Oncogene-induced senescence (OIS) is crucial for tumour suppression. Senescent cells implement a complex pro-inflammatory response termed the senescence-associated secretory phenotype (SASP). The SASP reinforces senescence, activates immune surveillance and paradoxically also has pro-tumorigenic properties. Here, we present evidence that the SASP can also induce paracrine senescence in normal cells both in culture and in human and mouse models of OIS in vivo. Coupling quantitative proteomics with small-molecule screens, we identified multiple SASP components mediating paracrine senescence, including TGF-β family ligands, VEGF, CCL2 and CCL20. Amongst them, TGF-β ligands play a major role by regulating p15^INK4b and p21^CIP1. Expression of the SASP is controlled by inflammasome-mediated IL-1 signalling. The inflammasome and IL-1 signalling are activated in senescent cells and IL-1α expression can reproduce SASP activation, resulting in senescence. Our results demonstrate that the SASP can cause paracrine senescence and impact on tumour suppression and senescence in vivo.

Senescent cells secrete a combination of factors collectively known as the senescence-associated secretory phenotype (SASP). The SASP reinforces senescence and activates an immune surveillance response, but it can also show pro-tumorigenic properties and contribute to age-related pathologies. In a drug screen to find new SASP regulators, we uncovered the mTOR inhibitor rapamycin as a potent SASP suppressor. Here we report a mechanism by which mTOR controls the SASP by differentially regulating the translation of the MK2 (also known as MAPKAPK2) kinase through 4EBP1. In turn, MAPKAPK2 phosphorylates the RNA-binding protein ZFP36L1 during senescence, inhibiting its ability to degrade the transcripts of numerous SASP components. Consequently, mTOR inhibition or constitutive activation of ZFP36L1 impairs the non-cell-autonomous effects of senescent cells in both tumour-suppressive and tumour-promoting contexts. Altogether, our results place regulation of the SASP as a key mechanism by which mTOR could influence cancer, age-related diseases and immune responses.

Somatic cells can be reprogrammed into induced pluripotent stem (iPS) cells by overexpressing combinations of factors such as Oct4, Sox2, Klf4, and c-Myc. Reprogramming is slow and stochastic, suggesting the existence of barriers limiting its efficiency. Here we identify senescence as one such barrier. Expression of the four reprogramming factors triggers senescence by up-regulating p53, p16^INK4a, and p21^CIP1. Induction of DNA damage response and chromatin remodeling of the INK4a/ARF locus are two of the mechanisms behind senescence induction. Crucially, ablation of different senescence effectors improves the efficiency of reprogramming, suggesting novel strategies for maximizing the generation of iPS cells.

Oncogene-induced senescence is a potent barrier to tumorigenesis that limits cellular expansion following certain oncogenic events. Senescent cells display a repressive chromatin configuration thought to stably silence proliferation-promoting genes while simultaneously activating an unusual form of immune surveillance involving a secretory program referred to as the senescence-associated secretory phenotype (SASP). Here, we demonstrate that senescence also involves a global remodeling of the enhancer landscape with recruitment of the chromatin reader BRD4 to newly activated super-enhancers adjacent to key SASP genes. Transcriptional profiling and functional studies indicate that BRD4 is required for the SASP and downstream paracrine signaling. Consequently, BRD4 inhibition disrupts immune cell–mediated targeting and elimination of premalignant senescent cells in vitro and in vivo. Our results identify a critical role for BRD4-bound super-enhancers in senescence immune surveillance and in the proper execution of a tumor-suppressive program. Significance: This study reveals how cells undergoing oncogene-induced senescence acquire a distinctive enhancer landscape that includes formation of super-enhancers adjacent to immune-modulatory genes required for paracrine immune activation. This process links BRD4 and super-enhancers to a tumor-suppressive immune surveillance program that can be disrupted by small molecule inhibitors of the bromo and extra terminal domain family of proteins. Cancer Discov; 6(6); 612–29. ©2016 AACR. See related commentary by Vizioli and Adams, p. 576. This article is highlighted in the In This Issue feature, p. 561

The INK4a/ARF tumor suppressor locus, a key executor of cellular senescence, is regulated by members of the Polycomb group (PcG) of transcriptional repressors. Here we show that signaling from oncogenic RAS overrides PcG-mediated repression of INK4a by activating the H3K27 demethylase JMJD3 and down-regulating the methyltransferase EZH2. In human fibroblasts, JMJD3 activates INK4a, but not ARF, and causes p16^INK4a-dependent arrest. In mouse embryo fibroblasts, Jmjd3 activates both Ink4a and Arf and elicits a p53-dependent arrest, echoing the effects of RAS in this system. Our findings directly implicate JMJD3 in the regulation of INK4a/ARF during oncogene-induced senescence and suggest that JMJD3 has the capacity to act as a tumor suppressor.