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Journal of Lipid Research, Vol. 46, 469-474, March 2005 Apolipoprotein L-I is positively associated with hyperglycemia and plasma triglycerides in CAD patients with low HDL
* Departments of Medicine, University of Washington, Seattle, WA Published, JLR Papers in Press, December 16, 2004. DOI 10.1194/jlr.M400304-JLR200
1 To whom correspondence should be addressed. e-mail: bgbrown{at}u.washington.edu
Apolipoprotein L-I (apoL-I) is present on a subset of HDL particles and is positively correlated with plasma triglycerides (TGs). We measured plasma apoL-I levels in coronary artery disease (CAD) subjects with low HDL who were enrolled in an angiographic CAD prevention trial. At baseline, apoL-I levels (n = 136; range, 2.264.1 µg/ml) were right skewed with a large degree of variability. Multivariate analysis for biological determinants of apoL-I revealed that the log of VLDL-TG (+0.17; P < 0.05) and hyperglycemia (HG; +0.26; P < 0.005) independently predicted apoL-I level. Hyperglycemic patients (n = 24) had mean apoL-I levels >50% higher than normoglycemic subjects (n = 112; 13.2 vs. 8.3 µg/ml, respectively; P < 0.001). No relationship between apoL-I level and change in CAD was found (r = 0.06, P = 0.49). Simvastatin-niacin therapy did not alter apoL-I levels (n = 34; P = 0.27), whereas antioxidant vitamins alone increased apoL-I by >50% (n = 36; P < 0.01). Genotyping of a known apoL-I polymorphism (Lys166Glu) did not independently account for any of the variability in apoL-I levels. In conclusion, we found TG and HG to be the strongest predictors of apoL-I within a dyslipidemic CAD population. These data provide further characterization of the novel HDL-associated apoL-I.
Supplementary key words diabetes mellitus coronary artery disease lipids antioxidant vitamins statins niacin high density lipoprotein
We originally identified, cloned, and characterized a novel apolipoprotein termed apolipoprotein L (apoL) in 1997 (1). Subsequently, our group and another have identified a cluster of apoL genes on chromosome 22 that encode up to six apoL proteins (apoL-I to apoL-VI) (2, 3). These genes have putative sterol response elements in their promoters, and sequence analysis has revealed conserved amphipathic helices, suggesting that they may be involved in lipid metabolism (2, 3). ApoL proteins are found in plasma and a variety of tissues, including liver, lung, brain, pancreas, placenta, and vascular endothelium (24). In addition, apoL-I was recently found to be the trypanosome lytic factor in human serum (5), suggesting that the apoL family may play an important role in biology.
ApoL-I, the original protein described in this family, is a 42 kDa protein that is found in the plasma mainly associated with large-diameter HDL particles (found on
Study design HATS (7) was a quantitative angiographic clinical trial aimed at assessing the effects of lipid-altering drugs and/or antioxidant vitamins on CAD progression and regression. HATS enrolled patients with clinical and anatomic CAD, low HDL-C ( 35 mg/dl for men, 40 mg/dl for woman), and LDL-C 145 mg/dl. There were four treatment arms: niacin plus simvastatin; antioxidant vitamin supplements (vitamins C and E, ß-carotene, and selenium); combination of niacin-simvastatin plus vitamins; or placebo for all drugs. Patients were followed on average for 3 years. The primary patient end point was the average change in percentage diameter stenosis for nine proximal coronary artery segments (% S). Of the 160 individuals who were enrolled in HATS, 146 completed the protocol. Plasma was available for on- and off-treatment apoL-I measurement for 137 subjects. At the time of enrollment in HATS, patients read and signed a consent form that had been approved by the university human subjects committee.
ApoL-I measurements
Lipid, lipoprotein, and metabolic measurements
Patient characteristics
ApoL-I genotyping
Statistical analysis
Baseline characteristics Table 1 shows the clinical and lipid/lipoprotein/metabolic characteristics of 136 HATS subjects at the time of their off-treatment apoL-I measurement. Overall, this group was similar to the whole HATS population (7). Approximately 18% of the patients had DM2 or IFG, 26% were active smokers, and 45% had a history of hypertension.
ApoL-I is right skewed with a bimodal appearance Figure 1 shows the right skewed, nonnormal distribution of apoL-I in the HATS population (all four of the tests examining for normality of log apoL-I produced P < 0.01). The GM of plasma off-therapy apoL-I concentration was 9.0 µg/ml (95% CI, 8.19.9; range, 2.264.1 µg/ml). As illustrated by Fig. 1B, there appeared to be two distinct modes in the apoL-I distribution. Using a variety of clustering methods, the predominant finding for log apoL-I was of two clusters, each containing one of the modes, whose boundary was between the observed apoL-I values of 16.7 and 23.1 µg/ml. Corresponding to those clusters, 88% (120 of 136) of the patients had apoL-I levels < 17 µg/ml ["low"; GM = 7.9 µg/ml (CI, 7.48.4)] and 12% (16 of 136) had levels > 23 µg/ml ["high"; GM = 36.4 µg/ml (CI, 31.441.4)].
VLDL-TG and HG independently predict apoL-I level Multivariate regression analysis of log apoL-I vs. 24 predictors revealed log VLDL-TG (r = +0.17, P = 0.042) and HG (r = +0.26, P = 0.003) to be the only significant, independent predictors of log apoL-I. Combined, they generated a multiple R = 0.34 with P < 0.0005. If HG was excluded from the model, FG became an independent predictor along with log VLDL-TG, but the model was weaker (multiple R = 0.32, P < 0.001).
ApoL-I in hyperglycemic subjects
ApoL-I levels and CAD progression Positive associations of in-treatment levels of a variable with change in CAD (% S) are suggestive that the variable, or another highly correlated variable, is proatherogenic (7, 8, 10). ApoL-I on-treatment levels did not correlate with % S in this population (n = 136; r = 0.06, P = 0.49).
Effect of lipid-altering and antioxidant vitamin therapy on apoL-I levels
The apoL-I Lys166Glu variant The Lys166Glu variant was genotyped in 142 HATS patients to assess for a relationship of this variant with apoL-I level. The Lys166 allele frequency was 0.796 and the Glu166 allele frequency was 0.204. The genotype frequencies were in Hardy-Weinberg equilibrium, with baseline patient characteristics by genotype shown in Table 3. ApoL-I levels were available for 124 of the 142 genotyped patients (87%). There was a tendency toward higher apoL-I levels in the Glu166 allele homozygotes, although these patients also had higher FG levels. The Glu166 allele was not more common in the cluster of patients with high apoL-I levels (frequency of Glu166 allele low cluster = 0.200 vs. high cluster = 0.269; P = 0.43), and the Lys166Glu genotype did not provide any independent predictive value when added to our multivariate apoL-I model (data not shown).
We present a series of interesting findings regarding the newly described apoL-I (13, 6) among patients enrolled in HATS (7). In this population, apoL-I levels were right skewed, with a bimodal-appearing distribution. Of the many metabolic variables measured, VLDL-TG and hyperglycemic state were the strongest independent predictors of apoL-I. No association was found between in-treatment apoL-I levels and progression of CAD. Interestingly, apoL-I levels were significantly increased by antioxidant vitamin therapy. Although a right skewed distribution of apoL-I has been seen previously in a population of normolipidemic individuals (6), apoL-I appeared to have a bimodal distribution in our population of dyslipidemic CAD patients. The presence of bimodality is supported by the fact that log transformation of apoL-I did not normalize the apoL-I distribution, as would be expected for most skewed variables with a single mode. In addition, cluster analysis suggested that there were two distinct clusters of apoL-I values. The fact that there may be two apoL-I modes may be biologically important, as there appeared to be a group of patients with mean apoL-I levels >4-fold higher than the rest of this population.
A genetic explanation for the observed apoL-I variability was investigated. Genetic variation is known to play a role in the distribution of some factors important in lipid metabolism (e.g., hepatic lipase, paraoxonase, cholesteryl ester transfer protein) (1115). The Lys166Glu apoL-I polymorphism did not, however, appear to account for the high degree of apoL-I variability that we observed. Although apoL-I levels were higher in the Glu homozygotes, this occurred in the setting of correspondingly higher FG levels, making an independent relationship difficult to support. In addition, the lack of an independent association of the Lys166Glu genotype with apoL-I levels in our multivariate model further suggests that a strong relationship between the Lys166Glu polymorphism and apoL-I plasma level did not exist in our study population. These findings, however, must be considered in light of the fact that our sample was significantly underpowered. Using our observed apoL-I variability, we calculate that As off-therapy apoL-I was measured at the end of the study, it is possible that some lingering effect of randomization might explain our findings of a skewed apoL-I distribution. This, however, is unlikely because randomization was proportionally similar between the high and low apoL-I groups (P = 0.88). In addition, measurement of pretreatment apoL-I levels in a subgroup (n = 28) of high and low apoL-I patients revealed that, even before randomization, there was a significant difference in apoL-I level between the two groups (data not shown). At this point, no clear explanation is available for our observation. The question of whether apoL-I has a bimodal distribution in the general population, or only in select populations, will need to be investigated with larger numbers of patients.
Multivariate analysis showed that there were significant associations between apoL-I and VLDL-TG and HG. Associations of apoL-I with total plasma TGs in normolipidemic individuals, and in those with dyslipidemias that include hypertriglyceridemia, have been reported previously (6). We now extend this finding to patients with low HDL-C and CAD and provide evidence that VLDL-TG is the specific TG component associated with apoL-I. Because a small fraction of total plasma apoL-I has been found on the VLDL particle (1), it is possible that the independent association we are seeing with VLDL-TG is directly related to changes in VLDL particle number or composition, or to another highly correlated variable. The lack of an association of apoL-I with HDL-C and/or plasma apoA-I content has been noted before (6). This may be related to its presence on only An association of apoL-I with HG has not been previously reported. This finding adds another lipid variable to the list of abnormalities seen in the dyslipidemia of diabetes and the metabolic syndrome (1619). Because the association between apoL-I and HG was stronger than that with FG, it is possible that the relationship between apoL-I and the hyperglycemic state is not directly related to glucose level itself but to some other metabolic abnormality associated with, or contributing to, the hyperglycemic phenotype. We tried to account for known predictors of HG by including many of them in our multivariate model (age, fasting insulin, BMI, and TGs), although we obviously could not account for all of them. The fact that almost 50% of patients with high apoL-I levels had a HG phenotype compared with <15% of those in the low apoL-I cluster further highlights the strong association between apoL-I level and a hyperglycemic phenotype. We did not find an association between apoL-I level and the progression of CAD in our population with low HDL-C. This suggests that apoL-I itself is not strongly proatherogenic. Our finding that apoL-I levels are positively associated with other known atherogenic factors (TG and HG), however, suggests that high apoL-I levels may be a novel marker of an atherogenic phenotype. The discovery that apoL proteins are present in human atherosclerotic vascular tissue (4, 20) is consistent with this hypothesis and argues for further investigation into apoL-I's role in lipid metabolism. Interestingly, the lipid-altering drugs simvastatin and niacin did not significantly alter apoL-I levels. This is surprising considering that these drugs significantly decreased TG and increased HDL-C in HATS (7). One possibility is that the simvastatin-niacin drug combination influences competing factors important in apoL-I regulation. Niacin itself has the potential to do this through its somewhat unusual effect of increasing insulin resistance while decreasing plasma TG levels. Another possibility is that the subset of apoL-I-containing HDL particles is not influenced by these lipid-altering drugs. The increase in apoL-I seen in the antioxidant vitamin group was unanticipated. These data suggest that antioxidant vitamins can alter factors important in apoL-I regulation. This finding is supportive of recent work showing that antioxidant vitamins can alter various aspects of lipid metabolism (21, 22). Cheung et al. (21) showed that antioxidant vitamins can interfere with the beneficial effects of niacin on HDL metabolism, a finding that has challenged the assumption that antioxidant vitamins are, at worst, benign. Our study again raises the question of whether antioxidant vitamins in their current formulations are benign by showing that they can increase levels of an apolipoprotein that is associated with known atherogenic factors. Although we are unable to provide a mechanistic explanation for this finding, we anticipate that this observation will be helpful in guiding further investigations of apoL-I metabolism. In conclusion, we present a series of interesting observations regarding apoL-I in patients with dyslipidemia and CAD. ApoL-I has previously been found to be right skewed and associated with plasma TG level (6). In our CAD population, we also found a right skewed distribution of apoL-I, but with a tendency toward a bimodal distribution. Although much of the variability in apoL-I was unaccounted for by the variables we measured, the HATS population showed a significant association of apoL-I with both hyperglycemic phenotype and VLDL-TG. In this study, lipid-altering therapies did not change apoL-I levels, whereas antioxidant vitamins increased apoL-I plasma levels. We feel that these data highlight some unique characteristics of the novel HDL-associated apoL-I and should facilitate further investigation into its role in lipid metabolism.
This work was supported by National Institutes of Health Grants HL-50782, HL-31210, AA-11205, and HL-50779, by a grant from the Joseph Drown Foundation, and by a gift from Donald Yellon. Studies were performed in part at the University of Washington Clinical Nutrition Research Unit (supported by National Institutes of Health Grant DK-35816), in part at the Diabetes Endocrinology Research Center (supported by National Institutes of Health Grant DK-17047), and in part at the Clinical Research Center (supported by National Institutes of Health Grant RR-37). Medications were supplied by Upsher-Smith Laboratories and Merck and Co. Manuscript received August 10, 2004 and in revised form December 13, 2004.
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