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We previously showed that endothelial cells (EC) from the vasculature of human solid tumors have a decreased expression of intercellular adhesion molecule-1 (ICAM-1) and ICAM-2 as compared with normal tissue EC. This effect is explained by EC exposure to angiogenic factors. It is known that upregulation of endothelial adhesion molecules (EAM) is a sign of EC activation in inflammatory responses. We therefore tested the effect of angiogenic factors on upregulation of EAM on tumor EC and human umbilical vein EC (HUVEC) by proinflammatory cytokines. Incubation of tumor-derived EC in tumor necrosis factor alpha (TNF alpha) did result in expression levels of only 20% of the level of similarly treated normal tissue-derived EC. Pretreatment of HUVEC with 10 ng/ml basic fibroblast growth factor (bFGF) for 3 days, before TNF alpha- or interleukin-1 alpha (IL-1 alpha) stimulation, resulted in ICAM-1 levels of only 30% to 60% of cells without pretreatment. Also, the induction of vascular EC adhesion molecule-1 (VCAM-1) and E- selectin by TNF alpha was significantly inhibited by prior exposure to bFGF. Vascular endothelial growth factor had similar but less prominent effects. The effect of transforming growth factor-beta and IL-8 was studied as well. The functional relevance of the finding of a decreased EC inflammatory response was confirmed by adhesion assays. Our results show that tumor angiogenesis induces EC anergy. This may serve as a tumor-protecting mechanism by impairing the development of an efficient leukocyte infiltrate in tumors.

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CD44 is described to be an activation molecule in a number of different cell types. We investigated the role of CD44 on human endothelial cells (EC) and in tumor angiogenesis. Using flow cytometry we showed that EC from the vasculature of human solid tumors display an enhanced expression of CD44 as compared to EC from normal tissue. This finding was confirmed by immunohistochemical studies on frozen tissue sections. Because tumors are dependent on angiogenesis, the role of angiogenic stimuli in the enhanced CD44 expression was investigated. We found that basic fibroblast growth factor (bFGF ) and vascular endothelial growth factor were able to efficiently upregulate CD44 expression on cultured human EC. The upregulation reached maximal levels after treatment for 3 days with 10 ng/mL bFGF. The physiological impact of this upregulation was shown by the enhanced binding of EC to hyaluronate after pretreatment with bFGF. In a next set of studies that were designed to unravel the regulation of CD44 expression on EC we concluded that CD44 is an activation antigen on human EC since (1) human umbilical vein derived endothelial cells, which in vivo do not express CD44, begin to express CD44 when plated and cultured, (2) CD44 expression is enhanced after subculture of confluent cultures, (3) CD44 is predominantly expressed on the BrdU incorporating subset of cultured EC. The specific expression of CD44 on activated and tumor EC prompted us to study the usefulness of CD44 as an endothelial target for therapy with immunotoxins. In vitro experiments showed that EC are efficiently killed after targeting immunotoxin to CD44.

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No abstract

CD44 is described to be an activation molecule in a number of different cell types. We investigated the role of CD44 on human endothelial cells (EC) and in tumor angiogenesis. Using flow cytometry we showed that EC from the vasculature of human solid tumors display an enhanced expression of CD44 as compared to EC from normal tissue. This finding was confirmed by immunohistochemical studies on frozen tissue sections. Because tumors are dependent on angiogenesis, the role of angiogenic stimuli in the enhanced CD44 expression was investigated. We found that basic fibroblast growth factor (bFGF ) and vascular endothelial growth factor were able to efficiently upregulate CD44 expression on cultured human EC. The upregulation reached maximal levels after treatment for 3 days with 10 ng/mL bFGF. The physiological impact of this upregulation was shown by the enhanced binding of EC to hyaluronate after pretreatment with bFGF. In a next set of studies that were designed to unravel the regulation of CD44 expression on EC we concluded that CD44 is an activation antigen on human EC since (1) human umbilical vein derived endothelial cells, which in vivo do not express CD44, begin to express CD44 when plated and cultured, (2) CD44 expression is enhanced after subculture of confluent cultures, (3) CD44 is predominantly expressed on the BrdU incorporating subset of cultured EC. The specific expression of CD44 on activated and tumor EC prompted us to study the usefulness of CD44 as an endothelial target for therapy with immunotoxins. In vitro experiments showed that EC are efficiently killed after targeting immunotoxin to CD44.