Differentiating brown and white adipose tissues by high-resolution diffusion NMR spectroscopy

Abstract

There are two types of fat tissues, white adipose tissue (WAT) and brown adipose tissue (BAT), which essentially perform opposite functions in whole body energy metabolism. There is a large interest in identifying novel biophysical properties of WAT and BAT by a quantitative and easy-to-run technique. In this work, we used high-resolution pulsed field gradient diffusion NMR spectroscopy to study the apparent diffusion coefficient (ADC) of fat molecules in rat BAT and WAT samples. The ADC of fat in BAT and WAT from rats fed with a chow diet was compared with that of rats fed with a high-fat diet to monitor how the diffusion properties change due to obesity-associated parameters such as lipid droplet size, fatty acid chain length, and saturation. Feeding a high-fat diet resulted in increased saturation, increased chain lengths, and reduced ADC of fat in WAT. The ADC of fat was lower in BAT relative to WAT in rats fed both chow and high-fat diets. Diffusion of fat was restricted in BAT due to the presence of small multilocular lipid droplets. Our findings indicate that in vivo diffusion might be a potential way for better delineation of BAT and WAT in both lean and obese states. There are two types of fat tissues, white adipose tissue (WAT) and brown adipose tissue (BAT), which essentially perform opposite functions in whole body energy metabolism. There is a large interest in identifying novel biophysical properties of WAT and BAT by a quantitative and easy-to-run technique. In this work, we used high-resolution pulsed field gradient diffusion NMR spectroscopy to study the apparent diffusion coefficient (ADC) of fat molecules in rat BAT and WAT samples. The ADC of fat in BAT and WAT from rats fed with a chow diet was compared with that of rats fed with a high-fat diet to monitor how the diffusion properties change due to obesity-associated parameters such as lipid droplet size, fatty acid chain length, and saturation. Feeding a high-fat diet resulted in increased saturation, increased chain lengths, and reduced ADC of fat in WAT. The ADC of fat was lower in BAT relative to WAT in rats fed both chow and high-fat diets. Diffusion of fat was restricted in BAT due to the presence of small multilocular lipid droplets. Our findings indicate that in vivo diffusion might be a potential way for better delineation of BAT and WAT in both lean and obese states. Obesity is a global health issue, resulting in cardiovascular diseases, diabetes, hypertension, stroke, and musculoskeletal disorders (1Haththotuwa R.N. Wijeyaratne C.N. Senarath U. Worldwide epidemic of obesity.in: Mahmood T. Arulkumaran S. Obesity. A Ticking Time Bomb for Reproductive Health. Elsevier, Oxford, UK2013: 3-11Crossref Scopus (15) Google Scholar, 2Mitchell S. Shaw D. The worldwide epidemic of female obesity.Best Pract. Res. Clin. Obstet. Gynaecol. 2015; 29: 289-299Crossref PubMed Scopus (78) Google Scholar). At least two types of fat tissues exist in mammals, white adipose tissue (WAT) and brown adipose tissue (BAT), and they have different colors, cell structures, and functional roles (3Cinti S. The role of brown adipose tissue in human obesity.Nutr. Metab. Cardiovasc. Dis. 2006; 16: 569-574Abstract Full Text Full Text PDF PubMed Scopus (114) Google Scholar, 4Frühbeck G. Becerril S. Sáinz N. Garrastachu P. García-Velloso M.J. BAT: a new target for human obesity?.Trends Pharmacol. Sci. 2009; 30: 387-396Abstract Full Text Full Text PDF PubMed Scopus (149) Google Scholar, 5Nedergaard J. Cannon B. How brown is brown fat? It depends where you look.Nat. Med. 2013; 19: 540-541Crossref PubMed Scopus (51) Google Scholar). WAT comprises the majority of body fat, which is utilized for energy storage, whereas BAT is a minor portion that is involved in thermogenesis, due to the presence of uncoupling protein 1 (UCP1) in the mitochondria. In addition to the differences in energy storage and expenditure, BAT is a highly heterogeneous, densely vascularized tissue with abundant oxygen, blood supply, and iron-rich mitochondria (6Strobel K. van den Hoff J. Pietzsch J. Localized proton magnetic resonance spectroscopy of lipids in adipose tissue at high spatial resolution in mice in vivo.J. Lipid Res. 2008; 49: 473-480Abstract Full Text Full Text PDF PubMed Scopus (66) Google Scholar, 7Hu H.H. Smith D.L. Nayak K.S. Goran M.I. Nagy T.R. Identification of brown adipose tissue in mice with fat-water IDEAL-MRI.J. Magn. Reson. Imaging. 2010; 31: 1195-1202Crossref PubMed Scopus (122) Google Scholar). WAT is composed of unilocular lipid droplets, whereas BAT is composed of multilocular lipid droplets scattered throughout the cytoplasm and surrounded by mitochondria. Positron emission tomography (PET) has been the gold standard for imaging BAT (8Cypess A.M. Lehman S. Williams G. Tal I. Rodman D. Goldfine A.B. Kuo F.C. Palmer E.L. Tseng Y-H. Doria A. et al.Identification and importance of brown adipose tissue in adult humans.N. Engl. J. Med. 2009; 360: 1509-1517Crossref PubMed Scopus (3137) Google Scholar, 9Virtanen K.A. Lidell M.E. Orava J. Heglind M. Westergren R. Niemi T. Taittonen M. Laine J. Savisto N-J. Enerbäck S. et al.Functional brown adipose tissue in healthy adults.N. Engl. J. Med. 2009; 360: 1518-1525Crossref PubMed Scopus (2315) Google Scholar) because of a selective image contrast of activated BAT by the increased uptake of 18F-deoxyglucose. However, PET is not suitable for longitudinal studies because it requires injection of exogenous radioactive tracers. Magnetic resonance (MR)-based methods are more promising for real-time and long-term observation of fat accumulation and consumption (10Hu H.H. Gilsanz V. Developments in the imaging of brown adipose tissue and its associations with muscle, puberty, and health in children.Front. Endocrinol. (Lausanne). 2011; 2: 33Crossref PubMed Scopus (17) Google Scholar). Traditional MR-based approaches exploit the differences in the water content in WAT and BAT using chemical shift imaging or the Dixon technique (11Chen Y.C. Cypess A.M. Palmer M. Kolodny G. Kahn C.R. Kwong K.K. Measurement of human brown adipose tissue volume and activity using anatomic MR imaging and functional MR imaging.J. Nucl. Med. 2013; 54: 1584-1587Crossref PubMed Scopus (73) Google Scholar, 12Cypess A.M. Haft C.R. Laughlin M.R. Hu H.H. Brown fat in humans: consensus points and experimental guidelines.Cell Metab. 2014; 20: 408-415Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar, 13Hu H.H. Perkins T.G. Chia J.M. Gilsanz V. Characterization of human brown adipose tissue by chemical-shift water-fat MRI.AJR Am. J. Roentgenol. 2013; 200: 177-183Crossref PubMed Scopus (93) Google Scholar). Imaging of water and fat permits quantitative assessment of the fat fraction. Additionally, the differences in iron content, perfusion, and vasculature have been exploited by T2 and T2* relaxation techniques (14Chen Y.I. Cypess A.M. Sass C.A. Brownell A-L. Jokivarsi K.T. Kahn C.R. Kwong K.K. Anatomical and functional assessment of brown adipose tissue by magnetic resonance imaging.Obesity (Silver Spring). 2012; 20: 1519-1526Crossref PubMed Scopus (73) Google Scholar, 15Hu H.H. Wu T.W. Yin L. Kim M.S. Chia J.M. Perkins T.G. Gilsanz V. MRI detection of brown adipose tissue with low fat content in newborns with hypothermia.Magn. Reson. Imaging. 2014; 32: 107-117Crossref PubMed Scopus (32) Google Scholar). However, although WAT and BAT are manually separable by color and texture, they cannot be easily distinguished by MR because of the similarity in their magnetic and chemical characteristics (14Chen Y.I. Cypess A.M. Sass C.A. Brownell A-L. Jokivarsi K.T. Kahn C.R. Kwong K.K. Anatomical and functional assessment of brown adipose tissue by magnetic resonance imaging.Obesity (Silver Spring). 2012; 20: 1519-1526Crossref PubMed Scopus (73) Google Scholar, 15Hu H.H. Wu T.W. Yin L. Kim M.S. Chia J.M. Perkins T.G. Gilsanz V. MRI detection of brown adipose tissue with low fat content in newborns with hypothermia.Magn. Reson. Imaging. 2014; 32: 107-117Crossref PubMed Scopus (32) Google Scholar). Applications of nontraditional MR methodologies for selectively detecting BAT have been attempted. Branca and Warren (16Branca R.T. Warren W.S. In vivo brown adipose tissue detection and characterization using water-lipid intermolecular zero-quantum coherences.Magn. Reson. Med. 2011; 65: 313-319Crossref PubMed Scopus (57) Google Scholar, 17Branca R.T. Warren W.S. In vivo NMR detection of diet-induced changes in adipose tissue composition.J. Lipid Res. 2011; 52: 833-839Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar) have studied focusing on intermolecular multiple-quantum coherences through dipolar interactions between water and fat methyl protons. A quantitative cross-peak originating from a flip-flip or flip-flop transition can be observed in the two-dimensional MR spectra. Furthermore, a combination of intermolecular double-quantum coherence with a chemical shift encoded Dixon method has recently been reported by Bao et al. (18Bao J. Cui X. Cai S. Zhong J. Cai C. Chen Brown adipose tissue in rats with intermolecular double-quantum coherence and Dixon water-fat 2013; PubMed Scopus (17) Google Scholar). The of intermolecular methodologies has been the low has to be for human imaging (16Branca R.T. Warren W.S. In vivo brown adipose tissue detection and characterization using water-lipid intermolecular zero-quantum coherences.Magn. Reson. Med. 2011; 65: 313-319Crossref PubMed Scopus (57) Google Scholar). Measurement of diffusion using pulsed field gradient NMR is a way to the D. tissue and PubMed Scopus Google Scholar). Diffusion of large such as lipids or in adipose tissue is by lipid droplet size, and chemical A. in Scopus Google Scholar, the between diffusion coefficient and A to diffusion in 2011; Scopus Google Scholar, diffusion coefficient of from different with 2011; Scopus Google Scholar, M. Scopus Google Scholar, P. Wu In vivo diffusion of molecules in Google Scholar). the diffusion properties of and lipids in the of P. A.M. Wu Diffusion magnetic resonance lipid droplet in Reson. Med. 2015; PubMed Scopus Google Scholar) and V. R. C. or diffusion of lipids in human by in vivo proton MR Reson. Med. 2012; PubMed Scopus (17) Google Scholar) have been and restricted diffusion has been observed in the restricted diffusion a way to the lipid droplet by using diffusion NMR with a for coefficient of Scopus Google Scholar, K. C. NMR studies of restricted I. in Sci. Scopus Google Scholar). has been utilized in vivo P. A.M. Wu Diffusion magnetic resonance lipid droplet in Reson. Med. 2015; PubMed Scopus Google Scholar) and in studies T. T. L. A. A. K. NMR studies on in water Scopus Google Scholar, V. V. droplet in by diffusion NMR Sci. 2011; PubMed Scopus Google Scholar). of the of diffusion on droplet size, with large unilocular lipid droplets, can be to have different diffusion properties compared with with multilocular lipid droplets. The fat of the adipose tissues depends on The of diet high-fat can the of adipose to such as and D. X. of (Silver Spring). 2009; PubMed Scopus Google Scholar). are with a small of and of and are through the combination of with fatty acid The of and fat can role in and D. X. of (Silver Spring). 2009; PubMed Scopus Google Scholar). It has been that the BAT and WAT in mice in the of and fatty (6Strobel K. van den Hoff J. Pietzsch J. Localized proton magnetic resonance spectroscopy of lipids in adipose tissue at high spatial resolution in mice in vivo.J. Lipid Res. 2008; 49: 473-480Abstract Full Text Full Text PDF PubMed Scopus (66) Google Scholar). Lipid droplets are involved in energy and cell in accumulation of lipid droplets and have been with to and M.S. V. The role of lipid droplets in in and Clin. 2011; PubMed Scopus Google Scholar). lipid droplets in are for A. R. P. A novel of Clin. PubMed Scopus Google Scholar). In this biophysical properties and the of the lipid droplets in to adipose tissue is for and the health In this we and the biophysical properties of BAT and WAT through quantitative diffusion by high-resolution diffusion NMR spectroscopy at two different and Furthermore, the of high-fat diet on diffusion properties of WAT and BAT and the restricted diffusion in BAT The droplet in BAT was by the the and to the by the and rats at of two chow diet and high-fat The high-fat diet was composed of and The chow diet protein and from both chow and high-fat rats at of by A BAT was from the whereas WAT was from the fat of BAT by WAT was by the WAT that be distinguished by the of WAT or BAT tissue was with and between G. in high-resolution NMR Magn. Reson. 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Scopus Google Scholar). is the diffusion coefficient that to the of a A of with gradient to and which resulted in and of and The was with to the of and diffusion of on the tissue with a diffusion and with gradient and gradient to the restricted gradient was used for the and of and The gradient was with a of in at R. in and water in the Scopus Google Scholar, M. in using and Magn. Reson. Scholar). the a of with and with gradient to which to the of A gradient was used for the and throughout diffusion NMR both at and at The the fat at was by using The apparent diffusion coefficient (ADC) was by the experimental to 1 by using the of A was to the by the water and lipid in the as a of chain length, saturation, and by using the as reported in the M. S. S. A. P. of adipose tissues in rats using in vivo and vivo magnetic resonance Lipid Res. 2011; 52: Full Text Full Text PDF PubMed Scopus Google Scholar). The BAT from the from the as by et al. S. J. T. P. X. K. of brown adipose tissue and adipose in 2012; PubMed Scopus Google Scholar). The WAT from the and the the tissue in for and in at and on and with and and at with was from the and by using the Lipid and with to by using a with to the real-time quantitative in by using the on a and to and which was used as The used for the and The in BAT and WAT from chow and high-fat diet tissues with to the in The lipid droplet can be by the restricted diffusion as by and coefficient of Scopus Google Scholar). the of the a in was by et al. T. T. L. A. A. K. NMR studies on in water Scopus Google Scholar) and utilized for P. A.M. Wu Diffusion magnetic resonance lipid droplet in Reson. Med. 2015; PubMed Scopus Google Scholar, V. V. droplet in by diffusion NMR Sci. 2011; PubMed Scopus Google where and are the with and diffusion gradient from the droplet of and is lipid diffusion the of the lipid droplets in the is to be where is the standard of The observed in diffusion is the the droplets in whole and is by the where the is a and and are the and droplet The NMR at diffusion of was to with a to the and the or of the BAT of of WAT was by using a of J. I. V. M. Pietzsch T. S. C. S. B. et for 2012; PubMed Scopus Google Scholar, M. J. A. N. C. for the of white adipose tissue in Lipid Res. 2012; Full Text Full Text PDF PubMed Scopus Google to the and of droplets. 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A of WAT droplets from the chow tissues and droplets from the high-fat tissues from the the of chow and high-fat tissues with a The droplet of WAT was for the chow diet and for the high-fat The adipose tissues are the storage for fat in the of BAT and WAT are the two types of adipose tissues that roles in the role of BAT in is a large interest in the biophysical and properties by have in BAT by imaging approaches M. K.A. T. A. P. Enerbäck S. Brown adipose tissue in humans: detection and functional using PET emission MRI resonance and energy 2014; PubMed Scopus Google Scholar, X. Kuo C. A. C. In vivo imaging of brown adipose tissue with 2013; PubMed Scopus Google Scholar). In the BAT and WAT from chow and high-fat rats by diffusion NMR was to restricted diffusion and droplet The of BAT a water resonance compared with due to the abundant and water WAT increased of fat compared with MRI and MR spectroscopy studies of BAT and WAT a in the fat of BAT compared with WAT H.H. Wu T.W. Yin L. Kim M.S. Chia J.M. Perkins T.G. Gilsanz V. MRI detection of brown adipose tissue with low fat content in newborns with hypothermia.Magn. Reson. Imaging. 2014; 32: 107-117Crossref PubMed Scopus (32) Google Scholar, G. Smith D.L. M. Nayak K.S. Hu H.H. MR properties of brown and white adipose Magn. Reson. Imaging. 2011; PubMed Scopus (93) Google Scholar, H.H. Smith D.L. in T2* and fat content of brown and white adipose tissues by chemical-shift Reson. Imaging. 2012; 30: PubMed Scopus Google Scholar, C. R. C. A. A. Magnetic resonance spectroscopy of brown adipose tissue and brown Lipid Res. Full Text PDF PubMed Google Scholar). The of fatty of tissue from BAT and WAT in the the of saturation, and chain M. S. S. A. P. of adipose tissues in rats using in vivo and vivo magnetic resonance Lipid Res. 2011; 52: Full Text Full Text PDF PubMed Scopus Google Scholar, C. R. C. A. A. Magnetic resonance spectroscopy of brown adipose tissue and brown Lipid Res. Full Text PDF PubMed Google Scholar). in that the of fat molecules in not by high-fat diet as as in WAT. The changes for BAT (WAT) for the for the and for the chain length, changes be due to the changes in activity of in WAT M. S. S. A. P. of adipose tissues in rats using in vivo and vivo magnetic resonance Lipid Res. 2011; 52: Full Text Full Text PDF PubMed Scopus Google Scholar). The of fat diffusion observed in this study was not by although the ADC increased due to energy A. in Scopus Google Scholar). The high-fat diet not the ADC of fat molecules in BAT at both and whereas it reduced the ADC in WAT is due to increased saturation, and increased chain The diffusion coefficient of fat molecules with in chain G. diffusion imaging of adipose tissue in the human lower at Reson. Med. 2011; 65: PubMed Scopus Google Scholar). with reduced are and and are to diffusion R. and Scholar). The differences in the and and the diffusion in of using a has that the apparent diffusion of lipid molecules the droplet and a diffusion is P. Wu In vivo diffusion of molecules in Google Scholar, P. A.M. Wu Diffusion magnetic resonance lipid droplet in Reson. Med. 2015; PubMed Scopus Google Scholar). Diffusion of fat molecules in WAT is whereas it is restricted in BAT restricted in BAT might be due in to the of the multilocular droplet The differences between BAT and WAT in the of ADC for both chow and high-fat The in ADC with a diffusion in the high-fat diet BAT in the chow diffusion in the due to changes in and A of the diffusion of fat molecules is by a A. in Scopus Google the diffusion coefficient in WAT at is the diffusion of the longitudinal relaxation of fat molecules in BAT was not the diffusion was and a or was that of the fat molecules a droplet in BAT the at least and the experimental at the at a diffusion 1 the droplet for BAT by the NMR diffusion on the The was and to a a the to The apparent is and that was to the diffusion of the In different from of fat molecules from the droplets exist because they are to be be fat molecules not be and in energy 1 that the apparent droplet increased by high-fat The presence of small brown droplets and large BAT was by the The NMR are with the diffusion due to restricted be observed in not in WAT. The of the restricted to droplet requires a large in the diffusion NMR and a diffusion to fat molecules to the droplet the of droplet in WAT increased in the high-fat diet due to increased fat storage, which was observed in the of WAT because of a high-fat diet is with A. V. to and 2008; PubMed Scopus Google Scholar). of is a potential way to and of adipose NMR diffusion methods can be to lipid droplet BAT with the in adipose tissues have high that their diffusion study requires large to with the However, from the and a MR be observed from BAT from WAT by of diffusion because the of the as gradient and diffusion cannot be by of relaxation because the relaxation of fat and water is in BAT in WAT and the differences in or T2* are not In we have the differences in the biophysical properties of BAT and WAT by diffusion NMR techniques and studied the of high-fat diet on the diffusion in two adipose The diffusion properties of WAT by the changes due to saturation, and chain The method can be easily on NMR with the field gradient and can be used for brown or adipose tissues with study and human fat in The of large for might be a for with the gradient