Tumor suppressor PDCD4 inhibits NF-κB-dependent transcription in human glioblastoma cells by direct interaction with p65

PDCD4 is a tumor suppressor induced by apoptotic stimuli that regulates both translation and transcription. Previously, we showed that overexpression of PDCD4 leads to decreased anchorage-independent growth in glioblastoma (GBM)-derived cell lines and decreased tumor growth in a GBM xenograft model. In inflammatory cells, PDCD4 stimulates tumor necrosis factor-induced activation of the transcription factor NF-kappa B, an oncogenic driver in many cancer sites. However, the effect of PDCD4 on NF-kappa B transcriptional activity in most cancers including GBM is still unknown. We studied the effect of PDCD4 on NF-kappa B-dependent transcriptional activity in GBM by stably overexpressing PDCD4 in U251 and LN229 cells. Stable PDCD4 expression inhibits NF-kappa B transcriptional activation measured by a luciferase reporter. The molecular mechanism by which PDCD4 inhibits NF-kappa B transcriptional activation does not involve inhibited expression of NF-kappa B p65 or p50 proteins. PDCD4 does not inhibit pathways upstream of NF-kappa B including the activation of IKK alpha and IKK beta kinases or degradation of I kappa B alpha, events needed for nuclear transport of p65 and p50. PDCD4 overexpression does inhibit localization of p65 but not p50 in the nucleus. PDCD4 protein interacts preferentially with p65 protein as shown by co-immunoprecipitation and confocal imaging. PDCD4 overexpression inhibits the mRNA expression of two NF-kappa B target genes in a p65-dependent manner. These results suggest that PDCD4 can significantly inhibit NF-kappa B activity in GBM cells by a mechanism that involves direct or indirect protein-protein interaction independent of the expected mRNA-selective translational inhibition. These findings offer novel opportunities for NF-kappa B-targeted interventions to prevent or treat cancer.