97 Down-regulation of these proteins by hypoxia was associated with a decreased level of HRR activity as measured by the internal reporter system. Furthermore, they observed that decreased levels of

these HRR proteins was due to reduced translation of corresponding mRNAs, suggesting involvement of the mTORC1 or UPR pathways described above. They also demonstrated that depressing HRR by chronic hypoxia increased the cells’ sensitivity to the DNA cross-linking agents mitomycin C and cisplatin, and to radiation.92 Recently, a new pathway to down-regulate one of HRR gene, RAD52, by hypoxia was reported. Crosby et al. showed that HIF1-dependent up-regulation of the micro-RNAs, mir-210 and mir-373, results in suppression of RAD52 transcription.98 They showed that both micro-RNAs interact with the 3′ untranslated region of RAD52, suggesting that mir-210 and mir-373 are responsible for the repression Enzalutamide cost of RAD52.98 Taken together, these Birinapant results suggest that

depression of HRR by hypoxia may force a cell to use error-prone NHEJ that generates many genetic alterations. Yuan et al. showed that hypoxia increases the UV-induced mutation rate in tissue culture cells, suggesting that hypoxia represses nucleotide excision repair (NER).99 Later, Crosby et al. showed that hypoxia up-regulates mir-373, which in turn degrades the RAD23B transcript, one of the genes involved in NER.98 RAD23B recognizes UV-induced DNA damage in association with XPC and this complex recruits proteins, including XPA, RPA, XPB and XPD, for DNA unwinding. A small patch of single-stranded DNA containing damage is excised by XPG and XPF/ERCC1, and repaired and sealed by polymerase and ligase, respectively.100 Thus, repression of RAD23B by hypoxia can impair NER. During replication, DNA polymerase sometimes incorporates a wrong base generating a mismatch or generates a single-stranded loop within a highly repetitive sequence,

for instance at a microsatellite locus. These mistakes are repaired prior to mitosis by the MMR pathway. There are six MMR proteins involved in this system in humans, MSH2, MSH6, MSH3, MLH1, PMS2 and MLH3. The recognition of mismatch or loops containing one nucleotide Doxorubicin supplier is mainly mediated by MutSα (a heterodimer of MSH2 and MSH6). Recognition of a loop containing two or more nucleotides is mediated by MutSβ.101 Excision of a mismatched base or a loop on a newly synthesized strand is initiated by recruited MutLα (a heterodimer of MLH1 and PMS2) or MutLβ (a heterodimer of MLH1 and MLH3) and followed by exonuclease (EXO1). Re-synthesis is done by DNA polymerase and the nick is sealed by DNA ligase.101 If one of six MMR proteins is disabled, the mutation frequency in the microsatellite sequences increases. The microsatellite slippage mutations described above are associated with hypoxia-induced repression of MMR proteins, including MSH2, MSH6, MSH3, MLH1 and PMS2.85–87 Mihaylova et al. first demonstrated that severe chronic hypoxia (<0.