PREPROPACAP-DERIVED PEPTIDES OCCUR IN VIP-PRODUCING TUMORS AND COEXIST WITH VIP

Pituitary adenylate cyclase activating polypeptide (PACAP) is a newly discovered neuropeptide which exists in two biologically active forms: PACAP-38 consisting of 38 amino acids and PACAP-27, a peptide corresponding to the N-terminal 27 amino acids of PACAP-38. Both PACAPs are derived from a 176 amino acid precursor (preproPACAP) which in addition gives rise to a 29 amino acid peptide, designated PACAP-related peptide (PRP). The presence of the three preproPACAP-derived peptides (PACAP-38, PACAP-27 and PRP) in tumour tissue from nine patients with VIP-producing tumours (pancreatic carcinoma, neuroblastoma, ganglioneuroma and pheochromocytoma) and eleven patients with non-VIP-secreting tumours (gastrinoma, glucagonoma, somatostatinoma, neuroblastoma) was examined by specific radioimmunoassays. In seven out of the nine VIP-secreting tumours elevated concentrations of all the three preproPACAP-derived peptides were found compared with normal tissue, while the concentrations in the non-VIP-secreting tumours were within the normal range. PACAP-38 was in all cases the dominating peptide, the concentration ranging from 41 to 3606 pmol/g. When tumour extracts were fractionated on Sephadex G50 column, tricine gel electrophoresis or reverse-phase HPLC immunoreactive components corresponding to synthetic PACAP-38, PACAP-27 and human PRP were identified, suggesting that preproPACAP was fully processed. Immunocytochemical examination showed PACAP-immunoreactive cells in the tumour tissue which also stained for VIP. This co-localization of PACAP and VIP was confirmed by double-staining experiments on the same sections, demonstrating PHM/VIP mRNA and PACAP-immunostaining in the same cells.