Background/Aims: Endoplasmic reticulum (ER) stress is an important event in atherosclerosis. Recent studies have shown that ER stress deregulates cholesterol metabolism via multiple pathways. This study aimed to determine the relationship between ER stress and lipid metabolism and to verify that upregulation of miR-33 is involved in this process. Methods: An atherosclerosis model was established in apolipoprotein E-deficient (ApoE/) mice fed a Western diet, and THP-1 derived macrophages were used in this study. Hematoxylin-eosin and Oil Red O staining were used to quantify the atherosclerotic plaques. 1,1'-Dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate labeled oxidized low-density lipoprotein binding assay and a Cholesterol Efflux Fluorometric Assay Kit were used to observe cholesterol uptake and efflux. The mRNA and protein levels of biomarkers associated with ER stress and cholesterol metabolism in atherosclerotic plaques and macrophages were evaluated by real-time PCR and western blotting, respectively. Immunofluorescence was used to observe alterations of ABCA1 localization. Small interfering RNAs were used to knock down CHOP and miR-33 in macrophages to alter CHOP and miR-33 expression. Results: Atherosclerotic lesions and systemic lipid levels were ameliorated after inhibition of ER stress (tauroursodeoxycholic acid) in vivo. In vitro studies confirmed that ER stress regulated the lipid catabolism of macrophages by promoting cholesterol uptake, inhibiting cholesterol efflux, and modulating the expression of related transporters. CHOP contributed to lipid metabolism disorder following ER stress. Furthermore, over-expression of miR-33 was involved in ER stress that induced lipid metabolism disorder in macrophages. These findings support a model of ER stress induction by oxidized low-density lipoprotein that affects macrophage lipid catabolism disorder. Conclusion: Our data shed new light on the relationship between ER stress and lipid metabolism in vivo and in vitro, and confirm that upregulation of miR-33 is involved in this process. The relationship between ER stress and miR-33 represents a novel target for the treatment of atherosclerosis. (C) 2018 The Author(s) Published by S. Karger AG, Basel.