© 1999 American Heart Association, Inc.
Thrombosis |
Storage of Tissue-Type Plasminogen Activator in Weibel-Palade Bodies of Human Endothelial Cells
From the Division of Angiology and Hemostasis (C.R., E.K.O.K.), Division of Clinical Biochemistry (U.M.V.) and Louis Jeantet Research Laboratories (J.-C.I., P.A.H.), University Hospital Geneva, Switzerland, and the Center for Transgene Technology and Gene Therapy, Leuven, Belgium (R.D.G.).
Correspondence to Egbert K. O. Kruithof, Division of Angiology and Hemostasis, University Hospital, 24, rue Micheli-du-Crest, CH-1211 Geneva 14, Switzerland.
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Abstract—Tissue-type plasminogen activator (t-PA) is acutely released by endothelial cells. Although its endothelial storage compartment is still not well defined, t-PA release is often accompanied by release of von Willebrand factor (vWf), a protein stored in Weibel-Palade bodies. We investigated, therefore, whether t-PA is stored in these secretory organelles. Under basal culture conditions, a minority of human umbilical vein endothelial cells (HUVEC) exhibited immunofluorescent staining for t-PA, which was observed only in Weibel-Palade bodies. To increase t-PA expression, HUVEC were infected with a t-PA recombinant adenovirus (AdCMVt-PA). Overexpressed t-PA was detected in Weibel-Palade bodies and acutely released together with endogenous vWf by thrombin or calcium ionophore stimulation. In contrast, plasminogen activator inhibitor type 1 and urokinase were not detected in Weibel-Palade bodies after adenovirus-mediated overexpression. Infection of HUVEC with proinsulin recombinant adenovirus resulted in the storage of insulin in Weibel-Palade bodies, indicating that these organelles can also store nonendothelial proteins that show regulated secretion. Infection of AtT-20 pituitary cells, a cell type with regulated secretion, with AdCMVt-PA resulted in the localization of t-PA in adrenocorticotropic hormone–containing granules, indicating that t-PA can be diverted to secretory granules independently of vWf. Coinfection of AtT-20 cells with AdCMVt-PA and proinsulin recombinant adenovirus resulted in the colocalization of t-PA and insulin in the same granules. Taken together, these results suggest that HUVEC have protein sorting mechanisms similar to those of other regulated secretory cells. Although the results did not exclude an alternative storage site for t-PA in HUVEC, they established that t-PA can be stored in Weibel-Palade bodies. This finding may explain the acute coordinate secretion of t-PA and vWf.
Key Words: tissue-type plasminogen activator • von Willebrand factor • Weibel-Palade bodies • endothelial cells • immunofluorescence
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Introduction |
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Tissue-type plasminogen activator (t-PA) is a key enzyme for the removal of incipient thrombi in the vascular system, and recombinant t-PA is now widely used for the thrombolytic therapy of myocardial infarction. Endothelial cells (EC) are the main source for plasma t-PA.1 2 3 4 Although a variety of stimuli such as venous occlusion, exercise, or injection of vasoactive substances are known to acutely increase plasma levels of t-PA, the precise mechanisms responsible for this increase are poorly defined. They may involve an acute release of t-PA from EC, variations in the rate of production of t-PA by EC, or changes in the clearance rate.5 6
Several in vivo and ex vivo studies give clear evidence for the occurrence of acute release of t-PA. Experimental induction of disseminated intravascular coagulation in chimpanzees or baboons results in a 50-fold increase in t-PA plasma levels within a few minutes.7 8 The rapidity and the magnitude of the increase in the t-PA concentration, as well as the rapid return to normal levels, are consistent only with a massive release of t-PA from storage pools. The findings that injection of vasoactive agents such as thrombin or calcium ionophore leads to an acute increase of t-PA in isolated vascular systems are also consistent with an endothelial t-PA storage pool.9 10 11
Acute release of t-PA in vivo or in vitro is often accompanied by the release of von Willebrand factor (vWf), an adhesive glycoprotein involved in primary hemostasis. Plasma vWf originates mainly from EC, which store the protein in specific rod-like secretory granules known as Weibel-Palade bodies.12 Concomitant release of t-PA and vWf has been observed in vivo after injection of 1-desamino-8-D-arginine vasopressin (DDAVP)13 or after experimental disseminated intravascular coagulation,7 8 14 ex vivo in isolated rat hindlegs10 11 and in vitro in human umbilical vein endothelial cells (HUVEC).15 Furthermore, patients with severe von Willebrand's disease are deficient in acute t-PA release.13 16 17 Taken together, these data suggest that t-PA and vWf are released either from the same storage granules or from distinct granules responsive to common stimuli. Support for the existence of distinct granules is provided by studies that have observed a discrepancy in the release of t-PA and vWf. In the rat hindleg model, adenosine diphosphate stimulated the release of t-PA but not of vWf.18 In cultured HUVEC, the thrombin-induced release of t-PA and vWf revealed subtle differences in calcium requirement and pertussis toxin sensitivity.19 Furthermore, cell fractionation and immunofluorescence suggest that t-PA and vWf are stored in distinct granules.20 However, the heterogeneity in t-PA expression by EC3 does not allow firm conclusions to be drawn as to whether t-PA and vWf have distinct cellular localization(s).
The key role of t-PA in protecting the vascular system from thrombotic occlusions makes it important to better understand the mechanisms of t-PA storage and release. The present immunofluorescence study was undertaken to investigate the intracellular sorting of t-PA in HUVEC. Because endogenous t-PA expression is very low in HUVEC, we enhanced t-PA expression by adenovirus-mediated t-PA gene transfer. This approach was chosen because primary human EC are known to be resistant to other transfection methods. We also investigated the localization of t-PA in AtT-20 cells, a model cell system that is used to study the sorting of proteins into the constitutive or the regulated secretory pathway.21 A further aim was to determine whether Weibel-Palade bodies can store non-EC proteins known to follow the regulated pathway of secretion. Our observations indicate that both t-PA and other regulated proteins can be sorted to Weibel-Palade bodies.
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Materials
Unless stated otherwise, all biochemical reagents and chemicals used in this study were from Sigma or Fluka and of the highest grade available. The following antibodies were used: monoclonal anti-human t-PA antibody PAM-3 (directed against the protease domain) was from Biopool; monoclonal anti-human t-PA antibody ESP-4 (kringle 2 domain) was from American Diagnostica; monoclonal anti-human u-PA antibodies 7C7, 2L3, and 4D1E822 23 were a gift from Dr Paul de Clerck (Institute for Pharmacological Sciences, University of Leuvan, Leuvan, Belgium); rabbit anti-human vWf antibodies were from Dako and immunopurified on immobilized vWf before use; rabbit anti-human adrenocorticotropic hormone (ACTH) antibodies and monoclonal anti-human insulin (used for insulin and vWf double label experiments) and anti-rat Golgi 58K protein antibodies were from Sigma; monoclonal anti-human Lamp-2 H4B4 used as a lysosomal marker was from the Developmental Studies Hybridoma Bank; monoclonal 6C4 antibody used as a marker for late endosomes24 was a gift from Dr Toshihide Kobayashi and Jean Gruenberg, (University of Geneva, Geneva, Switzerland); monoclonal anti-human PAI-1 antibody was from Monozyme; guinea pig anti-human insulin antibodies (used for insulin and t-PA double label experiments) were a gift from Dr Paolo Meda, (University of Geneva Medical School, Geneva, Switzerland); rhodamine-conjugated goat anti-mouse antibodies were from Jackson (these antibodies were passed over immobilized rabbit antibodies to remove cross-reactivity); fluorescein isothiocyanate (FITC)-conjugated goat anti-rabbit antibodies were from Organon Teknika Cappel; FITC-conjugated sheep anti-guinea pig antibodies were from Biosys.
Cell Culture
Human umbilical vein endothelial cells (HUVEC)
were isolated by collagenase digestion25 and
grown at 37°C in a humidified atmosphere containing 5%
CO2. Briefly, the umbilical vein was washed with
Krebs-Ringer–bicarbonate buffer (KRBH, 120 mmol/L NaCl, 4.75
mmol/L KCl, 1.2 mmol/L KH2PO4, 0.6
mmol/L MgSO4, 1.2 mmol/L
CaCl2, 25 mmol/L
NaHCO3, and 25 mmol/L HEPES, pH 7.4) and
incubated for 10 minutes with 1 mg/mL collagenase (CLS,
type 1, Worthington Biochemical). Cells were collected by flushing the
vein with 50 mL RPMI 1640 supplemented with 10% FBS (Life
Technologies) and then grown in RPMI 1640 containing 90 µg/mL heparin
(Boehringer Ingelheim), 15 µg/mL endothelial
cell growth supplement (Upstate Biotechnology), 10 mmol/L HEPES,
100 U/mL penicillin, and 100 µg/mL streptomycin and supplemented with
10% FBS. Cells were passaged by trypsin/EDTA (Biochrom KG,
Berlin, Germany) treatment at a split ratio of 1/3 and used at passage
1 or 2. Tissue culture dishes (Falcon Becton Dickinson Labware, Lincoln
Parc, NJ), 24-well plates (Costar, Cambridge, MA) and glass coverslips
were coated with 0.1% gelatin. Cultured cells were identified as
endothelial by the presence of vWf antigen and labeling
of Weibel-Palade bodies using indirect
immunofluorescence.
AtT-20 mouse pituitary tumor-derived cells were grown in Dulbecco's modified Eagle's medium containing 4,5 g/L glucose (DMEM, Life Technologies) and supplemented with 10% FBS. AtT-20 cells were passaged by trypsin/EDTA treatment at a split ratio of 1:10.
Cell Infection With Recombinant Adenoviruses
The following recombinant adenoviruses have been previously
described: t-PA recombinant adenovirus (AdCMVt-PA),26
PAI-1 recombinant adenovirus (AdCMVPAI-1),27 and
proinsulin recombinant adenovirus.28 Recombinant u-PA
adenovirus was prepared as follows: a plasmid containing the human
pro-urokinase cDNA, pu-PA, was made available by William Bennett of
Genentech Inc, San Francisco, CA. The uPA cDNA fragment from HindIII to
SspI, containing 75 bp of 5' untranslated sequence and 600 bp of 3'
untranslated sequence was inserted into the adenovirus construction
plasmid, pACskCMV2. This plasmid contains the left end of adenovirus
type 5 including the origin of replication and packaging sequences (nt
no. 1-454), an SV40 ori/hGH terminator, a polylinker to facilitate cDNA
insertion, the CMV promoter, and flanking adenovirus sequences (nt no.
3334-5779) serving as the target for homologous recombination. It was
kindly provided by Joseph Alcorn of UT Southwestern Medical
Center (Dallas, Texas). The resulting plasmid, pACskCMVu-PA, was
cotransfected with ClaI digested Ad5dL309 into 293 cells using standard
methods29 to yield AdCMVu-PA. The recombinant adenoviruses
were propagated on a monolayer of 293 cells (CRL 1573, ATCC) and
titrated by plaque assay.29
HUVEC or AtT-20 cells were grown to 50% confluency in 24-well plates or on 12-mm diameter glass coverslips. They were then infected for 1 hour at 37°C in 300 µL RPMI 1640/10% FBS or DMEM/10% FBS with t-PA recombinant adenovirus (AdCMVt-PA), with u-PA recombinant adenovirus (AdCMVu-PA), with PAI-1 recombinant adenovirus (AdCMVPAI-1), or with proinsulin recombinant adenovirus at a titer of 108 pfu/mL. These virus titers were chosen to obtain positive staining for the recombinant proteins in 30% to 50% of the cells. After infection, the cells were washed and incubated for 48 hours at 37°C before fixation or secretion studies.
Indirect Immunofluorescence Staining
HUVEC or AtT-20 cells grown on glass coverslips were fixed for
20 minutes in 4% freshly depolymerized
paraformaldehyde in phosphate buffered saline (PBS:
10 mmol/L
NaH2PO4/Na2HPO4, 136
mmol/L NaCl, and 4 mmol/L KCl, pH 7.4), washed with PBS, quenched
for 20 minutes with 0.27% NH4Cl/0.38% glycine
in PBS, pH 7.4 and permeabilized for 30 minutes with
0.1% saponin/0.1% BSA in PBS, pH 7.4. For insulin staining, HUVEC
were fixed for 30 minutes in Bouin solution (75% saturated picric acid
solution, 7.4% formaldehyde, and 5% acetic acid) and
permeabilized by progressive 3 minutes ethanol
dehydration (30%, 50%, 70%, and 90% ethanol in water) and
rehydration (70%, 50%, 30%, and 0% ethanol in water).
For single label analyses, cells were incubated sequentially with the primary antibody and the fluorescent second antibody. For double label analyses, cells were incubated sequentially with a mixture of primary antibodies and a mixture of fluorescent second antibodies. Antibodies were diluted in 0.1% BSA in PBS, pH 7.4. Monoclonal antibodies for primary staining were used at 10 µg/mL for detection of t-PA, u-PA, and PAI-1, 5 µg/mL for late endosomal membranes and at dilutions of 1/5000 for lysosomal membranes, 1/50 for insulin, and 1/100 for Golgi; polyclonal antibodies for primary staining were used at 2 µg/mL for detection of vWf, 1/100 for ACTH, and 1/200 for insulin. Rhodamine-conjugated goat anti-mouse and FITC-conjugated goat anti-rabbit antibodies were used at a dilution 1/100. FITC-conjugated sheep anti-guinea pig antibodies was used at a dilution of 1/400. All incubations were performed for 1 hour at room temperature. After washing, coverslips were mounted in polyvinyl alcohol. Pictures were taken with a Zeiss Axiophot (Carl Zeiss) photomicroscope equipped with epi-illumination and specific filters for fluorescein and rhodamine using a plan apochromatex63/1.40 objective and Tmax black and white film (Eastman Kodak).
Negative control experiments were performed by omitting the primary antibodies or by using an irrelevant primary antibody of the same species or IgG subclass (for monoclonal antibodies). For double label analyses, we verified that the FITC fluorescence gave no signal in the rhodamine channel and conversely. We also confirmed the absence of cross-reactivity between mouse antibodies and FITC-conjugated goat anti-rabbit antibodies as well as between rabbit antibodies and rhodamine-conjugated goat anti-mouse antibodies.
Secretion Studies
Forty-eight hours after infection with recombinant AdCMVt-PA,
HUVEC grown on 24-well plates were washed 3x with KRBH/BSA 0.1%, pH
7.4, preincubated for 30 minutes in 1 mL KRBH/BSA, and then incubated
again for 30 minutes in 0.5 mL fresh KRBH/BSA alone, KRBH/BSA
containing 1 NIH U/mL of human thrombin (Sigma), or KRBH/BSA containing
2 µmol/L calcium ionophore A23187 (Sigma). The cell supernatants
were centrifuged and kept at -20°C until determination of
t-PA and vWf antigen concentrations.
Antigen Determinations
Human t-PA antigen was determined by ELISA (Imulyse t-PA,
Biopool). vWf was measured by ELISA as described
elsewhere.14 Data are presented as
means±SEM. The significance of differences was calculated using the
Student t test.
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Results |
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Localization of t-PA in Endothelial Cells
To study the intracellular localization of t-PA in HUVEC grown under basal culture conditions, we performed double label indirect immunofluorescence analysis for t-PA and vWf. All data shown in this article concerning the immunolocalization of t-PA were obtained using monoclonal antibody ESP-4 (similar data were obtained using monoclonal antibody PAM-3). A minority of the cells (<2%) were weakly but distinctly stained by anti–t-PA antibodies. These antibodies stained elongated organelles spread throughout the cytoplasm and positive for vWf, which identifies them as Weibel-Palade bodies (Figure 1
). This pattern of
staining was observed in HUVEC originating from different umbilical
cords and using either monoclonal antibody. The presence of cells
positive for vWf but negative for t-PA provided an intra-image negative
control that ruled out the possibility of interference from the bright
vWf/FITC signal into the rhodamine channel and the possibility of
cross-reactivity between the rabbit anti-vWf and the
rhodamine-conjugated goat anti-mouse antibodies.
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We compared the staining pattern for t-PA with that for
plasminogen activator inhibitor-1
(PAI-1), the principal inhibitor of t-PA that is produced
in high amounts by HUVEC and is constitutively secreted by these cells.
Strong PAI-1 staining was observed in all cells (Figure 2, left panel). It was mainly restricted
to a distinct perinuclear region that was similar to that labeled with
an antibody against protein 58 K, a membrane protein of the Golgi
apparatus (data not shown). No staining of elongated
organelles was observed using anti–PAI-1 antibodies.
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Many cell types are able to internalize and degrade t-PA. To test
whether endocytosis of t-PA by some of the cells could have contributed
to the immunofluorescence signal for t-PA, we
compared the morphology of late endosomes (labeled with antibody 6C4,
Figure 2
, middle panel) and lysosomes (labeled
with anti–Lamp-2 antibody, Figure 2, right panel) with that of
the t-PA–labeled organelles. The morphology of the numerous
endocytotic/lysosomal structures stained in all cells by these
antibodies was clearly different from that of the t-PA–positive
organelles.
Localization of Overexpressed t-PA in Endothelial Cells
The results presented above for t-PA can be interpreted in
2 ways: 1) there is heterogeneity in the intracellular
sorting of t-PA with only a minority of EC able to target t-PA to
Weibel-Palade bodies, or 2) there is a heterogeneity in
t-PA expression levels, with only a minority of cells expressing
sufficient t-PA to be detected by our technique. To increase t-PA
expression in HUVEC we used a recombinant adenovirus vector expressing
human t-PA (AdCMVt-PA). A multiplicity of infection was chosen such
that approximately 30% to 50% of the cells over-expressed t-PA to
preserve some noninfected cells as negative controls in the immediate
vicinity of cells that overexpressed t-PA. Double label
immunofluorescence experiments for t-PA and vWf
were then performed. In infected cells, the staining pattern for
overexpressed t-PA, namely elongated organelles and a bright
perinuclear pattern, was identical to that for vWf (Figure 3). No staining for t-PA was detected in
areas devoid of vWf. The perinuclear staining for vWf was identical to
that previously described30 and similar to the pattern
seen in studies using antibodies specific for the rough endoplasmic
reticulum.31 There were no endosome or
lysosome-type structures labeled with anti–t-PA antibody in
AdCMVt-PA–infected HUVEC. This suggested that endocytosis of
overexpressed t-PA did not contribute to the
immunofluorescence signal. The presence of cells
positive for vWf but negative for t-PA again provided the intra-image
negative control. At higher AdCMVt-PA titers, almost all HUVEC
expressed high amounts of t-PA in Weibel-Palade bodies (data not
shown).
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To verify whether overexpressed t-PA follows the regulated pathway of
secretion, we measured t-PA release in AdCMVt-PA infected HUVEC after
stimulation with known secretagogues. Treatment with thrombin or the
calcium ionophore A23187 led to an acute increase in release of t-PA
(3- to 4-fold) and of vWf (6-fold) as compared with untreated HUVEC
(Table 1).
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Localization of Overexpressed PAI-1 and u-PA in Endothelial
Cells
To analyze whether adenovirus-mediated
overexpression of a protein would artifactually result in a detectable
signal in Weibel-Palade bodies, we infected HUVEC with recombinant
adenoviruses that encode PAI-1 or u-PA under control of the same
promoter (cytomegalovirus [CMV]) as that used for the t-PA
recombinant adenovirus. We then performed double
immunofluorescence analysis for vWf and
either PAI-1 or u-PA.
The staining pattern for overexpressed PAI-1 was similar to that for
endogenous PAI-1 and markedly different from that for vWf
(Figure 4). We observed strong staining
for PAI-1 in the perinuclear region corresponding the Golgi
apparatus and diffuse staining throughout the cell. As
observed in uninfected HUVEC, the anti–PAI-1 antibodies did not label
Weibel-Palade bodies.
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HUVEC cultured under basal conditions exhibited a weak
immunofluorescence signal for u-PA (data not
shown). Infection of these cells with AdCMVu-PA led to a marked
increase in u-PA expression. The staining pattern for overexpressed
u-PA, corresponding to the Golgi apparatus and to
focal contact points was distinct from that for vWf (Figure 5). The anti–u-PA antibodies did not
label Weibel-Palade bodies.
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Expression of t-PA in AtT-20 Cells
To test whether the sorting of t-PA into regulated secretory
granules requires the presence of vWf, we infected AtT-20 cells with
AdCMVt-PA. Double label immunofluorescence
analysis showed a strong signal for t-PA in approximately 50%
of the cells, whereas no t-PA was detected in uninfected cells. t-PA
was localized in round storage granules containing ACTH (Figure 6), an endogenous protein
with regulated secretion. Staining was particularly strong at the tips
of the extended processes of these cells. The presence of cells
positive for ACTH but negative for t-PA provided an intra-image
negative control.
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To test whether AtT-20 cells can be infected by more than 1 recombinant
adenovirus and then express and store different proteins in the same
organelle, these cells were concomitantly infected with both AdCMVt-PA
and proinsulin recombinant adenoviruses. The AtT-20 cells were then
analyzed by double label immunofluorescence
for t-PA and insulin (Figure 7). AtT-20
cells simultaneously infected by both t-PA and proinsulin
recombinant adenoviruses efficiently expressed these proteins.
Furthermore, staining for t-PA and insulin was observed within the same
granules.
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Expression of Insulin in Endothelial Cells
To determine whether Weibel-Palade bodies are able to store non-EC
proteins with known regulated secretion, we infected HUVEC with
proinsulin-recombinant adenovirus. Double-label
immunofluorescence analysis showed that
adenovirus-mediated proinsulin gene transfer in HUVEC resulted in the
targeting of insulin in Weibel-Palade bodies (Figure 8).
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Discussion |
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The regulated release of t-PA enables EC to acutely generate a strong, local fibrinolytic response. Indeed, in chimpanzees and baboons a thrombogenic stimulus leads to a massive increase in plasma concentrations of t-PA resulting in the rapid appearance of fibrin degradation products.7 8 From these results it appears that the regulated release of t-PA by EC is a major defense mechanism against thrombosis. However, other proteins stored by EC have a prothrombotic action. Thus, exocytosis from Weibel-Palade bodies leads to exposure of P-selectin at the cell surface, which stimulates leukocyte adhesion and release of vWf that contributes to platelet aggregation.12 Many factors—such as thrombin, fibrin, bradykinin, histamine, platelet activating factor, DDAVP, and the calcium ionophore A23187—are known to induce the release of vWf from EC in vitro and in vivo.5–11,14,15,19,32 A majority of these factors also induce the acute release of t-PA. These observations suggest that t-PA and vWf are stored either in the same granules or in distinct granules that respond to similar stimuli.
Our data provide strong evidence that EC are able to sort t-PA into Weibel-Palade bodies. In HUVEC cultured under basal conditions, only a small percentage of the cells had sufficient t-PA to be detected by immunofluorescence analysis. In these t-PA positive cells, t-PA was detected in Weibel-Palade bodies. To increase the expression of t-PA in HUVEC, we used an adenoviral vector containing the t-PA cDNA under control of the strong CMV promoter (AdCMVt-PA).26 Thrombin and the calcium ionophore A23187 induced the parallel acute release of vWf and of overexpressed t-PA, indicating that both proteins were released from preformed storage compartments. The secretion data obtained in our model of adenovirus-mediated t-PA overexpression are in agreement with previous findings in HUVEC cultivated under basal conditions and using the same secretagogues in which acute t-PA release could be observed using an ultra-sensitive t-PA antigen assay.15 19 By immunofluorescence analysis, we observed a clear localization of overexpressed t-PA in the Weibel-Palade bodies of AdCMVt-PA infected cells. At high titers of AdCMVt-PA, almost all HUVEC expressed t-PA in Weibel-Palade bodies. This showed that the ability to store t-PA in Weibel-Palade bodies is not restricted to a small subpopulation of the cells. Rather, it suggests that the presence of a t-PA signal in the Weibel-Palade bodies of some HUVEC cultured under basal conditions is due to differences in t-PA expression levels among cells.
The targeting of overexpressed t-PA to Weibel-Palade bodies was unlikely to be an artifact of overexpression because 1) a qualitatively similar staining pattern was observed in HUVEC cultured under basal conditions; 2) we observed no staining of Weibel-Palade bodies using anti–PAI-1 antibody, even though PAI-1 is endogenously expressed by HUVEC at much higher levels than t-PA after AdCMVt-PA treatment; and 3) no PAI-1 or u-PA could be detected in Weibel-Palade bodies after adenovirus-mediated overexpression of these proteins. This implies that protein over-expression by itself is not sufficient to result in protein sorting into Weibel-Palade bodies.
Our finding on the localization of t-PA in Weibel-Palade bodies is in contradiction to several recent studies that have suggested distinct storage compartments for t-PA and vWf. In 1 study using butyrate to increase t-PA expression, subtle differences in calcium requirement and pertussis toxin sensitivity were observed between t-PA and vWf release.19 However, in our hands, treatment of HUVEC with butyrate increased t-PA expression in only a fraction of the cells devoid of vWf (Rosnoblet et al, unpublished observations, 1996), and it cannot be excluded that butyrate has modified the secretory response of these cells. Another study reported that the density of t-PA storage granules was the same as that of Weibel-Palade bodies when analyzed in Nycodenz gradients but different in sucrose gradients.20 Furthermore, an immunoelectron microscopy analysis of HUVEC and of murine capillary EC detected t-PA in small electron dense granules distinct from Weibel-Palade bodies.20 To what extent the heterogeneity of t-PA expression in EC in culture (this study) and in vivo33 34 may explain these differences remains to be established. Also, our data do not rule out the possibility that t-PA is stored in 2 compartments, ie, small dense granules and Weibel-Palade bodies. Storage in Weibel-Palade bodies may be more prominent in organs/sites where t-PA is expressed in high amounts or at times of increased synthesis such as inflammation.8
The sorting of t-PA into Weibel-Palade bodies may be an intrinsic property of t-PA or be due to a noncovalent interaction with vWf. The relevance of this question is strengthened by the recent observation that transfected vWf can function as a targeting chaperone that diverts transfected coagulation factor VIII from a constitutive to a regulated secretory pathway in AtT-20 cells.35 Sorting of endogenous or transfected t-PA to the secretory granules of PC12 cells36 37 suggests that t-PA storage can occur independently of an interaction with vWf. In the present study, we expressed t-PA in the ACTH-producing AtT-20 cells, a model cell system that expresses neither t-PA nor vWf and that has been widely used to study the targeting of exogenous proteins toward constitutive or regulated secretory pathways. t-PA was found in round granules in colocalization with endogenous ACTH. This confirms that targeting of t-PA to the regulated pathway is not dependent on an interaction with vWf and strongly suggests that t-PA contains its own cis-acting sorting signal.
Coinfection of AtT-20 cells with adenoviral vectors expressing t-PA and proinsulin resulted in storage of the 2 proteins within the same granules. Although ACTH, t-PA, and proinsulin share no obvious structural homology, they probably each contain a signal that is appropriately recognized by the sorting machinery of the pituitary cells. The ability of different adenoviral vectors to infect the same cell is in keeping with our previous findings38 and suggests possibilities to study the elements that play a critical role in the process of storage and secretion. Indeed, complementation experiments in model cell systems may be achieved more easily by using simultaneous infection with different recombinant adenoviruses than by performing successive stable transfection experiments.
Weibel-Palade bodies are regulated secretory granules not only for vWf but also for EC proteins like vasoactive peptides39 and t-PA (this report). In addition, transfection of EC with coagulation factor VIII has demonstrated that factor VIII can be stored in Weibel-Palade bodies in a vWf-dependent fashion.35 The ability of Weibel-Palade bodies to function as storage granules for non-EC proteins with regulated secretion is illustrated by our finding that insulin is found in these organelles after adenovirus-mediated proinsulin gene transfer. This suggests that the mechanisms of sorting of proteins to Weibel-Palade bodies are not fundamentally different from those in other cells possessing regulated secretory mechanisms.
In conclusion, the present study demonstrates that EC are able to target t-PA to Weibel-Palade bodies. This finding may explain the acute coordinated secretion of t-PA and vWf observed in numerous in vitro and in vivo studies. The storage of t-PA within secretory granules after adenovirus-mediated t-PA gene transfer in AtT-20 cells that do not express vWf suggests that t-PA contains its own sorting signal. Adenovirus-mediated expression of insulin in HUVEC and storage of this protein in Weibel-Palade bodies suggests similar storage mechanisms for endothelial and neuroendocrine cells.
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Acknowledgments |
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We gratefully thank Katherine Meyer and Nicole Aebischer for their technical help. We are grateful to Dr Marie-Luce Bochaton-Piallat and Prof Giulio Gabbiani for assistance with the immunofluorescence technique and for access to the fluorescent microscope. This work was supported by grants from the Swiss National Fund for Scientific Research to E.K.O.K. (31.050645.97), to U.M.V. (32.052667.97), and to P.A.H. (31.50811.97), from the Office Fédéral de l'Education et de la Science (95.0155/CT96-1438 and 95.0654/CT96-0023) and from the Foundation for Research on Atherosclerosis and Thrombosis. E.K.O.K. is a recipient of a personal grant from the Association-Vaud-Genève and U.M.V. is a recipient of a SCORE subsidy from the Swiss National Fund for Scientific Research.
Received October 16, 1998; accepted December 22, 1998.
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