Extracellular Matrix–Bound Angiopoietin-Like 4 Inhibits Endothelial Cell Adhesion, Migration, and Sprouting and Alters Actin Cytoskeleton
Angiopoietin-like 4 (ANGPTL4) is a secreted protein that belongs to the angiopoietin family and is involved in angiogenesis and metabolism regulation. We previously reported the induction of angptl4 by hypoxia in endothelial cells and in human ischemic tissues from peripheral artery disease. We here observed in a mouse model of hindlimb ischemia that the mRNA upregulation in the vessels correlates with the accumulation of the full-length protein in ischemic tissues. We then investigated its functions in endothelial cells. In response to hypoxia, endogenous ANGPTL4 accumulates in the subendothelial extracellular matrix (ECM). Although the secreted protein undergoes proteolysis leading to truncated fragments present in the medium, only full-length ANGPTL4 interacts with the ECM. Competition and direct binding assays indicate that the strong interaction of ANGPTL4 with the ECM is heparin/heparan sulfate proteoglycan dependent. The balance between matrix-associated and soluble forms of ANGPTL4 points to the role of the ECM in the regulation of its bioavailability. The angiogenic function of the ECM-bound full-length protein was investigated using either the form associated with the conditioned ECM from ANGPTL4-transfected HEK293 cells or the purified immobilized protein. We show that matrix-associated and immobilized ANGPTL4 limit the formation of actin stress fibers and focal contacts in the adhering endothelial cells and inhibit their adhesion. Immobilized ANGPTL4 also decreases motility of endothelial cells and inhibits the sprouting and tube formation. Altogether, these findings show that hypoxic endothelial cells accumulate ANGPTL4 in the ECM, which in turn negatively regulates their angiogenic capacities through an autocrine pathway.
Cardiovascular disorders such as coronary and peripheral artery diseases lead to a deficient blood supply to tissues and a decrease in oxygen partial pressure, ie, hypoxia. Because of their location at the interface of circulating blood and peripheral tissues, endothelial cells are exposed to hypoxia. A critical adaptation to hypoxia is angiogenesis, which consists in the formation of new blood vessels extending from the preexisting vasculature.1 This phenomenon occurs through the activation of the endothelial cells by a multistep process including changes of cell/extracellular matrix (ECM) interactions. In ischemic cardiovascular pathologies, reactive angiogenesis is a beneficial event. Therefore, unraveling the interplay of multiple molecular signals and events that occur in hypoxia and lead to functional new blood vessels is a challenging issue.
We previously identified angiopoietin-like 4 gene (angptl4) as a hypoxia-induced target in vitro in the human microvascular endothelial cell line (HMEC-1) and in vivo in the vessels of ischemic tissues from peripheral artery disease.2 Human ANGPTL4 is a secreted glycoprotein that belongs to the angiopoietin family.3 It is composed of 406 amino acids and contains an amino-terminal signal sequence, a coiled-coil domain and a carboxy-terminal fibrinogen-like domain. ANGPTL4 oligomerizes and undergoes proteolysis mediated by a cell-associated protease.4
Angiopoietins are major regulators of the balance between destabilization and stabilization of the vasculature occurring during angiogenesis. Angiopoietin-1 tightens vessels by promoting interactions between cells of the vascular wall, whereas angiopoietin-2 loosens these interactions and stimulates the growth of immature vessels.5,6 Angiopoietin-like proteins also play a role in the modulation of angiogenesis, as shown for ANGPTL1,7,8 ANGPTL2,8,9 ANGPTL3,10 and ANGPTL4.2,3,11,12 ANGPTL4, previously known as hepatic fibrinogen/angiopoietin-related protein (HFARP),3 peroxisome proliferator-activated receptor-γ (PPAR-γ), angiopoietin-related gene (PGAR),13 or fasting-induced adipose factor (FIAF),14 is also involved in lipid and glucose metabolism.15–17
In the pathological context of ischemic diseases, ANGPTL4 could modulate angiogenesis by modifying the microenvironment in response to hypoxia. A crucial determinant of cell microenvironment is the ECM, whose regulated composition plays a pivotal role in neovessel formation, stability, and maturation.18 The ECM is mainly composed of fibrous proteins, such as collagen or fibronectin, and interstitial glycosaminoglycans covalently bound to core proteins to form proteoglycans, such as heparan sulfate proteoglycans (HSPGs). The interaction of growth factors, including angiopoietin-1 and vascular endothelial growth factor (VEGF), with the ECM in the vicinity of the production site regulates their bioavailability.19,20
In the present study, using in vivo and in vitro models, we investigated the expression of ANGPTL4, its interaction with the ECM, and its bioactivity on endothelial cells. We report that full-length ANGPTL4 accumulates in the mouse ischemic hindlimb after vascular ligature and in the ECM of hypoxic endothelial cells through heparin/HSPGs. ECM-bound ANGPTL4 reduces endothelial cell adhesion, prevents the organization of focal adhesions and actin stress fibers, and decreases cell migration and sprouting. Therefore, ANGPTL4, through its autocrine effect on endothelial cells, participates in the modulation of angiogenesis in a hypoxic microenvironment.
Materials and Methods
Cell culture, expression and purification of recombinant ANGPTL4, immunofluorescence, statistical analysis, antibodies, and reagents are described in the online data supplement, available at http://circres.ahajournals.org.
Extraction of ECM-Associated Proteins
ECM was prepared, according to a protocol adapted from Owensby et al,21 by incubating cells in 5 mmol/L EDTA, 1% Triton X-100, at 4°C. ECM proteins were extracted in Laemmli buffer for Western blot analysis.
Mouse Model of Hindlimb Ischemia
This study was conducted in accordance with both institutional guidelines and those formulated by the European Community for experimental animal use. Unilateral hindlimb ischemia was induced by ligation and excision of the femoral artery in C57BL/6 mice (Charles River, Saint Germain sur l’Arbresle, France), as previously described.22 Mice were euthanized at day 2 or 6 (3 mice per group). Tissues from both hindlimbs were either fixed in 4% paraformaldehyde and paraffin embedded, either snap-frozen and stored at −80°C until used. Probe labeling by in vitro transcription and in situ hybridization were performed as previously described on paraffin sections, with an antisense mouse ANGPTL4 probe.23 Total proteins were extracted from frozen tissues for Western blot analysis.
Conditioned ECM were prepared from human embryonic kidney 293 (HEK293) cells grown to confluence for 48 hours and lifted by 5 mmol/L EDTA. Alternatively, plates were coated overnight at 4°C with increasing concentrations of purified ANGPTL4. Plates were saturated with 1% BSA. Trypsinized endothelial cells were plated in complete medium. Adherent cells were fixed 1 hour after plating in 4% paraformaldehyde, stained with 0.1% crystal violet, and quantified by measuring the absorbance at 570 nm.
Transwell filters (8-μm pores) were coated on the lower side with fibronectin (10 μg/mL)±ANGPTL4 at various concentrations. Human umbilical vein endothelial cells (HUVECs) were seeded on the upper side of the membrane in ECBM2 with 0.2% FCS for 2 hours. Cells from the upper side were mechanically removed, and cells from lower side were fixed. Nuclei were stained with 4′-6 diamidino-2-phenylindole (DAPI) and counted.
HUVECs were detached by 10 mmol/L EDTA and seeded in complete medium on wells coated with fibronectin and/or purified ANGPTL4. After 40 minutes, cells were recorded at a 5 minutes lapse interval for 5 hours on inverted Leica microscope IRBE equipped with ×10/0.40 objective lenses, enclosed in a Life Imaging Services environmental incubator. Images were acquired using a Coolsnap HQ Roper Scientific camera. Motion analysis was performed with MetaMorph software.
Tube Formation Assay
Culture wells were coated on ice with Matrigel (Collaborative Biochemical Products, Bedford, Mass) alone or mixed with ANGPTL4 (1 or 5 μg/mL). After 30 minutes at 37°C, HMEC-1 cells were seeded on top and further incubated for 24 hours in the presence of ECBM2 with 0.5% FCS. The number of tubes was counted from 4 images acquired per condition with a ×4 objective.
Spheroids of 500 HUVECs were generated as previously described24 and embedded in collagen gels with or without ANGPTL4 (1 μg/mL). Endothelial sprouting was stimulated with 20 ng/mL VEGF for 24 hours. Sprouting intensity was quantitated by determining the cumulative sprout length per spheroid using an Olympus IX50 inverted microscope and Cell P software (Soft Imaging System, Münster, Germany). The mean of the cumulative sprout length of 10 randomly selected spheroids was analyzed as an individual data point.
ANGPTL4 Accumulates in Ischemic Hindlimb
Having previously shown that ANGPTL4 mRNA is upregulated in the vessel wall of human ischemic limbs,2 we addressed the issue in a mouse model of hindlimb ischemia. In situ hybridization at day 6 after ligature showed an upregulation of ANGPTL4 mRNA in the interstitial capillaries of the ischemic leg compared with the controlateral leg (Figure 1A) but not in the muscular fibers. Full-length mouse ANGPTL4 protein was also overexpressed in the ischemic limb (Figure 1B).
Hypoxia-Induced Endogenous ANGPTL4 Accumulates in the ECM of Endothelial Cells
Because the vessels are a site of expression of ANGPTL4 in ischemic tissues, the hypoxic regulation of the protein was further analyzed in primary cultures of HUVECs. The expression of full-length ANGPTL4 (55 kDa) was markedly induced in conditioned medium (CM) from HUVECs grown in hypoxia (1% O2) for 48 hours, as compared with cells grown in normoxia (21% O2) (Figure 2A, top). A high level of full-length ANGPTL4 was associated to the ECM and was greatly increased by hypoxia (Figure 2A, bottom). The binding properties of ANGPTL4 to the ECM have been compared with those of the matricellular proteins Cyr61/CCN1, which strongly binds the HSPGs, and Nov/CCN3, which displays a weak interaction with the matrix.25 As shown in Figure 2A, Nov was detected in the CM and was absent from the ECM extract, whereas Cyr61 was both detected in the CM and the ECM. Immunofluorescence analysis of unpermeabilized HUVEC in hypoxic condition showed that the distribution of ANGPTL4 in the ECM was diffuse without visible fibrillar organization, in a pattern similar to Cyr61 (Figure 2B). Nov was not detected in the ECM.
ANGPTL4 was also accumulated in the ECM of hypoxic endothelial cells from microvascular origin ie, primary culture of human dermal microvascular cells (HDMECs) and HMEC-1 cells (Figure 2C). ANGPTL4 accumulation in the matrix is regulated by the level and duration of hypoxia. Whereas 10% O2 for 2 days failed to upregulate matrix associated-ANGPTL4 in the HUVECs, a 5% O2 exposure induced a detectable increase of the ANGPTL4 level (Figure 2D). Thus, the amount of ECM-bound ANGPTL4 depends on the level of oxygen. Besides, ANGPTL4 accumulation is rapidly reversible. Reoxygenation of the HUVECs for 24 hours reverted the ECM-bound protein amount to its basal level (Figure 2E). Altogether, ANGPTL4 accumulation occurs in the endothelial ECM in mild to severe hypoxic conditions via a dynamic process.
Interaction of Full-Length ANGPTL4 With ECM Regulates Its Bioavailability
ANGPTL4 undergoes oligomerization and proteolytic processing in the CM of cell culture or in plasma.4,26 To determine which form interacts with the ECM, recombinant ANGPTL4 tagged with either myc-His or flag C-terminal epitope was expressed in dihydrofolate reductase–deficient Chinese hamster ovary (CHO-DHFR) cells, HEK293 cells, or HUVECs. Full-length ANGPTL4 was detected in the CM and the ECM of the 3 cell types (Figure 3). In nonreducing conditions, 2 bands of higher molecular weight corresponding to oligomeric forms were detected, indicating that ANGPTL4 monomers and oligomers interact with the ECM (Figure 3A).
To evaluate the ratio of matrix-bound versus soluble ANGPTL4, CHO-DHFR-ANGPTL4-myc cells were seeded in serum-free medium and grown for 24 hours. CM and ECM were prepared from an equal cell number. Most of the full-length ANGPTL4 was accumulated in the ECM (Figure 3B, left). A C-terminal proteolytic fragment was identified in the CM. This 35-kDa fragment was absent from the ECM fraction. Accordingly, HUVECs expressing ANGPTL4-myc displayed 2 bands of 55 and 35 kDa in the CM, indicating that endothelial cells are also able to process the soluble ANGPTL4. This phenomenon could not be revealed for the endogenous protein because the anti-ANGPTL4 antibody does not allow the recognition of the 35-kDa form. In agreement with the detection of endogenous ANGPTL4 in the ECM, only full-length 55-kDa recombinant ANGPTL4 interacted with the ECM from HUVECs (Figure 3B, right).
To further investigate the distribution of ANGPTL4 in the ECM and the medium, varying ratios of HEK293 and HEK293-ANGPTL4-flag cells were seeded in serum-free medium for 24 hours. Using a low ratio of ANGPTL4-expressing cells (Figure 3C, first lane in both panels), the full-length ANGPTL4 was detected in the ECM, whereas the C-terminal fragment alone was detected in the CM. With increasing proportions of HEK293-ANGPTL4, ECM-bound ANGPTL4 still consisted of the full-length protein and increasing amounts of both full-length and processed ANGPTL4 were detected in the CM. These experiments suggest that full-length ANGPTL4 is preferentially accumulated in the ECM until it reaches a saturation level.
ANGPTL4 Strongly Binds to the Endothelial ECM and Interacts With Heparin and Heparan Sulfate Proteoglycans
The strength of the interaction of ANGPTL4 with the ECM was then characterized. Recombinant ANGPTL4 was first allowed to interact with the ECM of normoxic HUVECs containing few endogenous protein. ECM-associated ANGPTL4 was analyzed after treatment with increasing concentrations of NaCl (Figure 4A and B). Full-length ANGPTL4 displayed a strong association with the ECM as 0.8 mol/L NaCl was required to release more than 50% of the bound protein from the ECM. Furthermore, ANGPTL4 was still detected in the presence of 1.2 mol/L NaCl.
ANGPTL4 could accumulate in the ECM through interaction with HSPGs, as previously reported for Cyr61.25 Increasing amounts of heparin added to the culture medium of HEK293-ANGPTL4 cells led to a dose-dependent displacement of the full-length protein from the ECM to the medium, suggesting a competition between soluble heparin and matrix HSPGs for recombinant ANGPTL4 binding (Figure 4C). Moreover in the presence of heparin (50 μg/mL), endogenous ANGPTL4 was shifted from the ECM of hypoxic HUVECs into the CM, as well as Cyr61 (Figure 4D). Similar results have been obtained with microvascular endothelial cells (data not shown). Full-length ANGPTL4 was applied on heparin/sepharose affinity column to further investigate a potential direct interaction. ANGPTL4 bound to heparin and it was eluted at 0.7 mol/L NaCl (Figure 4E). This strong affinity of ANGPTL4 for heparin was in a range similar to that previously reported for Cyr61.27 Moreover, in a solid-phase binding assay, purified full-length ANGPTL4 interacted with immobilized heparin as well as immobilized HSPGs, whereas it weakly interacted with BSA (Figure 4F). Altogether, our results demonstrate that both recombinant and endogenous full-length ANGPTL4 strongly interact with the ECM in a heparin/HSPGs dependent manner.
Matrix-Bound ANGPTL4 Inhibits Endothelial Cell Adhesion to the ECM
The functional role of the ECM-bound full-length ANGPTL4 was then investigated. Conditioned ECM from HEK293 or HEK293-ANGPTL4-flag cells was used to quantify the adhesion of endothelial cells 1 hour after plating. As compared with control ECM, HUVEC adhesion was decreased by 32.7±16.7% on ANGPTL4-containing ECM (Figure 5A). To determine the direct involvement of ANGPTL4 in the antiadhesive effect, endothelial adhesion was analyzed on increasing concentrations of purified immobilized protein (Figure 5B). HUVEC adhesion was reduced in a dose-dependent manner, reaching 39.5±9.7% of decrease on 5 μg/mL ANGPTL4. Adhesion of human umbilical artery endothelial cells (HUAECs) and HMEC-1 cells on ANGPTL4 was also decreased by 52.4±5.7% and 28.4±8.2%, respectively, when compared with control adhesion (Figure 5C).
Matrix-Bound ANGPTL4 Prevents the Formation of Actin Stress Fibers and the Organization of Focal Contacts of Adhering Endothelial Cells
Cell adhesion on ECM affects cytoskeletal dynamics. The organization of the actin cytoskeleton of endothelial cells was investigated during the adhesion process on ANGPTL4. HUVECs were plated on conditioned ECM from either HEK293 or HEK293-ANGPTL4-flag cells (Figure 6A). Two hours after plating on the control ECM, cells displayed a well-ordered array of transversal actin stress fibers extending to focal adhesions evidenced by vinculin. This organized cytoskeleton was similar to that from HUVECs adhering on fibronectin. In contrast, HUVECs adhering on ECM from HEK293-ANGPTL4 cells were almost devoid of actin stress fibers and focal adhesions. HUVECs displayed a similar disorganized cytoskeleton on purified immobilized ANGPTL4 compared with control coating. On wild-type ECM, 57.3±6.7% HUVECs displayed stress fibers versus 29.7±3.9% on ECM containing ANGPTL4. On control buffer, 67.4±3.7% cells displayed an organized cytoskeleton versus 38.7±5.8% on purified ANGPTL4 (Figure 6B, top). The alteration of actin organization and focal adhesion distribution induced by ANGPTL4 was dose dependent (Figure 6B, bottom).
A time course of the effect of ECM-associated ANGPTL4 was performed during the adhesion process (Figure 6C). The dynamic reorganization of actin fibers and focal adhesion progressed over the next 4 hours after adhesion on control ECM. To the contrary, stress fibers failed to form up to 4 hours after plating on ANGPTL4-containing ECM.
Immobilized ANGPTL4 Inhibits Endothelial Cell Migration
Endothelial cell migration is an essential step of the angiogenic process. A migration assay was performed in a Transwell system in which the lower side of the membrane was coated with fibronectin in the absence or presence of ANGPTL4 (Figure 7A). HUVECs showed a decreased locomotion toward fibronectin when immobilized ANGPTL4 was added. Furthermore, this effect was dose dependent, with a maximal reduction of 47% of the number of cells that had migrated (Figure 7A, right).
Cell migration was further analyzed by time-lapse videomicroscopy. The motility tracking of individual HUVECs plated on various coatings was monitored. Six representative paths are shown in Figure 7B (left). Immobilized ANGPTL4 decreased HUVEC migration speed when compared with plastic or fibronectin alone (control, 23.08±5.16 μm/h; ANGPTL4, 14.34±5.22 μm/h; fibronectin, 40.62±10.18 μm/h), as quantified in Figure 7B (right). Furthermore, ANGPTL4 immobilized in presence of fibronectin was able to counteract the fibronectin-induced high motility (ANGPTL4+fibronectin, 28.20±7.37 μm/h). Movies are shown in supplemental Figure I.
Matrix-Bound ANGPTL4 Inhibits Tube Formation and Sprouting
The ability of microvascular endothelial cells to form tubes was assessed in Matrigel containing increasing concentrations of full-length ANGPTL4. Tube formation of HMEC-1 cells was reduced on ANGPTL4 and Matrigel when compared with Matrigel alone (Figure 8A and 8B). To further characterize the role of ANGPTL4 in the sprouting process, we used a 3D spheroid model.24 HUVEC spheroids were stimulated to sprout in a collagen gel in the absence or presence of immobilized ANGPTL4. Figure 8C shows representative spheroids for each condition and the sprouting was quantified on Figure 8D. Whereas ANGPTL4 embedded in the collagen gel has no statistically significant effect in basal conditions, it decreased the sprouting of VEGF-stimulated HUVECs.
Previous in vivo studies reported a strong upregulation of angptl4 mRNA in human samples from critical leg ischemia2 and an inhibition of angiogenesis and vascular leakiness.11 In this study, we observed an accumulation of the protein in a mouse model of hindlimb ischemia. Altogether, these data suggest a pathophysiological role of ANGPTL4 in altering efficient neovascularization in ischemic tissues. Thus, deciphering the cellular mechanisms of the angiogenic responses to ANGPTL4 is an important issue.
We here demonstrate that ANGPTL4, a member of the angiopoietin family, strongly interacts with the ECM through HSPGs and that matrix-bound ANGPTL4 inhibits endothelial cell adhesion, cytoskeleton organization, migration, and sprouting. Angiopoietin-1 also interacts with the ECM, whereas angiopoietin-2 is more diffusible.19 These interactions provide different local concentrations, involve other proteins as coreceptors, and are thought to account in part for the differences in the biological effect of the angiopoietins. In addition to the transcriptional regulation by hypoxia,2,28 PPAR agonists, and nutritional factors,13,14,26 we here propose a new regulatory mechanism of the bioavailability of ANGPTL4 through ECM binding. This interaction with the ECM participates in concentrating the full-length protein near its production site. Consistently, we found an increase in full-length ANGPTL4 in the mouse ischemic muscle, whereas the interstitial vessels appear as a major expression site. Altogether, these findings suggest that the ECM from the vascular wall might constitute a dynamic reservoir of ANGPTL4. Its upregulation by hypoxia could be involved in microenvironment modifications occurring in ischemic events. This hypothesis cannot be fully tested in vivo, given the paucity of tools to detect endogenous ANGPTL4.
We show here a direct interaction of ANGPTL4 with heparin. However a higher salt concentration was required to release ANGPTL4 from subendothelial ECM than from the heparin/sepharose. This suggests that other ECM components are likely involved in the interaction between ANGPTL4 and the matrix. A putative heparin-binding motif has been found in ANGPTL3, the closest related known member of the family,29 although no consensus sequence exists in ANGPTL4. However, many heparin-binding proteins do not display a consensus sequence, suggesting rather the implication of a structural heparin-binding motif that remains to be identified for ANGPTL4.
ANGPTL4 is proteolyzed in vitro and in vivo and circulates in the peripheral blood mainly as truncated fragments,4,26 suggesting the relative instability of the full-length soluble form. Interestingly, we show here that full-length ANGPTL4 preferentially interacts with the ECM. This binding may protect ANGPTL4 from proteolysis and regulate its processing. Differences in the ratio of the full-length and truncated forms reported in human liver and adipose tissue26 could partly result from a variable ANGPTL4 binding capacity to the ECM in these organs.
Once incorporated in the ECM, full-length ANGPTL4 could be part of the guidance cues provided by the matrix to the endothelial cells and involved in vascular morphogenesis and angiogenesis.18 Cell adhesion on the ECM is a dynamic process involving attachment, spreading and formation of focal adhesions and stress fibers, which generate a strong adhesive state. A state of intermediate adhesion, characterized by a fairly spread shape of the cells with disruption of focal adhesions and disassembly of actin stress fibers, is thought to represent the physiological “de-adhesion” process, allowing tissue remodeling.30 Some ECM-associated proteins, the matricellular proteins, which include thrombospondin-1 (TSP-1), SPARC (Secreted Protein Acidic and Rich in Cysteine), and the CCN family of proteins (Cyr61, connective tissue growth factor, Nov) regulate cell/matrix interactions.31 In the present study, the antiadhesive activity induced by matrix-associated ANGPTL4 is similar to that observed for TSP-1 or SPARC. The cytoskeleton disorganization could explain the defective migration of endothelial cells on an attractive substratum as fibronectin.
The pathway leading to the inhibition of endothelial cell adhesion and migration and cytoskeleton disruption is likely to involve the integrin family, which comprises primary ECM receptors. ANGPTL4, similarly to the angiopoietins,32 or ANGPTL310 could directly bind an integrin, or inhibit cell-ECM interaction through a downstream impairment of integrin function. ANGPTL4 receptor has not yet been characterized, and these hypotheses need further investigation.
In summary, ANGPTL4 is a hypoxia-induced angiogenic modulator, whose bioavailability is regulated by a strong interaction with the ECM. It acts as a chemorepellent and disorganizes the actin cytoskeleton of endothelial cells. Thus, ECM-bound ANGPTL4 may be involved through an autocrine pathway in the highly regulated vascular morphogenesis occurring in response to hypoxia.
We thank Corinne Ardidie-Robouant, Alain Barret, and Eric Etienne for technical assistance; Dr Florence Ruggiero for helpful discussions; and Dr Vassilis Tsatsaris for umbilical cord samples.
Sources of Funding
This study was supported by a fellowship from the Fondation de France (to S.G.). S.G. belongs to the European Vascular Genomics Network (http://www.evgn.org) (contract LSHMCT-2003-503254). This work was also supported in part by a grant from Novartis. A.C. was the recipient of a Groupe de Reflexion sur la Recherche Cardiovasculaire and a Fondation Bettencourt Schueller grant; A.G. was financially supported by a Lefoulon-Delalande grant; S.L.J. was a recipient of a Fondation pour la Recherche Medicale grant.
Original received March 20, 2006; resubmission received September 19, 2006; revised resubmission received October 11, 2006; accepted October 12, 2006.
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