Cellular and molecular biological study of biosynthesis of human gastric mucin.

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We studied the biosynthesis of mucin in the human stomach using an anti-carboxyl terminal of the apomucin antibody. Human stomach mucosa was labeled with [^<35>S]methionine,and chased for 3 h. Approximately a 60 kilo-daltons subunit of human gastric mucin precursor protein,and its dimer,trimer,and tetramer(120,180,240kilo-daltons), and high molecular weight mature mucin were detected in the intracellular product. Extracellular products contained only the mature mucin. Inhibition of N-glycosylation with tunicamycin had no effects on the synthesis of 60 kilo-daltons subunit and its oligomers,and the secretion of the mature mucin. Some adenocarcinomas synthesize mucin or mucin-related products and secrete them as carcinoma-associated antigens. But very little is known about the intracellular transport and secretion mechanisms involved. We studied the biosynthesis and secretion of mucin-related products in a gastric cancer cell line(Hs746T) by pulse-chase experiments. Intracellular and extracellular products were immunoprecipitated with an anti-carboxyl terminal of the apomucin monoclonal antibody. In Hs746T cells,an approximately 55kilo-daltons precursor protein, 110kilo-daltons protein as the dimer,and a high molecular weight mucin-like product were detected among the intracellular products. The extracellular products included the dimer and a small amount of high molecular weight mucin. Treatment with tunicamycin or endo-b-N-acetylglucosaminidase H had no effect on the 55kilo-daltons protein and its dimer,but tunicamycin inhibited secretion of the dimer. These findings suggest that N-glycosylation may be involved in the secretory mechanism of the dimer protein as an immature product.