Background & aims

Radiation therapy in the pelvic area is associated with side effects that impact the quality of life of cancer survivors. Interestingly, the gastrointestinal tract is able to adapt to significant changes in oxygen availability, suggesting that mechanisms related to hypoxia sensing help preserve tissue integrity in this organ. However, hypoxia-inducible factor (HIF)-dependent responses to radiation-induced gut toxicity are unknown. Radiation-induced intestinal toxicity is a complex process involving multiple cellular compartments. Here, we investigated whether epithelial or endothelial tissue-specific HIF-1α deletion could affect acute intestinal response to radiation.

Methods

Using constitutive and inducible epithelial or endothelial tissue-specific HIF-1α deletion, we evaluated the consequences of epithelial or endothelial HIF-1α deletion on radiation-induced enteritis after localized irradiation. Survival, radiation-induced tissue injury, molecular inflammatory profile, tissue hypoxia, and vascular injury were monitored.

Results

Surprisingly, epithelium-specific HIF-1α deletion does not alter radiation-induced intestinal injury. However, irradiated VECad-Cre^+/-HIF-1α^FL/FL mice present with lower radiation-induced damage, showed a preserved vasculature, reduced hypoxia, and reduced proinflammatory response compared with irradiated HIF-1α^FL/FL mice.

Conclusions

We demonstrate in vivo that HIF-1α impacts radiation-induced enteritis and that this role differs according to the targeted cell type. Our work provides a new role for HIF-1α and endothelium-dependent mechanisms driving inflammatory processes in gut mucosae. Results presented show that effects on normal tissues have to be taken into account in approaches aiming to modulate hypoxia or hypoxia-related molecular mechanisms.

The pathophysiological mechanism involved in side effects of radiation therapy, and especially the role of the endothelium remains unclear. Previous results showed that plasminogen activator inhibitor-type 1 (PAI-1) contributes to radiation-induced intestinal injury and suggested that this role could be driven by an endothelium-dependent mechanism. We investigated whether endothelial-specific PAI-1 deletion could affect radiation-induced intestinal injury. We created a mouse model with a specific deletion of PAI-1 in the endothelium (PAI-1KO(endo)) by a Cre-LoxP system. In a model of radiation enteropathy, survival and intestinal radiation injury were followed as well as intestinal gene transcriptional profile and inflammatory cells intestinal infiltration. Irradiated PAI-1KO(endo) mice exhibited increased survival, reduced acute enteritis severity and attenuated late fibrosis compared with irradiated PAI-1(flx/flx) mice. Double E-cadherin/TUNEL labeling confirmed a reduced epithelial cell apoptosis in irradiated PAI-1KO(endo). High-throughput gene expression combined with bioinformatic analyses revealed a putative involvement of macrophages. We observed a decrease in CD68(+)cells in irradiated intestinal tissues from PAI-1KO(endo) mice as well as modifications associated with M1/M2 polarization. This work shows that PAI-1 plays a role in radiation-induced intestinal injury by an endothelium-dependent mechanism and demonstrates in vivo that the endothelium is directly involved in the progression of radiation-induced enteritis.