High dietary sodium intake increases white adipose tissue mass and plasma leptin in rats

Objective: Salt restriction has been reported to increase white adipose tissue (WAT) mass in rodents. The objective of this study was to investigate the effect of different sodium content diets on the lipogenic and lipolytic activities of WAT. Research Methods and Procedures: Male Wistar rats were fed on normal‐sodium (NS; 0.5% Na⁺), high‐sodium (HS; 3.12% Na⁺), or low‐sodium (LS; 0.06% Na⁺) diets for 3, 6, and 9 weeks after weaning. Blood pressure (BP) was measured using a computerized tail‐cuff system. At the end of each period, rats were killed and blood samples were collected for leptin determinations. The WAT from abdominal and inguinal subcutaneous (SC), periepididymal (PE) and retroperitoneal (RP) depots was weighed and processed for adipocyte isolation, rate measurement of lipolysis and d ‐[U‐¹⁴C]‐glucose incorporation into lipids, glucose‐6‐phosphate dehydrogenase (G6PDH) and malic enzyme activity evaluation, and determination of G6PDH and leptin mRNA expression. Results: After 6 weeks, HS diet significantly increased BP; SC, PE, and RP WAT masses; PE adipocyte size; plasma leptin concentration; G6PDH activity in SC WAT; and PE depots and malic activity only in SC WAT. The leptin levels correlated positively with WAT masses and adipocyte size. An increase in the basal and isoproterenol‐stimulated lipolysis and in the ability to incorporate glucose into lipids was observed in isolated adipocytes from HS rats. Discussion: HS diet induced higher adiposity characterized by high plasma leptin concentration and adipocyte hypertrophy, probably due to an increased lipogenic capacity of WAT.     

Depot-specific effects of the PPAR�� agonist rosiglitazone on adipose tissue glucose uptake and metabolism

We investigated mechanisms whereby peroxisome proliferator-activated receptor γ (PPARγ) agonism redistributes lipid from visceral (VF) toward subcutaneous fat (SF) by studying the impact of PPARγ activation on VF and SF glucose uptake and metabolism, lipogenesis, and enzymes involved in triacylglycerol (TAG) synthesis. VF (retroperitoneal) and SF (inguinal) of rats treated or not for 7 days with rosiglitazone (15 mg/kg/day) were evaluated in vivo for glucose uptake and lipogenesis and in vitro for glucose metabolism, gene expression, and activities of glycerolphosphate acyltransferase (GPAT), phosphatidate phosphatase-1 (or lipin-1), and diacylglycerol acyltransferase. Rosiglitazone increased SF glucose uptake, GLUT4 mRNA, and insulin-stimulated glucose oxidation, conversion to lactate, glycogen, and the glycerol and fatty acid components of TAG. In VF, only glucose incorporation into TAG-glycerol was stimulated by rosiglitazone and less so than in SF (1.5- vs. 3-fold). mRNA levels of proteins involved in glycolysis, Krebs cycle, glycogen synthesis, and lipogenesis were markedly upregulated by rosiglitazone in SF and again less so in VF. Rosiglitazone activated TAG-glycerol synthesis in vivo (2.8- vs. 1.9-fold) and lipin activity (4.6- vs. 1.5-fold) more strongly in SF than VF, whereas GPAT activity was increased similarly in both depots. The preferential increase in glucose uptake and intracellular metabolism in SF contributes to the PPARγ-mediated redistribution of TAG from VF to SF, which in turn favors global insulin sensitization.     

Dietary sucrose is essential to the development of liver injury in the methionine-choline-deficient model of steatohepatitis

Methionine-choline-deficient (MCD) diets cause steatohepatitis in rodents and are used to study the pathophysiology of fatty liver disease in human beings. The most widely used commercial MCD formulas not only lack methionine and choline but also contain excess sucrose and fat. The objective of this study was to determine whether dietary sucrose in the MCD formula plays a role in the pathogenesis of MCD-related liver disease. We prepared two custom MCD formulas, one containing sucrose as the principal carbohydrate and the other substituting sucrose with starch. Mice fed the sucrose-enriched formula developed typical features of MCD-related liver disease, including hepatic steatosis, hepatocellular apoptosis, alanine aminotransferase elevation, lipid peroxidation, and hepatic inflammation. In contrast, mice fed MCD-starch were significantly protected against liver injury. MCD-sucrose and MCD-starch mice displayed identical diet-related abnormalities in hepatic fatty acid uptake and triglyceride secretion. Hepatic de novo lipogenesis and triglyceride synthesis, however, were 2 times higher in MCD-sucrose mice than MCD-starch mice (P < 0.01). Hepatic lipid analysis revealed accumulation of excess saturated fatty acids in MCD-sucrose mice that correlated with hepatocellular injury. Overall, the results indicate that dietary sucrose is critical to the pathogenesis of MCD-mediated steatohepatitis. They suggest that saturated fatty acids, which are products of de novo lipogenesis, are mediators of hepatic toxicity in this model of liver disease.     

Fat metabolism is regulated by altered gene expression of lipogenic enzymes and regulatory factors in liver and adipose tissue but not in semimembranosus muscle of pigs during the fattening period

It has been shown previously that lipid metabolism is regulated by fatty acids (FA) and that thyroid hormones are important regulators of energy metabolism. The effects of weight, dietary fat level and dietary FA profile on thyroid hormone levels and expression of lipogenic genes and tissue FA composition were studied. Sixty-one crossbred gilts weighing 62 ± 5.2 kg BW average were either slaughtered at the beginning of the trial (n = 5) or fed one of seven diets (n = 8 pigs per diet): a semi-synthetic diet formulated to contain a very low level of fat (NF) and six diets based on barley-soybean meal supplemented with approximately 10% fat of different origin and slaughtered at 100 kg BW. The supplemental fats were tallow, high-oleic sunflower oil, sunflower oil (SFO), linseed oil, fat blend (55% tallow, 35% sunflower oil, 10% linseed oil) and fish oil blend (40% fish oil, 60% linseed oil). In general, the dietary FA profiles altered the FA composition of liver, semimembranosus muscle and adipose tissues. Pigs fed the NF diet had the highest free and total triiodothyronine (T3) values followed by pigs fed SFO. Total T3 levels were higher in pigs at 60 kg than in pigs at 100 kg. Correlations between thyroid hormones and genes encoding enzymes of fat synthesis in adipose tissue (acetyl CoA carboxylase (ACACA), fatty acid synthase and stearoyl CoA desaturase (SCD)) and the large differences in expression of lipogenic genes at different weights (60 and 100 kg BW), suggest a role for thyroid hormones and for T3, in particular, in regulating whole animal fat metabolism, with effects brought about by altered expression of lipogenic genes. Liver sterol receptor element binding protein-1 (SREBP1) mRNA content was affected by dietary treatment (P < 0.001) and was correlated with ACACA and SCD, whereas adipose tissue SREBP1 was not correlated with the mRNA abundance of any lipogenic enzyme. Weight and tissue factors showed greater influence on mRNA abundance of genes related with lipid metabolism than diet and tissue FA composition. In the pig, FA synthesis appear to be of greater magnitude in adipose tissue than in the liver as suggested by the higher expression of lipogenic genes in adipose tissue.     

Acetoacetate metabolism in AS-30D hepatoma cells

Metabolic characteristics of experimental hepatoma cells include elevated rates of glycolysis and lipid synthesis. However, pyruvate derived from glucose is not redily oxidized, and the source of acetly CoA for lipid synthesis in As-39D cells has not been characterized. In this study ketone bodies were examined as a possible source of acetyl CoA in AS-30D hepatoma cells.The major findings were:

Integration of lipid metabolism in the mammary gland and adipose tissue by prolactin during lactation

Prolactin deficiency, induced by bromocryptine treatment, brought about reciprocal changes in the ability of adipocytes and acini isolated from lactating rats to synthesize lipids. The capacity to synthesize fatty acids and phospholipids decreased in the mammary gland and increased in adipocytes by bromocryptine treatment. In the mammary gland, the maximum potential activity of the pentose shunt as well as the specific activities of the pathway dehydrogenases were significantly reduced by bromocryptine treatment. Simultaneously, adipose tissue increased its lipogenic capacity but neither the maximum potential of the shunt nor the specific activities of the pentose phosphate shunt dehydrogenases were significantly changed with respect to the control lactating rats. Thus, a differential regulatory mechanism(s) of the pentose phosphate shunt activity appears to operate in these two tissues. Adipocytes from lactating rats showed a poor responsiveness to insulin in terms of lipid synthesis from glucose. In contrast, in adipocytes from bromocryptine treated rats insulin was able to increase lipid synthesis (105%). Sheep prolactin administration in vivo partially reversed the effects of bromocryptine. These data suggest that prolactin mediates adipocytes resistance to insulin during lactation. Phospholipid synthesis, as occurred in fatty acid synthesis, is increased in adipose tissue and decreased in mammary gland by bromocryptine treatment. However, -adrenergic stimulation increases phosphatidylinositol turnover to about the same percentages in both mammary gland acini and adipocytes from lactating rats independently of bromocryptine treatment.     

Thematic review series: glycerolipids. Mammalian glycerol-3-phosphate acyltransferases: new genes for an old activity

Glycerol-3-phosphate acyltransferases (GPATs; EC2.3.1.15) catalyze the first step in the de novo synthesis of neutral lipids (triglycerides) and glycerophospholipids. The existence of multiple enzyme isoforms with GPAT activity was predicted many years ago when GPAT activities with distinct kinetic profiles and sensitivity to inhibitors were characterized in two subcellular compartments, mitochondria and microsomes. We now know that mammals have at least four GPAT isoforms with distinct tissue distribution and function. GPAT1 is the major mitochondrial GPAT isoform and is characterized by its resistance to sulfhydryl-modifying reagents, such as N-ethylmaleimide (NEM). GPAT2 is a minor NEM-sensitive mitochondrial isoform. The activity referred to as microsomal GPAT is encoded by two closely related genes, GPAT3 and GPAT4. GPAT isoforms are important regulators of cellular triglyceride and phospholipid content, and may channel fatty acids toward particular metabolic fates. Overexpression and knock-out studies suggest that GPAT isoforms can play important roles in the development of hepatic steatosis, insulin resistance, and obesity; GPAT isoforms are also important for lactation. This review summarizes the current state of knowledge on mammalian GPAT isoforms.     

‘Why do tumour cells glycolyse?’: From glycolysis through citrate to lipogenesis

The re-emergence of interest in intermediary metabolism and the development of metabolomics in relation to cancer and other diseases provide a timely reason to revisit issues of tumour cell metabolism. In this review, we address the issue of the role of high aerobic glycolysis, which is commonly associated with the metabolism of many tumour cells. The concept presented emphasises the importance of the glycolysis-citrate-lipogenesis pathway in providing the synthetic and bioenergetic requirements that are essential for the growth and proliferation of tumour cells. We hope that our discussion will be informative and instructive, and will stimulate interest and research regarding the intermediary metabolism and its regulation in tumour cells. We express our appreciation to the many pioneering and contemporary researchers whose studies provide much of the basis for this presentation. (Mol Cell Biochem xxx: 1–8, 2005)     

Regulation of SREBP-1 expression and transcriptional action on HKII and FAS genes during fasting and refeeding in rat tissues

The sterol regulatory element binding protein 1 (SREBP-1) is regarded as a major factor involved in the nutritional regulation of lipogenesis. The aim of the present work was to demonstrate its involvement in the response of key genes of glucose and lipid metabolism in liver, adipose tissue, and skeletal muscle during fasting and refeeding. The regulation of hexokinase-2 (HKII) was investigated as a marker of the glucose metabolic pathway and that of FAS was investigated as a marker of the lipogenic pathway. The in vivo association of SREBP-1 with the promoter regions of these genes was determined in the different tissues using chromatin immunoprecipitation assays. Fasting decreased, and refeeding restored, FAS and HKII mRNA and protein levels in each tissue. The concomitant measurement of SREBP-1a and SREBP-1c mRNA levels, of mature SREBP-1 protein abundance in nuclear extracts, and of SREBP-1 interaction with target promoters led to the conclusion that SREBP-1 plays a major role in the response of FAS and HKII genes to nutritional regulation in rodents. These data elucidate the important role of SREBP-1 not only in the regulation of lipid metabolism but also of glucose metabolism and energy homeostasis.