Chemokine-mediated migration of melanoma cells towards lymphatics--a mechanism contributing to metastasis

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.

Chemokine C-X-C motif receptor 6 contributes to cell migration during hypoxia

The chemokine and chemokine receptor families have important roles in tumorigenesis. Although CXCR4 and CCR7 have been reported to be associated with cancer metastasis, the role of other chemokine receptors in cancer is poorly understood. We explored the status of CXCR6 in hypoxia-induced cell migration. Breast cancer cells and human umbilical vein endothelial cells (HUVEC) expressed CXCR6, and showed appreciable chemotactic migration to CXCL16. Significant accumulation of CXCR6 mRNA and protein during hypoxia was observed. Overexpression of CXCR6 increased cell migration, and knockdown of CXCR6 attenuated hypoxia-mediated cell migration and MMP-2 secretion. To investigate possible mechanisms regulating CXCR6 expression during hypoxia, we detected the expression of HIFs and found that HIF-1alpha was involved in CXCR6 regulation. CXCR6 and HIF-1alpha were highly expressed in breast cancer lymph nodes metastases. Our data suggest CXCR6 contributes significantly to cell migration during hypoxia.

The good and the bad of chemokines/chemokine receptors in melanoma

Chemokine ligand/receptor interactions affect melanoma cell growth, stimulate or inhibit angiogenesis, recruit leukocytes, promote metastasis, and alter the gene expression profile of the melanoma associated fibroblasts. Chemokine/chemokine receptor interactions can protect against tumor development/growth or can stimulate melanoma tumor progression, tumor growth and metastasis. Metastatic melanoma cells express chemokine receptors that play a major role in the specifying the organ site for metastasis, based upon receptor detection of the chemokine gradient elaborated by a specific organ/tissue. A therapeutic approach that utilizes the protective benefit of chemokines involves delivery of angiostatic chemokines or chemokines that stimulate the infiltration of cytotoxic T cells and natural killer T cells into the tumor microenvironment. An alternative approach that tackles the tumorigenic property of chemokines uses chemokine antibodies or chemokine receptor antagonists to target the growth and metastatic properties of these interactions. Based upon our current understanding of the role of chemokine-mediated inflammation in cancer, it is important that we learn to appropriately regulate the chemokine contribution to the tumorigenic 'cytokine/chemokine storm', and to metastasis.

Vascular endothelial growth factor-C and C-C chemokine receptor 7 in tumor cell-lymphatic cross-talk promote invasive phenotype

Most carcinomas spread to distant sites through lymphatic vessels. Several preclinical and clinical studies have shown a positive correlation between the incidence of lymph node metastasis and secretion of the lymphatic growth factor vascular endothelial growth factor-C (VEGF-C) by tumor cells, suggesting tumor lymphangiogenesis as an escape mechanism. However, recent evidence has shown VEGF receptor-3 (VEGFR-3) expression on tumor cells and autocrine signaling, which increase metastatic potential. Furthermore, there is growing evidence implicating lymphatic-homing chemokine receptors, particularly C-C chemokine receptor 7 (CCR7), in lymph node metastasis. We report here that expressions of VEGF-C and CCR7 by tumor cells act synergistically to promote their invasion toward lymphatics. First, VEGF-C acts to increase lymphatic secretion of CCL21, which in turn drives CCR7-dependent tumor chemoinvasion toward lymphatics. Second, VEGF-C acts in an autocrine fashion to increase tumor invasiveness by increasing the proteolytic activity and motility of tumor cells in a three-dimensional matrix. Both of these effects are VEGFR-3 dependent and evident only in three-dimensional environments. We further verified that VEGF-C induces lymphatic CCL21 up-regulation in vivo by direct injection of VEGF-C protein intradermally in the mouse. Taken together, these results bridge the prometastatic functions of CCR7 and VEGF-C in tumors and show that, beyond lymphangiogenesis, VEGF-C promotes tumor invasion toward lymphatics by both autocrine and CCR7-dependent paracrine signaling mechanisms, which may be a significant cause of lymph node metastasis.

Inducible nitric oxide synthase expression in melanoma: implications in lymphangiogenesis

Cutaneous melanoma preferentially metastasizes via the lymphatic route. However, the mechanisms of lymphatic invasion and metastasis to regional lymph nodes are poorly understood. Nitric oxide is a free radical molecule synthesized from L-arginine by nitric oxide synthases that plays a critical role in various physiological and pathological processes, including tumor growth and angiogenesis. We have tested whether inducible nitric oxide synthase expression correlates with lymphatic vessel density identified with D2-40 antibody and/or blood microvessel density identified with CD105/endoglin in a series of melanocytic nevi (n=28) and cutaneous melanomas (n=38), representative of various pT. Inducible nitric oxide synthase expression was significantly lower in melanocytic nevi in comparison with primary and metastatic melanomas (P<0.001). Mean microvessel density was significantly higher in primary and metastatic melanomas in comparison with melanocytic nevi (P<0.001 for intratumoral and P=0.001 for peritumoral vessels). Vertical growth phase melanomas showed a higher intratumoral microvessel density in comparison with radial growth phase melanomas (P=0.02). The number of peritumoral lymphatics was significantly lower in nevi as compared with primary and metastatic melanomas (P=0.01). No correlation between microvessel or lymphatic vessel and clinical outcome was found in melanomas. A significant direct correlation was observed between inducible nitric oxide synthase immunostaining in melanocytic tumor cells and the density of lymphatic vessels (peritumoral: P=0.001; intratumoral: P=0.08), and the density of peritumoral blood microvessel (P=0.02). Our findings support the hypothesis that inducible nitric oxide synthase is implicated not only in blood, but also in lymphatic vascular neoformation in melanoma. Mechanistic studies are needed to address the possibility that inducible nitric oxide synthase controls lymphangiogenesis, dissemination and lymphatic borne metastases.

Chemokines and the microenvironment in neuroectodermal tumor-host interaction

Chemokines and chemokine receptors play an important role in immune homeostasis and surveillance. Altered or defective expression of chemokines and/or chemokine receptors could lead to a disease state including autoimmune disorder or cancer. Tumors from glioblastoma, melanoma, and neuroblastoma secrete high levels of chemokines that can promote tumor growth and progression or induce stromal cells present in the tumor microenvironment to produce cytokines or chemokines which, in turn, can regulate angiogenesis, tumor growth, and metastasis. On the other hand, chemokines secreted by tumor or stromal cells can also attract leukocytes such as dendritic cells, macrophages, neutrophils, and lymphocytes which may downmodulate tumor growth. New therapies that are aimed at limiting tumor growth and progression by attracting immune effector cells to the tumor site with chemokines may hold the key to the successful treatment of cancer, although this approach may be hampered by possible tumor growth-stimulating effects of chemokines.

Lymphatic invasion in cutaneous melanoma is associated with sentinel lymph node metastasis

BACKGROUND

Sentinel lymph node (SLN) metastasis is a major determinant for staging, prognostication and clinical management of patients with cutaneous melanoma. However, the role of lymphatic vs. vascular invasion (VI) for SLN spread remains unclear.

METHODS

We compared the frequency of lymphatic invasion (LI) vs. VI in melanoma sections from 94 patients with a mean three-year clinical follow up using immunostains for the lymphatic endothelial markers D2-40 (podoplanin) and LYVE-1 and the panvascular marker CD31.

RESULTS

LI occurred more frequently than VI (16 vs. 3%, respectively, p = 0.001) and correlated with higher American Joint Committee on Cancer stage at diagnosis (p = 0.0004). In a univariate analysis, LI was strongly associated with SLN metastasis (p = 0.008), independent of tumor thickness. In a multivariate analysis, LI was not a significant risk factor for SLN metastasis. The presence of intratumoral lymphatics (ITLs) was associated with distant metastasis, whereas VI was rare and did not correlate with SLN or distant metastasis. A combination of LI and ITL had higher positive and negative predictive values for the risk of developing SLN metastasis compared with routine histology and VI.

CONCLUSION

Detection of LI in the primary tumor may aid in identifying melanoma patients with the propensity to develop SLN metastasis.

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.

A novel gene expression profile in lymphatics associated with tumor growth and nodal metastasis

Invasion of lymphatic vessels is a key step in the metastasis of primary tumors to draining lymph nodes. Although the process is enhanced by tumor lymphangiogenesis, it is unclear whether this is a consequence of increased lymphatic vessel number, altered lymphatic vessel properties, or both. Here we have addressed the question by comparing the RNA profiles of primary lymphatic endothelial cells (LEC) isolated from the vasculature of normal tissue and from highly metastatic T-241/vascular endothelial growth factor (VEGF)-C fibrosarcomas implanted in C57BL/6 mice. Our findings reveal significant differences in expression of some 792 genes (i.e., >or=2-fold up- or down-regulated, P <or= 0.05) that code for a variety of proteins including components of endothelial junctions, subendothelial matrix, and vessel growth/patterning. The tumor LEC profile, validated by immunohistochemical staining, is distinct from that of normal, inflammatory cytokine, or mitogen-activated LEC, characterized by elevated expression of such functionally significant molecules as the tight junction regulatory protein endothelial specific adhesion molecule (ESAM), the transforming growth factor-beta coreceptor Endoglin (CD105), the angiogenesis-associated leptin receptor, and the immunoinhibitory receptor CD200, and reduced expression of subendothelial matrix proteins including collagens, fibrillin, and biglycan. Moreover, we show similar induction of ESAM, Endoglin, and leptin receptor within tumor lymphatics in a series of human head and neck and colorectal carcinomas, and uncover a dramatic correlation between ESAM expression and nodal metastasis that identifies this marker as a possible prognostic indicator. These findings reveal a remarkable degree of phenotypic plasticity in cancer lymphatics and provide new insight into the processes of lymphatic invasion and lymph node metastasis.

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.

Molecular control of lymphatic metastasis

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

Hypoxia increases breast cancer cell-induced lymphatic endothelial cell migration

Because tumors are characterized by hypoxic environments, we used a novel in vitro noninvasive magnetic resonance imaging assay to examine the influence of invasive MDA-MB-231 breast cancer cells on the invasion and migration of human dermal lymphatic microvascular endothelial cells (HMVEC-dLy) under normoxic and hypoxic conditions. Nonmalignant immortalized MCF-12A human mammary epithelial cells instead of cancer cells or chambers with HMVEC-dLy alone were used as controls for comparison. HMVEC-dLy cells were labeled with a T(2) contrast agent (Feridex), and their invasion and migration through extracellular matrix under normoxic and hypoxic conditions were monitored using magnetic resonance imaging. A significant increase in the invasion and migration of HMVEC-dLy cells was detected in the presence of cancer cells, which further increased significantly under hypoxic conditions. HMVEC-dLy cells formed interconnecting strands extending toward the cancer cells under normoxic but not under hypoxic conditions. Following reoxygenation, these interconnecting strands, extending from HMVEC-dLy cells toward the cancer cells, were observed. These data demonstrate the importance of hypoxia in lymphatic endothelial cell invasion and migration through extracellular matrix in the presence of cancer cells.

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.

G-protein coupled chemoattractant receptors and cancer

Chemoattractant receptors are a group of seven transmembrane, G protein coupled receptors (GPCRs). They were initially identified mainly on leukocytes to mediate cell migration in response to pathogen or host-derived chemotactic factors. During the past decade, chemoattractant GPCRs have been discovered not only to mediate leukocyte chemotaxis thus promoting innate and adaptive host immune responses, but also to play essential roles in development, homeostasis, HIV infection, angiogenesis and wound healing. A growing body of evidence further indicates that chemoattractant GPCRs contribute to tumor growth, invasion, angiogenesis/angiostasis and metastasis. The diverse properties of GPCRs in the progression of malignant tumors have attracted intense interest in their potential as novel anti-tumor pharmacological targets.

Lymphatic invasion revealed by multispectral imaging is common in primary melanomas and associates with prognosis

Lymphatic invasion by tumor cells has been noted infrequently in primary melanomas. Our primary hypotheses were that using immunohistochemical markers of lymphatic vessels and of tumor cells would improve detection of lymphatic invasion and that lymphatic invasion would correlate with regional nodal metastatic disease. This study included 106 patients who were diagnosed between 1972 and 1991 and who had 10 years or more of follow-up. We performed dual immunohistochemical stains for podoplanin (for lymphatic vessels) and S-100 (for melanoma cells). Lymphatic invasion was identified by light microscopy and confirmed by multispectral imaging analysis. Lymphatic invasion was detected by morphology alone in 5 cases (4.7%) in contrast to immunohistochemical staining augmented by multispectral imaging analysis where 35 cases (33%) were identified (P < .0001). Lymphatic invasion was significantly associated with time to regional nodal metastatic disease, as well as first metastasis and melanoma-specific death. "Local metastasis," defined by immunohistochemistry-detected lymphatic invasion, satellites, or neural invasion, identified 64% of those who had regional nodal metastatic disease within 5 years of diagnosis. Lymphatic invasion is an underobserved phenomenon in primary melanomas that can be better detected by immunohistochemical staining. The presence of lymphatic invasion may be a clinically useful predictor of regionally metastatic disease.

Membrane-type 1 matrix metalloproteinase regulates cell migration during zebrafish gastrulation: evidence for an interaction with non-canonical Wnt signaling

Key to invasiveness is the ability of tumor cells to modify the extracellular matrix, become motile, and engage in directed migration towards the vasculature. One significant protein associated with metastatic progression is membrane-type 1 matrix metalloproteinase (MT1-MMP/MMP14). How MMP14 activity is coordinated with other signaling pathways to regulate cell migration in vivo is largely unknown. Here we have used zebrafish embryogenesis as a model to understand the potential relationship between MMP14-dependent pericellular proteolysis, cell polarity, and motility. Knockdown of zebrafish Mmp14 function disrupted gastrulation convergence and extension cell movements and craniofacial morphogenesis. Using time-lapse imaging and morphometric analyses, we show that Mmp14 is required for proper cell polarity underlying the directed migration of mesodermal cells during gastrulation. We have identified a genetic interaction between mmp14 and non-canonical Wnt signaling, a pathway that also regulates cell polarity in embryonic tissues and is increasingly being linked with tumor cell migration. Finally, we demonstrate that Van Gogh-like 2, a key regulator of the non-canonical Wnt pathway, co-localizes with MMP14 and becomes redistributed towards the leading edge of polarized human cancer cells. Together, our results support the notion that pathways regulating pericellular proteolysis and cell polarity converge to promote efficient cell migration.

Purging metastases in lymphoid organs using a combination of antigen-nonspecific adoptive T cell therapy, oncolytic virotherapy and immunotherapy

In many common cancers, dissemination of secondary tumors via the lymph nodes poses the most significant threat to the affected individual. Metastatic cells often reach the lymph nodes by mimicking the molecular mechanisms used by hematopoietic cells to traffic to peripheral lymphoid organs. Therefore, we exploited naive T cell trafficking in order to chaperone an oncolytic virus to lymphoid organs harboring metastatic cells. Metastatic burden was initially reduced by viral oncolysis and was then eradicated, as tumor cell killing in the lymph node and spleen generated protective antitumor immunity. Lymph node purging of tumor cells was possible even in virus-immune mice. Adoptive transfer of normal T cells loaded with oncolytic virus into individuals with cancer would be technically easy to implement both to reduce the distribution of metastases and to vaccinate the affected individual in situ against micrometastatic disease. As such, this adoptive transfer could have a great therapeutic impact, in the adjuvant setting, on many different cancer types.

Lymphatic metastasis in breast cancer: importance and new insights into cellular and molecular mechanisms

Lymph node metastasis is the main prognosis factor in a number of malignancies, including breast carcinomas. The means by which lymph node metastases arise is not fully understood, and many questions remain about their importance in the further spread of breast cancer. Nevertheless, a number of key cellular and molecular mechanisms of lymphatic metastasis have been identified. These include induction of intra- or peri-tumoral lymphangiogenesis or co-option of existing lymphatic vessels to allow tumour cells to enter the lymphatics, although it remains to be established whether this is primarily an active or passive process. Gene expression microarrays and functional studies in vitro and in vivo, together with detailed clinical observations have identified a number of molecules that can play a role in the genesis of lymph node metastases. These include the well-recognised lymphangiogenic cytokines VEGF-C and VEGF-D as well as chemokine-receptor interactions, integrins and downstream signalling pathways. This paper briefly reviews current clinical and experimental evidence for the underlying mechanisms and significance of lymphatic metastasis in breast cancer and highlights questions that still need to be addressed.

Potential therapeutic strategies for lymphatic metastasis

Physiologically, the lymphatic system regulates fluid volume in the interstitium and provides a conduit for immune cells to travel to lymph nodes, but pathologically, the lymphatic system serves as a primary escape route for cancer cells. Lymphatic capillaries have a thin discontinuous basement membrane, lack pericyte coverage and often contain endothelial cell gaps that can be invaded by immune cells (or tumor cells). In addition, tumor cells and stromal cells in the tumor microenvironment secrete factors that stimulate lymphangiogenesis, the growth of lymphatic endothelial cells and the sprouting of lymphatic capillaries. As a result, many tumors are surrounded by large, hyperplastic, peri-tumoral lymphatic vessels and less frequently are invaded by intra-tumoral lymphatic vessels. Carcinoma cells commonly metastasize through these lymphatic vessels to regional lymph nodes. The presence of metastatic cells in the sentinel lymph node is a prognostic indicator for many types of cancer, and the degree of dissemination determines the therapeutic course of action. Lymphangiogenesis is currently at the frontier of metastasis research. Recent strides in this field have uncovered numerous signaling pathways specific for lymphatic endothelial cells and vascular endothelial cells. This review will provide an overview of tumor lymphangiogenesis and current strategies aimed at inhibiting lymphatic metastasis. Novel therapeutic approaches that target the tumor cells as well as the vascular and lymphatic endothelial compartments are discussed.

Expression of VEGF(xxx)b, the inhibitory isoforms of VEGF, in malignant melanoma

Malignant melanoma is the most lethal of the skin cancers and the UK incidence is rising faster than that of any other cancer. Angiogenesis – the growth of new vessels from preexisting vasculature – is an absolute requirement for tumour survival and progression beyond a few hundred microns in diameter. We previously described a class of anti-angiogenic isoforms of VEGF, VEGF_xxxb, that inhibit tumour growth in animal models, and are downregulated in some cancers, but have not been investigated in melanoma. To determine whether VEGF_xxxb expression was altered in melanoma, PCR and immunohistochemistry of archived human tumour samples were used. In normal epidermis and in a proportion of melanoma samples, VEGF_xxxb staining was seen. Some melanomas had much weaker staining. Subsequent examination revealed that expression was significantly reduced in primary melanoma samples (both horizontal and vertical growth phases) from patients who subsequently developed tumour metastasis compared with those who did not (analysis of variance (ANOVA) P<0.001 metastatic vs nonmetastatic), irrespective of tumour thickness, while the surrounding epidermis showed no difference in expression. Staining for total VEGF expression showed staining in metastatic and nonmetastatic melanomas, and normal epidermis. An absence of VEGF_xxxb expression appears to predict metastatic spread in patients with primary melanoma. These results suggest that there is a switch in splicing as part of the metastatic process, from anti-angiogenic to pro-angiogenic VEGF isoforms. This may form part of a wider metastatic splicing phenotype.

Autologous chemotaxis as a mechanism of tumor cell homing to lymphatics via interstitial flow and autocrine CCR7 signaling

CCR7 is implicated in lymph node metastasis of cancer, but its role is obscure. We report a mechanism explaining how interstitial flow caused by lymphatic drainage directs tumor cell migration by autocrine CCR7 signaling. Under static conditions, lymphatic endothelium induced CCR7-dependent chemotaxis of tumor cells through 3D matrices. However, interstitial flow induced strong increases in tumor cell migration that were also CCR7 dependent, but lymphatic independent. This autologous chemotaxis correlated with metastatic potential in four cell lines and was verified by visualizing directional polarization of cells in the flow direction. Computational modeling revealed that transcellular gradients of CCR7 ligand were created under flow to drive this response. This illustrates how tumor cells may be guided to lymphatics during metastasis.