Lp-PL[A.sub.2]: a new kid on the block.
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IS Lp-PL[A.sub.2] ALSO KNOWN AS PLATELET ACTIVATING FACTOR (PAF) ACETYLHYDROLASE] "FRIEND OR FOE"? High substrate specificity of Lp-PL[A.sub.2] toward polar phospholipids raises the question of whether hydrolysis of PAF-like phospholipids in oxidized LDL (and PAF itself) by Lp-PL[A.sub.2] attenuates their proinflammatory properties. Conversely, enzymatic Lp-PL[A.sub.2] activity toward polar phospholipids contributes to the formation of downstream products that could mediate biological effects of this enzyme (e.g., lysophosphatidylcholine). Interestingly, a broad range of effects elicited by lysophosphatidylcholine species are also observed in the setting of increased plaque vulnerability (e.g., the induction of inflammatory cell phenotype, apoptosis) (17). In the context of this debate (i.e., "friend or foe"), recent findings in patients with prior acute coronary syndrome (ACS) add to the growing evidence that higher concentrations of circulating Lp-PL[A.sub.2] activity are associated with increased risk of recurrent fatal and nonfatal cardiovascular event over the next 2 years, as opposed to lower cardiovascular risk (12,14).
Could increased Lp-PL[A.sub.2] concentrations among patients who go on to experience a recurrent event simply represent a compensatory mechanism for removing oxidized LDL? Although epidemiology studies cannot provide conclusive answers to this question, several studies in primary prevention populations (i.e., without manifested atherosclerotic vascular disease) illustrate a similar picture in which increased Lp-PL[A.sub.2] concentrations (either mass or activity) are predictive of future cardiovascular events for as many as 14 years (5). In addition, mechanistic studies also suggest that higher Lp-PL[A.sub.2] activity contributes to processes identified as pivotal to plaque vulnerability, including monocyte migration, proinflammatory effects of oxidized LDL, and macrophage death (18,19). Recent findings by Stafforini et al. (20) further indicate that human Lp-PL[A.sub.2] activity contributes to the release of F2-isoprostanes, products of lipid peroxidation in vivo that possess several noxious biologic effects. Collectively, these experimental observations indicate that increased Lp-PL[A.sub.2] activity has the potential to exert harmful effects, thereby supporting the conclusions from epidemiologic studies.
HOW CAN Lp-PL[A.sub.2] EPIDEMIOLOGY DATA BE RECONCILED WITH CONTRADICTORY RESULTS FROM STUDIES OF Lp-PL[A.sub.2] VARIANTS?
Several reports purported that Japanese carriers of the mutation in the PLA2G7 gene (V279 variant) are at increased risk of cardiovascular events, despite the absence of circulating enzyme. The subsequent discovery that carriers of this variant are unable to secrete Lp-PL[A.sub.2] from monocytes/macrophages suggests a more complex picture than previously suggested (21). Rather than the lack of circulating Lp-PL[A.sub.2] being responsible for cardiovascular risk, the unfolded protein syndrome in individuals with V279 variant, with its effect on monocyte/macrophages survival (e.g., apoptosis), could offer 1 explanation for the disconnect between findings from many epidemiologic studies and those from genetic studies of Lp-PL[A.sub.2] in Japanese.
IS Lp-PL[A.sub.2] SIMPLY A MARKER OF SMALLER AND DENSER LDL?
Although circulating concentrations of Lp-PL[A.sub.2] (mass and activity) correlate with several clinical characteristics [e.g., sex (concentrations tend to be higher in men than in women), age, smoking status] and laboratory variables (e.g., LDL), they contribute to only a relatively small portion of its variation in plasma (22). Recently, Corsetti et al. undertook an exploratory factor analysis to define the association between Lp-PL[A.sub.2] activity and clusters of lipid measurements, hemostatic biomarkers, and inflammatory and glycemic control measures in post-ACS patients (14). The fact that Lp-PL[A.sub.2] was associated with apoB and an atherogenic phenotype (high triglycerides, low concentrations of HDL cholesterol, and small LDL particle size) is not surprising, given that ~80% of the enzyme is bound to apoB and the activity of Lp-PL[A.sub.2] is mainly associated with smaller LDL particles (23, 24). However, how does one explain that only ~40% of the variance in Lp-PL[A.sub.2] could be attributed to lipid-related measurements? The answer is possibly related to the fact that Lp-PL[A.sub.2] expression is dependent on leukocyte activation, with lipoproteins serving as a functional reservoir for the enzyme in circulation (19). An additional pool of Lp-PL[A.sub.2] is produced by the activated inflammatory cells within atheroma in situ, where enzyme expression is particularly high within the lipid core and co-localizes with apoptotic cells (17, 25). These findings imply that circulating Lp-PL[A.sub.2] may reflect the lipoprotein profile (e.g., higher concentrations of LDL or number of smaller denser LDL carrying most of Lp-PL[A.sub.2]), the state of circulating leukocytes, and also the inflammatory composition of atheroma (Fig. 1). Consistent with this hypothesis, Lp-PL[A.sub.2] surpassed apoB as an independent predictor of cardiovascular risk in a multivariate model in the work by Corsetti et al. (14). Unfortunately, data from diabetic patients from the same cohort have not been provided, which is important information, because diabetics have higher concentrations of small size LDL particles, greater disease burden with plaques enriched with inflammatory cells, and carry increased risk of cardiovascular events.
Lp-PL[A.sub.2] MEASUREMENTS IN POST-ACS PATIENTS?
Analogous to LDL concentrations that lack predictive value when measured in the setting ACS, emerging data from FRISC-2, GUSTO IV, and PROVE-IT (TINA-22) indicate that Lp-PL[A.sub.2] is not a useful biomarker to assess the long-term cardiovascular risk when measured shortly after the acute event (12,15,16). The changes in variables that likely influence Lp-PL[A.sub.2] measurements (e.g., decreased LDL concentrations) are likely explanations why Lp-PL[A.sub.2] concentrations add little to the risk assessment when measured within hours or days after ACS. These findings should not be misinterpreted, however, as evidence that a therapeutic intervention aimed at such biomarkers would be futile. The results from statin trials in ACS, such as MIRACL and PROVE-IT, have illustrated that LDL lowering is effective for reducing cardiovascular events despite no association of baseline LDL values with outcomes in these patients (26, 27). Interestingly, when Lp-PL[A.sub.2] activity is measured at later time points, when the metabolic storm surrounding ACS has subsided [e.g., at 1 month: PROVE-IT (12) or at 2 months: Corsetti et al. (14)], the measurements emerge as an independent predictor of the long-term cardiovascular risk in the multivariate analysis. These recent data also raise a provocative question as to whether better adherence to statin therapy in post-ACS patients would sufficiently lower Lp-PL[A.sub.2] to abrogate the risk that was conferred by the reported activity levels. In the PROVE-IT study, in which all post-ACS patients were treated with 1 of 2 statin regimens, pravastatin failed to decrease Lp-PL[A.sub.2] activity despite lowering LDL by 12.5%. Even aggressive atorvastatin therapy lowered mean Lp-PL[A.sub.2] activity by only 20% after 1 month of treatment in spite of concomitant decrease in LDL by 42.5% (12). These data illustrate the limits of statin therapy in terms of modest effects on the levels of this enzyme mediated mainly by lowering LDL. Other studies suggest a similar effect between statins and fibrates on reducing plasma Lp-PL[A.sub.2] (i.e., ~25% reduction), again through effects on lowering LDL; (28-30) however, there are no robust data with respect to the effects of other pharmacologic interventions on Lp-PL[A.sub.2] mass or activity.
Lp-PL[A.sub.2] QUO VADIS AND CHALLENGES AHEAD?
To what extent residual plasma Lp-PL[A.sub.2] activity is causally responsible for death and recurring cardiovascular events in aggressively treated post ACS patients--rather than being merely associated with such risk--cannot be answered from epidemiology studies. This is more than an academic question, given that intensively treated patients in the PROVE-IT study still had >20% risk of death or coronary events over the next 2 years (27). Clearly the answer to this query must await future randomized trials exploring the therapeutic value of selective Lp-PL[A.sub.2] inhibition in conjunction with the standard of care (e.g., statins). Determining whether Lp-PL[A.sub.2] inhibition improves clinical outcomes beyond that which can be achieved with high-dose statin therapy will be helpful in addressing the clinical utility of this novel biomarker.
At this time, the role of Lp-PL[A.sub.2] measurements in routine clinical practice has not been fully established. Despite the growing number of epidemiologic studies assessing Lp-PL[A.sub.2] activity or mass in primary or secondary prevention populations, the transition of this biomarker from research to clinical practice arena will require further work (Table 1). An important issue is to determine whether to measure the mass of Lp-PL[A.sub.2] or to measure its enzyme activity. Several of the epidemiology studies measured only Lp-PL[A.sub.2] mass (2-5; 7-9; 15,16), whereas others measured only enzyme activity, (6,10,11,14) and a few have measured both in the same study population (12,13). The relatively low correlation (r = 0.36) between Lp-PL[A.sub.2] mass and activity in recent studies has raised the question of precisely what is being measured with the respective assays (12). The findings from one study (24), suggest that the activity assay measures enzyme associated with smaller, denser LDL, and the mass assay measures enzyme associated with both LDL and HDL. However, more research is needed to clarify potential differences between assays.
It is important to caution that neither hazard ratio (e.g., comparing the highest versus the lowest quartile or quintile of the study population) or accompanying P values provide definitive answers regarding clinical utility of measuring the biomarker of interest (31). Key additional issues yet to be resolved include quantifying the incremental value of Lp-PL[A.sub.2] measurements--over and beyond currently recommended multivariable tools used to quantify cardiovascular risk in primary prevention population--and defining the threshold values that warrant additional aggressive interventions in those with already manifested vascular disease. Recent observations from an ARIC study suggested that Lp-PL[A.sub.2] measurements yielded only a small increase in the area under the ROC curve when added to traditional risk factors and failed to restratify patients from the intermediate to high risk (32). These caveats notwithstanding, it should be underscored that Lp-PL[A.sub.2] is a modifiable risk marker and its utility as a therapeutic target can be addressed only through future clinical trials directly targeting this enzyme (Table 1). Addressing the remaining gaps in our understanding of the utility and pitfalls of measuring Lp-PL[A.sub.2] is needed before this promising biomarker could be fully integrated into daily practice for prognostic or therapeutic purposes (31, 32).
In summary, the efforts toward better understanding the predictors of clinical instability have resulted in numerous studies of cardiovascular biomarkers and other intermediate endpoints in populations at risk. In a broad sense, poor correlations between circulating inflammatory biomarkers (e.g., hsC-reactive protein and Lp-PL[A.sub.2]) and vascular imaging highlight biological variability of the analyte in a disease with a long period of an indolent course or technology-related challenges related to such measurements (33, 34). Results of several epidemiology investigations could suggest that novel inflammatory biomarkers reflect the activity of atherosclerotic vascular disease, rather than plaque size or lumen narrowing. The reported lack of correlation between Lp-PL[A.sub.2] activity and C-reactive protein levels also illustrates differences between the pathways, in which a growing number of inflammatory biomarkers are involved. The ultimate goal of these studies is to refine our current clinical tools for estimating cardiovascular risk and for selecting optimal therapy for individual patients. It is unlikely that a single test will overcome imprecision of looking into the future. Instead, the array of three biomarkers mentioned above: imaging (what type of plaque is present?), functional assessments (how abnormal is the vessel function?) and circulating biomarkers (how active is the disease process?) will be likely required to assess risk and guide treatment decisions.
Received March 23, 2006; accepted June 19, 2006.
Previously published online at DOI: 10.1373/clinchem.2006.070672
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ANDREW ZALEWSKI, [1,4 *] JEANENNE J. NELSON,  LISA HEGG,  and COLIN MACPHEE 
 GlaxoSmithKline,  Cardiovascular Medicine Development Centre;  Worldwide Epidemiology; and  Center of Excellence for Drug Discovery, Philadelphia, PA.  Thomas Jefferson University, Philadelphia, PA.  Nonstandard abbreviations: Lp-PL[A.sub.2], lipoprotein-associated phospholipase [A.sub.2]; PAF, platelet acfivafing factor; ACS, acute coronary syndromes; PROVE-IT, PRavastafin Or atorVastatin Evaluation and Infection Therapy; TIMI-22, Thrombolysis In Myocardial Infarction trial-22.
* Address correspondence to this author at: GlaxoSmithKline, 2301 Renaissance Blvd. RN0320, King of Prussia, PA 19406. E-maII firstname.lastname@example.org.
Table 1. Stepwise validation of cardiovascular biomarker clinical utility. Validation Steps Biomarker Characteristics Biomarker assay and High throughput standardized assay with low its variability variability over the range of measurements Population norms and Known effects of demographics, biological variability diurnal/seasonal variations, and common CV medications. Association with CV risk Portends significant risk independent of conventional risk factors in populations that reflect the intended use of the assay. Clinical value in Incremental prognostic value when primary prevention incorporated into risk assessment algorithms (e.g., FRS). Clinical value in Thresholds values that portend increased CV secondary prevention risk despite the standard of care. Causal relationship CV event reduction when the levels of a with CV risk biomarker are lowered with specific intervention added to the standard of care. CV, cardiovascular; FRS, Framingham risk score.
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|Author:||Zalewski, Andrew; Nelson, Jeanenne J.; Hegg, Lisa; Macphee, Colin|
|Date:||Sep 1, 2006|
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