Kidney Function Decline in the Elderly: Impact of Lipoprotein-Associated Phospholipase A2Peralta C.A.a, b · Katz R.c · Shlipak M.a, b · Dubin R.b · DeBoer I.c · Jenny N.d · Fitzpatrick A.c · Koro C.e · Kestenbaum B.c · Ix J.f · Sarnak M.g · Cushman M.d
aSan Francisco VA Medical Center, and bUniversity of California, San Francisco, San Francisco, Calif., cUniversity of Washington, Seattle, Wash., dUniversity of Vermont College of Medicine, Burlington, Vt., eGlaxoSmithKline, Collegeville, Pa., fUniversity of California, San Diego, La Jolla, Calif., and gTufts University School of Medicine, Boston, Mass., USA Corresponding Author
Background: Whether lipoprotein-associated phospholipase A2 (Lp-PLA2) levels are associated with kidney function decline has not been well studied. Methods: We investigated associations of Lp-PLA2 antigen and activity with kidney function decline and rapid decline over 5.7 years in the Cardiovascular Health Study (n = 4,359). We estimated kidney function by cystatin C (eGFRcys) in repeated measures, and defined rapid decline as ≧3 ml/min/1.73 m2 per year. We stratified by baseline preserved GFR (≧60 ml/min/1.73 m2). Results: Mean age was 72 ± 5 years. Average eGFRcys decline was –1.79 ml/min/1.73 m2 (SD = 2.60) per year. Among persons with preserved GFR, compared to the lowest quartile of Lp-PLA2 antigen, eGFRcys decline was faster among persons in the second, β –0.31 (95% CI –0.52, –0.10), third –0.19 (–0.41, 0.02) and fourth quartiles –0.26 (–0.48, –0.04) after full adjustment. Persons in the highest quartile of Lp-PLA2 antigen had increased odds of rapid decline 1.34 (1.03, 1.75), compared to the lowest. There was no significant association between levels of Lp-PLA2 activity and eGFRcys decline or rapid decline. Associations were not statistically significant among persons with low eGFR (<60 ml/min/1.73 m2) at baseline. Conclusion: Higher levels of Lp-PLA2 antigen but not activity were significantly associated with faster rates of kidney function decline. These findings may suggest a novel vascular pathway for kidney disease progression.
© 2011 S. Karger AG, Basel
Chronic kidney disease (CKD) is associated with increased risk for cardiovascular disease (CVD), [1,2] even at early stages of kidney dysfunction . In addition, clinical and subclinical CVDs are independent predictors of kidney function decline . Proposed mechanisms to explain these associations have included increased inflammation or oxidative stress associated with vascular damage. However, the associations of inflammatory markers and kidney function are modest, and the direction of the association remains unclear [5,6,7].
Lipoprotein-associated phospholipase A2 (Lp-PLA2) has been independently associated with CVD in several studies [8,9,10,11]. Lp-PLA2 is bound to low-density lipoprotein (LDL) cholesterol in the circulation, it is highly expressed in coronary plaques, and it is thought to have proinflammatory properties. Lp-PLA2 is also thought to promote endothelial dysfunction and increased inflammation within arterial walls [12,13,14,15]. Since endothelial dysfunction and increased arterial stiffness [16,17] have been associated with kidney dysfunction, Lp-PLA2 may be a plausible pathway associated with kidney dysfunction. The association between Lp-PLA2 and kidney function decline has not been evaluated.
Understanding whether Lp-PLA2 is associated with kidney function decline may help elucidate pathways that lead to kidney disease. Therefore, we studied the association of Lp-PLA2 antigen (mass) and activity with kidney function decline among participants in the Cardiovascular Health Study (CHS). We hypothesized that Lp-PLA2 would be associated with kidney function decline, independent of serum lipid concentrations.
We included participants from the CHS, a community-based longitudinal study designed to understand risk factors for development and progression of CVD. Details on design and recruitment have been previously published . Briefly, CHS recruited adults who were 65 years of age or older and living in the community by using Medicare eligibility lists. CHS recruited in four sites: Forsyth County, N.C.; Sacramento County, Calif.; Washington County, Md., and Pittsburgh, Pa. Participants were excluded if they were not expected to remain in the current community for 3 years or longer, were receiving treatment for cancer, or were unable to provide informed consent. The initial 5,201 participants were enrolled from January 1989 to June 1990; an additional 687 black participants (with race self-reported) were recruited and enrolled by June 1993. The institutional review boards at all participating centers approved these studies, and all participants gave informed consent.
For these analyses, we included all participants who had a measure of kidney function at baseline and at least one follow-up measure of kidney function and who had measures of Lp-PLA2 at baseline (n = 4,362).
Our two primary predictors of interest were Lp-PLA2 antigen (mass) and activity. In CHS, phlebotomy was performed after a 12-hour fast, and all assays were performed in frozen serum specimens that were stored at –70°C. Plasma Lp-PLA2 antigen (mass) was measured at the University of Vermont using a commercially available enzyme-linked immunosorbent assay (ELISA; second generation PLAC Test; diaDexus Inc., South San Francisco, Calif., USA). Plasma Lp-PLA2 activity was measured at GlaxoSmithKline (Research Triangle, N.C., USA) using a high-throughput radiometric assay, as previously reported. The interassay coefficients of variation were 6.3% for antigen and 7.5% for activity . In CHS, the correlation coefficient between antigen (mass) and activity was 0.51.
Kidney function measures included serum creatinine and cystatin C. Cystatin C was measured by means of a particle-enhanced immunonephelometric assay (N Latex Cystatin C; Dade Behring) with a nephelometer (BNII; Dade Behring). Serum creatinine was measured by a colorimetric method (Ektachem 700; Eastman Kodak). Kidney function was measured at baseline and at year 3 and 7 visits. Overall, 2,539 persons had 3 measures of kidney function and 1,823 had 2 measures. We estimated the glomerular filtration rate (eGFR) with the use of the CKD-EPI creatinine equation (eGFRcreat) and the CKD-EPI cystatin C equation (eGFRcys) without demographic coefficients: eGFRcys = 76.7 × cys C–1.19. Both formulae were developed from the pooling of several cohorts with GFR measured from iothalamate clearance [19,20].
The primary outcome of interest was kidney function decline, assessed by cystatin C using repeated measures of eGFRcys. A secondary outcome of interest was rapid kidney function decline, defined as eGFRcys decline of ≧3 ml/min/year, as has been defined in previous studies showing the association of this definition with adverse outcomes .
For these two outcomes, we stratified a priori by whether persons had preserved GFR or established CKD, defined as eGFRcreat <60 ml/min/1.73 m2. In particular, we chose to define CKD at baseline based on creatinine because it is still the clinical standard . We chose to study kidney function decline and rapid decline based on eGFRcys rather than eGFRcreat serial measures for several reasons. First, longitudinal measures of eGFRcys have been shown to better correlate with changes in kidney function by direct GFR measurement . Second, we have reported that eGFRcys is an important predictor of aging success in the elderly  and that eGFRcys detects kidney function decline in the elderly better than creatinine across the spectrum of GFR . Third, our group has recently shown that only CKD that is confirmed by eGFRcys <60 ml/min/1.73 m2 is associated with increased risk for death, CVD events, heart failure and end-stage renal disease, compared to CKD detected by creatinine alone .
All CHS participants underwent a comprehensive examination at each visit. Age, gender, race, and medication use were ascertained by questionnaire at the baseline visit. Diabetes was defined as a self-report of diabetes, the use of insulin or oral hypoglycemic agents or a fasting glucose ≧126 mg/dl. Three blood pressure measurements were obtained 5 min apart in the seated position. The mean of the second two measurements was used for analysis. Prevalent CVD was defined as having a history of coronary heart disease, heart failure or stroke. High-density lipoprotein (HDL) cholesterol was measured using the Olympus Demand System (Olympus, Lake Success, N.Y., USA). LDL cholesterol was calculated by the Friedewald formula after excluding participants with triglycerides >400 mg/dl. Details on assays have been previously published . C-reactive protein (CRP) was measured with an ELISA and interleukin-6 (IL-6) was measured by ELISA (R&D Systems, Minneapolis, Minn., USA).
We compared baseline characteristics by quartiles of Lp-PLA2 antigen (mass) using χ2 or ANOVA, as appropriate.
We first evaluated the association between Lp-PLA2 antigen and activity separately with kidney function decline. We used linear mixed models with random intercepts and slopes to estimate and compare linear trends in mean eGFR. This approach takes into account the correlation of observations by subject. Lp-PLA2 antigen and activity were modeled linearly per standard deviation (SD) and categorically as quartiles. We used nested models with serial adjustment to understand the role of potential confounders. Model 1 adjusted for age, gender, and race. Model 2 adjusted for model 1 plus diabetes, systolic blood pressure, use of antihypertensive medications, LDL cholesterol, HDL cholesterol, use of lipid lowering medications, and prevalent CVD. Finally, we were interested in understanding whether inflammation may play a role in explaining possible associations. Therefore, model 3 added CRP and IL-6.
We used multivariate logistic regression to study the association of Lp-PLA2 antigen and activity with rapid decline. We adjusted in nested models as above.
Earlier literature has suggested different strengths of association between Lp-PLA2 and cardiovascular outcomes among persons in different strata of lipids and CRP [10,28], so we also performed stratified analyses by LDL cholesterol (above and below 130 mg/dl), HDL cholesterol (above and below 40 and 50 mg/dl for men and women, respectively) and CRP (above and below 3 mg/l). All analyses were performed using S-Plus (release 8.0; Insightful, Inc., Seattle, Wash., USA) and SPPS (version 16.0.2; SPSS, Inc., Chicago, Ill., USA).
Among the 4,359 participants in this study, mean age was 72 (±5 SD) years, 13% were black and 60% female. Mean eGFRcys at baseline was 80 ml/min/1.73 m2 (SD = 19) and mean eGFRcreat was 74 ml/min/1.73 m2 (SD = 17). The mean for Lp-PLA2 antigen level was 341 ng/ml (SD = 118) and activity was 39 nmol/min/l (SD = 13).
Persons in the highest quartile of Lp-PLA2 antigen levels had higher baseline levels of LDL cholesterol, lower levels of HDL cholesterol, and were less likely to be on a lipid-lowering agent. Persons in the highest quartile of Lp-PLA2 antigen also had lower baseline eGFR by cystatin C and creatinine (table 1).
|Table 1. Baseline characteristics by Lp-PLA2 antigen|
The average decline in eGFRcys during a median follow up of 5.7 years was –1.79 ml/min/1.73 m2 (SD = 2.60) per year.
Among persons with eGFRcreat ≧60 ml/min/1.73 m2, higher levels of Lp-PLA2 antigen were significantly associated with faster kidney function decline, and this association was independent of demographics, comorbidities, CRP or IL-6 (table 2). Compared to persons in the lowest quartile of Lp-PLA2 antigen level, those in quartile 4 had approximately 15% faster rates of kidney function decline. These associations were not present among persons with eGFRcreat <60 ml/min/1.73 m2 at baseline (table 2).
|Table 2. Association of Lp-PLA2 with kidney function decline over 5.7 years among elderly persons by baseline CKD|
In contrast to findings with Lp-PLA2 antigen, Lp-PLA2 activity level was not significantly associated with kidney function decline (table 2).
A total of 1,084 (25%) subjects had a rapid decline in kidney function. Among persons with eGFRcreat ≧60 ml/min/1.73 m2 at baseline, 855 persons had a rapid decline, and 229 persons in the eGFRcreat <60 ml/min/ 1.73 m2 group. Lp-PLA2 antigen levels were also independently associated with higher odds of rapid kidney function decline among persons with eGFRcreat ≧60 ml/min/1.73 m2 at baseline. Each SD increase in Lp-PLA2 antigen was associated with 14% higher odds of rapid decline (table 3). When we categorized Lp-PLA2 antigen, only the highest quartile was significantly associated with higher odds of rapid decline compared to the lowest quartile (table 3). These associations were not significant among persons with eGFRcreat <60 ml/min/1.73 m2 at baseline. Lp-PLA2 activity was not statistically significantly associated with rapid decline (table 3).
|Table 3. Association of Lp-PLA2 with rapid kidney function decline among elderly persons by baseline CKD|
After full adjustment, the association between Lp-PLA2 antigen and kidney function decline was not significantly different by LDL or HDL level (p value for interaction 0.34 and 0.69, respectively). Moreover, there was no significant effect modification by CRP on the association between Lp-PLA2 antigen and kidney function decline (p value for interaction 0.79; fig. 1).
|Fig. 1. Association of Lp-PLA2 antigen with kidney function decline (in ml/min/1.73 m2 per year) among elderly persons by LDL, HDL and CRP levels at baseline.|
In this study, we found that Lp-PLA2 antigen, not activity, was significantly associated with kidney function decline and rapid kidney function decline among elderly persons without CKD (eGFR ≧60 ml/min/1.73 m2), independent of lipids, CRP and IL-6. Our findings have important implications for the understanding of potential pathways that may link CVD and kidney disease. Lp-PLA2 has emerged as an independent risk factor for heart failure, stroke, coronary heart disease and cardiovascular death [8,9,10]. Lp-PLA2 appears to be associated with the development of endothelial dysfunction and inflammation and disruption of the arterial intima by hydrolyzing oxidized phospholipids and oxidating LDL [12,13,14,15]. Since several studies have shown that subclinical CVD , endothelial dysfunction , and reduced arterial elasticity  are associated with faster rates of kidney function decline, it is possible that Lp-PLA2 is a novel common pathway of vascular injury in CKD and CVD.
Moreover, Lp-PLA2 may be involved in an important pathway for development of CKD. In particular, our findings may shed light on why previous literature on the association of serum concentration of lipids and inflammatory markers with kidney disease has been inconsistent [31,32]. Lp-PLA2 travels bound preferentially to small LDL, and it hydrolyzes phospholipids, thus promoting atherogenic activity of LDL. Studying total LDL cholesterol and kidney disease may not fully capture these associations. In fact, in a study from the Multi-Ethnic Study of Atherosclerosis, levels of small LDL particles, but not conventional LDL levels were associated with early kidney dysfunction . We found that inflammation did not attenuate our findings. Since the direction of the association between inflammation and kidney disease remains unclear [5,6], future studies on the interplay between Lp-PLA2, inflammation, and kidney function decline are warranted.
We are the first to report an independent association between Lp-PLA2 antigen (mass) and kidney function decline. Strengths of this study include the well-characterized population, repeated measures of kidney function and the prospective design. Moreover, the use of cystatin C as a kidney function marker improves prediction of kidney decline in the elderly. However, we are limited by no direct measure of GFR. The small observed changes in eGFR may have reduced our ability to detect anything less than large associations. Future studies should include persons with large changes in eGFR over time .
However, no large, community-based epidemiological studies include iothalamate GFR. Our findings that Lp-PLA2 antigen but not activity are associated with kidney function decline are in accordance with prior reports from CHS and a report on differing associations between antigen and activity and calcified coronary plaque in a young cohort [10,35]. These differences in associations between antigen and activity may be due to differences in assay design or variability. Alternatively, it is possible that the Lp-PLA2 antigen assay can better capture the complexity of the atherogenic process, particularly in this elderly cohort. Future studies are needed to elucidate these mechanisms. Our findings that the association of Lp-PLA2 and kidney function decline was not observed among persons with eGFR <60 ml/min/1.73 m2 should be interpreted with caution, as it is possible that survival bias and regression to the mean may account for these findings. Alternatively, it is possible that certain pathways may have different relative importance in the development versus progression of CKD. We are unable to account for any possible associations with albuminuria, as CHS did not measure albuminuria at baseline.
In summary, we found that Lp-PLA2 antigen (mass) is independently associated with kidney function decline and rapid kidney function decline among elderly persons with preserved GFR. Future studies should evaluate this novel pathway in other populations. Moreover, more studies are needed to elucidate causal pathways.
CHS is supported by contract numbers N01-HC-85079 through N01-HC-85086, N01-HC-35129, N01 HC-15103, N01 HC-55222, N01-HC-75150, N01-HC-45133, grant number U01 HL080295 from the National Heart, Lung, and Blood Institute, with additional contribution from the National Institute of Neurological Disorders and Stroke. This project is also supported by RO1 AG 027002 (Sarnak) and K23SK082793 (Peralta). A full list of principal CHS investigators and institutions can be found at http://www.chs-nhlbi.org/pi.htm.
This study is supported by a grant from GlaxoSmithKline (GSK). Dr. Koro is an employee at GSK. Drs. Peralta, Cushman, and Fitzpatrick have funding from GSK. The funding sources did not have a role in the design, analysis or approval of the manuscript.
Carmen A. Peralta, MD, MAS
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Received: July 13, 2011
Accepted: September 10, 2011
Published online: November 1, 2011
Number of Print Pages : 7
Number of Figures : 1, Number of Tables : 3, Number of References : 35
American Journal of Nephrology
Vol. 34, No. 6, Year 2011 (Cover Date: December 2011)
Journal Editor: Bakris G. (Chicago, Ill.)
ISSN: 0250-8095 (Print), eISSN: 1421-9670 (Online)
For additional information: http://www.karger.com/AJN