J Biol Chem, Vol. 274, Issue 26, 18463-18469, June 25, 1999

Rap1 GTPase-activating Protein SPA-1 Negatively Regulates Cell Adhesion^*

Noriyuki Tsukamoto, Masakazu Hattori, Hailin Yang, Johannes L. Bos, and Nagahiro Minato^§

From the Department of Immunology and Cell Biology, Graduate School of Medicine, Kyoto University, Kyoto 606-8501, Japan and the  Laboratory for Physiological Chemistry, Utrecht University, 2584 CG Utrecht, The Netherlands

	ABSTRACT

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

Rap1 GTPase is activated by a variety of stimulations in many types of cells, but its exact functions remain unknown. In this study we have shown that SPA-1 interferes with Rap1 activation by membrane-targeted C3G, C3G-F, in 293T cells through the GTPase activating protein (GAP) activity. SPA-1 transiently expressed in HeLa cells was mostly localized at the cortical cytoskeleton and induced rounding up of the cells, whereas C3G-F conversely induced extensive cell spreading. Conditional SPA-1 overexpression in HeLa cells by tetracycline-regulative system suppressed Rap1 activation upon plating on dishes coated with fibronectin and resulted in the reduced adhesion. When SPA-1 was conditionally induced after the established cell adhesion, the cells gradually rounded up and detached from the dish. Both effects were counteracted by exogenous fibronectin in a dose-dependent manner. Retroviral overexpression of SPA-1 in promyelocytic 32D cells also inhibited both activation of Rap1 and induction of cell adhesion by granulocyte colony stimulating factor without affecting differentiation. These results have indicated that Rap1 GTP is required for the cell adhesion induced by both extracellular matrix and soluble factors, which is negatively regulated by SPA-1.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

Rap1 is the closest member of Ras family small GTPases (1). Mammalian Rap1 was isolated by cross-hybridization with a Drosophila ras-related gene (2) and was reported to revert "malignant" morphology of oncogenic Ras-transformed 3T3 cells to "normal" phenotype (3). Activation of Rap1 can be regulated by specific guanine nucleotide exchange factors (GEF)¹ catalyzing the conversion from GDP- to GTP-bound forms and GTPase activating proteins (GAPs) accelerating the hydrolysis of bound GTP to GDP (4). C3G, originally isolated as a binding protein to v-crk oncogene product (5), has been shown to exhibit Rap1 GEF activity (6). Most recently, other Rap1 GEFs have been isolated including CalDAG GEFI and Epac, which are regulated by Ca²⁺ and diacylglycerol and cAMP, respectively (7-9). On the other hand, at least two distinct proteins are shown at present to exhibit specific Rap1 GAP activity in vitro, rapGAP (10) and SPA-1 (11). Expression profiles of rapGAP and Spa-1 genes are quite distinct, in that the former is selectively expressed in brain, pancreas, and kidney and the latter predominantly in lymphohematopoietic tissues (11). Recently, an arginine finger motif conserved in the shared catalytic domains of rapGAP and SPA-1 has been proposed to be essential for their GAP activity (12).

Based on the findings that Rap1 shares the effector domain with Ras and binds to several Ras effector molecules (13), it has been proposed that Rap1 GTP antagonizes the Ras signaling pathways (13, 14). This proposed function of Rap1 in normal cells, however, remains controversial (15). Rap1 is activated by the agonistic stimulation of various receptors coupled with tyrosine kinases or G proteins, including thrombin receptor in platelets (16), insulin receptor (17), antigen receptors in lymphocytes (18, 19), GM-CSF receptor and other serpentine receptors in neutrophils (20), and nerve cell growth factor receptor in PC12 cells (21). In some of these, it has been shown that Rap1 is activated by C3G recruited by the CRK adapter protein (18, 21). Rap1 has been shown also to be activated by cAMP (22, 23) and phospholipase C- pathway (19), as well as during the cell adhesion in NIH 3T3 cells (24). Together with the presence of multiple Rap1-regulatory proteins with unique signaling motifs and distinct tissue distribution patterns, these results suggest that Rap1 is activated in multiple signal transduction pathways depending on the cell types. The exact functions of Rap1 in the cells, however, still remain largely unknown.

In the present study, we first indicate that SPA-1 and C3G regulate Rap1 activation antagonistically to each other in the cells, in which the former interferes with the activation of endogenous Rap1 by the latter. Conditional overexpression of SPA-1 in HeLa cells resulted in the inhibition of Rap1 activation and reduced adhesion upon contact to the substrate and the detachment of the cells when induced after the establishment of cell adhesion, both of which could be overcome dose-dependently by exogenous fibronectin. Also, retroviral overexpression of SPA-1 in the nonadherent promyelocytic 32D cells inhibited the activation of Rap1 and the induction of cell adhesion by G-CSF stimulation. Thus, the present results have indicated that Rap1 activation is critically involved in the cell adhesion induced by both ECM and soluble factors and that SPA-1 functions as a negative regulator for the activation of Rap1, thereby setting a threshold for cell adhesion.

	EXPERIMENTAL PROCEDURES

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

Antibodies and Other Materials-- Rabbit anti-SPA-1 antibody has been described previously (11). Antibodies for VLA-4, LFA-1, and CD44 were provided by Dr. T. Kina, Institute for the Bioregeneration, Kyoto University, Kyoto, Japan. Other antibodies were purchased commercially: anti-Rap1 (Transduction Laboratories), anti-C3G (Santa Cruz Biotechnology), anti-FLAG (Sigma), and anti-CD29 and anti-CD11b (PharMingen). Recombinant human G-CSF was provided by Chugai Pharmaceutical Inc., Tokyo, Japan. Fibronectin was purchased from Life Technologies, Inc.

Cell Cultures-- HeLa/Tet-Off (CLONTECH Laboratories, Inc.) and 293T cells provided by Dr. M. Matsuda, Research Institute, International Medical Center of Japan, Tokyo, Japan, were maintained in the Dulbecco's minimal essential medium (DMEM) supplemented with 10% fetal calf serum (FCS). 32Dcl.3 cells were obtained from the ATCC (CRL 11346) and maintained in RPMI 1640 supplemented with 10% FCS and IL-3 (100 units/ml). An ecotropic cell line, GP+E86, was provided by Dr. A. Bank, Columbia University, NY, and maintained in DMEM containing 10% FCS, 15 µg/ml hypoxanthine, 250 µg/ml xanthine, and 25 µg/ml mycophenolic acid.

Plasmid Construction-- A BamHI site was introduced before the initiation codon of mouse Spa-1 cDNA in pBluescript SK(+) (pSK⁺Spa-1). A GRD mutant (deleted of GAP-related domain, residues 195-535) of Spa-1 cDNA was constructed by the ligation of a NcoI-KpnI fragment of the pSK⁺Spa-1 into the EcoRV-KpnI site of pBluescript SK(+) vector followed by the insertion of a BamHI-BglII fragment of the pSK⁺Spa-1 (pSK⁺ GRD-Spa-1). A FLAG epitope was introduced at the N termini by the ligation of synthetic oligonucleotides (5'-GATCCATGGACTACAAGGACGACGATGACAAA and 5'-GATCTTTGTCATCGTCGTCCTTGTAGTCCATG) into the BamHI sites of pSK⁺Spa-1 and pSK⁺GRD-Spa-1. Tet-off expression vector, pTRE/FLAG-Spa-1, was constructed by subcloning a BamHI fragment of pSK⁺FLAG-Spa-1 into a pTRE (pUHD-10.3) vector provided by Dr. S. Takeda, Faculty of Medicine, Kyoto University, Kyoto, Japan. Retrovirus expression vectors, pLXSN/FLAG-Spa-1 and pLXSN/FLAG-GRD-Spa-1, were constructed by subcloning the BamHI fragments of pSK⁺/FLAG-Spa-1 and pSK⁺/FLAG-GRD-Spa-1, respectively, into the pLXSN vector provided also by Dr. A. Bank. pCAGGS-C3G and pCAGGS-C3G-F (6) were provided also by Dr. M. Matsuda, and pLSV-hygB by Dr. T. Sudo, Toray Inc., Kamakura, Japan.

HeLa/Tet-off Cells Expressing Spa-1-- HeLa/Tet-off cells were cotransfected with pTRE/FLAG-Spa-1 (5 µg) and pLSV-hygB (0.4 µg) with the CaPO₄ precipitation method and cultured in DMEM supplemented with Dox (5 ng/ml) and hygromycin B (250 µg/ml). Two weeks later, hygromycin B-resistant colonies were individually isolated and checked for the expression of SPA-1 by immunoblotting with anti-FLAG antibody.

Production of Recombinant Retroviruses and Infection-- pLXSN/FLAG-Spa-1 and pLXSN/FLAG-GRD-Spa-1 plasmids were transfected into GP⁺E86 cells with a LipofectAMINE transfection kit (Life Technologies, Inc.). Virus titers in the supernatants of the G418-resistant clones were checked on NIH 3T3 cells. 32D cells were infected with the culture supernatant of virus producers in the presence of 10 µg/ml Polybrene for 2 h and cultured in the medium supplemented with IL-3 and G418 (400 µg/ml).

Detection of Rap1 GTP-- Intracellular Rap1 GTP was detected as described previously (25). Briefly, the cells were lysed with RBD buffer (150 mM NaCl, 50 mM Tris-HCl, pH 7.6, 0.5% Triton X-100, 1 mM phenylmethylsulfonyl fluoride, 1 mM Na₃VO₄, 10 mM NaF, 2 µg/ml leupeptin, 2 µg/ml aprotinin). The lysate was incubated with GST fusion protein of the Ras-binding domain (RBD) of RalGDS conjugated with glutathione-Sepharose at 2 °C for 60 min, washed, and analyzed by the immunoblotting with anti-Rap1 antibody.

Immunoblotting, Immunofluorescence Staining, and Flow Cytometry-- Immunoblotting, immunofluorescence staining, and flow cytometric analysis were performed as described previously (11), and analyzed with a confocal laser microscope (Olympus) and FACScan (Becton Dickinson).

Subcellular Fractionation-- Ten million HeLa cells were resuspended in 1 ml of hypotonic buffer (10 mM HEPES, pH 7.9, 5 mM KCl, 2 mM MgCl₂, 10 µg/ml aprotinin, 1 µg/ml leupeptin, 1 mM phenylmethylsulfonyl fluoride), incubated at 4 °C for 10 min, and homogenized with a Dounce homogenizer. The lysates were centrifuged at 500 × g for 5 min, and the supernatant was ultracentrifuged at 100,000 × g at 4 °C for 1 h to separate into soluble (S100) and insoluble (P100) fractions.

Cell Adhesion Assay-- The short term adhesion experiments were performed as described by Vouri et al. (26). Briefly, HeLa cells were starved for 20 h in DMEM containing 1% FCS, trypsinized, and then treated with trypsin inhibitor. After washing with DMEM containing 0.5% bovine serum albumin, cells were held in suspension for 40 min by gentle rotation and then allowed to adhere for 30 min by plating on plastic dishes coated with various concentrations of human fibronectin. For the cell detachment assay, HeLa/Tet-off cells were seeded at 1 × 10⁵ cells/60-mm tissue culture dishes coated with various concentrations of fibronectin, and cultured in DMEM containing 10% FCS in the presence or absence of Dox for the indicated number of days. 32D cells were seeded at 5 × 10⁵ cells/ml in 60-mm tissue culture dishes and cultured in the complete RPMI 1640 and G-CSF (2 ng/ml) for the indicated number of days. The cultured cells were rinsed five times with phosphate-buffered saline, including Ca²⁺and Mg²⁺, and the adherent or nonadherent cells were counted.

	RESULTS

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

SPA-1 Interferes with the Activation of Rap1 by C3G-F in 293T Cells-- We have investigated first whether SPA-1 antagonizes the Rap1 GEF activity of C3G in the cells. Transfection of C3G cDNA failed to activate Rap1 most likely because of its inefficient accessibility to Rap1 (6), and therefore C3G cDNA tagged with a membrane-anchoring CAAX motif at the C terminus (C3G-F) was employed, which induced efficient Rap1 activation in 293T cells (Fig. 1, A and B). As shown in Fig. 1A, cotransfection of varying amounts of Spa-1 (0.06-1 µg) with C3G-F cDNA (1 µg) suppressed the generation of Rap1 GTP in a dose-dependent manner. Conversely, in the presence of a constant level of SPA-1 (1 µg), the efficiency of GEF activity of C3G-F for the endogenous Rap1 was reduced significantly (Fig. 1B). A GRD mutant of Spa-1, which was defective for Rap1 GAP activity in vitro (data not shown), failed to affect the Rap1 GEF activity of C3G-F (Fig. 1A). The results have indicated that SPA-1 interferes with the activation of Rap1 in the cells through the GAP activity.

View larger version (43K): [in this window] [in a new window]   Fig. 1.   SPA-1 interferes with the activation of Rap1 by C3G-F in 293T cells. A, 293T cells (8 × 105 cells/60-mm dish) were transfected with the indicated doses of Spa-1 or GRD Spa-1 cDNA along with 1 µg of C3G-F cDNA with CaPO4 precipitation method. Total amount of plasmids was adjusted to 2 µg in each transfection with a control vector. Two days later, the cells in the dishes were directly lysed with 500 µl of RBD lysis buffer. Ten µl of each lysate was electrophoresed in SDS-polyacrylamide gel electrophoresis and immunoblotted with antibodies for Rap1, SPA-1, and C3G. Two hundred µl of each lysate was precipitated with GST-RalGDS-RBD and glutathione-Sepharose, electrophoresed in SDS-polyacrylamide gel electrophoresis, and blotted with anti-Rap1 to detect Rap1 GTP. B, 293T cells were transfected with the indicated doses of C3G-F cDNA with or without 1 µg of Spa-1. Total amount of plasmids was adjusted to 2 µg in each transfection with a control vector. Two days later, the cells were lysed and analyzed as above.

Overexpression of SPA-1 and C3G-F Differentially Affects the Cell Shape and Size and, SPA-1 Is Probably Associated with Cortical Cytoskeleton-- We then intended to examine the cellular effects of SPA-1 overexpression in HeLa cells, which marginally expressed SPA-1. HeLa cells were transfected with Spa-1, C3G, or C3G-F cDNA, stained with corresponding antibodies, and analyzed by confocal microscopy. As shown in Fig. 2A, a, SPA-1 was detected mostly at the cortical area of the cells. It was noted that Spa-1-transfected HeLa cells tended to become round and smaller as compared with uninfected neighboring cells. To see the possible relationship of SPA-1 with cytoskeleton, the effect of cytochalasin D (1 µg/ml) was examined. At the condition in which cytoskeletal actin organization was disrupted largely, a significant portion of SPA-1 staining was disrupted also into patchy aggregates, mostly colocalizing with F-actin (Fig. 2A, b, merged as yellow staining). C3G was distributed diffusely throughout the cytosol as expected, hardly affecting the cell shape and size (Fig. 2A, c). In contrast, HeLa cells transfected with C3G-F cDNA exhibited an enlarged and spread shape with extensive cytoplasmic protrusions (Fig. 2A, d, note the difference in magnification). A portion of C3G-F was distributed in the mottled pattern at the basal cell surface attached to the dish as revealed by the confocal picture focused on the basal surface (Fig. 2A, d). To confirm the intracellular localization of SPA-1, cell fractionation analysis was done. As shown in Fig. 2B, the majority of SPA-1 was associated with the fraction of hypotonic cell lysate precipitated with the light centrifugation (PPT fraction), which contained cytoskeleton. The majority of Rap1 was also detected in this fraction with a minor portion in the membranous fraction (P100). Conforming to the immunostaining analysis, the significant proportion of SPA-1 in the PPT fraction was released into the S100 and P100 fractions following the cytochalasin D treatment. Although not shown, a similar shift of actin localization was confirmed by immunoblotting using anti-actin antibody. C3G-F was also detected in the PPT and partly in P100, whereas the vast majority of C3G was found in the S100. Iron regulatory protein-1 (IRP-1), used as a control, was exclusively detected in the S100 as expected.

View larger version (61K): [in this window] [in a new window]   Fig. 2.   Overexpression of SPA-1 and C3G-F induces changes in cell shape and size, and SPA-1 is probably associated with cytoskeleton. A, HeLa cells were transfected with 2 µg of Spa-1 (a, b), C3G (c), or C3G-F (d) with Lipofectin. Two days after the transfection, cytochalasin D (1 µg/ml) was added to a portion of cultures transfected with Spa-1 cDNA (b) and incubated for 30 min. Then the cells were fixed, double stained with anti-SPA-1 (a, b) or anti-C3G (c, d) followed by the fluorescein isothiocyanate-conjugated anti-rabbit IgG and rhodamine-conjugated phalloidin, and analyzed with confocal laser microscopy. Note the difference in magnifications (a-c versus d). B, HeLa cells transfected with Spa-1 cDNA, either pretreated with cytochalasin D or not, were lysed with hypotonic buffer and precipitated with centrifugation at 500 × g (PPT). The supernatants were centrifuged again at 100, 000 × g to obtain soluble (S100) and precipitated (P100) fractions. SDS sample duffer was added to each fraction and analyzed by immunoblotting with antibodies for SPA-1, Rap1, C3G, and IRP-1.

Conditional Overexpression of SPA-1 in HeLa Cells after Establishment of Adhesion Causes Detachment of Cells, whereas That before Adhesion Results in Reduced Adhesion to Fibronectin-- The effect of SPA-1 overexpression was further investigated by stable transfectants. To avoid possible clonal variance, HeLa cells transfected with Spa-1 cDNA whose expression is regulated by tetracycline (SPA/HeLa) have been established. SPA/HeLa cells cultured in the presence of 1 ng/ml Dox expressed an undetectable level of SPA-1 and exhibited indistinguishable features from parental cells. When SPA/HeLa cells were shifted to the medium containing decreasing concentrations of Dox, SPA-1 expression was induced increasingly within a day (Fig. 3A). As shown in Fig. 3B, when the wild-type HeLa cells in suspension were plated onto the tissue culture dishes and allowed to adhere, Rap1GTP was induced above the basal level. In contrast, SPA/HeLa cells that had been induced for SPA-1 in Dox-free medium exhibited negligible generation of Rap1GTP upon plating on the dishes (Fig. 3B). When non-induced SPA-1/HeLa cells that had adhered to dishes were shifted to the medium without Dox, they rounded up progressively and finally detached from the dish within 3 days, whereas all the cells remained adherent in the presence of Dox (Fig. 3, C and D). Round detached cells were all viable (data not shown). Identical results were obtained by using independently isolated SPA/HeLa clones. The results have suggested that Rap1 GTP is required to maintain cell adhesion.

View larger version (75K): [in this window] [in a new window]   Fig. 3.   Conditional overexpression of SPA-1 in the adhered SPA/HeLa cells induces cell rounding and detachment from the dish. A, SPA/HeLa cells were cultured in the absence or presence of the indicated concentrations of Dox for 2 days, lysed with SDS sample buffer, electrophoresed in SDS-polyacrylamide gel electrophoresis, and immunoblotted with anti-FLAG antibody to detect SPA-1. B, control HeLa/Tet-off cells and SPA/HeLa cells that had been cultured in the absence of Dox for 24 h were trypsinized and washed. A portion of them was held in suspension by gentle rotation, while the rest was allowed to adhere on the tissue culture plates for 30 min in the culture medium with 0.5% bovine serum albumin in the absence of serum. The cells were harvested and lysed with RBD buffer. Total Rap1 and Rap1 GTP of each sample were detected as described in the legend of Fig. 1. C, control HeLa/Tet-off cells (, ) and SPA/HeLa cells (, ) were cultured at 105 cells/60-mm dish in the presence (, ) or absence (, ) of Dox (1 ng/ml). At the indicated days of culture, nonadherent floating cells were recovered by gentle rinsing with medium, and viable cell numbers were counted. Each point represents the mean of triplicate cultures. D, control HeLa/Tet-off (a-c) and SPA/HeLa (d-f) cells were cultured in the absence of Dox for 24 h (a, d), 48 h (b, e), and 72 h (c, f) in the complete medium supplemented with serum and observed with a phase-contrast microscope (× 400).

We then examined the effect of SPA-1 overexpression on the initiation of adherence. SPA/HeLa cells that had been induced in Dox-free medium were made into suspension by trypsinization. Upon plating on the fibronectin-coated dishes, significant generation of Rap1 GTP was detected in the SPA/HeLa cells that had been induced for SPA-1, but the level was much lower than in parental cells that exhibited close to maximal Rap1 GTP even on the plain tissue culture dishes (Fig. 4A). In the short-term adhesion assay at 30 min, the induced SPA/HeLa cells started to adhere to the fibronectin-coated dishes in a dose-dependent fashion, but again it was much less efficient as compared with non-induced SPA/HeLa cells (Fig. 4B). Similarly, detachment of the adhered SPA/HeLa cells following the induction of SPA-1 was prevented by fibronectin in a dose-dependent fashion (Fig. 4C). These results have indicated that SPA-1 interferes with both initiation and maintenance of cell adhesion, which can be counteracted by exogenous fibronectin.

View larger version (27K): [in this window] [in a new window]   Fig. 4.   Exogenous fibronectin induces Rap1 activation to counteract the effect of SPA-1 on cell adherence and detachment in SPA/HeLa cells. A, control HeLa and SPA/HeLa cells cultured in the absence of Dox for 2 days were trypsinized, washed, and held in suspension or plated on the dishes coated with the indicated concentrations of fibronectin for 30 min in the absence of serum. Rap1 GTP was assayed as described in the legend of Fig. 3B. At the same condition as above, cell adherence assay was performed. , control HeLa cells; , SPA/HeLa cells. The means of three independent experiments are indicated. Note that the concentrations of fibronectin are plotted in the log scale. C, control HeLa and SPA/HeLa cells (105 cells) cultured in the Dox-containing medium in the 60-mm dishes coated with the indicated concentrations of fibronectin were shifted to the Dox-free medium containing the serum. Four days later, the cells detached from the dishes were collected and counted as in Fig. 3C. Means of triplicate cultures and standard deviation are indicated.

Overexpression of SPA-1 Inhibits the G-CSF-induced Adhesion of Promyelocytic 32D Cells-- Promyelocitic 32D cells that express significant SPA-1 are nonadherent but are induced to adhere following the G-CSF stimulation. To see the effect of SPA-1 overexpression, 32D cells were infected with either Spa-1 or GRD-Spa-1 recombinant retrovirus. As shown in Fig. 5A, 32D cells infected with the Spa-1 retrovirus (32D/SPA-1) expressed by far more SPA-1 than wild type cells. Upon shift to the medium containing G-CSF, Rap1GTP was generated in the wild type cells within a day and increased steadily thereafter (Fig. 5B). As shown in Fig. 5, C and D, the cells concomitantly adhered to the dishes following the shift to G-CSF. The accumulation of Rap1 GTP apparently paralleled with the increased proportion of adherent cells. In contrast, negligible activation of Rap1 was induced in the 32D/SPA-1 following G-CSF stimulation (Fig. 5B), whereas those infected with the GRD-Spa-1 retrovirus (32D/GRD) exhibited comparable Rap-1 activation to wild type cells (data not shown). As also shown in Fig. 5, C and D, 32D/SPA-1 cells remained totally nonadherent in the presence of G-CSF, whereas those infected with 32D cells infected with pLXSN retrovirus (32D/cont) and 32D/GRD-Spa-1 cells became adherent similarly to wild type cells. Because G-CSF also induces the differentiation of 32D cells into granulocytes, we finally examined the effect of SPA-1 overexpression on the process. As shown in Fig. 6A, 32D/SPA-1 cells differentiated into granulocytes at a degree comparable with that of wild type cells in the presence of G-CSF. The proportions of granulocytes in the adherent and nonadherent fractions of 32D cells were also comparable, suggesting that the cell adhesion and morphological differentiation are independent events. Following G-CSF stimulation, the expression of several adhesion molecules such as VLA4, CD18, LFA-1, and CD44 was augmented indistinguishably in wild type and 32D/SPA-1 cells (Fig. 6C), indicating that SPA-1 overexpression barely affected their expression levels per se.

View larger version (51K): [in this window] [in a new window]   Fig. 5.   Overexpression of SPA-1 in 32D cells by recombinant retrovirus infection interferes with the activation of Rap1 and induction of cell adhesion by G-CSF. A, 32D cells were infected with pLXSN retrovirus (32D/cont) or pLXSN containing FLAG-tagged SPA-1 (32D/SPA-1) or FLAG-tagged GRD-Spa-1 (32D/GRD) and then cultured in the presence of IL-3 and G418. Two weeks later, expression of SPA-1 (130 kDa) and GRD-SPA-1 (85 kDa) was analyzed by immunoblotting with anti-FLAG or anti-SPA-1 antibody. B, 32D/cont and 32D/SPA-1 cells maintained in the IL-3-containing medium were shifted into the culture medium containing G-CSF. At the indicated number of days, the cells were harvested, and activation of Rap1 was examined as before. C, wild type 32D (), 32D/cont (), 32D/SPA-1 (), and 32D/GRD () cells maintained in the IL-3-containing medium were shifted into the culture medium containing G-CSF. At the indicated number of days in the culture, adherent and nonadherent cells were separately collected and counted, and proportions of the adherent cells were calculated. D, wild type 32D (a), 32D/cont (b), 32D/GRD (c), and 32D/SPA-1 (d) cells were cultured in the presence of G-CSF for 6 days and rinsed with warm buffer five times to deplete nonadherent cells. Cells that remained adhered to dishes were photographed under the phase-contrast microscope (×100).

View larger version (45K): [in this window] [in a new window]   Fig. 6.   Overexpression of SPA-1 in 32D cells by recombinant retrovirus infection does not affect cell differentiation by G-CSF nor expression levels of adhesion molecules. A, 32D/cont (), 32D/SPA-1 (), and 32D/GRD () cells maintained in the IL-3-containing medium were shifted into the culture medium containing G-CSF. At the indicated number of days in the culture, total cells were harvested and cytospinned, and the typical polymorphonuclear cells were differentially counted and indicated as percentage of total cells. Each point represents the mean of four independent experiments. B, wild type 32D (32D/wt) and 32D/SPA-1 cells were cultured for 7 days in the presence of G-CSF. Nonadherent and adherent cells were collected separately as above, cytospinned, and stained with Giemza's solution along with 32D/wt cells maintained in the presence of IL-3. Few adherent cells could be obtained in the 32D/SPA-1 cells. C, 32D/wt and 32D/SPA-1 cells maintained in IL-3-containing medium or cultured in the presence of G-CSF for 7 days were stained with the antibodies for indicated adhesion molecules followed by flow cytometric analysis (solid areas). Background binding was determined by staining with the second antibodies alone as indicated by lines. Because 32D/wt and 32D/SPA-1 cells maintained in IL-3-containing medium showed identical profiles, only the former is indicated.

	DISCUSSION

TOP ABSTRACT INTRODUCTION EXPERIMENTAL PROCEDURES RESULTS DISCUSSION REFERENCES

In the present study, we have indicated that SPA-1 interferes with the activation of Rap1 by C3G bearing a CAAX motif (C3G-F) in 293T cells. In the presence of excess SPA-1, efficiency of Rap1 activation by the transfected C3G-F was markedly reduced, strongly suggesting that SPA-1 can control the threshold against the activation of Rap1 in the cells. HeLa cells transiently transfected with Spa-1 cDNA tended to show smaller and round cell shape, whereas those transfected with C3G-F cDNA conversely exhibited more enlarged and flattened shape with extensive cytoplasmic protrusions. Recently, overexpression of v-Crk that can bind and recruit C3G has been reported to induce cell spreading and focal adhesion formation in PC12 cells (27). The effect was confirmed in HeLa cells that could conditionally overexpress SPA-1 by the tetracycline-regulative system (SPA/HeLa). SPA/HeLa cells were indistinguishable from parental cells, whereas, upon induction of SPA-1 by removing Dox from the culture, the tightly adherent cells gradually became round and detached from the dish. The results have strongly suggested that sustained activation of Rap1 is needed for cells to maintain the adherent state. When non-induced SPA/HeLa cells in suspension were plated on dishes and allowed to adhere, Rap1GTP was significantly increased. In contrast, no detectable activation of Rap1 was observed in SPA/HeLa cells that had been induced with SPA-1. When plated onto dishes coated with fibronectin, such induced SPA/HeLa cells showed significant generation of Rap1GTP, yet much less than in parental cells. Concomitantly, they showed reduced adhesion to exogenous fibronectin. Thus, it has been indicated that SPA-1 negatively regulates both initiation and maintenance of cell adhesion.

The effect of SPA-1 was further investigated in the nonadherent 32D cells, in which cell adhesion is induced by specific soluble factor. Following the stimulation with G-CSF, Rap1 GTP was generated within a day and increased thereafter. In an attempt to prevent the accumulation of Rap1 GTP following the G-CSF stimulation, we transduced Spa-1 cDNA into 32D cells using a recombinant retrovirus (32D/SPA-1). As expected, the generation of Rap1GTP following G-CSF stimulation was nearly completely suppressed in the 32D/SPA-1. Concomitantly, 32D/SPA-1 cells remained totally nonadherent in the presence of G-CSF. The effect was not observed in 32D cells transduced with GRD-Spa-1 retrovirus, indicating that Rap1 GAP activity was required for the effect. We have reported previously that the endogenous SPA-1 is markedly down-regulated in human HL60 promyeloid cells preceding the induction of cell adhesion and extensive spreading by 12-O-tetradecanoylphorbol-13-acetate (11). On the other hand, morphological differentiation into polymorphonuclear granulocytes by G-CSF occurred in 32D/SPA-1 comparably to the wild type cells, indicating that the G-CSF receptor per se functioned normally in 32D/SPA-1 cells. Expression levels of both integrins 1 and 2 following G-CSF stimulation were unaffected either by SPA-1 overexpression.

These results have indicated collectively that Rap1 is activated either by the adherence to substratum in intrinsically adherent HeLa cells or by the specific soluble factor G-CSF in nonadherent hematopoietic 32D cells, and in both types of cells Rap1GTP is required for the cell adherence and possibly for cell spreading. It appears that sustained presence of Rap1 GTP is required to maintain the cell adhesion. SPA-1 not only inhibits the activation of Rap1 upon contact with substrates but also reduces the accumulated Rap1 GTP pool in the adherent state, thereby interfering with both initiation and maintenance of cell adhesion. Cell adhesion to the ECM via integrins results in the activation of tyrosine kinases such as Src and FAK, which phosphorylate focal adhesion molecules including FAK itself, paxillin, and Cas (28). Because it has been shown that Crk adapter protein is recruited to the focal adhesion complex via SH2 domain (28), it seems possible that Rap1 can be activated there. In addition to C3G that can bind to Crk, distinct Rap1GEFs directly activated by the common second messengers have been recently identified (7-9), and it remains to be seen which Rap1GEF is primarily responsible for Rap1 activation by ECM in HeLa or by G-CSF in 32D cells. Rho GTPases involved in the cytoskeletal reorganization are also activated by both extracellular soluble factors and insoluble ECMs in fibroblasts (29). In human T cells, however, inactivation of Rho GTPase by Clostridium botulinum C3 exoenzyme was reported to show no effect on the cell adhesion to ECM per se, whereas it inhibited the costimulatory activity for T cell receptor-mediated activation (30).

How Rap1GTP is involved in the cell adhesion and spreading remains to be investigated. Recently, molecules that specifically bind to Rap1GTP have been identified, including a group of RalGEFs such as RalGDS (31) and Rgr (32, 33) that can activate Ral GTPase. Furthermore, one of the Ral GTP-binding proteins (RalBP1) has been shown to be a GAP for Rho family GTPases (34). These results may imply that ECM- or receptor-coupled Rap1 signaling pathway converges to other small GTPases including Rho family GTPases at the cytoskeletal compartment. In this aspect, it is particularly noted that a substantial portion of SPA-1 is located at the cytoskeletal compartment along with Rap1. It has been reported also that Rap1 is translocated rapidly to the cytoskeleton upon activation in platelets (35, 36). SPA-1 has a PDZ domain followed by leucine zipper motif (37), which potentially mediates the interaction with cytoskeletal elements (38). Our unpublished results, on the other hand, have indicated that overexpression of rapGAP, which lacks above domains and is located mostly in the cytosolic soluble fraction, hardly affects the cell adhesion in the same HeLa/Tet-off system.² These results altogether suggest that the cytoskeletal association of SPA-1 is crucially important for the regulation of cell adhesion by setting a threshold for Rap1 activation via various external signals.

We have indicated previously that the Spa-1 gene is expressed most abundantly in normal lymphohematopoietic cells (11), in particular mature peripheral lymphocytes.³ One of the most characteristic features of them is to recirculate in the blood and lymphatics. Under the particular situations such as antigenic stimulation and production of chemotaxic factors in inflammation, however, they are trapped rapidly at the secondary lymphoid tissues or inflammatory sites, where they are activated and/or proliferate. In such mobile cells, polarized regulation of cell adhesion within a cell is shown to play critical roles for cellular interactions and cell movement such as chemotaxis, phagocytosis, antigen-presentation, and cell-mediated cytotoxicity (39). Additionally, cell adhesion plays important roles in the signal transduction for both cell differentiation and proliferation (28). Analysis of the mice targeted for Spa-1 gene, which we have recently developed, should help to understand the roles Rap1 and SPA-1 play in vivo.

	ACKNOWLEDGEMENTS

We are grateful to Drs. M. Matsuda, A. Bank, T. Kina, S. Takeda, and T. Sudo for providing us with materials.

	FOOTNOTES

^* This work was supported by grants from the Ministry of Education, Science and Culture of Japan.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^§ To whom correspondence should be addressed. Tel.: +81-75-753-4659; Fax: +81-75-753-4403; E-mail: minato{at}med.kyoto-u.ac.jp.

² M. Hattori and N. Minato, unpublished observation.

³ N. Minato, unpublished observation.

	ABBREVIATIONS

The abbreviations used are: GEF, guanine nucleotide exchange factor; ECM, extracellular matrix; GAP, GTPase activating protein; G-CSF, granulocyte colony stimulating factor; GST, glutathione S-transferase; RBD, Ras binding domain; GRD, GAP-related domain; IL-3, interleukin 3; DMEM, Dulbecco's modified Eagle's medium; FCS, fetal calf serum; wt, wild type.

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