Date

September 2011

Document Type

Dissertation

Degree Name

Ph.D.

Department

Dept. of Cell and Developmental Biology

Institution

Oregon Health & Science University

Abstract

Members of the Myc family – N-Myc, c-Myc, and L-Myc – have prominent roles in embryonic development and tumorigenesis. Myc proteins are transcription factors that control the expression of genes involved in diverse cellular processes such as proliferation, growth, inhibition of differentiation, metabolism, and apoptosis. Overexpression of N-Myc, in particular, contributes to the malignant progression of the pediatric cancer neuroblastoma and is a strong predictor of poor prognosis. MYCN-gene amplification is one mechanism that contributes to N-Myc overexpression. However, mechanisms that deregulate N-Myc at the post-transcriptional level are less well understood. The purpose of this thesis is to understand how the small GTPase Ras controls N-Myc at the post-transcriptional level. Ras communicates extracellular signals to the intracellular portion of the cell by activating a number of signal transduction pathways. In turn, these signaling pathways regulate the expression and activity of multiple factors, including N-Myc. Mutations within Ras or, more commonly, alterations to upstream components that activate Ras, are observed in N-Myc-associated cancers. Therefore, understanding how Ras activation regulates N-Myc may provide points of intervention to suppress aberrant N-Myc levels and its oncogenic activity in cancer. Here I show that Ras activation promotes both the synthesis and degradation of N-Myc protein. The translational upregulation exceeds the destabilizing effect Ras has on N-Myc, such that higher levels of N-Myc protein are the net result. Interestingly, Ras-mediated proteolysis of N-Myc is associated with an increase in N-Myc transcriptional activity. These findings suggest that upregulation of N-Myc translation coupled with increased degradation is an underlying mechanism by which Ras stimulates N-Myc transcriptional and oncogenic activity. The current understanding of the process by which Ras controls Myc proteolysis has largely been determined using c-Myc as a model. However, this model cannot account for the observation that Ras destabilizes rather than stabilizes N-Myc. Two critical phosphorylation sites within c-Myc are involved in its proteolysis, Thr58 and Ser62. Ras activity modulates the phosphorylation of these sites to prevent proteolysis of c-Myc. In N-Myc, the equivalent sites, Thr50 and Ser54, are perfectly conserved. For this reason, it is generally thought that N-Myc is stabilized by Ras in a manner similar to c-Myc. Since my studies demonstrate that Ras does not stabilize, but rather destabilizes N-Myc, I directly tested whether Thr50 and Ser54 of N-Myc were involved in N-Myc degradation and found that they indeed serve the same proteolytic role as Thr58 and Ser62. Furthermore, the mechanism by which Ras destabilizes N-Myc does not require phosphorylation of N-Myc at Thr50. These studies show that N-Myc degradation is controlled by at least two separate mechanisms: one involving Ras and another involving phosphorylation of N-Myc at Thr50 and Ser54. Taken together, these studies provide mechanistic insight into how oncogenic Ras augments N-Myc expression and activity. Further, they suggest that upregulation of NMyc translation and transcriptional activity may be an important mechanism underlying the oncogenic activities of hyperactivated Ras.

Identifier

doi:10.6083/M4CJ8BG6

School

School of Medicine

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