Cellular senescence is a stress response that accompanies stable exit from the cell cycle. Classically, senescence, particularly in human cells, involves the p53 and p16/Rb pathways, and often both of these tumor suppressor pathways need to be abrogated to bypass senescence. In parallel, a number of effector mechanisms of senescence have been identified and characterized. These studies suggest that senescence is a collective phenotype of these multiple effectors, and their intensity and combination can be different depending on triggers and cell types, conferring a complex and diverse nature to senescence. Series of studies on senescence-associated secretory phenotype (SASP) in particular have revealed various layers of functionality of senescent cells in vivo. Here we discuss some key features of senescence effectors and attempt to functionally link them when it is possible.

Senescence, a persistent form of cell cycle arrest, is often associated with a diverse secretome, which provides complex functionality for senescent cells within the tissue microenvironment. We show that oncogene-induced senescence (OIS) is accompanied by a dynamic fluctuation of NOTCH1 activity, which drives a TGF-β-rich secretome, whilst suppressing the senescence-associated pro-inflammatory secretome through inhibition of C/EBPβ. NOTCH1 and NOTCH1-driven TGF-β contribute to ‘lateral induction of senescence’ through a juxtacrine NOTCH-JAG1 pathway. In addition, NOTCH1 inhibition during senescence facilitates upregulation of pro-inflammatory cytokines, promoting lymphocyte recruitment and senescence surveillance in vivo. Because enforced activation of NOTCH1 signalling confers a near mutually exclusive secretory profile compared to typical senescence, our data collectively indicate that the dynamic alteration of NOTCH1 activity during senescence dictates a functional balance between these two distinct secretomes: one representing TGF-β and the other pro-inflammatory cytokines, highlighting that NOTCH1 is a temporospatial controller of secretome composition.

Senescence is a stress-responsive form of stable cell cycle exit. Senescent cells have a distinct gene expression profile, which is often accompanied by the spatial redistribution of heterochromatin into senescence-associated heterochromatic foci (SAHFs). Studying a key component of the nuclear lamina lamin B1 (LMNB1), we report dynamic alterations in its genomic profile and their implications for SAHF formation and gene regulation during senescence. Genome-wide mapping reveals that LMNB1 is depleted during senescence, preferentially from the central regions of lamina-associated domains (LADs), which are enriched for Lys9 trimethylation on histone H3 (H3K9me3). LMNB1 knockdown facilitates the spatial relocalization of perinuclear H3K9me3-positive heterochromatin, thus promoting SAHF formation, which could be inhibited by ectopic LMNB1 expression. Furthermore, despite the global reduction in LMNB1 protein levels, LMNB1 binding increases during senescence in a small subset of gene-rich regions where H3K27me3 also increases and gene expression becomes repressed. These results suggest that LMNB1 may contribute to senescence in at least two ways due to its uneven genome-wide redistribution: first, through the spatial reorganization of chromatin and, second, through gene repression.