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Previous evidence has indicated a beneficial role for aldehyde dehydrogenase 2 (ALDH2) in suppressing atherosclerotic plaque progression and instability. However, the underlying mechanism remains somewhat elusive. This study was designed to examine the effect of ALDH2 deficiency on high-cholesterol diet-induced atherosclerotic plaque progression and plaque vulnerability in atherosclerosis-prone ApoE knockout (ApoE−/−) mice with a focus on foam cell formation in macrophages and senescence of vascular smooth muscle cells (VSMCs). Serum lipid profile, plaque progression, and plaque vulnerability were examined in ApoE−/− and ALDH2/ApoE double knockout (ALDH2−/−ApoE−/−) mice after high-cholesterol diet intake for 8 weeks. ALDH2 deficiency increased the serum levels of triglycerides while it decreased levels of total cholesterol and high-density lipoprotein cholesterol. Unexpectedly, ALDH2 deficiency reduced the plaque area by 58.9% and 37.5% in aorta and aortic sinus, respectively. Plaque instability was aggravated by ALDH2 deficiency along with the increased necrotic core size, decreased collagen content, thinner fibrous cap area, decreased VSMC content, and increased macrophage content. In atherosclerotic lesions, ALDH2 protein was located in both macrophages and VSMCs. Further results revealed downregulated ALDH2 expression in aorta of aged ApoE−/− mice compared with young mice. However, in vitro study suggested that ALDH2 expression was upregulated in bone marrow-derived macrophages (BMDMs) with an opposite effect in VSMCs following 80 μg/ml oxidized low-density lipoprotein (oxLDL) treatment. Interestingly, ALDH2 deficiency displayed little effect in oxLDL-induced foam cell formation from BMDMs, while ALDH2 knockdown by siRNA and ALDH2 overexpression by lentivirus infection promoted and retarded oxLDL-induced VSMC senescence, respectively. Mechanistically, ALDH2 mitigated oxLDL-induced overproduction of mitochondrial reactive oxygen species (mROS) and activation of downstream p53/p21/p16 pathway. Clearance of mROS by mitoTEMPO significantly reversed the promotive effect of ALDH2 knockdown on VSMC senescence. Taken together, our data revealed that ALDH2 deficiency suppressed atherosclerotic plaque area while facilitating plaque instability possibly through accelerating mROS-mediated VSMC senescence. This article is part of a Special Issue entitled: Genetic and epigenetic regulation of aging and longevity edited by Jun Ren & Megan Yingmei Zhang. • Although ALDH2 deficiency reduced the atherosclerotic plaque area, plaque instability was aggravated by ALDH2 deficiency. • ALDH2 knockdown and ALDH2 overexpression promoted and retarded oxLDL-induced VSMC senescence, respectively. • ALDH2 mitigated oxLDL-induced mROS overproduction and activation of downstream p53/p21/p16 pathway. • Clearance of mROS by mitoTEMPO reversed the aggra effect of ALDH2 knockdown on VSMC senescence. [ABSTRACT FROM AUTHOR]