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The development of atherosclerotic plaques is the result of a chronic inflammatory response coordinated by stromal and immune cellular components of the vascular wall. While endothelial cells and leukocytes are well-recognised mediators of inflammation in atherosclerosis, the role of smooth muscle cells (SMCs) remains incompletely understood. Here we aimed to address the role of canonical NF-κB signalling in SMCs in the development of atherosclerosis. We investigated the role of NF-κB signalling in SMCs in atherosclerosis by employing SMC-specific ablation of NEMO, an IKK complex subunit that is essential for canonical NF-κB activation, in ApoE−/− mice. We show that SMC-specific ablation of NEMO (NEMOSMCiKO) inhibited high fat diet induced atherosclerosis in ApoE−/− mice. NEMOSMCiKO/ApoE−/− mice developed less and smaller atherosclerotic plaques, which contained fewer macrophages, decreased numbers of apoptotic cells and smaller necrotic areas and showed reduced inflammation compared to the plaques of ApoE−/− mice. In addition, the plaques of NEMOSMCiKO/ApoE−/− mice showed higher expression of α-SMA and lower expression of the transcriptional factor KLF4 compared to those of ApoE−/− mice. Consistently, in vitro, NEMO-deficient SMCs exhibited reduced proliferation and migration, as well as decreased KLF4 expression and lower production of IL-6 and MCP-1 upon inflammatory stimulus (TNF or LPS) compared to NEMO-expressing SMCs. In conclusion, NEMO-dependent activation of NF-κB signalling in SMCs critically contributes to the pathogenesis of atherosclerosis by regulating SMC proliferation, migration and phenotype switching in response to inflammatory stimuli. [ABSTRACT FROM AUTHOR]