The mechanisms of drug resistance associated with advanced, hormone-independent prostate carcinoma are poorly understood. The human prostate carcinoma PC-3 cell line, derived from a metastatic tumor and lacking androgen receptors, represents a useful model to investigate drug resistance.The effects of oncostatin M (OM), antiinterleukin-6 (IL-6) treatment, or interference with the gp130-mediated signaling on etoposide- or cisplatin-mediated cytotoxicity were investigated.Both endogenous and exogenous IL-6 and exogenous OM up-regulated cell growth and enhanced resistance of PC-3 tumor cells to both etoposide and cisplatin. The influence of IL-6 is controlled by treating PC-3 tumor cells with anti-IL-6 neutralizing antibody and, more efficiently, by a mutated IL-6, Sant7. Sant7 has a high affinity binding to the IL-6 receptor-alpha (IL-6Ralpha) subunit, but does not bind to the signaling subunit gp130; therefore, it behaves as a receptor antagonist. Both IL-6- and OM-mediated effects are inhibited by the treatment of PC-3 with an antisense oligodeoxynucleotide against gp130, the protein kinase inhibitor genistein (GNS), or the monoterpene perillic acid (PA), a posttranslational inhibitor of p21ras isoprenylation.These results demonstrate the protective roles in drug sensitivity of IL-6 and OM through signaling of the common chain gp130 and, most likely, a downstream ras-dependent pathway in PC-3 tumor cells. These findings suggest the potential clinical application of anticytokine therapy or interference with gp130 signaling in the treatment of drug resistant prostate carcinoma.