Curcumin as a Natural Remedy for Atherosclerosis: A Pharmacological Review

Curcumin, a natural polyphenolic compound present in Curcuma longa L. rhizomes, shows potent antioxidant, anti-inflammatory, anti-cancer, and anti-atherosclerotic properties. Atherosclerosis is a comprehensive term for a series of degenerative and hyperplasic lesions such as thickening or sclerosis in large- and medium-sized arteries, causing decreased vascular-wall elasticity and lumen diameter. Atherosclerotic cerebro-cardiovascular disease has become a major concern for human health in recent years due to its clinical sequalae of strokes and heart attacks. Curcumin concoction treatment modulates several important signaling pathways related to cellular migration, proliferation, cholesterol homeostasis, inflammation, and gene transcription, among other relevant actions. Here, we provide an overview of curcumin in atherosclerosis prevention and disclose the underlying mechanisms of action of its anti-atherosclerotic effects.


Introduction
Atherosclerosis is a common cause of cerebro-cardiovascular disease and is an agerelated chronic large-artery condition that develops in adult and aged patients [1]. The pathogenesis of atherosclerosis is multifaceted. Numerous investigations have highlighted hyperlipidemia, diabetes, smoking, hypertension, and other cardiovascular risk factors which mediate oxidative stress causing damage to vascular endothelial cells. They also cause infiltration of low-density lipoproteins (LDL) into the sub-endothelial space, monocyte chemotaxis, aggregation below the endothelium, and platelet activation leading to chronic inflammatory responses in vascular walls [2][3][4][5]. Atherosclerosis is the pathological basis for many cerebro-cardiovascular diseases and acute cerebro-cardiovascular events such as myocardial infarction and ischemic stroke, making it a serious public health concern [6,7]. Anti-arteriosclerotic traditional Chinese medicines (TCM) are widely used in Chinese clinical practice with a good safety profile and lasting efficacy [8,9]. Many traditional medicines used in TCM and other traditional medicine systems such as Ayurveda including turmeric and ginseng have anti-atherosclerotic effects [10,11].
Turmeric prepared from the dried rhizomes of Curcuma longa L. (family, Zingiberaceae) is enriched with multiple bioactive chemical entities with multiple therapeutic applications. The roots and rhizomes of turmeric contain curcumin that has been used as a traditional drug to increase blood circulation and improve stasis [12]. Curcumin has lipid-lowering, antioxidative, anti-inflammatory, and anti-infective effects [13][14][15]. There is growing evidence that curcumin can regulate different signaling molecules to retard the

Atheroprotective Effects of Curcumin In Vitro
The potential of curcumin in protecting against various medical ailments, including atherosclerosis, has been widely assessed. Atherosclerosis is a chronic inflammatory disease resulting from arterial wall injury, sustained due to dyslipidemia, diabetes, hypertension, and other cardiovascular risk factors that leads to macrophage and VSMC-derived foam cell formation, endothelial cell dysfunction, immune cell activation, platelet activation, and thrombus formation [30][31][32][33]. Several studies have demonstrated curcumin's potent therapeutic potential in preventing foam cell formation, modulating macrophage polarization, tuning cholesterol efflux, and regulating pro-inflammatory responses [16,[34][35][36][37][38].
TLR4, an important signaling receptor, plays an important role in the pathogenesis of plaque formation and the development of atherosclerosis [73]. Furthermore, TLR4 activates a variety of signal transduction molecules as well as transcription factors. An important response of TLR4 activation is NF-κB and MAPK activation, which triggers nuclear transduction that simultaneously propels the gene expression profile of an inflammatory reaction. The amplified expression profile increases ROS production and the expression of inflammatory molecules, which causes the initiation of atherogenesis, leading ultimately to the clinically critical destabilization of atherosclerotic plaques [16]. Reports on curcumin supplementation fostering negative regulation not only on towards the TLR receptor but also on nuclear transduction molecules and inflammatory cytokines (TNF-α, IL-1β, VCAM-1, ICAM 1, etc.) are presented [74] (Figure 1). into the plaques. More recently, Zhang et al. [62] showed that curcumin inhibits aldosterone-induced production of CRP in VSMCs by reducing ROS production via limiting aberrant activation of the ERK1/2 signal pathway. LDL is another important pathological entity that contributes to the development of atherosclerotic lesions. ROS modifies LDL, thereby producing Ox-LDL. An increase in Ox-LDL concentration in plasma has long been recognized as a key factor in atherosclerosis. Ox-LDL, rather than binding to LDL receptor, binds to scavenger receptors (SRs). The major SR is CD36 that recognizes ox-LDL [76]. After binding to CD36 on cell membrane, ox-LDL can also trigger CD36 expression via PPAR-γ pathway [77]. Specifically, PPAR-γ, once activated, dimerizes with the retinoid X receptor (RXR) and triggers PPAR-response element (PPRE)-containing genes, which ultimately increases CD36 expression, resulting in increased ox-LDL influx [78].
Cholesterol accumulation in macrophages results in foam cell formation and fatty streak development via upregulating the expression/activity of several receptors, such as SR-AI/II, SRBI, CD36, and LOX-1. In contrast, various efflux transporters play an active role via ATP-binding cassette (ABC) transporters ABCA1, ABCG1, and SR-BI to facilitate reverse cholesterol transport from macrophages [79]. Fatty acid-binding protein (FABP)-4 or adipocyte protein 2 (aP2) coordinates cholesterol trafficking (efflux) but is also known to activate an inflammatory response. Lack of aP2 protein complex changes the cholesterol composition in macrophages, which concurrently amplifies CD36 expression and enhances oxLDL influx [80]. This cascade creates a disease state, whereby macrophages in- Curcumin has also been shown to inhibit ligand-induced and ligand-independent dimerization at the receptor level. LPS induces activation of both MyD88 and TRIFdependent signaling via the TLR4 receptor. Upon curcumin supplementation, TLR4 homodimerization was blocked [46], providing a novel mechanism for its anti-inflammatory effects. In a similar fashion, curcumin inhibits the NOD-like receptor (NLR) family, the pyrin domain containing 3 (NLRP3) inflammasome via suppressing TLR4/MyD88/NF-κB, the phosphorylation level of IkB-α, and purinergic 2X7 receptor (P2X7R) pathways in phorbol 12-myristate 13-acetate (PMA)-induced macrophages [55]. NLRP3 inflammasome is composed of a multiprotein complex having caspase and caspase 1 protein complex for apoptosis [75]. On NLRP3 complex stimulation, caspase-1 is activated, which cleaves the pro-forms of interleukin (IL)-1β and IL-18 into their mature forms. Once in fully mature form, IL-1β (a primary pro-inflammatory cytokine) mediates the development of atherosclerosis. Curcumin also inhibits VSMC migration by negatively regulating NLRP3 expression via an NF-κB-mediated response and decreasing IL-1 concentration [55].
In VSMCs, curcumin supplementation markedly reduces inflammatory responses induced by LPS acting at TLR4. LPS induced stimulation of TRL4 increases the phosphorylation of IκBα, NF-κB (p65), and MAPKs [59]. Concurrently, this increases the inflammatory cytokine expression profile of TLR4, MCP-1, iNOS, TNF-α, and NO production. In addition, Meng et al. (2013) [59] established that curcumin supplementation inhibits TLR4 activation and ERK1/2 and p38 MAPK phosphorylation, thereby preventing NF-κB nuclear translo-cation that mediates ROS production. Thus, inhibition of the expression profile may reduce atherosclerotic plaque formation and reduce inflammatory cell infiltration into the plaques. More recently, Zhang et al. [62] showed that curcumin inhibits aldosterone-induced production of CRP in VSMCs by reducing ROS production via limiting aberrant activation of the ERK1/2 signal pathway.
LDL is another important pathological entity that contributes to the development of atherosclerotic lesions. ROS modifies LDL, thereby producing Ox-LDL. An increase in Ox-LDL concentration in plasma has long been recognized as a key factor in atherosclerosis. Ox-LDL, rather than binding to LDL receptor, binds to scavenger receptors (SRs). The major SR is CD36 that recognizes ox-LDL [76]. After binding to CD36 on cell membrane, ox-LDL can also trigger CD36 expression via PPAR-γ pathway [77]. Specifically, PPAR-γ, once activated, dimerizes with the retinoid X receptor (RXR) and triggers PPAR-response element (PPRE)-containing genes, which ultimately increases CD36 expression, resulting in increased ox-LDL influx [78].
Cholesterol accumulation in macrophages results in foam cell formation and fatty streak development via upregulating the expression/activity of several receptors, such as SR-AI/II, SRBI, CD36, and LOX-1. In contrast, various efflux transporters play an active role via ATP-binding cassette (ABC) transporters ABCA1, ABCG1, and SR-BI to facilitate reverse cholesterol transport from macrophages [79]. Fatty acid-binding protein (FABP)-4 or adipocyte protein 2 (aP2) coordinates cholesterol trafficking (efflux) but is also known to activate an inflammatory response. Lack of aP2 protein complex changes the cholesterol composition in macrophages, which concurrently amplifies CD36 expression and enhances oxLDL influx [80]. This cascade creates a disease state, whereby macrophages induce the release of IL-1β, TNFα, ROS, and matrix metalloproteases coupled with the development of inflammation, cell migration, and plaque formation (Figure 1). Hence, genetic or pharmacological inhibition of aP2 and CD36 expression might offer potential remedies to atherosclerosis.
Several further lines of experimental evidence highlight the potent anti-atherogenic effects of curcumin (documented in Table 1). For example, Zhou et al. (2014) [36] demonstrated that curcumin treatment reduces the expression profile of oxLDL-induced thrombospondins-4 (THBS-4). THBS-4 was reported to influence important cellular responses such as cell migration, proliferation, and adhesion, leading to atherogenesis progression [81]. Curcumin further inhibits p38 MAPK activation and reduces PPAR-γ and CD36 expression in oxLDL-treated macrophages, leading to decreased foam cell formation [77]. In human umbilical vein endothelial cells (HUVECs), curcumin inhibits ROS production, NF-κB-dependent LOX-1 expression, and VCAM-1 and ICAM-1 expression. In addition, curcumin promotes NO production to confer vasodilatory effects [6,7]. Recent studies also suggest that curcumin could reduce oxidative stress, ER stress, and inflammatory response induced by acrolein (a toxin from tobacco smoke) and cytomegalovirus (CMV) infection in human endothelial cells [29,66]. The anti-inflammatory effects of curcumin is exerted through inhibiting COX-2 expression and prostaglandin production via reducing the phosphorylation of PKC, p38 MAPK, and cAMP response element-binding protein as well as inhibiting the HMGB1-TLRS-NF-κB signaling pathway [29,66]. The broad anti-inflammatory effects of curcumin underlie its effects on improving flow-mediated dilation in human subjects [82].

Atheroprotective Effects of Curcumin In Vivo
Numerous lines of experimental evidence (Table 2) support the actions of curcumin in reducing the cardiovascular risk associated with atherosclerosis. Table 2. In Vivoevaluation of the pharmacological properties of curcumin against atherosclerosis.

Clinical Studies of Curcumin
Few clinical trials involving double-blind placebo-controlled studies and randomized controlled trials have been undertaken. A 12-week randomized placebo-controlled trial of 118 participants showed that curcumin treatment reduced the risk of developing acute cardiovascular events in people with type 2 diabetes and dyslipidemia [95]. Another randomized controlled research with 87 patients found that taking 1 g of curcumin for eight weeks lowered TC, TG, and HDL-c levels following nonalcoholic fatty liver infections [96]. On the other hand, curcumin lowered LDL-c and Apo B and increased Apo A1 and HDL-c levels in healthy people, indicating anti-atherosclerosis efficacy [97]. In coronary bypass graft, curcumin (4 g/day) reduced acute myocardial infarction and significantly decreased malondialdehyde levels [98]. Further, in patients with chronic obstructive pulmonary disease, curcumin (Theracurmin ® 90 mg/day for 24 weeks) reduced the level of the α1-antitrypsin-low-density lipoprotein (AT-LDL) complex, which promotes arteriosclerosis [99]. In another randomized trial, curcumin usage at 80 mg per day ameliorated dyslipidemia in patients with reduced serum TG, salivary amylase, and β-amyloid levels and increased plasma nitric oxide level after four weeks of study [100]. Likewise, in a double-blind placebo-controlled study, curcumin (200 mg) supplementation improved endothelial function measured by flow-mediated dilation (FMD), thus decreasing the risk of cardiovascular diseases [101]. In another pilot study, curcumin (500 mg/day for 12 weeks) de-stiffened arteries in young, obese men with aortic stiffness [102]. Studies with curcumin have potential limitations due to factors such as limited sample sizes; therefore, large-scale clinical trials are required to characterize the actual potential and identify the direct molecular targets of curcumin in treating atherosclerosis.

Conclusions and Perspectives
Substantial experimental evidence suggests that curcumin prevents endothelial dysfunction, smooth muscle cell proliferation and migration, and foam cell formation and modulates macrophage polarization. Curcumin also counteracts inflammatory response, supporting its potential application in atherosclerosis treatment. The anti-atherosclerotic properties of curcumin occur through suppressing inflammatory response by skewing macrophage polarization from M1 to M2 or by inducing M2 polarization through regulating TLR4/MAPK/NF-κB pathways in macrophages and secretion of interleukins (IL-4 and/or IL-13). Similarly, curcumin concurrently regulates the expression and activity of the lipid transporter expression (CD36, CD38, ABCA1, aP2, etc.) responsible for cholesterol uptake and efflux, thus maintaining cell homeostasis. In addition, curcumin lowers the circulating level of ox-LDL and blocks oxLDL elicited pro-atherogenic events by decreasing the expression of MCP-1 and THBS-4 via the p38 MAPK and NF-κB pathways [52]. Likewise, curcumin suppresses TLR4 expression and macrophage infiltration in aortic tissues and protects against atherosclerotic plaque formation [16]. A recent study has suggested that curcumin blocks LPA-induced MCP-1 expression via TGFBR1/ROCK signaling pathway [103].Additional studies are required to improve or add meaningful insights into our understanding of the mechanism(s) of action of curcumin against atherosclerosis, especially in the clinical setting. In addition, the development of novel drug delivery systems, such as the creation of curcumin nanomicelles [104,105], is critical for improving the oral bioavailability of curcumin which may contribute to its clinical efficacy [106].  conduct the study. The authors are grateful to Peter J. Little (University of Sunshine Coast, Australia) for proofreading and editing of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.