Effect of tunicamycin on thyroglobulin secretion

Secretion of thyroglobulin was studied by incubating pig thyroid follicles, isolated by collagenase digestion and opened up by trypsinization. When followed over periods of 4 h, the secretion of [14C]leucine-labeled thyroglobulin into the medium was reduced by 60-95% in the presence of 1 microgram/ml and 5 micrograms/ml of tunicamycin. These concentrations of the antibiotic reduced incorporation of [3H]mannose into the follicle proteins by 70-80% but did not significantly influence the incorporation of [14C]leucine. Rat thyroid lobes were labeled with [3H]leucine for 20 min and chase-incubated for 0-4 h. In electron microscopic autoradiographs obtained immediately after labeling, the label was restricted to the follicle cells and concentrated over the endoplasmic reticulum both in controls and in specimens exposed to tunicamycin (5 micrograms/ml). After 4 h chase most radioactivity was located in the follicle lumen in controls whereas in tunicamycin-exposed lobes almost all labeled material was retained in the follicle cells. It is concluded that tunicamycin suppresses thyroglobulin secretion and that this is not due to inhibited protein synthesis.