Epithelial expression of the 75-kDa low-affinity neurotrophin receptor (p75 NTR ) is inversely associated with the malignant progression of the human prostate. To elucidate the function of p75 NTR in the prostate, the human prostate epithelial tumor cell line TSU-pr1, which does not express p75 NTR , was stably and transiently transfected with the cDNA for the receptor. The stably transfected cells were assessed for levels of p75 NTR expression and categorized into low, intermediate, and high receptor-expressing clones by immunocytochemical and immunoblot analyses. Incorporation of [ 3 H]thymidine was used to assess nerve growth factor (NGF)induced changes in cell proliferation. TSU-pr1 epithelial cells transfected with a neomycin-resistance vector alone demonstrated a dose-dependent increase in the rate of NGF-stimulated [ 3 H]thymidine uptake. Expression of p75 NTR decreased the dose-dependent NGF-mediated proliferation of the TSU-pr1 prostate epithelial cells. The greater the degree of expression of p75 NTR in the transfected clones, the less the stimulatory effect of exogenous NGF on cell proliferation. Furthermore, the ratio of p75 NTR to tropomyosin receptor kinase for each clone was inversely correlated with the ability of NGF to stimulate growth of the TSU-pr1 transfectants. To determine whether p75 NTR -mediated growth inhibition of prostate epithelia occurs by induction of programmed cell death, transiently transfected clones were analyzed by an in situ DNA nick-translation assay. NGF deprivation and anti-NGF treatment of transiently transfected TSU-pr1 cells significantly increased the proportion of epithelial cells undergoing programmed cell death by approximately fourfold above control levels. Conversely, addition of NGF was able to rescue p75 NTR -expressing clones from undergoing programmed cell death at levels not significantly different from those of mock-transfected clones. These results demonstrate that p75 NTR is a negative regulator of human prostate epithelial cell growth by induction of programmed cell death. Hence, loss of p75 NTR expression in human prostate epithelia eliminates a growth-inhibitory pathway, thereby contributing to the malignant progression of the prostate.