Lipoprotein-Associated Phospholipase A2 (Lp-PLA2)

What is Lp-PLA2?

Lp-PLA2, also known as platelet-activating factor acetylhydrolase (PAF-AH), was initially identified as the enzyme responsible for hydrolyzing and inactivating the inflammatory phospholipid PAF (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine).1-3

PAF is a biologically active phospholipid that expresses several inflammatory activities and is involved in various pathophysiological conditions including atherogenesis.1-3 PAF is hydrolyzed and inactivated by PAF-acetylhydrolase, a Ca2+-independent, phospholipase A2 (PLA2).3 PAF-acetylhydrolase circulates in plasma in active form bound to lipoproteins,4-6 and is thus known as lipoprotein-associated phospholipase A2 (Lp-PLA2).6

Approximately 80% of Lp-PLA2 circulates bound to low-density lipoprotein (LDL), whereas the other 20% is bound to high-density lipoprotein (HDL).7,8 Lp-PLA2 hydrolyzes the sn-2 ester bond in phospholipids of which the fatty acid moiety has been shortened or altered by oxidation to yield oxidized fatty acid and lysophosphatidylcholine (lysoPC).9 These metabolites have inflammatory properties,10 and lysoPC has been shown to have adverse effects on endothelial function.11-13

Lp-PLA2 could, therefore, play a direct role in the development of endothelial dysfunction and coronary artery disease (CAD). In addition, it may also serve as a useful biomarker for predicting coronary endothelial dysfunction.14

More recent evidence demonstrates that in addition to the lipoproteins, another carrier of Lp-PLA2 in human plasma is platelet-borne microparticles (PMPs).15,16 It is well-documented that Lp-PLA2 is a cardiovascular disease (CVD)-specific vascular inflammation biomarker which has been shown to be strongly predictive of CVD events, including stroke.

The precise pathophysiological role of the Lp-PLA2 enzyme in plasma as well as in the artery wall needs further clarification.

Several studies have suggested that the exact role of Lp-PLA2 may be differentiated according to the type of the lipoprotein carrier with which Lp-PLA2 circulates in plasma. Lp-PLA2 also plays a inflammatory role in coronary endothelial dysfunction and early atherosclerosis.14

Several population-based studies have shown that elevated lipoprotein-associated phospholipase A2 (Lp-PLA2) levels are associated with an increased risk of coronary heart disease and ischemic stroke.14,17-31

Rupture Prone Plaque & Lp-PLA2

Lp-PLA2 is a marker of vascular-specific inflammation and reflects the presence of rupture-prone plaque. Elevated levels of serum Lp-PLA2 are indicative of rupture-prone plaque and a strong independent predictor of cardiovascular risk, including CAD, myocardial infarction (MI), and stroke.18

Lp-PLA2 is clinically associated with an increased CHD risk, and there is a large body of published evidence from epidemiologic studies addressing the relationship of Lp-PLA2 and risk of cardiovascular disease.32-34

Although a small proportion of circulating Lp-PLA2 activity is associated with high-density lipoprotein (HDL), the majority (~80%) is associated with LDL. In particular, Lp-PLA2 is a potential marker for atherogenic small dense LDL (sdLDL), as most LDL-associated Lp-PLA2 (LDL–Lp-PLA2) is found complexed with sdLDL particles.16

The distribution of Lp-PLA2 between LDL and HDL is altered in various types of dyslipidemias (e.g., when plasma levels of lipoprotein (a) [Lp(a)] exceed 30 mg/dL)7,35; moreover, evidence suggests that the role of Lp-PLA2 in atherosclerosis may depend upon the type of lipoprotein particle with which it is associated.

For example, while several population-based studies have demonstrated independent association of plasma Lp-PLA2 levels—which largely reflect LDL–Lp-PLA2—with increased cardiovascular risk, HDL-associated Lp-PLA2 may be atheroprotective.7 Recent studies also indicate that oxidized phospholipids (oxPL) are preferentially sequestered by Lp(a), and subsequently degraded by Lp(a)-associated Lp-PLA2. These findings suggest that Lp(a) is a potential scavenger of oxPL, providing novel insight into the role of Lp(a) and Lp(a)-associated Lp-PLA2 in normal physiology as well as in inflammation and atherosclerosis.16

Elevated plasma Lp-PLA2 raises the risk of cardiovascular events by approximately 2 fold.16 Multiple prospective epidemiologic studies have demonstrated association of increased Lp-PLA2 levels with primary coronary or cardiovascular events, future coronary events, and stroke, suggesting that Lp-PLA2 has significant clinical utility as a cardiovascular risk marker.17

Further support for the proatherogenic role of LDL–Lp-PLA2 is derived from its preferential association with sdLDL,36 the most atherogenic LDL species.37 The enrichment of sdLDL with Lp- PLA2 enhances production of lysoPC during oxidation, in both normolipidemic and hypercholesterolemic patients.38 Other studies have demonstrated that Lp-PLA2 may significantly contribute to the atherogenicity of the electronegative LDL subfraction.39,40

Lp-PLA2 and Ethnicity

Emerging data suggest that metabolic and inflammatory factors impacting CVD risk differ across ethnic groups.26,32,33 There are several important differences between African-Americans and Caucasians with respect to Lp-PLA2. First, among subjects with CAD, Lp-PLA2 activity levels were higher among African- Americans. Second, the difference in Lp-PLA2 activity levels between CAD and non-CAD subjects was higher among African- Americans. Furthermore, the Lp-PLA2 index, a measure of enzymatic properties, was independently associated with the extent of CAD among African-Americans.41

Lp-PLA2 and Cardiovascular Disease

Functional medicine physicians seek the underlying problem. Cardiovascular disease is a complex process with over 400 different contributing risk factors. Understanding Lp-PLA2 helps us evaluate cardiovascular disease risk as well as where we should place our efforts to try to resolve the issue.

Lp-PLA2 is a vascular-specific proinflammatory enzyme that operates in the arterial intima.

Lp-PLA2 localizes to atherosclerotic plaque, particularly in those with a necrotic core and in ruptured plaques.42 High levels of Lp-PLA2 are found in rupture- prone plaques, and it appears that Lp-PLA2 is released from these plaques into the circulation. Lp-PLA2 is primarily produced by macrophages and then bound to various lipoproteins, including the apoB portion of LDLs and Lp(a).14

Staining of coronary and carotid tissue demonstrates the presence of Lp-PLA2 in the thin fibrous cap of rupture-prone plaques, but not in the early-stage plaques.43 Coronary and carotid tissue concentrations of Lp- PLA2 are notably very high in the rupture-prone shoulder region of thin fibrous cap atheromas, and histopathologic stains reveal that Lp-PLA2 colocalizes with macrophages and oxidized LDL in atherosclerotic coronary and carotid plaques.44

Lp-PLA2 hydrolyzes phospholipids on oxidized LDL particles in the subendothelial space. Lp-PLA2 hydrolyzes the center (n-2) ester bond of phospholipids, which yields oxidized fatty acids and lysoPC, a molecule with a range of potentially atherogenic effects, including chemoattraction of monocytes, increased expression of adhesion molecules, and inhibition of endothelial nitric oxide production.10,45

In this manner, a vicious cycle is set up that leads to the recruitment of monocytes to the intima, where they differentiate to become macrophages and, ultimately, foam cells, while at the same time locally producing more Lp-PLA2.

Furthermore, lysoPC has been found to be cytotoxic to vascular smooth muscle cells and can induce the local production of matrix metalloproteinases (MMPs), which can thin the fibrous cap and destabilize the architectural integrity of an atheromatous plaque, increasing its propensity to rupture.46

In terms of its utility as a circulating biomarker, Lp-PLA2 produced by activated macrophages and foam cells re- enters the bloodstream and can be measured. As reported by Lavi et al., Lp-PLA2 blood concentrations sampled simultaneously in the human coronary sinus demonstrated a net increase in Lp-PLA2 levels as blood traverses the coronary vascular bed with significant atherosclerotic plaque.14 However, when no coronary plaque is present, a decrease in Lp-PLA2 levels is found. This study also showed that the lysoPC produced by the Lp-PLA2-mediated hydrolysis of oxidized LDL is strongly associated with coronary artery endothelial dysfunction.

Summary of Lp-PLA2 Clinical Studies:

Lp-PLA2 is a Predictor of Cardiovascular and Stroke Events:

1. An independent risk factor for CVD and stroke events14,18
   • 2x risk for CVD events when elevated
   • 5.5x risk for stroke events when elevated
2. Predicts CVD in elderly men and women even with normal LDL-C levels19
3. Additive risk with CRP18,20 or systolic blood pressure (SBP)20
   • When both Lp-PLA2 and CRP are very high—4x risk for CVD events
   • When both Lp-PLA2 and SBP are very high—6.4x risk for ischemic stroke
   • When both Lp-PLA2 and CRP are very high—11.4x risk for ischemic stroke
4. Lp-PLA2 with CRP predicts recurrence and severity of second stroke event24
5. Additive risk from Lp-PLA2 and CRP beyond carotid IMT results25
6. Predicts angiographic finding of coronary atherosclerosis14,22
7. Predicts coronary death22
8. Predicts coronary endothelial dysfunction, which is a marker for early atherosclerosis and increased risk of ischemic cardiac events and stroke14
   • Lp-PLA2 is primarily associated with LDL (LDL-Lp-PLA2); a small amount of the enzyme activity is also associated with HDL.16
   • Most LDL-Lp-PLA2 is bound to atherogenic sdLDL particles and is a possible marker of sdLDL in plasma.16
   • Distribution of Lp-PLA2 between LDL and HDL is affected by various types of dyslipidemias.16
   • Lp-PLA2 levels may also be affected when Lp(a) plasma levels are greater than 30 mg/dL.16

How to reduce your Lp-PLA2:

A cross-sectional study of apparently healthy men and women demonstrated that Lp-PLA2 activity is influenced by a number of modifiable factors. Circulating levels of the enzyme were found to be positively associated with body weight and smoking, but inversely associated with increased consumption of alcohol and protein, and in women, with use of postmenopausal hormones.47

• Decrease body fat
• Do not smoke! Smoking increases Lp-PLA2
• Appropriate alcohol intake – 1-2 glasses per day for men, and 1 for women
• Consume appropriate amounts of protein
• Hormone replacement therapy
• Lp-PLA2 is lowered by statins and fenofibrate50-53
• Niacin added to established statin therapy lowers Lp-PLA2 levels by additional 20%54
• Lp-PLA2 cut-point values for patients with known CVD
  • Alert cut-point of > 235 ng/mL8,9,32
  • Goal cut-point of < 200 ng/mL8,9

References

1. Demopoulos CA, Karantonis HC, Antonopoulou S. Platelet activating factor — a molecular link between atherosclerosis theories. Eur J Lipid Sci Technol 2003;105(11):705-16.
2. Prescott SM, Zimmerman GA, Stafforini DM, et al. Platelet-activating factor and related lipid mediators. Annu Rev Biochem 2000;69:419-45.
3. Tselepis AD, John Chapman M. Inflammation, bioactive lipids and atherosclerosis: potential roles of a lipoprotein-associated phospholipase A2, platelet activating factor-acetylhydrolase. Atheroscler Suppl 2002;3(4):57-68.
4. Stafforini DM, McIntyre TM, Carter ME, et al. Human plasma platelet-activating factor acetylhydrolase. Association with lipoprotein particles and role in the degradation of platelet-activating factor. J Biol Chem 1987;262(9):4215-22.
5. Ostermann G, Kostner GM, Gries A, et al. The contribution of individual lipoproteins to the degradation of platelet-activating factor in human serum. Haemostasis 1989;19(3):160-8.
6. Tew DG, Southan C, Rice SQ, et al. Purification, properties, sequencing, and cloning of a lipoprotein-associated, serine-dependent phospholipase involved in the oxidative modification of low-density lipoproteins. Arterioscler Thromb Vasc Biol 1996;16(4):591-9.
7. Tsimihodimos V, Karabina SA, Tambaki AP, et al. Altered distribution of platelet-activating factor- acetylhydrolase activity between LDL and HDL as a function of the severity of hypercholesterolemia. J Lipid Res 2002;43(2):256-63.
8. Caslake MJ, Packard CJ, Suckling KE, et al. Lipoprotein-associated phospholipase A(2), platelet-activating factor acetylhydrolase: a potential new risk factor for coronary artery disease. Atherosclerosis 2000;150(2):413-9.
9. Caslake MJ, Packard CJ. Lipoprotein-associated phospholipase A2 (platelet-activating factor acetylhydrolase) and cardiovascular disease. Curr Opin Lipidol 2003;14(4):347-52.
10. MacPhee CH, Moores KE, Boyd HF, et al. Lipoprotein-associated phospholipase A2, platelet-activating factor acetylhydrolase, generates two bioactive products during the oxidation of low-density lipoprotein: use of a novel inhibitor. Biochem J 1999;338 ( Pt 2):479-87.
11. Murugesan G, Sandhya Rani MR, Gerber CE, et al. Lysophosphatidylcholine regulates human microvascular endothelial cell expression of chemokines. J Mol Cell Cardiol 2003;35(11):1375-84.
12. Wolfram Kuhlmann CR, Wiebke Ludders D, Schaefer CA, et al. Lysophosphatidylcholine-induced modulation of Ca(2+)-activated K(+)channels contributes to ROS-dependent proliferation of cultured human endothelial cells. J Mol Cell Cardiol 2004;36(5):675-82.
13. Chaudhuri P, Colles SM, Damron DS, et al. Lysophosphatidylcholine inhibits endothelial cell migration by increasing intracellular calcium and activating calpain. Arterioscler Thromb Vasc Biol 2003;23(2):218-23.
14. Lavi S, McConnell JP, Rihal CS, et al. Local production of lipoprotein-associated phospholipase A2 and lysophosphatidylcholine in the coronary circulation: association with early coronary atherosclerosis and endothelial dysfunction in humans. Circulation 2007;115(21):2715-21.
15. Mitsios JV, Vini MP, Stengel D, et al. Human platelets secrete the plasma type of platelet-activating factor acetylhydrolase primarily associated with microparticles. Arterioscler Thromb Vasc Biol 2006;26(8):1907-13.
16. Tellis CC, Tselepis AD. The role of lipoprotein-associated phospholipase A2 in atherosclerosis may depend on its lipoprotein carrier in plasma. Biochim Biophys Acta 2009;1791(5):327-38.
17. Garza CA, Montori VM, McConnell JP, et al. Association between lipoprotein-associated phospholipase A2 and cardiovascular disease: a systematic review. Mayo Clin Proc 2007;82(2):159-65.
18. Ballantyne CM, Hoogeveen RC, Bang H, et al. Lipoprotein-associated phospholipase A2, high-sensitivity C-reactive protein, and risk for incident coronary heart disease in middle-aged men and women in the Atherosclerosis Risk in Communities (ARIC) study. Circulation 2004;109(7):837-42.
19. Caslake MJ, Packard CJ, Robertson M, et al. Lipoprotein-associated phospholipase A(2), inflammatory biomarkers, and risk of cardiovascular disease in the Prospective Study of Pravastatin in the Elderly at Risk (PROSPER). Atherosclerosis 2010;210(1):28-34.
20. Ballantyne CM, Hoogeveen RC, Bang H, et al. Lipoprotein-associated phospholipase A2, high-sensitivity C-reactive protein, and risk for incident ischemic stroke in middle-aged men and women in the Atherosclerosis Risk in Communities (ARIC) study. Arch Intern Med 2005;165(21):2479-84.
21. McHugh VL, Barnhart SI, Schaper AM, et al.Improving risk prediction in primary prevention: the role of carotid ultrasound, hs-CRP, and Lp-PLA2. 46th Annual Conference on Cardiovascular Disease Epidemiology and Prevention. 2006. 113. e301-e81
22. May HT, Horne BD, Anderson JL, et al. Lipoprotein-associated phospholipase A2 independently predicts the angiographic diagnosis of coronary artery disease and coronary death. Am Heart J 2006;152(5):997-1003.
23. Yang EH, McConnell JP, Lennon RJ, et al. Lipoprotein-associated phospholipase A2 is an independent marker for coronary endothelial dysfunction in humans. Arterioscler Thromb Vasc Biol 2006;26(1):106-11.
24. Koenig W, Twardella D, Brenner H, et al. Lipoprotein-associated phospholipase A2 predicts future cardiovascular events in patients with coronary heart disease independently of traditional risk factors, markers of inflammation, renal function, and hemodynamic stress. Arterioscler Thromb Vasc Biol 2006;26(7):1586-93.
25. Koenig W, Khuseyinova N, Lowel H, et al. Lipoprotein-associated phospholipase A2 adds to risk prediction of incident coronary events by C-reactive protein in apparently healthy middle-aged men from the general population: results from the 14-year follow-up of a large cohort from southern Germany. Circulation 2004;110(14):1903-8.
26. Ballantyne C, Cushman M, Psaty B, et al. Collaborative meta-analysis of individual participant data from observational studies of Lp-PLA2 and cardiovascular diseases. Eur J Cardiovasc Prev Rehabil 2007;14(1):3-11.
27. Yusuf S, Hawken S, Ounpuu S, et al. Effect of potentially modifiable risk factors associated with myocardial infarction in 52 countries (the INTERHEART study): case-control study. Lancet 2004;364(9438):937-52.
28. Packard CJ, O’Reilly DS, Caslake MJ, et al. Lipoprotein-associated phospholipase A2 as an independent predictor of coronary heart disease. West of Scotland Coronary Prevention Study Group. N Engl J Med 2000;343(16):1148-55.
29. O’Donoghue M, Morrow DA, Sabatine MS, et al. Lipoprotein-associated phospholipase A2 and its association with cardiovascular outcomes in patients with acute coronary syndromes in the PROVE IT-TIMI 22 (PRavastatin Or atorVastatin Evaluation and Infection Therapy-Thrombolysis In Myocardial Infarction) trial. Circulation 2006;113(14):1745-52.
30. Brilakis ES, McConnell JP, Lennon RJ, et al. Association of lipoprotein-associated phospholipase A2 levels with coronary artery disease risk factors, angiographic coronary artery disease, and major adverse events at follow-up. Eur Heart J 2005;26(2):137-44.
31. Blake GJ, Dada N, Fox JC, et al. A prospective evaluation of lipoprotein-associated phospholipase A(2) levels and the risk of future cardiovascular events in women. J Am Coll Cardiol 2001;38(5):1302-6.
32. Pearson TA, Mensah GA, Alexander RW, et al. Markers of inflammation and cardiovascular disease: application to clinical and public health practice: A statement for healthcare professionals from the Centers for Disease Control and Prevention and the American Heart Association. Circulation 2003;107(3):499-511.
33. National Cholesterol Education Program (NCEP) Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel III). Third Report of the National Cholesterol Education Program (NCEP) Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel III) final report. Circulation 2002;106(25):3143-421.
34. Davidson MH, Corson MA, Alberts MJ, et al. Consensus panel recommendation for incorporating lipoprotein-associated phospholipase A2 testing into cardiovascular disease risk assessment guidelines. Am J Cardiol 2008;101(12A):51F-7F.
35. Karabina SA, Elisaf MC, Goudevenos J, et al. PAF-acetylhydrolase activity of Lp(a) before and during Cu(2+)-induced oxidative modification in vitro. Atherosclerosis 1996;125(1):121-34.
36. Gazi I, Lourida ES, Filippatos T, et al. Lipoprotein-associated phospholipase A2 activity is a marker of small, dense LDL particles in human plasma. Clin Chem 2005;51(12):2264-73.
37. Griffin BA. Lipoprotein atherogenicity: an overview of current mechanisms. Proc Nutr Soc 1999;58(1):163-9.
38. Zalewski A, Macphee C. Role of lipoprotein-associated phospholipase A2 in atherosclerosis: biology, epidemiology, and possible therapeutic target. Arterioscler Thromb Vasc Biol 2005;25(5):923-31.
39. Karabina SA, Elisaf M, Bairaktari E, et al. Increased activity of platelet-activating factor acetylhydrolase in low-density lipoprotein subfractions induces enhanced lysophosphatidylcholine production during oxidation in patients with heterozygous familial hypercholesterolaemia. Eur J Clin Invest 1997;27(7):595-602.
40. De Castellarnau C, Sanchez-Quesada JL, Benitez S, et al. Electronegative LDL from normolipemic subjects induces IL-8 and monocyte chemotactic protein secretion by human endothelial cells. Arterioscler Thromb Vasc Biol 2000;20(10):2281-7.
41. Anuurad E, Ozturk Z, Enkhmaa B, et al. Association of lipoprotein-associated phospholipase A2 with coronary artery disease in African- Americans and Caucasians. J Clin Endocrinol Metab 2010;95(5):2376-83.
42. Kolodgie FD, Burke AP, Skorija KS, et al. Lipoprotein-associated phospholipase A2 protein expression in the natural progression of human coronary atherosclerosis. Arterioscler Thromb Vasc Biol 2006;26(11):2523-9.
43. Berneis KK, Krauss RM. Metabolic origins and clinical significance of LDL heterogeneity. J Lipid Res 2002;43(9):1363-79.
44. Mannheim D, Herrmann J, Versari D, et al. Enhanced expression of Lp-PLA2 and lysophosphatidylcholine in symptomatic carotid atherosclerotic plaques. Stroke 2008;39(5):1448-55.
45. Vickers KC, Maguire CT, Wolfert R, et al. Relationship of lipoprotein-associated phospholipase A2 and oxidized low density lipoprotein in carotid atherosclerosis. J Lipid Res 2009;50(9):1735-43.
46. Safaya R, Chai H, Kougias P, et al. Effect of lysophosphatidylcholine on vasomotor functions of porcine coronary arteries. J Surg Res 2005;126(2):182-8.
47. Hatoum IJ, Nelson JJ, Cook NR, et al. Dietary, lifestyle, and clinical predictors of lipoprotein-associated phospholipase A2 activity in individuals without coronary artery disease. Am J Clin Nutr 2010;91(3):786-93.
48. Serruys PW, Garcia-Garcia HM, Buszman P, et al. Effects of the direct lipoprotein-associated phospholipase A(2) inhibitor darapladib on human coronary atherosclerotic plaque. Circulation 2008;118(11):1172-82.
49. McConnell JP, Jaffe AS. The spin stops here: inhibition of lipoprotein-associated phospholipase A2– a promising target but a negative initial trial? Clin Chem 2009;55(1):21-3.
50. Muhlestein JB, May HT, Jensen JR, et al. The reduction of inflammatory biomarkers by statin, fibrate, and combination therapy among diabetic patients with mixed dyslipidemia: the DIACOR (Diabetes and Combined Lipid Therapy Regimen) study. J Am Coll Cardiol 2006;48(2):396-401.
51. Albert MA, Glynn RJ, Wolfert RL, et al. The effect of statin therapy on lipoprotein associated phospholipase A2 levels. Atherosclerosis 2005;182(1):193-8.
52. Winkler K, Abletshauser C, Friedrich I, et al. Fluvastatin slow-release lowers platelet-activating factor acetyl hydrolase activity: a placebocontrolled trial in patients with type 2 diabetes. J Clin Endocrinol Metab 2004;89(3):1153-9.
53. Tsimihodimos V, Kakafika A, Tambaki AP, et al. Fenofibrate induces HDL-associated PAF-AH but attenuates enzyme activity associated with apoB-containing lipoproteins. J Lipid Res 2003;44(5):927-34.
54. Kuvin JT, Dave DM, Sliney KA, et al. Effects of extended-release niacin on lipoprotein particle size, distribution, and inflammatory markers in patients with coronary artery disease. Am J Cardiol 2006;98(6):743-5.