At diabetes-like concentration, glucose down-regulates the placental serotonin transport system in a cell-cycle-dependent manner

Serotonin-estrogen interactions: What can we learn from pregnancy?

We have reviewed the scientific literature related to four diseases in which to serotonin (5-HT) is involved in the etiology, herein named 5-HT-linked diseases, and whose prevalence is influenced by estrogenic status: depression, migraine, irritable bowel syndrome and eating disorders. These diseases all have in common a sex-dimorphic prevalence, with women more frequently affected than men. The co-occurrence between these 5-HT-linked diseases suggests that they have common physiopathological mechanisms. In most 5-HT-linked diseases (except for anorexia nervosa and irritable bowel syndrome), a decrease in the serotonergic tone is observed and estrogens are thought to contribute to the improvement of symptoms by stimulating the serotonergic system. Human pregnancy is characterized by a unique 5-HT and estrogen synthesis by the placenta. Pregnancy-specific disorders, such as hyperemesis gravidarum, gestational diabetes mellitus and pre-eclampsia, are associated with a hyperserotonergic state and decreased estrogen levels. Fetal programming of 5-HT-linked diseases is a complex phenomenon that involves notably fetal-sex differences, which suggest the implication of sex steroids. From a mechanistic point of view, we hypothesize that estrogens regulate the serotonergic system, resulting in a protective effect against 5-HT-linked diseases, but that, in turn, 5-HT affects estrogen synthesis in an attempt to retrieve homeostasis. These two processes (5-HT and estrogen biosynthesis) are crucial for successful pregnancy outcomes, and thus, a disruption of this 5-HT-estrogen relationship may explain pregnancy-specific pathologies or pregnancy complications associated with 5-HT-linked diseases.

RETRACTED: Placental serotonin systems in pregnancy metabolic complications associated with maternal obesity and gestational diabetes mellitus

This article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/our-business/policies/article-withdrawal). The publication was retracted by request of the authors following an investigation by Monash University performed following its Procedures for Investigating Code Breaches and in accordance with the Australian Code for the Responsible Conduct of Research. The University concluded on the balance of probability that a significant part of the text in the paper was included without knowledge, without consent and without correct attribution of the original author who, at the time, was a student at the University. The results discussed in the review article are still scientifically valid. ☆ This article is part of a Special Issue entitled: Membrane Transporters and Receptors in Pregnancy Metabolic Complications edited by Luis Sobrevia.

Post-translational modifications of serotonin transporter

The serotonin transporter (SERT) is an oligomeric glycoprotein with two sialic acid residues on each of two complex oligosaccharide molecules. Studies using in vivo and in vitro model systems demonstrated that diverse post-translational modifications, including phosphorylation, glycosylation, serotonylation, and disulfide bond formation, all favorably influences SERT conformation and allows the transporter to function most efficiently. This review discusses the post-translational modifications and their importance on the structure, maturation, and serotonin (5-HT) uptake ability of SERT. Finally, we discuss how these modifications are altered in diabetes mellitus and subsequently impairs the 5-HT uptake ability of SERT.

Association with serotonin transporter enables the phosphorylation of insulin receptor in placenta

Upon binding to insulin, the β-subunit of insulin receptor (IR) is phosphorylated and instantly activates intracellular signaling. A defect in this process causes the development of several metabolic disorders including non-insulin-dependent diabetes, such as type 2 and gestational diabetes mellitus (GDM). Under diabetic conditions the phosphorylation of IR in placenta, but not in platelets, is impaired. Interestingly the cellular distribution of the serotonin transporter (SERT), which utilizes the insulin signaling for posttranslational modification, shows tissue-type-dependent variation: SERT function is impaired in GDM-associated placenta, but not in platelets. In order to understand the correlation between IR, SERT and their tissue-type-dependent features, we tested an association between SERT and IR and whether this association affects the phosphorylation of IR. Using various approaches, we demonstrated a physical association between the Carboxyl terminal of SERT and the β-subunit of IR. This association was found on the plasma membrane of the placenta and the platelets. Next, the contribution of the SERT-IR association to the phosphorylation of IR was analyzed in heterologous and endogenous expression systems following insulin-treatment. The in vivo impact of SERT-IR association on the phosphorylation of IR was explored in placenta and platelets of SERT gene knockout (KO) mice. The IR phosphorylation was significantly downregulated only in the placenta, but not in platelets of SERT-KO mice. These findings are supported by time course experiments, which demonstrate that the phosphorylation of IR occurs vis-a-vis IR-SERT association, and at least one of the IR binding domains is identified as the carboxyl-terminus of SERT. These findings suggest an important role for IR-SERT association in maintaining the phosphorylation of IR and regulating the insulin signaling in placenta.

Epigenetic adaptation of the placental serotonin transporter gene (SLC6A4) to gestational diabetes mellitus

We tested the hypothesis that gestational diabetes mellitus (GDM) alters the DNA methylation pattern of the fetal serotonin transporter gene (SLC6A4), and examined the functional relevance of DNA methylation for regulation of the SLC6A4 expression in the human placenta. The study included 50 mother-infant pairs. Eighteen mothers were diagnosed with GDM and 32 had normal glucose tolerance (NGT). All neonates were of normal birth weight and born at term by planned Cesarean section. DNA and RNA were isolated from samples of tissue collected from the fetal side of the placenta immediately after delivery. DNA methylation was quantified at 7 CpG sites within the SLC6A4 distal promoter region using PCR amplification of bisulfite treated DNA and subsequent DNA sequencing. SLC6A4 mRNA levels were measured by reverse transcription—quantitative PCR (RT-qPCR). Functional SLC6A4 polymorphisms (5HTTLPR, STin2, rs25531) were genotyped using standard PCR-based procedures. Average DNA methylation across the 7 analyzed loci was decreased in the GDM as compared to the NGT group (by 27.1%, p = 0.037) and negatively correlated, before and after adjustment for potential confounder/s, with maternal plasma glucose levels at the 24th to 28th week of gestation (p<0.05). Placental SLC6A4 mRNA levels were inversely correlated with average DNA methylation (p = 0.010) while no statistically significant association was found with the SLC6A4 genotypes (p>0.05). The results suggest that DNA methylation of the fetal SLC6A4 gene is sensitive to the maternal metabolic state in pregnancy. They also indicate a predominant role of epigenetic over genetic mechanisms in the regulation of SLC6A4 expression in the human placenta. Longitudinal studies in larger cohorts are needed to verify these results and determine to which degree placental SLC6A4 changes may contribute to long-term outcomes of infants exposed to GDM.

Role of the ubiquitin-proteasome system and autophagy in regulation of insulin sensitivity in serum-starved 3T3-L1 adipocytes

The ubiquitin-proteasome system (UPS) and autophagy are two conserved intracellular proteolytic pathways, responsible for degradation of most cellular proteins in living cells. Currently, both the UPS and autophagy have been suggested to be associated with pathogenesis of insulin resistance and diabetes. However, underlying mechanism remains largely unknown. The purpose of the present study is to investigate the impact of the UPS and autophagy on insulin sensitivity in serum-starved 3T3-L1 adipocytes. Our results show that serum depletion resulted in activation of the UPS and autophagy, accompanied with increased insulin sensitivity. Inhibition of the UPS with bortezomib (BZM), a highly selective, reversible 26S proteasome inhibitor induced compensatory activation of autophagy but did not affect significantly insulin action. Genetic and pharmacological inhibition of autophagy dramatically mitigated serum starvation-elevated insulin sensitivity. In addition, autophagy inhibition compromised UPS function and led to endoplasmic reticulum (ER) stress and unfolded protein response (UPR). Inability of the UPS by BMZ exacerbated autophagy inhibition-induced ER stress and UPR. These results suggest that protein quality control maintained by the UPS and autophagy is required for preserving insulin sensitivity. Importantly, adaptive activation of autophagy plays a critical role in serum starvation-induced insulin sensitization in 3T3-L1 adipocytes.

GDM-associated insulin deficiency hinders the dissociation of SERT from ERp44 and down-regulates placental 5-HT uptake

Serotonin (5-HT) transporter (SERT) regulates the level of 5-HT in placenta. Initially, we found that in gestational diabetes mellitus (GDM), whereas free plasma 5-HT levels were elevated, the 5-HT uptake rates of trophoblast were significantly down-regulated, due to impairment in the translocation of SERT molecules to the cell surface. We sought to determine the factors mediating the down-regulation of SERT in GDM trophoblast. We previously reported that an endoplasmic reticulum chaperone, ERp44, binds to Cys200 and Cys209 residues of SERT to build a disulfide bond. Following this posttranslational modification, before trafficking to the plasma membrane, SERT must be dissociated from ERp44; and this process is facilitated by insulin signaling and reversed by the insulin receptor blocker AGL2263. However, the GDM-associated defect in insulin signaling hampers the dissociation of ERp44 from SERT. Furthermore, whereas ERp44 constitutively occupies Cys200/Cys209 residues, one of the SERT glycosylation sites, Asp208 located between the two Cys residues, cannot undergo proper glycosylation, which plays an important role in the uptake efficiency of SERT. Herein, we show that the decrease in 5-HT uptake rates of GDM trophoblast is the consequence of defective insulin signaling, which entraps SERT with ERp44 and impairs its glycosylation. In this regard, restoring the normal expression of SERT on the trophoblast surface may represent a novel approach to alleviating some GDM-associated complications.

Effects of intermittent high glucose on BeWo choriocarcinoma cells in culture

AIM

The aim of this study was to investigate the cellular effects of intermittent high glucose on the human BeWo placental choriocarcinoma cell line, used as a model of the effects of glucose fluctuation in diabetic pregnancies.

MATERIALS AND METHODS

BeWo cells were subjected to three different glucose conditions for 48 h: 7 mmol/L (control), 42 mmol/L (high glucose), or 7 and 42 mmol/L glucose (intermittent, alternated every 6 h). Cell viability was assessed using cell counts, a cell proliferation assay, and a cell viability assay. Apoptosis was also studied using a terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling assay and by immunocytochemistry of fractin, the N-terminal fragment of actin, which can distinguish apoptotic from necrotic cells. Furthermore, the expression of the well-known survival factors of trophoblast cells, heparin-binding epidermal growth factor-like growth factor and leptin, was evaluated by real-time polymerase chain reaction and Western blot analyses.

RESULTS

Intermittent high-glucose conditions significantly decreased cell viability and enhanced apoptosis compared with control or continuous high-glucose conditions. Furthermore, the expression of heparin-binding epidermal growth factor-like growth factor, but not that of leptin, was significantly increased under intermittent high-glucose conditions compared to its expression under either control or continuous high-glucose conditions.

CONCLUSIONS

These data indicate that intermittent high glucose is more deleterious to BeWo cells than continuous high-glucose conditions. Although further in vitro and in vivo study is necessary, excess fluctuation of glucose levels in the placental circulation might be involved in the impairment of placental development leading to the placental dysfunction.

Expression of placental serotonin transporter and 5-HT 2A receptor in normal and gestational diabetes mellitus pregnancies

Recent studies reveal that serotonin (5-hydroxytryptamine, 5-HT) might play a role in the aetiology of gestational diabetes mellitus (GDM). The involvement of the serotonin transporter (SERT) and 5-HT(2A) receptor (5-HT(2A)R) in diabetes has also been suggested. However, placental SERT and 5-HT(2A)R have never been studied in GDM-complicated pregnancies. The aim of this study was to investigate the effect of GDM on the expression of both placental SERT and 5-HT(2A)R. First, immunohistochemical analysis demonstrated the presence of SERT and 5-HT(2A)R proteins in the villous trophoblast and the fetal capillary endothelium of normal term placental tissue. Protein and mRNA expression of SERT and 5-HT(2A)R in the villous cytotrophoblastic and syncytiotrophoblastic cells was further confirmed in primary culture. A significantly (P < 0.05) decreased expression of SERT mRNA (56.3%) and protein (79.7%), and 5-HT(2A)R mRNA (79.1%) and protein (29.1%) was observed in placental tissues from GDM compared with non-GDM pregnancies. These data suggest that SERT and 5-HT(2A)R might be implicated in the aetiology of GDM. Moreover, the presence of SERT and 5-HT(2A)R in villous trophoblastic cells argues in favour of an important role of serotonin in human placental function.

Drug transporters in the human blood-placental barrier

Studies on the increasing number of transporters found in the placental barrier are gaining momentum, because of their tissue-specific expression, significance in physiology and disease, and the possible utilization of the emerging knowledge in pharmacology. In the placenta, both syncytiotrophoblast and fetal capillary endothelium express transporters. Fetal exposure is determined by the net effect of combination of transporters, their nature and localization in relation to placental cells and their substrate specificity. Although the significance of placental transporters on human fetal drug exposure is almost an unstudied field so far, their potential use to design drugs that do not cross the placenta is already being pursued. It is thus of interest to review the existing knowledge of human placental transporters. Transporters in all groups which take part in drug transport are found in human placenta. Especially, ATP-binding cassette transporters ABCG2/breast cancer resistance protein, ABCB1/P-glycoprotein and ABCC2/MRP2 are all expressed at the apical surface of syncytiotrophoblast facing maternal blood and are putatively important protective proteins both for placental tissue and the fetus, because they are efflux transporters and their substrates include many drugs and also environmental chemicals. Such protective effect has been shown in animals, but these results cannot be directly extrapolated to humans due to interspecies differences in placental structure and function. Experimental models utilizing human placental tissue, especially human placental perfusion, offer valuable possibilities, which have been insufficiently studied so far.

The effect of high glucose on SERT, the human plasmalemmal serotonin transporter

The aim of this work was to investigate the effect of short- and long-term high-glucose exposure on the plasmalemmal serotonin transporter (SERT)-mediated uptake of [(3)H]-serotonin (5-HT) by Caco-2 cells. Short-term exposure of Caco-2 cells to high apical glucose levels (30 mM for 2 h or 40 mM for 1 h) decreased the uptake of [(3)H]-5-HT by 20-30%. On the other hand, long-term (21-24 weeks) exposure of the cells to high (25 mM) glucose caused a 30% increase in the uptake of [(3)H]5-HT. Under these conditions, the affinity of the transporter for 5-HT and noradrenaline was not significantly changed, and the inhibitory potencies of fluoxetine and desipramine were also unchanged. In conclusion, high-glucose levels modulate SERT activity. A short-term exposure of the cells to a high concentration of glucose decreases the activity of the transporter, whereas a longer exposure of the cells to a high concentration of glucose increases the activity of SERT, without interfering with its affinity.