Low reversibility of intracellular cAMP accumulation in mouse Leydig tumor cells (MLTC-1) stimulated by human Luteinizing Hormone (hLH) and Chorionic Gonadotropin (hCG).

In order to study the intracellular cAMP response kinetics of Leydig cells to hormones with LH activity, we used MLTC-1 cells transiently expressing a chimeric cAMP-responsive luciferase so that real-time variations of intracellular cAMP concentration could be followed using oxiluciferin luminescence produced from catalyzed luciferin oxidation. The potencies of the different LHs and CGs were evaluated using areas under the curves (AUC) of their kinetics over 60 min stimulation. All mammalian LHs and CGs tested were found to stimulate cAMP accumulation in these cells. The reversibility of this stimulation was studied by removing the hormone from the culture medium after 10 min of incubation. The ratios of kinetics AUC after removing or not the hormone were used to evaluate the stimulation reversibility of each hormone. Natural and recombinant hLHs and hCGs were found to exhibit slowly reversible activation compared to pituitary rat, ovine, porcine, camel and equine LHs, serum-derived eCG (PMSG) and recombinant eLH/CGs. Carbohydrate side chains are not involved in this phenomenon since natural and recombinant homologous hormones exhibit the same reversibility rates. It is still unknown whether only one human subunit, α or ß, is responsible for this behaviour or whether it is due to a particular feature of the hLH and hCG quaternary structure.

Structure-function relationships of glycoprotein hormones and their subunits' ancestors.

Glycoprotein hormones (GPHs) are the most complex molecules with hormonal activity. They exist only in vertebrates but the genes encoding their subunits' ancestors are found in most vertebrate and invertebrate species although their roles are still unknown. In the present report, we review the available structural and functional data concerning GPHs and their subunits' ancestors.

Differential thermal stability of human, bovine and ovine Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH) quaternary structures.

Quaternary structure of human, bovine and ovine Follicle-Stimulating Hormones (hFSH, bFSH and oFSH) and Luteinizing Hormone was assessed in sandwich ELISAs using monoclonal anti-oFSHß or anti-oLHß antibodies, respectively, for capture and a biotinylated anti-hFSHα (α4 epitope) for detection. Neither free subunit gave any signal in this assay so that it was possible to measure the residual heterodimeric fraction after thermal treatment of the gonadotropins under study. The hormones were subjected to 5-min heating between 37 and 90 °C before rapid cooling in melting ice before ELISA. The data show half-dissociation of natural and recombinant human and ovine FSH preparations between 68 and 74 °C whereas bovine FSH preparations exhibited lower stability in these conditions with half-dissociation between 61 and 64 °C. Moreover, whereas all human and bovine as well as most ovine FSH preparations were fully dissociated at temperatures above 80 °C, one natural oFSH and one recombinant hLH preparations contained an important fraction that resisted dissociation even at 93 °C and retained in vitro bioactivity. This suggests the existence of gonadotropin αß heterodimer with covalently linked subunits. Similarly, about 20% of the recombinant hLH preparation was also found withstand heat denaturation and also probably to have cross-linked subunits. The origin and chemical nature of these inter-subunit bonds remain to be determined.

Straightforward isolation of phosphatidyl-ethanolamine-binding protein-1 (PEBP-1) and ubiquitin from bovine testis by hydrophobic-interaction chromatography (HIC).

Isolation of phosphatidyl-ethanolamine-binding protein-1 (PEBP-1) from bovine brain was described almost three decades ago but it required a large number of steps to reach high purity. After the fractionation of bovine testis proteins by ammonium sulfate precipitation we found that PEBP-1, detected by Western blotting, was among the very few proteins still soluble at 80% ammonium sulfate saturation (3.2M). This soluble fraction (S80) was directly loaded onto a phenyl sepharose column equilibrated at the same ammonium sulfate concentration (3.2M). A stepwise elution of the retained material at 1.0, 0.5, 0.2, 0.1M ammonium sulfate in ammonium hydrogen carbonate was performed and then with ammonium hydrogen carbonate alone and finally with 50% ethylene glycol. All fractions were analyzed by SDS-PAGE and Western blotting and the fractions containing PEBP-1 was further fractionated by size exclusion chromatography on a HR75 Superdex column permitting the isolation of ubiquitin in addition to PEBP-1 as demonstrated by Western blotting and mass spectrometry. This study shows the feasibility of hydrophobic interaction chromatography (HIC) on phenyl sepharose at a very high ammonium sulfate concentration (3.2M; 80% saturation) to efficiently purify the proteins that are still soluble in these extreme conditions.

Potentiation of the reductase activity of protein disulphide isomerase (PDI) by 19-nortestosterone, bacitracin, fluoxetine, and ammonium sulphate.

Protein disulphide isomerase (PDI) in the endoplasmic reticulum catalyzes the rearrangement of disulphide bridges during folding of secreted proteins. It binds various molecules that inhibit its activity. But here, we looked for molecules that would potentiate its activity. PDI reductase activity was measured in vitro using di-eosin-oxidized glutathione as substrate. Its classical inhibitor bacitracin was found to exert a biphasic effect: stimulatory at low concentrations (∼10(-6) M) and inhibitory only at higher concentrations (∼10(-4)-10(-3) M). The weak oestrogenic molecule bisphenol A was found to exert a weak inhibitory effect on PDI reductase activity relative to the strong oestrogens, ethynylestradiol, and diethylstilbestrol. Like 19-nortestosterone, fluoxetine was found to exert a potentiating effect on PDI reductase activity and their potentiating effects could be reversed by increasing concentrations of oestrogens. In conclusion, this paper provides the first identification of potentiators of PDI activity that are potential pharmaceuticals against pathologies affecting protein folding such as Alzheimer's disease.

Effect of pharmaceutical potential endocrine disruptor compounds on protein disulfide isomerase reductase activity using di-eosin-oxidized-glutathione.

BACKGROUND: Protein Disulfide Isomerase (PDI) in the endoplasmic reticulum of all cells catalyzes the rearrangement of disulfide bridges during folding of membrane and secreted proteins. As PDI is also known to bind various molecules including hormones such as estradiol and thyroxin, we considered the hypothesis that adverse effects of endocrine-disrupter compounds (EDC) could be mediated through their interaction with PDI leading to defects in membrane or secreted proteins. METHODOLOGY/PRINCIPAL FINDINGS: Taking advantage of the recent description of the fluorescence self quenched substrate di-eosin-oxidized-glutathione (DiE-GSSG), we determined kinetically the effects of various potential pharmaceutical EDCs on the in-vitro reductase activity of bovine liver PDI by measuring the fluorescence of the reaction product (E-GSH). Our data show that estrogens (ethynylestradiol and bisphenol-A) as well as indomethacin exert an inhibition whereas medroxyprogesteroneacetate and nortestosterone exert a potentiation of bovine PDI reductase activity. CONCLUSIONS: The present data indicate that the tested EDCs could not only affect endocrine target cells through nuclear receptors as previously shown, but could also affect these and all other cells by positively or negatively affecting PDI activity. The substrate DiE-GSSG has been demonstrated to be a convenient substrate to measure PDI reductase activity in the presence of various potential EDCs. It will certainly be usefull for the screening of potential effect of all kinds of chemicals on PDI reductase activity.

Comparative structure analyses of cystine knot-containing molecules with eight aminoacyl ring including glycoprotein hormones (GPH) alpha and beta subunits and GPH-related A2 (GPA2) and B5 (GPB5) molecules.

BACKGROUND: Cystine-knot (cys-knot) structure is found in a rather large number of secreted proteins and glycoproteins belonging to the TGFbeta and glycoprotein hormone (GPH) superfamilies, many of which are involved in endocrine control of reproduction. In these molecules, the cys-knot is formed by a disulfide (SS) bridge penetrating a ring formed by 8, 9 or 10 amino-acid residues among which four are cysteine residues forming two SS bridges. The glycoprotein hormones Follicle-Stimulating Hormone (FSH), Luteinizing Hormone (LH), Thyroid-Stimulating Hormone (TSH) and Chorionic Gonadotropin (CG) are heterodimers consisting of non-covalently associated alpha and beta subunits that possess cys-knots with 8-amino-acyl (8aa) rings. In order to get better insight in the structural evolution of glycoprotein hormones, we examined the number and organization of SS bridges in the sequences of human 8-aa-ring cys-knot proteins having 7 (gremlins), 9 (cerberus, DAN), 10 (GPA2, GPB5, GPHalpha) and 12 (GPHbeta) cysteine residues in their sequence. DISCUSSION: The comparison indicated that the common GPH-alpha subunit exhibits a SS bridge organization resembling that of DAN and GPA2 but possesses a unique bridge linking an additional cysteine inside the ring to the most N-terminal cysteine residue. The specific GPHbeta subunits also exhibit a SS bridge organization close to that of DAN but it has two additional C-terminal cysteine residues which are involved in the formation of the "seat belt" fastened by a SS "buckle" that ensures the stability of the heterodimeric structure of GPHs. GPA2 and GPB5 exhibit no cys residue potentially involved in interchain SS bridge and GPB5 does not possess a sequence homologous to that of the seatbelt in GPH beta-subunits. GPA2 and GPB5 are thus not expected to form a stable heterodimer at low concentration in circulation. SUMMARY: The 8-aa cys-knot proteins GPA2 and GPB5 are expected to form a heterodimer only at concentrations above 0.1 microM: this would be consistent with a short-term paracrine role but not with an endocrine role after dilution in circulation. Consequently, GPA2 and GPB5 could exert separate endocrine roles either during development and/or during adult life of both vertebrates and invertebrates.

Stability and biological activities of heterodimeric and single-chain equine LH/chorionic gonadotropin variants.

Recombinant equine LH/chorionic gonadotropin (eLH/CG) was expressed in the baculovirus-Sf9 insect cell system either as a single-chain with the C-terminus of the beta-subunit fused to the N-terminus of the alpha-subunit or as non-covalently linked heterodimers with or without a polyhistidine tag at various locations. All these non-covalently linked eLH/CG variants were secreted as stable heterodimers in the medium of infected Sf9 cells. To assess the influence of the presence and the position of polyhistidine tag on LH bioactivity, we expressed four non-covalently linked tagged heterodimeric eLH/CG variants that were secreted in threefold higher quantities than the single chain. Among them, only two exhibited full in vitro LH bioactivity, relative to untagged heterodimers, namely the one His-tagged at the N-terminus of alpha-subunit and the other at the C-terminus of the beta-subunit both of which are amenable to nickel-affinity purification. Furthermore, single-chain eLH/CG was found to be N- and O-glycosylated but nevertheless less active in in vitro LH bioassays than natural eCG and heterodimeric recombinant eLH/CG. The thermal stability of natural and recombinant hormones was assessed by the initial rates of dissociation from 20 to 90 degrees C. Heterodimeric eLH/CG from Sf9 cells was found to be as stable as pituitary eLH and serum eCG (T(1/2), 74-77 degrees C). Although Sf9 cells only elaborated short immature-type carbohydrate side chains on glycoproteins, recombinant eLH/CG produced in these cells exhibited stabilities similar to that of pituitary eLH. In conclusion, recombinant heterodimeric eLH/CG exhibits the same thermal stability as natural pituitary LH and its advantages over the single-chain eLH/CG include higher secretion, higher in vitro bioactivity, and reduced potential risk of immunogenicity.

Nitro-thiocyanobenzoic acid (NTCB) reactivity of cysteines beta100 and beta110 in porcine luteinizing hormone: metastability and hypothetical isomerization of the two disulfide bridges of its beta-subunit seatbelt.

Luteinizing hormone (LH) like all other glycoprotein hormones is composed of two dissimilar subunits, alpha and beta, that are non-covalently associated. The heterodimer is stabilized by a region of the beta-subunit called the "seatbelt" because it wraps around the alpha-subunit and it is fastened by a disulfide bridge between cysteines beta26 and beta110. Although all 22 cysteines of porcine LH (pLH) are engaged in disulfide bridges, we previously showed that the free cysteine-specific reagent NTCB could react with pLH: it slowly cyanylated two cysteines in pLH and there was a close relationship between NTCB reaction with pLH and association/dissociation kinetics of its subunits. Therefore, cysteines beta26 and beta110 were considered as the best candidates for NTCB reaction. In order to identify the NTCB-reactive cysteines in pLH we have performed a mass spectroscopic analysis of the peptides released after mild basic hydrolysis of S-cyanylated pLH and its subunits. Only cysteines beta100 and beta110 were found to react with NTCB. Since these residues are not linked by a disulfide bridge in the crystallographic 3D structure of gonadotropins, it is proposed that their respective counterparts (Cysbeta93 and beta26) do not react with NTCB either because they are shielded from solvent or because they form a transient bridge. In the first hypothesis, both seatbelt bridges would be independently metastable; in the second one, a fast reversible isomerization between bridges beta26-beta110 and beta93-beta100 would occur. Such a reaction could be catalyzed by the previously recognized intrinsic protein disulfide isomerase (PDI) activity of gonadotropins.

Involvement of equine chorionic gonadotropin (eCG) carbohydrate side chains in its bioactivity; lessons from recombinant hormone expressed in insect cells.

Natural eCG consists of as much as 45% carbohydrate side chains. The present paper deals with the analysis of the roles of the N- and O-linked saccharides of this hormone in the different steps of its activity and its possible replacement by recombinant eCG expressed in baculovirus-insect cell systems.

Mammalian-like nonsialyl complex-type N-glycosylation of equine gonadotropins in Mimic insect cells.

Recombinant equine luteinizing hormone/chorionic gonadotropin (eLH/CG) was expressed in Mimic insect cells, that are commercial stably transformed Spodoptera frugiperda (Sf9) cells expressing five mammalian genes encoding glycosyltransferases involved in the synthesis of complex-type monosialylated N-glycans. We previously showed that it exhibited no in vivo bioactivity although expressing full in vitro bioactivity, and it was suspected that this was because of insufficient sialylation of eLH/CG N-glycans. Lectin binding analyses were performed with recombinant dimeric eLH/CG or its alpha subunit, secreted in the serum-containing supernatant of infected Sf9 and Mimic cells. Two types of specific lectin affinity assays (blot analyses and enzyme-linked immunosorbent assay) were used to compare the ability or inability of natural and recombinant gonadotropins to bind to various lectins. In natural equine chorionic gonadotropin (eCG), complex-type N-glycans terminating with both Siaalpha2,3Gal (based on Maackia amurensis agglutinin [MAA] binding) and Siaalpha2,6Gal (based on Sambucus nigra agglutinin [SNA] binding) were found, but in the alpha subunit dissociated from natural eCG, we only detected Siaalpha2-6Gal. In eLH/CG and its alpha subunit produced by Sf9 cells, N-glycans were found to be terminated by mannosyl residues (based on Galanthus nivalis agglutinin [GNA] binding), whereas those produced in Mimic cells were terminated by galactoses (based on binding to Ricinus communis agglutinin I [RCA I] , but not to SNA or MAA). This is in agreement with the fact that the nucleotide donor substrate of sialic acid is not naturally synthesized in insect cells. On the basis of binding to Arachis Hypogaea agglutinin [PNA], O-glycans exhibited the Galbeta1-3GalNAc structure in recombinant-free alpha and eLH/CG from both Sf9 and Mimic cell lines. Both N- and O-linked carbohydrate side chains synthesized in Mimic cells should thus be amenable to further acellular sialylation.

Biological activities of recombinant equine luteinizing hormone/chorionic gonadotropin (eLH/CG) expressed in Sf9 and Mimic insect cell lines.

Equine luteinizing hormone (eLH) and chorionic gonadotropin (eCG) are composed of identical alpha and beta polypeptide chains, but eCG subunits are much more heavily glycosylated and sialylated. Consequently, eCG exhibits a much longer half-life than eLH in blood. Recombinant eLH/CG, expressed in Sf9 and Mimic insect cells, were compared with one another and to the natural hormones eCG and eLH. Mimic cells are stably-transformed Sf9 cells, expressing five mammalian genes encoding glycosyltransferases involved in the synthesis of complex N-carbohydrate chains. Recombinant eLH/CG expressed in Mimic cells exhibited a higher apparent molecular weight (MW) than that expressed in Sf9 cells, suggesting that its N-glycosylation was, as expected, more complete. Nevertheless, the two recombinant eLH/CG exhibited lower MW than natural eCG from pregnant mare plasma. The two eLH/CG produced in Sf9 and Mimic cells were found to be active in in vitro LH and FSH bioassays, with potencies similar to those of eCG. By contrast, they exhibited no significant in vivo bioactivity, neither in the specific follicle-stimulating hormone (FSH) assay nor in the specific eCG assay. Although recombinant eLH/CG produced in Mimic cells bears more elaborate carbohydrate chains than recombinant eLH/CG from Sf9 cells, it exhibits no significant in vivo bioactivity, probably because of insufficient terminal sialylation of its carbohydrate chains, leading to its rapid removal from blood.