## 4 Discussions
In this study, we demonstrated that treatment with magnolol reduced thrombin-induced CTGF expression (Figure 1C) via the PAR-1/JNK-1/AP-1 signaling cascade. In addition, VSMC proliferation (Figure 5A) and vascular restenosis caused by balloon angioplasty (Figure 5C) were ameliorated by magnolol treatment. It has been shown that CTGF contributes significantly to the proliferation of lung fibroblasts, adipose-derived stromal cells, and mesenchymal stromal cells [44; 45; 46]. CTGF also mentioned that it participates in the occurrence and development of restenosis through paracrine action, and overexpression of CTGF may weaken the stability of advanced atherosclerotic plaques [47]. The inhibition of CTGF expression improves cardiac repair and suppresses fibrosis after myocardial infarction [48]. Therefore, magnolol may be a new therapeutic agent for complications caused by myocardial fibrosis and CTGF-related diseases. Even though we focused on the role of magnolol in the PAR-1/JNK/AP-1 pathway, there were several mechanisms that should not be neglected in research regarding smooth muscle cell activation and restenosis. Magnolol has been reported to be an inhibitor of signal transducer and activator of transcription protein 3 (STAT3) [49]. thrombin activates STAT3 via PAR-1 pathway [50], and the STAT3 activation is an important molecular mechanism for restenosis and smooth muscle cell activation [51]. Furthermore, thrombin activated nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB) was reported to involve in the mechanism of VSMCs proliferation, atherosclerosis, and restenosis [52]. An inhibitor, such as magnolol, of NF-kB, has shown the ability to reduce the VSMCs proliferation and benefit restenosis [53]. In this study, we revealed a novel function of magnolol as a thrombin inhibitor and PAR-1 antagonist. Currently, vorapaxar is the only approved therapeutic agent targeting PAR-1 [54], but due to safety considerations, its clinical application is restricted [15]. According to the adoption of new strategies, many new reagents have been designed to circumvent the limitations of the previously developed PAR-1 modulation [15]. On the other hand, thrombin as an activator of PAR-1, inhibiting thrombin activity may be an alternative plan to regulate PAR-1 activity. Oral direct thrombin inhibitors (DTIs) such as Dabiqatran etexilate has considered to modulating the activity of PAR-1 many advantages, such as good safety and wide treatment range [55]. Magnolol is an important traditional Chinese and Japanese herbal plant and is still used in modern clinical practice. Similar to Varapraxa and dabigatran etexilate, magnolol may block the active site of PAR-1 (Figure 2D) and has the ability to inhibit thrombin activity (Figure 2A). Most importantly, magnolol did not harm VSMC in our study (Figure 1A,B). Therefore, magnolol has the potential as a new therapeutic agent against thrombin and PAR-1 related diseases. Inhibition of PAR-1 activity on platelets will affect the platelets aggregation, thereby causing the possibility of bleeding-particularly intracranial hemorrhage [56]. Previous studies have pointed out that Vorapaxary is at risk of bleeding in patients with certain diseases (e.g., diabetics) or concomitant use of other anticoagulants (such as aspirin, clopidogrel) [15]. However, in patients with PCI and CABG, the use of Vorapaxar does not significantly increase the side effects of bleeding [15]. Compared with Vorapaxar, magnolol has a variety of biological activities, such as anti-inflammatory, anti-free radical and neuroprotective ability [57]. In patients with cardiovascular diseases, it may have a better protective effect than Vorapaxar due to its multiple effects. In order to avoid bleeding by systemic effects, local administration of magnolol (for example, a drug-eluting balloon) may be considered. In addition, the applicable indications of Magnolol, the applicable blood concentration and whether it can be co-treated with other antiplatelet drugs are worthy of further research in the future.