Pituitary adenylate cyclase-activating polypeptide inhibits transforming growth factor-beta1-induced apoptosis in a human pituitary adenoma cell line

Pituitary adenylate cyclase-activating polypeptide (PACAP) was originally isolated from hypothalamic tissues based on its ability to stimulate cAMP production in cultured anterior pituitary cells. Recent studies have suggested a functional role for PACAP in the apoptosis of brain cells. However, the role of PACAP in regulating apoptosis in human pituitary adenomas has not previously been examined. Analysis of the cultured human pituitary adenoma cell line HP75, which expresses all three major PACAP receptors, showed that both PACAP-38 and PACAP-27 inhibited TGF-beta1-induced apoptosis. Treatment with the PACAP receptor antagonists PACAP 6-38 (PACAP type I receptor antagonist) and (p-chloro-D-Phe(6), Leu(17))-VIP (PACAP type II receptor antagonist) blocked the effects of PACAP-38 on the inhibition of transforming growth factor-beta1 (TGF-beta1)-induced apoptosis, confirming the specificity of the role of PACAP. Treatment with forskolin but not phorbol 12-myristate 13-acetate (PMA) also inhibited TGF-beta1-induced apoptosis. TGF-beta1 treatment was associated with an increase in mitogen-activated protein kinase (MAP kinase) when analyzed by Western blotting, but PACAP inhibition of TGF-beta1-induced apoptosis was not associated with activation of MAP kinase. Immunocytochemical analysis of the cell cycle cyclin-dependent kinase inhibitor p27 showed that treatment with TGF-beta1, forskolin, PMA, and PACAP increased p27 expression in cultured HP75 cells. These results indicate that PACAP is a highly specific inhibitor of TGF-beta1-induced apoptosis in the HP75 human pituitary adenoma cell line and that PACAP, TGF-beta1, forskolin, and PMA all stimulate expression of the TGF-beta-regulated cell cycle protein p27 in the HP75 human pituitary adenoma cell line. The HP75 cell line can be used as a model to study the regulation of apoptosis in human pituitary cells.