Proceedings of The Physiological Society
University of Cambridge (2008) Proc Physiol Soc 11, C27
Homocysteine inhibition of system L amino acid transport activity in human placenta
E. Tsitsiou1, C. P. Sibley1, S. W. D’Souza1, J. D. Glazier1
1. Maternal and Fetal Health Research Group, University of Manchester, Manchester, United Kingdom.
BACKGROUND: Elevated plasma levels of homocysteine (Hcy) are associated with vascular-related complications of pregnancy and adverse neonatal outcomes. Fetal plasma Hcy concentration at term is positively correlated with maternal Hcy concentration, suggesting placental transport of Hcy may be an important determinant of fetal plasma Hcy. The mechanisms involved in placental Hcy transport are uncharacterised. We tested the hypothesis that the heterodimeric amino acid transporter system L, comprising CD98 heavy chain linked to either LAT1 or LAT2 light chain, provides one mechanism for placental Hcy transport by measuring the ability of Hcy to inhibit system L activity. System L transports neutral amino acids in a Na+-independent manner, across the microvillous (MVM) and basal plasma membranes of human placenta. Both LAT1 and LAT2 light chains are expressed in the human placenta, with previous studies demonstrating that LAT1 is localised to the MVM, while LAT2 localisation is less clear with functional studies supporting both MVM and BM distribution. Here we focus on MVM, the first plasma membrane barrier to maternofetal transport. MATERIALS AND METHODS: MVM were isolated from placentas of normal pregnancies at term (n=6). System L activity was measured as Na+-independent 35S-methionine (0.2μM) uptake into MVM vesicles at 30sec (initial rate) in the absence (control) or presence of varying concentrations of L-Hcy, DL-Hcy or system L model substrates (L-methionine, L-Leucine, 2-amino-2-norbornane-carboxylic acid (BCH)). System A specific substrate methylaminoisobutyric acid (MeAIB) and system y+L substrate L-arginine were included as negative controls. Gene expression for the components of system L, CD98, LAT1 and LAT2 was examined by real-time quantitative PCR (n=6). RESULTS: Both L-Hcy and DL-Hcy caused a dose-dependent inhibition of 35S-methionine uptake into MVM in a comparable manner to system L substrates suggesting similar affinity of system L for Hcy, whereas as predicted, this was not observed for MeAIB and arginine. In all six placentas examined, gene expression for CD98, LAT1 and LAT2 was confirmed. CONCLUSION: Inhibition of methionine uptake by L-Hcy and DL-Hcy in a Na+-independent manner implies that Hcy can serve as a substrate for system L. Gene expression for LAT1 and LAT2 in human placenta allows for the possibility that either light chain links to CD98 to form a functional complex and mediate Hcy transport across MVM. We speculate that inhibition of placental amino acid uptake by Hcy could impact on fetal growth and development.
Where applicable, experiments conform with Society ethical requirements