The lymphoma transmembrane glycoprotein GP85 (CD44) is a novel guanine nucleotide-binding protein which regulates GP85 (CD44)-ankyrin interaction

Epitope Mapping of the Anti-CD44 Monoclonal Antibody (C44Mab-46) Using the REMAP Method

CD44 functions as a major hyaluronan receptor on most cell types, with roles in cell adhesion, migration, proliferation, differentiation, and survival. The CD44 gene comprises 20 exons, with alternative splicing producing many different isoforms. CD44 variant isoforms exhibit tissue-specific expression patterns and have been studied as therapeutic targets for several cancers; therefore, anti-CD44 monoclonal antibodies (mAbs) are useful for investigating CD44 expression in various cancers. Previously, we established an anti-CD44 mAb, C₄₄Mab-46 (IgG₁, κ), by immunizing mice with the CD44v3-10 ectodomain. Although C₄₄Mab-46 recognized all CD44 isoforms, the binding epitope of C₄₄Mab-46 has not been determined. In this study, we first checked the reactivity of C₄₄Mab-46 to several CD44v3-10 deletion mutants such as dN79, dN124, dN147, and dN224. We found the N-terminus of the C₄₄Mab-46-binding epitope between residues 147 and 224 of CD44v3-10. We next investigated this epitope using a novel mapping system: RIEDL insertion for epitope mapping (REMAP) method. We constructed 31 CD44 standard (CD44s) mutants where the RIEDL tag was inserted into the expected epitope region in CD44s. We observed that the C₄₄Mab-46 epitope constituted five amino acids: ₁₇₄-TDDDV-₁₇₈ of CD44s. Thus, the REMAP method could be used to determine mAb binding epitopes for membrane proteins.

Enforced hematopoietic cell E- and L-selectin ligand (HCELL) expression primes transendothelial migration of human mesenchymal stem cells

According to the multistep model of cell migration, chemokine receptor engagement (step 2) triggers conversion of rolling interactions (step 1) into firm adhesion (step 3), yielding transendothelial migration. We recently reported that glycosyltransferase-programmed stereosubstitution (GPS) of CD44 on human mesenchymal stem cells (hMSCs) creates the E-selectin ligand HCELL (hematopoietic cell E-selectin/L-selectin ligand) and, despite absence of CXCR4, systemically administered HCELL(+)hMSCs display robust osteotropism visualized by intravital microscopy. Here we performed studies to define the molecular effectors of this process. We observed that engagement of hMSC HCELL with E-selectin triggers VLA-4 adhesiveness, resulting in shear-resistant adhesion to ligand VCAM-1. This VLA-4 activation is mediated via a Rac1/Rap1 GTPase signaling pathway, resulting in transendothelial migration on stimulated human umbilical vein endothelial cells without chemokine input. These findings indicate that hMSCs coordinately integrate CD44 ligation and integrin activation, circumventing chemokine-mediated signaling, yielding a step 2-bypass pathway of the canonical multistep paradigm of cell migration.

Stable alterations of CD44 isoform expression in prostate cancer cells decrease invasion and growth and alter ligand binding and chemosensitivity

Background

Dysregulated CD44 expression characterizes most human cancers, including prostate cancer (PCa). PCa loses expression of CD44 standard (CD44s) that is present in benign epithelium, and overexpresses the novel splice variant isoform, CD44v7-10.

Methods

Using retroviral gene delivery to PC-3M PCa cells, we expressed luciferase-only, enforced CD44s re-expression as a fusion protein with luciferase at its C-terminus or as a protein separate from luciferase, or knocked down CD44v7-10 by RNAi. Invasion, migration, proliferation, soft agar colony formation, adhesion, Docetaxel sensitivity, and xenograft growth assays were carried out. Expression responses of merlin, a CD44 binding partner, and growth-permissive phospho-merlin, were assessed by western blot.

Results

Compared to luciferase-only PC-3M cells, all three treatments reduced invasion and migration. Growth and soft agar colony formation were reduced only by re-expression of CD44s as a separate or fusion protein but not CD44v7-10 RNAi. Hyaluronan and osteopontin binding were greatly strengthened by CD44s expression as a separate protein, but not a fusion protein. CD44v7-10 RNAi in PC-3M cells caused marked sensitization to Docetaxel; the two CD44s re-expression approaches caused minimal sensitization. In limited numbers of mouse subcutaneous xenografts, all three alterations produced only nonsignificant trends toward slower growth compared with luciferase-only controls. The expression of CD44s as a separate protein, but not a fusion protein, caused emergence of a strongly-expressed, hypophosphorylated species of phospho-merlin.

Conclusion

Stable re-expression of CD44s reduces PCa growth and invasion in vitro, and possibly in vivo, suggesting CD44 alterations have potential as gene therapy. When the C-terminus of CD44s is fused to another protein, most phenotypic effects are lessened, particularly hyaluronan adhesion. Finally, CD44v7-10, although it was not functionally significant for growth, may be a target for chemosensitization.

Glycosyltransferase-programmed stereosubstitution (GPS) to create HCELL: engineering a roadmap for cell migration

During evolution of the vertebrate cardiovascular system, the vast endothelial surface area associated with branching vascular networks mandated the development of molecular processes to efficiently and specifically recruit circulating sentinel host defense cells and tissue repair cells at localized sites of inflammation/tissue injury. The forces engendered by high-velocity blood flow commensurately required the evolution of specialized cell surface molecules capable of mediating shear-resistant endothelial adhesive interactions, thus literally capturing relevant cells from the blood stream onto the target endothelial surface and permitting subsequent extravasation. The principal effectors of these shear-resistant binding interactions comprise a family of C-type lectins known as 'selectins' that bind discrete sialofucosylated glycans on their respective ligands. This review explains the 'intelligent design' of requisite reagents to convert native CD44 into the sialofucosylated glycoform known as hematopoietic cell E-/L-selectin ligand (HCELL), the most potent E-selectin counter-receptor expressed on human cells, and will describe how ex vivo glycan engineering of HCELL expression may open the 'avenues' for the efficient vascular delivery of cells for a variety of cell therapies.

The CD44 standard/ezrin complex regulates Fas-mediated apoptosis in Jurkat cells

The transmembrane receptor CD44 conveys important signals from the extracellular microenvironment to the cytoplasm, a phenomena known as "outside-in" signaling. CD44 exists as several isoforms that result from alternative splicing, which differ only in the extracellular domain but yet exhibit different activities. CD44 is a binding partner for the membrane-cytoskeleton cross-linker protein ezrin. In this study, we demonstrate that only CD44 standard (CD44s) colocalizes and interacts with the actin cross-linkers ezrin and moesin using well-characterized cell lines engineered to express different CD44 isoforms. Importantly, we also show that the association CD44s-ezrin-actin is an important modulator of Fas-mediated apoptosis. The results highlight a mechanism by which signals from the extracellular milieu regulate intracellular signaling activities involved in programmed cell death.

Hyaluronan-CD44 interaction with IQGAP1 promotes Cdc42 and ERK signaling, leading to actin binding, Elk-1/estrogen receptor transcriptional activation, and ovarian cancer progression

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.

The effect of anti-CD44 monoclonal antibodies on differentiation and proliferation of human acute myeloid leukemia cells

Acute myeloid leukemia (AML) is a clonal malignant disease characterized by an increasing number of immature myeloid cells arrested at various stages of granulocytic and monocytic differentiation. The stage of the blockage defines distinct AML subtypes (AML1 to AML5 are the most frequent ones). There is increasing evidence that the malignant clone is maintained by rare AML stem cells endowed with self-renewal capacity, which through extensive proliferation coupled to partial differentiation, generate leukemic progenitors and blasts, of which the vast majority have limited proliferative capacity. Contrarily to chemotherapy alone, which is still unable to cure most AML patients, the differentiation therapy, which consists in releasing the differentiation blockage of leukemic blasts, has succeeded, when it is combined with chemotherapy, to greatly improve the survival of AML3 patients, using retinoic acid as differentiating agent. However, this molecule is ineffective in other AML subtypes, which are the most frequent. We have shown that specific monoclonal antibodies (mAbs, H90 and A3D8) directed to the CD44 cell surface antigen, that is strongly expressed on human AML blasts, are capable of triggering terminal differentiation of leukemic blasts in AML1 to AML5 subtypes. These results have raised the perspective of developing a CD44-targeted differentiation therapy in most AML cases. Interestingly, these anti-CD44 mAbs can also induce the differentiation of AML cell lines, inhibit their proliferation and, in some cases, induce their apoptotic death. These results suggest that H90 and/or A3D8 mAbs may be capable to inhibit the proliferation of leukemic progenitors, to promote the differentiation of the leukemic stem cells at the expense of their self-renewal, and, perhaps, to induce their apoptotic death, thereby contributing to decrease the size of the leukemic clone. The challenges of an anti-CD44 based differentiation therapy in AML, and its importance in relation to the new other therapies developed in this malignancy, are discussed in this review.

The role of the CD44/ezrin complex in cancer metastasis

CD44 is a cell adhesion molecule that was traditionally known as 'homing receptor'. This molecule is known to interact with the ezrin family (ERM family) members and form a complex that plays diverse roles within both normal and abnormal cells, particularly cancer cells. CD44 and ezrin and their respective complex have properties suggesting that they may be important in the process of tumour-endothelium interactions, cell migrations, cell adhesion, tumour progression and metastasis. This article reviews the role of CD44, ezrin family and the CD44/ezrin complex in cancer cells and their clinical impact in patients with cancer.

CD44: from adhesion molecules to signalling regulators

Cell-adhesion molecules, once believed to function primarily in tethering cells to extracellular ligands, are now recognized as having broader functions in cellular signalling cascades. The CD44 transmembrane glycoprotein family adds new aspects to these roles by participating in signal-transduction processes--not only by establishing specific transmembrane complexes, but also by organizing signalling cascades through association with the actin cytoskeleton. CD44 and its associated partner proteins monitor changes in the extracellular matrix that influence cell growth, survival and differentiation.

Hyaluronan promotes CD44v3-Vav2 interaction with Grb2-p185(HER2) and induces Rac1 and Ras signaling during ovarian tumor cell migration and growth

In this study we initially examined the interaction between CD44v3 (a hyaluronan (HA) receptor) and Vav2 (a guanine nucleotide exchange factor) in human ovarian tumor cells (SK-OV-3.ipl cell line). Immunological data indicate that both CD44v3 and Vav2 are expressed in SK-OV-3.ipl cells and that these two proteins are physically linked as a complex in vivo. By using recombinant fragments of Vav2 and in vitro binding assays, we have detected a specific binding interaction between the SH3-SH2-SH3 domain of Vav2 and the cytoplasmic domain of CD44. In addition, we have observed that the binding of HA to CD44v3 activates Vav2-mediated Rac1 signaling leading to ovarian tumor cell migration. Further analyses indicate that the adaptor molecule, growth factor receptor-bound protein 2 (Grb2) that is bound to p185(HER2) (an oncogene product), is also associated with the CD44v3-Vav2 complex. HA binding to SK-OV-3.ipl cells promotes recruitment of both Grb2 and p185(HER2) to the CD44v3-Vav2 complex leading to Ras activation and ovarian tumor cell growth. In order to determine the role of Grb2 in CD44v3 signaling, we have transfected SK-OV-3.ipl cells with Grb2 mutant cDNAs (e.g. Delta N-Grb2 that has a deletion in the amino-terminal SH3 domain or Delta C-Grb2 that has a deletion in the carboxyl-terminal SH3 domain). Our results clearly indicate that the SH3 domain deletion mutants of Grb2 (i.e. the Delta N-Grb2 (and to a lesser extent the Delta C-Grb2) mutant) not only block their association with p185(HER2) but also significantly impair their binding to the CD44v3-Vav2 complex and inhibit HA/CD44v3-induced ovarian tumor cell behaviors. Taken together, these findings strongly suggest that the interaction of CD44v3-Vav2 with Grb2-p185(HER2) plays an important role in the co-activation of both Rac1 and Ras signaling that is required for HA-mediated human ovarian tumor progression.

CD44 interaction with c-Src kinase promotes cortactin-mediated cytoskeleton function and hyaluronic acid-dependent ovarian tumor cell migration

In this study we have demonstrated that both CD44 (the hyaluronan (HA) receptor) and c-Src kinase are expressed in human ovarian tumor cells (SK-OV-3.ipl cell line), and that these two proteins are physically associated as a complex in vivo. Using a recombinant cytoplasmic domain of CD44 and an in vitro binding assay, we have detected a specific interaction between CD44 and c-Src kinase. Furthermore, the binding of HA to SK-OV-3.ipl cells promotes c-Src kinase recruitment to CD44 and stimulates c-Src kinase activity, which, in turn, increases tyrosine phosphorylation of the cytoskeletal protein, cortactin. Subsequently, tyrosine phosphorylation of cortactin attenuates its ability to cross-link filamentous actin in vitro. In addition, transfection of SK-OV-3.ipl cells with a dominant active form of c-Src (Y527F)cDNA promotes CD44 and c-Src association with cortactin in membrane projections, and stimulates HA-dependent/CD44-specific ovarian tumor cell migration. Finally, overexpression of a dominant-negative mutant of c-Src kinase (K295R) in SK-OV-3.ipl cells impairs the tumor cell-specific phenotype. Taken together, these findings strongly suggest that CD44 interaction with c-Src kinase plays a pivotal role in initiating cortactin-regulated cytoskeleton function and HA-dependent tumor cell migration, which may be required for human ovarian cancer progression.

Controlling duration of contact between T cells and antigen-presenting cells

A new method which allows precise control of the duration of contact between T cells and antigen-presenting cells (APCs) has been developed. A glass coverslip coated with poly-L-lysine, and then with T cells, was placed at the base of a cylindrical well, and the well was filled with liquid medium. A round coverslip, on which APCs were adhered, was supported on the surface of the medium by surface tension, cell-side down. By withdrawing medium from four capillary holes near the base of the well, the coverslip could be lowered to initiate contact between T cells and APCs at a defined time zero. The contact was broken at desired time points by re-introducing medium into the well in order to separate the two coverslips. Each cell type remained adherent to its original surface after separation for all contact times studied. The T cells were monitored for intracellular calcium mobilization using the fluorescent dye, Fura-2. Contact durations of less than 1 min did not trigger calcium signals. Contact durations of 3 and 5 min induced strong calcium signals. Breaking the contact caused a rapid decrease in intracellular calcium levels. This method of cell manipulation allows precise control of the duration of contact of T cells with APCs, while keeping the cells under continuous observation. The measurements so obtained provide a quantitative understanding of the dynamics of early T cell activation.

Homing-associated cell adhesion molecule (H-CAM/CD44) on human CD34+ hematopoietic progenitor cells

Human CD34+ hematopoietic progenitor cells (HPCs) express CD44 and can directly adhere to hyaluronate (HA) via CD44. Furthermore, CD44 may also be involved in the regulation of CD34+ HPC proliferation and development. The expression of CD44 molecules on CD34+ hematopoietic progenitor cells is significantly lower on bone marrow (BM) CD34+ cells compared with circulating CD34+ cells in cord blood and peripheral blood. Myeloid and erythroid progenitor cells are found predominantly in CD34+ CD44+ cell fractions. More interestingly, CD34+ CD44- cells expressing B-lymphocyte-associated CD10 and CD19 would represent unique B-lymphocyte committed precursors in the BM, which might undergo apoptotic cell death in the early steps of B-cell differentiation.

CD44v10 in hematopoiesis and stem cell mobilization

Transplantation of hematopoietic progenitor cells provides in many instances of malignant tumors an ultimate chance of curative therapy, whereby the transfer of peripheral blood stem cells (PBSC) may even be advantageous as compared to bone marrow cells. Yet, the transfer of PBSC requires mobilization of stem cells into the periphery, which is mostly achieved via hematopoietic growth factors like G-CSF. Although G-CSF has been found to efficiently mobilize stem cells in most instances, some patients do not or insufficiently respond to G-CSF treatment In addition, G-CSF treatment may by accompanied by maturation of the most primitive progenitors and this may have an impact on stem cell homing and recovery of hemopoiesis. Therefore, additional approaches for stem cell mobilization have been searched for, in particular mobilization via a blockade of an adhesion molecule expressed by CD34-positive cells, like VLA-4 (CD49d) and the hematopoietic isoform of CD44 (CD44s). We recently described that in the mouse one of the CD44 variant isoforms, CD44v10, is expressed on a subpopulation of bone marrow cells, whereas a CD44v10 receptor-globulin only binds to stromal elements. These features appeared promising for anti-CD44v10 as a means of stem cell mobilization. Indeed, treatment with anti-CD44v10 revealed promising results concerning the recovery of multilineage colony forming units in the spleen and the peripheral blood. We here summarize features of expression and function of CD44 in hematopoiesis an provide further evidence for anti-CD44v10 as a means to mobilize hematopoietic progenitor cells.

Ankyrin-Tiam1 interaction promotes Rac1 signaling and metastatic breast tumor cell invasion and migration

Tiam1 (T-lymphoma invasion and metastasis 1) is one of the known guanine nucleotide (GDP/GTP) exchange factors (GEFs) for Rho GTPases (e.g., Rac1) and is expressed in breast tumor cells (e.g., SP-1 cell line). Immunoprecipitation and immunoblot analyses indicate that Tiam1 and the cytoskeletal protein, ankyrin, are physically associated as a complex in vivo. In particular, the ankyrin repeat domain (ARD) of ankyrin is responsible for Tiam1 binding. Biochemical studies and deletion mutation analyses indicate that the 11-amino acid sequence between amino acids 717 and 727 of Tiam1 ((717)GEGTDAVKRS(727)L) is the ankyrin-binding domain. Most importantly, ankyrin binding to Tiam1 activates GDP/GTP exchange on Rho GTPases (e.g., Rac1). Using an Escherichia coli-derived calmodulin-binding peptide (CBP)-tagged recombinant Tiam1 (amino acids 393-728) fragment that contains the ankyrin-binding domain, we have detected a specific binding interaction between the Tiam1 (amino acids 393-738) fragment and ankyrin in vitro. This Tiam1 fragment also acts as a potent competitive inhibitor for Tiam1 binding to ankyrin. Transfection of SP-1 cell with Tiam1 cDNAs stimulates all of the following: (1) Tiam1-ankyrin association in the membrane projection; (2) Rac1 activation; and (3) breast tumor cell invasion and migration. Cotransfection of SP1 cells with green fluorescent protein (GFP)-tagged Tiam1 fragment cDNA and Tiam1 cDNA effectively blocks Tiam1-ankyrin colocalization in the cell membrane, and inhibits GDP/GTP exchange on Rac1 by ankyrin-associated Tiam1 and tumor-specific phenotypes. These findings suggest that ankyrin-Tiam1 interaction plays a pivotal role in regulating Rac1 signaling and cytoskeleton function required for oncogenic signaling and metastatic breast tumor cell progression.

CD44v6 cell surface expression is a common feature of macrophages and macrophage-like cells - implication for a natural macrophage extravasation mechanism mimicked by tumor cells

Soluble CD44standard (sCD44s) and CD44v6 (sCD44v6) cannot only be detected in sera of patients with pancreatic carcinoma but also of healthy blood donors. To investigate whether sCD44s and sCD44v6 are derived from white blood cells, we stimulated whole blood with phytohemagglutinin and interleukin-2, which induced expression of CD44v6 only on monocytes. For further investigations, we used the promyelocytic leukemia cell line Hl-60. Only Hl-60 cells differentiating along the macrophage pathway showed increased expression of CD44s and CD44v6. Furthermore, only macrophages showed increased secretion of sCD44s and sCD44v6. Our data suggest that CD44s and CD44v6 are common adhesion molecules on macrophages and macrophage-like cells.

Intracellular osteopontin is an integral component of the CD44-ERM complex involved in cell migration

Osteopontin (OPN) is a secreted glycoprotein with mineral- and cell-binding properties that can regulate cell activities through integrin receptors. Previously, we identified an intracellular form of osteopontin with a perimembranous distribution in migrating fetal fibroblasts (Zohar et al., J Cell Physiol 170:88-98, 1997). Since OPN and CD44 expression are increased in migrating cells, we analyzed the relationship of these proteins with immunofluorescence and confocal microscopy. A distinct co-localization of perimembranous OPN and cell-surface CD44 was observed in fetal fibroblasts, periodontal ligament cells, activated macrophages, and metastatic breast cancer cells. The co-localization of OPN and CD44 was prominent at the leading edge of migrating fibroblasts, where OPN also co-localized with the ezrin/radixin/moesin (ERM) protein ezrin, as well as in cell processes and at attachment sites of hyaluronan-coated beads. The subcortical location of OPN in these cells was verified by cell-surface biotinylation experiments in which biotinylated CD44 and non-biotinylated OPN were isolated from complexes formed with hyaluronan-coated beads and identified with immunoblotting. That perimembranous OPN represents secreted protein internalized by endocytosis or phagocytosis appeared to be unlikely since exogenous OPN that was added to cell cultures could not be detected inside the cells. A physical association with OPN, CD44, and ERM, but not with vinculin or alpha-actin, was indicated by immunoadsorption and immunoblotting of cell proteins in complexes extracted from hyaluronan-coated beads. The functional significance of OPN in this complex was demonstrated using OPN-/- and CD-/- mouse fibroblasts which displayed impaired migration and a reduced attachment to hyaluronan-coated beads. These studies indicate that OPN exists as an integral component of a hyaluronan-CD44-ERM attachment complex that is involved in the migration of embryonic fibroblasts, activated macrophages, and metastatic cells.

The ankyrin-binding domain of CD44s is involved in regulating hyaluronic acid-mediated functions and prostate tumor cell transformation

CD44 isoforms, such as CD44s (the standard form), contain at least one ankyrin-binding site within the 70-amino acid (aa) cytoplasmic domain and several hyaluronic acid (HA)-binding sites within the extracellular domain. To study the role of CD44s-ankyrin interaction in regulating human prostate tumor cells, we have constructed several CD44s cytoplasmic deletion mutants that lack the ankyrin-binding site(s). These truncated cDNAs were stably transfected into CD44-negative human prostate tumor cells (LNCaP). Our results indicate that a critical region of 15-amino acids (aa) between aa 304 and aa 318 of CD44s is required for ankyrin binding. Biochemical analyses, using competition binding assays with a synthetic peptide containing the 15 aa between aa 304 and aa 318 (NSGNGAVEDRKPSGL), further support the conclusion that this region contains the ankyrin-binding domain of CD44s. Deletion of this 15-aa ankyrin-binding sequence from CD44s results in a drastic reduction of HA-mediated binding/cell adhesion, Src p60 kinase(s) interaction and anchorage-independent growth in soft agar. These findings suggest that the binding of cytoskeletal proteins, such as ankyrin, to the cytoplasmic domain of CD44s plays a pivotal role in regulating HA-mediated functions as well as Src kinase activity and prostate tumor cell transformation.

Interaction between CD44 and the repeat domain of ankyrin promotes hyaluronic acid-mediated ovarian tumor cell migration

The adhesion molecule, CD44, interacts with ankyrin within its cytoplasmic domain and binds to hyaluronic acid (HA) at its extracellular domain. In this study, we focused on the functional domain in ankyrin (in particular, the ankyrin repeat domain [ARD]) responsible for CD44 binding and its role in regulating HA-mediated ovarian tumor cell function. Using recombinant fragments of ankyrin (e.g., ARD and subdomain 1 [S1, aa1-aa217], subdomain 2 [S2, aa218-aa381], subdomain 3 [S3, aa382-aa612], and subdomain 4 [S4, aa613-aa834]) and in vitro binding assays, we determined that the S2 but not S1, S3, or S4 of ARD is the primary ankyrin binding region for CD44. Microinjection of antiglutathione S-transferase (GST)-tagged S2 or GST-tagged ARD fusion protein into CD44-positive ovarian tumor cells (e.g., SKOV3 cell line) promotes ankyrin association with CD44 in plaque-like structures and membrane projections. Additionally, we demonstrated that transfection of SKOV3 cells with S2cDNA or ARD cDNA results in an upregulation of HA-mediated tumor cell migration. Taken together, we believe that the S2 of the ARD plays a pivotal role in the direct binding to CD44 and promotes the cytoskeleton activation required for HA-mediated function such as ovarian tumor cell migration.

Rho-kinase (ROK) promotes CD44v(3,8-10)-ankyrin interaction and tumor cell migration in metastatic breast cancer cells

Metastatic breast tumor Met-1 cells express CD44v(3,8-10), a major adhesion receptor that binds extracellular matrix components at its extracellular domain and interacts with the cytoskeletal protein, ankyrin, at its cytoplasmic domain. In this study, we have determined that CD44v(3,8-10) and RhoA GTPases are physically associated in vivo, and that CD44v(3,8-10)-bound RhoA displays GTPase activity, which can be inhibited by botulinum toxin C3-mediated ADP-ribosylation. In addition, we have identified a 160 kDa Rho-Kinase (ROK) as one of the downstream targets for CD44v(3,8-10)-bound RhoA GTPase. Specifically, RhoA (complexed with CD44v(3, 8-10)) stimulates ROK-mediated phosphorylation of certain cellular proteins including the cytoplasmic domain of CD44v(3,8-10). Most importantly, phosphorylation of CD44v(3,8-10) by ROK enhances its interaction with the cytoskeletal protein, ankyrin. We have also constructed two ROK cDNA constructs that encode for proteins consisting of 537 amino acids [designated as the constitutively active form of ROK containing the catalytic domain (CAT, also the kinase domain)], and 173 amino acids [designated as the dominant-negative form of ROK containing the Rho-binding domain (RB)]. Microinjection of the ROK's CAT domain into Met-1 cells promotes CD44-ankyrin associated membrane ruffling and projections. This membrane motility can be blocked by CD44 antibodies and cytochalasin D (a microfilament inhibitor). Furthermore, overexpression of a dominant-negative form of ROK by transfection of Met-1 cells with ROK's Rho-binding (RB) domain cDNA effectively inhibits CD44-ankyrin-mediated metastatic behavior (e.g., membrane motility and tumor cell migration). These findings support the hypothesis that ROK plays a pivotal role in CD44v(3,8-10)-ankyrin interaction and RhoA-mediated oncogenic signaling required for membrane-cytoskeleton function and metastatic tumor cell migration.

CD44 and adhesion of normal and leukemic CD34+ cells to bone marrow stroma

CD44 has long been implicated in the interaction between hematopoietic progenitors and bone marrow stroma. More recently it has become apparent that CD44 antibodies cannot only inhibit CD44 mediated adhesion to hyaluronic acid and cellular ligands but can stimulate adhesion to these ligands. The mechanism involved in CD44 antibody stimulated adhesion to cellular layers is still not known. While adhesion of T cells to keratinocytes is integrin mediated it appears that adhesion of hematopoietic progenitors to bone marrow stromal layers is the result of an antibody induced conformational change in the CD44 molecule similar to that seem for the augmentation of hyaluronic acid binding by some CD44 antibodies. The ligand for CD44 involved in this binding has not been identified but it does not appear to be hyaluronic acid.

CD44v(3,8-10) is involved in cytoskeleton-mediated tumor cell migration and matrix metalloproteinase (MMP-9) association in metastatic breast cancer cells

In the present study, we have employed a unique breast cancer cell line (Met-1, which was derived from a high metastatic potential tumor in transgenic mice expressing polyomavirus middle T oncogene) to study the role of CD44 variant isoform(s) in the regulation of metastatic breast tumor cell behavior. The results of reverse transcriptase-polymerase chain reaction, Southern blot, nucleotide sequencing, immunoprecipitation, and immunoblot analyses indicated that these cells express a major CD44 isoform (molecular weight approximately 260 kDa) containing a v3,8-10 exon insertion (designated as CD44v3,8-10). In addition, we have determined that CD44v3,8-10 binds specifically to the cytoskeletal proteins such as ankyrin. Biochemical analyses, using competition binding assays and a synthetic peptide identical to NGGNGTVEDRKPSEL (a sequence located between aa480 and aa494 of CD44v3,8-10) indicate that this 15-amino acid peptide binds specifically to the cytoskeletal protein ankyrin (but not to fodrin or spectrin). This peptide competes effectively for ankyrin binding to CD44v3,8-10. Therefore, we believe that the sequence 480NGGNGTVEDRKPSE494L, located at the cytoplasmic domain of CD44v3,8-10, is required for the ankyrin binding. We have also detected that CD44v3,8-10-containing Met-1 cells are capable of forming membrane spikes or "invadopodia" structures and undergo active migration processes. Treatments of Met-1 cells with certain agents including anti-CD44v3 antibody, cytochalasin D (a microfilament inhibitor), and W-7 (a calmodulin antagonist), but not colchicine (a microtubule disrupting agent) effectively inhibit "invadopodia" formation and subsequent tumor cell migration. Further analyses using zymography assays and double immunofluorescence staining indicated that CD44v3,8-10 is closely associated with the active form of matrix metalloproteinase, MMP-9, in a complex within "invadopodia" structures. These findings suggest that CD44v3,8-10 plays an important role in linking ankyrin to the membrane-associated actomyosin contractile system required for "invadopodia" formation (coupled with matrix degradation activities) and tumor cell migration during breast cancer progression.

Interaction between the adhesion receptor, CD44, and the oncogene product, p185HER2, promotes human ovarian tumor cell activation

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.

Coexpression of CD44 variant (v10/ex14) and CD44S in human mammary epithelial cells promotes tumorigenesis

CD44 is the major hyaluronan cell surface receptor and functions as an adhesion molecule in many different cell types, including human breast epithelial cells. The coexpression of certain CD44 variants (CD44v), such as CD44v (v10/ex14), with CD44s (standard form) appears to be closely associated with human breast tumor metastasis. In this study we have established a stable transfection of CD44v (v10/ex14) cDNA into nontumorigenic human breast epithelial cells (HBL100) which contain endogenous CD44s. Our results indicate that coexpression of both CD44v (v10/ex14) and CD44s alters the following important biological properties of these cells: 1) there is a significant reduction in hyaluronic acid (HA)-mediated cell adhesion; 2) there is an increased migration capability in collagen-matrix gel; and 3) these cells constitutively produce certain angiogenic factors and effectively promote tumorigenesis in athymic nude mice. These findings suggest that coexpression of CD44v (v10/ex14) and CD44s may trigger the onset of cell transformation required for breast cancer development.

CD44 and the adhesion of neoplastic cells

CD44 is a family of transmembrane glycoproteins that act mainly as a receptor for hyaluronan. It can also bind some other extracellular matrix ligands (chondroitin sulphate, heparan sulphate, fibronectin, serglycin, osteopontin) with lower affinity. CD44 is encoded by a single gene containing 20 exons, 10 of which (v1-v10) are variant exons inserted by alternative splicing. The standard, ubiquitously expressed isoform of CD44, does not contain sequences encoded by these variant exons. Numerous variant isoforms of CD44 containing different combinations of exons v1-v10 inserted into the extracellular domain can be expressed in proliferating epithelial cells and activated lymphocytes. CD44 plays a significant role in lymphocyte homing. Both alternative splicing and glycosylation influence receptor function of the molecule, usually reducing its affinity to hyaluronan. The cytoplasmic domain of CD44 communicates with the cytoskeleton via ankyrin and proteins belonging to the ezrin-moesin-radixin family. Relatively little is known about the intracellular events following interactions of CD44 with its ligands. Some variant isoforms, especially those containing sequences encoded by v6-v10, are overexpressed in both human and animal neoplasms. In a rat pancreatic adenocarcinoma model one of the variant CD44 isoforms was proved to be determinant in the metastatic process. For some human neoplasms (carcinomas of the digestive tract, non-Hodgkin's lymphomas, thyroid carcinomas, and others) correlations have been made between the particular pattern of CD44 variants produced by neoplastic cells and clinicopathological parameters of tumours, such as grade, stage, presence of metastases, and survival. In vitro studies indicate that modifications of CD44 expression result in different ligand recognition and influence cell motility, invasive properties, and metastatic potential of experimental tumours. Investigation of CD44 neoexpression can be useful both in early cancer diagnosis and in predicting tumour behaviour. It can also contribute to better understanding of molecular mechanisms leading to neoplastic transformation.

CD44: structure, function, and association with the malignant process

CD44 is a ubiquitous multistructural and multifunctional cells surface adhesion molecule involved in cell-cell and cell-matrix interactions. Twenty exons are involved in the genomic organization of this molecule. The first five and the last 5 exons are constant, whereas the 10 exons located between these regions are subjected to alternative splicing, resulting in the generation of a variable region. Differential utilization of the 10 variable region exons, as well as variations in N-glycosylation, O-glycosylation, and glycosaminoglycanation (by heparan sulfate or chondroitin sulfate), generate multiple isoforms (at least 20 are known) of different molecular sizes (85-230 kDa). The smallest CD44 molecule (85-95 kDa), which lacks the entire variable region, is standard CD44 (CD44s). As it is expressed mainly on cells of lymphohematopoietic origin, CD44s is also known as hematopoietic CD44 (CD44H). CD44s is a single-chain molecule composed of a distal extracellular domain (containing, the ligand-binding sites), a membrane-proximal region, a transmembrane-spanning domain, and a cytoplasmic tail. The molecular sequence (with the exception of the membrane-proximal region) displays high interspecies homology. After immunological activation, T lymphocytes and other leukocytes transiently upregulate CD44 isoforms expressing variant exons (designated CD44v). A CD44 isform containing the last 3 exon products of the variable region (CD44V8-10, also known as epithelial CD44 or CD44E), is preferentially expressed on epithelial cells. The longest CD44 isoform expressing in tandem eight exons of the variable region (CD44V3-10) was detected in keratinocytes. Hyaluronic acid (HA), an important component of the extracellular matrix (ECM), is the principal, but by no means the only, ligand of CD44. Other CD44 ligands include the ECM components collagen, fibronectin, laminin, and chondroitin sulfate. Mucosal addressin, serglycin, osteopontin, and the class II invariant chain (Ii) are additional, ECM-unrelated, ligands of the molecule. In many, but not in all cases, CD44 does not bind HA unless it is stimulated by phorbol esters, activated by agonistic anti-CD44 antibody, or deglycosylated (e.g., by tunicamycin). CD44 is a multifunctional receptor involved in cell-cell and cell-ECM interactions, cell traffic, lymph node homing, presentation of chemokines and growth factors to traveling cells, and transmission of growth signals. CD44 also participates in the uptake and intracellular degradation of HA, as well as in transmission of signals mediating hematopoiesis and apoptosis. Many cancer cell types as well as their metastases express high levels of CD44. Whereas some tumors, such as gliomas, exclusively express standard CD44, other neoplasms, including gastrointestinal cancer, bladder cancer, uterine cervical cancer, breast cancer and non-Hodgkin's lymphomas, also express CD44 variants. Hence CD44, particularly its variants, may be used as diagnostic or prognostic markers of at least some human malignant diseases. Furthermore, it has been shown in animal models that injection of reagents interfering with CD44-ligand interaction (e.g., CD44s- or CD44v-specific antibodies) inhibit local tumor growth and metastatic spread. These findings suggest that CD44 may confer a growth advantage on some neoplastic cells and, therefore, could be used as a target for cancer therapy. It is hoped that identification of CD44 variants expressed on cancer but not on normal cells will lead to the development of anti-CD44 reagents restricted to the neoplastic growth.

The cell adhesion molecule, GP116, is a new CD44 variant (ex14/v10) involved in hyaluronic acid binding and endothelial cell proliferation

In this study we have found that endothelial cells from different origins all contain a CD44-related transmembrane glycoprotein, named GP116. Using a bovine aortic endothelial cell line and a standard pulse-chase protocol, we show that GP116 is synthesized as a 52-kDa nascent polypeptide precursor (p52) which is processed to GP116 as follows, p52 --> p63/65 --> p82 --> p100 --> GP116. GP116 contains approximately 8 N- and approximately 11 O-linked oligosaccharide chains (but lacks glycosaminoglycans) and interacts directly with the cytoskeletal protein, ankyrin, both in vitro (Kd approximately 1.2 nM) and in vivo. The results of GP116 amino acid composition, reverse transcriptase-polymerase chain reaction, Southern blot, Northern blot, cloning, and sequence analyses indicate that endothelial cells express this new CD44 variant that contains an exon having significant homology with human CD44 exon 14 (ex14/v10). GP116, designated as CD44 (ex14/v10), has been shown to be a major hyaluronic acid (HA) receptor (Kd approximately 0.5-0.8 nM) responsible for cell adhesion. Most importantly, we have found that the interaction between CD44(ex14/v10) and HA or a small fragment of HA (10-15 disaccharide units) induces a mitogenic response in endothelial cells. These findings suggest that this CD44 variant plays an important role in regulating endothelial cell proliferation.

Mechanisms of recovery from mechanical injury of cultured rat hepatocytes

The mechanism(s) whereby hepatocytes restore denuded areas remains unknown. We therefore studied the recovery of denuded areas made in monolayers of primary cultures of rat hepatocytes. Minimal recovery occurred in cells plated on plastic. Plating on Matrigel produced modest recovery (25% at 24 h), whereas plating on a type I collagen substrate resulted in > 70% recovery at 24 h. The rate of recovery on collagen could be attenuated by a monoclonal antibody directed against the extracellular domain of the beta 1-integrin subunit. Monoclonal antibodies directed against CD44 (the hyaluron receptor) and E-cadherin did not influence the rate of recovery. Recovery could be stimulated, in a dose-dependent fashion, by epidermal and hepatocyte growth factors. The effects of epidermal and hepatocyte growth factors to promote recovery occurred in the absence of 5-bromo-2'-deoxyuridine uptake, suggesting a proliferation-independent mechanism. Transforming growth factor-beta 1 inhibited recovery. Exposure to selected cytokines (interleukins 1 and 2), an adenine nucleotide [adenosine 5'-O-(3-thiotriphosphate)], adenosine, pertussis toxin, and selected agents that bind to fibronectin and other matrix component adhesive sites (heparin and the RGD peptide) did not influence the rate of recovery of hepatocytes. However, the peptide DGEA, which can bind to collagen adhesive sites, attenuated recovery. These studies demonstrate that primary cultures of rat hepatocytes require a particular type of extracellular matrix to renew denuded areas and that the beta 1-integrin subunit may be involved in this recovery process. Hepatocyte recovery of denuded areas can be modulated by growth factors in both a stimulatory (epidermal and hepatocyte growth factors) and an inhibitory (transforming growth factor-beta 1) fashion.

Cellular interactions among marrow stromal and normal/neoplastic pre-B- and B-lymphoblastic cells

The direct interaction between B-cells, both normal and neoplastic, and marrow stromal cells (MSC) or MSC-secreted extracellular matrix proteins is believed to play a decisive role in lymphopoiesis as well as in the sustained proliferation and homing of lymphoid malignancies. Although information has been accruing on possible obligatory cell-cell and cell-extracellular matrix adhesion molecules, our current state of knowledge regarding B-cell interaction with the marrow microenvironment is rather limited. We discuss in this review what is currently known regarding adhesion molecules involved in the apposition between B-cells and MSC. In addition, we describe how the cellular interactions are modulated by cytokines present in the marrow microenvironment. Further, we present possible signal transduction pathways activated by B-cell/MSC apposition and attempt to integrate in vitro data with in vivo observations. We close this review by providing implications for novel clinical and therapeutic applications based on the current knowledge of B-cell/MSC adhesion mechanisms and signaling pathways.

HA receptors: regulators of signalling to the cytoskeleton

Hyaluronan (HA) is a ubiquitous component of the extracellular matrix (ECM) and occurs transiently in both the cell nucleus and cytoplasm. It has been shown to promote cell motility, adhesion, and proliferation and thus it has an important role in such processes as morphogenesis, wound repair, inflammation, and metastasis. These processes require massive cell movement and tissue reorganization and are always accompanied by elevated levels of HA. Many of the effects of HA are mediated through cell surface receptors, three of which have been molecularly characterized, namely CD44, RHAMM, and ICAM-1. Binding of the HA ligand to its receptors triggers signal transduction events which, in concert with other ECM and cytoskeletal components, can direct cell trafficking during physiological and pathological events. The HA mediated signals are transmitted, at least in part, by the activation of protein phosphorylation cascades, cytokine release, and the stimulation of cell cycle proteins. A variety of extracellular signals regulate the expression of both HA and the receptors necessitating that HA-receptor signalling is a tightly controlled process. Regulated production of soluble forms of the receptors, alternately spliced cell surface isoforms, and glycosylation variants of these receptors can dramatically modulate HA binding, ligand specificity, and stimulation of the signalling pathway. When these processes are deregulated cell behaviour becomes uncontrolled leading to developmental abnormalities, abnormal physiological responses, and tumorigenesis. The elucidation of the molecular mechanisms regulating HA-mediated events will not only contribute greatly to our understanding of a variety of disease processes but will also offer many new avenues of therapeutic intervention.

CD44 and hyaluronan binding by human myeloid cells

The CD44 cell surface molecule has been shown to be the principal cell surface receptor for hyaluronan (or hyaluronic acid), a glycosaminoglycan component of marrow extracellular matrix. However, its affinity for hyaluronan is not constitutive, since it depends on the cell type, the stage of differentiation and on activation by external stimuli including certain anti-CD44 antibodies and phorbol esters. Except for a few lymphoid cell lines, hematopoietic cells do not spontaneously bind hyaluronan and initial studies reported that, contrary to lymphocytes, myeloid cells could not be activated to bind hyaluronan. Because CD44 plays an important role in the initial phases of hematopoiesis, as shown by experiments using blocking anti-CD44 monoclonal antibodies, its capacity to mediate adhesion of primitive myeloid cells has been investigated. It was found that CD44 could mediate spontaneous adhesion to hyaluronan of immature myeloid cell lines KG1, KG1a, and TF1, which serve as a model for hematopoietic progenitors. However, despite expressing high amounts of CD44, no more than 15% of bone marrow progenitors could adhere to hyaluronan. Recent experiments have shown that a very important feature of CD44 is its capacity to be rapidly activated by certain antibodies and cytokines (GM-CSF and KL) from a low affinity to a high affinity state for hyaluronan. These data shed light on striking similarities in the functional regulation of CD44 and of the two integrin receptors VLA-4 (a4b1), and VLA-5 (a5b1), which are also expressed on hematopoietic progenitors. The relevance of these data to the regulation of normal hematopoiesis and mobilization of CD34+ progenitors in the view of cell grafting is analyzed. In addition, we show that in idiopathic myelofibrosis, the amount of hyaluronan is markedly increased in the extracellular matrix from the myeloproliferative spleen. Considering that the production of cytokines is enhanced in this disease, we discuss whether CD44-hyaluronan interaction may have a role in the pathophysiology of this myeloproliferative syndrome.

The effect of sodium hyaluronate on the growth of rabbit corneal epithelial cells in vitro

The effect of sodium hyaluronate (Na-HA) on the growth of rabbit corneal epithelial cells was investigated in vitro by using MTT assay and [3H]-thymidine incorporation. Na-HA, with a molecular weight of 86 x 10(4), enhanced the growth of the corneal epithelial cells cultured on a fibronectin (FN) coated dish, but when cells were cultured on a noncoated dish, Na-HA did not enhance the growth of the cells. Na-HA, labeled with 5-aminofluorescein (FA-HA), bound to the corneal epithelial cells, and the CD44-like molecule was identified on the cells by using a monoclonal antibody, OS/37. Furthermore, the binding of FA-HA to the cells was significantly inhibited by the monoclonal antibody. These results suggest that Na-HA binds to the CD44-like molecule associated with FN and then enhances the growth of the corneal epithelial cells.

Involvement of CD44 and its variant isoforms in membrane-cytoskeleton interaction, cell adhesion and tumor metastasis

CD44s (standard form of CD44) is a transmembrane glycoprotein whose external domain displays extracellular matrix adhesion properties by binding both hyaluronic acid (HA) and collagen. The cytoplasmic domain of CD44s interacts with the cytoskeleton by binding directly to ankyrin. It has been shown that post-translational modifications, such as phosphorylation (by protein kinase C), acylation (by acyl-transferase) and GTP-binding enhanced CD44's interaction with cytoskeletal proteins. Most importantly, the interaction between CD44s and the cytoskeletal protein, ankyrin, is required for the modulation of CD44s cell surface expression and its adhesion function. Recently, a number of tumor cells and tissues have been shown to express CD44 variant (CD44v) isoforms. Using RT-PCR and DNA sequence analyses, we have found that unique CD44 splice variant isoforms are expressed in both prostate and breast cancer cell lines and carcinomas. Most importantly intracellular ankyrin is preferentially accumulated underneath the patched/capped structures of CD44 variant isoform in both breast and prostate cancer cells attached to HA-coated plates. We propose that selective expression of CD44v isoforms unique for certain metastatic carcinomas and their interaction with the cytoskeleton may play a pivotal role in regulating tumor cell behavior during tumor development and metastasis.

Hyaluronan: RHAMM mediated cell locomotion and signaling in tumorigenesis

Extracellular matrix molecules and their receptors are important regulators of cell movement, adhesion and cytoskeletal organization. Adhesion molecules can also serve to mediate signal transduction and can influence, and sometimes direct, the events required for tumorigenesis. The extracellular matrix molecule, hyaluronan and its receptors have been implicated in transformation and metastasis, in particular the processes of tumor cell motility and invasion. RHAMM (receptor for hyaluronan mediated motility) is required for the cell locomotion of ras-transformed fibrosarcoma cells, cytokine stimulated fibrobasts and T lymphocytes, malignant B cells, and breast carcinoma cells. HA:RHAMM interactions promote cell locomotion via a protein tyrosine kinase signal transduction pathway that targets focal adhesions. The tyrosine kinase pp60c-src is associated with RHAMM in cells and is required for RHAMM mediated cell motility. It is possible that a RHAMM/src pathway induces focal adhesions to signal the cytoskeletal changes required for elevated cell motility seen in tumor progression, invasion and metastasis.

Are murine marginal-zone macrophages the splenic white pulp analog of high endothelial venules?

The entry of lymphocytes into the spleen, in contrast to lymph nodes, does not involve high endothelial venule (HEV) interaction. The precise point of entry, as well as the mechanism by which lymphocytes enter the lymphoid areas of the spleen, remains controversial. We examined in detail the effect of two agents, pertussis toxin (PT) and the sulfated polysaccharide fucoidan, on splenic lymphocyte entry and positioning. These have previously been shown to interfere with lymphocyte extravasation across HEV. PT prevents lymphocyte extravasation, but not binding, to HEV, whereas fucoidan prevents binding and thus subsequent extravasation. Studies presented here show that pretreatment of murine lymphocytes with PT does not numerically affect entry into spleen, but profoundly alters lymphocyte positioning within the spleen. When fluorescently labeled, PT-treated lymphocytes are injected intravenously, they initially accumulate in the marginal zone, in apparent association with the layer of marginal zone macrophages (MZM phi) which form a shell around the white pulp. They fail to traverse this layer into the white pulp, and subsequently localize in the red pulp. In contrast, untreated cells initially appear in the marginal zone, then continue to migrate into the white pulp after traversing the MZM phi layer. The localization of PT-pretreated lymphocytes adjacent to the MZM phi layer is disrupted by intravenous administration of fucoidan. Using a flow cytometric assay of aggregation between MZM phi and lymphocytes, we confirmed that fucoidan is also able to inhibit this association in vitro, whereas PT has no effect on this interaction. We propose that MZM phi in the mouse are the splenic analog of HEV, forming the port of entry of lymphocytes into the white pulp of the spleen.

Involvement of CD44 variant isoforms in hyaluronate adhesion by human activated T cells

The standard, 85-95-kDa form of the hyaluronic acid (HA) receptor CD44 and a number of CD44 mRNA splice variants play important roles in immune responses and tumor metastasis. Variants carrying exon 6 (v6), or 9 (v9) products are transiently expressed on activated human T cells. Here, modulation experiments with specific monoclonal antibodies (mAb) indicate that v6 and v9 are expressed independently on distinct sets of CD44 molecules, and that their combined expression is necessary for HA adhesion. Moreover, the finding that mAb-mediated cross-linking of v6 and v9 promoted cytosolic free Ca2+ mobilization and co-stimulated CD3-triggered T cell proliferation indicates that v6 and v9 possess signaling and effector function activation ability. Finally, HA-mediated signaling appears to be required for variant-dependent adhesion to HA. The observation that soluble HA promoted cytosolic free Ca2+ mobilization indicates that HA-induced Ca2+ mobilization can occur during T cell-HA interaction. Since Ca2+ mobilization was inhibited by pretreatment of cells with an anti-CD44 mAb directed against the HA-binding domain of CD44, CD44 receptors appear to be involved in HA-mediated signal transduction. The requirement of cytosolic free Ca2+ for adhesion is shown by the fact that ionomycin (a Ca2+ ionophore) stimulated, and EGTA (a Ca2+ chelator), inhibited HA adhesion. In addition, cytoskeletal functional activation is required for cell adhesion to HA, since drugs that block actin polymerization, such as cytochalasin B, or actomyosin contraction, such as the calmodulin antagonist W-7, inhibited cell adhesion to HA. As this adhesion is also ADP ribosylation-sensitive, it may involve a GTP-dependent function of CD44v, i.e. ankyrin binding. Our data indicate that there is a functional hierarchy among the CD44 molecules expressed on human peripheral blood T cells and that the splice variants, as compared to the standard form, exhibit a greater HA binding ability which involves CD44-mediated signaling and effector function activation.

Ryanodine receptor-ankyrin interaction regulates internal Ca2+ release in mouse T-lymphoma cells

In this study, we have identified and partially characterized a mouse T-lymphoma ryanodine receptor on a unique type of internal vesicle which bands at the relatively light density of 1.07 g/ml. Analysis of the binding of [3H]ryanodine to these internal vesicles reveals the presence of a single, low affinity binding site with a dissociation constant (Kd) of 200 nM. The second messenger, cyclic ADP-ribose, was found to increase the binding affinity of [3H]ryanodine to its vesicle receptor at least 5-fold (Kd approximately 40 nM). In addition, cADP-ribose appears to be a potent activator of internal Ca2+ release in T-lymphoma cells and is capable of overriding ryanodine-mediated inhibition of internal Ca2+ release. Immunoblot analyses using a monoclonal mouse antiryanodine receptor antibody indicate that mouse T-lymphoma cells contain a 500-kDa polypeptide similar to the ryanodine receptor found in skeletal muscle, cardiac muscle, and brain tissues. Double immunofluorescence staining and laser confocal microscopic analysis show that the ryanodine receptor is preferentially accumulated underneath surface receptor-capped structures. T-lymphoma ryanodine receptor was isolated (with an apparent sedimentation coefficient of 30 S) by extraction of the light density vesicles with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid (CHAPS) in 1 M NaCl followed by sucrose gradient centrifugation. Further analysis indicates that specific, high affinity binding occurs between ankyrin and this 30 S lymphoma ryanodine receptor (Kd = 0.075 nM). Most importantly, the binding of ankyrin to the light density vesicles significantly blocks ryanodine binding and ryanodine-mediated inhibition of internal Ca2+ release. These findings suggest that the cytoskeleton plays a pivotal role in the regulation of ryanodine receptor-mediated internal Ca2+ release during lymphocyte activation.

New CD44 splice variants associated with human breast cancers

Changes in the CD44 variant (CD44v) isoforms on the cell surface have been correlated with tumor metastasis. In this study we have examined the expression of CD44 variant isoforms in human breast carcinoma samples by a variety of techniques including immunohistochemistry, reverse transcriptase-polymerase chain reaction (RT-PCR), and nucleotide sequencing. Using RT-PCR, we have determined that normal human breast tissue contains primarily the CD44 epithelial (CD44E) form and very little CD44 standard (CD44s) form. However, metastatic breast carcinomas appear to overexpress both the CD44E and CD44s forms and also display multiple new species of CD44 variant isoforms. Histocytochemical staining using anti-CD44 antibody (recognizing a common determinant of the CD44 class of glycoproteins) confirms that the CD44 molecules are overexpressed and preferentially located in metastatic breast cancer tissues. Nucleotide sequencing analyses indicate that at least four new CD44 variant isoforms (i.e., displaying unique splicing via the insertion or the deletion of exons 7, 10, 11, and 14) may be closely associated with human metastatic breast cancers. These newly described CD44 variant isoforms may be useful for monitoring the progression of human breast cancer metastasis.

Epican, a heparan/chondroitin sulfate proteoglycan form of CD44, mediates cell-cell adhesion

Epican is a heparan/chondroitin sulfate proteoglycan form of CD44 and is expressed on the surface of keratinocytes from the basal layer to the granular layer of the epidermis. To analyze the adhesive properties of epican apart from the influence of other adhesive molecules found on keratinocytes, mouse L cell fibroblasts were transfected with CD44Epican cDNA. The epican expressed on the surface of transfected L cells was predominantly a heparan or chondroitin sulfate proteoglycan. The CD44Epican-transfected L cells acquired: (a) a self-aggregating phenotype that required hyaluronan but was calcium-independent; and (b) a new capacity to adhere to keratinocytes, a property that was blocked by an anti-epican antibody. Both aggregation and adhesion of CD44Epican-transfected cells were completely prevented by pretreatment with hyaluronidase, but were totally restored by the addition of exogenous hyaluronan. Aggregation of transfected L cells was minimally influenced by other glycosaminoglycans, but adhesion of transfected L cells to keratinocytes was substantially inhibited by heparin.

Stimulation of T cells via CD44 requires leukocyte-function-associated antigen interactions and interleukin-2 production

This article reports the results of the analysis of the activation signals delivered to T and B cells by means of the CD44 molecule and an agonistic mAb, i.e., CB05 mAb, which is able to induce cell activation and aggregation upon binding. The functional effects culminate in T-cell proliferation in the presence of autologous accessory cells. Such effects are barely detectable in thymocytes, while B cells prove refractory to the action of the agonistic mAb. All of these events have been followed by the expression of surface activation markers, by the transcription of selected cytokine genes (IFN-gamma, IL-4, and GM-CSF), and by the secretion of IL-2. Cell activation via CD44 has been evaluated as to its relationship with CD3 and CD2 activation pathways, proving synergistic with the latter. The CD44 signaling is protein kinase dependent. Furthermore, the role of surface molecules as cosignals in the CD44 pathway has been analyzed, showing that CD11a (and its ligand CD54), HLA class I, and CD25 are instrumental in the implementation of (a) efficient activation/proliferation signals and (b) a potent cytotoxic potential.