Glycosylation of thyroglobulin secreted by porcine cells cultured in chamber system: thyrotropin controls the number of oligosaccharides and their anionic residues.
Résumé
Porcine thyroid cells were cultured in porous bottom chambers in the presence or in the absence of TSH added to the basal medium. Radiolabeled-sugar (3H-mannose) was added to the basal medium on day 11 for 4 days and the glycosylation of thyroglobulin (Tg), the major glycoprotein secreted into the apical medium, was studied. The incorporation of 3H-mannose per molecule of Tg was increased 1.5-fold by a 50 microU/ml minimal concentration of TSH. The distribution of glycopeptides (after pronase digestion) on concanavalin A sepharose column was not modified by the presence of TSH. However this distribution was different from that observed for Tg extracted from gland (more multiantennary units than biantennary units and polymannose units). After desialylation and desulfation, the sizes of the oligosaccharide chains analyzed on HPLC appeared similar when cells were cultured under stimulation or not. Thus TSH enhanced sugar incorporation without modifying either the distribution of the different oligosaccharide moieties or their sizes. Consequently the effect of TSH was a 1.5-fold increase in oligosaccharide chains linked to asparagine residues. 3H-Mannose-oligosaccharide chains were then analyzed on ion-exchange HPLC before and after desialylation and desulfation. The number of anionic residues per oligosaccharide unit (particularly sulfate residues) was higher in the absence of TSH than in the presence of TSH. Nevertheless, since TSH increased the number of carbohydrate units per molecule of Tg 1.5-fold, the total content of anionic residues bound to oligosaccharide units per molecule of Tg seems not to be modified by TSH.