Hepatic VLDL secretion of genetically obese Zucker rats is inhibited by a high-fat diet

Regulation of fatty acid trafficking in liver by thioesterase superfamily member 1

Thioesterase superfamily member 1 (Them1) is an acyl-CoA thioesterase that is highly expressed in brown adipose tissue, where it functions to suppress energy expenditure. Lower Them1 expression levels in the liver are upregulated in response to high-fat feeding. Them1-/- mice are resistant to diet-induced obesity, hepatic steatosis, and glucose intolerance, but the contribution of Them1 in liver is unclear. To examine its liver-specific functions, we created conditional transgenic mice, which, when bred to Them1-/- mice and activated, expressed Them1 exclusively in the liver. Mice with liver-specific Them1 expression exhibited no changes in energy expenditure. Rates of fatty acid oxidation were increased, whereas hepatic VLDL triglyceride secretion rates were decreased by hepatic Them1 expression. When fed a high-fat diet, Them1 expression in liver promoted excess steatosis in the setting of reduced rates of fatty acid oxidation and preserved glycerolipid synthesis. Liver-specific Them1 expression did not influence glucose tolerance or insulin sensitivity, but did promote hepatic gluconeogenesis in high-fat-fed animals. This was attributable to the generation of excess fatty acids, which activated PPARα and promoted expression of gluconeogenic genes. These findings reveal a regulatory role for Them1 in hepatocellular fatty acid trafficking.

Differential effects of low-fat and high-fat diets on fed-state hepatic triacylglycerol secretion, hepatic fatty acid profiles, and DGAT-1 protein expression in obese-prone Sprague-Dawley rats

The purpose of this study was to compare the effects of short-term low-fat (LF) and high-fat (HF) diets on fed-state hepatic triacylglycerol (TAG) secretion, the content of proteins involved in TAG assembly and secretion, fatty acid oxidation (FAO), and the fatty acid profile of stored TAG. Using selectively bred obese-prone Sprague-Dawley rats, we directly measured fed-state hepatic TAG secretion, using Tyloxapol (a lipoprotein lipase inhibitor) and a standardized oral mixed meal (45% carbohydrate, 40% fat, 15% protein) bolus in animals fed a HF or LF diet for 2 weeks, after which the rats were maintained on their respective diet for 1 week (washout) prior to the liver being excised to measure protein content, FAO, and TAG fatty acid profiles. Hepatic DGAT-1 protein expression was ∼27% lower in HF- than in LF-fed animals (p < 0.05); the protein expression of all other molecules was similar in the 2 diets. The fed-state hepatic TAG secretion rate was ∼39% lower (p < 0.05) in HF- (4.62 ± 0.18 mmol·h(-1)) than in LF- (7.60 ± 0.57 mmol·h(-1)) fed animals. Hepatic TAG content was ∼2-fold higher (p < 0.05) in HF- (1.07 ± 0.15 nmol·g(-1) tissue) than in LF- (0.50 ± 0.16 nmol·g(-1) tissue) fed animals. In addition, the fatty acid profile of liver TAG in HF-fed animals closely resembled the diet, whereas in LF-fed animals, the fatty acid profile consisted of mostly de novo synthesized fatty acids. FAO was not altered by diet. LF and HF diets differentially alter fed-state hepatic TAG secretion, hepatic fatty acid profiles, and DGAT-1 protein expression.

Validity of the doubly labeled water method for estimating CO2 production in mice under different nutritional conditions

The doubly labeled water (DLW) technique is used to assess metabolic rate (MR) in free-living conditions. We investigated whether differences in the nutritional and body adiposity status affect validity of the assessment of CO2 production (rCO2) by the DLW technique. To serve this purpose, we compared calculated rCO2 by the DLW method to actual CO2 production concomitantly measured in an indirect calorimetry setup over a 3-day period in mice fed with a low-fat (LF) diet or an obesogenic high-fat/high-sucrose (HF) diet. To uncover a potential effect of body composition on DLW accuracy, the HF-fed group was further subdivided in a diet-induced obesity-prone (DIO) and diet-induced obesity-resistant (DR) group. Furthermore, we assessed the influence of different sampling protocols, duration, and methodology of calculation. An excellent match was found between rCO2 assessed by the two methods in the LF-fed mice (least discrepancy -0.5 ± 1.1%). In contrast, there was a consistent overestimation of rCO2 by the DLW technique in the HF-fed animals compared with actual CO2 production independent from body mass gain (least discrepancy DR +15.9 ± 2.2%, DIO +18.5 ± 3.2%). The least discrepancies were found when two-pool model equations and the intercept method were used to calculate the body water pool. Furthermore, the HF group presented different equilibration kinetics of (2)H and (18)O and a lower dilution space ratio between the two. We recommend particular caution when using the DLW method for MR assessment in HF-fed animals and potentially humans because of the overestimation of rCO2.

The opposite effects of high-sucrose and high-fat diet on Fatty Acid oxidation and very low density lipoprotein secretion in rat model of metabolic syndrome

Aims. To determine the effect of two different diets (high-sucrose (HS) and high-fat (HF)) on the main metabolic pathways potentially contributing to the development of steatosis: (1) activity of the liver lysosomal and heparin-releasable lipases; (2) fatty acid (FFA) oxidation; (3) FFA synthesis de novo; (4) VLDL output in vivo in a rat model of metabolic syndrome (MetS), hereditary hypertriglyceridemic (HHTg) rats fed HS or HF diets. Results. Both diets resulted in triacylglycerol (TAG) accumulation in the liver (HF > HS). The intracellular TAG lipolysis by lysosomal lipase was increased in both groups and positively correlated with the liver TAG content. Diet type significantly affected partitioning of intracellular TAG-derived fatty acids among FFA-utilizing metabolic pathways as HS feeding accentuated VLDL secretion and downregulated FFA oxidation while the HF diet had an entirely opposite effect. FFA de novo synthesis from glucose was significantly enhanced in the HS group (fed ≫ fasted) while being completely eradicated in the HF group. Conclusions. We found that in rats prone to the development of MetS associated diseases dietary-induced steatosis is not simply a result of impaired TAG degradation but that it depends on other mechanisms (elevated FFA synthesis or attenuated VLDL secretion) that are specific according to diet composition.

The increased activity of liver lysosomal lipase in nonalcoholic Fatty liver disease contributes to the development of hepatic insulin resistance

We tested the hypothesis that TAG accumulation in the liver induced by short-term high-fat diet (HFD) in rats leads to the dysregulation of endogenous TAG degradation by lysosomal lipase (LIPA) via lysosomal pathway and is causally linked with the onset of hepatic insulin resistance. We found that LIPA could be translocated between qualitatively different depots (light and dense lysosomes). In contrast to dense lysosomal fraction, LIPA associated with light lysosomes exhibits high activity on both intracellular TAG and exogenous substrate and prandial- or diet-dependent regulation. On standard diet, LIPA activity was upregulated in fasted and downregulated in fed animals. In the HFD group, we demonstrated an increased TAG content, elevated LIPA activity, enhanced production of diacylglycerol, and the abolishment of prandial-dependent LIPA regulation in light lysosomal fraction. The impairment of insulin signalling and increased activation of PKCε was found in liver of HFD-fed animals. Lipolysis of intracellular TAG, mediated by LIPA, is increased in steatosis probably due to the enhanced formation of phagolysosomes. Consequent overproduction of diacylglycerol may represent the causal link between HFD-induced hepatic TAG accumulation and hepatic insulin resistance via PKCε activation.

Apoprotein C-III deficiency markedly stimulates triglyceride secretion in vivo: comparison with apoprotein E

Apoprotein (apo) C-III plays an important role in the development of hypertriglyceridemia by inhibiting triglyceride (TG) removal. However, the effect of apo C-III on TG production remains unclear. We measured TG secretion rate (TGSR) in apo C-III gene-disrupted (apo C-III-null) mice to investigate the influence of this protein on TG turnover. TGSR measured by the Triton WR-1339 method was increased twofold in these mice compared with wild-type (WT) mice. Obesity was induced by the injection of gold-thioglucose (GTG), which made the WT mice hypertriglyceridemic due to a threefold increase of TGSR. However, GTG-induced obesity failed to increase TG in apo C-III-null mice, although TGSR was increased 10-fold, suggesting substantial stimulation of TG removal. Apo E-null mice were severely hypercholesterolemic but were not hypertriglyceridemic, and TGSR was rather decreased. GTG-induced obesity made these mice hypertriglyceridemic because of TG overproduction to an extent similar to that seen in WT mice. These results suggest that apo C-III deficiency potently enhances TG turnover, especially when TG production is stimulated, and that apo E deficiency is not always rate limiting for TG production.