A major contribution of the HIF-1α pathway in the oxygen effect

Oxygen plays an essential role in the response to ionizing radiation through the production of ROS induced by water radiolysis while hypoxia may lead to radioresistance. However, compared to photons, carbon ions have the advantage of a high relative biological efficiency (EBR) independently of the oxygen concentration. The protein HIF-1α (Hypoxia-Inducible Factor 1α) is considered as the major transcriptional regulator of the cellular response to oxygen homeostasis. HIF-1α is thus involved in the resistance to photons whereas its role in the response to carbon ions remains unclear. This work aims at clarifying the role of HIF-1 in the oxygen effect. Cancer Stem Cells (CSCs) isolated from Head-and-Neck-Squamous Cell Carcinoma (HNSCC) were irradiated with photons (250kV) and carbon ions (75Mev/n) in normoxic or hypoxic (1% O2) conditions. For two HNSCC cell lines and their CSC subpopulation, in response to photons under hypoxia, an OER (Oxygen Enhancement Ratio calculated as the ratio of dose inducing 10% survival after photon or carbon ion irradiation in hypoxic versus normoxic conditions) upper than 1.2 was associated with HIF-1α expression. This stabilization appears earlier in CSCs than in non-CSCs and is correlated with the variation of ROS levels, confirming the adaptive properties of CSCs (located in tumor hypoxic niches) to hypoxia. The diffuse ROS production by photons (measured by flow cytometry) is concomitant with HIF-1α expression and essential to its activation. Compared with photons, the oxygen effect is canceled after carbon ion exposure (OER=1) and no expression of HIF-1α was observed in normoxia, probably due to the ROS localization in the track, insufficient to stabilize HIF-1α. Inhibition of HIF-1α with a siRNA leads to the decrease of HNSCC-CSC survival, and subsequently radiosensitization, after both types of radiations in hypoxic conditions (OER<1). The signaling pathways leading to HIF-1α expression are currently investigated. These results demonstrate that HIF-1 plays a key role in the response of CSCs and non-CSCs to photon and carbon ion irradiations, participating to radioresistance by increasing cell survival. This makes the HIF-1α targeting an attractive therapeutic challenge. Supported by LabEx PRIMES (ANR11LABX0063), France Hadron (ANR11INBS0007) and Lyric (INCaDGOS4664)