Inhibitory effects of tunicamycin and 2-deoxyglucose on thyroglobulin synthesis

The kinetic incorporation of labelled sugars and amino acids by rat thyroid hemilobes was measured in the presence of 2-deoxyglucose and tunicamycin, inhibitors of the glycosylation of glycoproteins. With either inhibitor the carbohydrate content of exocytosed thyroglobulin was only slightly decreased (less than 20% of control) whereas the rate of exocytosis was strongly inhibited (by 60-80%). As no intracellular accumulation or proteolysis of non-glycosylated molecules was detected, the reduced rate of thyroglobulin release seems essentially due to a decrease in protein synthesis. In a whole cell system (hemilobes), it is impossible to uncouple glycosylation and protein synthesis by incubation with tunicamycin; 50 micrograms/ml tunicamycin for 270 min inhibited total [3H]-glucosamine and 14C-labelled amino acid incorporation by 65% and 33% respectively. This can be contrasted with cell-free incubation of thyroid rough microsomes where glycosylation was blocked by the same tunicamycin concentration (90% inhibition of N-[3H]acetylglucosamine transfer from UDP-N-[3H]acetylglucosamine) whilst ongoing protein synthesis was not significantly modified (less than 4% inhibition). This clearly suggests that, in thyroid follicular cells, a regulatory link exists between the synthesis of the peptide moiety of a glycoprotein and its glycosylation.