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

Investigation of the Molecular Mechanisms by Which Endothelin-3 Stimulates Preadipocyte Growth

Endothelins induce many biological responses, and they are composed of three peptides: ET-1, ET-2, and ET-3. Reports have indicated that ET-1 regulates cell proliferation, adipogenesis, and other cell responses and that ET-3 stimulates the growth of gastrointestinal epithelial cells and melanocytes. However, the signalling pathways of ET3 that mediate the growth of fat cells are still unclear. Using 3T3-L1 white preadipocytes, we found that ET-3 induced increases in both cell number and BrdU incorporation. Pretreatment with an ETAR antagonist (but not an ETBR antagonist) blocked the ET-3-induced increases in both cell number and BrdU incorporation. Additionally, BQ610 suppressed the ET-3-induced increases in phosphorylation of AMPK, c-JUN, and STAT3 proteins, and pretreatment with specific inhibitors of AMPK, JNK/c-JUN, or JAK/STAT3 prevented the ET-3-induced increases in phosphorylation of AMPK, c-JUN, and STAT3, respectively. Neither p38 MAPK inhibitor nor PKC inhibitor altered the effects of ET-3 on cell growth. These data suggest that ET-3 stimulates preadipocyte growth through the ETAR, AMPK, JNK/c-JUN, and STAT3 pathways. Moreover, ET-3 did not alter HIB1B brown preadipocyte and D12 beige preadipocyte growth, suggesting a preadipocyte type-dependent effect. The results of this study may help explain how endothelin mediates fat cell activity and fat cell-associated diseases.

Endothelin and the podocyte

In the past decade, research has advanced our understanding how endothelin contributes to proteinuria and glomerulosclerosis. Data from pre-clinical and clinical studies now provide evidence that proteinuric diseases such as focal segmental glomerulosclerosis and diabetic nephropathy as well as hypertension nephropathy are sensitive to treatment with endothelin receptor antagonists (ERAs). Like blockade of the renin-angiotensin system, ERA treatment-under certain conditions-may even cause disease regression, effects that could be achieved on top of renin-angiotensin-aldosterone system blockade, suggesting independent therapeutic mechanisms by which ERAs convey nephroprotection. Beneficial effects of ERAs on podocyte function, which is essential to maintain the glomerular filtration barrier, have been identified as one of the key mechanisms by which inhibition of the endothelin ETA receptor ameliorates renal structure and function. In this article, we will review pre-clinical studies demonstrating a causal role for endothelin in proteinuric chronic kidney disease (with a particular focus on functional and structural integrity of podocytes in vitro and in vivo). We will also review the evidence suggesting a therapeutic benefit of ERA treatment on the functional integrity of podocytes in humans.

Regulation of endothelin-1 expression and function by nutrient stress in mouse colon epithelia

OBJECTIVE

The endothelin (ET) system is influenced by a variety of stress conditions in many tissues. However, the effects of nutrient stress conditions on ET expression and its function are not well understood in the intestinal tract, while ET-1 gene expression and peptide were found in the intestinal tract. The aim of this study was to investigate the effect of feeding and fasting on the expression of ET-1 and short-circuit current (Isc) induced by ET-1 in mouse colon.

MATERIAL AND METHODS

Mice were fed freely, fasted for 48 h, and re-fed after fasting, respectively. ET-1 mRNA levels and peptide concentrations were analyzed using real-time polymerase chain reaction (PCR) and sandwich ELISA, respectively. Isc of epithelial tissue was measured under short-circuit conditions using a Ussing chamber.

RESULTS

ET-1 mRNA expression and peptide concentrations in epithelial colonic tissue were significantly increased 48 h after fasting, and decreased within 2 h of re-feeding after a 48-h fast. Furthermore, the addition of ET-1 to the serosal but not the mucosal side increased Isc in colonic epithelia. An increase in Isc was caused by chloride ion (Cl(-)) secretion because Isc induced by ET-1 was blocked by bumetanide and Cl(- -) free conditions. In addition, an increase in Isc induced by ET-1 in colon excised from fasted mice was much lower than that obtained from free-fed mice.

CONCLUSIONS

Gene expression, peptide concentration, and the function of ET-1 in mouse colonic epithelia are regulated by nutrient stress.

Reversal of proteinuric renal disease and the emerging role of endothelin

Proteinuria is a major long-term clinical consequence of diabetes and hypertension, conditions that lead to progressive loss of functional renal tissue and, ultimately, end-stage renal disease. Proteinuria is also a strong predictor of cardiovascular events. Convincing preclinical and clinical evidence exists that proteinuria and the underlying glomerulosclerosis are reversible processes. This Review outlines the mechanisms involved in the development of glomerulosclerosis--particularly those responsible for podocyte injury--with an emphasis on the potential capacity of endothelin receptor blockade to reverse this process. There is strong evidence that endothelin-1, a peptide with growth-promoting and vasoconstricting properties, has a central role in the pathogenesis of proteinuria and glomerulosclerosis, which is mediated via activation of the ET(A) receptor. Several antiproteinuric drugs, including angiotensin-converting-enzyme inhibitors, angiotensin receptor antagonists, statins and certain calcium channel blockers, inhibit the formation of endothelin-1. Preclinical studies have demonstrated that endothelin receptor antagonists can reverse proteinuric renal disease and glomerulosclerosis, and preliminary studies in humans with renal disease have shown that these drugs have remarkable antiproteinuric effects that are additive to those of standard antiproteinuric therapy. Additional clinical studies are needed.

POTENTIAL FOR ENDOTHELIN-1-MEDIATED IMPAIRMENT OF CONTRACTILE ACTIVITY IN HYPERTENSION

1. The present study examined the potential for reduced exercise capacity observed in hypertensive patients as a result of elevated levels of endothelin (ET)-1. We have previously reported that ET-1 exerts low-dose stimulatory or high-dose inhibitory effects on the metabolism of the rat isolated perfused hindlimb from its vasoconstrictor activity. 2. Herein, we determined whether there are similar effects on tension development by the rat isolated constant-flow hindlimb during ET-1-mediated vasoconstriction. 3. The dose-dependent vasoconstrictor effects of ET-1 on metabolism in contracting muscle were the same as those observed previously in resting muscle. Highest concentrations of ET-1 gave rise to a transient stimulation followed by a marked inhibition of tension development, consistent with a decrease in aerobic capacity of the muscle. The vasoconstriction due to the higher doses of ET-1 was not dilated by electrical stimulation. 4. In conclusion, the biphasic nature of the actions of ET-1 suggests that although lower concentrations of ET-1 do not affect exercise capacity, higher concentrations that may occur in hypertension are inhibitory to metabolism and aerobic capacity of muscle. The inhibitory effects of ET-1 appear to result from enhanced functional shunting.

Review: Insulin and endothelin: an interplay contributing to hypertension development?

CONTEXT

The aim of this article was to review the existing data on the interactions among insulin, insulin resistance, and endothelin and how those contribute to the development of hypertension in insulin-resistant states.

EVIDENCE ACQUISITION

A literature search of MEDLINE database was performed to identify English-language articles published during the last 20 yr. Search terms used were endothelin, insulin, insulin resistance, and hyperinsulinemia in combination with blood pressure and hypertension. Reference lists of retrieved articles were also evaluated for relevant information.

EVIDENCE SYNTHESIS

Several mechanisms connect insulin resistance and compensatory hyperinsulinemia with blood pressure elevation in the context of the metabolic syndrome, i.e. sodium retention, sympathetic activation, and impairment of endothelial nitric oxide production. Accumulating evidence suggests that activation of the endothelin system seems to be another important, yet less discussed, mechanism. In vitro studies have shown that insulin stimulates both endothelin-1 production and action on the vascular wall. In vivo, high levels of insulin result in increase in circulating endothelin-1 in healthy individuals, and this effect is also seen in insulin-resistant subjects, a relationship not observed with nitric oxide production. Moreover, endothelin receptor antagonism effectively reduces blood pressure in animal models of insulin resistance and hypertension. On the other hand, elevation of endothelin-1 levels can further increase insulin resistance, forming possibly a deleterious circle.

CONCLUSIONS

Endothelin-1 may play a crucial role in the pathogenesis of hypertension in insulin-resistant states. Future research should examine the potential of endothelin receptor antagonism to help blood pressure control in patients with insulin resistance.

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.

Metabolic and vascular actions of endothelin-1 are inhibited by insulin-mediated vasodilation in perfused rat hindlimb muscle

Endothelin-1 (ET-1) is a potent endothelium-derived vasoactive peptide and may be involved in the microvascular actions of insulin for the normal delivery of nutrients to muscle, although higher levels may be antagonistic. Our aim was to observe the interaction between ET-1 and insulin. Initially, we attempted to distinguish the vascular from the metabolic effects of ET-1 in the constant-flow pump-perfused rat hindlimb by using various doses of ET-1 and measuring changes in perfusion pressure (PP), oxygen consumption (VO(2)), glucose uptake (GU) and lactate release (LR). Sodium nitroprusside (SNP) was used to block vasoconstriction and to thus assess the relationship between vascular and metabolic effects. Insulin was included in later experiments to determine the interaction between insulin and ET-1 on the above parameters. ET-1 caused a dose-dependent increase in PP. Effects on VO(2) were biphasic, with low doses increasing VO(2), and higher doses leading to a net inhibition. GU and LR were increased at lower doses (ET-1 < or =1 nM), but this effect was lost at higher doses (> or =10 nM ET-1). SNP (50 microM) fully blocked the increase in pressure and metabolism due to low-dose ET-1 and partly blocked both pressure and metabolic responses by the high dose. ET-1 vasodilatory activity was minimal at high or low dose. Insulin (15 nM) alone caused GU, which was not affected by ET-1. Of the other parameters measured, insulin behaved essentially the same as SNP, inhibiting the pressure and oxygen effects. Overall, these results show that ET-1 has a biphasic dose-dependent vasoconstrictor effect on hindlimb blood vessels, able to modulate flow to cause both the stimulation and inhibition of metabolism, although these effects are blocked by insulin, which is able to vasodilate against both low and high doses of ET-1.

Phosphatidylinositol 4,5-bisphosphate reverses endothelin-1-induced insulin resistance via an actin-dependent mechanism

Phosphatidylinositol (PI) 4,5-bisphosphate (PIP(2)) plays a pivotal role in insulin-stimulated glucose transport as an important precursor to PI 3,4,5-trisphosphate (PIP(3)) and a key regulator of actin polymerization. Since endothelin (ET)-1 impairs insulin sensitivity and PIP(2) is a target of ET-1-induced signaling, we tested whether a change in insulin-stimulated PIP(3) generation and signaling, PIP(2)-regulated actin polymerization, or a combination of both accounted for ET-1-induced insulin resistance. Concomitant with a time-dependent loss of insulin sensitivity, ET-1 caused a parallel reduction in plasma membrane PIP(2). Despite decreased insulin-stimulated PI 3-kinase activity and PIP(3) generation, ET-1 did not diminish downstream signaling to Akt-2. Furthermore, addition of exogenous PIP(2), but not PIP(3), restored insulin-regulated GLUT4 translocation and glucose transport impaired by ET-1. Microscopic and biochemical analyses revealed a PIP(2)-dependent loss of cortical filamentous actin (F-actin) in ET-1-treated cells. Restoration of insulin sensitivity by PIP(2) add-back occurred concomitant with a reestablishment of cortical F-actin. The corrective effect of exogenous PIP(2) in ET-1-induced insulin-resistant cells was not present in cells where cortical F-actin remained experimentally depolymerized. These data suggest that ET-1-induced insulin resistance results from reversible changes in PIP(2)-regulated actin polymerization and not PIP(2)-dependent signaling.

Temporal profiling of the transcriptional basis for the development of corticosteroid-induced insulin resistance in rat muscle

Elevated systemic levels of glucocorticoids are causally related to peripheral insulin resistance. The pharmacological use of synthetic glucocorticoids (corticosteroids) often results in insulin resistance/type II diabetes. Skeletal muscle is responsible for close to 80% of the insulin-induced systemic disposal of glucose and is a major target for glucocorticoid-induced insulin resistance. We used Affymetrix gene chips to profile the dynamic changes in mRNA expression in rat skeletal muscle in response to a single bolus dose of the synthetic glucocorticoid methyl-prednisolone. Temporal expression profiles (analyzed on individual chips) were obtained from tissues of 48 drug-treated animals encompassing 16 time points over 72 h following drug administration along with four vehicle-treated controls. Data mining identified 653 regulated probe sets out of 8799 present on the chip. Of these 653 probe sets we identified 29, which represented 22 gene transcripts, that were associated with the development of insulin resistance. These 29 probe sets were regulated in three fundamental temporal patterns. 16 probe sets coding for 12 different genes had a profile of enhanced expression. 10 probe sets coding for eight different genes showed decreased expression and three probe sets coding for two genes showed biphasic temporal signatures. These transcripts were grouped into four general functional categories: signal transduction, transcription regulation, carbohydrate/fat metabolism, and regulation of blood flow to the muscle. The results demonstrate the polygenic nature of transcriptional changes associated with insulin resistance that can provide a temporal scaffolding for translational and post-translational data as they become available.

The effect of smoking on peripheral insulin sensitivity and plasma endothelin level

OBJECTIVES

The aim of the present study was to investigate the effect of smoking on peripheral insulin effectiveness.

METHODS

Seven healthy volunteers, nonsmokers, of mean age 39.6 +/- 7.1 Years and mean BMI 22.65 +/- 11.98 kg/m2, without family history of diabetes mellitus, with normal blood pressure participated in the study. All the parameters were studied twice - at baseline as well as after smoking (4 cigarettes per one hour). The study was performed in three days: at the first day we studied peripheral insulin effectiveness (M) in vivo by the artificial endocrine pancreas (Biostator), using the euglycaemic hyperinsulinaemic clamp technique, and insulin-receptor binding on circulating mononuclear blood cells; at the second day - the same parameters after one-hour smoking during the third hour of clamping; at the third day - plasma endothelin level, blood pressure and heart rate at baseline and after one-hour smoking.

RESULTS

There was a significant decrease in glucose utilization during the second clamp test, when the volunteers smoked during the third hour as compared to the test at baseline (p=0.04). This was accompanied by a significant decrease in insulin receptor affinity (p=0.04). Systolic blood pressure and heart rate increased significantly after one-hour smoking (p=0.03 and p=0.001, respectively). Plasma endothelin level increased significantly after smoking (from 0.62 +/- 0.15 pg/ml to 2.05 +/- 1.67 pg/ml, p=0.03).

CONCLUSION

Our results demonstrate that smoking decreases peripheral insulin sensitivity reducing insulin receptor affinity. We have confirmed that smoking increases plasma endothelin level, which probably by causing vasoconstriction and consequent tIssue hypoxaemia could decrease peripheral glucose utilization. We consider that smoking could also have a direct effect on insulin receptor affinity, thus leading to decreased peripheral insulin effectiveness.

Endothelin-1 inhibits TNF alpha-induced iNOS expression in 3T3-F442A adipocytes

1. Endothelin-1 (ET-1) and tumor necrosis factor alpha (TNFalpha) by their action on adipocytes have been independently linked to the pathogenesis of insulino-resistance. In isolated adipocytes, TNFalpha induces the expression of the inducible nitric oxide synthase (iNOS). The purpose of the present work was, in the 3T3-F442A adipocyte cell line, to characterise TNFalpha-induced iNOS expression and to determine whether or not ET-1 could influence TNFalpha-induced iNOS expression and NO production. 2. In differentiated 3T3-F442A, treatment with TNFalpha (20 ng ml(-1)) induced the expression of a functional iNOS as demonstrated by nitrite assay, Western blot, reverse transcription-polymerase chain reaction and Northern blot analysis. TNFalpha-induced iNOS expression requires nuclear factor kappaB activation, but does not necessitate the activation of the PI-3 kinase/Akt and P38-MAP kinase pathways. 3. ET-1, but not ET-3, inhibited the TNFalpha-induced expression of iNOS protein and mRNA as well as nitrite production. The effects of ET-1 were blocked by a specific ETA (BQ123, pA(2) 7.4) but not by a specific ETB receptor antagonist (BQ788). 3T3-F442A adipocytes express the mRNAs for prepro-ET-1 and the ET-A receptor subtype, but not for the ET-B subtype. 4. The inhibitory effect of ET-1 was not affected by bisindolylmaleimide, SB 203580 or indomethacin, inhibitors of protein kinase C, p38-MAP kinase and cyclooxygenase, respectively, and was not associated with cAMP production. However, the effect of ET-1 was partially reversed by wortmannin, suggesting the involvement of PI3 kinase in the transduction signal of ET-1. 5. Differentiated 3T3-F442A adipocytes did not release ET-1 with or without exposure to TNFalpha, although the mRNA for preproET-1 was detected in both pre- and differentiated adipocytes. 6. Thus, these results confirm that adipocytes are a target for circulating ET-1 and demonstrate that the activation of the ETA receptor subtype can prevent TNFalpha-induced iNOS expression.

Endothelin-1 inhibits resistin secretion in 3T3-L1 adipocytes

Resistin is an adipocyte-derived hormone whose role in the development of insulin resistance is controversial. Endothelin-1 (ET-1) is a 21 amino acid peptide demonstrated to possess vasoconstrictor, positive inotropic, mitogenic, and metabolic properties. In numerous disease states, including congestive heart failure, obesity, and diabetes, elevated levels of ET-1 have been reported and are thought to contribute to the pathology of the disease. A recent study demonstrated that ET-1 induces the expression and stimulates the secretion of the adipose tissue-derived hormone leptin. However, the effect of ET-1 on resistin secretion has not been determined. To characterize the effect of ET-1 on resistin secretion, 3T3-L1 fibroblasts were differentiated into adipocytes and allowed to mature for 14 days. Cells were incubated for 24h with ET-1 (1-100 nM), insulin (1-100 nM), insulin+ET-1 (100 nM I+E) or the appropriate vehicle or antagonist. At the end of the incubation period, resistin secretion was determined in the media by immunoblotting and densitometric analysis. ET-1 (1-100 nM) significantly decreased basal resistin secretion by 49% (1 nM), 43% (10nM), and 59% (100 nM). Insulin (1-100 nM) produced a concentration-dependent increase in resistin secretion from 3T3-L1 adipocytes (1 nM-42%, 10nM-55%, and 100 nM-86% vs. control). Insulin-stimulated resistin secretion (100 nM) was almost completely inhibited (94%) by ET-1 (100 nM). The effects of ET-1 on resistin protein secretion were inhibited by co-incubation with the ET(A) receptor antagonist BQ-610. In conclusion, our studies demonstrate that basal and hormonal stimulation of resistin secretion by insulin are inhibited by ET-1. Such findings demonstrate that resistin secretion is regulated in a similar manner to other adipose tissue factors, including leptin, in 3T3-L1 adipocytes. In addition, our findings suggest that vascular factors such as ET-1 may regulate whole body energy metabolism through adipocyte-derived hormones, including leptin and resistin.