12-O-tetradecanoyl-phorbol-13-acetate-induced ACTH secretion in pituitary tumor cells

The ability of the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) to stimulate secretion of immunoreactive ACTH from a clonal strain of mouse pituitary tumor cells (AtT-20/D16-16), was investigated. The secretory response to TPA was concentration-and time-dependent. TPAs action on AtT-20 cells was cyclic AMP independent. TPA, even at maximally effective concentrations, had an additive effect on ACTH secretion when co-applied with other agonists such as corticotropin releasing factor, vasoactive intestinal peptide, or (1)-isoproterenol. Secretion of ACTH in response to TPA was reduced by lowering extracellular calcium concentration or in the presence of the calcium channel blocker, nifedipine. Indomethacin and dexamethasone, both blocked TPA stimulated secretion; arachidonic acid, in high concentrations, had a small stimulatory effect on ACTH release. The data suggest that TPA induced secretion in pituitary tumour cells may result from the previously reported ability of the phorbol ester to stimulate the activity of phospholipase A2, and to the subsequent biosynthesis of prostaglandins.