Amino acid uptake by liver of genetically obese Zucker rats

High fat diet reveals sex-specific fecal and liver metabolic alterations in C57BL/6J obese mice

Obesity is a major health concern that poses significant risks for many other diseases, including diabetes, cardiovascular disease, and cancer. Prevalence of these diseases varies by biological sex. This study utilizes a mouse (C57BL/6J) model of obesity to analyze liver and fecal metabolic profiles at various time points of dietary exposure: 5, 9, and 12 months in control or high fat diet (HFD)-exposed mice. Our study discovered that the female HFD group has a more discernable perturbation and set of significant changes in metabolic profiles than the male HFD group. In the female mice, HFD fecal metabolites including pyruvate, aspartate, and glutamate were lower than control diet-exposed mice after both 9th and 12th month exposure time points, while lactate and alanine were significantly downregulated only at the 12th month. Perturbations of liver metabolic profiles were observed in both male and female HFD groups, compared to controls at the 12th month. Overall, the female HFD group showed higher lactate and glutathione levels compared to controls, while the male HFD group showed higher levels of glutamine and taurine compared to controls. These metabolite-based findings in both fecal and liver samples for a diet-induced effect of obesity may help guide future pioneering discoveries relating to the analysis and prevention of obesity in people, especially for females.

Dietary leucine requirement of older men and women is higher than current recommendations

BACKGROUND

Current national (34 mg . kg-1 . d-1) and international (39 mg kg-1 . d-1) recommendations for leucine in older adults are based on data from young adults. Evidence suggests that the leucine requirements of older adults are higher than those of young adults.

OBJECTIVE

The objective of the current study was to directly determine the leucine requirements in healthy older adult male and female study participants aged >60 y.

METHODS

Leucine requirement was determined using the indicator amino acid oxidation method (IAAO) with l-[1-13C]phenylalanine as the indicator. Sixteen older adults (n = 7 male and n = 9 female participants) were randomly assigned to receive 3 to 7 leucine intakes from 20 to 120 mg . kg-1 . d-1. The rate of release of 13CO2 from l-[1-13C]phenylalanine oxidation was measured, and breakpoint analysis was used to estimate the leucine requirement. The 95% CI was calculated using the parametric bootstrap method.

RESULTS

The mean leucine requirement for male participants was 77.8 mg . kg-1 . d-1 (upper 95% CI: 81.0) and for female participants, it was 78.2 mg . kg-1 . d-1 (upper 95% CI: 82.0) with no sex effect based on body weight. The data were therefore combined to yield a mean of 78.5 mg . kg-1 d-1 (upper 95% CI: 81.0 mg . kg-1 . d-1 ) for both sexes. On the basis of fat-free mass, the mean ± SEM leucine requirements were 115 ± 3.2 and 127 ± 2.4 mg . kg-1 . d-1 for male and female participants, respectively (P < 0.005).

CONCLUSIONS

The estimated leucine requirement of older adults is more than double the amount in current recommendations. These data suggest that leucine could be a limiting amino acid in the diet of older adults consuming the current RDA for protein and those consuming a plant-based diet. In view of the functional and structural role of leucine, especially its importance in muscle protein synthesis, current leucine recommendations of older adults should be revised. This trial was registered at clinicaltrials.gov as NCT03506126.

Roux-en-Y gastric bypass alters small intestine glutamine transport in the obese Zucker rat

The metabolic effects of Roux-en-Y gastric bypass (RYGB) are caused by postsurgical changes in gastrointestinal anatomy affecting gut function. Glutamine is a critical gut nutrient implicated in regulating glucose metabolism as a substrate for intestinal gluconeogenesis. The present study examines the effects of obesity and RYGB on intestinal glutamine transport and metabolism. First, lean and obese Zucker rats (ZRs) were compared. Then the effects of RYGB and sham surgery with pair feeding (PF) in obese ZRs were studied. Segments of small intestine (biliopancreatic limb, Roux limb, and common channel) mucosa were harvested and brush border membrane vesicles (BBMVs) were isolated on postoperative day 28. Glutamine transporter activity and abundance, B(0)AT1 protein, and mRNA levels were measured. Levels of glutaminase, cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C), and glucose-6-phosphatase (G6Pase) were measured to assess glutamine metabolism and intestinal gluconeogenesis. Obesity increased glutamine transport and B(0)AT1 expression throughout the intestine. RYGB increased glutamine transport activity in the biliopancreatic (3.8-fold) and Roux limbs (1.4-fold) but had no effect on the common channel. The relative abundance of B(0)AT1 mRNA and protein were increased in the biliopancreatic (6-fold) and Roux limbs (10-fold) after RYGB (P < 0.05 vs. PF), but not the common channel. Glutaminase levels were increased, whereas the relative abundance of PEPCK-C and G6Pase were decreased in all segments of intestine after RYGB. RYGB selectively increased glutamine absorption in biliopancreatic and Roux limbs by a mechanism involving increased B(0)AT1 expression. Post-RYGB glutaminase levels were increased, but the reductions in PEPCK-C and G6Pase suggest that RYGB downregulates intestinal gluconeogenesis.

The hepatic amino acid system A transport activity, is up-regulated in obese Zucker rats

The utilization of L-alanine by liver is dependent on amino acid uptake from blood. This uptake, mainly mediated by the A transport system, may be regulated by different nutritional and physiologic conditions. The regulation of this transport system by diets with different protein content was tested in lean and obese Zucker rats. High-protein (HP) and low-protein (LP) diets led to changes in the rats' growth patterns, especially in lean animals. However, homeostasis was relatively well maintained, as seen in plasma values, in spite of the increased urea production in the HP groups and increased triacylglycerides in the LP groups. The obese animals took up L-alanine at a higher rate than the lean animals. Obesity led to the emergence of a high-affinity component (K(M) approximately 0.1-0.2 mM) in the transport system, which was not dependent on the protein content of the diet. This component has a 10-fold increase in affinity for L-alanine, but with an approximately 3- to 5-fold reduction in maximal velocity of transport.

Ammonium uptake and urea production in hepatocytes from lean and obese Zucker rats

The metabolic differences in vitro between genetic and dietary obese rats in the uptake of ammonium and amino acids by the liver and their use for ureogenesis have been assayed using hepatocytes isolated from Lean, Obese Zucker (Genetic obese) rats and Dietary obese rats. The hepatocytes of genetic obese animals took up more ammonium and produced higher amounts of urea from ammonium and alanine than those of lean and dietary obese groups (2 and 5 times more respectively). In the lean and dietary obese groups urea synthesis accounted for almost all the nitrogen taken up as ammonium. Thus, dietary and genetic obesity show a widely different handling of nitrogen, and the genetic obese rats need to break down protein to maintain their hepatocyte function.

Na+-dependent nucleoside transport in liver: two different isoforms from the same gene family are expressed in liver cells

Hepatocytes show a Na+-dependent nucleoside transport activity that is kinetically heterogeneous and consistent with the expression of at least two independent concentrative Na+-coupled nucleoside transport systems (Mercader et al. Biochem. J. 317, 835-842, 1996). So far, only a single nucleoside carrier-related cDNA (SPNT) has been isolated from liver cells (Che et al. J. Biol. Chem. 270, 13596-13599, 1995). This cDNA presumably encodes a plasma membrane protein responsible for Na+-dependent purine nucleoside transport activity. Thus, the liver must express, at least, a second nucleoside transporter which should be pyrimidine-preferring. Homology cloning using RT-PCR revealed that a second isoform is indeed present in liver. This second isoform turned out to be identical to the 'epithelial-specific isoform' called cNT1, which shows in fact high specificity for pyrimidine nucleosides. Although cNT1 mRNA is present at lower amounts than SPNT mRNA, the amounts of cNT1 protein, when measured using isoform-specific polyclonal antibodies, were even higher than the SPNT protein levels. Moreover, partially purified basolateral plasma membrane vesicles from liver were enriched in the SPNT but not in the cNT1 protein, which suggests that the subcellular localization of these carrier proteins is different. SPNT and cNT1 protein amounts in crude membrane extracts from 6 h-regenerating rat livers are higher than in the preparations from sham-operated controls (3.5- and 2-fold, respectively). These results suggest that liver parenchymal cells express at least two different isoforms of concentrative nucleoside carriers, the cNT1 and SPNT proteins, which show differential regulation and subcellular localization.

Expression of sodium-dependent purine nucleoside carrier (SPNT) mRNA correlates with nucleoside transport activity in rat liver

The expression of sodium-dependent purine nucleoside transport (SPNT) mRNA has been studied in physiological situations in which Na+-dependent nucleoside uptake in plasma membrane vesicles from rat liver was induced. Sodium-dependent uridine transport rates were induced in genetically obese Zucker rats, during liver regeneration after partial hepatectomy, and under euglycemic-hyperinsulinemic clamp. A PCR-generated fragment, based on a published SPNT sequence cloned from rat liver, was used as a probe in Northern blot analysis. We show that the hepatic mRNA levels of the putative sodium-dependent transport system SPNT correlate with the sodium-dependent uridine transport rates in plasma membrane vesicles from rat liver. These results suggest that the induction of the sodium-dependent nucleoside transport expressed in liver parenchymal cells involves regulation of SPNT gene expression.

Enzymes of carbohydrate metabolism in young and adult rats fed diets differing in fat and carbohydrate

Glycogen content as well as glycolytic, gluconeogenic and fatty acid synthesis enzyme activities were monitored in young and adult male rats fed diets differing in fat content: 11% (low), 22% (medium) and 42% (high) of total energy from fat. The results showed significant differences in the responses of young and adult rats to changes in dietary fat and carbohydrate. In young animals, increasing dietary fat decreased total liver glycogen phosphorylase (GP), pyruvate kinase (PK), glycerol 3-phosphate dehydrogenase, glucose 6-phosphate dehydrogenase, malic enzyme (ME), ATP-citrate lyase (ATP-CL) and fatty acid synthase (FAS). Increasing dietary fat also affected enzyme levels in other tissues: hexokinase (HK) and pyruvate dehydrogenase (PDH) activities decreased whereas skeletal muscle PK activity increased. The pattern of enzyme changes was similar in livers of fed adults with the exception that liver GP was not affected by dietary manipulations. Overnight food deprivation decreased liver glucokinase (GK), ME, ATP-CL, and FAS activities and increased liver phosphoenolpyruvate carboxykinase (PEPCK) and phosphofructokinase in both young and adult animals. In young animals, food deprivation also: (i) reduced liver GK and PK, (ii) increased kidney PEPCK, (iii) decreased muscle PEPCK and (iv) decreased kidney PDH. Food-deprived adults had increased skeletal muscle PEPCK and kidney glycogen synthetase as well as decreased kidney PEPCK muscle GP activity. These differences suggest that young animals are somewhat more responsive to changes in dietary manipulations. They also show that overnight food restriction causes a more profound metabolic re-organization in younger than in older animals.

Coordinate induction of Na(+)-dependent transport systems and Na+,K(+)-ATPase in the liver of obese Zucker rats

Solute uptake into liver plasma membrane vesicles from either lean or obese Zucker rats was monitored. D-Glucose and L-leucine uptakes at physiological concentrations of the substrate were not different in lean and obese Zucker rats. In agreement with a previous report (Ruiz et al. (1991) Biochem. J. 280, 367-372) L-alanine uptake was significantly enhanced in those preparations from obese animals. Na(+)-coupled uridine transport was markedly enhanced also in obese rats. The effect was due to an increase in Vmax (5.5 +/- 0.6 vs. 2.1 +/- 0.2 pmol/mg protein per 3 s, P < 0.01) without any significant change in Km (11.0 +/- 2.8 vs. 9.0 +/- 2.7 microM for obese and lean rats, respectively). Na+,K(+)-ATPase activity was also higher in liver plasma membrane vesicles from rat liver and it correlated with a higher amount of alpha 1-subunit protein in both, plasma membrane vesicles and homogenates from obese rat livers. In summary, in the hypertrophic liver of obese Zucker rats a coordinate induction of several Na(+)-dependent transport systems occurs and, in order to sustain the metabolic pressure associated with this adaptation, a significant induction of the Na+,K(+)-ATPase expression is also found. These data also provide new evidence for regulation of the recently characterized Na(+)-dependent nucleoside transporter.

Splanchnic amino acid pattern in genetic and dietary obesity in the rat

The study of intestinal and hepatic uptake of amino acids by obese rats has been the main objective of this work. The obese animals used were either from genetic or from nutritional basis. In fed state, the intestinal release of amino acids was higher in obese animals than in lean ones (around the double values), but nutritionally and genetically obese rat showed a related pattern, specially for the case of alanine (increased release in relation to controls by a factor of 10). The higher alanine release by intestine is not reversed by 12-h food deprivation. The hepatic availability was also higher in obesity models than in lean animals (increases over 30%). However, the hepatic uptake was increased in genetically obese animals (more than 35%) and decreased in nutritionally obese animals (more than 40%), especially due to alanine uptake (2419, 1100 and 3794 nmols/min/g protein in lean, Diet-ob and fa/fa animals respectively). In obese animals the food deprivation tended to normalize the hepatic uptake of alanine. The differences in alanine uptake between both types of obesity may reflect the differences of urea synthesis.

Up-regulation of liver system A for neutral amino acid transport in euglycemic hyperinsulinemic rats

To determine the role of insulin on the in vivo modulation of liver system A activity, we used the euglycemic hyperinsulinemic clamp coupled to the measurement of solute uptakes into plasma membrane vesicles partially purified from livers of hyperinsulinemic rats and their saline-infused controls. The clamp was performed in chronically catheterized rats, either in the fasted state, 24 h after surgery (Group I), or after 3 days of recovery (Group II). System A activity, measured as the MeAIB-inhibitable L-alanine uptake, was selectively induced by hyperinsulinemia, although the effect was much greater in Group II than in Group I rats (137% vs. 24% over the basal values, respectively). This might be explained by the higher basal levels found in those liver plasma membrane vesicles from Group I fasted animals. Hyperinsulinemia also decreased blood amino acids but to a similar extent in both experimental groups. This suggests that amino acid depletion by itself may not cause up-regulation of system A. Other transport activities involved in neutral amino acid transport (Systems ASC, N and L) were not modified by the clamp. The induction of system A cannot be explained by changes in the dissipation rate of the Na+ transmembrane gradient, because the differences between insulin- and saline-infused rats remained even when the electrochemical Na+ gradient was disrupted in the presence of monensin. Thus, hyperinsulinemia might induce an increase in the number of transporters inserted into the plasma membrane.

Regulatory and molecular aspects of mammalian amino acid transport

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Characteristics of the obesity syndrome in Zucker-Brown Norway (ZBN) hybrid rats

The existence of a restriction fragment length polymorphism (RFLP) closely linked to the fatty locus between the Zucker (Z) and Brown Norway (BN) rat strains allows evaluation of early effects of the fatty (fa) gene using offspring of back-crosses (N2) between F1 females and Zucker obese males. We examined several metabolic characteristics of N2 animals to determine if these hybrid animals exhibited similar characteristics of the obese syndrome to those of Zucker rats. Females from crosses of obese male Zucker (fa/fa) and lean female BN (+/+) rats were back-crossed to their sires, resulting in twelve N2 litters. At 9 weeks of age, liver, spleen, interscapular brown fat (IBAT), and gonadal, retroperitoneal (RP), and inguinal fat depots were removed and weighed. Samples of the RP depot were analyzed for cell size and number. Obese N2 rats were hyperphagic, with body weights in the range of those of obese Zucker rats. Obese N2 rats were also hyperinsulinemic [mean +/- SEM, microU/ml: females, 7.9 +/- 0.6 vs. 82.1 +/- 8.4 (lean vs. obese); males, 10.5 +/- 1.6 vs. 128.5 +/- 13.4 (lean vs. obese)] and mildly hyperglycemic [mean +/- SEM, mg/dl: females, 104.1 +/- 2.0 vs. 139.0 +/- 14.7 (lean vs. obese); males, 100.9 +/- 2.6 vs. 132.0 +/- 2.8 (lean vs. obese) p < or = 0.05]. White fat depots in obese rats were 3 to 7 times heavier than those in lean rats; adipocyte numbers in RP depots were 50% greater in obese than in lean rats; and cell size was more than 3 times larger. IBAT, liver, and spleen were also heavier in obese vs. lean rats, while tail lengths were shorter. Percent lean carcass mass and % carcass protein were about 30% greater in lean vs. obese rats, while % carcass fat in obese rats was 5 times greater than that of lean rats. Thus, phenotypic expression of the fa gene in ZBN hybrid animals, with approximately 25% of their genetic background coming from the BN strain, appears to be similar to that in Zucker rats. Given the similarity of phenotypic expression of the fa gene between the Zucker strain and ZBN hybrids, it is plausible to consider using ZBN hybrids for studies of early manifestations of fa gene action prior to onset of detectable obesity.

Up-regulation of system A activity in the regenerating rat liver

System A activity for neutral amino acid transport, measured as the MeAIB-sensitive Na(+)-dependent L-alanine uptake, is induced 6 h after partial hepatectomy in plasma membrane vesicles from rat livers. Other Na(+)-dependent transporters, like system ASC (MeAIB-insensitive Na(+)-dependent L-alanine transport) and the nucleoside carrier show similar inductions. Up-regulation of system A is not explained by changes in the dissipation rate of the Na+ transmembrane gradient, as deduced from uptake measurements performed in the presence of monensin. To determine whether induced system A shared any similarity with the activity found in hepatoma cell lines, we analyzed the N-ethylmaleimide (NEM) sensitivity of system A in both regenerating and control rat liver plasma membrane vesicles. NEM treatment was equally effective in inhibiting system A in both experimental groups. Thus, during the prereplicative phase of liver growth, a transport activity similar to basal system A is up-regulated in liver parenchymal cells, by a stable mechanism that does not involve changes in the Na+ transmembrane gradient.

Early induction of Na(+)-dependent uridine uptake in the regenerating rat liver

Na(+)-dependent uridine transport into liver plasma membrane vesicles from partially hepatectomized and sham-operated rats was studied. Preparations purified 6 h after 70% hepatectomy exhibited an increased Vmax of uridine uptake (3.7 vs. 1.4 pmol/mg prot/3 s) without any change in Km (6 microM). Incubation of the vesicles in the presence of monensin decreased uridine uptake although the differences between both experimental groups remained identical. It is concluded that uridine transport is induced early after partial hepatectomy by a mechanism which does not involve changes in the transmembrane Na+ gradient. This is the first evidence in favor of modulation of nucleoside transport into liver cells.

Uridine transport in basolateral plasma membrane vesicles from rat liver

The characteristics of uridine transport were studied in basolateral plasma membrane vesicles isolated from rat liver. Uridine was not metabolized under transport measurements conditions and was taken up into an osmotically active space with no significant binding of uridine to the membrane vesicles. Uridine uptake was sodium dependent, showing no significant stimulation by other monovalent cations. Kinetic analysis of the sodium-dependent component showed a single system with Michaelis-Menten kinetics. Parameter values were KM 8.9 microM and Vmax 0.57 pmol/mg prot/sec. Uridine transport proved to be electrogenic, since, firstly, the Hill plot of the kinetic data suggested a 1 uridine: 1 Na+ stoichiometry, secondly, valinomycin enhanced basal uridine uptake rats and, thirdly, the permeant nature of the Na+ counterions determined uridine, transport rates (SCN- greater than NO3- greater than Cl- greater than SO4(2-)). Other purines and pyrimidines cis-inhibited and trans-stimulated uridine uptake.