Brown Adipose Tissue Fatty Acid Research Articles

1650-P: Characterizing the Mechanism of Action of the Lipokine 12,13-diHOME

Lipids play an essential role in physiology. In addition to serving as an energy source, bioactive lipids, also known as lipokines, function as signaling molecules regulating metabolism and inflammation. A major source of lipokines, brown adipose tissue (BAT) has been shown to have beneficial effects on cardiometabolic health by increasing energy expenditure and improving glucose tolerance, thus providing a potential target for therapeutic interventions. Recent studies have identified 12,13-dihydroxy-9Z-octadecenoic acid (12,13-diHOME, produced by cytochrome P450-epoxide hydrolase metabolism of linoleic acid (C18:2n-6), as a lipokine produced by BAT that modulates fatty acid uptake (FAU) into brown adipocytes and skeletal muscle cells in response to cold stimulation or exercising, respectively. Here, we characterized the molecular mechanism and structure-activity relationship (SAR) of 12,13-diHOME in BAT fatty acid uptake. Conversion of the terminal carboxylic acid of 12,13-diHOME to ethyl acetate and changes in saturation results in decreased stimulated FAU indicating 12,13-diHOME's activity is structurally dependent. In addition, we provide insight into a novel protein target responsible for enhancing FAU in BAT, in part, by enhancing intracellular calcium mobilization and influencing the expression of lipolytic genes. Furthermore, we have determined that the 12,13-diHOME-stimulated FAU is, in part, dependent on Gq-mediated calcium mobilization as determined by the utilization of Gq inhibitor YM-254890. Thus, the receptor of 12,13-diHOME could represent a target to augment BAT activity by modulating fatty acid metabolism, which could contribute to increased energy expenditure and, consequently, to the prevention of cardiometabolic diseases. Disclosure J.I.Senfeld: None. M.Lynes: None. S.Kodani: None. K.Lee: None. Y.Tseng: Consultant; Cellarity, LyGenesis.

Basal and cold-induced fatty acid uptake of human brown adipose tissue is impaired in obesity

Fatty acids (FA) are important substrates for brown adipose tissue (BAT) metabolism, however, it remains unclear whether there exists a difference in FA metabolism of BAT between lean and obese healthy humans. In this study we evaluated supraclavicular BAT fatty acid uptake (FAU) along with blood perfusion in lean and obese subjects during cold exposure and at room temperature using positron emission tomography (PET)/computed tomography (CT). Additionally, tissue samples were taken from supraclavicular region (typical BAT region) from a subset of subjects to evaluate histological presence of BAT. Non-shivering cold stress elevated FAU and perfusion of BAT in lean, but not in obese subjects. Lean subjects had greater FAU in BAT compared to obese subjects during cold exposure and interestingly also at room temperature. The higher BAT FAU was related to younger age and several indicators of superior systemic metabolic health. The subjects who manifested BAT histologically had several folds higher BAT FAU compared to subjects with no such histological manifestation. Together, obese subjects have less active tissue in supraclavicular region both in basal and cold-activated state and the FA metabolism of BAT is blunted in obesity.

Circadian lipid synthesis in brown fat maintains murine body temperature during chronic cold

Ambient temperature influences the molecular clock and lipid metabolism, but the impact of chronic cold exposure on circadian lipid metabolism in thermogenic brown adipose tissue (BAT) has not been studied. Here we show that during chronic cold exposure (1 wk at 4 °C), genes controlling de novo lipogenesis (DNL) including Srebp1, the master transcriptional regulator of DNL, acquired high-amplitude circadian rhythms in thermogenic BAT. These conditions activated mechanistic target of rapamycin 1 (mTORC1), an inducer of Srebp1 expression, and engaged circadian transcriptional repressors REV-ERBα and β as rhythmic regulators of Srebp1 in BAT. SREBP was required in BAT for the thermogenic response to norepinephrine, and depletion of SREBP prevented maintenance of body temperature both during circadian cycles as well as during fasting of chronically cold mice. By contrast, deletion of REV-ERBα and β in BAT allowed mice to maintain their body temperature in chronic cold. Thus, the environmental challenge of prolonged noncircadian exposure to cold temperature induces circadian induction of SREBP1 that drives fuel synthesis in BAT and is necessary to maintain circadian body temperature during chronic cold exposure. The requirement for BAT fatty acid synthesis has broad implications for adaptation to cold.

Chronic high-fat feeding impairs adaptive induction of mitochondrial fatty acid combustion-associated proteins in brown adipose tissue of mice

Since brown adipose tissue (BAT) is involved in thermogenesis using fatty acids as a fuel, BAT activation is a potential strategy for treating obesity and diabetes. However, whether BAT fatty acid combusting capacity is preserved in these conditions has remained unclear. We therefore evaluated expression levels of fatty acid oxidation-associated enzymes and uncoupling protein 1 (Ucp1) in BAT by western blot using a diet-induced obesity C57BL/6J mouse model. In C57BL/6J mice fed a high-fat diet (HFD) over 2–4 weeks, carnitine palmitoyltransferase 2 (Cpt2), acyl-CoA thioesterase (Acot) 2, Acot11 and Ucp1 levels were significantly increased compared with baseline and control low-fat diet (LFD)-fed mice. Similar results were obtained in other mouse strains, including ddY, ICR and KK-Ay, but the magnitudes of the increase in Ucp1 level were much smaller than in C57BL/6J mice, with decreased Acot11 levels after HFD-feeding. In C57BL/6J mice, increased levels of these mitochondrial proteins declined to near baseline levels after a longer-term HFD-feeding (20 weeks), concurrent with the accumulation of unilocular, large lipid droplets in brown adipocytes. Extramitochondrial Acot11 and acyl-CoA oxidase remained elevated. Treatment of mice with Wy-14,643 also increased these proteins, but was less effective than 4 week-HFD, suggesting that mechanisms other than peroxisome proliferator-activated receptor α were also involved in the upregulation. These results suggest that BAT enhances its fatty acid combusting capacity in response to fat overload, however profound obesity deprives BAT of the responsiveness to fat, possibly via mitochondrial alteration.

Foraging at wastewater treatment works affects brown adipose tissue fatty acid profiles in banana bats.

ABSTRACTIn this study we tested the hypothesis that the decrease in habitat quality at wastewater treatment works (WWTW), such as limited prey diversity and exposure to the toxic cocktail of pollutants, affect fatty acid profiles of interscapular brown adipose tissue (iBrAT) in bats. Further, the antioxidant capacity of oxidative tissues such as pectoral and cardiac muscle may not be adequate to protect those tissues against reactive molecules resulting from polyunsaturated fatty acid auto-oxidation in the WWTW bats. Bats were sampled at two urban WWTW, and two unpolluted reference sites in KwaZulu-Natal, South Africa. Brown adipose tissue (BrAT) mass was lower in WWTW bats than in reference site bats. We found lower levels of saturated phospholipid fatty acids and higher levels of mono- and polyunsaturated fatty acids in WWTW bats than in reference site bats, while C18 desaturation and n-6 to n-3 ratios were higher in the WWTW bats. This was not associated with high lipid peroxidation levels in pectoral and cardiac muscle. Combined, these results indicate that WWTW bats rely on iBrAT as an energy source, and opportunistic foraging on abundant, pollutant-tolerant prey may change fatty acid profiles in their tissue, with possible effects on mitochondrial functioning, torpor and energy usage.

Heart-type Fatty Acid-binding Protein Is Essential for Efficient Brown Adipose Tissue Fatty Acid Oxidation and Cold Tolerance

Brown adipose tissue has a central role in thermogenesis to maintain body temperature through energy dissipation in small mammals and has recently been verified to function in adult humans as well. Here, we demonstrate that the heart-type fatty acid-binding protein, FABP3, is essential for cold tolerance and efficient fatty acid oxidation in mouse brown adipose tissue, despite the abundant expression of adipose-type fatty acid-binding protein, FABP4 (also known as aP2). Fabp3(-/-) mice exhibit extreme cold sensitivity despite induction of uncoupling and oxidative genes and hydrolysis of brown adipose tissue lipid stores. However, using FABP3 gain- and loss-of-function approaches in brown adipocytes, we detected a correlation between FABP3 levels and the utilization of exogenous fatty acids. Thus, Fabp3(-/-) brown adipocytes fail to oxidize exogenously supplied fatty acids, whereas enhanced Fabp3 expression promotes more efficient oxidation. These results suggest that FABP3 levels are a determinant of fatty acid oxidation efficiency by brown adipose tissue and that FABP3 represents a potential target for modulation of energy dissipation.

Brown adipose tissue glyceroneogenesis is activated in rats exposed to cold

We have previously found that glyceroneogenesis is very active in brown adipose tissue (BAT) and increases in fasted, diabetic and high-protein-diet-fed rats, situations of reduced thermogenic activity. To understand better the role of glyceroneogenesis in BAT glycerol-3-phosphate (G3P) generation, we investigated its activity during cold exposure (10 days at 4 degrees C), a condition in which, in contrast to the above situations, BAT thermogenesis is markedly activated. Rates of total (from all sources) BAT fatty acid (FA) synthesis and rates of incorporation of glucose carbon into BAT glyceride-FA and -glycerol in vivo were markedly increased by cold exposure. Cold exposure induced a marked increase in BAT glyceroneogenic activity, evidenced by (1) increased rates of non-glucose carbon incorporation into glyceride-glycerol in vivo and of [1-14C]-pyruvate incorporation into glyceride-glycerol in vitro, and (2) a threefold increase in phosphoenolpyruvate carboxykinase activity. Most of the glyceride-glycerol synthesized by BAT via glyceroneogenesis or from glucose was used to esterify preformed FA. This use was markedly increased by cold exposure, in parallel with a pronounced activation of BAT lipoprotein lipase activity. In conclusion, during cold exposure BAT glyceroneogenesis is markedly activated, contributing to increase the generation of G3P, which is mostly used to esterify preformed FA.

The impact of dietary fats, photoperiod, temperature and season on morphological variables, torpor patterns, and brown adipose tissue fatty acid composition of hamsters, Phodopus sungorus.

We investigated how dietary fats and oils of different fatty acid composition influence the seasonal change of body mass, fur colour, testes size and torpor in Djungarian hamsters, Phodopus sungorus, maintained from autumn to winter under different photoperiods and temperature regimes. Dietary fatty acids influenced the occurrence of spontaneous torpor (food and water ad libitum) in P. sungorus maintained at 18 degrees C under natural and artificial short photoperiods. Torpor was most pronounced in individuals on a diet containing 10% safflower oil (rich in polyunsaturated fatty acids), intermediate in individuals on a diet containing 10% olive oil (rich in monounsaturated fatty acids) and least pronounced in individuals on a diet containing 10% coconut fat (rich in saturated fatty acids). Torpor in P. sungorus on chow containing no added fat or oil was intermediate between those on coconut fat and olive oil. Dietary fatty acids had little effect on torpor in animals maintained at 23 degrees C. Body mass, fur colour and testes size were also little affected by dietary fatty acids. The fatty acid composition of brown fat from hamsters maintained at 18 degrees C and under natural photoperiod strongly reflected that of the dietary fatty acids. Our study suggests that the seasonal change of body mass, fur colour and testes size are not significantly affected by dietary fatty acids. However, dietary fats influence the occurrence of torpor in individuals maintained at low temperatures and that have been photoperiodically primed for the display of torpor.

Muscle and brown adipose tissue fatty acid profiles in cold-exposed rats.

Effects of cold acclimation and cold exposure for 25 generations on triglyceride (TG)-, total phospholipid (PL)-, and cardiolipin (CL)-fatty acid (FA) compositions were studied in the different types of skeletal muscles (red and white muscles from quadriceps, and soleus) as well as brown adipose tissue (BAT) in rats. TG: in cold-acclimated rats (CA) and rats reared in cold for 25 generations (25G) white muscle showed the decreased saturated FA (SA) and increased polyunsaturated FA (PU). In soleus of 25G, monounsaturated FA (MU) and PU increased, and SA decreased. In BAT, SA increased and MU decreased in CA, and SA and PU decreased, and MU increased in 25G. PL: in red muscle of CA, SA increased and PU decreased. In red muscle of 25G the similar but more marked changes were found accompanied by lowered arachidonic acid and unsaturation index (UI). In BAT, SA, PU, arachidonic acid, and UI increased in CA, and arachidonic acid increased in 25G. CL: in red and white muscles of 25G, oleic acid and MU increased. In BAT, arachidonic acid increased in CA and 25G, and UI increased in 25G. These results suggest that FA unsaturation of CL in the muscles as well as BAT is involved in an enhanced thermogenesis in cold-exposed rats.

Effect of surcose overfeeding on brown adipose tissue lipogenesis and lipoprotein lipase activity in rats

During cold-induced nonshivering thermogenesis, interscapular brown adipose tissue (BAT) lipoprotein lipase (LPL) activity and lipogenesis are elevated. Because of the many similarities between cold- and diet-induced thermogenesis, we examined the effect of ad libitum access to a 32% sucrose solution on caloric intake, adiposity, and BAT enzyme activities in male rats. Daily caloric intakes of sucrose-fed animals were elevated by 20%–25%, and 8 wk of sucrose feeding doubled carcass fat content. This sucrose-feeding induced obesity was associated with increases in circulating triglyceride and insulin levels as well as increased retroperitoneal white adipose tissue LPL activity. However, the increased carcass lipid content accounted for less than half of the excess calories ingested by the sucrose-fed rats. Sucrose feeding stimulated in vivo lipogenesis in BAT and elevated BAT fatty acid synthetase and acetyl-CoA carboxylase activities but not LPL activity. These findings suggest that overeating enhances endogenous lipogenesis but not uptake of circulating triglyceride in BAT. Thus, both cold- and diet-induced thermogenesis increase BAT lipogenesis, while only cold-induced thermogenesis is associated with elevated LPL activity in BAT.

Effects of estradiol on tissue distribution of newly-synthesized fatty acids in rats and hamsters

Estradiol treatment decreases body weight and adiposity in ovariectomized (OVX) rats and hamsters partly by increasing energy expenditure. Other manipulations which increase energy expenditure (e.g., cold exposure or overfeeding) enhance thermogenesis in brown adipose tissue (BAT) and stimulate BAT fatty acid synthesis/uptake. We examined the effect of estradiol treatment on the in vivo distribution of newly-synthesized fatty acids in OVX rats and hamsters. In both species estradiol treatment increased BAT fatty acid synthesis/uptake (incorporation of tritium from (3H)2O into lipid), consistent with the possibility that enhanced thermogenesis in BAT may contribute to estradiol-induced energy expenditure. Estradiol treatment increased BAT lipoprotein lipase (LPL) activity in hamsters, but not in rats. Thus, hamsters may utilize fatty acids synthesized in other tissues as a fuel for BAT thermogenesis, whereas rats may rely more on in situ lipogenesis. Estradiol-induced decreases in carcass adiposity (white adipose tissue mass) may be accomplished by different means in rats and hamsters. Estradiol treatment reduced white adipose tissue LPL activity and fatty acid synthesis/uptake in rats, but not in hamsters. While there are some species differences in the effects of estradiol on lipid metabolism, it appears that in both rats and hamsters estradiol acts to direct metabolic fuels (especially lipids) away from white adipose tissue storage depots and into tissues where they are oxidized (e.g., BAT). Finally, cold acclimation and estradiol had similar effects in OVX hamsters including increases in BAT fatty acid synthesis/uptake, BAT LPL activity, and energy expenditure. These findings, too, are consistent with a role for BAT in estradiol-induced thermogenesis.