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Lipoprotein(a) [Lp(a)] plays an important role in atherosclerosis. The biological effects of Lp(a) have been attributed either to apolipoprotein(a) or to its low-density lipoprotein-like particle. Lp(a) contains platelet-activating factor acetylhydrolase, an enzyme that exhibits a Ca2+-independent phospholipase A2 activity and is complexed to lipoproteins in plasma; thus, it is also referred to as lipoprotein-associated phospholipase A2. Substrates for lipoprotein-associated phospholipase A2 include phospholipids containing oxidatively fragmented residues at the sn-2 position (oxidized phospholipids; OxPLs). OxPLs may play important roles in vascular inflammation and atherosclerosis. Plasma levels of OxPLs present on apolipoprotein B-100 particles (OxPL/apolipoprotein B) are correlated with coronary artery, carotid, and peripheral arterial disease. Furthermore, OxPL/apolipoprotein B levels in plasma are strongly correlated with Lp(a) levels, are preferentially sequestered on Lp(a), and thus are potentially subjected to degradation by the Lp(a)-associated lipoprotein-associated phospholipase A2. The present review article focuses specifically on the characteristics of the lipoprotein-associated phospholipase A2 associated with Lp(a) and discusses the possible role of this enzyme in view of emerging data showing that OxPLs in plasma are preferentially sequestered on Lp(a) and may significantly contribute to the increased atherogenicity of this lipoprotein.

Lipoprotein(a) [Lp(a)] may be an independent risk factor for coronary artery disease (CAD). Lp(a) is enriched in platelet activating factor acetylhydrolase (PAF-AH), an enzyme which hydrolyzes and inactivates platelet activating factor (PAF) and oxidized phospholipids that are implicated in atherogenesis. We determined the mass and catalytic properties of the Lp(a)-associated PAF-AH in 28 CAD patients in relation to the LDL-associated enzyme ones. Results were then compared to those of 30 control subjects and 16 unrelated patients with primary hypercholesterolemia (Type IIA dyslipidemia) before and after atorvastatin therapy. The mass, the specific activity and kinetic constants of the Lp(a)-associated PAF-AH were significantly lower in CAD patients compared to those of either controls or hypercholesterolemic patients, a phenomenon not observed for LDL-associated PAF-AH. The enzyme specific activity and kinetic constants were significantly increased after removal of apo(a) from Lp(a) by reductive cleavage, which was not found in the control population, suggesting that the apo(a) moiety of Lp(a) from CAD patients may play an important role in the observed lower catalytic efficiency of PAF-AH. The reduced PAF-AH mass and specific activity on Lp(a) is a feature characteristic of this lipoprotein in CAD patients and may lead to a diminished capability of Lp(a) to degrade proinflammatory phospholipids. The consequences of this phenomenon as regards the pathophysiological role of Lp(a) in atherosclerosis remain to be established.

Objective: Lipoprotein (a) [Lp(a)] is considered an atherogenic lipoprotein, which is also implicated in thrombosis. Lp(a) binds to platelets and modulates the effects of various platelet agonists. Platelet activating factor (PAF) is a potent platelet agonist degraded and inactivated by PAF-acetylhydrolase (PAF-AH), which in plasma is associated with lipoproteins. Lp(a) is enriched in PAF-AH, thus a functional characteristic of this lipoprotein is its capability to hydrolyze and inactivate PAF. In the present study, we investigated the effect of Lp(a) on PAF-induced platelet activation. The potential roles of the apo(a) moiety and especially of the PAF-AH content of Lp(a) on the above effect were also addressed., Methods: Lp(a) was isolated by affinity chromatography from plasma of apparently healthy fasting donors with serum Lp(a) concentrations >/=20 mg/dl. Reduced Lp(a) [Lp(a-)] was prepared by incubation of Lp(a) with dithiothreitol (DTT), whereas inactivation of Lp(a)-associated PAF-AH was performed by incubation of Lp(a) with pefabloc [pefa-Lp(a)]. Platelet-rich plasma (PRP) or washed platelets were prepared from peripheral venous blood samples of normolipidemic, apparently healthy fasting donors with their serum Lp(a) levels lower than 0.8 mg/dl. The surface expression of the platelet integrin-receptor alpha(IIb)beta3 and the fibrinogen binding to the activated alpha(IIb)beta3 was studied by flow cytometry., Results: Lp(a), at doses higher than 20 microg/ml, inhibits PAF-induced platelet activation in a dose-dependent manner. Pefa-Lp(a), lacking PAF-AH activity, exhibited a similar to Lp(a) inhibitory effect. Importantly, the Lp(a) inhibitory effect was not influenced by the apo(a) isoform size, whereas Lp(a-) was a more potent inhibitor compared to Lp(a). Similarly to PAF, Lp(a) inhibits platelet aggregation induced by ADP or Calcium ionophore A23187. Lp(a), pefa-Lp(a) or Lp(a-) effectively inhibited PAF- or ADP-induced surface expression of alphaIIbbeta3, the Lp(a-) being more potent compared to Lp(a) or to pefa-Lp(a). Finally, Lp(a) significantly inhibited fibrinogen binding to platelets activated with PAF., Conclusions: Lp(a) inhibits PAF-induced platelet activation in a non-specific manner. The Lp(a)-associated PAF-AH does not play any important role in this effect, whereas the apo(a) moiety of Lp(a) significantly reduces its inhibitory effect. The inhibition of alpha(IIb)beta3 activation and fibrinogen binding to the activated platelets may represent the major mechanism by which Lp(a) inhibits PAF-induced platelet aggregation.

The increased risk for ischemic heart disease (IHD) associated with subclinical hypothyroidism (SH) has been partly attributed to dyslipidemia. There is limited information on the effect of SH on lipoprotein (a) [Lp(a)], which is considered a significant predictor of IHD. Serum Lp(a) levels are predominantly regulated by apolipoprotein [apo(a)] gene polymorphisms. The aim of our study was to evaluate the Lp(a) levels and apo(a) phenotypes in patients with SH compared to healthy controls as well as the influence of levothyroxine substitution therapy on Lp(a) values in relation to the apo(a) isoform size. Lp(a) levels were measured in 69 patients with SH before and after restoration of a euthyroid state and in 83 age- and gender-matched healthy controls. Apo(a) isoform size was determined by sodium dodecyl sulfate (SDS) agarose gel electrophoresis followed by immunoblotting and development via chemiluminescence. Patients with SH exhibited increased Lp(a) levels compared to controls (median value 10.6 mg/dL vs. 6.0 mg/dL, p = 0.003]), but this was not because of differences in the frequencies of apo(a) phenotypes. There was no association between thyrotropin (TSH) and Lp(a) levels in patients with SH. In subjects with either low (LMW; 25 patients and 28 controls) or high (HMW; 44 patients and 55 controls) molecular weight apo(a) isoforms, Lp(a) concentrations were higher in patients than in the control group (median values 26.9 mg/dL vs. 21.8 mg/dL, p = 0.02 for LMW, and 6.0 mg/dL versus 3.3 mg/dL, p < 0.001 for HMW). Levothyroxine treatment resulted in an overall reduction of Lp(a) levels (10.6 mg/dL baseline vs. 8.9 mg/dL posttreatment, p = 0.008]). This effect was mainly evident in patients with LMW apo(a) isoforms associated with high baseline Lp(a) concentrations (median values 26.9 mg/dL vs. 23.2 mg/dL pretreatment and posttreatment, respectively; p = 0.03). In conclusion, even though a causal effect of thyroid dysfunction on Lp(a) was not clearly demonstrated in patients with SH, levothyroxine treatment is beneficial, especially in patients with increased baseline Lp(a) levels and LMW apo(a) isoforms.