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RESEARCH COMMUNICATION
1 Institute of Pharmacology, University of Heidelberg, 69120 Heidelberg, Germany; 2 Department of Virology, University of Heidelberg, 69120 Heidelberg, Germany; 3 Zentrum für Molekulare Biologie Heidelberg, University of Heidelberg, 69120 Heidelberg, Germany
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Abstract |
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[Keywords: Diaphanous-related formins; RhoA; LARG; actin polymerization; LPA; Rho-kinase]
Received January 11, 2007; revised version accepted May 8, 2007.
). DRFs are involved in essential cellular processes such as cytokinesis, cell movement, and polarity (Faix and Grosse 2006; Gomez et al. 2007), which are frequently deregulated during pathological situations like tumor cell transformation and metastasis (Sahai 2005). The dormant conformation of the DRF Dia1 is maintained by intramolecular association of its regulatory N terminus to the diaphanous autoregulatory domain (DAD), which is relieved through binding of active RhoA (Lammers et al. 2005; Otomo et al. 2005a). The catalytic FH2 domain is believed to become "exposed" by conformational changes in the DRFs to promote barbed end actin polymerization by forming a tethered dimer (Xu et al. 2004; Otomo et al. 2005a). The FH2 domain of Dia1 promotes stress fiber formation and transcriptional activation of the MAL/SRF pathway through its actin-polymerizing activity (Copeland and Treisman 2002; Grosse et al. 2003; Miralles et al. 2003). A Dia1 mutant defective in FH2 dimerization interferes with lysophosphatidic acid (LPA)-induced stress fiber formation and SRF activity (Copeland and Treisman 2002), suggesting that Dia1 is part of LPA signal transduction known to play an important role in cell proliferation and metastasis of a variety of human cancers (Mills and Moolenaar 2003). LPA receptors belong to the group of G-protein-coupled receptors that activate the heterotrimeric G-proteins G12 and G13, which can bind to RGS-containing Rho-GEFs such as leukemia-associated Rho-GEF (LARG), initially isolated from a patient with acute myeloid leukemia (AML) (Kourlas et al. 2000; Vazquez-Prado et al. 2004). Rho-dependent mechanisms have emerged as critical processes in tumor progression (Sahai and Marshall 2002; Lozano et al. 2003), and evidence exists that Rho/ROCK function is essential to promote a specific type of rounded bleb-associated mode of cell invasion (Sahai and Marshall 2003; Wyckoff et al. 2006). However, the role of DRFs in tumor cell behavior has not been investigated. In this study, we provide evidence for an essential role of the Rho-effector Dia1 in LPA-mediated Rho/ROCK activity for tumor cell morphology and invasion that involves LARG, thereby constituting a positive feedback loop toward RhoA.
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Results and Discussion |
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TopAbstractResults and DiscussionMaterials and methodsAcknowledgmentsReferences |
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). HEK293 cells were used to achieve transfection efficiencies >70% to determine whether versions of Dia1 lacking the regulatory N terminus (
DiaN) (Fig. 1A) required for autoinhibition could induce endogenous RhoA activity. Interestingly, DiaN caused substantial activation of RhoA (Fig. 1B). Active DAD-deletion mutants (DAD) of mouse Dia1, Dia2, and Dia3 also stimulated RhoA but not Cdc42 activity, indicating that this is a shared characteristic among DRFs of this family (Fig. 1C). Dia1 also activated Rac as previously suggested (Tsuji et al. 2002; data not shown). Further analysis revealed that the FH2 domain was sufficient to activate RhoA (Fig. 1D). Since the FH2 is also sufficient for actin assembly and activation of SRF, we tested their possible involvement, indicative of an indirect mechanism. For this we treated cells with Latrunculin B, which sequesters actin monomers and prevents SRF activation (Sotiropoulos et al. 1999). Although Latrunculin B effectively blocked FH2-induced SRF activation as expected (Fig. 1E), it failed to inhibit RhoA activation by FH2 and LARG, while transfection of C3 transferase as a control blocked RhoA activity (Fig. 1D). Interestingly and in agreement with previous findings (Copeland and Treisman 2002), we observed that FH2-induced SRF activity was significantly inhibited by C3 (Fig. 1E), suggesting that upstream RhoA contributes to FH2-exerted actin dynamics. The FH2 domain dimerizes involving its N-terminal lasso loop linker in order to nucleate actin filaments, whereas the core FH2 (FH2core) lacking the lasso behaves as a barbed end capper (Shimada et al. 2004; Otomo et al. 2005b). Therefore, we compared FH2core and FH2 for RhoA activity and actin polymerization. We found that both domains activate RhoA (Fig. 1F), but only the FH2 was able to stimulate SRF and actin polymerization, whereas the FH2core inhibited SRF and basal actin polymerization due to its capping activity (Fig. 1G,H). These data demonstrate that Dia1 stimulates RhoA activity independently of its actin nucleation activity, thus representing a novel function of this conserved protein.
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12/13 as well as by using a dominant-negative mutant of LARG (dnLARG) (Vogt et al. 2003; Tanabe et al. 2004; Vazquez-Prado et al. 2004); and (2) the Drosophila LARG homolog dRhoGEF2 colocalizes with Diaphanous at the furrow canal during cellularization (Grosshans et al. 2005). To investigate whether LARG is involved in Dia1-induced stimulation of RhoA, we used the previously characterized dnLARG lacking the DH/PH domains (Fig. 2A; Vogt et al. 2003). Despite relatively low expression levels, dnLARG partially but significantly inhibited Dia1–DAD-induced RhoA activity (Fig. 1J), indicating that LARG is involved in RhoA-GTP formation through Dia1.
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), but we did not detect dishevelled in the Dia1/LARG complex (data not shown).
LARG belongs to the RGS domain-containing RhoGEFs, consisting of two additional members, p115RhoGEF and PDZ-RhoGEF (PRG) (Vazquez-Prado et al. 2004). Hence, we assessed the specificity of the Dia1/LARG association. We found that Dia1 interaction with LARG is specific among this RhoGEF family since PRG or p115RhoGEF did not coimmunoprecipitate with Dia1 (Supplementary Fig. 1). However, we cannot exclude the possibility that Dia1 influences additional RhoGEFs or RhoGTPase-regulating proteins.
To determine the regions required for association with Dia1, a series of LARG truncation mutants was tested. Only LARG mutants containing the C terminus coimmunoprecipitated with Dia1 (Fig. 2C). Since the FH2core domain of Dia1 is sufficient to induce RhoA activity, we tested whether the LARG C terminus and the FH2core domain would associate. Indeed, we could coimmunoprecipitate the FH2core domain with the C terminus of LARG (Fig. 2D). To further define the region within the LARG C terminus, we coexpressed its N- or C-terminal regions (CT-N and CT-C, respectively) with FH2core. As shown in Figure 2D, only the C-terminal half of the LARG C terminus efficiently coimmunoprecipitated the FH2core domain, indicating that these regions are responsible for the association. These data were confirmed using protein overlay assays (Supplementary Fig. 2).
The association of FH2core with the C terminus of LARG suggested that activated Dia1 regulates the LARG C terminus. The release of Dia1 autoinhibition through binding of active RhoA to Dia1-NT (Fig. 1A) can be reconstituted in vitro by measuring FH2-DAD (CT) domain-induced actin polymerization (Fig. 3A; Li and Higgs 2005; Brandt et al. 2007). To address whether autoinhibition of Dia1 regulates its interaction with LARG, we performed GST pull-down assays with LARG CT-C and the Dia1-NT and Dia1-CT fragments in the absence or presence of active RhoAV14. Interestingly, association between Dia1-CT and LARG-CT-C was strongly inhibited by the addition of Dia1-NT (Fig. 3B). Moreover, this inhibition was overcome by the addition of active RhoA to the complex (Fig. 3C). These data demonstrate that the interaction between Dia1 and LARG is controlled by RhoA-induced release of Dia1 autoinhibition. This suggests the existence of a positive feedback loop involving RhoA, Dia1, and LARG. Interestingly, conformational changes of the C terminus of LARG (CT) appear to be mediated by the interaction of N- and C-terminal regions, while the FH2core domain can interfere with CT-N/CT-C binding in a dose-dependent manner (Supplementary Fig. 2), indicating that Dia1 may modulate LARG C-terminal conformation, known to control LARG activity in vivo (Chikumi et al. 2004).
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LARG was identified as a gene fusion rearranged in AML (Kourlas et al. 2000). Up-regulated activities of RhoA have been reported for a variety of human cancers (Sahai and Marshall 2002; Lozano et al. 2003). Here we show that Dia1 promotes RhoA activity via interaction with LARG and that Dia1 is required for LPA-mediated RhoA activity. Since LPA is known to play an important role in tumor development and progression promoting cancer cell metastasis and invasion (Mills and Moolenaar 2003), we tested the role of Dia1 in tumor cell invasion. One mode of tumor cell invasion in three-dimensional (3D) matrices is represented by the rounded mode of motility, for which Rho/ROCK signaling is necessary and sufficient (Sahai and Marshall 2003; Wyckoff et al. 2006). After optimizing the conditions for small interfering RNA (siRNA_ toward Dia1 (Fig. 4A,B), we used highly invasive MDA-MB-435 human cancer cells (Sellappan et al. 2004; Rae et al. 2006), which express Dia1, Dia2 (Fig. 4B), and Dia3 (data not shown), and tested their ability to migrate into LPA-containing 3D matrices when Dia1 or Dia2 expression was down-regulated. Interestingly, knockdown of Dia1 efficiently and significantly inhibited the total number of tumor cells that have invaded as well as the overall invasion distance (Fig. 4D,E; Supplementary Videos 1, 2), whereas random MDA-MB-435 cell migration under two-dimensional (2D) tissue culture conditions was slightly increased (Fig. 4C). Knockdown of Dia2 did not inhibit cell invasion (data not shown). Similar results were obtained in A375m2 melanoma cells (data not shown), which also use the rounded mode of motility (Sahai and Marshall 2003). We next determined the involvement of LARG in MDA-MB-435 cell invasion. For this, we transfected GFP-dnLARG, GFP alone, or cotransfected with LARG1–820 as a control (transfection efficiencies were 
1%) and assessed the GFP-positive cells invaded into the Matrigel. This revealed that dnLARG inhibited cell invasion (Fig. 4F) but not random 2D migration (Fig. 4C). Inhibition of ROCK with 10 µM Y27632 blocked invasion of MDA-MB-435 cells as expected, while treatment with the Src-kinase inhibitor PP1 had no effect (Fig. 4G). Consistently, usage of siRNAs against LARG significantly reduced cell invasion (Fig. 4H,I). Together, these data suggest that LARG is involved in cancer cell invasion.
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) to control contractile force through myosin activity (Riento and Ridley 2003). When basal as well as LPA-stimulated phosphorylation of myosin light chain (MLC), a ROCK substrate (Riento and Ridley 2003), was determined in MDA-MB-435 cells in the presence or absence of siRNA against LARG or Dia1, we found that MLC phosphorylation was strongly reduced (Fig. 5C–E) in contrast to Dia2 siRNA (Fig. 5F). To investigate whether Dia1 promotes ROCK activity through RhoA, we used HEK293 cells to efficiently express the FH2 domain with ROCK-I before determining in vitro kinase activities using either MBP or MLC as substrates. Interestingly, we found that active Dia1 increased ROCK activity in a Rho-dependent manner (Fig. 5G). These results clearly show that activation of RhoA by Dia1 leads to an increase in ROCK activity to promote the MLC phosphorylation required for bleb-associated cell invasion. Dia1 is a well-established downstream effector of active RhoA. Our results presented here reveal an unexpected and essential role for Dia1 in the activation of the Rho/ROCK signal transduction pathway and subsequent bleb-associated cancer cell motility. We provide evidence that Dia1 is required and sufficient for full LPA-induced activation of RhoA and downstream ROCK signaling. This effect can be mediated through interaction of Dia1 with LARG. The data shown here imply a novel signaling module by which Dia1, in addition to its role as a downstream RhoA effector, can function upstream of RhoA. This constitutes a positive feedback mechanism (Fig. 5H) amplifying signal-regulated cellular effects such as tumor cell invasion by activating RhoA and its effector ROCK.
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Materials and methods |
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TopAbstractResults and DiscussionMaterials and methodsAcknowledgmentsReferences |
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Plasmids were generated using standard procedures. All siRNA oligos were from IBA. GTPase pull-down assays, immunoprecipitations, and Rho-kinase and SRF activity assays were performed in HEK293 or NIH3T3 cells. See the Supplemental Material for details.
Protein purification, actin assembly, and guanyl-nucleotide exchange assays
Proteins were produced and purified in Escherichia coli strain DE3 as GST or His fusions. Actin polymerization was monitored as described (Brandt et al. 2007). Guanyl-nucleotide exchange assays were performed using GST-RhoA loaded with [8-3H]GDP. See the Supplemental Material for full details.
3D matrigel invasion and confocal analysis
Human MDA-MB-435 cancer cells were used for invasion assays. Assays were analyzed using confocal microscopy (Leica TCS SP2). See the Supplemental Material for full details.
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Acknowledgments |
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TopAbstractResults and DiscussionMaterials and methodsAcknowledgmentsReferences |
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Footnotes |
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E-MAIL robert.grosse{at}pharma.uni-heidelberg.de; FAX 49-06221-548549.
Supplemental material is available at http://www.genesdev.org.
Article is online at http://www.genesdev.org/cgi/doi/10.1101/gad.424807
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