Intestinal epithelial cell dysfunction is mediated by an endothelial-specific radiation-induced bystander effect.

The response of endothelial cells (EC) to high radiation doses leads to damage of normal tissue or tumor. The precise mechanisms of the endothelial-tissue linkage are still largely unknown. We investigated the possible involvement of a bystander effect, secondary to endothelial damage, in tissue response to radiation. Proliferating human intestinal epithelial T84 cells were grown in a non-contact co-culture with confluent primary human microvascular EC (HMVEC-L). The bystander response in unirradiated T84 cells co-cultured with irradiated EC was studied by evaluating cell growth, cell death and epithelial morphology. Twenty-four hours after exposure of EC to 15 Gy, unirradiated T84 cells showed a decreased cell number (29%) and percentage in mitosis (66%) as well as increased apoptosis (1.5-fold) and cell surface area (1.5-fold), highlighting the involvement of bystander effects on T84 cells after irradiation of EC. Furthermore, the responses of T84 cells were amplified when EC and T84 cells were irradiated together, indicating that the bystander response in T84 cells adds further to direct radiation damage. As opposed to direct irradiation, the T84 cell bystander response did not involve the cell cycle-related protein p21(Waf1) (CDKN1A) and pro-apoptosis protein BAX. The bystander effect was specific to EC since the irradiation of human colon fibroblasts did not induce bystander responses in unirradiated T84 cells. These results strengthen previous in vivo evidence of the role of EC in tissue damage by radiation. In addition, this study provides a suitable and useful model to identify soluble factors involved in bystander effects secondary to endothelial damage. Modulating such factors may have important clinical implications.