High level MycN expression in non-MYCN amplified neuroblastoma is induced by the combination treatment nutlin-3 and doxorubicin and enhances chemosensitivity

MYCN gene amplification is a negative prognostic indicator in neuroblastoma and high level MycN expression in Stage IV neuroblastoma is generally a hallmark of poor patient outcome. However, high level expression of the MycN protein in neuroblastoma cells lacking MYCN amplification suppresses growth and drives apoptosis; this, in part, explains the absence of clinical observations of high level MycN in neuroblastoma lacking MYCN amplification. In the current study, we found that combination treatment with nutlin-3 and doxorubicin upregulated MycN expression in non-MYCN-amplified neuroblastoma cells at both the protein and mRNA levels. The induced expression of MycN in non-MYCN-amplified cells inhibited cell proliferation and increased apoptosis. MycN induction also upregulated p53, p21 and Bax protein levels, as well as mRNA levels for the positive neuroblastoma prognostic factors CD44 and EFNB3. Blocking MycN reversed these effects. These results were corroborated by findings using a MycN-inducible system in SHEP cells, another MYCN non-amplified neuroblastoma cell line. Our results indicate that doxorubicin/nutlin-3 combination treatment both induces expression of MycN in a non-MYCN-amplified background and sensitizes neuroblastoma cells to chemotherapy. These findings support the idea that induction of MycN in non-MYCN-amplified cells drives neuroblastoma cells toward apoptosis and suggest that combination nutlin-3/doxorubicin treatment may be clinically important.