Leptin and Leptin Receptor Gene Polymorphisms and Changes in Glucose Homeostasis in Response to Regular Exercise in Nondiabetic Individuals: The HERITAGE Family Study

The HERITAGE Family Study is a multicenter exercise training study, the main objective of which is to assess the role of genetic factors in cardiovascular, metabolic, and hormonal responses to aerobic exercise training in sedentary families (30). The study design, sampling, and inclusion and exclusion criteria have been described in detail previously (30). In brief, the offspring were required to be aged ≥17 years and the parents aged ≤65 years. The subjects were also required to be sedentary, defined as not having engaged in regular physical activity over the previous 6 months, and free of diabetes, cardiovascular diseases, or other chronic diseases that would prevent their participation in an exercise training program. The study protocol was approved by each of the institutional review boards of the HERITAGE Family Study research consortium. Written informed consent was obtained from each participant. Data on measures of insulin and glucose metabolism were available for 596 individuals. The present analyses included 397 whites (200 men and 197 women) and 143 blacks (61 men and 82 women) for whom complete data were available on all variables.

The 20-week exercise training program has been described in detail previously (29,30). Briefly, the subjects trained three times per week on cycle ergometers in the laboratory. Exercise intensity was customized for each subject based on the relationship between heart rate and oxygen uptake measured at baseline. During the first 2 weeks, the subjects trained for 30 min per session at a heart rate corresponding to 55% of the baseline Vo_2max. Duration was gradually increased to 50 min per session, and heart rate was increased to the level corresponding to 75% of the baseline Vo_2max. This level was sustained for the last 6 weeks. Heart rate was monitored during all training sessions by a computerized cycle ergometer system, which adjusted ergometer resistance to maintain the target heart rate. The subjects were instructed not to change their diet during the intervention.

All phenotypes were measured at baseline and after the 20-week exercise training program. A frequently sampled intravenous glucose tolerance test (FSIVGT) was performed as described (31) in the morning after a 12-h overnight fast and no less than 24 h after the last exercise session. Insulin sensitivity index (S_i) and glucose effectiveness (S_g) were computed by a least-squares fitting of the temporal pattern of glucose and insulin throughout the FSIVGT using the MINMOD program (32). S_i measures the ability of an increment in plasma insulin to enhance the net disappearance of glucose from plasma. S_g measures the ability of glucose per se, independent of changes in plasma insulin, to increase glucose disposal and to suppress endogenous glucose output. Acute insulin response to intravenous glucose (AIR_g), computed as the incremental integrated area under the insulin curve for the first 10 min of the FSIVGT, was used as a measure of insulin secretion. Disposition index (D_i), the product of S_i and AIR_g, is a measure of the ability of the pancreatic β-cells to compensate for insulin resistance (33). Glucose disappearance index (K_g) estimates glucose disappearance (percent per minute) based on the slope of the line derived from least-squares regression of the natural logarithm of plasma glucose from 10 to 60 min during the FSIVGT and was used as a measure of overall glucose tolerance. We did not give a secretagogue at 20 min to provide an accurate measure of K_g. Measurement of fasting plasma insulin, glucose, and C-peptide has been described previously (29). BMI was computed as the ratio of body weight divided by body height squared (kilograms per meter squared). Body fat mass was measured using underwater weighing (34).

Genetic analyses have been described previously in detail (26). In brief, genomic DNA was isolated from lymphoblastoid cells. A single nucleotide polymorphism in the LEP gene, an alanine-to-glycine substitution in untranslated exon 1 (A19G), and three single nucleotide polymorphisms in the LEPR gene, a lysine-to-alanine substitution at codon 109 in exon 4 (K109R), a glutamine-to-arginine substitution at codon 223 in exon 6 (Q223R), and a lysine-to-asparagine substitution at codon 656 in exon 16 (K656N), were typed using PCR technology.

A χ² test was used to compare allele frequencies between whites and blacks and to test whether the genotype distributions were in Hardy-Weinberg equilibrium. Student’s t tests were used to compare means of measures of glucose homeostasis at baseline and after exercise training. The associations of the LEP and LEPR gene polymorphisms with the exercise-induced changes in glucose homeostasis, and the interactions among the variants, were analyzed using a MIXED model that takes into account nonindependence among family members. Data were adjusted for age, sex, baseline fat mass, and baseline measure of glucose homeostasis. Further adjustment for the change in fat mass did not affect the associations (data not shown). All analyses were performed with the SAS Statistical Software Package, separately in whites and blacks. Haplotype analyses were performed with the Quantitative Transmission Disequilibrium Test (35).

Baseline characteristics are shown in Table 1. The frequency of the LEP A19 allele was 0.42 in whites and 0.41 in blacks. The frequencies of the LEPR 109R, 223R, and 656N alleles were 0.26, 0.45, and 0.19 in whites and 0.13, 0.54, and 0.15 in blacks, respectively. The frequency of the LEPR 109R allele was significantly higher in whites than in blacks (P < 0.001). All genotype frequencies in both races were in Hardy-Weinberg equilibrium.

Physical activity increased S_i by 8.5% (P = 0.005), D_i by 9.5% (P = 0.004), S_g by 13.0% (<0.001), K_g by 3.2% (P = 0.030), and fasting glucose by 1.0% (P = 0.004) and decreased AIR_g by 6.6% (P < 0.001) and fasting insulin by 10.3% (P < 0.001). In blacks, exercise increased S_i by 19.0% (P = 0.008), D_i by 18.4% (P = 0.004), S_g by 20.7% (P = 0.004), and fasting glucose by 2.6% (P = 0.001) and decreased AIR_g by 6.7% (P = 0.016) and fasting insulin by 14.0% (P = 0.006). In whites, physical activity increased S_g by 9.6% (P = 0.010) and decreased AIR_g by 6.4% (P = 0.005) and fasting insulin by 8.7% (P < 0.001). Other changes in glucose homeostasis were not significant.

The LEPR K109R polymorphism was associated with exercise-induced changes in various measures of glucose homeostasis in whites (Fig. 1). Physical activity increased S_i (P = 0.041) and D_i (P = 0.046) in the 109R allele carriers but not in the K109K homozygotes. Exercise increased K_g more in the R109R homozygotes than in the K109 allele carriers (P = 0.039). Physical activity decreased fasting glucose only in the 109R allele carriers (P = 0.018). Exercise also reduced fasting insulin more in the R109R homozygotes (−12.7 ± 4.9 pmol/l [mean ± SE]) than in the K109 allele carriers (−8.4 ± 1.8 pmol/l), but the difference was not significant (P = 0.38). The LEPR Q223R polymorphism was not significantly associated with changes in glucose homeostasis in whites (P > 0.05). The haplotype consisting of the LEPR 109R allele and the LEPR 223R allele was associated with a greater exercise-induced increase in S_i (P = 0.041) and D_i (P = 0.016) than the other haplotypes in whites. However, the associations of the haplotype mainly reflected the effect of the 109R allele. The K109R and Q223R polymorphisms were not significantly related to changes in glucose homeostasis in blacks (P > 0.05).

In whites, exercise increased AIR_g and S_g and decreased fasting glucose in the LEPR N656N homozygotes (n = 11), but did not influence glucose homeostasis in the K656N heterozygotes (n = 129) or the K656K homozygotes (n = 256). The means (±SE) of the AIR_g changes in the N656N homozygotes, the K656N heterozygotes, and the K656K homozygotes were 11.3 ± 5.1, 2.4 ± 2.3, and −1.8 ± 1.4 μU/ml, respectively (P = 0.030). The means (±SE) of the S_g changes were 0.015 ± 0.005, 0.002 ± 0.001, and 0.004 ± 0.001%/min (P = 0.025), and the means (±SE) of the glucose changes were −2.8 ± 1.1, −0.4 ± 0.6, and 0.2 ± 0.5 mg/dl (P = 0.052). There were no N656N homozygotes in blacks, and data on the K656N polymorphism are not presented.

Physical activity decreased fasting insulin more in the LEP A19A homozygotes (n = 61, −12.6 ± 3.0 pmol/l [mean ± SE]) than in the A19G heterozygotes (n = 212, −7.8 ± 1.8 pmol/l) or the G19G homozygotes (n = 123, −7.2 ± 3.0 pmol/l) in whites, but the difference was not significant (P = 0.19). As shown in Fig. 2, however, there was a significant interaction between the LEP A19G and LEPR K109R polymorphisms on the exercise-induced change in fasting insulin in whites (P = 0.010), and the association between the LEP A19G polymorphism and insulin change was evident only in the LEPR 109R carriers (P = 0.019). Consistent with the results in whites, in blacks the LEP A19G polymorphism was not significantly associated with insulin change, but similar interaction between the LEP A19G and LEPR K109R polymorphisms was found (P = 0.046).

The present study shows that regular physical activity increases insulin sensitivity, but also suggests that regular exercise improves the ability of the pancreatic β-cells to compensate for insulin resistance and increases glucose effectiveness in nondiabetic individuals. A novel finding of the study is that exonic variants in the LEP and LEPR genes modified the effects of regular physical activity on glucose homeostasis in whites. These data support our recent observation (29) in the same nondiabetic population that a genomic region close to the LEP locus was linked to fasting insulin response to exercise training in whites.

The principal finding of this study is that regular physical activity improved insulin sensitivity, pancreatic β-cell compensation for insulin resistance, and glucose tolerance in the LEPR 109R carriers but not in the K109K homozygotes in whites. These results are potentially important from a public health point of view because the LEPR K109K homozygotes, who did not respond to regular exercise, represent >50% of the Caucasian populations (26). Regular physical activity had the strongest fasting insulin decreasing effect in the LEP A19A homozygotes who carried the LEPR 109R allele in whites. We observed similar interaction in blacks, but only three blacks had these genotypes, which complicates the interpretation of the results. These data suggest that both LEP and LEPR genes may contribute to improvements in glucose homeostasis in response to exercise training.

A possible explanation for our findings is that DNA sequence variations in the LEP and LEPR genes induce a mild dysfunction in the leptin-mediated signaling pathway and impair the peripheral effects of leptin (11–14), and thus attenuate the favorable effects of regular physical activity on glucose homeostasis. Although all nucleotide alterations in the LEP and LEPR genes of our study produce amino acid changes and may thus have functional consequences, no true evidence of their functionality is available. Yet, it is possible that the DNA sequence variants of our interest serve as genetic markers for nearby functional variants, which are in linkage disequilibrium with our markers and eventually explain the observed associations.

The differences in the exercise-induced changes among the genotypes were clear, but there was also considerable individual variation in responses even within genotypes. This is likely explained by the multifactorial and multigenic nature of the regulation of glucose homeostasis. The fact that most of the associations were evident only in whites may reflect a limited statistical power in blacks due to a small number of subjects in some genotype groups. Another reason for the lack of agreement may be that the frequency of the LEPR 109R allele was significantly higher in whites than in blacks.

The present study suggests that DNA sequence variations in the LEP and LEPR genes are associated with the magnitude of the effects of regular physical activity on glucose homeostasis in nondiabetic individuals. The screening of polymorphisms in the LEP and LEPR genes and other genes may be useful to identify individuals who are most likely to benefit from regular physical activity in terms of the prevention of type 2 diabetes and also those who do not respond to exercise training, for whom other interventions such as dietary changes or drug treatment might be preferable.

The HERITAGE Family Study is supported by the National Heart, Lung, and Blood Institute through grants HL45670 (to C.B.), HL47323 (to A.S.L.), HL47317 (to D.C.R), HL47327 (to J.S.S), and HL47321 (to J.H.W.). C.B. is partially supported by the George A. Bray Chair in Nutrition. A.S.L. is partially supported by the Henry L. Taylor endowed Professorship in Exercise Science and Health Enhancement. R.N.B. is supported by the National Institutes of Health (DK27619 and DK29867) and holds the Keck Chair in Medicine. T.A.L. was supported by grants from the Academy of Finland, the Yrjo Jahnsson Foundation, the Paavo Nurmi Foundation, and the University of Kuopio and is a Research Fellow of the Academy of Finland.

Gratitude is expressed to Dr. Andre Nadeau and the staff of the Diabetes Research Unit, Laval University Medical Center, Ste-Foy, Québec, Canada, for their contribution to these studies.