During mammalian development, a subpopulation of endothelial cells in the cardinal vein (CV) expresses lymphatic-specific genes and subsequently develops into the first lymphatic structures, collectively termed as lymph sacs. Budding, sprouting and ballooning of lymphatic endothelial cells (LECs) have been proposed to underlie the emergence of LECs from the CV, but the exact mechanisms of lymph vessel formation remain poorly understood. Applying selective plane illumination-based ultramicroscopy to entire wholemount-immunostained mouse embryos, we visualized the complete developing vascular system with cellular resolution. Here, we report emergence of the earliest detectable LECs as strings of loosely connected cells between the CV and superficial venous plexus. Subsequent aggregation of LECs resulted in formation of two distinct, previously unidentified lymphatic structures, the dorsal peripheral longitudinal lymphatic vessel (PLLV) and the ventral primordial thoracic duct (pTD), which at later stages formed a direct contact with the CV. Providing new insights into their function, we found vascular endothelial growth factor C (VEGF-C) and the matrix component CCBE1 indispensable for LEC budding and migration. Altogether, we present a significantly more detailed view and novel model of early lymphatic development.

Ultramicroscopy of wholemount mouse embryos uncovers the first, previously unknown lymphatic structures in mammals: the dorsal longitudinal lymphatic vessel and the ventral primordial thoracic duct, which eventually connect with the cardinal vein as previously described.

Peroxisome proliferator-activated receptor (PPAR) delta is an important regulator of fatty acid (FA) metabolism. Angiopoietin-like 4 (Angptl4), a multifunctional protein, is one of the major targets of PPAR delta in skeletal muscle cells. Here we investigated the regulation of Angptl4 and its role in mediating PPAR delta functions using human, rat and mouse myotubes. Expression of Angptl4 was upregulated during myotubes differentiation and by oleic acid, insulin and PPAR delta agonist GW501516. Treatment with GW501516 or Angptl4 overexpression inhibited both lipoprotein lipase (LPL) activity and LPL-dependent uptake of FAs whereas uptake of BSA-bound FAs was not affected by either treatment. Activation of retinoic X receptor (RXR), PPAR delta functional partner, using bexarotene upregulated Angptl4 expression and inhibited LPL activity in a PPAR delta dependent fashion. Silencing of Angptl4 blocked the effect of GW501516 and bexarotene on LPL activity. Treatment with GW501516 but not Angptl4 overexpression significantly increased palmitate oxidation. Furthermore, Angptl4 overexpression did not affect the capacity of GW501516 to increase palmitate oxidation. Basal and insulin stimulated glucose uptake, glycogen synthesis and glucose oxidation were not significantly modulated by Angptl4 overexpression. Our findings suggest that FAs-PPARdelta/RXR-Angptl4 axis controls the LPL-dependent uptake of FAs in myotubes, whereas the effect of PPAR delta activation on beta-oxidation is independent of Angptl4.

Fibroblast Growth Factor receptor (FGFR) activity plays crucial roles in tumor growth and patient survival. However, FGF (Fibroblast Growth Factor) signaling as a target for cancer therapy has been under-investigated compared to other receptor tyrosine kinases. Here, we studied the effect of FGFR signaling inhibition on tumor growth, metastasis and lymphangiogenesis by expressing a dominant negative FGFR (FGFR-2DN) in an orthotopic mouse mammary 66c14 carcinoma model. We show that FGFR-2DN-expressing 66c14 cells proliferate in vitro slower than controls. 66c14 tumor outgrowth and lung metastatic foci are reduced in mice implanted with FGFR-2DN-expressing cells, which also exhibited better overall survival. We found 66c14 cells in the lumen of tumor lymphatic vessels and in lymph nodes. FGFR-2DN-expressing tumors exhibited a decrease in VEGFR-3 (Vascular Endothelial Growth Factor Receptor-3) or podoplanin-positive lymphatic vessels, an increase in isolated intratumoral lymphatic endothelial cells and a reduction in VEGF-C (Vascular Endothelial Growth Factor-C) mRNA expression. FGFs may act in an autocrine manner as the inhibition of FGFR signaling in tumor cells suppresses VEGF-C expression in a COX-2 (cyclooxygenase-2) or HIF1-α (hypoxia-inducible factor-1 α) independent manner. FGFs may also act in a paracrine manner on tumor lymphatics by inducing expression of pro-lymphangiogenic molecules such as VEGFR-3, integrin α9, prox1 and netrin-1. Finally, in vitro lymphangiogenesis is impeded in the presence of FGFR-2DN 66c14 cells. These data confirm that both FGF and VEGF signaling are necessary for the maintenance of vascular morphogenesis and provide evidence that targeting FGFR signaling may be an interesting approach to inhibit tumor lymphangiogenesis and metastatic spread.

The coxsackie- and adenovirus receptor (CAR) is a cell adhesion molecule predominantly associated with epithelial tight junctions in adult tissues. CAR is also expressed in cardiomyocytes and essential for heart development up to embryonic day 11.5, but not thereafter. CAR is not expressed in vascular endothelial cells but was recently detected in neonatal lymphatic vessels, suggesting that CAR could play a role in the development of the lymphatic system. To address this, we generated mice carrying a conditional deletion of the CAR gene (Cxadr) and knocked out CAR in the mouse embryo at different time points during post-cardiac development. Deletion of Cxadr from E12.5, but not from E13.5, resulted in subcutaneous edema, hemorrhage and embryonic death. Subcutaneous lymphatic vessels were dilated and structurally abnormal with gaps and holes present at lymphatic endothelial cell-cell junctions. Furthermore, lymphatic vessels were filled with erythrocytes showing a defect in the separation between the blood and lymphatic systems. Regionally, erythrocytes leaked out into the interstitium from leaky lymphatic vessels explaining the hemorrhage detected in CAR-deficient mouse embryos. The results show that CAR plays an essential role in development of the lymphatic vasculature in the mouse embryo by promoting appropriate formation of lymphatic endothelial cell-cell junctions.

Background

Angiopoietin-2 (Ang2), a ligand for endothelial TEK (Tie2) tyrosine kinase receptor, is induced in hypoxic endothelial cells of tumors, where it promotes tumor angiogenesis and growth. However, the effects of Ang2 on tumor lymphangiogenesis and metastasis are poorly characterized.

Methods

We addressed the effect of Ang2 on tumor progression and metastasis using systemic Ang2 overexpression in mice carrying tumor xenografts, endothelium-specific overexpression of Ang2 in VEC-tTA/Tet-OS-Ang2 transgenic mice implanted with isogenic tumors, and administration of Ang2-blocking antibodies to tumor-bearing immunodeficient mice. Fisher's exact test was used for analysis of metastasis occurrence, and repeated measures one-way analysis of variance was used for the analysis of primary tumor growth curves. Unpaired t test was used for all other analyses. All statistical tests were two-sided.

Results

Adenoviral expression of Ang2 increased lymph node and lung metastasis in tumor xenografts. The metastatic burden in the lungs was increased in transgenic mice in which Ang2 expression was induced specifically in the vascular endothelium (tumor burden per grid, VEC-tTA/Tet-OS-Ang2 mice [n = 5] vs control mice [n = 4]: 45.23 vs 12.26 mm2, difference = 32.67 mm2, 95% confidence interval = 31.87 to 34.07, P < .001). Ang2-blocking antibodies reduced lymph node and lung metastasis, as well as tumor lymphangiogenesis, and decreased tumor cell homing to the lungs after intravenous injection. In the lung metastases, Ang2 overexpression decreased endothelial integrity, whereas the Ang2-blocking antibodies improved endothelial cell–cell junctions and basement membrane contacts of metastasis-associated lung capillaries. At the cellular level, the Ang2-blocking antibodies induced the internalization of Ang2-Tie2 receptor complexes from endothelial cell–cell junctions in endothelial–tumor cell cocultures.

Conclusion

Our results indicate that blocking Ang2 inhibits metastatic dissemination in part by enhancing the integrity of endothelial cell–cell junctions.

Angiogenesis, the growth of new blood vessels, involves specification of endothelial cells to tip cells and stalk cells, which is controlled by Notch signalling, whereas vascular endothelial growth factor receptor (VEGFR)-2 and VEGFR-3 have been implicated in angiogenic sprouting. Surprisingly, we found that endothelial deletion of Vegfr3, but not VEGFR-3-blocking antibodies, postnatally led to excessive angiogenic sprouting and branching, and decreased the level of Notch signalling, indicating that VEGFR-3 possesses passive and active signalling modalities. Furthermore, macrophages expressing the VEGFR-3 and VEGFR-2 ligand VEGF-C localized to vessel branch points, and Vegfc heterozygous mice exhibited inefficient angiogenesis characterized by decreased vascular branching. FoxC2 is a known regulator of Notch ligand and target gene expression, and Foxc2+/−; Vegfr3+/− compound heterozygosity recapitulated homozygous loss of Vegfr3. These results indicate that macrophage-derived VEGF-C activates VEGFR-3 in tip cells to reinforce Notch signalling, which contributes to the phenotypic conversion of endothelial cells at fusion points of vessel sprouts.

Twenty years after the discovery of the vascular endothelial Tie receptor tyrosine kinases and 15 years after the discovery of the Tie2 ligand, angiopoietin-1 (Angpt1, also known as Ang1), a study published in the current issue of the JCI reveals an unexpected loss-of-function phenotype of mice conditionally deleted of the Angpt1 gene. The results suggest that Angpt1 is needed as a vascular stabilizing factor that organizes and limits the angiogenesis response and protects from pathological consequences, such as tissue fibrosis.

Detection of recurrent somatic rearrangements routinely allows monitoring of residual disease burden in leukemias, but is not used for most solid tumors. However, next-generation sequencing now allows rapid identification of patient-specific rearrangements in solid tumors. We mapped genomic rearrangements in three cancers and showed that PCR assays for rearrangements could detect a single copy of the tumor genome in plasma without false positives. Disease status, drug responsiveness, and incipient relapse could be serially assessed. In future, this strategy could be readily established in diagnostic laboratories, with major impact on monitoring of disease status and personalizing treatment of solid tumors.

Background

Most bladder cancer patients experience lymphatic metastasis in the course of disease progression, yet the relationship between lymphangiogenesis and lymphatic metastasis is not well known. The aim of this study is to elucidate underlying mechanisms of how expanded lymphatic vessels and tumor microenvironment interacts each other and to find effective therapeutic options to inhibit lymphatic metastasis.

Results

The orthotopic urinary bladder cancer (OUBC) model was generated by intravesical injection of MBT-2 cell lines. We investigated the angiogenesis, lymphangiogenesis, and CD11b+/CD68+ tumor-associated macrophages (TAM) by using immunofluorescence staining. OUBC displayed a profound lymphangiogenesis and massive infiltration of TAM in primary tumor and lymphatic metastasis in lymph nodes. TAM flocked near lymphatic vessels and express higher levels of VEGF-C/D than CD11b- cells. Because VEGFR-3 was highly expressed in lymphatic vascular endothelial cells, TAM could assist lymphangiogenesis by paracrine manner in bladder tumor. VEGFR-3 expressing adenovirus was administered to block VEGF-C/D signaling pathway and clodronate liposome was used to deplete TAM. The blockade of VEGF-C/D with soluble VEGF receptor-3 markedly inhibited lymphangiogenesis and lymphatic metastasis in OUBC. In addition, the depletion of TAM with clodronate liposome exerted similar effects on OUBC.

Conclusion

VEGF-C/D are the main factors of lymphangiogenesis and lymphatic metastasis in bladder cancer. Moreover, TAM plays an important role in these processes by producing VEGF-C/D. The inhibition of lymphangiogenesis could provide another therapeutic target to inhibit lymphatic metastasis and recurrence in patients with invasive bladder cancer.

In individuals with mammary carcinoma, the most relevant prognostic predictor of distant organ metastasis and clinical outcome is the status of axillary lymph node metastasis. Metastases form initially in axillary sentinel lymph nodes and progress via connecting lymphatic vessels into postsentinel lymph nodes. However, the mechanisms of consecutive lymph node colonization are unknown. Through the analysis of human mammary carcinomas and their matching axillary lymph nodes, we show here that intrametastatic lymphatic vessels and bulk tumor cell invasion into these vessels highly correlate with formation of postsentinel metastasis. In an in vitro model of tumor bulk invasion, human mammary carcinoma cells caused circular defects in lymphatic endothelial monolayers. These circular defects were highly reminiscent of defects of the lymphovascular walls at sites of tumor invasion in vivo and were primarily generated by the tumor-derived arachidonic acid metabolite 12S-HETE following 15-lipoxygenase-1 (ALOX15) catalysis. Accordingly, pharmacological inhibition and shRNA knockdown of ALOX15 each repressed formation of circular defects in vitro. Importantly, ALOX15 knockdown antagonized formation of lymph node metastasis in xenografted tumors. Furthermore, expression of lipoxygenase in human sentinel lymph node metastases correlated inversely with metastasis-free survival. These results provide evidence that lipoxygenase serves as a mediator of tumor cell invasion into lymphatic vessels and formation of lymph node metastasis in ductal mammary carcinomas.

The role of lymphangiogenesis in inflammation has remained unclear. To investigate the role of lymphatic versus blood vasculature in chronic skin inflammation, we inhibited vascular endothelial growth factor (VEGF) receptor (VEGFR) signaling by function-blocking antibodies in the established keratin 14 (K14)–VEGF-A transgenic (Tg) mouse model of chronic cutaneous inflammation. Although treatment with an anti–VEGFR-2 antibody inhibited skin inflammation, epidermal hyperplasia, inflammatory infiltration, and angiogenesis, systemic inhibition of VEGFR-3, surprisingly, increased inflammatory edema formation and inflammatory cell accumulation despite inhibition of lymphangiogenesis. Importantly, chronic Tg delivery of the lymphangiogenic factor VEGF-C to the skin of K14-VEGF-A mice completely inhibited development of chronic skin inflammation, epidermal hyperplasia and abnormal differentiation, and accumulation of CD8 T cells. Similar results were found after Tg delivery of mouse VEGF-D that only activates VEGFR-3 but not VEGFR-2. Moreover, intracutaneous injection of recombinant VEGF-C156S, which only activates VEGFR-3, significantly reduced inflammation. Although lymphatic drainage was inhibited in chronic skin inflammation, it was enhanced by Tg VEGF-C delivery. Together, these results reveal an unanticipated active role of lymphatic vessels in controlling chronic inflammation. Stimulation of functional lymphangiogenesis via VEGFR-3, in addition to antiangiogenic therapy, might therefore serve as a novel strategy to treat chronic inflammatory disorders of the skin and possibly also other organs.

SUMMARY

Tie-2 is a member of the receptor tyrosine kinase family and is required for vascular remodeling and maintenance of mammalian vessel integrity. A number of mutations in the human TIE2 gene have been identified in patients suffering from cutaneomucosal venous malformations and ventricular septal defects. How exactly Tie-2 signaling pathways play different roles in both vascular development and vascular stability is unknown. We have generated a zebrafish line carrying a stop mutation in the kinase domain of the Tie-2 receptor. Mutant embryos lack Tie-2 protein, but do not display any defect in heart and vessel development. Simultaneous loss of Tie-1 and Tie-2, however, leads to a cardiac phenotype. Our study shows that Tie-1 and Tie-2 are not required for early heart development, yet they have redundant roles for the maintenance of endocardial-myocardial connection in later stages. Tie-2 and its ligand Angiopoietin-1 have also been reported to play an important role in vessel stability. We used atorvastatin and simvastatin, drugs that cause bleeding in wild-type zebrafish larvae, to challenge vessel stability in tie-2 mutants. Interestingly, recent clinical studies have reported hemorrhagic stroke as a side effect of atorvastatin treatment. Exposure of embryos to statins revealed that tie-2 mutants are significantly protected from statin-induced bleeding. Furthermore, tie-2 mutants became less resistant to bleeding after VE-cadherin knockdown. Taken together, these data show that atorvastatin affects vessel stability through Tie-2, and that VE-cadherin and Tie-2 act in concert to allow vessel remodeling while playing a role in vessel stability. Our study introduces an additional vertebrate model to study in vivo the function of Tie-2 in development and disease.

Tec family kinases have important roles in lymphocytes, however little is known about their function in monocytes/macrophages. Here we report that Tec family kinases are essential for macrophage colony stimulating factor (M-CSF)-induced signaling pathways that regulate macrophage survival. Compared to wildtype bone marrow-derived macrophages (BMM) cultures, Tec−/−Btk−/− BMM cultures displayed increased cell death that correlated with a severe drop in macrophage numbers. In addition, macrophages deficient in either Tec or Btk showed expression and activation of caspase-11. Elucidation of M-CSF receptor (M-CSFR) signaling pathways revealed that the total tyrosine phosphorylation pattern upon M-CSF stimulation was altered in Tec−/−Btk−/− macrophages despite normal expression and phosphorylation of the M-CSFR. Further, Tec and Btk are required for proper expression of GM-CSF receptor alpha (GM-CSFRα) chain in macrophages but not dendritic cells, implicating Tec family kinases in the lineage-specific regulation of GM-CSFRα expression. Taken together, our study shows that Tec and Btk regulate M-CSFR signaling-induced macrophage survival and provides a novel link between Tec family kinases and the regulation of caspase-11 and GM-CSFRα expression.

Background

The family of vascular endothelial growth factors (VEGF) contains key regulators of blood and lymph vessel development, including VEGF-A, -B, -C, -D, and placental growth factor. The role of VEGF-B during physiological or pathological angiogenesis has not yet been conclusively delineated. Herein, we investigate the function of VEGF-B by the generation of mouse models of cancer with transgenic expression of VEGF-B or homozygous deletion of Vegfb.

Methodology/Principal Findings

Ectopic expression of VEGF-B in the insulin-producing β-cells of the pancreas did not alter the abundance or architecture of the islets of Langerhans. The vasculature from transgenic mice exhibited a dilated morphology, but was of similar density as that of wildtype mice. Unexpectedly, we found that transgenic expression of VEGF-B in the RIP1-Tag2 mouse model of pancreatic neuroendocrine tumorigenesis retarded tumor growth. Conversely, RIP1-Tag2 mice deficient for Vegfb presented with larger tumors. No differences in vascular density, perfusion or immune cell infiltration upon altered Vegfb gene dosage were noted. However, VEGF-B acted to increase blood vessel diameter both in normal pancreatic islets and in RIP1-Tag2 tumors.

Conclusions/Significance

Taken together, our results illustrate the differences in biological function between members of the VEGF family, and highlight the necessity of in-depth functional studies of VEGF-B to fully understand the effects of VEGFR-1 inhibitors currently used in the clinic.

The phosphoinositide 3-kinase (PI3K) family has multiple vascular functions, but the specific regulatory isoform supporting lymphangiogenesis remains unidentified. Here we report that deletion of the Pik3r1 gene, encoding the regulatory subunits p85α, p55α, and p50α impairs lymphatic sprouting and maturation, and causes abnormal lymphatic morphology, without major impact on blood vessels. Pik3r1 deletion had the most severe consequences among gut and diaphragm lymphatics, which share the retroperitoneal anlage, initially suggesting that the Pik3r1 role in this vasculature is anlage-dependent. However, whereas lymphatic sprouting toward the diaphragm was arrested, lymphatics invaded the gut, where remodeling and valve formation were impaired. Thus, cell-origin fails to explain the phenotype. Only the gut showed lymphangiectasia, lymphatic up-regulation of the TGFβ co-receptor endoglin, and reduced levels of mature VEGF-C protein. Our data suggest that Pik3r1 isoforms are required for distinct steps of embryonic lymphangiogenesis in different organ microenvironments, whereas they are largely dispensable for hemangiogenesis.

If neuropilin-2 and the growth factor VEGF-C don’t come together, lymphatic vessels don’t branch apart.

Vascular sprouting is a key process-driving development of the vascular system. In this study, we show that neuropilin-2 (Nrp2), a transmembrane receptor for the lymphangiogenic vascular endothelial growth factor C (VEGF-C), plays an important role in lymphatic vessel sprouting. Blocking VEGF-C binding to Nrp2 using antibodies specifically inhibits sprouting of developing lymphatic endothelial tip cells in vivo. In vitro analyses show that Nrp2 modulates lymphatic endothelial tip cell extension and prevents tip cell stalling and retraction during vascular sprout formation. Genetic deletion of Nrp2 reproduces the sprouting defects seen after antibody treatment. To investigate whether this defect depends on Nrp2 interaction with VEGF receptor 2 (VEGFR2) and/or 3, we intercrossed heterozygous mice lacking one allele of these receptors. Double-heterozygous nrp2vegfr2 mice develop normally without detectable lymphatic sprouting defects. In contrast, double-heterozygote nrp2vegfr3 mice show a reduction of lymphatic vessel sprouting and decreased lymph vessel branching in adult organs. Thus, interaction between Nrp2 and VEGFR3 mediates proper lymphatic vessel sprouting in response to VEGF-C.

Background

In vivo studies demonstrate that the Prox1 transcription factor plays a critical role in the development of the early lymphatic system. Upon Prox1 expression, early lymphatic endothelial cells differentiate from the cardinal vein and begin to express lymphatic markers such as VEGFR-3, LYVE-1 and Podoplanin. Subsequent in vitro studies have found that differentiated vascular endothelial cells can be reprogrammed by Prox1 to express a lymphatic gene profile, suggesting that Prox1 can initiate the expression of a unique gene signature during lymphangiogenesis. While the in vitro data suggest that gene reprogramming occurs upon Prox1 expression, it is not clear if this is a direct result of Prox1 in vascular endothelial cells in vivo.

Results

Overexpression of Prox1 in vascular endothelial cells during embryonic development results in the reprogramming of genes to that of a more lymphatic signature. Consequent to this overexpression, embryos suffer from gross edema that results in embryonic lethality at E13.5. Furthermore, hemorrhaging and anemia is apparent along with clear defects in lymph sac development. Alterations in junctional proteins resulting in an increase in vascular permeability upon Prox1 overexpression may contribute to the complications found during embryonic development.

Conclusion

We present a novel mouse model that addresses the importance of Prox1 in early embryonic lymphangiogenesis. It is clear that there needs to be a measured pattern of expression of Prox1 during embryonic development. Furthermore, Prox1 reprograms vascular endothelial cells in vivo by creating a molecular signature to that of a lymphatic endothelial cell.

The therapeutic potential of vascular endothelial growth factor C (VEGF-C) and VEGF-D in skeletal muscle has been of considerable interest as these factors have both angiogenic and lymphangiogenic activities. Previous studies have mainly employed adenoviral gene delivery for short-term expression of VEGF-C and VEGF-D in pig, rabbit and mouse skeletal muscles. Here we have used the activated mature forms of VEGF-C and VEGF-D expressed via recombinant adeno-associated virus (rAAV), which provides stable, long-lasting transgene expression in various tissues including skeletal muscle. Mouse tibialis anterior muscle was transduced with rAAV encoding human or mouse VEGF-C or VEGF-D. Two weeks later, immunohistochemical analysis showed increased numbers of both blood and lymph vessels, and doppler ultrasound analysis indicated increased blood vessel perfusion. The lymphatic vessels further increased at the four-week time point were functional, as shown by FITC-lectin uptake and transport. Furthermore, receptor activation and arteriogenic activity were increased by an alanine substitution mutant of human VEGF-C (C137A) having an increased dimer stability and by a chimeric CAC growth factor that contained the VEGF receptor-binding domain flanked by VEGF-C propeptides, but only the latter promoted significantly more blood vessel perfusion when compared to the other growth factors studied. We conclude that long-term expression of VEGF-C and VEGF-D in skeletal muscle results in the generation of new functional blood and lymphatic vessels. The therapeutic value of intramuscular lymph vessels in draining tissue edema and lymphedema can now be evaluated using this model system.

Prostate cancer dissemination is difficult to detect in the clinic, and few treatment options exist for patients with advanced-stage disease. Our aim was to investigate the role of tumor lymphangiogenesis during metastasis. Further, we implemented a noninvasive molecular imaging technique to facilitate the assessment of the metastatic process. The metastatic potentials of several human prostate cancer xenograft models, LAPC-4, LAPC-9, PC3 and CWR22Rv-1 were compared. The cells were labeled with luciferase, a bioluminescence imaging reporter gene, to enable optical imaging. After tumor implantation the animals were examined weekly during several months for the appearance of metastases. Metastatic lesions were confirmed by immunohistochemistry. Additionally, the angiogenic and lymphangiogenic profiles of the tumors were characterized. To confirm the role of lymphangiogenesis in mediating metastasis, the low-metastatic LAPC-9 tumor cells were engineered to overexpress VEGF-C, and the development of metastases was evaluated. Our results show CWR22Rv-1 and PC3 tumor cell lines to be more metastatic than LAPC-4, which in turn disseminates more readily than LAPC-9. The difference in metastatic potential correlated with the endogenous production levels of lymphangiogenic growth factor VEGF-C and the presence of tumor lymphatics. In agreement, induced overexpression of VEGF-C in LAPC-9 enhanced tumor lymphangiogenesis leading to the development of metastatic lesions. Taken together, our studies, based on a molecular imaging approach for semiquantitative detection of micrometastases, point to an important role of tumor lymphatics in the metastatic process of human prostate cancer. In particular, VEGF-C seems to play a key role in prostate cancer metastasis.

The vascular endothelial growth factors VEGFA and VEGFC are crucial regulators of vascular development. They exert their effects by dimerization and activation of the cognate receptors VEGFR2 and VEGFR3. Here, we have used in situ proximity ligation to detect receptor complexes in intact endothelial cells. We show that both VEGFA and VEGFC potently induce formation of VEGFR2/-3 heterodimers. Receptor heterodimers were found in both developing blood vessels and immature lymphatic structures in embryoid bodies. We present evidence that heterodimers frequently localize to tip cell filopodia. Interestingly, in the presence of VEGFC, heterodimers were enriched in the leading tip cells as compared with trailing stalk cells of growing sprouts. Neutralization of VEGFR3 to prevent heterodimer formation in response to VEGFA decreased the extent of angiogenic sprouting. We conclude that VEGFR2/-3 heterodimers on angiogenic sprouts induced by VEGFA or VEGFC may serve to positively regulate angiogenic sprouting.

Lymph node involvement denotes a poor outcome for patients with prostate cancer. Our group, along with others, has shown that initial tumor cell dissemination to regional lymph nodes via lymphatics also promotes systemic metastasis in mouse models. The aim of this study was to investigate the efficacy of suppressive therapies targeting either the angiogenic or lymphangiogenic axis in inhibiting regional lymph node and systemic metastasis in subcutaneous and orthotopic prostate tumor xenografts. Both androgen-dependent and more aggressive androgen-independent prostate tumors were used in our investigations. Interestingly, we observed that the threshold for dissemination is lower in the vascular-rich prostatic microenvironment compared with subcutaneously grafted tumors. Both vascular endothelial growth factor-C (VEGF-C) ligand trap (sVEGFR-3) and antibody directed against VEGFR-3 (mF4-31C1) significantly reduced tumor lymphangiogenesis and metastasis to regional lymph nodes and distal vital organs without influencing tumor growth. Conversely, angiogenic blockade by short hairpin RNA against VEGF or anti–VEGFR-2 antibody (DC101) reduced tumor blood vessel density, significantly delayed tumor growth, and reduced systemic metastasis, although it was ineffective in reducing lymphangiogenesis or nodal metastasis. Collectively, these data clarify the utility of vascular therapeutics in prostate tumor growth and metastasis, particularly in the context of the prostate microenvironment. Our findings highlight the importance of lymphangiogenic therapies in the control of regional lymph node and systemic metastasis.

Bmx non-receptor tyrosine kinase has an established role in endothelial and lymphocyte signaling, however its role in the heart is unknown. To determine whether Bmx participates in cardiac growth, we subjected mice deficient in the molecule (Bmx KO mice) to transverse aortic constriction (TAC). In comparison to WT mice, which progressively developed massive hypertrophy following TAC, Bmx KO mice were resistant to TAC-induced cardiac growth at the organ and cell level. Loss of Bmx preserved cardiac ejection fraction and decreased mortality following TAC. These findings are the first to demonstrate a necessary role for the Tec family of tyrosine kinases in the heart and reveal a novel regulator (Bmx) of pressure overload-induced hypertrophic growth.

The mechanisms of blood vessel maturation into distinct parts of the blood vasculature such as arteries, veins, and capillaries have been the subject of intense investigation over recent years. In contrast, our knowledge of lymphatic vessel maturation is still fragmentary. In this study, we provide a molecular and morphological characterization of the major steps in the maturation of the primary lymphatic capillary plexus into collecting lymphatic vessels during development and show that forkhead transcription factor Foxc2 controls this process. We further identify transcription factor NFATc1 as a novel regulator of lymphatic development and describe a previously unsuspected link between NFATc1 and Foxc2 in the regulation of lymphatic maturation. We also provide a genome-wide map of FOXC2-binding sites in lymphatic endothelial cells, identify a novel consensus FOXC2 sequence, and show that NFATc1 physically interacts with FOXC2-binding enhancers. As damage to collecting vessels is a major cause of lymphatic dysfunction in humans, our results suggest that FOXC2 and NFATc1 are potential targets for therapeutic intervention.

Vascular Endothelial Growth Factor-B (VEGF-B) is poorly angiogenic but prominently expressed in metabolically highly active tissues, including the heart. We produced mice expressing a cardiac-specific VEGF-B transgene via the alpha myosin heavy chain promoter. Surprisingly, the hearts of the VEGF-B transgenic mice showed concentric cardiac hypertrophy without significant changes in heart function. The cardiac hypertrophy was due to an increased size of the cardiomyocytes. Blood capillary size was increased, while the number of blood vessels per cell nucleus remained unchanged. Despite the cardiac hypertrophy, the transgenic mice had lower heart rate and blood pressure than their littermates, and they responded similarly to angiotensin II-induced hypertension, confirming that the hypertrophy does not compromise heart function. Interestingly, the isolated transgenic hearts had less cardiomyocyte damage after ischemia. Significantly increased ceramide and decreased triglyceride levels were found in the transgenic hearts. This was associated with structural changes and eventual lysis of mitochondria, resulting in accumulation of intracellular vacuoles in cardiomyocytes and increased death of the transgenic mice, apparently due to mitochondrial lipotoxicity in the heart. These results suggest that VEGF-B regulates lipid metabolism, an unexpected function for an angiogenic growth factor.

Objective

The endogenous role of the VEGF family member vascular endothelial growth factor-B (VEGF-B) in pathological angiogenesis remains unclear.

Methods and Results

We studied the role of VEGF-B in various models of pathological angiogenesis using mice lacking VEGF-B (VEGF-B−/−) or overexpressing VEGF-B167. After occlusion of the left coronary artery, VEGF-B deficiency impaired vessel growth in the ischemic myocardium whereas, in wild-type mice, VEGF-B167 overexpression enhanced revascularization of the infarct and ischemic border zone. By contrast, VEGF-B deficiency did not affect vessel growth in the wounded skin, hypoxic lung, ischemic retina, or ischemic limb. Moreover, VEGF-B167 overexpression failed to enhance vascular growth in the skin or ischemic limb.

Conclusion

VEGF-B appears to have a relatively restricted angiogenic activity in the ischemic heart. These insights might offer novel therapeutic opportunities.