17 β-Estradiol Increases VEGF Receptor-2 and Promotes DNA Synthesis in Retinal Microvascular Endothelial Cells

Abstract

purpose. Estrogen is known to promote angiogenesis in gonads. The presence of estrogen receptors in the vascular endothelium of organs other than gonads has been reported. The goal of this study was to determine whether estrogen promotes the proliferation of retinal microvascular endothelial cells and to explore the mechanism of it.

methods. DNA was quantitated using primary cultures of bovine retinal endothelial cells that were incubated with different doses of 17β -estradiol (E₂), VEGF, or both. The changes in expression level of VEGF and VEGF receptor-2 (VEGFR2) were measured using northern blot analysis after treatment with E₂. The presence of estrogen receptors in the endothelial cells was studied by immunohistochemistry and western blot analysis.

results. 17 β-Estradiol (E₂) increased the DNA level in bovine retinal capillary endothelial cells (BRECs) by 177% at 1 nM (P < 0.05) and 150% at 10 nM (P < 0.05) by comparison with unstimulated BREC. One hundred nanomole tamoxifen completely blocked the E₂-induced DNA synthesis in BRECs. Ten nanomole E₂ augmented vascular endothelial growth factor (VEGF)–induced DNA synthesis in BRECs significantly (160%, P < 0.01). Ten nanomole E₂ also increased VEGF mRNA expression, which peaked after 24 hours (6.7 times, P < 0.05), and VEGF receptor-2 (VEGFR2) mRNA expression, which peaked after 9 hours (2.4 times, P < 0.05). The mRNA expression level of VEGFR2 peaked with 10 nM E₂ (P < 0.05) and that of VEGF reached maximum with 1 nM E₂ (15 times, P < 0.001). VEGFR2 and VEGF proteins increased in parallel with their mRNA levels. Immunocytochemistry showed estrogen receptor expression in BRECs, and western blot analysis indicated the presence of a 67-kDa protein that was compatible with the estrogen receptor.

conclusions. These findings suggest that E₂ may stimulate BREC growth by the receptor-mediated pathway and that E₂ may augment the VEGF-dependent angiogenesis partly through the upregulation of VEGFR2.

Estrogen is a member of steroid hormone well known for its classic roles in the homeostasis of sex steroid hormones and the maintenance of gonad function. ¹ Recently, the existence of the estrogen receptor in the vascular endothelium of organs other than gonads has been reported, and its role in both physiological and pathologic conditions has begun to interest many investigators, who have found that estrogen exerts its effect on nitric oxide synthase ² and lipid metabolism ³ ; affects the release of prostaglandins and leukotrienes, thereby inhibiting the chemotaxis of white blood cells ^{4 5} ; and inhibits leukocyte infiltration by reducing intercellular adhesion molecules. ⁶ Among the many roles of estrogen, one of the most important is its angiogenic property, principally in the gonadal organs; estrogen is involved in the physiological condition of menstruation and in some pathologic situations, such as in endometriosis. ⁷  

Diabetic retinopathy is characterized by proliferative neovascularization of the retina and is one of the leading causes of visual loss in adults in the United States. ⁸ In female patients, the diabetic retinopathy often deteriorates during pregnancy. ^{9 10 11} The serum level of estrogen during pregnancy is raised nearly 1000 times that of nonpregnancy. These suggest that estrogen may have some effect on vascular proliferation, but to our knowledge there has been no report on the presence of estrogen receptors or effects of estrogen on ocular vasculature. Thus, in the study described herein, we investigated the effects of 17β -estradiol (E₂), the major functional derivative of estrogen, on retinal endothelial cell proliferation using bovine retinal endothelial cells (BRECs). Furthermore, because elevated levels of vascular endothelial growth factor (VEGF) in patients with diabetic retinopathy seem to play an important role in neovascularization, ^{12 13 14} we also explored if estrogen could have some synergistic effect with VEGF on BREC proliferation. 

Materials and Methods

Materials

VEGF was purchased from Genzyme (Cambridge, MA); plasma-derived horse serum was obtained from Wheaton (Pipersville, PA); ³²P-dCTP and ³⁵S-methionine were obtained from Amersham (Buckinghamshire, UK); 17 β-estradiol, tamoxifen, fibronectin, sodium pyrophosphate, sodium fluoride, sodium orthovanadate, aprotinin, leupeptin, and phenylmethylsulfonyl fluoride were obtained from Sigma (St. Louis, MO); and protein A-Sepharose beads were from Pharmacia Biotech (Uppsala, Sweden). For immunocytochemistry, mouse monoclonal anti-estrogen receptor antibody was obtained from ABR Inc. (Golden, CO); for western blot analysis, mouse monoclonal anti-estrogen receptor antibody was purchased from NeoMarkers (Fremont, CA); and for immunoprecipitation analysis, rabbit polyclonal anti-human VEGF antibody and VEGF receptor-2 (VEGFR2) antibody were obtained from Santa Cruz Biotechnology (Santa Cruz, CA). Human VEGFR2 cDNA was a generous gift from Lloyd P. Aiello and George L. King (Joslin Diabetes Center, Boston, MA). 

Cell Culture

Primary cultures of BRECs were isolated from freshly isolated calf eyes obtained from a local abattoir by homogenization and a series of filtration steps, as described by King et al. ¹⁵ Primary BRECs were grown on fibronectin-coated dishes (Iwaki Glass, Tokyo, Japan) containing phenol red–free Dulbecco’s modified Eagle’s medium (DMEM, Sigma) with 5.5 mM glucose, charcoal/dextran-treated 10% plasma–derived horse serum, 50 mg/l heparin, and 50 U/l endothelial cell growth factor (Boehringer Mannheim, Indianapolis, IN). The cells were cultured in 5% CO₂ at 37°C; medium was changed every 3 days. Endothelial cell homogeneity was confirmed by positive immunostaining for anti–factor VIII antibodies analyzed by confocal microscopy. Cells were plated at a density of 2 × 10⁴ cells/cm² and passaged when confluent (approximately 1 × 10⁵ cells/cm²). The medium was again changed every 3 days, and cells from passages 3 through 10 were used for the experiments. 

DNA Content Assay in Retinal Endothelial Cells

BRECs were seeded sparsely (approximately 2500 cells/well) on 24-well culture plates (Iwaki Glass, Tokyo, Japan) and incubated for 24 hours with phenol red–free DMEM containing charcoal-filtered 10% calf serum (GIBCO Laboratories, Grand Island, NY). The medium was changed to each of the experimental conditioned media the following day, as follows: (1) to study the effect of estrogen on BREC proliferation, the medium were conditioned with 1 pM to 100 nM E₂ in either the presence or absence of VEGF, added at a concentration of 0.6 nM, which has been reported to induce the maximum proliferative effect on BRECs ¹⁶ ; (2) to observe augmented VEGF-dependent cell proliferation by E₂, VEGF was added at 0.0006 to 0.6 nM concentrations to 10 nM E₂; (3) to study the effect of estrogen receptor antagonist, a 10-fold excess of tamoxifen was added to the medium 3 hours before stimulation with E₂. In each of these experiments, the cells were incubated in the experimental medium at 37°C for 4 days and lysed in 0.1% sodium dodecyl sulfate; DNA content was measured by means of Hoechst-33258 dye and a fluorometer (model TKO-100; Hoefer Scientific Instruments, San Francisco, CA). It has previously been shown that total cellular DNA content measured in this manner correlates closely with actual cell number, as determined by hemocytometer counting of trypsinized cells. ¹⁶  

RNA Extraction

The medium was decanted, and the cells were lysed directly in the culture dishes with 600 μl guanidinium thiocyanate. RNA was extracted by adding 240 μl of chloroform and shaking for 10 seconds, then cooling at 4°C for 5 minutes. The suspension was centrifuged at 15,000 rpm for 15 minutes at 4°C, and the aqueous phase transferred to a new tube. The RNA was precipitated by adding 600 μl isopropanol and incubated on ice for 15 minutes at 4°C. The RNA pellets were washed once with 75% ethanol, dried, resuspended in 20 μl diethyl pyrocarbonate (DEPC)–treated water, and incubated for 10 minutes at 60°C. RNA purity was determined by the ratio of optical density (OD) measured at 260 and 280 nm (OD₂₆₀/OD₂₈₀), and RNA quantity was estimated at OD₂₆₀. 

Northern Blot Analysis

Northern blot analysis was performed on 15 μg total RNA after 1% agarose–2 M formaldehyde gel electrophoresis and subsequent capillary transfer to Biodyne nylon membranes (Pall BioSupport, East Hills, NY) and ultraviolet crosslinking using a FUNA-UV-LINKER (model FS-1500; Funakoshi, Tokyo, Japan). Radioactive probes were generated using an Amersham Megaprime labeling kit and ³²P-dCTP. Blots were prehybridized, hybridized, and washed 4 times in 0.5 × SSC, 5% sodium dodecyl sulfate (SDS), at 65°C for 1 hour in a rotating hybridization oven (TAITEC, Koshigaya, Japan). All signals were analyzed using a densitometer (model BAS-2000 II; Fuji Photograph Film, Tokyo, Japan), and the samples of each lane were normalized using the 36B4 cDNA probe. 

Immunoprecipitation

Confluent BRECs were serum-deprived for 24 hours and then treated with 10 nM E₂ or vehicle for 9 hours for VEGFR2 protein evaluation or for 24 hours for VEGF protein evaluation. Cells were then incubated with 100 μCi/ml of ³⁵S-methionine in methionine-free DMEM for 4 hours, washed 3 times with cold phosphate-buffered saline (PBS), and then solubilized in 1 ml of lysis buffer containing 1% Triton X-100, 50 mM HEPES, 10 mM EDTA, 10 mM sodium pyrophosphate, 100 mM sodium fluoride, 1 mM sodium orthovanadate, 1 μg/ml aprotinin, 1 μg/ml leupeptin, and 2 mM phenylmethylsulfonyl fluoride. After centrifugation at 12,000 rpm for 10 minutes, equal amounts of protein from each sample were preabsorbed with protein A–Sepharose beads and reacted with an excess amount of rabbit anti-human VEGFR2 antibody or rabbit anti-human VEGF antibody; immunoprecipitation was then done with protein A–Sepharose beads. The resin was washed 5 times with lysis buffer and boiled for 5 minutes in nonreducing SDS–polyacrylamide gel electrophoresis sample buffer. Each sample was electrophoresed on a 7.5% SDS–polyacrylamide gel, and all signals were analyzed with a densitometer (model BAS-2000 II; Fuji Photograph Film). 

Immunocytochemistry

BRECs were seeded sparsely (approximately 2500 cells/well) on 4-well chamber slides (Nalge Nunc International, Naperville, IL) and incubated for 4 days with phenol red–free DMEM containing charcoal-filtered 10% calf serum. The medium was then discarded, and the BRECs fixed in acetone for 10 minutes at 4°C. After a 10-minute wash with PBS, slides were incubated in a humidified chamber for 2 hours with primary antibody (1:50, mouse monoclonal anti-estrogen receptor antibody, ABR Inc. and then rinsed for 10 minutes with PBS at room temperature. Slides were next incubated for 1 hour with secondary antibody (goat anti-mouse IgG antibody; 1:200), and then incubated with streptavidine for 2 hours at room temperature. A confocal laser scanning microscope (model LSM 410 invert Laser Scan Microscope; Zeiss, Oberkochen, Germany) was used for the immunocytochemical analysis. Digitized images were captured by computer and stored on an optical disc for subsequent display. Photographic images were printed with a 35 mm film printer. 

Western Blot Analysis

Cells were washed 3 times with cold PBS and then solubilized in 100 μl lysis buffer (1% Triton X-100, 50 mM HEPES, 10 mM EDTA, 10 mM sodium pyrophosphate, 100 mM sodium fluoride, 1 mM sodium orthovanadate, 1 μg/ml aprotinin, 1 μg/ml leupeptin, and 2 mM phenylmethylsulfonyl fluoride). After centrifugation at 12,000 rpm for 10 minutes, 30 μg protein was subjected to 7.5% SDS gel electrophoresis and transferred electrically to a nitrocellulose membrane (Schleicher & Schuell, Keene, NH). The membrane was then soaked in the blocking buffer (PBS containing 0.1% Tween-20 and 3% bovine serum albumin) for 1 hour at room temperature and incubated with anti-estrogen receptor antibody (1:100) for 1.5 hours. The blots were washed with PBS containing Tween-20, and the signals detected by an ECL western blot analysis system (Amersham). 

Statistical Analysis

All determinations were performed in triplicate, and all experiments were repeated at least three times. Results are expressed as the mean ± SEM, unless otherwise indicated. Statistical analysis used Student’s t-test or ANOVA to compare quantitative data populations with normal distributions and equal variance. Data were analyzed using the Mann–Whitney rank sum test or the Kruskal–Wallis test for populations with nonnormal distributions or unequal variance. P < 0.05 was considered statistically significant. 

Results

Effect of E₂ on DNA Synthesis in BRECs

The effect of exogenous E₂ on DNA synthesis in BRECs is shown in Figure 1 . BREC DNA synthesis was increased by the addition of exogenous E₂; the maximal stimulatory effect was achieved with between 1 nM (177% ± 78.1%, P < 0.05) and 10 nM (150% ± 20.8%, P < 0.05). At concentrations above 10 nM, increased cell growth was not apparent, and its effect was eliminated entirely by a 10-fold excess dose of tamoxifen, an estrogen receptor antagonist (Fig. 1) . 

Effect of E₂ on VEGF-Induced DNA Synthesis of BRECs

The effect of E₂ on the VEGF-induced DNA synthesis in BRECs is shown in Figure 2 A. E₂ at concentrations of 1 to 10 nM, in combination with 0.6 nM VEGF, significantly enhanced VEGF-induced DNA synthesis in BRECs. The maximum response was obtained at a concentration of 10 nM E₂ (160% ± 25.8%, P < 0.01). Furthermore, 10 nM E₂ augmented the dose-dependent VEGF induction of DNA synthesis in BRECs at each concentration of VEGF tested (Fig. 2B) , but the synergistic effect was maximum with 0.6 nM VEGF (167% ± 9.3%, P < 0.001). 

Effect of E₂ on VEGFR2 and VEGF Gene Expression in BRECs

Because E₂ was shown to enhance VEGF-induced DNA synthesis in BRECs, using northern blot analysis we next investigated whether it regulates the expression of the VEGF receptor or VEGF. As shown in Figures 3 A and 3B, 10 nM of E₂ significantly increased VEGFR2 mRNA levels from 6 hours (2.2 ± 0.3 times, P < 0.05) through 24 hours (2.1 ± 0.4 times, P < 0.05), with a maximal increase at 9 hours (2.4 ± 0.3 times, P < 0.05) after treatment. As for VEGF, a large increase in its mRNA was observed at 24 hours (6.7 ± 0.6 times, P < 0.05) after E₂ treatment (Figs. 3C 3D) . 

We then studied the effect of different doses of E₂ on the increase in mRNA expressions at the time of maximal response. VEGFR2 expression was evaluated at 9 hours, and VEGF mRNA expression was determined at 24 hours after E₂ treatment. As shown in Figures 4 A and 4B, the expression of VEGFR2 mRNA was maximally increased with 10 nM E₂ (2.3 ± 0.5 times, P < 0.05). As for VEGF mRNA expression, a significant increase was observed with concentrations between 0.1 and 100 nM of E₂ (0.1 nM, 6.3 ± 0.06 times; 10 nM, 6.9 ± 1.8 times; 100 nM, 5.3 ± 1.4 times, P < 0.01), and peak response was observed with 1 nM (15 ± 1.9 times, P < 0.001) E₂ (Figs. 4C 4D) . 

E₂ Induction of VEGFR2 and VEGF Protein Production

To determine whether E₂ regulation of VEGFR2 and VEGF mRNA in BRECs is well correlated with their protein levels, VEGFR2 and VEGF protein expressions were assessed by immunoprecipitation using anti-VEGFR2 and anti-VEGF antibodies. As shown in Figures 5 A and 5B, incubation with 10 nM E₂ increased protein levels of VEGFR2 (∼205 kDa) after 13 hours and that of VEGF (∼46 kDa) after 28 hours. 

Estrogen Receptor Expression in BRECs

To confirm that an estrogen receptor is actually expressed in cultured BRECs, immunocytochemical staining was performed using mouse monoclonal anti-estrogen receptor antibody. Immunopositivity for the estrogen receptor was observed diffusely in the cytoplasm of BRECs and was especially prominent in the perinuclear region (Fig. 6 A). Estrogen receptor protein expression was also assessed by western blot analysis using anti-human estrogen receptor antibody. As shown in Figure 6B 6a single band was observed at ∼67 kDa, which is compatible with the classic estrogen receptor protein weight. 

Discussion

In the present study, we first investigated the possible role of estrogen in the angiogenic response of retinal endothelial cells, especially in direct relation to the presence of VEGF. We studied DNA content in the endothelial cells to estimate BREC proliferation. ¹⁶ The enhancement of DNA synthesis in BRECs by E₂ suggests that E₂ may promote proliferation of BRECs. Furthermore, this E₂-induced increase in DNA synthesis was inhibited completely by pretreatment with tamoxifen. Tamoxifen has been shown to influence cell behavior differently according to the concentration used; it inhibits protein kinase C at concentrations greater than 100 μM ¹⁷ and inhibits lung cancer and human melanoma cell growth by a non-estrogen receptor–specific mechanism at concentrations of 1 to 10 μM ^{18 19} ; at concentrations below 1 μM, tamoxifen works as an estrogen receptor antagonist. ^{20 21} The concentration used in our experiments (100 nM of tamoxifen) is thus low enough to act as an estrogen receptor antagonist, and our results support the hypothesis that the estrogen-dependent increase in DNA synthesis in BRECs is an estrogen receptor–mediated event. We also confirmed the presence of estrogen receptors in cultured retinal endothelium by both immunocytochemistry and western blot analysis. In our experiments, peak response in DNA synthesis in BRECs was achieved with between 1 and 10 nM of E₂. The physiological level of plasma estrogen ranges from picomolar to less than 100 nanomolar concentrations, the latter observed during pregnancy. ²² These findings suggest that estrogen receptors may be present in retinal vascular endothelium and that estrogen may actually promote receptor-mediated endothelial proliferation in the eye in vivo, especially during pregnancy when plasma estrogen levels reach nanomolar ranges. In our DNA content assay, we found that DNA synthesis decreased with E₂ at concentrations of 100 nM or more. A similar decline in proliferative response was observed in a number of reports. ^{23 24 25 26} This may suggest that E₂ may promote cell proliferation at a limited range of its concentration in vivo as well. 

In angiogenesis, a variety of growth factors are released that affect one another, and estrogen itself has been reported to have some features that could contribute to angiogenesis; in breast carcinoma cell lines, estrogen makes cells more responsive to the proliferative effect of insulinlike growth factors, basic fibroblast growth factor (b-FGF), transforming growth factor-α, and epidermal growth factor ²⁷ ; estrogen may also facilitate the release of angiogenic factors such as b-FGF by increasing 27-kDa heat shock protein ²⁸ ; and in the uterus, estrogen is reported to increase VEGF expression, which may accompany angiogenesis during menstruation. ^{7 29 30} These indicate that factors other than VEGF could also contribute to estrogen-induced endothelial proliferation; however, considering the importance of VEGF in ocular diseases such as proliferative diabetic retinopathy, ^{12 13} we concentrated on the effect of estrogen in relation to VEGF. As presented in Figure 2B , 10 nM of E₂ not only augmented BREC proliferation at each concentration of VEGF that we used but it also increased the sensitivity of BRECs to VEGF. These results clearly show the effect of E₂ on endothelial cells in direct relation to VEGF. 

We next studied how estrogen enhanced the effect of VEGF. VEGF has two functional high affinity tyrosine kinase receptors, fms-like tyrosine kinase (Flt, VEGFR1) and a kinase insert domain–containing receptor (KDR, VEGFR2). ^{31 32} In vitro studies have shown that VEGFR1 is expressed in both endothelial cells and pericytes, whereas VEGFR2 is expressed in microvascular endothelial cells. ^{33 34} In diabetic retinopathy, pericytes disappear during the early stages, ³⁵ and it has been suggested that VEGFR2 plays a major role in VEGF-dependent angiogenesis. ³⁶ In our experiments, a significant increase in VEGFR2 in BRECs was observed 6 hours after treatment with E₂. This prompt response suggests that E₂ may directly induce VEGFR2 mRNA. To our knowledge, this is the first quantitative report that VEGFR2 mRNA is increased by E₂ in vitro. This could be supported by a previous report that estrogen enhanced the immunohistochemical expression of VEGFR2 in the vascular endothelial cells of the rat pituitary. ³⁷ This may partly explain why E₂ continues to augment cell proliferation even with maximal doses of VEGF. VEGFR2 was increased significantly with E₂ at a concentration of 10 nM, which is compatible with the findings that E₂ enhancement of BREC proliferation in the presence of VEGF was most marked at this concentration. Additionally, we also studied the effect of estrogen on VEGF expression in BRECs, because the upregulation of VEGF mRNAs by estrogen has been shown by several investigators. ^{7 29} The increase in VEGF mRNAs in BRECs by estrogen may suggest that the effect of VEGF could also be enhanced by an autocrine manner. The expression of VEGF in endothelial cells, however, has never been documented in vivo, and the significance of the upregulation of the gene in BREC is still to be investigated. 

Considering that estrogen is constitutively present in the circulatory system in a rather wide range of concentrations, varying according to the time during the menstruation cycle or to the months of pregnancy, it will not be easy to implicate it in a specific pathologic event. Yet the consistency in the proliferative response of BRECs and the increased expression of VEGFR2 by E₂ at 10 nM indicate that the physiological level of estrogen such as that observed during pregnancy could enhance VEGF-induced proliferation of endothelial cells partly by increasing expression of VEGFR2. This may imply that during pregnancy VEGF may be effectively used for vascular construction in the fetus but could worsen angiogenic ocular diseases such as diabetic retinopathy. In conclusion, further understanding of the roles of estrogen in neovascularization at the cellular and molecular levels may contribute to our therapeutic and preventive strategies to ocular neovascularization. 

 Reprint requests: Michiko Mandai, Department of Ophthalmology and Visual Sciences, Kyoto University Graduate School of Medicine, Kyoto 606, Japan.

 Submitted for publication November 2, 1998; revised March 12, 1999; accepted April 21, 1999.

 Proprietary interest category: N.

 

Figure 1.

 

Stimulation of bovine retinal endothelial cell (BREC) growth by E₂. Estrogen was added to sparsely plated BRECs, and cell number or DNA content was measured after 4 days. Pretreatment with tamoxifen was also conducted to challenge the maximal response to E₂. Bars represent mean ± SD from five separate experiments performed in triplicate. *P < 0.05. T, tamoxifen.

Figure 2.

 

Combined stimulation of BREC growth by E₂ and VEGF. (A) Augmentative effect of E₂ on VEGF-induced retinal endothelial cell growth. Recombinant human VEGF (0.6 nM) was added to sparsely plated BRECs and cultured with various concentrations of E₂. DNA content was measured after 4 days. Bars represent mean ± SD from five separate experiments performed in triplicate. *P < 0.05. V, VEGF. (B) Comparison of BREC growth stimulated by VEGF and E₂ with that stimulated by VEGF alone. Endothelial cells were plated and cultured either in 10 nM E₂-replete or E₂-depleted medium with various concentrations of recombinant human VEGF. Results are displayed as mean ± SD from five separate experiments in triplicate. *P < 0.05. **P < 0.01. ***P < 0.001.

Figure 3.

 

The time course of the effect of E₂ treatment on VEGFR2 and VEGF mRNA expression in BRECs. Subconfluent BRECs were cultured with 10 nM E₂. At each time point, cells were collected and 20-μl RNA extracts were analyzed by northern blot analysis with VEGFR2 mRNA probe. (A) Results from representative northern blot analysis are shown. (B) Densitometric analysis of the northern blot analysis data. *P < 0.05. Subconfluent BRECs were cultured with 10 nM E₂. At each time point, cells were collected, and 20 μl RNA extracts were analyzed by Northern blot analysis with VEGF mRNA probe. (C) Results from representative northern blot analysis are shown. (D) Densitometric analysis of the northern blot analysis data. *P < 0.05.

Figure 4.

 

The effect of different doses of E₂ treatment on VEGFR2 (A, B) and VEGF (C, D) mRNA expression in BRECs. Subconfluent BRECs were cultured with various concentrations of E₂ for 9 hours. Cells were collected, and 20 μl RNA extracts were analyzed by Northern blot analysis with VEGFR2 mRNA probe. (A) Results from representative northern blot analysis are shown. (B) Densitometric analysis of the northern blot analysis data. *P < 0.05. (C, D) The effect of different doses of E₂ treatment on VEGF mRNA expression in BRECs. Subconfluent BRECs were cultured with various concentrations of E₂ for 24 hours. Cells were collected, and 20 μl RNA extracts were analyzed by Northern blot analysis with VEGF mRNA probe. (C) Results from representative northern blot analysis are shown. (D) Densitometric analysis of the northern blot analysis data.** P < 0.01; ***P < 0.001.

Figure 5.

 

The effect of E₂ treatment on and VEGFR2 (A) and VEGF (B) protein expression in BRECs. Subconfluent BRECs were cultured with E₂-depleted medium or 10-nM E₂–replete medium. Cells were collected after 9 hours for VEGFR2 protein detection and after 24 hours for VEGF protein detection. Protein extracts were reacted with anti-VEGFR2 antibody or anti-VEGF antibody and immunoprecipitated with protein A–Sepharose beads. Each sample was electrophoresed and analyzed using a densitometer.

Figure 6.

 

The expression of estrogen receptor in retinal endothelial cells. (A) Immunolocalization of estrogen receptor protein in BRECs. Significant positive staining was obtained with anti–estrogen receptor antibody (1:50). (B) Western blot analysis of estrogen receptor protein expression in BRECs. Cell extracts (30 μg protein) were subjected to SDS electrophoresis and blotted to nitrocellulose membranes. Membranes were incubated with estrogen receptor antibody solution.

Financial support for this work was provided by Grant-in-Aid for Scientific Research A2-10307042 and C-09671793 from the Ministry of Education, Science, and Culture of the Japanese Government.