Tumor-associated macrophages express lymphatic endothelial growth factors and are related to peritumoral lymphangiogenesis

Nodal lymphangiogenesis and metastasis: Role of tumor-induced lymphatic vessel activation in extramammary Paget's disease

Nodal lymphangiogenesis promotes distant lymph node (LN) metastasis in experimental cancer models. However, the role of nodal lymphangiogenesis in distant metastasis and in the overall survival of cancer patients remains unknown. Therefore, we investigated mechanisms that might facilitate regional and distant LN metastasis in extramammary Paget's disease (EMPD). We retrospectively analyzed the impact of tumor-induced lymphatic vessel activation on the survival of 116 patients, the largest cohort with EMPD studied to date. Nodal lymphangiogenesis was significantly increased in metastatic, compared with tumor-free, LNs (P = 0.022). Increased lymphatic invasion within regional LNs was significantly associated with distant metastasis in LN (P = 0.047) and organs (P = 0.003). Thus, invasion within regional LNs is a powerful indicator of systemic tumor spread and reduced patient survival in EMPD (P = 0.0004). Lymphatic vessels associated with tumors expressed stromal cell-derived factor-1 (SDF-1), whereas CXCR4 was expressed on invasive Paget cells undergoing epithelial-mesenchymal transition (EMT)-like process. A431 cells overexpressing Snail expressed increased levels of CXCR4 in the presence of transforming growth factor-beta1. Haptotactic migration assays confirmed that Snail-induced EMT-like process promotes tumor cell motility via the CXCR4-SDF-1 axis. Sinusoidal lymphatic endothelial cells and macrophages expressed SDF-1 in subcapsular sinuses of lymph nodes before Paget cell arrival. Our findings reveal that EMT-related features likely promote lymphatic metastasis of EMPD by activating the CXCR4-SDF-1 axis.

Endothelial cells from cord blood CD133+CD34+ progenitors share phenotypic, functional and gene expression profile similarities with lymphatics

The existence of endothelial progenitor cells (EPC) with high cell-cycle rate in human umbilical cord blood has been recently shown and represents a challenging strategy for therapeutic neovascularization. To enhance knowledge for future cellular therapy, we compared the phenotypic, functional and gene expression differences between EPC-derived cells generated from cord blood CD34⁺ cells, and lymphatic and macrovascular endothelial cells (EC) isolated from human foreskins and umbilical veins, respectively. Under appropriate culture conditions, EPC developed into fully matured EC with expression of similar endothelial markers as lymphatic and macrovascular EC, including CD31, CD36, von Willebrand factor FVIII, CD54 (ICAM-1), CD105 (endoglin), CD144 (VE-cadherin), Tie-1, Tie-2, VEGFR-1/Flt-1 and VEGFR-2/Flk-1. Few EPC-derived cells became positive for LYVE-1, indicating their origin from haematopoietic stem cells. However they lacked expression of other lymphatic cell-specific markers such as podoplanin and Prox-1. Functional tests demonstrated that the cobblestone EPC-derived cells up-regulated CD54 and CD62E expression in response to TNF-α, incorporated DiI-acetylated low-density liproprotein and formed cord- and tubular-like structures with capillary lumen in three-dimensional collagen culture – all characteristic features of the vascular endothelium. Structures compatible with Weibel-Palade bodies were also found by electron microscopy. Gene microarray profiling revealed that only a small percentage of genes investigated showed differential expression in EPC-derived cells and lymphatic EC. Among them were adhesion molecules, extracellular matrix proteins and cytokines. Our data point to the close lineage relationship of both types of vascular cells and support the theory of a venous origin of the lymphatic system.

Vascular endothelial growth factor-C protects prostate cancer cells from oxidative stress by the activation of mammalian target of rapamycin complex-2 and AKT-1

Recurrence and subsequent metastatic transformation of cancer develops from a subset of malignant cells, which show the ability to resist stress and to adopt to a changing microenvironment. These tumor cells have distinctly different growth factor pathways and antiapoptotic responses compared with the vast majority of cancer cells. Long-term therapeutic success can only be achieved by identifying and targeting factors and signaling cascades that help these cells survive during stress. Both microarray and immunohistochemical analysis on human prostate cancer tissue samples have shown an increased expression of vascular endothelial growth factor-C (VEGF-C) in metastatic prostate cancer. We have discovered that VEGF-C acts directly on prostate cancer cells to protect them against oxidative stress. VEGF-C increased the survival of prostate cancer cells during hydrogen peroxide stress by the activation of AKT-1/protein kinase Balpha. This activation was mediated by mammalian target of rapamycin complex-2 and was not observed in the absence of oxidative stress. Finally, the transmembrane nontyrosine kinase receptor neuropilin-2 was found to be essential for the VEGF-C-mediated AKT-1 activation. Indeed, our findings suggest a novel and distinct function of VEGF-C in protecting cancer cells from stress-induced cell death, thereby facilitating cancer recurrence and metastasis. This is distinctly different from the known function of VEGF-C in inducing lymphangiogenesis.

Chemokine CCL2/MCP-1 negatively regulates metastasis in a highly bone marrow-metastatic mouse breast cancer model

Bone is the most frequent site of breast cancer metastasis, and once such metastasis occurs, complete remission is extremely difficult to achieve. In an effort to define the mechanisms underlying metastatic spread of breast cancer to bone, we previously developed and characterized the highly bone metastatic 4T1E/M3 mouse breast cancer cells. We found that following injection into mice, 4T1E/M3 cells exhibited greater bone metastasis and greater in vitro anchorage-independent growth and cell migration than their parental cells (4T1E). We also found that expression of intracellular adhesion molecule-1 (ICAM-1) is crucially involved in these metastatic activities of 4T1E/M3 cells. In the present study, our analysis of gene and protein expression revealed that production of chemokine CCL2 (MCP-1) is dramatically reduced in 4T1E/M3 cells, and that restoration of CCL2 expression in 4T1E/M3 cells diminishes their metastasis to bone and lung. Overexpression of CCL2 in 4T1E/M3 cells significantly reduced not only in vitro anchorage-independent cell growth and cell migration, but also mRNA and cell surface expression of ICAM-1. Conversely, knocking down CCL2 in 4T1E parental cells augmented their metastatic spread to spine and lung. The expression of ICAM-1 was also upregulated in 4T1E-derived CCL2 knockdown cells. Taken together, these results suggest that CCL2 expression may negatively regulate breast cancer metastasis to bone marrow and lung in our model and that expression of ICAM-1 plays a crucial role in that process.

Alternatively activated RAW264.7 macrophages enhance tumor lymphangiogenesis in mouse lung adenocarcinoma

Tumor-associated macrophages (TAMs) have been implicated in promoting tumor progression and invasion. The onset and maintenance of tumor angiogenesis and lymphangiogenesis also seem to be partly driven by a group of polarized alternatively activated macrophages (aaMphi) in lung adenocarcinoma. Here, the aaMphi and classically activated macrophages (caMphi) were obtained using RAW264.7 cells via IL-4 and IFN-gamma + LPS treatment, respectively. Co-inoculation of aaMphi with Lewis lung carcinoma (LLC) cells promoted tumor growth, increased lymph node metastasis, and reduced the survival in C57BL/6 mice bearing LLC. Furthermore, the effects of the activated macrophages on the lymphangiogenesis-related properties of lymphatic endothelial cells (LECs) were investigated in vitro. When LECs were cultured in macrophages conditioned medium or in a co-culture system of macrophages and LECs, aaMphi significantly promoted proliferation, migration, and tube-like formation of LECs. We identified high VEGF-C expression in aaMphi and low expression in caMphi as well as unactivated macrophages by ELISA and Western blotting. In LECs, co-culture with aaMphi resulted in a significant increase of mRNA levels of specific lymphatic marker VEGF receptor-3 and the homeobox gene Prox-1, as well as lymphangiogenic factor VEGF-C rather than VEGF-D by quantitative RT-PCR. Furthermore, enhanced LECs migration and capillary formation by co-culture with aaMphi were significantly inhibited by rVEGF receptor-3/Fc chimera. In conclusion, these data show that aaMphi play a critical role in tumor-induced lymphangiogenesis through up-regulating VEGF-C and increasing lymphangiogenesis-related behavior of LECs, which may contribute to lymphatic invasion in lung adenocarcinoma.

Lymphangiogenesis and cancer metastasis

Metastasis is a characteristic trait of most tumour types and the cause for the majority of cancer deaths. Many tumour types, including melanoma and breast and prostate cancers, first metastasize via lymphatic vessels to their regional lymph nodes. Although the connection between lymph node metastases and shorter survival times of patients was made decades ago, the active involvement of the lymphatic system in cancer, metastasis has been unravelled only recently, after molecular markers of lymphatic vessels were identified. A growing body of evidence indicates that tumour-induced lymphangiogenesis is a predictive indicator of metastasis to lymph nodes and might also be a target for prevention of metastasis. This article reviews the current understanding of lymphangiogenesis in cancer anti-lymphangiogenic strategies for prevention and therapy of metastatic disease, quantification of lymphangiogenesis for the prognosis and diagnosis of metastasis and in vivo imaging technologies for the assessment of lymphatic vessels, drainage and lymph nodes.

CEACAM1+ myeloid cells control angiogenesis in inflammation

Local inflammation during cutaneous leishmaniasis is accompanied by accumulation of CD11b+ cells at the site of the infection. A functional role for these monocytic cells in local angiogenesis in leishmaniasis has not been described so far. Here, we show that CD11b+ cells express high levels of the myeloid differentiation antigen carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1). In experimental cutaneous leishmaniasis in C57BL/6 wild-type (B6.WT) and B6.Ceacam1−/− mice, we found that only B6.Ceacam1−/− mice develop edemas and exhibit impairment of both hemangiogenesis and lymphangiogenesis. Because CEACAM1 expression correlates with functional angiogenesis, we further analyzed the role of the CD11b+ population. In B6.Ceacam1−/− mice, we found systemic reduction of Ly-6Chigh/CD11bhigh monocyte precursors. To investigate whether CEACAM1+ myeloid cells are causally related to efficient angiogenesis, we used reverse bone marrow transplants (BMTs) to restore CEACAM1+ or CEACAM1− bone marrow in B6.Ceacam1−/− or B6.WT recipients, respectively. We found that angiogenesis was restored by CEACAM1+ BMT only. In addition, we observed reduced morphogenic potential of inflammatory cells in Matrigel implants in CEACAM1− backgrounds or after systemic depletion of CD11bhigh macrophages. Taken together, we show for the first time that CEACAM1+ myeloid cells are crucial for angiogenesis in inflammation.

Significance of M2-polarized tumor-associated macrophage in pancreatic cancer

BACKGROUND

The roles of infiltrating macrophages within the tumor microenvironment are complex because of their functional variety. The aim of this study is to examine the role and prognostic significance of tumor-associated macrophages (TAMs) that have an M2 polarized function in pancreatic cancer.

MATERIALS AND METHODS

Formalin-fixed, paraffin-embedded blocks were obtained from 76 patients with pancreatic head cancer. All patients underwent macroscopic curative resection. We assessed the number of infiltrating macrophages within the tumor invasive front by not only CD68 but also by CD163 and CD204, which are specific receptors on M2-polarized macrophages. Furthermore, to evaluate lymphangiogenesis, we measured the density of lymphatic vessels in the tumor invasive front by using D2-40.

RESULTS

High incidence of lymph node metastasis was shown in cases with a high number of CD163- or CD204-positive macrophages. Significantly increased lymphatic vessel density (LVD) was shown in cases with lymph node metastasis compared with cases without lymph node metastasis (P=0.0094). Significantly increased LVD (P=0.0175) and a poor prognosis (P=0.0171) were shown in cases with a high number of macrophages that express CD163 or CD204, however, there was no significant difference according to the number of CD68-positive macrophages.

CONCLUSIONS

M2-polarized TAMs in the invasive front of pancreatic cancer are associated with a poor prognosis due to accelerated lymphatic metastasis, and inhibition of the functional interaction between M2-polarized TAMs and tumor cells may improve the prognosis.

Cytokine genetic polymorphisms and prostate cancer aggressiveness

Prostate cancer (PCa) is one of the most common cancers in the world. Inflammation has been described as a risk factor for PCa and depends on the production of cytokines in response to tissue damage or the presence of stimuli that induces cellular stress. Interindividual variation in cytokine production is partially controlled by single-nucleotide polymorphisms (SNPs) that have been associated with differential production of cytokines. We have recently showed that SNP-SNP interactions of cytokine genes are associated with PCa risk. However, little is known about the association of cytokine SNPs and PCa aggressiveness. In this study, we evaluated the association of 15 SNPs in five cytokine genes and aggressiveness of PCa in African- and Caucasian-American individuals. Caucasian Americans with the genotypes IL10-1082GG or IL1B+3954TT had 2.31-fold [95% confidence interval (CI) = 1.13-4.72] and 3.11 (95% CI = 1.20-8.06)-fold risk, respectively, of developing aggressive PCa, as compared with individuals without those genotypes. We did not find any associations in the African-American group. Using Multivariate Adaptive Regression Splines modeling for exploratory SNP-SNP interactions, our results showed that more aggressive PCa in Caucasians Americans is associated with the CT genotype at IL8-47 [odds ratios (OR) = 3.50; 95% CI = 1.13-10.88] or combined genotypes of IL1B-511CC and IL10-1082GG (OR = 3.38; 95% CI = 1.70-6.71). Unfortunately, the same analysis could not be performed in the African-Americans due to limited number of individuals. With limited sample size, the results from this study suggest that SNPs in cytokine genes may be associated with PCa aggressiveness. More extensive studies are warranted to validate our findings.

Mechanisms of lymphatic metastasis in human colorectal adenocarcinoma

The invasion of lymphatic vessels by colorectal cancer (CRC) and its subsequent spread to draining lymph nodes is a key determinant of prognosis in this common and frequently fatal malignancy. Although tumoural lymphangiogenesis is assumed to contribute to this process, review of the current literature fails to support any notion of a simple correlation between lymphatic vessel density and CRC metastasis. Furthermore, attempts to correlate the expression of various lymphangiogenic growth factors, most notably VEGF-C and VEGF-D, with the lymphatic metastasis of CRC have provided contradictory results. Recent evidence from animal and human models of tumour metastasis suggests that complex functional and biochemical interactions between the microvasculature of tumours and other cell types within the tumour microenvironment may play a pivotal role in the behaviour of commonly metastasizing tumours. Indeed, previous insights into tumoural blood vessels have provided candidate markers of tumoural angiogenesis that are currently the subject of intense investigation as future therapeutic targets. In this review article we survey the current evidence relating lymphangiogenesis and lymphangiogenic growth factor production to metastasis by CRC, and attempt to provide some insight into the apparent discrepancies within the literature. In particular, we also discuss some new and provocative insights into the properties of tumoural lymphatics suggesting that they have specific expression profiles distinct from those of normal lymphatic vessels and that appear to promote metastasis. These findings raise the exciting prospect of future biomarkers of lymphatic metastasis and identify potential targets for new generation anti-tumour therapies.

Activated forms of VEGF-C and VEGF-D provide improved vascular function in skeletal muscle

The therapeutic potential of vascular endothelial growth factor (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 used 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 4-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.

Macrophages regulate salt-dependent volume and blood pressure by a vascular endothelial growth factor-C-dependent buffering mechanism

In salt-sensitive hypertension, the accumulation of Na(+) in tissue has been presumed to be accompanied by a commensurate retention of water to maintain the isotonicity of body fluids. We show here that a high-salt diet (HSD) in rats leads to interstitial hypertonic Na(+) accumulation in skin, resulting in increased density and hyperplasia of the lymphcapillary network. The mechanisms underlying these effects on lymphatics involve activation of tonicity-responsive enhancer binding protein (TonEBP) in mononuclear phagocyte system (MPS) cells infiltrating the interstitium of the skin. TonEBP binds the promoter of the gene encoding vascular endothelial growth factor-C (VEGF-C, encoded by Vegfc) and causes VEGF-C secretion by macrophages. MPS cell depletion or VEGF-C trapping by soluble VEGF receptor-3 blocks VEGF-C signaling, augments interstitial hypertonic volume retention, decreases endothelial nitric oxide synthase expression and elevates blood pressure in response to HSD. Our data show that TonEBP-VEGF-C signaling in MPS cells is a major determinant of extracellular volume and blood pressure homeostasis and identify VEGFC as an osmosensitive, hypertonicity-driven gene intimately involved in salt-induced hypertension.

An immunohistochemical study of the inflammatory infiltrate associated with nasal carcinoma in dogs and cats

The aims of this study were to characterize the inflammatory infiltrate associated with nasal carcinoma in dogs and cats and to determine whether this differed between the two species or with different types of carcinoma. Sections from fixed tissue biopsy samples of intranasal carcinoma from 31 dogs and six cats were labelled immunohistochemically to detect expression of the T-lymphocyte marker CD3, class II molecules of the major histocompatibility complex (MHC II), the myelomonocytic antigen MAC387 and immunoglobulin (Ig) G, IgA and IgM within the cytoplasm of plasma cells. All canine carcinomas were heavily infiltrated by MAC387(+) neutrophils, with smaller numbers of MAC387(+) macrophages. T cells were particularly prominent in the infiltrate associated with transitional carcinoma, and in such tumours were frequently mixed with MHC II(+) cells having macrophage or dendritic cell morphology. IgG(+) and IgA(+) plasma cells were detected at the peripheral margins of all types of canine carcinoma. In contrast, feline intranasal carcinoma was invariably associated with a marked infiltration of CD3(+) T cells. The feline tumour infiltrates contained sparse neutrophils and macrophages and few IgG(+) and IgA(+) plasma cells. These findings suggest that qualitatively different immune responses are induced in response to specific types of canine intranasal carcinoma, and that the canine and feline immune response to these neoplasms is also distinct.

Molecular targets for therapeutic lymphangiogenesis in lymphatic dysfunction and disease

The convergence of multiple disciplines upon the study of the lymphatic vasculature has invigorated a renaissance of research, using powerful investigative tools and an exponential growth of interest in this historically underappreciated system. Fundamental discoveries in lymphatic development have yielded relevant animal models for vexing clinical diseases that suffer from nonexistent of minimally effective treatments. Inherited and acquired lymphedema represent the current crux of research efforts to identify potential molecular therapies born from these early discoveries. The importance of the lymphatic system is, however, not limited to lymphedema but encompasses a diverse spectrum of human disease including inflammation and cancer metastasis. As the lymphatic vasculature continues to benefit from fruits of biomedical investigation, translation of mechanistic insights into targeted, rationally-conceived therapeutics will be become a reality.

The lymphatic system in health and disease

The lymphatic vascular system has an important role in the regulation of tissue pressure, immune surveillance and the absorption of dietary fat in the intestine. There is growing evidence that the lymphatic system also contributes to a number of diseases, such as lymphedema, cancer metastasis and different inflammatory disorders. The discovery of various molecular markers allowing the distinction of blood and lymphatic vessels, together with the availability of a increasing number of in vitro and in vivo models to study various aspects of lymphatic biology, has enabled tremendous progress in research into the development and function of the lymphatic system. This review discusses recent advances in our understanding of the embryonic development of the lymphatic vasculature, the molecular mechanisms mediating lymphangiogenesis in the adult, the role of lymphangiogenesis in chronic inflammation and lymphatic cancer metastasis, and the emerging importance of the lymphatic vasculature as a therapeutic target.

Corrupt policemen: inflammatory cells promote tumor angiogenesis

PURPOSE OF REVIEW

The last decade has seen a paradigm change in that tumor stroma contributes to malignant tumor progression in a manner comparable to genetic and epigenetic changes within cancer cells. This review summarizes recent novel insights into how inflammatory conditions stimulate the formation and expansion of blood and lymphatic vessels within tumors and, thus, allow tumors to grow, to gain invasive capabilities, and to finally seed metastasis in distant organs.

RECENT FINDINGS

Different cancer types have a highly defined microenvironment, which is composed of cancer-associated fibroblasts, blood vessel and lymphatic endothelial cells, pericytes, and a heterogeneous infiltrate of cells of the immune system. In addition to the local stimulation of tumor angiogenesis and tumor lymphangiogenesis, cytokines released by the primary tumor and by the immune cell infiltrate also instruct bone marrow-derived cells to colonize distant organs and to prepare these sites for future metastasis.

SUMMARY

Inflammatory reactions coinciding with carcinogenesis can be visualized by the presence of specific bone marrow-derived, inflammatory cells in patients' peripheral blood. Recent findings suggest that such inflammatory fingerprints may better define the inflammatory nature of the primary malignancy and, thus, allow the design of therapeutic strategies targeting the protumorigenic immune cell stroma compartment.

Tumor-associated macrophages: effectors of angiogenesis and tumor progression

Tumor-associated macrophages (TAMs) are a prominent inflammatory cell population in many tumor types residing in both perivascular and avascular, hypoxic regions of these tissues. Analysis of TAMs in human tumor biopsies has shown that they express a variety of tumor-promoting factors and evidence from transgenic murine tumor models has provided unequivocal evidence for the importance of these cells in driving angiogenesis, lymphangiogenesis, immunosuppression, and metastasis. This review will summarize the mechanisms by which monocytes are recruited into tumors, their myriad, tumor-promoting functions within tumors, and the influence of the tumor microenvironment in driving these activities. We also discuss recent attempts to both target/destroy TAMs and exploit them as delivery vehicles for anti-cancer gene therapy.

Microvessel density correlates with lymph node metastases and prognosis in hilar cholangiocarcinoma

BACKGROUND

Neovascularization was shown to be critically involved in the progression of multiple cancers, and treatment approaches targeting tumor-associated neovascularization provide convincing results in recent years in some tumor entities. However, little is known about the tumor-associated neovascularization in hilar cholangiocarcinoma. The present study was conducted to analyze tumor-associated neovascularization in hilar cholangiocarcinoma and to determine its influence on tumor growth, metastasis, recurrence, and prognosis.

METHODS

We analyzed tissue specimens of hilar cholangiocarcinoma (n = 60) by immunohistochemistry using the endothelial-specific antibody CD31 and subsequently quantified the microvessel density (MVD). The MVD was correlated with clinicopathological characteristics and recurrence pattern of the tumors as well as survival of patients.

RESULTS

Hilar cholangiocarcinoma revealed a high degree of vascularization, with a calculated mean MVD of 28.1 +/- 14.5 vessels. Tumors with a high MVD had a significant higher incidence of lymph node involvement (P = 0.009) and local recurrence (P < 0.001). Furthermore, a high MVD was identified to be a significant overall survival disadvantage (3-year, 28% vs. 93%; 5-year, 8% vs. 78%; P < 0.001) as well as disease-free survival disadvantage (3-year, 7% vs. 88%, 5-year, 7% vs. 72%; P < 0.001), with MVD representing an independent prognostic factor for survival.

CONCLUSIONS

Neovascularization is associated with nodal spread as well as local recurrence and serves as an independent prognostic factor for survival after curative resection of hilar cholangiocarcinoma. Therefore, tumor-associated neovascularization seems to be critically involved in the progression of this tumor entity. In addition, neovascularization may represent a potential target in he development of new therapeutic approaches in hilar cholangiocarcinoma.

The Yin-Yang of tumor-associated macrophages in neoplastic progression and immune surveillance

An intrinsic (oncogene-driven) pathway and an extrinsic (microenvironment-driven) pathway connect inflammatory reactions and cancer. M2-polarized tumor-associated macrophages and the related myeloid-derived suppressor cells are key prototypic components of smoldering inflammation driving neoplastic progression. However, mononuclear phagocytes can exert anti-tumor activity by killing tumor cells and eliciting tissue disruptive reactions (M1), a likely scenario in the early phases of carcinogenesis of immunogenic tumors and following therapeutic intervention. Shifting the macrophage balance represents a viable therapeutic target. Herein, the 'macrophage balance' is discussed in the context of the apparent paradox of tumor promotion by innate immunity-driven inflammation and the seemingly opposed tumor surveillance by adaptive immune responses.

Efficacy and safety of (32)P-nanocolloid for treatment of distant lymph node metastasis in VX2 tumor-bearing rabbits

BACKGROUND

Eradication of micrometastases present in lymph nodes of cancer patients improves their prognosis significantly. Radionuclide therapy possesses the potential to eliminate such metastases.

OBJECTIVE

This study was performed to evaluate the efficacy and safety of (32)P-nanocolloid therapy in the treatment of distant carcinoma cell metastases in lymph nodes of VX2 tumor-bearing rabbits.

METHODS

To obtain VX2 tumor micrometastases in right armpit lymph nodes of 12 male New Zealand white rabbits, VX2 tumors were implanted by hypodermal inoculation into the right anterior limb. Animals were randomly divided into therapy (n = 6) and control (n = 6) groups. (32)P-nanocolloid (0.5 mCi), 95% of which was >50 nm in diameter, was administered to the therapy group, and saline was administered to the control group. Injections were given once weekly for 4 weeks.

RESULTS

2-Deoxy-2[(18)F]-fluoro-D -glucose positron emission tomography revealed that the number of involved lymph nodes and the maximum standardized uptake value decreased in the (32)P-nanocolloid therapy group as compared with the baseline or saline control group (P < 0.05). The expression of the lymphangiogenesis factors vascular endothelial growth factors (VEGF)-C and VEGF-D by VX2 tumor cells present in lymph nodes was significantly lower in the therapy group as compared with the control group. Additionally, apoptotic VX2 tumor cell death was significantly greater in lymph nodes of the therapy as compared with the control group (P < 0.01). With the exception of a decrease in white blood cells of peripheral blood (P < 0.05), standard laboratory values were unaffected throughout the course of therapy with (32)P-nanocolloid.

CONCLUSIONS

These findings support treatment with (32)P-nanocolloid as a safe and effective approach for eradication of lymph node micrometastases.

Lymphatic vessels develop during tubulointerstitial fibrosis

Recent progress with specific markers of lymphatic vessel endothelium allowed recognition of lymphangiogenic events in various disease states; however, there is little information concerning this process in human chronic renal diseases. To determine this we measured expression of the lymphatic marker D2-40 and vascular endothelial growth factor-C (VEGF-C), a major growth factor in lymphangiogenesis, in 124 human renal biopsy specimens. In the kidneys of control subjects and in uninjured areas of pathologic specimens, lymphatic vessels were detected only around the arcuate and interlobular arteries. An increase in the number of lymphatic vessels was found at the site of tubulointerstitial lesions correlating with the degree of tissue damage and more strongly correlating with areas of fibrosis than inflammation. On serial sections, lymphatic vessel proliferation was found in the tubulointerstitial area at the site of tuft adhesions to Bowman's capsule. Lymphatic growth was associated with VEGF-C expression in inflammatory mononuclear cells and tubular epithelial cells, mainly of proximal tubules. Lymphangiogenesis and VEGF-C expression was elevated in diabetic nephropathy in comparison to other renal diseases. Our results indicate that lymphangiogenesis is a common feature in the progression of the tubulointerstitial fibrosis.

Increased density of tumor-associated macrophages is associated with decreased survival in advanced thyroid cancer

Thyroid cancers are infiltrated with tumor-associated macrophages (TAMs), yet their role in cancer progression is not known. The objectives of this study were to characterize the density of TAMs in well-differentiated (WDTC), poorly differentiated (PDTC), and anaplastic thyroid cancers (ATC) and to correlate TAM density with clinicopathologic parameters. Immunohistochemistry was performed on tissue microarray sections from WDTC (n=33), PDTC (n=37), and ATC (n=20) using macrophage-specific markers. Electronic medical records were used to gather clinical and pathologic data. Follow-up information of PDTC patients was available for 0-12 years. In total, 9 out of 33 WDTC (27%), 20 out of 37 PDTC (54%), and 19 out of 20 ATC (95%) had an increased density of CD68(+) TAMs (> or = 10 per 0.28 mm(2); WDTC versus PDTC, P=0.03; WDTC versus ATC, P<0.0001; PDTC versus ATC, P<0.002). Increased TAMs in PDTC was associated with capsular invasion (P=0.034), extrathyroidal extension (P=0.009), and decreased cancer-related survival (P=0.009) compared with PDTC with a low density of TAMs. In conclusion, the density of TAMs is increased in advanced thyroid cancers. The presence of a high density of TAMs in PDTC correlates with invasion and decreased cancer-related survival. These results suggest that TAMs may facilitate tumor progression. As novel therapies directed against thyroid tumor cell-specific targets are being tested, the potential role of TAMs as potential modulators of the thyroid cancer behavior will need to be considered.

FLT1 and its ligands VEGFB and PlGF: drug targets for anti-angiogenic therapy?

Less than 5 years ago, it was still not clear whether anti-angiogenic drugs would prove successful in the clinic. After numerous patients with cancer or age-related macular degeneration have been treated with these drugs, they have now become part of the standard range of therapeutic tools. Despite this milestone, anti-angiogenic therapy still faces a number of clinical hurdles, such as improving efficacy, avoiding escape and resistance, and minimizing toxicity. Hopefully, other agents with complementary mechanisms, such as those that target placental growth factor, will offer novel opportunities for improved treatment.

Induction of proinflammatory response in prostate cancer epithelial cells by activated macrophages

Emerging evidence indicates that chronic inflammation plays an important role in prostate carcinogenesis. Yet to date the precise molecular and cellular mechanisms linking inflammation to carcinogenesis remains unclear. The purpose of this study was to determine the local contribution of prostate epithelial cells to the inflammatory process. We characterized the inflammatory response elicited directly by prostate epithelial cells using an in vitro culture system in which androgen-dependent LNCaP prostate cancer epithelial cells were exposed to conditioned media from LPS-activated THP-1 macrophages. Upon exposure to activated macrophage conditioned media, LNCaP cells elicited a local proinflammatory response, as evidenced by NFkappaB activation, and the production of proinflammatory cytokines TNFalpha, IL-1beta, and IL-6. Furthermore, we observed a significant upregulation of the adhesion molecule VCAM-1 and nuclear estrogen receptor alpha (ERalpha) two biomarkers that correlate with tumor immune evasion and tumor progression. Our results suggest that prostate epithelial cells may play a significant role in sustaining and amplifying the inflammation process through NFkappaB activation and local production of proinflammatory cytokines that results in the recruitment and activation of additional immune cells in the prostate. At the same time, increased expression of VCAM-1 and ERalpha in prostate epithelial cells upon exposure to inflammatory conditions highlights the potential link between chronic inflammation and its involvement in promoting prostate cancer carcinogenesis.

Transgenic induction of vascular endothelial growth factor-C is strongly angiogenic in mouse embryos but leads to persistent lymphatic hyperplasia in adult tissues

Vascular endothelial growth factor-C (VEGF-C) is the quintessential lymphangiogenic growth factor that is required for the development of the lymphatic system and is capable of stimulating lymphangiogenesis in adults by activating its receptor, VEGFR-3. Although VEGF-C is a major candidate molecule for the development of prolymphangiogenic therapy for defective lymphatic vessels in lymphedema, the stability of lymph vessels generated by exogenous VEGF-C administration is not currently known. We studied VEGF-C-stimulated lymphangiogenesis in inducible transgenic mouse models in which growth factor expression can be spatially and temporally controlled without side effects, such as inflammation. VEGF-C induction in adult mouse skin for 1 to 2 weeks caused robust lymphatic hyperplasia that persisted for at least 6 months. VEGF-C induced lymphangiogenesis in numerous tissues and organs when expressed in the vascular endothelium in either neonates or adult mice. Very few or no effects were observed in either blood vessels or collecting lymph vessels. Additionally, VEGF-C stimulated lymphangiogenesis in embryos after the onset of lymphatic vessel development. Strikingly, a strong angiogenic effect was observed after VEGF-C induction in vascular endothelium at any point before embryonic day 16.5. Our results indicate that blood vessels can undergo VEGF-C-induced angiogenesis even after down-regulation of VEGFR-3 in embryos; however, transient VEGF-C expression in adults can induce long-lasting lymphatic hyperplasia with no obvious side effects on the blood vasculature.

TGF-β1 is a negative regulator of lymphatic regeneration during wound repair

Although clinical studies have identified scarring/fibrosis as significant risk factors for lymphedema, the mechanisms by which lymphatic repair is impaired remain unknown. Transforming growth factor -β1 (TGF-β1) is a critical regulator of tissue fibrosis/scarring and may therefore also play a role in the regulation of lymphatic regeneration. The purpose of this study was therefore to assess the role of TGF-β1 on scarring/fibrosis and lymphatic regeneration in a mouse tail model. Acute lymphedema was induced in mouse tails by full-thickness skin excision and lymphatic ligation. TGF-β1 expression and scarring were modulated by repairing the wounds with or without a topical collagen gel. Lymphatic function and histological analyses were performed at various time points. Finally, the effects of TGF-β1 on lymphatic endothelial cells (LECs) in vitro were evaluated. As a result, the wound repair with collagen gel significantly reduced the expression of TGF-β1, decreased scarring/fibrosis, and significantly accelerated lymphatic regeneration. The addition of recombinant TGF-β1 to the collagen gel negated these effects. The improved lymphatic regeneration secondary to TGF-β1 inhibition was associated with increased infiltration and proliferation of LECs and macrophages. TGF-β1 caused a dose-dependent significant decrease in cellular proliferation and tubule formation of isolated LECs without changes in the expression of VEGF-C/D. Finally, the increased expression of TGF-β1 during wound repair resulted in lymphatic fibrosis and the coexpression of α-smooth muscle actin and lymphatic vessel endothelial receptor-1 in regenerated lymphatics. In conclusion, the inhibition of TGF-β1 expression significantly accelerates lymphatic regeneration during wound healing. An increased TGF-β1 expression inhibits LEC proliferation and function and promotes lymphatic fibrosis. These findings imply that the clinical interventions that diminish TGF-β1 expression may be useful in promoting more rapid lymphatic regeneration.

Intratumoral lymphangiogenesis of esophageal squamous cell carcinoma and relationship with regulatory factors and prognosis

The clinical and pathological significance of intratumoral lymphangiogenesis (ITL) with human esophageal squamous cell carcinomas (ESCC) remains unclear, as does the role of signaling molecules such as vascular endothelial growth factor (VEGF)-A,C, platelet-derived growth factor (PDGF)-A, and p53, in the regulation of ITL. Lymphatic vessel density (LVD) was significantly increased in VEGF-A and VEGF-C immunohistochemical score 1 and 2-3 groups as compared to the score 0 group and also with high of VEGF-A, VEGF-C and PDGF-A mRNA expression. Both LVD and blood vessel density (BVD) were significantly greater in the p53 gene mutant group than in the wild-type group. Lymph node metastasis was significantly more frequent with than without ITL and Kaplan-Meier analysis indicated a significantly poorer prognosis. Multivariate analysis using Cox proportional hazard method showed that invasion depth, lymph node metastasis and ITL were independent prognostic factors.

Developmental and pathological lymphangiogenesis: from models to human disease

The lymphatic vascular system, the body's second vascular system present in vertebrates, has emerged in recent years as a crucial player in normal and pathological processes. It participates in the maintenance of normal tissue fluid balance, the immune functions of cellular and antigen trafficking and absorption of fatty acids and lipid-soluble vitamins in the gut. Recent scientific discoveries have highlighted the role of lymphatic system in a number of pathologic conditions, including lymphedema, inflammatory diseases, and tumor metastasis. Development of genetically modified animal models, identification of lymphatic endothelial specific markers and regulators coupled with technological advances such as high-resolution imaging and genome-wide approaches have been instrumental in understanding the major steps controlling growth and remodeling of lymphatic vessels. This review highlights the recent insights and developments in the field of lymphatic vascular biology.

AllergoOncology: the role of IgE-mediated allergy in cancer

Epidemiological studies have suggested inverse associations between allergic diseases and malignancies. As a proof of concept for the capability of immunoglobulin E (IgE) to destruct tumor cells, several experimental strategies have evolved to specifically target this antibody class towards relevant tumor antigens. It could be demonstrated that IgE antibodies specific to overexpressed tumor antigens have been superior to any other immunoglobulin class with respect to antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis (ADCP) reactions. In an alternative approach, IgE nonspecifically attached to tumor cells proved to be a powerful adjuvant establishing tumor-specific immune memory. Active Th2 immunity could also be achieved by applying an oral immunization regimen using mimotopes, i.e. epitope mimics of tumor antigens. The induced IgE antibodies could be cross-linked by live tumor cells leading to tumoricidic mediator release. Thus, IgE antibodies may not only act in natural tumor surveillance, but could possibly also be exploited for tumor control in active and passive immunotherapy settings. Thereby, eosinophils, mast cells and macrophages can be armed with the cytophilic IgE and become potent anti-tumor effectors, able to trace viable tumor cells in the tissues. It is strongly suggested that the evolving new field AllergoOncology will give new insights into the role of IgE-mediated allergy in malignancies, possibly opening new avenues for tumor therapy.

Three-dimensional context regulation of metastasis

Tumor progression ensues within a three-dimensional microenvironment that consists of cellular and non-cellular components. The extracellular matrix (ECM) and hypoxia are two non-cellular components that potently influence metastasis. ECM remodeling and collagen cross-linking stiffen the tissue stroma to promote transformation, tumor growth, motility and invasion, enhance cancer cell survival, enable metastatic dissemination, and facilitate the establishment of tumor cells at distant sites. Matrix degradation can additionally promote malignant progression and metastasis. Tumor hypoxia is functionally linked to altered stromal-epithelial interactions. Hypoxia additionally induces the expression of pro-migratory, survival and invasion genes, and up-regulates expression of ECM components and modifying enzymes, to enhance tumor progression and metastasis. Synergistic interactions between matrix remodeling and tumor hypoxia influence common mechanisms that maximize tumor progression and cooperate to drive metastasis. Thus, clarifying the molecular pathways by which ECM remodeling and tumor hypoxia intersect to promote tumor progression should identify novel therapeutic targets.

Impact of the hyaluronan-rich tumor microenvironment on cancer initiation and progression

Hyaluronan acts as a microenvironmental stimulus that can influence the malignant phenotype of cancer cells. During cancer progression, hyaluronan assembles an extracellular matrix that is favorable for both the motility and proliferation of cancer cells and the recruitment of inflammatory and bone marrow-derived progenitor cells. The varied roles of this polysaccharide are regulated via multiple mechanisms involving biosynthesis, degradation, binding with other extracellular molecules, and activation of signaling pathways. Recent animal studies have provided evidence that aberrant biosynthesis of hyaluronan accelerates tumor growth through a diverse repertoire of host-tumor interactions, such as stromal cell recruitment, angiogenesis, lymphangiogenesis, and inflammation. Hyaluronan in the tumor microenvironment thus significantly impacts cancer initiation and progression via stroma-cancer cell interactions.

Cytokines as a key component of cancer-related inflammation

Inflammatory conditions in some tissues increase the risk of cancer. Cytokines and chemokines are components of an intensive dialog promoting angiogenesis, metastasis, subversion of adaptive immunity and changing response to hormones and to chemotherapeutic agents. Cytokines involved in cancer-related inflammation represent a target for innovative diagnostic and therapeutic strategies, and a future challenge for scientists and clinicians.

Anti- and proinflammatory cytokine gene polymorphism and genetic predisposition: association with smoking, tumor stage and grade, and bacillus Calmette-Guérin immunotherapy in bladder cancer

Cytokines mediate many immune and inflammatory responses contributing to tumorigenesis. The present study evaluated polymorphisms of IL4, IL6, and TNF (previously TNFA) genes influencing risk in development of transitional cell carcinoma of bladder and recurrence after bacillus Calmette-Guérin (BCG) immunotherapy. The study included 136 unrelated histopathologically confirmed cases and 200 population-based controls. Genomic DNA was extracted from peripheral leukocytes and genotyped for polymorphism in IL4 intron 3, with point mutations identified by amplification refractory mutation system polymerase chain reaction (ARMS-PCR) in IL6-174 G/C and by PCR-restriction fragment length polymorphism analysis in TNF-308 G/A. The IL6 variant C/C exhibited significant association with bladder cancer risk (odds ratio OR = 2.811, P = 0.004), but IL4 and TNF genetic variants did not. Significant association was observed for IL4 (B1/B2+B2/B2) with high-grade or late-stage tumor for TaG3+T1 and T2+ (OR = 5.950, and 6.342 respectively) and with smoking (P = 0.004, OR = 4.202). Low recurrence risk was observed in BCG-treated patients carrying C/C genotype of IL6 (hazard ratio = 0.298, P = 0.03), and also higher recurrence-free survival (log rank P = 0.021). TNF and IL4 demonstrated no association of bladder cancer risk and BCG therapy. The low-producing variant C/C of IL6 may be a risk factor for bladder cancer, whereas high-producing genotypes of IL4 (B1/B2+B2/B2) may predispose to higher risk in patients with high-grade or late-stage tumor and smoking habits. The low-producing C/C IL6 genotype, which favors Th1 response, may be a beneficial prognostic indicator for treatment and survival of BCG-treated patients.

Differential response of primary tumor versus lymphatic metastasis to VEGFR-2 and VEGFR-3 kinase inhibitors cediranib and vandetanib

Blood vessels are required for a tumor to grow and functional lymphatic vessels are required for it to disseminate to lymph nodes. In an attempt to eradicate both the primary tumor and its lymphatic metastasis, we targeted both blood and lymphatic vessels using two different tyrosine kinase inhibitors (TKIs): cediranib and vandetanib, which block vascular endothelial growth factor receptor (VEGFR)-2 and -3 in enzymatic assays. We found that although both cediranib and vandetanib slowed the growth rate of primary tumors and reduced blood vessel density, neither agent was able to prevent lymphatic metastasis when given after tumor cells had seeded the lymph node. However, when given during tumor growth, cediranib reduced the diameters of the draining lymphatic vessels, the number of tumor cells arriving in the draining lymph node, and the incidence of lymphatic metastasis. On the other hand, vandetanib had minimal effect on any of these variables, suggesting that vandetanib did not effectively block VEGFR-3 on lymphatic endothelial cells in our animal model. Collectively, these data indicate that the response of lymphatic vessels to a TKI can determine the incidence of lymphatic metastasis, independent of the effect of the TKI on blood vessels.

The role of VEGF-C staining in predicting regional metastasis in melanoma

Sentinel lymph node status is the most important prognostic factor in primary melanoma. The number of melanoma-associated lymphatic vessels has been associated with sentinel lymph node status and survival. Vascular endothelial growth factor-C (VEGF-C) is found to promote tumour-associated lymphatic vessel growth. In many human neoplasms, VEGF-C expression in neoplastic cells or tumour-associated macrophages (TAMs) has been linked to lymphatic dissemination of tumour cells. Recent studies have suggested a correlation between VEGF-C expression in primary melanoma and the presence of lymph node metastasis. We performed VEGF-C immunohistochemical staining on melanoma tissues of 113 patients with known sentinel lymph node status. We showed that both high VEGF-C expression in melanoma cells and TAMs are positively associated with the presence of a positive sentinel lymph node. No correlation with Breslow thickness, Clark invasion level or ulceration could be detected. VEGF-C expression in melanoma cells was predictive of a shorter overall and disease-free survival, without being an independent predictor of survival. Our results confirm that VEGF-C expression in primary cutaneous melanoma plays a role in the lymphatic spread of the tumour.

Lymphatic vessel activation in cancer

Most cancerous lesions metastasize through the lymphatic system and the status of regional lymph nodes is the most important indicator of a patient's prognosis. The extent of lymph node involvement with cancer is also an important parameter used for determining treatment options. Although the importance of the lymphatic system for metastasis has been well recognized, traditionally, the lymphatic vessels have not been considered actively involved in the metastatic process. Recent evidence, however, indicates that the activation of the lymphatic system is an important factor in tumor progression to metastasis. Tumor lymphangiogenesis has been associated with increased propensity for metastasis, and lymphatic vessel density has emerged as another promising prognostic indicator. More recently, lymphangiogenesis in the sentinel lymph nodes has been shown to contribute to malignant progression. In addition to its role as a transport system for tumor cells, the lymphatic system may also be more actively involved in metastases by directly facilitating tumor cell recruitment into the lymphatic vessels. This review highlights recent advances in our understanding of the mechanisms by which lymphatic vessels participate in metastasis.

The link between lymphatic function and adipose biology

Despite observations of a link between lymphatic vessels and lipids that date as far back as 300, a link between lymphatic vessels and adipose tissue has only recently been recognized. This review will summarize documented evidence that supports a close relationship between lymphatic vessels and adipose tissue biology. Lymphatic vessels mediate lipid absorption and transport, share an intimate spatial association with adipose tissue, and regulate the traffic of immune cells that rely on specialized adipose tissue depots as a reservior of energy deployed to fight infection. Important links between inflammation and adipose tissue biology will also be discussed in this article, as will recent evidence connecting lymphatic vascular dysfunction with the onset of obesity. There seems little doubt that future research in this topical field will ensure that the link between lymphatic vascular function and adipose tissue is firmly established.

VEGF-A and VEGFR-3 correlate with nodal status in operable non-small cell lung cancer: inverse correlation between expression in tumor and stromal cells

BACKGROUND

Lymph node metastasis is an essential determinant for stage and clinical management of non-small cell lung cancer (NSCLC). The vascular endothelial growth factors (VEGFs) and receptors (VEGFRs) are fundamental molecules in angiogenesis and lymphangiogenesis. We aimed to explore the correlations between nodal metastasis and the expression of VEGFs and VEGFRs in tumor cells and in tumor-related stroma.

PATIENTS AND METHODS

Tumor tissue samples from 335 resected patients with stage I-IIIA NSCLC were obtained and tissue microarrays were constructed from duplicate cores of tumor cells and surrounding stromal tissue from each resected specimen. Immunohistochemistry was used to evaluate the expression of VEGF-A, VEGF-C, and VEGF-D and VEGFR-1, VEGFR-2 and VEGFR-3.

RESULTS

There were 232 N0 and 103 N+ patients (76 N1, 27 N2). In multivariate analyses, low stromal VEGF-A expression (P=0.018) is associated with N+ status. In tumor cells, strong correlations exist between high VEGF-A expression (P=0.032) and N+ status, and high VEGFR-3 expression (P<0.001) and N2-status.

CONCLUSION

The converse impact by stromal VEGF-A versus tumor cell VEGF-A expression on nodal metastasis may allude the importance of the tumor-stroma interaction when trying to understand lymphatic metastasis in NSCLC.

Modification of the primary tumor microenvironment by transforming growth factor alpha-epidermal growth factor receptor signaling promotes metastasis in an orthotopic colon cancer model

The transforming growth factor alpha (TGFalpha)/epidermal growth factor receptor (EGFR) signaling pathway appears to play a critical role in colon cancer progression, but the cellular and molecular mechanisms that contribute to metastasis remain unknown. KM12C colon cancer cell clones expressing high (C9) or negligible (C10) levels of TGFalpha were implanted into the cecal walls of nude mice. C9 tumors formed autocrine and paracrine EGFR networks, whereas C10 tumors were unable to signal through EGFR. The tumor microenvironment of C9, but not C10, contained cells enriched in vascular endothelial growth factor (VEGF) A, interleukin-8, and matrix metalloproteinases-2 and -9 and had a high vascular surface area. C9 tumors recruited a macrophage population that co-expressed F4/80 and lymphatic vessel endothelial hyaluronic acid receptor and produced VEGFC. The mean lymphatic density of C9 tumors was threefold higher than that of C10 tumors. C9, but not C10, tumor cells metastasized to regional lymph nodes in all mice and to the liver in 5 of 10 mice. Forced expression of TGFalpha in C10 tumor cells led to the generation of autocrine and paracrine EGFR signaling, macrophage recruitment, enhanced blood and lymphatic vascular surface areas, and increased lymphatic metastasis. Collectively, these data show that activation of TGFalpha-EGFR signaling in colon cancer cells creates a microenvironment that is conducive for metastasis, providing a rationale for efforts to inhibit EGFR signaling in TGFalpha-positive colon cancers.

Pathogenesis of lymphangiomas

Based on various hypotheses concerning lymphangiogenesis published in the literature, different putative mechanisms of lymphangioma development are discussed including failure of the lymphatic system to connect with or separate from the venous system, abnormal budding of the lymphatic system from the cardinal vein, or acquired processes such as traumata, infections, chronic inflammations, and obstructions. Increasingly, the possible influence of lymphangiogenic growth factors on the development of lymphangiomas is discussed. The proved expression of different growth factors in the endothelium of lymphangiomas leads to new hypotheses regarding the pathogenesis of lymphangiomas. Thus, further studies on the lymphangiogenesis and the development of lymphangiomas will have to clarify as to whether lymphangiomas are true malformations or neoplastic in nature.

Molecular biology and pathology of lymphangiogenesis

The lymphatic vasculature is essential for the maintenance of tissue fluid balance, immune surveillance, and adsorption fatty acids in the gut. The lymphatic vessels are also crucially involved in the pathogenesis of diseases such as tumor metastasis, lymphedema, and various inflammatory conditions. Attempts to control or treat these diseases have drawn a lot of interest to lymphatic vascular research during the past few years. Recently, several markers specific for lymphatic endothelium and models for lymphatic vascular research have been characterized, enabling great technical progress in lymphatic vascular biology, and many critical regulators of lymphatic vessel growth have been identified. Despite these significant achievements, our understanding of the lymphatic vessel development and pathogenesis is still rather limited. Several key questions remain to be resolved, including the relative contributions of different pathways targeting lymphatic vasculature, the molecular and cellular processes of lymphatic maturation, and the detailed mechanisms of tumor metastasis via the lymphatic system.