Effect of (1-34) parathyroid hormone-related peptide on the composition and turnover of phospholipids in syncytiotrophoblast brush border and basal plasma membranes of human placenta

STC1 and PTHrP Modify Carbohydrate and Lipid Metabolism in Liver of a Teleost Fish

Stanniocalcin 1 (STC1) and parathyroid hormone-related protein (PTHrP) are calciotropic hormones in vertebrates. Here, a recently hypothesized metabolic role for these hormones is tested on European sea bass treated with: (i) teleost PTHrP(1-34), (ii) PTHrP(1-34) and anti-STC1 serum (pro-PTHrP groups), (iii) a PTHrP antagonist PTHrP(7-34) or (iv) PTHrP(7-34) and STC1 (pro-STC1 groups). Livers were analysed using untargeted metabolic profiling based on proton nuclear magnetic resonance (¹H-NMR) spectroscopy. Concentrations of branched-chain amino acid (BCAA), alanine, glutamine and glutamate increased in pro-STC1 groups suggesting their mobilization from the muscle to the liver for degradation and gluconeogenesis from alanine and glutamine. In addition, only STC1 treatment decreased the concentrations of succinate, fumarate and acetate, indicating slowing of the citric acid cycle. In the pro-PTHrP groups the concentrations of glucose, erythritol and lactate decreased, indicative of gluconeogenesis from lactate. Taurine, trimethylamine, trimethylamine N-oxide and carnitine changed in opposite directions in the pro-STC1 versus the pro-PTHrP groups, suggesting opposite effects, with STC1 stimulating lipogenesis and PTHrP activating lipolysis/β-oxidation of fatty acids. These findings suggest a role for STC1 and PTHrP related to strategic energy mechanisms that involve the production of glucose and safeguard of liver glycogen reserves for stressful situations.

Bone development and mineral homeostasis in the fetus and neonate: roles of the calciotropic and phosphotropic hormones

Mineral and bone metabolism are regulated differently in utero compared with the adult. The fetal kidneys, intestines, and skeleton are not dominant sources of mineral supply for the fetus. Instead, the placenta meets the fetal need for mineral by actively transporting calcium, phosphorus, and magnesium from the maternal circulation. These minerals are maintained in the fetal circulation at higher concentrations than in the mother and normal adult, and such high levels appear necessary for the developing skeleton to accrete a normal amount of mineral by term. Parathyroid hormone (PTH) and calcitriol circulate at low concentrations in the fetal circulation. Fetal bone development and the regulation of serum minerals are critically dependent on PTH and PTH-related protein, but not vitamin D/calcitriol, fibroblast growth factor-23, calcitonin, or the sex steroids. After birth, the serum calcium falls and phosphorus rises before gradually reaching adult values over the subsequent 24-48 h. The intestines are the main source of mineral for the neonate, while the kidneys reabsorb mineral, and bone turnover contributes mineral to the circulation. This switch in the regulation of mineral homeostasis is triggered by loss of the placenta and a postnatal fall in serum calcium, and is followed in sequence by a rise in PTH and then an increase in calcitriol. Intestinal calcium absorption is initially a passive process facilitated by lactose, but later becomes active and calcitriol-dependent. However, calcitriol's role can be bypassed by increasing the calcium content of the diet, or by parenteral administration of calcium.

A wave of IP3 production accompanies the fertilization Ca2+ wave in the egg of the frog, Xenopus laevis: theoretical and experimental support

The fertilization Ca2+ wave in Xenopus laevis is a single, large wave of elevated free Ca2+ that is initiated at the point of sperm-egg fusion and traverses the entire width of the egg. This Ca2+ wave involves an increase in inositol-1,4,5-trisphosphate (IP3) resulting from the interaction of the sperm and egg, which then results in the activation of the endoplasmic reticulum Ca2+ release machinery. The extraordinarily large size of this cell (1.2 mm diameter) together with the small surface region of sperm-receptor activation makes special demands on the IP3-dependent Ca2+ mobilizing machinery. We propose a detailed model of the fertilization Ca2+ wave in Xenopus eggs that requires an accompanying wave of IP3 production. While the Ca2+ wave is initiated by a localized increase of IP3 near the site of sperm-egg fusion, the Ca2+ wave propagates via IP3 production correlated with the Ca2+ wave-possibly via Ca(2+)-mediated PLC activation. Such a Ca(2+)-mediated IP(3) production wave has not been required previously to explain the fertilization Ca2+ wave in eggs; we argue this is necessary to explain the observed IP3 dynamics in Xenopus eggs. To test our hypothesis, we have measured the IP3 levels from 20 nl "sips" of the egg cortex during wave propagation. We were unable to detect the low IP3 levels in unfertilized eggs, but after fertilization, [IP3] ranged from 175 to 430 nM at the sperm entry point and from 120 to 700 nM 90 degrees away once the Ca2+ wave passed that region about 2 min after fertilization. Prior to the Ca2+ wave reaching that region the IP3 levels were undetectable. Since significant IP3 could not diffuse to this region from the sperm entry point within 2 min, this observation is consistent with a regenerative wave of IP3 production.

Phospholipase C axis is the preferential pathway leading to PKC activation following PTH or PTHrP stimulation in human term placenta

Parathyroid-related peptide (PTHrP) is abundant in human syncytiotrophoblast where it was suggested to play an important role in maternal-fetal calcium homeostasis. On the other hand, parathyroid hormone (PTH), another hypercalcemic factor, would be implicated in the maintenance of the mother's calcium balance. In many cells, these hormones are associated to G-coupled receptors and activate protein kinase (PKC). Thus, the first aim of this study was to determine the cellular pathway (phospholipase; PLC and phosphatidyl-inositol-3 kinase; PI3K) leading to the activation of PKC following a PTH or PTHrP stimulation in brush border (BBM) and basal plasma membranes (BPM) of human term placenta. Both peptides were shown to be potent modulators of the PKC activity in these membranes with optimal concentrations of 10(-8)M and 10(-9)M for hPTH and hPTHrP, respectively. Furthermore, the use of bisindolylmaleimide (BIM), a non-selective PKC inhibitor, serves to demonstrate the specificity of the PKC-dependent MARCKS-psd phosphorylation. While LY-294002, a PI3K inhibitor failed to counteract the hPTH- and hPTHrP-induced PKC stimulation in BBM and BPM, U-73122, a PLC inhibitor, totally abolished the PKC stimulation by hPTH and hPTHrP. Taken together, these data suggest that the activation of PKC by hPTH or hPTHrP, in BBM and BPM, is preferentially associated to the PLC pathway rather than the PI3K's.

Corticotropin-releasing hormone binding to the syncytiotrophoblast membranes

The human placenta secretes large amounts of corticotropin-releasing hormone (CRH) which was thought to exert a paracrine action in the placenta. We have recently characterized high-affinity binding sites for CRH in the human placenta. However, our studies utilized whole placental membranes, which did not identify the site of binding of CRH in the plasma membrane. In this study we investigated the characteristics of CRH binding to purified mother-facing, brush border membranes (BBM) and fetus-facing, basal plasma membranes (BPM) of the syncytiotrophoblast. The two membranes were separated by a series of differential and density-gradient centrifugations. The purity of the membranes was determined by measuring alkaline phosphatase, as a marker of BBM and Na+/K+ATPase as a marker of BPM. Each membrane showed specific and high-affinity binding. Scatchard analysis revealed a high-affinity binding site for CRH with Kd of 1.0 +/- 0.15 and 1.3 +/- 0.176 for BBM and BPM, respectively. The maximal number of binding sites was significantly different (P < 0.01) in the two plasma membranes: Bmax of 79 +/- 6.4 fmol/mg protein for BBM and 23 +/- 3.9 fmol/mg protein for BPM. Both the mother-facing and fetus-facing membranes of the syncytiotrophoblast contain binding proteins for CRH, with significantly more binding sites on the mother-facing membranes. The functional consequences of CRH binding could be different for the two polar membranes due to differential localization of second messenger systems between the two membrane types. It is proposed that partial purification of BBM and BPM provides a better system to study CRH action in the placenta, than whole placental membrane preparations.

Implication of ATP and sodium in arachidonic acid incorporation by placental syncytiotrophoblast brush border and basal plasma membranes in the human

The human placental syncytiotrophoblast is the main site of exchange of nutrients and minerals between the mother and her fetus. In order to characterize the placental transport of some fatty acids, we studied the incorporation of arachidonic acid, a fetal primordial fatty acid, in purified bipolar syncytiotrophoblast brush border (BBM) and basal plasma membranes (BPM) from human placenta. The basal arachidonic acid incorporation in BBM and BPM was time dependent and reached maximal values of 0.75+/-0.10 and 0.48+/-0.18 pmol/mg protein, respectively, after 2.5 min. The presence of adenosine triphosphate (ATP) (3 m m) increases significantly the maximal incorporation of arachidonic acid by sixfold (4.75+/-0.35 pmol/mg) and ninefold (4.40+/-0.84 pmol/mg) in BBM and BPM, respectively. Moreover, an increase in the arachidonic acid incorporation was also obtained in the presence of sodium where the values achieved 7.68+/-0.98 (10x) and 6.53 pmol/mg (13.6x) for BBM and BPM, respectively. We also showed that the combination of both Na(+)and ATP increases significantly the maximal incorporation of arachidonic acid in BPM to 7.89+/-0.15 pmol/mg protein, while in BBM it did not modify its incorporation (8.18+/-0.25 pmol/mg protein), as compared to the presence of sodium alone. Our results demonstrate that arachidonic acid is incorporated by both placental syncytiotrophoblast membranes, and is ATP and sodium-linked. However, different mechanisms seem to be involved in this fatty acid incorporation through BBM and BPM, since the presence of Na(+)or ATP increased it, while the association of these two elements increased it only in BPM. We also demonstrated by osmolarity experiments that both membranes bind arachidonic acid, potentially involving one or more fatty acids binding proteins.

Calcitonin gene-related peptide receptor in human placental syncytiotrophoblast brush-border and basal plasma membranes

Minerals, such as calcium and potassium, are essential for fetal development, but their transplacental transport, and in particular, the effect of hormones on this process has not been extensively studied. Human alpha-calcitonin gene-related peptide (h alpha CGRP), a hormone constituted of 37 amino acids, is obtained by the alternative splicing of the mRNA from the calcitonin gene, and could be implicated in placental ion transport. In order to study the presence of this receptor, brush-border and basal plasma membranes were purified, and membrane binding studies were conducted using [125I]h alpha CGRP. The initiation of binding of [125I]h alpha CGRP to both membranes was rapid and reached maximal value after 10 min of incubation at 37 degrees C. Scratchard analysis revealed single-affinity binding sites for h alpha CGRP with Kd equal to 4412.45 +/- 604.81 pM and 2673.24 +/- 552.51 pM for brush-border and basal plasma membranes, respectively, which were significantly different. Moreover, the maximal number of receptors was significantly different (P < 0.001) in both membranes, with Bmax of 627.94 +/- 31.40 fmol/mg protein for brush-border membranes and 343.70 +/- 43.52 fmol/mg protein in basal-plasma membranes. Competitive displacement of [125I]h alpha CGRP with other ligands showed the following potencies; h alpha CGRP approximately h beta CGRP approximately Cys (acm)2,7 CGRP > CGRP (8-37), but no competition was observed with human and salmon calcitonin. Half-maximal displacement for human alpha- and beta CGRP was reached at approximately 10(-10)M for brush-border and basal-plasma membranes. alpha- and beta CGRP, and their fragment and analogue, stimulated cyclic AMP production in placental homogenate ranging from 143-163 per cent. Thus, our results show the presence of CGRP-specific receptors in both the syncytiotrophoblast membranes of human placenta. The role(s) of this related peptide in placenta remains to be investigated.

Asymmetrical distribution of G proteins in syncytiotrophoblastic brush-border and basal-plasma membranes of human term placenta

In human placental syncytiotrophoblast brush-border (BBM, facing the mother) and basal-plasma membranes (BPM, facing to fetus) we have recently demonstrated the presence of calcaemic hormone-specific receptors for parathyroid hormone and calcitonin, which could be implicated in calcium transport from the mother to the fetus. It is well recognized that signal transducing G proteins (guanosinc nucleotide-binding proteins) can associate with various transmembrane receptors and effector proteins, and regulate a variety of second-messenger systems and ion channels. In this present paper, we investigated the presence of a variety of alpha and beta subunits of G proteins in both syncytiotrophoblast, BBM and BPM by Western blot technique. For the first time, we were able to demonstrate the presence of G proteins in the bipolar syncytiotrophoblast membranes, which were evaluated by immunoblotting using affinity purified antiserum raised against the alpha subunits of Gi1, Gi1/i2, Gi3, G0, Gq, Gs, G7 and against the beta subunits. In BBM, we identified the alpha subunits of Gi1, Gi3, G0, Gq, Gs (42, 46 kDa), Gz and beta subunits. The same alpha subunits of G proteins were found in BPM, although alpha subunits of Gi1, Gq, Gs (46 kDa) were located predominantly in the BBM, and the alpha subunit of G0 was found preferentially in BPM. Moreover, in BBM and BPM, a purified antisera raised against the alpha subunits of Gi1 and Gs, detected a 105 kDa protein and a 67 kDa protein, respectively. Interestingly, the 67 kDa protein was preferentially located in BBM, and none of these proteins were detectable in membranes prepared from brain (control). The asymmetrical distribution of the alpha subunits of G proteins among the two different placental bipolar membranes might reflect the very specialized function of these syncytiotrophoblast membranes in ions and nutrients transport from the mother to the fetus.

Linoleic acid transport by human placental syncytiotrophoblast membranes

The placenta syncytiotrophoblast is the site of exchange of nutrients, lipids and minerals between the mother and the fetus. In order to characterize the transport of fatty acids by the placenta, we purified bipolar syncytiotrophoblast brush border and basal plasma membranes from human placenta. These purified brush border and basal plasma membranes enriched 3-fold and 22-fold, respectively, in sodium/potassium-ATPase and 27-fold and 6-fold in alkaline phosphatase activity, compared with the placental homogenates. Fatty acid transport was performed at different fatty acid/albumin ratios to evaluate the optimal uptake conditions. The maximal transport efficiency, for linoleic acid bound to albumin by sonication, was obtained with a 6:1 fatty acid/albumin ratio in brush border and basal plasma membranes. The linoleic acid transport observed with brush border membranes followed Michaelis-Menten kinetics, with a Michaelis constant of 7.89 +/- 0.01 microM and a maximal incorporation rate of 30.80 +/- 6.39 pmol.mg-1.min-1. Linoleic acid transport was very low in basal plasma membranes and we obtained a Michaelis constant of 0.95 +/- 0.01 microM and a maximal incorporation rate of 1.62 +/- 5.06 pmol.mg-1.min-1. In order to show that linoleic acid accumulated within brush border and plasma membrane vesicles, and to eliminate the possibility of a non-specific binding of fatty acid to these membranes, we demonstrated by an osmolarity experiment, the decrease of the linoleic acid transport in brush border and basal plasma membranes obtained in the presence of 455 microM essential fatty acid at 23 degrees C for 180 min. The results presented in this study suggest that linoleic acid is transported significantly by syncytiotrophoblast brush border membranes and basal plasma membranes. Thus, it may represent a unidirectional transport from mother to fetus through the brush border membranes facing the mother, followed by transport at a slower rate through basal plasma membranes facing the fetus.

Calcitonin receptor in human placental syncytiotrophoblast brush border and basal plasma membranes

The physiology of calcium transport through the placenta has not been studied thoroughly. In particular, the effect of calcaemic hormones on this process has never been reported. In this paper we questioned if calcitonin, a hypocalcaemic hormone, is also implicated in the regulation of calcium transport by one of the placental syncytiotrophoblast bipolar membranes. In order to investigate the implication of calcitonin on calcium transport, we first studied whether this hormone binds to any of these bipolar membranes, i.e. purified syncytiotrophoblast brush border (facing the mother) and basal plasma membranes (facing the fetus). The initiation of binding of human [125I]calcitonin to the two types of membranes was rapid and reached a steady state after 10 min of incubation at 37 degrees C. The number of binding sites and the affinity of these receptors for the hormone were studied for each type of membrane, with concentrations of [125I]calcitonin varying from 0.01 to 1.8 nM. Scatchard analysis revealed a single affinity binding site for human calcitonin with Kds of 0.83 +/- 0.09 nM and 0.67 +/- 0.26 nM for brush border and basal plasma membranes respectively. The maximal number of receptors was significantly different (p < 0.001) in the two membranes: Bmax of 66.64 +/- 9.15 fmol/mg protein for brush border membranes and 19.66 +/- 2.80 fmol/mg protein for basal plasma membranes. Competitive displacement of [125I]calcitonin with other ligands showed the following potencies between human calcitonin > salmon calcitonin > calcitonin gene-related peptides and segments analogues but no competition with some human calcitonin gene-related peptides fragments.(ABSTRACT TRUNCATED AT 250 WORDS)