Transformed growth of human small cell lung cancer (SCLC) is mediated by autocrine signaling through multiple G protein-coupled neuropeptide receptors. To define the role of Gq and its effector, phospholipase Cbeta (PLCbeta), in SCLC growth, we expressed a COOH-terminal fragment of PLCbeta1 (PLCbetaCT) that is catalytically inactive and is predicted to behave as a competitive inhibitor of Gq signaling. Using endogenous muscarinic receptors as indicators of Gq-coupled receptor signaling status, we observed that stable expression of PLCbetaCT in NCI-H345 SCLC cells significantly inhibited muscarinic receptor-mediated phospholipase C activation and intracellular Ca2+ mobilization. In addition, PLCbetaCT expression reduced the basal activity of protein kinase C as well as the receptor-stimulated activity of the extracellular signal-regulated kinases, consistent with the sequential requirement for Gq, PLCbeta, and protein kinase C in the regulation of the extracellular signal-regulated kinases by neuropeptide and muscarinic receptors in SCLC. By contrast, muscarinic agonist stimulation of the c-Jun NH2-terminal kinases was not inhibited in SCLC cells expressing PLCbetaCT, indicating that other G proteins such as the G12,13 family members mediate c-Jun NH2-terminal kinase activation by neuropeptides and muscarinic agonists. Finally, soft agar colony formation by the SCLC cells expressing PLCbetaCT, but not growth in suspension culture, was markedly reduced, indicating that signaling through Gq and PLCbeta by autocrine-signaling neuropeptide receptors is a dominant pathway involved in the transformed growth of SCLC.