Placental amino acid transfer : regulation of amino acid transport system A in the BeWo cell line
In this thesis regulation of amino acid transport system A in the placenta is investigated. Firstly, a suitable in vitro placental model is identified from two clones of the BeWo choriocarcinoma cell line. Both the ATCC and b30 clones show the phenotypic and genotypic characteristics of the parent cell line. The b30 clone forms a polarised monolayer when cultured on permeable filters suitable for studying transcellular transport. The b30 clone was chosen as the model to be used for all further studies in this thesis. The regulation of amino acid transport system A by substrate restriction was demonstrated over 30 years ago in the placenta. However, studies were limited to MeAIB uptake and did not include transcellular or molecular studies. This thesis demonstrates that transcellular 4C-MeAIB transport is increased following amino acid restriction and that this increase is linked to the relocalisation of existing system A SNAT2 transporters to the plasma membrane and increased SNAT2 expression following chronic restriction. In contrast, SNAT1 does not appear to be involved in this response and is down regulated by non-essential amino acid deprivation. Amino acid transport system A is highly regulated by many factors and it was suggested that increased system A activity may be a general nutrient stress response. This thesis tested this hypothesis by studying the effects of micronutrient deprivation on transcellular 4C-MeAIB transfer. Both iron and copper deficiency caused significant increases in transcellular transfer that could be reversed upon micronutrient replenishment. Finally, the effects of endocrine stress on amino acid transport system A were investigated.