Increased susceptibility to OVX-associated metabolic dysfunction in UCP1-null mice

Abstract

Premenopausal females are protected against adipose tissue inflammation and insulin resistance, until loss of ovarian hormone production (e.g., menopause). There is some evidence that females have greater brown adipose tissue (BAT) thermogenic capacity. Because BAT mass correlates inversely with insulin resistance, we hypothesized that increased uncoupling protein 1 (UCP1) expression contributes to the superior metabolic health of females. Given that UCP1 transiently increases in BAT following ovariectomy (OVX), we hypothesized that UCP1 may 'buffer' OVX-mediated metabolic dysfunction. Accordingly, female UCP1-knockout (KO) and WT mice received OVX or sham (SHM) surgeries at 12 weeks of age creating four groups (n = 10/group), which were followed for 14 weeks and compared for body weight and adiposity, food intake, energy expenditure and spontaneous physical activity (metabolic chambers), insulin resistance (HOMA-IR, ADIPO-IR and glucose tolerance testing) and adipose tissue phenotype (histology, gene and protein expression). Two-way ANOVA was used to assess the main effects of genotype (G), OVX treatment (O) and genotype by treatment (GxO) interactions, which were considered significant when P ≤ 0.05. UCP1KO mice experienced a more adverse metabolic response to OVX than WT. Whereas OVX-induced weight gain was not synergistically greater for KO compared to WT (GxO, NS), OVX-induced insulin resistance was significantly exacerbated in KO compared to WT (GxO for HOMA-IR, P < 0.05). These results suggest UCP1 is protective against metabolic dysfunction associated with loss of ovarian hormones and support the need for more research into therapeutics to selectively target UCP1 for prevention and treatment of metabolic dysfunction following ovarian hormone loss.

Keywords: adipose tissue; estrogen receptor; insulin resistance; ovarian function; whole animal physiology.

Figure 1.. Influence of UCP1 ablation and OVX on % weight gained, body weight, and adiposity.

UCP1 null and wild-type female mice were subject to ovariectomy or sham surgery at 12 weeks of age, then assessed for 14 weeks for: (A) % body weight gained from baseline; (B) body weight; (C) body fat percentage; (D) body composition; (E) fat pad mass. WT, wild-type; KO, UCP1 knock-out; SHM, sham surgery; OVX, ovariectomy; 2×2 analysis of variance (ANOVA) was performed to assess main effects of genotype (G), ovariectomy (O), and genotype and ovariectomy interactions (GxO). GxO were followed by Tukey’s post hoc tests. Where such tests revealed significant differences between groups, those differences are indicated. Data are expressed as means ± standard error (SE); n=10/group, significance was accepted at P<0.05.

Figure 2.. Influence of UCP1 ablation and OVX on energy expenditure, spontaneous physcial activity and energy intake.

UCP1 null and wild-type female mice were subject an ovariectomy or sham surgery at 12 weeks of age, then placed in indirect calorimetry chambers and assessed for metabolic activity parameters: (A) total energy expenditure; (B) spontaneous physical activity; (C) resting energy expenditure; (D) energy intake; (E) metabolic efficiency. WT, wild-type; KO, UCP1 knock-out; SHM, sham surgery; OVX, ovariectomy; 2×2 analysis of variance (ANOVA) was performed to assess main effects of genotype (G), ovariectomy (O), and genotype and ovariectomy interactions (GxO). GxO were followed by Tukey’s post hoc tests. Where such tests revealed significant differences between groups, those differences are indicated. Data are expressed as means ± standard error (SE); n=10/group, significance was accepted at P<0.05.

Figure 3.. Influence of UCP1 ablation and OVX on glucose metabolism in female mice.

UCP1 null and wild-type female mice were subject to an ovariectomy or sham surgery, then administered a glucose tolerance test 12wks post-surgery: (A) glucose tolerance test and area under the curve (AUC); (B) fasting insulin; (C) fasting glucose; (D) homeostasis model of assessment of insulin resistance (HOMA-IR); (E) adipose tissue insulin resistance (ADIPO-IR); (F) circulating adiponectin; (G) circulating leptin; (I) leptin:adiponectin ratio. WT, wild-type; KO, UCP1 knock-out; SHM, sham surgery; OVX, ovariectomy; 2×2 analysis of variance (ANOVA) was performed to assess main effects of genotype (G), ovariectomy (O), and genotype and ovariectomy interactions (GxO). GxO were followed by Tukey’s post hoc tests. Where such tests revealed significant differences between groups, those differences are indicated; * P<0.05 compared to all other groups. Data are expressed as means ± standard error (SE); n=10/group, significance was accepted at P<0.05.

Figure 4.. Influence of UCP1 ablation and OVX on brown adipose tissue phenotype and immunometabolism.

UCP1 null and wild-type female mice were subject to an ovariectomy or sham surgery at 12 weeks of age, then sacrificed after 14 weeks. Brown adipose tissue samples were collected to assess phenotype, gene expression and protein content: (A) interscapular brown adipose tissue histology; (B) brown gene expression; (C) western blot representative images; (D) brown adipocyte size; (E) protein expression. WT, wild-type; KO, UCP1 knock-out; SHM, sham surgery; OVX, ovariectomy; 2×2 analysis of variance (ANOVA) was performed to assess main effects of genotype (G), ovariectomy (O), and genotype and ovariectomy interactions (GxO). GxO were followed by Tukey’s post hoc tests. Where such tests revealed significant differences between groups, those differences are indicated. Data are expressed as means ± standard error (SE); n=10/group, significance was accepted at P<0.05.

Figure 5.. Influence of UCP1 ablation and OVX on perigonadal adipose tissue immunometabolism in female mice.

UCP1 null and wild-type female mice were subject to an ovariectomy or sham surgery at 12 weeks of age, then sacrificed after 14 weeks. Perigonadal adipose tissue was harvested and processed for adipocyte size, gene expression and protein content: (A) perigonadal adipose tissue (PGAT) histology; (B) perigonadal gene expression; (C) western blot representative images; (D) perigonadal adipocyte size; (E) protein expression. WT, wild-type; KO, UCP1 knock-out; SHM, sham surgery; OVX, ovariectomy; 2×2 analysis of variance (ANOVA) was performed to assess main effects of genotype (G), treatment (O), and genotype and treatment interactions (GxO). GxO were followed by Tukey’s post hoc tests. Where such tests revealed significant differences between groups, those differences are indicated. Data are expressed as means ± standard error (SE); n=10/group, significance was accepted at P<0.05.