Time-dependent transition from H(2)O(2)-extracellular signal-regulated kinase- to O(2)-nitric oxide-dependent mechanisms in the stimulatory effect of leptin on renal Na+/K+/-ATPase in the rat

Dysregulated Epicardial Adipose Tissue as a Risk Factor and Potential Therapeutic Target of Heart Failure with Preserved Ejection Fraction in Diabetes

Cardiovascular (CV) disease and heart failure (HF) are the leading cause of mortality in type 2 diabetes (T2DM), a metabolic disease which represents a fast-growing health challenge worldwide. Specifically, T2DM induces a cluster of systemic metabolic and non-metabolic signaling which may promote myocardium derangements such as inflammation, fibrosis, and myocyte stiffness, which represent the hallmarks of heart failure with preserved ejection fraction (HFpEF). On the other hand, several observational studies have reported that patients with T2DM have an abnormally enlarged and biologically transformed epicardial adipose tissue (EAT) compared with non-diabetic controls. This expanded EAT not only causes a mechanical constriction of the diastolic filling but is also a source of pro-inflammatory mediators capable of causing inflammation, microcirculatory dysfunction and fibrosis of the underlying myocardium, thus impairing the relaxability of the left ventricle and increasing its filling pressure. In addition to representing a potential CV risk factor, emerging evidence shows that EAT may guide the therapeutic decision in diabetic patients as drugs such as metformin, glucagon-like peptide‑1 (GLP-1) receptor agonists and sodium-glucose cotransporter 2 inhibitors (SGLT2-Is), have been associated with attenuation of EAT enlargement.

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.

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.

Reduced nicotinamide adenine dinucleotide phosphate oxidase mediates fibrotic and inflammatory effects of leptin on hepatic stellate cells

Although leptin induces fibrotic activity in hepatic stellate cells (HSCs), the mechanisms are not entirely understood. To investigate the potential role of reduced nicotinamide adenine dinucleotide phosphate oxidase (NADPH) and reactive oxygen species (ROS) in leptin signaling in HSCs, we analyzed leptin-induced intracellular signaling pathways in primary wild-type (WT), p47(phox(-/-) ), and signal transducer and activator of transcription protein 3 (STAT3)-deleted HSCs. Leptin-stimulated ROS production was attenuated in human and mouse HSCs by the NADPH oxidase inhibitor diphenylene-iodonium (DPI) and in HSCs lacking the NADPH component p47(phox). Leptin-induced phosphorylation of extracellular signal-regulated kinase (ERK) and AKT, but not of STAT3, was blocked by NADPH oxidase inhibition. Moreover, leptin-induced ROS production was inhibited by the Janus kinase (JAK) inhibitor, AG490, but normal ROS production was observed in STAT3-deleted HSCs. Pharmacologic or genetic inhibition of NADPH in HSCs not only resulted in a reduction of leptin-mediated HSC proliferation but also reduced the leptin-mediated up-regulation of the fibrogenic markers collagen alpha1(I) and alpha-smooth muscle actin and of the inflammatory mediators monocyte chemotactic protein-1 (MCP-1), macrophage inflammatory protein 1 (MIP-1), and macrophage inflammatory protein 2 (MIP-2). In vivo, leptin enhanced chemokine expression induced by chemokine (C-C motif) ligand 4 (CCl(4)) in WT mice, but a blunted response was observed in p47(phox-/-) mice. In conclusion, NADPH oxidase is a crucial mediator of proliferative, fibrogenic, and inflammatory actions of leptin. Leptin-induced NADPH oxidase acts downstream of JAK activation but is independent of STAT3. Our results, in conjunction with previous studies on angiotensin II and platelet-derived growth factor (PDGF), place NADPH in the center of the fibrogenic signaling response in HSCs and demonstrate its potential role as a pharmacological target for antifibrotic therapies.

Role of leptin in hepatic ischemia/reperfusion-induced intestinal injury of rats

AIM: To explore the changes of leptin in intestinal tract following hepatic ischemia/reperfusion (H-I/R), to investigate the association between these changes and H-I/R-induced intestinal injury, and to find out the role of leptin in H-I/R-induced intestinal injury.

METHODS: A 70% H-I/R model of rats was established, forming 5 groups including sham-operation and injury ones based on different reperfusion time. Enzyme-colorimetry was used to detect serum diamine oxidase activity after injury, hematoxylin-eosin staining and immunohistochemistry were applied to investigate pathological variations and leptin protein expressions in duodenum after injury, respectively, while reverse transcriptase-PCR was used to detect leptin mRNA expressions in duodenum after injury.

RESULTS: Compared with sham-operation group after injury, the four reperfusion groups showed no significant difference in serum diamine oxidase activity, but serum diamine oxidase level was significantly higher in 60 min ischemia/60 min reperfusion (I60'R60') group than in I60'R360' group (P = 0.0077). Pathological investigation suggested that duodenal impairments at the early phase of H-I/R were more serious, while the impairments at the later phase lessened gradually. Compared with leptin protein expression in duodenum of sham-operation group after injury, that of I60'R240' and I60'R360' groups increased significantly (0.126503 ± 0.005873, 0.129458 ± 0.003755 vs 0.079269 ± 0.001995, both P < 0.01), and the levels of reperfusion groups decreased in such order as I60'R360', I60'R240', I60'R60' and I60'R150' groups. Compared with leptin mRNA expression in duodenum of sham-operation group after injury, that of I60'R150' group decreased significantly (0.944 ± 0.033 vs 1.022 ± 0.011, P = 0.049), and it was significantly lower than the level of I60'R360' group.

CONCLUSION: The expression changes of leptin in intestinal tract after H-I/R are closely associated with intestinal injury, suggesting that leptin may be a protective factor of resisting H-I/R-induced intestinal injury.

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.

Role of nicotinamide adenine dinucleotide phosphate-oxidase family in liver fibrogenesis

NADPH oxidase (nicotinamide adenine dinucleotide phosphate-oxidase, NOX) is a multi-protein complex producing reactive oxygen species (ROS), present both in phagocytes, being essential in host defense and in non-phagocytic cells, regulating intracellular signaling. In liver, NADPH oxidase plays a central role in fibrogenesis. A functionally active form of NADPH oxidase is expressed not only in Kupffer cells (phagocytic cell type) but also in hepatic stellate cells (HSCs) (non-phagocytic cell type), suggestive of its role the non-phagocytic NADPH oxidase in HSCs activation. This paper reviewed effects of NOX in liver fibrogenesis.

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).

Reactive oxygen species and the regulation of renal Na+-K+-ATPase in opossum kidney cells

Culture of opossum kidney (OK) cells over 40 passages resulted in marked overexpression of α1-subunit and β1-subunit Na+-K+-ATPase, which paralleled with increases in Na+-K+-ATPase activity, and enhanced availability in hydrogen peroxide (H2O2) ( J Membr Biol 212: 163–175, 2006). The present study evaluated the mechanisms involved in increased H2O2 production in OK cells cultured over 40 passages (up to passage 80) and whether H2O2 plays a role in the regulation of Na+-K+-ATPase expression and activity. The accumulation of H2O2 in the extracellular medium over 24 h (236.5 ± 2.7 vs. 319.3 ± 2.9 nM) and the rate of H2O2 production (5.21 ± 0.02 vs. 7.37 ± 0.02 nM/min) in cells with 40 passages was lower ( P < 0.05) than in cells with 80 passages. The increase in H2O2 production in cells with 80 passages was accompanied by overexpression of NOX1, but not of NOX2, and overexpression of SOD1, SOD2, and SOD3. Apocynin, a NADPH inhibitor, markedly attenuated the increase in H2O2 production in cells with 80 passages and significantly decreased Na+-K+-ATPase activity (26% reduction) and α1-subunit Na+-K+-ATPase overexpression. Increases in Na+-K+-ATPase α1-subunit mRNA abundance were also observed in OK cells with increased number of cell passages, but apocynin did not affect transcript abundance. In conclusion, the increased availability of H2O2 in the intracellular milieu plays an important role in the long-term regulation of Na+-K+-ATPase expression and activity. These cells may constitute an interesting model to study events related to oxidative stress, in which adaptation to increases in H2O2 had progressively adapted in a more natural manner.

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.

NOX in liver fibrosis

NADPH oxidase is a multi-protein complex producing reactive oxygen species (ROS) both in phagocytic cells, being essential in host defense, and in non-phagocytic cells, regulating intracellular signalling. In the liver, NADPH oxidase plays a central role in fibrogenesis. A functionally active form of the NADPH oxidase is expressed not only in Kupffer cells (phagocytic cell type) but also in hepatic stellate cells (HSCs) (non-phagocytic cell type), suggesting a role of the non-phagocytic NADPH oxidase in HSC activation. Consistent with this concept, profibrogenic agonists such as Angiotensin II (Ang II) and platelet derived growth factor (PDGF), or apoptotic bodies exert their activity through NADPH oxidase-activation in HSCs. Both pharmacological inhibition with DPI and genetic studies using p47(phox) knockout mice provided evidence for a central role of NADPH oxidase in the regulation of HSC-activity and liver fibrosis. In addition to the p47(phox) component, only Rac1 has been identified as a functional active component of the NADPH oxidase complex in HSCs.

Oxyl radicals, redox-sensitive signalling cascades and antioxidants

Oxidative stress is an increase in the reduction potential or a large decrease in the reducing capacity of the cellular redox couples. A particularly destructive aspect of oxidative stress is the production of reactive oxygen species (ROS), which include free radicals and peroxides. Some of the less reactive of these species can be converted by oxidoreduction reactions with transition metals into more aggressive radical species that can cause extensive cellular damage. In animals, ROS may influence cell proliferation, cell death (either apoptosis or necrosis) and the expression of genes, and may be involved in the activation of several signalling pathways, activating cell signalling cascades, such as those involving mitogen-activated protein kinases. Most of these oxygen-derived species are produced at a low level by normal aerobic metabolism and the damage they cause to cells is constantly repaired. The cellular redox environment is preserved by enzymes and antioxidants that maintain the reduced state through a constant input of metabolic energy. This review summarizes current studies that have been regarding the production of ROS and the general redox-sensitive targets of cell signalling cascades.