Insulin regulation of branched chain alpha-keto acid dehydrogenase in adipose tissue

Regulation of adipose branched-chain amino acid catabolism enzyme expression and cross-adipose amino acid flux in human obesity

Elevated blood branched-chain amino acids (BCAA) are often associated with insulin resistance and type 2 diabetes, which might result from a reduced cellular utilization and/or incomplete BCAA oxidation. White adipose tissue (WAT) has become appreciated as a potential player in whole body BCAA metabolism. We tested if expression of the mitochondrial BCAA oxidation checkpoint, branched-chain α-ketoacid dehydrogenase (BCKD) complex, is reduced in obese WAT and regulated by metabolic signals. WAT BCKD protein (E1α subunit) was significantly reduced by 35-50% in various obesity models (fa/fa rats, db/db mice, diet-induced obese mice), and BCKD component transcripts significantly lower in subcutaneous (SC) adipocytes from obese vs. lean Pima Indians. Treatment of 3T3-L1 adipocytes or mice with peroxisome proliferator-activated receptor-γ agonists increased WAT BCAA catabolism enzyme mRNAs, whereas the nonmetabolizable glucose analog 2-deoxy-d-glucose had the opposite effect. The results support the hypothesis that suboptimal insulin action and/or perturbed metabolic signals in WAT, as would be seen with insulin resistance/type 2 diabetes, could impair WAT BCAA utilization. However, cross-tissue flux studies comparing lean vs. insulin-sensitive or insulin-resistant obese subjects revealed an unexpected negligible uptake of BCAA from human abdominal SC WAT. This suggests that SC WAT may not be an important contributor to blood BCAA phenotypes associated with insulin resistance in the overnight-fasted state. mRNA abundances for BCAA catabolic enzymes were markedly reduced in omental (but not SC) WAT of obese persons with metabolic syndrome compared with weight-matched healthy obese subjects, raising the possibility that visceral WAT contributes to the BCAA metabolic phenotype of metabolically compromised individuals.

Effect of hyperinsulinemia on amino acid utilization and oxidation independent of glucose metabolism in the ovine fetus

We studied the effect of acute hyperinsulinemia on amino acid (AA) utilization and oxidation rates independent of insulin-enhanced glucose metabolism in fetal sheep. Metabolic studies were conducted in each fetus (n = 11) under three experimental periods. After control period (C) study, a fetal hyperinsulinemic-euglycemic-euaminoacidemic (HI-euG-euAA) clamp was established, followed by a hyperinsulinemic-hypoglycemic-euaminoacidemic (HI-hypoG-euAA) clamp to decrease glucose metabolic rates toward C values. Infusions of (3)H(2)0, L-[1-(13)C]leucine, and [(14)C(U)]glucose were administered to measure blood flow, leucine oxidation, and fetal glucose uptake, utilization, and oxidation in each period. Fetal glucose utilization rate increased 1.7-fold with hyperinsulinemia (C 5.8 +/- 0.8 mg.kg(-1).min(-1), HI-euG-euAA 10 +/- 1.3 mg.kg(-1).min(-1), P < 0.0001), returning to rates not different from C with hypoglycemia (HI-hypoG-euAA 7.1 +/- 0.9 mg.kg(-1).min(-1) vs. C value, P = 0.15). Fetal glucose oxidation rate increased 1.7-fold with hyperinsulinemia (C 3.1 +/- 0.2 mg.kg(-1).min(-1), HI-euG-euAA 5.4 +/- 0.4 mg.kg(-1).min(-1), P < 0.0001) and decreased to near control rates with hypoglycemia (4.0 +/- 0.3 HI-hypoG-euAA vs. C value, P = 0.006). AA utilization rates increased with hyperinsulinemia for all essential and most nonessential AAs (P < 0.001) and did not change when insulin-induced increases in glucose utilization returned to control rates. Leucine oxidation rate increased 1.7-fold with hyperinsulinemia (C 1.0 +/- 0.3 micromol.min(-1).kg(-1), HI-euG-euAA 1.7 +/- 0.3 micromol.min(-1).kg(-1), P < 0.002) and did not change when glucose oxidation rate was decreased with hypoglycemia. These results demonstrate that, in fetal sheep, insulin promotes AA utilization and oxidation independent of its simultaneous effects on glucose metabolism. In acute hyperinsulinemic conditions, AA oxidation does not change when insulin-induced glucose utilization is prevented.

Effects of insulin and amino acids on milk protein concentration and yield from dairy cows

Our study investigated the effect of insulin on the regulation of milk protein synthesis in well-fed cows (n = 4) with or without additional amino acids (AA). The design was a two-way crossed factorial with two 12-d periods involving abomasal infusions of either water or a mixture of casein (500 g/d) plus branched-chain AA (88 g/d). During the last 4 d of each period a hyperinsulinemic-euglycemic clamp was performed; insulin was infused at 1.0 microgram.kg of BW-1.h-1 to increase circulating levels fourfold, and euglycemia was maintained by infusion of glucose. Cows were fed a diet formulated to exceed requirements for metabolizable energy and protein. During abomasal water infusion, the insulin clamp increased milk protein yields by 15% (+128 g/d); when combined with abomasal infusion of casein plus branched-chain AA, milk protein yield was increased by 25% (+213 g/d). These increases resulted from equivalent increases in milk protein concentration and milk yield. Concentrations of casein and whey proteins in milk were increased by insulin clamp treatments; however, there were no major changes in the relative proportions of individual casein and whey proteins. Plasma concentrations of essential AA were reduced (-33%) during the insulin clamp treatments; effects were most dramatic for the branched-chain AA (-41%) and their keto acids (-45%). Results confirm the important regulatory role of the endocrine system in milk protein synthesis and demonstrate this potential to produce milk protein is not fully expressed.

Nutritional status affects branched-chain oxoacid dehydrogenase activity during exercise in humans

We examined the effect of glycogen availability and branched-chain amino acid (BCAA) supplementation on branched-chain oxoacid dehydrogenase (BCOAD) activity during exercise. Six subjects cycled at approximately 75% of their maximal oxygen uptake to exhaustion on three occasions under different preexercise conditions: 1) low muscle glycogen (LOW), 2) low muscle glycogen plus BCAA supplementation (LOW+BCAA), and 3) high muscle glycogen (CON). The LOW trial was performed first, followed by the other two conditions in random order, and biopsies for all trials were obtained at rest, after 15 min of exercise (15 min), and at the point of exhaustion during the LOW trial (49 min). BCOAD activity was not different among the three conditions at rest; however, at 15 min BCOAD activity was higher (P < or = 0.05) for the LOW (31 +/- 5%) and LOW+BCAA (43 +/- 11%) conditions compared with CON (12 +/- 1%). BCOAD activity at 49 min was not different from respective values at 15 min for any condition. These data indicate that BCOAD is rapidly activated during submaximal exercise under conditions associated with low carbohydrate availability. However, there was no relationship between BCOAD activity and glycogen concentration or net glycogenolysis, which suggests that factors other than glycogen availability are important for BCOAD regulation during exercise in humans.

Leucine metabolism in rats with cirrhosis

BACKGROUND/AIMS

This study aimed to investigate the pathogenesis of reduced plasma levels of branched-chain amino acids leucine, isoleucine and valine in cirrhosis.

METHODS

Cirrhosis was induced by intragastric administration of 36 doses of carbon tetrachloride in olive oil over a period of 12 weeks. Rats treated with oil alone served as controls. The rates of leucine turnover, clearance, oxidation and incorporation into proteins were evaluated using [1-14C]leucine, [4,5-3H]leucine and alpha-keto[1-14C]isocaproate 3 days after the last intragastric treatment in vivo and in the isolated perfused liver.

RESULTS

In animals with cirrhosis we observed a profound fall in plasma branched-chain amino acid levels and significant decreases in leucine turnover, oxidation and incorporation into tissue proteins. A more pronounced fall in leucine incorporation in proteins resulted in a significant increase in the oxidized leucine fraction in rats with cirrhosis as compared to controls. Leucine clearance was higher in the cirrhosis group. Concomitant to the fall of whole body leucine turnover, decreases of leucine incorporation into protein and of ketoisocaproic acid decarboxylation were observed in the isolated perfused liver of rats with cirrhosis. However, leucine oxidation was increased compared with control rats.

CONCLUSIONS

Our results indicate that the predominant mechanism of the decrease in plasma leucine levels in rats with cirrhosis is an increase in the oxidized leucine fraction associated with a decrease in leucine turnover. An increase in leucine oxidation in the cirrhotic liver is one of the mechanisms involved.

Euglycemic hyperinsulinemia augments amino acid uptake by human leg tissues during hyperaminoacidemia

The effect of insulin on leg and whole body protein turnover was determined by leg exchange and plasma kinetics of [15N]phenylalanine and [1-13C]leucine during amino acid (AA) sufficiency. Eight healthy subjects were studied during AA infusion alone and during infusion of glucose and insulin (0.29 nmol.m-2.min-1) with additional AA. Insulin strongly stimulated the positive leg AA balance seen with AA (AA alone, 2.6 +/- 6.1 vs. insulin + AA, 33.1 +/- 5.8 nmol phenylalanine . 100 g leg-1.min-1; P less than 0.001). Phenylalanine uptake by leg tissues rose during insulin plus AA (47.3 +/- 11.5 vs. 73.1 +/- 7.3 nmol. 100 g-1.min-1; P = 0.022) but with only a slight reduction in leg phenylalanine release (44.7 +/- 8.1 vs. 40.0 +/- 7.9 nmol.100 g-1.min-1). Leg nonoxidative leucine plus alpha-ketoisocaproate (KIC) uptake was increased slightly with insulin (129 +/- 26 vs. 146 +/- 21 nmol.100 g-1. min-1), but leg leucine oxidation increased fourfold (P = 0.012). Leg leucine plus KIC release was reduced by insulin (120 +/- 17 vs. 84 +/- 10 nmol.100 g-1.min-1; P = 0.005); endogenous leucine appearance of leucine and phenylalanine decreased with insulin (leucine, 1.97 +/- 0.08 vs. 1.65 +/- 0.10; phenylalanine, 0.76 +/- 0.03 vs. 0.54 +/- 0.08 mumols.kg-1.min-1; P less than 0.02). The results suggest that insulin, given with sufficient amino acids, may stimulate leg and whole body protein balance by mechanisms including stimulation of protein synthesis and inhibition of protein breakdown.

The effect of amino acid infusion on leg protein turnover assessed by L-[15N]phenylalanine and L-[1-13C]leucine exchange

A stable isotope technique depending on the use of [15N]phenylalanine and [1-13C]leucine to assess exchange was utilized to measure the components of protein turnover of the human leg and the effects of amino acid infusion. Eight healthy subjects (28.5 +/- 2.5 years) were studied when post-absorptive in the basal state and again during infusion of a mixed amino acid solution (55 g l-1, 1.52 ml kg-1 h-1). During the basal period leucine oxidation by the leg was 4.4 +/- 2.0 nmol 100 g-1 min-1 and this increased threefold during amino acid infusion (13.6 +/- 3.1 nmol 100 g-1 min-1, mean +/- SEM, P = 0.003). Amino acid infusion abolished the net negative balance between incorporation of leucine into, and release from, protein (basal, -31.8 +/- 5.8; during infusion, +3.1 +/- 7.1 nmol 100 g-1 P = 0.001). Phenylalanine exchange showed a similar pattern (basal, -13.7 +/- 1.8; during infusion, -0.8 +/- 3.0 nmol 100 g-1 min-1, P = 0.003). Basal entry of leucine into leg protein (i.e. protein synthesis) was 70.0 +/- 10.8 nmol 100 g-1 min-1 and this increased during amino acid infusion to 87.3 +/- 14.1 nmol 100 g-1 min-1 (P = 0.11). Phenylalanine entry to protein also increased with amino acid infusion (29.1 +/- 4.5 vs. 38.3 +/- 5.8 nmol 100 g-1 min-1, P = 0.09). Release from protein of leucine (101.8 +/- 9.1 vs. 84.2 +/- 9.1 nmol 100 g-1 min-1, P = 0.21) and of phenylalanine (42.8 +/- 4.2 vs. 39.1 +/- 4.2 nmol 100 g-1 min-1, P = 0.50) was unchanged by amino acid infusion. The results suggest that, in the post-absorptive state in man, infusion of mixed amino acids, without additional energy substrates; reverses negative amino acid balance by a mechanism which includes stimulation of muscle protein synthesis but which does not alter protein breakdown. Interpretation of the results obtained concurrently on whole-body protein turnover suggests that the increase in muscle protein synthesis contributes substantially to the whole-body increase, but the fall in whole-body breakdown with exogenous amino acids is independent of changes in muscle.

Inability to stimulate skeletal muscle or whole body protein synthesis in type 1 (insulin-dependent) diabetic patients by insulin-plus-glucose during amino acid infusion: studies of incorporation and turnover of tracer L-[1-13C]leucine

Despite its anabolic effects on protein balance, acute administration of insulin has been reported to have no effect on skeletal muscle or whole body protein synthesis in man. However, insulin also reduces plasma and intramuscular amino acid availability, which may limit protein synthesis. We have therefore measured the acute effects of insulin on skeletal muscle (anterior tibialis) protein synthesis and whole body leucine turnover in eight insulin-withdrawn Type 1 (insulin-dependent) diabetic patients. They were studied initially when insulin deficient, but during infusion of mixed amino acids at a rate sufficient to raise plasma amino acids by 30% i.e. to 4 mmol/l in total; measurements were continued when insulin was infused together with an increased rate of amino acids to maintain insulinopoenic plasma amino acid concentrations. Using 13C-alpha-ketoisocaproate in plasma as an index of the intracellular precursor labelling, incorporation of [1-13C]leucine into skeletal muscle protein was 0.068 +/- 0.007%/h during insulin withdrawal and was unaltered during insulin infusion. The value is higher than observed in muscle of healthy man, possibly because of a stimulatory effect of endogenous intramuscular amino acids. Also, calculated on the basis of alpha-ketoisocaproate labelling, non-oxidised whole body leucine disappearance (i.e. whole body protein synthesis) was 110 +/- 4 mumol.kg-1.h-1 during insulin withdrawal; this also was unchanged during insulin infusion. Despite stable or increased plasma concentrations of most amino acids, the intramuscular concentrations of a number of amino acids decreased during insulin infusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Intra- and extracellular amino acid concentrations in portacaval-shunted rabbits. Role of hyperammonemia and effects of branched-chain amino acid-enriched parenteral nutrition

Intra- and extracellular amino acid concentrations were measured in rabbits in order to elucidate the possible role of hyperammonemia in lowering the postabsorptive plasma levels of branched-chain amino acids (BCAA) and to assess the effects of BCAA-enriched total parenteral nutrition (TPN) on the amino acid pattern of muscle. The pathophysiological part of this paper deals with portacaval anastomosis (PCA) and is aimed at substantiating or rejecting our hypothesis that excessive ammonia-by stimulating glutamine synthesis-reduces the intracellular glutamate pool which is then restored, at least in part, by an intensified BCAA degradation. Regarding infusion therapy, we were mainly interested in whether an amino acid solution adapted to the metabolism in liver cirrhosis causes an accumulation of BCAA in muscle or modifies the intracellular content of glutamate and glutamine. Eighteen rabbits did not undergo surgery and served as controls (group A), while 30 were given a portacaval end-to-side anastomosis (group B). Two weeks after creating the PCA, venous blood samples were taken and muscle biopsies (Bergström's technique) were performed postabsorptively. An 18-h TPN was then started, the regimen administered included dextrose, fat and, in addition, either a conventional (group B1, n = 15) or an adapted amino acid solution (group B2, n = 15). We obtained second blood specimens and muscle biopsies at the end of the infusion period. With the control animals, the same time schedule for blood sampling and muscle biopsies was followed. Fourteen days after the operation, the PCA rabbits displayed a mean plasma ammonia level 5.1 times higher than that measured in the controls (p less than or equal to 0.001). Conventional blood chemistry did not reveal any impairment of liver cell integrity or over-all hepatic function, whereas the nutritional state of the shunted animals worsened, as indicated by body weight and biochemical variables. Since in the PCA rabbits, the total amino acid pools of muscle and plasma were seen to be increased and decreased, respectively, the results concerning the individual amino acids are given in terms of both the absolute and percentage values, the latter more often revealing high levels of statistical significance. PCA induced a marked rise in the intra- and extracellular concentrations of glutamine, while the values of glutamate and alanine showed a decline in muscle and plasma. The extracellular levels of methionine, phenylalanine, and tyrosine were raised, while those of the BCAA were diminished.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of hepatic branched-chain 2-oxoacid dehydrogenase by rat liver cytosolic supernatant

Hepatic branched-chain 2-oxoacid dehydrogenase is inactivated by nutritional alterations. Reactivation occurs during preincubation of intact mitochondria in the presence of rat liver cytosolic supernatant. Cytosolic supernatant contains two factors capable of reactivating the enzyme. On gel-filtration (Sephadex G-100), one factor (AF1) elutes in the molecular range of 35,000-40,000 and the other factor (AF2) elutes slightly later than inorganic phosphate. AF2 is stable against heat denaturation and treatment with proteinases. It is destroyed by alkaline phosphatase and in the presence of Ap5A, atractyloside, CaCl2 and NaF its stimulatory effect on branched-chain 2-oxoacid dehydrogenase activity is abolished. Inhibition of activation by NaF suggests that a phosphatase might be involved in the activation process.

Ketone body synthesis from leucine by adipose tissue from different sites in the rat

Leucine is catabolized to ketone bodies in adipose tissue, but the contribution of this output to overall ketone metabolism is not known. The intent of the present study was to determine the capacity of different adipose tissues to synthesize ketone bodies from leucine. The amino acid was readily converted into acetoacetate in epididymal, perirenal, and omental fat tissues. In rats fed ad libitum, the rate of acetoacetate synthesis in omental fat (about 2 mumol g tissue-1h-1) was at least 8 times higher than in epididymal or perirenal fat. In omental fat, the rates of acetoacetate formation from alpha-ketoisocaproic acid were 47-55% lower than from leucine at all concentrations examined. There was no significant synthesis of beta-hydroxybutyrate from leucine or alpha-ketoisocaproic acid. After oxidative decarboxylation, a greater proportion (about three-fourths) of leucine in omental fat was metabolized to acetoacetate than to CO2 production through the Krebs cycle. Although addition of glucose, pyruvate, or carnitine did not affect the production of acetoacetate, fasting for 24 h stimulated acetoacetate synthesis from leucine and alpha-ketoisocaproic acid in omental fat. The high rate of leucine conversion to acetoacetate in omental fat was related to high activities of leucine aminotransferase and branched-chain alpha-keto acid dehydrogenase. Moreover, protein content and cytochrome c oxidase activity of omental mitochondria were, respectively, 13 and 12 times higher than in epididymal mitochondria. In contrast, fat content of epididymal adipose tissue was 21 times that of omental adipose tissue. Epididymal depot consisted of 2.0% protein and 75.8% fat, whereas omental depot contains 17.2% protein and 3.6% fat, resembling that of liver and muscle. The results suggest that the high ketogenic capacity of omental fat stems in part from an augmented mitochondrial mass and high activity of branched-chain alpha-keto acid dehydrogenase.

Branched-chain ketoacid dehydrogenase activity and growth of normal and mutant human fibroblasts: the effect of branched-chain amino acid concentration in culture medium

We investigated changes in cell growth and branched chain ketoacid dehydrogenase (BCKD) activity by varying the concentrations of branched-chain amino acids (BCAAs) in culture medium of diploid fibroblasts from humans with normal BCKD and with impaired enzyme function. For logarithmic growth the two cell populations required similar minimal concentrations (0.05 mM) for each of leucine, isoleucine, and valine tested together. At confluency (saturation density) mutant cells grew less well to the extent of 30 to 40% in the highest concentrations of BCAAs that could be tested, 20.8 mM. BCKD activity was not changed by growth of normal or mutant cells in the absence of BCAAs. This enzyme activity was increased in normal but not mutant cells by growth in 20.8 mM BCAAs. These studies suggest the following: (1) BCKD mutant fibroblasts in culture slow their growth rate in response to high concentrations of BCAAs; (2) the growth disadvantage for mutant cells in high concentrations of BCAAs may be useful to select for enzyme normal hybrids derived when cells with two different mutations affecting BCKD are fused; (3) the increase of BCKD activity in normal but not mutant cells grown in high concentrations of BCAAs can distinguish these phenotypes more precisely in humans; and (4) the mechanism of BCKD stimulation in normal cells grown in high concentrations of BCAAs remains to be explained but can be pursued further with the cell culture conditions described.

Influence of insulin on blood levels of branched chain keto and amino acids in man

Branched chain keto acids, their corresponding amino acids, glucose, glucagon, growth hormone, C-peptide and gastric inhibitory polypeptide were determined in 8 healthy subjects after an intravenous bolus injection of 0.1 U/kg insulin. Branched chain keto acids declined within 60 min, the corresponding amino acids within 20 min or later. Amino acids tended to return towards normal earlier than their keto acids. Blood glucose levels were normal 2 hr after insulin injection while keto and amino acids remained diminished for more than 3 hr. In 8 healthy controls, given physiological saline instead of insulin, the branched chain keto acids did not decline throughout the test. It is suggested that insulin diminishes blood levels of branched chain keto acids, that the intraorgan flux of branched chain keto acids is different from the flux of branched chain amino acids and that branched chain keto acids may serve to correct for hypoglycemia.

Leukocyte valine dehydrogenase activity in Friedreich's ataxia

We studied the activity of valine dehydrogenase (VDH) in leukocytes of 14 Friedreich's ataxia patients and of 14 normal control subjects. There was a significant 26% mean decrease in enzyme activity in the patients, a finding which could be responsible for the chronic accumulation of some alpha-keto acids with toxic metabolic consequences in that disease. However the deficiency was not present in all patients with the typical symptoms, nor was its magnitude sufficient to be considered the primary genetic defect in Friedreich's Ataxia.

Effects of hypophysectomy and thyroidectomy on leucine metabolism in adipose tissue

Hypophysectomy doubled the rate of oxidation of L-[1-14C]leucine to 14CO2 by segments of rat epididymal adipose tissue. Thyroidectomy, but not adrenalectomy, produced identical results. Acceleration of leucine oxidation occurred even in the presence of glucose and saturating concentrations of insulin and leucine, suggesting that thyroidectomy increased the capacity to degrade leucine. Treatment of thyroidectomized rats with triiodothyronine (T3) decreased leucine oxidation, but at least 4 days were required. Treatment of hypophysectomized rats with T3 for 6 days was ineffective unless growth hormone was also given. A similar acceleration was also seen in the rate of oxidation of alpha-keto[1-14C]isocaproate, the deaminated analogue of leucine, but neither hypophysectomy nor thyroidectomy accelerated the rate of oxidation of isovalerate, the next metabolite in the degradative sequence. These observations suggested that hypothyroidism, whether primary or secondary, might increase the activity of the mitochondrial reaction responsible for the decarboxylation of alpha-ketoisocaproate. Because thyroidectomy failed to modify the rate of oxidation of [1-14C]pyruvate that occurs by an analogue reaction and requires the same cofactors, an effect of thyroidectomy on cofactor availability was ruled out. Direct assay in a cell-free homogenate revealed a nearly twofold increase in the activity of the alpha-ketoisocaproate dehydrogenase enzyme complex. The findings support the conclusion that hypothyroidism increases the amount or activity of the mitochondrial enzyme complex responsible for decarboxylation of branched-chain alpha-keto acids.