Role of Cbl in shear-activation of PI 3-kinase and JNK in endothelial cells

Relationship between hemodynamic alteration and sympathetic nerve activation following a single oral dose of cinnamtannin A2

We previously found that a single dose of B-type procyanidin mixture increase in skeletal muscle blood flow (BF). We compared BF changes following administration of (-)-epicatechin (EC, monomer) and the B-type procyanidins procyanidin B2 (B2, dimer), procyanidin C1 (C1, trimer), and cinnamtannin A2 (A2, tetramer). Each chemical was administered orally to rats, followed by BF measurement in cremaster arteriole for 60 min. About 10 and 100 µg/kg of B2 and C1 elicited BF increase, the effect was potent at 100 µg/kg. BF also increased significantly after administration of 10 µg/kg A2, but not with the administration at 100 µg/kg. EC yielded no BF changes. Co-treatment with the nonselective adrenaline blocker carvedilol attenuated the BF increase seen with 10 µg/kg A2 treatment. This outcome suggested the involvement of sympathetic nerve activation in the BF increase by this dose of A2. Co-treatment of 100 µg/kg A2 with the α2 blocker yohimbine exhibited an increase of BF significantly. The α2 adrenaline receptor in the vasomotor centre is an inhibitory receptor and it regulates hemodynamics. This result suggested that high doses of A2 did not alter BF because of activating the α2 adrenergic receptor. Phosphorylation of aortic endothelial nitric oxide synthase (eNOS) increased with 10 µg/kg A2 alone or co-treatment with 100 µg/kg A2 and yohimbine, but not with co-treatment of 10 µg/kg A2 and carvedilol or 100 µg/kg A2 alone. These results imply that A2 does not directly activate eNOS, but that shear stress from the increased BF might be associated with eNOS phosphorylation.

Engineering Porous β-Tricalcium Phosphate (β-TCP) Scaffolds with Multiple Channels to Promote Cell Migration, Proliferation, and Angiogenesis

Inadequate oxygen and nutrient diffusion in a porous scaffold often resulted in insufficient formation of branched vasculatures, which hindered bone regeneration. In this study, interconnected porous β-tricalcium phosphate (β-TCP) scaffolds with different geometric designs of channels were fabricated and compared to discover the functionality of structure on facilitating nutrient diffusion for angiogenesis. In vitro fluid transportation and degradation of the scaffolds were performed. Cell infiltration, migration, and proliferation of human umbilical vein endothelial cells (HUVECs) on the scaffolds were carried out under both static and dynamic culture conditions. A computational simulation model and a series of immunofluorescent staining were implemented to understand the mechanism of cell behavior in response to different types of scaffolds. Results showed that geometry with multiple channels significantly accelerated the release of Ca²⁺ and increased the fluid diffusion efficiency. Moreover, multiple channels promoted HUVECs' infiltration and migration in vitro. The ex vivo implantation results showed that the channels promoted cells from the rats' calvarial bone explants to infiltrate into the implanted scaffold. Multiple channels also stimulated HUVECs' proliferation prominently at both static and dynamic culturing conditions. The expression of both cell migration-related protein α5 and angiogenesis-related protein CD31 on multiple-channeled scaffolds was upregulated compared to that on the other two types of scaffolds, implying that multiple channels reinforced cell migration and angiogenesis. All the findings suggested that the geometric design of multiple channels in the porous β-TCP scaffold has promising potential to promote cell infiltration, migration, and further vascularization when implanted in vivo.

Acylated anthocyanins derived from purple carrot (Daucus carota L.) induce elevation of blood flow in rat cremaster arteriole

Acylated anthocyanins are more stable than monomeric anthocyanins, but little is known about their physiological effects.

Onset of a hypotensive effect following ingestion of flavan 3-ols involved in the activation of adrenergic receptors

A lot of epidemiological and intervention studies support the hypotensive action resulting from ingestion of foods rich in flavan 3-ols. However, the mechanisms of this action remain unclear. We have reported previously on the alteration of the micro- and systemic circulations after administration of a flavan 3-ol fraction (FL) derived from cocoa in mammals. We also confirmed that blood catecholamine levels increase significantly after administration of FL. In the present study, we examined whether adrenaline receptors are involved in the hemodynamic changes using several adrenaline receptor (AR) blockers. First, we confirmed that mean blood pressure (MBP) decreased significantly and aortic endothelial nitric oxide synthase (eNOS) levels increased significantly following oral treatment of 10mg/kg FL for 2 weeks in normal rats compared with vehicle administration. However, these changes were not observed with treatment of 1mg/kg (-)-epicatechin (EC), which contains nearly equivalent amount of 10mg/kg FL. Secondly, we observed that a single dose of FL produced different hemodynamic changes, such as a transient elevation in heart rate (HR) after ingestion of 1-100mg/kg FL, but not with 1mg/kg EC. Furthermore, although MBP rose transiently after 1 and 10mg/kg FL, this effect was not observed with 100mg/kg or 1mg/kg EC. The increases in HR, MBP, and aortic phosphorylated eNOS (p-eNOS) induced by 10mg/kg FL were prevented completely by pretreatment with the AR blocker, carvedilol. Combination treatment with 100mg/kg FL and an α1AR blocker, prazosin, significantly reduced MBP, whereas the elevation in HR was enhanced. In addition, after pretreatment with the β2AR blocker, butoxamine, we observed no significant hemodynamic changes with or without 100mg/kg FL. Moreover, the combination of 100mg/kg FL and the α2AR blocker, yohimbine, markedly increased MBP, HR and aortic p-eNOS level. These results suggested that the postprandial hemodynamic changes after a single oral dose of FL were induced by an adrenergic effect. This adrenomimetic activity suggested the involvement of a hypotensive effect of FL.

The role of focal adhesion kinase in the regulation of cellular mechanical properties

The regulation of mechanical properties is necessary for cell invasion into connective tissue or intra- and extravasation through the endothelium of blood or lymph vessels. Cell invasion is important for the regulation of many healthy processes such as immune response reactions and wound healing. In addition, cell invasion plays a role in disease-related processes such as tumor metastasis and autoimmune responses. Until now the role of focal adhesion kinase (FAK) in regulating mechanical properties of cells and its impact on cell invasion efficiency is still not well known. Thus, this review focuses on mechanical properties regulated by FAK in comparison to the mechano-regulating protein vinculin. Moreover, it points out the connection between cancer cell invasion and metastasis and FAK by showing that FAK regulates cellular mechanical properties required for cellular motility. Furthermore, it sheds light on the indirect interaction of FAK with vinculin by binding to paxillin, which then impairs the binding of paxillin to vinculin. In addition, this review emphasizes whether FAK fulfills regulatory functions similar to vinculin. In particular, it discusses the differences and the similarities between FAK and vinculin in regulating the biomechanical properties of cells. Finally, this paper highlights that both focal adhesion proteins, vinculin and FAK, synergize their functions to regulate the mechanical properties of cells such as stiffness and contractile forces. Subsequently, these mechanical properties determine cellular invasiveness into tissues and provide a source sink for future drug developments to inhibit excessive cell invasion and hence, metastases formation.

Common genetic denominators for Ca++-based skeleton in Metazoa: role of osteoclast-stimulating factor and of carbonic anhydrase in a calcareous sponge

Calcium-based matrices serve predominantly as inorganic, hard skeletal systems in Metazoa from calcareous sponges [phylum Porifera; class Calcarea] to proto- and deuterostomian multicellular animals. The calcareous sponges form their skeletal elements, the spicules, from amorphous calcium carbonate (ACC). Treatment of spicules from Sycon raphanus with sodium hypochlorite (NaOCl) results in the disintegration of the ACC in those skeletal elements. Until now a distinct protein/enzyme involved in ACC metabolism could not been identified in those animals. We applied the technique of phage display combinatorial libraries to identify oligopeptides that bind to NaOCl-treated spicules: those oligopeptides allowed us to detect proteins that bind to those spicules. Two molecules have been identified, the (putative) enzyme carbonic anhydrase and the (putative) osteoclast-stimulating factor (OSTF), that are involved in the catabolism of ACC. The complete cDNAs were isolated and the recombinant proteins were prepared to raise antibodies. In turn, immunofluorescence staining of tissue slices and qPCR analyses have been performed. The data show that sponges, cultivated under standard condition (10 mM CaCl₂) show low levels of transcripts/proteins for carbonic anhydrase or OSTF, compared to those animals that had been cultivated under Ca²⁺-depletion condition (1 mM CaCl₂). Our data identify with the carbonic anhydrase and the OSTF the first two molecules which remain conserved in cells, potentially involved in Ca-based skeletal dissolution, from sponges (sclerocytes) to human (osteoclast).

Role of shear stress and stretch in vascular mechanobiology

Blood vessels are under constant mechanical loading from blood pressure and flow which cause internal stresses (endothelial shear stress and circumferential wall stress, respectively). The mechanical forces not only cause morphological changes of endothelium and blood vessel wall, but also trigger biochemical and biological events. There is considerable evidence that physiologic stresses and strains (stretch) exert vasoprotective roles via nitric oxide and provide a homeostatic oxidative balance. A perturbation of tissue stresses and strains can disturb biochemical homeostasis and lead to vascular remodelling and possible dysfunction (e.g. altered vasorelaxation, tone, stiffness, etc.). These distinct biological endpoints are caused by some common biochemical pathways. The focus of this brief review is to point out some possible commonalities in the molecular pathways in response to endothelial shear stress and circumferential wall stretch.

Shear Stress Regulates the Flk-1/Cbl/PI3K/NF-κB Pathway Via Actin and Tyrosine Kinases

Vascular endothelial cells (ECs) are continuously exposed to mechanical stimuli (e.g., shear stress). Our previous study has shown that the shear-induced nuclear factor-κB (NF-κB) activation is mediated by integrins [Bhullar, I. S., Y. S. Li, H. Miao, E. Zandi, M. Kim, et al. J. Biol. Chem. 273:30544-30549, 1998]. The shear-activated integrins can also transactivate Flk-1 (a receptor for vascular endothelial growth factor (VEGF)) [Wang, Y., H. Miao, S. Li, K. D. Chen, Y. S. Li, et al. Am. J. Physiol. Cell Physiol. 283:C1540-C1547, 2002], which subsequently recruits Casitas B-lineage lymphoma (Cbl) to regulate inhibitor of κB protein kinase (IKK) [Wang, Y., J. Chang, Y. C. Li, Y. S. Li, J. Y. Shyy, and S. Chien. Am. J. Physiol. Heart Circ. Physiol. 286:H685-H692, 2004], an upstream molecule of NF-κB. Therefore, shear stress may likely utilize the Flk-1/Cbl pathway in regulating NF-κB. In this paper, we confirmed that the inhibition of Flk-1 by its specific inhibitor SU1498 blocked the shear-induced NF-κB translocation. The inhibition of Cbl (an adaptor protein which binds to Flk-1 upon shear) by using a negative mutant (Cbl(nm)) also blocked the promoter activity of NF-κB, and the inhibition of the Cbl-downstream molecule phosphatidylinositol-3-kinase (PI3K) abolished the NF-κB translocation. Further experiments revealed that the disruption of actin cytoskeleton inhibited the Flk-1 and Cbl interaction and NF-κB translocation. The inhibition of focal adhesion kinase (FAK) and Src family kinases, which are involved in the integrin-mediated focal adhesion complex, also blocked the shear-induced NF-κB translocation. Together with our previous findings that integrins mediate the shear-induced activation of Flk-1 and NF-κB [Bhullar, I. S., Y. S. Li, H. Miao, E. Zandi, M. Kim, et al. J. Biol. Chem. 273:30544-30549, 1998; Wang, Y., H. Miao, S. Li, K. D. Chen, Y. S. Li, et al. Am. J. Physiol. Cell Physiol. 283:C1540-C1547, 2002], the present results suggest that Flk-1, Cbl, and PI3K act upstream to NF-κB in response to shear stress. This Flk-1/Cbl/PI3K/NF-κB signaling pathway may be originated from integrins and transmitted by key tyrosine kinases and actin cytoskeleton. These results shed new lights on the molecular mechanism by which mechanical shear stress activates the NF-κB signaling pathway, which is critical for vascular inflammatory responses and atherosclerosis.

Effect of shear stress on vascular inflammation and plaque development

PURPOSE OF REVIEW

This review describes evidence that shear stress acts through modulation of inflammation and by that process affects atherogenesis and plaque composition.

RECENT FINDINGS

In low shear stress regions antiatherogenic transcription factors are downregulated and pro-atherogenic transcription factors are upregulated. Consequently, inflammatory cells may home low shear stress regions more easily to the plaques because of increased expression of adhesion factors, a decreased rolling speed and an increased expression of chemokines, thereby changing the composition of the plaques into a more vulnerable phenotype. In contrast, in advanced plaque development vascular lumen decreases and shear stress increases, especially upstream of the plaques. The predominant upstream location of lipids induces a prevalent upstream location of inflammatory cells leading to localized plaque rupture.

SUMMARY

Shear stress has been shown to play a role in plaque induction, plaque progression and plaque rupture. The mechanism for plaque induction seems to differ from the role of shear stress for plaque rupture, whereby the former mechanism is induced by low shear stress and the latter by high shear stress.

Epigallocatechin Gallate, a Green Tea Polyphenol, Mediates NO-dependent Vasodilation Using Signaling Pathways in Vascular Endothelium Requiring Reactive Oxygen Species and Fyn

Green tea consumption is associated with reduced cardiovascular mortality in some epidemiological studies. Epigallocatechin gallate (EGCG), a bioactive polyphenol in green tea, mimics metabolic actions of insulin to inhibit gluconeogenesis in hepatocytes. Because signaling pathways regulating metabolic and vasodilator actions of insulin are shared in common, we hypothesized that EGCG may also have vasodilator actions to stimulate production of nitric oxide (NO) from endothelial cells. Acute intra-arterial administration of EGCG to mesenteric vascular beds isolated ex vivo from WKY rats caused dose-dependent vasorelaxation. This was inhibitable by L-NAME (NO synthase inhibitor), wortmannin (phosphatidylinositol 3-kinase inhibitor), or PP2 (Src family kinase inhibitor). Treatment of bovine aortic endothelial cells (BAEC) with EGCG (50 microm) acutely stimulated production of NO (assessed with NO-specific fluorescent dye DAF-2) that was inhibitable by l-NAME, wortmannin, or PP2. Stimulation of BAEC with EGCG also resulted in dose- and time-dependent phosphorylation of eNOS that was inhibitable by wortmannin or PP2 (but not by MEK inhibitor PD98059). Specific knockdown of Fyn (but not Src) with small interfering RNA inhibited both EGCG-stimulated phosphorylation of Akt and eNOS as well as production of NO in BAEC. Treatment of BAEC with EGCG generated intracellular H(2)O(2) (assessed with H(2)O(2)-specific fluorescent dye CM-H(2)DCF-DA), whereas treatment with N-acetylcysteine inhibited EGCG-stimulated phosphorylation of Fyn, Akt, and eNOS. We conclude that EGCG has endothelial-dependent vasodilator actions mediated by intracellular signaling pathways requiring reactive oxygen species and Fyn that lead to activation of phosphatidylinositol 3-kinase, Akt, and eNOS. This mechanism may explain, in part, beneficial vascular and metabolic health effects of green tea consumption.

FAK association with multiple signal proteins mediates pressure-induced colon cancer cell adhesion via a Src-dependent PI3K/Akt pathway

Cancer cell adhesion is traditionally viewed as random, occurring if the cell's receptors match the substrate. Cancer cells are subjected to pressure and shear during growth against a constraining stroma, surgical manipulation, and passage through the venous and lymphatic system. Cells shed into a cavity such as the abdomen postoperatively also experience increased pressure from postoperative edema. Increased extracellular pressure stimulates integrin-mediated cancer cell adhesion via FAK and Src. PI 3-kinase (PI3K) inhibitors (LY294002 or wortmannin), Akt inhibitors, or Akt1 siRNA blocked adhesion stimulated by 15 mmHg pressure in SW620 or primary human malignant colonocytes. Pressure activated PI3K, tyrosine-phosphorylated and membrane-translocated the p85 subunit, and phosphorylated Akt. PI3K inhibitor (LY294002) prevented pressure-stimulated Akt Ser473 and FAK Tyr397, but not FAK576 or Src416 phosphorylation. PP2 inhibited PI3K activity and Akt phosphorylation. FAK siRNA did not affect pressure-induced PI3K activation but blocked Akt phosphorylation. Pressure also stimulated FAK or FAKY397F mutant translocation to the membrane. Akt inhibitor IV blocked pressure-induced Akt and FAK translocation. Pressure activated Src- and PI3K-dependently induced p85 interaction with FAK, and FAK with beta1 integrin. These results delineate a novel force-activated inside-out Src/PI3K/FAK/Akt pathway by which cancer cells regulate their own adhesion. These signals may be potential targets for inhibition of metastatic adhesion.

The Cbl family proteins: ring leaders in regulation of cell signaling

The proto-oncogenic protein c-Cbl was discovered as the cellular form of v-Cbl, a retroviral transforming protein. This was followed over the years by important discoveries, which identified c-Cbl and other Cbl-family proteins as key players in several signaling pathways. c-Cbl has donned the role of a multivalent adaptor protein, capable of interacting with a plethora of proteins, and has been shown to positively influence certain biological processes. The identity of c-Cbl as an E3 ubiquitin ligase unveiled the existence of an important negative regulatory pathway involved in maintaining homeostasis in protein tyrosine kinase (PTK) signaling. Recent years have also seen the emergence of novel regulators of Cbl, which have provided further insights into the complexity of Cbl-influenced pathways. This review will endeavor to provide a summary of current studies focused on the effects of Cbl proteins on various biological processes and the mechanism of these effects. The major sections of the review are as follows: Structure and genomic organization of Cbl proteins; Phosphorylation of Cbl; Interactions of Cbl; Localization of Cbl; Mechanism of effects of Cbl: (a) Ubiquitylation-dependent events: This section elucidates the mechanism of Cbl-mediated downregulation of EGFR and details the PTK and non-PTKs targeted by Cbl. In addition, it addresses the functional requirements for E3 Ubiquitin ligase activity of Cbl and negative regulation of Cbl-mediated downregulation of PTKs, (b) Adaptor functions: This section discusses the mechanisms of adaptor functions of Cbl in mitogen-activated protein kinase (MAPK) activation, insulin signaling, regulation of Ras-related protein 1 (Rap1), PI-3' kinase signaling, and regulation of Rho-family GTPases and cytoskeleton; Biological functions: This section gives an account of the diverse biological functions of Cbl and includes the role of Cbl in transformation, T-cell signaling and thymus development, B-cell signaling, mast-cell degranulation, macrophage functions, bone development, neurite growth, platelet activation, muscle degeneration, and bacterial invasion; Conclusions and perspectives.

PilT is required for PI(3,4,5)P3-mediated crosstalk between Neisseria gonorrhoeae and epithelial cells

The retractile type IV pilus participates in a number of fundamental bacterial processes, including motility, DNA transformation, fruiting body formation and attachment to host cells. Retraction of the N. gonorrhoeae type IV pilus requires a functional pilT. Retraction generates substantial force on its substrate (> 100 pN per retraction event), and it has been speculated that epithelial cells sense and respond to these forces during infection. We provide evidence that piliated, Opa non-expressing Neisseria gonorrhoeae activates the stress-responsive PI-3 kinase/Akt (PKB) pathway in human epithelial cells, and activation is enhanced by a functional pilT. PI-3 kinase inhibitors wortmannin and LY294002 reduce cell entry by 81% and 50%, respectively, illustrating the importance of this cascade in bacterial invasion. PI-3 kinase and its direct downstream effectors [PI(3,4,5)P3] and Akt are concentrated in the cell cortex beneath adherent bacteria, particularly at the periphery of the bacterial microcolonies. Furthermore, [PI(3,4,5)P3] is translocated to the outer leaflet of the plasma membrane. Finally, we show that [PI(3,4,5)P3] stimulates microcolony formation and upregulates pilT expression in vitro. We conclude that N. gonorrhoeae activation of PI-3 kinase triggers the host cell to produce a lipid second messenger that influences bacterial behaviour.

Effects of flow patterns on the localization and expression of VE-cadherin at vascular endothelial cell junctions: in vivo and in vitro investigations

Atherosclerosis occurs preferentially at vascular curvature and branch sites where the vessel walls are exposed to fluctuating shear stress and have high endothelial permeability. Endothelial permeability is modulated by intercellular adhesion molecules such as VE-cadherin. This study was designed to elucidate the effects of different flow patterns on the localization and expression of VE-cadherin in endothelial cells (ECs) both in vivo and in vitro. VE-cadherin staining at EC borders was much stronger in the descending thoracic aorta and abdominal aorta, where the pulsatile flow has a strong net forward component than in the aortic arch and the poststenotic dilatation site beyond an experimental constriction, where the flow near the wall is complex and reciprocating with little net flow. With the use of flow chambers the effects of pulsatile flow (12 +/- 4 dyn/cm2 at 1 Hz) and reciprocating flow (0.5 +/- 4 dyn/cm2 at 1 Hz) on VE-cadherin organization in endothelial monolayers were studied in vitro. VE-cadherin staining was continuous along cell borders in static controls. Following 6 h of either pulsatile or reciprocating flow, the VE-cadherin staining at cell borders became intermittent. When the pulsatile flow was extended to 24, 48 or 72 h the staining around the cell borders became continuous again, but the staining was still intermittent when the reciprocating flow was similarly extended. Exposure to pulsatile or reciprocating flow for 6 and 24 h neither change the expression level of VE-cadherin nor its distribution between membrane and cytosol fractions as determined by Western blot and compared with static controls. These findings suggest that the cell junction remodeling induced by different flow patterns may result from a redistribution of VE-cadherin within the cell membrane. Both the in vivo and in vitro data indicate that pulsatile and reciprocating flow patterns have different effects on cell junction remodeling. The lack of junction reorganization in regions of reciprocating flow in vivo and in vitro may provide a mechanistic basis for the high permeability and the preferential localization of atherosclerosis in regions of the arterial stress with complex flow patterns and fluctuating shear stress.

The effect of insulin and exercise on c-Cbl protein abundance and phosphorylation in insulin-resistant skeletal muscle in lean and obese Zucker rats

AIMS/HYPOTHESIS

Recruitment of the protein c-Cbl to the insulin receptor (IR) and its tyrosine phosphorylation via a pathway that is independent from phosphatidylinositol 3'-kinase is necessary for insulin-stimulated GLUT4 translocation in 3T3-L1 adipocytes. The activation of this pathway by insulin or exercise has yet to be reported in skeletal muscle.

METHODS

Lean and obese Zucker rats were randomly assigned to one of three treatment groups: (i). control, (ii). insulin-stimulated or (iii). acute, exhaustive exercise. Hind limb skeletal muscle was removed and the phosphorylation state of IR, Akt and c-Cbl measured.

RESULTS

Insulin receptor phosphorylation was increased 12-fold after insulin stimulation ( p<0.0001) in lean rats and threefold in obese rats. Acute exercise had no effect on IR tyrosine phosphorylation. Similar results were found for serine phosphorylation of Akt. Exercise did not alter c-Cbl tyrosine phosphorylation in skeletal muscle of lean or obese rats. However, in contrast to previous studies in adipocytes, c-Cbl tyrosine phosphorylation was reduced after insulin treatment ( p<0.001).

CONCLUSIONS/INTERPRETATION

We also found that c-Cbl associating protein expression is relatively low in skeletal muscle of Zucker rats compared to 3T3-L1 adipocytes and this could account for the reduced c-Cbl tyrosine phosphorylation after insulin treatment. Interestingly, basal levels of c-Cbl tyrosine phosphorylation were higher in skeletal muscle from insulin-resistant Zucker rats ( p<0.05), but the physiological relevance is not clear. We conclude that the regulation of c-Cbl phosphorylation in skeletal muscle differs from that previously reported in adipocytes.

Vascular endothelial growth factor-dependent down-regulation of Flk-1/KDR involves Cbl-mediated ubiquitination. Consequences on nitric oxide production from endothelial cells

Ligand-stimulated degradation of receptor tyrosine kinase (RTK) is an important regulatory step of signal transduction. The vascular endothelial growth factor (VEGF) receptor Flk-1/KDR is responsible for the VEGF-stimulated nitric oxide (NO) production from endothelial cells. Cellular mechanisms mediating the negative regulation of Flk-1 signaling in endothelial cells have not been investigated. Here we show that Flk-1 is rapidly down-regulated following VEGF stimulation of bovine aortic endothelial cells (BAECs). Consequently, VEGF pretreatment of endothelial cells prevents any further stimulation of Flk-1, resulting in decreased NO production from subsequent VEGF challenges. Ubiquitination of RTKs targets them for degradation; we demonstrate that activation of Flk-1 by VEGF leads to its polyubiquitination in BAECs. Furthermore, VEGF stimulation of BAECs or COS-7 cells transiently transfected with Flk-1 results in the phosphorylation of the ubiquitin ligase Cbl, the enhanced association of Cbl with Flk-1, and the relocalization of Cbl to vesicular structures in BAECs. Overexpression of Cbl in COS-7 cells enhances VEGF-induced ubiquitination of Flk-1, whereas a Cbl mutant lacking the ubiquitin ligase RING finger domain, 70Z/3-Cbl, does not. Moreover, expression of Cbl in contrast to 70Z/3-Cbl inhibits the Flk-1-dependent activation of eNOS and, thus, NO release. In BAEC overexpressing Cbl, the degradation of Flk-1 upon VEGF stimulation is accelerated compared with cells transfected with a control vector (green fluorescent protein). Our findings demonstrate that Flk-1 is rapidly down-regulated following sustained VEGF stimulation and identify Cbl as a negative regulator of Flk-1 signaling to eNOS. Cbl thus plays a role in the regulation of VEGF signaling by mediating the stimulated ubiquitination and, consequently, degradation of Flk-1 in endothelial cells.

Interplay between integrins and FLK-1 in shear stress-induced signaling

Blood flow can modulate vascular cell functions. We studied interactions between integrins and Flk-1 in transducing the mechanical shear stress due to flow. This application of a step shear stress caused Flk-1. Casitas B-lineage lymphoma (Cbl) activation (Flk-1. Cbl association, tyrosine phosphorylation of the Cbl-bound Flk-1, and tyrosine phosphorylation of Cbl) in bovine aortic endothelial cells (BAECs). The activation of integrins by plating BAECs on vitronectin or fibronectin also induced this Flk-1. Cbl activation. The shear-induced Flk-1. Cbl activation was blocked by inhibitory antibodies for alphavbeta3- or beta1-integrin, suggesting that it is mediated by integrins. Inhibition of Flk-1 by SU1498 also abolished this shear-induced Flk-1. Cbl activation. In contrast to the requirement of integrins for Flk-1. Cbl activation, the Flk-1 blocker SU1498 had no detectable effect on the shear-induced integrin activation, suggesting that integrins and Flk-1 play sequential roles in the signal transduction hierarchy induced by shear stress. Integrins are essential for the mechanical activation of Flk-1 by shear stress but not for the chemical activation of Flk-1 by VEGF.