S100A8/A9: A Janus-faced molecule in cancer therapy and tumorgenesis

European Journal of Pharmacology 625 (2009) 73–83

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European Journal of Pharmacology

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Review

S100A8/A9: A Janus-faced molecule in cancer therapy and tumorgenesis

Saeid Ghavami a,b,c, Seth Chitayat d, Mohammad Hashemi e, Mehdi Eshraghi f,Walter J. Chazin d, Andrew J. Halayko a,b,c, Claus Kerkhoff g,⁎a Department of Physiology, University of Manitoba, Winnipeg, Canadab MICH, University of Manitoba,Winnipeg, Canadac National Training Program of Asthma and Allergy, University of Manitoba, Winnipeg, Canadad Departments of Biochemistry and Chemistry, Center for Structural Biology, Vanderbilt University, Nashville, TN 37232-8725, USAe Department of Clinical Biochemistry, School of Medicine, Zahedan University of Medical Sciences, Zahedan, Iranf MICB, CancerCare Manitoba, Department of Biochemistry and Medical Genetics, University of Manitoba,Winnipeg, Canadag Institute of Immunology, University of Muenster, Germany

⁎ Corresponding author. Institute of Immunology, UnE-mail address: [email protected] (C. Kerkh

0014-2999/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.ejphar.2009.08.044

a b s t r a c t
a r t i c l e i n f o
Article history:Received 24 April 2009Received in revised form 31 July 2009Accepted 19 August 2009Available online 14 October 2009

Keywords:Apoptotic agentChemoattractanceGrowth arrestImmune escapeMetastasisReactive oxygen speciesStress responseTumorgenesis

Correlations exist between the abundance of S100 proteins and disease pathologies. Indeed, this is evidencedby the heterodimeric S100 protein complex S100A8/A9 which has been shown to be involved ininflammatory and neoplastic disorders. However, S100A8/A9 appears as a Janus-faced molecule in thiscontext. On the one hand, it is a powerful apoptotic agent produced by immune cells, making it a veryfascinating tool in the battle against cancer. It spears the risk to induce auto-immune response and may serveas a lead compound for cancer-selective therapeutics. In contrast, S100A8/A9 expression in cancer cells hasalso been associated with tumor development, cancer invasion or metastasis. Clearly, there is a dichotomyand future investigations into the role of S100A8/A9 in cancer biology need to consider both sides of thesame coin.

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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 742. S100A8 and S100A9 are members of the S100 protein family . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

2.1. S100 gene and protein structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 742.2. S100A8 and S100A9 are abundant in myeloid cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 742.3. Localization and trafficking of S100A8/A9 in myeloid cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 752.4. Intracellular activities of S100A8/A9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 752.5. Extracellular activities of S100A8/A9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

3. Extracellular S100A8/A9 induces apoptosis in target cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763.1. S100A8/A9 induces apoptosis not simply by zinc sequestration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763.2. Molecular mechanisms of S100A8/A9-induced apoptosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 773.3. S100A8/A9 as apoptosis-inducing factor that might be useful in the immune therapy of malignant cells . . . . . . . . . . . . . . . . 77

4. S100 expression in epithelial cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 774.1. Stress response-induced S100 expression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 774.2. S100A8/A9 Mediates Growth Arrest in Epithelial Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 774.3. S100 Gene expression in cancer cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 784.4. S100A8/A9 induces cancer cell proliferation via RAGE ligation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794.5. S100A8/A9 serves as chemotactic factor for tumor cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

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74 S. Ghavami et al. / European Journal of Pharmacology 625 (2009) 73–83

5. Conclusions and future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

1. Introduction

S100 proteins are a group of multigenic, non-ubiquitous cytoplas-mic Ca2+-binding proteins that are differentially expressed in a widevariety of cell types. S100A8 and S100A9were originally discovered asimmunogenic proteins expressed and specifically released fromphagocytes. High S100A8/A9 plasma levels are measured in a numberof inflammatory disorders, such as chronic bronchitis, cystic fibrosis,and rheumatoid arthritis (Nacken et al., 2003). Interestingly, there arehigh correlations between S100A8/A9 plasma concentrations andclinical and laboratory markers of inflammation, as well as the rapidnormalization following clinical improvement, suggesting that theseproteins track disease activity.

Although a number of distinct functions have been attributed tothe S100 proteins, their biological functions still remain unclear. Intra-as well as extracellular roles have been proposed (Table 1). Forexample, intracellular S100A8/A9 promotes NADPH oxidase and NF-κB activation. The secreted form has chemotactic and chemorepulsiveproperties; exerts apoptosis-inducing activity on cancer cells and,remarkably, cancer cells utilize these S100 proteins as guidance forthe adhesion and invasion of disseminating malignant cells. In theextracellular milieu, S100A8/A9 can also bind cell surface receptor(s)such as the receptor for adavanced glycation end products (RAGE) andinduces the expression of cytokine(s).

The promiscuity of S100A8/A9 has direct implications on a widerange of potential effects on human health and is the basis of thisreview. Clearly, S100A8/A9 has gained interest in many fields ofmedicine due to its deregulated epidermal expression as a response tostress and in association with neoplastic disorders. Although severalexcellent general articles on S100 proteins have been publishedrecently (Donato, 2007; Heizmann et al., 2007; Schaub and Heizmann,2008), in this review we will only include details on the cell growth-modulating and apoptosis-inducing activity of S100A8/A9.

2. S100A8 and S100A9 are members of the S100 protein family

The members of the S100 protein family compose a multigenicfamily of non-ubiquitous cytoplasmic Ca2+-binding proteins of EF-

Table 1Intra- and extracellular roles of S100A8 and S100A9.

Proposed function References

Antimicrobial properties Steinbakk et al. (1990), Sohnle et al. (1991),Loomans et al. (1998), Corbin et al. (2008)

Calcium binding Teigelkamp et al. (1991), Sturchler et al. (2006),Korndörfer et al. (2007)

Chemotactic activity Ryan and Geczy (1986), Lackmann et al. (1992),Ryckman et al. (2003), Vandal et al. (2003)

Fugetactic activity Sroussi et al. (2007)Inhibition of caseinkinases I and II

Murao et al. (1990)

Interaction with cytoskeleton Dianoux et al. (1992), Guignard et al. (1995),Vogl et al. (2004)

Growth inhibitory activities Yui et al. (1993), Yui et al. (2003),Ghavami et al. (2004)

Binding of polyunsaturatedfatty acids

Kerkhoff et al. (1999, 2001)

Enhancement of NADPHoxidases

Kerkhoff et al. (2005), Benedyk et al. (2007)

Growth-promoting activity Ghavami et al. (2008b), Gebhardt et al. (2008)Metastasis development Hiratsuka et al. (2006, 2008),

Rafii and Lyden (2006), Yong and Moon (2007)

hand type. They are small acidic proteins (10–12 kDa) that are foundexclusively in vertebrates (Schafer and Heizmann, 1996) and aredifferentially expressed in a wide variety of cell types. They have beenimplicated in the regulation of many diverse processes such as signaltransduction, cell growth and motility, cell-cycle regulation, tran-scription, differentiation and cell survival (Donato, 2001; Heizmann,2002; Donato, 2003; Salama et al., 2008).

Most S100 genes are located in a gene cluster near a region onhuman chromosome 1q21, which is responsible for a number ofchromosomal abnormalities and has been frequently rearranged inhuman cancer (Heizmann et al., 2002; Emberley et al., 2004; Carlssonet al., 2005; Cross et al., 2005; Luthra et al., 2007; Liu et al., 2008). Inaddition, the S100 gene cluster is close to the epidermal differenti-ation complex (Mischke et al., 1996) as well as to a psoriasis suscep-tibility region, the PSORS4 locus (Hardas et al., 1996; Semprini et al.,2002). These data are important indications for the involvement ofS100 genes in inflammatory as well as neoplastic disorders. Theserearrangements may result in a deregulated expression of S100 genesassociated with neoplasias.

2.1. S100 gene and protein structure

The S100 genes share significant structural similarities at bothgenomic and protein levels. Most S100 genes consist of three exonsthat are separated by two introns. S100 proteins are encoded bysequences in exon 2 and exon3, encoding an N-terminal and a C-terminal EF-handmotif, respectively. In each gene, exon 1 encodes theuntranslated region.

S100 proteins comprise two helix-loop–helix EF-hand motifs thattogether form a stable four-helix domain with a distinct hydrophobiccore. The C-terminal EF-hand contains a canonical Ca2+-binding loopof 12 amino acids. Conversely, the N-terminal EF-hand contains aCa2+-binding loop of 14 residues that binds Ca2+ mostly through themain-chain carbonyl groups that is specific to S100 proteins. In isola-tion, S100 proteins have a weaker Ca2+ affinity than the typical Ca2+

sensors such as calmodulin, with KD values of approximately 200–500 µM (Donato, 1986; Sturchler et al., 2006; Korndörfer et al., 2007).

Overall, the S100 proteins share significant sequential homology inthe EF-hand motifs, but are least conserved in the hinge region. Thisregion is proposed to provide for specific interaction with target pro-teins (Gimbrone et al., 1990; Groves et al., 1998; Zimmer et al., 2003;Santamaria-Kisiel et al., 2006; Rintala-Dempsey et al., 2006; Fernan-dez-Fernandez et al., 2008; van Dieck et al., 2009). The availability ofhigh-resolution S100-target structures has highlighted importantstructural features that contribute to S100 protein functional specific-ity (Rety et al., 1999; Rustandi et al., 2000; Bhattacharya et al., 2003).

The functional diversification of S100 proteins is achieved by theirspecific cell- and tissue-expression patterns, structural variations,different metal ion binding properties (Ca2+, Zn2+ and Cu2+) aswell as their ability to form homo-, hetero- and oligomeric assemblies(Hunter and Chazin, 1998; Osterloh et al., 1998; Pröpper et al., 1999;Tarabykina et al., 2001; Moroz et al., 2003). Although the function ofS100 proteins in cancer cells in most cases is still unknown, thespecific expression patterns of these proteins are a valuable diagnostictool.

2.2. S100A8 and S100A9 are abundant in myeloid cells

S100A8 and S100A9 are predominantly expressed in myeloid cells(Kerkhoff et al., 1998; Nacken et al., 2003), probably driven by a

75S. Ghavami et al. / European Journal of Pharmacology 625 (2009) 73–83

recently-characterized regulatory element (Kerkhoff et al., 2002).Except for inflammatory conditions, the expressions of S100A8 andS100A9 are restricted to a specific stage of myeloid differentiation,since both proteins are expressed in circulating neutrophils andmonocytes but are absent in normal tissue macrophages and lym-phocytes (Odink et al., 1987; Lagasse and Clerc, 1988; Zwadlo et al.,1988; Hogg et al., 1989). Under chronic inflammatory conditions, suchas psoriasis and malignant disorders, they are also expressed in theepidermis (Brandtzaeg et al., 1987; Wilkinson et al., 1988; Madsenet al., 1992).

Previous studies have reported that S100A8 and S100A9 arefrequently co-expressed and their expression appears to be coordi-nately regulated (Teigelkamp et al., 1991;Marionnet et al., 2003). Viceversa, it has been demonstrated that either gene knock-out (Manitzet al., 2003; Hobbs et al., 2003) or gene silencing (Kerkhoff et al.,2005) of one S100 gene caused the absence of the other at proteinlevel. Therefore, it is nowadays widely accepted that the heterodimeris the most functionally relevant form to study.

S100A8 and S100A9 show a strong tendency to form heteromericprotein complexes. This preference for the formation of the hetero-dimeric protein complexes over S100A8 and S100A9 homodimers inboth the apo- and holo-states has been demonstrated by a batteryof biochemical, biophysical and structural methods (Hunter andChazin, 1998; Vogl et al., 2006). Complex formation is not calcium-dependent as shown in various studies (Kerkhoff et al., 1999; Voglet al., 1999). However, binding of calcium to the S100A8/A9heterodimer results in structural changes of the hinge region, helix3 and second calcium-binding loop regions. These conformationalchanges lead to the exposure of hydrophobic surfaces upon calciumbinding. The involvement of critical residues within the C-terminaldomain to the heterodimer formation of S100A8 and S100A9 has beenconfirmed using the yeast two-hybrid system and site-directedmutagenesis (Pröpper et al., 1999). In the presence of calcium,tetrameric complexes as well as higher order species are formed inaddition to the heterodimeric complex (Teigelkamp et al., 1991;Kerkhoff et al., 1999; Vogl et al., 1999).

2.3. Localization and trafficking of S100A8/A9 in myeloid cells

The S100A8/A9 complex is located in the cytosol of restingphagocytes and exhibits two independent translocation pathwayswhen the cells are activated.

Upon elevation of the intracellular calcium level they are trans-located from the cytosol to cytoskeleton and to plasma membrane(Roth et al., 1993). At a later time point, they appear as non covalentlyassociated S100A8/A9 heterodimers on the surface of monocytes(Bhardwaj et al., 1992). The mechanism by which the S100A8/A9heterodimer penetrates the plasma membrane, and how the S100A8/A9 protein complex is anchored into the cell membrane, remainsunclear since both proteins lack a transmembrane signaling region.

Upon activation of protein kinase C S100A8/A9 heterodimers arereleased from human monocytes by a novel secretion pathway whichis energy-consuming and depends on an intact microtubule network(Murao et al., 1990; Rammes et al., 1997).

Therefore, it has been assumed that membrane-associated andsoluble S100A8/A9 may have distinct cellular functions. In view of thenumber of tertiary structures formed by S100A8 and S100A9 (Nackenet al., 2003) and with respect to the data above it is tempting tospeculate that this variability may account for the promiscuity of thetwo S100 proteins.

2.4. Intracellular activities of S100A8/A9

In the intracellular milieu, S100 proteins are considered as calciumsensors changing their conformation in response to calcium influxand then mediating calcium signals by binding to other intracellular

proteins. Recent data indicate that intracellular S100A8/A9 interactswith subunits of the NADPH oxidase complex, thereby promotingthe NADPH oxidase activation. The mode of action is suggested tocomprise on the transfer of arachidonic acid to membrane-boundgp91phox during interactions with two cytosolic oxidase activationfactors, p67phox and Rac-2 (Kerkhoff et al., 2005). Arachidonic acid isan essential factor in the formation of active NADPH oxidase since itsbinding to gp91phox induces structural changes in cytochrome b558(Doussiere et al., 1999). These data are supported by the findings that1) S100A8/A9 binds polyunsaturated fatty acids, such as arachidonicacid, γ- and α-linolenic acid, in a calcium-dependent manner(Kerkhoff et al., 1999; Sopalla et al., 2002), while the individualS100 proteins do not bind fatty acids; 2) upon cellular activation,S100A8/A9 is associated with low density detergent-resistant mem-branes (Nacken et al., 2004), and NADPH oxidase complex assemblytakes place at cholesterol-enriched membrane microdomains (lipidrafts) (Vilhardt and van Deurs, 2004), and 3) the phagocyte NADPHoxidase activation is impaired in neutrophil-like NB4 cells, after spe-cifically blocking S100A9 expression, and employing bone marrow-derived PMNs from S100A9−/− mice (Kerkhoff et al., 2005).

In areas of acute inflammation polymorphonuclear leukocytes(PMN), expressing the membrane-associated heterodimer S100A8/A9, are the predominant cell type. These cells have been shown torelease high amounts of TNF-α and IL-1β, indicating that S100A8/A9surface expression is restricted to activated or recruited phagocytes(Lagasse and Clerc, 1988). These phagocytes perform several host-defense functions, such as phagocytosis of invading microorganismsand cell debris, release of proteolytic enzymes, and generation ofreactive oxygen metabolites. In addition, they release a number ofarachidonic acid-derived eicosanoids which amplify or perpetuate theacute inflammatory response. This subset of phagocytes is presentingin acute but absent in chronic inflammatory disorders (Bhardwajet al., 1992). These findings have led to the assumption that S100A8and S100A9 affect leukocyte trafficking and display a propagating rolein inflammatory responses. Recently, in migrating monocytes theS100A8/A9 complex has been found to be associatedwith cytoskeletaltubulin and to modulate transendothelial migration (Vogl et al.,2004). The putative pro-inflammatory functions of S100A8 andS100A9 have recently been investigated in two different mouseknock-out models. S100A9 deficiency did not result in an obviousphenotype (Manitz et al., 2003; Hobbs et al., 2003). However, reducedmigration of S100A9-deficient neutrophils and decreased surfaceexpression of CD11b, which belongs to the integrin family, wereobserved upon in vitro stimulation. In addition, chemokine-induceddown regulation of the cytosolic Ca2+-level was detected. Obviously,these in vitro effects are compensated by alternative pathways in vivo.

2.5. Extracellular activities of S100A8/A9

Once S100A8/A9 is released from activated phagocytes into theextracellular space, it is an effective antimicrobial agent by deprivingbacterial pathogens of essential trace metals such as Zn2+ and Mn2+

(Steinbakk et al., 1990; Sohnle et al., 1991;Miyasaki et al., 1993;Murthyet al., 1993; Clohessy andGolden, 1995; Sohnle et al., 2000; Corbin et al.,2008). In fact, S100A8/A9 provides the major activity associated withgrowth inhibition of pathogenic microorganisms in neutrophil cyto-plasm and abscess fluids (Loomans et al., 1998). In the context ofinflammation, it has been proposed that S100A8/A9 is massivelyreleased when neutrophils die to provide a growth-inhibitory type ofhost defense that is adjunctive to the usual microbicidal functionsby binding metals other than Ca2+ (Corbin et al., 2008).

S100A8/A9 also exhibits cytokine-like functions, including activa-tion of the receptor for advanced glycation endproducts (RAGE)(Hofmann et al., 1999; Herold et al., 2007), enhancing leukocyterecruitment to inflammatory sites (Ryckman et al., 2003; Vandal et al.,2003), and arachidonic acid transport to target cells (Kerkhoff et al.,

Fig. 1. S100 cell death pathway. S100A8/A9 induces cell death in cancer cells viadifferent pathways: A) S100A8/A9 suppresses intracellular zinc levels, thereby inducingthe cleavage of pro-caspase 3. B) S100A8/A9 apoptosis binds to a yet unknown lowaffinity receptor, causes Bax and Bak dimerization and their translocation tomitochondria with subsequent mitochondrial damage in the absence of cytochrome crelease. S100A8/A9 also inhibits mitochondrial fission machinery which inducesselective release of Smac/Diablo and Omi/HtrA2. These eliminate the inhibitory effect ofthe inhibitor-of-apoptosis proteins (IAPs) thereby inducing the activity of caspases(Ghavami et al. 2008a).


2001). S100A8/A9 also binds and activates cells via heparin sulfateproteoglycans (Srikrishna et al., 2001), heparan sulphate glycosami-noglycans present on endothelial surfaces (Robinson et al., 2002), orCD36 (Kerkhoff et al., 2001).

As a result of the various pro-inflammatory properties of S100A8/A9, strategies targeting the protein complex by administration of S100antibodies represent a novel option for anti-inflammatory therapies.Another approach is the inhibition of the release of these cytokine-likemolecules at sites of inflammation. In fact, recent evidence has shownthat the use of S100A8/A9 asmarkers of synovial inflammation is evensuperior to C-reactive protein and erythrocyte sedimentation rate(Kane et al., 2003). For this reason, tests have been developed todetect S1008 and S100A9 in body fluids of patients with rheumatoidarthritis for the prejudice of active and non-active osteoarthritis fromrheumatoid arthritis. The prevalence of S100A8/A9 in the extracellu-lar milieu and its important role in the innate immune responseprovide strong motivation for continued biochemical, structural andfunctional studies to gain a greater understanding of its many physio-logical functions.

3. Extracellular S100A8/A9 induces apoptosis in target cells

The extracellular S100A8/A9 protein complex has been shown toexert apoptotic/cytotoxic effects against various tumor cells. S100A8/A9 exhibits growth-inhibitory activity against mouse embryonicfibroblasts and human dermal fibroblasts (Yui et al., 1997), andmany tumor cells with broad specificity such as L-929 mousefibrosarcoma (Yui et al., 1995) MH-134 mouse hepatoma, B16mouse melanoma, J774.1 mouse macrophage-like cells, Ros17/2.8,rat osteosarcoma, and MCF-7 human mammary adenocarcinoma (Yuiet al., 2003). In addition S100A8/A9 exerts apoptosis-inducing activityagainst HT29/219 and SW742 colon carcinoma cells (Ghavami et al.,2004), human gastric adenocarcinoma cancer cells (Zali et al., 2008),EL-4 lymphoma cells, mouse MM46 mammary carcinoma (Nakataniet al., 2005), murine C2C12myoblasts, mouse EL-4 lymphoma, humanMOLT-4 leukemia cells (Kerkhoff and Ghavami, 2005; Yui et al., 1995;2002), MCF-7 (human estrogen receptor-positive breast cancer),MDA–MB231 (human estrogen receptor-negative breast cancer),Jurkat (human T cell leukemia), BJAB (murine B cell leukemia), L929(murine fibrosarcoma), human embryo kidney (HEK)-293, SHEP, andKELLY (human neuroblastoma) (Ghavami et al., 2008a).

The effective concentration for the apoptosis-inducing activity ofS100A8/A9 was found to be in the range of 50–250 μg/ml. For example,rat S100A8/A9 partially inhibited [3H]-thymidine incorporation intomouse EL-4 lymphoma and human MOLT-4 leukemia cells after 24 h,and cell growth was almost completely blocked after 48 h at concen-trations of 100–200 μg/ml (Yui et al., 1995). Rat S100A8/A9 was lesseffective against normal fibroblasts, however, S100A8/A9 clearlyinduced apoptoticmorphology infibroblasts after prolonged cultivation(Yui et al., 1997). Treatmentof colon carcinomacell lines (HT29/219andSW742) with human S100A8/A9 resulted in significantly reduced cellviability at concentrations above 120 μg/ml (corresponding to 5 μMS100A8/A9) within 30 hrs (Ghavami et al., 2004).

3.1. S100A8/A9 induces apoptosis not simply by zinc sequestration

It has been reported that apoptosis-inducing activity of S100A8/A9is reversed in the presence of zinc (Yui et al., 1997, 2002). Theapoptotic morphology in mouse EL-4 lymphoma and human MOLT-4leukemia lines induced by rat S100A8/A9 was completely abrogatedin the presence of 10 µM zinc, whereas it was not affected by 5 µMcalcium ormagnesium (Yui et al., 1995). Similar results were obtainedfor MM46 cells (Yui et al., 1995). The apoptosis-inducing activity wasalso significantly abrogated by other divalent cations, such as Cu2+,Mn2+, and Fe2+, whereas the trivalent cations Al3+ and Fe3+ had noeffect (Yui et al., 1997). When EL-4 lymphoma cells were cultured

in divalent cation-depleted medium, the dose-response curve forS100A8/A9 was shifted to about a 10-fold lower concentration rangethan that in standard medium (Yui et al., 2002). Exclusion of zinc byS100A8/A9 from target cells has been suggested to be one mechanismby which this protein induces apoptosis. This statement is inaccordance with other reports showing that apoptosis was inducedby the chelation of intra and extracellular cellular zinc with either acell membrane-permeable zinc chelator TPEN (N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine) or the membrane impermeable zincchelator DTPA (diethylenetriaminepentacetic acid) (Kerkhoff andGhavami, 2005; Hashemi et al., 2007).

In accordance with this finding, apoptosis-inducing activity ofS100A8/A9 was retained if the physical contact between S100A8/A9and the cells was precluded by a dialysis membrane (Yui et al., 2002).This result has been confirmed by membrane impermeable zincchelator DTPA that also induces cell death similar to S100A8/A9(Hashemi et al., 2007). Extracellular chelation of zinc by S100A8/A9 orDTPA might decrease the intracellular pool of this ion resulting in theactivation of caspase-3 because caspase-3 zymogen (pro-caspase-3) isstabilized in the presence of zinc ions, either directly through bindingto Zn2+ or indirectly through the effect of Zn2+ on redox controlledprocesses (Fig. 1).

However, there are several reports showing that S100A8/A9 hasapoptosis/cytotoxicity-inducing activity against cancer cell lines,whereas DTPA was ineffective. For example, MM46 cells were insen-sitive to the action of DTPA, but were sensitive to S100A8/A9, in spiteof the fact that the S100A8/A9 effect on MM46 cells was also inhibitedby zinc (Yui et al., 2003). In our previous studies on the molecularmechanisms of S100A8/A9-induced apoptosis in SW742 and HT29/219 cell lines (Ghavami et al., 2004), we demonstrated that intra-cellular zinc concentrations decreased in a concentration dependentmanner if cells were treated with either DTPA or S100A8/A9.However, we observed that (i) Zn2+ and Cu2+ did not completelyreverse the apoptotic effect of S100A8/A9; (ii) the changes in themorphology of both cell lines treated by S100A8/A9 were differentfrom those observed with DTPA; (iii) the time course of S100A8/A9-induced apoptosis differed from that induced by DTPA, and (iv) DTPAinduced caspase-3 in both colon carcinoma cell lines at similar levels,althoughDTPA showed significant different effects on the intracellular


zinc level. These data are indicative of a zinc-independentmechanism.Nonetheless, intracellular Zn2+ depletion causes significant cellularstress by itself, since these divalent cations are critical for the functionof several transcription factors and enzymes.

Cellular stress is known to activate the mitochondrial dependent(intrinsic) pathway. Thus, these observations imply that S100A8/A9induces apoptosis by a mechanism that is not simply due to zincsequestration. In addition, although the other S100 proteins alsobind zinc, S100A8/A9 is a much more potent inducer of apoptosis(Ghavami et al., 2004).

3.2. Molecular mechanisms of S100A8/A9-induced apoptosis

In a recent study, we could demonstrate that S100A8/A9 exertsapoptosis-inducing activity cell death through the mitochondrialpathway (Ghavami et al., 2008a). This pathway has been investigatedby exploring the response in certain cell lines either deficient for orover expressing components of the death signaling machinery. Wefound that over-expression of the mitochondrial transmembranedeficient form of the pro-apoptotic BNIP3 (ΔTM–BNIP3) partiallyreversed the cytotoxicity of S100A8/A9. In further detailed analyses,we demonstrated that S100A8/A9 i) rapidly dropped the mitochon-drial membrane potential; ii) caused selective translocation of Smac/Diablo and Omi/HtrA2 without cytochrome c release because ofinhibition of mitochondrial fission machinery Drp1, iii) induces celldeath by modulating the balance of pro- and anti-apoptotic Bcl2family towards pro-apoptotic one, and iv) induces Bax mitochondrialtranslocation and Bak dimerization (Fig. 1). This property was notmediated by binding to RAGE as demonstrated by knock down andblocking RAGE-specific antibody experiments (Ghavami et al., 2008a).

3.3. S100A8/A9 as apoptosis-inducing factor that might be useful in theimmune therapy of malignant cells

The apoptotic activity of S100A8/A9 against cancer cells togetherwith the demonstrations that S100A8 and S100A9 positive cells,macrophages and polymorphonuclear leukocytes, accumulate alongthe invasive margin of carcinoma (Stulik et al., 1999), and activatedphagocytes specifically release S100A8/A9 (Rammes et al., 1997)indicate that PMNs are potential mediators of antitumor effects andplay an important role in the antitumor response. Vice versa, S100A8/A9 might be a useful target for the immune therapy of malignantcells inasmuch 1) S100A8/A9 is a native agent and it is not likelythat S100A8/A9 induces immune responses itself; 2) on the basis ofS100A8/A9 ‘small molecule analogs’ could be developed that wouldretain its anticancer activity while having superior pharmacokineticsand stability; 3) the knowledge of the S100 cell death pathway mayprovide novel targets for cancer-selective therapeutics that aremore effective and trigger less side effects; and 4) S100A8/A9-basedtherapy is not genotoxic, thus it would be devoid of a risk to causesecondary cancer by survivors.

However, further investigations are necessary since it has beenshown that cancer cells utilize S100A8 and S100A9 as guidance for theadhesion and invasion of disseminating malignant cells (Hiratsukaet al., 2006) and S100A8/A9 promotes tumor cell proliferation inspecific conditions by RAGE ligation (Ghavami et al., 2008a,b). Thesefindings indicate that S100A8/A9 is a target of immune escapemechanism by tumors (for review see Igney and Krammer, 2002).

4. S100 expression in epithelial cells

4.1. Stress response-induced S100 expression

In normal epidermis S100A8 and S100A9 are expressed at onlyminimal levels. However, their expression is induced in response tostress at specific conditions. For example, they are significantly up-

regulated in differentiating suprabasal wound keratinocytes (Thoreyet al., 2001), especially in the first 12 to 24 h after injury, with agradual return to baseline expression over a 2-week period (Sooet al., 2002). Similarly, they are transiently induced in keratinocytesafter UVB irradiation (Dazard et al., 2003), and they are massivelyexpressed in psoriatic keratinocytes (Madsen et al., 1992). Then bothproteins are found in the granular layer and in the basal and spinouslayers ((Broome et al., 2003) and references therein).

In addition, their expression is induced by pro-inflammatorycytokines such as TNFα and IL1β. In view of these findings, the twoS100 proteins have been characterized as stress-induced proteins(Eckert et al., 2004). Recently, we found evidence for a complex ofpoly (ADP-ribose) polymerase (PARP-1) and Ku70/80 driving thestress-specific S100 gene expression (Grote et al., 2006).

It is worthwhile mentioning that the PSORS4 psoriasis suscepti-bility region has been mapped to chromosome 1q21 (Semprini et al.,2002), where the two S100 genes are located within the epidermaldifferentiation complex together with other members of the S100family, as well as epidermal differentiation markers such as severalcytokeratins, profilaggrin and involucrin (Mischke et al., 1996; Hardaset al., 1996). In addition, the S100 proteins are aberrantly overexpressed in experimental models of cultured keratinocytes exhibit-ing an abnormal psoriasis-like phenotype (Nagpal et al., 1996). Thesedata indicate a prominent role of the S100 protein in skin physiology.

4.2. S100A8/A9 Mediates Growth Arrest in Epithelial Cells

The ability of keratinocytes to proliferate or to differentiate isregulated by a number of biological signals derived from cell-cell orcell-matrix interactions that act through downstream signalingpathways. Among the pathways that are particularly important forthe regulation of keratinocyte growth and differentiation are thoseinvolving the NF-κB pathway. NF-κB activation has been shown toinduce growth arrest in epithelial cells opposite to immune cells(Seitz et al., 1998). An important role of S100A8/A9 in the regulationof proliferation and differentiation was shown by the demonstrationthat S100A8/A9 also promotes epithelial NADPH oxidase(s) subse-quently followed by the enhancement of NF-κB activation (Benedyket al., 2007). As a cellular consequence S100A8/A9-over expres-sion was shown to affect proliferation and differentiation of HaCaTkeratinocytes. S100A8/A9-positive cells showed both a decreased celldivision rate compared to S100A8/A9-negative cells as analyzed bythe CFDA dilution in flow cytometry and an increased expression ofthe differentiation markers involucrin and filaggrin (Bode G, Voss A,Varga G, Sopalla C, Benedyk M, Böhm M, Nacken W, Kerkhoff C;unpublished observation). Importantly, this effect did not rely on therelease of S100A8/A9 followed by its binding to cell surface receptors.HaCaT keratinocytes over expressing S100A8/A9 wild-type displayenhanced phosphorylation of NF-κB p65, had a significantly reducedcell division rate, an enhanced expression of the two differentiationmarkers involucrin and filaggrin, an increased cleavage of the apopto-sis marker poly (ADP-ribose) polymerase-1 (PARP-1), and enhancedphosphorylation of Akt/protein kinase B, a kinase involved in cellsurvival. Finally, we demonstrated that S100A8/A9-overexpressionmodulated resistance against UVB exposition.

In view of the stress response-induced expression of the two S100proteins in keratinocytes our findings have great implications forseveral cellular processes. Maintaining the dynamic balance ofkeratinocyte proliferation, differentiation, and cell death is necessaryto achieve a homeostatic thickness and function of normal skin. Aftercutaneous wounding, keratinocytes acquire an activated state inwhich proliferation is favored over differentiation in order to replenishthe lost material and rapidly close the site of injury. Subsequently,tissue architecture within the healing neo-epidermis has to be re-established, which is achieved by the switch from keratinocyteproliferation to the apoptosis-like program of terminal differentiation.


The prevention of UV-induced skin carcinogenesis is thought tobe mediated through correction of the mutations or, alternatively,elimination of the damaged cells by driving them toward apoptosis,senescence, or terminal differentiation. In these cellular processesthe expression of S100A8/A9 is transiently induced, and therefore,S100A8/A9 may play an important role in their regulation.

In addition, the observed activation of Akt/protein kinase B mightbe of great importance since enhanced Akt/protein kinase B activationplays a crucial role in acquired apoptotic resistance and perhapsmalignant transformation induced by chronic UVA exposure. Similar,it has been demonstrated that transgenic mice engineered to haveselective inhibition of NF-κB signaling (using a mutant IκBα cDNA)are prone to develop squamous cell carcinoma.

It is worthwhile mentioning that the S100A8/A9-mediated NF-κBactivation may also function as a reversible signal for cellularproliferation with relevance for tumorgenesis. In particular, S100A8/A9 over expression induced the phosphorylation of Akt/protein kinaseB, also referred to as PKB (protein kinase B), that belongs to a family ofserine/threonine kinases. It is known that phospho-Akt/proteinkinase B promotes cell survival by inhibiting apoptosis. Specifically,phospho-Akt1 has been shown to phosphorylate Bad, amember of theBcl-2 family that promotes cell death. This phosphorylation results inthe inactivation of the pro-apoptotic function of Bad. Therefore, wesuggest that S100A8/A9-enhanced Akt/protein kinase B activationmay play a crucial role in acquired apoptotic resistance and perhapsmalignant transformation induced by chronic UVA exposure.

4.3. S100 Gene expression in cancer cells

Comparative and functional genomics are powerful tools toadvance the understanding of the molecular basis of cancer. It isbelieved that genes are epigenetically regulated and, thus, each tumortype and stage will be characterized by a gene expression fingerprint.Interestingly, a number of S100 proteins are found to be differentiallyexpressed in cancer cells. Interestingly, S100 proteins have beenreported as being involved in the regulation of a number of cellularprocesses such as cell-cycle progression and differentiation (Donato,2003). Several of these have been associated with tumor develop-ment, cancer invasion or metastasis in recent studies (for review seereference Salama et al., 2008).

The examination of various S100 proteins in normal and carcino-genic human tissues and cell lines by immunohistochemical methodsrevealed that S100A1 and S100A2 can be detected in a few normaltissues only, whereas S100A4, S100A6, and S100B are expressed athigher levels in cancer tissues (Ilg et al., 1996). It has been postulatedthat the latter ones have a tumor suppressor function (Wicki et al.,1997; Salama et al., 2008). S100A2, S100A3, S100A4, S100A7 andS100A10 are over expressed in gastric cancer. In particular, theexpression of S100A3 correlates with tumor differentiation and stageof gastric cancer (Liu et al., 2008). Elevated intratumoral levels ofS100B have been detected in malignant melanoma and to a lesserextent in thyroid carcinoma and renal cell carcinoma (Molina et al.,2002). There is evidence that S100A14 is over expressed in ovary,breast, and uterus tumors but under expressed in kidney, rectum, andcolon tumors (Pietas et al., 2002). Another S100 gene S100Z has beenidentified with deregulated expression in some tumors (Gribenkoet al., 2001).

Several S100 proteins have been reported to be associated withdifferent types of cancers including lung adenocarcinomas (Arai et al.,2001; Beer et al., 2002; Bartling et al., 2007). Additionally, some S100proteins have also been shown to be associated with metastasis(Maelandsmo et al., 1997; Ebralidze et al., 1989; Diederichs et al.,2004; Helfman et al., 2005; Garrett et al., 2006;Wang et al., 2006; Tianet al., 2007). In particular, S100A4 has been shown to be involved inregulating the metastatic behavior of tumor cells (Ebralidze et al.,

1989). For S100P and S100A2, mRNA over expression was found inmetastasizing tumors (Diederichs et al., 2004).

Concerning S100A2, conflicting results have been published.Expression of S100A2 is suppressed early during lung carcinogenesisand its loss may be a contributing factor in lung cancer developmentor a biomarker of early changes in this process (Feng et al., 2001).Other groups described an association between the down regulationof S100A2 and the development of melanoma (Maelandsmo et al.,1997) and other malignant cells (Liu et al., 2000).

S100A4 and S100B are thought to inhibit p53 transcriptionalactivity by inhibition of its phosphorylation and compromising p53tumor suppressor activity (Grigorian et al., 2001) Recent studydemonstrates that the expressions of S100A4 and p53were significantpredictive factors of relapse in gastric cancer after curative resectionand adjuvant chemotherapy (Kim et al., 2008). In addition, it has beenreported that S100A4 enhanced p53-dependent apoptosis whileS100A2 promotes p53 transcriptional activity (Mueller et al., 2005).There is also some evidence that in vitro S100B inhibits calcium-dependent phosphorylation of p53 by protein kinase C which can leadsuppression of the p53 tumor suppressor mechanism and resulting inuncontrolled tumor growth (Donato, 1991; Wilder et al., 1998).

Differential expression of S100A8 and S100A9 has been shownto contribute to the development and progression of various typesof cancer. For example, S100A8 and S100A9 are over expressed inpancreatic adenocarcinoma (Shen et al., 2004), bladder cancers (Yaoet al., 2007), and breast cancers (Cross et al., 2005). S100A9 expres-sion is significantly linked to dedifferentiation of thyroid carcinoma(Ito et al., 2005).

Concerning skin cancer, conflicting results have been published.In a mouse model of chemically induced skin carcinogenesis hightranscript level of S100A8 is detected (Hummerich et al., 2006).However, S100A8 and S100A9 were significantly down regulated inhuman esophageal squamous cell carcinoma versus the normalcounterparts. (Zhi et al., 2003; Ji et al., 2004).

Several studies have attempted to correlate the expression ofS100A8 and S100A9 with the degree of noninvasive/invasivebehavior. The non-invasive MCF-7 breast cancer cells do not expressS100A9, and its gene expression is induced by cytokine oncostatin Mthrough the STAT3 signaling cascade (Li et al., 2004). However, inthe non-invasive MDA–MB-468 cells both S100 proteins are highlyexpressed (Bode et al., 2008). The invasive breast cancer cells MDA–MB-231 show only a low transcript level of S100A9 (Nagaraja et al.,2006), while S100A9 is over-expressed in invasive ductal carcinomaof the breast (Seth et al., 2003; Arai et al., 2004).

Of note, S100A8 and S100A9 have been suggested to representnovel diagnostic markers when measured in the serum of patientswith prostate cancer and benign prostate hyperplasia (Hermani et al.,2005).

An additional important indication for the involvement in inflam-matory and neoplastic disorders is that most S100 genes are found neara break-point region on human chromosome 1q21 which, if affected, isresponsible for a number of genetic abnormalities related to autoim-mune pathologies or cancer (Mischke et al., 1996). Although thefunction of S100 proteins in cancer cells in most cases is still unknown,the specific expression patterns of these proteins are a valuableprognostic tool (Heizmann et al., 2002). The link between S100 proteinsand tumorgenesis is also given by the recent finding that a complex ofPARP-1 and Ku70/80 drives the stress-specific S100 gene expression(Grote et al., 2006). Since these candidates, poly(ADP-ribose)polymer-ase-1 (PARP-1) and the heterodimeric complex Ku70/Ku80, are knownto participate in inflammatory disorders as well as tumorgenesis, thedata may indicate a possible link between S100 and inflammation-associated cancer. Further investigations are needed to resolve thephysiological role of the S100 proteins in tumor progression.

Recently, a novel link between S100 proteins and myeloid-derivedsuppressor cells (MDSCs) has been reported (Cheng et al., 2008).

Fig. 2. S100A8/A9 cell growth-promoting activity. S100A8/A9 induces cell proliferationby binding to RAGE. RAGE ligation causes activation of the PI3K-Akt–NF-κB survivalpathway including reactive oxygen species generation. This pathway in turn inducesthe production of growth factors, cytokines and sRAGE leading to cell proliferation.Soluble RAGE may play a role as competitor for S100A8/A9 binding to RAGE and thusdecrease S100A8/A9-induced proliferation in target cells (Ghavami et al. 2008b). PI3K= Phosphoinositid-3-Kinase; Akt = Akt/protein kinase B; CREB = cAMP responseelement-binding protein; MAPK = mitogen-activated protein kinase; ROS = reactiveoxygen species; sRAGE = soluble receptor of advanced glycation endproducts.


Myeloid-derived suppressor cells (MDSCs) are a heterogeneous pop-ulation of immune cells that accumulates in tumor-bearing hosts andin response to inflammation. The findings that up-regulation of theS100 proteins results in inhibition of DC differentiation and accu-mulation of MDSCs and the levels of S100 proteins were increased inmice with colon cancer than in tumor-free mice gave new insightsinto the underlying mechanisms by which MDSCs may account forthe limited effectiveness of cancer vaccines and anti-growth factortherapies.

4.4. S100A8/A9 induces cancer cell proliferation via RAGE ligation

In a previous study S100A8/A9 at concentration below 25 µg/mlpromotes proliferation of various human cancer cells (MCF-7, MDA–MB231, SHEP, and Kelly) (Ghavami et al., 2008b). This growth-promoting activity was demonstrated to be mediated by the receptor

Fig. 3. S100A8/A9 chemotactic signaling guide tumor cells to their target tissue. Tumor ceS100A8/A9 secretion, a chemical gradient is formed leading the tumor cells to the target tisdeprivation of soluble factors such as VEGF-A, TNFα, and TGFβ (Hiratsuka et al. 2006).

of advanced glycation end products (RAGE) using an in-vitroproteolysis assay. The growth-promoting activity was blocked by aRAGE specific siRNA and RAGE blocking antibody. Furthermore, it wasrevealed that S100A8/A9 provoked p38 and p44/42 mitogen-activated protein kinase phosphorylation while JNK phosphorylationwas unaffected, and S100A8/A9 induced sRAGE (soluble RAGE)release into the media. This finding is of potential interest becausesRAGE may play a role as competitor for S100A8/A9 binding to RAGEand thus decrease S100A8/A9-induced proliferation in target cells(Ghavami et al., 2008b) (Fig. 2).

4.5. S100A8/A9 serves as chemotactic factor for tumor cells

Chronic inflammation increases the risk for the expansion oftransformed cells and aggravates the development of many recog-nizedmalignancies (Balkwill andMantovani, 2001; Ruegg, 2006) in asmuch as inflammation promotes angiogenesis (Ruegg, 2006), inducesthe expression of tumor growth-promoting cytokines (Luo et al.,2004) and anti-apoptotic genes (Karin and Greten, 2005), and feedsforward signaling in tumor cells through cell surface receptors such asRAGE (Gebhardt et al., 2008). Moreover, reactive oxygen species areliberated during inflammation and they are powerful DNA-damagingagents. This is of importance since cell division is increased inresponse to inflammation putting more cells at risk of mutations asthey replicate their DNA during S phase. Finally, apoptosis of damagedcells is suppressed in inflamed tissue. So cells with precancerousgenetic mutations, which should have committed suicide, may escapethe apoptotic pathways resulting to promotion of tumor progression.

Notably, primary tumors release soluble factors that induce theexpression of members of S100 protein family, primarily in lung andonly minimally in liver or kidneys (Rafii and Lyden, 2006). Based onthe cell growth-promoting activity of S100A8/A9 it could beenvisioned that the selective up-regulation of S100 proteins in lungscould facilitate the survival and proliferation of metastasizing cancercells. This assumption supports the concept that S100A8 and S100A9play an important role in tumor growth and malignancy.

Another mechanism represents the finding that cancer cells utilizeS100A8 and S100A9 as guidance for the adhesion and invasion ofdisseminating malignant cells (Hiratsuka et al., 2006). In the contextof malignancy it was reported that S100A8/A9 attracts Mac-1+

myeloid cells to the lung tissue. Recruited Mac-1+ myeloid cells inlung in turn produce S100A8/A9 in response to primary malignant

lls release soluble factors that induce S100 gene expression in the target tissue. Aftersue. This model is sustained by the fact that S100A8/A9 expression is eliminated after


cells in a so called “premetastatic phase”. This phase shows thegeneral characteristics of an inflammation state which facilitates themicro-environmental changes required for the migration and im-plantation of primary tumor cells to lung tissue. After preparation ofthe target tissue for accepting the malignant cells, tumor cells mimicMac-1+myeloid cells in response to S100A8/A9 chemotactic signalingandmigrate to lung. So, it seems that tumor cells andMac-1+myeloidcells utilize a common pathway for migration to lung which involvesthe activation of mitogen-activated protein kinase pathway (Hirat-suka et al., 2006). These findings suggest S100A8/A9 as an attractivetarget for the development of strategies counteracting tumormetastasizing to certain organs (Fig. 3).

5. Conclusions and future perspectives

S100A8/A9 appears as a Janus-faced molecule. It is a powerfulapoptotic agent produced by immune cells, making it a veryfascinating tool in the battle against cancer. In fact, S100A8/A9promotes apoptosis in a number of cancer cells. S100A8/A9-positivephagocytes accumulate at the edge of malignant tissue, and after theirrelease S100A8/A9 may be involved in the immune response againsttumor by inducing apoptosis and regression of tumor cells.

In epithelial cells S100A8/A9 is expressed in response to stress. Itsexpression is transiently induced in keratinocytes after epidermalinjury and after UVB irradiation. After cutaneous wounding, kerati-nocytes acquire an activated state in which proliferation is favoredover differentiation in order to replenish the lost material and rapidlyclose the site of injury. Subsequently, tissue architecture within thehealing neo-epidermis has to be re-established, which is achieved bythe switch from keratinocyte proliferation to the apoptosis-likeprogram of terminal differentiation. The prevention of UV-inducedskin carcinogenesis is thought to be mediated through correction ofthe mutations or, alternatively, elimination of the damaged cells bydriving them toward apoptosis, senescence, or terminal differen-tiation. In these cellular processes the expression of S100A8/A9 istransiently induced, and therefore, S100A8/A9 may play an importantrole in their regulation. With regard to our data we hypothesizethat the S100A8/A9-mediated growth arrest via NF-κB activation isrequired for the upcoming cell fate decision of keratinocytes, i.e.for a survival phase to be followed by differentiation, proliferation, orapoptosis.

On the other hand, epithelial S100A8/A9 also mediated Akt/protein kinase B activation that functions as a reversible signal forcellular proliferation with relevance for tumorgenesis. As a result,primary tumors release soluble factors that induce the expression ofS100A8/A9. In view of the cell growth-promoting activity of S100A8/A9 it is likely to speculate that the selective up-regulation of S100proteins may be of importance for survival and proliferation ofmetastasizing cancer cells. Moreover, cancer cells utilize S100A8 andS100A9 as a guide for the adhesion and invasion of disseminatingmalignant cells. These events represent putative immune escapemechanisms by which cancer cells reprogram the immune responseand by which inflammatory cells contribute to tumor progressionby production of tumor growth factors and metastasis-promotingfactors.

Cancer is not a single disease; the word cancer refers to approxi-mately 150 diseases. These diseases share two characteristics incommon, uncontrolled cell growth and the ability to damage normaltissue. However, depending upon the particular mutations accumu-lated and the location of the tumor, cancers can be, in many respects,quite diverse. The diversity of cancer applies to their immune evasionstrategies as well. The mechanisms that cancers use to escape hostimmune responses may vary among different types of cancers andeven different cancers of a particular type. Therefore, the knowledgeof the particular evasion strategies that tumors employ is necessary todesign vaccines that specifically target the ability of tumors to escape

immunity and may also be useful to predict responsiveness to avail-able radiation or chemotherapy.

S100A8/A9 is a native, not genotoxic, potent apoptosis-inducingfactor against tumor cells. It spears the risk to induce auto-immuneresponse and may serve as lead compound for cancer-selectivetherapeutics but it bears the risk for immune escape mechanisms bytumors. Further investigations should be aware of these two sides ofthe same coin.

Acknowledgements

This work was in part funded by a CLA/GSK/CIHR postdoctoralfellowship (S.G. and S.C.) and a CIHR National Training Programin Allergy and Asthma fellowship (to S.G.), by NIH RO1 GM62112 (toW.J.C.), and by the “Deutsche Forschungsgemeinschaft (DFG)”, projectKE 820/2-4 and project KE 820/6-1 (both to C.K.). S.G. is currently aParker B. Francis Fellow in Pulmonary Disease. A.J.H. is supported bythe Canada Research Chairs Program.

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S100A8/A9: A Janus-faced molecule in cancer therapy and tumorgenesis

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