AbstractThe type I insulin‐like growth factor receptor (IGF‐IR) is involved in tumour cell proliferation, invasion, and cancer cell survival. Several studies indicate that the IGF axis contributes to prostate cancer pathogenesis, but there is no consensus regarding the relative expression of the IGF‐IR in benign and malignant prostate epithelium. In this study, endogenous IGF‐IR gene expression was reduced in stably transfected PC‐3 cells by employing an antisense RNA strategy which resulted in significant suppression of both PC‐3 cell invasion and proliferation in vitro. Furthermore, it was demonstrated that a direct correlation exists between the inhibition of IGF‐IR gene expression and either up‐regulation of IGF binding protein (BP)‐3 or down‐regulation of matrix metalloproteinase (MMP)‐2 expression in androgen‐independent PC‐3 cells. Moreover, inhibition of IGF‐IR gene expression in transfected PC‐3 cells leads to an enhanced rate of spontaneous apoptosis. In addition, expression analyses by quantitative RT‐PCR on RNA from laser microdissected matched normal prostate and prostate tumour samples revealed that IGF‐IR gene expression was up‐regulated in nine of 12 prostate cancers, whereas IGFBP‐3 gene expression was down‐regulated in all 12 prostate carcinomas analysed. These results indicate an important role for IGF‐IR and IGFBP‐3 in the homeostasis of prostate carcinoma cells and provide a further basis for targeting IGF‐IR or IGFBP‐3 gene expression in order to improve understanding of the IGF‐IR‐activated signalling pathways and as a potential treatment for prostate cancer. Copyright © 2004 John Wiley & Sons, Ltd.