Bombesin-like peptides, including the mammalian homologue gastrin-releasing peptides, are highly expressed and secreted by neuroendocrine cells in prostate carcinoma (PCa) tissues and are likely to be related to the progression of this disease. In the present study, we show that bombesin enhances the migration of androgen-independent PCa cells (PC-3) in vitro, while not affecting their adhesion to extracellular matrix proteins. The bombesin-increased motility of PC-3 cells occurs through its receptor, and, as shown with inhibitors, it likely requires activation of both protein tyrosine kinases (PTKs) and protein kinases C (PKCs). Because the focal adhesion kinase pp125 FAK plays a key role in adhesion/ motility and is highly expressed in advanced PCa, we examined whether in PC-3 cells bombesin signal transduction triggers the tyrosine phosphorylation of this PTK and of associated integrins and signaling proteins likely to be present in focal adhesion plaques. pp125 FAK tyrosine phosphorylation was stimulated by bombesin and mimicked by PKC activation with the tumor-promotor phorbol 12-myristate-13-acetate (PMA). Moreover, this effect of bombesin on pp125 FAK tyrosine phosphorylation requires the presence of both active PKC and cytoskeleton integrity since this signal was abolished by down-regulating PKCs induced by prolonged PMA treatment or by PKC inhibition with GF 109203X, as well as by disruption of the cytoskeleton with cytochalasin D. We also show that bombesin increases the tyrosine phosphorylation of a 95-kDa protein (pp95) which was co-immunoprecipitated with the av and b (3 and 5) subunits, forming integrin receptors with av in PC-3 cells. The protein pp95 is distinct from the endogenously tyrosine-phosphorylated b3 subunit. In addition, upon bombesin treatment, the b1, b3 and b5 integrin subunits co-immunoprecipitated with pp125 FAK and major phosphotyrosine (pY)-containing proteins of 125 and 68-70 kDa, likely corresponding to pp125 FAK and paxillin. Together our data suggest that, in addition to PKC activation, tyrosine phosphorylation of pp125 FAK and integrin-associated proteins may play an important role in bombesin signaling, triggering the processes of PCa cell motility and invasion. Int.