Studies on lipogenesis in vivo. Lipogenesis during extended periods of re-feeding after starvation

1. Lipogenesis was studied in mice re-fed for up to 21 days after starvation. At appropriate times [U-(14)]glucose was given by stomach tube and incorporation of (14)C into various lipid fractions measured. 2. In mice starved for 48hr. and then re-fed for 4 days with a diet containing 1% of corn oil, incorporation of (14)C from [U-(14)C]glucose into liver fatty acids and cholesterol was respectively threefold and eightfold higher than in controls fed ad libitum. The percentages by weight of fatty acids and cholesterol in the liver also increased and reached peaks after 7 days. Both the radioactivity and weights of the fractions returned to control values after 10-14 days' re-feeding. These changes could be diminished by re-feeding the mice with a diet containing 20% of corn oil. Incorporation of (14)C from [U-(14)C]glucose into extrahepatic fatty acids (excluding those of the epididymal fat pads) was not elevated during re-feeding with a diet containing either 1% or 20% of corn oil. However, incorporation of (14)C from [U-(14)C]glucose into the fatty acids of the epididymal fat pads was increased in mice re-fed with either diet, as compared with non-starved controls. 3. Lipogenesis was also studied in mice alternately fed and starved. Mice given a diet containing 1% of corn oil for 6hr./day for 4 weeks lost weight initially and never attained the weight or carcass fat content of controls fed ad libitum. Incorporation of (14)C from dietary [U-(14)C]-glucose into the fatty acids of the epididymal fat pads was elevated threefold in the mice allowed limited access to food, although the incorporation into the remainder of the extrahepatic fatty acids was not different from that found for controls. Mice given a diet containing 20% of corn oil for 6hr./day adapted to the limited feeding regimen quicker and in 4 weeks did attain the weight and carcass fat content of controls. Incorporation of (14)C from [U-(14)C]glucose into the fatty acids of the epididymal fat pads and the remainder of the extrahepatic fatty acids was respectively fivefold and threefold higher than in controls fed ad libitum. 4. The elevation in liver lipogenesis during re-feeding was greatest on a diet containing 1% of corn oil, whereas in extrahepatic tissues the increase in lipogenesis was greater when the mice were re-fed or were allowed limited access to a diet containing 20% of corn oil. These results suggest that the causes of the increased rate of incorporation of (14)C from [U-(14)C]glucose into fatty acids during re-feeding may be different in liver from that in extrahepatic tissues.     

Fat pad triacylglycerol fatty acid loss and oxidation as indices of total body triacylglycerol fatty acid mobilization and oxidation in starving mice

We tested our hypothesis that, kinetically, triacylglycerol fatty acids in heterogeneously labeled adipocytes behave similarly to the whole fat pad triacylglycerol fatty acid during starvation in mice. Adipose triacylglycerol fatty acids were labeled with [1-14C]palmitate (complexed to albumin) by injection of a small bolus (2-5 microliter) into either epididymal or inguinal fat pads. Both 14C-labeled triacylglycerol fatty acid spec. act. and breath 14CO2 spec. act. were monitored 30 min after tracer injection and after 24-72 h starvation. Adipose triacylglycerol fatty acid spec. act. remained approximately constant during fasting, i.e., tracer and mass disappeared at similar rates. Negligible translocation of labeled triacylglycerol fatty acid from the injection site to other parts of the same fat pad or to distant fat pads occurred. Triacylglycerol fatty acid was mobilized more slowly from epididymal than from inguinal fat pads in two of three studies. Triacylglycerol fatty acid disappearance (loss) from inguinal fat pads was more replicable than from epididymal fat pads and more closely reflected the fall in whole body total lipid during starvation. The estimated percent of breath CO2-carbon derived from adipose triacylglycerol fatty acid increased from an average of approx. 32% in the postabsorptive state to about 77% after 48 h starvation. The data help to validate the direct tracer injection technique as a means of studying adipose triacylglycerol fatty acid turnover and oxidation. This approach should be particularly useful for studying the fate of adipose triacylglycerol fatty acid when it is mobilized. e.g., during states of inanition and starvation and in response to hormones and cancer-induced cachexia.     

Studies on lipogenesis in vivo. Effects of starvation and re-feeding, and studies on cholesterol synthesis

1. Studies in vivo have been carried out on hepatic and extrahepatic cholesterol synthesis and also on the effects of starvation and re-feeding on both cholesterol and fatty acid synthesis. 2. In rats and mice fed on a stock diet, extrahepatic tissues accounted for about 4 times as much newly synthesized cholesterol as did the liver. The liver appeared to be somewhat more important in the rat than the mouse. Feeding with cholesterol greatly decreased and cholestyramine greatly increased hepatic cholesterol synthesis without much effect on extrahepatic synthesis. 3. Mice starved for up to 7hr. did not lose any of the ability to convert a [U-(14)C]glucose meal into fat, whereas 18hr. of starvation resulted in an 80% loss of fatty acid synthesis in liver and carcass, an 80% loss in liver cholesterol synthesis and a 65% decrease in carcass cholesterol synthesis; 18hr. of food deprivation also decreased the proportion of counts in epididymal fat pads present as fat and increased the proportion present as glyceride glycerol. 4. Re-feeding for up to 7hr. restored fatty acid synthesis from a [U-(14)C]glucose meal to about 50% of the values for non-starved mice but had no effect on hepatic cholesterol synthesis. The altered distribution of counts in the epididymal fat pads caused by starvation was restored to normal after feeding for 1hr.     

A comparison of extrahepatic lipogenesis from a small glucose meal in obob and gold thioglucose obese mice fed low- or high-fat diets with or without the addition of Delta22-5beta-taurocholenic acid

Extrahepatic fatty acid synthesis from a 250 mg meal of [U-(14)C]-glucose was measured in epidymal fat pads and the remaining carcass of hyperglycemic obese (obob), gold thioglucose obese, and nonobese controls under conditions of maximum and minimum lipogenesis. Also assessed was the effect of Delta(22)-5beta-taurocholenic acid, previously shown to inhibit hepatic fatty acid synthesis. Both types of obese and nonobese mice were fed for 6 weeks glucose-based diets containing either 1% corn oil or 40% lard with or without the addition of 0.05% taurocholenic acid. In mice fed 1% corn oil, incorporation of labeled glucose into carcass fatty acids was 25% greater in nonobese than obese mice of either type of obesity. On this diet incorporation of labeled glucose into epididymal fatty acids was reduced 83% in hyperglycemic obese mice compared with nonobese littermates. The corresponding reduction in lipogenesis in gold thioglucose obese mice was only 23% compared with nonobese controls. Feeding 40% lard reduced incorporation of labeled glucose into epididymal and carcass fatty acid 67 to 95% compared with mice fed 1% corn oil in both types of obese and nonobese mice whether or not taurocholenic acid had been fed. Both types of obesity or feeding 40% lard reduced lipogenesis in fat pads to a greater extent than glucose uptake by the pads with the reductions additive. Feeding taurocholenic acid reduced pad weight 30% across strain and obesity status, increased uptake of labeled glucose into epididymal fat pads and increased the percentage of the labeled glucose in the pad recovered as fatty acid in both types of obese and nonobese mice when the diet was 1% corn oil. Similarities and differences between the two obesity models are discussed.     

Studies on lipogenesis in vivo. Effect of dietary fat or starvation on conversion of [14C]glucose into fat and turnover of newly synthesized fat

1. Lipogenesis was studied in vivo by giving mice 250mg. meals of [U-(14)C]glucose and measuring the disposition and incorporation of label. About 48% of the (14)C dose was eliminated as (14)CO(2) in the first 2hr. At 60min. after administration, 1.0, 1.9 and 11.9% of the dose was recovered as liver glycogen, liver fatty acid and carcass fatty acid respectively. Of the [(14)C]glucose converted into fat in the epididymal pads about 90% was present as glyceride fatty acid and 10% as glyceride glycerol. 2. Hepatic synthesis of fatty acid was depressed by dietary fat to a much greater extent than was synthesis outside the liver. Both feeding with fat and starvation decreased the proportion of the label taken up by adipose tissue present as fat (triglyceride) and increased the proportion of triglyceride label present as glyceride glycerol. These results are consistent with the hypothesis that the primary action of both these conditions in decreasing fat synthesis is to inhibit synthesis of fatty acids. 3. Turnover of body fat labelled in vivo from [U-(14)C]glucose was estimated from the decline in radioactivity measured over the first 24hr. of the experiment. The half-life of liver and extrahepatic fatty acids (excluding epididymal fat) was 16hr. and 3 days respectively. In contrast, no measurable decrease in radioactivity of the fatty acids of epididymal fat was observed for 7 days after administration of the [U-(14)C]glucose.     

Melatonin inhibits fatty acid transport in inguinal fat pads of hepatoma 7288CTC-bearing and normal Buffalo rats via receptor-mediated signal transduction.

Melatonin inhibits fatty acid uptake and linoleic acid-dependent growth in hepatoma 7288CTC in vivo in Buffalo rats. In this study we measured the effects of melatonin on arteriovenous differences for fatty acids across inguinal fat pads in fed and fasted rats to determine if fatty acid transport in white adipose tissue was also affected by melatonin. Intravenous infusion of melatonin in fasted tumor-bearing rats in vivo simultaneously and rapidly inhibited both fatty acid release from fat pads and fatty acid uptake by the tumors. Perfusion of fat pads in situ in normal rats with melatonin (0.1 nM) inhibited fatty acid release (fasted rats) and uptake (fed rats). Fatty acid transport was restored by addition of any of the following: a melatonin receptor antagonist (S 20928, 1.0 nM), pertussis toxin (0.5 microg/ml), forskolin (1 microM) or 8-Br-cAMP (10 microM). We conclude that fatty acid transport in inguinal fat pads requires cAMP and that melatonin inhibits this transport via a melatonin receptor-mediated, Gi protein-coupled signal transduction pathway. Melatonin has both anticachectic and lipid homeostatic actions in the white adipose tissue of inguinal fat pads.     

Protein turnover in adipose tissue from fasted or diabetic rats

Protein synthesis and degradation in vitro were compared in epididymal fat pads from animals deprived of food for 48 h or treated 6 or 12 days prior with streptozotocin to induce diabetes. Although both fasting and diabetes led to depressed (-24% to -57%) protein synthesis, the diminution in protein degradation (-63% to -72%) was even greater, so that net in vitro protein balance improved dramatically. Insulin failed to inhibit protein degradation in fat pads of these rats as it does for fed animals. Although insulin stimulated protein synthesis in fat pads of fasted and 12 day diabetic rats, the absolute change was much smaller than that seen in the fed state. The inhibition of protein degradation by leucine also seems to be less in fasted animals, probably because leucine catabolism is slower in fasting. These results show that fasting and diabetes may improve protein balance in adipose tissue but diminish the regulatory effects of insulin.     

Factors influencing the utilization of ketone bodies by mouse adipose tissue

Factors influencing the utilization of ketone bodies by mouse adipose tissue in vitro were studied. Epididymal fat pads can oxidize DL-Beta-hydroxybutyrate-3-(14)C and acetoacetate-3-(14)C to (14)CO(2) as well as convert these compounds to fatty acid-(14)C. An increased output of (14)CO(2) from Beta-hydroxybutyrate-3-(14)C was noted in response to glucose plus insulin, succinate, oxaloacetate, L-asparate, and L-malate. Fatty acid synthesis from Beta-hydroxybutyrate was enhanced by glucose plus insulin, L-aspartate, L-malate, oxaloacetate, and citrate. Nicotinamide stimulated the oxidation of Beta-hydroxybutyrate but not of acetoacetate to CO(2), and did not affect fatty acid synthesis from either ketone body. Nicotinamide increased NAD(+) and NADP(+) levels in epididymal fat pads without affecting the concentration of NADH and NADPH. "Superlipogenesis" caused by fasting the mice for 48 hr and re-feeding them for 24 hr sharply enhanced CO(2) output and lipogenesis from Beta-hydroxybutyrate. The activities of glucose-6-phosphate dehydrogenase, 6-phosphogluconic dehydrogenase, NADP-malic dehydrogenase, and citrate cleavage enzyme from mouse adipose tissue were increased during "superlipogenesis." Free fatty acid release by epididymal fat pads in vitro was slightly increased by Beta-hydroxybutyrate. The relationship of ketone body metabolism and lipogenesis in adipose tissue is discussed.     

Effect of methoxyindole 2-carboxylic acid and 4-pentenoic acid on adipose tissue metabolism.

1. Administration of methoxyindole 2-carboxylic acid to rats caused an increase in circulating free fatty acids which was associated with rapid hypoglycemia in fasted rats and liver glycogenolysis without hypoglycemia in fed rats. 2. The incorporation of labeled glucose, pyruvate and acetate carbons into triacylglycerol-glycerol, triacylglycerol-fatty acids and CO2 was inhibited in epididymal fat pads from methoxyindole 2-carboxylic acid-treated rats and by the addition of methoxyindole 2-carboxylic acid in vitro. In contrast, palmitate esterification and oxidation were enhanced by methoxyindole 2-carboxylic acid. 3. The activity of enzymes associated with fatty acid synthesis was reduced to a varying degree in the presence of methoxyindole 2-carboxylic acid in the reaction mixture in concentrations lower than those used to inhibit glucose and pyruvate metabolism in the intact tissue in vitro. 4. 4-Pentenoic acid, a potent inhibitor of pyruvate and palmitate metabolism in the liver, was considerably less effective in adipose tissue. 5. The effect of the two hypoglycemic substances investigated on adipose tissue metabolism seems to be different.     

Hepatic triglyceride secretion in relation to lipogenesis and free fatty acid mobilization in fasted and glucose-refed rats

Plasma triglyceride concentrations were significantly lowered by a single feeding of glucose to rats that had been fasted for 22 hr. Three feedings of glucose produced a similar effect. In the glucose-refed animals mobilization of free fatty acids from adipose tissue was impaired more rapidly than hepatic lipogenesis was restored from its low fasting level. These effects of glucose were shown by both a 50% fall in plasma free fatty acid concentration and an 84% decrease in free fatty acid release by isolated epididymal fat pads within 30 min after a single refeeding of glucose. Hepatic lipogenesis from either acetate-1-(14)C or glucose-U-(14)C was not restored even after glucose had been fed three times at hourly intervals. Triton-induced hypertriglyceridemia was used to measure the hepatic triglyceride secretory rate; it was found that glucose refeeding decreased this rate in all but one of several experiments. This decreased secretion rate was sufficient to account for the nearly complete disappearance of triglyceride in very low density lipoproteins (d < 1.019) that occurred within 1 hr after a single glucose intubation.     

Effects of starvation, refeeding, and fat feeding on adipocyte ghost adenyl cyclase activity

Basal adenyl cyclase activity and its response to epinephrine and glucagon were studied in isolated adipocyte ghosts obtained from fed, starved, refed, and fat-diet-adapted rats. Epinephrine stimulation of adenyl cyclase was significantly increased in fasted rats, but the glucagon response did not change. Rats fasted for 48 hr and refed a high carbohydrate, low fat diet for 48 or 96 hr showed no differences from chow-fed animals in either basal or hormone-stimulated adenyl cyclase activity. Rats adapted to a high fat, low carbohydrate diet showed an initial and transitory increase in basal activity but a progressive loss of epinephrine- and glucagon-stimulated enzyme activities. The loss in hormone responsiveness correlated well with a decrease in hormone-stimulated lipolysis of fat pads and was associated with a significant increase in fat cell diameter.     

Dietary protein and the control of fatty acid synthesis in rat adipose tissue

Fatty acid synthesis in adipose tissue normally proceeds at a high rate when fasted animals are refed a diet containing carbohydrate, protein, and low levels of fat. This study investigated the effect of omitting protein from the refeeding diet. Rats were fasted for 48 hr and refed either a protein-free diet or a balanced diet, and the rate of fatty acid synthesis from glucose, pyruvate, lactate, and aspartate was measured. Refeeding the animals a diet devoid of protein resulted in a low rate of fatty acid synthesis from each of these substrates as well as a reduction in carbon flow over the citrate cleavage pathway. The activities of glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, NADP-malate dehydrogenase, and ATP-citrate lyase were also reduced in epididymal fat pads from these rats. On the other hand, adipose tissue phosphoenolpyruvate carboxykinase activity was five times as great as that in tissue from animals refed a balanced diet. This difference could be eliminated if actinomycin D was injected coincident with refeeding. Refeeding rats diets high in carbohydrate is not, therefore, capable of inducing high rates of fatty acid synthesis in adipose tissue in the absence of dietary proteins. Thus, liver and adipose tissue respond differently to dietary protein.     

The mobilization of free fatty acids in relation to adipose tissue triglyceride fatty acids in the rat

Three diets, consisting respectively of formulations high in oleic and stearic acid, linolenic acid, and lauric acid, were fed to rats until the adipose tissue TGFA largely reflected the dietary pattern of fatty acids. The composition of the serum FFA under basal conditions and following noradrenaline-stimulated lipolysis, were examined in relation to the respective adipose tissue TGFA. It was found in both in vivo and in vitro studies that lauric acid appeared to be less easily mobilized than longer chain acids. The in vitro studies indicated that this could not be explained either by positional preference of the shorter chain acids for the alpha-position of esterification or by increased reesterification of the shorter chain acids. The possibility remains that the difference is due to some specificity of tissue lipases for certain ester linkages.     

Trafficking of dietary oleic, linolenic, and stearic acids in fasted or fed lean rats

Increasing evidence supports the notion that there are significant differences in the health effects of diets enriched in saturated, as opposed to monounsaturated or polyunsaturated fat. However, the current understanding of how these types of fat differ in their handling by relevant tissues is incomplete. To examine the effects of fat type and nutritional status on the metabolic fate of dietary fat, we administered (14)C-labeled oleic, linolenic, or stearic acid with a small liquid meal to male Sprague-Dawley rats previously fasted for 15 h (fasted) or previously fed ad libitum (fed). (14)CO(2) production was measured for 8 h after tracer administration. The (14)C content of gastrointestinal tract, serum, liver, skeletal muscle (soleus, lateral, and medial gastrocnemius), and adipose tissue (omental, retroperitoneal, and epididymal) was measured at six time points (2, 4, 8, 24, and 48 h and 10 days) after tracer administration. Plasma levels of glucose, insulin, and triglyceride were also measured. Oxidation of stearic acid was significantly less than that of either linolenic or oleic acid in both the fed and fasted states. This reduction was in part explained by a greater retention of stearic acid within skeletal muscle and liver. Oxidation of oleate and stearate were significantly lower in the fed state than in the fasted state. In the fasted state, liver and skeletal muscle were quantitatively more important than adipose tissue in the uptake of dietary fat tracers during the immediate postprandial period. In contrast, adipose tissue was quantitatively more important than skeletal muscle or liver in the fed state. The movement of carbons derived from dietary fat between tissues is a complex time-dependent process, which varies in response to the type of fat ingested and the metabolic state of the organism.     

Enzymes catalyzing the hydrolysis of long-chain monoacyglycerols in rat adipose tissue

Acetone-ether preparations of epididymal fat pads from fasted or fed rats contained two enzymes catalyzing the hydrolysis of long-chain monoacylglycerols. The enzymes were identified as monoacylglycerol lipase (Tornqvist, H. and Belfrage, P., (1976) J. Biol Chem. 251, 813--819) and lipoprotein lipase by their apparent pI values after electrofocusing in non-ionic detergent, selective inhibition properties, substrate specificity and positional specificity. It was estimated that monoacylglycerol lipase accounted for about 90% of the total monoacylglycerol-hydrolyzing activity in acetone-ether preparations from fasted and 70% from fed rats. Its enzyme activity did not change with the nutritional state in contrast to that of lipoprotein lipase. The latter enzyme hydrolyzed 2-monoacylglycerols at a much lower rate than the 1(3)-isomers. Monoacylglycerol lipase was located almost entirely in the adipocytes, thus most of the enzyme activity towards monoacylglycerols in the adipose tissue was found in this site. Fractionated sucrose homogenates of rat epididymal fat pads also contained a third enzyme with monoacylglycerol-hydrolyzing activity, identified as hormone-sensitive lipase by its pI, selective inhibition properties and substrate specificity. It was estimated that hormone-sensitive lipase accounted for less than 20% of the total activity against monoacylglycerols in these tissue preparations from fasted rats. Over-all quantitative estimations emphasized the dominant role of monoacylglycerol lipase over the other two enzymes in the hydrolysis of monoacylglycerols.     

Alteration of rat adipose tissue lipolytic response to norepinephrine by dietary fatty acid manipulation.

The present study clearly shows that, by feeding rats a semi-synthetic diet of known composition enriched with saturated fatty acids, the epididymal fat pad responsiveness to norepinephrine $\text{in}\text{vitro}$ can be abolished relative to fat pads from animals fed a similar diet but enriched with polyunsaturated fatty acids. Addition of varying concentrations of norepinephrine to the incubation medium produced a clear dose-response relationship in fat pads from animals fed diet enriched with polyunsaturated fatty acids while no effect of norepinephrine was apparent at any dose level in fat tissue from animals fed saturated fatty acids. These changes in lipolytic responsiveness were concurrent with alterations in fatty acid compositions of adipose tissue phospholipids and triglycerides as well as in total tissue contents of phospholipids and cholesterol.     

Calculation of lipolysis and esterification from glycerol metabolism in rat adipose tissue

Pieces of epididymal fat pad from fed and 48-hr-fasted rats were incubated for various periods of time in Krebs-Ringer bicarbonate containing [1-(14)C]glycerol. The radioactive substrate taken up by the tissue increased linearly with time in both groups and was mainly converted to glyceride-glycerol and CO(2). The slopes of the regressions of (14)C-labeled glyceride-glycerol with time were not different between the groups, while those of (14)CO(2) were smaller in the fasted than in the fed animals. Because the radioactive glycerol in the medium is being continuously diluted with the glycerol coming out of the tissue, it is necessary to take account of this factor in calculating the actual amount of glycerol utilized by the tissue. The glycerol produced by the tissues is higher in the fasted than in the fed animals, and in both groups it increases hyperbolically with time. As negligible amounts of the (14)C-labeled glycerol taken up by the tissue recirculates to the medium, the rates of glycerol release (lipolysis), esterification, and oxidation to CO(2) were calculated.     

Meal-feeding studies in mice: effects of diet on blood lipids and energy expenditure

To identify optimal study-design conditions to investigate lipid metabolism, male, C57BL/6J mice (age, 59 +/- 3 days) were allotted to eight groups, with six animals per group that were stratified by three factors: diet type (high fat [HF]: 60% of energy from fat versus that of a standard rodent diet, 14% fat, fed for 7 weeks), feeding regimen (ad libitum [ad lib] versus meal fed), and metabolic state (data collected in fasted or fed states). Serum free fatty acids (FFA) and triacylglycerols (TAG) concentrations, and energy expenditure (EE) were assessed. Mice gained 0.30 +/- 0.11 g of body weight/day when allowed ad lib access to HF diet, similar weight when meal-fed the HF or ad lib-fed the standard diet (0.10 +/- 0.03 g/day), and no weight when meal-fed the standard diet (0.01 +/- 0.02 g/day). Fed-state TAG concentration was 88 to 100% higher (P < 0.02) than that of the fasted state, except when animals were ad lib-fed the HF diet. When the standard diet was meal fed, FFA concentration was 30% higher in the fasted compared with the fed state (P = 0.003). Mice had 33% higher postprandial EE when either diet was meal fed (P = 0.01). Mice adapted to meal feeding developed transitions in metabolism consistent with known physiologic changes that occur from fasting to feeding. When fed the standard diet, a 6-h per day meal-feeding regimen was restrictive for normal growth. These data support use of a meal-feeding regimen when HF diets are used and research is focused on metabolic differences between fasted and fed states. This protocol allows study of the metabolic effects of an HF diet without the confounding effects of over-consumption of food and excess body weight gain.     

Alterations of lipid metabolism in mice injected with endotoxin.

Some alterations in lipid metabolism in mice were observed by the intraperitoneal injection of endotoxin from Salmonella typhimurium. The content of serum triglyceride increased markedly in poisoned mice 16-24 hr postintoxication. The level of free fatty acid (FFA) in the serum of endotoxin-administered mice decreased in inverse proportion to an increase in the injected dose of endotoxin. The electrophoretic analysis of the serum lipoprotein on cellulose acetate membrane showed that pre beta-lipoprotein increased markedly and that FFA fraction in the poisoned mice sera disappeared 18 hr postintoxication. The activity of hormone-sensitive lipase in adipose tissue was elevated appreciably 2 hr after injection, but decreased more significantly after 18 hr than that in fasted control mice. On the other hand, the activity of lipoprotein lipase decreased in the post-heparin serum and adipose tissue 3 hr postintoxication, and decreased significantly after 16 hr. There were no significant differences between changes in the formation of active glycerol (alpha-GP) and in the activity of alpha glycerophosphate dehydrogenase (alpha-GPDH) in the mice liver with or without administration of endotoxin, and after 16 hr levels of both hepatic alpha-GP content and alpha-GPDH activity in poisoned mice showed a tendency to be slightly lower than those in fasted control mice.     

Increased production of very-low-density lipoproteins in transgenic mice overexpressing human apolipoprotein A-II and fed with a high-fat diet

We investigated the mechanisms that lead to combined hyperlipidemia in transgenic mice that overexpress human apolipoprotein (apo) A-II (line 11.1). The 11.1 transgenic mice develop pronounced hypertriglyceridemia, and a moderate increase in free fatty acid (FFA) and plasma cholesterol, especially when fed a high-fat/high-cholesterol diet. Post-heparin plasma lipoprotein lipase and hepatic lipase activities (using artificial or natural autologous substrates), the decay of plasma triglycerides with fasting, and the fractional catabolic rate of the radiolabeled VLDL-triglyceride (both fasting and postprandial) were similar in 11. 1 transgenic mice and in control mice. In contrast, a 2.5-fold increase in hepatic VLDL-triglyceride production was observed in 11. 1 transgenic mice in a period of 2 h in which blood lipolysis was inhibited. This increased synthesis of hepatic VLDL-triglyceride used preformed FFA rather than FFA of de novo hepatic synthesis. The 11.1 transgenic mice also presented reduced epididymal/parametrial white adipose tissue weight (1.5-fold), increased rate of epididymal/parametrial hormone-sensitive lipase-mediated lipolysis (1.2-fold) and an increase in cholesterol and, especially, in triglyceride liver content, suggesting an enhanced mobilization of fat as the source of preformed FFA reaching the liver. Increased plasma FFA was reverted by insulin, demonstrating that 11.1 transgenic mice are not insulin resistant. We conclude that the overexpression of human apoA-II in transgenic mice induces combined hyperlipidemia through an increase in VLDL production. These mice will be useful in the study of molecular mechanisms that regulate the overproduction of VLDL, a situation of major pathophysiological interest since it is the basic mechanism underlying familial combined hyperlipidemia.