Cholera toxin inhibits signal transduction by several mitogens and the in vitro growth of human small-cell lung cancer

Cholera toxin (CT) inhibited the in vitro growth of three of four human small-cell lung carcinoma (SCLC) cell lines with a 50% inhibitory concentration of 27-242 ng/ml. Loss of surface membrane ruffling and the capacity of [Tyr⁴]-bombesin, vasopressin, and fetal calf serum to stimulate increases in intracellular free calcium clearly preceded effects on cellular metabolic activity and cell growth. ¹²⁵I-[Tyr⁴]-bombesin binding was unaffected by CT treatment but [Tyr⁴]-bombesin stimulated phospholipase C activity was decreased in membranes from CT-treated SCLC cells. CT stimulated a rapid but transient increase in intracellular cyclic AMP ([cAMP](i)) in SCLC. The effect of CT on susceptible SCLC were not reproduced by elevations of [cAMP](i) induced by forskolin or cyclic AMP analogues. G(M1) ganglioside, the cellular binding site for CT, was highly expressed in the CT-sensitive but not the CT-resistant SCLC cell lines. In contrast, expression of guanine nucleotide binding protein substrates for ADP-ribosylation by CT was similar. These data demonstrate the existence of a CT-sensitive growth inhibitory pathway in SCLC-bearing G(M1) ganglioside. Addition of CT results in decreased responsiveness to several mitogenic stimuli. These results suggest novel therapeutic approaches to humans SCLC.