Phosphorylation of the C-terminal sites of human p53 reduces non-sequence-specific DNA binding as modeled with synthetic peptides

Roles of p53 Family Structure and Function in Non-Canonical Response Element Binding and Activation

The p53 canonical consensus sequence is a 10-bp repeat of PuPuPuC(A/T)(A/T)GPyPyPy, separated by a spacer with up to 13 bases. C(A/T)(A/T)G is the core sequence and purine (Pu) and pyrimidine (Py) bases comprise the flanking sequence. However, in the p53 noncanonical sequences, there are many variations, such as length of consensus sequence, variance of core sequence or flanking sequence, and variance in number of bases making up the spacer or AT gap composition. In comparison to p53, the p53 family members p63 and p73 have been found to have more tolerance to bind and activate several of these noncanonical sequences. The p53 protein forms monomers, dimers, and tetramers, and its nonspecific binding domain is well-defined; however, those for p63 or p73 are still not fully understood. Study of p63 and p73 structure to determine the monomers, dimers or tetramers to bind and regulate noncanonical sequence is a new challenge which is crucial to obtaining a complete picture of structure and function in order to understand how p63 and p73 regulate genes differently from p53. In this review, we will summarize the rules of p53 family non-canonical sequences, especially focusing on the structure of p53 family members in the regulation of specific target genes. In addition, we will compare different software programs for prediction of p53 family responsive elements containing parameters with canonical or non-canonical sequences.

Critical role of p53 in histone deacetylase inhibitor-induced Epstein-Barr virus Zta expression

The tumor suppressor gene p53 plays a central role in the maintenance of normal cell growth and genetic integrity, while its impact on the Epstein-Barr virus (EBV) life cycle remains elusive. We found that p53 is important for histone deacetylase inhibitor-induced EBV lytic gene expression in nasopharyngeal carcinoma cells. Restoration of p53 in p53-null, EBV-infected H1299 cells augments the potential for viral lytic cycle initiation. Evidence from reporter assays demonstrated that p53 contributes to the expression of the immediate-early viral Zta gene. Further analysis indicated that the DNA-binding ability of p53 and phosphorylation of Ser392 may be critical. This study provides the first evidence that p53 is involved in the regulation of EBV lytic cycle initiation.

Indole-3-carbinol mediated cell cycle arrest of LNCaP human prostate cancer cells requires the induced production of activated p53 tumor suppressor protein

Indole-3-carbinol (I3C), a dietary compound found naturally in cruciferous vegetables of the Brassica genus such as broccoli and brussels sprouts, induces a G1 growth arrest of human reproductive cancer cells. We previously reported that in LNCaP prostate cancer cells, I3C down-regulated cyclin-dependent kinase (CDK) 2 activity. In our current study, Western blotting and quantitative RT-PCR demonstrated that I3C treatment increased both the transcripts and protein levels of the CDK2 inhibitor p21(waf1/cip1) (p21). Transfection of luciferase reporter plasmids containing wild-type and mutated p21 promoter fragments revealed that I3C induced p21 gene transcription through a p53 DNA binding element. Oligonucleotide precipitation showed that I3C increased the level of activated p53 nuclear protein that is competent to bind its DNA target site on the p21 promoter. Ablation of p53 production using short interfering RNA (siRNA) prevented that the I3C induced G1 arrest and up-regulation of p21 expression. Western blots using p53 phospho-specific antibodies revealed that I3C treatment increased the levels of three phosphorylated forms of p53 (Ser15, Ser37, Ser392) that are known to contribute to p53 protein stability and greater transactivation potential. Taken together, our results establish that the I3C induced G1 arrest of human prostate cancer cells requires the induced production of the activated phosphorylated forms of p53, which stimulate transcription of the CDK2 inhibitor p21.

The extracellular matrix, p53 and estrogen compete to regulate cell-surface Fas/Apo-1 suicide receptor expression in proliferating embryonic cerebral cortical precursors, and reciprocally, Fas-ligand modifies estrogen control of cell-cycle proteins

BACKGROUND

Apoptosis is important for normal cerebral cortical development. We previously showed that the Fas suicide receptor was expressed within the developing cerebral cortex, and that in vitro Fas activation resulted in caspase-dependent death. Alterations in cell-surface Fas expression may significantly influence cortical development. Therefore, in the following studies, we sought to identify developmentally relevant cell biological processes that regulate cell-surface Fas expression and reciprocal consequences of Fas receptor activation.

RESULTS

Flow-cytometric analyses identified two distinct neural sub-populations that expressed Fas on their cell surface at high (FasHi) or moderate (FasMod) levels. The anti-apoptotic protein FLIP further delineated a subset of Fas-expressing cells with potential apoptosis-resistance. FasMod precursors were mainly in G0, while FasHi precursors were largely apoptotic. However, birth-date analysis indicated that neuroblasts express the highest levels of cell-surface Fas at the end of S-phase, or after their final round of mitosis, suggesting that Fas expression is induced at cell cycle checkpoints or during interkinetic nuclear movements. FasHi expression was associated with loss of cell-matrix adhesion and anoikis. Activation of the transcription factor p53 was associated with induction of Fas expression, while the gonadal hormone estrogen antagonistically suppressed cell-surface Fas expression. Estrogen also induced entry into S-phase and decreased the number of Fas-expressing neuroblasts that were apoptotic. Concurrent exposure to estrogen and to soluble Fas-ligand (sFasL) suppressed p21/waf-1 and PCNA. In contrast, estrogen and sFasL, individually and together, induced cyclin-A expression, suggesting activation of compensatory survival mechanisms.

CONCLUSIONS

Embryonic cortical neuronal precursors are intrinsically heterogeneous with respect to Fas suicide-sensitivity. Competing intrinsic (p53, cell cycle, FLIP expression), proximal (extra-cellular matrix) and extrinsic factors (gonadal hormones) collectively regulate Fas suicide-sensitivity either during neurogenesis, or possibly during neuronal migration, and may ultimately determine which neuroblasts successfully contribute neurons to the differentiating cortical plate.

Recognition of DNA modified by antitumor cisplatin by "latent" and "active" protein p53

Tumor suppressor protein p53 possesses two DNA-binding sites. One that is located within its core domain is responsible for sequence-specific DNA binding of the protein, non-specific binding to internal segments of single- or double-stranded DNA, and to certain kinds of non-B DNA structures. The other that is contained in the C-terminus of the protein binds to damaged DNA. Binding of active, latent, and in vitro-activated p53 protein to DNA fragments modified by antitumor cisplatin was studied using electrophoretic mobility shift assay in agarose gels and immunoblotting analysis. We found that both latent and active p53 forms bound to random sequences of DNA globally modified by cisplatin with a higher affinity than to unmodified DNA. Interestingly, the latent form exhibited a more pronounced selectivity for platinated DNA than the active p53. Consistently with this observation, the preference of the latent form for platinated DNA decreased as a consequence of the activation of latent p53 by phosphorylation at the protein kinase C site within its C-terminus or by binding of the monoclonal antibody Bp53-10.1. Competition experiments involving a 20-bp consensus sequence of p53 suggested that the p53 core domain was a primary binding site of the active p53 when it bound to DNA fragments lacking consensus sequence, but modified by cisplatin. In addition, the latent protein was found to selectively interact with DNA modified by cisplatin probably via its C-terminus.

Synthesis of complex phosphopeptides as mimics of p53 functional domains

A complete set of mono-, di- and triphosphorylated peptides comprising amino acids 10-27, the Mdm2 and p300 binding site(s) of p53, with and without a fluorescein label at the N-terminus, was synthesized by step-by-step solid phase synthesis. Fluorescence polarization analysis revealed that phosphorylation at Thr18 decreased binding to recombinant Mdm2 protein compared with the unphosphorylated and the two other single phosphorylated analogues. Unlabelled multiply phosphorylated peptides corresponding to this amino-terminal transactivation domain proved to be powerful tools in analysing the phosphate specificity of existing anti-p53 monoclonal and polyclonal antibodies using direct ELISA. The tetramerization domain of human p53 protein was modelled with a 53 residue-long unlabelled unphosphorylated and Ser315-phosphorylated peptide pair. CD analysis showed similar alpha-helical structures for both peptides and no major difference in the secondary structure could be observed upon phosphorylation. Size-exclusion HPLC indicated that these synthetic oligomerization domain mimics underwent a pH-dependent tetramerization process, but the presence of a phosphate group at Ser315 did not modify the oligomeric state of the 308-360 p53 fragments. Nevertheless, the fluorescein-labelled Ser315 phosphorylated peptide bound to the downstream signalling ligand DNA topoisomerase I protein with slightly higher affinity than did the unphosphorylated analogue.

Identification of crucial residues for the antibacterial activity of the proline-rich peptide, pyrrhocoricin

Members of the proline-rich antibacterial peptide family, pyrrhocoricin, apidaecin and drosocin appear to kill responsive bacterial species by binding to the multihelical lid region of the bacterial DnaK protein. Pyrrhocoricin, the most potent among these peptides, is nontoxic to healthy mice, and can protect these animals from bacterial challenge. A structure-antibacterial activity study of pyrrhocoricin against Escherichia coli and Agrobacterium tumefaciens identified the N-terminal half, residues 2-10, the region responsible for inhibition of the ATPase activity, as the fragment that contains the active segment. While fluorescein-labeled versions of the native peptides entered E. coli cells, deletion of the C-terminal half of pyrrhocoricin significantly reduced the peptide's ability to enter bacterial or mammalian cells. These findings highlighted pyrrhocoricin's suitability for combating intracellular pathogens and raised the possibility that the proline-rich antibacterial peptides can deliver drug leads into mammalian cells. By observing strong relationships between the binding to a synthetic fragment of the target protein and antibacterial activities of pyrrhocoricin analogs modified at strategic positions, we further verified that DnaK was the bacterial target macromolecule. Inaddition, the antimicrobial activity spectrum of native pyrrhocoricin against 11 bacterial and fungal strains and the binding of labeled pyrrhocoricin to synthetic DnaK D-E helix fragments of the appropriate species could be correlated. Mutational analysis on a synthetic E. coli DnaK fragment identified a possible binding surface for pyrrhocoricin.

A fluorescence polarization assay for the identification of inhibitors of the p53-DM2 protein-protein interaction

Improper function of the tumor suppressor protein p53 is a contributing factor in many human cancers. In normal cells, p53 acts to arrest the cell cycle in response to DNA damage or nucleotide depletion. One mechanism of regulating the amount of p53 in the cell is through the action of the Double Minute 2 protein, DM2 (also known as MDM2), which ubiquitinates p53 and targets it for proteosomal degradation. In a number of human cancers, the DM2 gene is amplified or overexpressed, leading to inadequate levels of p53 for cell cycle arrest or apoptosis. With the goal of restoring p53 function in cancers that overexpress DM2, we are developing inhibitors of the p53-DM2 protein-protein interaction that structurally mimic the N-terminal segment of p53 that binds to DM2. To assist this effort, we have devised a fluorescence polarization assay that quantifies the interaction between the N-terminal regions of both proteins in 384-well microtiter plates. Using this assay, we have demonstrated that a peptide with a nonhydrolyzable beta-amino acid substitution binds DM2 with an affinity comparable to a p53 peptide that is composed of only alpha-amino acids.

NMR spectroscopy reveals the solution dimerization interface of p53 core domains bound to their consensus DNA

The p53 protein is a transcription factor that acts as the major tumor suppressor in mammals. The core DNA-binding domain is mutated in about 50% of all human tumors. The crystal structure of the core domain in complex with DNA illustrated how a single core domain specifically interacts with its DNA consensus site and how it is inactivated by mutation. However, no structural information for the tetrameric full-length p53-DNA complex is available. Here, we present novel experimental insight into the dimerization of two p53 core domains upon cooperative binding to consensus DNA in solution obtained by NMR. The NMR data show that the p53 core domain itself does not appear to undergo major conformational changes upon addition of DNA and elucidate the dimerization interface between two DNA-bound core domains, which includes the short H1 helix. A NMR-based model for the dimeric p53 core-DNA complex incorporates these data and allows the conclusion that the dimerization interface also forms the actual interface in the tetrameric p53-DNA complex. The significance of this interface is further corroborated by the finding that hot spot mutations map to the H1 helix, and by the binding of the putative p53 inhibitor 53BP2 to this region via one of its ankyrin repeats. Based on symmetry considerations it is proposed that tetrameric p53 might link non-contiguous DNA consensus sites in a sandwich-like manner generating DNA loops as observed for transcriptionally active p53 complexes.

Strategies for manipulating the p53 pathway in the treatment of human cancer

Human cancer progression is driven in part by the mutation of oncogenes and tumour-suppressor genes which, under selective environmental pressures, give rise to evolving populations of biochemically altered cells with enhanced tumorigenic and metastatic potential. Given that human cancers are biologically and pathologically quite distinct, it has been quite surprising that a common event, perturbation of the p53 pathway, occurs in most if not all types of human cancers. The central role of p53 as a tumour-suppressor protein has fuelled interest in defining its mechanism of function and regulation, determining how its inactivation facilitates cancer progression, and exploring the possibility of restoring p53 function for therapeutic benefit. This review will highlight the key biochemical properties of p53 protein that affect its tumour-suppressor function and the experimental strategies that have been developed for the re-activation of the p53 pathway in cancers.

Human topoisomerase IIalpha and IIbeta interact with the C-terminal region of p53

The p53 tumor suppressor protein is a critical regulator of cell cycle progression and apoptosis following exposure of cells to DNA damaging agents such as ionizing radiation or anticancer drugs. An important group of anticancer drugs, including compounds such as etoposide and doxorubicin (Adriamycin), interacts with DNA topoisomerase II (topo II), causing the accumulation of enzyme-DNA adducts that ultimately lead to double-strand breaks and cell death via apoptosis. Human topo IIbeta has previously been shown to interact with p53, and we have extended this analysis to show that both topo IIalpha and IIbeta interact with p53 in vivo and in vitro. Furthermore, we show that the regulatory C-terminal basic region of p53 (residues 364-393) is necessary and sufficient for interaction with DNA topo II.

Overlapping and divergent actions of estrogen and the neurotrophins on cell fate and p53-dependent signal transduction in conditionally immortalized cerebral cortical neuroblasts

The developing cerebral cortex undergoes overlapping periods of neurogenesis, suicide, and differentiation to generate the mature cortical plate. The following experiments examined the role of the gonadal hormone estrogen in comparison to the neurotrophins, in the regulation of p53-dependent cortical cell fate. To synchronize choices between neurogenesis, apoptosis, and neural differentiation, embryonic rat cerebral cortical neuroblasts were conditionally immortalized with the SV40 large T antigen containing the tsA58/U19 temperature-sensitive mutations. At the nonpermissive temperature, cessation of large T antigen expression was accompanied by induction of p53, as well as the p53-dependent proteins, wild-type p53-activated fragment-1/Cdk (cyclin-dependent kinase)-interacting protein-1 (p21/Waf1), Bcl (B-cell lymphoma)-associated protein (Bax), and murine double minute 2 (MDM2), that lead to cell cycle-arrest, suicide, and p53 inhibition, respectively. Simultaneously, neuroblasts exit cell cycle and die apoptotically or differentiate primarily into astrocytes and immature postmitotic neuroblasts. At the nonpermissive temperature, estrogen specifically induced an antagonist-independent increase in phosphorylated p53 expression, while increasing p21/Waf1 and decreasing Bax. Coincidentally, estrogen rapidly increased and then decreased MDM2 relative to controls, suggesting temporal modulation of p53 function. Both estrogen and neurotrophins prevented DNA fragmentation, a marker for apoptosis. However, estrogen also induced a transient increase in released lactate dehydrogenase, suggesting that estrogen simultaneously induced rapid cell death in a subpopulation of cells. In contrast to the neurotrophins, estrogen also increased cell proliferation. Both estrogen and the neurotrophins supported neuronal differentiation. However, in contrast to the neurotrophins, estrogen only supported the expression of a subset of oligodendrocytic markers. These results suggest that estrogen and the neurotrophins support overlapping and distinct aspects of differentiation in the developing cerebral cortex.