Inhibition of p53 transcriptional activity by the S100B calcium-binding protein

Proteins of the S100 family regulate the oligomerization of p53 tumor suppressor

S100B protein is elevated in the brains of patients with early stages of Alzheimer's disease and Down's syndrome. S100A4 is correlated with the development of metastasis. Both proteins bind to p53 tumor suppressor. We found that both S100B and S100A4 bind to the tetramerization domain of p53 (residues 325-355) only when exposed in lower oligomerization states and so they disrupt the tetramerization of p53. In addition, S100B binds to the negative regulatory and nuclear localization domains, which results in a very tight binding to p53 protein sequences that exposed the tetramerization domain in their C terminus. Because the trafficking of p53 depends on its oligomerization state, we suggest that S100B and S100A4 could regulate the subcellular localization of p53. But, the differences in the way these proteins bind to p53 could result in S100B and S1004 having different effects on p53 function in cell-cycle control.

S100B increases proliferation in PC12 neuronal cells and reduces their responsiveness to nerve growth factor via Akt activation

S100B is a Ca2+-modulated protein of the EF-hand type expressed in high abundance in a restricted set of cell types including certain neuronal populations. S100B has been suggested to participate in cell cycle progression, and S100B levels are high in tumor cells, compared with normal parental cells. We expressed S100B in the neuronal cell line PC12, which normally does not express the protein, by the Tet-Off technique, and found the following: (i) proliferation was higher in S100B+ PC12 cells than in S100B- PC12 cells; (ii) nerve growth factor (NGF), which decreased the proliferation of S100B- PC12 cells, was less effective in the case of S100B+ PC12 cells; (iii) expression of S100B made PC12 cells resistant to the differentiating effect of NGF; and (iv) interruption of S100B expression did not result in an immediate restoration of PC12 cell sensitivity to the differentiating effect of NGF. Expression of S100B in PC12 cells resulted in activation of Akt; increased levels of p21WAF1, an inhibitor of cyclin-dependent kinase (cdk) 2 and a positive regulator of cdk4; increased p21WAF1-cyclin D1 complex formation; and increased phosphorylation of the retinoblastoma suppressor protein, Rb. These S100B-induced effects, as well as the reduced ability of S100B+ PC12 cells to respond to NGF, were dependent on Akt activation because they were remarkably reduced or abrogated in the presence of LY294002, an inhibitor of the Akt upstream kinase phosphatidylinositol 3-kinase. Thus, S100B might promote cell proliferation and interfere with NGF-induced PC12 cell differentiation by stimulating a p21WAF1/cyclin D1/cdk4/Rb/E2F pathway in an Akt-mediated manner.

Inhibiting S100B restores p53 levels in primary malignant melanoma cancer cells

S100 calcium-binding proteins such as S100B are elevated in primary malignant melanoma and are used as markers for this and numerous other cancers. Wild-type p53 protein levels are relatively low in these cancer cells (i.e. when compared with cells without S100B) but are elevated when RNA antisense to S100B is introduced. This result implicates S100B in the down-regulation of p53 and is consistent with the large decreases in p53 protein levels observed previously in transient co-transfections of p53 and S100B (Lin, J., Blake, M., Tang, C., Zimmer, D., Rustandi, R. R., Weber, D. J., and Carrier, F. (2001) J. Biol. Chem. 276, 35037-35041). Down-regulation of p53 in primary malignant melanoma cells is likely the result of a direct interaction with S100B, which was observed by co-immunoprecipitation experiments. Furthermore, p53 binds regions of the S100B promoter, one of which matches the 20-nucleotide p53-binding consensus DNA sequence perfectly. Therefore, when p53 levels increase, it contributes to its own demise by up-regulating the transcription of S100B as part of a negative feedback loop. This is analogous to what is found for another protein that down-regulates p53, namely hdm2 (human double mutant 2).

Polo-like kinase 1 (Plk1) inhibits p53 function by physical interaction and phosphorylation

Polo-like kinase 1 (Plk1) has an important role in the regulation of M phase of the cell cycle. In addition to its cell cycle-regulatory function, Plk1 has a potential role in tumorigenesis. Here we found for the first time that Plk1 physically binds to the tumor suppressor p53 in mammalian cultured cells, and inhibits its transactivation activity as well as its pro-apoptotic function. During the cisplatin-induced apoptosis in human neuroblastoma SH-SY5Y cells, the expression level of Plk1 was significantly decreased both at mRNA and protein levels, whereas cisplatin treatment caused a remarkable stabilization of p53. Systematic immunoprecipitation analyses using a series of deletion mutants of p53 revealed that a sequence-specific DNA-binding region of p53 is required and sufficient for the physical interaction with Plk1. The ectopically overexpressed Plk1 was co-localized with the endogenous p53 in mammalian cell nucleus, as shown by confocal laser microscopy. Expression of exogenous Plk1 and p53 in p53-deficient lung carcinoma H1299 cells greatly decreased the p53-mediated transcription from the p53-responsive p21(WAF1), MDM2, and BAX promoters, whereas the kinase-deficient mutant form of Plk1 failed to reduce the transcriptional activity of p53. Consistent with the luciferase reporter analysis, Plk1 had an ability to block the p53-dependent induction of the endogenous p21(WAF1). In addition, Plk1 inhibited the pro-apoptotic function of p53 in H1299 cells. Intriguingly, Plk1-mediated repression of p53 was attenuated with ATM. Thus, our present findings strongly suggest that p53 is a critical target of Plk1, and its function is abrogated through the physical interaction with Plk1.

Differential expression of S100 gene family in human esophageal squamous cell carcinoma

PURPOSE

To study the differential expression of the S100 gene family at the RNA level in human esophageal squamous cell carcinoma (ESCC), and to find the relationship of the S100 gene family with ESCC.

METHODS

Firstly, the specific primers were designed for the different S100 genes with Software Primer 3, which required that both primer sequences of each S100 gene were from two different exons respectively. Then, the differential expression of 16 S100 genes was examined by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) in 62 cases of ESCC versus the corresponding normal esophageal mucosa. All RT-PCR products were analyzed by 1.5% agarose gel. With Fluor-S MultiImager and Multi-Analyst software, the electrophoresis images were evaluated with statistics analysis using SAS 8.1 software.

RESULTS

Eleven out of 16 S100 genes were significantly downregulated ( p<0.05) in ESCC versus the normal counterparts such as S100A1, S100A2, S100A4, S100A8, S100A9, S100A10, S100A11, S100A12, S100A14, S100B, and S100P genes. Only the S100A7 gene in the S100 family was markedly upregulated ( p<0.05). Moreover, the S100B gene was significantly correlated with histological differentiation of ESCC ( p=0.0247), and the deregulation of some S100 genes was closely correlated ( p<0.05), such as S100A10/S100A11, S100A2/S100A8, S100A2/S100A14, S100A8/S100A14, and S100A2/S100P etc.

CONCLUSIONS

The S100 gene family is closely associated with ESCC.

Nucleophosmin sets a threshold for p53 response to UV radiation

Because activation of p53 can trigger cell cycle arrest and apoptosis, it is necessary for a cell to suppress this activation until it is absolutely required for survival. The mechanisms underlying this important regulatory event are poorly understood. Here we show that nucleophosmin (NPM) acts as a natural repressor of p53 by setting a threshold for p53 activation in response to UV radiation. NPM binds to the p53 N terminus and inhibits p53 transcriptional activity by more than 70%. Our data indicate that the levels of NPM in a cell determine the UV dose at which the tumor suppressor p53 can be phosphorylated on Ser15. Moreover, we show that NPM is a substrate for the UV-activated protein kinase ATR and inhibits the UV-induced p53 phosphorylation at Ser15. In addition, NPM forms a complex with p53 and ATR in vivo. These data suggest that NPM is an early responder to DNA damage that prevents premature activation of p53. In normal cells, NPM could contribute to suppressing p53 activation until its functions are absolutely required while in cancer cells overexpression of NPM could contribute to p53 inactivation and tumor progression.

S100B in brain damage and neurodegeneration

S100B is a calcium-binding peptide produced mainly by astrocytes that exert paracrine and autocrine effects on neurons and glia. Some knowledge has been acquired from in vitro and in vivo animal experiments to understand S100B's roles in cellular energy metabolism, cytoskeleton modification, cell proliferation, and differentiation. Also, insights have been gained regarding the interaction between S100B and the cerebral immune system, and the regulation of S100B activity through serotonergic transmission. Secreted glial S100B exerts trophic or toxic effects depending on its concentration. At nanomolar concentrations, S100B stimulates neurite outgrowth and enhances survival of neurons during development. In contrast, micromolar levels of extracellular S100B in vitro stimulate the expression of proinflammatory cytokines and induce apoptosis. In animal studies, changes in the cerebral concentration of S100B cause behavioral disturbances and cognitive deficits. In humans, increased S100B has been detected with various clinical conditions. Brain trauma and ischemia is associated with increased S100B concentrations, probably due to the destruction of astrocytes. In neurodegenerative, inflammatory and psychiatric diseases, increased S100B levels may be caused by secreted S100B or release from damaged astrocytes. This review summarizes published findings on S100B regarding human brain damage and neurodegeneration. Findings from in vitro and in vivo animal experiments relevant for human neurodegenerative diseases and brain damage are reviewed together with the results of studies on traumatic, ischemic, and inflammatory brain damage as well as neurodegenerative and psychiatric disorders. Methodological problems are discussed and perspectives for future research are outlined.

Intracellular and extracellular roles of S100 proteins

S100, a multigenic family of non-ubiquitous Ca(2+)-modulated proteins of the EF-hand type expressed in vertebrates exclusively, has been implicated in intracellular and extracellular regulatory activities. Members of this protein family have been shown to interact with several effector proteins within cells thereby regulating enzyme activities, the dynamics of cytoskeleton constituents, cell growth and differentiation, and Ca(2+) homeostasis. Structural information indicates that most of S100 proteins exist in the form of antiparallelly packed homodimers (in some cases heterodimers), capable of functionally crossbridging two homologous or heterologous target proteins in a Ca(2+)-dependent (and, in some instances, Ca(2+)-independent) manner. In addition, extracellular roles have been described for several S100 members, although secretion (via an unknown mechanism) has been documented for a few of them. Extracellular S100 proteins have been shown to exert regulatory effects on inflammatory cells, neurons, astrocytes, microglia, and endothelial and epithelial cells, and a cell surface receptor, RAGE, has been identified as a potential S100A12 and S100B receptor transducing the effects of these two proteins on inflammatory cells and neurons. Other cell surface molecules with ability to interact with S100 members have been identified, suggesting that RAGE might not be a universal S100 protein receptor and/or that a single S100 protein might interact with more than one receptor. Collectively, these data indicate that members of the S100 protein family are multifunctional proteins implicated in the regulation of a variety of cellular activities.

A search for candidate genes for lipodystrophy, obesity and diabetes via gene expression analysis of A-ZIP/F-1 mice

Genome scans for diabetes have identified many regions of the human genome that correlate with the disease state. To identify candidate genes for type 2 diabetes, we examined the transgenic A-ZIP/F-1 mouse. This mouse model has no white fat, resulting in abnormal levels of glucose, insulin, and leptin, making the A-ZIP/F-1 mice a good model for lipodystrophy and insulin resistance. We used cDNA-based microarrays to find differentially expressed genes in four tissues of these mice. We examined these results in the context of human linkage scans for lipodystrophy, obesity, and type 2 diabetes. We combined 199 known human orthologs of the misregulated mouse genes with 33 published human genome scans on a genome map. Integrating expression data with human linkage results permitted us to suggest and prioritize candidate genes for lipodystrophy and related disorders. These genes include a cluster of 3 S100A genes on chromosome 1 and SLPI1 on chromosome 20.

Solution NMR structure of S100B bound to the high-affinity target peptide TRTK-12

The solution NMR structure is reported for Ca(2+)-loaded S100B bound to a 12-residue peptide, TRTK-12, from the actin capping protein CapZ (alpha1 or alpha2 subunit, residues 265-276: TRTKIDWNKILS). This peptide was discovered by Dimlich and co-workers by screening a bacteriophage random peptide display library, and it matches exactly the consensus S100B binding sequence ((K/R)(L/I)XWXXIL). As with other S100B target proteins, a calcium-dependent conformational change in S100B is required for TRTK-12 binding. The TRTK-12 peptide is an amphipathic helix (residues W7 to S12) in the S100B-TRTK complex, and helix 4 of S100B is extended by three or four residues upon peptide binding. However, helical TRTK-12 in the S100B-peptide complex is uniquely oriented when compared to the three-dimensional structures of other S100-peptide complexes. The three-dimensional structure of the S100B-TRTK peptide complex illustrates that residues in the S100B binding consensus sequence (K4, I5, W7, I10, L11) are all involved in the S100B-peptide interface, which can explain its orientation in the S100B binding pocket and its relatively high binding affinity. A comparison of the S100B-TRTK peptide structure to the structures of apo- and Ca(2+)-bound S100B illustrates that the binding site of TRTK-12 is buried in apo-S100B, but is exposed in Ca(2+)-bound S100B as necessary to bind the TRTK-12 peptide.

A novel p53 transcriptional repressor element (p53TRE) and the asymmetrical contribution of two p53 binding sites modulate the response of the placental transforming growth factor-beta promoter to p53

Previous studies in our laboratory and others identified placental transforming growth factor-beta (PTGF-beta) as an important downstream mediator of DNA damage signaling and a transcriptional target of p53. Here we show that accumulation of PTGF-beta mRNA in response to p53 overexpression is delayed relative to p21(WAF1), whereas the promoters of these genes respond to p53 with similar kinetics. Mutational analyses of two p53 binding sites within the PTGF-beta promoter revealed that site p53-1 (+29 bp) is responsible for as much as 80% of the transcriptional response to p53. This is consistent with electrophoretic mobility shift assays showing that site p53-1 binds p53 with a much higher affinity than site p53-2 (-850 bp). We also describe for the first time a novel 21-bp element (-222 to -242 bp) that acts to down-regulate the PTGF-beta promoter response to p53. Termed the p53 transcriptional repressor element (p53TRE), this sequence was shown to suppress p53 transactivation in a position- and promoter-independent fashion and to associate with a 28-kDa protein expressed in several tumor cell lines. A p53 suppressor element and asymmetric p53 binding sites may participate determining the activation thresholds of p53-responsive promoters in a cell- and context-specific manner.

The role of calcium-binding proteins in the control of transcription: structure to function

Transcriptional regulation is coupled with numerous intracellular signaling processes often mediated by second messengers. Now, growing evidence points to the importance of Ca(2+), one of the most versatile second messengers, in activating or inhibiting gene transcription through actions frequently mediated by members of the EF-hand superfamily of Ca(2+)-binding proteins. Calmodulin and calcineurin, representative members of this EF-hand superfamily, indirectly regulate transcription through phosphorylation/dephosphorylation of transcription factors in response to a Ca(2+) increase in the cell. Recently, a novel EF-hand Ca(2+)-binding protein called DREAM has been found to interact with regulatory sequences of DNA, thereby acting as a direct regulator of transcription. Finally, S100B, a dimeric EF-hand Ca(2+)-binding protein, interacts with the tumor suppressor p53 and controls its transcriptional activity. In light of the structural studies reported to date, this review provides an overview of the structural basis of EF-hand Ca(2+)-binding proteins linked with transcriptional regulation.

Ca2+-dependent translocation of the calcyclin-binding protein in neurons and neuroblastoma NB-2a cells

The calcyclin-binding protein (CacyBP) binds calcyclin (S100A6) at physiological levels of [Ca(2+)] and is highly expressed in brain neurons. Subcellular localization of CacyBP was examined in neurons and neuroblastoma NB-2a cells at different [Ca(2+)](i). Immunostaining indicates that CacyBP is present in the cytoplasm of unstimulated cultured neurons in which resting [Ca(2+)](i) is known to be approximately 50 nm. When [Ca(2+)](i) was increased to above 300 nm by KCl treatment, the immunostaining was mainly apparent as a ring around the nucleus. Such perinuclear localization of CacyBP was observed in untreated neuroblastoma NB-2a cells in which [Ca(2+)](i) is approximately 120 nm. An additional increase in [Ca(2+)](i) to above 300 nm by thapsigargin treatment did not change CacyBP localization. However, when [Ca(2+)](i) in NB-2a cells dropped to 70 nm, because of BAPTA/AM treatment, perinuclear localization was diminished. Ca(2+)-induced translocation of CacyBP was confirmed by immunogold electron microscopy and by fluorescence of NB-2a cells transfected with an EGFP-CacyBP vector. Recombinant CacyBP can be phosphorylated by protein kinase C in vitro. In untreated neuroblastoma NB-2a cells, CacyBP is phosphorylated on a serine residue(s), but exists in the dephosphorylated form in BAPTA/AM-treated cells. Thus, phosphorylation of CacyBP occurs in the same [Ca(2+)](i) range that leads to its perinuclear translocation.