Specific Low/Endogenous Replication Stress Response Protects Genomic Stability via Controlled ROS Production in an Adaptive Way and Is Dysregulated in Transformed Cells

Abstract

Cells are assaulted daily by stresses that jeopardize genome integrity. Primary human cells adapt their response to the intensity of replication stress (RS) in a diphasic manner: below a stress threshold, the canonical DNA damage response (cDDR) is not activated, but a noncanonical cellular response, low-level stress-DDR (LoL-DDR), has recently been described. LoL-DDR prevents the accumulation of premutagenic oxidized bases (8-oxoguanine) through the production of ROS in an adaptive way. The production of RS-induced ROS (RIR) is tightly controlled: RIR are excluded from the nucleus and are produced by the NADPH oxidases DUOX1/DUOX2, which are controlled by NF-κB and PARP1; then, RIR activate the FOXO1-detoxifying pathway. Increasing the intensity of RS suppresses RIR via p53 and ATM. Notably, LoL-DDR is dysregulated in cancer cell lines, in which RIR are not produced by NADPH oxidases, are not detoxified under high-level stress, and favor the accumulation of 8-oxoguanine. LoL-DDR dysregulation occurred at an early stage of cancer progression in an in vitro model. Since, conversely, ROS trigger RS, this establishes a vicious cycle that continuously jeopardizes genome integrity, fueling tumorigenesis. These data reveal a novel type of ROS-controlled DNA damage response and demonstrate the fine-tuning of the cellular response to stress. The effects on genomic stability and carcinogenesis are discussed here.

Keywords: DNA damage response; NF-κB; PARP1; ROS; genetic instability; replication stress.

Figure 1

(A). The biphasic response model to DNA damage [25]. Primary cells adapt their response to the intensity of replication stress in two distinct phases: the low-level/endogenous stress response and the high-level stress response. Below a threshold of stress (red arrow), cells engage in a low-level response (LoL-DDR), which does not arrest DNA synthesis and cell cycle progression. The LoL-DDR regulates the production of extranuclear ROS (RIR) under cellular control via the following cascades: PARP1 activates NF-κB, which induces the expression of the DUOX1 and DUOX2 genes. Furthermore, NF-κB induces the expression of proinflammatory cytokines. Then DUOX1 and DUOX2 proteins are translated in the cytoplasm and produce ROS (RIR) in the cytoplasm. RIR produced by DUOX1 and DUOX 2 induce the detoxification program FOXO1, which protects against the accumulation of premutagenic lesions, such as 8-oxoguanine, in an adaptive-type detoxification response. Above this threshold, cells accumulate DSBs and activate the cDDR, which detoxifies RIR. (B). Effects of replication stress on RIR production (top blue line) and the accumulation of 8-oxoguanine (bottom red line) [25]. Below the stress threshold (red arrow), the cells are not blocked, and RIR accumulate; concomitantly, 8-oxoguanine levels in the genome decrease. Beyond the stress threshold (red arrow), RIR decrease, and 8-oxoguanine accumulation returns to basal levels.

Figure 2

Accumulation of RIR and 8-oxoG in immortalized/transformed cell lines. (A). Accumulation of RIR in immortalized/transformed cell lines. (B). RIR accumulation in telomerase-immortalized human fibroblasts (BJ-hTERT). (C). 8-oxoG accumulation in immortalized/transformed cell lines. (D). 8-oxoG accumulation in telomerase-immortalized human fibroblasts (BJ-hTERT).

Figure 3

Networks of ROS/replication stress in primary versus cancer cells.