Antitumor effect of an HER2-specific antibody-toxin fusion protein on human prostate cancer cells

BACKGROUND. HER2/neu has been implicated in the oncogenesis of human prosta te cancer. Clinical studies have suggested that overexpression of HER2 may be one of the indicators of poor prognosis in prostate cancer patients. METHODS. We used Western blot analysis to examine the expression of HER2 in a panel of established human prostate cancer cell lines and used an MTT as say to evaluate the cytotoxicity on these cells of a recombinant fusion pro tein consisting of an HER2-specific single-chain antibody and the Pseudomon as exotoxin A, scFv(FRP5)-ETA. RESULTS. LNCaP cells express high levels of HER2 protein. Exposure of LNCaP cells to scFv(FRP5)-ETA caused remarkable cell death. In contrast, PC3M ce lls, which express an undetectable level of HER2 protein, were resistant to scFv(FRP5)-ETA-induced cytotoxicity. MDA PCa 2a, MDA PCa 2b, and DU145 cel ls express low-to-medium levels of HER2 protein and showed an HER2 level-de pendent response to scFv(FRP5)-ETA-induced cytotoxicity. The scFv(FRP5)-ETA -induced cytotoxicity of LNCaP cells could be inhibited by an anti-HER2 mon oclonal antibody (mAb), which downregulated the levels of HER2 protein, ind icating the specificity of scFv(FRP5)-ETA in inducing cytotoxicity in LNCaP cells. Using an apoptosis ELISA, we demonstrated that scFv(FRP5)-ETA induc ed apoptosis in LNCaP cells. The apoptosis was inhibited by the presence of dihydrotestosterone (DHT) in culture medium. Exposure of LNCaP cells to sc Fv(FRP5)-ETA caused reduction in the level of the prostate-specific antigen (PSA). CONCLUSIONS. Our data suggest that scFv(FRP5)-ETA might be a useful agent f or the treatment of human prostate cancer cells with high levels of HER2 ex pression. Prostate 47: 21-28, 2001. (C) 2001 Wiley-Liss, Inc.