The exocrine protein trypsinogen is targeted into the secretory granules of an endocrine cell line: studies by gene transfer.

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Abstract
The exocrine protein rat anionic trypsinogen was expressed and is secreted from the murine anterior pituitary tumor cell line AtT-20. Which secretory pathway trypsinogen takes to the surface of this endocrine-derived cell line was examined. The "constitutive" pathway externalizes proteins rapidly and in the absence of an external stimulus. In the alternate, "regulated" pathway, proteins are stored in secretory granules until the cells are stimulated to secrete with 8-Br [8-bromo]-cAMP. On the basis of indirect immunofluorescence localization, stimulation of release, and subcellular fractionation, we find that trypsinogen is targeted into the regulated secretory pathway in AtT-20 cells. Laminin, an endogenous secretory glycoprotein, is shown to be secreted constitutively. Evidently the transport apparatus for the regulated secretory pathway in endocrine cells can recognize not only endocrine prohormones, but also the exocrine protein trypsinogen, which suggests that a similiar sorting mechanism is used by endocrine and exocrine cells.