Opioid alkaloids and casomorphin peptides decrease the proliferation of prostatic cancer cell lines (LNCaP, PC3 and DU145) through a partial interaction with opioid receptors

Opioid agonists (ethylketocyclazocine, etolphine, [D-Ala(2),D-Leu(5)]enkephalin (DADLE), [D-Ala(2), N-Me-Phe(4)-Gly-ol]enkephalin (DAGO), [D-Ser(2),Leu(5)]enkepharin-Thr(6) (DSLET) and morphine were found to inhibit the proliferation of human prostate cancer cell lines (LNCaP, DU145, and PC3), in a dose-dependent manner. The 50% inhibitory concentrations (IC50) were in the picomolar range. In many cases, this effect was antagonized by the general opioid antagonist, diprenorphine, indicating the existence of specific opioid binding sites. Saturation binding experiments with selective ligands and effecters showed no opioid sites on the LNCaP cell line, kappa(1) and mu sites on the PC3 cell line, and kappa(1), kappa(3) and mu sites on the DU145 cell line. In other cases, the opioid effect was not antagonized by diprenorphine, indicating that the action of opioids might be mediated through other membrane receptors. Furthermore, casomorphin peptides, issued from bovine alpha-(alpha-casein-90-95 and alpha-casein-90-96) and beta-caseins (beta-casomorphin and beta-casomorphin-1-5), and human alpha(S1)-casein (alpha(S1)-casomorphin and alpha(S1)-casomorphin amide) inhibited cell proliferation of human prostate cell lines, also by a mechanism partly involving opioid receptors. As opioid neurons can be found in the prostate gland, and casomorphin peptides might reach the gland through the general circulation, the above findings indicate a putative role of opioids in prostate cancer cell growth. (C) 1997 Elsevier Science B.V.