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Contribution of Vasoconstriction to the Origin of Atherosclerosis
BRIEF REVIEWS Contribution of Vasoconstriction to the Origin of Atherosclerosis A Conceptual Study William H. Gutstein* and Cristian A. Pe´rez
Research during the past century has clearly shown that endothelial injury (EI) and/or endothelial dysfunction (ED) are among the major events determining the onset of atherosclerosis. Included in the events that may elicit endothelial damage, vasoconstriction (VC) has received relatively little attention. This conceptual review attempts to show that in elastic and conduit arteries, VC is not only capable of producing EI/ ED, but is also closely associated with many recognized proatherogenic stimuli. Of related interest is the observation that a number of suspected antiatherogenic stimuli oppose VC by their vasodilatory effects, lending further support to this relationship. In addition, recent developments in the knowledge of the molecular basis of VC (including the role of specific inhibitors) are discussed, and their potential for preventing lesion formation and thus becoming novel therapeutic alternatives against the onset of atherosclerosis are highlighted. (Trends Cardiovasc Med 2004;14:257–261) D 2004, Elsevier Inc.
It is well known that endothelial injury (EI) and endothelial dysfunction (ED) each play a major role in the onset of atherosclerosis (AS). This damage is usually focal, may have various sources (e.g., hyperlipidemia, oxidative stress, etc.), can be purely functional (ED) or
William H. Gutstein is at the New York Medical College, Department of Pathology, Valhalla, New York, USA. Cristian A. Pe´rez is at The Rockefeller University, Laboratory of Molecular Genetics, New York, New York, USA. *Address correspondence to: William H. Gutstein, 47 East 87th Street, New York, NY 10128, USA. Tel and fax: (+1) 212-831-8674; e-mail: [email protected]. D 2004, Elsevier Inc. All rights reserved. 1050-1738/04/$-see front matter
TCM Vol. 14, No. 7, 2004
manifest structural damage (EI), and is usually silent as it develops into an initial atherosclerotic lesion. Currently, there is no clear understanding of how a normal endothelium can be transformed into either of these conditions. The objective of the present review is to call attention to the concept that vasoconstriction (VC) is an early event that could lead to such endothelial damage and therefore to atherogenesis.
Mechanisms of Injury: Relationship to Degree and Parameters of Force Associated with VC
During VC, vascular smooth muscle cells (VSMCs) of the tunica media contract and generate a circumferential stress that is transmitted inwardly to the
enclosed intima and its adjacent endothelial layer. When VC occurs, the endothelial cells may be lifted from their underlying basement membranes and become separated at their intercellular junctions, depending on the parameters of the generated force such as magnitude or severity, duration, and frequency of occurrence. Although we believe this to be the major mechanism by which injury occurs, other sources of EI/ED may also develop during VC. For example, when contraction occurs focally in the tunica media, the arterial wall becomes compact at that site. This change in wall density may have consequences for mass transport and result in interference with intramural traffic, ultimately compromising the integrity of the endothelium (Caro et al. 1971). Similarly, obstruction of the vasa vasorum may occur, producing hypoxemic conditions as far lumenally as the intima, with potential injury to the endothelium (Martin et al. 1991).
Histology of the Endothelium and Intima Following VC
In several studies that ranged over various species—including living human subjects—we and others have clearly shown that spontaneous or experimentally induced VC resulted in histologic injury to the endothelium and intima, which was focal in nature and consisted of severe intimal and often medial damage (reviewed in detail in Gutstein 1999). At times, considerable cell death also occurred in the latter, but only the intima is described here. A striking observation was that the endothelium demonstrated a variety of degenerative features that colocalized with the region of VC, including lifting of the cells from their basement membranes into the lumen, attachment of neutrophils and monocytes, separation at intercellular junctions, and degenerative and necrotizing changes leading to frank denudation with clear portals of entry for the
257
Figure 1. Vasoconstriction (VC)-induced arterial cellular changes. (A) Semi-thin light microscopic section of normal left coronary artery of the rat near its origin, stained with toluidine blue in a control animal in which VC had not been induced. The endothelial cells (E) form a flattened layer; internal elastic lamina and media are of normal thickness. (B) Semi-thin light microscopic section of left coronary artery of the rat near its origin, in which VC has been induced by hypothalamic stimulation (Gutstein 1988). The vessel is markedly contracted (see inset), showing reduction in diameter, buckling of the internal elastic lamina (IEL), and thickening of the wall. Endothelial cells (E) can be seen projecting into the lumen with creation of large vacuolated spaces beneath them. The medial vascular smooth muscle cell (VSMC) layer shows disarray and the diameters of these cells appear to be increased. Calibration marks in (A, B, and inset) are 25 Am. (C, D) Schematic representations of transverse microscopic sections through arterial wall and one exposed to focal VC—equivalent to (A) and (B), respectively. (Parts A and B reprinted from Atherosclerosis, 70, WH Gutstein, b The central nervous system and atherogenesis: endothelial injury,Q 145–154, copyright 1988, with permission of Elsevier.)
possible inflow of cellular and molecular components from the circulating plasma. In some bbare areas,Q platelet aggregates and microthrombi were incorporated and at times attached to subendothelial collagen. In a few studies, the subendothelial space contained fibrin, erythrocytes, or tracer substances previously injected into the lumen to assess permeability of the endothelial barrier. Occasional monocytes—some with apoptotic appearance—were also observed in this location. Intimal thickening, due to VSMC proliferation and their extracellular products, was frequently observed and marked at times. The internal elastic lamina was usually ruffled and often disrupted. Figure 1 depicts transitions from normal to altered endothelium in both light microscopic observations and diagrammatic representations. In essence, these histologic alterations may be seen with varying degrees of VC and demonstrate that EI/ED is frequently present. Although often severe, they are not generally conceived of as occurring with VC and therefore have escaped detection as a possible starting point for the onset of atherogenesis. Some possible reasons why histologic changes due to VC may not be appreciated include the fact that arteries are not generally examined fol-
258
lowing an episode of VC. Additionally, inspection at the microscopic level is required and even should such observations be employed, the rapid rate of endothelial regeneration may obscure the finding of EI if there is a delay between VC and examination, because newly regenerated cells may already be in place and, although appearing morphologically normal, are nevertheless
functionally disturbed; that is, in a state resembling ED.
Supporting Evidence for the Role of VC in Atherogenesis
Although VC is a common cardiovascular event, its potential contribution to the pathogenesis of AS has seldom been considered. Table 1 lists several
Table 1. Proatherogenic factors that enhance vasoconstriction Proatherogenic factors
Reference
Hypercholesterolemia Essential hypertension Diabetes mellitus Cigarette smoking Angiotensin II Endothelin Testosterone Thromboxane A2 Oxidized low-density lipoprotein/ lysophosphatidylcholine Epidermal and platelet-derived growh factors (EGF, PDGF) Homocysteinemia Emotional stress Ergonovine sensitivity
McCalden and Nath 1989 Luscher 1992 Gries and Koschinsky 1991 Noma et al. 2003 Galle et al. 2003 Bauersachs et al. 2000 Matsuda et al. 1994 Matsuda et al. 1994 Galle et al. 2003, Yokoyama et al. 1990 Berk and Alexander 1989
Low wall shear stress
Cook et al. 2002 Kumari et al. 2003 Hackett et al. 1987, Kaski et al. 1989 and 1992 Gimbrone 1999, Malek et al. 1999
TCM Vol. 14, No. 7, 2004
conditions deemed important for the development of AS, all of which—despite their varying effects on the arterial wall— have in common the capacity for producing or enhancing VC. Consequently, this group of seemingly unrelated conditions, likely to be linked by the contraction of VSMCs to EI/ED, may provide significant reasons for believing that VC contributes to the onset of early atherosclerotic lesions (see Table 1). Although limited in their number, they include important influences such as risk factors for the development of new lesions (both those that are traditional and those more recently discovered) and other conditions, and thus suggest the possibility of a general relationship between VC and atherogenesis. Parenthetically, just as induction of VC may be proatherogenic, there is evidence to show that influences that favor vasodilation may be antiatherogenic. In Table 2 are listed several conditions that support this possibility. In this context, it may be noted that vasodilation can arise directly (e.g., due to an increase in endothelial cell levels of dilators such as nitric oxide) or indirectly (e.g., by antagonism of receptors for contracting substances such as angiotensin II or endothelin I) (Babaei et al. 2000, Strawn et al. 2000), shifting the balance between contracting and relaxing factors to the dilated state (Lamping 1997). Despite these relationships, it is important to take note of certain caveats concerning the physiology of VC. The literature shows that the physiology, biochemistry, and molecular biology are not the same in every vascular bed. Consequently, arterial segments that respond to certain stimuli in one regional
location may fail to do so in another (Luscher et al. 1990) or even in an adjacent segment (el-Tamimi et al. 1994, Penny et al. 1995). Responses at a given site may also vary in sensitivity, efficacy, magnitude, duration, and frequency to a given stimulus (Hill et al. 2001, Maseri and Chierchia 1982). Additionally, a distinction between the vasoactive effects of a given substance versus its VSMC proliferative effect has to be considered. For example, in vitro studies indicate that platelet-derived growth factor is mitogenic for VSMCs in culture (Raines 2004), whereas animal experiments have established its vasoactivity (Berk and Alexander 1989). Moreover, interaction of vasoactive substances may occur with synergistic or opposing effects. For example, low concentrations of endothelin 1 potentiated the effects of norepinephrine and U-46619 (a thromboxane A2 mimetic) in animal experiments (Pang et al. 2001). On the other hand, angiotensin II—a potent vasoconstrictor—has been reported to offer paradoxical cerebroprotection against stroke (Fournier et al. 2004), the explanation of which apparently lies in the simultaneous activation of the receptors AT-1 and AT-2 and the subsequently AT-2mediated production of the vasodilators nitric oxide and prostacyclin. Consequently, exploration of such possible interactions is necessary before conclusions are drawn concerning activity of vasoconstrictor substances and their mediators in any given situation.
Molecular Biology of VC
Because Table 1 reveals many factors linking VC to the possible development of AS, knowledge of the molecular biol-
Table 2. Antiatherogenic factors that suppress vasoconstriction Antiatherogenic factors Endogenous Nitric oxide Prostacyclin EDHF High-density lipoprotein Estradiol Exogenous Antioxidants (e.g., probucol, vitamin E) Rho-kinase inhibitors (e.g., fasudil Y27632) Flavonoids (French paradox)
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Reference Russo et al. 2002 Fink et al. 1999 McGuire et al. 2001 Kuvin et al. 2002 Calkin et al. 2002 Chan 1998, Jiang et al. 2002 Shimokawa 2002, Shimokawa et al. 2002 Rosenkranz et al. 2002
ogy of the contraction of VSMCs may provide opportunities for important therapeutic strategies of intervention or prevention of early lesions. Indeed, some of these are currently in use, without the realization, perhaps, that their efficacy may depend on the success rate with which they are able to counteract or oppose underlying VC. The basic molecular principles of the contractile process have been reviewed elsewhere (Lamping 1997), and are generally associated with phosphorylation/dephosphorylation of lightchain myosin and calcium-independent and -dependent events. An additional mechanism, termed calcium sensitivity— generation of a greater force development for any given calcium concentration (Savineau and Marthan 1997)— relies on the activity of the relatively novel enzyme rho kinase, a member of the ras family of small guanosine triphophatase molecules (Shimokawa 2002, Shimokawa et al. 2002). Based on these and other molecular interactions, the results of animal experiments and clinical investigations have provided several potential strategies that are currently available for controlling and modifying the development and progression of lesions, such as application of nitrogen donors (Cooke and Tsao 1994); calcium channel antagonists (Borcherding et al. 1993); converting enzyme inhibitors of constrictor agents (Mancini 1996); receptor blockers for specific contractile agonists (Luscher and Barton 2000); and, more recently, rho-kinase inhibitors—such as fasudil— for VC prevention (Batchelor et al. 2001, Seto et al. 1999) as well as for vasospasm, its more severe form of contraction. Because these therapeutic applications intervene with primary events in the contractile process of VSMCs, and reduce VC as a result, we believe that they lend further support to the concept proposed in this article.
Conclusion
Taken together, the weight of evidence reveals that a number of established risk factors and conditions that are important for the development of AS also possess the ability to induce or enhance VC. Similarly, various endogenous and exogenous agents that tend to suppress lesion formation increase dilation. Even observations
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such as those that patients with Prinzmetal’s variant angina—sensitive to ergonovine administration by constriction of the coronary arteries and exhibiting subsequent development of organic lesions in precisely the same location (see Table 1 and Kaski et al. 1989 and 1992)—reflect such an association. Another example consistent with this concept is the bunorthodoxQ atheroprotective effect found in the French paradox (lower incidence of cardiovascular disease in geographic regions with red wine consumption above average), which may be due to vasorelaxant effects of red wine flavonoids inhibiting the platelet-derived growth factor receptor (see Table 2). These opposing vasomotor activities suggest the existence of a general relationship between the contractile behavior of elastic and conduit arteries and atherogenesis. Although there may be exceptions, the fact that VC is so ubiquitous a cardiovascular event and appears to be such an effective cause of EI further strengthens this relationship. A point of interest concerning the development of endothelial damage as a function of VSMC contraction is the question of variation in individual contributions of the separate parameters of force that may bring it about. For example, is it possible that multiple episodes of a moderate degree of force at a given site, over time, create as much as, less, or a greater amount of EI/ED than does a single but severe episode? If this is indeed true, because of the potential diversity of these events, it could help to explain the presence of lesions in different stages of development at any given time, as well as the pleiotropic nature of initial lesions reported over the years—for example, fatty streaks, focal intimal edema, intramural microthrombi, and so forth (Geer and Haust 1972). Because there is no animal model at present to address this issue, some help in differentiating acute effects from those of a chronic nature might be achieved by inducing VC in animals over different periods of time. Force parameters could be varied with appropriate dose concentrations of the contractile agonists, as could duration of the effect and frequency of administration. Histologic examination following such an investigation may provide definitive information. There is, in addition, a practical value associated with an understanding that
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VC may contribute to the initiation of atherogenesis, as discussed above. Information concerning the molecular biology of contraction of VSMCs may lead to new strategies for intervention in the development of lesions. Even though the focus of this article is on development of the early lesions of AS, it is also interesting to note that in terms of clinical studies with human subjects, as well as with experimental animals, counteracting contraction of VSMCs with agents such as rho-kinase inhibitors may also be beneficial in later stages (Shimokawa et al. 2002) and possibly help to bring about regression as well. Future research should be directed toward validating the concept that VC can contribute to the onset of atherogenesis. With the help of biomedical engineering science, methods may— and should—be developed to quantify force parameters that induce EI; for example, modification of the method of Helmke et al. (2003) or that of Zulliger et al. (2004). Animal experiments employing vasoactive substances and their inhibitors should target development of different types of lesions and their suppression.
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Caro CG, Fitz-Gerald JM, Schroter RC: 1971. Atheroma and arterial wall shear. Observation, correlation and proposal for a shear dependent mass transfer mechanism for atherogenesis. Proc R Soc Lond Biol Sci 177:109–159. Chan AC: 1998. Vitamin E and atherosclerosis. J Nutr 128:1593–1596. Cook JW, Taylor LM, Orloff SL, et al.: 2002. Homocysteine and arterial disease. Experimental mechanisms. Vascul Pharmacol 38:293–300. Cooke JP, Tsao PS: 1994. Is NO an endogenous antiatherogenic molecule? Arterioscler Thromb 14:653–655. el-Tamimi H, Mansour M, Wargovich TJ, et al.: 1994. Constrictor and dilator responses to intracoronary acetylcholine in adjacent segments of the same coronary artery in patients with coronary artery disease. Endothelial function revisited. Circulation 89:45–51. Fink AN, Frishman WH, Azizad M, et al.: 1999. Use of prostacyclin and its analogues in the treatment of cardiovascular disease. Heart Dis 1:29–40. Fournier A, Achard JM, Boutitie F, et al.: 2004. Is the angiotensin II type 2 receptor cerebroprotective? Curr Hypertens Rep 6: 182–189. Galle J, Mameghani A, Bolz SS, et al.: 2003. Oxidized LDL and its compound lysophosphatidylcholine potentiate AngII-induced vasoconstriction by stimulation of rhoA. J Am Soc Nephrol 14:1471–1479. Geer JC, Haust MD: 1972. Early lesions of atherosclerosis. In Pollak OJ, Simms HS, Kirk JE. eds. Smooth Muscle Cells in Atherosclerosis. Basel, Switzerland; Karger, 75–76. Gimbrone MA: 1999. Vascular endothelium, hemodynamic forces, and atherogenesis. Am J Pathol 155:1–5. Gries FA, Koschinsky T: 1991. Diabetes and arterial disease. Diabet Med 8:S82–S87. Gutstein WH: 1988. The central nervous system and atherogenesis: endothelial injury. Atherosclerosis 70:145–154. Gutstein WH: 1999. Vasospasm, vascular injury, and atherogenesis: a perspective. Hum Pathol 30:365–371.
Berk BC, Alexander RW: 1989. Vasoactive effects of growth factors. Biochem Pharmacol 38:219–225.
Hackett D, Larkin S, Chierchia S, et al.: 1987. Induction of coronary artery spasm by a direct local action of ergonovine. Circulation 75:577–582.
Borcherding SM, Meeves SG, Klutman NE, et al.: 1993. Calcium-channel antagonists for prevention of atherosclerosis. Ann Pharmacother 27:61–67.
Helmke BP, Rosen AB, Davies PF: 2003. Mapping mechanical strain of an endogenous cytoskeletal network in living endothelial cells. Biophys J 84:2691–2699.
Calkin AC, Sudhir K, Honisett S, et al.: 2002. Rapid potentiation of endotheliumdependent vasodilation by estradiol in postmenopausal women is mediated via cyclooxygenase 2. J Clin Endocrinol Metab 87:5072–5075.
Hill CE, Phillips JK, Sandow SL: 2001. Heterogeneous control of blood flow amongst different vascular beds. Med Res Rev 21: 1–60. Jiang JL, Li NN, Li YJ, et al.: 2002. Probucol preserves endothelial function by reduction
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of the endogenous nitric oxide synthase inhibitor level. Br J Pharmacol 135:1175–1182. Kaski JC, Maseri A, Vejar M, et al.: 1989. Spontaneous coronary artery spasm in variant angina is caused by a local hyperreactivity to a generalized constrictor stimulus. J Am Coll Cardiol 14:1456–1463. Kaski JC, Tousoulis D, McFadden E, et al.: 1992. Variant angina pectoris. Role of coronary spasm in the development of fixed coronary obstructions. Circulation 85:619–626. Kumari M, Grahame-Clarke C, Shanks N, et al.: 2003. Chronic stress accelerates atherosclerosis in the apolipoprotein E deficient mouse. Stress 6:297–299. Kuvin JT, Ramet ME, Patel AR, et al.: 2002. A novel mechanism for the beneficial vascular effects of high-density lipoprotein cholesterol: enhanced vasorelaxation and increased endothelial nitric oxide synthase expression. Am Heart J 144:165–172. Lamping KG: 1997. Hypercontractility of vascular muscle in atherosclerosis. Circulation 96:4131–4132. Luscher TF: 1992. Heterogeneity of endothelial dysfunction in hypertension. Eur Heart J 13(Suppl D):50–55. Luscher TF, Barton M: 2000. Endothelins and endothelin receptor antagonists: therapeutic considerations for a novel class of cardiovascular drugs. Circulation 102: 2434–2440. Luscher TF, Richard V, Tschudi M, et al.: 1990. Endothelial control of vascular tone in large and small coronary arteries. J Am Coll Cardiol 15:519–527. Malek AM, Alper SL, Izumo S: 1999. Hemodynamic shear stress and its role in atherosclerosis. JAMA 282:2035–2042. Mancini GB: 1996. Emerging concepts: angiotensin-converting enzyme inhibition in coronary artery disease. Cardiovasc Drugs Ther 10:609–612.
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Martin JF, Booth RF, Moncada S: 1991. Arterial wall hypoxia following thrombosis of the vasa vasorum is an initial lesion in atherosclerosis. Eur J Clin Invest 21: 355–359. Maseri A, Chierchia S: 1982. Coronary artery spasm: demonstration, definition, diagnosis, and consequences. Prog Cardiovasc Dis 25:169–192. Matsuda K, Ruff A, Morinelli TA, et al.: 1994. Testosterone increases thromboxane A2 receptor density and responsiveness in rat aortas and platelets. Am J Physiol 267: H887–H893. McCalden TA, Nath RG: 1989. Mechanisms of vascular supersensitivity in hypercholesterolemia. Stroke 20:238–241. McGuire JJ, Ding H, Triggle Endothelium-derived relaxing focus on endothelium-derived rizing factors. Can J Physiol 79:443–470.
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Noma K, Higashi Y, Jitsuiki D, et al.: 2003. Smoking activates rho-kinase in smooth muscle cells of forearm vasculature in humans. Hypertension 41:1102–1105. Pang CY, Xu H, Huang N, et al.: 2001. Amplification effect and mechanism of action of ET-1 in U-46619-induced vasoconstriction in pig skin. Am J Physiol Regul Integr Comp Physiol 280:R713–R720. Penny WF, Rockman H, Long J, et al.: 1995. Heterogeneity of vasomotor response to acetylcholine along the human coronary artery. J Am Coll Cardiol 25:1046–1055. Raines EW: 2004. PDGF and cardiovascular disease. Cytokine Growth Factor Rev 15: 237–254. Rosenkranz S, Knirel D, Dietrich H, et al.: 2002. Inhibition of the PDGF receptor by red wine flavonoids provides a molecular explanation for the bFrench paradox.Q FASEB J 16:1958–1960.
Russo G, Leopold JA, Loscalzo J: 2002. Vasoactive substances: nitric oxide and endothelial dysfunction in atherosclerosis. Vascul Pharmacol 38:259–269. Savineau JP, Marthan R: 1997. Modulation of the calcium sensitivity of the smooth muscle contractile apparatus: molecular mechanisms, pharmacological and pathophysiological implications. Fundam Clin Pharmacol 11:289–299. Seto M, Takuwa Y, Sasaki Y: 1999. The molecular mechanism of vasospasm and the attenuation by fasudil. Nippon Yakurigaku Zasshi 114:66P–70P. Shimokawa H: 2002. Rho-kinase as a novel therapeutic target in treatment of cardiovascular diseases. J Cardiovasc Pharmacol 39:319–327. Shimokawa H, Hiramori K, Iinuma H, et al.: 2002. Anti-anginal effect of fasudil, a Rhokinase inhibitor, in patients with stable effort angina: a multicenter study. J Cardiovasc Pharmacol 40:751–761. Strawn WB, Chappell MC, Dean RH, et al.: 2000. Inhibition of early atherogenesis by losartan in monkeys with diet-induced hypercholesterolemia. Circulation 101: 1586–1593. Yokoyama M, Hirata K, Miyake R, et al.: 1990. Lysophosphatidylcholine: essential role in the inhibition of endothelium-dependent vasorelaxation by oxidized low density lipoprotein. Biochem Biophys Res Commun 168:301–308. Zulliger MA, Fridez P, Hayashi K, et al.: 2004. A strain energy function for arteries accounting for wall composition and structure. J Biomech 37:989–1000.
PII S1050-1738(04)00112-4
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Research during the past century has clearly shown that endothelial injury (EI) and/or endothelial dysfunction (ED) are among the major events determining the onset of atherosclerosis. Included in the events that may elicit endothelial damage, vasoconstriction (VC) has received relatively little attention. This conceptual review attempts to show that in elastic and conduit arteries, VC is not only capable of producing EI/ ED, but is also closely associated with many recognized proatherogenic stimuli. Of related interest is the observation that a number of suspected antiatherogenic stimuli oppose VC by their vasodilatory effects, lending further support to this relationship. In addition, recent developments in the knowledge of the molecular basis of VC (including the role of specific inhibitors) are discussed, and their potential for preventing lesion formation and thus becoming novel therapeutic alternatives against the onset of atherosclerosis are highlighted. (Trends Cardiovasc Med 2004;14:257–261) D 2004, Elsevier Inc.
It is well known that endothelial injury (EI) and endothelial dysfunction (ED) each play a major role in the onset of atherosclerosis (AS). This damage is usually focal, may have various sources (e.g., hyperlipidemia, oxidative stress, etc.), can be purely functional (ED) or
William H. Gutstein is at the New York Medical College, Department of Pathology, Valhalla, New York, USA. Cristian A. Pe´rez is at The Rockefeller University, Laboratory of Molecular Genetics, New York, New York, USA. *Address correspondence to: William H. Gutstein, 47 East 87th Street, New York, NY 10128, USA. Tel and fax: (+1) 212-831-8674; e-mail: [email protected]. D 2004, Elsevier Inc. All rights reserved. 1050-1738/04/$-see front matter
TCM Vol. 14, No. 7, 2004
manifest structural damage (EI), and is usually silent as it develops into an initial atherosclerotic lesion. Currently, there is no clear understanding of how a normal endothelium can be transformed into either of these conditions. The objective of the present review is to call attention to the concept that vasoconstriction (VC) is an early event that could lead to such endothelial damage and therefore to atherogenesis.
Mechanisms of Injury: Relationship to Degree and Parameters of Force Associated with VC
During VC, vascular smooth muscle cells (VSMCs) of the tunica media contract and generate a circumferential stress that is transmitted inwardly to the
enclosed intima and its adjacent endothelial layer. When VC occurs, the endothelial cells may be lifted from their underlying basement membranes and become separated at their intercellular junctions, depending on the parameters of the generated force such as magnitude or severity, duration, and frequency of occurrence. Although we believe this to be the major mechanism by which injury occurs, other sources of EI/ED may also develop during VC. For example, when contraction occurs focally in the tunica media, the arterial wall becomes compact at that site. This change in wall density may have consequences for mass transport and result in interference with intramural traffic, ultimately compromising the integrity of the endothelium (Caro et al. 1971). Similarly, obstruction of the vasa vasorum may occur, producing hypoxemic conditions as far lumenally as the intima, with potential injury to the endothelium (Martin et al. 1991).
Histology of the Endothelium and Intima Following VC
In several studies that ranged over various species—including living human subjects—we and others have clearly shown that spontaneous or experimentally induced VC resulted in histologic injury to the endothelium and intima, which was focal in nature and consisted of severe intimal and often medial damage (reviewed in detail in Gutstein 1999). At times, considerable cell death also occurred in the latter, but only the intima is described here. A striking observation was that the endothelium demonstrated a variety of degenerative features that colocalized with the region of VC, including lifting of the cells from their basement membranes into the lumen, attachment of neutrophils and monocytes, separation at intercellular junctions, and degenerative and necrotizing changes leading to frank denudation with clear portals of entry for the
257
Figure 1. Vasoconstriction (VC)-induced arterial cellular changes. (A) Semi-thin light microscopic section of normal left coronary artery of the rat near its origin, stained with toluidine blue in a control animal in which VC had not been induced. The endothelial cells (E) form a flattened layer; internal elastic lamina and media are of normal thickness. (B) Semi-thin light microscopic section of left coronary artery of the rat near its origin, in which VC has been induced by hypothalamic stimulation (Gutstein 1988). The vessel is markedly contracted (see inset), showing reduction in diameter, buckling of the internal elastic lamina (IEL), and thickening of the wall. Endothelial cells (E) can be seen projecting into the lumen with creation of large vacuolated spaces beneath them. The medial vascular smooth muscle cell (VSMC) layer shows disarray and the diameters of these cells appear to be increased. Calibration marks in (A, B, and inset) are 25 Am. (C, D) Schematic representations of transverse microscopic sections through arterial wall and one exposed to focal VC—equivalent to (A) and (B), respectively. (Parts A and B reprinted from Atherosclerosis, 70, WH Gutstein, b The central nervous system and atherogenesis: endothelial injury,Q 145–154, copyright 1988, with permission of Elsevier.)
possible inflow of cellular and molecular components from the circulating plasma. In some bbare areas,Q platelet aggregates and microthrombi were incorporated and at times attached to subendothelial collagen. In a few studies, the subendothelial space contained fibrin, erythrocytes, or tracer substances previously injected into the lumen to assess permeability of the endothelial barrier. Occasional monocytes—some with apoptotic appearance—were also observed in this location. Intimal thickening, due to VSMC proliferation and their extracellular products, was frequently observed and marked at times. The internal elastic lamina was usually ruffled and often disrupted. Figure 1 depicts transitions from normal to altered endothelium in both light microscopic observations and diagrammatic representations. In essence, these histologic alterations may be seen with varying degrees of VC and demonstrate that EI/ED is frequently present. Although often severe, they are not generally conceived of as occurring with VC and therefore have escaped detection as a possible starting point for the onset of atherogenesis. Some possible reasons why histologic changes due to VC may not be appreciated include the fact that arteries are not generally examined fol-
258
lowing an episode of VC. Additionally, inspection at the microscopic level is required and even should such observations be employed, the rapid rate of endothelial regeneration may obscure the finding of EI if there is a delay between VC and examination, because newly regenerated cells may already be in place and, although appearing morphologically normal, are nevertheless
functionally disturbed; that is, in a state resembling ED.
Supporting Evidence for the Role of VC in Atherogenesis
Although VC is a common cardiovascular event, its potential contribution to the pathogenesis of AS has seldom been considered. Table 1 lists several
Table 1. Proatherogenic factors that enhance vasoconstriction Proatherogenic factors
Reference
Hypercholesterolemia Essential hypertension Diabetes mellitus Cigarette smoking Angiotensin II Endothelin Testosterone Thromboxane A2 Oxidized low-density lipoprotein/ lysophosphatidylcholine Epidermal and platelet-derived growh factors (EGF, PDGF) Homocysteinemia Emotional stress Ergonovine sensitivity
McCalden and Nath 1989 Luscher 1992 Gries and Koschinsky 1991 Noma et al. 2003 Galle et al. 2003 Bauersachs et al. 2000 Matsuda et al. 1994 Matsuda et al. 1994 Galle et al. 2003, Yokoyama et al. 1990 Berk and Alexander 1989
Low wall shear stress
Cook et al. 2002 Kumari et al. 2003 Hackett et al. 1987, Kaski et al. 1989 and 1992 Gimbrone 1999, Malek et al. 1999
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conditions deemed important for the development of AS, all of which—despite their varying effects on the arterial wall— have in common the capacity for producing or enhancing VC. Consequently, this group of seemingly unrelated conditions, likely to be linked by the contraction of VSMCs to EI/ED, may provide significant reasons for believing that VC contributes to the onset of early atherosclerotic lesions (see Table 1). Although limited in their number, they include important influences such as risk factors for the development of new lesions (both those that are traditional and those more recently discovered) and other conditions, and thus suggest the possibility of a general relationship between VC and atherogenesis. Parenthetically, just as induction of VC may be proatherogenic, there is evidence to show that influences that favor vasodilation may be antiatherogenic. In Table 2 are listed several conditions that support this possibility. In this context, it may be noted that vasodilation can arise directly (e.g., due to an increase in endothelial cell levels of dilators such as nitric oxide) or indirectly (e.g., by antagonism of receptors for contracting substances such as angiotensin II or endothelin I) (Babaei et al. 2000, Strawn et al. 2000), shifting the balance between contracting and relaxing factors to the dilated state (Lamping 1997). Despite these relationships, it is important to take note of certain caveats concerning the physiology of VC. The literature shows that the physiology, biochemistry, and molecular biology are not the same in every vascular bed. Consequently, arterial segments that respond to certain stimuli in one regional
location may fail to do so in another (Luscher et al. 1990) or even in an adjacent segment (el-Tamimi et al. 1994, Penny et al. 1995). Responses at a given site may also vary in sensitivity, efficacy, magnitude, duration, and frequency to a given stimulus (Hill et al. 2001, Maseri and Chierchia 1982). Additionally, a distinction between the vasoactive effects of a given substance versus its VSMC proliferative effect has to be considered. For example, in vitro studies indicate that platelet-derived growth factor is mitogenic for VSMCs in culture (Raines 2004), whereas animal experiments have established its vasoactivity (Berk and Alexander 1989). Moreover, interaction of vasoactive substances may occur with synergistic or opposing effects. For example, low concentrations of endothelin 1 potentiated the effects of norepinephrine and U-46619 (a thromboxane A2 mimetic) in animal experiments (Pang et al. 2001). On the other hand, angiotensin II—a potent vasoconstrictor—has been reported to offer paradoxical cerebroprotection against stroke (Fournier et al. 2004), the explanation of which apparently lies in the simultaneous activation of the receptors AT-1 and AT-2 and the subsequently AT-2mediated production of the vasodilators nitric oxide and prostacyclin. Consequently, exploration of such possible interactions is necessary before conclusions are drawn concerning activity of vasoconstrictor substances and their mediators in any given situation.
Molecular Biology of VC
Because Table 1 reveals many factors linking VC to the possible development of AS, knowledge of the molecular biol-
Table 2. Antiatherogenic factors that suppress vasoconstriction Antiatherogenic factors Endogenous Nitric oxide Prostacyclin EDHF High-density lipoprotein Estradiol Exogenous Antioxidants (e.g., probucol, vitamin E) Rho-kinase inhibitors (e.g., fasudil Y27632) Flavonoids (French paradox)
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Reference Russo et al. 2002 Fink et al. 1999 McGuire et al. 2001 Kuvin et al. 2002 Calkin et al. 2002 Chan 1998, Jiang et al. 2002 Shimokawa 2002, Shimokawa et al. 2002 Rosenkranz et al. 2002
ogy of the contraction of VSMCs may provide opportunities for important therapeutic strategies of intervention or prevention of early lesions. Indeed, some of these are currently in use, without the realization, perhaps, that their efficacy may depend on the success rate with which they are able to counteract or oppose underlying VC. The basic molecular principles of the contractile process have been reviewed elsewhere (Lamping 1997), and are generally associated with phosphorylation/dephosphorylation of lightchain myosin and calcium-independent and -dependent events. An additional mechanism, termed calcium sensitivity— generation of a greater force development for any given calcium concentration (Savineau and Marthan 1997)— relies on the activity of the relatively novel enzyme rho kinase, a member of the ras family of small guanosine triphophatase molecules (Shimokawa 2002, Shimokawa et al. 2002). Based on these and other molecular interactions, the results of animal experiments and clinical investigations have provided several potential strategies that are currently available for controlling and modifying the development and progression of lesions, such as application of nitrogen donors (Cooke and Tsao 1994); calcium channel antagonists (Borcherding et al. 1993); converting enzyme inhibitors of constrictor agents (Mancini 1996); receptor blockers for specific contractile agonists (Luscher and Barton 2000); and, more recently, rho-kinase inhibitors—such as fasudil— for VC prevention (Batchelor et al. 2001, Seto et al. 1999) as well as for vasospasm, its more severe form of contraction. Because these therapeutic applications intervene with primary events in the contractile process of VSMCs, and reduce VC as a result, we believe that they lend further support to the concept proposed in this article.
Conclusion
Taken together, the weight of evidence reveals that a number of established risk factors and conditions that are important for the development of AS also possess the ability to induce or enhance VC. Similarly, various endogenous and exogenous agents that tend to suppress lesion formation increase dilation. Even observations
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such as those that patients with Prinzmetal’s variant angina—sensitive to ergonovine administration by constriction of the coronary arteries and exhibiting subsequent development of organic lesions in precisely the same location (see Table 1 and Kaski et al. 1989 and 1992)—reflect such an association. Another example consistent with this concept is the bunorthodoxQ atheroprotective effect found in the French paradox (lower incidence of cardiovascular disease in geographic regions with red wine consumption above average), which may be due to vasorelaxant effects of red wine flavonoids inhibiting the platelet-derived growth factor receptor (see Table 2). These opposing vasomotor activities suggest the existence of a general relationship between the contractile behavior of elastic and conduit arteries and atherogenesis. Although there may be exceptions, the fact that VC is so ubiquitous a cardiovascular event and appears to be such an effective cause of EI further strengthens this relationship. A point of interest concerning the development of endothelial damage as a function of VSMC contraction is the question of variation in individual contributions of the separate parameters of force that may bring it about. For example, is it possible that multiple episodes of a moderate degree of force at a given site, over time, create as much as, less, or a greater amount of EI/ED than does a single but severe episode? If this is indeed true, because of the potential diversity of these events, it could help to explain the presence of lesions in different stages of development at any given time, as well as the pleiotropic nature of initial lesions reported over the years—for example, fatty streaks, focal intimal edema, intramural microthrombi, and so forth (Geer and Haust 1972). Because there is no animal model at present to address this issue, some help in differentiating acute effects from those of a chronic nature might be achieved by inducing VC in animals over different periods of time. Force parameters could be varied with appropriate dose concentrations of the contractile agonists, as could duration of the effect and frequency of administration. Histologic examination following such an investigation may provide definitive information. There is, in addition, a practical value associated with an understanding that
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VC may contribute to the initiation of atherogenesis, as discussed above. Information concerning the molecular biology of contraction of VSMCs may lead to new strategies for intervention in the development of lesions. Even though the focus of this article is on development of the early lesions of AS, it is also interesting to note that in terms of clinical studies with human subjects, as well as with experimental animals, counteracting contraction of VSMCs with agents such as rho-kinase inhibitors may also be beneficial in later stages (Shimokawa et al. 2002) and possibly help to bring about regression as well. Future research should be directed toward validating the concept that VC can contribute to the onset of atherogenesis. With the help of biomedical engineering science, methods may— and should—be developed to quantify force parameters that induce EI; for example, modification of the method of Helmke et al. (2003) or that of Zulliger et al. (2004). Animal experiments employing vasoactive substances and their inhibitors should target development of different types of lesions and their suppression.
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