METTL3 regulates glucose transporter expression in placenta exposed to hyperglycemia through the mTOR signaling pathway.

BACKGROUND: Alterations in the placental expression of glucose transporters (GLUTs), the crucial maternal-fetal nutrient transporters, have been found in women with hyperglycemia in pregnancy (HIP). However, there is still uncertainty about the underlying effect of the high-glucose environment on placental GLUTs expression in HIP.

METHODS: We quantitatively evaluated the activity of mammalian target of rapamycin (mTOR) and expression of GLUTs (GLUT1, GLUT3, and GLUT4) in the placenta of women with normal pregnancies (CTRL, n = 12) and pregnant women complicated with poorly controlled type 2 diabetes mellitus (T2DM, n = 12) by immunohistochemistry. In addition, BeWo cells were treated with different glucose concentrations to verify the regulation of hyperglycemia. Then, changes in the expression of GLUTs following the activation or suppression of the mTOR pathway were also assessed using MHY1485/rapamycin (RAPA) treatment or small interfering RNA (siRNA)-mediated silencing approaches. Moreover, we further explored the alteration and potential upstream regulatory role of methyltransferase-like 3 (METTL3) when exposed to hyperglycemia.

RESULTS: mTOR, phosphorylated mTOR (p-mTOR), and GLUT1 protein levels were upregulated in the placenta of women with T2DM compared with those CTRL. In BeWo cells, mTOR activity increased with increasing glucose concentration, and the expression of GLUT1, GLUT3, and GLUT4 as well as GLUT1 cell membrane translocation were upregulated by hyperglycemia to varying degrees. Both the drug-mediated and genetic depletion of mTOR signaling in BeWo cells suppressed GLUTs expression, whereas MHY1485-induced mTOR activation upregulated GLUTs expression. Additionally, high glucose levels upregulated METTL3 expression and nuclear translocation, and decreasing METTL3 levels suppressed GLUTs expression and mTOR activity and vice versa. Furthermore, in METTL3 knockdown BeWo cells, the inhibitory effect on GLUTs expression was eliminated by activating the mTOR signaling pathway using MHY1485.

CONCLUSION: High-glucose environment-induced upregulation of METTL3 in trophoblasts regulates the expression of GLUTs through mTOR signaling, contributing to disordered nutrient transport in women with HIP.