The crystal structure at 2A resolution of the Ca2+ -binding protein S100P

Targeted Destruction of S100A4 Inhibits Metastasis of Triple Negative Breast Cancer Cells

Most patients who die of cancer do so from its metastasis to other organs. The calcium-binding protein S100A4 can induce cell migration/invasion and metastasis in experimental animals and is overexpressed in most human metastatic cancers. Here, we report that a novel inhibitor of S100A4 can specifically block its increase in cell migration in rat (IC50, 46 µM) and human (56 µM) triple negative breast cancer (TNBC) cells without affecting Western-blotted levels of S100A4. The moderately-weak S100A4-inhibitory compound, US-10113 has been chemically attached to thalidomide to stimulate the proteasomal machinery of a cell. This proteolysis targeting chimera (PROTAC) RGC specifically eliminates S100A4 in the rat (IC50, 8 nM) and human TNBC (IC50, 3.2 nM) cell lines with a near 20,000-fold increase in efficiency over US-10113 at inhibiting cell migration (IC50, 1.6 nM and 3.5 nM, respectively). Knockdown of S100A4 in human TNBC cells abolishes this effect. When PROTAC RGC is injected with mouse TNBC cells into syngeneic Balb/c mice, the incidence of experimental lung metastases or local primary tumour invasion and spontaneous lung metastasis is reduced in the 10-100 nM concentration range (Fisher's Exact test, p ≤ 0.024). In conclusion, we have established proof of principle that destructive targeting of S100A4 provides the first realistic chemotherapeutic approach to selectively inhibiting metastasis.

Lanthanides as Calcium Mimetic Species in Calcium-Signaling/Buffering Proteins: The Effect of Lanthanide Type on the Ca2+/Ln3+ Competition

Lanthanides, the 14 4f-block elements plus Lanthanum, have been extensively used to study the structure and biochemical properties of metalloproteins. The characteristics of lanthanides within the lanthanide series are similar, but not identical. The present research offers a systematic investigation of the ability of the entire Ln3+ series to substitute for Ca2+ in biological systems. A well-calibrated DFT/PCM protocol is employed in studying the factors that control the metal selectivity in biological systems by modeling typical calcium signaling/buffering binding sites and elucidating the thermodynamic outcome of the competition between the “alien” La3+/Ln3+ and “native” Ca2+, and La3+ − Ln3+ within the lanthanide series. The calculations performed reveal that the major determinant of the Ca2+/Ln3+ selectivity in calcium proteins is the net charge of the calcium binding pocket; the more negative the charge, the higher the competitiveness of the trivalent Ln3+ with respect to its Ca2+ contender. Solvent exposure of the binding site also influences the process; buried active centers with net charge of −4 or −3 are characterized by higher Ln3+ over Ca2+ selectivity, whereas it is the opposite for sites with overall charge of −1. Within the series, the competition between La3+ and its fellow lanthanides is determined by the balance between two competing effects: electronic (favoring heavier lanthanides) and solvation (generally favoring the lighter lanthanides).

Calcium-Bound S100P Protein Is a Promiscuous Binding Partner of the Four-Helical Cytokines

S100 proteins are multifunctional calcium-binding proteins of vertebrates that act intracellularly, extracellularly, or both, and are engaged in the progression of many socially significant diseases. Their extracellular action is typically mediated by the recognition of specific receptor proteins. Recent studies indicate the ability of some S100 proteins to affect cytokine signaling through direct interaction with cytokines. S100P was shown to be the S100 protein most actively involved in interactions with some four-helical cytokines. To assess the selectivity of the S100P protein binding to four-helical cytokines, we have probed the interaction of Ca²⁺-bound recombinant human S100P with a panel of 32 four-helical human cytokines covering all structural families of this fold, using surface plasmon resonance spectroscopy. A total of 22 cytokines from all families of four-helical cytokines are S100P binders with the equilibrium dissociation constants, K_d, ranging from 1 nM to 3 µM (below the K_d value for the S100P complex with the V domain of its conventional receptor, receptor for advanced glycation end products, RAGE). Molecular docking and mutagenesis studies revealed the presence in the S100P molecule of a cytokine-binding site, which overlaps with the RAGE-binding site. Since S100 binding to four-helical cytokines inhibits their signaling in some cases, the revealed ability of the S100P protein to interact with ca. 71% of the four-helical cytokines indicates that S100P may serve as a poorly selective inhibitor of their action.

Erythropoietin Interacts with Specific S100 Proteins

Erythropoietin (EPO) is a clinically significant four-helical cytokine, exhibiting erythropoietic, cytoprotective, immunomodulatory, and cancer-promoting activities. Despite vast knowledge on its signaling pathways and physiological effects, extracellular factors regulating EPO activity remain underexplored. Here we show by surface plasmon resonance spectroscopy, that among eighteen members of Ca2+-binding proteins of the S100 protein family studied, only S100A2, S100A6 and S100P proteins specifically recognize EPO with equilibrium dissociation constants ranging from 81 nM to 0.5 µM. The interactions occur exclusively under calcium excess. Bioinformatics analysis showed that the EPO-S100 interactions could be relevant to progression of neoplastic diseases, including cancer, and other diseases. The detailed knowledge of distinct physiological effects of the EPO-S100 interactions could favor development of more efficient clinical implications of EPO. Summing up our data with previous findings, we conclude that S100 proteins are potentially able to directly affect functional activities of specific members of all families of four-helical cytokines, and cytokines of other structural superfamilies.

Specific cytokines of interleukin-6 family interact with S100 proteins

Cytokines of interleukin-6 (IL-6) family are important signaling proteins involved in various physiological and pathological processes. Earlier, we described interactions between IL-11 and S100P/B proteins from the family of S100 proteins engaged in the pathogenesis of numerous diseases. We probed here interactions between seven IL-6 family cytokines (IL-6, IL-11, OSM, LIF, CNTF, CT-1, and CLCF1) and fourteen S100 proteins (S100A1/A4/A6/A7/A8/A9/A10/A11/A12/A13/A14/A15/B/P). Surface plasmon resonance spectroscopy revealed formation of calcium-dependent complexes between IL-11, OSM, CNTF, CT-1, and CLCF1 and distinct subsets of S100A1/A6/B/P proteins with equilibrium dissociation constants of 19 nM - 12 µM. The existence of a network of interactions between Ca²⁺-loaded S100 proteins and IL-6 family cytokines suggest regulation of these cytokines by the extracellular forms of S100 proteins.

The Role of the C-Terminal Lysine of S100P in S100P-Induced Cell Migration and Metastasis

S100P protein is a potent inducer of metastasis in a model system, and its presence in cancer cells of patients is strongly associated with their reduced survival times. A well-established Furth Wistar rat metastasis model system, methods for measuring cell migration, and specific inhibitors were used to study pathways of motility-driven metastasis. Cells expressing C-terminal mutant S100P proteins display markedly-reduced S100P-driven metastasis in vivo and cell migration in vitro. These cells fail to display the low focal adhesion numbers observed in cells expressing wild-type S100P, and the mutant S100P proteins exhibit reduced biochemical interaction with non-muscle myosin heavy chain isoform IIA in vitro. Extracellular inhibitors of the S100P-dependent plasminogen activation pathway reduce, but only in part, wild-type S100P-dependent cell migration; they are without effect on S100P-negative cells or cells expressing C-terminal mutant S100P proteins and have no effect on the numbers of focal adhesions. Recombinant wild-type S100P protein, added extracellularly to S100P-negative cells, stimulates cell migration, which is abolished by these inhibitors. The results identify at least two S100P-dependent pathways of migration, one cell surface and the other intracellularly-linked, and identify its C-terminal lysine as a target for inhibiting multiple migration-promoting activities of S100P protein and S100P-driven metastasis.

S100 Calcium-Binding Protein P Secreted from Megakaryocytes Promotes Osteoclast Maturation

Megakaryocytes (MKs) differentiate from hematopoietic stem cells and produce platelets at the final stage of differentiation. MKs directly interact with bone cells during bone remodeling. However, whether MKs are involved in regulating bone metabolism through indirect regulatory effects on bone cells is unclear. Here, we observed increased osteoclast differentiation of bone marrow-derived macrophages (BMMs) cultured in MK-cultured conditioned medium (MK CM), suggesting that this medium contains factors secreted from MKs that affect osteoclastogenesis. To identify the MK-secreted factor, DNA microarray analysis of the human leukemia cell line K562 and MKs was performed, and S100 calcium-binding protein P (S100P) was selected as a candidate gene affecting osteoclast differentiation. S100P was more highly expressed in MKs than in K562 cells, and showed higher levels in MK CM than in K562-cultured conditioned medium. In BMMs cultured in the presence of recombinant human S100P protein, osteoclast differentiation was promoted and marker gene expression was increased. The resorption area was significantly larger in S100P protein-treated osteoclasts, demonstrating enhanced resorption activity. Overall, S100P secreted from MKs promotes osteoclast differentiation and resorption activity, suggesting that MKs indirectly regulate osteoclast differentiation and activity through the paracrine action of S100P.

Discovery of novel small molecule inhibitors of S100P with in vitro anti-metastatic effects on pancreatic cancer cells

S100P, a calcium-binding protein, is known to advance tumor progression and metastasis in pancreatic and several other cancers. Herein is described the in silico identification of a putative binding pocket of S100P to identify, synthesize and evaluate novel small molecules with the potential to selectively bind S100P and inhibit its activation of cell survival and metastatic pathways. The virtual screening of a drug-like database against the S100P model led to the identification of over 100 clusters of diverse scaffolds. A representative test set identified a number of structurally unrelated hits that inhibit S100P-RAGE interaction, measured by ELISA, and reduce in vitro cell invasion selectively in S100P-expressing pancreatic cancer cells at 10 μM. This study establishes a proof of concept in the potential for rational design of small molecule S100P inhibitors for drug candidate development.

Mouse S100G protein exhibits properties characteristic of a calcium sensor

Bovine S100 G (calbindin D_9k, small Ca²⁺-binding protein of the EF-hand superfamily) is considered as a calcium buffer protein; i.e., the binding of Ca²⁺ practically does not change its general conformation. A set of experimental approaches has been used to study structural properties of apo- and Ca²⁺-loaded forms of mouse S100 G (81.4% identity in amino acid sequence with bovine S100 G). This analysis revealed that, in contrast to bovine S100 G, the removal of calcium ions increases α-helices content of mouse S100 G protein and enhances its accessibility to digestion by α-chymotrypsin. Furthermore, mouse apo-S100 G is characterized by a decreased surface hydrophobicity and reduced tendency for oligomerization. Such behavior is typical of calcium sensor proteins. Apo-state of mouse S100 G still has rather compact structure, which can be cooperatively unfolded by temperature and GdnHCl. Computational analysis of amino acid sequences of S100 G proteins shows that these proteins could be in a disordered state upon a removal of the bound calcium ions. The experimental data show that, although mouse apo-S100 G is flexible compared to the Ca²⁺-loaded state, the apo-form is not completely disordered and preserves some cooperatively meting structure. The origin of the unexpectedly high stability of mouse S100 G can be rationalized by an exceptionally strong association of its N- and C-terminal parts containing the EF-hands I and II, respectively.

Monomeric state of S100P protein: Experimental and molecular dynamics study

S100 proteins constitute a large subfamily of the EF-hand superfamily of calcium binding proteins. They possess one classical EF-hand Ca²⁺-binding domain and an atypical EF-hand domain. Most of the S100 proteins form stable symmetric homodimers. An analysis of literature data on S100 proteins showed that their physiological concentrations could be much lower than dissociation constants of their dimeric forms. It means that just monomeric forms of these proteins are important for their functioning. In the present work, thermal denaturation of apo-S100P protein monitored by intrinsic tyrosine fluorescence has been studied at various protein concentrations within the region from 0.04-10 μM. A transition from the dimeric to monomeric form results in a decrease in protein thermal stability shifting the mid-transition temperature from 85 to 75 °C. Monomeric S100P immobilized on the surface of a sensor chip of a surface plasmon resonance instrument forms calcium dependent 1 to 1 complexes with human interleukin-11 (equilibrium dissociation constant 1.2 nM). In contrast, immobilized interleukin-11 binds two molecules of dimeric S100P with dissociation constants of 32 nM and 288 nM. Since effective dissociation constant of dimeric S100P protein is very low (0.5 μM as evaluated from our data) the sensitivity of the existing physical methods does not allow carrying out a detailed study of S100P monomer properties. For this reason, we have used molecular dynamics methods to evaluate structural changes in S100P upon its transition from the dimeric to monomeric state. 80-ns molecular dynamics simulations of kinetics of formation of S100P, S100B and S100A11 monomers from the corresponding dimers have been carried out. It was found that during the transition from the homo-dimer to monomer form, the three S100 monomer structures undergo the following changes: (1) the helices in the four-helix bundles within each monomer rotate in order to shield the exposed non-polar residues; (2) almost all lost contacts at the dimer interface are substituted with equivalent and newly formed interactions inside each monomer, and new stabilizing interactions are formed; and (3) all monomers recreate functional hydrophobic cores. The results of the present study show that both dimeric and monomeric forms of S100 proteins can be functional.

Analyzing the structural and functional roles of residues from the 'black' and 'gray' clusters of human S100P protein

Two highly conserved structural motifs observed in members of the EF-hand family of calcium binding proteins. The motifs provide a supporting scaffold for the Ca2+ binding loops and contribute to the hydrophobic core of the EF-hand domain. Each structural motif represents a cluster of three amino acids called cluster I ('black' cluster) and cluster II ('grey' cluster). Cluster I is more conserved and mostly incorporates aromatic amino acids. In contrast, cluster II is noticeably less conserved and includes a mix of aromatic, hydrophobic, and polar amino acids of different sizes. In the human calcium binding S100 P protein, these 'black' and 'gray' clusters include residues F15, F71, and F74 and L33, L58, and K30, respectively. To evaluate the effects of these clusters on structure and functionality of human S100 P, we have performed Ala scanning. The resulting mutants were studied by a multiparametric approach that included circular dichroism, scanning calorimetry, dynamic light scattering, chemical crosslinking, and fluorescent probes. Spectrofluorimetric Ca2+-titration of wild type S100 P showed that S100 P dimer has 1-2 strong calcium binding sites (K₁ = 4 × 106 M^-1) and two cooperative low affinity (K₂ = 4 × 104 M^-1) binding sites. Similarly, the S100 P mutants possess two types of calcium binding sites. This analysis revealed that the alanine substitutions in the clusters I and II caused comparable changes in the S100 P functional properties. However, analysis of heat- or GuHCl-induced unfolding of these proteins showed that the alanine substitutions in the cluster I caused notably more pronounced decrease in the protein stability compared to the changes caused by alanine substitutions in the cluster II. Opposite to literature data, the F15 A substitution did not cause the S100 P dimer dissociation, indicating that F15 is not crucial for dimer stability. Overall, similar to parvalbumins, the S100 P cluster I is more important for protein conformational stability than the cluster II.

A subset of calcium-binding S100 proteins show preferential heterodimerization

The assembly of proteins into dimers and oligomers is a necessary step for the proper function of transcription factors, muscle proteins, and proteases. In uncontrolled states, oligomerization can also contribute to illnesses such as Alzheimer's disease. The S100 protein family is a group of dimeric proteins that have important roles in enzyme regulation, cell membrane repair, and cell growth. Most S100 proteins have been examined in their homodimeric state, yet some of these important proteins are found in similar tissues implying that heterodimeric molecules can also be formed from the combination of two different S100 members. In this work, we have established co-expression methods in order to identify and quantify the distribution of homo- and heterodimers for four specific pairs of S100 proteins in their calcium-free states. The split GFP trap methodology was used in combination with other GFP variants to simultaneously quantify homo- and heterodimeric S100 proteins in vitro and in living cells. For the specific S100 proteins examined, NMR, mass spectrometry, and GFP trap experiments consistently show that S100A1:S100B, S100A1:S100P, and S100A11:S100B heterodimers are the predominant species formed compared to their corresponding homodimers. We expect the tools developed here will help establish the roles of S100 heterodimeric proteins and identify how heterodimerization might alter the specificity for S100 protein action in cells.

Reviewing the Crystal Structure of S100Z and Other Members of the S100 Family: Implications in Calcium-Regulated Quaternary Structure

This paper takes the cue from the previously solved crystal structure of human apo-S100Z and compares it with that of the calcium-bound S100Z from zebrafish in order to stress, for this particular S100, the significant role of the presence of calcium in promoting supramolecular assemblies with likely biological meaning. This consideration is then expanded through a wider review on analogous situations concerning all other S100s for which there is crystallographic o biochemical evidence of how the presence of calcium promotes the formation of quaternary complexes.The paper also deals with some considerations on the quality of the crystals obtained for the solved members of this family and on the need for experimental phasing for solving some of the structures where the good general sequence homology among the members of the family would have suggested molecular replacement (MR) as the easiest way to solve them.These considerations, along with the PCA analysis carried out on all the known S100s, further demonstrate that calcium plays a fundamental role in triggering quaternary structure formation for several members of this family of proteins.

S100P and Ezrin promote trans-endothelial migration of triple negative breast cancer cells

PURPOSE

Triple negative breast cancer (TNBC) patients generally have an adverse clinical outcome because their tumors often recur and metastasize to distant sites in the first 3 years after surgery. Therefore, it has become pivotal to identify potential factors associated with metastasis. Here, we focused on the effects of S100P and Ezrin on the trans-endothelial migration (TEM) of TNBC cells, as they have both been suggested to play a role in this process in other malignancies.

METHODS

The expression of S100P and Ezrin was examined by immunohistochemistry in 58 primary TNBC samples. The mRNA and protein levels of S100P and Ezrin were assessed in breast cancer-derived cell lines using qRT-PCR and Western blotting, respectively. Proliferation and migration assays were performed using TNBC-derived MFM-223 and SUM-185-PE cells transfected with S100P and Ezrin siRNAs. Two different timeframes were employed for TEM assays using TNBC-derived cells and human umbilical vein endothelial-derived cells, respectively. Correlations between the status of Ezrin^Thr-567 expression and various clinicopathological features were analyzed by immunohistochemistry.

RESULTS

We found that S100P and Ezrin double negative TNBC cases were significantly associated with a better disease-free survival. We also found that single and double siRNA-mediated knockdown of S100P and Ezrin in TNBC-derived cells significantly inhibited their TEM and destabilized the intercellular junctions of endothelial cells. In addition, we found that Ezrin^Thr-567 immunoreactivity significantly correlated with vascular invasion in TNBC patients.

CONCLUSIONS

From our data we conclude that S100P, Ezrin and Ezrin^Thr-567 are involved in the trans-endothelial migration of TNBC cells and that they may serve as potential targets in TNBC patients.

S100P as a Marker for Urothelial Histogenesis: A Critical Review and Comparison With Novel and Traditional Urothelial Immunohistochemical Markers

S100P, or placental S100, is a member of a large family of S100 proteins and considered to be a promising immunohistochemical marker to support urothelial differentiation. This review synthesizes published data regarding the expression of S100P in urothelial carcinoma across histological grade and variant patterns, and in other malignancies, in an effort to summarize the state of understanding of this marker and evaluate its potential. We provide also a broad comparison of S100P with other contemporary and traditional urothelial markers and outline the potential utility of S100P in various diagnostically challenging scenarios. Taken in context, we recommend that to provide immunohistochemical support for consideration of urothelial differentiation, S100P may be included in a panel of markers (due to its high sensitivity), with better established (GATA3) and more specific (uroplakin 2) markers, for comparison with corresponding markers of other primary sites under consideration, depending on the clinical context. We emphasize that the overall most appropriate panel for any given case depends on the differential diagnosis engendered by the morphology encountered, and the constellation of clinical findings. As always with immunohistochemical panels, expected positive and negative markers for each diagnostic consideration should be included. Finally, since as of date there are no optimally sensitive or specific markers of urothelial differentiation, all final diagnoses relying on immunohistochemical support should be made in the appropriate clinical and histological context.

Solving the crystal structure of human calcium-free S100Z: the siege and conquer of one of the last S100 family strongholds

The X-ray structure of human apo-S100Z has been solved and compared with that of the zebrafish calcium-bound S100Z, which is the closest in sequence. Human apo-S100A12, which shows only 43% sequence identity to human S100Z, has been used as template model to solve the crystallographic phase problem. Although a significant buried surface area between the two physiological dimers is present in the asymmetric unit of human apo-S100Z, the protein does not form the superhelical arrangement in the crystal as observed for the zebrafish calcium-bound S100Z and human calcium-bound S100A4. These findings further demonstrate that calcium plays a fundamental role in triggering quaternary structure formation in several S100s. Solving the X-ray structure of human apo-S100Z by standard molecular replacement procedures turned out to be a challenge and required trying different models and different software tools among which only one was successful. The model that allowed structure solution was that with one of the lowest sequence identity with the target protein among the S100 family in the apo state. Based on the previously solved zebrafish holo-S100Z, a putative human holo-S100Z structure has been then calculated through homology modeling; the differences between the experimental human apo and calculated holo structure have been compared to those existing for other members of the family.

Identification of miRNAs and differentially expressed genes in early phase non-small cell lung cancer

To explore the potential therapeutic targets of early‑stage non-small cell lung cancer (NSCLC), gene microarray analysis was conducted. The microarray data of NSCLC in stage IA, IB, IIA, and IIB (GSE50081), were downloaded from the Gene Expression Omnibus (GEO) database. The differentially expressed genes (DEGs) in IB vs. IA, IIA vs. IB, IIB vs. IIA were screened out via R. ToppGene Suite was used to get the enriched Gene Ontology (GO) terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways of the DEGs. The GeneCoDis3 database and Cytoscape software were used to construct the transcriptional regulatory network. In total, 25, 17 and 14 DEGs were identified in IB vs. IA, IIA vs. IB, IIB vs. IIA of NSCLC, respectively. Some GO terms and pathways (e.g., extracellular space, alveolar lamellar body, bioactivation via cytochrome P450 pathway) were found significantly enriched in DEGs. Genes S100P, ALOX15B, CCL11, NLRP2, SERPINA3, FoxO4 and hsa-miR-491 may play important roles in the development of early-stage NSCLC. Thus, by bioinformatics analysis the key genes and biological processes involving in the development of early-stage NSCLC could be established, providing more potential references for the therapeutic targets.

A novel truncated form of S100P predicts disease-free survival in patients with lymph node positive breast cancer

The calcium-binding protein S100P is overexpressed in various cancers and may contribute to the oncogenic phenotype. This study used mass spectrometry to characterize a novel 9.2-kDa C-terminally truncated form of S100P (t-S100P), and to investigate its potential prognostic value in breast cancer. Univariate analysis demonstrated the association between breast tissue t-S100P levels (n = 148) and conventional pathological markers. Across all tumor samples, high t-S100P was strongly prognostic for poor disease-free survival (P = 0.005), its efficacy confined to lymph node-positive tumors (n = 74, P = 0.007). Matrix-assisted laser desorption/ionization imaging mass spectrometry confirmed differential t-S100P abundance between breast cancer and unaffected adjacent tissue. t-S100P was exclusively located in the cell nucleus of breast cancer tissue, and full-length S100P was essentially undetectable by mass spectrometry. We conclude that t-S100P is the predominant form of S100P in breast cancer tissue and is strongly prognostic for disease-free survival in women with lymph node-positive disease.

Expression and Distribution of Calcium-Binding Protein S100P in Human Placenta during Pregnancy

Background

S100P is a member of the S100 family of calcium-binding proteins, and it participates in pathophysiological events, such as tumor growth and invasion. Based on the striking similarities between trophoblast cells and tumor cells with regard to proliferative and invasive properties, we raised the question of whether and how S100P expresses in trophoblast cells during development. This study aimed to investigate the expression pattern of S100P in the human placenta during pregnancy development.

Materials and Methods

In this experimental study, we collected 16 first-trimester placental tissues, 10 second-trimester placental tissues, and 12 term placentas. The mRNA expression levels of S100P were detected by reverse-transcription-polymerase chain reaction (RT-PCR) and quantitative real-time PCR, the protein expression levels were detected by western blot, and the localization of S100P was measured by immunohistochemical staining. The values obtained from PCR and western blot analysis were expressed as the mean ± SD. Levene’s test was used to test equal variances, and one-way analysis of variance (ANOVA) was used to evaluate differences between groups.

Results

Protein and mRNA expression of S100P could be detected in placenta during pregnancy, with minor higher levels in first-trimester (p>0.05). Immunohistochemical staining revealed that S100P protein was strongly expressed in syncytiotrophoblasts, and moderate expression was detected in villous cytotrophoblasts and cytotrophoblast columns. The S100P protein was localized to both cytoplasm and nuclei in syncytiotrophoblasts, while it only existed in the cytoplasm of cytotrophoblasts.

Conclusion

S100P was strongly detected in human placenta during pregnancy. The specific expression and distribution of S100P in human placenta throughout gestation suggested that S100P function might vary with its location in the placenta.

Interaction between S100P and the anti-allergy drug cromolyn

The S100P protein has been known to mediate cell proliferation by binding the receptor for advanced glycation end products (RAGE) to activate signaling pathways, such as the extracellular regulated kinase (ERK) and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathways. S100P/RAGE signaling is involved in a variety of diseases, such as cancer, metastasis, and diabetes. Cromolyn is an anti-allergy drug that binds S100P to block the interaction between S100P and RAGE. In the present study, we characterized the properties of the binding between cromolyn and calcium-bound S100P using various biophysical techniques. The binding affinity for S100P and cromolyn was measured to be in the millimolar range by fluorescence spectroscopy. NMR-HSQC titration experiments and HADDOCK modeling was employed to determine the spatial structure of the proposed heterotetramer model of the S100P-cromolyn complex. Additional MD simulation results revealed the important properties in the complex stability and conformational flexibility of the S100P-cromolyn complex. This proposed model has provided an understanding of the molecular level interactions of S100P-cromolyn complex.

Structural insights into calcium-bound S100P and the V domain of the RAGE complex

The S100P protein is a member of the S100 family of calcium-binding proteins and possesses both intracellular and extracellular functions. Extracellular S100P binds to the cell surface receptor for advanced glycation end products (RAGE) and activates its downstream signaling cascade to meditate tumor growth, drug resistance and metastasis. Preventing the formation of this S100P-RAGE complex is an effective strategy to treat various disease conditions. Despite its importance, the detailed structural characterization of the S100P-RAGE complex has not yet been reported. In this study, we report that S100P preferentially binds to the V domain of RAGE. Furthermore, we characterized the interactions between the RAGE V domain and Ca²⁺-bound S100P using various biophysical techniques, including isothermal titration calorimetry (ITC), fluorescence spectroscopy, multidimensional NMR spectroscopy, functional assays and site-directed mutagenesis. The entropy-driven binding between the V domain of RAGE and Ca⁺²-bound S100P was found to lie in the micromolar range (K_d of ∼6 µM). NMR data-driven HADDOCK modeling revealed the putative sites that interact to yield a proposed heterotetrameric model of the S100P-RAGE V domain complex. Our study on the spatial structural information of the proposed protein-protein complex has pharmaceutical relevance and will significantly contribute toward drug development for the prevention of RAGE-related multifarious diseases.

Joining S100 proteins and migration: for better or for worse, in sickness and in health

The vast diversity of S100 proteins has demonstrated a multitude of biological correlations with cell growth, cell differentiation and cell survival in numerous physiological and pathological conditions in all cells of the body. This review summarises some of the reported regulatory functions of S100 proteins (namely S100A1, S100A2, S100A4, S100A6, S100A7, S100A8/S100A9, S100A10, S100A11, S100A12, S100B and S100P) on cellular migration and invasion, established in both culture and animal model systems and the possible mechanisms that have been proposed to be responsible. These mechanisms involve intracellular events and components of the cytoskeletal organisation (actin/myosin filaments, intermediate filaments and microtubules) as well as extracellular signalling at different cell surface receptors (RAGE and integrins). Finally, we shall attempt to demonstrate how aberrant expression of the S100 proteins may lead to pathological events and human disorders and furthermore provide a rationale to possibly explain why the expression of some of the S100 proteins (mainly S100A4 and S100P) has led to conflicting results on motility, depending on the cells used.

S100P dissociates myosin IIA filaments and focal adhesion sites to reduce cell adhesion and enhance cell migration

S100 proteins promote cancer cell migration and metastasis. To investigate their roles in the process of migration we have constructed inducible systems for S100P in rat mammary and human HeLa cells that show a linear relationship between its intracellular levels and cell migration. S100P, like S100A4, differentially interacts with the isoforms of nonmuscle myosin II (NMIIA, K(d) = 0.5 μM; IIB, K(d) = 8 μM; IIC, K(d) = 1.0 μM). Accordingly, S100P dissociates NMIIA and IIC filaments but not IIB in vitro. NMIIA knockdown increases migration in non-induced cells and there is no further increase upon induction of S100P, whereas NMIIB knockdown reduces cell migration whether or not S100P is induced. NMIIC knockdown does not affect S100P-enhanced cell migration. Further study shows that NMIIA physically interacts with S100P in living cells. In the cytoplasm, S100P occurs in discrete nodules along NMIIA-containing filaments. Induction of S100P causes more peripheral distribution of NMIIA filaments. This change is paralleled by a significant drop in vinculin-containing, actin-terminating focal adhesion sites (FAS) per cell. The induction of S100P, consequently, causes significant reduction in cellular adhesion. Addition of a focal adhesion kinase (FAK) inhibitor reduces disassembly of FAS and thereby suppresses S100P-enhanced cell migration. In conclusion, this work has demonstrated a mechanism whereby the S100P-induced dissociation of NMIIA filaments leads to a weakening of FAS, reduced cell adhesion, and enhanced cell migration, the first major step in the metastatic cascade.

Intrinsic disorder in S100 proteins

Although the members of the largest subfamily of the EF-hand proteins, S100 proteins, are evolutionarily young, their functional diversity is extremely broad, partly due to their ability to adapt to various targets. This feature is a hallmark of intrinsically disordered proteins (IDPs), but none of the S100 proteins are recognized as IDPs. S100 are predicted to be enriched in intrinsic disorder, with 62% of them being predicted to be disordered by at least one of the predictors: 31% are recognized as 'molten globules' and 15% are shown to be in extended disordered form. The disorder level of predicted disordered S100 regions is conserved compared to that of more structured regions. The central disordered stretch corresponds to the major part of pseudo EF-hand loop, helix II, hinge region, and an initial part of helix III. It contains about half of known sites of enzymatic post-translational modifications (PTMs), confirming that this region can be flexible in vivo. Most of the internal residues missing in tertiary structures belong to the hinge. Both hinge and pseudo EF-hand loop correspond to the local maxima of the PONDR® VSL2 score and are shown to be evolutionary hotspots, leading to gain of new functional properties. The action of PTMs is shown to be destabilizing, in contrast with the effect of metal-binding or S100 dimerization. Formation of the S100 heterodimers relies on the interplay between the structural rigidity of one of the S100 monomers and the flexibility of another monomer. The ordered regions dominate in the S100 homodimerization sites. Target-binding sites generally consist of distant regions, drastically differing in their disorder level. The disordered region comprising most of the hinge and the N-terminal half of helix III is virtually not involved into dimerization, being intended solely for target recognition. The structural flexibility of this region is essential for recognition of diverse target proteins. At least 86% of multiple interactions of S100 proteins with binding partners are attributed to the S100 proteins predicted to be disordered. Overall, the intrinsic disorder is inherent to many S100 proteins and is vital for activity and functional diversity of the family.

The Calcium-Dependent Interaction of S100B with Its Protein Targets

S100B is a calcium signaling protein that is a member of the S100 protein family. An important feature of S100B and most other S100 proteins (S100s) is that they often bind Ca²⁺ ions relatively weakly in the absence of a protein target; upon binding their target proteins, Ca²⁺-binding then increases by as much as from 200- to 400-fold. This manuscript reviews the structural basis and physiological significance of increased Ca²⁺-binding affinity in the presence of protein targets. New information regarding redundancy among family members and the structural domains that mediate the interaction of S100B, and other S100s, with their targets is also presented. It is the diversity among individual S100s, the protein targets that they interact with, and the Ca²⁺ dependency of these protein-protein interactions that allow S100s to transduce changes in [Ca²⁺]_{intracellular} levels into spatially and temporally unique biological responses.

Modulation of quaternary structure of S100 proteins by calcium ions

It is well established that calcium binding leads to conformational changes in S100 proteins. These conformational changes are thought to activate the protein and render a protein conformation that is capable of binding other proteins. The basic quaternary structural motif of S100 proteins is a homodimer, however there is little information if higher order non-covalent oligomers are also formed and whether these oligomers are of functional relevance. To this end we performed equilibrium analytical ultracentrifugation experiments for 16 S100 proteins (S100A1, S100A2, S100A3, S100A4, S100A5, S100A6, S100A7, S100A8, S100A9, S100A10, S100A11, S100A12, S100A13, S100B, S100P, and S100Z) under reducing conditions in the absence and presence of calcium ions. We show that the addition of calcium promotes the formation of tetrameric structures which could be further enhanced under in vivo conditions where there is an additional effect of molecular crowding.

S100P: a novel therapeutic target for cancer

S100P expression is described in many different cancers, and its expression is associated with drug resistance, metastasis, and poor clinical outcome. S100P is member of the S100 family of small calcium-binding proteins that have been reported to have either intracellular or extracellular functions, or both. Extracellular S100P can bind with the receptor for advanced glycation end products (RAGE) and activate cellular signaling. Through RAGE, S100P has been shown to mediate tumor growth, drug resistance, and metastasis. S100P is specifically expressed in cancer cells in the adult. Therefore, S100P is a useful marker for differentiating cancer cells from normal cells, and can aid in the diagnosis of cancer by cytological examination. The expression of S100P in cancer cells has been related to hypomethylation of the gene. Multiple studies have confirmed the beneficial effects of blocking S100P/RAGE in cancer cells, and different blockers are being developed including small molecules and antagonist peptides. This review summarizes the role and significance of S100P in different cancers.

Generation and characterization of a novel, permanently active S100P mutant

S100 proteins function as Ca2+ signal transducers by regulating cellular targets in their Ca2+ bound conformation. S100P is a member of the S100 protein family that can activate the membrane and F-actin binding protein ezrin in a Ca2+ dependent manner at least in vitro. Here we generated a novel tool to elucidate directly the S100P-ezrin interaction in vivo. This was achieved by constructing a S100P derivative that contained mutations in the two EF hand loops predicted to lock the protein in a permanently active state. The resulting S100P mutant, termed here S100P pa, could be purified as a soluble protein and showed biochemical properties displayed by wild-type S100P only in the presence of Ca2+. Importantly, S100P pa bound to the N-terminal domain of ezrin in the absence of Ca2+ showing an affinity only slightly reduced as compared to that of Ca2+-bound WT S100P. In line with this permanent complex formation, S100P pa colocalized with ezrin to plasma membrane protrusions of epithelial cells even in the absence of intracellular Ca2+ transients. Thus, S100P pa is a novel type of S100 protein mutant locked in a permanently active state that shows an unregulated complex formation with its cellular target ezrin.

Characterization of the Ca2+ -regulated ezrin-S100P interaction and its role in tumor cell migration

Ezrin is a multidomain protein providing regulated membrane-cytoskeleton contacts that play a role in cell differentiation, adhesion, and migration. Within the cytosol of resting cells ezrin resides in an autoinhibited conformation in which the N- and C-terminal ezrin/radixin/moesin (ERM) association domains (ERMADs) interact with one another. Activation of the ezrin membrane-cytoskeleton linker function requires an opening of this interdomain association that can result from phosphatidylinositol 4,5-bisphosphate binding to the N-ERMAD and threonine 567 phosphorylation in the C-ERMAD. We have shown that ezrin can also be activated by Ca(2+)-dependent binding of the EF-hand protein S100P. We now provide a quantitative analysis of this interaction and map the respective binding sites to the F2 lobe in the ezrin N-ERMAD and a stretch of hydrophobic residues in the C-terminal extension of S100P. Phospholipid binding assays reveal that S100P and phosphatidylinositol 4,5-bisphosphate compete to some extent for at least partially overlapping binding sites in N-ERMAD. Using interaction-competent as well as interaction-incompetent S100P derivatives and permanently active ezrin mutants, we also show that the protein interaction and a resulting activation of ezrin promote the transendothelial migration of tumor cells. Thus, a prometastatic role of ezrin and S100P that had been proposed based on their overexpression in highly metastatic cancers is probably due to a direct interaction between the two proteins and the S100P-mediated activation of ezrin.

Identification of a dimeric intermediate in the unfolding pathway for the calcium-binding protein S100B

The S100 proteins comprise 25 calcium-signalling members of the EF-hand protein family. Unlike typical EF-hand signalling proteins such as calmodulin and troponin-C, the S100 proteins are dimeric, forming both homo- and heterodimers in vivo. One member of this family, S100B, is a homodimeric protein shown to control the assembly of several cytoskeletal proteins and regulate phosphorylation events in a calcium-sensitive manner. Calcium binding to S100B causes a conformational change involving movement of helix III in the second calcium-binding site (EF2) that exposes a hydrophobic surface enabling interactions with other proteins such as tubulin and Ndr kinase. In several S100 proteins, calcium binding also stabilizes dimerization compared to the calcium-free states. In this work, we have examined the guanidine hydrochloride (GuHCl)-induced unfolding of dimeric calcium-free S100B. A series of tryptophan substitutions near the dimer interface and the EF2 calcium-binding site were studied by fluorescence spectroscopy and showed biphasic unfolding curves. The presence of a plateau near 1.5 M GuHCl showed the presence of an intermediate that had a greater exposed hydrophobic surface area compared to the native dimer based on increased 4,4-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid fluorescence. Furthermore, (1)H-(15)N heteronuclear single quantum coherence analyses as a function of GuHCl showed significant chemical shift changes in regions near the EF1 calcium-binding loop and between the linker and C-terminus of helix IV. Together these observations show that calcium-free S100B unfolds via a dimeric intermediate.

The calcium-binding protein S100P in normal and malignant human tissues

Background

S100P is a Ca²⁺ binding protein overexpressed in a variety of cancers, and thus, has been considered a potential tumor biomarker. Very little has been studied about its normal expression and functions.

Methods

We examined S100P expression in normal human tissues by quantitative reverse transcription polymerase chain reaction and immunohistochemistry. S100P protein expression was also studied in a series of tumors, consisting of 74 ovarian, 11 pancreatic, 56 gastric, 57 colorectal, 89 breast and 193 prostate carcinomas using a novel anti-S100P monoclonal antibody.

Results

Among the normal tissues, the highest S100P mRNA levels were observed in the placenta and esophagus. Moderate signals were also detected in the stomach, duodenum, large intestine, prostate and leukocytes. At the protein level, the highest reactions for S100P were seen in the placenta and stomach. Immunostaining of tumor specimens showed that S100P protein is expressed in all the tumor categories included in the study, being most prevalent in gastric tumors.

Conclusion

Based on our observations, S100P is widely expressed in both normal and malignant tissues. The high expression in some tumors suggests that it may represent a potential target molecule for future diagnostic and therapeutic applications.

Crystal structure study on human S100A13 at 2.0 A resolution

The S100 protein family is the largest group of calcium-binding protein families, which consists of at least 25 members. S100A13, which is widely expressed in a variety of tissues, is a unique member of the S100 protein family. Previous reports showed that S100A13 might be involved in the stress-induced release of some signal peptide-less proteins (such as FGF-1 and IL-1alpha) and also associated with inflammatory functions. It was also reported that S100A13 is a new angiogenesis marker. Here we report the crystal structure of the Ca(2+)-bound form of S100A13 at 2.0 A resolution. S100A13 is a homodimer with four EF-hand motifs in an asymmetric unit, displaying a folding pattern similar to other S100 members. However, S100A13 has the unique structural feature with all alpha-helices being amphiphilic, which was not found in other members of S100s. We propose that this characteristic structure of S100A13 might be related to its ability to mediate the release of FGF-1 and IL-1alpha.

Pathologies involving the S100 proteins and RAGE

The S100 proteins are exclusively expressed in vertebrates and are the largest subgroup within the superfamily of EF-hand Ca2(+)-binding proteins Generally, S100 proteins are organized as tight homodimers (some as heterodimers). Each subunit is composed of a C-terminal, 'canonical' EF-hand, common to all EF-hand proteins, and a N-terminal, 'pseudo' EF-hand, characteristic of S100 proteins. Upon Ca2(+)-binding, the C-terminal EF-hand undergoes a large conformational change resulting in the exposure of a hydrophobic surface responsible for target binding A unique feature of this protein family is that some members are secreted from cells upon stimulation, exerting cytokine- and chemokine-like extracellular activities via the Receptor for Advanced Glycation Endproducts, RAGE. Recently, larger assemblies of some S100 proteins (hexamers, tetramers, octamers) have been also observed and are suggested to be the active extracellular species required for receptor binding and activation through receptor multimerization Most S100 genes are located in a gene cluster on human chromosome 1q21, a region frequently rearranged in human cancer The functional diversification of S100 proteins is achieved by their specific cell- and tissue-expression patterns, structural variations, different metal ion binding properties (Ca2+, Zn2+ and Cu2+) as well as their ability to form homo-, hetero- and oligomeric assemblies Here, we review the most recent developments focussing on the biological functions of the S100 proteins and we discuss the presently available S100-specific mouse models and their possible use as human disease models In addition, the S100-RAGE interaction and the activation of various cellular pathways will be discussed. Finally, the close association of S100 proteins with cardiomyopathy, cancer, inflammation and brain diseases is summarized as well as their use in diagnosis and their potential as drug targets to improve therapies in the future.

Calcium-dependent and -independent interactions of the S100 protein family

The S100 proteins comprise at least 25 members, forming the largest group of EF-hand signalling proteins in humans. Although the proteins are expressed in many tissues, each S100 protein has generally been shown to have a preference for expression in one particular tissue or cell type. Three-dimensional structures of several S100 family members have shown that the proteins assume a dimeric structure consisting of two EF-hand motifs per monomer. Calcium binding to these S100 proteins, with the exception of S100A10, results in an approx. 40 degrees alteration in the position of helix III, exposing a broad hydrophobic surface that enables the S100 proteins to interact with a variety of target proteins. More than 90 potential target proteins have been documented for the S100 proteins, including the cytoskeletal proteins tubulin, glial fibrillary acidic protein and F-actin, which have been identified mostly from in vitro experiments. In the last 5 years, efforts have concentrated on quantifying the protein interactions of the S100 proteins, identifying in vivo protein partners and understanding the molecular specificity for target protein interactions. Furthermore, the S100 proteins are the only EF-hand proteins that are known to form both homo- and hetero-dimers, and efforts are underway to determine the stabilities of these complexes and structural rationales for their formation and potential differences in their biological roles. This review highlights both the calcium-dependent and -independent interactions of the S100 proteins, with a focus on the structures of the complexes, differences and similarities in the strengths of the interactions, and preferences for homo- compared with hetero-dimeric S100 protein assembly.

Induction of metastasis by S100P in a rat mammary model and its association with poor survival of breast cancer patients

S100P, an EF-hand calcium-binding protein, has been reported to be associated with the progression of many types of cancers. Transfection of an expression vector for S100P into a benign, nonmetastatic rat mammary cell line causes a 4- to 6-fold increase in its level in all four transformant cell clones. When the resultant transformant cell lines are introduced in turn into the mammary fat pads of syngeneic Furth-Wistar rats, there is a significant 3-fold increase in local muscle invasion and a significant induction of metastasis in 64% to 75% of tumor-bearing animals. In a group of 303 breast cancer patients followed for up to 20 years, antibodies to S100P immunocytochemically stain 161 primary tumors. Survival of patients with S100P-positive carcinomas is significantly worse by about 7-fold than for those with negatively stained carcinomas. There is also a significant association between the class level of immunocytochemical staining of the carcinoma cells and decreased patient survival. Positive staining for S100P is significantly associated with that for two other metastasis-inducing proteins, S100A4 and osteopontin. Patients with tumors that stained positively for both S100P and S100A4 have a significantly reduced survival of 1.1% over patients with either S100 protein alone. Multivariate regression analysis identifies S100P, S100A4, and osteopontin as the most significant independent indicators of death in this group of patients. These results suggest that stratification of patients into groups according to expression of multiple metastasis-inducing proteins may lead to a more accurate prediction of patient survival.

Principal Component Analysis of the Conformational Freedom within the EF-Hand Superfamily

A database of nonredundant structures of EF-hand domains--i.e., pairs of helix-loop-helix motifs--has been assembled, and the six angles among the four helices re-determined. A principal component analysis of these angles allows us to use two such components (PC1 and PC2) to describe the system retaining 80% of the total variance. A PC2 against PC1 plot representation allows us to represent in a compact way the full range of structural diversity of EF-hand domains, their grouping into protein families, and the variation for each family upon calcium and peptide binding.

Molecular basis of the complex formation between the two calcium-binding proteins S100A8 (MRP8) and S100A9 (MRP14)

S100 proteins form characteristic homo- and/or heterodimers that play a role in calcium-mediated signaling. We characterized the formation of the human S100A8/S100A9 heterodimer using the yeast two-hybrid system. Employing site-directed mutagenesis we found that distinct hydrophobic amino acids of helix I/I′ are located at a crucial site of the S100A8/S100A9 dimer interface, whereas conserved residues within helix IV/IV′ are not important for heterodimerization. Furthermore, amino acids Y16 and F68 prevent homodimerization of human S100A8. These data demonstrate for the first time the functional relevance of distinct hydrophobic amino acids for human S100A8/S100A9 complex formationin vivo.

Heterodimeric interaction and interfaces of S100A1 and S100P

With the widespread use of yeast two-hybrid systems, many heterodimeric forms of S100 proteins have been found, although their biological significance is unknown. In the present study, S100A1 was found to interact with another S100 protein, S100P, by using the yeast two-hybrid system. The binding parameters of the interaction were obtained using an optical biosensor and show that S100P has a slightly higher affinity for S100A1 (K(d)=10-20 nM) when compared with that for self-association (K(d)=40-120 nM). The physical interaction of S100A1 and S100P was also demonstrated in living mammalian cells using a fluorescence resonance energy transfer technique. Preincubation of recombinant S100P with S100A1, before the biosensor assay, reduced by up to 50% the binding of S100P to a recombinant C-terminal fragment of non-muscle myosin A, one of its target molecules. Site-specific mutations of S100P and S100A1, combined with homology modelling of an S100P/S100A1 heterodimer using known S100P and S100A1 structures, allowed the hydrophobic interactions at the dimeric interface of the heterodimer to be defined and provide an explanation for the heterodimerization of S100P and S100A1 at the molecular level. These results have revealed the similarities and the differences between the S100P homodimer and the S100A1/S100P heterodimer.

Survey of the year 2003 commercial optical biosensor literature

In the year 2003 there was a 17% increase in the number of publications citing work performed using optical biosensor technology compared with the previous year. We collated the 962 total papers for 2003, identified the geographical regions where the work was performed, highlighted the instrument types on which it was carried out, and segregated the papers by biological system. In this overview, we spotlight 13 papers that should be on everyone's 'must read' list for 2003 and provide examples of how to identify and interpret high-quality biosensor data. Although we still find that the literature is replete with poorly performed experiments, over-interpreted results and a general lack of understanding of data analysis, we are optimistic that these shortcomings will be addressed as biosensor technology continues to mature.

S100P stimulates cell proliferation and survival via receptor for activated glycation end products (RAGE)

S100P is a member of the S100 protein family that is expressed in several malignant neoplasms. Currently the effects of this molecule on cell function are unknown. In the present study we investigated the biological effects and mechanisms of action of S100P using NIH3T3 cells. Expression of S100P in NIH3T3 cells led to the presence of S100P in the culture medium, increased cellular proliferation, and enhanced survival after detachment from the culture substrate or after exposure to the chemotherapeutic agent 5-flurouracil. The proliferation and survival effects of S100P expression were duplicated in a time- and concentration-dependent manner by the extracellular addition of purified S100P to wild-type NIH3T3 cells and correlated with the activation of extracellular-regulated kinases (Erks) and NF-kappaB. To determine the mechanisms involved in these effects, we tested the hypothesis that S100P activated RAGE (receptor for activated glycation end products). We found that S100P co-immunoprecipitated with RAGE. Furthermore, the effects of S100P on cell signaling, proliferation, and survival were blocked by agents that interfere with RAGE including administration of an amphoterin-derived peptide known to antagonize RAGE activation, anti-RAGE antibodies, and by expression of a dominant negative RAGE. These data suggest that S100P can act in an autocrine manner via RAGE to stimulate cell proliferation and survival.