[Show abstract][Hide abstract] ABSTRACT: The viral G protein-coupled receptor (vGPCR) is proposed to act as one of the predominant mediators of Kaposi’s sarcoma (KS), a human herpes virus 8 (HHV8)-elicited disease. The actions of vGPCR manifest pathogenesis, in part, through increased permeability of endothelial cells. Endothelial cell-cell junctions have indeed emerged as an instrumental target involved in the vasculature defects observed within the tumor microenvironment. The pathway leading to adherens junction destabilization has been shown to involve the activation of the small GTPase Rac, in the context of either latent infection or the sole expression of vGPCR. However, the precise molecular mechanisms governed by vGPCR in vascular leakage require further elucidation.
Guanine exchange factors (GEFs) function as critical molecular switches that control the activation of small GTPases. We therefore screened the effects of 80 siRNAs targeting GEFs on vGPCR-driven endothelial permeability and identified switch-associated protein 70 (SWAP70) as necessary for its elevating effects. Pull-down experiments further showed that Rac activation by vGPCR was dependent on SWAP70. Examination of tissues and cells from HHV8-positive patients revealed that SWAP70 was ubiquitously expressed. Furthermore, SWAP70 was found to be crucial for vGPCR-driven endothelial tube formation and endothelial sprouting in vitro.
SWAP70 appears to act as a molecular intermediate between vGPCR and endothelial activation. Because of the important role of vGPCR-mediated endothelial plasticity in KS pathogenesis, inhibition of SWAP70 function could be of interest for blocking vGPCR-driven activities in HHV8-defined diseases.
Preview · Article · Dec 2015 · Cell Communication and Signaling
[Show abstract][Hide abstract] ABSTRACT: Glioblastoma are malignant highly vascularized brain tumours, which feature large oedema resulting from tumour-promoted vascular leakage. The pro-permeability factor Semaphorin3A (Sema3A) produced within glioblastoma has been linked to the loss of endothelial barrier integrity. Here, we report that extracellular vesicles (EVs) released by patient-derived glioblastoma cells disrupt the endothelial barrier. EVs expressed Sema3A at their surface, which accounted for in vitro elevation of brain endothelial permeability and in vivo vascular permeability, in both skin and brain vasculature. Blocking Sema3A or its receptor Neuropilin1 (NRP1) hampered EV-mediated permeability. In vivo models using ectopically and orthotopically xenografted mice revealed that Sema3A-containing EVs were efficiently detected in the blood stream. In keeping with this idea, sera from glioblastoma multiforme (GBM) patients also contain high levels of Sema3A carried in the EV fraction that enhanced vascular permeability, in a Sema3A/NRP1-dependent manner. Our results suggest that EV-delivered Sema3A orchestrates loss of barrier integrity in glioblastoma and may be of interest for prognostic purposes.Oncogene advance online publication, 14 September 2015; doi:10.1038/onc.2015.317.
[Show abstract][Hide abstract] ABSTRACT: Background
The interleukin-8 chemokine (IL-8) G-protein coupled receptor CXCR2 governs pro-inflammatory and pro-angiogenic responses in leukocytes and endothelial cells. At a molecular standpoint, CXCR2 is widely reported to operate through calcium flux, phosphoinoisitide 3 kinase (PI3K) and mitogen-activated protein kinase (MAPK). While CXCR2 trafficking is suspected to be intertwined with its signaling, the exact mechanism is not fully elucidated.
Here, we identified the lysine 327 within the CXCR2 C-terminal tail as a key residue for ubiquitination, internalization, and signaling. First, the substitution to an arginine of K327 mutation was associated with a reduction in CXCR2 poly-ubiquitination. While WT CXCR2 was rapidly internalized following IL-8 administration, K327R mutant remained at the plasma membrane. Finally, K327R mutant failed to promote the recruitment of β-arrestin2, as estimated by imagery and bioluminescence resonance transfer. As a consequence, the activation of intracellular signaling, including both early events such as ERK phosphorylation and the increase in calcium flux, and the latter activation of the AP1 and NF-κB transcription factors, was blunted.
Overall, our results demonstrate that CXCR2 ubiquitination on K327 residue modulates agonist-activated CXCR2 cell sorting and intracellular signaling. Thus, the inhibition of K327 ubiquitination might emerge as an effective mean to curb exacerbated CXCR2 signaling in several pathological conditions, such as inflammatory diseases and cancer.
[Show abstract][Hide abstract] ABSTRACT: The nuclear factor κB (NF-κB) family members p65 and c-Rel chiefly orchestrate lymphocytes activation following T-cell receptor (TCR) engagement. In contrast to p65, which is rapidly mobilized, c-Rel activation occurs subsequently as it involves a nuclear factor of activated T-cells (NFAT)-dependent upregulation step. However, how TCR ligation drives p65 and c-Rel activation is not fully understood. Because several ubiquitylated components of NF-κB signaling cascade accumulate in close proximity to membranes, we screened a siRNA library against E3-ligases that contain transmembrane domains on TCR-mediated NF-κB activation. Here, we report the identification of the endoplasmic reticulum resident TRIM13 protein as an enhancer of NF-κB promoter activity. We found that knocking down TRIM13 by RNA interference reduced the activation of p65, while the translocation of c-Rel into the nucleus was blunted. We further observed that c-Rel induction was diminished without TRIM13, as NFAT activation was compromised. These results unveil that TRIM13 is a selective regulator of p65 and of c-Rel activation.
No preview · Article · Jul 2014 · The International Journal of Biochemistry & Cell Biology
[Show abstract][Hide abstract] ABSTRACT: Rapidly growing and highly vascularized tumors, such as glioblastoma multiforme, contain heterogeneous areas within the tumor mass, some of which are inefficiently supplied with nutrients and oxygen. While the cell death rate is elevated in such zones, tumor cells are still suspected to grow and survive independently of extracellular growth factors. In line with this, glioblastoma stem-like cells (GSCs) are found closely associated with brain vasculature in situ, and as such are most likely in a protected microenvironment. However, the behavior of GSCs under deprived conditions has not been explored in detail. Using a panel of 14 patient-derived GSCs, we report that ex vivo mitogen deprivation impaired self-renewal capability, abolished constitutive activation of the mTor pathway, and impinged on GSC survival via the engagement of autophagic and apoptotic cascades. Moreover, pharmacological inhibition of the mTor pathway recapitulated the mitogen deprivation scenario. In contrast, blocking either apoptosis or autophagy, or culturing GSCs with endothelial-secreted factors partly restored mTor pathway activation and rescued GSC survival. Overall, our data suggest that GSCs are addicted to mTor, as their survival and self-renewal are profoundly dependent on this signaling axis. Thus, as mTor governs the fate of GSCs under both deprivation conditions and in the presence of endothelial factors, it could be a key target for therapeutic purposes.
[Show abstract][Hide abstract] ABSTRACT: Antigen receptor-mediated nuclear factor κB (NF-κB) activation relies on the formation of a large multi-protein complex that contains CARMA1, BCL10 and MALT1 (CBM complex). This signalosome is pirated in the activated B-cell like subgroup of diffuse large B cell lymphoma (ABC DLBCL) to drive aberrant NF-κB activation, thereby promoting cell survival and propagation. Using an unbiased proteomic approach, we screened for additional components of the CBM in lymphocytes. We found that the linear ubiquitin chain assembly complex (LUBAC), which was previously linked to cytokine-mediated NF-κB activation, dynamically integrates the CBM and marshals NF-κB optimal activation following antigen receptor ligation independently of its catalytic activity. The LUBAC also participates to preassembled CBM complex in cells derived from ABC DLBCL. Silencing the LUBAC reduced NF-κB activation and was toxic in ABC DLBCL cell lines. Thus, our findings reveal a role for the LUBAC during lymphocyte activation and in B cell malignancy.
[Show abstract][Hide abstract] ABSTRACT: Kaposi sarcoma (KS) and primary effusion lymphoma (PEL) are two pathologies associated with KS herpes virus (KSHV/HHV-8) infection. KSHV genome contains several oncogenes, among which, the viral G-protein-coupled receptor (vGPCR open reading frame 74) has emerged as a major factor in KS pathogenicity. Indeed, vGPCR is a constitutively active receptor, whose expression is sufficient to drive cell transformation in vitro and tumour development in mice. However, neither the role of vGPCR in KSHV-infected B-lymphocytes nor the molecular basis for its constitutive activation is well understood. Here, we show that vGPCR expression contributes to nuclear factor-kB (NF-kB)-dependent cellular survival in both PEL cells and primary B cells from HIV-negative KS patients. We further identified within vGPCR an AP2 consensus binding motif, Y 326 GLF, that directs its localization between the plasma membrane and clathrin-coated vesicles. The introduction of a mutation in this site (Y 326 A) increased NF-kB activity and proinflammatory cytokines production. This correlated with exacerbated morphological rearrangement, migration and proliferation of non-infected monocytes. Collectively, our work raises the possibility that KSHV-infected B-lymphocytes use vGPCR to impact ultimately the immune response and communication within the tumour microenvironment in KSHV-associated pathologies. Oncogene advance online publication, 2 December 2013; doi:10.1038/onc.2013.503
[Show abstract][Hide abstract] ABSTRACT: The innate and adaptive immune responses involve the stimulation of nuclear factor κB (NF-κB) transcription factors through the Lys(63) (K(63))-linked ubiquitylation of specific components of NF-κB signaling pathways. We found that ubiquitylated components of the NF-κB pathway accumulated on the cytosolic leaflet of the endoplasmic reticulum (ER) membrane after the engagement of cell-surface, proinflammatory cytokine receptors or antigen receptors. Through mass spectrometric analysis, we found that the ER-anchored protein metadherin (MTDH) was a partner for these ubiquitylated activators of NF-κB and that it directly bound to K(63)-linked polyubiquitin chains. Knockdown of MTDH inhibited the accumulation of ubiquitylated NF-κB signaling components at the ER, reduced the extent of NF-κB activation, and decreased the amount of proinflammatory cytokines produced. Our observations highlight an unexpected facet of the ER as a key subcellular gateway for NF-κB activation.
[Show abstract][Hide abstract] ABSTRACT: Background
The endothelial specific cell-cell adhesion molecule, VE-cadherin, modulates barrier function and vascular homeostasis. In this context, we have previously characterized that VEGF (vascular endothelial growth factor) leads to VE-cadherin phosphorylation, β-arrestin2 recruitment and VE-cadherin internalization in mouse endothelial cells. However, exactly how this VE-cadherin/β-arrestin complex contributes to VEGF-mediated permeability in human endothelial cells remains unclear. In this study, we investigated in-depth the VE-cadherin/β-arrestin interactions in human endothelial cells exposed to VEGF.
First, we demonstrated that VEGF induces VE-cadherin internalization in a clathrin-dependent manner in human umbilical vein endothelial cells (HUVEC). In addition to the classical components of endocytic vesicles, β-arrestin1 was recruited and bound to phosphorylated VE-cadherin. Molecular mapping of this interaction uncovered that the C-terminus tail of β-arrestin1, that comprises amino acids 375 to 418, was sufficient to directly interact with the phosphorylated form of VE-cadherin. Interestingly, the expression of the C-terminus tail of β-arrestin1 induced loss of surface exposed-VE-cadherin, promoted monolayer disorganization and enhanced permeability. Finally, this effect relied on decreased VE-cadherin expression at the transcriptional level, through inhibition of its promoter activity.
Altogether, our results demonstrate that β-arrestin1 might play multiple functions collectively contributing to endothelial barrier properties. Indeed, in addition to a direct implication in VE-cadherin endocytosis, β-arrestin1 could also control VE-cadherin transcription and expression. Ultimately, understanding the molecular mechanisms involved in VE-cadherin function might provide therapeutic tools for many human diseases where the vascular barrier is compromised.
Full-text · Article · May 2013 · Cell Communication and Signaling
[Show abstract][Hide abstract] ABSTRACT: Background
NF-κB is a master gene regulator involved in plethora of biological processes, including lymphocyte activation and proliferation. Reversible ubiquitinylation of key adaptors is required to convey the optimal activation of NF-κB. However the deubiquitinylases (DUBs), which catalyze the removal of these post-translational modifications and participate to reset the system to basal level following T-Cell receptor (TCR) engagement continue to be elucidated.
Here, we performed an unbiased siRNA library screen targeting the DUBs encoded by the human genome to uncover new regulators of TCR-mediated NF-κB activation. We present evidence that knockdown of Ubiquitin-Specific Protease 34 (USP34) selectively enhanced NF-κB activation driven by TCR engagement, similarly to siRNA against the well-characterized DUB cylindromatosis (CYLD). From a molecular standpoint, USP34 silencing spared upstream signaling but led to a more pronounced degradation of the NF-κB inhibitor IκBα, and culminated with an increased DNA binding activity of the transcription factor.
Collectively, our data unveils USP34 as a new player involved in the fine-tuning of NF-κB upon TCR stimulation.
Full-text · Article · Apr 2013 · Cell Communication and Signaling
[Show abstract][Hide abstract] ABSTRACT: The prototypical death receptor Fas (also known as CD95 or Apo-1) plays an essential role in the maintenance of lymphocyte homeostasis. Propagation of cell death through Fas relies on the formation of a multiprotein complex at the receptor level known as the death-inducing signaling complex (DISC). Here, we describe an immunoprecipitation-based protocol to study DISC assembly in activated human T lymphocytes. This procedure is a useful tool to visualize proteins associated with Fas.
No preview · Article · Feb 2013 · Methods in molecular biology (Clifton, N.J.)
[Show abstract][Hide abstract] ABSTRACT: Glioblastoma constitutes the most aggressive and deadly of brain tumors. As yet, both conventional and molecular-based therapies have met with limited success in treatment of this cancer. Among other explanations, the heterogeneity of glioblastoma and the associated microenvironment contribute to its development, as well as resistance and recurrence in response to treatments. Increased vascularity suggests that tumor angiogenesis plays an important role in glioblastoma progression. However, the molecular crosstalk between endothelial and glioblastoma cells requires further investigation. To examine the effects of glioblastoma-derived signals on endothelial homeostasis, glioblastoma cell secretions were collected and used to treat brain endothelial cells. Here, we present evidence that the glioblastoma secretome provides pro-angiogenic signals sufficient to disrupt VE-cadherin-mediated cell-cell junctions and promote endothelial permeability in brain microvascular endothelial cells. An unbiased angiogenesis-specific antibody array screen identified the chemokine, interleukin-8, which was further demonstrated to function as a key factor involved in glioblastoma-induced permeability, mediated through its receptor CXCR2 on brain endothelia. This underappreciated interface between glioblastoma cells and associated endothelium may inspire the development of novel therapeutic strategies to induce tumor regression by preventing vascular permeability and inhibiting angiogenesis.
[Show abstract][Hide abstract] ABSTRACT: VE-cadherin-mediated cell-cell junction weakening increases paracellular permeability in response to both angiogenic and inflammatory stimuli. Although Semaphorin 3A has emerged as one of the few known anti-angiogenic factors to exhibit pro-permeability activity, little is known about how it triggers vascular leakage. Here we report that Semaphorin 3A induced VE-cadherin serine phosphorylation and internalization, cell-cell junction destabilization, and loss of barrier integrity in brain endothelial cells. In addition, high-grade glioma-isolated tumour initiating cells were found to secrete Semaphorin 3A, which promoted brain endothelial monolayer permeability. From a mechanistic standpoint, Semaphorin 3A impinged upon the basal activity of the serine phosphatase PP2A and disrupted PP2A interaction with VE-cadherin, leading to cell-cell junction disorganization and increased permeability. Accordingly, both pharmacological inhibition and siRNA-based knockdown of PP2A mimicked Semaphorin 3A effects on VE-cadherin. Hence, local Semaphorin 3A production impacts on the PP2A/VE-cadherin equilibrium and contributes to elevated vascular permeability.
Full-text · Article · Jun 2012 · Journal of Cell Science
[Show abstract][Hide abstract] ABSTRACT: During a viral infection, the intracellular RIG-I-like receptors (RLRs) sense viral RNA and signal through the mitochondrial antiviral signaling adaptor MAVS (also known as IPS-1, Cardif and VISA) whose activation triggers a rapid production of type I interferons (IFN) and of pro-inflammatory cytokines through the transcription factors IRF3/IRF7 and NF-κB, respectively. While MAVS is essential for this signaling and known to operate through the scaffold protein NEMO and the protein kinase TBK1 that phosphorylates IRF3, its mechanism of action and regulation remain unclear.
We report here that RLR activation triggers MAVS ubiquitination on lysine 7 and 10 by the E3 ubiquitin ligase TRIM25 and marks it for proteasomal degradation concomitantly with downstream signaling. Inhibition of this MAVS degradation with a proteasome inhibitor does not affect NF-κB signaling but it hampers IRF3 activation, and NEMO and TBK1, two essential mediators in type I IFN production, are retained at the mitochondria.
These results suggest that MAVS functions as a recruitment platform that assembles a signaling complex involving NEMO and TBK1, and that the proteasome-mediated MAVS degradation is required to release the signaling complex into the cytosol, allowing IRF3 phosphorylation by TBK1.
[Show abstract][Hide abstract] ABSTRACT: The two major cytopathic factors in human immunodeficiency virus type 1 (HIV-1), the accessory proteins viral infectivity factor (Vif) and viral protein R (Vpr), inhibit cell-cycle progression at the G2 phase of the cell cycle. Although Vpr-induced blockade and the associated T-cell death have been well studied, the molecular mechanism of G2 arrest by Vif remains undefined. To elucidate how Vif induces arrest, we infected synchronized Jurkat T-cells and examined the effect of Vif on the activation of Cdk1 and CyclinB1, the chief cell-cycle factors for the G2 to M phase transition. We found that the characteristic dephosphorylation of an inhibitory phosphate on Cdk1 did not occur in infected cells expressing Vif. In addition, the nuclear translocation of Cdk1 and CyclinB1 was disregulated. Finally, Vif-induced cell cycle arrest was correlated with proviral expression of Vif. Taken together, our results suggest that Vif impairs mitotic entry by interfering with Cdk1-CyclinB1 activation.
[Show abstract][Hide abstract] ABSTRACT: Glioma stem-cells are associated with the brain vasculature. However, the way in which this vascular niche regulates stem-cell renewal and fate remains unclear. Here, we show that factors emanating from brain endothelial cells positively control the expansion of long-term glioblastoma stem-like cells. We find that both pharmacological inhibition of and RNA interference with the mammalian target of rapamycin (mTOR) pathway reduce their spheroid growth. Conversely, the endothelial secretome is sufficient to promote this mTOR-dependent survival. Thus, interfering with endothelial signals might present opportunities to identify treatments that selectively target malignant stem-cell niches.
[Show abstract][Hide abstract] ABSTRACT: Beside their established function in shaping cell architecture, some cell polarity proteins were proposed to participate to lymphocyte migration, homing, scanning, as well as activation following antigen receptor stimulation. Although PALS1 is a central component of the cell polarity network, its expression and function in lymphocytes remains unknown. Here we investigated whether PALS1 is present in T cells and whether it contributes to T Cell-Receptor (TCR)-mediated activation.
By combining RT-PCR and immunoblot assays, we found that PALS1 is constitutively expressed in human T lymphocytes as well as in Jurkat T cells. siRNA-based knockdown of PALS1 hampered TCR-induced activation and optimal proliferation of lymphocyte. We further provide evidence that PALS1 depletion selectively hindered TCR-driven activation of the transcription factor NF-κB.
The cell polarity protein PALS1 is expressed in T lymphocytes and participates to the optimal activation of NF-κB following TCR stimulation.
[Show abstract][Hide abstract] ABSTRACT: T-cell-receptor (TCR) signalling to NFkappaB requires the assembly of a large multiprotein complex containing the serine/threonine kinase CK1alpha, the scaffold protein CARMA1, the heterodimer BCL10-MALT1 (the CBM complex) and the IkappaB kinase complex (IKK). Although the mechanisms regulating recruitment and activation of IKK within the CBM microenvironment have been extensively studied, there is little understanding of how IKK subsequently binds and phosphorylates IkappaBalpha, the inhibitor of NFkappaB, to promote IkappaBalpha ubiquitylation and proteasomal degradation. Here, we show that BCL10, MALT1 and IKK inducibly associate with IkappaBalpha in a complex that is physically distinct from the early CK1alpha-CBM signalosome. This IkappaBalpha-containing complex probably maturates from the CBM, because siRNA-based knockdown of CARMA1, CK1alpha and BCL10 hampered its assembly, leading to a reduction in NFkappaB activation. By contrast, CK1alpha normally recruited both BCL10 and ubiquitylated species of MALT1 when IkappaBalpha levels were reduced. However, knockdown of IkappaBalpha led to an altered ubiquitylation profile of BCL10-MALT1 combined with a defect in MALT1 reorganisation within large cytoplasmic structures, suggesting that, following stimulation, IkappaBalpha might also participate in MALT1 recycling. Altogether, our data suggest a two-step mechanism to connect active IKK to IkappaBalpha, and further unveil a potential role for IkappaBalpha in resetting TCR-mediated signalling.
[Show abstract][Hide abstract] ABSTRACT: The transcription factor NF-kappaB is required for lymphocyte activation and proliferation as well as the survival of certain lymphoma types. Antigen receptor stimulation assembles an NF-kappaB activating platform containing the scaffold protein CARMA1 (also called CARD11), the adaptor BCL10 and the paracaspase MALT1 (the CBM complex), linked to the inhibitor of NF-kappaB kinase complex, but signal transduction is not fully understood. We conducted parallel screens involving a mass spectrometry analysis of CARMA1 binding partners and an RNA interference screen for growth inhibition of the CBM-dependent 'activated B-cell-like' (ABC) subtype of diffuse large B-cell lymphoma (DLBCL). Here we report that both screens identified casein kinase 1alpha (CK1alpha) as a bifunctional regulator of NF-kappaB. CK1alpha dynamically associates with the CBM complex on T-cell-receptor (TCR) engagement to participate in cytokine production and lymphocyte proliferation. However, CK1alpha kinase activity has a contrasting role by subsequently promoting the phosphorylation and inactivation of CARMA1. CK1alpha has thus a dual 'gating' function which first promotes and then terminates receptor-induced NF-kappaB. ABC DLBCL cells required CK1alpha for constitutive NF-kappaB activity, indicating that CK1alpha functions as a conditionally essential malignancy gene-a member of a new class of potential cancer therapeutic targets.