The induction of vascular endothelial growth factor (VEGF) expression by 17β-estradiol (E₂) 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/KDR-expressing capillaries in the human endometrium during the proliferative and mid-secretory phases, strongly suggestive of an E₂ 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 E₂ in human endothelial cells from the microcirculation (HMEC-1) or macrocirculation (HUVEC), even in endothelial cells overexpressing ERα or ERβ after ER-mediated adenovirus infection. In contrast, Flk-1/KDR expression was up-regulated by VEGF itself, in a time- and dose-dependent manner, with the maximal response at 10 ng/ml. Thus, we suggest that E₂ 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.

Abstract

Previous studies have shown a heterogeneous expression of LH receptors in various structures of the porcine ovary. Specially striking was the existence in the preovulatory follicle of inner layers of theca interna cells devoid of LH receptor and the confinement in the corpus luteum of the LH receptor to the external cellular layers. In the present study, we have compared the steroidogenic capabilities of LH receptor-positive and -negative cells using immunocytochemistry for side-chain cleavage P450, 3β-hydroxysteroid-dehydrogenase, 17α-hydroxylase P450 and aromatase P450. We have also examined, using the same methods, the evolution of the various cell types after ovulation and during the development of the corpus luteum.

In preovulatory follicles the inner layers of theca cells which were not labelled with anti-LH receptor antibodies appeared to express the steroidogenic enzymes in a way similar to that of the outer LH receptor-positive cell layers. Ovulation per se did not change the distribution of LH receptors (present in the outer luteal cells and in the granulosa) or of steroidogenic enzymes. However, 48 h after follicular rupture there was a marked decrease in overall labelling with anti-LH receptor antibody, and especially a disappearance of immunostaining in the luteal cells of granulosa origin. In the mid-luteal phase (6 days after ovulation), the receptor content seemed to increase in the peripheral luteal cells derived from the theca but the receptor did not reappear in the granulosa-derived luteal cells. Thus the down-regulation of LH receptor appeared to be reversible in the external thecal layers but irreversible in the granulosa cells. Furthermore, the distribution of the various steroidogenic enzymes in the corpora lutea delineated granulosa-derived from theca-derived cells and showed that only the external layers of the latter expressed the LH receptor.

These results showed the existence in the preovulatory follicle of two theca interna regions expressing the same steroidogenic enzymes but possibly submitted to a different hormonal control. Furthermore, the cells derived from these two regions as well as the cells of granulosa origin showed a distinct pattern of variation of LH receptivity during the development of the corpus luteum.

During these studies we also observed that, in the interstitial tissue, only a minority of cells which derived from remnants of atretic follicles expressed both the LH receptor and the steroidogenic enzymes.

Journal of Endocrinology (1996) 148, 435–446