Mechanistic target of rapamycin (mTOR) regulates trophoblast folate uptake by modulating the cell surface expression of FR-α and the RFC

Abstract

Folate deficiency in fetal life is strongly associated with structural malformations and linked to intrauterine growth restriction. In addition, limited availability of methyl donors, such as folate, during pregnancy may result in abnormal gene methylation patterns and contribute to developmental programming. The fetus is dependent on placental transfer of folate, however the molecular mechanisms regulating placental folate transport are unknown. We used cultured primary human trophoblast cells to test the hypothesis that mechanistic target of rapamycin complex 1 (mTORC1) and 2 (mTORC2) regulate folate transport by post-translational mechanisms. Silencing raptor (inhibits mTORC1) or rictor (inhibits mTORC2) markedly decreased basal folate uptake. Folate uptake stimulated by insulin + IGF-1 was mediated by mTORC2 but did not involve mTORC1. mTORC1 or mTORC2 silencing markedly decreased the plasma membrane expression of FR-α and RFC transporter isoforms without affecting global protein expression. Inhibition of the ubiquitin ligase Nedd4-2 had no effect on folate transport. In conclusion, we report for the first time that mTORC1/C2 are positive regulators of cellular folate uptake by modulating the cell surface abundance of specific transporter isoforms. We propose that regulation of placental folate transport by mTOR signaling provide a direct link between placental function, gene methylation and fetal programming.

Folate transporter expression in placenta from pregnancies complicated with birth defects

“…Localization of folate transporters within placental tissue has been described in first trimester and term placentas (Solanky et al, ). FR‐α, RFC and PCFT are expressed in MVM of term placental tissue (Carter et al, ; Rosario, Powell, & Jansson, ; Solanky et al, ). In line with these studies, we observed expression of folate transporters in MVM from placentas with a gestational age between 25 and 41 weeks.…”

Effects of maternal obesity on placental function and fetal development

“…Additionally, mTOR has been mechanistically linked to the regulation of placental nutrient transport in cultured primary human trophoblast cells. Specifically, mTOR signaling functions as a positive regulator of trophoblast System A and L amino acid transporters (Rosario et al 2016a; Rosario et al 2016c; Rosario et al 2013) and folate transporters (Rosario et al 2016b). In human pregnancy, placental mTOR is activated in pregnancies complicated by maternal obesity (Jansson et al 2013) and inhibited in IUGR (Roos et al 2007; Chen et al 2015).…”

Glucocorticoid regulation of amino acid transport in primary human trophoblast cells

“…often subjected to posttranslational regulation and these proteins mediate transport only when inserted in the plasma membrane (Rosario et al 2013(Rosario et al , 2016b.…”