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Inhibition of Phosphatidylinositol 3-Kinase Destabilizes Mycn Protein and Blocks Malignant Progression in Neuroblastoma

Departments of ¹ Neurology, ² Pediatrics, ³ Pathology, and ⁴ Biostatistics Core, Comprehensive Cancer Center, and ⁵ Department of Neurological Surgery, University of California at San Francisco, San Francisco, California and ⁶ Dana-Farber Cancer Institute, Boston, Massachusetts

Requests for reprints: Louis Chesler, Pediatric Hematology Oncology, Department of Pediatrics, University of California at San Francisco, Room U-441K, 533 Parnassus Avenue, San Francisco, CA 94143. Phone: 415-502-1695; Fax: 415-476-0133; E-mail: cheslerl{at}peds.ucsf.edu.

Amplification of MYCN occurs commonly in neuroblastoma. We report that phosphatidylinositol 3-kinase (PI3K) inhibition in murine neuroblastoma (driven by a tyrosine hydroxylase-MYCN transgene) led to decreased tumor mass and decreased levels of Mycn protein without affecting levels of MYCN mRNA. Consistent with these observations, PI3K inhibition in MYCN-amplified human neuroblastoma cell lines resulted in decreased levels of Mycn protein without affecting levels of MYCN mRNA and caused decreased proliferation and increased apoptosis. To clarify the importance of Mycn as a target of broad-spectrum PI3K inhibitors, we transduced wild-type N-myc and N-myc mutants lacking glycogen synthase kinase 3ß phosphorylation sites into human neuroblastoma cells with no endogenous expression of myc. In contrast to wild-type N-myc, the phosphorylation-defective mutant proteins were stabilized and were resistant to the antiproliferative effects of PI3K inhibition. Our results show the importance of Mycn as a therapeutic target in established tumors in vivo, offer a mechanistic rationale to test PI3K inhibitors in MYCN-amplified neuroblastoma, and represent a therapeutic approach applicable to a broad range of cancers in which transcription factors are stabilized through a PI3K-dependent mechanism. (Cancer Res 2006; 66(16): 8139-46)

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