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

Chronic high-sucrose diet increases fibroblast growth factor 21 production and energy expenditure in mice

Excess carbohydrate intake causes obesity in humans. On the other hand, acute administration of fructose, glucose or sucrose in experimental animals has been shown to increase the plasma concentration of anti-obesity hormones such as glucagon-like peptide 1 (GLP-1) and Fibroblast growth factor 21 (FGF21), which contribute to reducing body weight. However, the secretion and action of GLP-1 and FGF21 in mice chronically fed a high-sucrose diet has not been investigated. To address the role of anti-obesity hormones in response to increased sucrose intake, we analyzed mice fed a high-sucrose diet, a high-starch diet or a normal diet for 15 weeks. Mice fed a high-sucrose diet showed resistance to body weight gain, in comparison with mice fed a high-starch diet or control diet, due to increased energy expenditure. Plasma FGF21 levels were highest among the three groups in mice fed a high-sucrose diet, whereas no significant difference in GLP-1 levels was observed. Expression levels of uncoupling protein 1 (UCP-1), FGF receptor 1c (FGFR1c) and β-klotho (KLB) mRNA in brown adipose tissue were significantly increased in high sucrose-fed mice, suggesting increases in FGF21 sensitivity and energy expenditure. Expression of carbohydrate responsive element binding protein (ChREBP) mRNA in liver and brown adipose tissue was also increased in high sucrose-fed mice. These results indicate that FGF21 production in liver and brown adipose tissue is increased in high-sucrose diet and participates in resistance to weight gain.

Sarcolipin is a novel regulator of muscle metabolism and obesity

Obesity is increasing at an alarming rate, both in adults and adolescents, across the globe due to increased consumption of caloric rich diet. Obesity and its associated complications appear to be major contributing factors not only to diabetes/heart disease but also to cancer, and neurological diseases causing a huge burden on the health care system. To date, there are no effective treatments to reduce weight gain, other than caloric restriction and exercise which are often difficult to enforce. There are very few drugs available for treating obesity and those that are available only reduce obesity by ∼ 10%. Identifying mechanisms to increase energy expenditure, on top of the increase elicited by exercise, would be more beneficial to control weight gain. The purpose of this review is to highlight the role of sarcolipin (SLN), a regulator of SERCA pump, in muscle thermogenesis and metabolism. We will further discuss if enhancing SLN activity could be an effective mechanism to increase energy expenditure and control weight gain. We will also discuss the merits of adaptive thermogenesis in muscle and brown fat as potential mechanisms to increase energy expenditure during caloric overload. That said, there is still a great need for further research into the mechanism of diet induced thermogenesis and its relevance to overall metabolism and obesity.

Does overfeeding enhance genotype effects on energy metabolism and lipid deposition in breast muscle of ducks?

We evaluated the effects of genotype (Muscovy, Pekin and their crossbreed hinny and mule ducks) and feeding levels (overfeeding between 12 and 14 weeks of age vs ad libitum feeding) on energy metabolism and lipid deposition in breast muscle of ducks. Samples of breast muscle (Pectoralis major) were collected at 14 weeks of age from 8 birds per group. Overfeeding induced an accumulation of lipids in breast muscle (1.5- to 1.7-fold, depending on genotype) mainly induced by triglyceride deposition. It also induced a considerable increase in the amounts (expressed as g/100 g of tissue) of saturated and mono-unsaturated fatty acids (SFA, MUFA), while the amounts of poly-unsaturated fatty acids (PUFA) remained unchanged in hinny and Muscovy ducks or slightly increased in Pekin and mule ducks. In breast muscle, overfeeding decreased the activity of the main enzymes involved in lipogenesis from glucose (glucose-6-phosphate dehydrogenase, G6PDH, malic enzyme, ME, acetyl CoA carboxylase, ACX). Lipoprotein lipase (LPL) activity in Pectoralis major muscle was also significantly decreased (-21%). The ability of muscle tissues to catabolize long-chain fatty acids, as assessed by beta-hydroxyacyl CoA dehydrogenase (HAD) activity, was increased in Pectoralis major muscle, as was cytochrome-c oxidase (COX) activity. Hybrid and Pekin ducks exhibited higher levels of ACX and LPL activity in Pectoralis major muscle than Muscovy ducks, suggesting a greater ability to synthesise lipids in situ, and to take up circulating lipids. Total lipid content in breast muscle of hybrid and Pekin ducks was higher than in that of Muscovy ducks. In hybrid and Pekin ducks, lipid composition of breast muscle was characterized by higher amounts of triglycerides, SFA and MUFA than in Muscovy ducks. Finally, oxidative metabolism was greater in Pectoralis major muscles of hybrid and Pekin ducks than in Muscovy ducks, suggesting an adaptative strategy of muscle energy metabolism according to lipid level.

Voluntary sucrose ingestion, like corticosterone replacement, prevents the metabolic deficits of adrenalectomy

We tested whether corticosterone replacement causes increased sucrose drinking in adrenalectomized (ADX) rats compared to sham-ADX (sham) rats. ADX rats given high doses of corticosterone drank as much sucrose as sham rats, whereas at three lower doses of corticosterone, drinking was similar between groups and was only approximately 40% of that ingested by shams. Compared to sham rats, ADX rats drinking saline, or saline and saccharin, gain weight more slowly, contain less white adipose tissue, and have higher sympathetic outflow as assessed by uncoupling protein content in brown adipose tissue. Allowing sucrose as well as saline to drink restored all of these variables to normal in ADX rats with no- or low-corticosterone. All endpoints from sucrose-drinking ADX rats with no-or low-corticosterone were indistinguishable from those in water-drinking shams. By contrast, sucrose-drinking ADX rats that were given high doses of corticosterone exhibited the usual catabolic effects of corticosterone on body weight gain and, unlike sucrose-drinking shams, were obese. We conclude that (i) high corticosterone stimulates the potability of sucrose and inhibits sympathetic stimulation of uncoupling protein; (ii) sucrose, without corticosterone, normalizes metabolic deficits in ADX rats probably through actions mediated both peripherally and by the central nervous system; and (iii) ADX rats have a distinct sucrose appetite.

Mineral content of the diet alters sucrose-induced obesity in rats

The effects of dietary mineral levels on caloric intake, nutrient choice, body weight, adipose tissue weight, interscapular brown adipose tissue (IBAT) weight, and thermogenic capacity, and plasma insulin and glucose levels were examined in adult male Sprague-Dawley rats. In Experiments 1 and 2, rats were fed a purified diet with zinc (Zn), chromium (Cr), and selenium (Se) added, or the same diet without the addition of these minerals. In Experiment 3, the effects of Zn and Cr were examined separately. In all experiments, half of the rats in each diet group were given a 32% sucrose solution in addition to their standard diet and water. Rats given sucrose consumed more calories and gained more weight than rats not given sucrose. However, mineral levels altered the effects of sucrose on these measures. Added minerals increased percent sucrose intake, reduced weight gain and feed efficiency, increased GDP binding in IBAT mitochondria, improved glucose tolerance, and reduced plasma insulin levels. The reduction in weight gain and increased feed efficiency found when Zn alone was added to the diet was independent of sucrose condition. In comparison, the alterations observed in these measures when Cr alone was added to the diet varied as a function of sucrose availability.

Regulation of expression of the lipoprotein lipase gene in brown adipose tissue

The regulation of lipoprotein lipase gene expression in brown adipose tissue was studied. Rats were preacclimated to 21 degrees C. Exposure to cold (4 degrees C) resulted in a rapid increase in the level of lipoprotein lipase mRNA in the tissue. The level peaked (expressed per microgram total RNA) after approximately 8 h and then slowly declined. The increased lipoprotein lipase mRNA level was not due to an increased stability of the mRNA, but, in a transition event from a high to a low expression of the lipoprotein lipase gene, a transcription-dependent process was recruited that accelerated the breakdown of lipoprotein lipase mRNA. Norepinephrine injections increased lipoprotein lipase mRNA levels in the tissue; this effect was mediated via a beta-adrenergic receptor. The effect of cold could be mimicked by norepinephrine injections, and these two effects were not additive, indicating that the cold effect was mediated by norepinephrine. The lipoprotein lipase mRNA level was also increased by insulin injections (into fasted animals); thus an increase in lipoprotein lipase gene expression in brown adipose tissue may be induced via two different stimuli, which, intracellularly, would be mediated via different signaling systems. In all investigated conditions, the changes in lipoprotein lipase mRNA levels observed here were parallelled by alterations in lipoprotein lipase activity reported earlier from this laboratory. It was therefore concluded that, under the conditions studied, lipoprotein lipase activity in brown adipose tissue was primarily regulated at the transcriptional level.

Dietary modulation of the anorectic potency of amphetamine

The effects of intake of an amphetamine solution on food and fluid intakes, body weight (b.wt.) and feed efficiency (FE) were examined in rats fed either a high-carbohydrate (HC) (65% of total calories) or high-fat (HF) (65% of total calories) diet. During a 17-day predrug period, neither caloric intake, fluid intake, b.wt. nor feed efficiency (FE) differed as a function of diet. When given a 0.1 mg/ml amphetamine sulfate (AMPH) solution as their sole source of fluid, rats in both diet groups decreased fluid intake by an equivalent amount. While diet did not influence AMPH intake, it did alter the drug's effects on caloric intake, b.wt. and FE. In both diet groups, rats drinking AMPH decreased caloric intake, b.wt. gain and FE relative to rats which drank water. However, rats fed the HC diet decreased caloric intake less, but lost more weight than rats fed the HF diet. Further, rats fed the HC diet displayed a rapid tolerance to the anorectic effects of AMPH, with no tolerance to the drug's effect on b.wt. In contrast, rats fed the HF diet suppressed caloric intake throughout the drug period, but weighed more than rats fed the HC diet. Thus, when drinking the AMPH solution, FE of rats fed the HC diet was significantly lower than that of rats fed the HF diet. When the drug was removed, rats eating the HC diet failed to alter caloric intake and b.wt. In contrast, rats fed the HF diet increased caloric intake and gained weight. These data indicate that dietary factors must be considered when evaluating the anorectic actions of psychopharmacological agents.

Refeeding and insulin increase lipoprotein lipase activity in rat brown adipose tissue

Induction of lipoprotein lipase activity in brown adipose tissue (BAT) in response to cold stress has earlier been shown to be regulated by a beta-adrenergic mechanism and to be dependent on mRNA synthesis. In the present study, we have investigated the acute effects of refeeding after a short starvation period and the hormonal mechanism underlying the observed effects. Refeeding was found to rapidly increase tissue wet weight and lipoprotein lipase activity. The increase in enzyme activity could be blocked by the RNA synthesis inhibitor actinomycin D, indicating a gene activation. beta-Adrenergic blockade had no effect on this elevation of enzyme activity, but the increase could be mimicked by insulin injection. The results suggest that BAT contains two different pathways for regulation of lipoprotein lipase activity, both involving mRNA synthesis.

Norepinephrine turnover in iron deficiency: effect of two semi-purified diets

The effects of two dietary treatments on norepinephrine turnover in iron deficiency were examined. These studies were designed to bridge the gap between previous studies of poor thermoregulation in iron deficiency which used a diet (HMW, Hubbel-Mendel-Wakeman formulation) relatively high in fat (46% of calories) and moderate in carbohydrate (46% of calories) and the more recent studies of thermogenesis in iron deficiency which use the AIN-76 recommended diet which is relatively low in fat (11% of calories) and high in carbohydrate (67% of calories). Iron deficient rats grew less well and had depressed thyroid hormone concentrations regardless of dietary treatment group. The HMW diet significantly increased norepinephrine turnover in heart in iron deficient animals relative to AIN diet but had no effect in controls. Brown adipose tissue norepinephrine turnover was threefold higher in HMW rats fed a low iron diet, and only 67 percent higher in control rats. This study demonstrates that certain modest macronutrient manipulations affect norepinephrine content and turnover more in iron deficient than controls. However, abnormalities in thyroid hormone concentrations persist in iron deficient animals regardless of these dietary treatments.

The effects of meal-feeding and the diurnal cycle on lipogenesis in brown adipose tissue of rats

A significant diurnal variation in the rates of lipogenesis in vivo in brown adipose tissue occurred in both virgin and lactating rats. On a meal-feeding regime of either a chow, high-sucrose, or high-lipid diet, there was a very large increase in BAT lipogenesis following the meal. The rates observed after the sucrose meal are the highest so far reported. There was no significant difference in BAT lipogenesis between lactating and virgin rats, contrary to previous reports by others. The pattern of stimulation of BAT lipogenesis by these feeding regimes was different from that for white adipose tissue and liver and was not correlated with plasma insulin levels.

When does sucrose increase appetite and adiposity?

Two methods of sucrose feeding have been employed in studies with rodents. In the nutritional method, part or all of the starch in a diet is replaced with sucrose. In the solution method, animals maintained on a nutritionally complete diet are given a sucrose solution to drink. The solution method is generally a more effective and reliable method of producing obesity except for weanling rodents. These two methods yield different results with regard to interactions with the fat and protein content of the diet, efficiency of weight gain, disaccharide effects and effects of meal feeding. It is suggested that for the nutritional method, sucrose alters food intake and adiposity via its effects on fat oxidation. For the solution method, the critical factor may be presenting a wet source of calories rather than sucrose per se. Differences in the way sucrose is fed do not account for all divergent results. Different investigators conducting similar experiments have often obtained different results. For these and other reasons, animal studies do not support the idea that sucrose intake causes obesity in humans.

Effects of dietary cholesterol on adipose tissue lipoprotein lipase in the baboon

The effects of infant diet (breast milk or formula containing 2, 30 or 60 mg/dl cholesterol) and subsequent dietary cholesterol (0.02, 1.0 or 1.7 mg/kcal) and fat (saturated or unsaturated) on heparin-releasable lipolytic activity from omental adipose tissue was estimated from 99 baboons of 5-8 years of age. This lipase activity was characterized as lipoprotein lipase based on salt inhibition and apolipoprotein C-II activation. Lipoprotein lipase activity released from adipose tissue by heparin was significantly (P less than 0.002) lower in high cholesterol-fed baboons than in those fed low cholesterol. Most of this difference was due to impaired long-term heparin release of lipoprotein lipase. Adipose tissue lipoprotein lipase increased with increasing fat cell size regardless of diet, but there was no effect of diet on adipocyte size. There were no significant effects of infant cholesterol intake nor adult saturated or unsaturated fat on lipoprotein lipase activity. Adult baboons breast fed as infants had lower adipose tissue lipoprotein lipase activity (P less than 0.07) than adults fed formula as infants.

Lipoprotein lipase in adipose tissues of exercise-trained, cold-acclimated rats

The combined effects of exercise training and cold acclimation on serum lipids and on the activity of lipoprotein lipase (LPL) in epididymal white (WAT) and interscapular brown adipose tissues (BAT) of the rat were evaluated. Sedentary and exercise-trained (treadmill) rats were housed and trained at either 24 or 4 degrees C for 45 days. Although neither treatment given alone influenced serum total cholesterol, exercise training in the cold resulted in a 54% increase in this variable. Serum triglycerides were lowered to 80% of control values by exercise training and to 44% of control by cold acclimation. Both treatments resulted in lower insulin and thyroxine concentrations, while triiodothyronine levels were unaffected. Total LPL activity in WAT was increased twofold by exercise training at both temperatures, whereas cold acclimation stimulated WAT LPL activity to a lesser extent. Exercise training had no overall effect on LPL activity of BAT, whereas cold acclimation increased the latter in both sedentary (217%) and trained (420%) animals. These results emphasize the potential importance of LPL-mediated lipid assimilation in the metabolic events that lead to energy production in response to environmental stresses and lend support to the notion that the regulation of LPL activity is tissue specific.

Modifications in food intake and energy metabolism in rats as a function of chronic naltrexone infusions

The effects of chronic naltrexone infusions on food intake and energy balance were examined in male rats. Animals were fed either Purina Chow, or chow plus a 32% sucrose solution. After one week of being maintained on these diets, animals were implanted (intrascapularly) with osmotic minipumps infusing either 200 micrograms/kg/hr naltrexone hydrochloride or saline. Sucrose + chow-fed animals exhibited increased O2 consumption, increased CO2 production and an elevation in the respiratory quotient (RQ) relative to chow-fed controls. When infused with naltrexone, sucrose + chow-fed animals decreased food intake and body weight gain. While chow-fed animals also suppressed food intake and body weight gain, these decreases were not as great as those observed in sucrose + chow-fed animals. As a function of naltrexone administration, both chow-fed and sucrose + chow-fed animals altered their metabolism as reflected by decreased RQ and adiposity as determined by skinfold measurements. In addition, sucrose feeding led to a hyperthermia which was reversed by naltrexone infusions. Thus, chronic naltrexone administration depressed appetite, reduced energy production and induced hypothermia in rats. As naltrexone is thought to block the endogenous opioid system, this suggests that the endorphins are involved in the regulation of food intake and thermogenesis.

Effects of sucrose feeding and denervation on lipogenesis in brown adipose tissue

Rats were fed standard laboratory chow or sucrose + chow for 2-3 weeks. Sucrose feeding significantly increased the size and norepinephrine content of interscapular brown adipose tissue (BAT). Sucrose feeding stimulated in vivo lipogenesis in BAT, white adipose tissue, and liver. Unilateral denervation reduced BAT norepinephrine content by 90%, and significantly reduced BAT protein and DNA content. Prior surgical denervation of BAT had no effect on lipogenesis in chow-fed rats, but reduced the sucrose-induced lipogenesis in BAT by 75% and increased lipogenesis in retroperitoneal white adipose tissue of sucrose-fed rats. These data demonstrate that the optimal stimulation of lipogenesis in BAT by sucrose feeding requires innervation of this tissue.

Sucrose and polysaccharide induced obesity in the rat

Adult male rats were fed, in addition to chow and water, solutions (32%) of either sucrose, Polycose (a bland-tasting polysaccharide), or Polycose sweetened with 0.2% saccharin. The solutions were available for two 30-day periods when the rats were 90-120, and 180-210 days of age. A control group received only chow and water during these periods. During the second 30-day test the Polycose and Saccharin- Polycose groups were hyperphagic and gained as much excess weight as did the Sucrose group. The sucrose-fed rats, however, did not overeat relative to the control animals. Adding saccharin to the Polycose failed to increase total caloric intake or weight gain, but did increase Polycose intake and percent carbohydrate intake. The intakes of both Polycose solutions were greater than that of the sucrose solution, although in brief two-bottle preference tests the sucrose solution was preferred over the Polycose and saccharin- Polycose solutions. A similar pattern of results was obtained during the first test period, but the group differences were less pronounced. The findings demonstrate that carbohydrate-induced obesity is not unique to sweet-tasting sugars, but can also be produced by bland-tasting polysaccharides. Sweet taste does increase polysaccharide preference and intake, however.

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.