Characterization of a sialic-acid-rich mucus glycoprotein secreted by a premalignant human colorectal adenoma cell line.

The human colonic cell line PC/AA, derived from an adenoma, retains in vitro colonic cell differentiation, notably the production of mucus glycoproteins. The PC/AA adenoma cells produce an extracellular gel layer in culture. The PC/AA gel could be isolated by extraction of the cell cultures with guanidine hydrochloride. The extracted material was purified by gel filtration and caesium chloride density-gradient centrifugation and showed properties typical of mucus glycoproteins, namely, a carbohydrate content above 60% of dry weight rich in N-acetylgalactosamine and sialic acid and low in mannose; an amino acid composition with high serine threonine and proline content; a molecular weight above 1,000 kDa on Sepharose CL 4B chromatography and on SDS-polyacrylamide gel electrophoresis under reducing conditions (greater than 200 kDa); a buoyant density of approximately 1.48 g/ml and the release of oligosaccharides by the alkaline beta-elimination reaction. Comparison of the gel mucus glycoprotein purified from premalignant PC/AA cells with normal human colon mucin showed that it has a higher sialic acid content. This suggests that higher sialic acid levels may precede the development of malignancy.