Role of Cannabinoid Signaling in Cardiovascular Function and Ischemic Injury

Abstract

Cardiovascular disease represents a leading cause of death, morbidity, and societal economic burden. The prevalence of cannabis use has significantly increased due to legalization and an increased societal acceptance of cannabis. Therefore, it is critically important that we gain a greater understanding of the effects and risks of cannabinoid use on cardiovascular diseases as well as the potential for cannabinoid-directed drugs to be used as therapeutics for the treatment of cardiovascular disease. This review summarizes our current understanding of the role of cannabinoid receptors in the pathophysiology of atherosclerosis and myocardial ischemia and explores their use as therapeutic targets in the treatment of ischemic heart disease. Endocannabinoids are elevated in patients with atherosclerosis, and activation of cannabinoid type 1 receptors (CB₁Rs) generally leads to an enhancement of plaque formation and atherosclerosis. In contrast, selective activation of cannabinoid type 2 receptors (CB₂Rs) appears to exert protective effects against atherosclerosis. Endocannabinoid signaling is also activated by myocardial ischemia. CB₂R signaling appears to protect the heart from ischemic injury, whereas the role of CB₁R in ischemic injury is less clear. This narrative review serves to summarize current research on the role of cannabinoid signaling in cardiovascular function with the goal of identifying critical knowledge gaps and future studies to address those gaps in a way that facilitates the development of new treatments and better cardiovascular health.

SIGNIFICANCE STATEMENT

Cardiovascular diseases, including atherosclerosis and myocardial infarction, are a leading cause of death. Cannabinoid drugs have well known acute effects on cardiovascular function, including tachycardia and orthostatic hypotension. The recent legalization of marijuana and cannabinoids for both medical and recreational use has dramatically increased their prevalence of use. This narrative review on the role of cannabinoid signaling in cardiovascular disease contributes to a better understanding of this topic by integrating current knowledge and identifying critical gaps.

Introduction

Cardiovascular disease is the leading cause of death in the United States. More than 20 million people in the United States have cardiovascular disease, and 697,000 people died (20% of all deaths) from cardiovascular diseases in 2020 (Tsao et al., 2022). Approximately 805,000 people in the United States experience a heart attack each year (Tsao et al., 2022). Recent changes in state laws and increased societal acceptance of cannabis have significantly increased the prevalence of cannabis use in the United States despite its schedule I classification by the Drug Enforcement Administration. However, the cardiovascular risks of cannabinoid use and the potential for cannabinoid receptors to be used as therapeutic targets for the treatment of atherosclerosis, myocardial ischemia, and other cardiovascular disorders remain unclear. This review summarizes our current understanding of the role of cannabinoid receptors in the pathophysiology of atherosclerosis and myocardial ischemia and explores their use as therapeutic targets in the treatment of ischemic heart disease.

Endocannabinoid Signaling System

Although the medicinal and recreational effects of cannabis have been known for thousands of years and were reported by Chinese Emperor Shen Nung in 2737 BC, a clear understanding of the mechanisms responsible for the effects of cannabis on the human body have only emerged over the last 50 years. In 1964, pioneering work by Gaoni and Mechoulam (1965) identified delta-9-tetrahydrocannabinol (Δ⁹-THC) as the psychoactive component in hashish. Two cannabinoid receptors were identified in subsequent years. These cloned receptors include the neuronal cannabinoid type 1 receptor (CB₁R), which is responsible for mediating the psychoactive effects of Δ⁹-THC and is highly expressed in the brain (Matsuda et al., 1990). A second cannabinoid type 2 receptor (CB₂R) was cloned and is highly expressed in cells of the immune system (Munro et al., 1993). Both CB₁R and CB₂R are expressed in cardiovascular tissues, including the heart, smooth muscle, and endothelial cells of the vasculature (Liu et al., 2000, 2003; Pacher et al., 2005; Weis et al., 2010). These receptors are typically coupled to Gα_i/o proteins in neurons where activation of presynaptic CB₁R results in the activation of mitogen-activated protein kinases (MAPKs) and G protein–coupled inwardly rectifying potassium channels (GIRKs) (Mackie et al., 1995) as well as inhibition of adenylyl cyclase (Howlett and Fleming, 1984) and voltage-gated calcium channels (VGCCs) (Mackie and Hille, 1992). Similarly, cannabinoid agonists have been shown to activate MAPK signaling pathways, including p38 and c-Jun N-terminal kinases (JNKs), in cardiomyocytes and vascular endothelial cells (Mukhopadhyay et al., 2010; Rajesh et al., 2010) and decrease cAMP accumulation in the isolated rat heart (Krylatov et al., 2005).

Two primary endocannabinoids, N-arachidonoylethanolamine (AEA; anandamide) (Devane et al., 1992) and 2-arachidonoylglycerol (2-AG) (Sugiura et al., 1995; Stella et al., 1997), have been identified and exert effects on cardiac function through CB₁R and CB₂R (Pacher et al., 2005). Endocannabinoids are produced from plasma membrane phospholipids by endocannabinoid synthesizing enzymes, whereas endocannabinoid signaling is terminated by endocannabinoid hydrolysis enzymes (Piomelli, 2003). Synthesis of anandamide can occur through at least three separate biosynthetic pathways, whereas the majority of 2-AG is produced through the conversion of diaglycerol to 2-AG by sn-1-diaglyerol lipase alpha and beta (DAGLα/β) (Bisogno et al., 2003). Breakdown of AEA is mediated by fatty acid amide hydrolase (FAAH) (Cravatt et al., 1996), whereas 2-AG is hydrolyzed predominately through the activity of monoacylglycerol lipase (MAGL) (Dinh et al., 2002). These biosynthetic and metabolic pathways may provide opportunities for the development of therapeutic targets.

Cannabinoid Signaling in Cells of the Cardiovascular System

The adult myocardium is composed of approximately 56% cardiomyocytes, 27% fibroblasts, 10% vascular smooth muscle cells, and 7% endothelial cells (Banerjee et al., 2007). Likewise, the vasculature is also composed of multiple cell types, including vascular smooth muscle cells, endothelial cells, fibroblasts, and perivascular adipocytes. In addition, macrophages can be found throughout the cardiovascular system. CB₁R and CB₂R are found in many of these cell types, where they mediate the effects of both endogenous and exogenous cannabinoids.

Cardiomyocytes

CB₁R and CB₂R are expressed at very low levels in the mouse and human myocardium (Rajesh et al., 2012, 2022; Valenta et al., 2018). However, expression of these receptors is significantly upregulated under some pathologic conditions, including obesity (Valenta et al., 2018), heart failure (van Esbroeck et al., 2020) cardiomyopathy (Matyas et al., 2020), and type I diabetes (Rajesh et al., 2022). Under conditions of doxorubicin-induced cardiomyopathy (Mukhopadhyay et al., 2010) or diabetic cardiomyopathy (Rajesh et al., 2012), CB₁R signaling promotes increased oxidative/nitrosative stress, activation of p38 and JNK kinases, and increased apoptosis and cardiomyocyte death. CB₁R receptor signaling also decreases isoproterenol and forskolin-induced cAMP levels (Liao et al., 2013) and suppresses the activation of L type calcium channels in isolated cardiomyocytes (Li et al., 2009). CB₁R signaling results in suppression of cardiac contractile function (Bonz et al., 2003). CB₂R has been reported to protect isolated cardiomyocytes from oxidative stress and to protect the heart from ischemia/reperfusion injury (Defer et al., 2009). However, CB₂R has little or no impact on cardiomyocyte function under basal conditions.

Vascular Smooth Muscle Cells

CB₁R and CB₂R have opposing roles in regulating vascular smooth muscle cells. CB₁R activation promotes smooth muscle cell proliferation and migration in vitro (Rajesh et al., 2008b) and also increases smooth muscle cell proliferation in vivo after carotid balloon injury (Molica et al., 2013). In contrast, CB₂R activation suppresses smooth muscle cell proliferation after balloon injury and also suppresses proinflammatory signaling pathways [Ras, p38, MAPK, extracellular signal-regulated kinase (ERK), JNK, and protein kinase B (Akt)] in smooth muscle cells (Rajesh et al., 2008a). These data suggest that CB₁R blockade or CB₂R stimulation may provide novel therapeutic strategies to suppress vascular smooth muscle cell proliferation, stenosis, and vascular remodeling.

CB₁R located in vascular smooth muscle does not play a major role in regulating blood pressure. Wang et al. (2022) recently reported that mice in which CNR1 was deleted from vascular smooth muscle exhibit no changes in basal blood pressure. However, these animals had significantly larger infarct volumes after ischemic stroke (Wang et al., 2022). These data indicate that CB₁R expression in vascular smooth muscle is protective (perhaps via CB₁R-mediated vasodilation) under conditions of cerebral ischemia. Previous work demonstrated that CB₁R on cerebral vascular smooth muscle cells regulates the tone of cerebral arteries by decreasing calcium influx and promoting cerebral vasodilation (Gebremedhin et al., 1999). Others have suggested that cannabinoid receptors may play a role in cerebral vascular dysfunction associated with subarachnoid hemorrhage and traumatic brain injury (Benyó et al., 2016). The cerebrovascular actions of endocannabinoids and cannabinoid receptors were recently the topic of a detailed review (Benyó et al., 2016).

Endothelial Cells

Multiple studies have demonstrated that CB₁R stimulation promotes inflammation in the vasculature. CB₁R signaling in coronary artery endothelial cells is coupled to proinflammatory signaling pathways [p38, JNK, nuclear factor kappa B (NF-κB)], increased generation of reactive oxygen species, apoptosis, and cell death (Rajesh et al., 2007, 2010). More recent work demonstrated that Δ⁹-THC concentration dependently decreases cell viability in cultured endothelial cells and increases the synthesis of proinflammatory cytokines and production of reactive oxygen species (nitric oxide synthase-2 and NADPH oxidase) (Wei et al., 2022). These effects of CB₁R signaling lead to dysfunction of vascular endothelial cells in vivo. These Δ⁹-THC–induced changes were blocked by CRISPR-based suppression of CB₁R expression and by CB₁R blockade with genistein (Wei et al., 2022), demonstrating that CB₁R plays an important role in Δ⁹-THC–induced endothelial inflammation. This is consistent with endothelial dysfunction that occurs after exposure to marijuana smoke (Wang et al., 2016; Wei et al., 2022). CB₁R signaling also enhances neointima formation after balloon-induced injury and promotes the formation of atherosclerotic lesions in apoprotein E knockout mice (Molica et al., 2013). These proinflammatory and proatherosclerotic effects of CB₁R signaling are consistent with the identification of cannabis use as a risk factor for the cardiovascular disease (Skipina et al., 2022b).

In contrast to CB₁R, CB₂R signaling suppresses atherosclerosis by attenuating the endothelial expression of cell adhesion molecules that enable monocytes to migrate through the endothelium (Rajesh et al., 2007; Zhao et al., 2010b). CB₂R signaling also inhibits endothelial production of monocyte chemoattractant protein-1 and monocyte binding to cultured human coronary artery endothelial cells (Rajesh et al., 2007). CB₂R agonists also suppress tumor necrosis factor alpha (TNF-α)-induced activation of a variety of proinflammatory signaling proteins (Ras, p38, ERK, JNK, and Akt) in endothelial cells (Rajesh et al., 2008a). Thus, CB₂R suppresses inflammatory processes in endothelial cells that contribute to cardiovascular disease.

Fibroblasts

CB₁R signaling enhances interstitial fibrosis in the heart and worsens cardiac dysfunction under a variety of pathologic conditions, including diabetes (Rajesh et al., 2012), doxorubicin-induced cardiomyopathy (Mukhopadhyay et al., 2010), and after myocardial infarction (Slavic et al., 2013). Fibrosis is alleviated under these conditions by genetic deletion or pharmacological blockade of CB₁R (Rajesh et al., 2007, 2012; Slavic et al., 2013). Rimonabant blocks interleukin-1–induced upregulation of matrixmetalloprotease-9 in isolated cardiac fibroblasts and decreases hydroxyproline and collagen content in the heart after ischemic injury (Slavic et al., 2013). These CB₁R signaling events occurring in fibroblasts are associated with detrimental cardiac remodeling.

CB₂R signaling in fibroblasts plays a protective role in the heart. Diabetes-induced cardiac fibrosis is attenuated by CB₂R agonists and enhanced by genetic deletion of CB₂R (Rajesh et al., 2022). CB₂R also decreases collagen and fibronectin synthesis in the heart after an ischemic insult (Li et al., 2016). CB₂R also suppressed collagen secretion, Akt phosphorylation, and oxidative stress in isolated cardiac fibroblasts after hypoxic injury (Li et al., 2016). Thus, CB₁R and CB₂R have opposing roles in the regulation of cardiac fibroblast function.

Macrophages

Macrophages express both CB₁R and CB₂R. CB₁R activation on macrophages potentiates NLRP3-mediated inflammation (Jourdan et al., 2013), increases the production of proinflammatory cytokines (TNF-α, interleukin-6, MCP-1) (Mai et al., 2015), and promotes macrophage migration (Mai et al., 2015). In contrast, CB₂R signaling suppresses inflammation (Denaës et al., 2016; Kumawat and Kaur, 2023) and may protect the vasculature against the formation of atherosclerotic lesions. Oxidized low-density lipoprotein stimulates the production of endocannabinoids and the expression of both CB₁R and CB₂R in macrophages (Jiang et al., 2009), and CB₁R promotes the accumulation of intracellular cholesterol in macrophages (Jiang et al., 2009). Thus, inflammatory processes are enhanced by CB₁R signaling and attenuated by CB₂R signaling in macrophages.

Acute Cardiovascular Effects of Cannabinoids

Studies have demonstrated that Δ⁹-THC acts as a partial agonist at CB₁R and CB₂R, whereas the endocannabinoid 2-AG and many synthetic cannabinoids such as CP55,940 and WIN55,212-2 are full agonists at these receptors (Govaerts et al., 2004). Research done in the 1970s demonstrated that oral and inhaled administration of Δ⁹-THC or cannabis produces a number of acute cardiovascular effects, including dose-dependent tachycardia (Renault et al., 1971), reduced peripheral vascular resistance (Benowitz et al., 1979), and episodes of orthostatic hypotension (Benowitz and Jones, 1975) and syncope (Benowitz and Jones, 1975). Since these early studies, the Δ⁹-THC content in cannabis has increased dramatically from ∼3% to 6% in the 1970s to greater than 30% in cannabis strains that are available for recreational and medical use today (ElSohly et al., 2016; Pennypacker et al., 2022). Additional research from more recent studies also demonstrates robust acute effects of oral Δ⁹-THC or inhaled cannabis containing Δ⁹-THC on cardiovascular function, including blood pressure and heart rate (Martin-Santos et al., 2012; Kayser et al., 2020). These effects of Δ⁹-THC on heart rate that have been observed in adults have also been found to occur in adolescents (Murray et al., 2022). Pharmacokinetic and pharmacodynamic modeling found that the half-life for the effect of Δ⁹-THC on elevated heart rate is ∼8 minutes (Strougo et al., 2008). These studies demonstrate that Δ⁹-THC rapidly increases heart rate and can decrease blood pressure in adults and adolescents.

Early work suggested that 18–20 days of oral Δ⁹-THC administration results in a decrease in the magnitude of orthostatic hypotension, possibly due to the development of tolerance (Benowitz and Jones, 1975). However, more recent work in a carefully controlled inpatient setting found that tolerance developed to the subjective “high” effects of oral Δ⁹-THC but that tolerance was not observed for the cardiovascular effects (Gorelick et al., 2013). Multiple studies have unequivocally demonstrated that CB₁R antagonists such as rimonabant, surinabant, or AVE1625 block the cardiovascular effects of Δ⁹-THC (Gorelick et al., 2006; Huestis et al., 2007; Zuurman et al., 2010; Klumpers et al., 2013). It is unclear why tolerance develops to the psychoactive effects produced by CB₁R signaling but does not develop to the CB₁R-mediated cardiovascular effects.

Cannabinoid-induced tachycardia increases myocardial oxygen demand, and exercise tolerance is decreased in patients with angina after inhalation of a single marijuana cigarette (Aronow and Cassidy, 1974). A case crossover study of 124 patients admitted for myocardial infarction found that inhaled cannabis transiently increases the relative risk of myocardial infarction by 4.8-fold for 1 hour after cannabis consumption (Mittleman et al., 2001). Cannabis consumption has also been found to cause transient changes in the electrocardiogram, including ST segment elevation, T wave flattening and inversions, increased P wave width, and decreased P wave amplitude (Beaconsfield et al., 1972; Kochar and Hosko, 1973). These electrophysiological changes are consistent with case reports of cardiac arrhythmias associated with cannabis smoking (Singh, 2000).

Since the actions of CB₁R agonists (including Δ⁹-THC) in the brain cause psychoactive effects that are associated with substance abuse, the development of peripherally restricted agonists represents a drug discovery approach that might circumvent abuse liability associated with direct-acting CB₁R agonists. AZD1940 and AZD1704 are two peripherally restricted CB₁R agonists that were developed by AstraZeneca as novel pain therapeutics. However, the use of these compounds failed in clinical trials due to serious cardiovascular and metabolic side effects, including heart rate changes, hypotension, weight gain, and liver toxicity (Kalliomäki et al., 2013; Pacher et al., 2018). Some synthetic cannabinoids are 100 times more potent than Δ⁹-THC at CB₁R, suggesting that the adverse cardiovascular effects of these synthetic cannabinoids are likely to be much more severe than those of Δ⁹-THC (Marusich et al., 2022). These cardiometabolic effects represent a significant challenge to the therapeutic use of CB₁R agonists.

Under pathologic conditions involving shock (such as myocardial infarction, endotoxin exposure, and liver cirrhosis), endocannabinoids acting at vascular CB₁R have been shown to contribute to vasodilation and hypotension in rodent models (Wagner et al., 1997, 2001; Varga et al., 1998). As described earlier in this review, both AEA and 2-AG are rapidly degraded by hydrolytic enzymes to terminate signaling. Degradation of AEA by FAAH produces ethanolamine and arachidonic acid (AA), an important precursor in the production of eicosanoids involved in inflammation, vasodilation, and vasoconstriction, including prostaglandins, epoxyeicosatrienoic acids, hydroxyeicosatetraenoic acids, and leukotrienes (Maccarrone, 2017; Marusich et al., 2022). AA can also be generated by degradation of 2-AG by MAGL, which causes the production of AA and glycerol (Maccarrone, 2017). These AA-derived signals can exert anti- or proinflammatory effects on the cardiovascular system that are not mediated by CB₁R or CB₂R and can be either cardioprotective or damaging depending on the context and the specific AA-derived signaling molecule produced (Pacher et al., 2018; Beccacece et al., 2023).

Atherosclerosis

Role of the Endocannabinoid System in Atherosclerosis

The process by which atherosclerosis develops was the subject of a recent review (Jebari-Benslaiman et al., 2022). Atherosclerotic plaque formation is initiated when low-density lipoprotein (LDL) that is trapped beneath the endothelium becomes oxidized. Oxidized LDL stimulates the synthesis of cell adhesion molecules on the overlying endothelial cells, which subsequently attract monocytes and macrophages. These immune cells move through the endothelium into the subendothelial space where they engulf oxidized LDL, resulting in the formation of lipid-filled “foam cells.” The attraction of additional macrophages and neutrophils results in the formation of a “necrotic core,” which becomes covered by a fibrous cap that is composed primarily of smooth muscle cells and collagen. The resulting atheroma is prone to rupture, resulting in platelet aggregation, activation of blood clotting proteins, and potential occlusion of the vessel.

Endocannabinoids have been implicated in multiple steps of atherogenesis (Fig. 1). Jehle et al. (2016) reported that deletion of DAGLα (the enzyme that produces 2-AG in macrophages) decreased the formation of atherosclerotic plaque and infiltration of macrophages into arterial walls. Consistent with this observation, increasing endogenous 2-AG concentrations in vascular tissues, either by genetic deletion or pharmacological inhibition of MAGL, resulted in increased plaque formation and an increase in monocyte and macrophage infiltration into the vessel wall (Vujic et al., 2016; Jehle et al., 2018). Enhancement of anandamide concentrations in mice, either by genetic deletion of FAAH (Lenglet et al., 2013) or by FAAH inhibition using URB597 (Hoyer et al., 2014), resulted in the formation of plaques with increased neutrophil infiltration, increased matrix metalloproteinase 9 expression, and decreased collagen content. These changes in plaque composition resulted in plaques that were more vulnerable to rupture compared with plaques that developed in mice with normal anandamide concentrations. Finally, elevated levels of circulating anandamide and 2-AG in patients with coronary artery disease (compared with patients without coronary artery disease) suggest a role for endocannabinoids in atherosclerotic plaque formation (Sugamura et al., 2009).

Fig. 1.

Role of endocannabinoids and CB₁R signaling in atherosclerotic disease. Increasing levels of 2-AG or AEA by inhibiting their hydrolytic enzymes resulted in increased atherosclerotic plaque formation and infiltration of macrophages into arterial walls. Δ⁹-THC exposure induces the formation of atherosclerotic lesions in the vasculature, whereas inhibition or genetic deletion of CB₁R has protective effects on the severity of atherosclerotic plaque formation.

Role of CB₁R

CB₁R expression is upregulated in monocytes during their differentiation into macrophages, and activation of CB₁R promotes the release of proinflammatory cytokines, including interleukin-1β, interleukin-8, and TNF-α (Sugamura et al., 2009). Recent work assessing blood samples from recreational marijuana smokers confirmed a role for CB₁R in marijuana-induced increases in proinflammatory cytokines (Wei et al., 2022). Δ⁹-THC exposure also induces endothelial cell dysfunction, oxidative stress, inflammation, and the formation of atherosclerotic lesions in the vasculature (Wei et al., 2022). These effects of Δ⁹-THC were attenuated by CB₁R blockade with genistein, small interfering RNA (siRNA)-induced knockdown of CB₁R, and CRISPR-mediated deletion of CB₁R expression (Wei et al., 2022), implicating CB₁R in these Δ⁹-THC–induced vascular changes.

Studies using atherosclerotic-prone mouse models also point to a role of CB₁R in atherogenesis. Rimonabant inhibits the formation of atherosclerotic lesions and attenuates the production of proinflammatory cytokines in LDL receptor knockout (KO) mice fed a western diet (Dol-Gleizes et al., 2009). Rimonabant also decreased atherosclerotic lesion development and produced favorable changes in the serum lipid profile [decreased plasma triglycerides, increased high-density lipoprotein (HDL) cholesterol] in dyslipidemic ApoE3-Leiden-cholesteryl ester transfer protein (CETP) mice fed a western diet (van Eenige et al., 2021).

Human studies provide further evidence for the involvement of CB₁R in atherosclerosis. CB₁R expression is upregulated in human coronary atheromas isolated from coronary arteries of patients with unstable angina when compared with atheromas from patients with stable angina (Sugamura et al., 2009). This is consistent with the finding that CB₁R expression was increased in unstable lipid-rich plaques that are more prone to rupture compared with stable fibrous plaques (Sugamura et al., 2009). A randomized, double-blind clinical trial found that rimonabant improved cardiometabolic risk factors such as body weight and waste circumference in obese patients while producing favorable changes in lipid profile (increased HDL and decreased triglyceride levels) and decreased C reactive protein levels (Nissen et al., 2008). These data suggest a facilitative role for CB₁R in vascular inflammation and atherosclerosis and raise the possibility that peripherally restricted CB₁R antagonists, without psychiatric side effects associated with action at CB₁R in the brain, might be a possible therapeutic option for slowing the progression of atherosclerosis.

Role of CB₂R

In contrast to the proatherogenic effects of CB₁R signaling (Fig. 1), activation of CB₂R suppresses atherogenesis (Fig. 2). Zhao et al. (2010b) reported that activation of CB₂R suppresses the expression of cell adhesion molecules such as intercellular adhesion molecule 1, vascular cell adhesion molecule 1, and P selectin in endothelial cells. This decreased the infiltration of macrophages into the vascular wall and significantly reduced atherosclerotic plaque formation in the mouse aorta (Zhao et al., 2010b). Other work found that treatment with WIN55212-2, a mixed CB₁R/CB₂R agonist, decreased the number of macrophages, decreased NF-κB signaling, and decreased the expression of proinflammatory genes in atherosclerotic plaques from ApoE−/− mice. Importantly, these effects of WIN55,212-2 on plaque formation and macrophage infiltration were blocked by AM630, suggesting that they were mediated through CB₂R (Zhao et al., 2010a). Furthermore, genetic deletion of CB₂R in atherosclerosis-prone ApoE−/− mice resulted in increased infiltration of leukocytes into atherosclerotic lesions and increased the production of reactive oxygen species in the arterial wall of mice fed a high-cholesterol diet (Hoyer et al., 2011). In addition, CB₂R in circulating immune cells suppresses immune cell infiltration into atherosclerotic lesions (Hoyer et al., 2011). Also, Steffens et al. (2005) identified CB₂R in macrophages and T lymphocytes in atherosclerotic plaques from apoprotein E knockout mice. They found that orally administered Δ⁹-THC suppressed the progression of atherosclerotic plaques in these animals and that this effect was blocked by a CB₂R-selective antagonist (SR144528). Finally, CB₂R signaling alters the extracellular matrix composition of atherosclerotic plaques such that they have greater collagen content, decreased smooth muscle content, and increased structural stability (Netherland et al., 2010). Taken together, these studies suggest that CB₂R signaling exerts protective effects on atherosclerosis by suppressing the recruitment of immune cells in the vascular wall and by promoting changes in plaque composition that increase stability and decrease the likelihood of atherosclerotic lesion rupture. This raises the possibility that CB₂R-selective agonists might be useful therapeutic agents to slow the progression of atherosclerosis. This possibility is intriguing since CB₂R agonists do not exert negative neuropsychiatric side effects due to limited expression of this receptor in the brain. To date, no human clinical trials have assessed the effect of CB₂R-selective agonists on atherosclerosis. Human clinical trials should be performed to address this critical gap in our understanding surrounding a therapeutic role for CB₂R signaling in the development of atherosclerosis in humans.

Fig. 2.

Role of CB₂R in atherosclerotic disease. Activation of CB₂R signaling exerts protective effects on atherogenesis by suppressing the infiltration of macrophages into the vascular wall. Genetic deletion or pharmacological inhibition of CB₂R in atherosclerosis-prone ApoE−/− mice increased infiltration of leukocytes into atherosclerotic lesions.

Impact of Recreational Cannabinoids on Atherosclerosis

The impact of recreational cannabis use on atherosclerosis-related cardiovascular disease was recently the topic of a detailed review (Pacher et al., 2018). Some investigators have found that cannabis use is associated with an increase in atherosclerotic cardiovascular risk score (Skipina et al., 2022a) and an elevated risk for atherosclerosis-related cardiovascular disorders, including myocardial infarction (Chami and Kim, 2019; Wei et al., 2022), acute coronary syndrome (Richards et al., 2019), stroke (Hemachandra et al., 2016; Zhao et al., 2021), transient ischemic attack (Hemachandra et al., 2016), and chronic cardiovascular diseases (Richards et al., 2019). In contrast, other work has found no association between cannabis use and clinical markers of atherosclerosis such as carotid intima-media thickness (Jakob et al., 2021) and coronary or abdominal aorta calcium scores (Auer et al., 2018) and no association between marijuana use and stroke, transient ischemic attacks, coronary artery disease, or cardiovascular mortality (Reis et al., 2017; Dutta et al., 2021). Mahtta et al. (2021) reported that recreational use of alcohol, tobacco, amphetamine, and cannabis were each independently associated with early-onset atherosclerotic cardiovascular disease, suggesting that the increased risk of cardiovascular disease may result from other lifestyle factors associated with recreational drug use. Indeed, the American Heart Association recently published a position statement that current evidence on cardiovascular outcomes associated with recreational cannabis use is inconclusive. This report cited confounding limitations in the analysis and interpretation of current clinical research on the cardiovascular impact of cannabis use, including concurrent use of cannabis with tobacco and other recreational drugs, variations in cannabinoid content in cannabis, and biases associated with the use of hospitalized cannabis users (and hospitalized nonuser control patients) (Page et al., 2020). The scheduling of cannabis as a schedule I controlled substance has made it difficult for researchers to conduct appropriately controlled and sufficiently powered clinical studies. Most federally funded research using cannabis in the United States has used strains containing between 2% and 10% Δ⁹-THC cultivated at the University of Mississippi National Center for Natural Products Research. These strains are not necessarily representative of strains containing 20%–30% Δ⁹-THC that are commonly used by recreational cannabis users. The cardiovascular impact of the consumption of cannabis strains that have high Δ⁹-THC concentrations represent a critical gap in our understanding and is an area requiring further research.

In summary, CB₁R and CB₂R signaling plays important (and often opposing) roles in atherogenesis by modulating proinflammatory cytokine production, proatherosclerotic lipid profiles, infiltration of immune cells into the atherosclerotic wall, adhesion of monocytes to the endothelium, lipid accumulation in foam cells, and altering the composition and structural stability of atheromas. Although CB₁R activation tends to enhance the development of atherosclerosis (Fig. 1), signaling by CB₂R exerts a protective effect (Fig. 2) that could potentially be exploited therapeutically using treatment with CB₂R-selective agonists.

Myocardial Ischemia

Endocannabinoid Signaling in the Ischemic Heart

Endocannabinoid signaling has been shown to play a role in protecting the heart from ischemic injury (Fig. 3). 2-AG and anandamide concentrations are elevated in plasma from patients with coronary artery disease compared with healthy control patients (Sugamura et al., 2009). Anandamide concentrations in blood collected directly from the coronary arteries of patients with acute coronary syndrome are 10 times higher than anandamide concentrations in blood collected from systemic circulation, suggesting that anandamide is released locally from the ischemic heart (Maeda et al., 2009). Similarly, 2-AG concentrations were found to be significantly elevated in blood isolated from coronary arteries in myocardial infarction patients compared with blood collected from the coronary arteries of patients without coronary blockage (Wang et al., 2012). Consistent with these findings in the human heart, ischemia induced by ligation of the left anterior descending coronary artery elevated 2-AG concentrations in the mouse heart and plasma (Schloss et al., 2019). The increased 2-AG detected in infarcted hearts from these mice resulted from increased cardiac expression of DAGL, the biosynthetic enzyme responsible for 2-AG production, and decreased expression of MAGL, the hydrolytic enzyme that metabolizes 2-AG (Schloss et al., 2019). The increased expression of 2-AG leads to an increase in the number of neutrophils and monocytes in the blood and infarcted heart. An opposite pattern of increased DAGL mRNA expression and decreased MAGL gene expression was observed in bone marrow, suggesting that myocardial infarction might lead to a 2-AG gradient driving the recruitment of neutrophils and monocytes from bone marrow into the bloodstream and to the infarcted heart. Interestingly, the effects of 2-AG on neutrophil and monocyte recruitment were blunted in CB₂R KO mice, suggesting that these protective effects of 2-AG are CB₂R mediated. These human and animal studies suggest that endocannabinoids are released in response to a cardiac ischemic insult. Other work has found that anandamide is released from the ischemic brain (Muthian et al., 2004) and that 2-AG is released from the ischemic kidney, suggesting that activation of the endocannabinoid system might be a conserved and universal protective response to ischemic injury in both cardiac and noncardiac tissues.

Fig. 3.

Endocannabinoid signaling in cardiac ischemia. Endocannabinoid signaling exerts a protective effect on ischemic injury in the heart, and both 2-AG and anandamide concentrations are elevated in plasma from patients with coronary artery disease. Cardiac ischemic injury is worsened by genetic deletion or pharmacological inhibition of CB₂R. CB₂R-selective agonists cause a reduction in the size of myocardial infarction. Ischemic injury elevated 2-AG concentrations in the mouse heart and plasma leading to an increase in the number of neutrophils and monocytes in the blood and infarcted heart. CB₁R blockade can enhance the postischemic recovery of contractile function after myocardial infarction. Several clinical studies suggest that cannabis use is associated with an increased risk of myocardial infarction.

Impact of CB₁R and CB₂R Signaling on the Ischemic Heart

Evidence from rodent models indicates that CB₂R signaling protects the heart from ischemic injury. This is supported by work demonstrating that ischemic cardiac injury is worsened by genetic deletion of CB₂R (Defer et al., 2009; Duerr et al., 2015; Hu et al., 2019) or pharmacological inhibition of CB₂R using selective antagonists (Hajrasouliha et al., 2008; González et al., 2011; Yu et al., 2019). CB₂R KO mice exhibit attenuated expression of antioxidant enzymes and increased infiltration of macrophages into the myocardium after an ischemic insult, indicating that endocannabinoid signaling through CB₂R attenuates ischemia-induced inflammation (Duerr et al., 2015). Administration of CB₂R agonists prior to ischemia or at the onset of reperfusion decreased infarct size, apoptosis, release of enzymatic markers of myocardial injury (lactate dehydrogenase and troponin), fibrosis, and inflammation after an ischemic insult (Di Filippo et al., 2004; Montecucco et al., 2009; Wang et al., 2012; Li et al., 2013b; Li et al., 2016; Yu et al., 2019; Liu et al., 2021). CB₂R is coupled to an Akt–phosphatidylinositol-3-kinase–dependent signaling pathway that has been shown to inhibit opening of the mitochondrial permeability transition pore. This preserves mitochondrial integrity and reduces apoptosis and infarction size (Li et al., 2013b, 2014). CB₂R signaling also protects cardiomyocytes from calcium overload–induced injury by suppressing the influx of calcium that occurs during an ischemic insult (Li et al., 2013a). In contrast to cardioprotective effects of CB₂R on the myocardium, 2-AG released from the ischemic heart acts at CB₂R on nearby myeloid tissues to recruit neutrophils and monocytes from bone marrow to the site of ischemic injury. Disruption of normal 2-AG signaling through CB₂R in mice using systemic treatment with JZL-184, an MAGL inhibitor, exacerbated myocardial fibrosis and myocardial contractile function (Schloss et al., 2019). When considered collectively, the currently available data indicate that endocannabinoids act through CB₂R-mediated mechanisms that are protective against myocardial ischemic injury.

Studies investigating the impact of CB₁R have generated mixed results. In vivo studies with rimonabant demonstrated that CB₁R blockade enhances the postischemic recovery of contractile function by decreasing cardiac fibrosis and facilitating cardiac remodeling after myocardial infarction (Lim et al., 2009; Slavic et al., 2013). The protective effect of rimonabant was absent in CB₁R KO mice, confirming that this effect of rimonabant was mediated by CB₁R. However, genetic deletion of CB₁R had no impact on infarct size, suggesting that acute versus chronic disruption of CB₁R signaling might have different effects on cardiac ischemic injury. Others have suggested that CB₁R signaling within the heart may be cardioprotective, whereas stimulation of CB₁R located in unidentified tissue(s) outside of the heart might worsen myocardial ischemic injury (Lim et al., 2009). Thus, the impact of CB₁R signaling on myocardial ischemic injury remains unclear. Several avenues of future research could provide additional clarity. For example, experimental approaches that allow inducible deletion of CB₁R in mice would allow investigators to parse out the temporal effects of CB₁R signaling on myocardial ischemic injury, and studies with cardiac-specific CB₁R KO animals would help to differentiate the roles of cardiac versus noncardiac CB₁R signaling after a cardiac ischemic insult. Furthermore, a great deal of progress has been made generating positive and negative allosteric modulators of CB₁R that have yet to be studied in the context of ischemic injury.

Impact of Recreational Cannabis on Myocardial Infarction

The impact of recreational cannabis on the risk of myocardial infarction was recently the topic of a detailed review (Pacher et al., 2018). Several large clinical studies suggest that cannabis use is associated with an increased risk of myocardial infarction, especially in the first hour after cannabis use when the well known tachycardic effects of cannabinoids occur (Mittleman et al., 2001; Patel et al., 2020; Ladha et al., 2021; Ma et al., 2021; Wei et al., 2022). In addition, numerous case reports have documented elevated ST segment and elevated troponin levels indicative of myocardial infarction in young healthy individuals using cannabis despite the absence of coronary atherosclerosis or other risk factors for ischemic heart disease. The use of synthetic cannabinoids, with potencies that are often more than 100 times greater than naturally occurring phytocannabinoids in cannabis, pose unique risks for cardiac events such as chest pain, dyspnea, and myocardial infarction (Armenian et al., 2018). Nevertheless, an unequivocal cause-effect relationship between cannabis use and the risk of myocardial infarction remains controversial due to concurrent tobacco use, obesity, and other cardiovascular risk factors (Mittleman et al., 2001) and also bias that is inherent to studies using hospitalized patients to study the impact of cannabis on the risk of experiencing a myocardial infarction (Page et al., 2020).

A recently published review on the mechanisms by which cannabinoids might induce myocardial infarction posited that Δ⁹-THC increases myocardial oxygen demand by increasing cardiac contractile force and heart rate while simultaneously decreasing the coronary flow rate and limiting blood flow to the myocardium (Weresa et al., 2022). Smoked cannabis, but not vaporized or oral cannabinoids, leads to the formation of carboxyhemoglobin, which limits the capacity of red blood cells to transport oxygen to tissues, including the heart. Thus, cannabinoids could act through multiple mechanisms to increase the risk of myocardial ischemia.

Cannabinoid System As a Therapeutic Target for Cardiovascular Diseases

Correlation between Plasma Endocannabinoid Levels and Cardiovascular Risk

Plasma 2-AG concentrations in obese men are positively correlated with cardiometabolic risk factors, including abdominal obesity, body mass index, waist girth, and plasma triglyceride and insulin levels, and negatively correlated with plasma high-density lipoprotein levels (Côté et al., 2007), In addition, AEA concentrations are significantly elevated in obese patients and correlate with coronary endothelial and circulatory dysfunction (Quercioli et al., 2011, 2012). These data suggest that increased production of endocannabinoids may provide a mechanistic link between obesity and coronary circulatory dysfunction (Al Suwaidi et al., 2001; Schindler et al., 2006).

Role of Endocannabinoids in Cardiomyopathy

Doxorubicin is a topoisomerase inhibitor that is commonly used for cancer chemotherapy. Therapeutic use of this agent is often limited by doxorubicin-induced toxicity, which is characterized by oxidative/nitrative stress, apoptosis, and declining cardiac function. Genetic deletion of FAAH, the enzyme that metabolizes AEA, enhances doxorubicin-induced oxidative/nitrative stress, increases apoptosis and cell death, and increases doxorubicin-induced cardiac dysfunction and mortality in mice (Mukhopadhyay et al., 2011). Doxorubicin-induced toxicity is attenuated by CB₁R blockade both in human cardiomyocytes (Mukhopadhyay et al., 2010) and in mice (Mukhopadhyay et al., 2007). Similarly, myocardial AEA levels, CB₁R expression, oxidative/nitrative stress, and apoptosis are increased in the heart under conditions of diabetic cardiomyopathy (Rajesh et al., 2012). Pharmacological blockade or genetic deletion of the CB₁R suppresses diabetes-induced oxidative stress, apoptosis, and inflammation in the heart and preserves myocardial contractile function. These data provide evidence that endogenous cannabinoids acting on CB₁R worsen doxorubicin- and diabetes-induced cardiomyopathy. In contrast, deletion of the CB₂R worsens oxidative/nitrative stress, inflammation, apoptosis, and contractile dysfunction associated with diabetic cardiomyopathy, indicating that endocannabinoid signaling via CB₂R has a cardioprotective effect under these pathologic conditions (Rajesh et al., 2022).

Clinical Impact of CB₁R Blockade on Cardiovascular Risk

Dysfunction of the endocannabinoid system plays an important role in obesity. Plasma 2-AG concentrations are significantly greater in obese individuals than lean subjects and are positively correlated with visceral fat mass (Blüher et al., 2006). In addition, the number of mRNA transcripts encoding CB₁R and FAAH in visceral adipose tissue is significantly lower in obese individuals compared with lean subjects (Blüher et al., 2006). Variations in genes involved in endocannabinoid signaling may predispose individuals to weight gain. A single nucleotide polymorphism in the gene encoding fatty acid amide hydrolase (P129T) results in decreased FAAH expression and activity (Chiang et al., 2004). Sipe et al. (2005) reported that people who are homozygous for this polymorphism exhibit a significantly greater body mass index than those who are heterozygotes or inherit the wild-type version of this gene, suggesting that this heritable polymorphism may play a role in obesity.

Rimonabant was approved in Europe in 2006 for the treatment of obesity. Clinical trials on rimonabant found significant improvements in cardiovascular risk factors, including decreased body weight, improved dyslipidemias, and decreased fasting glucose and insulin levels in overweight patients (Hollander, 2007; Nissen et al., 2008; Van Gaal et al., 2008b). The Rimonabant in Obesity (RIO) study found that obese patients who were prescribed rimonabant in combination with a modestly hypocaloric diet (600 kcal per day deficit) lost significantly more weight after 1 year than patients taking a placebo in concert with the hypocaloric diet. Rimonabant produced significant improvements (compared with placebo) in several cardiometabolic risk factors, including waist circumference, insulin resistance, prevalence of metabolic syndrome, A1C, HDL cholesterol, and triglycerides (Van Gaal et al., 2005, 2008a). These benefits were sustained after a second year of rimonabant treatment, indicating that chronic rimonabant therapy could produce long-term improvements in these cardiometabolic risk factors (Van Gaal et al., 2008b). The Comprehensive Rimonabant Evaluation Study of Cardiovascular Endpoints and Outcomes (CRESCENDO) trial was established to determine whether long-term rimonabant treatment decreased the risk of myocardial infarction, stroke, and other cardiovascular causes of death. This study was a double-blind, placebo-controlled clinical trial involving 974 hospitals in 42 countries and was intended to follow patients taking rimonabant or placebo for a minimum of 33 months. However, due to an increase in psychiatric side effects, including suicidal ideation, this study was terminated prematurely after 14 months with no significant cardiovascular findings (Topol et al., 2010). The prevalence and severity of these adverse psychiatric effects led to rimonabant’s removal from the European market in 2008. Two other clinical studies found no impact on clinical indicators of the progression of atherosclerosis, including atheroma volume in the coronary vasculature (Nissen et al., 2008) or carotid intima-media thickness (O’Leary et al., 2011), in patients taking rimonabant for 12–30 months. The investigators suggested that the benefits of rimonabant may have been masked by the fact that most patients had lipid profiles that were well controlled by statins prior to initiation of the study (Nissen et al., 2008). Similar to the CRESCENDO trial, both of these studies found adverse psychiatric effects in patients using rimonabant.

In light of the finding that adverse psychiatric effects are significantly increased by rimonabant, peripherally acting CB₁R antagonists or monoclonal antibodies that do not enter the central nervous system (CNS) may be more therapeutically useful. Wei et al. (2022) recently identified genistein as a CB₁R antagonist with minimal penetration into the CNS. Genistein reduced Δ⁹-THC–induced atherogenesis in vivo, providing evidence that this ligand may provide an alternative approach to targeting peripheral CB₁Rs without inducing adverse effects in the CNS. Another enticing therapeutic possibility involving CB₁R is the use of negative allosteric modulators that might dampen endocannabinoid signaling tone in the context of cardiovascular disease. The use of CB₁R negative allosteric modulators to reduce endogenous CB1R signaling (as opposed to antagonists directed to the orthosteric binding site) is an approach that might reduce the likelihood of adverse psychiatric side effects.

Current data indicate that CB₂R signaling exerts a protective effect in the context of atherosclerosis and myocardial ischemia. Therefore, the prophylactic use of selective CB₂R agonists in patients with cardiovascular risk factors is a therapeutic option that warrants more study. Interestingly, CB₂R is virtually nonexistent in the uninjured brain under normal conditions, and the use of CB₂R agonists has not been linked to psychoactivity or adverse psychiatric effects in either rodent models or human studies. Another therapeutic possibility warranting further study is the use of inhibitors of endocannabinoid hydrolysis enzymes to enhance levels of anandamide and 2-AG. Since chronic genetic and pharmacological blockade of MAGL has been shown to desensitize and downregulate CB₁R in the brain, MAGL inhibitors may be useful for treating human diseases, including cardiovascular conditions (Schlosburg et al., 2010). Similar to the use of allosteric modulators, therapeutic approaches to increase basal endocannabinoid signaling tone are less likely to produce CB₁R-associated negative side effects compared with direct acting orthosteric agonists. Finally, to fully realize the potential of different cannabinoid-directed therapeutics, it is important that we gain a clear understanding of the temporal requirements for cannabinoid signaling in cardiovascular disease. For example, it is important to understand the capacity of each cannabinoid-directed therapeutic compound to prevent the establishment of cardiovascular disease as opposed to their abilities to reverse disease once it has occurred. This knowledge will shed insight on whether treatments need to be given prophylactically to patients at risk for cardiovascular disease or whether they might also be useful in treating established disease.

Abbreviations

AA

arachidonic acid

AEA

anandamide

2-AG

2-arachidonoylglycerol

Akt

protein kinase B

CB₁R

cannabinoid type 1 receptor

CB₂R

cannabinoid type 2 receptor

CNS

central nervous system

DAGL

sn-1-diaglyerol lipase

FAAH

fatty acid amide hydrolase

HDL

high-density lipoprotein

JNK

c-Jun N-terminal kinase

KO

knockout

LDL

low-density lipoprotein

MAGL

monoacylglycerol lipase

MAPK

mitogen-activated protein kinase

Δ⁹-THC

delta-9-tetrahydrocannabinol

TNF-α

tumor necrosis factor alpha