Role of Vascular Endothelial Growth Factor (VEGF) in Thymus of Mice under Normal Conditions and with Tumor Growth.

In our study, we for the first time investigated a role for VEGF as a factor regulating transendothelial migration of murine thymocytes in vitro. Effects of VEGF were examined in a model of thymocyte migration across a monolayer of EA.hy 926 endothelial cells. We showed that VEGF enhanced transendothelial migration of murine thymocytes and their adhesion to endothelial cells in a dose-dependent manner. VEGF did not influence thymocytes, but rather acted on endothelial cells by upregulating surface expression of adhesion molecule ICAM-1 and downregulating activity of 5'-nucleotidase. Effects from VEGF were comparable with those from TNF-α. Because it is known that administration of VEGF to intact animals results in thymic atrophy, it was assumed that it might play a role in developing thymic involution during tumor growth. Enhanced egress of thymocytes to the periphery was considered as a plausible mechanism underlying effects of VEGF. However, we revealed no difference in parameters of in vitro transendothelial migration for thymocytes from animals bearing a transplantable hepatoma 22a compared to control animals. VEGF mRNA expression in lysates of thymic stroma was found to be upregulated in mice with grafted tumors, whereas at the protein level the amount of VEGF did not differ. While examining expression of VEGF receptors on thymocytes by flow cytometry, both VEGFR-1 and VEGFR-2 were not detected, whereas the percentage of Nrp-1-positive thymocytes in animals with hepatoma 22a was as high as in the control group. Thus, we were unable to confirm a hypothesis regarding participation of VEGF in developing thymic involution during progression of experimental hepatoma. However, a set of novel data concerning a role for VEGF in stimulating transendothelial migration of thymocytes in vitro was obtained, and it may be of significance for understanding mechanisms underlying thymus functioning as well as a role of this cytokine in preparing endothelial cells for egress of thymocytes to the periphery.

REGULATION OF VASCULAR ENDOTHELIAL GROWTH FACTOR (VEGF) PRODUCTION BY MOUSE THYMIC EPITHELIAL CELL LINES.

Vascular endothelial growth factor (VEGF) is synthesized in small amounts by thymus epithelial cells in adults and plays a role in supporting vascular homeostasis. However its role becomes dramatically important during the process of thymus reparation after involution caused by chemo-, x-ray or hormonal therapy. The aim of the study was to evaluate the influence of different factors on VEGF production by mouse thymic epithelial cells in vitro. As a model two cell lines were used: cortical cTEC1-2 and medullar mTEC3-10 cells. These cells were characterized by their ability to synthesize VEGF mRNA and protein as well as by their expression of VEGF receptors. VEGFR1 and VEGFR2 mRNA expression in these cells were absent while NRP-1 mRNA revealed low level of expression. It was shown by ELISA that cTEC1-2 cells produced VEGF about 30 times more than mTEC3-10. When cultivated in the presence of cytokines, hormonal factors or thymocytes, both cell lines responded differently. Introduction of keratinocyte growth factor (KGF) induced VEGF mRNA expression as well as VEGF production in medullar cells but simultaneously down-regulated VEGF mRNA expression in cortical cells. Dexamethasone suppressed mRNA VEGF expression and VEGF production in cortical cells while in medullar cells only VEGF production was reduced. Introduction of IL-7, IL-1b or murine thymocytes increased while addition of Semaphorin 3A, SDF-1a or ACTH decreased VEGF production by cortical epithelial cells with no influence on medullar cells. We suggest that our data obtained in vitro can be used for further development of special programs for directed regulation of VEGF synthesis in the thymus epithelial cells in the vivo.