Increased transfer of oligosaccharide from oligosaccharide pyrophosphoryl dolichol to protein acceptors upon estrogen-induced chick oviduct differentiation

Dolichol phosphate mannose synthase is differentially expressed in male and female worms of Schistosoma mansoni

The cDNA encoding the Schistosoma mansoni dolichol phosphate mannose synthase was completely sequenced, displaying the highest homology with Cricetulus griseus and Saccharomyces pombe genes. The Schistosome enzyme had a K(m) of 0.127 microM, a value that is within the range of those reported for several other species. Thin-layer chromatography of the radiolabelled schistosome lipid intermediate showed it was identical to dolichol-phosphate (C80-C105). Expression of dolichol phosphate mannose synthase of S. mansoni (SmDPMS) was analysed by Northern blot and quantified by semi-quantitative RT-PCR with cDNA from mature and immature male and female worms. Northern blot analysis revealed a single 1-kb band. Both approaches confirmed a higher level of expression in mature female worms, as compared to immature and male worms.

Thyrotropin controls dolichol-linked sugar pools and oligosaccharyltransferase activity in thyroid cells

We previously showed that thyroglobulin (Tg) glycosylation is enhanced 1.5-fold under thyrotropin (TSH) stimulation, corresponding to an increased number of oligosaccharide chains per molecule of Tg. Now the steps involving dolichol components and oligosaccharyltransferase activity have been studied. Porcine thyroid cells were cultured on porous bottom filters with or without TSH and incubated with [14C]mevalonate. Under TSH regulation, the level of the whole of dolichol components was increased 1.25-fold without modifying their distribution. Dolichol, and free and monosaccharide-linked dolichyl-phosphate, represented respectively 40% and 45% of total dolichol components while dolichyl-pyrophosphate-oligosaccharide represented 3% only. A marked enhancement (4.2-fold) of oligosaccharyltransferase activity occurred in stimulated cells, which could correspond to the addition of the two TSH effects: stimulation of Tg synthesis (3-fold) and of Tg glycosylation (1.5-fold). The amount of lipid carriers appeared to be insufficiently increased but no component is a limiting step, suggesting that the turnover of dolichol derivatives may be increased under TSH control through their use by higher amounts of Tg.

Incorporation of mevalonate into dolichol and other isoprenoids during estrogen-induced chick oviduct differentiation

Incorporation of [14C]mevalonate into dolichol and other isoprenoid compounds by chick oviduct explants has been studied. A reliable assay of dolichol biosynthesis employing several chromatographic procedures, including two-dimentional TLC, was developed. Incorporation of [14C]mevalonate into dolichol by oviduct explants was linear for at least 6 h. The effect of estrogen-induced differentiation was studied by incubation of explants obtained from chicks treated for various periods of time with diethylstilbestrol. Mevalonate incorporation into dolichol, when expressed as cpm per g of tissue, was not affected by estrogen treatment, but since the oviduct increased about 100-fold in mass during differentiation, each oviduct synthesizes about 100-fold more dolichol. In most tissues, the major product of mevalonate incorporation is cholesterol. However, although approx. 90% of the non-saponifiable 14C-labeled compounds were in the so-called 'cholesterol fraction', oviduct explants from estrogenized chicks synthesized little, if any, cholesterol. A number of cholesterol biosynthetic intermediates were observed, with compounds comigrating with squalene and lanosterol accounting for about 50% of the total. Since the estrogenized chick has serum cholesterol levels in the range of 800-900 mg/dl, these results suggest that oviduct has secondary control points which allow it to inhibit cholesterol synthesis when mevalonate is used as the precursor. In support of this hypothesis is the observation that explants from untreated chicks can incorporate mevalonate into cholesterol.

Developmental regulation of glycosyltransferases involved in biosynthesis of asparagine-linked glycoproteins in mouse mammary gland

Three glycosyltransferases, namely N-acetylglucosaminyl-1-phosphotransferase, mannosyltransferase and glucosyltransferase, involved in the biosynthesis of asparagine-linked glycoproteins in the mouse mammary gland, were identified by characterization of the products formed by these enzymes. The tissue capacities of the glycosyltransferases, measured as pmol product formed g tissue-1 min-1, increase during developmental cycle of the gland until, at late lactational stage, they reach more than 34, 14 and 60 times, respectively, the basal level found in the tissue of virgin animals. Among the three enzymes at late lactational stage the specific activities of glucosyltransferase and N-acetylglucosaminyl-1-phosphotransferase increase 7-fold and 4-fold respectively, whereas mannosyltransferase shows a mere 1.4-fold increase during tissue development. All of the enzymes, both in terms of tissue capacity and specific activity, return to the basal levels of the virgin gland one month after the end of lactation. The activities of the three enzymes in the lactating gland decrease precipitously following treatment of mice with bromocriptine, a drug that interferes with the release of prolactin from the pituitary gland and thereby inhibits milk synthesis and secretion. These results indicate that the three glycosyltransferases are developmentally regulated during the growth and differentiation of the mouse mammary gland.

Localization of the lipid intermediate pathway of protein glycosylation in oviduct cell types

Oviduct tissue slices were incubated with [3H]-leucine or [3H]-mannose in the presence and absence of tunicamycin, a specific inhibitor of lipid-mediated protein glycosylation. Conditions were established where tunicamycin had maximal effect on [3H]-mannose incorporation (greater than 90% inhibition) but a minimal effect on [3H]-leucine incorporation (less than 10% inhibition) into total TCA-insoluble products. Analysis of incubated tissues by SDS-polyacrylamide gel electrophoresis revealed that in the absence of tunicamycin, [3H]-mannose was incorporated into only a few proteins, of which ovalbumin represented the major radiolabeled component. Tunicamycin markedly reduced the incorporation of [3H]-mannose into ovalbumin and other oviduct glycoproteins. In contrast, analysis by SDS-polyacrylamide gel electrophoresis showed that [3H]-leucine was incorporated into a variety of proteins in the absence of tunicamycin. The radioactivity profile of some of these proteins was shifted toward lower Mr when oviduct slices were incubated in the presence of tunicamycin, with only a minimal decrease in protein labeling. Light microscopic autoradiograms of tissue incubated with [3H]-leucine in either the presence or absence of tunicamycin exhibited extensive labeling of tubular gland and epithelial cells. In the absence of tunicamycin, these cell types also become markedly labeled with [3H]-mannose; however, incorporation of label in both cell types was substantially reduced in the presence of tunicamycin. Qualitatively, labeling of tubular gland cells appeared greater than that of epithelial cells, largely due to the concentration of silver grains over the dense population of secretory vesicles in the tubular gland cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of dolichyl phosphate-mediated protein glycosylation: estrogen effects on glucosyl transfers in oviduct membranes

Regulation of Glc transfer from UDP-Glc via Glc-P-Dolichol to form Glc3-Man9-oligosaccharide-lipid has been studied during estrogen-induced chick oviduct differentiation. The process was studied as two distinct reactions: transfer of Glc from UDP-Glc to Dol-P, forming Glc-P-Dol; and transfer of Glc from Glc-P-Dol to Man9-OL (oligosaccharide-lipid), forming a series of glucosylated oligosaccharide-lipids. Kinetic analysis of [14C]Glc transfer from UDP-[14C]Glc to endogenous Dol-P shows that Dol-P is limiting in membrane preparations and that, concomitant with estrogen-induced differentiation, there is an increase in Dol-P available for Glc transfers. There is also greater glucosyl transferase activity present in membranes from mature hens and estrogenized chicks than in membranes from immature chicks. In order to study the second phase of glucosylation, transfer to the oligosaccharide, it was necessary to develop an assay in which membranes could be reacted with exogenously added substrates, [14C]Glc-P-Dol and [3H]Man9-OL. This reaction is dependent on detergent (0.02% NP-40 was used) and is stimulated by EDTA. The apparent Km for Glc-P-Dol was about 1.5 microM. A series of double-labeled oligosaccharides having sizes consistent with Glc1-, Glc2-, and Glc3-Man9-OL were formed. Chemical and enzymatic analysis of [14C]Glc oligosaccharides formed by incubation with the exogenous substrates, or by incubation with UDP-[14C]Glc and endogenous acceptors, indicated that the glucosylated oligosaccharides were similar. Assays of membranes from estrogenized chicks, mature hens, and hormone-withdrawn chicks showed increased glucosyl transferase activity upon hormone treatment. Similar assays in the absence of exogenous Man9-OL indicated that hormone treatment was also accompanied by increased levels of endogenous oligosaccharide-lipid acceptors.

Developmental regulation of glycosyltransferases involved in synthesis of N-linked glycoproteins in sea urchin embryos

Previous in vivo studies using drugs that inhibit the N-glycosylation of proteins have demonstrated that newly synthesized N-linked glycoproteins are required for gastrulation in embryos of two species of sea urchins, Strongylocentrotus purpuratus and Arbacia punctulata. To understand the biochemical events regulating glycoprotein synthesis during gastrulation in S. purpuratus embryos, we examined the in vitro activities of enzymes catalyzing several of the early steps in N-linked glycoprotein synthesis. The activities of glycosyl transferases responsible for production of N,N-diacetylchitobiosylpyrophosphoryldolichol and glucosylphosphoryldolichol, two intermediates in the formation of oligosaccharylpyrophosphoryldolichol (the carbohydrate donor for N-glycosylation), were low but detectable in membranes from eggs. After fertilization these activities remained constant or increased slowly up to the blastula stage and thereafter increased rapidly at gastrulation. In agreement with these in vitro findings, in vivo labeling experiments revealed that the rate of incorporation of [3H]Man into oligosaccharylpyrophosphoryldolichol and into protein increased three- to fourfold prior to gastrulation and then slightly more at the prism stage. In contrast, in vitro activity of mannosylphosphoryldolichol synthase, another enzyme in the pathway of N-linked glycosylation, was maximal in membranes from egg and embryos in the early stages of development and declined prior to gastrulation. Furthermore, the level of this activity was at least 100-fold greater than that for enzymes involved in the formation of the chitobiosyl and glucosyl lipids. With the exception of mannosylphosphoryldolichol synthase activity, these data indicate that there is a general activation of the glycosylation apparatus before gastrulation in sea urchin embryos. Possible explanations for the decrease in mannosylphosphoryldolichol synthase activity are discussed.

Oviduct dolichyl phosphate phosphatase: estrogen effects and a possible biosynthetic role

Estrogen-induced chick oviduct differentiation is accompanied by an increased capacity for protein glycosylation. A portion of this increase has been attributed to increased levels of dolichyl phosphate (Dol-P). Hormone withdrawal leads to an apparent decrease in Dol-P. Dol-P metabolism in the oviduct has been studied, and one of the enzymes having a direct effect on Dol-P, Dol-P phosphatase is herein described. Dol-P phosphatase has a pH optimum of 6.0, does not require a metal ion, and is inhibited by Mn2+ at concentrations greater than 5 mM. Inhibitor studies indicate that Dol-P hydrolysis is inhibited by polyprenyl phosphates having both saturated and unsaturated alpha-isoprene residues, but not by the corresponding alcohols. The enzyme is also inhibited by phosphatidic acid unless 2 mM Mn2+ is included in the incubations. Under these conditions Dol-P hydrolysis is only slightly inhibited (less than 10%), but phosphatidate inhibition is totally eliminated. Oviduct membranes also possess phosphatidate phosphatase, but this enzyme is distinct from Dol-P phosphatase based on thermolability, metal ion sensitivity, and sulfhydryl reagent sensitivity. Studies of enzyme activity in response to estrogen treatment reveal that both Dol-P phosphatase and phosphatidate phosphatase have maximal specific activity early in the differentiation process (peaking after 3 days of treatment), and low specific activity in fully differentiated oviducts, including laying hen oviduct. Hormone withdrawal elicits a small increase in specific activity of both phosphatases. The hormone effects suggest that Dol-P phosphatase may be a biosynthetic enzyme.

Differential effects of thyrotropin on various glycosyltransferases in porcine thyroid cells

Porcine thyroid cells were cultured with or without thyrotropin for 9 days. It is known that the hormone enhances the synthesis of thyroglobulin, with an increase in the content and stability of its m-RNA. We show in the present work that thyrotropin also stimulates a number of glycosyltransferases diversely situated along the process of N-glycosylation. The most increased was oligosaccharyltransferase, responsible for attachment to nascent peptides of preformed core carbohydrate sequences. The relatively low activity level of oligosaccharyltransferase and its preferential responsiveness to thyrotropin supports the possibility of a regulation point at this enzyme.