Paula Velasco

Induction of tumor lymphangiogenesis by VEGF-C promotes breast cancer metastasis

Metastasis of breast cancer occurs primarily through the lymphatic system, and the extent of lymph node involvement is a key prognostic factor for the disease. Whereas the significance of angiogenesis for tumor progression has been well documented, the ability of tumor cells to induce the growth of lymphatic vessels (lymphangiogenesis) and the presence of intratumoral lymphatic vessels have been controversial. Using a novel marker for lymphatic endothelium, LYVE-1, we demonstrate here the occurrence of intratumoral lymphangiogenesis within human breast cancers after orthotopic transplantation onto nude mice. Vascular endothelial growth factor (VEGF)-C overexpression in breast cancer cells potently increased intratumoral lymphangiogenesis, resulting in significantly enhanced metastasis to regional lymph nodes and to lungs. The degree of tumor lymphangiogenesis was highly correlated with the extent of lymph node and lung metastases. These results establish the occurrence and biological significance of intratumoral lymphangiogenesis in breast cancer and identify VEGF-C as a molecular link between tumor lymphangiogenesis and metastasis.

Molecular characterization of lymphatic endothelial cells

The lymphatic microvasculature is uniquely adapted for the continuous removal of interstitial fluid and proteins and is an important entry point for leukocytes and tumor cells. Specialized functions of lymphatics suggest differences in the molecular composition of the lymphatic and blood vascular endothelium. However, the extent to which the two cell types differ is still unclear, and few molecules that are truly specific to lymphatic endothelial cells have been identified to date. We have isolated primary lymphatic and blood microvascular endothelial cells from human skin by immunoselection with the lymphatic marker LYVE-1 and demonstrate that the two cell lineages express distinct sets of vascular markers and respond differently to growth factors and extracellular matrix. Comparative microarray analysis of gene-expression profiles revealed a number of unique molecular properties that distinguish lymphatic and blood vascular endothelium. The molecular profile of lymphatic endothelium seems to reflect characteristic functional and structural features of the lymphatic capillaries. Classification of the differentially expressed genes into functional groups revealed particularly high levels of genes implicated in protein sorting and trafficking, indicating a more active role of lymphatic endothelium in uptake and transport of molecules than previously anticipated. The identification of a large number of genes selectively expressed by lymphatic endothelium should facilitate the discovery of hitherto unknown lymphatic vessel markers and provide a basis for the analysis of the molecular mechanisms accounting for the characteristic functions of lymphatic capillaries.

VEGF-A promotes tissue repair-associated lymphatic vessel formation via VEGFR-2 and the α1β1 and α2β1 integrins

Vascular endothelial growth factor‐A (VEGF‐A) is strongly up‐regulated in wounded cutaneous tissue and promotes repair‐associated angiogenesis. However, little is known about its role in lymphatic regeneration of the healing skin. We studied wound healing in transgenic mice that overexpress VEGF‐A specifically in the epidermis and in wild‐type mice in the absence or presence of inhibitors of VEGF‐A signaling. Surprisingly, transgenic overexpression of VEGF‐A in the skin promoted lymphangiogenesis at the wound healing site, whereas systemic blockade of VEGFR‐2 prevented lymphatic vessel formation. Studies in cultured lymphatic endothelial cells revealed that VEGF‐A induced expression of the α1 and α2 integrins, which promoted their in vitro tube formation and their haptotactic migration toward type I collagen. VEGF‐A‐induced lymphatic endothelial cord formation and haptotactic migration were suppressed by anti‐α1 and anti‐α2 integrin blocking antibodies, and systemic blockade of the α1 and α2 integrins inhibited VEGF‐A‐driven lymphangiogenesis in vivo. We propose that VEGF‐A promotes lymphatic vasculature formation via activation of VEGFR‐2 and that lineage‐specific differences of integrin receptor expression contribute to the distinct dynamics of wound‐associated angiogenesis and lymphangiogenesis.

Overexpression of Thrombospondin-1 Decreases Angiogenesis and Inhibits the Growth of Human Cutaneous Squamous Cell Carcinomas

The function of the endogenous angiogenesis inhibitor thrombospondin-1 (TSP-1) in epithelial tumor development has remained controversial. We studied the in vitro growth characteristics and the in vivo tumor xenograft growth of the human squamous cell carcinoma cell lines A431 and SCC-13, stably transfected to overexpress human TSP-1. Overexpression of TSP-1 inhibited tumor growth of A431 xenotransplants, and completely abolished tumor formation by SCC-13 cells. TSP-1 overexpressing A431 tumors were characterized by extensive areas of necrosis and by decreased tumor vessel number and size. The effects of TSP-1 on tumor cell growth were indirect since tumor cell proliferation rates in vivo and in vitro, anchorage-dependent and -independent growth in vitro, and susceptibility to induction of apoptosis by serum withdrawal were unchanged in TSP-1 overexpressing tumor cells. However, TSP-1 overexpression up-regulated the TSP-1 receptor CD36, leading to enhanced adhesion of A431 cells to TSP-1. These findings establish TSP-1 as a potent inhibitor of angiogenesis and tumor growth in carcinomas of the skin.

Thrombospondin‐1 suppresses wound healing and granulation tissue formation in the skin of transgenic mice

The function of the endogenous angiogenesis inhibitor thrombospondin‐1 (TSP‐1) in tissue repair has remained controversial. We established transgenic mice with targeted overexpression of TSP‐1 in the skin, using a keratin 14 expression cassette. TSP‐1 transgenic mice were healthy and fertile, and did not show any major abnormalities of normal skin vascularity, cutaneous vascular architecture, or microvascular permeability. However, healing of full‐thickness skin wounds was greatly delayed in TSP‐1 transgenic mice and was associated with reduced granulation tissue formation and highly diminished wound angiogenesis. Moreover, TSP‐1 potently inhibited fibroblast migration in vivo and in vitro. These findings demonstrate that TSP‐1 preferentially interfered with wound healing‐associated angiogenesis, rather than with the angiogenesis associated with normal development and skin homeostasis, and suggest that therapeutic application of angiogenesis inhibitors might potentially be associated with impaired wound vascularization and tissue repair.

Activation of the tie2 receptor by angiopoietin-1 enhances tumor vessel maturation and impairs squamous cell carcinoma growth.

The distinct roles of angiopoietin (Ang)-1 and Ang2, counteracting ligands for the endothelium-specific Tie2 receptor, in tumor development and progression have remained poorly understood. We investigated the expression of Ang1 and Ang2 during multistep mouse skin carcinogenesis and in human squamous cell carcinoma (SCC) xenografts. Expression of Ang2, but not of Ang1, was up-regulated in angiogenic tumor vessels already in early stages of skin carcinogenesis and was also strongly increased in SCCs. Stable overexpression of Ang1 in human A431 SCCs resulted in a more than 70% inhibition of tumor growth, associated with enhanced Tie2 phosphorylation levels, as compared with low levels in control transfected tumors. No major changes in the vascular density, vascular endothelial growth factor mRNA and protein expression, and vascular endothelial growth factor receptor-2 phosphorylation levels were observed in Ang1-expressing tumors. However, the fraction of tumor blood vessels with coverage by alpha-smooth muscle actin-positive periendothelial cells was significantly increased, indicative of an increased vascular maturation status. These findings identify an inhibitory role of Ang1/Tie2 receptor-mediated vessel maturation in SCC growth and suggest that up-regulation of its antagonist, Ang2, during early-stage epithelial tumorigenesis contributes to the angiogenic switch by counteracting specific vessel-stabilizing effects of Ang1.

Thrombospondin-2 plays a protective role in multistep carcinogenesis: a novel host anti-tumor defense mechanism.

The angiogenic switch during tumorigenesis is thought to be induced by a change in the balance of pro‐ angiogenic and anti‐angiogenic factors. To elucidate the biological role of the endogenous angiogenesis inhibitor thrombospondin‐2 (TSP‐2) during multistep carcinogenesis, we subjected TSP‐2‐deficient and wild‐type mice to a chemical skin carcinogenesis regimen. Surprisingly, TSP‐2 expression was strongly upregulated in the mesenchymal stroma of wild‐type mice throughout the consecutive stages of tumorigenesis whereas the angiogenesis factor, vascular endothelial growth factor, was induced predominantly in tumor cells. TSP‐2 deficiency dramatically enhanced susceptibility to skin carcinogenesis and resulted in accelerated and increased tumor formation. The angiogenic switch occurred in early stages of pre‐malignant tumor formation, and tumor angiogenesis was significantly enhanced in TSP‐2‐deficient mice. While TSP‐2 deficiency did not affect tumor differentiation or proliferation, tumor cell apoptosis was signific antly reduced. These results reveal upregulation of an endogenous angiogenesis inhibitor during multi step tumorigenesis and identify enhanced stromal TSP‐2 expression as a novel host anti‐tumor defense mechanism.

Expression of Vascular Endothelial Growth Factor Induces an Invasive Phenotype in Human Squamous Cell Carcinomas

Inhibition of the vascular endothelial growth factor (VEGF) receptor Flk-1 has been shown to prevent invasion of experimental squamous cell carcinomas (SCC). To directly investigate the role of VEGF in tumor invasion, we stably transfected human SCC-13 cells, which are characterized by a noninvasive phenotype in vivo, with expression vectors containing murine VEGF(164) in sense (SCC/VEGF+) or antisense (SCC/VEGF-) orientation or with vector alone (SCC/vec). SCC/vec cells formed slowly growing, well-differentiated tumors with well-defined borders between tumor and stroma, after intradermal or subcutaneous injection. In contrast, SCC/VEGF+ tumors were characterized by rapid tumor growth, with small cell groups and single cells invading into the surrounding tissue, and by admixture of blood vessels and tumor cells in areas of tumor invasion. We detected an increase in tumor vessel density and size in VEGF-overexpressing tumors, resulting in a more than fourfold increase in total vascular areas. In contrast, SCC/VEGF- clones formed noninvasive, sharply circumscribed tumors with reduced vascular density. These findings demonstrate that selective VEGF overexpression was sufficient to induce tumor invasiveness, and they provide further evidence for an active role of the tumor stroma in cancer progression.

The Angiogenesis Inhibitor Thrombospondin-1 Inhibits Acute Cutaneous Hypersensitivity Reactions

There is increasing evidence that vascular remodeling and endothelial cell activation promote acute and chronic inflammation. Thrombospondin 1 (TSP-1) is a potent endogenous angiogenesis inhibitor thought to play an important role in maintaining cutaneous vascular quiescence. We first investigated TSP-1 expression in human and contact hypersensitivity (CHS) reactions and found that TSP-1 was upregulated in the inflamed skin of patients and in mice. To elucidate the function of TSP-1 in cutaneous inflammation, we induced CHS reactions in the skin of mice with targeted epidermal TSP-1 overexpression in TSP-1-deficient mice and in wild-type mice. We found decreased edema formation, angiogenesis, and inflammatory infiltrate in the inflamed skin of TSP-1 transgenic mice. Conversely, TSP-1-deficient mice exhibited an enhanced and prolonged inflammation, characterized by increased edema formation, enhanced vascular remodeling, and increased neutrophilic infiltrate, when compared with wild-type mice. Moreover, we found strong upregulation of the proinflammatory cytokines IL-1beta, macrophage inflammatory protein 2, and tumor necrosis factor-alpha in the inflamed skin of TSP-1-deficient mice. Our results indicate that TSP-1 downregulates cutaneous delayed-type hypersensitivity reactions by acting on several distinct pathways mediating skin inflammation.

Isolation and culture of microvascular endothelial cells.

The cultivation of endothelial cells from large vessels, predominantly from human umbilical veins (1,2), has become a routine procedure in many laboratories and has contributed to the development of modern vascular biology. However, there is convincing evidence that microvascular endothelial cells display a number of important functional differences, compared to large vessel-derived endothelial cells (3), in particular, with regard to their growth factor response (4,5) and their regulation of adhesion molecule expression (6-8). Since endothelial cells involved in the pathogenesis of tumor angiogenesis, wound healing, and acute and chronic inflammation are predominantly of microvascular origin, techniques have been developed to isolate endothelial cells from small vessels, most frequently from the skin (5,9-13). The culture of human dermal microvascular endothelial cells (HDMEC) has remained problematic because of difficulties in cell isolation, low cell yields, and short lifespans of the isolated cells. In particular, potential contamination of HDMEC cultures with fibroblasts required time-consuming density-gradient centrifugations (5,12) or mechanical removal of fibroblasts (10), and remained problematic after several cell passages. We established a simplified protocol that allows the rapid and reliable immunomagnetic isolation of a well characterized, 100% pure population of HDMEC from neonatal human foreskins. This technique is based on the endothelial cell-specific induction of E-selectin by tumor necrosis factor-alpha (TNF-α) (14), predominantly in postcapillary venule endothelial cells (15), and selection of E-selectin-expressing cells by Dynabeads coupled with an anti-E-selectin monoclonal antibody.