Homocysteine induces programmed cell death in human vascular endothelial cells through activation of the unfolded protein response

Severe hyperhomocysteinemia is associated with endothelial cell injury that may contribute to an increased incidence of thromboembolic disease. In thi s study, homocysteine induced programmed cell death in human umbilical vein endothelial cells as measured by TdT-mediated dUTP nick end labeling assay , DNA ladder formation, induction of caspase 3-like activity, and cleavage of procaspase 3. Homocysteine-induced cell death was specific to homocystei ne, was not mediated by oxidative stress, and was mimicked by inducers of t he unfolded protein response (UPR), a signal transduction pathway activated by the accumulation of unfolded proteins in the lumen of the endoplasmic r eticulum. Dominant negative forms of the endoplasmic reticulum-resident pro tein kinases IRE1 alpha and -beta, which function as signal transducers of the UPR prevented the activation of glucose-regulated protein 78/immunoglob ulin chain-binding protein and C/EBP homologous protein/growth arrest and D NA damage-inducible protein 153 in response to homocysteine. Furthermore, o verexpression of the point mutants of IRE1 with defective RNase more effect ively suppressed the cell death than the kinase-defective mutant. These res ults indicate that homocysteine induces apoptosis in human umbilical vein e ndothelial cells by activation of the UPR and is signaled through IRE1. The studies implicate that the UPR may cause endothelial cell injury associate d with severe hyperhomocysteinemia.