Small cell lung cancer (SCLC), which accounts for 25% of the 150,000 new cases of primary lung cancer diagnosed each year, exhibits neuroendocrine features, synthesizes mitogenic neuropeptides such as bombesin and vasopressin and express their specific G protein-coupled receptors, potentially leading to autocrine-stimulated growth. These receptor systems are clearly mitogenic in specific cell types and "overactivity" of G proteins leads to human hyperplasias of endocrine origin. while much work has focused on the bombesin-lime peptides, the majority of SCLC lines are vasopressin-responsive and elevated plasma concentrations of vasopressin occur in 30 to 70 percent of SCLC patients, often leading to the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). The contribution of ectopically-produced vasopressin to autocrine growth of SCLC is poorly defined. Also the identity of the SCLC vasopressin receptors relative to vascular smooth muscle and hepatic vasopressin receptors, the G proteins and the effector enzymes that mediate mitogenic signal transduction ar largely undefined. This proposal explores the hypothesis that ectopically-expressed vasopressin significantly contributes to growth of SCLC by signalling through specific receptors, G proteins and effector enzymes. To test this hypothesis, the vasopressin-responsiveness for the various sCLC lines will be characterized and the expressed vasopressin receptors will be molecularly defined. A panel of SClC lines will be screened for vasopressin synthesis. Ultimately, the contribution of ectopically-expressed vasopressin to growth of these SCLC lines will be defined with V1 and V3- specific receptor antagonists and compared to other specific and broad- spectrum neuropeptide antagonists. The G proteins and effector enzymes that couple the vasopressin receptor(s) to mitogenic pathways will be defined in the panel of human SCLC lines. Biochemical studies with pertussis toxin will be complemented with a molecular genetic approach whereby mutant G protein alpha-subunit genes (alphai2, alphao, and alphaq) that exhibit constitutive-active phenotypes will be expressed in SCLC to define the dominant G protein pathways involved in neuropeptide- stimulated growth of SCLC. Vasopressin-sensitive effector enzyme systems (phospholipase C, phospholipase A2, phospholipase D, protein kinase C, p42/44 MAP kinase and protein tyrosine kinase activity) will be monitored throughout. Together, this proposal will examine the role of vasopressin synthesis and define the G protein-coupled signal transduction system in a common human lung cancer where interaction with the kidney occurs leading to SIADH. The work will increase the general understanding of neuropeptide signalling in SCLC and may eventually contribute to the definition of a strategy for pharmacologic intervention using receptor antagonists and inhibitors of involved effector systems, (PLA2, cyclooxygenase, etc.) activated by vasopressin and bombesin receptors.