MYCN enhances P-gp/MDR1 gene expression in the human metastatic neuroblastoma IGR-N-91 model.

Despite intensive high-dose chemotherapy and autologous hematopoietic stem cell transplantation, disseminated neuroblastoma (NB) frequently proves to be chemosensitive but not chemocurable, and more often so in NB-presenting MYCN amplification. To assess the direct relationship between the MYCN oncogene and chemoresistance acquisition during NB metastatic dissemination, we have studied MYCN and MDR1 genes using the human IGR-N-91 ectopic xenograft metastatic model. This characterized experimental in vitro model includes human neuroblasts derived from a subcutaneous primary tumor xenograft, disseminated blood cells, myocardium, and bone marrow (BM) metastatic cells. All IGR-N-91-derived neuroblasts harbor a consistent MYCN genomic content but, unlike primary tumor xenograft, BM, and myocardium, human neuroblasts elicit a concomitant increase in MYCN and MDR1 transcripts levels, consistent with chemoresistance phenotype and active P-gp. In contrast, no variation of MRP1 transcript level was associated with the metastatic process in this model. Using an MDR1 promoter-CAT construct, we have shown that the MycN protein activates MDR1 transcription both in exogenous transient MYCN-transfected SK-N-SH cells and in endogenous BM metastatic neuroblasts with an increase in the MYCN transcript level. Band-shift experiments indicate that IGR-N-91 cells enriched with the MycN transcription factor do bind to two E-box motifs localized within the MDR1 promoter. Overall, our data indicate that MYCN overexpression increment contributes to the acquired drug resistance that occurs throughout the NB metastatic process.