REGULATED AND UNREGULATED PATHWAYS FOR MUC2 MUCIN SECRETION IN HUMAN COLONIC LS180 ADENOCARCINOMA CELLS ARE DISTINCT

We have shown previously [McCool, Forstner and Forstner (1994) Biochem . J. 302, 111-118] using pulse-chase labelling of mucin with [H-3]thre onine that LS180 colonic tumour cells synthesize and secrete MUC2 with out the addition of secretagogues. Treatment of the LS180 cells with m onensin to disrupt Golgi function was also found to inhibit baseline s ecretion almost completely. In this paper we show that addition of noc odazole to inhibit microtubule assembly reduced baseline secretion by 53% over a 6 h chase period. In contrast, cytochalasin D did not affec t the rate of unstimulated mucin synthesis or secretion, suggesting th at baseline secretion is not influenced by disruption of actin microfi laments. In addition, regulated mucin secretion by LS180 cells was stu died in response to carbachol, phorbol 12-myristate 13-acetate and A23 187. Mucin released in response to secretagogues behaved identically o n SDS/PAGE to that secreted under baseline conditions. T84 cells and t he B6 subclone of the HT29 cell line responded in a similar manner to LS180 cells and secreted high-molecular-mass mucin which included MUC2 and behaved like LS180 mucin on SDS/PAGE. Neither monensin nor nocoda zole significantly affected secretagogue-stimulated mucin secretion. S ince these compounds inhibited secretion of labelled mucin under basel ine conditions, mucin released by secretagogues must have come from a separate, unlabelled mucin pool in stored granules. Cytochalasin D, on the other hand, caused the release of small amounts of stored mucin, suggesting that actin microfilaments participate in regulated exocytos is. Thus two kinds of mucin secretion occur in LS180 cells. Unregulate d secretion depends upon continuous transport of mucin granules from G olgi vesicles to the cell surface and does not utilize stored mucin, w hereas regulated secretion involves the release of mucin from storage granules and is not affected by microtubule or Golgi disruption.