Nerve Growth Factor Regulation of Transcription Factor p53 Activity

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Title: Nerve Growth Factor Regulation of Transcription Factor p53 Activity
Author: Brynczka, Christopher
Advisors: Robert C. Smart, PhD, Committee Co-Chair
B. Alex Merrick, PhD, Committee Co-Chair
Lloyd N. Fleisher, PhD, Committee Member
Yoshiaki Tsuji, PhD, Committee Member
Abstract: p53 is recognized as a critical regulator of the cell cycle and apoptosis. Mounting evidence also suggests a role for p53 in the differentiation of several cell types including neuronal precursors. Using the pheochromocytoma PC12 cell model, we studied the role of p53 during nerve growth factor (NGF)-mediated neuronal differentiation and how p53 activity is modulated by NGF during nitric oxide (NO)-induced apoptosis. p53 protein was transcriptionally activated and contributed to NGF-mediated neurite outgrowth during differentiation of PC12 cells. Using a genome-wide chromatin immunoprecipitation cloning technique, we identified and validated 14 novel p53-regulated genes following NGF treatment, including the wnt7b secreted morphogen. Wnt7b expression was p53-dependent and its overexpression was sufficient to restore neurite outgrowth in p53-silenced cells, demonstrating that wnt7b is a p53-regulated neuritogenic factor that in conjunction with NGF signaling is capable of eliciting potent induction of neurite outgrowth in PC12 cells. NGF is also recognized for its role in neuronal differentiation and maintenance. We investigated NGF influence over p53 activity during NO-induced apoptosis by sodium nitroprusside in differentiated and mitotic PC12 cells. NGF-differentiation produced increased p53 levels, nuclear localization and sequence-specific DNA binding. Apoptosis in mitotic cells also produced these events but the accompanying activation of caspases 1-10 and mitochondrial depolarization were inhibited during NGF differentiation and could be reversed in p53-silenced cells. Transcriptional regulation of PUMA and survivin expression were not inhibited by NGF, although NO-induced mitochondrial depolarization was dependent upon de novo gene transcription and only occurred in mitotic cells. Therefore, NGF mediates prosurvival signaling through factors such as Bcl-2 and p21Waf1⁄Cip1 without altering p53 transcriptional activity to inhibit apoptosis. These studies demonstrate that NGF potently activates p53 transcriptional regulation of target genes involved in cell cycle arrest and neurite outgrowth. Although NGF potentiates p53 DNA-binding activity, it also inhibits p53-dependent apoptosis. In summary, receptor-mediated NGF signaling represents a comprehensive mechanism through which many functions of p53 activity may be regulated in responsive neuronal cells.
Date: 2007-07-10
Degree: PhD
Discipline: Toxicology

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