Background: Previous studies from our group showed that low-intensity sonodynamic therapy (SDT) has protective effects on atherosclerosis (AS). However, because the intensity of ultrasound passing through tissue is attenuated, the consequences of very low-intensity SDT, referred to as non-lethal SDT (NL-SDT), on atherosclerotic plaques are unclear. The aim of this study was to determine whether NL-SDT affects atherosclerotic plaques and to elucidate the possible underlying mechanisms. Methods: An AS model was established using ApoE-/- mice fed a western diet. En face Oil Red O staining was used to measure atherosclerotic plaque size. Hematoxylin and eosin staining and immunohistochemical staining were used to observe plaque morphology and assess the location of macrophages and heme oxygenase 1 (HO-1). HO-1 mRNA and protein levels in AS plaques were evaluated by real-time PCR and western blotting. Human THP-1 cells and mouse peritoneal macrophages were used in this study. Western blotting was used to investigate the expression of cellular proteins after NL-SDT. Macrophage apoptosis was evaluated by TUNEL assays and flow cytometry with Annexin V/PI double staining. Intracellular reactive oxygen species (ROS) and mitochondrial membrane potential (MMP) were measured with 2′-7′-dichlorofluorescein diacetate (DCFH-DA) and 5,5′,6,6′-tetrachloro-1,1′,3,3′-tetraethyl benzimidazolyl carbocyanine iodide (JC-1) staining, respectively. Results: NL-SDT significantly inhibited AS progression and reduced the necrotic core area. NL-SDT induced HO-1 expression in lesional macrophages and in cultured macrophages. NL-SDT activated the protein kinase B (AKT) and extracellular signal-related protein kinase (ERK) pathways and the transcription factor NF-E2-related factor 2 (Nrf2).NL-SDT significantly reduced oxidized LDL (ox-LDL)-induced macrophage MMP collapse, ROS production and cell apoptosis. Zinc protoporphyrin (ZnPP), a HO-1-specific inhibitor, reversed the protective effects of NL-SDT. Conclusion: NL-SDT inhibits atherosclerotic plaque progression and increases plaque stability. In vitro, NL-SDT has a protective effect on ox-LDL-induced macrophage impairment via HO-1.

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