Glucocorticoid regulation of amino acid transport in primary human trophoblast cells

Abstract

Excess maternal glucocorticoids reduce placental amino acid transport and fetal growth, but whether these effects are mediated directly on the syncytiotrophoblast remains unknown. We hypothesised that glucocorticoids inhibit mechanistic target of rapamycin (mTOR) signaling and insulin-stimulated System A amino acid transport activity in primary human trophoblast (PHT) cells. Syncytialised PHTs, isolated from term placentas (n = 15), were treated with either cortisol (1 μM) or dexamethasone (1 μM), ± insulin (1 nM) for 24 h. Compared to vehicle, dexamethasone increased mRNA expression, but not protein abundance of the mTOR suppressor, regulated in development and DNA damage response 1 (REDD1). Dexamethasone enhanced insulin receptor abundance, activated mTOR complex 1 and 2 signaling and stimulated System A activity, measured by Na+-dependent 14C-methylaminoisobutyric acid uptake. Cortisol also activated mTORC1 without significantly altering insulin receptor or mTORC2 read-outs or System A activity. Both glucocorticoids downregulated expression of the glucocorticoid receptor and the System A transporter genes SLC38A1, SLC38A2 and SLC38A4, without altering SNAT1 or SNAT4 protein abundance. Neither cortisol nor dexamethasone affected System L amino acid transport. Insulin further enhanced mTOR and System A activity, irrespective of glucocorticoid treatment and despite downregulating its own receptor. Contrary to our hypothesis, glucocorticoids do not inhibit mTOR signaling or cause insulin resistance in cultured PHT cells. We speculate that glucocorticoids stimulate System A activity in PHT cells by activating mTOR signaling, which regulates amino acid transporters post-translationally. We conclude that downregulation of placental nutrient transport in vivo following excess maternal glucocorticoids is not mediated by a direct effect on the placenta.

Dexamethasone acutely suppresses the anabolic SNAT2/SLC38A2 amino acid transporter protein in L6‐G8C5 rat skeletal muscle cells

“…Even though a marked decrease in System A activity was observed in L6‐G8C5 cells (Figure 1A), this was not seen either in wild‐type or in SNAT2‐eGFP transfected HEK293A cells (Figure 6A). These observations, and reports elsewhere of the divergent effects of glucocorticoid on System A activity in nonmuscle cells, notably the stimulation reported in response to DEX in placenta 56,57 indicate that, in addition to driving SNAT2 proteolysis, glucocorticoid may also exert additional (possibly stimulatory) effects on the active System A transporter pool in the plasma membrane. The mechanism of this additional effect is unknown.…”

miR-373-3p Regulates the Proliferative and Migratory Properties of Human HTR8 Cells via SLC38A1 Modulation

“…In the current study, we report three main findings: first, smaller sIUGR fetuses had lower placental SLC38A1 expression, contrasting with the upregulation 9 Disease Markers seen with miR-373-3p; second, introducing the miR-373-3p mimic or inhibitor to trophoblast cells led to SLC38A1 downregulation or upregulation, respectively; and third, we identified an interaction between miR-373-3p and the 3′-UTR of SLC38A1, experimentally verifying the regulation of the latter by the former. Other studies have also recognized the important contribution of SLC38A1 as an amino acid transporter in placental and fetal development [42][43][44][45][46]. Similarly, research has already described the relationship of SLC38A1 with miR-150-5p, miR-593-3p, and miR-4317 in specific pathological conditions [47][48][49].…”

Metabolic Consequences of Glucocorticoid Exposure before Birth

“…In pregnant mice fed ad libitum, the decrease in transplacental MeAIB clearance seen in response to corticosterone administration is associated with upregulation of the mTOR inhibitor, DNA damage responses 1 (REDD1) and with reductions in the downstream readouts of mTOR activity [ 59 ]. Cortisol also upregulates REDD1 at the mRNA level in human trophoblast cells in vitro [ 117 ]. Therefore, maternal glucocorticoids may suppress placental amino acid transport in vivo by reducing amino acid transporter abundance and/or translocation to the trophoblast plasma membrane, via mTOR.…”