**1. Introduction**

Diabetes in pregnancy is considered a high-risk condition for maternal and neonatal morbidity and remains a significant medical challenge. Women with diabetes have a higher risk of preeclampsia, caesarean section, and fetal outcomes including congenital anomalies, stillbirth, and macrosomia. Moreover, women with diabetes at the time of conception have more adverse outcomes (i.e., birth defects, perinatal mortality, and morbidity) than women who develop gestational diabetes during pregnancy [1]. The adverse outcomes associated with diabetes in pregnancy are substantially associated with hyperglycemia. However, the molecular mechanism underlying how maternal hyperglycemia leads to abnormal fetal growth remains unclear.

Placental transfer of amino acids via amino acid transporters is essential for optimal fetal growth and development. Decrease in amino acid transport by the placenta is implicated as a potential cause of fetal growth restriction. There are a variety of such transporters, selective for various classes of amino acids. Na+-dependent system A amino acid transporter is specific for neutral amino acids with short side chains such as glycine and alanine. System L amino acid transporter is Na+-independent exchanger specific for large neutral amino acids. Studies have shown that placental system A activity is lower in intrauterine growth restriction (IUGR) compared with normal pregnancies [2,3]. However, alterations in the placental amino acid transporters in pregnancies complicated by diabetes are largely unknown.

The mammalian target of rapamycin (mTOR) signaling pathway has newly been suggested to be a nutrient sensor in the placenta. mTOR is a ubiquitously expressed serine/threonine protein kinase that exists in two complexes, mTOR complexes 1 (mTORC1) and 2 (mTORC2). Placental nutrient transporter expression is mainly regulated by mTORC1. We previously reported that mTORC1 might regulate placental glucose transport by altering the glucose transporter isoform-3 expression in placental trophoblast [4]. In addition, evidence has shown that mTORC1 could regulate placental amino acid transport by modulating the cell surface abundance of system A and system L transporter isoforms without affecting global protein expression in primary human trophoblast cells [5]. Alterations of placental mTORC1 activity has been found as being associated with adverse pregnancy outcomes. For example, studies have shown that placental mTORC1 activity was decreased in human IUGR [6] or animal model of IUGR [7], suggesting placental mTORC1 plays an important role in the development of abnormal fetal growth. Although dysregulation of placental mTOR has been well documented in IUGR pregnancies, it has rarely been studied in pregnancy complicated with diabetes.

It is speculated that mTORC1 may be activated in diabetes in association with increased placental amino acid transport [8]. Thus, we aimed to investigate the alteration of mTORC1 pathway and system L amino acid transporters in diabetic placentas and to study the impacts of high glucose on mTORC1 activity and system L amino acid transporter expression in placental trophoblast cells.

### **2. Results**
