Abstract
Diabetes mellitus (DM) is a chronic and complex metabolic disorder and also an important cause of cardiovascular (CV) disease (CVD). Patients with type 2 DM (T2DM) and obesity show a greater propensity for visceral fat deposition (and excessive fat deposits elsewhere) and the link between adiposity and CVD risk is greater for visceral than for subcutaneous (SC) adipose tissue (AT). There is growing evidence that epicardial AT (EAT) and pericardial AT (PAT) play a role in the development of DM-related atherosclerosis, atrial fibrillation (AF), myocardial dysfunction, and heart failure (HF). In this review, we will highlight the importance of PAT and EAT in patients with DM. We also consider therapeutic interventions that could have a beneficial effect in terms of reducing the amount of AT and thus CV risk. EAT is biologically active and a likely determinant of CV morbidity and mortality in patients with DM, given its anatomical characteristics and proinflammatory secretory pattern. Consequently, modification of EAT/PAT may become a therapeutic target to reduce the CV burden. In patients with DM, a low calorie diet, exercise, antidiabetics and statins may change the quantity of EAT, PAT or both, alter the secretory pattern of EAT, improve the metabolic profile, and reduce inflammation. However, well-designed studies are needed to clearly define CV benefits and a therapeutic approach to EAT/PAT in patients with DM.
Keywords
Introduction
Diabetes mellitus (DM) and obesity are related multifactorial and complex metabolic disorders that significantly increase the risk for cardiovascular (CV) disease (CVD). 1,2 Obesity is a pro-inflammatory state that enhances insulin resistance (IR) and contributes to the risk of developing hypertension, dyslipidemia, DM, atherosclerosis and CVD. 3 The joint position statement from the International Chair on Cardiometabolic Risk Working Group on Visceral Obesity and the International Atherosclerosis Society highlighted the role of ectopic adipose tissue (AT) as emerging risk factors (RFs) for type 2 DM (T2DM), atherosclerosis, and CVD. 4
In the narrowest sense, obesity implies the expansion of AT (excess fat), however the type of AT as well as its distribution in the body can influence CV risk. Weight-centric management of T2DM plays a role in the prevention of CVD 5 and certain fat depots may also be relevant to CV risk. 6 In this context, the link between adiposity and CVD risk is greater for visceral than for subcutaneous (SC) fat. 6 Excess fat could manifest itself as “orthotopic” fat when one refers to organs where some fat is usually present but its quantity and function are abnormal. 7 -9 The term “ectopic” fat is inappropriate since organs like the liver and skeletal muscle have some fat present in the “normal” state. 8 There may of course be organs where there is no fat present in the “normal” state. In that case, new fat would indeed be ectopic. Patients with T2DM and obesity show a greater propensity for visceral fat deposition and excess fat in other organs. 10 Regardless of body mass index (BMI), DM is related to significant hepatic and cardiac steatosis and abnormalities in cardiac energetics and structure. 10
Although obesity per se has been viewed as a RF for CVD, the fact that fat depots are heterogeneous has led to the idea that fat depots close to coronary arteries and myocardium, epicardial AT (EAT) and pericardial AT (PAT), are more important for coronary atherosclerosis and myocardial dysfunction than excesss AT in other, more distal, organs. 11 The accumulation of toxic lipid metabolites in the endothelium of arteries and myocardium, whose source is these excess fat depots (through their lipolytic and secretory hyperactivity) may accelerate atherosclerosis. 11 There is growing evidence that EAT plays an active role in the development of coronary artery disease (CAD) and DM-related atherosclerosis, very likely due to its anatomical proximity to the myocardium. Atrial myopathy can also occur due to EAT close to the left atrium and can clinically manifest as atrial fibrillation (AF), 12 and some authors concluded that EAT accumulation is associated with adverse events associated with AF. 13 EAT releases various inflammatory cytokines and chemokines, such as interleukin (IL)-1β, IL-6, monocyte chemotactic protein [MCP]-1, and tumor necrosis factor (TNF)-α messenger ribonucleic acid (mRNA) and protein, and the levels of these biomarkers, which are important in DM, 14 are significantly higher in epicardial adipose stores compared with SC AT. 15 The transcriptome of EAT is unique when compared with SC fat; it is already clear that in patients with CAD, EAT (compared with SC AT) is highly enriched with genes that determine coagulation, immune signaling, potassium transport, inflammation and apoptosis. 16 The anti-inflammatory potential of EAT cannot be ruled out, through the secretion of adiponectin and adrenomedulin, 17 because their secretion is usually altered in patients with DM and this will probably reduce their protective effects. 18
A big step forward will be establishing the clinical significance of EAT and PAT, as well as their importance in determining CV morbidity and mortality, especially in patients with DM. 11 EAT possesses proinflammatory activity that could be considered as a feature of DM, and this is supported by recent genetic investigations (RNA sequencing analysis) which point out that the EAT transcriptome in patients with DM (mainly enriched in inflammatory genes, such as colony stimulating factor 3 (CSF3), IL-1β, IL-6) differs substantially compared with that in patients without DM. 19 Patients with multivessel CAD and DM also revealed an increased volume of EAT and more dysfunctional profile of gene expression in epicardial fat. 20 Regardless of type 1 DM (T1DM), 21,22 or T2DM status or the EAT measurement method used, the amount of EAT is greater in patients with DM compared with those without DM. 23 Furthermore, an abnormal increase in EAT may be an independent predictor of new DM and may become a new therapeutic target. 23
If the importance of EAT and PAT in CV complications is confirmed, these fat depots will become the target of both old and new therapeutic modalities, with the aim of improving CV outcomes. 11 Furthermore, pharmacologically targeting EAT may promote beneficial metabolic and CV effects. 24 In this article, we will discuss these issues.
Methods
For this narrative review we searched English-language publications in PubMed published mostly in the last decade, with emphasis on EAT and PAT in patients with DM. Our search terms were: “diabetes mellitus, visceral fat, epicardial, pericardial, risk factors, cardiovascular disease, drugs, fat depots.” Reference lists of the identified publications were also searched. We highlighted the importance of PAT and EAT in patients with DM, the ability to measure them and their relevance in terms of predicting vascular risk. We have also considered therapeutic interventions that can reduce the amount of adipose tissue and thus CV risk. English language articles related to the topic were considered if they discussed one of the issues of interest and were peer reviewed.
What Is EAT/PAT?
EAT is visceral fat, located between the myocardium and the visceral pericardium, that directly surrounds the coronary arteries. 25 Pericardial fat is anterior to EAT and is located on the parietal pericardium, 25 and has several functions; it serves as a lipid-storing depot, as an inflammatory tissue that secretes chemokines and cytokines, as well as an endocrine organ that secretes hormones. 26 Splanchnopleuric mesoderm associated with the gut is the origin of EAT, mesenteric and omental AT. 27 In contrast, PAT originates from the primitive thoracic mesenchyme. 28 Vascularization of these depots is also different, EAT is supplied by the coronary arteries, while PAT is supplied from non-coronary sources (pericardiacophrenic branch of the internal mammary artery). 29 Furthermore, PAT does not share the same microcirculation with the myocardium, which is the case for EAT. All this leads to the conclusion that EAT is a true visceral fat depot of the heart. 29 The myocardium and EAT are not separated by a fascia and share the same microcirculation indicating a close interaction between these tissues. 30
Brown AT generates heat, non-shivering thermogenesis, in response to cold temperatures and activation of the autonomic nervous system, and dissipates energy through uncoupling protein (UCP)-1-mediated heat production. 31 Because UCP-1 is highly expressed in EAT compared with other fat depots, the possibility should be considered that EAT functions like brown fat to defend the myocardium and coronary vessels against hypothermia. 32 In addition, brown adipocyte differentiation transcription factors PR-domain-missing 16 (PRDM16) and peroxisome proliferator-activated receptor (PPAR)-γ coactivator-1 alpha (PGC-1α), are also highly expressed in human EAT. 32 Some authors 33 considered that the high metabolic activity of brown AT could be beneficial for increasing the clearance and utilization of circulating lipids and glucose, hence potentially ameliorating systemic metabolic homeostasis and CV benefits; this is discussed elsewhere.
The most important functions of EAT are mechanical (protect the coronary artery against excessive distortion due to its elasticity and compressibility), metabolic (higher rate of free fatty acid [FFA] release and uptake compared with SC fat), thermogenic (large amounts of UCP1 to generate heat in response to cold exposure), 32 and endocrine/paracrine function (under normal physiological conditions produces anti-inflammation or antiatherosclerotic cytokines, such as adiponectin and adrenomedullin, while in disease conditions can be harmful, producing inflammatory factors like IL-1, IL-6, MCP-1, or TNF-α). 34 Anti-inflammatory adipokines also include C1q/TNF-related proteins (CTRPs), omentin, and secreted frizzled-related protein 5 (SFRP5), while pro-inflammatory adipokines include leptin, resistin, chemerin, visfatin, retinol binding protein 4 (RBP4), and lipocalin 2 (LCN2). 35
Healthy individuals possess a balance between anti- and pro-inflammatory adipokines. In patients with obesity and MetS, EAT volume expands and consequently this balance shifts in favor of proinflammatory mediators, while simultaneously the anti-inflammatory potential of EAT decreases. 35 Furthermore, these adipokines influence the activation state, differentiation, and proliferation of AT-resident and infiltrating immune cells. This adipokine profile dysregulation has been associated with an increased risk of metabolic dysfunction, T2DM and CVDs. 35
Ansaldo et al 36 stated that EAT is characterized by a complex secretome, which is influenced by many factors (epigenetic, genetic and environmental); these mediators are directly released into the vasa vasorum of the coronary arteries, a mechanism referred to as “vasocrine.”
Regarding molecular changes, DM is associated with decreased mRNA relative expression of fibroblast growth factor 21 (FGF21), increased expression of adrenomedullin and increased expression of RAGE (receptor for advanced glycation end products) in EAT suggesting their unique role in the atherosclerotic pathway in DM. 20
In terms of clinical relevance, the role of EAT is important due to a potentially unfavorable metabolic and CV risk profile. 25 Therefore, quantification of EAT is important.
How to Measure EAT and PAT; What Are the Limitations?
EAT and PAT volumes are variable and to date there is no consensus on the upper limit of normal cardiac AT volume. 37 Echocardiography, computed tomography (CT) and magnetic resonance imaging (MRI) are used to assess and measure the amount of EAT/PAT, with their advantages and limitations.
Iacobelis et al introduced EAT measurement by transthoracic echocardiography (TTE) in 2003; this method could be an accurate, easy, and reliable option to visualize visceral AT (VAT). 38 EAT measurement by TTE is described elsewhere. 39 Echocardiographically measured EAT thickness varies from a minimum of 1 mm to almost a maximum of 25 mm. 40 These results came from a study (n = 246 white subjects, median age 46 years, mean BMI 32 kg/m2), in which patients with metabolic syndrome (MetS) showed significantly greater median values of EAT thickness of 9.5 mm in men and 7.5 mm in women, compared with patients without MetS (4.5 and 3.5 mm, respectively) (P < 0.001). 40 Furthermore, EAT thickness of 9.5 and 7.5 mm maximize the sensitivity and specificity to predict MetS, in men and women, respectively, using receiver operating characteristics (ROC) analysis. 40 There are also data that in a specific group of patients, EAT may be used as predictor of new-onset DM. In patients with CAD treated with statins, EAT thickness at systole >5.0 mm had sensitivity of 52.5%, and specificity of 80.8% for predicting new-onset DM (hazard ratio (HR) 3.402, 95% CI: 1.751-6.611, P < 0.001). 41 Results from a prospective cohort study (n = 314 pregnant women with gestational DM (GDM) and 1832 pregnant women without GDM) showed that EAT thickness was significantly associated with the presence of GDM (odds ratio [OR] 2.87, 95% CI: 2.49-3.31, P < 0.001), using multivariate regression analysis. 42
The advantages of using TTE, in assessing EAT thickness, are the ability to directly measure visceral AT, simplicity, noninvasiveness, repeatability, and correlation with myocardial fat content. In contrast, the inability to asses EAT volume and to measure regional epicardial fat content, high variability and interindividual difference related to the investigator, are disadvantages. 31 EAT differs from PAT using TTE; PAT thickness is presented as the hypoechoic space anterior to the EAT and parietal pericardium, and does not deform substantially during cardiac cycles. 29 In cardiology patients, exposure to extensive cumulative radiation doses (an increasingly relevant safety issue) from noninvasive testing is important. 43 This favors the widespread use of TTE, which is free of ionizing radiation, compared with CT. 43 TTE only partially measures EAT compared with CT and MRI, which are much more accurate and not only measure EAT thickness but also its volume. 39 If we compare these methods, another problem is reproducibility, which favors CT and MRI. Other limitations of echocardiography include poor inter- and intra-observer agreement, the lack of threshold values to predict pathologies and EAT thickness (these values could be influenced by gender, age and ethnicity). 39 Another study 38 (n = 60 healthy subjects, mean age 49.5 ± 16.2 years) showed a strong correlation between EAT and waist circumference (r = 0.895, P = 0.01) and MRI abdominal VAT (r = 0.864, P = 0.01). After multiple regression analysis, EAT thickness was the strongest independent variable that correlated with MRI VAT (r = 0.442, P = 0.02). 38
CT provides a 3-dimensional (3D) assessment and high spatial resolution allowing the accurate quantification of EAT volume; consequently, using this radiological method may sometimes be unavoidable. 44 CT provides 3D views of the heart and its epicardial surface, advantages of high temporal and spatial resolution and submillimeter collimation. 37 Furthermore, in CT images, PAT and EAT can be quantified separately, and there is no need to use contrast. 37 The proposed methodology for assessing EAT/PAT using a CT scan is described elsewhere. 45 The interval between −190 < Hounsfield Unit (HU) < −30 is considered as the normal CT attenuation range for fat tissues, 46 but this information alone is not enough to obtain accurate segmentation of fat depots. 47 In addition, this interval depends on the individual and the CT scanner, so adjusting these limits is necessary to suit the population involved. 47 Automatic measurements of PAT can be performed accurately and reliably using non-contrast CT images and equipment acquired for routine coronary calcium screening. 48 In addition to PAT volume and pericardial fat ratio (PAT volume normalized to cardiac volume) correlate strongly with abdominal visceral fat, a known CV RF. 48 The widespread use of CT scans is not available or affordable, especially for screening purposes (where echocardiography is preferred) and ionizing radiation must be considered. Furthermore, currently, there are no clearly defined physiological limits of EAT/PAT volume. It is also not known how specific conditions/comorbidities (such as DM) can further affect changes in these fat depots.
Using CT or MRI, it is possible to quantify the area of cardiac AT, distinguishing between EAT and PAT. 49 Cardiac MRI provides a noninvasive and precise assessment of both EAT thickness and volume. MRI with a standardized protocol is used for heart evaluation; oblique axial orientation in diastole is used to estimate the amount of cardiac AT. 49 Compared with TTE, MRI is operator independent, not limited by the acoustic windows, and more accurate; MRI use is limited due to longer imaging and analysis time, less availability and 3-fold higher cost. 49
New methods like proton magnetic resonance spectroscopy (1H-MRS) is the gold standard to measure myocardial triglyceride (TG) content which is independently correlated with EAT thickness. 50
Although there is no consensus on the upper limit of normal cardiac AT volume, Bertaso et al suggested that EAT thicknesses >5 mm, or a volume >125 mL or 68 mL/m2 might be considered abnormal. 51
Measuring EAT and PAT, using different imaging modalities, is described in greater detail elsewhere. 29,52 -56
EAT and PAT as Predictors of CVD Risk
Although it is well-known that obesity and high caloric intake are undoubtedly the most important RFs for the accumulation of periorgan fat, it must also be noted that patients with IR, MetS and T2DM are susceptible to more pronounced accumulation of this fat. 57 In patients with DM, the amount of epicardial fat is significantly increased, compared with non-DM subjects, regardless of the type of DM or the method used to quantify EAT. 23 This evidence has opened up new ideas regarding the use of EAT/PAT for diagnosis and treatment. Li et al 23 showed in their meta-analysis (n = 1102 patients with DM and 813 healthy controls, 13 studies) that patients with DM vs control group had significantly greater EAT (standard mean difference [SMD] 1.23, 95% CI 0.98, 1.48; P = 0.000; trial sequential analysis [TSA]-adjusted 95% CI 0.91, 2.13; P < 0.0001). The correlation between EAT thickness and fasting plasma glucose (FPG) was evaluated in a study (n = 115 consecutive non-DM Caucasian subjects; EAT measured using TTE) and results showed significant positive correlation (r = 0.60, P < 0.001). 58 In addition, EAT thickness in patients with impaired fasting glucose (IFG) (8.0 ± 3.0 in men and 7.1 ± 4 mm in women) was higher compared with patients with normal fasting glucose (NFG) (6.0 ± 2.0 in men 5.8 ± 3.0 mm in women) (for both P < 0.001). 58 EAT volume was associated with increased risk for T2DM (OR 2.02, 95% CI: 0.68-6.57 for the second tertile [EAT volume 117-150 cm3] and OR 4.82, CI 95%: 1.55-16.58 for the third tertile [EAT volume >150 cm3]), using multiple logistic regression. 59 In addition, in patients with T2DM, the calculated visceral adiposity index (VAI) was associated with increase EAT. In a cross-sectional study (n = 128 T2DM patients and 32 control subjects; mean age 50.09 ± 7.81 years; EAT measured by TTE), EAT was positively correlated with VAI (rS = 0.207, P = 0.019), waist circumference (rS = 0.215, P = 0.015) and body weight (rs = 0.220, P = 0.013). 60 Also, in patients with T2DM and obesity, EAT volume was higher compared with lean T2DM patients (96 ± 40 vs 71 ± 21 cm3; P = 0.04). 10
The secretory pattern of EAT/PAT in patients with DM is also important. Greulich et al 18 reported that media generated from EAT biopsies obtained from patients with T2DM causes myocardial dysfunction and induces IR in primary rat cardiomyocytes. They also demonstrated that the secretory profile of EAT is considerably different from SC AT and PAT, and this secretion pattern differs between T2DM (activin A, angiopoietin-2, and CD14 are selectively accumulated in CM-EAT-T2DM) and non-DM patients; this has been discussed elsewhere. 18
There is also evidence that prostaglandin E2 (PGE2) biosynthetic enzyme (PTGES-2) (samples obtained by EAT biopsies) correlates with echocardiographic parameters of left ventricular (LV) enlargement: LV diameters, LV end diastolic volume, and LV masses (measured by TTE). 61 Moreover, PTGES-2 is directly associated with EPAC2 (exchange protein directly activated by cAMP isoform 2) gene (r = 0.70, P < 0.0001), a molecular inducer of ST2/IL-33 mediators involved in maladaptive heart remodeling. 61
Azar et al evaluated the association between EAT thickness and the levels of heart-type fatty acid binding protein (hFABP) in patients with obstructive sleep apnea syndrome (OSAS) (n = 99 OSAS patients compared with 50 control subjects). 62 hFABP levels in the OSAS group were significantly higher compared with the control group (2.65 ± 2.1 vs 1.62 ± 0.90 ng/mL; P = 0.002). EAT thickness in the OSAS and control group were 4.3 ± 1.79 and 5.3 ± 2.04 mm, respectively (P = 0.019). Their study suggested that EAT thickness and hFABP could predict CV risk. 62
Christensen et al 63 showed in their study (n = 200 patients with T2DM and elevated urinary albumin excretion rate [UAER] and no clinical features of CAD) that incident CVD and all-cause mortality were associated with a total cardiac AT, rather than EAT or PAT alone. This association remained even after adjusting for traditional CVD RFs. Patients with low cardiac AT had a lower risk of CVD and mortality than patients with high cardiac AT in unadjusted (HR 1.9, CI: 1.1-3.4, P = 0.027) and adjusted (HR 2.0, CI: 1.1-3.7, P = 0.017) models. 63 Encouraging results led to another study [n = 1030 patients with T2DM; follow-up 4.7 years; 248 patients experienced the composite end-point (incident CVD and all-cause mortality)] with the aim to evaluate the predictive potential of total cardiac AT and EAT/PAT (measured by TTE) in patients with T2DM. 64 In the unadjusted model, patients with high EAT (> median level) had an increased risk of the composite end-point (HR 1.46, 95% CI: 1.13-1.88, P = 0.004), and these results were maintained after adjustment for age and sex (HR 1.31, 95% CI: 1.01-1.69, P = 0.038) but were attenuated after adjustment for age, sex, BMI, low-density lipoprotein cholesterol (LDL-C), smoking, DM duration, systolic blood pressure and glycated hemoglobin (HbA1c) (HR 1.32, 95% CI: 0.99-1.77, P = 0.058). 64
Association Between EAT/PAT and CAD
The use of TTE for EAT quantification may provide additional information for assessing CAD risk, presence, severity and activity of CAD. 65,66 Using multiple logistic analysis, TTE measured EAT thickness ≥3.0 mm was an independent RF for CAD (OR 3.357; 95% CI: 2.17-5.17, P < 0.001), in a study that included 527 patients who underwent their first coronary angiography. 65 In addition, TTE-measured EAT thickness was higher in patients with CAD compared with those without CAD (4.0 vs 1.5 mm, P < 0.001). Also, in patients with atypical chest pain, the thickness of EAT was lower compared with those with stable and unstable angina pectoris (1.5, 3.0 and 4.0 mm, respectively, P < 0.001). 65 Similar results were obtained by Eroglu et al (n = 100 patients with CAD and 50 patients with normal coronary arteries; mean age 55.7 ± 7.4 years); patients with normal coronary arteries had lower EAT thickness compared with those who had CAD (4.4 ± 0.8 vs 6.9 ± 1.5 mm; P < 0.001). 66
Data from the MESA (Multi-Ethnic Study of Atherosclerosis, n = 183, mean age 61 ± 9 years) showed that PAT volume (measured using CT) significantly correlated with the degree of plaque eccentricity (ratio of maximal to minimal coronary artery wall thickness, determined by MRI) (P < 0.05), and maintained significance in men (P < 0.01) after adjustments for traditional RFs, BMI, C-reactive protein and coronary artery calcification (CAC). 67 Nevertheless, the volume of PAT is more related to atherosclerotic plaque burden than to lesion severity; very weak correlations were observed between PAT and CAD index, and between PAT and BMI (r = 0.27, P = 0.02 and r = 0.33, P = 0.004, respectively). 68 The presence of high-risk plaque (HRP) is associated with EAT volume independently of traditional CAD RFs (P = 0.003). 69 This study [n = 275 patients who underwent multidetector CT (MDCT) for the evaluation of CAD; mean age 65 ± 10 years] also showed a significant negative correlation between estimated glomerular filtration rate (eGFR) and EAT volume (r = −0.34, P < 0.001). 69
Uygur et al 70 in a retrospective study (n = 157 patients with DM; evaluated by CT angiography) showed that in patients with DM, both left atrioventricular (AV) groove EAT (6.9 ± 3.18 vs 4.85 ± 2.03 mL, P = 0001) and total EAT (131.36 ± 53.66 vs 95.84 ± 27.38 mL, P = 0.0001) volumes were significantly higher in patients with CAD vs non-CAD patients. Also, in patients with T2DM after adjusting for other RFs, left AV groove EAT volume (OR 1.263; 95% CI: 1.009-1.581, P = 0.041) was an independent predictor of coronary atherosclerosis, using multivariate regression analysis, while the same was not shown for EAT volume. 70 In a study (n = 17 DM and 43 non-DM patients with end-stage renal disease [ESRD] and 20 healthy subjects; evaluated by MDCT), total CAC score and EAT volume were significantly higher in patients with DM and ESRD compared with non-DM ESRD patients (202 vs 0.1 and 215.5 vs 116.0 cm3, respectively; P < 0.001 for both. 71 In patients with T2DM (n = 95 Native American patients with T2DM, 38 patients diagnosed prior to age 20 years), EAT volume (measured by CT) was a predictor of CAC (β = 0.05 ± 0.02 cm3, P = 0.03) and IL-6 concentrations (β = 0.05 ± 0.01 pg/ml/cm3, P = 0.002) in early adult onset T2DM. 72 A prospective study, which evaluated asymptomatic patients with DM (n = 333 patients without prior history of CAD, non-contrast CT at baseline and followed by a repeat scan after 2.7 ± 0.3 years; median age 54 years), showed that the probability of CAC progression went up by 12%, for each 10 cm3 increase in EAT volume. 73 Also, EAT volume was associated with both, baseline CAC scores and CAC progression (OR 1.13, 95% CI: 1.04-1.22, P = 0.04 and OR 1.12, 95% CI: 1.05-1.19, P < 0.001, respectively), after adjustment for CV RFs. 73 Groves et al in their study (n = 203 patients with T2DM) showed that EAT volume >120 cm3 was associated with the presence of significant CAD (OR 4.47, 95% CI: 1.35-14.82), after adjusting for gender, race, age, race, insulin use, BMI, hypertension and CAC score. 74 In asymptomatic patients with T2DM, Kim et al showed (cross sectional study, n = 100 T2DM patients; EAT measured by MRI) that increased EAT thickness was an independent RF for significant coronary artery stenosis (OR 1.403, P = 0.026), after adjusting for traditional RFs. 75 In patients with stable chest pain (n = 83 T2DM patients, 118 with IFG and 209 controls), those with T2DM as well as those with IFG had higher EAT volumes vs controls (98 ± 41, 92 ± 39 vs 75 ± 34 cm3, P < 0.001 respectively). However, EAT was not an independent predictor for the presence (verified using MDCT angiography) or extent (which was expressed as the number of affected segments) of CAD (OR 1.00, P = 0.88 and B −0.11, P = 0.68, respectively). 76
The Heinz Nixdorf Recall Study (n = 4093 participants, age 59.4 years, 130 subjects developed a fatal or nonfatal coronary event, follow-up period 8.0 ± 1.5 years) was conducted to determine whether EAT volume predicts coronary events; they concluded that, in the general population, EAT was associated with nonfatal and fatal coronary events independent of traditional CV RFs. 77 After adjusting for CV RFs, doubling the EAT thickness leads to a 1.5 higher risk of CV event (HR 1.54, 95% CI: 1.09-2.19), which did not change after additional adjustment for CAC score (HR 1.50, 95% CI: 1.07-2.11), and incidence of CAD events increased by EAT quartiles (4.7% for fourth vs 0.9% for first quartile, respectively, P < 0.001). 77
Two meta-analyses examined the association between EAT and coronary atherosclerosis and its consequences. 78,79 In a meta-analysis (n = 3772 patients, 9 studies), Nerlekar et al showed that increasing EAT is associated with the presence of HRP (OR 1.26, 95% CI: 1.11-1.43]; P < 0.001), and the association of EAT-v (EAT measured by volumetric assessment, CT) with HRP is significant compared with EAT-t (EAT measured by linear thickness, TEE) (P < 0.001). 78 Mancio et al 79 in their meta-analysis (n = 41534 participants, 70 studies) showed that EAT volume (assessed by CT) was not significantly associated with the presence of CAC (OR 1.007, 95% CI: 1.000-1.011, I2 = 75.8%), but was associated with significant coronary stenosis (OR 1.514, 95% CI: 1.262-1.815; I2 = 51.8%), obstructive coronary stenosis (OR 1.055, 95% CI: 1.033-1.078; I2 = 63.5%), myocardial ischemia (OR 1.062, 95% CI: 1.006-1.122; I2 = 86.9%), and major adverse CV events (MACE) (HR 1.040, 95% CI: 1.024-1.056; I2 = 64.7%).
In a study (n = 66 patients with acute coronary syndrome [ACS]; 46 patients with stable angina pectoris [SAP] who underwent percutaneous coronary intervention [PCI]), patients with ACS had a significantly higher EAT volume (measured using 64-slice CT) compared with patients with SAP (118 ± 44 vs 101 ± 41 mL, P = 0.019). 80 Also, in patients with ACS, EAT volume correlated with the percentage of lipid plaque and total plaque volume (r = 0.31 and 0.27, respectively; P < 0.05), while in SAP patients, EAT volume was positively correlated with BMI (P < 0.01) and abdominal visceral fat area (P < 0.01) but not with plaque characteristics (P > 0.05). 80
Although most studies support the importance of measuring EAT/PAT volume in CAD patients, one study found contradictory results. 81 This study (n = 380 patients with known or suspected CAD) did not show a significant association between epicardial fat volume and CAC scanning, the presence of severity of ≥50% stenosis by quantitative coronary angiography, or abnormal myocardial perfusion by single-photon emission CT (SPECT). 81 One of the main limitations of this study is that they did not differentiate between epicardial and perivascular fat nor did they differentiate between fat types (e.g. brown fat vs other).
Overall, most of the evidence is unambiguous and leads to the conclusion that quantification of EAT may be useful to identify patients at risk for CAD. 82 Furthermore, the evidence suggests a similar pattern in patients with DM. 63,64,72,73
Association Between EAT/PAT and AF
BMI, as a measure of overall obesity, is a strong predictor of AF. 83 In recent years, the importance of VAT, especially EAT, has become more evident. The physiology and pathophysiology of EAT and implications in arrhythmogenesis are described elsewhere. 84 Several mechanisms may explain the relationship between EAT and AF, including direct myocardial fatty infiltration and secretory products of EAT with fibrotic and inflammatory properties, which most likely induce arrhythmogenesis. 84 It is necessary to mention that inflammation and expansion of EAT in patients with T2DM and obesity affects both the atrium and the ventricle. 85
In a study (n = 273 patients, 71 patients with persistent and 126 with paroxysmal AF, 76 patients in sinus rhythm; PAT volume measured using CT), patients in sinus rhythm had significantly lower volume of PAT compared with those with paroxysmal and persistent AF (76.1 ± 36.3 vs 93.9 ± 39.1 and 115.4 ± 49.3 ml, respectively, P = 0.02 and P = 0.001). 86 PAT volume is significantly associated with paroxysmal (OR 1.11, 95% CI: 1.01-1.23, P = 0.04) and persistent AF (OR 1.18, 95% CI: 1.05-1.33, P = 0.004) independent of traditional RFs (age, hypertension, sex, left atrial enlargement, valvular heart disease, LV ejection fraction [LVEF], DM, and BMI). 86 In another study (n = 1288 patients who underwent CAC scanning for coronary risk stratification), PAT correlated with persistent AF and left atrium size (P < 0.001), but not with paroxysmal AF. 87 Heckbert et al 88 based on the results of their study, pointed out potential differences between race/ethnic groups, when it comes to the link between EAT and AF. In the Multi-Ethnic Study of Atherosclerosis (10.0 years of follow-up) and Jackson Heart Study (4.5 years of follow-up) (n = 7991 patients; PAT volume quantified by CT; 756 incident AF cases were identified), after adjustment for BMI, a greater PAT volume was associated with incident AF in Hispanics (HR 1.24, 95% CI: 1.05-1.46) but not overall (HR 1.06, 95% CI: 0.97-1.15). 88 In multivariable-adjusted models (n = 3217 participants from the Framingham Heart Study who underwent MDCT; mean age 50.6 ± 10.1 years), PAT but not VAT or intrathoracic AT was associated with prevalent AF (OR per standard deviation [SD] of PAT volume 1.28; 95% CI: 1.03-1.58). 89 Al-Rawahi et al 90 described several possible mechanisms that may explain the association between AF and PAT; the most significant are cardiac structural changes, local inflammation, and modulation of the autonomic nervous system. However, specific studies in patients with DM on this topic are lacking.
Results from a meta-analysis showed that in patients with AF, both EAT volume surrounding the left atrium (LA-EAT; 16.35 ml, 95% CI: 12.73-19.98, P < 0.00001) and total-EAT volume (24.23 ml, 95% CI: 19.40-29.06, P < 0.00001) were significantly increased vs controls. 91 In another meta-analysis (7 studies), patients with AF compared with controls reported an epicardial fat difference of 32.0 ml (95% CI: 21.5-42.5) showing that EAT volume is higher in AF cases. 92 A statistically significant difference in EAT volume was observed when comparing both persistent and paroxysmal AF subtypes with controls (EAT difference 48.0 ml [95% CI: 25.2-70.8] and 15.7 ml [95% CI: 10.1-21.4] for persistent and paroxysmal AF, respectively). 92
It is important to point out that most of the presented data (regarding AF) do not refer specifically to patients with DM, which indicates the need for research in this specific population.
Association of EAT/PAT With Heart Failure (HF)
Adipokines released from EAT can alter myocardial function after transfer to the arterial wall and lumen, as well as the myocardium. 93 This paracrine and vasocrine action of epicardial EAT can lead to cardiac dysfunction, myocardial lipotoxic cardiomyopathy (CMP) and HF. 94
Data related to the link between EAT and HF in patients with DM are limited. In patients with T2DM, EAT thickness was associated with decreased diastolic function, while PAT thickness was associated with subtle reductions in systolic function. 95 In addition, in patients with newly diagnosed T2DM (n = 126 patients, age 45 ± 10 years, BMI 33 ± 7 kg/m2), EAT thickness was an independent predictor of diastolic dysfunction (OR 1.9, 95% CI: 1.3-2.9, P = 0.002). 96 Topuz and Dogan 97 evaluated association between EAT thickness and LV diastolic dysfunction (using TTE and coronary angiography) in 85 patients in the CAD group (with significant coronary lesion: ≥50% stenosis), 82 patients in the non-significant CAD group (coronary lesion: <50% stenosis) and 83 patients with normal coronary arteries [NCA]. EAT thickness was significantly associated with LV diastolic dysfunction in subjects with NCA (OR 1.019, 95% CI: 1.012-1.027, P < 0.001) after multivariate analysis. 97 Fontes-Carvalho et al 98 evaluated the effect of EAT and visceral VAT on LV systolic and diastolic function, in 225 patients 1 month after myocardial infarction (MI). In this study, patients with LV diastolic dysfunction showed higher EAT volume (116.7 ± 67.9 vs 93.0 ± 52.3 ml, P = 0.01), and there was a progressive increase in EAT volume according to LV diastolic dysfunction grades (P = 0.001). Furthermore, increasing EAT volume was associated with increased E/E “ratio (adjusted β 0.19, 95% CI: 0.07 to 0.31, P < 0.01) and decreased E” velocity (adjusted β −0.11, 95% CI: −0.19 to −0.03; P < 0.01), using multivariate analysis. 98
Levelt et al 10 carried out a small study (n = 27 patients with T2DM and obesity, 15 lean patients with T2DM, and 12 normal-weight control). EAT volume correlated negatively with systolic (r = −0.53; P = 0.004) and diastolic strain (r = −0.59; P = 0.001) rates. Ng et al 99 assessed 42 DM patients (median age 59 years, duration of DM 4 years) and found a correlation between echocardiographic parameters (measured by TTE) and myocardial TG content (quantified by MRI spectroscopy). Patients with high vs those with low myocardial TG content had greater impairment of right ventricle (RV) free wall strain (−24.5 [−19.0 to −27.7] vs −27.7 [−25.1 to −30.2] %, P = 0.016), LV myocardial global strain (−17.1% [−16.3 to −17.7] vs −19.3[−18.5 to −20.1] %, P = 0.001), RV free wall strain rate (for systolic P = 0.005 and for diastolic P = 0.001) and LV global strain rate (for systolic P < 0.001 and for diastolic P = 0.003). 99 In a cross-sectional study (n = 176 T2DM patients with asymptomatic HF and 62 healthy controls; EAT thickness measured using TTE; peak oxygen uptake [peakVO2] was measured using cardiopulmonary exercise testing), peak VO2 was associated with EAT thickness (β = −0.207, P = 0.004), using multiple regression analysis. 100 EAT was thicker in healthy control compared with T2DM patients with asymptomatic HF (5.5 ± 1.2 vs 9.3 ± 1.0 mm, respectively, P < 0.001). Sugita et al 100 concluded that EAT may be associated with exercise intolerance, LV functional and structural abnormalities. Endoplasmic reticulum stress (which could be linked to both, physiological and pathological states in the CV system) and autophagy pathways in EAT might be associated with heart disease (e.g. cardiomyopathies). 101 Burgeiro et al demonstrated that both the unfolded protein response (e.g. inositol-requiring enzyme 1α, glucose-regulated protein 94 KDa, glucose-regulated protein 78 KDa) and the autophagy pathways were significantly increased in EAT compared with SC AT. 101
The relation between EAT and the severity of HF in non-ischemic dilated CMP (NICMP) was evaluated using TTE 102 and cardiac MRI. 103 In 93 patients with NICMP, there was a significant correlation between EAT thickness, LV ejection fraction (r = 0.540, P < 0.001), and B-type natriuretic peptide (BNP) levels (r = −0.695, P < 0.001). 102 When EAT thickness was corrected for BMI, EAT thickness/BMI was higher in the control group than in patients with NYHA (New York Heart Association) functional class I-II and NYHA class III-IV (0.23 ± 0.04, 0.16 ± 0.02, 0.13 ± 0.01, respectively; P < 0.001). 102 In a study (n = 112 consecutive patients with NICMP and 48 healthy controls underwent cardiac MRI examination), in patients with NICMP, an increase in LV end-diastolic mass index (r = 0.417, P < 0.0001) and LV end-diastolic volume index (r = 0.251, P = 0.01) was associated with a significant increase in EAT mass. 103 The results of a meta-analysis (n = 22 studies) showed that EAT was associated with diastolic function (weighted mean difference [WMD] 24.43 mL; 95% CI: 18.5-30.4 mL; P < 0.001); furthermore, EAT was an effect modifier for chamber size (r-value range, 0.22-0.42; P < 0.05) but not LV systolic function. 104
Other vascular beds cannot be neglected. Kocaman et al evaluated the role of EAT in carotid artery atherosclerosis. 105 Their study (n = 252 patients; cross-sectional and prospective observational design; EAT thickness measured by echocardiography) showed that EAT thickness significantly correlated with carotid intima media thickness (cIMT) (r = 0.623, P < 0.001). In addition, cIMT was significantly increased with rising EAT thickness (0.72 ± 0.15, 0.85 ± 0.16 and 0.95 ± 0.12 mm in patients with EAT <5, 5-7 and >7 mm, P < 0.001, respectively). Independent predictors for the presence of carotid plaque, in this study, were LDL-C (OR 1.013; 95% CI: 1.002-1.013, P = 0.02) and EAT (OR 1.386; 95% CI: 1.203-1.597, P < 0.001), using logistic regression analysis. The authors 105 concluded that LDL-C plays a role in the later stages of atherosclerosis, while EAT thickness appears to play a role in the earliest stages of atherosclerosis. In patients with T2DM (n = 76 patients without clinical atherosclerotic CVD [ASCVD] and 30 sex- and age-matched controls, EAT thickness measured using TTE), cIMT and EAT thickness were greater (0.77 ± 0.150 vs 0.58 ± 0.08 mm, P < 0.001, and 6.23 ± 1.27 vs 4.6 ± 1.03 mm, P < 0.001, respectively) vs controls. Furthermore, EAT thickness correlated with cIMT (r = 0.572, P < 0.001), using bivariate correlation analysis. 106
Taking all this evidence into account, the importance of EAT and PAT in the development of subclinical atherosclerosis and CVD cannot be ignored, in both those with DM and those without DM. Although the results are convincing, they must still be considered with some reservation in patients with DM because of a lack of published studies. In general, there is considerable evidence to suggest that EAT contributes to the risk of CVD, but in patients with DM this issue needs additional confirmation. 11 To date, there is a lack of studies that would indicate the importance of EAT in CVD in patients with DM; this field warrants future research.
EAT as a Treatment Target in Patients With DM
CVD risk is likely to be influenced by EAT/PAT quality and quantity. Consequently, modification of EAT/PAT may become a treatment target, with the aim to reduce CV morbidity and mortality. In recent years in metabolic diseases, including DM, periorgan fat has become both a diagnostic and therapeutic target, with an emphasis on quality (dysfunctional fat) rather than quantity (obesity). 107 Although we always keep in mind the impact on atherosclerosis, it is important to consider therapeutic modalities that, by reducing the peri-atrial EAT volume, would have positive effects on reducing the incidence and recurrence of AF. 108 Several interventional studies have shown that EAT could be considered as a possible modifiable CV RF. The effects of lifestyle interventions, bariatric surgery and antidiabetic agents on EAT and PAT has been evaluated.
EAT/PAT and Lifestyle Interventions/Exercise
In a small study (n = 14 patients with T2DM; mean age 53 ± 2 years; BMI 35 ± 1 kg/m2; PAT measured using MRI; 16-week duration) a very low calorie diet [VLCD] (450 kcal, 50 g protein, 50-60 g carbohydrates, and 6 g lipids per day) reduced PAT, SC AT and VAT to 83%, 53% and 40% of baseline values (31 ± 2 vs 39 ± 4 ml, 701 ± 108 vs 1194 ± 105 ml, 228 ± 46 vs 553 ± 37 ml, respectively, P < 0.05), and after a 14-month follow-up without dietary interventions the reduction in PAT volume was sustained. 109 High-intensity interval endurance training (3 times/week for 45 min) reduced EAT mass (32%, 95% CI, 10%-53%, P < 0.001), but not PAT mass (11%, 95% CI: −5% to 27%; P = 0.17) compared with patients who did not exercise (n = 50 participants, 12-week follow-up; EAT/PAT measured using MRI). 110 In contrast, the same study showed that resistance training (3 times/week for 45 min) reduced both, EAT and PAT mass (24%, 95% CI: 1 to 46% and 31%, 95% CI: 16% to 47%; respectively, P < 0.001 for both) in individuals with abdominal obesity. 110 Exercise (6 months of moderate-intensity exercise, followed by a high-altitude trekking expedition with exercise of long duration) induced a decrease in PAT volume (from 4.6 ± 0.9 to 3.7 ± 0.8 mL, P = 0.02), but did not change EAT volume (P = 0.9), intramyocellular lipid content (P = 0.3), myocardial TG content (P = 0.9), or cardiac function (P = 0.5), measured by MRI in patients with T2DM (n = 12 patients; mean age 46 ± 2 years). 111
Honkala et al evaluated the effect of the high-intensity interval training (HIIT) and moderate-intensity continuous training (MICT) on EAT/PAT volume, in patients with defective glucose tolerance (DGT) (n = 16 patients; 13 had T2DM and 3 IFG and/or impaired glucose tolerance (IGT); BMI 23.8-33.5 kg/m2, age 43-53 years old) and in healthy controls (n = 28; BMI 20.7-30.0 kg/m2, age 40-55 years old). 112 At baseline, the DGT subjects had higher EAT and PAT volumes than the healthy subjects (41% and 12%, respectively, P < 0.001 for both). Training decreased EAT and PAT volumes by 5% in both healthy and DGT subjects (P < 0.001 for all). 112
There is also evidence that in patients with MetS, physical activity could affect EAT thickness. 113,114 In a study (n = 60 postmenopausal women with MetS; 45-55 year old; randomly allocated to an intervention or an age, BMI and gender matched control group; EAT thickness was assessed by TTE), Fornieles González et al showed that a supervised home-based 16-week treadmill training program (3 sessions/week) reduced EAT thickness in the intervention group compared with the control group who did not perform training (6.4 ± 1.1 vs 7.7 ± 1.2 mm; P < 0.05). 113 In patients with MetS and major depressive disorder (n = 30 patients; 20 received supervised exercise training, and 10 received no specific training; EAT volume measured by MRI), exercise training reduced the amount of EAT (P = 0.017), VAT (P = 0.023) and the number of MetS factors (P = 0.018). 114
EAT/PAT and Bariatric Surgery
Bariatric surgery has also promoted a reduction in EAT volume. 115 -117 Hunt et al 118 have shown a much more important fact related to the long term (up to 14 years) effects of bariatric surgery. In a study, which also included patients with DM (n = 261 who underwent bariatric surgery; 86% gastric bypass, 243 patients in control group; AT depots quantified by non-contrast CT), EAT, SC AT and VAT volume were 30%, 20% and 42% lower in the bariatric surgery group compared with the non-surgery group (all P < 0.01). 118
In 10 patients with insulin-dependent T2DM and obesity (mean age 53.7 ± 8.9 years; 40% male; EAT volume assessed with MRI and myocardial TG content with MR spectroscopy before and 16 weeks after Roux-en-Y gastric bypass [RYGB] surgery), surgical-induced weight loss led to a larger decrease in PAT than EAT (−17.3 ± 17.2 vs −6.4% ± 6.0%), while myocardial TG content did not change. 119
EAT/PAT and Metformin
Metformin monotherapy significantly decreases EAT thickness and BMI, in patients with IR 120 and T2DM. 121 In children with obesity (n = 30 patients with IR; mean age 14.3 ± 2.1 years; BMI 34.0 ± 7.6 kg/m2; EAT measured by TTE), treatment with metformin (after 3 months) reduced EAT thickness (6.4 ± 2.1 vs 4.7 ± 2.0 mm; P = 0.008) and BMI (34.0 ± 7.6 vs 31.8 ± 7.3, P < 0.001). 120 In a study (n = 40 newly diagnosed patients with T2DM, EAT thickness was measured echocardiographically, follow up 3 months), treatment with metformin significantly decreased EAT thickness (5.07 ± 1.33 vs 4.76 ± 1.32 mm; P < 0.001). 121 The limitations of this study are the small sample and lack of data on inflammatory markers. More detailed studies should be designed to clarify the mechanisms underlying the effects of metformin on EAT thickness. 121 In contrast, metformin did not affect PAT volume (29.2 ± 1.5 vs 29.6 ± 1.6 ml, P = 0.7) measured by MRI, in 78 men with T2DM (randomized, double blind, intervention study; aged 56.5 ± 0.6 years; HbA1c 7.1% ± 0.1%) without structural heart disease. 122 In canine models, treatment with metformin also attenuated EAT adipokine production (IL-6 110.25 ± 24.58 vs 80.25 ± 19.25 pg/mL, P < 0.01; adiponectin 166.34 ± 51.48 vs 447.33 ± 53.67 pg/mL, P < 0.01) in cell cultures. 123
EAT/PAT and Pioglitazone
Jonker el al. showed, in the above mentioned study, that pioglitazone increased PAT volume (30.5 ± 1.7 [baseline] vs 33.1 ± 1.8 ml], while metformin did not alter PAT volume (between groups, P < 0.02). 122 In contrast, EAT thickness was decreased after treatment with pioglitazone (n = 97 patients with T2DM divided into 2 groups according to baseline EAT thickness; 9 months follow-up), with more prominent improvement in patients that had a greater EAT depot before treatment. 124
Furthermore, treatment with pioglitazone in patients with T2DM was associated with decreased expression of IL-1 receptor antagonists, IL-10, and IL-1b mRNA in EAT (quantified using reverse transcription polymerase chain reaction [RT-PCR]). 125 In a study which evaluated the anti-inflammatory actions of pioglitazone and simvastatin on EAT in patients with MetS and multivessel CAD (n = 73 who underwent elective bypass grafting, and EAT samples were obtained during surgery), treatment with pioglitazone decreased levels of IL-6, TNF-α, resistin and matrix metalloproteinase-9 (P < 0.001 for all). 126
This evidence leads to the conclusion that in patients at high CV risk, pioglitazone improves the metabolic profile and reduces inflammation, although it shows inconsistent changes in EAT volume.
EAT/PAT and Dipeptidyl Peptidase-4 (DPP-4) Inhibitors
DPP4 and apelin mRNA were more expressed by human adipocytes isolated from EAT compared with the SC fat (P < 0.05). 127 In patients with T2DM inadequately controlled on metformin monotherapy, the addition of sitagliptin produced a significant and rapid reduction of EAT thickness. 128 In a study (n = 26 patients with T2DM, 24-week interventional study, average age of 43.8 ± 9.0 years, HbA1c ≥7% on metformin monotherapy; EAT was measured by echocardiography), addition of sitagliptin (100 mg daily) led to EAT reduction (from 9.98 ± 2.63 to 8.10 ± 2.11 mm, P = 0.001 after 24 weeks. 128
Glucagon Like Peptide-1 Receptor Agonists (GLP-1 RAs)
The results of studies that evaluated the impact of GLP-1 RA on EAT and PAT are controversial. In an initial study (n = 95, T2DM patients with BMI >27 kg/m2, HbA1c ≤8%, 6-month randomized, controlled, open-label), treatment with liraglutide caused a rapid and substantial EAT reduction. 129 Patients were randomized to receive liraglutide (1.8 mg SC once-daily [OD]) or to remain on metformin (2000 mg/day). EAT thickness was measured by TTE before liraglutide treatment as well as 3 and 6 months after. EAT decreased from 9.6 ± 2 to 6.8 ± 1.5 mm (−29% at 3 months, P < 0.001) and 6.2 ± 1.5 mm (−36% at 6 months, P < 0.001), whereas remaining on metformin was not associated with a change in EAT thickness. 129 However, Munoz et al criticized this study stating that the result should be viewed as part of the weight loss, which was significantly more pronounced in this study than in other similar studies. 130 Furthermore, van Eyk et al 131 in their study (placebo-controlled trial, n = 47 South Asian patients with T2DM assigned to treatment with liraglutide [1.8 mg OD SC] or placebo added to standard care, AT volume measured using MRI), showed that most of the AT compartments (abdominal SC AT, VAT, EAT and PAT) were not affected by treatment with liraglutide, in the intention-to-treat analysis (P > 0.05). Interestingly, the reduction of HbA1c was associated with the decrease of VAT volume (β: 0.165 mmol/mol per 1 cm2 decrease of VAT volume; 95% CI: 0.062 to 0.267). 131 Bizino et al 132 showed similar results; liraglutide primarily reduced SC AT but not VAT, EAT, hepatic or myocardial AT. 132
In a controlled, parallel study (n = 80 patients with obesity and T2DM, follow-up 12 weeks), Iacobellis et al reported a dose-dependent reduction of EAT thickness with both semaglutide (9.5 ± 2.6 vs 7.5 ± 2 mm, P < 0.01; 1 mg SC once-weekly [OW]) and dulaglutide (9.3 ± 2.2 vs 7.7 ± 2.2 mm, P < 0.01; 1.5 mg SC OW), while there was no EAT reduction in the metformin group, which served as the control group (7.1 ± 2.1 vs 7.1 ± 2.2, NS). 133 This study had limitations. First, it was not a randomized clinical trial. Second, EAT was measured by TTE as a linear measurement at a single location and therefore may not reflect the variability of total EAT volume or fat thickness as measured by CT. 133 In a prospective randomized controlled study (n = 44 patients with T2DM and obesity; EAT volume was assessed using MRI before and after 26 weeks of treatment), treatment with exenatide reduced EAT volume (−8.8 ± 2.1 vs −1.2% ± 1.6%; P = 0.003) compared with reference treatment (sulphonylureas or repaglinide alone or in combination with metformin). 134 Morano et al 135 in their study (n = 25 patients with T2DM; mean age 63.5 ± 8.8 years; 3-months follow-up), treatment with exenatide [5.0 µg SC twice daily for the first month and with 10 µg twice daily thereafter, n = 12] and liraglutide [1.2 mg SC OD, n = 13], significantly reduced EAT thickness (9.4 ± 1.6 vs 8.0 ± 1.9 mm, P = 0.003), compared with baseline values. Furthermore, SC AT changes showed no significant correlation (P = 0.064) with those of deep fat (epicardial, peri-renal, and pre-aortic deposits). 135
In addition to the known effects on weight loss, GLP-1 RAs treatment in patients with T2DM also affects the redistribution of fat depots, which may determine a better CV risk profile. 135 Furthermore, genes involved in white-to-brown AT differentiation and fatty acid oxidation are associated with GLP-1 receptor expression, which suggests why GLP-1 RAs can affect EAT. 136 Through interaction with GLP-1 receptors in EAT, GLP-1 RAs affect brown AT differentiation, local adipogenesis, and AT utilization. 136 Due to the continuity between EAT and coronary arteries, these positive effects of GLP-1 RAs can extend to the heart, 136 and in part shed light on their CV benefits. The effect of GLP-1 RAs on EAT reduction could be considered a class effect and a part of the “puzzle” of CV benefits.
Sodium-Glucose Co-Transporter 2 Inhibitors (SGLT2i)
Several small studies have evaluated the effect of SGLT2i on EAT. In a study (n = 40 patients with CAD, mean age 67.2 ± 5.4 years, follow-up 6 months), treatment with dapagliflozin significantly decreased body weight (BW) (−2.9 ± 3.4 vs 0.2 ± 2.4 kg, P = 0.01) and EAT volume (−16.4 ± 8.3 vs 4.7 ± 8.8 cm3, P = 0.01) compared with conventional therapy (α-glucosidase inhibitor, DPP-4 inhibitor, metformin, sulfonylurea or glinide). 137 Treatment with dapagliflozin improved the differentiation of stromal vascular cells (obtained from EAT). 138 Díaz-Rodríguez et al in their study (n = 52, adipose samples were obtained from patients undergoing heart surgery) showed that dapagliflozin reduced the secretion of pro-inflammatory chemokines and enhanced wound healing in endothelial cells (0.21 ± 0.05 vs 0.38 ± 0.08 open wound; P < 0.05), and increased glucose uptake (20.95 ± 4.4 vs 12.97 ± 4.1 mg/dL; P < 0.001) and glucose transporter type 4 (2.09 ± 0.3 fold change; P < 0.01) in EAT. 138 In another study (n = 49 patients who underwent open-heart surgery; paired EAT and SC AT biopsies were cultured and treated with dapagliflozin), SGLT2i dapagliflozin reduced the released lactate and acidosis in EAT (P < 0.05), in terms of overall lactate secretion in patients with CAD. 139 Their results showed that dapagliflozin reduced the released lactate and acidosis in epicardial fat (P < 0.05) without changes in lipid storage-involved genes.
In a small study (n = 13 patients with T2DM, follow-up 6 months) treatment with canagliflozin (100 mg daily) decreased EAT thickness (9.3 ± 2.5 to 7.3 ± 2.0 mm, P < 0.001). 140 Bouchi et al 141 in their study (n = 19 T2DM patients; HbA1c 6.5%-9.0% and BMI ≥25.0 kg/m2; EAT volume measured by MRI) showed that treatment with luseogliflozin (2.5-5.0 mg daily) significantly reduced EAT volume at 12 weeks (117 [96-136] to 111 [88-134] cm3, P = 0.048]; this reduction correlated with the reduction of serum C-reactive protein levels (r = 0.493, P = 0.019).
Fukuda et al in a pilot study 142 (n = 9 non-obese Japanese T2DM patients; MRI measured EAT volume; 12-week follow-up) showed that treatment with ipragliflozin (50 mg OD) significantly reduced EAT volume at 12 weeks (102 [79-126] to 89 [66-109] cm3, P = 0.008).
Insulin
Only one study 143 evaluated the effect of insulin on EAT/PAT. Elisha et al evaluated in a pilot study (6-month, open-label, interventional randomized; n = 36 patients in the detemir group and 20 in the glargine group; EAT measured by TTE) insulin-related weight gain, weight difference in terms of body composition and the impact on EAT thickness. In the detemir and glargine group, EAT thickness significantly decreased compared with baseline levels (7.5 ± 2.7 vs 5.9 ± 2.2 mm; P = 0.013 and 7.5 ± 3.5 vs 6.3 ± 3.3 mm; P = 0.028, respectively). 143 EAT thickness change revealed a significant correlation with total fat mass (r = 0.74; P = 0.022) and truncal fat mass (r = 0.77; P = 0.016), measured using dual-energy X-ray absorptiometry (DXA), only in the detemir group. 143
Most of the described studies related to therapeutic interventions that could affect EAT and PAT in patients with DM are summarized in Table 1.
Studies Related to Therapeutic Interventions That Could Affect EAT and PAT in Patients With DM.
Abbreviations: AT, adipose tissue; BMI, body mass index; CAD, coronary artery disease; CT, computed tomography; DM, diabetes mellitus; DPP4i, dipeptidyl peptidase-4 inhibitor; EAT, epicardial adipose tissue; GLP-1 RA, glucagon-like peptide-1 receptor agonist; HbA1c, glycated haemoglobin; IL, interleukin; MetS, metabolic syndrome; MMP-9, matrix metalloproteinase-9; MRI, magnetic resonance imaging; mRNA, messenger ribonucleic acid; MRS, magnetic resonance spectroscopy; PAT, pericardial adipose tissue; RYGB, Roux-en-Y gastric bypass; SC, subcutaneous: SGLT2i, sodium-glucose co-transporter-2 inhibitor; TG, triglyceride; TNF-α, tumor necrosis factor alpha; TTE, transthoracic echocardiography; T2DM, type 2 DM; VLCD, very low calorie diet.
Statins
Independently of lowering LDL-C levels, treatment with statins induces a decrease in metabolic activity in EAT by reduction in inflammation, cellularity, or vascularity. 144 In patients with T2DM, EAT overexpression of very low-density lipoprotein receptor (VLDLR) and low-density lipoprotein receptor-related protein 1 (LRP1) may play a key role in the lipid metabolism alterations. 145 In a study (n = 420 postmenopausal women; evaluated chest CT scans before and 1 year after statin treatment; randomized to either 80 mg of atorvastatin or 40 mg of pravastatin daily) EAT HUs value decreased significantly (5.4 ± 29.7 HU [−6% change]; P < 0.001), but equally in the patients given pravastatin and atorvastatin (−4.55+28 and −6.35+31 HU; P = 0.55). 144 In patients with multivessel CAD who underwent elective bypass surgery, treatment with simvastatin decreased levels of IL-6, resistin, leptin, and monocyte chemoattractant protein-1 (P < 0.001 for all); addition of pioglitazone to simvastatin treatment further decreased plasma levels of TNF-α, IL-6, metalloproteinase-9, resistin and asymmetric dimethylarginine vs control group (P < 0.001 for all). 126 It is important to point out that the intensity of statin therapy is also relevant. Moderate-intensity therapy (pravastatin 40 mg/day) was less effective than intensive statin therapy (atorvastatin 80 mg/day) in EAT volume reduction (−0.83% vs −3.38%, P = 0.025), in a substudy of the BELLES trial (Beyond Endorsed Lipid Lowering With Electron Beam Tomography Scanning). 146 Furthermore, in another study, atorvastatin (20 mg, n = 82 patients) was also superior to the simvastatin/ezetimibe combination (10 mg/10 mg, n = 63) in reducing EAT thickness (EAT thickness change 0.47 ± 0.65 vs 0.12 ± 0.52 mm, respectively; P = 0.001). 147
To the best of our knowledge, studies evaluating the effect of statins on EAT in patients with DM have not been published to date. However, the significant place of statin treatment in patients with DM requires the inclusion of non-DM studies in this review.
The Possible Effect of Exercise, Lifestyle Interventions, and Antidiabetics on the Molecular Background of EAT Inflammation
In the light of the different treatment options that could alter the function of EAT/PAT in patients with DM (exercise, lifestyle interventions, antidiabetics), it is important to emphasize the impact of treatment options on the balance of pro- and anti-inflammatory adipokines (Figure 1).
How EAT becomes dysfunctional and how drugs could ameliorate this phenotype.
In patients with obesity and other dysmetabolic states, such as DM, EAT shifts its profile of adipokine synthesis, which is reflected in decreased release of adiponectin and increased release of a family of proinflammatory adipokines (TNF-α, leptin, IL-1β, IL-6, and resistin). 148,149 These changes further promote macrophage infiltration, destroy microvascular systems, and activate profibrotic pathways. 150 Mesenchymal stem cells that are derived from the epicardium can migrate into the heart muscle and transform into fibroblasts, and alter the function of the LV. 151 The critically important paracrine functions of EAT shift from a source of nourishment to a nidus for inflammation, dysfunction and fibrosis. 148
Exercise training reduces AT inflammation by suppressing infiltration of inflammatory macrophages and CD8 T cells. 152 In mice models, the levels of TNF-α and IL6 mRNA in the high-fat diet (HFD) sedentary mice were higher than those in the normal diet (ND) sedentary and the HFD exercise mice (TNF-α, P < 0.01; IL-6, P < 0.05). Furthermore, exercise leads to the inhibition of macrophage infiltration and the induction of AT macrophages phenotypic switch toward the M2 phenotype. The mRNA levels of MCP-1 and 2 and macrophage inflammatory proteins 1α and 1β in AT were lower in the HFD exercise mice than those in the HFD sedentary mice. 152
There is also evidence in mice models that short term feeding of ketogenic diet modulated AT immune cells, and reduced macrophage infiltration and the expansion of γδ T-cells in VAT. 153
Metformin decreases MCP-1 in isolated human AT cultures, 154 alters C-reactive protein (CRP), nuclear factor kappa B (NF-kB) expression 155 and activates anti-inflammatory macrophage polarization. 156 Furthermore, metformin lowers the pro-inflammatory cytokine production through elevating the % of M2 macrophages and lowering the % of M1 macrophages. 156 In patients with T2DM, treatment with metformin or acarbose after 6 months, significantly decreased IL-1β levels compared with baseline levels (P < 0.05), and after 12 months of treatment the IL-6 level was also significantly decreased (P < 0.05). 157 Treatment with metformin combined with lifestyle intervention showed significant decrease of TNF-α levels compared with baseline in patients with pre-DM and DM. 158
Thiazolidinediones (TZDs) repress NF-kB and thus restore M2 macrophage phenotype, and prompt the recruitment of regulatory T-cells in various ATs. 159 The pioglitazone-associated reduction in the expression of inflammatory markers (IL-6, IL-1b, and inducible nitric oxide synthase) in patients with obesity is likely to be beneficial in term of CVD prevention. 160 In the THP-1 human monocytic cell line, S100A12 mRNA levels were decreased by ∼25% by adding pioglitazone, in a dose-dependent fashion. 161
RNA-sequencing analysis showed that human EAT expresses both GLP-1 R and GLP-2 R genes, and EAT can serve as a therapeutic target for GLP-1 RAs. 24 Furthermore, GLP-1 stimulates EAT thermogenesis and adipocyte browning, 162 improves insulin sensitivity, 163 and promotes EAT preadipocyte differentiation. 164 Liraglutide had an anti-inflammatory effect due to inhibition of NF-kB pathways, down-regulating TNF-α. 165 In addition, treatment with liraglutide improved adiponectin levels. 166
Levels of pro-inflammatory cytokines (IL-6 and IL-18) significantly decreased compared with baseline levels, after 12-weeks treatment with sitagliptin (P < 0.001 for both) or vildagliptin (P < 0.01 for both). 167 Furthermore, treatment with DPP4i significantly reduced TNF-α levels (P = 0.001) in DM patients. 168 DDP4 inhibition increased the levels of uncoupling proteins in brown AT in mice with diet-induced obesity. 169 Furthermore, DPP4i treatment improved adiponectin levels in patients with T2DM. 170
AT function may be improved by SGLT2i treatment, inducing decreases in serum TNF-α levels 171 and increases in serum adiponectin levels. 172 Treatment with dapagliflozin significantly attenuated the mRNA expression of IL-6 (by ∼44% in HepG2 cells and by ∼63% in 3T3-L1 adipocytes). 173 In the post-hoc analysis of the CANTATA-SU (CANagliflozin Treatment And Trial Analysis-Sulfonylurea) study, treatment with canagliflozin at week 52 reduced IL-6 levels by 22% (95% CI: −34%, −10%) compared with glimepiride. 174
Novel EAT/PAT Treatment Concepts in DM?
There are considerations that S100A12 (EN-RAGE), due to interaction with RAGE, might play a role in the development of atherosclerosis and DM, through the initiation of a proinflammatory cytokine response. 175 A useful therapeutic approach in patients with DM could be the blockade of the ligand-RAGE axis and/or exogenous FGF21 therapy. 176 Results from a study (n = 66 patients, 33 with DM and multivessel CAD, EAT volume measured by MRI), showed that higher EAT volume in patients with DM compared with non-DM (mean [SD], 105.6 [38.5] vs 84 [29.2] ml; P = 0.02), and DM was also associated with decreased expression of cardioprotective FGF21 compared with non-DM. 20 In a study, patients (n = 239) with T2DM with CAD (angiographically verified) and those with T2DM without CAD had significantly higher serum S100A12 levels compared with the controls (157.27 ± 43.24 vs 64.03 ± 18.99 ng/mL; P < 0.01 and 123.95 ± 39.56 vs 64.03 ± 18.99 ng/mL; P < 0.01, respectively). 177 In patients with CAD and DM, increased expression of RAGE was only observed in EAT (P = 0.03), but not in PAT (P = 0.61) and periarterial fat (P = 0.71). 20
Although these results justify the possible use of these therapeutic approaches in patients with DM, they certainly need further research.
Abnormal Peri-Organ or Intra-Organ Fat (APIFat)
It is important to consider that increased EAT and PAT are unlikely to occur in isolation. In other words, excess APIFat is also likely to be present in other organs (e.g. liver, kidney, pancreas and perivascular). 178 -180 APIFat in the pancreas and non-alcoholic fatty liver disease (NAFLD) are likely to increase the risk of developing T2DM. In turn, APIFat increases with obesity. Therefore, is the evidence that EAT can predict T2DM (see text above) related to some action of EAT or just “guilt by association”? Clearly, more studies need to assess the interaction between various types of APIFat.
A very speculative suggestion is “debulking (i.e. removing excess EAT and PAT)” when surgery is performed on the heart (e.g. coronary artery bypass graft). Is that a surgically feasible option (we do not think that such a procedure has been assessed)? Will such “debulking” reduce the adverse effects attributed to excess EAT and PAT? The concept of “debulking” we propose originates from cancer treatment where it has potential benefits. 181
More work is needed to assess what EAT and PAT measurements could add to conventional risk assessments. For example, predicting AF risk? Will measuring EAT and PAT be confined to research only?
Conclusions
In patients with DM, EAT and PAT are significantly increased compared with non-DM patients, regardless of the type of DM. Most studies have dealt with EAT/PAT quantity rather than quality. However, the transcriptome of EAT is unique and is enriched with genes that determine proinflammatory activity of the “EAT secretome.”
EAT and PAT are likely to be determinants of CV morbidity and mortality in patients with DM, given their anatomical characteristics and secretory proinflammatory pattern. Drugs with clear CV benefits are currently the focus of DM care. Consequently, modification of EAT/PAT may become a target, with the aim to reduce CV burden. It is important to define treatment modalities that could reduce EAT/PAT and its proatherogenic activity.
In patients with DM, VLCD, exercise, metformin, pioglitazone, DPP-4 inhibitors, GLP-1 RAs, SGLT2is and statins could change the quantity of EAT, PAT or both. Metformin, pioglitazone, GLP-1 RAs, SGLT2is and statins could alter the secretory pattern of EAT, improve the metabolic profile, and reduce inflammation. Bariatric surgery on EAT/PAT in patients with obesity and DM may also help.
Well-designed studies are needed to clearly define the CV benefits and therapeutic approach to EAT/PAT in patients with DM.
Footnotes
Author Contributions
Emir Muzurović contributed to interpretation, conception and design, wrote the manuscript, contributed to the research data discussion, and reviewed the data and manuscript. Snežana Vujošević contributed to the research data and its interpretation. Dimitri Mikhailidis contributed to interpretation, conception and design, drafting and critical review of the manuscript. All authors approved the final version of the text.
Declaration of Conflicting Interests
The author(s) declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: EM has given talks or attended conferences sponsored by Novo Nordisk, SANOFI, Boehringer Ingelheim, AstraZeneca, SERVIER and Merck. SV has given talks or attended conferences sponsored by Novo Nordisk, SANOFI, Boehringer Ingelheim and AstraZeneca. DPM has given talks, acted as a consultant or attended conferences sponsored by Amgen, Novo Nordisk and Libytec.
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.
