Protecting p53 from degradation

Inactivation of the p53 function is a common event in cancer. Approx. 50% of human tumours express mutant p53 and there is evidence that in others, including many childhood tumours, p53 function is impaired in other ways. These defects on p53 function may be due to the alteration of cellular factors that modulate p53 or to the expression of viral oncoproteins. Radiotherapy and many of the chemotherapeutic drugs currently used in cancer treatment are potent activators of p53. However, most of these therapies have a serious drawback; that is, the long-term consequences of their DNA-damaging effects. Understanding the mechanisms regulating p53 stability is crucial for the development of new strategies to activate p53 non-genotoxically. Here we describe the effect of a potent activator of the p53 response, the nuclear export inhibitor leptomycin B, on Mdm2 degradation and we provide evidence for the oligomerization of the p14ARF tumour suppressor and Mdm2 inhibitor in response to oxidative stress.     

Numblike and Numb differentially affect p53 and Sonic Hedgehog signaling

Numb serves as a key regulator of Notch and Sonic Hedgehog signaling and also modulates p53 protein levels. Numblike is a highly conserved homolog to mammalian Numb, but considerably less is known about its function. To address the role of Numblike, we have generated a mouse embryonic stem (ES) cell line, Nbl^TetOn, in which expression of Numblike can be induced and analyzed the effect of activation of Numblike. Induction of Numblike, similar to Numb, reduced the amount of Notch receptor, whereas Numblike differed from Numb with regard to p53 and Shh signaling. In contrast to Numb, Numblike did not elevate the level of p53 protein and Numblike potentiated, rather than reduced, the immediate downstream response of Shh signaling. In keeping with a role for Numblike in potentiating Shh signaling, Shh and Numblike synergistically increased the proportion of ES cells expressing pluripotency markers. In conclusion, the data demonstrate that Numb and Numblike have evolved to acquire at least partially distinct functions.     

p53 guards the metabolic pathway less travelled

Glucose is an important source of energy and carbon, and is required for cell growth. As such, glucose utilization is increased in rapidly dividing cancer cells. The tumour suppressor p53 has now been reported to block a metabolic pathway (the pentose phosphate pathway) that diverts glucose away from bioenergetic into biosynthetic routes.     

The p53-signaling pathway and colorectal cancer: Interactions between downstream p53 target genes and miRNAs

IntroductionWe examined expression of genes in the p53-signaling pathway. We determine if genes that have significantly different expression in carcinoma tissue compared to normal mucosa also have significantly differentially expressed miRNAs. We utilize a sample of 217 CRC cases.MethodsWe focused on fold change (FC) > 1.50 or <0.67 for genes and miRNAs, that were statistically significant after adjustment for multiple comparisons. We evaluated the linear association between the differential expression of miRNA and mRNA. miRNA:mRNA seed-region matches also were determined.ResultsEleven dysregulated genes were associated with 37 dysregulated miRNAs; all were down-stream from the TP53 gene. MiR-150-5p (HR = 0.82) and miR-196b-5p (HR 0.73) significantly reduced the likelihood of dying from CRC when miRNA expression increased in rectal tumors.ConclusionsOur data suggest that activation of p53 from cellular stress, could target downstream genes that in turn could influence cell cycle arrest, apoptosis, and angiogenesis through mRNA:miRNA interactions.     

Measuring a cell's response to stress: the p53 pathway

The characterization of complex cellular responses to diverse stimuli can be studied by the use of emerging chip-based technologies.     

A PTCH1 Homolog Transcriptionally Activated by p53 Suppresses Hedgehog Signaling

The p53-mediated responses to DNA damage and the Hedgehog (Hh) signaling pathway are each recurrently dysregulated in many types of human cancer. Here we describe PTCH53, a p53 target gene that is homologous to the tumor suppressor gene PTCH1 and can function as a repressor of Hh pathway activation. PTCH53 (previously designated PTCHD4) was highly responsive to p53 in vitro and was among a small number of genes that were consistently expressed at reduced levels in diverse TP53 mutant cell lines and human tumors. Increased expression of PTCH53 inhibited canonical Hh signaling by the G protein-coupled receptor SMO. PTCH53 thus delineates a novel, inducible pathway by which p53 can repress tumorigenic Hh signals.     

Analysis of molecular interactions of the p53-family p51(p63) gene products in a yeast two-hybrid system: homotypic and heterotypic interactions and association with p53-regulatory factors

p51 in the p53 tumor suppressor family, also referred to as p63, encodes multiple isoforms including p51A (TAp63gamma) and p51B (TAp63alpha). The p53 protein forms a tetramer, and its stability and activity are regulated by molecular association with viral and cellular proteins and by biochemical modifications. Using a yeast two-hybrid system, the p51A and p51B isoforms were examined for homotypic and heterotypic interactions in the p53 family proteins and for their affinity to the p53-regulatory factors. Results indicate a homotypic interaction dependent on the presumed oligomerization domain of the p51 proteins. The possibility of a weak heterotypic interaction between p51 and p73 proteins was suggested, while association between p51 and p53 appeared improbable. Furthermore, unlike p53, the p51 proteins failed to display an affinity to SV40 large T antigen or MDM2-family proteins. Having several features in common with p53, the p51 proteins may function in biological processes apart from p53.     

UVB-induced experimental carcinogenesis: dysregulation of apoptosis and p53 signalling pathway

Abstract

UVB irradiation is a major cause of human skin cancer. In addition to inducing neoplastic changes, UVB irradiation also causes immune suppression which may impair local defence reactions allowing abnormal cells to proliferate and develop into skin tumours.¹ The immuno-suppressive events induced by UVB irradiation are mediated by a number of pathways involving altered DNA,² cis-urocanic acid (cis-UCA),³ IL-10,⁴ and mast cells.⁵ UVB-damaged DNA can result in mutations including those of p53, allowing the proliferation of aberrant cells and, ultimately, local tumour invasion. In p53 gene knockout mice, development of skin tumours increases more rapidly after exposure to UVB irradiation.⁶ p53 as a target for UV-induced mutations in developing murine and human skin cancer involves C→T and CC→TT transitions at dipyrimidine sites.⁷ This paper summarises the adaptive responses in mouse skin following acute and chronic UV irradiation.     

A p53-dependent checkpoint pathway prevents rereplication

Eukaryotic cells control the initiation of DNA replication so that origins that have fired once in S phase do not fire a second time within the same cell cycle. Failure to exert this control leads to genetic instability. Here we investigate how rereplication is prevented in normal mammalian cells and how these mechanisms might be overcome during tumor progression. Overexpression of the replication initiation factors Cdt1 and Cdc6 along with cyclin A-cdk2 promotes rereplication in human cancer cells with inactive p53 but not in cells with functional p53. A subset of origins distributed throughout the genome refire within 2-4 hr of the first cycle of replication. Induction of rereplication activates p53 through the ATM/ATR/Chk2 DNA damage checkpoint pathways. p53 inhibits rereplication through the induction of the cdk2 inhibitor p21. Therefore, a p53-dependent checkpoint pathway is activated to suppress rereplication and promote genetic stability.     

Involvement of stromal p53 in tumor-stroma interactions

p53 is a major tumor-suppressor gene, inactivated by mutations in about half of all human cancer cases, and probably incapacitated by other means in most other cases. Most research regarding the role of p53 in cancer has focused on its ability to elicit apoptosis or growth arrest of cells that are prone to become malignant owing to DNA damage or oncogene activation, i.e. cell-autonomous activities of p53. However, p53 activation within a cell can also exert a variety of effects upon neighboring cells, through secreted factors and paracrine and endocrine mechanisms. Of note, p53 within cancer stromal cells can inhibit tumor growth and malignant progression. Cancer cells that evolve under this inhibitory influence acquire mechanisms to silence stromal p53, either by direct inhibition of p53 within stromal cells, or through pressure for selection of stromal cells with compromised p53 function. Hence, activation of stromal p53 by chemotherapy or radiotherapy might be part of the mechanisms by which these treatments cause cancer regression. However, in certain circumstances, activation of stromal p53 by cytotoxic anti-cancer agents might actually promote treatment resistance, probably through stromal p53-mediated growth arrest of the cancer cells or through protection of the tumor vasculature. Better understanding of the underlying molecular mechanisms is thus required. Hopefully, this will allow their manipulation towards better inhibition of cancer initiation, progression and metastasis.     

Regulation of the p53 pathway by ubiquitin and related proteins

The p53 tumour suppressor protein is subject to many levels of control, including modification with ubiquitin and related proteins such as SUMO and NEDD8. These modifications regulate p53 at a number of levels, including control of protein turnover, alterations in sub-cellular localization and changes in the ability to regulate gene expression. Numerous E3 ligases that can mediate these modifications of p53 have been described, some of which promote conjugation with more than one ubiquitin-like protein. Understanding the complexity of this mechanism of p53 regulation will help in the development of therapeutic drugs that function to modulate these events.     

Targeting protein-protein interactions: lessons from p53/MDM2

The tremendous challenge of inhibiting therapeutically important protein-protein interactions has created the opportunity to extend traditional medicinal chemistry to a new class of targets and to explore nontraditional strategies. Here we review a widely studied system, the interaction between tumor suppressor p53 and its natural antagonist MDM2, for which both traditional and nontraditional approaches have been reported. This system has been a testing ground for novel proteomimetic scaffold-based strategies, i.e., for attempts to mimic the recognition surface displayed by a folded protein with unnatural oligomers. Retroinverso peptides, peptoids, terphenyls, beta-hairpins, p-oligobenzamides, beta-peptides, and miniproteins have all been explored as inhibitors of the p53/MDM2 interaction, and we focus on these oligomer-based efforts. Traditional approaches have been successful as well, and we briefly review small molecule inhibitors along with other strategies for reactivation of the p53 pathway, for comparison with oligomer- based approaches. We close with comments on an emerging dichotomy among protein-protein interaction targets.