alpha1,4-N-acetylglucosaminyltransferase (alpha4GnT) is a glycosyltransferase that mediates transfer of GlcNAc to betaGal residues with alpha1,4-linkage, forming GlcNAcalpha1--> 4Galbeta-->R structures. In normal human tissues, glycoproteins having GlcNAcalpha1-->4Galbeta-->R structures at non-reducing terminals are exclusively limited to the mucins secreted from glandular mucous cells of gastric mucosa, Brunner's gland of duodenum, and accessory gland of pancreaticobiliary tract. Recently, we have isolated a cDNA encoding human alpha4GnT by expression cloning. Although alpha4GnT plays a key role in producing this unique glycan in vitro, the actual localization of alpha4GnT was not determined. In this study we examined the localization of alpha4GnT in various human tissues, including gastrointestinal mucosa, using a newly developed antibody against human alpha4GnT. The specificity of the antibody was confirmed by analyses of human gastric adenocarcinoma AGS cells transfected by alpha4GnT cDNA. Expression of alpha4GnT was largely associated with the Golgi region of mucous cells that produce the mucous glycoproteins having GlcNAcalpha1-->4Galbeta-->R, such as the glandular mucous cells of stomach and Brunner's gland. An immunoprecipitation experiment disclosed that two distinct mucin proteins, MUC5AC and MUC6 present in gastric mucin, carried the GlcNAcalpha1-->4Galbeta-->R structures. These results indicate that alpha4GnT is critical to form the mucous glycoproteins having GlcNAcalpha1-->4Galbeta-->R on MUC6 and MUC5AC in vivo.(J Histochem Cytochem 49:587-596, 2001)

S U M M A R YGastric gland mucous cells produce class III mucin, which is also found in Brunner's glands and mucous glands along the pancreaticobiliary tract, and in metaplasia and adenocarcinomas differentiating towards gastric mucosa. Recently, we showed that class III mucin possesses GlcNAc ␣ 1 → 4Gal ␤ → R, formed by ␣ 1,4-N -acetylglucosaminyltransferase ( ␣ 4GnT). Examining the tissue-specific expression of mucin epitopes is useful to clarify celllineage differentiation and to identify the site of origin of metastatic carcinomas in histological specimens. Formalin-fixed, paraffin-embedded tissue sections from esophagus, stomach, colon, liver, pancreas, lung, kidney, prostate, breast, and salivary gland resected for carcinoma, as well as salivary gland adenoma, colon adenoma, and metastatic adenocarcinoma of lymph nodes from stomach, pancreas, colon, and breast, were immunostained for MUC6, ␣ 4GnT, and GlcNAc ␣ 1 → 4Gal ␤ → R. These were all expressed in normal, metaplastic, and adenocarcinoma tissues of stomach, pancreas, and bile duct, and in pulmonary mucinous bronchioloalveolar carcinomas. Cells expressing ␣ 4GnT uniformly expressed GlcNAc ␣ 1 → 4Gal ␤ → R. Only MUC6 was expressed in normal salivary glands, pancreas, seminal vesicles, renal tubules, and colon adenomas, and in normal tissue and adenocarcinomas of prostate and breast. No tissues showed immunoreactivity for ␣ 4GnT alone. Immunohistochemistry (IHC) profiles were similar for metastatic carcinomas and primary carcinoma tissues. The IHC profiles for MUC6, ␣ 4GnT, and GlcNAc ␣ 1 → 4Gal ␤ → R may be diagnostically relevant.

In addition to the synthesis of GMC mucins increasing reversibly, their metabolism or release may also increase reversibly in H. pylori-associated gastritis. The up-regulation of the expression of gastric GMC mucins may be involved in defense against H. pylori infection in the gastric surface mucous gel layer and on the gastric mucosa.