6.3.1 Two distinct metabolic groups of obesity-discordant MZ pairs
A completely novel finding in the second study of the thesis was that the obesity-discordant MZ twin pairs could be divided into two distinct metabolic groups according to the amount of their liver fat. In half of the pairs (group 1, n=8), the obese co-twins had as low liver fat percentages as their leaner co-twin (Δ liver fat = 8%, p=0.21), whereas in the other group (group 2, n=8) the obese co- twins had a strikingly increased liver fat content compared with their leaner co-twins (Δ liver fat = 718%, p = 0.012) (Figure 13). These two groups did not differ in the measure overall fatness (weight, BMI, fat kg, fat %, SAT), but group 2 had increased ΔVAT content compared with group 1 (Figure 15 b, c). In group 1, there were no differences in the insulin or glucose curves during OGTT, and no differences in HOMA-index, Matsuda-index or circulating lipids (LDL, HDL) between the co-twins. However in group 2, the obese co-twins had significantly higher AUC for glucose (23%, p = 0.028) and insulin (78%, p = 0.028) during the OGTT (Figure 14, 119%, p = 0.018), higher HOMA-index and 55% (p = 0.028) lower Matsuda index (Figure 15 d, e) as well as significantly higher LDL- and lower HDL-cholesterol levels (Table 1, Article II) than their leaner co-twins.
Figure 13: Division of twins into two subgroups of obesity according to the amount of liver fat in the obese co-twins
Figure 13: The obese and the lean co-twins could be divided into two subgroups of obesity, according to the amount of liver fat in the obese co-twin. In group 1 (n=8 pairs, liver fat % ≤ 2), the obese co-twins were very similar to their leaner counterparts in the liver fat measures. In group 2 (n=8, liver fat % ≥2), the obese co-twins had significantly increased liver fat amounts compared to the lean co-twins.
Figure 14: Glucose and insulin during the OGTT (oral glucose tolerance test) in the obese and lean twins of groups 1 and 2
Figure 14: Glucose (a) and insulin (b) during the OGTT in group 1. Glucose (c) and insulin (d) during the OGTT in group 2. Obese twins in group 2 presented with significantly augmented responses compared with their leaner co-twins in both glucose and insulin during the test.
Figure 15: Metabolic characteristics of the twins divided in two subgroups on the basis of their liver fat content
Figure 15: Metabolic characteristics of the BMI-discordant co-twins stratified into two subgroups according to the amount of liver fat in the obese co-twin. White bars present lean twins and black bars obese twins. BMI-concordant pairs are presented as reference. ***P<0.001, **P<0.01, *P<0.05. Data is presented as mean ± SE. In group 1 (n=8 twin pairs) the obese co-twins did not significantly differ from their leaner counterparts in their metabolic measures. In group 2 (n=8), the obese twins had more liver fat, subcutaneous and visceral fat, higher hs-CRP levels and were more insulin resistant than their leaner co-twins.
The mean adipocyte cell size was increased in obese co-twins of both groups, but Δadipocyte number differed between the groups (p = 0.037). In group 1 the obese co-twins had more adipocytes than their leaner counterparts (11%, p = 0.069), whereas in group 2, the obese co-twins had even less adipocytes (8%, p = 0.13) than their lean co-twins. Interestingly, the result of two different groups of adipocyte hyperplasia or hypertrophy was reported in article I, where the twins divided in the hyperplastic group had insignificant difference in their liver fat between the obese and the lean co-twin, while in the hypertrophic group, there was a significant difference in the liver fat profiles of the co-twins, the heavier co-twins having much more liver fat than their leaner counterparts.
6.3.2. Distinct gene expression profiles in the two metabolically different obesity groups
When the gene expression profiles of the two groups were compared, in group 2 the obese and the lean co-twins differed significantly for mitochondrial pathways. ‘Oxidoreductase activity’ (GO: 0016491) (FDR corrected p = 0.045) and ‘Cofactor binding’ (GO: 0048037) (p = 0.0096), the latter of which also included several mitochondrial genes, were lower in the obese co-twins and restricted to group 2. In group 1, the obese and lean co-twins differed only for ‘structural constituent of ribosome’ (GO: 0003735) (FDR corrected p =0.016). Based on these findings, additional pathways were analyzed and three mitochondrial pathways selected to represent different aspects of energy handling; oxidative phosphorylation pathway (p = 0.48 in group 1, p = 0.028 in group 2) (Figure 16a); BCAA catabolism pathway (p = 0.48 in group 1, p = 0.018 in group 2) (Figure 16b) and fatty acid β oxidation pathway (p = 1.0 in group1, p = 0.018 in group 2) (Figure 16c) were all significantly different and downregulated in the obese co-twins compared with their lean twins in SAT of group 2 but not group 1. Also, the activity of the chronic inflammatory response pathway (CIRP) was observed to be significantly elevated in the obese co-twins of group 2 (p = 0.028) but not group 1 (p = 0.31). (Figure 16f) Two pathways representing SAT enlargement were also analyzed (triacylglycerol synthesis and adipocyte cell differentiation). The triacylglycerol synthesis pathways did not differ between the obese and lean co-twins’ SAT in either of the groups (Figure 16e). In contrast, the adipocyte differentiation pathway was significantly downregulated in group 2 obese co-twins (p = 0.004), whereas the expression pathways in group 1 co-twins were similar (p = 0.33), (Figure 16d).
Figure 16: Gene expression profiles of twins from the two metabolically different
subgroups
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Figure 16: Gene expression profiles of the two metabolically different subgroups in twins, separated according to the amount of liver fat in the obese co-twin. White bars represent lean twins and black bars obese twins. BMI-concordant pairs are presented as reference. **P<0.01, *P<0.05. Expression values represent the mean activation of the pathway by mean centroid values. Error bars indicate standard error. Oxidative phosphorylation, fatty-acid beta oxidation, BCAA catabolism and adipose tissue differentiation pathways were downregulated and chronic inflammatory response pathway (CIRP) upregulated in the obese twins of the group 2 with increased liver fat content in the obese co-twin.