Therefore, hypoxic cancer

cells have to deal with the toxic effect of ROS; however, if cancer cells have already acquired gene mutations, for instance mutated p53, which overcomes apoptosis signals triggered by H/R,45 these cells have an increased probability of gaining additional mutations. Although learn more ROS can generate various types of modified bases in DNA, 7,8-dihydro-8-oxoguanine (8-oxo-G) is frequently generated.46 For example, the hypoxic human cervical cancer cells, HeLa, placed under 1% oxygen for 24 h, produced excessive amounts of ROS at 30 min after reoxygenation.47 This overproduction of ROS was transient and lasted for 2 h after re-oxygenation. Simultaneously, the same cell population generating ROS also exhibited extensive DNA damage with 8-oxoguanine.47 The 8-oxo-G:C

pair, if not repaired, generates G:C > T:A or A:T > C:G transversions. These mutations are frequently found in sporadic human cancers, including lung, breast, ovarian, gastric and colon cancers.48 In in vivo and in vitro hypoxia models, an increase in transversion mutations, RG7204 order such as G:C > T:A and A:T > G:C, has been reported,10 suggesting an important carcinogenic role of ROS generated by H/R in tumor tissues. Reactive oxygen species also induce DNA slippage mutations at microsatellite sequences in human cells. When human lung cancer cells carrying plasmid vector with cytosine-adenine (CA) repeats were treated with ROS generating chemicals, paraquat and H2O2, a significant increase Farnesyltransferase in deletion or insertion mutations was observed within CA repeats.49 Similarly, Gasche et al. showed that the frequency of microsatellite mutations (CA repeats) in transfected plasmids was increased by H2O2 treatment in human colon cancer cells.50 Yamada et al. examined the effect of H2O2 treatment on mutation frequencies of mononucleotide (A or G repeats) and di-nucleotide repeats (CA repeats) in non-cancer human diploid cell lines. They found that H2O2 treatment decreased the mutation

frequency of mononucleotide repeats, but increased the mutation frequency of di-nucleotide repeats in non-cancer diploid human cells. They speculated that ROS induces low levels of mutations in di-nucleotide repeats.51 In accordance with the effect of ROS on microsatellite loci in human cells, Chang et al. reported that non-toxic levels of H2O2 impair mismatch repair activity,52 which leads to DNA slippage mutations at microsatellite loci (see below). In order to faithfully transmit genetic information to a progenitor cell, the cell is equipped with mechanisms that sense DNA damage in the genome (sensor), transmit a DNA damage-signal to repair system and cell cycle machinery (signal), and target a cell for apoptosis if damage is not repaired (effector). There is some evidence that H/R activates DNA damage response.