Abstract
The neurotrophin (NTR) receptor (p75^NTR^) is a cell‐surface glycoprotein that binds to the neurotrophin family of growth factors, of which the prototypic member is nerve growth factor (NGF). This receptor was previously shown to retard cell‐cycle progression by inducing accumulation of cells in G~1~ with a concomitant reduction of cells in the S phase of the cell cycle. Furthermore, p75^NTR^ was shown to be an effective tumor suppressor of bladder cancer cell growth in vivo. In order to investigate the mechanism of p75^NTR^‐dependent suppression of cell‐cycle progression, we utilized transgenic clones of bladder tumor cells that express p75^NTR^ in increasing concentrations to demonstrate an effect of p75^NTR^ on the levels of cell‐cycle regulatory proteins that modulate proliferation of tumor cells. A rank‐order (dose‐dependent) increase in p75^NTR^ protein expression was associated with a decrease in cell proliferation. This p75^NTR^‐dependent suppression of proliferation was rescued with NGF. In the absence of ligand, a dose‐dependent increase in p75^NTR^ protein expression was associated with reduced expression of cyclin D1, cyclin E, and cyclin‐dependent kinase 2 (cdk2) as well as decreased cdk2 activity. There was also a decrease in the expression of hyper‐phosphorylated retinoblastoma protein, the transcription factor E2F1, and proliferating cell nuclear antigen, and there was an increase in expression of hypophosphorylated Rb and the cdk inhibitor p16^Ink4a^ with increasing p75^NTR^ expression. Treatment of tumor cells with NGF ameliorated these p75^NTR^‐dependent changes in the levels of cell‐cycle regulatory proteins and rescued the tumor cells from p75^NTR^‐dependent inhibition of proliferation. Hence, it can be concluded that p75^NTR^ inhibits proliferation by altering the expression of cell‐cycle regulatory proteins and that NGF ameliorates this effect. © 2003 Wiley‐Liss, Inc.