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Original Articles |
Gaubius Laboratory TNO-PG (P.K., K.K., B.M., E.v.S., B.v.d.V., V.W.M.v.H.), 2333 CK Leiden, The Netherlands; Department of Obstetrics, Gynecology, and Reproductive Medicine, Leiden University Medical Center (K.K., B.M., E.v.S., F.M.H.), 2300 CK Leiden, The Netherlands; and Department of Physiology, Institute for Cardiovascular Research, Vrije Universiteit (V.W.M.v.H.), 1081 BT Amsterdam, The Netherlands
Address all correspondence and requests for reprints to: Dr. P. Koolwijk, Gaubius Laboratory TNO-PG, Zernikedreef 9, 2333 CK Leiden, The Netherlands. E-mail: p.koolwijk{at}pg.tno.nl
Abstract
The endometrium is a tissue unique for its cyclic destruction and rapid
regeneration of blood vessels. Angiogenesis, indispensable for the
regeneration process, provides a richly vascularized, receptive
endometrium fundamental for implantation, placentation, and
embryogenesis. Human endometrial microvascular endothelial cells
(hEMVEC) were isolated to better understand the properties and
angiogenic behavior of these cells. Unlike human foreskin microvascular
endothelial cells (hFMVEC), which proliferated better upon stimulation
by basic fibroblast growth factor, hEMVEC were much more sensitive to
vascular endothelial growth factor A (VEGF-A) stimulation, probably due
to enhanced VEGF receptor 2 expression. In addition, hEMVEC displayed
an enhanced expression of the urokinase-type plasminogen activator
(u-PA) compared with hFMVEC. No differences were found in tissue-type
PA, PA inhibitor-1, and u-PA receptor expression. The high expression
of u-PA by hEMVEC was also found in tissue sections. hEMVEC formed
capillary-like structures when cultured in 20% human serum on top of
three-dimensional fibrin matrices, and VEGF-A or basic fibroblast
growth factor increased this tube formation. This is in contrast with
hFMVEC, which formed tubes only after simultaneous stimulation by a
growth factor and tumor necrosis factor-
. The high basal level of
u-PA contributes to and may explain the higher angiogenic properties of
hEMVEC (in vitro).
HUMAN REPRODUCTION depends on the rapid cyclical development of a receptive maternal environment necessary for implantation and placentation. Indispensable to this physiological process is angiogenesis, the forming of new blood vessels. It is required for supporting the proliferation and differentiation of glandular and surface epithelial cells and stromal cells, of which the endometrium is mainly composed (1).
Studies have indicated that there are three successive episodes of physiological angiogenesis in the endometrium during the menstrual cycle (2). The first episode can be seen as postmenstrual repair and occurs during the early proliferative phase, the second episode takes place during the midproliferative phase under the influence of estradiol, and the third occurs during the estradiol- and progesterone-mediated secretory phase, when the coiled arteries grow. Together with the changes in vascular permeability throughout the menstrual cycle, transformation of a thin, dense endometrium into a thick, highly edematous secretory endometrium takes place (3).
Regulation of the outgrowth of new vessels is the result of a delicate balance between stimulators and inhibitors and involves several steps. After stimulation of the endothelial cells by angiogenic factors, the basement membrane is degraded by proteolytic enzymes, in particular matrix-degrading metalloproteinases (MMPs) and enzymes of the plasminogen activator system (4). The cells will then invade, migrate, and proliferate under the influence of angiogenic factors into the underlying interstitial matrix and will form new capillary structures (5, 6). It has been suggested that angiogenesis in the endometrium may occur by a process of elongation and expansion of preexisting vessels (7), a process that differs from the traditional concept of angiogenesis (5, 6).
It is generally assumed that urokinase-type plasminogen activator (u-PA) and its inhibitor, the plasminogen activator inhibitor I (PAI-1), are involved in regulation of the first steps of angiogenesis, i.e. local proteolytic remodeling of matrix proteins and migration of endothelial cells (6, 8, 9). u-PA converts plasminogen into the broadly acting serine protease plasmin, which, in turn, is able to both degrade matrix proteins and activate several MMPs (10, 11, 12).
Studies have shown organ-specific characteristics among microvascular endothelial cells (13). Physiology and pathology involving the endothelium could therefore be best addressed by studies of endothelial cells of the affected organ. As the endometrium is a tissue unique for its cyclic destruction and rapid regeneration of blood vessels, the angiogenic behavior of its endothelial cells is expected to differ from that of endothelial cells in other tissues [e.g. human microvascular foreskin endothelial cells (hFMVEC)]. To date, only a few reports describe the isolation and characterization of endothelial cells of the endometrium (14, 15, 16). These isolated human endometrial microvascular endothelial cells (hEMVEC) express estradiol and progesterone receptors and display an enhanced expression of the vascular endothelial growth factor (VEGF) receptor type 2 (VEGFR-2) (14). Furthermore, the expression of the extracellular matrix proteins elastin, collagens, and fibronectin by hEMVEC was not detectable, whereas endothelial cells from the human umbilical vein (HUVEC) do express these proteins (15).
The aim of this study was to examine the growth characteristics of hEMVEC and to study the fibrinolytic capacity of these cells and their ability to form capillary-like tubular structures in a three-dimensional (3-D) fibrin matrix. A comparison was made with hFMVEC and HUVEC.
Materials and Methods
Materials
Penicillin/streptomycin, L-glutamine, and medium 199
(M199) with or without phenol red and supplemented with 20
mM HEPES were obtained from BioWhittaker, Inc.
(Verviers, Belgium); newborn calf serum (NBCS) and collagenase type II
were obtained from Life Technologies, Inc. (Grand Island,
NY). Tissue culture plastics were purchased from
Costar/Corning (Cambridge, MA) and Falcon (Becton Dickinson and Co., Bedford, MA). A crude preparation of
endothelial cell growth factor (ECGF) was prepared from bovine
hypothalamus as described by Maciag et al.
(17). Human serum was obtained from a local blood bank;
serum was prepared from fresh blood from 10–20 healthy donors, pooled,
and stored at 4 C. It was heat inactivated before use. Heparin and
thrombin were obtained from Leo Pharmaceutics Products (Weesp, The
Netherlands). Human fibrinogen was obtained from Chromogenics AB
(Mölndal, Sweden). Drs. H. Metzner and. G. Seemann (Aventis
Behring, Marburg, Germany) provided factor XIII (fibrogammin-P).
Fibronectin was a gift from Dr. J. van Mourik (CLB, Amsterdam, The
Netherlands). In addition, the mouse antihuman CD31 antibodies (clone
CLB-HEC/75) and human serum albumin (HSA) were obtained from CLB.
Recombinant human VEGF-A and human placenta growth factor-2 (PlGF-2)
were commercially obtained from RELIATech (Braunschweig, Germany), and
tumor necrosis factor- (TNF), containing 2.45 x
107 U/mg protein and less than 40 ng
lipopolysaccharide/mg protein, was a gift from Dr. J. Travernier
(Biogent, Gent, Belgium). Recombinant human basic fibroblast growth
factor (bFGF) was purchased from PeproTech (Rocky Hill, NJ).
Tosyl-activated Dynabeads M450 and goat antimouse IgG-coated Dynabeads
were obtained from Dynal (Oslo, Norway). Ulex
europaeus agglutinin-1 (UEA-1) was obtained from
Sigma (St. Louis, MO), and UEA-1 conjugated with
fluorescein isothiocyanate (UEA-1-FITC) was purchased from Vector Laboratories, Inc. (Burlingame, CA). Monoclonal antibodies
(mAbs) against cytokeratin 8 (mAb M20) and cytokeratin 18 (mAb M9) were
gifts from Dr. G. van Muijen (Academic Hospital, Nijmegen, The
Netherlands), and the u-PA receptor-blocking mAb H-2 was a gift from
Dr. U. Weidle (Roche Molecular Biochemicals, Penzberg,
Germany) (18). Rabbit polyclonal anti-u-PA antibodies were
prepared in our laboratory. Mouse mAb against smooth muscle cell actin
was purchased from Progen Biotechnik GmbH (Heidelberg, Germany). Rabbit
antihuman von Willebrand factor (vWF) antibodies, FITC-conjugated swine
antirabbit Ig, FITC-conjugated rabbit antimouse Ig, and horseradish
peroxidase-conjugated goat antirabbit Ig were obtained from DAKO Corp. (Glostrup, Denmark). The rabbit polyclonal antibodies
specific for u-PA were prepared in our laboratory
(19).
Complementary DNA (cDNA) probes
The following cDNA fragments were used as probes in the hybridization experiments: a 1.02-kb fragment of the human u-PA cDNA (20), a 1.2-kb PstI fragment of rat glyceraldehyde-3-phosphate dehydrogenase cDNA (provided by Dr. R Offringa, Leiden University, Leiden, The Netherlands), a 1.05-kb fragment of the human VEGFR-1 cDNA (21), and a 1.4-kb fragment of the human VEGFR-2 cDNA (22).
Cell culture
HUVEC and hFMVEC were isolated and characterized as previously described (23, 24), then cultured on fibronectin- or gelatin-coated dishes in M199 supplemented with 20 mM HEPES (pH 7.3), 10% human serum, 10% heat-inactivated NBCS, 150 mg/mL ECGF, 5 U/mL heparin, 100 IU/mL penicillin, and 100 mg/mL streptomycin (i.e. culture medium). hEMVEC were isolated from endometrial tissue as described below and maintained in the above-described culture medium supplemented with 10% human serum and 5 ng/mL VEGF-A (i.e. hEMVEC culture medium). Cells were cultured on fibronectin-coated wells in 5% CO2/95% air until confluence was reached and were subsequently detached with 0.05% trypsin/0.025% ethylenediamine tetraacetate (EDTA) and transferred into fibronectin-coated or gelatin-coated dishes at a split ratio of 1:3. Fresh medium was given three times a week, twice at 2-day intervals and once after a weekend interval. All of the experiments described below were performed in M199 and 20% human serum with HUVEC between passage 1–3, hFMVEC between passages 9–11, and hEMVEC between passage 3–7, respectively.
Isolation and purification of hEMVEC
Endometrial tissue was obtained from premenopausal women who had had their uteri removed for benign pathology. All of the patients gave their informed consent according to the guidelines of the medical ethical review boards of Leiden University Medical Center (Leiden, The Netherlands), Bronovo Hospital (The Hague, The Netherlands), and St. Franciscus Gasthuis (Rotterdam, The Netherlands). After removal of the uterus, the endometrial tissue was scraped off and stored in ice-cold storage buffer (140 mmol/L NaCl, 4 mmol/L KCl, 11 mmol/L D-glucose, 10 mmol/L HEPES, 100 IU/mL penicillin, and 0.10 mg/mL streptomycin, pH 7.3) at 4 C overnight. The endometrium was minced and incubated in M199/penicillin/streptomycin containing 0.2% collagenase type II at 37 C for 2 h. Adding the same amount of culture medium stopped the reaction, and all remaining tissue was dissolved by powerful resuspension, resulting in a homogenous solution. After centrifugation (1200 rpm for 5 min at room temperature) the pellet obtained was resuspended in culture medium and transferred into a fibronectin-coated culture dish. Two to 4 h later, the nonadhered cells were removed, and the adherent cells were cultured in hEMVEC culture medium.
The primary heterogeneous cell population was grown until near confluence before selection of the endothelial cells using UEA-1-coated Dynabeads. After detachment using trypsin and centrifugation, the cells were resuspended in M199 containing 0.1% HSA with the UEA-1-coated beads (20 beads/target cell). A 15- to 30-min end-over-end rotation was performed at 4 C before the cells that bound to the beads were selected by the use of a magnet (Dynal). The positively selected cell population was cultured in hEMVEC culture medium until confluence and then further isolated using mouse antihuman CD31 antibodies and goat antimouse IgG-coated Dynabeads. Trypsinized cells were incubated with antihuman CD31 antibodies (2 µg/mL in M199/0.1% HSA) for 30 min and kept on ice while being stirred occasionally. The nonbound antibodies were washed away with M199/0.1% HSA before the addition of goat antimouse IgG-coated Dynabeads. After 15–30 min of incubation with the beads at 4 C, the cells were separated by the use of a magnet. After this selection the CD31-positive cells were cultured in hEMVEC culture medium in fibronectin-coated culture dishes till confluence. The isolation procedure with anti-CD31 antibodies and antimouse Dynabeads was repeated until a homogeneous culture of endometrial endothelial cells was obtained (as determined after immunofluorescent characterization; see below).
Characterization of the isolated hEMVEC
Immunofluorescent characterization of the hEMVEC was performed on cell monolayers that had been cultured on special optics 96-well black plates with an ultrathin clear bottom (Costar/Corning). After washing with M199 containing penicillin/streptomycin, the cells were fixed by the addition of 80% (vol/vol) acetone or 4% formaldehyde for 10 min. The cells were washed with PBS before they were incubated for 30 min with various primary monoclonal or polyclonal antibodies diluted in PBS and 0.3% HSA (PBS/HSA). The control wells were incubated with PBS/HSA only. After washing with PBS/HSA, the cells were incubated with the appropriate second antibody, either FITC-labeled rabbit antimouse (50 µg/mL in PBS/HSA) or FITC-labeled swine antirabbit IgG (20 µg/mL in PBS/HSA).
Incorporation of [3H]thymidine
Incorporation of [3H]thymidine in DNA was determined as the measurement of endothelial cell proliferation. Confluent cultures of endothelial cells were detached by trypsin/EDTA solution and allowed to adhere and spread at a density of 104 cells/cm2 on gelatin-coated dishes in M199-HEPES medium supplemented with 10% heat-inactivated NBCS and penicillin/streptomycin for 18 h. Then the cells were stimulated with bFGF, VEGF-A, or PlGF-2. After an incubation period of 42 h, a tracer amount of [3H]thymidine (0.5 µCi/2 cm2 well, added in a 10-µL volume) was added to the wells, and the cells were incubated for another 6-h period. Subsequently, the cells were washed with PBS and fixed with 100% methanol, 3H-labeled DNA was precipitated in 5% trichloroacetic acid, and the cells were dissolved in 0.5 mL (0.3 mol/L) NaOH and counted in a liquid scintillation counter. The stimulation index was calculated as follows: stimulation index = [(dpmstimulated condition) - (dpmbackground)]/[(dpmcontrol condition) - (dpmbackground)].
Determination of specific u-PA binding
Determination of specific u-PA binding was determined as
previously described by Kroon et al. (25). In
short, cells were incubated for 10 min on ice with 50 mmol/L
glycine/HCl buffer (pH 3.0) to remove receptor-bound endogenous u-PA.
Subsequently, the cells were incubated on ice with 8 nmol/L
125I-labeled
diisopropylfluorophosphate-treated (DIP)-u-PA in endothelial
cell-conditioned medium (M199 supplemented with 1% human serum
albumin, conditioned for 24 h) for 3 h. After the incubation
period, unbound ligand was removed by extensive washing with ice-cold
M199. Cell-bound ligand was solubilized with 0.3 mol/L NaOH, and the
radioactivity was determined in a 
-counter (Cobra Auto , Packard,
Meriden, CT). Specific binding was calculated by the subtraction of
nonspecific binding from total binding.
Northern blotting
Total ribonucleic acid (RNA) from hFMVEC and hEMVEC (30 cm2/condition) was isolated 8 and 24 h after stimulation using the isothiocyanate/phenol acid extraction method described by Chomczynski et al. (26). The RNA was dissolved in formamide, and the concentration was determined spectrophotometrically. Equal amounts (7.5 µg) of RNA were separated on a formaldehyde/agarose gel. Subsequently, the separated RNA was transferred to a Hybond-N membrane through capillary force according to the instructions of the manufacturer (Amersham Pharmacia Biotech, Arlington Heights, IL). Hybridization was performed in 7% (wt/vol) SDS, 1 mmol/L EDTA, and 0.5 mol/L NaH2PO4/Na2HPO4 buffer (pH 7.2) overnight at 63 C with 25 ng probe labeled with a random primer (Megaprime kit, Amersham Pharmacia Biotech). Thereafter the Hybond membrane was washed twice for 20 min each time with 2 x SSC/1% SDS (wt/vol) and three times with 1 x SSC/1% SDS (SSC contains 0.15 mol/L NaCl and 0.015 mol/L sodium citrate). Finally, the filters were exposed to a phosphorimager screen and analyzed using a computer.
Enzyme-linked immunosorbent assays
u-PA, tissue-type PA (t-PA), and PAI-1 antigen determinations were performed using commercially available immunoassay kits: u-PA EIA HS Taurus (Leiden, The Netherlands), Thrombonostika t-PA (Organon-Teknika, Turnhout, Belgium), and IMULYSE PAI-1 (Biopool, Umea, Sweden).
Immunohistochemistry
Human endometrial tissues were embedded in paraffin and cut in sections of 4 µm. The cross-sections were prewashed in PBS containing 1% NaN3 and 0.3% H2O2 solution. Subsequently, they were incubated for 15 min in a block buffer (5% BSA in PBS) to reduce background staining. After three wash steps in PBS, the polyclonal anti-u-PA antibody was added (0.01 µg/mL in PBS supplemented with 0.05% Tween-20 and 0.1% BSA), followed by an overnight incubation at 4 C. The next day, the sections were washed in PBS, horseradish peroxidase-conjugated goat antirabbit Ig (1:1000 in PBS supplemented with 1% BSA) was added, and the sections were incubated for 1 h at 37 C. Thereafter the sections were washed, and peroxidase activity was developed with diaminobenzidine. The sections were counterstained with Mayer’s hematoxylin and mounted in malinol.
In vitro angiogenesis model
Human fibrin matrices were prepared by the addition of 0.1 U/mL thrombin to a mixture of 2.5 U/mL factor XIII (final concentrations), 2 mg fibrinogen, 2 mg sodium citrate, 0.8 mg NaCl, and 3 µg plasminogen/mL M199. Three hundred microliters of this mixture were added to the wells of 48-well (1-cm2) plates. After clotting at 37 C, the fibrin matrices were soaked with M199 supplemented with 10% human serum and 10% NBCS for 2 h at 37 C to inactivate the thrombin.
Type I collagen was solubilized by stirring adult rat tail tendons for 48 h at 4 C in a sterile 1:1,000 (vol/vol) acetic solution (300 mL for 1 g collagen). The resulting solution was extensively dialyzed against 1:10,000 (vol/vol) acetic acid and stored at 4 C (27). For the collagen gels, 8 vol rat tail collagen type I were mixed with 1 vol 10 x M199 and 1 vol 2% (wt/vol) Na2CO3 (mixture pH 7.4). Three hundred-microliter aliquots were added to each well and allowed to gel at 37 C in the absence of CO2.
Highly confluent hFMVEC and hEMVEC were detached, seeded in a split ratio of 1.25:1 and 2.5:1, respectively, on the surface of the fibrin or type I collagen matrices, and cultured for 24 h in M199 without indicator supplemented with 20% human serum, 10% NBCS, and penicillin/streptomycin. Then the endothelial cells were cultured with the mediators indicated for 3–7 days. The culture medium was collected and replaced every 2 or 3 days. Invading cells and the formation of tubular structures of endothelial cells in the 3-D fibrin or collagen matrix were analyzed by phase contrast microscopy.
Statistics
Data for three experiments per wells are expressed as the mean ± SEM, and data for duplicate experiments per well are expressed as the mean, with the range between the error bars. Statistical analyses of the data (paired sample t tests) were calculated using the statistic program SPSS (version 10.0, SPSS, Inc., Chicago, IL).
Results
Isolation and characterization of human endothelial cells from endometrium tissue
hEMVEC were isolated by repeated selection of UEA-1- and
CD31-positive cells by means of lectin- or antibody-coated magnetic
beads. After two to four rounds of selection, all growing cells were
characterized as endothelial cells on the basis of expression of the
classical endothelial markers CD31 and vWF and the capacity to bind
UEA-1 (Fig. 1
). In addition, the cells
were negative when stained with antibodies recognizing the epithelial
cell markers cytokeratin 8 and 18 and smooth muscle cell actin (data
not shown).
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Using the method described, we succeeded in isolating 13 different hEMVEC isolations in 33 attempts. The phase of the menstrual cycle of the women who underwent hysterectomy did not influence the success rate. We succeeded in isolating hEMVEC from proliferative phase tissue as well as from secretory tissue. We never succeeded in isolating hEMVEC from menstruation phase endometrium. Probably the amount of tissue, especially that obtained from the thin (basal) endometrial layer toward the end of the menstruation, was too little to isolate hEMVEC.
Growth characteristics of hEMVEC
The growth characteristics of hEMVEC were compared with those of
hFMVEC. hFMVEC (as well as HUVEC, data not shown) were stimulated to
proliferate by the addition of bFGF and VEGF-A. These types of human EC
react better to bFGF compared with VEGF-A, as determined by the
incorporation of [3H]thymidine (Fig. 2B). The overall stimulation indexes for
HUVEC were 10.9 ± 1.2 and 5.3 ± 0.8 induced by 2.5 ng/mL
bFGF and 6.25 ng/mL VEGF-A, respectively (P = 0.00004;
n = 16), and those for hFMVEC were 12.1 ± 2.0 and 3.9
± 0.5 (P = 0.006; n = 11). In contrast, VEGF-A
was more potent in stimulating hEMVEC to proliferate compared with bFGF
(Fig. 2A). The mean stimulation index of hEMVEC was 16.8 ± 4.8
using 2.5 ng/mL bFGF and 30.0 ± 8.3 using 6.25 ng/mL VEGF-A
(P = 0.017; n = 7, performed with hEMVEC from four
different donors). The stimulation index for hEMVEC was higher than
those of HUVEC and hFMVEC due to the lower proliferative capacity of
the hEMVEC under control conditions (only in the presence of 10%
NBCS). Neither hEMVEC nor hFMVEC responded to PlGF-2 (Fig. 2).
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). Densitometric analysis of the blots
revealed a 2.4-fold increase in VEGFR-2 mRNA in hEMVEC compared with
hFMVEC. The expression of VEGFR-1 mRNA was very low and comparable in
the different endothelial cell types (data not shown).
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Compared with other types of human endothelial cells, unstimulated
hEMVEC produced considerably more u-PA. In 24 h, they accumulated
33.8 ± 6.9 ng/105 cells (n = 5)
compared with 0.1 ± 0.1 in HUVEC (n = 4; data not shown).
This was not due to a difference in reuptake of u-PA:PAI-1 complexes by
the cellular u-PA receptor, because after blockade of the u-PA/u-PAR
interaction by monoclonal antibody H-2, the levels of u-PA antigen were
43.2 ± 7.0 (Table 1) and 6.2
± 2.3 ng/105 cells for hEMVEC and hFMVEC (data
not shown), respectively. The overall production of u-PA by hEMVEC was
enhanced by 1.3-, 1.5-, and 1.8-fold after a 24-h incubation with bFGF,
VEGF-A, or their combination (Table 1). TNF further enhanced
the production of u-PA antigen by hEMVEC. The relatively high u-PA
production by hEMVEC compared with hFMVEC was also found at the
mRNA level (Fig. 3
).
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(Table 1).
Binding of [125I]DIP-u-PA to cellular u-PA
receptor on hEMVEC and hFMVEC revealed comparable binding of u-PA to
its receptor and a similar increase induced by VEGF-A, bFGF, and
simultaneous addition of bFGF/VEGF-A, bFGF/TNF, or VEGF/TNF (Fig. 4). The average basal u-PA binding to
hEMVEC was 4.2 ± 1.4 (n = 3) fmol/105
cells, whereas that to hFMVEC was 3.3 ± 0.8
fmol/105 cells (triplicate wells; n = 1;
Fig. 4B).
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Immunolocalization of u-PA in human endometrium tissue samples
To compare the high amount of u-PA accumulation in
vitro with the in vivo situation, sections of
endometrium and myometrium were studied. Immunostaining for u-PA was
found on the vessels of the endometrium and myometrium (Fig. 5) and in the stroma of the
endometrium, whereas surface and glandular epithelial cells were
negative for the u-PA antigen (Fig. 5). A negative staining procedure
(the same staining procedure but without the primary antibody) showed
no staining of the vessels and stroma (data not shown).
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Subsequently, we evaluated the ability of hEMVEC to form
capillary-like tubular structures in an in vitro
angiogenesis assay, consisting of human endothelial cells cultured on
3-D fibrin matrices (28). As we had previously found,
hFMVEC form capillary-like tubular structures when stimulated with the
combination of VEGF-A (or bFGF) and TNF after a culture period of 7
days (Fig. 6E) (28, 29). The addition of growth factor alone did not result in the
formation of tube-like structures, but induced holes in the
monolayer of endothelial cells due to uncontrolled lysis of the fibrin
matrix (Fig. 6D) (29)). When hEMVEC were cultured on top
of the fibrin matrix in the presence of 20% human serum, they began
forming capillary-like tubes within 3 days (Fig. 6A). This spontaneous
tube formation was enhanced by the addition of VEGF-A (Fig. 6B).
Further enhancement and stabilization of the tube-like structures were
seen when the growth factors were added in combination with TNF
(data not shown). Maximal induction of the formation of tube-like
structures by hEMVEC was seen on days 2–3, whereas those formed by
hFMVEC were maximal on days 7–10. The extent of tube formation was
markedly inhibited by polyclonal anti-u-PA antibodies under both
unstimulated as well as VEGF-A-stimulated (71.2 ± 5.3%; n =
4) conditions.
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(data not shown). However,
those few hFMVEC that were able to invade the matrix did not form
tube-like structures with lumens surrounded by endothelial cells, but
formed sprouts at or just beneath the collagen surface (Kroon,
M. E., P. Koolwijk, M. L. J. Van Sohie, B. van der Vecht, and V. W. M.
Hinsbergh, manuscript in preparation). hEMVEC displayed an increased
invasion and sprout formation in the collagen matrix that was increased
by the addition of VEGF-A alone. However, sprouts of this type of human
endothelial cell did not contain clear lumen-like structures (data not
shown). Discussion
Here we describe the isolation and characterization of hEMVEC from
various donors. These endothelial cells displayed an enhanced
responsiveness to VEGF-A compared with hFMVEC due to an enhanced
expression of VEGFR-2. In addition, hEMVEC are more angiogenic when
cultured in the presence of 20% human serum on top of a 3-D fibrin
matrix or a 3-D collagen matrix compared with hFMVEC. The hEMVEC formed
tube-like structures within 2–4 days that were enhanced by the
addition of VEGF-A alone. This is in contrast to hFMVEC, which had to
be stimulated with the combination of VEGF-A and TNF for a period of
7 days to form tubes. The enhanced angiogenic behavior of the hEMVEC
was probably due to an increase in the expression of u-PA, facilitating
an enhanced proteolytic capacity to the hEMVEC.
Overt angiogenesis takes place in the endometrium throughout the reproductive life of the female during each menstrual cycle. Therefore, the endothelial cells of the endometrium must be able to respond quickly to changes in steroids and environmental conditions and to angiogenic mediators to form a new vascular bed to allow proper implantation and placentation. To study the process of angiogenesis in the endometrium we isolated hEMVEC using a standard isolation procedure of endothelial cells consisting of digestion of the endometrial tissue and selection of endothelial cells using UEA-1- or anti-CD31-coated magnetic beads. hEMVEC were difficult to maintain in culture. Only in the presence of 20% human serum and VEGF-A (and/or ECGF) were the cells able to proliferate and to be passed up to passages 6–15. The sensitivity of the cells to the culture condition may be a reflection of the in vivo situation, where rapid apoptosis of the cells in the endometrium starts in the secretory phase and peaks in the menstrual phase (30).
The family of VEGF growth factors is thought to play an important role in the process of angiogenesis (31, 32). Both the expression of VEGF (3, 33, 34) and the specific VEGF receptors VEGFR-1 and VEGFR-2 are found in the endometrium during the three stages of the menstrual cycle (35, 36). In particular, the expression of VEGFR-2 on the vessels was increased during the proliferative phase (35). Cultured hEMVEC also display enhanced expression of VEGFR-2 compared with hFMVEC or HUVEC, whereas the expression of VEGFR-1 is comparable among these EC types. As it is generally accepted that VEGF-A-induced endothelial proliferation is mediated via VEGFR-2 (37, 38), the enhanced expression of this VEGFR may be an explanation for why hEMVEC are more reactive toward VEGF-A than to bFGF compared with hFMVEC. These data are in accordance with recently published data by Iruela-Arispe et al. (14), who also showed that hEMVEC expressed increased levels of VEGFR-2 and increased proliferation in response to VEGF-A.
Most striking was the finding of the relatively high u-PA expression by
hEMVEC, whereas the levels of the other compounds of the plasminogen
system, t-PA and PAI-1, were not significantly enhanced compared with
those in hFMVEC or HUVEC (28, 39). The average u-PA
binding to the u-PAR was also similar [4.2 ± 1.4 (n = 3)
fmol/105 cells vs. 6.4 ± 3.2
(n = 17) (28) and 2.6 ± 1.8 (n = 8)
(28) fmol/105 cells for hEMVEC,
HUVEC, and hFMVEC, respectively]. Under basal conditions, hFMVEC do
not express such high levels of u-PA (0.2 ± 0.1
ng/105 cells; n = 9) (28, 40).
Only when stimulated with the inflammatory mediator TNF, but not
with VEGF-A or bFGF, do both hFMVEC and HUVEC start to secrete
considerable levels of u-PA (up to 1 ng/105
cells) (28, 39, 41). However, production of u-PA by
activated hFMVEC and HUVEC is still a magnitude lower than basal or
TNF-stimulated u-PA production by hEMVEC. In contrast to that by
hFMVEC (28), u-PA production by hEMVEC increased after
addition of the angiogenic growth factors bFGF, VEGF-A, or the
combination of these mediators with TNF. The high expression of u-PA
by hEMVEC in vitro was confirmed by immunohistochemical
staining of endometrial tissue obtained from healthy premenopausal
women. The endothelial cells of the vessels in the myometrium as well
as the vessels of the endometrium in vivo showed expression
of the u-PA antigen. In normal human tissue the expression of
endothelium-associated u-PA is hardly detectable, but there is an
increase in endothelial cell expression of u-PA detectable in
inflamed tissues, such as during appendicitis (42) and
tumor angiogenesis (43), in atherosclerotic vessels
(44), and in vessels in atherosclerotic plaques
(25).
Simultaneously with the enhanced u-PA expression,
hEMVEC displayed an enhanced capacity to form tube-like
structures in 3-D fibrin matrices and sprout formation in collagen
matrices compared with hFMVEC. The formation of tube-like structures of
hFMVEC in fibrin matrices depends on u-PAR-localized u-PA activity
(25) and occurs only after stimulation of the cells with
the combination of a growth factor (either bFGF or VEGF-A) and the
inflammatory mediator TNF (28), which is a potent
stimulator of u-PA production by human EC. It is possible that the
spontaneous tube formation by hEMVEC may be due to a response of the
hEMVEC (mediated via, for instance, the increased VEGFR-2 expression)
to growth factors in human serum in combination with the enhanced basal
u-PA expression, as shown by the inhibition of tube formation after the
addition of neutralizing anti-u-PA antibodies. The enhanced
u-PA/plasmin activity, which is also able to activate several MMPs,
such as MMP-1, MMP-3, and MMP-9 in vitro
(10, 11, 12), may provide endothelial cells in the endometrium
with enhanced angiogenic capacity, as shown in vivo.
In conclusion, we show that human endometrium-derived endothelial cells display an enhanced proteolytic capacity and an enhanced angiogenic capacity. These data provide us with a better understanding of the regulation, production, and physiological responses of the vasculature in the endometrium and may lead to new insight into pathology during pregnancy, which may be related to diseases later in life and therapeutic strategies in the future.
Acknowledgments
We thank Dr. R. M. F. van der Weiden (St. Franciscus Gasthuis, Rotterdam, The Netherlands) for valuable discussions and the endometrial tissues, and Dr. R. A. Verwey and colleagues of the Bronovo Hospital (The Hague, The Netherlands), who provided us with endometrial tissues.
Received October 27, 2000.
Revised February 1, 2001.
Accepted March 3, 2001.
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), bFGF
(•), or PlGF-2 (
) in M199 supplemented with 10% NBCS. After
48 h, a tracer amount of [3H]thymidine was added to
the medium, the incubation was continued in the same medium for another
6 h, and [3H]thymidine incorporation was determined
as described in Materials and Methods. The data are
expressed as the mean ± SEM of triplicate wells and
are representative of four experiments performed with different hEMVEC
isolations.
