The close relationship between coronary artery disease (CAD) and type 2 diabetes mellitus (T2DM) has been established by current research. Patients with T2DM are more likely to experience CAD earlier in life and in a more severe and widespread manner compared to non-diabetic patients [1]. The progression of arteriosclerosis in this population occurs earlier and at a faster rate compared to non-diabetic individuals [2]. The significant public health impact caused by ischemic heart disease is projected to rise as the population ages and the incidence of comorbidities such as obesity and diabetes increases. According to projections, the global prevalence of diabetes among adults aged 20–79 years is expected to surge in the coming decades, reaching 783.2 million by 2045 [3].

Two significant trials have been reported: DCCT (the Diabetes Control and Complications Trial) [4] in type 1 diabetes (T1DM) and ADVANCE (Action in Diabetes and Vascular Disease: Preterax and Diamicron MR Controlled Evaluation) [5] in T2DM. The results from both trials showed that improved glycemic control reduces microvascular disease and can effectively lower long-term cardiovascular complications. The United Kingdom Prospective Diabetes Study [6] suggested that intensive glycemic control can significantly reduce the occurrence of diabetes-related events. However, the impacts on cardiovascular events and mortality were not substantial. Another study revealed that the intensive-therapy regimen resulted in an unexpected increase in mortality [7]. As a result, the Food and Drug Administration (FDA) established guidelines that any anti-hyperglycemic prescription medications must demonstrate the safety of their effects on the heart and blood vessels before they can be marketed. This policy was revised and strengthened in March 2020 [8].

Fortunately, several new pharmacological agents, such as glucagon-like peptide-1 (GLP-1) receptor agonists, dipeptidyl peptidase-4 (DPP-4) inhibitors, and sodium-glucose co-transporter 2 (SGLT2) inhibitors have obtained acceptance among medical practitioners and patients due to their reliable cardiovascular profiles as demonstrated in cardiovascular outcome studies (CVOTs) [9, 10]. Despite this, controversies surrounding their risk of cardiovascular events persist. In this review, we aim to summarize and evaluate the available evidence from pre-clinical studies and clinical trials regarding the effects and underlying mechanisms of GLP-1-based therapies on CAD.

GLP-1 was first discovered as a hormone that stimulates insulin production and is secreted by intestinal endocrine L-cells. GLP-1 is released within minutes of consuming food and helps to speed up the metabolism of nutrients. One of the defining characteristics of GLP-1 is its extremely brief half-life of approximately 2 minutes, making it highly susceptible to decomposition by DPP-4. The predominant biologically active variant of GLP-1, known as GLP-1 (7-36), initiates signaling cascades through the GLP-1 receptor (GLP-1R). Yet, it undergoes rapid conversion into GLP-1 (9-36) [11]. The GLP-1R is a protein consisting of 463 amino acids belonging to the Class B family of 7-transmembrane-spanning receptors [12]. GLP-1 and GLP-1 receptor agonists exert multiple physiological effects, including increased insulin production and release, decreased glucagon secretion, delayed gastric emptying, and increased satiety by acting on the GLP-1R.

The initial approval of the first GLP-1R agonist for the management of T2DM in 2005 was attributed to the therapeutic impact of GLP-1 on glucose metabolism [13]. Since then, the FDA has approved various GLP-1R agonist products with diverse structures and pharmacokinetics (as shown in Table 1). These drugs are commonly recommended as combination therapy in addition to metformin for patients who have not achieved their glycemic goals, particularly those who need to lose weight and reduce the risk of hypoglycemia [14]. The expression of GLP-1R is not limited to pancreatic islet cells but is also observed in multiple other anatomical locations, including the lung, brain, kidney, stomach, and liver. Considerable evidence suggests that the expression of GLP-1R has been further localized in cardiomyocytes, microvascular endothelium, the endocardium, the coronary arteries, and the smooth muscle cells [15].

While GLP-1R agonism is primarily recognized for its insulin-stimulating and weight-reducing properties, studies have also demonstrated its several positive effects on the cardiovascular system in animal models [16]. Importantly, the improvement in cardiovascular function brought about by GLP-1 is not solely dependent on its ability to enhance energy metabolism and reduce body weight. GLP-1 may enhance cardiovascular function through direct action on GLP-1R and indirect mechanisms that are unrelated to its mode of action in the heart.

Inflammation, especially in those with diabetes, plays an important role in the development of atherosclerosis. Systemic inflammation can speed up the advancement of the disease and heighten the susceptibility to other CAD, such as acute coronary syndromes (ACS) and myocardial infarction (MI). Proinflammatory stimuli can cause atherosclerosis, which is marked by elevated levels of Tumor Necrosis Factor-$\alpha$ (TNF-$\alpha$), Interleukin 6 (IL-6), C-Reactive Protein (CRP), or endotoxin [17]. Anti-inflammatory drugs were postulated to have beneficial effects on the natural progression of CAD by slowing down plaque expansion, reducing the risk of acute plaque changes, and improving the outcome of MI events [18].

Both endogenous GLP-1 and GLP-1R agonists can diminish cardiovascular inflammation via direct and indirect means. GLP-1 infusion decreases the elevation in microvascular permeability during inflammation caused by lipopolysaccharide (LPS), and the positive impact may be due to GLP-1R/cAMP pathways [19]. Liraglutide has a protective effect against TNF$\alpha$-induced inflammation in human aortic endothelial cells by inhibiting nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, Protein kinase C (PKC-$\alpha$), Nuclear factor-kappaB (NF-$\kappa$B) signaling [20].

Sirtuin 6 (SIRT6), a histone deacetylase, has been recognized as a promising therapeutic target for the treatment of inflammatory vascular diseases due to its effects on NF-$\kappa$B in HUVECs [21]. Plaques in individuals with diabetes have more inflammation and higher collagen content, along with increased SIRT6 expression, compared to those in non-diabetic individuals. GLP-1-based therapy can reduce the inflammation in diabetic plaques by regulating SIRT6/NF-$\kappa$B signaling [21].

Liraglutide reduces the level of E-selectin and vascular cell adhesion molecule (VCAM-1) in response to endotoxin stimulation. This inhibitory effect was nullified by inhibiting calmodulin-dependent protein kinase kinase-$\beta$ (CAMKK$\beta$). In addition, knocking down 5 adenosine monophosphate-activated protein kinase (AMPK) abolished the liraglutide-stimulated phosphorylation of AMPK and its anti-inflammatory effect. This finding indicates that the activation of CaMKK$\beta$ and AMPK is correlated with the anti-inflammatory effect of liraglutide in endothelial cells [22]. A one-week administration of liraglutide had an anti-inflammatory effect in C57BL/6 mice with obesity produced by a high-fat diet, independent of changes in body weight. These obese mice exhibited a decrease in the level of phosphorylated AMP-activated protein kinase (pAMPK), along with cardiac insulin resistance. Additionally, there was an increase in the expression of TNF-$\alpha$ and NF-$\kappa$B [18]. Our previous research also showed that recombination of GLP-1 and liraglutide can suppress the expression of matrix metalloproteinase 1 (MMP1) by inhibiting the ERK1/2-NF-$\kappa$B signaling pathway, reversing vascular remodeling [23]. Additionally, studies have demonstrated that the administration of exenatide elicits a strong and rapid anti-inflammatory action, regardless of weight loss, and this effect becomes evident within the first two hours of treatment [24]. These findings suggest that the direct anti-inflammatory properties of GLP-1 may be separate from its hypoglycemic effects.

Intriguingly, in a mouse model of atherosclerosis, it has been demonstrated that GLP-1 (9-37) and (28-37), which are breakdown products of GLP-1 (7-37), have the potential to decrease inflammation within plaques and enhance their stability, despite their inability to control glucose metabolism [25]. Future studies are required to clarify the exact impact of GLP-1-based treatment on atherosclerosis and coronary plaque stability. It is speculated that the mechanism may be associated with the indirect control of weight loss, glucose levels, blood pressure, inhibition of inflammation, and platelet activation [26]. However, a more detailed analysis of these elements falls outside the scope of this review.

Abnormal coronary blood flow is believed to contribute to ischemic heart disease. Insufficient coronary blood flow leads to fibrosis and necrosis of the heart muscle. Reviving the balance between oxygen consumption and demand is critical to preserving heart muscle in the face of obstructed coronary arteries [27]. While coronary intervention is an effective solution, finding effective drugs is also important.

Previous research has demonstrated that GLP-1R agonists can increase coronary flow in aerobically perfused mouse hearts through GLP-1R-dependent and GLP-1R-independent mechanisms [28]. The administration of GLP-1 through infusion has been observed to result in an augmentation in microvascular blood volume and flow velocity. This effect is believed to be facilitated by a process that relies on nitric oxide, as evidenced by the utilization of contrast-enhanced ultrasound imaging [29]. Additionally, another study has demonstrated that GLP-1 administration leads to an augmentation of coronary blood flow in conscious dogs with dilated cardiomyopathy produced by rapid pacing [30].

In a post-resuscitation swine model, continuous GLP-1 infusion increased coronary flow reserve, as estimated by intracoronary Doppler flow measurements [31]. Another study found that the administration of human GLP-1 by intravenous infusion following cardiac arrest and resuscitation resulted in enhanced coronary microvascular function via reducing oxidative stress [32]. Our team has also reported that GLP-1 exhibits a dosage-dependent inhibition of thromboxane receptor agonist-induced contraction in rat coronary artery rings, and this effect may be due to stimulation of potassium-ATP currents in the vascular endothelium [33]. Additionally, the presence of the relaxant effect of GLP-1 was not detected following the removal of the endothelial layer, indicating the potential involvement of the vascular endothelium. Putative mechanisms may include stimulation of the potassium-ATP currents [33]. A study on a rat model of metabolic syndrome found that chronic administration of liraglutide improved the capacity for nitric oxide-mediated dilation in coronary vessel internal diameter. These improvements were observed irrespective of any changes in body mass or control of blood glucose levels [34].

Clinical studies, however, have produced mixed results. A small, short-term clinical study revealed that infusion of GLP-1 resulted in a notable enhancement of resting myocardial blood flow (MBF) by about 24% in patients with T2DM but without heart failure or CAD, as seen by quantitative positron emission tomography (PET) [35]. Comparable findings were found in healthy participants using semi-quantitative contrast-enhanced echocardiography [36]. Conversely, a study of patients with non-diabetic chronic heart failure found that liraglutide administration for 24 weeks did not affect myocardial blood flow despite reducing weight, HbA1c, and 2-hour glucose values compared to placebo [37].

Although the relationship between GLP-1 and its receptor agonists and the nature of coronary arteries has been studied, the exact mechanisms linking GLP-1 to improved coronary blood flow are still unclear. One possible explanation is that GLP-1 may indirectly improve myocardial capillaries through the inhibition of myocardial hypertrophy [38, 39]. Currently, large randomized controlled trials investigating the influence of GLP-1 on coronary blood flow in CAD patients are lacking, and the available data is derived from small-sample studies with varying inclusion criteria. Additional studies are warranted to examine the role of GLP-1-based therapies in addressing issues such as coronary spasm, slow flow, and no flow, which are increasingly becoming angiographic phenomena in the treatment of CAD [40].

Ischemia-reperfusion (I/R) injury is the main pathological manifestation of CAD that causes myocardial damage. Currently, there is no effective medication to defend the cardiovascular system against I/R. Rapid increases in intracellular cAMP and calcium levels brought on by GLP-1 activation trigger PKA activation and phosphorylation of cyclic AMP response element binding protein (CREB). GLP-1R agonists can modulate the activity of CREB by stimulating cytoplasmic-to-nuclear translocation of target of rapamycin (TOR) complex-2 (TORC2) [16]. Moreover, GLP-1 receptor activation also activates PKC, extracellular regulated kinase 1/2 (ERK1/2), AMPK, and phosphoinositide 3-kinase (PI3K) [41]. These pro-survival intracellular signaling pathways and proteins protect against I/R injury.

The impact of GLP-1-based therapy on ischemia-reperfusion (I/R)-induced cardiac damage has been largely demonstrated in animal models [28, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52]. However, these studies have exhibited variable findings, with some reporting positive outcomes, while others have shown neutral or negative effects (as shown in Table 2, Ref. [28, 42, 44, 45, 47, 48, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59]). For example, Kavianipour et al. [53] showed that the administration of recombinant GLP-1 did not limit infarct size during I/R when utilizing a porcine heart model. Similarly, in other studies using porcine ischemia-reperfusion models induced by balloon occlusion, liraglutide also exerts neutral properties on infarct size [54, 55]. Timmers et al. [56] demonstrated that exenatide treatment reduced myocardial infarct size in Dalland Landrace pig I/R models. The variation in the consequences of GLP-1 on I/R injury may also be attributed to variations in dosing times, whether it was given before or after the injury or the duration and methods used to artificially obstruct coronary blood flow.

Moreover, the protective effects of GLP-1 seem to be independent of its receptors [28]. In the C57BL/6J mice I/R model, pre-treatment with GLP-1 (28-36) reduces myocardial infarct size through the suppression of inhibiting mitochondrial trifunctional protein-$\alpha$, and the protective property was preserved in Glp1r${}^{-}$/${}^{-}$ mice [57]. Ban and colleagues [58] revealed that the protective effects of exendin-4 were abolished in cardiomyocytes treated with exendin (9–39), as well as in cardiomyocytes isolated from GLP-1r${}^{-}$/${}^{-}$ mice. Also, GLP-1 (9-36), a GLP-1 metabolite made by enzymatic cleavage that was thought to be biologically inactive at first, reduced the size of an infarct after I/R in a rat model and protected cardiomyocytes from GLP-1r${}^{-}$/${}^{-}$ mice. In a clinical trial conducted by Besch et al. [59], the researchers did not observe additional cardioprotective effects on I/R injury with exenatide versus insulin in patients undergoing coronary artery bypass grafting (CABG) surgery.

The disparities between the results mentioned above can be attributed to several factors. Firstly, different dosing phases may have led to varying results. Several studies have suggested that early drug administration exhibits the greatest protection against myocardial I/R injury. Secondly, differences in results may be due to inadequate doses. Clinical studies of drugs often use off-label doses, which presents challenges in establishing comparability between clinical and pre-clinical studies. Finally, other interfering or confounding factors may have overshadowed the beneficial impact of GLP-1-based therapy. For instance, the aforementioned randomized controlled trial utilized insulin in both the control and study groups, and it is well-established that insulin protects against I/R injury [60, 61]. Therefore, more research is needed to evaluate the potential benefits of various GLP-1R agonists in the context of I/R damage.

In the LEADER trial (Liraglutide and Cardiovascular Outcomes in Type 2 Diabetes), liraglutide 1.8 mg (Victoza) reduced the risk of the primary outcome (cardiac death, myocardial infarction, or stroke) by 13% in diabetics with high cardiovascular risk compared to placebo [86]. Based on these significant cardiovascular benefits, in 2017, the European Medicines Agency incorporated the outcomes derived from the LEADER trial into the prescription label for the 3 mg dosage of liraglutide (Saxenda) [87]. Although some studies suggest GLP-1 receptor agonists demonstrate a dose-response relationship in HbA1c reduction [88] and weight loss [89], whether higher doses can further reduce cardiovascular events needs further investigation.

Results from the SUSTAIN 6 trial (Semaglutide Unabated Sustainability in Treatment of Type 2 Diabetes) demonstrated that injectable semaglutide, when compared to placebo, resulted in a 26% relative and a 2.3% absolute reduction in the risk of the 3-points MACE composite endpoint in diabetics with CAD or at high atherosclerotic cardiovascular disease (ASCVD) risk [90]. In the PIONEER 6 trial (Peptide InnOvatioN for Early diabEtes tReatment), treatment with oral semaglutide in diabetics with high cardiovascular risk reduced the occurrence of MACE by 21% and significantly reduced cardiovascular death by 53% compared to placebo [91]. With the FDA approval of Wegovy (semaglutide) injection for chronic weight management in obese or overweight adults in 2021 [92], semaglutide appears to have the trend of becoming one of the most widely used GLP-1 receptor agonists. Another ongoing SOUL (Semaglutide cardiOvascular oUtcomes triaL) trial enrolled 9650 participants, with 70.7% having a history of coronary heart disease, the results of which will provide evidence on the cardiovascular effects of oral semaglutide in patients with T2DM and established ASCVD [93].

A multitude of studies of GLP-1-based interventions have shown beneficial effects on cardiovascular parameters that go well beyond its classical identification as a hypoglycemic drug. Several GLP-1R agonists, such as liraglutide, albiglutide, semaglutide, and dulaglutide, have been recommended to reduce the risk of major adverse cardiovascular events in patients with T2DM and established ASCVD or multiple risk factors for ASCVD [94]. Therefore, larger and longer-term randomized trials with clinical endpoints, such as cardiovascular morbidity and mortality, should be performed to verify the cardioprotective benefit of adjunctive GLP-1-based therapies during all periods of heart attack. However, the translation of experimentally promising results into clinical therapy has proven to be challenging. A new paradigm is required to overcome the barrier of translating pre-clinical findings on GLP-1-based therapies into practical applications.

Three elements are more crucial. First, patients, especially with STEMI, must be able to survive malignant arrhythmias. GLP-1 may provide an anti-arrhythmic effect, including in ischemia/reperfusion-induced ventricular arrhythmias [95]. Secondly, infarct size must be restricted. The best way to treat STEMI is with primary percutaneous coronary intervention to get the blood flow back to the coronary artery as soon as possible. However, this is paradoxically associated with reperfusion injury due to the generation of reactive oxygen metabolites and proinflammatory neutrophil infiltrates, which can exacerbate apoptosis and cell death. There is no standard therapy for myocardial protection following reperfusion in acute myocardial infarction. Even though different ways that GLP-1 protects the heart are biologically plausible and have shown promising results in experiments, they still need to be confirmed in large-scale clinical trials. Thirdly, the prognosis and quality of life of patients with CAD must be improved. A general goal in management includes improved symptoms with hemodynamic stabilization and increased use of evidence-based therapies to reduce recurrent hospitalization and mortality. Due to their inotropic and chronotropic properties [96], GLP-1R agonists might be selected as an add-on therapy to improve myocardial function and remodeling in CAD patients with or without diabetes.

In conclusion, currently, GLP-1 is a promising option with benign cardiovascular safety for the treatment of T2DM and coexistent CAD. Nevertheless, larger and longer-term well-designed randomized control trials of both cardiovascular morbidity and mortality are needed to evaluate GLP-1R agonist as a cardioprotective agent and the population in whom it will work best.

QX and JW prepared the manuscript and tables. KH helped with references collection. LZ and WL designed manuscript outline and revised manuscript. All authors have read and agreed to the published version of the manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work. All authors contributed to editorial changes in the manuscript.

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This study was supported by the Science and Technology Plan Project of Jiangxi Provincial Health Commission (NO. 202312257).

The authors declare no conflict of interest.