Georgia Mavria

Conditional ROCK Activation In vivo Induces Tumor Cell Dissemination and Angiogenesis

Progression of tumors to invasive and metastatic forms requires that tumor cells undergo dramatic morphologic changes, a process regulated by Rho GTPases. Elevated expression of RhoA and RhoC, as well as the Rho effector proteins ROCK I and ROCK II, are commonly observed in human cancers and are often associated with more invasive and metastatic phenotypes. To examine how ROCK contributes to the progression of solid tumors, we established a conditionally activated form of ROCK II by fusing the kinase domain to the estrogen receptor hormone-binding domain (ROCK:ER). ROCK:ER-expressing colon carcinoma cells grown as tumors in immunocompromised nude mice organized into discrete clusters surrounding blood vessels. However, ROCK:ER activation resulted in the aggressive dissemination of tumor cells into the surrounding stroma, indicating that increased ROCK signaling is sufficient to promote invasion from solid tumors. In addition, tumors in which ROCK:ER was activated were more highly vascularized, indicating that ROCK contributes to tumor angiogenesis. ROCK:ER activation resulted in changes to epithelial morphology and organization that facilitated motility in vitro, likely by inducing the redistribution of proteins such as ezrin, as well as adherens junction and extracellular matrix-binding proteins. These results suggest that ROCK inhibitors would be useful antimetastatic and antiangiogenic chemotherapeutic agents in tumors associated with elevated RhoA, RhoC, ROCK I, or ROCK II expression.

VE-Cadherin-Mediated Cell-Cell Interaction Suppresses Sprouting via Signaling to MLC2 Phosphorylation

During new blood vessel formation, the cessation of angiogenic sprouting is necessary for the generation of functional vasculature. How sprouting is halted is not known, but it is contemporaneous with the development of stable intercellular junctions [1]. We report that VE-cadherin, which is responsible for endothelial adherens junction organization [2, 3], plays a crucial role in the cessation of sprouting. Abrogating VE-cadherin function in an organotypic angiogenesis assay and in zebrafish embryos stimulates sprouting. We show that VE-cadherin signals to Rho-kinase-dependent myosin light-chain 2 phosphorylation, leading to actomyosin contractility [4], which regulates the distribution of VE-cadherin at cell-cell junctions. VE-cadherin antagonizes VEGFR2 signaling, and consequently, inhibition of VE-cadherin, Rho-kinase, or actomyosin contractility leads to VEGF-driven, Rac1-dependent sprouting. These findings suggest a novel mechanism by which cell-cell adhesion suppresses Rac1-dependent migration and sprouting by increasing actomyosin contractility at cell junctions.

RhoJ/TCL Regulates Endothelial Motility and Tube Formation and Modulates Actomyosin Contractility and Focal Adhesion Numbers

Objective—RhoJ/TCL was identified by our group as an endothelial-expressed Rho GTPase. The aim of this study was to determine its tissue distribution, subcellular localization, and function in endothelial migration and tube formation. Methods and Results—Using in situ hybridization, RhoJ was localized to endothelial cells in a set of normal and cancerous tissues and in the vasculature of mouse embryos; endogenous RhoJ was localized to focal adhesions by immunofluorescence. The proangiogenic factor vascular endothelial growth factor activated RhoJ in endothelial cells. Using either small interfering (si)RNA-mediated knockdown of RhoJ expression or overexpression of constitutively active RhoJ (daRhoJ), RhoJ was found to positively regulate endothelial motility and tubule formation. Downregulating RhoJ expression increased focal adhesions and stress fibers in migrating cells, whereas daRhoJ overexpression resulted in the converse. RhoJ downregulation resulted in increased contraction of a collagen gel and increased phospho–myosin light chain, indicative of increased actomyosin contractility. Pharmacological inhibition of Rho-kinase (which phosphorylates myosin light chain) or nonmuscle myosin II reversed the defective tube formation and migration of RhoJ knockdown cells. Conclusion—RhoJ is endothelial-expressed in vivo, activated by vascular endothelial growth factor, localizes to focal adhesions, regulates endothelial cell migration and tube formation, and modulates actomyosin contractility and focal adhesion numbers.

Uses of the in vitro endothelial-fibroblast organotypic co-culture assay in angiogenesis research

Angiogenesis is a complex process that involves multiple cellular events. In addition to receiving inputs from a range of stimulatory and inhibitory factors, endothelial cells undergoing angiogenesis make multiple interactions with the extracellular matrix and with other cell types in the stroma. Recreating angiogenesis in vitro is probably an impossible goal; however, a number of assays have been developed that recapitulate many of the key events of the process. These assays are indispensible tools for investigating the signalling pathways that control the formation of new blood vessels. In the present paper, we review the organotypic co-culture assay of angiogenesis - until recently, a comparatively underemployed assay, but one with a number of powerful advantages for angiogenesis research. We give a set of optimized protocols for its use, including protocols for siRNA (small interfering RNA)-based screens, and we discuss appropriate methods for obtaining quantitative data from the assay.

Generation of a high titre retroviral vector for endothelial cell-specific gene expression in vivo.

Tumour growth is dependent upon a blood supply and is associated with the switch to the angiogenic phenotype. We are developing strategies for targeting gene expression to endothelial cells in the tumour vasculature. Recombinant retroviruses have been generated that incorporate regulatory sequences of the prepro-endothelin-1 (ppET1) promoter. Following reverse transcription and integration these modifications are duplicated in the proviral 5′ LTR for transcription of the internal β-galactosidase reporter gene. The titres and endothelial specificity of retroviral vectors harbouring different modifications have been analysed. In the optimal strategy, replacing the MLV enhancer with ppET1 promoter sequences containing the GATA and AP1 elements whilst maintaining sequences from the viral promoter resulted in endothelial cell-specific expression of the reporter gene, and viral titres comparable to those of the unmodified vector. A panel of endothelial and non-endothelial cells infected with the modified virus from a high titre producer clone showed a pattern of expression consistent with the activity of the endogenous ppET1 promoter. The modified LTR retained specificity in vivo, in subcutaneous tumours arising from the co-injection of tumour cells and irradiated virus producer cells. This simple model achieves high efficiency of transduction and can be used routinely for the screening of targeted retroviral vectors.

A Rac/Cdc42 exchange factor complex promotes formation of lateral filopodia and blood vessel lumen morphogenesis

During angiogenesis, Rho-GTPases influence endothelial cell migration and cell–cell adhesion; however it is not known whether they control formation of vessel lumens, which are essential for blood flow. Here, using an organotypic system that recapitulates distinct stages of VEGF-dependent angiogenesis, we show that lumen formation requires early cytoskeletal remodelling and lateral cell–cell contacts, mediated through the RAC1 guanine nucleotide exchange factor (GEF) DOCK4 (dedicator of cytokinesis 4). DOCK4 signalling is necessary for lateral filopodial protrusions and tubule remodelling prior to lumen formation, whereas proximal, tip filopodia persist in the absence of DOCK4. VEGF-dependent Rac activation via DOCK4 is necessary for CDC42 activation to signal filopodia formation and depends on the activation of RHOG through the RHOG GEF, SGEF. VEGF promotes interaction of DOCK4 with the CDC42 GEF DOCK9. These studies identify a novel Rho-family GTPase activation cascade for the formation of endothelial cell filopodial protrusions necessary for tubule remodelling, thereby influencing subsequent stages of lumen morphogenesis.

In vivo efficacy of HSV-TK transcriptionally targeted to the tumour vasculature is augmented by combination with cytotoxic chemotherapy

Retroviral vectors are suitable for targeting endothelial cells in the tumour neovasculature because of their intrinsic selectivity for proliferating cells. Previously, we inserted regulatory elements of the endothelial‐specific prepro‐endothelin‐1 (ppET1) promoter in retroviral vectors to generate high‐titre, replication‐defective recombinant retroviruses that restricted gene expression to the vascular compartment of tumours.

Reduced growth in response to ganciclovir treatment of subcutaneous xenografts expressing HSV-tk in the vascular compartment

Using a recombinant retrovirus with ecotropic envelope we have achieved high efficiency of transduction of endothelial cells in the vasculature of subcutaneous xenografts arising from the co-injection of tumour cells and irradiated virus producers. We have used this experimental system to assess the efficacy of the herpes simplex virus-thymidine kinase (HSV-tk)/ganciclovir (GCV) prodrug activation system in anti-vascular therapy. Treatment of KSY-1 xenografts with HSV-tk transduction in the vascular compartment with the S-phase-dependent drug GCV resulted in extensive haemorrhagic necrosis, indicative of vascular damage. Therapeutic potential in tumours with transduced endothelial cells comprising 5% or less of the total tumour mass was similar to that of tumours with HSV-tk expression in over 46% of tumour cells. GCV treatment of animals bearing MDA-MB-361 breast carcinoma, SW620 and CACO2 colon carcinomas with HSV-tk expression in the vascular compartment also resulted in reduced tumour growth. We conclude that HSV-tk/GCV prodrug activation is an effective strategy for eradicating tumour vasculature, and that direct targeting of proliferating endothelial cells in established vasculature results in reduced tumour growth. The therapeutic potential observed with the slow-growing CACO2 colon and MDA-MB-361 breast carcinomas supports the notion that anti-vascular therapy targeted at proliferating endothelium is likely to prove efficacious in human cancers that generally grow at a lower rate than experimental tumours.

Characterization and structural determination of a new anti-MET function-blocking antibody with binding epitope distinct from the ligand binding domain

The growth and motility factor Hepatocyte Growth Factor/Scatter Factor (HGF/SF) and its receptor, the product of the MET proto-oncogene, promote invasion and metastasis of tumor cells and have been considered potential targets for cancer therapy. We generated a new Met-blocking antibody which binds outside the ligand-binding site, and determined the crystal structure of the Fab in complex with its target, which identifies the binding site as the Met Ig1 domain. The antibody, 107_A07, inhibited HGF/SF-induced cell migration and proliferation in vitro and inhibited growth of tumor xenografts in vivo. In biochemical assays, 107_A07 competes with both HGF/SF and its truncated splice variant NK1 for MET binding, despite the location of the antibody epitope on a domain (Ig1) not reported to bind NK1 or HGF/SF. Overlay of the Fab-MET crystal structure with the InternalinB-MET crystal structure shows that the 107_A07 Fab comes into close proximity with the HGF/SF-binding SEMA domain when MET is in the “compact”, InternalinB-bound conformation, but not when MET is in the “open” conformation. These findings provide further support for the importance of the “compact” conformation of the MET extracellular domain, and the relevance of this conformation to HGF/SF binding and signaling.

Scleroderma fibroblasts suppress angiogenesis via TGF-β/caveolin-1 dependent secretion of pigment epithelium-derived factor

Objectives Systemic sclerosis (SSc) is characterised by tissue fibrosis and vasculopathy with defective angiogenesis. Transforming growth factor beta (TGF-β) plays a major role in tissue fibrosis, including downregulation of caveolin-1 (Cav-1); however, its role in defective angiogenesis is less clear. Pigment epithelium-derived factor (PEDF), a major antiangiogenic factor, is abundantly secreted by SSc fibroblasts. Here, we investigated the effect of TGF-β and Cav-1 on PEDF expression and the role of PEDF in the ability of SSc fibroblasts to modulate angiogenesis. Methods PEDF and Cav-1 expression in fibroblasts and endothelial cells were evaluated by means of immunohistochemistry on human and mouse skin biopsies. PEDF and Cav-1 were silenced in cultured SSc and control fibroblasts using lentiviral short-hairpin RNAs. Organotypic fibroblast–endothelial cell co-cultures and matrigel assays were employed to assess angiogenesis. Results PEDF is highly expressed in myofibroblasts and reticular fibroblasts with low Cav-1 expression in SSc skin biopsies, and it is induced by TGF-β in vitro. SSc fibroblasts suppress angiogenesis in an organotypic model. This model is reproduced by silencing Cav-1 in normal dermal fibroblasts. Conversely, silencing PEDF in SSc fibroblasts rescues their antiangiogenic phenotype. Consistently, transgenic mice with TGF-β receptor hyperactivation show lower Cav-1 and higher PEDF expression levels in skin biopsies accompanied by reduced blood vessel density. Conclusions Our data reveal a new pathway by which TGF-β suppresses angiogenesis in SSc, through decreased fibroblast Cav-1 expression and subsequent PEDF secretion. This pathway may present a promising target for new therapeutic interventions in SSc.