Notch signalling mediates secondary senescence

Abstract

Oncogene induced senescence (OIS) is a tumour suppressive response to oncogene activation that can be transmitted to neighbouring cells through secreted factors of the senescence 20 associated secretory phenotype (SASP). Using single-cell transcriptomics we observed two distinct endpoints, a primary marked by Ras and a secondary by Notch. We find that secondary senescence in vitro and in vivo requires Notch, rather than SASP alone as previously thought.Currently, primary and secondary senescent cells are not thought of as functionally distinct endpoints. A blunted SASP response and the induction of fibrillar collagens in secondary 25 senescence compared to OIS point towards a functional diversification.2 One Sentence Summary: Notch signalling is an essential driver of secondary senescence with primary and secondary senescence being distinct molecular endpoints. Main Text:Cellular senescence is a stress response, resulting in a stable cell cycle arrest, and has been 5 implicated in tumour suppression, ageing and wound healing (1-4). Aberrant activation of the Ras oncogene triggers oncogene-induced senescence (OIS), conferring a precancerous state (5,6). OIS is an in vivo tumour suppressor mechanism preventing transformation and halting tumour growth (7,8) with the p53 and Rb/p16 pathways acting as major mediators of senescence induction and maintenance (6,9). OIS is characterised by multiple changes in 10 phenotype, such as heterochromatic foci (9-13) and the senescence-associated secretory phenotype (SASP, 14-16). Through the secretion of extracellular matrix proteases and interleukins and chemokines, OIS cells recruit immune cells, mediating their own clearance. However, SASP has been implicated in cancer initiation (17) by creating an inflammatory pro-tumorigenic microenvironment. Moreover, SASP factors play a role in cellular 15 reprogramming (18,19) and contribute to ageing and tissue degeneration (20-21). SASP acts in a paracrine fashion to induce secondary senescence in surrounding cells and mediates tissue re-organisation. Secondary senescence occurs in cells not directly receiving the stress insult 22 with paracrine secondary senescence being mediated by secreted SASP factors. Paracrine secondary senescence is thought to enhance immune surveillance and to act as a 20 fail-safe mechanism minimising chances of retaining damaged cells (16,22,23). More recently, ectopic Notch pathway activation has been implicated as an intermediate, unstable phenomenon during primary senescence induction, resulting in a distinct secretome.However, Notch as an endpoint for primary senescence and in secondary senescence