Hormonal regulation of hepatic amino acid transport

The sodium-coupled neutral amino acid transporter SNAT2 mediates an anion leak conductance that is differentially inhibited by transported substrates

The sodium-coupled neutral amino acid transporter SNAT2 mediates cellular uptake of glutamine and other small, neutral amino acids. Here, we report the existence of a leak anion pathway associated with SNAT2. The leak anion conductance was increased by, but did not require the presence of, extracellular sodium. The transported substrates L-alanine, L-glutamine, and alpha-(methylamino)isobutyrate inhibited the anion leak conductance, each with different potency. A transporter with the mutation H-304A did not catalyze alanine transport but still catalyzed anion leak current, demonstrating that substrate transport is not required for anion current inhibition. Both the substrate and Na+ were able to bind to the SNAT2H-304A transporter normally. The selectivity sequence of the SNAT2H-304A anion conductance was SCN->>NO3->I->Br->Cl->Mes-. Anion flux mediated by the more hydrophobic anion SCN- was not saturable, whereas nitrate flux demonstrated saturation kinetics with an apparent Km of 29 mM. SNAT2, which belongs to the SLC38 family of transporters, has to be added to the growing number of secondary, Na+-coupled transporters catalyzing substrate-gated or leak anion conductances. Therefore, we can speculate that such anion-conducting pathways are general features of Na+-transporting systems.

Impact of forskolin and amino acid depletion upon System A activity and SNAT expression in BeWo cells

Amino acid transport System A (SysA) plays an important role in mediating the transplacental transfer of neutral amino acids from mother to fetus. Given that prior work has demonstrated that SysA activity is regulated, both over gestation and in response to dietary restriction during pregnancy, we examined the response of SysA activity and sodium-dependent neutral amino acid transporter (SNAT; responsible for SysA activity) expression to cAMP analogues and amino acid deprivation in BeWo cells, an accepted model of placental syncytia. SysA activity was unaffected by forskolin, a cAMP agonist, at 48 and 72 h. Amino acid depletion was associated with an up-regulation of SysA activity, largely mediated through an enhancement of SNAT2 (Slc38a2) expression at both the protein and mRNA level. SNAT1 (Slc38a1) expression did not change in response to amino acid depletion, while SNAT4 (Slc38a4) could not be detected. In summary, SysA activity in BeWo cells responds to amino acid depletion through the differential regulation of SNAT subtypes.

Selective up-regulation of system a transporter mRNA in diabetic liver

The transport of alanine by system A is an important source of carbons for the synthesis of glucose in the liver. Here, we show that the mRNA encoding the ubiquitously expressed isoform of the rat system A transporter (SAT2) is dramatically increased in liver following streptozotocin-induced diabetes. This increase in SAT2 mRNA is intensified in the gluconeogenic periportal hepatocytes and also in hepatocytes surrounding the central vein. SAT3, the more abundant system A mRNA isoform present in liver, is restricted to perivenous hepatocytes and is also increased following this treatment but to a much lesser extent than SAT2 mRNA. SN1, an abundant system N mRNA isoform expressed in both perivenous and periportal hepatocytes, is not affected by streptozotocin treatment. A pharmacological dose of glucagon also increased both SAT2 and SAT3 mRNA levels in liver while SN1 mRNA levels remained unaffected. These results indicate that the increase in system A activity observed in liver following experimentally induced diabetes or glucagon treatment is due to the selective increase in mRNAs encoding system A transporters.

The role of system A for neutral amino acid transport in the regulation of cell volume

System A is a secondary active, sodium dependent transport system for neutral amino acids. Strictly coupled with Na,K-ATPase, its activity determines the size of the intracellular amino acid pool, through a complex network of metabolic reaction and exchange fluxes. Many hormones and drugs affect system A activity in specific cell models or tissues. In all the cell models tested thus far the activity of the system is stimulated by amino acid starvation, cell cycle progression, and the incubation under hypertonic conditions. These three conditions produce marked alterations of cell volume. The stimulation of system A activity plays an important role in cell volume restoration, through an expansion of the intracellular amino acid pool. Under normal conditions, system A substrates represent a major fraction of cell compatible osmolytes, organic compounds that exert a protein stabilizing effect. It is, therefore, likely that the activation of system A represents a portion of a more complex response triggered by exposure to stresses of various nature. Since system A transporters have been recently cloned, the molecular bases of these regulatory mechanisms will probably be elucidated in a short time.

No evidence for a tumor necrosis factor alpha stimulated 2-methylaminoisobutyric acid uptake in hepatocyte monolayer

This study investigates the short-term effects of glucagon and human recombinant tumor necrosis factor alpha (TNF alpha) singly and in association on 2-methylaminoisobutyric acid (MeAIB) transport in hepatocyte monolayers. As expected, glucagon induced a time-dependent stimulation of MeAIB transport. In our experimental conditions, TNF alpha did not induce cytolysis. A 2 hour exposure to TNF alpha (0.05-500 ng/l) with or without glucagon (10(-9) to 10(-6) M) did not modify the basal or glucagon-stimulated MeAIB transport. Varying the duration of exposure to TNF alpha 5 ng/l up to 6 h was equally ineffective. The presence of hydrocortisone potentiated the glucagon-stimulated transport, but TNF alpha remained ineffective. Finally, the association of interferon (IFN gamma) with TNF alpha and/or glucagon was unable to modify the transport activity. These data demonstrate that TNF alpha does not exert a direct effect on MeAIB transport in hepatocytes, at least on a short-term basis.

Effect of alanine concentration independent of changes in insulin and glucagon on alanine and glucose homeostasis in the conscious dog

The effect of an alanine load per se on hepatic alanine balance and hepatic glucose production is unclear. To examine this question, alanine was infused into six postabsorptive dogs at a rate of 6 mumol/kg-min, while maintaining insulin and glucagon levels using the pancreatic clamp technique. The arterial alanine concentration rose from a basal level of 227 +/- 16 mumol/L to 497 +/- 40 mumol/L during alanine infusion (P less than .01). The net hepatic fractional extraction of alanine remained unchanged, while hepatic alanine uptake increased from 3.0 +/- 0.3 to 6.0 +/- 0.4 mumol/kg-min (P less than .01). Conversion of alanine into glucose increased 87% to 2.7 +/- 0.3 mumol/kg-min during alanine infusion (P less than .01) while gluconeogenic efficiency remained essentially unchanged. Despite the increased gluconeogenic rate, the total rate of glucose production was unchanged. These data suggest that an increase in the alanine load to the liver causes a proportional increase in net hepatic alanine uptake and the gluconeogenic rate, but that in an overnight fasted animal this increase is insufficient to significantly increase glucose production.

Ionic dependence of amino-acid transport in the exocrine pancreatic epithelium: calcium dependence of insulin action

Rapid unidirectional transport (15 sec) of L-serine and 2-methylaminoisobutyric acid (MeAIB) was studied in the isolated perfused rat pancreas using a dual-tracer dilution technique. Time-course experiments in the presence of normal cation gradients revealed a time-dependent transstimulation of L-serine influx and transinhibition of MeAIB influx. Transport of the model nonmetabolized System A analog MeAIB was Na+ dependent and significantly inhibited during perfusion with 1 mM ouabain. Although transport of L-serine was largely Na+ independent, ouabain caused a time-dependent inhibition of transport. Influx of both amino acids appeared to be inhibited by the ionophore monensin but unaffected by a lowered extracellular potassium concentration. Removal of extracellular calcium had no effect on influx of the natural substrate L-serine, whereas stimulation of transport by exogenous insulin (100 microU/ml) was entirely dependent upon extracellular calcium and unaffected by ouabain. Paradoxically, exogenous insulin had no effect on the time-course of MeAIB influx. The characteristics of L-serine influx described in earlier studies together with our present findings suggest that insulin may modulate the activity of System asc in the exocrine pancreatic epithelium by a calcium-dependent mechanism.

The roles of insulin and glucagon in the regulation of amino acid turnover rate and pool size: in vivo study with [15N]glycine and gas chromatography--mass spectrometry

Gas chromatography--mass spectrometry analysis of plasma amino acid derivatives has been used to determine the 15N enrichment time decay curves of plasma glycine following a single dose administration of [15N]glycine in untreated and insulins-, glucagon-, and cycloheximide-treated rabbits. The present study indicated the following: (a) Increases of 80 and 50% in plasma glycine disappearance rate constants occurred in insulin- and glucagon-treated rabbits as compared with control postabsorptive rabbits; (b) The hormones in the intact rabbits caused a significant depletion in glycine pool size, which led to a moderate reduction in the fluxes of glycine. (c) A significant reduction in glycine turnover rate constants and pool size was noted at 3 and 24 hr following the administration of a sublethal dose of cycloheximide and a restoration towards control postabsorptive values was observed 48 hr after cycloheximide administration. (d) Sublethal doses of cycloheximide inhibited by 60 and 90% the stimulatory action of insulin and glucagon on plasma glycine disappearance, respectively. The present data suggest that both insulin and glucagon may act directly on plasma glycine disappearance rates. The stimulatory action of insulin differs from the action of glucagon in that it is not completely blocked by cycloheximide. Presumably glucagon and insulin modify the glycine transport system at different sites or by a different mechanism.

Distribution of some non-physiological amino acids (AIB, DAB, MeAIB) in mice

The distribution of labelled 2-aminoisobutyric acid (AIB), 2,4-diaminobutyric acid (DAB) and 2-(methylamino)-isobutyric acid (MeAIB) was studied by whole body autoradiography in mice. All the amino acids were rapidly cleared from the blood and were taken up in most of the organs in the body. The results suggest the presence of operating transport systems for these amino acids into the majority of different cell types in mice. In the central nervous system, small amounts could be registered and in the pancreas there was a marked difference between DAB, which was not taken up, and the other two amino acids which were heavily accumulated. In an early embryo only MeAIB was observed in a high concentration.

Amino acid transport in isolated rat hepatocytes

Improvements in the collagenase perfusion techniques have made isolated rat hepatocytes a popular model in which to study hepatic function. Our knowledge of hepatic amino acid transport has been advanced as a result of this methodology. Translocation across the hepatocyte plasma membrane can, in some instances, represent the rate-limiting step in the overall metabolism of certain amino acids. Furthermore, regulation of amino acid uptake by hepatocytes appears to play a role in diabetes, and perhaps in malignant transformation. Comparisons between normal adult hepatocytes and several hepatoma cell lines show basic differences in amino acid transport. There are at least eight distinct systems in normal hepatocytes for transport of the hormones. Systems A and N exhibit enhanced uptake rates after the cells have been maintained in the absence of extracellular amino acids, a phenomenon termed adaptive control. Further studies using isolated hepatocytes will increase our basic understanding of membrane transport processes and their regulation.

Calcium-dependent hormonal regulation of amino acid transport and cyclic AMP accumulation in rat hepatocyte monolayer cultures

The effect of glucagon, epinephrine, norepinephrine, dexamethasone, insulin, and dexamethasone plus glucagon on the transport of 2-aminoisobutyric acid (AIB) and that of glucagon on the production of cyclic AMP were examined in rat hepatocyte monolayer cultures under three different culture conditions involving calcium. The hepatocytes were studied in calcium-contaning medium after treatment with or without 0.033% dimethyl sulfoxide, the solvent for the calcium ionophore A23187 (calcium controls); calcium-free medium after treatment with A23187 (calcium-depleted); and calcium-containing medium after treatment with ionophore (calcium-restored). The basal and hormonally regulated rates of AIB transport for hepatocytes in calcium control and calcium-depleted cultures were comparable. The restoration of calcium in calcium-restored cultures increased the basal and the hormonally stimulated transport of AIB when compared to the other conditions. Calcium markedly enhanced the stimulation of AIB transport in cultures treated with glucagon, catecholamines, and dexamethasone plus glucagon. The level of cyclic AMP production in response to glucagon in calcium control and calcium-depleted cultures was the same and it was conspicuously higher than the level in calcium-restored cultures. Varying the concentration of calcium in the medium used to maintain the hepatocytes in calcium control cultures did not affect the stimulation of AIB transport or cyclic AMP production by glucagon. However, in calcium-restored cultures, increasing the calcium concentration of the medium resulted in increased stimulation of AIB transport and decreased production of cyclic AMP by glucagon. In the calcium-restored cultures, calcium in the absence of glucagon enhanced AIB transport but had no effect on cyclic AMP production. Cultures maintained for 6 hr in calcium-free medium after the depletion of calcium showed a 6- to 7-fold increase in the production of cyclic AMP in response to glucagon, but no stimulation of AIB transport. We suggest that mobilization of cellular calcium by glucagon either directly or through cyclic AMP mediates its stimulation of amino acid transport.

Hormonal regulation of amino acid transport and cAMP production in monolayer cultures of rat hepatocytes

The effects of insulin, glucagon of Dexamethasone (DEX) and of glucagon with insulin or DEX were examined on the uptake of 2-amino [1-14C]isobutyric acid (AIB) and N-Methyl-2-amino [1-14C]isobutyric acid (NMe AIB) in monolayer cultures of rat hepatocytes. Insulin and glucagon stimulated the uptake of both the amino acids and DEX inhibited it, showing that all three of these hormones regulate the A system (the sodium-dependent system that permits the transport of NMe AIB) for amino acid transport in these cultures. Experiments investigating the transport of aminocyclopentane-1-carboxylic acid, 1- [carboxyl-14C] in the presence of excess AIB or in the absence of sodium showed that insulin had no effect on the activity of the L system (the sodium-independent system that prefers leucine). Experiments on the uptake of AIB in the presence of excess NMe AIB showed insulin had no effect on the transport activity of the ASC system (the sodium-dependent system that does not transport NMe AIB). Insulin concentrations ranging from 0.1 nM to 100 nM did not antagonize the stimulatory effect of optimum or suboptimum concentrations of glucagon on the uptake of either AIB or NMe AIB. Similarly, glucagon did not antagonize the stimulatory effect of optimum or suboptimum concentrations of insulin on the uptake of both the amino acids. The combined effect of insulin and glucagon was additive on the rate as well as the cumulative uptake of both AIB and NMe AIB. DEX alone inhibited the transport of both AIB and NMe AIB by about 25%, while glucagon caused a 2--3-fold increase; however, the addition of glucagon to cultures containing DEX caused a 7--8-fold increase in the uptake of both AIB and NMe AIB when compared to cultures containing DEX alone. The effect of insulin on the levels of cAMP was also investigated. Insulin had no effect on the cAMP levels in cultures treated or untreated with optimum or suboptimum concentrations of glucagon.

Dicarboxylic amino acid uptake in normal, Friedreich's ataxia, and dicarboxylic aminoaciduria fibroblasts

Glutamic and aspartic acid uptake was measured in skin fibroblasts from patients with Friedreich's Ataxia, dicarboxylic aminoaciduria, and normal individuals. The results showed no difference in uptake kinetics of either dicarboxylic amino acids between Friedreich's Ataxia and normal cells, but reduced uptake velocities in dicarboxylic aminoaciduria fibroblasts. Friedreich's Ataxia fibroblasts were, however, less calcium-dependent and more magnesium and phosphate-dependent than controls in glucose-free incubation mixture. This difference might be related to some degree of glucose intolerance by Friedreich's Ataxia fibroblasts in culture.