OBJECTIVES: Valproic acid (VPA) and trichostatin A (TSA) exert antitumor 
activity as histone deacetylase inhibitors, whereas ellipticine action is based 
mainly on DNA intercalation, inhibition of topoisomerase II and formation of 
cytochrome P450 (CYP)- and peroxidase-mediated covalent DNA adducts. This is the 
first report on the molecular mechanism of combined treatment of human 
neuroblastoma UKF-NB-3 and UKF-NB-4 cells with these compounds.
METHODS: HPLC with UV detection was employed for the separation and 
characterization of ellipticine metabolites formed by microsomes and 
peroxidases. Covalent DNA modifications by ellipticine in neuroblastoma cells 
and in incubations with microsomes and peroxidases were detected by 
32P-postlabeling. Expression of CYP enzymes, peroxidases and cytochrome b5 was 
examined by Western blot.
RESULTS: The cytotoxicity of ellipticine to neuroblastomas was increased by 
pre-treating these cells with VPA or TSA. A higher sensitivity of cells to 
ellipticine correlated with an increase in formation of covalent 
ellipticine-derived DNA adducts in these cells. To evaluate the mechanisms of 
this finding, we investigated the modulation by VPA and TSA of CYP- and 
peroxidase-mediated ellipticine-derived DNA adduct formation in vitro. The 
effects of ellipticine in the presence of VPA and TSA on expression of CYPs and 
peroxidases relevant for ellipticine activation and levels of cytochrome b5 and 
P-glycoprotein in neuroblastoma cells were also investigated. Based on these 
studies, we attribute most of the enhancing effects of VPA and TSA on 
ellipticine cytotoxicity to enhanced ellipticine-DNA adduct formation caused by 
an increase in levels of cytochrome b5, CYP3A4 and CYP1A1 in neuroblastoma 
cells. A lower sensitivity of UKF-NB-4 cells to combined effects of ellipticine 
with VPA and TSA than of UKF-NB-3 cells is also attributable to high levels of 
P-glycoprotein expressed in this cell line.
CONCLUSION: The results found here warrant further studies and may help in the 
design of new protocols geared to the treatment of high risk neuroblastomas.