Transplantation-induced endothelial dysfunction as studied in rat aorta allografts

BACKGROUND

Clinical evidence indicates that vascular endothelial cell (EC) dysfunction occurs early after transplantation (Tx) and initiates chronic graft vasculopathy. This study explored this phenomenon in rat aorta Tx using the stringent Dark Agouti (DA)-to-Lewis (LEW) and the weak Fischer 344 (F344)-to-LEW strain combinations.

METHODS

Donor abdominal aortae were orthotopically grafted into LEW rats. At post-Tx days 7, 14, 28, and 56, grafts were collected to assess changes in EC morphology (en face silver staining) and EC function, i.e., vasodilatory response to acetylcholine (ACH) after phenylephrine (PHE) precontraction; changes in vascular smooth muscle cell (VSMC) alpha-actin (western blotting), degree of apoptosis (caspase-3 activity), and morphology.

RESULTS

In DA allografts, VSMC and EC dysfunctions developed concomitantly and were completed at 14 days post-Tx, most likely due to the EC and alpha-actin-positive VSMC loss. Meanwhile, allografts revealed markedly increased caspase-3 activity. Neointima formation, restricted to the edges of allografts at day 28, covered the entire allografts by day 56 post-Tx. In F344-allografts, VSMC function was maintained up to day 14 post-Tx, whereas ACH-induced relaxation was reduced by 50% at day 7 and abolished at day 14. EC denudation was not seen up to 56 days post-Tx, despite prominent leukocyte adhesion. Neointima formation was not detected at day 28 post-Tx but appeared along the entire allografts at day 56 post-Tx.

CONCLUSIONS

These results confirm that Tx-induced EC dysfunction precedes the development of vasculopathy in rat aorta allografts and suggest that this early phenomenon can be best studied in the F344-to-LEW strain combination.

Species-specific and conserved epitopes on mouse and human E-selectin important for leukocyte adhesion

Selectins are C-type, cell surface lectins that are key players in leukocyte adhesion to the blood vessel wall endothelium. We describe here epitopes for a series of novel monoclonal antibodies (moAbs), UZ4-UZ7, directed against mouse E-selectin. All four antibodies specifically bind to mouse E-selectin, but not to P- or L-selectin, and all inhibit the adhesion of granulocytes, peripheral blood lymphocytes, and promyelocytic HL-60 cells to cytokine-activated mouse endothelium. Three moAbs, UZ5, UZ7, and UZ6, specifically inhibit mouse E-selectin-mediated adhesion by binding to epitopes in domains CR1 or CR2. moAb UZ4 inhibits leukocyte adhesion to both human and murine endothelium activated with IL-1 or other proinflammatory stimuli. UZ4 is the first described moAb that detects an epitope in the lectin domain which is conserved in both murine and human E-selectin (CXKKKL), but is not present in the other members of the selectin family, P- and L-selectin. Interestingly, UZ5, UZ6, and UZ7 more efficiently interfere with lymphocyte than with granulocyte adhesion to cytokine-activated endothelium, while UZ4 completely blocks adhesion of PMN, lymphocytes, and HL-60 and U937 cell lines. The data suggest that E-selectin-ligand engagement differs between lymphocytes and PMN, and that these differences may be accentuated by the CR1 and CR2 domains in the E-selectin cell adhesion molecule.

Endothelial dysfunction and denudation in rat aortic allografts

Clinical evidence suggests that early endothelial cell (EC) dysfunction may predict the development of graft vascular disease. We wished to assess the early functional and morphological changes in the graft endothelium in a commonly used animal model of graft vascular disease, the rat aortic interposition allograft model. To assess graft EC function, regulation of vascular tone by ECs was monitored in aortic rings from grafts harvested at various times after transplantation (Tx). EC morphology was assessed by silver staining, which was followed by en face inspection of the luminal side of the grafts. Acetylcholine-induced EC-dependent vasorelaxation was reduced in allografts at post-Tx days 7 and 14, whereas in syngeneic grafts EC-dependent relaxation was unaffected at any time after Tx. In separate grafts collected at the same time points, massive leukocyte adhesion at post-Tx day 7 and EC denudation at days 14 and 28 were evident in allografts but not in syngeneic grafts. At post-Tx day 56 (a time at which vessel wall remodeling is pronounced in this model), an intact EC layer covered the grafts. EC dysfunction and morphological changes were prevented by immunosuppression of recipient rats with cyclosporine. Our study shows that Tx-induced EC dysfunction in rat aortic allografts can be observed within 1 week of Tx in rat aortic allografts and that this is occurring concomitantly with enhanced leukocyte adhesion to the graft ECs. These changes occur before any other morphological or functional changes described thus far in this model and appear to be immune-driven. Taken together, these results show that Tx-induced early EC dysfunction, as described in patients, may be studied in the model of rat aortic Tx.

NF-kappaB-dependent fractalkine induction in rat aortic endothelial cells stimulated by IL-1beta, TNF-alpha, and LPS

Fractalkine is an endothelial cell-derived CX3C chemokine that is chemotactic mainly to mononuclear cells. Fractalkine was induced in rat aortic endothelial cells (RAEC) by interleukin-1beta (IL-1beta), tumor necrosis factor alpha (TNF-alpha), and lipopolysaccharide (LPS) transcriptionally and translationally. This induction correlated with increased NF-kappaB DNA binding activity as determined by gel mobility shift assay. Supershift assays revealed that the NF-kappaB subunits p50 and p65 were responsible for kappaB binding. Accordingly, we examined the role of NF-kappaB in fractalkine induction in RAEC through the use of an adenovirus-mediated mutant IkappaB as a specific inhibitor. Delivery of a dominant-negative form of IkappaBalpha in RAEC dramatically reduced the induction of fractalkine by these stimuli, suggesting a role for NF-kappaB activation in fractalkine induction. The inhibition of fractalkine expression by two potent NF-kappaB inhibitors, sulfasalazine and sanguinarine, further supported the central role of NF-kappaB in fractalkine transcription regulation and suggested a novel therapeutic target aimed at modulating leukocyte endothelial cell interaction.

Complete loss of functional smooth muscle cells precedes vascular remodeling in rat aorta allografts

BACKGROUND

The functional consequences of vascular remodeling in rat aorta allografts were studied at different times after transplantation (Tx).

METHODS

At days 1, 3, 7, 14, 28, and 56 after Tx, rat aorta allografts (Dark Agouti [DA]-to-Lewis) were mounted as isolated organs, and their contractile properties tested with phenylephrine, KCl, or endothelin-1. Controls were native DA-aortae and DA-syngeneic grafts. Changes in alpha smooth muscle actin and morphology were assessed by immunoblotting and histology.

RESULTS

PostTx syngeneic grafts presented similar functional and morphological properties to native aortae. In allografts, no morphological changes was detected at day 7 after Tx, but phenylephrine-induced vasoconstriction was reduced by 60%. Signs of medial smooth muscle cell (SMC) loss and adventitial inflammation were observed at day 14 after Tx, without neointima formation. A complete loss of contractile property was observed at day 28 after Tx in association with a 75% decrease in alpha-SMC actin, severe adventitial inflammation, and reduced medial cellularity. At this time, neointima was restricted to both edges of allografts. At day 56 after Tx, allografts were also not functional and exhibited neointima on their entire length. All these changes were prevented by treating recipients with cyclosporine (7.5 mg/kg/day).

CONCLUSION

These results indicate that, after Tx, the contractile property of rat aorta allografts is altered before manifest vascular remodeling. Because this can be prevented by cyclosporine, it most likely reflects an acute rejection of SMC. These results also show that vascular graft dysfunction can be used to monitor the development of rejection in the rat aorta allograft model.

The kinetics of association and phosphorylation of IkappaB isoforms by IkappaB kinase 2 correlate with their cellular regulation in human endothelial cells

Activation of the transcription factor NF-kappaB depends on the specific dual phosphorylation of its inhibitor protein IkappaB by the homologous cytokine-inducible IkappaB kinases 1 and 2 (IKK1/2). Various IkappaB isoforms exist: IkappaBalpha, IkappaBbeta1/2 (two alternative splice variants), and IkappaBepsilon. However, the individual relevance and the specific regulation of these isoforms is not well-understood. We have studied the direct interaction of recombinant IkappaBalpha, IkappaBbeta1, IkappaBbeta2, and IkappaBepsilon with the recombinant homodimeric IKK2. Fluorescence-based active site titration revealed that each IKK2 dimer contains two binding sites for IkappaB. By using surface plasmon resonance analysis, we found that all IkappaB proteins interact with the IKK2 dimer following a noncooperative binding mechanism. Further, the four IkappaB proteins bind to the kinase with equilibrium dissociation constants (KD) in the range of 50-300 nM; the association rate constants for all IkappaB isoforms with IKK2 were between 6.0 x 10(3) and 22.5 x 10(3) M-1 s-1, and the dissociation rate constants were between 1.25 x 10(-3) and 1.75 x 10(-3) s-1. This high-affinity binding suggests that the previously observed preassociation of all analyzed IkappaB proteins with the biochemically purified 700 kDa IkappaB kinase (IKK) complex is based on a direct enzyme-substrate association between the various IkappaB isoforms and the IKK proteins. The apparent catalytic efficiencies (kcat/KM) of IKK2 for IkappaBalpha, IkappaBbeta1, IkappaBbeta2, and IkappaBepsilon were 22 x 10(3), 10 x 10(3), 5.4 x 10(3), and 8.5 x 10(3) s-1 M-1, respectively, with KM values ranging between 1.7 x 10(-6) and 3.2 x 10(-6) M and kcat values ranging between 1.5 x 10(-2) and 3.7 x 10(-2) s-1. The relative affinities and catalytic efficiencies of IKK2 for the IkappaB isoforms were also reflected by the kinetics observed for the TNF-induced, phosphorylation-dependent degradation of the alpha, beta1, beta2, and epsilon isoforms of IkappaB in human umbilical vein endothelial cells. Therefore, differential regulation of the IkappaB isoforms in some cell types is not a direct result of the IKK activity, but appears to be due to parallel events.

Reduced microvascular thrombosis and improved outcome in acute murine stroke by inhibiting GP IIb/IIIa receptor-mediated platelet aggregation

Treatment options in acute stroke are limited by a dearth of safe and effective regimens for recanalization of an occluded cerebrovascular tributary, as well as by the fact that patients present only after the occlusive event is established. We hypothesized that even if the site of major arterial occlusion is recanalized after stroke, microvascular thrombosis continues to occur at distal sites, reducing postischemic flow and contributing to ongoing neuronal death. To test this hypothesis, and to show that microvascular thrombosis occurs as an ongoing, dynamic process after the onset of stroke, we tested the effects of a potent antiplatelet agent given both before and after the onset of middle cerebral arterial (MCA) occlusion in a murine model of stroke. After 45 min of MCA occlusion and 23 h of reperfusion, fibrin accumulates in the ipsilateral cerebral hemisphere, based upon immunoblotting, and localizes to microvascular lumena, based upon immunostaining. In concordance with these data, there is a nearly threefold increase in the ipsilateral accumulation of 111In-labeled platelets in mice subjected to stroke compared with mice not subjected to stroke. When a novel inhibitor of the glycoprotein IIb/IIIa receptor (SDZ GPI 562) was administered immediately before MCA occlusion, platelet accumulation was reduced 48%, and fibrin accumulation was reduced by 47% by immunoblot densitometry. GPI 562 exhibited a dose-dependent reduction of cerebral infarct volumes measured by triphenyltetrazolium chloride staining, as well as improvement in postischemic cerebral blood flow, measured by laser doppler. GPI 562 caused a dose-dependent increase in tail vein bleeding time, but intracerebral hemorrhage (ICH) was not significantly increased at therapeutic doses; however, there was an increase in ICH at the highest doses tested. When given immediately after withdrawal of the MCA occluding suture, GPI 562 was shown to reduce cerebral infarct volumes by 70%. These data support the hypothesis that in ischemic regions of brain, microvascular thrombi continue to accumulate even after recanalization of the MCA, contributing to postischemic hypoperfusion and ongoing neuronal damage.

SDZ RAD, a new rapamycin derivative: pharmacological properties in vitro and in vivo

BACKGROUND

This report describes the preclinical pharmacological profile of the new rapamycin analog, SDZ RAD, i.e., 40-O-(2-hydroxyethyl)-rapamycin.

METHODS

The pharmacological effects of SDZ RAD were assessed in a variety of in vitro and in vivo models, which included an autoimmune disease model as well as kidney and heart allotransplantation models using different rat strain combinations.

RESULTS

SDZ RAD has a mode of action that is different from that of cyclosporine or FK506. In contrast to the latter, SDZ RAD inhibits growth factor-driven cell proliferation in general, as demonstrated for the in vitro cell proliferation of a lymphoid cell line and of vascular smooth muscle cells. SDZ RAD is immunosuppressive in vitro as demonstrated by the inhibition of mouse and human mixed lymphocyte reactions and the inhibition of antigen-driven proliferation of human T-cell clones. The concentrations needed to achieve 50% inhibition in all of these assays fall into the subnanomolar range. SDZ RAD is effective in the in vivo models when given by the oral route in doses ranging between 1 mg/kg/day and 5 mg/kg/day. When compared with rapamycin, the in vitro activity of SDZ RAD is generally about two to three times lower; however, when administered orally, SDZ RAD is at least as active in vivo as rapamycin.

CONCLUSIONS

In conclusion, SDZ RAD is a new, orally active rapamycin-derivative that is immunosuppressive and that efficiently prevents graft rejection in rat models of allotransplantation. SDZ RAD has therefore been selected for development for use in combination with cyclosporine A to prevent acute and chronic rejection after solid organ allotransplantation.

L-selectin from human, but not from mouse neutrophils binds directly to E-selectin

L-Selectin on neutrophils as well as inducible E- and P-selectin on endothelium are involved in the recruitment of neutrophils into inflamed tissue. Based on cell attachment assays, L-selectin was suggested to function as a carbohydrate presenting ligand for E- and P-selectin. However, previous affinity isolation experiments with an E-selectin-Ig fusion protein had failed to detect L-selectin among the isolated E-selectin ligands from mouse neutrophils. We show here that L-selectin from human neutrophils, in contrast to mouse neutrophils, can be affinity-isolated as a major ligand from total cell extracts using E-selectin-Ig as affinity probe. Binding of human L-selectin to E-selectin was direct, since purified L-selectin could be reprecipitated with E-selectin-Ig. Recognition of L-selectin was abolished by sialidase-treatment, required Ca2+, and was resistant to treatment with endoglycosidase F. Binding of L-selectin to a P-selectin-Ig fusion protein was not observed. In agreement with the biochemical data, the anti-L-selectin mAb DREG56 inhibited rolling of human neutrophils on immobilized E-selectin-Ig but not on P-selectin-Ig. No such inhibitory effect was seen with the anti-mouse L-selectin mAb MEL14 on mouse neutrophils. Rolling of E-selectin transfectants on purified and immobilized human L-selectin was inhibited by mAb DREG56. We conclude that L-selectin on human neutrophils is a major glycoprotein ligand among very few glycoproteins that can be isolated by an E-selectin affinity matrix. The clear difference between human and mouse L-selectin suggests that E-selectin-binding carbohydrate moieties are attached to different protein scaffolds in different species.

Role of the glycoprotein Ib-binding A1 repeat and the RGD sequence in platelet adhesion to human recombinant von Willebrand factor

To assess the relative importance of the glycoprotein (GP) Ib binding domain and the RGDS binding site in platelet adhesion to isolated von Willebrand factor (vWF) and to collagen preincubated with vWF, we deleted the A1 domain yielding delta A1-vWF and introduced an aspartate-to-glycine substitution in the RGDS sequence by site-directed mutagenesis (RGGS-vWF). Recombinant delta A1-vWF and RGGS-vWF, purified from transfected baby hamster kidney cells, were compared with recombinant wild-type vWF (WT-vWF) in platelet adhesion under static and flow conditions. Purified mutants were coated on glass or on a collagen type III surface and exposed to circulating blood in a perfusion system. Platelet adhesion under static condition, under flow conditions, and in vWF-dependent adhesion to collagen has an absolute requirement for GPIb-vWF interaction. The GPIIb/IIIa-vWF interaction is required for adhesion to coated vWF under flow conditions. Under static condition and vWF-dependent adhesion to collagen, platelet adhesion to RGGS-vWF is similar as to WT-vWF, but platelet spreading and aggregation are abolished.

Decreased lumen size after balloon injury despite inhibition of neointimal thickening and antivasospastic treatment

OBJECTIVE

Both calcium antagonists and angiotensin converting enzyme (ACE) inhibitors are known to diminish the development of intimal thickening after a balloon catheter lesion. It was previously shown that narrowing of carotid artery lumen induced by balloon injury was not influenced by treatment, even though the two ACE inhibitors used inhibited neointimal thickening. The aim of the present study was to include a calcium antagonist as well, in order to investigate whether vasospasm contributes to the persistence of lumen narrowing in ACE inhibitor treated rats after a balloon lesion.

METHODS

Six groups of 10 rats were subject to balloon lesion of the left carotid artery. They received spirapril (10 mg.kg-1 x d-1) or isradipine (30 or 100 mg.kg-1 x d-1) or both, given throughout the study in the food. Controls received no drug. Neointimal thickening was measured histologically two weeks after injury. The cross sectional carotid lumen area was measured in vivo by nuclear magnetic resonance imaging, both before and two weeks after balloon injury, and also postmortem by histological techniques.

RESULTS

Two weeks after injury, the lumen area of the left carotid artery was significantly reduced following balloon injury, as measured by both techniques. Treatment did not modify the stenosis process as assessed by either method for measuring lumen size. Neointimal thickening, however, was inhibited by between 4% (low dose isradipine) and 59% (combined spirapril + high dose isradipine) in the various treatment groups.

CONCLUSIONS

Since calcium antagonist treatment was not able to influence the reduction of lumen size, it is unlikely that the narrowing is due to reversible spasm of the carotid artery in the first two weeks after inducing a balloon lesion. Alternatively, chronic vasospasm of neointimal smooth muscle might not be very sensitive to calcium antagonists.

Spirapril and cilazapril inhibit neointimal lesion development but cause no detectable inhibition of lumen narrowing after carotid artery balloon catheter injury in the rat

Five groups of 12 rats were subject to balloon lesion of the left carotid artery and neointimal thickening was measured histologically 2 weeks after injury. Rat groups received either spirapril (3, 10 or 30 mg/kg/day, administered throughout the study in the food), cilazapril (10 mg/kg/day) or placebo. Spirapril caused a dose-dependent inhibition of the neointimal thickening of the rat carotid artery. The degree of inhibition with 10 mg/kg/d spirapril and cilazapril was similar (-44% and -42% respectively). The carotid lumen area was measured in vivo by nuclear magnetic resonance (NMR) imaging both before and 2 weeks after balloon injury and also postmortem by histological techniques. Two weeks after injury, the lumen area of the left carotid artery was significantly reduced following balloon injury, as measured by both techniques. Treatment did not detectably modify this stenosis process despite the use of two independent methods for assessing lumen size, even though neointimal thickening was strongly attenuated by both angiotensin converting enzyme inhibitors. This dissociation between inhibition of neointimal lesion development and decrease of lumen size provides a new view of the role of angiotensin converting enzyme inhibitors in vascular damage situations. Our results suggest that the focus, particularly in clinical studies, on lumen size, may mean that potentially beneficial effects of these drugs on other parts of the vascular wall be overlooked.

Platelet aggregation and fibrinogen binding in human, rhesus monkey, guinea-pig, hamster and rat blood: activation by ADP and a thrombin receptor peptide and inhibition by glycoprotein IIb/IIIa antagonists

Platelet aggregation and fibrinogen binding in whole blood, induced either by ADP or by a 14 amino acid peptide mimicking an N-terminal region of the platelet thrombin receptor (TRP, thrombin receptor activating peptide), have been studied with blood from different species. Aggregation was assessed by counting the number of single platelets in blood before und after addition of the agonist with an automated cell counter. Both ADP (0.02-0.5 microM) and TRP (1-10 microM) were found to be potent agonists of platelet aggregation in human, rhesus monkey and guinea-pig blood, causing a near-maximal aggregatory response within 2 min of agonist addition. In contrast, hamster and rat platelets were much less responsive to both ADP and TRP in terms of the whole blood aggregation. Echistatin, RGDW and a synthetic glycoprotein (GP) IIb/IIIa antagonist, Ro 43-8857, inhibited fibrinogen binding to purified immobilized human GP-IIb/IIIa with IC50's of 1.6, 88 and 11.4 nM, respectively. In whole human blood, the respective IC50's (as determined by flow cytometric analysis of platelet fibrinogen binding) were 4.4, 1700 and 29.5 nM. The affinities of these three compounds for inhibiting fibrinogen binding in whole blood from rhesus monkeys and guinea-pigs were similar to the affinities for human platelets, but with rat blood echistatin, RGDW and Ro 43-8857 were all around 100-fold less potent. Ro 43-8857 was a potent inhibitor of ADP- or TRP-induced platelet aggregation in human, rhesus monkey and guinea-pig whole blood (IC50 of 69-320 nM) but was much less active in hamster blood.(ABSTRACT TRUNCATED AT 250 WORDS)

Time course of spirapril-induced structural and functional changes after myocardial infarction in rats followed with magnetic resonance imaging

Structural alterations after myocardial infarction (MI) in rats are usually examined only after death of the experimental animal. Magnetic resonance imaging (MRI) allows repeated and noninvasive measurements of important structural [left ventricular (LV) mass, LV wall thickness, LV chamber radius] as well as function [LV end-systolic and LV end-diastolic volume, stroke volume (SV), ejection fraction (EF)] parameters for a prolonged period. We describe our experience in a series of experiments in rats. Three weeks after MI, infarct size (IS) was determined by MRI and the rats were divided into two groups with equal IS. Three weeks later, treatment with the angiotensin-converting enzyme (ACE) inhibitor spirapril (10 mg/kg in food) or placebo was started. In both groups, the first MRI scan taken before the treatment showed moderately dilated left ventricles and signs of impaired LV function, i.e., an increase in LV end-systolic and end-diastolic volume and decreased EF. After 3-week treatment, no significant differences with respect to heart structure and function were detected as compared with those of untreated animals. Prolonged treatment for 10 weeks with spirapril resulted in significant reduction of LV dilatation, LV mass, and LV end-systolic and end-diastolic volume, which was accompanied by improved EF. Hemodynamic examinations after treatment for 6 months showed, in contrast to control animals, no increase in right ventricular systolic pressure in animals receiving spirapril. Furthermore, histologic examination of perfusion-fixed hearts at the end of the study demonstrated more pronounced LV dilatation in control animals, thus confirming the in vivo MRI data. Delayed treatment with spirapril proved to have beneficial effects on structure and function of infarcted hearts within 10 weeks. Spirapril limited LV dilatation, reduced LV weight and LV end-systolic and end-diastolic volumes, and improved EF.

Integrin and Arg-Gly-Asp dependence of cell adhesion to the native and unfolded triple helix of collagen type VI

Pepsin-solubilized collagen VI in triple-helical and heat-denatured, unfolded form was shown to promote Mg(2+)- and Mn(2+)-dependent attachment and spreading of various cell lines. On the triple-helical substrate no inhibition of cell adhesion was observed with several synthetic RGD peptides except in the case of A375 melanoma cells. In contrast, adhesion to the unfolded substrate was highly sensitive to RGD inhibition. Nine synthetic peptides were designed according to 10 RGD sequences present in the triple-helical sequence of human collagen alpha 1(VI), alpha 2(VI), and alpha 3(VI) chains. Only one peptide, corresponding to the C-terminal end of alpha 3(VI) chain, showed substantial inhibitory activity, whereas several peptides were active in direct adhesion assays when used as albumin conjugates. Inhibition tests with antibodies to integrin subunits, affinity chromatography, and ligand binding with purified integrins (alpha 1 beta 1, alpha 2 beta 1, alpha V beta 3, and alpha IIb beta 3) were used to identify collagen VI receptors. Binding to the triple-helical substrate is mediated by alpha 1 beta 1 and alpha 2 beta 1 integrins. Binding of both integrins to collagen VI was weaker than that to collagens I and/or IV. Recognition of the denatured substrate is mediated by beta 1 and beta 3 integrins. Activity was shown for alpha 5 beta 1 and alpha V beta 3 and weakly for alpha IIb beta 3 but not all alpha subunits possibly involved were identified. Distinct sets of receptors were also involved in A375 cell binding to triple-helical (beta 1-mediated) and denatured (beta 3-mediated) collagen VI, even though in this case both interactions could be efficiently inhibited by RGD peptides.

Two-step binding mechanism of fibrinogen to alpha IIb beta 3 integrin reconstituted into planar lipid bilayers

The platelet integrin alpha IIb beta 3 binds to fibrinogen and thus mediates platelet aggregation after stimulation. This integrin was isolated from human platelets and reconstituted into lipid vesicles. As judged by electron microscopy the integrin incorporated adequately only into 1,2-dimyristoylglycero-3-phosphocholine/1,2-dimyristoylphosphatidy lglycerol vesicles after removal of the detergent by adsorption to Bio-Beads. These vesicles were then used to generate planar lipid bilayers. The binding of fluorochrome labeled fibrinogen or the peptide ligand Gly-Arg-Gly-Asp-Ser-Pro-Cys (GRGDSPC) was monitored by total internal reflection fluorescence microscopy and a solid phase binding assay. Analysis of the kinetics revealed fast reversible formation of a fibrinogen/integrin precomplex (KD = 50 nM) followed by formation of a stable irreversible complex. This transition was monitored by measuring the fraction of precomplex which could be dissociated by addition of excess Gly-Arg-Gly-Asp-Ser (GRGDS). For the peptide, the KD was 1200 nM, and the rates of association and dissociation were faster than the time resolution of the method. Similar KD values were found by inhibition of fibrinogen binding to alpha IIb beta 3 in the immobilized receptor assay. Since the binding of fibrinogen was irreversible, KD values were dependent on the time period between fibrinogen incubation and peptide addition. These and results by other authors point to the biological importance of the biphasic binding process of fibrinogen to its receptor on platelets.

Endothelioma cells expressing the polyoma middle T oncogene induce hemangiomas by host cell recruitment

Mouse endothelioma cells expressing the polyoma middle T oncogene induced hemangiomas in a variety of species such as mice, rats, chicks, and quails. In embryos and newborn mice the hemangiomas expanded within 10-18 hr of injection, disrupting the vasculature and causing the death of the animal. In contrast, the hemangiomas formed a stable structure reminiscent of benign human hemangiomas in adult mice within 5 days. Analysis of the cells comprising the hemangioma revealed that over 95% of the endothelial cells were host derived. No induction of host cell proliferation was detected, and no endothelial mitogens were secreted by the endothelioma cells in vitro. The maintenance of the hemangioma appeared to require the continuous presence of endothelioma cells. The results indicate that these endothelioma cells act as a potent stimulating agent in the rapid formation of hemangiomas by recruiting nonproliferating host endothelial cells.

Vasculogenesis and angiogenesis in embryonic-stem-cell-derived embryoid bodies

Embryonic stem cells (ESC) have been established previously from the inner cell mass cells of mouse blastocysts. In suspension culture, they spontaneously differentiate to blood-island-containing cystic embryoid bodies (CEB). The development of blood vessels from in situ differentiating endothelial cells of blood islands, a process which we call vasculogenesis, was induced by injecting ESC into the peritoneal cavity of syngeneic mice. In the peritoneum, fusion of blood islands and formation of an in vivo-like primary capillary plexus occurred. Transplantation of ESC and ESC-derived complex and cystic embryoid bodies (ESC-CEB) onto the quail chorioallantoic membrane (CAM) induced an angiogenic response, which was directed by nonyolk sac endoderm structures. Neither yolk sac endoderm from ESC-CEB nor normal mouse yolk sac tissue induced angiogenesis on the quail CAM. Extracts from ESC-CEB stimulated the proliferation of capillary endothelial cells in vitro. Mitogenic activity increase during in vitro culture and differentiation of ESC. Almost all growth factor activity was associated with the cells. The ESC-CEB derived endothelial cell growth factor bound to heparin-sepharose. The identification of acidic fibroblast growth factor (FGF)in heparin-sepharose-purified material was accomplished by immunoblot experiments involving antibodies against acidic and basic FGF. We conclude that vasculogenesis, the development of blood vessels from in situ differentiating endothelial cells, and angiogenesis, the sprouting of capillaries from preexisting vessels are very early events during embryogenesis which can be studied using ESC differentiating in vitro. Our results suggest that vasculogenesis and angiogenesis are differently regulated.

Polarized secretion of a platelet-derived growth factor-like chemotactic factor by endothelial cells in vitro

Cultured bovine aortic endothelial cells secrete a potent migration-stimulating factor for vascular smooth muscle cells (SMCs) and adventitial fibroblasts. Vascular pericytes are 20-fold less responsive, and endothelial cells themselves do not respond at all. Checkerboard analysis of SMC migration in a micro-chemotaxis chamber assay shows that the factor is chemotactic. Chemotactic activity for SMCs and adventitial fibroblasts is specifically inhibited by antibodies against platelet-derived growth factor. Endothelial cells cultured on nitrocellulose filters secrete the platelet-derived growth factor-like factor almost exclusively into the basal compartment. We suggest that this factor plays an important role in the recruitment of vascular wall cells during the morphogenesis of blood vessels and pathological conditions, such as atherosclerosis.