Synthesis and secretion of glycoproteins by mucosa of rat gastric antrum in organ culture

Adaptation of the gastric epithelium to injury is maintained in vitro and is associated with increased TGF-alpha expression

Adaptation is the name given to the progressive decrease in gastric mucosal damage following repeated dosing with damaging agents. This study aimed to determine whether adaptation is an intrinsic property of the gastric epithelium and the role in the development of this process of TGF alpha. Rats were given either one or six daily doses of 10 mg/kg diclofenac or six daily doses of vehicle only (1% methylcellulose). On the 7th day, antral mucosa was taken for organ culture and loaded with [51Cr]. Explants were challenged with ethanol and damage quantified by [51Cr] release. In a separate experiment, rats were dosed as above and the gastric mucosa was extracted and TGF alpha quantified by RIA. The rate of [51Cr] release was significantly lower after ethanol injury in explants from rats previously adapted to diclofenac (9.2 +/- 2.5%) compared with those exposed to a single damaging dose of diclofenac (25.9 +/- 3.5%) or vehicle only (26.4 +/- 3.3%; P < 0.01; ANOVA). The concentration of TGF alpha was significantly higher in the gastric epithelium of rats adapted to diclofenac than other groups (P < 0.05; t-test). Cross adaptation of the gastric mucosa to injury has therefore been demonstrated in antral mucosal explants in organ culture while TGF alpha peptide expression is elevated in the adapted gastric mucosa. These findings suggest that adaptation is an inherent property of the gastric epithelium and it is likely that TGF alpha may play a role in its maintenance.

Maturation of human fetal stomach in organ culture

BACKGROUND

This investigation was undertaken to establish a serum-free organ culture technique allowing for the morphological and physiological maintenance of human fetal stomach in vitro.

METHODS

Explants from gastric corpuses (12-17 weeks of gestation) were cultured in serum-free medium for periods of up to 15 days.

RESULTS

After 15 days of culture, surface mucous cells were more mature, gastric glands were numerous and well developed, and all epithelial cell types were morphologically very well preserved. Morphometric measurements of the glands revealed an accelerated development in culture compared with that found in utero. Even though the incorporation of [3H]thymidine into total DNA decreased, the labeling indices determined by radioautography confirmed that epithelial cell proliferation was maintained especially in the pit/neck portion and at the base of the glandular compartments. A significant increase in total glycoprotein synthesis, as evaluated by the incorporation of [3H]glucosamine, was observed and correlated with the differentiation of the mucous cells.

CONCLUSIONS

This investigation establishes for the first time that human gastric mucosa can be maintained up to 15 days in organ culture and that maturation of the gastric mucosa can be reproduced in chemically defined media.

Glycoprotein synthesis and secretion by cultured small intestinal mucosa in coeliac disease

Glycoprotein biosynthesis by jejunal mucosa was examined during culture in vitro in 26 patients with coeliac disease and 19 controls. The incorporation rates of tritiated glucosamine into tissue and secreted glycoproteins were determined using established techniques. The total glucosamine incorporation in untreated coeliac patients was significantly greater than that of histologically normal mucosa (p less than 0.001) and jejunal tissue from patients with treated coeliac disease (p less than 0.01). Enhanced secretion of in vitro labelled glycoproteins was observed in untreated coeliac patients. The total incorporation of tritiated glucosamine in intestinal tissues was correlated with goblet cell numbers. These results indicate that quantitative changes in glycoprotein synthesis and secretion occur in coeliac disease.

Effects of endogenous and exogenous prostaglandins on glycoprotein synthesis and secretion in isolated rabbit gastric mucosa

We studied gastric glycoprotein synthesis and secretion in organ culture before and during cyclooxygenase inhibition and replacement with exogenous prostaglandins (16,16-dimethyl prostaglandin E2 and prostaglandin F2 alpha). Isolated rabbit antral and fundic mucosal explants incorporated [14C]N-acetylglucosamine and [3H]leucine in a linear fashion and steadily secreted labeled proteins and glycoproteins during the 24-h incubation period. On sepharose 4B, greater than 90% of the secreted protein-bound [14C]N-acetylglucosamine was found in the high molecular weight peak. Incorporation of tracer was not influenced by cyclooxygenase inhibition with indomethacin or the addition of exogenous prostaglandins. Secretion of newly formed glycoprotein, however, was significantly inhibited by indomethacin and stimulated by both tested prostaglandins in a concentration-dependent manner. 16,16-Dimethyl prostaglandin E2 caused significant stimulation in concentrations that are well in the physiologic range for endogenous prostaglandin E2, whereas prostaglandin F2 alpha stimulated in 100 times higher concentrations. We conclude that in the isolated gastric mucosa both endogenous and exogenous prostaglandins stimulate mucus secretion. For prostaglandin E2, but not prostaglandin F2 alpha, a role in the physiologic regulation of gastric mucus secretion is probable.

In vitro mucus glycoprotein production by colonic tissue from patients with ulcerative colitis

Colonic mucus production was measured in vitro by means of incorporation of tritiated glucosamine using biopsy material from patients with ulcerative colitis and compared with data from patients with Crohn's disease, colonic carcinoma, colonic polyps and patients with apparently normal colonic mucosae. Mucus production was significantly decreased (p less than 0.03) in all patients with ulcerative colitis, and in particular in patients with inactive disease when compared with normal subjects. In patients with active disease mucus production was not significantly different from normal subjects. The radiolabelled material was characterised by gel filtration and ion exchange liquid chromatography as mainly high molecular weight glycoproteins. These results indicate that the quantitative character of colonic mucus is abnormal in inactive ulcerative colitis.

Ethanol damage to rat gastric mucosa is unlikely to be mediated by ethanol

During injury to the gastric mucosa, lysosomes become more fragile and lysosomal enzymes, which are activated at acid pH, leak into the surrounding environment. It is not clear whether these changes contribute to the mechanism of damage or are merely a secondary result of it. To test whether lysosomes modulate gastric mucosal damage, we pretreated rats with a lysosomal labilizing agent, Triton WR 1339 (1.5 g/kg) and histologically assessed mucosal damage in vivo after challenge with 30% ethanol. No significant differences were found in the length or depth of eroded mucosa: mean erosion length, Triton 23.9 +/- 6.6% vs. control 19.7 +/- 5.2%; mean depth (micron), Triton 19 +/- 4 vs. control 20 +/- 7. After a similar pretreatment regimen, rat antral mucosa was cultured, challenged with ethanol and damage assessed by release into media of previously incorporated mucosal 51chromium. With 15% ethanol challenge, no change in 51chromium release was seen: after Triton, 9.8 +/- 1.4% vs. control 10.3 +/- 1.0%. Triton pretreatment perturbed gastric lysosomes as shown in organ culture by significantly raised tissue lysosomal enzyme activities and increased lysosomal enzyme release into culture media after ethanol challenge. The lack of effect of this pretreatment regimen suggests that lysosomes do not have a major pathogenetic role in ethanol-induced gastric damage.

Biosynthesis of proteins by human gastric mucosa in vitro

Incorporation of L-[U-14C]leucine and of D[U-14C]glucose into proteins of fresh human gastric mucosa in vitro was studied after incubation of homogenized tissue and of intact mucosal pieces. CsCl centrifugation was used to separate high-density mucus glycoproteins from other mucosal proteins, and the macromolecular nature of radioactive mucosal glycoprotein fractions was confirmed by SDS/polyacrylamide-gel electrophoresis and autoradiography of the polyacrylamide gels. In all experiments a substantial proportion of total incorporated radioactivity was associated with gastric-mucosal glycoprotein fractions (CsCl fraction L3), indicating their biosynthesis. Radioactivity of these fractions was shown to co-chromatograph with carbohydrates when fractionated either directly or after reduction and alkylation (1) Sephadex G-200 chromatography in the excluded fractions and (2) by DEAE-cellulose ion-exchange chromatography. On incubation of intact mucosa, the major portion of radioactivity associated with the glycoprotein fractions of both leucine- and glucose-labelled specimens was secreted into the mucosal media during the course of the experiment. It is suggested that biosynthesis of mucus in vivo by gastric mucosa may be associated with rapid secretion of the synthesized macromolecules into the lumen of the stomach and that investigations of the metabolic processes within the mucosa should consider the products of secretion of the tissue. Incorporation of L-[U-14C]leucine implies biosynthesis of the polypeptide components of the macromolecules.

Ethanol-induced cell damage in cultured rat antral mucosa assessed by chromium-51 release

We have developed an in vitro method for studying ethanol-induced injury to gastric mucosa using organ culture of rat antrum. Cell damage was assessed by measurement of the release of [51Cr]sodium chromate from preloaded cells, a method adapted from a standard immunologic technique. This system provided rapid and highly reproducible quantitation of tissue injury as assessed by 51Cr release into the culture medium. The threshold concentration for ethanol-induced damage was between 10 and 15% v/v, similar to in vivo thresholds observed by others. 51Cr release could also be induced by very short exposure to ethanol (5-15 min), and then continued despite ethanol removal. Interestingly, after continuous ethanol exposure, a plateau of maximum 51Cr release was reached 60 min after exposure to ethanol over the concentration range 20-50%, suggesting tissue adaptation to ethanol damage. This organ culture system, which allows precise control of experimental conditions, may be useful for studying mechanisms of gastric mucosal injury and protection.

Reserpine-induced alterations in mucus production and calmodulin-binding proteins in a human epithelial cell line

The characterization of a mucus-producing human cell line (HC-84) derived from a colon carcinoma and its response to in vitro reserpine treatment is reported. Mucous granules were demonstrated within these cells on the basis of electron microscopic examination and incorporation of [3H]glucosamine with subsequent autoradiographic analysis. Fluorographic analysis of total HC-84 cell protein after incubation with [3H]glucosamine indicated that the majority of tritium was incorporated into two proteins with molecular weights of 115 and 120 kD. When total HC-84 protein was subjected to immuno-blot analysis utilizing rabbit antibody against human intestinal mucus, only these two proteins (115K and 120K) reacted positively, indicating a direct correlation between [3H]glucosamine incorporation and mucus production. Immunofluorescence localization of mucus within HC-84 cells utilizing this same antibody resulted in a punctate pattern of fluorescence within the cytoplasm. Treatment of HC-84 cells with 30 microM reserpine for 7 days resulted in a three-fold increase in mucus production compared with controls. There was also a concomitant loss of a 30K calmodulin-binding protein in cells treated with reserpine. These cells represent a useful system for studying the effect of reserpine on the processes of mucus synthesis and secretion.

Changes in volume of stored mucus in pit and surface cells during their maturation and migration in the antral mucosa of the mouse

In antral mucosa of the mouse stomach, the volume of mucus in mucous cells was measured morphometrically to determine whether this value changes during cell migration from the base of the pit to the surface. Both the volume density of mucous granules (the fraction of cell volume occupied by the granules) and the volume of intracellular mucus were reduced to about half in surface cells compared with those of upper pit cells. This indicates that mucus secretion is substantial during the later part of the lifespan of these cells, and is not due simply to the shedding of senescent cells.