Natural products are well-known for their therapeutic value [1]. Throughout human history, they have played an important role in traditional medicine by treating a wide range of pathological ailments, offering effective alternatives to modern medicine [2]. Thanks to recent advances in scientific research and growing environmental awareness [3], there has been a significant increase in the use of natural and plant-based products to promote synergistic biological health benefits ^[4][5][6][4,5,6]. This is particularly important in western society where a shift in the modern dietary regimen towards a “Western diet” and an increase in sedentary lifestyles is believed to be, in part, responsible for the substantial increase in the prevalence of cardiovascular complications ^[7][8][7,8].

Cardiovascular diseases (CVDs) refer to a broad range of conditions affecting the heart and blood vessels [9]. They register a global annual death toll of more than 19.1 million and are the most prevalent cause of morbidity and mortality [10]. Alarmingly, the World Health Organization estimates that CVDs will contribute to over 22 million deaths in 2030 [11]. CVDs encompass a variety of diseases, including hypertension, atherosclerosis, coronary artery disease (CAD), heart failure (HF), myocardial infarction (MI) and stroke [9]. Atherosclerosis and hypertension are the leading causes of CVDs and are the most common underlying diatheses for most deaths [12]. Independent risk factors for CVDs include hypertension, hyperhomocysteinemia (HHcy), dyslipidemia, type-2 diabetes, and cholesterol ^[12][13][14][12,13,14]. The pathology of CVDs is complex, and oxidative stress, hyper-inflammation, endothelial dysfunction, and hyperlipidemia have been reported as common phenotypes in the pathogenesis of CVDs and have become the main targets of therapeutic intervention ^[15][16][15,16]. Conventional therapy commonly used to treat CVDs can be costly and may produce adverse side effects, unwanted drug–drug interactions and life-threatening toxicities [17], and thus, alternative approaches have been considered, including physical activity, as well as the use medicinal natural product supplementation, for CVD treatment ^[18][19][20][18,19,20].

Taurine, a natural product, is a non-toxic, semi-essential sulfur-containing amino acid (Figure 1A) that is abundant in human cardiac and vascular tissues ^[21][22][21,22]. Although synthesized endogenously from methionine and cysteine [23], the majority of taurine is obtained through dietary intake (e.g., seafood, meat, poultry and eggs) [21]. It is well-established that taurine has diverse physiological functions in humans and is seen to protect against several pathophysiological conditions ^[22][24][25][22,24,25]. Most of the effects mediated by taurine are reported in the cardiovascular system and include modulating ion transporters, membrane stabilization, anti-oxidation, anti-hyperlipidemia, anti-platelet, and anti-inflammatory activity, as well as modulation of blood pressure and vascular tone ^[22][24][25][22,24,25].

Piper nigrum, commonly known as black pepper, has been used to treat a variety of conditions [26]. The therapeutic value of black pepper is attributed to its constituents, terpenes or terpenoids, which are a diverse class of organic compounds present in essential oils obtained from plants ^[26][27][26,27]. Terpenes consist of over 30,000 compounds whose functional groups include alcohols, aldehydes, or ketones [28]. Abundant terpenes found in black pepper consist of β-caryophyllene (BCP) (Figure 1B), α-pinene (Figure 1C), β-pinene (Figure 1D), limonene (Figure 1E), α-humulene (Figure 1F), and sabinene (Figure 1G) [29]. The therapeutic merit of black pepper has been attributed to the ability of BCP to specifically activate cannabinoid 2 receptor (CB₂R), resulting in downstream signaling pathways that mediate anti-inflammatory, anti-atherosclerotic and anti-oxidative properties, which have been correlated with beneficial cardiovascular effects [30]. Unlike BCP, studies involving α-pinene ^[31][32][31,32], β-pinene ^[31][32][31,32] and α-humulene [33] have demonstrated the inability of these terpenes to interact with CB₂R. Several studies have noted the significant cardiovascular-protective effects of black pepper and its terpenes against a range of CVDs, including hypertension, atherosclerosis, CAD, and HF due to their anti-inflammatory, anti-oxidative, anti-hyperlipidemia and blood pressure-lowering abilities ^[34][35][34,35]. As such, black pepper provides novel therapeutic strategies to target CVD initiation and progression.

Acute and chronic inflammation play key roles in the pathogenesis, progression, and severity of CVDs [36]. Hypertension, atherosclerosis and CAD are characterized as chronic inflammatory diseases and are clinically associated with increased expression of important pro-inflammatory receptors (e.g., toll-like receptors (TLR) and receptor for advanced glycation end-products (RAGE)) and molecules (e.g., high-mobility group box-1 (HMGB-1), interleukin (IL)-1β, IL-6, tumor necrosis factor-alpha (TNF-α), monocyte chemoattractant protein-1 (MCP-1), nitric oxide (NO), prostaglandin E₂ (PGE₂), inducible nitric oxide synthase (iNOS), and cyclooxygenase (COX)-2) ^[8][37][8,37]. Thus, their downregulation and inhibition have been reported in literature as potential therapeutic targets against CVDs [38]. Taurine and terpenes contain general anti-inflammatory effects (Table 1), which may be useful in combination therapy as due to their abilities to suppress the expression of pro-inflammatory cytokines and alleviate cell and tissue damage in multiple disease states ^{[39][40][41][42]}[39,40,41,42]. The two natural compounds significantly mitigate the expression of pro-inflammatory mediators (e.g., IL-1α, IL-1β, IL-6, IL-13, IL-17, TNF-α, interferon-γ, MCP-1, and vascular endothelial growth factor) in order to reverse the severity of inflammation-mediated injury ^[42][43][42,43]. Administration of taurine (100 mM) in Wistar rats 30 days after coronary artery occlusion-induced ischemia significantly decreased myocardial infarct size and reduced expression of IL-6 and TNF-α, suggesting that taurine provides anti-inflammatory cardio-protective effects [44]. Understanding the anti-inflammatory targets of taurine and terpenes may provide further evidence of their therapeutic use for CVDs (Figure 2, Table 1), as well as other conditions such as inflammatory bowel disease [45], rheumatoid arthritis [45], and traumatic brain injury where inflammation is a well-known critical event [43].

Nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) is a central transcription regulator that plays a critical role in the activation of inflammatory responses by inducing transcription of pro-inflammatory genes [46]. During basal conditions, NF-κB is bound to the inhibitory protein nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor alpha (IκBα) within the cytosol [47]. Upon stimulation, the inhibitor of NF-κB subunit beta (IκKβ) phosphorylates and degrades IκBα, allowing NF-κB dimers p50 and p65 to translocate towards the nucleus where they upregulate the activation of pro-inflammatory genes [48]. The phosphorylation of IκBα is triggered by a diverse range of stimuli, including various ligands for IL-6 receptors, TNF receptor (TNFR), and pattern-recognition receptors (PRRs) (e.g., TLR and RAGE) ^[49][50][51][49,50,51], which can activate kinase-signaling transduction pathways consisting of p38-mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase (ERK) and Janus kinase (JAK) [52].

Taurine and terpenes both exert anti-inflammatory effects through inhibition of NF-κB and kinase-signaling pathways ^[53][54][55][53,54,55]. An increase in taurine metabolites, chloro-taurine (TauCl) and taurine-bromamine (TauBr) have been shown to modulate the myeloid-differentiation factor-88 (MyD88)-dependent TLR/NF-κB inflammatory signaling pathway in both human and mouse macrophages to disrupt the phosphorylation and degradation of IκBα ^[56][57][56,57], as well as prevent the activation and subsequent translocation of NF-κB ^[56][58][56,58]. Additionally, taurine can significantly reduce the expression signal transducer and activator of transcription 3 (STAT) 3 in the JAK-STAT signaling pathway [44], which is shown to play a vital role in cardio-protection, as inflammation associated with myocardial ischemia is mitigated through inhibition of JAK-STAT3-signaling pathways [44]. Black pepper terpenes have also been shown to attenuate pro-inflammatory cytokine expression through downregulation of kinase signaling pathways and inhibition of IκBα phosphorylation ^[44][59][44,59]. Limonene blocks the phosphorylation of IκBα and NF-κB to suppress lipopolysaccharide (LPS)-induced inflammation in acute lung injury [59]. α-pinene has been shown to inhibit IκKβ activation and prevent NF-κB translocation [60]. In human chondrocytes, α-pinene (200 µg/mL) suppressed IL-1β-induced inflammation through suppression of NO, iNOS, matrix metallopeptidase (MMP)-1, and IL-13 pro-inflammatory mediators [61]. This anti-inflammatory effect was produced by the inhibition of c-Jun N-terminal kinase (JNK) phosphorylation. Additionally, α-pinene has also been shown to attenuate MAPK and NF-κB activation, resulting in the decreased expression of pro-inflammatory factors IL-6, TNF-α, NO, iNOS, and COX-2 [44]. The terpenes BCP, α-humulene, and α-pinene have been shown to inhibit the expression of NO synthase and COX-2 through inhibition of MAPK action through IKK-B inhibition [38], while limonene was shown to mitigate pro-inflammatory cytokines secretion as well as rescue iNOS, MMP-1, and MMP-3 gene transcription through inhibition of MAPK/JNK-dependent NF-κB activation pathways [62]. Pre-treatment with limonene (25, 50 and 75 mg/kg) inhibited release of pro-inflammatory cytokines (i.e., TNF-α, IL-1β and IL-6) and prevented phosphorylation of p38-MAPK, IκBα, NF-κB p65, JNK and ERK in mice with lipopolysaccharide (LPS)-induced acute lung injury [59]. Likewise, both α-pinene and β-pinene reduce the expression of genes associated with LPS-induced inflammation [63], suggesting that terpenes may mediate anti-inflammatory effects through regulation of kinase and NF-κB gene signaling.

BCP has been identified as a CB₂R agonist [64], which is primarily expressed in immune tissues and is known to modulate immune cell function to provide anti-inflammatory effects [65]. Exogenous administration of CB₂R agonists is observed to inhibit inflammation by reducing the production of pro-inflammatory cytokines and attenuating oxidative stress in various inflammatory diseases ^[66][67][66,67]. BCP has been observed to inhibit the activation of NF-κB, thus attenuating the production of inflammatory mediators. Furthermore, this effect disappears with CB₂R^−/− knockout expression, suggesting that BCP exerts its anti-inflammatory responses through activation of CB₂R [68]. The cardioprotective effects of BCP on isoproterenol (ISO)-induced myocardial injury in Wister rats through activation of the CB₂R has been observed. BCP modulated MAPK/ERK/JNK/NF-κB signaling to significantly suppress the expression of IL-1β, IL-6, TNF-α, iNOS, and COX-2 [69]. Additionally, this comprehensive study reported that BCP also attenuated mitochondrial dysfunction and improved the atherogenic index via a CB₂R-independent reduction in dyslipidemia [69]. BCP has been shown to inhibit the expression of vascular cell adhesion molecule-1 [70], which is known to promote the adhesion of macrophages to the vascular endothelium in the development of atherogenic plaque [71]. These studies suggest that both taurine and terpene extracts present in black pepper may combine to exert effective anti-inflammatory responses and thereby provide beneficial cardioprotective effects in CVDs.

The nucleotide-binding domain, leucine-rich containing family, pyrin domain-containing 3 (NLRP3) inflammasome is a critical component of the innate immune system and is responsible for the production of pro-inflammatory cytokines in response to cellular damage [72]. Recently, NLRP3 has been shown to play an indispensable role in the development of vascular diseases, including hypertension, atherosclerosis, and CAD, as well as diabetes and neurological disorders [73]. Increasing evidence supports NLRP3 as a new target to inhibit inflammatory diseases [74]. NLRP3 is regulated by NF-κB signaling pathways; therefore, taurine and terpenes have the ability to inhibit NF-κB as researchers have previously highlighted a pathway for preventing NLRP3 activation. Although taurine has been shown to attenuate in vitro and ex vivo inflammation through inhibition of NF-κB-dependent NLRP3 activation ^[75][76][75,76], studies on the isolated effect of black pepper terpenes on inflammasome inhibition are limited. A study conducted on the essential oil of pomelo peel, in which limonene (55.92%) was the most abundant compound, found that it significantly inhibited inflammation mediated by NLRP3 activation in rats [77]. Additionally, Makauy leaf ethanol extract consisting of limonene (5.6%) and BCP (4.9%) inhibited the in vitro expression of NLRP3 in J774A.1 mouse macrophage cells stimulated with LPS-induced inflammation [78]. Lastly, CB₂R agonist JWH-133 has been shown to produce cardioprotective effects against MI in rats by suppressing NLPR3-related mechanisms, suggesting that BCP, a CB₂R agonist, may also suppress inflammation through NLPR2 inhibition [79].

Collectively, these studies highlight the effects that taurine and black pepper terpenes have on inhibiting NF-κB, kinase- and inflammasome-signaling pathways and suppressing the expression of pro-inflammatory mediators. The potential combined use of taurine and terpenes may have synergistic effects in exerting effective anti-inflammatory responses, potentially providing effective treatment of CVDs.

HMGB-1 is an evolutionarily preserved, architectural, non-histone chromosome binding peptide that is found abundantly within the nucleus bound to DNA ^[80][81][80,81]. HMGB-1 facilities nuclear homeostasis by coordinating important nuclear functions, such as maintaining stability of the genome, acting as a chaperone for DNA, and regulating DNA transcription, replication, recombination, and repair ^[80][81][80,81]. Outside the nucleus, HMGB-1 drives cell stress responses by acting as a dynamic alarmin signal (which can also be referred to as a danger associated molecular pattern (DAMP)) that engages with PRRs of the immune system (e.g., TLR1/2/6, TLR4 and RAGE) to promote inflammation through the canonical NF-κB signaling pathway (discussed above) ^{[81][82][83][84]}[81,82,83,84]. Elevated levels of HMGB-1 have been demonstrated in animal models of atherosclerosis, hypertension, CAD, MI, and HF [84]. Clinical studies have also reported increased HMGB-1 serum levels in patients with CAD ^[85][86][85,86], peripheral artery [87] and HHcy [88], suggesting that HMGB-1 has a critical pathogenic role during development and progression of CVDs. While the correlation between taurine and HMGB-1 has yet to be investigated in the literature, a study involving patients with bladder cancer noted elevated levels of taurine upregulate gene 1 (TUG1; long non-coding RNA transcripts that require taurine for gene upregulation) and the reversal of TUG1 knockdown by overexpression of HMGB-1 [89], thus suggesting a possible unknown mechanism between taurine, TUG1 and HMGB-1. The role that taurine and BCP have in attenuating HMGB-1-mediated inflammation and dysfunction remains elusive; however, there is evidence to suggest that there may be an unknown interlinking mechanism between taurine and HMGB-1 [90]. In mice with HMGB-1 deletion in intestinal epithelial cells, a diet high in fat, cholesterol and fructose increased taurine concentration in serum collected from portal blood samples when compared with wild type controls [90].

Moreover, BCP and CB₂R activation may lower HMGB-1, contributing to a reduction in inflammation. CB₂R knockout mice challenged with LPS to induce sepsis displayed reduced survival rate and significantly elevated serum IL-6, TNF-α and HMGB-1 levels when compared with CB₂R expressing littermates [91]. Furthermore, treatment with GW405833 (10 mg/kg, a CB₂R agonist) was able to significantly reduce serum levels of TNF-α, IL-6, and HMGB-1 6 h after LPS injection (5 mg/kg) [91]. Furthermore, activation of CB₂R with GW405833 has been shown to induce intracellular HMGB-1 degradation via the autophagy-lysosome pathway in macrophages without modulating HMGB-1 mRNA expression [92], thus potentially preventing its release into the extracellular milieu where it can engage with immune system receptors and activate pro-inflammatory downstream signalling pathways. BCP’s anti-inflammatory properties may also be due to its ability to reduce HMGB-1 and its endogenous receptors as BCP was able to attenuate increased serum levels of HMGB-1 and suppressed protein expression of TLR4 and RAGE and their downstream inflammatory molecules (i.e., ERK, p38 and JNK phosphorylation, NF-κB, early growth response protein-1, and macrophage inflammatory protein-2 protein) in Kupffer cells from mice with galactosamine and LPS-induced hepatic injury [93].

Although the relationship between taurine, terpenes and HMGB1 remains elusive, these findings suggest that targeting HMGB-1-mediated inflammation through a combination of taurine and black pepper terpenes may be a potential mechanism to reduce inflammation associated with CVDs.

TLRs are integral to immunity as they provide host surveillance by detecting molecular signatures present on pathogens (referred to as pathogen-associated molecular patterns) and launch mechanisms of inflammation to neutralize and eliminate infection [94]. Importantly, TLRs also provide protection against tissue injury in the absence of pathogenic infiltration by identifying DAMPs (e.g., HMGB-1) secreted by damaged, injured, and dying cells ^[94][95][94,95]. However, unregulated TLR activation results in a vicious cycle of unresolving inflammation and has been demonstrated to be fundamental in the development of CVDs. Thus, they have become a target to attenuate disease development ^[95][96][95,96]. Specifically, taurine has been shown to suppress inflammation via modulation of the TLR2 and TLR4 ^[97][98][99][97,98,99] signaling pathways. TLR2 and 4 are able to recognize bacterial cell wall components (e.g., lipopeptides, peptidoglycan and LPS), and stimulation of TLR4 causes receptor homodimerization of the MyD88- and Toll/IL-1 receptor domain-containing adaptor-inducing interferon-β (TRIF)-dependent signaling cascade, whereas TLR2 activation requires recruitment of TLR1 or 6 for normal receptor function via the MyD88-dependent pathway ^[100][101][100,101]. Male Sprague-Dawley rats challenged with LPS (25 mg/kg; TLR4 activating ligand) to induce acute lung injury showed that those which were pre-treated with taurine supplemented drinking water had decreased infiltration and activation of neutrophils and macrophages. Further, they lowered levels of TLR4, MyD88, NF-κB and p65, reduced pro-inflammatory cytokines (IL-6, IL-18) and increased anti-inflammatory cytokines (IL-4, IL-10) in serum and lung tissues of the treated rats [97]. Similarly, taurine provides hepatic protection by targeting the TLR4/MyD88/NF-κB pathway in rat models of chronic alcohol-fed [98] and thioacetamide-induced [99] liver injury, resulting in decreased lymphocyte infiltration and reduced inflammation (TLR4, MyD88, IκB, NF-κB, iNOS, IL-6, IL-1β, and TNF-α). Regarding TLR2, taurine has been shown to reduce neutrophil activation of MAPK signaling following Streptococcus uberis (TLR2 activating ligand) infection [102]. A supporting study also showed that pre-treatment of taurine in rat models with Streptococcus uberis mastitis had significantly decreased expression of iNOS and TNF-α, which was associated with the downregulation of TLR2 and NF-κB mRNA expression, as well as inhibition of the DNA binding activity of NF-κB [103].

Like taurine, terpenes may also exert anti-inflammatory effects by modulation of the TLR signaling cascade. In LPS activated RBL-2H3 cells (derived from a basophilic leukemia cell line), α-pinene, β-pinene and limonene displayed anti-inflammatory abilities by reducing the expression of genes associated with LPS-mediated inflammation [63]. Likewise, α-pinene attenuated activation of the MAPK and NF-κB pathways and reduced production of IL-6, TNF-α, NO, iNOS and COX-2 following LPS stimulated mouse intraperitoneal macrophages [104]. However, in human chondrocytes, α-pinene elicited a more potent inhibition of IL-1β-induced inflammation, NF-κB and JNK activation and expression of iNOS compared with β-pinene and limonene [61]. Importantly, BCP has been shown to ameliorate ISO-induced (85 mg/kg) MI through modulation of the heat shock protein-60/TLR2/4/MyD88/NF-κB pathway. This study reported that BCP treatment was able to markedly reduce infarct size, normalized electrocardiogram and blood pressure parameters and significantly reduced protein expression of MyD88, TLR2, TLR4, and TIR-domain-containing adaptor-inducing interferon-β (accessory molecule of TLR signal transduction) and levels of inflammatory markers (i.e., IL-1β and TNF-α) [105]. These findings suggest a direct relationship between the anti-inflammatory abilities of terpenes and TLR-mediated pathways involved in CVDs.

These results suggest that combination therapy of taurine and black pepper terpenes may hold therapeutic cardiovascular-protective value for attenuating inflammation through modulation of TLR2 and 4 signaling pathways, which play a significant role in the development of CVDs.