The Role of Brown Adipose Tissue (BAT) Activity inMice Devoid of Receptor for Advanced Glycation End Product (RAGE)

Objectives: Brown adipose tissue (BAT) is one of the primary tissues responsible for adaptive non-shivering thermogenesis, mediated by mitochondria uncoupling protein-1 (UCP1), and contributes to energy expenditure. BAT has been shown to play a role in adiposity, insulin resistance and hyperlipidaemia. We and others1-2 have shown that genetic deficiency of RAGE (receptor for advanced glycation end products) prevented the effects of a high-fat diet (HFD) on energy expenditure, weight gain, adipose tissue inflammation, and insulin resistance. The aim of this study is to compare and quantitate the BAT activity and BAT volume in RAGE null (RKO, Ager ^-/-) and wild type (WT) mice in response to HFD and low-fat diets (LFD) using in vivo PET imaging studies with [¹⁸F]FDG. Since BAT is not readily visible with [¹⁸F]FDG at room temperature based on our previous BAT imaging study3, all comparative studies were conducted under identical mild- cold stimulation conditions with [¹⁸F]FDG.

Methods: PET/CT imaging (Siemens, Inveon) of 4 groups of male C57/BL6 mice were compared (Ager ^-/--LFD, Ager ^-/--HFD, WT-LFD and WT-HFD). Before each scan, all mice were fasted for at least 4 h and then exposed to a cold-pack (0- 4°C) for 30 min. For each study, a dynamic PET scan was carried out for 1 h, followed by a CT scan (6 min) for the purpose of attenuation correction and co-registration. SUV_mean (mean standard uptake values), SUVR (ratio of SUV_iBAT/SUV_muscle), and %ID/g for the interscapular BAT were determined.

Results: Preliminary results are summarized here: 1) For LFD mice, the uptake was significantly higher in WT-LFD (SUVR: 4.03±0.4) than that in Ager ^-/--LFD (SUVR: 1.4±0.1); 2) The SUVR were similar for both WT-HFD and Ager ^-/--HFD (1.4±0.1 and 1.52±0.1, respectively) (Figure 1); 3) Among Ager ^-/- types, the uptake was similar in both Ager ^-/--HFD (SUVR: 1.52±0.1) and Ager ^-/--LFD (SUVR: 2.14±0.1), and the %ID/g values were similar in both as well. These findings corroborate our previous findings on UCP1 mRNA2; that is, (1) UCP1 mRNA transcripts in BAT are almost halved in Ager ^-/--LFD than in WTLFD; (2) in HFD, UCP1-mRNA transcripts did not differ in BAT between WT and Ager ^-/-mice; (3) a bigger change was observed in UCP1 mRNA between WT-LFD and WT-HFD, while a smaller change was seen between Ager ^-/--LFD and Ager ^-/- -HFD (Table 1).

Conclusion: In WT mice, BAT activity was significantly reduced after HFD as compared to LFD; however, BAT activity was not affected by HFD in Ager ^-/- mice. The consistent findings between the current in vivo PET imaging study of BAT and our previous study of UCP1 mRNA further support our hypothesis that RAGE may contribute to altered energy expenditure, and provide a protective effect against HFD by Ager deletion. Research Support: References: 1. Fromme T, Klingenspor M. Uncoupling protein 1 expression and high‐fat diets. Am J Physiol Regul Integr Comp Physiol. 2011; 300 (1): R1‐8. 2. Song F, Hurtado del Pozo C, Rosario R, et al. RAGE regulates the metabolic and inflammatory response to high‐fat feeding in mice. Diabet. 2014; 63 (6): 1948‐65. 3. Lin S‐F, Fan X, Yeckel CW, et al. Ex vivo and in vivo evaluation of the norepinephrine transporter ligand [11C]MRB for brown adipose tissue imaging. Nucl Med Biol. 2012; 39 (7): 1081‐6.