N-Myc Differentially Regulates Expression of MXI1 Isoforms in Neuroblastoma

• Published in: Neoplasia (New York, N.Y.) Vol. 15; no. 12; pp. 1363 - 1370
• Main Authors: Armstrong, Michael B, Mody, Rajen J, Ellis, D. Christian, Hill, Adam B, Erichsen, David A, Wechsler, Daniel S
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01-12-2013; Elsevier
• Subjects: Basic Helix-Loop-Helix Transcription Factors - genetics; Basic Helix-Loop-Helix Transcription Factors - metabolism; Cell Line, Tumor; Cell Proliferation; Gene Amplification; Gene Expression; Gene Expression Regulation, Neoplastic; Humans; N-Myc Proto-Oncogene Protein; Neuroblastoma; Nuclear Proteins - physiology; Oncogene Proteins - physiology; Oncology; Promoter Regions, Genetic; Protein Isoforms - genetics; Protein Isoforms - metabolism; RNA, Messenger - genetics; RNA, Messenger - metabolism; Tumor Suppressor Proteins - genetics; Tumor Suppressor Proteins - metabolism; NB; neuroblastoma
• ISSN: 1476-5586; 1522-8002; 1476-5586; 1522-8002
• DOI: 10.1593/neo.11606

Abstract Amplification of the MYCN proto-oncogene is associated with a poor prognosis in patients with metastatic neuroblastoma (NB). MYCN encodes the N-Myc protein, a transcriptional regulator that dimerizes with the Max transcription factor, binds to E-box DNA sequences, and regulates genes involved in cell growth and apoptosis. Overexpression of N-Myc leads to transcriptional activation and an increase in NB cell proliferation. Mxi1, a member of the Myc family of transcriptional regulators, also binds to Max. However, Mxi1 is a transcriptional repressor and inhibits proliferation of NB cells, suggesting that Mxi1 functions as an N-Myc antagonist. Our laboratory previously identified Mxi1-0, an alternatively transcribed Mxi1 isoform. Mxi1-0 has properties distinct from those of Mxi1; in contrast to Mxi1, Mxi1-0 is unable to suppress c-Myc-dependent transcription. We now show that Mxi1-0 expression increases in response to MYCN overexpression in NB cells, with a positive correlation between MYCN and MXI1-0 RNA levels. We also show that N-Myc expression differentially regulates the MXI1 and MXI1-0 promoters: Increased MYCN expression suppresses MXI1 promoter activity while enhancing transcription through the MXI1-0 promoter. Finally, induction of Mxi1-0 leads to increased proliferation, whereas expression of Mxi1 inhibits cell growth, indicating differential roles for these two proteins. These data suggest that N-Myc differentially regulates the expression of MXI1 and MXI1-0 and can alter the balance between the two transcription factors. Furthermore, MXI1-0 appears to be a downstream target of MYCN-dependent signaling pathways and may contribute to N-Myc-dependent cell growth and proliferation.