Vascular smooth muscle cells (VSMCs) are key participants in both early and late-stage atherosclerosis. VSMCs invade the early atherosclerotic lesion from the media, expanding lesions, but also forming a protective fibrous cap rich in extracellular matrix to cover the ‘necrotic’ core. Hence, VSMCs have been viewed as plaque-stabilizing, and decreased VSMC plaque content—often measured by expression of contractile markers—associated with increased plaque vulnerability. However, the emergence of lineage-tracing and transcriptomic studies has demonstrated that VSMCs comprise a much larger proportion of atherosclerotic plaques than originally thought, demonstrate multiple different phenotypes in vivo, and have roles that might be detrimental. VSMCs down-regulate contractile markers during atherosclerosis whilst adopting alternative phenotypes, including macrophage-like, foam cell-like, osteochondrogenic-like, myofibroblast-like, and mesenchymal stem cell-like. VSMC phenotypic switching can be studied in tissue culture, but also now in the media, fibrous cap and deep-core region, and markedly affects plaque formation and markers of stability. In this review, we describe the different VSMC plaque phenotypes and their presumed cellular and paracrine functions, the regulatory mechanisms that control VSMC plasticity, and their impact on atherogenesis and plaque stability.

Rationale: Vascular smooth muscle cell (VSMC) senescence promotes atherosclerosis and features of plaque instability, in part, through lipid-mediated oxidative DNA damage and telomere dysfunction. SIRT6 (Sirtuin 6) is a nuclear deacetylase involved in DNA damage response signaling, inflammation, and metabolism; however, its role in regulating VSMC senescence and atherosclerosis is unclear. Objective: We examined SIRT6 expression in human VSMCs, the role, regulation, and downstream pathways activated by SIRT6, and how VSMC SIRT6 regulates atherogenesis. Methods and Results: SIRT6 protein, but not mRNA, expression was markedly reduced in VSMCs in human and mouse atherosclerotic plaques, and in human VSMCs derived from plaques or undergoing replicative or palmitate-induced senescence versus healthy aortic VSMCs. The ubiquitin ligase CHIP (C terminus of HSC70-interacting protein) promoted SIRT6 stability, but CHIP expression was reduced in human and mouse plaque VSMCs and by palmitate in a p38- and c-Jun N-terminal kinase-dependent manner. SIRT6 bound to telomeres, while SIRT6 inhibition using shRNA or a deacetylase-inactive mutant (SIRT6 ^H133Y ) shortened human VSMC lifespan and induced senescence, associated with telomeric H3K9 (histone H3 lysine 9) hyperacetylation and 53BP1 (p53 binding protein 1) binding, indicative of telomere damage. In contrast, SIRT6 overexpression preserved telomere integrity, delayed cellular senescence, and reduced inflammatory cytokine expression and changes in VSMC metabolism associated with senescence. SIRT6, but not SIRT6 ^H133Y , promoted proliferation and lifespan of mouse VSMCs, and prevented senescence-associated metabolic changes. ApoE ^−/− (apolipoprotein E) mice were generated that overexpress SIRT6 or SIRT6 ^H133Y in VSMCs only. SM22α-hSIRT6/ApoE ^−/− mice had reduced atherosclerosis, markers of senescence and inflammation compared with littermate controls, while plaques of SM22α-hSIRT6 ^H133Y /ApoE ^−/− mice showed increased features of plaque instability. Conclusions: SIRT6 protein expression is reduced in human and mouse plaque VSMCs and is positively regulated by CHIP. SIRT6 regulates telomere maintenance and VSMC lifespan and inhibits atherogenesis, all dependent on its deacetylase activity. Our data show that endogenous SIRT6 deacetylase is an important and unrecognized inhibitor of VSMC senescence and atherosclerosis.

Autophagy is a subcellular process that plays an important role in the degradation of proteins and damaged organelles such as mitochondria (a process termed “mitophagy”) via lysosomes. It is crucial for regulating protein and mitochondrial quality control and maintaining cellular homeostasis, whereas dysregulation of autophagy has been implicated in a wide range of diseases including atherosclerosis. Recent evidence has shown that the autophagic process becomes dysfunctional during the progression of atherosclerosis, regardless of whether there are many autophagy‐stimulating factors (e.g., reactive oxygen species, oxidized lipids, and cytokines) present within the atherosclerotic plaque. This review highlights the recent insights into the causes and consequences of defective autophagy in atherosclerosis, with a special focus on the role of autophagy and mitophagy in plaque macrophages, vascular smooth muscle cells (VSMCs), and endothelial cells (ECs). It has been shown that defective autophagy can promote apoptosis in macrophages but that it accelerates premature senescence in VSMCs. In the ECs, defective autophagy promotes both apoptosis and senescence. We will discuss the discrepancy between these three cell types in their response to autophagy deficiency and underline the cell type‐dependent role of autophagy, which may have important implications for the efficacy of autophagy‐targeted treatments for atherosclerosis.

Cellular senescence has been implicated in many age-related pathologies including atherosclerosis, heart failure, age-related cardiac remodeling, diabetic cardiomyopathy and the metabolic syndrome. Here, we will review the characteristics of senescent cells and their endogenous regulators, and summarize the metabolic stressors that induce cell senescence. We will discuss the evidence of cell senescence in the onset and progression of several cardiometabolic diseases and the therapeutic potential of anti-senescence therapies.

Apoptosis of macrophages and vascular smooth muscle cells (VSMCs) in advanced atherosclerotic plaques contributes to plaque progression and instability. Caspase‐3, a key executioner protease in the apoptotic pathway, has been identified in human and mouse atherosclerotic plaques but its role in atherogenesis is not fully explored. We therefore investigated the impact of caspase‐3 deletion on atherosclerosis by crossbreeding caspase‐3 knockout (Casp3^−/−) mice with apolipoprotein E knockout (ApoE^−/−) mice. Bone marrow‐derived macrophages and VSMCs isolated from Casp3^−/−ApoE^−/− mice were resistant to apoptosis but showed increased susceptibility to necrosis. However, caspase‐3 deficiency did not sensitize cells to undergo RIP1‐dependent necroptosis. To study the effect on atherosclerotic plaque development, Casp3^+/+ApoE^−/− and Casp3^−/−ApoE^−/− mice were fed a western‐type diet for 16 weeks. Though total plasma cholesterol, triglycerides, and LDL cholesterol levels were not altered, both the plaque size and percentage necrosis were significantly increased in the aortic root of Casp3^−/−ApoE^−/− mice as compared to Casp3^+/+ApoE^−/− mice. Macrophage content was significantly decreased in plaques of Casp3^−/−ApoE^−/− mice as compared to controls, while collagen content and VSMC content were not changed. To conclude, deletion of caspase‐3 promotes plaque growth and plaque necrosis in ApoE^−/− mice, indicating that this antiapoptotic strategy is unfavorable to improve atherosclerotic plaque stability.