Control Of P53 Activity Research Articles

Differential regulation of p63 and p73 expression.

Two homologs of the tumor suppressor p53, named p63 and p73, are each expressed from at least two start sites of mRNA synthesis, yielding full-length, transactivating (TA) isoforms, and also aminoterminally truncated (DeltaN) isoforms that act as antagonists to p53. The expression of TAp73-transcripts is induced by E2F and negatively regulated by transforming growth factor beta (TGFbeta). The DeltaNp73 promoter is induced by p53, resulting in negative feedback to control p53 activity. Here, we have analysed the expression of p63 in comparison with p73. In contrast to the induction of DeltaNp73, the expression of DeltaNp63 was reduced by p53 particularly in human keratinocytes, at the mRNA and protein levels. Accordingly, the 3' promoter of p73, but not that of p63, was activated by p53 in reporter assays. DeltaNp73 mRNA and DeltaNp73 protein, but not the p63 gene products, also accumulated when HaCat cells (lacking functional p53) were grown to high density. TAp73, but not TAp63, expression was suppressed by TGFbeta in these cells, and the TAp73, but not the TAp63, promoter was induced by E2F-1. Thus, in contrast to the functional similarities of their respective products, the expression levels of p63 and p73 are regulated by different mechanisms. This might be responsible for the discordant biological roles of p63 and p73 in development, as well as their deviant expression characteristics in cancer.

The evolution of diverse biological responses to DNA damage: insights from yeast and p53.

The cellular response to ionizing radiation provides a conceptual framework for understanding how a yeast checkpoint system, designed to make binary decisions between arrest and cycling, evolved in a way as to allow reversible arrest, senescence or apoptosis in mammals. We propose that the diversity of responses to ionizing radiation in mammalian cells is possible because of the addition of a new regulatory control module involving the tumour-suppressor gene p53. We review the complex mechanisms controlling p53 activity and discuss how the p53 regulatory module enables cells to grow, arrest or die by integrating DNA damage checkpoint signals with the response to normal mitogenic signalling and the aberrant signalling engendered by oncogene activation.

The complexity of p53 modulation: emerging patterns from divergent signals.

Functional inactivation of p53 by gene mutation and deletion, protein degradation, or viral oncogene binding renders a mammalian cell susceptible to oncogenic stimuli and environmental insults that promote growth deregulation and malignant progression. Although a variety of mechanisms have been proposed for how p53 protects cells against neoplastic transformation, it is becoming clear that p53 integrates signals from the cell’s internal and external environment to respond to inappropriate growth promoting or growth inhibiting conditions (for review, see Gottleib and Oren 1995; Ko and Prives 1996; Levine 1997). This ‘‘sensor’’ function of p53 makes it unusual in the tumor suppressor gene family. The list of stimuli that alter p53 activity is increasing and our understanding of the signal transduction pathways used to signal to p53 are starting to become elucidated. The predominant regulation of p53 occurs at the protein level. Mutations in p53 that affect its conformation typically increase its half-life, in part by inhibiting degradation by the ubiquitin complex (Maki et al. 1996; Haupt et al. 1997; Kubbutat et al. 1997; Midgley and Lane 1997), and the majority of human tumor mutations decrease the sequence-specific DNA binding and transcriptional activity of p53 protein (Cho et al. 1994). In unstressed cells, p53 appears to be present at low levels and exists in a latent, inactive form that requires modification to become active. The types of modification that p53 is subjected to seem to be stress-, speciesand celltype-specific. Levels and/or activity of p53 increase in response to DNA damaging agents (Maltzman and Czyzyk 1984; Kastan et al. 1991; Nelson and Kastan 1994), decreased oxygen (Graeber et al. 1994), oncogenic stimuli (Debbas and White 1993; Lowe and Ruley 1993; Hermeking and Eick 1994; Wanger et al. 1994; Serrano et al. 1997), cell adhesion (Nigro et al. 1997), altered ribonucleotide pools (Linke et al. 1996), and redox stress (Hainaut and Milner 1993a; Hupp et al. 1993). Although all of these stresses signal the activation of p53 protein, unique pathways appear to be utilized by the different stresses. Is p53 protein accumulation needed for p53 activation or are modifications necessary for p53 activation separable from the modifications required for p53 protein accumulation? Both the modifiers and the types of modification will be important to sort out to understand the relationship between accumulation and activation. Although the two appear to be separable (Chernov et al. 1998; Hupp et al. 1995), it is possible that both are essential for full tumor suppressor function. The importance of p53 and modifications that affect its functions are not limited to malignant disease. The activity of p53 can increase in normal tissues when undergoing pathophysiological changes that result in oxidative or redox stress, such as ischemia and reperfusion injury of the brain, heart, and other tissues. Thus both oxidative stress generated by hydrogen peroxide, as well as reducing stresses generated by the lack of oxygen, appear to be potent stimulators of p53 activity. In addition, the link between p53 function and the modulation of angiogenesis further implicates p53 pathways in the processes of wound healing (Antoniades et al. 1994) and ischemic injury responses (Banasiak and Haddad 1998). At present little is known about the pathways that control p53 activity in response to ischemia and reperfusion in normal tissues, and this area should be fertile ground for research in future years. In this review, we discuss what is known about how various stressors signal to p53 and the various mechanisms utilized to modulate p53 activity. Because of the focus on signaling to p53, we will not attempt to discuss the ‘‘downstream’’ physiologic effects of p53 activation such as cell-cycle perturbations or cell death.

Mdm‐2 is not induced by p53 in human keratinocytes in vivo

Normal p53 protein protects the genome after DNA damage by delaying replication and allowing DNA repair. mdm-2 is a recently discovered protein that controls p53 activity by binding to and inactivating p53 protein. A negative feedback loop has been suggested in which p53 induces mdm-2 expression. We have shown that doses of ultraviolet B radiation that cause mild sunburn clinically, produce thymine dimers in keratinocytes detectable by immunocytochemistry. This causes an elevation of p53 protein without a concomitant p53-mediated induction of mdm-2.