Promyelocytic Leukemia Protein Sensitizes Tumor Necrosis Factor α-Induced Apoptosis by Inhibiting the NF-κB Survival Pathway

The promyelocytic leukemia protein (PML) is a growth/tumor suppressor essential for induction of apoptosis by diverse apoptotic stimuli. The mechanism by which PML regulates cell death remains unclear. In this study we found that ectopic expression of PML potentiates cell death by apoptosis in the tumor necrosis factor alpha (TNF alpha )-resistant cell line U2OS and other cell lines. Treatment with TNF alpha significantly sensitized these cells to apoptosis in a p53-independent manner. PML/TNF alpha -induced cell death is associated with DNA fragmentation, activation of caspase-3, -7, and -8, and degradation of DNA fragmentation factor/inhibitor of CAD. PML/TNF alpha -induced cell death could be blocked by the caspase-8 inhibitors CrmA and c-FLIP but not by Bcl-2. These findings indicate that this cell death event is initiated through the death receptor-dependent apoptosis pathway. PML is a transcriptional repressor of NF- Kappa B by interacting with RelA/p65 and prevents its binding to the cognate enhancer through the C terminus. Coimmunoprecipitation and double-color immunofluorescence staining demonstrated that PML physically interacts with RelA/p65 in vivo and the two proteins colocalized at the endogenous levels. Overexpression of NF- Kappa B rescued cell death induced by PML/TNF alpha . Furthermore, PML super(-/-) mouse embryo fibroblasts are more resistant to TNF alpha -induced apoptosis. Together this study defines a novel mechanism by which PML induces apoptosis through repression of the NF- Kappa B survival pathway.