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A Novel Role for Extracellular Signal-Regulated Kinase 5 and Myocyte Enhancer Factor 2 in Medulloblastoma Cell Death

¹ Program in Neuroscience, Department of Neurology, Children's Hospital Boston, Harvard Medical School, Boston, Massachusetts and ² Texas Children's Cancer Center, Baylor College of Medicine, Houston, Texas

Requests for reprints: Scott L. Pomeroy, Department of Neurology, Enders 260, Children's Hospital, 300 Longwood Avenue, Boston, MA 02115. Phone: 617-355-6874; Fax: 617-730-0242; E-mail: scott.pomeroy{at}childrens.harvard.edu.

Expression of the neurotrophin-3 receptor, tyrosine kinase C (TrkC), is associated with favorable prognosis in medulloblastoma patients. This may be due to increased tumor apoptosis induced by TrkC activation. Neurotrophin-3/TrkC–induced apoptosis is inhibited by the mitogen-activated protein (MAP) kinase (MAPK) pharmacologic antagonists SB203580 and PD98059. In addition to extracellular signal-regulated kinase (ERK)-1/2, PD98059 also inhibits the more recently identified neurotrophin-responsive MAPK, ERK5 (big MAPK 1). In the present study, we investigate the contribution of ERK5 and its target myocyte enhancer factor 2 (MEF2) to neurotrophin-3/TrkC–induced medulloblastoma cell death. Neurotrophin-3 not only enhanced ERK5 phosphorylation but also significantly enhanced the transcriptional activity of MEF2, a specific target of ERK5. Overexpression of both ERK5 and MEF2 induced a statistically significant increase in cell death of neurotrophin-3–responsive and nonresponsive medulloblastoma cell lines (Daoy-trkC and Daoy) and primary cultures of patched heterozygous mouse medulloblastomas. Only those cells expressing MAP/ERK kinase 5 (MEK5) plus ERK5 or MEF2 constructs underwent apoptosis, indicating that overexpression of either is sufficient to induce medulloblastoma cell death. Expression of a dominant-negative MEF2 or small interfering RNA for the ERK5 activator, MEK5, significantly inhibited neurotrophin-3–induced cell death. The dominant-negative MEF2 construct also blocked MEK5/ERK5-induced cell death, supporting a role for MEF2 downstream of ERK5. Coimmunoprecipitation studies revealed direct interaction of phosphorylated ERK5 with MEF2 in response to neurotrophin-3. Our investigation of the mechanism of neurotrophin-3/TrkC–induced apoptosis has identified a novel role for both MEK5/ERK5 and MEF2 in cell death, suggesting that these molecules can be exploited to induce apoptosis in both TrkC-expressing and nonexpressing medulloblastoma cells.

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