Phosphorylation of a conserved integrin alpha 3 QPSXXE motif regulates signaling, motility, and cytoskeletal engagement

KRAS Addiction Promotes Cancer Cell Adaptation in Harsh Microenvironment Through Macropinocytosis

KRAS is the most frequently mutated oncogene in cancer and despite intensive studies, attempts to develop effective therapies targeting KRAS or its downstream signaling have failed mostly due to the complexity of KRAS activation and function in cancer initiation and progression. Over the years, KRAS has been involved in several biological processes including cell survival, proliferation, and metabolism by promoting not only a favorable tumor environment but also a cell-microenvironment dialog to allow cancer cells to adapt to tumor microenvironment scarcity. One of the mechanisms involved in this adaption is KRAS-mediated macropinocytosis. Macropinocytosis is an evolutionarily conserved, large-scale, and nonselective form of endocytosis involving actin-driven cell membrane remodeling to engulf large amounts of extracellular fluids and proteins from the local environment. While macropinocytosis process has been known for decades, recent gain interest due to its regulation of KRAS-driven tumor growth in adverse microenvironments. By promoting extracellular protein and other macromolecules internalization, macropinocytosis provides a survival mechanism under nutrient scarce conditions and the potential for unrestricted tumor growth. Thus, a better understanding of macropinocytotic process is needed to develop alternative therapeutic strategies.

β2 Integrin CD11d/CD18: From Expression to an Emerging Role in Staged Leukocyte Migration

CD11d/CD18 is the most recently discovered and least understood β2 integrin. Known CD11d adhesive mechanisms contribute to both extravasation and mesenchymal migration – two key aspects for localizing peripheral leukocytes to sites of inflammation. Differential expression of CD11d induces differences in monocyte/macrophage mesenchymal migration including impacts on macrophage sub-set migration. The participation of CD11d/CD18 in leukocyte localization during atherosclerosis and following neurotrauma has sparked interest in the development of CD11d-targeted therapeutic agents. Whereas the adhesive properties of CD11d have undergone investigation, the signalling pathways induced by ligand binding remain largely undefined. Underlining each adhesive and signalling function, CD11d is under unique transcriptional control and expressed on a sub-set of predominately tissue-differentiated innate leukocytes. The following review is the first to capture the nearly three decades of CD11d research and discusses the emerging role of CD11d in leukocyte migration and retention during the progression of a staged immune response.

Quantitative phosphoproteomic analysis identifies the potential therapeutic target EphA2 for overcoming sorafenib resistance in hepatocellular carcinoma cells

Limited therapeutic options are available for advanced-stage hepatocellular carcinoma owing to its poor diagnosis. Drug resistance to sorafenib, the only available targeted agent, is commonly reported. The comprehensive elucidation of the mechanisms underlying sorafenib resistance may thus aid in the development of more efficacious therapeutic agents. To clarify the signaling changes contributing to resistance, we applied quantitative phosphoproteomics to analyze the differential phosphorylation changes between parental and sorafenib-resistant HuH-7 cells. Consequently, an average of ~1500 differential phosphoproteins were identified and quantified, among which 533 were significantly upregulated in resistant cells. Further bioinformatic integration via functional categorization annotation, pathway enrichment and interaction linkage analysis led to the discovery of alterations in pathways associated with cell adhesion and motility, cell survival and cell growth and the identification of a novel target, EphA2, in resistant HuH-7^R cells. In vitro functional analysis indicated that the suppression of EphA2 function impairs cell proliferation and motility and, most importantly, overcomes sorafenib resistance. The attenuation of sorafenib resistance may be achieved prior to its development through the modulation of EphA2 and the subsequent inhibition of Akt activity. Binding analyses and in silico modeling revealed a ligand mimic lead compound, prazosin, that could abate the ligand-independent oncogenic activity of EphA2. Finally, data obtained from in vivo animal models verified that the simultaneous inhibition of EphA2 with sorafenib treatment can effectively overcome sorafenib resistance and extend the projected survival of resistant tumor-bearing mice. Thus our findings regarding the targeting of EphA2 may provide an effective approach for overcoming sorafenib resistance and may contribute to the management of advanced hepatocellular carcinoma.

Tetraspanin CD9: A Key Regulator of Cell Adhesion in the Immune System

The tetraspanin CD9 is expressed by all the major subsets of leukocytes (B cells, CD4⁺ T cells, CD8⁺ T cells, natural killer cells, granulocytes, monocytes and macrophages, and immature and mature dendritic cells) and also at a high level by endothelial cells. As a typical member of the tetraspanin superfamily, a prominent feature of CD9 is its propensity to engage in a multitude of interactions with other tetraspanins as well as with different transmembrane and intracellular proteins within the context of defined membranal domains termed tetraspanin-enriched microdomains (TEMs). Through these associations, CD9 influences many cellular activities in the different subtypes of leukocytes and in endothelial cells, including intracellular signaling, proliferation, activation, survival, migration, invasion, adhesion, and diapedesis. Several excellent reviews have already covered the topic of how tetraspanins, including CD9, regulate these cellular processes in the different cells of the immune system. In this mini-review, however, we will focus particularly on describing and discussing the regulatory effects exerted by CD9 on different adhesion molecules that play pivotal roles in the physiology of leukocytes and endothelial cells, with a particular emphasis in the regulation of adhesion molecules of the integrin and immunoglobulin superfamilies.

Molecular Basis of Laminin-Integrin Interactions

Laminins are composed of three polypeptide chains, designated as α, β, and γ. The C-terminal region of laminin heterotrimers, containing coiled-coil regions, short tails, and laminin globular (LG) domains, is necessary and sufficient for binding to integrins, which are the major laminin receptor class. Laminin recognition by integrins critically requires the α chain LG domains and a glutamic acid residue of the γ chain at the third position from the C-terminus. Furthermore, the C-terminal region of the β chain contains a short amino acid sequence that modulates laminin affinity for integrins. Thus, all three of the laminin chains act cooperatively to facilitate integrin binding. Mammals possess 5 α (α1-5), 3 β (β1-3), and 3 γ (γ1-3) chains, combinations of which give rise to 16 distinct laminin isoforms. Each isoform is expressed in a tissue-specific and developmental stage-specific manner, exerting its functions through binding of integrins. In this review, we detail the current knowledge surrounding the molecular basis and physiological relevance of specific interactions between laminins and integrins, and describe the mechanisms underlying laminin action through integrins.

Tetraspanins as regulators of the tumour microenvironment: implications for metastasis and therapeutic strategies

One of the hallmarks of cancer is the ability to activate invasion and metastasis. Cancer morbidity and mortality are largely related to the spread of the primary, localized tumour to adjacent and distant sites. Appropriate management and treatment decisions based on predicting metastatic disease at the time of diagnosis is thus crucial, which supports better understanding of the metastatic process. There are components of metastasis that are common to all primary tumours: dissociation from the primary tumour mass, reorganization/remodelling of extracellular matrix, cell migration, recognition and movement through endothelial cells and the vascular circulation and lodgement and proliferation within ectopic stroma. One of the key and initial events is the increased ability of cancer cells to move, escaping the regulation of normal physiological control. The cellular cytoskeleton plays an important role in cancer cell motility and active cytoskeletal rearrangement can result in metastatic disease. This active change in cytoskeletal dynamics results in manipulation of plasma membrane and cellular balance between cellular adhesion and motility which in turn determines cancer cell movement. Members of the tetraspanin family of proteins play important roles in regulation of cancer cell migration and cancer-endothelial cell interactions, which are critical for cancer invasion and metastasis. Their involvements in active cytoskeletal dynamics, cancer metastasis and potential clinical application will be discussed in this review. In particular, the tetraspanin member, CD151, is highlighted for its major role in cancer invasion and metastasis.

LINKED ARTICLES

This article is part of a themed section on Cytoskeleton, Extracellular Matrix, Cell Migration, Wound Healing and Related Topics. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-24.

The CD9/CD81 tetraspanin complex and tetraspanin CD151 regulate α3β1 integrin-dependent tumor cell behaviors by overlapping but distinct mechanisms

Integrin α3β1 potently promotes cell motility on its ligands, laminin-332 and laminin-511, and this may help to explain why α3β1 has repeatedly been linked to breast carcinoma progression and metastasis. The pro-migratory functions of α3β1 depend strongly on lateral interactions with cell surface tetraspanin proteins. Tetraspanin CD151 interacts directly with the α3 integrin subunit and links α3β1 integrin to other tetraspanins, including CD9 and CD81. Loss of CD151 disrupts α3β1 association with other tetraspanins and impairs α3β1-dependent motility. However, the extent to which tetraspanins other than CD151 are required for specific α3β1 functions is unclear. To begin to clarify which aspects of α3β1 function require which tetraspanins, we created breast carcinoma cells depleted of both CD9 and CD81 by RNA interference. Silencing both of these closely related tetraspanins was required to uncover their contributions to α3β1 function. We then directly compared our CD9/CD81-silenced cells to CD151-silenced cells. Both CD9/CD81-silenced cells and CD151-silenced cells showed delayed α3β1-dependent cell spreading on laminin-332. Surprisingly, however, once fully spread, CD9/CD81-silenced cells, but not CD151-silenced cells, displayed impaired α3β1-dependent directed motility and altered front-rear cell morphology. Also unexpectedly, the CD9/CD81 complex, but not CD151, was required to promote α3β1 association with PKCα in breast carcinoma cells, and a PKC inhibitor mimicked aspects of the CD9/CD81-silenced cell motility defect. Our data reveal overlapping, but surprisingly distinct contributions of specific tetraspanins to α3β1 integrin function. Importantly, some of CD9/CD81's α3β1 regulatory functions may not require CD9/CD81 to be physically linked to α3β1 by CD151.

CD81 regulates cell migration through its association with Rac GTPase

Data presented here provide evidence for a new direct interaction of the GTPase Rac with the C-terminal cytoplasmic domain of tetraspanin CD81. Tetraspanin-enriched, microdomain-dependent compartmentalization is a novel regulatory mechanism of Rac activity turnover, which provides a novel mechanism for regulation of cell motility by tetraspanins.

CD81 is a member of the tetraspanin family that has been described to have a key role in cell migration of tumor and immune cells. To unravel the mechanisms of CD81-regulated cell migration, we performed proteomic analyses that revealed an interaction of the tetraspanin C-terminal domain with the small GTPase Rac. Direct interaction was confirmed biochemically. Moreover, microscopy cross-correlation analysis demonstrated the in situ integration of both molecules into the same molecular complex. Pull-down experiments revealed that CD81-Rac interaction was direct and independent of Rac activation status. Knockdown of CD81 resulted in enhanced protrusion rate, altered focal adhesion formation, and decreased cell migration, correlating with increased active Rac. Reexpression of wild-type CD81, but not its truncated form lacking the C-terminal cytoplasmic domain, rescued these effects. The phenotype of CD81 knockdown cells was mimicked by treatment with a soluble peptide with the C-terminal sequence of the tetraspanin. Our data show that the interaction of Rac with the C-terminal cytoplasmic domain of CD81 is a novel regulatory mechanism of the GTPase activity turnover. Furthermore, they provide a novel mechanism for tetraspanin-dependent regulation of cell motility and open new avenues for tetraspanin-targeted reagents by the use of cell-permeable peptides.

Extracellular phosphorylation and phosphorylated proteins: not just curiosities but physiologically important

Mining of the literature and high-throughput mass spectrometry data from both healthy and diseased tissues and from body fluids reveals evidence that various extracellular proteins can exist in phosphorylated states. Extracellular kinases and phosphatases (ectokinases and ectophosphatases) are active in extracellular spaces during times of sufficiently high concentrations of adenosine triphosphate. There is evidence for a role of extracellular phosphorylation in various physiological functions, including blood coagulation, immune cell activation, and the formation of neuronal networks. Ectokinase activity is increased in some diseases, including cancer, Alzheimer's disease, and some microbial infections. We summarize the literature supporting the physiological and pathological roles of extracellularly localized protein kinases, protein phosphatases, and phosphorylated proteins and provide an analysis of the available mass spectrometry data to annotate potential extracellular phosphorylated proteins.

Tetraspanin CD82 inhibits protrusion and retraction in cell movement by attenuating the plasma membrane-dependent actin organization

To determine how tetraspanin KAI1/CD82, a tumor metastasis suppressor, inhibits cell migration, we assessed which cellular events critical for motility are altered by KAI1/CD82 and how KAI1/CD82 regulates these events. We found that KAI1/CD82-expressing cells typically exhibited elongated cellular tails and diminished lamellipodia. Live imaging demonstrated that the polarized protrusion and retraction of the plasma membrane became deficient upon KAI1/CD82 expression. The deficiency in developing these motility-related cellular events was caused by poor formations of actin cortical network and stress fiber and by aberrant dynamics in actin organization. Rac1 activity was reduced by KAI1/CD82, consistent with the diminution of lamellipodia and actin cortical network; while the growth factor-stimulated RhoA activity was blocked by KAI1/CD82, consistent with the loss of stress fiber and attenuation in cellular retraction. Upon KAI1/CD82 expression, Rac effector cofilin was not enriched at the cell periphery to facilitate lamellipodia formation while Rho kinase exhibited a significantly lower activity leading to less retraction. Phosphatidylinositol 4, 5-biphosphate, which initiates actin polymerization from the plasma membrane, became less detectable at the cell periphery in KAI1/CD82-expressing cells. Moreover, KAI1/CD82-induced phenotypes likely resulted from the suppression of multiple signaling pathways such as integrin and growth factor signaling. In summary, at the cellular level KAI1/CD82 inhibited polarized protrusion and retraction events by disrupting actin reorganization; at the molecular level, KAI1/CD82 deregulated Rac1, RhoA, and their effectors cofilin and Rho kinase by perturbing the plasma membrane lipids.

Loss of adenomatous poliposis coli-α3 integrin interaction promotes endothelial apoptosis in mice and humans

RATIONALE

Pulmonary hypertension (PH) is characterized by progressive elevation in pulmonary pressure and loss of small pulmonary arteries. As bone morphogenetic proteins promote pulmonary angiogenesis by recruiting the Wnt/β-catenin pathway, we proposed that β-catenin activation could reduce loss and induce regeneration of small pulmonary arteries (PAs) and attenuate PH.

OBJECTIVE

This study aims to establish the role of β-catenin in protecting the pulmonary endothelium and stimulating compensatory angiogenesis after injury.

METHODS AND RESULTS

To assess the impact of β-catenin activation on chronic hypoxia-induced PH, we used the adenomatous polyposis coli (Apc(Min/+)) mouse, where reduced APC causes constitutive β-catenin elevation. Surprisingly, hypoxic Apc(Min/+) mice displayed greater PH and small PA loss compared with control C57Bl6J littermates. PA endothelial cells isolated from Apc(Min/+) demonstrated reduced survival and angiogenic responses along with a profound reduction in adhesion to laminin. The mechanism involved failure of APC to interact with the cytoplasmic domain of the α3 integrin, to stabilize focal adhesions and activate integrin-linked kinase-1 and phospho Akt. We found that PA endothelial cells from lungs of patients with idiopathic PH have reduced APC expression, decreased adhesion to laminin, and impaired vascular tube formation. These defects were corrected in the cultured cells by transfection of APC.

CONCLUSIONS

We show that APC is integral to PA endothelial cells adhesion and survival and is reduced in PA endothelial cells from PH patient lungs. The data suggest that decreased APC may be a cause of increased risk or severity of PH in genetically susceptible individuals.

Tetraspanin CO-029 inhibits colorectal cancer cell movement by deregulating cell-matrix and cell-cell adhesions

Alterations in tetraspanin CO-029 expression are associated with the progression and metastasis of cancers in the digestive system. However, how CO-029 promotes cancer metastasis is still poorly understood. To determine the mechanism, we silenced CO-029 expression in HT29 colon cancer cells and found that the CO-029 knockdown significantly reduced cell migratory ability. The diminished cell migration was accompanied by the upregulation of both integrin-dependent cell-matrix adhesion on laminin and calcium-dependent cell-cell adhesion. The cell surface levels of laminin-binding integrin α3β1 and fibronectin-integrin α5β1 were increased while the level of CD44 was decreased upon CO-029 silencing. These changes contribute to the altered cell-matrix adhesion. The deregulated cell-cell adhesion results, at least partially, from increased activity of cadherins and reduced level of MelCAM. In conclusion, CO-029 functions as a regulator of both cell-matrix and cell-cell adhesion. During colon cancer progression, CO-029 promotes cancer cell movement by deregulating cell adhesions.

The C-terminal tail of tetraspanin protein CD9 contributes to its function and molecular organization

Tetraspanin protein CD9 supports sperm-egg fusion, and regulates cell adhesion, motility, metastasis, proliferation and signaling. The large extracellular loop and transmembrane domains of CD9 engage in functionally important interactions with partner proteins. However, neither functional nor biochemical roles have been shown for the CD9 C-terminal tail, despite it being highly conserved throughout vertebrate species. To gain new insight into the CD9 tail, three C-terminal amino acids (Glu-Met-Val) were replaced with residues corresponding to C-terminal amino acids from tetraspanin protein CD82 (Pro-Lys-Tyr). Wild-type and mutant CD9 were then stably expressed in MOLT-4, K562, U937, RD and HT1080 cells. Whereas wild-type CD9 inhibited cell adhesion and spreading on fibronectin, mutant CD9 did not. Wild-type CD9 also promoted homotypic cell-cell aggregation and microvilli formation, whereas mutant CD9 did not. Protein interactions of wild-type and mutant CD9 were compared quantitatively using stable isotope labeling with amino acids in cell culture (SILAC) in conjunction with liquid-chromatography-tandem mass spectrometry (LC-MS/MS) technology. SILAC results showed that, despite wild-type and mutant CD9 having identical expression levels, mutant CD9 and its major transmembrane interacting partners were recovered in substantially reduced amounts from 1% Brij 96 lysates. Immunoprecipitation experiments confirmed that mutant CD9 recovery was decreased in Brij 96, but not in more stringent Triton X-100 detergent. Additionally, compared with wild-type CD9 complexes, mutant CD9 complexes were larger and more oligomerized in Brij 96 detergent, consistent with decreased Brij 96 solubility, perhaps due to more membrane domains packing more tightly together. In conclusion, multiple CD9 functions depend on its C-terminal tail, which affects the molecular organization of CD9 complexes, as manifested by their altered solubilization in Brij 96 and organization on the cell surface.

Integrin {alpha}3, but not {beta}1, regulates islet cell survival and function via PI3K/Akt signaling pathways

β1-integrin is a well-established regulator of β-cell activities; however, the role of its associated α-subunits is relatively unknown. Previously, we have shown that human fetal islet and INS-1 cells highly express α3β1-integrin and that collagens I and IV significantly enhance their survival and function; in addition, blocking β1 function in the fetal islet cells decreased adhesion on collagen I and increased apoptosis. The present study investigates the effect of blocking α3. Using α3 blocking antibody or small interfering RNA, the effects of α3-integrin blockade were examined in isolated human fetal or adult islet cells or INS-1 cells, cultured on collagens I or IV. In parallel, β1 blockade was analyzed in INS-1 cells. Perturbing α3 function in human islet or INS-1 cells resulted in significant decreases in cell function (adhesion, spreading, proliferation and Pdx1 and insulin expression/secretion), primarily on collagen IV. A significant decrease in focal adhesion kinase and ERK1/2 phosphorylation and increased caspase3 cleavage were observed on both collagens. These effects were similar to changes after β1 blockade. Interestingly, only α3 blockade reduced expression of phospho-Akt and members of its downstream signaling cascades (glycogen synthase kinase β and X-linked inhibitor of apoptosis), demonstrating a specific effect of α3 on the phosphatidylinositol 3-kinase/Akt pathway. These results suggest that α3- as well as β1-integrin-extracellular matrix interactions are critical for modulating β-cell survival and function through specialized signaling cascades and enhance our understanding of how to improve islet microenvironments for cell-based treatments of diabetes.

Functional and biochemical studies of CD9 in fibrosarcoma cell line

CD9, a member of the tetraspanin family, plays important roles in a variety of cell activities. Fibrosarcoma is a malignant tumor that arises from fibroblasts. Low CD9 expression is found in fibrosarcoma tumor, but function of CD9 in fibrosarcoma has been rarely studied. In this study, stable cell lines for CD9 overexpression and vector were generated in HT1080, a human fibroscarcoma cell line, and cellular functions were widely investigated. In CD9-HT1080 cells, CD9 mainly localized in the membrane and co-localized with F-actin in the filopodia of cell surface. In functional assays, we demonstrated that CD9 could up-regulate total and active caspase-3 expression and induce cell apoptosis, but cell proliferation remained unchanged. CD9 overexpression inhibited HT1080 cell adhesion to FN but promoted cell spreading on FN. We also observed CD9 reduced cell migration using FN a chemoattractant and inhibited cell colony formation in soft agar medium. To explore the biochemical mechanism for functional changes, we investigated the effects of CD9 overexpression on cellular pathways and protein association. CD9 overexpression induced Akt phosphorylation on FN but did not change total Akt expression. Phosphorylation of p38 but not ERK was increased by CD9 overexpression, total p38 and ERK were not affected. CD9 overexpression did not affect the expression of TGFα, EGFR, β1, and EWI-2, but EWI-F expression was up-regulated. Moreover, CD9 could associate with TGFα, EGFR, β1, EWI-2, and EWI-F in HT1080 cell line. Take together, CD9 overexpression had promoting effects on cell apoptosis and cell spreading, but had inhibitory effects on cell adhesion, migration, and cell colony formation. These effects might be ascribed to CD9 associations with EWI-2/EWI-F/β1 complex and EGFR pathway, and the activation of Akt and p38 signalings as well.

Glycoprotein capture and quantitative phosphoproteomics indicate coordinated regulation of cell migration upon lysophosphatidic acid stimulation

The lipid mediator lysophosphatidic acid (LPA) is a serum component that regulates cellular functions such as proliferation, migration, and survival via specific G protein-coupled receptors. The underlying signaling mechanisms are still incompletely understood, including those that operate at the plasma membrane to modulate cell-cell and cell-matrix interactions in LPA-promoted cell migration. To explore LPA-evoked phosphoregulation with a focus on cell surface proteins, we combined glycoproteome enrichment by immobilized lectins with SILAC-based quantitative phosphoproteomics. We performed biological replicate analyses in SCC-9 squamous cell carcinoma cells and repeatedly quantified the effect of 1.5- and 5-min LPA treatment on more than 700 distinct phosphorylations in lectin-purified proteins. We detected many regulated phosphorylation events on various types of plasma membrane proteins such as cell adhesion molecules constituting adherens junctions, desmosomes, and hemidesmosomes. Several of these LPA-regulated phosphorylation sites have been characterized in a biological context other than G protein-coupled receptor signaling, and the transfer of this functional information suggests coordinated and multifactorial cell adhesion control in LPA-induced cell migration. Additionally, we identified LPA-mediated activation loop phosphorylation of the serine/threonine kinase Wnk1 and verified a role of Wnk1 for LPA-induced cell migration in knock-down experiments. In conclusion, the glycoproteome phosphoproteomics strategy described here sheds light on incompletely understood mechanisms in LPA-induced cell migratory behavior.

Discovery and characterization of a small molecule inhibitor of the PDZ domain of dishevelled

Dishevelled (Dvl) is an essential protein in the Wnt signaling pathways; it uses its PDZ domain to transduce the Wnt signals from the membrane receptor Frizzled to downstream components. Here, we report identifying a drug-like small molecule compound through structure-based ligand screening and NMR spectroscopy and show the compound to interact at low micromolar affinity with the PDZ domain of Dvl. In a Xenopus testing system, the compound could permeate the cell membrane and block the Wnt signaling pathways. In addition, the compound inhibited Wnt signaling and reduced the levels of apoptosis in the hyaloid vessels of eye. Moreover, this compound also suppressed the growth of prostate cancer PC-3 cells. These biological effects suggest that by blocking the PDZ domain of Dvl, the compound identified in our studies effectively inhibits the Wnt signaling and thus provides a useful tool for studies dissecting the Wnt signaling pathways.

Transmembrane interactions are needed for KAI1/CD82-mediated suppression of cancer invasion and metastasis

In transmembrane (TM) domains, tetraspanin KAI1/CD82 contains an Asn, a Gln, and a Glu polar residue. A mutation of all three polar residues largely disrupts the migration-, invasion-, and metastasis-suppressive activities of KAI1/CD82. Notably, KAI1/CD82 inhibits the formation of microprotrusions and the release of microvesicles, while the mutation disrupts these inhibitions, revealing the connections of microprotrusion and microvesicle to KAI1/CD82 function. The TM polar residues are needed for proper interactions between KAI1/CD82 and tetraspanins CD9 and CD151, which also regulate cell movement, but not for the association between KAI1/CD82 and alpha3beta1 integrin. However, KAI1/CD82 still efficiently inhibits cell migration when either CD9 or CD151 is absent. Hence, KAI1/CD82 interacts with tetraspanin and integrin by different mechanisms and is unlikely to inhibit cell migration through its associated proteins. Moreover, without significantly affecting the glycosylation, homodimerization, and global folding of KAI1/CD82, the TM interactions maintain the conformational stability of KAI1/CD82, evidenced by the facts that the mutant is more sensitive to denaturation and less associable with tetraspanins and supported by the modeling analysis. Thus, the TM interactions mediated by these polar residues determine a conformation either in or near the tightly packed TM region and this conformation and/or its change are needed for the intrinsic activity of KAI1/CD82. In contrast to immense efforts to block the signaling of cancer progression, the perturbation of TM interactions may open a new avenue to prevent cancer invasion and metastasis.

Controlling cell surface dynamics and signaling: how CD82/KAI1 suppresses metastasis

The recent identification of metastasis suppressor genes, uniquely responsible for negatively controlling cancer metastasis, are providing inroads into the molecular machinery involved in metastasis. While the normal function of a few of these genes is known; the molecular events associated with their loss that promotes tumor metastasis is largely not understood. KAI1/CD82, whose loss is associated with a wide variety of metastatic cancers, belongs to the tetraspanin family. Despite intense scrutiny, many aspects of how CD82 specifically functions as a metastasis suppressor and its role in normal biology remain to be determined. This review will focus on the molecular events associated with CD82 loss, the potential impact on signaling pathways that regulate cellular processes associated with metastasis, and its relationship with other metastasis suppressor genes.

Tetraspanin CD151 promotes cell migration by regulating integrin trafficking

Regulation of cell migration is an important feature of tetraspanin CD151. Although it is well established that CD151 physically associates with integrins, the mechanism by which CD151 regulates integrin-dependent cell migration is basically unknown. Given the fact that CD151 is localized in both the plasma membrane and intracellular vesicles, we found that CD151 and its associated alpha3beta1, alpha5beta1, and alpha6beta1 integrins undergo endocytosis and accumulate in the same intracellular vesicular compartments. CD151 contains a YRSL sequence, a YXXvarphi type of endocytosis/sorting motif, in its C-terminal cytoplasmic domain. Mutation of this motif markedly attenuated CD151 internalization. The loss of CD151 trafficking completely abrogated CD151-promoted cell migration on extracellular matrices such as laminin and diminished the internalization of its associated integrins, indicating a critical role for integrin trafficking in regulating cell motility. In conclusion, the YXXvarphi motif-mediated internalization of CD151 promotes integrin-dependent cell migration by modulating the endocytosis and/or vesicular trafficking of its associated integrins.

Integrin alpha3beta1 interacts with I1PP2A/lanp and phosphatase PP1

Integrin alpha3beta1 is a receptor for the extracellular matrix component laminin 5. To elucidate possible signaling pathways induced by integrin alpha3beta1, we looked for proteins that interact with the cytoplasmic part of the alpha3A integrin subunit. We identified several multifunctional proteins by affinity chromatography and subsequent MALDI-TOF-MS and focused on the inhibitor 1 of serine/threonine phosphatase PP2A (I1PP2A, synonym: lanp) which also plays a role during the development of the mouse cerebellum. I1PP2A/lanp colocalizes with the alpha3A integrin subunit in differentiated PC12 cells in the cell body and in neurites as well as in Purkinje cells of mouse cerebellum. Overexpression of GFP-I1PP2A/lanp in PC12 cells leads to markedly reduced neurite length on laminin 5 after induction with nerve growth factor. By affinity chromatography the protein phosphatase PP1 can also be identified as a alpha3A/cyto-binding protein. PP1 and integrin alpha3beta1 can be pulled down by GST-I1PP2A/lanp from cell lysates of differentiated and undifferentiated PC12 cells. The phosphatase binds to the cytoplasmic membrane-proximal conserved GFFKR motif of the alpha integrin subunit, whereas I1PP2A/lanp requires a longer sequence for binding. PP1 but not PP2A is able to dephosphorylate precipitated integrin alpha3beta1 in vitro. Furthermore, PP1 releases phosphate from T1046 of phosphopeptides that mimic the phosphorylation consensus sequence in the cytoplasmic part of the alpha3A integrin subunit. These data suggest that I1PP2A/lanp forms a complex with PP1 and the alpha3A integrin subunit and might possibly regulate the phosphorylation status of integrin alpha3beta1 and/or integrin downstream targets.

Stepped changes of monovalent ligand-binding force during ligand-induced clustering of integrin alphaIIB beta3

Recent evidence demonstrated that conformational changes of the integrin during receptor activation affected its binding to extracellular matrix; however, experimental assessment of ligand-receptor binding following the initial molecular interaction has rarely been carried out at a single-molecule resolution. In the present study, laser tweezers were used to measure the binding force exerted by a live Chinese hamster ovary cell that expressed integrin alphaIIb beta3 (CHO alphaIIb beta3), to the bead carrier coated with the snake venom rhodostomin that served as an activated ligand for integrin alphaIIb beta3. A progressive increase of total binding force over time was noticed when the bead interacted with the CHO alphaIIb beta3 cell; such an increase was due mainly to the recruitment of more integrin molecules to the bead-cell interface. When the binding strength exerted by a single ligand-receptor pair was derived from the "polyvalent" measurements, surprisingly, a stepped decrease of the "monovalent binding force" was noted (from 4.15 to 2.54 piconewtons (pN)); such decrease appeared to occur during the ligand-induced integrin clustering process. On the other hand, the mutant rhodostomin defective in clustering integrins exhibited only one (1.81 pN) unit binding strength.

A critical role for tetraspanin CD151 in alpha3beta1 and alpha6beta4 integrin-dependent tumor cell functions on laminin-5

The basement membrane protein laminin-5 supports tumor cell adhesion and motility and is implicated at multiple steps of the metastatic cascade. Tetraspanin CD151 engages in lateral, cell surface complexes with both of the major laminin-5 receptors, integrins alpha3beta1 and alpha6beta4. To determine the role of CD151 in tumor cell responses to laminin-5, we used retroviral RNA interference to efficiently silence CD151 expression in epidermal carcinoma cells. Near total loss of CD151 had no effect on steady state cell surface expression of alpha3beta1, alpha6beta4, or other integrins with which CD151 associates. However, CD151-silenced carcinoma cells displayed markedly impaired motility on laminin-5, accompanied by unusually persistent lateral and trailing edge adhesive contacts. CD151 silencing disrupted alpha3beta1 integrin association with tetraspanin-enriched microdomains, reduced the bulk detergent extractability of alpha3beta1, and impaired alpha3beta1 internalization in cells migrating on laminin-5. Both alpha3beta1- and alpha6beta4-dependent cell adhesion to laminin-5 were also impaired in CD151-silenced cells. Reexpressing CD151 in CD151-silenced cells reversed the adhesion and motility defects. Finally, loss of CD151 also impaired migration but not adhesion on substrates other than laminin-5. These data show that CD151 plays a critical role in tumor cell responses to laminin-5 and reveal promotion of integrin recycling as a novel potential mechanism whereby CD151 regulates tumor cell migration.

Modulation of Kv3.1b potassium channel phosphorylation in auditory neurons by conventional and novel protein kinase C isozymes

In fast-spiking neurons such as those in the medial nucleus of the trapezoid body (MNTB) in the auditory brainstem, Kv3.1 potassium channels are required for high frequency firing. The Kv3.1b splice variant of this channel predominates in the mature nervous system and is a substrate for phosphorylation by protein kinase C (PKC) at Ser-503. In resting neurons, basal phosphorylation at this site decreases Kv3.1 current, reducing neuronal ability to follow high frequency stimulation. We used a phospho-specific antibody to determine which PKC isozymes control serine 503 phosphorylation in Kv3.1b-tranfected cells and in auditory neurons in brainstem slices. By using isozyme-specific inhibitors, we found that the novel PKC-delta isozyme, together with the novel PKC-epsilon and conventional PKCs, contributed to the basal phosphorylation of Kv3.1b in MNTB neurons. In contrast, only PKC-epsilon and conventional PKCs mediate increases in phosphorylation produced by pharmacological activation of PKC in MNTB neurons or by metabotropic glutamate receptor activation in Kv3.1/mGluR1-cotransfected cells. We also measured the time course of dephosphorylation and recovery of basal phosphorylation of Kv3.1b following brief high frequency electrical stimulation of the trapezoid body, and we determined that the recovery process is mediated by both novel PKC-delta and PKC-epsilon isozymes and by conventional PKCs. The association between Kv3.1b and PKC isozymes was confirmed by reciprocal coimmunoprecipitation of Kv3.1b with multiple PKC isozymes. Our results suggest that the Kv3.1b channel is regulated by both conventional and novel PKC isozymes and that novel PKC-delta contributes specifically to the maintenance of basal phosphorylation in auditory neurons.

The cytoplasmic tail of the alpha3 integrin subunit promotes neurite outgrowth in PC12 cells

Binding of integrins to proteins of the extracellular matrix (ECM) provides structural and signaling information for biological processes such as cell proliferation, migration, neurite outgrowth, and differentiation. Integrins represent a family of heterodimeric transmembrane cell surface receptors. Besides connecting the ECM with the cytoskeleton, integrins also induce various signaling pathways in response to ligand binding. Integrin ligation leads to cytoplasmic protein-protein interactions requiring both integrin cytoplasmic tails. These sequences are initiation points for focal adhesion formation and subsequent signal transduction cascades. In this study, we addressed the question of whether the short cytoplasmic tail of the alpha(3) integrin subunit of alpha(3)beta(1) integrin is required for alpha(3)beta(1) integrin-dependent processes. For this purpose, cDNA representing the extracellular and transmembrane domain of the interleukin 2 receptor (IL2R) alpha subunit and the cytoplasmic sequence of the alpha(3) integrin subunit was transfected into PC12 cells. Autonomous expression of the cytoplasmic alpha(3) tail does not affect attachment but leads to inhibition of neuronal differentiation on laminin 5. This indicates that the cytoplasmic alpha(3) sequence is not required for cell attachment but is necessary for long-term adhesion and for the reorganization of the cytoskeleton that precedes neuronal differentiation. Inhibition of neurite outgrowth by chimeric IL2R-alpha(3) can be rescued by treatment of transfected cells with the pharmacological inhibitor Y27632, which inhibits the RhoA downstream effector Rho kinase alpha.

Tetraspanin CD82 Attenuates Cellular Morphogenesis through Down-regulating Integrin α6-Mediated Cell Adhesion

Tetraspanin CD82 has been implicated in integrin-mediated functions such as cell motility and invasiveness. Although tetraspanins associate with integrins, it is unknown if and how CD82 regulates the functionality of integrins. In this study, we found that Du145 prostate cancer cells underwent morphogenesis on the reconstituted basement membrane Matrigel to form an anastomosing network of multicellular structures. This process entirely depends on integrin alpha6, a receptor for laminin. After CD82 is expressed in Du145 cells, this cellular morphogenesis was abolished, indicating a functional cross-talk between CD82 and alpha6 integrins. Interestingly, antibodies against other tetraspanins expressed in Du145 cells such as CD9, CD81, and CD151 did not block this integrin alpha6-dependent morphogenesis. We further found that CD82 significantly inhibited cell adhesion on laminin 1. Notably, the level of alpha6 integrins on the cell surface was down-regulated upon CD82 expression, although total cellular alpha6 protein levels remained unchanged in CD82-expressing cells. This down-regulation indicates that the diminished cell adhesiveness of CD82-expressing Du145 cells on laminin likely resulted from less cell surface expression of alpha6 integrins. As expected, CD82 physically associated with the integrin alpha6 in Du145-CD82 transfectant cells, suggesting that the formation of the CD82-integrin alpha6 complex reduces alpha6 integrin cell surface expression. Finally, the internalization of cell surface integrin alpha6 is significantly enhanced upon CD82 expression. In conclusion, our results indicate that 1) CD82 attenuates integrin alpha6 signaling during a cellular morphogenic process; 2) the decreased surface expression of alpha6 integrins in CD82-expressing cells is likely responsible for the diminished adhesiveness on laminin and, subsequently, results in the attenuation of alpha6 integrin-mediated cellular morphogenesis; and 3) the accelerated internalization of integrin alpha6 upon CD82 expression correlates with the down-regulation of cell surface integrin alpha6.

The palmitoylation of metastasis suppressor KAI1/CD82 is important for its motility- and invasiveness-inhibitory activity

The cancer metastasis suppressor protein KAI1/CD82 is a member of the tetraspanin superfamily. Recent studies have demonstrated that tetraspanins are palmitoylated and that palmitoylation contributes to the organization of tetraspanin webs or tetraspanin-enriched microdomains. However, the effect of palmitoylation on tetraspanin-mediated cellular functions remains obscure. In this study, we found that tetraspanin KAI1/CD82 was palmitoylated when expressed in PC3 metastatic prostate cancer cells and that palmitoylation involved all of the cytoplasmic cysteine residues proximal to the plasma membrane. Notably, the palmitoylation-deficient KAI1/CD82 mutant largely reversed the wild-type KAI1/CD82's inhibitory effects on migration and invasion of PC3 cells. Also, palmitoylation regulates the subcellular distribution of KAI1/CD82 and its association with other tetraspanins, suggesting that the localized interaction of KAI1/CD82 with tetraspanin webs or tetraspanin-enriched microdomains is important for KAI1/CD82's motility-inhibitory activity. Moreover, we found that KAI1/CD82 palmitoylation affected motility-related subcellular events such as lamellipodia formation and actin cytoskeleton organization and that the alteration of these processes likely contributes to KAI1/CD82's inhibition of motility. Finally, the reversal of cell motility seen in the palmitoylation-deficient KAI1/CD82 mutant correlates with regaining of p130(CAS)-CrkII coupling, a signaling step important for KAI1/CD82's activity. Taken together, our results indicate that palmitoylation is crucial for the functional integrity of tetraspanin KAI1/CD82 during the suppression of cancer cell migration and invasion.

p51/p63 Controls Subunit α3 of the Major Epidermis Integrin Anchoring the Stem Cells to the Niche

p51/p63, a member of the tumor suppressor p53 gene family, is crucial for skin development. We describe here identification of ITGA3 encoding integrin alpha(3) as a target of its trans-activating function, proposing that p51/p63 allows epidermal stem cells to express laminin receptor alpha(3)beta(1) for anchorage to the basement membrane. When activated by genotoxic stress or overexpressed ectopically in non-adherent cells, p51/p63 transduced a phenotype to attach to extracellular matrices, which was accompanied by expression of ITGA3. Motifs matching the p53-binding consensus sequence were located in a scattered form in intron 1 of human ITGA3, and served as p51/p63-responsive elements in reporter assays. In addition to the trans-activating ability of the TA isoform, we detected a positive effect of the DeltaN isoform on ITGA3. The high level alpha(3) production in human keratinocyte stem cells diminished upon elimination of p51/p63 by small interfering RNA or by Ca(2+)-induced differentiation. Furthermore, a chromatin immunoprecipitation experiment indicated a physical interaction of p51/p63 with intron 1 of ITGA3. This study provides a molecular basis for the standing hypothesis that p51/p63 is essential for epidermal-mesenchymal interactions.

Physiological and Pathological Roles of α3β1 Integrin

Alpha3beta1 integrin has been considered to be a mysterious adhesion molecule due to the pleiotropy in its ligand-binding specificity. However, recent studies have identified laminin isoforms as high-affinity ligands for this integrin, and demonstrated that alpha3beta1 integrin plays a number of essential roles in development and differentiation, mainly by mediating the establishment and maintenance of epithelial tissues. Furthermore, alpha3beta1 integrin is also implicated in many other biological phenomena, including cell growth and apoptosis, angiogenesis and neural functions. This integrin receptor forms complexes with various other membrane proteins, such as the transmembrane-4 superfamily proteins (tetraspanins), cytoskeletal proteins and signaling molecules. Recently, lines of evidence have been reported showing that complex formation regulates integrin functions in cell adhesion and migration, signal transduction across cell membranes, and cytoskeletal organization. In addition to these roles in physiological processes, alpha3beta1 integrin performs crucial functions in various pathological processes, especially in wound healing, tumor invasion and metastasis, and infection by pathogenic microorganisms.

The basement membrane protein laminin-5 acts as a soluble cell motility factor

One of the basement membrane (BM) proteins, laminin-5 (LN5), is known to support efficient cell adhesion and migration through interaction with integrins on the basal plasma membrane. Here, we show that a soluble form of LN5 induced migration of human epithelial cells and carcinoma cells by interacting with integrins on the apical cell surface. Although both LN5 and laminin-10/11 (LN10/11) promoted cell migration when coated onto a plastic surface as insoluble substrata, only LN5 stimulated cell migration in its soluble form on other substrata such as fibronectin (FN), vitronectin (VN) and collagen. Soluble LN5 interacted with integrins alpha3beta1 and alpha6beta1 on the apical cell surface and stimulated cell migration, while the cell morphology was largely dependent on the underlying substratum. Thus, integrin signals from the apical surface and the basal surface synergistically regulated cytoskeletal organization and cell motility. Soluble and insoluble LN5 induced cell motility by activating signal pathways via protein kinase C (PKC), phosphoinositide 3-OH kinase (PI3-K) and MAP kinase. The PKC dependency was more prominent for soluble LN5 than insoluble LN5, and was absent in the stimulation by insoluble LN10/11. In vitro scratch assays with keratinocytes, self-produced soluble LN5 bound to the apical cell surface of migrating cells at the scratched edges, suggesting that soluble LN5 may contribute to cell migration in pathological conditions such as wound healing and tumor invasion.

KAI1 COOH-Terminal Interacting Tetraspanin (KITENIN), a Member of the Tetraspanin Family, Interacts with KAI1, a Tumor Metastasis Suppressor, and Enhances Metastasis of Cancer

We cloned recently an alternatively spliced variant of KAI1 mRNA that lacked exon 7 at the COOH-terminal region and showed differences in metastasis suppression when compared with the wild-type KAI1. These findings indicated that the COOH-terminal region of KAI1 is critical for its metastasis suppressor function. In this study, we isolated a cDNA clone of VANGL1, a member of the tetraspanin protein family, which interacted specifically with the COOH-terminal cytoplasmic domain of KAI1 in the yeast two-hybrid system. We renamed it KAI1 COOH-terminal interacting tetraspanin (KITENIN). We found that KITENIN-overexpressing CT-26 mouse colon cancer cells showed increased tumorigenicity and early hepatic metastasis in vivo, as well as increased invasiveness and adhesion to fibronectin in vitro compared with parental cells. Moreover, increased levels of KITENIN were observed in a human gastric tumor and its metastatic tissues, compared with the normal adjacent mucosa. Our results indicate that KITENIN promotes adhesion and invasion of cancer cells in vitro and in vivo, and suggest that KITENIN participates in the regulation of the tumor formation and metastasis by interacting with KAI1, a metastasis suppressor and antisense KITENIN strategy that can be used to inhibit metastasis in various cancers.

Dynamic regulation of a GPCR-tetraspanin-G protein complex on intact cells: central role of CD81 in facilitating GPR56-Galpha q/11 association

By means of a variety of intracellular scaffolding proteins, a vast number of heterotrimeric G protein-coupled receptors (GPCRs) may achieve specificity in signaling through a much smaller number of heterotrimeric G proteins. Members of the tetraspanin family organize extensive complexes of cell surface proteins and thus have the potential to act as GPCR scaffolds; however, tetraspanin-GPCR complexes had not previously been described. We now show that a GPCR, GPR56/TM7XN1, and heterotrimeric G protein subunits, Galpha(q), Galpha(11), and Gbeta, associate specifically with tetraspanins and CD81, but not with other tetraspanins. CD9 Complexes of GPR56 with CD9 and CD81 remained intact when fully solubilized and were resistant to cholesterol depletion. Hence they do not depend on detergent-insoluble, raft-like membrane microdomains for stability. A central role for CD81 in promoting or stabilizing a GPR56-CD81-Galpha(q/11) complex was revealed by CD81 immunodepletion and reexpression experiments. Finally, antibody engagement of cell surface CD81 or cell activation with phorbol ester revealed two distinct mechanisms by which GPR56-CD81-Galpha(q/11) complexes can be dynamically regulated. These data reveal a potential role for tetraspanins CD9 and CD81 as GPCR scaffolding proteins.

Positional control of cell fate through joint integrin/receptor protein kinase signaling

Cells adhere to the extracellular matrix throughout most of their lifetime. This close, intimate contact with the matrix exerts an extraordinary control on the behavior of cells, determining whether they move or stay put, proliferate or remain quiescent, and even live or die. Attachment to the matrix not only enables cells to respond to soluble growth factors and cytokines but also determines the nature of the response. The integrins are a large family of receptors that attach cells to the matrix, organize their cytoskeleton, and cooperate with receptor protein tyrosine kinases to regulate cell fate. Research on integrin signaling is beginning to explain the complex and specific effects that the extracellular matrix exerts on cells.

Suprabasal alpha6beta4 integrin expression in epidermis results in enhanced tumourigenesis and disruption of TGFbeta signalling

Inappropriate alpha6beta4 integrin expression correlates with a high risk of tumour progression in stratified squamous epithelia. Targeted expression of alpha6beta4 in the suprabasal layers of transgenic mouse epidermis dramatically increased the frequency of papillomas, carcinomas and metastases induced by chemical carcinogenesis, independent of the beta4 cytoplasmic domain. Suprabasal alpha6beta4 also perturbed transforming growth factor beta (TGFbeta) signalling as demonstrated by decreased nuclear Smad2 in transgenic epidermis and tumours. In cultured keratinocytes, suprabasal alpha6beta4 relieved TGFbeta-mediated growth inhibition and blocked nuclear translocation of activated Smad2/3. Responsiveness to TGFbeta could be restored by inhibiting cadherin-mediated cell-cell adhesion or phosphoinositide 3-kinase (PI3-K) activity, but not by inhibiting mitogen-activated protein kinase (MAPK) activity. These data suggest that suprabasal alpha6beta4 promotes tumourigenesis by preventing TGFbeta from suppressing clonal expansion of initiated cells in the epidermal basal layer.

Integrin alpha 4 beta 1-dependent T cell migration requires both phosphorylation and dephosphorylation of the alpha 4 cytoplasmic domain to regulate the reversible binding of paxillin

alpha 4 integrins mediate increased cell migration and decreased cell spreading because the alpha 4 cytoplasmic domain (tail) binds tightly to paxillin, a signaling adaptor protein. Paxillin binding to the alpha 4 tail is blocked by alpha 4 phosphorylation at Ser988. To establish the biological role of alpha 4 phosphorylation, we reconstituted alpha 4-deficient Jurkat T cells with phosphorylation-mimicking (alpha 4(S988D)) or non-phosphorylatable (alpha 4(S988A)) mutants. alpha 4(S988D) disrupted paxillin binding and also inhibited cell migration and promoted cell spreading. In contrast, the non-phosphorylatable alpha 4(S988A) resulted in a further reduction in cell spreading; however, this mutation led to an unexpected suppression of cell migration. The suppression of cell migration by alpha 4(S988A) was ascribable to enhanced alpha 4-paxillin association, because enforced association by an alpha 4-paxillin fusion led to a phenotype similar to that of the non-phosphorylatable alpha 4(S988A) mutant. These data establish that optimal alpha 4-mediated cell migration requires both phosphorylation and dephosphorylation of the alpha 4 cytoplasmic domain to regulate the reversible binding of paxillin.

Spatial restriction of alpha4 integrin phosphorylation regulates lamellipodial stability and alpha4beta1-dependent cell migration

Integrins coordinate spatial signaling events essential for cell polarity and directed migration. Such signals from alpha4 integrins regulate cell migration in development and in leukocyte trafficking. Here, we report that efficient alpha4-mediated migration requires spatial control of alpha4 phosphorylation by protein kinase A, and hence localized inhibition of binding of the signaling adaptor, paxillin, to the integrin. In migrating cells, phosphorylated alpha4 accumulated along the leading edge. Blocking alpha4 phosphorylation by mutagenesis or by inhibition of protein kinase A drastically reduced alpha4-dependent migration and lamellipodial stability. alpha4 phosphorylation blocks paxillin binding in vitro; we now find that paxillin and phospho-alpha4 were in distinct clusters at the leading edge of migrating cells, whereas unphosphorylated alpha4 and paxillin colocalized along the lateral edges of those cells. Furthermore, enforced paxillin association with alpha4 inhibits migration and reduced lamellipodial stability. These results show that topographically specific integrin phosphorylation can control cell migration and polarization by spatial segregation of adaptor protein binding.

Requirement of the p130CAS-Crk coupling for metastasis suppressor KAI1/CD82-mediated inhibition of cell migration

KAI1/CD82 protein is a member of the tetraspanin superfamily and has been rediscovered as a cancer metastasis suppressor. The mechanism of KAI1/CD82-mediated suppression of cancer metastasis remains to be established. In this study, we found that migration of the metastatic prostate cancer cell line Du145 was substantially inhibited when KAI1/CD82 was expressed. The expression of focal adhesion kinase (FAK) and Lyn, a Src family tyrosine kinase and substrate of FAK, was up-regulated at both RNA and protein levels upon KAI1/CD82 expression. The activation of FAK and Lyn, however, remained unchanged in Du145-KAI1/CD82 cells. As a downstream target of FAK-Lyn signaling, the p130CAS (Crk-associated substrate) protein was decreased upon the expression of KAI1/CD82. Consequently, less p130CAS-CrkII complex, which functions as a "molecular switch" in cell motility, was formed in Du145-KAI1/CD82 cells. To confirm that the p130CAS-CrkII complex is indeed important for the motility inhibition by KAI1/CD82, overexpression of p130CAS in Du145-KAI1/CD82 cells increased the formation of p130CAS-CrkII complex and largely reversed the KAI1/CD82-mediated inhibition of cell motility. Taken together, our studies indicate the following: 1) signaling of FAK-Lyn-p130CAS-CrkII pathway is altered in KAI1/CD82-expressing cells, and 2) p130CAS-CrkII coupling is required for KAI1/CD82-mediated suppression of cell motility.

Functional domains in tetraspanin proteins

Exciting new findings have emerged about the structure, function and biochemistry of tetraspanin proteins. Five distinct tetraspanin regions have now been delineated linking structural features to specific functions. Within the large extracellular loop of tetraspanins, there is a variable region that mediates specific interactions with other proteins, as well as a more highly conserved region that has been suggested to mediate homodimerization. Within the transmembrane region, the four tetraspanin transmembrane domains are probable sites of both intra- and inter-molecular interactions that are crucial during biosynthesis and assembly of the network of tetraspanin-linked membrane proteins known as the 'tetraspanin web'. In the intracellular juxtamembrane region, palmitoylation of cysteine residues also contributes to tetraspanin web assembly, and the C-terminal cytoplasmic tail region could provide specific functional links to cytoskeletal or signaling proteins.

Comparison of the mechanisms of nongenomic actions of thyroid hormone and steroid hormones

Steroids and thyroid hormone are thought primarily to act via binding to hormone-specific nuclear receptor superfamily members. The nuclear ligand-receptor complexes then initiate transcriptional activity. Actions of steroids and iodothyronines that are nongenomic or extranuclear in mechanism have been recognized recently and new insights into such mechanisms are available. Despite their distinct structures and biologic effects, the two families of hormones have similarities in the mechanisms of their nongenomic actions. That is, both steroids and thyroid hormone appear to interact with specific cell surface G protein-coupled receptors and to activate signal transducing kinases such as those involved in the mitogen-activated protein kinase (MAPK) pathway. Much is known about the ability of certain steroids such as estrogen and mineralocorticoids to increase [Ca2+]i acutely and stimulation of the MAPK cascade by L-T4 appears to depend upon a hormone-induced increase in [Ca2+]i via phosphoinositide pathway activation. At least in the case of iodothyronines, hormone activation of the MAPK pathway modulates the cellular activities of certain cytokines and growth factors. One of the two cell surface estrogen receptors (ERs) may be an expression of the same transcript as that for nuclear ER, whereas the mineralocorticoid and progesterone-binding proteins in the plasma membrane appear to be products of genes different from those of nuclear receptors. Iodothyronine structure-activity relationships at the plasma membrane binding site for thyroid hormone suggest that the cell surface receptor for T4 that also binds 3,5,3'-triiodo-L-T3 is different from the nuclear T3 receptor (TR). There are interfaces of nongenomic and genomic mechanisms for both steroids and thyroid hormone. For example, by nongenomic mechanisms, estrogen and thyroid hormone can promote serine phosphorylation, respectively, of nuclear ER and TR. Transcriptional activity of the nuclear receptor proteins can be altered by such phosphorylation.

Vascular endothelial growth factor-induced migration of multiple myeloma cells is associated with beta 1 integrin- and phosphatidylinositol 3-kinase-dependent PKC alpha activation

In multiple myeloma (MM), migration is necessary for the homing of tumor cells to bone marrow (BM), for expansion within the BM microenvironment, and for egress into the peripheral blood. In the present study we characterize the role of vascular endothelial growth factor (VEGF) and beta(1) integrin (CD29) in MM cell migration. We show that protein kinase C (PKC) alpha is translocated to the plasma membrane and activated by adhesion of MM cells to fibronectin and VEGF. We identify beta(1) integrin modulating VEGF-triggered MM cell migration on fibronectin. We show that transient enhancement of MM cell adhesion to fibronectin triggered by VEGF is dependent on the activity of both PKC and beta(1) integrin. Moreover, we demonstrate that PKC alpha is constitutively associated with beta(1) integrin. These data are consistent with PKC alpha-dependent exocytosis of activated beta(1) integrin to the plasma membrane, where its increased surface expression mediates binding to fibronectin; conversely, catalytically active PKC alpha-driven internalization of beta(1) integrin results in MM cell de-adhesion. We show that the regulatory subunit of phosphatidylinositol (PI) 3-kinase (p85) is constitutively associated with FMS-like tyrosine kinase-1 (Flt-1). VEGF stimulates activation of PI 3-kinase, and both MM cell adhesion and migration are PI 3-kinase-dependent. Moreover, both VEGF-induced PI 3-kinase activation and beta(1) integrin-mediated binding to fibronectin are required for the recruitment and activation of PKC alpha. Time-lapse phase contrast video microscopy (TLVM) studies confirm the importance of these signaling components in VEGF-triggered MM cell migration on fibronectin.

Function of the tetraspanin CD151-alpha6beta1 integrin complex during cellular morphogenesis

Upon plating on basement membrane Matrigel, NIH3T3 cells formed an anastomosing network of cord-like structures, inhibitable by anti-alpha6beta1 integrin antibodies. For NIH3T3 cells transfected with human CD151 protein, the formation of a cord-like network was also inhibitable by anti-CD151 antibodies. Furthermore, CD151 and alpha6beta1 were physically associated within NIH3T3 cells. On removal of the short 8-amino acid C-terminal CD151 tail (by deletion or exchange), exogenous CD151 exerted a dominant negative effect, as it almost completely suppressed alpha6beta1-dependent cell network formation and NIH3T3 cell spreading on laminin-1 (an alpha6beta1 ligand). Importantly, mutant CD151 retained alpha6beta1 association and did not alter alpha6beta1-mediated cell adhesion to Matrigel. In conclusion, the CD151-alpha6beta1 integrin complex acts as a functional unit that markedly influences cellular morphogenesis, with the CD151 tail being of particular importance in determining the "outside-in" functions of alpha6beta1-integrin that follow ligand engagement. Also, antibodies to alpha6beta1 and CD151 inhibited formation of endothelial cell cord-like networks, thus pointing to possible relevance of CD151-alpha6beta1 complexes during angiogenesis.

Complexes of tetraspanins with integrins: more than meets the eye

The transmembrane proteins of the tetraspanin superfamily are implicated in a diverse range of biological phenomena, including cell motility, metastasis, cell proliferation and differentiation. The tetraspanins are associated with adhesion receptors of the integrin family and regulate integrin-dependent cell migration. In cells attached to the extracellular matrix, the integrin-tetraspanin adhesion complexes are clustered into a distinct type of adhesion structure at the cell periphery. Various tetraspanins are associated with phosphatidylinositol 4-kinase and protein kinase C isoforms, and they may facilitate assembly of signalling complexes by tethering these enzymes to integrin heterodimers. At the plasma membrane, integrin-tetraspanin signalling complexes are partitioned into specific microdomains proximal to cholesterol-rich lipid rafts. A substantial fraction of tetraspanins colocalise with integrins in various intracellular vesicular compartments. It is proposed that tetraspanins can influence cell migration by one of the following mechanisms: (1) modulation of integrin signalling; (2) compartmentalisation of integrins on the cell surface; or (3) direction of intracellular trafficking and recycling of integrins.

Phosphorylation of the integrin alpha 4 cytoplasmic domain regulates paxillin binding

alpha4 integrins are essential for embryogenesis, hematopoiesis, inflammation, and immune response possibly because alpha4 integrins have distinct signaling properties from other integrins. Specifically, the alpha4 cytoplasmic domain binds tightly to paxillin, a signaling adaptor protein, leading to increased cell migration and an altered cytoskeletal organization that results in reduced cell spreading. The alpha4 tail contains potential phosphorylation sites clustered in its core paxillin binding region. We now report that the alpha4 tail is phosphorylated in vitro and in vivo. Furthermore, Ser(988) is a major phosphorylation site. Using antibodies specific for Ser(988)-phosphorylated alpha4, we found the stoichiometry of alpha4 phosphorylation varied in different cells. However, >60% of alpha4 was phosphorylated in Jurkat T cells. Phosphorylation at Ser(988) blocked paxillin binding to the alpha4 tail. A phosphorylation-mimicking mutant of alpha4 (alpha4S988D) blocked paxillin binding and reversed the inhibitory effect of alpha4 on cell spreading. Consequently, alpha4 phosphorylation is a biochemical mechanism to modulate paxillin binding to alpha4 integrins with consequent regulation of alpha4 integrin-dependent cellular functions.

Transmembrane-4 superfamily proteins associate with activated protein kinase C (PKC) and link PKC to specific beta(1) integrins

Translocation of conventional protein kinases C (PKCs) to the plasma membrane leads to their specific association with transmembrane-4 superfamily (TM4SF; tetraspanin) proteins (CD9, CD53, CD81, CD82, and CD151), as demonstrated by reciprocal co-immunoprecipitation and covalent cross-linking experiments. Although formation and maintenance of TM4SF-PKC complexes are not dependent on integrins, TM4SF proteins can act as linker molecules, recruiting PKC into proximity with specific integrins. Previous studies showed that the extracellular large loop of TM4SF proteins determines integrin associations. In contrast, specificity for PKC association probably resides within cytoplasmic tails or the first two transmembrane domains of TM4SF proteins, as seen from studies with chimeric CD9 molecules. Consistent with a TM4SF linker function, only those integrins (alpha(3)beta(1), alpha(6)beta(1), and a chimeric "X3TC5" alpha(3) mutant) that associated strongly with tetraspanins were found in association with PKC. We propose that PKC-TM4SF-integrin structures represent a novel type of signaling complex. The simultaneous binding of TM4SF proteins to the extracellular domains of the integrin alpha(3) subunit and to intracellular PKC helps to explain why the integrin alpha3 extracellular domain is needed for both intracellular PKC recruitment and PKC-dependent phosphorylation of the alpha(3) integrin cytoplasmic tail.