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 aut ocrine and paracrine growth loops involving neuropeptides. The bombesi n family of peptides are autocrine growth factors in H345 SCLC cells a nd provide a paradigm for the study of growth factors and mitogenic si gnaling in SCLC cells, We show that bombesin (and other neuropeptides) stimulates protein tyrosine phosphorylation (particularly focal adhes ion kinase) and protein tyrosine kinase (PTK) activity in intact SCLC cells, Furthermore, the broad spectrum neuropeptide receptor antagonis t [D-Arg(1), D-Phe(5), D.Trp(7,9), Leu(11)]substance P inhibits all ne uropeptide-mediated signals (including PTK activation), SCLC cell grow th in vivo and in vitro, and also increases the natural rate of apopto sis seen in growing SCLC cell lines, Hence the effect of selective PTK inhibition on SCLC cell growth and apoptosis was examined, We show th at selective inhibition of PTK activity, with genistein and 4,5-tri-hy droxyphenyl)methylene(-propanedinitrile) tyrphostin-25 inhibits basal and neuropeptide-stimulated SCLC cell growth, Genistein and tyrphostin -25 also stimulate apoptosis in SCLC cells, Inhibition of proliferatio n in these cells is intimately linked to apoptosis, because these chan ges occurred without any effect on SCLC cell cycle kinetics, suggestin g that apoptosis occurs independently of the cell cycle and that failu re to progress through the cell cycle results in apoptosis, Because ty rphostin-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 p hosphorylation is a mitogenic signal in SCLC cells and suggest that re gulation 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 therapeuti c option in SCLC that has become resistant to conventional chemotherap eutic agents.