Lwoff-CNRS, Villejuif, France The phosphoinositide 3-kinase related kinases (PIKKs) family mainly comprised the ATR ATM and DNA-PK proteins. These large

proteins initiate cellular stress responses when genome integrity is compromised. Emerging evidence suggest that hypoxia led to activation of these stress kinases in severe hypoxic conditions. For example, stalled replication forks contribute to ATR activation. ATM is also activated in severe hypoxia (less than 0.1% O2) through alternate mechanisms that do not involve DNA breaks. However, the role of this DDR –like response on hypoxia S63845 research buy tolerance remains unknown. We first demonstrated here that the third member of the PI3KK family, DNA-PK (that AMN-107 order comprises a DNA binding sub-unit Ku and a catalytic

sub-unit DNA-PKcs) is activated by mild hypoxia conditions (0.1 to 1% O2). This was shown by Ku/DNA-PK mobilization from a soluble nucleoplasmic compartment to a less extractable nuclear fraction and its autophosphorylation on serine 2056. This activation was independent selleck kinase inhibitor FER of DNA double strand breaks (DSBs) and probably relies on the chromatin modification observed in hypoxic cells according to our preliminary results. Importantly, DNA-PK nuclear activation positively regulates

HIF-1α accumulation and its subsequent target gene expression as shown using DNA-PK deficient cells. This effect is dependent of the kinase activity of the whole DNA-PK complex since a strong decrease in HIF-1α expression was observed in cells deficient in its regulatory sub-unit Ku and in presence of a selective inhibitor of the kinase activity of DNA-PK, Nu7026. Finally, the reduced half-life of HIF-1α in DNA-PK deficient cells upon hypoxia provided a mechanistic explanation for the observed effects. In conclusion, our results demonstrate that a new nuclear and DNA dependent stress response pathway contributes to the adaptative response of hypoxic tumours cells and shed a new light on the interest of DNA-PK inhibitors to down-regulate HIF-1α expression in human tumours. Poster No.