Alternatively spliced vascular endothelial growth factor receptor-2 is an essential endogenous inhibitor of lymphatic vessel growth

The endogenous soluble VEGF receptor-2 isoform suppresses lymph node metastasis in a mouse immunocompetent mammary cancer model

BACKGROUND

Cancer metastasis contributes significantly to cancer mortality and is facilitated by lymphangiogenesis and angiogenesis. A new splicing variant, endogenous soluble vascular endothelial growth factor receptor-2 (esVEGFR-2) that we recently identified is an endogenous selective inhibitor of lymphangiogenesis. To evaluate the antimetastatic potential of esVEGFR-2, gene therapy with vector expressing esVEGFR-2 (pesVEGFR-2) or endostatin (pEndo) as a positive control was conducted on murine metastatic mammary cancer.

METHODS

Syngeneic inoculated metastatic mammary cancers received direct intratumoral injection of pesVEGFR-2, pEndo or pVec as control, once a week for six weeks. In vivo gene electrotransfer was performed on the tumors after each injection.

RESULTS

Deaths from metastasis were much lower in the pesVEGFR-2 and pEndo groups than in those of the pVec. Tumor volume was significantly lower in the pesVEGFR-2 and the pEndo groups throughout the study. Multiplicity of lymph node and lung metastatic nodules was significantly suppressed in the pesVEGFR-2 and pEndo groups. Moreover, the total number of overall metastasis including the other organs was also decreased in these groups. However, pesVEGFR-2 was not able to decrease the number of lungs, ovaries, kidneys and adrenals with metastasis as counted by unilateral or bilateral metastasis. The number of CD34+/Lyve-1⁻ blood microvessels was significantly decreased in the pEndo group, while the number of CD34⁻/Lyve-1+ lymphatic vessels was significantly decreased in the pesVEGFR-2 and pEndo groups. In addition, a significant reduction in the number of dilated lymphatic vessels containing intraluminal cancer cells was observed in the pesVEGFR-2 and pEndo groups. Levels of apoptosis were significantly increased in the pEndo group, whereas the rates of cell proliferation were significantly decreased in the pesVEGFR-2 and pEndo groups.

CONCLUSIONS

Our data demonstrate that esVEGFR-2 can inhibit mainly lymph node metastasis. The antimetastatic activity of esVEGFR-2 may be of high clinical significance in the treatment of metastatic breast cancer because lymph node involvement is a most important prognostic factor in cancer patients.

Endostatin inhibits tumour lymphangiogenesis and lymphatic metastasis via cell surface nucleolin on lymphangiogenic endothelial cells

Endostatin has potent anti-endothelial and anti-angiogenic functions. Endostatin was reported to reduce lymphangiogenesis by down-regulating the level of VEGF-C in tumour tissues. However, there is little evidence for the direct function of endostatin on lymphangiogenic endothelial cells and lymphangiogenic vessels. Here, we report that cell surface nucleolin, which was reported as an endostatin receptor mediating its anti-angiogenic and anti-tumour functions, is also selectively expressed on the cell surface of lymphangiogenic endothelial cells both in vitro and in vivo. Treatment of primary mouse lymphatic endothelial cells (mLECs) by endostatin inhibits mLEC migration, tubule formation, and activation of the Erk pathway in mLECs, while neutralization of cell surface nucleolin or nucleolin knockdown results in loss of the anti-lymphatic endothelial activities of endostatin. Also, anti-nucleolin antibody or lentivirus delivered nucleolin siRNA abolishes the anti-lymphangiogenic function of endostatin in the Matrigel plug assay. Endostatin remarkably inhibits tumour-associated lymphangiogenesis, leading to reduced lymphatic metastasis. Systemic blockade of nucleolin notably abolishes the anti-lymphangiogenic and anti-lymphatic metastatic functions of endostatin. Importantly, endostatin does not affect quiescent lymphatics in normal organs, which is consistent with the lack of expression of cell surface nucleolin in quiescent lymphatics. Taken together, our results demonstrate that endostatin directly acts on lymphangiogenic endothelial cells via cell surface nucleolin, which provides a novel mechanism for the inhibition of tumour lymphangiogenesis and lymphatic metastasis by endostatin.

Angiogenesis and breast cancer

Angiogenesis is an essential step for breast cancer progression and dissemination. The development of new blood vessels in cancer setting (angiogenesis) is conducted by numerous physiological and pathological stimuli, where the main stimulus is hypoxia. The knowledge of different molecular pathways regulating angiogenesis is constantly growing. An increased and complex scenario of angiogenesis is nowadays available in breast cancer, specifically, and permits not only to understand most of the important phases of neoplastic growth but also offer an exciting perspective for new therapeutic proposals based on blocking new blood vessels sprouting. This review focused on historical and recent understanding of angiogenesis occurrence in breast cancer.

Podoplanin-Fc reduces lymphatic vessel formation in vitro and in vivo and causes disseminated intravascular coagulation when transgenically expressed in the skin

Podoplanin is a small transmembrane protein required for development and function of the lymphatic vascular system. To investigate the effects of interfering with its function, we produced an Fc fusion protein of its ectodomain. We found that podoplanin-Fc inhibited several functions of cultured lymphatic endothelial cells and also specifically suppressed lymphatic vessel growth, but not blood vessel growth, in mouse embryoid bodies in vitro and in mouse corneas in vivo. Using a keratin 14 expression cassette, we created transgenic mice that overexpressed podoplanin-Fc in the skin. No obvious outward phenotype was identified in these mice, but surprisingly, podoplanin-Fc-although produced specifically in the skin-entered the blood circulation and induced disseminated intravascular coagulation, characterized by microthrombi in most organs and by thrombocytopenia, occasionally leading to fatal hemorrhage. These findings reveal an important role of podoplanin in lymphatic vessel formation and indicate the potential of podoplanin-Fc as an inhibitor of lymphangiogenesis. These results also demonstrate the ability of podoplanin to induce platelet aggregation in vivo, which likely represents a major function of lymphatic endothelium. Finally, keratin 14 podoplanin-Fc mice represent a novel genetic animal model of disseminated intravascular coagulation.

Blood vessel endothelial VEGFR-2 delays lymphangiogenesis: an endogenous trapping mechanism links lymph- and angiogenesis

Angio- and lymphangiogenesis are inherently related processes. However, how blood and lymphatic vessels regulate each other is unknown. This work introduces a novel mechanism explaining the temporal and spatial relation of blood and lymphatic vessels. Vascular endothelial growth factor-A (VEGF-A) surprisingly reduced VEGF-C in the supernatant of blood vessel endothelial cells, suggesting growth factor (GF) clearance by the growing endothelium. The orientation of lymphatic sprouting toward angiogenic vessels and away from exogenous GFs was VEGF-C dependent. In vivo molecular imaging revealed higher VEGF receptor (R)-2 in angiogenic tips compared with normal vessels. Consistently, lymphatic growth was impeded in the angiogenic front. VEGF-C/R-2 complex in the cytoplasm of VEGF-A-treated endothelium indicated that receptor-mediated internalization causes GF clearance from the extracellular matrix. GF clearance by receptor-mediated internalization is a new paradigm explaining various characteristics of lymphatics.

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.

Glomerular structure and function require paracrine, not autocrine, VEGF-VEGFR-2 signaling

VEGF is a potent vascular growth factor produced by podocytes in the developing and mature glomerulus. Specific deletion of VEGF from podocytes causes glomerular abnormalities including profound endothelial cell injury, suggesting that paracrine signaling is critical for maintaining the glomerular filtration barrier (GFB). However, it is not clear whether normal GFB function also requires autocrine VEGF signaling in podocytes. In this study, we sought to determine whether an autocrine VEGF-VEGFR-2 loop in podocytes contributes to the maintenance of the GFB in vivo. We found that induced, whole-body deletion of VEGFR-2 caused marked abnormalities in the kidney and also other tissues, including the heart and liver. By contrast, podocyte-specific deletion of the VEGFR-2 receptor had no effect on glomerular development or function even up to 6 months old. Unlike cell culture models, enhanced expression of VEGF by podocytes in vivo caused foot process fusion and alterations in slit diaphragm-associated proteins; however, inhibition of VEGFR-2 could not rescue this defect. Although VEGFR-2 was dispensable in the podocyte, glomerular endothelial cells depended on VEGFR-2 expression: postnatal deletion of the receptor resulted in global defects in the glomerular microvasculature. Taken together, our results provide strong evidence for dominant actions of a paracrine VEGF-VEGFR-2 signaling loop both in the developing and in the filtering glomerulus. VEGF produced by the podocyte regulates the structure and function of the adjacent endothelial cell.

Phage-derived fully human monoclonal antibody fragments to human vascular endothelial growth factor-C block its interaction with VEGF receptor-2 and 3

Vascular endothelial growth factor C (VEGF-C) is a key mediator of lymphangiogenesis, acting via its receptors VEGF-R2 and VEGF-R3. High expression of VEGF-C in tumors correlates with increased lymphatic vessel density, lymphatic vessel invasion, sentinel lymph node metastasis and poor prognosis. Recently, we found that in a chemically induced skin carcinoma model, increased VEGF-C drainage from the tumor enhanced lymphangiogenesis in the sentinel lymph node and facilitated metastatic spread of cancer cells via the lymphatics. Hence, interference with the VEGF-C/VEGF-R3 axis holds promise to block metastatic spread, as recently shown by use of a neutralizing anti-VEGF-R3 antibody and a soluble VEGF-R3 (VEGF-C/D trap). By antibody phage-display, we have developed a human monoclonal antibody fragment (single-chain Fragment variable, scFv) that binds with high specificity and affinity to the fully processed mature form of human VEGF-C. The scFv binds to an epitope on VEGF-C that is important for receptor binding, since binding of the scFv to VEGF-C dose-dependently inhibits the binding of VEGF-C to VEGF-R2 and VEGF-R3 as shown by BIAcore and ELISA analyses. Interestingly, the variable heavy domain (V_H) of the anti-VEGF-C scFv, which contains a mutation typical for camelid heavy chain-only antibodies, is sufficient for binding VEGF-C. This reduced the size of the potentially VEGF-C-blocking antibody fragment to only 14.6 kDa. Anti-VEGF-C V_H-based immunoproteins hold promise to block the lymphangiogenic activity of VEGF-C, which would present a significant advance in inhibiting lymphatic-based metastatic spread of certain cancer types.

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.

Immune privilege of corneal allografts

Corneal transplantation has been performed successfully for over 100 years. Normally, HLA typing and systemic immunosuppressive drugs are not utilized, yet 90% of corneal allografts survive. In rodents, corneal allografts representing maximal histoincompatibility enjoy >50% survival even without immunosuppressive drugs. By contrast, other categories of transplants are invariably rejected in such donor/host combinations. The acceptance of corneal allografts compared to other categories of allografts is called immune privilege. The cornea expresses factors that contribute to immune privilege by preventing the induction and expression of immune responses to histocompatibility antigens on the corneal allograft. Among these are soluble and cell membrane molecules that block immune effector elements and also apoptosis of T lymphocytes. However, some conditions rob the corneal allograft of its immune privilege and promote rejection, which remains the leading cause of corneal allograft failure. Recent studies have examined new strategies for restoring immune privilege to such high-risk hosts.

Dendritic cell physiology and function in the eye

The eye and the brain are immunologically privileged sites, a property previously attributed to the lack of a lymphatic circulation. However, recent tracking studies confirm that these organs have good communication through classical site-specific lymph nodes, as well as direct connection through the blood circulation with the spleen. In addition, like all tissues, they contain resident myeloid cell populations that play important roles in tissue homeostasis and the response to foreign antigens. Most of the macrophage and dendritic cell (DC) populations in the eye are restricted to the supporting connective tissues, including the cornea, while the neural tissue (the retina) contains almost no DCs, occasional macrophages (perivascularly distributed), and a specialized myeloid cell type, the microglial cell. Resident microglial cells are normally programmed for immunological tolerance. The privileged status of the eye, however, is relative, as it is susceptible to immune-mediated inflammatory disease, both infectious and autoimmune. Intraocular inflammation (uveitis and uveoretinitis) and corneal graft rejection constitute two of the more common inflammatory conditions affecting the eye leading to considerable morbidity (blindness). As corneal graft rejection occurs almost exclusively by indirect allorecognition, host DCs play a major role in this process and are likely to be modified in their behavior by the ocular microenvironment. Ocular surface disease, including allergy and atopy, also comprise a significant group of immune-mediated eye disorders in which DCs participate, while infectious disease such as herpes simplex keratitis is thought to be initiated via corneal DCs. Intriguingly, some more common conditions previously thought to be degenerative (e.g. age-related macular degeneration) may have an autoimmune component in which ocular DCs and macrophages are critically involved. Recently, the possibility of harnessing the tolerizing potential of DCs has been applied to experimental models of autoimmune uveoretinitis with good effect. This approach has considerable potential for use in translational clinical therapy to prevent sight-threatening disease caused by ocular inflammation.

Soluble forms of VEGF receptor-1 and -2 promote vascular maturation via mural cell recruitment

Two soluble forms of vascular endothelial growth factor (VEGF) receptors, sVEGFR-1 and sVEGFR-2, are physiologically released and overproduced in some pathologies. They are known to act as anti-VEGF agents. Here we report that these soluble receptors contribute to vessel maturation by mediating a dialogue between endothelial cells (ECs) and mural cells that leads to blood vessel stabilization. Through a multidisciplinary approach, we provide evidence that these soluble VEGF receptors promote mural cell migration through a paracrine mechanism involving interplay in ECs between VEGF/VEGFR-2 and sphingosine-1-phosphate type-1 (S1P)/S1P1 pathways that leads to endothelial nitric oxyde synthase (eNOS) activation. This new paradigm is supported by the finding that sVEGFR-1 and -2 perform the following actions: 1) induce an eNOS-dependent outgrowth of a mural cell network in an ex vivo model of angiogenesis, 2) increase the mural cell coverage of neovessels in vitro and in vivo, 3) promote mural cell migration toward ECs, and 4) stimulate endothelial S1P1 overproduction and eNOS activation that promote the migration and the recruitment of neighboring mural cells. These findings provide new insights into mechanisms regulating physiological and pathological angiogenesis and vessel stabilization.

Non-peptide small molecule regulators of lymphangiogenesis

Adrenomedullin (AM) and gastrin releasing peptide (GRP) are neuroendocrine peptides that have been previously implicated as regulators of angiogenesis and lymphangiogenesis. Using an immortalized human dermal microvascular lymphatic endothelial cell line stably transfected with red fluorescent protein (LEC/RFP), we demonstrate the ability of AM and GRP to augment tube formation complexity of this target cell in a dose-dependent manner. Maximum tube density was initiated at 1 nM for both peptides, and as concentrations exceeded 10 nM a decrease in tube formation was noted, hence following a classic rise/fall biological response curve. In addition, we show that appropriate small molecule mimetics to neutralizing monoclonal antibodies of AM or GRP, at 1 microM concentration, can function to either inhibit (antagonist) or enhance (super agonist) peptide-induced tube formation of LEC/RFP. Our small molecule reagents by themselves have no activity, but in the presence of their respective peptides can mediate a positive or negative response, hence the super agonist designation. These compounds represent new regulatory drugs of the lymphatic system with possible patient application in the clinical management of edema and metastatic disease.

Lymphangiogenesis: Molecular mechanisms and future promise

The growth of lymphatic vessels (lymphangiogenesis) is actively involved in a number of pathological processes including tissue inflammation and tumor dissemination but is insufficient in patients suffering from lymphedema, a debilitating condition characterized by chronic tissue edema and impaired immunity. The recent explosion of knowledge on the molecular mechanisms governing lymphangiogenesis provides new possibilities to treat these diseases.

Neuroblastoma progression correlates with downregulation of the lymphangiogenesis inhibitor sVEGFR-2

PURPOSE

Tumor progression correlates with the induction of a dense supply of blood vessels and the formation of peritumoral lymphatics. Hemangiogenesis and lymphangiogenesis are potently regulated by members of the vascular endothelial growth factor (VEGF) family. Previous studies have indicated the upregulation of VEGF-A and -C in progressed neuroblastoma, however, quantification was performed using semiquantitative methods, or patients who had received radiotherapy or chemotherapy were studied.

EXPERIMENTAL DESIGN

We have analyzed primary neuroblastoma from 49 patients using real-time reverse transcription-PCR and quantified VEGF-A, -C, and -D and VEGF receptors (VEGFR)-1, 2, 3, as well as the soluble form of VEGFR2 (sVEGFR-2), which has recently been characterized as an endogenous inhibitor of lymphangiogenesis. None of the patients had received radiotherapy or chemotherapy before tumor resection.

RESULTS

We did not observe upregulation of VEGF-A, -C, and -D in metastatic neuroblastoma, but found significant downregulation of the lymphangiogenesis inhibitor sVEGFR-2 in metastatic stages III, IV, and IVs. In stage IV neuroblastoma, there were tendencies for the upregulation of VEGF-A and -D and the downregulation of the hemangiogenesis/lymphangiogenesis inhibitors VEGFR-1 and sVEGFR-2 in MYCN-amplified tumors. Similarly, MYCN transfection of the neuroblastoma cell line SH-EP induced the upregulation of VEGF-A and -D and the switching-off of sVEGFR-2.

CONCLUSION

We provide evidence for the downregulation of the lymphangiogenesis inhibitor sVEGFR-2 in metastatic neuroblastoma stages, which may promote lymphogenic metastases. Downregulation of hemangiogenesis and lymphangiogenesis inhibitors VEGFR-1 and sVEGFR-2, and upregulation of angiogenic activators VEGF-A and VEGF-D in MYCN-amplified stage IV neuroblastoma supports the crucial effect of this oncogene on neuroblastoma progression.

A placental growth factor variant unable to recognize vascular endothelial growth factor (VEGF) receptor-1 inhibits VEGF-dependent tumor angiogenesis via heterodimerization

Angiogenesis is one of the crucial events for cancer development and growth. Two members of the vascular endothelial growth factor (VEGF) family, VEGF-A and placental growth factor (PlGF), which are able to heterodimerize if coexpressed in the same cell, are both required for pathologic angiogenesis. We have generated a PlGF1 variant, named PlGF1-DE in which the residues Asp72 and Glu73 were substituted with Ala, which is unable to bind and activate VEGF receptor-1 but is still able to heterodimerize with VEGF. Here, we show that overexpression in tumor cells by adenoviral delivery or stable transfection of PlGF1-DE variant significantly reduces the production of VEGF homodimer via heterodimerization, determining a strong inhibition of xenograft tumor growth and neoangiogenesis, as well as significant reduction of vessel lumen and stabilization, and monocyte-macrophage infiltration. Conversely, the overexpression of PlGF1wt, also reducing the VEGF homodimer production comparably with PlGF1-DE variant through the generation of VEGF/PlGF heterodimer, does not inhibit tumor growth and vessel density compared with controls but induces increase of vessel lumen, vessel stabilization, and monocyte-macrophage infiltration. The property of PlGF and VEGF-A to generate heterodimer represents a successful strategy to inhibit VEGF-dependent angiogenesis. The PlGF1-DE variant, and not PlGF1wt as previously reported, acts as a "dominant negative" of VEGF and is a new candidate for antiangiogenic gene therapy in cancer treatment.