Role of Endoplasmic Reticulum Calcium Disequilibria in the Mechanism of Homocysteine-Induced ER Stress
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Our laboratory demonstrated that hyperhomocysteinemia accelerates atherosclerosis in mouse models through ER stress and activation of the unfolded protein response (UPR). In this study, we tested the hypothesis that homocysteine-induced ER stress may arise from ER-Ca(2+) disequilibria. We found that homocysteine-induced cytosolic Ca(2+) transients in T24/83 cells and human aortic smooth muscle cells (HASMCs). These calcium effects occurred at concentrations of homocysteine in the external medium (1-5 mM) that increase intracellular homocysteine in these cell types. Prolonged homocysteine treatment (5 h) at these exogenous concentrations reduced ER-Ca(2+) emptying evoked by thapsigargin. However, these homocysteine-induced effects on ER-Ca(2+) emptying were of a much smaller magnitude than those evoked by A23187 or thapsigargin (ER stressors known to induce ER stress through ER-Ca(2+) depletion). T24/83 cells stably overexpressing the Ca(2+)-binding ER chaperone GRP78 showed diminished cytosolic Ca(2+) transients induced by homocysteine and reduced ER-Ca(2+) emptying evoked by thapsigargin. Prevention of the homocysteine-induced UPR by cycloheximide pretreatment normalized GRP78 expression and ER-Ca(2+) emptying evoked by thapsigargin. These results are inconsistent with a mechanism of ER stress induction by homocysteine through ER-Ca(2+) depletion.
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