Adenovirus encoding vascular endothelial growth factor-D induces tissue-specific vascular patterns in vivo

Vascular Endothelial Growth Factor-D (VEGF-D): An Angiogenesis Bypass in Malignant Tumors

Vascular endothelial growth factors (VEGFs) are the key regulators of vasculogenesis in normal and oncological development. VEGF-A is the most studied angiogenic factor secreted by malignant tumor cells under hypoxic and inflammatory stress, which made VEGF-A a rational target for anticancer therapy. However, inhibition of VEGF-A by monoclonal antibody drugs led to the upregulation of VEGF-D. VEGF-D was primarily described as a lymphangiogenic factor; however, VEGF-D's blood angiogenic potential comparable to VEGF-A has already been demonstrated in glioblastoma and colorectal carcinoma. These findings suggested a role for VEGF-D in facilitating malignant tumor growth by bypassing the anti-VEGF-A antiangiogenic therapy. Owing to its high mitogenic ability, higher affinity for VEGFR-2, and higher expression in cancer, VEGF-D might even be a stronger angiogenic driver and, hence, a better therapeutic target than VEGF-A. In this review, we summarized the angiogenic role of VEGF-D in blood vasculogenesis and its targetability as an antiangiogenic therapy in cancer.

Distinct Characteristics of VEGF-D and VEGF-C to Predict Mortality in Patients With Suspected or Known Coronary Artery Disease

Background

VEGF‐D (vascular endothelial growth factor D) and VEGF‐C are secreted glycoproteins that can induce lymphangiogenesis and angiogenesis. They exhibit structural homology but have differential receptor binding and regulatory mechanisms. We recently demonstrated that the serum VEGF‐C level is inversely and independently associated with all‐cause mortality in patients with suspected or known coronary artery disease. We investigated whether VEGF‐D had distinct relationships with mortality and cardiovascular events in those patients.

Methods and Results

We performed a multicenter, prospective cohort study of 2418 patients with suspected or known coronary artery disease undergoing elective coronary angiography. The serum level of VEGF‐D was measured. The primary outcome was all‐cause death. The secondary outcomes were cardiovascular death and major adverse cardiovascular events defined as a composite of cardiovascular death, nonfatal myocardial infarction, and nonfatal stroke. During the 3‐year follow‐up, 254 patients died from any cause, 88 died from cardiovascular disease, and 165 developed major adverse cardiovascular events. After adjustment for possible clinical confounders, cardiovascular biomarkers (N‐terminal pro‐B‐type natriuretic peptide, cardiac troponin‐I, and high‐sensitivity C‐reactive protein), and VEGF‐C, the VEGF‐D level was significantly associated with all‐cause death and cardiovascular death but not with major adverse cardiovascular events.. Moreover, the addition of VEGF‐D, either alone or in combination with VEGF‐C, to the model with possible clinical confounders and cardiovascular biomarkers significantly improved the prediction of all‐cause death but not that of cardiovascular death or major adverse cardiovascular events. Consistent results were observed within patients over 75 years old.

Conclusions

In patients with suspected or known coronary artery disease undergoing elective coronary angiography, an elevated VEGF‐D value seems to independently predict all‐cause mortality.

Increased Lymphangiogenesis and Lymphangiogenic Growth Factor Expression in Perivascular Adipose Tissue of Patients with Coronary Artery Disease

Experimental and human autopsy studies have associated adventitial lymphangiogenesis with atherosclerosis. An analysis of perivascular lymphangiogenesis in patients with coronary artery disease is lacking. Here, we examined lymphangiogenesis and its potential regulators in perivascular adipose tissue (PVAT) surrounding the heart (C-PVAT) and compared it with PVAT of the internal mammary artery (IMA-PVAT). Forty-six patients undergoing coronary artery bypass graft surgery were included. Perioperatively collected C-PVAT and IMA-PVAT were analyzed using histology, immunohistochemistry, real time PCR, and PVAT-conditioned medium using cytokine arrays. C-PVAT exhibited increased PECAM-1 (platelet endothelial cell adhesion molecule 1)-positive vessel density. The number of lymphatic vessels expressing lymphatic vessel endothelial hyaluronan receptor-1 or podoplanin was also elevated in C-PVAT and associated with higher inflammatory cell numbers, increased intercellular adhesion molecule 1 (ICAM1) expression, and fibrosis. Significantly higher expression of regulators of lymphangiogenesis such as vascular endothelial growth factor (VEGF)-C, VEGF-D, and VEGF receptor-3 was observed in C-PVAT compared to IMA-PVAT. Cytokine arrays identified angiopoietin-2 as more highly expressed in C-PVAT vs. IMA-PVAT. Findings were confirmed histologically and at the mRNA level. Stimulation of human lymphatic endothelial cells with recombinant angiopoietin-2 in combination with VEGF-C enhanced sprout formation. Our study shows that PVAT surrounding atherosclerotic arteries exhibits more extensive lymphangiogenesis, inflammation, and fibrosis compared to PVAT surrounding a non-diseased vessel, possibly due to local angiopoietin-2, VEGF-C, and VEGF-D overexpression.

KLK3/PSA and cathepsin D activate VEGF-C and VEGF-D

Vascular endothelial growth factor-C (VEGF-C) acts primarily on endothelial cells, but also on non-vascular targets, for example in the CNS and immune system. Here we describe a novel, unique VEGF-C form in the human reproductive system produced via cleavage by kallikrein-related peptidase 3 (KLK3), aka prostate-specific antigen (PSA). KLK3 activated VEGF-C specifically and efficiently through cleavage at a novel N-terminal site. We detected VEGF-C in seminal plasma, and sperm liquefaction occurred concurrently with VEGF-C activation, which was enhanced by collagen and calcium binding EGF domains 1 (CCBE1). After plasmin and ADAMTS3, KLK3 is the third protease shown to activate VEGF-C. Since differently activated VEGF-Cs are characterized by successively shorter N-terminal helices, we created an even shorter hypothetical form, which showed preferential binding to VEGFR-3. Using mass spectrometric analysis of the isolated VEGF-C-cleaving activity from human saliva, we identified cathepsin D as a protease that can activate VEGF-C as well as VEGF-D.

The lymphatic system is composed of networks of vessels that drain fluids from the body’s tissues and filter it back into the blood. Growing these vessels depends on a factor known as VEGF-C, which is released in an inactive form and must be cut by enzymes before it can work. One enzyme that is known to activate the VEGF-C signal when the early embryo is developing is ADAMTS3. If this signal fails to switch on this can result in a condition known as lymphedema – whereby problems in the lymphatic system cause tissues to swell due to insufficient drainage. However, it is unknown whether the VEGF-C signal can be activated by enzymes other than ADAMTS3.

To investigate this Jha, Rauniyar et al. tested a specific family of proteins commonly found in the human prostate, which have previously been predicted to act on VEGF-C. This revealed that the lymphatic vessel growth factor can also be activated by an enzyme found in seminal fluid called prostate specific antigen, or PSA for short. To see if enzymes in other bodily fluids could switch on VEGF-C, different components of human saliva were separated and tested to see which could cut inactive VEGF-C. This showed that VEGF-C could be converted to an active form by another enzyme called cathepsin D.

Unexpectedly, Jha, Rauniyar et al. found that VEGF-C was also present in semen. For conception to occur PSA must liquify the semen following ejaculation. It was discovered that PSA activates VEGF-C just as the semen starts to liquify, suggesting that the lymphatic vessel growth factor might also play an important role in reproduction. In addition to VEGF-C, both PSA and cathepsin D were found to activate another growth factor called VEGF-D, which has an unknown role in the human body.

VEGF-C helps the spread of tumors, and blocking the two enzymes that activate this growth factor may be a new therapeutic approach for cancer. However, more work is needed to validate which types of tumor, if any, use these enzymes to activate VEGF-C. In addition, understanding the relationship between PSA and VEGF-C could help improve our knowledge of human reproduction.

The Role of Limbal Epithelial Stem Cells in Regulating Corneal (Lymph)angiogenic Privilege and the Micromilieu of the Limbal Niche following UV Exposure

The cornea is a clear structure, void of blood, and lymphatic vessels, functioning as our window to the world. Limbal epithelial stem cells, occupying the area between avascular cornea and vascularized conjunctiva, have been implicated in tissue border maintenance, preventing conjunctivalisation and propagation of blood and lymphatic vessels into the cornea. Defects in limbal epithelial stem cells are linked to corneal neovascularisation, including lymphangiogenesis, chronic inflammation, conjunctivalisation, epithelial abnormalities including the presence of goblet cells, breaks in Bowman's membrane, persistent epithelial defects and ulceration, ocular surface squamous neoplasia, lipid keratopathy, pain, discomfort, and compromised vision. It has been postulated that pterygium is an example of focal limbal deficiency. Previous reports showing changes occurring in limbal epithelium during pterygium pathogenesis suggest that there is a link to stem cell damage. In this light, pterygium can serve as a model disease of UV-induced stem cell damage also characterised by corneal blood and lymphangiogenesis. This review focuses on the role of corneal and limbal epithelial cells and the stem cell niche in maintaining corneal avascularity and corneal immune privilege and how this may be deregulated following UV exposure. We present an overview of the PUBMED literature in the field as well as recent work from our laboratories.

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.

Roles of signaling and transcriptional networks in pathological lymphangiogenesis

Lymphangiogenesis, the generation of new lymphatic vessels, plays important roles in cancer metastasis. Outstanding progress during the past decade has dramatically increased the novel knowledge and insights of the mechanisms underlying the generation of new lymphatic vessels, the roles of transcription factors and lymphangiogenic growth factors during physiological development and pathological processes such as cancer and inflammation. Furthermore, an understanding of the molecular consequences during tumor lymphangiogenesis has provided chances to develop better diagnostic and therapeutic approaches that aim to limit the progression of cancer. In this article, we will explain the current knowledge of how lymphatic function is altered in various pathological conditions including cancer progression.

Yes-associated protein regulates endothelial cell contact-mediated expression of angiopoietin-2

Angiogenesis is regulated by the dynamic interaction between endothelial cells (ECs). Hippo-Yes-associated protein (YAP) signalling has emerged as a key pathway that controls organ size and tissue growth by mediating cell contact inhibition. However, the role of YAP in EC has not been defined yet. Here, we show expression of YAP in the developing front of mouse retinal vessels. YAP subcellular localization, phosphorylation and activity are regulated by VE-cadherin-mediated-EC contacts. This VE-cadherin-dependent YAP phosphorylation requires phosphoinositide 3-kinase-Akt activation. We further identify angiopoietin-2 (ANG-2) as a potential transcriptional target of YAP in regulating angiogenic activity of EC in vitro and in vivo. Overexpression of YAP-active form in EC enhances angiogenic sprouting, and this effect is blocked by ANG-2 depletion or soluble Tie-2 treatment. These findings implicate YAP as a critical regulator in angiogenesis and provide new insights into the mechanism coordinating junctional stability and angiogenic activation of ECs.

Poxue Huayu and Tianjing Busui Decoction for cerebral hemorrhage (Upregulation of neurotrophic factor expression): Upregulation of neurotrophic factor expression

This study established a rat model of cerebral hemorrhage by injecting autologous anticoagulated blood. Rat models were intragastrically administered 5, 10, 20 g/kg Poxue Huayu and Tianjing Busui Decoction, supplemented with Hirudo, raw rhubarb, raw Pollen Typhae, gadfly, Fructrs Trichosanthis, Radix Notoginseng, Rhizoma Acori Talarinowii, and glue of tortoise plastron, once a day, for 14 consecutive days. Results demonstrated that brain water content significantly reduced in rats with cerebral hemorrhage, and intracerebral hematoma volume markedly reduced after treatment. Immunohistochemical staining revealed that brain-derived neurotrophic factor, tyrosine kinase B and vascular endothelial growth factor expression noticeably increased around the surrounding hematoma. Reverse transcription-PCR revealed that brain-derived neurotrophic factor and tyrosine kinase B mRNA expression significantly increased around the surrounding hematoma. Neurologic impairment obviously reduced. These results indicated that Poxue Huayu and Tianjing Busui Decoction exert therapeutic effects on cerebral hemorrhage by upregulating the expression of brain-derived neurotrophic factor.

Impact of the prolymphangiogenic crosstalk in the tumor microenvironment on lymphatic cancer metastasis

Lymphangiogenesis is a very early step in lymphatic metastasis. It is regulated and promoted not only by the tumor cells themselves, but also by cells of the tumor microenvironment, including cancer associated fibroblasts, mesenchymal stem cells, dendritic cells, or macrophages. Even the extracellular matrix as well as cytokines and growth factors are involved in the process of lymphangiogenesis and metastasis. The cellular and noncellular components influence each other and can be influenced by the tumor cells. The knowledge about mechanisms behind lymphangiogenesis in the tumor microenvironmental crosstalk is growing and offers starting points for new therapeutic approaches.

Anti-lymphangiogenesis effects of a specific anti-interleukin 7 receptor antibody in lung cancer model in vivo

Interleukin 7 (IL-7) is known to promote lymphangiogenesis. To study the relationship between IL-7 and the lymphangiogenesis in lung cancer cells xenograft tumors, we investigated how IL-7 regulates lymphangiogenesis by Quantitative real-time reverse transcriptase-polymerase chain reaction, Western blot, co-immunoprecipitation, chromatin immunoprecipitation, and immunohistochemistry methods. We found that, in lung cancer cells xenograft tumors IL-7/IL-7 receptor (IL-7R) increase the expression of VEGF-D and lymphangiogenesis, induce c-Fos and c-Jun heterodimer formation, and enhance c-Fos/c-Jun DNA binding activity to regulate VEGF-D. Taken together, our results provided evidence that IL-7/IL-7R induce VEGF-D upregulation and promote lymphangiogenesis via c-Fos/c-Jun pathway in lung cancer.

Novel anti(lymph)angiogenic treatment strategies for corneal and ocular surface diseases

The cornea is one of the few tissues which actively maintain an avascular state, i.e. the absence of blood and lymphatic vessels (corneal [lymph]angiogenic privilege). Nonetheless do several diseases interfere with this privilege and cause pathologic corneal hem- and lymphangiogenesis. The ingrowths of pathologic blood and lymphatic vessels into the cornea not only reduce transparency and thereby visual acuity up to blindness, but also significantly increases the rate of graft rejections after subsequent corneal transplantation. Therefore great interest exists in new strategies to target pathologic corneal (lymph)angiogenesis to promote graft survival. This review gives an overview on the vascular anatomy of the normal ocular surface, on the molecular mechanisms contributing to the corneal (lymph)angiogenic privilege and on the cellular and molecular mechanisms occurring during pathological neovascularization of the cornea. In addition we summarize the current preclinical and clinical evidence for three novel treatment strategies against ocular surface diseases based on targeting pathologic (lymph)angiogenesis: (a) modulation of the immune responses after (corneal) transplantation by targeting pathologic (lymph)angiogenesis prior to and after transplantation, (b) novel concepts against metastasis and recurrence of ocular surface tumors such as malignant melanoma of the conjunctiva by anti(lymph)angiogenic therapy and (c) new ideas on how to target ocular surface inflammatory diseases such as dry eye by targeting conjunctival and corneal lymphatic vessels. Based on compelling preclinical evidence and early data from clinical trials the novel therapeutic concepts of promoting graft survival, inhibiting tumor metastasis and dampening ocular surface inflammation and dry eye disease by targeting (lymph)angiogenesis are on their way to translation into the clinic.

Widespread evidence of viral miRNAs targeting host pathways

Background

MicroRNAs (miRNA) are regulatory genes that target and repress other RNA molecules via sequence-specific binding. Several biological processes are regulated across many organisms by evolutionarily conserved miRNAs. Plants and invertebrates employ their miRNA in defense against viruses by targeting and degrading viral products. Viruses also encode miRNAs and there is evidence to suggest that virus-encoded miRNAs target specific host genes and pathways that may be beneficial for their infectivity and/or proliferation. However, it is not clear whether there are general patterns underlying cellular targets of viral miRNAs.

Results

Here we show that for several of the 135 known viral miRNAs in human viruses, the human genes targeted by the viral miRNA are enriched for specific host pathways whose targeting is likely beneficial to the virus. Given that viral miRNAs continue to be discovered as technologies evolve, we extended the investigation to 6809 putative miRNAs encoded by 23 human viruses. Our analysis further suggests that human viruses have evolved their miRNA repertoire to target specific human pathways, such as cell growth, axon guidance, and cell differentiation. Interestingly, many of the same pathways are also targeted in mice by miRNAs encoded by murine viruses. Furthermore, Human Cytomegalovirus (CMV) miRNAs that target specific human pathways exhibit increased conservation across CMV strains.

Conclusions

Overall, our results suggest that viruses may have evolved their miRNA repertoire to target specific host pathways as a means for their survival.

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.

Roles of transcriptional network during the formation of lymphatic vessels

The lymphatic vascular system, also known as the second vascular system in vertebrates, plays crucial roles in various physiological and pathological processes. It participates in the maintenance of normal tissue fluid balance, trafficking of the immune cells and absorption of fatty acids in the gut. Furthermore, lymphatic system is associated with the pathogenesis of a number of diseases, including lymphedema, inflammatory diseases and tumour metastasis. Lymphatic vessels are comprised of lymphatic endothelial cells (LECs), which are differentiated from blood vascular endothelial cells. This review highlights recent advances in our understanding of the transcriptional control of LEC fate determination and reflects on efforts to understand the roles of transcriptional networks during this discrete developmental process.

Dilated thin-walled blood and lymphatic vessels in human endometrium: a potential role for VEGF-D in progestin-induced break-through bleeding

Progestins provide safe, effective and cheap options for contraception as well as the treatment of a variety of gynaecological disorders. Episodes of irregular endometrial bleeding or breakthrough bleeding (BTB) are a major unwanted side effect of progestin treatment, such that BTB is the leading cause for discontinued use of an otherwise effective and popular medication. The cellular mechanisms leading to BTB are poorly understood. In this study, we make the novel finding that the large, dilated, thin walled vessels characteristic of human progestin-treated endometrium include both blood and lymphatic vessels. Increased blood and lymphatic vessel diameter are features of VEGF-D action in other tissues and we show by immunolocalisation and Western blotting that stromal cell decidualisation results in a significant increase in VEGF-D protein production, particularly of the proteolytically processed 21 kD form. Using a NOD/scid mouse model with xenografted human endometrium we were able to show that progestin treatment causes decidualisation, VEGF-D production and endometrial vessel dilation. Our results lead to a novel hypothesis to explain BTB, with stromal cell decidualisation rather than progestin treatment per se being the proposed causative event, and VEGF-D being the proposed effector agent.

Photodynamic ablation of lymphatic vessels and intralymphatic cancer cells prevents metastasis

The dissemination of tumor cells to sites far from the primary tumor (metastasis) is the principal cause of death in cancer patients. Tumor-associated lymphatic vessels are a key conduit for metastatic tumor cells, which typically first colonize the lymph nodes. Although the primary tumor and affected lymph nodes can be removed during surgery, tumor cells inside lymphatic vessels are left behind. Here, we show that in-transit tumor cells inside lymphatic vessels in mice bearing mouse melanomas or human lung tumors give rise to metastases. Using photodynamic therapy with the benzoporphyrin derivative verteporfin, we selectively destroyed lymphatic vessels in mice and pigs. Destruction of tumor-associated lymphatic vessels also eradicated intralymphatic tumor cells and prevented metastasis of mouse melanoma cells and subsequent relapse. Photodynamic therapy, when combined with anti-lymphangiogenic therapy, prevented further tumor invasion of lymphatic vessels. These findings highlight the potential of targeting in-transit tumor cells in patients.

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.

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.

Autocrine effects of VEGF-D on endothelial cells after transduction with AD-VEGF-D(DeltaNDeltaC)

Endothelial cells in tumor vessels display unusual characteristics in terms of survival and angiogenic properties which result from the increased expression of VEGF-D and its autocrine effect. To evaluate mechanisms by which VEGF-D leads to such abnormal phenotype, we searched for proteins with modified expression in HUVECs enriched in the recombinant mature VEGF-D (VEGFD(DeltaNDeltaC)) delivered by adenovirus. Expression of membrane proteins in endothelial cells was characterized by FACS using anti-human IT-Box-135 antibodies. HUVECs transduced with Ad-VEGF-D(DeltaNDeltaC) revealed markedly increased expression of proteins involved in adhesion and migration such as (a) integrins (alphaVbeta5, alpha2beta1, alpha5beta1, alphaMbeta2, alphaLbeta2), (b) matrix metalloproteinases (MMP-2, MMP-9, and MMP-14), (c) components of fibrinolytic system (PAI-1, u-PAR), and (d) CD45, CD98, CD147. Interestingly, there also were numerous proteins with significantly reduced expression, particularly among surface exposed membrane proteins. Thus, it can be concluded that to induce proangiogenic phenotype and facilitate migration of HUVECs, VEGF-D(DeltaNDeltaC) not only upregulates expression of proteins known to participate in the cell-matrix interactions but also silences some membrane proteins which could interfere with this process.

Lymphatic development

The lymphatic system is essential for fluid homeostasis, immune responses, and fat absorption, and is involved in many pathological processes, including tumor metastasis and lymphedema. Despite its importance, progress in understanding the origins and early development of this system has been hampered by lack of defining molecular markers and difficulties in observing lymphatic cells in vivo and performing genetic and experimental manipulation of the lymphatic system. Recent identification of new molecular markers, new genes with important functional roles in lymphatic development, and new experimental models for studying lymphangiogenesis has begun to yield important insights into the emergence and assembly of this important tissue. This review focuses on the mechanisms regulating development of the lymphatic vasculature during embryogenesis.

Cooperation between integrin alphavbeta3 and VEGFR2 in angiogenesis

The cross-talk between receptor tyrosine kinases and integrin receptors are known to be crucial for a number of cellular functions. On endothelial cells, an interaction between integrin alphavbeta3 and VEGFR2 seems to be particularly important process during vascularization. Importantly, the functional association between VEGFR2 and integrin alphavbeta3 is of reciprocal nature since each receptor is able to promote activation of its counterpart. This mutually beneficial relationship regulates a number of cellular activities involved in angiogenesis, including endothelial cell migration, survival and tube formation, and hematopoietic cell functions within vasculature. This article discusses several possible mechanisms reported by different labs which mediate formation of the complex between VEGFR-2 and alphavbeta3 on endothelial cells. The pathological consequences and regulatory events involved in this receptor cross-talk are also presented.

Integrin and growth factor receptor alliance in angiogenesis

A sequence of events in vascular and stromal cells maintained in a highly coordinated manner regulates angiogenesis and tissue remodeling. These processes are mediated by the ability of cells to respond to environmental cues and activate surface integrins. Physiological and pathological processes in vascular biology are dependent on the specificity of important signaling mechanisms that are activated through the association between growth factors, their receptors, integrins, and their specific extracellular matrix ligands. A large body of evidence from in vitro and in vivo models demonstrates the importance of coordination of signals from the extracellular environment that activates specific tyrosine kinase receptors and integrins in order to regulate angiogenic processes in vivo. In addition to complex formation between growth factor receptors and integrins, growth factors and cytokines also directly interact with integrins, depending upon their concentration levels in the environment, and differentially regulate integrin-related processes. Recent studies from a number of laboratories including ours have provided important novel insights into the involvement of many signaling events that improve our existing knowledge on the cross-talk between growth factor receptors and integrins in the regulation of angiogenesis. In this review, our focus will be on updating the recent developments in the field of integrin-growth factor receptor associations and their implications in the vascular processes.

Human telomerase reverse transcriptase expression correlates with vascular endothelial growth factor-promoted tumor cell proliferation in prostate cancer

OBJECTIVE

To investigate the correlation between the expressions of human telomerase reverse transcriptase (hTERT) and vascular endothelial growth factor (VEGF) in prostate cancer (PCa) and benign prostatic hyperplasia (BPH), and to determine if hTERT was correlated with VEGF-promoted tumor cell proliferation in prostate cancer.

MATERIALS AND METHODS

Immunohistochemistry was used to analyze the expressions of hTERT and VEGF in 60 cases of PCa and 60 cases of benign prostatic hyperplasia (BPH). Then their correlation in PCa was analyzed by Spearman correlative analysis.

RESULTS

The expressions of hTERT were detected in 38 cases of PCa and 10 cases of BPH. The expressions of VEGF were detected in 46 cases of PCa and 28 cases of BPH. The expressions of hTERT and VEGF in PCa were significantly higher than those in BPH (P < 0.05). As a result of correlation analysis, it was found that with an increase of the expression of VEGF, the expression of hTERT also increased in PCa. Significant correlation was observed between the expressions of hTERT and VEGF in PCa (r = 0.8333, P < 0.05). But there was no significant correlation between the expressions of hTERT and VEGF in BPH (r = 0.3156, P > 0.05).

CONCLUSIONS

All experiences above indicate that hTERT was one of the important proteins in the proliferation-promoting effect of VEGF on tumor cells in PCa.

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.

Interleukin-1beta-mediated inhibition of the processes of angiogenesis in cardiac microvascular endothelial cells

Angiogenesis, the formation of new capillaries from preexisting vessels, plays an essential role in revascularization of the myocardium following myocardial infarction (MI). Interleukin-1beta (IL-1beta), a proinflammatory cytokine increased in the heart following MI, is shown to be essential for angiogenesis in the invasiveness of tumor cells, the progression of arthritic conditions and endometriosis, and the promotion of wound healing. Here we studied the steps of angiogenesis in response to IL-1beta in cardiac microvascular endothelial cells (CMECs) and aortic tissue. Cell cycle progression analysis using flow cytometry indicated a G0/G1 phase cell cycle arrest in IL-1beta-stimulated cells. IL-1beta significantly reduced levels of fibrillar actin in the cytoskeleton, a pre-requisite for tube formation, as indicated by phalloidin-FITC staining. Wound healing assays demonstrated IL-1beta prevents cell-to-cell contact formation. On the other hand, vascular endothelial growth factor-D (VEGF-D) initiated restoration of the cell monolayer. IL-1beta significantly inhibited in vitro tube formation as analyzed by three-dimensional collagen matrix assay. Aortic ring assay demonstrated that IL-1beta inhibits basal and VEGF-D-stimulated microvessel sprouting from aortic rings. The data presented here are novel and of significant interest, providing evidence that IL-1beta impedes the process of angiogenesis in myocardial endothelial cells.

Vascular endothelial growth factor attenuates hepatic sinusoidal capillarization in thioacetamide-induced cirrhotic rats

AIM: To investigate the effect of vascular endothelial growth factor (VEGF) transfection on hepatic sinusoidal capillarization.

METHODS: Enhanced green fluorescent protein (EGFP)/VEGF transfection was confirmed by immunofluorescence microscopy and immunohistoche-mistry both in primary hepatocytes and in normal liver. Cirrhotic rats were generated by thioacetamide (TAA) administration and then divided into a treatment group, which received injections of 400 &mgr;g of plasmid DNA encoding an EGFP-VEGF fusion protein, and a blank group, which received an equal amount of normal saline through the portal vein. The portal vein pressure was measured in the normal and cirrhotic state, in treated and blank groups. The average number of fenestrae per hepatic sinusoid was determined using transmission electron microscopy (TEM), while the relative abundance of VEGF transcripts was examined by Gene array.

RESULTS: Green fluorescent protein was observed in the cytoplasms of liver cells under immunofluorescence microscopy 24 h after transfection with EGFP/VEGF plasmid in vitro. Staining with polyclonal antibodies against VEGF illustrated that hepatocytes expressed immunodetectable VEGF both in vitro and in vitro. There were significant differences in the number of fenestrae and portal vein pressures between normal and cirrhotic rats (7.40 ± 1.71 vs 2.30 ± 1.16 and 9.32 ± 0.85 cmH₂O vs 17.92 ± 0.90 cmH₂O, P < 0.01), between cirrhotic and treated rats (2.30 ± 1.16 cmH₂O vs 4.60 ± 1.65 and 17.92 ± 0.90 cmH₂O vs 15.52 ± 0.93 cmH₂O, P < 0.05) and between the treatment group and the blank group (4.60 ± 1.65 cmH₂O vs 2.10 ± 1.10 cmH₂O and 15.52 ± 0.93 cmH₂O vs 17.26 ± 1.80 cmH₂O, P < 0.05). Gene-array analysis revealed that the relative abundance of transcripts of VEGF family members decreased in the cirrhotic state and increased after transfection.

CONCLUSION: Injection of a plasmid encoding VEGF through the portal vein is an effective method to induce the formation of fenestrae and decrease portal vein pressure in cirrhotic rats. Therefore, it may be a good choice for treating hepatic cirrhosis and portal hypertension.

Umbilical cord blood-derived progenitor cells enhance muscle regeneration in mouse hindlimb ischemia model

Progenitor cell therapy is a potential new treatment option for ischemic conditions in the myocardium and skeletal muscles. However, it remains unclear whether umbilical cord blood (UCB)-derived progenitor cells can provide therapeutic effects in ischemic muscles and whether ex vivo gene transfer can be used for improving the effect. In this study, the use of a lentiviral vector led to efficient transduction of both UCB-derived hematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs). Our method resulted in a long-term transgene expression and did not alter the differentiation potential of either HSCs or MSCs. In addition, we studied the therapeutic potential of CD133(+) and MSC progenitor cells transduced ex vivo with lentiviruses encoding the mature form of vascular endothelial growth factor D (VEGF-D(DeltaNDeltaC)) or the enhanced green fluorescent protein (eGFP) marker gene in a nude mouse model of skeletal muscle ischemia. Progenitor cells enhanced the regeneration of ischemic muscles without a detectable long-term engraftment of either CD133(+) or MSC progenitor cells. Our results show that, rather than directly participating in angiogenesis or skeletal myogenesis, UCB-derived progenitor cells indirectly enhance the regenerative capacity of skeletal muscle after acute ischemic injury. However, VEGF-D gene transfer of progenitor cells did not improve the therapeutic effect in ischemic muscles.

Akt1 signaling regulates integrin activation, matrix recognition, and fibronectin assembly

Akt, a serine-threonine kinase, regulates multiple cellular processes in vascular cells. We have previously documented that Akt activates integrins and Akt1 deficiency results in matrix abnormalities in skin and blood vessels in vivo. Based on these observations, we hypothesized that Akt1 is necessary for integrin activation and matrix assembly by fibroblasts. In this study, using various cell systems, we show that Akt1 is essential for the inside-out activation of integrins in endothelial cells and fibroblasts, which in turn, mediates matrix assembly. Fibronectin is a major extracellular matrix component of the skin and the vascular basement membrane, which possesses binding sites for many integrins and extracellular matrix proteins. Akt1(-/-) fibroblasts and NIH fibroblasts expressing dominant negative Akt1 (K179M-Akt1) showed impaired fibronectin assembly compared with control fibroblasts. In contrast, expression of constitutively active Akt1 (myrAkt1) resulted in enhanced fibronectin assembly. Although increased fibronectin assembly by myrAkt1-expressing human foreskin fibroblasts was abolished by treatment with anti-integrin beta(1) blocking antibodies, treatment with beta(1)-stimulating antibodies rescued the impaired fibronectin assembly that was due to lack of Akt activity. Finally, expression of myrAkt1 corrected the phenotype of Akt1(-/-) fibroblasts thus showing that Akt1 regulates fibronectin assembly through activation of integrin alpha(5)beta(1).

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.

Akt1 governs breast cancer progression in vivo

The serine threonine kinase Akt1 has been implicated in the control of cellular metabolism, survival and growth. Here, disruption of the ubiquitously expressed member of the Akt family of genes, Akt1, in the mouse demonstrates a requirement for Akt1 in ErbB2-induced mammary tumorigenesis. Akt1 deficiency delayed tumor growth and reduced lung metastases, correlating with a reduction in phosphorylation of the Akt1 target, tuberous sclerosis 2 (TSC2) at Ser-939. Akt1-deficient mammary epithelial tumor cells (MEC) were reduced in size and proliferative capacity, with reduced cyclin D1 and p27(KIP1) abundance. Akt1 deficiency abrogated the oncogene-induced changes in polarization of MEC in three-dimensional culture and reverted oncogene-induced relocalization of the phosphorylated ezrin-radixin-moesin proteins. Akt1 increased MEC migration across an endothelial cell barrier, enhancing the persistence of migratory directionality. An unbiased proteomic analysis demonstrated Akt1 mediated MEC migration through paracrine signaling via induction of expression and secretion of CXCL16 and MIP1gamma. Akt1 governs MEC polarity, migratory directionality and breast cancer onset induced by ErbB2 in vivo.

Vascular endothelial growth factor family of ligands and receptors: review

VEGF signaling often represents a critical rate-limiting step in physiological angiogenesis. The VEGF family comprises seven secreted glycoproteins that are designated VEGF-A, VEGF-B, VEGF-C, VEGF-D, VEGF-E, placental growth factor (PlGF) and VEGF-F. The VEGF family members bind their cognate receptors. The receptors identified so far are designated VEGFR-1, VEGFR-2, VEGFR-3 and the neuropilins (NP-1 and NP-2). We review in this article the biology of the VEGF ligands and the receptors.

Adenovirus-Mediated Gene Transfer of Human Vascular Endothelial Growth Factor-D Induces Transient Angiogenic Effects in Mouse Hind Limb Muscle

We evaluated the therapeutic potential of adenovirus (Ad)-mediated human vascular endothelial growth factor-D (hVEGF-D) gene delivery in mice. Hind limbs of hypercholesterolemic mice ( n = 120) were injected with AdhVEGF-D, AdhVEGF-A, control AdLacZ (all at 1x10(11)viral particles) or saline. Animals were killed at 4, 7, 14, 28, and 42 days. Newly formed vessels were characterized for their quantity, sprouting, angiogenic versus lymphangiogenic phenotype, and arterial versus venous phenotype by endothelial enzymes markers, pericyte coverage, and electron microscopy. Perfusion was measured by power Doppler ultrasound and edema by magnetic resonance imaging (MRI). AdhVEGF-D induced significant formation of new blood vessels, which featured lumenal enlargement, branching, and sprouting. Branching originated mainly from arterioles. The highest vessel density was present on days 4-7 and the effect lasted up to 28 days. Endothelial marker enzyme activity indicated the predominance of arterial capillaries and arterioles. Forty percent of the neovessels were positive for desmin, indicating that VEGF-D increased pericyte coverage. However, branching vessels were highly positive for smooth muscle actin pericyte marker but negative for desmin. Maximal perfusion was measured during the first week after AdhVEGF-D gene transfer. Ultrastructural analysis showed endothelial cells enriched with vesiculo-vacuolar organelles and cytoplasmic protrusions. Modest lymphangiogenic activity was also detected, which could contribute to the relatively low level of edema detected by MRI. In conclusions, AdhVEGF-D has a strong angiogenic effect and a modest lymphangiogenic effect in mouse skeletal muscle. VEGF-D also increases the presence of pericytes/smooth muscle cells in neovessels. AdhVEGF-D is a potential new agent for the induction of therapeutic vascular growth in skeletal muscle.

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.

Localisation of lymphatic vessels and vascular endothelial growth factors-C and -D in human and mouse skeletal muscle with immunohistochemistry

The present study was aimed to localise lymphatic vessels and their growth factors in human and mouse skeletal muscle with immunohistochemistry and specific antibodies (VEGFR-3, LYVE-1, VEGF-C and VEGF-D). The largest lymphatic vessels were found in perimysial connective tissue next to the arteries and veins, as has been shown earlier with electron microscopy. As a new finding, we also found small LYVE-1 positive vessels in the capillary bed between muscle fibres. These vessels were located next to CD31 positive blood capillaries and were of the same size, but fewer in number. In addition, we described the localisation of the two main lymphangiogenic growth factor proteins, vascular endothelial growth factor-C and -D. Both proteins were expressed in skeletal muscle at mRNA and protein levels. VEGF-D was located under the sarcolemma in some of the muscle fibres, in the endothelia of larger blood vessels and in fibroblasts. VEGF-C protein was localised to the nerves and muscle spindles, to fibroblasts and surrounding connective tissue, but was not found in muscle fibres or endothelial cells. Our results are the first to suggest the presence of lymphatic capillaries throughout the skeletal muscle, and to present the localisation of VEGF-C and -D in the muscles.

Lymphangiogenesis does not occur in breast cancer

Breast cancer metastasis predominantly occurs via lymphatic vessels. However, the study of lymphatic vessels and lymphangiogenesis has been hampered by lack of specific markers. Recently, antibodies directed against M2A (D2-40), Podoplanin, and Prox-1 that specifically mark lymphatic vessels in paraffin-embedded sections have become available. These were used to study lymphangiogenesis in archival paraffin sections of normal breast (n = 23), fibrocystic disease (n = 7), ductal carcinoma in situ (n = 32), invasive ductal carcinoma (n = 50), and invasive lobular carcinoma (n = 5). In addition, endothelial proliferation in lymphatic vessels was analyzed by dual-color immunohistochemistry with D2-40 and proliferating cell nuclear antigen (PCNA). Expression of D2-40, Prox-1, and Podoplanin was seen in lymphatic vessels but not in blood vessels. Lymphatic vessels were seen in the peritumoral area and as "entrapped" intratumoral vessels adjacent to preexisting normal lobules and ducts. Unlike angiogenesis, there was no increase of lymphatic vessel density in association with neoplastic transformation. On the contrary, a marked reduction in intratumoral lymphatic vessel density was seen in comparison to normal breast tissue, fibrocystic disease, and ductal carcinoma in situ (P = 0.0001). There was an increase in peritumoral lymphatic vessel density as compared with normal breast (P = 0.0001). However, the endothelial cells in the "entrapped" or the peritumoral lymphatic vessels did not show any expression of PCNA indicating minimal or no proliferative activity. This was in contrast to the strong expression seen in adjacent tumor cells and blood vessel endothelial cells. Thus, lymphangiogenesis was not evident when studied by lymphatic vessel density or by lymph vessel endothelial proliferation.

The lymphatic vasculature: recent progress and paradigms

The field of lymphatic research has been recently invigorated by the identification of genes and mechanisms that control various aspects of lymphatic development. We are beginning to understand how, starting from a subgroup of embryonic venous endothelial cells, the whole lymphatic system forms in a stepwise manner. The generation of genetically engineered mice with defects in different steps of the lymphangiogenic program has provided models that are increasing our understanding of the lymphatic system in health and disease. This knowledge, in turn, should lead to the development of better diagnostic methods and treatments of lymphatic disorders and tumor metastasis.

Lymphangiogenesis by blind-ended vessel sprouting is concurrent with hemangiogenesis by vascular splitting

Development of effective vascular therapies requires the understanding of all modes of vessel formation involved in angiogenesis (here termed "hemangiogenesis") and lymphangiogenesis. Two major modes of vessel morphogenesis include sprouting of a new vessel from a preexisting vessel and splitting of a preexisting parent vessel into two offspring vessels. In the quail chorioallantoic membrane (CAM) during mid-development (embryonic days E6-E9), lymphangiogenesis progressed primarily via blind-ended vessel sprouting. Isolated lymphatic endothelial progenitor cells were recruited to the tips of growing vessels. During concurrent hemangiogenesis, parent blood vessels expanded from the capillary network and split into offspring vessels, accompanied by transient capillary expression of alpha smooth muscle actin (alphaSMA) and recruitment of polarized mural progenitor cells. Lymphatics and blood vessels were identified by confocal/fluorescence microscopy of vascular endothelial growth factor (VEGF) receptor VEGFR-2, alphaSMA (specific to CAM blood vessels), homeobox transcription factor Prox1 (specific to lymphatics), and the quail hematopoetic marker, QH-1. VEGFR-2 was expressed intensely in isolated cells and lymphatics, and moderately in blood vessels. Prox1 was absent from isolated progenitor cells prior to lymphatic recruitment. Exogenous vascular endothelial growth factor-165 (VEGF165) increased blood vessel density and anastomotic frequency without changing endogenous modes of vascular/lymphatic vessel formation or marker expression. Although VEGF165 is a key cellular regulator of hemangiogenesis and vasculogenesis, the role of VEGF165 in lymphangiogenesis is less clear. Interestingly, VEGF165 increased lymphatic vessel diameter and density as measured by novel Euclidean distance mapping, and the antimaturational dissociation of lymphatics from blood vessels, accompanied by lymphatic reassociation into homogeneous networks.

Molecular lymphangiogenesis: new players

The function of the lymphatic vascular system as a conduit for immune cells and excess tissue fluid has been known for over a century, but the molecular players involved in lymphangiogenesis--the formation of lymphatic vessels--have remained unknown until recently. Signals mediated by vascular endothelial growth factor-C, the homeodomain transcription factor PROX1 and the forkhead transcription factor FOXC2 have been implicated in the growth, morphogenesis and hierarchic organization of the lymphatic vascular network. Recent results have also shown the importance of the angiopoietin-Tie and ephrin-Eph signaling systems in lymphangiogenesis, suggesting that these pathways regulate the lymphatic vascular system in a manner similar to, yet distinct from, their regulation of angiogenesis. This review provides an overview of the molecular players involved in lymphangiogenesis, with special emphasis on recently discovered molecular mechanisms.

Akt1 regulates pathological angiogenesis, vascular maturation and permeability in vivo

Akt kinases control essential cellular functions, including proliferation, apoptosis, metabolism and transcription, and have been proposed as promising targets for treatment of angiogenesis-dependent pathologies, such as cancer and ischemic injury. But their precise roles in neovascularization remain elusive. Here we show that Akt1 is the predominant isoform in vascular cells and describe the unexpected consequences of Akt1 knockout on vascular integrity and pathological angiogenesis. Angiogenic responses in three distinct in vivo models were enhanced in Akt1(-/-) mice; these enhanced responses were associated with impairment of blood vessel maturation and increased vascular permeability. Although impaired vascular maturation in Akt1(-/-) mice may be attributed to reduced activation of endothelial nitric oxide synthase (eNOS), the major phenotypic changes in vascular permeability and angiogenesis were linked to reduced expression of two endogenous vascular regulators, thrombospondins 1 (TSP-1) and 2 (TSP-2). Re-expression of TSP-1 and TSP-2 in mice transplanted with wild-type bone marrow corrected the angiogenic abnormalities in Akt1(-/-) mice. These findings establish a crucial role of an Akt-thrombospondin axis in angiogenesis.

Thrombospondin-1 up-regulates expression of cell adhesion molecules and promotes monocyte binding to endothelium

Expression of cell adhesion molecules (CAM) responsible for leukocyte-endothelium interactions plays a crucial role in inflammation and atherogenesis. Up-regulation of vascular CAM-1 (VCAM-1), intracellular CAM-1 (ICAM-1), and E-selectin expression promotes monocyte recruitment to sites of injury and is considered to be a critical step in atherosclerotic plaque development. Factors that trigger this initial response are not well understood. As platelet activation not only promotes thrombosis but also early stages of atherogenesis, we considered the role of thrombospondin-1 (TSP-1), a matricellular protein released in abundance from activated platelets and accumulated in sites of vascular injury, as a regulator of CAM expression. TSP-1 induced expression of VCAM-1 and ICAM-1 on endothelium of various origins, which in turn, resulted in a significant increase of monocyte attachment. This effect could be mimicked by a peptide derived from the C-terminal domain of TSP-1 and known to interact with CD47 on the cell surface. The essential role of CD47 in the cellular responses to TSP-1 was demonstrated further using inhibitory antibodies and knockdown of CD47 with small interfering RNA. Furthermore, we demonstrated that secretion of endogenous TSP-1 and its interaction with CD47 on the cell surface mediates endothelial response to the major proinflammatory agent, tumor necrosis factor alpha (TNF-alpha). Taken together, this study identifies a novel mechanism regulating CAM expression and subsequent monocyte binding to endothelium, which might influence the development of anti-atherosclerosis therapeutic strategies.