Bombesin and gastrin-releasing peptide (GRP) are potent neuropeptides expressed by prostate cancer neuroendocrine cells and are related to the progression of this malignancy. This study characterizes bombesin receptors in human prostate cancer cell lines (PC-3, DU-145, LNCaP) and assesses the in vitro effect of bombesin on signal transduction and cell proliferation. [125I]Tyr4-bombesin binding assays (37 °C) and Scatchard analyses revealed the presence of a single class of high-affinity receptors with similar K d values (1·5, 1·1 and 3·6 × 10−10 m in PC-3, DU-145 and LNCaP cells respectively) but with significant differences in the number of binding sites per cell (47·6, 1·5 and 0·1 × 103 in PC-3, DU-145 and LNCaP cells respectively). Molecular characterization of the binding sites performed in PC-3 cells by cross-linking experiments and SDS/PAGE revealed a single radioactive band of 85 kDa. To determine which of the three known bombesin receptor subtypes (GRP receptor (GRP-R), neuromedin B receptor, bombesin receptor subtype-3) were expressed in the cell lines, reverse transcription/PCR analysis of cellular RNA followed by hybridization with receptor-specific cDNA was performed. This revealed the presence of GRP-R transcript in all cell lines, while neither of the other two receptor transcripts were expressed. When intracellular calcium mobilization was measured by Fura-2/AM cell labeling and spectrofluorometric monitoring, bombesin (100 nm) induced rapid calcium mobilization in both PC-3 (>200% of baseline) and DU-145 (>100% of baseline) cells, but not in LNCaP cells. However, as measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and [3H]thymidine incorporation, no growth modulation was observed with bombesin or bombesin receptor antagonist at various concentrations (0-500 nm). Our data indicate that bombesin is a potent inducer of signal transduction via GRP-R receptors in androgen-insensitive PC-3 and DU-145 prostate cancer cells. This suggests that the bombesin/GRP family of neuropeptides may play a regulatory role in the biology of androgen-independent prostate cancer.