ERM family members as molecular linkers between the cell surface glycoprotein CD44 and actin-based cytoskeletons

Hippo-YAP signaling pathway: A new paradigm for cancer therapy

In the past decades, the Hippo signaling pathway has been delineated and shown to play multiple roles in the control of organ size in both Drosophila and mammals. In mammals, the Hippo pathway is a kinase cascade leading from Mst1/2 to YAP and its paralog TAZ. Several studies have demonstrated that YAP/TAZ is a candidate oncogene and that other members of the Hippo pathway are tumor suppressive genes. The dysregulation of the Hippo pathway has been observed in a variety of cancers. This review chronicles the recent progress in elucidating the function of Hippo signaling in tumorigenesis and provide a rich source of potential targets for cancer therapy.

Structural basis of DDB1-and-Cullin 4-associated Factor 1 (DCAF1) recognition by merlin/NF2 and its implication in tumorigenesis by CD44-mediated inhibition of merlin suppression of DCAF1 function

Merlin, a tumor suppressor encoded by the neurofibromatosis type 2 gene, has been shown to suppress tumorigenesis by inhibiting the Cullin 4-RING E3 ubiquitin ligase CRL4(DCAF) (1) in the nucleus. This inhibition is mediated by direct binding of merlin to DDB1-and-Cullin 4-associated Factor 1 (DCAF1), yet the binding mode of merlin to DCAF1 is not well defined. Here, we report structural and biophysical studies of the merlin binding to DCAF1 and its interference with CD44 binding. The crystal structure of the merlin FERM domain bound to the DCAF1 C-terminal acidic tail reveals that the hydrophobic IILXLN motif located at the C-terminal end of DCAF1 binds subdomain C of the FERM domain by forming a β-strand. The binding site and mode resemble that of merlin binding to the CD44 cytoplasmic tail. Competition binding assay showed that CD44 and DCAF1 compete for binding to the merlin FERM domain in solution. The CD44 cytoplasmic tail is known to be cleaved for nuclear translocation by regulated intra-membrane proteolysis (RIP). Our structure implies that, in the nucleus, the CD44 cytoplasmic tail cleaved by RIP could release DCAF1 from merlin by competing for binding to the merlin FERM domain, which results in the inhibition of merlin-mediated suppression of tumorigenesis.

Class I myosins in B-cell physiology: functions in spreading, immune synapses, motility, and vesicular traffic

Myosins comprise a family of motor proteins whose role in muscle contraction and motility in a large range of eukaryotic cells has been widely studied. Although these proteins have been characterized extensively and much is known about their function in different cellular compartments, little is known about these molecules in hematopoietic cells. Myosins expressed by cells from the immune response are involved in maintaining plasma membrane tension, moving and secreting vesicles, endo- and exocytotic processes, and promoting the adhesion and motility of cells. Herein, we summarize our current understanding of class I myosins in B cells, with an emphasis on the emerging roles of these molecular motors in immune functions.

Molecular insights into NF2/Merlin tumor suppressor function

The FERM domain protein Merlin, encoded by the NF2 tumor suppressor gene, regulates cell proliferation in response to adhesive signaling. The growth inhibitory function of Merlin is induced by intercellular adhesion and inactivated by joint integrin/receptor tyrosine kinase signaling. Merlin contributes to the formation of cell junctions in polarized tissues, activates anti-mitogenic signaling at tight-junctions, and inhibits oncogenic gene expression. Thus, inactivation of Merlin causes uncontrolled mitogenic signaling and tumorigenesis. Merlin's predominant tumor suppressive functions are attributable to its control of oncogenic gene expression through regulation of Hippo signaling. Notably, Merlin translocates to the nucleus where it directly inhibits the CRL4(DCAF1) E3 ubiquitin ligase, thereby suppressing inhibition of the Lats kinases. A dichotomy in NF2 function has emerged whereby Merlin acts at the cell cortex to organize cell junctions and propagate anti-mitogenic signaling, whereas it inhibits oncogenic gene expression through the inhibition of CRL4(DCAF1) and activation of Hippo signaling. The biochemical events underlying Merlin's normal function and tumor suppressive activity will be discussed in this Review, with emphasis on recent discoveries that have greatly influenced our understanding of Merlin biology.

The role of key genes and pathways involved in the tumorigenesis of Malignant Mesothelioma

Malignant Mesothelioma (MM) is a very aggressive cancer with low survival rates and often diagnosed at an advanced stage. Several players have been implicated in the development of this cancer, such as asbestos, erionite and the simian virus 40 (SV40). Here, we have reviewed the involvement of erionite, SV40, as well as, the role of several genes (p16(INK4a), p14(ARF), NF2, LATS2, SAV, CTNNB1 and among others), the pathways (RAS, PI3K, Wnt, BCL and Hippo), and their respective roles in the development of MM.

Role of ezrin in osteosarcoma metastasis

The cause of death for the vast majority of cancer patients is the development of metastases at sites distant from that of the primary tumor. For most pediatric sarcoma patients such as those with osteosarcoma (OS), despite successful management of the primary tumor through multimodality approaches, the development of metastases, commonly to the lungs, is the cause of death. Significant improvements in long-term outcome for these patients have not been seen in more than 30 years. Furthermore, the long-term outcome for patients who present with metastatic disease is grave [1-5]. New treatment options are needed.Opportunities to improve outcomes for patients who present with metastases and those at-risk for progression and metastasis require an improved understanding of cancer progression and metastasis. With this goal in mind we and others have identified ezrin as a metastasis-associated protein that associated with OS and other cancers. Ezrin is the prototypical ERM (Ezrin/Radixin/Moesin) protein family member. ERMs function as linker proteins connecting the actin cytoskeleton and the plasma membrane. Since our initial identification of ezrin in pediatric sarcoma, an increasing understanding the role of ezrin in metastasis has emerged. Briefly, ezrin appears to allow metastatic cells to overcome a number of stresses experienced during the metastatic cascade, most notably the stress experienced as cells interact with the microenvironment of the secondary site. Cells must rapidly adapt to this environment in order to survive. Evidence now suggests a connection between ezrin expression and a variety of mechanisms linked to this important cellular adaptation including the ability of metastatic cells to initiate the translation of new proteins and to allow the efficient generation of ATP through a variety of sources. This understanding of the role of ezrin in the biology of metastasis is now sufficient to consider ezrin as an important therapeutic target in osteosarcoma patients. This chapter reviews our understanding of ezrin and the related ERM proteins in normal tissues and physiology, summarizes the expression of ezrin in human cancers and associations with clinical parameters of disease progression, reviews reports that detail a biological understanding of ezrin's role in metastatic progression, and concludes with a rationale that may be considered to target ezrin and ezrin biology in osteosarcoma.

Conserved sequence repeats of IQGAP1 mediate binding to Ezrin

Mammalian IQGAP proteins all feature multiple ∼50 amino acid sequence repeats near their N-termini, and little is known about the function of these "Repeats". We have expressed and purified the Repeats from human IQGAP1 to identify binding partners. We used mass spectrometry to identify 42 mouse kidney proteins that associate with the IQGAP1 Repeats including the ERM proteins ezrin, radixin, and moesin. ERM proteins have an N-terminal FERM domain (4.1, ezrin, radixin, moesin) through which they bind to protein targets and phosphatidylinositol 4,5-bisphosphate (PIP2) and a C-terminal actin-binding domain and function to link the actin cytoskeleton to distinct locations on the cell cortex. Isothermal titration calorimetry (ITC) reveals that the IQGAP1 Repeats directly bind to the ezrin FERM domain, while no binding is seen for full-length "autoinhibited" ezrin or a version of full-length ezrin intended to mimic the activated protein. ITC also indicates that the ezrin FERM domain binds to the Repeats from IQGAP2 but not the Repeats from IQGAP3. We conclude that IQGAP1 and IQGAP2 are positioned at the cell cortex by ERM proteins. We propose that the IQGAP3 Repeats may likewise bind to FERM domains for signaling purposes.

Overexpression of c-Met and CD44v6 receptors contributes to autocrine TGF-β1 signaling in interstitial lung disease

The hepatocyte growth factor (HGF) and the HGF receptor Met pathway are important in the pathogenesis of interstitial lung disease (ILD). Alternatively spliced isoforms of CD44 containing variable exon 6 (CD44v6) and its ligand hyaluronan (HA) alter cellular function in response to interaction between CD44v6 and HGF. TGF-β1 is the crucial cytokine that induces fibrotic action in ILD fibroblasts (ILDFbs). We have identified an autocrine TGF-β1 signaling that up-regulates both Met and CD44v6 mRNA and protein expression. Western blot analysis, flow cytometry, and immunostaining revealed that CD44v6 and Met colocalize in fibroblasts and in tissue sections from ILD patients and in lungs of bleomycin-treated mice. Interestingly, cell proliferation induced by TGF-β1 is mediated through Met and CD44v6. Further, cell proliferation mediated by TGF-β1/CD44v6 is ERK-dependent. In contrast, action of Met on ILDFb proliferation does not require ERK but does require p38(MAPK). ILDFbs were sorted into CD44v6(+)/Met(+) and CD44v6(-)/Met(+) subpopulations. HGF inhibited TGF-β1-stimulated collagen-1 and α-smooth muscle cell actin expression in both of these subpopulations by interfering with TGF-β1 signaling. HGF alone markedly stimulated CD44v6 expression, which in turn regulated collagen-1 synthesis. Our data with primary lung fibroblast cultures with respect to collagen-1, CD44v6, and Met expressions were supported by immunostaining of lung sections from bleomycin-treated mice and from ILD patients. These results define the relationships between CD44v6, Met, and autocrine TGF-β1 signaling and the potential modulating influence of HGF on TGF-β1-induced CD44v6-dependent fibroblast function in ILD fibrosis.

CD44 is prognostic for overall survival in the NCI randomized trial on breast conservation with 25 year follow-up

CD44 is a transmembrane glycoprotein involved in numerous cellular functions, including cell adhesion and extracellular matrix interactions. It is known to be functionally diverse, with alternative splice variants increasingly implicated as a marker for tumor-initiating stem cells associated with poor prognosis. Here, we evaluate CD44 as a potential marker of long-term breast cancer outcomes. Tissue specimens from patients treated on the National Cancer Institute 79-C-0111 randomized trial of breast conservation versus mastectomy between 1979 and 1987 were collected, and immunohistochemistry was performed using the standard isoform of CD44. Specimens were correlated with patient characteristics and outcomes. Survival analysis was performed using the log rank test. Fifty-one patients had evaluable tumor sections and available long-term clinical follow up data at a median follow up of 25.7 years. Significant predictors of OS were tumor size (median OFS 25.4 years for ≤2 cm vs. 7.5 years for >2 cm, p = 0.001), nodal status (median OS 17.2 years for node-negative patients vs. 6.7 years for node positive patients, p = 0.017), and CD44 expression (median OS 18.9 years for CD44 positive patients vs. 8.6 years for CD44 negative patients, p = 0.049). There was a trend toward increased PFS for patients with CD44 positive tumors (median PFS 17.9 vs. 4.3 years, p = 0.17), but this did not reach statistical significance. These findings illustrate the potential utility of CD44 as a prognostic marker for early stage breast cancer. Subgroup analysis in patients with lymph node involvement revealed CD44 positivity to be most strongly associated with increased survival, suggesting a potential role of CD44 in decision making for axillary management. As there is increasing interest in CD44 as a therapeutic target in ongoing clinical trials, the results of this study suggest additional investigation regarding the role CD44 in breast cancer is warranted.

Characterization of a Slow-Migrating Component of the Rabies Virus Matrix Protein Strongly Associated with the Viral Glycoprotein

We investigated multiple forms of rabies virus matrix (M) protein. Under non-reducing electrophoretic conditions, we detected, in addition to major bands of monomer forms (23- and 24-kDa) of M protein, an M antigen-positive slow-migrating minor band (about 54 kDa) in both the virion and infected cells. Relative contents of the 54-kDa and monomer components in the virion were about 20-30% and 70-80% of the whole M protein, respectively, while the content of the 54-kDa component was smaller (about 10-20% of the total M protein) in the cell than in the virion. The 54-kDa components could be extracted from the infected cells with sodium deoxycholate, but they were quite resistant to extraction with 1% nonionic detergents by which most monomer components were solubilized. The 54-kDa component was precipitated more efficiently than the monomer by a monoclonal antibody (mAb; #3-9-16), which recognized a linear epitope located at the N-terminal of the M protein. The mAb #3-9-16 coprecipitated the viral glycoprotein (G), which was demonstrated to be due to strong association between the G and 54-kDa component of the M protein. Monomers and the 54-kDa polypeptide migrated to the same isoelectric point (pI) in twodimensional (2-D) gel electrophoresis, implicating that the 54-kDa component was composed of component(s) of the same pI as that of the M protein monomers. From these results, we conclude that the M antigen-positive 54-kDa polypeptide is a homodimer of M protein, taking an N-terminal-exposed conformation, and is strongly associated with the viral glycoprotein. Possible association with a membrane microdomain of the cell will be discussed.

Design, synthesis and biological evaluation of ezrin inhibitors targeting metastatic osteosarcoma

Respiratory failure due to pulmonary metastasis is the major cause of death for patients with osteosarcoma. However, the molecular basis for metastasis of osteosarcoma is poorly understood. Recently, ezrin, a member of the ERM family of proteins, has been associated with osteosarcoma metastasis to the lungs. The small molecule NSC 668394 was identified to bind to ezrin, inhibit in vitro and in vivo cell migration, invasion, and metastatic colony survival. Reported herein are the design and synthesis of analogues of NSC 668394, and subsequent functional ezrin inhibition studies. The binding affinity was characterized by surface plasmon resonance technique. Cell migration and invasion activity was determined by electrical cell impedance methodology. Optimization of a series of heterocyclic-dione analogues led to the discovery of compounds 21k and 21m as potential novel antimetastatic agents.

Monoclonal Antibody #3-9-16 Recognizes One of the Two Isoforms of Rabies Virus Matrix Protein That Exposes Its N-Terminus on the Virion Surface

We investigated behaviors of the rabies virus matrix (M) protein using a monoclonal antibody (mAb), #3-9-16, that recognized a linear epitope located at the N-terminus of the protein. Based on the reactivity with this mAb, M proteins could be divided into at least two isoforms; an ordinary major form (Malpha) whose 3-9-16 epitope is hidden, and an N-terminal-exposed epitope-positive form (Mbeta). The Mbeta protein accounted for about 25-30% of the total M proteins in the virion, while its content in the cell ranged from 10 to 15% of total M protein. Fluorescent antibody (FA) staining showed that the Mbeta antigen distributed in the Golgi area where the colocalized viral glycoprotein antigen was also detected. Mbeta antigen was shown to be exposed on the surface of infected cells by both immunoprecipitation and FA staining with the mAb, whereby the cells might have become sensitive to the mAb-dependent complement-mediated cytolysis. Similarly, the Mbeta antigen was shown to be exposed on the virion surface, and the infectivity of the virus was destroyed by the mAb in the presence of a complement. Together with these results, we think that the M protein molecule takes either of two conformations, one (Mbeta) of which exposes the 3-9-16 epitope located in the N-terminal region of the M protein, that are also exposed on the surface of the virion and infected cells, whereby it might play a certain important role(s) in the virus replication process differently from the other form (Malpha), probably through its intimate association with the Golgi area and/or the cell membrane.

Two appendages homologous between basal bodies and centrioles are formed using distinct Odf2 domains

Analysis of Odf2 deletion mutants reveals regions important for the formation of basal body transition fibers and centriole distal appendages and distinct regions required for basal feet and subdistal appendages.

Ciliogenesis is regulated by context-dependent cellular cues, including some transduced through appendage-like structures on ciliary basal bodies called transition fibers and basal feet. However, the molecular basis for this regulation is not fully understood. The Odf2 gene product, ODF2/cenexin, is essential for both ciliogenesis and the formation of the distal and subdistal appendages on centrioles, which become basal bodies. We examined the effects of Odf2 deletion constructs on ciliogenesis in Odf2-knockout F9 cells. Electron microscopy revealed that ciliogenesis and transition fiber formation required the ODF2/cenexin fragment containing amino acids (aa) 188–806, whereas basal foot formation required aa 1–59 and 188–806. These sequences also formed distal and subdistal appendages, respectively, indicating that the centriole appendages are molecularly analogous to those on basal bodies. We used the differential formation of appendages by Odf2 deletion constructs to study the incorporation and function of molecules associated with each appendage type. We found that transition fibers and distal appendages were required for ciliogenesis and subdistal appendages stabilized the centrosomal microtubules.

Merlin sumoylation is required for its tumor suppressor activity

Merlin, encoded by the Neurofibromatosis 2 (NF2) gene, is a multifunctional tumor suppressor that integrates and regulates extracellular cues and intracellular signaling pathways, both at the plasma membrane and in the nucleus, to control cell proliferation, migration and invasion. Molecular mechanisms regulating merlin's tumor-suppressive activity have not been clearly defined. Here we report that merlin can be sumoylated on Lysine residue (K76) in vitro and in vivo. Sumoylation mediates merlin's intramolecular and intermolecular binding activities and regulates its cytoplasm/nucleus trafficking. Interestingly, sumoylation of merlin is regulated by its phosphorylation via Akt and PAK2 kinases. Mutation of K76 into arginine (R) abolishes its sumoylation, disrupts merlin cortical cytoskeleton residency and attenuates its stability. Using a K76R mutant merlin in a subcutaneous U87MG xenograft model, we demonstrate that merlin sumoylation is required for tumor-suppressive activity. Taken together, our findings indicate that merlin is sumoylated and that this post-translational modification is essential for tumor suppression.

SC35 promotes splicing of the C5-V6-C6 isoform of CD44 pre-mRNA

CD44 is a cell membrane glycoprotein that mediates the response of cells to their cellular microenvironment and regulates growth, survival, differentiation and motility. CD44 pre-mRNA contains 20 exons, 10 of which are alternatively spliced. Among the CD44 spliced variants, one of the V6 exon-containing isoforms, the V4-7 variant which contains variable exons 4, 5, 6 and 7, confers metastatic potential to non-metastatic cells. However, the splicing regulation of the V6 exon is not completely understood. SC35 is an arginine-serine rich protein that regulates alternative splicing of various pre-mRNAs. In the present study, we established a stable cell line which indicates inclusion or skipping of the V6 exon with the RFP or GFP signal. Using this stable cell line, we found that the V6 exon and flanking introns of CD44 pre-mRNA contained SC35 response elements that regulate V6 splicing. RT-PCR analyses of the endogenous CD44 splicing showed that SC35 promotes the production of the C5-V6-C6 isoform. shRNA knockdown of SC35 showed that reduced expression of SC35 decreased expression of the V6 exon-containing isoforms. Our results reveal a novel mechanism of CD44V6 splicing.

Interactome analysis reveals ezrin can adopt multiple conformational states

Ezrin, a member of the ezrin-radixin-moesin family (ERM), is an essential regulator of the structure of microvilli on the apical aspect of epithelial cells. Ezrin provides a linkage between membrane-associated proteins and F-actin, oscillating between active/open and inactive/closed states, and is regulated in part by phosphorylation of a C-terminal threonine. In the open state, ezrin can bind a number of ligands, but in the closed state the ligand-binding sites are inaccessible. In vitro analysis has proposed that there may be a third hyperactivated form of ezrin. To gain a better understanding of ezrin, we conducted an unbiased proteomic analysis of ezrin-binding proteins in an epithelial cell line, Jeg-3. We refined our list of interactors by comparing the interactomes using quantitative mass spectrometry between wild-type ezrin, closed ezrin, open ezrin, and hyperactivated ezrin. The analysis reveals several novel interactors confirmed by their localization to microvilli, as well as a significant class of proteins that bind closed ezrin. Taken together, the data indicate that ezrin can exist in three different conformational states, and different ligands "perceive" ezrin conformational states differently.

Clathrin-independent endocytosis: a cargo-centric view

Clathrin-independent endocytosis occurs in all cells and interest in this mode of cellular entry has grown. Although this form of endocytosis was first described for entry of bacterial toxins, here we focus our attention on the endogenous cell surface "cargo" proteins that enter cells by this mechanism. The cargo proteins entering by this mechanism are varied and include nutrient transporters, ion channels, cell adhesion molecules and proteins associated with the immune system. Despite the apparent lack of selection at the cell surface, we provide some examples of specific sorting of these cargo proteins after entry, leading to distinct itineraries and cellular fates.

Intracellular domain fragment of CD44 alters CD44 function in chondrocytes

The hyaluronan receptor CD44 undergoes sequential proteolytic cleavage at the cell surface. The initial cleavage of the CD44 extracellular domain is followed by a second intramembranous cleavage of the residual CD44 fragment, liberating the C-terminal cytoplasmic tail of CD44. In this study conditions that promote CD44 cleavage resulted in a diminished capacity to assemble and retain pericellular matrices even though sufficient non-degraded full-length CD44 remained. Using stable and transient overexpression of the cytoplasmic domain of CD44, we determined that the intracellular domain interfered with anchoring of the full-length CD44 to the cytoskeleton and disrupted the ability of the cells to bind hyaluronan and assemble a pericellular matrix. Co-immunoprecipitation assays were used to determine whether the mechanism of this interference was due to competition with actin adaptor proteins. CD44 of control chondrocytes was found to interact and co-immunoprecipitate with both the 65- and 130-kDa isoforms of ankyrin-3. Moreover, this interaction with ankyrin-3 proteins was diminished in cells overexpressing the CD44 intracellular domain. Mutating the putative ankyrin binding site of the transiently transfected CD44 intracellular domain diminished the inhibitory effects of this protein on matrix retention. Although CD44 in other cells types has been shown to interact with members of the ezrin/radixin/moesin (ERM) family of adaptor proteins, only modest interactions between CD44 and moesin could be demonstrated in chondrocytes. The data suggest that release of the CD44 intracellular domain into the cytoplasm of cells such as chondrocytes exerts a competitive or dominant-negative effect on the function of full-length CD44.

Expression of moesin and CD44 is associated with poor prognosis in gastric adenocarcinoma

AIMS

CD44 has been reported as a negative prognostic marker in gastric cancer. It interacts with moesin in epithelial-mesenchymal transition. To date, to our knowledge, there has been no clinical study dealing with the relationship between moesin and gastric adenocarcinoma. We analysed the expression of moesin and CD44 in gastric adenocarcinoma tissue, and correlations with clinicopathological factors.

METHODS AND RESULTS

A retrospective analysis was made of 430 patients who had undergone gastrectomy at the Korea University Guro Hospital between 2002 and 2005 for gastric adenocarcinoma. Using tissue microarray and immunohistochemical staining, moesin expression was observed in 192 (44.7%) cases; it was associated significantly with poorly differentiated histology, invasion depth, lymph node metastasis, lymphatic invasion and advanced pathological TNM stage. CD44 expression was not correlated with clinicopathological features or moesin expression. Moesin expression was a strong predictor of lymph node metastasis in logistic regression analysis. Both moesin expression and CD44 expression were associated significantly with poor overall survival in univariate analysis. Furthermore, in multivariate analysis, moesin and CD44 were independent markers of poor prognosis, along with pathological TNM stage and older patient age.

CONCLUSION

Moesin expression and CD44 expression might be useful markers of poor prognosis in gastric adenocarcinoma.

A link between two tumorigenic proteins, CD44 and p21WAF1: CD44 increases phorbol ester-induced expression of p21WAF1 by stabilizing its mRNA and extending protein half-life

The cell surface glycoprotein CD44 enhances phorbol-12-myristate 13-acetate (TPA)-induced expression of p21WAF1 by stabilizing its mRNA and enhancing the protein's half-life in several cell lines. Only the plasma membrane-anchored cytoplasmic tail of CD44 and its interacting ezrin, radixin, moesin (ERM) proteins are required for this effect. A mitogen activated kinase (MEK) inhibitor abolishes the action of CD44 on p21. Down-regulation of p21 dramatically decreased anchorage-independence of a cancer cell line, whereas CD44 expression in this background could partially rescue the phenotype.

The expanding family of FERM proteins

Our understanding of the FERM (4.1/ezrin/radixin/moesin) protein family has been rapidly expanding in the last few years, with the result that many new physiological functions have been ascribed to these biochemically unique proteins. In the present review, we will discuss a number of new FRMD (FERM domain)-containing proteins that were initially discovered from genome sequencing but are now being established through biochemical and genetic studies to be involved both in normal cellular processes, but are also associated with a variety of human diseases.

Desmoglein 3 promotes cancer cell migration and invasion by regulating activator protein 1 and protein kinase C-dependent-Ezrin activation

Desmoglein 3 (Dsg3), the pemphigus vulgaris antigen, has recently been shown to be upregulated in squamous cell carcinoma (SCC) and has been identified as a good tumor-specific marker for clinical staging of cervical sentinel lymph nodes in head and neck SCC. However, little is known about its biological function in cancer. The actin-binding protein Ezrin and the activator protein 1 (AP-1) transcription factor are implicated in cancer progression and metastasis. Here, we report that Dsg3 regulates the activity of c-Jun/AP-1 as well as protein kinase C (PKC)-mediated phosphorylation of Ezrin-Thr567, which contributes to the accelerated motility of cancer cells. Ectopic expression of Dsg3 in cancer cell lines caused enhanced phosphorylation at Ezrin-Thr567 with concomitant augmented membrane protrusions, cell spreading and invasive phenotype. We showed that Dsg3 formed a complex with Ezrin at the plasma membrane that was required for its proper function of interacting with F-actin and CD44 as Dsg3 knockdown impaired these associations. The increased Ezrin phosphorylation in Dsg3-overexpressing cells could be abrogated substantially by various pharmacological inhibitors for Ser/Thr kinases, including PKC and Rho kinase that are known to activate Ezrin. Furthermore, a marked increase in c-Jun S63 phosphorylation, among others, was found in Dsg3-overexpressing cells and the activation of c-Jun/AP-1 was further supported by a luciferase reporter assay. Taken together, our study identifies a novel Dsg3-mediated c-Jun/AP-1 regulatory mechanism and PKC-dependent Ezrin phosphorylation that could be responsible for Dsg3-associated cancer metastasis.

CD44 regulates vascular endothelial barrier integrity via a PECAM-1 dependent mechanism

Vascular integrity is a critical parameter in normal growth and development. Loss of appropriate vascular barrier function is present in various immune- and injury-mediated pathological conditions. CD44 is an adhesion molecule expressed by multiple cell types, including endothelial cells (EC). The goal of the present study was to examine how loss of CD44 affected vascular permeability. Using C57BL/6 WT and CD44-KO mice, we found no significant permeability to Evan's Blue in either strain at baseline. However, there was significantly increased histamine-induced permeability in CD44-deficient mice compared to WT counterparts. Similar results were observed in vitro, where CD44-deficient endothelial monolayers were also impermeable to 40kD-FITC dextran in the absence of vasoactive challenge, but exhibited enhanced and prolonged permeability following histamine. However, CD44-KO monolayers have reduced baseline barrier strength by electrical resistance, which correlated with increased permeability, at baseline, to smaller molecular weight 4-kD FITC-dextran, suggesting weakly formed endothelial junctions. The CD44-KO EC displayed several characteristics consistent with impaired barrier function/dysfunctional EC junctions, including differential expression, phosphorylation, and localization of endothelial junction proteins, increased matrix metalloprotease expression, and altered cellular morphology. Reduced platelet endothelial cell adhesion molecule-1 (PECAM-1) expression by CD44-KO EC in vivo and in vitro was also observed. Reconstitution of murine CD44 or PECAM-1 restored these defects to near WT status, suggesting CD44 regulates vascular permeability and integrity through a PECAM-1 dependent mechanism.

Mechanisms underlying cancer progression caused by ezrin overexpression in tongue squamous cell carcinoma

BACKGROUND

Ezrin is a member of the ezrin, radixin, and moesin family that provides a functional link between the plasma membrane and the cortical actin cytoskeleton. A correlation between ezrin overexpression and aggressive cancer behavior has been recently reported in various tumor types. However, its roles in the mechanisms underlying progression of tongue squamous cell carcinoma (SCC) are unclear.

METHOD

We used human tongue SCC and noncancerous tissue microarrays to immunohistochemically analyze the ezrin expression level and its relationship with proliferative activity. The human tongue SCC cell line HSC-3 was used to determine the effects of ezrin RNA interference (RNAi) on cancer cells during MTT; wound healing and invasion assays; immunofluorescence of the actin cytoskeleton; and western blotting of E-cadherin, N-cadherin, β-catenin, and the active and total RhoA/Rac1/cdc42.

RESULTS

Ezrin was overexpressed in 46.4% of the tumors examined in human tongue SCC tissue microarrays. Ezrin expression was correlated with the Ki-67 index. Ezrin depletion by RNAi in the HSC-3 cells significantly reduced cell proliferation, migration, and invasiveness and disturbed actin reorganization during podia formation. Its effects on RhoA/Rac1/cdc42 expression were not significant, whereas it enhanced E-cadherin and β-catenin expression and decreased N-cadherin expression.

CONCLUSIONS

Ezrin is often overexpressed in primary tongue SCCs and may have an important role in their growth, migration, and invasiveness possibly via its relationship with the E-cadherin/β-catenin complex and the cadherin switch. Thus, ezrin could be a therapeutic target in tongue SCC.

Expression of CD44 in articular cartilage is associated with disease severity in knee osteoarthritis

OBJECTIVES

To investigate CD44 levels in articular cartilage of knee osteoarthritis (OA) and the relationship between CD44 and severity of the disease.

METHODS

All 50 cartilage tissues included normal and OA cartilage, and were ascribed to the following four groups on the basis of modified Mankin score: normal, mild lesions, moderate lesions and severe lesions. CD44 levels in articular cartilage were assessed by immunohistochemical methods.

RESULTS

CD44 levels were detected in all four groups. The difference in average gray value of CD44 expression showed statistical significance when compared between each group (P < 0.05). In addition, CD44 expression in each group correlated with disease severity, according to the modified Mankin score (ρ = -0.848, P < 0.01).

CONCLUSIONS

CD44 in articular cartilage is associated with progressive knee OA joint damage.

Phosphorylation of moesin by c-Jun N-terminal kinase is important for podosome rosette formation in Src-transformed fibroblasts

Podosomes are actin-based membrane protrusions that facilitate extracellular matrix degradation and invasive cell motility. Podosomes can self-organize into large rosette-like structures in Src-transformed fibroblasts, osteoclasts, and some highly invasive cancer cells. However, the mechanism of this assembly remains obscure. In this study, we show that the suppression of c-Jun N-terminal kinase (JNK) by the JNK inhibitor SP600125 or short-hairpin RNA inhibited podosome rosette formation in SrcY527F-transformed NIH3T3 fibroblasts. In addition, SrcY527F was less potent to induce podosome rosettes in JNK1-null or JNK2-null mouse embryo fibroblasts than in their wild-type counterparts. The kinase activity of JNK was essential for promoting podosome rosette formation but not for its localization to podosome rosettes. Moesin, a member of the ERM (ezrin, radixin, and moesin) protein family, was identified as a substrate of JNK. We show that the phosphorylation of moesin at Thr558 by JNK was important for podosome rosette formation in SrcY527F-transformed NIH3T3 fibroblasts. Taken together, our results unveil a novel role of JNK in podosome rosette formation by phosphorylating moesin.

Control of actin dynamics by allosteric regulation of actin binding proteins

The regulated assembly and organization of actin filaments allows the cell to construct a large diversity of actin-based structures specifically suited to a range of cellular processes. A vast array of actin regulatory proteins must work in concert to form specific actin networks within cells, and spatial and temporal requirements for actin assembly necessitate rapid regulation of protein activity. This chapter explores a common mechanism of controlling the activity of actin binding proteins: allosteric autoinhibition by interdomain head-tail interactions. Intramolecular interactions maintain these proteins in a closed conformation that masks protein domains needed to regulate actin dynamics. Autoinhibition is typically relieved by two or more ligand binding and/or posttranslational modification events that expose key protein domains. Regulation through multiple inputs permits precise temporal and spatial control of protein activity to guide actin network formation.

Regulation of ROCK activity in cancer

Cancer-associated changes in cellular behavior, such as modified cell-cell contact, increased migratory potential, and generation of cellular force, all require alteration of the cytoskeleton. Two homologous mammalian serine/threonine kinases, Rho-associated protein kinases (ROCK I and II), are key regulators of the actin cytoskeleton acting downstream of the small GTPase Rho. ROCK is associated with cancer progression, and ROCK protein expression is elevated in several types of cancer. ROCKs exist in a closed, inactive conformation under quiescent conditions, which is changed to an open, active conformation by the direct binding of guanosine triphosphate (GTP)–loaded Rho. In recent years, a number of ROCK isoform-specific binding partners have been found to modulate the kinase activity through direct interactions with the catalytic domain or via altered cellular localization of the kinases. Thus, these findings demonstrate additional modes to regulate ROCK activity. This review describes the molecular mechanisms of ROCK activity regulation in cancer, with emphasis on ROCK isoform-specific regulation and interaction partners, and discusses the potential of ROCKs as therapeutic targets in cancer.

Lipid raft association restricts CD44-ezrin interaction and promotion of breast cancer cell migration

Cancer cell migration is an early event in metastasis, the main cause of breast cancer-related deaths. Cholesterol-enriched membrane domains called lipid rafts influence the function of many molecules, including the raft-associated protein CD44. We describe a novel mechanism whereby rafts regulate interactions between CD44 and its binding partner ezrin in migrating breast cancer cells. Specifically, in nonmigrating cells, CD44 and ezrin localized to different membranous compartments: CD44 predominantly in rafts, and ezrin in nonraft compartments. After the induction of migration (either nonspecific or CD44-driven), CD44 affiliation with lipid rafts was decreased. This was accompanied by increased coprecipitation of CD44 and active (threonine-phosphorylated) ezrin-radixin-moesin (ERM) proteins in nonraft compartments and increased colocalization of CD44 with the nonraft protein, transferrin receptor. Pharmacological raft disruption using methyl-β-cyclodextrin also increased CD44-ezrin coprecipitation and colocalization, further suggesting that CD44 interacts with ezrin outside rafts during migration. Conversely, promoting CD44 retention inside lipid rafts by pharmacological inhibition of depalmitoylation virtually abolished CD44-ezrin interactions. However, transient single or double knockdown of flotillin-1 or caveolin-1 was not sufficient to increase cell migration over a short time course, suggesting complex crosstalk mechanisms. We propose a new model for CD44-dependent breast cancer cell migration, where CD44 must relocalize outside lipid rafts to drive cell migration. This could have implications for rafts as pharmacological targets to down-regulate cancer cell migration.

The ezrin metastatic phenotype is associated with the initiation of protein translation

We previously associated the cytoskeleton linker protein, Ezrin, with the metastatic phenotype of pediatric sarcomas, including osteosarcoma and rhabdomyosarcoma. These studies have suggested that Ezrin contributes to the survival of cancer cells after their arrival at secondary metastatic locations. To better understand this role in metastasis, we undertook two noncandidate analyses of Ezrin function including a microarray subtraction of high-and low-Ezrin-expressing cells and a proteomic approach to identify proteins that bound the N-terminus of Ezrin in tumor lysates. Functional analyses of these data led to a novel and unifying hypothesis that Ezrin contributes to the efficiency of metastasis through regulation of protein translation. In support of this hypothesis, we found Ezrin to be part of the ribonucleoprotein complex to facilitate the expression of complex messenger RNA in cells and to bind with poly A binding protein 1 (PABP1; PABPC1). The relevance of these findings was supported by our identification of Ezrin and components of the translational machinery in pseudopodia of highly metastatic cells during the process of cell invasion. Finally, two small molecule inhibitors recently shown to inhibit the Ezrin metastatic phenotype disrupted the Ezrin/PABP1 association. Taken together, these results provide a novel mechanistic basis by which Ezrin may contribute to metastasis.

Modulation of T cell immune functions by the prostaglandin E(2) - cAMP pathway in chronic inflammatory states

Cyclic AMP is the intracellular second messenger for a variety of immunoregulatory inflammatory mediators such as prostaglandin E2, adenosine and histamine that signal to effector T cells from monocytes, macrophages and regulatory T cells. Protein kinase A (PKA) type I localizes to lipid rafts in effector T cells during T cell activation and directly modulates proximal signal events including phosphorylation of C-terminal Src kinase (Csk), which initiates a negative signal pathway that fine-tunes the T cell activation process. The PKA-Csk immunoregulatory pathway is scaffolded by the A kinase anchoring protein ezrin, the Csk binding protein phosphoprotein associated with glycosphingolipid-enriched membrane microdomains and the linker protein ezrin/radixin/moesin binding protein of 50 kDa. This pathway is hyperactivated in chronic infections with an inflammatory component such as HIV, other immunodeficiencies and around solid tumours as a consequence of local inflammation leading to inhibition of anti-tumour immunity. LINKED ARTICLES This article is part of a themed section on Novel cAMP Signalling Paradigms. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.166.issue-2.

Therapeutic targeting of hyaluronan in the tumor stroma

The tumor stroma, consisting of non-malignant cells and the extracellular matrix, undergoes significant quantitative and qualitative changes throughout malignant transformation and tumor progression. With increasing recognition of the role of the tumor microenvironment in disease progression, stromal components of the tumor have become attractive targets for therapeutic intervention. Stromal accumulation of the glycosaminoglycan hyaluronan occurs in many tumor types and is frequently associated with a negative disease prognosis. Hyaluronan interacts with other extracellular molecules as well as cellular receptors to form a complex interaction network influencing physicochemical properties, signal transduction, and biological behavior of cancer cells. In preclinical animal models, enzymatic removal of hyaluronan is associated with remodeling of the tumor stroma, reduction of tumor interstitial fluid pressure, expansion of tumor blood vessels and facilitated delivery of chemotherapy. This leads to inhibition of tumor growth and increased survival. Current evidence shows that abnormal accumulation of hyaluronan may be an important stromal target for cancer therapy. In this review we highlight the role of hyaluronan and hyaluronan-mediated interactions in cancer, and discuss historical and recent data on hyaluronidase-based therapies and the effect of hyaluronan removal on tumor growth.

Merlin: a tumour suppressor with functions at the cell cortex and in the nucleus

Inhibition of proliferation by cell-to-cell contact is essential for tissue organization, and its disruption contributes to tumorigenesis. The FERM domain protein Merlin, encoded by the NF2 tumour suppressor gene, is an important mediator of contact inhibition. Merlin was thought to inhibit mitogenic signalling and activate the Hippo pathway by interacting with diverse target-effectors at or near the plasma membrane. However, recent studies highlight that Merlin pleiotropically affects signalling by migrating into the nucleus and inducing a growth-suppressive programme of gene expression through its direct inhibition of the CRL4DCAF1 E3 ubiquitin ligase. In addition, Merlin promotes the establishment of epithelial adhesion and polarity by recruiting Par3 and aPKC to E-cadherin-dependent junctions, and by ensuring the assembly of tight junctions. These recent advances suggest that Merlin acts at the cell cortex and in the nucleus in a similar, albeit antithetic, manner to the oncogene β-catenin.

Clinicopathological significance of fascin and CD44v6 expression in endometrioid carcinoma

Background

Fascin and CD44v6 may have significant roles as biomarkers in tumour progression and metastasis. In endometrioid carcinomas, the fascin expression profile is less defined, and the significance of CD44v6 is uncertain. We aimed to investigate the expressions of both fascin and CD44v6 in endometrioid carcinomas and to evaluate their inter-relation with clinicopathological parameters.

Methods

Fascin and CD44v6 expressions were evaluated, individually and in combination, in a series of 47 endometrioid carcinomas and 10 proliferative endometrium samples. The staining extent and intensity of both markers in tumour cells were scored semiquantitatively. The relationship between immunoexpressions and clinicopathological variables was assessed.

Results

The expression rates of fascin and CD44v6 in endometrioid carcinoma were 72.34% and 46.80%, respectively. Although these expression rates were higher than those in proliferative endometrial samples, fascin expression showed a statistically significant difference from the normal group (p = 0.02), but CD44v6 did not differ (p = 0.54). Fascin expression was significantly correlated with tumour grade (p = 0.003) and neural invasion (p = 0.036) in a univariate analysis. In contrast, no significant correlation was found between CD44v6 and any of the clinicopathological parameters.

Conclusions

Our findings suggest that fascin might be an independent prognostic indicator in the different steps of extracellular matrix invasion. On the other hand, CD44v6 was not a predictive factor in endometrioid cancer.

Virtual Slides

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/8511594927206899.

Thrombomodulin is an ezrin-interacting protein that controls epithelial morphology and promotes collective cell migration

Adhesive interactions between cells are needed to maintain tissue architecture during development, tissue renewal and wound healing. Thrombomodulin (TM) is an integral membrane protein that participates in cell-cell adhesion through its extracellular lectin-like domain. However, the molecular basis of TM-mediated cell-cell adhesion is poorly understood. Here, we demonstrate that TM is linked to the actin cytoskeleton via ezrin. In vitro binding assays showed that the TM cytoplasmic domain bound directly to the N-terminal domain of ezrin. Mutational analysis of the TM cytoplasmic domain identified (522)RKK(524) as important ezrin-binding residues. In epidermal epithelial A431 cells, TM colocalized with ezrin and actin filaments at cell-cell contacts. Knockdown of endogenous TM expression by RNA interference induced morphological changes and accelerated cell migration in A431 cells. Moreover, epidermal growth factor, upstream of ezrin activation, stimulated the interaction between ezrin and TM. In skin wound healing of mice, TM and ezrin were highly expressed in neoepidermis, implying that both proteins are key molecules in reepithelialization that requires collective cell migration of epithelial cells. Finally, exogenous expression of TM in TM-deficient melanoma A2058 cells promoted collective cell migration. In summary, TM, which associates with ezrin and actin filaments, maintains epithelial morphology and promotes collective cell migration.

Tunneling nanotubes provide a unique conduit for intercellular transfer of cellular contents in human malignant pleural mesothelioma

Tunneling nanotubes are long, non-adherent F-actin-based cytoplasmic extensions which connect proximal or distant cells and facilitate intercellular transfer. The identification of nanotubes has been limited to cell lines, and their role in cancer remains unclear. We detected tunneling nanotubes in mesothelioma cell lines and primary human mesothelioma cells. Using a low serum, hyperglycemic, acidic growth medium, we stimulated nanotube formation and bidirectional transfer of vesicles, proteins, and mitochondria between cells. Notably, nanotubes developed between malignant cells or between normal mesothelial cells, but not between malignant and normal cells. Immunofluorescent staining revealed their actin-based assembly and structure. Metformin and an mTor inhibitor, Everolimus, effectively suppressed nanotube formation. Confocal microscopy with 3-dimensional reconstructions of sectioned surgical specimens demonstrated for the first time the presence of nanotubes in human mesothelioma and lung adenocarcinoma tumor specimens. We provide the first evidence of tunneling nanotubes in human primary tumors and cancer cells and propose that these structures play an important role in cancer cell pathogenesis and invasion.

Characterization of the effects of cross-linking of macrophage CD44 associated with increased phagocytosis of apoptotic PMN

Control of macrophage capacity for apoptotic cell clearance by soluble mediators such as cytokines, prostaglandins and lipoxins, serum proteins, and glucocorticoids may critically determine the rate at which inflammation resolves. Previous studies suggested that macrophage capacity for clearance of apoptotic neutrophils was profoundly altered following binding of CD44 antibodies. We have used a number of different approaches to further define the mechanism by which CD44 rapidly and specifically augment phagocytosis of apoptotic neutrophils. Use of Fab' fragments unequivocally demonstrated a requirement for cross-linking of macrophage surface CD44. The molecular mechanism of CD44-augmented phagocytosis was shown to be opsonin-independent and to be distinct from the Mer/protein S pathway induced by glucocorticoids and was not functional for clearance of apoptotic eosinophils. CD44-cross-linking also altered macrophage migration and induced cytoskeletal re-organisation together with phosphorylation of paxillin and activation of Rac2. Investigation of signal transduction pathways that might be critical for CD44 augmentation of phagocytosis revealed that Ca²⁺ signalling, PI-3 kinase pathways and altered cAMP signalling were not involved, but did implicate a key role for tyrosine phosphorylation events. Finally, although CD44 antibodies were able to augment phagocytosis of apoptotic neutrophils by murine peritoneal and bone marrow-derived macrophages, we did not observe a difference in the clearance of neutrophils following induction of peritonitis with thioglycollate in CD44-deficient animals. Together, these data demonstrate that CD44 cross-linking induces a serum opsonin-independent mechanism of macrophage phagocytosis of apoptotic neutrophils that is associated with reduced macrophage migration and cytoskeletal reorganisation.

Enhancement of metastatic ability by ectopic expression of ST6GalNAcI on a gastric cancer cell line in a mouse model

ST6GalNAcI is a sialyltransferase responsible for the synthesis of sialyl Tn (sTn) antigen which is expressed in a variety of adenocarcinomas including gastric cancer especially in advanced cases, but the roles of ST6GalNAcI and sTn in cancer progression are largely unknown. We generated sTn-expressing human gastric cancer cells by ectopic expression of ST6GalNAcI to evaluate metastatic ability of these cells and prognostic effect of ST6GalNAcI and sTn in a mouse model, and identified sTn carrier proteins to gain insight into the function of ST6GalNAcI and sTn in gastric cancer progression. A green fluorescent protein-tagged human gastric cancer cell line was transfected with ST6GalNAcI to produce sTn-expressing cells, which were transplanted into nude mice. STn-positive gastric cancer cells showed higher intraperitoneal metastatic ability in comparison with sTn-negative control, resulting in shortened survival time of the mice, which was mitigated by anti-sTn antibody administration. Then, sTn-carrying proteins were immunoprecipitated from culture supernatants and lysates of these cells, and identified MUC1 and CD44 as major sTn carriers. It was confirmed that MUC1 carries sTn also in human advanced gastric cancer tissues. Identification of sTn carrier proteins will help understand mechanisms of metastatic phenotype acquisition of gastric cancer cells by ST6GalNAcI and sTn.

Dysregulation of ezrin phosphorylation prevents metastasis and alters cellular metabolism in osteosarcoma

Ezrin links the plasma membrane to the actin cytoskeleton where it plays a pivotal role in the metastatic progression of several human cancers; however, the precise mechanistic basis for its role remains unknown. Here, we define transitions between active (phosphorylated open) and inactive (dephosphorylated closed) forms of Ezrin that occur during metastatic progression in osteosarcoma. In our evaluation of these conformations we expressed C-terminal mutant forms of Ezrin that are open (phosphomimetic T567D) or closed (phosphodeficient T567A) and compared their biologic characteristics to full-length wild-type Ezrin in osteosarcoma cells. Unexpectedly, cells expressing open, active Ezrin could form neither primary orthotopic tumors nor lung metastases. In contrast, cells expressing closed, inactive Ezrin were also deficient in metastasis but were unaffected in their capacity for primary tumor growth. By imaging single metastatic cells in the lung, we found that cells expressing either open or closed Ezrin displayed increased levels of apoptosis early after their arrival in the lung. Gene expression analysis suggested dysregulation of genes that are functionally linked to carbohydrate and amino acid metabolism. In particular, cells expressing closed, inactive Ezrin exhibited reduced lactate production and basal or ATP-dependent oxygen consumption. Collectively, our results suggest that dynamic regulation of Ezrin phosphorylation at amino acid T567 that controls structural transitions of this protein plays a pivotal role in tumor progression and metastasis, possibly in part by altering cellular metabolism.

SOX2-RNAi attenuates S-phase entry and induces RhoA-dependent switch to protease-independent amoeboid migration in human glioma cells

BACKGROUND

SOX2, a high mobility group (HMG)-box containing transcription factor, is a key regulator during development of the nervous system and a persistent marker of neural stem cells. Recent studies suggested a role of SOX2 in tumor progression. In our previous work we detected SOX2 in glioma cells and glioblastoma specimens. Herein, we aim to explore the role of SOX2 for glioma malignancy in particular its role in cell proliferation and migration.

METHODS

Retroviral shRNA-vectors were utilized to stably knockdown SOX2 in U343-MG and U373-MG cells. The resulting phenotype was investigated by Western blot, migration/invasion assays, RhoA G-LISA, time lapse video imaging, and orthotopic xenograft experiments.

RESULTS

SOX2 depletion results in pleiotropic effects including attenuated cell proliferation caused by decreased levels of cyclinD1. Also an increased TCF/LEF-signaling and concomitant decrease in Oct4 and Nestin expression was noted. Furthermore, down-regulation of focal adhesion kinase (FAK) signaling and of downstream proteins such as HEF1/NEDD9, matrix metalloproteinases pro-MMP-1 and -2 impaired invasive proteolysis-dependent migration. Yet, cells with knockdown of SOX2 switched to a RhoA-dependent amoeboid-like migration mode which could be blocked by the ROCK inhibitor Y27632 downstream of RhoA-signaling. Orthotopic xenograft experiments revealed a higher tumorigenicity of U343-MG glioma cells transduced with shRNA targeting SOX2 which was characterized by increased dissemination of glioma cells.

CONCLUSION

Our findings suggest that SOX2 plays a role in the maintenance of a less differentiated glioma cell phenotype. In addition, the results indicate a critical role of SOX2 in adhesion and migration of malignant gliomas.

The biology of CD44 and HCELL in hematopoiesis: the 'step 2-bypass pathway' and other emerging perspectives

PURPOSE OF REVIEW

The homing and egress of hematopoietic stem and progenitor cells (HSPCs) to and from marrow, respectively, and the proliferation and differentiation of HSPCs within marrow are complex processes critically regulated by the ordered expression and function of adhesion molecules that direct key cell-cell and cell-matrix interactions. The integral membrane molecule CD44, known primarily for its role in binding hyaluronic acid, is characteristically expressed on HSPCs. Conspicuously, human HSPCs uniquely display a specialized glycoform of CD44 known as hematopoietic cell E-/L-selectin ligand (HCELL), which is the most potent ligand for both E-selectin and L-selectin expressed on human cells. This review focuses on recent advances in our understanding of the biology of CD44 and HCELL in hematopoiesis.

RECENT FINDINGS

New data indicate that CD44-mediated events in hematopoiesis are more complex than previously imagined. Ex-vivo glycan engineering has established that HCELL serves as a 'bone marrow homing receptor'. Moreover, biochemical studies now show that CD44 forms bimolecular complexes with a variety of membrane proteins, one of which is VLA-4. Engagement of CD44 or of HCELL directly induces VLA-4 activation via G-protein-dependent signaling, triggering a 'step 2-bypass pathway' of cell migration, and extravascular lodgment, in absence of chemokine receptor engagement.

SUMMARY

Recent studies have further clarified the roles of CD44 and its glycoform HCELL in hematopoietic processes, providing key insights on how targeting these molecules may be beneficial in promoting hematopoiesis and in treating hematologic malignancies.

EphrinA1-EphA2 signal induces compaction and polarization of Madin-Darby canine kidney cells by inactivating Ezrin through negative regulation of RhoA

The epithelial cells exhibit either a columnar or a flat shape dependent on extracellular stimuli or the cell-cell adhesion. Membrane-anchored ephrinA stimulates EphA receptor tyrosine kinases as a ligand in a cell-cell contact-dependent manner. The mechanism through which ephrinA1/EphA2 signal regulates the cell morphology remains elusive. We demonstrate here that ephrinA1/EphA2 signal induces compaction and enhanced polarization (columnar change) of Madin-Darby canine kidney epithelial cells by regulating Ezrin, a linker that connects plasma membrane and actin cytoskeleton. Activation of EphA2 resulted in RhoA inactivation through p190RhoGAP-A and subsequent dephosphorylation of Ezrin on Thr-567 phosphorylated by Rho kinase. Consistently, the cells expressing an active mutant of Ezrin in which Thr-567 was replaced with Asp did not change their shape in response to ephrinA1. Furthermore, depletion of Ezrin led to compaction and enhanced polarization without ephrinA1 stimulation, suggesting the role for active Ezrin in keeping the flat cell shape. Ezrin localized to apical domain irrespective of ephrinA1 stimulation, whereas phosphorylated Ezrin on the apical domain was reduced by ephrinA1 stimulation. Collectively, ephrinA1/EphA2 signal negatively regulates Ezrin and promotes the alteration of cell shape, from flat to columnar shape.