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Circulation Research. 2005;97:1182-1189
Published online before print October 20, 2005, doi: 10.1161/01.RES.0000191541.06788.bb
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(Circulation Research. 2005;97:1182.)
© 2005 American Heart Association, Inc.


Integrative Physiology

Ischemic Neoangiogenesis Enhanced by ß2-Adrenergic Receptor Overexpression

A Novel Role for the Endothelial Adrenergic System

Guido Iaccarino, Michele Ciccarelli, Daniela Sorriento, Gennaro Galasso, Alfonso Campanile, Gaetano Santulli, Ersilia Cipolletta, Vincenzo Cerullo, Vincenzo Cimini, Giovanna Giuseppina Altobelli, Federico Piscione, Ornella Priante, Lucio Pastore, Massimo Chiariello, Francesco Salvatore, Walter J. Koch, Bruno Trimarco

From the Dipartimento di Medicina Clinica e Scienze Cardiovascolari ed Immunologiche (G.I., M.C., D.S., G.G., A.C., G.S., E.C., F.P., O.P., M.C., B.T.), Università Federico II, Napoli, Italy; Dipartimento di Biochimica e Biotecnologie Mediche (V.C., L.P., F.S.), Università Federico II, Napoli, Italy; CEINGE-Biotecnologie Avanzate (V.C., L.P., F.S.), Napoli, Italy; Dipartimento di Scienze Biomorfologiche e Funzionali (V.C., G.G.A.), Università Federico II, Napoli, Italy; Center for Translational Medicine (W.J.K.), Thomas Jefferson University, Philadelphia, Pa; and IRCCS Neuromed (B.T.), Pozzilli, Italy.

Correspondence to Guido Iaccarino, MD, PhD, Medicina Clinica, Scienze Cardiovascolari ed Immunologiche, Federico II University, Via Pansini 5, Edificio 2, 80131 Naples, Italy. E-mail guiaccar{at}unina.it


*    Abstract
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ß2-Adrenergic receptors (ß2ARs) are widely expressed, although their physiological relevance in many tissues is not yet fully understood. In vascular endothelial cells, they regulate NO release and vessel tone. Here we provide novel evidence that ß2ARs can regulate neoangiogenesis in response to chronic ischemia. We used in vivo adenoviral-mediated gene transfer of the human ß2AR to the endothelium of the rat femoral artery and increased ß2AR signaling resulting in ameliorated angiographic blood flow and hindlimb perfusion after chronic ischemia. Histological analysis confirmed that ß2AR overexpression also produced benefits on capillary density. The same maneuver partially rescued impaired angiogenesis in spontaneously hypertensive rats (SHR), whereas gene delivery of the G-protein–coupling defective mutant Ile164 ß2AR failed to provide ameliorations. Stimulation of endogenous and overexpressed ß2AR on endothelial cells in vitro was found to regulate cell number by inducing proliferation and [3H]-thymidine incorporation through means of extracellular receptor-activated kinase and vascular endothelial growth factor. The ß2AR also has novel effects on endothelial cell number through stimulation of proapoptosis and antiapoptosis pathways involving p38 mitogen-activated protein kinase and PI3-kinase/Akt activation. Therefore, ß2ARs play a critical role in endothelial cell proliferation and function including revascularization, suggesting a novel and physiologically relevant role in neoangiogenesis in response to ischemia.


Key Words: angiogenesis • rats • polymorphism • hypertension • in vivo digital angiography


*    Introduction
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*Introduction
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The endothelium controls several vascular functions, including vascular tone and permeability, thrombosis, hemostasis, and angiogenesis.1 It is noteworthy that all these functions can be regulated by activation of receptors, and often, the same receptor can activate multiple endothelial functions.

Adult angiogenesis only occurs in particular conditions such as wound healing, tumorogenesis, hypoxia, and chronic ischemia.2 It is a phenomenon intimately associated with endothelial cell (EC) proliferation, which appears to be under control of the extracellular receptor-activated kinase (ERK)/mitogen-activated protein kinase (MAPK)–mediated signaling cascades.2 The most important system regulating angiogenesis is the cytokine vascular endothelial growth factor (VEGF), although a number of other cytokines and hormones acting through various tyrosine kinase and G-protein–coupled receptors are also implicated in this process.

The adrenergic system is the major regulator of cardiac and vascular function, and evidence is mounting for the relevance of this system in the control of endothelial vasodilation through means of {alpha}2- and ß-adrenergic receptors (ßARs). In particular, ß2ARs, the most abundant ßARs in the vasculature,3,4 modulate the release of NO, causing endothelium-dependent vasodilation.5 ß2ARs are G-protein–coupled receptors activated by adrenergic catecholamines and promote a series of intracellular signal transduction pathways, leading to multiple cell-specific responses.6,7 Recently, it was proposed that ß2ARs modulate cell proliferation, at least in fibroblasts, by activating ERK/MAPK through pathways dependent on ß-arrestins.8 The physiological implications of such an in vitro observation have not yet been fully investigated in vivo.

Altogether, the above considerations provide the background for the current investigation into the role of ß2ARs in the control of angiogenesis. Accordingly, we evaluated whether ß2ARs on ECs could enhance ischemia-induced angiogenesis in vivo. Also, in ECs, we evaluated the in vitro effects of ß2AR overexpression on cell signaling and biology.


*    Materials and Methods
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*Materials and Methods
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In Vitro and In Vivo Procedures
Experimental procedures were performed as described previously.5,9–13 Extended details are provided in the online data supplement, available at http://circres.ahajournals.org.


*    Results
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*Results
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Effects of In Vivo ß2AR Gene Transfer to the Endothelium During Chronic Ischemia in Wistar Kyoto Rats
We studied neoangiogenesis in a well-characterized rat model of chronic hindlimb ischemia in the rat.14 Our control group consisted of pooled data collected from sham-treated and AdEmpty virus–treated rats because no differences were found between these two treatments (supplemental Table I). In this model, 15 days after resection of the femoral artery, we observed a reduction in ßAR density within the ischemic hindlimb, which was attributable to a selective downregulation of ß2ARs, the major ßAR subtype found in this tissue (Table). The numbers included in the Table reflect the sum of ßARs expressed on all cell types present in the hindlimb area, including, but not solely, ECs. Also, 15 days after femoral artery resection, there was impairment in limb perfusion assessed by digital angiography (Figure 1A; supplemental video 1). These hemodynamic changes were paralleled by the rarefaction in capillary (<5 µm) density in the anterior tibial muscle compared with the contralateral normal limb (Figure 1C and 1D). In a subset of rats, we injected the Adß2AR-wt into the femoral artery at the time of surgery. Consistent with previous results,15 this resulted in ß2AR overexpression in muscle, arterial capillaries, and venous (Figure 1E), which was double that of endogenous ßAR density found in the rat hindlimb (Table). After 15 days of ischemia, rats receiving Adß2AR-wt had significantly improved hindlimb perfusion with increased blood flow compared with untreated rats (Figure 1A and 1B; supplemental video 2). Similarly, histological analysis of the anterior tibial muscle revealed restored capillary density because of Adß2AR-wt treatment (Figure 1C and 1D). These positive benefits in vivo appear to be attributable to enhanced ß2AR signaling because delivery of Adß2AR-Ile164 (Figure 1A through 1D) resulted in no beneficial effects on hindlimb hemodynamic or histology. The Ile164ß2AR mutation is a naturally occurring human polymorphism that causes a severe impairment in the G-protein coupling.16–19


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Table 1. ßAR Density in Rat Hindlimbs



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Figure 1. Increased neoangiogenetic responses by ß2AR-wt gene transfer during chronic ischemia in vivo. A, TIMI frames count (FC). After 15 days of chronic ischemia, digital angiographies evidenced a reduced number of TIMI FCs in ischemic hindlimbs treated with Adß2AR-wt with respect to control rats (*P<0.05). Adß2AR-Ile164 did not modify the number of TIMI FCs respect to controls. SHR have impaired angiogenesis because TIMI FC was significantly higher compared with control WKY (§P<0.05). Also in SHR, Adß2AR-wt significantly improved ischemia (#P<0.05 vs controls). B, Dyed beads dilution assay, where Adß2AR-wt but not Adß2AR-Ile164 attenuated blood perfusion in ischemic hindlimb respect to controls. Shown is the ischemic to nonischemic ratio of dyed beads content per mg of hindlimb muscle tissue (*P<0.05). In SHR, there is reduced blood flow under control conditions compared with WKY rats (§P<0.05) and Adß2AR-wt treatment restores blood flow in the ischemic hindlimb (#P<0.05 vs control). C and D, Lectin BS-I staining of capillaries in the rat hindlimb. Magnification x40; bar=10 µm. Chronic ischemia in WKY produced a rarefaction on the capillary density of anterior tibial muscle evaluated as number of capillary corrected for number of muscle fibers (!P<0,05 vs nonischemic). Adß2AR-wt corrected capillary density respect to control (*P<0.05), whereas Adß2AR-Ile164 did not produce any significant effect. In SHR, capillary density is lower in nonischemic tissue compared with WKY (§P<0.05), and ischemia further reduces this score (*P<0.05 vs nonischemic). Adß2AR-wt enhances the capillary density in the ischemic district also in this rat strain (#P<0.05 vs control). E, Immunohistochemistry of overexpressed ß2AR using a specific antibody (see supplemental data). ß2ARs were visualized as a red reaction product. In ischemic muscle, the intra-arterial infusion of Adß2AR-wt produces an increase in ß2AR expression, which is most localized in the perivascular spaces (arrows). F, Systemic levels of VEGF on serum. Systemic VEGF-165 was used as marker of ischemia and assessed on serum by ELISA. Fifteen days after femoral artery resection, serum VEGF-165 levels were significantly lower in the Adß2AR-wt–treated rats (*P<0.05) vs controls. G, Systemic levels of VEGF on nonischemic contralateral muscle. We evaluated VEGF levels on contralateral hindlimbs by WB (inset) as an indicator of systemic VEGF. Using muscle of rats that were not subjected to femoral artery resection as non ischemic reference (no Ischemia), we found that ischemia (control) caused an increase in systemic levels of VEFG in WKY (!P<0.02 vs nonischemic) and in SHR (§P<0.05 vs nonischemic). In WKY rats, Adß2AR-wt but not Adß2AR-Ile164 delivery leads to a limitation of the ischemic insult and consequently to a reduction of systemic levels of VEGF after 15 days (*P<0.05 vs control). In SHR, Adß2AR-wt also reduced VEGF levels, indicating an ameliorated ischemic hindlimb perfusion (#P<0.05 vs control; ADU indicates arbitrary densitometry units).

Chronic ischemia leads to increased systemic VEGF levels, a potent proangiogenesis cytokine, that typically return to baseline values once the ischemia is eliminated.20,21 We examined VEGF levels in our model in the blood and in the nonischemic contralateral hindlimb. In both samples, VEGF-165 levels were significantly lower in ischemic rats treated with Adß2AR-wt compared with control and Adß2AR-Ile164 rats, suggesting that ß2AR overexpression significantly lessens ischemia (Figure 1F and 1G). Overall, these results suggest that ß2ARs can regulate key endothelial cellular functions in vivo in response to chronic ischemia.

Impaired Angiogenesis Ameliorated by In Vivo ß2AR Gene Transfer to the Endothelium During Chronic Ischemia in Spontaneously Hypertensive Rats
To further explore the role of endothelial ß2AR in pathophysiological conditions, we repeated the in vivo experiment using the angiogenesis-impaired spontaneously hypertensive rat (SHR).22 Previously in this model, we have shown that reduced vascular and endothelial ßAR signaling and vasodilation is corrected by ß2AR gene transfer.5 Fifteen days after resection of the femoral artery in SHR, there was a dramatic impairment of hindlimb perfusion, evidenced by the elevated occurrence of blistering, necrosis, or self-inflicted amputation of the ischemic paw (occurrence of necrosis, amputation, and blistering: SHR 35%; Wistar Kyoto [WKY] 0%; P<0.05; {chi}2 test). This loss of perfusion was confirmed by digital angiography (Figure 1A), dyed microsphere dilution (Figure 1B), and by histological analysis (Figure 1D). In SHR treated with the ADß2AR-wt, we observed a partial correction of this phenotype indicated by the reduced occurrence of blistering (10%; P<0.05; {chi}2 test) and the ameliorated hemodynamic and histological parameters (Figure 1A, 1B, and 1D).

Finally, the analysis by Western blot of VEGF performed on the contralateral nonischemic hindlimb showed that systemic levels of this cytokine were reduced in the Adß2AR-wt SHR compared with the nontreated group, suggesting that local ischemia was resolved (Figure 1G).

Effects of ß2AR Signaling on EC Proliferation
Our in vivo results do not let us discern whether the protective effects of ß2AR overexpression in the rat hindlimb arterial endothelium after ischemic damage are attributable to an effect on cell proliferation or cell survival. These mechanistic questions were addressed in vitro using ECs in culture. The overall angiogenic properties of ß2AR were recapitulated under conditions in which stimulated human ECs organize in network forming tubules. The ßAR agonist isoproterenol (ISO) increased the number of connections among endothelial tubules in vitro, and this response was magnified by Adß2AR-wt but not Adß2AR-Ile164 (Figure 2A). Importantly, ISO-mediated tubule formation was similar to that induced by VEGF, used as positive control.



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Figure 2. In vitro effects of ß2AR stimulation on EC biology. A, Angiogenic properties of ß2ARs. Human ECs were exposed to the Adß2AR-wt or the Adß2AR-Ile164 and cocultured with other human cells in a specially designed medium in a 24-well plate. Every three days, ISO (10–8 mol/L) was added to the cultures. On the eleventh day, cells were visualized by staining for anti-human CD31 (platelet-endothelial cell adhesion molecule-1). In this setting, cells tend to form a tubule-like shape and organize in a network. Images were digitalized and scored with an image analysis software to count the number of cellular connections and the total tubule length. In control cells, ISO stimulation produces an increased number of connections, and this response was enhanced by Adß2AR-wt but not Adß2AR-Ile164. VEGF-165 (100 nmol/L) was used as positive control (*P<0.05 vs basal; §P<0.05 vs the respective condition in control; n=2 in duplicate). B, Proliferative response in subconfluent BAECs. Adß2AR-wt but not Adß2AR-Ile164 produced a biphasic response, which was initially proliferative with a maximal effect at 24 hours, and then cell availability reduced at 36 and 48 hours (*P<0.05 vs control). ISO (10–8 mol/L) caused an increase of cells number, followed by a drop in cell number observed at 36 and 48 hours. The ß2AR antagonist ICI (10–6 mol/L) prevented the changes in cell number induced by ISO (§P<0.02 vs control), whereas Adß2AR-wt but not Adß2AR-Ile164 enhanced ISO response (*P<0.05 vs control; n=3 to 5 in triplicate). C, DNA synthesis by [3H]-thymidine incorporation. The activation of ßAR by ISO (10–8 mol/L) caused increased DNA synthesis. The same effect was obtained in Adß2AR-wt but not Adß2AR-Ile164. The maximal response was observed in ISO-stimulated Adß2AR-wt cells (*P<0.05 vs basal; #P<0.05 vs control; n=3 to 5 in duplicate). D, Progression in cell cycle assessed by Rb phosphorylation. This protein regulates cell cycle progression through the restriction point within the G1 phase. By Western blot at 12 and 24 hours of stimulation (inset), ISO (10–8 mol/L), caused Rb phosphorylation, antagonized by the ß2AR antagonist ICI (10–6 mol/L; ADU indicates arbitrary densitometry units; *P<0.05 vs control; #P<0.05 vs ISO; n=3 in duplicate) E, ß2AR overexpression in BAECs. To test the level of overexpression induced by Adß2AR-wt and Adß2AR-Ile164 in BAECs, we performed a Western blot of ß2AR on membrane extracts (inset). Densitometric analysis (bar graph) showed a similar 3-fold increase of the ß2AR density induced by both adenoviruses (*P<0.05 vs control; n=3 in triplicate). F, Role of ERK/MAPK activation and VEGF production on ISO (10–8 mol/L) induced cell proliferation. The MAPK kinase inhibitor UO126 (10–6 mol/L), the antibody to VEGF (0.2 µg/mL) as well as the VEGF receptor inhibitor ZD6474 (10–6 mol/L), attenuated to a similar extent the proliferation induced by ISO (10–8 mol/L). Also, the cAMP antagonist Rp-cAMP (10–5 mol/L) similarly prevented ISO proliferation (*P<0.05 vs control; ANOVA; n=3 to 5 experiments, repeated in triplicate). G, ERK/MAPK activation. Western blot of activated (phosphorylated; p-ERK) ERK1/2 after ISO treatment. Equal amount of proteins were confirmed via blotting for total ERK/MAPK. Representative blots are presented in the inset. Densitometric analysis (bar graph) show that ßAR stimulation caused ERK/MAPK activation with a maximal effect at 10–7 mol/L ISO. Adß2AR-wt treatment induced ERK activation the ISO (10–7 mol/L) response was enhanced (*P<0.05 vs control; #P<0.05 vs basal; n=3 in duplicate). H, VEGF production in BAECs measured by Western blot (inset) of VEGF-165. Shown are VEGF-165 levels after 6 hours of serum starvation. Equal amount of proteins were verified by blotting for actin. Adß2AR-wt and ISO (10–8 mol/L) increased VEGF-165 production with a maximal effect in presence of both conditions (*P<0.05 vs control; #P<0.05 vs basal; n=3 in duplicate).

Because angiogenesis is intimately associated to EC proliferation, we explored whether stimulation of ß2ARs in ECs can increase cell number. In subconfluent bovine aorta ECs (BAECs), the addition of ISO caused a biphasic response with an initial increase, followed by a drop in cell number after 36 hours (Figure 2B). Loss of cell viability is probably related to multiple effects of ISO, which are dependent on time and dosage, because it can be observed after 24 hours, when using higher doses of ISO (>10–8 mol/L; supplemental Figure I). The response to ßAR stimulation is largely dependent on endogenous ß2AR because cell proliferation can be prevented by the selective ß2AR inhibitor ICI 118 551 (ICI; Figure 2B). Consistent with an effect on cell proliferation, ßAR stimulation by ISO resulted in an increase in [3H]-thymidine incorporation (Figure 2C), an index of DNA synthesis, and phosphorylation of retinoblastoma (Rb) protein, which removes the inhibition of E2F in the nucleus causing the cell cycle progression from G1 to S23. This response in BAEC was attributable to ß2AR signaling because it was attenuated by the ß2AR antagonist ICI (Figure 2D).

For in vitro analysis of the effects of added wild-type or mutant (Ile164) ß2AR density on EC function, adenoviral-mediated ß2AR overexpression was obtained (Figure 2E), causing an increase in total ßAR density (from 29.7±0.9 to 55.5±1.4 fmol/mg and 55.2±1.4 fmol/mg protein in Adß2AR-wt and Adß2AR-Ile164, respectively). Overexpression of the wild-type but not the Ile164 mutant ß2AR resulted in increased cell proliferation and DNA synthesis as measured by [3H]-thymidine incorporation, and ISO further enhanced these mitogenic responses (Figure 2B and 2C). The need of an intact ß2AR signaling through the cAMP second messenger to achieve cell proliferation was verified by the antagonist Rp-cAMP because the presence of this inhibitor attenuated the proliferative response after 24 hours of ISO treatment (Figure 2F).

Angiogenesis is largely dependent on ERK/MAPK activation24–26 because it can promote EC proliferation and expression of VEGF, which, in turn, sustains the proangiogenic phenotype.27 Using BAECs, we found that ISO leads to significant ERK activation (Figure 2G) and VEGF production (Figure 2H). Moreover, overexpression of the wild-type ß2AR enhanced ERK/MAPK activation (Figure 2G). The ß2AR-Ile164 failed to induce the potentiation of ERK/MAPK observed with the ß2AR-wt (supplemental Figure II). To further assess the role of ß2AR-mediated ERK/MAPK activation in BAEC proliferation, we used an inhibitor of ERK/MAPK activation, U0126 (10–6 mol/L), and observed reduced BAEC proliferation in response to ISO (Figure 2F). Because ß2ARs can stimulate VEGF production, we evaluated whether this cytokine is responsible for the proliferative effects of ECs after ISO. VEGF signaling was inhibited by using either an antibody to VEGF (200 ng/mL) or an inhibitor of the VEGF receptor. In both cases, BAEC proliferation in response to ISO was reduced (Figure 2F). These results suggest that ß2AR-mediated EC proliferation is dependent, at least in part, on VEGF production and release.

Effects of ß2AR Signaling on EC Apoptosis
Our cell proliferation data demonstrate that chronic exposure to ISO causes a loss in cell viability after 36 hours (Figure 2B). This is probably initiated by serum deprivation but is clearly further sustained by chronic ßAR activation. EC number may also be the result of cellular apoptosis under the control of ßARs. Indeed, a previous report has shown that the ß2AR in neonatal cardiac myocytes produces proapoptotic and antiapoptotic effects, and this signaling involves the p38 MAPK and PI3-kinase/Akt pathways.28 We have recently shown that in endothelium, ß2ARs activate Akt.29 Other reports have documented the ability of ßARs to stimulate p38 MAPK,28,30 which is considered an important mechanism of receptor-mediated apoptosis.31,32 Using BAECs, we first determined whether ß2AR stimulation could result in Akt or p38 MAPK activation, and indeed, this was found with endogenous ßARs as well as overexpressed ß2ARs (Figure 3A and 3B). Next, we examined apoptosis using caspase-3 cleavage as a marker. Serum starvation and ISO reproducibly activated caspase-3 in BAECs, and both responses were significantly greater in ECs overexpressing the wild-type ß2AR (Figure 3C). The involvement of p38 MAPK is suggested by the negative regulation of the p38/MAPK inhibitor SB203580 on ß2AR-mediated caspase-3 cleavage (Figure 3D). Overall, these apoptosis data correlate with the observed reduction in cell number at 36 and 48 hours observed in cell proliferation experiments (Figure 2B).



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Figure 3. ß2AR regulation of apoptosis in BAEC. A, Akt activation assessed by Western blotting (p-Akt). Blots were corrected by total Akt (insets). ISO (10–7 mol/L) induced Akt activation with a maximal effect in Adß2AR-wt cells (ADU indicates arbitrary densitometry units; *P<0.05 vs control; #P<0.05 vs basal; n=3 in duplicate). B, p38/MAPK activation determined by Western blot of phosphorylated (activated) p38/MAPK using cell lysates of BAECs and corrected for blotting of total p38 MAPK. ISO (10–7 mol/L) induced p38 activation with a maximal effect in Adß2AR-wt cells (*P<0.05 vs control; #P<0.05 vs basal; n=3 in duplicate). C, Caspase-3 cleavage assessed as a marker of cell apoptosis. Densitometric analysis of Western blots for cleaved caspase-3 was corrected by actin blotting. ISO (10–8 mol/L) and Adß2AR-wt induced caspase-3 cleavage. The presence of both conditions produced a maximal effect (*P<0.05 vs control; #P<0.05 vs basal; n=3 in triplicate). D, Effects of Akt inhibition on caspase-3 cleavage. Wortmannin (10–5 mol/L), an inhibitor of Akt/PKB activation, enhanced caspase-3 cleavage in response to serum starvation and ISO. The maximal activation of caspase-3 was observed in Adß2AR-wt either with or without ISO (*P<0.05 vs control; #P<0.05 vs base; n=3 in triplicate). A similar result was observed using another inhibitor of Akt activation: LY 294002 (supplemental Figure III). E, Effects of p38 MAPK inhibition on caspase 3 cleavage. SB 203580 (10–5 mol/L) attenuated caspase-3 cleavage in a similar manner in control and Adß2AR-wt cells, with and without ISO (10–8 mol/L; n=3 in triplicate).

Importantly, we also examined whether ß2ARs have any effect on prosurvival pathways mediated by PI3-kinase/Akt. We repeated the above apoptosis assays in the presence of inhibitors of Akt activation and found that caspase-3 cleavage was significantly enhanced after ISO in Adß2AR-wt and control cells (Figure 3D; supplemental Figure III). Overall, our data show that ß2ARs exert a dualistic effect on cell survival, with activation and inhibition of apoptosis through means of p38 MAPK and PI3-kinase/Akt, respectively.


*    Discussion
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*Discussion
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We verified that the ß2AR is an endogenous mediator of angiogenesis in vivo, using a rat model of neovascularization and adrenergic activation attributable to hindlimb ischemia.14 The observed downregulation ß2AR density suggests a role for this receptor in the compensatory response to chronic ischemia and adrenergic activation. Adenoviral-mediated overexpression of wild-type ß2AR in vivo improves neoangiogenesis activity documented by digital imaging and perfusion techniques. Moreover, histological analysis showed that ß2AR signaling preserves and enhances the number of capillaries in the ischemic area. The implications of this phenomenon were further tested in SHR, which experience impaired ßAR signaling and angiogenesis. In this model, the removal of the femoral artery causes severe perfusion deficit with necrosis and self-inflicted amputations of the ischemic limb. These alterations were all attenuated after intrafemoral artery delivery of the Adß2AR-wt. Mechanistic studies show that ß2ARs control EC proliferation as well as stimulating proapoptotic and antiapoptotic pathways. These novel effects of ß2ARs in ECs are in addition to their recently discovered role on NO release and regulation of vascular tone.5,29 ßAR stimulation activates proliferative and cell death and survival pathways, but the ß2AR-mediated EC proliferative mechanism appears to be relevant for therapeutic angiogenesis.

It is noteworthy that ß2ARs can induce cell proliferation and promote cell survival in other tissues; in particular, Kim et al recently described that ß2ARs expressed on cardiac fibroblasts can induce cell proliferation through ERK/MAPK-dependent mechanisms.8 Our current data demonstrate that this signaling paradigm also exists in ECs because ß2AR stimulation increased EC number, and this cellular proliferative effect is blocked by ERK/MAPK inhibition. Although we did not investigate the whole signal transduction involved in the proangiogenic phenotype induced by ß2AR in ECs, our data are suggestive that cAMP production is important in this response.

ß2ARs exert a positive effect on EC ERK/MAPK activation by at least two mechanisms. First, stimulation of endothelial ßARs and wild-type ß2AR overexpression directly activate ERK/MAPK. Second, ß2AR stimulation can induce the release of VEGF, which can also activate ERK/MAPK.26 ERK/MAPK inhibition totally prevents EC proliferation, demonstrating that this kinase is critical for ßAR-mediated cell mitogenesis and proliferation. The ability of ßARs to induce VEGF production and release was reported previously for ß1ARs and ß3ARs in adipose tissue.33 Here, we show that this effect can also be demonstrated for the ß2ARs found on ECs. Our in vitro results may appear in contrast with the in vivo observation of lower systemic VEGF in the Adß2AR-wt treated rats. On the contrary, both results testify the facilitation of angiogenesis mediated by an early boost of VEGF production that chronically causes a faster resolution of ischemia with the consequent return of systemic VEGF toward lower levels.

Concerning cell survival, Morisco et al have also shown in neonatal cardiomyocytes that ß2AR signaling can prevent apoptosis through Akt activation.34 Zhu et al have confirmed this observation in adult cardiomyocytes.28 In ECs, too, ß2AR activation leads to Akt-mediated apoptosis protection. Furthermore, in ECs, ß2ARs can also stimulate apoptosis through a p38/MAPK mechanism. The net effect of proapoptotic and antiapoptotic signaling results in an eventual loss of cell number when ßARs are chronically activated, whereas in the short term, there is an increase in cell number.

The effects of ß2AR signaling on EC biology are clearly dependent on the overall integrity of the total signal transduction pathway. Indeed, overexpression of the naturally occurring Ile164 ß2AR mutant failed to induce the enhancement of neoangiogenesis observed in vivo in the ischemic hindlimb. This mutant receptor is largely nonfunctional and poorly activates downstream G-protein–mediated signaling events. In transgenic mice with cardiac overexpression (&45-fold over total ßAR density) of this receptor, the changes in cardiac contractility induced by ßAR stimulation were not different from those of nontransgenic littermate.17 Furthermore, in patients harboring this polymorphism in heterozygosity, ß2AR-dependent responses to exercise (cardiac index, stroke volume, systemic vascular resistance) were all reduced by ≥50%.19 In vitro, the EC50 of the mutant receptor to epinephrine is 5x higher than that of the wild type.16 Our results add to these data, showing that its overexpression did not alter the angiogenic response of ischemic hindlimb, nor in vitro EC proliferation, thymidine incorporation, and apoptosis. These results work also as a further control, ruling out the hypothesis of a "mass effect" because of overexpression-induced transactivation of non-ß2AR via heterodimerization.35 Our results imply that patients carrying this polymorphism may have an impairment of angiogenic response, in particular, in those conditions that are associated to sympathetic activation, such as chronic ischemia or cardiac dysfunction. Indeed, Liggett et al showed reduced survival in heart failure patients harboring the Ile164 mutant of the ß2AR gene,36 thus suggesting a more profound inadequacy of the adaptive responses of these patients to the challenged hemodynamic condition. Our data provide the ground to interpret these findings as the result of the impaired angiogenesis associated to this polymorphism, leading to a reduced myocardial perfusion in a situation in which angiogenesis largely determines cardiac function and growth.37

The sympathetic nervous system regulates in vivo blood vessel growth, although this process was believed to be mediated by {alpha}ARs and limited to vascular smooth muscle cell proliferation.38 Recently, another sympathetic neurotransmitter, the neuropeptide Y, has been demonstrated to be a potent angiogenic factor in vivo through means of the endothelial Y2 receptors.39 It is therefore emerging that the sympathetic nervous system is an important determinant of neoangiogenesis, acting through means of catecholamines and neuropeptides. As further support to this notion, mice lacking the ability to produce catecholamines are not able to fully develop during fetal life.40

In conclusion, we provide evidence that ß2ARs are involved in the control of EC biology with implications in neoangiogenesis in response to ischemia. This is a novel finding carrying critical relevance in the current and future treatment of chronic ischemia. Therapeutic angiogenesis is the ultimate rescue of ischemic tissue, pursued using soluble growth factors such as VEGF, with the limitation of the potential facilitation of tumor genesis or growth. Our data suggest that therapeutic angiogenesis in ischemic tissue might be more efficiently and selectively achieved when associated to activation of ß2AR signaling by means of available classical drugs or novel molecular tools such as adenoviral-mediated ß2AR gene transfer.


*    Acknowledgments
 
This work was supported by grants from Italian Ministry of University and Research (B.T., G.I.), Fondazione Telethon Grant n. GGP04039 (L.P.) and National Institutes of Health grant R01 HL65360 (W.J.K.). The authors are indebted to Alberto Frezza and Giuseppe Rusciano for their valuable technical support.


*    Footnotes
 
Original received December 13, 2004; resubmission received June 16, 2005; accepted October 10, 2005.


*    References
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up arrowAbstract
up arrowIntroduction
up arrowMaterials and Methods
up arrowResults
up arrowDiscussion
*References
 
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