The retinoic acid analogue 4HPR, IFN-gamma, and the demethylating agent 5-aza-cytidine activate this promoter in NB cells that lack endogenous caspase-8, indicating that this element may regulate both constitutive and inducible CASP8 expression.
The authors found that interferon-gamma induces caspase-8 expression in neuroblastoma cells irrespective of the gene silenced by hypermethylation of caspase-8 promoter.
These results indicate that the profile of caspase 8 expression is an important determinant of the response of neuroblastoma cells to Fas-mediated cell death.
However, approximately 20% of the neuroblastoma cell lines with methylated CASP8 alleles are also highly resistant to staurosporine (STS)- and radiation-induced cell death, presumably because cytochrome c is not released from mitochondria.
Caspase-8 silenced N-type invasive NB cell lines LAN-1 and IMR-32 were investigated for their sensitivity to dox, and compared to S-type noninvasive SH-EP NB cells expressing caspase-8.
A missense mutation was detected at codon 96, GCT (Alanine) to GTT (Valine), of the caspase 8 gene in one of the NB cell lines lacking caspase 8 expression.
These data indicate that Fas-mediated apoptosis in neuroblastoma cells is mitochondria-dependent and inhibited both at the mitochondrial level and at the level of caspase 8 activation.
As frequent methylation of the CASP8 gene has also been reported in neuroblastoma, we investigated whether RASSF1A and CASP8 methylation were independent or related events.
Subsequently others have also shown that caspase-8 is silenced by methylation in neuroblastoma and peripheral neural ectodermal tumors, and that the caspase-9 regulator Apaf-1 is silenced by methylation in melanoma cell lines and patient samples.
Caspase 8-null neuroblastoma cells were resistant to death receptor- and doxorubicin-mediated apoptosis, deficits that were corrected by programmed expression of the enzyme.