Biological role of the regulation of the expressions of transpersonal DNA polymereases by hypoxia

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AIM : It has been demonstrated that hypoxia/reoxygenation increases mutation frequency in tumor cells, leading to genetic instability. However, its molecular mechanism remains to be obscure. It is known that a large amount of reactive oxygen species is produced in cells after hypoxia/reoxygenation which generate highly mutagenic base 8-oxoG. In this study, we evaluated the possible involvement of error-prone DNA polymerases that can bypass 8-oxoG and proceed translesional DNA replication in hypoxia/reoxygenation-induced mutagenesis. RESULTS : The expressions of error-prone DNA polymerases,η,ι,κ and μ, in HeLa cells were examined by RT-PCR after culturing them under hypoxic conditions (1% O_2) for 6-24 h. The results showed that the expression of DNA polymerase ι (pol ι) gene was significantly up-regulated by hypoxia. Hypoxia mimetic desferrioxamine and CoCl_2 also increased the expression of pol ι gene. Hypoxia up-regulated the pol ι gene in all of 8 human cell lines examined. Luciferase reporter assays revealed that the transcriptional activity of the pol ι gene promoter region (-1345/+419) was increased in response to hypoxia and constitutive active HIF-1 α and that the activity was suppressed by dominant negative HIF-1 α. Furthermore, gel shift and chromatin immunoprecipitation assays demonstrated the importance of the ACGTG sequence in the intron 1 of pol ι gene for HIF-1 responsiveness. On the other hand, it was found that the frequency of mutation was increased in MCF7 cells by hypoxia/reoxygenation, but it was suppressed by treatment of the cells with pol ι -specific small interfering RNA. CONCLUSION : The results suggest that Pol ι is involved in the hypoxia/reoxygenation-induced mutagenesis.