Radiation therapy attenuates lymphatic vessel repair by reducing VEGFR-3 signalling

Introduction: Surgery and radiotherapy are key cancer treatments and the leading causes of damage to the lymphatics, a vascular network critical to fluid homeostasis and immunity. The clinical manifestation of this damage constitutes a devastating side-effect of cancer treatment, known as lymphoedema. Lymphoedema is a chronic condition evolving from the accumulation of interstitial fluid due to impaired drainage via the lymphatics and is recognised to contribute significant morbidity to patients who survive their cancer. Nevertheless, the molecular mechanisms underlying the damage inflicted on lymphatic vessels, and particularly the lymphatic endothelial cells (LEC) that constitute them, by these treatment modalities, remain poorly understood. Methods: We used a combination of cell based assays, biochemistry and animal models of lymphatic injury to examine the molecular mechanisms behind LEC injury and the subsequent effects on lymphatic vessels, particularly the role of the VEGF-C/VEGF-D/VEGFR-3 lymphangiogenic signalling pathway, in lymphatic injury underpinning the development of lymphoedema. Results: We demonstrate that radiotherapy selectively impairs key LEC functions needed for new lymphatic vessel growth (lymphangiogenesis). This effect is mediated by attenuation of VEGFR-3 signalling and downstream signalling cascades. VEGFR-3 protein levels were downregulated in LEC that were exposed to radiation, and LEC were therefore selectively less responsive to VEGF-C and VEGF-D. These findings were validated in our animal models of radiation and surgical injury. Discussion: Our data provide mechanistic insight into injury sustained by LEC and lymphatics during surgical and radiotherapy cancer treatments and underscore the need for alternative non-VEGF-C/VEGFR-3-based therapies to treat lymphoedema.

Key signaling networks are dysregulated in patients with the adipose tissue disorder, lipedema

Objectives

Lipedema, a poorly understood chronic disease of adipose hyper-deposition, is often mistaken for obesity and causes significant impairment to mobility and quality-of-life. To identify molecular mechanisms underpinning lipedema, we employed comprehensive omics-based comparative analyses of whole tissue, adipocyte precursors (adipose-derived stem cells (ADSCs)), and adipocytes from patients with or without lipedema.

Methods

We compared whole-tissues, ADSCs, and adipocytes from body mass index–matched lipedema (n = 14) and unaffected (n = 10) patients using comprehensive global lipidomic and metabolomic analyses, transcriptional profiling, and functional assays.

Results

Transcriptional profiling revealed >4400 significant differences in lipedema tissue, with altered levels of mRNAs involved in critical signaling and cell function-regulating pathways (e.g., lipid metabolism and cell-cycle/proliferation). Functional assays showed accelerated ADSC proliferation and differentiation in lipedema. Profiling lipedema adipocytes revealed >900 changes in lipid composition and >600 differentially altered metabolites. Transcriptional profiling of lipedema ADSCs and non-lipedema ADSCs revealed significant differential expression of >3400 genes including some involved in extracellular matrix and cell-cycle/proliferation signaling pathways. One upregulated gene in lipedema ADSCs, Bub1, encodes a cell-cycle regulator, central to the kinetochore complex, which regulates several histone proteins involved in cell proliferation. Downstream signaling analysis of lipedema ADSCs demonstrated enhanced activation of histone H2A, a key cell proliferation driver and Bub1 target. Critically, hyperproliferation exhibited by lipedema ADSCs was inhibited by the small molecule Bub1 inhibitor 2OH-BNPP1 and by CRISPR/Cas9-mediated Bub1 gene depletion.

Conclusion

We found significant differences in gene expression, and lipid and metabolite profiles, in tissue, ADSCs, and adipocytes from lipedema patients compared to non-affected controls. Functional assays demonstrated that dysregulated Bub1 signaling drives increased proliferation of lipedema ADSCs, suggesting a potential mechanism for enhanced adipogenesis in lipedema. Importantly, our characterization of signaling networks driving lipedema identifies potential molecular targets, including Bub1, for novel lipedema therapeutics.

Three-dimensional CRISPR screening reveals epigenetic interaction with anti-angiogenic therapy

Angiogenesis underlies development, physiology and pathogenesis of cancer, eye and cardiovascular diseases. Inhibiting aberrant angiogenesis using anti-angiogenic therapy (AAT) has been successful in the clinical treatment of cancer and eye diseases. However, resistance to AAT inevitably occurs and its molecular basis remains poorly understood. Here, we uncover molecular modifiers of the blood endothelial cell (EC) response to a widely used AAT bevacizumab by performing a pooled genetic screen using three-dimensional microcarrier-based cell culture and CRISPR–Cas9. Functional inhibition of the epigenetic reader BET family of proteins BRD2/3/4 shows unexpected mitigating effects on EC survival and/or proliferation upon VEGFA blockade. Moreover, transcriptomic and pathway analyses reveal an interaction between epigenetic regulation and anti-angiogenesis, which may affect chromosomal structure and activity in ECs via the cell cycle regulator CDC25B phosphatase. Collectively, our findings provide insight into epigenetic regulation of the EC response to VEGFA blockade and may facilitate development of quality biomarkers and strategies for overcoming resistance to AAT.

Through three-dimensional CRISPR screening, He et al. report that functional inhibition of BET family of proteins BRD2/3/4 shows mitigating effects on blood endothelial cell (EC) survival and/or proliferation upon VEGFA blockade. An interaction between epigenetic regulation and anti-angiogenesis, which may affect chromosomal structure and activity in ECs through CDC25B phosphatase, is potentially involved with EC resistance to anti-angiogenic therapy.

Control of Gene Expression by Exosome-Derived Non-Coding RNAs in Cancer Angiogenesis and Lymphangiogenesis

Abstract: Tumour angiogenesis and lymphangiogenesis are hallmarks of cancer and have been associated with tumour progression, tumour metastasis and poor patient prognosis. Many factors regulate angiogenesis and lymphangiogenesis in cancer including non-coding RNAs which are a category of RNAs that do not encode proteins and have important regulatory functions at transcriptional and post-transcriptional levels. Non-coding RNAs can be encapsulated in extracellular vesicles called exosomes which are secreted by tumour cells or other cells in the tumour microenvironment and can then be taken up by the endothelial cells of blood vessels and lymphatic vessels. The "delivery" of these non-coding RNAs to endothelial cells in tumours can facilitate tumour angiogenesis and lymphangiogenesis. Here we review recent findings about exosomal non-coding RNAs, specifically microRNAs and long non-coding RNAs, which regulate tumour angiogenesis and lymphangiogenesis in cancer. We then focus on the potential use of these molecules as cancer biomarkers and opportunities for exploiting ncRNAs for the treatment of cancer.

Evolutionary Differences in the Vegf/Vegfr Code Reveal Organotypic Roles for the Endothelial Cell Receptor Kdr in Developmental Lymphangiogenesis

Lymphatic vascular development establishes embryonic and adult tissue fluid balance and is integral in disease. In diverse vertebrate organs, lymphatic vessels display organotypic function and develop in an organ-specific manner. In all settings, developmental lymphangiogenesis is considered driven by vascular endothelial growth factor (VEGF) receptor-3 (VEGFR3), whereas a role for VEGFR2 remains to be fully explored. Here, we define the zebrafish Vegf/Vegfr code in receptor binding studies. We find that while Vegfd directs craniofacial lymphangiogenesis, it binds Kdr (a VEGFR2 homolog) but surprisingly, unlike in mammals, does not bind Flt4 (VEGFR3). Epistatic analyses and characterization of a kdr mutant confirm receptor-binding analyses, demonstrating that Kdr is indispensible for rostral craniofacial lymphangiogenesis, but not caudal trunk lymphangiogenesis, in which Flt4 is central. We further demonstrate an unexpected yet essential role for Kdr in inducing lymphatic endothelial cell fate. This work reveals evolutionary divergence in the Vegf/Vegfr code that uncovers spatially restricted mechanisms of developmental lymphangiogenesis.

CCL27/CCL28-CCR10 Chemokine Signaling Mediates Migration of Lymphatic Endothelial Cells

Metastasis via the lymphatic vasculature is an important step in cancer progression. The formation of new lymphatic vessels (lymphangiogenesis), or remodeling of existing lymphatics, is thought to facilitate the entry and transport of tumor cells into lymphatic vessels and on to distant organs. The migration of lymphatic endothelial cells (LEC) toward guidance cues is critical for lymphangiogenesis. While chemokines are known to provide directional navigation for migrating immune cells, their role in mediating LEC migration during tumor-associated lymphangiogenesis is not well defined. Here, we undertook gene profiling studies to identify chemokine-chemokine receptor pairs that are involved in tumor lymphangiogenesis associated with lymph node metastasis. CCL27 and CCL28 were expressed in tumor cells with metastatic potential, while their cognate receptor, CCR10, was expressed by LECs and upregulated by the lymphangiogenic growth factor VEGFD and the proinflammatory cytokine TNFα. Migration assays demonstrated that LECs are attracted to both CCL27 and CCL28 in a CCR10-dependent manner, while abnormal lymphatic vessel patterning in CCR10-deficient mice confirmed the significant role of CCR10 in lymphatic patterning. In vivo analyses showed that LECs are recruited to a CCL27 or CCL28 source, while VEGFD was required in combination with these chemokines to enable formation of coherent lymphatic vessels. Moreover, tumor xenograft experiments demonstrated that even though CCL27 expression by tumors enhanced LEC recruitment, the ability to metastasize was dependent on the expression of VEGFD. These studies demonstrate that CCL27 and CCL28 signaling through CCR10 may cooperate with inflammatory mediators and VEGFD during tumor lymphangiogenesis. SIGNIFICANCE: The study shows that the remodeling of lymphatic vessels in cancer is influenced by CCL27 and CCL28 chemokines, which may provide a future target to modulate metastatic spread.

Modulation of Immunity by Lymphatic Dysfunction in Lymphedema

The debilitating condition known as secondary lymphedema frequently occurs after lymphadenectomy and/or radiotherapy for the treatment of cancer. These therapies can damage lymphatic vessels leading to edema, fibrosis, inflammation and dysregulated adipogenesis, which result in profound swelling of an affected limb. Importantly, lymphedema patients often exhibit impaired immune function which predisposes them to a variety of infections. It is known that lymphadenectomy can compromise the acquisition of adaptive immune responses and antibody production; however the cellular mechanisms involved are poorly understood. Here we discuss recent progress in revealing the cellular and molecular mechanisms underlying poor immune function in secondary lymphedema, which has indicated a key role for regulatory T cells in immunosuppression in this disease. Furthermore, the interaction of CD4⁺ T cells and macrophages has been shown to play a role in driving proliferation of lymphatic endothelial cells and aberrant lymphangiogenesis, which contribute to interstitial fluid accumulation in lymphedema. These new insights into the interplay between lymphatic vessels and the immune system in lymphedema will likely provide opportunities for novel therapeutic approaches designed to improve clinical outcomes in this problematic disease.

Abstract 1: A kinome-wide CRISPR screen reveals BET inhibition-associated endothelial cell resistance to anti-angiogenic therapy

Angiogenesis inhibition is a useful strategy for treating cancer. However, the efficacy of anti-angiogenic therapy (AAT) in clinical oncology has been limited largely by highly variable patient response and the inevitable occurrence of resistance. The modest patient benefits have underscored a pressing need for qualified biomarkers and better knowledge of resistance mechanisms. Blood endothelial cells (ECs) are one of the main targets of AAT in the tumor microenvironment. Hence, understanding the EC response to AAT will provide insight into tumor response. To identify molecular modifiers of the EC response to AAT (in this study, bevacizumab, a humanized neutralizing anti-VEGF-A monoclonal antibody), we developed a high-throughput genetic screening platform. This involved a three-dimensional microcarrier-based culture system, CRISPR-Cas9-driven gene loss-of-function (LOF) and VEGF-A-dependent serum-free culture conditions for response modifiers. A pooled kinome-wide CRISPR-Cas9-based screen of 763 genes (with four single guide RNAs/sgRNAs targeting each gene) identified 18 candidate genes that upon LOF (represented by sgRNAs) were significantly enriched or depleted in the bevacizumab versus control treatment arm (P ≤ 0.005, FDR ≤ 0.3). Further candidate evaluation using siRNAs validated six genes whose knockdown conferred EC resistance or sensitization to bevacizumab (P < 0.05). Of these, knockdown of BRD2 or BRD3, which encode the epigenetic reader bromodomain-containing protein 2 or 3 (BRD2 or BRD3), respectively, conferred EC resistance to bevacizumab. The bromodomain and extraterminal domain (BET) inhibitors JQ1 and I-BET762, which selectively target the BET family of proteins (BRD2, BRD3, BRD4 and BRDT), reproduced the results of BRD2 or BRD3 LOF, with a more prominent phenotype (P < 0.05). Drug dose-response assessment indicated an anti-angiogenic effect of BET inhibitors regardless of the presence of bevacizumab. This inhibitory effect was unexpectedly attenuated when cells were co-treated with bevacizumab under VEGF-A-dependent conditions. Experiments to investigate the mechanistic basis for this phenotype of resistance are ongoing and include differential gene expression analysis using RNA-Seq and in vivo evaluation. Applying a non-biased approach to identify molecular modifiers of the EC response to AAT, we demonstrate in this study that BET inhibition is unexpectedly associated with resistance to bevacizumab, despite BET inhibition alone having an anti-angiogenic effect. These observations prompt further evaluation of epigenetic regulation in tumor angiogenesis, particularly in the context of interaction between BET inhibition and VEGF blockade. Clinically, these findings may facilitate development of potential predictive and/or response biomarkers and strategies to overcome resistance to AAT and/or BET inhibitors.

Citation Format: Michael Y. He, Michael M. Halford, Marc G. Achen, Steven A. Stacker. A kinome-wide CRISPR screen reveals BET inhibition-associated endothelial cell resistance to anti-angiogenic therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1.

Exit Stage Left: A Tumor Cell's Journey from Lymph Node to Beyond

Even though we have known for over 250 years that cancers spread to regional lymph nodes (LNs) and distant organs, the fundamental question of which anatomical routes are taken by tumor cells has remained a mystery. Two recently published papers in Science, by Pereira et al. and Brown et al., directly address this important issue in tumor biology by assessing the capacity of tumor cells in LNs to spread to distant sites.

The fibrinolysis inhibitor α2-antiplasmin restricts lymphatic remodelling and metastasis in a mouse model of cancer

Remodelling of lymphatic vessels in tumours facilitates metastasis to lymph nodes. The growth factors VEGF-C and VEGF-D are well known inducers of lymphatic remodelling and metastasis in cancer. They are initially produced as full-length proteins requiring proteolytic processing in order to bind VEGF receptors with high affinity and thereby promote lymphatic remodelling. The fibrinolytic protease plasmin promotes processing of VEGF-C and VEGF-D in vitro, but its role in processing them in cancer was unknown. Here we explore plasmin's role in proteolytically activating VEGF-D in vivo, and promoting lymphatic remodelling and metastasis in cancer, by co-expressing the plasmin inhibitor α₂-antiplasmin with VEGF-D in a mouse tumour model. We show that α₂-antiplasmin restricts activation of VEGF-D, enlargement of intra-tumoural lymphatics and occurrence of lymph node metastasis. Our findings indicate that the fibrinolytic system influences lymphatic remodelling in tumours which is consistent with previous clinicopathological observations correlating fibrinolytic components with cancer metastasis.

Systematic high-content genome-wide RNAi screens of endothelial cell migration and morphology

Many cell types undergo migration during embryogenesis and disease. Endothelial cells line blood vessels and lymphatics, which migrate during development as part of angiogenesis, lymphangiogenesis and other types of vessel remodelling. These processes are also important in wound healing, cancer metastasis and cardiovascular conditions. However, the molecular control of endothelial cell migration is poorly understood. Here, we present a dataset containing siRNA screens that identify known and novel components of signalling pathways regulating migration of lymphatic endothelial cells. These components are compared to signalling in blood vascular endothelial cells. Further, using high-content microscopy, we captured a dataset of images of migrating cells following transfection with a genome-wide siRNA library. These datasets are suitable for the identification and analysis of genes involved in endothelial cell migration and morphology, and for computational approaches to identify signalling networks controlling the migratory response and integration of cell morphology, gene function and cell signaling. This may facilitate identification of protein targets for therapeutically modulating angiogenesis and lymphangiogenesis in the context of human disease.

Vegfd modulates both angiogenesis and lymphangiogenesis during zebrafish embryonic development

Vascular endothelial growth factors (VEGFs) control angiogenesis and lymphangiogenesis during development and in pathological conditions. In the zebrafish trunk, Vegfa controls the formation of intersegmental arteries by primary angiogenesis and Vegfc is essential for secondary angiogenesis, giving rise to veins and lymphatics. Vegfd has been largely thought of as dispensable for vascular development in vertebrates. Here, we generated a zebrafish vegfd mutant by genome editing. vegfd mutants display significant defects in facial lymphangiogenesis independent of vegfc function. Strikingly, we find that vegfc and vegfd cooperatively control lymphangiogenesis throughout the embryo, including during the formation of the trunk lymphatic vasculature. Interestingly, we find that vegfd and vegfc also redundantly drive artery hyperbranching phenotypes observed upon depletion of Flt1 or Dll4. Epistasis and biochemical binding assays suggest that, during primary angiogenesis, Vegfd influences these phenotypes through Kdr (Vegfr2) rather than Flt4 (Vegfr3). These data demonstrate that, rather than being dispensable during development, Vegfd plays context-specific indispensable and also compensatory roles during both blood vessel angiogenesis and lymphangiogenesis.

The Role of the Tumor Vasculature in the Host Immune Response: Implications for Therapeutic Strategies Targeting the Tumor Microenvironment

Recently developed cancer immunotherapy approaches including immune checkpoint inhibitors and chimeric antigen receptor T cell transfer are showing promising results both in trials and in clinical practice. These approaches reflect increasing recognition of the crucial role of the tumor microenvironment in cancer development and progression. Cancer cells do not act alone, but develop a complex relationship with the environment in which they reside. The host immune response to tumors is critical to the success of immunotherapy; however, the determinants of this response are incompletely understood. The immune cell infiltrate in tumors varies widely in density, composition, and clinical significance. The tumor vasculature is a key component of the microenvironment that can influence tumor behavior and treatment response and can be targeted through the use of antiangiogenic drugs. Blood vascular and lymphatic endothelial cells have important roles in the trafficking of immune cells, controlling the microenvironment, and modulating the immune response. Improving access to the tumor through vascular alteration with antiangiogenic drugs may prove an effective combinatorial strategy with immunotherapy approaches and might be applicable to many tumor types. In this review, we briefly discuss the host's immune response to cancer and the treatment strategies utilizing this response, before focusing on the pathological features of tumor blood and lymphatic vessels and the contribution these might make to tumor immune evasion.

Differential Receptor Binding and Regulatory Mechanisms for the Lymphangiogenic Growth Factors Vascular Endothelial Growth Factor (VEGF)-C and -D

VEGF-C and VEGF-D are secreted glycoproteins that induce angiogenesis and lymphangiogenesis in cancer, thereby promoting tumor growth and spread. They exhibit structural homology and activate VEGFR-2 and VEGFR-3, receptors on endothelial cells that signal for growth of blood vessels and lymphatics. VEGF-C and VEGF-D were thought to exhibit similar bioactivities, yet recent studies indicated distinct signaling mechanisms (e.g. tumor-derived VEGF-C promoted expression of the prostaglandin biosynthetic enzyme COX-2 in lymphatics, a response thought to facilitate metastasis via the lymphatic vasculature, whereas VEGF-D did not). Here we explore the basis of the distinct bioactivities of VEGF-D using a neutralizing antibody, peptide mapping, and mutagenesis to demonstrate that the N-terminal α-helix of mature VEGF-D (Phe⁹³–Arg¹⁰⁸) is critical for binding VEGFR-2 and VEGFR-3. Importantly, the N-terminal part of this α-helix, from Phe⁹³ to Thr⁹⁸, is required for binding VEGFR-3 but not VEGFR-2. Surprisingly, the corresponding part of the α-helix in mature VEGF-C did not influence binding to either VEGFR-2 or VEGFR-3, indicating distinct determinants of receptor binding by these growth factors. A variant of mature VEGF-D harboring a mutation in the N-terminal α-helix, D103A, exhibited enhanced potency for activating VEGFR-3, was able to promote increased COX-2 mRNA levels in lymphatic endothelial cells, and had enhanced capacity to induce lymphatic sprouting in vivo. This mutant may be useful for developing protein-based therapeutics to drive lymphangiogenesis in clinical settings, such as lymphedema. Our studies shed light on the VEGF-D structure/function relationship and provide a basis for understanding functional differences compared with VEGF-C.

VEGF-D promotes pulmonary oedema in hyperoxic acute lung injury

Leakage of fluid from blood vessels, leading to oedema, is a key feature of many diseases including hyperoxic acute lung injury (HALI), which can occur when patients are ventilated with high concentrations of oxygen (hyperoxia). The molecular mechanisms driving vascular leak and oedema in HALI are poorly understood. VEGF‐D is a protein that promotes blood vessel leak and oedema when overexpressed in tissues, but the role of endogenous VEGF‐D in pathological oedema was unknown. To address these issues, we exposed Vegfd‐deficient mice to hyperoxia. The resulting pulmonary oedema in Vegfd‐deficient mice was substantially reduced compared to wild‐type, as was the protein content of bronchoalveolar lavage fluid, consistent with reduced vascular leak. Vegf‐d and its receptor Vegfr‐3 were more highly expressed in lungs of hyperoxic, versus normoxic, wild‐type mice, indicating that components of the Vegf‐d signalling pathway are up‐regulated in hyperoxia. Importantly, VEGF‐D and its receptors were co‐localized on blood vessels in clinical samples of human lungs exposed to hyperoxia; hence, VEGF‐D may act directly on blood vessels to promote fluid leak. Our studies show that Vegf‐d promotes oedema in response to hyperoxia in mice and support the hypothesis that VEGF‐D signalling promotes vascular leak in human HALI. © 2016 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

A Simple Bioassay for the Evaluation of Vascular Endothelial Growth Factors

The analysis of receptor tyrosine kinases and their interacting ligands involved in vascular biology is often challenging due to the constitutive expression of families of related receptors, a broad range of related ligands and the difficulty of dealing with primary cultures of specialized endothelial cells. Here we describe a bioassay for the detection of ligands to the vascular endothelial growth factor receptor-2 (VEGFR-2), a key transducer of signals that promote angiogenesis and lymphangiogenesis. A cDNA encoding a fusion of the extracellular (ligand-binding) region of VEGFR-2 with the transmembrane and cytoplasmic regions of the erythropoietin receptor (EpoR) is expressed in the factor-dependent cell line Ba/F3. This cell line grows in the presence of interleukin-3 (IL-3) and withdrawal of this factor results in death of the cells within 24 hr. Expression of the VEGFR-2/EpoR receptor fusion provides an alternative mechanism to promote survival and potentially proliferation of stably transfected Ba/F3 cells in the presence of a ligand capable of binding and cross-linking the extracellular portion of the fusion protein (i.e., one that can cross-link the VEGFR-2 extracellular region). The assay can be performed in two ways: a semi-quantitative approach in which small volumes of ligand and cells permit a rapid result in 24 hr, and a quantitative approach involving surrogate markers of a viable cell number. The assay is relatively easy to perform, is highly responsive to known VEGFR-2 ligands and can accommodate extracellular inhibitors of VEGFR-2 signaling such as monoclonal antibodies to the receptor or ligands, and soluble ligand traps.

The proteolytic activation of angiogenic and lymphangiogenic growth factors in cancer--its potential relevance for therapeutics and diagnostics

The growth of blood vessels (angiogenesis) and lymphatic vessels (lymphangiogenesis) in and around solid tumors is central to the growth and metastatic spread of cancer. The therapeutic targeting of angiogenesis has become an established modality of cancer treatment, however, more comprehensive targeting of angiogenic signaling pathways may be required to enhance the clinical benefits of this approach. Angiogenesis and lymphangiogenesis are driven, or modulated, by a range of secreted glycoproteins including vascular endothelial growth factors, platelet-derived growth factors, and transforming growth factor-β. These key regulatory growth factors are subject to proteolytic activation, involving highly specific cleavage events, which can occur at the cell surface or in the extracellular milieu. These cleavage events are catalysed by a variety of enzymes including proprotein convertases. This proteolysis can regulate the activity of these growth factors by enhancing binding affinities for cell surface receptors and co-receptors, or by altering their interactions with heparan sulfate proteoglycans, thereby modulating bioavailability. The proteolytic processing of these growth factors complicates strategies for targeting them for diagnostic and/or therapeutic purposes in cancer, as processing can generate various forms with distinct biological properties. Hence it has been important to determine which forms are biologically active for promoting angiogenesis and lymphangiogenesis in cancer, so as to indicate clinical relevance. Here we review the regulation of tumor angiogenesis and lymphangiogenesis by proteolytic activation of growth factors, and the potential therapeutic and diagnostic strategies arising from our understanding of this process.

Intestinal-specific activatable Myb initiates colon tumorigenesis in mice

Transcription factor Myb is overexpressed in most colorectal cancers (CRC). Patients with CRC expressing the highest Myb are more likely to relapse. We previously showed that mono-allelic loss of Myb in an Adenomatous polyposis coli (APC)-driven CRC mouse model (Apc^Min/+) significantly improves survival. Here we directly investigated the association of Myb with poor prognosis and how Myb co-operates with tumor suppressor genes (TSGs) (Apc) and cell cycle regulator, p27. Here we generated the first intestinal-specific, inducible transgenic model; a MybER transgene encoding a tamoxifen-inducible fusion protein between Myb and the estrogen receptor-α ligand-binding domain driven by the intestinal-specific promoter, Gpa33. This was to mimic human CRC with constitutive Myb activity in a highly tractable mouse model. We confirmed that the transgene was faithfully expressed and inducible in intestinal stem cells (ISCs) before embarking on carcinogenesis studies. Activation of the MybER did not change colon homeostasis unless one p27 allele was lost. We then established that MybER activation during CRC initiation using a pro-carcinogen treatment, azoxymethane (AOM), augmented most measured aspects of ISC gene expression and function and accelerated tumorigenesis in mice. CRC-associated symptoms of patients including intestinal bleeding and anaemia were faithfully mimicked in AOM-treated MybER transgenic mice and implicated hypoxia and vessel leakage identifying an additional pathogenic role for Myb. Collectively, the results suggest that Myb expands the ISC pool within which CRC is initiated while co-operating with TSG loss. Myb further exacerbates CRC pathology partly explaining why high MYB is a predictor of worse patient outcome.

A phase Ib/II translational study of sunitinib with neoadjuvant radiotherapy in soft-tissue sarcoma

Background:

Preoperative radiotherapy (RT) is commonly used to treat localised soft-tissue sarcomas (STS). Hypoxia is an important determinant of radioresistance. Whether antiangiogenic therapy can ‘normalise' tumour vasculature, thereby improving oxygenation, remains unknown.

Methods:

Two cohorts were prospectively enrolled. Cohort A evaluated the implications of hypoxia in STS, using the hypoxic tracer ¹⁸F-azomycin arabinoside (FAZA-PET). In cohort B, sunitinib was added to preoperative RT in a dose-finding phase 1b/2 design.

Results:

In cohort A, 13 out of 23 tumours were hypoxic (FAZA-PET), correlating with metabolic activity (r²=0.85; P<0.001). Two-year progression-free (PFS) and overall (OS) survival were 61% (95% CI: 0.44–0.84) and 87% (95% CI: 0.74–1.00), respectively. Hypoxia was associated with radioresistance (P=0.012), higher local recurrence (Hazard ratio (HR): 10.2; P=0.02), PFS (HR: 8.4; P=0.02), and OS (HR: 41.4; P<0.04). In Cohort B, seven patients received sunitinib at dose level (DL): 0 (50 mg per day for 2 weeks before RT; 25 mg per day during RT) and two patients received DL: −1 (37.5 mg per day for entire period). Dose-limiting toxicities were observed in 4 out of 7 patients at DL 0 and 2 out of 2 patients at DL −1, resulting in premature study closure. Although there was no difference in PFS or OS, patients receiving sunitinib had higher local failure (HR: 8.1; P=0.004).

Conclusion:

In STS, hypoxia is associated with adverse outcomes. The combination of sunitinib with preoperative RT resulted in unacceptable toxicities, and higher local relapse rates.

Lymphangiogenesis and lymphatic vessel remodelling in cancer

The generation of new lymphatic vessels through lymphangiogenesis and the remodelling of existing lymphatics are thought to be important steps in cancer metastasis. The past decade has been exciting in terms of research into the molecular and cellular biology of lymphatic vessels in cancer, and it has been shown that the molecular control of tumour lymphangiogenesis has similarities to that of tumour angiogenesis. Nevertheless, there are significant mechanistic differences between these biological processes. We are now developing a greater understanding of the specific roles of distinct lymphatic vessel subtypes in cancer, and this provides opportunities to improve diagnostic and therapeutic approaches that aim to restrict the progression of cancer.

Ccbe1 regulates Vegfc-mediated induction of Vegfr3 signaling during embryonic lymphangiogenesis

The VEGFC/VEGFR3 signaling pathway is essential for lymphangiogenesis (the formation of lymphatic vessels from pre-existing vasculature) during embryonic development, tissue regeneration and tumor progression. The recently identified secreted protein CCBE1 is indispensible for lymphangiogenesis during development. The role of CCBE1 orthologs is highly conserved in zebrafish, mice and humans with mutations in CCBE1 causing generalized lymphatic dysplasia and lymphedema (Hennekam syndrome). To date, the mechanism by which CCBE1 acts remains unknown. Here, we find that ccbe1 genetically interacts with both vegfc and vegfr3 in zebrafish. In the embryo, phenotypes driven by increased Vegfc are suppressed in the absence of Ccbe1, and Vegfc-driven sprouting is enhanced by local Ccbe1 overexpression. Moreover, Vegfc- and Vegfr3-dependent Erk signaling is impaired in the absence of Ccbe1. Finally, CCBE1 is capable of upregulating the levels of fully processed, mature VEGFC in vitro and the overexpression of mature VEGFC rescues ccbe1 loss-of-function phenotypes in zebrafish. Taken together, these data identify Ccbe1 as a crucial component of the Vegfc/Vegfr3 pathway in the embryo.

The VEGF signaling pathway in cancer: the road ahead

The vascular endothelial growth factor (VEGF) family of soluble protein growth factors consists of key mediators of angiogenesis and lymphangiogenesis in the context of tumor biology. The members of the family, VEGF-A (also known as VEGF), VEGF-B, VEGF-C, VEGF-D, and placenta growth factor (PIGF), play important roles in vascular biology in both normal physiology and pathology. The generation of a humanized neutralizing antibody to VEGF-A (bevacizumab, also known as Avastin) and the demonstration of its benefit in numerous human cancers have confirmed the merit of an anti-angiogenesis approach to cancer treatment and have validated the VEGF-A signaling pathway as a therapeutic target. Other members of the VEGF family are now being targeted, and their relevance to human cancer and the development of resistance to anti-VEGF-A treatment are being evaluated in the clinic. Here, we discuss the potential of targeting VEGF family members in the diagnosis and treatment of cancer.

The propeptides of VEGF-D determine heparin binding, receptor heterodimerization, and effects on tumor biology

VEGF-D is an angiogenic and lymphangiogenic glycoprotein that can be proteolytically processed generating various forms differing in subunit composition due to the presence or absence of N- and C-terminal propeptides. These propeptides flank the central VEGF homology domain, that contains the binding sites for VEGF receptors (VEGFRs), but their biological functions were unclear. Characterization of propeptide function will be important to clarify which forms of VEGF-D are biologically active and therefore clinically relevant. Here we use VEGF-D mutants deficient in either propeptide, and in the capacity to process the remaining propeptide, to monitor the functions of these domains. We report for the first time that VEGF-D binds heparin, and that the C-terminal propeptide significantly enhances this interaction (removal of this propeptide from full-length VEGF-D completely prevents heparin binding). We also show that removal of either the N- or C-terminal propeptide is required for VEGF-D to drive formation of VEGFR-2/VEGFR-3 heterodimers which have recently been shown to positively regulate angiogenic sprouting. The mature form of VEGF-D, lacking both propeptides, can also promote formation of these receptor heterodimers. In a mouse tumor model, removal of only the C-terminal propeptide from full-length VEGF-D was sufficient to enhance angiogenesis and tumor growth. In contrast, removal of both propeptides is required for high rates of lymph node metastasis. The findings reported here show that the propeptides profoundly influence molecular interactions of VEGF-D with VEGF receptors, co-receptors, and heparin, and its effects on tumor biology.

Remodeling of the lymphatic vasculature during mouse mammary gland morphogenesis is mediated via epithelial-derived lymphangiogenic stimuli

Despite the key roles of lymphatic vessels in homeostasis and disease, the cellular sources of signals that direct lymphatic vascular growth and patterning remain unknown. Using high-resolution imaging in two and three dimensions, we demonstrated that postnatal mouse mammary gland lymphatic vessels share an intimate spatial association with epithelial ducts and large blood vessels. We further demonstrated that the lymphatic vasculature is remodeled together with the mammary epithelial tree and blood vasculature during postnatal mouse mammary gland morphogenesis. Neither estrogen receptor α nor progesterone receptor were detected in lymphatic endothelial cells in the mouse mammary gland, suggesting that mammary gland lymphangiogenesis is not likely regulated directly by these steroid hormones. Epithelial cells, especially myoepithelial cells, were determined to be a rich source of prolymphangiogenic stimuli including VEGF-C and VEGF-D with temporally regulated expression levels during mammary gland morphogenesis. Blockade of VEGFR-3 signaling using a small-molecule inhibitor inhibited the proliferation of primary lymphatic endothelial cells promoted by mammary gland conditioned medium, suggesting that lymphangiogenesis in the mammary gland is likely driven by myoepithelial-derived VEGF-C and/or VEGF-D. These findings provide new insight into the architecture of the lymphatic vasculature in the mouse mammary gland and, by uncovering the proximity of lymphatic vessels to the epithelial tree, suggest a potential mechanism by which metastatic tumor cells access the lymphatic vasculature.

Vascular endothelial growth factor-D: signaling mechanisms, biology, and clinical relevance

Vascular endothelial growth factor-D (VEGF-D) is a secreted glycoprotein that promotes growth of blood vessels (angiogenesis) and lymphatic vessels (lymphangiogenesis), and can induce remodeling of large lymphatics. VEGF-D enhances solid tumor growth and metastatic spread in animal models of cancer, and in some human cancers VEGF-D correlates with metastatic spread, poor patient outcome, and, potentially, with resistance to anti-angiogenic drugs. Hence, VEGF-D signaling is a potential target for novel anti-cancer therapeutics designed to enhance anti-angiogenic approaches and to restrict metastasis. In the cardiovascular system, delivery of VEGF-D in animal models enhanced angiogenesis and tissue perfusion, findings which have led to a range of clinical trials testing this protein for therapeutic angiogenesis in cardiovascular diseases. Despite these experimental and clinical developments, our knowledge of the signaling mechanisms driven by VEGF-D is still evolving--here we explore the biology of VEGF-D, its signaling mechanisms, and the clinical relevance of this growth factor.

The connection between lymphangiogenic signalling and prostaglandin biology: a missing link in the metastatic pathway

Substantial evidence supports important independent roles for lymphangiogenic growth factor signaling and prostaglandins in the metastatic spread of cancer. The significance of the lymphangiogenic growth factors, vascular endothelial growth factor (VEGF)-C and VEGF-D, is well established in animal models of metastasis, and a strong correlation exits between an increase in expression of VEGF-C and VEGF-D, and metastatic spread in various solid human cancers. Similarly, key enzymes that control the production of prostaglandins, cyclooxygenases (COX-1 and COX-2, prototypic targets of Non-steroidal anti-inflammatory drugs (NSAIDs)), are frequently over-expressed or de-regulated in the progression of cancer. Recent data have suggested an intersection of lymphangiogenic growth factor signaling and the prostaglandin pathways in the control of metastatic spread via the lymphatic vasculature. Furthermore, this correlates with current clinical data showing that some NSAIDs enhance the survival of cancer patients through reducing metastasis. Here, we discuss the potential biochemical and cellular basis for such anti-cancer effects of NSAIDs through the prostaglandin and VEGF signaling pathways.

Towards the biomarker-guided rational use of antiangiogenic agents in the treatment of metastatic colorectal cancer

SUMMARY Clinical oncology experience with recently marketed antiangiogenic agents, which inhibit proteins important for tumor angiogenesis, has exposed significant limitations to their efficacy. Bevacizumab, a humanized neutralizing anti-VEGF-A monoclonal antibody, used in combination with cytotoxic chemotherapy for the treatment of metastatic colorectal cancer, represents the best-studied clinical example of targeted antiangiogenic therapy. In this context, bevacizumab provides modestly improved progression-free and overall survival in unselected patient populations via poorly understood mechanisms. Here we review concepts central to the identification and development of biomarkers in order to refine clinical use of bevacizumab in treating colorectal cancer and outline a phenotype-driven strategy for the discovery of high-value candidate biomarkers based on large-scale screening by molecular perturbation.

VEGF-D promotes tumor metastasis by regulating prostaglandins produced by the collecting lymphatic endothelium

Lymphatic metastasis is facilitated by lymphangiogenic growth factors VEGF-C and VEGF-D that are secreted by some primary tumors. We identified regulation of PGDH, the key enzyme in prostaglandin catabolism, in endothelial cells of collecting lymphatics, as a key molecular change during VEGF-D-driven tumor spread. The VEGF-D-dependent regulation of the prostaglandin pathway was supported by the finding that collecting lymphatic vessel dilation and subsequent metastasis were affected by nonsteroidal anti-inflammatory drugs (NSAIDs), known inhibitors of prostaglandin synthesis. Our data suggest a control point for cancer metastasis within the collecting lymphatic endothelium, which links VEGF-D/VEGFR-2/VEGFR-3 and the prostaglandin pathways. Collecting lymphatics therefore play an active and important role in metastasis and may provide a therapeutic target to restrict tumor spread.

A role for bone morphogenetic protein-4 in lymph node vascular remodeling and primary tumor growth

Lymph node metastasis, an early and prognostically important event in the progression of many human cancers, is associated with expression of VEGF-D. Changes to lymph node vasculature that occur during malignant progression may create a metastatic niche capable of attracting and supporting tumor cells. In this study, we sought to characterize molecules expressed in lymph node endothelium that could represent therapeutic or prognostic targets. Differential mRNA expression profiling of endothelial cells from lymph nodes that drained metastatic or nonmetastatic primary tumors revealed genes associated with tumor progression, in particular bone morphogenetic protein-4 (BMP-4). Metastasis driven by VEGF-D was associated with reduced BMP-4 expression in high endothelial venules, where BMP-4 loss could remodel the typical high-walled phenotype to thin-walled vessels. VEGF-D expression was sufficient to suppress proliferation of the more typical BMP-4-expressing high endothelial venules in favor of remodeled vessels, and mechanistic studies indicated that VEGF receptor-2 contributed to high endothelial venule proliferation and remodeling. BMP-4 could regulate high endothelial venule phenotype and cellular function, thereby determining morphology and proliferation responses. Notably, therapeutic administration of BMP-4 suppressed primary tumor growth, acting both at the level of tumor cells and tumor stromal cells. Together, our results show that VEGF-D-driven metastasis induces vascular remodeling in lymph nodes. Furthermore, they implicate BMP-4 as a negative regulator of this process, suggesting its potential utility as a prognostic marker or antitumor agent.

Proteolytic processing of vascular endothelial growth factor-D is essential for its capacity to promote the growth and spread of cancer

VEGF-D is a mitogen for endothelial cells that promotes tumor growth and metastatic spread in animal models, and expression of which correlates with lymph node metastasis in some human cancers. It is secreted from the cell as a full-length form with propeptides flanking a central region containing binding sites for VEGFR-2 and VEGFR-3, receptors that signal for angiogenesis and lymphangiogenesis. The propeptides can be cleaved from VEGF-D, enhancing affinity for VEGFR-2 and VEGFR-3 in vitro; however, the importance of this processing in cancer is unclear. To explore the necessity of processing for the effects of VEGF-D in cancer, we use a mutant full-length form that cannot be processed, and show that, in contrast to full-length VEGF-D that is processed, this mutant does not promote tumor growth and lymph node metastasis in a mouse tumor model. Processing of VEGF-D is required for tumor angiogenesis, lymphangiogenesis, and recruitment of tumor-associated macrophages. These observations may be explained by the requirement of processing for VEGF-D to bind neuropilin receptors and activate VEGFR-2. Our results indicate that proteolytic processing is necessary for VEGF-D to promote the growth and spread of cancer, and suggest that enzymes catalyzing this processing could be targets for antimetastatic therapeutics.

The VD1 neutralizing antibody to vascular endothelial growth factor-D: binding epitope and relationship to receptor binding

Vascular endothelial growth factor-D (VEGF-D) is a secreted protein that promotes tumor growth and metastatic spread in animal models of cancer. Expression of VEGF-D in prevalent human cancers was reported to correlate with lymph node metastasis and patient outcome-hence, this protein is a potential target for novel anticancer therapeutics designed to restrict tumor growth and spread. Here, we define the binding site in VEGF-D of a neutralizing antibody, designated VD1, which blocks the interaction of VEGF-D with its cell surface receptors vascular endothelial growth factor receptor (VEGFR)-2 and VEGFR-3 and is being used for the development of therapeutic antibodies. We show by peptide-based mapping and site-directed mutagenesis that the VD1 binding site includes the five residues (147)NEESL(151) and that immunization with a synthetic peptide containing this motif generates antibodies that neutralize VEGF-D. The tertiary structure of VEGF-D indicates that the (147)NEESL(151) epitope is located in the L2 loop of the growth factor, which is important for receptor binding. Mutation of any of these five residues influences receptor binding; for example, mutations to E148, which abolished binding to VD1, impaired the interaction with VEGFR-2 but enhanced binding to VEGFR-3. This structure/function study indicates that the VD1 binding epitope is part of the receptor binding site of VEGF-D, identifies a region of VEGF-D critical for binding of receptors and explains why VD1 does not bind other members of the VEGF family of growth factors.

Targeting lymphatic vessel functions through tyrosine kinases

The lymphatic vascular system is actively involved in tissue fluid homeostasis, immune surveillance and fatty acid transport. Pathological conditions can arise from injury to the lymphatics, or they can be recruited in the context of cancer to facilitate metastasis. Protein tyrosine kinases are central players in signal transduction networks and regulation of cell behavior. In the lymphatic endothelium, tyrosine kinases are involved in processes such as the maintenance of existing lymphatic vessels, growth and maturation of new vessels and modulation of their identity and function. As such, they are attractive targets for both existing inhibitors and the development of new inhibitors which affect lymphangiogenesis in pathological states such as cancer. RNAi screening provides an opportunity to identify the functional role of tyrosine kinases in the lymphatics. This review will discuss the role of tyrosine kinases in lymphatic biology and the potential use of inhibitors for anti-lymphangiogenic therapy.

Vascular endothelial growth factor-D over-expressing tumor cells induce differential effects on uterine vasculature in a mouse model of endometrial cancer

BACKGROUND

It has been hypothesised that increased VEGF-D expression may be an independent prognostic factor for endometrial cancer progression and lymph node metastasis; however, the mechanism by which VEGF-D may promote disease progression in women with endometrial cancer has not been investigated. Our aim was to describe the distribution of lymphatic vessels in mouse uterus and to examine the effect of VEGF-D over-expression on these vessels in a model of endometrial cancer. We hypothesised that VEGF-D over-expression would stimulate growth of new lymphatic vessels into the endometrium, thereby contributing to cancer progression.

METHODS

We initially described the distribution of lymphatic vessels (Lyve-1, podoplanin, VEGFR-3) and VEGF-D expression in the mouse uterus during the estrous cycle, early pregnancy and in response to estradiol-17beta and progesterone using immunohistochemistry. We also examined the effects of VEGF-D over-expression on uterine vasculature by inoculating uterine horns in NOD SCID mice with control or VEGF-D-expressing 293EBNA tumor cells.

RESULTS

Lymphatic vessels positive for the lymphatic endothelial cell markers Lyve-1, podoplanin and VEGFR-3 profiles were largely restricted to the connective tissue between the myometrial circular and longitudinal muscle layers; very few lymphatic vessel profiles were observed in the endometrium. VEGF-D immunostaining was present in all uterine compartments (epithelium, stroma, myometrium), although expression was generally low. VEGF-D immunoexpression was slightly but significantly higher in estrus relative to diestrus; and in estradiol-17beta treated mice relative to vehicle or progesterone treated mice. The presence of VEGF-D over-expressing tumor cells did not induce endometrial lymphangiogenesis, although changes were observed in existing vessel profiles. For myometrial lymphatic and endometrial blood vessels, the percentage of profiles containing proliferating endothelial cells, and the cross sectional area of vessel profiles were significantly increased in response to VEGF-D in comparison to control tumor cells. In contrast, no significant changes were noted in myometrial blood vessels. In addition, examples of invading cells or tumor emboli were observed in mice receiving VEGF-D expressing 293EBNA cells.

CONCLUSIONS

These results illustrate that VEGF-D over-expression has differential effects on the uterine vasculature. These effects may facilitate VEGF-D's ability to promote endometrial cancer metastasis and disease progression.

Molecular control of lymphatic metastasis

The metastatic spread of tumor cells is the most lethal aspect of cancer and often occurs via the lymphatic vasculature. Both experimental tumor models and human clinicopathologic data indicate that growth of lymphatic vessels (lymphangiogenesis) near solid tumors is often associated with lymph node metastasis. Changes in the adhesive properties of lymphatic endothelium near tumors may also facilitate metastatic spread via the lymphatics. Lymphangiogenic growth factors have been identified that promote formation of tumor lymphatics and metastatic spread of tumor cells to lymph nodes. These include the secreted glycoproteins vascular endothelial growth factor-C (VEGF-C) and VEGF-D, which act via their cognate receptor tyrosine kinase VEGF receptor-3 (VEGFR-3) located on lymphatic endothelial cells. Other signaling molecules that have been reported to promote lymphangiogenesis and/or lymphatic metastasis in cancer include VEGF-A, platelet-derived growth factor-BB, and hepatocyte growth factor. However, the quantitative contribution of these proteins to tumor lymphangiogenesis and lymphatic metastasis in different tumor types requires further investigation. In addition, chemokines are thought to play a role in attracting tumor cells and lymphatic vessels to each other. Moreover, it has recently been shown that lymphangiogenic growth factors secreted from a primary tumor can induce lymphangiogenesis in nearby lymph nodes, even before arrival of tumor cells, which may facilitate further metastasis. This article provides an overview of the molecular mechanisms that control lymphatic metastasis and discusses potential therapeutic approaches for inhibiting this process in human cancer.

A system for quantifying the patterning of the lymphatic vasculature

The lymphatic vasculature is critical for immunity and interstitial fluid homeostasis, playing important roles in diseases such as lymphedema and metastatic cancer. Animal models have been generated to explore the role of lymphatics and lymphangiogenic growth factors in such diseases, and to study lymphatic development. However, analysis of lymphatic vessels has primary been restricted to counting lymphatics in two-dimensional tissue slices, due to a lack of more sophisticated methodologies. In order to accurately examine lymphatic dysfunction in these models, and analyse the effects of lymphangiogenic growth factors on the lymphatic vasculature, it is essential to quantify the morphology and patterning of the distinct lymphatic vessels types in three-dimensional tissues. Here, we describe a method for performing such analyses, integrating user-operated image-analysis software with an approach that considers important morphological, anatomical and patterning features of the distinct lymphatic vessel subtypes. This efficient, reproducible technique is validated by analysing healthy and pathological tissues.

Distinct roles of vascular endothelial growth factor-D in lymphangiogenesis and metastasis

In many human carcinomas, expression of the lymphangiogenic factor vascular endothelial growth factor-D (VEGF-D) correlates with up-regulated lymphangiogenesis and regional lymph node metastasis. Here, we have used the Rip1Tag2 transgenic mouse model of pancreatic beta-cell carcinogenesis to investigate the functional role of VEGF-D in the induction of lymphangiogenesis and tumor progression. Expression of VEGF-D in beta cells of single-transgenic Rip1VEGF-D mice resulted in the formation of peri-insular lymphatic lacunae, often containing leukocyte accumulations and blood hemorrhages. When these mice were crossed to Rip1Tag2 mice, VEGF-D-expressing tumors also exhibited peritumoral lymphangiogenesis with lymphocyte accumulations and hemorrhages, and they frequently developed lymph node and lung metastases. Notably, tumor outgrowth and blood microvessel density were significantly reduced in VEGF-D-expressing tumors. Our results demonstrate that VEGF-D induces lymphangiogenesis, promotes metastasis to lymph nodes and lungs, and yet represses hemangiogenesis and tumor outgrowth. Because a comparable transgenic expression of vascular endothelial growth factor-C (VEGF-C) in Rip1Tag2 has been shown previously to provoke lymphangiogenesis and lymph node metastasis in the absence of any distant metastasis, leukocyte infiltration, or angiogenesis-suppressing effects, these results reveal further functional differences between VEGF-D and VEGF-C.

Molecular Pathways for Lymphangiogenesis and their Role in Human Disease

The lymphatic network functions to return fluid, cells and macromolecules to the circulation. Recent characterization of growth factors that control the growth and development of the lymphatics, and markers which specify lymphatic endothelial cells have enhanced our understanding of this system. Members of the VEGF family of factors are key regulators of these vessels with VEGF-C/VEGF-D and VEGFR-3 being the best validated signalling pathways in lymphangiogenesis. The study of these molecules in various pathologies has shown that they are important in the processes of cancer metastasis and in the formation of lymphoedema. Knowledge of these molecular pathways allows for the generation of modulators of these pathways which could form the basis of novel therapeutic approaches.

Proprotein convertases promote processing of VEGF-D, a critical step for binding the angiogenic receptor VEGFR-2

Vascular endothelial growth factor (VEGF)-D is a secreted glycoprotein that induces angiogenesis and lymphangiogenesis. It consists of a central domain, containing binding sites for VEGF receptor-2 (VEGFR-2) and VEGFR-3, and N- and C-terminal propeptides. It is secreted from the cell as homodimers of the full-length form that can be proteolytically processed to remove the propeptides. It was recently shown, using adenoviral gene delivery, that fully processed VEGF-D induces angiogenesis in vivo, whereas full-length VEGF-D does not. To better understand these observations, we monitored the effect of VEGF-D processing on receptor binding using a full-length VEGF-D mutant that cannot be processed. This mutant binds VEGFR-2, the receptor signaling for angiogenesis, with approximately 17,000-fold lower affinity than mature VEGF-D, indicating the importance of processing for interaction with this receptor. Further, we show that members of the proprotein convertase (PC) family of proteases promote VEGF-D processing, which facilitates the VEGF-D/VEGFR-2 interaction. The PCs furin and PC5 promote cleavage of both propeptides, whereas PC7 promotes cleavage of the C-terminal propeptide only. The finding that PCs promote activation of VEGF-D and other proteins with roles in cancer such as matrix metalloproteinases, emphasizes the importance of these enzymes as potential regulators of tumor progression and metastasis.

Tumor lymphangiogenesis and metastatic spread-new players begin to emerge

The metastatic spread of tumor cells is the most lethal aspect of cancer and can occur via various routes, including the lymphatic vasculature. Studies of tumor models in animals and clinicopathological data have indicated that growth of lymphatic vessels (lymphangiogenesis) in the vicinity of solid tumors may contribute to lymphatic metastasis. Research over the past 5 years has identified a range of lymphangiogenic growth factors that could conceivably play a role in promoting tumor lymphangiogenesis and lymphatic metastasis. The most extensively studied signaling system that promotes lymphangiogenesis in tumors involves the secreted lymphangiogenic proteins vascular endothelial growth factor-C (VEGF-C) and VEGF-D, and their cognate receptor on lymphatic endothelium VEGF receptor-3 (VEGFR-3). More recent studies have identified other signaling molecules that can also promote lymphangiogenesis in vivo, including hepatocyte growth factor and members of the fibroblast growth factor, angiopoietin, platelet-derived growth factor and insulin-like growth factor families of secreted proteins. This article provides an overview of the molecular mechanisms that control lymphangiogenic signaling, emphasizing the more recently identified lymphangiogenic growth factors and the roles they may play in cancer biology. Molecular approaches for inhibiting lymphangiogenic signaling in cancer, designed to restrict tumor metastasis, are also examined.

Lymphangiogenic growth factor responsiveness is modulated by postnatal lymphatic vessel maturation

Lymphatic vessel plasticity and stability are of considerable importance when attempting to treat diseases associated with the lymphatic vasculature. Development of lymphatic vessels during embryogenesis is dependent on vascular endothelial growth factor (VEGF)-C but not VEGF-D. Using a recombinant adenovirus encoding a soluble form of their receptor VEGFR-3 (AdVEGFR-3-Ig), we studied lymphatic vessel dependency on VEGF-C and VEGF-D induced VEGFR-3 signaling in postnatal and adult mice. Transduction with AdVEGFR-3-Ig led to regression of lymphatic capillaries and medium-sized lymphatic vessels in mice under 2 weeks of age without affecting collecting lymphatic vessels or the blood vasculature. No effect was observed after this period. The lymphatic capillaries of neonatal mice also regressed partially in response to recombinant VEGFR-3-Ig or blocking antibodies against VEGFR-3, but not to adenovirus-encoded VEGFR-2-Ig. Despite sustained inhibitory VEGFR-3-Ig levels, lymphatic vessel regrowth was observed at 4 weeks of age. Interestingly, whereas transgenic expression of VEGF-C in the skin induced lymphatic hyperplasia even during embryogenesis, similar expression of VEGF-D resulted in lymphangiogenesis predominantly after birth. These results indicate considerable plasticity of lymphatic vessels during the early postnatal period but not thereafter, suggesting that anti-lymphangiogenic therapy can be safely applied in adults.

Targeting lymphangiogenesis to prevent tumour metastasis

Recent studies involving animal models of cancer and clinicopathological analyses of human tumours suggest that the growth of lymphatic vessels (lymphangiogenesis) in or nearby tumours is associated with the metastatic spread of cancer. The best validated molecular signalling system for tumour lymphangiogenesis involves the secreted proteins vascular endothelial growth factor-C (VEGF-C) and VEGF-D that induce growth of lymphatic vessels via activation of VEGF receptor-3 (VEGFR-3) localised on the surface of lymphatic endothelial cells. In this review, we discuss the evidence supporting a role for this signalling system in the spread of cancer and potential approaches for blocking this system to prevent tumour metastasis.

Lymphatic vessels in cancer metastasis: bridging the gaps

Distant organ metastasis is the most important factor in determining patient survival in cancer. This is thought to occur via the body's own systems for transporting fluid and cells, the blood vascular and lymphatic systems. Cancer cells may exploit these vascular systems by expressing growth factors, which alter the normal pattern of angiogenesis and lymphatic vessel growth (lymphangiogenesis), thus creating conduits for tumour metastasis. With respect to lymphatic metastasis, techniques which allow the mapping of a tumour's lymphatic drainage and sampling of the 'sentinel node' from the regional lymph node group provide crucial prognostic information, determine further treatment and offer a window into tumour-host immune interactions. Aberrant drainage patterns so identified are both clinically significant, and highlight important anatomical and molecular complexities not explained by existing models of lymphatic development or anatomy. The molecular controls of tumour lymphangiogenesis and factors determining which lymphatic vessel subtypes are induced may be targets for novel therapeutics designed to restrict cancer metastasis. Furthermore, analyses of these control mechanisms will enhance our understanding of the interactions between the tumour cells and the lymphatic vasculature. For many years, disparate groups of clinical researchers and basic scientists have been working to unravel the mysteries of the lymphatic system. This review aims to summarize these contributions, in terms of the history, identification, structure and function of lymphatic vessels in cancer and the role they play in tumour metastasis. Current ideas about the roles of lymphangiogenic growth factors, their signalling pathways in lymphatic metastasis and therapeutic opportunities to restrict this spread will also be explored.

Current Strategies for Modulating Lymphangiogenesis Signalling Pathways in Human Disease

The recent discovery that members of the vascular endothelial growth factor (VEGF) family of secreted glycoproteins can mediate lymphatic vessel growth (lymphangiogenesis) via cell surface receptor tyrosine kinases expressed on endothelial cells has opened the way for therapeutic intervention for pathologies involving dysregulated lymphatic vessel function. At least two members of this family, VEGF-C and VEGF-D, have been shown to induce lymphangiogenesis in vivo. Lymphatic vessels and their specific growth factors have been directly implicated in a number of significant human pathologies. In cancer, VEGF-C and VEGF-D appear to correlate with tumor metastasis and poor patient outcome in a range of prevalent human cancers. Experimental studies have demonstrated that expression of the lymphangiogenic growth factors in tumor models induces increased lymphangiogenesis and results in spread of tumor cells via the lymphatics. In contrast, conditions such as lymphedema, where lymphatic vessels fail to clear fluid from interstitial spaces, are opportunities for which the application of growth factors to generate new lymphatic vessels may be a viable therapeutic option. The list of molecules that control lymphangiogenesis is now expanding, allowing more opportunities for the development of drugs with which to manipulate the relevant signalling pathways. Modulating these pathways and other molecules with specificity to the lymphatic endothelium could offer alternative treatments for a number of important clinical conditions.