Role of protein glycosylation in the cAMP-mediated induction of alkaline phosphatase in mouse L-cells

Lectin histochemistry in the human placenta of pregnancies complicated by intrauterine growth retardation based on absent or reversed diastolic flow

The oligosaccharide distribution of the glycoconjugates was investigated in placental tissue of pregnancies complicated by intrauterine growth retardation (IUGR) with absent or reversed flow in the umbilical artery (ARED), between the 29 and the 37 weeks of pregnancy. Placentae of a gestational age-matched group of normally grown pregnancies was also selected as control group. For this purpose a battery of seven HRP-conjugated lectins was used (DBA, SBA, PNA, ConA, WGA, LTA and UEA I). Our data showed that alpha-D-mannose (ConA), N-acetyl-D-glucosamine (WGA), beta-N-acetyl-D-galactosamine (SBA), alpha-L-fucose (LTA and UEA I) were present in less amount or were not present in the trophoblast and/or in the endothelial cells of the pathological group compared to the control one. The trophoblast basement membrane and/or basal plasma membrane of the pathological placentae were characterized by the presence of alpha-D-mannose (ConA), N-acetyl-D-glucosamine (WGA), sialic acid and D-galactose-(beta1-->3)-N-acetyl-D-galactosamine (neuraminidase-PNA), only in some tracts, in all the weeks of gestation. In the control placentae these sugar residues were present in the whole basement membrane and/or basal plasma membrane from 31 or 35 weeks. The Hofbauer cells of the pathological placental tissue showed a less amount or lack of alpha-D-mannose (ConA), beta-N-acetyl-D-galactosamine (SBA) and alpha-L-fucose (UEA I) compared to the control ones. These results suggest that a less amount or lack of some sugar residues may contribute to restricted placenta growth and development and thus reduced efficiency in maternal-fetal exchanges of gases and metabolites.

Lectinhistochemical and cytometrical evaluation of the corpus luteum of the rat at the end of pregnancy

Most studies on the biochemistry and structure of the corpus luteum have focused on elucidating the processes of progesterone synthesis and release. In the present work, the histochemical composition of the corpus luteum of the rat was evaluated using lectinhistochemistry on rats at the end of pregnancy (days 18-23). We also analysed the morphology of the luteal cells, to characterize the changes attributable to regression in this organ. Seven biotinylated lectins were used (CON-A, WGA, DBA, SBA, PNA, RCA and UEA-I) following pre-set protocols (ABC method). The average diameter and area of the cells and their nuclei were measured. High reactivity of the luteal cells was observed with CON-A and a lower reactivity with WGA. The capillary endothelium gave positive reactivity with WGA and to a certain extent with SBA, PNA and RCA. Vesicular structures were intensely stained with DBA, and were more abundant in sections from animals with more advanced pregnancy, which could be attributable to cellular debris, on the basis of their morphologic characteristics. There were no significant differences among the cytometric variables analysed in comparisons of the values corresponding to the different days of gestation. These observations, together with previous research, suggest that, on the day of delivery, the corpus luteum of the rat is in the very early stages of structural regression, with no changes at the morphological level, but with changes at the molecular level.

Changes in expression of the oligosaccharides in the human fetal skin

Lectin histochemistry was used to investigate the glycoconjugate saccharidic moieties in the skin of human fetuses ranging in age from the 8th to the 13th week of gestation. For this purpose, seven HRP-conjugated lectins were employed (Con A, WGA, PNA, DBA, SBA, UEAI and LTA). The distribution and changes of the sugar residues of glycoconjugates at the level of the various layers of the fetal epidermis, as well of the underlying mesenchyme were studied. We have shown that UEAI reactivity is a characteristic finding of the cells of the intermediate layer at the 11th and 12th week of gestation. LTA reactivity was detected only at the free border of the peridermal cells from the 10th to the 13th week of gestation. We demonstrated the presence of PNA binding sites at both the basal and peridermal cells from the 8th week of gestation, whereas other authors have stated that the appearance of PNA reactivity coincides with the onset of fetal skin stratification (11th-12th week of gestation). Moreover, some typical features in lectin reactivity at the level of the mesenchymal cells and fibres have been pointed out.

Role of carbohydrate content on the properties of galactose oxidase from Dactylium dendroides

The stability of intracellular, extracellular, and deglycosylated forms of galactose oxidase was compared with respect to the denaturing effects of heat, pH, and guanidine hydrochloride. The highly glycosylated forms were found to be more stable to pH and thermal inactivation. All forms were reversibly denaturated by guanidine hydrochoride, but the extent was dependent on the carbohydrate content. Deglycosylation did not affect the affinity of the enzyme for dihydroxyacetone and galactose. Exposure of different forms of galactose oxidase to proteases like pronase and trypsin resulted in a rapid degradation of the glycoenzymes with the formation of stable products. After pronase digestion of intra- and extracellular forms of galactose oxidase catalytic species were isolated by gel filtration. The species (61 and 42 kDa) isolated from pronase-digested extracellular enzyme lost their ability to oxidize primary alcohols. Species (67 and 46 kDa) obtained from the intracellular enzyme kept the specificity of the original enzyme. Active pronase-derived peptides (42 and 46 kDa, respectively) had a higher carbohydrate content than the inactive ones.

Protein glycosylation regulates the externalization of two distinct classes of glucocorticoid-induced glycoproteins in rat hepatoma cells

The relationship of protein glycosylation to the externalization of glucocorticoid inducible alpha1-acid glycoprotein and mouse mammary tumor virus glycoproteins was examined in M1.54, a clonal population of mouse mammary tumor virus-infected rat hepatoma cells. Multiple freeze-thaw of isolated microsomes revealed that while alpha 1-acid glycoprotein is carried through the cell as a soluble component of vesicles, extracellular viral glycoproteins are initially membrane-associated. At concentrations of tunicamycin that specifically inhibited N-linked protein glycosylation, alpha 1-acid glycoprotein fractionated between the cellular and extracellular compartments. Thus, approximately one half of the newly synthesized, nonglycosylated (22,000 Mr) alpha 1-acid glycoprotein was rapidly secreted with kinetics similar to its glycosylated counterpart (release half-time of 60 min), while the remaining species first localized in an undefined intracellular compartment prior to its slow secretion (release half-time of 24 h). The same distribution of nonglycosylated alpha 1-acid glycoprotein was observed at various absolute levels of polypeptide, suggesting that this was not due simply to the saturation of an efficient secretory pathway at high polypeptide levels. In contrast to alpha 1-acid glycoprotein, no labeled viral antigens were released by tunicamycin-treated M1.54, while a nonglycosylated viral precursor glycopolyprotein was expressed intracellularly. Taken together, these results suggest that carbohydrate attachment strongly regulates the externalization of both alpha 1-acid glycoprotein and mouse mammary tumor virus species, which represent two distinct classes of extracellular glycoproteins.

Alkaline phosphatase and 5'-nucleotide phosphodiesterase from bovine intestine are cross-reactive

Polyclonal antibodies to native alkaline phosphatase and to native 5'-nucleotide phosphodiesterase were found to strongly cross-react with both enzymes. The antibodies also cross-react with both denatured enzymes, with glycopeptides from 5'-nucleotide phosphodiesterase, and with the oligosaccharides remaining after Pronase E digestion of the phosphodiesterase. They do not cross-react with either enzyme after their oligosaccharides have been modified or removed by periodate or trifluoromethanesulfonic acid treatment. Antibodies to denatured 5'-nucleotide phosphodiesterase do not bind to the native phosphodiesterase or alkaline phosphatase but do cross-react with denatured alkaline phosphatase even after removal or modification of the carbohydrate moieties. These results suggest that antibodies to denatured 5'-nucleotide phosphodiesterase may recognize amino acid sequence homology between alkaline phosphatase and 5'-nucleotide phosphodiesterase. However, antibodies to native enzymes apparently recognize cross-reactive determinants of the native enzymes which are carbohydrate in nature. This is the first report of antimammalian alkaline phosphatase antibodies which recognize the carbohydrate moieties of the enzyme.

Synergistic stimulation of alkaline phosphatase activity in bovine aortic endothelial cells grown in the presence of retinoids and glucocorticoids

Low concentrations of retinol (10 nM-10 microM) and dexamethasone (0.1 nM-1.0 microM) elevated activity of alkaline phosphatase (E.C. 3.1.3.1) in bovine endothelial cells in culture. The effect was apparent after 48 hr of growth in the presence of either of the two compounds, prior to any growth stimulatory effects. A synergistic stimulation of alkaline phosphatase was achieved in the presence of both retinol and dexamethasone implying different mechanisms of induction. Retinoic acid and retinyl acetate also elevated alkaline phosphatase but the retinol analogue reduced in the side chain (perhydroretinol) was inactive. The ability of steroids to elevate alkaline phosphatase activity was associated with the structure commonly required for glucocorticoid activity; however, competitors for the steroid receptor binding failed to prevent the elevation by dexamethasone or the synergism in the presence of retinol and dexamethasone.

Epidermal fucosylation of cell surface glycoprotein

When Ulex europeus agglutinin I (UEA) conjugated with fluorescein isothiocyanate is applied to tissue sections from the cutaneous epidermis of the newborn rat, the lectin binds to the surfaces of cells in the layer immediately above the basal layer but not to the cells in the basal layer itself. The latter cells bind the isolectin I-B4, from Griffonia simplicifolia (GS I-B4). The addition of a fucosyl residue to the oligosaccharide of the glycoprotein found on the surface of the basal cell can account for the change in lectin-binding specificity which occurs as the basal cell moves toward the cutaneous surface and becomes a spinous cell. The epidermis of the newborn rat has the necessary transferase to convert a glycoprotein with binding-specificity for GS I-B4 to binding specificity for UEA by adding a fucosyl residue from GDP-L-fucose.

Glycosylation-dependent regulation of opiate (enkephalin) receptors in neurotumor cells

Electron inactivation analysis revealed that the opiate (enkephalin) binding site in neurotumor cell lines NG108-15 and NCB-20 had an apparent target size of 200,000 daltons. Expression of functional opiate receptors in neurotumor cells appeared to require glycosylation, as treatment of such cells with tunicamycin (TM; under conditions where de novo glycosylation of asparagine residues in protein was reduced by 80%, but overall protein and DNA synthesis were inhibited by less than 10%) resulted in the loss of 50% of the opiate binding sites. The loss of binding sites could not be prevented by addition of protease inhibitors to cell cultures, but binding sites were partially restored 48-60 h after removal of the TM. In addition, the number of enkephalin binding sites in TM-treated cells was also restored to near-normal levels by addition of physiological concentrations (1-10 mM) of manganese ions to the in vitro receptor binding incubation mixture. TM treatment resulted in receptor supersensitivity to manganese ions for both opiate agonists and antagonists, no change in the sodium effect for either agonists or antagonists, and subsensitivity to GTP for both agonists and antagonists. However, opiate binding to cell membranes was not substantially inhibited by either neuraminidase treatment or short-term incubation with lectins such as wheat germ agglutinin, ricin, or concanavalin A. Thus, the data suggest that oligosaccharide units are not directly involved in opiate receptor-ligand interactions, but protein glycosylation is required for functional expression of receptors.