Characteristics of L-lactic acid transport in basal membrane vesicles of human placental syncytiotrophoblast

Disposition of glycolic acid into the embryo following oral administration of ethylene glycol during placentation in the rat and rabbit

In order to evaluate the role of the placenta in the etiology of ethylene glycol (EG) developmental toxicity, the distribution of EG and its main metabolites, glycolic acid (GA) and oxalic acid (OX), into the conceptus was determined at the beginning and completion of placentation in the rat and rabbit. Two groups (n = 28) of timed-pregnant Wistar rats were administered EG (1000 mg/kg bw/day, oral gavage) from gestation day (GD) 6 to either GD 11 or GD 16; similarly, two groups (n = 28) of timed-pregnant New Zealand White rabbits were administered EG from GD 6 to either GD 10 or GD 19. Four animals from each group were sacrificed at 1, 3, 6, 9, 12, 18, or 24 h after the final administration, and maternal blood, extraembryonic fluid, and embryonic tissue were removed for analysis of EG, GA, and OX. The three analytes were predominantly cleared from all compartments in both species within 24 h. Neither EG nor OX preferentially accumulated into the conceptus compartments, compared with the maternal blood, in either species. Critically, GA was preferentially accumulated from the maternal blood only into the rat embryo at GD 11, but not at GD 16 and not into the rabbit embryo at either GD 10 or GD 19. The accumulation of GA into the rat embryo, and its decline over the course of placentation, is discussed in relation to the expression of monocarboxylate transporter isoforms across the syncytiotrophoblast.

Mitochondrial ROS Accumulation Contributes to Maternal Hypertension and Impaired Remodeling of Spiral Artery but Not IUGR in a Rat PE Model Caused by Maternal Glucocorticoid Exposure

Increased maternal glucocorticoid levels have been implicated as a risk factor for preeclampsia (PE) development. We found that pregnant rats exposed to dexamethasone (DEX) showed hallmarks of PE features, impaired spiral artery (SA) remodeling, and elevated circulatory levels of sFlt1, sEng IL-1β, and TNFα. Abnormal mitochondrial morphology and mitochondrial dysfunction in placentas occurred in DEX rats. Omics showed that a large spectrum of placental signaling pathways, including oxidative phosphorylation (OXPHOS), energy metabolism, inflammation, and insulin-like growth factor (IGF) system were affected in DEX rats. MitoTEMPO, a mitochondria-targeted antioxidant, alleviated maternal hypertension and renal damage, and improved SA remodeling, uteroplacental blood flow, and the placental vasculature network. It reversed several pathways, including OXPHOS and glutathione pathways. Moreover, DEX-induced impaired functions of human extravillous trophoblasts were associated with excess ROS caused by mitochondrial dysfunction. However, scavenging excess ROS did not improve intrauterine growth retardation (IUGR), and elevated circulatory sFlt1, sEng, IL-1β, and TNFα levels in DEX rats. Our data indicate that excess mitochondrial ROS contributes to trophoblast dysfunction, impaired SA remodeling, reduced uteroplacental blood flow, and maternal hypertension in the DEX-induced PE model, while increased sFlt1 and sEng levels and IUGR might be associated with inflammation and an impaired energy metabolism and IGF system.

Maternal obesity-associated disruption of polarized lactate transporter MCT4 expression in human placenta

Maternal obesity is associated with an increased risk of adverse pregnancy outcomes including stillbirth, and their etiology is thought to be related to placental and fetal hypoxia. In this study, we sought to investigate the levels of lactate in maternal and umbilical cord blood, a well characterized biomarker for hypoxia, and expression of plasma membrane lactate transporter MCT1 and MCT4 in the placental syncytiotrophoblast (STB), which are responsible for lactate uptake and extrusion, respectively, from pregnant women with a diagnosis of obesity following a Cesarean delivery at term. With use of approaches including immunofluorescence staining, Western blot, RT-qPCR and ELISA, our results revealed that in controls the expression of MCT1 was equally observed between basal (fetal-facing, BM) and microvillous (maternal-facing, MVM) membrane of the STB, whereas MCT4 was predominantly expressed in the MVM but barely detected in the BM. However, obese patients demonstrated significant decreased MCT4 abundance in the MVM coupled with concurrent elevated expression in the BM. We also found a linear trend toward decreasing MCT4 expression ratio of MVM to BM with increasing maternal pre-pregnancy BMI. Furthermore, our data showed that the lactate ratios of fetal cord arterial to maternal blood were remarkably reduced in obese samples compared to their normal counterparts. Collectively, these results suggest that the loss of polarization of lactate transporter MCT4 expression in placental STB leading to disruption of unidirectional lactate transport from the fetal to the maternal compartment may constitute part of mechanisms linking maternal obesity and pathogenesis of stillbirth.

Lactic Acid Transport Mediated by Aquaporin-9: Implications on the Pathophysiology of Preeclampsia

Aquaporin-9 (AQP9) expression is significantly increased in preeclamptic placentas. Since feto-maternal water transfer is not altered in preeclampsia, the main role of AQP9 in human placenta is unclear. Given that AQP9 is also a metabolite channel, we aimed to evaluate the participation of AQP9 in lactate transfer across the human placenta. Explants from normal term placentas were cultured in low glucose medium with or without L-lactic acid and in the presence and absence of AQP9 blockers (0.3 mM HgCl₂ or 0.5 mM Phloretin). Cell viability was assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay and lactate dehydrogenase release. Apoptotic indexes were analyzed by Bax/Bcl-2 ratio and Terminal Deoxynucleotidyltransferase-Mediated dUTP Nick-End Labeling assay. Heavy/large and light/small mitochondrial subpopulations were obtained by differential centrifugation, and AQP9 expression was detected by Western blot. We found that apoptosis was induced when placental explants were cultured in low glucose medium while the addition of L-lactic acid prevented cell death. In this condition, AQP9 blocking increased the apoptotic indexes. We also confirmed the presence of two mitochondrial subpopulations which exhibit different morphologic and metabolic states. Western blot revealed AQP9 expression only in the heavy/large mitochondrial subpopulation. This is the first report that shows that AQP9 is expressed in the heavy/large mitochondrial subpopulation of trophoblasts. Thus, AQP9 may mediate not only the lactic acid entrance into the cytosol but also into the mitochondria. Consequently, its lack of functionality in preeclamptic placentas may impair lactic acid utilization by the placenta, adversely affecting the survival of the trophoblast cells and enhancing the systemic endothelial dysfunction.

Revisiting preeclampsia: a metabolic disorder of the placenta

Preeclampsia (PE) is a leading cause of maternal and neonatal mortality and morbidity worldwide, impacting the long-term health of both mother and offspring. PE has long been characterized by deficient trophoblast invasion into the uterus and consequent placental hypoperfusion, yet the upstream causative factors and effective interventional targets for PE remain unknown. Alterations in the metabolism of preeclamptic placentas are thought to result from placental ischemia, while disturbances of the metabolism and of metabolites in PE pathogenesis are largely ignored. In fact, as one of the largest fetal organs at birth, the placenta consumes a considerable amount of glucose and fatty acid. Increasing evidence suggests glucose and fatty acid exist as energy substrates and regulate placental development through bioactive derivates. Moreover, recent findings have revealed that the placental metabolism adapts readily to environmental changes, altering its response to nutrients and endocrine signals; this adaptability optimizes pregnancy outcomes by diversifying available carbon sources for energy production, hormone synthesis, angiogenesis, immune activation, and tolerance, and fetoplacental growth. These observations raise the possibility that carbohydrate and lipid metabolism abnormalities play a role in both the etiology and clinical progression of PE, sparking a renewed interest in the interrelationship between PE and metabolic dysregulation. This review will focus on key metabolic substrates and regulatory molecules in the placenta and aim to provide novel insights with respect to the metabolism's role in modulating placental development and functions. Further investigations from this perspective are poised to decipher the etiology of PE and suggest potential therapies.

Lactic Acid: A Novel Signaling Molecule in Early Pregnancy?

Aerobic glycolysis is a recognized feature shared by tumors, leading to the accumulation of lactic acid in their local microenvironments. Like the tumors, the blastocysts, placenta, trophoblasts and decidual immune cells can also produce a large amount of lactic acid through aerobic glycolysis during the early pregnancy. Moreover, the placenta expresses the transporters of the lactic acid. While several studies have described the role of lactic acid in the tumor microenvironment, especially lactic acid's modulation of immune cells, the role of lactic acid produced during pregnancy is still unclear. In this paper, we reviewed the scientific evidence detailing the effects of lactic acid in the tumor microenvironment. Based on the influence of the lactic acid on immune cells and tumors, we proposed that lactic acid released in the unique uterine environment could have similar effects on the trophoblast cells and immune cells during the early pregnancy.

Complex, coordinated and highly regulated changes in placental signaling and nutrient transport capacity in IUGR

The most common cause of intrauterine growth restriction (IUGR) in the developed world is placental insufficiency, a concept often used synonymously with reduced utero-placental and umbilical blood flows. However, placental insufficiency and IUGR are associated with complex, coordinated and highly regulated changes in placental signaling and nutrient transport including inhibition of insulin and mTOR signaling and down-regulation of specific amino acid transporters, Na⁺/K⁺-ATPase, the Na^+/H⁺-exchanger, folate and lactate transporters. In contrast, placental glucose transport capacity is unaltered and Ca²⁺-ATPase activity and the expression of proteins involved in placental lipid transport are increased in IUGR. These findings are not entirely consistent with the traditional view that the placenta is dysfunctional in IUGR, but rather suggest that the placenta adapts to reduce fetal growth in response to an inability of the mother to allocate resources to the fetus. This new model has implications for the understanding of the mechanisms underpinning IUGR and for the development of intervention strategies.

Placental Lactate Transporter Activity and Expression in Intrauterine Growth Restriction

OBJECTIVES

To compare lactate uptake in the microvillous plasma membrane (maternal facing [MVM]) in term and preterm placentas in intrauterine growth restriction (IUGR) and appropriate weight for gestational age (AGA) controls, and in the basal plasma membrane (fetal facing [BM]) at term. In addition, we examine the expression of monocarboxylate transporters (MCT1 and MCT4).

METHODS

We measured [14C] L-lactate uptakes into vesicles prepared from MVM and BM, stimulated by an inwardly directed H+ gradient. MCT expression was examined by Western blotting.

RESULTS

In term placentas, mean (+/- SE) [14C] L-lactate uptake into MVM vesicles of the IUGR (n = 6) and AGA (n = 11) groups at initial rate was similar (15.4 +/- 2.3 versus 15.0 +/- 1.1 pmol/mg protein/20 s). In preterm placentas, in IUGR (n = 3) and AGA (n = 3) groups, [14C] l-lactate uptake into MVM was also not significantly different. In BM vesicles from term placentas, [14C] L-lactate uptake was significantly lower in IUGR (n = 5) than in AGA (n = 6) controls (3.6 +/- 0.4 versus 5.6 +/- 0.6 pmol/mg protein/20 s, P <.05). MCT1 and MCT4 were expressed in BM vesicles, but there was no difference in expression between the IUGR and AGA groups.

CONCLUSIONS

These findings suggest that in IUGR placental lactate transport capacity in the BM is reduced, which may adversely affect placental lactate clearance.

Lactacidemia in intrauterine growth restricted (IUGR) pregnancies: relationship to clinical severity, oxygenation and placental weight

The aim of the study was to evaluate the impact of clinical severity and placental weight upon fetal lactacidemia in intrauterine growth restricted (IUGR) pregnancies. Seventy pregnancies complicated by IUGR were compared with 70 normal (appropriate for gestational age, AGA) pregnancies at the time of elective cesarean section. IUGR pregnancies were divided according to clinical severity in three groups: Group 1 had normal fetal heart rate (FHR) and normal pulsatility index of the umbilical artery (PI); Group 2 had normal FHR and abnormal PI; and Group 3 had abnormal FHR and PI. No cases with severe lactacidemia had placental weights >or=250 g. Forty-four fetuses had placental weight <250 g: Twenty-four had severe lactacidemia and all were Group 2 and 3, whereas 20 had normal umbilical artery lac (lactate) (and small placentas) regardless of the clinical severity. Gestational age, fetal and placental weights, F/P ratios, uv (umbilical vein) and ua lac and ua oxygen content and pH were significantly decreased in fetuses with small placentas and high lactate. There was a significant relationship between fetal and placental weight in AGA and IUGR. However, IUGR fetuses with placental weight >or=250 g exhibited an F/P ratio significantly lower than that in AGA fetuses suggesting that IUGR may be due to a reduction of placental function per gram of tissue.

Functional analysis of organic cation transporter 3 expressed in human placenta

The aim of this study is to investigate the placental transport mechanism of cationic compounds by comparison of the uptake of an organic cation into human placental basal membrane vesicles (BLMVs) with that into organic cation transporter 3 (OCT3)-expressing cells. Reverse transcription-polymerase chain reaction analysis demonstrated that OCT3 is the only OCT isoform expressed in the human placenta. The function of OCT3 was investigated by measuring the uptake of 1-methyl-4-phenylpyridinium (MPP(+)) into human embryonic kidney (HEK)293 cells stably expressing OCT3 (HEK/OCT3 cells). The OCT3-mediated uptake of MPP(+) was sodium- and chloride-independent and saturable, with a Michaelis constant (K(m)) of 82.5 microM. The OCT3-mediated uptake was inhibited by various cationic drugs in a concentration-dependent manner but not by anionic compounds, such as p-aminohippuric acid and captopril, or a zwitterion, carnitine. Western blotting analysis of membrane vesicles prepared from human term placenta revealed that OCT3 is expressed only in BLMVs but not in microvillous membrane vesicles. The uptake of MPP(+) into BLMVs was membrane potential-dependent and saturable, with a K(m) value of 51.8 muM, which is similar to that in HEK293/OCT3 cells. The inhibitory spectrum of various compounds on MPP(+) uptake by BLMVs was also similar to that in HEK293/OCT3 cells. These results suggest that OCT3 is expressed on the basal membrane of human trophoblast cells and plays an important role in the placental transport of cationic compounds.

Activity and expression of Na+/H+ exchanger isoforms in the syncytiotrophoblast of the human placenta

The purpose of this study was to compare Na+/H+ exchanger (NHE) activity in the microvillous (MVM) and basal (BM) plasma membrane of the human placental syncytiotrophoblast and to determine the relative contribution of various NHE isoforms to this activity. Uptake of 22Na into isolated MVM vesicles in the presence of a H+ gradient, at initial rate, was four- to fivefold higher than that by BM vesicles (214+/-28 vs. 49+/-9 pmol/mg protein per 30 s, respectively, means+/-SEM, n=8, 6, P<0.001). The 22Na uptake by MVM, but not by BM, was reduced in the absence of a H+ gradient and in the presence of 500 microM amiloride. To determine the contribution of NHE1, NHE2 and NHE3 isoforms to NHE activity in MVM, we investigated the effect of amiloride analogues which show isoform selectivity. HOE 694, an analogue selective for NHE1 at low concentrations, inhibited 22Na uptake with an EC50 of 0.13+/-0.05 microM (n=6), whereas S3226, an analogue selective for NHE3 at low concentrations had an EC50 of 3.01+/-0.85 microM (n=5). To investigate this further, we measured recovery of syncytiotrophoblast intracellular pH (pHi) from an acid load using a H+-selective, fluorescent dye (BCECF) loaded into isolated intact placental fragments. This recovery was blocked in the absence of Na+ and the presence of amiloride (500 microM) and concentrations of HOE 694 and S3226 were comparable to those used in vesicle experiments. Overall these data show that under the conditions used NHE activity in the term placental syncytiotrophoblast is absent from BM. NHE activity in the MVM is attributable predominantly to NHE1.

Polarized lactate transporter activity and expression in the syncytiotrophoblast of the term human placenta

We investigated the polarization of l-lactate transport in human syncytiotrophoblast by measuring uptake of [(14)C] l-lactate by both microvillous (maternal-facing; MVM) and basal (fetal-facing; BM) plasma membranes. [(14)C] l-lactate uptake by MVM and BM was stimulated in the presence of an inwardly directed H(+)gradient, with a significantly higher uptake in MVM than in BM at initial rate (15.4+/-2.3 vs 5.6+/-0.6 pmol/mg protein/20 sec). Stereospecific inhibition was observed in MVM, with a higher affinity for l-lactate compared with d-lactate. In BM, there was no difference in the inhibition by these two stereoisomers. Inhibition of lactate uptake in both MVM and BM by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), an inhibitor of monocarboxylate transporter (MCT) activity, indicated MCT-mediated mechanisms across both membranes. Kinetic modelling supported a two-transporter model as the best fit for both MVM and BM, the K(m)of the major component being 6.21 mm and 25.01 mm in MVM and BM respectively. Western blotting and immunolocalization examining the distribution of MCT1 and MCT4, showed that MCT expression was polarized, MCT1 being predominantly localized to BM and MCT4 showing greater abundance on MVM. CD147, a chaperone protein for MCT1 and MCT4, was equally expressed by both membranes. These studies demonstrate that the opposing plasma membranes of human syncytiotrophoblast are polarized with respect to both MCT activity and expression.

l-Carnitine transport in human placental brush-border membranes is mediated by the sodium-dependent organic cation transporter OCTN2

Maternofetal transport of l-carnitine, a molecule that shuttles long-chain fatty acids to the mitochondria for oxidation, is thought to be important in preparing the fetus for its lipid-rich postnatal milk diet. Using brush-border membrane (BBM) vesicles from human term placentas, we showed that l-carnitine uptake was sodium and temperature dependent, showed high affinity for carnitine (apparent Km= 11.09 ± 1.32 μM; Vmax= 41.75 ± 0.94 pmol·mg protein−1·min−1), and was unchanged over the pH range from 5.5 to 8.5. l-Carnitine uptake was inhibited in BBM vesicles by valproate, verapamil, tetraethylammonium, and pyrilamine and by structural analogs of l-carnitine, including d-carnitine, acetyl-d,l-carnitine, and propionyl-, butyryl-, octanoyl-, isovaleryl-, and palmitoyl-l-carnitine. Western blot analysis revealed that OCTN2, a high-affinity, Na+-dependent carnitine transporter, was present in placental BBM but not in isolated basal plasma membrane vesicles. The reported properties of OCTN2 resemble those observed for l-carnitine uptake in placental BBM vesicles, suggesting that OCTN2 may mediate most maternofetal carnitine transport in humans.

Pyruvate shuttle in muscle cells: high-affinity pyruvate transport sites insensitive to trans-lactate efflux

The specificity of the transport mechanisms for pyruvate and lactate and their sensitivity to inhibitors were studied in L6 skeletal muscle cells. Trans- and cis-lactate effects on pyruvate transport kinetic parameters were examined. Pyruvate and lactate were transported by a multisite carrier system, i.e., by two families of sites, one with low affinity and high capacity (type I sites) and the other with high affinity and low capacity (type II). The multisite character of transport kinetics was not modified by either hydroxycinnamic acid (CIN) or p-chloromercuribenzylsulfonic acid (PCMBS), which exert different types of inhibition. The transport efficiency (TE) ratios of maximal velocity to the trans-activation dissociation constant (Kt) showed that lactate and pyruvate were preferentially transported by types I and II sites, respectively. The cis-lactate effect was observed with high Ki values for both sites. The trans-lactate effect on pyruvate transport occurred only on type I sites and exhibited an asymmetric interaction pattern (Kt of inward lactate > Kt of outward lactate). The inability of lactate to trans-stimulate type II sites suggests that intracellular lactate cannot recruit these sites. The high-affinity type II sites act as a specific pyruvate shuttle and constitute an essential relay for the intracellular lactate shuttle.

Basal Membrane Localization of MRP1 in Human Placental Trophoblast

The placental trophoblast is considered to act as a barrier between mother and fetus, mediating the exchange of various materials across the placenta. ATP-binding cassette (ABC) transporters such as P-glycoprotein (P-gp) and multidrug-resistance protein (MRP) are expressed in the placenta and function as efflux transport systems for xenobiotics. In the present study, we aimed to determine the localization of MRP1 in the human placenta in comparison with that of P-gp. Western blotting analysis with human placental membrane vesicles indicated that P-gp and MRP1 are localized on the brush-border membranes and basal membranes, respectively. Immunohistochemical analysis with human normal full-term placenta showed that anti-P-gp monoclonal antibody F4 stained the brush-border side of the trophoblast cells, whereas anti-MRP1 monoclonal antibody MRPr1 stained the basal side. These results confirm that P-gp and MRP1 are located on the brush-border membranes and basal membranes, respectively, of human full-term placental trophoblast. MRP1 was also detected on the abluminal side of blood vessels in the villi. Accordingly, MRP1 may play a role distinct from that of P-gp, which is considered to restrict the influx of xenobiotics into the fetus.