Targeting hypoxic cells through the DNA damage response

Abstract

Exposure to hypoxia-induced replication arrest initiates a DNA damage response that includes both ATR- and ATM-mediated signaling. DNA fiber analysis was used to show that these conditions lead to a replication arrest during both the initiation and elongation phases, and that this correlated with decreased levels of nucleotides. The DNA damage response induced by hypoxia is distinct from the classical pathways induced by damaging agents, primarily due to the lack of detectable DNA damage, but also due to the coincident repression of DNA repair in hypoxic conditions. The principle aims of the hypoxia-induced DNA damage response seem to be the induction of p53-dependent apoptosis or the preservation of replication fork integrity. The latter is of particular importance should reoxygenation occur. Tumor reoxygenation occurs as a result of spontaneous changes in blood flow and also therapy. Cells experiencing hypoxia and/or reoxygenation are, therefore, sensitive to loss or inhibition of components of the DNA damage response, including Chk1, ATM, ATR, and poly(ADP-ribose) polymerase (PARP). In addition, restoration of hypoxia-induced p53-mediated signaling may well be effective in the targeting of hypoxic cells. The DNA damage response is also induced in endothelial cells at moderate levels of hypoxia, which do not induce replication arrest. In this situation, phosphorylation of H2AX has been shown to be required for proliferation and angiogenesis and is, therefore, an attractive potential therapeutic target.

Figure 1. Hypoxia-induced replication arrest triggers a DNA damage response

Chronic exposure to levels of oxygen below 0.1% lead to replisome disassembly therefore preventing any possible replication restart during reoxygenation. In contrast, hypoxic cells arrested for only short periods of time do undergo replication restart but do so in the presence of reoxygenation-induced DNA damage and hypoxia-repressed DNA repair. Highlighted in red are the potential therapeutic targets for targeting cells cycling through hypoxia/reoxygenation or angiogenesis discussed in the text. Specific targets which have inhibitors in clinical trials are indicated in italics.

Figure 2. Schematic representation of the DDR to hypoxia

The DDR to hypoxia is both oxygen and cell type specific, for example endothelial cells have been shown to be reliant on γH2AX for proliferation and angiogenesis. At severe levels of hypoxia nucleotide levels fall leading to stalled replication and a DDR. Highlighted in red are the potential therapeutic targets for targeting cells cycling through hypoxia/reoxygenation or angiogenesis discussed in the text.