Regulation of the Transport and Metabolism of Amino Acids in Isolated Fat Cells

Vanadium Complexes with Thioanilide Derivatives of Amino Acids: Inhibition of Human Phosphatases and Specificity in Various Cell Models of Metabolic Disturbances

In the text, the synthesis and characteristics of the novel ONS-type vanadium (V) complexes with thioanilide derivatives of amino acids are described. They showed the inhibition of human protein tyrosine phosphatases (PTP1B, LAR, SHP1, and SHP2) in the submicromolar range, as well as the inhibition of non-tyrosine phosphatases (CDC25A and PPA2) similar to bis(maltolato)oxidovanadium(IV) (BMOV). The ONS complexes increased [14C]-deoxy-D-glucose transport into C2C12 myocytes, and one of them, VC070, also enhanced this transport in 3T3-L1 adipocytes. These complexes inhibited gluconeogenesis in hepatocytes HepG2, but none of them decreased lipid accumulation in the non-alcoholic fatty liver disease model using the same cells. Compared to the tested ONO-type vanadium complexes with 5-bromosalicylaldehyde and substituted benzhydrazides as Schiff base ligand components, the ONS complexes revealed stronger inhibition of protein tyrosine phosphatases, but the ONO complexes showed greater activity in the cell models in general. Moreover, the majority of the active complexes from both groups showed better effects than VOSO4 and BMOV. Complexes from both groups activated AKT and ERK signaling pathways in hepatocytes to a comparable extent. One of the ONO complexes, VC068, showed activity in all of the above models, including also glucose utilizatiand ONO Complexes are Inhibitors ofon in the myocytes and glucose transport in insulin-resistant hepatocytes. The discussion section explicates the results within the wider scope of the knowledge about vanadium complexes.

Serum modulation of insulin action: effects on rat gingival fibroblast metabolism

Serum is reported to reduce the sensitivity of cells in culture to insulin. The effect of serum concentration in the growth medium on the responsiveness of control (C) and streptozotocin diabetic (D) rat gingival fibroblasts to insulin was measured by monitoring cellular DNA, RNA, total protein and medium hydroxyproline (collagen) levels, as well as the cellular uptake of C14-alpha-NH2-isobutyrate (alpha-AIB) and H3-2-deoxyglucose (2DG). The cells were grown in alpha-MEM at 5, 10, 15 or 20% FCS with 0, 10(-12), 10(-10), 10(-8) and 10(-6) M insulin used at each serum level. Insulin effects in the absence of serum were not assessed. For both the C and D rat cells, the DNA increased proportionately with increasing serum and insulin levels. In contrast, RNA and total cell protein increased with increase in insulin and decrease in serum, the magnitude of the effect being greater in C than in D cells. The insulin stimulation of both 2DG and alpha-AIB uptake and of collagen secretion varied inversely with serum concentrations. The magnitude of the insulin-serum interaction on metabolite uptake was greater for the D rat cells. These data indicate that serum significantly reduced the cell response to insulin stimulated metabolite uptake and collagen secretion, but was without apparent effect on the intracellular insulin responsive parameters. They suggest that serum factor(s) interfere with the availability of insulin to the cell and that the D rat cells are most affected.

Regulation of protein turnover by glucose, insulin, and amino acids in adipose tissue

Protein synthesis and degradation were measured simultaneously in epididymal fat pads of rats by use of the incorporation of [14C]phenylalanine into protein and the sum of net protein breakdown and protein synthesis, respectively. Neither glucose nor insulin altered protein synthesis, but together they promoted this process; pyruvate could be substituted for glucose. Separately, glucose or insulin diminished proteolysis, and these effects were additive. In the presence of glucose and insulin, leucine, alanine, glutamine, glutamate, and aspartate lowered protein degradation to varying degrees but did not alter protein synthesis. Glutamate, but not leucine or alanine, was inhibitory without glucose and insulin present. When aminooxyacetic acid was provided to decrease the rate of transamination of amino acids, the inhibitory effects of leucine, alanine, and aspartate, but not of glutamate, appeared to be diminished. alpha-Ketoglutarate, but neither alpha-ketoisocaproate nor pyruvate, could diminish proteolysis. Inhibition of proteolysis was associated with a higher tissue content of glutamate and a greater production of glutamate and glutamine. These results suggest that glutamate itself may inhibit proteolysis in adipose tissue and mediate, at least in part, the effects of other amino acids.

A comparison of the effects of sodium nitroprusside and insulin on the control of metabolism in rat isolated adipocytes

Sodium nitroprusside, a known activator of guanylate cyclase within cells, was used as a probe to investigate the possible role of cyclic GMP in the control of metabolism within rat isolated white adipocytes. Over the concentration range 0-0.1 mM, it increased intracellular cyclic GMP concentrations up to 6-fold within 2 min. Over the same concentration range, it increased the incorporation of 14C from D-[U-14C]glucose into triacylglycerol and of L-[14C]leucine into protein. It also inhibited adrenalin -stimulated lipolysis in the cells, but had no effect on the transport of glucose into the cells. The effects of sodium nitroprusside were compared with those elicited by insulin under identical conditions, as this hormone was shown to cause a similar, but transient, rise in intracellular cyclic GMP concentrations within these cells. Nor insulin, neither sodium nitroprusside were able to increase cyclic AMP levels in adipocytes, whereas adrenalin (0.3 microM) stimulated this production. It is suggested that cyclic GMP may have a role in the control of some part of metabolism 'glucose or amino acids' in adipocytes, and that sodium nitroprusside is a useful probe to investigate this. The limitation of its use are discussed.

Insulin regulation of fat cell ribosomes, protein synthesis, and lipoprotein lipase

Ribosomes of high purity were isolated from fat cells by discontinuous sucrose gradient centrifugation. Approximately 23% of the ribosomes were membrane bound. A reproducible fraction of ribosomes was recovered as polysomes capable of incorporating [3H]leucine into peptides in a cell-free system. Insulin (0.1-1.0 mU/ml) produced an increase in polysomal activity. Linear sucrose gradient profiles also revealed an increase in the ratio of polysomes to total ribosomes. Coincident with this effect, insulin increased the lipoprotein lipase activity fraction inhibitable by cycloheximide (0.01 mg/ml), and the immunotitratable enzyme activity. Insulin also enhanced the incorporation of labeled amino acids into adipose tissue immunoprecipitable lipoprotein lipase and total fat cell proteins. Cordycepin (0.1 mg/ml) or alpha-amanitin (10 micrograms/ml) partially inhibited insulin effects on ribosomes and protein synthesis but not on lipoprotein lipase. EGTA (1mM) prevented all of the insulin effects, whereas the calcium ionophore A-23187 (2 microM) augmented the hormone actions. The ionophore alone partially mimicked the insulin effects. In adipocytes, insulin increased the size of the protein-synthesizing polysomal pool by a mechanism which, in part, requires nuclear mRNA processing. Insulin, however, increased lipoprotein lipase synthesis independent of nuclear events. Calcium ions may be important for the expression of these insulin effects.

Effect of hyperinsulinemia on acid cholesterol ester hydrolase activity in liver, heart and epididymal fat pad preparations from rats and mice

The effect of insulin on lysosomal acid cholesterol ester hydrolase activity was studied in liver, heart and fat pad preparations from rats and mice. Hyperinsulinemia was induced for a period of 6 days in rats by the subcutaneous administration of exogenous insulin by an osmotic minipump. The effect of more chronic endogenous hyperinsulinemia was studied using genetic strains of diabetic (db/db) mice at 12 weeks of age. Mouse liver and heart preparations were characterized as having an acid pH optimum of 4.5-5 for cholesterol ester hydrolase activity; a smaller but distinct pH optimum could also be observed at pH 7. In contrast, hydrolase activity in mouse fat pad preparations had only one distinct pH optimum of 6.5. Hyperinsulinemia in rats and mice resulted in a significant decrease in acid cholesterol ester hydrolase activity in heart preparations, but had no consistent effect on acid hydrolase activity observed in liver and fat pad preparations. This decrease in lysosomal acid cholesterol ester hydrolase activity in cardiac tissue due to hyperinsulinemia cannot be related to any changes in lipoprotein turnover caused by insulin or diabetes.

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.

Insulin inhibition of protein degradation in cell monolayers

Protein degradation has been measured in confluent monolayers of eleven lines of contact-inhibited cells and ten transformed lines as the rate of release of trichloroacetic acid-soluble radioactivity after prelabeling cell protein with [3H]leucine. Insulin, at contrations from 10(-12) M to 10(-6) M, has been added at the beginning of the 4-hour degradation period to detect selective effects of this hormone as an inhibitor of the inducible proteolysis occurring in serum-free medium. In addition insulin binding measurements have been performed on selected cell lines in an attempt to relate receptor properties to insulin action. Substantial effects of insulin are found in most cells with a selective inhibition at low insulin concentrations noted in several of the transformed lines. The difference in insulin sensitivity is not entirely definitive because temperature-sensitive transformation mutants of NRK cells are not more sensitive to insulin at a temperature where they show the transformed phenotype. Although insulin receptors on different cell lines have similar binding properties, two of the hepatomas used, H35 and MH1C1, show inhibition of protein degradation at insulin concentrations where receptor occupancy is extremely low. Calvarial osteoblast-like cells have a high rate of protein degradation which can be reduced by growth factors but not by insulin. The lack of an insulin response is a consequence of poor insulin binding to the cells. Insulin binds to the osteogenic sarcoma cells in substantial amounts. However, its normal action to inhibit the induced proteolysis is restricted because with these cells no increase of proteolysis occurs in serum-free medium. Generally higher rates of protein degradation are observed in the contact-inhibited lines than the transformed cells. We suggest that this difference may provide a selective growth advantage to transformed cells.

Measurement of membrane potentials (psi) of erythrocytes and white adipocytes by the accumulation of triphenylmethylphosphonium cation

The accumulation of the lipophilic cation, triphenylmethylphosphonium, has been employed to determine the resting membrane potential in human erythrocytes, turkey erythrocytes, and rat white adipocytes. The triphenylmethylphosphonium cation equilibrates rapidly in human erythrocytes in the presence of low concentrations of the hydrophobic anion, tetraphenylborate. Tetraphenylborate does not accelerate the uptake of triphenylmethylphosphonium ion by adipocytes. The cell associated vs. extracellular distribution of the triphenylmethylphosphonium ion is proportional to changes in membrane potential. The distribution of this ion reflects the membrane potential determining concentration of the ion with dominant permeability in a "Nernst" fashion. The resting membrane potentials for the human erythrocyte, turkey erythrocyte, and rat white adipocyte were found to be -8.4 +/- 1.3, -16.8 +/- 1.1, and -58.3 +/- 5.0 mV, respectively, values which compare favorably with values obtained by other methods. In addition, changes in membrane potential can be assessed by following triphenylmethylphosphonium uptake without determining the intracellular water space. The method has been successfully applied to a study of hormonally induced changes in membrane potential of rat white adipocytes.

Evidence against the involvement of cyclic GMP in the insulin-stimulation of lipoprotein lipase activity in fat cells

Under in vitro experimental conditions in which insulin increases adipose tissue lipoprotein lipase, cyclic GMP or dibutyryl cyclic GMP has no effect on this enzyme in rat adipose tissue fragments, or on either the intra- or extracellular forms of this enzyme in isolated fat cells. These results do not support the involvement of cyclic GMP in the insulin-stimulation of lipoprotein lipase in adipose tissue.

The effects of insulin on the turnover of glucose-6-phosphate dehydrogenase in epididymal adipose tissue of the rat

The effects of insulin on the turnover of glucose-6-phosphate dehydrogenase in rat epididymal adipose tissue were studied by immunochemical technique in in vitro incubations. Insulin increased the relative rate of synthesis of glucose-6-phosphate dehydrogenase by two-fold in tissue obtained from normal rats. Insulin also had an effect on the rate of degradation of this enzyme. In the absence of insulin in the incubation medium the rate constant of degradation was 0.11 h-1 (half-life, 6.3 h). When insulin was added to the medium degradation of this enzyme was slowed. The new rate constant of degradation was 0.04 h-1 (half-life, 17 h). In the presence of insulin, the rate constant of degradation of total protein in adipose tissue was unchanged; therefore the effects of insulin on the degradation of glucose-6-phosphate dehydrogenase are specific to that protein and perhaps to a few other specific proteins.

Regulation of cardiac protein balance by hydrocortisone: interaction with insulin

In fetal mouse hearts in organ culture the rate of protein synthesis was substantially reduced and the rate of protein degradation slightly increased by hydrocortisone in the absence of insulin, but in the presence of insulin the steroid caused a small increase in protein synthesis and a significant reduction in protein degradation. Hydrocortisone promoted the net uptake (or reduced the net release) of branched-chain amino acids independent of insulin and independent of simultaneous changes in protein balance. The specific activities of the lysosomal enzymes cathepsin D and glucosaminidase were reduced by hydrocortisone in all media, whereas the specific activity of creatine kinase increased when the medium contained insulin but decreased in the absence of insulin. It is concluded that hydrocortisone regulates cardiac protein balance via alterations both in synthesis and in degradation. Some of the hormone's myocardial effects are influenced by insulin so that hydrocortisone is anabolic in its presence but catabolic in its absence.

The effects of bioregulators upon amino acid transport and protein synthesis in isolated rat hepatocytes

Isolated rat hepatocytes prepared by an enzyme perfusion technique possess a functional amino acid transport system and retain the capacity to synthesize protein. Amino acid transport was studied using the non-metabolizable amino acid analog alpha-aminoisobutyric acid. The transport process was time, temperature and concentration dependent. Similarly, leucine incorporation into protein was time and temperature dependent being optimal at 3m degrees C. Amino acid, fetal calf serum, growth hormone and glucose all produced small, reproducible increases in protein synthesis rates. Bovine serum albumin diminished the uptake of alpha-aminoisobutyric acid and leucine incorporation into protein. The amino acid content on either side of the cell membrane was found to affect transport into or out of the cellular compartment (transconcentration effects). High cell concentrations decreased transport and protein synthesis as a result of isotopic dilution of labelled amino acids with those released by the hepatocytes. This was consistent with the capacity of naturally occurring amino aicds to compete with alpha-aminoisobutyric acid for uptake into the hepatocyte. In order to define more precisely the effects of bioregulators on transport and protein synthesis it will be necessary to define and subfractionate cellular compartments and proteins which are the specific targets of cellular regulation.

Determination of plasma amino acids in small samples with the use of Dansyl-chloride

A radiochemical method based on dansylation of plasma samples with Dansyl-chloride and thin alyer chromatography on polyamide sheets is presented for the quantitative individual determination of plasma amino acids in very small blood samples. The results are first corrected with a norvaline internal standard and secondly with specific recovery factors for each amino acid. The results agree slosely with other methods and with already published plasma normal amino acids values in healthy adult rats.

An established preadipose cell line and its differentiation in culture. II. Factors affecting the adipose conversion

When cells of the established preadipose line 3T3-L1 enter a resting state, they accumulate triglyceride and convert to adipose cells. The adipose conversion is brought about by a large increase in the rate of triglyceride synthesis, as measured by the incorporation rate of labeled palmitate, acetate, and glucose. In a resting 3T3 subline which dose not undergo the adipose conversion, the rate of triglyceride synthesis from these precursors is very low, and similar to that of growing 3T3-L1 cells, before their adipose conversion begins. If 3T3-L1 cells incorporate bromodeoxyuridine during growth, triglyceride synthesis does not increase when the cells reach a stationary state, and triglycerides do not accumulate. As would be expected from their known actions on tissue adipose cells, lipogenic and lipolytic hormones and drugs affect the rate of synthesis and accumulation of triglyceride by 3T3-L1 cells, but in contrast to bromodeoxyuridine, these modulating agents do not seem to affect the proportion of cells which undergoes the adipose conversion. Insulin markedly increases the rate of synthesis and accumulation of triglyceride by fatty 3T3-L1 cells, and produces a related increase in cell protein content. Of 20 randomly selected clones isolated from the original 3T3 stock, 19 are able to convert to adipose cells. The probability of such a conversion varies greatly among the different clones, in most cases being much lower than for 3T3-L1; but once the conversion takes place, the adipose cells produced from all of the 19 clones appear similar. The adipose conversion would seem to depend on an on-off switch, which is on with a different probability in different clones. This probability is quasistably inherited by the clonal progeny.