Transport of 2-aminoisobutyric acid in cultured endothelial cells

Reduced Na+ K+ -ATPase activity may reduce amino acid uptake in intrauterine growth restricted fetal sheep muscle despite unchanged ex vivo amino acid transporter activity

KEY POINTS

Fetuses with intrauterine growth restriction (IUGR) have reduced muscle mass that persists postnatally, which may contribute to their increased risk for adult onset metabolic diseases, such as diabetes and obesity. Amino acid transporter-mediated histidine uptake and system L amino acid transporter activity were similar in sarcolemmal membranes isolated from control and IUGR hindlimb skeletal muscle. Activity of Na⁺ K⁺ -ATPase, which is responsible for establishing the sodium gradient necessary for system A and N amino acid transporter function, was significantly reduced in IUGR skeletal muscle sarcolemma compared to control. ATP content was lower in IUGR skeletal muscle. Expression and phosphorylation of proteins in the mechanistic target of rapamycin pathway were similar in control and IUGR skeletal muscle homogenate. Our data suggest that lower Na⁺ K⁺ -ATPase activity, which reduces the driving force for active amino acid transport, and lower ATP availability contribute to reduced amino acid uptake and protein synthesis in IUGR fetal skeletal muscle.

ABSTRACT

Fetuses with intrauterine growth restriction (IUGR) have lower muscle mass that persists postnatally. Using a sheep model of placental insufficiency and IUGR, we have previously demonstrated lower net total uptake of amino acids by the fetal hindlimb and lower skeletal muscle protein synthesis rates. To investigate the mechanisms underlying these changes, we tested the hypothesis that ex vivo amino acid transporter and Na⁺ K⁺ -ATPase activity is reduced, and ex vivo ATP levels are lower in hindlimb skeletal muscle of the IUGR fetus. We developed a novel protocol to measure transporter-mediated histidine uptake, system L amino acid transporter activity and Na⁺ K⁺ -ATPase activity using sarcolemmal membranes isolated from hindlimb muscle of control (CON, n = 11-12) and IUGR (n = 12) late gestation fetal sheep. We also determined ATP content and the activity of insulin and mechanistic target of rapamycin (mTOR) signalling, which are involved in regulating cellular amino acid uptake and protein synthesis, by measuring the expression and phosphorylation of AKT, 4E-BP1, eIF2α, AMPKα, p70 S6 kinase and rpS6 in muscle homogenates. Transporter-mediated histidine uptake and system L activity were similar in control and IUGR sarcolemma, although ex vivo Na⁺ K⁺ -ATPase activity was lower by 64% (P = 0.019) in IUGR sarcolemma. ATP content was lower by 25% (P = 0.007) in IUGR muscle. Insulin, AMPK, and mTOR signalling activity was similar in control and IUGR muscle. We speculate that reduced muscle sarcolemmal Na⁺ K⁺ -ATPase activity and lower ATP content diminishes the sodium gradient in vivo, resulting in a reduced driving force for sodium-dependent transporters and subsequently lower muscle amino acid uptake.

Regulation of amino acid and glucose transporters in endothelial and smooth muscle cells

While transport processes for amino acids and glucose have long been known to be expressed in the luminal and abluminal membranes of the endothelium comprising the blood-brain and blood-retinal barriers, it is only within the last decades that endothelial and smooth muscle cells derived from peripheral vascular beds have been recognized to rapidly transport and metabolize these nutrients. This review focuses principally on the mechanisms regulating amino acid and glucose transporters in vascular endothelial cells, although we also summarize recent advances in the understanding of the mechanisms controlling membrane transport activity and expression in vascular smooth muscle cells. We compare the specificity, ionic dependence, and kinetic properties of amino acid and glucose transport systems identified in endothelial cells derived from cerebral, retinal, and peripheral vascular beds and review the regulation of transport by vasoactive agonists, nitric oxide (NO), substrate deprivation, hypoxia, hyperglycemia, diabetes, insulin, steroid hormones, and development. In view of the importance of NO as a modulator of vascular tone under basal conditions and in disease and chronic inflammation, we critically review the evidence that transport of L-arginine and glucose in endothelial and smooth muscle cells is modulated by bacterial endotoxin, proinflammatory cytokines, and atherogenic lipids. The recent colocalization of the cationic amino acid transporter CAT-1 (system y(+)), nitric oxide synthase (eNOS), and caveolin-1 in endothelial plasmalemmal caveolae provides a novel mechanism for the regulation of NO production by L-arginine delivery and circulating hormones such insulin and 17beta-estradiol.

Hyperosmotic stress up-regulates amino acid transport in vascular endothelial cells

Cultured vascular endothelial cells take up L-proline by sodium-dependent transport. Cells incubated in medium made hyperosmotic by addition of sucrose showed a dose-dependent increase in Na+/proline cotransport. Studies with alpha-(methylamino)isobutyric acid revealed that the up-regulation was specific for amino acid transport system A. Up-regulation was blocked by actinomycin D and cycloheximide, indicating roles for gene transcription and protein synthesis. Up-regulation was maximum after five to six hours of hyperosmotic treatment, but returned to control levels when osmotic stress was maintained for 24 hours. The decline at 24 hours was accompanied by a significant increase in Na+/gamma-aminobutyric acid cotransport. The activity of this system, which also transports betaine, remained unchanged after just five hours of hyperosmotic stress. Inclusion of betaine in the hyperosmotic medium reduced up-regulation of system A. Na/Pi cotransport also was up-regulated by five hours of hyperosmotic stress. Up-regulation of system A, but not Na/Pi cotransport, was detected in isolated membrane fractions indicating that increased activity of this membrane transport system may be one mechanism by which vascular endothelial cells accumulate amino acids. The amino acids may act as organic osmolytes to help maintain normal cell volume during the early phase of hyperosmotic stress.