Quantitative studies of the metabolism of chylomicron triglycerides and cholesterol by liver and extrahepatic tissues of sheep and dogs

Animal models of gestational diabetes: characteristics and consequences to the brain and behavior of the offspring

Gestational diabetes (GD) is the glucose intolerance that occurs during pregnancy. Mothers who develop diabetes during gestation are at increased risk of developing type 2 diabetes mellitus (T2DM) later in life, and the risk of adverse fetal and neonatal outcomes are also increased as a function of maternal hyperglycemia. Infants who are exposed to fetal hyperglycemia show an increased risk of becoming obese and developing T2DM later in life. Due to the need of new research on this field, and the difficulty of performing studies in human brain, studies using experimental models are necessary to suggest possible ways to avoid or inhibit offspring brain damage or harmful metabolic alterations. Here, it was made a review about the characteristics of the main animal models of GD, and what are the consequences to the brain and behavior of the offspring. In many experimental models, either by pharmacological induction, diet manipulation, or in the use of transgenic animals, glycemic conditions are severe. S961, a selective insulin receptor antagonist, revealed an increased fasting blood glucose level and glucose intolerance during mid-gestation, which returned to basal levels postpartum in mice. GD contributes to offspring neuroinflammation, influences neuronal distribution in central nervous system (CNS), and apoptosis during embryogenesis, which in turn may contribute to changes in behavior and memory in adult life and aging. The usage of animal models to study GD allows to examine extensively the characteristics of this condition, the molecular mechanisms involved and the consequences to the brain and behavior of the offspring.

Advances in fatty acids nutrition in dairy cows: from gut to cells and effects on performance

High producing dairy cows generally receive in the diet up to 5-6% of fat. This is a relatively low amount of fat in the diet compared to diets in monogastrics; however, dietary fat is important for dairy cows as demonstrated by the benefits of supplementing cows with various fatty acids (FA). Several FA are highly bioactive, especially by affecting the transcriptome; thus, they have nutrigenomic effects. In the present review, we provide an up-to-date understanding of the utilization of FA by dairy cows including the main processes affecting FA in the rumen, molecular aspects of the absorption of FA by the gut, synthesis, secretion, and utilization of chylomicrons; uptake and metabolism of FA by peripheral tissues, with a main emphasis on the liver, and main transcription factors regulated by FA. Most of the advances in FA utilization by rumen microorganisms and intestinal absorption of FA in dairy cows were made before the end of the last century with little information generated afterwards. However, large advances on the molecular aspects of intestinal absorption and cellular uptake of FA were made on monogastric species in the last 20 years. We provide a model of FA utilization in dairy cows by using information generated in monogastrics and enriching it with data produced in dairy cows. We also reviewed the latest studies on the effects of dietary FA on milk yield, milk fatty acid composition, reproduction, and health in dairy cows. The reviewed data revealed a complex picture with the FA being active in each step of the way, starting from influencing rumen microbiota, regulating intestinal absorption, and affecting cellular uptake and utilization by peripheral tissues, making prediction on in vivo nutrigenomic effects of FA challenging.

Hepatic transcriptomic adaptation from prepartum to postpartum in dairy cows

The transition from pregnancy to lactation is the most challenging period for high-producing dairy cows. The liver plays a key role in biological adaptation during the peripartum. Prior works have demonstrated that hepatic glucose synthesis, cholesterol metabolism, lipogenesis, and inflammatory response are increased or activated during the peripartum in dairy cows; however, those works were limited by a low number of animals used or by the use of microarray technology, or both. To overcome such limitations, an RNA sequencing analysis was performed on liver biopsies from 20 Holstein cows at 7 ± 5d before (Pre-P) and 16 ± 2d after calving (Post-P). We found 1,475 upregulated and 1,199 downregulated differently expressed genes (DEG) with a false discovery rate adjusted P-value < 0.01 between Pre-P and Post-P. Bioinformatic analysis revealed an activation of the metabolism, especially lipid, glucose, and amino acid metabolism, with increased importance of the mitochondria and a key role of several signaling pathways, chiefly peroxisome proliferators-activated receptor (PPAR) and adipocytokines signaling. Fatty acid oxidation and gluconeogenesis, with a likely increase in amino acid utilization to produce glucose, were among the most important functions revealed by the transcriptomic adaptation to lactation in the liver. Although gluconeogenesis was induced, data indicated decrease in expression of glucose transporters. The analysis also revealed high activation of cell proliferation but inhibition of xenobiotic metabolism, likely due to the liver response to inflammatory-like conditions. Co-expression network analysis disclosed a tight connection and coordination among genes driving biological processes associated with protein synthesis, energy and lipid metabolism, and cell proliferation. Our data confirmed the importance of metabolic adaptation to lipid and glucose metabolism in the liver of early Post-P cows, with a pivotal role of PPAR and adipocytokines.

Impact of liver fat on the differential partitioning of hepatic triacylglycerol into VLDL subclasses on high and low sugar diets

Dietary sugars are linked to the development of non-alcoholic fatty liver disease (NAFLD) and dyslipidaemia, but it is unknown if NAFLD itself influences the effects of sugars on plasma lipoproteins. To study this further, men with NAFLD (n = 11) and low liver fat ‘controls’ (n = 14) were fed two iso-energetic diets, high or low in sugars (26% or 6% total energy) for 12 weeks, in a randomised, cross-over design. Fasting plasma lipid and lipoprotein kinetics were measured after each diet by stable isotope trace-labelling.

There were significant differences in the production and catabolic rates of VLDL subclasses between men with NAFLD and controls, in response to the high and low sugar diets. Men with NAFLD had higher plasma concentrations of VLDL₁-triacylglycerol (TAG) after the high (P<0.02) and low sugar (P<0.0002) diets, a lower VLDL₁-TAG fractional catabolic rate after the high sugar diet (P<0.01), and a higher VLDL₁-TAG production rate after the low sugar diet (P<0.01), relative to controls. An effect of the high sugar diet, was to channel hepatic TAG into a higher production of VLDL₁-TAG (P<0.02) in the controls, but in contrast, a higher production of VLDL₂-TAG (P<0.05) in NAFLD. These dietary effects on VLDL subclass kinetics could be explained, in part, by differences in the contribution of fatty acids from intra-hepatic stores, and de novo lipogenesis. The present study provides new evidence that liver fat accumulation leads to a differential partitioning of hepatic TAG into large and small VLDL subclasses, in response to high and low intakes of sugars.

Increasing palmitic acid intake enhances milk production and prevents glucose-stimulated fatty acid disappearance without modifying systemic glucose tolerance in mid-lactation dairy cows

Feeding saturated fatty acids may enhance milk yield in part by decreasing insulin sensitivity and shifting glucose utilization toward the mammary gland. Our objective was to evaluate the effects of palmitic acid (C16:0) on milk production and insulin sensitivity in cows. Twenty multiparous mid-lactation Holstein cows were enrolled in a study consisting of a 5-d covariate, 49-d treatment, and 14-d posttreatment period. All cows received a common sorghum silage-based diet and were randomly assigned to a diet containing no supplemental fat (control; n=10; 138±45d in milk) or C16:0 at 4% of ration DM (PALM; 98% C16:0; n=10; 136±44d in milk). Blood and milk were collected at routine intervals. Intravenous glucose tolerance tests (300mg/kg of body weight) were performed at d -1, 24, and 49 relative to start of treatment. Data were analyzed as repeated measures using a mixed model with fixed effects of treatment and time, and milk yield served as a covariate. The PALM treatment increased milk yield by wk 7. Furthermore, PALM increased milk fat yield and energy-corrected milk at wk 3 and 7. Changes in milk production occurred in parallel with enhanced energy intake. Increased milk fat yield during PALM treatment was due to increased C16:0 and C16:1 incorporation; PALM had no effect on concentration of milk components, BW, or body condition score. Two weeks posttreatment, energy-corrected milk and milk fat yield remained elevated in PALM-fed cows whereas yields of milk were similar between treatments. Increased milk fat yield after PALM treatment was due to increased de novo lipogenesis and uptake of preformed fatty acids. The basal concentration of nonesterified fatty acids (NEFA) in plasma increased by d 4, 6, and 8 of PALM treatment, a response not observed thereafter. Although PALM supplementation did not modify insulin, glucose, or triacylglycerol levels in plasma, total cholesterol in plasma was elevated by wk 3. Estimated insulin sensitivity was lower during the first week of PALM treatment; however, glucose disposal following glucose tolerance tests was not modified. In contrast, C16:0 feeding reduced glucose-stimulated NEFA disappearance by wk 7. Results demonstrate that increasing dietary energy from C16:0 for 7wk improves milk yield and milk composition without modifying systemic glucose tolerance. Reduced glucose-stimulated NEFA disappearance with C16:0 feeding and elevated circulating NEFA may reflect changes in adipose tissue insulin sensitivity.

A Pressure-dependent Model for the Regulation of Lipoprotein Lipase by Apolipoprotein C-II

Apolipoprotein C-II (apoC-II) is the co-factor for lipoprotein lipase (LPL) at the surface of triacylglycerol-rich lipoproteins. LPL hydrolyzes triacylglycerol, which increases local surface pressure as surface area decreases and amphipathic products transiently accumulate at the lipoprotein surface. To understand how apoC-II adapts to these pressure changes, we characterized the behavior of apoC-II at multiple lipid/water interfaces. ApoC-II adsorption to a triacylglycerol/water interface resulted in large increases in surface pressure. ApoC-II was exchangeable at this interface and desorbed on interfacial compressions. These compressions increase surface pressure and mimic the action of LPL. Analysis of gradual compressions showed that apoC-II undergoes a two-step desorption, which indicates that lipid-bound apoC-II can exhibit at least two conformations. We characterized apoC-II at phospholipid/triacylglycerol/water interfaces, which more closely mimic lipoprotein surfaces. ApoC-II had a large exclusion pressure, similar to that of apoC-I and apoC-III. However, apoC-II desorbed at retention pressures higher than those seen with the other apoCs. This suggests that it is unlikely that apoC-I and apoC-III inhibit LPL via displacement of apoC-II from the lipoprotein surface. Upon rapid compressions and re-expansions, re-adsorption of apoC-II increased pressure by lower amounts than its initial adsorption. This indicates that apoC-II removed phospholipid from the interface upon desorption. These results suggest that apoC-II regulates the activity of LPL in a pressure-dependent manner. ApoC-II is provided as a component of triacylglycerol-rich lipoproteins and is the co-factor for LPL as pressure increases. Above its retention pressure, apoC-II desorbs and removes phospholipid. This triggers release of LPL from lipoproteins.

Experimental dog model for assessment of fasting and postprandial fatty acid metabolism: pitfalls and feasibility

The dog is a widely-used model for conducting metabolic studies. This is mainly due to its large size and its physiology which is relatively similar to that of humans. Here, we attempted to optimize a postprandial metabolic study protocol used in dogs. Following acclimatization, female mongrel dogs underwent 9 h profiling for time-course baseline plasma data on triglyceride, adrenocorticotropic hormone (ACTH) and cortisol levels. One week later, carotid and jugular catheters were surgically inserted for sampling and infusions. Initial post-operative care, based on the literature (Protocol 1), consisted of analgesia (buprenorphine every 8-12 h and 2-3 doses/day of acepromazine), restriction by Pavlov harness within cages, and a two- to three-day recovery period. Throughout the experiment, dogs received a lipid tracer diluted in 5% bovine serum albumin (BSA). Compared with baseline, animals vomited (n = 6/6) and exhibited high ACTH + cortisol levels (stress biomarkers), resulting in blunted triglyceride peak levels. To avoid these undesirable effects, post-operative care was modified (Protocol 2) as follows: animals (n = 19) were given a single dose of buprenorphine and no acepromazine, were unrestrained and free to move within cages, the recovery period was extended to seven days, and the lipid tracer was diluted in 0.002% versus 5% BSA. Using this modified protocol, postprandial plasma-triglyceride and ACTH/cortisol patterns were similar to baseline values. Controlling for stressors, as well as for factors which may alter proper digestion, is critical for all postprandial metabolic studies. Our results show that an optimized postprandial metabolic protocol used in dogs reduces experimental variability, while improving animal care and comfort.

Maternal obesity or weight loss around conception impacts hepatic fatty acid metabolism in the offspring

OBJECTIVE

To determine the impact of maternal obesity or weight loss during the periconceptional period on programming of lipid metabolism in the liver of the offspring.

METHODS

An embryo transfer model was used to investigate the effects of exposure to either maternal obesity and/or weight loss before and for 1-week post-conception on the abundance of key molecules regulating hepatic fatty acid oxidation and lipid synthesis in the 4-month-old lamb.

RESULTS

Periconceptional maternal obesity resulted in decreased hepatic PPARα, PGC1α and GCN5 abundance and increased hepatic SIRT1 and AMPKα1, AMPKα2 and SREBP1 abundance in the offspring. Maternal weight loss in obese ewes did not ablate all of these effects of maternal obesity on hepatic metabolism in the lamb. Weight loss in normal weight ewes also resulted in decreased hepatic PGC1α and GCN5 and increased AMPKα2 abundance in the offspring.

CONCLUSIONS

Exposure of the oocyte/embryo to either maternal obesity or weight loss during the periconceptional period has long term consequences for hepatic lipid metabolism. These findings highlight the sensitivity of the early embryo to maternal nutrition and the need for dietary interventions which maximize metabolic benefits and minimize metabolic costs for the next generation.

Spillover of Fatty acids during dietary fat storage in type 2 diabetes: relationship to body fat depots and effects of weight loss

Spillover of lipoprotein lipase-generated fatty acids from chylomicrons into the plasma free fatty acid (FFA) pool is an important source of FFA and reflects inefficiency in dietary fat storage. We measured spillover in 13 people with type 2 diabetes using infusions of a [(3)H]triolein-labeled lipid emulsion and [U-(13)C]oleate during continuous feeding, before and after weight loss. Body fat was measured with dual energy X-ray absorptiometry and computed tomography. Participants lost ∼14% of body weight. There was an ∼38% decrease in meal-suppressed FFA concentration (P < 0.0001) and an ∼23% decrease in oleate flux (P = 0.007). Fractional spillover did not change (P = NS). At baseline, there was a strong negative correlation between spillover and leg fat (r = -0.79, P = 0.001) and a positive correlation with the trunk-to-leg fat ratio (R = 0.56, P = 0.047). These correlations disappeared after weight loss. Baseline leg fat (R = -0.61, P = 0.027) but not trunk fat (R = -0.27, P = 0.38) negatively predicted decreases in spillover with weight loss. These results indicate that spillover, a measure of inefficiency in dietary fat storage, is inversely associated with lower body fat in type 2 diabetes.

Biochemistry and pathophysiology of intravascular and intracellular lipolysis

All organisms use fatty acids (FAs) for energy substrates and as precursors for membrane and signaling lipids. The most efficient way to transport and store FAs is in the form of triglycerides (TGs); however, TGs are not capable of traversing biological membranes and therefore need to be cleaved by TG hydrolases ("lipases") before moving in or out of cells. This biochemical process is generally called "lipolysis." Intravascular lipolysis degrades lipoprotein-associated TGs to FAs for their subsequent uptake by parenchymal cells, whereas intracellular lipolysis generates FAs and glycerol for their release (in the case of white adipose tissue) or use by cells (in the case of other tissues). Although the importance of lipolysis has been recognized for decades, many of the key proteins involved in lipolysis have been uncovered only recently. Important new developments include the discovery of glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1), the molecule that moves lipoprotein lipase from the interstitial spaces to the capillary lumen, and the discovery of adipose triglyceride lipase (ATGL) and comparative gene identification-58 (CGI-58) as crucial molecules in the hydrolysis of TGs within cells. This review summarizes current views of lipolysis and highlights the relevance of this process to human disease.

Intravenous niacin acutely improves the efficiency of dietary fat storage in lean and obese humans

Spillover of fatty acids released by lipoprotein lipase hydrolysis of meal triglycerides may be a major contributor to the free fatty acid (FFA) pool. We studied lean (n = 6) and overweight and obese (n = 5) subjects during continuous feeding on two occasions: during intravenous infusion of niacin (2.8 mg/min) and saline. After establishment of steady-state chylomicronemia and suppression of adipose tissue lipolysis with a liquid meal, spillover was measured with infusions of [U-(13)C]oleate and [(3)H]triolein. Total FFA concentrations were lower during niacin infusion in both lean (50 ± 4 vs. 102 ± 7 μmol/L; P < 0.002) and obese (75 ± 6 vs. 143 ± 13 μmol/L; P < 0.01) subjects. Oleate appearance was lower during niacin infusion than during saline infusion in both lean (21 ± 2 vs. 32 ± 5 μmol/min; P = 0.07) and obese subjects (25 ± 3 vs. 46 ± 8 μmol/min; P < 0.02). Spillover was lower during niacin infusion than during saline infusion in lean (21 ± 4 vs. 29 ± 3%) and obese (21 ± 2 vs. 29 ± 5%) subjects (P < 0.03 for both). In summary, during meal absorption, niacin produces additional suppression of lipolysis and a reduction in fractional spillover compared with saline in both normal and obese subjects. Infusion of intravenous niacin provides a model for acutely improving dietary fat storage, perhaps by suppressing lipolysis in visceral adipose tissue.

Triglyceride-rich lipoproteins and plasma lipid transport

This memoir provides a history of the triglyceride-rich lipoproteins of blood plasma over the last half-century. As precursors of low-density lipoproteins and in their own right, triglyceride-rich lipoproteins are essential to the formation of atherosclerotic plaques and to consequent ischemic vascular disease. The author recounts research at the National Heart Institute during 1953 to 1956 and continuing thereafter at the University of California San Francisco. Emphasis is placed on key insights arising from investigations of human disease, the interplay of fatty acid and triglyceride-transport involving the liver, small intestine, adipose tissue and muscle, and the role of the liver in the synthesis and catabolism of atherogenic lipoproteins.

Triglyceride uptake and lipoprotein lipase-generated fatty acid spillover in the splanchnic bed of dogs

The action of lipoprotein lipase on triglyceride-rich lipoproteins generates fatty acids that are either transported into tissues or mix with circulating free fatty acids (FFAs) via a process known as spillover. In the present study, arterial, portal vein, and hepatic vein sampling catheters were surgically placed in nine mongrel dogs. The animals were subsequently studied after a 42-h fast during infusion of [14C]oleate and a lipid emulsion containing [3H]triolein; the emulsion was used as a surrogate for the study of chylomicron metabolism. More than one-half of splanchnic [3H]triglyceride uptake occurred in the liver, and substantial fractional spillover of [3H]oleate was observed in both liver and nonhepatic tissues (approximately 50% each). There was a significant correlation between FFA release from nonhepatic tissues (presumably visceral fat) and nonhepatic fractional spillover (R = 0.81, P < 0.01), consistent with a model in which the rate of intracellular lipolysis influences spillover by determining the direction of net fatty acid flow between the cell and the interstitium. There was a significant correlation between "true" and "net" splanchnic spillover (R = 0.84, P < 0.005), the latter representing calculation of spillover between arterial and hepatic venous blood without portal venous data. Metabolism of chylomicron triglycerides in visceral fat may be an important source of portal venous FFAs.

Increased dietary substrate delivery alters hepatic fatty acid recycling in healthy men

Sources of fatty acids flowing to the liver may be used for triacylglycerol (TAG) synthesis. Our objective was to quantify contributions of nonesterified fatty acids (NEFAs), de novo lipogenesis, and dietary fatty acids to VLDL-TAG in the fed state after meal feeding in healthy subjects (n = 6). The effect of substrate delivery rate was also determined by comparison with data obtained under a continuous-feeding regimen. A liquid diet was administered by mouth or via feeding tube. Contributions of NEFAs, de novo lipogenesis, and dietary fatty acids to VLDL-TAG were quantified using stable isotopes and gas chromatography-mass spectrometry. Contribution of NEFAs to VLDL-TAG was similar under meal feeding and continuous feeding, although insulin area under the curve (AUC) was greater under meal feeding (1,597 +/- 455 vs. 471 +/- 484 pmol . h . l(-1), P < 0.004). Lipogenesis achieved a higher AUC with meal feeding versus continuous feeding (88.7 +/- 84.4 vs. 1.9 +/- 19.3 mumol . h . l(-1), P = 0.03) supporting greater stimulation of de novo lipogenesis from increased glucose delivery rate. The contribution of dietary fatty acids to VLDL-TAG was also greater with meal feeding. These data demonstrate for the first time in humans the well-coordinated use of fatty acids by the liver during the transition from fasted to fed states and highlight the dominant role of NEFAs for VLDL-TAG synthesis in both states.

Triacilglicerol intramuscular: um importante substrato energético para o exercício de endurance

Os ácidos graxos são uma importante fonte de energia para exercício de endurance. Os ácidos graxos plasmáticos encontram-se disponíveis para as fibras musculares sob a forma de ácidos graxos associados à albumina ou agregados à molécula de triacilglicerol (TAG) encontrada nas lipoproteínas. Entretanto, além dessas fontes plasmáticas, a hidrólise do TAG encontrado no músculo também pode contribuir com a oferta de ácidos graxos durante o exercício de endurance. O objetivo do presente trabalho foi realizar uma extensa revisão da literatura sobre a importância do TAG intramuscular como substrato energético. A revisão da literatura sugere que a contribuição dos estoques endógenos de TAG durante a realização do exercício de endurance é bastante relevante. Além disso, pode-se concluir que uma adaptação induzida pelo treinamento de endurance é o aumento dos estoques intramusculares de TAG. Após o treinamento de endurance, também é observado aumento na capacidade de utilização desses estoques. Apesar de parecer importante, a contribuição do TAG intramuscular ainda é motivo de controvérsia na literatura. Essa discrepância de resultados está relacionada às metodologias empregadas para estimar a sua oxidação no exercício. A fim de esgotar este assunto de maneira apropriada, mais pesquisas, com novos métodos (ex.: utilização de isótopos, ressonância magnética nuclear e microscopia eletrônica), precisam ser conduzidas.

Splanchnic metabolism of dairy cows during the transition from late gestation through early lactation

Blood flow and net nutrient fluxes for portal-drained viscera (PDV) and liver (total splanchnic tissues) were measured at 19 and 9 d prepartum and at 11, 21, 33, and 83 d in milk (DIM) in 5 multiparous Holstein-Friesian cows. Cows were fed a grass silage-based gestation ration initially and a corn silage-based lactation ration peripartum and postpartum. Meals were fed at 8-h intervals and hourly (n = 8) measures of splanchnic metabolism were started before (0730 h and 0830 h) feeding at 0830 h. Dry matter intakes (DMI) at 19 and 9 d prepartum were not different. Metabolism changes measured from 19 to 9 d prepartum were lower arterial insulin and acetate, higher arterial nonesterified fatty acids and increased net liver removal of glycerol. After calving, PDV and liver blood flow and oxygen consumption more than doubled as DMI and milk yield increased, but 85 and 93% of the respective increases in PDV and liver blood flow at 83 DIM had occurred by 11 DIM. Therefore, factors additional to DMI must also contribute to increased blood flow in early lactation. Most postpartum changes in net PDV and liver metabolism could be attributed to increases in DMI and digestion or increased milk yield and tissue energy loss. Glucose release was increasingly greater than calculated requirements as DIM increased, presumably as tissue energy balance increased. Potential contributions of lactate, alanine, and glycerol to liver glucose synthesis were greatest at 11 DIM but decreased by 83 DIM. Excluding alanine, there was no evidence of an increased contribution of amino acids to liver glucose synthesis is required in early lactation. Increased net liver removal of propionate (69%), lactate (20%), alanine (8%), and glycerol (4%) can account for increased liver glucose release in transition cows from 9 d before to 11 d after calving.

ADSA Foundation Scholar Award. Biology of dairy cows during the transition period: the final frontier?

The transition period, from 3 wk before to 3 wk after parturition, is critically important to health, production, and profitability of dairy cows. Most health disorders occur during this time. Compared with other stages of the lactation cycle, relatively little is known about fundamental biological processes during the transition period. The regulation and coordination of lipid metabolism among adipose tissue, liver, gut, and mammary gland are key components of the adaptations to lactation. Lipid accumulation in liver may contribute to health disorders and decreased milk production. Knowledge of key control points in hepatic metabolism of long-chain fatty acids is lacking, as is an understanding of the metabolic effects of hormones, growth factors, and cytokines that mediate stress. Recent evidence indicates that supplemental fats or restricted intakes before parturition can induce a coordinated set of metabolic changes in metabolism of long-chain fatty acids, including peroxisomal beta-oxidation, perhaps mediated by peroxisome proliferator-activated receptors. Estimates of the mixture of fuels constituting metabolizable energy in cows during the early postpartum period suggest that supply of amino acids and glucogenic compounds may be under proposed optima, whereas ketogenic and lipogenic compounds and long-chain fatty acids may be in excess. Because dietary fat does not suppress body lipid mobilization, during the early postpartum period supplemental fat may further imbalance the mixture of fuels and lead to decreased dry matter intake. Increased understanding of the biology of the transition period should decrease health problems and increase profitability of dairy cows.

Effects of an oral and intravenous fat load on adipose tissue and forearm lipid metabolism

We have studied the fate of lipoprotein lipase (LPL)-derived fatty acids by measuring arteriovenous differences across subcutaneous adipose tissue and skeletal muscle in vivo. Six subjects were fasted overnight and were then given 40 g of triacylglycerol either orally or as an intravenous infusion over 4 h. Intracellular lipolysis (hormone-sensitive lipase action; HSL) was suppressed after both oral and intravenous fat loads (P < 0.001). Insulin, a major regulator of HSL activity, showed little change after either oral or intravenous fat load, suggesting that suppression of HSL action occurred independently of insulin. The rate of action of LPL (measured as triacylglycerol extraction) increased with both oral and intravenous fat loads in adipose tissue (P = 0.002) and skeletal muscle (P = 0.001). There was increased escape of LPL-derived fatty acids into the circulation from adipose tissue, shown by lack of reesterification of fatty acids. There was no release into the circulation of LPL-derived fatty acids from skeletal muscle. These results suggest that insulin is not essential for HSL suppression or increased triacylglycerol clearance but is important in reesterification of fatty acids in adipose tissue but not uptake by skeletal muscle, thus affecting fatty acid partitioning between adipose tissue and the circulation, postprandial nonesterified fatty acid concentrations, and hepatic very low density lipoprotein secretion.

Measurement of postprandial remnant-like particles (RLPs) following a fat-loading test

To assess the usefulness of remnant-like particles (RLPs), which represent potentially atherogenic lipoprotein remnants, as an index of postprandial hyperlipidemia, RLPs and other lipids were measured in 19 healthy adults (23-44 years old) after ingestion of a test meal (fat content, 32.9%). The lipoprotein derivatives, with a marked increase after fat-loading, were triglycerides, retinyl palmitate and RLPs. The retinyl palmitate peak lagged behind the RLP and triglyceride peaks, indicating that it may have different kinetics than RLP and triglyceride. While triglycerides are considered as an index of atherosclerotic risk factor, RLPs also may be a suitable index for evaluating postprandial hyperlipidemia. RLPs remained high even 8 h after fat-loading, suggesting that the postprandial state persists for almost the whole day. Accordingly, it may be important to assess postprandial remnant concentrations.

Abnormal metabolism of free fatty acids in hypertriglyceridaemic men: apparent insulin resistance of adipose tissue

OBJECTIVE

There is growing evidence that endogenous hypertriglyceridaemia is frequently accompanied by a state of insulin resistance. The present study was performed to determine whether patients with primary endogenous hypertriglyceridaemia commonly have abnormalities in plasma concentrations and turnover rates of free fatty acids (FFA), which could reflect a state of insulin resistance in adipose tissue and could account for raised plasma triglycerides.

DESIGN

Hypertriglyceridaemic and normotriglyceridemic control patients underwent measurements of plasma concentrations and turnover rates of FFA. Fat weights in both groups were determined by hydrodensitometry, and fat distribution was assessed by skin-folds and measurement of waist and hip circumferences. Other measurements included plasma glucose, insulin, lipids, and lipoproteins.

SUBJECTS

Fifteen men with normal plasma triglycerides and 21 men with primary endogenous hypertriglyceridaemia were studied. Men in both groups varied in body weights and total fat weights, but total fat weights were entirely overlapping for the two groups. Waist-to-hip ratios and waist circumferences also were similar for the two groups.

RESULTS

For any total body fat content or waist circumference, most hypertriglyceridaemia patients had higher mean plasma concentrations of FFA and higher turnover rates (flux) for FFA than did normotriglyceridemic patients. Hypertriglyceridaemic patients also had higher fasting insulin concentrations for a given body fat content. In general, both FFA flux and plasma insulin levels were positively correlated with plasma concentrations of triglyceride and inversely with high density lipoprotein (HDL) cholesterol.

CONCLUSIONS

These studies indicate that many patients with primary endogenous hypertriglyceridaemia have increased flux of FFA and hyperinsulinemia that cannot be explained either by increased total body fat content or by greater waist circumferences than observed in normotriglyceridemic patients.

The effect of triacylglycerol-fatty acid positional distribution on postprandial metabolism in subcutaneous adipose tissue

We hypothesized that fatty acids at the sn-2 position of chylomicron triacylglycerol are preferentially released into the venous plasma (rather than being taken up and stored in the adipocytes) after hydrolysis by lipoprotein lipase (EC 3.1.1.34) in adipose tissue. Arteriovenous differences across adipose tissue were studied in eight healthy subjects on two occasions for 6 h after ingestion of different structured triacylglycerols rich in palmitic acid either at the sn-2 or the sn-1,3 positions. In particular the specific fatty acids making up lipoprotein fractions and plasma non-esterified fatty acids were analysed. After the different meals there were no differences between either postprandial arterialized or venous plasma metabolite concentrations. Chylomicron triacylglycerol extraction in adipose tissue was the same following the two types of fat. There was no difference between the specific fatty acid composition of the postprandial non-esterified fatty acid release from adipose tissue after ingestion of the two triacylglycerols, indicating that there was no preferential release of a saturated fatty acid at the sn-2 position.

Kinetics of retinyl esters during postprandial lipemia in man: a compartmental model

Orally ingested vitamin A (retinol) is incorporated into intestinal chylomicrons (CHYLO) in the form of retinyl esters (RE) along with newly absorbed dietary triglycerides (TG). As the intestinal lipoproteins undergo hydrolysis in the circulation, the majority of the RE remain with the secreted intestinal particles and have been used as a marker for intestinally derived lipoproteins during the early phase of the postprandial state. A multicompartmental model was developed for the kinetics of RE during postprandial lipemia in individuals with normal lipid levels (n = 16) and in patients with hyperlipidemia (n = 44). The assumptions used in the development of the model are presented in this report. Some of the key findings include (1) as much as 50% of the newly synthesized RE may be secreted by the intestine as very-low-density lipoprotein (VLDL)-sized particles of S(f) 20 to 400 following consumption of a test meal containing a moderate amount of fat (20 to 30 g); (2) in most individuals, approximately 50% of the RE secreted in S(f) greater than 400 are converted to smaller, less buoyant fractions, and 50% are irreversibly removed directly from the plasma; (3) as much as 5% to 20% of the ingested retinol may be secreted as small intestinal lipoproteins with the buoyance of low-density lipoprotein (LDL) in some individuals; and (4) less than 5% of RE flux through S(f) 20 to 400 is converted to S(f) less than 20, and the primary catabolic pathway for RE in this fraction is direct uptake. Comparable estimates can be obtained for the kinetic parameters when repeat studies are made in the same subjects under comparable conditions.

Triacylglycerol synthesis in the different skeletal muscle fiber sections of the rat

Triacylglycerol (TG) synthesis rates of low-oxidative fast-twitch white and high-oxidative fast- and slow-twitch red skeletal muscle fiber sections of adult rats were measured by the incorporation of perfusate-derived palmitate into the neutral lipid fraction by use of a perfused hindquarter preparation under high-flow conditions. The perfusion medium consisted of 95% O2-5% CO2 Krebs-Henseleit buffer, pH 7.4, containing 5 g/100 ml bovine serum albumin, 100 microU/ml insulin, 5 mmol/l glucose, amino acids, and added fatty acids (FA), including 0.1 microCi/ml [3H]palmitate. FA incorporation was linear with time. TG synthesis rates correlated (r > or = 0.90) with the oxidative capacity of each of the different fiber type sections and increased in proportion to the perfusate FA concentration (0.25, 0.5, 1.0, 1.5, and 2.0 mmol/l). TG synthesis rates among different muscle fiber sections were related (r > or = 0.90) to perfusate flow during high-flow conditions; however, this was not causal, because TG synthesis rates within each muscle fiber section were independent of flow rate when experimentally varied over an approximately threefold range. Thus the relatively high TG synthesis rates observed in the high-oxidative muscle sections are not uniquely related to their high-flow capacities but are inherent to the TG synthesis process, probably events associated with FA uptake and/or capacity of the TG synthesis pathway.

The transfer of docosahexaenoic acid from the yolk to the tissues of the chick embryo

Changes in the amounts of the major fatty acids present in the lipids of the yolk complex and the embryo were delineated during embryogenesis of the chicken. The rates of transfer of palmitic, oleic, linoleic, linolenic and arachidonic acids from the lipids of the yolk complex were essentially identical. In contrast, docosahexaenoic acid (DHA) was preferentially transferred from the yolk complex at a rate which was significantly higher than that exhibited by the other major fatty acids. The rates of accumulation of both arachidonic acid and DHA in the lipids of the whole embryo were significantly greater than the rates observed for the C16 and C18 fatty acids, particularly between days 12 and 16 of the 21-day embryonic period. Analysis of the fatty acid composition of plasma lipid throughout development indicated that the triacylglycerol fraction contained relatively high proportions (up to approx. 14% w/w of total fatty acids) of DHA, but much lower proportions (approx. 3%) of arachidonic acid. In contrast, plasma phospholipid was enriched in arachidonic acid (up to approx. 18%), but contained much lower proportions (generally less than 3%) of DHA. A considerable amount of DHA was incorporated into adipose tissue triacylglycerol, so that by the time of hatching, the tissue represented a major store of this fatty acid. Over the hatching period, the amount of DHA in adipose triacylglycerol decreased dramatically, by up to 85%, whereas there was little or no change in the amounts of the other major fatty acyl components in this tissue. The amount of DHA as a component of brain phospholipid increased continuously throughout the developmental period studied. However, by the time of hatching, the amount of DHA in brain phospholipid represented less than 10% of the amount of this fatty acid originally present in the lipids of the yolk.

Regulation of fatty acid movement in human adipose tissue in the postabsorptive-to-postprandial transition

There is net outward flow of fatty acids from adipose tissue in the fasted state but net inward flow and storage in the postprandial state. We investigated how this is regulated. Arteriovenous differences were measured across a subcutaneous adipose depot in six normal subjects before and for 5 h after a meal containing 80 g fat and 80 g carbohydrate. In five further experiments, insulin was infused at 40 mU.m-2.min-1 from 30 min after the meal, clamping the plasma glucose. Net transcapillary fatty acid flow changed from negative (outward flow from tissue to capillaries) in the postabsorptive state to consistently positive (net inward flow, implying fat storage) after the meal despite continued net efflux of fatty acids into venous blood. In the "clamped" experiments (with additional insulin), net fatty acid efflux in the venous blood was suppressed and positive transcapillary flux (storage) was more marked. Regulation of fatty acid flow appeared to depend on coordinated changes in hormone-sensitive lipase (HSL) and lipoprotein lipase (LPL) action and fatty acid esterification. Additional insulin caused no further suppression of HSL or activation of LPL but markedly stimulated fatty acid retention (presumed to represent esterification). In the absence of additional insulin, a high proportion of the fatty acids liberated by LPL are released into the venous plasma in both postabsorptive and postprandial states. We hypothesize that this "loss" of fatty acids is necessary to give precise control to the pathway of fat storage.

Effects of exogenous apo E-3 and of cholesterol-enriched meals on the cellular metabolism of human chylomicrons and their remnants

The effects of exogenous apo E-3 and of cholesterol-enriched meals on the binding, cell association and proteolytic degradation of human chylomicrons and their remnants were determined in cultured human skin fibroblasts. Chylomicrons were prepared from plasma of normolipemic humans 4 h after a fat meal with normal or high cholesterol content. Remnants were obtained after incubation of chylomicrons with lipoprotein lipase in vitro. Cellular metabolism of chylomicrons was minimal, less than 10% that of LDL. Exogenous apo E-2 enhanced chylomicron metabolism by 3-4-fold. The cellular metabolism of remnants was 2.5-3.5-fold higher as compared to intact chylomicrons but their response to exogenous apo E-3 was considerably lower. The cellular metabolism of chylomicrons and chylomicron remnants obtained from subjects eating cholesterol-enriched fat meal was the highest either without or with added exogenous apo E-3. Yet, even in the preparation that exhibits the highest metabolic activity (apo E-3 enriched remnants from cholesterol-enriched meals) the absolute proteolytic degradation was about two-thirds that of LDL. We conclude that although LDL-receptors take up and degrade chylomicron remnants, the rate of catabolism of remnants by this route can not explain the rapid and complete remnant removal process as observed in vivo.

Serum lipoprotein and apoprotein concentrations in 4-(4-chlorophenyl)-2-hydroxytetronic acid and clofibrate-treated cholesterol and cholic acid-fed rats

Influence of clofibrate and an aci-reductone, 4-(4-chlorophenyl)-2-hydroxytetronic acid (CHTA) on lipoproteins and apoproteins was studied in cholesterol- plus cholic acid-fed rats. CHTA (0.4 mmol/kg body wt, twice daily) significantly lowered serum total cholesterol and triglyceride concentrations at both 10 and 16 days, whereas clofibrate at the same dose did not alter serum cholesterol levels, but elevated serum triglyceride concentrations at 16 days. The abnormal cholesterol-rich very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL) and low density lipoproteins (LDL) produced by cholesterol plus cholic acid were significantly reduced in their cholesterol content by treatment with CHTA, a compound having an oxidation reduction potential. Conversely, clofibrate administration increased VLDL-cholesterol with concomitant decreases in IDL- and LDL-cholesterol concentrations. Administration of CHTA to cholesterol- plus cholic acid-fed rats significantly increased concentrations of VLDL and IDL, but had no effect on HDL protein. Both CHTA and clofibrate administration to cholesterol- plus cholic acid-fed rats significantly lowered IDL protein concentrations. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) studies of apoproteins revealed that clofibrate treatment significantly reduced apoC-III and C-II in VLDL, C-II in IDL, and apoA-IV and A-I in HDL. Rats treated with CHTA significantly raised apoC-II and C-III in HDL.(ABSTRACT TRUNCATED AT 250 WORDS)

Absorption and transport of fat in mammals with emphasis on n-3 polyunsaturated fatty acids

The current state of knowledge concerning the absorption and transport of dietary fat with emphasis on long-chain polyunsaturated n-3 fatty acids in mammals is reviewed. It is apparent that long-chain polyunsaturated n-3 fatty acids, either as free acids or as part of triglycerides, are readily absorbed in the gut and transported by the circulatory system. Indeed, it would appear that long-chain polyunsaturated n-3 fatty acids are digested, absorbed and transported similarly to other long-chain fatty acids with only minor variations, although there is much that is still not understood about these processes. The main unresolved issues in the area of the absorption and transport of long-chain polyunsaturated n-3 fatty acids appear to be: 1) If they, when located in the 2-position of triglycerides, have unique metabolic pathways; and 2) whether the unnatural forms, i.e., methyl or ethyl ester derivatives, are suitable vehicles for administration as dietary supplements. The effect in man of dietary, long-chain polyunsaturated n-3 fatty acids on blood serum lipid and lipoprotein levels, particularly the low density lipoproteins, remains controversial, except for the well-documented reduction in serum triglyceride levels. Also, there is uncertainty regarding their distribution and metabolism in tissues. Finally, if the consumption of long-chain polyunsaturated n-3 fatty acids has beneficial health consequences, what is the appropriate therapeutic dose? In view of these important, unresolved issues and uncertainties, it would seem prudent to direct additional research toward a better understanding of the overall process by which fat is digested, absorbed and transported.

Hepatocytic lipoprotein receptors and intracellular lipoprotein catabolism

Hepatocytes, as the major site of synthesis and terminal catabolism of plasma lipoproteins, exert the major regulatory influence on the concentration of atherogenic lipoproteins in blood plasma and may thereby influence the rate of atherogenesis. The LDL receptor on the microvillous sinusoidal surface of hepatocytes mediates the catabolism of remnants of triglyceride-rich lipoproteins and LDL. Binding of VLDL remnants to the receptor, mediated by apo E, is of very high affinity and presumably multivalent, whereas binding of LDL, mediated by apo B-100, is monovalent and of lower affinity, accounting for the much longer residence time of the latter in the blood. The magnitude of the influx of lipoprotein particles into hepatocytic endosomal compartments dwarfs that of other macromolecules undergoing receptor-mediated endocytosis and terminal catabolism in lysosomes of these cells. The intracellular compartments and processing steps in hepatocytic lipoprotein uptake and degradation are essentially the same as those described for other ligands in the liver and other cells. Receptors with bound lipoproteins migrate into coated pits which become coated vesicles. These vesicles uncoat and fuse to form CURL vesicles and tubules near the cell surface where most receptors are recycled, presumably via receptor-rich appendages that become separated from the vesicles. CURL vesicles become mature MVBs as they migrate to the Golgi/bile canalicular pole of hepatocytes, where they fuse with putative Golgi-derived primary lysosomes and are transformed into heterophagic secondary lysosomes. MVBs also contain a receptor-rich appendage that may recycle some receptors directly to the cell surface or through adjacent Golgi compartments. Dilated ends of trans-Golgi cisternae contain nascent VLDL undergoing packaging for secretion following their synthesis and assembly in the endoplasmic reticulum. Because these "forming secretory vesicles" resemble remnant-filled MVBs, occur in a similar location in the Golgi area of hepatocytes and coisolate in centrifugal fractions of liver homogenates, there has been considerable confusion about the identity of these compartments. With the aid of specific endocytic and exocytic markers, highly purified and morphologically intact endosomal and Golgi compartments can now be obtained from rat liver homogenates. The availability of these and similar fractions of defined purity should facilitate investigation of the hepatocytic processing of endocytosed and secreted macromolecules. Although chylomicron remnants are also taken up by receptor-mediated endocytosis, the nature of the hepatocytic remnant receptor remains elusive.(ABSTRACT TRUNCATED AT 400 WORDS)

Triglyceride accumulation and very low density lipoprotein secretion by rat and goat hepatocytes in vitro

Utilization of [1-14C] oleate by freshly isolated rat and goat hepatocytes was compared. Intracellular [14C] triglyceride accumulation by hepatocytes did not differ between species. At 2 h of incubation, rat hepatocytes secreted approximately 25 times more [14C] triglyceride than goat hepatocytes. Very low density lipoprotein secretion was greatest by hepatocytes incubated in media containing 4:1 oleate:bovine serum albumin. Rat hepatocytes converted three to four times more [1-14C] oleate to 14CO2 and acid-soluble products than goat hepatocytes. Rate of 14CO2 formation by both rat and goat hepatocytes increased as incubation time increased and as rate of cellular triglyceride accumulation decreased. The ratio of 14CO2:[14C] acid-soluble products formed was greater for rat than goat hepatocytes, which indicated rat hepatocytes may oxidize fatty acid more completely. Differences in metabolic rate, based on oxygen consumption, between isolated goat and rat hepatocytes were minor and could not account for marked differences in very low density lipoprotein secretion. Goat hepatocytes did not incorporate detectable quantities of labeled fatty acid into low or high density lipoproteins. Ruminants may be susceptible to fatty liver when the liver takes up large amount of nonesterified fatty acid due to an inability to efficiently export fatty acid as very low density lipoprotein triglyceride.

Production and characterization of a monoclonal antibody to dog hepatic lipase

Partially purified dog hepatic lipase was used as antigen to produce monoclonal antibodies in mice. In addition to enzyme-linked immunosorbent assay (ELISA), a reliable and efficient procedure for screening antibodies reacting to hepatic lipase has been developed. A method to distinguish antibodies directing to active site or non-active site epitopes has also been described. We obtained three positive clones that survived after subcloning and expansion. All three monoclonal antibodies possess gamma one (gamma 1) heavy chains and kappa (kappa) light chains. Specificity of monoclonal antibody LDHL No. 537 to dog hepatic lipase was demonstrated by passing post-heparin plasma through its immunoaffinity column. Only dog hepatic lipase was removed by LDHL No. 537 from post-heparin plasma. The immunoaffinity chromatography also demonstrated the co-existence of three enzyme activities (mono- and triacylglycerol lipase and phospholipase A1) on the dog hepatic lipase molecule. The subunit weight of dog hepatic lipase has been estimated at 57500 +/- 600 (n=3) by using immunoaffinity chromatography and the combination of immunoprecipitation and autoradiography methods.