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Advances in Brief |
Department of Biological Regulation, Weizmann Institute of Science, Rehovot 76100 Israel
| ABSTRACT |
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| Introduction |
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Being endocrine-dependent carcinoma, the development of breast cancer necessitates hormones, in particular, estrogen and progesterone (5 , 6) . Estrogen mediates its activities through nuclear receptors, so-called ERs, which activate transcription and, hence, up-regulate expression of genes that are important for cell growth (7) . Multiple lines of evidence indicate that estrogen regulates angiogenesis in the female reproductive system and in breast cancer, although the mechanism of the regulation has not been defined (8) . Moreover, the association between VEGF expression and estrogen was studied extensively yielding, however, controversial results, e.g., the expression of VEGF in MCF7 human breast cancer cells was shown to increase on incubation with estrogen, as well as with tamoxifen (9) .
The essential functions of angiogenesis and estrogen in the progression of breast cancer, in addition to the widespread pharmacological use of estrogenic and antiestrogenic drugs, bring up the need for acquiring a comprehensive knowledge regarding hormonal regulation of angiogenesis. We have thus investigated VEGF expression in MCF7 breast cancer during hormonal treatments with estrogen and tamoxifen, in vivo, ex vivo, and in vitro. Parametric maps of vascular permeability were obtained from model-based analysis of contrast-enhanced 1H MRI. The results indicate that tamoxifen exerts its antitumor effects through affecting VEGF expression and vascular permeability in an opposite manner to their regulation by estrogen. In addition to characterizing the hormonal regulation of angiogenesis in breast cancer, this study presents a noninvasive methodology to assess VEGF expression via determining its effect on vascular permeability. This methodology can be further extended to monitor the efficacy of therapy, including hormonal treatments.
| Materials and Methods |
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6 weeks old) after s.c. inoculation of MCF7 cells (
8 x 106 cells/0.5 ml PBS) into a mammary gland. Tumors were allowed to develop to
1 cm3 (38 weeks). Tamoxifen treatment was applied by replacing the estrogen pellet with a tamoxifen one (5 mg/pellet, 21-day release time; Innovative Research of America). For the MRI measurements, mice were anesthetized by exposure to 1% isoflurane in 3:7 O2/N2O mixture (Medeva Pharmaceuticals, Rochester, NY). Approval for all animal procedures was obtained according to the guidelines of the Committee on Animals of the Weizmann Institute of Science.
Contrast-enhanced 1H MRI and Image Analysis.
1H-MR images were recorded with a 4.7 T Biospec spectrometer (Bruker Analytik, Karlsruhe, Germany). The dynamic T1-weighted spin-echo experiment applied on a central slice of each tumor began with acquisition of a precontrast image, followed by a bolus injection of GdDTPA-dimeglumine (0.4 mmol/kg weight and 100 µl of total volume) and then acquisition of sequential images for
40 min. Acquisition parameters were: echo time/repetition time = 15/250 ms, two averages, spatial resolution = 0.19 x 0.39 mm2, slice thickness = 1 mm, and time resolution = 1.1 min. The dynamic experiments were applied twice on each animal (n = 3), firstly under estrogen before initiating tamoxifen treatment and secondly after 2 weeks of tamoxifen treatment. The dynamic images were analyzed using model-based algorithms, as was described previously (11
, 12)
. The analysis produced parametric images of rate constant of permeability to GdDTPA (kPS, min-1) and a correlation coefficient, which reflects the quality of the fit to the model (R2).
Immunohistochemistry.
Mice, treated with estrogen (control, n = 3) or treated for 2 weeks with tamoxifen (n = 3), were euthanized, and the left atrium was perfused with PBS and 2.5% formaldehyde (50 ml each). Tumors were then removed, fixed, and maintained in paraffin blocks. Microsections from each tumor (5 µm) were stained with H&E and immunostained for VEGF. Slides for immunostaining were incubated in peroxide solution (300 µl of H2O2, 100 µl of concentrated HCl, 5 ml of methanol, and 5 ml of PBS; 15 min). Antigen retrieval was achieved in steaming Tris buffer [0.05 M (pH 10); 10 min]. The primary antibody, rabbit antihuman polyclonal against VEGF (BioGenex, San Ramon, CA) diluted 1:10 in fresh blocking solution (20% nonimmune goat serum, 0.5% Triton, PBS; 1 h), was added for overnight incubation at 4°C and visualized by avidin-biotin-peroxidase kit (Zymed Laboratories, South San Francisco, CA), after nuclei counterstaining with hematoxilin. Immunostaining procedure was identical for both estrogen and tamoxifen treatments. The fraction of VEGF staining (brown color) was determined in 10 fields (1002 µm2) within viable tissue of one microsection per tumor, by dividing the area stained brown color by the total field area, using Image-Pro Plus software (Media Cybernetics, Silver Spring, MD).
RT-PCR.
RNA was extracted using TriReagent (MRC, Cincinnati, OH). Semiquantitative RT-PCR reactions were performed with Ready-To-Go RT-PCR beads (Pharmacia Biotech, Piscataway, NJ) with the following conditions: (a) 42°C, 30 min; (b) 94°C, 5 min; (c) 28 cycles of 94°C, 1 min; 56°C, 1 min; 72°C, 1 min; and (d) 72°C, 7 min. VEGF121 and VEGF165 were amplified with specific primers A (sense; 5'-GCCTTGCTGCTCTACCTC-3') and B (antisense; 5'-GCTTGTCACATTTTTCTTGTC-3'), or A and C (antisense; 5'-CCACA GGGATTTTCTTGTCT-3'), respectively. S16 rRNA served as an internal control that is not regulated by hormones and was amplified with primers D (sense; 5'-TCCAA GGGTCCGCTGCAGT-3') and E (antisense; 5'-TCACGATGGGCTTATCGGTA-3'). The conditions for a linear assay were determined in preliminary experiments (data not shown). Each experiment included two negative control samples: (a) lacking RNA; and (b) with inactivated reverse transcription. Products were separated on 2% agarose gels and visualized by ethidium bromide staining after densitometry with Quantity One (Bio-Rad Laboratories, Hercules, CA). cDNA products obtained from the different bands were sequenced to verify their identity.
Statistical Analysis.
Data from multiple experiments or groups were expressed as mean ± SE. The Ps were calculated by unpaired two-tailed Students t test (unless otherwise stated). A P < 0.05 was regarded as statistically significant.
| Results |
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0.6. The spatial distribution of kPS was found highest at peripheral areas of the tumors and decayed gradually toward the center of the tumors (Fig. 1)
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| Discussion |
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To our knowledge, this is the first time that estrogen is shown to regulate angiogenesis at the molecular level in coherence with the physiological level, by suppressing excessive expression of VEGF and, thus, of vascular permeability. Therefore, we suggest the following hypothesis in ER-positive breast cancer: estrogen maintains VEGF expression at moderate levels that enable angiogenesis, the formation of functional microcapillaries, and consequently tumor growth. Accordingly, tamoxifen treatment removes the estrogen regulation leading to increased VEGF expression in parallel to an extreme elevation in vascular permeability. Increased permeability to extreme values interferes with vessel functionality by reducing its perfusion capacity and is therefore destructive to the tumors. The fact that tamoxifen carried out the reverse effect of estrogen also suggests that the estrogenic effect was exerted via the ER and that the therapeutic effect of tamoxifen is in line with its specific antiestrogenic activity, in ER-positive breast cancer (14) .
The gradual decrease observed in VEGF mRNA levels during incubation with estrogen (Fig. 3)
, as well as the significant increase of VEGF protein expression during tamoxifen treatment (Fig. 2)
, strongly imply that estrogen does not regulate VEGF expression via conventional ER-induced transcription of this growth factor. This hypothesis is supported by recent findings that of the two estrogen-responsive element sequences found in the 3'- and 5'-untranslated regions of the rat VEGF gene, the latter sequence is unusual as it mediates transcriptional activation in one orientation and a significant repression in the opposite orientation (15)
. Moreover, overexpression of ER
in tumors initiated from Ishikawa human endometrial cancer cells was found to inhibit the expression of VEGF and
vß3 integrin, as well as to suppress the degree of vascularization and, hence, to limit tumor growth (16)
. These results indicated that high levels of ER
might be beneficial in controlling female cancer because of its inhibitory effect on angiogenic pathways.
Finally, the positive correlation that was established herein between VEGF expression ex vivo and vascular permeability in vivo enables to use the latter noninvasive approach for monitoring angiogenesis during anticancer therapy, particularly targeted anti-VEGF and antivascular therapies. Extension to this methodology is currently being clinically evaluated for the characterization of breast lesion according to estimation of vascular permeability to GdDTPA (17) . Assessment of vascular permeability can be also used to determine malignant potential that was shown to correlate with VEGF content (18) .
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported by NIH Grant CA42238 and the Susan G. Komen Breast Cancer Foundation. Hadassa Degani is the incumbent of the Fred and Andrea Fallek Professorial Chair for Breast Cancer Research and heads the Willner Family Center for Vascular Biology. ![]()
2 To whom requests for reprints should be addressed, at Department of Biological Regulations Weizmann Institute of Science Rehovot, 76100 Israel. Phone: 972-8-9343920; Fax: 972-8-9344186; E-mail: hadassa.degani{at}weizmann.ac.il ![]()
3 The abbreviations used are: VEGF, vascular endothelial growth factor; ER, estrogen receptor; MRI, magnetic resonance imaging; GdDTPA, gadolinium-diethylenetriamine pentaacetate; RT-PCR, reverse transcription-PCR. ![]()
Received 2/ 1/02. Accepted 2/14/02.
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