Investigation of the p53-dependent apoptosis versus arrest decision through analysis of Prep gene regulation

Abstract

The p53 tumor suppressor gene is thought to play a critical role in the maintenance of genetic integrity by virtue of its ability to check the growth of abnormal or damaged cells through initiation of the cell cycle arrest and apoptosis pathways. p53 itself is mutated in roughly half of all human tumors and is likely to be inactivated through mutations in other components of the stress response pathway in the remaining cases, highlighting the centrality of p53 function to effective tumor suppression. The vast majority of p53 mutations disrupt its sequence-specific DNA binding function, compromising its ability to activate a large array of target genes involved in apoptosis, arrest, DNA repair, senescence, differentiation and other cellular functions. While the catalog of target genes is rapidly expanding, our understanding of the effector pathways downstream of p53 and the regulatory networks that govern their expression remains incomplete. Subtle changes in cellular environment and cell type specific factors can drastically affect the decision to pause and repair accumulated damage, or to commit cellular suicide to prevent the propagation of damaged genetic material. Our understanding of this life or death decision is still rudimentary. The work presented here represents an attempt to uncover mechanisms underlying the decision to initiate apoptosis or cell cycle arrest in a simple model system. Mouse embryo fibroblasts normally respond to DNA damage by undergoing cell cycle arrest, but can be sensitized to initiate p53-dependent apoptosis by the oncoprotein E1A. Through a subtractive hybridization screen, the Perp gene was identified as a transcript strongly upregulated in cells initiating p53-dependent apoptosis compared to those undergoing cell cycle arrest.

(cont.) This pattern of expression is strikingly different from those of other p53 target genes, many of which are induced to similar levels under both conditions. This finding suggested that Perp might be involved in initiation of apoptosis and that analysis of Perp regulation by p53 could provide insight into how p53 activity is modulated to affect two very disparate responses. In our examination of the Perp locus we have identified four putative p53 response elements with differing characteristics in several in vitro and in vivo assays and have shown that p53 can distinguish Perp from other target genes at the level DNA binding to these sites. These findings demonstrate that p53 plays an active role in determining the fate of the cell during the stress response and that intrinsic differences in p53 response elements of target genes determine which effectors are activated under particular conditions. A model for p53-dependent apoptosis and arrest and the implications for future research will be discussed.