The effect of exenatide (a GLP-1 analogue) and sitagliptin (a DPP-4 inhibitor) on asymmetric dimethylarginine (ADMA) metabolism and selected biomarkers of cardiac fibrosis in rats with fructose-induced metabolic syndrome

Asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide (NO) synthesis, is a risk factor for endothelial dysfunction, a common pathophysiological denominator for both atherogenesis and cardiac fibrosis. We aimed to investigate whether the cardioprotective and antifibrotic effects of incretin drugs, exenatide and sitagliptin, may be associated with their ability to affect circulating and cardiac ADMA metabolism. Normal and fructose-fed rats were treated with sitagliptin (5.0/10 mg/kg) or exenatide (5/10 µg/kg) for 4 weeks. The following methods were used: LC-MS/MS, ELISA, Real-Time-PCR, colorimetry, IHC and H&E staining, PCA and OPLS-DA projections. Eight-week fructose feeding resulted in an increase in plasma ADMA and a decrease in NO concentration. Exenatide administration into fructose-fed rats reduced the plasma ADMA level and increased NO level. In the heart of these animals exenatide administration increased NO and PRMT1 level, reduced TGF-ß1, α-SMA levels and COL1A1 expression. In the exenatide treated rats renal DDAH activity positively correlated with plasma NO level and negatively with plasma ADMA level and cardiac α-SMA concentration. Sitagliptin treatment of fructose-fed rats increased plasma NO concentration, reduced circulating SDMA level, increased renal DDAH activity and reduced myocardial DDAH activity. Both drugs attenuated the myocardial immunoexpression of Smad2/3/P and perivascular fibrosis. In the metabolic syndrome condition both sitagliptin and exenatide positively modulated cardiac fibrotic remodeling and circulating level of endogenous NOS inhibitors but had no effects on ADMA levels in the myocardium.

Hydrogen sulfide in the experimental models of arterial hypertension

Hydrogen sulfide (H₂S) is the third member of gasotransmitter family together with nitric oxide and carbon monoxide. H₂S is involved in the regulation of blood pressure by controlling vascular tone, sympathetic nervous system activity and renal sodium excretion. Moderate age-dependent hypertension and endothelial dysfunction develop in mice with knockout of cystathionine γ-lyase (CSE), the enzyme involved in H₂S production in the cardiovascular system. Decreased H₂S concentration as well as the expression and activities of H₂S-producing enzymes have been observed in most commonly used animal models of hypertension such as spontaneously hypertensive rats, Dahl salt-sensitive rats, chronic administration of NO synthase inhibitors, angiotensin II infusion and two-kidney-one-clip hypertension, the model of renovascular hypertension. Administration of H₂S donors decreases blood pressure in these models but has no major effects on blood pressure in normotensive animals. H₂S donors not only reduce blood pressure but also end-organ injury such as vascular and myocardial hypertrophy and remodeling, hypertension-associated kidney injury or erectile dysfunction. H₂S level and signaling are modulated by some antihypertensive medications as well as natural products with antihypertensive activity such as garlic polysulfides or plant-derived isothiocyanates as well as non-pharmacological interventions. Modifying H₂S signaling is the potential novel therapeutic approach for the management of hypertension, however, more experimental clinical studies about the role of H₂S in hypertension are required.

The effect of exenatide (a GLP-1 analog) and sitagliptin (a DPP-4 inhibitor) on plasma platelet-activating factor acetylhydrolase (PAF-AH) activity and concentration in normal and fructose-fed rats

Inflammation and oxidative stress are the two processes crucial in atherogenesis. Platelet-activating factor acetylhydrolase (PAF-AH), a plasma lipoprotein-associated enzyme, degrades pro-inflammatory lipids generated within oxidatively modified lipoproteins. Extensive evidence shows that incretin-based drugs, a new class of anti-diabetic agents, can provide cardiovascular protection that cannot be attributed to their glucose-lowering effects. The present study was undertaken to determine whether the antiatherogenic effects of the GLP-1(glucagon-like peptide-1) receptor agonist (exenatide) and DPP-4(dipeptidyl peptidase-4) inhibitors (sitagliptin) may occur via the regulation of platelet-activating factor acetylhydrolase (PAF-AH) activity/mass and inhibition of low-density lipoprotein (LDL) oxidation in the fructose-fed rats. Normal and fructose-fed rats (8 wk) were treated (4 wk) with sitagliptin (5 and 10 mg/kg p.o.) or with exenatide (5 and 10 µg/kg, s.c.). Plasma PAF-AH activity and phosphatidylcholine (PC) concentration were measured colorimetrically. Plasma PAF-AH concentration, oxidized LDL (oxLDL), hexanoyl-Lys adduct (HEL), lyso-PC, apolipoprotein A-I (apoA-I), apoB, platelet-activating factor (PAF), monocyte chemoattractant protein-1 (MCP-1) and endothelin-1 (ET-1) were measured by ELISA. The four-week exenatide (5 µg/kg, sc.) treatment of fructose fed-rats significantly increased plasma PAF-AH activity (+33%, P < 0.001) and decreased the level of circulating oxLDL (-42%, P < 0.05) and MCP-1 (-23%, P < 0.01). These changes were accompanied by the decrease in plasma PC/lyso-PC (-47%, P < 0.001) and apoB/apoA-I ratio (-75%, P < 0.001). The effect of exenatide on enzyme activity was associated with only a minor effect on metabolic parameters and was independent of weight reduction. Exenatide but not sitagliptin inhibits oxidative modification of LDL probably due to favorable effect on plasma PAF-AH activity.

Synthesis, Metabolism, and Signaling Mechanisms of Hydrogen Sulfide: An Overview

In addition to nitric oxide (NO) and carbon monoxide (CO), hydrogen sulfide (H₂S) has recently emerged as the novel gasotransmitter involved in the regulation of the nervous system, cardiovascular functions, inflammatory response, gastrointestinal system, and renal function. H₂S is synthesized from L-cysteine and/or L-homocysteine by cystathionine β-synthase, cystathionine γ-lyase, and cysteine aminotransferase together with 3-mercaptopyruvate sulfurtransferase. In addition, H₂S is enzymatically metabolized in mitochondria by sulfide:quinone oxidoreductase, persulfide dioxygenase, and sulfite oxidase to thiosulfate, sulfite, and sulfate which enables to regulate its level by factors such as oxygen pressure, mitochondria density, or efficacy of mitochondrial electron transport. H₂S modifies protein structure and function through the so-called sulfuration or persulfidation, that is, conversion of cysteine thiol (-SH) to persulfide (-SSH) groups. This, as well as other signaling mechanisms, is partially mediated by more oxidized H₂S-derived species, polysulfides (H₂S_n). In addition, H₂S is able to react with reactive oxygen and nitrogen species to form other signaling molecules such as thionitrous acid (HSNO), nitrosopersulfide (SSNO^-), and nitroxyl (HNO). All H₂S-synthesizing enzymes are expressed in the vascular wall, and H₂S has been demonstrated to regulate vascular tone, endothelial barrier permeability, angiogenesis, vascular smooth muscle cell proliferation and apoptosis, and inflammatory reaction. H₂S-modifying therapies are promising approach for diseases such as arterial hypertension, diabetic angiopathy, and atherosclerosis.

Metabolic Effects of Metformin in the Failing Heart

Accumulating evidence shows that metformin is an insulin-sensitizing antidiabetic drug widely used in the treatment of type 2 diabetes mellitus (T2DM), which can exert favorable effects on cardiovascular risk and may be safely used in patients with heart failure (HF), and even able to reduce the incidence of HF and to reduce HF mortality. In failing hearts, metformin improves myocardial energy metabolic status through the activation of AMP (adenosine monophosphate)-activated protein kinase (AMPK) and the regulation of lipid and glucose metabolism. By increasing nitric oxide (NO) bioavailability, limiting interstitial fibrosis, reducing the deposition of advanced glycation end-products (AGEs), and inhibiting myocardial cell apoptosis metformin reduces cardiac remodeling and hypertrophy, and thereby preserves left ventricular systolic and diastolic functions. While a lot of preclinical and clinical studies showed the cardiovascular safety of metformin therapy in diabetic patients and HF, to confirm observed benefits, the specific large-scale trials configured for HF development in diabetic patients as a primary endpoints are necessary.

Serum paraoxonase 1 activity and protein N-homocysteinylation in primary human endometrial cancer

Paraoxonase 1 plays an important role in protection from oxidative stress and also decomposes homocysteine thiolactone, the toxic metabolite of homocysteine. A limited number of reports evaluated the role of paraoxonase 1 in women affected by female genital tract neoplasms, including endometrial cancer. This study aimed to analyze the paraoxonase activity in the group of endometrial cancer patients (n = 48) who underwent primary surgery and to compare the data available with a well-matched control group (n = 30). Due to the role of paraoxonase 1 in the metabolism of homocysteine (Hcy) thiolactone, the amount of Hcy-thiolactone as well as total serum Hcy concentrations was also measured. Serum paraoxonase 1 activity toward synthetic substrates, paraoxon and phenyl acetate, in the study group was significantly lower compared to the control one. The mean paraoxonase 1 activity toward homocysteine thiolactone tended to be lower in the endometrial cancer group but this difference was not significant. There was no relationship between endometrial cancer and Q192R polymorphism of PON1 assessed by the dual substrate method. No differences in paraoxonase 1 activity between endometrial cancer subgroups according to clinico-pathological features were detected. Total serum homocysteine and protein-bound homocysteine thiolactone did not differ between control and cancer groups. In conclusion, reduced paraoxonase 1 activity suggests diminished important antioxidant mechanisms during the development of primary endometrial cancers in humans. PON1 Q192R polymorphism is not associated with the risk of endometrial cancer. Despite lower paraoxonase 1 activity, homocysteine concentration, and protein N-homocysteinylation in endometrial cancers do not differ from matched controls.

Progranulin, a New Adipokine at the Crossroads of Metabolic Syndrome, Diabetes, Dyslipidemia and Hypertension

BACKGROUND

Progranulin is a multifunctional regulatory protein with growth-promoting, neuroprotective and antiinflammatory activities. Recent studies indicate that progranulin is one of the adipose tissue hormones (adipokines). Progranulin expression in visceral adipose tissue and circulating progranulin concentration are increased in obesity and hyperprogranulinemia is involved in the pathogenesis of obesity-associated insulin resistance. Progranulin impairs insulin signaling and reduces insulin-induced glucose uptake both in vitro and in vivo whereas progranulin deficiency protects from high fat diet-induced insulin resistance. Several studies, including some prospective ones, have demonstrated the association between high progranulin and type 2 diabetes and its complications such as nephro- and retinopathy as well as non-alcoholic fatty liver disease. It is quite well established that progranulin contributes to insulin resistance and resulting deterioration of carbohydrate metabolism. In addition, progranulin may be associated with the development of diabetic microangiopathy, fatty liver disease and possibly with the increased risk of cancer in subjects with the metabolic syndrome. On the other hand, progranulin augments vasorelaxation, inhibits inflammatory reaction, is neuroprotective and reduces ischemiareperfusion injury.

CONCLUSION

Progranulin has both detrimental and beneficial effects. More clinical studies including prospective ones are needed to clarify the role of progranulin in obesity-associated pathologies such as diabetes, hyperlipidemia, hypertension and atherosclerosis.

Processes of plasma protein N-homocysteinylation in multiple sclerosis

BACKGROUND

Homocysteine thiolactone (HTL) is a cyclic thioester of homocysteine (Hcy) contributing to the toxicity of this amino acid. HTL spontaneously reacts with protein lysine residues leading to altered properties of target proteins and induction of immune response. HTL is hydrolyzed to Hcy by plasma enzyme, paraoxonase 1 (PON1). Although both Hcy and PON1 may be involved in the pathogenesis of multiple sclerosis (MS), protein modification by HTL in this disease has not been studied so far. Purpose/Aim: The aim of this study was to assess the level of Hcy, HTL and autoantibodies against N-homocysteinylated proteins as well as PON1 activity in patients with MS.

METHODS

The studies were performed in 61 MS patients with relapsing-remitting (RR group, n = 25) and secondary-progressive type of MS (SP group, n = 36), and in healthy people (C - control group, n = 44).

RESULTS

Homocysteine level was significantly higher in MS patients comparing to control group (C vs. RR p < 0.01; C vs. SP p < 0.05). The level of HTL tended to be higher in RR-MS in comparison to control group, but it did not reach the level of significance. The level of antibodies against N-homocysteinylated proteins did not differ significantly between studied groups. PON1 activity was significantly lower in SP type of MS (SP vs. C p < 0.05; SP vs. RR p < 0.05).

CONCLUSIONS

Although plasma Hcy concentration is higher in MS patients and PON1 activity is reduced in the SP form, MS is associated with minor or no changes in protein-attached HTL and anti-homocysteinylated protein immune response.

Hydrogen-sulfide-mediated vasodilatory effect of nucleoside 5'-monophosphorothioates in perivascular adipose tissue

Hydrogen sulfide (H2S) is synthesized in perivascular adipose tissue (PVAT) and induces vasorelaxation. We examined whether the sulfur-containing AMP and GMP analogs AMPS and GMPS can serve as the H2S donors in PVAT. H2S production by isolated rat periaortic adipose tissue (PAT) was measured with a polarographic sensor. In addition, phenylephrine-induced contractility of aortic rings with (+) or without (-) PAT was examined. Isolated PAT produced H2S from AMPS or GMPS in the presence of the P2X7 receptor agonist BzATP. Phenylephrine-induced contractility of PAT(+) rings was lower than of PAT(-) rings. AMPS or GMPS had no effect on the contractility of PAT(-) rings, but used together with BzATP reduced the contractility of PAT(+) rings when endogenous H2S production was inhibited with propargylglycine. A high-fat diet reduced endogenous H2S production by PAT. Interestingly, AMPS and GMPS were converted to H2S by PAT of obese rats, and reduced contractility of PAT(+) aortic rings isolated from these animals even in the absence of BzATP. We conclude that (i) AMPS and GMPS can be hydrolyzed to H2S by PAT when P2X7 receptors are activated, (ii) a high-fat diet impairs endogenous H2S production by PAT, (iii) AMPS and GMPS restore the anticontractile effects of PAT in obese animals without P2X7 stimulation.

Nucleoside monophosphorothioates as the new hydrogen sulfide precursors with unique properties

Hydrogen sulfide (H2S) is the gasotransmitter enzymatically synthesized in mammalian tissues from l-cysteine. H2S donors are considered as the potential drugs for the treatment of cardiovascular, neurological and inflammatory diseases. Recently, it has been demonstrated that synthetic nucleotide analogs, adenosine- and guanosine 5'-monophosphorothioates (AMPS and GMPS) can be converted to H2S and AMP or GMP, respectively, by purified histidine triad nucleotide-binding (Hint) proteins. We examined if AMPS and GMPS can be used as the H2S donors in intact biological systems. H2S production by isolated rat kidney glomeruli was measured by the specific polarographic sensor. H2S production was detected when glomeruli were incubated with AMPS or GMPS and ionotropic purinergic P2X7 receptor/channel agonist, BzATP. More H2S was generated from GMPS than from equimolar amount of AMPS. Nucleoside phosphorothioates together with BzATP relaxed angiotensin II-preconstricted glomeruli. In addition, infusion of AMPS or GMPS together with BzATP into the renal artery increased filtration fraction and glomerular filtration rate but had no effect on renal vascular resistance or renal blood flow. AMPS but not GMPS was converted to adenosine by isolated glomeruli, however, adenosine was not involved in AMPS-induced H2S synthesis because neither adenosine nor specific adenosine receptor agonists had any effect on H2S production. AMPS, but not GMPS, increased phosphorylation level of AMP-stimulated protein kinase (AMPK), but AMPK inhibitor, compound C, had no effect on AMPS-induced H2S production. In conclusion, nucleoside phosphorothioates are converted to H2S which relaxes isolated kidney glomeruli in vitro and increases glomerular filtration rate in vivo. AMPS and GMPS can be used as the H2S donors in experimental studies and possibly also as the H2S-releasing drugs.

Endogenous hydrogen sulfide in perivascular adipose tissue: role in the regulation of vascular tone in physiology and pathology

Hydrogen sulfide (H2S) is synthesized from L-cysteine by cystathionine β-synthase (CBS) or cystathionine γ-lyase (CSE), and is enzymatically metabolized in mitochondria by sulfide:quinone oxidoreductase (SQR). Recent studies have indicated that H2S is synthesized by CSE in perivascular adipose tissue (PVAT), and is responsible for the anticontractile effect of PVAT on adjacent vessels. The lipophilic statin atorvastatin increases PVAT-derived H2S by suppressing its mitochondrial oxidation; the effect that results from statin-induced depletion of ubiquinone. Experimental obesity induced by a highly palatable diet has a time-dependent effect on H2S in PVAT. Adipose tissue hypoxia suppresses H2S oxidation and increases its level in short-term obesity not associated with insulin resistance. In contrast, in long-term obesity, insulin resistance and (or) hyperinsulinemia result in the down-regulation of CSE and H2S deficiency, which is corrected by treatment with the insulin sensitizer rosiglitazone. In addition, cannabinoid CB1 receptor agonist administered for 2 weeks increases H2S by impairing mitochondria biogenesis. This indicates that the rate of mitochondrial H2S oxidation plays an important role in the regulation of H2S level in PVAT. Up-regulation of H2S signaling in short-term obesity and (or) by elevated endocannabinoids may be a compensatory mechanism that maintains vascular tone, despite endothelial dysfunction.

Thiazolidinedione-induced fluid retention: recent insights into the molecular mechanisms

Peroxisome proliferator-activated receptor- γ (PPAR γ ) agonists such as rosiglitazone and pioglitazone are used to improve insulin sensitivity in patients with diabetes mellitus. However, thiazolidinediones induce fluid retention, edema, and sometimes precipitate or exacerbate heart failure in a subset of patients. The mechanism through which thiazolidinediones induce fluid retention is controversial. Most studies suggest that this effect results from the increase in tubular sodium and water reabsorption in the kidney, but the role of specific nephron segments and sodium carriers involved is less clear. Some studies suggested that PPAR γ agonist stimulates Na(+) reabsorption in the collecting duct by activating epithelial Na(+) channel (ENaC), either directly or through serum and glucocorticoid-regulated kinase-1 (SGK-1). However, other studies did not confirm this mechanism and even report the suppression of ENaC. Alternative mechanisms in the collecting duct include stimulation of non-ENaC sodium channel or inhibition of chloride secretion to the tubular lumen. In addition, thiazolidinediones may augment sodium reabsorption in the proximal tubule by stimulating the expression and activity of apical Na(+)/H(+) exchanger-3 and basolateral Na(+)-HCO3 (-) cotransporter as well as of Na(+),K(+)-ATPase. These effects are mediated by PPAR γ -induced nongenomic transactivation of the epidermal growth factor receptor and downstream extracellular signal-regulated kinases (ERK).

Leptin and the regulation of endothelial function in physiological and pathological conditions

Obesity and the accompanying metabolic syndrome are among the most important causes of cardiovascular pathologies associated with endothelial dysfunction, such as arterial hypertension and atherosclerosis. This detrimental effect of obesity is mediated, in part, by excessive production of the adipose tissue hormone leptin. Under physiological conditions leptin induces endothelium-dependent vasorelaxation by stimulating nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF). Leptin activates endothelial NO synthase (eNOS) through a mechanism involving AMP-activated protein kinase (AMPK) and protein kinase B/Akt, which phosphorylates eNOS at Ser(1177) , increasing its activity. Under pathological conditions, such as obesity and metabolic syndrome, the NO-mediated vasodilatory effect of leptin is impaired. Resistance to the acute NO-mimetic effect of leptin is accounted for by chronic hyperleptinaemia and may result from different mechanisms, such as downregulation of leptin receptors, increased levels of circulating C-reactive protein, oxidative stress and overexpression of suppressor of cytokine signalling-3. In short-lasting obesity, impaired leptin-induced NO production is compensated by EDHF; however, in advanced metabolic syndrome, the contribution of EDHF to the haemodynamic effect of leptin becomes inefficient. Resistance to the vasodilatory effects of leptin may contribute to the development of arterial hypertension owing to unopposed stimulation of the sympathetic nervous system by this hormone.

Modulation of h(2)s metabolism by statins: a new aspect of cardiovascular pharmacology

SIGNIFICANCE

Statins (3-hydroxy-3-methylglutarylcoenzyme A reductase inhibitors) are commonly used in the treatment of cardiovascular diseases. Statins reduce plasma low-density lipoproteins, inhibit inflammatory reaction, improve endothelial function, ameliorate oxidative stress, and reduce platelet activity. Consequently, statins markedly decrease the risk of acute cardiovascular events. H(2)S is synthesized in all layers of the vascular wall, including the endothelium, smooth muscle cells, and perivascular adipose tissue (PVAT).

RECENT ADVANCES

Recent studies demonstrate that PVAT-derived H(2)S decreases vascular tone by activating K(ATP) and/or KCNQ potassium channels in smooth muscle cells. Lipophilic atorvastatin, but not hydrophilic pravastatin, increases net H(2)S production in PVAT by inhibiting its mitochondrial oxidation, and augments the anticontractile effect of PVAT. Inhibition of H(2)S metabolism results from atorvastatin-induced decrease in coenzyme Q, which is a cofactor of H(2)S oxidation by sulfide:quinone oxidoreductase. In contrast to H(2)S, statins do not impair mitochondrial oxidation of organic substrates.

CRITICAL ISSUES

Taking into account antiatherosclerotic and anti-inflammatory effect of H(2)S, the gas may mediate some of the beneficial effects of statins on the cardiovascular system. In addition, specific statins differ in their ability to enhance H(2)S signaling.

FUTURE DIRECTIONS

Since both statins and H(2)S reduce ischemia-reperfusion injury, the possible effect of statins on H(2)S oxidation in other tissues such as the heart and the kidney needs to be examined. Inhibition of H(2)S metabolism may be a new therapeutic strategy to improve H(2)S signaling, especially in the mitochondrial compartment.

Opposite effects of pravastatin and atorvastatin on insulin sensitivity in the rat: role of vitamin D metabolites

OBJECTIVE

Recent studies indicate that pravastatin improves whereas other statins impair glucose homeostasis in humans, but the underlying mechanisms are not clear. We examined the effect of pravastatin and atorvastatin on insulin sensitivity in a rat model.

METHODS

Pravastatin (40 mg/kg/day) or atorvastatin (20mg/kg/day) were administered for 3 weeks and insulin sensitivity was assessed by measuring fasting plasma insulin, HOMA-IR, non-esterified fatty acids (NEFA) and glycerol levels, as well as by the hyperinsulinemic euglycemic clamp.

RESULTS

Pravastatin had no effect on fasting insulin and HOMA-IR but significantly reduced plasma NEFA and glycerol levels and increased glucose infusion rate (GIR) during the hyperinsulinemic clamp. Increase in GIR induced by pravastatin was not abolished by NO synthase inhibitor, l-NAME, indicating that this effect did not result from the improvement of endothelial function. Atorvastatin increased fasting insulin, HOM-IR, NEFA and glycerol levels as well as reduced GIR. Statins had no effect on leptin, HMW adiponectin, resistin, visfatin, interleukin-6 and TNF-α. Pravastatin increased plasma concentrations of 25-hydroxy- and 1,25-dyhydroxyvitamin D(3) (25-OH-D(3) and 1,25-(OH)(2)-D(3)), and its effect on insulin sensitivity was mimicked by exogenous 1,25-(OH)(2)-D(3). Atorvastatin reduced plasma 25-OH-D(3) but had no effect on 1,25-(OH)(2)-D(3). Decrease in insulin sensitivity induced by atorvastatin was not corrected by supplementation of vitamin D(3) despite normalization of plasma 25-OH-D(3) level.

CONCLUSIONS

Pravastatin and atorvastatin have opposite effects on insulin sensitivity and vitamin D(3) status. Pravastatin-induced increase in insulin sensitivity is mediated by elevation of 1,25-(OH)(2)-D(3). In contrast, atorvastatin-induced decrease in insulin sensitivity is independent of lowering 25-OH-D(3).

Leptin and the Regulation of Renal Sodium Handling and Renal Na-Transporting ATPases: Role in the Pathogenesis of Arterial Hypertension

Leptin, an adipose tissue hormone which regulates food intake, is also involved in the pathogenesis of arterial hypertension. Plasma leptin concentration is increased in obese individuals. Chronic leptin administration or transgenic overexpression increases blood pressure in experimental animals, and some studies indicate that plasma leptin is elevated in hypertensive subjects independently of body weight. Leptin has a dose- and time-dependent effect on urinary sodium excretion. High doses of leptin increase Na(+) excretion in the short run; partially by decreasing renal Na(+),K(+)-ATPase (sodium pump) activity. This effect is mediated by phosphatidylinositol 3-kinase (PI3K) and is impaired in animals with dietary-induced obesity. In contrast to acute, chronic elevation of plasma leptin to the level observed in patients with the metabolic syndrome impairs renal Na(+) excretion, which is associated with the increase in renal Na(+),K(+)-ATPase activity. This effect results from oxidative stress-induced deficiency of nitric oxide and/or transactivation of epidermal growth factor receptor and subsequent stimulation of extracellular signal-regulated kinases. Ameliorating "renal leptin resistance" or reducing leptin level and/or leptin signaling in states of chronic hyperleptinemia may be a novel strategy for the treatment of arterial hypertension associated with the metabolic syndrome.

Hydrogen sulfide and its modulation in arterial hypertension and atherosclerosis

Apart from nitric oxide (NO) and carbon monoxide (CO), hydrogen sulfide (H₂S) is the third gaseous mediator in mammals. H₂S is synthesized from L-cysteine by cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE), or by sequential action of alanine aminotransferase and 3-mercaptopyruvate sulfurtransferase. In the cardiovascular system, H₂S is involved in the regulation of vascular tone and blood pressure, inhibits atherogenesis, and protects myocardium from ischemia-reperfusion injury. Recently, the first organic, water-soluble H₂S donor, GYY4137, has been synthesized. In addition, H₂S-releasing derivatives of several currently used drugs such as sildenafil, diclofenac, aspirin and mesalamine were obtained. Such compounds may be used in the future treatment of cardiovascular diseases. In this article, I describe the role of H₂S in the regulation of blood pressure and in the pathogenesis of arterial hypertension and atherosclerosis which are two most common cardiovascular disorders.

Differential effects of statins on endogenous H2S formation in perivascular adipose tissue

Hydrogen sulfide (H(2)S) is a new gasotransmitter synthesized enzymatically from l-cysteine in cytosol and is oxidized in mitochondria. In the cardiovascular system, H(2)S regulates vascular tone, inhibits atherogenesis, and protects against myocardial ischemia-reperfusion injury. We examined the effect of statins on vascular H(2)S production. Male Wistar rats received pravastatin (40mg/kg/day) or atorvastatin (20mg/kg/day) for 3 weeks and then H(2)S formation was measured in aortic media, periaortic adipose tissue (PAAT) and the liver. Only atorvastatin increased H(2)S production in PAAT whereas both statins stimulated its formation in the liver. Neither statin affected H(2)S production in aortic media. H(2)S formation in post-mitochondrial supernatant was higher than in mitochondria-containing supernatant and was not influenced by statins in any tissue. In addition, oxidation of exogenous H(2)S in isolated liver mitochondria was slower in statin-treated than in control rats. These data indicate that statins increase net H(2)S production by inhibiting its mitochondrial oxidation. Statins had no effect on the activity of H(2)S-metabolizing enzyme, sulfide:quinone oxidoreductase, measured at saturating coenzyme Q concentration. Both statins reduced CoQ(9) concentration in plasma and liver, but only atorvastatin decreased CoQ(9) in PAAT. Atorvastatin attenuated phenylephrine-induced contraction of PAAT+ but not of PAAT- aortic rings. Effects of atorvastatin on net H(2)S production, mitochondrial H(2)S oxidation and aortic contractility were abolished by supplementation of exogenous CoQ(9). In conclusion, lipophilic atorvastatin, but not hydrophilic pravastatin, increases net H(2)S production in perivascular adipose tissue by inhibiting its mitochondrial oxidation. This effect is mediated by statin-induced CoQ(9) deficiency and results in the augmentation of anticontractile effect of perivascular adipose tissue.

The differentiating effect of glimepiride and glibenclamide on paraoxonase 1 and platelet-activating factor acetylohydrolase activity

AIMS

The present study was designed to examine the effect of sulphonylureas, glimepiride (GM) and glibenclamide (GB), on paraoxonase 1 (PON1) and platelet activating factor acetylohydrolase (PAF-AH) activity in normal and streptozotocin (STZ)-induced (50 mg/kg) diabetic rats.

MAIN METHODS

In treated groups, glimepiride (0.1 mg/kg) or glibenclamide (2 mg/kg) was given orally for 4 weeks. A PON1 and PAF-AH activity were estimated by spectrophotometric method.

KEY FINDINGS

Hyperglycemia was accompanied by a significant decrease in plasma PON1 activity toward paraoxon (P < 0.001) and phenyl acetate (P < 0.01) and increase in plasma PAF-AH activity (P < 0.01). In STZ-induced diabetic rats the administration of both GM and GB had no effect on plasma PON1 activity but reversed elevated plasma PAF-AH activity (GM: P < 0.05, GB: P < 0.01). In non-diabetic rats after either GM or GB administration the decreased PON1 activity in the plasma was observed (GM: P < 0.001, GB: P < 0.05), but plasma PAF-AH activity remained unchanged. Both GM and GB had no effect on total plasma antioxidant capacity in diabetic and control treated groups. Additionally, both drugs increased PON1 activity toward phenyl acetate in the liver, in diabetic rats (GM: P < 0.05, GB:ns) as well as in non-diabetic rats (GM: P < 0.001, GB: P < 0.001), and reduced lipid peroxidation in the liver.

SIGNIFICANCE

These results demonstrate that in streptozotocin-induced diabetic rats as well as in normal rats glimepiride and glibenclamide have no beneficial effects on circulating PON1 and PAF-AH activities, but both drugs increase PON1 activity in the liver.

Liver X receptor (LXR) and the reproductive system--a potential novel target for therapeutic intervention

Liver X receptor (LXR) alpha and beta are ligand-activated transcription factors that regulate the expression of genes involved in the removal of cholesterol from cells by high-density lipoproteins, the transport of cholesterol to the liver and the biliary excretion of cholesterol. LXRs are activated by oxygenated cholesterol derivatives such as 24(S),25-epoxycholesterol or 24(S)-, 25- and 27-hydroxycholesterol. In this review, we will discuss the role of LXR in the reproductive system and perspectives on the application of LXR agonists in the treatment of reproductive pathologies. Interestingly, progressive age-related impairment of fertility is observed in both female and male LXR knockout mice. Reduced fertility in female LXR knockout mice is associated with resistance to follicular fluid meiosis-activating sterol (FF-MAS), the intermediate of cholesterol synthesis generated in the ovaries that is responsible for stimulating oocyte meiosis partially in a LXR-dependent manner. Female mice lacking both LXR isoforms exhibit symptoms of ovarian hyperstimulation syndrome when treated with pharmacological doses of gonadotropins. LXR agonists have mainly been considered as potential anti-atherosclerotic medications. However, experimental studies suggest that natural or synthetic LXR agonists may also effectively treat some reproductive pathologies, such as infertility, impaired uterine contractility, hormone-dependent cancers and insulin resistance in patients with polycystic ovarian syndrome. However, the specific adverse effects of LXR agonists on the reproductive system must also be considered. Adverse effects of LXR agonists include impaired trophoblast invasion, excessive transplacental cholesterol transport from the mother to the fetus leading to fetal hypercholesterolemia, and augmented estrogen deficiency after menopause.

Chronic hyperleptinemia induces resistance to acute natriuretic and NO-mimetic effects of leptin

Apart from controlling energy balance, leptin, secreted by adipose tissue, is also involved in the regulation of cardiovascular function. Previous studies have demonstrated that acutely administered leptin stimulates natriuresis and vascular nitric oxide (NO) production and that these effects are impaired in obese animals. However, the mechanism of resistance to leptin is not clear. Because obesity is associated with chronically elevated leptin, we examined if long-term hyperleptinemia impairs acute effects of leptin on sodium excretion and NO production in the absence of obesity. Hyperleptinemia was induced in lean rats by administration of exogenous leptin at a dose of 0.5mg/kg/day for 7 days, and then acute effect of leptin (1mg/kg i.v.) was studied under general anesthesia. Leptin increased fractional sodium excretion and decreased Na(+),K(+)-ATPase activity in the renal medulla. In addition, leptin increased the level of NO metabolites and cyclic GMP in plasma and aortic wall. These acute effects of leptin were impaired in hyperleptinemic animals. In both control and hyperleptinemic groups the effect of leptin on Na(+) excretion and renal Na(+),K(+)-ATPase was abolished by phosphoinositide 3-kinase (PI3K) inhibitor, wortmannin, but not by protein kinase B/Akt inhibitor, triciribine,. In contrast, acute effect of leptin on NO metabolites and cGMP was abolished by triciribine but not by wortmannin. Leptin stimulated Akt phosphorylation at Ser(473) in aortic tissue but not in the kidney, and this effect was comparable in control and hyperleptinemic groups. These results suggest that hyperleptinemia may mediate "renal" and "vascular" leptin resistance observed in obesity.

Resistance to acute NO-mimetic and EDHF-mimetic effects of leptin in the metabolic syndrome

AIMS

We examined mechanisms leading to the impairment of nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF)-dependent vasorelaxation in response to acutely administered leptin in rats with the metabolic syndrome.

MAIN METHODS

Effects of leptin on blood pressure and NO and cGMP in the aortic wall were studied in four groups of rats: (1) lean control, (2) obese, fed "cafeteria diet" for 3months (hyperleptinemia and hyperinsulinemia), (3) hyperleptinemia induced by administration of exogenous leptin for 8days, and (4) fructose-fed, receiving 20% fructose in the drinking water for 8weeks (hyperinsulinemia with slightly elevated leptin).

KEY FINDINGS

Stimulatory effect of leptin on NO and cGMP production in the aortic wall was impaired in obese and hyperleptinemic groups but not in the fructose group. In contrast, EDHF-mimetic effect of leptin was impaired in obese and fructose-fed but not in the hyperleptinemic group. Leptin increased tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) in the aortic wall, and this effect was impaired in obese and fructose-fed animals. The EDHF-mimetic effect of leptin was abolished by phosphoinositide 3-kinase inhibitor, wortmannin, whereas its effect on NO was not. In addition, IRS-1 phosphorylation at Ser(307) and Ser(612) was enhanced in obese and fructose-fed but not in hyperleptinemic rats.

SIGNIFICANCE

These results indicate that: (1) long-term hyperleptinemia induces resistance to acute vascular NO-mimetic effect of leptin in obesity/metabolic syndrome, (2) leptin stimulates EDHF in IRS-1 and PI3K-dependent manner, and this effect is impaired in obesity due to excessive serine phosphorylation of IRS-1.

EGF receptor as a drug target in arterial hypertension

Drugs which inhibit epidermal growth factor receptor (EGFR) are used in the treatment of cancers. EGFR may contribute to the development of hypertension by regulating vascular tone and renal Na+ handling. Synthetic EGFR inhibitors reduce blood pressure in some experimental models of hypertension suggesting that this receptor is a novel target for antihypertensive therapy.

Renal antioxidant enzymes and glutathione redox status in leptin-induced hypertension

Previously, we have demonstrated that leptin increases blood pressure (BP) in the rats through two oxidative stress-dependent mechanisms: stimulation of extracellular signal-regulated kinases (ERK) by H(2)O(2) and scavenging of nitric oxide (NO) by superoxide (O(2-.)). Herein, we examined if renal glutathione system and antioxidant enzymes determine the mechanism of prohypertensive effect of leptin. Leptin administered at 0.5 mg/kg/day for 4 or 8 days increased BP and renal Na(+),K(+)-ATPase activity and reduced fractional sodium excretion; these effects were prevented by NADPH oxidase inhibitor, apocynin. Superoxide scavenger, tempol, abolished the effect of leptin on BP and renal Na(+) pump in rats receiving leptin for 8 days, whereas ERK inhibitor, PD98059, was effective in animals treated with leptin for 4 days. Leptin administered for 4 days decreased glutathione (GSH) and increased glutathione disulfide (GSSG) in the kidney. In animals receiving leptin for 8 days GSH returned to normal level, which was accompanied by up-regulation of gamma-glutamylcysteine synthetase (gamma-GCS), a rate-limiting enzyme of the GSH biosynthetic pathway. In addition, superoxide dismutase (SOD) activity was decreased, whereas glutathione peroxidase (GPx) was increased in rats receiving leptin for 8 days. Cotreatment with gamma-GCS inhibitor, buthionine sulfoximine (BSO), accelerated, whereas GSH precursor, N-acetylcysteine (NAC), attenuated leptin-induced changes in gamma-GCS, SOD, and GPx. In addition, coadministration of BSO changed the mechanism of BP elevation from H(2)O(2)-ERK to (O(2-.))-NO dependent in animals receiving leptin for 4 days, whereas NAC had the opposite effect in rats treated with leptin for 8 days. These results suggest that initial change in GSH redox status induces decrease in SOD/GPx ratio, which results in greater amount of (O)2-.)) versus H(2)O(2) in later phase of leptin treatment, thus shifting the mechanism of BP elevation from H(2)O(2)-ERK to (O(2-.))-NO dependent.

Adiponectin and its role in cardiovascular diseases

Studies performed during the last decade indicate that adipose tissue is not only a site of triglyceride storage but also an active endocrine organ which secretes many biologically active mediators referred to as "adipokines". In contrast to many adipokines which are overproduced in obese individuals and exert deleterious effects on insulin sensitivity, lipoprotein metabolism and cardiovascular system, such as leptin, tumor necrosis factor-alpha, plasminogen activator inhibitor-1, resistin, etc., adiponectin seems to be a unique adipokine which is produced in lower amounts in obese than in lean subjects and possesses predominantly beneficial activities, i.e. increases insulin sensitivity, stimulates fatty acid oxidation, inhibits inflammatory reaction and induces endothelium-dependent nitric oxide-mediated vasorelaxation. Adiponectin binds two receptors, AdipoR1 and AdipoR2. Adiponectin knockout mice exhibit various manifestations of the metabolic syndrome such as insulin resistance, glucose intolerance, hyperlipidemia, impaired endothelium-dependent vasorelaxation and hypertension, as well as augmented neointima formation after vascular injury. Clinical studies indicate that plasma adiponectin concentration is lower in patients with essential hypertension and ischemic heart disease. Raising endogenous adiponectin level or increasing the sensitivity to this hormone may be a promising therapeutic strategy for patients with metabolic and cardiovascular diseases. Among currently used drugs, thiazolidinediones (peroxisome proliferator activated receptor gamma agonists) are most effective in elevating adiponectin level.

Role of extracellular signal-regulated kinases (ERK) in leptin-induced hypertension

We investigated if extracellular signal-regulated kinases (ERK) and oxidative stress are involved in the pathogenesis of arterial hypertension induced by chronic leptin administration in the rat. Leptin was administered at a dose of 0.25 mg/kg twice daily s.c. for 4 or 8 days. Blood pressure (BP) was higher in leptin-treated than in control animals from the third day of the experiment. The superoxide dismutase (SOD) mimetic, tempol, normalized BP in leptin-treated rats on days 6, 7 and 8, whereas the ERK inhibitor, PD98059, exerted a hypotensive effect on days 3 through 6. Leptin increased ERK phosphorylation level in renal and aortic tissues more markedly after 4 than after 8 days of treatment. In addition, leptin reduced urinary Na(+) excretion and increased renal Na(+),K(+)-ATPase activity, and these effects were abolished on days 4 and 8 by PD98059 and tempol, respectively. The levels of NO metabolites and cGMP were reduced in animals receiving leptin for 8 days. Markers of oxidative stress (H(2)O(2) and lipid peroxidation products) were elevated to a greater extent after 4 than after 8 days of leptin treatment. In contrast, nitrotyrosine, a marker of protein nitration by peroxynitrite, was higher in animals receiving leptin for 8 days. NADPH oxidase inhibitor, apocynin, prevented leptin's effect on BP, ERK, Na(+),K(+)-ATPase/Na(+) excretion and NO formation at all time points. SOD activity was reduced, whereas glutathione peroxidase (GPx) activity was increased in the group treated with leptin for 8 days. These data indicate that: (1) ERK, activated by oxidative stress, is involved only in the early phase of leptin-induced BP elevation, (2) the later phase of leptin-induced hypertension is characterized by excessive NO inactivation by superoxide, (3) the time-dependent shift from ERK to O(2)(-)-NO dependent mechanism may be associated with reduced SOD/GPx ratio, which favors formation of O(2)(-) instead of H(2)O(2).

Liver X receptors (LXRs). Part I: structure, function, regulation of activity, and role in lipid metabolism

Liver X receptors (LXRs) alpha and ss belong to a family of nuclear receptors which form heterodimers with the retinoid X receptor and, upon ligand binding, stimulate the expression of target genes. LXRs were initially described as orphan receptors and oxidized cholesterol derivatives (oxysterols) were later identified as their natural ligands. In addition, several synthetic LXR agonists such as T0901317 and GW3965 were synthesized. Oxysterols are formed in amounts proportional to cholesterol content in the cell and therefore the LXRs operate as cholesterol sensors which protect from cholesterol overload by: 1) inhibiting intestinal cholesterol absorption, 2) stimulating cholesterol efflux from cells to high-density lipoproteins through the ATP-binding cassette transporters ABCA1 and ABCG1, 3) activating the conversion of cholesterol to bile acids in the liver, and (4) activating biliary cholesterol and bile acid excretion. In addition, LXR agonists activate de novo fatty acid synthesis by stimulating the expression of a lipogenic transcription factor, sterol regulatory element-binding protein-1c (SREBP-1c), leading to the elevation of plasma triglycerides and liver steatosis. Here we describe the structure and function of the LXRs, their endo- and exogenous agonists and antagonists, the regulation of LXR expression and activity, and their role in the regulation of cholesterol and lipid metabolism. In the accompanying article we characterize other effects of LXRs, alterations in LXR expression, and changes in the level of their endogenous agonists in pathological conditions as well as therapeutic implications.

Liver X receptors (LXRs). Part II: non-lipid effects, role in pathology, and therapeutic implications

Liver X receptors (LXRs) alpha and ss belong to a group of nuclear receptors which, after ligand binding, regulate gene transcription. Their natural agonists are oxidized cholesterol derivatives (oxysterols). The main function of LX receptors is the regulation of cholesterol metabolism. In the first part of this work we discussed the structure and mechanism of action of LXRs, their agonists and antagonists, the regulation of LXR expression, and their role in cholesterol and lipid metabolism. In the present article we describe other roles of LXRs. Agonists of these receptors increase insulin sensitivity and stimulate insulin secretion. Activation of LXRs inhibits inflammation and autoimmune reactions. Moreover, pharmacological studies and genetic manipulations indicate that these receptors inhibit atherogenesis. LX receptors are also involved in the regulation of renin secretion, inhibit the formation of amyloid ss in the central nervous system, regulate gonadal function and steroidogenesis both in gonads and in adrenals, influence the proliferation and differentiation of keratinocytes, and inhibit the proliferation of tumor cells. Changes in the expression of these receptors and in the level of their agonists are observed in many diseases. Taking into account the multiple roles of LX receptors, their agonists may be applied in the future in the treatment of many disorders, including diabetes, inflammatory diseases, atherosclerosis, Alzheimer's disease, and hypogonadism. However, possible side effects should be taken into account, including enhancement of lipogenesis, hypertriglyceridemia, and liver steatosis. The function of LX receptors is also modulated by many currently used drugs such as statins, fibrates, and thazolidinedione derivatives.

Regulation of renal ouabain-resistant Na+-ATPase by leptin, nitric oxide, reactive oxygen species, and cyclic nucleotides: implications for obesity-associated hypertension

This study examined the effect of leptin on renal ouabain-resistant Na(+)-ATPase, which drives the reabsorption of about 10% of sodium transported in the proximal tubule. Chronic leptin administration (0.25 mg/kg s.c. twice daily for seven days) increased Na(+)-ATPase activity by 62.9%. This effect was prevented by the coadministration of superoxide dismutase mimetic, tempol, or the NADPH oxidase inhibitor, apocynin (2 mM in the drinking water). Acutely administered NO donors decreased Na(+)-ATPase activity. This effect was abolished by soluble guanylate cyclase inhibitor, ODQ, but not by protein kinase G inhibitors. Exogenous cGMP reduced Na(+)-ATPase activity, but its synthetic analogues, 8-bromo-cGMP and 8-pCPT-cGMP, were ineffective. The inhibitory effect of NO donors and cGMP was abolished by EHNA, an inhibitor of cGMP-stimulated phosphodiesterase (PDE2). Exogenous cAMP analogue and dibutyryl-cAMP increased Na(+)-ATPase activity and abolished the inhibitory effect of cGMP. Finally, the administration of superoxide-generating mixture (xanthine oxidase+hypoxanthine) increased Na(+)-ATPase activity. The results suggest that nitric oxide decreases renal Na(+)-ATPase activity by stimulating cGMP, which in turn activates PDE2 and decreases cAMP concentration. Increased production of reactive oxygen species may lead to the elevation of Na(+)-ATPase activity by scavenging NO and limiting its inhibitory effect. Chronic hyperleptinemia is associated with increased Na(+)-ATPase activity due to excessive oxidative stress.

Role of PI3K and PKB/Akt in acute natriuretic and NO-mimetic effects of leptin

Apart from controlling energy balance, leptin, a peptide hormone secreted by white adipose tissue, is also involved in the regulation of cardiovascular function. Previous studies have documented that leptin stimulates natriuresis and nitric oxide (NO) production, but the mechanism of these effects is incompletely elucidated. We examined whether phosphoinositide 3-kinase (PI3K) and its downstream effector, protein kinase B/Akt are involved in acute natriuretic and NO-mimetic effects of leptin in anaesthetized rats. Leptin (1 mg/kg i.v.) induced a marked increase in natriuresis and this effect was abolished by pretreatment with either wortmannin (15 microg/kg) or LY294002 (0.6 mg/kg), two structurally different PI3K inhibitors. Moreover, leptin increased plasma concentration and urinary excretion of NO metabolites, nitrites+nitrates (NO(x)), and of NO second messenger, cyclic GMP. In addition, leptin increased NO(x) and cGMP in aortic tissue. The stimulatory effect of leptin on NO(x) and cGMP was prevented by PKB/Akt inhibitor, triciribine, but not by either wortmannin or LY294002. Triciribine had no effect on leptin-induced natriuresis. Leptin stimulated Akt phosphorylation at Ser(473) in aortic tissue but not in the kidney. These results suggest that leptin-induced natriuresis is mediated by PI3K but not Akt, whereas NO-mimetic effect of leptin results from PI3K-independent stimulation of Akt.

Hydrogen sulfide (H2S) - the third gas of interest for pharmacologists

Nitric oxide (NO) and carbon monoxide (CO) synthesized from L-arginine by NO synthase and from heme by heme oxygenase, respectively, are the well-known neurotransmitters and are also involved in the regulation of vascular tone. Recent studies suggest that hydrogen sulfide (H(2)S) is the third gaseous mediator in mammals. H(2)S is synthesized from L-cysteine by either cystathionine beta-synthase (CBS) or cystathionine gamma-lyase (CSE), both using pyridoxal 5'-phosphate (vitamin B(6)) as a cofactor. H(2)S stimulates ATP-sensitive potassium channels (K(ATP)) in the vascular smooth muscle cells, neurons, cardiomyocytes and pancreatic beta-cells. In addition, H(2)S may react with reactive oxygen and/or nitrogen species limiting their toxic effects but also, attenuating their physiological functions, like nitric oxide does. In contrast to NO and CO, H(2)S does not stimulate soluble guanylate cyclase. H(2)S is involved in the regulation of vascular tone, myocardial contractility, neurotransmission, and insulin secretion. H(2)S deficiency was observed in various animal models of arterial and pulmonary hypertension, Alzheimer's disease, gastric mucosal injury and liver cirrhosis. Exogenous H(2)S ameliorates myocardial dysfunction associated with the ischemia/reperfusion injury and reduces the damage of gastric mucosa induced by anti-inflammatory drugs. On the other hand, excessive production of H(2)S may contribute to the pathogenesis of inflammatory diseases, septic shock, cerebral stroke and mental retardation in patients with Down syndrome, and reduction of its production may be of potential therapeutic value in these states.

H2O2 and Src-dependent transactivation of the EGF receptor mediates the stimulatory effect of leptin on renal ERK and Na+, K+-ATPase

We examined the mechanism through which leptin increases Na(+), K(+)-ATPase activity in the rat kidney. Leptin was infused under anaesthesia into the abdominal aorta proximally to the renal arteries and then Na(+), K(+)-ATPase activity was measured in the renal cortex and medulla. Leptin (1mug/kgmin) increased Na(+), K(+)-ATPase activity after 3h of infusion, which was accompanied by the increase in urinary H(2)O(2) excretion and phosphorylation level of extracellular signal regulated kinase (ERK). The effect of leptin on ERK and Na(+), K(+)-ATPase was abolished by catalase, specific inhibitors of epidermal growth factor (EGF) receptor, AG1478 and PD158780, as well as by ERK inhibitor, PD98059, and was mimicked by both exogenous H(2)O(2) and EGF. The effect of leptin was also prevented by the inhibitor of Src tyrosine kinase, PP2. Leptin and H(2)O(2) increased Src phosphorylation at Tyr(418). We conclude that leptin-induced stimulation of renal Na(+), K(+)-ATPase involves H(2)O(2) generation, Src kinase, transactivation of the EGF receptor, and stimulation of ERK.

Apelin and visfatin: unique "beneficial" adipokines upregulated in obesity?

Recent studies suggest that adipose tissue hormones ("adipokines") are involved in the pathogenesis of various complications of obesity, including hyperlipidemia, diabetes mellitus, arterial hypertension, atherosclerosis, and heart failure. Apelin and visfatin are two recently described adipokines, although they are also synthesized outside adipose tissue. Apelin exists in at least three forms, consisting of 13, 17, or 36 amino acids, all originating from a common 77-amino-acid precursor. In the cardiovascular system, apelin elicits endothelium-dependent, nitric oxide-mediated vasorelaxation and reduces arterial blood pressure. In addition, apelin demonstrates potent and long-lasting positive inotropic activity which is preserved even in injured myocardium and is not accompanied by myocardial hypertrophy. Apelin synthesis in adipocytes is stimulated by insulin, and plasma apelin level markedly increases in obesity associated with insulin resistance and hyperinsulinemia. In addition to regulating cardiovascular function, apelin inhibits water intake and vasopressin production. Visfatin, previously recognized as a pre-B cell colony-enhancing factor (PBEF), is abundantly expressed in visceral adipose tissue and is upregulated in some, but not all, animal models of obesity. Preliminary studies suggest that plasma visfatin concentration is also increased in humans with abdominal obesity and/or type 2 diabetes mellitus. Visfatin binds to the insulin receptor at a site distinct from insulin and exerts hypoglycemic effect by reducing glucose release from hepatocytes and stimulating glucose utilization in peripheral tissues. Thus, apelin and visfatin are unique among adipose tissue hormones in that they are upregulated in the obese state and both exert primarily beneficial effects.

Role of leptin in blood pressure regulation and arterial hypertension

Leptin is a 16-kDa protein secreted by white adipose tissue that is primarily involved in the regulation of food intake and energy expenditure. Plasma leptin concentration is proportional to the amount of adipose tissue and is markedly increased in obese individuals. Recent studies suggest that leptin is involved in cardiovascular complications of obesity, including arterial hypertension. Acutely administered leptin has no effect on blood pressure, probably because it concomitantly stimulates the sympathetic nervous system and counteracting depressor mechanisms such as natriuresis and nitric oxide (NO)-dependent vasorelaxation. By contrast, chronic hyperleptinemia increases blood pressure because these acute depressor effects are impaired and/or additional sympathetic nervous system-independent pressor effects appear, such as oxidative stress, NO deficiency, enhanced renal Na reabsorption and overproduction of endothelin. Although the cause-effect relationship between leptin and high blood pressure in humans has not been demonstrated directly, many clinical studies have shown elevated plasma leptin in patients with essential hypertension and a significant positive correlation between leptin and blood pressure independent of body adiposity both in normotensive and in hypertensive individuals. In addition, leptin may contribute to end-organ damage in hypertensive individuals such as left ventricular hypertrophy, retinopathy and nephropathy, independent of regulating blood pressure. Here, current knowledge about the role of leptin in the regulation of blood pressure and in the pathogenesis of arterial hypertension is presented.

Asymmetric dimethylarginine (ADMA) as a target for pharmacotherapy

Asymmetric dimethylarginine (ADMA) is synthesized during the methylation of protein arginine residues by protein arginine methyltransferases (PRMT) and is released during proteolysis. ADMA is a competitive inhibitor of nitric oxide synthase and may decrease NO availability. ADMA is eliminated by renal excretion or is metabolized by dimethylarginine dimethylaminohydrolase (DDAH) to citruline and dimethylamine. Two other endogenous methylarginines are also synthesized by PRMT: N-monomethyl-L-arginine (L-NMMA) and symmetric dimethylarginine (SDMA). L-NMMA inhibits NO synthase but its concentrations in circulation are much lower than ADMA whereas SDMA is inactive. Plasma concentration of ADMA is markedly increased in patients with chronic renal failure and moderately increased in patients with many other diseases including hyperlipidemia, diabetes mellitus, arterial hypertension, hyperhomocysteinemia and heart failure. The increased concentration of ADMA is positively correlated with markers of atherosclerosis, such as carotid artery intima-media thickness and has a predictive value for acute cardiovascular events in prospective studies. Angiotensin-converting enzyme inhibitors, angiotensin AT1 receptor antagonists, vitamin E and, according to some studies, estrogens used in hormonal replacement therapy reduce plasma ADMA concentration, which may contribute to their beneficial effect on NO synthesis and endothelial function. However, in some states associated with excess of NO, such as septic shock or excitotoxic neuronal injury ADMA may be protective by limiting toxic effect of high concentrations of NO. This article reviews the effect of pharmacotherapy on ADMA metabolism and its possible clinical implications.