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Advances in Brief |
Edwin L. Steele Laboratory, Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114
| ABSTRACT |
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Experimental Design: ZR75-1, a human estrogen-dependent mammary carcinoma, was implanted into the mammary fat pad (primary site) of ovariectomized SCID female mice carrying estrogen pellets. The same tumor line was also grown in the cranial window (metastasis site). When tumors reached the diameter of 2.5 mm, angiogenesis, hemodynamics, and vascular permeability were measured by intravital microscopy, and expression of angiogenic growth factors was determined by quantitative reverse transcription-PCR.
Results: ZR75-1 tumors grown in the mammary fat pad had higher microvascular permeability but lower vascular density than the same tumors grown in the cranial window (2.5- and 0.7-fold, respectively). There was no significant difference in RBC velocity, vessel diameter, blood flow rate, and shear rate between two sites. The levels of vascular endothelial growth factor (VEGF), its receptors VEGFR1 and VEGFR2, and angiopoietin-1 mRNA tended to be higher in the mammary fat pad tumors than in the cranial tumors (1.5-, 1.5-, 3-, and 2-fold, respectively).
Conclusions: The primary breast cancer exhibited higher vascular permeability, but the cranial tumor showed more angiogenesis, suggesting that the cranial environment is leakage resistant but proangiogenic. Collectively, host microenvironment is an important determinant of tumor gene expression and microvascular functions, and, thus, orthotopic breast tumor models should be useful for obtaining clinically relevant information.
| Introduction |
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| Materials and Methods |
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25 g, were used following institutional guidelines. They were anesthetized by 90 mg of ketamine hydrochloride and 9 mg of xylazine/kg s.c. Cells (3 x 106; final volume of 30 µl in PBS) were injected using a 30-gauge needle under a dissecting microscope into the mammary fat pad just inferior to the nipple of ovariectomized female SCID mice, while avoiding leakage to s.c. space. Mice were ovariectomized and implanted with control release pellets containing 0.75 mg of estrogen for 60-day release (Innovative Research, Sarasota, FL) 1 week before tumor implantation; this allows enough time to recover from estrogen depletion-induced changes in hemodynamics (13)
and avoids the effect of endogenous estrogen, which may vary between animals. For intravital microscopy (under the same anesthetics), a midline incision was made through skin and fascia, and a flap was gently elevated by blunt dissection, not disrupting the vasculature and avoiding irritation of the tumor vessels. The flap with mammary tumor was then placed on a specially designed stage developed originally for the liver preparation (3)
, and a glass coverslip was placed over the tumor to allow intravital microscopy and analysis of microvascular parameters (Fig. 1)
2.5 mm (
4 weeks after the tumor implantation). Tumor volume doubling times during the exponential growth phase were
5 and
8 days for cranial window and mammary fat pad models, respectively.
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Microvascular Permeability Measurement.
Mice were injected with a bolus (100 µl) of 1% tetramethylrhodamine-labeled BSA (Molecular Probes, Eugene, OR) in saline via the tail vein. Fluorescence intensity of the tumor tissue was measured every 2 min for a total of 20 min by a photomultiplier (9203B; EMI, Rockaway, NJ) in a well-perfused area using a x20 objective lens. The microvascular permeability to albumin was then calculated as described previously (17)
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Preparation of RNA.
Tumors were excised, placed on 1.5-ml microcentrifuge tubes, immediately frozen in liquid nitrogen, and stored at -70°C until further analysis. Total RNAs were isolated from each tumor using RNAzol B (Tel-Test, Inc., Friendswood, TX). Slight modifications were introduced to the protocol recommended by the manufacturer to eliminate sample contamination during the homogenization step. Mainly, tumors were turned into a powder with sterile ceramic mortar and pestles cooled with liquid nitrogen and then homogenized to completion in RNAzol B with sterile disposable mini-pellet pestles (Kontes Pellet Pestle) attached to a Cordless Motor (Fisher Scientific, Pittsburgh, PA). The homogenization was done at low temperatures by keeping the microcentrifuge tubes containing the tumor on a metal block precooled for 30 min on dry ice. For complete removal of DNA contamination, each RNA preparation was treated subsequently with DNase I using the Message Clean system (GenHunter Corp., Nashville, TN). RNAs obtained by this procedure were typically of high quality, as assessed by A260/A280 ratios and by size analysis by denaturing gel electrophoresis using the RNA Millennium marker (Ambion, Inc., Austin, TX). RNAs were stored in aliquots at -70°C at concentrations > 1 µg/ml in diethyl pyrocarbonate-treated water.
Reverse Transcription.
RNAs were reverse transcribed into cDNA using the conditions specified in the TaqMan Reverse Transcription Reagents system (PE Applied Biosystems, Foster City, CA). Typically, 1 µg of total RNA was reverse transcribed with oligo d(T)16 primers in a final reaction volume of 100 µl.
Quantitative PCR for Angiogenic Growth Factor Expression.
Primers followed the requirements specified in the manual for SYBR Green Real Time PCR (PE Applied Biosystems) and were designed on the basis of the known sequences and exon-intron structures (when available) of the mouse and human genes reported in the GenBank and with the help of Primer 3 [Whitehead Institute for Biomedical Research, Steve Rozen, Helen J. Skaletsky (1996, 1997) obtainable as Freeware].8
Primer sequences were as follows: VEGF-A (FP: 5'-CCT TGC TGC TCT ACC TCC AC-3'; RP: 5'-CAC ACA GGA TGG CTT GAA GA-3'), VEGFR1 (FP: 5'-AAA AAT GGC CAC CAC TCA AG-3'; RP: 5'-GGA GAT CCG AGA GAA AAT GG-3'), VEGFR2 (FP: 5'-GGA GAA GAA TGT GGT TAA GAT CTG TGA-3'; RP: 5'-ACA CAT CGC TCT GAA TTG TGT ATA CTC-3'), Ang-1 (FP: 5'-AGG CTT GGT TTC TCG TCA GA-3'; RP: 5'-TCT GCA CAG TCT CGA AAT GG-3'), HPRT (normalizer for RNA content; FP: 5'- TTA CCA GTG TCA ATT ATA TCT TCA ACA ATC-3'). Primers were synthesized by Integrated DNA Technologies, Inc. (Coralville, IA), and the optimal conditions for their use were determined according to the Taq-Man PCR Reagents protocol. Nontumor human and mouse total RNAs used as positive controls (lung, heart, ovary, liver, and brain) were obtained from Ambion, Inc. RNA samples that had not been reverse transcribed (minus-reverse transcription control) were included in the PCR reactions as negative controls. The size of the PCR products, as well as the presence of a single band, was assessed by electrophoresis of the RT-PCR reactions on NuSieve 3:1 agarose gels (FMC Bioproducts, Rockland, ME) and comparison with several DNA ladders (PCR Marker 502000 bp; Parfect DNA 100-bp ladder, Novagen, Madison, WI). Quantification of each cDNA was done relatively to calibrator samples (mouse heart), after the exact instructions from the manufacturer (PE Applied Biosystems).
Statistical Analysis.
Data were expressed as mean ± SD. The difference between mammary tumors and cranial tumors was analyzed by Mann-Whitney U test. P < 0.05 was considered to be statistically significant.
| Results |
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Vascular Permeability in Breast Cancer.
Vascular hyperpermeability is a hallmark of tumor vessels. ZR75-1 tumors grown in the mammary fat pad had a significantly higher vascular permeability than tumors grown in the cranial window (3.2 ± 1.3 x 10-7 cm/s; 1.3 ± 0.5 x 10-7 cm/s; P = 0.04; Fig. 2
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Angiogenic Factor Expression in Breast Cancer.
To gain mechanistic insight into the vascular phenotype of ZR75-1 tumors grown in different sites, we determined angiogenic factor/receptor expression by quantitative RT-PCR. VEGFR2 (Flk-1) and Ang-1 mRNA expression in the mammary fat pad tumors were statistically significantly higher than those in the cranial tumors (Table 3)
. VEGF and VEGFR1 (flt-1) also tended to be higher in the mammary tumors. Ang-2 expression was not detectable in either site in the conditions used.
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| Discussion |
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Using these two breast cancer models, we found that breast cancer grown in different sites has different angiogenic gene expression and microvascular characteristics. VEGF is one of the most potent angiogenic and vascular permeability factors. It is expressed in most human malignant tumors. VEGF has been shown to increase angiogenesis, tumor growth, and experimental metastasis in breast cancer (27) . Angiopoietins play important roles in a later phase of developmental angiogenesis, such as remodeling and maturation of vessel network. Ang-1 induces angiogenesis and reduces vascular permeability in the skin vessels (28) . Receptors to VEGF and Angs, such as Flt-1/Flk-1 and Tie-2, respectively, are predominantly expressed on vascular endothelial cells. VEGF is known to induce its receptors expression. Our quantitative RT-PCR data showed that both VEGF and Ang-1, as well as VEGF receptors, tend to be expressed at a higher level in tumors grown in the mammary fat pad (primary site) than in the cranial window (metastasis site). Increased expression of VEGF, VEGF receptors, Flt-1, and Flk-1 has also been reported in breast cancer tissue from patients (29) . However, vascular density in the mammary fat pad tumors is lower than in the cranial tumors. This is counterintuitive and suggests that we should be cautious about predicting functional outcome of different organ sites simply from the concentration of putative growth factors. We have shown previously that angiogenesis in collagen gels containing known concentrations of VEGF or basic fibroblast growth factor is significantly faster in the cranial window than in the dorsal skin chamber (2) . Thus, higher vessel density and blood flow rate underlying pial tissue may potentiate the angiogenic response to tumor factors. In addition, the response of host vascular endothelial cells to the stimuli, or the recruitment of circulating endothelial cells, may be host organ dependent. Finally, other angiogenic or antiangiogenic factors may also be involved. Tissue level of mRNA should be interpreted carefully as well. Protein level expression and/or localization is more relevant to the functional outcome.
Contrary to the vascular density (angiogenesis), vascular permeability was significantly higher in tumors grown in the mammary fat pad than those grown in the cranial window. Higher VEGFR2 expression in the mammary tumors may explain, in part, the increased permeability. However, Ang-1 was also higher in the mammary tumors, but it was 40 times lower than VEGF. Although Ang-1 is known to induce resistance to leakiness in the skin vessels, this may not be the case for the tumor vessels. Blood vessels in the CNS are known to have tight junctions and form blood brain barrier. Tumors in the brain also show similar selectivity in vascular permeability (30) . We reported previously that vessels of tumors grown in cranial windows are less permeable to albumin and exhibit smaller pore size compared with tumors grown in s.c. space (4, 5, 6 , 14) . Furthermore, vascular permeability change in response to VEGF is much lower in pial vessels compared with that in s.c. vessels (6) . Collectively, vessels in the cranial environment or those originating from endothelial cells with tight junctions exhibit less leakiness to macromolecules.
To summarize, vascular permeability in the primary breast cancer was higher than that in the brain metastasis. On the other hand, cranial tumors showed more angiogenesis. These data suggest that cranial environment is leakage resistant but proangiogenic. Collectively, the present study underscores the importance of the host microenvironment in gene expression and physiological functions and suggests that the orthotopic breast tumor models be used to obtain clinically relevant information.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported by the U.S. Army Medical Research and Material Command under DAMD 17-98-1-8088 and Program Project Grant PO1-CA80124 from the National Cancer Institute (to D. F. and R. K. J.). ![]()
2 Present address: Department of Radiobiological Sciences, Beth Israel-Deaconess Medical Center, Boston, MA 02115. ![]()
3 Present address: Center for Molecular Medicine, Maine Medical Center Research Institute, Scarborough, ME 04174. ![]()
4 Present address: Department of Internal Medicine II, National Defense Medical College, Saitama 359-8513, Japan. ![]()
5 Present address: Department of Surgery, Tsukuba Gakuen Hospital, Ibaraki 305-8558, Japan. ![]()
6 To whom request for reprints should be addressed, at Department of Radiation Oncology, Massachusetts General Hospital, 100 Blossom Street, Cox-736, Boston MA 02114. Phone: (617) 726-8143; Fax: (617) 724-5841; E-mail: dai{at}steele.mgh.harvard.edu ![]()
7 The abbreviations used are: VEGF, vascular endothelial growth factor; RT-PCR, reverse transcription-PCR; CNS, central nervous system; Ang-1, angiopoietin-1. ![]()
8 Internet address: http://www-genome.wi.mit.edu/cgi-bin/primer/primer3.cgi. ![]()
Received 10/ 5/01; revised 12/ 5/01; accepted 12/ 5/01.
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