Role of MYCN on ERAP1 and ERAP2 expression in neuroblastoma

164

In this study we have investigated the molecular mechanism involved in the expression of two major components of the antigen processing machinery by MYCN in the most aggressive forms of neuroblastoma.
 Neuroblastoma (NB) is the most common neuroectodermal cancer of childhood. The most aggressive forms are immunologically characterized by a severe reduction in major histocompatibility complex (MHC) class I molecules associated with amplification of the MYCN oncogene. The MHC class I presented on the cell surface plays an important role in the killer T lymphocyte-mediated immune response. We have recently shown that human cell lines from various tumours show de-regulated expression of two ER aminopeptidases, ERAP1 and ERAP2, that are closely involved in the generation of HLA class I binding peptides. HLA class I expression in such cell lines correlates significantly with ERAP1 expression. Here, in order to explore the possibility that the enhanced expression of MYCN oncogene may affect expression of ERAP1 and ERAP2, twelve NB cell lines were analysed for expression of MYCN, ERAP1, ERAP2 and HLA class I molecules. All NB cell lines expressing moderate to high levels of MYCN expression were low in HLA class I, ERAP1 and ERAP2 expression. Conversely, all NB cell lines low in MYCN expression were high in ERAP1 and/or HLA class I and ERAP2 expression. In the MYCN amplified NB cell lines, the expression of ERAP1, ERAP2 and HLA class I could be restored by IFN-γ, suggesting that these defects are non-structural in nature, but rather reflect abnormalities in regulatory mechanisms involved in the expression of these molecules.
 MYCN contributes to oncogenesis by activating and repressing a large repertoire of genes. Because the repressor function of N-Myc has been found critical for cell transformation, to verify whether MYCN affects the expression of ERAP1 and ERAP2, we studied a NB cell line transfected with a MYCN vector regulated by tetracycline. Our data indicate that MYCN regulates the expression of these molecules.
 MYCN performs its repressive function by interfering with positive transcription factors. To identify the transcription factor(s) involved in the ERAPs expression, several potential candidates were overexpressed in MYCN amplified (SH-EP21N) and not amplified (SH-EP) cell lines and ERAP1 and ERAP2 were analysed. Overexpression of two transcription factors enhanced expression of ERAP1 and ERAP2 in not amplified MYCN SH-EP cell line, but not in MYCN amplified SH-EP21N cell line. These data suggest that MYCN interaction with these transcription factors reduces their transcriptional activity.
 We show that NB is characterized by complicated mechanisms correlated to antigen processing components deficiency through MYCN overexpression. Such mechanisms play an important role in immune evasion of neuroblastoma tumor.