Melatonin Stabilizes Rupture‐Prone Vulnerable Plaques via Regulating Macrophage Polarization in a Nuclear Circadian Receptor <scp>ROR</scp> α‐Dependent Manner

Rupture of vulnerable plaques is the main trigger of acute cardio-cerebral vascular events, but mechanisms responsible for transforming a stable atherosclerotic into a vulnerable plaque remain largely unknown. Melatonin, an indoleamine hormone secreted by the pineal gland, plays pleiotropic roles in the cardiovascular system; however, the effect of melatonin on vulnerable plaque rupture and its underlying mechanisms remain unknown. Here, we generated a rupture-prone vulnerable carotid plaque model induced by endogenous renovascular hypertension combined with low shear stress in hypercholesterolemic ApoE-/- mice. Melatonin (10mg/kg/day by oral administration for 9 weeks) significantly prevented vulnerable plaque rupture, with lower incidence of intraplaque hemorrhage (42.9% vs. 9.5%, P=0.014) and of spontaneous plaque rupture with intraluminal thrombus formation (38.1% vs. 9.5%, P=0.029). Mechanistic studies indicated that melatonin ameliorated intraplaque inflammation by suppressing the differentiation of intraplaque macrophages toward the proinflammatory M1 phenotype, and circadian nuclear receptor retinoid acid receptor-related orphan receptor-α (RORα) mediated melatonin-exerted vasoprotection against vulnerable plaque instability and intraplaque macrophage polarization. Further analysis in human monocyte-derived macrophages confirmed the role of melatonin in regulating macrophage polarization by regulating the AMPKα-STATs pathway in a RORα-dependent manner. In summary, our data provided the first evidence that melatonin-RORα axis acts as a novel endogenous protective signaling pathway in the vasculature, regulates intraplaque inflammation, and stabilizes rupture-prone vulnerable plaques.