Atherosclerosis (AS) is considered as a chronic inflammation of blood vessels that leads to many disabling or fatal complications, such as myocardial infarction and ischemic strokes (Fan and Watanabe, 2022, Kong et al., 2022). Vascular endothelial cells (VECs) are located in the innermost layer of the vessel wall and constitute the interface between circulating blood components and the vessel wall. Endothelial dysfunction results in sustained and chronic vascular inflammation, which is central to atherosclerotic diseases (Jia et al., 2022). VECs inflammatory injury may increase cell permeability. As a result, lipids, monocytes, phagocytes and platelets in the blood are more likely to infiltrate the vessel wall and increase adhesion to injured VECs. Among them, low-density lipoprotein is modified to oxidized low-density lipoprotein (ox-LDL), which enters macrophages and eventually evolves into foam cells to form atherosclerotic plaques. Eventually, these results lead to the accumulation of secondarily necrotic macrophages and the formation of an advanced lesion with a necrotic lipid core (Bäck et al., 2019). Therefore, exploring the mechanisms of vascular inflammation will help to develop better therapeutic options for AS.
Dysfunctions in the gut microbiota or dysbiosis are closely associated with the development of AS (Jie et al., 2017). In clinical studies, it has been shown that the gut of AS patients exhibited a higher abundance of Enterobacteriaceae and Streptococcus (Jie et al., 2017). These genera are involved in the metabolism and transport of several important cardiovascular health molecules, suggesting that dysbiosis of the gut microbiota is closely related to AS. Many findings in animal models and humans have identified that the gut microbiota may mediate AS development through endogenous metabolites, including short-chain fatty acids (SCFAs), lipopolysaccharides (LPS) and trimethylamine-N-oxide (TMAO) (Verhaar et al., 2020). SCFAs have various effects on different parts of AS development (Hu et al., 2022). For example, butyrate can improve endothelial function and prevent AS (Tian et al., 2021). The critical role of propionate in AS by regulating cholesterol metabolism (Haghikia et al., 2022). Acetate is involved in the development of AS by affecting the production of IL-6 and IL-8 (Li et al., 2018). LPS is known to induce systemic inflammation and has been shown to have both metabolic and cardiovascular effects (Verhaar et al., 2020). TMAO is considered as an AS promoter (Liu et al., 2018), which could enhance foam cells formation (Wang et al., 2015), directly activate inflammatory responses (Chen et al., 2017) and interfere with the reverse transport of cholesterol (Koeth et al., 2013). In brief, gut microbiota metabolites play an important role in the occurrence and development of AS. Therefore, further exploration into microbial metabolites associated with AS is warranted, along with clarification of their mechanisms.
In this study, we investigated the anti-AS mechanism of Paeonol (2′-Hydroxy-4′-methoxyacetophenone, C9H10O3, Pae), an active ingredient in Paeonia Suffruticosa Andr. (Liu et al., 2021, Liu et al., 2023). A large number of experiments have confirmed the anti-atherogenic effect of Pae. (1) Pae possessed potent antioxidant capacity, which effectively reduced vascular inflammation by scavenging free radicals within the body. As a result, Pae could inhibit the release of inflammatory cytokines and mitigate the risk of AS formation. (Higashi et al., 2009, Yu et al., 2020, Yuan et al., 2011). (2) Pae exerted an anti-AS effect by restoring the ecological balance of the gut microbiota and ameliorating the abnormal metabolic profile of the gut microbiota. (Liu et al., 2023). (3) From the previous metabolomics results, we found a differential metabolite in the serum of AS mice, aminoadipic acid (AAA) (Liu et al., 2023), which had been reported as a potential small-molecule marker of oxidative stress (Zeitoun-Ghandour et al., 2011). AAA is a metabolite present in the major biochemical pathways of lysine. Studies in rats had shown that AAA levels were high during pre-diabetes (Wijekoon et al., 2004). In addition, endothelial cells stimulated by prolonged hyperglycemia could lead to increased levels of AAA, which was considered as a sign of lysine breakdown through oxidative stress (Yuan et al., 2011). However, whether AAA could promote AS vascular inflammation and the effects of Pae on AAA levels were still unclear.
In this study, we explored the relationship between AAA and AS vascular inflammation through clinical experiments and animal experiments. We also elucidated the mechanisms by which AAA promoted AS and the intervention effects of Pae. Our study creatively proposed AAA as a potential biomarker for clinical judgment of AS and provided a new treatment strategy for AS.