Inhibition of neuropeptide-stimulated tyrosine phosphorylation and tyrosine kinase activity stimulates apoptosis in small cell lung cancer cells.

Small cell lung cancer (SCLC) cell growth is sustained by multiple autocrine and paracrine growth loops involving neuropeptides. The bombesin family of peptides are autocrine growth factors in H345 SCLC cells and provide a paradigm for the study of growth factors and mitogenic signaling in SCLC cells. We show that bombesin (and other neuropeptides) stimulates protein tyrosine phosphorylation (particularly focal adhesion kinase) and protein tyrosine kinase (PTK) activity in intact SCLC cells. Furthermore, the broad spectrum neuropeptide receptor antagonist [D-Arg, D = Phe, D-Trp, Leu11]substance P inhibits all neuropeptide-mediated signals (including PTK activation), SCLC cell growth in vivo and in vitro, and also increases the natural rate of apoptosis seen in growing SCLC cell lines. Hence the effect of selective PTK inhibition on SCLC cell growth and apoptosis was examined. We show that selective inhibition of PTK activity, with genistein and (3,4,5-tri-hydroxyphenyl)-methylene(-propanedinitrile) tyrphostin-25 inhibits basal and neuropeptide-stimulated SCLC cell growth. Genistein and tyrphostin-25 also stimulate apoptosis in SCLC cells. Inhibition of proliferation in these cells is intimately linke to apoptosis, because these changes occurred without any effect on SCLC cell cycle kinetics, suggesting that apoptosis occurs independently of the cell cycle and that failure to progress through the cell cycle results in apoptosis. Because tyrphostin-25 fails to influence p53 or Bcl-2 expression in these cells, this mode of programmed cell death appears to be via a p53- and Bcl-2-independent mechanism. These results provide evidence that tyrosine phosphorylation is a mitogenic signal in SCLC cells and suggest that regulation of the level of protein tyrosine phosphorylation represents a critical determinant of whether SCLC cells survive and proliferate or die by apoptosis. Thus PTK inhibition may provide a novel therapeutic option in SCLC that has become resistant to conventional chemotherapeutic agents.