Expression and role of SNAT3 in the placenta

The SLC38A5/SNAT5 amino acid transporter: from pathophysiology to pro-cancer roles in the tumor microenvironment

SLC38A5/SNAT5 is a system N transporter that can mediate net inward or outward transmembrane fluxes of neutral amino acids coupled with Na+ (symport) and H+ (antiport). Its preferential substrates are not only amino acids with side chains containing amide (glutamine and asparagine) or imidazole (histidine) groups, but also serine, glycine, and alanine are transported by the carrier. Expressed in the pancreas, intestinal tract, brain, liver, bone marrow, and placenta, it is regulated at mRNA and protein levels by mTORC1 and WNT/β-catenin pathways, and it is sensitive to pH, nutritional stress, inflammation, and hypoxia. SNAT5 expression has been found to be altered in pathological conditions such as chronic inflammatory diseases, gestational complications, chronic metabolic acidosis, and malnutrition. Growing experimental evidence shows that SNAT5 is overexpressed in several types of cancer cells. Moreover, recently published results indicate that SNAT5 expression in stromal cells can support the metabolic exchanges occurring in the tumor microenvironment of asparagine-auxotroph tumors. We review the functional role of the SNAT5 transporter in pathophysiology and propose that, due to its peculiar operational and regulatory features, SNAT5 may play important pro-cancer roles when expressed either in neoplastic or in stromal cells of glutamine-auxotroph tumors.NEW & NOTEWORTHY The transporter SLC38A5/SNAT5 provides net influx or efflux of glutamine, asparagine, and serine. These amino acids are of particular metabolic relevance in several conditions. Changes in transporter expression or activity have been described in selected types of human cancers, where SNAT5 can mediate amino acid exchanges between tumor and stromal cells, thus providing a potential therapeutic target. This is the first review that recapitulates the characteristics and roles of the transporter in physiology and pathology.

Contributions of system A subtypes to α-methylaminoisobutyric acid uptake by placental microvillous membranes of human and rat

System A consists of three subtypes, sodium-coupled neutral amino acid transporter 1 (SNAT1), SNAT2, and SNAT4, which are all expressed in the placenta. The aim of this study was to evaluate the contributions of each of the three subtypes to total system A-mediated uptake in placental MVM of human and rat, using betaine and L-arginine as subtype-selective inhibitors of SNAT2 and SNAT4, respectively. Appropriate concentrations of betaine and L-arginine for subtype-selective inhibition in SNAT-overexpressing cells were identified. It was found that 10 mM betaine specifically and almost completely inhibited human and rat SNAT2-mediated [¹⁴C]α-methylaminoisobutyric acid ([¹⁴C]MeAIB) uptake, while 5 mM L-arginine specifically and completely inhibited [³H]glycine uptake via human SNAT4, as well as [¹⁴C]MeAIB uptake via rat SNAT4. In both human and rat placental MVM vesicles, sodium-dependent uptake of [¹⁴C]MeAIB was almost completely inhibited by 20 mM unlabeled MeAIB. L-Arginine (5 mM) partly inhibited the uptake in humans, but hardly affected that in rats. Betaine (10 mM) partly inhibited the uptake in rats, but hardly affected it in humans. These results suggest that SNAT1 is most likely the major contributor to system A-mediated MeAIB uptake by human and rat MVM vesicles and that the remaining uptake is mainly mediated by SNAT4 in humans and SNAT2 in rats. Thus, inhibition studies using betaine and L-arginine are useful to characterize the molecular mechanisms of system A-mediated transport.

The SLC38 family of sodium-amino acid co-transporters

Transporters of the SLC38 family are found in all cell types of the body. They mediate Na(+)-dependent net uptake and efflux of small neutral amino acids. As a result they are particularly expressed in cells that grow actively, or in cells that carry out significant amino acid metabolism, such as liver, kidney and brain. SLC38 transporters occur in membranes that face intercellular space or blood vessels, but do not occur in the apical membrane of absorptive epithelia. In the placenta, they play a significant role in the transfer of amino acids to the foetus. Members of the SLC38 family are highly regulated in response to amino acid depletion, hypertonicity and hormonal stimuli. SLC38 transporters play an important role in amino acid signalling and have been proposed to act as transceptors independent of their transport function. The structure of SLC38 transporters is characterised by the 5 + 5 inverted repeat fold, which is observed in a wide variety of transport proteins.

Relevance of syndecan-1 in the trophoblastic BeWo cell syncytialization

PROBLEM

To investigate the role of syndecan-1 in the differentiation of the BeWo cells into syncytiotrophoblast.

METHOD OF STUDY

BeWo cells were stimulated with forskolin to form syncytia, and the expression of syndecan-1, desmoplakin I+II, human chorionic gonadotrophin (hCG) and angiogenesis-associated factors was analyzed. Syndecan-1 was silenced by siRNA to evaluate its involvement in the forskolin-mediated syncytia formation.

RESULTS

Treatment of the BeWo cells with forskolin led to a significant increase in the syncytia formation. It was associated with an increase in the expression of syndecan-1 with a concomitant decrease in the expression of desmoplakin I+II. Forskolin treatment of the BeWo cells also led to an increase in the secretion of soluble endoglin, whereas no change was observed in the soluble fms-like tyrosine kinase-1. Silencing of the syndecan-1 expression in BeWo cells led to a significant decrease in cell fusion both in the presence and in the absence of forskolin. It was associated with a significant decrease in hCG level in the conditioned medium.

CONCLUSION

Syndecan-1 is up-regulated in BeWo cells during differentiation and its silencing inhibits syncytialization and thus could be a useful biomarker for syncytiotrophoblast formation.