Regulation of Homocysteine Transport in Vascular Cells.

Increased levels of plasma homocysteine is an independent risk factor for cardiovascular disease and has cell-type distinct proatherosclerotic effects on vascular cells. In this study, we characterized L- homocysteine transport in cultured human aortic endothelial and aortic smooth muscle cells. L-homocysteine was transported into vascular cells in a time-dependent fashion. L-homocysteine transport activity was about 2-fold higher in aortic smooth muscle cells. In addition, L-homocysteine transport in both cell types was mediated by sodium-dependent and independent carrier systems. Competition studies revealed that the neutral amino acids cysteine, glycine, serine, tyrosine, alanine, leucine, and methionine, and inhibitors of the cysteine transport systems inhibited L-homocysteine uptake in both cell types, but the inhibition was greater in endothelial cells. Eadie-Hofstee plots demonstrated that L-Hcy transport in endothelial cells had a Michaelis constant (Km) of 79mM and a maximum transport velocity (Vmax) of 873 pmol/mg protein/min. In contrast, homocysteine transport in aortic smooth muscle cells had a lower affinity (Km=212mM) but a higher transport capacity (Vmax=4192 pmol/mg protein/min). Interestingly, increases in pH (pH 6.5–8.2) only inhibited L-homocysteine uptake in endothelial cells. Moreover, L-homocysteine transport in endothelial cells was partially inhibited by lysosomal inhibitors. Our studies indicate that L-homocysteine shares transporter systems with cysteine and can be inhibited for transport by multiple neutral amino acids in vascular cells, and that L-homocysteine transport involves lysosomal transport in endothelial cells. The specific lysosomic feature of L-homocystein transport in endothelial cells may contribute to cell type specific growth inhibitory effects and therefore play a role in homocysteine atherogenic potential.