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Journal of Clinical Endocrinology & Metabolism , doi:10.1210/jc.2007-2585
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The Journal of Clinical Endocrinology & Metabolism Vol. 93, No. 8 3158-3164
Copyright © 2008 by The Endocrine Society

Intravascular Kinetics of C-Reactive Protein and Their Relationships with Features of the Metabolic Syndrome

Jean-François Mauger, Josée Lévesque, Marie-Eve Paradis, Nathalie Bergeron, André Tchernof, Patrick Couture and Benoît Lamarche

Institute on Nutraceuticals and Functional Foods (J.-F.M., J.L., M.-E.P., B.L.), Laval University, Québec, Canada G1K 7P4; College of Pharmacy (N.B.), Touro University-California, Vallejo, California 94592; Molecular Endocrinology and Oncology Research Center (A.T.) and Lipid Research Clinic (P.C.), Laval University Medical Research Center, Québec, Canada G1V 4G2

Address all correspondence and requests for reprints to: Benoît Lamarche, Ph.D., Institute on Nutraceuticals and Functional Foods, Pavillon des Services, Laval University, Québec, 2440 Hochelaga Boulevard, Québec, Canada G1K 7P4. E-mail: benoit.lamarche{at}inaf.ulaval.ca.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
Objective: The objective of the study was to describe, for the first time, the intravascular kinetics of C-reactive protein (CRP), using stable isotopes, and its relationship with features of the metabolic syndrome.

Methods: Sixteen men and 16 women [aged 49 ± 9 years, body mass index (BMI) 28.7 ± 4.5 kg/m2] underwent a 12-h primed-constant infusion of 5,5,5–2H3-L-leucine. CRP was purified from the plasma fraction {rho} greater than 1.25 g/ml by affinity chromatography, and isotopic enrichment over time was determined by gas chromatography/mass spectrometry.

Results: The CRP fractional catabolic rate was 60% higher in men than women (0.49 ± 1.83 vs. 0.30 ± 1.80 pool/d, P = 0.03), but this difference was no longer significant in a multivariate model that included several features associated with the metabolic syndrome. The CRP production rate (PR) and pool size were not statistically different between sexes. Plasma CRP concentrations were more strongly correlated with the PR (r = 0.80, P < 0.0001) than with the fractional catabolic rate of CRP (r = 0.39, P < 0.05). The PR of CRP was positively correlated with waist girth (r = 0.53, P < 0.01), plasma low-density lipoprotein apolipoprotein B-100 (r = 0.42, P = 0.07), triglyceride (r = 0.41, P = 0.06), and IL-6 concentrations (r = 0.61, P = 0.0008) and inversely correlated with high-density lipoprotein-cholesterol (r = –0.47, P = 0.03) and adiponectin (r = –0.63, P < 0.0005) after adjustment for sex. Blood pressure and low-density lipoprotein-cholesterol showed no association with CRP kinetics.

Conclusion: The PR of CRP appeared as the main determinant of CRP concentrations and showed significant associations with features of the metabolic syndrome as well as with adipose tissue-derived cytokines such as IL-6 and adiponectin.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
C-reactive protein (CRP) has been used extensively as a nonspecific marker of acute phase response in humans for decades (1). Inflammation is being increasingly suggested to play a determinant role in the initiation and development of atherosclerosis, the hallmark of cardiovascular disease (CVD) in industrial countries (2). The predictive value of plasma CRP concentrations measured by a high-sensitivity assay as a risk factor for CVD has been investigated in several epidemiological studies. These studies have been relatively consistent in showing that individuals with high baseline CRP levels (>3.0 mg/liter) were at greater risk of CVD, compared with individuals with lower (<1.0 mg/liter) CRP levels, independent of sex (3, 4, 5, 6). However, recent studies have produced less conclusive data on the added value of CRP to predict CVD risk, compared with traditional risk factors such as low-density lipoprotein (LDL) cholesterol (7, 8).

The metabolic syndrome (MetSyn) is another emerging paradigm in the management of CVD risk and latest estimates showed that individuals affected by the MetSyn have a 3-fold increased risk of CVD (9). The most recent definition of MetSyn by the International Diabetes Federation includes elevated waist girth (various cut points, depending on ethnicity and gender) plus any two of the following: elevated plasma triglyceride (>1.7 mmol/liter), low high-density lipoprotein (HDL) cholesterol (>1.03 mmol/liter for men, > 1.29 mmol/liter for women), high blood pressure (>130/85 mm Hg), and high fasting plasma glucose (>5.6 mmol/liter) (10). Interestingly, most patients with MetSyn also display low-grade chronic inflammation, as reflected by elevated plasma CRP concentrations (11, 12, 13, 14, 15). This relationship may be explained, at least partly, by concurrent variations in a novel adipose tissue exclusive endocrine factor named adiponectin. Early reports on adiponectin showed its strong insulin sensitizing and antiinflammatory effects. Consistent with these observations, reduced plasma adiponectin concentrations have been repeatedly correlated with several of the MetSyn features, including increased plasma CRP concentrations (16).

Whereas novel endocrine factors responsible for the association between inflammation and the MetSyn are beginning to be identified, the physiological mechanisms sustaining plasma CRP concentrations in this metabolic condition are still virtually unknown. Kinetic studies of proteins using stably labeled isotopomer precursors provide considerable precision about in vivo protein synthesis rates and have become the gold standard to investigate such mechanisms in humans. In vivo CRP kinetics have never been studied using endogenous-labeling methods. Furthermore, data on the relationship between CRP kinetics in human and traditional and emerging CVD risk factors associated with the MetSyn are utterly lacking.

The aim of the present study was therefore to investigate, for the first time, the intravascular kinetics of CRP using endogenous protein labeling with stable isotopes in men and women with CRP levels corresponding to different strata of CVD risk according to current guidelines. We also sought to examine the association between features of the MetSyn and the in vivo kinetics of CRP in men and women.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Study subjects

Sixteen Caucasian men and 16 Caucasian women, nonsmokers, normoglycemic, and free of any thyroid, endocrine, cardiovascular, hepatic, or renal disorders were recruited in the Québec City area. Subjects who experienced significant gain or loss of weight (>3 kg) in the 2 months preceding the study, who had excessive alcohol intake (>30 g/d), had taken drugs, or had unusual dietary habits were excluded. CRP kinetics could not be determined in one male subject due to extremely low plasma CRP concentrations. Thus, final analyses are based on a sample size of 15 male and 16 female subjects. Women who were postmenopausal (11 of 15) were not taking hormone replacement therapy. All subjects gave written informed consent to participate, and the study protocol was accepted by the Clinical Research Ethical Committee of Laval University. The most recent definition from the International Diabetes Federation was used to identify men and women with the MetSyn (10). However, the waist girth cutoff values suggested by the National Cholesterol Education Program Adult Treatment Panel III (men: > 102 cm; women: > 88 cm) were used as proposed by the International Diabetes Federation in a recent publication because they have been the reference values used in most studies conducted in North American Caucasians subjects (10).

Anthropometric measurements

Body weight and waist circumferences were measured according to standardized procedures (17).

Lipid and plasma protein measurements

Plasma very low-density lipoprotein (VLDL; {rho} < 1.006 g/ml) were isolated by ultracentrifugation, and the HDL fraction was obtained after precipitation of LDL in the infranatant ({rho} > 1.006 g/ml) with heparin and MnCl2. The cholesterol, triglyceride (TG), and phospholipid concentrations in the various plasma fractions were measured by enzymatic methods on a Technicon RA-500 analyzer (Bayer Corp., Tarrytown, NY) as previously described (18). Plasma apolipoprotein (apo) A-I and VLDL-apoB-100 concentrations were measured by nephelometry (19). LDL cholesterol concentrations were estimated using the Friedwald equation (20) whereas LDL-apoB-100 levels were obtained by subtraction. Commercial high-sensitivity immunosorbent assays were used to measure plasma IL-6 (R&D Systems, Minneapolis, MN) and CRP concentrations (BioCheck Inc., Foster City, CA). Adiponectin was measured using a commercial RIA (Linco Research Inc., St. Charles, MO) (21).

Kinetic studies

At 0700 h, after a 12-h fast, study participants were admitted at the clinical investigation unit. They immediately started consuming half-hourly small snacks that each accounted for one thirtieth of their estimated daily caloric intake to achieve a constantly fed steady state (22). After 3 h of constant feeding, the primed-constant infusion of [5, 5, 5,-2H3]L-leucine was initiated and carried out for 12 h, with participants kept in the constantly fed steady state. The bolus dose and injection rate of [5, 5, 5,-2H3]L-leucine were calculated based on the subjects’ weight (10 µmol/kg and 10 µmol/kg·h, respectively). Blood samples (20 ml) were collected via a second iv line into vacutainer tubes containing EDTA (to a final concentration of 0.1%) at time 0.0, 0.5, 1, 2, 3, 4, 6, 8, 10, and 12 h.

CRP isolation

The density of plasma (2.0 ml) was adjusted to 1.25 g/ml with Potassium bromide and centrifuged in a 100 Ti rotor (Beckmann, Fullerton, CA) at 75,000 rpm for 24 h. All plasma lipoproteins ({rho} < 1.25 g/ml) were removed by discarding the supernatant, and CRP was purified by affinity chromatography from plasma density fraction {rho} greater than 1.25 mg/ml. Briefly, EDTA was first removed from samples by an overnight dialysis step against Tris-buffered saline. The EDTA-free {rho} greater than 1.25 g/ml plasma protein fraction was then incubated with 75 µl of phosphoethanolamine covalently immobilized on activated CH-Sepharose beads (activated CH Sepharose 4B; GE Healthcare Bio-Sciences Corp., Piscataway, NJ) in a Tris-calcium buffer [140 mM NaCl, 10 mM Tris, 10 mM CaCl2 (pH 8)] for 45 min at room temperature. Afterward, beads were washed thoroughly with Tris-calcium buffer (2 ml) and CRP was specifically eluted using 10 mM phosphocholine in Tris-calcium buffer. Recovered proteins were then subjected to electrophoresis on SDS-polyacrylamide gel (16%) for 3 h at 150 V. Coomassie blue staining and pure human CRP (Fitzgerald Industries International, Concord, MA) in a control lane on the gel were used to identify the band corresponding to isolated CRP monomers. CRP immunoblotting and peptide mass fingerprint analyses were used to respectively confirm the absence of CRP in other plasma density fractions and the purity of isolated CRP (data not shown). Concentrations of CRP in the {rho} greater than 1.25 g/ml plasma fraction were systematically lower than plasma CRP concentrations (mean recovery 58 ± 22%), but a strong correlation was observed between both concentrations (R2 = 0.934, P < 0.0001). Assuming that the loss in recovery from the {rho} greater than 1.25 g/ml was not related to a loss of a specific intravascular pool of CRP, a linear regression equation was used to adjust values from the {rho} greater than 1.25 g/ml fraction back to plasma concentrations: corrected CRPplasma (milligrams per liter) = 1.558CRP{rho}>1.25 + 0.117, where 1.558 and 0.117 represent, respectively, the slope and intercept of the regression between CRP measured in the plasma and in the {rho} greater than 1.25 g/ml density fraction.

Isotopic enrichment measurements

Gel bands containing CRP were hydrolyzed in HCl 6 N at 110 C for 24 h. Recovered amino acids were derivatized using trifluoroacetic acid/trifluoroactice acid anhydride and leucine isotopic ratio was determined at each kinetic time point by gas chromatography/mass spectrometry using single ion monitoring (m/z 209 and 212).

Determination of CRP kinetics

By fitting the CRP tracer enrichment data [m/z 212/(209 + 212)] over time from each individual to a monoexponential function using the SAAMII software (Department of Bioengineering, University of Washington, Seattle, WA) (23), an estimate of the fractional catabolic rate (FCR) of CRP was derived for each study participant. Enrichment data were fitted to the mathematical function Z(t) = Zp(1 – e-k(t-d)] where Z(t) is the enrichment ratio at time t, Zp is the enrichment ratio corresponding to the plateau of the curve representing the precursor amino acid pool, d is the delay time in hours, and k is the FCR in pools per hour. We used the enrichment ratio [m/z 212/(209 + 212)] rather than the tracer to tracee ratio (m/z 212/209) because recent mathematical modeling of enrichment data has shown that the latter tends to overestimate the actual fractional synthetic rate, particularly when enrichment values are low, such as with CRP (23). The enrichment plateau of VLDL apoB-100 with deuterium-labeled leucine was used as the forcing function for precursor pool enrichment because CRP synthesis is believed to occur mostly in the liver. CRP levels remained in steady state over the 12 h of our study protocol (data not shown), which is a prerequisite for the calculation of FCR. Absolute production rate (PR) was calculated (in milligrams per day) using the formula: PR = FCR (pools per day) x pool size (milligrams per pool).

Pool size was calculated as the plasma concentration of CRP (milligrams per liter) multiplied by the estimated plasma volume [value fixed at 0.045 liter per kilogram body weight (24, 25)].

Statistical analysis

Statistical analyses were undertaken using SAS (version 9.0, SAS Institute Inc., Cary, NC). ANOVA for repeated measures with main effect of time as the repeated measure was used to assess steady state of CRP concentrations over the 12-h kinetic time course (data not shown). Unpaired t tests were used to compare CRP kinetic values between sexes and individuals with and without MetSyn. CRP concentrations, pool size (PS), and PR as well as IL-6 data were log(e) transformed before analysis because of the skewness of their distribution. Pearson correlation analysis was used to assess relationship between CRP kinetic data and metabolic and anthropometric variables. In analyses including all subjects, generalized linear modeling and regression analyses were conducted to assess the contribution of sex to the associations between CRP kinetics and metabolic and anthropometric variables. Regression analyses were also used to dissect out the respective association between the FCR and PR of CRP and metabolic and anthropometric variables. Finally, the correlations between the PR of CRP and CRP plasma concentrations and between the FCR of CRP and CRP plasma concentrations were compared using the CALIS procedure.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Subject characteristics are presented in Table 1Go. Men as a group were significantly younger than women and were characterized by a less favorable metabolic profile. As a result, more men were characterized by the MetSyn (eight men vs. one woman). The most frequent features of the MetSyn in men were high blood pressure (56%), waist girth greater than 102 cm (44%), and HDL cholesterol less than 1.03 mmol/liter (38%). Most frequent features of the MetSyn among women were HDL cholesterol less than 1.30 mmol/liter (50%) and waist girth greater than 88 cm (31%). Plasma CRP concentrations were not statistically different between men and women.


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  		TABLE 1. Characteristics of study participants

 
The pool size and PR of CRP tended to be higher among men, although the difference did not reach statistical significance (Fig. 1Go). Men had a 60% higher mean FCR of CRP than women (P = 0.03), but this difference was no longer significant (P = 0.553) in a multivariate model that included waist girth, HDL cholesterol, TG, apoB100, adiponectine, and IL-6 concentrations. The PR and FCR of CRP were strongly correlated (r = 0.73 P < 0.0001, data not shown). In an analysis in which participants were grouped according to current CRP cutoff values used to estimate the risk of CVD (Fig. 2Go), variations in circulating concentrations of CRP were more strongly associated with variations in the PR rather than the FCR of CRP. Plasma CRP concentrations also showed stronger correlation with its PR (r = 0.80, P < 0.0001) than with its FCR (r = 0.39, P < 0.05, P < 0.0001 for the comparison of the correlation coefficients).


Figure 1
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  		FIG. 1. Difference in CRP kinetic between men (n = 15) and women (n = 16). Mean values (shown within bars) correspond to geometric mean ± SE of the mean for the PS and PR of CRP and mean ± SEM for the FCR of CRP.

 

Figure 2
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  		FIG. 2. Mean PR of CRP (top panel) and FCR of CRP (bottom panel) among subjects grouped according to proposed CRP cutoff values for CVD risk management (CRP < 1.0 mg/liter, low risk; CRP between 1.0 and 3.0 mg/liter, intermediate risk; CRP > 3.0 mg/liter, high risk). Bars represent geometric means ± SEM for the PR of CRP and mean ± SEM for the FCR of CRP. m, Men; w, women.

 
Associations between CRP kinetics and metabolic and anthropometric variables were investigated within each sex separately and in all subjects combined. In men, positive trends were observed between the PR of CRP and LDL apoB-100 concentrations (r = 0.50, P < 0.056), IL-6 concentrations (r = 0.48, P < 0.068), waist girth (r = 0.44, P = 0.10), and body mass index (BMI) (r = 0.45, P = 0.096). In women, the PR of CRP was inversely associated with HDL cholesterol (r = –0.52, P < 0.05), apoA-I (r = –0.51, P < 0.05), and adiponectin concentrations (r = –0.70 P = 0.003), positively associated with IL-6 concentrations (r = 0.66, P < 0.01), waist girth (r = 0.52, P = 0.04), and BMI (r =0.56, P = 0.02), and a positive trend was observed between the PR of CRP and plasma TG concentrations (r = 0.49, P < 0.056). Combined analysis of men and women adjusted for sex yielded positive associations between CRP PR and waist girth (Fig. 3Go), BMI (r = 0.56, P = 0.004, not shown), and IL-6 concentrations (r = 0.61, P = 0.0008, not shown) and negative associations between CRP PR and HDL cholesterol (r = –0.47, P = 0.03, not shown) and adiponectin concentrations (Fig. 4Go). Weaker associations were also observed between the PR of CRP and LDL apoB-100 (r = 0.42, P = 0.08), total to HDL cholesterol ratio (r = 0.43, P = 0.06), plasma TG (r = 0.41, P = 0.06), and apoA-I (r = –0.43, P = 0.06). The relationships between CRP PR and BMI, waist girth, and plasma adiponectin concentrations remained significant after further adjustment for sex and the FCR of CRP. Interestingly, these three relationships also remained significant even after inclusion of IL-6 as covariate in the multiple regression analysis (BMI: P = 0.0003; waist girth: P = 0.005; and adiponectin: P = 0.01, respectively). In general, the FCR of CRP showed weaker associations with metabolic and anthropometric variables and adjustment for either sex, waist girth, or the PR of CRP completely nullified all of these relationships.


Figure 3
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  		FIG. 3. Relationship between the PR of CRP and waist girth. Gray circles represent men, open circles represent women, dotted lines represent 95% confidence interval. P value adjusted for sex and FCR of CRP.

 

Figure 4
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  		FIG. 4. Relationship between CRP kinetics and the number of MetSyn-related features. Gray circles represent men, open circles represent women, dotted lines represent 95% confidence interval. P value adjusted for sex and FCR of CRP.

 
CRP kinetic was finally investigated according to the cumulative number of MetSyn features. As shown in Fig. 5Go, there was a gradual increase in the PR of CRP with the number of features of the MetSyn, whereas no significant difference was observed in the FCR of CRP among the three groups. Adjustment for sex had no impact on these results.


Figure 5
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  		FIG. 5. PR (top panel) and FCR (lower panel) of CRP with increasing number of features of the MetSyn. P values for between-group comparisons are adjusted for sex. Bars represent geometric mean ± SEM for the PR of CRP and mean ± SEM for the FCR of CRP. m, Men; w, women.

 

   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
The aim of this study was to investigate for the first time the intravascular kinetic of CRP in men and women using endogenous labeling methods and its relationship with features of the MetSyn and other risk factors for CVD.

We described a new method allowing the investigation of the in vivo kinetic of CRP in humans using stable isotopes, in which pure CRP was isolated from the plasma fraction {rho} greater than 1.25 g/ml obtained at predetermined time points during a 12-h primed-constant infusion of a 5,5,5-2D3-L-leucine isotopomer. The {rho} greater than 1.25 g/ml plasma fraction allowed the use of relatively high concentrations of calcium (10 mM) to facilitate the calcium-dependent interaction between CRP and its natural ligands without the occurrence of clotting. This plasma fraction was shown to contain essentially all recoverable plasma CRP as assessed by Western blot analysis and was free of lipoprotein-associated phospholipids that could have interfered with the binding of CRP to the phosphoethanolamine covalently linked to activated CH-Sepharose beads. The purity of CRP obtained with the purification protocol used was confirmed by its electrophoretic properties and peptide mass fingerprint that perfectly matched that of commercially available human pure CRP (data not shown). Together, these observations confirmed that the present CRP purification protocol using the {rho} greater than 1.25 g/ml plasma fraction was suitable for the kinetic study of endogenously labeled CRP. The use of a 12-h primed-constant infusion was justified by two reasons. First, in the only report to date of in vivo CRP kinetics in humans (26), the turnover rate of CRP was shown to be approximately 3 times faster than that of LDL apoB-100 (27), which have been extensively studied in the past using 12-h tracer infusions. Our study showed that a primed-constant infusion of 5,5,5-2D3-L-leucine over 12 h yields CRP enrichment that allow the determination of intravascular kinetic parameters. Second, CRP is thought to be mainly synthesized by hepatocytes, and it was possible, using the plateau enrichment values of VLDL apoB-100, to estimate the intrahepatic precursor pool, which is a prerequisite in multicompartmental modeling of kinetic data.

The only previous study of human CRP kinetics has been conducted using exogenously radiolabeled CRP (26). The mean FCR of plasma CRP reported by Vigushin et al. (26) was substantially higher than values in our study using prime-constant infusion of stable isotope in the fed state (0.9 ± 0.2 vs. 0.46 ± 0.27 pool/d). Previous studies have shown that exogenous labeling of protein may yield slightly different FCRs, compared with endogenous labeling, probably due to subtle protein alteration during in vitro iodination, resulting in accelerated or reduced catabolism (28, 29, 30). Despite the fact that radioiodinated CRP was reported to exhibit similar physicochemical properties and kinetics in mice and rabbit, compared with unlabeled human CRP (26), it is still possible that iodination of CRP led to modifications that have resulted in an accelerated catabolism in humans. It is unlikely that the fed state used in the present study may have been responsible for differences in CRP FCR, compared with the study of Vigushin et al., because it has been shown that acute intake of calories has essentially no impact on plasma CRP levels (31, 32). Finally, we and Vigushin et al. have consistently shown that the PR of CRP, not its FCR, was the key determinant of CRP pool size.

The relatively large differences in CRP PR and PS between sexes were not significant. The apparent difference in the FCR of CRP between men and women was also no longer significant in a multivariate model that included several features associated with the MetSyn. This suggests that differences in the risk profile between sexes confounded to a significant extent the difference in the FCR of CRP between men and women. Based on this, it appears unlikely that gender has an independent influence on intravascular CRP kinetics, but additional studies are needed to further investigate this possibility.

The positive correlation between the PR and FCR of CRP was of interest. Several hypotheses can explain this apparent artifact. First, the FCR is used directly in the calculation of the PR. Second, we currently know very little on the in vivo kinetic of CRP, and it is possible that there are compensatory mechanisms through which an increased PR of CRP is counterbalanced by an increased clearance. However, this possibility is not entirely supported by the shape of the association between CRP concentrations and its FCR, which appeared to plateau at CRP concentrations greater than 1.0 mg/liter. Rather, this suggests that CRP clearance may reach a maximum rate and remain stable in the presence of higher CRP concentrations. Whereas the hypothesis of a maximal clearance rate would be consistent with the previous demonstration of an unchanged FCR of CRP during acute phase when CRP concentrations are dramatically increased (26), this issue will have to be further clarified in future studies.

Among both sexes, but more strongly so in women, CRP PR was positively associated with indices of obesity such as BMI and waist girth. A strong positive correlation between CRP PR and IL-6 and a strong negative association between CRP PR and plasma adiponectin concentrations were also observed. This is consistent with the hypothesis according to which the inflammatory state associated with the MetSyn may be the result of a combination of two key factors: 1) an increased secretion of proinflammatory cytokines such as IL-6 by the adipose tissue or infiltrating macrophages (33) and 2) a reduced secretion of antiinflammatory factors such as adiponectin attributed to excessive adiposity or, perhaps more importantly, to reduced adipocyte metabolic efficiency (34). Interestingly, the correlations between the PR of CRP and BMI and waist girth remained significant, even after adjustment for the FCR or CRP, sex, and plasma IL-6 and adiponectin concentrations, thereby suggesting independent contributions of both obesity and endocrine factors to sustaining low-grade chronic inflammation.

This study has apparent limitations, two of which being the study sample size and the relatively low proportion of individuals with MetSyn, especially among women. First, it should be emphasized that our sample size of 31 comprising both men and women is rather large, compared with most in vivo kinetic studies conducted in humans to date. It should also be noted that despite the low prevalence of the MetSyn in women, some of its key features such as low HDL cholesterol and elevated waist girth were present in high proportions (50 and 31%, respectively). Associations between the PR of CRP and metabolic variables were also stronger in females than males. Based on this, we hypothesize that a higher prevalence of the MetSyn among female participants would most likely have strengthened the observed associations because MetSyn features are generally thought to be synergistically interrelated. Finally, statistical adjustments for sex generally had no impact on the purported associations between the PR of CRP and features of the MetSyn.

In conclusion, this first investigation of CRP kinetic using endogenous labeling of proteins with stable isotopes in men and women brings evidence that plasma CRP concentrations appear to be determined mainly by its rate of production rather than by its rate of clearance. Our observations also emphasized the importance of obesity as well as adipose tissue-derived cytokines such as adiponectin and IL-6 as key correlates of in vivo CRP production. Interestingly, plasma LDL cholesterol concentrations and hypertension were not related to variations in in vivo CRP kinetics. Finally, further studies are warranted to investigate how the MetSyn as an entity, rather than its individual features, contributes to variations in intravascular CRP kinetic, especially in women.


   Acknowledgments
 
The authors thank Dr. Mark B. Pepys and Dominique Michaud for their technical advice on CRP purification. The authors are also indebted to study participants as well as Amélie Charest, Danielle Aubin, Johanne Marin, George Cousineau, Dominique Michaud, and Karine Coenen for their most valuable technical assistance.


   Footnotes
 
This work was supported in part by the Canada Research Chair in Nutrition and Cardiovascular Health and the Fonds Québécois de Recherche en Nature et Technologies. J.-F.M. is a recipient of a Student’ship from the Canadian Institutes of Health Research, and A.T. is the recipient of a Canadian Institutes of Health Research New Investigator Scholarship.

Disclosure Statement: The authors have nothing to disclose.

First Published Online May 20, 2008

Abbreviations: apo, Apolipoprotein; BMI, body mass index; CRP, C-reactive protein; CVD, cardiovascular disease; FCR, fractional catabolic rate; HDL, high-density lipoprotein; LDL, low-density lipoprotein; MetSyn, metabolic syndrome; PR, production rate; PS, pool size; TG, triglyceride; VLDL, very low-density lipoprotein.

Received November 26, 2007.

Accepted May 14, 2008.


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

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