Acute multi-level response to defective de novo chromatin assembly in S-phase.

Long-term perturbation of de novo chromatin assembly during DNA replication has profound effects on epigenome maintenance and cell fate. The early mechanistic origin of these defects is unknown. Here, we combine acute degradation of Chromatin Assembly Factor 1 (CAF-1), a key player in de novo chromatin assembly, with single-cell genomics, quantitative proteomics, and live-microscopy to uncover these initiating mechanisms in human cells. CAF-1 loss immediately slows down DNA replication speed and renders nascent DNA hyperaccessible. A rapid cellular response, distinct from canonical DNA damage signaling, is triggered and lowers histone mRNAs. As a result, histone variants usage and their modifications are altered, limiting transcriptional fidelity and delaying chromatin maturation within a single S-phase. This multi-level response induces a cell-cycle arrest after mitosis. Our work reveals the immediate consequences of defective de novo chromatin assembly during DNA replication, explaining how at later times the epigenome and cell fate can be altered.

Senescent cells limit p53 activity via multiple mechanisms to remain viable.

Super-enhancers regulate genes with important functions in processes that are cell type-specific or define cell identity. Mouse embryonic fibroblasts establish 40 senescence-associated super-enhancers regardless of how they become senescent, with 50 activated genes located in the vicinity of these enhancers. Here we show, through gene knockdown and analysis of three core biological properties of senescent cells that a relatively large number of senescence-associated super-enhancer-regulated genes promote survival of senescent mouse embryonic fibroblasts. Of these, Mdm2, Rnase4, and Ang act by suppressing p53-mediated apoptosis through various mechanisms that are also engaged in response to DNA damage. MDM2 and RNASE4 transcription is also elevated in human senescent fibroblasts to restrain p53 and promote survival. These insights identify key survival mechanisms of senescent cells and provide molecular entry points for the development of targeted therapeutics that eliminate senescent cells at sites of pathology.

p21 produces a bioactive secretome that places stressed cells under immunosurveillance.

Immune cells identify and destroy damaged cells to prevent them from causing cancer or other pathologies by mechanisms that remain poorly understood. Here, we report that the cell-cycle inhibitor p21 places cells under immunosurveillance to establish a biological timer mechanism that controls cell fate. p21 activates retinoblastoma protein (Rb)­dependent transcription at select gene promoters to generate a complex bioactive secretome, termed p21-activated secretory phenotype (PASP). The PASP includes the chemokine CXCL14, which promptly attracts macrophages. These macrophages disengage if cells normalize p21 within 4 days, but if p21 induction persists, they polarize toward an M1 phenotype and lymphocytes mount a cytotoxic T cell response to eliminate target cells, including preneoplastic cells. Thus, p21 concurrently induces proliferative arrest and immunosurveillance of cells under duress.