The Secretory Granule Protein 7B2 is Secreted in Parallel with Proopiomelanocortin and its End‐Products by the Mouse Corticotroph Tumour Cells AtT‐20

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Abstract
First isolated in porcine pituitary glands the protein 7B2 subsequently proved to be a specific biochemical marker of the secretory granules. Likewise 7B2 was detected in almost all normal and tumoral endocrine tissues. Unexpectedly, several authors failed to demonstrate its presence in rat and human adrenocorticotrophin (ACTH)-secreting cells. In order to definitely establish whether this cell type also produces 7B2 we chose the mouse pituitary corticotroph tumour cell line AtT-20 as a model. Serial dilutions of the mouse culture medium generated displacement curves parallel to that of the standard in a specific 7B2 RIA directed against the human 7B2(23-39) fragment. Under basal secretory conditions immunoreactive 7B2 accumulated in the culture medium in parallel with proopiomelanocortin (POMC) and its fragments N-terminal-joining peptide (NT-JP), joining peptide (JP), beta-lipotropin (beta-LPH), and beta-endorphin (beta-end), although at a much lower (approximately 100-fold) molar concentration. As expected mouse corticotroph cells responded to the stimulatory action of cyclic AMP (3.5 mM) with a preferential increase in the release of POMC end-products, JP and beta-end, which was accompanied by a parallel increase in immunoreactive 7B2 release.