Calcitonin is a substrate for oligosaccharyltransferase in vitro

Tumor-derived, large molecular weight forms of calcitonin, have been postulated to result from glycosylation of the hormone. To address this question we have examined the glycosylation of calcitonin in vitro and in cultured thyroidal C-cells. We show that native, undenatured calcitonin is an active substrate for oligosaccharyltransferase and that glycosylation of calcitonin by the transferase is inhibited by tunicamycin. In addition, calcitonin is an effective competitive inhibitor of the glycosylation of a known peptide substrate for oligosaccharyltransferase. Pulse-labelling of cultured medullary thyroid carcinoma cells with [3H]-mannose indicate that detectable quantities of carbohydrate-containing forms of calcitonin are produced in these cells. These data indicate that glycosylation of calcitonin is one mechanism whereby tumor cells could produce higher molecular weight forms of the hormone.