Inhibition of Tumor Angiogenesis and Melanoma Growth by Targeting Vascular E-Selectin

Objectives: Aggressive human melanomas express, C-X-C chemokine receptor 4 (CXCR4), the receptor for the chemokine, stromal cell-derived factor-1alpha (SDF-1 alpha). The CXCR4-SDF-1 alpha axis has been postulated to increase melanoma invasiveness. We discovered that SDF-1 alpha specifically up-regulates E-selectin on endothelial cells, thus tethering circulating endothelial progenitor cells (EPC) and facilitating homing. We investigated the hypothesis that small interfering ribonucleic acid (siRNA)-mediated E-selectin blockade inhibits melanoma angiogenesis and tumor growth. Methods: Human melanoma cells overexpressing SDF-1 alpha were xenografted on severe combined immunodeficiency (SCID) mice. SDF-1 alpha expression in cells was measured by enzyme-linked immunosorbent assay (ELISA). In vitro melanoma cell growth was examined by cell proliferation assay. In vivo vascular E-selectin knockdown was achieved by administration of high-volume E-selectin siRNA(100 pmol/180 mu L/week x 3 times) and inhibition was validated by immunostaining (N= 6/group, E-Selectin siRNA vs control siRNA). Tumor angiogenesis was quantified (DiI-perfusion and LASER confocal microscopy). EPC homing to tumor vasculature was detected by immunostaining. Explanted in vivo tumor size and weight were measured. Results: Three melanoma cells tested expressed undetectable levels of SDF-1 alpha. Additional enforced overexpression of SDF-1 alpha (by Lenti-SDF-1 alpha) increased melanoma cell growth both in vitro and in vivo, enhanced EPC homing to tumor tissue, and increased tumor angiogenesis. Knocking-down vascular E-selectin significantly inhibited SDF-1 alpha-induced EPC homing, tumor angiogenesis, and decreased melanoma growth in vivo. Conclusions: Downregulation of vascular E-selectin profoundly inhibits EPC homing, tumor angiogenesis, and tumor growth in human melanoma xenograft murine model, potentially by suppression of E-selectin-mediated EPC-endothelial cells interactions/homing. These findings identify E-selectin as a novel target for inhibition of melanoma angiogenesis and tumor growth.