The main demographic, anthropometric, clinical, and biochemical characteristics of the 10 patients (5 CABG and 5 VR) are reported in Table S1 (Supplementary Information). The mean age was 65.7±12.7 years. Even if CABG patients were older than VR patients (71.6±6.8 years vs. 59.8±14.3 years, respectively), no statistical difference was seen. The majority of patients were males (n=9).
No significant difference in the measurements of weight, height, body mass index (BMI), and waist circumferences was observed between CABG and VR patients.
According to BMI, an indicator of general fatness, eight patients were affected by obesity and two by overweight; while adopting waist circumference as an indicator of abdominal fat distribution, all patients were affected by abdominal obesity.
No difference was observed in smoking status, and only one patient in the CABG group was affected by type 2 diabetes mellitus. The percentages of antiplatelet, statins, angiotensinogen-converting enzyme inhibitors/angiotensin receptor blockers and oral glucose-lowering drugs were similar in the two groups; no difference was observed in echocardiographic EAT thickness between the two groups. Adipocytes have a dynamic endocrine role by expressing and secreting many factors, including hormones and cytokines, that depend on cell size . Several studies have recently paid attention to EAT because its specific anatomical location. In this way, regional differences in adipocyte hypertrophy and inflammatory function might suggest a different metabolic response in patients with cardiovascular disease. The size of adipocytes in EAT is significantly smaller than in other adipose tissues .
In line with data from other studies (6), in our study, the size of adipocytes of both groups was around 3000 µm2 (2940 ± 188 vs. 3260 ± 299 VR vs. CABG group respectively, p= 0.39 ns). Thus, the size was about one-half found in visceral fat (omentum) of patients with obesity of a different case series but measured with the same methods .
Inflamed visceral adipose tissue is marked by the presence of CLSs, where scavengers’ macrophages surround dead adipocytes [3, 4]. Although all patients had abdominal (visceral) obesity, in EAT samples, no CLS were detected in the VR group who did not show any sign of CAD in the pre-operative coronary angiographic examination; in contrast, CLS were detected in 3 patients of the CABG group (about 17 CLS/104 adipocytes vs. 0.0 CLS/104 adipocytes, CABG vs. VR group respectively), although no significant differences in adipocyte size were seen in the two groups (Figure 1). Interestingly, the density of CLS was about three times higher than that found in visceral fat (omentum) of patients with obesity in the above-cited study . One of the CABG patients (SL) had an extraordinary density of CLS (Figure 2); therefore, to avoid a false picture of the degree of inflammation in the CABG group, his data were not included. CLS density in SL was about 100 times the average value observed in other CABG patients. We observed such a density of CLS only in visceral fat of mice with severe obesity [11 and unpublished data], but never in humans . Of note, the clinical data of LS differed from all other cases mainly for one important clinical aspect: he had a recent episode of myocardial infarction and a quite rapid (few years) weight gain of about 13 kg, almost entirely in the abdominal region, with an increase in abdominal circumference of 9 cm.
The specific unique nature of EAT with intermediate characteristics between white and brown adipocytes, also known as beige or brite adipose tissue, [6,7] could explain the higher inflammation found in EAT when compared with omental fat of another case series studied with the same methods . In addition, the age of LS (78 years) must also be taken into account as, with ageing, epicardial adipocytes become more susceptible to environmental, metabolic, and haemodynamic factors, which gradually change the function of EAT from thermogenesis to energy storage .
Indeed, EAT brown fat-like activity decreases substantially with age. The changes are not only functional but also structural. The proportion of brown adipocytes decreases in favour of more unilocular white adipocytes in older individuals. This finding suggests that the transition from brown fat to beige fat is a feature of EAT in adults . In line with these data, we recently showed that whitening of brown fat induces a high degree of inflammation with a high density of CLS in this depot (12)
The absence of CLS in the VR group could be explained by the small adipocytes size difference (about 10%), but mainly by the different ages of the two study groups, as the CABG group was more than ten years older than the VR group, who did not show any sign of CAD in the pre-operative coronary angiographic examination. Aging has been recently reported as an important factor worsening EAT chronic inflammation (13).
An alternative, plausible explanation of the higher presence of CLS in EAT from CABG than VR patients is the downregulation of the EAT housekeeping genes transcriptome that we observed and reported previously (14). The hypothesis might be that end/stage or advanced coronary artery disease (CAD) causes downregulation of EAT genes due to fibrotic and apoptotic changes or, better, to the mounting pyroptotic changes following the continuous and chronic inflammatory insult (2). These processes would lead to adipocytes’ death and consequent CLS formation. Based on this observation, we suggest that the EAT adipocytes are “burned-out” (pyroptosis) in advanced CAD. CAD more than obesity plays a role in the higher EAT inflammation and related morphological changes. This study clearly confirms this.
In line with previous data showing that EAT plays a role in the progression and development of CAD, inflammation is the main feature of EAT in patients with obesity and CAD, showing dense macrophage infiltrates [5,6,8], as observed among adipocytes in all our studied patients with abdominal obesity and CAD (not shown).
To the best of our knowledge, only one recent paper described CLS in EAT of patients with obesity, CAD and type 2 diabetes mellitus . They did not quantify the CLS density in each patient but described only if CLS were present or not. Surprisingly, in their case series of EAT biopsies studied by immunohistochemistry (n=16 patients affected by obesity and n=28 patients without obesity), the CLS were found only in 14% of patients without obesity, but with CAD; interestingly the prevalent phenotype of macrophages was the pro-inflammatory M1 . It would be interesting to know if the patients without obesity but with CLS in EAT, in that case series, had an excess of visceral fat. Data presented show only an average (including patients without CLS) waist circumference within normal values.