The ability of cells to adhere to each other and to their surrounding extracellular matrices is essential for a multicellular existence. Adhesion provides physical support for cells, regulates cell positioning and enables microenvironmental sensing. The integrins and the syndecans are two adhesion receptor families that mediate adhesion, but their relative and functional contributions to cell-extracellular matrix interactions remain obscure. Recent advances have highlighted connections between the signalling networks that are controlled by these families of receptors. Here we survey the evidence that synergistic signalling is involved in controlling adhesive function and the regulation of cell behaviour in response to the external environment.

Proteoglycans, key molecular effectors of cell surface and pericellular microenvironments, perform multiple functions in cancer and angiogenesis by virtue of their polyhedric nature and their ability to interact with both ligands and receptors that regulate neoplastic growth and neovascularization. Some proteoglycans such as perlecan, have pro- and anti-angiogenic activities, whereas other proteoglycans, such as syndecans and glypicans, can also directly affect cancer growth by modulating key signalling pathways. The bioactivity of these proteoglycans is further modulated by several classes of enzymes within the tumour microenvironment: (i) sheddases that cleave transmembrane or cell-associated syndecans and glypicans, (ii) various proteinases that cleave the protein core of pericellular proteoglycans and (iii) heparanases and endosulfatases which modify the structure and bioactivity of various heparan sulphate proteoglycans and their bound growth factors. In contrast, some of the small leucine-rich proteoglycans, such as decorin and lumican, act as tumour repressors by physically antagonizing receptor tyrosine kinases including the epidermal growth factor and the Met receptors or integrin receptors thereby evoking anti-survival and pro-apoptotic pathways. In this review we will critically assess the expanding repertoire of molecular interactions attributed to various proteoglycans and will discuss novel proteoglycan functions modulating cancer progression, invasion and metastasis and how these factors regulate the tumour microenvironment.

CONCLUSIONS

Glypicans are heparan sulfate proteoglycans that are bound to the outer surface of the plasma membrane by a glycosyl-phosphatidylinositol anchor. Homologs of glypicans are found throughout the Eumetazoa. There are six family members in mammals (GPC1 to GPC6). Glypicans can be released from the cell surface by a lipase called Notum, and most of them are subjected to endoproteolytic cleavage by furin-like convertases. In vivo evidence published so far indicates that the main function of membrane-attached glypicans is to regulate the signaling of Wnts, Hedgehogs, fibroblast growth factors and bone morphogenetic proteins (BMPs). Depending on the context, glypicans may have a stimulatory or inhibitory activity on signaling. In the case of Wnt, it has been proposed that the stimulatory mechanism is based on the ability of glypicans to facilitate and/or stabilize the interaction of Wnts with their signaling receptors, the Frizzled proteins. On the other hand, GPC3 has recently been reported to inhibit Hedgehog protein signaling during development by competing with Patched, the Hedgehog receptor, for Hedgehog binding. Surprisingly, the regulatory activity of glypicans in the Wnt, Hedgehog and BMP signaling pathways is only partially dependent on the heparan sulfate chains.

Hyaluronan (HA), a major component of the extracellular matrix (ECM), is enriched in many types of tumors. In cancer patients HA concentrations are usually higher in malignant tumors than in corresponding benign or normal tissues, and in some tumor types the level of HA is predictive of malignancy. HA is often bound to CD44 isoforms which are ubiquitous, abundant, and functionally important cell surface receptors. This article reviews the current evidence for HA/CD44-mediated activation of the ankyrin-based cytoskeleton and RhoGTPase signaling during tumor progression. A special focus is placed on the role of HA-mediated CD44 interaction with unique downstream effectors (e.g., the cytoskeletal protein, ankyrin and/or various GTPases (e.g., RhoA, Rac1 and Cdc42)) in coordinating intracellular signaling pathways (e.g., Ca(2+) mobilization, Rho signaling, PI3 kinase-AKT activation, NHE1-mediated cellular acidification, transcriptional upregulation and cytoskeletal function) and generating the concomitant onset of tumor cell activities (e.g., tumor cell adhesion, growth, survival, migration and invasion) and tumor progression. I believe this information will provide valuable new insights into poorly understood aspects of solid tumor malignancy. Furthermore, the new knowledge concerning HA/CD44-mediated oncogenic signaling events will have potentially important clinical utility, and could establish CD44 and its associated signaling molecules as important tumor markers for the early detection and evaluation of oncogenic potential. It could also serve as ground work for the future development of new drug targets to inhibit HA/CD44-mediated tumor metastasis and cancer progression.

Decorin, a member of the small leucine-rich proteoglycan gene family, impedes tumor cell growth by down-regulating the epidermal growth factor receptor. Decorin has a complex binding repertoire, thus, we predicted that decorin would modulate the bioactivity of other tyrosine kinase receptors. We discovered that decorin binds directly and with high affinity (K(d) = approximately 1.5 nM) to Met, the receptor for hepatocyte growth factor (HGF). Binding of decorin to Met is efficiently displaced by HGF and less efficiently by internalin B, a bacterial Met ligand. Interaction of decorin with Met induces transient receptor activation, recruitment of the E3 ubiquitin ligase c-Cbl, and rapid intracellular degradation of Met (half-life = approximately 6 min). Decorin suppresses intracellular levels of beta-catenin, a known downstream Met effector, and inhibits Met-mediated cell migration and growth. Thus, by antagonistically targeting multiple tyrosine kinase receptors, decorin contributes to reduction in primary tumor growth and metastastic spreading.

The mechanisms linking immune responses and inflammation with tumor development are not well understood. Here, we show that the soluble form of the extracellular matrix proteoglycan decorin controls inflammation and tumor growth through PDCD4 (programmed cell death 4) and miR-21 (microRNA-21) by two mechanisms. First, decorin acted as an endogenous ligand of Toll-like receptors 2 and 4 and stimulated production of proinflammatory molecules, including PDCD4, in macrophages. Second, decorin prevented translational repression of PDCD4 by decreasing the activity of transforming growth factor-β1 and the abundance of oncogenic miR-21, a translational inhibitor of PDCD4. Moreover, increased PDCD4 abundance led to decreased release of the anti-inflammatory cytokine interleukin-10, thereby making the cytokine profile more proinflammatory. This pathway operates in both pathogen-mediated and sterile inflammation, as shown here for sepsis and growth retardation of established tumor xenografts, respectively. Decorin was an early response gene evoked by septic inflammation, and protein concentrations of decorin were increased in the plasma of septic patients and mice. In cancer, decorin reduced the abundance of anti-inflammatory molecules and increased that of proinflammatory molecules, thereby shifting the immune response to a proinflammatory state associated with reduced tumor growth. Thus, by stimulating proinflammatory PDCD4 and decreasing the abundance of miR-21, decorin signaling boosts inflammatory activity in sepsis and suppresses tumor growth.

Decorin, an archetypal member of the small leucine-rich proteoglycan gene family, has a broad binding repertoire that encompasses matrix structural components, such as collagens, and growth factors, particularly those that belong to the transforming growth factor-β ligand superfamily. Within the tumor microenvironment, stromal decorin has an inherent proclivity to directly bind and down-regulate several receptor tyrosine kinases, which are often overexpressed in cancer cells. The decorin interactome commands a powerful antitumorigenic signal by potently repressing and attenuating tumor cell proliferation, survival, migration, and angiogenesis. This collection of interacting molecules also regulates key downstream signaling processes indirectly via the sequestration of growth factors or directly via the antagonism of receptor tyrosine kinases. We propose that decorin can be considered a "guardian from the matrix" because of its innate ability to oppose pro-tumorigenic cues.

Syndecan-1 (Sdc1) is a matrix receptor shown to associate via its extracellular domain with the alpha(v)beta(3) and alpha(v)beta(5) integrins, potentially regulating cell adhesion, spreading, and invasion of cells expressing these integrins. Using Sdc1 deletion mutants expressed in human mammary carcinoma cells, we identified the active site within the Sdc1 core protein and derived a peptide inhibitor called synstatin (SSTN) that disrupts Sdc1's interaction with these integrins. Because the alpha(v)beta(3) and alpha(v)beta(5) integrins are critical in angiogenesis, a process in which a role for Sdc1 has been uncertain, we used human vascular endothelial cells in vitro to show that the Sdc1 regulatory mechanism is also required for integrin activation on these cells. We found Sdc1 expressed in the vascular endothelium during microvessel outgrowth from aortic explants in vitro and in mouse mammary tumors in vivo. Moreover, we show that SSTN blocks angiogenesis in vitro or when delivered systemically in a mouse model of angiogenesis in vivo, and impairs mammary tumor growth in an orthotopic mouse tumor model. Thus, Sdc1 is a critical regulator of these two important integrins during angiogenesis and tumorigenesis, and is inhibited by the novel SSTN peptide.

In this study we have examined CD44 (a hyaluronan (HA) receptor) interaction with a RhoA-specific guanine nucleotide exchange factor (p115RhoGEF) in human metastatic breast tumor cells (MDA-MB-231 cell line). Immunoprecipitation and immunoblot analyses indicate that both CD44 and p115RhoGEF are expressed in MDA-MB-231 cells and that these two proteins are physically associated as a complex in vivo. The binding of HA to MDA-MB-231 cells stimulates p115RhoGEF-mediated RhoA signaling and Rho kinase (ROK) activity, which, in turn, increases serine/threonine phosphorylation of the adaptor protein, Gab-1 (Grb2-associated binder-1). Phosphorylated Gab-1 promotes PI 3-kinase recruitment to CD44v3. Subsequently, PI 3-kinase is activated (in particular, alpha, beta, gamma forms but not the delta form of the p110 catalytic subunit), AKT signaling occurs, the cytokine (macrophage-colony stimulating factor (M-CSF)) is produced, and tumor cell-specific phenotypes (e.g. tumor cell growth, survival and invasion) are up-regulated. Our results also demonstrate that HA/CD44-mediated oncogenic events (e.g. AKT activation, M-CSF production and breast tumor cell-specific phenotypes) can be effectively blocked by a PI 3-kinase inhibitor (LY294002). Finally, we have found that overexpression of a dominant-negative form of ROK (by transfection of MBA-MD-231 cells with the Rho-binding domain cDNA of ROK) not only inhibits HA/CD44-mediated RhoA-ROK activation and Gab-1 phosphorylation but also down-regulates oncogenic signaling events (e.g. Gab-1.PI 3-kinase-CD44v3 association, PI 3-kinase-mediated AKT activation, and M-CSF production) and tumor cell behaviors (e.g. cell growth, survival, and invasion). Taken together, these findings strongly suggest that CD44 interaction with p115RhoGEF and ROK plays a pivotal role in promoting Gab-1 phosphorylation leading to Gab-1.PI 3-kinase membrane localization, AKT signaling, and cytokine (M-CSF) production during HA-mediated breast cancer progression.

Anchorage of cells to "heparin"--binding domains that are prevalent in extracellular matrix (ECM) components is thought to occur primarily through the syndecans, a four-member family of transmembrane heparan sulfate proteoglycans that communicate environmental cues from the ECM to the cytoskeleton and the signaling apparatus of the cell. Known activities of the syndecans trace to their highly conserved cytoplasmic domains and to their heparan sulfate chains, which can serve to regulate the signaling of growth factors and morphogens. However, several emerging studies point to critical roles for the syndecans' extracellular protein domains in tumor cell behavior to include cell adhesion and invasion. Although the mechanisms of these activities remain largely unknown, one possibility involves "co-receptor" interactions with integrins that may regulate integrin function and the cell adhesion-signaling phenotype. Thus, alterations in syndecan expression, leading to either overexpression or loss of expression, both of which take place in tumor cells, may have dramatic effects on tumor cell invasion.

Fibroblast growth factor-2 (FGF-2) is a member of a large family of proteins that bind heparin and heparan sulfate and modulate the function of a wide range of cell types. FGF-2 stimulates the growth and development of new blood vessels (angiogenesis) that contribute to the pathogenesis of several diseases (i.e. cancer, atherosclerosis), normal wound healing and tissue development. FGF-2 contains a number of basic residues (pI 9.6) and consists of 12 anti-parallel beta-sheets organized into a trigonal pyrimidal structure. FGF-2 binds to four cell surface receptors expressed as a number of splice variants. Many of the biological activities of FGF-2 have been found to depend on its receptor's intrinsic tyrosine kinase activity and second messengers such as the mitogen activated protein kinases. However, considerable evidence suggest that intracellular FGF-2 might have a direct biological role particularly within the nucleus. In addition, heparan sulfate proteoglycans have been demonstrated to enhance and inhibit FGF-2 activity. The possibility that FGF-2 activity can be manipulated through alterations in heparan sulfate-binding is currently being exploited in the development of clinical applications aimed at modulating either endogenous or administered FGF-2 activity.

Syndecan-1 is a cell surface heparan sulfate proteoglycan that binds to many mediators of disease pathogenesis. Through these molecular interactions, syndecan-1 can modulate leukocyte recruitment, cancer cell proliferation and invasion, angiogenesis, microbial attachment and entry, host defense mechanisms, and matrix remodeling. The significance of syndecan-1 interactions in disease is underscored by the striking pathological phenotypes seen in the syndecan-1 null mice when they are challenged with disease-instigating agents or conditions. This review discusses the key molecular functions of syndecan-1 in modulating the onset, progression, and resolution of inflammatory diseases, cancer, and infection.

High levels of heparanase are an indicator of poor prognosis in myeloma patients, and up-regulation of the enzyme enhances tumor growth, angiogenesis, and metastasis in animal models. At least part of the impact of heparanase in driving the aggressive tumor phenotype is due to its effect on increasing the expression and shedding of the heparan sulfate proteoglycan syndecan-1, a molecule known to promote myeloma progression. The present work demonstrated that elevation in heparanase expression in myeloma cells stimulates sustained ERK phosphorylation that in turn drives MMP-9 expression. In addition, urokinase-type plasminogen activator (uPA) and uPA receptor expression levels increased, and blocking the proteolytic activation of either MMP-9 or uPA inhibited the heparanase-induced increase in syndecan-1 shedding. Together these data provide a mechanism for heparanase-induced syndecan-1 shedding and, more importantly, demonstrate that heparanase activity in myeloma cells can lead to increased levels of proteases that are known to play important roles in the aggressive behavior of myeloma tumors. This in addition to its other known biological roles, indicates that heparanase acts as a master regulator of the aggressive tumor phenotype by up-regulating protease expression and activity within the tumor microenvironment.

Decorin, the prototype member of the small leucine-rich proteoglycans, resides in the tumor microenvironment and affects the biology of various types of cancer by downregulating the activity of several receptors involved in cell growth and survival. Decorin binds to and modulates the signaling of the epidermal growth factor receptor and other members of the ErbB family of receptor tyrosine kinases. It exerts its antitumor activity by a dual mechanism: via inhibition of these key receptors through their physical downregulation coupled with attenuation of their signaling, and by binding to and sequestering TGFbeta. Decorin also modulates the insulin-like growth factor receptor and the low-density lipoprotein receptor-related protein 1, which indirectly affects the TGFbeta receptor pathway. When expressed in tumor xenograft-bearing mice or injected systemically, decorin inhibits both primary tumor growth and metastatic spreading. In this review, we summarize the latest reports on decorin and related molecules that are relevant to cancer and bring forward the idea of decorin as an anticancer therapeutic and possible prognostic marker for patients affected by various types of tumors. We also discuss the role of lumican and LRIG1, a novel cell growth inhibitor homologous to decorin.

Decorin inhibits the epidermal growth factor receptor (EGFR) by down-regulating its tyrosine kinase activity, thereby blocking the growth of a variety of transformed cells and tumor xenografts. In this study we provide evidence that decorin directly binds to the EGFR causing its dimerization, internalization, and ultimately its degradation. Using various pharmacological agents to disrupt clathrin-dependent and -independent endocytosis, we demonstrate that decorin evokes a protracted internalization of the EGFR primarily via caveolar-mediated endocytosis. In contrast to EGF, decorin targets the EGFR to caveolae, but not to early or recycling endosomes. Ultimately, however, both EGF- and decorin-induced pathways converge into late endosomes/lysosomes for final degradation. Thus, we have discovered a novel biological mechanism for decorin that could explain its anti-proliferative and anti-oncogenic mode of action.

In this study, we have examined the interaction of hyaluronan (HA)-CD44 with IQGAP1 (one of the binding partners for the Rho GTPase Cdc42) in SK-OV-3.ipl human ovarian tumor cells. Immunological and biochemical analyses indicated that IQGAP1 (molecular mass of approximately 190 kDa) is expressed in SK-OV-3.ipl cells and that IQGAP1 interacts directly with Cdc42 in a GTP-dependent manner. Both IQGAP1 and Cdc42 were physically linked to CD44 in SK-OV-3.ipl cells following HA stimulation. Furthermore, the HA-CD44-induced Cdc42-IQGAP1 complex regulated cytoskeletal function via a close association with F-actin that led to ovarian tumor cell migration. In addition, the binding of HA to CD44 promoted the association of ERK2 with the IQGAP1 molecule, which stimulated both ERK2 phosphorylation and kinase activity. The activated ERK2 then increased the phosphorylation of both Elk-1 and estrogen receptor-alpha (ER alpha), resulting in Elk-1- and estrogen-responsive element-mediated transcriptional up-regulation. Down-regulation of IQGAP1 (by treating cells with IQGAP1-specific small interfering RNAs) not only blocked IQGAP1 association with CD44, Cdc42, F-actin, and ERK2 but also abrogated HA-CD44-induced cytoskeletal function, ERK2 signaling (e.g. ERK2 phosphorylation/activity, ERK2-mediated Elk-1/ER alpha phosphorylation, and Elk-1/ER alpha-specific transcriptional activation), and tumor cell migration. Taken together, these findings indicate that HA-CD44 interaction with IQGAP1 serves as a signal integrator by modulating Cdc42 cytoskeletal function, mediating Elk-1-specific transcriptional activation, and coordinating "cross-talk" between a membrane receptor (CD44) and a nuclear hormone receptor (ER alpha) signaling pathway during ovarian cancer progression.

In this study we have examined the interaction between CD44s (the standard form) and the p185(HER2) proto-oncogene in the ovarian carcinoma cell line. Surface biotinylation followed by wheat germ agglutinin column chromatography and anti-CD44-mediated immunoprecipitation indicate that both CD44s and p185(HER2) are expressed on the cell surface and most importantly, that these two molecules are physically linked to each other via interchain disulfide bonds. We have also determined that hyaluronic acid stimulates CD44s-associated p185(HER2) tyrosine kinase activity, leading to an increase in the ovarian carcinoma cell growth. After transfection of the ovarian carcinoma cell line with the adenovirus 5 E1A gene, which is known to repress p185(HER2) expression, we observed that both surface CD44s expression and CD44s-mediated cell adhesion to hyaluronic acid are significantly reduced in the transfectant cells compared with the control cells. These data suggest that down-regulation of p185(HER2) blocks CD44s expression and subsequent adhesion function. Our findings also indicate that the CD44s-p185(HER2) interaction is both functionally coupled and biosynthetically regulated. We believe that direct "cross-talk" between these two surface molecules (i.e. CD44s and the p185(HER2)) may be one of the most important signaling events in human ovarian carcinoma development.

Heparan sulfate proteoglycans (HSPGs) play a crucial role in growth regulation by assembling signaling complexes and presenting growth factors to their cognate receptors. Within the immune system, expression of the HSPG syndecan-1 (CD138) is characteristic of terminally differentiated B cells, ie, plasma cells, and their malignant counterpart, multiple myeloma (MM). This study explored the hypothesis that syndecan-1 might promote growth factor signaling and tumor growth in MM. For this purpose, the interaction was studied between syndecan-1 and hepatocyte growth factor (HGF), a putative paracrine and autocrine regulator of MM growth. The study demonstrates that syndecan-1 is capable of binding HGF and that this growth factor is indeed a potent stimulator of MM survival and proliferation. Importantly, the interaction of HGF with heparan sulfate moieties on syndecan-1 strongly promotes HGF-mediated signaling, resulting in enhanced activation of Met, the receptor tyrosine kinase for HGF. Moreover, HGF binding to syndecan-1 promotes activation of the phosphatidylinositol 3-kinase/protein kinase B and RAS/mitogen-activated protein kinase pathways, signaling routes that have been implicated in the regulation of cell survival and proliferation, respectively. These results identify syndecan-1 as a functional coreceptor for HGF that promotes HGF/Met signaling in MM cells, thus suggesting a novel function for syndecan-1 in MM tumorigenesis.

Decorin, a small leucine-rich proteoglycan, inhibits tumor growth by antagonizing multiple receptor tyrosine kinases including EGFR and Met. Here, we investigated decorin during normoxic angiogenic signaling. An angiogenic PCR array revealed a profound decorin-evoked transcriptional inhibition of pro-angiogenic genes, such as HIF1A. Decorin evoked a reduction of hypoxia inducible factor (HIF)-1α and vascular endothelial growth factor A (VEGFA) in MDA-231 breast carcinoma cells expressing constitutively-active HIF-1α. Suppression of Met with decorin or siRNA evoked a similar reduction of VEGFA by attenuating downstream β-catenin signaling. These data establish a noncanonical role for β-catenin in regulating VEGFA expression. We found that exogenous decorin induced expression of thrombospondin-1 and TIMP3, two powerful angiostatic agents. In contrast, decorin suppressed both the expression and enzymatic activity of matrix metalloprotease (MMP)-9 and MMP-2, two pro-angiogenic proteases. Our data establish a novel duality for decorin as a suppressor of tumor angiogenesis under normoxia by simultaneously down-regulating potent pro-angiogenic factors and inducing endogenous anti-angiogenic agents.

A theme emerging during the past few years is that members of the small leucine-rich proteoglycan gene family affect cell growth by interacting with multiple receptor tyrosine kinases (RTKs), mostly by a physical down-regulation of the receptors, thereby depriving tumor cells of pro-survival signals. Decorin binds and down-regulates several RTKs, including Met, the receptor for hepatocyte growth factor. Here we demonstrate that decorin blocks several biological activities mediated by the Met signaling axis, including cell scatter, evasion, and migration. These effects were mediated by a profound down-regulation of noncanonical β-catenin levels. In addition, Myc, a downstream target of β-catenin, was markedly down-regulated by decorin, whereas phosphorylation of Myc at threonine 58 was markedly induced. The latter is known to destabilize Myc and target it for proteasomal degradation. We also discovered that systemic delivery of decorin using three distinct tumor xenograft models caused down-regulation of Met and a concurrent suppression of β-catenin and Myc levels. We found that decorin protein core labeled with the near infrared dye IR800 specifically targeted the tumor cells expressing Met. Even 68-h post-injection, decorin was found to reside within the tumor xenografts with little or no binding to other tissues. Collectively, our results indicate a role for a secreted proteoglycan in suppressing the expression of key oncogenic factors required for tumor progression.

A family of proteins known as IQGAPs have been identified in yeast, amebas and mammals. IQGAPs are multidomain molecules that contain several protein-interacting motifs which mediate binding to target proteins. Mammalian IQGAP1 is a component of signaling networks that are integral to maintaining cytoskeletal architecture and cell-cell adhesion. Published data suggest that IQGAP1 is a scaffolding protein that modulates cross-talk among diverse pathways in complex regulatory circuits. These pathways include modulating the actin cytoskeleton, mediating signaling by Rho family GTPases and calmodulin, regulating E-cadherin and beta-catenin function and organizing microtubules.

The small leucine-rich proteoglycan, decorin, has incrementally been shown to be a powerful inhibitor of growth in a wide variety of tumour cells, an effect specifically mediated by the interaction of decorin core protein with the epidermal growth factor receptor (EGFR) and other ErbB family proteins. Nowadays, this matrikine has become the main focus of various cancer studies. Decorin is an important component of the cellular microenvironment or extracellular matrix (ECM). Its interactions with matrix and cell membrane components have been implicated in many physiological and pathophysiological processes including matrix organisation, signal transduction, wound healing, cell migration, inhibition of metastasis, and angiogenesis. This review summarises recent findings on decorin's interactions and behaviour related to cancer. Highlighted are key functions of decorin such as interaction with cell surface receptors, as well as with ECM components, and the therapeutic potential of this multifunctional molecule.

Glypican-3 (GPC3) is a member of the glypican family. Glypicans are proteoglycans that are attached to the cell surface by a glycosyl-phosphatidylinositol anchor. They regulate the signaling activity of several growth factors, including Wnts. This regulation is based on the ability of glypicans to stimulate or inhibit the interaction of these growth factors with their respective signaling receptors. It has been clearly established that whereas GPC3 is expressed by most hepatocellular carcinomas (HCCs), this glypican is not detected in normal and cirrhotic liver, or in benign hepatic lesions. Consequently, immunostaining of liver biopsies for GPC3 is currently being used by clinical pathologists to confirm HCC diagnosis when the malignant nature of the lesion is difficult to establish. In addition to being a marker of HCC, GPC3 plays a role in the progression of the disease. GPC3 promotes the growth of HCC by stimulating canonical Wnt signaling. It has been proposed that this stimulation is based on the ability of GPC3 to increase the binding of Wnt to its signaling receptor, Frizzled. Two therapeutic approaches for HCC that target GPC3 are currently being tested in phase II clinical trials. One of them is based on the use of a humanized GPC3 monoclonal antibody that inhibits the in vivo growth of HCC xenografts by inducing antibody-dependent cellular cytotoxicity. The second approach employs a vaccine that consists of two GPC3-derived peptides that induce cytotoxic T lymphocytes against these peptides. Targeting of GPC3 might offer a new tool for the treatment of HCC.