REGULATION OF THE VASCULAR ENDOTHELIAL GROWTH FACTOR (VEGF) RECEPTOR FLK-1/KDR BY ESTRADIOL THROUGH VEGF IN UTERUS

The induction of vascular endothelial growth factor (VEGF) expression by 17-estradiol (E 2 ) in many target cells, including epithelial cells, fibroblasts and smooth muscle cells, suggests a role for this hormone in the modulation of angiogenesis and vascular permeability. We have already described a cyclic increase in Flk-1/KDRexpressing capillaries in the human endometrium during the proliferative and mid-secretory phases, strongly suggestive of an E 2 effect on Flk-1/KDR expression in the endometrial capillaries. However, it is unclear whether these processes are due to a direct effect of E2 on endothelial cells. Using immunohistochemistry, we report an increase in Flk-1/KDR expression in endometrial capillaries of ovariectomized mice treated with E2, or both E2 and progesterone. This process is mediated through estrogen receptor (ER) activation. In vitro experiments using quantitative RT-PCR analysis demonstrate that Flk-1/KDR expression was not regulated by E 2 in human endothelial cells from the microcirculation (HMEC-1) or macrocirculation (HUVEC), even in endothelial cells overexpressing ER or ER after ER-mediated adenovirus infection. In contrast, Flk-1/KDR expression was up-regulated by VEGF itself, in a time- and dosedependent manner, with the maximal response at 10 ng/ ml. Thus, we suggest that E 2 up-regulates Flk-1/KDR expression in vivo in endothelial cells mainly through the modulation of VEGF by a paracrine mechanism. It is currently unknown whether or not the endothelial origin might account for differences in the E2-modulation of VEGF receptor expression, particularly in relation to the vascular bed of sex steroid-responsive tissues.