The p53 protein is an indispensable tumor suppressor that is involved in numerous cellular processes, such as regulation of the cell cycle and metabolism. About two decades ago, two proteins with significant homology to p53 were discovered, and termed p63 and p73. To date, these three proteins make up the p53 family of transcription factors. Each gene has the capacity to form multiple protein isoforms through the use of various promoters and alternative splicing, with p73 forming the most isoforms. At the 5` end of the p73 gene, transcription initiation from promoters 1 and 2 form the TAp73 and DNp73 N-terminal isoforms, respectively. At the 3` end, alternative splicing in p73 exons 11 through 13 give rise to seven known C-terminal isoforms (a, b, g, d, e, z, h). The function of a majority of the C-terminal isoforms is not well-studied. As such, the work in this dissertation focuses on characterizing the function of the p73 C-terminal isoforms.
Chapter one serves as an introduction to the p53 family of proteins, while comprehensively compiling the literature that describes how these three proteins are involved in regulating lipid and iron metabolism. This chapter highlights the numerous ways in which p53 regulates lipid and iron metabolism, while revealing the limited literature on the role of p63 and p73 in these processes.
Chapter two lays the foundation for the rest of the dissertation, which focuses on characterizing the function of a novel p73 C-terminal isoform that we discovered. This novel isoform, termed p73a1, results from the exclusion of p73 exon 12 (E12). By using CRIPSR to remove E12 from multiple cancer cell lines, the role of p73a1 in oncogenesis was studied. To study the physiological function of p73a1, E12 heterozygous (E12+/-) mice were generated and life-span, tumor burden, and histopathology were analyzed. It was found that p73a1 functions as a tumor suppressor, both in vitro and in vivo. Moreover, E12+/- mice exhibited widespread systemic inflammation. As a result, the findings from this study identified Notch1 as a direct target of p73a1-mediated tumor suppression and inflammation.
Chapter three integrates the previous two chapters to investigate the role of p73a1 in regulating lipid metabolism. A multi-omic approach was taken and integrated with molecular biology techniques to identify lipid classes and lipid metabolism-associated genes that were altered by p73a1. Specifically, p73a1 was found to directly inhibit expression of Stearoyl-CoA Desaturase 1 (SCD1), leading to altered lipid profiles. Furthermore, the tumor suppressive function of p73a1 was found to be mediated in part through SCD1. Through these findings, a previously unidentified p73 target, SCD1, was established and a role for p73a1 in lipid metabolism was determined.
