Background <p>Oxidized low-density lipoprotein (ox-LDL) provokes endothelial dysfunction central to atherogenesis. The mechanosensitive channel Piezo1 regulates Ca²⁺ influx and may intersect with extracellular cues such as cartilage oligomeric matrix protein (COMP). We investigated whether COMP-associated Piezo1 signaling is involved in endothelial injury and EndMT-like changes in vitro and in vivo.</p> Methods <p>Human umbilical vein endothelial cells were exposed to ox-LDL with Piezo1 inhibition (GsMTx-4) or activation (Yoda1), recombinant COMP dosing, or COMP overexpression. Apoptosis, nuclear morphology, and viability were assessed by Annexin V-FITC/PI flow cytometry, Hoechst staining, and MTT; nitric oxide (NO) by Griess; intracellular Ca²⁺ by Fura-2 AM; and Ca²⁺-Mg²⁺-ATPase and Na⁺-K⁺-ATPase activities by micro-colorimetry. Co-immunoprecipitation was performed to assess the association between COMP and Piezo1. ATP2B1 and ATP1A1 protein expression, intracellular ATP levels, CD31 and α-SMA expression, and EndMT-related transcription factors were additionally evaluated by immunoblotting, ATP assay, immunofluorescence, and RT-qPCR. Expression of COMP, Piezo1, eNOS, and related targets was quantified by RT-qPCR and immunoblotting. In ApoE^−/− mice with diet-induced atherosclerosis, Piezo1 was pharmacologically modulated; cardiac performance (echocardiography), hemorheology, aortic histology, α-SMA immunofluorescence, NO and ATPase activities, and aortic transcripts/proteins (including KATP subunits) were evaluated.</p> Results <p>Ox-LDL triggered endothelial apoptosis, loss of viability, NO suppression, and elevated intracellular Ca²⁺ with inverse changes in membrane ATPase activities; GsMTx-4 treatment mitigated these effects, whereas Yoda1 treatment intensified them. Co-immunoprecipitation supported a biochemical association between COMP and Piezo1. Ox-LDL also reduced ATP2B1 and ATP1A1 expression and intracellular ATP levels, decreased CD31, and increased α-SMA and the EndMT-related transcription factors Snail, Slug, Twist, and ZEB1; these changes were partially reversed by GsMTx-4 and further aggravated by Yoda1. Recombinant COMP and COMP overexpression aggravated ox-LDL injury and aligned with higher COMP/Piezo1 and lower eNOS at mRNA and protein levels. In atherosclerotic mice, GsMTx-4 treatment was associated with improved cardiac indices and hemorheology, increased NO and ATPase-related readouts, reduced aortic α-SMA-associated changes and histologic remodeling, and shifted vascular expression toward lower COMP/Piezo1 and higher eNOS; Yoda1 treatment produced the opposite pattern, while KATP-related changes were directionally associated with pharmacologic Piezo1 modulation.</p> Conclusion <p>COMP-associated Piezo1 signaling is linked to Ca²⁺ loading, ATPase-related dysfunction, and impaired eNOS/NO signaling in association with endothelial injury and EndMT-like changes. Piezo1 modulation confers partial vascular protection in these experimental settings and supports further investigation of this pathway in atherosclerosis.</p>