Incorporation in vitro of [3H]glucosamine or [3H]glucose and [35S]SO42- into rat gastric mucosa. Presence of N-acetylhexosamine mono- and disulfates and galactose monosulfate in glycoprotein

Rebamipide increases the amount of mucin-like substances on the conjunctiva and cornea in the N-acetylcysteine-treated in vivo model

PURPOSE

Rebamipide increases the amount of mucin-like substances in the stomach. We aimed to determine the effects of rebamipide on the amount of mucin-like substances in the conjunctiva and cornea of N-acetylcysteine-treated eyes. Furthermore, we attempted to evaluate the effects of rebamipide on the wound healing of N-acetylcysteine-treated eyes.

METHODS

The model was created by instilling 10% N-acetylcysteine solutions into rabbit eyes. Rebamipide was then applied on the day following the completion of N-acetylcysteine treatment. The amount of mucin-like substances on the conjunctiva and cornea was measured using the Alcian-blue binding method. The degree of damage was evaluated using scores based on the areas and densities of the cornea and conjunctival after staining using a rose Bengal solution under blind conditions.

RESULTS

Rebamipide increased the level of mucin-like substances on the conjunctiva of N-acetylcysteine-treated eyes when instilled at concentrations of 0.3% or higher, and 1% rebamipide increased the amount of mucin-like substances covering the cornea. Moreover, 1% rebamipide improved the rose Bengal scores of the cornea and conjunctiva in N-acetylcysteine-treated eyes.

CONCLUSIONS

Rebamipide increased mucin-like substances on the cornea and conjunctiva of N-acetylcysteine-treated eyes. In accordance with the mucin-increasing effects, rebamipide improved the rose Bengal scores for the cornea and conjunctiva of N-acetylcysteine-treated eyes. However, the relevance of these findings to dry eyes is unclear because it is not known whether the change in mucus expression in the N-acetylcysteine model is similar to what occurs in aqueous tear deficiency. Consequently, it may be worth trying on an animal model of keratoconjunctivitis sicca.

Synthesis and utility of sulfated chromogenic carbohydrate model substrates for measuring activities of mucin-desulfating enzymes

A chromogenic substrate, 4-nitrophenyl 2-acetamido-2-deoxy-beta-D-glucopyranoside 6-sodium sulfate was synthesized and used in combination with beta-N-acetylhexosaminidase for detection of the sulfatase, MdsA, by release of 4-nitrophenol. MdsA was originally isolated from the bacterium Prevotella strain RS2 and is believed to be involved in desulfation of sulfomucins, major components of the mucus barrier protecting the human colon surface. The exo nature of the MdsA sulfatase was indicated by its inability to de-esterify the disaccharide 4-nitrophenyl beta-D-galactopyranosyl-(1-->4)-2-acetamido-2-deoxy-beta-D-glucopyranoside 6-sodium sulfate. This latter compound was prepared from monosaccharide precursors by two different methods, the shorter requiring just six steps from 4-nitrophenyl 2-acetamido-2-deoxy-beta-D-glucopyranoside and giving an overall yield of 26.4%. The syntheses of 4-nitrophenyl beta-D-galactopyranoside 3-triethylammonium sulfate and 6-triethylammonium sulfate and their use in combination with beta-galactosidase as chromogenic substrates for detecting Bacteroides fragilis sulfatases with different specificities was also demonstrated.

Sulphated macromolecules produced by in vivo labelling in the rat gastric mucosa

The aim of this study was to investigate the nature and distribution of sulphated macromolecules of the extracellular matrix in rat gastric mucosa. This was achieved by developing an in vivo labelling system. An intraperitoneal injection of 1 mCi [35S]-sulphate was given for either 4 h (0.01% incorporation into macromolecular fraction) or 8 h (0.13% incorporation). At the end of the labelling period the stomach was removed and the mucosa and submucosa was either taken as a single combined sample or separated into four layers by blunt dissection. Each sample was papain digested and analysed by ion-exchange chromatography. This analysis revealed sulphated species of differing charge existing in differing proportions throughout the mucosa. These sulphated species eluted at NaCl concentrations of approximately 0 (A), 0.19 (B), 0.34 (C) and 0.78 mol/L (D) from a Q-Sepharose ion exchange column. Further analysis by size exclusion chromatography and chemical and enzymatic digestion showed that peaks B and C had molecular weights of 2.4 x 10(5) and 2.8 x 10(5), respectively and were resistant to chondroitinase ABC, heparitinase and nitrous acid digestion. Peak D was found to contain a polydisperse population of molecules with a molecular weight range of approximately 1 x 10(4) to 6 x 10(4). This sample was susceptible to nitrous acid and chondroitinase ABC digestion and was found predominantly in the sample isolated from deeper in the tissue. We have thus developed an in vivo labelling technique for sulphated macromolecules that can be used in the further study of injury to the gastric mucosa.

Colorectal mucin histochemistry in health and disease: a critical review

Neoplastic, inflammatory and regenerative processes affecting colorectal mucosa are associated with alterations in structure of epithelial mucin. This review collates mucin-, lectin-, and immuno-histochemical observations on colorectal mucins and introduces recent molecular genetic insights into the structure of the protein backbone of mucins. The numerous structural modifications uncovered by the various technical approaches have been reduced to a few manageable principles that are of relevance to both researcher and diagnostic pathologist. Particular attention is drawn to the need to appreciate the limited specificities of probes, the confounding influences of anatomical site and genetic factors (necessitating the use of appropriate positive and negative control tissues) and the precise location of secretory material. In the past, insufficient attention has been given to the effects of altered differentiation including metaplasia and differing lineage expression in epithelial disorders of growth. It is likely that certain changes loosely ascribed to goblet cell mucin, such as neo-expression of blood group antigens and anomalous expression of core carbohydrate structures, do not occur at all. Critical examination of available data point to only two consistent and unequivocal changes affecting goblet cell mucin in pathological processes: loss of O-acetyl substituents at sialic acid C4 and C7,8,9 and increased sialylation. Furthermore, there are no neoplasia-specific alterations in mucins documented to date. All neoplasia-associated changes have been described in non-neoplastic lesions also.

Effect of ebrotidine on the synthesis and secretion of gastric sulfomucin

1. The effect of a new antiulcer agent, ebrotidine, on the synthesis and secretion of sulfomucin in gastric mucosa was investigated. Rat gastric mucosal segments were incubated in DMEM containing [3H]proline, [3H]glucosamine and [35S]Na2SO4 as markers for mucin synthesis, glycosylation and sulfation, in the presence of 0-150 microM ebrotidine. 2. The drug, while showing no discernible effect on the apomucin synthesis, evoked a dose-dependent increase in mucin glycosylation and sulfation, which at 100 microM ebrotidine, attained its maximum of 2.4 and 2.7-fold stimulation, respectively. 3. The analysis of mucin secretory responses revealed that ebrotidine caused a concentration-dependent enhancement in sulfomucin secretion which attained its maximum increase of 3.3-fold at 100-120 microM ebrotidine. Furthermore, the sulfomucin elaborated in the presence of ebrotidine exhibited a higher content of a large molecular-weight mucus glycoprotein form, the assembly of which is intimately associated with the sulfation event. 4. The results suggest that the ability of ebrotidine to enhance gastric sulfomucin synthesis and secretion may play an important role in the gastroprotective mechanism of action of this agent.

Nitecapone effect on the synthesis and secretion of gastric sulfomucin

1. The effect of a new antiulcer agent, nitecapone, on the synthesis and secretion of sulfomucin in gastric mucosa was investigated using mucosal segments incubated in the presence of [3H]proline, [3H]glucosamine and [35S]sulfate. 2. The drug, while showing no discernible effect on the apomucin synthesis, evoked a dose-dependent increase in mucin glycosylation and sulfation, which at 225 microM nitecapone, attained its maximum of 1.8 and 2.2-fold stimulation, respectively. 3. The analysis of mucin secretory responses revealed that nitecapone caused a concentration-dependent enhancement in sulfomucin secretion attaining maximum increase of 1.5-fold at 150 microM nitecapone. 4. The stimulatory effect of nitecapone on sulfomucin secretion was accompanied by 1.4-fold increase in mucosal cAMP level, and showed sensitivity to protein kinase A inhibitors, thus pointing towards the involvement of protein kinase A in mediation of gastric sulfomucin secretory responses to nitecapone. 5. The ability of nitecapone to enhance sulfomucin synthesis and secretion could be of importance to the gastroprotective action of this agent.

Role of sulfation in post-translational processing of gastric mucins

1. Gastric mucosal segments were incubated in MEM supplemented with various sulfate concentrations in the presence of [3H]glucosamine, [3H]proline and [35S]Na2SO4, with and without chlorate, an inhibitor of 3'-phosphoadenosine-5'-phosphosulfate formation. 2. Incorporation of glucosamine and sulfate depended upon the sulfate content of the medium and reached a maximum at 300 microM sulfate. Introduction of chlorate into the medium, while having no effect on protein synthesis as evidenced by proline incorporation, caused, at its optimal concentration of 2 mM, a 90% decrease in mucin sulfation and a 40% drop in glycosylation. 3. At low sulfate content in the medium and in the presence of chlorate, the incorporation of sulfate and glucosamine was mainly into the low molecular-weight form of mucin. An increase in sulfate in the medium caused an increase in the high molecular-weight form of mucin and in the extent of sulfation in its carbohydrate chain. 4. The results suggest that the sulfation process is an early event taking place at the stage of mucin subunit assembly and that sulfate availability is essential for the formation of the high molecular-weight mucin polymer.

Glycosulfatase activity of helicobacter pylori toward gastric mucin

A glycosulfatase activity toward sulfated gastric mucus glycoprotein was identified in the extracellular material elaborated by H. pylori, a bacteria implicated in the etiology of gastric disease. Upon acetone precipitation, an active enzyme fraction at 64% acetone was obtained which on SDS-PAGE gave a major 30kDa protein band. The H. pylori glycosulfatase exhibited maximum activity (314.8 pmol/mg protein/h) at pH 5.7 in the presence of Triton X-100 and CaCl2, and was capable of removal of the sulfate ester groups situated at C-6 of N-acetylglucosamine, galactose and glucose. However, the enzyme was ineffective toward galactosylceramide and lactosylceramide sulfates which contain the sulfate ester group on C-3 of galactose. The results suggest that H. pylori is capable of overcoming the interference by sulfated mucus glycoprotein with its colonization of gastric mucosa.

Mucin-type glycoproteins

Considerable advances have been made in recent years in our understanding of the biochemistry of mucin-type glycoproteins. This class of compounds is characterized mainly by a high level of O-linked oligosaccharides. Initially, the glycoproteins were solely known as the major constituents of mucus. Recent studies have shown that mucins from the gastrointestinal tract, lungs, salivary glands, sweat glands, breast, and tumor cells are structurally related to high-molecular-weight glycoproteins, which are produced by epithelial cells as membrane proteins. During mucin synthesis, an orchestrated sequence of events results in giant molecules of Mr 4 to 6 x 10(6), which are stored in mucous granules until secretion. Once secreted, mucin forms a barrier, not only to protect the delicate epithelial cells against the extracellular environment, but also to select substances for binding and uptake by these epithelia. This review is designed to critically examine relations between structure and function of the different compounds categorized as mucin glycoproteins.

Nonenzymatic modifications of amino sugars in vitro

Browning reactions of amino sugars were observed in a variety of sterile pH buffers at 25-37 degrees C. These reactions were signaled by an increase in absorbance at 273 nm, followed by an increase in absorbance at 320-360 nm. The reactions were maximal at pH 7.0 in phosphate buffer. Acidic solutions (pH less than 2.2) of 50 mM D-glucosamine hydrochloride gave only a negligible reaction and 2-acetamido-2-deoxy-D-glucose was unreactive. Half of the D-glucosamine in a 100 mM solution in sterile 0.2 M sodium phosphate buffer, pH 7.4, at 37 degrees C decomposed or was transformed in 27 h. A comparison of reactivity in generating A273 and A340 chromophores showed D-mannosamine greater than D-galactosamine greater than D-glucosamine. Permanganate oxidation of incubated glucosamine solutions afforded a compound which chromatographed like 2,5-pyrazinedicarboxylic acid and gave the same ultraviolet absorption spectrum. This, together with fractionating and thin-layer chromatography of the products of glucosamine incubation, suggests that 2,5-bis(tetrahydroxybutyl)pyrazine is formed as one of the products of autocondensation of D-glucosamine in accord with the report of Candiano et al. (1988, Carbohydr. Res. 184, 67-75) on products formed in glucosamine-lysine incubation mixtures. Formation of products absorbing at 325-360 nm was inhibited by the chelator diethylene-triaminepentaacetic acid. This suggests that the later reactions may be mediated by a metal-stimulated free radical mechanism. After 4 days incubation high molecular weight products with absorbance maxima at 273 nm and 325-360 nm were detected. Some of these were retained by dialysis membranes of molecular weight cut-off greater than 3500 and greater than 12,000.

Sulphation causes heterogeneity of gastric mucins

The synthesis of mucus glycoprotein in rat stomach was studied in stomach segments, which were pulse-labelled with both [3H]galactose and [35S]sulphate and chased for various times. The radioactive glycoproteins were analyzed by CsCl centrifugation and by agarose gel electrophoresis. After a pulse-labelling for 15 min with [3H]galactose, a possible intermediate with an Mr of 200,000 and a buoyant density of 1.60 g/ml could be demonstrated. Following chase periods of 1 and 4 h, [3H]galactose and [35S]sulphate were present in glycoproteins with a mean buoyant density of 1.50 g/ml. This is clearly different from the main density of glycoproteins isolated from mucosal scrapings (1.46 g/ml). Another difference is the high electrophoretic mobility on gel electrophoretic analysis of newly synthesized glycoproteins compared to that of the major portion of the glycoproteins from mucosal scrapings. When sulphation of glycoproteins was inhibited by sodium chlorate, electrophoretic mobility and buoyant density both decreased. Sodium chlorate had no effect on glycoprotein synthesis nor on glycoprotein secretion. We conclude from our data that the heterogeneity in electrophoretic mobility and buoyant density can be attributed to a different degree of sulphation of the same glycoprotein.

Prostaglandin effect on the enzymatic sulfation of mucus glycoprotein in gastric mucosa

The effect of 16,16-dimethyl prostaglandin E2 (DMPGE2) on the sulfation of mucus glycoprotein in gastric mucosa was investigated. The enzymatic activity which catalyzes the transfer of the sulfate ester group from 3'-phosphoadenosine-5'-phosphosulfate to gastric mucus glycoprotein was located in the detergent extracts of Golgi-rich membrane fraction of antral and body mucosa of rat stomach. The sulfotransferase activity of this fraction from body mucosa, however, was 35% higher than that from the antrum. The enzyme exhibited optimum activity at pH 6.8 using 0.5% Triton X-100 and 30 mM NaF. The apparent Km of the enzyme for sulfation of mucus glycoprotein was 10.5 microM, and the sulfate ester was found incorporated into the carbohydrate chains of the glycoprotein. Introduction of DMPGE2 to the reaction mixtures led to an enhancement in the rate of mucus glycoprotein sulfation. The rate of enhancement was proportional to the concentration of DMPGE2 up to 1.0 x 10(-4) M and was of the competitive type, with an apparent Km value of 6.7 microM. Since sulfated mucins play an important role in gastric mucosal defense and the increase in their sulfation occurred at levels of prostaglandin present in gastric mucosa, the observed effect may be of significance to gastric mucosal defense in vivo.

Changes in the Incorporating Activity of 35S-Sulfate into Gastric Sulfated Glycoproteins in the Rat with Erosions by Restraint and Water Immersion Stress

The synthetic activity of rat gastric sulfated glycoproteins (SGP) in vitro was investigated at various time intervals after water immersion stress using 35S-sulfate as a precursor. More than 90 percent of the total radioactivity was incorporated into mucosal SGP, and the rest was incorporated into glycosaminoglycans in the gastric muscular layer. The incorporation of 35S-sulfate into SGP increased at 2 hr and decreased at 6 hr after the onset of stress. The incorporating activity again increased markedly at 12 hr and then recovered to the normal level at 24 hr after the onset of stress. An anti-ulcer agent, N-(N-acetyl-beta-alanyl)-L-histidine aluminum complex (AAHA), significantly increased the SGP synthetic activity at 12 hr and at 24 hr after the onset of stress. It was indicated from the elution patterns on the DEAE-cellulose column that AAHA increased the amount of highly sulfated glycoproteins compared with the stress control at 12 hr after the onset of stress. The uronic acid content in the gastric muscular layer of the rat was unchanged with stress. These results in the in vitro experiment indicate that the SGP synthetic activity does not decrease with stress load, but rather increases at 2 hr and at 12 hr after the onset of stress when a sufficient amount of 35S-sulfate is supplied. Accordingly, it is suggested that SGP facilitates the restoration of the gastric mucosal damage caused by stress.

Isolation of three species of proteoglycan synthesized by cloned bone cells

Three proteoglycan fractions have been isolated from clonal populations of osteoblast-like cells derived from fetal rat calvaria. One of these is secreted into the culture medium, is of apparent Mr 350 000, and has a glycosaminoglycan (GAG) composition of 77% chondroitin sulfate (CS) and 20% dermatan sulfate (DS). The remaining two proteoglycan fractions are associated with the cell layer. One of these has an apparent molecular weight of approximately 250 000 and a GAG composition of 54% CS and 40% DS. Both this species and the secreted proteoglycan have GAG chains of Mr 25 000. The other cell-associated proteoglycan contains heparan sulfate (HS), is solubilized by detergents, and appears to be contaminated with a CS proteoglycan. This HS-containing species may be similar to plasma membrane proteoglycans that have been isolated from several other cell types. Rat calvarial clones also synthesize hyaluronic acid and a number of glycoproteins.

The site of sulfated glycoprotein biosynthesis in rat gastric mucosa

The site of the synthesis of the sulfated glycoproteins in rat gastric mucosa using organ culture system has been studied. About 98% of the 35S radioactivity associated with the glycoproteins was found in corpus region. The sulfated glycoproteins were synthesized mainly in corpus, poorly in antrum and none in forestomach. The synthesized sulfated glycoproteins were heterogeneous by equilibrium centrifugation in CsCl density gradients. The 35S-labeled oligosaccharides released from the glycoproteins under alkaline condition were composed of 4-8 sugar residues.