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Departments of Pathology (I.F.-L., A.B.) and Urology and Andrology (S.H., J.-E.D.), Umeå University, 901 87 Umeå, Sweden
Address all correspondence and requests for reprints to: Anders Bergh, Department of Pathology, Umeå University, 901 87 Umeå, Sweden. E-mail: anders.bergh{at}pathol.umu.se
| Abstract |
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| Introduction |
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Previous studies have suggested that castration reduces endothelial cell numbers and the endothelial cell proliferation rate in the adult rat ventral prostate, and that they are normalized by testosterone treatment (8). Similarly, castration decreases and testosterone treatment rapidly normalizes blood flow to the adult rat ventral prostate (7). These studies indicate that the vasculature could be regulated, directly or indirectly, by androgens. If this is the case, castration-induced involution and testosterone-stimulated regrowth could be used to study factors regulating angiogenesis, vascular growth, and local blood flow in the normal prostate, and to examine the role of vascular factors in prostate physiology and pathology. The aim of this study therefore was to examine, in detail, how the prostate vasculature responds to castration and testosterone treatment. An understanding of this may expand our knowledge of factors regulating growth in the normal prostate and in prostate tumors.
| Materials and Methods |
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Stereology
Four-micrometer thick sections of the whole ventral prostate
lobe were stained with hematoxylin and eosin, with toluidine blue (to
stain mast cells), and by immunohistochemistry (as described below),
and examined in a light microscope equipped with a square lattice (121
points) in the eyepiece. Using point counting morphometry as described
by Weibel (10), i.e. counting the number of grid
intersections (hits) falling on the measured tissue compartment and
reference space, the volume density (percentage of tissue volume
occupied by the defined tissue compartment) of various parts of the
prostate was determined. In randomly chosen areas, the following
measurements were performed. First, the volume density of stroma,
glandular lumen, and glandular epithelium was obtained by counting the
number of hits falling on each of these tissue compartments,
respectively, at x100 magnification. Second, the volume density of
blood vessel lumina, endothelial cells, blood vessel walls (endothelium
+ pericyte + muscular coat), mast cells, and immunostained macrophages
in the stroma was measured at x400 magnification by counting hits
falling on the respective tissue component and on stroma.
The total weight (= volume) of the different stroma components per ventral prostate was determined by total lobe weight x volume density of stroma x volume density of the respective stroma component. In these calculations, we assume that the specific gravity of prostate tissue is 1.0 (8), and that changes in tissue volume during fixation and tissue processing influence all groups in the same way.
Immunohistochemistry
Bromodeoxyuridine. Four-micrometer thick paraffin sections
were immunostained with a monoclonal antibody against BrdU (Dako,
Älvsjö, Sweden) using biotinylated goat antimouse IgG and a
peroxidase-labeled ABC reagent (Vector Labs., Burlingame, CA). The
percentage of BrdU-labeled epithelial and endothelial cells were
measured in each ventral prostate; 800-1000 cells of each type were
examined in each organ.
Macrophages. Four-micrometer thick sections were deparaffinated, rehydrated, heated in a microwave oven (600 W for 2 x 5 min in citrate buffer 0.01 M, pH 6) (11), and immunostained with a monoclonal antibody against macrophages (ED-1; Serotec Ltd., Oxford, UK). The ED-1 antibody recognizes tissue macrophages and monocytes and can be used on paraffin- embedded tissue (12).
Statistical analyses
Values are presented as means ± SEM of 510
observations. Groups were compared using the Mann-Whitney U
test. A P value <0.05 was considered significant.
| Results |
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| Discussion |
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Endothelial cell proliferation and regrowth of the vasculature are stimulated by testosterone in the regressed ventral prostate. The effect is rapid and occurs quicker than that of the glandular epithelium. Vascular weights are normalized after 2 days, but epithelial cell, stromal, and organ weights are not normalized until several days later (8). Endothelial cell proliferation starts earlier than the proliferation in the epithelium. In addition, testosterone apparently induces a rapid vasodilatation, because the total weight of blood vessel lumina (present study) and blood flow (7) are normalized after only 1 day of testosterone substitution. The rapid normalization of blood flow and vascular tissue weights after 12 days, respectively, can probably not be explained only by proliferation of vascular cells (peaking after 24 days). Our data therefore suggests that testosterone treatment induces, with different kinetics, vasodilators, factors increasing the size of vascular wall cells, and endothelial cell mitogens. The increased metabolic demand of the testosterone-stimulated prostate is apparently first met by vasodilatation of existing blood vessels and increased blood flow, and later by proliferation and growth of vascular cells. The observation that growth of the glandular epithelium is preceded by growth in the vasculature suggests that vascular growth could be a prerequisite for organ growth. It remains to be shown whether growth of a prostate tumor is preceded by growth of its vasculature. The observation of an increased vascular density in relation to prostatic intraepithelial neoplasia lesions do, however, suggest that this may be the case (13).
In line with English et al. (8), we observed that 12% of the endothelial cells in the intact ventral prostate are proliferating. This is a rather high figure compared with other normal tissues in which the labeling indices ranges from 0.011% (14). The relatively high endothelial cell proliferation rate may be related to the observation that the potent endothelial cell mitogens vascular endothelial cell growth factor (VEGF) (15), VEGF-B (16), and VEGF-C (17, 18) are all produced locally in the prostate. Endothelial cell proliferation is probably balanced by endothelial cell death. Previous investigators have, however, not detected apoptosis in prostatic blood vessels in normal or castrated animals (19). However, castration-induced death of prostatic endothelial cells is, although difficult to detect, not unlikely, because tissue involution generally is accompanied by endothelial cell death (20, 21).
The present observations suggest that testosterone treatment induces the synthesis of an angiogenic factor (endothelial cell mitogen), or that testosterone up-regulates the expression of endothelial receptors for angiogenic factors, or that testosterone decreases the synthesis of an angiogenesis inhibitor. In line with the first suggestion, we recently showed that castration decreases and testosterone treatment increases VEGF messenger RNA expression in the ventral prostate (S. Häggström, our unpublished observations). In the prostate, VEGF is expressed only in glandular epithelial cells (15) (our unpublished observations). These observations may suggest that the angiogenic effect of testosterone could be mediated via VEGF synthesis in the epithelial cells. Another substance that could be involved is epidermal growth factor (22), which is decreased by castration and stimulated by testosterone (23). Several other factors that stimulate angiogenesis in other tissues (22), such as basic fibroblast growth factor (b-FGF), transforming growth factor-ß1 (TGF-ß1), and hepatocyte growth factor, are produced in the prostate but they are all up-regulated by castration and down-regulated by testosterone treatment (23), suggesting that they are not involved in the testosterone-dependent changes observed in the present study.
Apart from the glandular epithelial cells, other cell types could also be involved in the vascular control of the prostate. Numerous mast cells are observed in the stroma of the rat ventral prostate, and they are often observed close to blood vessels (24). In other tissues, mast cells are involved in angiogenesis (25) and in the control of local blood flow and inflammation (26). In this study, we observe a castration-induced increase and a transient but more pronounced increase in mast cells in the ventral prostate 1 day after testosterone treatment. Mast cells are often observed in close relation to proliferating endothelial cells. It is therefore not unlikely that they could be involved in the initial phase of the testosterone-induced vascular response in the ventral prostate. The mast cell products involved and the mechanisms attracting mast cell to the prostate are at present unknown, but interestingly, VEGF, TGF-ß, and b-FGF (TGF-ß and b-FGF are increased by castration, see above) are all chemotactic for mast cells (27, 28).
Tissue macrophages play an important role for angiogenesis in wound healing and in tumors (29, 30), and inhibition of macrophage influx and tumor necrosis factor synthesis inhibit angiogenesis and growth in rat prostatic tumors (31). Castration increases the number of macrophages in the prostate, but their numbers return to intact values after testosterone treatment. This may suggest that this cell type is a less likely source of the testosterone-induced vascular stimulators than epithelial and/or mast cells. However, because numerous macrophages are present in the prostate at the start of testosterone-induced vascular regrowth, it cannot be excluded that they could play a role in this process.
In summary, the present study suggests that castration-induced involution and testosterone-stimulated regrowth of the vasculature can be used as a model to study the kinetics and origin of the molecular factors that regulate vascular growth, angiogenesis, and local blood flow in the prostate. It is not unlikely that similar factors could be of importance for the growth of androgen-dependent prostate tumors.
| Acknowledgments |
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| Footnotes |
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Received June 23, 1997.
| References |
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