CELL-SURFACE CARBOHYDRATES OF RAT ALVEOLAR TYPE-II CELLS IN PRIMARY CULTURE

Cell surface carbohydrates have been shown to be altered during cellular differentiation. Alveolar type II (ATII) cells in culture gradually lose their differentiated phenotype. Therefore, the aim of this study was (1) to characterize changes in terminal carbohydrates of cell surface glycoproteins of rat ATII cells cultured for 1 to 5 days on plastic, and (2) to assess the concomitant changes in sialidase and sialyltransferase activity of ATII cell homogenates. Cells were surface-labeled with potassium-[H-3]-borohydride after oxidation by sodium periodate at millimolar concentrations, galactose oxidase or neuraminidase plus galactose oxidase, allowing for the specific labeling of terminal sialic acids, terminal galactose/N-acetylgalactosamine (Gal/GalNAc), or terminal and penultimate Gal/GalNAc residues, respectively. Glycoproteins were separated by SDS-PAGE. On day 1, cells were heavily coated with sialic acids, since no labeling could be introduced with galactose oxidase alone. From day 1 to day 5, we observed a selective and progressive desialylation of two glycoproteins (200 and 165 kD). At the same time, the ATII cells' sialidase activity (pH 4.2) exhibited an 8-fold increase (60.3 +/- 4.0 pmol/min/mg protein on day 1 versus 406.9 +/- 3.7 pmol/min/mg protein on day 5), whereas the sialyltransferase activity increased 2-fold (212 +/- 8 fmol/min/mg protein on day 1 versus 395 +/- 82 fmol/min/mg protein on day 5) and the supernatant sialidase activity was unchanged (2.8 +/- 0.7 pmol/min/ml on day 5). Thus, the phenotypic changes of ATII cells in primary culture are accompanied by a partial cell surface desialylation and an increase in intracellular sialidase activity.