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Title

Selective Translational Control and Its Role in Cancer

Date of Award

Spring 5-15-2013

Author's School

Graduate School of Arts and Sciences

Author's Department

Biology & Biomedical Sciences (Molecular Cell Biology)

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

Cancer initiation is hallmarked by the inability of cells to properly control their proliferation and growth. Tumor suppressor genes often function to restrain improper cell growth and division, as well as to stimulate cell death under certain circumstances to keep our cells in proper balance and prevent cellular transformation. The alternative reading frame (ARF) tumor suppressor is critical in this sense. In response to hyperproliferative stimuli, ARF acts as a key regulator of ribosome biogenesis and induces cell cycle arrest through p53-dependent and -independent mechanisms. The main objectives of my dissertation research are to identify novel molecular targets of ARF that contribute to its tumor surveillance activities, to characterize the mechanisms through which this regulation occurs, and to understand how these targets abrogate/promote tumorigenesis. Defining new regulatory pathways linked to ARF function will further our insight into how cells evade these safeguards during cellular transformation.

Cancer cells require robust protein synthesis, and in many cases, cancer initiation is accompanied by an increase in the translational efficiency of a select group of mRNAs. At the onset of my dissertation research, it had been demonstrated that ARF not only negatively modulates ribosome maturation, in part through repressing DDX5 activity, but also that loss of ARF alters the population of ribosome-bound transcripts in a selective manner. However, it was not known whether ARF was able to regulate other proteins associated with ribosome biogenesis to further limit cell growth in the presence of oncogenic cues. Moreover, the mechanisms driving this sophisticated translational repression network in the absence of ARF remained unclear. Given DDX5's known interaction with the microprocessor component, Drosha, a protein previously linked with rRNA processing, I hypothesized that Drosha may be repressed by ARF in a similar manner to DDX5. I also hypothesized that miRNAs, a class of non-coding RNAs commonly associated with blocking the translation of select mRNAs, may serve as downstream effector molecules in ARF-mediated tumor suppression and that loss of Arf prompts a change in the miRNA signature of a cell ultimately promoting cell growth and proliferation.

The data presented in this dissertation reveals that Drosha protein expression is enhanced in the absence of ARF and that this induction relies solely on the increased translation of existing Drosha mRNAs. Through this regulatory network, ARF is able to restrict rRNA synthesis at the processing stage and protect cells from oncogene-induced transformation as elevated Drosha expression is critical in maintaining RasV12-induced cellular transformation. A closer examination of Drosha in the context of human breast cancer demonstrated that its locus is frequently amplified and that the resulting heighted Drosha expression confers a proliferative and tumorigenic advantage for certain cells. Lastly, loss of Arf impacts mature miRNA expression, albeit not globally, and restoration of a subset of these miRNAs partially restores growth control and protection from neoplastic transformation even in the absence of ARF. Taken together these data suggest novel and critical roles of miRNAs and Drosha in ARF tumor suppression and the involvement of the latter in facilitating mammary tumorigenesis.

Language

English (en)

Chair and Committee

Jason D Weber

Committee Members

Elaine R Mardis, Sheila A Stewart, Michael H Tomasson, Zhongsheng You, John R Edwards

Comments

Permanent URL: https://doi.org/10.7936/K7HH6H0J

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