Ionizing radiation activates PERK/eIF2α/ATF4 signaling via ER stress-independent pathway in human vascular endothelial cells

Purpose: Perturbations in protein folding induce endoplasmic reticulum (ER) stress, which elicits coordinated response, namely the unfolded protein response (UPR), to cope with the accumulation of misfolded proteins in ER. In this study, we characterized mechanisms underlying ionizing radiation (IR)-induced UPR signaling pathways. Materials and methods: We analyzed alterations in UPR signaling pathways in human umbilical vein endothelial cells (HUVEC) and human coronary artery endothelial cells (HCAEC) irradiated with 15 Gy IR. Results: IR selectively activated the eIF2 alpha/ATF4 branch of the UPR signaling pathway, with no alterations in the IRE1 and ATF6 branches in HUVEC and HCAEC. Phosphorylation of PERK was enhanced in response to IR, and the IR-induced activation of the eIF2 alpha/ATF4 signaling pathway was completely inhibited by PERK knockdown with siRNA. Surprisingly, chemical chaperones, which inhibit the formation of misfolded proteins and sequential protein aggregates to reduce ER stress, failed to prevent the IR-induced phosphorylation of PERK and the subsequent activation of the eIF2 alpha/ATF4 signaling pathway. Conclusions: PERK mediates the IR-induced selective activation of the eIF2 alpha/ATF4 signaling pathway, and the IR-induced activation of PERK/eIF2 alpha/ATF4 signaling in human vascular endothelial cells is independent of alterations in protein-folding homeostasis in the ER.