Characterization of a novel novobiocin analogue as a putative C-terminal inhibitor of heat shock protein 90 in prostate cancer cells

PURPOSE Hsp90 is important in the folding, maturation and stabilization of pro‐tumorigenic client proteins and represents a viable drug target for the design of chemotherapies. Previously, we reported the development of novobiocin analogues designed to inhibit the C‐terminal portion of Hsp90, which demonstrated the ability to decrease client protein expression. We now report the characterization of the novel novobiocin analogue, F‐4, which demonstrates improved cytotoxicity in prostate cancer cell lines compared to the N‐terminal inhibitor, 17‐AAG. MATERIALS AND METHODS LNCaP and PC‐3 cells were treated with 17‐AAG or F‐4 in anti‐proliferative, apoptosis, cell cycle and cytotoxicity assays. Western blot and prostate specific antigen (PSA) ELISAs were used to determine client protein degradation, induction of Hsp90 and to assess the functional status of the androgen receptor (AR) in response to F‐4 treatment. Surface plasmon resonance (SPR) was also used to determine the binding properties of F‐4 to Hsp90. RESULTS F‐4 demonstrated improved potency and efficacy compared to novobiocin in anti‐proliferative assays and decreased expression of client proteins. PSA secretion was inhibited in a dose‐dependent manner that paralleled a decrease in AR expression. The binding of F‐4 to Hsp90 was determined to be saturable with a binding affinity (Kd) of 100 µM. In addition, superior efficacy was demonstrated by F‐4 compared to 17‐AAG in experiments measuring cytotoxicity and apoptosis. CONCLUSIONS These data reveal distinct modes of action for N‐terminal and C‐terminal Hsp90 inhibitors, which may offer unique therapeutic benefits for the treatment of prostate cancer. Prostate 70: 27–36, 2010. © 2009 Wiley‐Liss, Inc.