Lipogenesis at the suckling-weaning transition in liver and brown adipose tissue of the rat

Glucose availability but not changes in pancreatic hormones sensitizes hepatic AMPK activity during nutritional transition in rodents

The cellular energy sensor AMP-activated protein kinase (AMPK) is a metabolic regulator that mediates adaptation to nutritional variations to maintain a proper energy balance in cells. We show here that suckling-weaning and fasting-refeeding transitions in rodents are associated with changes in AMPK activation and the cellular energy state in the liver. These nutritional transitions were characterized by a metabolic switch from lipid to glucose utilization, orchestrated by modifications in glucose levels and the glucagon/insulin ratio in the bloodstream. We therefore investigated the respective roles of glucose and pancreatic hormones on AMPK activation in mouse primary hepatocytes. We found that glucose starvation transiently activates AMPK, whereas changes in glucagon and insulin levels had no impact on AMPK. Challenge of hepatocytes with metformin-induced metabolic stress strengthened both AMPK activation and cellular energy depletion under limited-glucose conditions, whereas neither glucagon nor insulin altered AMPK activation. Although both insulin and glucagon induced AMPKα phosphorylation at its Ser^485/491 residue, they did not affect its activity. Finally, the decrease in cellular ATP levels in response to an energy stress was additionally exacerbated under fasting conditions and by AMPK deficiency in hepatocytes, revealing metabolic inflexibility and emphasizing the importance of AMPK for maintaining hepatic energy charge. Our results suggest that nutritional changes (i.e. glucose availability), rather than the related hormonal changes (i.e. the glucagon/insulin ratio), sensitize AMPK activation to the energetic stress induced by the dietary transition during fasting. This effect is critical for preserving the cellular energy state in the liver.

Lipid metabolism related gene-expression profiling in liver, skeletal muscle and adipose tissue in crossbred Duroc and Pietrain Pigs

Body-weight differences in animals may be ascribed to genetic and environmental factors. Here we utilized two divergent porcine genotypes, the highly muscled, leaner PietrianxYorkshire pigs and less muscled, fatter DurocxYorkshire growing pigs (75-110 kg), to examine the role of genetic background on expression of genes associated with anabolic (Fatty acid synthase, FAS; glucose transporter 4, GLUT-4; stearoyl CoA desaturase, SCD; Sterol regulatory binding protein-1, SREBP-1; leptin) and catabolic lipid metabolism (Carnitine palmitoyltransferase-1B, CPT-1B; acyl-CoA dehydrogenase, ACDH) in adipose tissue (AT), liver (L) and skeletal muscle (SKM). Pietrain pigs had lower mRNA abundance for FAS, SREBP-1, SCD and leptin in AT and L, but higher mRNA abundance for L ACDH and SKM ACDH and CPT-1B than Durocs. Duroc pigs exhibited higher expression of FAS, SREBP-1, SCD, leptin in AT and FAS in L and lower expression of ACDH and CPT-1B in L SKM. GLUT-4 expression did not differ in SKM between the two genotypes. Feeding of a beta adrenergic agonist (Paylean) for 52 days lowered expression of lipid anabolic and enhanced lipid catabolic genes expressions similarly in both genotypes. Overall, the lipid metabolism genes differential expression patterns documented here showed that in Pietrain pigs mRNA abundances of synthesis genes were lower and of catabolic genes were higher than in Duroc pigs.

Ethanol intake during lactation. II. Effects On pups' liver and brain metabolism

Lactating rats, with litters adjusted to 8 pups on day 1, were divided into 4 groups: control animals (C), which received water and Nuvilab chow ad libitum, and ethanol animals (E), which received 20% (E20), 10% (E10), or 5% (E5) ethanol diluted in the drinking water and Nuvilab chow ad libitum. On day 12 of life, the pups were weighed and decapitated. The intake of 10% and 20% ethanol solutions by the lactating rats decreased the pups' body weight and liver weight. The pups' liver ATP-citrate lyase activity was decreased in all ethanol groups. The pups' brain weight decreased in E20 only. Glucose metabolism and lactate production were studied in the pups' brain slices, which were incubated at 37 degrees C in Krebs-Henseleit buffer under carbogen in the presence of glucose (5 mM) plus 14C-glucose (0.04 microCi) with or without beta-hydroxybutyrate or insulin. Study of the incubated pups' brain slices showed that the intake of the 20% ethanol solution by the dams increased glucose consumption, oxidation, lactate production, and lipogenesis rate from glucose in all media studied, as compared with findings in the C group. In the pups' brain slices, the lactate production and lipogenesis rate from glucose were higher in E10 than in the C group. The addition of beta-hydroxybutyrate to the incubation medium caused a decrease in glucose oxidation in C, E5, and E20 and an increase in glucose consumption in E10. Ingestion of the 5% ethanol solution by dams decreased the pups' brain lipogenesis rate from glucose in all media studied. We concluded that the effects of maternal alcohol intake on the pups' development and metabolism are dose-dependent. High amounts of ethanol intake (10% or 20%) caused a great impairment in the pups' growth, as well as their liver and brain metabolism. The low dose (5%) did not affect the pups' body weight gain or their brain and liver weight, but it did alter brain glucose metabolism.

Developmental changes of lipogenic enzyme activities and lipogenesis in brown adipose tissue and liver of the rat

1. Brown adipose tissue (BAT) and liver lipogenesis in vivo estimated by using 3H2O as tracer was very low and did not change significantly between 10 and 20 days after birth. Lipogenesis increased dramatically in both tissues by weaning at 20 days, peaking between 25 and 30 days of age. Since that time the rate of fatty acid synthesis in BAT decreased gradually to reach adult level after 2 months, whereas in the liver there was a sharp decrease of lipogenesis. 2. The activities of fatty acid synthase, citrate cleavage enzyme, malic enzyme and glucose 6-phosphate dehydrogenase essentially followed a similar course of developmental changes as lipogenesis. 3. In contrast to the enzymes listed above NADP-linked isocitrate dehydrogenase remained unaltered over the period studied, whereas lactate and malate dehydrogenases exhibited very high activity at 10 days after birth and from then decreased to reach adult level at the age of about 20 days. 4. The data obtained indicate that no substantial differences could be detected in the developmental pattern of lipogenesis and lipogenic enzyme activities between BAT and liver up to 30 days of age but after this time these processes were not co-ordinated in both tissues. Beyond this time the BAT was characterized by a much higher rate of lipogenesis than the liver. 5. The results are discussed in terms of the nutrient changes and the relationship between thermogenesis and lipogenesis in BAT.

Pre-translational control of hepatic malic enzyme expression during the development of the rat

The expression of hepatic cytosolic malic enzyme in the developing rat has been studied by molecular-biological techniques. Malic enzyme mRNA was barely detectable throughout the neonatal period, but increased to significant levels immediately before weaning. Northern-blot analysis demonstrated that the two major malic enzyme mRNA species displayed non-co-ordinate control during development, with the 2.0 kb form accumulating to a greater extent than the 3.1 kb form. A novel 1.6 kb mRNA species was found to predominate in foetal samples. Tri-iodothyronine treatment of neonatal rats caused premature induction of all three malic enzyme mRNA species. Dietary studies also showed precocious induction of the mRNA with diets high in carbohydrate, but not with those high in fat.

Hormonal control of specific gene expression in the rat liver during the suckling-weaning transition

In the rat, the suckling-weaning transition is accompanied by marked changes in nutrition. During the suckling period, the pups are fed with milk which is a high-fat low-carbohydrate diet. At weaning, milk is progressively replaced by the rat chow which is a high-carbohydrate low-fat diet. This is accompanied by considerable hormonal modifications: an increase in plasma insulin and a decrease in plasma glucagon concentrations, as well as by marked changes in metabolic pathways in liver: decrease in hepatic gluconeogenesis, increase in lipogenesis, and appearance of liver glucokinase. Most of the data concerning these changes are related to maximal activity of enzymes. The recent availability of specific cDNA probes for phosphoenolpyruvate carboxykinase, acetyl-CoA carboxylase, fatty acid synthase and glucokinase has allowed study of the role of pancreatic hormones and of nutrition in the changes of the expression of these genes at weaning in the rat.

Rat hepatic mRNA-S14 and lipogenic enzymes during weaning: role of S14 in lipogenesis

The rapid and marked response of hepatic mRNA-S14 sequence to both triiodothyronine and carbohydrate intake has made this sequence an attractive model for studying the action of hormonal and dietary factors. Because it is highly expressed and regulated only in lipogenic tissues, we have suggested that it plays a role in some aspect of lipid synthesis, transport, or metabolism. To provide more precise information regarding the function of S14 we have measured lipogenesis, lipogenic enzymes, beta-oxidation, and mRNA-S14 levels in spontaneously weaning neonatal rats and in rats prematurely weaned to a laboratory diet on postnatal day 17. After birth, the levels of lipogenesis, mRNA-S14, and the lipogenic enzymes malic enzyme (ME) and fatty acid synthase (FAS) were almost undetectable but increased with the onset of spontaneous weaning. Coincident with these changes, beta-oxidation decreased. Premature weaning beginning on day 17 resulted in an earlier and even more marked increase in lipogenesis, ME, FAS, and mRNA-S14. On day 19, ME and FAS activities were 6- to 19-fold more than activities in control suckling pups, whereas mRNA-S14 levels had risen to greater than 100 times the control values. Thus directional shifts in mRNA-S14 corresponded with indices of lipogenesis and were opposite to indices reflecting beta-oxidation. The response of mRNA-S14 therefore suggests that it may be related to the synthesis of fatty acids. On the other hand, the level of lipogenesis in the fetus was high despite the fact that the levels of both mRNA-S14 and ME were low. This dissociation raises the possibility that the S14 protein participates in lipogenesis in the neonate and adult but not in the fetus.

Glycolysis and lipid synthesis in brown adipose tissue during ageing in the rat

The maximal activity of the enzymes hexokinase and 6-phosphofructokinase were measured in interscapular brown adipose tissue during ageing in the rat. The activity of these glycolytic enzymes increased markedly during the suckling-weaning transition (20-22 days of age), but from 30 to 100 days of age the activities per gram wet weight and per mg protein decreased. The rate of lipogenesis in brown adipose tissue was also increased over the suckling-weaning transition and the contribution of glucose to this fatty acid synthesis was greater in weaned animals especially under the influence of insulin.

Regulation of lipogenesis and of non-saponifiable-lipid synthesis in vivo at birth and after prolonged starvation in the newborn rat

The rate of lipogenesis in the liver was increased by glucose injection at birth, mediated by the insulin secretion. In addition, glucagon decreased the rates of lipogenesis and non-saponifiable-lipid synthesis after birth. These rates decreased after prolonged starvation in the newborn rat. Tri-iodothyronine injection increased the rates of lipogenesis enhanced in response to glucose administration after prolonged starvation in liver and brown adipose tissue. Dexamethasone, however, increased the rates of lipogenesis enhanced in response to glucose in liver and prevented the increase in the rates of lipogenesis in brown adipose tissue.

Development of fatty acid-synthetic capacity in interscapular brown adipose tissue during suckling in genetically obese Zucker rats

The development of the lipogenic capacity in brown adipose tissue was studied in suckling lean (Fa/fa) and obese (fa/fa) Zucker pups aged from 7 to 22 days. In both lean and obese pups, activities of the two key lipogenic enzymes, fatty acid synthetase and acetyl-CoA carboxylase, and of citrate cleavage enzyme rose from the early to the late suckling period. Compared with lean pups, 7-day-old fa/fa pups showed a 35% increase in fat accumulation in interscapular brown adipose tissue and a 25% increase in fatty acid synthetase activity. By 10 days of age, fat deposition, lipogenesis in vivo (assessed by the incorporation of 3H from 3H2O into fatty acids) and fatty acid synthetase activity were 1.5-2-fold higher in pre-obese than in lean pups. Compared with lean pups, the increased lipogenesis in vivo observed in brown adipose tissue of 10-day-old pre-obese pups could not entirely account for the difference in fat deposition observed in this tissue, suggesting that additional mechanisms are at play to explain the increased fat content of this tissue.

Adaptive changes in lipid composition of rat liver plasma membrane during postnatal development following maternal ethanol ingestion

The fatty acid composition of constituent phospholipids and the cholesterol content of rat liver plasma membranes were determined subsequent to maternal alcohol ingestion during pregnancy and lactation. The alcoholic group was given a liquid Metrecal diet containing 37% ethanol-derived calories. The control group was pair-fed an isocaloric sucrose/Metrecal diet. Litters were killed for lipid analyses at days 5, 15 and 25 after birth. These studies revealed that the total phospholipid phosphorus was similar and increased significantly with age in both groups. Cholesterol also increased significantly with age in both groups but was greater in the alcoholic pups, resulting in a higher cholesterol/phospholipid molar ratio. While the phosphatidylethanolamine (PE) content increased with age in both groups, that of sphingomyelin decreased. Phosphatidylserine + phosphatidylinositol (PS + PI) was significantly higher in the control group at all ages studied. A consistent increase of C22:6 in phosphatidylcholine (PC), sphingomyelin, PS + PI and in the total phospholipid fraction from alcoholic pups was observed. Although other fatty acid changes were found in PC, PS + PI and sphingomyelin, PE was not affected. These results suggest that specific adaptive changes were induced in the liver plasma membrane lipids of the progeny from alcoholic rats.

Fatty acid synthesis and ketone body utilization by brown adipose tissue of the rat. Response to cold or nutritional state?

The effects of cold adaptation (exposure to 2-4 degrees C for 2-3 weeks) on the incorporation of 3H from 3H2O and of D-3-hydroxy [3- 14C]butyrate into fatty acid in vivo in interscapular brown adipose tissue of the rat were examined. Fatty acid synthesis, measured with 3H2O, was increased in brown adipose tissue of cold-adapted rats pair-fed ad libitum but not in cold-adapted rats pair-fed to the same food intake as rats maintained at 18-20 degrees C. Incorporation of D-3-hydroxybutyrate into fatty acid was increased in brown adipose tissue of cold-adapted rats, both when rats were fed ad libitum and when they were deprived of food for 48 h. Ketone bodies may be an important substrate for brown adipose tissue in the cold-adapted rat.

Triiodothyronine-dietary interrelationships in the modulation of brown adipose tissue and liver lipogenesis in the rat

The influence of diet and triiodothyronine on "in vivo" lipogenesis has been compared in liver and brown adipose tissue of the rat at weaning and in adult life. At both ages studied, hyperthyroidism increases lipogenesis only in the liver. However brown adipose tissue accumulates lipid (probably derived from white adipose tissue) thus helping to explain previous observations of low rates of lipogenesis in this tissue during the late suckling period when thermogenesis is high and plasma triiodothyronine levels rise. Suppression of lipogenesis by dietary polyunsaturated fat in both tissues at both ages studied confirms previous studies on adult animals. Malic enzyme and glucose-6-phosphate dehydrogenase activities correlate well with changes in lipogenesis in brown adipose tissue but not in liver and the hepatic enzymes exhibit a diminished response to dietary and hormonal factors with age.