Early Imaging Biomarker of Myocardial Glucose Adaptations in High-Fat-Diet-Induced Insulin Resistance Model by Using 18F-FDG PET and [U-13C]glucose Nuclear Magnetic Resonance Tracer

Abstract

Background: High-fat diet (HFD) induces systemic insulin resistance leading to myocardial dysfunction. We aim to characterize the early adaptations of myocardial glucose utility to HFD-induced insulin resistance.

Methods: Male Sprague-Dawley rats were assigned into two groups, fed a regular chow diet or HFD ad libitum for 10 weeks. We used in vivo imaging of cardiac magnetic resonance (CMR), ¹⁸F-FDG PET, and ex vivo nuclear magnetic resonance (NMR) metabolomic analysis for the carbon-13-labeled glucose ([U-¹³C]Glc) perfused myocardium.

Results: As compared with controls, HFD rats had a higher ejection fraction and a smaller left ventricular end-systolic volume (P < 0.05), with SUV_max of myocardium on ¹⁸F-FDG PET significantly increased in 4 weeks (P < 0.005). The [U-¹³C]Glc probed the increased glucose uptake being metabolized into pyruvate and acetyl-CoA, undergoing oxidative phosphorylation via the tricarboxylic acid (TCA) cycle, and then synthesized into glutamic acid and glutamine, associated with overexpressed LC3B (P < 0.05).

Conclusions: HFD-induced IR associated with increased glucose utility undergoing oxidative phosphorylation via the TCA cycle in the myocardium is supported by overexpression of glucose transporter, acetyl-CoA synthase. Noninvasive imaging biomarker has potentials in detecting the metabolic perturbations prior to the decline of the left ventricular function.

Figure 1

Flow chart of animal experiments. The age and number of animals used in each experiment are displayed. Rats were divided into two groups by the high-fat diet and regular chow diet, following the biochemical assessment, in vivo imaging study, Langendorff perfusion NMR, and western blotting.

Figure 2

Characterization of HFD-induced obesity model. Rats fed high-fat diet became significantly obese in the time course of body weight (a). Slightly increased glucose concentration in plasma in the HFD group after 10 weeks of high-fat diet feeding (b). Significantly increased insulin (c), HOMA-IR (d), triglyceride (e), and total cholesterol (f) concentration in the HFD group after 10 weeks of high-fat diet feeding. Slightly decreased HDL-C in the HFD group after 10 weeks of high-fat diet feeding (g). Taken together, the HFD rats developed insulin resistance in 10 weeks after high-fat diet feeding. ^∗ P < 0.05; ^∗∗ P < 0.01; ^∗∗∗ P < 0.001 versus control.

Figure 3

The representative short axis of CMR images in end-systolic and end-diastolic phases. The myocardium CMR images of control and HFD rats in end-systolic and end-diastolic phases are shown. The ejection fraction of the HFD rats significantly increased, and the left ventricle volume of the HFD rats significantly decreased, as compared with control rats based on the QMass analysis.

Figure 4

¹⁸F-FDG PET/CT imaging. (a) Representative midventricular transversal images in a time course. The right image is a corresponding coronal plane ¹⁸F-FDG PET/CT. (b) Quantitative myocardial glucose uptake by micro-PET after intravenous injection of ¹⁸F-FDG in the control (n=3) and HFD (n=3) rats. Significantly increased myocardial ¹⁸F-FDG uptake in 4 weeks after HFD feeding is noted. ^∗ P < 0.05 versus control. Ctrl = control.

Figure 5

[U-¹³C]-labeled downstream glucose metabolism. (a) The measurement of downstream glucose metabolism by [U-¹³C]-labeled glucose and NMR technique. The carbon atoms of glutamine (Glu45) are from the upstream of glucose metabolism. (b) The ¹³C NMR full spectrum of the myocardium in the HFD rats is demonstrated. After the citric acid cycle, the 4th labeled carbon position in glutamate showed a higher signal than that of the other positions.

Figure 6

Western blotting and mRNA analysis of myocardium: AceCS1, Glut4, PARP, and LC3B protein expression and Slc2a4 and Acss2 gene expression in the perfusion hearts in HFD and control rats in 18 weeks and 36 weeks after high-fat diet feeding. AceCS1 protein expression and Acss2 gene expression did not show a significant difference between control and HFD in early and late stages. However, mRNA levels change in Glut4, albeit different in trend. Nonetheless, the protein expression of Glut4 increases in accordance with the ¹⁸F-FDG PET/CT. Decreased cleaved/prototype PARP and increased LC3B-autophagy activation in the myocardium of the HFD rats are noted.