Cells are continuously subjected to exogenous or endogenous stresses that compromise genome stability, leading to inflammation, premature aging or cancer. To counteract these stresses, the DNA damage response (DDR) coordinates a network of metabolic pathways that control the cell cycle, DNA replication and repair: following genotoxic stress, activation of the DDR results in the arrest of cell cycle progression and genome replication. This allows sufficient time and full access to essential cofactors (ATP and nucleotides) for the DNA repair machinery. This coordinated response allows surviving cells to resume replication on an intact DNA template. However, even in the absence of exogenous stress, cells are regularly subjected to unavoidable endogenous stresses, such as replicative stress and oxidative stress, which also jeopardize genome integrity. Despite continuous exposure to these chronic endogenous stresses, these untreated cells continue to proliferate and replicate their genome, suggesting that the DDR is not, or is not fully, activated. This raises the question of whether the cells actually respond to low-level stresses (corresponding to endogenous stresses) or whether they have developed specific alternative responses.
Researchers from the Institut Cochin, in collaboration with the Institut Gustave Roussy and the CEA, show here that primary human cells respond to replicative stress in two distinct phases, adapting the response to the severity of the stress. After low-intensity replicative stress, which does not lead to the activation of the canonical DDR and the complete arrest of replication, the cells produce ROS (reactive oxygen species). These replication stress-induced ROS (RIR) are synthesized by the cellular NADPH oxidase DUOX1 and DUOX2, the expression of which is induced by the transcription factor NF-κB, which is activated by the DNA damage sensing protein PARP1 (Figure). Paradoxically, this production of RIRs prevents the accumulation of pre-mutagenic oxidative damage in DNA. Indeed, in addition to being tightly controlled by the cell, RIRs are excluded from the nucleus, and activate the general ROS detoxification pathway FOXO1 that induces the expression of detoxification genes, such as SEPP1, catalase, GPX1 and SOD2 (Figure). This response also protects cells from exogenous exposure to hydrogen peroxide (H2O2), defining an adaptive-type response. Of note, treatment of patients suffering of chronic myelomonocytic leukemia with hydroxyurea, which generates replicative stress, activates the NF-κB and FOXO1 pathways in their proliferating cells, revealing activation of this pathway in vivo.
Increasing the intensity of replicative stress induces the canonical DDR, which leads to the arrest of DNA replication and suppresses RIRs in a p53 and ATM-dependent manner. These data highlight that the cellular response to replicative stress can be subdivided into two phases: a low-stress DDR (LoL-DDR: Low-Level DDR), which is adaptive, protecting against the accumulation of premutagenic lesions, but does not stop DNA replication, and a high-stress DDR, which stops replication, controls the cell cycle and DNA repair (canonical DDR), and detoxifies RIRs (figure).
In parallel, activation of the PARP1/NF-κB axis by replicative stress induces the expression of inflammatory cytokines (Figure) that could activate the innate immune response, which allows the elimination of cells carrying DNA damages. However, a chronic activation of this pathway, could lead to inflammation.
These data reveal a specific cellular defense response to low-level / endogenous stress, underlining the fine-tuning of the cellular responses to stress severity.
A noncanonical response to replication stress protects genome stability through ROS production, in an adaptive manner. Sandrine Ragu, Nathalie Droin, Gabriel Matos-Rodrigues, Aurélia Barascu, Sylvain Caillat, Gabriella Zarkovic, Capucine Siberchicot, Elodie Dardillac, Camille Gelot, Josée Guirouilh-Barbat, J. Pablo Radicella, Alexander A. Ishchenko, Jean-Luc Ravanat, Eric Solary and Bernard S. Lopez. Cell Death & Differenciation. Online on March 3, 2023. DOI: 10.1038/s41418-023-01141-0