IGF regulation of neutral amino acid transport in the BeWo choriocarcinoma cell line (b30 clone): evidence for MAP kinase-dependent and MAP kinase-independent mechanisms

OBJECTIVE

IGF-1 and IGF-1 receptors are major determinants of fetal growth and are expressed primarily on the maternal-facing surface of the syncytiotrophoblast cell membrane in the human placenta. IGF-1 regulates fetal growth, in part, by regulating amino acid transport across the placenta. The objective of these studies was to study the role of IGF-1 and its signaling pathway in regulating neutral amino acid transport in a human trophoblast cell culture model.

DESIGN

The regulation of neutral amino acid transport by IGF-1 was studied in cultured BeWo(b30) choriocarcinoma cells using the non-metabolizing amino acid analog, [(3)H]-alpha-aminoisobutyric acid (AIB). Transport in the absence of Na was used to distinguish system L from total AIB transport. Similarly, Na-dependent transport in the presence of excess methyl-AIB (MeAIB) permitted discrimination of systems A (MeAIB-sensitive) and ASC (MeAIB-insensitive). Specific inhibitors of intracellular signaling pathways were then used to determine the signaling pathway utilized by IGFs to regulate each amino acid transport system. Specificity of inhibition was assessed using specific markers of p70 S6 kinase activity and MAP kinase activation.

RESULTS

Maximal stimulating concentrations of IGF-I (100 ng/ml) stimulated AIB transport by 30-40% exclusively through system A. Wortmannin (100 nM), an inhibitor of PI-3-kinase activity, inhibited all IGF-I-stimulated transport. Rapamycin (100 ng/ml), an inhibitor of p70 S6 kinase, and bisindolylmaleimide, an inhibitor of protein kinase C (PKC), had no effect. PD-098059 (50 miccroM), an inhibitor of MAP kinase activation, inhibited 20-30% of basal AIB transport but did not inhibit IGF-I-stimulated transport under the conditions studied. IGF-1 did not increase steady state mRNA levels of the system A transporters, SNAT1 and SNAT2, suggesting IGF-1 stimulates transport via post-transcriptional mechanisms.

CONCLUSIONS

These data demonstrate that IGF-I stimulates neutral amino acid transport system A by a PI3-kinase dependent, post-transcriptional pathway in the BeWo(b30) cell line. Additionally, system A activity appear to be sensitive to MAP kinase-dependent pathways not regulated by IGFs.

Insulin and insulin-like growth factors in human development: implications for the perinatal period

The physiologic and cellular mechanisms regulating fetal growth cannot be adequately described by regulatory mechanisms important postnatally. This review summarizes recent advances in clinical medicine, cell and molecular biology, and physiology showing the central and essential roles of insulin and the insulin-like growth factor family of peptides in regulating fetal growth. Moreover, the importance of insulin-like growth factors in tissue-specific growth regulation during critical periods of development suggest that these mechanisms may also be relevant to the pathogenesis of tissue injury in the preterm infant, and may offer therapeutic strategies aimed at reducing morbidity associated with prematurity. Illustrations of how the insulin-like growth factor axis may represent potential therapeutic targets for specific clinical problems facing the newborn are briefly discussed.

IGF binding protein-1 (IGFBP-1) is preferentially associated with the fetal-facing basal surface of the syncytiotrophoblast in the human placenta

IGF receptors are expressed in a spatially polarized manner on the syncytiotrophoblast cell membrane. We therefore examined the hypothesis that IGFBPs expressed at the maternal-fetal interface interact with distinct surfaces of the syncytiotrophoblast membrane to modulate IGF function. Membrane vesicles were prepared specifically from the maternal-facing, microvillous membrane (MVM) and the fetal-facing, basal membrane (BM) surfaces of the syncytiotrophoblast. The association of IGFBPs with each membrane preparation was determined by ligand blot analysis. A doublet migrating at 38/42 kD was detected in both MVM and BM preparations. Selective immunoprecipitation followed by ligand blot analysis identified this IGF binding species as IGFBP-3. Additionally, a protein migrating at approximately 29 kD was associated primarily with the BM. This protein was identified as IGFBP-1 by both immunoprecipitation and ligandblotting techniques. Non-denaturing PAGE revealed five distinct bands corresponding to different degrees of phosphorylation. The phosphorylation pattern of BM-associated IGFBP-1 was identical to that of native IGFBP-1 in amniotic fluid. Immunohistological analysis of term placenta revealed IGFBP-1-specific staining of the syncytiotrophoblast and the fetal capillary/pericapillary bed. The localization of IGFBP-1 to a distinct compartment within the fetal placenta, not in proximity to the syncytiotrophoblast type I IGF receptor, suggests it may play a role in regulating/targeting IGF activity within the stromal compartment or by exerting IGF-independent effects on the basal surface of the syncytiotrophoblast. The nature of its binding to the BM has not been determined.

Soluble HLA-G in human placentas: synthesis in trophoblasts and interferon-gamma-activated macrophages but not placental fibroblasts

The HLA class Ib antigen, HLA-G, is highly expressed in early gestation placentas where it is believed to modulate maternal-fetal immunological interactions. In this study, soluble isoforms (sHLA-G) encoded by intron 4-retaining transcripts were identified in first trimester placentas by immunohistochemistry using a mAb specific for the C-terminus of sHLA-G. Immunoreactive sHLA-G protein was localized to trophoblast cells and to villous mesenchymal cells with the morphological features of macrophages. Reverse transcriptase polymerase chain reaction analysis which used primers specific for intron 4 and the 3' untranslated region of the HLA-G gene showed that transcripts encoding sHLA-G were present in the trophoblast-derived Jeg-3 cells as well as interferon-gamma-activated myelomonocytic U937 cells but were absent and uninducible in placental fibroblasts. These results indicate that placental sHLA-G is synthesized in trophoblast cells and activated placental macrophages and support the postulate that placenta-derived sHLA-G modulates maternal and fetal immune cell functions during pregnancy.

Spatial polarization of insulin-like growth factor receptors on the human syncytiotrophoblast

The expression of IGF receptors on the maternal-facing, microvillous membrane (MVM) surface and the fetal-facing, basal membrane (BM) surface of the syncytiotrophoblast was studied using standard ligand binding assays, covalent cross-linking techniques, and immunoblot analysis. Scatchard analysis of [125I]IGF-I and -II binding revealed the presence of both high and low affinity binding sites associated with each membrane preparation that did not clearly distinguish between the two membrane preparations. Cross-linking analysis, however, demonstrated type I and type II IGF receptors associated primarily with MVM, suggesting that nonreceptor binding sites may contribute to total membrane binding. Ligand blot analysis revealed that BM are uniquely associated with 29- and 24-kD IGF binding proteins (IGFBPs). [125I]QAYL-IGF-I, having reduced affinity for IGFBPs, was therefore used to study receptor-specific binding. Approximately 5-fold more type I IGF receptors were shown to be associated with MVM than BM by Scatchard and cross-linking analyses. This was confirmed by immunoblot analysis. By contrast, immunoblot analysis revealed approximately 50-100% more type II IGF receptor protein associated with BM, whereas cross-linking to [125I]IGF-II revealed a MVM predominance. In the presence of 5 mM mannose 6-phosphate, however, a substantial increase in [125I]IGF-II cross-linked to the type II IGF receptor was observed in BM but not MVM consistent with immunoblot analysis. These data demonstrate that type 1 and unoccupied type II IGF receptors are expressed primarily on the maternal-facing. MVM surface of the syncytiotrophoblast.

Overexpression of insulin-like growth factor-II induces accelerated myoblast differentiation

Previous studies have shown that exogenous insulin-like growth factors (IGFs) can stimulate the terminal differentiation of skeletal myoblasts in culture and have established a correlation between the rate and the extent of IGF-II secretion by muscle cell lines and the rate of biochemical and morphological differentiation. To investigate the hypothesis that autocrine secretion of IGF-II plays a critical role in stimulating spontaneous myogenic differentiation in vitro, we have established C2 muscle cell lines that stably express a mouse IGF-II cDNA under control of the strong, constitutively active Moloney sarcoma virus promoter, enabling us to study directly the effects of IGF-II overproduction. Similar to observations with other muscle cell lines, IGF-II overexpressing myoblasts proliferated normally in growth medium containing 20% fetal serum, but they underwent enhanced differentiation compared with controls when incubated in low-serum differentiation medium. Accelerated differentiation of IGF-II overexpressing C2 cells was preceded by the rapid induction of myogenin mRNA and protein expression (within 1 h, compared with 24-48 h in controls) and was accompanied by an enhanced proportion of the retinoblastoma protein in an underphosphrylated and potentially active form, by a marked increase in activity of the muscle-specific enzyme, creatine phosphokinase, by extensive myotube formation by 48 h, and by elevated secretion of IGF binding protein-5 when compared with controls. These results confirm a role for IGF-II as an autocrine/paracrine differentiation factor for skeletal myoblasts, and they define a model cell system that will be useful in determining the biochemical mechanisms of IGF action in cellular differentiation.

Developmentally regulated expression of IGF binding protein-3 (IGFBP-3) in human placental fibroblasts: effect of exogenous IGFBP-3 on IGF-1 action

Preterm, human, placental fibroblasts exhibit growth rates, in vitro, that vary with gestational age. The observed increase in proliferation rate is associated with enhanced mitogenic responsiveness to IGF-I. IGFBPs can either potentiate or inhibit IGF action at the cellular level. The production of IGFBPs by placental fibroblasts was studied as potential modulators of their responsiveness to IGFs. Human placental fibroblasts were obtained at various gestational ages and maintained in culture. IGFBP-3 protein and mRNA expression were assessed by Northern and ligand blot analyses. First, media conditioned by fibroblasts, in culture, were subjected to ligand blot analysis. Multiple species of IGFBPs were present in each cell line tested. IGFBP-3, migrating as a doublet at approx. 38/42 kDa, was the predominant IGFBP species present. Other IGFBPs of 22-35 kDa were also present. The secretion of IGFBP-3 exhibited a marked decrease at 10-15 weeks gestation relative to 8-9 week fibroblasts but began to increase again by 19 weeks. We next studied the expression of IGFBP-3 mRNA. Total cellular RNA was obtained from rapidly growing cells and subjected to Northern analysis. Placental fibroblasts exhibited decreased steady state levels of IGFBP-3 mRNA at 10-15 weeks gestation consistent with its decreased protein expression. The ability of IGFBP-3 to influence IGF-1 stimulated DNA synthesis was studied in 10 week placental fibroblasts as measured by [3H]thymidine incorporation. IGFBP-3 inhibited IGF-1 (3.3 nM) stimulated DNA synthesis in a dose-dependent manner when added simultaneously with IGF-1 or preincubated with the cells for 48 h prior to the addition of IGF-1. By contrast, maximum effective concentrations of IGFBP-3 (52 nM) potentiated the effect of IGF-1 50-200% when preincubated with bovine fibroblasts for 48 h prior to the addition of IGF-1. These data suggest that IGFBP-3 production is developmentally regulated in human placental fibroblasts and inhibits their mitogenic response to IGF-1. The regulated expression of IGFBP-3 may contribute to the altered growth rate and IGF responsiveness exhibited by placental fibroblasts, in vitro.

Human placental endothelin: expression of endothelin-1 mRNA by human placental fibroblasts in culture

Endothelin-1 (ET-1) mRNA is expressed by the human placenta in a developmentally regulated manner and has been shown to stimulate the growth of placental mesenchymal cells. The ability of placental fibroblasts to express preproET-1 mRNA was studied to determine if ET-1 could potentially participate via autocrine mechanisms in the proliferation of placental fibroblasts. Fibroblasts were isolated from normal placentae at various gestational ages (7-19 weeks and term) and their abilities to express preproET-1 mRNA in culture evaluated by Northern analysis. Sparse, rapidly growing cultures of placental fibroblasts expressed preproET-1 mRNA at each gestational age in the presence of 10% FBS. The regulation of preproET-1 expression in placental fibroblasts was studied by exposing cells to known mitogenic stimuli. Quiescent, confluent monolayers of placental fibroblasts expressed no detectable levels of preproET-1 mRNA under basal conditions. Epidermal growth factor (EGF, 10 mg/ml), transforming growth factor-beta 1 (TGF-beta 1, 5 ng/ml), or interleukin 1 beta (IL-1 beta) alone, had no significant effect on steady state preproET-1 mRNA levels. Cycloheximide, an inhibitor of protein synthesis, increased the steady state levels of preproET-1 mRNA at a concentration of 10 micrograms/ml. In the presence cycloheximide, IL-1 beta markedly stimulated preproET-1 mRNA expression, whereas EGF was less effective. TGF-beta 1 had no effect in the presence or absence of cycloheximide. In contrast, 12-O-tetradecanoylphorbol 13-acetate (TPA, 20 nM) exerted a small stimulatory effect on preproET-1 mRNA expression which was not influenced by cycloheximide.(ABSTRACT TRUNCATED AT 250 WORDS)

A potential role for endothelin-1 in human placental growth: interactions with the insulin-like growth factor family of peptides

Endothelin-1 (Et-1) stimulated DNA synthesis in placental fibroblasts in a dose-dependent manner, as measured by [3H]thymidine incorporation (ED50, 0.2-0.3 ng/mL). Insulin-like growth factor-I (IGF-I) interacted synergistically with Et-1 to potentiate the stimulation of DNA synthesis. Additionally, Et-1 stimulated the turnover of phosphoinositides in a time- and concentration-dependent manner (ED50, 1 ng/mL), as measured by a 2- to 3-fold increase in the total accumulation of [3H]inositol phosphates. This was accompanied by a 2- to 3-fold rise in intracellular calcium, as measured by fura-2 fluorescence. IGF-I, however, had no potentiating effect on Et-1-stimulated phosphoinositide turnover or increase in cytosolic Ca2+. The ability of Et-1 to stimulate the production of IGF-II and IGFBPs by placental fibroblasts was then studied. Western ligand blot analysis using an [125I]IGF-II probe revealed the presence of six major binding proteins corresponding to 42, 38, 35, 32, 31, and 24 kilodaltons. Et-1 (50 ng/mL) stimulated all binding fractions concordantly. This was accompanied by a similar increase in immunoreactive IGF-II secretion, as assessed by a specific RIA. No increase in immunoreactive IGF-I was observed. The ability of the placenta to produce Et-1 was examined by Northern blot analysis. Placentae at 14 and 17 weeks gestation expressed small amounts Et-1 mRNA, whereas significantly higher levels of mRNA were expressed at term. These data suggest that the human placenta produces Et-1 in a developmentally regulated manner that may act via paracrine and/or autocrine mechanisms to regulate placental growth.

In vitro growth rate of placental fibroblasts is developmentally regulated

Placental cells of mesenchymal origin were used to study the regulation of fetal growth at the cellular level. A significant difference in the in vitro growth rates of placental fibroblasts was observed as a function of gestational age. Cells derived from 10-19-wk placentae exhibited proliferative rates two to three times greater than cells derived from 7-9-wk placentae (16-30 h vs. 30-60 h, P less than 0.001). The proliferation rate remained stable throughout multiple passages in culture. Additionally, these two groups of cell strains exhibited marked differences in their responsiveness to mitogenic stimuli. Using maximal effective concentrations, insulin-like growth factor I interacted synergistically with epidermal growth factor and fibroblast growth factor to stimulate DNA synthesis in cells derived from 10-19-wk placentae. By contrast, the interaction of insulin-like growth factor 1 with epidermal growth factor and fibroblast growth factor exhibited significantly less synergy in 7-9-wk cells. These findings argue that the accelerated growth rate of human fetal cells results primarily from developmental events intrinsic to the cells and is associated with enhanced responsiveness to the mitogenic action of peptide growth factors.

Evidence for carrier-mediated transport of glucocorticoids by human placental membrane vesicles

Glucocorticoid uptake by isolated placental membrane vesicles has been studied in an attempt to identify a membrane-mediated carrier mechanism. A preliminary communication from this laboratory has reported that uptake of the glucocorticoid corticosterone by these vesicles was a time-dependent, saturable, osmotically sensitive process (Fant, M.E., Harbison, R.D. and Harrison, R.W. (1979) J. Biol. Chem. 254, 6218-6221), but did not conclusively demonstrate a carrier mechanism. Further studies of labeled corticosterone uptake by placental vesicles are described herein which indicate that steroid uptake by these vesicles is a carrier-mediated process. We found that corticosterone uptake was temperature-sensitive, and an apparent phase-transition effect on the rate of uptake was seen to occur at approximately 16 degrees C. Treatment of the vesicles with phospholipase A2 and the sulfhydryl group attacker, p-chloromercuriphenylsulfonate, inhibited corticosterone uptake. In contrast to our previous findings in intact cells, neuraminidase treatment of membranes did not inhibit steroid uptake, perhaps indicating a species variation. Lastly, it was possible to show that corticosterone movement across the membrane exhibited countertransport, a phenomenon common only to carrier-mediated transport mechanisms. These studies show that placental vesicles accumulate corticosterone by a carrier-mediated mechanism.

Perturbation of alpha-aminoisobutyric acid transport in human placental membranes: direct effects by HgCl2, CH3HgCl, and CdCl2

Mercuric chloride, methylmercuric chloride, and cadmium chloride directly affect the human placental syncytiotrophoblast microvillous membrane. These heavy metals alter the facilitated diffusion of alpha-aminoisobutyric acid (AIB) into vesicles of this membrane in microM concentrations. Mercuric chloride abolishes temporal kinetics of AIB transport, inducing an initial increase in AIB transport (27% at 100 microM) but subsequently lowering equilibrium values when compared to equilibrium time points in control. Methylmercuric chloride and cadmium chloride inhibited the initial rate of AIB transport (40% and 21%, respectively, at 200 microM), but did not affect the equilibrium value of AIB transported when compared to equilibrium levels in control. These effects were concentration dependent. Methylmercuric chloride was more potent in inhibiting AIB transport than cadmium chloride. Methylmercuric chloride and cadmium chloride effects on AIB transport were observed with minimal preincubation with placental vesicles. However, preincubation was necessary for mercuric chloride-induced perturbation of AIB transport. Cysteine protects against mercuric chloride- and methylmercuric chloride-induced effects on AIB transport but did not reverse these perturbations. Mercury- and cadmium-induced placental membrane toxicity result from interactions of these heavy metals with the placental plasma membranes.

Syncytiotrophoblast membrane vesicles: a model for examining the human placental cholinergic system

A membrane vesicle preparation was used to examine characteristics of the human placental cholinergic system. Plasma membrane vesicles were prepared from the microvillous surface of the human placental syncytiotrophoblast. Membranes were purified 18 -to 20-fold as indicated by 5'-nucleotidase activity. Vesicle cholinesterase activity was enriched and had a substrate preference consistent with that of acetylcholinesterase (acetylcholine greater than acetyl-beta-methylcholine greater than butyryl-choline). Choline acetyltransferase specific activity was reduced 80%. The synthetic muscarinic ligand, [3H]-quinuclidinyl benzilate (QNB), was used to identify two classes of muscarinic cholinergic binding sites. The dissociation constant of QNB binding was 80 pM and 30 nM for the two sites. The sites were saturable and bound 9 fmoles and 910 fmoles per mg protein for the high and low affinity sites, respectively. Specific binding was inhibited by scopolamine, atropine, carbamylcholine (CCH), and diphenhydramine, but not by non-muscarinic ligands-i.e. GABA, glycine, d-amphetamine, kappa-bungarotoxin and nicotine. The cholinergic agonist CCh had no effect on active AIB transport, although pharmacologic doses (lmM) of atropine, scopolamine and lidocaine reduced Na-gradient active transport of kappa-aminoisobutyric acid (AIB). No effect on Na-independent AIB transport was observed. Thus, these drugs apparently reduced AIB uptake through their shared local anesthetic activity and not through a central cholinergic mechanism. In contrast, CCh was able to stimulate Ca2+ uptake by the vesicles in a dose-dependent manner paralleling its ability to inhibit QNB binding. The CCh-stimulated Ca2+ uptake was inhibited by scopolamine, implying its mediation via cholinergic-type binding sites. The membrane vesicle preparation therefore provides a useful model for examination of the role of the human placental cholinergic system.