title: Differential regulation of homocysteine transport in vascular endothelial and smooth muscle cells.
abstract: OBJECTIVE
  We previously reported that homocysteine (Hcy) inhibits endothelial cell (EC) growth and promotes vascular smooth muscle cell (VSMC) proliferation. This study characterized and directly compared Hcy transport in cultured human aortic ECs (HAECs) and smooth muscle cells (HASMCs).
  
  
  RESULTS
  Hcy (10 micromol/L) was transported into both cell types in a time-dependent fashion but was approximately 4-fold greater in HASMCs, and is nonstereoenantiomer specific. Hcy transport in HAECs had a Michaelis-Menten constant (Km) of 39 micromol/L and a maximal transport velocity (Vmax) of 873 pmol/mg protein/min. In contrast, Hcy transport in HASMCs had a lower affinity (Km = 106 micromol/L) but a higher transport capacity (Vmax = 4192 pmol/mg protein/min). Competition studies revealed that the small neutral amino acids tyrosine, cysteine, glycine, serine, alanine, methionine, and leucine inhibited Hcy uptake in both cell types, but the inhibition was greater for tyrosine, serine, glycine, and alanine in HAECs. Sodium-depletion reduced Hcy transport to 16% in HAECs and 56% in HASMCs. Increases in pH from 6.5 to 8.2 or lysosomal inhibitors blocked Hcy uptake only in HAECs. In addition, Hcy shares carrier systems with cysteine, in a preferable order of alanine-serine-cysteine (ASC) > aspartate and glutamate (X(AG)) = large branched-chain neutral amino acids (L) transporter systems in HAECs and ASC > L > X(AG) in HASMCs. The sodium-dependent system ASC plays a predominant role for Hcy transport in vascular cells.
  
  
  CONCLUSIONS
  Transport system ASC predominantly mediates Hcy transport in EC and is lysosomal dependent.
authors: Temple University School of Medicine, Department of Pharmacology, 3420 North Broad Street, Philadelphia, PA 19140, USA.
authors: Xiaohua
authors: X
authors: Jiang
authors: Fan
authors: F
authors: Yang
authors: Eugen
authors: E
authors: Brailoiu
authors: Hieronim
authors: H
authors: Jakubowski
authors: Nae J
authors: NJ
authors: Dun
authors: Andrew I
authors: AI
authors: Schafer
authors: Xiaofeng
authors: X
authors: Yang
authors: William
authors: W
authors: Durante
authors: Hong
authors: H
authors: Wang
mesh_terms: D026962:Amino Acid Transport System ASC / Q000502:physiology*; D001011:Aorta / Q000166:cytology; D001693:Biological Transport, Active / Q000502:physiology; D002478:Cells, Cultured; D042783:Endothelial Cells / Q000502:physiology*; D006710:Homocysteine / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008247:Lysosomes; D009131:Muscle, Smooth, Vascular / Q000166:cytology; D032389:Myocytes, Smooth Muscle / Q000502:physiology*
chemical_list: D026962:Amino Acid Transport System ASC; D006710:Homocysteine
references: Chem Pharm Bull (Tokyo). 2002 Aug;50(8):1081-5
references: 12192140
references: FASEB J. 2000 Sep;14(12):1775-83
references: 10973927
references: Pflugers Arch. 1996 Nov-Dec;433(1-2):180-7
references: 9019721
references: J Clin Invest. 1996 Jan 1;97(1):146-53
references: 8550827
references: Arterioscler Thromb Vasc Biol. 2006 Jun;26(6):1397-404
references: 16574890
references: Acta Paediatr Suppl. 2006 Apr;95(451):77-82
references: 16720471
references: J Neuropathol Exp Neurol. 2004 Aug;63(8):821-30
references: 15330337
references: Nucleic Acids Res. 1981 Jul 10;9(13):3105-17
references: 7024910
references: Circ Res. 2006 Sep 15;99(6):598-606
references: 16931800
references: Sheng Li Xue Bao. 2005 Apr 25;57(2):103-14
references: 15830093
references: Am J Physiol. 1995 Mar;268(3 Pt 2):H1158-64
references: 7535004
references: Blood. 2002 Feb 1;99(3):939-45
references: 11806997
references: J Nutr. 2006 Jun;136(6 Suppl):1741S-1749S
references: 16702349
references: Cardiovasc Res. 2006 Jan;69(1):253-62
references: 16226235
references: J Biol Chem. 1997 Oct 3;272(40):25380-5
references: 9312159
references: J Biol Chem. 2000 Jul 21;275(29):21813-6
references: 10829011
references: Arch Biochem Biophys. 2006 Feb 15;446(2):119-30
references: 16455044
references: N Engl J Med. 1995 Feb 2;332(5):286-91
references: 7816063
references: J Clin Invest. 2001 May;107(10):1263-73
references: 11375416
references: Proc Natl Acad Sci U S A. 1997 May 27;94(11):5923-8
references: 9159176
references: Biochim Biophys Acta. 1990 Sep 24;1054(3):263-6
references: 2207177
references: JAMA. 1992 Aug 19;268(7):877-81
references: 1640615
references: J Biol Chem. 2006 Aug 11;281(32):22485-92
references: 16769724
references: Exp Mol Pathol. 1988 Feb;48(1):103-15
references: 3335248
references: J Biol Chem. 2006 Jun 9;281(23):15923-8
references: 16595650