The use of drugs to dissect the pathway for secretion of the glycoprotein hormone chorionic gonadotropin by cultured human trophoblastic cells

Modulation of endocrine and transport functions in human trophoblasts by saquinavir and nelfinavir

OBJECTIVES

The distribution of drugs to the maternal-fetal interface is influenced by the expression of various efflux transporters. Among these transporters, P-glycoprotein (P-gp) is responsible for the efflux of a great number of drugs such as protease inhibitors of the human immunodeficiency virus, thus reducing the chemical exposure of the fetus.

STUDY DESIGN

The effects of saquinavir and nelfinavir were evaluated on human trophoblast functions and integrity by investigating their effect on human chorionic gonadotropin (hCG) secretion and on P-gp expression and functionality.

RESULTS

Nelfinavir significantly reduced hCG secretion by 30% after a 48-h treatment but it had no effect on syncytia formation. Saquinavir had no effect on hCG secretion but significantly increased both expression (to a 2-fold extent) and functionality (by 17.9%) of P-gp, whereas nelfinavir only increased functionality (by 23.1%) with a dissociation of P-gp from caveolin-1.

CONCLUSION

These results suggest that the effects of saquinavir and nelfinavir differ on trophoblast functions.

Secretion of pro-apoptotic intron 4-retaining soluble HLA-G1 by human villous trophoblast

One major materno-fetal interface in the human placenta is constituted by the syncytiotrophoblast, in contact with maternal blood of the intervillous space, which derives from differentiation and fusion of the villous cytotrophoblast (vct). In the present work, we purified vct from term placenta by depleting HLA class I- and class II-positive cells. We found by RT-PCR that both soluble intron 4-retaining HLA-G1 (sHLA-G1) and HLA-G2 isoforms were transcribed in purified vct. Using different HLA-G-specific mAb, we demonstrated by intracellular flow cytometry, Western blotting and ELISA, that sHLA-G1 but no soluble HLA class Ia molecule was secreted by vct. We then purified sHLA-G1 from vct culture supernatant and found that it exhibited an unusual glycosylation pattern. Finally, we showed that such trophoblast-derived sHLA-G1 triggered specific apoptosis of activated CD8+ T cells. Taken together, these results demonstrated that vct did secrete functional sHLA-G1 in primary culture and suggested that, in vivo, sHLA-G1 might be an important immunomodulatory molecule controlling the activity of maternal immune effector CD8+ cells circulating in the blood that immerses chorionic villi.

Diflubenzuron stimulates phosphorylation of a 39 kDa integumental protein from newly molted American cockroach (Periplaneta americana)

We identified a 39 kilodalton (kDa) protein from newly molted American cockroaches (Periplaneta americana), isolated from a 100,000 g precipitate of a 1000 g supernatant from an integument homogenate, which exhibited increased phosphorylation following diflubenzuron (DFB) treatment. This 39 kDa phosphoprotein was partially purified from an intracellular membrane vesicle containing fractions obtained by discontinuous sucrose density centrifugation. Both the interfaces of the 30/40% and 40/50% sucrose density layers were found to be enriched in the 39 kDa phosphoprotein. Treatments with various chemicals such as the ionophores valinomycin and A23187, the protonophore carbonylcyanide p-trifluoromethoxy-phenylhydrazone (FCCP), and the vacuolar H(+)-ATPase inhibitor N-ethylmaleimide (NEM), like DFB, were also found to stimulate phosphorylation of 39 kDa protein. These results suggest that by disrupting the normal pH gradient occurring in certain acidic vacuolar-type membrane vesicles, phosphorylation of the 39 kDa protein will be increased. In addition, we found that by decreasing the external pH to 5.0 from about neutral, it was possible to stimulate the phosphorylation activity of this particular protein, and thus stimulate the action of DFB. We concluded the DFB's action is very probably related to its ability to disrupt the normal ionic balance of these vacuolar-type vesicles, leading to eventual disruption of the proton gradient within the vesicles.

Biosynthesis of mucin derived from a 60-kDa precursor protein in the human stomach

We studied the biosynthesis of mucin in the human stomach using an anti-mucin core peptide monoclonal antibody, 3G12. Human stomach mucosa was labeled with [35S]methionine, and chased for 3 h. An approximately 60-kDa subunit of human gastric mucin precursor protein was detected in the intracellular product. Under nonreducing conditions, dimer, trimer, and tetramer mucin precursor protein (120, 180, 240 kDa) were detected. Treatment with tunicamycin or endo-beta-N-acetylglucosaminidase H had no effect on the 60-kDa subunit and its oligomers. Extracellular products contained only the high molecular weight mucin, and the secretion was not affected by tunicamycin. By treatment with monensin or brefeldin A, the mature mucin was not secreted extracellularly. These findings suggested that a 60-kDa subunit of the mucin precursor protein was biosynthesized into mature mucin after oligomerization to tetramers, and that neither the oligomerization nor the intracellular transport of the mucin in the human stomach was associated with N-glycosylation.

Evidence for posttranscriptional stimulation of monoclonal antibody secretion by l-glutamine during slow hybridoma growth

The addition of 5-40 mM L-glutamine to batch cultures of a murine hybridoma following the cessation of rapid growth significantly stimulated monoclonal antibody (mAb) synthesis and secretion per cell. Stimulation of mAb secretion following the cessation of rapid growth was also observed in response to addition of mitochondrial intermediates of glutamate oxidation and was not found to be the result of release of transiently stored mAb. Less than 1% of the secreted mAb was detected by ELISA in isolated hybridoma lysosomes. This stimulation was posttranscriptional and not the result of enhancement of levels of mAb mRNAs or stabilization of heavy (H) or light (L) chain encoding message. Sub-inhibitory levels of lysosomotrophic weak bases stimulated release of lysosomal contents but did not result in release of intact or partially degraded mAb. Inhibition of aspartic proteinase activity secreted by the hybridoma did not enhance mAb secretion even though a high level of mAb degrading proteinase activity was continuously secreted during both rapid and slow growth. These responses indicate that during slow growth, the addition of L-glutamine increases the availability of cellular ATP generated by mitochondrial respiration which stimulates some posttranscriptional step in the pathway of mAb secretion such as the rate of H or L chain translation, chain assembly, interorganelle transport or vesicular transport from the Golgi to the cell membrane.

Environmental effects on protein glycosylation

Cultured mammalian cells are being used to produce proteins for therapeutic and diagnostic use because of their ability to perform complex post-translational modifications, including glycosylation. The oligosaccharide moieties can play an important role in defining several biological properties of glycoproteins, including clearance rate, immunogenicity, and biological specific activity. There is a growing interest in defining the factors that influence glycosylation, including the cell culture environment. In this review we organize the published data from in vitro cell culture and tissue culture studies that demonstrate direct effects of the culture environment on N-linked glycosylation.

Alteration of intracellular traffic by monensin; mechanism, specificity and relationship to toxicity

Monensin, a monovalent ion-selective ionophore, facilitates the transmembrane exchange of principally sodium ions for protons. The outer surface of the ionophore-ion complex is composed largely of nonpolar hydrocarbon, which imparts a high solubility to the complexes in nonpolar solvents. In biological systems, these complexes are freely soluble in the lipid components of membranes and, presumably, diffuse or shuttle through the membranes from one aqueous membrane interface to the other. The net effect for monensin is a trans-membrane exchange of sodium ions for protons. However, the interaction of an ionophore with biological membranes, and its ionophoric expression, is highly dependent on the biochemical configuration of the membrane itself. One apparent consequence of this exchange is the neutralization of acidic intracellular compartments such as the trans Golgi apparatus cisternae and associated elements, lysosomes, and certain endosomes. This is accompanied by a disruption of trans Golgi apparatus cisternae and of lysosome and acidic endosome function. At the same time, Golgi apparatus cisternae appear to swell, presumably due to osmotic uptake of water resulting from the inward movement of ions. Monensin effects on Golgi apparatus are observed in cells from a wide range of plant and animal species. The action of monensin is most often exerted on the trans half of the stacked cisternae, often near the point of exit of secretory vesicles at the trans face of the stacked cisternae, or, especially at low monensin concentrations or short exposure times, near the middle of the stacked cisternae. The effects of monensin are quite rapid in both animal and plant cells; i.e., changes in Golgi apparatus may be observed after only 2-5 min of exposure. It is implicit in these observations that the uptake of osmotically active cations is accompanied by a concomitant efflux of H+ and that a net influx of protons would be required to sustain the ionic exchange long enough to account for the swelling of cisternae observed in electron micrographs. In the Golgi apparatus, late processing events such as terminal glycosylation and proteolytic cleavages are most susceptible to inhibition by monensin. Yet, many incompletely processed molecules may still be secreted via yet poorly understood mechanisms that appear to bypass the Golgi apparatus. In endocytosis, monensin does not prevent internalization. However, intracellular degradation of internalized ligands may be prevented.(ABSTRACT TRUNCATED AT 400 WORDS)

O-glycosylation of leukosialin in K562 cells. Evidence for initiation and elongation in early Golgi compartments

The O-glycosylation of leukosialin, a major sialoglycoprotein found on leukocytes, has been studied in the human erythroleukemic cell line K562. The appearance of its O-linked chains has been followed in pulse-chase experiments with [35S]methionine by immunoprecipitation with an anti-peptide antiserum as well as with a lectin from Salvia sclarea seeds (SSA) specific for GalNAc-Ser/Thr and the peanut (Arachis hypogaea) agglutinin (PNA) which recognizes Gal beta 1----3GalNAc-Ser/Thr structures. An O-glycan-free precursor was converted into the fully O-glycosylated mature form within the 10-min labeling period and no intermediates carrying only GalNAc-Ser/Thr structures could be detected. The ionophore monensin was used in order to slow down intracellular traffic and thus O-glycan synthesis. The drug partly inhibited the transport from rough endoplasmic reticulum (RER) to the Golgi and also the cell-surface expression of leukosialin. It was found to have a marked effect on the synthesis of O-linked carbohydrate structures of leukosialin since the amount of O-glycans containing only GalNAc or NeuNAc alpha 2----6GalNAc was significantly increased after monensin treatment. Under these conditions the biosynthesis of the N-glycan on leukosialin was completely arrested in an endoglycosidase-H-sensitive step of processing, whereas the O-glycans already contained galactose and sialic acid although at a reduced level. On the other hand, the small amounts of leukosialin expressed on the cell surface of monensin-treated cells carried the same glycans as those remaining blocked inside the cell. In addition, immunocytochemical studies using SSA and PNA on untreated K562 cells suggested the absence of detectable amounts of GalNAc-Ser/Thr-bearing glycoproteins in the RER as well as in the Golgi. In contrast Gal beta 1----3GalNAc structures could be detected on intracellular membranes which were tentatively identified as the cis-Golgi. Together these results lead us to the following conclusions: N-glycan transfer occurs in the RER before the initiation of O-glycans which takes place at the entrance of the protein into the Golgi; further elongation of O-glycans with galactose and sialic acid follows very rapidly, probably before the final processing of N-glycans to complex-type structures.

Purification and characterization of the glycoprotein hormone alpha-subunit-like material secreted by HeLa cells

The protein secreted by HeLa cells that cross-reacts with antiserum developed against the alpha-subunit of human chorionic gonadotropin (hCG) has been purified approximately 30,000-fold from concentrated culture medium by organic solvent fractionation followed by ion exchange, gel filtration, and lectin affinity chromatography. The final preparation had a specific activity (by RIA) of 6.8 x 10(5) ng of alpha/mg of protein and appeared homogeneous by electrophoresis on reducing/denaturing polyacrylamide gels (SDS-PAGE). Amino acid analysis indicated that HeLa-alpha had a composition very similar to that of the urinary hCG alpha-subunit. Peptide fingerprints of the HeLa protein and hCG-alpha revealed that several of the Tyr-, Met-, and Cys-containing tryptic peptides were held in common, thus identifying the tumor protein as a glycoprotein hormone alpha-subunit with a primary structure similar to that of hCG-alpha. However, comparison of hCG-alpha and HeLa-alpha demonstrated that the tumor-associated subunit was not identical with its normal counterpart. Only two of the three Tyr-containing tryptic peptides present in hCG-alpha could be detected in HeLa-alpha after iodination with 125I. HeLa-alpha eluted prior to hCG-alpha during Sephadex G-75 chromatography, but the subunits coeluted when the tumor protein was first subjected to mild acid hydrolysis. The purified tumor protein had an apparent molecular weight greater than that of the urinary alpha-subunit when analyzed by SDS-PAGE (Coomassie blue staining), and this difference was even greater when a partially purified preparation was examined by an immunoblot technique (Western). Isoelectric focusing of the HeLa and hCG subunits demonstrated that the tumor protein had a lower pI (4.7-5.5 compared to 6.5-7.8), and removal of sialic acid by mild acid hydrolysis did not entirely eliminate this difference. Immunoprecipitation and electrophoresis of alpha-subunit from HeLa cultures labeled with [3H]fucose indicated that the tumor subunit was fucosylated, whereas analysis of hCG-alpha hydrolysates by HPLC confirmed previous reports that the placental subunit does not contain fucose. HeLa alpha-subunit was unable to combine with hCG beta-subunit to form holo-hCG under conditions where the hCG alpha-subunit was able to do so. The results indicate that, regardless of whether or not a single alpha-subunit gene is being expressed in both normal and neoplastic tissues, posttranslational modifications lead to a highly altered subunit in the tumor. The differences observed may be useful in diagnosing neoplastic vs hyperplastic conditions and may lend insight into the mechanism of ectopic hormone production by tumors

Distribution of O-linked sugar units on hCG and its free alpha-subunit

hCG, a glycoprotein hormone produced by the trophoblast in pregnancy, is composed of two dissimilar subunits, alpha and beta, joined non-covalently. hCG has four O-linked sugar units, all attached to the beta-subunit. The trophoblast also produces a free form of the alpha-subunit, which unlike the alpha-component of hCG, can contain an O-linked sugar unit. The structures of the O-linked sugar units were examined. Four structures were identified on urinary hCG. A hexasaccharide, NeuAc alpha 2-3Gal beta 1-3(NeuAc alpha 2-3Gal beta 1-4GlcNAc beta 1-6)GalNAc- accounting for 13%, a tetrasaccharide, NeuAc alpha 2-3Gal beta 1-3(NeuAc alpha 2-6)GalNAc-, for 34%, a trisaccharide, NeuAc alpha 2-3Gal beta 1-3GalNAc-, for 43% and a disaccharide, NeuAc alpha 2-6GalNAc- for 10% of the total O-linked sugar structures. Similar mixtures were found on peptides containing one, three or four sugar units suggesting a random distribution among attachment sites. The distribution of O-linked sugar structures on hCG and free alpha-subunit from trophoblast explant cultures was compared. The mixture of structures attached at the single site on the free alpha-subunit paralleled that at the four sites on the hCG.

Regulation of alpha 1-acid glycoprotein externalization and intracellular accumulation in glucocorticoid-induced rat hepatoma cells

alpha 1-Acid glycoprotein (alpha 1-AGP) is a glucocorticoid inducible gene product that is synthesized and secreted by certain rat hepatoma tissue culture (HTC) cell lines such as M1.54. Exposure to monensin, a Na+-K+ ionophore, causes a significant redistribution of alpha 1-AGP into two distinct fractions; immunoprecipitation of [35S]methionine-labeled proteins revealed that a 27% decrease in secretion accounts for a sixfold increase in accumulation of a stable intracellular species. The new intracellular alpha 1-AGP is more heterogeneous than normal while the extra-cellular form is 6000 Da smaller than normal. These effects are due to selective alterations in carbohydrate maturation; endo-beta-N-acetylglucosaminidase H (endo H) digestion demonstrated that both alpha 1-AGP species contain variable numbers of endo H-resistant oligosaccharide side chains ranging between zero and five. Ricin affinity chromatography revealed that the attachment of galactose residues is strikingly correlated with alpha 1-AGP externalization while neuraminidase digestions demonstrated that sialic acid attachment appears unessential for its secretion. Taken together, our results suggest that in the presence of monensin the cellular transport of intracellular destined and externalized alpha 1-AGP proceeds in common through the early segments of the Golgi and at a point prior to or at the compartment containing galactosyl transferase, alpha 1-AGP becomes committed for secretion.

Influence of monensin on biosynthesis, processing and secretion of proteodermatan sulfate by skin fibroblasts

The influence of monensin on biosynthesis, processing and secretion of proteodermatan sulfate from human skin fibroblasts was studied with the aid of a specific immunological procedure. Double-labeling experiments with [3H]leucine and [35S]sulfate indicated that monensin caused a dose-dependent parallel decrease of sulfate incorporation into total and of secretion of 3H-labeled proteodermatan sulfate. Compared with the untreated control, a greater proportion of incorporated [35S]sulfate than of incorporated [3H]leucine became secreted. Other monensin effects were a moderate intracellular accumulation of glycosaminoglycan-free core protein, a reduced chain length and a greatly reduced epimerization of D-glucuronic to L-iduronic acid residues. In contrast to the formation of N-acetylgalactosamine 4-sulfate residues 6-sulfation was not affected. Conversion of high-mannose-type oligosaccharides to complex-type N-glycans which normally occurred concomitantly with glycosaminoglycan biosynthesis was inhibited. Withdrawal of monensin made possible an additional sulfation of intracellularly accumulated proteodermatan sulfate. The newly formed sulfate esters did not cluster at the non-reducing ends of the glycosaminoglycan chains. Cells preexposed to monensin and labeled with [3H]glucosamine either in the absence or continuous presence of the drug incorporated similar amounts of 3H radioactivity into proteodermatan sulfate. The results suggest that epimerization of D-glucuronic acid residues and 4-sulfation occur predominantly in the trans cisternae of the Golgi apparatus whereas chain polymerisation and 6-sulfation take place predominantly in the cis Golgi complex.

Differential processing and regulation of thyroid-stimulating hormone subunit carbohydrate chains in thyrotropic tumors and in normal and hypothyroid pituitaries

Thyroid-stimulating hormone (TSH) alpha- and beta-subunit glycosylation was investigated in mouse thyrotropic tumor and in normal and hypothyroid pituitary cells for various periods of time in the presence of [3H]mannose or [3H]galactose. After sequential precipitation with anti-alpha and anti-beta sera, subunits were treated with Pronase followed by endo-beta-N-acetylglucosaminidase H (Endo H) and analyzed by paper chromatography. In primary cultures of thyrotropic tumor cells incubated for 60 min with [3H]mannose, primarily Man9GlcNAc and Man8GlcNAc were found on TSH + alpha subunits, whereas Glc1Man9GlcNAc and Man9GlcNAc were prominent on free beta subunits. After preincubation of cells for 16 h in the presence or absence of glucose followed by a 60-min pulse of [3H]mannose, there was an 8-fold increase in labeled TSH + alpha but only a minimal change in free beta or total proteins. In the absence of glucose, there was a selective accumulation of Man8GlcNAc on TSH + alpha but not on free beta or total proteins; however, there was no detectable accumulation of Endo H resistant forms during glucose starvation on TSH subunits or total proteins. Normal mouse and rat pituitary minces incubated for 60 min with either [3H]mannose or [3H]galactose showed no glucose-containing species on TSH subunits, but equal amounts of Man9GlcNAc and Man8GlcNAc on TSH + alpha, and mostly Man9GlcNAc on free beta subunits. In contrast, hypothyroid mouse and rat pituitaries exhibited an increase in Glc1Man9NAc and Glc1Man8GlcNAc on free beta but not on TSH + alpha or total proteins.(ABSTRACT TRUNCATED AT 250 WORDS)

The structures of the serine-linked sugar chains on human chorionic gonadotropin

The human chorionic gonadotropin beta-subunit tryptic COOH-terminal peptide (residues 123-145) which contains 3 serine-linked sugar chains was isolated. The sugar chains were cleaved by beta-elimination and then separated by gel filtration. The peaks were pooled and their compositions determined. The products of serial glycosidase digestion and periodate oxidation of the intact glycopeptide were also characterized. Of the serine-linked sugar chains, 13% were the hexasaccharide NeuAc alpha 2,3 Gal beta 1,3 (NeuAc alpha 2,3 Gal beta 1,4 GlcNAc beta 1,6) GalNAc, 34% the tetrasaccharide NeuAc alpha 2,3 Gal beta 1,3 (NeuAc alpha 2,6) GalNAc, 43% the trisaccharide NeuAc alpha 2,3 Gal beta 1,3 GalNAc and 10% the disaccharide NeuAc alpha 2,6 GalNAc.

An oligosaccharide of the O-linked type distinguishes the free from the combined form of hCG alpha subunit

JAR malignant trophoblast cells produce a free alpha subunit in addition to an alpha combined with beta subunit as hCG. The free alpha is larger by gel chromatography and SDS-PAGE than combined alpha and is unable to associate with beta subunit to form hCG. A tryptic fragment, representing amino acid residues 36-42, derived from free alpha was larger than the corresponding fragment from combined alpha. After neuraminidase treatment, the fragment from free alpha bound peanut lectin agarose, which is specific for Gal beta 1-3GalNAc as found in O-linked oligosaccharides. The fragment also contained Gal and GalNAc (and a lesser amount of GlcNAc) as determined by glycosidase sensitivity and amino sugar analyses. Removal of this tryptic fragment ablated the size difference between free and combined alpha subunits.