Fatty acid oxidation in relation to cholesterol biosynthesis in rats

The Adaptive Characteristics of Cholesterol and Bile Acid Metabolism in Nile Tilapia Fed a High-Fat Diet

Since high-fat diet (HFD) intake elevates liver cholesterol and enhanced cholesterol-bile acid flux alleviates its lipid deposition, we assumed that the promoted cholesterol-bile acid flux is an adaptive metabolism in fish when fed an HFD. The present study investigated the characteristic of cholesterol and fatty acid metabolism in Nile tilapia (Oreochromis niloticus) after feeding an HFD (13% lipid level) for four and eight weeks. Visually healthy Nile tilapia fingerlings (average weight 3.50 ± 0.05 g) were randomly distributed into four treatments (4-week control diet or HFD and 8-week control diet or HFD). The liver lipid deposition and health statue, cholesterol/bile acid, and fatty acid metabolism were analyzed in fish after short-term and long-term HFD intake. The results showed that 4-week HFD feeding did not change serum alanine transaminase (ALT) and aspartate transferase (AST) enzyme activities, along with comparable liver malondialdehyde (MDA) content. But higher serum ALT and AST enzyme activities and liver MDA content were observed in fish fed 8-week HFD. Intriguingly, remarkably accumulated total cholesterol (mainly cholesterol ester, CE) was observed in the liver of fish fed 4-week HFD, along with slightly elevated free fatty acids (FFAs) and comparable TG contents. Further molecular analysis in the liver showed that obvious accumulation of CE and total bile acids (TBAs) in fish fed 4-week HFD was mainly attributed to the enhancement of cholesterol synthesis, esterification, and bile acid synthesis. Furthermore, the increased protein expressions of acyl-CoA oxidase 1/2 (Acox1 and Acox2), which serve as peroxisomal fatty acid β-oxidation (FAO) rate-limiting enzymes and play key roles in the transformation of cholesterol into bile acids, were found in fish after 4-week HFD intake. Notably, 8-week HFD intake remarkably elevated FFA content (about 1.7-fold increase), and unaltered TBAs were found in fish liver, accompanied by suppressed Acox2 protein level and cholesterol/bile acid synthesis. Therefore, the robust cholesterol-bile acid flux serves as an adaptive metabolism in Nile tilapia when fed a short-term HFD and is possibly via stimulating peroxisomal FAO. This finding enlightens our understanding on the adaptive characteristics of cholesterol metabolism in fish fed an HFD and provides a new possible treatment strategy against metabolic disease induced by HFD in aquatic animals.

Peroxisomal β-oxidation stimulates cholesterol biosynthesis in the liver in diabetic mice

Although diabetes normally causes an elevation of cholesterol biosynthesis and induces hypercholesterolemia in animals and human, the mechanism linking diabetes to the dysregulation of cholesterol biosynthesis in the liver is not fully understood. As liver peroxisomal β-oxidation is induced in the diabetic state and peroxisomal oxidation of fatty acids generates free acetate, we hypothesized that peroxisomal β-oxidation might play a role in liver cholesterol biosynthesis in diabetes. Here, we used erucic acid, a specific substrate for peroxisomal β-oxidation, and 10,12-tricosadiynoic acid, a specific inhibitor for peroxisomal β-oxidation, to specifically induce and suppress peroxisomal β-oxidation. Our results suggested that induction of peroxisomal β-oxidation increased liver cholesterol biosynthesis in streptozotocin-induced diabetic mice. We found that excessive oxidation of fatty acids by peroxisomes generated considerable free acetate in the liver, which was used as a precursor for cholesterol biosynthesis. In addition, we show that specific inhibition of peroxisomal β-oxidation decreased cholesterol biosynthesis by reducing acetate formation in the liver in diabetic mice, demonstrating a crosstalk between peroxisomal β-oxidation and cholesterol biosynthesis. Based on these results, we propose that induction of peroxisomal β-oxidation serves as a mechanism for a fatty acid-induced upregulation in cholesterol biosynthesis and also plays a role in diabetes-induced hypercholesterolemia.

The effect of dietary oleic, linoleic, and linolenic acids on fat oxidation and energy expenditure in healthy men

Studies have shown that the long chain fatty acid composition of a dietary fat influences whether it will be partitioned for either energy or storage. The objective of this study was to compare the effects of 3 oils differing in fatty acid composition on postprandial energy expenditure and macronutrient oxidation in healthy normal-weight men. Using a randomized crossover design, 15 subjects consumed breakfast meals containing 60% of energy as fat. The principal source of fat was (a) olive oil rich in oleic acid (18:1n-9), (b) sunflower oil rich in linoleic acid (18:2n-6), or (c) flaxseed oil rich in linolenic acid (18:3n-3). Measurements of resting metabolic rate, thermic effect of food, and postprandial energy expenditure were conducted with indirect calorimetry that recorded O(2) consumed and CO(2) produced one-half hour before meal consumption and 6 hours after meal consumption. Fat and carbohydrate oxidation rates were calculated from nonprotein gaseous exchange. Olive oil feeding showed a significant overall increase in energy expenditure compared with flaxseed oil (P < .0006) and a trend to increased energy expenditure compared with sunflower oil (P < .06). None of the 3 treatments exhibited significant effects on fat or carbohydrate oxidation. In conclusion, diets rich in oleic acid derived from olive oil may offer increased oxidation translating into increased energy expenditure postprandially.

Effect of fatty acid chain length and saturation on the gastrointestinal handling and metabolic disposal of dietary fatty acids in women

The gastrointestinal handling and metabolic disposal of [1-13C]palmitic acid, [1-13C]stearic acid and [1-13C]oleic acid administered within a lipid–casein–glucose–sucrose emulsion were examined in normal healthy women by determining both the amount and nature of the13C label in stool and label excreted on breath as13CO2. The greatest excretion of13C label in stool was in the stearic acid trial (9.2 % of administered dose) whilst comparatively little label was observed in stool in either the palmitic acid (1.2 % of administered dose) or oleic acid (1.9 % of administered dose) trials. In both the palmitic acid and oleic acid trials, all of the label in stool was identified as being present in the form in which it was administered (i.e. [13C]palmitic acid in the palmitic acid trial and [13C]oleic acid in the oleic acid trial). In contrast, only 87 % of the label in the stool in the stearic acid trial was identified as [13C]stearic acid, the remainder was identified as [13C]palmitic acid which may reflect chain shortening of [1-13C]stearic acid within the gastrointestinal tract. Small, but statistically significant, differences were observed in the time course of recovery of13C label on breath over the initial 9 h of the study period (oleic acid = palmitic acid > stearic acid). However, when calculated over the 24 h study period, the recovery of the label as13CO2was similar in all three trials (approximately 25 % of absorbed dose). These results support the view that chain length and degree of unsaturation may influence the gastrointestinal handling and immediate metabolic disposal of these fatty acids even when presented within an emulsion.

Metabolism of polyunsaturated fatty acids and ketogenesis: an emerging connection

This paper summarizes the emerging literature indicating that at least two polyunsaturated fatty acids (PUFA; linoleate, alpha-linolenate) are moderately ketogenic and that via ketone bodies significant amounts of carbon are recycled from these fatty acids into de novo synthesis of lipids including cholesterol, palmitate, stearate and oleate. This pathway (PUFA carbon recycling) is particularly active in several tissues during the suckling period when, depending on the tissue, >200 fold more carbon from alpha-linolenate can be recycled into newly synthesized lipids than is used to make docosahexaenoate. At least in rats, PUFA carbon recycling also occurs in adults and even during extreme linoleate deficiency. Hence, this pathway should be considered an obligatory component of PUFA metabolism. It is still speculative but part of the clinical benefit of the very high fat ketogenic diet in intractable seizures may be achieved by raising plasma levels of PUFA that have anti-seizure effects, especially arachidonate and docosahexaenoate. Hence, in addition to some PUFA being ketogenic substrates, the state of ketosis involves potentially beneficial changes in PUFA homeostasis. Both the molecular controls on these pathways and their clinical significance still need elucidation.

Problems with essential fatty acids: time for a new paradigm?

The term 'essential fatty acid' is ambiguous and inappropriately inclusive or exclusive of many polyunsaturated fatty acids. When applied most rigidly to linoleate and alpha-linolenate, this term excludes the now well accepted but conditional dietary need for two long chain polyunsaturates (arachidonate and docosahexaenoate) during infancy. In addition, because of the concomitant absence of dietary alpha-linolenate, essential fatty acid deficiency is a seriously flawed model that has probably led to significantly overestimating linoleate requirements. Linoleate and alpha-linolenate are more rapidly beta-oxidized and less easily replaced in tissue lipids than the common 'non-essential' fatty acids (palmitate, stearate, oleate). Carbon from linoleate and alpha-linolenate is recycled into palmitate and cholesterol in amounts frequently exceeding that used to make long chain polyunsaturates. These observations represent several problems with the concept of 'essential fatty acid', a term that connotes a more protected and important fatty acid than those which can be made endogenously. The metabolism of essential and non-essential fatty acids is clearly much more interconnected than previously understood. Replacing the term 'essential fatty acid' by existing but less biased terminology, i.e. polyunsaturates, omega3 or omega6 polyunsaturates, or naming the individual fatty acid(s) in question, would improve clarity and would potentially promote broader exploration of the functional and health attributes of polyunsaturated fatty acids.

Why is carbon from some polyunsaturates extensively recycled into lipid synthesis?

We summarize here the evidence indicating that carbon from alpha-linolenate and linoleate is readily recycled into newly synthesized lipids. This pathway consumes the majority of these fatty acids that is not beta-oxidized as a fuel. Docosahexaenoate undergoes less beta-oxidation and carbon recycling than do alpha-linolenate or linoleate, but is it still actively metabolized by this pathway? Among polyunsaturates, arachidonate appears to undergo the least beta-oxidation and carbon recycling, an observation that may help account for the resistance of brain membranes to loss of arachidonate during dietary deficiency of n-6 polyunsaturates. Preliminary evidence suggests that de novo lipid synthesis consumes carbon from alpha-linolenate and linoleate in preference to palmitate, but this merits systematic study. Active beta-oxidation and carbon recycling of 18-carbon polyunsaturates does not diminish the importance of being able to convert alpha-linolenate and linoleate to long-chain polyunsaturates but suggests that a broad perspective is required in studying the metabolism of polyunsaturates in general and alpha-linolenate and linoleate in particular.

Carbon recycling into de novo lipogenesis is a major pathway in neonatal metabolism of linoleate and alpha-linolenate

Recent reports indicate that recycling of the beta-oxidized carbon skeleton of linoleate and alpha-linolenate into newly synthesized cholesterol and fatty acids in the brain is quantitatively significant in both suckling rats and pre- and postnatally in rhesus monkeys. The recycling appears to occur via ketones which are not only readily produced from these 18 carbon polyunsaturates but are also the main lipogenic precursors for the developing mammalian brain. Since the neonatal rat brain appears not to acquire cholesterol or long chain saturated or monounsaturated fatty acids from the circulation, ketones and ketogenic precursors seem to be crucial for normal brain synthesis of these lipids. Cholesterol is plentiful in brain membranes and it has also been discovered to be the essential lipid adduct of the 'hedgehog' family of proteins, the appropriate expression of which determines normal embryonic tissue patterning and neurological development. Insufficient cholesterol or inappropriate expression of 'sonic hedgehog' has major adverse neurodevelopmental consequences typified in humans by Smith-Lemli-Optiz syndrome. Hence, we propose that the importance of alpha-linolenate and linoleate for normal neural development arises not only from being precursors to longer chain polyunsaturates incorporated into neuronal membranes but, perhaps equally importantly, by being ketogenic precursors needed for in situ brain lipid synthesis.

[3-13C] gamma-linolenic acid: a new probe for 13C nuclear magnetic resonance studies of arachidonic acid synthesis in the suckling rat

Our objective was to develop a suitable probe to study metabolism of polyunsaturated fatty acids by 13C nuclear magnetic resonance (NMR) in the suckling rat pup. [3-13C] gamma-Linolenic acid was chemically synthesized, and a 20 mg (Experiment 1) or 5 mg (Experiment 2) dose was injected into the stomachs of 6-10-day-old suckling rat pups that were then killed over a 192 h (8 d) time course. 13C NMR showed that 13C in gamma-linolenate peaked in liver total lipids by 12-h post-dosing and that [5-13C]-arachidonic acid peaked in both brain and liver total lipids 48-96 h post-dosing. 13C enrichment in brain gamma-linolenic acid was not detected by NMR, but gas chromatography-combustion-isotope ratio mass spectrometry showed that its mass enrichment in brain phospholipids at 48-96 h post-dosing was 1-2% of that in brain arachidonic acid. 13C was present in liver and brain cholesterol and in perchloric acid-extractable water-soluble metabolites in the brain, liver and carcass. We conclude that low but measurable amounts of exogenous gamma-linolenic acid do access the suckling rat brain in vivo. The slow time course of [5-13C] arachidonic acid appearance in the brain suggests most of it was probably transported there after synthesis elsewhere, probably in the liver. Some carbon from gamma-linolenic acid is also incorporated into lipid products other than n-6 long-chain polyunsaturated fatty acids.

Dietary linoleic, alpha-linolenic and oleic acids are oxidized at similar rates in rats fed a diet containing these acids in equal proportions

The objective of this study was to examine whether whole body oxidation rates of dietary linoleic, alpha-linolenic and oleic acids differ when the acids are provided in identical quantities. Male rats were fed for 10 wk a 15% fat (w/w) diet containing equal amounts of linoleic, alpha-linolenic and oleic acids (22.7, 23.0 and 23.2% of total fatty acids, respectively). At week 10, after overnight fasting, rats were intragastrically administered 20 microCi of either [1-14C]-labelled linoleic, alpha-linolenic or oleic acid in a 200-microL bolus of oil containing equal quantities of each fatty acid. The appearance of 14CO2 in expired air was then monitored hourly for 12 h for each animal. A preliminary study had shown that growth and food consumption patterns in animals consuming the oil containing equal quantities of each of the fatty acids paralleled the patterns of animals that were self-selecting among separate diets, each of which contained one of the component oils. The appearance of 14C, expressed as percent dose administered, peaked at 2-3 h post-dose for 14C-labelled linoleic (5.28 +/- 0.37%/h), alpha-linolenic (6.92 +/- 0.51%/h) and oleic (5.98 +/- 0.44%/h) acids. Statistically these values were not significantly different. Cumulative 14CO2 excretion rates over 12 h were also similar for linoleic (27.2 +/- 0.9%), alpha-linolenic (26.8 +/- 1.2%) and oleic (25.9 +/- 1.2%) acids. The results suggest that the rat's capacity to oxidize 18-carbon unsaturated fatty acids is not affected by fatty acid unsaturation when these fatty acids are provided at equal dietary levels.