Protein kinase activity is associated with CD63 in melanoma cells
© Iida et al; licensee BioMed Central Ltd. 2005
Received: 12 August 2005
Accepted: 30 November 2005
Published: 30 November 2005
The tetraspan protein CD63, originally described as a stage-specific melanoma antigen but also present in a number of normal cells, regulates melanoma cell growth in nude mice, motility in serum containing media, and adhesion to several extracellular matrix proteins. CD63 has been reported to associate with β1 and β2 integrins, but the mechanism of signal transduction by CD63 is not clear. This study examined whether CD63 is associated with protein kinase and can transmit signals in melanoma cells.
Immunoprecipitation and radiolabeling were used to test for association of protein kinase activity with CD63. Adhesion of cells to monoclonal antibodies immobilized to microtiter plates was used to examine the ability of CD63 to transmit signals.
CD63 was capable of transmitting a signal in melanoma cells that required extracellular calcium. In the absence of extracellular calcium at the time of binding to the CD63 mAb, the cell was no longer responsive to stimulation by CD63. Immunoprecipitation studies demonstrated protein kinase activity associated with CD63, and phosphoamino acid analysis revealed that most of this protein kinase activity was due to serine kinase activity.
The current study suggests that serine protein kinase activity associated with CD63 may play a role in signaling by CD63 in melanoma cells.
CD63 was initially described as the ME491 antigen of melanoma cells [1–8]. CD63 is not expressed in normal melanocytes, but is expressed in nevi and many melanoma cells. CD63 is also expressed in a number of normal cells [1–5, 9–18]. A number of studies have suggested that CD63 may regulate melanoma cell functions. Transfection with CD63 of NIH-3T3 cells transformed with H-ras, resulted in cells with lower growth rates following subcutaneous injection in nude mice . Transfection of the CD63 negative melanoma cell line KM3 with genomic CD63 resulted in cells with similar in vitro growth rates as control cells, but much slower growth rates in vivo when cells were injected intradermally in nude mice . In addition, intravenous injection of CD63 transfected KM3 cells resulted in fewer peritoneal and subcutaneous metastases . These data, therefore, suggest that CD63 may regulate in vivo growth and metastatic capability . Finally, transfection of the highly motile KM3 cells with CD63 resulted in suppression of in vitro motility in serum-containing media that was potentiated by CD63 mAbs, and increased adhesion and migration on fibronectin, laminin, and collagen . In contrast, transfection of antisense CD63 cDNA into melanoma cells endogenously expressing CD63, resulted in increased cell motility and invasiveness in vitro. Thus, CD63 appears capable of regulating melanoma cell functions, although the mechanism of this regulation is unclear.
CD63 is a member of the tetraspan family, traversing the membrane four times and having a major extracellular loop and a small extracellular loop, with short intracellular amino and carboxy termini [1, 2, 4, 5, 14, 18, 23–25]. The intracellular domains of CD63 are small and have no clear motif known to be involved in signal transmission.
In this study, the properties of the adhesion of melanoma cells to immobilized CD63 mAb suggested that CD63 was capable of transmitting a signal that requires extracellular calcium. Protein kinase activity, predominantly in the form of serine kinase activity, was found to be associated with CD63 in these cells. The association of serine protein kinase activity with CD63 may play a role in signaling by CD63 in melanoma.
Highly metastatic human melanoma cells, A375SM, which were selected by in vivo experimental metastasis assays of parent A375P cells in nude mice  were kindly provided by Dr. I. J. Fidler (M.D. Anderson Hospital Cancer Center, Houston, TX). The cells were maintained in Eagle's Minimum Essential Medium (EMEM) supplemented with 10% FBS, vitamin solution, 50 mg/ml gentamycin, and 1 mM sodium pyruvate. Cells were routinely used after fewer than 15 passages from frozen stocks in order to minimize phenotypic drift.
Monoclonal antibodies (mAbs)
The CD63 mAbs, AHN-16 (IgG2a), AHN-16.1 (IgG1), AHN-16.2 (IgG2b), AHN-16.3 (IgG2b), and AHN-16.5 (IgG1) were previously described . mAb U131 (IgG1) was previously described . mAb 9.2.27 that recognizes the core protein of melanoma associated antigen (MCSP)  was a gift of Dr. Ralf Reisfeld (Scripps Clinic, La Jolla, CA). Fab fragments of AHN-16 were prepared by using immobilized papain according to the manufacturer's instruction (Pierce, Rockford, IL), and their purity was confirmed by SDS-PAGE analysis.
Cell adhesion assays
Cell adhesion assays to immobilized mAbs were performed as described previously . Briefly, 96-well plates were coated with 50 ul of goat anti-mouse IgG Fc (Organon Teknika Corp., Durham, NC) overnight at 37°C and then blocked with PBS containing 2 mg/ml BSA for 3 h at 37°C. The purified mAbs were diluted in PBS, dispersed into each well at 1 ug/ml in 50 ul, incubated for 3 to 4 h at 37°C, and then washed 2 times with PBS. Subconfluent A375SM cells that had been radiolabeled overnight with 3H-thymidine (3H-TdR, specific activity 6.7 Ci/mmol; NEN Research Products, Boston, MA), were harvested by rinsing with PBS containing 1 mM EDTA, washed two times with EMEM/BSA (EMEM containing 2 mg/ml BSA and 15 mM Hepes, pH 7.2), and adjusted to a concentration of 105 cells/ml in the same medium. When cell adhesion assays were carried out in the presence of 10 mM EDTA or EGTA, cells were resuspended in EMEM/BSA containing 10 mM EDTA or EGTA. An aliquot of 100 ul of the cell suspension was dispersed into each well and incubated for 60 min at 37°C. For inhibiting cell adhesion to the immobilized AHN-16, cells were preincubated with Fab fragments of AHN-16 or of normal mouse IgG for 20 min at room temperature. Cell adhesion assays were carried out in the continuous presence of these Fab fragments for 60 min at 37°C. The assays were terminated by aspirating loosely bound and unbound cells from the wells. Bound radioactivity was determined in a liquid scintillation counter (Beckman Model 3801 Liquid Scintillation Counter).
Cells were harvested as described above, and washed 3 times with RPMI 1640 containing 1% heat-inactivated goat serum, 20 mM HEPES, and 0.02% NaN3(FACS buffer). Cells were resuspended in the FACS buffer at a concentration of 105 cells/ml with 10 ug/ml normal goat IgG for 30 min at 4°C and then washed 3 times with the FACS buffer. An aliquot of 1 ml of cell suspension was incubated with 5 ug/ml of mAbs for 30 min at 4°C with mixing every 5 min. Cells were washed 3 times with the FACS buffer and then incubated with FITC conjugated goat anti-mouse IgG (final dilution of 1: 500) at 4°C for 40 min with mixing every 5 min. The binding of the primary and the secondary mAbs were maximal under these experimental conditions. Cells were washed 3 times with the FACS buffer and resuspended in 200 ul of PBS containing 2% formaldehyde. Cells were analyzed on a FACS Star (Becton-Dickinson, Mt. View, CA).
Protein kinase assay
Immunoprecipitation was performed as previously described with minor modifications . Briefly, 2 × 107 cells were suspended in 1.1 ml of Brij solubilization buffer [20 mM Tris-HCl, pH 8.2, 150 mM NaCl, 1 mM PMSF, 2 mM MgCl2, 0.02% NaN3, and 1.0% Brij 58 (Pierce)], and incubated on ice for 15 min. The suspensions were then centrifuged at 9,800 × g for 15 min at 4°C. The resulting supernatants were used for immunoprecipitation or analyzed by SDS-PAGE directly.
For immunoprecipitations, 1 ml of 10% Staphylococcus aureus (Pansorbin A, Calbiochem, La Jolla, CA) was mixed with 100 μl of rabbit anti-mouse IgGH+L (Organon Teknika, Westchester, PA), and the mixture was incubated at 4°C for 1 hr. One ml of buffer A (1 mg/ml BSA, 0.05% NP-40, 20 mM Tris-HCl, pH 7.6, 100 mM NaCl, 1 mM EDTA, 0.05% NaN3) was added and the Pansorbin A-antibody complex was recovered by centrifuging at 2000 × g for 10 min at 4°C and resuspended in 390 μl of buffer A and 40 μl of ascites containing the indicated monoclonal antibody, or normal mouse serum (NMS) was added, and the mixtures were incubated for 2 hr at 4°C. The Pansorbin A-antibody complex was recovered by centrifuging at 2000 × g for 10 min at 4°C, washed once with 1 ml of buffer A, and resuspended in 1 ml of Brij-SA buffer (1 mg/ml BSA, 0.05% Brij 58, 20 mM Tris-HCl, pH 7.6, 100 mM NaCl, 1 mM EDTA, 0.05% NaN3). Cell proteins were then immunoprecipitated in reaction mixtures containing the cell extract, Pansorbin A-bound antibody complex, 20 mM Tris-HCl, pH 8.2, 100 mM NaCl, 0.5% Brij 58, 1 mM EDTA, 0.125 mg/ml gelatin, and 1 mM PMSF in a total volume of approximately 350 μl in 10 × 75 mm glass tubes. After 1 hr at 4°C, the mixture was washed three times with Brij wash buffer (20 mM Tris-HCl, pH 8.2, 150 mM NaCl, 1 mg/ml BSA, 0.5% Brij 58, 2 mM MgCl2, 0.125 mg/ml gelatin, 1 mM PMSF, and 0.02% NaN3) and then once with NaCl-HEPES (145 mM NaCl, 20 mM HEPES, pH 7.3). Immunoprecipitates were suspended in 30 μl of NaCl-HEPES. Thirty ul of labeling buffer (0.1% Brij 58, 6 mM MnCl2, 40 mM MgCl2, 200 uM Na3VO4, 200 uM Na2MoO4, and 10 μCi of [γ-32P]ATP) was then added, and the mixtures were incubated for 10 min at 23°C. The reaction was stopped by adding 4 × Laemmli sample buffer, incubated at 100°C for 2 min, and analyzed by SDS-PAGE . Molecular weight standards were purchased from Sigma. Gel slabs were stained, dried, and subjected to autoradiography by using Dupont Cronex film.
Phosphoamino acid analysis
Phosphoamino acid analyses were performed as previously described [32, 33]. Briefly, radiolabeled proteins resolved by SDS-PAGE were transblotted onto Immobilon-P (PVDF) paper (Millipore Corp, Bedford, MA), localized by autoradiography, and excised. The proteins on the PVDF were then hydrolyzed in vacuo in constantly boiling 6 M HCl for 2 hr at 110°C. The HCl was removed by evaporation using a SpeedVac (Savant Instruments). The dried, partially hydrolyzed samples were then dissolved in water containing phosphoserine (P-S), phosphothreonine (P-T), and phosphotyrosine (P-Y) (Sigma) each at 1 mg/ml. The samples were then spotted on aluminum sheets precoated with silica gel (0.2 mm layer thickness, 11 cm height, Merck Laboratories, EM Science, Gibbstown, NJ) and resolved by one-dimensional thin layer chromatography in ethanol: 25% NH4OH (3.5:1.6). The chromatography cycle was repeated three times to achieve optimal separation. Between cycles, plates were air dried followed by chromatography in the same solvent. The radiolabeled phosphoamino acids were detected by autoradiography using X-Omat AR film.
Human melanoma cells adhere to immobilized CD63 mAbs
Human melanoma cell adhesion to immobilized CD63 mAbs
Cell adhesion (%)b
Human melanoma cell adhesion to immobilized CD63 mAb is inhibited by chelerythrine
Human melanoma cell adhesion to immobilized CD63 mAb requires external Ca2+
In some cells, for example human neutrophils, the binding of a stimulus to its receptor on the cell surface results in the activation of a signal transduction system that is transient, and whose function is dependent on extracellular Ca2+ [35, 36]. When calcium is absent during a critical period following ligand-receptor coupling, the subsequent addition of calcium does not result in cell activation, and the cells may be partially desensitized to subsequent stimulation in the presence of Ca2+ [35, 36]. The role of extracellular Ca2+ was therefore further evaluated. When calcium (10 mM) was added to the cells in medium with EGTA immediately after adding them to the plate, adhesion was normal. However, if the cells were allowed to interact with the immobilized CD63 mAbs for 20 min before repleting the calcium, adhesion was not restored by the calcium (not shown).
Identification of protein kinase activity associated with CD63
Immunohistochemistry studies have characterized CD63 as the stage-specific melanoma-associated antigen ME491 [6–8]. Over expression of CD63 partially suppressed malignant phenotypes of H-ras-transformed fibroblasts in vivo, and transfection of melanoma cell lines also suggested that CD63 can regulate melanoma cell function [20, 21]. The current study also demonstrates that CD63 can transduce signals in melanoma cells and alter melanoma cell behavior. In addition, this study demonstrates that CD63 is capable of transducing signals in melanoma cells that requires extracellular Ca2+ and is inhibited by the protein kinase C inhibitor chelerythrine. Signaling by tetraspans, including the induction of calcium-dependent cell-cell adhesion, has been reported in other systems [37–39].
To examine potential signal transducing pathways through which CD63 may act, an in vitro assay measuring melanoma cell adhesion to immobilized mAbs was used. We previously utilized this method for characterizing molecular mechanisms of signal transduction from proteoglycan as well as integrin receptors in melanoma cells [29, 40]. Melanoma cells specifically adhered to immobilized CD63 mAb, and the adhesion was inhibited by chelerythrine but not by genistein, suggesting that signaling pathways involving protein kinase C activity, but not genistein-sensitive tyrosine kinases, play a role in the stimulation by CD63. The inhibitory activity of chelerythrine was not due to cytotoxicity, since the chemicals did not affect cell adhesion to immobilized mAb 9.2.27. The IC50 value of chelerythrine is close to the value that inhibits the activity of protein kinase C in vitro, supporting the conclusion that the inhibitory effect of chelerythrine on cell adhesion to immobilized CD63 mAb is of a specific nature.
CD63 may interact with different signaling molecules in different cell types. In human neutrophils, tyrosine protein kinase activity has been reported to be associated with CD63; most of this tyrosine kinase activity was found to be due to the src family tyrosine kinases lyn and hck which coimmunoprecipitated with CD63 in neutrophils, suggesting that these kinases may play a role in signaling via CD63 . In the current study, we found serine protein kinase activity, but could not detect tyrosine kinase activity, in immunoprecipitates using CD63 mAbs. Recent studies have reported that CD63 in platelets modulates spreading and tyrosine kinase activity . Thus, these results indicate that the interaction between CD63 and these kinases is dependent upon the cell type. These data also support a role for protein kinase C in CD63 signaling in melanoma cells. CD63 has also been found to associate with the β2 integrin CD18 in neutrophils and the β1 integrin in several other cells, including melanoma cells, and this may also play a role in signaling by CD63 [15, 42–45]
Melanoma cell adhesion to immobilized CD63 mAb required extracellular calcium, indicating that extracellular Ca2+ plays an important role in signaling by CD63 in these cells. Addition of calcium to cells 20 min after interaction of cells with immobilized CD63 mAb resulted in no adhesion. Thus, the interaction of melanoma cells expressing CD63 to immobilized CD63 mAbs resulted in the generation of a transient signal that required extracellular calcium to effect melanoma cell adhesion. These observations are similar to those reported with human neutrophils where ligation of CD63 results in a transient activation signal that requires extracellular calcium .
The current study demonstrates that CD63 is capable of transducing signals in melanoma cells that requires extracellular Ca2+ and is inhibited by the protein kinase C inhibitor chelerythrine, and that CD63 is associated with serine protein kinase activity in melanoma cells. This associated protein kinase activity may play a role in the mechanism whereby CD63 exerts its previously reported effects on melanoma cell function [20, 21].
- Abbreviations used in this paper:
EMEM, Eagle's minimum essential medium
normal mouse serum
- FACS buffer:
RPMI 1640 containing 1% heat-inactivated goat serum, 20 mM HEPES, and 0.02% NaN3
145 mM NaCl, 20 mM HEPES, pH 7.3
- Brij solubilization buffer:
(20 mM Tris-HCl, pH 8.2, 150 mM NaCl, 1 mM PMSF, 2 mM MgCl2, 0.02% NaN3, and 1.0% Brij 58)
- Brij wash buffer:
(20 mM Tris-HCl, pH 8.2, 150 mM NaCl, 1 mg/ml BSA, 0.5% Brij 58, 2 mM MgCl2, 0.125 mg/ml gelatin, 1 mM PMSF, and 0.02% NaN3)
- labeling buffer:
(0.1% Brij 58, 6 mM MnCl2, 40 mM MgCl2, 200 uM Na3VO4, 200 uM Na2MoO4, and 10 μCi of [γ-32P]ATP)
- buffer A:
(1 mg/ml BSA, 0.05% NP-40, 20 mM Tris-HCl, pH 7.6, 100 mM NaCl, 1 mM EDTA, 0.05% NaN3)
- buffer B:
(PBS, pH 7.4, 0.2% BSA, 0.05% NaN3)
- Brij SA buffer:
(1 mg/ml BSA, 0.05% Brij 58, 20 mM Tris-HCl, pH 7.6, 100 mM NaCl, 1 mM EDTA, 0.05% NaN3, 1 mM Na3VO4, and 1 mM Na2MoO4)
We thank I. Fidler for providing cell lines, K. Campbell for assistance with protein kinase assays, K. Ahmed and A. Davis for assistance with the phosphoamino acid analysis, and R. Reisfeld for providing mAb 9.2.27. Supported in part by the American Heart Association, Minnesota Affiliate, the Office of the Vice President for Research and Dean of the Graduate School of the University of Minnesota, the Minnesota Medical Foundation, and the Masonic Memorial Hospital Fund, Inc.
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