Changes in lipoprotein lipase modulate tissue energy supply during stress.

We studied the variations caused by stress in lipoprotein lipase (LPL) activity, LPL-mRNA, and local blood flow in LPL-rich tissues in the rat. Stress was produced by body immobilization (Immo): the rat's limbs were taped to metal mounts, and its head was placed in a plastic tube. Chronic stress (2 h daily of Immo) decreased total LPL activity in mesenteric and epididymal white adipose tissue (WAT) and was accompanied by a weight reduction of these tissues. In limb muscle, heart, and adrenals, total LPL activity and mRNA levels increased, and, in plasma, LPL activity and mass also increased. Acute stress (30-min Immo) caused a decrease in total LPL activity only in retroperitoneal WAT and an increase in preheparin plasma active LPL, but the overall weight of this tissue did not vary significantly. We propose an early release of the enzyme from this tissue into the bloodstream by some unknown extracellular pathways or other local mechanisms. These changes in this key energy-regulating enzyme are probably induced by catecholamines. They modify the flow of energy substrates between tissues, switching the WAT from importer to exporter of free fatty acids and favoring the uptake by muscle of circulating triacylglycerides for energy supply. Moreover, we found that acute stress almost doubled blood flow in all WAT studied, favoring the export of free fatty acids.     

Isoproterenol increases active lipoprotein lipase in adipocyte medium and in rat plasma

White adipose tissue (WAT) lipoprotein lipase (LPL) activity channels diet fat towards storage in adipocytes. Adrenaline (ADR) is accepted to reduce WAT or adipocyte LPL activity (LPLa), but available data are not clear-cut regarding long exposure to ADR in vitro or in vivo. We studied the effects of long exposures to ADR or beta-adrenergic agonist on LPL: in isolated rat adipocytes (3 h) and in rats (>1 day). Isoproterenol (ISO) (1 microM) did not alter LPLmRNA nor LPLa in adipocytes, but increased LPLa in medium more than twofold (3.58 +/- 0.35 vs. 1.32 +/- 0.35 mU/10(6) adipocytes, P < 0.001). Effect was time (not present at 1 h, clear at 2 h) and concentration dependent (high sensitivity from 10 to 100 nM, max at 1 microM). Adenylate cyclase activator or cyclic AMP (cAMP) analogue produced a similar increase. Thus in adipocytes ISO produced an increase in LPLa release and/or a decrease in extracellular LPLa degradation. ADR or ISO treated rats had a two to fourfold decrease in WAT LPLa vs. unchanged LPLmRNA. This decrease was 10-fold in WAT heparin-releasable LPLa (5.7 +/- 0.6 vs. 57.3 +/- 10.2 mU/g, P < 0.001), which represents peri/extracellular LPLa. Plasma LPLa was increased 11-fold by ADR (3.30 +/- 0.58 vs. 0.32 +/- 0.08 mU/ml, P < 0.001) whereas only threefold by ISO (P > 0.01). We suggest that in vivo ADR increased release of active LPL to plasma from endothelial cells of LPL-rich tissue(s)-WAT was probably one of these tissues releasing LPL since it lost 90% of its peri/extracellular LPLa-and/or decreased degradation of plasma active LPL. Since liver LPLa was not increased, plasma active LPL might be kept away from hepatic degradation by binding to stabilising entities in plasma (fatty acids (FA), lipoproteins or soluble heparan sulphates (HS)). In conclusion, we believe this is the first report stating that: (a) ISO increases LPLa in isolated adipocyte medium, and (b) ADR administration to rats decreases WAT extracellular active LPL and increases preheparin plasma active LPL.     

Effects of cold acclimation on the activity of lipoprotein lipase in adipose tissues of genetically obese Zucker rats

The activity of lipoprotein lipase (LPL) was studied in interscapilar brown adipose tissue (BAT), epididymal white adipose tissue (WAT) and in the heart of lean and obese adult Zucker rats maintained at 22 degrees C or adapted to cold (10 degrees C). In WAT the specific activity per gram of tissue was lower in obese than in lean rats but the total activity within the tissue was three-fold higher. Cold acclimation did not modify total activity in either lean or obese rats. In BAT, but not in the heart, both specific and total activities were lower in obese than in lean animals. They were enhanced in both tissues following cold acclimation. Six-hour fasting led to a decrease in specific activity in WAT of lean rats but had no effect in obese animals; an increase was observed in BAT and heart of both genotypes. Insulin administration has no effect on activities in WAT in either 22 or 10 degrees C adapted obese rats. Norepinephrine administration stimulates LPL activity in BAT and heart of all groups. It is concluded that the lack of development of obesity previously observed in obese rats following cold acclimation is not due to a decreased capacity of lipid uptake by WAT. It might in part be due to an increased lipid oxidation in BAT.     

Lipoprotein lipase in adipose tissues of rats running during cold exposure

This study evaluated the individual and combined effects of exercise training and intermittent cold exposure of similar energy cost on serum lipids and lipoprotein lipase (LPL) activity on epididymal white (WAT) and interscapular brown (BAT) adipose tissues of the rat. The animals were subjected daily to 2 h of treadmill running at 24 degrees C or for the same period of time at -5 degrees C, with or without exercise, for 28 days. Exercise training lowered serum triglycerides (P less than 0.01), whereas serum cholesterol was reduced by cold exposure (P less than 0.05). Cholesterol lowering occurred in the lipoproteins of lower densities. WAT weight was diminished by both treatments. Exercise training had an overall lowering effect on WAT total LPL activity (P less than 0.05), whereas cold exposure did not affect enzyme activity significantly. Exercise and intermittent cold interacted on BAT weight. Cold increased total BAT LPL activity (P less than 0.03), whereas simultaneous exercise in the cold greatly diminished this effect. Serum insulin levels were not affected by either treatment. Thus, in WAT, intermittent exposure to cold did not have any lasting effect on LPL activity, whereas exercise training decreased the latter. In contrast, exercise did not influence LPL in BAT of rats not exposed to cold but prevented the stimulation of enzyme activity induced by repeated cold exposure. These results support the notion that the regulation of LPL is tissue specific.     

Thermogenic effect of triiodothyroacetic acid at low doses in rat adipose tissue without adverse side effects in the thyroid axis

Triiodothyroacetic acid (TRIAC) is a physiological product of triiodothyronine (T(3)) metabolism, with high affinity for T(3) nuclear receptors. Its interest stems from its potential thermogenic effects. Thus this work aimed 1) to clarify these thermogenic effects mediated by TRIAC vs. T(3) in vivo and 2) to determine whether they occurred predominantly in adipose tissues. To examine this, control rats were infused with equimolar T(3) or TRIAC doses (0.8 or 4 nmolx100 g body wt(-1) x day(-1)) or exposed for 48 h to cold. Both T(3) doses and only the highest TRIAC dose inhibited plasma and pituitary thyroid-stimulating hormone (TSH) and thyroxine (T(4)) in plasma and tissues. Interestingly, the lower TRIAC dose marginally inhibited plasma T(4). T(3) infusion increased plasma and tissue T(3) in a tissue-specific manner. The highest TRIAC dose increased TRIAC concentrations in plasma and tissues, decreasing plasma T(3). TRIAC concentrations in tissues were <10% those of T(3). Under cold exposure or high T(3) doses, TRIAC increased only in white adipose tissue (WAT). Remarkably, only the lower TRIAC dose activated thermogenesis, inducing ectopic uncoupling protein (UCP)-1 expression in WAT and maximal increases in UCP-1, UCP-2, and lipoprotein lipase (LPL) expression in brown adipose tissue (BAT), inhibiting UCP-2 in muscle and LPL in WAT. TRIAC, T(3), and cold exposure inhibited leptin secretion and mRNA in WAT. In summary, TRIAC, at low doses, induces thermogenic effects in adipose tissues without concomitant inhibition of TSH or hypothyroxinemia, suggesting a specific role regulating energy balance. This selective effect of TRIAC in adipose tissues might be considered a potential tool to increase energy metabolism.     

Major involvement of mTOR in the PPARγ-induced stimulation of adipose tissue lipid uptake and fat accretion

Evidence points to a role of the mammalian target of rapamycin (mTOR) signaling pathway as a regulator of adiposity, yet its involvement as a mediator of the positive actions of peroxisome proliferator-activated receptor (PPAR)γ agonism on lipemia, fat accretion, lipid uptake, and its major determinant lipoprotein lipase (LPL) remains to be elucidated. Herein we evaluated the plasma lipid profile, triacylglycerol (TAG) secretion rates, and adipose tissue LPL-dependent lipid uptake, LPL expression/activity, and expression profile of other lipid metabolism genes in rats treated with the PPARγ agonist rosiglitazone (15 mg/kg/day) in combination or not with the mTOR inhibitor rapamycin (2 mg/kg/day) for 15 days. Rosiglitazone stimulated adipose tissue mTOR complex 1 and AMPK and induced TAG-derived lipid uptake (136%), LPL mRNA/activity (2- to 6-fold), and fat accretion in subcutaneous (but not visceral) white adipose tissue (WAT; 50%) and in brown adipose tissue (BAT; 266%). Chronic mTOR inhibition attenuated the upregulation of lipid uptake, LPL expression/activity, and fat accretion induced by PPARγ activation in both subcutaneous WAT and BAT, which resulted in hyperlipidemia. In contrast, rapamycin did not affect most of the other WAT lipogenic genes upregulated by rosiglitazone. Together these findings demonstrate that mTOR is a major regulator of adipose tissue LPL-mediated lipid uptake and a critical mediator of the hypolipidemic and lipogenic actions of PPARγ activation.     

Effects of fasting on lipoprotein lipase activity in different depots of white and brown adipose tissues in diet-induced overweight rats

The aim of the present study was to evaluate the effects of 24 hours of starvation on lipoprotein lipase (LPL) activity in various depots of white and brown adipose tissues in control rats and in rats with two different degrees of overweight, both induced by dietary treatment. In control rats, no changes in LPL immunoreactive mass were observed in either white or brown adipose tissues after fasting, whereas the effects of food deprivation on enzyme activity were opposite in white versus brown adipose tissues. The LPL activity response to fasting was impaired by obesity: White adipose depots of cafeteria obese rats showed a lower ability to downregulate LPL during fasting and the increased LPL activity induced by fasting in brown adipose depots was less intense in the obese rats compared with control animals. When the degree of overweight was reduced, the differences between obese and control rats were also attenuated.     

Release of lipoprotein lipase to plasma by triacylglycerol emulsions. Comparison to the effect of heparin.

It was previously known that lipoprotein lipase (LPL) activity in plasma rises after infusion of a fat emulsion. To explore the mechanism we have compared the release of LPL by emulsion to that by heparin. After bolus injections of a fat emulsion (Intralipid) to rats, plasma LPL activity gradually rose 5-fold to a maximum at 6-8 min. During the same time the concentration of injected triacylglycerols (TG) decreased by about half. Hence, the time-course for plasma LPL activity was quite different from that for plasma TG. The disappearance of injected 125I-labelled bovine LPL from circulation was retarded by emulsion. This effect was more marked 30 min than 3 min after injection of the emulsion. The data indicate that the release of LPL into plasma is not solely due to binding of the lipase to the emulsion particles as such, but involves metabolism of the particles. Emulsion increased the fraction of labelled LPL found in adipose tissue, heart and the red muscle studied, but had no significant effect on the fraction found in liver. The effects of emulsion were quite different from those of heparin, which caused an immediate release of the lipase to plasma, decreased uptake of LPL in most extrahepatic tissues by 60-95%, and increased the fraction taken up in the liver.     

Increased Oxidative Stress Impairs Adipose Tissue Function in Sphingomyelin Synthase 1 Null Mice

Sphingomyelin synthase 1 (SMS1) catalyzes the conversion of ceramide to sphingomyelin. Here, we found that SMS1 null mice showed lipodystrophic phenotype. Mutant mice showed up-regulation of plasma triglyceride concentrations accompanied by reduction of white adipose tissue (WAT) as they aged. Lipoprotein lipase (LPL) activity was severely reduced in mutant mice. In vivo analysis indicated that fatty acid uptake in WAT but not in liver decreased in SMS1 null compared to wild-type mice. In vitro analysis using cultured cell revealed that SMS1 depletion reduced fatty acid uptake. Proteins extracted from WAT of mutant mice were severely modified by oxidative stress, and up-regulation of mRNAs related to apoptosis, redox adjustment, mitochondrial stress response and mitochondrial biogenesis was observed. ATP content of WAT was reduced in SMS1 null mice. Blue native gel analysis indicated that accumulation of mitochondrial respiratory chain complexes was reduced. These results suggest that WAT of SMS1 null mice is severely damaged by oxidative stress and barely functional. Indeed, mutant mice treated with the anti-oxidant N-acetyl cysteine (NAC) showed partial recovery of lipodystrophic phenotypes together with normalized plasma triglyceride concentrations. Altogether, our data suggest that SMS1 is crucial to control oxidative stress in order to maintain WAT function.     

The response of lipoprotein lipase to feeding and fasting. Evidence for posttranslational regulation

The regulation of adipose tissue lipoprotein lipase (LPL) was examined in rats fed or fasted overnight, and was found to be controlled posttranslationally. LPL catalytic activity decreased by 50% after fasting while LPL mRNA levels and rates of synthesis increased nearly 2-fold; enzyme mass remained unchanged. The distribution of LPL within the endoplasmic reticulum (ER) and Golgi/post-Golgi secretory pathway was assessed by differentiating between LPL high mannose and complex forms. After fasting, the majority of LPL is in the high mannose ER form (65%, 0.97 micrograms/g wet weight tissue), whereas the LPL complex form comprises only 35% (or 0.52 micrograms/g). After refeeding, however, the Golgi-derived LPL complex form predominates (65%, 1.03 micrograms/g) over the high mannose ER form (35%, 0.55 micrograms/g). Kinetic analysis suggests that high mannose LPL disappears with a half-life of t0.5 = 40 min in both fed and fasted rats, indicating that the redistribution of LPL mass during feeding/fasting does not arise by differential retention within ER. Instead, the fractional catabolic rate of complex LPL within the Golgi/post-Golgi secretory compartment can be calculated to be 3.5-fold greater in fasting. In heart, changes in LPL activity in response to feeding/fasting are also not due to differences in mRNA levels or rates of synthesis. Based on these findings, a model of LPL posttranslational regulation is proposed and discussed.     

Adipose, muscle and lung tissue lipoprotein lipase activities in young streptozotocin treated rats

Lipoprotein lipase (LPL) activity was studied in adipose, muscle and lung tissues of post-weanling rats 48 and 96 hours after experimentally induced diabetes by streptozotocin administration. Weight gain was reduced, and blood glucose level increased about 3-4 fold above the control level as an indication of the diabetic state. LPL activity in brown and white adipose tissues decreased in diabetic rats to 10-30% of the control level. In soleus muscle the LPL activity was slightly enhanced 96 hours after the streptozotocin injection. In cardiac muscle the LPL activity was markedly increased already 48 hours after the administration of streptozotocin and the increase remained significant until 96 hours. There was in the pulmonary tissue also an increase of LPL activity of diabetic rats, although this was significant only 96 hours after streptozotocin treatment. The results suggest marked tissue specific variation in the LPL activity. Moreover, tissue responses to experimentally induced diabetes vary. In adipose tissue the decrease in the LPL activity suggests that lipid transport to adipocytes is decreased while an increase in skeletal and cardiac muscles and in lung tissue proposes that their lipid utilization is enhanced.     

Lipoprotein lipase and cholesterol transfer activities of lean and obese Zucker rats.

Adult female lean and obese Zucker rats maintained under standard conditions were used for the estimation of plasma, liver and white adipose tissue (WAT) activity of lipoprotein lipase, plasma and liver hepatic lipase and plasma lecithin-cholesterol acyltransferase. No differences in plasma or tissue levels of lipoprotein lipase between lean and obese rats were detected, but the larger WAT size of the obese rats resulted in higher lipase activity per unit of rat weight. Hepatic lipase levels in plasma were higher in the obese, but in liver, the higher activity was found in lean rats. No significant differences were found for lecithin-cholesterol acyltransferase activity, except when the levels in the HDL fraction were expressed per unit of protein weight, showing lower activity in the obese rats. In conclusion, the essentially maintained enzyme activities in obese rat tissues suggest that they cannot explain the deficient lipoproteins processing of obese rats, and, consequently their dislipidaemia.