Novel Molecular Mechanisms that Modulate the Cellular Response to p53 Activation

The p53 tumor suppressor protein is the most frequently mutated gene in human tumors and is amongst the most intensely studied molecules in modern biology. p53 acts primarily as a stress-responsive transcription factor that directly regulates the expression of over one hundred target genes, the products of which mediate such tumor suppressive responses as cell cycle arrest, apoptosis, senescence and DNA damage repair. It is well established that p53-dependent cellular stress responses can vary, depending on both the nature of the p53-activating stimulus as well as the cell type experiencing the stress; however, the molecular mechanisms that drive cells to adopt one p53-dependent cellular response over another in a context-specific manner remain largely elusive and are the subject of my dissertation research. First, using an experimental design wherein cell cycle arrest or apoptosis is elicited in a single cell type using non-genotoxic or genotoxic p53 activation, respectively, I investigated potential mechanisms driving stimulus-specific cellular responses. Combining gene expression studies with loss-of-function experiments, I discovered that the death receptor DR4 is a novel factor modulating p53-dependent cell fate choice. I found that while DR4 protein is expressed in dying cells and is required for apoptosis, DR4 fails to accumulate in cells undergoing cell cycle arrest. Lastly, my studies reveal that stimulus-specific DR4 expression results, at least in part, from p53-independent stabilization of the DR4 mRNA in cells undergoing apoptosis. Second, in a screen to identify novel constituents of the p53 transcriptional network, I identified the miR-34a locus as the first known example of a microRNA under the direct transcriptional control of p53 whose expression is cell type-specific. Mature miR-34a levels negatively correlate with the degree of p53-induced apoptosis across a range of cancer cell types, and functional studies in HCT116 cells revealed that miR-34a functions in an anti-apoptotic manner in the context of non-genotoxic p53 activation. Lastly, my data suggest that cell-type specific processing defects in the miR-34a primary transcript may affect mature miR-34a levels and thus the cellular response to p53 activation