A novel function of differentiation revealed by cDNA microarray profiling of p75NTR-regulated gene expression

The expression of the p75 neurotrophin receptor (p75NTR) is diminished in epithelial cells during progression of prostate cancer in vivo and in vitro. Previous studies have demonstrated a role for p75NTR as a tumor suppressor in prostate growth. To better understand the molecular mechanism of p75NTR on tumor suppression, we utilized a complementary deoxyribonucleic acid microarray composed of ∼6,000 human cancer-related genes to determine the gene expression pattern altered by re-introduction of p75NTR into PC-3 prostate tumor cells. Comparison of the transcripts in the neo and p75NTR-transfected cells revealed 52 differentially expressed genes, of which 21 were up-regulated and 31 were down-regulated in the presence of p75NTR. Based on the known biological functions of the p75NTR-regulated genes, we observed that p75NTR modulated the expression of genes that are critically involved in the regulation of differentiation as well as cell adhesion, signal transduction, apoptosis, tumor cell invasion, and metastasis. Several differentially expressed genes identified by microarray were selected for confirmation using quantitative real-time polymerase chain reaction. Immunoblot analysis further confirmed increased cellular retinoic acid-binding protein I (CRABPI) and IGFBP5 protein levels and decreased level of PLAUR protein with increasing p75NTR protein expression. As CRABPI was elevated far more than any other genes, we observed that the retinoids, all-trans retinoic acid and 9-cis retinoic acid, that bind CRABPI, promoted nitroblue tetrazolium-associated functional cell differentiation in p75NTR PC-3 cells, but not in neo control PC-3 cells. Subsequent examination of the retinoic acid receptors (RARs) expression levels demonstrated an absence of RAR-β in the neo control cells and re-expression in the p75NTR expressing cells, consistent with previous findings where RAR-β is believed to play a critical role as a tumor suppressor gene that is lost during de-differentiation of prostate epithelial cells. Whereas the RAR-α and -γ protein levels remained unchanged, retinoid X receptor (RXR)-α and -β also exhibited increasing protein levels with re-expression of the p75NTR protein. Moreover, the ability of p75NTR siRNA to knockdown levels of RAR-β, RXR-α, and RXR-β supports the specificity of the functional involvement of p75NTR in differentiation. Hence, re-expression of the p75NTR appears to partially reverse de-differentiation of prostate cancer cells by up-regulating the expression of CRABPI for localized sequestration of retinoids that are available to newly up-regulated RAR-β, RXR-α, and RXR-β.