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U.S. Department of Agriculture/Agricultural Research Service Childrens Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine (N.F.B., N.R.M., C.A.B.), Houston, Texas 77030; and Department of Genetics, Southwest Foundation for Biomedical Research (A.G.C., G.C., S.A.C.), San Antonio, Texas 78245
Address all correspondence and requests for reprints to: Dr. Nancy F. Butte, Childrens Nutrition Research Center, 1100 Bates Street, Houston, Texas 77030. E-mail: nbutte{at}bcm.tmc.edu.
| Abstract |
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Objectives: The aims of these analyses were: 1) to estimate the heritability of fasting serum adiponectin; 2) to evaluate the effects of age, sex, and body composition on fasting serum adiponectin; 3) to test for associations between fasting serum adiponectin and diet, fitness, energy expenditure, and fat oxidation; and 4) to determine the relationships between fasting serum adiponectin, insulin and lipids, and blood pressure in Hispanic children.
Design: Genetic and environmental factors influencing fasting serum adiponectin were investigated in a cohort of children participating in the VIVA LA FAMILIA Study in 20002005.
Setting: This study was performed at the Childrens Nutrition Research Center.
Participants: The study participants were 805 Hispanic nonoverweight and overweight children, ages 419 yr.
Main Measure: The main measure of the study was fasting serum adiponectin.
Results: The heritability of serum adiponectin was 0.93 ± 0.10 (P = 2.4 x 1040). Adiponectin differed by age (P = 0.001), sex (P = 0.04), and weight (P = 0.001) status. Adiponectin levels declined with age, in association with changes in sex hormones and growth factors. Adiponectin was not associated with macronutrient intake, fitness, 24-h energy expenditure, or fat oxidation. Controlling for age, sex, and percent fat mass, adiponectin was inversely associated with homeostasis model of insulin resistance, triglycerides (TG)/high-density lipoprotein cholesterol (HDL-C), and systolic blood pressure (P = 0.001). Significant positive genetic correlations were detected between adiponectin and total cholesterol (
G = 0.19), HDL-C (
G = 0.32), low-density lipoprotein cholesterol (
G = 0.24), and IGF-binding protein-1 (
G = 0.39), and negative genetic correlations were detected between adiponectin and leptin (
G = 0.30), TG (
G = 0.21), TG/HDL-C (
G = 0.33), and IGF-binding protein-3 (
G = 0.32), indicating shared genetic components in their expression.
Conclusion: The high heritability of adiponectin and pleiotropy seen between adiponectin and leptin, growth factors, and lipids may play a role in the pathogenesis of cardiovascular disease and type 2 diabetes in overweight Hispanic children.
| Introduction |
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, adiponectin is reduced in obesity. Dysregulation of the expression and/or secretion of adiponectin may play a role in the pathogenesis of cardiovascular disease (CVD) and type 2 diabetes (T2D) associated with obesity. Genetic linkage for the metabolic syndrome, a constellation of metabolic risk factors for CVD and T2D including abdominal obesity, dyslipidemia, hypertension, and insulin resistance, was detected in the region of chromosome 3 containing the structural gene for adiponectin (2). In adults, adiponectin is positively correlated with age, insulin sensitivity, and high-density lipoprotein cholesterol (HDL-C) and is negatively correlated with body mass index (BMI), sc adipose tissue, visceral adipose tissue, and serum triglycerides (TG) (3). Decreased adiponectin appears to be an independent risk factor for progression to T2D. Adiponectin expression in sc adipose tissue was 45% lower in 22 healthy, first-degree relatives of T2D patients, although serum adiponectin levels were similar to control values (4). In Japanese adults, those in the lowest tercile were 9.3 times more likely to develop diabetes than those in the highest tercile (5). Adiponectin has been measured in a few pediatric studies with conflicting results regarding age, sex, and other determinants (6, 7, 8, 9).
Numerous experimental studies in rodents and adult humans suggest a protective role for adiponectin against the development of insulin resistance and dyslipidemia (10). Adiponectin injected into mice accelerates the oxidation of nonesterified fatty acids, decreases TG storage in muscle and liver, improves hyperglycemia, and decreases plasma TG and nonesterified fatty acid concentrations. Using a euglycemic hyperinsulinemic clamp in adults, Mohlig et al. (11) demonstrated that insulin decreased adiponectin levels from 30.4 to 26.7 µg/ml (11). However, the function(s) and directionality of the relationship between adiponectin expression and insulin action are not completely understood. Adiponectin may enhance insulin sensitivity by blocking TNF-
signals, which block insulin action through inhibition of insulin receptor substrate-1 and tyrosine kinase activity (9). Adiponectin also may inhibit TNF-
-induced expression of adhesion molecules in endothelial cells and secretion of TNF-
from monocyte macrophages (6).
Given the paucity of pediatric studies in this area, we decided to examine genetic and environmental factors influencing fasting serum adiponectin in Hispanic children participating in the VIVA LA FAMILIA study, which was designed to genetically map childhood obesity in approximately 300 nuclear Hispanic families. The specific aims of these analyses were 1) to estimate the heritability of fasting serum adiponectin; 2) to evaluate the effects of age, sex, and body composition on fasting serum adiponectin; 3) to test for associations between fasting serum adiponectin and diet, fitness, energy expenditure, and fat oxidation; and 4) to determine the relationships between fasting serum adiponectin, insulin, and lipids and blood pressure in Hispanic children.
| Subjects and Methods |
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Genetic and environmental factors influencing fasting serum adiponectin were investigated in 805 of the 1030 children enrolled in the VIVA LA FAMILIA study, which was designed to genetically map childhood obesity in the Hispanic population. The age, weight, height, and BMI z-score of the 225 children enrolled later in the study did not differ from the 805 children described here. Each family was ascertained on an overweight proband between yr 4 and 19 of age using a bivariate ascertainment scheme [i.e. >95th percentile for BMI (12) and >85th percentile for fat mass (FM) (13, 14)]. Once identified, the overweight proband and all siblings, 419 yr of age, and their parents were invited to the Childrens Nutrition Research Center for a tour and full explanation of the study. All children and their parents gave written informed consent. Separate approvals were obtained from the institutional review boards for Human Subject Research for Baylor College of Medicine and Affiliated Hospitals and Southwest Foundation for Biomedical Research.
The overweight probands and all siblings were then characterized for body size, body composition, and endophenotypes associated with the development of obesity, including hormones and metabolites, energy expenditure, physical fitness, and diet. In this study, we report our findings on the endophenotype, adiponectin, in 422 overweight and 383 nonoverweight siblings.
Demographics and family history
A sociodemographic interview was conducted with the parents in their dominant language, English or Spanish. Included in the interview was a family history of chronic diseases, including T2D, among parents and grandparents.
Anthropometry and body composition
Body weight to the nearest 0.1 kg was measured with a digital balance, and height to the nearest 1 mm was measured with a stadiometer. Body composition was determined by dual energy x-ray absorptiometry using a Delphi-A whole-body scanner (Hologic, Inc., Waltham, MA). Total body estimates of FM and fat-free mass (FFM) were obtained.
Clinical signs
Blood pressure, heart rate, and temperature were determined in triplicate using a DINAMAP Vital Signs Monitor (8100T, Critikon, Inc., Tampa, FL). Each child was seated quietly for at least 5 min before measurement. The arm was supported at the heart level, and an appropriate cuff size was used. The presence or absence of acanthosis nigricans in the skin around the neck was assessed and recorded by a trained nurse. Tanner stages of sexual maturation, based on pubic hair and breast and genital development illustrated with drawings, were determined by self-report (15, 16, 17, 18).
Fasting blood biochemistries
A fasting blood sample was drawn for biochemical determinations. Fasting serum concentrations of glucose, TG, total cholesterol (TC), and HDL-C were measured by enzymatic-colorimetric techniques using the GM7 analyzer (Analox Instruments, Lundeburg, MA) and the Microquant Platereader (Biotek Instruments, Winooski, VT). Glucose was assayed using glucose oxidase. TG was determined using lipase, glycerol kinase, glycerol phosphate oxidase, and peroxidase. TC and HDL-C were determined using cholesterol esterase, cholesterol oxidase, and peroxidase. Low-density lipoprotein cholesterol (LDL-C) was calculated as: TC HDL-C (TG/5). Serum adiponectin, leptin, and insulin were measured by RIA (Linco Research, Inc., St. Charles, MO). The homeostasis model of insulin resistance [fasting insulin (microunits per milliliter) x fasting glucose (millimoles per liter)/22.5)] was used as an indicator of insulin resistance. RIA was used to measure testosterone (Diagnostic Systems Laboratory, Webster, TX). Estradiol was measured by electrochemiluminescence (Elecsys 1010, Roche, Indianapolis, IN). Free and bound IGF-I, IGF-binding protein-1 (IGFBP-1), and IGFBP-3 were determined using ELISA kits (Diagnostic Systems Laboratory).
Room respiration calorimetry
Oxygen consumption, carbon dioxide production, and respiratory quotient were measured for 24 h in a room respiration calorimeter. The operation and calibration of the calorimeters were described previously in detail (19). Fat oxidation was computed using oxygen consumption, carbon dioxide production, and 24-h urinary excretion of nitrogen, determined by the Kjeldahl method (Tecator, Hoganas, Sweden) (20).
Fitness
The oxygen consumption (VO2) peak was measured to assess fitness by collecting expired gases with a metabolic cart (model 2900, SensorMedics Corp., Yorba Linda, CA) during an exercise test on a treadmill (model Q55, Quinton Instrument Co., Seattle, WA). The protocol involved a constant speed of 2.5 mph at an initial 0% grade for the first 4 min. The grade was then increased to 10%. Every minute thereafter, the grade was increased by 2.5% until a maximum grade of 22.5%, when speed was then increased incrementally by 0.6 mph. VO2 peak was determined by standard criterion, specifically a heart rate greater than 195 beats/min or a respiratory quotient greater than 1.0 at maximum (21).
Dietary intake
A multiple-pass, 24-h dietary recall was recorded on two occasions by a registered dietitian using Nutrition Data System software on a laptop computer (22). This system automates interviewing, editing and coding of dietary intake data. The multiple-pass, 24-h recall uses three distinct passes to garner information about a subjects food intake during the preceding 24 h. Children, ages 7 yr or younger, were assisted by their mothers.
Statistical analysis
Data are summarized as the mean ± SD. ACCESS (version 9, Microsoft Corp., Redmond, WA) was used for database management. If the data were not normally distributed (kurtosis, >1.9), a log transformation was performed and used in additional analyses (23). Descriptive statistics, generalized estimating equations, and general least squares regression were performed using STATA (version 8.2, STATA Corp., College Station, TX) and MINITAB (version 13, Minitab, Inc., State College, PA). To account for correlated data within families, the family identification number was used as the cluster variable. Statistical significance was set at P < 0.05.
Quantitative genetic analysis
Univariate quantitative genetic analysis was used to partition the phenotypic variance for adiponectin into additive genetic and environmental variance components. The additive genetic heritability (h2) of a trait represents the portion of the total phenotypic variance accounted for by the additive genetic variance (i.e. h2 =
2g/
2p). The heritabilities were estimated using the maximum likelihood variance decomposition method (24, 25) implemented in the computer program SOLAR 2.0 (Southwest Foundation for Biomedical Research, San Antonio, TX) (26). An ascertainment correction is routinely performed in all genetic analyses in SOLAR. Bivariate analyses were conducted to partition the phenotypic relationships between two traits into genetic and environmental correlations. Evidence of pleiotropy (a common set of genes influence more than one trait) is indicated by a genetic correlation significantly different from zero. A shared environmental effect is implied by a significant environmental correlation. The phenotypic correlation (
P) between two traits can be expressed in terms of genetic (
G) and environmental correlations (
E), as described in the following equation:
P =
G
h12
h22 +
E
(1 h12)
(1 h22), where h12 and h22 correspond to the heritabilities of traits 1 and 2, respectively.
| Results |
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Fasting serum insulin was associated with both fasting serum adiponectin and percent FM, as shown in Fig. 2
(adiponectin x percent FM interaction, P = 0.001). The combination of low adiponectin and high percent FM was associated with the highest insulin level. The negative association between adiponectin and fasting insulin was evident at each percent FM quartile. Similarly, fasting serum adiponectin and percent FM were related to fasting TG/HDL-C, as shown in Fig. 3
(adiponectin x percent FM interaction, P = 0.001). At each percent FM quartile, adiponectin was inversely associated with TG/HDL-C. Although their interaction was not statistically significant, adiponectin and percent FM exerted independent effects on systolic blood pressure (Fig. 4
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The heritability (h2) of serum adiponectin in this cohort of Hispanic siblings was 0.93 ± 0.10 (P = 2.4 x 1040). Using variance decomposition techniques, significant positive genetic correlations were detected between adiponectin and HDL-C, LDL-C, and IGFBP-1, and negative genetic correlations were seen between adiponectin and TG, TG/HDL, and IGFBP-3, indicating pleiotropy or shared genetic components in their expression.
| Discussion |
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Heritability (h2) is defined as the relative proportion of the total phenotypic variance in a complex trait that is attributable to the additive effects of genes. Using quantitative genetic analysis, the total phenotypic variance is decomposed into its genetic and environmental components. In the broad sense, the genetic component represents the variance attributable to additive genetic effects, dominance, and epistasis, whereas the environmental component is attributable to measured environmental factors and random, unmeasured factors. In the narrow sense, heritability (h2) is defined and computed as the proportion of the variation attributed to the additive genetic effects only; therefore, the other genetic effects are subsumed under environmental factors.
The heritability (h2) of adiponectin in these Hispanic children is higher than our observations in 1100 northern European adults (h2 = 0.93 vs. 0.46), indicating that a greater portion of the variance in circulating levels in children is due to the additive genetic effect (27). The heritability (h2) of adiponectin in this cohort of Hispanic children is higher than our observations in 1100 northern European adults (h2 = 0.93 vs. 0.46), indicating that a greater portion of the variance in circulating levels in these children is due to the additive genetic effect (27). In children, the cumulative effect of environmental factors may be less than in adults. The ascertainment of our cohort was based on an obese proband that theoretically could introduce bias into our estimate of heritability. However, we routinely correct for ascertainment bias in all of our genetic analyses. Therefore, this estimate of heritability is generalizable to Hispanic children and, therefore, substantiates a strong genetic contribution to circulating levels of adiponectin in Hispanic children.
In our northern European adult study, the genomic scan revealed a maximum LOD score for adiponectin of 4.06, located 35 cM from pter on chromosome 5; the second highest LOD score (3.2) was on chromosome 14, 29 cM from pter. Evidence for pleiotropy was detected between adiponectin and HDL-C and TG (genetic correlations
G = 0.32, and 0.36, respectively), similar to the genetic correlations observed in the Hispanic children. Shared genetic regulation of adiponectin and lipids may contribute to the clustering of metabolic comorbidities seen with obesity. In the genomic scan examining metabolic syndrome and obesity traits in these northern European families, a quantitative trait locus was found on chromosome 3q27 in the region of the adiponectin gene, with LOD scores of 2.43.5 (2). Single nucleotide polymorphisms in the adiponectin gene have been associated with BMI, insulin sensitivity, and T2D in a number of cross-sectional studies (28, 29, 30, 31, 32, 33, 34, 35, 36, 37). In a prospective study, variations in the adiponectin gene affected weight gain, body fat distribution, and onset of hyperglycemia (37).
In the Hispanic children, age, sex, and overweight status exerted independent effects on the expression of serum adiponectin. Adiponectin per kilogram of FM declined precipitously during the prepubertal period and then plateaued, in parallel, but always lower in the overweight than nonoverweight children. Serum adiponectin and adiponectin per kilogram of FM were negatively influenced by developmental changes in growth and sexual maturation associated with increasing age; in addition, independent, negative effects were exerted by obesity and insulin resistance. Serum adiponectin was inversely associated with serum insulin, TG/HDL-C, and systolic blood pressure, independent of age, sex, and percent FM. The association with adiponectin was apparent at each level of adiposity, but was especially striking at the higher levels of adiposity. The cross-sectional nature of our data, however, does not allow us to make any conclusions regarding the directionality of associations with adiponectin. Serum adiponectin may prove to be a useful marker for identifying children at risk for the development of CVD and T2D, but prospective studies will be required to prove its clinical significance.
Adiponectin has been measured in a few other studies in children (6, 7, 8, 9). In 30 Hispanic and Asian American children, aged 1214 yr, adiponectin values were higher than those reported for adults and were higher in boys than in girls (6). These investigators reported a negative correlation between VO2 peak and adiponectin; however, VO2 peak was not standardized for body size. In our study, VO2 peak adjusted for weight or FFM using covariate analysis was not associated with adiponectin. In 10-yr-old Pima children, the percent FM, but not fasting plasma insulin, was independently associated with adiponectin (8). In contrast, in another study of 14 obese adolescents and eight nonobese controls, adiponectin was negatively correlated with percent FM, intramyocellular lipid content, and plasma TG and was positively correlated with insulin sensitivity (7).
Adiponectin purportedly is a regulator of energy homeostasis (38, 39). Attempts to demonstrate a relationship between adiponectin and whole body energy metabolism have failed in humans to date. Plasma adiponectin was not associated with fat oxidation in 75 nondiabetic Pima adults (40). Consistent with our findings, no significant correlations were detected among adiponectin, 24-h energy expenditure, and respiratory quotient. We did not detect any effect of diet on fasting serum adiponectin.
In conclusion, age, sex, and overweight status exerted independent effects on the expression of fasting serum adiponectin in these Hispanic children. Serum adiponectin was inversely associated with serum insulin, TG/HDL-C, and systolic blood pressure, independent of age, sex, and percent FM. The high heritability of adiponectin and pleiotropy seen among adiponectin and leptin, growth factors, and lipids may play a role in the pathogenesis of CVD and T2D in overweight Hispanic children.
| Acknowledgments |
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| Footnotes |
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First Published Online April 12, 2005
Abbreviations: BMI, Body mass index; CVD, cardiovascular disease; FFM, fat-free mass; FM, fat mass; HDL-C, high-density lipoprotein cholesterol; IGFBP, IGF-binding protein; LDL-C, low-density lipoprotein cholesterol; TC, total cholesterol; T2D, type 2 diabetes; TG, triglycerides; VO2, oxygen consumption.
Received November 30, 2004.
Accepted April 5, 2005.
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