Lipid-lowering actions of imidazoline antihypertensive agents in metabolic syndrome X

Increased monoamine oxidase activity and imidazoline binding sites in insulin-resistant adipocytes from obese Zucker rats

BACKGROUND

Despite overt insulin resistance, adipocytes of genetically obese Zucker rats accumulate the excess of calorie intake in the form of lipids.

AIM

To investigate whether factors can replace or reinforce insulin lipogenic action by exploring glucose uptake activation by hydrogen peroxide, since it is produced by monoamine oxidase (MAO) and semicarbazide-sensitive amine oxidase (SSAO) in adipocytes.

METHODS

³H-2-deoxyglucose uptake (2-DG) was determined in adipocytes from obese and lean rats in response to insulin or MAO and SSAO substrates such as tyramine and benzylamine. ¹⁴C-tyramine oxidation and binding of imidazolinic radioligands [³H-Idazoxan, ³H-(2-benzofuranyl)-2-imidazoline] were studied in adipocytes, the liver, and muscle. The influence of in vivo administration of tyramine + vanadium on glucose handling was assessed in lean and obese rats.

RESULTS

2-DG uptake and lipogenesis stimulation by insulin were dampened in adipocytes from obese rats, when compared to their lean littermates. Tyramine and benzylamine activation of hexose uptake was vanadate-dependent and was also limited, while MAO was increased and SSAO decreased. These changes were adipocyte-specific and accompanied by a greater number of imidazoline I₂ binding sites in the obese rat, when compared to the lean. In vitro, tyramine precluded the binding to I₂ sites, while in vivo, its administration together with vanadium lowered fasting plasma levels of glucose and triacylglycerols in obese rats.

CONCLUSION

The adipocytes from obese Zucker rats exhibit increased MAO activity and imidazoline binding site number. However, probably as a consequence of SSAO down-regulation, the glucose transport stimulation by tyramine is decreased as much as that of insulin in these insulin-resistant adipocytes. The adipocyte amine oxidases deserve more studies with respect to their putative contribution to the management of glucose and lipid handling.

Imidazoline Receptor System: The Past, the Present, and the Future

Imidazoline receptors historically referred to a family of nonadrenergic binding sites that recognize compounds with an imidazoline moiety, although this has proven to be an oversimplification. For example, none of the proposed endogenous ligands for imidazoline receptors contain an imidazoline moiety but they are diverse in their chemical structure. Three receptor subtypes (I₁, I₂, and I₃) have been proposed and the understanding of each has seen differing progress over the decades. I₁ receptors partially mediate the central hypotensive effects of clonidine-like drugs. Moxonidine and rilmenidine have better therapeutic profiles (fewer side effects) than clonidine as antihypertensive drugs, thought to be due to their higher I₁/α ₂-adrenoceptor selectivity. Newer I₁ receptor agonists such as LNP599 [3-chloro-2-methyl-phenyl)-(4-methyl-4,5-dihydro-3H-pyrrol-2-yl)-amine hydrochloride] have little to no activity on α ₂-adrenoceptors and demonstrate promising therapeutic potential for hypertension and metabolic syndrome. I₂ receptors associate with several distinct proteins, but the identities of these proteins remain elusive. I₂ receptor agonists have demonstrated various centrally mediated effects including antinociception and neuroprotection. A new I₂ receptor agonist, CR4056 [2-phenyl-6-(1H-imidazol-1yl) quinazoline], demonstrated clear analgesic activity in a recently completed phase II clinical trial and holds great promise as a novel I₂ receptor-based first-in-class nonopioid analgesic. The understanding of I₃ receptors is relatively limited. Existing data suggest that I₃ receptors may represent a binding site at the Kir6.2-subtype ATP-sensitive potassium channels in pancreatic β-cells and may be involved in insulin secretion. Despite the elusive nature of their molecular identities, recent progress on drug discovery targeting imidazoline receptors (I₁ and I₂) demonstrates the exciting potential of these compounds to elicit neuroprotection and to treat various disorders such as hypertension, metabolic syndrome, and chronic pain.

Development of insulin resistance in Nischarin mutant female mice

BACKGROUND/OBJECTIVES

NISCH-STAB1 is a newly identified locus correlated to human waist-hip ratio (WHR), which is a risk indicator of developing obesity-associated diabetes. Our previous studies have shown that Nisch mutant male mice increased glucose tolerance in chow-fed conditions. Thus we hypothesized that Nisch mutant mice will have changes in insulin resistance, adipocytes, hepatic steatosis when mice are fed with high-fat diet (HFD).

METHODS

Insulin resistance was assessed in Nisch mutant mice and WT mice fed with high-fat diet (60% by kCal) or chow diet. Whole-body energy metabolism was examined using an indirect calorimeter. Adipose depots including inguinal white adipose tissue (WAT), perigonadal WAT, retroperitoneal WAT, and mesenteric WAT were extracted. Area and eqdiameter of each adipocyte were determined, and insulin signaling was examined as well. Paired samples of subcutaneous and omental visceral adipose tissue were obtained from 400 individuals (267 women, 133 men), and examined the expression of Nischarin.

RESULTS

We found that insulin signaling was impaired in major insulin-sensitive tissues of Nisch mutant female mice. When mice were fed with HFD for 15 weeks, the Nisch mutant female mice not only developed severe insulin resistance and decreased glucose tolerance compared with wild-type control mice, but also accumulated more white fat, had larger adipocytes and developed severe hepatic steatosis than wild-type control mice. To link our animal studies to human diseases, we further analyzed Nischarin expression in the paired human samples of visceral and subcutaneous adipose tissue from Caucasians. In humans, we found that Nischarin expression is attenuated in adipose tissue with obesity. More importantly, we found that Nischarin mRNA inversely correlated with parameters of obesity, fat distribution, lipid and glucose metabolism.

CONCLUSIONS

Taken together, our data revealed sexual dimorphism of Nischarin in body fat distribution, insulin resistance, and glucose tolerance in mice.

Beneficial effect of coenzyme Q10 on increased oxidative and nitrative stress and inflammation and individual metabolic components developing in a rat model of metabolic syndrome

Metabolic syndrome (MetS) is a group of cardiovascular risk factors, including visceral obesity, glucose intolerance, hypertension, and dyslipidemia. Increased oxidative and nitrative stress and inflammation and decreased endothelial function occur in an animal model of metabolic syndrome, SHR/NDmcr-cp (SHR/cp) rats. The present study investigated the effects of coenzyme Q10 (CoQ10), one of the important antioxidants, on the abnormal oxidative condition and characteristic components of metabolic syndrome in SHR/cp rats by maintaining them on a diet supplemented with 0.07% - 0.7% CoQ10 for 26 weeks. We determined serum levels of oxidatively modified low-density lipoprotein (Ox-LDL) and 8-hydroxy-2'-deoxyguanosine (8-OHdG) as oxidative stress markers, 3-nitrotyrosine as a nitrative stress marker, 3-chlorotyrosine as a marker of myeloperoxidase (MPO)-catalyzed oxidation and high-sensitivity C-reactive protein (hsCRP) as an inflammatory marker. The administration of CoQ10 significantly attenuated the increase of oxidative and nitrative stress markers and inflammatory markers in a dose-dependent manner. CoQ10 prevented the elevated serum insulin levels, although it did not affect the elevated glucose level and dyslipidemia. CoQ10 also reduced elevated blood pressure, but did not affect body weight gain. In addition, CoQ10 improved endothelial dysfunction in the mesenteric arteries. These findings suggest that the antioxidant properties of CoQ10 can be effective for ameliorating cardiovascular risk in MetS.

Peroxynitrite is Involved in the dysfunction of vasorelaxation in SHR/NDmcr-cp rats, spontaneously hypertensive obese rats

SHR/NDmcr-cp (SHR-cp) rats display typical symptoms and features of the metabolic syndrome. We previously reported that endothelium-dependent relaxation decreases in the thoracic aortas of SHR-cp rats, despite increased nitric oxide (NO) production from the endothelium. In the present study, to search for the reasons for this contradiction, we investigated whether vascular abnormality could be reduced by treatment of SHR-cp rats with antihypertensive drugs; a calcium channel blocker (amlodipine), an alpha 2 and imidazoline receptor agonist (moxonidine), and an angiotensin II type 1 (AT1) receptor antagonist (telmisartan). Telmisartan but not amlodipine and moxonidine ameliorated the impairment of relaxation in response to acetylcholine and the increased protein expression of endothelium NO synthase in thoracic aortas. All three drugs significantly lowered the blood pressure. Telmisartan decreased the serum levels of lipid peroxide and 8-hydroxy-2'-deoxyguanosine, oxidative stress markers, and also the aortic levels of the protein expression of gp91, a component of NADPH oxidase, and 3-nitrotyrosine, a biomarker of peroxynitrite. These findings suggest that NADPH oxidase-derived superoxide, probably produced due to stimulation of AT1 receptors, reacts with NO to form peroxynitrite, and consequently decreases active NO, leading to attenuation of endothelium-dependent relaxation. Angiotensin receptor antagonists may be effective for preventing endothelial dysfunction in metabolic syndrome.

Therapeutic actions of allylmercaptocaptopril and captopril in a rat model of metabolic syndrome

BACKGROUND

Hypertension often coexists with hyperlipidemia, insulin resistance, and glucose intolerance in metabolic syndrome. Allylmercaptocaptopril is a conjugate of the angiotensin-converting enzyme inhibitor captopril with allicin, an active principle in garlic with multiple beneficial actions on metabolic-syndrome abnormalities. We sought to test the hypothesis that the conjugation of allicin to captopril may confer additional therapeutic actions in metabolic disease.

METHODS

We compared allylmercaptocaptopril (53.5 mg/kg/day orally for 60 days) to an equimolar dose of captopril (40 mg/kg/day) in the spontaneously hypertensive, obese rat (SHROB) model.

RESULTS

Allylmercaptocaptopril prevented progressive weight gain, without a detectable effect on food intake. Both captopril and allylmercaptocaptopril lowered blood pressure, but allylmercaptocaptopril was more effective. Allylmercaptocaptopril, but not captopril, improved cardiac hypertrophy, as indicated by heart weight and ventricular-wall thickness. Allylmercaptocaptopril improved, whereas captopril impaired, oral glucose tolerance after a fast. Triglycerides were decreased by both captopril and allylmercaptocaptopril. Total cholesterol and non-HDL cholesterol were reduced by captopril but not by allylmercaptocaptopril. The SHROB rats developed severe glomerulosclerosis and renal failure. Allylmercaptocaptopril showed significant nephro-protection, as indicated by reductions in urinary protein loss, urinary protein-to-creatinine ratio, and plasma creatinine. Captopril showed the same trends and also prevented the decline of creatinine clearance. Finally, both allylmercaptocaptopril and captopril reduced the basal level of lipolysis in isolated abdominal adipocytes, and restored the response to catecholamine stimulation.

CONCLUSIONS

Both captopril and allylmercaptocaptopril are effective in attenuating multiple abnormalities of metabolic syndrome. Allylmercaptocaptopril may have additional effectiveness on improving glucose tolerance, further lowering blood pressure, reducing cardiac hypertrophy, preventing weight gain, and protecting against renal disease.

Marked Insulin Resistance in Obese Spontaneously Hypertensive Rat Adipocytes Is Ameliorated by in Vivo but Not in Vitro Treatment with Moxonidine

The obese spontaneously hypertensive rat (SHROB) is a model of marked insulin resistance with normoglycemia. We sought to determine whether insulin resistance extends to adipocytes and the impact of an insulin-sensitizing imidazoline, moxonidine (4 mg/kg/days for 21 days). Gonadal adipocytes were isolated from SHROB and lean spontaneously hypertensive rat (SHR) littermates. In lean SHR adipocytes, Akt activation by 100 nM insulin peaked at 3 min at 25-fold, whereas SHROB adipocytes showed only 4-fold activation. In dose-response experiments, the maximal response (E(max)) was markedly reduced 18.8 +/- 2.3 versus 3.7 +/- 0.8. Insulin sensitivity was also attenuated, with higher concentrations required for responses (EC(50) = 3.5 +/- 0.5 versus 29 +/- 3.8 nM). Glucose uptake as determined with [(3)H]2-deoxyglucose was also less responsive to insulin in SHROB relative to lean SHR. Moxonidine had little or no effect when applied acutely in vitro, but adipocytes isolated from SHROB treated with moxonidine in vivo showed significantly improved responses to insulin, both in terms of Akt activation and facilitation of glucose uptake. Chronic but not acute moxonidine treatment partially restores insulin sensitivity in SHROB adipocytes, suggesting an indirect action of this agent.

The imidazoline I2-site ligands BU 224 and 2-BFI inhibit MAO-A and MAO-B activities, hydrogen peroxide production, and lipolysis in rodent and human adipocytes

Numerous imidazolinic agents exhibit antihyperglycaemic properties and have been described to promote insulin secretion, however their effects on adipose tissue development have been poorly investigated. Since white adipose tissue (WAT) plays an important role in glucose homeostasis and expresses imidazoline (I(2)) binding sites abundantly, this work aimed at studying extrapancreatic actions of two I(2)-site ligands, BU 224 and 2-BFI in adipocytes. Interaction with monoamine oxidase (MAO) was investigated by measuring the ability to modulate [(14)C]tyramine oxidation and hydrogen peroxide production. Direct influence on glucose uptake or on lipolytic activity was tested on mouse, rat, rabbit and human adipocytes. BU 224 and 2-BFI behaved as reversible inhibitors of both MAO-A and -B, as demonstrated by total inhibition of tyramine oxidation in human adipocytes and platelets or in liver from rats previously treated with selective MAO-inhibitors. Moreover, they weakly inhibited semicarbazide-sensitive amine oxidase. Like classical MAO-inhibitors, they were unable to produce hydrogen peroxide and to activate glucose uptake but prevented tyramine to do so in rodent or human adipocytes. BU 224 and 2-BFI also differed from MAO-inhibitors since they inhibited lipolysis at millimolar concentrations via a still undefined pathway independent of alpha(2)-adrenoceptor stimulation, beta-adrenergic antagonism and MAO activation. However, chronic treatment of obese Zucker rats with 2-BFI did not modify the maximal lipolytic capacity or the mild insulin resistance status of their adipocytes. Taken together, our observations demonstrate on WAT novel effects of BU 224 and 2-BFI different from their already reported actions on brain or endocrine pancreas.

Metabolic effects of antihypertensive agents: role of sympathoadrenal and renin-angiotensin systems

Reports of beneficial, neutral and adverse impacts of antihypertensive drug classes on glucose and lipid metabolism can be found in human data. Furthermore, mechanisms for these diverse effects are often speculative and controversial. Clinical trial data on the metabolic effects of antihypertensive agents are highly contradictory. Comparisons of clinical trials involving different agents are complicated by differences in the spectrum of metabolic disturbances that accompany hypertension in different groups of patients. Two physiological systems are predominant at the interface between metabolic and cardiovascular regulation: the sympathetic nervous system (SNS) and the renin-angiotensin system (RAS). These two systems are major targets of antihypertensive drug actions, and also mediate many of the beneficial and adverse effects of antihypertensive agents on glucose and lipid metabolism. Thiazides and beta-adrenergic antagonists can adversely affect glucose and lipid metabolism, which are frequently compromised in human essential hypertension, and increase the incidence of new cases of diabetes. Laboratory studies confirm these effects, and suggest that compensatory activation of the SNS and RAS may be one mechanism. Other antihypertensives directly targeting the SNS and RAS may have beneficial effects on glucose and lipid metabolism, and may prevent diabetes. Resolution of the controversies surrounding the metabolic effects of antihypertensive agents can only be resolved by further laboratory studies, in addition to controlled clinical trials.