Smad7 Protein Induces Interferon Regulatory Factor 1-Dependent Transcriptional Activation of Caspase 8 to Restore Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand (Trail)-Mediated Apoptosis.

Background: Smad7 may have biological roles other than inhibition of TGF-β signaling. Results: Smad7 activates caspase 8 expression by recruiting IRF1, thereby inducing TRAIL-mediated apoptosis. Conclusion: Smad7 functions as a transcriptional coactivator of IRF1 to regulate the expression of target genes. Significance: Smad7-inducing reagents can be used as anti-cancer drugs for tumors that have defects in caspase 8 expression. Smad7 has been known as a negative regulator for the transforming growth factor-β (TGF-β) signaling pathway through feedback regulation. However, Smad7 has been suspected to have other biological roles through the regulation of gene transcription. By screening differentially regulated genes, we found that the caspase 8 gene was highly up-regulated in Smad7-expressing cells. Smad7 was able to activate the caspase 8 promoter through recruitment of the interferon regulatory factor 1 (IRF1) transcription factor to the interferon-stimulated response element (ISRE) site. Interaction of Smad7 on the caspase 8 promoter was confirmed with electrophoretic mobility shift assay and chromatin immunoprecipitation experiment. Interestingly, Smad7 did not directly interact with the ISRE site, but it increased the binding activity of IRF1 with ISRE. These results support that Smad7 recruits IRF1 protein on the caspase 8 promoter and functions as a transcriptional coactivator. To confirm the biological significance of caspase 8 up-regulation, we tested tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)-mediated cell death assay in breast cancer cells. Smad7 in apoptosis-resistant MCF7 cells markedly sensitized the cells to TRAIL-induced cell death by restoring the caspase cascade. Furthermore, restoration of caspase 8-mediated apoptosis pathway repressed the tumor growth in the xenograft model. In conclusion, we suggest a novel role for Smad7 as a transcriptional coactivator for caspase 8 through the interaction with IRF1 in regulation of the cell death pathway.