Endothelin-1 induces insulin resistance in conscious rats

Adipocyte endothelin B receptor activation inhibits adiponectin production and causes insulin resistance in obese mice

AIMS

Endothelin-1 (ET-1) is elevated in patients with obesity and adipose tissue of obese mice fed high-fat diet (HFD); however, its contribution to the pathophysiology of obesity is not fully understood. Genetic loss of endothelin type B receptors (ETB) improves insulin sensitivity in rats and leads to increased circulating adiponectin, suggesting that ETB activation on adipocytes may contribute to obesity pathophysiology. We hypothesized that elevated ET-1 in obesity promotes insulin resistance by reducing the secretion of insulin sensitizing adipokines, via ETB receptor.

METHODS

Male adipocyte-specific ETB receptor knockout (adETBKO), overexpression (adETBOX), or control littermates were fed either normal diet (NMD) or high-fat diet (HFD) for 8 weeks.

RESULTS

RNA-sequencing of epididymal adipose (eWAT) indicated differential expression of over 5500 genes (p < 0.05) in HFD compared to NMD controls, and changes in 1077 of these genes were attenuated in HFD adETBKO mice. KEGG analysis indicated significant increase in metabolic signaling pathway. HFD adETBKO mice had significantly improved glucose and insulin tolerance compared to HFD control. In addition, adETBKO attenuated changes in plasma adiponectin, insulin, and leptin that is observed in HFD versus NMD control mice. Treatment of primary adipocytes with ET-1 caused a reduction in adiponectin production that was attenuated in cells pretreated with an ETB antagonist.

CONCLUSION

These data indicate elevated ET-1 in adipose tissue of mice fed HFD inhibits adiponectin production and causes insulin resistance through activation of the ETB receptor on adipocytes.

Endothelin-1 impairs skeletal muscle myogenesis and development via ETB receptors and p38 MAPK signaling pathway

Myopenia is a condition marked by progressive decline of muscle mass and strength and is associated with aging or obesity. It poses the risk of falling, with potential bone fractures, thereby also increasing the burden on family and society. Skeletal muscle wasting is characterized by a reduced number of myoblasts, impaired muscle regeneration and increased muscle atrophy markers (Atrogin-1, MuRF-1). Endothelin-1 (ET-1) is a potent vasoconstrictor peptide. Increased circulating levels of ET-1 is noted with aging and is associated with muscular fibrosis and decline of strength. However, the regulatory mechanism controlling its effect on myogenesis and atrophy remains unknown. In the present study, the effects of ET-1 on myoblast proliferation, differentiation and development were investigated in C2C12 cells and in ET-1-infused mice. The results show that ET-1, acting via ETB receptors, reduced insulin-stimulated cell proliferation, and also reduced MyoD, MyoG and MyHC expression in the differentiation processes of C2C12 myoblasts. ET-1 inhibited myoblast differentiation through ETB receptors and the p38 mitogen-activated protein kinase (MAPK)-dependent pathway. Additionally, ET-1 decreased MyHC expression in differentiated myotubes. Inhibition of proteasome activity by MG132 ameliorated the ET-1-stimulated protein degradation in differentiated C2C12 myotubes. Furthermore, chronic ET-1 infusion caused skeletal muscle atrophy and impaired exercise performance in mice. In conclusion, ET-1 inhibits insulin-induced cell proliferation, impairs myogenesis and induces muscle atrophy via ETB receptors and the p38 MAPK-dependent pathway.

Endothelin antagonism reduces hemoglobin A1c in patients with pulmonary hypertension

Our lab recently reported that the blockade of endothelin-1 (ET-1) receptors attenuates insulin resistance in obese mice; therefore, we hypothesized that patients taking ET-1 receptor antagonists (ERAs) will have improved glycemic control. University of Mississippi Medical Center (2013-2020) electronic health record (EPIC) data were extracted from patients ≥18 years old with a clinical diagnosis of pulmonary hypertension (Food and Drug Administration indication for ERA use) and at least two clinical visits within 2 years. Patients prescribed ERAs (n = 11) were similar in age (61 ± 14 years vs. 60 ± 14 years), body mass index (BMI) (34 ± 8 kg/m ² vs. 35 ± 11 kg/m²), diabetes prevalence (73% vs. 80%, p = 0.59), and follow-up time (209 ± 74 days vs. 283 ± 180 days) compared with patients not taking ERAs (n = 137). There was a small but similar decrease in BMI at follow-up in the ERA (-1.9 ± 3 kg/m²) and control patients (-1.6 ± 5 kg/m²). At follow-up, hemoglobin A1c (HbA1c) significantly decreased -12% ± 11% of baseline in patients taking ERAs, while this did not occur in the control patients (2% ± 20% increase in HbA1c). In the whole population, baseline HbA1c and ERA prescription predicted the fall in HbA1c, while there was no significant association with demographics, diabetes prevalence, and diabetic treatment. These data suggest a potential role of ET-1 in promoting insulin resistance and warrant further investigation into using these drugs for glycemic control.

Endothelin-1 induces lipolysis through activation of the GC/cGMP/Ca2+/ERK/CaMKIII pathway in 3T3-L1 adipocytes

Endothelin-1 (ET-1) is a potent vasoconstrictive peptide produced and secreted mainly by endothelial cells. Recent studies indicate that ET-1 can regulate lipid metabolism, which may increase the risk of insulin resistance. Our previous studies revealed that ET-1 induced lipolysis in adipocytes, but the underlying mechanisms were unclear. 3T3-L1 adipocytes were used to investigate the effect of ET-1 on lipolysis and the underlying mechanisms. Glycerol levels in the incubation medium and hormone-sensitive lipase (HSL) phosphorylation were used as indices for lipolysis. ET-1 significantly increased HSL phosphorylation and lipolysis, which were completely inhibited by ERK inhibitor (PD98059) and guanylyl cyclase (GC) inhibitor (LY83583). LY83583 reduced ET-1-induced ERK phosphorylation. A Ca2+-free medium and PLC inhibitor caused significant decreases in ET-1-induced lipolysis as well as ERK and HSL phosphorylation, and IP3 receptor activator (D-IP3) increased lipolysis. ET-1 increased cGMP production, which was not affected by depletion of extracellular Ca2+. On the other hand, LY83583 diminished the ET-1-induced Ca2+ influx. Transient receptor potential vanilloid-1 (TRPV-1) antagonist and shRNA partially inhibited ET-1-induced lipolysis. ET-1-induced lipolysis was completely suppressed by CaMKIII inhibitor (NH-125). These results indicate that ET-1 stimulates extracellular Ca2+ entry and activates the intracellular PLC/IP3/Ca2+ pathway through a cGMP-dependent pathway. The increased cytosolic Ca2+ that results from ET-1 treatment stimulates ERK and HSL phosphorylation, which subsequently induces lipolysis. ET-1 induces HSL phosphorylation and lipolysis via the GC/cGMP/Ca2+/ERK/CaMKIII signaling pathway in 3T3-L1 adipocytes.

Endothelin-1 stimulates preadipocyte growth via the PKC, STAT3, AMPK, c-JUN, ERK, sphingosine kinase, and sphingomyelinase pathways

Endothelin (ET)-1 regulates adipogenesis and the endocrine activity of fat cells. However, relatively little is known about the ET-1 signaling pathway in preadipocyte growth. We used 3T3-L1 preadipocytes to investigate the signaling pathways involved in ET-1 modulation of preadipocyte proliferation. As indicated by an increased number of cells and greater incorporation of bromodeoxyuridine (BrdU), the stimulation of preadipocyte growth by ET-1 depends on concentration and timing. The concentration of ET-1 that increased preadipocyte number by 51-67% was ~100 nM for ~24-48 h of treatment. ET-1 signaling time dependently stimulated phosphorylation of ERK, c-JUN, STAT3, AMPK, and PKCα/βII proteins but not AKT, JNK, or p38 MAPK. Treatment with an ET_AR antagonist, such as BQ610, but not ET_BR antagonist BQ788, blocked the ET-1-induced increase in cell proliferation and phosphorylated levels of ERK, c-JUN, STAT3, AMPK, and PKCα/βII proteins. In addition, pretreatment with specific inhibitors of ERK1/2 (U0126), JNK (SP600125), JAK2/STAT3 (AG490), AMPK (compound C), or PKC (Ro318220) prevented the ET-1-induced increase in cell proliferation and reduced the ET-1-stimulated phosphorylation of ERK1/2, c-JUN, STAT3, AMPK, and PKCα/β. Moreover, the SphK antagonist suppressed ET-1-induced cell proliferation and ERK, c-JUN, STAT3, AMPK, and PKC phosphorylation, and the SMase2 antagonist suppressed ET-1-induced cell proliferation. However, neither the p38 MAPK antagonist nor the CerS inhibitor altered the effect of ET-1. The results indicate that ET_AR, JAK2/STAT3, ERK1/2, JNK/c-JUN, AMPK, PKC, SphK, and SMase2, but not ET_BR, p38 MAPK, or CerS, are necessary for the ET-1 stimulation of preadipocyte proliferation.

Chronic endothelin-1 infusion causes adipocyte hyperplasia in rats

OBJECTIVE

The aim of this study was to investigate the regulatory mechanism of endothelin-1 (ET-1), an endothelium-derived vasoconstrictor, on adipogenesis in vitro and in vivo.

METHODS

3T3-L1 preadipocytes were used to explore the mechanisms mediating ET-1 actions on preadipocyte proliferation and adipocyte differentiation. To investigate the in vivo effect of ET-1, male Sprague-Dawley rats were infused with ET-1 or saline for 4 weeks via intraperitoneally implanted osmotic pumps, and the fat pad weight and adipocyte size of adipose tissues were measured.

RESULTS

ET-1 stimulated preadipocyte proliferation and increased the cell number at the mitotic clonal expansion stage of adipocyte differentiation via the endothelin A receptor (ETAR) and activation of the protein kinase C (PKC) pathway. ET-1, via ETAR, inhibited adipocyte differentiation partially through an ERK-dependent pathway. Furthermore, no significant difference in the body weight and fat pad weight was observed in either ET-1- or saline-infused rats. Compared with saline-infused rats, the adipocyte cell number was significantly increased but the adipocyte size was significantly decreased in ET-1-infused rats.

CONCLUSIONS

Chronic ET-1 infusion increased the number of small adipocytes without the change of white adipose tissue mass in rats, which were associated with ET-1-stimulated preadipocyte proliferation, but not ET-1-suppressed adipocyte differentiation.

Synergistic induction of insulin resistance by endothelin-1 and cAMP in 3T3-L1 adipocytes

Both endothelin-1 (ET-1) and cAMP are implicated for inducing insulin resistance. Since we have shown previously that there is a crosstalk between ET-1 and cAMP signaling pathways in regulating glucose uptake in 3T3-L1 adipocytes, we extended our investigation in this study on whether they may have a synergistic effect on inducing insulin resistance. Our results showed that it was indeed the case. Insulin-stimulated glucose uptake, phosphorylation of PKB, IRS-1-associated PI3K, and IRS-1 tyrosine phosphorylation were all inhibited by ET-1 and 8-bromo cAMP in a synergistic manner. IRS-1 protein levels were similarly decreased by ET-1 and 8-bromo cAMP, attributable to suppressed mRNA expression. In addition, after correction for the loss in IRS-1 protein, the inhibition of insulin-stimulated IRS-1 tyrosine phosphorylation or IRS-1-associated PI3K was mainly caused by cAMP. Moreover, whereas IRS-2 protein levels were increased by cAMP or ET-1 plus cAMP, insulin-stimulated IRS-2-associated PI3K activities were abolished by both treatments. Furthermore, ET-1 and β-adrenergic agonists had similar synergistic inhibition on insulin-stimulated glucose uptake. In conclusion, we have shown that ET-1 and cAMP may synergistically induce insulin resistance in adipocytes via inhibiting IRS-1 expression as well as insulin-stimulated IRS-1/IRS-2 activities.

Endothelial dysfunction in cirrhosis: Role of inflammation and oxidative stress

This review describes the recent developments in the pathobiology of endothelial dysfunction (ED) in the context of cirrhosis with portal hypertension and defines novel strategies and potential targets for therapy. ED has prognostic implications by predicting unfavourable early hepatic events and mortality in patients with portal hypertension and advanced liver diseases. ED characterised by an impaired bioactivity of nitric oxide (NO) within the hepatic circulation and is mainly due to decreased bioavailability of NO and accelerated degradation of NO with reactive oxygen species. Furthermore, elevated inflammatory markers also inhibit NO synthesis and causes ED in cirrhotic liver. Therefore, improvement of NO availability in the hepatic circulation can be beneficial for the improvement of endothelial dysfunction and associated portal hypertension in patients with cirrhosis. Furthermore, therapeutic agents that are identified in increasing NO bioavailability through improvement of hepatic endothelial nitric oxide synthase (eNOS) activity and reduction in hepatic asymmetric dimethylarginine, an endogenous modulator of eNOS and a key mediator of elevated intrahepatic vascular tone in cirrhosis would be interesting therapeutic approaches in patients with endothelial dysfunction and portal hypertension in advanced liver diseases.

ADP-ribosylation factor 6 regulates endothelin-1-induced lipolysis in adipocytes

Endothelin-1 (ET-1) induces lipolysis in adipocytes, where ET-1 chronic exposure results in insulin resistance (IR) through suppression of glucose transporter (GLUT)4 translocation to the plasma membrane and consequently glucose uptake. ARF6 small GTPase, which plays a vital role in cell surface receptors trafficking, has previously been shown to regulate GLUT4 recycling and thereby insulin signalling. ARF6 also plays a role in ET-1 promoted endothelial cell migration. However, ARF6 involvement in ET-1-induced lipolysis in adipocytes is unknown. Therefore, we investigated the role of ARF6 in ET-1-induced lipolysis in 3T3-L1 adipocytes. This was achieved by studying the effect of inhibitors for the activation of ARF6 and other signalling proteins on ET-1 induced lipolysis and ARF6 activation in the adipocytes. Our results indicate that ET-1 induces, through endothelin type A receptor (ETAR), lipolysis, the ARF6 activation and extracellular-signal regulated kinase (ERK) phosphorylation in adipocytes, further ET-1 stimulated lipolysis is inhibited by the inhibitors of ARF6 activation, ERK phosphorylation and dynamin, which is essential for endocytosis. Our studies also revealed that ARF6 acts upstream of ERK in ET-1-indcued lipolysis. In summary, we determined that ET-1 activation of ETAR signalled through ARF6, which is crucial for lipolysis.

Endothelin Type A Receptor Genotype is a Determinant of Quantitative Traits of Metabolic Syndrome in Asian Hypertensive Families: A SAPPHIRe Study

Co-heritability of hypertension and insulin resistance (IR) within families not only implies genetic susceptibility may be responsible for these complex traits but also suggests a rational that biological candidate genes for hypertension may serve as markers for features of the metabolic syndrome (MetS). Thus we determined whether the T323C polymorphism (rs5333) of endothelin type A (ETA) receptor, a predominant receptor evoking potent vasoconstrictive action of endothelin-1, contributes to susceptibility to IR-associated hypertension in 1694 subjects of Chinese and Japanese origins. Blood pressures (BPs) and biochemistries were measured. Fasting insulin level, insulin-resistance homeostasis model assessment (HOMAIR) score, and area under curve of insulin concentration (AUCINS) were selected for assessing insulin sensitivity. Genotypes were obtained by methods of polymerase chain reaction-restriction fragment length polymorphism. Foremost findings were that minor allele frequency of the T323C polymorphism was noticeable lower in our overall Asian subjects compared to multi-national population reported in gene database; moreover both the genotypic and allelic frequencies of the polymorphism were significantly different between the two ethnic groups we studied. The genotype distributions at TT/TC/CC were 65, 31, 4% in Chinese and 51, 41, 8% in Japanese, respectively (p < 0.0001). Additionally, carriers of the C homozygote revealed characteristics of IR, namely significantly higher levels of fasting insulin, HOMAIR score, and AUCINS at 29.3, 35.3, and 39.3%, respectively, when compared to their counterparts with TT/TC genotypes in Chinese. Meanwhile, the CC genotype was associated with a higher level of high density lipoprotein cholesterol in Japanese. No association of the polymorphism with BP was observed. This study demonstrated for the first time that T323C polymorphism of ETA receptor gene was associated with an adverse insulin response in Chinese and a favorite atherogenic index in Japanese.

A novel cross-talk between endothelin-1 and cyclic AMP signaling pathways in the regulation of GLUT1 transcription in 3T3-L1 adipocytes

We showed previously that chronic exposure to both endothelin-1 (ET-1) and cAMP resulted in a synergistic increase in Glut1 transcription in 3T3-L1 adipocytes via a protein kinase C (PKC)-dependent mechanism. In the present study, we further examined the molecular mechanism involved. Employing transient transfections with Glut1 promoter/enhancer -luciferase reporter and several dominant negative or constitutively active PKC mutants, we identified PKCε as the responsible PKC. Investigation with deletion and mutation mutants of the promoter/enhancer reporter suggested that Sp1, CREB and AP-1 responsive elements on enhancer 2 were involved. Furthermore, chromatin immunoprecipitation and co-immunoprecipitation analysis were applied to characterize the interactions between these transcription factors and their bindings to enhancer 2 in vivo. The results indicate that there are both negative and positive interactions between ET-1 and cAMP signaling pathways. On the one hand, cAMP inhibits ET-1 induced NF-κB activation required for ET-1-stimulated Glut1 transcription; on the other hand, cAMP, via sustained CREB phosphorylation, may activate AP-1 and cooperate with ET-1-activated PKCε to enhance Sp1 expression and consequently to generate a stable enhancer 2-bound Sp1/pCREB/AP-1 complex, which can strongly facilitate Glut1 transcription more than the additive effect of ET-1 and cAMP alone.

Endothelin-1 suppresses long-chain fatty acid uptake and glucose uptake via distinct mechanisms in 3T3-L1 adipocytes

Endothelin-1 (ET-1) has been demonstrated to induce insulin resistance (IR) and lipolysis, raising the possibility that ET-1 may also contribute to the elevated fatty acid levels in IR-associated comorbidities. We attempted to evaluate whether ET-1 also affects the long-chain fatty acid (LCFA) utilization in 3T3-L1 adipocytes. The effects of chronic ET-1 exposure on basal and insulin-stimulated LCFA uptake, and LCFA uptake kinetics were examined in 3T3-L1 adipocytes. Chronic exposure to ET-1 induced IR and suppressed basal and insulin-stimulated LCFA uptake. Given that insulin acutely stimulates LCFA uptake, there was dramatically similar trend of dose-response curves for ET-1-suppressed LCFA uptake, and also similar corresponding IC₅₀ values, between basal and insulin-stimulated states, reflecting that ET-1 predominantly suppresses basal LCFA uptake. Results of LCFA kinetics, western blots, and CD36 inhibition using sulfosuccinimidyl oleate (SSO) revealed that suppression of LCFA uptake by ET-1 is associated with downregulation of CD36. ET type A receptor (ET(A)R) antagonist BQ-610 reversed the IR induction and the ET-1-suppressed LCFA uptake. Exogenous replenishment of phosphatidylinositol (PI) 4, 5-bisphosphate (PIP₂) prevented IR induction, but not the suppression of LCFA uptake by ET-1. Pharmacological inhibition of the activation of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) completely blocked the ET-1-suppressed LCFA uptake. Serving as an inducer of IR, ET-1 also chronically suppresses LCFA uptake via PIP₂-independent and ERK-dependent pathway. The interplay between impaired glucose disposal and diminished LCFA utilization, induced by ET-1, could worsen the dysregulation of adipose metabolism and energy homeostasis in insulin-resistant states.

Endogenous ghrelin released in response to endothelin stimulates growth hormone secretion in cattle

The purpose of this study was to evaluate whether circulating ghrelin and growth hormone (GH) concentrations in cattle are regulated by endothelin-1 (ET-1), endothelin-3 (ET-3), and secretin. Six Holstein steers (242+/-1 d old, 280.5+/-4.4 kg body weight [BW]; mean+/-SEM) were allocated randomly in an incomplete Latin square design to receive each of 4 treatment compounds (vehicle, ET-1, ET-3, and secretin) with 1-d intervals between successive treatments. The treatment compounds were injected intravenously via a catheter inserted into the external jugular vein of each steer. Blood was sampled from the indwelling catheter at -30, -15, 0, 5, 10, 15, 20, 30, 45, 60, 90, 120, 150, and 180 min. Plasma ghrelin and GH responses to the treatment compounds were measured by a double-antibody radioimmunoassay system. Data were analyzed by using a MIXED procedure of SAS, version 9.1. Plasma acyl ghrelin, total ghrelin, and GH concentrations were increased by both ET-1 and ET-3 injection (ET-1 injection: 311+/-15 pg/mL vs 245+/-15 pg/mL, 2.4+/-0.2 ng/mL vs 1.61+/-0.05 ng/mL, 4.73+/-0.92 ng/mL vs 1.17+/-0.09 ng/mL for acyl ghrelin, total ghrelin, and GH, respectively; ET-3 injection: 337+/-27 pg/mL vs 245+/-15 pg/mL, 2.6+/-0.1 ng/mL vs 1.61+/-0.05 ng/mL, 5.56+/-0.97 ng/mL vs 1.17+/-0.09 ng/mL for acyl ghrelin, total ghrelin, and GH, respectively; P<0.01). Ghrelin and GH concentrations were not changed by secretin injection throughout the experimental periods. These results indicate that ET-1 and ET-3 stimulate ghrelin and GH secretion in cattle and demonstrate for the first time that endogenous ghrelin released in response to endothelin injection stimulates GH secretion in vivo in cattle.

Contribution of insulin resistance to vascular dysfunction

Insulin is a vascular hormone, able to influence vascular cell responses. In this review, we consider the insulin actions on vascular endothelium and on vascular smooth muscle cells (VSMC) both in physiological conditions and in the presence of insulin resistance. In particular, we focus the relationships between activation of insulin signalling pathways of phosphatidylinositol-3 kinase (PI3-K) and mitogen-activated protein kinase (MAPK) and the different vascular actions of insulin, with a particular attention to the insulin ability to activate the pathway nitric oxide (NO)/cyclic GMP/PKG via PI3-K, owing to the peculiar relevance of NO in vascular biology. We also discuss the insulin actions mediated by the MAPK pathway (such as endothelin-1 synthesis and secretion and VSMC proliferation and migration) and by the interactions between the two pathways, both in insulin-sensitive and in insulin-resistant states. Finally, we consider the influence of free fatty acids, cytokines and endothelin on vascular insulin resistance.

Endothelin-1 stimulates interleukin-6 secretion from 3T3-L1 adipocytes

BACKGROUND

Since both endothelin-1 (ET-1) and interleukin-6 (IL-6) may induce insulin resistance and adipose tissue is a major contributor of circulating IL-6, we examined the effects of ET-1 on IL-6 secretion from 3T3-L1 adipocytes.

METHODS

IL-6 release was measured by ELISA. RT-PCR and real-time PCR analyses were used to determine cellular IL-6 mRNA levels. A luciferase reporter driven by promoter (-1310/+198) of mouse IL-6 gene was transfected into 3T3-L1 adipocytes to monitor IL-6 transcription.

RESULTS

Treatment of adipocytes with ET-1 dose- and time-dependently increased IL-6 secretion. The stimulatory effect of ET-1 on IL-6 secretion was abolished by actinomycin D and ET-1 induced an increase in IL-6 mRNA levels. ET-1 was able to enhance the IL-6 promoter activity and its stimulatory effect was inhibited by GF109203X, U0126, salicylate, dominant negative CREB and mithramycin A. Thus it appears that ET-1 may stimulate IL-6 secretion mainly through an enhanced IL-6 transcription, by a mechanism involving both protein kinase C and p42/p44 mitogen-activated protein kinase, and probably downstream NF-kappaB, CREB and Sp1 transcription factors.

GENERAL SIGNIFICANCE

This study demonstrates that ET-1 is able to increase IL-6 secretion from adipocytes and raises the possibility that ET-1-induced insulin resistance may be mediated by IL-6.

Mechanisms of ET-1-induced endothelial dysfunction

There is now increasing evidence that endothelial dysfunction is an early event in the pathophysiology of cardiovascular diseases and can be corrected with certain therapies such as angiotensin converting enzyme inhibitors angiotensin type I receptor antagonists and stains independently of blood pressure lowering effects. Restoring endothelial function appears to be a crucial target since endothelial dysfunction predicts cardiovascular events in various situations such as coronary artery disease peripheral artery disease, or hypertension and in patients undergoing vascular surgery. Preclinical and clinical data strongly support that endothelin receptor antagonists belong to this restricted class of pharmacological agents able to act on the endothelium, and offer a potential therapeutic approach for numerous diseases associated with endothelial dysfunction. The purpose of this review will be therefore, 1) to propose mechanisms by which ET-1 can cause endothelial dysfunction; 2) to provide an overview of pathological situations associated with endothelial dysfunction related to ET-1; and 3) to assemble evidence on efficacy of endothelin receptor antagonists for improvement of endothelial function.

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.

Endothelin-1 impairs glucose transporter trafficking via a membrane-based mechanism

Endothelin-1 (ET-1) disrupts insulin-regulated glucose transporter GLUT4 trafficking. Since the negative consequence of chronic ET-1 exposure appears to be independent of signal disturbance along the insulin receptor substrate-1/phosphatidylinositol (PI) 3-kinase (PI3K)/Akt-2 pathway of insulin action, we tested if ET-1 altered GLUT4 regulation engaged by osmotic shock, a PI3K-independent stimulus that mimics insulin action. Regulation of GLUT4 by hyperosmotic stress was impaired by ET-1. Because of the mutual disruption of both insulin- and hyperosmolarity-stimulated GLUT4 translocation, we tested whether shared signaling and/or key phosphatidylinositol 4,5-bisphosphate (PIP2)-regulated cytoskeletal events of GLUT4 trafficking were targets of ET-1. Both insulin and hyperosmotic stress signaling to Cbl were impaired by ET-1. Also, plasma membrane PIP2 and cortical actin levels were reduced in cells exposed to ET-1. Exogenous PIP2, but not PI 3,4,5-bisphosphate, restored actin structure, Cbl activation, and GLUT4 translocation. These data show that ET-1-induced PIP2/actin disruption impairs GLUT4 trafficking elicited by insulin and hyperosmolarity. In addition to showing for the first time the important role of PIP2-regulated cytoskeletal events in GLUT4 regulation by stimuli other than insulin, these studies reveal a novel function of PIP2/actin structure in signal transduction.

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.

Role of PYK2 in the development of obesity and insulin resistance

Non-receptor proline-rich tyrosine kinase-2 (PYK2), which is activated by phosphorylation of one or more of its tyrosine residues, has been implicated in the regulation of GLUT4 glucose transporter translocation and glucose transport. Some data favor a positive role of PYK2 in stimulating glucose transport, whereas other studies suggest that PYK2 may participate in the induction of insulin resistance. To ascertain the importance of PYK2 in the setting of obesity and insulin resistance, we (1) evaluated the regulation of PYK2 in mice fed a high-fat diet and (2) characterized body and glucose homeostasis in wild type (WT) and PYK2(-/-) mice on different diets. We found that both PYK2 expression and phosphorylation were significantly increased in liver and adipose tissues harvested from high-fat diet fed mice. Wild type and PYK2(-/-) mice were fed a high-fat diet for 8 weeks to induce insulin resistance/obesity. Surprisingly, in response to this diet PYK2(-/-) mice gained significantly more weight than WT mice (18.7+/-1.2g vs. 9.5+/-0.6g). Fasting serum leptin and insulin and blood glucose levels were significantly increased in high-fat diet fed mice irrespective of the presence of PYK2 protein. There was a close correlation between serum leptin and body weight. Intraperitoneal glucose tolerance tests revealed that as expected, the high-fat diet resulted in increased blood glucose levels following glucose administration in wild type mice compared to those fed normal chow. An even greater increase in blood glucose levels was observed in PYK2(-/-) mice compared to wild type mice. These results demonstrate that a lack of PYK2 exacerbates weight gain and development of glucose intolerance/insulin resistance induced by a high-fat diet, suggesting that PYK2 may play a role in slowing the development of obesity, insulin resistance, and/or frank diabetes.

Endothelin inhibition delays onset of hyperglycemia and associated vascular injury in type I diabetes: evidence for endothelin release by pancreatic islet beta-cells

This study investigated the role of endothelin-1 for hyperglycemia, vascular, and pancreatic injury in early type I diabetes in non-obese-diabetic (NOD) mice. Endothelium dependent relaxation to acetylcholine and vascular gene expression of endothelin converting enzyme (ECE) isoforms 1 and 2 were studied as indicators of vascular injury. Endothelial NO bioactivity in the aorta was reduced in diabetic NOD mice while vascular expression of ECE-1 and ECE-2 mRNA was increased compared with controls (all p<0.05). Vascular histology was normal in all animals. Unexpectedly, treatment of prediabetic NOD mice for 6 weeks with the orally active ET(A) receptor antagonist BSF461314 prevented onset of diabetes without affecting insulitis severity. ET(A) receptor blockade also restored abnormal endothelial NO bioactivity and reduced ECE-1 and ECE-2 gene expression in NOD mice to levels comparable with healthy controls (p<0.05). Moreover, secretion of endothelin-1 in a time-dependent fashion was observed by pancreatic islet beta-cells cultured in vitro. These data suggest a critical role for ET(A) receptor signaling in the development of autoimmune forms of diabetes and the early vascular injury associated with it.

Endothelin-1 induces lipolysis in 3T3-L1 adipocytes

Endothelin-1 (ET-1) affects glucose uptake in adipocytes and may play an important role in adipose physiology. One of the principal functions of adipose tissue is the provision of energy substrate through lipolysis. In the present study, we investigated the effects of ET-1 on lipolysis in 3T3-L1 adipocytes. When glycerol release in the culture medium was measured as an index of lipolysis, the results showed that ET-1 caused a significant increase that was time and dose dependent. With a concentration of 10 nM ET-1, stimulation of glycerol release plateaued after 4 h of exposure. This effect was inhibited by the ETA receptor antagonist BQ-610 (10 microM) but not by the ETB receptor antagonist BQ-788 (10 microM). To further explore the underlying mechanisms of ET-1 action, we examined the involvement of the cAMP-dependent protein kinase A-mediated, phospholipase A2 (PLA2)-mediated, protein kinase C (PKC)-mediated, phosphatidylinositol 3 (PI 3)-kinase-mediated, and the mitogen-activated protein kinase (MAPK)-mediated pathways. Inhibition of adenylyl cyclase activation by SQ-22536 (100 microM) did not block ET-1-induced lipolysis. Pretreatment of adipocytes with the PLA2 inhibitor dexamethasone (100 nM), the PKC inhibitor H-7 (6 microM), or the PI 3-kinase inhibitor wortmannin (100 nM) also had no effect. ET-1-induced lipolysis was blocked by inhibition of extracellular signal-regulated kinase (ERK) activation using PD-98059 (75 microM), whereas a p38 MAPK inhibitor (SB-203580; 20 microM) had no effect. Results of Western blot further demonstrated that ET-1 induced ERK phosphorylation. These data show that ET-1 induces lipolysis in 3T3-L1 adipocytes via a pathway that is different from the conventional cAMP-dependent pathway used by isoproterenol and that involves ERK activation.

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.

Angiotensin II enhances insulin sensitivity in vitro and in vivo

The renin-angiotensin system plays a critical role in the pathogenesis of obesity, obesity-associated hypertension, and insulin resistance. However, the biological actions of angiotensin II (AII) on insulin sensitivity remain controversial. Because angiotensinogen and AII receptors are expressed on adipose tissue, we investigated the effect of AII on the insulin sensitivity of isolated rat adipocytes. The results of a receptor binding assay showed the maximal AII binding capacity of adipocytes to be 8.3 +/- 0.9 fmol/7 x 10(6) cells and the dissociation constant to be 2.72 +/- 0.11 nM. Substantial expression of both type 1 and 2 AII (AT1 and AT2) receptors was detected by RT-PCR. AII had no effect on basal glucose uptake, but significantly potentiated insulin-stimulated glucose uptake; this effect was abolished by the AT1 antagonist, losartan. In addition, AII did not alter the insulin binding capacity of adipocytes, but increased insulin-stimulated tyrosine phosphorylation of the insulin receptor beta-subunit, Akt phosphorylation, and translocation of glucose transporter 4 to the plasma membrane. AII potentiated insulin-stimulated glucose uptake through the AT1 receptor and by alteration of the intracellular signaling of insulin. Intraperitoneal injection of Sprague Dawley rats with AII increased insulin sensitivity in vivo. In conclusion, we have shown that AII enhances insulin sensitivity both in vitro and in vivo, suggesting that dysregulation of the insulin-sensitizing effect of AII may be involved in the development of insulin resistance.

Insulin infusion induces endothelin-1-dependent hypertension in rats

We previously showed that chronic insulin infusion induces insulin resistance, hyperendothelinemia, and hypertension in rats (C. C. Juan, V. S. Fang, C. F. Kwok, J. C. Perng, Y. C. Chou, and L. T. Ho. Metabolism 48: 465-471, 1999). Endothelin-1 (ET-1), a potent vasoconstrictor, is suggested to play an important role in maintaining vascular tone and regulating blood pressure, and insulin increases ET-1 production in vivo and in vitro. In the present study, BQ-610, a selective endothelin A receptor antagonist, was used to examine the role of ET-1 in insulin-induced hypertension in rats. BQ-610 (0.7 mg/ml; 0.5 ml/kg body wt) or normal saline was given intraperitoneally two times daily for 25 days to groups of rats infused with either saline or insulin (2 U/day via sc-implanted osmotic pumps), and changes in plasma levels of insulin, glucose, and ET-1 and the systolic blood pressure were measured over the experimental period, whereas changes in insulin sensitivity were examined at the end of the experimental period. Plasma insulin and ET-1 levels were measured by RIA, plasma glucose levels using a glucose analyzer, systolic blood pressure by the tail-cuff method, and insulin sensitivity by an oral glucose tolerance test. Our studies showed that insulin infusion caused sustained hyperinsulinemia in both saline- and BQ-610-injected rats over the infusion period. After pump implantation (2 wk), the systolic blood pressure was significantly higher in insulin-infused rats than in saline-infused rats in the saline-injected group (133 +/- 3.1 vs. 113 +/- 1.1 mmHg, P < 0.05) but not in the BQ-610-injected group (117 +/- 1.2 vs. 117 +/- 1.8 mmHg). Plasma ET-1 levels in both sets of insulin-infused rats were higher than in saline-infused controls (2.5 +/- 0.6 and 2.5 +/- 0.8 vs. 1.8 +/- 0.4 and 1.7 +/- 0.3 pmol/l, P < 0.05). Oral glucose tolerance tests showed that BQ-610 treatment did not prevent the insulin resistance caused by chronic insulin infusion. No significant changes were found in insulin sensitivity and blood pressure in saline-infused rats treated with BQ-610. In a separate experiment, insulin infusion induced the increase in arterial ET-1 content, hypertension, and subsequent plasma ET-1 elevation in rats. These results suggest that, in the insulin infusion rat model, ET-1 plays a mediating role in the development of hypertension, but not of insulin resistance.

Thapsigargin and EGTA inhibit endothelin-1-induced glucose transport

We have previously demonstrated that ET-1 may enhance glucose transport in 3T3-L1 adipocytes, secondarily to its stimulatory effect on GLUT1 gene expression by a mitogen-activated protein kinase (MAPK)-dependent pathway. In the present study, we further tested the involvement of Ca(2+) in glucose uptake in response to ET-1. Among a variety of Ca(2+)-related agents tested, EGTA and thapsigargin were found to suppress both the glucose uptake and intracellular Ca(2+) mobilization induced by ET-1, as determined by Fura-2 analysis. However, a phospholipase C inhibitor, U73122, also eliminated the intracellular calcium mobilization induced by ET-1, but had no effect on ET-1-stimulated glucose uptake. The finding that neither EGTA nor thapsigargin had any influence on ET-1-induced MAPK activation implies that some mechanism downstream of MAPK activation is involved. Further investigation showed that both agents exerted global inhibitory effects on protein and RNA syntheses. Since both thapsigargin and EGTA may deplete endoplasmic reticulum (ER) Ca(2+) stores, our results suggest that (1) ET-1-induced glucose transport is independent of ET-1's effect on Ca(2+) mobilization and (2) depletion of ER Ca(2+) stores per se may interfere with ET-1's effect on GLUT1 expression.

Chronic endothelin-1 treatment leads to insulin resistance in vivo

We determined whether chronic endothelin-1 (ET-1) treatment could lead to in vivo insulin resistance. Like insulin, ET-1 acutely stimulated glucose transport in isolated soleus muscle strips of WKY rats. ET-1 pretreatment (1 h) decreased insulin-stimulated glucose transport in muscle strips (-23%). Both ET-1-mediated effects were generated through ET(A) receptors, because a specific ET(A) receptor antagonist (BQ610) blocked these effects of ET-1. Osmotic minipumps were used to treat normal rats with ET-1 for 5 days. Subsequent hyperinsulinemic-euglycemic clamps showed that ET-1 treatment led to an approximately 30% decrease in insulin-stimulated glucose disposal rates in male and female rats. In addition, ex vivo study of soleus muscle strips showed decreased glucose transport into muscle from ET-1-treated animals. With respect to insulin signaling, chronic in vivo ET-1 treatment led to a 30-40% decrease in IRS-I protein content, IRS-I-associated p110(alpha), and AKT activation. In summary, 1) in vitro ET-1 pretreatment leads to decreased insulin-stimulated glucose transport in skeletal muscle strips; 2) chronic ET-1 administration in vivo leads to whole-body insulin resistance, with decreased skeletal muscle glucose transport and impaired insulin signaling; and 3) elevated ET-1 levels may be a cause of insulin resistance in certain pathophysiologic states.

Pathophysiological implications of insulin resistance on vascular endothelial function

BACKGROUND

Insulin resistance is a key component of the insulin resistance syndrome and is a crucially important metabolic abnormality in Type 2 diabetes. Insulin-resistant individuals are at significantly increased risk of cardiovascular disease, although the underlying mechanisms remain incompletely understood. The endothelium is thought to play a critical role in maintaining vascular homeostasis, a process dependent on the balance between the production of nitric oxide, superoxide and other vasoactive substances. Endothelial dysfunction has been demonstrated in insulin-resistant states in animals and humans and may represent an important early event in the development of atherosclerosis. Insulin resistance may be linked to endothelial dysfunction by a number of mechanisms, including disturbances of subcellular signalling pathways common to both insulin action and nitric oxide production. Other potential unifying links include the roles of oxidant stress, endothelin, the renin angiotensin system and the secretion of hormones and cytokines by adipose tissue. Lifestyle measures and drug therapies which improve insulin sensitivity and ameliorate endothelial dysfunction may be important in delaying the progression to overt cardiovascular disease in at risk individuals.

METHODS

We conducted a literature search using Medline, restricted to articles published in the English language between 1966 and the present, and reviewed bibliographies of relevant articles. An initial search strategy employing combinations of the MeSH terms: insulin resistance; endothelium, vascular; insulin; nitric oxide or hyperinsulinaemia produced over 300 references. Focused searches using keywords relevant to the molecular aspects of endothelial function and insulin signalling, and lifestyle or pharmacological interventions relevant to insulin resistance or endothelial function, produced over 300 further references. Abstracts of all references were screened before selecting those relevant to this review.

Tissue- and time-dependent effects of endothelin-1 on insulin-stimulated glucose uptake

Hyperendothelinaemia is associated with various insulin-resistant states, e.g., diabetes, obesity and heart failure, but whether endothelin-1 (ET-1) has a direct effect on insulin-mediated glucose uptake is unclear because the interpretation of in vivo metabolic studies is complicated by ET-1 effects on muscle blood flow and insulin secretion. This study investigated the effects of ET-1 (1-10 nM) on basal and insulin-stimulated 2-deoxy-D-[3H]glucose (2-DOG) uptake in cultured L6 myoblasts and 3T3-adipocytes. RT-PCR analysis showed that both cell types express ET(A) but not ET(B) receptors. ET-1 had no effect on basal (non-insulin-mediated) glucose transport, but there was evidence of a tissue- and time-dependent inhibitory effect of ET-1 on insulin-stimulated glucose uptake. Specifically, ET-1 10 nM had a transient (0.5 h) inhibitory effect on glucose uptake in 3T3 cells (C(I-150) [dose of insulin required to increase glucose uptake by 50%, relative to control 100%] increased from 89 +/- 14 nM to 270 +/- 12 nM at 30 mins, P < 0.05) but no effect on insulin sensitivity in L6 myoblasts (C(I-150) was 56 +/- 14 nM [control], 43 +/- 14 [30 mins] and 26 +/- 16 [2 h]). In conclusion, the inhibitory effect of ET-1 on insulin-stimulated glucose uptake is transient and occurs in 3T3-L1 adipocytes but not skeletal muscle-derived cells, perhaps reflecting tissue differences in ET(A)-receptor signaling. It is therefore unlikely that chronic hyperendothelinaemia has a direct insulin-antagonist effect contributing to peripheral (ie muscle/fat) insulin resistance in vivo.

Endothelin-1 increases glucose transporter glut1 mRNA accumulation in 3T3-L1 adipocytes by a mitogen-activated protein kinase-dependent pathway

The mechanism of enhancing glucose transport by prolonged endothelin-1 (ET-1) treatment of 3T3-L1 adipocytes was examined. Western and Northern blot analyses indicated that ET-1 increased the amount of both GLUT1 protein and mRNA. The degradation rate of GLUT1 mRNA as measured in the presence of actinomycin D, nevertheless, was not significantly altered by ET-1. Whereas various inhibitors for distinct signalling pathways were tested, only the mitogen-activated protein kinase (MAPK) kinase inhibitor, PD98059, was found to decrease significantly the enhancing effect of ET-1. Similar extent of inhibition was observed in cells pretreated with pertussis toxin (PT). Immunoblot analysis revealed that ET-1 may stimulate a transient phosphorylation of p42/p44 MAPK and both PT and PD98059 inhibited this stimulation. In addition, the effect of ET-1 on GLUT1 mRNA accumulation was inhibited by PD98059 and cycloheximide, implying that a trans-activation was involved. Taken together, these results suggest that ET-1 may induce GLUT1 gene expression by a MAPK-dependent mechanism.

Chronic endothelin-1 treatment leads to heterologous desensitization of insulin signaling in 3T3-L1 adipocytes

We recently reported that insulin and endothelin-1 (ET-1) can stimulate GLUT4 translocation via the heterotrimeric G protein G alpha q/11 and through PI3-kinase--mediated pathways in 3T3-L1 adipocytes. Because both hormones stimulate glucose transport through a common downstream pathway, we determined whether chronic ET-1 pretreatment would desensitize these cells to acute insulin signaling. We found that ET-1 pretreatment substantially inhibited insulin-stimulated 2-deoxyglucose uptake and GLUT4 translocation. Cotreatment with the ETA receptor antagonist BQ 610 prevented these effects, whereas inhibitors of G alpha i or G beta gamma were without effect. Chronic ET-1 treatment inhibited insulin-stimulated tyrosine phosphorylation of G alpha q/11 and IRS-1, as well as their association with PI3-kinase and blocked the activation of PI3-kinase activity and phosphorylation of AKT: In addition, chronic ET-1 treatment caused IRS-1 degradation, which could be blocked by inhibitors of PI3-kinase or p70 S6-kinase. Similarly, expression of a constitutively active G alpha q mutant, but not the wild-type G alpha q, led to IRS-1 degradation and inhibited insulin-stimulated phosphorylation of IRS-1, suggesting that the ET-1-induced decrease in IRS-1 depends on G alpha q/11 and PI3-kinase. Insulin-stimulated tyrosine phosphorylation of SHC was also reduced in ET-1 treated cells, resulting in inhibition of the MAPK pathway. In conclusion, chronic ET-1 treatment of 3T3-L1 adipocytes leads to heterologous desensitization of metabolic and mitogenic actions of insulin, most likely through the decreased tyrosine phosphorylation of the insulin receptor substrates IRS-1, SHC, and G alpha q/11.

Insulin preincubation effects on rat vessel contractile responses: role of the sarcoplasmic reticulum

In the present work, we studied the possible mechanisms involved in the insulin-induced acceleration of ET1 contractions. We observed a shortening of the half-life needed to achieve maximal developed force (t(1/2)) at 10(-7) M ET1 in rat aortic rings preincubated for 120 min with 3 nM insulin (control 380 +/- 15 s vs. 319 +/- 8 s with insulin, n = 28, p < 0.05). A tyrosine kinase linked receptor was involved in this effect because it was abolished by 30 microM genistein. Endothelium denudation and 10 microM indomethacin treatment did not effect this insulin effect, suggesting its independence of endothelial-derived factors. The effect was still present when the only source of Ca2+ was intracellular (t(1/2) values in the absence of external Ca2+: control 467 +/- 68 s vs. 213 +/- 28 s with insulin, n = 16, p < 0.05), but was blunted if the sarcoplasmic reticulum (SR) Ca2+ source was suppressed by exposure to 10 microM thapsigargin or 10 microM ryanodine. Preincubation with insulin did not potentiate either SR 45Ca2+ uptake or contractions evoked by caffeine-sensitive SR Ca2+ release. Since 30 microM cheleritrine abolished insulin-induced acceleration of ET1 contractions, we propose that the hormone might enhance a signal pathway related to PKC in order to produce a faster Ca2+ release from the SR.

The acute and chronic stimulatory effects of endothelin-1 on glucose transport are mediated by distinct pathways in 3T3-L1 adipocytes

We have recently shown that pretreatment with endothelin-1 (ET-1) for 20 min stimulates GLUT4 translocation in a PI3-kinase-dependent manner in 3T3-L1 adipocytes (Imamura, T. et al., J Biol Chem 274:33691-33695). This study presents another pathway by which ET-1 potentiates glucose transport in 3T3-L1 adipocytes. ET-1 treatment (10 nM) leads to approximately 2.5-fold stimulation of 2-deoxyglucose (2-DOG) uptake within 20 min, reaching a maximal effect of approximately 4-fold at approximately 6 h, and recovering almost to basal levels after 24 h. Insulin treatment (3 ng/ml) results in an approximately 5-fold increase in 2-DOG uptake at 1 h, and recovering to basal levels after 24 h. The ETA receptor antagonist, BQ 610, inhibited ET-1 induced glucose uptake both at 20 min and 6 h, whereas the ETB receptor antagonist, BQ 788, was without effect. Interestingly, ET-1 stimulated 2-DOG uptake at 6 h, not at 20 min, was almost completely blocked by the protein-synthesis inhibitor, cycloheximide and the RNA-synthesis inhibitor, actinomycin D, suggesting that the short-term (20 min) and long-term (6 h) effects of ET-1 involve distinct mechanisms. GLUT4 translocation assay showed that 20 min, but not 6 h, exposure to ET-1 led to GLUT4 translocation to the plasma membrane. In contrast, 6 h, but not 20 min, exposure to ET-1 increased expression of the GLUT1 protein, without affecting expression of GLUT4 protein. ET-1 induced 2-DOG uptake and GLUT1 expression at 6 h were completely inhibited by the MEK inhibitor, PD 98059, and partially inhibited by the PI3-kinase inhibitor, LY 294002, and the G alpha i inhibitor, pertussis toxin. The PLC inhibitor, U 73122, was without effect. These findings suggest that ET-1 induced GLUT1 protein expression is primarily mediated via MAPK, and partially via PI3K in 3T3-L1 adipocytes.

Combined use of insulin and endothelin-1 causes decrease of protein expression of beta-subunit of insulin receptor, insulin receptor substrate-1, and insulin-stimulated glucose uptake in rat adipocytes

Previously, we reported that insulin-stimulated glucose uptake (ISGU) can be inhibited by endothelin (ET-1). However, the mechanism by which ET-1 impairs ISGU in adipocytes remains unclear. This study investigated the effects of ET-1 on insulin action in rat adipocytes in order to elucidate the molecular mechanism of action of ET-1 on ISGU. The results show that ISGU was increased fivefold after 3-h treatment with 1 nM insulin. Treatment with 100 nM ET-1 had no effect on basal glucose uptake. However, ET-1 inhibited approximately 25% of ISGU and 20% of insulin binding after 3-h treatment in the presence of 1 nM insulin. Expression of the beta-subunit of the insulin receptor (IRbeta) and the insulin receptor substrate-1 (IRS-1) in adipocytes was not significantly affected by 1 nM insulin or by 100 nM ET-1, even after 3-h treatment. However, expressions of IRbeta and IRS-1 were dramatically decreased in a dose- and time-dependent manner when adipocytes were treated with both insulin and ET-1. Approximately 50% of IRbeta and 65% of IRS-1 expression levels were suppressed when adipocytes were simultaneously treated with both 1 nM insulin and 100 nM ET-1 for 3 h. These results suggest that the inhibitory effect of ET-1 on ISGU may be mediated via the insulin receptor and suppression of IRbeta/IRS-1 expression.

Effects of endothelin-1 and nitric oxide on glucokinase activity in isolated rat hepatocytes

To test the hypothesis that endothelin-1 (ET-1) and nitric oxide (NO) influence glucokinase (GK) activity in an opposite manner, we evaluated the effects of ET-1, L-NAME, an inhibitor of NO synthase, and L-arginine, a substrate for NO synthase, on GK activity and glycogen content in isolated rat hepatocytes. Moreover, to understand the receptor involved in the process, the effects of BQ 788, a specific antagonist of ETB receptor, and PD 142893, an antagonist of ETA-ETB receptors, were also evaluated. GK activity, cyclic guanosine monophosphate (cGMP), and glycogen intracellular content were measured on isolated hepatocytes, while glucose levels and NO as NO2-/NO3- were determined in the medium. High ET-1 levels induced a 20% decrease of NO2-/NO3- levels and cGMP intracellular content, followed by a 49% reduction of GK activity and a 15% decrease of glycogen. In parallel, a 10% increase of glucose in the medium was observed. In the presence of L-NAME, GK activity and glycogen levels showed analogous decrements as observed with ET-1. Also in this case, a significant decrease of the intracellular content of cGMP was observed. No synergistic effects of ET-1 and L-NAME were observed. L-Arginine was able to counteract the inhibitory effect of ET-1 on cGMP and GK activity. Glycogen content was slightly but not significantly reduced, and under those conditions, a significant decrease of glucose in the medium was observed. When hepatocytes were incubated with ET-1 plus BQ 788 or ET-1 plus PD 142893, GK activity was unchanged. Interestingly, no changes were observed in NO2-/NO3- levels and the intracellular content of cGMP was not modified when the antagonists of ET-1 receptors were added to the medium. In conclusion, the present study shows that the NO pathway seems to be an important regulator of GK activity and glycogen content through cGMP activity. In addition, ET-1 seems to be not active per se, but its activity seems mediated by a simultaneous decrease of NO levels.

Modulation of insulin receptor substrate-1 tyrosine phosphorylation by an Akt/phosphatidylinositol 3-kinase pathway

Serine/threonine phosphorylation of insulin receptor substrate 1 (IRS-1) has been implicated as a negative regulator of insulin signaling. Prior studies have indicated that this negative regulation by protein kinase C involves the mitogen-activated protein kinase and phosphorylation of serine 612 in IRS-1. In the present studies, the negative regulation by platelet-derived growth factor (PDGF) was compared with that induced by endothelin-1, an activator of protein kinase C. In contrast to endothelin-1, the inhibitory effects of PDGF did not require mitogen-activated protein kinase or the phosphorylation of serine 612. Instead, three other serines in the phosphorylation domain of IRS-1 (serines 632, 662, and 731) were required for the negative regulation by PDGF. In addition, the PDGF-activated serine/threonine kinase called Akt was found to inhibit insulin signaling. Moreover, this inhibition required the same IRS-1 serine residues as the inhibition by PDGF. Finally, the negative regulatory effects of PDGF and Akt were inhibited by rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), one of the downstream targets of Akt. These studies implicate the phosphatidylinositol 3-kinase/Akt kinase cascade as an additional negative regulatory pathway for the insulin signaling cascade.

Endothelin stimulates glucose uptake and GLUT4 translocation via activation of endothelin ETA receptor in 3T3-L1 adipocytes

Endothelin-1 (ET-1) is a 21-amino acid peptide that binds to G-protein-coupled receptors to evoke biological responses. This report studies the effect of ET-1 on regulating glucose transport in 3T3-L1 adipocytes. ET-1, but not angiotensin II, stimulated glucose uptake in a dose-dependent manner with an EC50 value of 0.29 nM and a 2.47-fold stimulation at 100 nM. ET-1 stimulated glucose uptake in differentiated 3T3-L1 cells but had no effect in undifferentiated cells, although ET-1 stimulated phosphatidylinositol hydrolysis to a similar degree in both. The 3T3-L1 cells expressed approximately 560,000 sites/cell of ETA receptor, which was not altered during differentiation. Western blot analysis and immunofluorescence staining show that ET-1 stimulated the translocation of insulin-responsive aminopeptidase and GLUT4 to the plasma membrane. The effect of ET-1 on glucose uptake was blocked by A-216546, an antagonist selective for the ETA receptor. ET-1 treatment did not induce phosphorylation of insulin receptor beta-subunit, insulin receptor substrate-1, or Akt but stimulated the tyrosyl phosphorylation of a 75-kDa protein. Genistein (100 microM), an inhibitor of tyrosine kinases, inhibited ET-1-stimulated glucose uptake. Our results show that ET-1 stimulates GLUT4 translocation and glucose uptake in 3T3-L1 adipocytes via activation of ETA receptor.

Evidence that endothelin-1 (ET-1) inhibits insulin-stimulated glucose uptake in rat adipocytes mainly through ETA receptors

The specificity of endothelin (ET) receptors involved in the inhibition of insulin-stimulated glucose uptake (ISGU) in rat adipocytes was investigated. Adipocytes were isolated from the epididymal fat pads of Sprague-Dawley rats. To determine receptor subtypes, we used three ET isopeptides, ET-1 and ET-2, both of which are nonselective agonists, and ET-3, a selective agonist for ETC receptors, to displace [125I]ET-1 binding from the fat cells. The efficiency of displacement was ET-1 > ET-2 >> ET-3, indicating that the primary receptors involved belonged to the ETA subtype. At an equal concentration of 1 micromol/L, BQ-610, a selective ETA antagonist, displaced [125I]ET-1 from binding to fat cells, whereas IRL-1038, a selective ETB antagonist, did not. Using [3H]2-deoxy-D-1-glucose ([3H]2-DG) as a tracer in studies of glucose uptake, we found that equimolar BQ-610 completely reversed the inhibitory effect of ET-1 on ISGU, whereas IRL-1038 was ineffective. Northern blot analysis of adipocyte receptors showed abundant mRNA for ETA, but no ETB subtype. These results clearly demonstrate that ETA is the predominant receptor in rat adipocytes.

Overexpression of vascular endothelin-1 and endothelin-A receptors in a fructose-induced hypertensive rat model

OBJECTIVE

To examine the temporal relationship between hyperinsulinemia and hypertension in the fructose-hypertensive rat model and to study the function of endothelin-1 (ET-1) in fructose-induced hypertension.

DESIGN

Since ET-1 induces insulin resistance in conscious rats, we tested the hypothesis that both hyperinsulinemia and hypertension developed in the fructose-hypertensive rat model might be the sequelae of an elevated tissue content of ET-1 and ET(A) receptors.

MATERIALS AND METHODS

Systolic hypertension was induced within 3 weeks in male Sprague-Dawley rats fed on a fructose-rich diet. After continual monitoring of blood pressure and plasma insulin concentrations, the animals were killed at the end of experiment to determine plasma levels of ET-1, the contractile response of aortic rings to ET-1, and ET-1 and ET(A) receptor gene expressions. In a separate experiment, BQ-610 was administered to lower the effect of ET-1 in rats with fructose-induced hypertension.

RESULTS

Compared with control rats given normal chow, the fructose-fed rats developed systolic hypertension after 3 weeks of the diet (127+/-3.7 versus 110+/-5.5 mmHg, P < 0.01) and hyperinsulinemia both before (1 07.1+/-32.5 versus 48.5+/-14.3 pmol/l, P < 0.005) and after (96.6+/-63.7 versus 50.4+/-5.6 pmol/l, P< 0.05) they became hypertensive. Although plasma ET-1 levels did not differ between the rat groups, aortic ring contraction-concentration curves, indicating vessel contractility in response to ET-1, were significantly greater in these rats than in controls (F1,72 = 12.34, P< 0.00077). Messenger RNA extracted from the tail arteries and blotted with both ET-1 and ET(A) probes showed that fructose-fed rats had greater ET-1 and ET(A)-receptor gene expression than control rats. Concomitant administration of BQ-610 to rats fed on a fructose diet significantly reduced the hypertension. Conclusions These findings suggest that elevated vascular expression of ET-1 and ET(A) receptor genes may mediate the development of hypertension and hyperinsulinemia in rats fed a fructose-rich diet

Is captopril-induced improvement of insulin sensitivity mediated via endothelin?

Angiotensin-converting enzyme (ACE) inhibitors have been reported to improve insulin sensitivity during either short-term or long-term administration. Recent studies indicate that endothelin-1 (ET-1) has potent glycogenolytic effects in rat hepatocytes and may cause insulin resistance in rat adipocytes. In addition, ET may also have a role in stimulation of the hypothalamic-pituitary-adrenal axis. To test the hypothesis that part of the effect of captopril in enhancing insulin sensitivity may be mediated via ET and/or by glucocorticoids, we measured 24-h urinary excretion of ET and free cortisol before and after short-term treatment with captopril. The 24-h urinary immunoreactive endothelin (IR-ET) excretion decreased significantly (p < 0.05) from 65 +/- 4 ng at baseline to 42 +/- 3 ng after captopril treatment, whereas no significant change in the 24-h urinary free cortisol excretion was observed. Moreover, no significant change in the 24-h urinary IR-ET and free cortisol excretions was noted in the placebo-treated group. We speculate that ACE inhibitors may exert their effect on insulin sensitivity not only by blocking the renin-angiotensin and kinin systems but also by inhibiting production and/or release of ET.

Endothelin-1 regulates tone of isolated small arteries in the rat: effect of hyperendothelinemia

Chronic elevation of plasma endothelin-1 (ET-1) levels has been reported in several pathological conditions. To investigate the consequences of increased circulating ET-1 on vascular responsiveness, Sprague-Dawley rats (n=16) were chronically instrumented with a minipump delivering ET-1 at a constant dose for 7 days. Plasma ET-1 levels were more than doubled in treated (0.98+/-0.09 pmol/L; P<.05) versus untreated sham-operated rats (0.43+/-0.04 pmol/L), whereas systolic arterial blood pressure increased (139+/-5 versus 128+/-4 mm Hg in untreated rats; P<.05). After rats were killed, segments of middle cerebral (MCA) and mesenteric (MES) arteries were mounted on an isometric myograph. ET-induced contraction was shifted to the right in ET-1-treated animals and not modified by BQ123 (an ET(A) receptor antagonist); bosentan (ET(A/B) receptor antagonist) prevented ET-1-induced contraction in both groups. After inhibition of nitric oxide synthase with N(omega)-nitro-L-arginine (L-NNA), both phenylephrine and oxymetazoline (an alpha2-adrenoceptor agonist) induced MCA contraction. The sensitivity to phenylephrine was decreased in ET-1-treated compared with control rats (P<.05). Sensitivity to phenylephrine-induced contraction was decreased by BQ123 in control rats only. In contrast, L-NNA revealed greater oxymetazoline-induced contractions in treated compared with control MCA rings (P<.05); this potentiation was blunted by bosentan but unaffected by BQ123. Removal of the endothelium revealed a direct constrictor effect of oxymetazoline that was insensitive to L-NNA alone or combined with bosentan; however, oxymetazoline induced significantly lower constriction in treated rat MCA segments. Responses to oxymetazoline were also blunted in treated compared with untreated denuded MES arteries. In conclusion, chronic elevated plasmatic ET-1 decreases smooth muscle cell sensitivity to contractile agonists both in MCA and MES rings. In cerebral vessels, endothelial alpha2-adrenoceptor-dependent stimulation induced greater contractile responses in treated rats which were sensitive to bosentan, suggesting that oxymetazoline stimulates ET-1 release from the endothelium. This may represent a compensatory mechanism for the loss of smooth muscle sensitivity.

Amelioration of insulin resistance and hypertension in a fructose-fed rat model with fish oil supplementation

In type II diabetic patients, one can detect several pathologic changes including insulin resistance and hypertension. Sprague-Dawley rats fed a fructose-rich diet (group F) exhibited these characteristic abnormalities within 2 weeks and were an excellent laboratory animal model for research on insulin action and development of hypertension. Since fish oils containing omega-3 fatty acids have a beneficial effect in preventing atherosclerotic diseases, we performed repeated experiments to test the effects of fish oil supplementation in group F rats. Compared with control rats on a normal diet (group C), group F consistently developed hypertriglyceridemia without elevated plasma free fatty acid (FFA), fasting hyperinsulinemia together with fasting hyperglycemia (insulin resistance syndrome), and systolic hypertension within 3 weeks. Insulin-stimulated glucose uptake and insulin binding of adipocytes were significantly reduced. Rats fed the same high-fructose diet but supplemented with fish oil (group O) had alleviation of all of these metabolic defects and a normalized insulin sensitivity and blood pressure. beta-Cell function as shown by plasma glucose and insulin responses to oral glucose remained intact in group F and group O. The plasma endothelin-1 (ET-1) level and ET-1 binding to adipocytes were not different among the three groups. Based on these results, we suggest that dietary high fructose induced hypertriglyceridemia and insulin resistance with normal islet function, and that the induced hypertension was not associated with plasma ET-1 abnormalities and was probably caused by other undefined pathologic changes that can be prevented by dietary omega-3 fatty acids.