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Epigenetic Regulation of Tumor Endothelial Cell Anergy: Silencing of Intercellular Adhesion Molecule-1 by Histone Modifications

¹ Angiogenesis Laboratory and ² Cancer Biology Laboratory, Research Institute for Growth and Development, Department of Pathology, Maastricht University and University Hospital; ³ Laboratory for Microcirculation, Cardiovascular Research Institute Maastricht, Department of Physiology, Maastricht University, Maastricht, The Netherlands; ⁴ Laboratory of Molecular Virology, Faculty of Medicine, Free University of Brussels, Brussels, Belgium; ⁵ Department of Biochemistry, University of Minnesota Academic Health Center, Minneapolis, Minnesota; and ⁶ Department of Pathology and Laboratory Medicine, University Medical Center Groningen, University of Groningen, Groningen, the Netherlands

Requests for reprints: Arjan W. Griffioen, Angiogenesis Laboratory, Department of Pathology, Maastricht University and University Hospital, P.O. Box 5800, 6202 AZ Maastricht, the Netherlands. Phone: 31-43-3874630; Fax: 31-43-3876613; E-mail: aw.griffioen{at}path.unimaas.nl.

Tumors can escape from immunity by repressing leukocyte adhesion molecule expression on tumor endothelial cells and by rendering endothelial cells unresponsive to inflammatory activation. This endothelial cell anergy is induced by angiogenic growth factors and results in reduced leukocyte-vessel wall interactions, thereby attenuating infiltration of leukocytes into the tumor. This report describes a novel mechanism of endothelial cell anergy regulation. We recently reported that DNA methyltransferase (DNMT) and histone deacetylase (HDAC) inhibitors have angiostatic activity. Here, we studied whether epigenetic mechanisms regulate this angiogenesis-mediated escape from immunity. We found that DNMT inhibitors 5-aza-2'-deoxycytidine and zebularine, as well as HDAC inhibitor trichostatin A, reexpressed intercellular adhesion molecule-1 (ICAM-1) on tumor-conditioned endothelial cells in vitro, resulting in restored leukocyte-endothelial cell adhesion. In addition, treatment with DNMT or HDAC inhibitors in vivo also restored ICAM-1 expression on tumor endothelial cells from two different mouse tumor models. Furthermore, leukocyte-vessel wall interactions in mouse tumors were increased by these compounds, as measured by intravital microscopy, resulting in enhanced leukocyte infiltration. We show that ICAM-1 down-regulation in tumor endothelial cells is associated with ICAM-1 promoter histone H3 deacetylation and loss of histone H3 Lys⁴ methylation but not with DNA hypermethylation. In conclusion, our data show that ICAM-1 is epigenetically silenced in tumor endothelial cells by promoter histone modifications, which can be overcome by DNMT and HDAC inhibitors, suggesting a new molecular mechanism based on which novel therapeutic approaches for cancer can be pursued. (Cancer Res 2006; 66(22): 10770-7)

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