Overproduction of endothelin-1 impairs glucose tolerance but does not promote visceral adipose tissue inflammation or limit metabolic adaptations to exercise

Abstract

Endothelin-1 (ET-1) is a potent vasoconstrictor and proinflammatory peptide that is upregulated in obesity. Herein, we tested the hypothesis that ET-1 signaling promotes visceral adipose tissue (AT) inflammation and disrupts glucose homeostasis. We also tested if reduced ET-1 is a required mechanism by which exercise ameliorates AT inflammation and improves glycemic control in obesity. We found that 1) diet-induced obesity, AT inflammation, and glycemic dysregulation were not accompanied by significantly increased levels of ET-1 in AT or circulation in wild-type mice and that endothelial overexpression of ET-1 and consequently increased ET-1 levels did not cause AT inflammation yet impaired glucose tolerance; 2) reduced AT inflammation and improved glucose tolerance with voluntary wheel running was not associated with decreased levels of ET-1 in AT or circulation in obese mice nor did endothelial overexpression of ET-1 impede such exercise-induced metabolic adaptations; 3) chronic pharmacological blockade of ET-1 receptors did not suppress AT inflammation in obese mice but improved glucose tolerance; and 4) in a cohort of human subjects with a wide range of body mass indexes, ET-1 levels in AT, or circulation were not correlated with markers of inflammation in AT. In aggregate, we conclude that ET-1 signaling is not implicated in the development of visceral AT inflammation but promotes glucose intolerance, thus representing an important therapeutic target for glycemic dysregulation in conditions characterized by hyperendothelinemia. Furthermore, we show that the salutary effects of exercise on AT and systemic metabolic function are not contingent on the suppression of ET-1 signaling.

Keywords: adipose tissue; endothelin-1; exercise; glucose control; inflammation.

Fig. 1.

Overexpression of endothelin-1 (ET-1) impairs glucose tolerance but does not cause visceral adipose tissue (AT) inflammation in female mice. A: body weight growth curves over 21 wk of diet intervention. B: plasma ET-1 concentrations. C: ET-1 concentrations in perigonadal AT. D: glucose tolerance test (GTT) with area under the curve (AUC) conducted at 19 wk of intervention in chow-fed mice. E: GTT with an AUC at 19 wk of intervention in Western diet-fed mice. F: energy expenditure throughout 24 h. G: macrophage marker MAC-2 immunostained perigonadal adipose sections. Magnification ×20 with ×10 objective. H: average MAC-2-positive stained area. I: average adipocyte size per 100 cells. J: expression of inflammatory genes in perigonadal AT, represented as fold differences from wild-type chow. Values are means ± SE; n = 9–11/group. “D” indicates P < 0.05, main effect of diet. “G” indicates P < 0.05, main effect of genotype. “D × G” indicates P < 0.05, diet × genotype interaction. *P < 0.05 vs. wild-type counterpart. A two-way ANOVA (diet and genotype as factors) with Fisher’s least-significant difference post hoc for pairwise comparisons was used. In addition, a repeated measures two-way ANOVA was used to assess group × time interactions for weekly body weight. A.U., arbitrary units.

Fig. 2.

Downregulation of endothelin-1 (ET-1) is not compulsory for exercise-induced amelioration of visceral adipose tissue (AT) inflammation and improvements in glucose control. A: body weight growth curves over 12 wk of the intervention. B: plasma ET-1 concentrations. C: ET-1 concentrations in epididymal AT. D and E: glucose tolerance test with area under the curve (AUC) conducted at 10 wk of intervention in sedentary and exercise-trained male mice, respectively. F: mean wheel running distance throughout the 12-wk intervention. G: macrophage marker MAC-2 immunostained epididymal adipose sections. Magnification ×20 with ×10 objective. H: average positive MAC-2-stained area. I: average adipocyte size per 100 cells. J: expression of inflammatory genes in epididymal AT (exercise × genotype interaction P values: IL-1β, P = 0.064; TNFα, P = 0.165; MCP-1, P = 0.146; CD68, P = 0.8186; EdnrA, P = 0.621; EdnrB, P = 0.051). Values are means ± SE; n = 7–12/group. “E” indicates P < 0.05, main effect of exercise. “G” indicates P < 0.05, main effect of genotype. “G × E” indicates P < 0.05, genotype × exercise interaction. *P < 0.05 vs. wild-type. A two-way ANOVA (exercise and genotype as factors) with Fisher’s least-significant difference post hoc for pairwise comparisons was used. In addition, a repeated measures two-way ANOVA was used to assess group × time interactions for weekly body weight.

Fig. 3.

Inhibition of endothelin-1 signaling via oral bosentan administration improves glycemic control but does not reduce visceral adipose tissue (AT) inflammation in male LDL receptor knockout mice. A: body weight growth curves over the 8-wk intervention. B: glucose tolerance test (GTT) conducted at 6 wk of the intervention. C: glucose area under the curve (AUC) during GTT. D: macrophage marker MAC-2-immunostained epididymal adipose sections. Magnification ×20 with ×10 objective. E: percent body fat as determined by EchoMRI. F: epididymal AT weight. G: average adipocyte size per 100 cells. H: mRNA expression in epididymal AT. Values are means ± SE; n = 7–12/group. *P < 0.05 vs. lean. #P < 0.05 vs. obese untreated. A one-way ANOVA with Fisher’s least-significant difference post hoc for pairwise comparisons was used.

Fig. 4.

Endothelin-1 (ET-1) levels in circulation or visceral adipose tissue (AT) do not correlate with markers of inflammation in AT of humans with a wide range of body mass indexes (BMIs). A: relationship between BMI and plasma ET-1 concentrations. B: relationship between BMI and AT ET-1 concentrations. C, E, and G: relationship between plasma ET-1 concentrations and inflammatory gene expression. D, F, and H: relationship between AT ET-1 concentrations and inflammatory gene expression. Pearson correlations were performed to regress BMI and inflammatory gene expression with ET-1 concentrations; n = 55–66.