Receptor subtype mediating the action of circulating endothelin on glucose metabolism and hemodynamics in perfused rat liver

Hepatic Knockdown of Endothelin Type A Receptor (ETAR) Ameliorates Hepatic Insulin Resistance and Hyperglycemia Through Suppressing p66Shc-Mediated Mitochondrial Fragmentation in High-Fat Diet-Fed Mice

BACKGROUND

Emerging evidence from animal studies and clinical trials indicates that systemic inhibition of endothelin1 (ET1) signaling by endothelin receptor antagonists improves pathological features of diabetes and its complications. It is indicated that endothelin type A receptor (ETAR) plays a major role in ET1-mediated pathophysiological actions including diabetic pathology. However, the effects as well as the mechanistic targets of hepatic ET1/ETAR signaling inhibition on the pathology of metabolic diseases remain unclear. This study aimed to investigate the beneficial effects as well as the underlying mechanisms of hepatic ETAR knockdown on metabolism abnormalities in high-fat diet (HFD)-fed mice.

METHODS

Mice were fed a HFD to induce insulin resistance and metabolism abnormalities. L02 cells were treated with ET1 to assess the action of ET1/ETAR signaling in vitro. Liver-selective knockdown of ETAR was achieved by tail vein injection of adeno-associated virus 8 (AAV8). Systemic and peripheral metabolism abnormalities were determined in vivo and in vitro. Mitochondrial fragmentation was observed by transmission electron microscope (TEM) and mitoTracker red staining.

RESULTS

Here we provided in vivo and in vitro evidence to demonstrate that liver-selective knockdown of ETAR effectively ameliorated hepatic insulin resistance and hyperglycemia in HFD-fed mice. Mechanistically, hepatic ETAR knockdown alleviated mitochondrial fragmentation and dysfunction via inactivating 66-kDa Src homology 2 domain-containing protein (p66Shc) to recover mitochondrial dynamics, which was mediated by inhibiting protein kinase Cδ (PKCδ), in the livers of HFD-fed mice. Ultimately, hepatic ETAR knockdown attenuated mitochondria-derived oxidative stress and related liver injuries in HFD-fed mice. These ETAR knockdown-mediated actions were confirmed in ET1-treated L02 cells.

CONCLUSION

This study defined an ameliorative role of hepatic ETAR knockdown in HFD-induced metabolism abnormalities by alleviating p66Shc-mediated mitochondrial fragmentation and consequent oxidative stress-related disorders and indicated that hepatic ETAR knockdown may be a promising therapeutic strategy for metabolic diseases.

Endothelin antagonism improves hepatic insulin sensitivity associated with insulin signaling in Zucker fatty rats

In the present study, we investigated the effects of long-term treatment with the endothelin (ET) antagonist atrasentan, an ET(A)-selective antagonist, on whole body glucose metabolism and insulin signaling in a commonly used model of insulin resistance, the Zucker fatty rat. Zucker lean and fatty rats were maintained for 6 weeks on either control or atrasentan-treated water. Euglycemic-hyperinsulinemic clamps (4 mU/kg per minute) were performed at the end of the 6-week treatment on a subset of rats (n=10/treatment). In another subset (n=5/treatment), an insulin tolerance test was performed; liver and muscle tissues were harvested 10 minutes following the challenge for further analysis. Results of the clamps demonstrated that long-term atrasentan treatment significantly increased whole body glucose metabolism in fatty rats compared with vehicle control subjects. Insulin-induced insulin receptor substrate 1 tyrosine and protein kinase B serine phosphorylation were significantly reduced in the liver and muscle of fatty animals compared with their lean littermates. This reduction was overcome with atrasentan treatment in the liver but not in the muscle. There was no difference between lean and fatty animals, however, in insulin receptor substrate 1 and protein kinase B protein expression in the liver and muscle and no effect by atrasentan. In contrast, expression of the regulatory subunit of PI-3 kinase (p85alpha) was significantly increased in the liver but not in the muscle of fatty animals compared with their lean littermates and this was normalized to levels of lean animals with atrasentan treatment. These findings indicate that long-standing ET antagonism improves whole body glucose metabolism in Zucker fatty rats through improvements in insulin signaling in the liver. These results indicate that therapeutic ET antagonism may assist in correcting the insulin-resistant state.

Metabolic responses with endothelin antagonism in a model of insulin resistance

Atrasentan, an endothelin antagonist, would have beneficial effects on metabolic responses in a model of insulin resistance. Zucker lean or fatty rats were maintained either on regular (lean and fatty control, n = 12) or atrasentan-treated water (5 mg/kg/d, fatty atrasentan, n = 13) for 6 weeks. There was no significant difference in water intake and body weight with the atrasentan-treated group compared with fatty controls. Although atrasentan had no effect on 3-hour fasting glucose levels, it reduced fasting insulin levels between weeks 2 and 4 of treatment by 53% (fatty control vs fatty atrasentan, P < .01). Atrasentan decreased the incremental area under the plasma glucose response curve ( Delta AUC) after a nutritionally complete meal tolerance test (MTT), by 28% in the atrasentan-treated group compared with fatty controls ( P < .05), and decreased the MTT-induced insulin Delta AUC by 63% in treated animals compared with the fatty control group ( P < .01). In addition, atrasentan significantly decreased the MTT-induced glucose-insulin index Delta AUC by 58% in treated rats compared with fatty controls ( P < .01). In summary, in the Zucker fatty rat, atrasentan significantly reduces (1) 3-hour fasting insulin levels at 4 weeks, (2) glucose and insulin MTT-induced Delta AUCs, and (3) the MTT-induced glucose-insulin index Delta AUC. These results demonstrate an improvement in hyperinsulinemia as well as in glucose tolerance and insulin sensitivity with chronic endothelin antagonism in a model of insulin resistance and suggest that chronic endothelin antagonism may have benefits in the treatment of insulin resistance and/or diabetes.