title: Differential regulation of amino acid transporter SNAT3 by insulin in hepatocytes.
abstract: The liver is a metabolism and transfer center of amino acids as well as the prime target organ of insulin. In this report, we characterized the regulation of system N/A transporter 3 (SNAT3) in the liver of dietary-restricted mice and in hepatocytes treated with serum starvation and insulin. The expression of SNAT3 was up-regulated in dietary-restricted mice. The expression of SNAT3 protein was detected on the plasma membrane of hepatocyte-like H2.35 cells with a half-life of 6-8 h. When H2.35 cells were depleted of serum, the expression of SNAT3 was increased. An increased concentration of insulin, however, suppressed SNAT3 expression. Interestingly, the down-regulation of SNAT3 expression by insulin was blocked by the specific phosphoinositide 3-kinase inhibitor LY294002 and mammalian target of rapamycin inhibitor, but not by MAPK inhibitor PD98059, suggesting that insulin exerts its effect on SNAT3 through phosphoinositide 3-kinase-mammalian target of rapamycin signaling. Surface biotinylation assay showed an increased level of SNAT3 on the cell surface after 0.5 h of insulin treatment, although no effect was observed after 24 h of treatment. Consistently, the transport of the substrate l-histidine was increased with short, but not long, treatment by insulin in both H2.35- and SNAT3-transfected COS-7 cells. The L-histidine uptake was inhibited significantly by L-histidine followed by 2-endoamino-bicycloheptane-2-carboxylic acid and L-cysteine and to a lesser extent by L-alanine and aminoisobutyric acid, but was not inhibited by alpha-(methylamino)isobutyric acid, implying that uptake of L-histidine in H2.35 cells is primarily mediated by system N transporters. In conclusion, differential regulation of SNAT3 by insulin and serum starvation reinforces the functional significance of this transporter in liver physiology.
authors.affiliation: Department of Biochemistry, University of Texas Health Science Center, San Antonio, Texas 78229-3900, USA.
authors.forename: Sumin
authors.initials: S
authors.lastname: Gu
authors.forename: Carla J
authors.initials: CJ
authors.lastname: Villegas
authors.forename: Jean X
authors.initials: JX
authors.lastname: Jiang
mesh_terms: D026921:Amino Acid Transport Systems, Neutral / Q000737:chemistry / Q000378:metabolism*; D000818:Animals; D001667:Binding, Competitive; D001692:Biological Transport; D019921:Biotinylation; D015152:Blotting, Northern; D019556:COS Cells; D002462:Cell Membrane / Q000378:metabolism; D002867:Chromones / Q000494:pharmacology; D016895:Culture Media, Serum-Free / Q000494:pharmacology; D018076:DNA, Complementary / Q000378:metabolism; D004032:Diet; D004305:Dose-Response Relationship, Drug; D015536:Down-Regulation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005419:Flavonoids / Q000494:pharmacology; D022781:Hepatocytes / Q000378:metabolism*; D006639:Histidine / Q000494:pharmacology; D006801:Humans; D015151:Immunoblotting; D047468:Immunoprecipitation; D007328:Insulin / Q000378:metabolism*; D008099:Liver / Q000378:metabolism; D051379:Mice; D018613:Microscopy, Confocal; D008856:Microscopy, Fluorescence; D009025:Morpholines / Q000494:pharmacology; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D000081082:Phosphoinositide-3 Kinase Inhibitors; D015398:Signal Transduction; D013997:Time Factors; D014162:Transfection; D015854:Up-Regulation
chemical_list: D026921:Amino Acid Transport Systems, Neutral; D002867:Chromones; D016895:Culture Media, Serum-Free; D018076:DNA, Complementary; D004791:Enzyme Inhibitors; D005419:Flavonoids; D006639:Histidine; D007328:Insulin; D009025:Morpholines; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; C403222:system N protein 1; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one; C093973:2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one