Knockout of the Endothelin B receptor in adipocytes improves insulin sensitivity and the metabolic profile of male mice fed a High fat Diet

Abstract

Endothelin-1 (ET-1) is elevated in patients who are overweight and suffer from obesity; however, its contribution to the pathophysiology related to obesity is not fully understood. Obesity is associated with dyslipidemia and insulin resistance, which may in part be mediated adipocyte hypertrophy and dysfunction leading to a reduction in circulating insulin sensitizing adipokines. Pharmacological blockade of ETB receptors improves insulin sensitivity in mice fed a high fat diet for 8 weeks, which is associated with increased Peroxisome proliferator-activated receptor gamma (pparg) levels in visceral adipose and increased circulating adiponectin, suggesting a role for ETB receptor activation on adipocytes in pathophysiology related to obesity. We hypothesized that elevated ET-1 in obesity promotes insulin resistance by reducing adipose pparg leading to a reduction in the secretion of adipokines, such as adiponectin. To test this hypothesis, adipocyte-specific ETB receptor knockout mice (adETBKO) were generated by crossing adiponectin Cre mice with ETB floxed mice. AdETBKO or wild type (WT) littermates were fed either normal diet (NMD) or high fat diet (HFD) for 8 weeks. WT HFD fed mice had significantly higher body weight and fat mass compared to WT NMD, although there was no difference between WT HFD and adETBKO HFD mice. KEGG pathway analysis of differentially expressed genes determined by RNA-Sequencing of gonadal adipose indicated that compared to WT HFD fed mice, adETBKO HFD mice have an attenuation of over 500 genes enriched within insulin signaling and fatty acid metabolism pathways including adiponectin, insulin receptor substrate 1, Glucose transporter 4, fatty acid synthase, and adipolin. In addition, adETBKO HFD mice had significantly improved glucose tolerance (4510±395 vs 6610±582 AUC, p<0.05)and insulin tolerance (-6867±517 vs -2927±584 AUC, p<0.05)compared to insulin resistant HFD fed mice. Plasma adiponectin was reduced by 2-fold in WT HFD compared to WT NMD mice (6.82±0.253 vs 10.8±0.624 ug/ml, p<0.05), and this was attenuated in adETBKO HFD fed mice (6.82±0.253 vs 9.90±0.352 ug/ml, p<0.05). Plasma insulin and leptin levels were significantly increased in WT HFD mice, an effect that was attenuated in adETBKO HFD mice (1.59±0.276 vs 0.809±0.064 ng/ml insulin, p<0.05, 22.6±1.08 vs 16.4±1.52 ng/ml leptin, p<0.05). Blood chemistry analysis showed that adETBKO HFD mice had significantly improved ALT (15.5±1.26 vs 46.5±6.16 U/L, p<0.05), HDL (98.3±3.68 vs 84.6±1.76 mg/dl, p<0.05), LDL (19.6±0.945 vs 29.2±2.63 mg/dl, p<0.05) and NEFA (0.377±0.035 vs 0.751±0.076 mEq/L, p<0.05) levels compared to WT HFD mice. These data indicate that loss of the ETB receptor in adipocytes improves peripheral glucose homeostasis, dyslipidemia and the metabolic/cholesterol profile in mice fed a HFD. Coupled with the transcriptomic profile of visceral adipose tissue in adETBKO HFD mice, which reflects a more insulin sensitive phenotype, these data suggest a role for the adipocyte ETB receptor in the development of insulin resistance exhibited by overweight and obese individuals.