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Sustained Induction of ERK, Protein Kinase B, and p70 S6 Kinase Regulates Cell Spreading and Formation of F-actin Microspikes Upon Ligation of Integrins by Galectin-8, a Mammalian Lectin*

  • Yifat Levy
    Affiliations
    Department of Molecular Cell Biology, The Weizmann Institute of Science, Rehovot 76100, Israel
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  • Denise Ronen
    Affiliations
    Department of Molecular Cell Biology, The Weizmann Institute of Science, Rehovot 76100, Israel
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  • Alexander D. Bershadsky
    Affiliations
    Department of Molecular Cell Biology, The Weizmann Institute of Science, Rehovot 76100, Israel
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  • Yehiel Zick
    Correspondence
    To whom correspondence should be addressed. Tel.: 972-8-9342-380; Fax: 972-8-9344-125
    Affiliations
    Department of Molecular Cell Biology, The Weizmann Institute of Science, Rehovot 76100, Israel
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  • Author Footnotes
    * This work was supported by grants from the CaPCure Israel Foundation, the Moross Center for Cancer Research, and the Israel Cancer Association.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Open AccessPublished:April 18, 2003DOI:https://doi.org/10.1074/jbc.M207380200
      Galectin-8, a member of the galectin family of mammalian lectins, is a secreted protein that promotes cell adhesion and migration upon binding to a subset of integrins through sugar-protein interactions. Ligation of integrins by galectin-8 triggers a distinct pattern of cytoskeletal organization, including formation of F-actin-containing microspikes. This is associated with activation of integrin-mediated signaling cascades (ERK and phosphatidylinositol 3 kinase (PI3K)) that are much more robust and are of longer duration than those induced upon cell adhesion to fibronectin. Indeed, formation of microspikes is enhanced 40% in cells that overexpress protein kinase B, the downstream effector of PI3K. Inhibition of PI3K activity induced by wortmannin partially inhibits cell adhesion and spreading while largely inhibiting microspike formation in cells adherent to galectin-8. Furthermore, the inhibitory effects of wortmannin are markedly accentuated in cells overexpressing PKB or p70S6K (CHOPKB and CHOp70S6Kcells), whose adhesion and spreading on galectin-8 (but not on fibronectin) is inhibited ∼25–35% in the presence of wortmannin. The above results suggest that galectin-8 is an extracellular matrix protein that triggers a unique repertoire of integrin-mediated signals, which leads to a distinctive cytoskeletal organization and microspike formation. They further suggest that downstream effectors of PI3K, including PKB and p70 S6 kinase, in part mediate cell adhesion, spreading, and microspike formation induced by galectin-8.
      Extracellular matrix (ECM)
      The abbreviations used are: ECM
      extracellular matrix
      FCS
      fetal calf serum
      PBS
      phosphate-buffered saline
      GST
      glutathione S-transferase
      RBD
      Ras binding domain of Raf
      FAK
      focal adhesion kinase
      PKB
      protein kinase B
      p70S6K
      p70 S6 kinase
      PI3K
      phosphatidylinositol 3-kinase
      MAPK
      mitogen-activated protein kinase
      CHO
      Chinese hamster ovary
      CHOPKB and CHOp70S6K
      CHO-P cells overexpressing PKB or p70S6K, respectively
      ERK
      extracellular-regulated kinase
      HE
      human endothelial
      TRITC
      tetramethylrhodamine isothiocyanate
      WT
      wild-type
      1The abbreviations used are: ECM
      extracellular matrix
      FCS
      fetal calf serum
      PBS
      phosphate-buffered saline
      GST
      glutathione S-transferase
      RBD
      Ras binding domain of Raf
      FAK
      focal adhesion kinase
      PKB
      protein kinase B
      p70S6K
      p70 S6 kinase
      PI3K
      phosphatidylinositol 3-kinase
      MAPK
      mitogen-activated protein kinase
      CHO
      Chinese hamster ovary
      CHOPKB and CHOp70S6K
      CHO-P cells overexpressing PKB or p70S6K, respectively
      ERK
      extracellular-regulated kinase
      HE
      human endothelial
      TRITC
      tetramethylrhodamine isothiocyanate
      WT
      wild-type
      proteins have important functions in providing structural integrity to tissues and in presenting proper environmental cues for cell adhesion, migration, growth, and differentiation (
      • Aplin A.E.
      • Howe A.
      • Alahari S.K.
      • Juliano R.L.
      ,
      • Hynes R.O.
      ,
      • Schwartz M.A.
      • Ginsberg M.H.
      ,
      • Geiger B.
      • Bershadsky A.
      • Pankov R.
      • Yamada K.M.
      ,
      • Miranti C.K.
      • Brugge J.S.
      ). These functions rely on spatio-temporal expression of adhesive as well as anti-adhesive components of the ECM proteins (
      • Chiquet-Ehrismann R.
      ). ECM proteins like fibronectin (
      • Yamada K.M.
      • Kennedy D.W.
      ,
      • Faull R.J.
      • Kovach N.L.
      • Harlan J.M.
      • Ginsberg M.H.
      ,
      • Woods M.L.
      • Cabanas C.
      • Shimizu Y.
      ), collagen (
      • Heino J.
      ), and laminin (
      • Calof A.L.
      • Campanero M.R.
      • O'Rear J.J.
      • Yurchenco P.D.
      • Lander A.D.
      ) are best characterized, though other types of proteins, including mammalian lectins, also function as modulators of cell adhesion. Selectins mediate cell-cell interactions (
      • Juliano R.L.
      ) through calcium-dependent recognition of sialylated glycans (
      • Vestweber D.
      • Blanks J.E.
      ,
      • Feizi T.
      • Galustian C.
      ), whereas galectins, animal lectins that specifically bind β-galactoside residues (
      • Barondes S.H.
      • Castronovo V.
      • Cooper D.N.
      • Cummings R.D.
      • Drickamer K.
      • Feizi T.
      • Gitt M.A.
      • Hirabayashi J.
      • Hughes C.
      • Kasai K.
      • Leffler H.
      • Liu F.T.
      • Lotan R.
      • Mercurio A.M.
      • Monsigny M.
      • Pillai S.
      • Poirer F.
      • Raz A.
      • Rigby P.W.J.
      • Rini J.M.
      • Wang J.L.
      ), were implicated as modulators of cell-matrix interactions. Although lacking a signal peptide and found mainly in the cytosol, galectins are externalized by an atypical secretory mechanism (
      • Hughes R.C.
      ) to regulate cell growth, cell transformation, embryogenesis, and apoptosis (reviewed in Refs.
      • Perillo N.L.
      • Pace K.E.
      • Seilhamer J.J.
      • Baum L.G.
      and
      • Rabinovich G.A.
      • Rubinstein N.
      • Fainboim L.
      ). In accordance with their proposed functions, galectins enhance or inhibit cell-matrix interactions (reviewed in Ref.
      • Hughes R.C.
      ).
      Cellular adhesion to extracellular matrix proteins is mediated by a diverse class of cell surface αβ heterodimeric receptors known as integrins (
      • Hynes R.O.
      ,
      • Giancotti F.G.
      • Ruoslahti E.
      ,
      • Schwartz M.A.
      ). In addition to mediating cell adhesion, integrins induce multiple signal transduction pathways that regulate cytoskeletal rearrangements, cell spreading, migration, differentiation, survival, and cell growth. These processes are associated with activation of a number of signaling elements, (
      • Geiger B.
      • Bershadsky A.
      • Pankov R.
      • Yamada K.M.
      ), most prominent of which is focal adhesion kinase (FAK), which undergoes integrin-stimulated autophosphorylation. Tyr-phosphorylated FAK recruits Grb2-Sos complexes, which activate the Ras-MAPK signaling pathway. FAK also phosphorylates p130Cas, which binds Crk and generates further signals through c-Jun NH2-terminal kinase. P-Tyr397 of FAK serves as a binding site for the SH2 domain of p85α, the regulatory subunit of PI3K that propagates signals to protein kinase B (PKB) and p70 S6 kinase (p70S6K) (reviewed in Refs.
      • Geiger B.
      • Bershadsky A.
      • Pankov R.
      • Yamada K.M.
      and
      • Hood J.D.
      • Cheresh D.A.
      ). Stimulation of integrins also activates the Rho-family GTPases Rho, Rac, and Cdc42, which mediate the formation of stress fibers, lamellipodia, and filopodia, respectively (
      • Ridley A.J.
      ).
      We have recently shown that different cell types adhere and spread when cultured on immobilized galectin-8, a mammalian β-galactoside-binding protein (
      • Levy Y.
      • Arbel-Goren R.
      • Hadari Y.R.
      • Ronen D.
      • Bar-Peled O.
      • Elhanany E.
      • Geiger B.
      • Zick Y.
      ). Galectin-8 (
      • Hadari Y.R.
      • Paz K.
      • Dekel R.
      • Mestrovic T.
      • Accili D.
      • Zick Y.
      ,
      • Hadari Y.R.
      • Eisenstein M.
      • Zakut R.
      • Zick Y.
      ,
      • Hadari Y.R.
      • Goren R.
      • Levy Y.
      • Amsterdam A.
      • Alon R.
      • Zakut R.
      • Zick Y.
      ), a member of the galectin family, is a secreted protein that is widely expressed. It is made of two homologous carbohydrate-recognition domains linked by a short (∼26 amino acids) peptide. Upon secretion, galectin-8 binds to a subset of cell surface integrins, which include integrin α3β1 or α6β1but not α4β1 (
      • Hadari Y.R.
      • Goren R.
      • Levy Y.
      • Amsterdam A.
      • Alon R.
      • Zakut R.
      • Zick Y.
      ). Immobilized galectin-8 is equipotent to fibronectin in promoting cell adhesion and spreading, effects that involve sugar-protein interactions of integrins with galectin-8 (
      • Levy Y.
      • Arbel-Goren R.
      • Hadari Y.R.
      • Ronen D.
      • Bar-Peled O.
      • Elhanany E.
      • Geiger B.
      • Zick Y.
      ). Accordingly, cell adhesion to galectin-8 is potentiated in the presence of Mn2+, whereas adhesion is interrupted in the presence of soluble galectin-8, integrin β1 inhibitory antibodies, EDTA, or thiodigalactoside but not RGD peptides (
      • Levy Y.
      • Arbel-Goren R.
      • Hadari Y.R.
      • Ronen D.
      • Bar-Peled O.
      • Elhanany E.
      • Geiger B.
      • Zick Y.
      ). Whereas immobilized galecin-8 promotes cell adhesion, soluble galectin-8 interacts both with cell surface integrins and other soluble ECM proteins and inhibits cell-matrix interactions (
      • Hadari Y.R.
      • Goren R.
      • Levy Y.
      • Amsterdam A.
      • Alon R.
      • Zakut R.
      • Zick Y.
      ). These observations suggest that galectin-8 is a matrix protein that can positively or negatively modulate cell adhesion (
      • Levy Y.
      • Arbel-Goren R.
      • Hadari Y.R.
      • Ronen D.
      • Bar-Peled O.
      • Elhanany E.
      • Geiger B.
      • Zick Y.
      ).
      In the present study we undertook to characterize the signaling cascades downstream of FAK, which are activated by galectin-8 and confer upon cells adherent to this lectin a unique cytoskeletal organization. Our results indicate that ligation of integrins by galectin-8 is associated with GTP loading onto Ras as well as sustained and potent activation of ERK, PKB, and p70S6K. These downstream effectors of PI3K modulate cell adhesion and spreading on galectin-8 and account for the extensive F-actin-containing microspikes that are formed when cells adhere onto galectin-8. Our findings therefore implicate galectin-8 as an ECM protein that triggers a unique repertoire of integrin-mediated signals, leading to a distinctive cell adhesion, spreading, and cytoskeletal organization.

      DISCUSSION

      In the present study we provide evidence that galectin-8, a mammalian lectin (
      • Levy Y.
      • Arbel-Goren R.
      • Hadari Y.R.
      • Ronen D.
      • Bar-Peled O.
      • Elhanany E.
      • Geiger B.
      • Zick Y.
      ,
      • Hadari Y.R.
      • Goren R.
      • Levy Y.
      • Amsterdam A.
      • Alon R.
      • Zakut R.
      • Zick Y.
      ), functions as an ECM protein that triggers a unique spectrum of signaling events upon ligation of sugar moieties of integrins. Although cell adhesion onto galectin-8 or fibronectin activates to the same extent FAK and p130Cas, the signaling cascades triggered upon adhesion to galectin-8 are characterized by a robust and sustained activation of ERK-1 and -2, which contrasts with the transient nature of ERK activation upon cell adhesion to fibronectin. Similarly, activation of PKB and p70S6K, which serve as downstream effectors of PI3K, is several -folds more intense and sustained when cells adhere to galectin-8 than fibronectin. The unique signaling pattern triggered upon cell adhesion to galectin-8 is associated with faster cell spreading and with a distinctive organization of cytoskeletal elements. Prominent stress fibers that traverse the cell body are readily observed in cells adherent to fibronectin, but they are less abundant in cells adherent to galectin-8. Similarly, formation of focal contacts is limited when cells adhere onto galectin-8 (
      • Levy Y.
      • Arbel-Goren R.
      • Hadari Y.R.
      • Ronen D.
      • Bar-Peled O.
      • Elhanany E.
      • Geiger B.
      • Zick Y.
      ). Instead, adhesion to galectin-8 triggers sustained formation of F-actin microspikes. This is rather a general phenomenon observed in a number of cell lines; still, it is not a ubiquitous phenomenon because certain cell types, such as B16 murine melanoma cells, fail to produce microspikes upon adhesion to galectin-8.
      Inhibitors of PI3K impair cell adhesion and spreading on galectin-8 and the formation of microspikes but have no effects on cells adherent to fibronectin, indicating that downstream effectors of PI3K selectively regulate cytoskeletal rearrangements that occur when cells adhere to and spread on galectin-8. Indeed, overexpression of PKB potentiates the formation of microspikes, whereas overexpression of PKB or p70S6K accentuates the sensitivity of cells adherent to and spread on galectin-8 to inhibitors of PI3K. Hence, the differences in cytoskeletal organization observed when cells adhere to galectin-8 or fibronectin can be attributed to differences in the robustness and duration of the PI3K-mediated signals emitted upon adhesion to the two matrices.
      Several lines of evidence support these conclusions. First, we could demonstrate that cell adhesion onto galectin-8 induces signaling cascades that are being utilized by integrins upon ligation by other ECM proteins. Common signaling elements, activated to about the same extent by galectin-8 and fibronectin, include FAK (
      • Levy Y.
      • Arbel-Goren R.
      • Hadari Y.R.
      • Ronen D.
      • Bar-Peled O.
      • Elhanany E.
      • Geiger B.
      • Zick Y.
      ) and p130Cas (Fig. 3), indicating that upstream elements of integrin signal transduction, such as FAK and p130Cas, are activated irrespective of the mode of ligation and clustering of integrins, which might involve either protein-protein interactions, in the case of fibronectin, or protein-sugar interactions, in the case of galectin-8. Hence, the restricted lateral mobility of integrins at the plane of the membrane upon binding of a bivalent lectin to their extracellular domains is sufficient to induce a conformational change that is conveyed to the cytoplasmic domains of integrins and triggers the recruitment and activation of FAK. However, the overall ligand-induced conformational change of integrins differs, depending on whether integrin clustering is induced upon protein-protein or protein-sugar interactions, because the cytoskeletal organization and the nature of the distal signals emitted downstream of FAK and p130Cas differ when cells adhere onto galectin-8 or fibronectin. Whereas cell adhesion to fibronectin leads to transient activation of MAPK, engagement of integrins by galectin-8 leads to sustained activation of ERK-1 and -2. Similarly, ligation of integrins by galectin-8 results in more robust and sustained activation of PKB and p70S6K. How is a similar extent of activation of FAK and p130Cas by galectin-8 or fibronectin translated into differences in the state of activation of their downstream effectors (Ras, ERK-1,2, PKB, and p70S6K)? One possibility is that a different set of integrins is ligated by galectin-8 or fibronectin. We have already shown that galectin-8 preferentially ligates α3β1 or α6β1, but not α4β1 integrins (
      • Hadari Y.R.
      • Goren R.
      • Levy Y.
      • Amsterdam A.
      • Alon R.
      • Zakut R.
      • Zick Y.
      ), whereas the repertoire of integrins ligated by fibronectin is much broader (
      • Danen E.H.
      • Yamada K.M.
      ). As a result, the composition of signaling complexes formed between the different cytoplasmic tails of integrins and their downstream effectors might differ, depending on whether integrins were clustered by galectin-8 or fibronectin.
      The robustness and duration of the activation of a given signaling pathway has far reaching biological consequences. For example, it is well established that transient activation of the MAPK cascade (e.g. by epidermal growth factor) leads to enhanced growth of PC-12 cells, whereas stimulation of these cells with nerve growth factor induces sustained activation of the MAPK cascade, which leads to cellular differentiation (
      • Qui M.S.
      • Green S.H.
      ). Accordingly, the sustained and robust activation of the MAPK and PI3K signaling pathway upon cell adhesion to galectin-8 might account for the unique cytoskeletal organization and biological functions of cells adherent to this lectin. Activation of ERK was inhibited in the presence of wortmannin, suggesting that PI3K is an upstream regulator of ERK signaling, triggered upon cell adhesion to galectin-8 or fibronectin. This conclusion is in accordance with previous studies that have demonstrated that PI3K may function upstream of Raf-1 but downstream of Ras upon integrin ligation by fibronectin (
      • King W.G.
      • Mattaliano M.D.
      • Chan T.O.
      • Tsichlis P.N.
      • Brugge J.S.
      ). These results further support the role of integrins as ligands for galectin-8 because, unlike integrin signaling, activation of the ERK pathway by other means, such as ligation of growth factor receptors, is most often insensitive to inhibitors of PI3K (
      • Liu Y.F.
      • Paz K.
      • Herschkovitz A.
      • Alt A.
      • Tennenbaum T.
      • Sampson S.R.
      • Ohba M.
      • Kuroki T.
      • LeRoith D.
      • Zick Y.
      ).
      An interesting outcome of the present study is the finding that PKB and p70S6K are actively involved in mediating cell adhesion and spreading on galectin-8. Their positive role is particularly evident in cells overexpressing PKB or p70S6K (CHOPKB and CHOp70S6K cells), whose adhesion and spreading is inhibited ∼30% in the presence of PI3K inhibitors. The role of PKB or p70S6K as positive regulators of cell adhesion is in accordance with the fact that ligation of growth factor receptors, which stimulates the activity of PKB or p70S6K (
      • Belham C.
      • Wu S.
      • Avruch J.
      ), potentiates the adhesive process in a number of cell types (
      • Eliceiri B.P.
      • Cheresh D.A.
      ). At present, the signaling pathways regulating cell adhesion and spreading are not fully understood. Still, our results suggest that inhibition of the PI3K activity, induced by wortmannin, does not directly affect the active conformation of integrins because wortmannin does not inhibit cell adhesion to fibronectin. Instead, our results suggest that protein substrates for PKB or p70S6K are phosphorylated to a higher extent in cells adherent to galectin-8, in particular CHOPKB and CHOp70S6K cells, and this enables them to replace other signaling molecules that promote cell adhesion and spreading on galectin-8. The displacement of the native signaling elements with downstream effectors of PI3K is an irreversible process, because inhibition of PI3K activity does not enable the original participants to resume their positions within the integrin signaling complex and to turn the adhesive process less sensitive to PI3K inhibitors, as in non-transfected CHO cells. PKB has already been implicated as a mediator of cell adhesion (
      • Chou M.M.
      • Blenis J.
      ), but the role of p70S6K in this process is less obvious. There is little evidence that p70S6K is required for the processes of cell adhesion, and activation of p70S6K was shown to be independent of pathways that regulate formation of focal adhesions (
      • Malik R.K.
      • Parsons J.T.
      ). Our results suggest that p70S6K under certain conditions might modulate integrin activation by selective ECM proteins such as galectin-8, although the direct targets of this kinase within cell adhesion complexes remain to be determined. Our findings further suggest that cells that overexpress specific signaling elements, such as downstream effectors of PI3K, are bound to utilize these new elements not only for the promotion of cellular growth but also for remodeling of their “inside out” signaling elements and integrins function.
      An immediate consequence of cell adhesion is cell spreading. In the present study we provide evidence that cells spread much faster on galectin-8, compared with fibronectin. Interestingly, although the variance of areas of cells adherent to fibronectin at early time points is rather limited, a much wider variance characterizes cells adherent to galectin-8. The reason for this variability is presently unknown, but it cannot be attributed to variable induction of protein synthesis because inclusion of inhibitors of protein synthesis did not alter the rate or phenotype of cells adherent to galectin-8. In contrast, cellular attachment to galectin-8 is inhibited ∼50%, whereas spreading hardly takes place at 4 °C, indicating that energy-consuming processes and active protein trafficking are required to mediate these events. In that respect, cell adhesion to galectin-8 resembles cell adhesion to other ECM proteins, which is largely inhibited at low temperatures (compare Ref.
      • Adler R.
      • Jerdan J.
      • Hewitt A.T.
      ).
      Cellular attachment and spreading on fibronectin involves an initial requirement for Cdc42 in the formation of filopodial protrusions and the subsequent involvement of both Cdc42 and Rac during cell spreading and organization of the actin cytoskeleton (
      • Price L.S.
      • Leng J.
      • Schwartz M.A.
      • Bokoch G.M.
      ). In galectin-8-adherent cells, focal contacts poorly assemble (
      • Levy Y.
      • Arbel-Goren R.
      • Hadari Y.R.
      • Ronen D.
      • Bar-Peled O.
      • Elhanany E.
      • Geiger B.
      • Zick Y.
      ) and microspikes containing F-actin are formed instead. These structures have been functionally implicated in cell migration (
      • Adams J.C.
      • Schwartz M.A.
      ), which is readily induced by galectin-8 (
      • Levy Y.
      • Arbel-Goren R.
      • Hadari Y.R.
      • Ronen D.
      • Bar-Peled O.
      • Elhanany E.
      • Geiger B.
      • Zick Y.
      ). Microspikes are readily formed when cells adhere to a variety of other ECM proteins such as thrombospondin-I (
      • Adams J.C.
      • Schwartz M.A.
      ), laminin-5 (
      • Kawano K.
      • Kantak S.S.
      • Murai M.
      • Yao C.C.
      • Kramer R.H.
      ), and Tenascin-C splice variants (
      • Fischer D.
      • Tucker R.P.
      • Chiquet E.R.
      • Adams J.C.
      ). On fibronectin, Cdc42- and Rac-dependent formation of microspikes is involved in early steps of cell adhesion, but these events are transient and microspikes are rapidly replaced by focal contacts (
      • Adams J.C.
      • Schwartz M.A.
      ). In contrast, microspikes are stabilized when cells adhere to galectin-8, and the cells do not proceed to form highly developed focal contacts (
      • Levy Y.
      • Arbel-Goren R.
      • Hadari Y.R.
      • Ronen D.
      • Bar-Peled O.
      • Elhanany E.
      • Geiger B.
      • Zick Y.
      ). Whereas Cdc42 leads to the formation of elongated projections containing F-actin, Rac leads to the formation of ribbons of short spikes (
      • Adams J.C.
      • Schwartz M.A.
      ). The microspikes formed when cells adhere to galectin-8 are short and radial and in that respect resemble microspikes formed when C2C12 cells, overexpressing a constitutively active Rac, adhere onto thrombospondin-I (
      • Adams J.C.
      ). We can therefore suggest that formation of microspikes upon cell adhesion to galectin-8 presumably involves Rac activation. Still, additional signaling elements, induced by galectin-8, are likely to be involved. Potential candidates are elements of the PI3K/PKB pathway, which are activated to a much greater extent by galectin-8 than fibronectin. In accordance with this idea, addition of wortmannin, a potent inhibitor of PI3K, effectively inhibits PKB activity and formation of microspikes when cells adhere onto galectin-8. Furthermore, overexpression of PKB promotes formation of microspikes in cells adherent onto galectin-8. The possible involvement of PI3K and its downstream effectors in galectin-8-mediated formation of microspikes is supported by recent findings implicating the signaling pathway from the insulin-like growth factor-I receptor through PI3K in the rapid organization of microspikes at cell-cell junctions (
      • Guvakova M.A.
      • Boettiger D.
      • Adams J.C.
      ,
      • Guvakova M.A.
      • Adams J.C.
      • Boettiger D.
      ). PKB, the downstream effector of PI3K, activates a number of kinases, including the p21-activated kinase (PAK) (
      • Manser E.
      • Leung T.
      • Salihuddin H.
      • Zhao Z.S.
      • Lim L.
      ) that has been implicated as playing a role in actin organization. PAK inhibits the activity of coffilin (reviewed in Refs.
      • Bar-Sagi D.
      • Hall A.
      and
      • Ridley A.J.
      ) and in such a way may inhibit actin depolymerization and promote formation of F-actin microspikes induced by galectin-8.
      Finally, it should be noted that prostate carcinoma tumor antigen-1, the human isoform of galectin-8, is highly expressed in certain forms of prostate carcinomas (
      • Su Z.-Z.
      • Lin J.
      • Shen R.
      • Fisher P.E.
      • Goldstein N.I.
      • Fisher P.B.
      ) and other tumors (
      • Camby I.
      • Belot N.
      • Rorive S.
      • Lefranc F.
      • Maurage C.A.
      • Lahm H.
      • Kaltner H.
      • Hadari Y.
      • Ruchoux M.M.
      • Brotchi J.
      • Zick Y.
      • Salmon I.
      • Gabius H.J.
      • Kiss R.
      ). In contrast, galectin-8 expression decreases in human colon cancer when compared with normal and dysplastic colon tissues (
      • Nagy N.
      • Bronckart Y.
      • Camby I.
      • Legendre H.
      • Lahm H.
      • Kaltner H.
      • Hadari Y.
      • Van H.P.
      • Yeaton P.
      • Pector J.C.
      • Zick Y.
      • Salmon I.
      • Danguy A.
      • Kiss R.
      • Gabius H.J.
      ). This is associated with reduced migration of the colon cancer cells on immobilized galectin-8. Because interactions of soluble galectin-8 with cell surface integrins inhibit cell adhesion (
      • Hadari Y.R.
      • Goren R.
      • Levy Y.
      • Amsterdam A.
      • Alon R.
      • Zakut R.
      • Zick Y.
      ), whereas immobilized galectin-8 has the potential to promote cell attachment and spreading (
      • Levy Y.
      • Arbel-Goren R.
      • Hadari Y.R.
      • Ronen D.
      • Bar-Peled O.
      • Elhanany E.
      • Geiger B.
      • Zick Y.
      ), galectin-8 may modulate cell-matrix interactions under a variety of physiological and pathological conditions, depending on the repertoire, duration, and robustness of signals emitted when cells interact with this lectin.

      Acknowledgments

      We thank Drs. Ronit Sagi-Eisenberg and Benjamin Geiger for helpful discussions and a critical review of this manuscript.

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