deacetylase inhibitors (13). During investigation of the mechanism of transcriptional repression by the TRD of MBD1, we have recently found that a transcriptional mediator, MBD1-containing chromatin associated factor (MCAF), binds the TRD of MBD1 to form the repressive complex (16). In addition, the MBD of MBD1 binds a symmetrically methylated CpG sequence (17), but transcriptional roles of this domain have not been investigated. Little is understood about the state of histone modifications within MBD1-containing heterochromatin on methylated gene promoters. In this study, we show that H3 (Lys-9) methylase Suv39h1 and the methyl lysine-binding protein HP1 interact with MBD of MBD1. Suv39h1 enhances MBD1-mediated transcriptional repression via MBD but not the C-terminal TRD of MBD1. Furthermore, MBD1 associates with histone deacetylases through Suv39h1. Our data suggest that MBD1 tethers the Suv39h1-HP1 complex to methylated DNA regions. We discuss the possible pathway from DNA