Medium-Chain Fatty Acid Feeding Reduces Oxidation and Causes Panacinar Steatosis in Livers of Neonatal Pigs

BACKGROUND

Medium-chain fatty acids (MCFAs) are commonly used to enhance the caloric content of infant formulas. We previously reported that pigs fed MCFA developed hepatic steatosis when compared to those fed isocaloric long-chain fatty acid (LCFA) rich formula.

OBJECTIVES

The objectives of this study were to investigate: 1) whether MCFA and LCFA feeding affect hepatic fatty acid oxidation, and 2) how fat type alters the expression of hepatic fatty acid metabolic genes.

METHODS

Twenty-six, 7-d-old pigs were fed a low-energy control (CONT) formula, or 2 isocaloric high-energy formulas rich in LCFA or MCFA for 22 days. Livers were collected for examining ex vivo fatty acid oxidation, fatty acid content, and mRNA expression of fatty acid metabolic genes.

RESULTS

Liver fat was 20% for pigs in the MCFA compared with 2.9% and 4.6% for those in the CONT and LCFA groups (P < 0.05). MCFA-fed pigs had greater amounts of hepatic laurate, myristate, palmitate, and palmitoleate (14, 34, 49, and 9.3 mg · g-1) than those fed LCFA and CONT (1.8, 1.9, 19, 1.5 mg · g-1) formulas (P ≤ 0.05). Hepatic laurate and palmitate oxidation was reduced for pigs fed MCFA (29 mmol · mg-1 · h-1) compared with those fed CONT (54 mmol · mg-1 · h-1) and LCFA (51 mmol · mg-1 · h-1) formulas (P < 0.05). Expression of fatty acid synthase 3 (FASN-3), fatty acid binding protein 1 (FABP-1), and acetyl-CoA carboxylase 1 (ACACA-1) were 8-, 6-, and 2-fold greater for pigs in the MCFA than those in the LCFA and CONT groups (P < 0.05).

CONCLUSIONS

Feeding MCFA resulted in hepatic steatosis compared with an isocaloric formula rich in LCFA. Steatosis occurred concomitantly with reduced fatty acid oxidation but greater mRNA expression of fatty acid synthetic and catabolic genes.

Feeding medium-chain fatty acid-rich formula causes liver steatosis and alters hepatic metabolism in neonatal pigs

Medium-chain fatty acids (MCFA) and long-chain fatty acids (LCFAs) are often added to enhance the caloric value of infant formulas. Evidence suggests that MCFAs promote growth and are preferred over LCFAs due to greater digestibility and ease of absorption. Our hypothesis was that MCFA supplementation would enhance neonatal pig growth to a greater extent than LCFAs. Neonatal pigs (n = 4) were fed a low-energy control (CONT) or two isocaloric high-energy formulas containing fat either from LCFAs, or MCFAs for 20 days. Pigs fed the LCFAs had greater body weight compared with CONT- and MCFA-fed pigs (P < 0.05). In addition, pigs fed the LCFAs and MCFAs had more body fat than those in the CONT group. Liver and kidney weights as a percentage of body weight were greater (P ≤ 0.05) for pigs fed the MCFAs than those fed the CONT formula, and in those fed LCFAs, liver and kidney weights as a percentage of body weight were intermediate (P ≤ 0.05). Pigs in the CONT and LCFA groups had less liver fat (12%) compared with those in the MCFA (26%) group (P ≤ 0.05). Isolated hepatocytes from these pigs were incubated in media containing [13C]tracers of alanine, glucose, glutamate, and propionate. Our data suggest alanine contribution to pyruvate is less in hepatocytes from LCFA and MCFA pigs than those in the CONT group (P < 0.05). These data suggest that a formula rich in MCFAs caused steatosis compared with an isocaloric LCFA formula. In addition, MCFA feeding can alter hepatocyte metabolism and increase total body fat without increasing lean deposition.NEW & NOTEWORTHY Our data suggest that feeding high-energy MCFA formula resulted in hepatic steatosis compared with isoenergetic LCFA or low-energy formulas. Steatosis coincided with greater laurate, myristate, and palmitate accumulation, suggesting elongation of dietary laurate. Data also suggest that hepatocytes metabolized alanine and glucose to pyruvate, but neither entered the tricarboxylic acid (TCA) cycle. In addition, the contribution of alanine and glucose was greater for the low-energy formulas compared with the high-energy formulas.

Rumen-protected choline and methionine during the periparturient period affect choline metabolites, amino acids, and hepatic expression of genes associated with one-carbon and lipid metabolism

Feeding supplemental choline and Met during the periparturient period can have positive effects on cow performance; however, the mechanisms by which these nutrients affect performance and metabolism are unclear. The objective of this experiment was to determine if providing rumen-protected choline, rumen-protected Met, or both during the periparturient period modifies the choline metabolitic profile of plasma and milk, plasma AA, and hepatic mRNA expression of genes associated with choline, Met, and lipid metabolism. Cows (25 primiparous, 29 multiparous) were blocked by expected calving date and parity and randomly assigned to 1 of 4 treatments: control (no rumen-protected choline or rumen-protected Met); CHO (13 g/d choline ion); MET (9 g/d DL-methionine prepartum; 13.5 g/d DL-methionine, postpartum); or CHO + MET. Treatments were applied daily as a top dress from ~21 d prepartum through 35 d in milk (DIM). On the day of treatment enrollment (d -19 ± 2 relative to calving), blood samples were collected for covariate measurements. At 7 and 14 DIM, samples of blood and milk were collected for analysis of choline metabolites, including 16 species of phosphatidylcholine (PC) and 4 species of lysophosphatidylcholine (LPC). Blood was also analyzed for AA concentrations. Liver samples collected from multiparous cows on the day of treatment enrollment and at 7 DIM were used for gene expression analysis. There was no consistent effect of CHO or MET on milk or plasma free choline, betaine, sphingomyelin, or glycerophosphocholine. However, CHO increased milk secretion of total LPC irrespective of MET for multiparous cows and in absence of MET for primiparous cows. Furthermore, CHO increased or tended to increase milk secretion of LPC 16:0, LPC 18:1, and LPC 18:0 for primi- and multiparous cows, although the response varied with MET supplementation. Feeding CHO also increased plasma concentrations of LPC 16:0 and LPC 18:1 in absence of MET for multiparous cows. Although milk secretion of total PC was unaffected, CHO and MET increased secretion of 6 and 5 individual PC species for multiparous cows, respectively. Plasma concentrations of total PC and individual PC species were unaffected by CHO or MET for multiparous cows, but MET reduced total PC and 11 PC species during wk 2 postpartum for primiparous cows. Feeding MET consistently increased plasma Met concentrations for both primi- and multiparous cows. Additionally, MET decreased plasma serine concentrations during wk 2 postpartum and increased plasma phenylalanine in absence of CHO for multiparous cows. In absence of MET, CHO tended to increase hepatic mRNA levels of betaine-homocysteine methyltransferase and phosphate cytidylyltransferase 1 choline, α, but tended to decrease expression of 3-hydroxy-3-methylglutaryl-coenzyme A synthase 2 and peroxisome proliferator activated receptor α irrespective of MET. Although shifts in the milk and plasma PC profile were subtle and inconsistent between primi- and multiparous cows, gene expression results suggest that supplemental choline plays a probable role in promoting the cytidine diphosphate-choline and betaine-homocysteine S-methyltransferase pathways. However, interactive effects suggest that this response depends on Met availability, which may explain the inconsistent results observed among studies when supplemental choline is fed.

Branched-chain amino acids alter cellular redox to induce lipid oxidation and reduce de novo lipogenesis in the liver

Metabolic and molecular interactions between branched-chain amino acid (BCAA) and lipid metabolism are evident in insulin-resistant tissues. However, it remains unclear whether insulin resistance is a prerequisite for these relationships and whether BCAAs or their metabolic intermediates can modulate hepatic lipid oxidation and synthesis. We hypothesized that BCAAs can alter hepatic oxidative function and de novo lipogenesis, independent of them being anaplerotic substrates for the mitochondria. Mice (C57BL/6NJ) were reared on a low-fat (LF), LF diet plus 1.5X BCAAs (LB), high-fat (HF) or HF diet plus 1.5X BCAAs (HB) for 12 wk. Hepatic metabolism was profiled utilizing stable isotopes coupled to mass spectrometry and nuclear magnetic resonance, together with fed-to-fasted changes in gene and protein expression. A greater induction of lipid oxidation and ketogenesis on fasting was evident in the BCAA-supplemented, insulin-sensitive livers from LB mice, whereas their rates of hepatic de novo lipogenesis remained lower than their LF counterparts. Onset of insulin resistance in HF and HB mice livers blunted these responses. Whole body turnover of BCAAs and their ketoacids, their serum concentrations, and the ketogenic flux from BCAA catabolism, all remained similar between fasted LF and LB mice. This suggested that the impact of BCAAs on lipid metabolism can occur independent of them or their degradation products fueling anaplerosis through the liver mitochondria. Furthermore, the greater induction of lipid oxidation in the LB livers accompanied higher mitochondrial NADH/NAD+ ratio and higher fed-to-fasting phosphorylation of AMPKα and ACC. Taken together, our results provide evidence that BCAA supplementation, under conditions of insulin sensitivity, improved the feeding-to-fasting induction of hepatic lipid oxidation through changes in cellular redox, thus providing a favorable biochemical environment for flux through β-oxidation and lower de novo lipogenesis.NEW & NOTEWORTHY Branched-chain amino acids (BCAAs) have been shown to modulate lipid metabolic networks in various tissues, especially during insulin resistance. In this study we show that the dietary supplementation of BCAAs to normal, insulin-sensitive mice resulted in higher mitochondrial NADH:NAD+ ratios and AMPK activation in the liver. This change in the cellular redox status provided an optimal metabolic milieu to increase fatty acid oxidation while keeping the rates of de novo lipogenesis lower in the BCAA-supplemented mice livers.

Remodeling of Hepatocyte Mitochondrial Metabolism and De Novo Lipogenesis During the Embryonic-to-Neonatal Transition in Chickens

Embryonic-to-neonatal development in chicken is characterized by high rates of lipid oxidation in the late-term embryonic liver and high rates of de novo lipogenesis in the neonatal liver. This rapid remodeling of hepatic mitochondrial and cytoplasmic networks occurs without symptoms of hepatocellular stress. Our objective was to characterize the metabolic phenotype of the embryonic and neonatal liver and explore whether these metabolic signatures are preserved in primary cultured hepatocytes. Plasma and liver metabolites were profiled using mass spectrometry based metabolomics on embryonic day 18 (ed18) and neonatal day 3 (nd3). Hepatocytes from ed18 and nd3 were isolated and cultured, and treated with insulin, glucagon, growth hormone and corticosterone to define hormonal responsiveness and determine their impacts on mitochondrial metabolism and lipogenesis. Metabolic profiling illustrated the clear transition from the embryonic liver relying on lipid oxidation to the neonatal liver upregulating de novo lipogenesis. This metabolic phenotype was conserved in the isolated hepatocytes from the embryos and the neonates. Cultured hepatocytes from the neonatal liver also maintained a robust response to insulin and glucagon, as evidenced by their contradictory effects on lipid oxidation and lipogenesis. In summary, primary hepatocytes from the embryonic and neonatal chicken could be a valuable tool to investigate mechanisms regulating hepatic mitochondrial metabolism and de novo lipogenesis.

Dietary Macronutrient Composition Differentially Modulates the Remodeling of Mitochondrial Oxidative Metabolism during NAFLD

Diets rich in fats and carbohydrates aggravate non-alcoholic fatty liver disease (NAFLD), of which mitochondrial dysfunction is a central feature. It is not clear whether a high-carbohydrate driven ‘lipogenic’ diet differentially affects mitochondrial oxidative remodeling compared to a high-fat driven ‘oxidative’ environment. We hypothesized that the high-fat driven ‘oxidative’ environment will chronically sustain mitochondrial oxidative function, hastening metabolic dysfunction during NAFLD. Mice (C57BL/6NJ) were reared on a low-fat (LF; 10% fat calories), high-fat (HF; 60% fat calories), or high-fructose/high-fat (HFr/HF; 25% fat and 34.9% fructose calories) diet for 10 weeks. De novo lipogenesis was determined by measuring the incorporation of deuterium from D₂O into newly synthesized liver lipids using nuclear magnetic resonance (NMR) spectroscopy. Hepatic mitochondrial metabolism was profiled under fed and fasted states by the incubation of isolated mitochondria with [¹³C₃]pyruvate, targeted metabolomics of tricarboxylic acid (TCA) cycle intermediates, estimates of oxidative phosphorylation (OXPHOS), and hepatic gene and protein expression. De novo lipogenesis was higher in the HFr/HF mice compared to their HF counterparts. Contrary to our expectations, hepatic oxidative function after fasting was induced in the HFr/HF group. This differential induction of mitochondrial oxidative function by the high fructose-driven ‘lipogenic’ environment could influence the progressive severity of hepatic insulin resistance.

Branched chain amino acids and carbohydrate restriction exacerbate ketogenesis and hepatic mitochondrial oxidative dysfunction during NAFLD

Mitochondrial adaptation during non-alcoholic fatty liver disease (NAFLD) include remodeling of ketogenic flux and sustained tricarboxylic acid (TCA) cycle activity, which are concurrent to onset of oxidative stress. Over 70% of obese humans have NAFLD and ketogenic diets are common weight loss strategies. However, the effectiveness of ketogenic diets toward alleviating NAFLD remains unclear. We hypothesized that chronic ketogenesis will worsen metabolic dysfunction and oxidative stress during NAFLD. Mice (C57BL/6) were kept (for 16-wks) on either a low-fat, high-fat, or high-fat diet supplemented with 1.5X branched chain amino acids (BCAAs) by replacing carbohydrate calories (ketogenic). The ketogenic diet induced hepatic lipid oxidation and ketogenesis, and produced multifaceted changes in flux through the individual steps of the TCA cycle. Higher rates of hepatic oxidative fluxes fueled by the ketogenic diet paralleled lower rates of de novo lipogenesis. Interestingly, this metabolic remodeling did not improve insulin resistance, but induced fibrogenic genes and inflammation in the liver. Under a chronic "ketogenic environment," the hepatocyte diverted more acetyl-CoA away from lipogenesis toward ketogenesis and TCA cycle, a milieu which can hasten oxidative stress and inflammation. In summary, chronic exposure to ketogenic environment during obesity and NAFLD has the potential to aggravate hepatic mitochondrial dysfunction.

Lipid Intake Enhances Muscle Growth But Does Not Influence Glucose Kinetics in 3-Week-Old Low-Birth-Weight Neonatal Pigs

BACKGROUND

Low-birth-weight (LBWT) neonates grow at a slower rate than their normal-birth-weight (NBWT) counterparts and may develop hypoglycemia postnatally.

OBJECTIVE

We investigated whether dietary lipid supplementation would enhance growth and improve glucose production in LBWT neonatal pigs.

METHODS

Twelve 3-d-old NBWT (1.606 kg) crossbred pigs were matched to 12 LBWT (1.260 kg) same-sex littermates. At 6 d of age, 6 pigs in each group were fed a low-energy (LE) or a high-energy (HE) isonitrogenous formula containing 5.2% and 7.3% fat, respectively. Body composition was assessed using dual-energy X-ray absorptiometry; plasma glucose and glycerol kinetics were assessed using stable isotope tracers. After killing, weights of skeletal muscles and visceral organs were measured. Data were analyzed by ANOVA for a 2 × 2 factorial design; temporal effects were investigated using repeated-measures analysis.

RESULTS

Lipid supplementation did not affect body weight of LBWT or NBWT pigs. However, liver and longissimus dorsi weights as a percentage of body weight were greater for pigs fed an HE diet than for those fed an LE diet (4.3% compared with 3.4% and 1.5% compared with 1.2%, respectively) but remained less for LBWT than for NBWT pigs (3.8% compared with 3.9% and 1.3% compared with 1.5%, respectively) (P < 0.05). In addition, hepatic fat content increased (7.9 compared with 2.6 g) in pigs fed the HE compared with those fed the LE formula (P < 0.05). Lipid supplementation did not influence plasma glucose concentration which remained lower in the LBWT than in the NBWT group (4.1 compared with 4.5 mmol/L) (P < 0.05).

CONCLUSIONS

Our data suggest that lipid supplementation modestly improved growth of skeletal muscle and the liver but did not affect glucose homeostasis in all groups, and glucose concentration remained lower in LBWT than in NBWT pigs. These data suggest that the previously reported hyperglycemic effect of lipid supplementation may depend on the route of administration or age of the neonatal pig.

Impact of exenatide on mitochondrial lipid metabolism in mice with nonalcoholic steatohepatitis

Exenatide (Exe) is a glucagon-like peptide (GLP)-1 receptor agonist that enhances insulin secretion and is associated with induction of satiety with weight loss. As mitochondrial dysfunction and lipotoxicity are central features of nonalcoholic steatohepatitis (NASH), we tested whether Exe improved mitochondrial function in this setting. We studied C57BL/6J mice fed for 24 weeks either a control- or high-fructose, high-trans-fat (TFD)-diet (i.e., a NASH model previously validated by our laboratory). For the final 8 weeks, mice were treated with Exe (30 µg/kg/day) or vehicle. Mitochondrial metabolism was assessed by infusion of [13C3]propionate, [3,4-13C2]glucose and NMR-based 13C-isotopomer analysis. Exenatide significantly decreased fasting plasma glucose, free fatty acids and triglycerides, as well as adipose tissue insulin resistance. Moreover, Exe reduced 23% hepatic glucose production, 15% tri-carboxylic acid (TCA) cycle flux, 20% anaplerosis and 17% pyruvate cycling resulting in a significant 31% decrease in intrahepatic triglyceride content (P = 0.02). Exenatide improved the lipidomic profile and decreased hepatic lipid byproducts associated with insulin resistance and lipotoxicity, such as diacylglycerols (TFD: 111 ± 13 vs Exe: 64 ± 13 µmol/g protein, P = 0.03) and ceramides (TFD: 1.6 ± 0.1 vs Exe: 1.3 ± 0.1 µmol/g protein, P = 0.03). Exenatide lowered expression of hepatic lipogenic genes (Srebp1C, Cd36) and genes involved in inflammation and fibrosis (Tnfa, Timp1). In conclusion, in a diet-induced mouse model of NASH, Exe ameliorates mitochondrial TCA cycle flux and significantly decreases insulin resistance, steatosis and hepatocyte lipotoxicity. This may have significant clinical implications to the potential mechanism of action of GLP-1 receptor agonists in patients with NASH. Future studies should elucidate the relative contribution of direct vs indirect mechanisms at play.

Delayed Feeding Alters Transcriptional and Post-Transcriptional Regulation of Hepatic Metabolic Pathways in Peri-Hatch Broiler Chicks

Hepatic fatty acid oxidation of yolk lipoproteins provides the main energy source for chick embryos. Post-hatching these yolk lipids are rapidly exhausted and metabolism switches to a carbohydrate-based energy source. We recently demonstrated that many microRNAs (miRNAs) are key regulators of hepatic metabolic pathways during this metabolic switching. MiRNAs are small non-coding RNAs that post-transcriptionally regulate gene expression in most eukaryotes. To further elucidate the roles of miRNAs in the metabolic switch, we used delayed feeding for 48 h to impede the hepatic metabolic switch. We found that hepatic expression of several miRNAs including miR-33, miR-20b, miR-34a, and miR-454 was affected by delaying feed consumption for 48 h. For example, we found that delayed feeding resulted in increased miR-20b expression and conversely reduced expression of its target FADS1, an enzyme involved in fatty acid synthesis. Interestingly, the expression of a previously identified miR-20b regulator FOXO3 was also higher in delayed fed chicks. FOXO3 also functions in protection of cells from oxidative stress. Delayed fed chicks also had much higher levels of plasma ketone bodies than their normal fed counterparts. This suggests that delayed fed chicks rely almost exclusively on lipid oxidation for energy production and are likely under higher oxidative stress. Thus, it is possible that FOXO3 may function to both limit lipogenesis as well as to help protect against oxidative stress in peri-hatch chicks until the initiation of feed consumption. This is further supported by evidence that the FOXO3-regulated histone deacetylase (HDAC2) was found to recognize the FASN (involved in fatty acid synthesis) chicken promoter in a yeast one-hybrid assay. Expression of FASN mRNA was lower in delayed fed chicks until feed consumption. The present study demonstrated that many transcriptional and post-transcriptional mechanisms, including miRNA, form a complex interconnected regulatory network that is involved in controlling lipid and glucose molecular pathways during the metabolic transition in peri-hatch chicks.

Effect of canagliflozin treatment on hepatic triglyceride content and glucose metabolism in patients with type 2 diabetes

AIM

To evaluate the impact of the sodium glucose co-transporter 2 inhibitor canagliflozin on intrahepatic triglyceride (IHTG) accumulation and its relationship to changes in body weight and glucose metabolism.

MATERIALS AND METHODS

In this double-blind, parallel-group, placebo-controlled, 24-week trial subjects with inadequately controlled type 2 diabetes mellitus (T2DM; HbA1c = 7.7% ± 0.7%) from two centres were randomly assigned (1:1) to canagliflozin 300 mg or placebo. We measured IHTG by proton-magnetic resonance spectroscopy (primary outcome), hepatic/muscle/adipose tissue insulin sensitivity during a 2-step euglycaemic insulin clamp, and beta-cell function during a mixed meal tolerance test. Analyses were per protocol.

RESULTS

Between 8 September 2014-13 June 2016, 56 patients were enrolled. Canagliflozin reduced HbA1c (placebo-subtracted change: -0.71% [-1.08; -0.33]) and body weight (-3.4% [-5.4; -1.4]; both P ≤ 0.001). A numerically larger absolute decrease in IHTG occurred with canagliflozin (-4.6% [-6.4; -2.7]) versus placebo (-2.4% [-4.2; -0.6]; P = 0.09). In patients with non-alcoholic fatty liver disease (n = 37), the decrease in IHTG was -6.9% (-9.5; -4.2) versus -3.8% (-6.3; -1.3; P = 0.05), and strongly correlated with the magnitude of weight loss (r = 0.69, P < 0.001). Body weight loss ≥5% with a ≥30% relative reduction in IHTG occurred more often with canagliflozin (38% vs. 7%, P = 0.009). Hepatic insulin sensitivity improved with canagliflozin (P < 0.01), but not muscle or adipose tissue insulin sensitivity. Beta-cell glucose sensitivity, insulin clearance, and disposition index improved more with canagliflozin (P < 0.05).

CONCLUSIONS

Canagliflozin improves hepatic insulin sensitivity and insulin secretion and clearance in patients with T2DM. IHTG decreases in proportion to the magnitude of body weight loss, which tended to be greater and occur more often with canagliflozin.

Pioglitazone improves hepatic mitochondrial function in a mouse model of nonalcoholic steatohepatitis

Pioglitazone is effective in improving insulin resistance and liver histology in patients with nonalcoholic steatohepatitis (NASH). Because dysfunctional mitochondrial metabolism is a central feature of NASH, we hypothesized that an important target of pioglitazone would be alleviating mitochondrial oxidative dysfunction. To this end, we studied hepatic mitochondrial metabolism in mice fed high-fructose high-transfat diet (TFD) supplemented with pioglitazone for 20 wk, using nuclear magnetic resonance-based ¹³C isotopomer analysis. Pioglitazone improved whole body and adipose insulin sensitivity in TFD-fed mice. Furthermore, pioglitazone reduced intrahepatic triglyceride content and fed plasma ketones and hepatic TCA cycle flux, anaplerosis, and pyruvate cycling in mice with NASH. This was associated with a marked reduction in most intrahepatic diacylglycerol classes and, to a lesser extent, some ceramide species (C22:1, C23:0). Considering the cross-talk between mitochondrial function and branched-chain amino acid (BCAA) metabolism, pioglitazone's impact on plasma BCAA profile was determined in a cohort of human subjects. Pioglitazone improved the plasma BCAA concentration profile in patients with NASH. This appeared to be related to an improvement in BCAA degradation in multiple tissues. These results provide evidence that pioglitazone-induced changes in NASH are related to improvements in hepatic mitochondrial oxidative dysfunction and changes in whole body BCAA metabolism.

Mitochondrial Adaptation in Nonalcoholic Fatty Liver Disease: Novel Mechanisms and Treatment Strategies

Nonalcoholic fatty liver disease (NAFLD) is prevalent in patients with obesity or type 2 diabetes. Nonalcoholic steatohepatitis (NASH), encompassing steatosis with inflammation, hepatocyte injury, and fibrosis, predisposes to cirrhosis, hepatocellular carcinoma, and even cardiovascular disease. In rodent models and humans with NAFLD/NASH, maladaptation of mitochondrial oxidative flux is a central feature of simple steatosis to NASH transition. Induction of hepatic tricarboxylic acid cycle closely mirrors the severity of oxidative stress and inflammation in NASH. Reactive oxygen species generation and inflammation are driven by upregulated, but inefficient oxidative flux and accumulating lipotoxic intermediates. Successful therapies for NASH (weight loss alone or with incretin therapy, or pioglitazone) likely attenuate mitochondrial oxidative flux and halt hepatocellular injury. Agents targeting mitochondrial dysfunction may provide a novel treatment strategy for NAFLD.

Mitochondrial ATP transporter depletion protects mice against liver steatosis and insulin resistance

Non-alcoholic fatty liver disease (NAFLD) is a common metabolic disorder in obese individuals. Adenine nucleotide translocase (ANT) exchanges ADP/ATP through the mitochondrial inner membrane, and Ant2 is the predominant isoform expressed in the liver. Here we demonstrate that targeted disruption of Ant2 in mouse liver enhances uncoupled respiration without damaging mitochondrial integrity and liver functions. Interestingly, liver specific Ant2 knockout mice are leaner and resistant to hepatic steatosis, obesity and insulin resistance under a lipogenic diet. Protection against fatty liver is partially recapitulated by the systemic administration of low-dose carboxyatractyloside, a specific inhibitor of ANT. Targeted manipulation of hepatic mitochondrial metabolism, particularly through inhibition of ANT, may represent an alternative approach in NAFLD and obesity treatment.

Adenine nucleotide translocase (ANT) 2 promotes ADP/ATP exchange across the mitochondrial inner membrane. Cho et al. show that liver specific Ant2 deletion increases uncoupled respiration and protects mice against fatty liver and obesity-induced insulin resistance.

Cold adaptation increases rates of nutrient flow and metabolic plasticity during cold exposure in Drosophila melanogaster

Metabolic flexibility is an important component of adaptation to stressful environments, including thermal stress and latitudinal adaptation. A long history of population genetic studies suggest that selection on core metabolic enzymes may shape life histories by altering metabolic flux. However, the direct relationship between selection on thermal stress hardiness and metabolic flux has not previously been tested. We investigated flexibility of nutrient catabolism during cold stress in Drosophila melanogaster artificially selected for fast or slow recovery from chill coma (i.e. cold-hardy or -susceptible), specifically testing the hypothesis that stress adaptation increases metabolic turnover. Using (13)C-labelled glucose, we first showed that cold-hardy flies more rapidly incorporate ingested carbon into amino acids and newly synthesized glucose, permitting rapid synthesis of proline, a compound shown elsewhere to improve survival of cold stress. Second, using glucose and leucine tracers we showed that cold-hardy flies had higher oxidation rates than cold-susceptible flies before cold exposure, similar oxidation rates during cold exposure, and returned to higher oxidation rates during recovery. Additionally, cold-hardy flies transferred compounds among body pools more rapidly during cold exposure and recovery. Increased metabolic turnover may allow cold-adapted flies to better prepare for, resist and repair/tolerate cold damage. This work illustrates for the first time differences in nutrient fluxes associated with cold adaptation, suggesting that metabolic costs associated with cold hardiness could invoke resource-based trade-offs that shape life histories.

Lipotoxicity in steatohepatitis occurs despite an increase in tricarboxylic acid cycle activity

The hepatic tricarboxylic acid (TCA) cycle is central to integrating macronutrient metabolism and is closely coupled to cellular respiration, free radical generation, and inflammation. Oxidative flux through the TCA cycle is induced during hepatic insulin resistance, in mice and humans with simple steatosis, reflecting early compensatory remodeling of mitochondrial energetics. We hypothesized that progressive severity of hepatic insulin resistance and the onset of nonalcoholic steatohepatitis (NASH) would impair oxidative flux through the hepatic TCA cycle. Mice (C57/BL6) were fed a high-trans-fat high-fructose diet (TFD) for 8 wk to induce simple steatosis and NASH by 24 wk. In vivo fasting hepatic mitochondrial fluxes were determined by(13)C-nuclear magnetic resonance (NMR)-based isotopomer analysis. Hepatic metabolic intermediates were quantified using mass spectrometry-based targeted metabolomics. Hepatic triglyceride accumulation and insulin resistance preceded alterations in mitochondrial metabolism, since TCA cycle fluxes remained normal during simple steatosis. However, mice with NASH had a twofold induction (P< 0.05) of mitochondrial fluxes (μmol/min) through the TCA cycle (2.6 ± 0.5 vs. 5.4 ± 0.6), anaplerosis (9.1 ± 1.2 vs. 16.9 ± 2.2), and pyruvate cycling (4.9 ± 1.0 vs. 11.1 ± 1.9) compared with their age-matched controls. Induction of the TCA cycle activity during NASH was concurrent with blunted ketogenesis and accumulation of hepatic diacylglycerols (DAGs), ceramides (Cer), and long-chain acylcarnitines, suggesting inefficient oxidation and disposal of excess free fatty acids (FFA). Sustained induction of mitochondrial TCA cycle failed to prevent accretion of "lipotoxic" metabolites in the liver and could hasten inflammation and the metabolic transition to NASH.

Plasma thyroid hormone concentration is associated with hepatic triglyceride content in patients with type 2 diabetes

The underlying mechanisms responsible for the development and progression of non-alcoholic fatty liver disease (NAFLD) in patients with type 2 diabetes mellitus (T2DM) are unclear. Since the thyroid hormone regulates mitochondrial function in the liver, we designed this study in order to establish the association between plasma free T4 levels and hepatic triglyceride accumulation and histological severity of liver disease in patients with T2DM and NAFLD. This is a cross-sectional study including a total of 232 patients with T2DM. All patients underwent a liver MR spectroscopy ((1)H-MRS) to quantify hepatic triglyceride content, and an oral glucose tolerance test to estimate insulin resistance. A liver biopsy was performed in patients with a diagnosis of NAFLD. Patients were divided into 5 groups according to plasma free T4 quintiles. We observed that decreasing free T4 levels were associated with an increasing prevalence of NAFLD (from 55% if free T4≥1.18 ng/dL to 80% if free T4<0.80 ng/dL, p=0.016), and higher hepatic triglyceride accumulation by (1)H-MRS (p<0.001). However, lower plasma free T4 levels were not significantly associated with more insulin resistance or more severe liver histology (ie, inflammation, ballooning, or fibrosis). Decreasing levels of plasma free T4 are associated with a higher prevalence of NAFLD and increasing levels of hepatic triglyceride content in patients with T2DM. These results suggest that thyroid hormone may play a role in the regulation of hepatic steatosis and support the notion that hypothyroidism may be associated with NAFLD. No NCT number required.

Mitochondrial metabolism mediates oxidative stress and inflammation in fatty liver

Mitochondria are critical for respiration in all tissues; however, in liver, these organelles also accommodate high-capacity anaplerotic/cataplerotic pathways that are essential to gluconeogenesis and other biosynthetic activities. During nonalcoholic fatty liver disease (NAFLD), mitochondria also produce ROS that damage hepatocytes, trigger inflammation, and contribute to insulin resistance. Here, we provide several lines of evidence indicating that induction of biosynthesis through hepatic anaplerotic/cataplerotic pathways is energetically backed by elevated oxidative metabolism and hence contributes to oxidative stress and inflammation during NAFLD. First, in murine livers, elevation of fatty acid delivery not only induced oxidative metabolism, but also amplified anaplerosis/cataplerosis and caused a proportional rise in oxidative stress and inflammation. Second, loss of anaplerosis/cataplerosis via genetic knockdown of phosphoenolpyruvate carboxykinase 1 (Pck1) prevented fatty acid-induced rise in oxidative flux, oxidative stress, and inflammation. Flux appeared to be regulated by redox state, energy charge, and metabolite concentration, which may also amplify antioxidant pathways. Third, preventing elevated oxidative metabolism with metformin also normalized hepatic anaplerosis/cataplerosis and reduced markers of inflammation. Finally, independent histological grades in human NAFLD biopsies were proportional to oxidative flux. Thus, hepatic oxidative stress and inflammation are associated with elevated oxidative metabolism during an obesogenic diet, and this link may be provoked by increased work through anabolic pathways.

Cross-talk between branched-chain amino acids and hepatic mitochondria is compromised in nonalcoholic fatty liver disease

Elevated plasma branched-chain amino acids (BCAA) in the setting of insulin resistance have been relevant in predicting type 2 diabetes mellitus (T2DM) onset, but their role in the etiology of hepatic insulin resistance remains uncertain. We determined the link between BCAA and dysfunctional hepatic tricarboxylic acid (TCA) cycle, which is a central feature of hepatic insulin resistance and nonalcoholic fatty liver disease (NAFLD). Plasma metabolites under basal fasting and euglycemic hyperinsulinemic clamps (insulin stimulation) were measured in 94 human subjects with varying degrees of insulin sensitivity to identify their relationships with insulin resistance. Furthermore, the impact of elevated BCAA on hepatic TCA cycle was determined in a diet-induced mouse model of NAFLD, utilizing targeted metabolomics and nuclear magnetic resonance (NMR)-based metabolic flux analysis. Insulin stimulation revealed robust relationships between human plasma BCAA and indices of insulin resistance, indicating chronic metabolic overload from BCAA. Human plasma BCAA and long-chain acylcarnitines also showed a positive correlation, suggesting modulation of mitochondrial metabolism by BCAA. Concurrently, mice with NAFLD failed to optimally induce hepatic mTORC1, plasma ketones, and hepatic long-chain acylcarnitines, following acute elevation of plasma BCAA. Furthermore, elevated BCAA failed to induce multiple fluxes through hepatic TCA cycle in mice with NAFLD. Our data suggest that BCAA are essential to mediate efficient channeling of carbon substrates for oxidation through mitochondrial TCA cycle. Impairment of BCAA-mediated upregulation of the TCA cycle could be a significant contributor to mitochondrial dysfunction in NAFLD.

Nonalcoholic fatty liver disease: current issues and novel treatment approaches

Nonalcoholic fatty liver disease (NAFLD) is considered the most common liver disorder in the Western world. It is commonly associated with insulin resistance, obesity, dyslipidaemia, type 2 diabetes mellitus (T2DM) and cardiovascular disease. Nonalcoholic steatohepatitis (NASH) is characterized by steatosis with necroinflammation and eventual fibrosis, which can lead to end-stage liver disease and hepatocellular carcinoma. Its pathogenesis is complex, and involves a state of 'lipotoxicity' in which insulin resistance, with increased free fatty acid release from adipose tissue to the liver, play a key role in the onset of a 'lipotoxic liver disease' and its progression to NASH. The diagnosis of NASH is challenging, as most affected patients are symptom free and the role of routine screening is not clearly established. A complete medical history is important to rule out other causes of fatty liver disease (alcohol abuse, medications, other). Plasma aminotransferase levels and liver ultrasound are helpful in the diagnosis of NAFLD/NASH, but a liver biopsy is often required for a definitive diagnosis. However, there is an active search for plasma biomarkers and imaging techniques that may non-invasively aid in the diagnosis. The treatment of NASH requires a multifaceted approach. The goal is to reverse obesity-associated lipotoxicity and insulin resistance via lifestyle intervention. Although there is no pharmacological agent approved for the treatment of NAFLD, vitamin E (in patients without T2DM) and the thiazolidinedione pioglitazone (in patients with and without T2DM) have shown the most consistent results in randomized controlled trials. This review concentrates on our current understanding of the disease, with a focus on the existing therapeutic approaches and potential future pharmacological developments for NAFLD and NASH.

PEPCK-M expression in mouse liver potentiates, not replaces, PEPCK-C mediated gluconeogenesis

BACKGROUND & AIMS

Hepatic gluconeogenesis helps maintain systemic energy homeostasis by compensating for discontinuities in nutrient supply. Liver-specific deletion of cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C) abolishes gluconeogenesis from mitochondrial substrates, deregulates lipid metabolism and affects TCA cycle. While the mouse liver almost exclusively expresses PEPCK-C, humans equally present a mitochondrial isozyme (PEPCK-M). Despite clear relevance to human physiology, the role of PEPCK-M and its gluconeogenic potential remain unknown. Here, we test the significance of PEPCK-M in gluconeogenesis and TCA cycle function in liver-specific PEPCK-C knockout and WT mice.

METHODS

The effects of the overexpression of PEPCK-M were examined by a combination of tracer studies and molecular biology techniques. Partial PEPCK-C re-expression was used as a positive control. Metabolic fluxes were evaluated in isolated livers by NMR using (2)H and (13)C tracers. Gluconeogenic potential, together with metabolic profiling, was investigated in vivo and in primary hepatocytes.

RESULTS

PEPCK-M expression partially rescued defects in lipid metabolism, gluconeogenesis and TCA cycle function impaired by PEPCK-C deletion, while ∼10% re-expression of PEPCK-C normalized most parameters. When PEPCK-M was expressed in the presence of PEPCK-C, the mitochondrial isozyme amplified total gluconeogenic capacity, suggesting autonomous regulation of oxaloacetate to phosphoenolpyruvate fluxes by the individual isoforms.

CONCLUSIONS

We conclude that PEPCK-M has gluconeogenic potential per se, and cooperates with PEPCK-C to adjust gluconeogenic/TCA flux to changes in substrate or energy availability, hinting at a role in the regulation of glucose and lipid metabolism in the human liver.

Elevated TCA cycle function in the pathology of diet-induced hepatic insulin resistance and fatty liver

The manner in which insulin resistance impinges on hepatic mitochondrial function is complex. Although liver insulin resistance is associated with respiratory dysfunction, the effect on fat oxidation remains controversial, and biosynthetic pathways that traverse mitochondria are actually increased. The tricarboxylic acid (TCA) cycle is the site of terminal fat oxidation, chief source of electrons for respiration, and a metabolic progenitor of gluconeogenesis. Therefore, we tested whether insulin resistance promotes hepatic TCA cycle flux in mice progressing to insulin resistance and fatty liver on a high-fat diet (HFD) for 32 weeks using standard biomolecular and in vivo (2)H/(13)C tracer methods. Relative mitochondrial content increased, but respiratory efficiency declined by 32 weeks of HFD. Fasting ketogenesis became unresponsive to feeding or insulin clamp, indicating blunted but constitutively active mitochondrial β-oxidation. Impaired insulin signaling was marked by elevated in vivo gluconeogenesis and anaplerotic and oxidative TCA cycle flux. The induction of TCA cycle function corresponded to the development of mitochondrial respiratory dysfunction, hepatic oxidative stress, and inflammation. Thus, the hepatic TCA cycle appears to enable mitochondrial dysfunction during insulin resistance by increasing electron deposition into an inefficient respiratory chain prone to reactive oxygen species production and by providing mitochondria-derived substrate for elevated gluconeogenesis.

Excessive hepatic mitochondrial TCA cycle and gluconeogenesis in humans with nonalcoholic fatty liver disease

Approximately one-third of the U.S. population has nonalcoholic fatty liver disease (NAFLD), a condition closely associated with insulin resistance and increased risk of liver injury. Dysregulated mitochondrial metabolism is central in these disorders, but the manner and degree of dysregulation are disputed. This study tested whether humans with NAFLD have abnormal in vivo hepatic mitochondrial metabolism. Subjects with low (3.0%) and high (17%) intrahepatic triglyceride (IHTG) were studied using (2)H and (13)C tracers to evaluate systemic lipolysis, hepatic glucose production, and mitochondrial pathways (TCA cycle, anaplerosis, and ketogenesis). Individuals with NAFLD had 50% higher rates of lipolysis and 30% higher rates of gluconeogenesis. There was a positive correlation between IHTG content and both mitochondrial oxidative and anaplerotic fluxes. These data indicate that mitochondrial oxidative metabolism is ~2-fold greater in those with NAFLD, providing a potential link between IHTG content, oxidative stress, and liver damage.

FGF15/19 regulates hepatic glucose metabolism by inhibiting the CREB-PGC-1α pathway

Regulation of hepatic carbohydrate homeostasis is crucial for maintaining energy balance in the face of fluctuating nutrient availability. Here, we show that the hormone fibroblast growth factor 15/19 (FGF15/19), which is released postprandially from the small intestine, inhibits hepatic gluconeogenesis, like insulin. However, unlike insulin, which peaks in serum 15 min after feeding, FGF15/19 expression peaks approximately 45 min later, when bile acid concentrations increase in the small intestine. FGF15/19 blocks the expression of genes involved in gluconeogenesis through a mechanism involving the dephosphorylation and inactivation of the transcription factor cAMP regulatory element-binding protein (CREB). This in turn blunts expression of peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) and other genes involved in hepatic metabolism. Overexpression of PGC-1α blocks the inhibitory effect of FGF15/19 on gluconeogenic gene expression. These results demonstrate that FGF15/19 works subsequent to insulin as a postprandial regulator of hepatic carbohydrate homeostasis.

Uncoupling hypoxia signaling from oxygen sensing in the liver results in hypoketotic hypoglycemic death

As the ultimate electron acceptor in oxidative phosphorylation, oxygen plays a critical role in metabolism. When oxygen levels drop, heterodimeric hypoxia-inducible factor (Hif) transcription factors become active and facilitate adaptation to hypoxia. Hif regulation by oxygen requires the protein von Hippel-Lindau (pVhl) and pVhl disruption results in constitutive Hif activation. The liver is a critical organ for metabolic homeostasis, and Vhl inactivation in hepatocytes results in a Hif-dependent shortening in life span. While albumin-Cre;Vhl^F/F mice develop hepatic steatosis and impaired fatty acid oxidation, the variable penetrance and unpredictable life expectancy has made the cause of death elusive. Using a system in which Vhl is acutely disrupted and a combination of ex vivo liver perfusion studies and in vivo oxygen measurements, we demonstrate that Vhl is essential for mitochondrial respiration in vivo. Adenovirus-Cre mediated acute Vhl disruption in the liver caused death within days. Deprived of pVhl, livers accumulated tryglicerides and circulating ketone and glucose levels dropped. The phenotype was reminiscent of inborn defects in fatty acid oxidation and of fasted PPARα-deficient mice and while death was unaffected by pharmacologic PPARα activation, it was delayed by glucose administration. Ex vivo liver perfusion analyses and acylcarnitine profiles showed mitochondrial impairment and a profound inhibition of liver ketone and glucose production. By contrast, other mitochondrial functions, such as ureagenesis, were unaffected. Oxygen consumption studies revealed a marked suppression of mitochondrial respiration, which, as determined by magnetic resonance oximetry in live mice, was accompanied by a corresponding increase in liver pO₂. Importantly, simultaneous inactivation of Hif-1β suppressed liver steatosis and rescued the mice from death. These data demonstrate that constitutive Hif activation in mice is sufficient to suppress mitochondrial respiration in vivo and that no other pathway exists in the liver that can allow oxygen utilization when Hif is active precluding thereby metabolic collapse.

Progressive adaptation of hepatic ketogenesis in mice fed a high-fat diet

Hepatic ketogenesis provides a vital systemic fuel during fasting because ketone bodies are oxidized by most peripheral tissues and, unlike glucose, can be synthesized from fatty acids via mitochondrial beta-oxidation. Since dysfunctional mitochondrial fat oxidation may be a cofactor in insulin-resistant tissue, the objective of this study was to determine whether diet-induced insulin resistance in mice results in impaired in vivo hepatic fat oxidation secondary to defects in ketogenesis. Ketone turnover (micromol/min) in the conscious and unrestrained mouse was responsive to induction and diminution of hepatic fat oxidation, as indicated by an eightfold rise during the fed (0.50+/-0.1)-to-fasted (3.8+/-0.2) transition and a dramatic blunting of fasting ketone turnover in PPARalpha(-/-) mice (1.0+/-0.1). C57BL/6 mice made obese and insulin resistant by high-fat feeding for 8 wk had normal expression of genes that regulate hepatic fat oxidation, whereas 16 wk on the diet induced expression of these genes and stimulated the function of hepatic mitochondrial fat oxidation, as indicated by a 40% induction of fasting ketogenesis and a twofold rise in short-chain acylcarnitines. Together, these findings indicate a progressive adaptation of hepatic ketogenesis during high-fat feeding, resulting in increased hepatic fat oxidation after 16 wk of a high-fat diet. We conclude that mitochondrial fat oxidation is stimulated rather than impaired during the initiation of hepatic insulin resistance in mice.

Gluconeogenesis differs in developing chick embryos derived from small compared with typical size broiler breeder eggs

We hypothesized that, as the supply of preformed glucose diminishes during development, the embryo would transition to a greater rate of gluconeogenesis (GNG) and that GNG would be greater in embryos from small vs. typical size eggs. Gluconeogenesis by embryos from small (51.1 +/- 3.46 g) and typical size (65 +/- 4.35 g) broiler breeder eggs was measured by dosing [(13)C(6)]glucose (15 mgxegg(-1)) into the chorio-allantoic fluid for 3 consecutive days to achieve isotopic steady-state before blood collection on embryonic day (e) 12, e14, e16, and e18 (4 to 5 eggsxsize(-1)xd(-1)). The (13)C-Mass isotopomer enrichment of blood glucose was determined by gas chromatography-mass spectrometry. On e14, e16, and e18, but not on e12, embryos from small eggs weighed less (P < 0.05) than typical size eggs. For both sizes of eggs, blood glucose concentration, glucose entry rate (g.d(-1)), and Cori cycling and glucose (13)C-recycling (% of entry rate) increased (P < 0.05) with development. On e12 and e14, rates of glucose entry and Cori cycle flux were greater (P < 0.05) for embryos from small eggs. When standardized to BW (g.100 g of BW(-1)xd(-1)), glucose entry and Cori and non-Cori cycle fluxes were greater for embryos from small eggs. From e12 through e18, blood concentrations of gluconeogenic AA (threonine, glutamine, arginine, proline, isoleucine, and valine) were 25 to 48% less (P < 0.01) in embryos from small eggs. In conclusion, embryos from small eggs exhibit greater rates of GNG earlier in development compared with typical size eggs and, perhaps as a consequence, their reduced embryonic growth may result from diverting greater supplies of AA toward GNG.

Glutamate is the major anaplerotic substrate in the tricarboxylic acid cycle of isolated rumen epithelial and duodenal mucosal cells from beef cattle

In this study, we aimed to determine the contribution of substrates to tricarboxylic acid (TCA) cycle fluxes in rumen epithelial cells (REC) and duodenal mucosal cells (DMC) isolated from Angus bulls (n = 6) fed either a 75% forage (HF) or 75% concentrate (HC) diet. In separate incubations, [(13)C(6)]glucose, [(13)C(5)]glutamate, [(13)C(5)]glutamine, [(13)C(6)]leucine, or [(13)C(5)]valine were added in increasing concentrations to basal media containing SCFA and a complete mixture of amino acids. Lactate, pyruvate, and TCA cycle intermediates were analyzed by GC-MS followed by (13)C-mass isotopomer distribution analysis. Glucose metabolism accounted for 10-19% of lactate flux in REC from HF-fed bulls compared with 27-39% in REC from HC and in DMC from bulls fed both diets (P < 0.05). For both cell types, as concentration increased, an increasing proportion (3-63%) of alpha-ketoglutarate flux derived from glutamate, whereas glutamine contributed <3% (P < 0.05). Although leucine and valine were catabolized to their respective keto-acids, these were not further metabolized to TCA cycle intermediates. Glucose, glutamine, leucine, and valine catabolism by ruminant gastrointestinal tract cells has been previously demonstrated, but in this study, their catabolism via the TCA cycle was limited. Further, although glutamate's contribution to TCA cycle fluxes was considerable, it was apparent that other substrates available in the media also contributed to the maintenance of TCA fluxes. Lastly, the results suggest that diet composition alters glucose, glutamate, and leucine catabolism by the TCA cycle of REC and DMC.

Salvage of blood urea nitrogen in sheep is highly dependent on plasma urea concentration and the efficiency of capture within the digestive tract1,2

The aims of this study were 1) to determine whether transfer of blood urea to the gastrointestinal tract (GIT) or the efficiency of capture of urea N within the GIT is more limiting for urea N salvage, and 2) to establish the relationship between plasma urea concentration and recycling of urea N to the GIT. We used an i.v. urea infusion model in sheep to elevate the urea entry rate and plasma concentrations, thus avoiding direct manipulation of the rumen environment that otherwise occurs when feeding additional N. Four growing sheep (28.1 +/- 0.6 kg of BW) were fed a low-protein (6.8% CP, DM basis) diet and assigned to 4 rates of i.v. urea infusion (0, 3.8, 7.5, or 11.3 g of urea N/d; 10-d periods) in a balanced 4 x 4 Latin square design. Nitrogen retention (d 6 to 9), urea kinetics([(15)N2]urea infusion over 80 h), and plasma AA were determined. Urea infusion increased apparent total tract digestibility of N (29.9 to 41.3%) and DM (47.5 to 58.9%), and N retention (1.45 to 5.46 g/d). The plasma urea N entry rate increased (5.1 to 21.8 g/d) with urea infusion, as did the amount of urea N entering the GIT (4.1 to 13.2 g/d). Urea N transfer to the GIT increased with plasma urea concentration, but the increases were smaller at greater concentrations of plasma urea. Anabolic use of urea N within the GIT also increased with urea infusion (1.43 to 2.98 g/d; P = 0.003), but anabolic use as a proportion of GIT entry was low and decreased (35 to 22%; P = 0.003) with urea infusions. Consequently, much (44 to 67%) of the urea N transferred to the GIT returned to the liver for resynthesis of urea (1.8 to 9.2 g/d; P < 0.05). The present results suggest that transfer of blood urea to the GIT is 1) highly related to blood urea concentration, and 2) less limiting for N retention than is the efficiency of capture of recycled urea N by microbes within the GIT.

Intestinal protein supply alters amino acid, but not glucose, metabolism by the sheep gastrointestinal tract

This study was intended to establish the extent which amino acids (AAs) and glucose are net metabolized by the gastrointestinal tract (GIT) of ruminant sheep when intestinal protein supply is varied. Wether sheep (n = 4, 33 +/- 2.0 kg) were fitted with catheters for measurement of net absorption by the mesenteric (MDV) and portal-drained (PDV) viscera and a catheter inserted into the duodenum for casein infusions. Sheep received a fixed amount of a basal diet that provided adequate metabolizable energy (10.9 MJ/d) but inadequate metabolizable protein (75 g/d) to support 300-g gain per day. Four levels of casein infusion [0 (water), 35, 70, and 105 g/d], each infused for 5.5 d, were assigned to sheep according to a 4 x 4 Latin square design. [methyl-(2)H(3)]leucine was infused (8 h) into the duodenum while [1-(13)C]leucine plus [6-(2)H(2)]glucose were infused (8 h) into a jugular vein. With the exception of glutamate and glutamine, net absorption of AAs increased linearly (P < 0.05, R(2) = 0.46-1.79 for MDV; P < 0.05, R(2) = 0.6-1.58 for PDV) with casein infusion rate. Net absorption by the PDV accounted for <100% of the additional supplies of leucine, valine, and isoleucine (0.6-0.66, P < 0.05) from casein infusion, whereas net absorption by the MDV accounted for 100% of the additional essential AA supply. Glucose absorption (negative) and utilization of arterial glucose supply by the GIT remained unchanged. There was a positive linear (P < 0.05) relation between transfer of plasma urea to the GIT and arterial urea concentration (MDV, P < 0.05, r = 0.90; PDV, P < 0.05, r = 0.93). The ruminant GIT appears to metabolize increasing amounts of the branched-chain AAs and certain nonessential AAs when the intestinal supply of protein is increased.

Application of stable isotopes and mass isotopomer distribution analysis to the study of intermediary metabolism of nutrients1

Genomic investigations in animals have begun to reveal the metabolic and physiological functions of genes and protein products. However, a thorough understanding of the genomic roadmaps will require investigative approaches that yield qualitative and quantitative information on the activities, fluxes, and connectivity of pathways involved in nutrient use in farm animals; that is, the metabolic phenotype. Recently, the commercial availability of stable isotope (13C, 15N, 2H)-labeled compounds and highly accurate mass spectrometers has made it possible to probe the details of metabolic pathways involved in macronutrient use. For years, the biological sciences have exploited uniformly 13C-labeled substrates (e.g., glucose, amino acids, nucleic acids) and 13C-mass isotopomer distribution (MID) in their metabolic investigations, whereas their use in the animal sciences is very limited. When [U-13C] substrates are fed, infused, or added to cell incubations, the 13C-skeletons distribute throughout metabolic networks. 13C-Mass isotopomer distribution in intermediates and end products of the pathways provides a signature of the fluxes and activities of pathway enzymes traveled by the precursor molecule. This paper highlights aspects of animal nutrition and metabolism in which [U-13C] substrates and MID can be applied to investigations of amino acid, carbohydrate, and fat metabolism. We will focus on [U-13C] glucose as a tracer in chickens, fish, sheep, and cell cultures to investigate the interconnectivity of the pathways of macronutrient and nucleic acid metabolism, and provide demonstration of the central position of the Krebs cycle in preserving metabolic flexibility via anaplerotic and cataplerotic sequences. Exploitation of this approach in animal sciences offers endless opportunities to provide missing details of the biochemical networks of nutrient use that may prove to be strictly under genomic control.