doi: 10.1161/CIRCRESAHA.108.181610
© 2008 American Heart Association, Inc.
Editorials |
Sphingosine-1-Phosphate in the Regulation of Vascular Tone
A Finely Tuned Integration System of S1P Sources, Receptors, and Vascular Responsiveness
From the Institute of Pathophysiology, University Hospital Essen, Germany.
Correspondence to Bodo Levkau, Institute of Pathophysiology, University Hospital Essen, Hufelandstrasse 55, 45122 Essen, Germany. E-mail levkau{at}uni-essen.de
See related article, pages 315–324
Key Words: sphingosine-1-phosphate • smooth muscle cells • endothelium • vasodilation • vasoconstriction
Sphingosine-1-phosphate (S1P) is a biologically active lysophospholipid that plays an important role in the physiology of the cardiovascular, nervous and immune systems.1 Red blood cells are the major source of plasma S1P, where it mainly associates with High-density lipoproteins (HDL). There is a steep concentration gradient of S1P between plasma and the interstitial compartment, with S1P being present in the low micromolar range in plasma and at least 1 order of magnitude lower in the interstitial fluid. S1P exerts numerous effects on the vascular cells regulating arterial tone—the smooth muscle cell and the endothelial cell —via a set of 5 cognate G protein–coupled receptors (S1P1 to 5) expressed differentially in the different cell types.2 In addition, S1P has negative effects on cardiac rate, contractility, and output. All of this makes S1P a candidate for a bona fide modulator of vascular tone.
Direct vasoactive effects of S1P have been studied both in isolated vessel strips ex vivo and different vascular territories in vivo.2 The consensus from all these studies is that S1P induces vasoconstriction in resistance vessels such as mesenteric, cerebral, and coronary arteries but has little or no effect on conduit vessels such as aorta, carotid, and femoral arteries. This implies S1P as a player in the control of blood flow in the periphery. However, these studies have been all performed by using exogenously added S1P, making it difficult to draw conclusions on the role of endogenous S1P sources in the homoeostatic regulation of resistance artery tone. In a previous study, this has been addressed by Bolz et al by overexpressing sphingosine kinase (Sphk)1 in hamster resistance arteries, showing that endogenously produced S1P participates in the regulation of basal tone, as well as that of the myogenic response,3 the unique intrinsic ability of resistance arteries to react through vasoconstriction to elevation in transmural pressure caused by raises in systemic blood pressure to keep a constant blood flow to the supplied tissues.
In this issue of Circulation Research, Peter et al4 expand their previous studies by showing that a balance between S1P-generating and -degrading enzymes inside the vascular wall is responsible for the regulation of basal and myogenic resistance artery tone in the hamster gracilis muscle resistance artery. Overexpression of the S1P degrading enzyme S1P-phosphohydrolase (SPP)1 reduced basal tone and myogenic vasoconstriction and antagonized the increase in myogenic tone induced by overexpression of S1P-generating Sphk1. Moreover, SPP1 was also able to counteract the vasoconstriction induced by exogenously added S1P, suggesting that it was able to efficiently degrade extracellular S1P. As SPP1 is known to be localized in the endoplasmic reticulum (ER) rather than the plasma membrane Peter et al hypothesized that it must have sufficient access to the pool of extracellular S1P for degradation if it were to restrain its vasoconstrictive action. Indeed, the study shows that such access was granted by cystic fibrosis transmembrane conductance regulator (CFTR) (also known as ATP-binding cassette subfamily C member 7 [ABCC7]), the only known outside-in transporter for S1P that had also been proven capable of preventing interaction of S1P with its cell surface receptors by diverting sufficient quantities of extracellular S1P to the inside of the cell (Figure 1).5 Using an inhibitor of CFTR [CFTR(inh)-172], Peter et al could reverse the reduced resting tone and myogenic vasoconstriction caused by SPP1 overexpression. Consequently, a conclusive model of resistance artery tone regulation by endogenous, smooth muscle cell–derived S1P emerges, in which S1P production by Sphk1 and its degradation by SPP1 after CFTR-mediated import together determine the amount of biologically active S1P concentration presented at the S1P receptor level for vasoconstriction (Figure).
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Much work has been done to identify which S1P receptors are responsible for its vasoconstrictive effect. S1P1–3 are expressed in smooth muscle cells. However, smooth muscle cells from different arterial beds appear to have different relative expression levels of the 3 receptors, which has been proposed as an explanation for the different potency of S1P-mediated vasoconstriction between, eg, aorta and cerebral arteries.6 The most informative studies with respect to which S1P receptor(s) influence the physiological regulation of vascular tone and blood pressure/blood flow relationship have emerged from studies using S1P2- and S1P3-deficient mice. In S1P3-deficient mice, S1P did not affect basilar artery vascular tone, while inducing regular vasoconstriction in wild-type and S1P2-deficient mice, suggesting that it constricts cerebral arteries via S1P3.7 In contrast, S1P2-deficient mice were shown to exhibit normal blood pressure but clearly decreased resting vascular tone and contractile responsiveness to 
-adrenergic stimulation, resulting in elevation of regional blood flow and a decrease in vascular resistance.8 Thus S1P2 appears to have a crucial role in the regulation of normal physiological hemodynamics. The fact that the contractile response to phenylephrine (PE) was blunted both in vivo and in isolated artery strips from S1P2-deficient mice suggests that the functional consequence of S1P2 deficiency was present even in the absence of exogenously added S1P. This happens in agreement with a scenario where S1P generated within the arterial wall is sufficient for S1P2 engagement. Peter et al4 also argue in favor of intramural S1P production being a determinant of vascular tone and suggest S1P2 as the mediator. However, they did not observe differences in the vasomotor responses to noradrenaline after inhibiting SPP1, S1P2, or CTPR, together with a still substantial residual S1P vasoconstriction after S1P2 inhibition. Although the agent that they have used to inhibit S1P2, JTE-013, is nonspecific, because it inhibits not only S1P-induced vasoconstriction but also KCl-, U46619-, and endothelin-1–induced constriction,7 the antisense approach for S1P2 and SPP1 is valid, as well as the pharmacological CTPR inhibition. One possible explanation may be that S1P2 action in the smooth muscle cell maintains a certain basal tone that is necessary for optimal PE effect and that is completely abolished in the genetic knockout but remains intact in the study by Peter et al4 for insufficient interference with S1P availability and S1P2 activity. Although the decreased vascular resistance in S1P2-deficient mice and the blunted PE response in isolated S1P2-deficient arteries support this, there is a clear contradiction to the regular PE response and increased maximal force generation in S1P2-deficient arteries denuded of endothelium. A significant unanswered question in the study by Peter et al is to what extent the activity and function of SPP1, Sphk1, and S1P2 have been altered, or the S1P concentration has been changed by antisense treatments, because neither expression levels nor enzymatic activity nor the S1P levels have ever been measured.
Beyond these considerations and the obvious difficulty in comparing vessels from different regions and species, the explanation may lie in the next level of complexity inherent in the S1P vasoregulatory system: despite all data on consistent, although not always equipotent, vasoconstriction by S1P dependent on the vascular bed, there is ample evidence that S1P is also an effective activator of the endothelial nitric oxide synthase with the ability to dilate arteries dependent on the circumstances.9 These circumstances always refer to the underlying arterial tone. Aortic strips precontracted with PE dilate in response to S1P in an NO-dependent manner,10 whereas the inhibitor of S1P1 and S1P3 VPC23019 boosts S1P-induced vasoconstriction of the basilar artery.7 Also in vivo, intraarterial S1P application abolishes the elevation of arterial blood pressure caused by endothelin administration.10 This suggests the existence of an S1P-sensing module in the artery wall that integrates endothelial and smooth muscle effects of S1P with the net product determining the actual arterial tone. This offers an explanation to the apparent contradiction above: SlP generated in the vascular wall by Sphkl cannot engage the contractile S1P2 missing in smooth muscle cells of S1P2-deficient arteries, but activates S1P1 and S1P3 in the overlying endothelium with consecutive NO-dependent decrease in vascular tone and increase in EC50 for PE-mediated vasoconstriction. If this were the case, there would still have to be enough functional S1P2 receptors present in the study by Peter et al4 to avoid loss of S1P2-mediated tone. The endothelial S1P receptors S1P1 and S1P3 responsible for NO-dependent vasodilation would then not only respond to luminal plasma S1P but also to S1P produced in the vessel wall by the smooth muscle cells (Figure).
Many more exciting questions remain. What is the stimulus for Sphk1 activation for proper myogenic response: tensegrity-based mechanosensing of tension? If so, via which molecular mechanisms? Is the myogenic response altered in arteries of Sphk1 knockout mice? Their normal blood pressure in vivo does not exclude a blunted contractile response similar to the scenario present in S1P2-deficient mice. What is the response to S1P in arteries deficient for S1P receptors exclusively in the endothelium or smooth muscle? What is the role of other S1P-generating or -degrading enzymes such as Sphk2 and S1P lyase, respectively? How much S1P is synthesized and degraded in the context of endogenous arterial tone regulation? Is there a S1P gradient in the vessel wall? To what extent does the plasma S1P packaged in HDL regulate not only endothelial but also smooth muscle cell function? If HDL can pass the endothelial cell lining for reverse cholesterol transport, then it can certainly deliver its S1P cargo to the smooth muscle cells as well (Figure). Does the endothelial S1P synthesis machinery recently discovered as an important source of plasma S1P11 also release S1P abluminally to stimulate smooth muscle cells?
Defective vascular tone is a key feature of cardiovascular diseases such as hypertension, coronary heart disease, and atherosclerosis. Not much is currently known about the role vascular tone regulation by S1P may play in these pathologies. However, data on S1P receptor agonists inducing hypertension on intravenous infusion in conscious rats12 and the attenuated vasoconstriction response to S1P in spontaneously hypertensive rats13 suggest that there may be much more to come. Understanding the physiological mechanisms of vascular tone regulation by S1P will be the key to their future therapeutic targeting.
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Acknowledgments |
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Sources of Funding
The author is supported by grants LE940/3-1 and LE 940/4-1 from the Deutsche Forschungsgemeinschaft.
Disclosures
None.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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