Antioxidant and Anticancer Functions of Protein Acyltransferase DHHC3

Silencing of DHHC3, an acyltransferase enzyme in the DHHC family, extensively upregulates oxidative stress (OS). Substrates for DHHC3-mediated palmitoylation include several antioxidant proteins and many other redox regulatory proteins. This helps to explain why DHHC3 ablation upregulates OS. DHHC3 also plays a key role in cancer. DHHC3 ablation leads to diminished xenograft growth of multiple cancer cell types, along with diminished metastasis. Furthermore, DHHC3 protein is upregulated on malignant/metastatic cancer samples, and upregulated gene expression correlates with diminished patient survival in several human cancers. Decreased primary tumor growth due to DHHC3 ablation may be partly explained by an elevated OS → senescence → innate immune cell recruitment mechanism. Elevated OS due to DHHC3 ablation may also contribute to adaptive anticancer immunity and impair tumor metastasis. In addition, DHHC3 ablation disrupts antioxidant protection mechanisms, thus enhancing the efficacy of OS-inducing anticancer drugs. A major focus has thus far been on OS regulation by DHHC3. However, remaining to be studied are multiple DHHC3 substrates that may affect tumor behavior independent of OS. Nonetheless, the currently established properties of DHHC3 make it an attractive candidate for therapeutic targeting in situations in which antioxidant protections need to be downmodulated, and also in cancer.

Antioxidant functions of DHHC3 suppress anti-cancer drug activities

Ablation of protein acyltransferase DHHC3 selectively enhanced the anti-cancer cell activities of several chemotherapeutic agents, but not kinase inhibitors. To understand why this occurs, we used comparative mass spectrometry-based palmitoyl-proteomic analysis of breast and prostate cancer cell lines, ± DHHC3 ablation, to obtain the first comprehensive lists of candidate protein substrates palmitoylated by DHHC3. Putative substrates included 22-28 antioxidant/redox-regulatory proteins, thus predicting that DHHC3 should have antioxidant functions. Consistent with this, DHHC3 ablation elevated oxidative stress. Furthermore, DHHC3 ablation, together with chemotherapeutic drug treatment, (a) elevated oxidative stress, with a greater than additive effect, and (b) enhanced the anti-growth effects of the chemotherapeutic agents. These results suggest that DHHC3 ablation enhances chemotherapeutic drug potency by disabling the antioxidant protections that contribute to drug resistance. Affirming this concept, DHHC3 ablation synergized with another anti-cancer drug, PARP inhibitor PJ-34, to decrease cell proliferation and increase oxidative stress. Hence, DHHC3 targeting can be a useful strategy for selectively enhancing potency of oxidative stress-inducing anti-cancer drugs. Also, comprehensive identification of DHHC3 substrates provides insight into other DHHC3 functions, relevant to in vivo tumor growth modulation.

Integrin-independent support of cancer drug resistance by tetraspanin CD151

Tetraspanin protein CD151 has typically been studied as binding partner and functional regulator of laminin-binding integrins. However, we show here that CD151 supports anti-cancer drug resistance independent of integrins. CD151 ablation sensitized multiple tumor cell types to several anti-cancer drugs (e.g., gefitinib and camptothecin), thus increasing apoptosis, as seen using cleaved caspase-3, cleaved PARP (poly (ADP-ribose) polymerase), annexin V, and propidium iodide staining assays. Drug sensitization due to CD151 ablation is integrin-independent, because, (1) effects occurred in cells when integrins were unengaged with ligand, (2) integrin ablation (α3 and α6 subunits) did not mimic effects of CD151 ablation, (3) the CD151^QRD mutant, with diminished integrin association, and CD151^WT (unmutated CD151) similarly reconstituted drug protection, and (4) treatment with anti-cancer drugs selectively upregulated intracellular nonintegrin-associated CD151 (NIA-CD151), consistent with its role in drug resistance. Together, these results suggest that upregulated CD151 expression may support not only typical integrin-dependent functions, but also integrin-independent survival of circulating (and possibly metastatic) cancer cells during anti-cancer drug therapy.

Protein Acyltransferase DHHC3 Regulates Breast Tumor Growth, Oxidative Stress, and Senescence

DHHC-type protein acyltransferases may regulate the localization, stability, and/or activity of their substrates. In this study, we show that the protein palmitoyltransferase DHHC3 is upregulated in malignant and metastatic human breast cancer. Elevated expression of DHHC3 correlated with diminished patient survival in breast cancer and six other human cancer types. ZDHHC3 ablation in human MDA-MB-231 mammary tumor cell xenografts reduced the sizes of both the primary tumor and metastatic lung colonies. Gene array data and fluorescence dye assays documented increased oxidative stress and senescence in ZDHHC3-ablated cells. ZDHHC3-ablated tumors also showed enhanced recruitment of innate immune cells (antitumor macrophages, natural killer cells) associated with clearance of senescent tumors. These antitumor effects were reversed upon reconstitution with wild-type, but not enzyme-active site-deficient DHHC3. Concomitant ablation of the upregulated oxidative stress protein TXNIP substantially negated the effects of ZDHHC3 depletion on oxidative stress and senescence. Diminished DHHC3-dependent palmitoylation of ERGIC3 protein likely played a key role in TXNIP upregulation. In conclusion, DHHC3-mediated protein palmitoylation supports breast tumor growth by modulating cellular oxidative stress and senescence. Cancer Res; 77(24); 6880-90. ©2017 AACR.

Novel impact of EWI-2, CD9, and CD81 on TGF-β signaling in melanoma

Cell surface transmembrane protein IGFS8 (herein called EWI-2) negatively regulates melanoma TGF-β signaling and is well positioned to control the transition in TGF-β signaling from cytostatic (in early melanoma stages) to pro-invasion/metastasis (in later stages). EWI-2 functions by sequestering the tetraspanin proteins CD9 and CD81, thereby making them unavailable to support the association of TGFβ receptor 1 with TGFβ receptor 2.

Tetraspanin TSPAN12 regulates tumor growth and metastasis and inhibits β-catenin degradation

Ablation of tetraspanin protein TSPAN12 from human MDA-MB-231 cells significantly decreased primary tumor xenograft growth, while increasing tumor apoptosis. Furthermore, TSPAN12 removal markedly enhanced tumor-endothelial interactions and increased metastasis to mouse lungs. TSPAN12 removal from human MDA-MB-231 cells also caused diminished association between FZD4 (a key canonical Wnt pathway receptor) and its co-receptor LRP5. The result likely explains substantially enhanced proteosomal degradation of β-catenin, a key effecter of canonical Wnt signaling. Consistent with disrupted canonical Wnt signaling, TSPAN12 ablation altered expression of LRP5, Naked 1 and 2, DVL2, DVL3, Axin 1, and GSKβ3 proteins. TSPAN12 ablation also altered expression of several genes regulated by β-catenin (e.g. CCNA1, CCNE2, WISP1, ID4, SFN, ME1) that may help to explain altered tumor growth and metastasis. In conclusion, these results provide the first evidence for TSPAN12 playing a role in supporting primary tumor growth and suppressing metastasis. TSPAN12 appears to function by stabilizing FZD4-LRP5 association, in support of canonical Wnt-pathway signaling, leading to enhanced β-catenin expression and function.

Tetraspanin proteins promote multiple cancer stages

An abundance of evidence shows supporting roles for tetraspanin proteins in human cancer. Many studies show that the expression of tetraspanins correlates with tumour stage, tumour type and patient outcome. In addition, perturbations of tetraspanins in tumour cell lines can considerably affect cell growth, morphology, invasion, tumour engraftment and metastasis. This Review emphasizes new studies that have used de novo mouse cancer models to show that select tetraspanin proteins have key roles in tumour initiation, promotion and metastasis. This Review also emphasizes how tetraspanin proteins can sometimes participate in tumour angiogenesis. These recent data build an increasingly strong case for tetraspanins as therapeutic targets.

Tetraspanin CD151 plays a key role in skin squamous cell carcinoma

Here we provide the first evidence that tetraspanin CD151 can support de novo carcinogenesis. During two-stage mouse skin chemical carcinogenesis, CD151 reduces tumor lag time and increases incidence, multiplicity, size and progression to malignant squamous cell carcinoma (SCC), while supporting both cell survival during tumor initiation and cell proliferation during the promotion phase. In human skin SCC, CD151 expression is selectively elevated compared with other skin cancer types. CD151 support of keratinocyte survival and proliferation may depend on activation of transcription factor STAT3 (signal transducers and activators of transcription), a regulator of cell proliferation and apoptosis. CD151 also supports protein kinase C (PKC)α-α6β4 integrin association and PKC-dependent β4 S1424 phosphorylation, while regulating α6β4 distribution. CD151-PKCα effects on integrin β4 phosphorylation and subcellular localization are consistent with epithelial disruption to a less polarized, more invasive state. CD151 ablation, while minimally affecting normal cell and normal mouse functions, markedly sensitized mouse skin and epidermoid cells to chemicals/drugs including 7,12-dimethylbenz[α]anthracene (mutagen) and camptothecin (topoisomerase inhibitor), as well as to agents targeting epidermal growth factor receptor, PKC, Jak2/Tyk2 and STAT3. Hence, CD151 'co-targeting' may be therapeutically beneficial. These findings not only support CD151 as a potential tumor target, but also should apply to other cancers utilizing CD151/laminin-binding integrin complexes.

Palmitoylation by DHHC3 is critical for the function, expression, and stability of integrin α6β4

The laminin-binding integrin α6β4 plays key roles in both normal epithelial and endothelial cells and during tumor cell progression, metastasis, and angiogenesis. Previous cysteine mutagenesis studies have suggested that palmitoylation of α6β4 protein supports a few integrin-dependent functions and molecular associations. Here we took another approach and obtained strikingly different results. We used overexpression and RNAi knockdown in multiple cell types to identify protein acyl transferase DHHC3 as the enzyme responsible for integrin β4 and α6 palmitoylation. Ablation of DHHC3 markedly diminished integrin-dependent cellular cable formation on Matrigel, integrin signaling through Src, and β4 phosphorylation on key diagnostic amino acids (S1356 and 1424). However, unexpectedly, and in sharp contrast to prior α6β4 mutagenesis results, knockdown of DHHC3 accelerated the degradation of α6β4, likely due to an increase in endosomal exposure to cathepsin D. When proteolytic degradation was inhibited (by Pepstatin A), rescued α6β4 accumulated intracellularly, but was unable to reach the cell surface. DHHC3 ablation effects were strongly selective for α6β4. Cell-surface levels of ~10 other proteins (including α3β1) were not diminished, and the appearance of hundreds of other palmitoylated proteins was not altered. Results obtained here demonstrate a new substrate for the DHHC3 enzyme and provide novel opportunities for modulating α6β4 expression, distribution, and function.

Tetraspanin CD151 protects against pulmonary fibrosis by maintaining epithelial integrity

RATIONALE

Idiopathic pulmonary fibrosis (IPF) is a chronic pulmonary disorder of unknown etiology with few treatment options. Although tetraspanins are involved in various diseases, their roles in fibrosis have not been determined.

OBJECTIVES

To investigate the role of tetraspanin CD151 in pulmonary fibrosis.

METHODS

CD151 knockout (KO) mice were studied by histological, biochemical, and physiological analyses and compared with wild-type mice and CD9 KO mice. Further mechanistic analyses were performed in vitro, in vivo, and on samples from patients with IPF.

MEASUREMENTS AND MAIN RESULTS

A microarray study identified an enrichment of genes involved in connective tissue disorders in the lungs of CD151 KO mice, but not in CD9 KO mice. Consistent with this, CD151 KO mice spontaneously exhibited age-related pulmonary fibrosis. Deletion of CD151 did not affect pulmonary fibroblast functions but instead degraded epithelial integrity via attenuated adhesion strength on the basement membrane; CD151-deleted alveolar epithelial cells exhibited increased α-SMA expression with activation of p-Smad2, leading to fibrotic changes in the lungs. This loss of epithelial integrity in CD151 KO lungs was further exacerbated by intratracheal bleomycin exposure, resulting in severe fibrosis with increased mortality. We also observed decreased numbers of CD151-positive alveolar epithelial cells in patients with IPF.

CONCLUSIONS

CD151 is essential for normal function of alveolar epithelial cells; loss of CD151 causes pulmonary fibrosis as a result of epithelial disintegrity. Given that CD151 may protect against fibrosis, this protein represents a novel target for the treatment of fibrotic diseases.

Abstract 2704: Tetraspanin CD151 drives mammary tumor onset and metastasis by coordinating integrin-dependent cell adhesion, motility, survival and signaling

The ErbB2+ subtype of human breast tumors remains a significant threat to women's health, as they are highly prone to recurrence or metastasis, and development of drug resistance. Here we report that CD151, a member of the tetraspanin family and a close partner of the laminin-binding (LB) integrins (≥3α1 and α6β4), contributes to the onset and malignancy of ErbB2-induced mammary tumors. By crossing gene-targeted mice onto a clinically-relevant MMTV-ErbB2 transgenic model, we demonstrated that deletion of CD151 significantly impaired the onset and metastasis of mammary tumors in mice. Our subsequent analyses of cell lines derived from mouse primary tumors showed that loss of CD151 decreased integrin-mediated cell-ECM adhesion and cell-cell contacts. Also, for ErbB2-overexpressing MCF-10A cells cultured in 2D or 3D, ablation of CD151 markedly reduced tumor cell motility, invasion and survival. However, only MAPK and FAK kinases among an array of signaling molecules downstream of integrins and ErbB receptors, exhibited a decreased activation in response to CD151 disruption. Importantly, there were corresponding changes in the activation and signaling of α6β4 integrin, as reflected by decreased phosphorylation of multiple serine residues at its cytoplasmic domain. Also, the cross-talk between integrins and ErbB receptors was strongly inhibited. Meanwhile, our analyses of clinical dataset revealed a strong correlation between CD151 gene expression and patients’ metastasis-free survival. Taken together, our results demonstrate that tetraspanin CD151 drives ErbB2-induced mammary tumorigenesis by coordinating the function and signaling of LB integrins. This newly-identified crucial role of CD151 offers a strong therapeutic potential for the treatment of breast cancer patients.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2704. doi:1538-7445.AM2012-2704

Transcriptional profiling of stroma from inflamed and resting lymph nodes defines immunological hallmarks

Lymph node stromal cells (LNSCs) closely regulate immunity and self-tolerance, yet key aspects of their biology remain poorly elucidated. Here, comparative transcriptomic analyses of mouse LNSC subsets demonstrated the expression of important immune mediators, growth factors and previously unknown structural components. Pairwise analyses of ligands and cognate receptors across hematopoietic and stromal subsets suggested a complex web of crosstalk. Fibroblastic reticular cells (FRCs) showed enrichment for higher expression of genes relevant to cytokine signaling, relative to their expression in skin and thymic fibroblasts. LNSCs from inflamed lymph nodes upregulated expression of genes encoding chemokines and molecules involved in the acute-phase response and the antigen-processing and antigen-presentation machinery. Poorly studied podoplanin (gp38)-negative CD31(-) LNSCs showed similarities to FRCs but lacked expression of interleukin 7 (IL-7) and were identified as myofibroblastic pericytes that expressed integrin α(7). Together our data comprehensively describe the transcriptional characteristics of LNSC subsets.

CD151 restricts the α6 integrin diffusion mode

Laminin-binding integrins (α3β1, α6β1, α6β4, α7β1) are almost always expressed together with tetraspanin CD151. In every coexpressing cell analyzed to date, CD151 makes a fundamental contribution to integrin-dependent motility, invasion, morphology, adhesion and/or signaling. However, there has been minimal mechanistic insight into how CD151 affects integrin functions. In MDA-MB-231 mammary cells, tetraspanin CD151 knockdown impairs α6 integrin clustering and functions without decreasing α6 integrin expression or activation. Furthermore, CD151 knockdown minimally affects the magnitude of α6 integrin diffusion, as measured using single particle tracking. Instead, CD151 knockdown has a novel and unexpected dysregulating effect on the mode of α6 integrin diffusion. In control cells α6 integrin shows mostly random-confined diffusion (RCD) and some directed motion (DMO). In sharp contrast, in CD151-knockdown cells α6 integrin shows mostly DMO. In control cells α6 diffusion mode is sensitive to actin disruption, talin knockdown and phorbol ester stimulation. By contrast, CD151 knockdown cell α6 integrin is sensitive to actin disruption but desensitized to talin knockdown or phorbol ester stimulation, indicating dysregulation. Both phorbol ester and EGF stimulate cell spreading and promote α6 RCD in control cells. By contrast, CD151-ablated cells retain EGF effects but lose phorbol-ester-stimulated spreading and α6 RCD. For α6 integrins, physical association with CD151 promotes α6 RCD, in support of α6-mediated cable formation and adhesion. By comparison, for integrins not associated with CD151 (e.g. αv integrins), CD151 affects neither diffusion mode nor αv function. Hence, CD151 support of α6 RCD is specific and functionally relevant, and probably underlies diverse CD151 functions in skin, kidney and cancer cells.

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.

Disruption of laminin-integrin-CD151-focal adhesion kinase axis sensitizes breast cancer cells to ErbB2 antagonists

Resistance to anti-ErbB2 agents is a significant problem in the treatment of human ErbB2+ breast cancers. We show here that adhesion of human ErbB2+ breast cancer cells to basement membrane laminin-5 provides substantial resistance to trastuzumab and lapatinib, agents that respectively target the extracellular and kinase domains of ErbB2. Knockdown of laminin-binding integrins (alpha6beta4, alpha3beta1) or associated tetraspanin protein CD151 reversed laminin-5 resistance and sensitized ErbB2+ cells to trastuzumab and lapatinib. CD151 knockdown, together with trastuzumab treatment, inhibited ErbB2 activation and downstream signaling through Akt, Erk1/2, and focal adhesion kinase (FAK). Hence, ErbB2 function in mammary tumor cells is promoted by integrin-mediated adhesion to laminin-5, with strong support by CD151, leading to signaling through FAK. Consequently, removal or inhibition of any of these components (laminin-5, integrin, CD151, FAK) markedly sensitizes cells to anti-ErbB2 agents. These new insights should be useful when devising strategies for overcoming drug resistance in ErbB2+ cancers.

Tetraspanin12 regulates ADAM10-dependent cleavage of amyloid precursor protein

Using mass spectrometry, we identified ADAM10 (a membrane-associated metalloproteinase) as a partner for TSPAN12, a tetraspanin protein. TSPAN12-ADAM10 interaction was confirmed by reciprocal coimmunoprecipitation in multiple tumor cell lines. TSPAN12, to a greater extent than other tetraspanins (CD81, CD151, CD9, and CD82), associated with ADAM10 but not with ADAM17. Overexpression of TSPAN12 enhanced ADAM10-dependent shedding of amyloid precursor protein (APP) in MCF7 (breast cancer) and SH-SY5Y (neuroblastoma) cell lines. Conversely, siRNA ablation of endogenous TSPAN12 markedly diminished APP proteolysis in both cell lines. Furthermore, TSPAN12 overexpression enhanced ADAM10 prodomain maturation, whereas TSPAN12 ablation diminished ADAM10 maturation. A palmitoylation-deficient TSPAN12 mutant failed to associate with ADAM10, inhibited ADAM10-dependent proteolysis of APP, and inhibited ADAM10 maturation, most likely by interfering with endogenous wild-type TSPAN12. In conclusion, TSPAN12 serves as a novel and robust partner for ADAM10 and promotes ADAM10 maturation, thereby facilitating ADAM10-dependent proteolysis of APP. This novel mode of regulating APP cleavage is of relevance to Alzheimer's disease therapy.

Tetraspanin proteins regulate membrane type-1 matrix metalloproteinase-dependent pericellular proteolysis

Membrane type-1 matrix metalloproteinase (MT1-MMP) supports tumor cell invasion through extracellular matrix barriers containing fibrin, collagen, fibronectin, and other proteins. Here, we show that simultaneous knockdown of two or three members of the tetraspanin family (CD9, CD81, and TSPAN12) markedly decreases MT1-MMP proteolytic functions in cancer cells. Affected functions include fibronectin proteolysis, invasion and growth in three-dimensional fibrin and collagen gels, and MMP-2 activation. Tetraspanin proteins (CD9, CD81, and TSPAN2) selectively coimmunoprecipitate and colocalize with MT1-MMP. Although tetraspanins do not affect the initial biosynthesis of MT1-MMP, they do protect the newly synthesized protein from lysosomal degradation and support its delivery to the cell surface. Interfering with MT1-MMP-tetraspanin collaboration may be a useful therapeutic approach to limit cancer cell invasion and metastasis.

Targeting of tetraspanin proteins--potential benefits and strategies

The tetraspanin transmembrane proteins have emerged as key players in malignancy, the immune system, during fertilization and infectious disease processes. Tetraspanins engage in a wide range of specific molecular interactions, occurring through the formation of tetraspanin-enriched microdomains (TEMs). TEMs therefore serve as a starting point for understanding how tetraspanins affect cell signalling, adhesion, morphology, motility, fusion and virus infection. An abundance of recent evidence suggests that targeting tetraspanins, for example, by monoclonal antibodies, soluble large-loop proteins or RNAi technology, should be therapeutically beneficial.

Overexpression of fatty acid synthase is associated with palmitoylation of Wnt1 and cytoplasmic stabilization of beta-catenin in prostate cancer

Fatty acid synthase (FASN), a key metabolic enzyme for liponeogenesis highly expressed in several human cancers, displays oncogenic properties such as resistance to apoptosis and induction of proliferation when overexpressed. To date, no mechanism has been identified to explain the oncogenicity of FASN in prostate cancer. We generated immortalized prostate epithelial cells (iPrECs) overexpressing FASN, and found that (14)C-acetate incorporation into palmitate synthesized de novo by FASN was significantly elevated in immunoprecipitated Wnt-1 when compared to isogenic cells not overexpressing FASN. Overexpression of FASN caused membranous and cytoplasmic beta-catenin protein accumulation and activation, whereas FASN knockdown by short-hairpin RNA resulted in a reduction in the extent of beta-catenin activation. Orthotopic transplantation of iPrECs overexpressing FASN in nude mice resulted in invasive tumors that overexpressed beta-catenin. A strong significant association between FASN and cytoplasmic (stabilized) beta-catenin immunostaining was found in 862 cases of human prostate cancer after computerized subtraction of the membranous beta-catenin signal (P<0.001, Spearman's rho=0.33). We propose that cytoplasmic stabilization of beta-catenin through palmitoylation of Wnt-1 and subsequent activation of the pathway is a potential mechanism of FASN oncogenicity in prostate cancer.

CD151 accelerates breast cancer by regulating alpha 6 integrin function, signaling, and molecular organization

CD151, a master regulator of laminin-binding integrins (alpha(6)beta(4), alpha(6)beta(1), and alpha(3)beta(1)), assembles these integrins into complexes called tetraspanin-enriched microdomains. CD151 protein expression is elevated in 31% of human breast cancers and is even more elevated in high-grade (40%) and estrogen receptor-negative (45%) subtypes. The latter includes triple-negative (estrogen receptor, progesterone receptor, and HER2 negative) basal-like tumors. CD151 ablation markedly reduced basal-like mammary cell migration, invasion, spreading, and signaling (through FAK, Rac1, and lck) while disrupting epidermal growth factor receptor (EGFR)-alpha(6) integrin collaboration. Underlying these defects, CD151 ablation redistributed alpha(6)beta(4) integrins subcellularly and severed molecular links between integrins and tetraspanin-enriched microdomains. In a prototypical basal-like mammary tumor line, CD151 ablation notably delayed tumor progression in ectopic and orthotopic xenograft models. These results (a) establish that CD151-alpha(6) integrin complexes play a functional role in basal-like mammary tumor progression; (b) emphasize that alpha(6) integrins function via CD151 linkage in the context of tetraspanin-enriched microdomains; and (c) point to potential relevance of CD151 as a high-priority therapeutic target, with relative selectivity (compared with laminin-binding integrins) for pathologic rather than normal physiology.

DHHC2 affects palmitoylation, stability, and functions of tetraspanins CD9 and CD151

Although palmitoylation markedly affects tetraspanin protein biochemistry and functions, relevant palmitoylating enzymes were not known. There are 23 mammalian "DHHC" (Asp-His-His-Cys) proteins, which presumably palmitoylate different sets of protein substrates. Among DHHC proteins tested, DHHC2 best stimulated palmitoylation of tetraspanins CD9 and CD151, whereas inactive DHHC2 (containing DH-->AA or C-->S mutations within the DHHC motif) failed to promote palmitoylation. Furthermore, DHHC2 associated with CD9 and CD151, but not other cell surface proteins, and DHHC2 knockdown diminished CD9 and CD151 palmitoylation. Knockdown of six other Golgi-resident DHHC proteins (DHHC3, -4, -8, -17, -18, and -21) had no effect on CD9 or CD151. DHHC2 selectively affected tetraspanin palmitoylation, but not the palmitoylations of integrin beta4 subunit and bulk proteins visible in [(3)H]palmitate-labeled whole cell lysates. DHHC2-dependent palmitoylation also had multiple functional effects. First, it promoted physical associations between CD9 and CD151, and between alpha3 integrin and other proteins. Second, it protected CD151 and CD9 from lysosomal degradation. Third, the presence of DHHC2, but not other DHHC proteins, shifted cells away from a dispersed state and toward increased cell-cell contacts.

A novel cysteine cross-linking method reveals a direct association between claudin-1 and tetraspanin CD9

Tetraspanins serve as molecular organizers of multiprotein microdomains in cell membranes. Hence to understand functions of tetraspanin proteins, it is critical to identify laterally interacting partner proteins. Here we used a novel technical approach involving exposure and cross-linking of membrane-proximal cysteines coupled with LC-MS/MS protein identification. In this manner we identified nine potential tetraspanin CD9 partners, including claudin-1. Chemical cross-linking yielded a CD9-claudin-1 heterodimer, thus confirming direct association and adding claudin-1 to the short list of proteins that can directly associate with CD9. Interaction of CD9 (and other tetraspanins) with claudin-1 was supported by subcellular colocalization and was confirmed in multiple cell lines, although other claudins (claudin-2, -3, -4, -5, and -7) associated to a much lesser extent. Moreover claudin-1 was distributed very similarly to CD9 in sucrose gradients and, like CD9, was released from A431 and A549 cells upon cholesterol depletion. These biochemical features of claudin-1 are characteristic of tetraspanin microdomain proteins. Although claudins are major structural components of intercellular tight junctions, CD9-claudin-1 complexes did not reside in tight junctions, and depletion of key tetraspanins (CD9 and CD151) by small interfering RNA had no effect on paracellular permeability. However, tetraspanin depletion did cause a marked decrease in the stability of newly synthesized claudin-1. In conclusion, these results (a) validate a technical approach that appears to be particularly well suited for identifying protein partners directly associated with tetraspanins or with other proteins that contain membrane-proximal cysteines and (b) provide insight into how non-junctional claudins may be regulated in the context of tetraspanin-enriched microdomains.

Deletion of tetraspanin Cd151 results in decreased pathologic angiogenesis in vivo and in vitro

Tetraspanin protein CD151 is abundant on endothelial cells. To determine whether CD151 affects angiogenesis, Cd151-null mice were prepared. Cd151-null mice showed no vascular defects during normal development or during neonatal oxygen-induced retinopathy. However, Cd151-null mice showed impaired pathologic angiogenesis in other in vivo assays (Matrigel plug, corneal micropocket, tumor implantation) and in the ex vivo aortic ring assay. Cd151-null mouse lung endothelial cells (MLECs) showed normal adhesion and proliferation, but marked alterations in vitro, in assays relevant to angiogenesis (migration, spreading, invasion, Matrigel contraction, tube and cable formation, spheroid sprouting). Consistent with these functional impairments, and with the close, preferential association of CD151 with laminin-binding integrins, Cd151-null MLECs also showed selective signaling defects, particularly on laminin substrate. Adhesion-dependent activation of PKB/c-Akt, e-NOS, Rac, and Cdc42 was diminished, but Raf, ERK, p38 MAP kinase, FAK, and Src were unaltered. In Cd151-null MLECs, connections were disrupted between laminin-binding integrins and at least 5 other proteins. In conclusion, CD151 modulates molecular organization of laminin-binding integrins, thereby supporting secondary (ie, after cell adhesion) functions of endothelial cells, which are needed for some types of pathologic angiogenesis in vivo. Selective effects of CD151 on pathologic angiogenesis make it a potentially useful target for anticancer therapy.

In vitro interaction between hepatitis C virus (HCV) envelope glycoprotein E2 and serum lipoproteins (LPs) results in enhanced cellular binding of both HCV E2 and LPs

Hepatitis C virus (HCV) particles in serum associate with lipoproteins (LPs), and the low-density lipoprotein receptor (LDLr) has been implicated in virus attachment and entry into cells. To clarify the basis of interactions between virus and LPs, we determined whether HCV interacts with human LPs via its envelope glycoprotein E2. The binding of serum-derived virus-like particles, HCV E2, and HCV E2-LP complexes to CD81 and LDLr was studied. Incubation of HCV E2 protein with human and bovine LPs (very low density, low density, and high density) enhanced the binding of both HCV E2 and LPs to CD4+ lymphoblastoid (MOLT-4) cells, foreskin fibroblasts, and hepatocytes. The binding of HCV E2 to MOLT-4 cells was not enhanced when it was preincubated with lipid-free apoprotein B, which suggests that E2 interacts with the lipid moiety of human lipoproteins. The LP interaction was specific for HCV E2--incubation of HIV gp120 with LPs did not enhance gp120 binding to MOLT-4 cells. The enhanced HCV E2 binding required expression of both human CD81 and LDLr. These data suggest that HCV E2 associates with LDL and that the resulting complex enhances binding of both ligands to cells, which may contribute to the finding that HCV-infected individuals have significantly lower levels of LDL than control subjects.

EWI-2 and EWI-F link the tetraspanin web to the actin cytoskeleton through their direct association with ezrin-radixin-moesin proteins

EWI-2 and EWI-F, two members of a novel subfamily of Ig proteins, are direct partners of tetraspanins CD9 (Tspan29) and CD81 (Tspan28). These EWI proteins contain a stretch of basic charged amino acids in their cytoplasmic domains that may act as binding sites for actin-linking ezrin-radixin-moesin (ERM) proteins. Confocal microscopy analysis revealed that EWI-2 and EWI-F colocalized with ERM proteins at microspikes and microvilli of adherent cells and at the cellular uropod in polarized migrating leukocytes. Immunoprecipitation studies showed the association of EWI-2 and EWI-F with ERM proteins in vivo. Moreover, pulldown experiments and protein-protein binding assays with glutathione S-transferase fusion proteins containing the cytoplasmic domains of EWI proteins corroborated the strong and direct interaction between ERMs and these proteins. The active role of ERMs was further confirmed by double transfections with the N-terminal domain of moesin, which acts as a dominant negative form of ERMs, and was able to delocalize EWIs from the uropod of polarized leukocytes. In addition, direct association of EWI partner CD81 C-terminal domain with ERMs was also demonstrated. Functionally, silencing of endogenous EWI-2 expression by short interfering RNA in lymphoid CEM cells augmented cell migration, cellular polarity, and increased phosphorylation of ERMs. Hence, EWI proteins, through their direct interaction with ERM proteins, act as linkers to connect tetraspanin-associated microdomains to actin cytoskeleton regulating cell motility and polarity.

Contrasting effects of EWI proteins, integrins, and protein palmitoylation on cell surface CD9 organization

CD9, a tetraspanin protein, makes crucial contributions to sperm egg fusion, other cellular fusions, epidermal growth factor receptor signaling, cell motility, and tumor suppression. Here we characterize a low affinity anti-CD9 antibody, C9BB, which binds preferentially to homoclustered CD9. Using mAb C9BB as a tool, we show that cell surface CD9 homoclustering is promoted by expression of alpha3beta1 and alpha6beta4 integrins and by palmitoylation of the CD9 and beta4 proteins. Conversely, CD9 is shifted toward heteroclusters upon expression of CD9 partner proteins (EWI-2 and EWI-F) or other tetraspanins, or upon ablation of CD9 palmitoylation. Furthermore, unpalmitoylated CD9 showed enhanced EWI-2 association, thereby demonstrating a previously unappreciated role for tetraspanin palmitoylation, and underscoring how depalmitoylation and EWI-2 association may collaborate to shift CD9 from homo- to heteroclusters. In conclusion, we have used a novel molecular probe (mAb C9BB) to demonstrate the existence of multiple types of CD9 complex on the cell surface. A shift from homo- to heteroclustered CD9 may be functionally significant because the latter was especially obvious on malignant epithelial tumor cells. Hence, because of its specialized properties, C9BB may be more useful than other anti-CD9 antibodies for monitoring CD9 during tumor progression.

Tetraspanin functions and associated microdomains

Cell-surface proteins of the tetraspanin family are small, and often hidden by a canopy of tall glycoprotein neighbours in the cell membrane. Consequently, tetraspanins have been understudied and underappreciated, despite their presence on nearly all cell and tissue types. Important new genetic evidence has now emerged, and is bolstered by new insights into the cell biology, signalling and biochemistry of tetraspanins. These new findings provide a framework for better understanding of these mysterious molecules in the regulation of cellular processes, from signalling to motility.

Structural organization and interactions of transmembrane domains in tetraspanin proteins

Background

Proteins of the tetraspanin family contain four transmembrane domains (TM1-4) linked by two extracellular loops and a short intracellular loop, and have short intracellular N- and C-termini. While structure and function analysis of the larger extracellular loop has been performed, the organization and role of transmembrane domains have not been systematically assessed.

Results

Among 28 human tetraspanin proteins, the TM1-3 sequences display a distinct heptad repeat motif (abcdefg)_n. In TM1, position a is occupied by structurally conserved bulky residues and position d contains highly conserved Asn and Gly residues. In TM2, position a is occupied by conserved small residues (Gly/Ala/Thr), and position d has a conserved Gly and two bulky aliphatic residues. In TM3, three a positions of the heptad repeat are filled by two leucines and a glutamate/glutamine residue, and two d positions are occupied by either Phe/Tyr or Val/Ile/Leu residues. No heptad motif is apparent in TM4 sequences. Mutations of conserved glycines in human CD9 (Gly25 and Gly32 in TM1; Gly67 and Gly74 in TM2) caused aggregation of mutant proteins inside the cell. Modeling of the TM1-TM2 interface in CD9, using a novel algorithm, predicts tight packing of conserved bulky residues against conserved Gly residues along the two helices. The homodimeric interface of CD9 was mapped, by disulfide cross-linking of single-cysteine mutants, to the vicinity of residues Leu14 and Phe17 in TM1 (positions g and c) and Gly77, Gly80 and Ala81 in TM2 (positions d, g and a, respectively). Mutations of a and d residues in both TM1 and TM2 (Gly25, Gly32, Gly67 and Gly74), involved in intramolecular TM1-TM2 interaction, also strongly diminished intermolecular interaction, as assessed by cross-linking of Cys80.

Conclusion

Our results suggest that tetraspanin intra- and intermolecular interactions are mediated by conserved residues in adjacent, but distinct regions of TM1 and TM2. A key structural element that defines TM1-TM2 interaction in tetraspanins is the specific packing of bulky residues against small residues.

Metabolic activation-related CD147-CD98 complex

Cell surface CD147 protein promotes production of matrix metalloproteinases and hyaluronan, associates with monocarboxylate transporters and integrins, and is involved in reproductive, neural, inflammatory, and tumor functions. Here we combined covalent cross-linking, mass spectrometric protein identification, and co-immunoprecipitation to show selective CD147 association with three major types of transporters (CD98 heavy chain (CD98hc)-L-type amino acid transporter, ASCT2, and monocarboxylate transporters) as well as a regulator of cell proliferation (epithelial cell adhesion molecule). In the assembly of these multicomponent complexes, CD147 and CD98hc play a central organizing role. RNA interference knock-down experiments established a strong connection between CD147 and CD98hc expression and a strong positive association of CD147 (and CD98hc) with cell proliferation. As the CD147-CD98hc complex and proliferation diminished, AMP-activated protein kinase (a cellular "fuel gauge") became activated, indicating a disturbance of cellular energy metabolism. Our data point to a CD147-CD98 cell surface supercomplex that plays a critical role in energy metabolism, likely by coordinating transport of lactate and amino acids. Furthermore we showed how covalent cross-linking, together with mass spectrometry, can be used to identify closely associated transmembrane proteins. This approach should also be applicable to many other types of transmembrane proteins besides those associated with CD98hc and CD147.

Palmitoylation supports assembly and function of integrin-tetraspanin complexes

As observed previously, tetraspanin palmitoylation promotes tetraspanin microdomain assembly. Here, we show that palmitoylated integrins (α3, α6, and β4 subunits) and tetraspanins (CD9, CD81, and CD63) coexist in substantially overlapping complexes. Removal of β4 palmitoylation sites markedly impaired cell spreading and signaling through p130Cas on laminin substrate. Also in palmitoylation-deficient β4, secondary associations with tetraspanins (CD9, CD81, and CD63) were diminished and cell surface CD9 clustering was decreased, whereas core α6β4–CD151 complex formation was unaltered. There is also a functional connection between CD9 and β4 integrins, as evidenced by anti-CD9 antibody effects on β4-dependent cell spreading. Notably, β4 palmitoylation neither increased localization into “light membrane” fractions of sucrose gradients nor decreased solubility in nonionic detergents—hence it does not promote lipid raft association. Instead, palmitoylation of β4 (and of the closely associated tetraspanin CD151) promotes CD151–α6β4 incorporation into a network of secondary tetraspanin interactions (with CD9, CD81, CD63, etc.), which provides a novel framework for functional regulation.

Links between CD147 function, glycosylation, and caveolin-1

Cell surface CD147 shows remarkable variations in size (31-65 kDa) because of heterogeneous N-glycosylation, with the most highly glycosylated forms functioning to induce matrix metalloproteinase (MMP) production. Here we show that all three CD147 N-glycosylation sites make similar contributions to both high and low glycoforms (HG- and LG-CD147). l-Phytohemagglutinin lectin binding and swainsonine inhibition experiments indicated that HG-CD147 contains N-acetylglucosaminyltransferase V-catalyzed, beta1,6-branched, polylactosamine-type sugars, which account for its excess size. Therefore, CD147, which is itself elevated on invasive tumor cells, may make a major contribution to the abundance of beta1,6-branched polylactosamine sugars that appear on invasive tumor cells. It was shown previously that caveolin-1 associates with CD147, thus inhibiting CD147 self-aggregation and MMP induction; now we show that caveolin-1 associates with LG-CD147 and restricts the biosynthetic conversion of LG-CD147 to HG-CD147. In addition, HG-CD147 (but not LG-CD147) was preferentially captured as a multimer after treatment of cells with a homobifunctional cross-linking agent and was exclusively recognized by monoclonal antibody AAA6, a reagent that selectively recognizes self-associated CD147 and inhibits CD147-mediated MMP induction. In conclusion, we have 1) determined the biochemical basis for the unusual size variation in CD147, 2) established that CD147 is a major carrier of beta1,6-branched polylactosamine sugars on tumor cells, and 3) determined that caveolin-1 can inhibit the conversion of LG-CD147 to HG-CD147. Because it is HG-CD147 that self-aggregates and stimulates MMP induction, we now have a mechanism to explain how caveolin-1 inhibits these processes. These results help explain the previously established tumor suppressor functions of caveolin-1.

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.

Tetraspanin proteins mediate cellular penetration, invasion, and fusion events and define a novel type of membrane microdomain

This review summarizes key aspects of tetraspanin proteins, with a focus on the functional relevance and structural features of these proteins and how they are organized into a novel type of membrane microdomain. Despite the size of the tetraspanin family and their abundance and wide distribution over many cell types, most have not been studied. However, from studies of prototype tetraspanins, information regarding functions, cell biology, and structural organization has begun to emerge. Genetic evidence points to critical roles for tetraspanins on oocytes during fertilization, in fungi during leaf invasion, in Drosophila embryos during neuromuscular synapse formation, during T and B lymphocyte activation, in brain function, and in retinal degeneration. From structure and mutagenesis studies, we are beginning to understand functional subregions within tetraspanins, as well as the levels of connections among tetraspanins and their many associated proteins. Tetraspanin-enriched microdomains (TEMs) are emerging as entities physically and functionally distinct from lipid rafts. These microdomains now provide a context in which to evaluate tetraspanins in the regulation of growth factor signaling and in the modulation of integrin-mediated post-cell adhesion events. Finally, the enrichment of tetraspanins within secreted vesicles called exosomes, coupled with hints that tetraspanins may regulate vesicle fusion and/or fission, suggests exciting new directions for future research.

Evidence for specific tetraspanin homodimers: inhibition of palmitoylation makes cysteine residues available for cross-linking

It is a well-established fact that tetraspanin proteins, a large family of integral membrane proteins involved in cell motility, fusion and signalling, associate extensively with one another and with other transmembrane and membrane-proximal proteins. In this study, we present results strongly suggesting that tetraspanin homodimers are fundamental units within larger tetraspanin complexes. Evidence for constitutive CD9 homodimers was obtained using several cell lines, utilizing the following four methods: (1) spontaneous cross-linking via intermolecular disulphide bonds, (2) use of a cysteine-reactive covalent cross-linking agent, (3) use of an amino-reactive covalent cross-linking agent, and (4) covalent cross-linking via direct intermolecular disulphide bridging between unpalmitoylated membrane-proximal cysteine residues. In the last case, incubation of cells with the palmitoylation inhibitor 2-bromopalmitate exposed membrane-proximal cysteine residues, thus effectively promoting 'zero-length' cross-linking to stabilize homodimers. Similar to CD9, other tetraspanins (CD81 and CD151) also showed a tendency to homodimerize. Tetraspanin homodimers were assembled from newly synthesized proteins in the Golgi, as evidenced by cycloheximide and Brefeldin A inhibition studies. Importantly, tetraspanin homodimers appeared on the cell surface and participated in typical 'tetraspanin web' interactions with other proteins. Whereas homodimers were the predominant cross-linked species, we also observed some higher-order complexes (trimers, tetramers or higher) and a much lower level of cross-linking between different tetraspanins (CD81-CD9, CD9-CD151, CD81-CD151). In conclusion, our results strongly suggest that tetraspanin homodimers, formed in the Golgi and present at the cell surface, serve as building blocks for the assembly of larger, multicomponent tetraspanin protein complexes.

Caveolin-1 regulates matrix metalloproteinases-1 induction and CD147/EMMPRIN cell surface clustering

CD147, a regulator of matrix metalloproteinase (MMP) production, showed highly specific association with caveolin-1 on the surface of multiple cell types. CD147-caveolin-1 complex formation was temperature and cholesterol dependent, reminiscent of associations seen within caveolae/lipid rafts. However, the subset of caveolin-1 associated with CD147 appeared exclusively within intermediate density sucrose gradient fractions, rather than in the low density fractions containing the bulk of caveolin-1. Mutagenesis experiments revealed that CD147 Ig domain 2 was required for caveolin-1 association. In contrast to CD147-caveolin-1 complexes, CD147-alpha(3) integrin association was not disrupted upon cholesterol depletion, occurred in high density sucrose fractions, and did not involve CD147 Ig domain 2. Overexpression of caveolin-1 caused a specific decrease in clustering of cell surface CD147, as detected by "cluster specific" mAb M6/13. Conversely, a mutant CD147 deficient in caveolin-1 association showed enhanced spontaneous cell surface clustering (detected by mAb M6/13), and did not show decreased clustering in response to caveolin-1 overexpression. Furthermore, the same CD147 mutant yielded an elevated induction of MMP-1. In conclusion, caveolin-1 associates with CD147, in a complex distinct from CD147-alpha(3) integrin complexes, thereby diminishing both CD147 clustering and CD147-dependent MMP-1-inducing activity.

EWI-2 modulates lymphocyte integrin alpha4beta1 functions

The most prominent cell-surface integrin alpha4beta1 partner, a 70-kDa protein, was isolated from MOLT-4 T leukemia cells, using anti-alpha4beta1 integrin antibody-coated beads. By mass spectrometry, this protein was identified as EWI-2, a previously described cell-surface partner for tetraspanin proteins CD9 and CD81. Wild-type EWI-2 overexpression had no effect on MOLT-4 cell tethering and adhesion strengthening on the alpha4beta1 ligand, vascular cell adhesion molecule-1 (VCAM-1), in shear flow assays. However, EWI-2 markedly impaired spreading and ruffling on VCAM-1. In contrast, a mutant EWI-2 molecule, with a different cytoplasmic tail, neither impaired cell spreading nor associated with alpha4beta1 and CD81. The endogenous wild-type EWI-2-CD81-alpha4beta1 complex was fully soluble, and highly specific as seen by the absence of other MOLT-4 cell-surface proteins. Also, it was relatively small in size (0.5 x 10(6) Da to 4 x 10(6) Da), as estimated by size exclusion chromatography. Overexpression of EWI-2 in MOLT-4 cells caused reorganization of cell-surface CD81, increased the extent of CD81-CD81, CD81-alpha4beta1, and alpha4beta1-alpha4beta1 associations, and increased the apparent size of CD81-alpha4beta1 complexes. We suggest that EWI-2-dependent reorganization of alpha4beta1-CD81 complexes on the cell surface is responsible for EWI-2 effects on integrin-dependent morphology and motility functions.

EWI-2 regulates alpha3beta1 integrin-dependent cell functions on laminin-5

EWI-2, a cell surface immunoglobulin SF protein of unknown function, associates with tetraspanins CD9 and CD81 with high stoichiometry. Overexpression of EWI-2 in A431 epidermoid carcinoma cells did not alter cell adhesion or spreading on laminin-5, and had no effect on reaggregation of cells plated on collagen I (alpha2beta1 integrin ligand). However, on laminin-5 (alpha3beta1 integrin ligand), A431 cell reaggregation and motility functions were markedly impaired. Immunodepletion and reexpression experiments revealed that tetraspanins CD9 and CD81 physically link EWI-2 to alpha3beta1 integrin, but not to other integrins. CD81 also controlled EWI-2 maturation and cell surface localization. EWI-2 overexpression not only suppressed cell migration, but also redirected CD81 to cell filopodia and enhanced alpha3beta1-CD81 complex formation. In contrast, an EWI-2 chimeric mutant failed to suppress cell migration, redirect CD81 to filopodia, or enhance alpha3beta1-CD81 complex formation. These results show how laterally associated EWI-2 might regulate alpha3beta1 function in disease and development, and demonstrate how tetraspanin proteins can assemble multiple nontetraspanin proteins into functional complexes.

Tetraspanin CD151 regulates alpha6beta1 integrin adhesion strengthening

The tetraspanin CD151 molecule associates specifically with laminin-binding integrins, including alpha6beta1. To probe strength of alpha6beta1-dependent adhesion to laminin-1, defined forces (0-1.5 nN) were applied to magnetic laminin-coated microbeads bound to NIH 3T3 cells. For NIH 3T3 cells bearing wild-type CD151, adhesion strengthening was observed, as bead detachment became more difficult over time. In contrast, mutant CD151 (with the C-terminal region replaced) showed impaired adhesion strengthening. Static cell adhesion to laminin-1, and detachment of beads coated with fibronectin or anti-alpha6 antibody were all unaffected by CD151 mutation. Hence, CD151 plays a key role in selectively strengthening alpha6beta1 integrin-mediated adhesion to laminin-1.

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.

Beta1 integrins show specific association with CD98 protein in low density membranes

BACKGROUND

The CD98 (4F2, FRP-1) is a widely expressed cell surface protein heterodimer composed of a glycosylated heavy chain and a non-glycosylated light chain. Originally described as a T cell activation antigen, it was later shown to function in amino acid transport, cell fusion and homotypic cell aggregation. Several lines of evidence suggest its functional interaction with integrins but the biochemical basis for this interaction has been unclear.

RESULTS

We demonstrate that CD98 constitutively and specifically associates with beta1 integrins (alpha2beta1,alpha3beta1, alpha5beta1 and alpha6beta1), but minimally with alpha4beta1. Integrin-CD98 association was established by reciprocal immunoprecipitation experiments, and confirmed by CD98-induced clustering of alpha3beta1 but not alpha4beta1 on the surface of rhabdomyosarcoma cells. Integrin-CD98 association is independent of the alpha subunit cytoplasmic tail, is maintained in alpha3beta1 ligand-interaction deficient mutants, and is not inhibited by EDTA. Within the CD98 heavy chain, a C109S mutation (but not a C330S mutation) caused a loss of beta1 integrin association. The same C109S mutation also caused a loss of CD98 light chain association. Importantly, CD98 associated selectively with beta1 integrins present in low density "light membrane" fractions on a sucrose gradient. CD98 was not present in dense fractions that contained the majority of beta1 integrins. Notably, the C109S mutant of CD98, that did not associate with beta1 integrins, showed also a reduced localization into light membrane fractions.

CONCLUSIONS

We demonstrate that CD98 association with beta1 integrins is specific, occurs in the context of low density membranes, and may require the CD98 light chain.

An extracellular site on tetraspanin CD151 determines alpha 3 and alpha 6 integrin-dependent cellular morphology

The alpha 3 beta 1 integrin shows strong, stoichiometric, direct lateral association with the tetraspanin CD151. As shown here, an extracellular CD151 site (QRD(194-196)) is required for strong (i.e., Triton X-100-resistant) alpha 3 beta 1 association and for maintenance of a key CD151 epitope (defined by monoclonal antibody TS151r) that is blocked upon alpha 3 integrin association. Strong CD151 association with integrin alpha 6 beta 1 also required the QRD(194-196) site and masked the TS151r epitope. For both alpha 3 and alpha 6 integrins, strong QRD/TS151r-dependent CD151 association occurred early in biosynthesis and involved alpha subunit precursor forms. In contrast, weaker associations of CD151 with itself, integrins, or other tetraspanins (Triton X-100-sensitive but Brij 96-resistant) were independent of the QRD/TS151r site, occurred late in biosynthesis, and involved mature integrin subunits. Presence of the CD151-QRD(194-196)-->INF mutant disrupted alpha 3 and alpha 6 integrin-dependent formation of a network of cellular cables by Cos7 or NIH3T3 cells on basement membrane Matrigel and markedly altered cell spreading. These results provide definitive evidence that strong lateral CD151-integrin association is functionally important, identify CD151 as a key player during alpha 3 and alpha 6 integrin-dependent matrix remodeling and cell spreading, and support a model of CD151 as a transmembrane linker between extracellular integrin domains and intracellular cytoskeleton/signaling molecules.

Palmitoylation of tetraspanin proteins: modulation of CD151 lateral interactions, subcellular distribution, and integrin-dependent cell morphology

Here we demonstrate that multiple tetraspanin (transmembrane 4 superfamily) proteins are palmitoylated, in either the Golgi or a post-Golgi compartment. Using CD151 as a model tetraspanin, we identified and mutated intracellular N-terminal and C-terminal cysteine palmitoylation sites. Simultaneous mutations of C11, C15, C242, and C243 (each to serine) eliminated >90% of CD151 palmitoylation. Notably, palmitoylation had minimal influence on the density of tetraspanin protein complexes, did not promote tetraspanin localization into detergent-resistant microdomains, and was not required for CD151-alpha 3 beta 1 integrin association. However, the CD151 tetra mutant showed markedly diminished associations with other cell surface proteins, including other transmembrane 4 superfamily proteins (CD9, CD63). Thus, palmitoylation may be critical for assembly of the large network of cell surface tetraspanin-protein interactions, sometimes called the "tetraspanin web." Also, compared with wild-type CD151, the tetra mutant was much more diffusely distributed and showed markedly diminished stability during biosynthesis. Finally, expression of the tetra-CD151 mutant profoundly altered alpha 3 integrin-deficient kidney epithelial cells, such that they converted from a dispersed, elongated morphology to an epithelium-like cobblestone clustering. These results point to novel biochemical and biological functions for tetraspanin palmitoylation.

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.

Specific tetraspanin functions

Relatively little attention has been given to the large family of abundantly expressed transmembrane proteins known as tetraspanins. Now, the importance of tetraspanins is strongly supported by emerging genetic evidence, coupled with new insights into the biochemistry and functions of tetraspanin protein complexes.

Retooling of the beta 4 integrin in tumor cells--ligands lost and kinase gained

In carcinoma cells, the beta 4 integrin functions in a ligand-independent manner to promote proliferation, migration, and invasion. An interesting new paper describes a mechanism whereby the beta 4 integrin cytoplasmic tail becomes an integrin ligand-independent adaptor protein for the Met receptor tyrosine kinase, thereby enhancing the mitogenic, morphogenic, and motogenic properties of Met.

EWI-2 is a major CD9 and CD81 partner and member of a novel Ig protein subfamily

A novel Ig superfamily protein, EWI-2, was co-purified with tetraspanin protein CD81 under relatively stringent Brij 96 detergent conditions and identified by mass spectrometric protein sequencing. EWI-2 associated specifically with CD9 and CD81 but not with other tetraspanins or with integrins. Immunodepletion experiments indicated that EWI-2-CD9/CD81 interactions are highly stoichiometric, with approximately 70% of CD9 and CD81 associated with EWI-2 in an embryonic kidney cell line. The EWI-2 molecule was covalently cross-linked (in separate complexes) to both CD81 and CD9, suggesting that association is direct. EWI-2 is part of a novel Ig subfamily that includes EWI-F (F2alpha receptor regulatory protein (FPRP), CD9P-1), EWI-3 (IgSF3), and EWI-101 (CD101). All four members of this Ig subfamily contain a Glu-Trp-Ile (EWI) motif not seen in other Ig proteins. As shown previously, the EWI-F molecule likewise forms highly proximal, specific, and stoichiometric complexes with CD9 and CD81. Human and murine EWI-2 protein sequences are 91% identical, and transcripts in the two species are expressed in virtually every tissue tested. Thus, EWI-2 potentially contributes to a variety of CD9 and CD81 functions seen in different cell and tissue types.

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.

Evaluation of prototype transmembrane 4 superfamily protein complexes and their relation to lipid rafts

Recent literature suggests that tetraspanin proteins (transmembrane 4 superfamily; TM4SF proteins) may associate with each other and with many other transmembrane proteins to form large complexes that sometimes may be found in lipid rafts. Here we show that prototype complexes of CD9 or CD81 (TM4SF proteins) with alpha(3)beta(1) (an integrin) and complexes of CD63 (a TM4SF protein) with phosphatidylinositol 4-kinase (PtdIns 4-K) may indeed localize within lipid raft-like microdomains, as seen by three different criteria. First, these complexes localize to low density light membrane fractions in sucrose gradients. Second, CD9 and alpha(3) integrin colocalized with ganglioside GM1 as seen by double staining of fixed cells. Third, CD9-alpha3beta1 and CD81-alpha3beta1 complexes were shifted to a higher density upon cholesterol depletion from intact cells or cell lysate. However, CD9-alpha3beta1, CD81-alpha3beta1, and CD63-PtdIns 4-K complex formation itself was not dependent on localization into raftlike lipid microdomains. These complexes did not require cholesterol for stabilization, were maintained within well solubilized dense fractions from sucrose gradients, were stable at 37 degrees C, and were small enough to be included within CL6B gel filtration columns. In summary, prototype TM4SF protein complexes (CD9-alpha3beta1, CD81-alpha3beta1, and CD63-PtdIns 4-K) can be solubilized as discrete units, independent of lipid microdomains, although they do associate with microdomains resembling lipid rafts.

Regulation of MMP-1 and MMP-2 production through CD147/extracellular matrix metalloproteinase inducer interactions

Extracellular matrix metalloproteinase inducer (EMMPRIN; CD147) is a heavily glycosylated protein containing two immunoglobulin superfamily domains. It is enriched on the surface of tumor cells and stimulates the production of matrix metalloproteinases (MMPs) by adjacent stromal cells. Here we use CD147 transfectants and immobilized recombinant CD147-Fc fusion protein to show that CD147/FMMPRIN engages in a homophilic interaction, predominantly through the first immunoglobulin domain. Anti-CD147 antibody 8G6 and recombinant CD147-Fc fusion protein markedly inhibited not only homophilic interaction, but also the production of secreted MMP-2 by breast cancer cell line MDA-435 and the MMP-2-dependent invasion of MDA-435 cells through reconstituted basement-membrane Matrigel. Purified native CD147 induced the production of secreted MMP not only by dermal fibroblasts (MMP-1) but also by MDA-435 cells themselves (MMP-2), suggesting homophilic CD147-binding may occur in the context of both heterotypic and homotypic cell-cell interactions. Purified deglycosylated CD147 failed to induce MMP-1 or MMP-2, but instead antagonized the MMP-1-inducing activity of purified native CD147. Our results suggest that homophilic CD147 interactions may play a key role in MMP-2 production and tumor cell invasion, and that perturbation of this molecule may have potential therapeutic uses in the prevention of MMP-2 and MMP-1-dependent cancer metastasis.

FPRP, a major, highly stoichiometric, highly specific CD81- and CD9-associated protein

CD81 and CD9, members of the transmembrane-4 superfamily (TM4SF; tetraspanins), form extensive complexes with other TM4SF proteins, integrins, and other proteins, especially in mild detergents. In moderately stringent Brij 96 lysis conditions, CD81 and CD9 complexes are virtually identical to each other, but clearly distinct from other TM4SF complexes. One of the most prominent proteins within CD81 and CD9 complexes is identified here as FPRP, the 133-kDa prostaglandin F(2alpha) receptor regulatory protein. FPRP, a cell-surface Ig superfamily protein, associates specifically with CD81 or with CD81 and CD9, but not with integrins or other TM4SF proteins. In contrast to other CD81- and CD9-associating proteins, FPRP associates at very high stoichiometry, with essentially 100% of cell-surface FPRP on 293 cells being CD81- and CD9-associated. Also, CD81.CD9.FPRP complexes have a discrete size (<4 x 10(6) Da) as measured by gel permeation chromatography and remain intact after disruption of cholesterol-rich membrane microdomains by methyl-beta-cyclodextrin. Although CD81 associated with both alpha(3) integrin and FPRP in 293 cells, the alpha(3)beta(1).CD81 and CD81.CD9.FPRP complexes were distinct, as determined by immunoprecipitation and immunodepletion experiments. In conclusion, our data affirm the existence of distinct TM4SF complexes with unique compositions and specifically characterize FPRP as the most robust, highly stoichiometric CD81- and/or CD9-associated protein yet described.