Binding of pEL98 protein, an S100-related calcium-binding protein, to nonmuscle tropomyosin

Protein atlas of fibroblast specific protein 1 (FSP1)/S100A4

Fibroblast specific protein 1 (FSP1)/S100A4 is a calcium binding protein which has been linked to epithelial-mesenchymal transition, tissue fibrosis, pulmonary vascular disease, metastatic tumour development, increased tumour cell motility and invasiveness. This protein is reported to be also expressed in newly formed and differentiated fibroblasts and has been used in various studies to demonstrate epithelial-mesenchymal transition (EMT). We aimed to characterize S100A4 positive cells in different human tissue compartments, with the focus on fibroblasts/myofibroblast. We found S100A4 expression in a wide range of cells. Fibroblasts/myofibroblasts showed a broad spectrum of staining intensity, ranging from negative to strong expression of S100A4, with the strongest expression in smooth muscle actin positive myofibroblasts. Cells of haematopoietic lineage, namely CD4 and CD8 positive T-lymphocytes, but not B-lymphocytes expressed S100A4. All investigated monocytes, macrophages and specialised histiocytes were positive for S100A4. Even some epithelial cells of the kidney and bladder were positive for S100A4. Expression was also found in the vasculature. Here, cells of the subendothelial space, tunica adventitia and some smooth muscle cells of the tunica media were positive for S100A4. In summary, S100A4 is expressed in various cell types of different lineage and is not, as originally believed, specific for fibroblasts (FSP). Results attained under the premise of specificity of FSP1/S100A4 for fibroblasts, like the founding research on EMT type 2 in kidney and liver, therefore need to be reinterpreted.

Molecular and Genetic Biomarkers in Idiopathic Pulmonary Fibrosis: Where Are We Now?

Idiopathic pulmonary fibrosis (IPF) represents a chronic progressive fibrotic interstitial lung disease of unknown cause with an ominous prognosis. It remains an unprecedent clinical challenge due to its delayed diagnosis and unpredictable clinical course. The need for accurate diagnostic, prognostic and predisposition biomarkers in everyday clinical practice becomes more necessary than ever to ensure prompt diagnoses and early treatment. The identification of such blood biomarkers may also unravel novel drug targets against IPF development and progression. So far, the role of diverse blood biomarkers, implicated in various pathogenetic pathways, such as in fibrogenesis (S100A4), extracellular matrix remodelling (YKL-40, MMP-7, ICAM-1, LOXL2, periostin), chemotaxis (CCL-18, IL-8), epithelial cell injury (KL-6, SP-A, SP-D), autophagy and unfolded protein response has been investigated in IPF with various results. Moreover, the recent progress in genetics in IPF allows for a better understanding of the underlying disease mechanisms. So far, the causative mutations in pulmonary fibrosis include mutations in telomere-related genes and in surfactant-related genes, markers that could act as predisposition biomarkers in IPF. The aim of this review is to provide a comprehensive overview from the bench to bedside of current knowledge and recent insights on biomarkers in IPF, and to suggest future directions for research. Large-scale studies are still needed to confirm the exact role of these biomarkers.

Inhibition of the invasion and metastasis of mammary carcinoma cells by NBD peptide targeting S100A4 via the suppression of the Sp1/MMP‑14 axis

A synthetic peptide that blocks the interaction between the metastasis‑enhancing calcium‑binding protein, S100A4, and its effector protein, methionine aminopeptidase 2 (MetAP2) (the NBD peptide), was previously demonstrated to inhibit the angiogenesis of endothelial cells, leading to the regression of human prostate cancer in a xenograft model. However, the effects of the NBD peptide on the malignant properties of cancer cells that express S100A4 remain to be elucidated. The present study demonstrates that the NBD peptide inhibits the invasiveness and metastasis of highly metastatic human mammary carcinoma cells. The introduction of the peptide into MDA‑MB‑231 variant cells resulted in the suppression of matrix degradation in a gelatin invadopodia assay and invasiveness in a Matrigel invasion assay. In line with these results, the peptide significantly downregulated the expression of matrix metalloproteinase (MMP)‑14 (MT1‑MMP). Mechanistic analysis of the downregulation of MMP‑14 revealed the suppression of the expression of the transcription factor, specificity protein 1 (Sp1), but not that of nuclear factor (NF)‑κB, early growth response 1 (EGR1) or ELK3, all of which were reported to be involved in transcriptional regulation of the MMP‑14 gene. At the same time, evidence suggested that the NBD peptide also suppressed Sp1 and MMP‑14 expression levels in MDA‑MB‑468 cells. Importantly, the intravenous administration of the NBD peptide encapsulated in liposomes inhibited pulmonary metastasis from mammary gland tumors in mice with xenograft tumors. These results indicate that the NBD peptide can suppress malignant tumor growth through the suppression of the Sp1/MMP‑14 axis. Taken together, these results reveal that the NBD peptide acts on not only endothelial cells, but also on tumor cells in an integrated manner, suggesting that the peptide may prove to be a promising cancer therapeutic peptide drug.

Small Ones to Fight a Big Problem-Intervention of Cancer Metastasis by Small Molecules

Metastasis represents the most lethal attribute of cancer and critically limits successful therapies in many tumor entities. The clinical need is defined by the fact that all cancer patients, who have or who will develop distant metastasis, will experience shorter survival. Thus, the ultimate goal in cancer therapy is the restriction of solid cancer metastasis by novel molecularly targeted small molecule based therapies. Biomarkers identifying cancer patients at high risk for metastasis and simultaneously acting as key drivers for metastasis are extremely desired. Clinical interventions targeting these key molecules will result in high efficiency in metastasis intervention. In result of this, personalized tailored interventions for restriction and prevention of cancer progression and metastasis will improve patient survival. This review defines crucial biological steps of the metastatic cascade, such as cell dissemination, migration and invasion as well as the action of metastasis suppressors. Targeting these biological steps with tailored therapeutic strategies of intervention or even prevention of metastasis using a wide range of small molecules will be discussed.

Amlexanox Blocks the Interaction between S100A4 and Epidermal Growth Factor and Inhibits Cell Proliferation

The human S100A4 protein binds calcium, resulting in a change in its conformation to promote the interaction with its target protein. Human epidermal growth factor (EGF) is the target protein of S100A4 and a critical ligand of the receptor EGFR. The EGF/EGFR system promotes cell survival, differentiation, and growth by activating several signaling pathways. Amlexanox is an anti-inflammatory and anti-allergic drug that is used to treat recurrent aphthous ulcers. In the present study, we determined that amlexanox interacts with S100A4 using heteronuclear single quantum correlation titration. We elucidated the interactions of S100A4 with EGF and amlexanox using fluorescence and nuclear magnetic resonance spectroscopy. We generated two binary models (for the S100A4-EGF and S100A4-amlexanox complexes) and observed that amlexanox and EGF share a similar binding region in mS100A4. We also used a WST-1 assay to investigate the bioactivity of S100A4, EGF, and amlexanox, and found that amlexanox blocks the binding between S100A4 and EGF, and is therefore useful for the development of new anti-proliferation drugs.

Multilevel Changes in Protein Dynamics upon Complex Formation of the Calcium-Loaded S100A4 with a Nonmuscle Myosin IIA Tail Fragment

Dysregulation of Ca²⁺ -binding S100 proteins plays important role in various diseases. The asymmetric complex of Ca²⁺ -bound S100A4 with nonmuscle myosin IIA has high stability and highly increased Ca²⁺ affinity. Here we investigated the possible causes of this allosteric effect by NMR spectroscopy. Chemical shift-based secondary-structure analysis did not show substantial changes for the complex. Backbone dynamics revealed slow-timescale local motions in the H1 helices of homodimeric S100A4; these were less pronounced in the complex form and might be accompanied by an increase in dimer stability. Different mobilities in the Ca²⁺ -coordinating EF-hand sites indicate that they communicate by an allosteric mechanism operating through changes in protein dynamics; this must be responsible for the elevated Ca²⁺ affinity. These multilevel changes in protein dynamics as conformational adaptation allow S100A4 fine-tuning of its protein-protein interactions inside the cell during Ca²⁺ signaling.

Functional profile of S100A4-deficient T cells

The protein S100A4 is best known for its significant role in promoting motility and invasive capacity of cancer cells. Since S100A4 expression has been reported also in T cells, we analyzed its potential role in T cell motility and inflammation. Using S100a4(+/Gfp) mice, we show here that S100A4 is exclusively expressed by memory T cells of CD4(+) or CD8(+) subpopulations, predominantly of the effector memory T cell subtype. However, the protein was not required for in vitro memory T cell migration toward gradients of the inflammatory chemokine CXCL10. Moreover, T cell memory response was normal in S100A4-deficient mice and lack of S100a4 gene expression did not induce any defect in promoting the development of protective immunity or inflammatory reactions leading to autoimmunity. Taken together, our results demonstrate that S100A4 activity is dispensable for T cell motility/migration and inflammatory potential.

Efficient inhibition of tumor angiogenesis and growth by a synthetic peptide blocking S100A4-methionine aminopeptidase 2 interaction

The prometastatic calcium-binding protein, S100A4, is expressed in endothelial cells, and its downregulation markedly suppresses tumor angiogenesis in a xenograft cancer model. Given that endothelial S100A4 can be a molecular target for inhibiting tumor angiogenesis, we addressed here whether synthetic peptide capable of blocking S100A4-effector protein interaction could be a novel antiangiogenic agent. To examine this hypothesis, we focused on the S100A4-binding domain of methionine aminopeptidase 2, an effector protein, which plays a role in endothelial cell growth. Overexpression of the domain in mouse endothelial MSS31 cells reduced DNA synthesis, and the corresponding synthetic peptide (named NBD) indeed interacted with S100A4 and inhibited capillary formation in vitro and new blood vessel formation in vivo. Intriguingly, a single intra-tumor administration of the NBD peptide in human prostate cancer xenografts significantly reduced vascularity, resulting in tumor regression. Mechanistically, the NBD peptide enhanced assembly of nonmuscle myosin IIA filaments along with Ser1943 phosphorylation, stimulated formation of focal adhesions without phosphorylation of focal adhesion kinase, and provoked G1/S arrest of the cell cycle. Altogether, the NBD peptide is a potent inhibitor for tumor angiogenesis, and is the first example of an anticancer peptide drug developed on the basis of an endothelial S100A4-targeted strategy.

Extracellular S100A4 induces smooth muscle cell phenotypic transition mediated by RAGE

We identified S100A4 as a marker of rhomboid (R) smooth muscle cells (SMCs) in vitro (the synthetic phenotype, typical of intimal SMCs) in the porcine coronary artery and of intimal SMCs in vivo in both pigs and humans. S100A4 is an intracellular Ca²⁺ signaling protein and can be secreted; it has extracellular functions via the receptor for advanced glycation end products (RAGE). Our objective was to explore the role of S100A4 in SMC phenotypic change, a phenomenon characteristic of atherosclerotic plaque formation. Transfection of a human S100A4-containing plasmid in spindle-shaped (S) SMCs (devoid of S100A4) led to approximately 10% of S100A4-overexpressing SMCs, S100A4 release, and a transition towards a R-phenotype of the whole SMC population. Furthermore treatment of S-SMCs with S100A4-rich conditioned medium collected from S100A4-transfected S-SMCs induced a transition towards a R-phenotype, which was associated with decreased SMC differentiation markers and increased proliferation and migration by activating the urokinase-type plasminogen activator (uPA), matrix metalloproteinases (MMPs) and their inhibitors (TIMPs). It yielded NF-κB activation in a RAGE-dependent manner. Blockade of extracellular S100A4 in R-SMCs with S100A4 neutralizing antibody induced a transition from R- to S-phenotype, decreased proliferative activity and upregulation of SMC differentiation markers. By contrast, silencing of S100A4 mRNA in R-SMCs did not change the level of extracellular S100A4 or SMC morphology in spite of decreased proliferative activity. Our results show that extracellular S100A4 plays a pivotal role in SMC phenotypic changes. It could be a new target to prevent SMC accumulation during atherosclerosis and restenosis. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

S100A4 negatively regulates β-catenin by inducing the Egr-1-PTEN-Akt-GSK3β degradation pathway

S100A4, also known as the mts1 gene, has been reported as an invasive and metastatic marker for many types of cancers. S100A4 interacts with various target genes that affect tumor cell metastasis; however, little is known about cellular signaling pathways elicited by S100A4. In the current study, we demonstrate an inhibitory effect of S100A4 on β-catenin signaling in breast cancer cells. By overexpressing S100A4 in MCF-7, MDA-MB-231 and MDA-MB-453 breast cancer cells, we observed the down-regulation of β-catenin expression and β-catenin-dependent TCF/LEF transcriptional activities. The activity of GSK3β, which phosphorylates β-catenin and induces proteasomal degradation of β-catenin, was increased in S100A4-overexpressing cell lines. Blocking Glycogen Synthase Kinase (GSK3β) activity by lithium chloride or Dvl gene overexpression restored β-catenin expression. We also found that increased GSK3β activity was due to decrease in Akt activity resulting from Egr-1-induced phosphatase and tensin homolog (PTEN) expression. S100A4 induced Egr-1 nuclear localization by increasing the association between Egr-1 and importin-7 and this effect was reduced in S100A4 mutants that harbored a defect in nuclear localization signals. Collectively, we verify herein that S100A4 may act as a tumor suppressor in breast cancers by down-regulating the central signaling axis for tumor cell survival.

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.

Silencing of S100A4, a metastasis-associated protein, in endothelial cells inhibits tumor angiogenesis and growth

Endothelial cells express S100A4, a metastasis-associated protein, but its role in angiogenesis remains to be elucidated. Here we show that knockdown of S100A4 in mouse endothelial MSS31 cells by murine specific small interference RNA (mS100A4 siRNA) markedly suppressed capillary-like tube formation in vitro, in early stage after the treatment, along with down- and up-regulation of some of the pro-angiogenic and anti-angiogenic gene expression, respectively. Of particular note is that intra-tumor administration of the mS100A4 siRNA in a human prostate cancer xenograft significantly reduced tumor vascularity and resulted in the inhibition of tumor growth. These findings show that S100A4 in endothelial cells is involved in tube formation, and suggest its potential as a molecular target for inhibiting tumor angiogenesis, which warrants further development of endothelial S100A4-based strategies for cancer treatment.

Structure of the S100A4/myosin-IIA complex

Background

S100A4, a member of the S100 family of Ca²⁺-binding proteins, modulates the motility of both non-transformed and cancer cells by regulating the localization and stability of cellular protrusions. Biochemical studies have demonstrated that S100A4 binds to the C-terminal end of the myosin-IIA heavy chain coiled-coil and disassembles myosin-IIA filaments; however, the mechanism by which S100A4 mediates myosin-IIA depolymerization is not well understood.

Results

We determined the X-ray crystal structure of the S100A4Δ8C/MIIA^1908-1923 peptide complex, which showed an asymmetric binding mode for the myosin-IIA peptide across the S100A4 dimer interface. This asymmetric binding mode was confirmed in NMR studies using a spin-labeled myosin-IIA peptide. In addition, our NMR data indicate that S100A4Δ8C binds the MIIA^1908-1923 peptide in an orientation very similar to that observed for wild-type S100A4. Studies of complex formation using a longer, dimeric myosin-IIA construct demonstrated that S100A4 binding dissociates the two myosin-IIA polypeptide chains to form a complex composed of one S100A4 dimer and a single myosin-IIA polypeptide chain. This interaction is mediated, in part, by the instability of the region of the myosin-IIA coiled-coil encompassing the S100A4 binding site.

Conclusion

The structure of the S100A4/MIIA^1908-1923 peptide complex has revealed the overall architecture of this assembly and the detailed atomic interactions that mediate S100A4 binding to the myosin-IIA heavy chain. These structural studies support the idea that residues 1908–1923 of the myosin-IIA heavy chain represent a core sequence for the S100A4/myosin-IIA complex. In addition, biophysical studies suggest that structural fluctuations within the myosin-IIA coiled-coil may facilitate S100A4 docking onto a single myosin-IIA polypeptide chain.

A Calcium Binding Protein, S100A4, Mediates T Cell Dependent Cytotoxicity as a Transformation-Associated Antigen

The nature of the target molecule of TCR gamma delta T cell-mediated lysis remains to be determined. As we previously reported, #067 monoclonal antibody (mAb) recognizes one of the transformation-associated antigens, designated as #067 antigen. This antigen is expressed on the cell surface of rat fibrosarcoma W31 cells, which are established by transformation of fetal fibroblastic WFB cells with H-ras oncogene. It has been suggested that the #067 antigen is a target molecule for TCR gamma delta T cells since #067 mAb inhibited TCR gamma delta T cell-mediated lysis against #067 positive cells. In this study we attempted to identify the protein sequence of the #067 antigen. By using molecular cloning techniques, we demonstrated that a calcium binding protein, S100A4, was possibly one and the same molecule as the #067 antigen. It was shown that the expression of S100A4 was higher in W31 cells than in WFB cells at transcription and protein level. Flow cytometry and immunocytochemical studies showed that #067 antigen partially co-localized with S100A4 on the cell surface as well as the cytoplasm of W31 cells. Moreover, rabbit anti-S100A4 polyclonal antibodies (pAb) inhibited TCR gamma delta T cell-mediated lysis against #067 positive cells. Our results indicated that S100A4 may play a role as a possible target molecule for TCR gamma delta T cell-mediated lysis although how S100A4 is involved in TCR gamma delta T cell-mediated lysis remains to be determined.

Dynamic functions of RhoA in tumor cell migration and invasion

RhoA is one of the more extensively studied members of the Rho family of small GTPase where it is most readily recognized for its contributions to actin-myosin contractility and stress fiber formation. Accordingly, RhoA function during cell migration has been relegated to the rear of the cell where it mediates retraction of the trailing edge. However, RhoA can also mediate membrane ruffling, lamellae formation and membrane blebbing, thus suggesting an active role in membrane protrusions at the leading edge. With the advent of fluorescence resonance energy transfer (FRET)-based Rho activity reporters, RhoA has been shown to be active at the leading edge of migrating cells where it precedes Rac and Cdc42 activation. These observations demonstrate a remarkable versatility to RhoA signaling, but how RhoA function can switch between contraction and protrusion has remained an enigma. This review highlights recent advances regarding how the cooperation of Rho effector Rhotekin and S100A4 suppresses stress fiber generation to permit RhoA-mediated lamellae formation.

S100 calcium-binding protein A4 is a novel independent prognostic factor for the poor prognosis of gastric carcinomas

Overexpression of the S100 calcium-binding protein A4 (S100A4) is involved in epithelial-to-mesenchymal transition, oncogenic transformation, angiogenesis, cytoskeletal integrity and cancer metastasis. Here, we elucidated the role of S100A4 in tumorigenesis and progression of gastric carcinomas. S100A4 expression in gastric carcinomas, adenomas and adjacent non-neoplastic mucosa was analyzed by immunohistochemical, real-time reverse transcriptase (RT)-polymerase chain reaction (PCR) and western blot analyses, and was correlated with various clinicopathological parameters. S100A4 protein expression was increased gradually in the following order: gastritis (19.2%), intestinal metaplasia (IM; 23.3%), dysplasia (34.9%) and carcinoma (55.2%; P<0.001). S100A4 was positively correlated with tumor size, depth of invasion, lymphatic invasion, lymph node metastasis and tumor-node-metastasis (TNM) staging (P<0.05), but not with the age and gender of the carcinoma patients (P>0.05). Intestinal-type (IT) carcinomas showed a higher S100A4 expression than diffuse-type (DT) carcinomas (P<0.001). S100A4 mRNA expression also increased in the following order: gastritis < IM < dysplasia < carcinoma (P<0.05). S100A4 overexpression was observed in gastric carcinomas with a larger diameter, deeper invasion, lymph node metastasis and in IT carcinoma (P<0.05). Univariate analysis using the Kaplan-Meier method indicated a lower cumulative survival rate for patients with weak or moderate S100A4 expression compared with patients not expressing S100A4 (P<0.001). Multivariate analysis using Cox's proportional hazard model demonstrated that depth of invasion, lymphatic or venous invasion, lymph node metastasis, TNM staging and S100A4 expression were independent factors for poor patient prognosis (P<0.05). In conclusion, S100A4 upregulation is positively associated with the pathogenesis, growth, invasion, metastasis and differentiation of gastric carcinomas. S100A4 may be a promising marker indicative of the aggressive behavior and prognosis of gastric carcinomas.

Proteome analysis of distinct developmental stages of human natural killer (NK) cells

The recent Natural Killer (NK) cell maturation model postulates that CD34(+) hematopoietic stem cells (HSC) first develop into CD56(bright) NK cells, then into CD56(dim)CD57(-) and finally into terminally maturated CD56(dim)CD57(+). The molecular mechanisms of human NK cell differentiation and maturation however are incompletely characterized. Here we present a proteome analysis of distinct developmental stages of human primary NK cells, isolated from healthy human blood donors. Peptide sequencing was used to comparatively analyze CD56(bright) NK cells versus CD56(dim) NK cells and CD56(dim)CD57(-) NK cells versus CD56(dim)CD57(+) NK cells and revealed distinct protein signatures for all of these subsets. Quantitative data for about 3400 proteins were obtained and support the current differentiation model. Furthermore, 11 donor-independently, but developmental stage specifically regulated proteins so far undescribed in NK cells were revealed, which may contribute to NK cell development and may elucidate a molecular source for NK cell effector functions. Among those proteins, S100A4 (Calvasculin) and S100A6 (Calcyclin) were selected to study their dynamic subcellular localization. Upon activation of human primary NK cells, both proteins are recruited into the immune synapse (NKIS), where they colocalize with myosin IIa.

The unique cytoplasmic domain of human FcγRIIIA regulates receptor-mediated function

Ligand specificity characterizes receptors for Abs and many other immune receptors, but the common use of the FcR γ-chain as their signaling subunit challenges the concept that these receptors are functionally distinct. We hypothesized that elements for specificity might be determined by the unique cytoplasmic domain (CY) sequences of the ligand-binding α-chains of γ-chain-associated receptors. Among Fcγ receptors, a protein kinase C (PKC) phosphorylation consensus motif [RSSTR], identified within the FcγRIIIa (CD16A) CY by in silico analysis, is specifically phosphorylated by PKCs, unlike other FcRs. Phosphorylated CD16A mediates a more robust calcium flux, tyrosine phosphorylation of Syk, and proinflammatory cytokine production, whereas nonphosphorylatable CD16A is more effective at activation of the Gab2/PI3K pathway, leading to enhanced degranulation. S100A4, a specific protein-binding partner for CD16A-CY newly identified by yeast two-hybrid analysis, inhibits phosphorylation of CD16A-CY by PKC in vitro, and reduction of S100A4 levels in vivo enhances receptor phosphorylation upon cross-linking. Taken together, PKC-mediated phosphorylation of CD16A modulates distinct signaling pathways engaged by the receptor. Calcium-activated binding of S100A4 to CD16A, promoted by the initial calcium flux, attenuates the phosphorylation of CY, and, acting as a molecular switch, may both serve as a negative feedback on cytokine production pathways during sustained receptor engagement and favor a shift to degranulation, consistent with the importance of granule release following conjugate formation between CD16A(+) effector cells and target cells. This switch mechanism points to new therapeutic targets and provides a framework for understanding novel receptor polymorphisms.

S100A4 calcium-binding protein is key player in tumor progression and metastasis: preclinical and clinical evidence

The fatality of cancer is mainly bestowed to the property of otherwise benign tumor cells to become malignant and invade surrounding tissues by circumventing normal tissue barriers through a process called metastasis. S100A4 which is a member of the S100 family of calcium-binding proteins has been shown to be able to activate and integrate pathways both intracellular and extracellular to generate a phenotypic response characteristic of cancer metastasis. A large number of studies have shown an increased expression level of S100A4 in various types of cancers. However, its implications in cancer metastasis in terms of whether an increased expression of S100A4 is a causal factor for metastasis or just another after effect of several other physiological and molecular changes in the body resulting from metastasis are not clear. Here we describe the emerging preclinical and clinical evidences implicating S100A4 protein, in both its forms (intracellular and extracellular) in the process of tumorigenesis and metastasis in humans. Based on studies utilizing S100A4 as a metastasis biomarker and molecular target for therapies such as gene therapy, we suggest that S100A4 has emerged as a promising molecule to be tested for anticancer drugs. This review provides an insight in the (1) molecular mechanisms through which S100A4 drives the tumorigenesis and metastasis and (2) developments made in the direction of evaluating S100A4 as a cancer biomarker and drug target.

Crystal structure of the S100A4-nonmuscle myosin IIA tail fragment complex reveals an asymmetric target binding mechanism

S100A4 is a member of the S100 family of calcium-binding proteins that is directly involved in tumor metastasis. It binds to the nonmuscle myosin IIA (NMIIA) tail near the assembly competence domain (ACD) promoting filament disassembly, which could be associated with increasing metastatic potential of tumor cells. Here, we investigate the mechanism of S100A4-NMIIA interaction based on binding studies and the crystal structure of S100A4 in complex with a 45-residue-long myosin heavy chain fragment. Interestingly, we also find that S100A4 binds as strongly to a homologous heavy chain fragment of nonmuscle myosin IIC as to NMIIA. The structure of the S100A4-NMIIA complex reveals a unique mode of interaction in the S100 family: A single, predominantly α-helical myosin chain is wrapped around the Ca(2+)-bound S100A4 dimer occupying both hydrophobic binding pockets. Thermal denaturation experiments of coiled-coil forming NMIIA fragments indicate that the coiled-coil partially unwinds upon S100A4 binding. Based on these results, we propose a model for NMIIA filament disassembly: Part of the random coil tailpiece and the C-terminal residues of the coiled-coil are wrapped around an S100A4 dimer disrupting the ACD and resulting in filament dissociation. The description of the complex will facilitate the design of specific drugs that interfere with the S100A4-NMIIA interaction.

S100A4 downregulates filopodia formation through increased dynamic instability

Cell migration requires the initial formation of cell protrusions, lamellipodia and/or filopodia, the attachment of the leading lamella to extracellular cues and the formation and efficient recycling of focal contacts at the leading edge. The small calcium binding EF-hand protein S100A4 has been shown to promote cell motility but the direct molecular mechanisms responsible remain to be elucidated. In this work, we provide new evidences indicating that elevated levels of S100A4 affect the stability of filopodia and prevent the maturation of focal complexes. Increasing the levels of S100A4 in a rat mammary benign tumor derived cell line results in acquired cellular migration on the wound healing scratch assay. At the cellular levels, we found that high levels of S100A4 induce the formation of many nascent filopodia, but that only a very small and limited number of those can stably adhere and mature, as opposed to control cells, which generate fewer protrusions but are able to maintain these into more mature projections. This observation was paralleled by the fact that S100A4 overexpressing cells were unable to establish stable focal adhesions. Using different truncated forms of the S100A4 proteins that are unable to bind to myosin IIA, our data suggests that this newly identified functions of S100A4 is myosin-dependent, providing new understanding on the regulatory functions of S100A4 on cellular migration.

Rescue of tropomyosin deficiency in Drosophila and human cancer cells by synaptopodin reveals a role of tropomyosin α in RhoA stabilization

Tropomyosins are widespread actin-binding proteins that influence numerous cellular functions including actin dynamics, cell migration, tumour suppression, and Drosophila oocyte development. Synaptopodin is another actin-binding protein with a more restricted expression pattern in highly dynamic cell compartments such as kidney podocyte foot processes, where it promotes RhoA signalling by blocking the Smurf1-mediated ubiquitination of RhoA. Here, we show that synaptopodin has a shorter half-life but shares functional properties with the highly stable tropomyosin. Transgenic expression of synaptopodin restores oskar mRNA localization in Drosophila oocytes mutant for TmII, thereby rescuing germline differentiation and fertility. Synaptopodin restores stress fibres in tropomyosin-deficient human MDA-MB 231 breast cancer cells and TPMα-depleted fibroblasts. Gene silencing of TPMα but not TPMβ causes loss of stress fibres by promoting Smurf1-mediated ubiquitination and proteasomal degradation of RhoA. Functionally, overexpression of synaptopodin or RhoA(K6,7R) significantly reduces MDA-MB 231 cell migration. Our findings elucidate RhoA stabilization by structurally unrelated actin-binding proteins as a conserved mechanism for regulation of stress fibre dynamics and cell motility in a cell type-specific fashion.

Cysteine 81 is critical for the interaction of S100A4 and myosin-IIA

Overexpression of S100A4, a member of the S100 family of Ca(2+)-binding proteins, is associated with a number of human pathologies, including fibrosis, inflammatory disorders, and metastatic disease. The identification of small molecules that disrupt S100A4/target interactions provides a mechanism for inhibiting S100A4-mediated cellular activities and their associated pathologies. Using an anisotropy assay that monitors the Ca(2+)-dependent binding of myosin-IIA to S100A4, NSC 95397 was identified as an inhibitor that disrupts the S100A4/myosin-IIA interaction and inhibits S100A4-mediated depolymerization of myosin-IIA filaments. Mass spectrometry demonstrated that NSC 95397 forms covalent adducts with Cys81 and Cys86, which are located in the canonical target binding cleft. Mutagenesis studies showed that covalent modification of just Cys81 is sufficient to inhibit S100A4 function with respect to myosin-IIA binding and depolymerization. Remarkably, substitution of Cys81 with serine or alanine significantly impaired the ability of S100A4 to promote myosin-IIA filament disassembly. As reversible covalent cysteine modifications have been observed for several S100 proteins, we propose that modification of Cys81 may provide an additional regulatory mechanism for mediating the binding of S100A4 to myosin-IIA.

Two functional S100A4 monomers are necessary for regulating nonmuscle myosin-IIA and HCT116 cell invasion

S100A4, a member of the Ca(2+)-activated S100 protein family, regulates the motility and invasiveness of cancer cells. Moreover, high S100A4 expression levels correlate with poor patient survival in several cancers. Although biochemical, biophysical, and structural data indicate that S100A4 is a noncovalent dimer, it is unknown if two functional S100A4 monomers are required for the productive recognition of protein targets and the promotion of cell invasion. To address this question, we created covalently linked S100A4 dimers using a glycine rich flexible linker. The single-chain S100A4 (sc-S100A4) proteins exhibited wild-type affinities for calcium and nonmuscle myosin-IIA, retained the ability to regulate nonmuscle myosin-IIA assembly, and promoted tumor cell invasion when expressed in S100A4-deficient colon carcinoma cells. Mutation of the two calcium-binding EF-hands in one monomer, while leaving the other monomer intact, caused a 30-60-fold reduction in binding affinity for nonmuscle myosin-IIA concomitant with a weakened ability to regulate the monomer-polymer equilibrium of nonmuscle myosin-IIA. Moreover, sc-S100A4 proteins with one monomer deficient in calcium responsiveness did not support S100A4-mediated colon carcinoma cell invasion. Cross-linking and titration data indicate that the S100A4 dimer binds a single myosin-IIA target peptide. These data are consistent with a model in which a single peptide forms interactions in the vicinity of the canonical target binding cleft of each monomer in such a manner that both target binding sites are required for the efficient interaction with myosin-IIA.

Diagnostic and prognostic value of metastasis inducer S100A4 transcripts in plasma of colon, rectal, and gastric cancer patients

Early detection of tumors and metastases is critical for improving treatment strategies and patient outcomes. The development of molecular markers and simple tests that are clinically applicable for detection, prognostication, and therapy monitoring is strongly needed. The gene S100A4 has long been known to act as a metastasis inducer. High S100A4 levels in the primary tumor are prognostic for metachronous metastasis and correlate with reduced patient survival. We provide, for the first time, a plasma-based assay for transcript quantification of S100A4 in gastrointestinal patients' plasma. We conducted a study to define the diagnostic and prognostic power of S100A4 transcripts using 466 plasma samples from colon, rectal, and gastric cancer patients. Plasma was separated, RNA was isolated, and S100A4 mRNA was determined by quantitative RT-PCR. S100A4 transcripts were increased in cancer patients of each entity (P < 0.0001) and all disease stages (P < 0.05), compared with tumor-free volunteers (sensitivities of 96%, 74%, and 90% and specificities of 59%, 82%, and 71%, for colon, rectal, and gastric cancer patients, respectively). Prospectively analyzed follow-up patients who later experienced metastasis showed higher S100A4 levels than follow-up patients without metastasis. Disease-free survival was decreased in high S100A4-expressing follow-up colorectal cancer patients (P = 0.013). In summary, we developed a method for quantitative S100A4 transcript determination in plasma that allows clinical application routinely. We demonstrated the diagnostic and prognostic potential of this method for early defining cancer staging and patients' risk for metastasis.

Tropomyosin-binding properties of the CHASM protein are dependent upon its calponin homology domain

The calponin homology-associated smooth muscle protein (CHASM) can modulate muscle contractility, and its biological action may involve an interaction with the contractile filament. In this study, we demonstrate an interaction between CHASM and tropomyosin. Deletion constructs of CHASM were generated, and pull-down assays revealed a minimal deletion construct that could bind tropomyosin. Removal of the calponin homology (CH) domain or expression of the CH domain alone did not enable binding. The interaction was characterized by microcalorimetry with a dissociation constant of 2.0x10(-6) M. Confocal fluorescence microscopy also showed green fluorescent protein (GFP)-CHASM localization to filamentous structures within smooth muscle cells, and this targeting was dependent upon the CH domain.

S100A4 regulates macrophage chemotaxis

Using a targeted genetic deletion, we show that the S100A4 metastasis factor is required for macrophage recruitment to sites of inflammation in vivo. S100A4^−/− primary macrophages display defects in chemotaxis due to myosin-IIA overassembly and altered CSF-1 receptor signaling. These studies establish S100A4 as a regulator of macrophage motility.

S100A4, a member of the S100 family of Ca²⁺-binding proteins, is directly involved in tumor metastasis. In addition to its expression in tumor cells, S100A4 is expressed in normal cells and tissues, including fibroblasts and cells of the immune system. To examine the contribution of S100A4 to normal physiology, we established S100A4-deficient mice by gene targeting. Homozygous S100A4^−/− mice are fertile, grow normally and exhibit no overt abnormalities; however, the loss of S100A4 results in impaired recruitment of macrophages to sites of inflammation in vivo. Consistent with these observations, primary bone marrow macrophages (BMMs) derived from S100A4^−/− mice display defects in chemotactic motility in vitro. S100A4^−/− BMMs form unstable protrusions, overassemble myosin-IIA, and exhibit altered colony-stimulating factor-1 receptor signaling. These studies establish S100A4 as a regulator of physiological macrophage motility and demonstrate that S100A4 mediates macrophage recruitment and chemotaxis in vivo.

T cell-mediated biliary epithelial-to-mesenchymal transition in liver allograft rejection

Loss of bile duct epithelium is characteristic of early chronic rejection following liver transplantation. Recent studies have suggested that intrahepatic biliary epithelial cells can transform into myofibroblasts. This study examines the induction and molecular regulation of this transition during allograft rejection. Immortalized human cholangiocytes were stimulated with either transforming growth factor beta1 (TGFbeta1) or a T cell line, and they were examined for morphological, proteomic, and functional features. Posttransplant liver biopsy sections were also examined. Treatment of cholangiocytes with TGFbeta1 or TGFbeta-presenting T cells induced a bipolar morphology, reduced expression of E-cadherin and zona occludens 1 (ZO-1), and increased vimentin, fibronectin, matrix metalloproteinase 2 (MMP-2), MMP-9, and S100 calcium binding protein A4 (S100A4); treated cells invaded a model basement membrane. Chemokines induced T cell penetration of 3-dimensional, cultured bile duct-like structures and bile ducts in liver biopsy sections. A spatial association was observed between duct-infiltrating T cells and cholangiocyte expression of mesenchymal markers, including S100A4. Inhibition of S100A4 expression in vitro blocked TGFbeta1-mediated loss of E-cadherin and ZO-1 but did not reduce induction of fibronectin, MMP-2, or MMP-9. This study demonstrates the potential for T cells to induce an intrahepatic biliary epithelial-to-mesenchymal cell transition during chronic rejection. Furthermore, S100A4 expression by cholangiocytes was identified as a crucial regulator of this transition.

Phenothiazines inhibit S100A4 function by inducing protein oligomerization

S100A4, a member of the S100 family of Ca(2+)-binding proteins, regulates carcinoma cell motility via interactions with myosin-IIA. Numerous studies indicate that S100A4 is not simply a marker for metastatic disease, but rather has a direct role in metastatic progression. These observations suggest that S100A4 is an excellent target for therapeutic intervention. Using a unique biosensor-based assay, trifluoperazine (TFP) was identified as an inhibitor that disrupts the S100A4/myosin-IIA interaction. To examine the interaction of S100A4 with TFP, we determined the 2.3 A crystal structure of human Ca(2+)-S100A4 bound to TFP. Two TFP molecules bind within the hydrophobic target binding pocket of Ca(2+)-S100A4 with no significant conformational changes observed in the protein upon complex formation. NMR chemical shift perturbations are consistent with the crystal structure and demonstrate that TFP binds to the target binding cleft of S100A4 in solution. Remarkably, TFP binding results in the assembly of five Ca(2+)-S100A4/TFP dimers into a tightly packed pentameric ring. Within each pentamer most of the contacts between S100A4 dimers occurs through the TFP moieties. The Ca(2+)-S100A4/prochlorperazine (PCP) complex exhibits a similar pentameric assembly. Equilibrium sedimentation and cross-linking studies demonstrate the cooperative formation of a similarly sized S100A4/TFP oligomer in solution. Assays examining the ability of TFP to block S100A4-mediated disassembly of myosin-IIA filaments demonstrate that significant inhibition of S100A4 function occurs only at TFP concentrations that promote S100A4 oligomerization. Together these studies support a unique mode of inhibition in which phenothiazines disrupt the S100A4/myosin-IIA interaction by sequestering S100A4 via small molecule-induced oligomerization.

Self-association of calcium-binding protein S100A4 and metastasis

Elevated levels of the calcium-binding protein S100A4 promote metastasis and in carcinoma cells are associated with reduced survival of cancer patients. S100A4 interacts with target proteins that affect a number of activities associated with metastatic cells. However, it is not known how many of these interactions are required for S100A4-promoted metastasis, thus hampering the design of specific inhibitors of S100A4-induced metastasis. Intracellular S100A4 exists as a homodimer through previously identified, well conserved, predominantly hydrophobic key contacts between the subunits. Here it is shown that mutating just one key residue, phenylalanine 72, to alanine is sufficient to reduce the metastasis-promoting activity of S100A4 to 50% that of the wild type protein, and just 2 or 3 specific mutations reduces the metastasis-promoting activity of S100A4 to less than 20% that of the wild type protein. These mutations inhibit the self-association of S100A4 in vivo and reduce markedly the affinity of S100A4 for at least two of its protein targets, a recombinant fragment of non-muscle myosin heavy chain isoform A, and p53. Inhibition of the self-association of S100 proteins might be a novel means of inhibiting their metastasis-promoting activities.

S100A4 mediates endometrial cancer invasion and is a target of TGF-beta1 signaling

The molecular mechanisms of endometrial cancer invasion are poorly understood. S100A4, also known as FSP1 (fibroblast-specific protein 1), has long been known to be a molecular marker of fibrosis in a variety of different fibrotic diseases of the lungs, liver, kidney, and heart. We demonstrate here that increased expression of S100A4 is associated with advanced stage endometrial cancer and decreased recurrence free survival. To verify the essential role of S100A4 in invasiveness of endometrial cancer, S100A4 expression was downregulated by RNAi in HEC-1A cells, which resulted in undetectable S100A4 protein and significantly decreased migration and invasion. Owing to the established connection between TGF-beta1 and S100A4 induction in experimental models of kidney and liver fibrosis, we next examined whether TGF-beta1 could also regulate S100A4 in endometrial cancer cells. TGF-beta1 stimulated endometrial cancer cell migration and invasion with a concomitant increase in S100A4 protein. Induction of S100A4 was associated with the activation of Smads. TGF-beta1-mediated endometrial cancer cell motility was inhibited by S100A4 siRNA. In aggregate, these results suggest that S100A4 is a critical mediator of invasion in endometrial cancer and is upregulated by the TGF-beta1 signaling pathway. These results also suggest that endometrial cancer cell invasion and fibrosis share common molecular mechanisms.

S100A11: diverse function and pathology corresponding to different target proteins

S100A11, as a member of S100 protein family, while featuring the common identities as the other EF-hand Ca(2+)-binding family members, has its own individual characteristics. S100A11 is widely expressed in multiple tissues, and is located in cytoplasm, nucleus, and even cell periphery. S100A11 exists as a non-covalent homodimer with an antiparallel conformation. Ca(2+) binding to S100A11 would trigger conformational changes which would expose the hydrophobic cleft of S100A11 and facilitate its interaction with target proteins. Since S100A11 appears to lack enzymatic activity, in this article, corresponding to a variety of its target proteins, we systematically describe the biological roles of S100A11 and its possible mechanism in the processes of inflammation, regulation of enzyme activity, and cell growth regulation. As a dual cell growth mediator, S100A11 acts as either a tumor suppressor or promoter in many different types of tumors and would play respective roles in influencing the proliferation of the cancer cells. We intend to illustrate the biological function of the S100 protein, and shed light on the further research, which will provide us with a better understanding of it.

Binding of S100 proteins to RAGE: an update

The Receptor for Advanced Glycation Endproducts (RAGE) is a multi-ligand receptor of the immunoglobulin family. RAGE interacts with structurally different ligands probably through the oligomerization of the receptor on the cell surface. However, the exact mechanism is unknown. Among RAGE ligands are members of the S100 protein family. S100 proteins are small calcium binding proteins with high structural homology. Several members of the family have been shown to interact with RAGE in vitro or in cell-based assays. Interestingly, many RAGE ligands appear to interact with distinct domains of the extracellular portion of RAGE and to trigger various cellular effects. In this review, we summarize the modes of S100 protein-RAGE interaction with regard to their cellular functions.

The metastasis associated protein S100A4: a potential novel link to inflammation and consequent aggressive behaviour of rheumatoid arthritis synovial fibroblasts

The metastasis-associated protein S100A4 belongs to the large family of S100 calcium-binding proteins that appear to play regulatory roles in diverse biological activities. Moreover, a prognostic role of S100A4 has been suggested for patients with several types of cancer. Cancer promoting properties for S100A4 have been demonstrated, particularly through its regulation of cell motility, proliferation and apoptosis, as well as by stimulation of angiogenesis and remodelling of the extracellular matrix. Increased expression of S100A4 mRNA has been detected in proliferating synovial fibroblasts in rheumatoid arthritis. Furthermore, strong upregulation of the S100A4 protein in rheumatoid arthritis synovial tissue compared with osteoarthritis and control tissues has been demonstrated recently, especially at sites of joint invasion. Several immune and vascular cells were also identified to be producing S100A4 within the synovium. The local upregulation of S100A4 was accompanied by high plasma and synovial fluid concentrations of the S100A4 protein existing in the bioactive oligomeric form in patients with rheumatoid arthritis. Consistent with data from cancer studies, the extracellular S100A4 oligomer appears to be involved in regulation of several matrix-degrading enzymes and modulation of the transcriptional activation function of the tumour suppressor protein p53 in rheumatoid arthritis synovial fibroblasts. Taken together, one can speculate that increased S100A4 protein in circulation and locally at sites of inflammation, particularly at sites of joint destruction, might be linked to the process of aggressive fibroblast behaviour contributing to the pathogenesis of chronic autoinflammatory diseases such as rheumatoid arthritis.

The basic C-terminal amino acids of calcium-binding protein S100A4 promote metastasis

The calcium-binding protein S100A4 can induce a metastatic phenotype in animal model systems and its expression in various human cancers has been shown to be associated with metastasis and reduced patient survival. Using a series of nested deletion mutants, it is now shown that the two C-terminal lysine residues are required for the enhanced metastasis, invasion and migration abilities that S100A4 confers on cells in a model system of metastasis. Basic C-terminal residues enhance the affinity between S100A4 and its best characterized target, a recombinant C-terminal fragment of non-muscle myosin II heavy chain isoform A (NMMHC-IIA). In wild-type S100A4 protein, the presence of the C-terminal lysine, residue 101, enhances the rate of association between S100A4 and NMMHC-IIA. These results identify the amino acids of S100A4 that are involved in metastasis induction and show that the C-terminal region of S100A4 is a possible target for inhibitors of its metastatic action.

S100A4 overexpression proves to be independent marker for breast cancer progression

BACKGROUND

Breast cancer is the most common cancer and cause of deaths in women around the world. Oncogene amplification usually occurs late in tumor progression and correlates well with aggressiveness of tumor. In fact the function of the S100A4 protein and its role in metastasis is unclear at present. The purpose of the study was to determine the expression of S100A4 protein in the invasion status and metastatic potential of breast cancer by using tissue microarray and to determine its role in breast cancer based on the expression of S100A4 gene product.

METHODS

S100A4 protein expression was examined by immunohistochemistry (IHC) using commercially available tissue microarray containing malignant and normal breast tissue cores from 216 patients.

RESULTS

S100A4 was absent in normal breast tissues while positive in 45.1% of infiltrating ductal carcinoma (IDC) node negative and 48.8% of infiltrating lobular carcinoma node negative. In paired samples, S100A4 protein was expressed in 13.5% of IDC node positive cases and 35.1% of matched lymph node metastasis.

CONCLUSION

S100A4 protein expression appears widely expressed in early and advanced breast cancer stages compared with normal breast. Our study suggests S100A4 may play a role in breast cancer progression and may prove to be an independent marker of breast cancer which appears to be down regulated in more advanced stages of breast cancer.

Tumor expression of S100A6 correlates with survival of patients with stage I non-small-cell lung cancer

BACKGROUND

In a previously published in vitro study based on top-down proteomics we found that the calcium-binding proteins S100A6 and S100A4 were affected by exposure to ionizing radiation in a p53-dependent fashion. Both proteins showed post-translational modification changes, and S100A6 also showed increased expression and translocation in response to irradiation. Aim of the present study was to evaluate the expression of S100A6 and S100A4 in non-small-cell lung cancer (NSCLC).

METHODS

S100A6 expression on archival tumor cell lysates from 39 patients with radically resected NSCLC was assessed with SELDI-TOF-MS. S100A6 identity was confirmed using a SELDI-based antibody-capture method on lysates from the A549 lung cancer cell line, cell lysates from two freshly prepared NSCLC samples, four plasma samples and one pleural effusion sample. Immunostainings for S100A6, S100A4 and p53 were performed on tissue microarrays containing 103 stage I surgically resected NSCLC cases and 14 normal lung parenchyma specimens.

RESULTS

The presence of post-translationally modified S100A6 forms was confirmed with SELDI-MS on enriched tumor cell lysates, as well as in plasma and pleural effusion samples. In addition, high S100A6 peak intensity was associated with longer median survival (35 months vs. 18 months for high and low peak intensity, respectively; p=n.s.). The immunohistochemical analysis showed that 25% of tumors were S100A6 positive. S100A6 expression correlated directly with non-squamous histology (p<0.0001) and S100A4 expression (p=0.005), and inversely with p53 expression (p=0.01). S100A6-positive cases showed a trend of longer survival compared with S100A6-negative cases (p=0.07). This difference became significant when the analysis was restricted to p53-negative cases (n=72). In this subgroup of patients, whose tumors likely exhibit a functional p53, S100A6 was an independent prognostic factor of improved survival at multivariate analysis (HR 0.49, 95% CI 0.27-0.81, p=0.017).

CONCLUSIONS

In this study we have validated on clinical material our previous findings on cell lines in terms of S100A6 expression and post-translational modifications pattern in NSCLC. Moreover, the survival results obtained in p53-negative stage I NSCLC cases support the proposed pro-apoptotic function of S100A6 and suggest the hypothesis of a cross regulation between these two proteins.

The actin cytoskeleton in cancer cell motility

Cancer cell metastasis is a multi-stage process involving invasion into surrounding tissue, intravasation, transit in the blood or lymph, extravasation, and growth at a new site. Many of these steps require cell motility, which is driven by cycles of actin polymerization, cell adhesion and acto-myosin contraction. These processes have been studied in cancer cells in vitro for many years, often with seemingly contradictory results. The challenge now is to understand how the multitude of in vitro observations relates to the movement of cancer cells in living tumour tissue. In this review we will concentrate on actin protrusion and acto-myosin contraction. We will begin by presenting some general principles summarizing the widely-accepted mechanisms for the co-ordinated regulation of actin polymerization and contraction. We will then discuss more recent studies that investigate how experimental manipulation of actin dynamics affects cancer cell invasion in complex environments and in vivo.

Crystal structure of metastasis-associated protein S100A4 in the active calcium-bound form

S100A4 (metastasin) is a member of the S100 family of calcium-binding proteins that is directly involved in tumorigenesis. Until recently, the only structural information available was the solution NMR structure of the inactive calcium-free form of the protein. Here we report the crystal structure of human S100A4 in the active calcium-bound state at 2.03 A resolution that was solved by molecular replacement in the space group P6(5) with two molecules in the asymmetric unit from perfectly merohedrally twinned crystals. The Ca(2+)-bound S100A4 structure reveals a large conformational change in the three-dimensional structure of the dimeric S100A4 protein upon calcium binding. This calcium-dependent conformational change opens up a hydrophobic binding pocket that is capable of binding to target proteins such as annexin A2, the tumor-suppressor protein p53 and myosin IIA. The structure of the active form of S100A4 provides insight into its interactions with its binding partners and a better understanding of its role in metastasis.

Structure of Ca2+-bound S100A4 and its interaction with peptides derived from nonmuscle myosin-IIA

S100A4, also known as mts1, is a member of the S100 family of Ca2+-binding proteins that is directly involved in tumor invasion and metastasis via interactions with specific protein targets, including nonmuscle myosin-IIA (MIIA). Human S100A4 binds two Ca2+ ions with the typical EF-hand exhibiting an affinity that is nearly 1 order of magnitude tighter than that of the pseudo-EF-hand. To examine how Ca2+ modifies the overall organization and structure of the protein, we determined the 1.7 A crystal structure of the human Ca2+-S100A4. Ca2+ binding induces a large reorientation of helix 3 in the typical EF-hand. This reorganization exposes a hydrophobic cleft that is comprised of residues from the hinge region,helix 3, and helix 4, which afford specific target recognition and binding. The Ca2+-dependent conformational change is required for S100A4 to bind peptide sequences derived from the C-terminal portion of the MIIA rod with submicromolar affinity. In addition, the level of binding of Ca2+ to both EF-hands increases by 1 order of magnitude in the presence of MIIA. NMR spectroscopy studies demonstrate that following titration with a MIIA peptide, the largest chemical shift perturbations and exchange broadening effects occur for residues in the hydrophobic pocket of Ca2+-S100A4. Most of these residues are not exposed in apo-S100A4 and explain the Ca2+ dependence of formation of theS100A4-MIIA complex. These studies provide the foundation for understanding S100A4 target recognition and may support the development of reagents that interfere with S100A4 function.

A review of the S100 proteins in cancer

AIM

In the quest to reduce mortality and morbidity from cancer, there is continued effort to identify novel biomarkers to aid in the early detection and the accurate prediction of tumour behaviour. One group of proteins that is emerging as a potentially important group of markers in multiple tumour types is the S100 family. This review summarises the biological and clinical relevance of these proteins in relation to different tumour types.

METHODS

A literature search was performed using the PubMed database and the reference lists of relevant articles. Single case studies were excluded and only reports with a clinical relevance from 1961 to 2007 were included.

RESULTS

The search yielded over 1000 published articles and reports. Important reports and studies were reviewed, screened and tracked for further relevant publications. Only the most relevant publications are discussed with relation to individual members of the S100 family.

CONCLUSION

There is increasing evidence that altered expression of S100 family members is seen in many cancers including breast, lung, bladder, kidney, thyroid, gastric, prostate and oral cancers. S100 proteins are commonly up-regulated in tumours and this is often associated with tumour progression. In contrast S100A2, S100A11 and S100A9 have been documented as tumour suppressors in some cancers but as tumour promoters in others. This demonstrates the complexity of the family and variability of their functions. Although the precise roles of these proteins in cancer is still to be discovered many of the family are associated with promoting metastases through interactions with matrix metalloproteinases or by acting as chemoattractants. There is also evidence that some members can regulate transcription factors such as p53. S100B already has a role in a clinical setting in the diagnosis and therapeutic monitoring of malignant melanoma. As our understanding of this family develops it is likely that many more members will aid the diagnosis, monitoring and potential treatment of cancers in the future.

Intimal smooth muscle cells of porcine and human coronary artery express S100A4, a marker of the rhomboid phenotype in vitro

We reported that smooth muscle cell (SMC) populations isolated from normal porcine coronary artery media exhibit distinct phenotypes: spindle-shaped (S) and rhomboid (R). R-SMCs are recovered in higher proportion from stent-induced intimal thickening compared with media suggesting that they participate in intimal thickening formation. Our aim was to identify a marker of R-SMCs in vitro and to explore its possible expression in vivo. S- and R-SMC protein extracts were compared by means of 2-dimensional polyacrylamide gel electrophoresis followed by tandem mass spectrometry. S100A4 was found to be predominantly expressed in R-SMC extracts. Using a monoclonal S100A4 antibody we confirmed that S100A4 is highly expressed by R-SMCs and hardly detectable in S-SMCs. S100A4 was colocalized with alpha-smooth muscle actin in stress fibers of several quiescent cells and upregulated during migration. PDGF-BB, FGF-2 or coculture with endothelial cells, which modulate S-SMCs to a R-phenotype, increased S100A4 expression in both S- and R-SMCs. Silencing of S100A4 mRNA in R-SMCs decreased cell proliferation, suggesting a functional role for this protein. In vivo S100A4 was absent in normal porcine coronary artery media, but highly expressed by SMCs of stent-induced intimal thickening. In humans, S100A4 was barely detectable in coronary artery media and markedly expressed in SMCs of atheromatous and restenotic coronary artery lesions. Our results indicate that S100A4 is a marker of porcine R-SMCs in vitro and of intimal SMCs during intimal thickening development. It is also a marker of a large population of human atheromatous and restenotic SMCs. Clarifying S100A4 function might be useful to understand the evolution of atherosclerotic and restenotic processes.

Identification of metastasis candidate proteins among HCC cell lines by comparative proteome and biological function analysis of S100A4 in metastasis in vitro

Widespread metastasis of hepatocellular carcinoma (HCC) was a complex cascade of events, which is still beyond full appreciation. Screening key proteins, which play a critical role in metastasis, using high-throughput proteomics approach help discover valuable biomarkers and elucidate the mechanism of metastasis. This study was to find out some metastasis candidate proteins among HCC cell lines with various metastatic potential by comparative proteomics, and then further validate the biological function of these proteins in metastasis in vitro. The protein profiles of metastatic HCC cell lines (MHCC97H and MHCC97L) displayed obvious differences compared with nonmetastatic ones (Hep3B). Twenty-six metastasis candidate proteins, which were identified by on-line LC-ESI-MS/MS, such as S100 calcium-binding protein A4 (S100A4), annexin 1, etc., might have much application in diagnostic procedures and prognosis evaluation. S100A4, as a leading different metastasis candidate protein, which overexpressed only in the metastatic cells, was selected for further investigation. A series of assays related to invasion and metastasis in vitro, including cell motility, invasion, and matrix metalloproteinases (MMPs) secretion, were performed in MHCC97H/antisense recombinant plasmid to S100A4 (pcDNA3.1(+) AS S100A4) and the mock controls. All the data in the present study suggested that S100A4 might contribute to HCC invasion and metastasis through two paths of matrix metalloproteinase (MMP9) secretion regulation and strengthened motility and invasion properties.