Glucose metabolism in brown adipose tissue determined by deuterium metabolic imaging in rats.

BACKGROUND/OBJECTIVES: Brown adipose tissue (BAT) has gained growing interest as a potential target for treatment of obesity. Currently, the most widely used technique/method for in vivo measurements of BAT activity in humans is 18FDG PET/CT. To supplement these investigations novel radiation-free methods are warranted. Deuterium metabolic imaging (DMI) is a novel modality that combines magnetic resonance spectroscopic (MRS) imaging with deuterium-labelled glucose (2H-glucose). This allows for spatio-temporal and metabolic imaging beyond glucose uptake. We aimed to evaluate if DMI could discriminate glucose metabolism in BAT of cold-acclimatised and thermoneutral rats. SUBJECTS/METHODS: Male Sprague-Dawley rats were housed in a cold environment (9 °C, n = 10) or at thermoneutrality (30 °C, n = 11) for 1 week. For imaging rats were anaesthetized, received a 2H-glucose (1 M, 1.95 g/kg) bolus and DMI was acquired at baseline followed by 20 min time intervals up to 2 h. Furthermore, Dixon MRI was performed for anatomical determination of the interscapular BAT (iBAT) depot along with dynamic contrast enhanced (DCE) MRI to evaluate perfusion. RESULTS: 2H-glucose signal was higher in cold-acclimatised rats compared with thermoneutral rats (p ≤ 0.001) indicating an overall increase in glucose uptake and metabolism. This was in line with a lower fat/water threshold, higher perfusion and increased UCP1 mRNA expression in iBAT (ninefold increment) of cold-acclimatised rats compared with thermoneutral rats. CONCLUSIONS: We find that DMI can discriminate cold-acclimatised and thermoneutral BAT in rats. This is the first study to evaluate BAT activity by DMI, which may open up for the use of the non-radioactive DMI method for BAT measurements in humans.

Beta-1 and Not Beta-3 Adrenergic Receptors May Be the Primary Regulator of Human Brown Adipocyte Metabolism.

PURPOSE: Brown adipose tissue (BAT) activation in humans has gained interest as a potential target for treatment of obesity and insulin resistance. In rodents, BAT is primarily induced through beta-3 adrenergic receptor (ADRB3) stimulation, whereas the primary beta adrenergic receptors (ADRBs) involved in human BAT activation are debated. We evaluated the importance of different ADRB subtypes for uncoupling protein 1 (UCP1) induction in human brown adipocytes. METHODS: A human BAT cell model (TERT-hBA) was investigated for subtype-specific ADRB agonists and receptor knockdown on UCP1 mRNA levels and lipolysis (glycerol release). In addition, fresh human BAT biopsies and TERT-hBA were evaluated for expression of ADRB1, ADRB2, and ADRB3 using RT-qPCR. RESULTS: The predominant ADRB subtype in TERT-hBA adipocytes and BAT biopsies was ADRB1. In TERT-hBA, UCP1 mRNA expression was stimulated 11.0-fold by dibutyryl cAMP (dbcAMP), 8.0-fold to 8.4-fold by isoproterenol (ISO; a pan-ADRB agonist), and 6.1-fold to 12.7-fold by dobutamine (ADRB1 agonist), whereas neither procaterol (ADRB2 agonist), CL314.432, or Mirabegron (ADRB3 agonists) affected UCP1. Similarly, dbcAMP, ISO, and dobutamine stimulated glycerol release, whereas lipolysis was unaffected by ADRB2 and ADRB3 agonists. Selective knockdown of ADRB1 significantly attenuated ISO-induced UCP1 expression. CONCLUSION: The adrenergic stimulation of UCP1 and lipolysis may mainly be mediated through ADRB1. Moreover, ADRB1 is the predominant ADRB in both TERT-hBA and human BAT biopsies. Thus, UCP1 expression in human BAT may, unlike in rodents, primarily be regulated by ADRB1. These findings may have implications for ADRB agonists as future therapeutic compounds for human BAT activation.

Evaluation of Active Brown Adipose Tissue by the Use of Hyperpolarized [1-13C]Pyruvate MRI in Mice.

The capacity to increase energy expenditure makes brown adipose tissue (BAT) a putative target for treatment of metabolic diseases such as obesity. Presently, investigation of BAT in vivo is mainly performed by fluoro-d-glucose positron emission tomography (FDG PET)/CT. However, non-radioactive methods that add information on, for example, substrate metabolism are warranted. Thus, the aim of this study was to evaluate the potential of hyperpolarized [1-13C]pyruvate Magnetic Resonance Imaging (HP-MRI) to determine BAT activity in mice following chronic cold exposure. Cold (6 °C) and thermo-neutral (30 °C) acclimated mice were scanned with HP-MRI for assessment of the interscapular BAT (iBAT) activity. Comparable mice were scanned with the conventional method FDG PET/MRI. Finally, iBAT was evaluated for gene expression and protein levels of the specific thermogenic marker, uncoupling protein 1 (UCP1). Cold exposure increased the thermogenic capacity 3⁻4 fold (p < 0.05) as measured by UCP1 gene and protein analysis. Furthermore, cold exposure as compared with thermo-neutrality increased iBAT pyruvate metabolism by 5.5-fold determined by HP-MRI which is in good agreement with the 5-fold increment in FDG uptake (p < 0.05) measured by FDG PET/MRI. iBAT activity is detectable in mice using HP-MRI in which potential changes in intracellular metabolism may add useful information to the conventional FDG PET studies. HP-MRI may also be a promising radiation-free tool for repetitive BAT studies in humans.

Purinergic mechanisms and prostaglandin E receptors involved in ATP-induced relaxation of porcine retinal arterioles in vitro.

PURPOSE: Adenosine triphosphate (ATP) is involved in the tone regulation of retinal arterioles, and the effect may be direct, through ATP degradation or mediated by cyclo-oxygenase products. However, the relative contribution of these mechanisms and the extent to which the mechanisms are active in the retinal vascular wall or depend on the perivascular retinal tissue are unknown. METHODS: Porcine retinal arterioles with perivascular retinal tissue were mounted in a wire myograph for isometric tone recordings. The relaxing effects of ATP and the non-degradable analogue ATP-x03B3;S were studied in the presence of antagonists to ATP, adenosine and prostaglandin E (EP) receptors. The experiments were repeated after removal of the perivascular retinal tissue. RESULTS: ATP induced a significant concentration-dependent relaxation of retinal arterioles (p < 0.05) which was reduced after removal of perivascular retinal tissue. The effect was due to non-degraded ATP and a degradation product of ATP acting via adenosine receptors. Relaxation was reduced by ibuprofen and blocking of EP1 receptors. CONCLUSION: ATP-induced relaxation of retinal arterioles is mediated by ATP, ATP degradation products and by stimulation of EP1 receptors, involving both the perivascular retina and the vascular wall. The findings emphasize the complexity of purinergic effects in the regulation of retinal vascular tone.

Relaxation of porcine retinal arterioles exposed to hypercholesterolemia in vivo is modified by hepatic LDL-receptor deficiency and diabetes mellitus.

Metabolic disturbances in diabetes mellitus include changes in the type and concentration of lipids in the blood plasma which may contribute to the development of diabetic retinopathy. This disease is characterized by changes in retinal blood flow secondary to changes in the tone of retinal arterioles which is regulated by compounds such as adenosine, adenosine triphosphate (ATP), the glutamate agonist N-methyl-d-aspartate (NMDA) and prostaglandin E2 (PGE2). However, the relation between increased plasma low density lipoprotein (LDL) and tone regulation in retinal resistance vessels has not been studied in detail. Twelve male and nine female Yucatan minipigs overexpressing a gain-of-function mutant (D374Y) of the human gene PCSK9 that blocks LDL transport into the liver and twelve wild-type males were studied. The animals were fed a cholesterol rich diet from the age of 60 days, followed by induction of diabetes mellitus in twelve of the transgenic animals. The animals were sacrificed at a mean age of 51 weeks (range 26-60 weeks), followed by inspection and histological examination of retinal vessels, and examination of the changes in vascular tone induced by adenosine, ATP, NMDA and PGE2. In the transgenic pigs without diabetes mellitus ATP-induced relaxation was reduced in isolated arterioles, and a whitish infiltration in an arteriole was observed in 4/8 (50%) of the animals, whereas these changes were not found in the other groups. Histological examination of one of the infiltrations showed staining with Oil Red O representing foamy cells sub-endothelially in the vascular wall indicating atheromatosis. Adenosine, ATP and PGE2 induced a significant concentration-dependent relaxation of retinal arterioles in all groups. The presence of perivascular retinal tissue had no effect on the relaxing effect of adenosine, but increased the relaxing effect of ATP and PGE2 in the two transgenic animal groups, whereas NMDA had no significant effect on vascular tone in any of the groups. Relaxation of porcine retinal arterioles exposed to hypercholesterolemia in vivo is modified by hepatic LDL-receptor deficiency and diabetes mellitus. This suggests that transgenic animal models are suitable for studying the influence of systemic diseases on retinal vascular function.