Abstract
High levels of BN/GRP are present in classic SCLC and lung carcinoids, whereas BN immunoreactivity is absent in variant SCLC, adenocarcinoma, large cell carcinoma, squamous cell carcinoma, and mesothelioma cell lines. BN-like peptides are secreted from classic SCLC into the tissue culture medium. The secretion rate of BN-like peptides from cell line NCI-H345 was increased 3-fold by VIP (1 microM). Also, VIP increased the cAMP levels in cell line NCI-H345 by an order of magnitude. Therefore, SCLC cells have functional VIP receptors which regulate the secretion of BN-like peptides. Also, SRIF (100 nM) inhibits the VIP-stimulated increase in cAMP levels and secretion rate of BN-like peptides from SCLC cells. Because BN stimulates colony formation, VIP and/or SRIF may be able to alter the growth of SCLC cells. BN-like peptides are secreted from SCLC cells into the plasma. The levels of BN immunoreactivity in the plasma of SCLC patients with extensive disease is 2- to 40-fold greater than that of patients with limited disease. Secretin infusion into patients with extensive disease produces a transient increase (7-fold) in the plasma concentration of BN-like peptides. BN-like peptides are also present in the CSF of SCLC patients. When released from SCLC cells, BN-like peptides may interact with cell surface receptors. [Tyr4]BN binds with high affinity (Kd = 0.5 nM) to a single class of sites (1500/cell) on cell line NCI-H345. The carboxyl terminus of BN or GRP is essential for high-affinity binding activity. BN-like peptides elevate cytosolic Ca2+ levels as a result of increased phosphatidylinositol turnover. The putative BN receptor antagonist [D-Arg1, D-Pro2, D-Trp7,9, Leu11]substance P inhibits high-affinity [Tyr4]BN binding, the ability of BN to elevate cytosolic Ca2+ levels, and colony formation of SCLC cells. Therefore, BN receptor antagonists may serve as useful agents to inhibit the growth of SCLC.
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