Diminished Carcinogen Detoxification Is a Novel Mechanism for Hypoxia-inducible Factor 1-mediated Genetic Instability

The HIF1-pathway is induced in many tumours and associated with poorer outcome. The hypoxia responsive transcription factor HIF-1alpha dimerizes with the aryl hydrocarbon receptor nuclear translocator, which is also an important binding partner for the aryl hydrocarbon receptor (AhR). The AhR is an important mediator in the metabolic activation and detoxification of carcinogens, such as the environmental pollutant benzo[a]pyrene (BaP). We hypothesized that HIF-1alpha activation attenuates the BaP induced AhR mediated gene expression, which may lead to increased genetic instability and malignant progression. Human lung carcinoma cells (A549) were simultaneously stimulated with CoCl(2) which leads to HIF-1alpha stabilization and varying concentrations of BaP. Both quantitative PCR and immunoblotting analysis indicated that the induction of the hypoxic response pathway significantly reduced the levels of AhR downstream targets CYP1A1 and CYP1B1 and AhR protein binding to ARNT. We further demonstrated that the BaP-induced HPRT-mutation frequency and gammaH2AX foci were markedly amplified when the HIF1-pathway was induced. BaP-DNA adducts were only marginally increased and transient strand breaks were diminished by HIF-1 induction, indicating changes in DNA repair. These data indicate that concurrent exposure of tumour cells to hypoxia and exogenous genotoxins can enhance genetic instability.