Microtubules and the intracellular conversion of proparathyroid hormone to parathyroid hormone

The effects of several compounds which interfere with cellular microfilaments and microtubules on the conversion of proparathyroid hormone (ProPTH) to parathyroid hormone (PTH) were examined in slices of bovine parathyroid slices incubated in vitro with ³H-leucine. Inhibitors of microtubular function, vinblastine and colchicine, increased the ratio of ³H-labeled ProPTH to PTH in the tissue by 2- to 3-fold. Cytochalasin B, an inhibitor of microfilaments, was without effect. Concentrations of colchicine as low as 10^-6M maximally increased the ratio of ProPTH to PTH, whereas lumicolchicine, an analogue of colchicine which does not affect the function of microtubules, had no effect at concentrations as high as 10^-4M. The increased ratio of ProPTH to PTH was due partly to a stimulation by vinblastine and colchicine of the incorporation of ³H-leucine into ProPTH but not into general protein. However, after short incubations of parathyroid tissue with ³H-leucine, the amount of ³H-labeled PTH was less in colchicine-treated incubations than in control incubations. In the presence of vinblastine, after a 20-min incubation of parathyroid slices with ³H-leucine and vinblastine, ProPTH was not completely converted to PTH by an additional 90-min “chase”incubation with unlabeled leucine while a 20-min “chase”incubation is sufficient to convert essentially all the ProPTH to PTH in control incubations. These data suggest that the increased ratio of ProPTH to PTH is also due to a partial inhibition of the conversion ProPTHto PTH by vinblastine and colchicine. The data are consistent with the hypothesis that microtubules facilitate the transport of newly synthesized ProPTH to its intracellular site of cleavage to PTH.