Inhibition of N-myc expression and induction of apoptosis by iron chelation in human neuroblastoma cells

Neuroblastoma is the second most common solid malignancy of childhood. Enha nced expression of the amplified N-myc gene in the tumor tells may be assoc iated with poor patient prognosis and may contribute to tumor development a nd progression. The use of deferoxamine mesylate (DFO), an iron chelator, t o treat neuroblastoma is being investigated in national clinical studies. W e show here by TUNEL assay and DNA laddering that DFO induces apoptosis in cultured human neuroblastoma cells, which is preceded by a decrease in the expression of N-myc and the altered expression of some other oncogenes (up- regulating c-fos and down-regulating c-myb) but not housekeeping genes. The decrease in N-myc expression is iron-specific but does not result from inh ibition of ribonucleotide reductase, because specific inhibition of this ir on-containing enzyme by hydroxyurea does not affect N-myc protein levels. N uclear run-on and transient reporter gene expression experiments show that the decrease in N-myc expression occurs at the level of initiation of trans cription and by inhibiting N-myc promoter activity. Comparison across neuro blastoma fell lines of the amount of residual cellular N-myc protein with t he extent of apoptosis measured as pan-caspase activity after 48 h of iron chelation reveals no correlation, suggesting that the decrease in N-myc exp ression is unlikely to mediate apoptosis, In conclusion, chelation of cellu lar iron by DFO may alter the expression of multiple genes affecting the ma lignant phenotype by multiple pathways, Given the clinical importance of N- myc overexpression in neuroblastoma malignancy, decreasing N-myc expression by DFO might be useful as an adjunct to current therapy.