Inhibition of cell‐cycle effectors of proliferation in bladder tumor epithelial cells by the p75NTR tumor suppressor

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Abstract
The neurotrophin (NTR) receptor (p75NTR) 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 G1 with a concomitant reduction of cells in the S phase of the cell cycle. Furthermore, p75NTR was shown to be an effective tumor suppressor of bladder cancer cell growth in vivo. In order to investigate the mechanism of p75NTR-dependent suppression of cell-cycle progression, we utilized transgenic clones of bladder tumor cells that express p75NTR in increasing concentrations to demonstrate an effect of p75NTR on the levels of cell-cycle regulatory proteins that modulate proliferation of tumor cells. A rank-order (dose-dependent) increase in p75NTR protein expression was associated with a decrease in cell proliferation. This p75NTR-dependent suppression of proliferation was rescued with NGF. In the absence of ligand, a dose-dependent increase in p75NTR 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 p16Ink4a with increasing p75NTR expression. Treatment of tumor cells with NGF ameliorated these p75NTR-dependent changes in the levels of cell-cycle regulatory proteins and rescued the tumor cells from p75NTR-dependent inhibition of proliferation. Hence, it can be concluded that p75NTR inhibits proliferation by altering the expression of cell-cycle regulatory proteins and that NGF ameliorates this effect.