Activity and secretion of sialyltransferase in primary monolayer cultures of rat hepatocytes cultured with and without dexamethasone

B cells suppress medullary granulopoiesis by an extracellular glycosylation-dependent mechanism

The immune response relies on the integration of cell-intrinsic processes with cell-extrinsic cues. During infection, B cells vacate the marrow during emergency granulopoiesis but return upon restoration of homeostasis. Here we report a novel glycosylation-mediated crosstalk between marrow B cells and hematopoietic progenitors. Human B cells secrete active ST6GAL1 sialyltransferase that remodels progenitor cell surface glycans to suppress granulopoiesis. In mouse models, ST6GAL1 from B cells alters the sialylation profile of bone marrow populations, and mature IgD+ B cells were enriched in sialylated bone marrow niches. In clinical multiple myeloma, ST6GAL1 abundance in the multiple myeloma cells negatively correlated with neutrophil abundance. These observations highlight not only the ability of medullary B cells to influence blood cell production, but also the disruption to normal granulopoiesis by excessive ST6GAL1 in malignancy.

Circulating blood and platelets supply glycosyltransferases that enable extrinsic extracellular glycosylation

Glycosyltransferases, usually residing within the intracellular secretory apparatus, also circulate in the blood. Many of these blood-borne glycosyltransferases are associated with pathological states, including malignancies and inflammatory conditions. Despite the potential for dynamic modifications of glycans on distal cell surfaces and in the extracellular milieu, the glycan-modifying activities present in systemic circulation have not been systematically examined. Here, we describe an evaluation of blood-borne sialyl-, galactosyl- and fucosyltransferase activities that act upon the four common terminal glycan precursor motifs, GlcNAc monomer, Gal(β3)GlcNAc, Gal(β4)GlcNAc and Gal(β3)GalNAc, to produce more complex glycan structures. Data from radioisotope assays and detailed product analysis by sequential tandem mass spectrometry show that blood has the capacity to generate many of the well-recognized and important glycan motifs, including the Lewis, sialyl-Lewis, H- and Sialyl-T antigens. While many of these glycosyltransferases are freely circulating in the plasma, human and mouse platelets are important carriers for others, including ST3Gal-1 and β4GalT. Platelets compartmentalize glycosyltransferases and release them upon activation. Human platelets are also carriers for large amounts of ST6Gal-1 and the α3-sialyl to Gal(β4)GlcNAc sialyltransferases, both of which are conspicuously absent in mouse platelets. This study highlights the capability of circulatory glycosyltransferases, which are dynamically controlled by platelet activation, to remodel cell surface glycans and alter cell behavior.

Altered sialylation on the cell-surface proteins of dexamethasone-treated human macrophages contributes to augmented uptake of apoptotic neutrophils

Macrophages eliminate apoptotic granulocytes before their secondary necrosis during resolution of inflammation. A well-known glucocorticoid, the anti-inflammatory dexamethasone augments phagocytosis capacity of macrophages with a so far not fully clarified mechanism. We have found that sialylation of cell-surface proteins on human macrophages is markedly altered by dexamethasone. Compared to non-treated cells, dexamethasone-treated macrophages can bind significantly less Sambucus nigra lectin specific for sialic acids on their surfaces as a result of undersialylation of annexin-II and an HLA-II protein. Non-treated macrophages covered by S. nigra lectin had increased uptake of apoptotic cells; however, the significantly higher phagocytosis capacity of dexamethasone-treated macrophages could not be stimulated further this way. Our results suggest that dexamethasone treatment leads to decreased number of sialic acids on the surfaces of human macrophages promoting recognition and uptake of apoptotic cells.

The immunomodulatory action of dexamethasone on monoclonal antibody-producing hybridoma cells

As found in different studies, glucocorticoid hormones (GCs) as well as interleukin-6 (IL-6) are involved in the modulation of protein glycosylation. In this work we have investigated the immunomodulatory effect of dexamethasone by assessing in vitro IgG glycosylation by monoclonal antibody-producing hybridoma cells. As described in myeloma cell lines, cellular viability and proliferation rates of hybridoma 112D5 cells decrease when cultured with dexamethasone during 24 hours, in a dose-dependent way. Moreover, the corticosteroid triggered apoptosis of the hybridoma, which was observed as soon as 4 h after culturing cells in the presence of the drug. In line with these results, after 24 h, dexamethasone induced a drop in the anti-DNP level of antibodies synthesized by hybridoma 112D5. In previous works we described that asymmetric glycosylation of in vitro synthesized IgG correlated with induction of cell damage. Nevertheless, an increase in asymmetric IgG glycosylation was not observed here, but there was a decrease in the proportion of asymmetrically glycosylated IgG synthesized by the hybridoma after a 4-h culture with the drug. Finally, as results from assessing IL-6 production by ELISA, we conclude that the above described effects of dexamethasone on hybridoma 112D5 cells could not be due to the inhibition of IL-6 synthesis exerted by the corticoid but rather to a direct effect of the drug. Monoclonal antibody (MAb) producing hybridomas provide an excellent in vitro model for the study of the molecular mechanisms involved in immunoglobulin glycosylation.

Murine hepatic beta-galactoside alpha 2,6-sialyltransferase gene expression involves usage of a novel upstream exon region

ST6Gal I (beta-galactoside alpha 2,6-sialyltransferase, SiaT-1, ST6N, EC 2.4.99.1) mediates the attachment of the alpha 2,6-sialyl linkage common on N-linked glycans. Previous work suggests substantial inter-species conservation in SIAT1, the gene encoding ST6Gal I. In human and in rat, hepatic-specific SIAT1 transcription is initiated at Exon I. Here we report a surprising departure in the structural organization of the murine ST6Gal I gene. By a combination of primer extension analysis, 5'-RACE analysis, and analysis of genomic sequences, we show that the murine hepatic ST6Gal I mRNA contains a novel region 5' of Exon I. This novel sequence is encoded on a discrete upstream exon, Exon H. In contrast to human and rat hepatic ST6Gal I, the murine mRNA is transcriptionally initiated at the start of Exon H. Differential mRNA blot analysis indicates that transcripts containing Exon H sequences are preferentially expressed in liver.

Specific stimulation of alpha 2-6 sialyltransferase activity by a novel cytosolic factor from rat colon

A factor present in the 100,000 g supernatant from the homogenate of rat colon stimulated the activity of purified Gal beta 1-4GLcNAc alpha 2,6 sialyltransferase [alpha 2-6ST(N)] from rat liver and alpha 2-6ST(N) from either liver microsomes or Golgi membrane. The stimulation of alpha 2-6ST(N) activity by the colon factor using protein acceptors was about four-fold and highly reproducible when the reaction product of the alpha 2-6ST(N) was assayed by either precipitation or affinity chromatography. In contrast, the colon factor did not stimulate the Gal beta 1-4GlcNAc alpha 2,3 sialyltransferase [alpha 2-3ST (N)], from rat jejunum microsomes or purified Gal beta 1-3GalNAc alpha 2,4 sialyltransferase [alpha 2-3ST (O)] from porcine liver, of purified beta 1,4 galactosyltransferase (GT) from bovine milk. In addition to rat colon, the 100,00 g supernatant from the homogenates of rat brain and kidney also stimulated the alpha 2-6ST(N) activity. The stimulation of alpha 2-6ST(N) by the colon factor resulted in a decrease in the Km (by about two-fold) and an increase in Vmax (about 2- to 3-fold) for desialylated alpha 1 acid glycoprotein and CMP-[14C]N-acetylneuraminic acid. The stimulation of alpha 2-6ST(N) activity by the colon factor was temperature dependent, protease sensitive and was inhibited by CTP, but did not need the presence of either metal ions or detergent. The cytosolic factor was partially purified by ion-exchange chromatography with the retention of the activator activity in the peaks containing low molecular weight proteins, but the activity was lost on attempts to further purification. A specific marked stimulation of the alpha 2-6ST(N) activity by cytosolic factors in certain tissues might suggest a physiological role for these factors in the regulation of alpha 2-5ST(N) activity.

A missense mutation in the FUT6 gene results in total absence of alpha3-fucosylation of human alpha1-acid glycoprotein

The major alpha3-fucosyltransferase activity in human plasma is encoded by the gene for fucosyltransferase VI (FUT6). A missense mutation (Gly-739 --> Ala) in this gene is responsible for deficiency of enzyme activity in plasma. To examine whether this fucosyltransferase is the sole enzyme responsible for the alpha3-fucosylation of serum glycoproteins in the liver, we studied the fucosylation of three glycoproteins in sera of individuals with or without inactivated FUT3 and/or FUT6 gene(s) but with a functional FUT5 gene. alpha1-Acid glycoprotein was used as the principal reporter protein for liver alpha3-fucosyltransferase activity, because of its high fucose content. In all individuals with the FUT6 missense mutation Gly-739 --> Ala in double dose, no fucosylation of alpha1-acid glycoprotein was found. This alpha1-acid glycoprotein was not intrinsically resistant to fucosylation, since it was susceptible to in vitro fucosylation using an alpha3/4-fucosyltransferase isolated from human milk. The same result was found for alpha1-antichymotrypsin and alpha1-protease inhibitor. On the other hand in all individuals with alpha3-fucosyltransferase activity in plasma, alpha3-fucosylated glycoforms of the glycoproteins studied were found. The degree of fucosylation of alpha1-acid glycoprotein was correlated with alpha3-fucosyltransferase activity (Rs = 0.82). These data indicate that the product of FUT6, but not of FUT3 or of FUT5, is responsible for the alpha3-fucosylation of glycoproteins produced in liver and suggest that this organ is a major source of alpha3-fucosyltransferase activity in plasma.

Interleukin-6-type cytokine-induced changes in acute phase protein glycosylation

The plasma levels and the glycosylation of acute-phase proteins (APP) are subject to marked changes during acute and chronic inflammation. The pathophysiological variations in different glycoforms of APP in serum most likely result from changes in the glycosylation process during their biosynthesis in the parenchymal cells of the liver. This is suggested from in vitro studies with isolated hepatocytes and hepatoma cell lines. Inflammatory cytokines appear to regulate the changes in glycosylation independent from the rate of synthesis of the APP. In addition, other humoral factors like corticosteroids and growth factors are involved. The interplay of these factors is determined by the stage of the disease (as in rheumatoid arthritis) or the physiological situation (as in pregnancy). The changes in glycosylation of specific APP might affect the operation of the immune system.

Alpha 2,6-sialyltransferase predominates in cultured jejunum of suckling rats: it is up-regulated by dexamethasone and secreted during cultivation

The alpha 2,6-sialyltransferase (alpha 2,6-ST) acting on N-acetyllactosamine is the major sialyltransferase of suckling rat jejunum. Jejunal explants of 7-day-old rats maintained in serum-free or serum-containing organ culture secreted alpha 2,6-ST into the cultivation medium. Dexamethasone (80 nM) stimulates primarily the secreted pool of alpha 2,6-ST. Fetal calf serum promotes the stimulatory effect of dexamethasone also on the bound form of alpha 2,6-ST.

Role of antiproteolytic heparin-binding serum protein(s) in modulating the levels of sialyl- and galactosyltransferase activity released during the incubation of rat jejunal slices

1. Sialyltransferase released into the medium during the incubation of rat jejunal slices in serum-free buffer, was susceptible to proteolytic degradation. Heat inactivated horse serum or its antiproteolytic heparin-binding fraction was found to be necessary in determining the activity of sialyltransferase released (Nadkarni et al., 1991). 2. In the present study, we have shown that heat inactivated rat serum (HRS) or its antiproteolytic heparin-binding fraction (HBF) had a role in determining the sialyltransferase activity released during jejunal slice incubations. 3. Galactosyltransferase was also released during incubations, but was not proteolytically degraded and the presence of HRS or HBF in incubations did not alter the levels of galactosyltransferase activity released. 4. Trypsin activity in serum-free incubation medium was higher compared to medium containing HRS. 5. Addition of serum-free medium obtained from 4 hr incubations of the jejunal slices, to medium obtained from parallel incubations done in the presence of HRS, caused inhibition of sialyl- but not galactosyltransferase activity. 6. In jejunal homogenates stored at -20 degrees C, sialyltransferase activity was decreased during 0-45 days of storage, whereas galactosyltransferase activity remained fairly stable for upto 56 days. 7. Inclusion of HRS or HBF in homogenates resulted in higher sialyl- but not galactosyltransferase activity compared to serum-free homogenate samples. 8. The results suggest that HRS or its antiproteolytic heparin-binding proteins have a role in determining the sialyltransferase activity released from the jejunal slices. In contrast galactosyltransferase released was not susceptible to proteolysis, and HRS or HBF was not required to express its activity.

Sialyltransferase: a novel acute-phase reactant

1. Proteins that are released into the circulation in elevated amounts in injured mammals are referred to as acute-phase reactants. Most are liver synthesized glycoproteins of the secretable type. However, Gal-beta(1-->4)-GlcNAc-alpha(2-->6)-sialyltransferase (EC 2.4.99.1) is a novel acute-phase reactant since it is a Golgi membrane-bound enzyme rather than a secretable glycoprotein. 2. The role of glucocorticoids and cytokines in the control of synthesis and expression of acute-phase glycoproteins, including sialyltransferase, is discussed. 3. The acute-phase behaviour of Gal-beta(1-->4)-GlcNAc-alpha(2-->6)-sialyltransferase is dependent on the release of the enzyme from the Golgi in the acute-phase state. The mechanism of release of a catalytically active form of the enzyme is described.

Inflammation-induced expression of sialyl Lewis X-containing glycan structures on alpha 1-acid glycoprotein (orosomucoid) in human sera

The glycosylation of the acute phase glycoprotein alpha 1-acid glycoprotein (AGP) in human sera is subject to marked changes during acute inflammation as a result of the cytokine-induced hepatic acute phase reaction. The changes described thus far comprise alterations in the type of branching of the carbohydrate structures as revealed by increased reactivity of AGP with concanavalin A. We now report on acute inflammation-induced increases in alpha 1-->3-fucosylated AGP molecules, as detected by the reactivity of AGP towards the fucose-binding Aleuria aurantia lectin (AAL) in crossed affino-immunoelectrophoresis of human sera. Laparotomy of women, for the removal of benign tumors of the uterus, was used as a model for the development of the hepatic acute phase response. Hugh increases were detected in the amounts of strongly AAL-reactive fractions of AGP, presumably containing three or more fucosylated N-acetyllactosamine units. At least part of these Lewis X-type glycans (Gal beta 1-->[Fuc alpha 1-->3]GlcNAc-R) appeared to be substituted also with an alpha 2-->3-linked sialic acid residue. This was revealed by the laparotomy-induced abundant staining of AGP with an antisialyl Lewis X monoclonal antibody (CSLEX-1) on blots of sodium dodecyl sulfate-polyacrylamide gels containing AGP isolated from the sera of a patient at various days after operation. It is concluded that acute inflammation induces a strong increase in sialyl Lewis X-substituted AGP molecules that persists at a high level throughout the inflammatory period. We postulate that these changes represent a physiological feedback response on the interaction between leukocytes and inflamed endothelium, which is mediated via sialylated Lewis X structures and the selectin endothelial-leukocyte adhesion molecule 1.

Heparin-binding serum protein(s) is required for the protection of sialyltransferase released during the incubation of rat jejunal slices

Incubation of rat jejunal slices in Krebs-Ringer bicarbonate buffer (KRB) required the presence of heat-inactivated horse serum (HHS) in order to show time-dependent release of sialyltransferase into the medium. Sialyltransferase activity could not be detected in the medium when KRB alone or KRB supplemented with either albumin or glycerol was used in the incubations. The viability of the jejunal slices for up to 4 h of incubation was determined by studying the incorporation of glucosamine and leucine into acid-insoluble proteins. Supplementation of KRB with HHS had no beneficial effect on the rate of incorporation of leucine and glucosamine into proteins. KRB medium obtained after different periods of incubation contained higher trypsin-like activity than KRB medium containing HHS. Various antiproteases present as supplements to KRB resulted in the release of sialyltransferase activity from the jejunal slices. Among these antiproteases, alpha 1-proteinase inhibitor (alpha 1-PI) was the most effective. Also, HHS added to KRB immediately following incubation resulted in partial restoration of sialyltransferase activity in the medium, suggesting the presence of anti-proteolytic factors in HHS. The addition of increasing concentrations of heparin to incubations containing HHS caused a decrease in the medium sialyltransferase activity. The heparin-binding fraction (HBF) from HHS, when added to incubations, was able to protect the sialyltransferase released into medium. However, HHS depleted of its heparin-binding fraction by heparin-agarose affinity chromatography was unable to protect the sialyltransferase. HBF was separated into high- and low-molecular-mass fractions (fractions A and B respectively) by gel-filtration chromatography. The capacity to protect the released sialyltransferase was contained in fraction B. Fraction A contained multiple bands on SDS/PAGE and did not protect the enzyme. Fraction B contained a major protein band on the gel which corresponded to the migration of a similar band in human alpha 1-PI. HBF as well as fraction B isolated from HHS showed anti-trypsin-like activity. The results presented indicate that HHS contains a heparin-binding protein(s) similar to human alpha 1-PI which plays a role in the protection of sialyltransferase released from jejunal slices.

Stimulation of release of Gal beta 1-4GlcNAc alpha 2-6 sialyltransferase from the FAZA hepatoma cell line by dexamethasone and phorbol ester

1. Gal beta 1-4GlcNAc alpha 2-6 sialyltransferase was assayed in FAZA hepatoma cells and the cell culture medium following growth of cells in presence of dexamethasone and phorbol ester. 2. There was about a seven-fold increase in sialyltransferase activities in cells and medium in presence of dexamethasone with the maximum effect occurring at 10(-6)-10(-7) M dexamethasone. 3. The presence of 10(-6) M phorbol ester in the culture medium increased sialyltransferase activities in cells and medium by ca 40% over the values found with dexamethasone alone. 4. The use of the FAZA hepatoma cell line for studies on sialyltransferase is compared with the primary hepatocyte system reported on earlier (Woloski et al., 1986).

Effect of hydrocortisone on sialyltransferase activity in the rat small intestine during maturation. Changes along the villus-crypt axis and in fetal organ culture

Sialyltransferase activity was assayed in rat intestinal cells isolated as fractions reflecting the villus-crypt axis of differentiation. In 13-day-old rats both endo- and exogenous sialyltransferase activity reached their maximum in undifferentiated crypt cells and their peaks overlapped. In contrast, sialyltransferase of the adult intestine was 4-fold lower than that of sucklings in the crypts, with slight tendency to be transferred to the villus cells. Hydrocortisone applied to 10-day-old rats caused three days later a precocious drop of sialyltransferase activity in the crypt cells. Unlike in vivo, glucocorticoid responsiveness was accompanied by increased sialyltransferase activity in fetal small intestine cultivated for 17 days.