# Emerging Non-Canonical Functions and Regulation by p53: p53 and Stemess
## 1 Introduction
### The p53 Network and Its Role in Cancer
DNA damage such as double-strand breaks can activate upstream mediators including protein kinases ATM, ATR, Chk1, and Chk2 leading to phosphorylation of p53 and Mdm2. These post-translational modifications interfere with the formation of the p53-Mdm2 complex resulting in p53 stabilization, increased p53 protein levels and transcriptional activity, and activation of cell cycle arrest, senescence, DNA repair and apoptosis [19, 25, 36]. Figure 1: Functions of p53. In another example of p53 regulation through a negative feedback loop, ubiquitin ligases Cop1 and Pirh2 associate with p53 before undergoing ubiquitination and proteosomal degradation [37; 38]. The tumor suppressor PTEN can lead to activation of p53 through the downregulation of Mdm2 [39; 40; 41]. Additionally, Dapk1, c-Ha-Ras and DDR1 form positive feedback loops on p53 that can activate p19 ARF to inhibit Mdm2 [42; 43; 44]. Likewise, p21 inhibits cyclinE-cdk2 and increases formation of the Rb-Mdm2 complex and activation of p53 [39]. Cyclin G, another p53 target protein, can recruit protein phosphatase 2A (PP2A) and lead to Mdm2 and ATM dephosphorylation [45; 46]. Furthermore, p53 activity is regulated by interactions with Wnt/\(\beta\)-catenin, IGF-1-AKT, Rb-E2F, p14/19 ARF, cyclin-cdk, p38 MAP kinase, cyclin G-PP2A and p73 [39]. p53 mutations generated by missense mutations in the TP53 gene have been implicated in tumor progression as a result of \(wt\)p53 loss of functions as a tumor suppressor. Similarly, tumor-associated \(mut\)p53 acquires gain-of-function (GOF) oncogenic properties that are independent of \(wt\)p53. These mutations enable cancer-associated \(mut\)p53 with new properties that benefit the tumor at various levels of cancer progression. \(Mut\)p53 can exert its GOF properties through binding and inactivation of p53 associated proteins such as p53 family members p63 and p73. \(Mut\)p53 GOF processes are associated with a variety of physiological events: tumor cell proliferation, invasion, survival, metabolic changes, angiogenesis, metastasis, tissue modeling, enhanced chemoresistance, mitogenic defects, genomic instability and stem cell expansion [47; 48; 49; 50; 51]. Most p53 mutations associated with cancer are present in the p53 DNA binding domain (DBD) and are characterized as DNA contact mutants (hotspot mutant p53R273H) or conformational mutants (hotspot mutant p53R175H) [52]. \(Mut\)p53 has been shown to upregulate numerous genes, including MDR1, PCNA, EGFR, IGF1R, c-myc and IGF2 [53; 54; 55; 56; 57; 58]. \(Mut\)p53 can also downregulate pro-apoptotic genes such as caspase-3, and CD95/Fas/Apol, and \(wt\)p53 target genes including p21 and PTEN [59; 60; 61; 62]. The p53 family includes p63 and p73, which share similar gene structures and overlapping, but distinct functions cover a wide range of processes involved in development, apoptosis and cell cycle arrest, chemosensitivity and immortalization [31]. p63 isoforms include TAP63 (\(\alpha\),\(\beta\),\(\gamma\)) and Np63 (\(\alpha\),\(\beta\),\(\gamma\)), while p73 C- and N-terminal isoforms include TA73 (\(\alpha\), \(\beta\), \(\gamma\), \(\delta\), \(\varepsilon\), \(\zeta\), \(\eta\) and splice variants \(\Delta\)2, \(\Delta\)3, \(\Delta\)2/3). Additionally, internal promoter variants include \(\Delta\)Np73 (\(\alpha\), \(\beta\), \(\gamma\), \(\delta\), \(\varepsilon\), \(\zeta\), \(\eta\)) [63; 64; 65; 66; 67]. In response to stress, p53 activates cell cycle arrest and regulates genes, such as p21 to halt cell proliferation; however, in the induction of apoptosis, p53 requires p63 and p73 to activate apoptotic gene promoters. In normal development, p63 is involved in forming teeth, hair and glands of the mammary and prostate. p73 is also involved in normal development, including neurogenesis, and pheromonal signaling [30]. The tumor suppressors p53, p63 and p73 regulate miRNAs that are critical to tumor inhibition and downregulation of EMT, metastasis, and the proliferation of CSCs [68].