Project 1: Opposing Roles of Wild-type and Mutant p53 in Regulating Oncogenesis. The wild-type p53 protein is recognized as a major tumor suppressor protein. p53 sits at the hub of numerous upstream stress signaling and downstream gene regulatory pathways and works as a transcription factor whose varied transcriptional targets mediate numerous cellular outcomes. Depending on the context, these include cell cycle, cell death, senescence, metabolic changes and others that serve to protect cells from acquiring cancerous properties. By contrast, evidence continues to mount that the common mutant forms of p53 found in human tumors are actually contributing to their malignant properties. Both wild-type and mutant p53 have been shown to regulate expression over 1000 genes each. Interestingly, in many cases the genes and pathways that mutant p53 upregulates are actively inhibited by wild-type p53. One line of evidence for mutant p53 gain of oncogenic function that was supported by this program was the discovery that mutant p53 regulates the SREBP-2-dependent mevalonic acid (MVA) pathway by which cells synthesize cholesterol and that this is required for the malignant characteristics of breast cancer cells grown in 3D cultures. We now have evidence in collaboration with the Lowe laboratory that wild-type p53 actively represses the MVA pathway by a novel mechanism whereby p53 induces the ABCA1 cholesterol exporter leading to blocked maturation of the SREBP-2 transcription factor. With Scott Lowe we showed that p53 repression of the MVA pathway is required for Myc/p53 null liver tumors to develop. Interestingly, a key byproduct of the MVA pathway, namely protein geranylgeranylation, is required for mutant p53 to upregulate the MVA pathway and ablation of this byproduct plays a role in wild-type repression of this pathway. We also recently discovered VEGFR2, the receptor for the pro-angiogenic VEGF ligand, as another gene that is up-regulated by mutant p53 in breast cancer cells. Activation of VEGFR2 RNA expression involves mutant p53 cooperation with the SWI/SNF chromatin remodeling complex. Indeed, mutant p53 regulation of nearly half of its targets including the MVA pathway genes requires SWI/SNF. The goals of this project are to gain further insight into how mutant p53 works with SREBP2 to regulate both known (MVA and VEGFR2) as well as anticipated new target genes, to gain insight into the positive feedback relationship between mutant p53 and SWI/SNF, to determine how and when p53 activates ABCA1 to block maturation of the SREBP2 protein, to identify which geranylgeranyl targets are required for maintaining growth and malignancy in breast cancer cell cultures, and to identify factors that lead to dependency on the MVA pathway for viability. These experiments, which will potentially inform new treatments for cancer, will in many cases involve collaborative projects with the other members of this program.
The gene encoding the p53 protein is frequently mutated in many forms of human cancer. It is well established that the normal form of p53 serves many functions to protect cells, animals and humans from acquiring tumors, thereby defining p53 as an important suppressor of tumor formation. Some of the mutated forms of p53 found in tumors serve the opposite function in that they actually promote cancerous behavior of cells. The basis for many of the experiments in this project are to determine how cancer related mutant forms of p53 stimulate expression of the mevalonate pathway by which cells make cholesterol and key byproducts, and how the normal wild-type form of p53 actually dampens this same pathway. Results from the planned experiments could refine treatment options for breast cancer patients and might also contribute to novel therapies for liver cancer patients for whom there are currently no effective treatments.
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