Stimulation of calcium mobilization but not proliferation by bombesin and tachykinin neuropeptides in human small cell lung cancer cells

The tachykinin family of neuropeptides, including substance P and neurokinins A and B, induce a transient increase in intracellular free calcium concentration in human small cell lung carcinoma (SCLC) cells, as measured with a calcium indicator fura-2. The effects are dose dependent and even greater than that of bombesin at equimolar concentrations in these cells. The tachykinins, like bombesin, induce calcium mobilization mainly from intracellular store(s). None of the peptides, however, shows a stimulatory effect on DNA synthesis. In addition, exogenously applied bombesin does not stimulate DNA synthesis at any concentration tested. We also examined the effects of a recently reported bombesin antagonist [D-Arg1, D-Phe5, D-Trp7,9, Leu11]substance P in SCLC cells, and compared them to those in Swiss 3T3 fibroblasts in which the mitogenic effect of bombesin is well characterized. The antagonist at 10(-5) M completely abolishes the Ca2+-mobilizing effect of 10(-7) M bombesin in SCLC cells, and that of 10(-9) M but not 10(-7) M bombesin in Swiss 3T3 cells. The antagonist at this concentration effectively inhibits the mitogenic action of bombesin (10(-9) M) in Swiss 3T3 cells; however, much higher doses (approximately 10(-4) M) are needed to inhibit DNA synthesis in SCLC cells. Moreover, the antagonist inhibits DNA synthesis in bombesin/gastrin-releasing peptide-nonproducing cells with a similar dose dependency as in producing cells. These results indicate that bombesin/gastrin-releasing peptide and other calcium mobilizing peptides do not always act as a growth factor in SCLC cells, and that the bombesin antagonist could inhibit growth of SCLC cells through a mechanism other than bombesin antagonism.