Abstract 5635: Pegylated recombinant human hyaluronidase PH20 (PEGPH20) enhances Nab-Paclitaxel efficacy in BxPC-3 human pancreatic cancer xenografts

The interplay between epigenetic changes and the p53 protein in stem cells

This review by Levine and Berger discusses the cross-talk between the p53 protein and epigenetic programs. The p53 protein not only enforces the stability of the genome by the prevention of genetic alterations in cells but also plays an important role in regulating the epigenetic changes that occur in cells.

Epigenetic programs regulate the development and maintenance of organisms over a lifetime. These programs are carried out through chemical modifications of DNA and proteins such as histones and transcription factors. These epigenetic modifications are less stable than genetic alterations and even reversible under a variety of circumstances, such as developmental changes, regeneration of tissues, cell divisions, aging, and pathological conditions observed in many cancers. The p53 protein not only enforces the stability of the genome by the prevention of genetic alterations in cells but also plays a role in regulating the epigenetic changes that can occur in cells. The full-length p53 protein is largely inactive in stem cells but, when activated, helps to commit these cells to developmental lineages through a series of epigenetic changes. Just as p53 impacts epigenetic change, the enzyme activities that carry out epigenetic protein modifications act on the p53 protein and its splice variants in stem and progenitor cells to silence or activate its transcriptional activities. Thus, there is a great deal of cross-talk between the p53 protein and epigenetic programs. This review collects the diverse experimental evidence that leads to these conclusions. This in turn permits new ideas and directions for the treatment of cancers, reactivating developmental pathways for tissue regeneration and responses to the impact of aging.

Oncogenic Mutant p53 Gain of Function Nourishes the Vicious Cycle of Tumor Development and Cancer Stem-Cell Formation

More than half of human tumors harbor an inactivated p53 tumor-suppressor gene. It is well accepted that mutant p53 shows an oncogenic gain-of-function (GOF) activity that facilitates the transformed phenotype of cancer cells. In addition, a growing body of evidence supports the notion that cancer stem cells comprise a seminal constituent in the initiation and progression of cancer development. Here, we elaborate on the mutant p53 oncogenic GOF leading toward the acquisition of a transformed phenotype, as well as placing mutant p53 as a major component in the establishment of cancer stem cell entity. Therefore, therapy targeted toward cancer stem cells harboring mutant p53 is expected to pave the way to eradicate tumor growth and recurrence.

The Role of the p53 Protein in Stem-Cell Biology and Epigenetic Regulation

The p53 protein plays a passive and an active role in stem cells. The transcriptional activities of p53 for cell-cycle arrest and DNA repair are largely turned off in stem cells, but there is some indication that long-term stem-cell viability may require other p53-regulated functions. When p53 is activated in stem cells, it stops cell division and promotes the commitment to a differentiation pathway and the formation of progenitor cells. In the absence of any p53 activity, stem-cell replication continues and mistakes in the normal epigenetic pathway occur at a higher probability. In the presence of a functionally active p53 protein, epigenetic stability is enforced and stem-cell replication is regulated by commitment to differentiation. Over a lifetime of an organism, stem-cell clones compete in a tissue niche for Darwinian replicative advantages and in doing so accumulate mutations that permit stem-cell replication. Mutations in the p53 gene give stem cells this advantage, increase the clonal stem-cell population, and lower the age at which cancers can occur. Li-Fraumeni patients that inherit p53 mutations develop tumors in a tissue-type-specific fashion at younger ages. Throughout the life of a Li-Fraumeni patient, the tumor types that arise occur in tissues where stem cells are active and cell division is most rapid. Thus, p53 mutations that are inherited or occur during developmental life act in stem cells of the mesenchymal and epithelial lineages, whereas p53 mutations that occur in progenitor or differentiated (somatic) cells later in life function in tissues of endodermal origins, indicating that p53 may function differently in different developmental lineages.