Abnormal lipid metabolism in a rat model of arthritis: one possible pathway

Collagen-induced arthritis (CIA) animal model is associated with systemic manifestations, including alteration of lipid metabolism. In the present study, one possible pathway of altered lipid metabolism is proposed. Specimens of joint tissue and plasma were collected from the CIA and control rats, and quantitative analysis of lipid components was performed by nuclear magnetic resonance (NMR) spectroscopy technique. Correlation analysis was performed between the level of lipid components and antioxidant enzymes, lactate dehydrogenase (LDH), lipid peroxidation (LP), and cytokines in joint tissue and plasma. Differentiation between the CIA and control rats was established on the basis of the quantity of lipid components in the joint tissue and plasma. Positive correlation was observed for all the enzymes vs. lipid components as well as LP vs. lipid components in plasma and joint tissue. Positive correlation was observed for enzymes in plasma and joint tissue. A negative correlation was observed in between the plasma and joint tissue with the level of lipid components. Cytokine levels were also correlated with the level of lipid components and ratios of saturated fatty acids/unsaturated fatty acids in plasma and joint tissue. Inflammatory disease activity in CIA rats with synovitis brought about a significant change in lipid metabolism. Taken together, the results of our study are delineating a possible pathway of altered lipid metabolism in the CIA rat model, thereby contributing further to an understanding of the pathophysiology of rheumatoid arthritis (RA).     

Altered renal lipid metabolism and renal lipid accumulation in human diabetic nephropathy

Animal models link ectopic lipid accumulation to renal dysfunction, but whether this process occurs in the human kidney is uncertain. To this end, we investigated whether altered renal TG and cholesterol metabolism results in lipid accumulation in human diabetic nephropathy (DN). Lipid staining and the expression of lipid metabolism genes were studied in kidney biopsies of patients with diagnosed DN (n = 34), and compared with normal kidneys (n = 12). We observed heavy lipid deposition and increased intracellular lipid droplets. Lipid deposition was associated with dysregulation of lipid metabolism genes. Fatty acid β-oxidation pathways including PPAR-α, carnitine palmitoyltransferase 1, acyl-CoA oxidase, and L-FABP were downregulated. Downregulation of renal lipoprotein lipase, which hydrolyzes circulating TGs, was associated with increased expression of angiopoietin-like protein 4. Cholesterol uptake receptor expression, including LDL receptors, oxidized LDL receptors, and acetylated LDL receptors, was significantly increased, while there was downregulation of genes effecting cholesterol efflux, including ABCA1, ABCG1, and apoE. There was a highly significant correlation between glomerular filtration rate, inflammation, and lipid metabolism genes, supporting a possible role of abnormal lipid metabolism in the pathogenesis of DN. These data suggest that renal lipid metabolism may serve as a target for specific therapies aimed at slowing the progression of glomerulosclerosis.     

Altered oxidative metabolism in selenium-deficient rat granulocytes

Rats fed a selenium-deficient diet for 12 to 15 wk became selenium-depleted, measured by the selenium content of liver and granulocytes. The activity of granulocyte glutathione peroxidase, a selenoenzyme, in deficient rats was 11% of the activity in replete rat granulocytes. When stimulated with an H2O2 generating system, the HMPS activity of the deficient granulocytes was 50% of replete; however, when stimulated with methylene blue, the HMPS activity in deficient and replete granulocytes was the same. When granulocytes were incubated with PMA or OPZ, deficient granulocytes initially had the same O-2-generating activity as replete granulocytes; however, with increasing duration of stimulation, granulocytes from deficient rats generated less O-2 than replete rats. After 20 min in an H2O2-generating system, deficient granulocytes stimulated with PMA or OPZ generated less O-2 than replete granulocytes. These results indicate that deficient granulocytes did not metabolize H2O2 as well as replete granulocytes and that H2O2 caused damage to the O2-generating system. Measurement of O-2 generation in membrane-enriched particles showed the above effects were due to inactivation of the NADPH-dependent O2-generating system. Deficient granulocytes stimulated with OPZ for 20 min had 70% less membrane O-2-generating activity than controls. In addition, when membrane-enriched particles were made from cells that had been stressed with an H2O2-generating system, NADPH-dependent O-2-generating activity in deficient granulocytes was 50% of replete. In selenium-deficient granulocytes with low GSH-Px activity, prolonged incubation with stimulants and prior incubations with an H2O2-generating system caused loss of activity of the membrane-bound, NADPH-dependent, O-2-generating system.     

Altered mitochondrial apoptotic pathway in placentas from undernourished rat gestations

Maternal undernutrition (MUN) during pregnancy results in intrauterine growth-restricted (IUGR) fetuses and small placentas. Although reduced fetal nutrient supply has been presumed to be etiologic in IUGR, MUN-induced placental dysfunction may occur prior to detectable fetal growth restriction. Placental growth impairment may result from apoptosis signaled by mitochondria in response to reduced energy substrate. Therefore, we sought to determine the presence of mitochondrial-induced apoptosis under MUN and ad libitum diet (AdLib) pregnancies. Pregnant rats were fed an AdLib or a 50% MUN diet from embryonic day 10 (E10) to E20. At E20, fetuses and placentas from proximal- and mid-horns (extremes of nutrient/oxygen supply) were collected. Right-horn placentas were used to quantify apoptosis. Corresponding left-horn placentas were separated into basal (hormone production) and labyrinth (feto-maternal exchange) zones, and protein expression of the mitochondrial pathway was determined. Our results show that the MUN placentas had significantly increased apoptosis, with lower expression of cytosolic and mitochondrial anti-apoptotic Bcl2 and Bcl-X(L), and significantly higher expression of pro-apoptotic Bax and Bak especially in the labyrinth zone. This was paralleled by higher coimmunostaining with the mitochondrial marker manganese superoxide dismutase (MnSOD), indicating transition of pro-apoptotic factors to the mitochondrial membrane. Also, cytosolic cytochrome c and activated caspases-9 and -3 were significantly higher in all MUN. Conversely, peroxisome proliferator-activator receptor-γ (PPARγ), a member of the nuclear receptor family with anti-apoptotic properties, was significantly downregulated in both zones and horns. Our results suggest that MUN during rat pregnancy enhances mitochondria-dependent apoptosis in the placenta, probably due to the downregulation of PPARγ expression.     

In vitro lipid metabolism in the rat pancreas. I. Basal lipid metabolism.

1. The in vitro basal lipid metabolism of rat pancreatic fragments was compared with that in adipose tissue fragments and liver slices. 2. [1-14C]Acetate added to the media was mostly incorporated into palmitic acid and to a lesser extent into oleic acid. In addition, pancreatic tissue exhibited a marked capacity for elongation of polyunsaturated fatty acids by [1-14C]acetate and resulting desaturation when compared to adipose tissue and liver. 3. Data obtained in the presence of [U-14C]glucose, [1-14C]palmitate and 3H20 indicate that acetyl-CoA derived from glucose and from beta-oxidation of fatty acids contributed to de novo lipogenesis. 4. Oxidation of [1-14C]palmitic acid was 9-13 times higher in the pancreas than in adipose tissue or liver when expressed on a wet weight basis. 5. The fatty acid moiety of pancreatic glycerolipids could be derived from de novo synthesis, fatty acids added to the medium, or from fatty acids formed from the hydrolysis of endogenous lipids. The glycerol moiety could be derived either from glucose, or directly from glycerol through participation of glycerol kinase.     

Alterations in rat lipid metabolism following ecdysterone treatment

The influence of ecdysterone on the lipid metabolism in liver and adipose tissue from rat was investigated using 14C-acetate and 32P-orthophosphate as precursors. Ecdysterone produced an increase in 14C-acetate incorporation into triglycerides. A concomitant decrease in free fatty acids and diglycerides was observed. The effect of ecdysterone on triglyceride lipase activity was investigated and a significant decrease was found. Ecdysterone produced a significant increase in the specific activity of phosphatidylethanolamine and phosphatidylserine in liver. On the contrary, the specific activity of phosphatidylcholine was reduced. In adipose tissue, the most evident effect observed was the increase of specific activity of phosphatidylcholine. These results contribute to knowledge of the heterophylic action of ecdysterone.     

Abnormal hepatobiliary and circulating lipid metabolism in the Long-Evans Cinnamon rat model of Wilson's disease

Long-Evans Cinnamon (LEC) rats exhibit a genetic defect in Atp7b gene, which is homologous to the human Wilson's disease gene, resulting in an inability to mobilize copper from the liver. This study was undertaken to gain insight into the relationship between liver copper accumulation and plasma lipid profile, circulating lipoprotein composition, hepatic sterol metabolism and biliary lipid secretion rates in 12-week-old LEC rats compared to control Long-Evans rats. Concomitant with hepatic copper deposition, LEC rats displayed increased content of triglycerides (TGs), free cholesterol (FC) and cholesteryl ester (CE) in the liver. Hepatic concentrations of malondialdehyde (MDA), an index of lipid peroxidation were also significantly elevated in LEC rats (50%). This steatosis was associated with aberrant microsomal apolipoprotein (apo) B-100 and microsomal triglyceride transfer protein (MTP) content, hypotriglyceridemia, hypocholesterolemia and abnormalities in both circulating lipoprotein composition and size. Atypical hepatobiliary sterol metabolism was established by the assessment of the activity of key intracellular enzymes for cholesterol homeostasis, which demonstrated, with respect to controls, a 40% reduction in 3-hydroxy-3-methylglutaryl coenzyme A reductase, a 30% reduction in cholesterol 7alpha-hydroxylase, and a 54% reduction in acyl CoA:cholesterol acyltransferase. During a 6-h biliary drainage, a decline in the bile acid output was recorded and might be linked to the low protein expression of the bile salt export pump (BSEP or ABCB11). Our data emphasize the crucial role of copper balance in hepatic sterol homeostasis and lipoprotein metabolism in LEC rats. Additional studies are needed to delineate the mechanisms of these disorders.     

Altered expression of genes involved in lipid metabolism in obese subjects with unfavourable phenotype

Obesity (BMI ≥30 kg/m²) increases the risk of developing lifestyle-related diseases. A subgroup of obese individuals has been described as “metabolically healthy, but obese” (MHO). In contrast to at-risk obese (ARO), the MHO phenotype is defined by a favourable lipid profile and a normal or only slightly affected insulin sensitivity, despite the same amount of body fat. The objective was to characterize the metabolic phenotype of MHO subjects. We screened a variety of genes involved in lipid metabolism and inflammation in peripheral blood mononuclear cells (PBMC). Obese subjects (men and women; 18–70 years) with BMI ≥30 kg/m² were characterized as MHO (n = 9) or as ARO (n = 10). In addition, eleven healthy, normal weight subjects characterized as healthy by the same criteria as described for the MHO subjects were included. We found that with similar weight, total fat mass and fat mass distribution, the ARO subjects have increased plasma levels of gamma-glutamyl transpeptidase and free fatty acids. This group also has altered expression levels of a number of genes linked to lipid metabolism in PBMC with reduced gene expression levels of uncoupling protein 2, hormone-sensitive lipase and peroxisome proliferator-activated receptor δ compared with MHO subjects. The present metabolic differences between subgroups of obese subjects may contribute to explain some of the underlying mechanisms causing the increased risk of disease among ARO subjects compared with MHO subjects.     

Altered metabolism in cancer

Cancer cells have different metabolic requirements from their normal counterparts. Understanding the consequences of this differential metabolism requires a detailed understanding of glucose metabolism and its relation to energy production in cancer cells. A recent study in BMC Systems Biology by Vasquez et al. developed a mathematical model to assess some features of this altered metabolism. Here, we take a broader look at the regulation of energy metabolism in cancer cells, considering their anabolic as well as catabolic needs.     

Effect of Lipoprotein-X on lipid metabolism in rat kidney

Lipoprotein-X (Lp-X) is found in the plasma of patients with familiallecithin: cholesterol acyltransferase (LCAT) deficiency syndromes. Themajority of the patients with this disorder develop progressiveglomerulosclerosis. In this study, the effect of Lp-X on lipid metabolism inperfused rat kidney was investigated. Lp-X was isolated from plasma ofpatients with familial LCAT deficiency by sequential ultracentrifugation andgel filtration column chromatography. Rat kidneys were perfused for 1-2 hwith Krebs-Henseleit buffer containing 20 µM [1-14C]acetate or 20µM [Me-3H]choline. In the presence of Lp-X, no significant differencein the incorporation of radioactivity into triglycerides, cholesterol,phosphocholine, CDP-choline and sphingomyelin was observed. However,incorporation of radioactivity into cholesteryl esters andphosphatidylcholine was significantly elevated in Lp-X perfused kidneys. Thecontents of cholesterol, cholesteryl esters and phosphatidylcholine werealso significantly increased in Lp-X perfused kidneys. The increase in lipidcontent in the Lp-X perfused kidney is attributed to the direct depositionof Lp-X lipids into the organ. The increase in the labelling of cholesterylesters was attributed to the increase of available substrate (cholesterol)for the acyl-CoA:cholesterol acyltransferase (ACAT) reaction. The increasein phosphatidylcholine labelling was caused by a reduced turnover of thenewly synthesized labelled phosphatidylcholine during Lp-X perfusion.     

Effects of dexamethasone on lipid metabolism in rat organs

Injecting of dexamethasone (10 mg/kg body weight) for 8 days to rats decreased the body weight and feed intake by 29 and 50%, respectively. The increase in weights of liver, heart, kidneys and testes per 100 g body weight was 55, 37, 33 and 13%, respectively. Though, in general, the triglyceride content increased in all the organs, maximum increase (9-fold) was observed in the liver. The plasma showed elevated levels of triglycerides, cholesterol and phospholipids. In hepatic mitochondrial membranes, the content of protein, phospholipids and cholesterol decreased/g tissue. The percent 14C distribution, as a part of total incorporation in nonpolar lipids, of [14C]acetate into triglycerides of liver, kidneys and testes increased significantly. The increased turnover of phospholipids in liver and heart was mainly due to increased turnover of phosphatidyl choline (PC) and phosphatidyl ethanolamine in liver and PC in heart. Turnover of phospholipids of testes was not affected.     

Interleukin-1 and lipid metabolism in the rat.

By using [1-14C]butyrate, the fluxes of butyrate to acetate and fatty acids were measured in rat hepatocytes. Both fluxes were inhibited to a similar extent by (-)-hydroxycitrate, with no significant effect on butyrate uptake. These results indicate that acetate formation takes place in the cytoplasm, presumably via ATP-stimulated acetyl-CoA hydrolase. Since acetate formation occurred despite a net uptake of acetate, the results are also consistent with the operation of a substrate cycle between acetate and acetyl-CoA, recently proposed by other workers, and suggest that this cycle is cytoplasmic.     

Autophagy-lysosomal pathway is involved in lipid degradation in rat liver

We present data supporting the hypothesis that the lysosomal-autophagy pathway is involved in the degradation of intracellular triacylglycerols in the liver. In primary hepatocytes cultivated in the absence of exogenous fatty acids (FFA), both inhibition of autophagy flux (asparagine) or lysosomal activity (chloroquine) decreased secretion of VLDL (very low density lipoproteins) and formation of FFA oxidative products while the stimulation of autophagy by rapamycine increased some of these parameters. Effect of rapamycine was completely abolished by inactivation of lysosomes. Similarly, when autophagic activity was influenced by cultivating the hepatocytes in "starving" (amino-acid poor medium) or "fed" (serum-supplemented medium) conditions, VLDL secretion and FFA oxidation mirrored the changes in autophagy being higher in starvation and lower in fed state. Autophagy inhibition as well as lysosomal inactivation depressed FFA and DAG (diacylglycerol) formation in liver slices in vitro. In vivo, intensity of lysosomal lipid degradation depends on the formation of autophagolysosomes, i.e. structures bringing the substrate for degradation and lysosomal enzymes into contact. We demonstrated that lysosomal lipase (LAL) activity in liver autophagolysosomal fraction was up-regulated in fasting and down-regulated in fed state together with the increased translocation of LAL and LAMP2 proteins from lysosomal pool to this fraction. Changes in autophagy intensity (LC3-II/LC3-I ratio) followed a similar pattern.     

A proposed model of fat packaging by exchangeable lipid droplet proteins

Humans have evolved mechanisms of efficient fat storage to survive famine, but these mechanisms contribute to obesity in our current environment of plentiful food and reduced activity. Little is known about how animals package fat within cells. Five related structural proteins serve roles in packaging fat into lipid droplets. The proteins TIP47, S3-12, and OXPAT/MLDP/PAT-1 move from the cytosol to coat nascent lipid droplets during rapid fat storage. In contrast, perilipin and adipophilin constitutively associate with lipid droplets and play roles in sustained fat storage and regulation of lipolysis. Different tissues express different complements of these lipid droplet proteins. Thus, the tissue-specific complement of these proteins determines how tissues manage lipid stores.     

The pathway of glutamate metabolism in rat brain mitochondria

1. The pathway of glutamate metabolism in non-synaptic rat brain mitochondria was investigated by measuring glutamate, aspartate and ammonia concentrations and oxygen uptakes in mitochondria metabolizing glutamate or glutamine under various conditions. 2. Brain mitochondria metabolizing 10mm-glutamate in the absence of malate produce aspartate at 15nmol/min per mg of protein, but no detectable ammonia. If amino-oxyacetate is added, the aspartate production is decreased by 80% and ammonia production is now observed at a rate of 6.3nmol/min per mg of protein. 3. Brain mitochondria metabolizing glutamate at various concentrations (0-10mm) in the presence of 2.5mm-malate produce aspartate at rates that are almost stoicheiometric with glutamate disappearance, with no detectable ammonia production. In the presence of amino-oxyacetate, although the rate of aspartate production is decreased by 75%, ammonia production is only just detectable (0.3nmol/min per mg of protein). 4. Brain mitochondria metabolizing 10mm-glutamine and 2.5mm-malate in States 3 and 4 were studied by using glutamine as a source of intramitochondrial glutamate without the involvement of mitochondrial translocases. The ammonia production due to the oxidative deamination of glutamate produced from the glutamine was estimated as 1nmol/min per mg of protein in State 3 and 3nmol/min per mg of protein in State 4. 5. Brain mitochondria metabolizing 10mm-glutamine in the presence of 1mm-amino-oxyacetate under State-3 conditions in the presence or absence of 2.5mm-malate showed no detectable aspartate production. In both cases, however, over the first 5min, ammonia production from the oxidative deamination of glutamate was 21-27nmol/min per mg of protein, but then decreased to approx. 1-1.5nmol/min per mg. 6. It is concluded that the oxidative deamination of glutamate by glutamate dehydrogenase is not a major route of metabolism of glutamate from either exogenous or endogenous (glutamine) sources in rat brain mitochondria.     

Transaminative pathway of cysteine metabolism in rat tissues.

Enzyme activities of the transaminative pathway of cysteine metabolism in various rat tissues were examined. Liver was found the most active tissue, followed by kidney and heart. Liver and kidney were more pronounced in mercaptopyruvate sulfurtransferase activity than in cysteine transaminase activity; heart was more active in the latter. Red blood cells, which have pronounced sulfurtransferase activity, exhibited no transaminase activity, indicating the pathway is negligible in this tissue.