The human prostate gland is a well known source of H1 and H2 relaxin but information is lacking on the expression and potential role of the INSL3 peptide hormone within the prostate gland. In the present study we have investigated the expression of human INSL3 in patients with benign prostate hyperplasia (BPH), prostate intraepithelial neoplasia (PIN) and prostate carcinoma tissues. Of the prostate epithelial cells, strongest INSL3 expression was detected in the basal epithelial cell compartment. Weaker INSL3 mRNA expression and immunoreactive INSL3 production were observed in secretory epithelial cells and in interstitial smooth muscle cells. Prostate epithelial cells were also a source for transcripts encoding the INSL3 receptor LGR8 suggesting the presence of an autocrine/paracrine INSL3-LGR8 ligand-receptor system within the human prostate. Three human prostate carcinoma cell lines displayed differential gene activity for INSL3 and LGR8. While LNCaP was devoid of INSL3, the androgen-insensitive PC-3 and the stromal prostate cell line hPCP co-expressed INSL3 and LGR8 transcripts. In addition to expressing INSL3 mRNA, the LGR8-negative DU-145 also expressed an INSL3 splice form formerly demonstrated in thyroid carcinoma cells. When incubated with recombinant INSL3, PC-3 cells showed significantly increased intracellular cAMP levels indicating functional LGR8 receptors. INSL3 did not alter the proliferative or metabolic activity of PC-3 carcinoma cells. Instead, PC-3 responded to INSL3 with significantly enhanced tumor cell motility and a transcriptional down-regulation of ErbB receptors and EGF. All-trans-retinoic acid was demonstrated in PC-3 to up-regulate LGR8 gene activity in a dose- and time-dependent manner while having no effect on INSL3 gene activity. In conclusion, we have identified a functional INSL3-LGR8 ligand-receptor system in human prostate carcinoma cells.