FOXO3/FKHRL1 is activated by 5-aza-2-deoxycytidine and induces silenced caspase-8 in neuroblastoma

It is shown for the first time that FOXO3/FKHRL1 induces caspase-8 expression via the ATM-CREB pathway independent of caspase-8 gene methylation status. Induction of caspase-8 by the DNA-methylation inhibitor 5-azadC also depends on FOXO3, suggesting that 5-azadC triggers gene expression via the FOXO3-ATM-CREB pathway.
Forkhead box O (FOXO) transcription factors control diverse cellular functions, such as cell death, metabolism, and longevity. We analyzed FOXO3/FKHRL1 expression and subcellular localization in tumor sections of neuroblastoma patients and observed a correlation between nuclear FOXO3 and high caspase-8 expression. In neuroblastoma caspase-8 is frequently silenced by DNA methylation. Conditional FOXO3 activated caspase-8 gene expression but did not change the DNA-methylation pattern of regulatory sequences in the caspase-8 gene. Instead, FOXO3 induced phosphorylation of its binding partner ATM and of the ATM downstream target cAMP-responsive element binding protein (CREB), which was critical for FOXO3-mediated caspase-8 expression. Caspase-8 levels above a critical threshold sensitized neuroblastoma cells to tumor necrosis factor–related apoptosis-inducing ligand–induced cell death. The DNA-demethylating drug 5-Aza-2-deoxycytidine (5-azadC) induced rapid nuclear accumulation of FOXO3, ATM-dependent CREB phosphorylation, and caspase-8 expression in a FOXO3-dependent manner. This indicates that 5-azadC activates the FOXO3-ATM-CREB signaling pathway, which contributes to caspase-8 expression. The combined data suggest that FOXO3 is activated by 5-azadC treatment and triggers expression of caspase-8 in caspase-8–negative neuroblastoma, which may have important implication for metastasis, therapy, and death resistance of this childhood malignancy.