The p75NTR tumor suppressor induces caspase‐mediated apoptosis in bladder tumor cells

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Abstract
P75NTR was identified as a tumor and metastasis suppressor that functions in part via induction of apoptosis in tumor cells. To examine p75NTR-dependent apoptosis in tumor cells, we demonstrated that a dose-dependent increase in p75NTR expression was associated with a concomitant increase in the mitochondrial proapoptotic effector proteins Bad, Bax and Bik and a decrease in the mitochondrial prosurvival effector proteins phospho-Bad, Bcl-2 and Bcl-xL. Significantly, p75NTR-dependent induction of cytochrome c release from the mitochondria occurred during CHX potentiation of apoptosis. Furthermore, p75NTR expression largely suppressed expression of IAP-1 and induced cleavage of procaspase-9 and procaspase-7 but not of procaspases 2, 3, 6, 8 and 10. A specific peptide inhibitor of procaspase-9 cleavage also inhibited cleavage of procaspase-7, indicating that caspase-7 is downstream of caspase-9. As end points of apoptosis, we observed p75NTR-dependent annexin V binding to the plasma membrane, an indicator of early apoptotic events, and Hoechst staining of DNA nuclear fragmentation, an indicator of late apoptotic events, whereas control tumor cells that lack expression of the p75NTR protein did not exhibit either of these apoptotic markers. Together, these results delineate the mitochondria-mediated apoptotic pathway of the p75NTR tumor-suppressor gene product.