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The anti-atherogenic properties of HDL particles
International Congress Series 1303 (2007) 103 – 110
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The anti-atherogenic properties of HDL particles G. Zuliani ⁎, G.B. Vigna, R. Fellin Department of Clinical and Experimental Medicine, Section of Internal Medicine, Gerontology and Geriatrics, University of Ferrara, Italy
Abstract. High density lipoproteins (HDLs) are a class of lipoproteins characterized by small diameter and high density. Epidemiological studies have demonstrated that HDL cholesterol levels are inversely and independently related to the incidence of coronary heart disease. The mechanisms involved in HDL protection against atherosclerosis are uncertain, but the beneficial effect of HDLs might be the consequence of its properties. The best known of them is related to the “reverse cholesterol transport”, i.e. the transfer of cholesterol from non-hepatic cells to the liver. Second, HDLs exert anti-inflammatory activity by inhibiting the expression of adhesion molecules by endothelial cells and the subsequent transmigration of monocytes. Third, HDLs have antioxidant activity through the anti-oxidative properties of apoprotein A-I, and the presence of enzymes such as paraoxonase, glutathione-peroxidase, and PAF acetylhydrolase. Fourth, HDLs display an antithrombotic effect by inhibiting platelets aggregation, reducing von Willebrand factor levels, and enhancing the activity of activated protein C and S. Fifth, HDLs have a beneficial effect on endothelial function by activating endothelial NO synthase and enhancing NO release. Finally, high HDL-C has been associated with “longevity syndrome” while low HDL-C has been related to the “frailty syndrome”, suggesting a possible role of HDL in the phenomenon of successful aging. © 2007 Elsevier B.V. All rights reserved. Keywords: HDL; Cholesterol; Atherosclerosis; Endothelium; Inflammation
1. Introduction High-density lipoprotein (HDL) encopasses a class of heterogeneous lipoproteins which have in common high density (N 1.063 g/mL), small size (diameter between 5 and 17 nm),
⁎ Corresponding author. Tel.: +39 0532 247409; fax: +39 +0532 210884. E-mail address: [email protected] (G. Zuliani). 0531-5131/ © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ics.2007.04.003
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and absence of apoprotein B. On average, lipids constitute about 50% of the total mass of HDL (30% phospholipids, 10–20% cholesterol, and 5% triglycerides). HDL particles also contain a large amount of apolipoproteins (principally apo A-I and apo A-II), and some enzymes which are probably relevant to metabolic pathways and anti-atherogenic properties of these particles. A large number of epidemiological studies have demonstrated that HDL cholesterol (HDL-C) levels are inversely correlated with coronary heart disease (CHD). In particular, both the Framingham study and the PROCAM study show that low HDL plasma levels are strongly associated with higher incidence of CHD, independently of low-density lipoprotein cholesterol (LDL-C) levels [1,2]. Moreover, it has been reported that low HDL-C might be the most prevalent lipid abnormality (about 36%) in men affected by CHD [3]. Nevertheless, contrary to LDL-C, a cause–effect relationship between low HDL-C values and CHD incidence has not been demonstrated for certain. As a matter of fact, the U-shaped association between HDL-C and CHD, and the strong the association of low HDL-C with both metabolic syndrome and systemic inflammation do not support a causal association [4]. Despite these uncertainty, the inverse relationship between HDL-C and cardiovascular risk is supported by several anti-atherogenic properties of these particles (Table 1) including: (1) the reverse cholesterol transport; (2) the anti-inflammatory and anti-oxidant activity; (3) the anti-thrombotic activity and the endothelial protection exerted by HDL particles. Moreover, the association between HDL-C levels and longevity and/or good functional status suggests the hypothesis of a role of this lipoprotein in the phenomenon of successful aging (i.e. aging without chronic diseases and functional impairment).
Table 1 The principal anti-atherosclerotic properties of HDL particles Reverse cholesterol transport • Transport cholesterol from peripheral tissues and macrophages to the liver for disposal into the bile • Exchange apoproteins with other lipoproteins Anti-inflammatory • Inhibit the expression of adhesion molecules (e.g. VCAM, ICAM, E selectin) • Inhibit the expression of MCP-1 in response to oxidized LDLs • Inhibit the production and enhance the degradation of PAF Antioxidant • Remove hydroperoxides from LDLs by apo A-I • Destroy lipid hydroperoxides of LDLs by paraoxonase and GPX • Remove oxidized phospholipids from LDLs by PAF acyhydrolase Endothelial protection • Activate eNOS thus inducing NO production • Enhance NO-dependent peripheral vasodilation • Increase PGI2 release by endothelial cells • Inhibit in vitro the secretion of endothelin 1 Antithrombotic • Increase the production of NO and PGI2 • Inhibit PAF production • Inverse correlation with vW factor plasma concentration • Enhance the activity of protein C and S
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2. The reverse cholesterol transport The “reverse cholesterol transport” describes a specific process in which cholesterol is transferred from non-hepatic cells to the liver (see Fig. 1), being the only organ capable of removing cholesterol from the body through its secretion into the bile [4]. Briefly, lipid free apo A-I or nascent HDLs, produced by the liver or intestine or shed during lipolysis of triglycerides (TG) rich lipoproteins, initiate the efflux of cholesterol from cell membranes. The “gatekeeper” of this pathway is an ATP-binding cassette (ABC) transporter called ABCA1. ABC transporters are a superfamily of proteins that use ATP as a source of energy on order to transport substrates between different cellular compartments and from the cell. ABCA1 transports cellular cholesterol and phospholipids (mostly phosphatidylcholine) to cell surface-bound apolipoproteins. This protein therefore represents the first and rate-controlling step in the reverse cholesterol transport pathway. Discoidal HDLs are then esterified by Lecithin-Cholesterol Acyl Tranferase (LCAT) becoming spherical as cholesteryl esters (CE) move to the core of the particles. The exchange of CE for TG mediated by Cholesteryl Ester Transfer Protein (CETP) moves CE to lipoprotein remnants which are cleared by the liver the apo B/apo E receptor. The joint action of CEPT and Hepatic Lipase [i.e. the transfer of CE to apo B containing lipoproteins and the hydrolysis of TG and phospholipids (PL)] leads to the formation of smaller HDLs that are the favorite ligand for scavenger receptor type B1 (SR-B1). SR-B1 is a multi-ligand receptor (member of the CD36 proteins family) expressed by hepatocytes and steroid-hormone producing tissues, that interacts with both HDL and LDL
Fig. 1. The HDL mediated reverse cholesterol transport.
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particles. It mediates the flux of free-cholesterol from HDLs in a bidirectional way that is depending on the cholesterol gradient; moreover, it mediates the unidirectional efflux of CE, PL and TG from HDLs, thus promoting the depletion of their lipid core. Finally, the combined action of CETP, HL, and PL transfer protein (PLTP) brings again to the formation of lipid-free apo A-I or lipid poor HDLs; of consequence, new acceptors of cellular cholesterol are perpetually regenerated. Interestingly, it has been shown that the disruption of one or more steps in this cycle results in accelerate atherosclerosis, and this is independent from the decreasing or increasing of HDL-C plasma levels. 3. The anti-inflammatory and antioxidant actions of HDL It has been shown that atherosclerosis (ATS) is an inflammatory disorder characterized by the progressive accumulation of macrophages and T lymphocytes in the arterial intima; this process is associated with an increase in the plasma levels of several inflammatory markers. The early step in this inflammatory disease is the adhesion of monocytes to the “dysfunctional” endothelium, mediated by adhesion molecules such as VCAM-1, ICAM-1 and E-selectin. Successively, the monocytes are recruited into the sub-endothelial space by specific chemokines such as MCP-1. In vitro studies suggest that HDLs inhibit the expression of these adhesion molecules [5] by inhibiting endothelial sphingosine kinase; this enzyme is fundamental in the pathway by which TNF-α stimulates the nuclear translocation of NF-kb, which in turn activates the expression of adhesion molecules [5]. Furthermore, HDLs inhibit in vitro the transmigration of monocytes in response to oxidized LDLs [6], and this activity seems to be related to paraoxonase and PAF acylhydrolase transported by HDL particles [7]. HDLs also inhibit the expression of MCP-1 in response to oxidized LDLs [6], and this process has been related to paraoxonase activity. HDLs might exert their anti-atherosclerotic activity also by reducing the phenomenon of LDL oxidation [8]. Apo A-I, the principal apoprotein of HDLs, is capable of removing lipid hydroperoxides from LDLs; successively, these products are selectively removed by liver cells. HDLs also contain enzymes such as paraoxonase 1 and 3, and GPX which are able to destroy lipid hydroperoxides that oxidize the PL of LDLs. In addition, HDLs contain other enzymes, namely PAF-acylhydrolase, that are able to remove oxidized PL. Interestingly, during inflammation HDL-C plasma levels decrease while HDL particles change their composition by losing apo A-I, paraoxonase and PAF-acylhydrolase [7], and acquiring serum amyloid A and ceruloplasmin [7,9]. The affinity of HDLs for hepatocytes is thus reduced, while the affinity for macrophages is much increased [9]; moreover, HDLs lose the ability to protect LDLs against oxidation [7]. On the whole these changes suggest that during acute inflammation HDLs lose most of their anti-inflammatory properties while possibly becoming “pro-inflammatory”. We have recently evaluated the relationship between markers of systemic inflammation and HDL-C levels by studying a large sample of community dwelling older Italian subjects [10]. We found that increased interleukin-6 plasma levels (III vs. I tertile, OR:2.10; 1.10– 3.75) are associated with low HDL-C levels, independent of a large number of confounders including the main traits of metabolic syndrome (triglycerides, fasting insulin, diabetes,
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hypertension, BMI, waist circumference), and life style habits (smoking, alcohol intake, physical activity). The adjusted attributable risk of low HDL-C in the “exposed group” (III tertile of IL-6) was 54%. We have thus provided the epidemiological evidence that inflammation (i.e. increased IL-6 levels) might be one of the main correlates of low HDL-C levels in the older population. 3.1. The endothelial protection and antithrombotic activity It has been shown in the last decades that vascular endothelium is very important in the maintenance of vascular homeostasis through the production of a number of molecules that modulate hemostasis, vascular tone, and inflammatory reactions. The injury to the endothelium, independently from the cause (e.g. hypertension, diabetes, hypercholesterolemia, smoking), produces an alteration of the endothelial physiology, the so called endothelial “dysfunction” (ED). This is considered a key early step not only in the development of the atherosclerotic lesion, but also in the progression, destabilization, and complication of the atherosclerotic plaque. Low HDL-C is associated with the ED [11], and that elevation of HDL-C levels leads to an improvement of the ED [12]. Nitric oxide (NO) is an essential signaling molecule that induces relaxation in vascular smooth muscle cells; it is produced in the endothelial cells by a costitutive NO synthase (eNOS), and a reduction in NO bioavailability is one of the permanent features of the ED. In vitro HDLs, but not lipid free apo A-I, activate eNOS in cultured endothelial cell [13]; it has been proposed that the interaction of HDLs with SR-B1 modifies the membrane cholesterol distribution and morphology thus influencing eNOS activity [14]. HDLs might also serve as a carrier of bioactive lysosphingolipids that are known to fully mimic the ability of HDLs in inducing vasorelaxation. These findings are indirectly supported by in vivo studies that have demonstrated a positive correlation between HDL-C plasma levels and NO-dependent peripheral vasodilation in healthy individuals[14], as well as in patients with diabetes or CHD. Prostacyclin (PGI2) is another vasodilator produced by endothelium that acts sinergically with NO to induce vascular smooth cells relaxation. HDLs, but not dilapidated HDLs, cause a dose-dependent increase of PGI2 release in cultured endothelial cells [15], and this is prevented by Cox inhibitors. This phenomenon might be dependent on the uptake and hydrolysis of HDL derived CE, mediated by SR-B1 [14]. HDLs also upregulate the basal and cytokine-induced expression of Cox 2 through a mechanism that is independent of NF-kb activation [16]. Interestingly, HDLs inhibit the in vitro the secretion of endothelin 1, a potent vasoconstrictor peptide that binds to specific receptors to reverse the response to NO, in endothelial cells [17]. Another interesting property of HDLs is their powerful antithrombotic activity. This action is mediated by the over-mentioned increase in the production of NO and PGI2 (both inhibit platelet aggregation), but also by a reduction of PAF production by the endothelial cells [18]. Indeed, it is known that HDLs transport both PAF acylhydrolase and other enzymes, such as LCAT and paraoxonase, which could degrade PAF. Furthermore, plasma concentrations of von Willebrand factor, a protein playing an essential role in platelets adhesion and aggregation, are inversely correlated with plasma HDL-C levels.
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On the whole, HDLs seem to exert a beneficial effect on endothelium function by modulating the activity and/or production of a number of molecules (NO, PGI2, endothelin, PAF, vWF), and affecting vascular tone and thrombogenicity. Finally, HDLs may beneficially influence the hemostatic balance; as a matter of fact, a positive correlation between HDL-C and TFPI (tissue factor pathway inhibitor) plasma levels has been reported; moreover, HDLs might also enhance the anticoagulant activity of activate protein C and S [14]. 3.2. HDL, longevity, and successful aging Since the early publications of Glueck et al. [19,20] increased levels of HDL-C (hyperalpha lipoproteinemia) and reduced levels of LDL-C (hypo-beta lipoproteinemia) had been associated with longevity in selected pedigree; these findings were justified by the antiatherosclerotic lipid profile typical of these particular subjects and to the low incidence of myocardial infarction. Epidemiological data from the Framingham successively showed that older individuals who subsequently become “healthy” octogenarias (i.e. without CHD and stroke) were not characterized by high HDL-C or low LDL-C levels; on the contrary they were exceedingly unlikely to have low HDL-C levels [21]. Similar data emerged also from the val Vibrata Aging Study; in this sample of Italian population, free-living healthy octo-nonagenarians (free from CHD and stroke) were characterized by a very low prevalence of low HDL-C (3.9%) in face of a high prevalence of high LDL-C (25%). Interestingly, in the same population we found that, for the same level of total cholesterol, HDL-C was significantly reduced in disabled compared to free-living older individuals [22]. The latter observation suggested the existence of an association between HDL-C and health status and/or functional impairment in the elderly. This finding was successively confirmed in a different population [23]; in the I.R.A. study we demonstrated that low HDL-C was independently associated with disability and, most important, that low HDL-C independently predicted the 2 years worsening in functional status. The strength of the association between HDL-C and health status was successively confirmed in a large population [24]. Volpato et al. found that, among community dwelling older individuals with hypocholesterolemia and normal serum albumin, HDL-C levels discriminated two groups of subjects with intermediate (29%) or low (17%) total mortality risk. On the whole, these data suggest the existence of a strong direct relationship between HDL-C and longevity and/or successful aging, defined as aging with a good health status Table 2 The reported associations between HDL plasma levels and longevity /successful AGING HDL-C and longevity-successful aging • High HDL-C is associated with longevity in selected kindreds • Low HDL-C is rare in older healthy individuals without functional impairment • Low HDL-C is very frequent in older individuals with disability • HDL-C is positively associated with physical function • Low HDL-C predicts the worsening in functional status in older individuals • HDL-C is inversely correlated with total mortality in older individuals with low total cholesterol levels
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and without disability (see Table 2); nevertheless, it has to be underlined that, at present time, a causal relationship between HDL and longevity and/or successful aging has not been demonstrated. References [1] T. Gordon, et al., High identity lipoprotein as a protective factor against coronary heart disease, Am. J. Med. 62 (1977) 707–714. [2] G. Assmann, et al., High-density lipoprotein cholesterol as a predictor of coronary heart disease risk. The PROCAM experience and pathophysiological implications for reverse cholesterol transport, Atherosclerosis (1996) S11–S20 (Suppl). [3] J. Genest Jr., et al., Lipoprotein cholesterol, apolipoprotein A-I and B and lipoprotein (a) abnormalities in men with premature coronary artery disease, J. Am. Coll. Cardiol. 19 (1992) 792–802. [4] A. von Eckardstein, J.-R. Nofer, G. Assmann, High density lipoprotein and arteriosclerosis. Role of cholesterol efflux and reverse cholesterol transport, Arterioscler. Thromb. Vasc. Biol. 21 (2001) 12–27. [5] P. Xia, et al., High density lipoprotein (HDL) interrupt the sphingosine kinase signaling pathway. A possible mechanism for protection against atherosclerosis by HDL, J. Biol. Chem. 274 (1999) 33143–33147. [6] M. Navab, et al., Monocyte transmigration induced by modification of low density lipoprotein in cocultures of human aortic wall cells is due to induction of monocyte chemotactic protein 1 synthesis and is abolished by high density lipoprotein, J. Clin. Invest. 88 (1991) 2039–2046. [7] B.J. Van Lenten, et al., Antiinflammatory HDL becomes pro-inflammatory during acute phase response. Loss of protective effect of HDL against LDL oxidation in aortic wall cell cocultures, J. Clin. Invest. 96 (1995) 2758–2767. [8] P.J. Barter, et al., Antiinflammatory properties of HDL, Circ. Res. 95 (2004) 764–772. [9] R. Kisilevski, L. Subrahmanyan, Serum amyloid A changes high density lipoprotein's cellular affinity, Lab. Invest. 66 (1992) 778–785. [10] G. Zuliani, et al., High interleukin-6 plasma levels are associated with low HDL-C levels in community dwelling older adults: the InChianti study, Atherosclerosis (in press) [Jun 18, Electronic publication ahead of print]. PMID: 16787648. [11] N.N. Chan, H.M. Colhoun, P. Vallance, Cardiovascular risk factors as determinants of endotheliumdependent and endothelium-independent vascular reactivity in the general population, J. Am. Coll. Cardiol. 38 (2001) 1814–1820. [12] J.T. Kuvin, et al., 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 (2002) 165–172. [13] I.S. Yuhanna, et al., High-density lipoprotein binding to scavenger receptor-BI activates endothelial nitric oxide synthase, Nat. Med. 7 (Jul 2001) 853–857. [14] L. Calabresi, M. Gomaraschi, G. Franceschini, Endothelial protection by high-density lipoproteins. From bench to bedside, Arterioscler. Thromb. Vasc. Biol. 23 (2003) 1724–1731. [15] L.N. Fleisher, et al., Stimulation of arterial endothelial cell prostacyclin synthesis by high density lipoproteins, J. Biol. Chem. 257 (1982) 6653–6655. [16] G.W. Cockerill, et al., High-density lipoproteins differentially modulate cytokine-induced expression of Eselectin and cyclooxygenase-2, Arterioscler. Thromb. Vasc. Biol. 19 (1999) 910–917. [17] H. Unoki, J. Fan, T. Watanabe, Low-density lipoproteins modulate endothelial cells to secrete endothelin-1 in a polarized pattern: a study using a culture model system simulating arterial intima, Cell Tissue Res. 295 (1999) 89–99. [18] J. Sugatami, et al., High-density lipoprotein inhibits the synthesis of platelet-activating factor in human vascular endothelial cells, J. Lipid Mediators Cell Signal. 13 (1996) 73–88. [19] C.J. Glueck, et al., Longevity syndromes: familial hypobeta and familial hyperalfpha lipoproteinemia, J. Lab. Clin. Med. 88 (1976) 941–957. [20] C.J. Glueck, et al., Hyperalpha- and hypobeta-lipoproteinemia in octogenarian kindreds, Atherosclerosis 27 (1977) 387–406.
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[21] E.J. Schaefer, et al., Plasma lipoproteins in healthy octogenarians: lack of reduced high density lipoprotein cholesterol levels: results from the Framingham heart study, Metabolism 4 (1989) 293–296. [22] G. Zuliani, et al., High-density lipoprotein cholesterol strongly discriminates between healthy free-living and disabled octo-nonagenarians. A cross sectional study, Aging Clin. Exp. Res. 9 (1997) 335–341. [23] G. Zuliani, et al., Low levels of high-density lipoprotein cholesterol are a marker of disability in the elderly, Gerontology 45 (1999) 317–322. [24] S. Volpato, et al., The value of serum albumin and high-density lipoprotein cholesterol in defining mortality risk in older persons with low serum cholesterol, J. Am. Geriatr. Soc. 49 (2001) 1142–1147.
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The anti-atherogenic properties of HDL particles G. Zuliani ⁎, G.B. Vigna, R. Fellin Department of Clinical and Experimental Medicine, Section of Internal Medicine, Gerontology and Geriatrics, University of Ferrara, Italy
Abstract. High density lipoproteins (HDLs) are a class of lipoproteins characterized by small diameter and high density. Epidemiological studies have demonstrated that HDL cholesterol levels are inversely and independently related to the incidence of coronary heart disease. The mechanisms involved in HDL protection against atherosclerosis are uncertain, but the beneficial effect of HDLs might be the consequence of its properties. The best known of them is related to the “reverse cholesterol transport”, i.e. the transfer of cholesterol from non-hepatic cells to the liver. Second, HDLs exert anti-inflammatory activity by inhibiting the expression of adhesion molecules by endothelial cells and the subsequent transmigration of monocytes. Third, HDLs have antioxidant activity through the anti-oxidative properties of apoprotein A-I, and the presence of enzymes such as paraoxonase, glutathione-peroxidase, and PAF acetylhydrolase. Fourth, HDLs display an antithrombotic effect by inhibiting platelets aggregation, reducing von Willebrand factor levels, and enhancing the activity of activated protein C and S. Fifth, HDLs have a beneficial effect on endothelial function by activating endothelial NO synthase and enhancing NO release. Finally, high HDL-C has been associated with “longevity syndrome” while low HDL-C has been related to the “frailty syndrome”, suggesting a possible role of HDL in the phenomenon of successful aging. © 2007 Elsevier B.V. All rights reserved. Keywords: HDL; Cholesterol; Atherosclerosis; Endothelium; Inflammation
1. Introduction High-density lipoprotein (HDL) encopasses a class of heterogeneous lipoproteins which have in common high density (N 1.063 g/mL), small size (diameter between 5 and 17 nm),
⁎ Corresponding author. Tel.: +39 0532 247409; fax: +39 +0532 210884. E-mail address: [email protected] (G. Zuliani). 0531-5131/ © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ics.2007.04.003
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and absence of apoprotein B. On average, lipids constitute about 50% of the total mass of HDL (30% phospholipids, 10–20% cholesterol, and 5% triglycerides). HDL particles also contain a large amount of apolipoproteins (principally apo A-I and apo A-II), and some enzymes which are probably relevant to metabolic pathways and anti-atherogenic properties of these particles. A large number of epidemiological studies have demonstrated that HDL cholesterol (HDL-C) levels are inversely correlated with coronary heart disease (CHD). In particular, both the Framingham study and the PROCAM study show that low HDL plasma levels are strongly associated with higher incidence of CHD, independently of low-density lipoprotein cholesterol (LDL-C) levels [1,2]. Moreover, it has been reported that low HDL-C might be the most prevalent lipid abnormality (about 36%) in men affected by CHD [3]. Nevertheless, contrary to LDL-C, a cause–effect relationship between low HDL-C values and CHD incidence has not been demonstrated for certain. As a matter of fact, the U-shaped association between HDL-C and CHD, and the strong the association of low HDL-C with both metabolic syndrome and systemic inflammation do not support a causal association [4]. Despite these uncertainty, the inverse relationship between HDL-C and cardiovascular risk is supported by several anti-atherogenic properties of these particles (Table 1) including: (1) the reverse cholesterol transport; (2) the anti-inflammatory and anti-oxidant activity; (3) the anti-thrombotic activity and the endothelial protection exerted by HDL particles. Moreover, the association between HDL-C levels and longevity and/or good functional status suggests the hypothesis of a role of this lipoprotein in the phenomenon of successful aging (i.e. aging without chronic diseases and functional impairment).
Table 1 The principal anti-atherosclerotic properties of HDL particles Reverse cholesterol transport • Transport cholesterol from peripheral tissues and macrophages to the liver for disposal into the bile • Exchange apoproteins with other lipoproteins Anti-inflammatory • Inhibit the expression of adhesion molecules (e.g. VCAM, ICAM, E selectin) • Inhibit the expression of MCP-1 in response to oxidized LDLs • Inhibit the production and enhance the degradation of PAF Antioxidant • Remove hydroperoxides from LDLs by apo A-I • Destroy lipid hydroperoxides of LDLs by paraoxonase and GPX • Remove oxidized phospholipids from LDLs by PAF acyhydrolase Endothelial protection • Activate eNOS thus inducing NO production • Enhance NO-dependent peripheral vasodilation • Increase PGI2 release by endothelial cells • Inhibit in vitro the secretion of endothelin 1 Antithrombotic • Increase the production of NO and PGI2 • Inhibit PAF production • Inverse correlation with vW factor plasma concentration • Enhance the activity of protein C and S
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2. The reverse cholesterol transport The “reverse cholesterol transport” describes a specific process in which cholesterol is transferred from non-hepatic cells to the liver (see Fig. 1), being the only organ capable of removing cholesterol from the body through its secretion into the bile [4]. Briefly, lipid free apo A-I or nascent HDLs, produced by the liver or intestine or shed during lipolysis of triglycerides (TG) rich lipoproteins, initiate the efflux of cholesterol from cell membranes. The “gatekeeper” of this pathway is an ATP-binding cassette (ABC) transporter called ABCA1. ABC transporters are a superfamily of proteins that use ATP as a source of energy on order to transport substrates between different cellular compartments and from the cell. ABCA1 transports cellular cholesterol and phospholipids (mostly phosphatidylcholine) to cell surface-bound apolipoproteins. This protein therefore represents the first and rate-controlling step in the reverse cholesterol transport pathway. Discoidal HDLs are then esterified by Lecithin-Cholesterol Acyl Tranferase (LCAT) becoming spherical as cholesteryl esters (CE) move to the core of the particles. The exchange of CE for TG mediated by Cholesteryl Ester Transfer Protein (CETP) moves CE to lipoprotein remnants which are cleared by the liver the apo B/apo E receptor. The joint action of CEPT and Hepatic Lipase [i.e. the transfer of CE to apo B containing lipoproteins and the hydrolysis of TG and phospholipids (PL)] leads to the formation of smaller HDLs that are the favorite ligand for scavenger receptor type B1 (SR-B1). SR-B1 is a multi-ligand receptor (member of the CD36 proteins family) expressed by hepatocytes and steroid-hormone producing tissues, that interacts with both HDL and LDL
Fig. 1. The HDL mediated reverse cholesterol transport.
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particles. It mediates the flux of free-cholesterol from HDLs in a bidirectional way that is depending on the cholesterol gradient; moreover, it mediates the unidirectional efflux of CE, PL and TG from HDLs, thus promoting the depletion of their lipid core. Finally, the combined action of CETP, HL, and PL transfer protein (PLTP) brings again to the formation of lipid-free apo A-I or lipid poor HDLs; of consequence, new acceptors of cellular cholesterol are perpetually regenerated. Interestingly, it has been shown that the disruption of one or more steps in this cycle results in accelerate atherosclerosis, and this is independent from the decreasing or increasing of HDL-C plasma levels. 3. The anti-inflammatory and antioxidant actions of HDL It has been shown that atherosclerosis (ATS) is an inflammatory disorder characterized by the progressive accumulation of macrophages and T lymphocytes in the arterial intima; this process is associated with an increase in the plasma levels of several inflammatory markers. The early step in this inflammatory disease is the adhesion of monocytes to the “dysfunctional” endothelium, mediated by adhesion molecules such as VCAM-1, ICAM-1 and E-selectin. Successively, the monocytes are recruited into the sub-endothelial space by specific chemokines such as MCP-1. In vitro studies suggest that HDLs inhibit the expression of these adhesion molecules [5] by inhibiting endothelial sphingosine kinase; this enzyme is fundamental in the pathway by which TNF-α stimulates the nuclear translocation of NF-kb, which in turn activates the expression of adhesion molecules [5]. Furthermore, HDLs inhibit in vitro the transmigration of monocytes in response to oxidized LDLs [6], and this activity seems to be related to paraoxonase and PAF acylhydrolase transported by HDL particles [7]. HDLs also inhibit the expression of MCP-1 in response to oxidized LDLs [6], and this process has been related to paraoxonase activity. HDLs might exert their anti-atherosclerotic activity also by reducing the phenomenon of LDL oxidation [8]. Apo A-I, the principal apoprotein of HDLs, is capable of removing lipid hydroperoxides from LDLs; successively, these products are selectively removed by liver cells. HDLs also contain enzymes such as paraoxonase 1 and 3, and GPX which are able to destroy lipid hydroperoxides that oxidize the PL of LDLs. In addition, HDLs contain other enzymes, namely PAF-acylhydrolase, that are able to remove oxidized PL. Interestingly, during inflammation HDL-C plasma levels decrease while HDL particles change their composition by losing apo A-I, paraoxonase and PAF-acylhydrolase [7], and acquiring serum amyloid A and ceruloplasmin [7,9]. The affinity of HDLs for hepatocytes is thus reduced, while the affinity for macrophages is much increased [9]; moreover, HDLs lose the ability to protect LDLs against oxidation [7]. On the whole these changes suggest that during acute inflammation HDLs lose most of their anti-inflammatory properties while possibly becoming “pro-inflammatory”. We have recently evaluated the relationship between markers of systemic inflammation and HDL-C levels by studying a large sample of community dwelling older Italian subjects [10]. We found that increased interleukin-6 plasma levels (III vs. I tertile, OR:2.10; 1.10– 3.75) are associated with low HDL-C levels, independent of a large number of confounders including the main traits of metabolic syndrome (triglycerides, fasting insulin, diabetes,
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hypertension, BMI, waist circumference), and life style habits (smoking, alcohol intake, physical activity). The adjusted attributable risk of low HDL-C in the “exposed group” (III tertile of IL-6) was 54%. We have thus provided the epidemiological evidence that inflammation (i.e. increased IL-6 levels) might be one of the main correlates of low HDL-C levels in the older population. 3.1. The endothelial protection and antithrombotic activity It has been shown in the last decades that vascular endothelium is very important in the maintenance of vascular homeostasis through the production of a number of molecules that modulate hemostasis, vascular tone, and inflammatory reactions. The injury to the endothelium, independently from the cause (e.g. hypertension, diabetes, hypercholesterolemia, smoking), produces an alteration of the endothelial physiology, the so called endothelial “dysfunction” (ED). This is considered a key early step not only in the development of the atherosclerotic lesion, but also in the progression, destabilization, and complication of the atherosclerotic plaque. Low HDL-C is associated with the ED [11], and that elevation of HDL-C levels leads to an improvement of the ED [12]. Nitric oxide (NO) is an essential signaling molecule that induces relaxation in vascular smooth muscle cells; it is produced in the endothelial cells by a costitutive NO synthase (eNOS), and a reduction in NO bioavailability is one of the permanent features of the ED. In vitro HDLs, but not lipid free apo A-I, activate eNOS in cultured endothelial cell [13]; it has been proposed that the interaction of HDLs with SR-B1 modifies the membrane cholesterol distribution and morphology thus influencing eNOS activity [14]. HDLs might also serve as a carrier of bioactive lysosphingolipids that are known to fully mimic the ability of HDLs in inducing vasorelaxation. These findings are indirectly supported by in vivo studies that have demonstrated a positive correlation between HDL-C plasma levels and NO-dependent peripheral vasodilation in healthy individuals[14], as well as in patients with diabetes or CHD. Prostacyclin (PGI2) is another vasodilator produced by endothelium that acts sinergically with NO to induce vascular smooth cells relaxation. HDLs, but not dilapidated HDLs, cause a dose-dependent increase of PGI2 release in cultured endothelial cells [15], and this is prevented by Cox inhibitors. This phenomenon might be dependent on the uptake and hydrolysis of HDL derived CE, mediated by SR-B1 [14]. HDLs also upregulate the basal and cytokine-induced expression of Cox 2 through a mechanism that is independent of NF-kb activation [16]. Interestingly, HDLs inhibit the in vitro the secretion of endothelin 1, a potent vasoconstrictor peptide that binds to specific receptors to reverse the response to NO, in endothelial cells [17]. Another interesting property of HDLs is their powerful antithrombotic activity. This action is mediated by the over-mentioned increase in the production of NO and PGI2 (both inhibit platelet aggregation), but also by a reduction of PAF production by the endothelial cells [18]. Indeed, it is known that HDLs transport both PAF acylhydrolase and other enzymes, such as LCAT and paraoxonase, which could degrade PAF. Furthermore, plasma concentrations of von Willebrand factor, a protein playing an essential role in platelets adhesion and aggregation, are inversely correlated with plasma HDL-C levels.
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On the whole, HDLs seem to exert a beneficial effect on endothelium function by modulating the activity and/or production of a number of molecules (NO, PGI2, endothelin, PAF, vWF), and affecting vascular tone and thrombogenicity. Finally, HDLs may beneficially influence the hemostatic balance; as a matter of fact, a positive correlation between HDL-C and TFPI (tissue factor pathway inhibitor) plasma levels has been reported; moreover, HDLs might also enhance the anticoagulant activity of activate protein C and S [14]. 3.2. HDL, longevity, and successful aging Since the early publications of Glueck et al. [19,20] increased levels of HDL-C (hyperalpha lipoproteinemia) and reduced levels of LDL-C (hypo-beta lipoproteinemia) had been associated with longevity in selected pedigree; these findings were justified by the antiatherosclerotic lipid profile typical of these particular subjects and to the low incidence of myocardial infarction. Epidemiological data from the Framingham successively showed that older individuals who subsequently become “healthy” octogenarias (i.e. without CHD and stroke) were not characterized by high HDL-C or low LDL-C levels; on the contrary they were exceedingly unlikely to have low HDL-C levels [21]. Similar data emerged also from the val Vibrata Aging Study; in this sample of Italian population, free-living healthy octo-nonagenarians (free from CHD and stroke) were characterized by a very low prevalence of low HDL-C (3.9%) in face of a high prevalence of high LDL-C (25%). Interestingly, in the same population we found that, for the same level of total cholesterol, HDL-C was significantly reduced in disabled compared to free-living older individuals [22]. The latter observation suggested the existence of an association between HDL-C and health status and/or functional impairment in the elderly. This finding was successively confirmed in a different population [23]; in the I.R.A. study we demonstrated that low HDL-C was independently associated with disability and, most important, that low HDL-C independently predicted the 2 years worsening in functional status. The strength of the association between HDL-C and health status was successively confirmed in a large population [24]. Volpato et al. found that, among community dwelling older individuals with hypocholesterolemia and normal serum albumin, HDL-C levels discriminated two groups of subjects with intermediate (29%) or low (17%) total mortality risk. On the whole, these data suggest the existence of a strong direct relationship between HDL-C and longevity and/or successful aging, defined as aging with a good health status Table 2 The reported associations between HDL plasma levels and longevity /successful AGING HDL-C and longevity-successful aging • High HDL-C is associated with longevity in selected kindreds • Low HDL-C is rare in older healthy individuals without functional impairment • Low HDL-C is very frequent in older individuals with disability • HDL-C is positively associated with physical function • Low HDL-C predicts the worsening in functional status in older individuals • HDL-C is inversely correlated with total mortality in older individuals with low total cholesterol levels
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and without disability (see Table 2); nevertheless, it has to be underlined that, at present time, a causal relationship between HDL and longevity and/or successful aging has not been demonstrated. References [1] T. Gordon, et al., High identity lipoprotein as a protective factor against coronary heart disease, Am. J. Med. 62 (1977) 707–714. [2] G. Assmann, et al., High-density lipoprotein cholesterol as a predictor of coronary heart disease risk. The PROCAM experience and pathophysiological implications for reverse cholesterol transport, Atherosclerosis (1996) S11–S20 (Suppl). [3] J. Genest Jr., et al., Lipoprotein cholesterol, apolipoprotein A-I and B and lipoprotein (a) abnormalities in men with premature coronary artery disease, J. Am. Coll. Cardiol. 19 (1992) 792–802. [4] A. von Eckardstein, J.-R. Nofer, G. Assmann, High density lipoprotein and arteriosclerosis. Role of cholesterol efflux and reverse cholesterol transport, Arterioscler. Thromb. Vasc. Biol. 21 (2001) 12–27. [5] P. Xia, et al., High density lipoprotein (HDL) interrupt the sphingosine kinase signaling pathway. A possible mechanism for protection against atherosclerosis by HDL, J. Biol. Chem. 274 (1999) 33143–33147. [6] M. Navab, et al., Monocyte transmigration induced by modification of low density lipoprotein in cocultures of human aortic wall cells is due to induction of monocyte chemotactic protein 1 synthesis and is abolished by high density lipoprotein, J. Clin. Invest. 88 (1991) 2039–2046. [7] B.J. Van Lenten, et al., Antiinflammatory HDL becomes pro-inflammatory during acute phase response. Loss of protective effect of HDL against LDL oxidation in aortic wall cell cocultures, J. Clin. Invest. 96 (1995) 2758–2767. [8] P.J. Barter, et al., Antiinflammatory properties of HDL, Circ. Res. 95 (2004) 764–772. [9] R. Kisilevski, L. Subrahmanyan, Serum amyloid A changes high density lipoprotein's cellular affinity, Lab. Invest. 66 (1992) 778–785. [10] G. Zuliani, et al., High interleukin-6 plasma levels are associated with low HDL-C levels in community dwelling older adults: the InChianti study, Atherosclerosis (in press) [Jun 18, Electronic publication ahead of print]. PMID: 16787648. [11] N.N. Chan, H.M. Colhoun, P. Vallance, Cardiovascular risk factors as determinants of endotheliumdependent and endothelium-independent vascular reactivity in the general population, J. Am. Coll. Cardiol. 38 (2001) 1814–1820. [12] J.T. Kuvin, et al., 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 (2002) 165–172. [13] I.S. Yuhanna, et al., High-density lipoprotein binding to scavenger receptor-BI activates endothelial nitric oxide synthase, Nat. Med. 7 (Jul 2001) 853–857. [14] L. Calabresi, M. Gomaraschi, G. Franceschini, Endothelial protection by high-density lipoproteins. From bench to bedside, Arterioscler. Thromb. Vasc. Biol. 23 (2003) 1724–1731. [15] L.N. Fleisher, et al., Stimulation of arterial endothelial cell prostacyclin synthesis by high density lipoproteins, J. Biol. Chem. 257 (1982) 6653–6655. [16] G.W. Cockerill, et al., High-density lipoproteins differentially modulate cytokine-induced expression of Eselectin and cyclooxygenase-2, Arterioscler. Thromb. Vasc. Biol. 19 (1999) 910–917. [17] H. Unoki, J. Fan, T. Watanabe, Low-density lipoproteins modulate endothelial cells to secrete endothelin-1 in a polarized pattern: a study using a culture model system simulating arterial intima, Cell Tissue Res. 295 (1999) 89–99. [18] J. Sugatami, et al., High-density lipoprotein inhibits the synthesis of platelet-activating factor in human vascular endothelial cells, J. Lipid Mediators Cell Signal. 13 (1996) 73–88. [19] C.J. Glueck, et al., Longevity syndromes: familial hypobeta and familial hyperalfpha lipoproteinemia, J. Lab. Clin. Med. 88 (1976) 941–957. [20] C.J. Glueck, et al., Hyperalpha- and hypobeta-lipoproteinemia in octogenarian kindreds, Atherosclerosis 27 (1977) 387–406.
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[21] E.J. Schaefer, et al., Plasma lipoproteins in healthy octogenarians: lack of reduced high density lipoprotein cholesterol levels: results from the Framingham heart study, Metabolism 4 (1989) 293–296. [22] G. Zuliani, et al., High-density lipoprotein cholesterol strongly discriminates between healthy free-living and disabled octo-nonagenarians. A cross sectional study, Aging Clin. Exp. Res. 9 (1997) 335–341. [23] G. Zuliani, et al., Low levels of high-density lipoprotein cholesterol are a marker of disability in the elderly, Gerontology 45 (1999) 317–322. [24] S. Volpato, et al., The value of serum albumin and high-density lipoprotein cholesterol in defining mortality risk in older persons with low serum cholesterol, J. Am. Geriatr. Soc. 49 (2001) 1142–1147.