Metabolic responses with endothelin antagonism in a model of insulin resistance

Endothelin receptor antagonism improves glucose tolerance and adipose tissue inflammation in an experimental model of systemic lupus erythematosus

Endothelin-1 (ET-1) is elevated in patients with systemic lupus erythematosus (SLE), an autoimmune disease characterized by high rates of hypertension, renal injury, and cardiovascular disease. SLE is also associated with an increased prevalence of obesity and insulin resistance compared to the general population. In the present study, we tested the hypothesis that elevated ET-1 in SLE contributes to obesity and insulin resistance. For these studies, we used the NZBWF1 mouse model of SLE, which develops obesity and insulin resistance on a normal chow diet. To test this hypothesis, we treated control (NZW) and SLE (NZBWF1) mice with vehicle, atrasentan (ETA receptor antagonist, 10 mg/kg/day), or bosentan (ETA/ETB receptor antagonist, 100 mg/kg/day) for 4 wk. Neither treatment impacted circulating immunoglobulin levels, but treatment with bosentan lowered anti-dsDNA IgG levels, a marker of SLE disease activity. Treatment with atrasentan and bosentan decreased glomerulosclerosis, and atrasentan lowered renal T-cell infiltration. Body weight was lower in SLE mice treated with atrasentan or bosentan. Endothelin receptor antagonism also improved hyperinsulinemia, homeostatic model assessment for insulin resistance, and glucose tolerance in SLE mice. Adipose tissue inflammation was also improved by endothelin receptor blockade. Taken together, these data suggest a potential therapeutic benefit for SLE patients with obesity and insulin resistance.NEW & NOTEWORTHY SLE is an autoimmune disease that is associated with obesity, insulin resistance, and elevated endothelin-1. The present study demonstrated that pharmacological inhibition of endothelin receptors decreased body weight, insulin resistance, and adipose tissue inflammation in a murine model of SLE. The therapeutic potential of endothelin receptor antagonists to treat obesity-related diseases and pathophysiological conditions, such as autoimmune diseases and insulin resistance, has become increasingly clear.

Increased endothelin-1 levels in coronary artery disease with diabetes mellitus in an Indonesian population

Diabetes mellitus (DM) increases risk of coronary artery disease (CAD). Endothelin-1 (ET-1) is a potential biomarker of endothelial dysfunction. This study aimed to evaluate ET-1 level in CAD patients and its relationship with DM. The cross-sectional design included subjects with angiographically proven CAD and controls among Indonesian. DM was defined by medical history and anti-diabetics use. Serum ET-1 level was measured in both subject groups. We recruited 305 subjects, 183 CAD patients and 122 controls. CAD subjects had higher percentage of males, DM, hypertension, dyslipidemia, smoking, family history of cardiovascular disease, and obesity. ET-1 level was significantly higher in CAD than in controls (2.44 ± 1.49 pg/mL vs. 1.76 ± 0.83 pg/mL; p < 0.001). Increased ET-1 level was significantly associated with DM and dyslipidemia. The highest ET-1 level was observed in CAD with DM, followed by CAD non-DM (2.79 ± 1.63 pg/mL vs. 2.29 ± 1.40 pg/mL; p = 0.023). Among controls, ET-1 level was the lowest in non-DM subjects. Female CAD had higher proportion of DM; however, ET-1 level was similar to male CAD with DM. In conclusion, an increased ET-1 level was significantly associated with DM in patients with CAD. Further research should investigate the potential role of ET-1 receptor antagonists in the secondary prevention of CAD with DM.

Blockade of Endothelin-1 Receptor Type B Ameliorates Glucose Intolerance and Insulin Resistance in a Mouse Model of Obstructive Sleep Apnea

Obstructive sleep apnea (OSA) is associated with insulin resistance (IR) and glucose intolerance. Elevated endothelin-1 (ET-1) levels have been observed in OSA patients and in mice exposed to intermittent hypoxia (IH). We examined whether pharmacological blockade of type A and type B ET-1 receptors (ETA and ETB) would ameliorate glucose intolerance and IR in mice exposed to IH. Subcutaneously implanted pumps delivered BQ-123 (ETA antagonist; 200 nmol/kg/day), BQ-788 (ETB antagonist; 200 nmol/kg/day) or vehicle (saline or propyleneglycol [PG]) for 14 days in C57BL6/J mice (10/group). During treatment, mice were exposed to IH (decreasing the FiO2 from 20.9% to 6%, 60/h) or intermittent air (IA). After IH or IA exposure, insulin (0.5 IU/kg) or glucose (1 mg/kg) was injected intraperitoneally and plasma glucose determined after injection and area under glucose curve (AUC) was calculated. Fourteen-day IH increased fasting glucose levels (122 ± 7 vs. 157 ± 8 mg/dL, PG: 118 ± 6 vs. 139 ± 8; both p < 0.05) and impaired glucose tolerance (AUCglucose: 19,249 ± 1105 vs. 29,124 ± 1444, PG AUCglucose: 18,066 ± 947 vs. 25,135 ± 797; both p < 0.05) in vehicle-treated animals. IH-induced impairments in glucose tolerance were partially ameliorated with BQ-788 treatment (AUCglucose: 21,969 ± 662; p < 0.05). Fourteen-day IH also induced IR (AUCglucose: 7185 ± 401 vs. 8699 ± 401; p < 0.05). Treatment with BQ-788 decreased IR under IA (AUCglucose: 5281 ± 401, p < 0.05) and reduced worsening of IR with IH (AUCglucose: 7302 ± 401, p < 0.05). There was no effect of BQ-123 on IH-induced impairments in glucose tolerance or IR. Our results suggest that ET-1 plays a role in IH-induced impairments in glucose homeostasis.

Endothelin receptor-A antagonist attenuates retinal vascular and neuroretinal pathology in diabetic mice

PURPOSE

We sought to determine the effects of atrasentan, a selective endothelin-A receptor antagonist, on the retinal vascular and structural integrity in a db/db mouse, an animal model of type 2 diabetes and diabetic retinopathy.

METHODS

Diabetic mice, 23 weeks old, were given either atrasentan or vehicle treatment in drinking water for 8 weeks. At the end of the treatment period, eyes underwent trypsin digest to assess the retinal vascular pathology focusing on capillary degeneration, endothelial cell, and pericyte loss. Paraffin-embedded retinal cross sections were used to evaluate retinal sublayer thickness both near the optic nerve and in the retinal periphery. Immunohistochemistry and TUNEL assay were done to evaluate retinal cellular and vascular apoptosis.

RESULTS

Compared with untreated db/db mice, atrasentan treatment was able to ameliorate the retinal vascular pathology by reducing pericyte loss (29.2% ± 0.4% vs. 44.4% ± 2.0%, respectively, P < 0.05) and capillary degeneration as determined by the percentage of acellular capillaries (8.6% ± 0.3% vs. 3.3% ± 0.41%, respectively, P < 0.05). A reduction in inner retinal thinning both at the optic nerve and at the periphery in treated diabetic mice was also observed in db/db mice treated with atrasentan as compared with untreated db/db mice (P < 0.05). TUNEL assay suggested that atrasentan may decrease enhanced apoptosis in neuroretinal layers and vascular pericytes in the db/db mice.

CONCLUSIONS

Endothelin-A receptor blockade using atrasentan significantly reduces the vascular and neuroretinal complications in diabetic mice. Endothelin-A receptor blockade is a promising therapeutic target in diabetic retinopathy.

Impaired insulin-mediated vasorelaxation in a nonobese model of type 2 diabetes: role of endothelin-1

Insulin resistance involves decreased phosphorylation of insulin receptor substrate (IRS) proteins and (or) Akt. In the vasculature, modulated Akt phosphorylation may cause impaired vasorelaxation via decreased eNOS activation. Diet-induced insulin resistance enhances endothelin-1(ET-1)-mediated vasoconstriction and prevents vasodilatation to insulin. Presently, we evaluated insulin-mediated vascular relaxation, assessed molecular markers of the insulin signaling pathway, and determined the involvement of ET-1 in response to insulin by using selective ETA- or ETB-receptor blockade in a lean model of type 2 diabetes. Dose-response curves to insulin (0.01-100 ng/mL) were generated with wire myograph using thoracic aorta rings from control Wistar or diabetic Goto-Kakizaki (GK) rats (n=3-11). Maximal relaxation (Rmax) to insulin was significantly impaired and insulin sensitivity was decreased in the GK group. Preincubation with 1 micromol/L BQ-123 or BQ-788 for ETA- and ETB-receptor blockade, respectively, resulted in improved insulin sensitivity. Immunoblotting for native and phosphorylated Akt and IRS-1 revealed a decrease in Akt activation in the GK group. In vivo hyperinsulinemic euglycemic clamp studies showed decreased glucose utilization in GK rats, indicative of insulin resistance. These findings provide evidence that vascular insulin resistance occurs in a nonobese model of diabetes and that both ET receptor subtypes are involved in vascular relaxation to insulin.

Efficacy and safety of atrasentan in patients with cardiovascular risk and early atherosclerosis

Endothelin plays an important role in the pathogenesis of atherosclerosis. The aim of the study was to evaluate the safety and hemodynamic and metabolic responses to 6 months treatment with atrasentan, the selective endothelin-A receptor antagonist. Seventy-two patients with multiple cardiovascular risk factors and nonobstructive coronary artery disease on coronary angiogram were randomly assigned in a double-blind manner to atrasentan or placebo. Mean aortic blood pressure decreased from 92+/-10 to 80+/-10 mm Hg (P<0.001) in the atrasentan group and did not change in the placebo group (93+/-10 and 92+/-11 mm Hg; P=0.84). The difference between the groups was significant (P<0.001). No effect on heart rate was observed. In a subgroup of patients not treated with angiotensin-converting enzyme inhibitor, creatinine level decreased in the atrasentan versus the placebo group (P=0.011). Fasting glucose (P=0.026), glycosylated hemoglobin level (P=0.041), triglyceride l (P=0.013), lipoprotein-A (P=0.046), and uric acid levels (P=0.048) decreased significantly in the atrasentan group compared with the placebo group. No progression of angiographic coronary disease was observed. The most common adverse effects with atrasentan were nasal stuffiness, headache, and edema. In conclusion, 6 months of treatment with atrasentan results in a reduction of blood pressure and improvement in glucose and lipid metabolism. These findings suggest the beneficial role of atrasentan in the treatment of hypertension and metabolic syndrome.

Endothelin limits insulin action in obese/insulin-resistant humans

The normal action of insulin to vasodilate and redistribute blood flow in support of skeletal muscle metabolism is impaired in insulin-resistant states. Increased endogenous endothelin contributes to endothelial dysfunction in obesity and diabetes. Here, we test the hypothesis that increased endogenous endothelin action also contributes to skeletal muscle insulin resistance via impairments in insulin-stimulated vasodilation. We studied nine lean and seven obese humans, measuring the metabolic and hemodynamic effects of insulin (300 mU . m(-2) . min(-1)) alone and during femoral artery infusion of BQ123 (an antagonist of type A endothelin receptors, 1 micromol/min). Endothelin antagonism augmented skeletal muscle responses to insulin in obese subjects through changes in both leg blood flow (LBF) and glucose extraction. Insulin-stimulated LBF was significantly increased in obese subjects only. These changes, combined with differential effects on glucose extraction, resulted in augmented insulin-stimulated leg glucose uptake in obese subjects (54.7 +/- 5.7 vs. 107.4 +/- 18.9 mg/min with BQ123), with no change in lean subjects (103.7 +/- 11.4 vs. 88.9 +/- 16.3, P = 0.04 comparing BQ123 across groups). BQ123 allowed augmented leg glucose extraction in obese subjects even in the face of NOS antagonism. These findings suggest that increased endogenous endothelin action contributes to insulin resistance in skeletal muscle of obese humans, likely through both vascular and tissue effects.

Dual endothelin receptor blockade acutely improves insulin sensitivity in obese patients with insulin resistance and coronary artery disease

OBJECTIVE

Endothelin (ET)-1 is a vasoconstrictor and proinflammatory peptide that may inhibit glucose uptake. The objective of the study was to investigate if ET (selective ET(A) and dual ET(A)+ET(B)) receptor blockade improves insulin sensitivity in patients with insulin resistance and coronary artery disease.

RESEARCH DESIGN AND METHODS

Seven patients (aged 58 +/- 2 years) with insulin resistance and coronary artery disease completed three hyperinsulinemic-euglycemic clamp protocols: a control clamp (saline infusion), during ET(A) receptor blockade (BQ123), and during combined ET(A) (BQ123) and ET(B) receptor blockade (BQ788). Splanchnic blood flow (SBF) and renal blood flow (RBF) were determined by infusions of cardiogreen and p-aminohippurate.

RESULTS

Total-body glucose uptake (M) differed between the clamp protocols with the highest value in the BQ123+BQ788 clamp (P < 0.05). The M value corrected by insulin was higher in the BQ123+BQ788 than in the control clamp (P < 0.01) or the BQ123 clamp (P < 0.05). There was no difference between the control clamp and the BQ123 clamp. Mean arterial pressure did not change during the control clamp, whereas it decreased during both the BQ123 (P < 0.01) and BQ123+BQ788 (P < 0.05) clamps. RBF increased and renal vascular resistance decreased in the BQ123+BQ788 clamp (P < 0.05) but not in the BQ123 clamp. There was no change in SBF in either clamp.

CONCLUSIONS

Dual ET(A)+ET(B) receptor blockade acutely enhances insulin sensitivity in patients with insulin resistance and coronary artery disease, indicating an important role for endogenous ET-1.

Cardioprotective effects of atrasentan, an endothelin-A receptor antagonist, but not of nitric oxide in diabetic mice with myocyte-specific overexpression of endothelial nitric oxide synthase

1. We investigated the roles of nitric oxide (NO) and endothelin-1 (ET-1) in organ dysfunction in diabetic mice with normal genotype (wild-type, WT) or myocyte-specific overexpression of endothelial NO synthase (eNOS) (transgenic, TG) after chronic oral treatment with the endothelin-A (ETA) receptor antagonist atrasentan. 2. Mice were rendered diabetic by injection of 200 mg kg-1 streptozotocin (STZ). Experimental groups were: untreated WT diabetic (n=9), untreated TG diabetic (n=9), atrasentan-treated WT diabetic (n=9), atrasentan-treated TG diabetic (n=8) and the four corresponding nondiabetic groups (n=5). Atrasentan was administered orally via drinking water at 3 mg kg-1 per day over 28 days. All diabetic mice developed similar hyperglycaemia (27-30 mmol l-1). 3. Atrasentan treatment significantly improved left ventricular systolic and diastolic function in response to exogenous norepinephrine, but there were no differences between genotypes. 4. Atrasentan antagonized the diabetic impairments in endothelium-dependent coronary relaxation and thromboxane-receptor mediated aortic constriction. Further, it improved cardiac and renal oxidant status as evident from reduced tissue malondialdehyde levels. 5. Atrasentan reduced diabetic urine flow, proteinuria and plasma creatinine levels, but creatinine clearance was not significantly altered. 6. These results suggest that in experimental type 1 diabetes, blocking ETA receptors ameliorates myocardial, coronary and renal function and improves tissue oxidant status, whereas raising myocardial NO levels has neither beneficial nor deleterious effects on diabetic cardiomyopathy in this transgenic model.

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.