doi: 10.1161/CIRCRESAHA.108.173021
© 2008 American Heart Association, Inc.
Editorials |
Physics Meets Molecules
Is Modulation of Shear Stress the Link to Vascular Prevention?
From Art.Net. (Arteriogenesis Network), Universitätsmedizin Berlin-Charité, Center for Cardiovascular Research, Department for Internal Medicine, CC 13 Cardiology, Berlin, and University Clinic Freiburg-Cardiology, Freiburg; and the Max Delbrück Center for Molecular Medicine, Department of Angiogenesis and Cardiovascular Pathology, Germany.
Correspondence to Universitätsmedizin Berlin-Charité, Center for Cardiovascular Research CCR, CC 13, Hessische Strasse 3-4, 10115 Berlin, Germany. E-mail ivo.buschmann{at}charite.de
See related article, pages 538–545
Key Words: shear stress • arteriogenesis • collateral arteries • cardiovascular prevention
Arteries and veins are permanently exposed to hemodynamic forces because of the pulsatile nature of blood pressure and flow. Hence, the endothelium is constantly detecting different biomechanical forces, cyclic stretch and shear stress in particular,1–2 and converts the latter stimuli into intra- and extracellular signals. Endothelial cells thereby modulate multiple of physiological and pathophysiological processes: production of growth-promoting and growth-inhibiting hormones, enzymes, cytokines, etc; mediation of inflammatory responses through the expression of chemotactic and adhesion molecules on the endothelial surface; modulation of hemostasis and thrombosis via secretion of procoagulant, anticoagulant, and fibrinolytic agents; and the regulation of vascular smooth muscle cell contraction through the release of vasodilators and vasoconstrictors.3–5
This being the case, the equilibrium between physiological levels of blood flow (shear stress) and the endothelium is tightly counterbalanced. Thereby, the lumen radius of an artery is the most important denominator, which signifies that the smaller the lumen the higher the shear stress. However, once physiological shear forces are reduced, several pathological conditions may arise: proatherogenic and/or prothrombotic states and hence atherosclerosis and/or thrombosis.6–7 Inversely, high levels of shear forces play a key role in adaptive phases of arteriogenesis (collateral artery growth), the most clinically relevant mechanism of vascular development.8 In case of an arterial stenosis, these arterial/arteriolar anastomoses are the only anastomosis to the low-pressure territory and perfuse the periphery with nutrient blood flow. Elevation of shear stress and concomitant cyclic stretch are currently discussed to be the strongest inducers of arteriogenesis. In a rabbit model of femoral artery ligation, Schaper and colleagues induced an abrupt increase in shear stress by means of an arterial–venous shunt (pressure drop across collateral arteries) and accelerated the speed of arteriogenesis, surpassing the conductance values of the occluded artery when shear stress is kept artificially high.9
In fact, these shunt experiments demonstrate the overall impact of shear forces to induce maximal arteriogenesis (Figure). Because of the shunt-induced peripheral pressure drop, the intraluminal pressure within collaterals is rather low, indicating only a secondary role of cyclic stretch in this phase of adaptive collateral growth.
|
Hence, this important role of shear stress and its modulation has stimulated researchers in the past to unravel the molecular mechanisms from physical forces to molecular medicine.10
In this issue of Circulation Research, Woo et al elegantly demonstrate a role for the posttranslational protein modifier SUMO (small ubiquitin-related modifier) and its substrate, the mitogen-activated protein kinase extracellular signal-regulated kinase (ERK)5, in the progression of diabetes-induced endothelial dysfunction. The authors used streptozotocin-treated diabetic mice and detected increased endogenous ERK5-SUMOylation in the aortas of these animals. Using mediators of endothelial dysfunction (reactive oxygen species [H2O2] and AGE [advanced glycosylation end products]), in an in vitro approach in primary endothelial cells, ERK5-Sumoylation was induced. This led to the inhibition of flow-induced activation of transcription factors myocyte enhancer factor (MEF)2 and Kruppel-like factor (KLF)2, as well as their downstream target endothelial NO synthase. KLF2 is well known as a transcriptional inhibitor of endothelial mediated inflammation. Woo et al provide a proof of concept by using specific SUMO site mutants (K6, K22), preventing ERK5 kinase activity and thus activation of downstream effectors1.11
Thus, the outstanding question in the field of biomechanical shear forces and vascular biology is: Does a single or specific mechanoreceptor exist, or is the cell itself, through cytoskeleton rearrangements (the concept of tensegrity), able to sense physical stimuli? Many kinds of molecules, including receptors, ion channels, caveolins, Src, focal adhesion kinase (FAK), G proteins, transcriptional factors, and specific shear-responsive elements in the promoter region associate with sensing mechanical stimulation. It is postulated that mechanical forces can modify the conformation of proteins directly, leading to functional switches (opening of ion channels) or changes in phosphorylation status, resulting in activation of downstream signaling cascades.
In a classic study by Resnick et al,12 it was shown that shear stress activates gene expression via transcription factor interaction with a common promoter element. The authors noted that shear stress–induced platelet-derived growth factor-B transcription requires an essential 12-bp component containing a core binding sequence that allows binding of transcription factors. This common promoter element was called shear stress–responsive element (SSRE), which was subsequently identified in other shear stress–regulated genes including intercellular adhesion molecule-1, and transforming growth factor-β. Such SSREs were also identified in endothelial NO synthase, another factor essential for flow-mediated vasodilation.13
The transcription factor KLF2 is rapidly upregulated by shear stress. A substantial number of in vitro and in vivo studies have shown that KLF2 is important in hemodynamic regulation in response to arterial shear stress, both in the embryo and adult, as well as pathophysiological conditions including atherosclerosis. Using a genetic approach in both zebrafish and mice, it was shown that loss of KLF2 results in heart failure.13 A detailed hemodynamic analysis revealed that this heart failure is caused by a high cardiac output state, in the absence of structural vascular defects, indicating loss of peripheral resistance. Consistent with this hypothesis, in both KLF2a morpholino knockdown zebrafish and KLF2-deficient mice, the addition of phenylephrine (a potent 
-adrenergic vasoconstrictor) could rescue these animals from lethal high-output heart failure.14 Based on these observations, it is hypothesized that fluid shear forces drive expression of endothelial KLF2 that regulates smooth muscle tone in the developing embryo.
At the structural level, it is suggested that adhesive receptors that physically link extracellular support scaffolds (the extracellular matrix) to the internal cytoskeleton may function as mechanoreceptors. The endothelial cytoskeleton is a candidate for shear stress transduction because of its fast and dramatic response to shear.15 At the protein level, integrins and adhesion receptors (including platelet endothelial cell adhesion molecule [PECAM]-1, E-selectin, cadherins) link the internal cytoskeleton with the extracellular matrix and provide mechanical coupling across the cell surface. Genetic deletion of fibronectin, 5β1 integrin, and components of the extracellular matrix, including thrombospondin-1 and -2 results in vascular remodeling and angiogenesis defects. A detailed analysis indicated that, in these models, lumenization of vessels and establishment of a functionally perfused vasculature were disturbed.
Signals from the extracellular matrix must be transduced to the signaling machinery inside the cell. Integrins accomplish this by recruiting multimolecular complexes of cytoskeletal and signaling molecules at focal adhesion sites. The activation of non–receptor tyrosine kinase FAK depends on this integrin clustering. Cells exposed to cyclic stretch or shear stress show enhanced FAK tyrosine phosphorylation, indicating that FAK activity links to mechanical stimuli.16 It is suggested that FAK activity is essential for flow-driven tubulogenesis.17
The endothelial cell–cell adhesion site is also implicated as a potential site for mechanosensing. VE-cadherin is a major adhesive protein for the adherens junction and expressed specifically in vascular endothelial cells.18 It interacts with the cytoskeleton via anchoring molecules, including β-catenin. In mice, genetic deletion of VE-cadherin results in severe vascular remodeling defects and abolishes vascular endothelial growth factor receptor (VEGFR)-2 signaling. Short intervals of shear stress stimulate the formation of the VEGFR-2/VE-cadherin/β-catenin complex in vascular endothelial cells.19 Exposure of endothelial cells to laminar shear stress results in induction of VEGFR-2 and promotes the formation of a complex containing VEGFR-2, VE-cadherin, and β-catenin. This intact complex appears essential for activating downstream SSRE-dependent gene transcription.
Adhesion molecules play a pivotal role in mediating mechanosensing upstream of the integrins.20 It could be demonstrated that the PECAM-1/VEGFR2/VE-cadherin complex acts as a mechanosensory complex in vitro. The phosphatidylinositol 3-kinase is essential for shear-driven activation of 5, β3 integrin involving c-Src. PECAM-1 is required for Src activation, whereas VE-cadherin is required for transmitting this signal to phosphatidylinositol 3-kinase. Neither VE-cadherin–/– nor PECAM-1–/– cells showed alignment of actin filaments in response to flow. Interestingly, in this setting, the involvement of VE-cadherin in shear stress–dependent signaling appeared independent of cell–cell signaling.
Taken together, these studies suggest that the PECAM-1/VE-cadherin/β-catenin/VEGFR2 pathway is critical for mediating shear stress responses in endothelial cells.
Here, Woo et al11 illustrate the importance of posttranslational modifications. Because SUMOylation appears to be involved in several cellular functions with a large number of cellular substrates and a highly diverse outcome, this study links shear stress–induced signal transduction with a clinical feature, diabetes.
In summary, beneficial effects of shear stress (antiatherosclerotic, proarteriogenic, improvement of endothelial dysfunction) receive more and more attention, both experimentally and clinically. The increased knowledge of the underlying molecular events of shear stress signaling is of key importance to understand and promote bench-to-bedside transfer of training and prevention programs.
 |
Acknowledgments |
|---|
Sources of Funding
Supported by the Volkswagen Foundation and Max-Planck-Society.
Disclosures
None.
|
Footnotes |
|---|
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
|
References |
|---|
 Top References |
|---|
- Davies PF, Zilberberg J, Helmke BP. Spatial microstimuli in endothelial mechanosignaling. Circ Res. 2003; 92: 359–370.
[Abstract/Free Full Text] - Skalak TC, Price RJ. The role of mechanical stresses in microvascular remodeling. Microcirculation. 1996; 3: 143–165.[Medline] [Order article via Infotrieve]
- Groenendijk BC, Van der Heiden K, Hierck BP. The role of shear stress on ET-1, KLF2, and NOS-3 expression in the developing cardiovascular system of chicken embryos in a venous ligation model. Physiology (Bethesda). 2007; 22: 380–389.[CrossRef][Medline] [Order article via Infotrieve]
- Slager CJ, Wentzel JJ, Gijsen FJ. The role of shear stress in the destabilization of vulnerable plaques and related therapeutic implications. Nat Clin Pract Cardiovasc Med. 2005; 2: 456–464.[CrossRef][Medline] [Order article via Infotrieve]
- Nichols WW, O’Rourke MF. McDonald’s Blood Flow in Arteries: Theoretical, Experimental and Clinical Principles. 5th ed. London, UK: A Hodder Arnold Publication; 2005.
- Badimon L, Badimon JJ, Penny W. Endothelium and atherosclerosis. J Hypertens Suppl. 1992; 10: S43–S50.[Medline] [Order article via Infotrieve]
- Quyyumi AA. Endothelial function in health and disease: new insights into the genesis of cardiovascular disease. Am J Med. 1998; 105 (1A): 32S–39S.[Medline] [Order article via Infotrieve]
- Gray C, Packham IM, Wurmser F. Ischemia is not required for arteriogenesis in zebrafish embryos. Arterioscler Thromb Vasc Biol. 2007; 27: 2135–2141.
[Abstract/Free Full Text] - Eitenmüller I, Volger O, Kluge A, Troidl K, Barancik M, Cai WJ, Heil M, Pipp F, Fischer S, Horrevoets AJ, Schmitz-Rixen T, Schaper W. The range of adaptation by collateral vessels after femoral artery occlusion. Circ Res. 2006; 99: 656–662.
[Abstract/Free Full Text] - Buschmann IR, Busch HJ, Mies G. Therapeutic induction of arteriogenesis in hypoperfused rat brain via granulocyte-macrophage colony-stimulating factor. Circulation. 2003; 108: 610–615.
[Abstract/Free Full Text] - Woo CH, Shishido T, McClain C, Lim JH, Li JD, Yang J, Yan C, Abe JI. Extracellular signal-regulated kinase 5 SUMOylation antagonizes shear stress–induced antiinflammatory response and endothelial nitric oxide synthase expression in endothelial cells. Circ Res. 2008; 102: 538–545.
[Abstract/Free Full Text] - Resnick N, Collins T, Atkinson W. Platelet-derived growth factor B chain promoter contains a cis-acting fluid shear-stress-responsive element. Proc Natl Acad Sci U S A. 1993; 90: 7908.
[Free Full Text] - Davis ME, Grumbach IM, Fukai T. Shear stress regulates endothelial nitric-oxide synthase promoter activity through nuclear factor kappaB binding. J Biol Chem. 2004; 279: 163–168.
[Abstract/Free Full Text] - Lee JS, Yu Q, Shin JT. Klf2 is an essential regulator of vascular hemodynamic forces in vivo. Dev Cell. 2006; 11: 845–857.[CrossRef][Medline] [Order article via Infotrieve]
- Ingber DE. Mechanical signaling and the cellular response to extracellular matrix in angiogenesis and cardiovascular physiology. Circ Res. 2002; 91: 877–887.
[Abstract/Free Full Text] - Ilic D, Kovacic B, McDonagh S, Jin F, Baumbusch C, Gardner DG, Damsky CH. Focal adhesion kinase is required for blood vessel morphogenesis. Circ Res. 2003; 92: 300–307.
[Abstract/Free Full Text] - Ilic D, Furuta Y, Kanazawa S. Reduced cell motility and enhanced focal adhesion contact formation in cells from FAK-deficient mice. Nature. 1995; 377: 539–544.[CrossRef][Medline] [Order article via Infotrieve]
- Carmeliet P, Lampugnani MG, Moons L. Targeted deficiency or cytosolic truncation of the VE-cadherin gene in mice impairs VEGF-mediated endothelial survival and angiogenesis. Cell. 1999; 98: 147–157.[CrossRef][Medline] [Order article via Infotrieve]
- Shay-Salit A, Shushy M, Wolfovitz E. VEGF receptor 2 and the adherens junction as a mechanical transducer in vascular endothelial cells. Proc Natl Acad Sci U S A. 2002; 99: 9462–9467.
[Abstract/Free Full Text] - Tzima E, Irani-Tehrani M, Kiosses WB. A mechanosensory complex that mediates the endothelial cell response to fluid shear stress. Nature. 2005; 437: 426–431.[CrossRef][Medline] [Order article via Infotrieve]
Related Article:
- Extracellular Signal-Regulated Kinase 5 SUMOylation Antagonizes Shear Stress–Induced Antiinflammatory Response and Endothelial Nitric Oxide Synthase Expression in Endothelial Cells
- Chang-Hoon Woo, Tetsuro Shishido, Carolyn McClain, Jae Hyang Lim, Jian-Dong Li, Jay Yang, Chen Yan, and Jun-ichi Abe
Circ. Res. 2008 102: 538-545.[Abstract] [Full Text] [PDF]
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||