In vitro release and biosynthesis of tumor ACTH in ectopic ACTH producing tumors.

Tumor tissues obtained from 4 patients with the ectopic ACTH syndrome were studied for release and synthesis of tumor ACTH, using an in vitro incubation system. The effect of various agents on release of tumor ACTH was evaluated in three cases; beta-MSH released and adenosine 3',5'-monophosphate (cyclic AMP) formed in the tissue were determined in one. Biosynthetic experiments using labeled amino acid incorporation were performed in two cases. Secretion of tumor ACTH was significantly stimulated in all cases by crude rat median eminence extract which was also effective in stimulating beta-MSH secretion associated with elevated tissue cyclic AMP levels in one. Addition of cyclic AMP and dibutyryl cyclic AMP caused a significant increase in release of both tumor ACTH and beta-MSH in one. Biogenic amines (norepinephrine and serotonin) markedly elevated tussie cyclic AMP levels without a corresponding increase of hormone release in one. Incorporation experiments revealed that 3H- or 14C-phenylalanine was incorporated into immunoreactive ACTH of a larger molecular size (big ACTH) in both cases by chromatographic procedures. However, biological activity of big ACTH was found to be undetectable by an in vivo steroidogenic assay. A mild tryptic digestion of the big forms resulted in the appearance of little ACTH to which the major radioactive peak shifted. These data suggest that the mechanism of release of tumor ACTH and beta-MSH is very similar to that of the pituitary, and that intracellular cyclic AMP may in part play some role in release of both hormones. It is also suggested that some ectopic ACTH producing tumors predominantly synthesize big ACTH, a possible precursor of ACTH, with less bioactivity.