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Rapid Communication |
agonist increases serum apolipoprotein A-V levels and the apolipoprotein A-V/apolipoprotein C-III ratio
Lilly Research Laboratories, Eli Lilly and Company, Indianapolis, IN 46285
Published, JLR Papers in Press, May 16, 2005. DOI 10.1194/jlr.C500010-JLR200
1 To whom correspondence should be addressed. e-mail: konrad_robert{at}lilly.com
ABSTRACT
Apolipoprotein A-V (apoA-V) first gained attention as a regulator of triglycerides through transgenic mouse studies. Furthermore, peroxisome proliferator-activated receptor
(PPAR
) agonists such as fenofibrate increase apoA-V mRNA expression. Our group recently developed the first assay to quantitate serum apoA-V levels. Therefore, we sought to determine whether administration of a PPAR
agonist would increase circulating apoA-V. Cynomolgus monkeys were dosed for 14 days with 0.3 mg/kg/day LY570977 L-lysine, a potent and selective PPAR
agonist. Blood samples were drawn throughout the treatment period and after a 2 week washout. Administration of the PPAR
agonist caused a 50% decrease in triglycerides that reversed at washout. Serum apoA-V concentrations increased 2-fold, correlated inversely with triglycerides, and were reversible at washout. The apoA-V/apoC-III ratio increased >2-fold, with this increase also reversible at washout.
These data demonstrate for the first time that a PPAR
agonist increases circulating apoA-V protein levels and the apoA-V/apoC-III ratio.
Supplementary key words triglycerides cholesterol enzyme-linked immunosorbent assay lipids cynomolgus monkeys peroxisome proliferator-activated receptor 
Apolipoprotein A-V (apoA-V) is now recognized as a key regulator of serum triglyceride levels (for a complete review, see 1, 2). The gene for this novel apolipoprotein was originally identified in experiments seeking new open reading frames in the ApoA1-ApoC3-ApoA4 gene cluster located on human chromosome 11q23 and at the molecular level in animal studies (3, 4). What emerged from this work was a new gene coding for an apolipoprotein with greatest homology to ApoA4; the new protein was named apoA-V (3, 4).
When the human gene for apoA-V was expressed in transgenic mice, triglyceride levels decreased by 5070%, and when the mouse ApoA5 gene itself was knocked out, triglyceride levels increased by
4-fold (4, 5). These data suggested that apoA-V expression may be highly and inversely correlated with triglyceride levels. In addition, polymorphisms in the human ApoA5 gene correlate with increased triglyceride levels (611).
Our group recently developed the first assay to measure apoA-V protein and demonstrated that apoA-V is present in human serum in specific lipoprotein particles (12). It was also shown recently that apoA-V mRNA expression is upregulated by peroxisome proliferator-activated receptor
(PPAR
) agonists such as fenofibrate, suggesting that increased apoA-V mRNA expression may contribute to decreases in serum triglycerides (13, 14). In light of these data and our ability to measure actual serum apoA-V protein levels, we sought to determine whether the administration of a potent and selective PPAR
agonist would increase circulating levels of apoA-V protein.
MATERIALS AND METHODS
Animals
Young adult, male, cynomolgus monkeys (Macaca fasciularis) weighing
2.55.0 kg were obtained from Charles River BRF, Inc. Monkeys were housed individually in aluminum cages with suspended floors under conditions of controlled temperature (22.2 ± 4.4°C), humidity (2080%), and light cycle (12/12 h light/dark). Monkeys were offered
820 biscuits (Certified Primate Diet 5048; PMI Nutrition International, Inc.) daily, with fresh fruit provided three times weekly and water supplied ad libitum.
Study design and institutional compliance statement
Six monkeys were given 0.3 mg/kg LY570977 L-lysine in a 2 ml/kg dose volume by nasogastric gavage daily for 14 consecutive days, followed by 14 days without treatment. Fasting blood samples were collected on days 0 (baseline), 1, 3, 7, 14, and 28 (washout). The EC50 of LY570977 L-lysine for human PPAR
is 1,082 nM. No activity is measurable for PPAR
or PPAR
. Based on these data as well as previous experience, a dose of 0.3 mg/kg LY570977 L-lysine was selected. Monkeys were housed in facilities at Eli Lilly and Co. (Greenfield, IN) that are accredited by the American Association for the Accreditation of Laboratory Animal Care, and all protocols were approved by the Eli Lilly Institutional Animal Care and Use Committee.
Blood sample collection and hematologic analysis
Monkeys were restrained in a chair and blood was drawn from the femoral vein into tubes containing EDTA for hematologic analysis or into tubes containing no anticoagulant for clinical chemistry analysis. EDTA anticoagulated blood samples were analyzed using Bayer Advia 120 (Bayer Corp., Tarrytown, NY). Morphology comments and differential leukocyte counts were made by direct microscopy of Modified Wright-stained blood smears.
Clinical chemistry, apolipoprotein, and lipoprotein analysis
Concentrations of all chemistry analytes, including triglycerides and apoA-I and apoB, were measured using Hitachi Chemistry Systems (Roche Diagnostics, Indianapolis, IN). Serum apoC-III levels were measured on a Roche Hitachi 717 instrument using reagents manufactured by Wako Chemicals. Lipoproteins were analyzed using NMR technology (Liposcience, Inc., Raleigh, NC).
Measurement of apoA-V levels
Recombinant apoA-V standard and anti-apoA-V antibodies were prepared as described previously (12). An apoA-V ELISA was used to measure apoA-V levels as described previously with minor modifications (12). Briefly, wells were coated overnight (Pierce carbonate-bicarbonate coating buffer, pH 9.40) with anti-N-terminal apoA-V antibody at a concentration of 5 µg/ml. The next day, wells were aspirated, washed three times with assay buffer (50 mM HEPES, pH 7.4, 150 mM NaCl, 1% Triton X-100, 5 mM EDTA, and 5 mM EGTA), and blocked for 1 h with TBS-casein blocking buffer (Pierce). Wells were washed three times with assay buffer. Next, 100 µl of human serum or recombinant apoA-V standard (varying concentrations of recombinant protein with 1.0% BSA as a carrier protein) diluted 1:10 in assay buffer was added to the wells and incubated for 2 h at room temperature. After aspiration, wells were washed six times with assay buffer, and 100 µl of a 1:1,000 dilution of conjugate antibody (HRP-labeled anti-C-terminal apoA-V antibody; 1 mg/ml) in assay buffer supplemented with 0.1% BSA was added to the wells for a 1 h incubation at room temperature. After aspiration, wells were washed six times with assay buffer. After the last aspiration, 100 µl of TMB development substrate (BioFX Laboratories) was added to the wells and allowed to incubate for 30 min at room temperature. The reaction was stopped with an equal volume of 2 N phosphoric acid, and plates were read at 450 nm.
Data analysis
SigmaPlot version 8.0 was used for fitting of the calibration curves. Data are expressed as means ± SEM using version 2.98 of the program FigP (Biosoft, St. Louis, MO). Statistical analysis was performed using the same program. Data were analyzed by one-way ANOVA followed by comparisons between the means using the least significant difference test. P < 0.05 was considered to indicate statistical significance.
RESULTS
No important compound-related alterations in complete blood counts occurred (data not shown). Minor compound-related chemistry alterations consisted of slight time-dependent decreases in the activity of alkaline phosphatase and
-glutamyltransferase (Table 1). All compound-related changes normalized at completion of the reversibility phase. None of the minor compound-related alterations was considered adverse to the monkeys' health.
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agonist. The mean baseline triglyceride level was 60.8 ± 5.3 mg/dl. On day 1 of dosing, triglycerides decreased to 27.3 ± 2.8 mg/dl (P < 0.05 compared with baseline, P < 0.05 compared with washout). This was followed by triglyceride levels of 26.8 ± 2.3 mg/dl (P < 0.05 compared with baseline, P < 0.05 compared with washout), 24.0 ± 2.1 mg/dl (P < 0.05 compared with baseline, P < 0.05 compared with washout), and 34.1 ± 4.0 mg/dl (P < 0.05 compared with baseline, P < 0.05 compared with washout) on days 3, 7, and 14 of dosing, respectively. On day 28, after a 14 day washout of the compound, triglyceride levels were 55.1 ± 4.7 mg/dl.
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Serum apoA-V levels were next measured via a dual-antibody sandwich ELISA (12) at corresponding time points. Figure 2A shows raw results from a representative apoA-V ELISA in which all samples were clearly on the standard curve and no sample required additional dilution for repeat analysis. This ELISA permits the measurement of apoA-V levels ranging from
10 to 1,000 ng/ml.
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agonist. The mean baseline apoA-V level was 34.3 ± 4.6 ng/ml. On day 1 of dosing, there was an increase in serum apoA-V levels to 43.0 ± 6.0 ng/ml (P = 0.2 compared with baseline, P < 0.05 compared with washout). This was followed by apoA-V levels of 77.8 ± 21.0 ng/ml (P = 0.07 compared with baseline, P < 0.05 compared with washout), 77.3 ± 22.6 ng/ml (P = 0.09 compared with baseline, P < 0.05 compared with washout), and 85.7 ± 28.6 ng/ml (P = 0.09 compared with baseline, P < 0.05 compared with washout) on days 3, 7, and 14 of dosing, respectively. On day 28, after a 14 day washout of the compound, apoA-V levels were 21.5 ± 2.9 ng/ml. An apoA-V/apoC-III ratio was also calculated for each data point. Figure 3 shows the changes that occurred in this ratio over time. The apoA-V/C-III ratio increased beginning on day 1, and this increase was sustained throughout the dosing period. The mean baseline apoA-V/C-III ratio was 0.86 ± 0.10 ng/µg. On day 1 of dosing, the ratio increased to 1.18 ± 0.11 ng/µg (P = 0.06 compared with baseline, P < 0.05 compared with washout). This was followed by apoA-V/C-III ratios of 2.07 ± 0.43 ng/µg (P < 0.05 compared with baseline, P < 0.05 compared with washout), 2.25 ± 0.44 ng/µg (P < 0.05 compared with baseline, P < 0.05 compared with washout), and 2.11 ± 0.62 ng/µg (P = 0.07 compared with baseline, P < 0.05 compared with washout) on days 3, 7, and 14 of dosing, respectively. On day 28, after a 14 day washout of the compound, the apoA-V/C-III ratio was 0.57 ± 0.06 ng/µg.
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agonist (P < 0.05 compared with baseline, P < 0.05 compared with washout at days 3, 7, and 14). Decreases in VLDL particle number were also significant and essentially paralleled those observed for VLDL-triglyceride. Minimal changes were observed in VLDL particle size.
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Our results demonstrate that administration of a potent and selective PPAR
agonist results in increased levels of serum apoA-V protein, which correlate inversely with decreased triglycerides, VLDL, and VLDL-triglyceride. The apoA-V/C-III ratio was also increased with administration of the PPAR
agonist. Upon washout of the compound, these changes were reversible, with apoA-V levels decreasing and serum triglycerides, VLDL, and VLDL-triglyceride increasing. Modest decreases were observed in serum levels of apoC-III, whose expression has been reported to decrease with PPAR
agonists (1518).
We previously described the first assay to measure apoA-V levels and found that apoA-V circulates at much lower concentrations than other apolipoproteins (12). These data explained why apoA-V was not discovered in serum, as were related apolipoproteins coded for on the human chromosome 11q23 locus, but was instead identified at the DNA level via a search for open reading frames (3, 4). Our current observations confirm this and are particularly important because they represent the first demonstration that administration of a PPAR
agonist causes increases in circulating levels of apoA-V protein.
As we reported previously, the lipoprotein particle distribution of apoA-V is most similar to that of apoC-III, an interesting observation in light of recent data suggesting that apoC-III and apoA-V influence triglyceride levels in opposite directions (1, 19). In the case of apoC-III, it is thought that the protein is an inhibitor of lipoprotein lipase. The mechanism by which apoA-V decreases triglyceride levels is unclear, although it has been demonstrated to inhibit VLDL production and to stimulate lipoprotein lipase-mediated VLDL-triglyceride hydrolysis (20, 21). Our current data suggest that decreases in VLDL-triglyceride observed in this study may have been attributable more to VLDL particle number reduction than to decreased VLDL particle size. What is clear, however, is that serum levels of apoA-V are minuscule compared with those of apoC-III. When differences in the molecular weights of the two proteins are taken into account, there is
4,0005,000-fold more apoC-III than apoA-V in serum on a molar basis.
There is significant current interest in better understanding the role of apoA-V in the regulation of serum lipids, with additional recent reports of apoA-V polymorphisms associated with hypertriglyceridemia (22, 23), with hyperlipidemia in patients with apoE2/2 phenotype (24), and of apoA-V mRNA expression being regulated via insulin (25). In light of our data describing increased serum levels of apoA-V in response to administration of a PPAR
agonist, it is very likely that assays for serum levels of apoA-V will emerge as important biomarkers for PPAR
agonists. We continue to anticipate that apoA-V measurement will be increasingly used in the selection and development of PPAR agonists as well as other types of small and large molecules developed to treat dyslipidemia.
ACKNOWLEDGMENTS
The authors thank Dr. Holger Schilske, Dr. David Robbins, Dr. Mel Prince, Nancy Hale, Paula Santa, and Jayne Talbot for their support.
Manuscript received March 17, 2005 and in revised form May 9, 2005.
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