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Cardiac MRI detects a reduced volume and anti-inflammatory fatty acid composition of epicardial adipose tissue in eplerenone-treated obese mice1University of Virginia, Charlottesville, VA, United States
Synopsis
Epicardial adipose tissue (EAT) volume and fatty acid composition (FAC) have significant roles in EAT-mediated coronary vascular inflammation. Using MRI of EAT volume and FAC, we tested the hypothesis that eplerenone reduces EAT volume and alters its FAC in a mouse model of obesity. We show that eplerenone significantly reduces EAT volume after 30 weeks on a high-fat high-sucrose diet compared to untreated mice. In addition, the EAT FAC is shifted from saturated fatty acid dominant in untreated mice to poly-unsaturated fatty acid dominant in eplerenone-treated mice. The results suggest that eplerenone has an anti-inflammatory role in obesity-related EAT.
Introduction
The accumulation of epicardial adipose tissue (EAT), as seen in obesity, enhances the risk of developing cardiovascular disease (CVD)1,2, potentially including coronary microvascular disease. EAT is comprised not only of adipocytes, but also immune cells, particularly macrophages3. An increased EAT volume promotes activation of the mineralocorticoid receptor (MLR)4–6 and a proinflammatory phenotype. Furthermore, the fatty acid composition (FAC) of adipose tissue contributes to the inflammatory state7,8. Specifically, saturated fatty acids (SFAs) promote an inflammatory macrophage phenotype and trigger inflammasome activation while poly-unsaturated fatty acids (PUFAs) promote an anti-inflammatory state9–11. Prior work has shown that MLR antagonism reduces fat accumulation and adipose inflammation in abdominal and epidydimal adipose tissue12,13. We hypothesized that cardiac MRI of EAT volume and FAC would show that MRL antagonism with eplerenone (EPL) reduces EAT volume and shifts the EAT FAC away from SFAs and toward PUFAs in mice fed a high-fat high-sucrose diet (HFHSD).Methods
Untreated (n=5) and EPL-treated (n=5) C57Bl/6 female mice fed a HFHSD were studied. HFHSD (40% kcal fat, 40% kcal sucrose) was initiated at 10 weeks of age and continued for 30 weeks. In the treatment group, EPL (100 mg/kg/day) was added to the HFHSD chow. MRI was performed after 30 weeks of diet using a 7T system (Clinscan, Bruker) and a 35mm diameter RF birdcage coil. For EAT volume imaging, an ECG-gated three-point Dixon gradient-echo sequence using the phase-offset multiplanar method14–16 was used to acquire 6 short-axis slices from base to apex. The three echo times (TEs) were 2.5, 3.0, and 3.5ms. For EAT FAC imaging, a similar sequence was used to acquire a single mid-ventricular short-axis slice using 9 TEs equally spaced from 2.0-4.4ms. Table 1 shows a full list of parameters for both sequences. Water and fat-separated images were computed from the three-point Dixon images for all slices17, and EAT volume was calculated from the fat-only images. EAT FAC was calculated using a voxel-by-voxel Iterative Decomposition of water and fat with Echo Asymmetry and Least‐squares estimation (IDEAL) based method18. As described in Figure 1, signals from each echo were fit to a signal model accounting for the ten proton resonances present in fatty acids, four fatty acid components (F1 to F4), and a complex field map summarizing the B0 and R2* effects. The signals from the fatty acid components were then combined into SFA, PUFA, and mono-unsaturated fatty acid (MUFA) fractions within each voxel19. A t-test was used to test for differences in EAT volume and SFA, PUFA, and MUFA% between untreated and EPL-treated mice.Results
Figure 2A shows example images with water and fat in-phase (TE 3.0ms) or out-of-phase (TE 2.5ms and 3.5ms), water-only, and fat-only in a mid-ventricular slice of an untreated mouse. EAT volume was reduced in EPL-treated vs untreated mice (2.65 ± 2.20 mm3 vs 6.92 ± 3.43 mm3, p<0.05, Figure 2B). Figure 3A shows example SFA and PUFA EAT colormaps in untreated and EPL-treated mice showing changes in the FAC between the two groups. Figure 3B shows that EAT SFA% was reduced (16 ± 7% vs 53 ± 10%, p<0.01) and PUFA% increased (74 ± 9% vs 40 ± 10%, p<0.01) in EPL-treated vs untreated mice. MUFA% remained unchanged (10 ± 5% vs 7 ± 1%).Conclusion and Discussion
To the best of our knowledge, this is the first report of FAC imaging of EAT as well as of the detection of shifts in EAT FAC via drug treatment. We have previously shown that: a) HFHSD mice develop coronary microvascular dysfunction, and b) EPL in HFHSD mice preserves coronary microvascular function20. Our current results suggest that a mechanism underlying EPL-induced preservation of coronary microvascular function likely involves the reduction of EAT volume and shifting EAT fatty acid composition towards PUFAs, which is known to reduce EAT and cardiovascular inflammation. MRI of EAT volume and fatty acid composition may be indicative of EAT inflammation and represent an important imaging biomarker in coronary microvascular dysfunction and other types of inflammation-related heart disease.Acknowledgements
This work was supported by NIH NIBIB R01 EB001763.References
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