The biochemistry of peptides and growth factors in lung cancer was investigated. High affinity neuromedin B (NMB) binding sites were detected in small cell lung cancer (SCLC) cells. Because NMB elevated the cytosolic Ca+2 in Fura-2 AM loaded NCI-H345 cells, NMB similar to gastrin realeasing peptide (GRP) may stimulate phosphatidylinositol (PI) turnover. GRP receptor antagonists such as (D-FPhe6, D-Ala11)BN6-13 methyl ester ((FA)BN6-13ME) only weakly antagonized the ability of NMB to elevate cytosolic Ca+2 or stimulate clonal growth. These data suggest that SCLC cells have distinct GRP and NMB receptors. Also, the GRP receptor was solubilized and purified using affinity chromatography techniques. A major 65 Kdalton band was isolated and the GRP receptor is a G-protein coupled receptor containing 384 amino acid residues and 7 hydrophobic domains. Vasoactive intestinal peptide (VIP) mRNA was detected in high concentrations in NCI-H727 cells. Also, immunoreactive VIP was detected in lung cancer cell extracts and conditioned media. Pituitary adenylate cyclase activating peptide (PACAP) and VIPhybrid inhibited 125I-VIP binding with high and moderate affinity respectively. VIP and PACAP elevated intracellular cAMP levels whereas VIPhybrid inhibited the increase in cAMP caused by VIP. Also, VIP and PACAP stimulated lung cancer clonal growth whereas VIPhybrid inhibited colony formation. VIP hybrid inhibited lung cancer xenograft formation in nude mice. These data indicate that VIPhybrid is a lung cancer VIP receptor antagonist. Also, specific 125I- PACAP binding was inhibited with high affinity by PACAP but moderate affinity by VIP. PACAP elevated the cytosolic Ca+2 in addition to elevating intracellular cAMP. These data suggest that PACAP binds to a unique receptor in addition to binding with high affinity to VIP receptors. GRP receptor antagonists may be useful for the treatment of SCLC. VIP receptor antagonists such as VIPhybrid have broader applicability in that they inhibit proliferation of both SCLC and NSCLC cells.
