To evaluate synthetic small interference RNA (siRNA) compounds targeting heat‐shock protein 27 (Hsp27) as an alternative approach to Hsp27 ‘knockdown’ in prostate cancer cells, as Hsp27 expression is highly up‐regulated in prostate cancer cells after androgen withdrawal or chemotherapy, to become uniformly highly expressed in androgen‐independent (AI) prostate cancer.

We recently showed that targeting Hsp27 by a 2′‐methoxyethyl modified phosphorothioate antisense oligonucleotide, OGX‐427, inhibits Hsp27 expression and enhances hormone‐ and chemotherapy in prostate cancer xenograft models. In the present study, a ‘gene walk’ screening different siRNAs was initially used in PC‐3 and LNCaP cells to determine the most potent sequence to down‐regulate Hsp27 mRNA and protein levels. The effects of Hsp27 silencing on in vitro growth rates were studied by tetrazolium‐blue and crystal violet assays. Apoptosis was determined by single‐stranded DNA nuclear and cleaved caspase‐3 immunostaining, as well as flow cytometry. Spotted microarrays with 14000 human oligonucleotides were used to examine changes in gene expression.

Low concentrations of 1 nm siRNA decreased Hsp27 mRNA levels by 19‐fold and suppressed protein expression to undetectable levels. Silencing of Hsp27 in prostate cancer cells by siRNA♯2 increased apoptotic rates 2.4–4 fold and caused 40–76% inhibition of cell growth in LNCaP and PC‐3 cells. Characteristic cleavage of caspase‐3 occurred after treatment with Hsp27 siRNA (1 nm). cDNA microarray analysis from LNCaP and PC‐3 cell lines revealed differential gene expression profiles after Hsp27 down‐regulation that could be used to identify various survival pathways involved in androgen‐dependent and AI growth.

These findings illustrate the potential utility of Hsp27‐silencing therapy and highlight Hsp27 siRNA strategies as a novel and highly effective tool, with the potential for future targeted therapy in enhancing the efficacy of chemotherapy in advanced prostate cancer.